Emerging and Conventional Pathways for Sustainable Ammonia Production: Technology Readiness, Economics, and Environmental Performance

Abstract

Ammonia is an essential high-volume chemical for fertilizer production and other industrial applications, and it is increasingly considered a potential energy carrier; however, its conventional manufacture remains highly energy- and carbon-intensive because it relies predominantly on fossil-based Haber–Bosch (HB) synthesis. This review compares sustainable ammonia-production pathways through the linked dimensions of technology readiness, environmental performance, and economic plausibility across renewable-H₂ HB, biomass- and waste-derived HB routes, electrochemical pathways, photocatalytic and photoelectrochemical systems, plasma-assisted synthesis, biological routes, and chemical looping ammonia synthesis. The analysis reveals a clear divide between pathways that benefit from established industrial infrastructure and those that still depend on unresolved catalytic, materials, or systems-level advances. Renewable-H₂ Haber–Bosch emerges as the most broadly scalable near-term option for large-scale ammonia decarbonization because it combines the highest maturity among low-carbon routes with the strongest techno-economic and life-cycle evidence base. Biomass- and waste-derived Haber–Bosch pathways may become cost-competitive regional complements when low-cost local residues, organic waste, or biomethane is available, feedstock logistics are favorable, and carbon, waste-treatment, or negative-emission credits are included. Overall, sustainable ammonia production is likely to advance through a portfolio of pathways, with near-term progress led by renewable-H₂ HB and longer-term development dependent on improved reactor integration, harmonized assessment methods, and scalable validation.

1. Introduction

Burning fossil fuel resources directly causes greenhouse gas (GHG) emissions (Figure 1) and contributes to increases in the Earth’s surface temperature [1]. Therefore, exploring clean alternatives for replacing fossil fuels has attracted significant attention. Among different solutions, ammonia is recognized as one of the key compounds in industrial and agricultural applications. From a broader chemical-industry perspective, ammonia is also one of the highest-volume commodity chemicals and is often reported among the top-produced chemicals worldwide (commonly ranked just after sulfuric acid by tonnage) [2]. It can be employed in diverse domains, including synthetic fertilizers, water treatment, refrigeration, mining, medicine, and textiles [3]. Moreover, ammonia is widely considered a practical hydrogen carrier for long-distance transport because it can be liquefied and stored under relatively mild conditions (moderate pressure or refrigeration), which simplifies bulk storage and shipping compared with hydrogen [3]. Ammonia can be produced through different feedstocks, the most important of which are shown in Figure 2 [4]. As illustrated, ammonia production is currently dependent on fossil sources (especially natural gas), with its associated CO₂ emissions, accounting for about 450 Mt CO₂/year of direct emissions [5]. Furthermore, Figure 3 shows projected ammonia demand from 2024 to 2034, broken down by region; overall demand rises markedly over time, with North America and Africa contributing a large share of the total increase [6]. Accordingly, shifting towards renewable energy instead of conventional fuels is essential for ammonia production to alleviate CO₂ emissions and meet the growing demand [7].

Among different renewable resources, wind and solar have attracted considerable attention, but they face challenges such as site selection and intermittency. In contrast, biomass can be introduced as one of the most promising renewable resources, widely employed in electricity and hydrogen production [9], with lower life-cycle CO₂ emissions compared to conventional fuels [10]. In this regard, various studies have investigated different techniques to produce ammonia, including bio-ammonia production through algae, wastewater recovery, and photocatalytic and electrocatalytic approaches [11]. Additional studies have proposed technologies such as carbon capture and storage (CCS) to decrease CO₂ emissions and reduce methane leakage [12]. Each approach should be evaluated through life cycle assessment (LCA) and techno-economic analysis to determine its feasibility, flexibility, and environmental impacts [13].

LCA is the most comprehensive tool for investigating the potential environmental impacts of all stages across a product’s life cycle. This method enables researchers to identify subprocesses with the highest environmental burden and improve systems to minimize overall impacts. According to ISO 14044 [14] standards, LCA consists of four stages: goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and interpretation [15]. In this regard, Bicer et al. [4] introduced ammonia production via electrolysis for hydrogen generation combined with the Haber–Bosch process for ammonia synthesis, employing four different resources for powering the system: nuclear, hydropower, municipal waste, and biomass. Similarly, Zhang et al. [16] investigated the resource consumption and environmental footprints of coal-to-ammonia technology from cradle to gate. Clearly, life cycle assessment plays a crucial role in analyzing the sustainability of chemical production industries.

Despite the rapidly growing literature on sustainable ammonia synthesis, the field remains fragmented across pathway-specific reviews that often focus on individual reaction classes, catalyst families, or selected process concepts in isolation. Recent high-quality roadmaps and reviews have provided important insights into ammonia-sector decarbonization scenarios, catalyst development, electrochemical nitrogen reduction, nitrate-to-ammonia conversion, light-driven routes, and green ammonia synthesis more broadly. However, these studies often emphasize either industrial transition pathways or specific reaction families, rather than comparing the full portfolio of ammonia-production routes using a common readiness, environmental, and economic assessment structure. This review addresses that gap by providing an integrated and critical assessment of the major sustainable ammonia pathways, including renewable-H₂ Haber–Bosch, biomass- and waste-derived Haber–Bosch, direct electrochemical N₂ reduction, photocatalytic and photoelectrochemical synthesis, plasma-assisted synthesis, chemical looping ammonia synthesis, biological routes, indirect electrochemical routes, and nitrate/nitrite-to-ammonia electroreduction. The unique contribution of this review is a unified TRL–environment–cost decision framework that compares these pathways not only by reaction chemistry, but by their realistic deployment readiness, environmental trade-offs, economic barriers, and most plausible near-, medium-, and long-term roles.

2. Review Scope and Methodology

This work was conducted as a structured critical narrative review of sustainable ammonia-production pathways. The aim was not only to summarize the principal reaction routes reported in the literature, but also to compare them through a consistent evaluative framework based on technology readiness, environmental performance, and economic plausibility. This review, therefore, focuses on studies that provide mechanistic insight, representative process configurations, life-cycle evidence, techno-economic results, or clear indicators of practical development status. The scope covers the major low-carbon and emerging ammonia pathways discussed in this article, including renewable-H₂ Haber–Bosch, biomass- and waste-derived Haber–Bosch routes, direct and indirect electrochemical approaches, photocatalytic and photoelectrochemical systems, plasma-assisted synthesis, biological pathways, and chemical looping ammonia synthesis.

The literature survey was performed using major scientific databases, including Scopus, Web of Science, Google Scholar, and publisher platforms such as ScienceDirect, with additional backward and forward citation tracking applied to identify influential earlier studies and recent developments. Search strings were constructed by combining pathway-specific and assessment-related keywords, including terms such as “ammonia synthesis”, “green ammonia”, “renewable hydrogen Haber-Bosch”, “biomass ammonia”, “waste-to-ammonia”, “electrochemical ammonia synthesis”, “nitrate reduction to ammonia”, “photocatalytic ammonia”, “photoelectrochemical nitrogen reduction”, “plasma-assisted ammonia”, “chemical looping ammonia synthesis”, “technology readiness level”, “life cycle assessment”, and “techno-economic analysis”. The literature survey emphasized recent studies in sustainable ammonia production while also incorporating earlier landmark publications that are essential for understanding the development of the major ammonia-synthesis pathways and their assessment frameworks. The search process was iterative rather than strictly linear, because additional keywords and landmark references were identified during full-text reading of review articles and primary studies.

The review prioritized peer-reviewed journal articles, authoritative review papers, and, where relevant for technology-readiness evaluation, credible industrial or demonstration-stage reports. Studies were included when they contributed at least one of the following: (i) description of the reaction principle or process configuration for a relevant ammonia-production pathway; (ii) quantitative environmental assessment, especially life-cycle greenhouse-gas or energy results; (iii) techno-economic metrics such as production cost, levelized cost, or key cost drivers; or (iv) evidence relevant to technology-readiness classification, such as laboratory validation, prototype demonstration, pilot operation, or commercial deployment. Duplicative studies, articles lacking sufficient technical detail, and sources not directly relevant to pathway assessment were excluded. Because the objective of this work was comparative synthesis rather than statistical meta-analysis, study selection also considered conceptual importance and relevance to the cross-pathway evaluation.

Data extraction and synthesis were structured to support a consistent cross-pathway comparison. For each pathway, information was compiled on reaction principle, representative feedstocks and process conditions, maturity indicators, environmental evidence, economic evidence, and major technical limitations. This information was then synthesized using a consistent evaluative framework in which all pathways were assessed through three linked dimensions: technology readiness, environmental performance, and economic plausibility. Technology readiness was evaluated using the nine-level TRL framework, with emphasis placed on the maturity of the integrated process rather than only on individual materials or catalyst performance. Environmental performance was interpreted primarily through life-cycle indicators reported in the literature, especially global warming potential, while recognizing that several studies used different system boundaries, electricity assumptions, co-product treatments, and functional units. Economic plausibility was interpreted in a similarly critical manner, distinguishing between conceptual process estimates, route-specific techno-economic analyses, and values derived under optimistic future-performance assumptions. Finally, each pathway was interpreted in terms of its likely role in the transition toward low-emission ammonia production, namely as a near-term deployable option, a region-specific or niche alternative, or a longer-term emerging technology (Figure 4).

Because the available literature is heterogeneous, the comparative results presented in this review should be interpreted as critical cross-pathway benchmarks rather than as fully harmonized one-to-one rankings. Wherever possible, environmental values were discussed on a common ammonia-production basis and economic values were compared using reported ammonia-production costs; however, exact comparability was often limited by differences in methodological assumptions, scale, energy source, plant integration, and downstream separation requirements. For this reason, the analysis places equal emphasis on the strengths and limitations of the available evidence, allowing the review to distinguish between pathways that are scientifically promising, regionally attractive, or realistically positioned for near-term deployment.

3. Ammonia Synthesis Methods

The primary goal of this section is to provide a comprehensive overview of the various technologies available for ammonia production. It provides a systematic overview of both conventional and emerging approaches, focusing on their fundamental principles, feedstock requirements, energy consumption, and environmental impacts. Therefore, it can help to build a foundation for understanding the current technological landscape, identifying the limitations of traditional processes, and exploring alternative routes that align with sustainability and decarbonization goals.

3.1. Haber–Bosch Ammonia Synthesis

More than 96% of ammonia is produced through the Haber–Bosch process [17]. This method was first invented in the early 20th century by Fritz Haber and Carl Bosch, who were German inventors. At first, ammonia production was mainly used for producing explosives during World War I. Its additional properties, particularly its role in improving agricultural productivity as a fertilizer and its function as a chemical intermediate in the chemical industry, have also attracted significant attention from researchers [18]. Generally, during this process, ammonia is produced from hydrogen, which is usually provided from fossil fuel resources, and nitrogen supplied via air separation, whose reaction is shown in Equation (1) [19]. The required ratio of hydrogen and nitrogen is stoichiometrically 3:1, which is commonly used as the feed ratio [19]. This reaction happens in the presence of metal-based catalysts, which are usually magnetite (Fe₃O₄) or Wustite (FeO).

$${\mathrm{N}}_2+3{\mathrm{H}}_2\to 2{\mathrm{NH}}_3\Delta H°=-92.44\mathrm{kJ}{\mathrm{mol}}^{-1}$$

(1)

Necessary pressure and temperature for this process are in the range of 15–25 MPa and 400–450 °C, respectively, because of the strong triple bond of nitrogen [20]. The overall conversion of this process can reach 97% by recycling unreacted gases. Under conventional HB conditions, the single-pass conversion is typically on the order of 10–20%, whereas overall conversion can exceed 95% due to continuous NH₃ removal and recycle of unreacted gases. The general schematic of the HB process is depicted in Figure 5.

Reaction pathways and equipment used in this process differ by feedstock selection. The majority of worldwide ammonia is produced from natural gas, in which hydrogen is produced through steam methane reforming and water–gas shift reactions, illustrated in the following equations [21]:

$${\mathrm{CH}}_4+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathrm{CO}+3{\mathrm{H}}_2\Delta H°=206\mathrm{kJ}{\mathrm{mol}}^{-1}$$

(2)

$$\mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CO}}_2+{\mathrm{H}}_2\Delta H°=-41.2\mathrm{kJ}{\mathrm{mol}}^{-1}$$

(3)

It is estimated that 3 to 5% of the world’s natural gas resources are allocated to the HB process, so CO₂ emissions from this process are significant, which are in the range of 1.6 to 2.86 tons of CO₂ per ton of ammonia production [22], contributing to 1.4% of global CO₂ emissions [23]. Therefore, replacing alternative resources for providing hydrogen are required. Another disadvantage of this process is its high energy consumption, which is approximately 60 GJ per ton of ammonia produced. The optimization efforts in the recent decade, such as implementing large centrifugal compressors, optimal heat integration, and efficient hydrogen recovery, can decrease the required input energy to the range of 27.4 to 31.8 GJ per ton of ammonia produced and increase the energy efficiency of the system to about 65% [17]. Furthermore, other improvements on developing the synthetic catalyst for ammonia production have been performed. It was observed that Ru-based catalysts can be a promising alternative to iron-based catalysts because they can operate under atmospheric pressure at a temperature of 300 to 400 °C with a higher conversion rate [24]. Despite this significant merit, ammonia produced from this process has a very low partial pressure, causing difficult condensation in the separation process. Although this issue can be solved by increasing the pressure to 2–3 MPa, overall capital cost is higher than the traditional HB approach [17]. Therefore, less than 5% of the world’s ammonia production is allocated to the ammonia synthesis with Ru-based catalysts. Nitrogen can be partially converted, ranging from 25% to 35% in a single pass through the catalyst because the ammonia synthesis is restricted by thermodynamic equilibrium [19]. Therefore, industrial-scale adiabatic ammonia reactors typically consist of three catalyst beds with cold gas injected between each stage to control temperature and prevent excessive heat buildup. This quench-cooling approach effectively removes heat between the catalyst layers to maintain optimal reaction conditions [25].

As noted previously, improving sustainability and reducing CO₂ emissions may be possible by replacing fossil feedstocks with renewable resources to provide the required hydrogen [26]. In this regard, biomass gasification and water electrolysis are discussed as promising technologies for this purpose. In power-to-ammonia, hydrogen is produced by water electrolysis fueled by renewable electricity [27]. The required nitrogen is provided from air separated through pressure swing adsorption and/or cryogenic distillation [28]. The first pilot plant was established at the West Central Research and Outreach Center at Minnesota, USA in 2013. The energy efficiency of this technology is assessed in the range of 50 to 60%, which is lower than the conventional HB processes because of higher energy consumption and energy losses in water electrolysis technology, such as atmospheric and high-pressure alkaline or proton exchange membrane (PEM) electrolysis employed for hydrogen production. It was shown that CO₂ emissions can be reduced from 1.5 to 0.38 tonnes of CO₂ per ton of ammonia produced when the required hydrogen is supplied through water electrolysis [4]. Hill et al. [17] concluded that CO₂ emissions can be reduced in conventional HB processes by 78% through supplying the energy required from renewable resources [29]. Between various technologies used for the electrolyzer, solid-oxide electrolysis (SOE) at high temperature offers more promising integration with the ammonia synthesis process, compared with alkaline and PEM electrolysis, because of effective heat integration with ammonia synthesis and higher electrical efficiency [30]. In this regard, an SOE cell with high operating temperature and energy efficiency above 70% is currently introduced for producing ammonia by Haldor Topsoe, where the required nitrogen is supplied through air separation at the cathode. This technology reduces power consumption compared to the conventional HB process.

Biomass can also be considered as the promising candidate for replacing fossil fuels because of its availability and life-cycle carbon neutrality [31]. Practically, it is logical to site gasification plants close to biomass sources because of the high cost of feedstock or gas product transportation [32]. Syngas, which mainly consists of H₂, CO, CO₂, and CH₄, is produced through biomass gasification. To increase the hydrogen content, the syngas is passed through the water–gas shift reactor. Then, acid gas removal and methanation processes are employed to obtain hydrogen with higher purity [33]. Generally, the efficiency of the biomass-to-ammonia processes is less than 50% [34]. Florez-Orrego et al. [35] compared the conventional ammonia production process based on natural gas with biomass-based ammonia production. The results demonstrated that the biomass-based process had an energy efficiency of about 41.3%, while the energy efficiency of natural-gas-based ammonia production was 65.8%. Among various technologies employed for the gasification process, the entrained-flow gasifier has attracted more attention in the recent decade [36] because of its higher reaction rate, more efficient biomass conversion with lower methane and CO₂, and no tar due to high working temperature and pressure. Also, a large air separation unit is required to provide the necessary oxygen used as the gasification agent, therefore also providing the nitrogen source for ammonia production. In this regard, the biomass-based production of ammonium nitrate fertilizer was investigated through life cycle assessment from cradle to gate [37]. The results demonstrated that by using biomass as the main feedstock, the global warming potential can be decreased by about 30%. In terms of economic analysis, the study about biomass-based ammonia production showed that the cost of the proposed system is competitive with the current market level when including carbon tax, while it can reduce 65% of GHG emissions compared to natural-gas-based ammonia plants [38].

These recent studies demonstrate that many endeavors are underway to make the renewable-based HB process feasible at a commercial scale. However, it still requires continuous effort on enhancing energy efficiency, especially the water electrolysis technologies, improving reliability, and being more cost-competitive with fossil-fuel-based systems for synthesizing green ammonia.

3.2. Electrochemical Ammonia Synthesis

To facilitate the ammonia synthesis process and overcome the energy-intensive and severe conditions of the HB process, several approaches have been developed to produce ammonia under mild conditions, including chemical looping, electrochemical, photochemical, photoelectrochemical, and plasma-assisted methods. Among these, electrochemical ammonia synthesis has attracted significant attention as a promising substitute for HB because it can operate under mild conditions, offers the potential for near-zero CO₂ emissions, and can convert renewable electricity into chemical energy [39].

In electrochemical systems, the nitrogen reduction reaction (NRR) competes with the hydrogen evolution reaction (HER) at the cathode. For acidic and basic electrolytes, the global reactions are typically written as follows [40]:

• Acidic electrolytes:

$${\mathrm{N}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to 2{\mathrm{NH}}_3{E}^0=0.148{\mathrm{V}}_{\mathrm{RHE}}(pH=7)$$

(4)

$$2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{E}^0=0{\mathrm{V}}_{\mathrm{SHE}}(pH=0)$$

(5)

• Basic electrolytes:

$${\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}\to 2{\mathrm{NH}}_3+6{\mathrm{OH}}^{-}{E}^0=-0.736{\mathrm{V}}_{\mathrm{SHE}}(pH=14)$$

(6)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2{\mathrm{OH}}^{-}{E}^0=-0.828{\mathrm{V}}_{\mathrm{SHE}}(pH=14)$$

(7)

The NRR encounters two main challenges. First, nitrogen gas is very stable due to its strong triple bond and poor solubility in water, making it hard to activate both thermodynamically and kinetically [41]. Additionally, NRR competes with HER, which is faster and more favorable because NRR requires six electrons and protons, whereas HER only needs two, making NRR much less selective and more energy-intensive [22]. These issues limit its efficiency and industrial potential, driving ongoing research into better catalysts, cell designs, and improved selectivity [42].

In electrochemical systems, nitrogen gas (N₂) is converted into ammonia through three basic steps: first, N₂ attaches to the surface of a catalyst; second, the strong triple bond in N₂ is weakened and broken gradually while hydrogen atoms are added; and third, the produced ammonia leaves the surface. Unlike the Haber–Bosch process, where N₂ first splits on the catalyst surface, electrochemical nitrogen reduction often adds hydrogen to N₂ step by step, forming N₂H_x intermediates, and the N–N bond weakens gradually as hydrogenation proceeds [43]. The overall electrochemical model for ammonia synthesis is illustrated in Figure 6. There are several types of associative pathways. In the distal pathway, hydrogen is first added to the nitrogen atom that is farthest from the catalyst [40]. In the alternating pathway, hydrogen atoms are added one by one to each nitrogen in turn [44]. The enzymatic pathway involves both nitrogen atoms being hydrogenated while the N₂ molecule is attached side-on to the catalyst [44]. For most electrochemical systems, the hardest part is starting the reaction (activating the N₂ molecule). However, recent research has found that using special catalysts like cobalt clusters can make it easier to start the reaction by changing the slowest step from nitrogen activation to a quicker step involving proton addition [45].

Several novel approaches do not require N₂ remaining adsorbed on a catalyst. In the Mars–van Krevelen (MvK) mechanism, nitrogen from inside a metal nitride material is hydrogenated to form ammonia, and then nitrogen gas from the air fills in the missing spots. This idea is still under study as several researchers are skeptical of its validity [46,47]. Another approach is the lithium-mediated process, where lithium metal reacts directly with nitrogen to form a compound (lithium nitride) that can easily turn into ammonia and lithium salt, which can be recycled to continue the reaction [22]. A representative two-step sequence is [48]:

$${\mathrm{N}}_2+6\mathrm{Li}\to 2{\mathrm{Li}}_3\mathrm{N}\Delta H°=-165\mathrm{kJ}{\mathrm{mol}}^{-1}$$

(8)

$${\mathrm{Li}}_3\mathrm{N}+3{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NH}}_3+3\mathrm{LiOH}\Delta H°=-486.15\mathrm{kJ}{\mathrm{mol}}^{-1}$$

(9)

Figure 6. Schematic of electrochemical model for ammonia synthesis. Reproduced from [49], under the Creative Commons CC BY 4.0 license.

Figure 6. Schematic of electrochemical model for ammonia synthesis. Reproduced from [49], under the Creative Commons CC BY 4.0 license.

3.2.1. Overview of Catalysts in Electrochemical Ammonia Production

Noble metals, such as gold, palladium, ruthenium, and rhodium, have been widely studied for their catalytic potential in NRR [50]. These elements are known for their high stability and strong catalytic properties [51]. However, their high cost and limited availability require strategies to maximize efficiency while minimizing material usage. To address this, researchers often work with noble metals in the form of nanostructures, single atoms, or alloys to improve surface area and atomic utilization. For example, nanoporous palladium hydride (np-PdH) was developed using a dealloying process of Al₈₀Pd₂₀. This material was shown to participate actively in NRR, with hydrogen atoms in the lattice structure playing a key role in electron transfer and intermediate adsorption. Similarly, modifying the crystal phase of palladium–copper alloys, from a face-centered cubic to a body-centered cubic structure, has led to enhanced nitrogen binding and greater ammonia yield [52]. In these studies, improvements were closely linked to electronic structure tuning, particularly the upward shift of the d-band center, which increases the interaction between nitrogen molecules and the catalyst surface. These approaches highlight how subtle changes in material design can dramatically enhance performance.

Non-noble metals like molybdenum and iron have gained significant attention due to their lower cost and natural occurrence in nitrogen-fixing enzymes such as nitrogenases. These metals offer a biologically inspired approach to catalyst design. Molybdenum, for instance, has been incorporated into materials such as MoS₂, Mo₂C, and thin films, with structural modifications aimed at exposing more active sites [53]. In pristine MoS₂, only edge-plane molybdenum atoms are catalytically active, while the basal planes are inactive due to sulfur coverage. To overcome this, researchers introduced cobalt atoms to generate sulfur vacancies, enabling more Mo atoms to participate in NRR [54]. The doped material showed significantly lower energy barriers for nitrogen activation. A similar concept was applied using single iron atoms anchored onto the MoS₂ surface [55]. These single-atom protrusions created strong localized electric fields that helped break the stable N₂ bond more easily. These approaches not only enhanced ammonia production but also demonstrated how they can effectively unlock the full potential of non-noble materials for nitrogen fixation.

Iron-based materials are among the most studied NRR catalysts due to their abundance and effectiveness. Fe₃O₄, FeOOH, and iron–nitrogen carbon complexes have all been explored [51]. One of the most promising designs is a Janus-type Fe–SnO₂ catalyst, which simultaneously supports ammonia synthesis and nitrogen oxidation reactions [56]. This dual functionality is enabled by combining lattice-doped Fe with Fe single atoms anchored to oxygen vacancies. Computational studies showed that the anchored Fe atoms were primarily responsible for NRR, while the lattice Fe improved conductivity and overall catalytic performance [57]. Meanwhile, copper-based catalysts are less commonly used due to their lower nitrogen adsorption strength and tendency to promote side reactions [58]. However, researchers have found ways to enhance copper’s activity by modifying its electronic structure. One strategy involves loading copper nanoparticles onto semiconducting nanostructures, which draw electrons from copper and create an electron-deficient state [58]. This not only increases nitrogen molecule polarization but also reduces hydrogen evolution. In another case, single copper atoms supported on nitrogen-doped carbon showed promising results in both acidic and alkaline environments, demonstrating that even less reactive metals like copper can be improved with the right support and design [59].

Rhenium has also emerged as a promising candidate for NRR, although it is less common than iron or molybdenum [60]. Researchers have modified rhenium-based materials using dopants such as iron or manganese, or by introducing structural defects like selenium vacancies [61]. These changes enhance the interaction between nitrogen molecules and the catalyst surface while also improving electrical conductivity. For example, doping rhenium disulfide with manganese led to significant charge transfer, creating strong metal–metal interactions and hybridized electronic states that facilitate nitrogen adsorption and reduction [60]. In parallel, early transition metals like scandium, yttrium, and titanium have shown potential due to their strong affinity for nitrogen atoms [62]. Single atoms of these metals, anchored onto nitrogen-doped carbon frameworks, have achieved notable performance in NRR [63]. Titanium-based catalysts, especially TiO₂ and Ti₃C₂Tx (MXene), have also been extensively studied [64]. Modifying TiO₂ by generating oxygen vacancies or doping with heteroatoms such as zirconium and iron has been shown to induce Ti³⁺ states, which are active for NRR [65]. With MXenes, strategies such as removing inactive surface groups or growing oxide layers have been used to increase the number of active edge sites, further enhancing NRR efficiency [66].

In addition to transition metals, main group elements like antimony and phosphorus have been explored for NRR catalysis [67]. For example, a composite material containing antimony phosphate and phosphorus-doped carbon demonstrated high ammonia production and good selectivity in neutral electrolytes; however, stability remained a challenge in acidic environments [67]. Metal-free catalysts, such as doped carbon-based materials and covalent organic frameworks (COFs), are also gaining traction due to their low cost and the potential to reduce impacts associated with critical-metal extraction and refining by avoiding scarce or toxic metals [67]. One notable example is fluorine-doped porous carbon derived from MOFs [67]. Fluorine’s high electronegativity creates Lewis acid sites on nearby carbon atoms, which can effectively bind nitrogen molecules while repelling protons, thus suppressing hydrogen evolution. Similarly, doping carbon with chalcogen elements like sulfur, selenium, or tellurium alters the charge distribution and magnetic properties of carbon atoms, enhancing their interaction with nitrogen [68]. Black phosphorus, due to its electronic structure similarity to nitrogen, has been engineered into few-layer nanosheets and quantum dots, both of which offer high surface area and strong N₂ binding. These results demonstrate that non-metal-based materials can compete with metals in catalytic performance when properly engineered [69].

Despite the diversity of catalysts developed for electrochemical nitrogen reduction, most still fall short of the performance needed for practical applications. Even the best-performing systems show ammonia yield rates and Faradaic efficiencies far below commercial targets. Furthermore, most studies rely on single-point isotopic labeling for validation, which limits the reliability of the data. To advance the field, researchers are exploring integrated strategies, such as combining structural tuning and surface modifications to boost activity and selectivity. There is also a growing focus on tailoring the electronic structure through careful selection of elemental composition, as well as improving control over catalyst synthesis at the atomic level. In addition, properties like conductivity, long-term stability, and cost must be considered alongside intrinsic catalytic activity. Finally, better cell design, electrode architecture, and local environment control are crucial to unlocking the full potential of these catalysts.

3.2.2. Overview of Electrolytes in Electrochemical Ammonia Synthesis

In electrochemical ammonia synthesis, the selection of electrolyte plays a key role in influencing both the efficiency and selectivity of the reaction. Electrolytes not only affect ionic conductivity and the reaction environment, but also impact how effectively nitrogen molecules are activated and reduced on the catalyst surface. Typically, a suitable electrolyte is paired with an electrocatalyst-coated working electrode to facilitate nitrogen reduction under mild conditions by lowering the activation energy [18]. A potentiostat is used to apply the desired voltage, allowing precise control of the electrochemical conditions while monitoring current and reaction time. Choosing the right electrolyte ensures compatibility with the catalyst and supports stable and selective ammonia production under the applied potential [69].

Solid electrolytes are among the earliest materials explored for electrochemical ammonia synthesis and are capable of operating across a wide temperature range, from ambient temperature up to 800 °C. These systems typically use ceramic proton conductors that transfer protons from the anode to the cathode, where nitrogen is reduced to ammonia [70]. One of the pioneering demonstrations was by Marnellos [71], who used SrCe_0.95Yb_0.05O_3−δ as the proton-conducting material. At 750 °C and atmospheric pressure, the single-chamber cell achieved 78% hydrogen conversion to ammonia. However, the single-chamber design caused oxidation and reduced efficiency [72]. To overcome this issue, dual-chamber cells were developed, which offer better control of the redox environment at each electrode [73]. Despite improvements, solid electrolytes often suffer from low proton conductivity at moderate temperatures, requiring higher temperatures to reduce ohmic losses [73]. This, however, raises the risk of ammonia decomposition. To mitigate this problem, water has been introduced as a proton donor, reducing the risk of catalyst poisoning from hydrogen gas impurities and enhancing system robustness [70]. Advanced configurations using Pd or Ru electrocatalysts with proton-conducting ceramics and steam have achieved production rates up to 5.57 × 10⁻³ $\mathsf{\mu }$g cm⁻² h⁻¹ at 650 °C [74]. Researchers have also experimented with introducing low-melting-point carbonates salts of lithium (Li), sodium (Na), and potassium (K), into oxide matrices to form composite electrolytes that allow ammonia synthesis at lower temperatures (e.g., 400 °C) [75]. Still, the electrolyte thickness plays a critical role in performance; thinner membranes improve conductivity and reduce energy losses [76]. A thin-film design operating at 530 °C achieved a rate of 250.92 $\mathsf{\mu }$g cm⁻² h⁻¹ [77], but structural integrity becomes a concern at very low thicknesses.

