Non-noble-metal catalysts for electrocatalytic ammonia oxidation

Abstract

Ammonia emerges as a high-energy density, carbon-free medium for the storage, transport, and utilization of green electricity. To unlock its full potential as a sustainable energy carrier, the development of efficient, scalable, and cost-effective electrocatalysts for the ammonia oxidation reaction (AOR) is critical. This review highlights the urgent demand for replacing noble-metal catalysts with earth-abundant, non-noble-metal alternatives to drive scalable AOR applications. We present a comprehensive analysis of AOR mechanisms, catalyst design principles and electrolyte effects, alongside analytical frameworks for reaction monitoring and product quantification. Beyond fundamentals, we also explore transformative applications of non-noble-metal AOR catalysts in three key domains: (1) ammonia electrolysis for on-demand hydrogen production, (2) direct ammonia fuel cells for decentralized power generation, and (3) electrocatalytic wastewater remediation. By bridging mechanistic insights with practical engineering, this review identifies unresolved challenges, such as sluggish kinetics, catalyst stability and system integration, and proposes a technical roadmap toward non-noble-metal driven ammonia energy systems.

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Mingchuan Luo

Mingchuan Luo is currently working as an assistant professor at School of Materials Science and Engineering, Peking University, China. He obtained his PhD degree from Beijing University of Chemical Technology in 2016. After graduation, he carried out postdoc research at Peking University (2016–2018), University of Toronto (2018–2020) and Leiden University (2020–2023, Marie Curie Fellow) before joining Peking University as a PI in 2023. His research focuses on understanding the chemistry of electrified interfaces and transferring the knowledge to advancing electrocatalysis for renewables-based conversions.

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1. Introduction

Integration of renewable energy into various sectors provides a pathway to sustainability.^1,2 The intermittency of renewable energy necessitates efficient, sustainable, and scalable solutions.³ Storing renewable electricity in the form of fuels and chemicals, such as those containing nitrogen and carbon, presents an attractive route.⁴ Among them, ammonia offers transmission and storage advantages. Ammonia couples several attributes: room-temperature liquefaction pressure of 10 atm;⁵ high volumetric energy density of 12.92–14.4 MJ L⁻¹;⁶ readily available transmission, dispensation, and storage network; and well-established handling processes.⁷ Ammonia – in light of these attributes and its appealing source-to-use techno-economics – stands out as a promising alternative to hydrogen and methanol. Ammonia can be produced from air and hydrogen. Efficient, scalable, and environmentally benign production of hydrogen can be achieved via various pathways, including water electrolysis. An example of this route includes the recent demonstration of ammonia production system that couples water electrolysis with the Haber–Bosch process, achieving an annual production capacity of 11 tons.⁸

Sustainable production of ammonia warrants efficient end-use scenarios. Such scenarios typically involve electrochemical or thermal oxidation processes. In particular, electrocatalytic conversion of ammonia can offer an efficient, sustainable, and environmentally benign ammonia oxidation process. Low-temperature ammonia oxidation can be particularly promising for two processes. Hydrogen production in an ammonia cracker presents an attractive pathway. Electricity generation in direct ammonia fuel cells (DAFCs) represents another promising route.⁹ Despite these processes possessing significant potential, they are currently challenged by the sluggish kinetics of ammonia oxidation reaction (AOR).¹⁰

The AOR can be catalyzed by noble- and earth-abundant elements. In particular, the overpotential of the Pt-catalyzed AOR is 0.4 V or more, which corresponds to a 30% voltage efficiency loss in fuel cells.¹¹ Nevertheless, among present-day AOR catalysts, noble metals remain the highest performing, energy efficient and stable materials for the AOR. Unfortunately, noble-metal AOR catalysts – due to their high cost – constrain system techno-economics, imposing scalability challenges.¹² Non-noble-metal catalysts provide a pathway to scalability and feasibility. However, the electrocatalytic performance, energy efficiency, and system stability are currently curtailed by the sluggish reaction rates of non-noble-metal AOR catalysts.¹³ Rationalizing the origins of poor electrocatalytic performance through clarifying the reaction mechanisms will be critical. Designing non-noble-metal catalysts that can reduce the energy barrier of rate-limiting steps through optimization of reaction mechanisms will be essential for reaction rate and energy efficiency improvements. Additionally, gaining further insights into the stability limiting challenges, such as deactivation of active sites by strongly adsorptive reaction intermediates (*N),¹⁴ will be critical to combining performance and operational stability in a single system.

Rationalizing the role of the micro-reaction environment will open the door for strategies that will enable precise tuning of reaction kinetics and pathways for performance and efficiency improvements. Despite the recent attempts to improve the reaction rates through optimization of operating conditions, current literature lacks clear understanding of the effects of operating conditions. In particular, gaining insights into electrolyte effects will be instrumental in understanding the role of cation identity and electrolyte concentration in the reaction kinetics and efficiencies, thereby optimizing them toward achieving performance and stability improvements. Transferring the mechanistic studies and optimization efforts to scalable systems is a critical milestone toward practical integration of the AOR into various electrocatalytic systems. Achieving this milestone will pave the way for the green ammonia scenario (Fig. 1). This review provides an overview of the AOR on non-noble-metal catalysts, with a particular focus on reaction mechanisms, catalyst designs, and electrolyte effects. The article also highlights the best practices for product analysis. It provides a synopsis of the recent progress in applications, such as hydrogen production, direct ammonia fuel cells, and wastewater remediation, where the AOR plays decisive roles in performance, efficiency, stability, and process techno-economics. The review ends with a summary of recent progress and an outlook for future research and development.

Fig. 1 Schematic illustration of sources and applications of ammonia.

2. Reaction mechanisms

Development of efficient AOR catalysts requires in-depth understanding of the reaction mechanisms and key reaction intermediates. The alkaline media AOR proceeds through adsorption of ammonia onto the catalyst surface, followed by its gradual dehydrogenation and oxidation. Ideally, the AOR produces nitrogen through a six-electron transfer, according to the following equations^4,12 (eqn (1)–(3)):

Anode:

2NH₃ + 6OH⁻ → N₂ + 6H₂O + 6e⁻, E⁰ = −0.77 V vs. SHE (1)

Cathode:

6H₂O + 6e⁻ → 3H₂ + 6OH⁻, E⁰ = −0.83 V vs. SHE (2)

Overall:

2NH₃ → N₂ + 3H₂, E = 0.06 V (3)

The AOR typically follows two reaction mechanisms: the Gerischer and Mauerer (G–M) mechanism and the Oswin–Salomon (O–S) mechanism. In the G–M mechanism,¹⁵ ammonia undergoes dehydration to form *NH_x (x = 1, 2), which then dimerizes into a hydrazine structure *N₂H_y (y = 2, 3, 4), followed by dehydrogenation to form nitrogen, also known as the partial dehydrogenation mechanism (Fig. 2, a1). In the O–S mechanism,¹⁶ ammonia progressively dehydrogenates to form N atoms, with two N atoms coupling to generate nitrogen, also referred to as the consecutive dehydrogenation mechanism (Fig. 2, b1). The classic G–M and O–S mechanisms are based on the reaction process from ammonia to nitrogen. Mavrikakis and colleagues¹⁷ conducted density functional theory (DFT) calculations for the AOR occurring at the surfaces of various metals, including Pt, Ir, Pd, Rh, Ru, Os, Re, Ni, Co, Au, Ag, and Cu. The results indicate that the G–M mechanism is kinetically more favorable than the O–S mechanism and that *N_ad possesses a poisoning effect on transition metal (TM) surfaces. A substantial body of research has demonstrated – using differential electrochemical mass spectrometry (DEMS) and infrared spectroscopy (IR) – that the G–M mechanism is more likely to occur on Pt surfaces due to the inhibition of N–N coupling by the strongly adsorbed poisoning species *N_ad.^18,19 Therefore, most Pt-based noble metals follow the G–M mechanism. Regarding the AOR mechanism on non-noblemetal surfaces, considerable efforts have been made to explore the pathways of AOR on various catalysts.

Fig. 2 A diagram of catalytic mechanism of the AOR. The red pathway represents the G–M mechanism. The blue pathway represents the O–S mechanism.

The distinction between the G–M and O–S mechanisms for the conversion of ammonia to nitrogen lies in the propensity of *NH₂ to either undergo sequential dehydrogenation to form *NH or dimerize to produce *N₂H₄. Consequently, comparing the energy barriers of the steps *NH₂ to *NH and *NH₂ to *N₂H₄ provides a means to ascertain the reaction pathway.²⁰ Nevertheless, coupling experimental and theoretical methodologies remains the current best practice for in-depth understanding of reaction mechanisms. Kang et al.²¹ – by using in situ IR – observed H–N–H bending, –NH₂ wagging, and N–N stretching vibrations corresponding to N₂H₄ intermediates at 1456, 1230 and 1109 cm⁻¹, respectively. Thus, the CuCo/CC surface phenomena corroborate the G–M mechanism. Wang et al.,²² by performing DEMS, observed NO, N₂O, and no N₂H₄ intermediate, corroborating the DFT findings that O–S is the more favorable pathway. Jia et al.²³ observed not only the 1093 and 1313 cm⁻¹ vibrations of the N₂H₄ adsorbed intermediate but also the N–O/NOH vibrations near 1509 cm⁻¹via IR. These findings suggest that nitrogen originates from *N₂H₄, while NO₂⁻ from *N, implying simultaneous occurrences of the G–M and O–S mechanisms. Therefore, the reaction mechanism of ammonia to nitrogen conversion on non-noble-metal catalyst surfaces typically proceeds through an N₂H₄ intermediate. However, the high applied voltage triggers NO_x⁻ formation, rendering the reaction mechanism complex. Despite the earlier studies consistently identified NO_x⁻ by *N (Fig. 2, b2), Koper and coworkers²⁴ revealed, by using DEMS and IR, that NO_x on the Pt surface comes from *NH₂OH, corroborating the G–M mechanism (Fig. 2, a2). Therefore, testing the validity of similar mechanisms or identifying the mechanistic discrepancy between noble and non-noble metals is critical for the design and optimization of AOR-active materials. Chen et al. numerically explored two different reaction pathways from ammonia to NO_x⁻ on β-Ni(OH)₂ and β-NiOOH surfaces.^25,26 The production of NO_x⁻via *NH₂OH or *NHOH intermediates offers kinetic benefits at high voltages. Burgess et al. combined DFT with ATR-SEIRAS, confirming Chen's conclusions and determining that the conversion of ammonia to NO_x⁻ on NiOOH in the OH vacancy proceeds mainly via the *NHOH intermediate.²⁷ Ming et al.²⁸ explored the possibility of NO₂⁻ production on the surface of NiCuCo–OOH via the G–M mechanism. The estimated energy barriers of *NH₂ to *NH (1.75 eV) and *NH₂ to *NH₂OH (1.1 eV) suggest that the deprotonation of ammonia toward *N is not the only enabler of nitrite and nitrate formation, and that the formation can also occur through hydroxylation, which proceeds via formation of oxygenated intermediates (i.e., *NH₂OH and *NHOH). Therefore, the difference between the G–M and O–S mechanisms for the conversion of ammonia to NO_x⁻ is that it is generated via either *NH₂OH and NHOH or *N intermediates.

Due to the low AOR potential of noble-metal catalysts, only the nitrogen generation step needs to be considered in most cases. More importantly, noble metals are susceptible to poisoning by *N intermediates, so noble-metal catalysts are preferred for the G–M mechanism. However, the diversity of surface environments on non-noble-metal catalysts and the generation of by-product NO_x⁻ at high voltages render the reaction mechanisms relatively more complex. As indicated by the above analysis, both the G–M and O–S mechanisms proceed through reaction pathways involving the formation of nitrogen and nitrogen oxides. Therefore, probing key intermediates (such as *N₂H₄ and *NH₂OH), via in situ IR or a suite of characterization techniques and DFT calculations, offers a means to identify the most probable reaction pathway.

Fig. 2 presents an updated map of AOR mechanisms guided by computational and characterization analyses. The red wireframe represents the G–M mechanism, including the reaction pathway from *N₂H_y to N₂ (Fig. 2, a1) and from *NH₂OH or *NHOH to NO_x⁻ (Fig. 2, a2). Meanwhile, the blue wireframe represents the O–S mechanism, including the reaction pathway from *N to N₂ (Fig. 2, b1) and from *N to NO_x⁻ (Fig. 2, b2).

