Advances in Layered Double Hydroxide (LDH)-Based Materials for Electrocatalytic Nitrogen Reduction to Ammonia: A Comprehensive Review

Abstract

Nitrogen (N₂), constituting the majority of Earth’s atmosphere, remains indispensable for biological systems and underpins modern agriculture and industry. Traditionally, the Haber–Bosch process has been essential for synthesizing ammonia (NH₃) from N₂ under high temperature and pressure, but it contributes significantly to global CO₂ emissions. Recently, carbon-free electrocatalytic nitrogen reduction (e-NRR) has emerged as a promising, eco-friendly, and cost-effective approach for green NH₃ production under mild conditions using renewable energy, offering a sustainable alternative to the fossil fuel dependent Haber–Bosch process. This work explores NRR by contrasting the limitations of Haber–Bosch with the advantages of electrocatalysis. Despite progress, electrochemical N₂ reduction to NH₃ production remains challenging due to low activity, poor selectivity, stability, efficiency, and detection issues. Developing efficient e-NRR electrocatalysts is crucial to enhance activity, suppress hydrogen evolution reaction (HER), boost NH₃ yield, and improve Faradaic efficiency. This review highlights the role of layered double hydroxide (LDH) catalysts in e-NRR, summarizing the fundamental process, reaction pathways, and synthesis strategies. Ammonia detection methods, key metrics, and potential contamination issues are compared to inform standard NRR measurement protocols. Lastly, we summarize key findings to synthesize and improve LDH electrocatalysts for NH₃ production and a sustainable, carbon-free N₂ economy.

1. Introduction

The 21st-century global energy crisis and mounting environmental concerns have catalyzed worldwide efforts to advance in energy storage and conversion technologies [1]. The rapid depletion of fossil fuels and the urgent need to combat climate change are compelling scientists and engineers to develop green energy carriers from renewable sources like wind, solar, hydro, and geothermal power [2,3,4,5,6]. Consequently, advanced energy conversion and, crucially storage systems will be indispensable, serving as the essential pillar for ensuring this energy is available where and when it is needed. A significant number of electrochemical processes that convert feedstocks CO₂, N₂, O₂, and H₂O into fuels and chemicals using renewable electricity have now reached high Technology Readiness Levels (TRLs), positioning them close to broad industrial application. The reduction of atmospheric nitrogen (N₂) into useful compounds is a cornerstone of modern chemistry that now represents a critical pathway to decarbonizing energy and agriculture [7,8].

The nitrogen, which is inert, colourless, and odourless, makes up around 78% of the earth’s atmosphere by volume. It is a fundamental building block of life, forming the backbone of amino acids, proteins, and nucleic acids [9]. It exhibits various valance states ranging from −3 to +5, accounting from its existence in diverse forms such as N₂ gas, NO and NO₂, NO₃⁻ and NO₂⁻, N₂H₄, and ammonium compounds. N₂ triple bond (N≡N) is the strongest covalent bond, with a strength of 945 kJ/mol at 298 K, which makes it chemically unreactive under normal conditions, yet this same stability presents challenges in converting it into biologically and industrially useful compounds [10,11,12]. Due to the inert nature of N₂, its reduction to NH₃ is crucial for fertilizer production and numerous chemical processes in industry. This can be achieved through the traditional Haber–Bosch process [13].

Ammonia is a carbon-free energy carrier (17.6% hydrogen content), with a large volumetric hydrogen energy density (10.7 kg H₂/100 L), which can be liquefied for handling, stored, and transported easily [14,15]. Ammonia plays a critical role in various industrial sectors, being indispensable for the production of fertilizers, pharmaceuticals, and a wide range of chemicals, and also in agriculture, it serves as the primary source of N₂ and for N₂-based fertilizers such as urea, ammonium nitrate, and ammonium sulphate, which are essential for enhancing soil fertility and increasing crop yields [16,17]. Ammonia production is projected to increase further in the coming years, particularly if NH₃ solidifies its role as a key energy carrier and a sustainable H₂ source [18,19]. This reflects the industrial outlook, where growing demand and expanding capacity are driven by NH₃’s emerging applications in energy conversion and H₂ markets. In the Haber–Bosch process, with high temperature and pressure, N₂ reacts with H₂ in the presence of a catalyst [20].

The development in the traditional Haber–Bosch process has immense benefits worldwide, with a million tons of NH₃ production [21]. German chemist, Fritz Haber in 1908 developed a method to synthesize NH₃ from H₂ and N₂ under high pressure and temperature. A large-scale industrial process was later developed by Carl Bosch, leading to the first NH₃ synthesis plant in 1931, for which they received the Nobel Prize in Chemistry [22,23]. NH₃ is synthesized by the Haber–Bosch process using fossil fuels as an energy source, in a direct reaction between N₂ and H₂. However, 2% of the global energy consumption and 1% of worldwide CO₂ emissions [24,25] have driven initiatives to seek out alternatives such as photocatalytic [26], electrocatalytic [27], and biological strategies [28] by N₂ reduction.

Biological N₂ fixation, catalyzed by nitrogenase enzymes, produces NH₃ under ambient conditions. This process involves multiple pathways for proton and electron transfer and is driven by the energy supplied by adenosine triphosphate. The efficiency of this natural reaction has inspired the recent development of electrocatalytic strategies for ambient NH₃ production [29]. Electrocatalytic nitrogen reduction reaction is one of the most promising ways which is powered by electricity, producing desired products at fixed potentials, owing to its environmental sustainability and economic viability, typically comparing it with the Haber–Bosch method [30,31]. The attractiveness of this strategy lies on the potential not only to decarbonize the NH₃ synthesis but also to disrupt the industrial model itself, enabling a decentralized production that circumvents large infrastructure investments. The electrocatalytic reduction of N₂ to NH₃ occurs at the cathode surface in aqueous electrolyte, where the hydrogen evolution reaction (HER) shows severe competition against NRR due to its theoretical potential less negative than that of e-NRR [12,32]. Considerable improvements in terms of catalytic activity, selectivity, low current density, high yield rate, and high faradic efficiency (FE) are necessary, which could be achieved by the desired catalyst [33].

The major challenges in advancing the e-NRR is catalyst development. Overcoming these hurdles is critical for industrial application and innovations in the fields of electrocatalysis and energy storage. A wide array of electrocatalysts for ambient NRR, including noble metals, non-noble metals, transition metal-based, carbon-based, alloys, etc., are researched [34,35,36,37]. Among these electrocatalysts, transition metal-based layered double hydroxides (TM-LDHs) have emerged as one of the most promising classes of materials for the NRR. Advancing catalytic potential from a versatile set of features, including highly tunable structural compositions, scalable synthesis roots, high anion exchange capacity, enhance redox flexibility, tunable physiochemical properties, and remarkable electrocatalytic performance [38,39].

This review gives a clear summary and critical evaluation of the recent progress in LDH-based e-NRR, encompassing mechanistic insights and design strategies, while emphasizing their potential to advance sustainable NH₃ production and contribute to the broader goals of energy transition and climate resilience. Figure 1 shows the conversion of N₂ to NH₃ on the surface of LDH catalyst by utilizing renewable energy sources. It begins with the fundamental aspects of e-NRR, highlighting the underlying processes, reaction mechanisms, and possible pathways. We have focused on electrocatalyst design and development strategies for NRR, with the main attention on the role of LDH catalysts. Furthermore, this review compiles current methods for electrochemically reduced N₂ to produce NH₃, and its detection as well as their corresponding activities are summarized to evaluate catalytic performance. The final section offers conclusions and perspectives on strategies to enhance electrocatalytic performance, to inspire new approaches, and expand the research landscape in this rapidly evolving field.

2. Fundamental Process and Mechanistic Understanding of NRR

2.1. Natural Occurring Pathways

The reduction of atmospheric N₂ to a usable compound is known as N₂ reduction, which naturally happens in plants and other microorganisms. Specifically, this process is catalyzed by the enzyme nitrogenase. Microbial organisms that possess nitrogenase convert N₂ to NH₃ in a process that is made possible by the presence of the iron, molybdenum, and sulphur cluster (FeMoS) used as a primary N₂ binding site. This process is represented by the following reaction Equation (1) [40].

N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi

(1)

where 8 electrons, 8 protons, and 16 equivalents of adenosine triphosphate are needed for the reaction. This procedure shows that converting one N₂ to two NH₃ is quite challenging [25].

2.2. Traditional Pathway: Haber–Bosch Process

The traditional pathway Haber–Bosch process, which includes reacting highly pure N₂ with hydrogen (H₂) at pressures (200–300 atm) and high temperatures (~500 °C) over iron- or ruthenium-based catalysts, remains a primary source for the production of industrial NH₃. This process requires direct reaction between N₂ and H₂ via a dissociative mechanism [41], given below in Equation (2)

N₂ + 3H₂ → 2NH₃

(2)

Besides relying on energy from fossil fuels to drive the reaction, the Haber–Bosch process also uses hydrogen obtained through steam methane reforming, Equation (3), or coal gasification, Equation (4), resulting in a large contribution to human-induced CO₂ emissions [42].

CH₄ + 2H₂O → 4H₂ + CO₂

(3)

CH₄ + 2H₂O →2H₂ + CO₂

(4)

CO₂ emissions intensify the greenhouse effect and creates significant environmental issues, emphasizing the crucial need for environmentally friendly and economically feasible alternatives to traditional NH₃ production [43]. These challenges have spurred significant research into sustainable and scalable approaches for NH₃ synthesis under ambient conditions, with growing attention on electrocatalysts as promising alternatives or complements to the traditional Haber–Bosch process [44].