Molten salts offer a unique approach to ammonia synthesis by operating at intermediate temperatures (300–500 °C) and enabling alternative reaction mechanisms [70]. A key innovation in this category is lithium-mediated nitrogen fixation [78]. Lithium, due to its strong reducing power, reacts readily with nitrogen gas even at ambient conditions to form lithium nitride (Li₃N), which can subsequently be hydrolyzed to produce ammonia [78]. Tsuneto et al. [79] were the first to demonstrate this route electrochemically, achieving 59% Faradaic efficiency under 5 MPa nitrogen pressure. However, these systems often rely on non-aqueous electron donors to reduce Li⁺ to metallic lithium, which complicates the process [79]. Murakami et al. [80] used a eutectic salt mixture (LiCl–KCl–CsCl) and directly fed nitride ions into the electrolyte to address pressure and electron donor limitations. The results indicated that ammonia production at atmospheric pressure and 400 °C, with rates up to 201.96 $\mathsf{\mu }$g cm⁻² h⁻¹ and efficiency of 27%, can be achieved. Methane and steam have also been explored as hydrogen sources [80]. However, methane decomposition at lower temperatures (e.g., 400 °C) was inefficient, limiting ammonia synthesis [80]. Pre-cracking methane or switching to steam has been shown to be more effective [80]. For example, steam-fed systems using molten salts achieved ammonia yields as high as 1224 $\mathsf{\mu }$g cm⁻² h⁻¹ at 300 °C [81]. A major challenge in this method is the corrosion of carbon anodes, which react with oxygen ions to form CO₂ [81]. To solve this issue, non-consumable materials like boron-doped diamond have been used as stable anodes [82]. Furthermore, McEnaney et al. [83] proposed a Li-mediated cycle with H₂ and H₂O, combining LiOH electrolysis, nitridation, and hydrolysis to produce ammonia with up to 88.5% efficiency. In this process, the reaction between Li and N₂ formed Li₃N, which then reacted with H₂O to form NH₃ and LiOH, as shown in Equations (8) and (9). The cycle can then be completed by reducing LiOH back to Li metal [83]. However, since lithium is molten at the required operating temperature (above 180 °C), effective collection strategies are needed. Kim et al. [78] addressed this by using a nickel substrate to anchor lithium and simplify the process. Figure 7 shows a schematic representation of a molten-salt electrochemical ammonia synthesis system.

Composite electrolytes combine multiple ionic conductors to enhance the mechanical stability, ionic conductivity, and overall performance of electrochemical ammonia synthesis systems [84,85]. These hybrid materials are designed to operate under high temperatures while resisting degradation and maintaining efficient ion transport [86]. A notable example is the work by Amar et al. [84], who developed a composite of LiAlO₂ and alkali metal carbonates (Li₂CO₃, Na₂CO₃, K₂CO₃) in a 1:1 weight ratio. This electrolyte, paired with a CoFe₂O₄ catalyst, facilitated ammonia synthesis from wet nitrogen and hydrogen at 400 °C. However, performance dropped significantly at 450 °C due to increased decomposition of ammonia, underscoring the importance of thermal balance. To improve performance, Amar et al. [85] used a cerium-based composite (Ce_0.8Sm_0.2O_2−δ with 30% carbonates), which achieved better proton conductivity and reached a peak ammonia production rate of 0.33 $\mathsf{\mu }$g cm⁻² h⁻¹ at 450 °C. Their findings also confirmed that raising the temperature improved performance up to a point, beyond which decomposition again became dominant [85]. In a separate study, Wang et al. [86] developed a composite of Y-doped ceria and binary phosphates (Ca₃(PO₄)₂ and K₃PO₄ in an 80:20 ratio) and used it to synthesize ammonia from natural gas and nitrogen at 650 °C, achieving a rate of 0.426 $\mathsf{\mu }$g cm⁻² h⁻¹. However, their gas analysis revealed that much of the methane remained unreacted, suggesting a need for improved catalytic activation or pre-treatment of the hydrocarbon feedstock [85].

Ammonia production at ambient temperature using aqueous electrolytes is gaining attention for its simplicity and cost-effectiveness [70]. In these systems, water acts both as the solvent and the hydrogen source. A PEM separates the anode and cathode, enhancing ammonia yield by minimizing oxidation losses [87]. The performance of these systems is highly dependent on the PEM properties, electrolyte pH, and the choice of electrocatalyst [87]. These low-temperature processes show significant potential for scalable, energy-efficient NH₃ synthesis, especially when integrated with renewable energy sources [70,87].

3.3. Photocatalytic Ammonia Synthesis

Ammonia production via photocatalytic nitrogen fixation has emerged as a promising alternative to conventional processes, especially for its potential as a sustainable method of producing ammonia using water as the hydrogen source, which has minimal environmental impact [40,88]. In contrast to the electrocatalysis approach, which depends on external voltage and electrodes to drive reactions, photocatalysis utilizes semiconductors that are activated by light energy [88]. When sunlight or ultraviolet light strikes the semiconductor, it absorbs photons, lifting electrons out of the valence band (VB) into the conduction band (CB) and leaving positively charged holes behind. This process is called charge-carrier separation [89]. These photo-excited electrons play a central role in reducing nitrogen molecules (N₂), while the holes facilitate the oxidation of water, enabling the simultaneous occurrence of both processes to yield ammonia [90]. The overall photocatalytic model for ammonia synthesis is demonstrated in Figure 8. Also, the main similarities and differences between the electro and photocatalytic ammonia synthesis are summarized in Figure 9. The main reactions involved in photocatalytic-based ammonia production can be expressed as follows [91]:

$${\mathrm{N}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to 2{\mathrm{NH}}_3{E}^0=0.092{\mathrm{V}}_{\mathrm{RHE}}$$

(10)

$$3{\mathrm{H}}_2\mathrm{O}\to \frac{3}{2}{\mathrm{O}}_2+6{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}{E}^0=1.229{\mathrm{V}}_{\mathrm{RHE}}$$

(11)

One of the core challenges in this approach lies in the nature of nitrogen gas itself. Being nonpolar and highly stable, N₂ possesses a triple bond with a very high dissociation energy (941 kJ mol⁻¹), making it exceptionally resistant to activation [93]. To weaken and eventually cleave this bond, electrons must be effectively transferred to nitrogen’s antibonding $\pi$ orbitals [94]. This electron transfer typically takes place at specific active sites on the photocatalyst surface, where nitrogen is adsorbed and undergoes successive protonation and reduction to form ammonia (Equation (10)) [95]. Therefore, the efficiency of ammonia synthesis largely depends on how well the catalyst facilitates charge carrier separation and transfer to nitrogen molecules [96].

For best performance, the photocatalyst should combine a small band gap that captures visible photons with strong electron–hole separation to minimize recombination, while also offering plenty of active sites where nitrogen can adsorb and react [97]. Titanium dioxide (TiO₂) has been one of the most studied materials due to its stability and cost-effectiveness [98]. However, its wide band gap limits light absorption [99]. To overcome this, strategies such as carbon and transition metal doping (e.g., Fe, Co, Mo, Ni) have been employed [100]. For instance, carbon-doped TiO₂ nanosheets with porous structures have shown significant improvements in ammonia production under visible light [99]. Similarly, Fe-doped catalysts have demonstrated enhanced charge separation, although with relatively modest conversion efficiency [101]. The strong nitrogen fixation ability of iron-based catalysts is mainly attributed to the higher electron concentration at the active site, where the additional electrons in the d-orbitals play a key role in facilitating the reaction [102].

Over the past decade, research into photocatalysts that outperform conventional metal-doped TiO₂ has expanded to a wider palette of semiconducting solids. Graphitic carbon nitride (g-C₃N₄), CdS, Ta₃N₅, BiVO₄, and, more recently, two-dimensional MXenes have all been evaluated as light harvesters for nitrogen reduction under ambient conditions [103]. A representative study by Liu et al. [103] illustrates this direction: when bulk and exfoliated g-C₃N₄ were impregnated with Ru and K, the resulting materials (Ru–K/B-g-C₃N₄ and Ru–K/E-g-C₃N₄) displayed markedly higher NH₃ formation rates than the parent carbon nitride, confirming the benefit of coupling well-dispersed noble-metal sites with a porous nitrogen-rich backbone. Utilizing sunlight for ammonia synthesis offers a sustainable alternative, but its success relies heavily on the development of well-designed and highly efficient photocatalysts. First, the electronic structure should enable absorption in the visible range (not only UV), because sunlight contains far more visible photons than UV; extending absorption into the visible therefore increases the usable solar photon flux and can substantially raise the achievable reaction rate under natural illumination. Second, once these photons are absorbed, electrons and holes must quickly separate and travel to the surface instead of recombining; only then can they participate in the redox chemistry that converts N₂ and water into NH₃. Third, the catalyst surface has to present an abundance of chemically receptive sites where nitrogen can anchor, accept electron density, and begin to relinquish its triple bond. Finally, the material must intrinsically discourage the competing hydrogen evolution reaction, ensuring that the photogenerated electrons preferentially reduce nitrogen rather than protons [104].

Parallel to progress in inorganic semiconductors, metal–organic frameworks (MOFs) have emerged as a versatile platform for both photocatalytic and electrocatalytic nitrogen fixation [90]. MOFs are crystalline lattices in which metal ions or clusters are bridged by multidentate organic linkers, generating large internal surface areas and highly ordered porosity [105]. These structural features lead to many Lewis-acidic metal sites that can bind nitrogen, while the organic linkers can be adjusted to improve light absorption or electronic conductivity [106]. Unsaturated metal nodes act as electrophilic sites that accept electron density from $\sigma$-orbitals of adsorbed nitrogen; back-donation from the metal into the ${\pi }^{*}$ orbitals of N₂ then weakens the formidable triple bond and initiates stepwise proton-coupled electron transfer to ammonia [94]. Because the inorganic metal centers and organic linkers in MOFs are modular, their structure can be adjusted after synthesis, commonly through ligand functionalization, metal substitution, or combining with other active materials, to improve their catalytic performance [96].

Initial demonstrations with pristine frameworks such as MIL-88(Fe), MIL-100(Fe), and MIL-101(Fe) confirmed that even unmodified MOFs possess measurable activity for N₂ reduction [107]. Subsequent ligand modification has extended the light absorption range and introduced new pathways for charge transfer. For example, amino-functionalized NH₂-MIL-125(Ti) showed effective ammonia production under simulated sunlight at room temperature, while thiol-modified UiO-66(SH)₂ exhibited similar improvements due to enhanced visible light absorption and faster electron transfer between zirconium sites and sulfur-containing linkers [108]. Moreover, heterojunctions formed by integrating MOFs with conductive carbon, metal sulfides, or other semiconductors accelerated interfacial charge separation, thereby suppressing electron–hole recombination and raising steady-state electron densities at the catalytic sites [109].

Although photocatalytic MOFs show great promise, several challenges still limit their practical use. Their tiny pores can make it difficult for nitrogen gas to reach the active sites deep inside the material, which reduces overall efficiency. In addition, many MOFs have poor electrical conductivity, making it harder for photogenerated charges to move freely and take part in the nitrogen reduction process. Producing MOFs often involves complex methods that require high temperature, high pressure, and harmful solvents, which are not ideal for large-scale production. To address these problems, researchers are exploring ways to introduce larger pores, improve charge transport by combining MOFs with materials like graphene or MXenes, and carefully tune the structure to allow better movement of nitrogen and electrons. Making MOFs in well-designed shapes, such as spheres, hollow particles, or pellets, can also improve their strength and make them easier to use in reactors, but this requires low-cost methods that do not harm their catalytic ability.

In the future, replacing the traditional HB process will depend on finding photocatalysts and electrocatalysts that are not only highly efficient and long-lasting but also made from materials that are inexpensive and widely available. MOFs are especially promising because their structure and chemistry can be easily adjusted. However, turning successful lab experiments into real-world applications will require significant improvements in how we make MOFs at a large scale and how well they conduct electricity. Moving forward, it will be necessary to combine experiments with computer modeling to better understand how the reaction works and how to design materials that can use sunlight or renewable electricity to produce ammonia without carbon emissions.

3.4. Photo-Electrocatalytic Ammonia Synthesis

Photoelectrochemical (PEC) technology combines principles from both photocatalysis and electrochemistry to enable environmentally friendly chemical transformations with reduced energy requirements [110]. This hybrid method is being explored for various sustainable applications such as splitting water [110], converting carbon dioxide [111], and capturing solar energy [112]. One of its emerging applications is in nitrogen fixation, where nitrogen gas (N₂) is converted to ammonia under light-driven conditions. The PEC nitrogen reduction reaction (PEC-NRR) has drawn attention as a potential alternative to the traditional HB process, offering the possibility of decentralized ammonia production using sunlight and water [113]. A typical PEC system designed for ammonia production includes several main components: a light-absorbing photocathode, an anode, an ion-conducting membrane (often a proton exchange membrane), a liquid electrolyte, and a nitrogen gas feed [114]. A DC power source is also used to apply bias across the electrodes [115]. During operation, the photocathode generates electron–hole pairs as electrons are excited from the valence band to the conduction band, leaving holes as electron vacancies within the semiconductor [116]. These excited electrons migrate to the surface of the photocathode, where they can interact with nitrogen molecules from the gas feed [117]. The holes remain as electron vacancies in the semiconductor and, driven by band bending and any applied bias, are transported within the solid to the oxidation site (typically the photoanode/oxidation catalyst interface), where they are consumed in the oxygen evolution reaction. This separation of charges is assisted by an internal electric field created at the semiconductor–electrolyte interface [118]. The relevant reactions are defined in Equations (10) and (11).

Once electrons reach the photocathode surface, they engage in the reduction of nitrogen molecules. The conversion of N₂ to NH₃ follows one of two main mechanisms: the associative or dissociative pathway [112]. In associative mechanisms, nitrogen molecules adsorb onto the photocathode and are hydrogenated step by step before the N≡N bond is completely broken. This can proceed via a distal path (where one nitrogen atom is fully hydrogenated before the second) or an alternating path (where hydrogen atoms are added alternately to each nitrogen atom) [112]. In the dissociative mechanism, the N≡N bond is first cleaved, and the resulting nitrogen atoms are then separately hydrogenated to form ammonia [119]. The differences between these two pathways are presented in Figure 10. These pathways highlight the complexity of breaking the strong triple bond in nitrogen and the importance of designing suitable active sites.

Despite its promise, PEC-based ammonia production still faces critical limitations. One major challenge is the inherent difficulty of activating nitrogen molecules, due to their strong triple bond and low solubility in water, which slows the reaction kinetics [120]. Another issue is competition with the HER, which can consume photogenerated electrons and reduce selectivity toward ammonia [121]. Additionally, the photocathode materials used in PEC cells often suffer from poor visible-light absorption, fast charge-carrier recombination, and instability under illumination [122]. Materials such as NiO, Cu₂O, and CuO have been studied due to their p-type semiconducting properties, but they typically exhibit limited efficiency due to issues like photocorrosion and low numbers of exposed catalytic sites [112]. To overcome these challenges, researchers have been developing advanced photocathode materials with better light absorption, stability, and catalytic activity. One approach involves constructing heterojunctions using MOFs or doping existing materials [123]. For example, coating Cu₂O with copper-based MOFs enhances ammonia production by creating more active sites and improving charge transfer [124]. Similarly, incorporating Ag-doped Ni-MOFs onto Cu₂O surfaces improves both the yield and Faradaic efficiency (FE) due to better charge separation and additional catalytic centers [125]. Other strategies include using semiconductors with engineered defects such as oxygen or nitrogen vacancies, which help in adsorbing and activating nitrogen molecules more effectively [126].

Recent studies have also explored novel materials like black silicon and black phosphorus for PEC-NRR applications. Black silicon, with its nanostructured surface, traps more sunlight and suppresses unwanted hydrogen production, making it more selective for ammonia formation [127]. When decorated with nanoparticles such as gold or silver, its performance further improves due to enhanced light absorption and catalytic activity through surface plasmon resonance (SPR) effects [128]. On the other hand, black phosphorus offers abundant catalytic sites and favorable electronic properties, making it a strong candidate for nitrogen reduction [112]. Combining photo- and electro-chemical forces, PEC technology offers a promising route for sustainable ammonia synthesis, but continued research is essential to optimize materials and design for real-world applications.

In conclusion, photoelectrochemical nitrogen reduction represents a promising route toward sustainable ammonia synthesis under ambient conditions. Unlike today’s industrial Haber–Bosch supply chain, where the carbon footprint is dominated by fossil-based hydrogen production (e.g., natural-gas reforming) and fossil energy used to provide heat and power, PEC-NRR uses sunlight to supply the driving energy, potentially lowering indirect CO₂ emissions when operated with low-carbon electricity and water as the proton source. To be viable for practical deployment, photocathode materials must satisfy several critical criteria. First, their conduction band edge must be more negative than the thermodynamic potential of nitrogen reduction ($-0.148{\mathrm{V}}_{\mathrm{RHE}}$), ensuring that photogenerated electrons possess sufficient energy to drive the N₂ to NH₃ conversion. Furthermore, efficient light absorption, particularly in the visible range, is necessary to generate an adequate population of charge carriers [129]. Equally important is the material’s ability to suppress charge recombination and support rapid charge transport, ensuring that electrons reach the active sites for the nitrogen reduction reaction before energy losses occur [130]. High catalytic activity and selectivity are also essential, given the inherent difficulty of N₂ activation and the competing hydrogen evolution reaction [131]. Photocathodes must expose sufficient active sites for N₂ adsorption and activation, while minimizing HER to improve Faradaic efficiency. Long-term operational stability is another key factor, requiring materials that are resistant to photocorrosion and maintain strong adhesion to their substrates. Strategies to address these requirements include engineering heterojunctions to enhance charge separation, introducing co-catalysts to improve reaction kinetics, and constructing 3D nanostructured architectures to increase surface area and light harvesting [132].

In addition to material design, efforts are being made to improve the reliability of NH₃ detection, as contamination or side reactions can produce signals that look like ammonia, even when real ammonia is not produced [133]. Tandem PEC systems and bias-free configurations that operate solely on solar energy, referred to as unassisted PEC, are also under development to improve overall energy efficiency [134]. Furthermore, PEC nitrate reduction is gaining attention as an alternative pathway with more favorable kinetics and the added benefit of addressing nitrate pollution. Overall, while PEC-NRR is still in its early stages, continued material innovation and system optimization hold strong potential to enable scalable green ammonia production.

3.5. Plasma-Assisted Ammonia Synthesis

Plasma-assisted ammonia synthesis, particularly through non-equilibrium low-temperature plasma, has emerged as an alternative route for ammonia production under comparatively mild operating conditions [135]. In this approach, plasma provides energetic electrons and reactive species that can activate otherwise inert molecules such as nitrogen, thereby facilitating ammonia formation without requiring the severe temperatures and pressures characteristic of the conventional Haber–Bosch process [136]. Low-temperature plasma contains a mixture of energetic electrons, ions, radicals, photons, and neutral particles, creating an active medium that can break molecular nitrogen’s strong triple bond [136]. When powered by renewable energy sources such as wind or solar, plasma-assisted systems provide a carbon-neutral pathway for ammonia production [137]. This makes the process not only energy-efficient but also compatible with decentralized and on-demand production, particularly suitable for small-scale applications where traditional methods fall short [138]. A variety of plasma types, including dielectric barrier discharge (DBD), plasma jets, and gliding arc discharges, have been investigated to improve reaction efficiency and catalyst compatibility.

Plasma-assisted ammonia synthesis can proceed with or without the involvement of a catalyst. In the non-catalytic or plasma-only route, reactions take place solely in the gas phase [139]. When a plasma discharge is applied to a mixture of nitrogen and hydrogen (or water), the energetic electrons in the plasma activate the N₂ and H₂ molecules by exciting or dissociating them into reactive species such as vibrationally or electronically excited N₂ (${\mathrm{N}}_2^{*}$), ionized nitrogen (${\mathrm{N}}_2^{+}$), atomic nitrogen (N), and corresponding hydrogen species like ${\mathrm{H}}_2^{+}$ and H atoms [138]. These reactions can be expressed in Equations (12)–(17):

$${\mathrm{N}}_2+{\mathrm{e}}^{-}\to {\mathrm{N}}_2^{*}+{\mathrm{e}}^{-}$$

(12)

$${\mathrm{N}}_2+{\mathrm{e}}^{-}\to {\mathrm{N}}_2^{+}+2{\mathrm{e}}^{-}$$

(13)

$${\mathrm{N}}_2+{\mathrm{e}}^{-}\to 2\mathrm{N}+{\mathrm{e}}^{-}$$

(14)

$${\mathrm{H}}_2+{\mathrm{e}}^{-}\to 2\mathrm{H}+{\mathrm{e}}^{-}$$

(15)

$${\mathrm{H}}_2+{\mathrm{e}}^{-}\to {\mathrm{H}}_2^{+}+2{\mathrm{e}}^{-}$$

(16)

$${\mathrm{N}}_2^{*}\to 2{\mathrm{N}}^{*}$$

(17)

These reactive intermediates then undergo gas-phase reactions. For instance, nitrogen atoms or excited N₂ species can react with hydrogen atoms to form NH, which is subsequently hydrogenated to NH₂ and finally NH₃ [140], which are described in Equations (18)–(22) [141]. This entire process happens in the gas phase without the aid of a catalyst, and while feasible, it generally suffers from low efficiency, limited selectivity, and undesired side reactions, including NH₃ decomposition [142].

$$\mathrm{N}+\mathrm{H}\to \mathrm{NH}$$

(18)

$${\mathrm{N}}_2^{*}+2\mathrm{H}\to 2\mathrm{NH}$$

(19)

$$\mathrm{N}+{\mathrm{H}}_2\to \mathrm{NH}+\mathrm{H}$$

(20)

$$\mathrm{NH}+\mathrm{H}\to {\mathrm{NH}}_2$$

(21)

$${\mathrm{NH}}_2+\mathrm{H}\to {\mathrm{NH}}_3$$

(22)

In contrast, plasma catalytic pathways utilize a catalyst to enhance the efficiency and selectivity of ammonia synthesis [139]. The plasma serves as an activation source, generating reactive nitrogen and hydrogen species that adsorb onto the catalyst surface [143]. The synergy between the plasma and the catalyst significantly improves ammonia production by lowering energy barriers and stabilizing intermediate species [144]. Typically, the plasma first activates N₂ and H₂ into reactive forms (N^*, ${\mathrm{N}}_2^{+}$, ${\mathrm{H}}^{*}$, etc.), and these species adsorb onto the catalyst surface, where stepwise hydrogenation occurs [140]. The presence of the catalyst allows for more controlled reaction pathways and offers two main surface reaction mechanisms: the Eley–Rideal (E-R) and the Langmuir–Hinshelwood (L-H) mechanisms [145]. In the E-R mechanism, a gas-phase reactive species directly reacts with an adsorbed species on the catalyst surface [145]. In the L-H mechanism, both reactive species are adsorbed onto the catalyst surface before reacting to form intermediates like NH and NH₂ [144]. These mechanisms can be described in Equations (23)–(29).

• E-R mechanism:

$${\mathrm{N}}_2\left(\mathrm{g}\right)\to 2{\mathrm{N}}^{*}$$

(23)

$${\mathrm{N}}^{*}+\mathrm{H}\left(\mathrm{g}\right)\to {\mathrm{NH}}^{*}$$

(24)

$${\mathrm{NH}}^{*}+\mathrm{H}\left(\mathrm{g}\right)\to {\mathrm{NH}}_2^{*}$$

(25)

$${\mathrm{NH}}_2^{*}+\mathrm{H}\left(\mathrm{g}\right)\to {\mathrm{NH}}_3\left(\mathrm{g}\right)$$

(26)

• L-H mechanism:

$${\mathrm{N}}_2\left(\mathrm{g}\right)\to 2{\mathrm{N}}^{*}$$

(27)

$${\mathrm{H}}_2\left(\mathrm{g}\right)\to 2{\mathrm{H}}^{*}$$

(28)

$${\mathrm{N}}^{*}+{\mathrm{H}}^{*}\to {\mathrm{NH}}^{*}\to {\mathrm{NH}}_2^{*}\to {\mathrm{NH}}_3$$

(29)

Further hydrogenation steps continue on the catalyst surface, where NH species convert to NH₂, and then to NH₃. NH radicals play a crucial role as intermediates, and their surface coverage correlates closely with final NH₃ yields [142]. The hydrogenation of NH₂ to form NH₃ can proceed via either the E-R or L-H pathways, depending on the system pressure [146]. However, competing reactions can occur, such as the decomposition of NH₃ under plasma conditions, which leads to the formation of NH₂ radicals and H or H₂, lowering overall ammonia yield [142]. Therefore, achieving a balance between formation and decomposition steps is key to maximizing efficiency in plasma-assisted systems.

Additionally, the type of catalyst support also affects the reaction mechanism and performance [138]. For instance, in the case of metal oxide supports like Al₂O₃ without any deposited metal catalysts, the reaction mechanism still involves surface and gas-phase species, but the absence of metal sites limits the activation and recombination rates of radicals [138]. Moreover, such supports tend to possess strong acidic sites that hinder NH₃ desorption [138]. As a result, NH₃ molecules may become trapped on the surface and further decompose under plasma exposure, decreasing overall NH₃ production efficiency. Thus, while plasma-only and metal oxide-supported systems are viable, incorporating effective metal catalysts (e.g., Ru, Ni) greatly enhances both selectivity and productivity [138].

Plasma-assisted ammonia synthesis using non-thermal plasma (NTP) typically achieves energy efficiencies in the range of 1.53 to 1.83 ${\mathrm{g}}_{{\mathrm{NH}}_3}$/kWh, which remains far below the industrial target of 150–200 ${\mathrm{g}}_{{\mathrm{NH}}_3}$/kWh [147]. Since improvement through plasma parameter adjustments alone is limited, recent research has increasingly focused on plasma catalytic systems. These systems combine plasma reactors with various catalysts such as tubular membrane structures [148], Ru-based mesoporous catalysts [149], supported microporous materials [150], Ni/Al₂O₃, zeolite 5A, and mesoporous silica frameworks like SBA-15 and MCM-41 [138,151,152,153] to enhance both ammonia yield and energy efficiency. Mehta et al. [154] demonstrated that vibrationally excited nitrogen generated in NTP can interact directly with catalyst surfaces, leading to reaction pathways that differ from those in thermal catalysis. A notable example is the study by Kim et al. [155], who achieved an efficiency of 35.7 ${\mathrm{g}}_{{\mathrm{NH}}_3}$/kWh using a nanosecond pulsed packed bed reactor with a Ru-Mg/$\gamma$-Al₂O₃ catalyst at 300 °C under atmospheric pressure and an N₂:H₂ ratio of 4:1.

In recent advancements toward sustainable ammonia production, plasma-assisted systems using nitrogen and water have emerged as a promising alternative to conventional N₂-H₂ processes [156]. Water replaces hydrogen as the proton donor, offering multiple advantages such as eliminating the need for hydrogen purification or methane reforming, both of which are energy-intensive and costly [157]. Although electrolysis can generate hydrogen from water, it still demands high electricity input. Conversely, directly utilizing water in plasma-driven systems is inherently safer and reduces logistical risks related to hydrogen storage and transport [157]. Nonetheless, challenges like low ammonia yield, poor selectivity, and high energy consumption remain critical obstacles in scaling up these technologies. The chemistry of ammonia formation in such systems is driven by complex interactions between plasma-generated reactive species and water molecules [158]. Electrons and UV and vacuum ultraviolet (VUV) photons initiate dissociation processes, breaking down water into H and OH radicals [141], as shown in Equations (30) and (31) [135].

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to \mathrm{OH}+\mathrm{H}+{\mathrm{e}}^{-}$$

(30)

$${\mathrm{H}}_2\mathrm{O}+\mathrm{UV}/\mathrm{VUV}\to \mathrm{OH}+\mathrm{H}$$

(31)

These reactive intermediates then interact with nitrogen species to form ammonia, though side reactions can lead to by-products like NO_x [158]. The nitrogen reduction reaction benefits from hydrogen radicals, while OH radicals tend to favor nitrogen oxidation, forming undesirable nitrogen oxides [159]. Importantly, the reactivity of different nitrogen species, such as atomic nitrogen, ionized nitrogen, and excited molecular nitrogen, varies based on their lifetimes and activation states [160]. Therefore, precise control over plasma properties is crucial to steering reactions toward ammonia rather than by-products.

Reactor design plays a critical role in determining the reaction pathways and energy efficiency of plasma-assisted N₂-H₂O systems. Three main configurations have been examined: gas interacting with an open liquid surface, gas-phase systems containing suspended liquid droplets, and systems based on submerged gas bubbles in a liquid medium. In gas–liquid configurations, the plasma forms and interacts at the liquid interface, and this category commonly includes reactor designs such as plasma jets and gliding arcs [161]. Experiments demonstrate that introducing water vapor into nitrogen before plasma exposure enhances ammonia formation [162]. Adding metals to the liquid phase further improves reaction rates by providing active sites or altering pH, with magnesium showing particularly strong catalytic behaviour [163]. When water exists as dispersed droplets in the gaseous phase, DBD reactors are frequently used [164]. These setups rely on aerosolized water reacting with excited nitrogen species [164]. The reactor’s geometry, discharge power, and inclusion of catalytic materials such as Ru supported on MgO strongly impact the outcome [164]. Water vapor typically outperforms droplets due to better interaction with plasma, but both forms are effective under the right thermal and flow conditions [165]. Studies have found that temperature and vapor concentration play a decisive role in reaction selectivity and energy efficiency, emphasizing the importance of precise control processes [165]. A less explored configuration involves injecting nitrogen gas into a liquid medium, forming bubbles that serve as plasma microreactors [166]. These underwater discharges generate localized high-energy zones where nitrogen and water interact [135]. Though this method achieves notable nitrogen fixation rates, the dominant products tend to be NO_x rather than ammonia [167]. However, combining this process with downstream electrochemical reduction offers a viable route for selective NH₃ production [167]. The conductivity of the liquid phase influences discharge stability, energy use, and product distribution [166]. Further research into catalyst integration, reactor miniaturization, and mechanistic modeling is necessary to optimize these submerged-phase approaches for practical ammonia synthesis.

The performance of NTP and plasma catalytic systems can be significantly improved when combined with auxiliary techniques such as photocatalysis and electrocatalysis, facilitating more efficient power-to-chemical conversions [168]. For instance, Abiev et al. [168] demonstrated that incorporating photocatalysis in NTP-assisted dry reforming of methane (DRM) provides an effective pathway for transforming natural gas into value-added chemicals under mild conditions. In this regard, Mei et al. [169] reported that coupling plasma with photocatalysis could boost CO₂ conversion and overall energy efficiency by up to 2.5 times in the DRM process. Likewise, Meng et al. [170] observed notable conversion rates of 15% for methane and 48.6% for propane when employing plasma-assisted photocatalysis to synthesize gasoline-range hydrocarbons. In a separate study, a dual-function strategy involving plasma-driven N₂ oxidation and subsequent electrochemical reduction of NO_x for ammonia production was explored [171]. These emerging approaches reveal that while plasma technologies hold promise for renewable chemical manufacturing, considerable gaps remain before they become industrially viable. Realizing the full potential of plasma-driven ammonia synthesis requires overcoming several fundamental and technical challenges. One of the main bottlenecks is the availability of robust and energy-efficient power supplies capable of generating stable plasma discharges under demanding operational conditions [146]. Designing a universal plasma reactor compatible with integrated systems, such as those involving electric fields, photocatalysis, or electrocatalysis cells, is also essential [146]. For commercial development, the literature has suggested that hybrid plasma–electrocatalytic systems should approach Faradaic efficiencies above 90% together with areal ammonia production rates above 10⁻⁴ mol cm⁻² h⁻¹, where the rate is normalized to the active electrode area [172]. Additionally, improving process selectivity, reducing equipment costs, and optimizing catalyst–plasma interactions are vital steps toward building scalable, high-performance power-to-chemical platforms.