3. Electrochemical procedures for AOR performance

To date, the evaluation of AOR electrocatalytic performance has not been fully harmonized, which complicates the reliable comparison of newly developed AOR electrocatalysts and systems. This section presents an overview of approaches for assessing the activity, selectivity, and stability of non-noble-metal AOR catalysts.

3.1 Activity evaluation

3.1.1 Onset potential. Onset potential is the potential at which the AOR initiates. It determines the direction and rate of the reaction and is a key indicator of catalyst's intrinsic activity. A lower onset potential is an indicator of catalyst's efficiency in terms of initiating the AOR at a smaller overpotential.²⁹

3.1.2 Current density. The AOR current density reflects the intrinsic activity and the reaction rate of the catalyst, and a higher current density indicates a higher reaction efficiency per unit area of catalytic sites. This index resembles the actual power density of DAFCs and the working efficiency of ammonia electrolyzers. Considering that the oxygen evolution reaction (OER) is an inevitable side reaction on non-noble-metal catalysts, the current density differences between 1 M KOH + 1 M NH₃ and 1 M KOH at the same potential can be used as the actual AOR current density.³⁰

3.1.3 Tafel slope. The Tafel slope calculated from the LSV curve reflects the kinetic information about the AOR. The calculation is performed according to the following equation: η = b log(j) + a, where η, b, and j denote the overpotential, Tafel slope, and kinetic current density, respectively. A smaller Tafel slope indicates faster reaction kinetics and higher catalytic activity.³¹ For the AOR on Ni, the AOR rate is assessed by measuring the Tafel slope at a lower voltage (to avoid the effect of the OER at higher voltages).

3.1.4 Electrochemical surface area (ECSA). The ECSA reflects the number of active sites over the catalyst and is related to its intrinsic catalytic efficiency. It can eliminate the difference in electrode geometry by comparing the catalytic performance per unit active site. The usual method of evaluating the ECSA involves fitting the non-Faraday currents at different sweep rates, which can be expressed as: i_c = C_dl × dν, with ν being the sweep rate. The sweep rate is plotted against the bilayer current, and the slope is the bilayer capacitance C_dl, which is proportional to the ECSA. The ECSA can be calculated according to the following equation: ECSA = C_dl/C_s, where C_s is the specific capacitance of a surface smoothened sample under the same conditions.²⁰

3.1.5 Electrochemical impedance spectroscopy (EIS). Measuring the EIS profiles of the electrodes at different potentials and analyzing parameters such as charge transfer resistance can provide insights into the electron transfer process and the nature of the catalyst–electrolyte interface. EIS can inform about the activity and stability of the catalyst. Probing the changes in the catalyst resistance and phase angle could provide insights into the potentials at which phase transition and the AOR occur.³²

3.2 Selectivity evaluation

The current in alkaline ammonia-containing electrolytes captures contributions from the oxidation of metals, AOR, and OER. The theoretical potential of the AOR is lower than that of the OER. Thus, at low potentials, the Ni-based catalyst can undergo only the Ni redox reaction and AOR, whereas at high overpotentials, it can also undergo the OER. Precise measurement of the catalyst's AOR activity requires exclusion of contributions from other side reactions. The meticulous design of experiments and measurement techniques is required to exclude the effects of side reactions.

3.2.1 Differential electrochemical mass spectrometry (DEMS). DEMS allows real-time detection of gas-phase products during the AOR process. DEMS offers practical characterization of AOR selectivity, which is particularly useful for the identification of oxygen. DEMS could also determine the onset potential of AOR, along with the AOR's contribution to the current at various potentials. DEMS could also capture some intermediates, such as *N₂H₄, which could be instrumental in revealing the reaction mechanism.¹⁹

3.2.2 Online gas chromatography (GC). Online GC allows for real-time detection and quantification of gaseous products (i.e., oxygen and nitrogen) during the AOR process. The selectivity can be evaluated by correlating product concentration with charge, which provides the knowledge of whether the reaction follows the desired path.³³

3.2.3 Ion chromatography (IC). IC is primarily used to quantify soluble cationic (NH₄⁺) or anionic (NO₂⁻, NO₃⁻) species in AOR systems. Accordingly, IC can determine ammonia conversion and product selectivity. In addition, the reaction paths can be inferred from the changes in the ion concentration.³³

3.2.4 UV-vis spectrophotometry. UV-vis spectrophotometry allows quantitative analysis of products in solution by characteristic absorption peaks. In general, for the reactants (i.e., NH₄⁺) and products (i.e., NO₂⁻ and NO₃⁻) of AOR, the specificity should be enhanced in combination with a colorimetric reaction prior to absorbance detection.³⁴

The selectivity analysis of AOR can be further broadened with the rotating ring disk electrode (RRDE) approach to distinguish the products by adjusting the ring electrode potential. Determining the oxygen reduction current – by applying the oxygen reduction potential to the ring electrode – distinguishes the AOR from the OER.³⁵

3.3 Stability evaluation

Stability reflects catalysts' ability to maintain its activity and structural integrity during extended operation. Stability is a key metric to evaluate the practicality of the AOR. Stability assessment is typically performed in a three-electrode system via either repetitive cyclic voltammetry (CV) or timed-current/timed-potential methods. Prolonged stability assessments provide insights into the durability of catalytic properties and techno-economics of the AOR process.

3.4 Device evaluation

Intrinsic activity, selectivity, and stability evaluations provide insights into the preliminary catalytic properties and facilitate the discovery of robust materials and systems. Such initial performance evaluations have been instrumental in the identification of numerous materials/systems under laboratory settings. Transferring the laboratory-scale research to industrial settings requires integrating promising AOR electrocatalysts into scalable flow-cells and membrane electrode assemblies (MEAs) at industrially relevant current density, efficiency, and stability.

4. Non-noble-metal catalysts

Non-noble-metal AOR catalysts – in light of their low cost – offer scalability advantages compared to their noble-metal counterparts. Realizing their full potential, however, requires efficient designs that can reduce the activation energy, stabilize reaction intermediates, and selectivity and efficiency of target products. This section will introduce the latest advancements in homogeneous and heterogeneous electrocatalysts.

4.1 Homogeneous electrocatalysts

In the field of electrocatalysis, homogeneous catalysts typically refer to molecular structures composed of metal ions and organic ligands. The metal center is responsible for providing catalytic activity, while the ligands modulate the electronic properties, stability, and selectivity of the metal center. Such a unique catalyst structure facilitates the adsorption of ammonia molecules onto the metal center. Subsequently, the electron and proton transfer processes dissociate the N–H bond, promoting reaction kinetics. Ultimately, nitrogen is generated, completing the catalytic cycle.³⁶ The cleavage of the N–H bond can occur via heterolytic and homolytic processes.³⁷ The removal of a proton by a base and an electron by a chemical oxidant or an electrocatalytic electrode is referred to as the heterolytic process. Conversely, the simultaneous removal of an electron and a proton by an organic molecular medium (such as the tri-tert-butylphenoxyl radical (ArO˙)) is termed as the homolytic N–H bond cleavage process. Additionally, the N–N bond formation can occur via two pathways.³⁶ The first is the bimetallic pathway that involves the coupling of two M–NH_x (x = 0, 1, or 2) species. The second is the monometallic pathway that involves the nucleophilic attack of ammonia and the intramolecular coupling of N–N. Generally, the monometallic pathway shows less sensitivity to the charge of the complex and the steric bulk of the ligands. Besides being stable in concentrated ammonia solutions, the monometallic pathway – compared to the bimetallic pathway – is more amenable, enabling greater reaction rates with lower overpotentials.³⁷ An ideal molecular catalyst for the AOR should balance three thermodynamic parameters: low and uniform bond dissociation free energies (N–H BDFEs), exergonic N–N coupling, and favorable nitrogen release. Therefore, the design and mechanistic studies on molecular complex catalysts typically consider the dissociation of N–H bonds and the formation of N–N bonds.

The common transition metals, such as Fe, Cu, and Ni, are used as active centers in molecular catalysts (Fig. 3a). Peters and co-workers³⁸ reported an Fe complex featuring a tetradentate polypyridine supporting ligand (TPA) and two cis-labile sites, [(TPA)Fe(MeCN)₂]²⁺. Under an applied bias of 1.1 V (vs. Fc/Fc⁺) in acetonitrile solution containing ammonia, 16 equivalents of nitrogen were produced after 18 hours, with a nitrogen faradaic efficiency (FE) of 80%. Despite the N–N bond formation is being proposed to occur after the generation of [(TPA)Fe^IV(NH₃)(NH₂)]³⁺, it remains uncertain whether the process is intermolecular or intramolecular. Later, the weak-field tertiary amine donors and pyridine arms of TPA were replaced with a bipyridine ligand containing a low-spin Fe complex with cis-coordination, [(bpyPy₂Me)Fe(MeCN)₂]²⁺.³⁹ The catalytic onset potential for the AOR over the [(bpyPy₂Me)Fe(MeCN)₂]²⁺ catalyst is 0.45 V (vs. Fc/Fc⁺), which is 250 mV lower than that over the control catalyst, along with a four-fold greater catalytic current (Fig. 3b). Under an applied bias of 0.85 V (vs. Fc/Fc⁺), 149 equivalents of nitrogen were produced after 48 hours, with a higher nitrogen FE of ∼87%. Simultaneously, different reaction pathways for N–N bond formation were explored through experimental and theoretical studies. The Fe-amine complexes exhibit an overpotential of 1.3 V, which is prohibitive considering that the thermodynamic potential for the interconversion of ammonia and nitrogen is −0.81 V vs. Fc^+/0. Later, Wang et al.⁴⁰ reported a novel Fe-amine complex [Cp*Fe-(1,2-Ph₂PC₆H₄NH)(NH₃)]⁺, which is capable of oxidizing ammonia at an overpotential of only 770 mV. The performance of the catalyst is primarily attributed to the non-innocent redox nature of the aniline ligand, facilitating the concerted transfer of one proton and two electrons. Inspired by Cu(I) complexes in the oxidation of aniline ArNH₂ to diazenes ArN=NAr, Warren and colleagues⁴¹ reported a novel β-diketiminato Cu complex, [ⁱPr₂NN_F6]Cu^I–NH₃. The AOR occurs at an overpotential of 700 mV, with an 84% nitrogen FE. Research on molecular catalysts has primarily focused on the ammonia-to-nitrogen conversion in non-aqueous electrolytes. However, Brudvig and colleagues⁴² reported a mononuclear Cu electrocatalyst, [Cu(bipyalkH)L](OTf)₂ (1; bipyalkH = 2-[(2,2′-bipyridin)-6-yl]propan-2-ol; L = methanol or acetonitrile), which can selectively oxidize ammonia to NO₂⁻ and NO₃⁻ in aqueous media. The FE reaches up to 94% (28.5% for NO₂⁻ and 65.2% for NO₃⁻). Additionally, single-metal catalysis and nucleophilic attack (WNA) mechanisms for N–O bond formation are proposed (Fig. 3c). Furthermore, Peters and coworkers⁴³ reported the evidence of intermolecular N–N bond formation in a Ni^III–NH₂ complex, representing a fully characterized Ni–NH₂NH₂–Ni complex.

Fig. 3 Molecular complexes that mediate the AOR. (a) Five representative molecular catalysts for the AOR with distinct metal centers. (I) Reproduced from ref. 47 with permission from American Chemical Society, copyright 2016. (II) Reproduced from ref. 45 with permission from Wiley, copyright 2018. (III) Reproduced from ref. 43 with permission from Wiley, copyright 2020. (IV) Reproduced from ref. 39 with permission from American Chemical Society, copyright 2021. (V) Reproduced from ref. 41 with permission from American Chemical Society, copyright 2022. (b) Cyclic voltammetry (CV) curves of MeCN solutions containing 0.2 M NH₃ (400 equivalents), 0.05 M NH₄OTf, and 0.5 mM [(TPA)Fe(MeCN)₂]OTf₂ or [(bpyPy₂Me)Fe(MeCN)₂]OTf₂. The CV curves were obtained by using BDD as the working electrode, Pt as the counter electrode, and 5 mM Ag/AgOTf as the reference electrode. Reproduced from ref. 39 with permission from American Chemical Society, copyright 2021. (c) Plausible pathways for N–O formation of [Cu(bipyalkH)L](OTf)₂. Reproduced from ref. 42 with permission from American Chemical Society, copyright 2022.

