Recent advances in metal single-atom catalysts for ammonia electrosynthesis

Abstract

Electrochemical ammonia synthesis is a promising alternative to the Haber–Bosch process, offering significant potential for sustainable agricultural production and the development of portable, carbon-free energy carriers. The development of electrocatalytic systems is currently dependent on the exploration of electrocatalysts with high activity, selectivity, and stability. Metal single-atom catalysts (SACs) have become a new attractive frontier for ammonia electrosynthesis, owing to their maximized atom utilization, unsaturated atom coordination, and tunable electronic structure. In this review, we focused on different metal sites inside the single-atom catalysts and summarized recent advances in SACs for ammonia electrosynthesis. The properties of small nitrogenous substances (including N₂, NO, NO₂⁻, and NO₃⁻) are summarized. In addition, the SACs for different catalytic systems are reviewed, with a particular focus on the special and common grounds of metal atom sites. Finally, the perspectives and challenges of SACs for ammonia electrosynthesis are comprehensively discussed, aspiring to provide insights into the development of electrochemical ammonia synthesis.

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Wider impact

The electrocatalytic conversion of nitrogenous substances (N₂, NO, NO₂⁻, and NO₃⁻) to ammonia represents a promising alternative to the Haber–Bosch process. Metal single-atom materials with high activity and ammonia selectivity have attracted significant attention in materials science and electrochemical ammonia synthesis due to the exposure of sufficient active sites and the adjustment of the electronic structure and microenvironment. This review provides a concise comparison of the physicochemical properties of diverse nitrogenous substances (including N₂, NO, NO₂⁻ and NO₃⁻) and offers a synopsis of the impact of these properties on ammonia electrosynthesis. Additionally, it summarizes the frequently used metal active sites in metal single-atom catalysts and their roles in ammonia electrosynthesis. Finally, the review comprehensively discusses the problems faced by metal single-atom catalysts in ammonia electrosynthesis and future research directions. These advancements have profound implications for the design of electrocatalytic materials at the atomic scale and the study of fundamental mechanisms of catalytic conversions of nitrogenous substances. This review offers potential solutions for green ammonia electrosynthesis and other electrochemical energy conversions.

1. Introduction

Ammonia (NH₃) plays a pivotal role in the global economy and has numerous applications in various fields, including fertilizer production. It also represents a future carbon–neutral hydrogen carrier with a high energy density.^1,2 Currently, approximately 90% of the global NH₃ production is produced through the century-old Haber–Bosch (H–B) process, which is regarded as the most significant innovation of the 20th century.³ However, the continued application of the existing energy- and capital-intensive H–B process NH₃ production is limited by high temperatures, extremely high pressures, and excessive CO₂ emissions.⁴ These limitations make the H–B process ineffective in meeting the future production demands of NH₃. Therefore, exploring a sustainable alternative to the conventional H–B process has attracted considerable attention from researchers.

NH₃ electrosynthesis has been identified as a potential solution to the dual challenges of energy crisis and environmental pollution (Fig. 1). This process will complement the H–B process as an additional means of NH₃ production. In natural nitrogen cycles, abundant and readily available main forms of nitrogen (N₂, NO, NO₂⁻, and NO₃⁻) are considered promising raw materials for NH₃ electrosynthesis.^5–8 Furthermore, NH₃ electrosynthesis is performed under environmental conditions, and it uses clean and renewable electric energy, making it a green and sustainable method.⁹ However, in the electrocatalytic process, the multitude of H⁺ and e⁻-transfer reactions, characterized by slow kinetics, ultimately result in an ultra-low NH₃ yield rate. Additionally, the competitive dynamics of the favorable two-electron hydrogen evolution reaction (HER) lead to low Faraday efficiency (FE) and selectivity in the NH₃ production.¹⁰

Fig. 1 The transformation of nitrogenous substances into NH₃.

Recently, the development of highly efficient electrocatalysts has been a crucial research focus for advancing “green NH₃” production technology. Several metals (including Pt, Ru, Cu, and Ti) and their compounds exhibit catalytic activity.^11–13 With further advancements in research, the design of metal active sites has become critical in optimizing the catalytic activity of electrocatalytic materials.¹⁴ Metal single-atom catalysts (SACs) have become an emerging research area, attracting much interest in the field of NH₃ electrosynthesis. Especially, SACs (in which metal atoms are atomically dispersed on supports) provide great potential to achieve high NH₃ selectivity and a high NH₃ yield rate due to the exposure of sufficient active sites and the adjustment of the electronic structure and microenvironment of the overall catalytic materials.¹⁵ They often exhibit higher selectivity than bulk metals due to their effective suppression of the HER through the ensemble effect.¹⁶ Furthermore, the well-defined atomic-level structure of SACs is promising as suitable candidates for studying the fundamental mechanisms of catalytic conversions of nitrogenous substrates, serving as a model for monitoring structural evolution at the atomic scale and the catalytic behavior of the reaction center.^17–21 The metal atoms in SACs usually have an unsaturated coordination environment, making their interactions with the reactants more flexible and efficient.²² The adsorption and activation of small nitrogenous substances can be enhanced through the engineering of the coordination environments of active single atoms. The energy barriers of different proton–electron transfer steps can be selectively adjusted, offering opportunities for further enhancing the selectivity of NH₃ production under various conditions.^23,24

In this review, we outline the importance of recent developments in these SACs for NH₃ electrosynthesis. First, we compare the characteristics of different nitrogenous substances (N₂, NO, NO₂⁻, and NO₃⁻), and summarize the advantages of the electroreduction process for NH₃ electrosynthesis and the technical difficulties associated with this process. Additionally, we summarize the advantages of SACs in each electrocatalytic reaction. Second, we provide a comprehensive overview of the research progress made by introducing different metal sites in different electrocatalytic reactions, and summarize the possible effect of frequently-used metal active sites in each reaction. Finally, we summarize the problems faced by SACs in NH₃ electrosynthesis and present future research directions. This review will provide insight into the development of advanced SACs for NH₃ electrosynthesis and other electrochemical energy conversion fields.

