Green Chemistry: Advanced Electrocatalysts and System Design for Ammonia Oxidation

The hazards associated with handling hydrogen fuels have driven people to consider alternative clean and sustainable fuel types. Ammonia shows significant potential for this task as both a direct fuel and hydrogen carrier due to its unique features of facile transportation and low cost. Regarding this, electrochemical ammonia oxidation reaction (AOR) is the essential process for utilizing ammonia for energy applications, either for hydrogen production via ammonia splitting or energy generation via direct ammonia fuel cells, which is highly commercially promising. On this basis, the development of high‐performance and economic electrocatalysts for AOR is critical. In this review, the kinetics and mechanism of ammonia electrooxidation are first discussed to provide a foundation to understand the current issues associated with this technology, and then a comprehensive presentation on the different types of electrocatalysts for AOR is illustrated. Afterward, an outlook is presented and the possible research directions for AOR electrocatalysis are proposed, which is expected to shed light on the future development of this promising technology.

DOI: 10.1002/sstr.202200266 The hazards associated with handling hydrogen fuels have driven people to consider alternative clean and sustainable fuel types. Ammonia shows significant potential for this task as both a direct fuel and hydrogen carrier due to its unique features of facile transportation and low cost. Regarding this, electrochemical ammonia oxidation reaction (AOR) is the essential process for utilizing ammonia for energy applications, either for hydrogen production via ammonia splitting or energy generation via direct ammonia fuel cells, which is highly commercially promising. On this basis, the development of high-performance and economic electrocatalysts for AOR is critical. In this review, the kinetics and mechanism of ammonia electrooxidation are first discussed to provide a foundation to understand the current issues associated with this technology, and then a comprehensive presentation on the different types of electrocatalysts for AOR is illustrated. Afterward, an outlook is presented and the possible research directions for AOR electrocatalysis are proposed, which is expected to shed light on the future development of this promising technology.
of ammonia fuel cells is still in its initial stage. This is mainly because the anodic AOR is more complex and sluggish than the hydrogen oxidation reaction. [2,32] Currently, a series of AOR-associated devices, such as DAFCs and ammonia-splitting plants, have emerged in both military and civil fields, [8,11] proving their high applicational feasibility. However, the widespread commercialization of these devices is still hampered by some major limitations, [4,13,16] including: 1) The sluggish reaction kinetics of AOR generally require high overpotentials, thus increasing the energy consumption for ammonia splitting and compromising the energy output of DAFCs. To overcome this, rationally designed and synthesized AOR electrocatalysts are highly demanded; 2) The development of low-cost and carbon-based catalyst materials is a highly desirable next stage of research; and 3) Catalyst recovery problem. Nowadays, most of the AOR catalysts are noble metal-based, which are difficult to recover. This is a shared problem with the current hydrogen-based fuel cells, which use Pt-based catalysts on both electrodes.
Based on these considerations, we propose a review that systematically summarizes the research on the electrocatalysts for AOR in recent years, aiming to provide new entry points for further research. First, we discuss the fundamental mechanisms of AOR. Subsequently, we analyze the catalysts of different materials, mainly including noble metal-based catalysts such as platinum, transition metal-based catalysts such as copper and nickel, and carbon material catalysts. Furthermore, through the analysis of alkaline electrolytes and organic electrolytes for AOR, we compare their advantages and potential. Finally, the challenges and perspectives for the development of AOR electrocatalysts and their practical applications are presented. It is expected this work could shed light on the future development of this promising technology.

