Recent Advances in Designing Efficient Electrocatalysts for Electrochemical Nitrate Reduction to Ammonia

As a raw material for producing chemical fertilizers, ammonia plays an essential role in human production and life. Due to the severe energy consumption and pollution caused by the industrial Haber–Bosch process of NH3 synthesis, developing the NH3 synthesis reaction under ambient conditions is vital. Electrochemical nitrogen reduction reaction (NRR) has recently emerged as a potential method. However, its limited NH3 yield and selectivity are unsatisfactory. NO3 −, as an oxidized form of nitrogen, universally exists in drinking water (<50 mg L−1) and industrial wastewater (2000‐4000 mg L−1). Electrochemical nitrate reduction reaction (NO3 −RR), with higher production and Faradaic efficiency, is a promising strategy for water treatment and NH3 production. In this review, a detailed overview of the recent progress in NO3 −RR for NH3 production with precious group metal (PGM) electrocatalysts, PGM‐free electrocatalysts, and nonmetal electrocatalysts is summarized. In addition, effective design strategies for efficient electrocatalysts, existing challenges, and research prospects for the next stage are also discussed. This review may provide some directions for improving efficient electrocatalysts in electrocatalytic NO3 −RR and offer inspiration for the electrochemical ammonia synthesis process.


Introduction
As an essential chemical, ammonia is the primary raw material of the fertilizer industry. It has many applications in many fields, including national defense, coatings, and explosives. [1][2][3] Also, NH 3 is an ideal intermediate for energy storage and a carbon-free energy carrier for the renewable energy industry. [4][5][6][7] Currently, the Haber-Bosch method is also the critical process for industrial NH 3 production, which realizes the conversion of nitrogen (N 2 ) and hydrogen (H 2 ) in the atmosphere into NH 3 . [8][9][10] Although the technology of this process continues to develop and progress, it still cannot solve the problems of massive energy consumption and huge greenhouse gas emissions. [11][12][13] Researchers have received extensive attention for electrochemical nitrogen reduction reaction (NRR) for ammonia production at ambient conditions via renewable energy. [14][15][16][17][18] However, because of the high dissociation energy of the N ≡ N (941 kJ mol À1 ) in N 2 and the critical challenges of the competitive hydrogen evolution reaction (HER), the achievable ammonia production rate and Faradaic efficiency (FE) in most studies are still constrained, and far from reaching the standard of industrial ammonia production. [19][20][21][22][23] Like electrochemical NRR, electrochemical NO 3 À reduction reaction (NO 3 À RR) can serve as a promising low-temperature ammonia synthesis strategy. [24,25] From an environmental standpoint, converting nitrate, a toxic pollutant in water, into the highvalue chemical ammonia is tempting. [26] From the energy point of view, the dissociation energy of the N ¼ O (204 kJ mol À1 ) in nitrate is significantly lower than the N ≡ N, which is favorable in reaction kinetics. [27,28] Attributed to the kinetics of the NO 3 À RR can be optimized, it exhibited an excellent selectivity against the HER. NO 3 À RR can effectively reduce the nitrate content in the water and reduce the risk of causing diseases such as methemoglobinemia. [29] Overall, electrochemical NO 3 À RR can effectively enhance energy and the environment and achieve the target of carbon neutrality. [30][31][32] Furthermore, although the NO 3 À RR cannot solve the ultimate goal of ammonia synthesis as the nitrate in the industry is derived from the oxidation of ammonia, it can reduce the energy consumption of the ammonia production process. Moreover, the NO 3 À RR catalyst design can provide a novel strategy and method for the design of NRR electrocatalysts to promote the progress of ammonia synthesis. [33,34] There are also some limitations of the electrochemical NO 3 À RR process (NO 3 À þ 9 H þ þ 8e À ! NH 3 þ 3H 2 O), which includes the transfer of 9 protons and 8 electrons, during which various undesired by-products (i.e., NO 2 À , N 2 and N 2 H 4 ) are inevitably generated. [35][36][37][38] Meanwhile, the selectivity of NO 3 À RR is affected by overpotential and current density, only under low potentials, excellent FE, and selectivity attained. [39,40] As a result, it is indispensable to design electrocatalysts that have high activity and selectivity of NO 3 RR.