2. Comparison of different nitrogenous substances for NH₃ electrosynthesis

The efficient NH₃ electrosynthesis is influenced by both thermodynamics and kinetics. Thus, the features of the nitrogenous reactants, reaction path, and reaction energy barrier need to be considered. The basic features of small nitrogenous substances are shown in Fig. 2. First, dinitrogen (N₂) is derived from the natural atmosphere and can be used without any limitations, making it the optimal reactant.²⁵ Nitrite ion (NO₂⁻) and nitrate ion (NO₃⁻) are major water pollutants.²⁶ The excessive use of nitrogen-rich fertilizers, the improper discharge of untreated municipal sewage and industrial wastewater, leachate from landfill sites, and poorly constructed septic tanks and leach pits mainly contribute to nitrate accumulation in water bodies.²⁷ Nitric oxide (NO) is recognized as a major component of atmospheric pollutants, and its primary anthropogenic source is fuel combustion in machinery, such as power plants, vehicles, aircraft, and internal combustion engines.²⁸ During the combustion process, N₂ in the fuel reacts with O₂ at high temperatures to form NO, which is subsequently released and then reacts with O₂ to form NO₂. Furthermore, the industrial manufacturing and agricultural activities associated with nitrogen-containing products, such as the production of nitric acid, nitrates, and gunpowder, also result in the emission of a certain quantity of nitric oxide gases. The uncontrolled release of NO₃⁻, NO₂⁻, and NO into the natural environment can significantly disrupt the nitrogen cycle, resulting in adverse environmental impacts and posing a threat to human health.²⁹ Therefore, converting these nitrogenous substances into NH₃ exemplifies the “waste-to-valuables conversion” concept.

Fig. 2 Comparison of the basic properties of nitrogenous substances and standard reduction potentials (E₀) for the major electrocatalytic NRR, NORR, NO₂⁻RR, NO₃⁻RR, and HER in an aqueous electrolyte under standard conditions.

Second, NO₃⁻ has the highest valence state of +5. Consequently, it entails a more intricate process involving multiple reactions, intermediates, and products that span a wide range of nitrogen valence states (from +5 to −3). Third, the highly inert N₂ has the highest bond energy (941 kJ mol⁻¹) from the N≡N triple bond (the first bond dissociation energy is 410 kJ mol⁻¹), followed by NO with a lower bond dissociation energy. Compared with NO, NO₂⁻ and NO₃⁻ have the lowest bond dissociation energy of 204 kJ mol⁻¹.^30,31 Therefore, the adsorption and activation of NO₂⁻ and NO₃⁻ on the catalyst surface may be more favorable because N–N separation is challenging. Finally, the water solubility of these nitrogenous substances has been compared. The results show that NO₂⁻ and NO₃⁻ are highly soluble, while N₂ and NO are insoluble. Therefore, NO₂⁻ and NO₃⁻ are more favorable for achieving a high reaction rate and rapid NH₃ electrosynthesis, and the mass transport limitation of NO and N₂ in the electrolyte is a critical issue.

For NH₃ electrosynthesis, the reactions and the corresponding standard reduction potentials (E₀) for the electroreduction of different nitrogenous substances to NH₃ are listed in Fig. 2. The E₀ values of the NO reduction reaction (NORR), the NO₂⁻ reduction reaction (NO₂⁻RR), and the NO₃⁻ reduction reaction (NO₃⁻RR) are more positive than those of the N₂ reduction reaction (NRR) [0.09 V vs. reversible hydrogen electrode (RHE)] and HER (0.00 V vs. RHE), indicating that they are thermodynamically easier to achieve than NRR.³² This indicates that the theoretical potential range of NORR is significantly larger than that of NRR in the absence of HER interference. Therefore, the electrocatalytic NORR, NO₂⁻RR, and NO₃⁻RR provide an excellent opportunity for NH₃ electrosynthesis.

In the electrochemical process, the key stages of the NH₃ electrosynthesis can be broadly divided into three categories: (1) the adsorption of nitrogenous substances, (2) the reaction of intermediates on the catalyst surface (multi-step reduction and oxidation), and (3) the desorption of NH₃ molecules. Based on the density functional theory (DFT), the Gibbs free energy of each reaction coordinate state represents the spontaneity of each reaction step.³³ Notably, because of the multistep proton–electron transfer process and multiple intermediates of these reactions, the reaction mechanisms are different. In the NRR process, the three main pathways are dissociative, associative distal, and associative altering pathways (Fig. 3a).^34,35 The dissociative pathway usually occurs in the H–B process. N₂ adsorption takes a side-on configuration on the active site, and then N₂ activation entails the N≡N bond breakage, generating an isolated active nitrogen (*N) atom on the active site. Since the N≡N bond is very stable, this reaction pathway requires significant energy consumption, which is not suitable for sustainable development. Comparatively, more NH₃ electrosynthesis reactions catalyzed by SACs have been reported to follow associative reaction mechanisms.³⁶ In the associative mechanism, the N₂ molecule is adsorbed on the active site, only breaking down in select hydrogenation stages. In addition, the hydrogenation process involves two steps: distal and altering pathways. The main bottlenecks in the electrochemical NRR arise from the thermodynamic constraints imposed by the intermediates of the reaction. Theoretically, when a sufficiently negative potential is applied to an electrode, the NRR is enabled at room temperature and atmospheric pressure. The equilibrium potential of the electrochemical NRR is comparable to that of the HER. Thus, H₂ is the primary byproduct of the NRR process. Nevertheless, the NRR process involves multiple proton–electron transfer reactions, leading to the formation of a multitude of intermediates. Especially, the redox potential of the initial step (which forms the *NNH intermediate) is more negative, indicating the inherent difficulty of the initial active hydrogen (*H) addition. Furthermore, the addition of a second *H is more challenging than the addition of a third *H, leading to higher redox potentials for two- and four-electron reduction processes than for six-electron reduction processes.¹⁰ These significant negative potentials of the intermediates demonstrate the thermodynamic difficulty of NRR.

Fig. 3 Schematic depiction of the as-proposed possible mechanisms for NH₃ electrosynthesis from (a) N₂, and (b) NO, NO₂⁻ and NO₃⁻.

The series of intermediate transformations involved in the electrochemical NORR, NO₂⁻RR, and NO₃⁻RR significantly differ from those observed in the NRR process. According to nitrogen chemistry, the electroreductions of NO, NO₂⁻ and NO₃⁻ are thermodynamically and dynamically more favorable than direct NRR-to-NH₃ production. Many studies have shown that the pathways for the electrocatalytic reduction of NO₂⁻ and NO₃⁻ to NH₃ and the reduction of NO to NH₃ are identical (Fig. 3b),³⁷ although their rate-limiting steps may be different. In the case of NO₃⁻RR, the electroreduction of NO₃⁻ to NO₂⁻ is typically regarded as the limiting step in the NO₃⁻RR process. Different from the NO₃⁻RR process, NO₂⁻RR directly commences with NO₂⁻ adsorption, indicating that NO₂⁻RR can facilitate the thermodynamic production of NH₃. The role of *H in the rapid reduction of intermediates is also significant. In addition to considering the effect of hydrogenation and the rate-determining step, the limited solubility of NO in water influences the performance. Furthermore, NO₂⁻RR and NO₃⁻RR depend on the key divergent center-adsorbed NO (*NO). The conversion of *NO to NH₃ involves complex reaction pathways. The hydrogenation process involves the following steps: the solvated protons first adsorb onto catalysts to form adsorbed *H, followed by surface hydrogenation (Tafel-type pathway), or a NO molecule and intermediates are protonated directly (Heyrovsky-type pathway).³⁸ The different reaction pathways determine the nature of the byproducts, which include N₂O, N₂, and NH₂OH. NH₂OH is reported as a common byproduct in metal single-atom catalysis systems, which is initially formed through the stepwise hydrogenation of *HNOH. It readily desorbs from the metal active sites when the adsorption is insufficient.^39,40 The formation of N₂ and N₂O byproducts usually requires the dimerization of the absorbed NO intermediate species with the same orientation on the adjacent metal sites.²³ Due to the high dispersion of metal sites, N₂ and N₂O byproducts are rarely found.