Mechanism and Kinetics of AOR
The study of the fundamental reaction mechanism of AOR is necessary for the development and design of optimal AOR catalysts. In acidic electrolytes, ammonia exists in the form of positively charged NH 4 þ , which is difficult to form chemical bonds on the surface of electrodes. It may even show a certain repulsion to the positively charged catalyst surface at the oxidation potential, leading to low AOR efficiency. AOR is, therefore, usually a process whereby ammonia is adsorbed onto the catalyst surface in an alkaline environment and subsequently combined with OH À in the electrolyte during electrooxidation. Ideally, this process produces only nitrogen and water via a six-electron pathway with a potential of only 0.056 V in relation to the reversible hydrogen electrode (RHE), and the overall equation is given in Equation (1). The binding of ammonia in the acidic environment is discussed later in Section 4. However, undesirable byproducts, such as hydrazine and nitrogen oxides, may also be yielded in the reaction due to some side reactions. Two commonly accepted mechanisms have been proposed for the AOR reaction pathways in an alkaline environment, namely, the continuous dehydrogenation mechanism and the partial dehydrogenation mechanism.
Overall reaction equation Oswin and Salomon (O-S) mechanism Ã NH 3 þ OH À ! Ã NH 2 þ H 2 O þ e À (2) Gerischer and Maurer (G-M) mechanism Ã NH 3 þ ð3 À xÞOH À ! Ã NH x þ ð3 À xÞH 2 O þ ð3 À xÞe À ðx ¼ 1, 2Þ Early in the 1960s, Oswin and Salomon, [33] based on the experimental study of ammonia oxidation over platinum black, suggested that ammonia removes three hydrogens continuously at the electrode surface to form N atoms, after which two N atoms were coupled to generate nitrogen molecules (i.e., the continuous dehydrogenation mechanism, called O-S mechanism; Equation (2)(3)(4)(5), Figure 2a). According to this, the combination of two *N to form N═N at high current density is the decisive step in the AOR, while the intermediate at low current density to generate *NH is the key to controlling the reaction rate. In the 1970s, a different partial dehydrogenation mechanism was proposed by Gerischer  and Maurer. [34] In this process, due to the influence of the adsorbed OH À , ammonia adsorbed on the catalyst surface first dehydrogenated to form *NH x (x = 1, 2) species, which dimerize to the hydrazine-like structures (i.e., *N 2 H y , y = 2, 3, and 4), which are then smoothly oxidized to N 2 after the transition to *N 2 , and the mechanism of partial dehydrogenation is also noted as the G-M mechanism (Equation (6-9), Figure 2b). Because the N atoms formed after continuous dehydrogenation of ammonia are often strongly chemisorbed on the catalyst surface, suppressing the subsequent dimerization press. Therefore, if N ads is generated during the reaction, the catalytic sites will be blocked, resulting in catalyst poisoning and irreversible deactivation. Generally, these two mechanisms are distinguished by the sequence of N-N coupling. In the O-S mechanism, the formation of N═N occurs only after the formation of adsorbed nitrogen atoms. In comparison, N-N coupling in the G-M mechanism typically occurs in the steps following the dehydrogenation of each amino group. In addition to N 2 , the products of AOR also include NO 2 À and NO 3 À , which are often caused by the overlap of the electrochemical windows of AOR and water oxidation. Though there is no clear and well-accepted explanation for the formation of these oxygenated nitrogen species, a proper understanding of the pathways through which these products are generated during AOR is still necessary. In 2005, Miura's group investigated the AOR behavior in alkaline environments and found the presence of reaction intermediates NH 2 OH at the anode, which could be further oxidized, and oxidation products such as nitrite and nitrate may be generated via Equation (10). [35] The effect of NH 2 OH on NO 2 À , NO 3 À formation was experimentally demonstrated by Rosca et al. [36] In addition to the formation of oxygenated nitriles by NH 2 OH oxidation, some groups have suggested that such peroxides can also be formed by the catalytic oxidation of PtO x and the oxidation of adsorbed nitrogen. [37,38] In addition to the above mechanisms, insights into the effect of pH values on AOR should also be taken into account. [39] In general, as pH increases, the peak current of AOR increases linearly with respect to pH, with a corresponding decrease in the onset potential. [40,41] Koper and Katsounaros et al. found that the value of the potential shift for NH 3 oxidation to N 2 caused by an increase in solution pH did not match the 59 mV obtained based on the Nernst equation, indicating a complex role of pH on AOR. [42] In order to explain this phenomenon, they proposed a new mechanism suggesting that the AOR process underwent a deprotonation step before the electron transfer, generating negatively charged surface adsorbates, which were further oxidized in the subsequent electron transfer step. The negatively charged species was the precursor for the formation of N 2 and NO. The proposed mechanism provides a new idea to further refine the mechanism of AOR. In addition to the direct pH dependence of the reaction activity of AOR, its product types also exhibit a strong dependence on pH values. In another work carried out by Wang et al. it was pointed out that high pH leads to the accumulation of NO 3 À during prolonged electrolysis, which was detrimental to the conversion of NH 3 to N 2 . [43] In another work, the Klinkova group concluded that the Faraday efficiency of NO 3 À formation increased with decreasing pH values. [44] Therefore, a reasonable balance of the pH of the AOR process is a topic worth investigating.
The O-S and G-M mechanisms may be in competition with one another during the actual ammonia oxidation process, according to the AOR kinetics, which is correlated with the strength of nitrogen's binding to the catalyst surface. In recent years, in order to further elucidate the reaction mechanism of AOR from ammonia to nitrogen, several research groups have also resorted to computational simulations by constructing models of Pt and other metal alloys to explore the reaction kinetics during ammonia oxidation. In a representative density functional theory (DFT) study by Ishikawa and coworkers, [45] they constructed the molecular configuration of low-index faceted Pt in an alkaline environment and found that the favorable  mechanism for electrochemical AOR on Pt(100) is the G-M mechanism at low potential (< þ0.5 V vs RHE) and the high potential (≥ þ0.5 V vs RHE) via the O-S mechanism. The low catalytic activity of Pt (111) and Pt (110) was attributed to the strong adsorption of nitrogen atoms in the hollow sites, and the high activation barrier makes it difficult for N atoms to dimerize into nitrogen. This conclusion is also in agreement with another computational work carried out by Yang Bo's group. [46] At present, some computational and experimental results have provided convincing evidence for the identification of N ads as inactive surface poisons, [47][48][49][50][51] justifying the more reasonable proposal of the G-M mechanism for N ads -induced catalyst poisoning.
The adsorption of OH À species and the dehydrogenation of adsorbed NH 3 species are respective charterers for the two mechanisms, and thus they become crucial for probing the nature of AOR. Recently, Xin and co-workers reported that the dehydrogenation of *NH 2 is dependent on the actual potential and that hydrogen bonding plays an important role in stabilizing *NH. [52] In addition, recent work by Yang Bo's group also investigated the dehydrogenation mechanism in electrochemical AOR as well as the kinetic process of reactive species using DFT. [53] In their work, two complex models (i.e., low potential [LP] and high potential [HP]) containing large amounts of water molecules were constructed and combined with free energy profiles. They determined the reaction trends of the OH in water (OH bulk ) and the OH adsorbed on the catalyst's surface (OH surf ) under different conditions (Figure 3a,b). OH bulk could effectively optimize the dehydrogenation kinetics in the HP model, while OH surf had a higher dehydrogenation reaction activity under LP conditions. The possible high dehydrogenation barrier of OH bulk at low potentials and the similarity of the AOR to the OH adsorption onset potential suggested the possibility of OH surf as an active species for the dehydrogenation reactions.
In addition, other experimental techniques have also been applied to investigate the AOR kinetics. For example, the AOR kinetics over Pt catalyst in alkaline media was studied in depth by Botte et al. using a rotating disc electrode (RDE). [54] Based on the correlation between the Tafel slope and peak current density at different sweep rates (Figure 3c), they concluded that the AOR was irreversible and the reaction kinetics was controlled by the mass transfer of the reactant (NH 3 ). The measured ammonia diffusion coefficient was also confirmed by the parallelism of the experimental results of the linear sweep voltammetry (LSV) with the theoretical Levich curve (Figure 3d). It is worth noting that the AOR kinetic is controlled by the diffusion of ammonia even at high rotational speeds and low scan rates so that the AOR can be performed rapidly and studied as a transient problem.

Evaluation for AOR Electrocatalysts
The activity and selectivity of many AOR catalysts were mostly assessed by cyclic voltammogram (CV) or chronoamperograms (CP) tests. However, direct comparison and evaluation of these data are not accurate enough because both the current of the water oxidation reaction and the oxidation of the catalyst would contribute to the actual current obtained in these measurements. It is, therefore, necessary to standardize the methods to evaluate ammonia oxidation. Johnston et al. by performing comparative Figure 3. a) Modeled systems with different potential conditions with 2 potassium atoms in the LP system and no potassium atoms in the HP system. b) Free energy barriers for OH surf or OH bulk assisted dehydrogenation of NH x (x = 1, 2, or 3) in the HP or LP structure. Reproduced with permission. [53] Copyright 2021, American Chemical Society. c) I P versus υ 1/2 plots for 0, 500, 1000, and 2000 rpm of rotation. d) A comparison of the experimental RDE results at various scan rates (5, 10, 30, and 50 mV s À1 ) with the Levich equation. Reproduced with permission. [54] Copyright 2012, Elsevier Ltd. voltammetric measurements on electrolytes with or without ammonia found that the degree of AOR activity could be characterized based on the current changes before and after the addition of ammonia. [55] As shown, the presence of ammonia in the electrolyte can cause a significant increase in peak intensity, due to the adsorption and oxidation of ammonia (Figure 4a,b). Therefore, it is concluded that the appearance of the new oxidation peak or the enhancement of an existing oxidation peak upon the addition of ammonia in the electrolyte is a valid marker for the material's AOR activity. A similar approach was taken by Zhu et al. who determined the Faraday efficiency during ammonia conversion by comparing the difference in charge accumulated over time between the AOR and the charge generated by the side reaction (Figure 4c,d). [56] In addition to standardized electrochemical properties, the various conflicting conclusions obtained by different researchers also call for more reliable and accurate measurement techniques to provide more direct and convincing evidence for evaluating the activity of AOR electrocatalysts.