Recently, researchers have devoted much energy to designing and preparing electrocatalysts, which are used in electrochemical NO 3 À RR to realize efficient NH 3 production, which has become the latest hot spot in the field of electrocatalysis. Therefore, the recent progress in applying various electrocatalysts in electrocatalytic NO 3 À RR needs to be interpreted from the atomic point of view. In this timely and targeted review, we provide a detailed summary of recent advances in precious group metal (PGM) electrocatalysts, PGM-free electrocatalysts, and nonmetal electrocatalysts for the NO 3 RR, and sort out the relationship between material structure and properties and provide experience and guidance for the further development of electrocatalytic NO 3 À RR to ammonia production.

Electrocatalysts for the NO 3
À RR 2.1. Noble Metal-Based electrocatalysts for the NO 3 À RR Among several noble metal materials, ruthenium (Ru) is the most reported electrocatalyst for electrochemical NRR. [41][42][43] Recently, the application of Ru in the electrocatalysis of NO 3 À RR to produce ammonia has gradually increased.
For example, the strained Ru nanocluster catalyst designed by Li et al. [44] could realize the electroreduction of nitrate to ammonia at room temperature with fast, high selectivity, and strong current density. Its excellent performance is contributed to the tensile lattice strain, which can improve the hydrogen-hydrogen coupling barrier to inhibit HER, and the generated hydrogen radicals can effectively promote the reaction process to produce NH 3 . Therefore, the strained nanostructures exhibit ultrahigh ammonia rates (5.56 AE 0.18 mol h À1 g cat

À1
) and maintain high selectivity over a wide range of applied potentials. Furthermore, as the firm binding of Ru with subsurface oxygen dopant, it exhibits excellent durability under high current. This work confirms the excellent potential of the concept of strain-directed radical evolution in NO 3 À RR (Figure 1a-e). By converting NO 3 À to NH 3 in a polymer electrolyte membrane (PEM) cell, Bunea et al. [45] found that the best performance is exhibited in the Ru-based catalysts among carbon-supported metal (Cu, Ru, Rh, and Pd) electrocatalysts. Specifically, they Reproducted with permission. [44] Copyright 2020, American Chemical Society.
can achieve high Faraday efficiency (94%) by regulating the number of protons the PEM transmits to inhibit HER. Furthermore, 93% nitrate conversion was attained in galvanostatic electrolysis for eight hours at 10 mA cm À2 . Through first-principles calculations, Wu et al. [46] systematically investigated the activity and selectivity of tetragonal M 2 P (M ¼ Ni, Co, Ru, and Pd) as NO 3 À RR electrocatalysts. They found that the constructed Rudoped Co 2 P monolayer showed excellent activity by adjusting the d-band center (ε d ) value approach to the upper of the volcano. Furthermore, Ru doping is favorable in thermodynamics, meaningfully repressing the HER. The work of Wu et al. provides an efficient descriptor for designing NO 3 À RR electrocatalysts for sustainable ammonia synthesis. In another work, through first-principles calculations, Lv et al. [47] researched the NO 3 À RR performance of the TM/g-C 3 N 4 (Ti, Cr, Mn, Ru, Os, and Pt as the supported TM) single-atom catalyst (SAC). They preliminarily selected 6 TM/g-C 3 N 4 electrocatalysts, among which Ru/g-C 3 N 4 was the most potential catalyst due to its lowest high energy barrier and excellent selectivity. Moreover, the HER has also been suppressed as the unavailable adsorption for H atoms. The work of Lv et al. screening of NO 3 À RR catalysts will provide a stimulating impetus for experimental exploration.
