Single-Atom Catalysts for Electrochemical Nitrate Reduction to Ammonia: Rational Design, Mechanistic Insights, and System Perspectives

Abstract

Ammonia serves as a critical industrial feedstock and a potential carbon-free energy carrier. However, its conventional synthesis method (the Haber–Bosch process) suffers from high energy consumption and substantial carbon emissions. The electrochemical nitrate reduction reaction (eNO₃RR) has emerged as a promising alternative pathway, capable of converting nitrate pollutants in water into high-value ammonia under mild conditions, enabling green synthesis while offering dual benefits of environmental remediation and energy conversion. Single-atom catalysts (SACs), with their maximal atom utilization efficiency, well-defined active sites, and highly tunable electronic structures, have demonstrated exceptional catalytic performance and selectivity in eNO₃RR. This review systematically summarizes recent advances of SACs in eNO₃RR, with a focus on reaction mechanisms, advanced in situ characterization techniques, theoretical calculation, and the catalytic behavior and structure–activity relationships of various non-noble metal centers (e.g., Cu, Fe, Co). Key strategies for enhancing SACs performance are elaborated, alongside an analysis of microenvironmental influences such as electrolyte composition, pH, and potential. Finally, we outlines current challenges in material design, dynamic active site identification, and the industrial application of SACs, and propose future research directions aimed at facilitating the practical implementation of eNO₃RR technology and contributing to the establishment of a sustainable ammonia economy.

1. Introduction

Ammonia (NH₃) is an important raw material for modern industry and agriculture, playing an indispensable role in global economic and social development. Beyond its critical function as the primary feedstock for nitrogen-based fertilizers, it is increasingly recognized as a promising carbon-free energy carrier, owing to its high hydrogen density (17.6 wt%) and favorable liquefaction properties [1,2,3]. Currently, the majority of global ammonia production is achieved through the Haber-Bosch process, which accounts for roughly 90% of the world’s supply [4]. This century-old method synthesizes NH₃ from nitrogen (N₂) and hydrogen (H₂) using iron-based catalysts under severe operating conditions (350–500 °C, 150–350 bar) [5]. Despite its revolutionary impact, the process is notoriously energy-intensive, consuming 1–2% of global energy annually and contributing approximately 1% of the total greenhouse gas emissions, thereby imposing a substantial ecological burden [6,7].

In response, the electrochemical nitrogen reduction reaction (eNRR) has attracted considerable attention as a potential sustainable alternative for ammonia synthesis under ambient conditions. However, its practical application is fundamentally constrained by the low aqueous solubility of N₂ and the exceptional stability of the N≡N triple bond (dissociation energy 941 kJ mol⁻¹), which collectively lead to poor reaction kinetics, low Faradaic efficiency (FE), and limited current density [8].

By contrast, nitrate (NO₃⁻), a ubiquitous nitrogen-oxygen anion occurring widely in wastewater, offers a compelling alternative nitrogen source for electrocatalytic NH₃ synthesis. NO₃⁻ contamination, largely resulting from excessive agricultural fertilization, industrial discharge, and domestic sewage, contributes to eutrophication of aquatic systems and poses direct risks to human health through its conversion to toxic nitrites [9,10]. Addressing NO₃⁻ pollution has thus become a pressing environmental priority. Conventional remediation strategies such as biological denitrification, reverse osmosis, ion exchange, and membrane filtration are often hampered by operational inefficiencies, high sludge production, and the generation of secondary waste streams [11,12,13].

Against this backdrop, the electrocatalytic nitrate reduction reaction (eNO₃RR) has emerged as a highly attractive technology, combining efficient NO₃⁻ removal with the simultaneous production of valuable NH₃. Driven by renewable electricity, eNO₃RR enables the direct conversion of aqueous NO₃⁻ to NH₃ under mild conditions, achieving the dual objectives of wastewater treatment and green chemical synthesis [14,15,16,17]. This integrated approach not only opens a sustainable route to NH₃ production but also represents a transformative strategy for water purification [18].

Although eNO₃RR is thermodynamically more favorable than eNRR, owing to the lower bond dissociation energy of N−O (204 kJ mol⁻¹) and the high solubility of NO₃⁻, the pathway from NO₃⁻ to NH₃ involves a complex network of proton-coupled electron transfers, traversing multiple nitrogen-containing intermediates (e.g., *NO₂, *NO, *NH₂OH, *NH₂) [19]. The adsorption strength of these intermediates critically dictates the reaction selectivity; suboptimal binding energies at any stage can divert the pathway toward undesirable by-products such as NO, N₂O, N₂, N₂H₄, or NH₂OH [20]. Moreover, the hydrogen evolution reaction (HER) presents a major competitive process, significantly compromising the Faradaic efficiency and energy conversion performance of eNO₃RR systems [21]. Thus, the rational design of electrocatalysts that combine high activity, selectivity, and durability for ammonia production through the eNO₃RR is essential to advance this technology toward practical implementation. To this end, a wide range of catalytic materials and modification strategies have been investigated in recent years [22].

A landmark development in this pursuit was the introduction of the “single-atom catalysis” concept by Zhang Tao and his colleagues in 2011, which inaugurated a new era of atomic-level catalyst design. In the years since, numerous single-atom catalysts (SACs) based on non-noble metals (e.g., Fe, Co, Ni, Cu) and noble metals (e.g., Ru, Pt, Ag) have been developed for electrochemical eNO₃RR, exhibiting exceptional performance. The SACs offer distinctive advantages for this reaction, including maximal atom utilization efficiency, uniform and well-defined active sites, and highly tunable electronic structures-properties that collectively enhance NH₃ selectivity and yield [23].

To advance a deeper understanding of SACs in eNO₃RR and accelerate the development of next-generation catalysts, a comprehensive and critical review of recent advances is urgently needed. This review systematically outlines the fundamental mechanisms and in situ characterization techniques of eNO₃RR, outlines the impact of the microenvironment on SACs for nitrate-to-ammonia conversion, and details the catalytic behavior and structure-activity relationships of various non-noble metal single-atom systems. Furthermore, it discusses the strategies for enhancing the performance of SACs in eNO₃RR. Finally, this study pinpoints prevailing challenges and proposes future research priorities, with the aim of providing a coherent conceptual framework to inspire innovative solutions and accelerate the development of sustainable ammonia synthesis technologies.

2. Brief Overview for eNO₃RR

2.1. Mechanisms of Electrocatalytic Nitrate to Ammonia

The eNO₃RR in aqueous solution is an intricate process involving multi-electron/proton transfers, capable of yielding a spectrum of products including the desired NH₃, as well as N₂, N₂O, NO, and NO₂⁻, among others [24]. The complicated thermodynamic relationship among these various products can be depicted in a Frost-Ebsworth diagram (Figure 1a) [25]. It is noted that among these intermediates and products, N₂ and NH₃ are the most common major products due to their high thermodynamic stability (Figure 1b) [26]. Due to the economic benefits, NH₃ has received more attention. The overall reaction for NH₃ is given in Equation (1).

NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O E^O = 0.69 V versus SHE

(1)

Mechanistically, the process of eNO₃RR can be categorized into direct reduction pathway and indirect reaction pathways, which is influenced by factors such as nitrate concentration, pH, and the nature of the electrocatalyst surface. The indirect reduction pathway occurs under high NO₃⁻ concentrations (>1.0 M) and strongly acidic conditions, wherein nitrate is not directly reduced but proceeds through critical intermediates such as NO⁺ and NO₂ [27]. The indirect reduction pathway can be summarized as the Vetter and Schmid pathways (Figure 1c). The primary products of both pathways are, respectively, NO₂ and HNO₂ [28]. In contrast, under most NO₃⁻ conditions (<1.0 M) relevant to SACs, the direct reduction pathway dominates, wherein NO₃⁻ is directly activated and reduced at the catalyst surface [29]. For SACs, this direct pathway can be conceptualized through two intertwined modes: electron reduction and adsorbed hydrogen reduction involving adsorbed hydrogen (*H), which are often concurrent and difficult to disentangle (Figure 1c) [30].

The electron reduction process, operative under acidic conditions, initiates with the adsorption of NO₃⁻ onto the active site (Equation (2)). The SACs facilitate this process through a well-defined electron donor-acceptor interaction between the metal site and NO₃⁻. Specifically, the unoccupied 3d orbitals of the metal center (e.g., Cu, Fe, Co) accept lone pair electrons from the oxygen atoms of NO₃⁻ (σ-donation), while the occupied metal d-orbitals back-donate electron density into the anti-bonding π* orbitals of NO₃⁻ (π-backdonation). This synergistic interaction not only strengthens the metal-oxo bond but, more importantly, it populates the anti-bonding orbitals of NO₃⁻, thereby weakening the N−O bonds and priming it for facile reduction.