Molecular catalysts with Mo and Mn as active centers are widely used in AOR studies (Fig. 3a). Chirik et al.⁴⁴ reported a library of molybdenum amine complexes with redox-active bis(imino)pyridine ligands. The catalysts simultaneously promote reversible N–H activation and hydrogen evolution from ammonia. Additionally, the interconversion of imido (Mo=NR) and amido (Mo–NHR) species in proton-coupled electron transfer (PCET) reactions was investigated using a terpyridine bis(phosphine) molybdenum aniline complex.⁴⁵ The objective of the study is to ascertain whether the reactivity and energetics of N–H bond formation and cleavage steps contribute to the molecular catalysts' AOR performance. Moreover, N–N coupling to generate nitrogen can also be achieved by extracting hydrogen from the amine ligand using organic molecules ArO˙.⁴⁶ The facile synthesis and high modularity of tetradentate salen ligands offer the flexibility of altering the ligands' electron-donating capabilities without changing the geometry of the metal center. This feature could offer greater redox activity at the ligand or metal center during single-electron oxidation. They are thus widely used in Mn-based complexes. Storr⁴⁷ and Nishibayashi⁴⁸ reported nitrogen generation from Mn^V(salen)(≡N) via single-electron transfer in the presence of the strong oxidant [(C₆H₃Br₂)₃N˙]⁺. Menard⁴⁹ also reported nitrogen evolution from Mn^V(salen)(≡N) and assessed the feasibility of each step in the ammonia oxidation cycle. When [Mn-(depe)₂(CO)(NH₃)]⁺ (depe = 1,2-bis(diethylphosphino)ethane) was treated with ArO˙, a rare cyclophosphonium cation was obtained, which significantly weakened the N–H bond and facilitated the overall reaction.

Homogeneous catalysts exhibit considerable potential in the conversion of ammonia to nitrogen. However, the practicality of homogeneous catalysts is presently curtailed by the high capital costs, intricate synthesis procedures, high toxicity, and complex molecular structures. Substantial catalytic performance improvements are thus needed to offset these drawbacks, warranting further insights into the kinetics of N–H bond cleavage and N–N bond formation.

4.2 Heterogeneous electrocatalysts

Heterogeneous catalysts remain a focal point in AOR research due to their advantages such as low cost, simple synthesis, and environmental friendliness. This section will provide an overview of single-metal and multi-metal catalysts with AOR activity.

4.2.1 Single-metal element catalysts. Early research on AOR electrocatalysts focused on single-metal element catalysts. Mavrikakis and co-workers¹⁷ investigated the AOR on Cu, Ni, and Co catalysts and compared the kinetics of two different reaction mechanisms (Fig. 4a). The calculated onset potential order for the G–M mechanism is Co < Ni < Cu. Although Co possesses a lower onset potential, the N–N bond formation barrier may limit its activity. Ni binds strongly with N but very weakly with H₂NNH₂, forming highly endothermic hydrazine from adsorbed NH₂ species. The NH₂ species can be oxidized and deactivated under alkaline conditions. A catalyst binding nitrogen weakly requires high overpotentials to lower the energy barrier of the rate-determining step (RDS); for example, the Cu (111) surface necessitates an overpotential of 1.22 V. Therefore, Cu is inactive for the AOR unless it is oxidized. Chen and colleagues⁵⁰ investigated the AOR using liquid ammonia on transition metal surfaces anchored to a g-CN monolayer. The interaction between the metal and the support results in a positively charged metal, rendering the corresponding d orbitals more polarizable and facilitating the adsorption of ammonia. Among them, Fe and Co exhibit lower limiting potentials of −0.47 V and −0.5 V, respectively, making them promising AOR catalysts. Hanada and colleagues⁵¹ investigated the AOR performance of Ni, Co, Fe, Ta, and Ti using liquid ammonia (Fig. 4b). At 0.3 V, the catalytic activity improved in the following order: Ni > Co > Fe > Ti ∼ Ta. The current density and the function of formation enthalpy of metal nitride (Δ_fH_MN, kJ mol⁻¹-N) showed a volcano correlation. The current densities of Ni, Co, and Fe decreased with the reduction of Δ_fH_MN, indicating that these metals offer high nitrogen affinity.

Fig. 4 Monometallic catalysts with AOR activity. (a) Activity as predicted by Sabatier analysis for both mechanisms at 0 V_RHE. Reproduced from ref. 17 with permission from American Chemical Society, copyright 2015. (b) Volcano plot of current density at 0.3 V vs. H₂/NH₃ as a function of formation enthalpy of metal nitride (Δ_fH_MN (kJ mol⁻¹-N)). Reproduced from ref. 51 with permission from Elsevier, copyright 2022. (c) Linear sweep voltammetry (LSV) curves of NiOOH–Ni and Ni in 0.2 M NH₃ + 0.1 M KOH electrolyte and 0.1 M KOH electrolyte, respectively, and the Tafel slopes (inset) associated with the AOR of NiOOH–Ni and Ni in the shaded area. Reproduced from ref. 54 with permission from Wiley, copyright 2024. (d) DFT-calculated bonding energy of NH₃ on Cu₂O (111), CuO (111), CuO (002), and Cu (111) facets, respectively. Reproduced from ref. 56 with permission from Elsevier, copyright 2023.

In the early stages of development of AOR nickel-based catalysts, Despić⁵² and Yao⁵³ demonstrated that metallic Ni is inactive for the AOR. The semiconductor properties of Ni oxides inhibit the adsorption of ammonia, deactivating the catalyst surface and suppressing the AOR. Klinkova et al.³³ investigated the product distribution of AOR on Ni(OH)₂ by adjusting the electrolyte and pH parameters. The results suggested that NiOOH is an active site for the AOR. A recent study also confirmed, through in situ Raman spectroscopy and synchrotron radiation, that high-valence Ni³⁺ facilitates the adsorption of ammonia and ensuing intermediates (Fig. 4c).⁵⁴ In nature, Cu(II) is the active center of ammonia monooxygenase enzyme, promoting the AOR. Mayer et al.⁵⁵ also discovered that CuO is the primary active phase in the AOR process, albeit the individual role of Cu₂O and CuO phases remains elusive. Wang et al.⁵⁶ sought to elucidate the contribution of each phase to the catalytic activity. Cu nanorod arrays were used as substrates to synthesize catalysts of distinct oxidation states: CuO and Cu₂O. The authors, via experiments and DFT calculations, found that CuO facilitates ammonia adsorption, enabling a near-unity NO_x⁻ FE of 98.7%. Moreover, CuO (002) was identified as the main active component (Fig. 4d). Huang et al.²² also suggested that CuO (111) is inert and that the introduction of oxygen vacancies via in situ electrochemical oxidation could extend AOR stability by up to 400 hours at a current density of 200 mA cm⁻². Additionally, Fe–SAs/N–C can achieve a high ammonia removal rate of >99% and a high nitrogen FE of 99.5% in chlorinated ammonia-containing wastewater.⁵⁷ However, further reaction rate improvements will be essential toward practicality. Promising AOR performance is also explored for various electrode configurations: CoO/G and defect-rich TiO₂ nanofiber self-supporting electrodes.^58,59

Overall, the theoretical and experimental studies reveal that single-metal catalysts, such as Ni, Cu, Co, Fe, and Ti, exhibit promising AOR activity. Additionally, the activity, selectivity, stability, and reaction mechanisms of other transition metal single-metal catalysts (i.e., Zn, Mn, and Sn) require comprehensive and systematic investigations involving experimental and theoretical aspects. However, the drawbacks of single-metal catalysts, such as their single active site, limited applicability, and constrained optimization potential, have motivated researchers to explore multi-metal catalysts.

4.2.2 Multi-metal element catalysts. Multi-metal catalysts, compared to single-metal catalysts, can provide more active sites. The interactions between multiple atoms can create synergistic effects, optimizing the electronic structure and reaction pathways. The synthesis methods of multi-metal catalysts can be precisely tuned to establish specific interactions between multiple elements. These interactions could, in turn, optimize the catalytic characteristics, typically enhancing catalytic activity and stability. The multi-element catalysts' ability to optimize multiple catalytic reactions renders them suitable for complex reaction systems. This section overviews various strategies that can enhance the activity, selectivity, and stability of catalysts via various mechanisms (Fig. 5).

Fig. 5 A summary of catalyst design strategies and their advantages.

4.2.2.1 Alloying effects. An alloy is a substance with unique physical/chemical properties, consisting of two or more metals or non-metals (Fig. 6). Synergistic interactions between different elements can modulate the electronic structure of the catalyst, provide more active sites, and improve the adsorption and desorption of reaction intermediates. These effects all together can enhance the kinetics of AOR. The synergistic effects between various elements, such as Cu and Ni, Cu and Co, or Cu, Ni and Fe, are some of the representatives of alloying strategy. For example, metallic Cu and Ni can adsorb and desorb nitrogen with various strengths, and the synergetic effects between Cu and Ni atoms could improve the activity and stability of AOR.⁶⁰ Furthermore, the unique electronic configuration of Cu can strongly interact with the molecular orbitals of oxygen-containing nitrogen products, providing effective active sites for the over-oxidation products. Liang et al.⁶¹ introduced Ni into the Cu lattice, and the resulting CuNi nanoalloy effectively modulated the charge distribution of Cu atoms and enhanced the electron transfer properties. CuNi alloy enabled a NO₂⁻ FE of 87% at 1.6 V. Later, the authors constructed a CuNi metallic glass electrocatalyst using an ultrarapid laser quenching strategy (Fig. 12a).⁶² The catalyst exhibits superior resistance to ammonia corrosion compared to traditional CuNi alloys. This specific alloying method allows for a higher Cu content, enabling a nitrite FE of 92% at 1.7 V and stable operation of 321 hours.

Fig. 6 A schematic illustration of the catalyst structure formed via alloying.

Moreover, the synergistic effects between Cu and Co can also accelerate the AOR. For example, Cu_0.5Co_0.5/NF enables 34% greater current efficiency compared to single-metal catalysts.⁶³ Similarly, Ni₄Cu₅Fe₁ ternary alloys⁶⁴ could enhance the AOR and oxygen reduction reaction (ORR) by lowering electronegativity through introduction of Fe³⁺ cations into Cu and Ni. When integrated into a symmetric DAFC, the cell enables an open circuit voltage of 0.62 V and a power density of 8.9 mW cm⁻² (at 0.3 V) at 80 °C. Phosphides, sulfides, and selenides are also used to form multi-element AOR catalysts. For example, the Ni₂P/NF catalyst achieves an ammonia removal rate of 54% for 24 hours at a current density of 10 mA cm⁻².⁶⁵ Similarly, the NiSe catalyst requires an overpotential of 1.52 V to drive the AOR at a practical current density of 100 mA cm⁻² – a voltage input substantially lower than the 1.79 V to drive the OER at similar rates.⁶⁶ Additionally, the NiSe catalyst exhibits improved mass transfer performance and mechanical robustness in ammonia alkaline electrolytes. In another study, an ethylene glycol-modulated Cu₂FeSnS₄ catalyst (EG-CFTS) is reported to achieve a stable operation of 20 hours at a current density of 150 mA cm⁻² (at 1.6 V).⁶⁷ The activity and stability enhancements herein were explained by the rough, porous, and ultrathin 2D nanosheet arrays, which ensure a higher electrochemically active area and faster electron transfer kinetics. When integrated into an EG-CFTS‖EG-CFTS electrolyzer, the cell can deliver hydrogen production rates of up to 10.8 mL while requiring only 50 mA cm⁻² during 30 minutes of continuous operation.