3. The characteristics of SACs in the NH₃ electrosynthesis

To date, various single metal atoms have been used as the active centers for NH₃ electrosynthesis. A periodic table shows the frequency of elements applied in SACs (Fig. 4). Most of these SACs consist of d-block transition metal sites. The underlying physical mechanism can be traced back to the parallelled d orbitals, which enable the electron “donation back donation” process for the molecule adsorption and activation.⁴¹ Conversely, s-block metal, alkali, and alkaline-earth metal atom sites have been rarely studied due to the lack of combined empty and occupied orbitals.⁴² Doping p-block atoms into the metal catalysts can modulate the reaction energetics and significantly enhance the catalytic activity, often surpassing the effects achieved with d-block dopants.⁴³ For NRR, metal Mo and Fe-based atom catalysts have been identified as highly active and are the focus of extensive research studies. In the NORR, Fe and Co are highly active.

Fig. 4 Frequency of elements applied in SACs towards NH₃ electrosynthesis.

The metal atoms influence the catalytic functionality in SACs, and depend on the synergy with the ligands incorporated in supports. For instance, metal sites are studied as active centers in carbon substrates.⁴⁴ In metal alloys or metal compounds, single metal sites are often considered to modulate the electronic structure of the host material, or to enable the single metal sites and metals/compounds to form synergistic interactions.⁴⁵ In either form, single-atom sites have demonstrated significant advantages for NH₃ electrosynthesis although some issues need to be resolved.

4. Small nitrogenous substances for NH₃ electrosynthesis

4.1 N₂-to-NH₃ electrosynthesis

The earth-abundant N₂ in the atmosphere is approximately 78%, making it highly abundant and essentially zero-cost raw materials. However, the negative properties of N₂, including formidable ionization energy (15.84 eV), highly stable triple N≡N bond, and zero dipole moment, hinder high-performance NH₃ electrosynthesis.⁴⁶ Biological N₂ fixation in many plants is facilitated by the enzyme nitrogenase under mild conditions, which directly converts N₂ to NH₃, providing growth nutrients.^35,47 However, the slow reaction rate of nitrogenase in the biological nitrogen fixation process has prompted significant efforts to explore alternative sustainable routes. Nitrogenase consists of two multi-subunit proteins: Fe protein and Fe–Mo protein.^48,49 Thus, in early studies, attempts were made to synthesize enzyme-like electrocatalysts with Fe and Mo sites to achieve N₂ electroreduction to NH₃.⁵⁰ With further research advancements, some other metal active centers have also been extensively studied (Table 1). To date, to the best of our knowledge, the reported SACs for N₂-to-NH₃ electrosynthesis include Au, Ru, Mo, Fe, and Co atoms supported on carbon substrates and metal compounds, in which metal atoms are anchored to supports exclusively through M–N/C/O/S coordination bonds or metal bonds.

Table 1 Summary of the recently reported SACs for NRR

Single metal atom	Catalyst	Electrolyte	FE	NH3 yield rate	Stability	Ref.
Noble metal atom	Ru@ZrO2/NC	0.1 M HCl	21%	3.665 mg h^−1 mgRu^−1	60 h	51
	Au1/C3N4	0.005 M H2SO4	11.1%	1.305 mg h^−1 mgAu^−1	∼2 h	52
	Ru SAs/N–C	0.05 M H2SO4	29.6%	120.9 μg h^−1 mgcat.^−1	12 h	53
	SA-Ag/NC	0.1 M HCl	21.9%	270.9 μg h^−1 mgcat.^−1	60 h	54
Mo	SA-Mo/NPC	0.1 M KOH	14.6 ± 1.6%	34.0 ± 3.6 μg h^−1 mgcat.^−1	—	55
	Mo-PTA@CNT	0.1 M K2SO4	83 ± 1%	51 ± 1 μg h^−1 mgcat.^−1	2 h	56
	Mo-SAs/AC	0.1 M Na2SO4	57.54 ± 6.98%	2.55 ± 0.31 mg h^−1 mgMo^−1	10 h	57
	MoSAs-Mo2C	0.005 M H2SO4 and 0.1 M K2SO4	7.1%	16.1 μg h^−1 mgcat.^−1	10 h	58
Fe	Fe–MoS2	0.1 M KCl	31.6 ± 2%	97.5 ± 6 μg h^−1 cm^−2	20 h	59
	Fe–B/N–C	0.1 M HCl	23.0%	100.1 μg h^−1 mgcat.^−1	30 h	60
	Fe–(O–C2)4	0.1 M KOH	51.0%	307.7 μg h^−1 mgcat.^−1	40 h	61
	FeMo/NC	0.1 M PBS	11.8 ± 0.8%	26.5 ± 0.8 μg h^−1 mgcat.^−1	100 000 s	62
Ni	Ni–Nx–C-700-3 h	0.1 M KOH	21 ± 1.9%	115 μg h^−1 cm^−2	20 h	63
	Ni–Mo2C/NC	0.5 M K2SO4	29.05%	46.49 μg h^−1 mg^−1	12 h	64
Co	Co/NC_500	0.1 M KOH	10.1%	5.1 μg h^−1 mgcat.^−1	6 h	65
Zn	Zn/NC NSs	0.1 M Na2SO4	95.8%	46.62 μg h^−1 mgcat.^−1	12 h	66
V	O–V2–NC	0.1 M HCl	77%	9.97 μg h^−1 mgcat.^−1	10 cycles	67
Al	Al–Co3O4/NF	0.1 M KOH	6.25%	6.48 × 10^−11 mol s^−1 cm^−2	24 h	68