The initial activation and reduction of NO₃⁻ to NO₂⁻ (Equation (3)) is widely recognized as the potential-determining step (PDS) due to the high energy barrier associated with breaking the first N−O bond [31]. The adsorption strength of NO₃⁻ on the catalyst surface is therefore a critical descriptor for activity. The resulting adsorbed NO₂⁻ (*NO₂⁻) is then rapidly reduced to a pivotal intermediate, nitric oxide (*NO) (Equation (4)). The fate of *NO represents the key branching point for selectivity. Subsequent hydrogenation of *NO leads to NH₃ through a series of intermediates (e.g., *NOH, *NHOH, *NH₂OH), while its dimerization or reaction with another *NO can lead to N₂O or N₂ [32]. On SACs, the unique coordination environment often favors the continuous hydrogenation pathway by stabilizing key intermediates like *NO and *NOH in an optimal energy window, thereby promoting NH₃ formation over N₂ [33]. The specific reactions of NO₃⁻ to NH₃ are as follows (Equations (2)–(8)):

NO₃⁻ ⇌ *NO₃⁻

(2)

*NO₃⁻ + 2H⁺ + 2e⁻ →*NO₂⁻ + H₂O

(3)

*NO₂⁻ + 2H⁺ + e⁻ → *NO + H₂O

(4)

*NO + e⁻ + H⁺ → *NOH

(5)

*NOH + e⁻ + H⁺ → *NHOH

(6)

*NHOH + e⁻ + H⁺ → *NH₂OH

(7)

*NH₂OH + 2e⁻ + 2H⁺ → *NH₃ + H₂O

(8)

The adsorbed hydrogen reduction occurs under both neutral and alkaline conditions [3]. The pathway involves the interaction of nitrate and its intermediates with chemically adsorbed hydrogen atoms (*H) generated through the Volmer step (Equation (10)). The role of *H is particularly significant on SACs, as their isolated nature can modulate *H binding energy, suppressing the competitive HER and enabling *H to act as a hydrogenating agent. Then, *H could assist in the reduction of adsorbed high-valence nitrate to low-valence NH₃ through intermediates with different valence states, including *NO₂, *NO, *N, *NH, and *NH₂ [34]. The specific reactions of NO₃⁻ to NH₃ are as follows (Equations (9)–(17)):

NO₃⁻ ⇌ *NO₃⁻

(9)

H₂O + e⁻→ *H+ OH⁻ (Volmer)

(10)

*NO₃⁻ + 2*H → *NO₂⁻ + H₂O

(11)

*NO₂⁻ + *H→ *NO + OH⁻

(12)

*NO + *H→ 2*N + H₂O

(13)

*N + *H → *NH

(14)

*NH + *H → *NH₂

(15)

*NH₂ + *H → *NH₃

(16)

*NH₃ → NH₃

(17)

While the aforementioned mechanistic framework provides a foundation for understanding the eNO₃RR, it must be acknowledged that a clear, atomic-level perception of the microscopic reaction network at the genuine catalytic interface remains elusive. Current research, particularly concerning SACs, still faces several severe challenges. Firstly, the experimental identification of key intermediates remains inadequate. Proposed intermediates such as *NOH and *NHOH are extremely short-lived, making them difficult to capture with conventional techniques; although in situ spectroscopic methods have advanced, the interpretation of their signals often relies on assistance from theoretical calculations, introducing a degree of uncertainty. Secondly, the “electron reduction” and “adsorbed hydrogen reduction” pathways are intricately intertwined in practical systems, making it difficult to quantify their respective contributions. The discussions often pertain to a macroscopic or thermodynamic propensity rather than a precise kinetic delineation. More importantly, most current mechanistic models stem from idealized computational simulations and gravely neglect the influence of the actual reaction environment, such as local pH, electric field effects, cation specificity, and dynamic changes in reactant concentration. These factors can profoundly alter the reaction pathway and the rate-determining step.

2.2. In Situ/Operando Characterizations

The eNO₃RR involves multiple proton-coupled electron transfer steps, generating a spectrum of transient and unstable intermediates that pose significant challenges for experimental characterization [35]. Conventional ex situ techniques, including electron microscopy and X-ray photoelectron spectroscopy, probe catalysts in a static, post-reaction state, rendering them inadequate for identifying true active sites or tracing dynamic reaction pathways [36]. Consequently, operando characterization techniques, which facilitate real-time monitoring under operational conditions, have become indispensable for elucidating the dynamic structural evolution of catalysts and reactants, thereby providing fundamental mechanistic insights [37]. This section critically reviews the application of pivotal operando spectroscopic and analytical techniques—including Fourier transform infrared (FTIR) spectroscopy, X-ray absorption spectroscopy (XAS), Raman spectroscopy, differential electrochemical mass spectrometry (DEMS), electron paramagnetic resonance (EPR)—in advancing our understanding of SACs for the nitrate-to-ammonia conversion.

2.2.1. Operando FTIR Spectroscopy

Operando FTIR spectroscopy is a highly sensitive vibrational technique exquisitely suited for detecting and identifying reaction intermediates adsorbed on the catalyst surface operational conditions. For instance, Xu et al. [38] employed synchrotron-radiation-based FTIR (SR-FTIR) to elucidate the eNO₃RR pathway on a Fe–N/P–C catalyst (Figure 2a,b). The SR-FTIR results demonstrated the initiation of NO₃⁻ reduction at a remarkably low overpotential (−0.2 V vs. RHE). The temporal evolution of the spectra revealed key mechanistic insights: positive peaks at 1390 cm⁻¹ and 1561 cm⁻¹ were assigned to NO₃⁻ (N–O stretching) and H₂O (O–H bending), respectively, while the progressive intensity increase in peaks at 1290, 1480, and 1630 cm⁻¹ indicated the deoxygenation and deep hydrogenation of the NO₂⁻ intermediate. Critically, the direct detection of NO₂⁻ and subsequent NHx species furnished compelling evidence for a reaction mechanism wherein NO₃⁻ is first reduced to NO₂⁻, followed by sequential hydrogenation steps to NH₃.

In a complementary study, Long et al. [39] applied operando FTIR to dynamically monitor reaction intermediates on copper single-atom catalysts with tailored coordination environments (Cu₁-N₂, Cu₁-N₃, Cu₁-N₄). Their monitoring revealed that the high-entropy Cu₁-N₄ configuration exhibited a faster deoxygenation rate, thereby minimizing the surface accumulation of NOx intermediates. This, coupled with a higher overall reduction rate, resulted in a substantial surface enrichment of NH₃ prior to desorption, directly linking the atomic-scale coordination environment to macroscopic catalytic performance.

2.2.2. Operando XAS

Operando XAS serves as a cornerstone technique for probing the atomic and electronic structure of SACs, providing unparalleled insight into the dynamic evolution of oxidation states, local coordination environments, and electronic configurations under realistic reaction conditions. Such information is crucial for establishing structure-activity relationships and elucidating the origins of stability [40,41].

In a study by Zhang et al. [42], operando XAS was employed to investigate a Fe-doped V₂O₅ catalyst (Fe-V₂O₅) during eNO₃RR. Under reaction conditions (−0.5 V vs. RHE with NO₃⁻), both the Fe and V K-edge XANES spectra (Figure 2c,e) exhibited shifts toward higher energies, accompanied by an intensified white-line peak. These features collectively indicated an elevation in the oxidation states of both metal centers, attributed to electron transfer from Fe/V sites to adsorbed NO₃⁻ for its activation. Concurrently, the corresponding EXAFS spectra (Figure 2d,f) displayed enhanced intensity and elongation of the Fe–O and V–O coordination peaks, confirming the bonding and stable adsorption of reaction intermediates. The synchronous evolution observed at both metal centers provided direct spectroscopic evidence for their synergistic role as co-active sites.

Figure 2. (a) Planar SR-FTIR spectra in the range of 800–2000 cm⁻¹ under different given potentials. (b) Time-dependent planar SR-FTIR spectra obtained at an applied potential of 0.4 V vs. RHE. Reproduced with permission [38]. Copyright 2023, Wiley. In situ Fe-edge XANES (c) and EXAFS spectra (d) of Fe-V₂O₅. Reproduced with permission. In situ V-edge XANES (e) and EXAFS spectra (f) of Fe-V₂O₅. Reproduced with permission [42]. Copyright 2023, Wiley.

Figure 2. (a) Planar SR-FTIR spectra in the range of 800–2000 cm⁻¹ under different given potentials. (b) Time-dependent planar SR-FTIR spectra obtained at an applied potential of 0.4 V vs. RHE. Reproduced with permission [38]. Copyright 2023, Wiley. In situ Fe-edge XANES (c) and EXAFS spectra (d) of Fe-V₂O₅. Reproduced with permission. In situ V-edge XANES (e) and EXAFS spectra (f) of Fe-V₂O₅. Reproduced with permission [42]. Copyright 2023, Wiley.

In another study on dual-atom catalysts, Lu et al. [43] utilized XAS to decipher the structure of a Fe/Cu-HNG catalyst. XANES analysis revealed significant charge redistribution between Fe and Cu, while the EXAFS data, particularly the absence of metal-metal scattering paths, provided definitive evidence for a hetero-binuclear Fe−Cu configuration, highlighting the power of XAS in resolving the precise structure of multi-atom active sites.

2.2.3. Operando Raman Spectroscopy

Operando Raman spectroscopy, a vibrational technique complementary to FTIR, probes chemical bond vibrations through inelastic light scattering and exhibits high sensitivity to changes in molecular polarizability [44]. It is widely valued for its rapid response, high spatial resolution, and non-destructive character [45].

Zhang et al. [46] employed operando Raman spectroscopy to investigate a Fe SAC (Figure 3a,b). The spectra revealed that at 0 V, the Fe sites were occupied by electrolyte sulfate ions (SO₄²⁻, ~980 cm⁻¹). Upon applying an eNO₃RR-driving potential (<−0.4 V), a pronounced increase in the NO₃⁻ signal (~1050 cm⁻¹) intensified markedly. This spectral evolution signified the potential-driven competitive adsorption of NO₃⁻ onto the active sites, a critical initial step for catalysis. Furthermore, the absence of spectral signatures attributable to iron nanoparticles or clusters throughout the process provided direct evidence for the exceptional structural stability of the Fe single atoms under operating conditions.