4.2.2.2 Element doping. The element doping strategy involves the introduction of small amounts of external elements into the bulk catalyst (Fig. 7). The addition of dopant can enhance the interaction with the active sites, modulate the electronic structure, and optimize the adsorption and desorption of intermediates and reaction pathways. As Cu²⁺ doping can form high electron density overhanging bands on O_O–Ni, it promotes ammonia adsorption. However, high Cu doping reduces the active surface area and decreases the AOR activity. Tao et al.⁶⁸ designed Ni–Cu nanowires with varying ratios and explored the optimal composition via CV measurements. In particular, Ni_0.8Cu_0.2 layered hydroxides (LHs) required 1.55 V to drive the AOR at 35 mA cm⁻². The authors further investigated the doping effects of Co, Fe, Zn, Mn, and Cr on the AOR activity of Ni LHs. The results showed that Cu is the only first-row transition metal that can effectively enhance the AOR activity of Ni(OH)₂. In particular, the Ni_0.8Cu_0.2 hydroxide/hydroxide catalyst achieves the best performance and stability with a Ni : Cu mass ratio of 4 : 1, delivering a NO₂⁻ FE of 98% for 3 hours at 1.53 V.²³ Additionally, Co sites are found to facilitate OH⁻ adsorption. The H–Ni₅₀Co₅₀/Ni catalyst synthesized via oxidative precipitation demonstrated 60% ammonia removal efficiency and 40% nitrogen selectivity.⁶⁹ The CoOOH phase lowers the charge transfer resistance, enhancing the catalytic kinetics on the NiCo surface. The authors sought to improve nitrogen FE by designing Cu-based LDH hydrotalcites (CuM) with various doping elements (M = Ni, Fe, and Co).²¹ Among them, CuCo LDH exhibited the highest nitrogen FE of 77.4%. Herein, Cu acts as an adsorption site for ammonia and Co catalyzes oxidation, achieving high oxidation efficiency. The Cu–Sn bimetallic hydroxide catalyst can also adsorb and oxidize ammonia efficiently with 84.5% nitrogen FE at 1.75 V.⁷⁰

Fig. 7 A schematic illustration of the catalyst structure formed via element doping.

There are also catalyst design considerations based on multiple doping elements. For example, Xue and colleagues⁷¹ demonstrated that the co-doping of Cu and Fe can optimize the electron distribution around oxygen atoms, promoting the AOR. In particular, the a-NiCuFe catalyst shows a high ammonia removal rate of 90% for 12 hours at 1.6 V. Co-doping of Cu and Co also promotes electron clustering around O atoms and reduces the adsorption and Gibbs free energy of the AOR intermediates.²⁸ Ni₄Cu₁Co_1.5-BP catalysts show higher AOR currents and lower onset potentials. When integrated into AEM-DAFCs, the cell enables an open circuit voltage of 0.75 V and a peak power density of 115.7 mW cm⁻² (at 0.3 V). High entropy spinel oxides (Mn, Fe, Co, Ni, and Cu)₃O₄ also achieve a high nitrogen FE of 90% in nonaqueous solutions.⁷² The improved nitrogen FE is ascribed to the Mn and Cu cations' ability to balance the metal d-bands. Doping non-metallic elements has also received considerable attention. Ye et al.⁷³ suggested that N doping can modify the electronic structure and improve the conductivity. Co and N co-doped NiCo₂N nanoneedles exhibited lower overpotential and Tafel slope compared to bare Ni. In a two-electrode system, the electrolytic potential of AOR was 1.0 V lower than that of water electrolysis, indicating that the catalyst possesses bifunctionality. The N-doped NiZnCu layered double hydroxide catalyst performs the AOR at 100 mA cm⁻² for 20 hours.⁷⁴ When integrated into a two-electrode cell, the N-NiZnCu LDH/rGO‖N-NiZnCu LDH/rGO electrolyzer outperforms Pt/C‖IrO₂ in terms of AOR performance. S doping can produce a lattice effect and promote hole generation. Wu et al.⁷⁵ concluded that Cu doping optimizes the internal electronic energy band structure and density, while S doping modulates the surface properties of the electrodes through exogenous sensing and electrochemical surface reconfiguration. Therefore, the Ni₁Cu₃-S-T/CP catalyst exhibits the highest AOR activity and good stability. Ni₁Cu₃-S-T/CP achieves an ammonia removal rate of 99% for 5 hours at 1.6 V and stable operation of 24 hours at 1.64 V. The authors also constructed a unique 3D catalyst with a nanoflower-like structure on a carbon paper substrate (Ni₁Cu₁Co_0.5-S-T/CP).⁷⁶ The Ni₁Cu₁Co_0.5-S-T/CP catalyst achieves an ammonia removal rate of 98% and a nitrogen FE of 81% at 1.5 V after 8 hours. Xu et al.⁷⁷ constructed B-NiFe-LDH/NF with a special crystalline–amorphous interface, where B nanoclusters acted as charge bridges to promote electron redistribution in LDH and generate highly active metal sites (Fig. 12b). The B-NiFe-LDH/NF catalyst achieves a stable AOR of 72 hours at a current density of 100 mA cm⁻² by requiring a cell voltage of 1.781 V (a voltage lower than the water splitting voltage of 1.985 V). The Cu and B co-doped dendritic Ni₄Cu₁BO_x−1/NF catalyst requires only 1.4 V to achieve 100 mA cm⁻² and remains stable for 24 hours.⁷⁸

4.2.2.3 Oxygen vacancies. Oxygen vacancies can be formed by making a defect in a position that could otherwise be occupied by an oxygen atom. The oxygen vacancies can be formed via electrochemical methods or temperature treatment (Fig. 8). Such defects can provide additional active sites that can improve the electronic structure and electrical conductivity of the catalyst. For example, Vo-rich CuO – enriched with oxygen vacancies through in situ electrochemical treatment – achieves a nitrogen FE of 62.8% at a current density of 200 mA cm⁻² for 400 hours (Fig. 12c).²² The performance improvements are attributed to the fact that the oxygen vacancies can regulate the d-band centers of the Cu sites closer to the Fermi energy level, thus stabilizing the *NH_x intermediates and promoting the conversion of ammonia to nitrogen. Defect-rich TiO nanofibers also exhibit high AOR efficiency and stability in alkaline ammonia solutions.⁵⁹ DFT calculations reveal that the oxygen vacancies on the TiO surface can reduce the energy barrier in the RDS of *HNNH₂ formation and promote the desorption of nitrogen. Additionally, CeO_x with vacancies exhibits a lower onset overpotential and a higher efficiency in non-aqueous electrolytes compared to commercial Pt/C.⁷⁹ The performance advances are primarily attributed to the oxygen vacancies, which facilitate the adsorption of ammonia and reduce the energy barrier for *NH_x dehydrogenation. Tao et al.⁸⁰ –inspired by the application of ABO₃-type perovskite oxides in the HER and ORR – designed B-site-doped LaNi_0.5Cu_0.5O_3−δ (LNCO55-Ar). The oxygen vacancies are formed through annealing in Ar and cationic substitution at the B-site, forming additional active sites. When integrated into a symmetric ammonia electrolyzer (SAE), the SAE-LNCO55-Ar achieved an ammonia removal rate of 32% at 1.23 V after 8 hours in 0.5 M KOH + 2210 ppm NH₄Cl. The authors further targeted LaCoO₃ by co-doping the B-site with Fe and Cr.⁸¹ The performance of DAFCs with LaCr_0.25Fe_0.25Co_0.5O_3−δ/C is comparable to that of DAFCs with Pt/C as the cathode. Ma et al.⁸² synthesized Sr_0.3La_0.7Mn_0.4Co_0.6O_3−δ perovskite with 98% ammonia removal efficiency in chlorine-containing wastewater by dual-site doping at the A and B sites of LaCoO₃. The authors proposed that co-doping could form mixed valence states, acting as redox couples and promoting electron transfer processes. Additionally, the generated oxygen vacancies could serve as new reactive sites. Tao et al.⁸³ designed an A-site and B-site co-doped A₂BO₄-type perovskite with high oxygen vacancies. The resulting La_0.5Sr_1.5Ni_0.9Cu_0.1O_4−δ-Ar catalyst achieved an ammonia removal efficiency of 95% in real landfill leachate.

Fig. 8 A schematic illustration of oxygen vacancies formed via electrochemical methods or temperature treatment.

4.2.2.4 Heterostructure construction. Heterostructure construction refers to the composite structure that couples different solid-state materials with interfacial interaction (Fig. 9). The built-in electric field, active sites, and synergistic effects at the heterojunction interface can optimize the adsorption and conversion of reactants. For example, Ni₃S₄@NiCo₂O₄/NF, as a heterostructure electrocatalyst, exhibits promising AOR activity (10 mA cm⁻² at 1.5 V) and stability (37 hours at 100 mA cm⁻²).⁸⁴ The high-density interfaces and strongly coupled electronic interactions between sulfide and oxide accelerate the catalyst's reconstruction process while reducing the energy barrier for the *NHNH₂ formation step. The combined effect of these two phenomena promotes ammonia adsorption and nitrogen desorption, promoting the AOR. Three-dimensional layered NiSe₂/Se-NiCuO_x/NF electrocatalysts with heterojunctions and oxygen vacancies reached an ammonia removal efficiency of 98% and a nitrogen FE of 98% at 1.5 V during the initial 5 hours of operation.²⁰ The abundant oxygen vacancies and heterojunctions facilitate the electron transfer at the interface and promote the formation of NiOOH active species. This unique structure also increases the adsorption capacity of *NH_x intermediates. A nanostructured Cu₂O wire-on-Ni(OH)₂ plate catalyst achieved 80 mA cm⁻² at 1.5 V and remained stable for 32 hours.⁸⁵ The unique wire-on-plate nanostructure enhanced catalytic activity through the Ni–Cu synergy while also protecting the Cu₂O nanowires from oxidation and dissolution (Fig. 12d). The Ni(OH)₂/SnO₂ composite catalyst achieved a nitrogen FE of 84% at 1.55 V.⁸⁶ The introduction of SnO₂ promotes the adsorption of ammonia on Ni sites while suppressing the OER. The NiCu/MnO₂ layered catalyst enabled a nitrogen FE of 97% at 1.5 V.⁸⁷ The synergistic effect of layered MnO₂ with Ni and Cu facilitated the dimerization of dehydrogenated ammonia and formation of N–N bonds. Similarly, the Cu₂O/Ni catalyst achieved an ammonia removal efficiency of 63% following 18 hours of AOR at 1.62 V.⁸⁸ The introduction of Cu₂O not only increased the electrochemical active surface area but also enabled efficient charge transfer. The three-dimensional CoS@NiCu catalyst required 1.5 V to achieve 100 mA cm⁻² in an ammonia-containing electrolyte.⁸⁹ The performance improvements were ascribed to the facilitated formation of electrocatalytically active CoOOH owing to the rapid charge transfer at the heterointerface. When integrated into an AOR electrolyzer, the cell enabled a 3.2-fold greater hydrogen production rate compared to benchmark systems.

Fig. 9 A schematic illustration of heterostructure construction.

4.2.2.5 Morphology engineering. Morphology engineering refers to the precise regulation of catalyst microforms and surface properties by adjusting catalyst synthesis parameters (e.g., pH, temperature, and surfactant) (Fig. 10). Morphology engineering could optimize critical catalytic properties, including active area, active site distribution, and mass transfer efficiency. Huang et al.⁹⁰ developed Cu₂O catalysts with distinct morphologies on Ni foam (Cu₂O/NF), such as flower, particle, and sheet structures. Among them, flower Cu₂O exhibited improved AOR activity, including an ammonia removal efficiency of 52%. The discrepancy in the AOR performance originates from the small Cu₂O size in the flower structure, providing the largest active surface area. The authors, via X-ray electron spectroscopy (XPS) measurements, discovered that the flower Cu₂O is prone to oxidation into Cu(II) and that the oxygen is donated to the adsorbed N, favoring the production of NO₃⁻. In contrast, the sheet-structured Cu₂O/NF catalyst maintains a stable Cu(I) structure, exhibiting the highest nitrogen FE of 60%. Cao et al.⁹¹ demonstrated that the hierarchical CuAl/carbon fiber-LDH composite (CuAl/CF-LDH) achieves an ammonia removal efficiency of 98.86%. The performance improvement is ascribed to the successful prevention of stacking and aggregation of highly uniform nanosheets enabled by the growth of LDH on carbon fibers. The highly porous structure of carbon fibers also facilitates ammonia diffusion and improves conductivity, endowing CuAl/CF-LDH with superior catalytic performance. Tang et al.⁹² synthesized NiCu double hydroxides (DHs) with tunable intercalation and morphologies [nanotyres (T), nanoflowers (F), and nanonets (N)] to enhance the conversion efficiency of Ni(OH)₂ to NiOOH in NiCu DHs. In particular, the porous NiCu double hydroxide nanotyres (NiCu DHTs) exhibit promising AOR performance, including low onset oxidation potential, high current density, and rapid AOR kinetics (Fig. 12e). The improved catalytic performance and stability are attributed to the intrinsic structural characteristics (i.e., hollow rings and porosity) and a unique hydroxyl-rich interface.