2.3. Emerging Pathway: Electrocatalytic Nitrogen Reduction Reaction (e-NRR)

Electrocatalytic nitrogen reduction reaction on the catalysts surface is adopting as a unified sustainable approach at ambient temperature and pressure, converting green power produced by renewable sources such as solar or wind energy by reducing N₂ and water H₂O into NH₃ with net zero CO₂ emission [45]. The reaction on the catalyst surface is divided into two half-reactions, each occurring at a different electrode. These electrodes are classified based on their reactions: oxidation, which occurs at the anode (O₂), while reduction takes place at the cathode (NH₃) [8]. The two most common cathodic (acidic) and anodic (basic) conditions are:

Anode (acidic conditions): 3H₂O → 3/2O₂ + 6H⁺ + 6e⁻

Cathode (acidic conditions): N₂ + 6H⁺ + 6e⁻ → 2NH₃

Anode (basic conditions): 6OH⁻ → 3H₂O + 3/2O₂ + 6e⁻

Cathode (basic conditions): N₂ + 6H₂O + 6e⁻ → 2NH₃ + 6OH⁻

Overall reaction:

N₂ + 3H₂O → 2NH₃ + 3/2O₂

(5)

Electrocatalytic nitrogen reduction reaction is a process where the initial dissociation of N₂ molecules takes place on the reactive catalysts surface, followed by a stepwise proton–electron transfer onto the adsorbed N₂ species. These sequential N₂ hydrogenation steps directly produce NH₃, which then desorbs from the catalyst surface. These potentials significantly influence the overall reaction kinetics and feasibility. Up to now, the reduction of N₂ to NH₃ on the catalyst is generally achieved through the N₂ reduction mechanism (Figure 2), which includes dissociative and associative N₂ reduction pathways. The associative pathway involves distal and alternating pathways [12,46,47].

2.3.1. Dissociative Pathways

The dissociative mechanism is one of the primary pathways for the e-NRR. In the dissociative mechanism, the e-NRR pathway is where the strong triple bond (N≡N) in the N₂ molecule is completely cleaved and broken before any hydrogenation (proton/electron transfer) steps occur. (Figure 2a) This results in two N₂ atoms being chemisorbed on the catalyst surface, which are then independently hydrogenates, and three consecutive hydrogenation processes are required to produce NH₃ from N* via *NH and *NH₂. Finally, NH₃ is desorbed from the catalyst surface to form two molecules of NH₃. (an asterisk, *, denotes a surface site).

However, the N₂ molecule has a very strong triple bond (941 kJ mol⁻¹), so breaking it directly is difficult, and the dissociative pathway is not suitable for electrocatalytic NRR.

N₂ + 2* → 2*N

(6)

2*N₂ + 2e⁻ + 2H⁺ → 2*NH

(7)

2*NH + 2e⁻ + 2H⁺ → 2*NH₂

(8)

2*NH₂ + 2e⁻ + 2H⁺ → 2*NH₃ + 2*

(9)

2.3.2. Associative Pathways

Under the mild conditions in aqueous electrochemistry, the NRR most commonly proceeds via an associative mechanism, where N₂ is hydrogenated before its bond is fully broken. In ambient circumstances, particularly in aqueous electrochemical environments, the associative pathway is typically regarded as the most prevalent mechanism for direct e-NRR. Particularly, dissolved N₂ molecules get adsorbed onto the electrode surface without immediately breaking the N≡N bond and go through several continuous hydrogenation steps primarily from H₂O molecules, generating intermediates such as *N₂H, *N₂H₂, and eventually NH₃. Then the NH₃ molecule desorbs simultaneously with the N≡N bond cleavage, enabling effective NH₃ production with less energy consumption than the dissociative method. This associative mechanism can be further classified into two distinct pathways: (i) distal pathway and (ii) alternative pathways.

Associative distal pathway

In the associative distal pathway, hydrogenation preferentially occurs at the N₂ atom furthest on the catalyst surface, leading to the formation and release of the first NH₃ molecule, followed by sequential hydrogenation step of the proximal N₂ which produces the second NH₃ molecule (Figure 2b).

*N₂ + e⁻ + H⁺ → *NNH

(10)

*NNH + e⁻ + H⁺ → *NNH₂

(11)

*NNH₂ + e⁻ + H⁺ → *N + NH₃

(12)

*N + e⁻ + H⁺ → *NH

(13)

*NH + e⁻ + H⁺ → *NH₂

(14)

*NH₂ + e⁻ + H⁺ → NH₃ + *

(15)

Associative alternative pathway

In the alternative pathway, hydrogenation occurs alternately between the two N₂ atoms of the adsorbed N₂ molecule, with the N≡N bond remaining intact until the final steps, leading to the near-simultaneous release of two NH₃ molecules (Figure 2c).

N₂ + * → *N₂

(16)

*N₂ + e⁻ + H⁺ → *NNH

(17)

*NNH + e⁻ + H⁺ → *NHNH

(18)

*NHNH + e⁻ + H⁺ → *NHNH₂

(19)

*NHNH₂ + e⁻ + H⁺ → *NH₂NH₂

(20)

*NH₂NH₂ + e⁻ + H⁺ → *NH₂ + NH₃

(21)

*NH₂ + e⁻ + H⁺ → NH₃ + *

(22)

3. Electrocatalyst Development and Design: N₂ Reduction to NH₃ Production

In the electrochemical NRR process, electrocatalysts plays a decisive role in determining selectivity, yield, as well as the Faradic efficiency. Thus, catalyst designs, particularly those which have abundant active sites suppressing HER and enhancing NRR activity, together with system optimization, are crucial for overcoming existing bottlenecks, and numerous types of catalysts have been developed in the past decade to advance electrochemical NH₃ synthesis [48].

In this section, we will be discussing the design of electrocatalysts toward NRR for achieving low overpotential and high FE, as well as high NH₃ yield rate (yNH₃).

3.1. Layered Double Hydroxide (LDH)

The majority of LDHs studied and applied today adopt the structure of hydrotalcite [Mg₆Al₂(OH)₁₆]CO₃·4H₂O, which was discovered in 1842 and first synthesized by Feitknecht in 1942, but the detailed structural characteristics of LDHs were described by Allmann, Taylor, and their coworkers in the late 1960s [48,49,50]. Layered double hydroxides are a class of anionic clays that are rare naturally occurring host–guest materials, especially when compared to cationic clays. Uniquely structured and positively charged metal hydroxide layers (with two metals as cations bridged by hydroxide groups) and charge-balancing interlayer anions result in the distinctive and versatile properties of LDH. The versatile characteristics of LDH materials, their simple and cost-effective synthesis, high surface area, and structural memory effect led to their adoption across their numerous applications. These include their primary use as catalysts and adsorbents in wastewater treatment, as well as roles in energy storage and conversion, biocatalyst, and photocatalysis [51,52,53,54,55].

3.2. Structure of LDH

The general formula is [M²⁺_1−x M³⁺_x (OH)₂]^x+ [Aⁿ⁻ _x/n]^x− .mH₂O], where M²⁺ and M³⁺ are divalent and trivalent metal ions (M²⁺ = Mg²⁺, Ni²⁺, Co²⁺, Zn²⁺, Fe ²⁺ Cu²⁺ etc. and M ³⁺ = Co³⁺, Al³⁺, Fe³⁺, etc., respectively. Aⁿ⁻ represents negatively charged interlayer guest anions (e.g., CO₃²⁻, NO³⁻, Cl⁻, SO₄²⁻), with a molar ratio of M³⁺ to (M²⁺ + M³⁺) denoted by (x), where x = 0.2–0.33 [56]. The structure of LDH is derived from brucite-like Mg (OH)₂ layers; these layers consist of infinite sheets of edge-sharing M(OH)₆ octahedra, where metal cations reside at the centre of each octahedron, surrounded by (OH)₆ ions at the corners (Figure 3). The host layer possesses a positive charge due to the incorporation of divalent and trivalent metal cations. This charge is balanced by anions, alongside water molecules, located in the interlayer region [57,58]. The basal spacing (d) in LDH materials is the distance from the centre of one host layer to the centre of the adjacent layer. It is therefore the sum of the brucite-like layer thickness and the interlayer space [59,60].

Layered double hydroxide exhibits 2D lamellar structure composed of stacked nanosheets, which provides the following advantages: (i) Metal cations in the host layers are highly tunable, as the M²⁺/M³⁺ composition can be readily adjusted during synthesis, allowing precise control over the material’s chemical and physical properties. (ii) The anions and spacing between layers in LDH can be changed through an anion exchange reaction, allowing LDH to be customized for specific adsorption, catalytic, or delivery functions. (iii) Layered double hydroxide exfoliation into ultrathin nanosheets by increasing interlayer spacing, often achieved with the suitable guest molecules or physical force, result in a much higher surface area and enhanced reactivity or accessibility for applications. Therefore, these aspects can efficiently regulate the physical and chemical properties of LDH [61]. Such unique lamellar structure of LDH allows precise tuning over chemical composition and interfacial electronic structure, which offer a versatile platform for optimizing electrochemically reduced N₂ activity. In the bimetallic LDH electrocatalysts, one transition metal favours N₂ adsorption, weakening the N≡N bond, while the other enhances NH₃ desorption in accordance with the “volcano curve” [62].