To provide a clearer comparison of representative catalyst and material performance,

Table 1. Representative quantitative comparison of catalysts and materials used in electrochemical, photoelectrochemical, and plasma-assisted ammonia synthesis.

Route	Catalyst/Material	NH3 Yield/Rate	FE or Energy Metric	Operating Conditions	Cost/Scalability
Electrochemical N2RR	Au hollow nanocages	3.9 $\mathsf{\mu }$g cm−2 h−1	FE: 30.2%	0.5 M LiClO4, ambient conditions	High material cost; useful benchmark but limited scalability.
Electrochemical N2RR	Bi nanosheet array	$6.89\times {10}^{-11}$ mol s−1 cm−2	FE: 10.26%	0.1 M HCl, $-0.50$ V vs. RHE	Lower-cost metal; still low yield and laboratory-scale.
Electrochemical molten-salt route	Li-mediated/molten-salt systems	201.96–1224 $\mathsf{\mu }$g cm−2 h−1	FE: 27–59%	300–400 °C; molten salts	Higher rate than aqueous N2RR, but complex electrolyte handling and regeneration are required.
Photoelectrochemical N2RR	Si-based aerophilic–hydrophilic photocathode	18.9 $\mathsf{\mu }$g cm−2 h−1	FE: 37.8%	Acid electrolyte, $-0.2$ V vs. RHE, illumination	Promising PEC benchmark, but device complexity and stability limit scale-up.
Photoelectrochemical Li-mediated N2RR	PdCu/TiO2/Si photocathode	43.09 $\mathsf{\mu }$g cm−2 h−1	FE: 46.15%	Pressurized N2, O2, and Li-mediated chemistry	Higher performance, but complex cell design and pressurized operation limit scalability.
Plasma-assisted synthesis	Ru-Mg/$\gamma$-Al2O3	Not usually reported as FE	35.7 gNH3 kWh−1	Nanosecond pulsed packed-bed reactor, 300 °C, 1 atm	High-performing plasma case, but Ru cost and energy efficiency remain concerns.
Plasma-assisted synthesis	Ru/AC and packed-bed DBD systems	Not usually reported as FE	0.6–2.3 gNH3 kWh−1	DBD or packed-bed plasma reactors	Modular and flexible, but energy yield and outlet NH3 concentration remain low.

summarizes selected quantitative indicators reported for electrochemical, photoelectrochemical, and plasma-assisted ammonia synthesis. Because the studies differ in reactor design, electrolyte, pressure, illumination, and normalization basis, the values should be interpreted as representative benchmarks rather than as a direct ranking.

As shown in Table 1, direct comparison remains difficult because the reported metrics are not standardized across routes. Electrochemical and PEC studies commonly report NH₃ yield and Faradaic efficiency, whereas plasma-assisted systems more often report energy yield in g_NH3 kWh⁻¹. Overall, the highest laboratory values still do not remove the main scale-up barriers, including low absolute NH₃ productivity, limited durability, costly or complex materials, and incomplete system-level validation.

3.6. Chemical Looping Ammonia Synthesis (CLAS)

Chemical looping ammonia synthesis splits the overall Haber–Bosch chemistry into two sequential steps [173]. In the first step, nitrogen from air reacts with a solid carrier to form a metal nitride, thereby storing activated nitrogen. In the second step, the solid is contacted with a hydrogen source (H₂ or H₂O) to produce ammonia and regenerate the metal carrier for the next cycle [174]. Metal nitrides can serve as both nitrogen carriers and active (electro) catalysts under synthesis conditions [13]. This approach is commonly labeled CLAS/CLAP/CLAG, all referring to the same concept, and systems are further differentiated by material properties into nitrogen-carrier (NC), oxygen-carrier, sulfur-carrier, carbon-carrier, and related classes. Adding metal hydrides can facilitate nitrogen reduction and hydrogen transfer, increasing ammonia formation rates. In chemical looping ammonia, nitrogen-carrier solids shuttle between a nitriding step and a hydrogenation step, allowing ammonia to be produced efficiently with reduced energy demand. Therefore, compared with Haber–Bosch, chemical looping may operate near atmospheric pressure and avoid large compressors, especially when paired with water electrolysis [174]. This enables modular, distributed plants powered by renewable power. Recent reviews further emphasize that temporal decoupling alleviates N₂/H₂ competitive adsorption and helps relax scaling-relation limits, enabling milder operation [175]. Remaining challenges include carrier stability, nitrogen capacity, and reaction kinetics [176].

At its core, CLAS decouples the classic reaction (Equation (1)) into two distinct reactions: (i) nitridation of a carrier and (ii) hydrogenation (or hydrolysis) of the nitride, which are defined in Equations (32) and (33), respectively.

$$\mathrm{M}+\frac{1}{2}{\mathrm{N}}_2\to \mathrm{M}-\mathrm{N}$$

(32)

$$\mathrm{M}-\mathrm{N}+x{\mathrm{H}}_2\to {\mathrm{NH}}_3+\mathrm{M}\mathrm{or}\mathrm{M}-\mathrm{N}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{NH}}_3+\mathrm{M}-\mathrm{O}$$

(33)

This temporal and spatial separation allows independent optimization of nitrogen activation and ammonia release steps, avoiding the competitive adsorption of N₂ and H₂ that limits conventional catalysts [177]. Recent reviews emphasize that decoupling nitridation and hydrogenation also helps circumvent the classic Brønsted-Evans-Polanyi scaling and the competitive N₂/H₂ adsorption that limit one-step catalysts, sharpening the theoretical case for CLAS under milder conditions [178].

Material choice is critical. Candidate carriers span alkali/alkaline-earth nitrides (Li₃N, Mg₃N₂, Ca₂NH-CaH₂), transition-metal nitrides (e.g., Mn₆N_2.58/Mn₄N, CrN/Cr₂O₃) and composite nitride–hydride or nitride–imide systems [179]. Recent work and reviews highlight imide–hydride composites and perovskite-type (oxy)nitride–hydride carriers with improved nitrogen uptake and hydrogenation kinetics [175]. For example, incorporating Zn into a LiH–Li₂NH-mediated loop increased the NH₃ formation rate by about nineteen-fold at approximately 350 °C and reduced the apparent activation energy, attributed to the formation of LiZn_x intermetallics and LiZnN as auxiliary nitrogen carriers [180]. Descriptors such as nitrogen-vacancy formation/mobility, lattice binding, and cyclic stability increasingly guide carrier discovery, supported by DFT screening [175]. Computational and experimental studies in 2023–2024 identify vacancy formation energy and solid-state N/H diffusion as key predictors; Li-Mg-N-H and Ca-Mn-N-H composites are frequently cited as promising, cyclable systems [175]. Studies also show that lattice strain and dopant incorporation (e.g., Zn, Al, Y) tune nitrogen mobility and hydrogenation rates, suggesting rational materials design strategies [180]. The overall schematic of CLAS mediated by metal nitride and alkali metal is illustrated in Figure 11.

Operating conditions for CLAS tend to be milder than Haber–Bosch: typical nitridation at 300–600 °C (often near 1 bar), followed by hydrogenation at 200–400 °C [177]; the modular nature allows smaller, decentralized units and compatibility with intermittent renewable heat or H₂ [179]. Recent analyses underline low-pressure operation and note that imide-type carriers can be faster yet more sensitive to CO₂/H₂O impurities [181]. Thermodynamic modeling indicates that CLAS configured for distributed operation could reduce compression and high-pressure infrastructure, improving scalability of low-carbon ammonia deployment [182]. However, continuous looping with integrated heat recovery, solids circulation, and long-term durability remains under development in laboratory settings [182].

Despite its promise, CLAS faces several significant challenges. Carrier durability remains a major issue: repeated nitridation/hydrogenation cycles can induce sintering, phase segregation, and impurity reactions (CO₂/H₂O) forming carbonates/hydroxides, lowering N-uptake kinetics [173,181]. Nitrogen capacity and space–time yield remain below industrial targets [183]. Heat management and reactor design pose obstacles: the cyclic nature requires careful thermal integration, switching valves, and robust solids-handling to avoid attrition and maintain throughput [182]. In addition, although low-pressure operation is possible, many carriers still need elevated temperatures or activated nitrogen species to reach practical rates, narrowing energy-savings compared to conventional routes [175,177]. Recent analyses highlight impurity sensitivity (CO₂/H₂O) for imide-rich carriers and the need for upstream gas polishing and loop-internal mitigation strategies (drying, CO₂ scrubbing, sweep-gas management, periodic regeneration) [181].

Innovation in recent work has sought to address these obstacles via advanced carrier design and reactor engineering [173]. Approaches include nanostructured/supported composites and photo- or plasma-assisted looping to accelerate nitrogen activation at lower bulk temperatures [184,185]. From a process viewpoint, three-reactor chemical-looping (TRCL) architectures provide greater operational flexibility and simpler heat management and have been evaluated via Aspen-based flowsheeting [186]. Coupling CLAS with electrolysis or biomass-derived H₂ enables self-sustaining green-ammonia systems [177]. Techno-economic and process studies indicate hybrid processes can minimize compression duty and CapEx while maintaining selectivity [187].

Overall, CLAS represents a promising pathway for sustainable, low-carbon ammonia production [173]. By temporally separating N₂ activation and hydrogenation, it allows step-wise optimization, enabling lower-pressure synthesis [182]. Ongoing priorities include carrier durability via interface/defect engineering, impurity tolerance, improved heat/mass transfer and solids handling, and integration with renewable heat/electricity [175]. Together, these advances bring CLAS closer to enabling distributed, low-carbon ammonia production compatible with a renewable-based energy economy [173].

3.7. Biological Ammonia Production

Biological ammonia synthesis emerges as a promising alternative, relying on nature-driven pathways that do not release harmful emissions. Multiple strategies fall under this umbrella, including biological nitrogen fixation (BNF), nitrate or nitrite reduction, hydrolysis of urea, and advanced microbial metabolic engineering [188]. Among them, BNF and synthetic modification of microorganisms to enhance ammonia output have gained the most traction. Another innovative approach involves fermenting protein-rich biomass using ruminal bacteria, offering a complementary method of biologically generating ammonia [189].

Biological ammonia-production routes are included in this review for completeness and as exploratory biohybrid concepts, not as near-term bulk ammonia-production technologies. At present, no continuous recoverable ammonia stream has been demonstrated at a practically relevant scale; therefore, their main value lies in understanding mild-condition nitrogen fixation, enzyme- or microbe-assisted pathways, and possible long-term biohybrid concepts.

At the heart of biological nitrogen fixation lie nitrogenase enzymes, which catalyze the conversion of inert atmospheric nitrogen into bioavailable ammonia [190]. Unlike the Haber–Bosch process, this reaction occurs at ambient temperature and pressure, though it demands a high amount of adenosine triphosphate (ATP) [191]. Nitrogenase is found in specific microbial groups such as Rhizobia (which form symbiotic relationships with legumes) and free-living nitrogen-fixing bacteria like Azotobacter and Klebsiella [192]. Efforts to replicate this biological machinery in synthetic systems have included expressing Klebsiella nitrogenase subunits in E. coli and yeast [193]. One notable line of research involves manipulating Bacillus subtilis through gene knockouts and overexpression to optimize ammonia production [194]. For instance, deletion of the codY gene, an important transcriptional repressor, leads to increased production of branched-chain amino acids and enhances the conversion of proteins into ammonia [194]. Additional removal of the BkdB gene, which is involved in fatty acid synthesis, further directs metabolic flux toward nitrogen-containing compounds [195]. The expression of genes like LeuDH and two-keto acid decarboxylase helps accelerate the breakdown of amino acids, thereby boosting precursor availability. Using this engineered strain, protein biomass from E. coli was successfully fermented, producing ammonia with a theoretical yield of 50% [194]. A separate study explored a similar approach using engineered E. coli, targeting genes associated with intracellular ammonia assimilation (glnA and gdhA) [196]. These genes normally help the cell to absorb ammonia, but their deletion allowed ammonia to accumulate externally, resulting in a 47.8% yield (458 mg/L) [197]. The researchers further evaluated the use of food processing residues, such as soy sauce cake and tomato peel, for ammonia generation [198]. They discovered that high glucose concentrations suppressed ammonia synthesis. To overcome this, the glucose transport gene ptsG and associated phosphotransferase system were knocked out using PCR-based gene editing [198]. The final strain successfully produced ammonia in a medium with both amino acids and glucose, achieving yields as high as 73% [198].

Bioelectrocatalytic ammonia synthesis can be considered a hybrid extension of biological nitrogen fixation, in which an electrode supplies reducing equivalents to a biological catalyst, purified enzyme, or living microorganism. In these systems, the role of the electrochemical component is not necessarily to reduce N₂ directly on an inorganic catalyst, but to provide electrons, mediators, or electrochemically generated H₂ to support nitrogenase-driven N₂ fixation. Three representative configurations can be identified: (i) enzymatic bioelectrocatalysis, where purified nitrogenase or nitrogenase-related components are coupled to an electrode through direct or mediated electron transfer; (ii) microbial electrosynthesis, where electroactive or diazotrophic microorganisms receive reducing equivalents from a cathode and convert N₂ to ${\mathrm{NH}}_3$/${\mathrm{NH}}_4^{+}$; and (iii) photo-bioelectrocatalytic systems, where light absorbers or photoelectrodes provide the driving force for electron delivery to biological nitrogen-fixation pathways. These configurations are summarized in

Table 2. Representative bioelectrocatalytic and biohybrid schemes for biological ammonia synthesis.

Scheme	Simplified Bioelectrocatalytic Pathway
Enzymatic bioelectrocatalysis	Cathode → mediator/direct electron transfer → purified nitrogenase system. The overall nitrogenase reaction can be represented as: ${\mathrm{N}}_2+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}+16\mathrm{ATP}\to 2{\mathrm{NH}}_3+{\mathrm{H}}_2+16\mathrm{ADP}+16{\mathrm{P}}_{\mathrm{i}}$
Microbial electrosynthesis	Anode/cathode pair → reducing equivalents, mediator, or H2 → diazotrophic microorganism → intracellular nitrogenase → NH3/${\mathrm{NH}}_4^{+}$ release.
Photo-bioelectrocatalysis	Light absorber or photoelectrode → photogenerated electrons → biological catalyst or microorganism → nitrogenase-assisted N2 fixation.

. Although these approaches are attractive because they combine mild biological catalysis with externally controllable electrical or solar energy input, they remain at proof-of-concept level because of low NH₃ titres, enzyme instability, inefficient electron transfer, ATP or energy-regeneration requirements, and difficult product recovery from dilute aqueous media.

In response to the problem of intracellular ammonia being absorbed by host cells, researchers have explored yeast-based systems to produce ammonia extracellularly [199]. One such strategy involved yeast cell surface engineering (YCSE), in which ammonia-generating enzymes, like L-amino acid oxidase, are displayed directly on the yeast cell membrane [199]. This was accomplished by attaching signal peptides to guide the enzyme to the cell surface. For example, a codon-optimized version of HcLAAO was inserted into the Saccharomyces cerevisiae strain BY4741/sedlA using a plasmid (pULDl-HcLAAO) [200]. The engineered yeast expressed up to 10⁶ target proteins on its surface, acting as immobilized catalysts. This platform enables extracellular conversion of amino acids to ammonia without subjecting the microbial host to ammonia toxicity, thereby improving overall process efficiency [200]. One promising approach utilizes soybean residues through the YCSE technique, where enzymes like ammonia lyases produce ammonia from glutamine without needing cofactors, unlike nitrogenase-based systems [201]. This method achieved an efficiency of 83.2% and a titer of 3.34 g/L [201]. However, its application is limited to glutamine. To address this, enzymes such as L-amino acid oxidase, which act on various amino acids, have been studied for broader applicability, though primarily on a lab scale [202].

Ammonia recovery is also possible through microbial fuel cells (MFCs) in wastewater treatment plants [203]. These systems rely on bacteria that oxidize ammonia while simultaneously producing electricity [204]. Wastewater is introduced into an anaerobic chamber, where bacteria degrade organic matter and generate electrons. These electrons facilitate ammonia oxidation, leading to nitrogen gas formation and purified water [204]. While innovative, this process still faces challenges in terms of scale and cost. Another route in wastewater treatment is ammonification, where nitrogen-containing compounds like food waste and human waste are converted to ammonia using anaerobic bacteria [205]. The resulting ammonia is then transformed into nitrate and nitrite via nitrification. Although this method is well studied, its industrial application remains limited by high energy demands. Ruminant animals possess a specialized digestive organ called the rumen, which serves as a natural bioreactor for ammonia production, breaking down dietary proteins into microbial biomass and ammonia [206]. However, excessive ammonia production driven by hyper-ammonia-producing bacteria (HABs) often exceeds microbial demand [207]. These bacteria, including species like Selenomonas ruminantium and Peptostreptococcus elsdenii, metabolize amino acids into ammonia, with production levels reaching up to 40 mM under favorable conditions [208]. Interestingly, hay-fed cattle show higher HAB activity due to the rumen’s more neutral pH compared to grain-fed animals [195]. It is worth mentioning that HABs first break down proteins through protease secretion, producing peptides and amino acids [209]. These hydrolysates are more bioavailable and support higher ammonia yields. Though some studies have investigated HAB fermentation with different protein sources like soy protein isolate or blood meal, more research is needed to compare hydrolyzed and non-hydrolyzed proteins. One study found that Clostridium aminophilum combined with soy protein isolate yielded up to 7.23 mM ammonia [210].

Protein hydrolysis is central to this process, as it improves substrate availability [211]. Soybean proteins, primarily glycinin and conglycinin, must be broken down into peptides and amino acids through enzymatic or thermochemical methods [212]. Proteases such as alcalase, pepsin, and flavourzyme have been shown to enhance hydrolysis under mild and eco-friendly conditions [213]. The degree of hydrolysis depends on the protein form and the enzyme used [195]. For instance, glycinin is more readily degraded than $\beta$-conglycinin by rumen microbes [195]. To further improve protein breakdown, multi-enzyme combinations are used [214]. These blends include endo- and exopeptidases that act at different points along the protein chain [214]. Industrial enzymes like flavourzyme, derived from Aspergillus oryzae, exhibit both activities [215]. Studies show that combining enzymes increases the degree of hydrolysis and the functionality of the resulting hydrolysates [195,216]. For example, higher alcalase concentration in enzyme blends significantly boosted hydrolysis of potato pulp and poultry meal proteins [216]. Several bacterial strains have demonstrated significant biological ammonia production potential [195]. Clostridium aminophilum ferments amino acids like glutamine and serine, producing ammonia, acetate, and butyrate [195]. Clostridium sticklandii utilizes pairs of amino acids via the Stickland reaction to generate ammonia, while Peptostreptococcus anaerobius degrades peptones and amino acids into various organic acids and ammonia [195]. Despite their promising activity, further research is needed to understand the genetics and improve conditions for large-scale ammonia biosynthesis using these microbes.

Parameters Influencing Biological Ammonia Production:

Biological ammonia production is influenced by several parameters, including pH, temperature, substrate type and concentration, and time, which are explained in this section.

• pH Effects
  pH plays a critical role in shaping ammonia yields during microbial fermentation, as it influences both the metabolic activity and population dynamics of HAB [207,217]. Fermentation studies have shown that initial pH not only impacts the growth environment but also affects by-product formation, directly altering the balance between ammonia (NH₃) and its ionic form, ammonium (${\mathrm{NH}}_4^{+}$) [217]. At lower pH, higher hydrogen ion concentrations favor ammonium formation, whereas alkaline conditions shift the equilibrium toward free ammonia [217]. Because ammonia production itself can cause pH fluctuations, this interaction can further influence microbial activity [217].
  Experiments conducted under pH ranges from 7 to 11 revealed that extreme alkaline levels (pH 9 and 11) led to fluctuating ammonia production, with an initial rise in the first 24 h followed by a decline between 24 and 48 h due to bacterial ammonia assimilation, and then a late resurgence after 72 h. Such a pattern is impractical for industrial operations because of prolonged processing time and contamination risks [195]. In contrast, pH 7 and 10 supported steady ammonia output over extended fermentation, with pH 10 maintaining high concentrations even after 120 h, while neutral pH (pH 7) produced the maximum concentration of 107.5 mg/L at 72 h before nutrient depletion and toxic metabolite accumulation reduced yields [218]. These observations align with findings from studies on ruminal microorganisms, where many anaerobic species, including monensin-sensitive HAB, exhibited optimal activity between pH 6.5–7.5, with ammonia production often measured at pH 7 using substrates such as casein, peptones, and soybean proteins [195]. However, other research has suggested that while pH can influence fermentation kinetics, it may not directly correlate with the proportion of HAB in rumen samples, indicating that pH effects may depend more on functional activity than microbial abundance [195].
• Time and Temperature Effects
  Temperature is a key determinant of ammonia production in HAB, as each species has a specific range for optimal activity [208]. Multiple studies have reported consistent mesophilic conditions, with growth and fermentation commonly maintained between 20 °C and 45 °C [195]. For instance, isolates from swine manure storage pits were cultured at 37 °C, while ruminal HAB were frequently grown anaerobically at 39 °C in basal media, including experiments using sheep-filtered ruminal fluid and soluble soybean protein solution [208]. This alignment across research suggests that HAB generally achieves peak performance within this moderate temperature range [195].
  Fermentation time also plays an important role in determining ammonia yields. Experiments have shown that Clostridium aminophilum produced its highest ammonia concentration when cultivated on soy protein isolate for 96 h, with time accounting for nearly 10% of the observed production variation [210]. In contrast, Peptostreptococcus anaerobius exhibited minimal growth and low ammonia output, remaining largely unaffected by incubation length [195]. In general, ammonia production often increases beyond 72 h, with 96 h emerging as a common benchmark for maximizing output in controlled fermentation trials.
• Substrate Type and Concentration Effects
  Substrate concentration is a critical parameter influencing the efficiency of ammonia formation during HAB fermentation. The response of HAB to substrate levels depends on factors such as strain-specific nutrient requirements, susceptibility to environmental stress, fermentation duration, and possible substrate inhibition at higher dosages [219]. Comparative trials using pure cultures of Clostridium aminophilum, C. sticklandii, and Peptostreptococcus anaerobius have evaluated multiple protein-rich nitrogen sources, including soy protein isolate (SPI), blood meal (BM), feather meal (FM), dried fish meal (DFM), and yeast extract (YE), both individually and in combination [210]. Across these tests, C. aminophilum consistently delivered the highest ammonia yields, reaching 7.23 mM when SPI was used alone, a result attributed to the favorable amino acid profile of SPI, which contains nine essential amino acids vital for microbial growth [210]. In contrast, P. anaerobius showed the lowest production rates under similar conditions [210].
  Experimental fermentations with varying SPI concentrations (2.5–20%) over 72 h revealed a clear optimum at 10%, producing 3500 mg/L of ammonia; higher concentrations resulted in declining yields, likely due to substrate inhibition and accumulation of inhibitory metabolites [220]. Similar patterns have been reported in other microbial systems, where excess substrate reduced product formation despite nutrient abundance [221]. Variations in substrate type and concentration also influence performance in vivo: ruminal studies have shown that cattle fed predominantly on hay-supported diets had higher HAB populations and ammonia production rates than grain-fed counterparts, in some cases doubling the yield, with mathematical models confirming greater maximal production velocities in hay-fed systems [195]. Collectively, these findings underscore the importance of optimizing both substrate type and concentration to enhance biological ammonia production while avoiding conditions that trigger inhibition.

To make the mechanistic comparison more quantitative,

Table 3. Representative energetic and kinetic constraints of selected emerging ammonia-synthesis pathways.

Pathway	Activation/Energy Indicator	Rate or Turn over Frequency (TOF) Indicator	Main Selectivity Constraint
Direct electrochemical N2RR	No universal activation energy; performance is governed by N2 adsorption/activation, proton-coupled electron transfer, and overpotential.	Usually reported as NH3 yield and FE rather than TOF.	Strong HER competition, low N2 solubility, trace NH3 detection, and possible contamination.
PEC ammonia synthesis	Controlled by photovoltage, charge separation, band alignment, and N2 activation at the photoelectrode surface.	Usually reported as NH3 yield, FE, or solar-to-ammonia efficiency.	HER competition, photoelectrode degradation, recombination losses, and low current density.
CLAS	Nitridation and hydrogenation barriers are carrier-dependent; hydride promoters can reduce the apparent activation barrier of ammonia-forming steps by about half in some Mn-nitride systems.	Representative CLAS rates include 56.3–128 $\mathsf{\mu }$mol NH3 ${\mathrm{mol}}_{\mathrm{metal}}^{-1}$ min−1 for selected metal nitride/hydride systems, and up to several thousand $\mathsf{\mu }$mol g−1 h−1 for some nitrogen carriers.	Limited lattice-nitrogen utilization, carrier regeneration, sintering, impurity sensitivity, and cycle stability.
Plasma-assisted synthesis	A single Arrhenius activation energy is not sufficient because plasma activates N2 and H2 through electrons, ions, radicals, and vibrationally excited species.	Often reported as energy yield, e.g., gNH3 kWh−1, rather than TOF. A reported high value is 35.7 gNH3 kWh−1 for Ru-Mg/$\gamma$-Al2O3 under pulsed plasma conditions.	Radical recombination, NH3 decomposition, NOx formation, low outlet NH3 concentration, and separation/recycle burden.
Biological nitrogen fixation	Requires at least 16 ATP per N2 reduced, equivalent to 8 ATP per NH3 and about 244 kJ mol−1 NH3 for ATP regeneration.	Nitrogenase TOF is approximately 1 N2 s−1, with $k_{\mathrm{cat}}/K_m\sim {10}^4$ M−1 s−1.	ATP/electron-supply burden, H2 evolution, feedback inhibition, cellular NH3 assimilation, and dilute-product recovery.

summarizes representative energetic and kinetic indicators for selected emerging ammonia-synthesis pathways. Since activation energies, turnover frequencies, and selectivity metrics are not reported in a uniform manner across the literature, the values should be interpreted as pathway-level benchmarks rather than directly equivalent performance rankings.

Table 3 shows that the emerging pathways are limited by different kinetic and selectivity bottlenecks. Direct electrochemical and PEC routes are mainly constrained by weak N₂ activation and competing HER. CLAS improves selectivity by separating nitridation and hydrogenation, but its performance depends strongly on carrier kinetics and cyclic stability. Plasma-assisted systems bypass part of the thermal activation barrier through reactive plasma species, but radical recombination, NH₃ decomposition, and low outlet concentration still limit efficiency. Biological systems operate under mild conditions, but the ATP and electron-supply requirements, together with dilute NH₃ recovery, make process-scale deployment difficult.

3.8. Nitrate/Nitrite-to-Ammonia Electrochemical Synthesis

Electrochemical nitrate-to-ammonia synthesis (electrochemical nitrate reduction reaction, eNO₃RR) converts dissolved nitrate (${\mathrm{NO}}_3^{-}$) into ammonia (${\mathrm{NH}}_3$) or ammonium (${\mathrm{NH}}_4^{+}$) using electricity, so it can simultaneously remove nitrate from water and recover a useful product [222,223]. Electrochemical nitrite-to-ammonia synthesis (electrochemical nitrite reduction reaction, ${\mathrm{eNO}}_2$RR) follows the same idea but starts from nitrite (${\mathrm{NO}}_2^{-}$), which is often an intermediate formed during nitrate reduction [223,224]. In many systems, nitrate is first reduced to nitrite and then further hydrogenated through adsorbed N-O intermediates until NH₃/${\mathrm{NH}}_4^{+}$ is formed, while side pathways can instead produce N₂ or other nitrogen products [222,224]. Because these reactions occur in aqueous electrolytes and compete with hydrogen evolution, high selectivity depends on controlling how strongly the catalyst binds nitrate/nitrite intermediates relative to H adsorption [223,225].

The overall cathodic half-reaction for nitrate reduction to ammonium is commonly expressed in acidic media as Equation (34).

$${\mathrm{NO}}_3^{-}+10{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\to {\mathrm{NH}}_4^{+}+3{\mathrm{H}}_2\mathrm{O}$$

(34)

which highlights that nitrate-to-ammonium conversion is an eight-electron process [222,223]. For nitrite, the analogous acidic half-reaction is defined in Equation (35).

$${\mathrm{NO}}_2^{-}+8{\mathrm{H}}^{+}+6{\mathrm{e}}^{-}\to {\mathrm{NH}}_4^{+}+2{\mathrm{H}}_2\mathrm{O}$$

(35)

showing the lower electron requirement once the N-O bonds are partially reduced [223,224]. In alkaline media, the same stoichiometry is often written to emphasize water as the proton source, which can be defined in Equations (36) and (37).

$${\mathrm{NO}}_3^{-}+9{\mathrm{H}}_2\mathrm{O}+8{\mathrm{e}}^{-}\to {\mathrm{NH}}_3+10{\mathrm{OH}}^{-}$$

(36)

$${\mathrm{NO}}_2^{-}+6{\mathrm{H}}_2\mathrm{O}+6{\mathrm{e}}^{-}\to {\mathrm{NH}}_3+7{\mathrm{OH}}^{-}$$

(37)

This alkaline form is convenient for discussing pH effects because it explicitly treats H₂O as the proton source and produces OH⁻, implying that nitrate/nitrite reduction can increase the local pH in the cathode interfacial region during operation [222,223]. In practice, the measured product distribution depends on the acid–base equilibrium between NH₃ and NH₄⁺ as well as the mass transport of nitrate/nitrite to the electrode surface [223,225].

