Ammonia from Hydrogen: A Viable Pathway to Sustainable Transportation?

Abstract

Addressing the critical need for sustainable, high-density hydrogen (H₂) carriers to decarbonize the global energy landscape, this paper presents a comprehensive critical review of ammonia’s pivotal role in the energy transition, with a specific focus on its application in the transportation sector. While H₂ is recognized as a future fuel, its storage and distribution challenges necessitate alternative vectors. Ammonia (NH₃), with its compelling advantages including high volumetric H₂ density, established global infrastructure, and potential for near-zero greenhouse gas emissions, emerges as a leading candidate. This review uniquely synthesizes the evolving landscape of sustainable NH₃ production pathways (e.g., green NH₃ from renewable electricity) with a systematic analysis of technological advancements to investigate its direct utilization as a transportation fuel. The paper critically examines the multifaceted challenges and opportunities associated with NH₃-fueled vehicles, refueling infrastructure development, and comprehensive safety considerations, alongside their environmental and economic implications. By providing a consolidated, forward-looking perspective on this complex energy vector, this paper offers crucial insights for researchers, policymakers, and industry stakeholders, highlighting NH₃’s transformative potential to accelerate the decarbonization of hard-to-abate transportation sectors and contribute significantly to a sustainable energy future.

1. Introduction

The continuous growth of the global population, combined with rapid urbanization, has imposed exceptional pressure on existing transportation systems, rendering the search for sustainable mobility more critical than ever. The main driver of greenhouse gas emissions is the reliance on fossil fuels, resulting in global warming and air pollution, particularly in rapidly growing megacities. In addition to environmental deterioration, the limited availability of traditional energy sources and the growing instability of global energy markets highlight the indispensable directive to transition towards cleaner, more durable, and environmentally friendly transportation approaches. The rising recognition of carbon footprints, energy security concerns, and the clear impact of vehicular emissions on public health jointly drive the rapid need for innovative and sustainable solutions to promote the reliable, efficient, and environmentally responsible flow of people and goods.

H₂ is known as an environmentally friendly energy carrier. It offers a major opportunity to decarbonize the transportation sector, thanks to its ability to produce zero emissions at the point of use—whether it is burned as fuel or used to generate electricity in fuel cells. It has a gravimetric energy density of 120 MJ/kg, which is higher than that of gasoline. This makes H₂ a compelling alternative for a range of mobility applications, especially in heavy-duty and long-range transport, where battery electric solutions often encounter performance and efficiency limitations. H₂ can be generated from several feedstocks and can be used in many ways, including direct use and combustion and as fuel cells. However, the main issue with H₂ is its low volumetric energy density in liquid (2.36 kWh/L) and gas (0.003–1.4 kWh/L) forms, which negatively affects its storage. Gaseous H₂ storage requires high-pressure vessels of up to 70 MPa, while liquid storage needs cryogenic tanks maintained at −253 °C. There are many ways to store H₂, including liquifying H₂ in cryogenic tanks, compressing H₂ in high-pressure cylinders, adsorbing H₂ on materials with high specific surface area, absorbing H₂ on interstitial sites in metal, oxidation of reactive metals, and chemical bonding in covalent and ionic compounds [1]. The low volumetric density of H₂ has led the industry to examine H₂-enriched compounds and use them as a convenient storage medium.

H₂-enriched compounds that are liquid at mild conditions, such as NH₃, methane, and methanol, have recently gained attention as a distribution medium or for the storage of H₂. In contrast to other forms of chemical storage, NH₃ is the only carbon-free H₂ carrier and can be synthesized from renewable sources, as demonstrated by the opening of a pilot plant by Siemens in Oxfordshire, UK, in June 2018 and the starting up of a “green NH₃” plant by Nel H₂ and Yara in Western Australia in 2022. In these projects, NH₃ is produced by combining H₂ from water splitting with nitrogen from the air. The ubiquitous abundance of nitrogen in the air—as opposed to carbon—supports the use of a carbon-free H₂ carrier for a future large-scale, sustainable energy storage cycle. NH₃ is one of the only materials that can be produced cheaply, transported efficiently, and transformed directly to yield H₂ and a non-polluting byproduct. The annual global NH₃ production is around 180 million tons, making it the second most commercially manufactured chemical worldwide, after sulfuric acid, owing to its versatility [2].

NH₃ is viewed as a key player in the decarbonization of the energy and transportation sectors. It could be used to export and transport large amounts of renewable energy, transformed to H₂, burned, or used as fuel cells to run vehicles and produce electricity [3]. This paper presents a comprehensive review and analysis of NH₃’s critical involvement in empowering sustainable transportation with H₂. The different pathways to generate NH₃ using H₂ derived from sustainable sources are presented in this paper. In addition, the direct applicability of NH₃ in powering diverse transportation modes is evaluated, along with an analysis of the properties applicable to fuel-related technologies. The latest technological evolution regarding NH₃-fueled vehicles and innovation in H₂ generation from NH₃ for onsite application are also discussed. Moreover, the key prerequisites for engineering a durable and effective refueling system and mitigating vital safety considerations attributed to NH₃ handling and use in transportation are presented. Lastly, the environmental effect and the economic feasibility of using NH₃ in the transportation sector are analyzed. This paper offers a distinct contribution to the existing literature by providing a holistic and coordinated synthesis of NH₃’s role, with the aim of resolving H₂’s operational challenges for transportation applications.

2. NH₃ as an H₂ Carrier

NH₃ is a colorless gas in its pure form at room temperature, formed of H₂ and N. It can be used in industries and for domestic cleaning. To liquefy NH₃, a pressure of 10 bar at room temperature is required to attain an energy density of 14 MJ/L; in comparison, 700 bar is required to compress H₂, and the energy density attained by cryogenically cooled liquid H₂ is 10 MJ/L.

Table 1. Comparison between liquid H₂ and NH₃ [5,6].

	Liquid H2	NH3
Gravimetric energy density (MJ/kg)	120	18.6
Volumetric energy density (MJ/L)	8.5	12.7
Boiling point (°C)	−252.87	−33.34
Flammability range	4–75%	15–28%
Health hazard	Non-toxic, risk of frostbite and cold burns because it has a low boiling point	Highly toxic, corrosive, and strong, with a pungent odor

displays a comparison between the properties of liquid H₂ and NH₃. Liquid H₂ has a significantly higher gravimetric energy density compared to NH₃, meaning it has more energy per unit mass. Even though the specific energy of NH₃ is 18.6 MJ/kg, compared to 120 MJ/kg for pure H₂, the energy density of H₂ falls to 8 MJ/kg after considering the mass of high-pressure tanks. In terms of volumetric density, NH₃ has a higher volumetric density compared to liquid H₂; thus, it can store more energy per unit volume. Liquid H₂ is more flammable compared to NH₃, while NH₃ is more hazardous to health compared to liquid H₂. NH₃ liquid has a volumetric H₂ density of 106 kg of H₂/m³ under a pressure of 1 MPa and a temperature of 300 K, compared to 70 kg of H₂/m³ for liquid H₂ at a temperature of 20 K [4].

NH₃ is considered the foundation of all mineral nitrogen fertilizers, creating a bridge between nitrogen and food. The main use of NH₃ is for fertilizers, accounting for around 70%, followed by several industrial applications, like explosives and plastics [7]. NH₃ has the potential to be used as fuel and could play a significant role with H₂ in the transition to clean energy. From a safety aspect, NH₃ is known as a toxic gas, and it emits NO_x and NH₃ residuals. It presents a considerable threat when used in the energy sector, and it has a low reactivity as fuel. NH₃ could be used safely in industrial applications like energy storage; however, its safety is not proved for use in vehicles and homes. NH₃ could be used to store energy with a round-trip efficiency that is the same as that of liquid H₂. The round-trip efficiency of liquid H₂ is 30% less than that of H₂ gas compressed at low pressure [8]. The current round trip of NH₃ ranges between 11 and 19%, with a possibility to reach 36–50% in the case of using waste heat for district heating [9]. NH₃ could be transported using the existing infrastructure of fossil fuels, which is not possible in the case of H₂ transportation. Thus, NH₃ has the potential to be an H₂ energy carrier and thus an H₂ supplier. After transporting NH₃, a decomposition process should be performed to release H₂ and nitrogen.

NH₃ is classified by colors based on its generation technology and related environmental impacts. This classification ranges from high emission to zero emission: gray, blue, green, turquoise, yellow, and pink.

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Gray NH₃: NH₃ is generated using natural gas or coal by the Haber–Bosch process. It is the most commonly used method, and it emits high CO₂ emissions.

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Blue NH₃: NH₃ is produced using natural gas through the Haber–Bosch process with carbon capture storage technology. It emits less CO₂ compared to gray NH₃, but it relies on fossil fuels.

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Green NH₃: Electrolysis of water, operated by renewable energy sources, is used to generate green H₂, which then combines with nitrogen from the air to generate NH₃. This process is considered the most sustainable method to generate NH₃.

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Turquoise NH₃: A thermal decomposition of methane through methane pyrolysis is used to generate H₂ and solid carbon. The generated H₂ combines with the nitrogen from the air to generate NH₃. This process generates solid carbon, which is easier to manage compared to gaseous CO₂.

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Yellow NH₃: This type of NH₃ is generated from yellow H₂ combined with nitrogen. The yellow H₂ is produced by water electrolysis operated by grid electricity that is supplied from a mix of renewable and non-renewable sources.

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Pink NH₃: The H₂ used to generate NH₃ is generated using water electrolysis powered by a nuclear energy source.

The development of blue and green NH₃ and direct generation of NH₃ is required to enhance the role of NH₃ and H₂ in the global decarbonization process [10]. The direct technology is aimed at immediately generating NH₃ through a process called electroreduction of N₂, avoiding the generation of H₂. The main challenges facing the generation of blue and green NH₃ are the high generation cost, while the direct generation faces technological feasibility issues regarding commercial deployment.