Rotating Disc Electrode System
The RDE system combines electrode theory with fluid dynamics to eliminate the diffusion layers effects, making it a widely used electrochemical analysis technique for accurately and quantitatively studying the kinetics of electrochemical processes and the associated formation intermediates/products. Before the utilization of RDE, the research of AOR electrocatalysis over various metals such as platinum or nickel was commonly carried out in conventional single-chamber three-electrode or three-chamber electrolytic systems. [57][58][59] In 2005, Miura et al. first investigated the mechanism of AOR on platinum electrodes using RDE, [60] and more studies on the characterization of the electrochemical properties and mechanisms of AOR have been conducted since then.
In 2017, Botte's group revisited a series of fundamental studies on AOR using RDE. [61] When ammonia was present in the electrolyte, cyclic voltammetry (CV) scans revealed that the hydrogen adsorption/desorption peak was shifted to a lower potential (from À0.59 to À0.7 V vs SHE), while the AOR initiated at roughly À0.46 V versus SHE (Figure 5a), possibly implying that the oxidation of ammonia occurred only after the desorption of hydrogen. The catalyst was preoxidized at 0.2 V versus SHE in 1 M KOH, and then the AOR performance of the electrocatalytic material after such oxidation as well as after a subsequent reduction was studied to determine the reason for catalyst deactivation at high potentials ( Figure 5b). It was found that only a very small AOR peak was presented in the oxidized Pt/C material, but it was fully recovered after reduction, indicating that the adsorption of OH À and the poisoning of AOR caused by the surface oxide of catalysts are reversible in the AOR testing potential range. The variation of hydrogen adsorption charge versus N ads surface coverage over time was then quantified because the N ads species would also hinder the hydrogen adsorption on the electrode surface ( Figure 5c). The similar trends of the decrease in hydrogen adsorption over the catalyst Reproduced with permission. [55] Copyright 2022, Wiley-VCH. c) CV profiles of activated NiCuFe (a-NiCuFe) electrode and activated NiCu (a-NiCu) electrode. d) Ammonia removal efficiency and Faradaic efficiency of activated NiCuFe electrode under different anode potential. Reproduced with permission. [56] Copyright 2021, Wiley-VCH.
surface and the decrease in the current density over the chronoamperometric AOR test also illustrated the contribution of N ads to catalyst deactivation.

Surface-Enhanced Raman Spectroscopy Study
Raman spectroscopy is another tool for characterizing the structure of molecular features, but the Raman scattering effect itself is very weak and requires the provision of a rough surface to achieve the enhancement effect, i.e., surface-enhanced Raman spectroscopy (SERS). In the study of ammonia electrooxidation intermediates over platinum in alkaline environments, Pe´rez's group attributed two distinct bands at 1338 and 2007 cm À1 in the Raman spectrum to adsorbed azide ( Figure 5d). It corresponds to the Raman-activated symmetric N-N-N stretching (ms) induced by the azide molecule and the Raman-forbidden asymmetric N-N-N stretching (mas), respectively, thus for the first time demonstrating the involvement of the adsorbed azide anion in AOR. [62] The observation of the azide intermediate possibly suggests a pathway through the reaction of N 2 H 4ads (N 2 H x ) and ammonia to generate azides and then convert them to dinitrogen.

Infrared Spectroscopy Study
Infrared (IR) spectroscopy unveils the molecular structure of materials through the selective absorption of certain wavelengths of IR light by molecules or functional groups. In situ spectroscopy can help us to get a real picture of the actual condition of the electrode surface, and it has been utilized in the study of AOR electrocatalysis. [63] Rosca's group combined electrochemical measurements with in situ IR spectroscopy to characterize the AOR on Pt (111) and Pt(100) facets. [64] It was found that N 2 O and NO were not produced during AOR in the 0.05-0.9 V range, in agreement with the conclusions of the Vidal-Iglesias group. Apart from this, Eguchi et al. also investigated the AOR behavior of Pt electrodes in aqueous alkaline solutions using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. [65] On this basis, it was found the intensity of the HNH bending characteristic band of NH 3 decreased continuously with increasing potential, while the intensity of the characteristic NH 2 wagging of N 2 H 4 band (1269 cm À1 , υ11) reached a local maximum at approximately the onset potential of hydrazine oxidation (Figure 5e,f ). This spectroscopic data was the first report of hydrazine as an intermediate product The AOR-related CV curves for the fresh Pt/C electrode, oxidized Pt/C electrode, and reduced oxidized Pt/C electrode. c) Integration of surface coverage for N ads and hydrogen adsorption peak charges as a function of respective reaction times. Reproduced with permission. [61] Copyright 2017, Elsevier Ltd. d) SER spectra of Pt nanoelectrodes in 0.2 M NaOH þ 0.1 M NH 3 solution at 0.35 V with an acquisition time of 180 s. Excitation lines: a) 514 nm; b) 632.8 nm. Reproduced with permission. [62] Copyright 2005, Elsevier B.V. e) Time-resolved IR spectra of the Pt surface captured concurrently with a linear sweep voltammogram in 0.1 M NH 3 À 1 M KOH. f ) The three typical bands' intensities at 1269, 1497-1508, and 1662-1674 cm À1 , as well as the relationship of the Pt electrode's current density on potential. Reproduced with permission. [65] Copyright 2015, American Chemical Society. in the AOR process over a Pt electrode, providing strong evidence for the G-M mechanism.

Differential Electrochemical Mass Spectrometry Study
Differential electrochemical mass spectrometry (DEMS) is a modern electrochemical testing technique that in situ detects the volatile gaseous intermediates as well as their structures in electrochemical reactions. Its application in AOR study can be traced back to 1994 and has been used by several research groups to study the reaction mechanisms and intermediates in AOR. In the early 21st century, the Vidal-Iglesias group carried out DEMS studies of AOR on the three basal planes of Pt(100), Pt (110), and Pt (111), and investigated the gaseous compounds produced during AOR. [66] They found ammonia oxidation almost exclusively occurred on the Pt(100) facet, and the main AOR product over this surface was N 2 as well as a small number of other nitrogen oxides (e.g., NO, N 2 O). Notably, they also found that the formation of toxic intermediates during ammonia oxidation would only inhibit the formation of N 2 , while the formation of N 2 O and NO was not inhibited at high potentials for surface oxide generation, which is consistent with the conclusion confirmed by Koper et al. through DEMS experiments that an oxynitride surface layer exists on the electrode surface when the electrode potential is sufficiently positive, and that oxygenated nitrogen-containing species (e.g., NO and N 2 O) are only formed when the electrode surface is oxidized. [47]

Gas Chromatography Study
Due to the competitive oxygen evolution reaction (OER) and the AOR at the anode, real-time monitoring of the gaseous products is necessary for a better understanding of the reaction process. Gas chromatography (GC) is a reliable measurement tool for evaluating and comparing the gaseous components produced during the ammonia oxidation process. Guntae Kim et al. constructed a closed liquid ammonia decomposition apparatus in which both AOR and hydrogen evolution reaction (HER) take place, and connected it directly to a gas chromatograph for systematically determining the composition of the gas product during AOR using quantitative gas chromatography under real condition ( Figure 6a). [67] They applied a constant current of 200 mA for 200 min, monitored the amount of gas produced every 20 min, and calculated the Faraday efficiency. In the initial stage of electrolysis, hydrogen and nitrogen were produced simultaneously with an average Faraday efficiency of 93%. After 100 min, however, the anodic reaction shifted from AOR to OER, and the reaction products changed to hydrogen and oxygen, and the change in the anodic reaction led to a significant increase in the energy consumption of the hydrogen production process (Figure 6b-d). Thus, it is often necessary for researchers  to distinguish AOR and OER, as the AOR operating potential overlaps with that of OER, especially for the non-noble metal catalysts.
In addition to the above characterization methods, microquantitative analysis of ammonia concentrations can be carried out by UV-vis spectrophotometry to provide a more intuitive understanding of the by-product formation during AOR, such as nitrates and nitrites. The use of a combination of characterization methods has been increasingly recognized and adopted as well, and the construction of performance evaluating systems/ platforms for AOR will not only enable direct comparison of the performance of various ammonia oxidation electrocatalysts but also provide a solid foundation for the exploration of this emerging research area of AOR.