Catalyst research has focused strongly on Cu-based materials because Cu can promote stepwise N-O bond hydrogenation while still allowing reasonably high NH₃ selectivity in many aqueous electrolytes [223,224]. Other catalyst families include transition metals (e.g., Ti), metal oxides/hydroxides (often with oxygen vacancies), and supported single-atom catalysts, each aiming to tune adsorption energies of key intermediates such as *NO₂, *NO, and *NH_x [222,226]. Recent mechanistic studies emphasize that controlling the *NO₂ stage is especially important because it can either continue toward NH₃ or divert toward undesired products depending on the surface chemistry [223,224]. Reactor-level designs (for example, structured electrodes and layered electrolyte concepts) have also been used to suppress the hydrogen evolution reaction and maintain high Faradaic efficiency at practical current densities [223,225].

Operating conditions strongly affect how fast ammonia is produced and how selectively it is formed, because nitrate/nitrite reduction depends on the solution pH, the applied voltage (or current density), the nitrate concentration, and mass transport conditions such as stirring/flow, electrode porosity, and diffusion distance [222,223].

High nitrate concentration often makes it easier to reach high current efficiency because nitrate competes more effectively against hydrogen evolution for active sites, while dilute streams place stronger demands on mass-transport engineering [223,225]. Energy consumption is commonly evaluated from the cell voltage and Faradaic efficiency and is frequently reported in kWh per kg NH₃ to link laboratory performance to process-scale expectations [227]. For example, flow-electrolyzer demonstrations have reported low power consumption when nitrate reduction is paired with a favorable anodic reaction, illustrating that system-level choices (anode reaction, membrane, and ohmic losses) can dominate the total electricity demand [224]. In wastewater-oriented systems that integrate nitrate removal with product recovery, reported energy consumption can be substantially higher when additional separations or resistive components are required, so reactor configuration and downstream handling must be considered alongside catalyst metrics [227,228].

4. Technology Readiness Level Assessment of Sustainable Ammonia Production Pathways

The reaction mechanisms, catalyst/material characteristics, and basic process principles of the ammonia-production pathways were discussed in Section 3. Therefore, the present section focuses on their technology readiness, environmental performance, economic evidence, and deployment-related challenges rather than repeating the mechanistic descriptions.

Technology Readiness Levels (TRLs) provide a standardized nine-stage framework for evaluating the maturity of a technology, ranging from basic scientific principles (TRL 1) to fully commercialized systems (TRL 9). Originally developed by NASA, the TRL approach is now widely adopted across energy, manufacturing, and chemical industries to assess how close a technology is to large-scale deployment.

In the context of ammonia production, TRLs offer a consistent benchmark to compare conventional industrial processes, such as the mature fossil-based HB route (TRL 9), with emerging low-carbon or carbon-neutral alternatives that are still in laboratory or pilot development stages. Each successive level in the TRL scale marks progress from theoretical formulation and laboratory validation to prototype demonstration, pilot operation, and eventual commercial deployment. For sustainable ammonia pathways, the TRL framework serves not only to classify technological maturity but also to contextualize economic scalability, environmental performance, and infrastructure readiness. This section applies the TRL methodology to all synthesis routes discussed in Section 3, systematically evaluating their current development stage, representative demonstration projects, technical and environmental challenges, and potential for large-scale, low-emission ammonia production.

Because several ammonia-production pathways include multiple configurations or subsystem maturity levels, some TRL values are reported as ranges, such as TRL 4–5 or TRL 5–6, rather than as single fixed values. In this review, the lower bound of a TRL range represents the conservative maturity level supported by reproducible experimental evidence, while the upper bound represents the most advanced reported configuration, subsystem, or prototype within that pathway. Because electrochemical ammonia-production routes require additional operational metrics, such as cell configuration, active area, operating time, current density, selectivity, product verification, and partial process integration, these criteria are also included in

Table 4. Criteria used to interpret TRL values and TRL ranges in this review, including additional operational criteria for electrochemical ammonia-production systems.

TRL Range	General Interpretation	Typical Evidence Required	Additional Electrochemical Criteria, Where Applicable
TRL 1–2	Basic principle or concept formulation	Scientific principle proposed; early mechanism identified; no validated device or process.	Reaction concept proposed, but no reproducible electrochemical cell performance.
TRL 3–4	Laboratory proof of concept	Laboratory-scale validation of a catalyst, material, enzyme, cell, or reaction concept.	Small laboratory cell, such as H-cell or batch cell; cm2-scale electrode; short tests, typically up to 10–24 h; NH3/${\mathrm{NH}}_4^{+}$ verified with controls; isotope labeling required for N2 reduction where applicable.
TRL 4–5	Laboratory-tested technology approaching relevant-environment validation	Subsystem, cell, reactor, or carrier cycle tested beyond isolated proof of concept, but without integrated pilot operation.	Transition from small static cells toward flow cells, gas-diffusion electrodes, membrane-electrode assemblies, or larger active areas, typically around or above 10 cm2; more stable operation, approximately 50–100 h; reported Faradaic efficiency/selectivity, current density, cell voltage, and contamination control.
TRL 5–6	Relevant-environment or early pilot validation	Integrated subsystem tested under more realistic conditions with some durability or scale-up evidence.	Electrochemical subsystem operated under more practical feed/electrolyte conditions; route-relevant current density maintained, approximately ≥10–50 mA cm−2 for Li-mediated N2 reduction and ≥50–100 mA cm−2 for nitrate/nitrite reduction; partial product recovery, electrolyte management, or membrane/separator validation demonstrated.
TRL 6–7	Pilot or prototype demonstration	Prototype or pilot-scale system demonstrated with meaningful process integration.	Multi-cell, stackable, or pilot-type operation with extended durability, product recovery, electrolyte recycle, and balance-of-plant considerations demonstrated.
TRL 8–9	Commercial or near-commercial deployment	Industrial-scale or commercially available process; remaining issues mainly relate to cost, optimization, or deployment conditions.	Commercial or near-commercial electrochemical system with validated long-term operation, product recovery, safety, and process integration.

.

4.1. Haber–Bosch with Renewable Hydrogen (Green Hydrogen)

The conventional Haber–Bosch synthesis loop and its integration with renewable hydrogen production were introduced in Section 3.1. This section, therefore, focuses on the technology readiness, environmental performance, economic feasibility, and deployment challenges of renewable-H₂ Haber–Bosch ammonia production.

4.1.1. Current TRL

Among sustainable ammonia-production routes, the renewable-hydrogen HB pathway is the most technologically mature. It relies on commercially proven subsystems, including electrolyzers, air-separation units, and ammonia-synthesis reactors, without introducing new chemical pathways. According to Cao et al. [174], the fully integrated green-HB system has effectively reached technology readiness level (TRL) 9, because both electrolysis and ammonia-synthesis units are already commercialized and widely deployed. Mohamed et al. [13] classify the same configuration at TRL 5–8, noting that complete integration under fully renewable operation remains at the demonstration stage. This difference mainly reflects whether maturity is assigned to the individual process units or to continuous integrated operation. Overall, the renewable-hydrogen HB pathway can reasonably be placed at TRL 8, representing the transition from demonstration to early commercialization [13,174].

Several industrial-scale projects demonstrate this maturity. In China, the Baotou (about 390,000 t NH₃ yr⁻¹), Da’an (about 180,000 t NH₃ yr⁻¹), and Chifeng (about 300,000 t NH₃ yr⁻¹) plants collectively target around 870,000 t NH₃ yr⁻¹ using wind- and solar-based electrolysis. The NEOM project in Saudi Arabia, one of the largest renewable-ammonia projects currently under development, aims to produce 600 t H₂ day⁻¹, corresponding to about 1.2 Mt NH₃ yr⁻¹, by 2026. Yara’s Yuri Project in Pilbara, with 10 MW electrolysis capacity, integrates renewable hydrogen with an existing ammonia plant, demonstrating feasibility in a solar-rich environment. In Norway, Yara Herøya operates a 24 MW electrolysis pilot estimated to supply 20,500 t NH₃ yr⁻¹ under renewable power. These projects confirm that multi-megawatt electrolyzers, air-separation units, and HB reactors can be effectively integrated, although maintaining stable operation under variable renewable supply remains a key engineering challenge.

Techno-economic analyses show that the cost of ammonia from the renewable-hydrogen HB route depends strongly on plant scale, electricity price, electrolyzer cost, financing structure, and whether the system is grid-connected, islanded, or highly decentralized [229,230,231,232]. Therefore, published values should be interpreted as a literature range under different scenarios rather than as a single representative cost. Under current or near-term assumptions, favorable large-scale or well-sited projects generally report levelized costs of ammonia (LCOA) of about USD 396–718 t⁻¹ ${\mathrm{NH}}_3$. Representative examples include USD 396.1 t⁻¹ ${\mathrm{NH}}_3$ for a favorable solar case in Egypt [229], USD 473 t⁻¹ ${\mathrm{NH}}_3$ for an optimized islanded system [230], about USD 556–680 t⁻¹ ${\mathrm{NH}}_3$ in a recent scale-sensitive assessment [233], and about USD 718 t⁻¹ ${\mathrm{NH}}_3$ for a base-case grid-powered Canadian system [234]. By contrast, smaller-scale or less favorable configurations are markedly more expensive. Lin et al. [235] reported USD 933–1030 t⁻¹ NH₃ for a 20,000 t yr⁻¹ green-ammonia plant, Sun et al. [236] reported USD 1169 t⁻¹ NH₃ for a renewable-energy-storage-oriented design, and Sousa et al. [231] estimated USD 1450 t⁻¹ ${\mathrm{NH}}_3$ for a 25,000 t yr⁻¹ hydropower-based electrolysis-to-ammonia plant at USD 0.05 kWh⁻¹ electricity. Accordingly, a defensible present-day plant-level literature range is approximately USD 396 to USD 1450 per tonne ${\mathrm{NH}}_3$, with the lower end associated with very favorable renewable resources and large-scale utilization, and the upper end with smaller plants or more conservative techno-economic assumptions. If broader decentralized supply-chain studies are also included, even higher costs are possible: Mingolla and Rosa [237] reported USD 659–1634 t⁻¹ ${\mathrm{NH}}_3$ for decentralized grid-powered systems and USD 1077–2266 t⁻¹ ${\mathrm{NH}}_3$ for decentralized solar-powered systems in the United States, although those results are not directly comparable with plant-gate production-cost studies.

Future projections are more favorable. Nayak-Luke et al. [230] estimated that multiple locations could fall below USD 350 t⁻¹ ${\mathrm{NH}}_3$ by 2030, Sun et al. [236] projected about USD 410.5 t⁻¹ ${\mathrm{NH}}_3$ in a 2050 scenario, and Cesaro et al. [238] suggested that many locations could achieve less than USD 400 t⁻¹ ${\mathrm{NH}}_3$ by 2040, with the potential to fall below USD 300 t⁻¹ ${\mathrm{NH}}_3$ under optimistic assumptions. Across essentially all studies, electricity cost and hydrogen production remain the dominant cost contributors. Consistent with this, Mersch et al. [232] concluded that low-carbon electricity prices of about USD 35 MWh⁻¹ or less are required for electrolytic ammonia to compete with blue ammonia under average gas-price conditions.

4.1.2. Challenges

Although the process exhibits high technological readiness, several challenges still limit its large-scale deployment:

• Intermittency and integration: Renewable electricity sources such as solar and wind are inherently variable, whereas the HB process operates most efficiently under steady-state conditions. Continuous operation, therefore, requires hybrid configurations, such as grid backup, energy storage, or hydrogen buffering, to stabilize the hydrogen supply and avoid inefficient plant design and operation [5].
• Hydrogen cost: Electrolytic hydrogen remains more expensive than fossil-based hydrogen in most current studies, and this cost gap remains the main barrier to green-ammonia competitiveness. Recent literature commonly places green-hydrogen production in the range of about USD 5–7 kg⁻¹ H₂ under present conditions, whereas fossil-based hydrogen is often reported at about USD 1–3 kg⁻¹ H₂, depending on natural-gas prices and carbon-policy assumptions [239,240]. Installed electrolyzer costs also vary substantially by region; the International Energy Agency (IEA) reports 2024 installed costs of roughly USD 2000–2600 kW⁻¹ outside China, compared with about USD 600–1200 kW⁻¹ for Chinese-made systems installed in China. Cost parity will require substantial reductions in electricity price and electrolyzer capital cost, ideally below about USD 0.02 kWh⁻¹ and USD 400 kW⁻¹, respectively, which are levels projected to become achievable through large-scale manufacturing [241]. Under such conditions, Argus estimates ammonia operating costs at USD 335–340 t⁻¹ ${\mathrm{NH}}_3$, comparable to conventional production when capital costs are excluded. Analyses by the IEA and the International Renewable Energy Agency (IRENA) indicate green-ammonia costs of about USD 300–500 t⁻¹ ${\mathrm{NH}}_3$ under optimistic scenarios, but above USD 1000 t⁻¹ ${\mathrm{NH}}_3$ where electricity exceeds about USD 70 MWh⁻¹. Collectively, these studies confirm that electricity price and electrolyzer capital intensity are the dominant levers for competitiveness.
• Electrolyzer scaling and efficiency: Alkaline electrolyzers and PEM electrolyzers, already commercial at TRL 8–9, are being deployed in modules exceeding 100 MW, such as Siemens Energy’s HyBlock systems [241]. Scaling toward the gigawatt level introduces challenges in supply chains, balance-of-plant (BOP), and mass manufacturing. Improving stack durability under cyclic loads and achieving system efficiencies of at least 75% remain essential objectives. Solid-oxide electrolyzers (SOE/SOEC) may reach efficiencies above 85% at laboratory scale, but their maturity remains lower, typically around TRL 5–6, because of material-stability, sealing, and high-temperature integration issues [242].
• Dynamic operation: Because the HB loop is optimized for steady-state conditions, fluctuating hydrogen supply requires advanced control strategies and buffer storage. Dynamic simulations show stable operation across a 10–100% load range with appropriately tuned pressure, flow, and cooling strategies and ramp rates near 3% min⁻¹. Some industrial systems also claim turndown ratios as low as 5–10% of nominal capacity, although detailed pilot-scale data remain limited [243].
• Siting and logistics: Renewable-ammonia projects are often situated in resource-rich yet remote regions, requiring costly transport by pipeline or ship, or long-distance electricity transmission [244,245]. Currently, only 10–13% of global ammonia output is traded internationally, implying that new export terminals, pipelines, and specialized vessels will be needed to support future market expansion [5].

4.1.3. Environmental Impact

Recent LCA and review studies consistently show that Haber–Bosch ammonia produced with renewable hydrogen can achieve major reductions in life-cycle GHG emissions relative to conventional natural-gas-based ammonia, provided that the electricity supplied to electrolysis and ammonia synthesis is predominantly renewable [13,246]. In this pathway, direct process CO₂ emissions from hydrogen production are essentially eliminated, and the remaining life-cycle impacts arise mainly from electricity-generation infrastructure, electrolyzer and plant construction, and auxiliary systems [12,247].

Recent quantitative studies provide a useful range of absolute GHG values for green ammonia. Song et al. [246] reported life-cycle emissions in the range of 120–530 kg ${\mathrm{CO}}_2$-eq t⁻¹ ${\mathrm{NH}}_3$ for low-pressure and ultra-low-pressure green-Haber–Bosch configurations, with wind-based cases reported in a narrower range of about 257–316 kg ${\mathrm{CO}}_2$-eq t⁻¹ ${\mathrm{NH}}_3$ under the assumptions of their study. They also quantified life-cycle ${\mathrm{NH}}_3$ emissions of about 0.046–0.056 kg ${\mathrm{NH}}_3$ t⁻¹ when purge-gas ${\mathrm{NH}}_3$ recovery was included, showing that non-CO₂ releases can also be explicitly controlled and assessed [246]. These values are consistent with broader recent literature indicating that green ammonia can reduce life-cycle GHG emissions by roughly an order of magnitude relative to fossil-based pathways when low-carbon electricity is used [12,13].

For comparison, conventional fossil-based ammonia remains substantially more carbon-intensive in most cradle-to-gate assessments. Reported values vary with system boundary and background datasets, but natural-gas-based Haber–Bosch production is typically on the order of about 1.6–2.8 t ${\mathrm{CO}}_2$-eq t⁻¹ ${\mathrm{NH}}_3$, while coal-based routes can be significantly higher [12,13]. Presenting these values in absolute terms is important because percentage reductions alone can obscure differences in assumptions and boundary definitions.

A central conclusion across recent studies is that the environmental performance of green ammonia is controlled primarily by the carbon intensity of electricity. Even when hydrogen is produced by electrolysis, the life-cycle GHG footprint can increase sharply if the electricity supply contains a substantial fossil share, and under unfavorable grid mixes the resulting emissions may approach or even exceed those of conventional ammonia production [12,13]. Consequently, claims of deep decarbonization are only robust when the electricity source is explicitly specified and demonstrably low-carbon [246,247].

Beyond climate change, recent multi-category LCAs also quantify important trade-offs across non-climate environmental indicators. In the blue-versus-green ammonia comparison by Mayer et al. [12] for a photovoltaic (PV)-based green-ammonia case, the green route shows lower impacts than the blue route for ozone depletion and photochemical ozone formation, but substantially higher impacts in several resource-related categories, including metals/minerals use, land use, and water use. Vinardell et al. [247] reached the same broader conclusion in a Spain-based assessment, reporting that green-ammonia scenarios reduced impacts in global warming, stratospheric ozone depletion, and fossil-resource scarcity, but increased impacts in land use, mineral-resource scarcity, freshwater eutrophication, and terrestrial acidification relative to gray ammonia. These results illustrate that green ammonia can deliver strong climate benefits while shifting burdens toward infrastructure- and material-intensive categories, which is why multi-category LCA reporting is essential for robust comparison [12,13,247].

Water use is another important consideration in electrolysis-based ammonia production. The stoichiometric water requirement for electrolysis is approximately 9 kg H₂O per kg H₂, which corresponds to roughly 1.6 t H₂O per t NH₃ for the hydrogen demand of ammonia synthesis. However, total site water demand can be considerably higher once cooling systems, purification losses, and, where applicable, desalination are included, and reported values depend on whether the study reports stoichiometric water demand, process make-up water, or total water consumption [13,247]. Distinguishing these definitions is essential for meaningful comparison across studies.

These non-GWP trade-offs are especially important because renewable-H₂ HB is the pathway most likely to be deployed at scale in the near term. The main source of these burdens is not the HB synthesis loop itself, but the upstream renewable-electricity and electrolysis infrastructure required to supply low-carbon hydrogen. For PV-powered systems, larger land occupation can arise from solar-field area requirements, while mineral and metal demand can increase because of PV modules, electrolyzer stacks, power electronics, catalysts, and balance-of-plant components. Depending on the background electricity mix and technology assumptions, these supply-chain burdens can contribute to mineral-resource scarcity, metal depletion, and toxicity-related categories. Water use is also more complex than the stoichiometric electrolysis requirement alone: deionized water preparation, cooling, and possible desalination can increase total water consumption, particularly in arid regions where solar resources are favorable. In addition, Vinardell et al. reported that green-ammonia scenarios can increase freshwater eutrophication and terrestrial acidification relative to gray ammonia, indicating that the environmental burden may shift from direct fossil-carbon emissions to infrastructure-, electricity-, and supply-chain-related categories [12,247]. Therefore, renewable-H₂ HB should be interpreted as the strongest near-term climate-mitigation option, but not as an impact-free pathway. Its environmental advantage is most robust when low-carbon electricity is coupled with high electrolyzer utilization, careful water sourcing, durable equipment, responsible mineral supply chains, and multi-category LCA reporting.

A useful longer-term perspective is provided by prospective LCA. Boyce et al. [248] showed that complete decarbonization of the ammonia industry is unlikely from a life-cycle perspective even by 2050 because residual supply-chain emissions remain, but that strong-policy scenarios could still reduce climate impacts by up to 70% per kilogram of ammonia. At the same time, they highlight likely shifts toward higher land, metal, and mineral use as renewable electricity and bioenergy with carbon capture expand. This reinforces the conclusion that deep decarbonization of green ammonia should be evaluated using multi-category environmental metrics rather than GWP alone.

Overall, the renewable-hydrogen Haber–Bosch route represents one of the lowest-carbon options for ammonia production when supplied by low-carbon electricity. However, its environmental performance remains strongly context-dependent, and robust comparison requires transparent reporting of the functional unit, system boundary, electricity source, water-accounting assumptions, and non-climate tradeoffs together with GWP.

4.1.4. Critical TRL Evaluation

The renewable-hydrogen HB process stands at the threshold of full commercialization, reflecting the maturity of its core components, namely electrolysis, air separation, and ammonia synthesis, together with the growing number of demonstration projects confirming complete system integration. The underlying chemistry and reactor design are already well established, and current efforts therefore focus primarily on achieving flexible operation under variable renewable supply and further cost reductions.

The remaining barriers are economic and systemic rather than scientific. The process already supports continuous operation, but achieving load-following flexibility without compromising catalyst lifetime or synthesis efficiency remains an active engineering challenge. Cost parity with fossil-based ammonia depends on continued improvements in electrolyzer efficiency, manufacturing scale, and renewable-power availability. Nevertheless, recent progress in dynamic control and large-capacity electrolyzers indicates that these challenges are being progressively reduced.

From a critical perspective, the renewable-powered HB process can be classified at about TRL 8, approaching TRL 9, indicating that it is technically proven and entering early commercial deployment. Future advancement depends more on infrastructure expansion, grid integration, and supportive policy frameworks than on fundamental scientific breakthroughs, positioning this pathway as the most mature and most immediately deployable route for low-carbon ammonia production.

The key maturity, advantages, limitations, and deployment role of renewable-H₂ Haber–Bosch ammonia synthesis are summarized in

Table 5. Concise summary of renewable-H₂ Haber–Bosch ammonia synthesis.

Aspect	Summary
TRL	High; about TRL 8–9.
Main strength	Most mature low-carbon route; compatible with existing HB infrastructure.
Main limitation	Strongly dependent on renewable electricity price, electrolyzer cost, and plant utilization.
Overall role	Primary near-term pathway for low-carbon ammonia production.

.

4.2. Haber–Bosch with Hydrogen from Biomass or Waste

The general Haber–Bosch synthesis framework and the role of alternative hydrogen sources are described in Section 3.1. Here, the discussion focuses on the maturity, environmental implications, economic drivers, and regional deployment potential of biomass- and waste-derived Haber–Bosch pathways.

4.2.1. Current TRL

Hydrogen generation from biomass and waste is technically feasible and has been demonstrated in several thermochemical and reforming processes, but it has not yet been widely commercialized for ammonia production. Biomass gasification is already established in combined heat-and-power and biofuel applications; however, its integration with the HB process remains largely at the pilot and demonstration stages. Reviews of low-carbon ammonia production generally classify biomass-based ammonia at about TRL 5, corresponding to prototype or pilot operation with limited industrial deployment [5]. Waste-gasification and pyrolysis-to-hydrogen systems exhibit slightly lower maturity, typically around TRL 4–6, because of greater feedstock variability and the absence of large-scale integrated installations [241].

Demonstrations exist for most steps in the biomass-to-ammonia chain, although not yet in a single continuously operated commercial process. Biomass gasification combined with hydrogen purification and ammonia-synthesis catalysts has been validated at pilot scale, with IEA Bioenergy reporting successful production of high-purity hydrogen together with integration of water–gas-shift and gas-cleanup units on pilot syngas streams [249]. However, long-duration stability, tar handling, and feedstock variability remain persistent barriers [250]. Bio-oil reforming studies have likewise confirmed the technical feasibility of catalytic steam reforming and autothermal reforming, but coke formation, catalyst deactivation, and impurity tolerance continue to constrain scalability [251,252]. Zhang et al. [252] emphasize that, although the thermochemical principles are well established, durability and scale-up remain key hurdles. Because each module, including gasification, shift, cleanup, hydrogen polishing, and ammonia synthesis, has been demonstrated only partially, long-term integrated operation under realistic biomass conditions has not yet been achieved. As of 2025, no commercial plant operates entirely on biomass-derived hydrogen for ammonia synthesis, supporting an overall readiness of about TRL 5–6.

A more advanced variant uses biomethane, that is, upgraded biogas, as the reforming or pyrolysis feedstock to produce hydrogen for ammonia synthesis. This pathway benefits from established natural-gas infrastructure and avoids the complexity associated with solid-biomass conversion. Yara’s Cubatão plant in Brazil, supplied by Raízen’s sugarcane-derived biomethane at approximately 20,000 Nm³ day⁻¹, has already produced ammonia using renewable biomethane and is equipped for full biomethane operation. Similarly, the Hazer Group-KBR demonstration achieved 450 h of continuous methane pyrolysis with biomethane, producing hydrogen and solid carbon suitable for downstream use. These industrial and pilot examples indicate that conversion of biomethane to hydrogen and subsequently to ammonia has reached the demonstration-to-early-commercial stage, approximately TRL 6–7, reflecting mature sub-processes but still limited evidence of full-scale continuous operation. In contrast, the biomass-gasification-to-hydrogen-to-ammonia pathway remains one step lower in maturity, primarily because of integration and long-term reliability constraints.

Techno-economic analyses indicate that the cost of HB ammonia produced from biomass- or waste-derived hydrogen spans a wide literature range and remains strongly dependent on plant scale, feedstock price and logistics, moisture and composition of the feedstock, co-product credits, and the degree of process integration [33,34,253]. Under favorable large-scale assumptions, recent biomass-to-ammonia studies report levelized or selling costs of about USD 425.85–468.13 t⁻¹ NH₃ at 2000 t day⁻¹ [253], while an industrial bio-oil-conversion study reported that the minimum selling price for the steam-reforming pathway could be as low as USD 0.39 kg⁻¹ NH₃, equivalent to about USD 390 t⁻¹ NH₃ [254]. Earlier large-scale biomass-gasification assessments likewise indicate costs near USD 500 t⁻¹ NH₃ under favorable woody-biomass assumptions [38]. At intermediate or smaller scales, the reported costs are higher: black-liquor gasification and co-gasification with pulp and waste sludges yield about USD 743–748 t⁻¹ NH₃ [33], sorption-enhanced biomass gasification about USD 692 t⁻¹ NH₃ [255], and solar-driven biomass gasification about USD 871.70 t⁻¹ NH₃ [256]. Small-scale biomass systems can exceed USD 1100 t⁻¹ NH₃, as shown by Arora et al. [31] who reported USD 1153–1172 t⁻¹ NH₃ for autothermal-reforming (ATR)- and steam-methane-reforming (SMR)-based biomass routes. Biomethane-based HB variants occupy an intermediate position: a recent European Union (EU) analysis of biomethane with CCS reported a net-zero production cost of about USD 0.65 kg⁻¹ NH₃ [257], whereas a landfill-biogas-derived biohydrogen route was found to be unprofitable under baseline assumptions, requiring ammonia prices of roughly USD 2.4 kg⁻¹ to become viable [258]. A further recent large-scale integrated study using unused lignocellulosic biomass and waste biogas reported a levelized ammonia cost of USD 1.11 kg⁻¹ NH₃ [259]. Taken together, the primary literature suggests a practical cost range of roughly USD 390 to above USD 2100 t⁻¹ NH₃ for biomass- and waste-based HB ammonia, while broader review-level compilations extend the upper bound to about USD 2416 t⁻¹ NH₃ for municipal-waste-derived cases [230].

Therefore, the combined biomass/waste-H₂ HB category should be interpreted as an umbrella category: solid biomass- and waste-gasification-to-ammonia pathways are assigned approximately TRL 5–6, whereas biomethane reforming or pyrolysis variants are more mature, approximately TRL 6–7, because they can use existing gas-reforming and HB infrastructure more directly.

4.2.2. Challenges

The challenges associated with biomass- or waste-based hydrogen can be grouped into feedstock constraints, process complexity, and scale-related limitations.

• Feedstock supply and consistency: Biomass and waste resources are geographically dispersed, heterogeneous, and often seasonally variable. Securing a reliable large-scale supply, typically on the order of hundreds of thousands of tonnes per year, is therefore more difficult than relying on pipeline natural gas. Moisture, ash, and contaminant contents vary widely, requiring pre-drying and cleaning. This variability increases the operation-and-maintenance burden and introduces uncertainty in system performance [260,261].
• Technological complexity and impurities: Thermochemical conversion of biomass or waste generates tars, soot, particulates, H₂S, chlorides, alkali vapors, and trace metals that can poison downstream catalysts in shift, reforming, or ammonia-synthesis units. To avoid catalyst deactivation, robust gas-cleanup systems, including cyclones, scrubbers, catalytic tar crackers, sulfur scavengers, and alkali traps, are required. These additional units increase both capital cost and parasitic energy consumption [262].
• Scale and economics: Biomass-based systems are often more feasible at modest scales than conventional world-scale ammonia plants, because scale-up is constrained not only by reactor design but also by feedstock collection, transport, storage, and densification. Recent reviews indicate that hydrogen from biomass gasification commonly falls in the range of about USD 2.0–5.0 kg⁻¹ H₂, while the 2025 IEA Bioenergy assessment estimates roughly USD 4.7 kg⁻¹ H₂ for a large 200 MW gasification plant at a biomass price of USD 23.2 MWh⁻¹ [249,263,264]. Under improved process performance and with CCS, this value could fall below USD 3.5 kg⁻¹ H₂, or to about USD 2.5–4.1 kg⁻¹ H₂ when additional CO₂ credits are assumed [249,263,264]. Consistent with the previous subsection, this translates into a wide ammonia-cost range: optimized large-scale biomass-to-ammonia systems can approach about USD 426–468 t⁻¹ NH₃, whereas smaller plants or less favorable feedstocks commonly fall in the range of roughly USD 450–970 t⁻¹ NH₃ and can exceed USD 1000 t⁻¹ NH₃ under more constrained conditions [253,255]. Accordingly, modular or decentralized plants located near biomass hubs may be more realistic than very large centralized facilities, although their higher unit costs remain a major challenge.
• Sustainability and land use: To avoid adverse land-use impacts, biomass feedstocks should originate primarily from residual or waste streams rather than from dedicated energy crops. Use of energy crops can create competition with food production, deforestation risk, or indirect land-use change. Transparent sustainability certification is therefore essential for validating life-cycle emissions and avoiding unintended environmental consequences [262,263].

4.2.3. Environmental Impact

Recent review literature confirms that ammonia production via HB using hydrogen derived from biomass or waste is among the most promising pathways for reducing life-cycle GHG emissions relative to conventional fossil-based ammonia, provided that feedstock sourcing and process integration are well managed [13,265]. At the same time, these reviews also emphasize that environmental conclusions are highly sensitive to methodological choices, including system boundaries, co-product allocation, treatment of biogenic carbon, and assumptions regarding avoided burdens in waste-based systems [13,265].