Alloying catalysts promise to enhance the electrode's activity, selectivity, and stability due to synergistic interactions between both metal components. [48,49] Based on the prediction of electrocatalyst activity from theoretical volcano plots, Wang et al. [50] prepared Pt x Ru y /C(x ¼ 48%-100%) catalysts with various components for NO 3 À RR by a simple liquid-phase reduction method. The experimental results showed that the carbon-supported Pt nanoparticles (Pt 100 /C) revealed the lowest activity, while Pt x Ru y /C alloy is more active than Pt/C. The electrocatalytic NO 3 À RR performance of Pt 78 Ru 22 /C is 6 times higher than that of Pt/C electrocatalyst at 0.1 V versus RHE, with FE of 93%-98%. Compared with Rh, the cost of Pt and Ru is lower, which makes Pt 78 Ru 22 /C more cost-effective for electrocatalytic NO 3 À RR. In another study, Cerrón-Calle et al. [51] developed a bimetallic Cu-Pt foam electrode to improve the electrochemical reduction of nitrate by electrodepositing a small amount of Pt (<0.50 wt%) nanoparticles on the surface of a foamed copper substrate. The results showed that different Pt loadings (%0.30, %0.33, %0.36, %0.47 wt% electrodeposited at 60, 120, 180, and 360 s, respectively) on Cu foam were able to elevate nitrate conversion. For the Cu-Pt (180 s) electrode, the nitrate conversion was 94% within 2 h, and the ammonia productivity reached a maximum of 194.4 mg L À1 g cat À1 with selectivity to the ammonia of 84%. These encouraging results underscore the potential of the Cu-Pt foam electrode for ammonia synthesis with nitrate electroreduction.
The Pd is a brilliant material for H adsorption, which is beneficial in promoting the electrocatalytic NO 3 À RR to synthesize NH 3 . [52,53] Recently, Han et al. [54] investigated the Pd nanocrystalline as NO 3 À RR electrocatalysts. Importantly, Pd (111) shows the most brilliant NO 3 À RR performance, with a FE of 79.91% and ammonia yield of 0.5485 mmol h À1 cm À2 , which are 1.4 and 1.9 times that of Pd (100) and Pd (110), respectively. The density functional theory (DFT) study reveals the reason for the brilliant NO 3 À RR performance of Pd (111), the smaller free energy change of the rate-limiting step ( * NH 3 !NH 3 ), and the suppression of HER. The study of Han et al. focuses on the potential of Pd-based nanocatalysts. It provides a new perspective for applying other nanomaterials with well-desired faces in the field of NO 3 À RR (Figure 2a-c). Inspired by zinc-air batteries, Guo et al. [55] first proposed and developed a Zn-nitrate battery system using Pd-doped TiO 2 nanoarrays (Pd/TiO 2 ) grown on the carbon cloth (CC) as the catalyst electrode, with a power density of 0.87 mW cm À2 and a high NH 3 FE of 81.3%. In addition, when the Pd/TiO 2 nanoarray was directly used as a NO 3 À RR electrocatalyst, the adsorption capacity of the intermediate was weakened due to the introduction of Pd. The catalyst achieved an exceptional NH 3 yield of 1.12 mg cm À2 h À1 , a high NH 3 FE of 92.1%, and an excellent NO 3 À conversion rate of 99.6%. This work of Guo et al. confirms the positive effect of Pd doping in TiO 2 in promoting NO 3 À RR, and proposes a Zn-NO 3 À battery system pioneering, which provides an efficient strategy for electrochemical synthesis of NH 3 and broadens the application of Zn-based batteries (Figure 2d-h). In a further example, Wang et al. [56] 3 À RR with high activity and selectivity ( Figure 2i). More recently, silver (Ag) and iridium (Ir) have been investigated as NO 3 À RR electrocatalysts for the electrochemical production of NH 3 . Liu et al. [57] found that nanocrystalline Ag (nano-Ag) was an efficient catalyst for reducing nitrate to ammonia. At À0.33 V versus RHE, nano-Ag electrocatalyst can provide high Faradaic efficiency of 96.4% at the À194.31 mA cm À2 current density, with an excellent NO 3 À conversion rate of 98.5% and a high ammonia selectivity as high as 89.6%. Moreover, it exhibits incredible stability, maintaining 91.1% of its original selectivity and approximately 100% of its original Faradaic efficiency after four cycles, indicating that nano-Ag is an efficient electrocatalyst for NO 3 À RR. Zhu et al. [58] investigated the electrochemical NO 3 À RR activity of the hollow Ir nanotubes (Ir NTs), which were successfully synthesized by the self-template method. The particular 1D porous structure endows Ir NTs with a large surface area, a high electrical conductivity, and optimal atom utilization efficiency. Therefore, compared with the commercial Ir nanocrystals, the electrocatalytic activity of Ir NTs toward NO 3 À RR was significantly enhanced, with an NH 3 yield as high as 921 μg h À1 mg cat À1 and a FE of 84.7% at 0.06 versus RHE. This study suggests that Ir NTs may be a promising electrocatalyst for the electrochemical synthesis of NH 3 from NO 3 À RR. Through the discussion on applying noble metal-based electrocatalysts in electrocatalytic NO 3 À RR to ammonia production, we found that the current research body is mainly dominated by noble metals Ru, Pd, and Pt, and related work is slightly insufficient, possibly due to their high cost and scarcity fatal flaw. In addition, the activity and selectivity of PGM electrocatalysts are not satisfied as expected. The main reason is the formation of byproducts and competition of completive reactions. Doping noble metals with non-noble metals is an effective strategy to reduce cost and enhance the NO 3 À RR activity. Overall, the research we discuss on PGM electrocatalysts has contributed to the development of electrocatalytic NO 3 À RR to ammonia production to a certain extent ( Table 1).