Layered double hydroxide catalysts are also known as competent catalysts that can mitigate the kinetic problems for N₂ molecule activation and dissociation due to the accessibility of d-orbital electrons for the π-back donation process, where it is clear that the empty d-orbitals of transition metals are capable of withdrawing electrons from nitrogen molecules [63,64]. Defect engineering via heteroatom doping can modify the intrinsic electronic structures of LDHs by incorporating P⁵⁺, which has a smaller ionic radius, into the lattice of LDH, thereby regulating its charge distribution and promoting the acceptor–donor interplay activities [65]. Based on Lewis acid–base theory, N₂ behaves as a soft base that preferentially reacts with soft acid. Experimental and theoretical evidence demonstrates that low valence 3d TMs could serve as Lewis acid sites, which is favourable for the adsorption and activation of N₂, promoting NRR performance [66]. The above-mentioned mechanistic insights of LDH structure, electronic configuration, and defect chemistry collectively provide a clear understanding of how N₂ adsorption and activation are influenced. For the activation of N₂, lots of research, including electrocatalyst design, electrolyte with different pH and tailored size, dopants, facet, nanostructure, etc., have focused on enhancing the rate of the NRR process while suppressing the competitive HER.

3.3. Properties of LDH

Layered double hydroxides have become a preferred catalyst in scientific research as they have excellent properties of surface area, tunability, memory effects, acidity, and alkalinity and thermal stability. When LDH is subjected to calcination, they are transformed into mixed metal oxides that exhibit attributes such as high surface area and significant surface basicity. At elevated temperatures, these metal oxides form a homogeneous mixture with uniformly distributed metal components, enhancing their catalytic and material properties [67]. The tunability of LDH allows for precise control of their crystallite size and also its distribution. By altering factors like nucleation, temperature, concentration, acidity, and supersaturation during synthesis, researchers can alter the nucleation sites and crystal growth rates to synthesis LDH with specific morphological properties [68]. The structural memory effect of LDH is their ability to be calcinated at 450–500 °C to turn into mixed metal oxides (MMOs or LDOs), while those oxides lose the original layered structure, they gain enhanced catalytic and adsorption capabilities [69]. The acidity as well as the alkalinity in LDHs arise from their unique composition and structure, which include metal hydroxide layers formed by divalent and trivalent metal cations and hydroxide (OH⁻) groups. The OH⁻ provides intrinsic basic sites, while the metal cations, particularly trivalent metals like Al³⁺ and Fe³⁺, act as Lewis acid sites. This combination creates surfaces possessing both acidic and basic active sites [70]. The composition of LDHs is a critical determinant of their thermal stability, governing a multi-stage decomposition process upon heating. Initially, below 200 °C, the structure remains intact as only interlayer water is removed. Upon further heating between 200–450 °C, the decomposition of hydroxyl groups and carbonate anions occurs, which slightly enhances the material’s surface area and microporosity. Finally, at temperatures of 450–550 °C, the layered structure undergoes a complete collapse, forming MMOs, LDOs characterized by significantly higher surface area, stability, and surface alkalinity. Exceeding 600 °C induces the irreversible transformation of LDOs into spinel structures, which exhibit diminished surface area, reduced alkalinity, and a complete loss in the originality of layered structure [71,72,73]. The high electrochemical activity of LDH originates from abundant surface oxygen vacancies and unsaturated metal cations, which serve as active sites. The electrocatalytic reduction of N₂ to NH₃ can be successfully achieved using LDH, as shown by recent studies.

3.4. Preparation Methods

Research efforts have been put for synthesizing LDH materials, which are mainly in various wet chemical strategies with tailored properties, such as co-precipitation, hydrothermal method, sol–gel method, electrodeposition, and anion exchange method. The main factors to consider during the synthesis of LDH include the selection of metal cation species (M²⁺ and M³⁺), alkaline source, cation ratios and reaction temperature, pH control, synthesis method, and concentration of precursors. The synthesis of nanosized LDH is generally classified as “bottom-up” and “top-down”. The “top-down” method is the extensively researched in synthesis route; however, its application is notoriously difficult for LDH because their exceptionally high charge density presents a greater challenge than with other lamellar materials [60]. An attractive alternative is the bottom-up approach, where 2D networks are constructed by initiating chemical reactions between selected molecular precursors. Its key advantages for producing 2D LDHs are its scalability and the ease of the preparation method [74]. Although LDH materials hold great promise for electrochemical energy storage and conversion, realizing their fascinating properties and broad applications requires overcoming the challenge of efficient and cost-effective electrode modification.

3.4.1. Co-Precipitation Method

The most common method for preparing LDH is co-precipitation, a simple and inexpensive “one-pot” synthesis that can produce powders with diverse cationic and anionic compositions [75]. The co-precipitation method offers two primary advantages: First, it enables the direct synthesis of nanoscale LDH powders with a homogeneous chemical composition through reactions in an aqueous solution. Second, it facilitates the preparation of these nanomaterials with a small and highly uniform particle size distribution. Sadavar et al. [76] synthesized cobalt-chromium-LDH hybridized with a graphene oxide, which led to anchored Co-Cr-LDH-GO (CCG) self-assembled high surface area, high electrical conductivity, high charge transfer kinetics, and mesoporous morphology. Following Reichle protocols, the nitrates intercalated Co-Cr-LDH was prepared by the co-precipitation method at ambient temperature. To overcome LDH contamination in the co-precipitation method, the synthesis was carried under N₂ atmosphere with decarbonated water at ambient temperature. Seftel et al. [77] used a simple co-precipitation method to obtain Zn-Al layered double hydroxides primarily due to its ability to produce homogeneous precursors. Zn-Al-LDH was varied from 1 to 4 (cationic ratios) using the co-precipitation method at constant pH of 7.5. This precise pH control was found crucial in the formation of the layered structure. Co-precipitation mechanism mainly depends on the typical characteristics of two metal hydrates and the two main metal hydrates, which easily formed brucite-like layers under the action of metallic cations and interlamellar anions in the alkaline environment. Pawar et al. [78] and co-workers synthesized cobalt–iron layered double hydroxide nanosheets (Co-Fe-LDHNSs) by a co-precipitation method, and then nanohybrids are prepared by the exfoliation and reassembly of positively charged 2D Co-Fe-LDH NSs and 1D negatively charged CNTs, resulting in self-assembled high surface area, mesoporous morphology, high electrical conductivity, and fast charge transfer kinetics.

In brief, the co-precipitation method involves the direct reaction of dissolved metal ions with a precipitating agent, where pH plays an important role in forming LDH. While this process is simple and offers easily controllable conditions, its main drawback is the tendency for uneven precipitation and particle agglomeration, which results from localized concentration gradients of the precipitating agent during its addition.

3.4.2. Hydrothermal Method

The hydrothermal method, originally developed by geologists to simulate natural mineralization processes, is now a fundamental technique for synthesizing crystalline functional materials, including LDH [79]. Environmentally friendly and low-temperature hydrothermal synthesis method is highly effective for engineering nanostructures through controlled crystallization under autogenous pressure [80]. This method involves reacting metal salt solutions (containing M²⁺ and M³⁺ cations) with a base in a sealed autoclave, where reactions proceed under elevated temperature and pressure. These conditions promote crystallization and minimize reactant volatilization, and finally the desired products are received. Thereafter, the products are centrifuged, washed, and dried. The hydrothermal method typically offers a lower yield than co-precipitation, but it provides superior control over the resulting LDH properties, which includes enhanced crystallinity, a well-defined microscopic morphology, and improved particle dispersion.

Kurnosenko et al. [81] successfully prepared ZnCr-LDH and NiCr-LDH by hydrothermal method, and they varied the hydrothermal synthesis conditions and the obtained desired product with a controllable crystallite size and a specific surface area. Preferably urea or hexamethylenetetramine (HMT) is used, whose slow thermal hydrolysis gradually elevates the pH in the reaction medium, thereby generating the desired conditions which are favourable for LDH crystal growth. Jeon et al. [82] reported the growth mechanism of NiAl-based layered nanostructures on rigid substrates by low temperature hydrothermal method, which featured LDH with wide variety of characteristics and can be producible by combinations of metal cations and anions. The growth of LDH was studied as a function of both source concentration and growth time to determine their respective effects on growth yield and kinetics.

The concentration of metal salts in hydrothermal synthesis can be adjusted to control the interlayer cations and structural properties of LDH. This approach is especially suitable for the preparation for LDH compounds with high uniformity, purity, and crystallinity. However, a crucial compromise is the difficult control of the two different processes of nucleation and growth during the formation of LDHs, which is caused by the one-pot process and the lack of a profound theoretical guideline. The huge advantage of this method over the co-precipitation method is that it effectively reduces the unwanted waste, which might be harmful for the environment [83,84]. The hydrothermal method has also been shown to be effective in the insertion of organic guest species into the LDH interlayers, as the input anions and only hydroxides with very low affinity to LDH is present as competing anions. Moreover, the final sizes and morphologies of the particles can also be accurately adjusted in this method. In some cases, it is used to improve the crystallinity of LDH to obtain better control of some properties.