Primary LCA studies provide the absolute values needed to quantify this advantage. A widely cited comparative analysis by Gilbert et al. [38] reports cradle-to-gate climate impacts of 1.64 kg CO₂-eq kg⁻¹ NH₃ for natural-gas-based ammonia versus 0.55 kg CO₂-eq kg⁻¹ NH₃ for a biomass-gasification route, corresponding to a reduction of about 1.09 kg CO₂-eq kg⁻¹ NH₃, or approximately 66%, under assumptions that include biogenic CO₂ uptake during biomass cultivation and effective heat integration. This result remains a useful benchmark and is consistent with more recent review-level summaries of biomass-based ammonia pathways [265].

Additional quantitative studies reinforce the same climate trend while showing sensitivity to process design and scale. Arora et al. [31] reported global warming potentials of 0.588 and 0.841 kg CO₂-eq kg⁻¹ NH₃ for two small-scale biomass-derived configurations, compared with 1.84 kg CO₂-eq kg⁻¹ NH₃ for a conventional natural-gas reference under their case-study assumptions. More recently, Jeong et al. [259] reported that an integrated large-scale system using unused lignocellulosic biomass, waste biogas, ATR, chemical looping, CCS, and HB could reduce GHG emissions by 95.3% relative to conventional ammonia production. These results confirm that biomass- and waste-derived hydrogen can substantially reduce the climate footprint of ammonia, particularly when combined with carbon capture and efficient heat recovery.

Beyond climate change, however, biomass- and waste-based ammonia pathways may exhibit trade-offs in other environmental impact categories, particularly those associated with feedstock cultivation, nutrient losses, and upstream logistics. In a detailed LCA of fertilizer production using biomass gasification with Salix and straw feedstocks versus natural gas, Ahlgren et al. [37] reported that the biomass-based systems reduced global warming potential (GWP) to about 0.75 and 0.55 kg CO₂-eq per functional unit, compared with about 2.44 kg CO₂-eq for the natural-gas case, but increased eutrophication from about 26.4 g O₂-eq to about 66.7 g O₂-eq for Salix and 39.7 g O₂-eq for straw, while acidification increased from about 1.95 g SO₂-eq to about 2.42 and 2.05 g SO₂-eq, respectively. This remains one of the clearest demonstrations that biomass-based ammonia can reduce fossil GHG emissions while shifting burdens toward eutrophication and, to a lesser extent, acidification.

Waste-derived hydrogen pathways can also perform favorably, particularly when assessments credit avoided disposal or alternative waste-treatment burdens. A recent LCA by Istrate et al. [257] showed that, in the EU context, ammonia production can achieve cradle-to-gate net-zero GHG emissions on average using a blend of 56% natural gas and 44% biomethane together with CCS, while a complete switch to biomethane with CCS can yield negative cradle-to-gate GHG emissions. These results strengthen the environmental case for waste- and residue-derived methane as a transitional feedstock for decarbonizing existing HB infrastructure. At the same time, broader sustainability analyses caution that some biomass-based ammonia pathways may perform worse in land-related or biosphere-related dimensions because biomass growth itself imposes land-use pressure [266].

Overall, the literature supports the conclusion that HB ammonia using biomass- or waste-derived hydrogen can offer a substantial climate advantage over fossil-derived hydrogen pathways, particularly when feedstocks are sustainably sourced and process integration is well designed. However, high-quality comparison still requires multi-category reporting, including eutrophication, acidification, land-related impacts, and explicit treatment of biogenic carbon and allocation assumptions, because the environmental benefits and trade-offs remain strongly pathway- and context-dependent [13,265].

4.2.4. Critical TRL Evaluation

The HB route using hydrogen from biomass or waste occupies an intermediate stage of technological maturity. The underlying conversion and synthesis steps are well established, but fully integrated operation at commercial scale remains unproven. The core technologies, including gasification, reforming, gas cleaning, and hydrogen separation, are technically mature when considered individually, yet their continuous integration with ammonia synthesis has been demonstrated only at pilot and demonstration scales. Current systems confirm the chemical feasibility of producing hydrogen from renewable carbon sources, but sustained operation under variable feedstock conditions and commercial settings has not yet been achieved. The pathway is therefore best described as TRL 5–6, reflecting established technical feasibility but limited industrial validation.

The major uncertainties are primarily operational rather than scientific. They include maintaining stable gas quality, managing ash and impurities, preserving catalyst performance, and securing reliable biomass or waste supply chains. Economic viability is also highly scenario-dependent and is shaped by plant scale, biomass or waste logistics, feedstock cost, and the availability of carbon credits or other policy incentives that reward low-carbon hydrogen and ammonia production. From a critical perspective, biomass-based ammonia should be viewed as a regionally flexible and complementary route rather than as a direct substitute for renewable-hydrogen HB systems in all contexts. Progress toward higher TRL values will depend less on new chemistry and more on system engineering, feedstock management, and policy frameworks that enable distributed, low-carbon ammonia production from locally available resources.

Table 6. Concise summary of biomass- and waste-derived Haber–Bosch ammonia synthesis.

Aspect	Summary
TRL	Moderate; about TRL 5–6 for biomass/waste gasification-to-ammonia routes and TRL 6–7 for biomethane-based variants.
Main strength	Uses renewable or waste carbon resources and can reduce fossil dependence.
Main limitation	Feedstock variability, logistics, gas cleaning, tar removal, and regional dependence.
Overall role	Complementary regional option where biomass, waste, or biomethane resources are available.

summarizes the main features, limitations, and likely role of biomass- and waste-derived Haber–Bosch ammonia synthesis.

4.3. Electrochemical Synthesis (Direct Nitrogen Reduction)

The fundamental mechanism, electrolyte environment, catalyst families, and HER competition associated with direct electrochemical N₂ reduction were introduced in Section 3.2. In this section, these issues are revisited from the perspective of technology readiness, reproducibility, environmental implications, and economic feasibility.

4.3.1. Current TRL

Electrochemical ammonia synthesis through direct electrochemical nitrogen reduction (N₂RR) remains confined to benchtop research. Most assessments place the technology at TRL 3–4, with a few optimistic prototypes approaching TRL 5 but still showing limited stability and reproducibility; broader comparative assessments likewise classify it as pre-pilot [13].

Typical aqueous N₂RR systems produce only microgram- to milligram-scale ammonia per hour with modest Faradaic efficiency. Early reports often suffered from contamination by background NH₃ or NO_x species in gases and electrolytes. Best-practice protocols therefore require rigorous blanks, ¹⁵N₂ labeling with impurity audits, and gas scrubbing to eliminate false positives [133,267]. Recent analyses have also quantified impurities in commercial ¹⁵N₂/¹⁵NH₃ cylinders, emphasizing the need for isotopic verification [268].

Reviews summarizing advances across catalysts and reactor designs conclude that practical, high-rate, and stable N₂-to-NH₃ electrocatalysis has not yet been achieved [225]. Lithium-mediated N₂ reduction in nonaqueous electrolytes offers improved selectivity, but remains limited by stability, current density, and energy efficiency and is still regarded as a laboratory research topic [269,270].

As of 2025, direct N₂RR remains a low-TRL pathway, and the literature on its economics is still dominated by techno-economic and process-modeling studies rather than pilot-demonstrated continuous systems [271,272,273,274,275]. Importantly, the reported ammonia costs span a wide range because they are based on substantially different assumptions regarding catalyst performance, electricity price, process scale, and system boundary. One of the earliest direct electrochemical nitrogen-reduction techno-economic studies, by Hochman et al. [271] reported modeled ammonia production costs of about USD 508 t⁻¹ NH₃ at 62.2% energy efficiency and a fixed electricity price of USD 50 MWh⁻¹, with more optimistic dynamic-electricity scenarios giving minima of about USD 284–547 t⁻¹ NH₃, depending on electricity price and operating strategy. A later large-scale analysis by Lazouski et al. [272] found that fully electrochemical ammonia production could approach about USD 0.9–1.0 kg⁻¹ NH₃, but only if the electrochemical reactor achieves ammonia partial current densities above about 400 mA cm⁻², energy efficiencies above about 30%, and operational lifetimes of several years. More recent studies using representative current experimental performance are substantially less optimistic. Kim et al. [275] reported that direct N₂RR still requires a levelized ammonia cost greater than USD 1.7 kg⁻¹ NH₃, making it the least economically favorable of the electrochemical ammonia pathways they examined. In parallel, Izelaar et al. [273] showed that aqueous N₂RR at ambient conditions becomes comparable with SMR–Haber–Bosch only under very optimistic assumptions, namely Faradaic efficiency above 80%, current density at or above 0.3 A cm⁻², and electricity cost below USD 0.024 kWh⁻¹, while Rix and Mitsos [274] concluded that even lower-bound competitiveness requires current densities and Faradaic efficiencies above about 100 mA cm⁻² and 60%, respectively, with actual requirements likely being stricter once neglected downstream burdens are included. Therefore, the most defensible interpretation is that direct N₂RR currently exhibits ammonia costs above about USD 1.7 kg⁻¹ NH₃ under present representative performance, whereas lower values in the range of roughly USD 0.28–1.0 kg⁻¹ NH₃ should be treated as aspirational or target-based future scenarios rather than as near-term realizable production costs.

4.3.2. Challenges

Direct N₂RR faces intertwined scientific and engineering barriers:

• Very low reaction rates and current density: The strong N≡N bond makes N₂ activation difficult, and in aqueous media the competing HER often dominates. Most reported current densities lie between about 0.1 and 10 mA cm⁻², far below the approximately 50–100 mA cm⁻² generally considered necessary for industrial relevance [276,277]. Techno-economic analyses indicate that competitive operation would require current densities above 100 mA cm⁻² with Faradaic efficiency above 60%, and in more stringent scenarios above 300 mA cm⁻² with Faradaic efficiency above 80% [273,274,276,277].
• Selectivity, false positives, and contamination control: Ambient NH₃/NO_x contamination from gases, electrolytes, membranes, laboratory consumables, or air exposure can produce spurious results, particularly when the true production rate is low. Reliable verification therefore requires rigorous blanks, impurity scrubbing, and ¹⁵N₂-labeling protocols with explicit impurity audits for ¹⁵N feed gases; omission of these controls remains a recurring problem in the literature [276,277].
• Catalyst limitations: Effective catalysts must bind and activate N₂, mediate successive proton/electron-transfer steps, suppress HER, and resist deactivation. Many materials bind N₂ either too weakly or too strongly. Approaches such as single-atom catalysts, nitrides, electrides, defect-engineered surfaces, and heterostructures show promise, but still lack the long-term stability and scalability required for practical application [276,278].
• Competing HER and electrolyte trade-offs: In aqueous electrolytes, HER preferentially consumes protons and electrons, sharply reducing selectivity at practical current densities. Moving to nonaqueous media, engineered proton donors, or Li-mediated schemes can improve selectivity, but often introduces new constraints, including electrolyte resistance, dendrite control, solid-electrolyte-interphase management, solvent stability, and safety, that limit durability and overall system efficiency [276,277].
• Membrane, electrolyte, and ammonia recovery: Ammonia can diffuse through membranes, remain dissolved in the electrolyte, or be lost with the gas stream. Efficient recovery from dilute solutions without imposing excessive energy demand remains unresolved [276,277].
• Scale-up and long-term operation: Scaling from laboratory electrodes to industrial dimensions introduces additional challenges in mass transport, fouling, thermal control, and mechanical integrity. Most catalysts deactivate within tens to hundreds of hours, which remains far short of the multi-year lifetimes required for commercial relevance [276,277].

4.3.3. Environmental Impact

In principle, direct N₂RR can avoid the high-temperature and high-pressure operation of the conventional Haber–Bosch process and eliminate direct process CO₂ emissions when powered by low-carbon electricity. However, the currently usable environmental evidence indicates that its life-cycle performance is governed primarily by electricity demand, which is in turn strongly controlled by ammonia partial current density, Faradaic efficiency, and downstream separation burden rather than by reactor concept alone [13,275,279].

Recent comparative assessment confirms this point. Kim et al. [275] conducted a techno-economic and environmental analysis of three emerging electrochemical ammonia pathways, namely N₂RR, Li-mediated NRR, and NO₃RR, under future grid scenarios. Their publicly accessible abstract reports a GWP of about 2 kg CO₂-eq kg⁻¹ NH₃ for NO₃RR and identifies direct N₂RR as the most environmentally and economically constrained route because low catalytic performance leads to high electricity demand and poor overall system efficiency [275]. Thus, although N₂RR is conceptually attractive, the present evidence does not yet establish a robust low-GWP benchmark for the pathway under current representative performance.

Additional quantitative insight comes from earlier cradle-to-gate work on electrochemical ammonia production using a proton-conducting membrane route, which is often treated as a proxy for direct electrochemical NH₃ concepts in environmental discussion, although it should not be interpreted as a strict benchmark for modern direct aqueous N₂RR [280]. In that GREET-based analysis, total energy use reached about 17 MWh t⁻¹ NH₃, and water consumption for the electrochemical route coupled with electrolysis-derived H₂ was about 2948 U.S. gallons per 1000 kg NH₃, compared with about 2044 U.S. gallons per 1000 kg NH₃ for an HB route using electrolysis-derived H₂ under the same assumptions [280]. The same study also reported inventories for pollutants beyond CO₂, including VOCs, NO_x, SO_x, CO, PM, CH₄, and N₂O, showing that non-climate impacts and resource burdens may also shift significantly with electricity source and upstream hydrogen supply [280].

A useful sustainability threshold is provided by broader comparative analyses of electrified ammonia production. One widely cited assessment concluded that direct electrochemical ammonia synthesis would need full-cell energy efficiency above about 40% at around 0.5 A cm⁻², together with electricity carbon intensity below roughly 180 g CO₂-eq kWh⁻¹, before clear climate benefits relative to incumbent Haber–Bosch production become likely [279]. This threshold is important because it links environmental competitiveness directly to reactor performance and grid decarbonization rather than to the conceptual appeal of mild-condition N₂ reduction alone.

Overall, direct N₂RR remains environmentally promising in principle, but the currently usable evidence shows that its realized advantage is still conditional rather than established. At present, the route is best described as a research-stage option whose climate and broader environmental performance depend critically on large improvements in current density, Faradaic efficiency, durability, and electricity use per unit NH₃, together with careful assessment of water use and non-climate emissions across the full system boundary.

4.3.4. Critical TRL Evaluation

Electrochemical N₂RR remains a low-maturity research technology at TRL 3–4. Despite rapid progress in mechanistic understanding and catalyst design, all demonstrations still occur under controlled laboratory conditions and produce microgram- to milligram-scale ammonia with low Faradaic efficiency and limited reproducibility. The pathway lacks prototype-scale validation and long-term stability; most tests last only short periods and use highly purified electrolytes. Advances in catalyst architecture, membrane design, and cell engineering have not yet yielded scalable and durable performance.

From a critical perspective, N₂RR currently represents a frontier of fundamental electrocatalysis rather than a near-term industrial route. Advancement to higher readiness will require reproducible long-duration operation, standardized testing methods, and verified energy and life-cycle metrics that demonstrate competitiveness with established ammonia-production technologies. A concise summary of the direct electrochemical N₂ reduction route is provided in

Table 7. Concise summary of direct electrochemical nitrogen reduction.

Aspect	Summary
TRL	Low; about TRL 3–4.
Main strength	Mild-condition and modular electricity-to-ammonia concept.
Main limitation	Low NH3 yield, low selectivity, HER competition, and reproducibility issues.
Overall role	Long-term research route rather than a near-term industrial option.

.

4.4. Photocatalytic Ammonia Synthesis

The fundamental light-driven mechanism, catalyst requirements, and charge-separation challenges of photocatalytic ammonia synthesis were introduced in Section 3.3. The present section evaluates these issues from the perspective of technology readiness, environmental performance, economic feasibility, and practical deployment limitations.

4.4.1. Current TRL

Photocatalytic nitrogen fixation remains at the fundamental research stage and is still limited to benchtop experiments and material-discovery studies. Reviews consistently describe it as an emerging pathway restricted to proof-of-concept demonstrations under controlled laboratory conditions.

Collado et al. [281] classify photocatalytic, light-driven ammonia synthesis at TRL 1, emphasizing that current systems only demonstrate the scientific principle of nitrogen photoreduction without verified scalability or durability [281]. They also note that most reported devices generate only trace amounts of ammonia and that even these results require confirmation through ¹⁵N₂ labeling to exclude contamination.

Similarly, Mohamed et al. [13] identify photocatalytic ammonia synthesis as a low-TRL pathway, approximately TRL 2–4, highlighting that despite rapid progress at the materials level, the route still lacks reactor-scale integration and validated energy efficiency. Their comparative assessment places photocatalysis alongside electrochemical nitrogen reduction among the least mature ammonia-production routes in current decarbonization strategies.

No pilot or prototype reactors have yet been reported; all published studies remain bench-scale tests yielding $\mathsf{\mu }$mol-level ammonia under ultraviolet or visible irradiation. Current research, therefore, continues to focus primarily on discovering and characterizing new photocatalyst materials, such as modified TiO₂, SrTiO₃, carbon nitride, and doped semiconductors, rather than on system-level optimization or continuous operation.

Available techno-economic analyses for direct photocatalytic ammonia synthesis remain very limited, and the reported costs should be interpreted as prospective scenario-based estimates rather than near-commercial values, because they depend strongly on assumed STA efficiency, reactor capital cost per illuminated area, catalyst durability, and whether nitrogen is supplied directly from air or through a prior separation step [281,282,283]. The main quantitative benchmark currently available remains the analysis of Liu et al. [282], which showed that, under highly optimistic assumptions of 10% STA efficiency, the LCOA could span approximately USD 30–660 t⁻¹ NH₃, depending on reactor configuration. In that study, low-capital slurry and fixed-panel reactors without air separation gave the lowest estimated costs, about USD 30 and USD 120 t⁻¹ NH₃, respectively, while membrane-based nitrogen separation increased these values to about USD 110 and USD 200 t⁻¹ NH₃; concentrator-array designs were substantially more expensive at about USD 580–660 t⁻¹ NH₃ [282]. The same analysis further showed that reaching an LCOA of USD 600 t⁻¹ NH₃ would require minimum STA efficiencies of roughly 1% for slurry reactors, 2.5% for fixed-panel systems, and 10% for concentrator arrays [282]. More recent reviews have not introduced a new independent direct-photocatalytic cost range, but instead confirm that techno-economic evidence remains limited and that current photocatalytic systems still operate far below the efficiencies assumed in these prospective models [281,283]. In particular, most reported photocatalysts still exhibit STA values below 0.1%, with only isolated best-case values approaching about 0.3% [281]. Therefore, although prospective TEA suggests that photocatalytic ammonia could become cost-competitive under favorable assumptions, the currently defensible interpretation is that USD 30–660 t⁻¹ NH₃ represents a forward-looking modeled range rather than a realizable near-term production cost.

4.4.2. Challenges

Photocatalytic ammonia synthesis faces both fundamental and engineering challenges similar to those encountered in direct electrochemical N₂ reduction, but compounded by the constraints of light harvesting:

• Low efficiency and quantum yield: Converting N₂ to NH₃ requires six electrons, and most absorbed photons are lost through charge-carrier recombination. Reported quantum efficiencies are typically well below 5%, far below the levels generally regarded as necessary for practical photocatalytic ammonia production [104,284].
• Competing reactions: H₂O splitting and HER often dominate under illumination. Many systems therefore rely on sacrificial agents to scavenge holes, which increases cost and generates additional waste. Achieving direct H₂O oxidation to O₂ while selectively reducing N₂ remains a major unresolved challenge [104].
• Nitrogen activation: Most photocatalyst surfaces adsorb N₂ only weakly. Designing active sites that both harvest light efficiently and bind or activate nitrogen, for example vacancy-engineered, FeMo-inspired, or plasmonic sites, remains difficult [284].
• Stability: Many semiconductor photocatalysts undergo photocorrosion, phase transformation, or gradual deactivation under sustained illumination, leading to rapid performance loss [284].
• Ammonia separation and production rate: The NH₃ concentration in aqueous suspension is usually in the ppm range, making downstream recovery and purification potentially energy-intensive [285].
• Reactor scaling: Efficient large-area illumination, heat management, and mass-transfer control remain unresolved at scale. Outdoor operation must also accommodate sunlight intermittency and environmental variability [284].

4.4.3. Environmental Impact

The environmental profile of direct photocatalytic ammonia synthesis has not yet been established through a robust route-specific cradle-to-gate life-cycle assessment. Recent reviews explicitly note that techno-economic studies and life-cycle analyses for photo(electro)catalytic nitrogen-reduction technologies are still lacking, so current discussion of environmental impact must rely mainly on performance-based proxies and broader system considerations rather than on a validated GWP benchmark [13,281].

In principle, photocatalytic NH₃ synthesis could become a very low-carbon route because it operates under mild conditions and uses sunlight as the primary energy input, potentially avoiding the high-temperature and high-pressure energy demand of the conventional Haber–Bosch process. However, this theoretical advantage is not yet realized in practice because present photocatalytic systems remain extremely inefficient. Recent assessments of light-driven nitrogen reduction show that the best reported STA efficiencies are only around 0.3%, while most photo(electro)catalysts still operate below 0.1% [281]. As a result, very large illuminated areas, high catalyst inventories, and long operating times would be required to produce meaningful NH₃ output, which would amplify the embodied impacts of reactor materials, supports, and module fabrication.

Land and area demand provide one of the few quantitative environmental indicators currently available. Under favorable assumptions and in regions with high solar flux, some analyses summarized in recent reviews estimate solar-capture requirements below 10% of farmland demand when photocatalysts with 0.1% STA are assumed. One cited estimate corresponds to about 100 m² ha⁻¹, or roughly 1% of arable land area, for a fertilizer demand of 50 kg N ha⁻¹ year⁻¹ [281]. Although these estimates suggest that land use may not be prohibitive under optimistic assumptions, actual area requirements would become much larger if the very low efficiencies typical of current photocatalysts persist.

Another likely contributor is downstream recovery. Because photocatalytic nitrogen fixation usually produces very low ammonia concentrations in solution, additional separation and concentration steps would be required before a usable fertilizer or ammonia product could be obtained. This means that environmental performance will depend not only on photon utilization, but also on the energy and materials required for product capture from dilute streams. In parallel, photocatalyst durability remains an environmental concern because photocorrosion, short lifetimes, and the use of potentially toxic or scarce materials can increase material consumption and waste generation [283,286].

Overall, photocatalytic ammonia synthesis remains one of the most environmentally attractive concepts in theory, but its practical environmental advantage has not yet been demonstrated quantitatively. The currently available evidence supports a cautious conclusion: until higher STA efficiency, longer catalyst lifetime, and low-energy ammonia recovery are achieved, the embodied impacts of materials, large illuminated-area requirements, and separation burdens are likely to dominate the life-cycle profile.

4.4.4. Critical TRL Evaluation

Photocatalytic ammonia synthesis remains at a very early stage of technological maturity, approximately TRL 2–3. Progress to date has focused mainly on catalyst discovery, band-gap engineering, and mechanistic investigation rather than on practical system development. Laboratory experiments produce only trace amounts of ammonia under controlled conditions, and reported efficiencies remain far below the levels required for prototype validation.

The primary challenges, namely weak N₂ activation, very low STA efficiency, and strong competition from HER, continue to prevent translation from materials research to device-scale implementation. No system has yet demonstrated stable, continuous operation or meaningful scalability, and product recovery at trace concentrations remains a major technical bottleneck.

From a critical perspective, photocatalytic nitrogen fixation is scientifically valuable but technologically immature. Its current significance lies mainly in advancing photocatalyst design and improving understanding of photochemical reaction pathways, whereas practical deployment will require integrated reactor design, efficient product separation, and long-term stability under realistic solar conditions. The main strengths, limitations, and development status of photocatalytic ammonia synthesis are summarized in

Table 8. Concise summary of photocatalytic ammonia synthesis.

Aspect	Summary
TRL	Very low; about TRL 2–3.
Main strength	Direct solar-driven operation under mild conditions.
Main limitation	Very low efficiency, photocorrosion, HER competition, and difficult NH3 detection.
Overall role	Fundamental materials and mechanism research platform.

.

4.5. Photoelectrochemical Ammonia Synthesis

The basic photoelectrochemical mechanism, photoelectrode operation, and catalyst/material considerations are described in Section 3.4. Here, the discussion focuses on how these features affect TRL, energy efficiency, stability, scalability, and the feasibility of photoelectrochemical ammonia production.

4.5.1. Current TRL

The PEC ammonia-synthesis pathway remains at a very early stage of development and is generally assessed at TRL 2–4. Laboratory-scale devices, typically with electrode areas on the order of a few square centimeters, have demonstrated proof-of-concept ammonia generation under illumination, but no pilot- or prototype-scale systems have been reported. Even PEC water splitting, which is a simpler and more mature technology, has advanced only modestly, further underscoring the immaturity of PEC N₂ reduction.

Recent reviews confirm that PEC N₂-to-NH₃ synthesis remains primarily at the research stage. Li et al. [119] emphasized that yield, selectivity, and stability remain major limitations. Likewise, Gnanasekar et al. [287] described PEC nitrogen reduction as an attractive but still highly challenging route, highlighting persistent issues related to photoelectrode stability, charge separation, and scalable reactor architecture. Considering the current limitations in device area, reaction rate, and stability, together with the absence of continuous or industrial operation, a readiness level of approximately TRL 3, corresponding to laboratory-scale demonstration without prototype validation, appears most appropriate.

Techno-economic evidence for PEC nitrogen-to-ammonia synthesis remains extremely limited, and the available figures should be interpreted as scenario-based projections rather than as costs demonstrated by existing PEC ammonia plants. Recent review literature explicitly notes that there is a lack of techno-economic studies centered on photo(electro)catalytic ammonia synthesis [281]. At present, the main PEC-related system-level estimate available in the literature is an exergoeconomic analysis of a hybrid solar concept in which PEC H₂ generation is coupled to a subsequent low-temperature ammonia-synthesis step; under the assumed large-scale design and financial inputs, hydrogen and ammonia costs of USD 3.24 kg⁻¹ H₂ and USD 0.84 kg⁻¹ NH₃ (approximately USD 840 t⁻¹ NH₃) were reported [281,288]. This value should therefore be interpreted as a projected cost for a PEC-assisted ammonia supply chain rather than for a stand-alone direct PEC-NRR reactor.

For context only, reviewed direct electrocatalytic N₂-reduction studies have reported modeled ammonia costs on the order of roughly USD 423–969 t⁻¹ NH₃ under various future renewable-power assumptions [281]. Likewise, separate photocatalytic nitrogen-fixation analyses have reported highly assumption-sensitive ammonia costs between about USD 30 and USD 660 t⁻¹ NH₃, assuming 10% solar-to-ammonia efficiency, depending on reactor type and N₂ supply strategy [281,282]. However, neither of these ranges should be interpreted as PEC-specific ammonia costs. Therefore, the present literature does not yet support a robust independent price range for direct PEC ammonia synthesis; instead, the most defensible economic benchmark remains the isolated PEC-assisted projection of about USD 0.84 kg⁻¹ NH₃, while the other reported values should be regarded only as non-PEC reference points.

4.5.2. Challenges

PEC ammonia synthesis faces combined obstacles inherent to both electrochemical N₂ reduction and solar-fuel device engineering:

• Catalyst and light-absorber integration: Efficient N₂ reduction requires coupling a stable light-absorbing semiconductor with a selective catalyst. Many semiconductors corrode in aqueous electrolytes or exhibit poor catalytic activity, making protective coatings and cocatalysts necessary. Balancing light absorption, photovoltage, stability, and selectivity, therefore, remains a central design challenge [289].
• Photovoltage requirements: Although the overall thermodynamic potential for N₂ reduction to NH₃ is close to 0 V versus the reversible hydrogen electrode (RHE), practical PEC cells must overcome substantial overpotentials at both the cathode and anode. This requires significant photovoltage from the absorber or the application of an external bias. Most reported PEC N₂-reduction experiments are therefore not bias-free and typically apply potentials of several tenths of a volt to drive the reaction. Tandem or multijunction photoelectrodes are being investigated to achieve bias-free operation, although this has been demonstrated mainly for nitrate-to-ammonia systems rather than for direct N₂ reduction [290,291,292].
• Low current density and selectivity: Verified PEC systems exhibit photocurrents in the $\mathsf{\mu }$A cm⁻² range, far below industrially relevant levels. In many configurations, most photogenerated carriers are consumed by HER or other parasitic reactions, leading to very low Faradaic efficiency and poor selectivity [293].
• Stability and durability: Photoelectrodes often degrade rapidly under ultraviolet or visible illumination combined with electrochemical stress. Many prototypes lose activity within only a few hours because of corrosion, delamination, or photocorrosion of the semiconductor [294].
• Product collection and crossover: The produced ammonia typically dissolves as ammonium in the catholyte and can diffuse to the anode or across membranes, reducing yield and complicating product recovery. Engineering efficient ammonia separation is therefore essential [295].
• Scale-up and engineering integration: Expanding from square-centimeter-scale cells to square-meter-scale modules requires uniform illumination, effective gas management, and thermal regulation. These engineering challenges, already well recognized in PEC water-splitting systems, remain unresolved and are even more demanding for PEC ammonia synthesis [296].

4.5.3. Environmental Impact

If operated efficiently under sunlight, PEC ammonia synthesis could in principle approach very low direct operational CO₂ emissions, since it is intended to couple solar energy input with nitrogen reduction under mild conditions. However, quantitative environmental assessment remains highly limited because direct cradle-to-gate life-cycle analyses for stand-alone PEC nitrogen-to-ammonia systems are still lacking. Recent review literature explicitly notes that life-cycle analyses of photo(electro)catalytic NRR technologies remain scarce, so environmental discussion must still rely partly on hybrid-system studies and analogies with PEC water splitting [281].

The most relevant ammonia-specific evidence currently available comes from a PEC-assisted hybrid route rather than from direct PEC-NRR itself. Bicer and Dincer evaluated an ambient electrochemical ammonia-production system supplied by PEC-generated hydrogen under concentrated sunlight and reported that its total environmental impact could be reduced to roughly 50% of the current steam-methane-reforming-based ammonia route [297]. This result supports the potential environmental advantage of solar-assisted low-temperature ammonia synthesis, but it should be interpreted carefully because it reflects a PEC-H₂-assisted ammonia supply chain rather than a dedicated direct PEC ammonia reactor. Accordingly, this value is treated as hybrid-system evidence only and is not used as a demonstrated GWP benchmark for stand-alone PEC ammonia synthesis.

Additional insight can be obtained from the PEC water-splitting literature, which provides the closest available life-cycle analogue. Recent assessments report global warming potentials of about 1.17 kg CO₂-eq kg⁻¹ H₂ for a PEC anion-exchange-membrane (AEM) reactor and about 1.052 kg CO₂-eq kg⁻¹ H₂ for a proposed PEC hydrogen system, while broader reviews conclude that PEC water splitting can exhibit one of the lowest environmental footprints among currently available hydrogen-production routes [296,298,299]. These findings suggest that, if PEC ammonia synthesis can eventually achieve comparable durability and material efficiency, its upstream climate burden could also be favorable.