Fig. 10 A schematic illustration of the catalyst structure formed by various morphology engineering methods.

4.2.2.6 Hybrid strategies. The hybrid strategy involves the complexation of transition metal catalysts with high surface area and highly conductive materials (Fig. 11). Carbon nanotubes (CNTs), graphene, and metal–organic frameworks (MOFs) are typically used as support materials for AOR catalysts. Such support materials typically enhance the specific surface area and electronic conductivity, accelerating AOR kinetics. Homogeneous dispersion of metals in the form of hydroxides and oxyhydroxides on single-walled CNTs is a typical hybrid catalyst design strategy. For example, the Ni–Cu–Co/CNT catalyst achieved a current density of 45.66 mA cm⁻² at 1.62 V (exceeding the 1.31 mA cm⁻² of NiCuCo at 1.62 V).³⁰ The enhanced AOR performance is attributed to the high specific surface area, high conductivity, and stability of CNTs. Yang et al.⁹³ synthesized a hybrid catalyst by electrodepositing NiCu on a B-doped diamond (BDD) electrode. The Ni₁₀Cu₁/BDD achieved an ammonia removal efficiency of 45% and a nitrogen FE of 89% at 2.1 V in 6 hours. The promising AOR performance here is ascribed to two phenomena: the activation of ˙OH radicals and the low surface adsorption properties of BDD electrode. The combination of these two effects accelerated the dehydration of ammonia and enhanced the AOR efficiency. Sathe et al.⁹⁴ prepared highly dispersed NiO nanoparticles on a polyaniline conductive polymer (NiO–PANI). The NiO–PANI catalyst achieved a current density of 50 mA cm⁻² at 1.35 V and remained stable for 15 hours (Fig. 12f). The high AOR activity is attributed to the synergistic interaction between NiO and polyaniline and high conductivity of the nanocomposite. Wu et al.⁹⁵ –inspired by the promising performance of single-metal Ni–N anchored to a porous carbon framework for the CO₂RR – designed a NiCu₃–N–C dual-atom catalyst (DAC) on N-doped porous carbon using melamine. The NiCu₃–N–C DAC catalyst exhibited an ammonia removal efficiency of 99.5% and a nitrogen FE of 98% following 6 hours of AOR at 1.5 V. The improved AOR performance was primarily attributed to the electronic coupling effect of the bimetallic active sites, which improved the adsorption balance of reaction intermediates. Sim et al.⁹⁶ used melamine as a template to synthesize Ni₂P nanoparticles on a N-doped carbon matrix (Ni₂P@N–C). The Ni₂P@N–C catalyst achieved a current density of 47.4 mA cm⁻² at 1.6 V – a notable productivity improvement compared to 16 mA cm⁻² obtained for the Ni₂P catalyst. Under similar operating conditions, the Ni₂P@N–C catalyst delivered an ammonia removal efficiency of 78% following 8 hours of AOR. The improved AOR performance results from the N-doped carbon formed on the surface of the Ni₂P nanoparticles, which improved the electronic conductivity and charge kinetics.

Fig. 11 A schematic illustration of hybrid strategies.

Luo et al.⁹⁷ sought to leverage high-density active sites formed in the cavities of MOFs to promote AOR. In particular, the authors used a terephthalic framework, Ni-BDC-NH₂, to synthesize Ni nanoparticles and nanoclusters encapsulated within mesoporous carbon nanoflakes. NiO/Ni/CNF exhibited an onset potential of 1.3 V (much lower than the OER potential of 1.7 V), along with an ammonia removal efficiency of >90% and a nitrogen FE of >90% following 96 hours of AOR at 5 mA cm⁻². The successful annealing of the Ni-MOF precursor can adjust the uniformity and size scale of Ni sites in mesoporous two-dimensional carbon, improving the nitrogen yield and reducing the required electrical energy. Anchoring Ni and Cu nanoparticles onto a Ni-MOF, the Ni₁Cu₃@Ni-NDC catalyst enabled a nitrogen FE of 90% at 1.75 V.⁹⁸ The performance improvement was primarily attributed to the suppression of oxygen-containing byproducts, which was enabled by the interaction between Cu and Ni nanoparticles.

A schematic of the non-noble-metal catalyst design strategy and the most representative transmission electron microscopy (TEM) images are shown in Fig. 12. A comparative performance summary of non-noble-metal AOR catalysts synthesized via various strategies is provided in Table 1.

Fig. 12 Schematic representation of design strategies for non-noble-metal catalysts, along with corresponding TEM images. (a) TEM image of CuNi-L@(−196). Reproduced from ref. 62 with permission from Wiley, copyright 2024. (b) TEM image of B-NiFe-LDH. Reproduced from ref. 77 with permission from Elsevier, copyright 2023. (c) TEM image of Vo-rich CuO. Reproduced from ref. 22 with permission from Springer Nature, copyright 2022. (d) TEM image of Ni(OH)₂–Cu₂O@CuO. Reproduced from ref. 85 with permission from American Chemical Society, copyright 2020. (e) TEM image of NiCu DHTs. Reproduced from ref. 92 with permission from Wiley, copyright 2021. (f) TEM image of NiO–PANI. Reproduced from ref. 94 with permission from American Chemical Society, copyright 2023.

Table 1 A performance summary of non-noble-metal AOR catalysts synthesized via various strategies^a

Catalyst	Electrolyte	Onset potential (V vs. RHE)	Current density (mA cm^−2)	Stability	Ammonia removal efficiency (%)	Ref.
a Current densities are obtained at 1.45 V vs. RHE. NH4^+–N removal efficiency: V vs. RHE.
Alloying effect
Cu8Ni2	1.0 M KOH + 55 mM NH4Cl	1.36	22	50 h @ 1.6 V vs. RHE	95%, 12 h @ 1.6 V	61
CuNi-L@(−196)	0.5 M KOH + 55 mM NH4Cl	1.38	20	312 h @ 1.7 V vs. RHE	94%, 8 h @ 1.7 V	62
Ni4Cu5Fe1/C	0.5 M KOH + 55 mM NH4Cl	1.44	<10	9 h @ 1.5 V vs. RHE	—	64
Ni2P/NF	0.1 M KOH + 5.0 g L^−1 NH3	1.44	<10	24 h @ 10 mA cm^−2	54%, 24 h @ 10 mA cm^−2	65
NiSe	1.0 M KOH + 1.0 M NH3	1.37	50	3 h @ 100 mA cm^−2	—	66
Cu2FeSnS4	1.0 M KOH + 0.5 M NH3	1.40	40	20 h @ 1.6 V vs. RHE	—	67
Element doping
Ni0.8Cu0.2 LHs	0.5 M NaOH + 55 mM NH4Cl	1.43	<10	3 h @ 1.55 V vs. RHE	—	68
a-NiCuFe	0.5 M NaOH + 55 mM NH4Cl	1.42	<10	12 h @ 1.6 V vs. RHE	90%, 12 h @ 1.6 V	71
Ni4Cu1Co1.5-BP	1.0 M KOH + 0.1 M NH3	1.32	52	5 h @ 1.57 V vs. RHE	—	28
N–NiZnCu LDH/rGO	1.0 M KOH + 0.3 M NH4Cl	1.40	20	20 h @ 100 mA cm^−2	—	74
Ni1Cu3-S-T/CP	1.0 M NaOH + 0.2 M NH4Cl	1.40	20	24 h @ 1.64 V vs. RHE	99%, 5 h @ 1.6 V	75
Ni1Cu1Co0.5-S-T/CP	1.0 M NaOH + 0.2 M NH4Cl	1.38	30	12 h @ 1.6 V vs. RHE	98%, 8 h @ 1.5 V	76
B-NiFe-LDH/NF	1.0 M KOH + 1.0 M NH3	1.35	350	72 h @ 100 mA cm^−2	79%, 24 h @ 100 mA cm^−2	77
NiCuBO/NF	1.0 M KOH + 0.5 M NH3	1.28	150	24 h @ 100 mA cm^−2	—	78
Oxygen vacancies
Vo-rich CuO	1.0 M KOH + 1.0 M NH3	1.36	60	400 h @ 1.52 V vs. RHE	—	22
LaNi0.5Cu0.5O3−δ	0.5 M KOH + 55 mM NH4Cl	1.41	<10	8 h @ 1.5 V vs. RHE	32%, 8 h @ 1.23 V	80
La0.5Sr1.5Ni0.9Cu0.1O4−δ	0.5 M KOH + 55 mM NH4Cl	1.38	<10	2 h @ 1.55 V vs. RHE	30%, 8 h @ 1.22 V	83
Heterostructure construction
Ni3S4@NiCo2O4/NF	1.0 M KOH + 1.0 M NH3	1.42	<10	37 h @ 100 mA cm^−2	—	84
NiSe2/Se-NiCuOx/NF	1.0 M NaOH + 0.5 M NH4Cl	1.38	40	50 h @ 100 mA cm^−2	98%, 5 h @ 1.5 V	20
Ni(OH)2–Cu2O@CuO	1.0 M KOH + 1.0 M NH3	1.35	25	32 h @ 1.5 V vs. RHE	—	85
NiCu/MnO2	0.5 M NaOH + 55 mM NH4Cl	1.36	12	48 h @ 1.5 V vs. RHE	—	87
Cu2O/Ni	1.0 M KOH + 0.1 M NH4Cl	1.45	<10	—	63%, 18 h @ 1.62 V	88
CoS@NiCu	1.0 M KOH + 2.0 g L^−1 NH3	1.35	60	4.5 h @ 1.6 V vs. RHE	—	89
Morphology engineering
Cu2O/NF	0.1 M Na2SO4 + 5 mM NH3	1.35	<10	3 h @ 1.70 V vs. RHE	55%, 3 h @ 1.70 V	90
NiCu-DHTs	0.1 M NaOH + 0.05 M NH3	1.30	<10	5000 s @ 1.50 V vs. RHE	—	92
Hybrid strategies
Ni–Cu–Co/CNT	1 M KOH+ 0.5 M NH3	1.38	18	2400 s @ 10 mA cm^−2	—	30
Ni10Cu1/BDD	0.5 M NaOH + 0.5 M NH3	1.35	15	—	45%, 6 h @ 2.1 V	93
NiO–PANI	1.0 M KOH + 0.5 M NH3	1.25	100	15 h @ 1.35 V vs. RHE	—	94
NiCu3–N–C	1.0 M NaOH + 0.2 M NH4Cl	1.40	15	5 h @ 1.5 V vs. RHE	99%, 6 h @ 1.5 V	95
Ni2P@N–C	1.0 M KOH + 0.5 M NH3	1.36	15	30 h @ 1.55 V vs. RHE	78%, 8 h @ 1.6 V	96
Ni1Cu3@Ni-NDC	1.0 M KOH + 0.5 M NH3	1.31	30	15 h @ 1.6 V vs. RHE	—	99

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5. Electrolytes

The AOR in aqueous solution is a pH-sensitive reaction. Under alkaline conditions (pH > 9.25), ammonia molecules are first adsorbed on the catalyst surface followed by their oxidation by OH⁻. Ammonia can effectively maintain its molecular form in alkaline electrolytes, typically resulting in higher AOR efficiency. However, a high concentration of OH⁻ may lead to competitive adsorption between ammonia and OH⁻, potentially triggering the OER as a side reaction.³³ Under neutral or acidic conditions (pH < 9.25), ammonia tends to ionize into ammonium ions, which can be indirectly oxidized by oxidants such as HOCl. The kinetics of indirect ammonia oxidation in acidic media are sluggish, primarily due to the repulsion of NH₄⁺ and electrode corrosion. Therefore, the AOR becomes thermodynamically and kinetically more favorable under alkaline conditions. Additionally, the application of non-aqueous electrolytes in the AOR has garnered significant attention. In non-aqueous electrolytes, ammonia exhibits high solubility, which avoids the competing OER and mitigates corrosion, enabling convenient transport and storage. However, challenges such as limited reactant and proton transport remain unsolved. Therefore, this section will provide an overview of the recent progress in the AOR in all-pH aqueous and non-aqueous electrolytes.