3.4.3. Electrodeposition Method

Electrodeposition, often referred to as the electrochemical method, is gaining attention as a practical way to synthesize semiconductor thin films and nanostructures, particularly LDH. While this technique has long been used in the industry for coating metals and alloys ranging from protective zinc layers to the production of electronic parts, its application to semiconducting materials is still fairly new and continues to develop. Electrodeposition proceeds through applying a cathodic potential in an aqueous electrolyte containing divalent and trivalent metal ions (e.g., Ni²⁺, Co²⁺, Al³⁺, Fe³⁺) where the cathodic reduction of nitrates or other anions generates hydroxide ions near the electrode surface, inducing the precipitation of LDH layers. A reaction proceeds if the applied potential falls below the redox potential for cathodic reactions or exceeds it for anodic reactions [85]. Yarger et al. [86] proposed electrodeposition conditions to obtain Zn-Al LDH films and used nitrate precursors, which contain Zn²⁺ and Al³⁺ ions, and is achieved by reducing the nitrate ions for the generation of hydroxide ions on the surface of the working electrode. The influence of deposition potential, solution pH, and Zn²⁺/Al³⁺ ratio on the crystallographic purity and crystallinity of electrodeposited Zn-Al LDH films was investigated. The pH conditions in the plating solution plays a critical role in electrodeposition, as increasing pH destabilizes the formation of the LDH phase while decreasing pH promoted deposition of other impurities. Han et al. [87] synthesized NiCo-LDH nanosheet networks on an activated carbon cloth with high mass loading by electrodeposition method by varying Ni/Co nitrate and acetate as reactants, which improved cyclic stability of the electrode material, which is denoted as NiCo(NA)-LDH. Desired nano- and microscale morphologies of LDH thin films (porosity, surface texture, domain size, packing orientation) enhance the desired reactivities and stabilities in LDH films.

Electrodeposition method is a quick, simple, and environmentally friendly method for synthesizing LDH. The thickness of LDH and performance can be modulated by controlling the concentration of the metal salt solution and also by varying the deposition time. In this process, the pH does not have to be raised with alkalis, avoiding the formation of extra hydroxide particles and yielding purer LDH [88].

3.4.4. Anion Exchange

The anion exchange method exploits the ability of LDH to replace their interlayer anions. This technique is particularly valuable when co-precipitation is unsuitable, such as when metal cations or target anions are unstable in alkaline conditions, when cations and guest anions react preferentially, or when soluble salts of the desired anion are unavailable. The exchange process is primarily governed by electrostatic interactions between the host layers and the incoming anions [89]. The anion exchange performance of LDH materials include their selectivity, capacity, and anion arrangement, which is governed by factors such as intercalation driving force, interlayer environment, and morphology. These characteristics can be tuned by varying the metal cations species, valence, size, and molar ratio, which directly adjust the host structure’s charge density and interlayer spacing. The modification process for anion exchange method of LDH materials is based on the tunability of interlayer anions. LDH exhibits exceptionally high anion exchange efficiencies for instance, with values up to 98% reported for the exchange of chloride with carbonate [90]. Crepaldi et al. [91] reported a new and easy way for the modification of LDHs, where LDH (CO₃) was converted to the LDH (NO₃) form, and subsequently NO₃ ions were exchanged with different surfactants in one pot and in short reaction times. Anion exchange directly modulates the interlayer spacing of LDH, a process dictated by the physicochemical properties of the guest anion. The basal spacing increases with the anion’s size and complexity but is also influenced by its charge and binding affinity to the host layers. Strong electrostatic interactions and hydrogen bonding can restrict layer expansion. Consequently, spacing follows the sequence: OH⁻ < CO₃²⁻ < Cl⁻ < Br⁻ < I⁻ < NO₃⁻< organic anions. The inherent flexibility of the LDH structure to accommodate this variety of anions is a fundamental property exploited to engineer materials for targeted functions [92]. These benefits make anion exchange a powerful tool for customizing LDH to meet the needs of diverse fields such as environmental remediation, catalysis, and biomedical engineering.

3.5. Layered Double Hydroxide-Based e-NRR Catalyst

Layered double hydroxides have emerged as promising non-noble metal electrocatalysts for the NRR, owing to their adjustable composition, large surface area, and distinctive layered architecture. To further boost NH₃ yield and FE, researchers have employed various strategies, including elemental doping, engineering three-dimensional nano-flower morphologies, integrating conductive materials such as carbon nanotubes (CNTs), reduced graphene oxide (rGO), and intercalating diverse anions within the LDH layers. A comparative summary of the electrocatalytic performance of representative LDH-based electrocatalysts is presented in Table 1.

Qiao et al. [62] hydrothermally fabricated a highly conductive CoFe-LDH film grown on a nickel foam substrate, employing both single- and double-chamber cells for electrochemical NRR. The catalyst achieved an NH₃ formation rate of 1.11 × 10⁻⁹ mol s⁻¹ cm⁻², with relatively low FE in a single-chamber cell, whereas a much higher FE of 14.18% was obtained in the double-chamber configuration. The improved selectivity was attributed to suppressed proton/electron supply in a concentrated 25 mol kg⁻¹ KOH electrolyte, as well as the binder-free three-dimensional (3D) nanoplates array that provided abundant N₂ adsorption sites and promoted synergistic interactions between Co and Fe atoms. From (Figure 4a), HR-TEM analysis demonstrated well-resolved lattice fringes with an interplanar spacing of 0.262 nm, indexed to the (012) plane of CoFe-LDH, while the SAED pattern confirmed its high crystallinity. The CoFe-LDH nanoplates grown vertically on the Ni foam substrate are interconnected to form open pores (~50 nm), with individual plate thicknesses of ~30 nm. Furthermore, (Figure 4b) shows EDX mapping, which confirms the homogeneous distribution of Co and Fe throughout the film. This unique 3D nanoarray structure enhanced the number of accessible active sites and facilitated mass transport, thereby significantly boosting the NRR catalytic performance.

Zhang et al. [93] employed a rotating disc electrode to investigate NiFe-based catalysts for electrochemical NRR to NH₃. They compared the activity of oxygen-deficient NiFe-LDH (Vo-NiFe-LDH) with fluorine-modified oxygen-deficient NiFe-LDH (VoF-NiFe-LDH). The Vo-NiFe-LDH exhibited the highest NH₃ yield of 19.44 μg·h⁻¹·mgcat⁻¹ at −0.2 V vs. RHE, which significantly surpassed that of the VoF-NiFe-LDH shown in Figure 4c. At the same potential in 0.1 M KOH electrolyte, the Vo-NiFe-LDH also achieved a remarkable FE of 19.41%, as shown in (Figure 4d). Stability assessments conducted over a continuous 12h operation shown in (Figure 4e) confirmed the durability of the Vo-NiFe-LDH, as only negligible current density loss was observed. Furthermore, density functional theory (DFT) calculations revealed that the electron-deficient sites in Vo-NiFe-LDH lowered the energy barrier of the potential-determining step to 0.76 eV, compared to 2.02 eV for pristine NiFe-LDH, thereby facilitating enhanced NRR activity.

Figure 4. (a) HRTEM images and SAED pattern of a CoFe-LDH nanoplate. (b) SEM and EDX mapping images of CoFe-LDH electrodes [62]. Copyright 2020 Elsevier. (c) A comparison of VoF-NiFe-LDH and Vo-NiFe-LDH catalyst-based yield rates for generating NH₃. (d) A comparison of Faradaic efficiency of VoF-NiFe-LDH and Vo-NiFe-LDH at different overpotentials. (e) Stability of Vo-NiFe-LDH demonstrated by chrono-potentiometry measurement for 12 h [93]. Copyright 2020 Springer Nature.

Figure 4. (a) HRTEM images and SAED pattern of a CoFe-LDH nanoplate. (b) SEM and EDX mapping images of CoFe-LDH electrodes [62]. Copyright 2020 Elsevier. (c) A comparison of VoF-NiFe-LDH and Vo-NiFe-LDH catalyst-based yield rates for generating NH₃. (d) A comparison of Faradaic efficiency of VoF-NiFe-LDH and Vo-NiFe-LDH at different overpotentials. (e) Stability of Vo-NiFe-LDH demonstrated by chrono-potentiometry measurement for 12 h [93]. Copyright 2020 Springer Nature.