3. Review Methods

A comprehensive and systematic review of the existing literature was conducted to support the key arguments presented in this manuscript. The search methodology was designed to ensure transparency, credibility, and reproducibility. The primary objective of the search was to identify a wide range of scholarly articles, review papers, conference proceedings, and technical reports on NH₃ as an energy carrier and its application in the transportation sector. The following electronic databases and academic search engines were consulted: Scopus, Web of Science, Google Scholar, and ScienceDirect. The search was conducted using a combination of keywords and phrases, including “NH₃ fuel”, “NH₃ transportation”, “NH₃ energy carrier”, “NH₃ utilization”, “NH₃ fuel cells”, “NH₃ combustion”, “green NH₃”, “NH₃ infrastructure”, and “NH₃ policy”. These terms were used in various combinations to capture all relevant publications.

The literature was screened according to the following criteria to ensure both relevance and quality:

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Inclusion: Peer-reviewed articles and conference papers; publications in English; studies focusing on the use of NH₃ as a fuel for transportation (e.g., maritime, road, and aviation); papers discussing NH₃ production pathways, infrastructure, safety, and policy implications related to its use as a fuel; and publications from the last two decades (2005–2025) to guarantee the review captures the most recent developments.

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Exclusion: Publications not directly related to NH₃ as a fuel or energy carrier and papers focused solely on NH₃’s traditional uses (e.g., fertilizer and chemical feedstock).

The chosen papers were systematically examined and analyzed to extract critical data and insights. The information was subsequently synthesized to identify key themes, highlight technological advancements, and compare various approaches to NH₃ utilization. Special attention was given to identifying the challenges, opportunities, and future research directions discussed in the literature.

4. Current and Future Use of NH₃

The global generation and use of NH₃ between 2016 and 2022 is displayed in Figure 1. The global generation of NH₃ reached around 180 million metric tons in 2022, making it the second most globally commercially manufactured chemical [11]. Most of this NH₃ is produced via the Haber–Bosch process. The agriculture sector is the highest consumer of NH₃, accounting for 80–85% of all usage [12]. NH₃ is mainly used to produce fertilizers like urea and ammonium nitrate. Additionally, it is also used in different industrial applications as refrigerant gas and to manufacture plastics, textiles, pesticides, explosives, and other chemicals. Due to the high generation of NH₃, its generation consumes around 2% of the global energy supply and releases about 400 Mt of CO₂, accounting for 1.6% of the world’s CO₂ emissions [13]. Many studies have been conducted to decarbonize the generation of NH₃ through different technologies such as “blue NH₃” using blue H₂ and “green NH₃” using green H₂ [14].

NH₃ has the potential to be used as an energy carrier and fuel. NH₃ is increasingly recognized as an energy carrier owing to its high energy density and its capability to store and transport H₂. It can be used for energy storage and for long-range transmission of renewable energy. Also, NH₃ could be used as fuel in the transportation and energy sectors. Many studies have been conducted to evaluate the possibility of using NH₃ as fuel in maritime transport and in buses and trucks. Some companies, like MAN Energy Solution and Wärtsilä, have already designed NH₃-fueled engines and are retrofitting existing ships to use NH₃. For instance, the global shipping giant Maersk has declared projects to investigate the use of NH₃ as an alternative eco-friendly fuel for its fleet. Norway has launched a project to design NH₃-powered ships. Similarly, several prototypes and engine fuel cells have been designed and tested for their abilities to use NH₃. In power generation, NH₃ has demonstrated its potential to be used to generate power either by dedicated NH₃ power plants or as a co-firing fuel with coal or natural gas to minimize greenhouse gas emissions. There is more global interest in generating green NH₃ using renewable energy resources. Many projects have already been launched in Europe and the Middle East to generate NH₃ with less environmental impact. For instance, NEOM Green H₂ Company (NGHC), situated in Oxagon, NEOM, in Saudi Arabia, aims to develop the world’s largest green H₂ facility. The plant will produce green H₂, which will be converted into green NH₃ for international export. Currently under construction, the project is expected to be operational by 2026.

5. Technologies Used to Generate NH₃

The production of NH₃ has traditionally depended on fossil fuel-based processes, especially the Haber–Bosch method. Although effective, these conventional methods are highly energy-intensive and account for a large share of global greenhouse gas emissions. New advanced technologies have been developed to generate NH₃ with a lower carbon footprint, such as blue and green NH₃. Additionally, some sophisticated methods, like photocatalysis and bio-based production, are gaining attention for their transformative potential for NH₃ synthesis through cleaner and more efficient techniques.

5.1. Haber–Bosch Process

The main process used to generate NH₃ is the Haber–Bosch process developed by the German physical chemist Fritz Haber and refined by Carl Bosch in the early 20th century. It is based on a direct synthesis of NH₃ from H₂ and nitrogen generated by air through a cryogenic distillation or pressure swing adsorption (PSA). This process is economically feasible and can be used for a large-scale process with the use of a catalyst and under high pressure needed to form NH₃. The working principle of this technology starts by combining nitrogen supplied from the air with H₂ under significantly high pressure and reasonably high temperature (Figure 2). N₂ and H₂ are precisely mixed and fed into the catalytic converter (reactor) under high temperature and pressure. Then, the hot gaseous mixture leaving the reactor contains NH₃, unreacted N₂, and H₂. NH₃ is separated, typically by cooling and condensing it into a liquid. The unreacted nitrogen and H₂ gases are recycled back into the reactor to maximize efficiency and conversion rates. H₂ is extracted from natural gas, coal, and waste, and the extraction process releases GHG emissions. The production of H₂ is considered as one of the most energy-intensive and carbon-emitting steps in the Haber–Bosch process [16].

The chemical reaction is reversible and exothermal and is expressed as

$${\mathrm{N}}_{2\left(\mathrm{g}\right)}+3{\mathrm{H}}_{2\left(\mathrm{g}\right)}\to 2\mathrm{N}{\mathrm{H}}_{3\left(\mathrm{g}\right)}$$

(1)

The reaction could achieve a better performance under low temperature, but by reason of kinetic reaction, high temperatures are needed for an acceptable generation. A catalyst, made predominately from iron, is required to speed up the reaction and to enable it to perform under lower temperatures. To enhance the ratio of NH₃ yielded in the mixture, the reaction should be performed under high pressure and low temperature. In the case of commercial generation, the reaction should be implemented under pressures starting from 200 to 400 atmospheres and under temperatures between 400 and 650° C [17].

5.2. Electrochemical Synthesis

In electrochemical synthesis, electricity is used to directly convert nitrogen and water into NH₃ through electrochemical cells. The N₂ reduction reaction takes place at the cathode side, and water oxidation occurs at the anode side under ambient pressure and temperature. The process begins by introducing nitrogen into the cathode to activate it owing to the catalyst at the cathode’s surface, which could also weaken the triple bond of nitrogen to make it reactive (Figure 3) [18]. Once activated, the N₂ molecule goes through a series of reduction steps by obtaining electrons (e⁻) supplied by the external electrical circuit and reacting with protons (H⁺). The protons can be supplied directly from the electrolyte or generated at the anode. The electrons generated at the anode side flow through the external circuit to the cathode, and the protons migrate through the electrolyte to the cathode. The chemical reaction of this process is

$${\mathrm{N}}_{2\left(\mathrm{g}\right)}+3{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2\mathrm{N}{\mathrm{H}}_{3\left(\mathrm{g}\right)}$$

(2)

This technology offers many advantages in terms of energy consumption, environmental impacts, and scalability. It can operate under ambient pressure and temperature, resulting in reduced energy consumption [20]. Waste heat from thermal or nuclear power plants or renewable energy sources could be used to generate NH₃ in high-temperature electrolytic conditions, making it a more eco-friendly process. Moreover, this process can be developed for small-scale and decentralized generation, making it appropriate for distributed NH₃ production. This technology encounters some barriers in terms of catalyst development, material stability, and reaction rates [20]. Exploring a performant, stable, and cost-efficient catalyst for the nitrogen reduction reaction is a barrier. Transition metal and non-metal barriers are candidate catalysts. Ensuring the long-term stability and performance of electrolyte and electrode under working circumstances is another barrier that should be addressed. Additionally, accomplishing high reaction rates to make the technology highly marketable is considered a challenge facing this process.

5.3. Solid-State NH₃ Synthesis (SSAS)

This is a novel technology to generate NH₃ using solid electrolyte. The working principle of this technology builds on transmitting protons or other ions through a solid electrolyte for easier nitrogen reduction (Figure 4) [21]. SSAS typically comprises an electrochemical cell that includes a solid electrolyte membrane separating an anode and a cathode. The H₂ source, which could be water, is split at the anode to provide protons (H⁺) or oxygen ions. Then, these ions are transported through the solid electrolyte. Nitrogen, supplied by the air, reacts with the transported ion at the cathode to form NH₃. This technology operates under atmospheric pressure and high temperatures (200–600 °C). The chemical reaction is given by

$${\mathrm{N}}_{2\left(\mathrm{g}\right)}+3{\mathrm{H}}_{2\left(\mathrm{g}\right)}\to 2\mathrm{N}{\mathrm{H}}_{3\left(\mathrm{g}\right)}$$

(3)

The advantages of this technology derive from its high energy efficiency and its low working temperature and pressure, resulting in energy savings, as well as the possibility to power the process using renewable energy, minimizing its carbon footprint [23]. Also, the SSAS process could be designed for modular and distributed NH₃ generation, leading to smaller-scale decentralized generation facilities. The SSAS process faces some barriers associated with assuring the long-term stability and performance of electrode materials and solid electrolyte under working circumstances [24]. Designing a scalable reactor and generation methods for industrial-scale generation is also considered a barrier that should be addressed. Additionally, the cost of solid electrolyte materials and electrode catalysts is a challenge that should be addressed to make this process competitive with existing technologies.