He et al. [47] utilized operando Raman spectroscopy to monitor the phase evolution of CuSP, CoSP, and CuCoSP catalysts. The dynamic spectral features offered direct evidence for a synergistic reaction pathway in the bimetallic catalyst, demonstrating the utility of Raman spectroscopy in deconvoluting complex tandem mechanisms.

2.2.4. Differential Electrochemical Mass Spectrometry (DEMS)

DEMS serves as an advanced analytical technique that combines electrochemistry with mass spectrometry, enabling the real-time identification and quantification of gaseous or volatile intermediates and products (e.g., NO, N₂, N₂O, NH₃) during electrocatalysis [48].

Zhu et al. [49] employed online DEMS to decipher the eNO₃RR pathway over a Cu-N₃ SAC. As shown in Figure 2b, the mass spectra displayed distinct signals corresponding to NH₂ (m/z = 16), NH₃ (m/z = 17), NO (m/z = 30), and H₂NOH (m/z = 33). The temporal evolution of these signals directly corroborated the hydrogenation pathway: *NO → *HNO → *H₂NO → *H₂NOH. More significantly, the consistent absence of a NO₂⁻ signal (m/z = 46) not only indicated a pathway bypassing NO₂⁻ but also underscores the exceptional selectivity of the well-defined Cu-N₃ site for NH₃ production.

In foundational research on model surfaces, Koper et al. [50] utilized DEMS to demonstrate high selectivity for N₂ formation on a Cu-modified Pt(100) electrode, highlighting the critical nitrate-to-ammonia step.

2.2.5. Electron Paramagnetic Resonance (EPR)

EPR, also referred to as Electron Spin Resonance (ESR), is a selective spectroscopic technique for detecting and characterizing paramagnetic species containing unpaired electrons, such as radical intermediates and certain metal ions [51,52].

To identify the origin of active hydrogen species, Wang et al. [53] performed operando EPR spectroscopy with DMPO as a spin trap. A characteristic nine-line signal corresponding to the DMPO–*H adduct was exclusively observed in the electrolyte surrounding the working Co-PBAs catalyst (Figure 3c). This result provided direct and unambiguous evidence for the electrocatalytic generation of *H by the catalyst. Similarly, operando EPR has also confirmed *H formation in other catalytic systems [54], highlighting its general utility in identifying crucial radical intermediates in eNO₃RR.

Figure 3. (a) In situ Raman spectra of the Fe SAC at varied potentials. (b) In situ DEMS measurements of Cu-N₃ SACs. Reproduced with permission [46]. Copyright 2023, Wiley-VCH GmbH. (c) ESR spectra of Co-PBAs and Co-PBA mixture in Ar atmosphere. Reproduced with permission [53]. Copyright 2022, American Chemical Society.

Figure 3. (a) In situ Raman spectra of the Fe SAC at varied potentials. (b) In situ DEMS measurements of Cu-N₃ SACs. Reproduced with permission [46]. Copyright 2023, Wiley-VCH GmbH. (c) ESR spectra of Co-PBAs and Co-PBA mixture in Ar atmosphere. Reproduced with permission [53]. Copyright 2022, American Chemical Society.

2.3. Theoretical Computations

Density functional theory (DFT) calculations have become an indispensable tool for deciphering the structure-activity relationships in SACs and the microscopic mechanisms of catalytic reactions. They can not only reveal the electronic structure of active sites and their interactions with reaction intermediates at the atomic scale but also predict catalytic performance and identify reaction pathways through quantitative computation, thereby guiding the rational design of high-performance catalysts.

2.3.1. Pre-Screening of Structural Stability and Active Sites

The DFT calculations provide the theoretical foundation for screening thermodynamically stable SACs structures by quantifying the interaction between metal atoms and the support. Key thermodynamic parameters, such as the adsorption energy and formation energy, directly reflect the stability of single-atom sites under synthesis and reaction conditions. For instance, Lim et al. [55] systematically calculated the adsorption energies of transition metal (TM) atoms on various nitrogen-doped graphene (NDG) sites to evaluate the binding strength between TM atoms and the support. Their computational results demonstrated that the PD2cGn double-vacancy structure, incorporating both pyridinic and graphitic nitrogen, could most robustly anchor TM single atoms. Electronic structure analysis further revealed the underlying stabilization mechanism: the introduction of graphitic nitrogen causes the conduction band to shift downwards across the Fermi level and induces a systematic downshift of the p-band center (from −4.63 eV to −4.86 eV) (Figure 4a). This modulation of the electronic structure enhanced the charge transfer and bonding strength between the support and the metal atom, providing crucial theoretical guidance for the pre-identification of highly stable TM-SACs/NDG structures. Similarly, Gao et al. [56] examined the eNO₃RR utilizing a defective titanium carbide MXene support (specifically, Ti₃C₂O₂ with an oxygen vacancy, denoted as TM/Ov-MXene). By employing DFT calculations to assess the formation energies of 22 distinct transition metal atoms on this support, they determined that Cu and Ag were exceptionally stable candidates.

2.3.2. Reaction Pathway Analysis and Identification of the Potential-Determining Step

By constructing free energy diagrams, DFT can accurately map the complete reaction network from NO₃⁻ to NH₃, identify key intermediates, and determine the PDS of the overall reaction, which corresponds to the elementary step associated with the theoretical limiting potential.

The study by Zhou et al. [57] served as a compelling demonstration of the predictive power of DFT. They first calculated the NO₃⁻ adsorption energies and the energy barriers of key steps for a series of TM/g-C₃N₄ catalysts, screening Ti/g-C₃N₄ as the optimal candidate due to its lowest theoretical limiting potential (−0.3 eV). The subsequently constructed free energy diagram clearly illustrated all reaction intermediates and elementary steps, clarifying the microscopic pathway for its efficient catalysis (Figure 4b). Likewise, Zuo et al. [58] employed DFT calculations to provide an in-depth elucidation of the intrinsic mechanism for the eNO₃RR on a p-CN-Cu^sLaⁿ-m catalyst. The researchers meticulously mapped out the complete reaction network, from the initial reactant NO₃⁻ through a series of key intermediates to the final product, NH₃. The calculations not only identified the crucial species along the reaction pathway but also revealed the existence of two competitive hydrogenation pathways following the formation of NO (generating NHO and *NOH, respectively), and confirmed that the p-CN-Cu^sLaⁿ-m catalyst preferentially selects the NHO pathway.

Figure 4. (a) Total density of states (TDOS) of PD2cGn (n = 1, 2, 3 and 4). Reproduced with permission [55]. Copyright 2023, Elsevier. (b) Gibbs free-energy step diagram for eNO₃RR on Ti/g-C₃N₄ catalyst. Reproduced with permission [57]. Copyright 2023, Elsevier. (c) Charge density difference in I adsorbed on Cu(100) in the I₁Cu₄ system and (d) Charge density difference in NO₃⁻ adsorbed on pure Cu and I₁Cu₄ sites. Reproduced with permission [59]. Copyright 2024, PNAS. (e) The correlation between the Bader charges of Cu on the electrocatalysts and the adsorption energy of *NO intermediates and (f) the correlation between the Bader charges of Cu on the abovementioned electrocatalysts and the adsorption energy of *H₂O intermediates. Reproduced with permission [56]. Copyright 2024, Royal Society of Chemistry.

Figure 4. (a) Total density of states (TDOS) of PD2cGn (n = 1, 2, 3 and 4). Reproduced with permission [55]. Copyright 2023, Elsevier. (b) Gibbs free-energy step diagram for eNO₃RR on Ti/g-C₃N₄ catalyst. Reproduced with permission [57]. Copyright 2023, Elsevier. (c) Charge density difference in I adsorbed on Cu(100) in the I₁Cu₄ system and (d) Charge density difference in NO₃⁻ adsorbed on pure Cu and I₁Cu₄ sites. Reproduced with permission [59]. Copyright 2024, PNAS. (e) The correlation between the Bader charges of Cu on the electrocatalysts and the adsorption energy of *NO intermediates and (f) the correlation between the Bader charges of Cu on the abovementioned electrocatalysts and the adsorption energy of *H₂O intermediates. Reproduced with permission [56]. Copyright 2024, Royal Society of Chemistry.

2.3.3. Deciphering Electronic Structure Modulation and Structure-Activity Relationships

To fundamentally understand the origin of catalytic performance, DFT is widely employed to analyze the electronic structure of catalysts, revealing how geometric coordination regulates electronic states and ultimately determines catalytic activity.

DFT calculations by Zhou et al. [59] precisely unveiled how I single atoms modulate the electronic structure of adjacent Cu sites. Charge density difference analysis indicated that I atoms withdraw 0.15 eV from Cu atom, resulting in a significantly positive charge on the Cu sites (Figure 4c). This electron redistribution further influenced the interaction between the catalyst and reactants: the adsorption of NO₃⁻ on the I₁Cu₄ site involved a charge transfer of 0.69 eV, substantially higher than that on the pure Cu site (0.58 eV) (Figure 4d). These computations confirmed at the electronic level that the introduction of I enhances the adsorption capacity of Cu for NO₃⁻ through a ligand effect, thereby optimizing the reaction pathway.

Similarly, employing DFT calculations, He et al. [56] verified that B atoms located in different coordination shells of the Cu-N₄ site could induce distinct strain effects, enabling “bidirectional regulation” of the Bader charge on the central Cu atom. By correlating reaction energy barriers with electronic descriptors (e.g., Bader charge) and constructing volcano plots, they successfully designed the Cu-N₄B₂ catalyst located at the peak of activity (Figure 4e,f). These profound electronic-level insights establish a design paradigm for optimizing catalytic performance by tailoring the electronic structure through coordination engineering.