Non-Noble Metal-Based electrocatalysts for the NO 3 À RR
PGM-free electrocatalysts have long been the focus of research in electrocatalysis as the low cost, high electrocatalytic activity, and potential for large-scale applications. [59,60] Recently, many PGM-free electrocatalysts have been investigated to promote electrocatalytic NO 3 À RR for ammonia production. [61] After a lot of research, classification, and summary, we will mainly introduce with permission. [54] Copyright 2021, Elsevier. d) Scanning electron microscope (SEM) images and energy dispersive X-ray spectroscopy spectrum (inset), e) the enlarged SEM image of Pd/TiO 2 nanoarrays; f ) 1 H NMR spectra of the electrolyte using 15 NO 3 À as the feedstock. g) NH 3 yield rates, h) NH 3 FEs for NO 3 À electroreduction into NH 3 over TiO 2 and Pd/TiO 2 ; i) the calculated adsorption energies for different intermediates on Pd/TiO 2 and TiO 2 surfaces.

Cu-Based electrocatalysts
Among the numerous nonprecious metals, Cu and Cu compounds are regarded as one of the most potential NO 3 À RR electrocatalysts, benefiting from their low cost and relatively high activity. [1,62] Through DFT calculations, Hu et al. [63] systematically investigated the pathway for NO 3 À RR on Cu surfaces with various pH values. The result showed that is the more thermodynamically favorable and the possible reaction pathway. Furthermore, the competition between HER and NO 3 À RR was found to be highly pH-dependent, and suitable pH ranges for NO 3 À RR on Cu (111), Cu(100), and Cu (110) were also calculated. Cu (100) and Cu(111) exhibit better NO 3 À RR performance. Specifically, Cu(111) exhibits the best NO 3 À RR performance in neutral or alkaline conditions, while Cu(100) is more efficient in strongly acidic environments. This work exhibits that the pH and surfaces are vital in NO 3 À RR. More recently, Fu et al. [64] researched the NO 3 À RR performance of Cu nanosheets under environmental conditions, achieving an NH 3 production of 390.1 μg mg À1 Cu h À1 and a FE of 99.7% at À0.15 V versus RHE. The brilliant NO 3 À RR performance of Cu (111) nanosheets is attributed to the suppressed HER activity and the enhanced rate of the rate-limiting step. This work of Fu et al. offers a powerful strategy for rationalizing robust and efficient catalysts through crystal facet engineering (Figure 3a-c). Wang et al. [65] constructed a novel 3D Cu nanobelt for NO 3 À RR. Notably, benefiting from the large surface area, the cathode significantly reduces the reaction rate of competing reactions, including HER and ORR, and enhances the charge transfer capability. Therefore, at À1.4 V (vs Ag/AgCl), the nitrate removal efficiency of the electrode reached 100% at 1 h, and the main product was determined to be ammonia.