5.4. Biological Nitrogen Fixation

In this process, a conversion of atmospheric nitrogen into NH₃ is based on a natural process effectuated by some bacteria and archaea. The enzyme, named nitrogenase and possessed by these microorganisms, catalyzes the conversion of nitrogen into NH₃ under ambient circumstances [25]. This enzyme can break the strong triple bonds in N₂ molecules and reduce it to NH₃. This process is crucial for the nitrogen cycle in nature, since it allows plants to obtain nitrogen needed for growth. The chemical reaction is expressed as

$${\mathrm{N}}_{2\left(\mathrm{g}\right)}+8{\mathrm{H}}^{+}+8{\mathrm{e}}^{-}\to 2\mathrm{N}{\mathrm{H}}_{3\left(\mathrm{g}\right)}+{\mathrm{H}}_2$$

(4)

This process is considered sustainable because it is a natural process, minimizing the use of synthetic fertilizers, which are highly energy intensive with a high carbon footprint [26]. Also, it requires low energy input, since it uses microorganisms, and the process occurs under ambient temperature and pressure. The main challenges facing this technology are biological limitations and yield limitation. The nitrogenase enzyme used in this technology is sensitive to oxygen, which can restrain its activity, and thus low-oxygen environments are required [27]. The quantity of nitrogen fixed by biological processes is lower than that of synthetic fertilizers, requiring a balance between chemical and biological inputs.

5.5. Thermochemical Looping

Thermochemical looping is considered a sophisticated technology to generate NH₃ from nitrogen and H₂ through multiple oxidation cycles and reduction reactions using metal oxides as oxygen carriers. The fundamental principle of thermochemical looping includes a solid oxygen carrier or reactive metal/nitride material experiencing a cyclic transformation between different oxidation/nitridation states (Figure 5) [28]. The process involves two main stages: nitrogen absorption/nitridation and NH₃ synthesis/H₂ generation [29]. In the first stage, a reduced metal (M) or metal oxide (M_xO_y) reacts with N₂ in the first reactor to form a metal nitride (M_xN_y) or a nitrogen-containing intermediate. Then, the metal nitride or nitrogen-containing intermediate is transferred to a second reactor to react with H₂ gas and to form NH₃ and regenerate the original reduced metal or metal oxide. This stage usually happens under different circumstances from the nitridation stage. After that, the regenerated solid material is returned to the first reactor to absorb more nitrogen and close the loop. The added heat could be obtained from renewable energy sources or waste heat. The separation of two stages is needed to prevent the direct high-pressure mixing of N₂ and H₂ inherent in Haber–Bosch, contributing to safety improvements and streamlining the entire process.

The chemical reactions are as follows:

Formation of metal nitride:

$$\mathrm{M}+{\displaystyle \raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{\mathrm{N}}_2\to \mathrm{M}\mathrm{N}$$

(5)

NH₃ production:

$$\mathrm{M}\mathrm{N}+3{\mathrm{H}}_{2\left(\mathrm{g}\right)}\to \mathrm{M}+2\mathrm{N}{\mathrm{H}}_{3\left(\mathrm{g}\right)}$$

(6)

This technology offers several advantages in terms of energy efficiency, integration with renewable energy, and environmental impacts [27]. A significant amount of energy could be saved in this process, since it operates at ambient pressure. This process could use green H₂, minimizing the carbon footprint of this process. However, this technology faces some challenges in assuring reactivity and long-term stability of the metal oxide over many cycles [30]. Designing scalable reactors and efficient heat management systems for industrial-scale applications is another barrier facing this technology. Many research studies are being conducted to optimize materials, processes, and reactor designs to deal with the challenges faced and promote this technology for large-scale NH₃ generation.

5.6. Plasma Synthesis

This technology is based on exposing nitrogen and H₂ to plasma to convert them into NH₃. The high-energy electrons of plasma facilitate the breaking and forming of chemical bonds. The core principle of plasma NH₃ synthesis is based on producing a non-thermal plasma, where a collision between an energetic electron and inert nitrogen and H₂ occurs, disrupting their strong chemical bonds or elevating them to highly reactive states, even at near-ambient gas temperatures and atmospheric pressure (Figure 6) [31]. These energetic electron collisions result in the formation of highly reactive species like atomic nitrogen (N*), atomic H₂ (H*), and excited molecular species (N₂*, H₂*). Then, a combination of these reactive species in the gas phase can be used to create NH₃ with the help of catalysts, thus avoiding the extreme temperature and pressure conditions necessary for the conventional Haber–Bosch process.

This process can operate under atmospheric conditions and different temperatures, which depend on the plasma source, reducing energy consumption [32]. This process does not require solid catalysts. The main challenge of this process is the requirement of high energy input for the ionization [33]. The complexity of management required for the highly reactive environment within the plasma reactor in order to reduce side reactions and optimize NH₃ generation is another challenge facing this technology.

5.7. Photocatalytic Synthesis

This technology provides an eco-friendly and decentralized solution for NH₃ production using light as the primary energy source. This technique depends on a photocatalyst, a material (typically a semiconductor) that, when exposed to light, can trigger a chemical reaction. The fundamental principle entails using solar energy to drive the nitrogen reduction reaction (NRR). When photons from sunlight hit the photocatalyst, they excite electrons, which then migrate to the catalyst’s surface. These energized electrons enable the cleavage of the strong triple bond of the nitrogen molecule (N₂) and its subsequent reaction with protons to form NH₃. This process eliminates the requirement for high temperatures and pressures in the traditional Haber–Bosch process, positioning it as a potentially greener and more decentralized approach [34]. The main challenge is centered on developing photocatalysts with high efficiency and stability that can effectively convert N₂ and water into NH₃ at ambient conditions, a task complicated by the low solubility of nitrogen in water and the strong competition from the H₂ evolution reaction [35].

5.8. Hybrid Plasma–Electrochemical Synthesis

Hybrid plasma–electrochemical synthesis integrates two distinct technologies to surmount the limitations inherent in each. This technique uses an electrochemical cell to perform the nitrogen reduction reaction but with the assistance of a plasma to activate the nitrogen molecules. In this system, a non-thermal plasma is generated near the cathode or in the gas phase, where it breaks the strong N₂ bonds into highly reactive nitrogen species (e.g., radicals or excited molecules) [36]. These activated species are subsequently more readily reduced on the catalyst surface within the electrochemical cell. The plasma significantly reduces the energy barrier for the nitrogen reduction reaction, enabling the electrochemical cell to function with enhanced efficiency and at a lower voltage. This hybrid method aims to accomplish high NH₃ yields without the extreme conditions of the Haber–Bosch process while concurrently boosting the efficiency of pure electrochemical methods that struggle with the inert nature of nitrogen [37]. The primary challenges include optimizing the plasma reactor design to work synergistically with the electrochemical cell and managing the system’s energy consumption.

5.9. A Comparison Between the Different Technologies Used to Generate NH₃

Table 2. A comparison between the different technologies used to generate NH₃.

Technology	Pros	Cons
Haber–Bosch	High generation rates, well established	High energy consumption, operation under high pressure and temperature, high carbon footprint
Electrochemical synthesis	Operates under ambient conditions, low carbon footprint	Low efficiency and generation rate, under research and development
Solid-state NH3 synthesis	Possibility for renewable energy integration, minimizes liquid handling problems	Operates under high temperature
Biological nitrogen fixation	Low energy input because it is based on microorganisms, eco-friendly	Lower production rate
Thermochemical looping	Possibility for use of renewable energy and waste heat, high efficiency	Operates under high temperature, a complex process with different stages
Plasma synthesis	Operate under atmospheric pressure, possibility for renewable energy integration	High energy input, scalability issues

displays a comparison between the different technologies used to generate NH₃, highlighting the main advantages and disadvantages. The Haber–Bosch technology is the most established technology but is also the most energy intensive and has the highest carbon footprint. Electrochemical synthesis is viewed as a promising technology, since it can operate under ambient conditions and be powered by renewable energy. Biological nitrogen can operate under ambient conditions, and it requires low energy inputs. Solid-state NH₃ synthesis and thermochemical looping operate at high temperatures, resulting in high energy demand.

6. NH₃ Utilization in Transportation

NH₃ has some properties that make it a good candidate to be used in the transportation sector. It requires less pressure to be stored, with a pressure of 10.2 atm under a temperature of 298 K, compared to a pressure of 245.0 atm to store liquid natural gas [38]. In terms of energy density and fuel density, NH₃ has lower energy density and fuel density compared to gasoline and diesel but higher energy density and fuel density compared to natural gas. Thus, NH₃ could be used as fuel with a comparable efficiency to conventional fuels owing to the easy storage and transportation. NH₃ could be stocked as a liquid state under low-temperature or moderate-pressure conditions, simplifying infrastructure development. NH₃ does not emit carbon dioxide (CO₂) during combustion, making it an eco-friendly alternative to traditional fossil fuels. The generation of green NH₃ using renewable energy can considerably minimize the total greenhouse gas emissions. In terms of infrastructure, existing NH₃ facilities, involving generation plants, storage facilities, and transmission systems, can be employed for its use as a fuel. Thus, the demand for significant new investment could be minimized in comparison to other alternative fuels, such as H₂. Despite the toxicity of NH₃, which necessitates cautious handling, it is less ignitable compared to other fuels, minimizing the threat of explosions and fires.

6.1. Ways of Using NH₃ in Transportation

NH₃ could be used in different ways in transportation, including direct combustion in internal combustion engines, conversion to electricity in fuel cells, and integration into hybrid power systems.