Analysis of AOR Products
Currently, the main products of ammonia oxidation for industrial applications include N 2 , NO x, and NO x À , and we will discuss the AOR from the aspects of product types in this section. Among the various production, N 2 is considered to be the most desirable one for energy conversion because the oxidation of ammonia into N 2 and water releases a significant amount of energy without the emission of any greenhouse gases. Therefore, the design of electrode materials for DAFCs often follows this idea. For example, Wojcik et al. developed an iron-based catalyst loaded on an Ag current collector for the catalytic AOR with ammonia as the sole fuel and nitrogen as the sole product. [68] Interestingly, this DAFC exhibited the same power density as a hydrogen fuel cell. Apart from this, a large number of other AOR catalysts for DAFCs electrodes, such as Pt-based, nickel-based, and Y-stabilized zirconia materials, have been developed and studied. [25,69] Apart from targeting N 2 formation, other potentially useful products such as NO 2 À and NO 3 À (collectively referred to as NO 2/3 À in the following section of this paper) have been paid less attention to. However, NO 2/3 À plays a significant role in the agricultural, chemical, and pharmaceutical industries. Consequently, it requires the development of a sustainable nitrite/nitrate production technology for these chemicals. Johnston et al. investigated the activity of copper metal electrodes on AOR and identified two different catalytic mechanisms. [70] Specifically, homogeneous catalysis of NH 3 electrooxidation by dissolved Cu 2þ/3þ species preferentially produced NO 2 À with a Faraday efficiency of 87%. By far, the developed AOR catalysts targeting NO 2/3 À generation are mainly limited to nickel-based, copperbased, and other non-noble metal-based materials.

Electrocatalysts for the AOR
As the core component involved in an electrocatalytic process, the catalyst has always been a hot spot for researchers to explore. In the catalytic process, the main factors affecting the performance of a catalyst are interface engineering, microstructure regulation, alloy design, and topography control, which determine the activity and efficiency of a catalyst. Therefore, these parameters must be strictly controlled via particular preparation conditions. Recent progress on both homogeneous and heterogeneous catalysts will be introduced in this section, and the associated issues will be discussed as well.

Homogeneous Catalysts
Metal complexes are used as the most important AOR homogeneous catalysts due to their unique structure, remarkable efficiency, and selectivity. Metal complexes can be adjusted to ensure tight binding of NH 3 molecules to the active site by adjusting the ligand structure and ligands to achieve 100% utilization of the catalyst. The formation of multibonded species of metal-imines and metal-nitride complexes is the key process in the electrocatalytic conversion of ammonia into nitrogen with transition metal complexes. Transition metal-imide and nitride complex have emerged as a hot topic of research in homogeneous catalysts because of their weak metal-nitrogen bonds, which are caused by d-electrons in the metal center occupying the antibonding orbitals of the metal-nitrogen bonds. [71,72] For homogeneous catalysts, the formation of multiple bonds between nitride complexes and metal imide is essential for the electrocatalytic conversion of ammonia. By far, metal-imide and -nitride complex systems are the most popular bimetallic systems because of the induced synergistic effect as well as the improved affinity for N ads and other intermediates (e.g., NH 2ad , N 2 H xads ). [71,73] Meyer's group first discovered metalorganic complexes (i.e., the dinuclear Ru-organic complexes) for catalyzing AOR in 1996 and proved that [(bpy) 2 (NH 3 ) RuORu(NH 3 )(bpy) 2 ] 4þ could be exchanged for N 2 via a hydrazine pathway (Figure 7a,b). [74] Subsequently, Smith et al. confirmed that the first step of NH 3 oxidation was the oxidation reaction at the single atomic center of Ru-organic complex by the isotope labeling technique, and this electrocatalyst can efficiently convert ammonia into N 2 in DMF. Some of these homogeneous catalysts might also lead to N-N coupling, as illustrated in the stoichiometric nitride pathway in Figure 7c. [75] Apart from Ru-organic complexes, other complexes based on non-noble metals, e.g., Fe and Ni, also showed good catalytic performance for AOR. These complexes can modulate various organic structures and show potential catalytic ability for AOR.  Figure 7d). [76] This report adds a firstrow transition metal (iron) complex to the Ru catalysts. They also suggest that catalysts composed of other transition metal coordination compounds also have the ability to oxidize ammonia to N 2 , either through the hydrazine or nitride pathways, without producing other nitrogen-containing by-products in the process.
Although the study of homogeneous organic catalysts in AOR has shown their potential, however, their development has been severely hindered by various disadvantages, such as high cost, high toxicity, and difficult recovery. Therefore, more research is needed for the practical applications of these catalysts in AOR. In addition, the controllable manufacture of catalysts with well-defined coordination structures and optimal components is also necessary.

Heterogeneous Catalysts
The heterogeneous catalysts have been the main focus in the research of AOR electrocatalysis due to their advantages of easy fabrication and relatively low cost. Heterogeneous catalysts include transition metal (compounds) and carbon-based in addition to common noble metal-based materials and derivatives. This section provides a summary of recent advances in the field.