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4.1.1 Noble metal sites. Various noble-metal single-atom materials have been initially developed as NRR electrocatalysts at aqueous electrolytes, showing favorable activities. Sun et al. first used Ru single sites supported on N-doped porous carbon for ambient aqueous N₂-to-NH₃ electrosynthesis.⁵¹ They found that the catalyst with a Ru–N–C₂ structure provided a high NH₃ yield rate of 3.665 mg_NH3 h⁻¹ mg_Ru⁻¹ at −0.21 V vs. RHE. Zeng et al. designed Ru single atoms distributed on N-doped carbon (Ru SAs/N–C) with a Ru–N₃/Ru–N₄ structure. The composite exhibited a high FE of 29.6% and a high NH₃ yield rate of 120.9 μg h⁻¹ mg⁻¹.⁵² Li et al. prepared atomically dispersed Au on carbon nitride (Au₁/C₃N₄) and investigated its NRR catalytic performance. Owing to the efficient utilization of Au atom, a high NH₃ yield rate of Au₁/C₃N₄ was obtained, which was approximately 22.5 times higher than that of Au nanoparticles.⁵³ Luo et al. synthesized a Ag single-atom catalyst (SA-Ag/NC) with the Ag–N₄ structure and compared the NRR performance with Ag nanoparticles/NC. The comparative analysis result showed that the SA-Ag/NC exhibited an NH₃ yield rate of 270.9 μg h⁻¹ mg⁻¹ and a FE of 21.9%, which was significantly higher than that of Ag nanoparticles/NC. Despite the excellent performance of these noble-metal single-atom catalysts, further advancement and widespread use of various non-noble metal single-atom catalysts continue to be an attractive option.

4.1.2 Fe site. To construct a bimetal-atom active center to mimic nitrogenase in nature, the Fe atom and its synergistic effect with the Mo atom have been widely investigated for N₂-to-NH₃ electrosynthesis. Yao et al. prepared Fe₂(MoO₄)₃, and found that it exhibited a high NH₃ yield rate of 7.5 μg h⁻¹ mg⁻¹ and an NH₃ FE of 1.0% at −0.7 V for 2 h under ambient conditions (0.1 M Na₂SO₄), which was attributed to the synergistic effect of Fe and Mo elements in the active center.⁶⁹ Zhao et al. prepared atomically dispersed Fe–(O–C₂)₄ sites on graphitic carbon. The N-free catalysts with the Fe–O active sites prevented interference caused by the presence of other NRR active coordination structures, especially Fe–N_x active sites.⁶¹ The Fe single-atom catalyst exhibited an NH₃ yield rate of 32.1 mg h⁻¹ mg⁻¹ and a FE of 29.3%. Zhang et al. found that the interfacial polarization of the Fe atom on MoS₂ efficiently enhances the N≡N fracture to boost the electrocatalytic NH₃ synthesis.⁵⁹ They prepared the protrusion-like Fe single-atom onto MoS₂ nanosheets, which triggered the interfacial polarization (Fig. 5a). The theoretical calculation results showed that the protrusion-shaped Fe single atoms engendered electric fields to polarize N₂, and the energy barrier was mitigated in a set for electron flow from the Fe sites to N₂ (Fig. 5b). The NH₃ FE and NH₃ yield rates closely depend on the Fe-atom concentration, confirming Fe atoms as the catalytic centers (Fig. 5c).

Fig. 5 (a) Side-view and perspective-view crystalline structures of Fe–MoS₂. (b) The activation energies and (c) performance comparison of various Fe–MoS₂ catalysts with different Fe loading amounts. Reproduced with permission from ref. 59 Copyright 2020, Elsevier.

4.1.3 Mo site. In 2017, Chen et al. systematically investigated the potential of transition metal atoms (Sc to Zn, Mo, Ru, Rh, Pd, and Ag) through DFT simulation to predict their immobilization on defective boron nitride monolayers as the NRR electrocatalysts.⁷⁰ The result showed that atomically dispersed Mo atoms bonded to N atoms exhibited good performances in activating N₂ molecules and stabilizing *N₂H, while destabilizing the *NH₂ species during the electrochemical NRR process. Furthermore, Xin et al. experimentally verified the catalytic activity of the Mo single atom.⁵⁵ Mo single atoms anchored to N-doped porous carbon (SA-Mo/NPC) were prepared as a cost-effective catalyst for N₂-to-NH₃ electrosynthesis. Owing to the optimized abundance of Mo active sites and the three-dimensional hierarchically porous feature, the SA-Mo/NPC catalyst exhibited a remarkable FE of up to 14.6% at −0.3 V vs. RHE in 0.1 M KOH. Jiang et al. prepared a Mo-PTA@CNT catalyst, in which Mo atoms were anchored onto the fourfold hollow sites of phosphotungstic acid and closely embedded in multi-walled carbon nanotubes (CNTs) (Fig. 6a).⁵⁶ The experimental results showed that Mo(IV) on the 4-H sites served as active sites for N₂ adsorption and activation. The optimal Mo-PTA@CNT exhibited a high NH₃ yield rate of approximately 51 μg h⁻¹ mg_cat.⁻¹ and an excellent FE of approximately 83% at −0.1 V vs. RHE under ambient conditions (Fig. 6b). Wang et al. revealed the role of Mo single atoms in the MoSAs-Mo₂C/NCNT electrocatalyst comprising Mo single atoms and Mo₂C nanoparticles grown on the N-doped CNTs.⁵⁸ The electrochemically experimental results showed that MoSAs-Mo₂C/NCNTs exhibited a high yield rate of 16.1 μg h⁻¹ cm_cat.⁻², which was approximately four times that of the Mo₂C/NCNTs at the same potential. Moreover, the FE of the MoSAs-Mo₂C/NCNTs is twice that of the Mo₂C/NCNTs. DFT calculations revealed that the HER-selective MoSAs in the combined entity provided a large *H coverage environment around the Mo₂C nanoparticles to activate the N₂ molecules, thus improving the overall NRR performance (Fig. 6c and d).

Fig. 6 (a) Probable crystalline structures of PTA and Mo-PTA. (b) NH₃ yield rates and FEs of Mo-PTA@CNT and PTA@CNT. Reproduced with permission from ref. 56 Copyright 2021, Wiley-VCH. The Gibbs free energy diagrams on the MoNC₂ SAC and the Mo₂C (101) surface along (c) the NRR pathways and (d) the HER. Reproduced with permission from ref. 58 Copyright 2020, Wiley-VCH.