6.1.1. Direct Use in Internal Combustion Engines

NH₃ could be used directly in modified internal combustion engines, providing a way to use existing technology and infrastructure. The operating principle is based on injecting NH₃ in the engine cylinders, mixing it with air, and igniting it to generate power. The direct combustion of pure NH₃ does not emit CO₂, and the storage and the transportation of liquid NH₃ are easier compared to highly compressed or cryogenic H₂. NH₃ could use a higher compression ratio in the engine owing to its high octane number, resulting in improved thermal efficiency [39]. The high latent heat of vaporization of NH₃ enables a cooling of the combustion chambers, despite potential issues in terms of complete combustion [40]. However, there are some challenges to the use of NH₃ in transportation. The high ignition, the narrow flammability, and the low flame speed of NH₃ compared to traditional fuels render the scalability and efficacy of combustion difficult [40]. Additionally, the volumetric energy density of NH₃ is low, almost half of the conventional fuels, requiring a bigger fuel tank for the same range. The combustion of NH₃ emits nitrogen oxides and nitrous oxides, which are considered to be greenhouse gases, offsetting the carbon-free advantages if not properly managed. Moreover, unburned NH₃, which is toxic with a strong odor, could be released with the exhaust, requiring strict emission control. The corrosivity of NH₃ for some engine materials necessitates consideration of material compatibility. The direct use of NH₃ in internal energy is promising for heavy-duty applications, including commercial trucks and marine shipping.

NH₃ dual-fuel engines originate from the necessity to address the limitations of NH₃’s properties, such as lower energy density. NH₃ could be integrated with another fuel that can offer better power output when required, leading to improved functionality of NH₃ as a vehicle fuel while preserving its environmental advantages [41]. Two designs of NH₃ dual engines are presented in Figure 7. In design A, the fuel, the air, and NH₃ are separately introduced into the internal combustion engine. However, the fuel and NH₃ are introduced together at a common point before entering the internal combustion engine, and air is introduced separately. Design A provides substantial advantages by enabling independent control of the injection of both the primary fuel and NH₃, allowing for precise optimization of combustion based on varying engine conditions. This flexibility is essential for handling NH₃’s challenging combustion characteristics, helping to minimize the likelihood of pre-ignition or knock in certain engine types. However, these benefits lead to greater system complexity, necessitating two different injection systems, thus adding to manufacturing overhead and ongoing maintenance. There is also a risk of poorer mixture uniformity when compared to pre-mixed systems, resulting in less efficient combustion and increased emissions concentrated in specific areas if not accurately regulated. In design B, the pre-mixing of fuel and NH₃ results in a more homogenous fuel–air mixture, which contributes to a more complete and more reliable combustion. This configuration streamlines the injection system by enabling a single point of entry for the mixed fuels [8]. Even so, the leading concern with this design arises from the serious safety implications of pre-mixing NH₃, a toxic and flammable gas, with another fuel before it enters the combustion chamber, leading to increased vulnerability to leaks or inadvertent ignition. Thus, the pre-mixed nature may restrict real-time adjustment of fuel ratios, and based on the chosen mixing strategy, challenges related to unburnt NH₃ may persist in the exhaust.

6.1.2. Use of NH₃ in Fuel Cells

The fuel cells convert chemical energy into electrical energy, combining high energy efficiency with a zero-emission footprint. NH₃ and H₂ fuel cells exhibit marked differences in their materials, reaction kinetics, and system integration [43]. Hydrogen fuel cells, like PEMFCs, rely on fast hydrogen oxidation kinetics and use platinum-based catalysts but face challenges with storing and transporting the low-density hydrogen fuel. Conversely, NH₃ fuel cells, such as SOFCs, benefit from the ease of storing and transporting liquid NH₃, which has a higher volumetric energy density [44]. However, their development is obstructed by the slow kinetics of the NH₃ oxidation reaction, which demands specific catalysts and often requires high operating temperatures to facilitate the in situ decomposition of NH₃. This variation in reaction speed and operating conditions results in unique system designs. Hydrogen systems are more mature for low-temperature applications, while NH₃ systems are better suited for high-temperature setups.

There are two options for using NH₃ in fuel cells: direct use or indirect use after cracking.

Direct NH₃ fuel cells: This is based on feeding NH₃ directly into the fuel cell, operating without relying on an external NH₃ cracking unit. The process begins by supplying NH₃ to the anode of the fuel cells, where it is electrochemically oxidized to be broken into nitrogen and protons/electrons and later reacts with oxygen supplied from the air at the cathode side to generate electricity. There are three types of direct NH₃ fuel cells: solid oxide fuel cells (SOFCs), alkaline fuel cells (AFCs), and NH₃ borane fuel cells. SOFCs are considered the most mature technology, working under high temperatures ranging from 600 to 850 °C, enabling an internal reforming/cracking NH₃ [45]. This leads to enhanced simplicity of the system architecture and high-power outputs. NH₃ borane fuel cells and AFCs are under consideration, providing lower power outputs and necessitating a more complicated NH₃ pretreatment as well as high NOx emission [46]. Direct NH₃ fuel cells are more efficient compared to internal combustion engines in converting chemical energy into electrical energy [39]. Additionally, they can simplify the system design, reduce the carbon footprint, and enhance the overall efficiency, since they do not require an external NH₃ cracker. Moreover, there is no direct emission of NOx or N₂O from the electrochemical reaction. However, direct NH₃ fuel cells encounter some challenges related to catalysts, material degradation, safety, and limited power density and efficiency. DAFC necessitates high active catalysts, and the working of SOFCs under high temperature results in some problems, such as sealing issues, nitride formation, and nickel coarsening [39]. In addition, the crossover of NH₃ to the cathode and securing system-wide safety are still critical. Moreover, attaining an equivalent or higher power density and high efficiency compared to H₂ fuel cells is still considered a challenge (Figure 8).

NH₃ cracking to H₂ for PEM fuel cells: In this process, NH₃ is thermally converted into H₂ and nitrogen over catalysts (e.g., ruthenium or nickel-based catalysts, typically at 400–600 °C) [47]. The H₂ steam should be subjected to a stringent purification process to eliminate uncracked NH₃ and nitrogen. This process is essential, since PEM fuel cells are extremely reactive to low-level presence of NH₃ (tolerance typically less than 0.1 ppm. Then, the purified H₂ is supplied to the PEM fuel cell to generate electricity. This method is mature and highly performant and can provide high-purity H₂ needed for PEMFCs. It could also maintain the benefits of easier NH₃ storage compared to H₂. The energy intensity of the cracking process leads to a considerable energy loss and to a decrease in the total well-to-wheel efficiency. This process is costly and complex because of the need for a cracking and purification unit [48]. Additionally, this process necessitates a more performant and durable cracking catalyst. Moreover, trace levels of residual NH₃ can be toxic and cause irreversible cell damage.

Catalyst design for NH₃ fuel cells depends on two main objectives: maximizing performance for the NH₃ oxidation reaction (AOR) at the anode and the oxygen reduction reaction (ORR) at the cathode while ensuring long-term stability. A fundamental design principle involves engineering a well-defined microstructure that facilitates efficient pathways for electrons, ions, and reactants to reach the active sites, often achieved using materials with high electrical and ionic conductivity [49]. For the anode, noble metals like platinum and ruthenium are highly active for AOR, but their high cost has led to research into non-platinum group metal (non-PGM) catalysts, such as transition metal alloys (e.g., NiCu), that provide a more economically viable alternative. Meanwhile, degradation mechanisms present a major obstacle to cell durability. NH₃ poisoning is a major issue, as uncracked NH₃ or its intermediate species can adsorb into the catalyst, obstructing active sites and markedly diminishing performance. Another critical mechanism is carbon corrosion, which compromises the integrity of the catalyst support structure, resulting in the detachment and loss of catalyst nanoparticles [50]. Sintering and agglomeration of metal nanoparticles also minimize the catalyst’s active surface area over time, further accelerating performance degradation [51]. Comprehending these degradation pathways is essential for developing more durable and efficient NH₃ fuel cell catalysts. Key innovations include a novel iron-based catalyst with an “inverse structure” that significantly increases ammonia production at low temperatures by boosting the density of active sites and enhancing nitrogen cleavage [52]. Another major development is the use of non-transition metal catalysts, such as a barium–silicon oxynitride–hydride catalyst that activates nitrogen using anion vacancies and performs better than expensive ruthenium-based alternatives [53]. Finally, copper-based catalysts are being explored for the electrochemical conversion of nitrate to ammonia, offering a way to use a more easily reduced nitrogen source and address environmental concerns related to nitrate pollution [54].

Beyond the development of new catalysts, recent research is focused on optimizing the overall system for ammonia synthesis to improve efficiency and safety. One approach involves using multibed reactor systems with a mixed catalyst configuration, such as combining iron- and ruthenium-based catalysts in series [55]. This method leverages the unique kinetic properties of each catalyst to increase ammonia yield and save energy. Additionally, advanced computational tools like machine learning and integrated process models are being used to optimize the entire production loop [56]. These models can predict optimal production conditions, ensure safety, and identify ways to reduce energy consumption, with one study showing a 3.2% energy reduction without sacrificing product quantity.

6.1.3. Hybrid Systems

Various power generation methods are combined and used in hybrid systems, resulting in optimal performance and emissions. Hybrid systems provide the ability to optimize different performance requirements, with the potential to mitigate some of the individual hurdles of direct NH₃ combustion or pure fuel cell systems. They can enhance the efficiency of the whole system, minimize specific emission, and improve power output and dynamic response, establishing a more practical foundation for NH₃ adoption in different transportation applications [57]. The incorporation of NH₃ in these systems could be carried out in many ways. An NH₃ fuel cell with a battery hybrid system is suitable for maritime applications, in which efficient power can be supplied by a direct NH₃ fuel cell, and management of load fluctuation can be offered by a battery tank, delivering energy during system fluctuations and providing backup capability. This configuration is undergoing evaluation for large vessels [58]. NH₃ ICE with a battery hybrid system is another configuration, where the main power is provided by ICE, and the battery aids in acceleration while capturing energy during regenerative braking and supports low-speed operation powered solely by zero-emission electricity [59]. This leads to enhanced performance and minimizes emission variability. A combination of NH₃ cracking with ICE and PEMFC is considered an advanced combination. In this configuration, H₂ is supplied to a PEM fuel cell through an onboard NH₃ cracker for effective energy production [57]. An ICE is used to supply peak power, operate as a standby, and utilize some of the cracker’s heat/off-gases. This system could achieve high energy and performance, with considerable carbon footprint reductions.