Pt-Based Catalysts
Although one of the major tasks of the AOR electrocatalyst exploration is seeking an efficient yet cheap one, however, Pt and its derivatives are still the most studied and show the best performances for AOR. Several studies have been conducted on the design of Pt-based monometallic catalysts with different structures for AOR, aiming at improving the inherent characteristic of Pt by interface engineering and optimization of alloy design. [19,77,78] As described in the previous section, platinum presents not only a lower AOR onset overpotential but also a high N ads oxidation potential and a lower NH 2ads dimerization activation energy. These characteristics make Pt an ideal model electrode for studying AOR. Mavrikakis et al. investigated the AOR mechanism on closepacked facets of Au, Ag, Cu, Pd, Pt, Ni, Ir, Co, Rh, Ru, Os, and Re, and evaluated the catalysts based on their energy efficiency and activity in fuel cells. [11] The theoretical calculation of the binding energies of the intermediates concluded that N-N bonds are formed by the combination of hydrogenated of NH x species rather than the adsorbed N atoms (Figure 8a). For Pt and Ir, in particular, the pathway with the lowest energy involves the dimerization of adsorbed NH 2 species to hydrazine, followed by its deprotonation to N 2 (Figure 8b). According to the above principle, the adsorption strength of nitrogen on different metals is as follows: Ru>Rh>Pd>Ir>Pt>Au, Ag, Cu (Figure 8c). Nevertheless, the formation of nitrogen or hydrazine on platinum is also closely related to the orientation of the metal crystal plane. Platinum-based catalysts usually expose (100), (110), and (111) crystal planes, but AOR almost exclusively takes place on the Pt(100) crystal plane, which undoubtedly increases the difficulty of the AOR reaction. To better understand the structural dependence of a Pt-based AOR electrocatalyst, and the AOR processes on Pt (111) and Pt(100) were studied by Koper et al. using voltammetry, chronoamperometry, and in situ IR spectroscopy (Figure 8d). [64] The oxidative adsorption of ammonia leads to the formation of NH x (x = 0-2) species. The oxidation of ammonia takes place in the double-layer area on Pt (111), leading to the formation of NH and possibly N adsorbates. On Pt(100), the adsorbed NH 2 species is the stable intermediate of AOR. In comparison, the oxidation of ammonia to dinitrogen on the Pt(111) hydrogen atoms are omitted for clarity. Reproduced with permission. [74] Copyright 1996, American Chemical Society. c) Hydrazine and nitride pathways in stoichiometric oxidations of NH 3 to N 2 . Reproduced with permission. [75] Copyright 2019, National Academy of Sciences. d) Molecular complexes that mediate AO. Reproduced with permission. [76] Copyright 2019, American Chemical Society. surface has extremely weak activity, thereby demonstrating that neither *NH nor the (strongly) adsorbed *N species are active for dinitrogen formation. As for the current research, the primary reasons for the outstanding AOR performances of Pt(100) crystal planes reside in 1) the relatively easy formation of *NH 2 intermediates on the crystal planes of Pt(100); 2) the highly coveted *NH 2 can be aggregated on the crystal planes of Pt(100); and 3) *OH adsorption promotes the reaction and *OH is thermodynamically stable in the Pt(100) crystal plane. [77] Despite their potentially high activity for AOR, the poisoning of Pt-based catalysts always exists, which should not be ignored. The OH ads and surface oxides may be the main reasons for the deactivation of a Pt-based AOR catalyst at a potential > À0.15 V versus RHE. [79] Katsounaros et al. investigated the reaction paths of AOR at different potentials on the Pt surface. [80] Figure 8e indicates that *N dimerization is present at high potential and *NH is a stable adsorbed species at 0.5-0.63 V. The coadsorption product *OH, which favors the dehydrogenation of intermediates, is present in the reaction path at high potentials. Thus, *NH 2 is Figure 8. a) Optimized geometry of H x NNH y species in their minimum-energy structures on Pt (111). b) Electrooxidation mechanism proposed. c) Estimated onset potential for close-packed facets of transition metals. Reproduced with permission. [11] Copyright 2015, American Chemical Society. d) Reactions on different crystal surfaces. Reproduced with permission. [64] Copyright 2006, The Royal Society of Chemistry. e) Proposed scheme for the most feasible steps during the electrochemical oxidation of ammonia on Pt(100). Reproduced with permission. [80] Copyright 2018, Elsevier. f ) TEM and SAED pattern image of Pt-Ir nanocubes. g) CVs measured on Pt-Ir nanocubes, polycrystalline Pt-Ir NPs, and Pt nanocubes, respectively, in 1 M KOH and 0.1 M ammonia solution at 0.01 V s À1 . h) CVs measured on Pt-Ir nanocubes, polycrystalline Pt-Ir NPs, and Pt nanocubes, respectively, in 0.5 M H 2 SO 4 solution at 0.05 V s À1 . Reproduced with permission. [82] Copyright 2014, American Chemical Society. easily dehydrogenated to *NH at 0.5 V, which can then dimerize easily at the fourfold hollow sites. The dimerization reactions involving *NH or *N species are kinetically and thermodynamically beneficial, suggesting that the adsorbed *N is only a precursor to the poisoning specie. Besides, the features that determine the effectiveness of AOR catalysts mainly include their affinity for nitrogen/ammonia and *OH. The high affinity for *N and *NH leads to a weak AOR initiation potential. Moreover, it leads to fast inactivation of the catalyst owing to the irreversible adsorption of *N. [10,19,81] To solve these problems of Pt catalysts, especially the rapid deactivation and high cost, binary or ternary Pt-based alloys have been developed. Alloying Pt with metallic species can adjust its surface electronic structure to alter the adsorption energy of the intermediates and reduce the reaction energy. Pt-Ir alloys present high catalytic performance in AOR and have attracted great attention in recent years. Zhong et al. reported Pt-Ir nanocubes with well-defined (100) crystal plane, and compared the AOR activity of Pt-Ir and polycrystalline Pt-Ir nanocubes for AOR (Figure 8f ). [82] The addition of Ir to Pt leads to electronic interactions between the Pt and Ir atoms in the Pt-Ir nanocubes while enhancing the lattice contraction in the crystal structure. Electrochemical measurements showed the shape and composition of the material related to the AOR activity. As shown in Figure 8g,h, the AOR activity of Pt-Ir nanocubes is much superior to that of polycrystalline Pt-Ir NPs and pure Pt nanocubes. This result demonstrates that the addition of Ir to Pt-Ir nanocubes does not destroy the highly active Pt(100) sites. Therefore, Pt-Ir nanocubes can achieve unique cooperative effects arising from possible bifunctional mechanisms and electronic effects. These results emphasize the significance of simultaneous control of the composition and shape of Pt-Ir NPs as an effective approach to enhancing electrocatalytic activity.
Besides Ir, Pt can also be alloyed by other metals, such as Cu, Ni, and Zn. These minor metal species, together with the carefully designed structures, contribute to the excellent catalytic performance of AOR. Through careful design of the structures of these alloys can contribute to the excellent catalytic performance of AOR. To date, such binary or ternary platinum-based alloys have been synthesized in a controlled manner into a variety of structures, and they have superior outstanding electrochemical properties. Lin et al. reported a hyperbranched concave octahedral nanodendrite containing PtIrCu alloy nanocrystals with high-index facets using a solvothermal method. This electrocatalytic material has abundant PtIr edge structures, strong lattice strain, and nanodendrites with high-index facets, resulting in high AOR activity and stability in alkaline electrolytes. Its high Figure 9. a) TEM image and electrochemical measurement of PtIrCu alloy nanocrystals (inset is corresponding 3D models). Reproduced with permission. [83] Copyright 2021, Elsevier. b) TEM image (inset is the elemental mapping). c) Electrochemical measurement of PtNi alloy nanoflowers. Reproduced with permission. [23] Copyright 2016, The Royal Society of Chemistry. d) SEM image. e) Electrochemical measurement of alloy nanofiber. Reproduced with permission. [84] Copyright 2019, The Royal Society of Chemistry. mass activity of 40.6 A g À1 at 0.5 V versus RHE is 10.3 times higher than that of commercial Pt/C (Figure 9a). [83] Another work from Liu et al. synthesized Pt-decorated and flower-like Ni particles that have a highly porous structure, exhibiting a high mass activity of 75.32 mA mg À1 , which is twofold higher than commercial Pt/C catalysts (Figure 9b,c). [23] Another Cu-Pt bimetallic alloy fiber with less than 3 wt% Pt was deposited on carbon filaments, which also showed a superior electrocatalytic performance to pure Pt. [84] For the Cu-Pt bimetallic alloy system, its onset potential was at À0.65 V versus Hg/HgO and reached the peak at À0.4 V versus Hg/HgO, and this outstanding AOR performance was attributed to the formation of an oxide layer on copper (Cu 2 O) in an alkaline KOH electrolyte (Figure 9d-e). Compared with pure Pt, Cu-Pt alloy fibers also suffered less from electrode poisoning and showed good mechanical binding to carbon fiber.
In addition to the chemical composition of the catalyst, its microstructure also indirectly affects the electrochemical activity by influencing factors such as the ion transfer efficiency and the number as well as the type of active sites. Therefore, such parameters must be strictly controlled via particular preparation conditions. Currently, various morphological catalysts (e.g., nanoflowers, [23] nanospheres, [85] nanofiber, [84] and nanocubes [82] ) have been developed to optimize catalytic performance by exposing certain active crystalline surfaces to increase the density of active sites on the material surface or to modulate the selectivity of metal nanoparticles. For example, Hu et al. used electrodeposition to form nanoporous platinum with coral-like nanowire structures on carbon fiber cloth using phytantriol lyotropic liquid crystals as a template. [86] The electrodes obtained consist of ordered in situ-grown Pt nanowires, of which individual Pt nanowires possess a large number of nanopores (Figure 10a-c). DFT calculations showed that Pt atoms are easier to be absorbed on the Pt(111) facet rather than carbon surface (Figure 10d), thus resulting in the radial growth of Pt nanowires away from the Pt nucleus and eventually the forming of coral-like platinum nanowires. The obtained coral-like Pt nanowires with nanoporous structure provided more electrocatalytic active sites due to their inherent large electrochemically active surface area (43.1 m 2 g À1 , almost twice of commercial Pt/C) and short ion/ electron transport pathways. Consequently, it displayed superior catalytic activity with a high AOR current up to 72.0 mA mg À1 , much higher than that of the commercial Pt/C (26.2 mA mg À1 ). In addition, after 100 cycles of CV testing, a high AOR peak current retention of 85.6% was achieved (Figure 10e-f ), which clearly demonstrates the effectiveness of interface regulation in promoting the material's catalytic stability.
Briefly, a range of Pt-based catalysts have been covered, and their high AOR catalytic performance is associated with the interface engineering, optimization of alloy design, and well-defined structures to facilitate AOR in various media. These structures (e.g., nanoflower, nanocubes, nanocrystals, and nanowires) have high porosity, high surface area, abundant active sites, and excellent stability. Besides, the alloy design can effectively solve the  (111) and carbon surfaces. e) CV curves of coral-like Pt nanowires and commercial Pt/C. f ) The 1st, 25th, 50th, 75th, and 100th cycle of CV curves of coral-like Pt nanowires. Reproduced with permission. [86] Copyright 2021, Wiley-VCH.
www.advancedsciencenews.com www.small-structures.com problem of Pt poisoning. Meanwhile, the rational design of multimetallic components brings about high index facets, electronic effect, geometric effect, and synergistic effect, which greatly improves the performance of the catalyst. Moreover, the continuity of the catalytic process can be well maintained by optimizing the specific crystal plane. Despite these, more investigations are still necessary to explore the low-cost catalysis process.