3. Research Progress of SACs for eNO₃RR

Noble metal-based SACs, including ruthenium (Ru), palladium (Pd), platinum, and silver (Ag), have been explored for eNO₃RR [60,61,62,63,64]. Their efficacy stems from optimal d-band center positions, which facilitate moderate binding of reactants and intermediates [65,66,67]. While noble metal-based SACs demonstrate promising activity for eNO₃RR, their practical viability is fundamentally challenged by intrinsic scarcity, high cost, and a predominant propensity for the competing hydrogen evolution reaction (HER) that often dominates over the NO₃⁻ reduction pathway [68,69,70]. Consequently, the research focus has shifted to the development of earth-abundant non-noble metal SACs, aiming to achieve a more favorable economic and catalytic profile [71,72]. In this context, 3d transition metal-based SACs stand out (

Table 1. The eNO₃RR performance for non-noble metal-based SACs.

Catalysts	Voltage	Electrolyte	FENH3 (%)	Ammonia Yield Rate	Ref.
BCN@Cu/CNT	−0.6 V	0.1 M KOH + 50 mg L−1 NO3−	95.32	10.13 mmol h−1 mgcat−1	[74]
Cu12-NDI-H	−1.1 V	0.1 M KHCO3 + 0.5 M KNO3	98.7	35.1 mg mgh−1 mgcat−1	[75]
Cu/CuAu SAA	−0.5 V	1 M KOH + 1 M KNO3	85.5	8470 m mol h−1 g−1	[76]
Cu-SACs	−0.6 V	0.1 M KOH + 100 mM NO3−	97.37	3.36 mg h−1 cm−2	[77]
Cu/Ni-NC	−0.7 V	0.5 M Na2SO4 + 100 ppm NO3−	97.28	322.35 mmol h−1 mgcat−1 cm−2	[78]
Fe-BCN	−0.3 V	0.1 M KOH + 0.5 M KNO3	97.48	0.128 mmol h−1 cm−2	[79]
Fe-MoS2-SACs	−0.48 V	0.1 M Na2SO4 + 0.1 M NaNO3	98	0.432 mg h−1 cm−2	[80]
Fe-N-C/PdNC	−0.5 V	1 M KOH + 0.5 M KNO3	98.6	392.16 mmol h−1 gcat−1	[81]
Fe-PPy SACs	−0.7 V/ −0.3 V	0.1 M KOH + 0.1 M KNO3	~100	0.162 mmol h−1 cm−2	[82]
Fe-SACs	−0.66 V	0.1 M K2SO4 + 0.5 M NO3−	75	0.46 mmol h−1 cm−2	[83]
Fe-V2O5-SACs	−0.7 V	1.0 M KOH + 0.1 M KNO3	98	12.5 mg h−1 cm−2	[84]
Co SAs/CNFs	−0.7 V	0.1 M K2SO4 + 0.1 M KNO3	91.3	0.79 mmol h−1 cm−2	[85]
Co1Ru	−0.7 V	0.5 M Na2SO4 + 0.1 M NaNO2	92	2379.2 μmol h−1 cm−2	[86]
Co2-P/NPG	−0.7 V	0.5 M K2SO4 + 0.1 M KNO3	93.8	0.506 mmol h−1 mgcat−1	[87]
Co-SACs	−0.69 V	100 mgL−1 NO3− + 0.02 M Na2SO4	92	0.433 mg h−1 cm−2	[88]

). The flexibility of their partially filled d-orbitals allows for precise modulation of the local electronic structure. This tunability is crucial for optimizing the adsorption free energy of critical reaction intermediates (e.g., *NO₂, *NH), steering the reaction pathway away from HER and toward high-selectivity NH₃ synthesis. As a result, these non-noble metal centers can exhibit eNO₃RR activity and selectivity that are comparable to, or even exceed, those of noble-metal benchmarks [73]. Here, we focus on common transition metals (Cu, Fe, Co) and their composites rather than noble metals for the research formulation of SACs in eNO₃RR applications.

3.1. Non-Noble Metal-Based SACs

3.1.1. Cu-Based SACs

Among non-noble d-block transition metals, Cu SACs have emerged as a principal focus for eNO₃RR, owing to a confluence of favorable electronic properties and structural versatility. The intrinsic activity of Cu stems from its d-orbital electronic configuration, which aligns synergistically with the π* orbital of the NO₃⁻, thereby facilitating efficient electron transfer for the critical initial nitrate activation [89]. Atomic dispersion on engineered supports not only maximizes active site exposure but also provides a versatile platform for precise manipulation of the local coordination environment to enhance performance [90,91].

A predominant strategy involves anchoring Cu single atoms within nitrogen-doped carbon (Cu-NC) matrices. The Cu-Nx coordination structure is pivotal for modulating electronic properties and intermediate adsorption. For instance, Yin et al. reported a Cu SA/NC catalyst that achieved an NH₃ FE of 100% and a remarkable yield rate of 7480 μg h⁻¹ mgcat⁻¹ [92]. The critical role of atomic dispersion was underscored by Zhu et al., whose Cu−N−C catalyst demonstrated that Cu-N₂ sites facilitate stronger adsorption of NO₃⁻ and NO₂⁻ intermediates compared to Cu nanoparticles, effectively suppressing nitrite release and promoting complete reduction to NH₃ [93]. Advancing beyond simple Cu-Nx identification, Long et al. [39] designed a Cu SAC (HE Cu₁-N₄) with high active site exposure and a locally electron-deficient environment through modulating the mass ratio between the Cu-ZIF-8 precursor and melamine (Figure 5a). The HE Cu₁–N₄ exhibited exceptional catalytic performance, achieving 100% NH₃ selectivity over a wide range of NO₃⁻ concentrations and an NH₃ production rate of 5.09 mg h⁻¹ mgcat⁻¹ nearly 7-fold higher than that of the conventional Cu₁-N₂ benchmark (Figure 5b,c). Synchrotron radiation characterization and DFT calculations revealed that this superior performance originated from the tailored coordination environment, which creates an electron-deficient Cu center. The resulting electronic polarization enhances the electrostatic affinity toward NO₃⁻, significantly boosting the nitrate adsorption and subsequent conversion (Figure 5d).

The electronic structure of Cu-Nx sites can be further refined by the incorporation of secondary heteroatoms, such as boron (B) and phosphorus (P), which break the local symmetry and induce charge redistribution [94]. Zhao et al. synthesized a Cu SAC embedded in boron-carbon nitride (BCN-Cu), which maintained >95% NH₃ FE over a wide potential window [77]. Theoretical analyses identified the adsorption of NO₃⁻ as the potential-determining step and revealed that B-doping enhances the intrinsic activity of the Cu center. Similarly, Wang et al. [95] reported a single-copper-site catalyst anchored on N,P-co-doped carbon (Cu-N₄/P) was developed for highly selective nitrate-to-ammonia conversion. The Cu-N₄/P catalyst achieved an NH₃ FE of 95.89% and 100 % conversion of NO₃-N was achieved after 5 h of electrolysis. DFT calculations showed that P doping thermodynamically promoted the *NO hydrogenation step to form *NOH, thereby steering the reaction pathway toward selective NH₃ formation.

Departing from conventional symmetric coordination spheres, Cheng et al. pioneered the synthesis of a Cu SAC with a defined cis-N₂O₂ coordination geometry [96]. This asymmetric configuration was shown to lower the local structural symmetry, creating a highly polarized Cu site that favors NO₃⁻ adsorption. The catalyst demonstrated exceptional durability, operating stably for over 2000 h, and achieved an industrial-scale NH₃ yield rate of 27.84 mg h⁻¹ cm⁻² at 366 mA cm⁻². This work underscores the profound impact of low-symmetry, asymmetric coordination environments on both activity and stability, opening a new dimension in SAC design.

A fundamental challenge in single-site catalysis is the inherent scaling relations between the adsorption energies of different intermediates, which can constrain the optimization of multi-step reactions like eNO₃RR [97]. To circumvent this limitation, dual-single-atom catalysts (DSACs) have recently been developed, wherein two different metal atoms are co-anchored within the support, creating unique bimetallic sites for synergistic catalysis [98,99]. Wang et al. constructed a Cu/Ni-NC catalyst, where adjacent Cu and Ni atoms engage in strong electronic interaction, with charge transferring from Ni to Cu [78]. The Cu/Ni-NC catalyst achieved an NH₃ FE of 97.28% and a yield rate of 5480 mg h⁻¹ mgcat⁻¹ cm⁻² at −0.7 V vs. RHE, significantly outperforming Cu-NC and Ni-NC (Figure 5e,f). This superior performance was attributed to the strong electronic interaction between adjacent Cu and Ni atoms, characterized by pronounced electron transfer from Ni to Cu. As illustrated in Figure 5g, the hybridizations between the Cu-Ni dual sites in Cu/Ni-NC and the O atoms of NO₃⁻ were stronger than those of Cu-NC and Ni-NC and O atom of NO₃⁻, which could effectively form the chemical bonds of Cu−O−NO₃⁻−O−Ni and then promoted the electrons transfer from Cu−Ni dual-single-atom to *NO₃⁻. This synergistic configuration was shown to concurrently lower the energy barrier of the rate-determining step and suppress competing reaction pathways, namely N−N coupling (which leads to N₂O and N₂) and the hydrogen evolution reaction, thereby significantly enhancing the overall activity and selectivity. In a similar vein, Li et al. [100] reported FeCu-NPCs, where the Fe-Cu pair acted synergistically to enhance NO₃⁻ adsorption and reduce the reaction barrier, leading to a high NH₃ yield rate of 82.72 mg h⁻¹ mgcat⁻¹. The emergence of DSACs represents a paradigm shift beyond single-metal-site design, offering a promising avenue to break scaling relations and achieve superior catalytic performance for complex reactions.