6.2. A Comparison Between the Various NH₃ Utilization Technologies in the Transportation Sector

A critical comparative analysis of NH₃ utilization technologies is presented in

Table 3. A critical comparative analysis of NH₃ utilization technologies [60,61,62,63].

Criteria	Direct Combustion (ICEs)	Fuel Cells (e.g., SOFCs)	Hybrid Systems (e.g., ICE + Cracker)
Costs	Lower initial capital cost due to engine adaptation	High initial cost (catalysts, complex systems)	High initial cost (multiple components)
Efficiency	Medium (typically < 50%)	High (up to 60%)	High (improved by blending with H2)
Emissions	High NOx emissions, requires aftertreatment	Zero direct NOx emissions	Moderate NOx emissions, requires aftertreatment
Infrastructure	Utilizes existing fuel handling but needs specialized storage.	Requires new, specialized fuel cell technology	Complex, requires onboard cracking and storage
Key challenges	Managing NOx and unburnt NH3 emissions	High cost, performance at varying loads	System integration and complexity

. It highlights specific trade-offs that will shape their future prioritization. While direct combustion in internal combustion engines (ICEs) is the most technologically mature and cost-effective solution for rapid deployment, its foremost challenge is reducing elevated nitrogen oxide (NOx) emissions. In contrast, fuel cells offer a pathway to zero tailpipe emissions and high efficiency but remain at a lower technology readiness level due to high material costs and operational complexities. Hybrid systems seek to bridge this gap by combining the benefits of both approaches, but their commercial viability depends on resolving issues related to system integration and cost. This analysis highlights several key research gaps: the need for cost-effective NOx reduction catalysts for direct combustion, the development of affordable and durable components for NH₃-fed fuel cells, and the optimization of onboard cracking technologies for hybrid powertrains. Addressing these research areas will be crucial for accelerating the commercial adoption of NH₃ as a transportation fuel and realizing its full potential for decarbonization.

6.3. The Current State of Technology for NH₃-Powered Vehicles

The use of NH₃ in different transportation types is discussed below.

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Marine transport: NH₃ has a promising potential to be used in internal combustion engines and fuel cells in the maritime industry with less environmental impact compared to conventional fuels [64]. In internal combustion engines, NH₃ could be used directly in a modified internal combustion engine or in dual-fuel systems, where NH₃ is mixed with other fuel to enhance its combustion efficiency. Also, NH₃ could be used as a fuel cell by implementing an onboard cracking system to decompose NH₃ into H₂ and nitrogen to generate electricity to power electric propulsion systems. Another way to use direct NH₃ fuel cells is to directly convert NH₃ into electricity without the need for an NH₃ cracking system, but this technology is still under development. The benefits and challenges facing the use of NH₃ in marine transport are the same as those in the aviation sector.

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Heavy-duty vehicles: The use of NH₃ in trucks and buses can be either in internal combustion engines or in fuel cells. The internal combustion engines should be designed to burn NH₃, and the fuel cell should be adjusted to convert NH₃ into electricity [65]. Ongoing research and development are being conducted to adjust the design of engines to deal with the different barriers associated with efficiencies and emissions. The cost of different vehicle fuels in terms of energy cost is displayed in Figure 9, including conventional NH₃, green NH₃, and H₂. Natural gas is the cheapest fuel with a narrow cost range, while green NH₃ and H₂ are more expensive, derived from the high cost of renewable energy. Conventional NH₃ is cheaper but more costly than fossil fuels.

Greenhouse gas emissions from different vehicle fuels are displayed in

Table 4. GHG emission per unit for different vehicle fuels [15,68].

Type of Vehicle Fuel		CO2 Emission
Gasoline		2.31 kg CO2/Liter
Diesel		2.68 kg CO2/Liter
Gray H2		10 kg CO2/kg
Blue H2		2 kg CO2/kg
Green H2		0.01 kg CO2/kg
Gray NH3		2.75 kg CO2/kg
Green NH3		0.01 kg CO2/kg
CNG		2.75 kg CO2/kg
Electricity	Renewable energy	Nearly zero kg CO2/kWh
	Coal	0.4–0.5 kg CO2/kWh
	Natural gas	0.82 kg CO2/kWh

, highlighting the significant differences in emissions based on fuel type and generation method. The amount of CO₂ emission from gasoline and diesel is high due to the direct combustion of fossil fuels. Natural gas emits less CO₂ compared to gasoline and diesel. The CO₂ emission from H₂ and NH₃ depends on the generation method of H₂.

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Aviation: NH₃ is being viewed as a potential aviation fuel owing to its high energy density, existing infrastructure, storage and handling, and smaller carbon footprint [69]. In terms of energy density, NH₃ has a higher energy density by volume compared to liquid H₂, making it more convenient for long-trip flights. The existing infrastructure to generate and distribute NH₃ for agriculture can be used for aviation. Additionally, NH₃ can be stocked as a liquid state under moderate temperatures and pressures, resulting in easier storage and handling. However, some challenges are encountered in the use of NH₃ in aviation, including toxicity and corrosivity, combustion efficiency, and the weight of the onboard cracking system. Stricter safety regulations are required to avoid any leak of NH₃ [70]. The combustion efficiency of NH₃ is lower compared to traditional jet fuels, requiring a substantial engine adaptation. The use of onboard cracking systems adds weight to the aircraft, negatively affecting the performance and the efficiency of the aircraft.

NH₃ could be used in jet engines with direct combustion or as an H₂ carrier. In the case of direct combustion, NH₃ can be used alone or in combination with other fuels in dual-fuel systems, like H₂. Jet engines should be extensively modified to allow a direct combustion of NH₃ and to assure a performant and stable combustion due to the low flame speed and combustion properties of jet engines. In the case of dual-fuel systems, the combination of NH₃ with other fuels improves the efficiency and the stability of combustion. In the case of using NH₃ as an H₂ carrier, NH₃ is cracked thermally to generate H₂ and nitrogen. Then, the generated H₂ can be utilized in fuel cells to produce electricity and power electric aircraft. Also, the cracking of NH₃ can be onboard by implementing an NH₃ cracking system in aircraft to ensure a continuous supply of H₂ for fuel cells [70].

The use of NH₃ in the aviation sector is still in the experimental stage. A comparison between different aircraft fuels in terms of energy consumption and fuel consumption is presented in Table 5.

Evaluating NH₃ as a carbon-free fuel for an Airbus A350-1000 class aircraft highlights a notable trade-off: while it offers up to a 75% reduction in global warming potential and supports more compact, efficient engine designs (e.g., a 9.37% reduction in engine weight), its low heating value significantly limits payload-range capability, requiring substantial aircraft redesign to maintain competitive performance [71] (Figure 10).

Table 5. Fuel and energy requirements of various aircraft fuels [72,73].

Fuel	Energy Consumption (MJ/km)	Fuel Consumption (kg/tkm)
Kerosene (jet fuel A)	343.5	0.21
Natural Gas	460	0.25
Methanol	360	0.49
Ethanol	360	0.33
H2	316.5	0.07
NH3	350	0.51

Table 5. Fuel and energy requirements of various aircraft fuels [72,73].

Fuel	Energy Consumption (MJ/km)	Fuel Consumption (kg/tkm)
Kerosene (jet fuel A)	343.5	0.21
Natural Gas	460	0.25
Methanol	360	0.49
Ethanol	360	0.33
H2	316.5	0.07
NH3	350	0.51

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Rail transport: NH₃ can be used to replace the use of diesel to power trains where electrification of railways is not practicable, offering a significant benefit. While promising, its broad implementation is challenged by engineering constraints, mainly attributed to its energy density; liquid NH₃ possesses a volumetric energy density of approximately 12.7 MJ/L, significantly less than diesel’s 36 MJ/L, implying that NH₃-powered locomotives would need either larger fuel tanks or more frequent refueling to achieve similar ranges [74]. NH₃ provides approximately 18.6 MJ/kg of energy, significantly lower than diesel’s 42 MJ/kg. Regarding emissions, while NH₃ combustion is carbon-free, it can lead to NOₓ emissions from the fuel’s nitrogen content. Nonetheless, its lower combustion temperatures may help limit thermal NOₓ from air nitrogen. Optimizing these variables necessitates sophisticated combustion technologies and exhaust after-treatment systems to control total NOx and potent nitrous oxide (N₂O) emissions. Although technical hurdles remain, companies including Fortescue Future Industries (FFI) and leading locomotive manufacturers are actively pursuing research and conceptual design work to explore NH₃’s integration into rail propulsion systems, frequently exploring both direct combustion in adapted engines and integration with high-efficiency fuel cells.

Table 6. Summary of TRL for NH₃ utilization in transportation [75].