4.1.4 Other metal sites. Research on other single metal atoms has mainly focused on the influence of metal sites on NRR performance under different substrates. Various metal–N–C materials have been investigated as NRR electrocatalysts, in which the metal sites include Ni, Co, Zn, and V. Similar to the Fe and Mo sites, single metal sites on carbon usually reduce HER and promote N₂ dissociation.^65,67 Wu et al. prepared atomically dispersed Ni sites on the carbon (Ni–N_x–C) catalyst with a NiN₃ structure. The result showed that the catalyst exhibited a high NH₃ yield rate of 115 μg cm⁻² h⁻¹ at −0.8 V vs. RHE and a maximum FE of 20% at −0.6 V vs. RHE.⁵² By comparison, Ni clusters supported on N-doped carbon had no significant NRR activity, confirming that atomically dispersed unsaturated Ni atoms were the real active sites. Shao et al. prepared atomically dispersed Zn supported on N-doped carbon nanosheets (Zn/NC NSs) with the Zn–N₄ structure as the NRR catalyst, achieving excellent NRR performance.⁶⁶ DFT calculations showed that the introduction of Zn sites effectively reduced the energy barrier for the formation of *NNH. In addition to the carbon substrates, metal compounds have been widely used as substrates to support single atoms. More studies have shown that single metal sites on metal compounds can generate the synergistic effect of active sites for the NRR process. Hu et al. prepared Ni atom-doped Mo₂C anchored on porous carbon, and found that the electronic configuration close to the Ni–Mo active sites was optimized. The N₂ adsorption was also promoted due to the increased electron transfer.⁶⁴ Yuan et al. introduced Al atoms into Co₃O₄ to effectively tune the electronic properties of the catalyst, and the increased surface oxygen vacancies induced by Al doping facilitated the N₂ activation.⁶⁸

4.2 NO-to-NH₃ electrosynthesis

NO is more active than N₂, thus making electrochemical NORR thermodynamically more feasible than NRR. NO, one of the major pollutants emitted during fossil fuel combustion and other chemical industrial processes, can be converted into valuable products through electrocatalysis, thereby alleviating the environmental load and reversing the anthropogenic global nitrogen-cycle imbalance. As mentioned in the previous section, *NO is a key intermediate that determines the NH₃ selectivity in the electroreduction process of these nitrogen oxides.⁷¹ Therefore, investigating NORR is crucial for gaining a fundamental understanding of the electroreduction process and achieving efficient NH₃ electrosynthesis. Liu et al. reported on single atomic Ce sites anchored on N-doped hollow carbon spheres (Ce-SA/NHCS) that could electrocatalyze NO reduction to NH₃ in an acidic solution.⁷² DFT calculations showed that Ce-SA/NHCS was not beneficial for adsorption of *H compared with *NO. When the SCN⁻ ions were added to the electrolyte due to the blocking effect, the NH₃ yield rate of Ce-SA/NHCS rapidly decreased, indicating that the Ce–N₄ moieties in Ce-SA/NHCS were active sites. In a typical NORR process, the adsorption configuration of NO was also crucial to modulating the bonding energy of N–O, which could further regulate the NH₃ selectivity. The bonding energy of N–O can be modulated through the adsorption configuration of NO on the surface-active sites of electrocatalysts. Our group constructed and compared the Co single-atom catalyst (Co SACs) and the Co nanoparticle (Co NPs) catalyst for NORR.⁷³ Under neutral conditions, Co NPs were inclined to generate NH₃, and Co SACs exhibited high NO-to-NH₂OH selectivity (Fig. 7a). DFT calculations revealed that the turnover frequencies (TOF) of NH₂OH were 50 times higher than the TOF of NH₃ on Co SACs (Fig. 7b). The linear adsorption of NO on isolated Co sites might maintain the N–O bond to produce NH₂OH through the intermediates of *HNO and *H₂NO. Additionally, Co NPs could break the N–O bond to form NH₃ through a dissociation mechanism (Fig. 7c).

Fig. 7 (a) FEs of NH₃ and NH₂OH on Co SACs and Co NPs at different potentials. (b) TOFs of NH₃ and NH₂OH over Co SACs obtained by microkinetic modeling. (c) Deduced electrocatalytic NORR pathway on Co SACs and Co NPs for NH₂OH and NH₃ production. Reproduced with permission from ref. 73 Copyright 2023, Wiley-VCH.

Based on this, well-designed single metal atoms anchored on metal-based supports might make it easier to obtain the optimal adsorption configuration of NO, leading to the generation of the target NH₃ product.⁷⁴ Studies have shown that single metal atoms, including Co, Ni, Mn, and Fe, supported on various metal/metal compound substrates are promising NO-to-NH₃ electrocatalysts with excellent activity and selectivity.^75–77 Zhu et al. successfully combined DFT and machine learning methods to examine Fe@MoS₂. They found that the Fe@MoS₂ composite was thermodynamically stable, and had strong resistance to electrochemical corrosion under local alloying conditions.⁷⁷ Chu et al. conducted various studies on NO-to-NH₃ electrosynthesis. They reported that atomically dispersed Co anchored on MoS₂ (Co₁/MoS₂) was an efficient NORR catalyst, highlighting Co as a critical element in the Co₁/MoS₂ catalyst for effective NORR and NO_x denitrification.⁷⁸ Structural characterizations and theoretical calculations unveiled the formation of active Co₁–S₃ moieties on Co₁/MoS₂, which could selectively adsorb NO molecules and promote the hydrogenation energetics of the NO-to-NH₃ electroreduction process (Fig. 8a, c and d). As a result, Co₁/MoS₂ exhibited excellent NORR performance with an NH₃ yield rate of 217.6 μmol h⁻¹ cm⁻² and an NH₃ FE of 87.7% at −0.5 V vs. RHE (Fig. 8b). They also designed p-block Sb single atoms confined in amorphous MoO₃ (Sb₁/a-MoO₃) evaluated as the NORR catalyst.⁷⁹ Compared with the a-MoO₃ catalyst, Sb₁/a-MoO₃ exhibited a high NH₃ yield rate of 273.5 μmol h⁻¹ cm⁻² and a NH₃ FE of 91.7% at −0.6 V vs. RHE. In situ spectroscopic characterizations and theoretical calculations showed that the isolated Sb₁ sites optimized the adsorption of *NO/*NHO to reduce the reaction energy barriers, and simultaneously exhibited a higher affinity to NO than to H₂O/H species. In another work, they designed single-atom Pd-alloyed Cu (Pd₁Cu) as the NORR catalyst, which exhibited excellent long-term durability for 200 h at industrial-level current densities (>0.2 A cm⁻²).⁸⁰

Fig. 8 (a) EXAFS fitting curve of Co₁/MoS₂. (b) NH₃ yield rate and FE of MoS₂ and Co₁/MoS₂. (c) Binding free energies of *NO and *H on Co₁–S₃. (d) Free energy profiles of the NORR process on MoS₂ and Co₁/MoS₂. Reproduced with permission from ref. 78 Copyright 2023, Elsevier.