Zou et al. [94] developed a NiCo-LDH hollow nanocages of (H-NiCo-NCs), with tunable Ni/Co ratios for the NRR applications. By varying the Ni²⁺ ion content in the hydrothermal synthesis, three distinct catalysts were obtained HNiCo-NC-L with low Co/Ni ratio, HNiCo-NC with intermediate ratio, and HNiCo-NC-H with high Co/Ni ratio. Among these, HNiCo-NC delivered the NH₃ yield 52.8 μg·h⁻¹·mg cat⁻¹ at −0.7 V, which was significantly higher than that of the HNiCo-NC-L (27.8 μg·h⁻¹·mg cat⁻¹) and HNiCo-NC-H (33.3 μg·h⁻¹·mg cat⁻¹). At the same potential, an FE of 11.5% was achieved as shown in Figure 5a. The reaction process was further probed using (in situ) time-dependent Raman spectroscopy in a modified electrolytic cell. At −0.7 V, a characteristic band at 235 cm⁻¹ (assigned to Co–N bonds) appeared after 20 min, confirming efficient N₂ adsorption, while a gradually intensifying band at 1644.4 cm⁻¹, corresponding to the H–NH stretching vibration, indicated NH₃ formation with prolonged reaction time, as shown in Figure 5b. Complementary DFT calculations demonstrated that varying the Ni/Co ratio effectively tuned the electronic structure and identified four potential active sites for NRR. While Co-based sites readily accepted electrons from adsorbed N₂, their strong affinity for *H adsorption hindered NRR selectivity. In contrast, the synergistic Co-Ni/Ni sites provided the most favourable balance between N₂ activation and HER suppression, establishing them as the optimal active centres for NRR. Wu et al. [65] constructed Co-Fe LDH/g-C₃N₄ heterointerface on a TiO₂ nanofibrous membrane (NM) via (in situ) polymerization and epitaxial growth. The TiO₂ NM, prepared by electrospinning and calcination, was coated with a thin g-C₃N₄ layer through melamine sublimation, followed by the epitaxial growth of Co-Fe LDH nanoneedle arrays and subsequent hydrothermal treatment to obtain LDH @g-C₃N₄ @TiO₂. TEM as well as HRTEM confirmed the uniform growth of Co-Fe LDH nanoneedles with strong interfacial bonding and a ~2 nm g-C₃N₄ coating, while lattice fringes of 0.26 nm corresponding to the (012) planes of Co-Fe LDH, verifying the heterostructure formation. Electrocatalytic tests (Figure 5c) of the composite obtained a maximum NH₃ yield of (2.07 × 10⁻⁹ mol·s⁻¹·cm⁻²) at −0.55 V vs. RHE and a peak FE of 25.3% at −0.45 V vs. RHE, both values substantially higher than those of pristine TiO₂ (0.45 × 10⁻⁹ mol·s⁻¹·cm⁻²) and g-C₃N₄ @TiO₂ (0.72 × 10⁻⁹ mol·s⁻¹·cm⁻²), as shown in Figure 5d. These results highlight the strong synergistic interaction among Co–Fe LDH, g-C₃N₄, and TiO₂. Notably, the catalyst, evaluated in 0.1 M Na₂SO₄ with a Nafion 212 separator, also outperformed several other reported noble-metal-free LDH-, g-C₃N₄-, and TiO₂-based systems. Arif et al. [64] decorated hollow hierarchical nanotubes (HHNTs) of CoVP@NiFeV-LDH heterostructures grown in situ on carbon paper via a hydrothermal route, where ternary NiFeV LDH was integrated on the hollow CoVP nanotubes. To assess their catalytic performance, different electrolytes were tested (0.1 M Na₂SO₄, 0.1 M KOH, 1.0 M KOH, and 0.05 M H₂SO₄), with the latter showing the best suppression of HER. UV–vis absorption spectra recorded after 2 h potentiostatic tests in 0.05 M H₂SO₄ revealed the potential-dependent NH₃ yield and FE, as shown in Figure 5e. Both parameters declined at −0.5 V due to the increasing dominance of HER, consistent with the rise in current density. For comparison, the individual components, NiFeV-LDHs and CoVP, were also tested (Figure 5f), yielding maximum FE values of 3.1% and 10.4% at −0.3 V, respectively (Figure 5g). In contrast, in 0.05 M H₂SO₄, the CoVP@NiFeV-LDH HHNTs exhibited superior performance and achieved an average NH₃ yield rate of 1.6 × 10⁻⁶ mol·h⁻¹·cm⁻² and an FE of 13.8% at −0.3 V vs. RHE. Notably, these results surpass many reported state-of-the-art NRR catalysts, including certain noble-metal-based systems. The enhanced activity was attributed to the synergistic effect of the hollow CoVP nanotubes and NiFeV-LDH heterostructures, which provided strong electronic interactions, a high surface area, abundant exposed active sites, and improved electrolyte diffusion, thereby promoting efficient NRR under ambient conditions.

Hua et al. [19] synthesized NiFe–Nb₂C composites by anchoring hydrothermally prepared NiFe-LDH with varying Ni/Fe ratios onto the Nb₂C MXene supported on glassy carbon electrode. The optimized catalyst delivered a high NH₃ yield of 61.16 μg·h⁻¹·cm⁻² with a Faradaic efficiency of 30.01% in 0.1 M KOH. Chronoamperometric measurements in Figure 5h confirmed stable current density over prolonged operation, indicating good durability, while the potential dependent activity plot in Figure 5i showed a volcano-type trend, with the best performance at −0.4 V. DFT calculations further revealed that the NiFe-Nb₂C-2 configuration effectively activated N₂ molecules through strong adsorption, where the synergistic interaction between NiFe-LDH and Nb₂C played a critical role in lowering energy barriers and boosting NRR activity. Liu et al. [95] proposed phosphorus-doped FeNi-LDH nanofibers (P–FeNi@C) as efficient NRR electrocatalysts. The incorporation of both Fe and Ni, known for their intrinsic NRR activity, provides a favourable dual-metal framework, while phosphorus doping further enhances performance by modulating the electronic structure and enriching the active sites for N₂ adsorption and activation. Using P–FeNi@C as the cathode and a graphite rod as the anode, the catalyst achieved an NH₃ production rate of 1.7 × 10⁻¹⁰ mol·s⁻¹·cm⁻² with a Faradaic efficiency of 23% at −0.5 V vs. RHE. Chronoamperometric analysis in Figure 6a demonstrated higher reduction current densities across −0.4 to −0.8 V, while Figure 6,c showed that the maximum NH₃ yield and FE were notably superior to those of undoped Fe-Ni@C nanofibers (0.95 × 10⁻¹⁰ mol·s⁻¹·cm⁻² and 8.4%, respectively), confirming the beneficial role of phosphorus doping in boosting NRR activity. Kong et al. [66] designed a strategy of substituting copper with ferrous iron to tune the electronic structure of layered double hydroxide (LDH), thereby synthesizing of Fe (II)Cu (II)Fe (III)-LDH via a modified solvothermal method for enhanced NRR activity. DFT studies suggested that NRR on Fe (II)Cu (II)Fe (III)-LDH (110) proceeds via a distal pathway, with a potential-determining step (PDS) energy barrier of 1.21 eV significantly lower than the 1.91 eV observed for Cu (II)Fe(III)-LDH (Figure 6d). The Fe (II)Cu (II)Fe(III)-LDH catalyst achieved a notably higher NH₃ yield rate of 33.1 ± 2.5 µg h⁻¹ mg cat⁻¹, with a Faradaic efficiency of 21.7 ± 1.8% at −0.5 V, compared to Cu(II)Fe(III)-LDH, which exhibited only 5.9 ± 0.8 µg h⁻¹ mg cat⁻¹ and 9.9 ± 0.7% FE under the same conditions (Figure 6e,f).

Yang et al. [96] verified the catalytic performance of NiFe LDH-derived selenides for electrocatalytic NRR in different electrolytes. NiFe LDH was synthesized hydrothermally using triethanolamine (TEA) as a chelating agent, followed by conversion to selenide in a tube furnace. Compared with pure NiSe₂ and FeSe₂, the NiFe selenide exhibited significantly enhanced NRR activity and selectivity, marking the first demonstration of a NiFe selenide catalyst synthesized through a facile approach for efficient N₂ fixation. As shown in the LSV curves, the catalyst displayed higher current density in N₂ saturated Li₂SO₄ solution than in Ar-saturated conditions (Figure 6g). The NRR activity was investigated across a potential window of 0 to −0.7 V vs. RHE, achieving an NH₃ yield of 5.64 μg h⁻¹ cm⁻², with a Faradaic efficiency of 12.3% at −0.1 V (Figure 6h). The superior performance in 0.1 M Li₂SO₄ electrolyte was attributed to effective HER suppression, which promoted N₂ adsorption and activation. Moreover, EIS Nyquist analysis in Figure 6i revealed a smaller semicircle diameter for NiFe-selenide compared to other catalysts, indicating higher conductivity due to reduced charge transfer resistance.

Figure 6. (a) Chronoamperometry curves at the corresponding potential for 2 h. (b) NH₃ yields and FE of doped Fe–Ni@C nanofibers at different potentials. (c) Comparison of NH₃ yields and FE of undoped and doped Fe–Ni@C nanofibers at −0.5 V [95]. Copyright 2020 Wiley-VCH. (d) Gibbs free energy diagram for N₂ reduction to NH₃ on Fe (II)Cu (II)Fe (III)-LDH (110) surface. (e) NH₃ yield rate and FE for Fe (II)Cu (II)Fe (III)-LDH at different potentials. (f) NH₃ yield rate and FE for Fe (II)Cu (II)Fe (III)-LDH and Cu (II)Fe (III)-LDH at −0.5 V [66]. Copyright 2022 Wiley-VCH. (g) LSV curves in the both N₂- and Ar-saturated electrolyte, respectively. (h) NH₃ yield (left axis) and FE (right axis) of Ni₀.₇₅ Fe₀.₂₅ Se₂ at different electrode potentials in 0.1 M Li₂SO₄ solution. (i) Nyquist plot from 0.1 Hz to 100 kHz of the catalysts loaded on GCE at 1.8 V [96]. Copyright 2021 Royal Society of Chemistry.