Technology	Transportation Sector	Description	Technology Readiness Level (TRL)
Direct combustion	Maritime, road, rail	NH3 is directly used as fuel in internal combustion engines (ICEs) or gas turbines. Requires modifications to the engine and aftertreatment systems to handle its low flammability and mitigate NOx and unburnt NH3 emissions.	6–7 (large-scale prototype or pre-commercial demonstration, particularly in the maritime sector).
Fuel cells	Maritime, heavy-duty transport	Solid oxide fuel cells (SOFCs): Use high temperatures to directly convert NH3 to electricity, considered more mature than other fuel cell types for NH3. Proton exchange membrane fuel cells (PEMFCs): Cannot use NH3 directly. Requires a reformer (cracker) to first convert NH3 to hydrogen, adding complexity and cost.	SOFCs: 5–6 (system-level validation in a relevant environment). PEMFCs: 3–5 (proof of concept to component and subsystem validation).
Hybrid systems	Maritime, heavy-duty transport	Combines different technologies, such as an internal combustion engine running on an NH3–hydrogen blend. Hydrogen is often produced onboard via an NH3 cracker, which improves the combustion properties of NH3 and increases efficiency.	5–7 (system-level validation to pre-commercial demonstration, often seen in maritime and power generation).

summarizes the technology readiness levels (TRLs) of various NH₃ utilization technologies for the transportation sector. TRL is a scale from 1 to 9 that evaluates the maturity of a technology, ranging from fundamental research to full-scale commercial deployment. While NH₃’s global production and transport infrastructure is mature (TRL 9), its use as a fuel in transportation is still in development. Direct combustion is the most mature, as it adapts existing engine technology and is already being demonstrated in maritime applications. Fuel cells are generally less mature, with solid oxide fuel cells (SOFCs) being further along because they can directly use NH₃ at high temperatures, whereas proton exchange membrane fuel cells (PEMFCs) require an additional, less-developed NH₃ cracking step. Hybrid systems, which combine aspects of direct combustion and fuel cell technologies, also demonstrate potential and are advancing toward demonstration.

7. Challenges and Potential Solutions for NH₃ as a Transportation Fuel

7.1. NOx Emissions Reduction

The burning of NH₃ in air, especially in internal combustion engines, could facilitate the creation of nitrogen oxides (NOx) given the existence of nitrogen in the fuel itself and nitrous oxide, an impactful greenhouse gas. This issue could be faced mainly in high-temperature SOFCs. Currently, there are many potential solutions to mitigate this challenge. Staged combustion is one of the solutions that is based on burning NH₃ in multiple, distinct steps to control the combustion process through managing the temperature and the presence of oxygen in order to hinder the development of NOx [76]. The second solution is the use of selective catalytic reduction systems to supply a chemical reductant (such as NH₃ solution or urea) into the exhaust steam in the presence of a catalyst to transform NOx into non-toxic nitrogen and water. Some studies have demonstrated that the emission of N₂O can be completely removed and the amount of NOx can be considerably minimized using a catalytic treatment [77,78]. Some advanced strategies can also be considered as solutions, including H₂ co-firing/pilot injection and moderate or intense low oxygen dilution (MILD) combustion. H₂ co-firing/pilot injection aims to mix NH₃ with a small quantity of more reactive fuel such as diesel (pilot fuel) or H₂ to enhance the stability of the combustion and minimize NOx [79]. In the moderate/intense low oxygen dilution combustion method, the combustion process is operated under highly diluted, fuel-lean conditions to minimize thermal NOx by enabling lower peak temperatures in the combustion room. [80] Lastly, the reduction of NOx formation can be achieved by a precise regulation of ignition timing, air–fuel mixing, and exhaust recirculation [81,82].

7.2. NH₃ Slip

NH₃ slip denotes the residual or unreacted NH₃ that escapes into the exhaust gas. This can occur in ICEs due to the unburned or partially reacted NH₃ from SCR systems used for NOx reduction. NH₃ is toxic and has a pungent odor, demanding stringent control. The potential solutions include NH₃ slip catalysts, optimized SCR control, engine design and tuning, and multistage fuel injection. NH₃ slip catalysts are targeted catalysts introduced downstream of the SCR system to convert any residual into nitrogen and water [83]. An accurate introduction of NH₃/urea in the SCR system could be a solution to optimize the conversion process of NOx without excessive NH₃ [84]. A rigorous monitoring of some factors, like NH₃ injection rate and catalyst temperature, should be carried out. In terms of design and tuning, some sophisticated engine designs and combustion tuning can accomplish a very low NH₃ slip without the need for an additional ASC, through allowing a more complete burning. For instance, WinGD’s X-DF-A has been crafted for marine engines [85]. Some solutions could include fuel injection by ensuring a precise injection of NH₃ in the combustion room with the aim of enhancing mixing and burning, resulting in less unburned NH₃ [86].

7.3. Engine/Fuel Cell Modifications

In the case of internal combustion engines, the use of NH₃ encounters some issues related to its high ignition temperature, its low flame speed, and its high minimum ignition energy compared to conventional fuels. These challenges could be mitigated through a dual operation using a pilot injection of diesel or H₂ or a considered increase in compression ratios using sophisticated ignition systems (such as multiple igniters and plasma-assisted ignition) and pre-combustion rooms [87,88]. The high latent heat of vaporization of NH₃ can cool the combustion room, inhibiting combustion. Preheating the NH₃ fuel or enhancing injection strategies for enhanced atomization and air–fuel mixing could be another solution. The corrosivity issues of NH₃ for some engine components through time is another challenge. This could be mitigated through using resistant materials for NH₃ and its residual in fuel lines and engine parts [89]. The enhancement of chamber design, air–fuel mixing, and fuel injection systems could ensure a stable and effective burning of NH₃ [90].

In the case of fuel cells, and for solid oxide fuel cells, there is a need to develop highly active and durable catalysts for direct electrochemical NH₃ oxidation at the anode in order to achieve efficient management of the transition of NH₃ to the cathode and enhance the resistance of materials to degradation under high operating temperatures. Currently, some novel catalyst materials, such as metal–organic frameworks, layered double hydroxides, spinel, and perovskite structures, are being explored to enhance electrode architectures and introduce DAFCs into hybrid energy systems to mitigate thermal swings and improve operational efficiency [91,92]. PEM fuel cells are strongly affected by even minor residues of NH₃, demanding high-performance purification. This could increase the demand for energy input that is already high. There are two options to solve this issue: one is based on cracking design, and the second is related to an efficient purification method. The cracking design modifications include the development of operationally efficient, durable, and economically feasible cracking catalysts, such as non-noble metal catalysts [93]. Additionally, heat exchangers to use the waste heat in the preheating and the cracking process could be incorporated to enhance the energy efficiency. The use of performant and robust H₂ purification methods, such as palladium membrane and pressure swing adsorption, could also enhance the purification process.

7.4. Fuel Efficiency

NH₃ has a low volumetric energy density, which is half of the volumetric energy density of diesel, requiring a bigger fuel tank to accommodate the same volume. Additionally, its combustion kinetics are slow, resulting in incomplete burning and lower thermal efficiency if inadequately controlled. To secure high combustion efficiency and completeness of NH₃, sophisticated combustion approaches should be implemented, including turbulence enhancement, high compression ratios, precise injection timing, and H₂ co-firing [94,95]. Moreover, the power needed for cracking NH₃ on board leads to an energy efficiency loss, minimizing the total well-to-wheel efficiency of NH₃-to-H₂-to-electricity pathways. The total energy generated could be reduced because of the high latent heat of vaporization of NH₃, meaning the need for higher energy input for vaporization. The vaporization of liquid NH₃ or providing energy for onboard cracking units could be performed using waste heat from the engine exhaust and cooling system, enhancing system performance [47]. The optimization of energy conversion efficiency requires an enhancement of the design of combustion rooms, fuel cell stacks, and fuel injection systems. Also, the improvement of the efficiency of the system could be achieved using H₂ systems. For instance, integrating batteries with fuel cells could enhance the total system efficiency through regulating the fluctuating operational loads, recovering the braking energy, and ensuring optimal operating conditions for each component [96]. An advanced control system should also be implemented for dynamic adjustment of fuel cell factors in real time to sustain peak performance across fluctuating operating conditions [97].

8. Infrastructure and Safety for NH₃ as a Transportation Fuel

The adoption of NH₃ as a carbon-free transportation fuel demands resilient infrastructure planning paired with rigorous safety measures. Despite the substantial ecological benefits of NH₃, its distinctive chemical properties necessitate a careful consideration regarding handling, storage, distribution, and overall safety management.

8.1. NH₃ Refueling Infrastructure Requirements

In terms of storage condition, NH₃ is usually stored in liquid form, necessitating pressurization or refrigeration. The pressurization should be up to 10 bars under ambient temperature, and the refrigeration could be up to −33 °C at atmospheric pressure [98]. Fully refrigerated storage is commonly favored owing to the lower toxicity of liquid NH₃ under low temperatures and cautious strategies adopted by regulatory agencies. The refueling infrastructure for NH₃ will vary from traditional fuels because of its physical properties and safety considerations. Refueling stations of NH₃ are lead-prone zones, necessitating an enclosed arrangement to avoid diffusion of leaked NH₃ [99]. Open space configurations could also be used with thorough risk evaluation and robust safety control in place. Some equipment is essential, such as drip trays, emergency pull-off valves, leakage detection systems, and dry quick connectors, in addition to fire extinguishing facilities and personal protective equipment (PPE). The bunkering process necessitates defined stages involving initial pre-cooling, hose connection, inert gas blanketing, flushing, transferring, unloading, and disconnecting [100]. Thus, there is a need to implement boil-off gas (BOG) management systems, since heat ingress can cause it. The materials used in the infrastructure components should be compatible with NH₃ to avoid corrosion and stress corrosion cracking, mainly with carbon steel surfaces. In terms of logistics and distribution, the transition to NH₃ could be facilitated by building on existing global NH₃ generation, storage, and transportation infrastructure. However, it is necessary to enhance production scales and adapt existing networks for fuel-specific demands.

8.2. Safety Considerations of Using NH₃ as a Transportation Fuel

NH₃ is a dangerous substance that is toxic, corrosive, and flammable, necessitating rigorous safety standards throughout its lifespan as a fuel. NH₃ is highly toxic, mainly in gaseous form. Exposure to high levels can pose an immediate threat to life, resulting in intense irritation and burns to skin, eyes, and respiratory tract, possibly causing pulmonary edema. Additionally, NH₃ has a strong, intense odor, warning of impending risks for leaks, but it should be taken seriously to detect hazardous exposure levels. Permissible exposure limits (PEL) are regulated (e.g., OSHA PEL of 50 ppm averaged over an 8 h work shift and NIOSH REL of 25 ppm over 10 h) [101].