At the same time, the main environmental limitation of PEC systems is not direct emissions, but the material and area demand associated with low practical conversion efficiency. Recent upscaling analysis for PEC water splitting indicates that, with current photoelectrodes, PEC devices may require at least 50 times larger active area than photovoltaic–electrolysis (PV-EC) systems to reach similar hydrogen output [296]. This implies potentially large embodied impacts from semiconductor absorbers, catalysts, transparent conductive layers, membranes, and protective coatings, as well as greater land-use pressure if solar-to-ammonia efficiency remains very low. Therefore, the practical environmental benefit of PEC ammonia synthesis will depend not only on renewable operation, but also on major improvements in conversion efficiency, durability, and scalable low-impact materials.

Overall, PEC ammonia synthesis remains one of the most environmentally attractive routes in theory, but its environmental advantage has not yet been demonstrated by a direct route-specific LCA. The currently usable evidence suggests that the pathway could become very low-carbon if high efficiency and long lifetime are achieved; however, under present laboratory performance, the embodied burdens of device manufacture, replacement, and large illuminated area are likely to dominate the life-cycle profile.

4.5.4. Critical TRL Evaluation

PEC ammonia synthesis remains at an early proof-of-concept stage, corresponding to approximately TRL 2–3. The concept of coupling light absorption with electrochemical nitrogen reduction is scientifically compelling, yet current devices operate only under controlled laboratory conditions and yield negligible amounts of ammonia, with photocurrents in the $\mathsf{\mu }$A range. These findings reflect steady progress in materials research, but only limited progress toward practical device-level maturity or meaningful energy efficiency.

Major barriers to higher readiness include semiconductor instability, insufficient photovoltage, weak N₂ activation, and dominant HER, all of which limit both selectivity and durability. Although bias-assisted configurations and nitrate-reduction analogues show partial feasibility, they remain model systems rather than genuine solar-driven nitrogen fixation.

To advance toward TRL 4, reproducible PEC cells capable of stable operation, measurable solar-to-ammonia efficiency, and verified isotopic nitrogen reduction under real sunlight are required. Until these milestones are achieved, PEC ammonia synthesis should be regarded as technologically immature but scientifically valuable, serving as a platform for the development of next-generation photocatalysts and integrated solar-chemical systems.

Table 9. Concise summary of photoelectrochemical ammonia synthesis.

Aspect	Summary
TRL	Very low; about TRL 2–3.
Main strength	Combines light absorption with electrochemical nitrogen reduction.
Main limitation	Low current density, weak N2 activation, photoelectrode degradation, and limited scale-up evidence.
Overall role	Exploratory solar-fuels platform with long-term potential.

summarizes the key features and current limitations of photoelectrochemical ammonia synthesis.

4.6. Plasma-Assisted Ammonia Synthesis

The plasma-activation mechanism and plasma–catalyst interactions are described in Section 3.5. The following discussion focuses on how these mechanistic features affect TRL, energy efficiency, reactor scalability, and deployment potential.

4.6.1. Current TRL

Plasma-assisted ammonia synthesis is at an early-to-intermediate stage of development and is typically positioned around TRL 3–5, depending on reactor type and system integration. Recent reviews classify plasma ammonia among promising but still under-developed electrified routes and describe DBD and microwave-plasma reactors as laboratory-validated systems that remain far from pilot or commercial deployment [13,300].

Recent work shows steady but still modest progress in energy yield and operational stability. Rouwenhorst et al. [301] demonstrated continuous ammonia formation in a plasma-catalytic configuration at ambient pressure, supporting the feasibility of decentralized operation under renewable electricity [301]. Comparable modular concepts are also being explored in the Netherlands, for example, by TNO and Innovatus, and in the United States, for example, in programs supported by ARPA-E, with emphasis on reactor architecture and power-electronics control to improve conversion and energy yield.

Bench-scale reactors operating at ambient or near-ambient pressure routinely produce ammonia, but the ammonia yield per unit electrical energy input remains low. Representative results include approximately 0.6 g NH₃ kWh⁻¹ for Ru/AC in DBD systems, 1.0–1.5 g NH₃ kWh⁻¹ in packed-bed DBD reactors, and up to 2.3 g NH₃ kWh⁻¹ when in situ adsorption suppresses plasma decomposition. Single-pass N₂ conversions are typically below a few percent, for example, about 0.38–2.7%, confirming laboratory feasibility but remaining well short of techno-economic targets [302].

Process-feasibility and economic analyses underline why TRL advancement remains constrained primarily by energy performance and low outlet NH₃ concentration. In a detailed small-scale feasibility study, Rouwenhorst and Lefferts [303] concluded that plasma-catalytic ammonia would need to reach a total energy consumption on the order of 80 GJ t⁻¹ NH₃ to become competitive with an electrolysis-based small-scale Haber–Bosch benchmark at about 10 kW. They reported that the best plasma value identified in their literature survey was about 95 GJ t⁻¹ NH₃ at an outlet concentration of only 0.2 mol.% NH₃, while most reported values span roughly ${10}^3$–${10}^6$ GJ t⁻¹ NH₃, depending on conversion level and operating conditions. The same study further showed that the capital investment of the state-of-the-art plasma-catalytic synthesis loop is about one order of magnitude higher than that of a comparable small-scale Haber–Bosch synthesis loop, largely because low single-pass conversion drives large recycle requirements and more demanding NH₃ separation at low partial pressure; even a best-case plasma loop was estimated to be only comparable in cost to the small-scale Haber–Bosch loop rather than clearly cheaper [303].

Recent roadmap literature is consistent with these feasibility conclusions. For example, Kosca et al. [300] summarize that current laboratory-scale plasma-catalytic ammonia synthesis can exhibit energy costs of about 26.4 MWh t⁻¹ NH₃ for ammonia synthesis alone, even before recycling and product-separation penalties are included. Together, these results indicate that plasma ammonia has clear proof-of-concept validation and modularity advantages, but remains limited by energy efficiency, low NH₃ outlet concentration, and durability, which jointly hinder progression toward TRL 5–6 without substantial performance gains.

Quantitative cost estimates for plasma-assisted ammonia synthesis are necessarily scenario-dependent because no commercial plasma-ammonia plants exist and because modeled costs are dominated by assumptions regarding (i) plasma energy yield in g NH₃ kWh⁻¹, (ii) electricity price and capacity factor, (iii) outlet NH₃ concentration, which drives recycle and separation duty, and (iv) whether the system is evaluated at plant gate or as a delivered supply cost including logistics. In a supply-chain-optimization study for Australia, they evaluated NTP ammonia synthesis coupled with renewable hydrogen supply via electrolysis across centralized and distributed configurations. From their reported scenario results, the centralized NTP-electrolysis option yields ammonia supply costs of approximately USD 565–825 t⁻¹ NH₃, whereas the distributed NTP-electrolysis options are substantially higher, about USD 1000–1432 t⁻¹ NH₃; the authors note that NTP-based ammonia is the most expensive option under their assumptions and remains far from competitiveness at present performance levels. By contrast, a more optimistic future-facing process-integration study that couples high-temperature plasma methane conversion for H₂ production with an improved NTP ammonia loop reported that, if the NTP energy yield approaches the best literature values under plausible improvement scenarios, the predicted ammonia cost could fall below USD 452 t⁻¹ NH₃ [304]. Taken together, the peer-reviewed TEA literature suggests that plasma-assisted ammonia is currently projected in the high-hundreds to above-USD 1000 t⁻¹ range in decentralized or supply-chain settings, while competitiveness near USD 450 t⁻¹ appears achievable only under substantially improved energy yield and favorable operating assumptions.

4.6.2. Challenges

Plasma-assisted ammonia synthesis faces several interconnected challenges:

• Excessive energy consumption: Current plasma-catalytic systems are highly energy-intensive, consuming one to two orders of magnitude more energy than the conventional Haber–Bosch process. Bench-scale DBD and microwave reactors produce sub-gram quantities of NH₃ per kilowatt-hour, reflecting major losses to nonselective electron heating and radiation rather than productive N₂ activation. Improving electron-energy selectivity and plasma–catalyst coupling is therefore essential for enhancing efficiency [147].
• Low single-pass conversion and separation complexity: In most plasma-catalytic systems, outlet ammonia concentrations are only about 0.2 mol.%, which means that N₂-to-NH₃ conversion per pass remains extremely low. This necessitates extensive gas recycling and compression, offsetting the benefits of near-atmospheric-pressure operation [303].
• Reactor and catalyst integration: The interaction between plasma species and catalyst surfaces is not yet fully understood. Catalyst sintering, nitridation, and plasma-induced degradation can shorten catalyst lifetime and reduce reproducibility [305].
• NO_x formation: Even trace oxygen impurities can lead to the formation of NO or N₂O in the plasma, reducing selectivity and complicating downstream purification [306].

4.6.3. Environmental Impact

Plasma-assisted ammonia synthesis can eliminate direct reactor-level CO₂ emissions when powered by renewable electricity, but current life-cycle evidence shows that its overall environmental performance is still dominated by electricity demand in plasma generation, gas recycle, and ammonia separation. Because present plasma systems generally operate at low single-pass NH₃ yield, the environmental burden depends strongly on the electricity source, plasma energy efficiency, and the extent of downstream recycle and product-capture requirements [13,307,308,309].

Among the available route-specific assessments, the cradle-to-site study of Osorio-Tejada et al. [308] provides the clearest quantitative benchmarks. In that work, the conventional centralized SMR-HB ammonia supply chain reached up to 2.96 kg CO₂-eq kg⁻¹ NH₃, with transport alone contributing as much as 29.3% of the total impact [308]. Distributed renewable pathways performed substantially better, and some plasma-related scenarios even produced negative net global-warming values once co-product credits, mainly oxygen and carbon black, and avoided transport were included. For example, the authors reported about $-0.65$ kg CO₂-eq kg⁻¹ NH₃ for a biogas-based plasma-related case and about $-1.07$ kg CO₂-eq kg⁻¹ NH₃ for a solar-powered farm-scale electrolyzer-non-thermal-plasma case with improved assumptions for plasma yield and separation [308]. These values are therefore scenario-specific net results rather than unconditional pathway emissions. The same study also showed that plasma-related pathways should not be judged by climate change alone: in credited renewable cases, freshwater-eutrophication impacts were reduced by up to 290%, whereas land-use and some toxicity-related impacts could worsen depending on the electricity source, particularly for solar- or biomass-based supply chains [308].

Process-level studies reach the same general conclusion. Anastasopoulou et al. [307] identified electricity consumption in plasma operation, together with downstream cooling and separation associated with low NH₃ yield, as the dominant source of environmental burden in small-scale plasma-ammonia systems. A related eco-efficiency analysis by the same research group further suggested that plasma-assisted NH₃ could show an improved environmental profile relative to the contemporary route only after substantial gains in performance, corresponding to about 17.2 g NH₃ kWh⁻¹ and 5% energy recovery in their prospective case study [307]. Riotto et al. [309] independently reinforced this hotspot diagnosis in a detailed process-design and environmental-impact assessment of an atmospheric-pressure DBD plasma NH₃ synthesis-and-separation flowsheet, reporting substantially larger environmental impacts across the board for the plasma-based process because of the increased electricity consumption required by plasma generation and associated separations.

Overall, the available environmental evidence indicates that plasma-assisted ammonia synthesis can become environmentally attractive only under favorable conditions, namely, low-impact electricity, substantially improved plasma energy yield, and better integration of recycling and separation. Otherwise, its impacts can remain above those of conventional or mature renewable-H₂ + Haber–Bosch routes, especially once non-climate categories and supply-chain assumptions are included. Thus, current plasma-ammonia LCAs support a conditional rather than universal environmental advantage.

4.6.4. Critical TRL Evaluation

Plasma-assisted ammonia synthesis remains a low-to-intermediate-readiness technology and is best characterized near the upper end of TRL 4, spanning roughly TRL 3–5 depending on reactor type and degree of system integration. Laboratory studies have repeatedly confirmed the fundamental feasibility of plasma-driven nitrogen activation, and modular prototypes increasingly integrate plasma discharge with catalytic surfaces. However, present performance is still constrained by low energy yield, low single-pass conversion, and limited operational lifetime, which together impede scale-up and sustained operation under realistic conditions.

Recent advances in plasma–catalyst coupling, DBD optimization, and power-electronics control demonstrate steady technical progress, yet continuous operation under industrially relevant conditions has not been demonstrated. In addition, the most recent scenario-based TEA results reinforce that current performance limitations translate into high projected ammonia costs: centralized configurations are typically estimated in the high-hundreds of USD per tonne, while decentralized supply-chain cases can exceed USD 1000 t⁻¹ NH₃, with substantially lower costs, on the order of about USD 450 t⁻¹ NH₃, appearing only in future-facing scenarios that assume major improvements in plasma energy yield and favorable operating assumptions.

To progress toward TRL 5–6, the technology must demonstrate sustained plasma-catalytic activity in an integrated loop, including recycle and separation, together with materially higher energy efficiency, higher effective NH₃ outlet concentration, minimal NO_x/N₂O formation under realistic impurity levels, and stable long-duration performance, on the order of hundreds to thousands of hours, with quantified degradation and maintenance requirements.

In critical assessment, plasma-assisted ammonia synthesis is scientifically validated but technologically premature. At present, it functions primarily as a flexible research platform for decentralized or renewable-powered ammonia production rather than as a near-term industrial alternative to the Haber–Bosch process.

The main characteristics, challenges, and possible role of plasma-assisted ammonia synthesis are summarized in

Table 10. Concise summary of plasma-assisted ammonia synthesis.

Aspect	Summary
TRL	Low to moderate; about TRL 3–5.
Main strength	Near-ambient operation, modularity, and compatibility with intermittent electricity.
Main limitation	High energy demand, low single-pass conversion, low NH3 concentration, and separation burden.
Overall role	Possible niche route if energy yield and integration improve substantially.

.

4.7. Biological Routes (Enzymatic or Microbial Synthesis)

The biological nitrogen-fixation mechanism and nitrogenase-related constraints were introduced in Section 3.7. This section evaluates their implications for process maturity, productivity, energy demand, and ammonia recovery.

4.7.1. Current TRL

Biological ammonia synthesis remains at the fundamental research stage, approximately TRL 1–3. Although biological nitrogen fixation is well established in nature through symbiotic and free-living microorganisms, harnessing this mechanism to generate a separable ammonia stream remains largely conceptual. Mohamed et al. [13] classify microbial and algal nitrogen fixation as early-TRL biological pathways with no present industrial deployment potential.

Natural nitrogen-fixing microorganisms such as Rhizobium and Azotobacter efficiently convert N₂ to ammonium for cellular metabolism, but they do not accumulate free NH₃, because it is rapidly assimilated through the glutamine-synthetase pathway [310]. Engineering efforts have produced ammonium-excreting diazotrophs, yet yields remain extremely low and confined to laboratory conditions. Commercial biofertilizers such as Pivot Bio’s strains deliver nitrogen directly to crops rather than producing recoverable ammonia [311,312].

Purified nitrogenase can catalyze N₂ reduction to NH₃ in vitro using ATP and reductants, but continuous ATP regeneration and system stability remain unresolved at scale. Electro- or photobiocatalytic hybrids achieve only micromolar-level NH₃ production in proof-of-concept cells, far below process relevance [313]. Photobioreactors fix N₂ but primarily assimilate nitrogen into biomass; reports of free NH₃ in solution are minimal, and no continuous product-capture system has been demonstrated [314].

Overall, no continuous process has yet achieved measurable, recoverable ammonia generation, and the pathway remains confined to bench-scale biochemical validation without prototype systems, consistent with an overall readiness of about TRL 2–3.

Because no biological ammonia route has yet demonstrated continuous, scalable recovery of a separable NH₃ product, rigorous plant-level techno-economic assessments remain extremely scarce. The available literature is therefore better interpreted as providing order-of-magnitude, scenario-based cost indicators rather than full minimum selling prices or levelized production costs. Wiskich and Rapson reported that a sugar-powered microbial route would require about 10.6 t of glucose per tonne NH₃, implying ammonia costs above approximately USD 3000 t⁻¹ NH₃ for a representative long-run sugar price of about USD 300 t⁻¹ [315]. For a biohybrid route based on electrolysis-derived H₂ and Xanthobacter, the same study estimated electricity requirements greater than 1000 MWh t⁻¹ NH₃, corresponding to electricity-cost contributions above approximately USD 30,000 t⁻¹ NH₃ at USD 30 MWh⁻¹; they further noted a minimum CO₂ feed cost of about USD 3574 t⁻¹ NH₃, excluding capital, nutrient supply, mixing, gas handling, and product recovery [315]. At the more optimistic end, Rouwenhorst et al. [176] estimated that even if nitrogenase-based ammonia synthesis could reach an overall efficiency of about 10–15%, the corresponding energy demand would still be about 150–225 GJ t⁻¹ NH₃, equivalent to roughly 42–63 MWh t⁻¹ NH₃ and thus approximately USD 1250–1875 t⁻¹ NH₃ in electricity alone at USD 30 MWh⁻¹. Collectively, these studies indicate that biological ammonia synthesis currently implies at least multi-thousand-USD-per-tonne costs under optimistic assumptions and can readily exceed tens of thousands of USD t⁻¹ NH₃ under current biohybrid performance, while a robust full-process LCOA has not yet been established.

4.7.2. Challenges

Biological routes face coupled biological and process-engineering bottlenecks:

• Low productivity: Even highly active biological systems fix nitrogen far more slowly than industrial ammonia synthesis. In agricultural settings, symbiotic fixation is typically reported on the order of 50–200 kg N ha⁻¹ yr⁻¹, equivalent to roughly 60–240 kg NH₃ ha⁻¹ yr⁻¹, which implies very low areal productivity compared with industrial plants and highlights the scale challenge for any bioprocess intended to generate a separable NH₃ stream [316,317].
• High energy requirement for N₂ fixation: In the canonical molybdenum-nitrogenase pathway, nitrogenase requires at least 16 ATP per N₂ reduced, that is, 8 ATP per NH₃, in addition to strong reductants. Any engineered microbial system or biohybrid configuration must therefore supply substantial energy for ATP, or an equivalent driving force, and replacing biological ATP regeneration with external electrical or chemical energy can impose a large indirect energy and carbon penalty unless very low-carbon power and high overall efficiency are achieved [318,319].
• Product inhibition and metabolic burden: Accumulated NH₃/${\mathrm{NH}}_4^{+}$ suppresses nitrogenase activity and downregulates nitrogen-fixation gene expression. Engineered strains designed to excrete NH₃ often suffer from substantial metabolic burdens, reduced viability, and fitness penalties, for example glutamine auxotrophy or impaired growth, which limit sustained ammonia release and long-term productivity [310,320].
• Recovery and separation from dilute aqueous media: In aqueous systems, produced ammonia predominantly exists as ${\mathrm{NH}}_4^{+}$, and reported titres are typically low. Recovering NH₃ therefore requires pH control and stripping, membrane-based extraction, or other separation methods, which can dominate total energy use and strongly influence life-cycle impacts and cost [321,322].
• Process stability, genetic drift, and contamination control: Maintaining stable long-term performance is difficult because engineered ammonia-excreting phenotypes are often unstable. Strains can lose the engineered trait, experience evolutionary drift, or be outcompeted by wild-type organisms, leading to rapid performance degradation. Achieving reliable, scalable operation will therefore require robust strain design, strong genetic-containment and stability strategies, and reactor operation that maintains selective pressure without excessive operating complexity [323,324].

4.7.3. Environmental Impact

Biological nitrogen fixation, whether enzymatic or microbial, proceeds under ambient temperature and pressure and therefore avoids the intrinsically high-temperature and high-pressure operation of the conventional Haber–Bosch process. However, quantitative environmental comparison remains highly uncertain because route-specific cradle-to-gate LCAs for engineered microbial or enzymatic NH₃ production systems are still lacking, and the available literature is dominated by proof-of-concept studies with limited data on productivity, stability, and downstream recovery. Recent ammonia-route LCA reviews likewise indicate that environmental evidence for emerging non-Haber–Bosch pathways remains much less developed than for conventional and electrolysis-based Haber–Bosch systems [13].

Several quantitative benchmarks nonetheless help bound the likely environmental performance. At the biochemical level, nitrogenase requires at least 16 ATP per N₂ reduced, corresponding to 8 ATP per NH₃ and a theoretical minimum ATP-regeneration demand of about 244 kJ mol⁻¹ NH₃. In practice, however, overall system losses associated with electron delivery, overpotentials, cellular maintenance, gas handling, and reactor operation substantially increase this value. Rouwenhorst estimated that even an optimistic nitrogenase-based route operating at about 10–15% overall efficiency would still require roughly 150–225 GJ t⁻¹ NH₃, indicating a very large energy burden even before downstream separation is considered [176].

Available scenario analyses further suggest that resource-use burdens may be severe. Wiskich and Rapson estimated that one microbial sugar-based route would require about 10.6 t of glucose per tonne NH₃, implying substantial upstream land, water, and agricultural-input burdens associated with sugar production. For a biohybrid route based on electrolysis-derived H₂ and Xanthobacter, they estimated electricity requirements above 1000 MWh t⁻¹ NH₃ and noted that the reported laboratory productivity would imply approximately 2.1 ML of bacterial culture volume per tonne NH₃ per year, while biomass formation would also require a minimum CO₂ input corresponding to about USD 3574 t⁻¹ NH₃ in feed cost under their assumptions [315]. Although these values are not direct LCA metrics, they strongly indicate that present biological concepts remain resource-intensive.

A second major contributor is downstream recovery from dilute aqueous media. Here, the most useful evidence comes not from biological ammonia-synthesis studies themselves, but from adjacent ammonium-recovery literature that provides bounding estimates for separation burdens. Conventional stripping-based recovery has been reported to consume about 17–26 kWh kg⁻¹ of ammonium removed, while some bioelectrochemical recovery systems report approximately 1.6 kWh kg⁻¹ of removed nitrogen, excluding aeration, and about 2.9 kWh kg⁻¹ of recovered nitrogen under favorable conditions [325]. These values should be interpreted as separation analogues rather than direct environmental results for biological ammonia synthesis, but they illustrate how strongly dilute product streams can dominate total impacts.

Overall, biological nitrogen fixation remains environmentally intriguing because it can, in principle, operate under mild conditions, but its life-cycle performance has not yet been established on a system basis. The currently usable quantitative evidence suggests that environmental performance is likely to be dominated by three factors: the high effective energy demand associated with ATP and electron supply, the upstream burden of feedstocks such as glucose or externally supplied H₂/CO₂, and the large energy penalty associated with recovering NH₃ from dilute media. Therefore, any future climate advantage will require major gains in overall efficiency, high product titres, low-energy separation, and low-carbon electricity supply.

4.7.4. Critical TRL Evaluation

Biological ammonia synthesis through microbial or enzymatic nitrogen fixation remains at a very early research stage, approximately TRL 1–3. Current work is dominated by laboratory cultures and in vitro enzyme assays that confirm the biochemical feasibility of nitrogenase-mediated N₂ reduction under mild conditions, but they do not yet constitute an engineered process capable of producing a separable NH₃ stream. Reported yields remain extremely low and transient, and no continuous process has demonstrated measurable, recoverable ammonia generation, consistent with an overall readiness of about TRL 2–3 for process deployment.

Progress toward higher TRL values is constrained primarily by coupled biological and systems-level barriers rather than by the basic chemistry of nitrogenase. Key limitations include the absence of an efficient, scalable energy supply for ATP and electron regeneration, feedback inhibition and metabolic burdens that restrict sustained ammonia excretion, and the need for low-energy recovery from dilute aqueous media, where NH₃ is present mainly as ${\mathrm{NH}}_4^{+}$. Long-duration operation is further limited by genetic drift and instability of engineered phenotypes, as well as by the difficulty of maintaining stable performance in non-ideal, contamination-prone environments. These constraints are reflected in scenario-based TEA results, which currently imply ammonia costs in the multi-thousand to tens-of-thousands of USD t⁻¹ range under present performance assumptions, highlighting the large gap between biochemical feasibility and economic or process viability.

In critical assessment, microbial and enzymatic ammonia synthesis is scientifically promising but technologically embryonic. At present, it serves primarily as an exploratory platform for biocatalysis and biohybrid or electrobiocatalytic integration rather than as a practical pathway for near-term industrial ammonia production. Advancement beyond TRL 3 will require reproducible long-duration operation, demonstrably higher NH₃ titres with continuous capture, and validated energy and life-cycle metrics under realistic operating conditions. A concise summary of biological ammonia synthesis routes is presented in

Table 11. Concise summary of biological ammonia synthesis routes.

Aspect	Summary
TRL	Very low; about TRL 1–3.
Main strength	Mild-condition nitrogen fixation using enzymes or microorganisms.
Main limitation	Low recoverable NH3 titres, ATP/electron supply needs, and difficult product separation.
Overall role	Early-stage biocatalytic research route, not near-term industrial production.

.

4.8. Chemical Looping Ammonia Synthesis

The nitridation–hydrogenation mechanism and nitrogen-carrier concept of CLAS are described in Section 3.6. Here, the discussion focuses on the maturity of the integrated cycle, carrier stability, regeneration, heat management, and process-scale feasibility.

4.8.1. Current TRL

CLAS remains an early-stage laboratory technology, and no integrated, continuous pilot plants have yet been reported. Reviews consistently classify it as an emerging thermochemical pathway validated mainly in benchtop systems operating with milligram- to gram-scale solids in batch or short cyclic modes. On this basis, a reasonable classification is approximately TRL 2–3, corresponding to early proof of concept and laboratory validation, and at most approaching TRL 4 where limited subsystem integration is demonstrated [175,179,183].

Laboratory studies have verified the elementary reaction steps, namely nitridation of the solid carrier under N₂, followed by hydrogenation or steam treatment to release NH₃. These results span several carrier families, but sustained closed-loop operation with reliable heat and mass management has not yet been achieved. Reported NH₃ outputs are typically in the $\mathsf{\mu }$mol-to-mmol range per cycle, with stability demonstrated over only tens of cycles, confirming the benchtop status of current systems [181,183].

No industrial- or pilot-scale CLAS units have been built. Process optimization, particularly with respect to heat management, carrier regeneration, and continuous operation, remains largely theoretical. Consequently, CLAS should presently be regarded as a low-maturity technology, approximately TRL 2–3, with encouraging chemical feasibility but without established engineering validation.

In parallel with its low technological maturity, the techno-economic evidence for CLAS remains limited and must be interpreted carefully because the available studies do not use a common system boundary. Early work on the Al₂O₃/AlN thermochemical cycle included only a preliminary economic assessment rather than a modern minimum selling price or levelized-cost benchmark [326]. Later, Pereira et al. [327] showed through process analysis that CLAS economics depend strongly on achievable ammonia yield, reporting that, at 150 bar, the process would require a yield above 26% to achieve cost parity with a commercial synthesis unit and above 38% to reach similar energy performance. A recent CLAS-specific techno-economic analysis by Burrows and Bollas reported very low levelized costs of ammonia for the synthesis loop only, namely USD 63.9 t⁻¹ NH₃ for a conventional Haber–Bosch loop and USD 9.95–9.73 t⁻¹ NH₃ for optimized low-temperature Ni-BaH₂-based CLAS configurations, corresponding to a substantial reduction in synthesis-section cost [187]. However, these values should not be interpreted as full plant-level ammonia production costs. On a broader process basis, a recent scale-up assessment of microwave-heating-assisted CLAS, which can be regarded as a closely related CLAS variant, estimated an ammonia-production cost of approximately USD 700 t⁻¹ NH₃, with a favorable-case potential near USD 400 t⁻¹ NH₃ and a wider sensitivity range of about USD 470–1200 t⁻¹ NH₃, driven mainly by hydrogen price [328]. Therefore, although the economic database is still sparse, the most defensible present conclusion is that CLAS shows strong promise for reducing the cost of the ammonia-synthesis section, whereas a cautious pathway-level cost range for CLAS-type ammonia production presently lies on the order of USD 400–1200 t⁻¹ NH₃.

4.8.2. Challenges

CLAS faces several interrelated scientific and engineering challenges:

• Multi-step integration and heat management: The chemical-looping approach separates ammonia synthesis into nitridation and hydrogenation or hydrolysis stages, requiring multiple reactors, gas-switching systems, and control of thermal swings between exothermic and endothermic steps. Efficient heat recuperation between stages is essential for maintaining overall energy efficiency, whereas inadequate thermal integration can increase both energy consumption and process cost during cyclic operation [173,183].
• Carrier performance and durability: Nitrogen-carrier materials must rapidly activate N₂, release NH₃ efficiently, and remain stable over many cycles. However, many candidate carriers suffer from sintering, phase segregation, loss of active surface area, or side reactions with H₂O and CO₂, leading to hydroxide or carbonate formation, reduced nitrogen uptake, and progressive deactivation [175,181,329].
• Coupling with hydrogen or steam and carrier regeneration: In CLAS systems, ammonia release often requires either H₂ or steam, and the spent carrier must then be regenerated in a separate step. This creates an additional systems-level challenge because hydrogen supply, steam management, regeneration strategy, and thermal integration must all be coordinated efficiently. As a result, the apparent chemical simplicity of looping can be offset by greater process complexity at the plant level if regeneration is not carefully designed [330].
• Kinetics and productivity under mild conditions: Although CLAS aims to avoid the severe conditions of conventional Haber–Bosch synthesis, many nitride carriers still require elevated temperature, activated nitrogen, or long residence times to achieve meaningful reaction rates. Improving both reaction kinetics and ammonia productivity under milder conditions remains a central challenge for practical deployment [173].
• Solid handling and reactor design: In configurations involving circulating solids, attrition, abrasion, agglomeration, and compatibility with reactive nitriding and hydriding environments pose major scale-up challenges. Fixed-bed or swing-reactor systems avoid solid transport, but they require frequent gas switching, complex valve networks, and often parallel reactor trains to approach continuous operation, which increases engineering complexity and capital cost [175,178].
• Limited continuous validation and scale-up evidence: Despite promising laboratory demonstrations, CLAS has not yet been validated in an integrated continuous pilot-scale system. Reported ammonia outputs remain generally low, stability is often demonstrated over only a limited number of cycles, and the long-term coupling of carrier performance, heat management, and reactor operation remains largely unproven. This lack of sustained engineering validation is one of the main barriers preventing advancement beyond the current low TRL.

4.8.3. Environmental Impact

CLAS is often discussed as a potentially low-emission route because it decouples nitridation and hydrogenation or hydrolysis steps and could, in principle, be driven by renewable electricity or high-temperature solar heat. However, quantitative environmental comparison remains uncertain because most CLAS configurations are still at the laboratory or conceptual process-design stage, and published results are strongly scenario- and carrier-dependent.