5.1 Alkaline electrolytes

Since ammonia molecules can maintain their stability in alkaline aqueous solutions, the AOR is generally carried out in alkaline ammonia solutions. Accordingly, DAFCs and ammonia electrolyzers typically employ alkaline aqueous solutions. While optimum OH⁻ availability promotes ammonia dehydrogenation, excessive OH⁻ concentration could trigger the competing OER. Such conditions could cause high reactive potentials on non-noble-metal catalysts, decreasing the AOR efficiency. Therefore, the alkaline-media AOR research sought to explore the effects of electrolyte pH on reaction kinetics. Tao et al.⁹⁹ found that increasing electrolyte pH enables greater AOR current densities and ammonia removal rates (reaching 78% at pH = 12). The mechanism behind these improvements is the positive role of OH⁻ concentration in the formation of Ni_xCu_1−xOOH, enhancing the electrocatalytic activity of the catalyst toward the AOR. A similar phenomenon is observed on the a-NiCuFe catalyst.⁷¹ The authors explained this behavior using the Nernst equation: E = −0.77 + 0.0592 (14 − pH). A higher pH results in a more negative potential, rendering the AOR thermodynamically more favorable. Additionally, a stronger interaction between Ni and ammonia is observed on the NiCu/BDD catalyst at high pHs.⁹³ While higher pHs favor NO_x⁻ production, lower pHs promote nitrogen selectivity up to 100%. Klinkova et al.³³ investigated the effect of pH (ranging from 9 to 14) on the products of AOR. The main product becomes NO₃⁻ at pH > 12 and NO₂⁻ at pH < 12. At pH > 12, the competing OER starts dominating. The authors sought to elucidate the potential effects of electrolyte identity on the AOR performance of Ni(OH)₂/NF. The constant potential electrolysis measurements show that replacement of 0.1 M Na₂SO₄ with 0.1 M NaOH could enable the AOR without competing OER over a wide range of potentials (Fig. 13a–d). When the AOR is performed using 0.1 M Na₂SO₄, nitrogen, nitrate, and nitrite become the main products. However, the use of 0.1 M NaOH triggers the OER by suppressing the AOR efficiency. These results indicate that the electrolyte identity, electrolyte composition, and electrolyte pH play a key role in the AOR selectivity by modulating the competition between the AOR and OER. Our literature review shows that Na₂SO₄ could be a more suitable electrolyte to steer the AOR toward nitrate and that highly alkaline electrolytes are necessary to steer the AOR to nitrite. Meanwhile, electrolyte optimization should be performed by also considering the activity of the competing OER.

Fig. 13 (a–d) The influence of hydroxide and sulfate ions on the activity and selectivity of the AOR in an alkaline electrolyte. Reproduced from ref. 33 with permission from Royal Society of Chemistry, copyright 2022. (e) Schematic diagram of the Cl⁻-mediated indirect AOR. Reproduced from ref. 102 with permission from Elsevier, copyright 2018. (f) CV curves of as-synthesized metal nitride, Ni(OH)₂, graphite paper and 20% Pt/C in NH₃ saturated 0.1 M KPF₆ acetonitrile solution. Reproduced from ref. 14 with permission from Elsevier, copyright 2021. (g) LSV curves of Y zeolite, CeO₂, CeO₂-400, 0.12CeO₂/Y, 0.12CeO_x/Y-400, and 0.2Pt/C in NH₃ or Ar-saturated ammonium benzenesulfonate DMF solution at 5 mV s⁻¹. (h) Calculated Gibbs free energy profile for the AOR process on CeO₂ and CeO₂-Vo following the G–M mechanism (adsorption sites, *). Color code: gray, Ce; red, O; blue, N; green, H (U = 0 V, pH = 7). Reproduced from ref. 79 with permission from American Chemical Society, copyright 2024.

5.2 Neutral or acidic electrolytes

The direct AOR requires an electrolyte pH greater than the pKa of the NH₄⁺/NH₃ acid–base pair (i.e., pH > 9.25). In practice, performing wastewater treatment under such conditions is economically unviable. Thus, Cl⁻-mediated indirect AOR in neutral/acidic electrolytes presents an efficient and cost-effective alternative to the direct AOR. However, the efficiency of indirect AOR depends on the availability of sufficient Cl⁻ ions in wastewater.¹⁰⁰ The indirect AOR occurs through a mechanism akin to that of breakpoint chlorination (Fig. 13e). First, highly oxidative active chlorine species (HClO or ClO⁻) are electrochemically generated (eqn (4) and (5)), followed by the stepwise reaction of ammonia with these species to produce nitrogen or NO₃⁻ (eqn (6) and (7)).¹⁰¹

2Cl⁻ → Cl₂ + 2e⁻ (4)

Cl₂ + H₂O → HClO + H⁺ + Cl⁻ (5)

2NH₄⁺ + 3HClO → N₂ + 3H₂O + 5H⁺ + 3Cl⁻ (6)

NH₄⁺ + 4HClO → NO₃⁻ + H₂O + 6H⁺ + 4Cl⁻ (7)

When the active chlorine/ammonia molar ratio gradually reaches 1.5,¹⁰² chloramine species are formed, namely monochloramine (eqn (8)), dichloramine (eqn (9)), and trichloramine (eqn (10)), ultimately leading to the production of nitrogen (eqn (11)).

NH₄⁺ + HClO → NH₂Cl + H₂O + H⁺ (8)

NH₂Cl + HClO → NHCl₂ + H₂O (9)

NHCl₂ + HClO → NCl₃ + H₂O (10)

NCl₃ + NHCl₂ + 3OH⁻ → N₂ + 2HClO + 3Cl⁻ + H₂O (11)

Thus, the concentration of Cl⁻ is crucial in the indirect AOR. After ammonia is oxidized to nitrogen or nitrate, the oxidant is regenerated back to Cl⁻.

Waite et al.¹⁰² suggest that high reaction rates and Cl⁻ concentrations accelerate the formation of active chlorine and increase the steady-state concentration of active chlorine. This, in turn, facilitates the degradation of ammonia. During the indirect AOR, the concentration of active chlorine first decreases rapidly. Upon complete degradation of ammonia, the concentration of active chlorine increases to a relatively higher value. In contrast, the impact of pH on ammonia removal is much less significant compared to that of current density or Cl⁻ concentration. A recent study also corroborates the minimal influence of pH on the AOR.¹⁰³ The ammonia concentration remains constant when the solution contains no Cl⁻. The presence of Cl⁻ ions causes gradual oxidation of ammonia. At Cl⁻ concentrations of 7500 and 10 000 mg L⁻¹, ammonia completely oxidizes within 150 minutes. The rising concentration of Cl⁻ causes an increase in the NO₃⁻ concentration, and the nitrogen remains the primary product. The presence of Cl⁻ is critical for the AOR. However, excessive concentrations of Cl⁻ can also trigger the formation of harmful by-products, such as chlorates and perchlorates.

Research on aqueous systems concentrates on the effect of OH⁻, SO₄²⁻, and Cl⁻ ions on AOR performance. However, the limited solubility of NH₃ in conventional aqueous electrolytes represents a significant challenge, reducing the yield and productivity of AOR. The interaction between the soft base N₂ and the soft acid Sn²⁺ can promote the oxidation of N₂. OH⁻, as a hard base, requires high overpotential for its adsorption on Sn²⁺, inhibiting the OER.¹⁰⁴ Noting that NH₃ and N₂ are soft bases and Cu⁺ is a soft acid, the interactions between them could be the root cause of lower OER performance on Cu-based catalysts during the AOR. In addition, unlike other ions, BF₃ interacts strongly with N₂, enhancing the concentration of N₂ in the electrolyte. The Lewis acid effect provided by free BF₃ can also promote the N₂ activation and protonation steps, enhancing the efficiency of AOR.¹⁰⁵ However, research on electrolyte effects of AOR is still in the early stages, and other possible components, such as BF₃, should also be comprehensively investigated for potential improvements in the solubility of NH₃via enhanced interaction.

5.3 Non-aqueous electrolytes

Non-aqueous electrolytes, compared to aqueous electrolytes, provide higher ammonia solubility. Additionally, non-aqueous electrolytes are less corrosive and typically cause no side reactions. Such electrolytes are also effective in preventing the formation of by products, such as NO and O₂. Non-aqueous electrolytes are used typically along with organic cations. Such organic cations are highly tunable, and they can block direct interactions between reaction intermediates and alkali metal cations. Therefore, numerous studies have explored the AOR in non-aqueous electrolytes. Liu et al.¹⁴ utilized ammonia saturated in 0.1 M KPF₆ acetonitrile solution as a non-aqueous electrolyte for the AOR. The prepared NiCo₂N nanosheets exhibited an AOR onset potential of 0.55 V vs. NHE and achieved a nitrogen FE of >90% (Fig. 13f). The voltage window of the NiCo₂N nanosheets in the non-aqueous solution was at least 0.40 V wider than that in the aqueous solution. The authors also designed a high-entropy oxide (Mn, Fe, Co, Ni, or Cu)₃O₄ that achieves a nitrogen FE of 90% at 1.0 V vs. NHE.⁷² Additionally, molecular organic complexes are often employed for the AOR in ammonia-containing acetonitrile solutions. In ammonia-saturated ammonium benzenesulfonate DMF non-aqueous electrolyte, the Sm₂O_3−x electrode exhibits good AOR stability with a low onset potential.¹⁰⁶ In aqueous electrolytes, the rate of nitrogen evolution is only 12.5% of that in non-aqueous electrolytes. Furthermore, the OER becomes dominant in aqueous electrolyte. Notably, in aqueous systems, protons generated from water dissociation limit the effectiveness of ammonia utilization for high-density storage capacity. 0.12CeO_x/Y-400 materials are also employed as the anode for the AOR in ammonia-saturated ammonium benzenesulfonate DMF non-aqueous electrolyte (Fig. 13g).⁷⁹ The 0.12CeO_x/Y-400 catalyst exhibits a nitrogen FE of 70%, doubling the nitrogen FE of commercial Pt/C catalysts. The 0.12CeO_x/Y-400 catalyst achieves a current retention rate of 78% during 10 hours of stability test, along with stable nitrogen and hydrogen FEs. The experimental, computational, and characterization studies, including in situ infrared spectroscopy, ¹H NMR spectroscopy, and DFT calculations, suggest a reaction pathway that aligns closely with the G–M mechanism (Fig. 13h).

However, research on non-aqueous electrolytes is still in its infancy. Research efforts have been mostly dedicated to enhancing the selectivity of nitrogen. The complexity of components of non-aqueous electrolytes renders the identification of specific reaction mechanisms rather complex. Additionally, non-aqueous electrolytes face challenges, such as low ionic conductivity, high cost and toxicity, limited reactant diffusion, and cumbersome product separation. Such challenges require substantial optimization efforts to meet the performance, efficiency, and stability requirements of practical applications.

6. Product analysis

Product analysis presents a means to assess the selectivity of the AOR overvarious catalysts. This, in turn, enables optimization of catalyst design, electrolyte type and concentration, and other operating conditions to maximize the productivity and efficiency of AOR by suppressing the competing OER. The AOR can produce nitrogen, NO₂⁻, and NO₃⁻ as the primary products, along with other potential products, such as NO, N₂O, and NO₂. Constant current/potential methods provide insights into the activity and stability of AOR on various catalysts. However, the time-dependent stabilization of catalysts causes varying current responses in constant potential methods, imposing total charge calculation errors. The challenges associated with the constant potential methods motivate researchers to use constant current methods.³³ The ammonia removal rate is an indicator of ammonia oxidation activity of catalysts. The ammonia removal rate can be obtained based on the detection of initial and post-reaction ammonia concentrations (eqn (12)).²⁰

The analysis and quantification of ammonia can be performed via three common methods: Nessler's reagent method, indophenol blue method, and ion chromatography.³⁴ Nessler's reagent and indophenol blue methods are cost-effective and widely used spectrophotometric techniques. Nessler's reagent is a solution of K₂HgI₄ and KOH. Iodine and mercury ions react with ammonia under alkaline conditions to produce a reddish brown complex, which exhibits strong absorption at 420 nm (eqn (13)).