Defect engineering is an adaptable method for modifying surface properties and electronic structures of electrocatalysts. [66,67] For example, Xu et al. [68] synthesized the defect-rich Cu nanoplates (dr-Cu NPs) by an in situ electroreduction of CuO nanoplates for efficient and selective electrocatalytic NO 3 À reduction to produce NH 3 . Defective nanoplates can advance the adsorption of NO 3 À and related intermediates on the electrocatalyst surface, and inhibit side reactions. Consequently, the as-prepared dr-Cu NPs exhibited a significant NH 3 yield of 781.25 μg h À1 mg À1 , an excellent NO 3 À conversion rate of 93.26%, outstanding ammonia selectivity of 81.99%, and a high FE of 85.47%, superior to the defect-free Cu nanoplates. This study sheds new light on efficient NO 3 À RR electrocatalyst design by exploiting defect engineering (Figure 3d-g). Moreover, Wang et al. [69] constructed the Cu/oxygen vacancy-rich Cu-Mn 3 O 4 heterostructured ultrathin nanosheet arrays on Cu foam to form Cu/Cu-Mn 3 O 4 NSAs/CF by a facile one-step hydrothermal method. In particular, the structure has a rich Cu/Cu-Mn 3 O 4 interface and abundant O v . Therefore, Cu/Cu-Mn 3 O 4 NSAs/CF showed brilliant NO 3 À RR performance, with excellent nitrate conversion rate (95.8%), good NH 3 selectivity (87.6%), ideal NH 3 yield (0.21 mmol h À1 cm À2 ), and high faraday efficiency (92.4%) at À1.3 V (vs SCE). This research demonstrates that an electrocatalyst constructed by a combined interface and defect engineering strategy can efficiently convert nitrate to ammonia. In another example, Hu et al. [70] proposed a method to synthesize Cu(100)-rich rugged Cu-nanobelt (Cu-NBs-100), during which the NO 3 À RR intermediate had a strong interaction with the Cu(100) facets, which could significantly facilitate the exposure of the Cu(100) surface. They found that the rough surface of Cu NBs resulted in many defects on the Cu(100) plane. DFT calculations and adsorption experiments show that Cu(100) facets and surface defects can synergistically enhance the adsorption strength of NO 3 * and H * , thereby reducing the reaction barrier of NO 3 À RR and inhibiting HER, respectively. As a result, the as-prepared Cu-NBs-100 exhibits excellent NO 3 À RR performance, with a high FE of 95.3% and a maximum NH 3 yield of 650 mmol h À1 g cat À1 at À0.15 V versus RHE. This finding provides a new way to use the synergistic effect of Cu(100) and defects to promote NO 3 À RR to NH 3 generation. Benefiting from the maximum atom utilization efficiency and brilliant catalytic performance, SACs have recently attracted wide attention. [71,72] The preparation of single-atom Cu catalysts by reducing the size of Cu is a promising strategy. By pyrolyzing a Cu-based metal-organic framework (MOF), Zhu et al. [73] successfully synthesized a Cu-N-C electrocatalyst. They found that the Cu-N x sites were crucial in promoting NO 3 À and NO 2 adsorption. Therefore, the Cu-N-C catalyst can effectively promote NO 3 À RR at a significantly higher rate. This study demonstrates the promise of Cu-SAC electrocatalysts for NO 3 À RR (Figure 4a-c).
The rational engineering of the metal-oxide interface can enhance the performance of NO 3 À RR. [74][75][76] Based on this, [77] a simple surface engineering strategy successfully synthesized the Cu nanowires with concave-convex surface Cu 2þ1 O layers  To deeply investigate the influence of the Cu oxidation state on the electrocatalytic performance of the NO 3 À RR, [78] the CuO nanowire arrays (CuO NWAs) as an effective catalyst for the NO 3 À RR showed excellent electrochemical performance. Specifically, at À0. 85 (Figure 5a-c).
In a further example, Qin et al. [79] explored the effect of crystal interface interactions on the NO 3 À RR by synthesizing Cu 2 O with different characteristics of (100) and (111) facets. The results show that Cu 2 O(100) exhibits an excellent NH 3 production of 743 μg h À1 mg cat À1 with a high FE of 82.3% at À0.6 V versus RHE, which was attributed to a relatively lower energy barrier (0.18 eV) for NH 3 production than the Cu 2 O(111) surface (1.43 eV). This study provided a novel approach to enhancing the selectivity of NO 3 À RR by surface engineering (Figure 6a-c). In another work, Yuan et al. [80] reported that oxidized Cu formed on Cu electrodes could improve the electrocatalytic activity and selectivity for NO 3 À RR to produce ammonia. Specifically, they fabricated an oxide-derived Cu (OD-Cu) electrode, which  [64] Copyright 2021, Elsevier. d) Linear sweep voltammetry (LSV) curves of dr-Cu NPs; e) FE and NH 3 yield at different potentials; f ) NH 3 yield over different catalysts. g) The standard curve of the integral area. Reproducted with permission. [68] Copyright 2021, Royal Society of Chemisty.  (Figure 6d,e). In addition, Geng et al. [81] prepared CuO x nanoparticles rich in oxygen vacancy(O v ) through laser irradiation and used them in a fluidized electrocatalytic system for ammonia production from NO 3 À RR.