Where system-level CO₂ values are reported, they typically reflect specific integrated-process assumptions rather than demonstrated renewable-powered operation. For example, Fang et al. [331] evaluated an integrated chemical looping ammonia generation (CLAG) system that explicitly relies on carbon-based energy as a raw material and co-produces steam, CO, and urea. Under that integrated configuration, they reported a total CO₂ emission of 2.05 kg CO₂ kg⁻¹ NH₃, while direct CO₂ emissions were reduced to 0.43 kg CO₂ kg⁻¹ NH₃; when urea co-production was included, the reported direct CO₂ balance became negative, down to $-0.86$ kg CO₂ kg⁻¹ NH₃, because of carbon utilization and crediting within the integrated system.

Non-climate impacts are even less well quantified for CLAS itself, but available process-level evidence indicates that auxiliary utilities can dominate several categories. For CLAS routes that form NH₃ via hydrolysis and therefore require separation of NH₃-H₂O mixtures, Pereira et al. [327] reported that the required separation power can be at least about twice that of conventional processes in the relevant configurations, implying potentially higher burdens in electricity- and materials-sensitive impact categories unless the process achieves higher effective NH₃ concentration or intensified separation.

Evidence from adjacent chemical-looping-based ammonia supply chains also suggests that reductions in GWP can coincide with increases in other impact categories. In an LCA of European ammonia-production pathways, Chisalita et al. [332] reported that adding chemical-looping hydrogen production produced the largest reduction in GWP, while six out of nine non-GWP impact indicators increased by approximately 30–60% relative to the reference case. Although this result concerns chemical-looping hydrogen integrated with ammonia production rather than CLAS carrier cycling, it highlights a key implication for CLAS development: embodied impacts of looping materials, auxiliary utilities, and separation or recycle requirements can shift burdens toward non-climate categories even when CO₂ emissions are reduced.

Overall, CLAS remains environmentally promising in principle, but its sustainability advantage has not yet been demonstrated at system scale. The limited published quantitative evidence shows that environmental performance depends critically on (i) low-carbon heat or electricity supply, where applicable; (ii) high carrier utilization and circularity; and (iii) minimization of separation and recycle penalties. Until these conditions are demonstrated and validated in multi-category LCAs for mature CLAS process designs, the environmental benefits should be regarded as conditional and scenario-dependent rather than established.

4.8.4. Critical TRL Evaluation

CLAS remains at an early stage of development and is most reasonably classified at TRL 2–3, at most approaching TRL 4 where limited subsystem integration is demonstrated. Laboratory studies have confirmed the thermochemical feasibility of alternating nitrogen fixation and ammonia release using several carrier families, including lithium-, magnesium-, and calcium-based materials. However, these demonstrations remain largely restricted to small-scale batch or short-cycle experiments, typically with low NH₃ output per cycle, limited cyclic stability, and incomplete regeneration of the active carrier.

The principal barriers are not only materials-related but also strongly linked to process engineering. Slow kinetics at moderate temperature, carrier deactivation, high energy demand for regeneration, heat-management constraints, and the difficulty of coordinating gas switching, solids handling, and reactor integration all limit scale-up. In addition, no integrated continuous pilot-scale CLAS system has yet demonstrated stable long-term operation, and the coupling between carrier durability, thermal efficiency, and reactor design remains largely unvalidated under practical conditions.

From a critical perspective, CLAS is scientifically credible but still technologically immature. Its main appeal lies in its conceptual flexibility, including the possibility of modular operation and partial decoupling of reaction steps, but these advantages have not yet been translated into engineering maturity. Therefore, CLAS should presently be regarded as a low-TRL laboratory technology with promising chemical feasibility but without sufficient systems-level validation for pilot-scale readiness.

Table 12. Concise summary of chemical looping ammonia synthesis.

Aspect	Summary
TRL	Low; about TRL 2–4.
Main strength	Decouples nitrogen activation and hydrogenation steps.
Main limitation	Carrier deactivation, regeneration complexity, heat management, and lack of continuous demonstration.
Overall role	Promising laboratory concept requiring integrated validation.

summarizes the maturity, strengths, limitations, and potential role of chemical looping ammonia synthesis.

4.9. Indirect Electrochemical Routes

Indirect electrochemical ammonia synthesis encompasses processes in which nitrogen fixation proceeds through chemical mediation rather than by direct catalytic reduction of N₂ at the electrode surface. The most prominent and experimentally reproducible approach is the lithium-mediated nitrogen reduction reaction (Li-NRR). In this system, Li⁺ ions are electrochemically reduced to metallic lithium at the cathode, where the deposited Li reacts spontaneously with N₂ to form lithium nitride (Li₃N). Subsequent protonation of Li₃N by a weak proton donor, such as ethanol or trace H₂O, releases NH₃ and regenerates Li⁺, thereby completing a closed redox cycle.

4.9.1. Current TRL

Li-NRR represents the most advanced of the non-HB electrochemical routes and is currently positioned between laboratory validation and early prototyping, approximately TRL 4–5. The concept originated with Tsuneto et al., who first demonstrated ammonia formation in nonaqueous lithium systems with measurable Faradaic efficiency. The recent revival of Li-NRR clarified its mechanism, namely, lithium plating, nitridation to Li₃N, and protonation to form NH₃, and established reproducible benchmarks through rigorous ¹⁵N isotope labeling and improved electrolyte formulations [20,333].

Multiple groups have since achieved stable operation with Faradaic efficiencies of about 35–61% and ammonia partial current densities of about 5–9 mA cm⁻² under optimized nonaqueous conditions. These advances were enabled by improved control of lithium deposition, oxygen or proton-shuttle additives, and formation of a robust solid-electrolyte interphase (SEI). Key milestones include Suryanto et al. [333] followed by detailed mechanistic studies and continuous-flow demonstrations that achieved reproducible and sustained performance [173,334].

Commercially, Jupiter Ionics, a Monash University spin-out, has reported progress toward continuous Li-mediated modules through public updates, invited talks, and press coverage, signaling a transition from benchtop studies to early prototype development. Despite this progress, operation remains limited to small nonaqueous systems running for only hours to tens of hours, with persistent SEI and durability challenges. No pilot-scale or industrially relevant systems have yet been realized. Li-NRR should therefore presently be regarded as a TRL 4–5 pathway, reflecting strong laboratory validation with emerging prototypes but no large-scale or long-duration demonstration.

From a techno-economic perspective, the available evidence for indirect electrochemical ammonia synthesis, primarily the Li-NRR route, remains limited but suggests a wide and assumption-sensitive cost range. An early preliminary analysis by Gomez and Garzon [335] estimated that ammonia could be produced for less than USD 700 t⁻¹ NH₃ under low-temperature lithium-mediated operation powered by inexpensive renewable electricity. More recently, Kani et al. [336] reported that high-pressure lithium-mediated ammonia synthesis could achieve an all-inclusive production cost of about USD 456 t⁻¹ NH₃ at an ammonia current density of 100 mA cm⁻² and a cell voltage of 6 V, indicating that improved operating pressure and cell performance can substantially strengthen the economic outlook. In contrast, a broader process-level techno-economic assessment by Kim et al. [275] estimated a levelized cost of ammonia of about USD 1130 t⁻¹ NH₃ for Li-NRR, while also showing strong sensitivity to hydrogen source and separation strategy. Consistent with this caution, Izelaar et al. [273] concluded that, at a small scale of 91 t day⁻¹, Li-mediated NRR was not economically comparable with SMR-based Haber–Bosch within the tested parameter space because of high lithium-plating potentials, ohmic losses, low energy efficiency, and the need for external hydrogen. Therefore, although the economic database remains sparse, the currently reported cost of ammonia from the indirect electrochemical route can be cautiously placed in the range of about USD 456–1130 t⁻¹ NH₃, with lower values relying on favorable high-pressure and cell-voltage assumptions and higher values reflecting more conservative integrated process analyses.

4.9.2. Challenges

Although indirect electrochemical ammonia synthesis is represented most convincingly at present by Li-NRR, this route still faces several interrelated challenges that limit its scalability and practical deployment:

• Handling and stability of metallic lithium: The process relies on repeated deposition and stripping of highly reactive metallic lithium in nonaqueous media. Even trace amounts of O₂ or H₂O can trigger parasitic reactions, surface passivation, dendritic growth, and loss of active lithium. Maintaining a suitable balance between lithium reactivity, proton availability, and suppression of side reactions remains essential for stable operation [337,338].
• Electrolyte, SEI, and membrane durability: Most Li-mediated systems employ ether-based electrolytes and lithium salts that are prone to decomposition under strongly reducing conditions. As a result, the composition and stability of the SEI become critical, because SEI growth can both protect and hinder the process. Electrolyte degradation, solvent instability, membrane fatigue, and impurity accumulation remain major barriers to long-duration operation [339,340,341].
• Energy efficiency and cell voltage: A major limitation of Li-mediated ammonia synthesis is its high energy demand. Lithium plating requires highly negative potentials, and practical cells operate at elevated voltages because of kinetic overpotentials and ohmic losses. Consequently, the energy efficiency remains substantially below that of mature ammonia-production routes, and reducing cell voltage through improved electrolyte design, interphase control, and faster nitridation/protonation kinetics remains a central challenge [269,337,342,343].
• Dependence on hydrogen or proton-shuttle management: Although the route is electrochemical, ammonia formation still depends strongly on how proton delivery is managed in the system. In many cases, this requires carefully controlled proton shuttles or external hydrogen-related steps, which increases process complexity and weakens the practical advantage of the route unless it is integrated very efficiently [22,339].
• Scalability and continuous operation: Most demonstrations of Li-mediated NRR remain limited to batch or short-duration flow systems. Extending the process to stable continuous operation requires simultaneous control of lithium morphology, gas transport, heat removal, ammonia separation, and impurity buildup over long time periods. Preserving reproducibility while avoiding progressive SEI thickening and lithium loss remains a major scale-up challenge [334,344,345].
• Safety and process control: Li-mediated systems employ metallic lithium together with flammable organic solvents, requiring strictly oxygen- and moisture-free environments. This creates significant safety and containment requirements for scale-up, including inert handling systems, tight process monitoring, and highly controlled electrolyte purity [337,339].
• Economic competitiveness: In addition to technical barriers, the current route remains economically challenged because of high cell voltage, limited energy efficiency, materials sensitivity, and system complexity. Present techno-economic studies suggest that the cost of ammonia remains highly sensitive to operating pressure, current density, hydrogen management, and balance-of-plant assumptions, indicating that substantial performance improvements are still required before commercial competitiveness can be achieved [273,275].

4.9.3. Environmental Impact

When powered by low-carbon electricity, Li-NRR can in principle avoid the large direct emissions associated with fossil-based Haber–Bosch ammonia, but its environmental profile remains strongly governed by electricity demand, hydrogen sourcing, and the upstream burdens of lithium-containing materials and organic electrolytes. A recent pathway-level techno-economic and environmental assessment explicitly quantified the GWP of Li-NRR and showed that its environmental performance is highly sensitive to the hydrogen source, indicating that upstream hydrogen provision remains a decisive factor in the overall life-cycle footprint [275]. For comparison, fossil-based ammonia production via steam-methane reforming is commonly reported at about 2.16–2.9 kg CO₂-eq kg⁻¹ NH₃, while coal-based ammonia can reach about 5.2 kg CO₂-eq kg⁻¹ NH₃ [12,346]. In contrast, electrolysis-based green ammonia has been reported in the range of 0.09–0.70 kg CO₂-eq kg⁻¹ NH₃, with fossil-depletion potentials of 3.62–213.56 kg oil-eq t⁻¹ NH₃ and ozone-depletion potentials of 0.001–0.082 g CFC-11-eq t⁻¹ NH₃ [346]. Even at the cell level, the electricity burden of Li-NRR remains substantial: in one of the most advanced continuous-flow demonstrations, the route reached an energy efficiency of only $13\pm 1$% at 1 bar, despite achieving a Faradaic efficiency of $61\pm 1$% [347].

The upstream lithium supply chain also contributes non-negligible environmental burdens. A recent regionalized life-cycle assessment of Li₂CO₃ production from brines reported climate-change impacts of 2.4–8.4 kg CO₂-eq kg⁻¹ Li₂CO₃ for conventional brine-based production, but 30–51 kg CO₂-eq kg⁻¹ Li₂CO₃ for direct-lithium-extraction routes; the corresponding water-scarcity impacts were 0.8–16.3 and 4.2–22 ${\mathrm{m}}_{\mathrm{world-eq}}^3$ kg⁻¹ Li₂CO₃, respectively [348]. On a market basis, currently producing brine sites average about 15 kg CO₂-eq kg⁻¹ Li₂CO₃ and 4.0 ${\mathrm{m}}_{\mathrm{world-eq}}^3$ kg⁻¹ Li₂CO₃, while late-stage and early-stage exploration sites rise to about 24 and 37 kg CO₂-eq kg⁻¹ Li₂CO₃, with corresponding water-scarcity impacts of 6.8 and 9.1 ${\mathrm{m}}_{\mathrm{world-eq}}^3$ kg⁻¹ Li₂CO₃ [348]. Therefore, the environmental viability of Li-mediated ammonia synthesis depends not only on renewable electricity, but also on low-carbon hydrogen supply, improved cell energy efficiency, longer electrolyte and SEI lifetimes, solvent recovery, and closed-loop lithium recycling [275,348].

4.9.4. Critical TRL Evaluation

Li-mediated ammonia synthesis currently represents the most mature of the non-Haber–Bosch electrochemical nitrogen-fixation pathways and is reasonably positioned at TRL 4–5. The fundamental reaction sequence, lithium plating, nitridation to Li₃N, and protonation to form NH₃, has been demonstrated reproducibly across multiple laboratories, and recent studies have established a much stronger mechanistic and experimental foundation than for most other alternative electrochemical routes. In this sense, the chemistry is no longer speculative.

However, the main barriers are now primarily related to engineering integration and practical operation rather than to basic proof of concept. High cell voltage and limited energy efficiency, instability of electrolytes and the SEI, dependence on carefully controlled proton-delivery or hydrogen-coupled schemes, and the safety requirements associated with metallic lithium and flammable organic solvents all continue to constrain scale-up. In addition, current systems still operate only for limited durations and have not yet demonstrated continuous, long-term, energy-efficient ammonia production at industrially relevant current density and scale. The route also remains challenged by uncertain economic competitiveness and by environmental burdens linked to electricity demand, hydrogen sourcing, lithium supply, and solvent management.

From a critical TRL perspective, Li-mediated ammonia synthesis is scientifically validated but still technologically immature. It has progressed beyond exploratory laboratory discovery and now stands as an emerging prototype-level technology, but substantial advances in durability, integrated reactor design, energy efficiency, safety, and system-level validation are still required before it can credibly approach pilot-scale readiness. Therefore, Li-mediated ammonia synthesis should presently be regarded as a promising but not yet commercially mature route that bridges fundamental electrochemistry and applied ammonia process engineering.

The main features and system-level challenges of indirect electrochemical ammonia synthesis routes are summarized in

Table 13. Concise summary of indirect electrochemical ammonia synthesis routes.

Aspect	Summary
TRL	Low to moderate (TRL 4–5); depends strongly on the mediator cycle.
Main strength	Can bypass direct N2 reduction limitations through mediated reaction steps.
Main limitation	Complex cycling, separation, regeneration losses, and limited integrated operation.
Overall role	Emerging route with potential, but still dependent on system-level proof.

.

4.10. Nitrate/Nitrite-to-Ammonia Electrochemical Synthesis

The basic reaction concept of nitrate/nitrite-to-NH₃ electroreduction is described in Section 3.8. The following discussion focuses on its maturity, environmental relevance, economic feasibility, feedstock dependence, product recovery, and practical role as a resource-recovery pathway.

4.10.1. Current TRL

Electrochemical nitrate-to-ammonia conversion, commonly referred to as the nitrate reduction reaction (NO₃RR), has emerged as one of the more advanced alternative ammonia-production pathways and is currently positioned at about TRL 4–5. The most notable example is the electrodialysis-and-nitrate-reduction (EDNR) process developed at Stanford University, which integrates electrodialysis with NO₃RR to recover ammonia from real agricultural runoff. In this demonstration, an ammonium-fertilizer solution (>100 mM ${\mathrm{NH}}_4^{+}$) was produced from an 8.2 mM nitrate-plus-ammonium feed, with reported energy consumption of approximately 245 MJ kg⁻¹ NH₃-N for simulated wastewater and about 920 MJ kg⁻¹ NH₃-N for real runoff [349].

Parallel laboratory studies have established high-selectivity catalysts, such as Fe single-atom systems and Cu-based nanostructures, that deliver 70–90% Faradaic efficiency and achieve current densities in flow-cell configurations exceeding hundreds of mA cm⁻², thereby confirming the kinetic and materials feasibility of NO₃RR at practically relevant rates [350,351,352,353].

With successful translation from bench to field through EDNR and an expanding catalyst library exhibiting high selectivity at practical current densities, nitrate/nitrite-to-ammonia conversion resides at the upper end of laboratory readiness, about TRL 4–5. It has been validated in relevant environments through modular continuous unit operations that combine separation and electroreduction, while broader deployment will depend on improved energy efficiency and feed pretreatment at industrially relevant scale [349].

From a techno-economic perspective, nitrate/nitrite-to-ammonia electrochemical synthesis is among the more promising alternative ammonia pathways, although the reported costs remain highly sensitive to nitrate concentration, reactor architecture, electricity price, and whether wastewater-treatment benefits are credited. Importantly, the literature does not use a uniform system boundary. For example, a preliminary techno-economic assessment of a porous solid-electrolyte (PSE) reactor treating a 2000 ppm nitrate point source reported a total ammonia-production cost of USD 2.17 kg⁻¹ NH₃, including both capital expenditure (CAPEX) and operating expenditure (OPEX), with electricity accounting for the largest cost share [225]. A later membrane-electrode-assembly (MEA) study based on Ru-Co(OH)₂ reported an estimated total cost of approximately USD 1.995 kg⁻¹ NH₃ and noted that the process could fall below the current Haber–Bosch ammonia market price under sufficiently low electricity cost and favorable operating conditions [354]. Likewise, a three-step pulsed MEA strategy designed for dilute 10 mM nitrate feeds reported an estimated cost of USD 1.73 kg⁻¹ NH₃, with the authors further arguing that coupling ammonia production with nitrate-removal credits can make the process economically attractive [355]. By contrast, some recent catalyst studies report substantially lower ammonia “production costs,” such as USD 0.57 kg⁻¹ NH₃ for hcp-Ru₁Co, but these values are calculated only from electricity consumption and assumed power price and therefore should not be interpreted as full plant-level costs [356]. Overall, a cautious and directly comparable present-day cost range for this pathway is about USD 1.73–2.17 kg⁻¹ NH₃ for process-inclusive studies, whereas lower values near USD 0.57 kg⁻¹ NH₃ should be treated as best-case electricity-only estimates rather than complete levelized production costs.

4.10.2. Challenges

Although nitrate/nitrite reduction is kinetically more accessible than direct N₂ reduction, several important technical and systems-level barriers still limit practical deployment.

• Product selectivity and nitrogen loss: Depending on catalyst composition, reactor design, and applied potential, nitrate may be only partially reduced to nitrite or diverted to by-products such as N₂O and N₂, thereby lowering ammonia yield and reducing nitrogen-recovery efficiency. In particular, N₂O formation is undesirable not only because it represents nitrogen loss, but also because it is a potent greenhouse gas, making selectivity control essential from both process and environmental perspectives [357].
• pH and electrolyte management: Careful pH control is critical because proton availability strongly influences both reaction kinetics and product distribution. Under more acidic conditions, the product remains mainly as ${\mathrm{NH}}_4^{+}$ in solution, whereas at higher pH a larger fraction shifts toward dissolved NH₃, which can facilitate stripping but complicates in-cell retention and continuous operation. Variations in wastewater composition can further alter local pH, buffering behavior, and catalyst performance [224,358,359].
• Product recovery and concentration: Even when ammonia is formed selectively, it is usually recovered first in aqueous form and often at relatively low concentration, so downstream separation remains necessary. Processes such as air stripping, membrane separation, or conversion to concentrated ammonium salts add energy demand and system complexity. Integrated concepts such as EDNR help reduce this burden by combining reduction and recovery, but efficient product concentration remains a major practical challenge [349,360].
• Catalyst durability and fouling: Cu-, Fe-, and other transition-metal-based catalysts can undergo corrosion, restructuring, poisoning, or loss of activity in real wastewaters containing competing ions, dissolved organics, and suspended solids. In coupled systems such as electrodialysis-assisted reactors, membrane scaling and fouling also reduce performance and increase cleaning and replacement requirements [349,352,361].
• Resource distribution and scalability: The recoverable nitrogen resource in wastewater is substantial, but it is geographically diffuse and distributed across municipal effluents, agricultural runoff, and industrial streams. This dispersion makes feed collection, pretreatment, and decentralized deployment major scale-up challenges, especially for low-concentration and highly variable nitrate sources [349,362].
• Energy demand and dilute-feed penalty: Although nitrate is easier to activate than N₂, the overall process can still become energy- and cost-intensive when treating dilute feeds, because large water volumes must be processed, transported, or pre-concentrated. As a result, the practical economics depend strongly on nitrate concentration, electricity price, reactor efficiency, and whether wastewater-treatment or nutrient-recovery credits are included.
• Competition with biological denitrification: Conventional wastewater treatment already relies heavily on mature biological denitrification systems that remove nitrate at relatively low cost and with established operational stability. Therefore, electrochemical nitrate-to-ammonia technologies must demonstrate clear additional value, such as selective nitrogen recovery, modular operation, or lower environmental impact, in order to justify adoption over existing treatment infrastructure [363,364].

4.10.3. Environmental Impact

Electrochemical nitrate/nitrite-to-ammonia synthesis offers an intrinsically attractive environmental proposition because it combines reactive-nitrogen removal with ammonia recovery. Instead of fixing N₂ under harsh HB conditions, this route valorizes nitrate- and nitrite-containing waste streams from agricultural runoff, municipal wastewater, and industrial effluents, thereby coupling pollution abatement with resource recovery. A recent review noted that, in principle, nitrate-to-ammonia electroreduction could reduce carbon emissions by as much as 420 Mt and energy consumption by about 2.6 EJ yr⁻¹ relative to the incumbent HB-centered nitrogen cycle, although such system-level benefits depend strongly on nitrate availability, recovery efficiency, and process design [365].

Recent quantitative assessments confirm that this pathway can be environmentally competitive, but the advantage is not automatic. In a comparative techno-economic and environmental assessment of emerging electrochemical ammonia routes, Kim et al. [275] reported a global warming impact of approximately 2 kg CO₂-eq kg⁻¹ NH₃ for electrochemical NO₃RR, identifying it as the most favorable of the electrochemical pathways considered, especially when wastewater-derived nitrate is used as the nitrogen source. For context, conventional fossil-based ammonia production via steam-methane reforming is typically reported at about 2.16–2.9 kg CO₂-eq kg⁻¹ NH₃, while coal-based ammonia can reach about 5.2 kg CO₂-eq kg⁻¹ NH₃ [12]. By comparison, electrolysis-based green ammonia produced through renewable-powered HB has been reported in the range of about 0.09–0.70 kg CO₂-eq kg⁻¹ NH₃, with fossil-depletion potentials of 3.62–213.56 kg oil-eq t⁻¹ NH₃ and ozone-depletion potentials of 0.001–0.082 g CFC-11-eq t⁻¹ NH₃ [346]. These comparisons indicate that nitrate electroreduction can be climate-competitive with conventional ammonia production, but it has not yet clearly surpassed the best renewable-HB scenarios on a fully consistent life-cycle basis.

Process-level data further show that feed composition and nitrate concentration strongly affect environmental performance. In the EDNR process demonstrated by Guo et al. [349], the energy consumption was about 245 MJ kg⁻¹ NH₃-N for simulated wastewater, but increased sharply to about 920 MJ kg⁻¹ NH₃-N for real agricultural runoff. This nearly fourfold increase highlights an important practical limitation: for dilute and compositionally complex waste streams, the energy penalty associated with ion transport, concentration, and recovery can dominate the environmental footprint. Thus, the sustainability of nitrate electroreduction depends not only on catalyst performance, but also on feed pretreatment, nitrate concentration, transport losses, and the degree of in situ product recovery.

Additional evidence from agricultural drainage systems confirms that nitrate-recovery efficiency is a decisive environmental lever. Samaroo and Hickey [366] reported that, at very low overall nitrate recovery, specifically 1.6% total mass nitrate recovery, the reduction in global warming potential relative to purchased Haber–Bosch fertilizer was only about 0.2–3.1%. When the recovery increased to 32%, however, the reduction rose to about 14–32%, depending on system configuration and whether electricity was supplied by the local grid or by on-site agrophotovoltaic systems. These results show that the environmental case for this route depends strongly on whether nitrate can actually be captured and converted at sufficiently high recovery from real distributed waste streams.

An additional issue that deserves explicit attention is the possible formation of N₂O. The U.S. EPA, reflecting IPCC AR6 values, reports a 100-year global warming potential of 273 for N₂O relative to CO₂. Earlier electrochemical nitrate-reduction studies have also shown that N₂O and NO can appear among the volatile products under some operating conditions. For example, Kato et al. [367] used online electrochemical mass spectrometry to quantify N₂, N₂O, and NO during nitrate reduction on Sn-modified Pt/Pd-based electrodes in acidic media, confirming that N₂O formation is chemically plausible even when NH₃ is not the target by-product. To illustrate the significance of this issue, an emission of only 1 g N₂O per kg NH₃ would add about 0.273 kg CO₂-eq kg⁻¹ NH₃ to the climate footprint. Therefore, future environmental assessments of nitrate/nitrite electroreduction should quantify N₂O explicitly rather than assuming it is negligible.

Overall, nitrate/nitrite-to-ammonia electrochemical synthesis remains one of the most environmentally promising emerging ammonia pathways because it can simultaneously remediate nitrate pollution and recover a valuable nitrogen product. However, the current quantitative literature shows that its benefit is highly conditional: it improves markedly when nitrate is recovered from concentrated or well-managed streams and when low-carbon electricity is used, but it deteriorates for dilute feeds, energy-intensive separations, or incomplete nitrogen recovery. Published environmental assessments for this route are still dominated by climate and energy indicators rather than by a broad, consistent set of midpoint categories, so further life-cycle work is needed to quantify impacts such as eutrophication, acidification, and water use on a common per-kg-NH₃ basis [275,365,366].

4.10.4. Critical TRL Evaluation

Electrochemical nitrate-to-ammonia synthesis has clearly progressed beyond proof of concept and is reasonably positioned at TRL 4–5, where laboratory and early prototype systems have validated the core chemistry under both controlled conditions and selected real-water environments. Integrated systems such as electrodialysis-assisted nitrate reduction have demonstrated that nitrate removal and ammonia recovery can be coupled in continuous operation, thereby confirming the practical relevance of the pathway beyond catalyst-scale experiments.

However, the route remains constrained by systems-level challenges that prevent transition to pilot-scale readiness. High energy demand, strong sensitivity to feed composition and nitrate concentration, incomplete nitrogen recovery, catalyst degradation, membrane fouling, and limited long-term operational stability all remain significant barriers. In addition, the practical value of the technology depends not only on ammonia selectivity, but also on efficient product recovery, control of undesired by-products, and the ability to maintain performance under sustained operation with variable real-world waste streams.

From a critical TRL perspective, nitrate-to-ammonia electroreduction is scientifically credible and among the more advanced alternative electrochemical ammonia pathways, but it remains pre-commercial. Its principal strength lies in combining pollutant removal with resource recovery, yet its environmental and economic advantages are strongly conditional on nitrate-capture efficiency, feed quality, energy consumption, and local system design. Therefore, although the pathway shows clear promise, it should presently be regarded as an emerging prototype-level technology rather than as an industrially validated process.

Table 14. Concise summary of nitrate/nitrite-to-ammonia electrochemical synthesis.

Aspect	Summary
TRL	Moderate for emerging electrochemical routes; about TRL 4–5.
Main strength	Combines ammonia recovery with nitrate/nitrite pollution removal.
Main limitation	Sensitive to feed concentration, energy use, membrane fouling, and nitrogen recovery efficiency.
Overall role	Promising resource-recovery route for suitable waste or nitrate-rich streams.

summarizes the key strengths, limitations, and potential application of nitrate/nitrite-to-ammonia electrochemical synthesis.

5. Comparative Assessment

This section applies the evaluative framework established in the review methodology to compare the major sustainable ammonia-production pathways on a consistent basis. The comparison is organized around three linked dimensions: technology readiness, environmental performance, economic plausibility and reactor Design and scale-Up considerations for emerging routes. Likely deployment role is then interpreted as a synthesis of these four perspectives rather than treated as an independent comparison dimension. At present, renewable-H₂ HB is the only low-carbon route that combines high readiness with a comparatively robust environmental and techno-economic evidence base. Most other pathways remain at laboratory or early-prototype stages, and their reported costs and environmental metrics are still strongly scenario-dependent. For this reason, the comparisons in this chapter should be interpreted as indicative cross-pathway benchmarks rather than as fully harmonized one-to-one rankings. In particular, reported environmental values are drawn from studies with different system boundaries and assumptions, and several emerging pathways still lack a reliable route-specific life-cycle benchmark.

5.1. Comparative Technology Readiness and Deployment Maturity

The technology-readiness dimension of the comparison shows a clear divide between pathways that rely on already commercial process hardware and those that still depend on unresolved catalytic, materials, or systems-level breakthroughs. Renewable-H₂ HB remains the only clearly near-commercial low-carbon option. Biomass- or waste-derived HB, Li-mediated synthesis, nitrate/nitrite electroreduction, and some plasma-assisted concepts occupy an intermediate position, with partial prototype or demonstration evidence but no broad industrial deployment. By contrast, direct electrochemical N₂ reduction, photocatalytic, PEC, biological, and CLAS routes remain earlier-stage technologies whose present value lies more in scientific development and future option generation than in near-term industrial implementation. To make this cross-pathway readiness contrast explicit,

Table 15. Comparative summary of representative TRL ranges, key strengths, principal limitations, and likely deployment roles across ammonia-production pathways.