Figure 6. (a) Chronoamperometry curves at the corresponding potential for 2 h. (b) NH₃ yields and FE of doped Fe–Ni@C nanofibers at different potentials. (c) Comparison of NH₃ yields and FE of undoped and doped Fe–Ni@C nanofibers at −0.5 V [95]. Copyright 2020 Wiley-VCH. (d) Gibbs free energy diagram for N₂ reduction to NH₃ on Fe (II)Cu (II)Fe (III)-LDH (110) surface. (e) NH₃ yield rate and FE for Fe (II)Cu (II)Fe (III)-LDH at different potentials. (f) NH₃ yield rate and FE for Fe (II)Cu (II)Fe (III)-LDH and Cu (II)Fe (III)-LDH at −0.5 V [66]. Copyright 2022 Wiley-VCH. (g) LSV curves in the both N₂- and Ar-saturated electrolyte, respectively. (h) NH₃ yield (left axis) and FE (right axis) of Ni₀.₇₅ Fe₀.₂₅ Se₂ at different electrode potentials in 0.1 M Li₂SO₄ solution. (i) Nyquist plot from 0.1 Hz to 100 kHz of the catalysts loaded on GCE at 1.8 V [96]. Copyright 2021 Royal Society of Chemistry.

Table 1. Summary of recently reported LDH-based catalysts for the e-NRR to NH₃.

Sr No.	Catalysts	Synthesis Method	Electrolyte	NH3 Yield Rate	F.E.	Potential (V vs. RHE)	NH3 Detection Method	Ref.
1	Co-Fe-LDH	Hydrothermal	0.01 KOH	2 × 10−10 mol s−1 cm−2	14.18%	−0.75 V	Nessler reagent	[62]
2	Ni-Fe-LDH	Hydrothermal	0.1 M KOH	19.44 µg h−1 mg−1	19.41%	−0.20 V	Indophenol blue	[93]
3	Ni-Co-LDH	Hydrothermal	0.1 M Na2SO4	52.8 μg h−1 mg cat−1	11.5 %	−0.7 V	Indophenol blue	[94]
4	Co-Fe LDH and g-C3N4	Hydrothermal	0.1 M Na2SO4	2.07 × 10−9 mol s−1 cm−2	25.3%	−0.45 V	Indophenol blue	[65]
5	CoVP @Ni-Fe/ V-LDH	Hydrothermal	0.05 M H2SO4	1.6 × 10−6 mol h−1 cm−2	13.8%	−0.3 V	Indophenol blue	[64]
6	NiFe–Nb2C2-LDH	Hydrothermal	0.1 M KOH	61.16 μg h−1 cm−2	30.01%	−0.4 V	Indophenol blue	[19]
7	P-doped Fi-Ni -C LDH	Hydrothermal	0.1 M Na2SO4	1.72 × 10−10 mol s−1 cm−2	23%	−0.5 V	Indophenol blue	[95]
8	Fe (II)Cu (II)Fe (III)-LDHs	Solvothermal	0.1 M Na2SO4	33.1 μg h−1 mg−1	21.7%	−0.5 V	Indophenol blue	[66]
9	Ni-Fe-Se-LDH	Hydrothermal	0.1 M Li2SO4	5.64 μg h−1 cm cat −2	12.3%	−0.1 V	Indophenol blue	[96]

Table 1. Summary of recently reported LDH-based catalysts for the e-NRR to NH₃.

Sr No.	Catalysts	Synthesis Method	Electrolyte	NH3 Yield Rate	F.E.	Potential (V vs. RHE)	NH3 Detection Method	Ref.
1	Co-Fe-LDH	Hydrothermal	0.01 KOH	2 × 10−10 mol s−1 cm−2	14.18%	−0.75 V	Nessler reagent	[62]
2	Ni-Fe-LDH	Hydrothermal	0.1 M KOH	19.44 µg h−1 mg−1	19.41%	−0.20 V	Indophenol blue	[93]
3	Ni-Co-LDH	Hydrothermal	0.1 M Na2SO4	52.8 μg h−1 mg cat−1	11.5 %	−0.7 V	Indophenol blue	[94]
4	Co-Fe LDH and g-C3N4	Hydrothermal	0.1 M Na2SO4	2.07 × 10−9 mol s−1 cm−2	25.3%	−0.45 V	Indophenol blue	[65]
5	CoVP @Ni-Fe/ V-LDH	Hydrothermal	0.05 M H2SO4	1.6 × 10−6 mol h−1 cm−2	13.8%	−0.3 V	Indophenol blue	[64]
6	NiFe–Nb2C2-LDH	Hydrothermal	0.1 M KOH	61.16 μg h−1 cm−2	30.01%	−0.4 V	Indophenol blue	[19]
7	P-doped Fi-Ni -C LDH	Hydrothermal	0.1 M Na2SO4	1.72 × 10−10 mol s−1 cm−2	23%	−0.5 V	Indophenol blue	[95]
8	Fe (II)Cu (II)Fe (III)-LDHs	Solvothermal	0.1 M Na2SO4	33.1 μg h−1 mg−1	21.7%	−0.5 V	Indophenol blue	[66]
9	Ni-Fe-Se-LDH	Hydrothermal	0.1 M Li2SO4	5.64 μg h−1 cm cat −2	12.3%	−0.1 V	Indophenol blue	[96]

4. Techniques for Detecting and Quantifying Ammonia

4.1. Ammonia Detection Method

The accurate detection of NH₃ synthesized by e-NRR is limited by multiple factors, which includes low production yields, ambient contamination, complex electrolyte separation, pH variation, and chemical interferents. The standard techniques for the detection of NH₃ are spectrophotometry, ion chromatography, and ¹H NMR. Although these methods are highly accurate for simple solutions, their performance in complex systems is less certain and must be rigorously validated. Recently, gas chromatography has emerged as a promising advanced method for in situ quantification, offering a path to greater analytical precision across diverse catalytic environments [95,97]. Detection methods: we will be systematically comparing these techniques in terms of their accuracy and reliability for post-reaction analysis.

4.1.1. Spectrophotometric Method

The spectrophotometric method, which is also known as the colorimetric method, is used for NH₃ detection due to its high precision and cost-effectiveness method, where the absorbance or transmittance of light is measured. In research concerning NH₃, spectrophotometric method was initially used to quantify NH₃ in water quality control [98]. Spectrophotometric method can be divided into three categories to measure NH3—the Nessler’s reagent method, the salicylate method, and the indophenol blue method—which are discussed below.

Nessler’s Reagent Method

Nessler’s reagent method is a solution containing K₂HgI₄ and KOH, named after Julius Nessler, who was the first to synthesize it. Iodide and mercury ions react with NH₃ in alkaline conditions to form a reddish-brown complex with a strong absorbance peak at 410–425 nm (Figure 7a,b) [99]. The absorbance of the complex is directly proportional to NH₃ concentration in the absence of interfering substances; therefore, the highly alkaline nature of Nessler’s reagent promotes the precipitation of metal hydroxide complexes. The resulting turbidity introduces significant error into the colorimetric detection of NH₃. Turbidity is often remediated by precipitating interferents through the addition of zinc sulphate and sodium hydroxide solutions, followed by filtration to remove the coagulated solids. To reduce interference from other ions (such as Fe³⁺, Co²⁺, Ni²⁺, Cr³⁺, Ag⁺, S²⁻, and others), Rochelle salt (KNaC₄H₄O₆·4H₂O) solution is usually added to remove residual hardness cations that might react with Nessler’s reagent [100]. Equation (23) illustrates the overall reaction.

2K₂HgI₄ + NH₃ + 3KOH → HgO· Hg (NH₂) I + 7KI + 2H₂O

(23)

Nessler’s reagent presents several significant limitations. Firstly, the reagent contains highly toxic mercury, necessitating stringent safety precautions during handling and disposal. Second, it has a short shelf life and must be used within approximately three weeks of preparation. Finally, for accurate results, the colorimetric detection of NH₃ must be conducted within 10 to 30 min of adding the reagent, as prolonged reaction times can lead to unreliable measurements [101].

Indophenol Blue Method

The indophenol blue (or phenate) method, based on the Berthelot reaction, is a standard technique for determining low-concentration NH₃ (0–0.6 mg NH₃-N L⁻¹) in water and wastewater, where NH₃ reacts with phenol and hypochlorite under alkaline conditions to produce a blue-coloured indophenol, which is then quantified by measuring its absorbance, typically between 630–660 nm [102]. The indophenol blue method proceeds through a series of reaction steps. First, monochloramine is formed from the reaction of NH₃ and hypochlorite at a pH range of 9.7–11.5, where the monochloramine and phenol combine to form quinone chloramine. The presence of sodium nitroprusside (Na₂[Fe(CN)₅NO]₅) facilitates quinone chloramine formation and boosts the indophenol colour reaction or sodium nitroferricyanide (Na₂[Fe (CN)₅NO] ·2H₂O) as catalysts, with citrate buffer maintaining pH stability in the reaction medium. A further reaction between the quinone chloramine and phenol produces a yellow intermediate. This intermediate then dissociates under alkaline conditions to yield the final blue indophenol product, the intensity of which is quantitatively measured using a UV–Vis spectrophotometer (Figure 7c,d) [103]. The indophenol blue method is carried out via the overall reaction process as shown in (Equation (24)) [104].