4.3 NO₂⁻/NO₃⁻-to-NH₃ electrosynthesis

Electrochemical NO₂⁻RR and NO₃⁻RR usually exhibit faster reaction rates, owing to the relatively lower dissociation energy of the N=O bond and higher solubility of NO₂⁻/NO₃⁻ in the aqueous electrolytes. In addition, converting NO₂⁻/NO₃⁻ pollutants into high-value-added products provides a promising approach for both wastewater treatment and NH₃ synthesis. In recent years, significant progress has been made on SACs for the NO₂⁻RR and NO₃⁻RR. NO₂⁻ is the main byproduct of NO₃⁻ electroreduction at low overpotential and has been reported in many cases under a two-electron reduction process. Zhi et al. compared the four mentioned nitrogenous substances. Their results showed that NO₂⁻ exhibited the highest calculated reduction potential of 0.897 V vs. RHE at a pH of 0, and KNO₂ exhibited the highest solubility (Fig. 9).⁸¹ Therefore, using NO₂⁻ as the raw material for NH₃ electrosynthesis can provide faster reaction thermodynamics and kinetics, and a higher NH₃ yield rate. Chu et al. designed atomically dispersed Pd on defective BN nanosheets (Pd₁/BN) as a potential NO₂⁻RR catalyst, achieving a high NH₃ FE of 91.7% and an NH₃ yield rate of 347.1 μmol h⁻¹ cm⁻² at −0.6 V vs. RHE.⁸² The theoretical calculations revealed that isolated Pd atoms served as catalytic centers, selectively adsorbing NO₂⁻ and enhancing the hydrogenation process with a minimized reaction barrier. In another work, they revealed that Mo single atoms in the Mo₁–ZrO₂ electrocatalyst effectively activated NO₂⁻ and facilitated the NO₂⁻RR protonation process.⁸³

Fig. 9 (a) Calculated potentials and chemical reactions of N₂, NO, NO₂⁻ and NO₃⁻ reduction to NH₄⁺. (b) The solubility of N₂(g), NO(g), KNO₂, and KNO₃ in 100 mL of water under one atmosphere and at 298.15 K. Reproduced with permission from ref. 81 Copyright 2022, The Royal Society of Chemistry.

Although NO₂⁻ has many theoretical advantages, its relatively unstable decomposition and toxicity limit its development in the field of NH₃ electrosynthesis. Considerable attention has been paid to the electroreduction of NO₃⁻ to NH₃. Currently, metal atoms, including Cu, Fe, Ru, and Bi, anchored on a set of supports (including N-doped carbon, metal, and metal compounds) have been extensively investigated in NO₃⁻RR.⁸⁴

4.3.1 Cu site. Cu-based electrocatalysts have been extensively reported for NO₃⁻RR, owing to the benefit of NO₃⁻ adsorption and matching electronic structure with the molecular orbital of NO₃⁻.^85–87 Due to its weak adsorption, the accumulation of NO₂⁻ results in sluggish kinetics of the subsequent hydrogenation steps and the formation of N₂ and N₂O byproducts.⁸⁷ The isolated Cu single-atom sites can effectively minimize the possibility for the formation of N₂ and N₂O byproducts during the NO₃⁻RR process. Li et al. compared a series of Cu_s+n/NC with mixed Cu single atoms (Cu SAs) and nanoparticles incorporated in N-doped carbon, and their counterpart Cu nanoparticles (Cu NPs) for NO₃⁻-to-NH₃ electrosynthesis (Fig. 10a–d).⁸⁸ They systematically elucidated the activity trends of Cu SA/NC, Cu_s+n/NC, and Cu NPs. The result showed that Cu SA/NC decreased NH₃ FE as the Cu single-atom content decreased. However, the production of N₂ and N₂O gradually increased, which reached the maximum on pure Cu NPs. This result showed that the high and low proportions of Cu SAs in Cu_s+n/NC facilitates the formation of NH₃ and N–N coupled products (N₂ and N₂O), respectively. Theoretical calculations revealed that the higher NH₃ FE of Cu SA/NC was attributed to the lower free energy of the rate-limiting step (*ONH → *N) and effective inhibition for the N–N coupled process.

Fig. 10 (a) Schematics of the NO₃⁻RR on Cu SAs and Cu nanoparticles. FEs of (b) Cu(0.1 wt%)/NC and (c) Cu(1.6 wt%)/NC at different densities. (d) Free energy diagram of the reaction coordinates of Cu SA/NC and Cu NPs. Reproduced with permission from ref. 88 Copyright 2023, American Chemical Society. (e) Cu K-edge XANES spectra and (f) Cu K-edge FT-EXAFS spectra at different potentials. (g) Proposed mechanisms of aggregation and re-dispersion. Reproduced with permission from ref. 89 Copyright 2022, American Chemical Society.

In addition to the symmetrical four N coordination structure, Cu single-atom catalysts with different coordination structures for NO₃⁻RR were investigated to further regulate the NO₃⁻RR performance.⁸⁹ Zhang et al. found that the coordination environment of the Cu single-atom sites (Cu(I)–N₃C₁) localized the charge around the central Cu atoms, and adsorbed *NO₃ and *H onto neighboring Cu and C sites with balanced adsorption energy.⁹⁰ Zhu et al. designed and synthesized a Cu single-atom catalyst with a low-coordinated Cu–N₃ structure on high-curvature hierarchically porous N-doped CNTs. The catalyst exhibited enhanced NO₃⁻RR activity with an NH₃ yield rate of 30.1 mg h⁻¹ mg⁻¹ and an NH₃ FE of 89.6%.⁹¹ Theoretical calculations showed that low-coordinated Cu–N₃ sites and high-curvature carbon support contributed to the fast reaction dynamics and low rate-determining step barrier for NO₃⁻RR. The Gibbs free energies of the intermediates for the Cu–N₃-plane were generally lower than those of the Cu–N₄-plane, indicating that the Cu–N₃ structure had better reaction dynamics than the Cu–N₄ structure. Lu et al. designed the symmetry-broken Cu-cis-N₂O₂ single-atom catalysts in the cis-configuration.⁹² The low coordination symmetry rendered the active site more polar and accumulated more NO₃⁻ near the electrocatalyst surface. Theoretical calculations revealed that the cis-coordination splits the Cu 3d orbitals generated an orbital-symmetry-matched π-complex of the key intermediate *ONH, and reduced the energy barrier when compared with Cu–N₄ and Cu-trans-N₂O₂. In addition, an average NH₃ yield rate of 27.84 mg h⁻¹ cm⁻² at an industrial level current density of 366 mA cm⁻² was retained for more than 2000 h. Wang et al. identified the real active site of the Cu–N₄ single-atom catalyst in NO₃⁻RR using operando X-ray absorption spectroscopy (Fig. 10e and f).⁹³ They found that the synthesized Cu–N₄ structure successively transformed to Cu–N₃, near-free Cu₀ single atoms, and eventually to aggregated Cu₀ nanoparticles with the applied potential ranging from 0.0 to −1.0 V vs. RHE. After electrolysis, the aggregated Cu₀ nanoparticles were disintegrated into single atoms, and then restored to the Cu–N₄ structure after being exposed to an ambient atmosphere (Fig. 10g).