Ni-Based Electrodes
Compared with the noble metal, the non-noble metal ones are more adequate, cheaper, and easier to produce on a large scale, making them quite promising for a variety of applications. However, the current of oxidation of the non-noble metal or the carbon catalysts can influence the assessment of AOR when evaluating AOR activity with CV curves. To avoid such interferences, researchers have calculated the Faraday efficiency of AOR by testing the oxidation current of the catalyst itself to eliminate the interference. [85] Ni is one of the most studied AOR electrocatalysts, which has shown strong catalytic activity for oxidative urea degradation as well. [87] Moreover, Ni alloys and composites exhibit excellent AOR catalytic properties due to synergistic effects and electronic structure changes. However, it is difficult to finely adjust the microstructures precisely during synthesis. Therefore, the controllable preparation of these alloys remains challenging. In this section, we report the latest advances in this field. An early study involving Ni-based catalysts used pure nickel plates. As ammonia concentration increased, the oxidation peak shifted positively, and the current density decreased. Whereafter, the adsorption and oxidation processes of ammonia on the Ni electrode were analyzed by semiconductor electrochemistry theory and point defect model. This work thus provided an insight into the feasibility of nickel as an electrocatalyst for AOR, and later the corresponding Ni-based AOR catalytic materials began to undergo a large-scale derivative era. [59] Table 1 compares the properties of noble metal and nonmetal catalysts. Till now, for AOR, Ni-based catalysts have been able to produce excellent performance and are gradually replacing noble metal-based catalysts materials.
Theoretically, copper is considered to be an excellent catalyst for AOR. [11] In the investigated first-row transition elements, NO 3 À produced during AOR and ammonia in the electrolyte will corrode the copper-based catalyst. Therefore, if one wants to obtain a non-noble metal-based catalyst with good stability in the AOR process, another metal is often added to enhance the corrosion resistance of the materials. [88] As mentioned in the previous section, Ni noble metal-based alloy materials have made good progress in the research of poisoning resistance and corrosion resistance. Therefore, it is reasonable to incorporate Ni into the Cu-based catalysts to alleviate their intrinsic disadvantages. For example, Xu et al. simultaneously deposited Ni and Cu metals at the surface of carbon paper by electrodeposition (Figure 11a-c). The stability and activity of the catalyst are largely improved in comparison with sole Ni or Cu catalysts. Moreover, this noble metal-free NiCu catalyst even performed better than Pt/C catalyst, as NiCu is not easily poisoned by ammonia (ammonia removal efficiency of %80% and coulombic efficiency up to %92%, Figure 11d,e). [85] In addition to alloy engineering, research has also been focused on the microstructure design of Ni-based alloys. For example, Liu et al. developed a macroporous Ni foam electrode integrated with vertically aligned and mesoporous Ni 2 P nanosheets, which is supposed to create sufficient exposure of active centers (Figure 11f-h). [89] The AOR mechanism results from direct oxidation by the high-valence Ni, which is significantly different from the conventional indirect active-chlorinespecies-mediated oxidation (Figure 11i). Liu et al. proposed an intermediate-free AOR mechanism, in which NH 3 was first adsorbed on the NiOOH species at the surface of Ni 2 P nanosheets, and then the H atoms of the adsorbed NH 3 were cleared by interacting with OH À groups near the catalyst surface. Then, the neighboring *NH 2 species readily binds to the freshly formed NH x (x = 1 or 2) group and is gradually dehydrogenated by OH À , which then produces/desorbs N 2 and regenerates the Ni (II) species ( Figure 11j). Thus, the AOR on Ni surface tends to adhere to www.advancedsciencenews.com www.small-structures.com the hydrazine route of formation, i.e., the route of minimum energy. However, a portion of (NH 3 ) ads would react with OH À and be over oxidized to form NO 2 À and NO 3 À species, which might gradually convert the NiP into inactive NiO x species and compromise the material's catalytic performance for AOR. In response to restraining the activity loss of nickel-based catalysts, the contribution of metal compounds (e.g., MO x , [65] M(OH) x , [90] MC x [91] ) to their doping modification was also studied. Huang et al. reported a nanostructured catalyst of Cu 2 O wire-in-Ni(OH) 2 plate passivated by a thin CuO surface, which can stably electrolyze alkaline ammonia solution into hydrogen and nitrogen at a high current density of 80 mA cm À2 (Figure 12a,f ). [92] The excellent performance obtained is attributed to the unique wire-wrapped plate nanostructures with Ni(OH) 2 -Cu 2 O and Cu 2 O@CuO double interfaces. The former induces a Ni-Cu synergies and improves the catalytic activity, whereas the latter provides protection of Cu 2 O nanowires from oxidation and dissolution in the electrolyte and enhances durability (Figure 12b-d,g).
Although the modification of Ni-based catalysts has made great progress, the nature of catalytic AOR by nickel-based materials remains ambiguous and difficult to understand. In the future, researchers will focus on the analysis of electrode materials from the following aspects, e.g., the properties of electrode intermediates. The relationship between catalyst structure and catalytic performance and corrosion protection of electrode were studied. To achieve this goal, in situ technologies also have the potential to provide insight into the changes in reactions and provide valuable mechanistic information.