The corrosion issues of NH₃ that could worsen under moist conditions could lead to stress corrosion cracking in some materials, such as carbon steel, and it may respond chemically to galvanized steel, copper, and brass [102]. Thus, the materials should be carefully selected for storage tanks, valves, and engine components to avoid leaks and structural breakdowns.

NH₃ is flammable, but it has a limited flammability range in air (15–28% by volume) and a high auto-ignition temperature (651 °C) compared to hydrocarbons [103]. Thus, it needs a considerable amount of energy to ignite and combusts more slowly than many other fuels. In the case of extensive leakage and accumulation of NH₃ in an enclosed space, explosion risk represents the main safety concern, thus demanding strong airflow management and gas leak monitoring.

The processing and management of NH₃ for fertilizer and chemical generation in industrial settings has maintained safety standards and regulatory frameworks for more than a century. Established industry experience provides a solid basis for formulating specific fuel regulations. In the case of marine applications, provisional regulations and procedural guidelines for NH₃ have been established by organizations such as the International Maritime Organization (IMO) and classification societies (including ABS and ClassNK), encompassing system design, safety mechanisms, arrangement, and operational protocols [104].

8.3. Strategies for Safe Handling, Storage, and Distribution of NH₃

Comprehensive safety strategies are essential to manage NH3’s risks. The different safety strategies for handling, storage, and distribution of NH₃ are displayed in Figure 11. Leak prevention and detection include the use of strong, corrosion-resistant materials with double-walled pipes to reduce the use of threaded joints and utilize high-integrity equipment. In addition, NH₃ gas detectors should be installed in sensitive zones with active alarm systems and automatic shutdown systems [104]. Moreover, mechanical ventilation systems should be installed to avoid NH₃ accumulation and support rapid mixing to minimize leak impact. NH₃ is highly soluble in water, requiring the integration of drip trays and a water spray system as protocols for the safe handling of NH₃–water effluent. The use of personal protective equipment (PPE) should be mandatory, particularly in bunkering operations or emergency situations. Additionally, robust emergency response plans are essential, including workforce preparedness for handling NH₃-specific hazards and controlling leaks, first aid for exposure, and emergency evacuation protocols. To reduce spill risk during emergencies, emergency release systems (ERSs) and breakaway couplings are integrated into bunkering hoses [105]. Regarding training and competency, robust training schemes for all team members responsible for NH3 handling, bunkering, operation, and maintenance are essential, encompassing its characteristics, associated hazards, and response protocols. Continuous qualitative hazard identification (HAZID) and quantitative risk assessments (QRAs) are essential for recognizing potential hazards and assessing risks to deploy mitigation actions throughout the fuel’s lifetime.

9. Environmental and Economic Analysis of NH₃ as a Transportation Fuel

The widespread adoption of NH₃ in the transportation sector depends on a comprehensive grasp of its environmental effects throughout its entire life cycle and its economic viability compared to conventional and alternative fuels.

9.1. Environmental Analysis of NH₃

A life cycle assessment (LCA) presents a detailed evaluation of the environmental consequences of a fuel, considering every phase from raw material extraction to end-use. LCA includes “well-to-tank” (WtT) and “tank-to-wheel” (TtW) (or “tank-to-wake” for marine) [8].

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Well-to-tank (WtT): This phase accounts for emissions resulting from H₂ generation and NH₃ synthesis through the Haber–Bosch process. The H₂ source determines its environmental effect. The generation of brown or gray H₂ is energy intensive and emits a significant amount of CO₂ emissions, since it is generated using natural gas and coal through steam methane reforming and coal gasification. This emission is reduced in the case of blue H₂ owing to the use of carbon capture and storage. However, green H, generated from water using water electrolysis powered by renewable energy, provides the most promising pathway to near-zero lifecycle emissions. The synthesis of NH₃ through the Haber–Bosch process necessitates substantial energy even when using green H₂ [106]. Thus, the entire process should rely on renewable energy to enable a low carbon footprint.

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Tank-to-wheel (TtW): This phase addresses combustion-related emissions or using NH₃ in vehicles. The combustion of NH₃ does not contribute to CO₂ emissions, but it emits nitrogen oxides (NOx), nitrous oxide (N₂O), and unburned NH₃ [107]. N₂O is a major heat-retaining gas in the atmosphere, and nitrogen oxide (NOx) leads to air quality degradation and acid rain formation. The engine technology and the functional settings, including fuel mix, air–fuel ratio, and combustion chamber design, highly influence the emission levels. Additionally, the unburned NH₃ and NOx may result in the creation of fine particulate matter (PM 2.5) in the atmosphere, presenting health hazards, particularly arising from pure NH₃ engines in the absence of cutting-edge emission management solutions [108].

NH₃ holds promise for a considerable reduction of GHG emissions compared to fossil fuels, mainly if generated as green NH₃. A notable reduction in the overall greenhouse gas emissions could be achieved through transitioning away from fossil fuels to green NH₃, contributing to the achievement of ambitious decarbonization milestones. Realizing this climate benefit depends on reducing slip emissions of unreacted NH₃ and N₂O during the burning stage and ensuring carbon-efficient production methods. Even though the transition from heavy fuel to NH₃ could minimize some GHG emissions, the largest emissions cuts are achieved through the generation of blue, or optimally green, NH₃. For example, certain studies estimate that NH₃’s climate impact accounts for 5.8% of that of current fossil fuel fleets if N₂O emissions are effectively managed [109].

9.2. Economic Analysis of NH₃ as a Transportation Fuel

The economic viability of NH₃ for powering vehicles and transport systems is essential to achieving large-scale deployment, including generation, distribution, vehicle, and fuel costs.

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Production costs: The production costs of brown and gray NH₃ are the most cost-effective, implementing mature and efficient procedures through natural gas or coal. However, the production costs of green NH₃ are more expensive due to considerable upfront capital investment (CAPEX) in renewable energy infrastructure (e.g., wind farms and solar panels) and electrolyzers. Advancements in technology, economies of scale, and supportive renewable energy policies are anticipated to drive down these costs. The current production costs of green NH3 range from USD 600 to 800 per ton, with forecasts indicating a reduction to approximately USD 250 per ton by 2030 if green H₂ production scales effectively [110] (Figure 12).

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Distribution costs: NH₃ takes advantage of a worldwide infrastructure already in place for its role as a fertilizer and chemical. NH₃ could be stored and transported as a liquid under moderate operating conditions (−33 °C or ~10 bar), requiring substantially less energy and expense than liquefying and transporting H₂ (which requires −253 °C or very high pressures) [112]. The volumetric energy density of NH₃ is higher compared to liquid H₂ (12,822 MJ/m³ for liquid NH₃ vs. 8496 MJ/m³ for liquid H₂) [113]. These lead to lower spatial requirements for storage, resulting in reduced distribution costs per energy unit. The transportation of H₂ as NH₃ is significantly more affordable than transporting gaseous H₂ across extended distances.

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Vehicle costs: Changes in internal combustion engines or the design of new NH₃-specific fuel cells are needed to adopt NH₃ as fuel. These adjustments, mainly for internal combustion engines, involve modification of fuel injection systems, higher compression ratios, and material selection to avoid corrosion. This could result in increasing initial investment in vehicles; additionally, dual-fuel engine scenarios, which are based on mixing different fuels like NH₃ with diesel or with H₂, are being evaluated to improve combustion and control emission, potentially impacting vehicle system complexity and expenses.

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Fuel costs: The total cost will encompass production, distribution, and requisite onboard processing. The cost of renewable energy and electrolyzer technology significantly increases the cost of green NH₃.

9.3. Comparison of Costs with Other Alternative Fuels

The comparison between NH₃ and H₂ as energy carriers shows that NH₃ is more cost-effective, mainly for transporting over long distances and storing at scale [114]. This is largely attributed to the markedly lower costs linked to storage and distribution of liquid NH₃ compared to highly compressed or cryogenic liquid H₂. Additionally, NH₃ offers a benefit in terms of energy density and operational ease compared to H₂, potentially increasing its competitiveness for applications necessitating greater energy storage capacity. Batteries are considered as direct storage for electric vehicles, especially for short distances. However, batteries face many difficulties in the case of heavy-duty transportation due to the energy density limitations and charging time of existing battery technologies, which undermine their ability to compete with liquid fuels, including NH₃. In terms of levelized cost of energy (LCOE), NH₃ offers an effective option for seasonal and long-duration storage compared to batteries [115].

9.4. Economies of Scale Driving Cost Reduction

The production costs of green NH₃ could be considerably reduced through economies of scale in renewable energy production and large-scale production of electrolyzers. Even though economies of scale diminish beyond 1 MMTPA (million metric tons per annum) for green NH₃ production facilities, as suggested by some studies, advantages may extend up to 10 MMTPA when integrated with new port facilities or extended pipeline networks [116]. Additionally, a cost reduction and an enhancement of efficiency are anticipated owing to the ongoing research and development in NH₃ synthesis processes, electrolyzer technologies, and engine technologies. The economic competitiveness of green NH₃ could be enhanced by government subsidies, carbon taxation systems, and policies supporting low-carbon fuels [117]. Moreover, building upon the existing global NH₃ generation and distribution networks, regardless of needed adjustments for fuel-specific applications, minimizes the requirement for completely new infrastructure, supporting a decrease in overall expenses.