In summary, the research landscape of Cu-SACs for eNO₃RR has matured from mapping fundamental Cu-Nx sites to the precise engineering of electronic and coordination environments. While these advances have yielded spectacular activities in laboratory settings, a significant gap persists between catalytic excellence and practical applicability. A central, unresolved challenge is the unambiguous identification of the active site’s dynamic structure under operating conditions, which often differs from its static, pre-catalytic state. This ambiguity, compounded by the lack of standardized performance metrics and stability assessments across studies, hinders the rational design of next-generation catalysts.

3.1.2. Fe-Based SACs

Among various reported metal-based SACs, Fe-based SACs are regarded as promising electrocatalysts for eNO₃RR due to their abundance, ease of preparation, and moderate adsorption ability of oxygen and nitrogen to iron [82,101]. Fe-based SACs maximize the exposure and uniformity of active sites by anchoring iron atoms in isolated form on specific supports, fundamentally addressing the issues of active site blocking and deactivation caused by metal agglomeration in conventional nanocatalysts. Their core advantage stems from the unique and tunable electronic structure and coordination microenvironment of the Fe centers, enabling precise optimization of the reaction pathway in eNO₃RR for simultaneous achievement of high activity and high selectivity towards NH₃ synthesis [102]. Seminal work by Wu et al. [83] reported the first construction of Fe-SACs with a well-defined Fe-N₄ configuration (Figure 6a), which achieved an NH₃ FE of approximately 75% and a yield rate of 5245 μg h⁻¹ mgcat⁻¹ (Figure 6b,c). Combined theoretical calculations revealed that the Fe-N₄ sites facilitate the adsorption and activation of NO₃⁻ effectively through a charge “acceptance-donation” mechanism. Concurrently, the spatial confinement effect inherent to isolated sites significantly suppresses undesirable N−N coupling side reactions, thereby preferentially steering the pathway toward NH₃ generation.

Dictated by the local coordination environment, the performance of Fe-SACs can be optimized through precise control over the coordinating atoms’ type, number, and geometry. This control enables optimized adsorption of key intermediates and lowered reaction energy barriers [103]. For instance, Song et al. [104] synthesized a Fe single-atom catalyst supported on g-C₃N₄ (FeSAs/g–C₃N₄) through a straightforward one-step hydrothermal method, achieving the embedding of isolated Fe atoms within the g–C₃N₄ matrix. X-ray absorption fine structure spectroscopy confirmed the formation of a Fe–N₄ coordination structure, which was identified as the active site responsible for the remarkable nitrate removal capacity of 9857.5 mgN g⁻¹ Fe in eNO₃RR. The Fe-N₄ centers exhibited enhanced NO₃⁻ dissociation, suppressed hydrogen evolution activity, and a lower energy barrier in the potential-determining step, collectively contributing to the highly efficient NO₃⁻ removal performance. Reducing the coordination number presents another viable tuning strategy. Liu et al. [105] reported a thermal-modulation strategy to engineer the coordination geometry of Fe single-atom catalysts for eNO₃RR. The optimized Fe₁/NC-900 catalyst, with a pyramidal Fe-N₃ structure, demonstrated outstanding performance, achieving a high yield of 18.8 mg h⁻¹ mgcat⁻¹ and an NH₃ FE of 86% (Figure 6d,e). Theoretical analyses revealed that the Fe-N₃ sites possess abundant lone-pair electrons, which strengthened the Fe–O bonding with oxygen atoms in NO₃⁻, enabled stronger charge transfer, and significantly reduced the energy barrier of the rate-determining step (Figure 6f). Furthermore, the introduction of heteroatoms into the coordination sphere can profoundly alter the electronic properties of the central metal site. Xu et al. [38] reported an iron source was introduced into the ZIF-8 support through electrostatic adsorption, followed by high-temperature pyrolysis to obtain Fe-N/P-C with a porous structure and abundant defects. Aberration-corrected electron microscopy and synchrotron radiation analyses confirmed an asymmetric Fe₁-N₃P configuration, featuring approximately one oxygen atom in the axial direction. DFT calculations revealed that the introduction of heteroatoms for synergistic coordination disrupts the symmetric electronic structure of the Fe center, which optimized the adsorption of nitrate and nitrogen-containing intermediates. The Fe-N/P-C catalysts exhibited a high FE of 90.3% and a yield of 17,980 μg h⁻¹ mgcat⁻¹, greatly outperforming the reported Fe-based catalysts.

Figure 6. (a) Schematic illustration of the synthesis of Fe SAC. (b) NH₃ FE of Fe SAC at each given potential. Red dot is FE estimated by three independent NMR tests. (c) NH₃ yield rate and partial current density of Fe SAC, FeNP/NC, and NC. Reproduced with permission [100]. Copyright 2021, Nature Communications. (d) NH₃ FE of the Fe1/NC-X at different potentials, (e) The corresponding NH₃ yield rate of the Fe₁/NC-X at different potentials. (f) Reaction free energies for different intermediates on Fe1/NC-800, Fe1/NC-900, and Fe1/NC-1000. Reproduced with permission [105]. Copyright 2023, Elsevier. (g) Schematic illustration of catalyst construction. (h) Crystal orbital Hamilton population (-COHP) and its integrated value (ICOHP) of *NO adsorption on different metal sites. (i) DEMS analyses of hydrogenation intermediates after the *NO adsorption step during the NO₃RR. Reproduced with permission [43]. Copyright 2023, Nature Communications.

Figure 6. (a) Schematic illustration of the synthesis of Fe SAC. (b) NH₃ FE of Fe SAC at each given potential. Red dot is FE estimated by three independent NMR tests. (c) NH₃ yield rate and partial current density of Fe SAC, FeNP/NC, and NC. Reproduced with permission [100]. Copyright 2021, Nature Communications. (d) NH₃ FE of the Fe1/NC-X at different potentials, (e) The corresponding NH₃ yield rate of the Fe₁/NC-X at different potentials. (f) Reaction free energies for different intermediates on Fe1/NC-800, Fe1/NC-900, and Fe1/NC-1000. Reproduced with permission [105]. Copyright 2023, Elsevier. (g) Schematic illustration of catalyst construction. (h) Crystal orbital Hamilton population (-COHP) and its integrated value (ICOHP) of *NO adsorption on different metal sites. (i) DEMS analyses of hydrogenation intermediates after the *NO adsorption step during the NO₃RR. Reproduced with permission [43]. Copyright 2023, Nature Communications.

Beyond direct modulation of the central Fe atom’s coordination environment, carrier engineering and the introduction of bimetallic synergistic effects provide crucial pathways for enhancing the performance of Fe-SACs [106,107]. The design of the carrier structure can create unique physicochemical environments. For instance, Zhang et al. [42] doped Fe single atoms into V₂O₅ to form an Fe-V₂O₅ catalyst. The Lewis acid sites (Fe–V pairs) in this catalyst strongly interact with NO₃⁻, which acts as a Lewis base, significantly promoting the activation and dissociation of NO₃⁻. Simultaneously, electrostatic repulsion suppresses the competing hydrogen evolution reaction, thereby achieving an NH₃ FE of 97.1%. Furthermore, constructing diatomic sites enables synergistic regulation of the electronic structure. Zhang et al. [43] fabricated a Fe/Cu diatomic catalyst (Fe/Cu-HNG) on holey nitrogen-doped graphene(Figure 6g), which exhibited high catalytic activities and selectivity for ammonia production. Theoretical analysis indicated that there was an appropriate interaction between NO₃⁻ and Fe/Cu, which facilitated the adsorption and release of NO₃⁻. As shown in Figure 6h, the relatively more positive ICOHP value (−14.04 eV) of NO molecules adsorbed at Fe/Cu sites suggested a pronounced weakening of the N−O bond, which can be attributed to the hetero-atomic structure. Furthermore, Owing to the significant activation at these Fe/Cu sites, the NH₂OH/NH₃ yield over Fe/Cu-HNG was markedly enhanced, reaching a level 8–10 times higher than that achieved with homogeneous Cu/Cu or Fe/Fe catalysts (Figure 6i). These experimental and theoretical results confirm that the strong coupling between NO₃⁻ and the d-orbitals of the bimetallic atoms reduced the energy barrier for anion adsorption, while the bimetallic atoms further weakened the N–O bond, resulting in a lower reaction energy barrier for NH₃ production.

3.1.3. Co-Based Catalysts

In contrast with other transition metals such as Fe and Cu, the electron-rich 3d orbitals of Co not only facilitate the adsorption and initial activation of NO₃⁻ but also effectively promote the cleavage of the N–O bond, thereby steering the reaction pathway efficiently and selectively toward the target product, NH₃ [108].