(24)

Salicylate Method

A notable modification of the indophenol blue method involves substituting phenol with sodium salicylate or salicylic acid. This “salicylate method” offers a safer alternative by suppressing the formation of a toxic and highly volatile by-product, o-chlorophenol, thereby reducing associated hazards. A key trade-off is the lower reactivity of salicylate, which necessitates the application of higher reagent concentrations to attain sensitivity equivalent to the traditional phenol-based protocol [105]. The salicylate method involves the formation of monochloramine from NH₃ and hypochlorite, like the indophenol blue method. However, this intermediate reacts with salicylate to generate blue-green 5-aminosalicylate, as depicted in (Equation (25)). Firstly oxidizing 5-Aminosalicylate is firstly and then combines with salicylate producing a chromophoric compound. Increasing NH₃ concentration produces a progressive colour shift in the salicylate method, transitioning from yellow to green to blue [106].

(25)

Figure 7. (a) UV–vis spectra of standard solutions with different concentrations and corresponding calibration curve. The absorbance values for the calibration curve were obtained from a wavelength of 655 nm. Inset of (b) is the photograph of the standard solutions after 2 h of reaction with the indophenol reagent. NH₃ determination using Nessler’s method [100]. Copyright 2020 Royal Society of Chemistry. (c) UV–vis spectra of the standard solution with different concentrations and corresponding calibration curve. The absorbance values for the calibration curve were obtained from a wavelength of 420 nm. Inset of (d) is the photograph of the standard solution after 20 min of reaction with Nessler’s reagent [106]. Copyright 2021 Royal Society of Chemistry.

Figure 7. (a) UV–vis spectra of standard solutions with different concentrations and corresponding calibration curve. The absorbance values for the calibration curve were obtained from a wavelength of 655 nm. Inset of (b) is the photograph of the standard solutions after 2 h of reaction with the indophenol reagent. NH₃ determination using Nessler’s method [100]. Copyright 2020 Royal Society of Chemistry. (c) UV–vis spectra of the standard solution with different concentrations and corresponding calibration curve. The absorbance values for the calibration curve were obtained from a wavelength of 420 nm. Inset of (d) is the photograph of the standard solution after 20 min of reaction with Nessler’s reagent [106]. Copyright 2021 Royal Society of Chemistry.

4.1.2. Ion Chromatography Method

Ion chromatography relies on the specific interaction of ions or polar molecules and an ion-exchange stationary phase, with a suitable eluent as the mobile phase, achieving separation [103]. In e-NRR and NO₃RR, scientists normally monitor the yield of NH₃. Because NH₃ is present in solution as the ammonium ion (NH₄⁺), ion chromatography with a cation exchange column is the preferred analytical method for this. The column is charged with a specially negatively charged substance. It is not very high in ion-exchange capacity, and it lets us employ highly dilute salt solutions in order to force the ions through [106]. Due to their extremely low concentrations, these kinds of solutions will necessarily have low electrical conductivity, which is important for what comes next. The detection may go in either of two ways. In a regular non-suppressed system, there is the potential for direct measurement of the solution conductivity as it comes out of the column. In an even greater sensitivity, however, a suppressed system is used where the flushing solution is chemically neutralized upon detection, hence enabling the ammonium ions to be especially emphasized against a very noiseless background [107]. One of the biggest advantages of this method is that it is fast and comprehensive. Rather than checking for single ions one at a time, it can separate and count several distinct cations simultaneously in one sample run that takes only minutes, achieving detection to milligram-per-litre levels. Separation occurs because each kind of positively charged ion interacts differently with the negatively charged column material, causing them to move at various rates and emerge from the column at differing times. A detector of conductivity monitors these ions as they leave. We determine what each ion is by seeing precisely when it leaves (its retention time) and comparing this to a sample of known composition. We measure its concentration by comparing the height of the peak it produces to a calibration curve. Even with the availability of older spectrophotometric techniques, ion chromatography is regarded by most scientists as a more efficient workhorse in this application because it is efficient and dependable. Its real strength is in combining efficiency and simplicity, offering a tool to separate, identify, and quantify multiple ions concurrently within a plausible time frame with immense confidence [108]. Its greatest strength is precision. The technique is highly selective, i.e., by suitably adjusting the settings, not only are you able to analyze common inorganic cations but you can also identify and measure organic ones with accuracy [109]. Its sensitivity is very high and provides accurate measurement in a broad window of concentration (a few µg L⁻¹ to several hundred mg L⁻¹). Because of pH stability in its column packings, ion chromatography is extremely stable and compatible and possesses strong acid eluents and has the ability to provide a broader application range [12]. While this method has significant benefits, some disadvantages are also observed. It is expensive and requires high-technology equipment and is less suitable for electrolytes that have Li⁺ and Na⁺ ions with short retention times. In NH₄⁺ determination, in particular, peak interference by other cations of comparable retention times (Na⁺, K⁺, and Li⁺) generally excludes detection, hence its usefulness to electrolytes that have these ions [110]. In order to circumvent these problems, column-switching techniques can be used, such as choosing proper columns and eluents or working with columns having greater cation-exchange capacity, allowing accurate determination of trace level cations for enhancing the separation of strongly entangled peaks like NH₄⁺ and Na⁺ in high-ionic-strength matrices [107,111].

4.1.3. ¹H NMR Method

Nuclear magnetic resonance (NMR) spectroscopy has many useful applications including determining the amount of NH₃ present in a sample through the magnetic visualization of ¹H, ¹⁴N, and ¹⁵N. When detecting NH₃ from NRR and nitrate reduction reactions (NO₃⁻RR) and trying to detect the ¹⁵NH₃, NMR of ¹H is highly preferred to ¹⁵N NMR. The proton has a much higher sensitivity to those NMR frequencies. The method is able to detect NH₃ at a concentration as low as a micromole in its more stable NH₄⁺ form. Ammonium NMR has the additional valuable appraisal of differentiating N₂ isotopes and determining the extent of uranium due to the ammonium ion’s effect on the surrounding protons. The creating of signal for scalar (J-coupling) ammonium NMR has distinct patterns, which are a result of the complex interactions of protons and N₂ nuclei. For ¹⁴NH₄⁺, the ¹H resonance is displayed as 2 symmetric peaks that are 53 h apart, whilst for ¹⁵NH₄⁺ the signal is displayed as 3 symmetric peaks with larger spacing (73Hz). This demonstrates clear isotopic detection and expands the composition of the copper NH₄⁺ solution [112].

Nuclear magnetic resonance (NMR) method is highly sensitive, highly reproducible, and able to discriminate and ignore contaminating NH₃ from all other extraneous sources. Also, pulse sequences can be tailored to provide accurate and precise measurements of concentrations in a variety of dissimilar media, even in non-deuterated, aqueous, and non-aqueous solutions. Yet, two distinct practical problems do arise. Access to an NMR spectrometer is not ubiquitous when compared to other laboratory instruments such as a spectrophotometer, ion chromatograph, and other basic scientific apparatus [113]. Also, when expensive ¹⁵N₂ gas is used in isotopic labelling experiments, a sophisticated gas circulation apparatus is necessary. This is one of the most important features of such a setting because the apparatus is essential to conserve the gas and, just as importantly, to masterfully purify the feed gas to eliminate the rest of the N₂ species that would compromise the labelled experiment changes [114].

4.1.4. Isotope Labelling (¹⁵N₂)

Isotopic labelling with ¹⁵N₂ is also a critical technique for uniquely establishing that the NH₃ arises from the provided N₂ gas instead of from some N₂-containing species of the electrocatalyst itself. Qualitative observation of reaction intermediates and quantitative analysis of product yields both require this technique [115]. Qualification of the NH₃ yield also requires carrying out a control experiment by feeding ¹⁵N₂ under the same conditions to the standard ¹⁴N₂ reduction test. A successful experiment is indicated by the synthesis of ¹⁵NH₃, and its yield is expected to be theoretically equal to that of ¹⁴NH₃ synthesized in the comparator experiment using ¹⁴N₂. The direct comparison hence determines the catalytic origin of the NH₃ change [116].

4.2. Ammonia Activity Metrics

Progress in NRR research is contingent upon the objective and accurate assessment of electrocatalysts, which must be rigorously evaluated based on three fundamental metrics: their activity, selectivity, and stability. Electrocatalysts activate N₂ at a low overpotential with high activity, which mainly produces NH₃ with very few by-products, such as hydrazine, which shows strong selectivity by keeping its performance steady over time, which describes its good stability. For NRR, activity and selectivity are typically evaluated through NH₃ yield and FE, which together provide the primary metrics of electrochemical performance.

4.2.1. Ammonia Yield Rate (yNH₃)

The NH₃ yield rate, i.e., the yield rate of NH₃ produced per unit time and normalized to the geometric area of the electrode or the mass of catalyst, is a simple performance indicator for reporting the yield rate of the e-NRR and is calculated using the following Equation/formula

yNH₃ = (CNH3 × V)/(t × ∆)

(26)

where CNH3 = Concentration of NH₃;

V = Volume of electrolyte (mL);

t = Reduction reaction time (seconds, minutes, or hours);

∆ = catalyst reaction area (mg _cat, cm⁻², or mg _metal).

The yNH₃ is primarily in the forms of µmol h⁻¹ cm⁻², µmol h⁻¹ mg⁻¹, µg ⁻¹ cm⁻², µg h⁻¹ mg⁻¹, µmol s⁻¹ mg⁻¹, µmol h⁻¹ g⁻¹ µmol s⁻¹ cm⁻², µmol s⁻¹ g⁻¹, mol s⁻¹ cm⁻², mol s⁻¹ g⁻¹, and mol s⁻¹ mg⁻¹, which represents the amount of NH₃ produced per hour (or per second) per unit of catalyst. cm⁻² represents the area-normalized yield rate of the active catalyst on the working electrode, and mg⁻¹ represents the mass-normalized yield rate on the working electrode [117].