9.5. Commercialization Path and Cost Reduction Potential of Green NH₃ in Different Markets

The viability and cost-effectiveness of green NH₃ for transportation heavily rely on regional conditions. The regions with abundant renewable energy resources, such as the Middle East, Australia, and North Africa, hold a competitive edge in producing cheaper green NH₃ owing to the high solar irradiance. The estimated cost of green NH₃ in these regions is among the lowest globally, with some projects in the Middle East and Australia aiming for a levelized cost of NH₃ (LCOA) below USD 800 per ton by 2025 [118]. This is favorable compared to Europe’s higher projected costs, which are driven by limited renewable resources. The EU’s Emissions Trading System (ETS) and other carbon taxes provide a strong financial incentive for industries to transition to low-carbon fuels such as green NH₃ [119]. The Carbon Border Adjustment Mechanism (CBAM), which entered its transitional phase in 2023, aims to tax carbon-intensive imports, including NH₃. This regulatory pressure increases the cost of conventional fossil fuel-derived (“gray”) NH₃. While the cost of green NH₃ in Northwest Europe is projected to be higher (over USD 850 per ton by 2025), the economic viability is secured by the cost of carbon emissions. For instance, with an ETS price of EUR 80 per ton of CO₂, the cost of conventional gray NH₃ rises significantly, narrowing the price gap and making green alternatives more competitive [118]. The market of regions with robust industrial infrastructure, such as the USA and East Asia, has the industrial expertise and infrastructure to promptly increase the capacity of NH₃ generation and utilization. The U.S. Inflation Reduction Act (IRA) offers substantial tax credits, such as the 45V Clean Hydrogen Production Tax Credit, which can lower the cost of green hydrogen (the primary component of green NH₃) by up to USD 3 per kilogram [120]. This substantially enhances the domestic competitiveness of green NH₃. The commercialization strategy follows a “hub-and-spoke” model, with green NH₃ produced at centralized hubs and distributed to surrounding industrial clusters. In these regions, cost reductions are expected to result from advancements in technology, including optimizing electrolyzer efficiency, and by leveraging existing distribution networks. Government support, in the form of tax breaks and infrastructure funding, can significantly speed up the transition.

10. Public Policy Perspective and Practical Implications

From a public policy perspective, the use of NH₃ as a transportation fuel offers both substantial opportunities and intricate challenges. The main catalyst for its adoption is its potential as a carbon-free fuel. Policymakers are progressively concentrating on decarbonizing the transportation sector, particularly heavy-duty applications like shipping, which are challenging to electrify [121]. NH₃, which can be produced from renewable electricity (“green NH₃”), provides a pathway to achieve climate goals and lower greenhouse gas emissions. Policies could promote this transition by offering incentives for the advancement of green NH₃ production, tax credits for companies adopting NH₃-fueled ships or trucks, and the creation of “green corridors” in maritime trade, where exclusively zero-carbon fuels are permitted [122]. Furthermore, policies are necessary to standardize safety regulations for the handling and storage and bunkering of liquid NH₃, as it is toxic and corrosive.

The real-world application of NH₃ as a transport fuel necessitates substantial infrastructure development and technological advancements. A major obstacle is the need for a completely new NH₃ bunkering and refueling network [121]. In contrast to existing fuels, NH₃ demands specialized materials and safety protocols because of its unique properties. This requires a significant investment in port facilities, pipelines, and specialized tanks. From a technological perspective, the engines themselves need to be re-engineered to handle NH₃’s lower combustion speed and higher nitrogen oxide (NOx) emissions, which are a major pollutant. While NH₃ combustion is a mature technology, its use in vehicle engines remains at the demonstration stage. Therefore, the transition is expected to be gradual and incremental, with a focus on high-power, long-haul applications like ocean shipping, where the high energy density of liquid NH₃ offers a significant advantage [123].

11. Future Prospects

The promotion of NH₃ in the transportation sector requires technological advancement, policy incentives, industry collaboration, and demonstration projects.

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Technological innovations: Several research studies will need to be conducted to enhance NH₃ combustion technologies, emission control systems, and fuel cells with the aim of boosting the feasibility of NH₃ as a transportation fuel and minimizing its emission. The main research areas are NH₃ cracking and direct NH₃ fuel cell technologies, including single-step conversion, elimination of cracking infrastructure, and new cracking technologies [124].

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Policy support and impulse: National incentives and policies to foster carbon-neutral fuels and minimize emissions will speed up the adoption of using NH₃ as fuel in the transportation sector.

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Green NH₃ generation: Shifting to green NH₃, generated using renewable energy resources, is essential for the sustainability aspect of NH₃ as fuel. Currently, several research studies are focusing on enhancing the performance and cost-effectiveness of green NH₃ generation methods [125].

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Infrastructure development: There is a need to invest in infrastructure for generation, storage, and distribution of NH₃ to promote its use as fuel in the transportation sector.

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Safety protocols and standards: The public widespread adoption of NH₃ requires the development of sturdy safety protocols and standards.

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Collaboration between different stakeholders: Cooperation and partnership between different stakeholders, including government, research centers, and industry players, will be essential to address the different economic and technical challenges and promote the expansion of NH₃-based transportation solutions.

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Pilot projects: Demonstration projects could supply useful data and perception, leading to cultivated technologies and boosted confidence in NH₃ as a feasible transportation fuel.

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Investment and funding: The development of NH₃-based energy requires a rise in investment and funding in research, pilot projects, infrastructure development, and technological innovations.

NH₃ is positioned to play a pivotal role as a fuel in decarbonizing the global shipping industry. This sector is a key focus due to its strong dependence on fossil fuels and the presence of a mature, globally integrated infrastructure for NH₃ production and transport. The ability to use NH₃ directly in large, two-stroke marine engines with minimal modifications offers a near-term solution for shipowners seeking to comply with upcoming carbon reduction mandates. Additionally, NH₃’s high energy density and potential for production from renewable sources (“green NH₃”) position it as an ideal candidate for long-haul voyages where battery electric alternatives are not feasible. Beyond maritime, other niche, hard-to-abate sectors like long-haul trucking and rail freight could also adopt NH₃, leveraging a distributed refueling network that mirrors the existing diesel infrastructure, though this would require more advanced onboard cracking or direct combustion technologies.

NH₃ demonstrates potential as a carbon-free alternative for aviation and railways, primarily because it serves as an efficient medium for transporting hydrogen and possesses a well-established global supply chain. For aviation, a significant technological challenge is its low combustion efficiency. Researchers are working on onboard catalytic cracking to convert NH₃ into a more combustible hydrogen–NH₃ fuel blend using waste heat from the jet engine. Although this enables the modification of existing engine designs, it simultaneously introduces challenges associated with NOx emissions and the need for larger fuel tanks due to NH₃’s lower volumetric energy density, which could affect an aircraft’s range.

In the railway sector, NH₃ is being investigated as a clean alternative to diesel for long-haul locomotives, where battery and hydrogen solutions face certain constraints. The main technological developments involve dual-fuel engine systems and NH₃ crackers to improve combustion. A critical challenge is managing NH₃ slip (unburned fuel) and controlling emissions of NOx. The use of solid oxide fuel cells (SOFCs) to convert NH₃ directly into electricity is another promising, although less commercially mature, avenue. Like aviation, the railway industry will be required to modify its infrastructure to ensure safe handling and storage, as NH₃ is toxic and corrosive.

12. Conclusions

The push toward the decarbonization of the global transportation sector, a leading contributor to greenhouse gas emissions, requires the pursuit of varied, eco-friendly fuel alternatives. This paper has thoroughly evaluated the significant promise of NH₃ as a versatile fuel, able to sharply cut the carbon footprint of shipping, heavy-duty road transport, and potentially aviation. Its high H₂ content, combined with straightforward liquefaction at around –33 °C and an extensive global infrastructure for storage and transport, establishes it as a frontrunner among future energy carriers.

The analysis underscores that the ongoing transition to green NH₃ is crucial for attaining true decarbonization. This delivers an almost zero lifecycle carbon footprint, differentiating it from traditional, fossil fuel-derived NH₃. In the transportation sector, NH₃ could be used directly in modified internal combustion engines and gas turbines and as fuel technologies. While direct NH₃ fuel cells (DAFCs) provide an optimal solution by eliminating the requirement for H₂ cracking, the use of NH₃ as an H₂ carrier for conventional H₂ fuel cells (via onboard cracking) also continues to be a viable choice, capitalizing on the current maturity of fuel cell technology.

The transition to widespread alternative NH₃ adoption is impeded by several significant challenges. In technical terms, these comprise enhancing the efficiency and robustness of NH₃ cracking technologies, improving the efficiency and lifespan of direct NH₃ fuel cells, and enhancing engine configurations to deliver superior efficiency with reduced nitrogen oxide (NOx) emissions. Regarding safety, NH₃’s toxicity and flammability range require strict handling procedures and highly sophisticated leak detection solutions, along with thorough crew training, especially in the maritime sector. The environmental analysis highlights the essential requirement for effective NOx abatement during combustion, guaranteeing that the transition from carbon emissions does not unintentionally cause new air quality issues. Additionally, the economic analysis indicates that although the long-term cost competitiveness of green NH₃ looks favorable, substantial initial investments are necessary for production facilities, bunkering infrastructure, and vehicle/vessel modifications. The present cost differential of green NH₃ relative to conventional fuels or even alternative fuels still poses a challenge to immediate large-scale uptake.

To summarize, NH₃ represents a very promising—though complex—approach to decarbonizing the transportation sector. Its distinctive attributes as a carbon-free, easily transportable, and high-energy-density fuel position it as a compelling option. However, realizing its full potential depends on sustained and significant research and development efforts to address existing technical challenges, targeted infrastructure investments aimed at strengthening supply chain resilience, and the establishment of comprehensive safety regulations and training programs. A coordinated international initiative spanning industry, academia, and policymaking will be critical in addressing these complexities and guaranteeing that NH₃ plays a critical role in shaping a cleaner, more sustainable future for global mobility.