Pathway	TRL	Key Strengths	Main Limitations/Critical Barriers	Likely Deployment Role
Renewable-H2 HB	8–9	Highest maturity among low-carbon routes; based on commercially proven subsystems; strongest near-term deployment potential	High capital intensity; strong dependence on electricity price and electrolyzer cost; integration challenges under variable renewable supply	Primary near-term decarbonization pathway
Biomass/Waste-H2 HB (gasification and biomethane variants)	5–6 for biomass/waste gasification; 6–7 for biomethane variants	Can use renewable carbon feedstocks and existing synthesis infrastructure; potentially very low-carbon or even net-negative under favorable biomass/biomethane + CCS assumptions	Feedstock variability and logistics; tar, ash, and impurity management; scale limitations; strong regional dependence	Regional complementary route where residue or biomethane resources are available
Direct electrochemical N2RR	3–4	Mild-condition modular concept; direct electricity-to-ammonia route without separate H2 production	Very low rates and selectivity; contamination sensitivity; poor reproducibility; high electricity demand under current performance	Long-term research route, not near-term deployment
Photocatalytic ammonia synthesis	2–3	Direct solar utilization; ambient-temperature operation; strong long-term conceptual sustainability appeal	Extremely low STA efficiency; photocorrosion; parasitic H2 evolution; trace NH3 production and difficult recovery	Fundamental materials and mechanism research
PEC ammonia synthesis	2–3	Integrates light absorption and electrochemical reduction; potential long-term route to low-bias solar-driven ammonia synthesis	Low photovoltage and current density; weak N2 activation; photoelectrode degradation; no route-specific scale-up evidence	Exploratory solar-fuels platform
Plasma-assisted ammonia synthesis	3–5	Near-ambient operation; modularity; compatibility with intermittent electricity; no need for a conventional high-pressure synthesis loop	High energy consumption; low single-pass conversion; low NH3 outlet concentration; NOx formation risk; recycle/separation burden	Possible niche modular role if energy yield improves materially
CLAS	2–4	Decoupled reaction steps; conceptual flexibility; potential compatibility with renewable heat and modular operation	Carrier deactivation; regeneration and heat-management complexity; solid handling; lack of continuous integrated demonstration	Promising laboratory concept, but still too immature for pilot deployment
Biological routes	1–3	Ambient operation; biocatalytic specificity; long-term potential for low-severity nitrogen fixation	Extremely low productivity; ATP/electron-supply burden; genetic instability; difficult NH3 recovery from dilute media	Exploratory route for long-term biohybrid development
Li-mediated synthesis	4–5	Best-developed indirect electrochemical route; relatively reproducible performance; stronger mechanistic foundation than most electrochemical alternatives	High cell voltage and energy demand; lithium handling and safety; SEI and electrolyte instability; limited long-duration operation	Most plausible indirect electrochemical candidate for early prototype development
Nitrate/Nitrite-to-NH3 electroreduction	4–5	Combines ammonia production with pollutant removal; high selectivity achievable; strongest near-term electrochemical application relevance	Strong dependence on feed concentration and composition; product recovery burden; catalyst and membrane fouling; competition with biological denitrification	Promising niche route where wastewater treatment and nutrient recovery are co-valued

summarizes the representative TRL ranges, key strengths, principal barriers, and likely deployment roles of the major ammonia-production pathways.

From a comparative perspective, the main divide is between pathways that inherit maturity from established synthesis infrastructure and those that still depend on unresolved catalytic or materials breakthroughs. Renewable-H₂ HB and, to a lesser extent, biomass- or waste-derived HB benefit from this infrastructure inheritance. Among non-HB routes, nitrate/nitrite electroreduction and Li-mediated synthesis currently show the strongest evidence of progression toward prototype-level validation, although both remain far from broad industrial deployment.

5.2. Comparative Environmental Performance and Evidence Robustness

Environmental comparison across ammonia pathways must be interpreted carefully because both reported impacts and the robustness of the available evidence vary substantially across routes and studies. Nonetheless, several robust patterns emerge. First, renewable-H₂ HB and well-designed biomass- or waste-derived HB routes currently provide the strongest route-specific evidence for major GHG reduction. Second, nitrate/nitrite electroreduction is particularly attractive because its environmental value can include both ammonia production and pollution mitigation. Third, several emerging routes remain environmentally promising mainly in principle, but their route-specific GWP or broader life-cycle advantages have not yet been established because current efficiencies remain too low or the available evidence is still limited to proxies and scenario analyses.

To avoid conflating demonstrated life-cycle performance with future environmental potential, this review distinguishes between route-specific LCA evidence and speculative low-carbon promise. For very low-maturity pathways, particularly TRL 1–3 and early TRL 3–4 concepts, no reliable cradle-to-gate GWP benchmark is assigned unless a route-specific LCA has been reported. Therefore, statements about low direct emissions, solar operation, biological operation, or electricity-driven synthesis for direct electrochemical N₂RR, photocatalytic, PEC, and biological routes should be interpreted as future potential rather than demonstrated life-cycle performance. Hybrid or analogue studies, such as PEC-H₂-assisted ammonia production or PEC water splitting, are useful for identifying possible impact-reduction pathways, but they are not treated here as proof of low life-cycle emissions for stand-alone direct ammonia synthesis.

To support direct comparison,

Table 16. Comparative environmental perspective across ammonia-production pathways. For low-TRL routes without route-specific cradle-to-gate LCA, reported environmental benefits indicate future potential or proxy evidence rather than demonstrated low life-cycle emissions.

Pathway	Indicative CO2-eq/GWP	Main Environmental Benefits	Principal Concerns/Trade-Offs	Environmental Evidence Base
Renewable-H2 HB	120–530 kg CO2-eq t−1 NH3; wind-based cases about 257–316 kg CO2-eq t−1 NH3 [246]	Strongest route-specific climate-reduction evidence among low-carbon routes; no direct fossil-derived H2 emissions; extensive multi-category evidence base [12,247]	Environmental performance remains highly sensitive to electricity carbon intensity; burdens can shift toward land use, minerals/metals use, water use, freshwater eutrophication, and terrestrial acidification [12,247]	Strong route-specific LCA evidence
Biomass/Waste-H2 HB	About 550–841 kg CO2-eq t−1 NH3 in representative biomass cases; potentially net-zero or negative in some biomethane + CCS cases [31,38,257]	Can provide major climate benefits and, in some configurations, net-zero or net-negative emissions; can valorize residues and waste streams	Land-use and biosphere trade-offs; eutrophication and acidification can increase; results are sensitive to allocation, biogenic-carbon treatment, and feedstock logistics [37,266]	Moderate-to-strong route-specific LCA evidence
Direct electrochemical N2RR	No robust route-specific GWP benchmark established [275,280]	Potential future low-carbon operation under low-carbon electricity; not yet demonstrated by robust route-specific cradle-to-gate LCA	Very high electricity intensity at present performance; environmental viability depends on major gains in current density, Faradaic efficiency, durability, and energy efficiency [275,279]	Limited; proxy and threshold-based evidence only. Low-carbon performance remains speculative
Photocatalytic ammonia synthesis	No robust route-specific GWP benchmark established [13,281]	Potential future solar-driven route with low direct operational emissions; life-cycle advantage not yet demonstrated	Very low STA efficiency; large illuminated-area and materials burden; photocorrosion; NH3 recovery from dilute media likely dominates impacts [281,283,286]	Limited; mostly review- and proxy-based evidence. GWP claims remain speculative.
PEC ammonia synthesis	No direct route-specific LCA benchmark established. One PEC-H2-assisted hybrid ammonia case reported about 50% of the impact of SMR-based ammonia, but this is not a stand-alone PEC-NRR benchmark [297]	Potential future solar-driven low-carbon route; current evidence is based on hybrid or analogue systems rather than demonstrated direct PEC ammonia production [296,298,299]	Large embodied burden from photoelectrodes and large active areas; route-specific environmental performance remains unproven [281,296]	Limited; hybrid-assisted and analogue evidence only. Low-carbon direct PEC-NRR remains unproven
Plasma-assisted ammonia synthesis	Route-specific values span from about 2960 kg CO2-eq t−1 NH3 for conventional centralized supply to negative net values in highly credited renewable cases [308]	Can eliminate direct reactor-level CO2 emissions under renewable power; compatible with intermittent electricity	Electricity demand, low NH3 yield, and recycle/separation dominate impacts; some non-climate categories can worsen sharply; negative values are credit-dependent net results [307,308,309]	Moderate route-specific LCA evidence, strongly scenario-dependent
CLAS	Scenario-specific values reported; e.g., about 2050 kg CO2 t−1 NH3 in one integrated carbon-based case, with lower direct-emission figures under crediting assumptions [331]	Potential compatibility with renewable heat and modular operation; may reduce direct emissions in some integrated configurations	Environmental evidence is sparse; regeneration and separation burdens may be large; route-specific multi-category LCAs remain lacking [327,332]	Sparse route-specific evidence
Biological routes	No route-specific cradle-to-gate GWP benchmark established [13]	Potential low-severity operation under mild biological conditions; no demonstrated cradle-to-gate low-GWP benchmark	Very high effective energy burden for ATP/electron supply; high feedstock and recovery burdens; current concepts are resource-intensive and dilute-product limited [176,315]	Sparse; no route-specific cradle-to-gate benchmark. Environmental promise remains speculative
Li-mediated synthesis	No single robust public GWP benchmark exposed; pathway-level evidence indicates strong dependence on electricity and hydrogen source [275]	Potentially low direct emissions under low-carbon electricity; compact electrochemical architecture	High electricity demand; upstream lithium extraction burden; solvent and SEI lifetime issues; water-scarcity and mineral impacts from Li supply chains [348]	Limited route-specific evidence; pathway-level and upstream-material evidence
Nitrate/Nitrite-to-NH3 electroreduction	About 2000 kg CO2-eq t−1 NH3 reported for one comparative pathway-level case [275]	Combines nitrogen recovery with pollution mitigation; may reduce eutrophication and support circular nitrogen management [365]	Strong dependence on nitrate concentration and recovery rate; energy penalty for dilute feeds; possible N2O formation; broader midpoint-category evidence remains limited [349,366,367]	Moderate route-specific evidence, but still limited in category coverage

summarizes the main environmental benefits and principal concerns for each pathway and reports indicative CO₂-eq values where defensible route-specific numbers are available. Where no robust route-specific value has yet been established, this is stated explicitly rather than replaced by speculative interpolation.

Taken together, the environmental comparison shows that maturity and environmental certainty are not the same thing. Some pathways, such as photocatalytic, PEC, and biological synthesis, remain environmentally attractive mainly in principle, whereas renewable-H₂ HB and some biomass- or waste-derived HB configurations already possess a much stronger route-specific evidence base. Nitrate/nitrite electroreduction stands out because its environmental value can include both fertilizer production and nitrogen-pollution abatement, although its benefit remains highly conditional on concentration, recovery, and electricity supply.

5.3. Comparative Economic Plausibility and Cost Uncertainty

Economic comparison also requires caution because both reported costs and the robustness of the underlying estimates vary across studies in terms of scale, system boundaries, and modeling assumptions. Several robust patterns nevertheless emerge. Renewable-H₂ HB currently defines the main near-term commercial benchmark for low-carbon ammonia, particularly in regions with low-cost electricity and large plant scales. Biomass- and waste-derived HB can also be competitive in favorable regional contexts, especially when residue feedstocks, biomethane, carbon credits, or CCS are available. By contrast, most non-HB emerging routes remain economically immature, and their lower reported values are often scenario-based, electricity-only, subsystem-only, or otherwise conditional rather than representative of a full plant-level cost.

To make the cross-pathway comparison clearer,

Table 17. Comparative economic perspective across ammonia-production pathways. Cost values are indicative literature benchmarks and reflect differing assumptions, scales, and system boundaries. Electricity-only, subsystem-only, and target-based values are not directly comparable with full plant-level ammonia-production costs.

Pathway	Indicative Cost Range	Estimate Type/Basis	Main Caveats/Cost Drivers
Renewable-H2 HB	About USD 396–1248 t−1 NH3 for plant-level studies; broader decentralized cases up to about USD 2266 t−1 NH3 [135,229,230,231,233,234,235,237]	Mostly plant-level TEA; some decentralized supply-chain studies included	Dominated by electricity price, electrolyzer capital cost, financing structure, and utilization; decentralized cases include additional supply-chain assumptions
Biomass/Waste-H2 HB	About USD 390 to above USD 2100 t−1 NH3; review-level upper bound about USD 2416 t−1 NH3 [31,33,253,254,255,256,258,259]	Mostly full process/pathway-level studies across biomass, biomethane, sludge, and landfill-gas cases	Highly sensitive to feedstock type, moisture, logistics, plant scale, co-product credits, and CCS integration; some waste-gas routes remain unprofitable without credits
Direct electrochemical N2RR	Current representative performance: >USD 1700 t−1 NH3 [271,272,273,274,275]	Current-performance-informed TEA	Reflects present or near-current integrated performance; still limited by low current density, Faradaic efficiency, energy efficiency, durability, and NH3 recovery
	Aspirational future cases about USD 284–1000 t−1 NH3 [271,272,273,274,275]	Future target-based scenarios	Prospective benchmark only; assumes major improvements in current density, Faradaic efficiency, cell voltage, and full-cell energy efficiency
Photocatalytic ammonia synthesis	About USD 30–660 t−1 NH3 in prospective modeled scenarios at 10% STA [281,282]	Prospective TEA framework rather than current plant-level cost	Strongly assumption-sensitive; current photocatalysts remain far below the efficiencies used in the model, so these values are forward-looking rather than near-term realizable
PEC ammonia synthesis	About USD 840 t−1 NH3 for a PEC-assisted hybrid route [281,288]	Hybrid PEC-assisted supply-chain estimate, not direct stand-alone PEC-NRR plant-gate cost	No robust independent direct PEC cost range yet; value should be treated as a benchmark rather than a deployment-ready cost
Plasma-assisted ammonia synthesis	Typically about USD 565–1432 t−1 NH3 in current modeled cases [304]	Current decentralized or supply-chain modeled cases	Dominated by plasma energy yield, electricity cost, outlet NH3 concentration, and recycle/separation burden
	Optimistic future case below about USD 452 t−1 NH3 [304]	Future-facing integration scenario	Prospective benchmark only; depends on improved energy yield, better plasma–catalyst coupling, higher NH3 concentration, and favorable electricity assumptions
CLAS	Pathway-level estimates about USD 400–1200 t−1 NH3 [187,327,328]	Full pathway-level or process-level estimate	Economic outlook depends strongly on carrier regeneration, yield, heat integration, hydrogen cost, and continuous operation
	Synthesis-loop-only values much lower [187,327,328]	Subsystem-only estimate	Not directly comparable with full pathway-level ammonia costs because upstream hydrogen, utilities, heat integration, separation, and balance-of-plant costs may be excluded
Biological routes	At least multi-thousand USD t−1 NH3; some biohybrid scenarios exceed tens of thousands USD t−1 NH3 [176,315]	Scenario-based cost indicators or energy/feedstock floors rather than full plant-level LCOA	Dominated by ATP/electron-supply burden, glucose or H2/CO2 requirements, and NH3 recovery from dilute streams
Li-mediated synthesis	About USD 456–1130 t−1 NH3 [275,335,336]	Highly assumption-sensitive TEA; includes optimistic pressure and cell-voltage assumptions	Small-scale TEAs still find it uncompetitive in many cases; sensitive to operating pressure, current density, hydrogen management, and balance of plant [273]
Nitrate/Nitrite-to-NH3 electroreduction	About USD 1730–2170 t−1 NH3 [225,354,355,356]	Process-inclusive TEA	Economics depend strongly on nitrate concentration, recovery credit, reactor design, separation, recovery efficiency, and whether wastewater-treatment value is monetized
	About USD 570 t−1 NH3 [225,354,355,356]	Electricity-only best-case estimate	Partial benchmark only; excludes major capital, separation, operating, and balance-of-plant costs, and should not be interpreted as a realistic plant-level ammonia-production cost

summarizes the main reported cost values in normalized units of USD t⁻¹ NH₃ where possible. However, the reported values do not always share the same system boundary. Therefore, Table 17 explicitly distinguishes full-process estimates from subsystem-only, electricity-only, and hybrid-assisted estimates in the basis column. Electricity-only and subsystem-only values are included only as optimistic or partial benchmarks and should not be interpreted as directly comparable to full plant-level ammonia production costs.

From a deployment perspective, these cost ranges suggest a likely phased transition. Renewable-H₂ HB is positioned to lead near-term industrial defossilization because it already combines high readiness with a growing project base. Biomass- and waste-derived HB may provide complementary regional solutions, particularly where suitable residues, biomethane, or carbon credits are available. Among non-HB routes, nitrate/nitrite electroreduction and Li-mediated synthesis appear to be the most plausible candidates for early niche deployment because their value propositions extend beyond bulk ammonia cost alone: nitrate electroreduction couples fertilizer production with pollution control, while Li-mediated systems have progressed further toward reproducible prototype-level operation than most alternative electrochemical routes. Plasma-assisted synthesis and CLAS may also become relevant for modular or flexible operation if their efficiency improves substantially. By contrast, direct N₂RR, photocatalytic, PEC, and biological routes remain too immature for meaningful bulk-market deployment under current performance.

More specifically, biomass- and waste-derived HB may be preferred over renewable-H₂ HB when low-cost and locally concentrated biomass residues, organic waste, or biomethane are available; feedstock transport and preprocessing costs are low; sufficient regional feedstock supply supports an economically meaningful plant scale; gas cleaning and syngas conditioning are reliable; and policy incentives such as carbon credits, waste-treatment credits, CCS incentives, or negative-emission credits are available. Under these favorable conditions, reported biomass/waste-HB costs of approximately USD 426–468 t⁻¹ NH₃ can become competitive with renewable-H₂ HB. In contrast, renewable-H₂ HB is generally preferred when low-cost renewable electricity is available at large scale, electrolyzer costs are low, sustainable biomass supply is limited or dispersed, or biomass conversion introduces major logistics, impurity-control, and gas-cleaning risks. Therefore, biomass/waste-HB is best interpreted as a regionally attractive complementary pathway, while renewable-H₂ HB remains the most broadly scalable near-term option.

Taken together, Table 15, Table 16 and Table 17 show that low-carbon ammonia is unlikely to be dominated by a single universal replacement pathway. Instead, the emerging landscape is more likely to consist of a portfolio of routes matched to different electricity conditions, feedstock bases, infrastructures, and end uses. When evaluated jointly in terms of technology readiness, environmental performance, and economic plausibility, renewable-H₂ HB remains the clearest anchor pathway for near-term deployment because it combines the highest maturity among low-carbon routes with the strongest route-specific environmental and techno-economic evidence base. Biomass- and waste-derived HB routes may provide important regional complements, whereas most non-Haber–Bosch alternatives remain earlier-stage or more specialized options along the research-to-commercialization spectrum.

5.4. Reactor Design and Scale-Up Considerations for Emerging Routes

In addition to TRL, environmental performance, and economic feasibility, emerging ammonia-production routes must also be evaluated from an engineering scale-up perspective.

Table 18. Engineering and scale-up constraints for emerging ammonia-synthesis routes.

Route	Reactor and Mass-Transfer Issue	Heat/Energy and Electricity Sensitivity	Modularity	Main Scale-Up Constraint
Direct electrochemical N2 reduction	Low N2 solubility, slow gas–liquid–solid transport, HER competition, and difficult NH3 recovery from dilute electrolyte.	Climate benefit requires high cell efficiency and low-carbon electricity; reported targets include >40% full-cell efficiency at about 0.5 A cm−2.	Potentially modular, but still mostly laboratory-scale.	Requires higher current density, stable membranes/electrodes, reliable NH3 quantification, and long-duration operation.
Photoelectrochemical ammonia synthesis	Limited illuminated electrode area, weak N2 activation, charge recombination, and low productivity per illuminated area.	Performance depends on photovoltage, solar-to-ammonia efficiency, photoelectrode stability, and light-management efficiency.	Conceptually modular for solar-driven operation.	Scale-up is limited by photoelectrode degradation, low current density, and lack of integrated outdoor demonstrations.
Plasma-assisted ammonia synthesis	Low single-pass conversion and low outlet NH3 concentration increase recycle and separation requirements.	Energy demand remains high; some reported systems require around 95 GJ t−1 NH3 at only about 0.2 mol.% NH3.	Highly modular and compatible with intermittent electricity.	Requires higher energy yield, improved plasma–catalyst coupling, higher NH3 concentration, and durable reactor operation.
Chemical looping ammonia synthesis	Requires repeated nitridation and hydrogenation of solid nitrogen carriers; gas–solid contact and carrier circulation are critical.	Heat management, carrier regeneration, and cyclic operation remain unresolved for continuous process integration.	Potentially modular, but not yet proven at large scale.	Requires stable carriers, high lattice-nitrogen utilization, continuous operation, and reliable heat/mass management.
Biological ammonia synthesis	Low volumetric productivity and dilute NH3 formation make recovery difficult from biological media.	Nitrogenase requires at least 16 ATP per N2 reduced; optimistic process estimates still imply high energy demand.	Possible for decentralized or niche applications, but not bulk production.	Limited by ATP/electron supply, slow productivity, biological stability, inhibition effects, and costly product recovery.
Nitrate/nitrite-to-NH3 electroreduction	Performance depends on nitrate/nitrite concentration, water composition, membrane fouling, competing reactions, and NH3 recovery.	Electricity and separation energy become more significant for dilute streams.	Promising modular resource-recovery option.	Most suitable for concentrated or well-managed waste streams where pollution removal and NH3 recovery are both valuable.

summarizes the main reactor-design, mass-transfer, heat-management, modularity, and electricity-sensitivity constraints for the major emerging non-Haber–Bosch pathways.

As shown in Table 18, the main barrier for emerging ammonia-production routes is not only catalyst activity but also integrated reactor performance. Electrochemical and PEC systems require improved mass transfer, higher current density, stable electrodes or membranes, and reliable ammonia recovery. Plasma-assisted systems offer modularity and compatibility with intermittent electricity, but their low outlet NH₃ concentration and high energy demand create major recycle and separation penalties. CLAS and biological routes avoid some limitations of direct electrochemical N₂ reduction, but they introduce other challenges related to carrier cycling, heat management, slow productivity, and dilute-product recovery. Therefore, future development should be assessed at the reactor and system level rather than only through laboratory catalyst performance.

To integrate the preceding pathway-specific comparisons,

Table 19. Integrated decision matrix comparing ammonia-production pathways across TRL, energy performance, environmental evidence, material/cost sensitivity, and CO₂-abatement potential.

Pathway	TRL	Energy Efficiency and Demand	Environmental Evidence	Material and Cost Sensitivity	CO2-Abatement Potential
Renewable-H2 HB	8–9	Medium-high Limited mainly by electrolysis	Strong LCA evidence Low GWP with low-carbon electricity	Electricity price Electrolyzer CAPEX H2 storage/integration	High and demonstrated Best broadly scalable near-term route
Biomass/Waste-H2 HB	5–7	Moderate Feedstock-dependent	Conditional LCA evidence Sensitive to logistics, CCS, and carbon accounting	Feedstock cost Transport distance Gas cleaning	High under favorable local conditions Regional complement
Direct electrochemical N2RR	3–4	Low at present Low current density HER losses	No robust GWP benchmark Mostly speculative	Catalyst cost varies Poor productivity dominates	Long-term potential only No demonstrated LCA abatement
Photocatalytic synthesis	2–3	Very low STA efficiency Low NH3 rate	No route-specific LCA Proxy evidence only	Photocatalyst cost Stability Scale-up synthesis	Speculative solar route Fundamental research stage
PEC synthesis	2–3	Low photocurrent Recombination losses Photoelectrode degradation	No direct PEC-NRR LCA Hybrid/proxy evidence only	Photoelectrode cost Lifetime Illuminated area	Possible future solar route Not yet demonstrated
Plasma-assisted synthesis	3–5	High energy demand Low single-pass conversion Separation/recycle burden	Limited LCA evidence Strong electricity dependence	Electricity cost Catalyst cost for Ru systems Recycle/separation cost	Conditional abatement Requires major efficiency gains
CLAS	2–4	Process-scale energy unclear Carrier regeneration required	No robust LCA benchmark Environmental benefit uncertain	Carrier cost Regeneration Sintering/solid handling	Future potential Needs continuous validation
Biological routes	1–3	Very low productivity High ATP/electron burden Dilute product	No reliable low-GWP benchmark Conceptual/lab evidence	Recovery cost Biological stability Low productivity	Not near-term practical Exploratory biohybrid route
Li-mediated synthesis	4–5	Better than direct N2RR Still energy-intensive High cell voltage	Limited route-specific LCA Li/electrolyte impacts uncertain	Li management Electrolyte/solvent cost SEI control	Promising indirect route Abatement depends on efficiency and clean electricity
Nitrate/Nitrite-to-NH3	4–5	Favorable for concentrated feeds Separation penalty for dilute feeds	Moderate but limited evidence Adds pollution-mitigation value	Catalyst/membrane stability Fouling Feed composition	High niche potential Best for waste-stream recovery

provides a compact decision matrix. The table summarizes each route using short qualitative descriptors because the underlying studies differ in scale, system boundary, functional unit, and performance metric.

5.5. Limitations of This Review

This review provides a comparative synthesis of sustainable ammonia-production pathways based on technology readiness, environmental performance, economic feasibility, reactor-design considerations, and scale-up potential. However, several limitations should be noted. First, this work is not a formal systematic meta-analysis and does not include statistical heterogeneity testing across the reviewed studies. The available literature spans different reactor types, scales, operating conditions, functional units, system boundaries, electricity mixes, feedstock prices, and policy assumptions, which limits the extent to which reported values can be directly pooled or statistically compared.

Second, the LCA and TEA values reported in the literature were not independently recalculated using a single harmonized model, common functional unit, or uniform system boundary. Instead, this review reports values where defensible route-specific estimates are available and explicitly identifies cases where evidence is based on proxy studies, partial systems, target-based scenarios, electricity-only estimates, hybrid-assisted configurations, or early-stage laboratory data. As a result, numerical comparisons should be interpreted as indicative benchmarks rather than fully normalized rankings.

Third, although this review adds reactor-design and scale-up considerations for emerging routes, the analysis is based on reported literature evidence rather than independent reactor simulation, process intensification modeling, or pilot-plant design. Therefore, the discussion of mass-transfer limitations, heat management, modularity, electricity sensitivity, product recovery, and separation requirements should be interpreted as a qualitative engineering assessment. More rigorous comparison would require harmonized reactor models, standardized current-density and productivity targets, common product-concentration assumptions, and experimentally validated scale-up data.

Fourth, TRL assignments involve some degree of interpretation because the type and quality of evidence differ substantially across pathways. Mature routes such as Haber–Bosch ammonia synthesis have industrial-scale evidence, while emerging routes such as direct electrochemical N₂ reduction, photocatalytic synthesis, PEC synthesis, plasma-assisted synthesis, CLAS, and biological routes often rely on laboratory-scale demonstrations, short-duration tests, subsystem-level studies, or indirect performance indicators. To reduce subjectivity, this review defines the criteria used to interpret TRL ranges and distinguishes between demonstrated performance and future potential.

Finally, the conclusions are sensitive to the assumptions and quality of the source studies, especially for low-TRL technologies where performance, durability, ammonia recovery, system integration, and scale-up evidence remain limited. Therefore, the pathway rankings and deployment roles proposed in this review should be interpreted as a structured assessment of the current literature rather than a definitive techno-economic, life-cycle, or reactor-scale optimization. Future work would benefit from harmonized cradle-to-gate LCAs, standardized TEA assumptions, common reactor-performance metrics, long-duration pilot data, and direct comparisons using shared functional units, regional boundary conditions, and scale-up assumptions.

6. Conclusions and Future Outlook

This review assessed conventional and emerging ammonia-production pathways from the combined perspectives of technology readiness, environmental performance, and economic feasibility. The comparison shows that sustainable ammonia production cannot be concluded only from the reaction pathway itself. Instead, the practical value of each route depends on the maturity of the full process, the source of energy and feedstock, the need for separation and purification, and the assumptions used in techno-economic and life-cycle assessments.

Among the evaluated options, renewable-H₂ Haber–Bosch remains the most practical route for near-term large-scale ammonia decarbonization. Its main advantage is that it uses the already established Haber–Bosch synthesis platform while replacing fossil-derived hydrogen with hydrogen from low-carbon electricity. However, its overall performance is still strongly linked to the electricity mix, electrolyzer cost, plant utilization factor, and the ability to operate reliably with variable renewable energy. Therefore, future deployment should focus not only on increasing electrolyzer capacity, but also on improving system integration, flexibility, hydrogen storage, and renewable-power management.

Biomass- and waste-derived Haber–Bosch pathways can provide useful complementary options, especially in regions with abundant residues, waste streams, biogas, or biomethane. These routes may reduce fossil-fuel dependence and support circular-resource use, but their performance is highly site-specific. Feedstock availability, collection distance, seasonal variation, gas cleaning, tar removal, and co-product treatment can strongly affect both cost and environmental outcomes. Future studies should therefore evaluate these systems under regional conditions rather than presenting them as universal alternatives to fossil-based ammonia.

Most non-Haber–Bosch routes, including direct electrochemical N₂ reduction, photocatalytic and photoelectrochemical synthesis, plasma-assisted systems, biological routes, and chemical looping ammonia synthesis, remain at lower technology readiness levels. These pathways are scientifically important because they explore milder operating conditions, modular production, direct renewable-energy use, or alternative nitrogen activation mechanisms. However, their current limitations, including low ammonia yield, limited selectivity, stability issues, difficult product separation, high energy demand, and lack of long-duration integrated demonstrations, still prevent near-term bulk deployment.

From an environmental perspective, future comparisons should move beyond reporting only greenhouse-gas emissions. Several low-carbon pathways may reduce CO₂ emissions while increasing other burdens such as land use, water demand, mineral consumption, eutrophication, or acidification. Therefore, future life-cycle assessments should use transparent system boundaries, consistent functional units, harmonized electricity assumptions, and multi-category environmental indicators. This is especially important for emerging routes, where many reported environmental benefits are still based on laboratory-scale data or optimistic future assumptions.

From an economic perspective, more complete plant-level techno-economic analyses are needed. Many reported cost values are not directly comparable because they are based on different scales, electricity prices, operating hours, separation requirements, and performance assumptions. Future work should include realistic capital and operating costs, ammonia recovery and purification, catalyst or carrier replacement, plant lifetime, financing assumptions, and sensitivity analysis. For emerging routes, cost estimation should be linked to experimentally demonstrated performance rather than only target values.

Overall, the transition toward sustainable ammonia production will likely proceed through different roles for different pathways. Renewable-H₂ Haber–Bosch is expected to lead near-term industrial decarbonization, biomass- and waste-derived routes can support region-specific deployment, and emerging non-Haber–Bosch technologies should continue to be developed as longer-term or niche options. Future progress will depend on moving from isolated laboratory performance toward integrated, stable, scalable, and transparently assessed systems.