The catalytic performance of Co-SACs is highly dependent on their local coordination structure. Precisely tuning the atom type and coordination number within the first coordination shell can effectively optimize the electronic properties of the Co active center, consequently modulating its adsorption strength towards key reaction intermediates [109]. For instance, introducing phosphorus (P) atoms with lower electronegativity to partially replace nitrogen in the conventional Co-N₄ structure is an effective strategy for regulating the electronic structure of the Co center [88]. Li et al. successfully constructed Co-SACs with a CoP₄N₃ coordination configuration through solid-state grinding combined with high-temperature pyrolysis (Figure 7a). The discontinuous active sites formed by P doping demonstrated superior N–O bond activation capability compared to the traditional Co-N₄ structure, achieving a remarkable NH₃ FE of 92.0% and an NH₃ yield rate of 433.3 μg h⁻¹ cm⁻² (Figure 7b). Theoretical calculations further revealed that P incorporation induces asymmetric charge distribution and electronic reconstruction, significantly lowering the energy barrier of the rate-determining step. Furthermore, the coordination number also plays a critical role in catalytic activity (Figure 7c).

Beyond the modulation of the active sites themselves, support engineering plays a crucial role in stabilizing Co single atoms and tuning their electronic structure. Metal-organic frameworks (MOFs) serve as ideal precursors for constructing well-defined Co-SACs. To address the issue of mass transfer limitations predominant in microporous traditional MOF–derived carbon materials, constructing three-dimensional (3D) interconnected pore structures has proven to be an effective strategy. For example, the 3D mesopore-rich Co-SACs (MR Co-NC) prepared using polystyrene as a sacrificial template possessed a higher specific surface area (273.74 m² g⁻¹) and interconnected hierarchical pore channels (Figure 7d). This significantly enhanced the accessibility of active sites and the mass transport of electrolytes, collectively promoting the efficiency of NH₃ generation with a FE of 95.35 ± 1.75% and an NH₃ yield rate of rate of 1.25 ± 0.023 mmol h⁻¹ cm⁻² (Figure 7e,f). Theoretical calculations revealed a lower energy barrier of the rate-determining step on the MR Co–NC over β–Co(OH)₂ formed by the reconstruction of carbon-free Co nanoparticles (Figure 7g). Moreover, constructing diatomic sites enables synergistic catalysis through electronic interplay between metals [110]. Liu et al. [111] reported a bimetallic conductive MOF (CuxCoyHHTP), where the Co sites act as electronic modulators, effectively optimizing the electronic structure of adjacent Cu sites. This not only enhances NO₃⁻ adsorption but also lowers the energy barrier of the potential-determining step (*NO → *NOH). Concurrently, this synergistic effect promotes the conversion of *NO₂ to *NO, effectively suppressing the accumulation of the nitrite (NO₂⁻) byproduct. Ultimately, the catalyst achieved a high NH₃ FE of 96.4% and an NH₃ yield rate of 299.9 μmol h⁻¹ cm⁻² in eNO₃RR.

In summary, through various strategies including coordination engineering (e.g., P doping and low-coordination structure design) and support engineering (e.g., 3D mesoporous carbon construction and diatomic site introduction), the catalytic activity and product selectivity of Co–SACs for NH₃ production through eNO₃RR have been significantly enhanced. However, the further development of this system still faces challenges: On one hand, compared to the more resource-abundant Fe and Cu, the higher cost and relatively limited crustal abundance of Co may constrain its economic feasibility for large-scale practical applications. On the other hand, a deeper understanding of the dynamic evolution of the electronic structure of Co–SACs under realistic reaction conditions and the associated catalytic mechanisms still relies on the further integration of advanced in situ/operando characterization techniques and theoretical simulations. Future research should focus on developing more economical and greener synthesis routes for Co–SACs and systematically elucidating the intrinsic “structure-performance” relationship to promote the practical application of these catalysts in the fields of green ammonia synthesis and nitrate wastewater treatment.

3.2. Criteria for Selection of Catalysts

Several parameters are employed to evaluate the performance of catalysts in eNO₃RR, including the NO₃⁻ removal rate (R($\mathrm{N}{\mathrm{O}}_3^{-}$)), NH₃ selectivity (S(NH₃)), Faradaic efficiency of NH₃ production (FE(NH₃)), and NH₃ yield rate (Y(NH₃)). The following equations (Equations (18)–(22)) are used to calculate these parameters.

$$\mathrm{R}(\mathrm{N}{\mathrm{O}}_3^{-})=\left[{\displaystyle \frac{\left({\mathrm{C}}_0\left(\mathrm{N}{\mathrm{O}}_3^{-}\right) - {\mathrm{C}}_{\mathrm{t}}(\mathrm{N}{\mathrm{O}}_3^{-})\right)}{{\mathrm{C}}_0(\mathrm{N}{\mathrm{O}}_3^{-})}}\right]\times 100\%$$

(18)

$$\mathrm{S}\left({\mathrm{N}\mathrm{H}}_3\right)=\left[{\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}}\left({\mathrm{N}\mathrm{H}}_3\right)}{{\mathrm{C}}_0(\mathrm{N}{\mathrm{O}}_3^{-}) - {\mathrm{C}}_{\mathrm{t}}(\mathrm{N}{\mathrm{O}}_3^{-})}}\right]\times 100\%$$

(19)

$$\mathrm{FE}({\mathrm{N}\mathrm{H}}_3)=\left[(8\times \mathrm{F}\times {\displaystyle \frac{{\mathrm{C}}_{\mathrm{t}}\left({\mathrm{N}\mathrm{H}}_3\right)\times \mathrm{V}}{{\mathrm{M}}_{\mathrm{N}{\mathrm{H}}_3}\times \mathrm{Q}}})\right]\times 100\%$$

(20)

$$\mathrm{Y}({\mathrm{N}\mathrm{H}}_3)={\displaystyle \frac{({\mathrm{C}}_{\mathrm{t}}\left({\mathrm{N}\mathrm{H}}_3\right)\times \mathrm{V})}{(\mathrm{t}\times \mathrm{A})}}$$

(21)

or

$$\mathrm{Y}({\mathrm{N}\mathrm{H}}_3)={\displaystyle \frac{({\mathrm{C}}_{\mathrm{t}}\left({\mathrm{N}\mathrm{H}}_3\right)\times \mathrm{V})}{(\mathrm{t}\times \mathrm{m})}}$$

(22)

where c₀(NO₃⁻) is the initial concentration of NO₃⁻(mol L⁻¹), c_t(NO₃⁻) is final concentration of NO₃⁻ (mol L⁻¹), c_t(NH₃) is the molar concentration of produced NH₃ (mol L⁻¹), F is the Faraday constant (96,485 C mol⁻¹), V is the total volume of the electrolyte (L), ${\mathrm{M}}_{\mathrm{N}{\mathrm{H}}_3}$ is the molecular mass of NH₃ (17 g mol⁻¹), Q is the total charge passed through the electrode, t is the electrolysis time (s or h), V is the volume of electrolytes (L), A is the geometric area of the electrode (cm²), and m is the mass loading of the catalyst (mg).

3.3. Strategies for Enhancing SACs Performance

Prevailing strategies for augmenting the performance of SACs primarily encompass support engineering, coordination environment optimization, dual atomic synergy, and interface engineering. These methodologies are dedicated to the systematic design and customization of high-performance catalysts at the atomic scale [112].

3.3.1. Support Engineering

In SAC design, the support transcends its conventional role as a passive physical scaffold for mitigating metal atom migration and agglomeration. It functions as an active, functional matrix that directly participates in and modulates the catalytic microenvironment through strong electronic interactions and reactant enrichment effects. Engineering the support through defect engineering and heteroatom doping represents a principal avenue towards this objective [113].

(1)

Defect Engineering: Vacancy Anchoring and Electronic Structure Modulation

The precise construction of vacancies (e.g., carbon vacancies, oxygen vacancies) within carbon-based or metal oxide supports generates numerous unsaturated coordination sites, acting as effective “traps” for anchoring single atoms [114]. Beyond enhancing single-atom stability through strong interactions, these defects fundamentally reshape the local electronic environment of the active sites. Vacancies can capture and transfer charge to the anchored metal atoms or directly modulate the electronic properties of the support itself, thereby influencing the d-band structure of the metal centers through strong metal-support interactions (SMSI) [113].

For instance, Ren et al. [115] achieved stable anchoring of Cu single atoms by introducing Zn vacancies into a NiFe-LDH support. Their study revealed that the Zn vacancies served not only as potent anchoring sites but also significantly enhanced the catalyst’s eNO₃RR performance through induced SMSI, underscoring the promise of defect engineering in non-carbon-based supports. In the context of the oxygen reduction reaction (ORR), Tian et al. [116] demonstrated that carbon vacancies adjacent to Fe-N₄ sites in Fe–N–C catalysts effectively modulate the Fe center’s electronic structure, optimizing the adsorption free energy of oxygenated intermediates and consequently boosting catalytic activity and stability.

(2)

Heteroatom Doping: Charge Redistribution and Reaction Pathway Optimization

Incorporating heteroatoms (e.g., B, P, S) into the support lattice, particularly for carbon-based supports, constitutes another powerful strategy to disrupt the inherent charge neutrality and induce charge redistribution for fine-tuning the electronic structure [117]. The introduction of heteroatoms creates a locally polarized environment, altering the electron density of neighboring single-atom active sites. This optimizes their adsorption energies for reactants, intermediates, and *H, ultimately steering the reaction pathway kinetically toward the target product, ammonia. Boron doping serves as a prominent example. Huang et al. [118] engineered the second coordination shell of a Cu–N–C model catalyst with B atoms (forming a Cu-N₄B₂ configuration), successfully introducing compressive strain and elevating the Cu center’s valence state. This optimized electronic structure positioned the catalyst near the peak of the volcano curve for the adsorption energies of the key intermediate NO and reactant H₂O, simultaneously lowering the energy barrier for the rate-determining step (*NO → *NOH) and accelerating water dissociation to supply protons. Similarly, phosphorus doping exhibits excellent regulatory prowess.