2[HgI₄]²⁻ + NH₃ + 3OH⁻ → Hg₂ONH₂I + 7I⁻ + 2H₂O (13)

The mercury ions in Nessler's reagent are toxic and environmentally hazardous, requiring careful handling and disposal procedures. The life of Nessler's reagent is relatively short, requiring completion of the measurements within a specified period. Additionally, any organic substances and metal ions in the sample can interfere with the detection results. The indophenol blue method follows the Berthelot reaction,¹⁰⁷ in which ammonia reacts with phenol and hypochlorite in an alkaline solution to produce a blue indophenol product (eqn (14)).

Zhang et al.³⁴ investigated the pH effects on ammonia quantification and proposed a criterion to select the most appropriate detection approach. Under neutral or alkaline conditions, both Nessler's reagent and indophenol blue methods can detect ammonia concentrations with high accuracy. However, only the Nessler’s reagent method is suitable for the detection/quantification of ammonia under acidic conditions, due to the instability of NaOCl under acidic conditions. Ion chromatography also provides a means to determine the nitrogen concentration of ammonia. Ion chromatography, compared to spectrophotometric methods, offers advantages, including high sensitivity, high selectivity, simple sample pretreatment, and environmental friendliness.

The detection of nitrite is typically performed using the diazo coupling spectrophotometric method.^56,95 The color-developing agent (Griess reagent) is prepared by mixing N-(1-naphthyl)-ethylenediamine dihydrochloride (NED), sulfanilamide, H₂PO₄, and DI water. Under acidic conditions, nitrite first reacts with sulfanilamide to form a diazonium salt, which then reacts with NED to produce a purple-red azo compound. By measuring the absorbance of the azo compound at a wavelength of 540 nm, the nitrite concentration can be indirectly determined.⁶¹ The method for nitrate detection is relatively more convenient. The diluted electrolyte is directly mixed with hydrochloric acid, and the quantification is achieved by measuring the absorption at 220 nm. However, organic substances also exhibit absorption at this wavelength. To eliminate interference errors, a second measurement is taken at 275 nm, where nitrate exhibits no adsorption. The nitrate nitrogen content is then calculated by substituting the values from the two measurements (A = A₂₂₀ − 2A₂₇₅).²⁰

Due to interference from air, the detection of gaseous products is challenging. Some studies have qualitatively analyzed gaseous products of AOR, such as nitrogen, N₂O, and NO₂ using DEMS.^22,85 However, the low concentrations of N₂O and NO₂ impose quantification challenges. Therefore, quantification of gaseous products relies on detection of nitrogen. The detection of nitrogen typically involves two methods.^54,95 The first method involves measuring the concentrations of NO₂⁻–N, NO₃⁻–N, and NH₄⁺–N before and after the reaction. The nitrogen concentration is then calculated using the mass balance before and after the reaction: C_N2–N = C_{(NH4⁺–N)0} − C_{(NH4⁺–N)t} − C_(NO2⁻–N) − C_(NO3⁻–N).²⁰ Despite its facileness, this quantification methodology is not highly accurate, as the presence of O₂ in the product stream cannot be accounted for, rendering it difficult to identify the FE of each product. An alternative quantification approach is online gas chromatography (GC).³³ The GC measurements are typically more accurate. However, the GC method requires elimination of oxygen and nitrogen present in the system via 1 hour of argon purging prior to measurements. Additionally, facile tail gas treatment using 1 M H₂SO₄ solution is required to prevent ammonia vapor from entering the GC column and damaging the instrument. Therefore, for accurate product detection and analysis, gaseous products need to be detected using online GC to eliminate interference from air. The liquid products, however, can be analyzed using ion chromatography to avoid a series of errors introduced by complex processing steps.

7. Applications

7.1 Ammonia electrolysis for hydrogen production

Hydrogen, as a clean, efficient, and versatile energy carrier, could play a critical role in global energy transition and sustainable development. Renewable-electricity-powered electrocatalytic hydrogen production provides a means to zero carbon emissions while addressing the intermittency of renewable energy. However, the practicality of water electrolysis is presently curtailed by sluggish kinetics and prohibitive overpotentials concomitant with the anodic reaction (OER). To overcome these challenges, the field presently pursues two major research streams: high-performance OER catalysts and alternative low-overpotential oxidation reactions.

The AOR stands out as an alternative reaction with energy efficiency advantages compared to the OER. The ammonia electrolysis technology involves the decomposition of ammonia into hydrogen and nitrogen where the theoretical voltage is much lower than that of water electrolysis (0.06 V vs. 1.23 V).⁶⁰ Present-day AOR research focuses on exploring active and low-overpotential materials, including hydroxides, phosphides, and sulfides. In particular, high-performance dual-functional catalysts, which can simultaneously catalyze the AOR and HER, offer significant potential for less-energy-intensive water electrolysis. For example, Chen et al.⁷⁸ synthesized a dendritic NiCuBO/NF-1 catalyst that drives the AOR at a current density of 25 mA cm⁻² at 1.301 V, approximately 180 mV lower than the OER potential of 1.481 V. The two-electrode system (NiCuBO/NF-1‖NiCuBO/NF-1) catalyzes ammonia decomposition at 10 mA cm⁻² at a cell voltage of 1.56 V, which is lower than the water splitting potential of 1.67 V. Accordingly, Ni₂P@N–C‖Ni₂P@N–C can decompose ammonia at a current density of 50 mA cm⁻² by requiring a cell voltage of 1.55 V, which is significantly lower than the 1.75 V needed for water electrolysis.⁹⁶ Xu et al.⁷⁷ designed a B-NiFe-LDH/NF catalyst by depositing amorphous boron nanoclusters on NiFe layered double hydroxide nanosheets. The B-NiFe-LDH/NF catalyst exhibits a synergistic effect of structural modification and electronic state regulation, which facilitates the adsorption–desorption of ammonia and water molecules on the catalytically active sites, demonstrating dual-functional catalytic activity for the HER and AOR. The B-NiFe-LDH/NF‖B–NiFe-LDH/NF catalyst-based ammonia electrolyzer shows ∼1.5-fold lower energy consumption in hydrogen production after 24 hours at 50 mA cm⁻². The common observation in various experimental studies is that the presence of ammonia could lower the potential of hydrogen production.^66,73,74

Overall, ammonia electrolysis for hydrogen production holds advantages in terms of energy efficiency, storage, transportation, and environmental friendliness, rendering it one of the critical technologies in future clean energy systems. Through further catalyst and electrolyzer optimizations and scaling-up studies, ammonia electrolyzers are expected to play a critical role in hydrogen supply and renewable energy storage, driving global energy transition and sustainable development.

7.2 Direct ammonia fuel cells

Fuel cells are energy-conversion systems that convert the chemical energy of a fuel and an oxidant into electricity with high energy-conversion efficiency. Fuel cells, unlike traditional power generation processes, are not limited by the Carnot cycle, typically yielding high energy conversion efficiency and high-power density. Under ideal conditions, fuel cells can achieve an energy conversion efficiency of 85–90%.¹

In particular, proton exchange membrane fuel cells (PEMFCs) are successfully used in stationary, portable, and transportation applications. PEMFCs utilize hydrogen as the fuel for the hydrogen oxidation reaction (HOR) and oxygen as the fuel for the OER. DAFCs and PEMFCs share similar operating principles.⁴ However, DAFCs utilize liquid-phase ammonia as the fuel, offering handling, storage, and transportation superiorities compared to PEMFCs. These advantages improve the practicality of DAFCs. In the cathode chamber, oxygen (from air or other diluted sources) undergoes the ORR, producing OH⁻ ions. The OH⁻ ions are then dragged to the anode through the AEM where they participate in the oxidation reaction with ammonia, generating gaseous nitrogen (N₂), water (H₂O), and electrons (e⁻). The anodic, cathodic, and overall reactions in DAFCs are as follows (eqn (15)–(17)):

Anode:

2NH₃ + 6OH⁻ → N₂ + 6H₂O + 6e⁻ (15)

Cathode:

2H₂O + O₂ + 4e⁻ → 4OH⁻ (16)

Overall:

4NH₃ + 3O₂ → 2N₂ + 6H₂O (17)

Tao et al. developed a Ni₄Cu₅Fe₁/C catalyst with dual-functional activity for the AOR and ORR.⁶⁴ The Ni₄Cu₅Fe₁/C catalyst was used as the anode and cathode of a symmetric DAFC. Such a configuration enabled cost reductions by simplifying the overall device structure. The effect of operating temperatures on the DAFC performance was investigated using 3 M KOH + 7 M NH₃ as the electrolyte. At lower temperatures of 40–60 °C, the cell yielded poor open-circuit voltage (OCV) and peak power density (PPD), due to the high resistance and sluggish reaction kinetics of the inactivated cell. At higher temperatures of 80 °C, the cell delivered a maximum OCV of 0.62 V and a maximum PPD of 8.9 mW cm⁻² at 0.3 V. In another work, Ming et al.²⁸ constructed a DAFC equipped with a Ni₄Cu₁Co_1.5-BP anode and a Mn-Co-BP2000 cathode. When operated at 80 °C using 3 M KOH + 7 M NH₃, the DAFC enabled a peak power density of 115.7 mW cm⁻² at 0.3 V and an OCV of 0.75 V. Systematic investigations into the effect of ammonia concentrations reveal that the peak power density increases with rising ammonia concentrations, attributable to the reduction in activation polarization and ohmic polarization losses. In particular, under diffusion-controlled conditions, increasing ammonia concentration enhances the contact between ammonia and the anode catalyst, providing abundant active sites to participate in the AOR, ultimately enabling smaller activation polarization losses. When the ammonia concentration increases from 7 M to 9 M, the AOR is primarily governed by the reaction kinetics. At such high ammonia concentrations, the active sites for AOR reach saturation, reducing local OH⁻ concentration and limiting product diffusion, which ultimately causes poor AOR productivity. This ultimately exacerbates the activation polarization losses in DAFCs.

Overall, DAFCs performing the AOR offer a means to promising, clean, efficient, and scalable energy production for off-grid power generation, aerospace, and other fields. Despite the remaining challenges, ongoing research contributes to the development of active, low overpotential, efficient, and scalable AOR catalysts. Moving forward, intensifying the catalyst exploration along with system integration and scalability will be critical toward large-scale deployment of DAFCs.

7.3 Wastewater remediation

Ammonia-containing wastewater primarily originates from chemical industry effluents, agricultural fertilizers, and landfill leachate. If these wastewaters are discharged without treatment, they can cause severe harm to aquatic ecosystems, such as water eutrophication and fish poisoning. Therefore, effective treatment of ammonia-containing wastewater is a crucial task for environmental protection.¹⁰² Present-day ammonia-containing wastewater treatment methods include physical, chemical, biological, or hybrid processes. Among them, electrocatalytic oxidation stands out as a promising approach for the treatment of ammonia-containing wastewater. The electrocatalytic oxidation process involves removing ammonia from water by electrochemically oxidizing it to produce gaseous nitrogen and water. The process combines several advantages, including high efficiency, environmental friendliness, low energy consumption, and resource recovery.

Tao et al. designed a LaNi_0.5Cu_0.5O_3−δ (LNCO55-Ar) catalyst with excellent AOR and HER activity. A symmetric ammonia electrolyzer (SAE), based on the LNCO55-Ar catalyst, exhibited a removal efficiency of 100% (100 hours) from ammonia solution (2210 ppm) or a removal efficiency of 70% from real landfill leachate (48 hours).⁸⁰ These metrics were both obtained at 1.23 V. The relatively lower removal efficiency from wastewater is attributable to the suspended solids and other side reactions caused by impurity ions in real wastewater. Later, Tao et al. designed a Sr and Cu dual-doped La_0.5Sr_1.5Ni_0.9Cu_0.1O_4−δ-Ar (LSNC-Ar) catalyst.⁸³ The catalyst exhibited excellent AOR and HER activity, owing to the improved conductivity and synergistic effects between metals. The symmetric ammonia electrolyzer (SAE-LSNC-Ar) based on La_0.5Sr_1.5Ni_0.9Cu_0.1O_4−δ-Ar catalyst demonstrated performance comparable to that of an ammonia electrolyzer based on the PtIr/C anode and Pt/C cathode at low voltages. At 1.22 V, the ammonia removal efficiency reached 100% from wastewater with an ammonia concentration of 2177 ppm (70 hours). In real landfill leachate, the ammonia removal efficiency reached 95% after 170 hours, representing the highest level of electrochemical ammonia removal from landfill leachate.