Carbon-Based Electrodes
Carbon-based catalysts are the most environment-friendly catalysts at present. With the in-depth study of the catalytic mechanism of AOR, graphene is considered one of the suitable candidates to replace metal-based catalysts. In recent years, Zhou et al. reported a novel free-standing 3D porous N-doped graphene aerogel (NGA). [93] This novel free-standing 3D porous NGA monolith is expected to be a potential and promising material for AOR electrocatalysis. Approximately 3.9 mL of N 2 and 8.5 mL of H 2 were obtained using this 3D porous NGA monolith loaded with Pt nanoparticles as both the cathode and anode at a cell voltage of 0.8 V for 3 h, corresponding to a cell efficiency of 68.5%. Furthermore, this 3D porous Pt/NGA monolith anode offered high resistance to the poisoning effect and ensured the sustainable and stable generation of hydrogen in AOR. Subsequently, Udert et al. found that the AOR process over the NGA catalyst occurred at moderate potentials (1-1.6 V vs SHE), which effectively prevented graphite from being oxidized. [16] Potentiostatic electrolysis also confirmed the antitoxic capability of graphite against N ads , which resulted in a high ammonia removal rate on the graphite surface that was even slightly higher than the traditional biological surface-based nitrogen removal system.
Briefly, homogeneous catalysts represented by metal complexes and heterogeneous catalysts represented by noble metal-based, non-noble metal-based, and carbon-based materials were summarized in this part, which all showed certain capabilities for AOR electrocatalysis. However, the existing studies are still preliminary, and alternative catalysts with sufficiently high Reproduced with permission. [85] Copyright 2018, Elsevier. SEM image of the as-prepared flow-through electrode with f ) macrochannel, g) mesochannel, h) microchannel, i) operando Raman spectra, and j) proposed mechanism for Ni 2 P-s/NF during the AOR process. Reproduced with permission. [89] Copyright 2021, American Chemical Society.
www.advancedsciencenews.com www.small-structures.com AOR catalytic activity are still necessary. Therefore, developing heterogeneous catalysts with controllable structure, better activity, stability, and low cost still requires attention. On the other hand, the study of the reaction mechanism over these catalysts is also helpful in solving the problem of catalyst poisoning.

Electrolytes for AOR
In general, alkaline aqueous electrolytes are still the most widely utilized in the research of AOR electrocatalysis because ammonia can effectively maintain its molecular form in alkaline aqueous electrolytes, generally resulting in a higher AOR efficiency. In comparison, the indirect oxidation of ammonia in acidic media is a very slow process due to the repulsion of NH 4 þ from the electrodes in acidic electrolytes and the presence of electrode corrosion. It is also worth mentioning that nonaqueous electrolytes have shown great potential for AOR because they can avoid issues of narrow AOR potential windows, undesirable water oxidation side reactions, and the formation of by-products such as nitrates. In this section, we give a brief overview of the acidic aqueous and nonaqueous electrolytes that will be presented to provide researchers with a more comprehensive perspective.
It has been shown that the electrochemical AOR in acidic media is an indirect process with various intermediates. Kapałka and co-workers studied the AOR behavior in perchlorate electrolytes on boron-doped diamond (BDD) electrodes. [94] During ammonia electrolysis at low pH in the presence of chlorine, the active chlorine and chlorate maintained a low concentration distribution, and the nitrated product was detected. This suggests the possible participation of chlorine in the AOR and the absence of chlorate-related by-products. Another study by  [92] Copyright 2020, American Chemical Society. Gendel's group also supports the formation of mediators in an acidic environment during the indirect oxidation of ammonia. [95] In their report, the NCl 3 mediator was determined as the main reactive substance at the anode, and the actual AOR involved the continuous decomposition of NCl 3 to form N 2 , NH 2 Cl, and NHCl 2 , which was, in turn, generated by the oxidation of NH 2 Cl and NHCl 2 . Nonaqueous electrolytes can be divided into two main categories: ammonia-saturated organic solvent electrolytes and liquid ammonia electrolytes. Because they are generally less corrosive to the transport devices and do not induce water oxidation side reactions, they can effectively prevent the generation of oxygenated nitrogen by-products and have gradually become an alternative to the conventional alkaline aqueous electrolytes. At present, developing high-performance nonaqueous electrolytes for AOR is challenging due to the fact that ammonia oxidation mechanisms in different nonaqueous electrolytes are often quite different as well. The earliest attempt for nonaqueous electrolytes can be traced back to the work published by the Compton group in 2004, [96] in which the ammonia oxidation pathway of Pt electrodes in dimethylformamide (DMF) and the room temperature ionic liquid (1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) was investigated. Based on the appearance of oxidation peaks in the CV curve and a completely new reduction peak (Figure 13a), they concluded that ammonia oxidation proceeded via a pathway of preferential oxidation to ammonium ions and then deprotonation to form ammonia with protons in this electrolyte (Equation (11)-(13)).
The ammonia-saturated acetonitrile solution is one of the most studied and applied nonaqueous electrolytes. [97,98] Li Xiao et al. investigated the surface states of different electrodes in acetonitrile solutions and obtained completely different conclusions from those in alkaline aqueous electrolytes. [99] They found that Pt electrodes consistently exhibited N 2 -producing AOR activity due to the absence of water oxidation. More interestingly, it was found that Pd was equally active for AOR in this nonaqueous media, which is very different from the results obtained in potassium hydroxide solutions. They attributed this to the passivation of the electrode surface caused by the alkaline aqueous solution. It is easy to see from the above studies that nonaqueous electrolytes are effective for alleviating the catalyst poisoning phenomenon. However, we still have not yet achieved a common understanding of the mechanism of nonaqueous electrolyte AOR, and further investigation in this field is necessary.

Direct Ammonia Fuel Cells
In the previous sections, the mechanism of AOR over various electrocatalysts has been studied. To utilize this clean and sustainable energy source, the development of the corresponding devices is also essential. DAFCs using ammonia as fuel were first proposed in the 1960s, with a maximum power density of 50 mW cm À2 under an operation temperature of 120°C. The current theoretical research on DAFCs is mainly focused on ammoniafed solid oxide fuel cells, and we will focus on this part below.