4.2.2. Faraday Efficiency (FE%)

In e-NRR systems, where side reactions such as HER and hydrazine formation (N₂H₄) are the most likely suspects, selectivity towards NH₃ must be determined with precision.

This selectivity is quantified by calculating the FE,

F.E (%) = (3F × CNH₃ × V)/(M × Q)

(27)

where F = Faraday constant (96,485.30 C mol⁻¹);

CNH₃ = is the measured NH₃ concentration;

V = volume of the electrolyte (mL);

M = relative molecular mass of NH₃ (M = 17);

Q = total charge passed through electrodes.

A critical consideration in NRR analysis is the formation of N₂H₄ as a reaction byproduct. Its presence in the electrolyte can be quantified experimentally using the spectrophotometric method pioneered by Watt and Chrisp [12,118]. The pathways for NRR to NH₃ synthesis are the associative and dissociative mechanisms on heterogeneous surfaces. And in the alternating pathway, N₂ undergoes single hydrogenation at the two N₂ centres, which can possibly result in N₂H₄ as a byproduct. The FE of N₂H₄ production can be calculated as follows:

F.E (%) = (4F × CN₂H₄ × V)/(M × Q)

(28)

where CN₂H₄ = N₂H₄ concentration;

M = relative molecular mass of N₂H₄.

Faradaic efficiency (FE) quantifies the impact of competing side reactions on NH₃ generation, providing a critical metric for the development and screening of highly selective NRR catalysts. Currently, the NH₃ selectivity of the e-NRR remains relatively low. Notably, NH₃ yield and FE often exhibit a leverage effect, which is often manifested as low FE at high NH₃ yield or high FE at low NH₃ yield (based on the available reports). The total NRR FE is the sum of these two reactions as follows:

FE _NRR (%) = FE NH₃ + FE N₂H₄

(29)

4.2.3. Selectivity

The modulation of NRR activity by the N₂ environment surrounding the active site highlights the need to evaluate NRR selectivity by taking into account the competing HER as a significant side reaction [119]. A key performance metric for NRR catalysts is selectivity, which measures the efficiency by converting N₂ to the desired NH₃ while minimizing the generation of undesired byproducts, primarily N₂H₄. Hydrazine is highly hazardous, although it is a highly valued chemical, which is a high-energy liquid fuel that has been typically utilized as a feedstock, deoxidizer, and reducer. Therefore, a water-soluble NH₃ solution containing even a small amount of N₂H₄ cannot be used as a nutrient-rich liquid fertilizer. In the e-NRR, N₂H₄ occurs if intermediate species follows an alternative associative pathway, which mainly involves energetically unfavourable (endergonic) steps. Further, most catalysts inherently do not produce N₂H₄ because they lack a reaction pathway that supports the stabilization and conversion of the N₂H₄ intermediate. These characteristics are advantageous for NH₃ production as the aqueous NH₃ product can be utilized immediately by eliminating any N₂H₄ intermediates. Furthermore, producing N₂H₄ via electrochemical means is generally not practical due to both economic and environmental concerns. This highlights that efficient NH₃ synthesis benefits from catalysts that avoid N₂H₄ formation pathways, favouring selective NH₃ generation [120].

4.2.4. Stability

The stability of a catalyst plays a crucial role in evaluating the practical viability of the e-NRR compared to other electrocatalytic reactions like the oxygen evolution reaction, hydrogen evolution reaction, and oxygen reduction reaction. The stability of the catalyst during use directly influences its lifespan. Electrode surfaces get poisoned or inactive due to the adsorption of oxygen, water, and hydrogen molecules, which directly block active sites required for N₂ adsorption. Typically, after the e-NRR catalyst is reused over 24 times or operated continuously for more than 24 h, it should maintain or only show slight fluctuations in its original NH₃ yield and FE. Therefore, designing electrocatalysts with high stability is essential for reliable, long-term NH₃ production [121]. Currently, only a limited number of studies focus on the stability of NRR catalysts. Understanding the mechanisms behind catalyst deactivation is crucial to designing catalysts that are both durable and highly efficient for N₂ reduction. Therefore, employing in situ or operando characterization techniques is highly recommended, as they allow real-time monitoring of catalytic processes and degradation pathways, providing valuable insights for developing robust NRR catalysts.

4.2.5. The Effects of pH Value in Different Electrolytes

One of the main reasons for developing e-NRR technology is to decrease the dependence on gaseous hydrogen as the main reactant for NH₃ synthesis. In these systems, hydrogen or protons typically come from the electrolyte. Therefore, electrolytes play a vital role by not only providing the necessary protons as raw materials but also by serving as the medium that enables ion transport throughout the electrochemical cell, making them essential to the overall reaction process. The proton-transfer rate can be limited by reducing the concentration of protons in the bulk electrolyte, using a nonaqueous electrolyte, or increasing the barrier for proton transfer to the catalyst [122]. Aqueous electrolytes with different pH values, such as acidic (0.05 M H₂SO₄, HCl pH = 1), neutral (0.1 M Na₂SO₄, 0.1 M PBS, pH = 7), and alkaline (0.1 M NaOH, 0.1 M KOH pH = 13) highlight the strong dependency of NRR activity and selectivity on electrolyte pH of electrocatalytic NRR [32]. The HER rates at the same reversible hydrogen electrode (RHE) potential follows the trend—j_HER (acidic) > j_HER (alkaline) > j_HER (neutral)—leading to a very high HER rate under acidic conditions due to the high availability of protons (H₃O⁺), the slowest in neutral environments, and intermediate rates in alkaline media, influenced by the shift in proton donors and reaction mechanisms with pH. HER activity is governed by proton availability. It proceeds most rapidly in acidic media, moderately in alkaline media, and slowest in neutral media. HER is a dominant side reaction; it consumes the vast majority of the electrical current. Therefore, in environments where HER is most aggressive (acidic), the selectivity for NRR is the lowest. Conversely, in environments where HER is naturally suppressed (neutral), a larger fraction of the total current can be diverted to the N₂ reduction pathway, resulting in the highest NRR Faradaic efficiency—NRR: FE_NRR (neutral) > FE_NRR (alkaline) > FE_NRR (acidic) [123]. Liquid electrolytes offer several advantages, such as simplicity, cost-effectiveness, ease of addition and recycling, and very high fluidity, making them suitable for a wide range of electrolytic cells. Generally, aqueous electrolytes can enhance proton exposure and transfer efficiency, but they also promote HER. However, proper modification of aqueous solutions can increase the selectivity for the e-NRR [124].

5. Conclusions and Future Perspective

Electrocatalytically, nitrogen reduced for NH₃ synthesis is a promising technology to replace the traditional Haber–Bosch industrial ammonia production method due to the mild and environmentally friendly reaction conditions. In this review, we highlighted that e-NRR provides NH₃ for the storage and utilization of electricity in next-generation energy storage and conversion systems due to its low cost and combination with renewable energy sources, such as hydro, solar, and wind. In this work, we summarized the fundamental process and reaction mechanism of e-NRR. We also summarized the catalyst development and design, where LDH structure, properties, and their preparation are discussed. Most importantly, recent works have focused on how to achieve efficient nitrogen reduction through LDH-based e-NRR catalyst, and the advantages for efficient N₂ reduction of these strategies are also mentioned in these reported works. We also summarized techniques for detecting and quantifying NH₃.

Tremendous advancements and discoveries have been made, yet there are still issues and difficulties with the current e-NRR systems. It should be noted that great effort needs to be made before the technology can be commercially applied for sustainable ammonia production at ambient conditions.

In the future, for e-NRR ammonia production, LDH-based catalysts for peroxymonosulfate activation, reasonable design of active sites exposure, electron conductivity, and synergistic doping or co-catalyst will be required. In terms of materials, LDHs are one of the most investigated types that exhibited great potential as solid precursors being a result of their highly structured arrangement in addition to remarkable surface area and adjustable compositions. Secondly, by combining it in situ and operando characterization approaches (e.g., X-ray absorption spectroscopy, Raman spectroscopy, and electrochemical impedance spectroscopy), a better understanding of the reaction mechanism and the dynamic structure evolution during NRR can be acquired. In addition, computational modelling and DFT calculations can be employed in the prediction and screening of new compositions of LDHs with desirable values for binding energies and for reactions laid down. In addition, morphology and composition can be precisely fine-tuned via various preparation techniques, including co-precipitation, hydrothermal synthesis, and exfoliation to promote the catalytic activity. To accurately assess catalyst activity, reliable methods for NH₃ detection without contamination and quantification are required. Finally, reliable quantitation means are made available by spectrophotometric methods (indophenol, salicylate, Nessler’s reagent), ion chromatography, ¹H NMR spectroscopy, and isotope labelling with ¹⁵N₂. Incorporating these methods and principal activity measures such as the turnover frequency of NH₃ formation, Faradic Efficiency, selectivity, stability, and pH-dependent behaviour permits a comprehensive benchmarking of NRR catalysts. Collaboration between experimentalists and theorists, as well as between the academia and industry, will surely expedite the translation of lab-scale discoveries into technologies.