RR.
Reproducted with permission. [73] (Figure 7a-c). In addition, they [86] also obtained Cu nanoparticles encapsulated in a vesicle-like BCN matrix (BCN@Cu  [88] reported a strategy to combine nitrate electroreduction to produce NH 3 with storage by carefully synthesizing a single-site Cull-bipyridine-based thorium MOF (Cu@Th-BPYDC). The coordination structure of Cu was determined to be a square coordination structure as measured by the single-crystal X-ray diffraction, indicating that the coordination of Cu was unsaturated. Therefore, Cu@Th-BPYDC exhibits brilliant performance in electroreduction of NO 3 À to produce NH 3 , achieving a high FE of 92.5% and an NH 3 production yield of 225.3 μmol h À1 cm À2 . Furthermore, the Cu@Th-BPYDC material can be used to efficiently capture the NH 3 produced by NO 3 À RR, providing up to 20.55 mmol g À1 .
( Figure 8a-f ). Chen et al. were inspired by enzymes with unique structures (highly active metal centers and protein scaffolds). [89] They studied the direct electrocatalytic NO 3 À RR to produce NH 3 with high selectivity by Cu-incorporated crystalline PTCDA (Cu-PTCDA).   (Figure 8g-j). Alloying Cu with second metals such as Pd and Ni can remarkably enhance the catalytic performance for NO 3 À RR to ammonia production because of the synergistic effect between elements of the catalyst and the modulated electronic structure. [91,92] For instance, Xu et al. [93] (Figure 9a-e). In a further example, Fang et al. [95] prepared a variety of Cu-Ni alloy nanoparticles with different Cu:Ni ratios dispersed on porous N-doped carbon film (Cu x Ni y /NC), and evaluated the electrocatalytic activity and corresponding durability of NO 3 À RR. The Cu 0.43 Ni 0.57 /NC exhibits the highest catalytic activity compared to Cu/NC and other ratios of Cu x Ni y /NC. Furthermore, they explain that introducing Ni lowers the energy barrier from *NO 2 to *NO, which improves the catalytic performance. Simultaneously, the porous and encapsulated structure enables Cu 0.43 Ni 0.57 /NC to expose active sites and maintain good long-term durability effectively. This research demonstrates that the electronic structure and morphology of the catalyst have an essential effect on the NO 3 À RR activity (Figure 9f,g).
Wang et al. [96] demonstrated the enhanced performance of the NO 3 À RR over the Cu 50 Ni 50 alloy catalyst. The performance improvement contains an upshift in the half-wave potential of 0.12 V and a decrease in the overpotential of 0.2 V for the optimal NH 3 FE. A 6 times increase in NO 3 À RR activity on the Cu 50 Ni 50 alloy catalysts compared with the pure Cu catalyst at 0 V. DFT calculations found that the introduction of Ni atoms to form the alloy catalyst moves the PDS from NO 3 À adsorption to * NH 2 hydrogenation because of the improved E ad of NO 3 À on the CuNi catalyst surface, thereby reducing the overpotential ( Table 2).

Fe-Based electrocatalysts
Among all the transition metal elements on the earth, Fe is one of the most abundant ones, which is the active center of all the nitrogenase enzymes, playing an essential function in the electron transfer process for enzyme catalysis. [97,98] Recently, Fe has been proved active for the electrocatalytic NO 3 À RR. [99] Inspired by the single site of Fe, Li et al. used a ferric acetylacetonate/ Reproducted with permission. [90] Copyright 2021, Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com polypyrrole hydrogel precursor, [100] and successfully prepared a nitrogen-coordinated Fe single-atom catalyst uniformly dispersed on carbon (Fe-PPy SACs) through a polymer-hydrogel strategy, which exhibited outstanding selectivity and activity for electrochemical NO 3 À RR in an alkaline environment.