3.3.2. Coordination Environment Optimization

The catalytic behavior of a single-atom center is directly dictated by its first coordination shell (comprising the identity of coordinating atoms, coordination number, and spatial geometry) [119,120]. Precise, atomic-level tuning of the coordination environment enables the targeted “tailoring” of key reaction intermediate adsorption behavior, representing the most direct strategy for enhancing the catalyst’s intrinsic activity.

(1)

Coordinating Atom Modulation

Employing heteroatoms with differing electronegativities (e.g., N, S, C, O) as coordinating atoms enables significant modulation of the central metal’s d-band electronic structure, thereby dictating its binding strength with reaction intermediates [121]. For the eNO₃RR, an ideal catalyst should strike an optimal balance in the adsorption strength of intermediates (e.g., *NO₂, *NO, *NOH), achieving both effective molecular activation and facile product desorption without causing active-site poisoning.

Exemplifying this, Xue et al. [122] reported a Cu SAC featuring a Cu(I)-N₃C₁coordination structure. Compared to the conventional, more symmetric Cu(II)-N₄ configuration, substituting one N atom with a less electronegative C atom localized charge around the Cu center, stabilizing the +1 oxidation state. This unique Cu(I)-N₃C₁ structure facilitated the cooperative adsorption of NO₃⁻ and H⁺ on adjacent Cu and C sites, respectively, achieving a balanced adsorption profile. Theoretical and experimental evidence confirmed that this structure significantly reduced the energy barriers for two key rate-controlling steps (*NO₃⁻ → *NO₂ and *NH₂ → *NH₃), rendering the reaction pathway thermodynamically more favorable for NH₃ generation.

(2)

Coordination Number Modulation

Reducing the coordination number of the central metal (e.g., from M-N₄ to M-N₃ or M-N₂) is a classical strategy for generating coordinatively unsaturated metal centers [123,124]. Low-coordination active sites typically exhibit heightened reactivity and enhanced adsorption capacity for reactants, as they expose a greater number of vacant orbitals, facilitating interactions with reactant molecules. This approach generates highly polarized active centers that effectively capture anionic NO₃⁻ reactants through strong electrostatic interactions, thereby promoting the initial adsorption and activation step.

Although the large-scale, stable synthesis of low-coordination SACs remains a formidable challenge, their significant potential has been illuminated by theoretical calculations. For instance, DFT studies by Leverett et al. [121] indicated that low-coordination Cu-N₂ sites are thermodynamically more favorable for NO₃⁻ adsorption compared to common Cu-N₄ sites, a factor considered crucial for enhancing the overall reaction rate. These theoretical insights offer compelling motivation for developing novel synthetic routes to achieve controllable preparation of low-coordination architectures.

3.3.3. Dual Single-Atom and Alloying Strategies

For complex cascade reactions like eNO₃RR, which involve multiple proton-electron transfer steps, single-metal sites are often hampered by scaling relationships between the adsorption energies of different intermediates, impeding simultaneous optimization of all key steps [125]. Constructing dual single-atom pairs (DSACs) or SAAs, which introduce a second metal center to engender synergistic effects, provides a new paradigm to circumvent this limitation.

(1)

Dual Single-Atom Catalysts: Synergistic Catalysis

Proximity anchoring of two complementary metal single atoms enables a synergistic catalytic effect. For instance, Suh et al. [126] developed a nitrogen-coordinated cobalt-copper hybrid single-atom/cluster catalyst (Co-Cu SCC). Electrochemical characterization revealed that the Co-Cu SCC outperforms both Co SCC and Cu SCC in terms of onset potential, FE, and yield rate, confirming the synergistic effect between the Co and Cu active sites. Theoretical calculations elucidated that the Cu sites primarily catalyze the conversion of NO₃⁻ to NO₂⁻, while the Co sites preferentially facilitate the reduction of NO₂⁻ to NH₃. This synergistic electroreduction within the Co−Cu SCC enabled the bypassing of the rate-determining step intrinsic to each individual active site, thereby lowering the overall reaction energy barrier.

(2)

SAA Strategy: Host Regulation and HER Suppression

SAA strategy involves dispersing one metal (the guest) as isolated atoms within the lattice or surface of another metal (the host). The catalytic properties of the guest single atoms are profoundly influenced by the host metal lattice through both geometric (ensemble effect) and electronic (ligand effect) contributions. This strategy demonstrates considerable promise in modulating catalytic selectivity, particularly for suppressing the competing HER.

A typical application entails using a base metal host to regulate the behavior of a noble metal guest. For instance, forming an SAA by embedding Pt single atoms into Cu nanoparticle surfaces can markedly weaken the adsorption energy of *H on the Pt sites. This effectively suppresses the HER while preserving high activity for NO₃⁻ conversion, thereby steering the reaction pathway preferentially toward NH₃ generation [125,127].

3.4. Interface Engineering

Achieving stable eNO₃RR at high current densities is pivotal for industrial application but imposes stringent demands on the catalyst’s electron transfer efficiency and structural integrity. Interface engineering, particularly constructing interfaces characterized by strong SMSI, is regarded as a potential strategy to address this challenge.

SMSI is central to interface engineering. Anchoring single atoms onto specific defect-rich metal oxide supports (e.g., TiO₂, CeO₂) can induce electron transfer from the support to the metal center, establishing a potent electronic coupling. For example, in photocatalytic NO₃RR studies, Hirakawa et al. [128] identified Ti³⁺ sites associated with defects (oxygen vacancies) on the TiO₂ surface as active centers for the selective production of NH₃. Hiramatsu et al. [129] observed that Cu doping in TiO₂ generated abundant, stable surface oxygen vacancies, where adjacent Ti³⁺ and Cu²⁺ sites constituted a synergistic catalytic center that promoted the reaction. These findings strongly suggest that precise anchoring of eNO₃RR-active single atoms (e.g., Co, Cu) onto reducible oxide supports rich in oxygen vacancies could form robust SMSI interfaces (e.g., Ti³⁺-Ov-Co³⁺), potentially establishing efficient electron “highways”. This would facilitate operation at high current densities while guaranteeing structural stability. The development of such SMSI-based oxide-supported SACs represents a critical and pressing frontier for exploration within the eNO₃RR field.

4. Effect of Microenvironment on eNO₃RR to NH₃

The microenvironment at the electrode–electrolyte interface—including the local pH, ion distribution, interfacial electric field, and the structure of the electrical double layer—is pivotal in dictating the reaction pathway, selectivity, and stability of eNO₃RR. This interfacial regime exerts its influence by directly modulating mass transport of reactants, the adsorption/desorption behavior of key intermediates, and the kinetics of proton-coupled electron transfer processes. Critically, these interfacial parameters do not operate in isolation but exhibit strong synergistic effects with the intrinsic electronic structure and coordination environment of SACs, collectively governing the overall catalytic performance [130].

4.1. Electrolyte pH

The bulk pH of the electrolyte is a key microenvironmental parameter that governs the thermodynamic and kinetic balance of the eNO₃RR. It directly affects the energy barriers of critical protonation steps and the progression of the competing HER by regulating proton availability and the catalyst surface charge state [131].

Under alkaline conditions, the high concentration of hydroxide ions (OH⁻) enhances the adsorption and activation of NO₃⁻ on the single-atom catalyst surface, facilitating more efficient electron transfer for its reduction to NH₃. Furthermore, the alkaline environment stabilizes key reaction intermediates (such as adsorbed hydrogen), thereby promoting NH₃ generation while effectively suppressing competing side reactions like the HER [132]. In acidic media, although the abundant proton supply facilitates PCET processes, it also promotes the leaching and deactivation of metal-based SACs and intensifies the HER, thereby severely compromising eNO₃RR selectivity [133].

Experimental studies strongly support the decisive influence of bulk pH. Guo et al. [134] reported that the peak current density on a Cu foil electrode in a strongly alkaline solution (pH = 13) was substantially higher than that in a strongly acidic solution (pH = 1) at the same NaNO₃ concentrations (0.01 M and 0.1 M). Similarly, Wang et al. [135] demonstrated that the ammonia production rate over Co@NC and Co/NC-800 catalysts in 0.1 M KOH reached 500 mmol h⁻¹ g⁻¹, far exceeding the 44 mmol h⁻¹ g⁻¹ observed in 0.1 M Na₂SO₄, underscoring the promoting role of an alkaline microenvironment.

In practical catalytic scenarios, the local pH at the catalyst surface often proves more critical than the bulk pH. Catalyst surface and interface engineering strategies, such as modulating surface hydrophobicity/hydrophilicity, can effectively regulate the diffusion of protons and hydroxide ions near the active sites, thereby creating a favorable local microenvironment [136,137]. A prominent example is the work by Shen et al. [138], who developed a Cu@hNCNC catalyst encapsulated within hydrophilic, hierarchical nitrogen-doped carbon nanocages. The hydrophilic hNCNC shell effectively traps electrogenerated OH⁻, creating a self-reinforcing local high-pH zone around the Cu nanoparticles. This ingenious design enabled outstanding eNO₃RR performance in a neutral bulk electrolyte, rivaling the activity of a comparable catalyst in an alkaline medium while achieving markedly superior stability, with an NH₃ yield rate of 4.0 mol h⁻¹ g⁻¹ and a high FE of 99.7% at optimum.