Ammonia-Fed Solid Oxide Fuel Cells
Ammonia-fed solid oxide fuel cells (ammonia-fed SOFCs) possess a high-level energy conversion efficiency, low environmental impact, and excellent fuel flexibility, and have been shown to perform in the same way as hydrogen fuel. The process of energy conversion is accompanied by the splitting of ammonia into N 2 and H 2 at the anode, which often requires a high temperature, thus often resulting in the low efficiency of the catalyst.
Wu et al. classified SOFCs into proton exchange membrane SOFCs (SOFCs-H) and anion exchange membrane SOFCs (SOFCs-O) according to the type of membrane used in the cell. [10] The hydrogen produced at the anode of SOFCs-H splits into H þ and electrons, and the H þ crosses the proton exchange membrane and reacts with oxygen at the cathode to produce water. In contrast, for SOFCs-O, oxygen is catalytically reduced into O 2À , which then crosses the anion exchange membrane and reacts with H 2 at the anode to form water (Figure 13b,c). Commonly, proton exchange membrane possesses a higher ionic conductivity compared to anion exchange membrane at low temperatures, resulting in higher power output of SOFCs-H than SOFCs-O. Consequently, SOFCs-H has attracted more attention than SOFCs-O at the current stage. [88,100] To better understand the mechanism of SOFCs-H, Ni et al. established an electrochemical model of SOFCs using NH 3 and H 2 as the fuel. The difference in power output between the NH 3 -fed and the H 2 -fed was experimentally observed to be small in SOFCs-H. In the SOFC-H, H 2 O is produced in the cathode, which enables complete fuel utilization on the one hand, but dilutes the concentration of O 2 and impedes the diffusion of O 2 to the reaction sites on the other hand. Thus, the cathode concentration overpotential is an important contributor to the voltage loss in the NH 3 -fed SOFC-H.

Sustainable Energy Network for DAFCs
Combined with the current research background and development of DAFCs, a carbon-free and sustainable energy network with the AOR as the core should be established in the future ( Figure 14). The energy produced by DAFCs can be directly transported, stored, and accessible to terminal users, such as automobiles, residential production, etc. And the AOR product N 2 can be resynthesized into ammonia to complete this cycle. Furthermore, the ammonia-rich wastewater from the waste gas of factories, as well as domestic sewage, is utilized as raw materials to prepare ammonia fuel. The entire cycle is energy recycling and utilization.

Conclusion and Outlook
The energy and environment crisis caused by the massive consumption of fossil fuels is increasingly encouraging the development of a clean and sustainable energy carrier. Ammonia is an ideal candidate for this task due to its unique characteristics, including high energy density, facile storage and transport, mature synthesis, and well-established worldwide infrastructure, which gives it significant potential as a new carbon-free energy carrier for solving the current energy-related issues together with a serial of environmental problems. To fully utilize the advantages of ammonia-based energy sources, electrochemical AOR is one of the core processes, which requires high-performance, durable, and low-cost electrocatalysts.
An in-depth understanding of the AOR mechanism can further guide the synthesis of high-performance catalysts. Currently, researchers generally agree that the AOR mechanism should follow either the O-S or the G-M pathway. In them, the  G-M mechanism is more strongly supported by experimental data, as evidenced by the catalyst poisoning caused by the high adsorption strength of *N. In addition, through extensive computational research and experimental work, platinum has been identified as the best monometallic catalyst for AOR, and a series of Pt-based high-performance AOR catalysts have been designed by microscopic morphology optimization, alloying, and surface treatment. [101][102][103][104] Other newly emerged catalysts, such as nonnoble metal catalysts, carbon-based catalysts, and homogeneous catalysts, have also been explored because of their low-cost, corrosion resistance, and high atomic utilization. In addition to energy-related applications, AOR also holds great promise for DAFCs and ammonia removal system applications. Despite these research progresses, challenges still exist. Below we briefly summarized the current issues and the corresponding strategies, and the outlook for future research on the efficient exploration of AOR catalysts is also presented.

The Reaction Mechanism is Still not Fully Uncovered
At present, although many research groups have made great efforts to understand the nature of AOR, a shared agreement about the actual AOR mechanism has still not yet been reached, and there is still even controversial conclusion regarding the intermediates of AOR, the mechanism of catalyst poisoning, and the reaction mechanism of selective N 2 production. Meanwhile, most of the current studies on the mechanism are based on the study of the reaction process over Pt or other noble metals. However, only a few studies have been conducted on non-noble metals, such as Ni and Cu, [39] which can hardly provide a sufficiently clear and comprehensive perspective of the AOR mechanism. In view of this situation, it is not only necessary to summarize and consider the existing research on the reaction mechanism in greater depth but also essential to collect more data on the AOR over non-noble metal catalysts and nonaqueous electrolytes using a combination of computational simulation and advanced characterization techniques ( Figure 15). This should be the key to establishing an intrinsic link of the AOR mechanisms over noble metal and non-noble metal systems, or in aqueous and nonaqueous systems, and to provide theoretical guidance for the optimal design of catalysts.

Electrocatalysts Need to be Further Developed
In the last decade or so, most of the research on AOR catalysts has been focused on Pt and Pt-based alloys, with relatively fewer research on Ir, Ni, Cu, Fe, and metal-free systems, which have only gained more attention in recent years. [76,105,106] The high cost of noble metals and the problem of rapid deactivation have limited their application for AOR electrocatalysis in real life, while those non-noble metals, such as Cu and Ni, often suffer from severe side reactions with by-products such as nitrates and nitrites as well as low ammonia oxidation activity. So the exploration for cheap, stable, and durable AOR catalysts is still the focus of current research. Numerous studies have reported that Pt(100) is a highly selective and active AOR. Therefore, for Pt-containing materials, surface engineering and templating for oriented crystal growth and designing catalysts with shorter ion/electron transport pathways but more active sites should be paid attention to. In addition, the Pt loading may also be reduced by designing high-entropy Pt alloys. For the non-noble metal systems, fabricating metal phosphides, sulfides, and nitrides as well as rationally manipulating their structure to obtain core-shell materials or multivacancy nanomaterials may be potentially beneficial for AOR applications ( Figure 15). Currently, theoretical calculations have been carried out investigating the AOR process over metalloporphyrins, [107] and it has been found further tuning of their coordination structures and optimization of composition may provide new pathways for AOR electrocatalysis. In addition, AOR catalysts that are specifically designed for nonaqueous electrolyte systems should receive more attention as well.
It is of great practical significance to build a sustainable energy network with AOR as the core reaction ( Figure 15). Unfortunately, very rare research has been conducted in this regard. Specifically, DAFCs using low-temperature polymerbased anion exchange membrane electrolytes tend to suffer from low reaction rates, while high-temperature ones using solid oxide electrolytes have long starting times and can lead to severe thermal degradation of the material. In addition, other critical factors, such as the operation temperature, are often not clarified in the current work, which also seriously affects the application of DAFCs. Consequently, it is necessary to construct a sustainable green ammonia cycling system in the large view as well as suitable DAFCs devices and ammonia removal systems too.
In summary, AOR holds great promise for DAFCs and ammonia removal system applications. In this article, a systematic review and summary of the reaction mechanism, catalyst design, and DAFCs applications are presented to provide a comprehensive understanding of the latest development in this important  field. Research into the use of ammonia oxidation for building devices to address energy and environmental issues is, however, still at an early stage, and significant challenges still exist. It is expected that this review and the proposed strategies will shed light on the roadmap for the exploration of novel electrocatalysts for AOR and other critical reactions.
Yunrui Tian received her master's degree from Taiyuan University of Technology. Currently, she is a Ph.D. student at the Institute of Materials Science and Engineering, Tianjin University, supervised by Prof. Ji Liang. She is focusing on the design and fabrication of advanced nanomaterials for renewable energy conversion and storage.
Zixian Mao is currently pursuing her master's degree at the Institute of Advanced Ceramics, Tianjin University, under the supervision of Prof. Ji Liang. Her research interests focus on the design and fabrication of advanced nanomaterials for electrochemical catalysis.