Overall, ammonia electrolysis offers an innovative and sustainable approach to wastewater treatment. The process could play a significant role in industrial wastewater treatment, agricultural wastewater management, and urban wastewater treatment, driving the green and sustainable development of water treatment technologies. The approach offers the potential to efficiently remove ammonia, reduce energy consumption, and produce hydrogen as a valuable byproduct. Toward large-scale implementation, active, low-overpotential, efficient, stable, and impurity-tolerant catalyst, electrode, and system design will be critical.

8. Conclusions and prospects

This review provides an overview of the latest progress in non-noble-metal catalysts for the AOR from various aspects, including reaction mechanisms, catalysts, electrolytes, products, and applications. The article summarizes the recent progress and findings in reaction mechanisms, structure–activity relationships of non-noble-metal catalysts, electrolyte effects, best practices in product analysis, and state-of-the-art AOR applications. Below, we underscore the recent progress and remaining technical and scientific challenges in critical aspects and provide an outlook for each aspect for practicality advancements (Fig. 14).

Fig. 14 Major applications and challenges of non-noble-metal catalysts for the AOR.

8.1 Current challenges

8.1.1 Electrolysis efficiency. The efficiency of the AOR process is governed by various factors, including electrode materials, electrolyte composition, and operating conditions. Coupling the AOR with HER provides a low-voltage pathway to water electrolysis, rendering the AOR theoretically more efficient.⁷⁴ However, over non-noble-metal catalysts, particularly over Ni, the AOR occurs at a high overpotential of 1.4 V, due to the redox potential of Ni.⁵⁴ Such overpotentials cause prohibitively low energy conversion efficiencies. Therefore, exploring transition metals with a low AOR onset potential and combining them with Ni to develop highly efficient AOR catalysts will be key for efficiency and practicality breakthroughs.

8.1.2 Electrocatalysts. Present-day non-noble-metal AOR catalysts encounter three main challenges: activity, selectivity, and stability. Ni- and Cu-based catalysts remain the ones that can, to a certain extent, couple electrocatalytic performance, stability, and scalability. However, such catalysts require prohibitive overpotentials to drive the AOR at practical rates, typically requiring an overpotential of 1.4 V.³³ Suppressing the overpotentials to practical levels requires catalyst design strategies, such as forming secondary or ternary alloys by doping the base element with metals or non-metals. Optimization of the doping or alloying strategy will be critical to optimizing the local crystal structure and improving the electrical conductivity, thereby increasing the intrinsic catalyst activity.^28,71 The doping strategy might be relatively more suitable for materials with dual-functional catalytic activity. For example, Ni-based catalysts exhibit intrinsic activity toward both the OER and AOR, and the competition between ammonia molecules and OH⁻ typically suppresses the selectivity of the AOR by triggering OER activity. Therefore, doping Ni with transition metals (i.e., Cu) possessing poor OER activity can steer the reaction toward the AOR by suppressing the OER.¹⁰⁸ Alternative approaches, such as non-metal element (i.e., P) doping, could also present a promising route to efficiency breakthroughs.^76,77 Additionally, three-dimensional and high-surface-area materials, or their combinations could present a route to augment the availability of catalytic sites, enhancing the catalytic activity and reducing the overpotential.⁹⁴ Another critical aspect is activity and selectivity loss due to structural changes. The catalysts operating under alkaline conditions and high potentials typically suffer structural changes, involving the oxidation of metal components and the formation of surface hydroxides and oxyhydroxides.⁵⁴ These unpredictable and often uncontrollable structural changes pose a major challenge, triggering catalyst aggregation and collapse and eventually causing activity loss. Therefore, current research focuses on maintaining a relatively stable interface during prolonged operation to enhance catalyst stability. Catalyst design strategies, such as in situ growth, reconstruction treatments, and heterogeneous structures, can be employed to mitigate catalyst dissolution and extend electrocatalytic stability.

8.1.3 Mechanism. The current understanding of the AOR mechanisms primarily involves experimental and theoretical investigations on Pt surfaces. The mechanistic studies on non-noble-metals are relatively limited. Additionally, the G–M and O–S mechanisms are currently under debate.^12,13 Gaining in-depth understanding of the reaction intermediates, fundamental reaction kinetics, and structure–activity–stability relationships of non-noble-metal catalysts presents a critical milestone toward further performance, efficiency, and stability advancements. In-depth mechanistic understanding requires a synergy between efficient and precise catalyst design and synthesis, advanced sampling and characterization techniques, and computational modelling. Due to the multiple electron transfer steps and the formation of various intermediates in the AOR, a single active site is often insufficient to optimize all reaction steps. Thus, constructing synergistic catalytic centers stands out as an effective strategy to optimize the adsorption and desorption of different intermediates. Theoretical calculations and characterization techniques can provide valuable insights into catalyst structures and intermediate adsorption, further enhancing the understanding of the reaction mechanism.

8.1.4 Reaction conditions. Operating conditions, including electrolyte concentration and pH, play a decisive role in catalytic performance.³³ Reaction conditions modulate the reaction kinetics and selectivity by tuning the micro-reaction environment. All these changes, in turn, impact the conversion efficiency and selectivity of the AOR. For example, a higher OH⁻ concentration lowers the onset potential of AOR and increases the current density. However, excessive OH⁻ concentration can also trigger side reactions (i.e., OER). Conversely, excessive ammonia concentrations can suppress local OH⁻ concentration and hinder product diffusion, negatively impacting the AOR. Rationalizing the impact of reaction conditions for non-noble-metal catalysts for various reaction durations will be critical to maximizing electrocatalytic performance, efficiency, and stability. Additionally, optimizing the operating conditions for end-use applications is critical. Thus, research efforts should also be directed at optimizing the reaction conditions considering the demands of practical applications, including application of catalysts in complex wastewater, such as landfill leachate, to overcome the limitations of laboratory-scale operations.

8.1.5 Characterization and computational studies. Although ex situ characterization methods provide static information, they typically fail to provide in-depth insights into the dynamic nature of the AOR. Operando characterization techniques can offer precise understanding of the interactions between the catalyst surface and ammonia. In situ infrared and in situ Raman spectroscopies are widely used to investigate the intermediates and evolution of active species in the AOR. In addition, methods such as temperature programmed desorption (TPD) can be used to characterize the ammonia adsorption and desorption capabilities of various non-noble-metal catalysts.¹⁰⁹ Other in situ techniques, such as in situ X-ray absorption spectroscopy (XAS), can provide detailed information on the catalyst's local coordination structure and average oxidation state,¹¹⁰ while in situ transmission electron microscopy (TEM) enables real-time observation of structural changes on the catalyst surface.¹¹¹ A detailed understanding of the catalyst structure will enable construction of more realistic structural models. Combining operando techniques with computational studies (DFT calculations) will widen our understanding of the energy barriers for each reaction step, intermediate adsorption/desorption, and electronic distribution around the active sites. The improved understanding of micro-reaction environment and reaction mechanisms will, in turn, be critical to optimizing catalytic properties, electrolyte identity, operating conditions for specific end-use applications – all guided by operando spectroscopies and computational insights.

8.1.6 Device design. Non-noble-metal catalysts overcome the scalability barriers concomitant with noble-metal AOR catalysts, providing a path to practicality. However, non-noble-metal catalysts currently suffer from poor AOR and ORR performance in DAFCs.⁶⁴ The poor AOR activity typically causes significant overpotentials at practical current densities, accelerating not only the degradation of catalysts but also the degradation of fuel cell components. In the context of electrocatalysts, designing bifunctional catalysts that could show satisfactory AOR and ORR performance under alkaline conditions will be essential to take technology a step closer to viability. In addition, conventional cation exchange membranes show high NH₃ permeability, leading to cathode contamination and efficiency loss. On the other hand, anion exchange membranes suffer poor chemical stability. Designing membranes that could mitigate these drawbacks would improve system practicality. Owing to their improved chemical stability and ability to block ammonia permeations, composite membranes offer significant potential. Another practicality limiting issue is the lack of understanding of the inherent complexity of DAFCs. This complexity requires optimization of operating conditions, such as operating temperature, ammonia feed, electrolyte concentration, and back pressure. Toward industrial-scale implementation, mitigating system-level challenges, such as corrosion of key system components including double electrodes, membranes, and catalytic interfaces, will also be critical.

8.1.7 Scale-up applications. Many successful catalysts, systems, and mechanistic and characterization studies are performed in laboratory settings. Transferring the learnings from the laboratory-scale research to scaled-up systems will be critical to identifying scaling challenges and developing effective mitigation strategies. Practicality and scalability-limiting challenges,^80,102 such as cost of catalysts, impurities in wastewater, design of reaction equipment, and energy consumption, require effective mitigation strategies. This, in turn, requires considerations from all aspects, including robust, stable, low-cost, and scalable catalysts, electrodes, systems, and process optimizations. Optimizing the proximity between the components and operating conditions will be critical to minimizing voltage losses in scaled-up systems that perform the AOR under industrially relevant conditions from industrially relevant ammonia sources.

8.1.8 Environment and safety. Despite holding practical value, the AOR, if not performed under controlled conditions with proper management strategies, might cause environmental pollution, including unwanted emissions of NO and N₂O, ammonia leakage, alkaline electrolyte treatment, organic and inorganic waste, and energy consumption.³ Effective management strategies – via optimizing catalysts, improving reaction conditions, utilizing clean energy, enhancing waste treatment, and recycling and reusing electrolytes – will be essential to mitigate environmental and sustainability issues, promoting the green and sustainable development of ammonia electrolysis technology.

8.2 Industrial production

When the AOR half-reaction meets the above challenges, industrialized development will be rapid. However, the AOR half-reaction is still in the early stage of research and development. There exists a significant gap between laboratory-scale applied performance and performance targets for large-scale industrial adaptation.⁵ It is imperative to bridge the gap between laboratory research and industrialization, which requires multidimensional synergistic innovation from catalyst design, reaction engineering, system integration, and economic evaluation.

8.2.1 Catalyst design. While NiCu is currently the best performing non-noble-metal AOR catalyst, it fails to meet the performance, stability, and scalability required for commercial applications.⁶⁸ The most urgent task is to develop large-scale synthesis approaches that could enable mass production of low-cost, high-activity, and stable NiCu-based commercial catalysts.

8.2.2 Reaction engineering. Performance evaluation of non-noble-metal catalysts is still conducted in single/two-compartment electrolytic cells. Despite being practical for rapid screening of preliminary catalytic performance, such cells intrinsically suffer from low current densities due to mass transport limitations, failing to meet industrially relevant current densities. Thus, there is an urgent need to transfer the catalyst design know-how to practical and scalable devices (i.e., flow cells and MEA electrolyzers) to thoroughly assess the practicality of new catalyst materials.⁹ Such system level investigations should also be coupled with reaction engineering approaches for practicality improvements.

8.2.3 System integration. The difference between the continuity of industrial production and intermittency of experimental production requires intelligent control systems. Such systems could be particularly useful for detecting the product distribution of AOR and the concentration of ammonia in real time. This knowledge could then guide the design of stacks and units for industrial applications.

8.2.4 Economic evaluation. Since wastewater treatment increases the cost of industrialization, there is a need to develop AOR reactants as well as product recycling and reuse technologies, aiming for a closed-loop electrolyte system.

If these challenges are properly addressed, we foresee significant potential for the application of AOR technology in large-scale installations – essential for decarbonizing the energy sector and strengthening the cornerstone of “ammonia economy”.³

Data availability

No primary research results, software or code has been included and no new data were generated or analysed as part of this review.

Author contributions

Ying Ji: conceptualization, formal analysis, investigation, methodology, writing – original draft, writing – review & editing. Adnan Ozden: investigation, methodology, writing – review & editing. Shuangqiang Chen: writing – review & editing. Chao Yang: investigation, methodology, writing – review & editing. Mingchuan Luo: conceptualization, supervision, funding acquisition, writing – review & editing. All authors reviewed and edited the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 92472121) and the Energy Revolution S&T Program of Yulin Innovation Institute of Clean Energy (No. YIICE E411090316).

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