4.3.2 Fe site. Zero-valence Fe has been widely reported as an effective catalyst for removing NO₃⁻ in groundwater to manage nitrogenous pollution.⁹⁴ In addition, Fe-dependent nitrite reductase with Fe–N₄ can actively produce NH₃ through the photosynthetic nitrate assimilation pathway in nature.⁹⁵ Jia et al. investigated electrochemical NO₃⁻RR on traditional metal–N₃ and –N₄ (traditional metal = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) doped graphene using first-principles calculations.⁹⁶ The result showed that Fe–N-doped graphene with Fe–N₄ structure exhibited excellent NO₃⁻RR performance, which was attributed to the effective adsorption and activation of NO₃⁻ through the charge “acceptance-donation” mechanism and its moderate binding due to the occupation of the d–p antibonding orbital. Inspired by the similar coordination structure, atomically dispersed Fe on the carbon substrates with various configurations were investigated as the NO₃⁻RR catalysts.^97,98 In 2021, Wang et al. reported on the Fe–N₄ structure as an active and selective site for NO₃⁻-to-NH₃ electrosynthesis.⁹⁷ They found that the Fe–N₄ single-atom catalyst effectively prevented the N–N coupling step required for N₂ due to the insufficient neighboring metal sites. Atanassov et al. investigated the NO₃⁻RR mechanism of Fe–N₄ active sites in Fe–Mo dual single-atom catalysts.⁹⁹ They found that one of the Fe–N₄ sites facilitated the transfer of NO₂⁻ to the NH₃ transfer pathway with approximately 100% NH₃ FE and a high yield rate in the NO₃⁻RR process (Fig. 11a–f). The NO₃⁻RR mechanism of the Fe–N₄ coordination structure was also identified in another Fe–Mo dual single-atom catalyst system reported by Li et al.¹⁰⁰ Yu et al. fabricated atomically dispersed N-coordinated Fe sites on carbon (Fe-PPy SACs) for NO₃⁻-to-NH₃ electrosynthesis.³⁹ The Fe-PPy SACs exhibited nearly 100% NH₃ selectivity, and a TOF that was over 12 times that of Fe nanoparticles. The mechanistic study revealed the exclusive existence of NO₃⁻-preoccupied Fe(II)–N_x sites, which effectively eliminated the competing H₂O adsorption under aqueous conditions; thus, no noticeable H₂ byproduct was produced (Fig. 11g–i). Li et al. compared the NO₃⁻RR performance of a zeolitic imidazolate framework-derived Fe single-atom catalyst with Fe–N₃S₁ structure (Fe–N₃S₁ SACs) and commercial FeS.¹⁰¹ The Fe–N₃S₁ SACs exhibited a higher NH₃ FE of 93.9% than FeS (67.2%). DFT calculations revealed the easier adsorption of NO₃⁻ by forming the *NO₃ intermediate on Fe–N₃S₁. Gu et al. prepared another Fe single-atom catalyst with a Fe–N₂O₄ coordination structure through pyrolysis of metal–organic frameworks.¹⁰² DFT calculations revealed that the Fe–N₂O₂ structure exhibited higher conductivity and NH₃ selectivity than the Fe–N₄ structure, which spontaneously triggered the transformation of *NOH to *N. Another metal atom that was close to the Fe atom might produce different NO₃⁻RR properties. Lu et al. synthesized a Fe/Cu diatomic catalyst on holey N-doped graphene, and the original coordination structure of the Fe sites was broken.¹⁰³ Compared with the Fe/Fe and Cu/Cu configurations, the Fe/Cu diatomic sites provided medium interactions toward most other intermediates and reduced the overall energy barriers for the NO₃⁻RR process.

Fig. 11 (a) Associative adsorption of NO₃⁻ on the Fe–N₄ site and (b) dissociative adsorption of NO₃⁻ into *O and NO₂⁻ on the Mo–N₄ site. (c) Proposed catalytic cascade for synergized NO₃⁻RR on the Mo–N₄ and Fe–N₄ sites. NH₃ and NO₂⁻ concentrations over a 24 h period of NO₃⁻RR electrolysis catalyzed by (d) Fe–N₄, (e) Mo–N₄, and (f) FeMo–N–C catalysts. Reproduced with permission from ref. 99 Copyright 2022, American Chemical Society. (g) The proposed preoccupied NO₃⁻RR mechanism for the single-site center and classical competitive mechanism for the bulk surface. (h) TOFs of the Fe-PPy SACs and Fe NPs for NH₃ production. (i) Gibbs free-energy diagram of NO₃⁻RR and HER. Reproduced with permission from ref. 39 Copyright 2021, The Royal Society of Chemistry.

When the Fe site is anchored on metal-based substrates, it may not serve as the direct active site for the NO₃⁻RR process. As a result, it effectively tunes the d-band center of the metal substrates and modulates the electronic structure of the overall catalysts. Zhi et al. reported a Fe-doped Ni₂P nanosheet as the NO₃⁻RR catalyst, which showed larger NH₃ yield rates and NH₃ FE when compared with Ni₂P at each given potential.¹⁰⁴ Theoretical calculations revealed that the Fe site caused a downshift of the d-band center of Ni atoms to the Fermi level, optimizing the Gibbs free energies for the NO₃⁻RR intermediates. Chu et al. reported on a Fe-doped V₂O₅ (Fe-V₂O₅), in which Fe dopants were in isolated single-atom states, and a high density of Lewis acid Fe–V pairs was readily created.¹⁰⁵ Theoretical calculations revealed that Lewis acid Fe–V pairs in Fe-V₂O₅ synergistically enhanced the NO₃⁻ activation and hydrogenation, and concurrently suppressed the HER.

4.3.3 Ru site. Ru exhibits suitable adsorption/desorption energy toward hydrogen intermediates, and it is considered one of the outstanding active sites for alkaline–water splitting to produce H₂.¹⁰⁶ Recent studies have shown that Ru-based NO₃⁻RR catalysts exhibit low overpotentials and high performance in the electrochemical reduction of NO₃⁻ to NH₃.^107–110 Wang et al. designed a Ru₁Cu catalyst, which showed an industrial-level current density of approximately 1.0 A cm⁻² with an NH₃ FE of 93%. Theoretical investigations unveiled that the excellent NO₃⁻RR performance of Ru₁Cu was attributed to the synergy of Ru and Cu, where Ru sites activated NO₃⁻ while the surrounding Cu sites suppressed the HER. Our group incorporated Ru atoms into the octahedral sites of Co₃O₄ to achieve an NH₃ yield rate of 24.6 mg h⁻¹ cm⁻².¹¹¹ Electrochemical in situ spectroscopic analyses and theoretical calculations revealed that Ru sites improved the water molecule coverage and facilitated the *H production, leading to stable and orderly NH₃ production.