4.2. Electrolyte Composition and Interfacial Ion Behavior

The electrolyte composition critically governs eNO₃RR performance by shaping the interfacial environment beyond its role as a mere charge carrier. Specifically, the identity of ions (both cations and anions) and the concentration of the reactant directly modulate mass transport, reactant adsorption kinetics, and the structure of the electrical double layer, thereby exerting a profound influence on the activity and selectivity of SACs [139].

The identity of alkali metal cations (e.g., Li⁺, Na⁺, K⁺, Cs⁺) is pivotal in generating interfacial electric fields of varying strengths, dictated by their hydrated radius and charge density [140]. Smaller cations like Li⁺, with a high charge density, act as strong structure-making ions, leading to a more ordered water network and a stronger electrostatic field at the outer Helmholtz plane. This intensified field promotes the accumulation of NO₃⁻ anions at the catalyst interface, either through enhanced electrostatic attraction or direct ion-pairing, which concomitantly boosts both reaction activity and NH₃ selectivity. This effect is exemplified by the work of Fajardo et al. [141], where Li⁺-containing electrolytes minimized NO₂⁻ byproduct formation and achieved the highest NH₃ selectivity. On Cu-N₄ SAC sites, the superior interfacial field induced by Li⁺ resulted in an NH₃ yield rate 35% higher than that in K⁺-containing electrolytes.

Anions play a dual and often competing role in the eNO₃RR [142]. Multivalent anions such as SO₄²⁻ and PO₄³⁻ can significantly suppress activity, primarily due to electrostatic shielding of the catalyst surface and direct competitive adsorption with NO₃⁻ [143]. In contrast, certain monovalent anions like Cl⁻ can exhibit a concentration-dependent promotional effect. For instance, Cl⁻ doping can modulate the electronic states of catalytic active sites. In Cu SACs, Cl⁻ stabilizes electron-deficient Cu species, promoting NO₃⁻ adsorption and activation, lowering the reaction energy barrier, and enhancing both NH₃ selectivity and yield [144]. Furthermore, the presence of Cl⁻ can enhance the catalyst’s ability to dissociate interfacial water molecules, facilitating the generation of *H. This provides an ample supply of protons for the NO₃⁻ reduction process, suppresses the competing HER, and improves the FE of NH₃ synthesis [145].

The NO₃⁻ concentration defines the mass transport threshold, which is essential for sustaining a high-performance reaction microenvironment on SACs. A stark performance contrast underscores this principle: Ye et al. [146] achieved an ultrahigh NH₃ yield rate of 9.56 mmol h⁻¹ cm⁻² with 100% FE in a concentrated 1.0 M NaNO₃ solution. Conversely, Zhang et al. [147] reported a drastic decline to 0.018 mmol h⁻¹ cm⁻² and an FE of only 65% at a very dilute 0.005 M NaNO₃ concentration. These results collectively highlight that an insufficient reactant supply fails to maintain a high local NO₃⁻ concentration at the catalytic interface, especially under high-rate operation, thereby becoming a primary limiting factor.

4.3. Potential Regulation and Interfacial Charge Dynamics

The applied electrode potential serves as a powerful external driving force that, by coupling with the Fermi level of SACs, directly modulates the electron occupancy and valence state of active sites, thereby steering the reaction pathway and selectivity [148].

Using Cu SACs as a representative example, the reduction pathway and catalytic activity are intimately linked to the applied potential. During a cathodic sweep from 0.00 V to −1.00 V vs. RHE, the reduction of Cu²⁺ to Cu⁺ and subsequently to Cu⁰, alongside the aggregation of Cu⁰ single atoms, coincides with a significant enhancement in the NH₃ yield rate. A maximum production rate of 4.5 mg cm⁻² h⁻¹ (equivalent to 12.5 mol NH₃ gCu⁻¹ h⁻¹) with a high FE of 84.7% has been achieved at −1.00 V vs. RHE, a performance surpassing most previously reported Cu-based catalysts under comparable conditions [149].

Beyond static potential control, dynamic potential modulation strategies, such as pulsed electrolysis, offer an advanced means to optimize the interfacial reaction process. This technique operates by periodically switching the potential between distinct windows. During the lower potential phase, NO₃⁻ is preferentially reduced to NO₂⁻ intermediates, while the higher potential phase facilitates the further reduction of these intermediates to NH₃. This cascade reaction mechanism effectively suppresses the HER and minimizes byproduct formation, thereby significantly enhancing both the FE and yield rate of NH₃. For instance, when employing a Cu@Co/NC catalyst, pulse electrolysis achieved an approximately 50% improvement in the NH₃ yield rate compared to constant potential electrolysis [150].

5. Conclusions and Outlook

The eNO₃RR represents a paradigm-shifting technology that synergistically addresses the dual challenges of environmental remediation and sustainable chemical synthesis. SACs recognized for their exceptional atom utilization and tunable coordination structures, have emerged as a pivotal platform in this field, demonstrating exceptional activity and selectivity that often surpass noble metal benchmarks. This review has systematically chronicled the rapid advancements in SACs for eNO₃RR, spanning from fundamental reaction mechanisms and state-of-the-art in situ/operando characterization to the intricate structure-activity relationships of non-noble metal centers (Cu, Fe, Co) and sophisticated performance-enhancement strategies.

Notwithstanding these spectacular laboratory achievements, the journey from fundamental understanding to widespread practical deployment is fraught with multi-scale challenges. The transition of SAC-mediated eNO₃RR into a commercially viable and environmentally beneficial technology necessitates a holistic, systems-level perspective that bridges atomic-scale design with process engineering, economics, and lifecycle management.

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From Atomic Precision to Macroscopic Synthesis: Bridging the Materials Gap

At the material level, a primary challenge lies in the scalable fabrication of high-density, thermodynamically stable single-atom sites. Conventional synthesis routes often suffer from limited metal loadings and inadequate control over the local coordination environment, making the resulting isolated sites prone to migration, agglomeration, or leaching under operational conditions. Achieving atomic-level precision over the metal coordination sphere (e.g., ligand identity, coordination number, local geometry) remains a formidable task. Future research must prioritize the development of scalable methodologies, such as continuous-flow impregnation, atomic layer deposition, or solid-state mechanochemical synthesis, that can guarantee uniform active site distribution and high metal loadings (>5 wt%) without compromising stability. Concurrently, establishing standardized protocols for assessing the dynamic structural evolution and dissolution resistance of SACs under harsh electrochemical conditions is paramount for durability prediction.

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Decoding the Dynamic Interface: Integrating Multi-modal Operando Insights

On the mechanistic front, a fundamental bottleneck concerns the unambiguous identification of the dynamic active site structure and the complex reaction network it governs under working conditions. The true coordination and electronic states of SACs, along with their dynamic synergy with the support, remain inadequately resolved. The future lies in the simultaneous application of complementary operando techniques (e.g., combining XAS with Raman spectroscopy or online DEMS) coupled with microelectrode arrays to deconvolute the complex interplay between the “electron reduction” and “adsorbed hydrogen reduction” pathways. Furthermore, integrating these rich experimental datasets with multi-scale theoretical simulations, including machine learning potentials and grand canonical DFT, will be crucial for building predictive models that account for the dynamic electrode-electrolyte interface, electric field effects, and cation specificity.

(3)

System Integration and Techno-Economic Viability: The Path to Industrialization.

The ultimate benchmark for eNO₃RR technology is its performance in integrated systems, not just in H-cell reactors.

Coupling with Renewable Energy and Grid Services: The intermittent nature of solar and wind power demands eNO₃RR systems that are resilient to variable input. Research should explore pulsed electrolysis and intelligent energy management systems that not only enhance NH₃ selectivity and catalyst stability but also allow eNO₃RR plants to act as demand-side managers, consuming excess renewable energy and providing grid stabilization services, thereby improving overall economics.

Synergy with Wastewater Treatment Infrastructures: Integrating eNO₃RR modules into existing wastewater treatment plants offers a compelling pathway for decentralized ammonia production and nutrient pollution abatement. However, this requires catalysts and reactors resistant to fouling from complex matrices containing Cl⁻, SO₄²⁻, and organic matter. Pilot-scale demonstrations using real wastewater streams are urgently needed to validate long-term performance and refine system design.

Life-Cycle Assessment (LCA) and Techno-Economic Analysis (TEA): A critical, and currently underdeveloped, research area is the rigorous LCA and TEA of SAC-based eNO₃RR. Future work must quantify the environmental footprint from cradle-to-grave, including catalyst synthesis and end-of-life, to ensure a genuine advantage over the Haber-Bosch process. Concurrently, TEA models must identify key cost drivers, targeting critical benchmarks such as catalyst lifetime (>10,000 h), energy efficiency (<40 kWh kg⁻¹ NH₃), and operating current density (>500 mA cm⁻² in flow cells or MEAs) to achieve economic competitiveness. The dual revenue streams from ammonia production and wastewater treatment fees could significantly enhance the business case.

In summary, SACs have reinvigorated the field of eNO₃RR, offering a unique platform for sustainable nitrate-to-ammonia conversion. While persistent challenges in synthesis precision, mechanistic understanding, and system integration require continued focus, progress through interdisciplinary collaboration is charting a clear path forward. By uniting fundamental discovery with engineering principles and sustainability science, SAC-mediated eNO₃RR is poised to evolve from a promising laboratory phenomenon into a cornerstone technology for a decentralized green ammonia economy and sustainable nitrogen management, thereby contributing decisively to a carbon-neutral future.