4.3.4 Other metal sites. The p-block metals (such as Bi, Sn, In) are poorly bound to H-adatoms.¹¹² Bi, as a typical metal, has relatively low catalytic activity for the HER and strong adsorption/activation ability toward the nitrogen-containing intermediates, making it widely used in small molecule catalytic activation including NO₃⁻ electroreduction.^113–115 Oschatz et al. reported on a porous Bi single-atom decorated, N-doped carbon with a BiN₂C₂ coordination structure.¹¹⁶ DFT calculations revealed that the Bi active center exhibited a relatively low charge, resulting in an unfavorable NO₃⁻ adsorption that was nonetheless beneficial for stabilizing the hydrogenated intermediates. Recent studies have shown that the incorporation of a p-block metal atom into d-block metal compounds can trigger a fascinating p–d orbital hybridization effect, effectively overcoming the limitations of d-metal compounds in facilitating the HER.^117,118 Chu et al. reported that Bi single atoms significantly improved the NO₃⁻RR performance when alloyed with the Pd metal (Fig. 12a and b). DFT calculations revealed that doping Bi single atoms into the Pd metal reduced the energy barrier for the *NO to *NOH hydrogenation process (Fig. 12d).¹¹⁹ This enhancement was attributed to the inhibitory effect of Bi on HER, which prevented *H coupling, resulting in more *H, and the strong interaction between the Bi 6p orbitals and N 2p orbitals, which increased the adsorption capacity of nitrogen-containing reaction intermediates (Fig. 12c). Bader charge analysis revealed the presence of a downshifted d-band center of Pd₁ (−1.55 eV) relative to the Pd atom (−1.48 eV) of pristine Pd, effectively destabilizing *NO while retaining *NOH stabilization (Fig. 12e). In another work, Chu et al. designed Bi-doped FeS₂, triggering a fascinating p–d orbital hybridization effect.¹²⁰ The electronic structure of the active centers and the adsorption/desorption behaviors of the N-containing intermediates were optimized, thus enhancing the NO₃⁻RR catalytic properties.

Fig. 12 (a) NH₃ yield rate and (b) NH₃ FE of Pd and Bi₁Pd at various potentials. (c) Fitted lines of the EPR intensity/reaction time versus NO₃⁻ concentration. (d) Free energy diagram of NO₃⁻RR pathways on Pd and Bi₁Pd; inset: atomic configurations of *NO on Pd and Bi₁Pd. (e) PDOS of Pd atom on Pd and Pd₁, Pd₂ and Bi₁ atoms on Bi₁Pd. Reproduced with permission from ref. 119 Copyright 2023, Wiley-VCH.

In addition to these single metal atoms, some other metal atom sites have similar advantages, in which one of the typical metal sites is Co.¹²¹ Zhu et al. synthesized Co single atoms anchored on carbon nanofibers (Co SAs/CNFs) with a Co–C₃O₁ coordination structure.¹²² The experimental and theoretical results revealed that the synthesized Co SAs/CNF electrocatalyst exhibited excellent electrocatalytic NO₃⁻RR activity, a smaller energy barrier for the rate-determining step of *NO to *NOH, and a stronger capability for inhibiting HER than Co nanoparticles. This regulation mechanism was also confirmed in another study which compared the Co–P₁N₃ and Co–N₄ reported by Liu et al.¹²³ Zhao et al. also developed the N, O trans-coordination Ag single atoms on carbon nanotubes, which inhibited the HER, optimized the adsorption of intermediates, and facilitated the potential-determining step of *NO → *NOH in the NO₃⁻RR process.¹²⁴

5. Conclusion and outlook

Electroreduction of small nitrogenous substances (N₂, NO, NO₂⁻, and NO₃⁻) for NH₃ electrosynthesis has recently attracted considerable attention. Metal single-atom catalysts (SACs) have made significant advancements in the field of NH₃ electrosynthesis, particularly in terms of enhancing catalytic performance and elucidating fundamental mechanisms. This review concisely compares the features of diverse nitrogenous substances and provides a synopsis of the impact of these properties on the NH₃ electrosynthesis. This review also summarizes the frequently used metal single-atom sites in SACs and explores their role in various reaction processes involved in the NH₃ electrosynthesis. The reported computational predictions and experimental findings on various single metal sites for electrochemical NH₃ synthesis are summarized. Regarding the N₂-to-NH₃ electroreduction synthesis, Mo, Fe, and V sites, which are the metal centers in nitrogenase enzymes, have been widely studied and exhibit excellent performance. Electroreduction of NO/NO₂⁻/NO₃⁻ to NH₃ has a similar reaction path, where NO, as the reaction center of divergence, is often used to study the mechanism. Experimental reports on the electroreduction of NO and NO₂⁻ to NH₃ are currently limited. In the electroreduction of NO₃⁻-to-NH₃ reaction, Cu, Fe, Ru, Bi, and other single-atom sites have been extensively studied. The coordination structure around them and supports around the metal sites play a regulating role in the rate-determining step, water splitting, and NH₃ selectivity in this reaction.

Although SACs have successfully improved NH₃ electrosynthesis, several challenges still need to be addressed. Several perspectives have been outlined to advance nitrogen-containing substances to NH₃ electrosynthesis, focusing on enhancing the catalytic activity and providing fundamental insights to guide future research. (1) The preparation process of SACs is relatively complex, requiring precise control of reaction conditions and the dispersion and anchoring of single metal atoms on the catalytic supports. This requirement increases the difficulty and cost of preparation, limiting the possibility of large-scale application. Therefore, universal and simple synthesis methods should be encouraged in the future. (2) The long-term stability of SACs may be compromised by environmental factors and reaction conditions, such as humidity, gas concentration, applied voltage, and pH. These factors can affect the stability of single metal atoms, reducing or deactivating catalyst activity. Notably, the stability of the previous single-atom structure under highly reducing conditions remains an open question, as changes in the metal valence may alter the catalyst structure. Additionally, compared with metal and metal compound catalysts, SACs have a more complex and diverse structure, making the structure–activity mechanisms and relationships in NH₃ electrosynthesis more complex. Unraveling the dynamic evolution of the SACs during electrolysis is crucial to identifying the actual active site. Therefore, further extensive research and development efforts are needed to fully harness the potential of SACs, especially using advanced technical methods (such as in situ spectroscopic techniques and theoretical calculations). (3) Although SACs have broad application prospects, there are still some challenges in complex continuous electrocatalytic hydrogenation reactions. Therefore, the design and development of new types of SACs are necessary. For instance, bi-/multi-metal-atom catalysts with multiple active centers may potentially provide higher activity than SACs through the synergistic action of two or more atoms.

Author contributions

Zhaole Lu: investigation, writing – original draft, and writing – review & editing. Jijie Zhang: conceptualization, writing – review & editing, and supervision. Yuting Wang: writing – review & editing. Yifu Yu: supervision. Lingjun Kong: supervision.

Data availability

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

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the National Natural Science Foundation of China (no. 22205156 and 22475146).

Notes and references

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