Specifically, the prepared Fe-PPy SACs catalyst acquires a maximum NH 3 yield of 2.75 mg h À1 cm À2 with nearly 100% FE. Furthermore, the turnover frequency of atomically isolated Fe catalytic sites in Fe-PPy SACs catalysts is 12 times higher than that of the Fe nanoparticles. Experimental result proposes that the Fe single-site catalyst would experience a nitrate-preoccupied transition center, barring the water adsorption (Figure 10a-d).
In another study, Wu et al. [101] successfully synthesized Fe SAC by a transition metal-assisted carbonization method with SiO 2 powers as the hard templates in the preparation process. The as-prepared Fe SAC catalyst was found to efficiently enhance the electrocatalytic NO 3 À RR with a maximum FE of %75% and an ammonia yield as high as %20 000 μg h À1 mg cat À1 . Through DFT calculations, the reaction mechanism for reducing NO 3 À to NH 3 at Fe-SAC sites was revealed, and the reduction of  NO * to HNO * and HNO * to N * was found to be potential limiting steps. Iron carbide (Fe 3 C) possesses high electrical conductivity and unique electronic structures akin to noble metals and exhibits excellent stability in alkaline solutions. [102] Recently, Wang et al. [103] fabricated N-doped carbon nanosheet-supported Fe 3 C nanoflakes (Fe 3 C/NC) with a large surface area (860.024 m 2 g À1 ). Its NO 3 À RR performances exhibited a volcano-like connection with the Fe 3þ /Fe 2þ ratios and the center of the d-band. Specifically, at À0.5 V versus RHE, Fe 3 C/NC attained a brilliant NO 3 À RR performance, with the NH 3 production of 0.476 mmol h À1 cm À2 , FE of 96.7%, selectivity of 79.0%, and current density of 85.6 mA cm À2 , respectively, which was better than most reported results. This remarkable performance is mainly due to the optimized electronic structure, which facilitates the NO 3 À adsorption and reaction kinetics with the Tafel slope of 56.7 mV dec À1 (Figure 10e,f ).
With plentiful surface vacancies and hydrophilic functional groups, the metal-cyano polymer is considered to be an available design method of superhydrophilic material to enhance the NO 3 À RR performance. Fang et al. [104] developed a 2D Fe-cyano polymer nanosheet (Fe-cyano NS) electrocatalyst using the in situ electrochemical reductions for NO 3 À RR. Contributing to its strong adsorption of NO 3 À on the Fe 0 site and the superhydrophilic surface with abundant electrocatalytic active sites, Fe-cyano NSs exhibit an outstanding ammonia yield of 42.1 mg h À1 mg cat À1 and a high FE of 90.2% at À0.5 V versus RHE. Furthermore, the 2D interconnected structure alleviates the problem of nanosheet restacking during the electrocatalysis process, resulting in efficient and stable NO 3 À RR catalytic activity. This research provides an alternative approach to the topological transformation of TM nanosheets for efficient NO 3 À RR (Figure 11a-e). Motivated by the structure of enzyme's active sites, Li et al. [105] investigate a new heterogeneous Fe SAC supported on 2D MoS 2 (Fe-MoS 2 ) for efficient electrocatalytic NO 3 À RR to synthesize ammonia. Specifically, Fe-MoS 2 has the highest Faradaic efficiency (98%) for converting NO 3 À RR to NH 3 at À0.48 V versus RHE. DFT studies indicate that the improved selectivity for NH 3 yield by a Fe SAC supported on MoS 2 is because of a 0.38 eV decrease of the reaction pathway energy barrier associated with deoxidation of * NO intermediate to * N intermediate. Furthermore, when the Fe-MoS 2 catalyst is assembled into an InGaP/GaAs/Ge triple-junction solar cell, a solar-driven process with solar-to-ammonia CE of 3.4% and a NH 3 production of 510 μg h À1 cm À2 can be achieved. This work of Li et al. exposed a novel avenue to design a SAC for solar-driven ammonia production (Figure 11f-h). Fan et al. [106] investigated in situ grown Fe 3 O 4 particles on stainless steel (Fe 3 O 4 /SS) for efficient electrocatalytic NO 3 À RR. The in situ growth strategy guarantees an efficient electron transfer path due to the direct contact of Fe 3 O 4 with the metal substrate. The test results show that Fe 3 O 4 /SS achieves a high FE of 91.5% and an exceptional NH 3 production of 10 145 μg h À1 cm À2 at -0.5 V versus RHE, significantly better than many previously reported electrocatalysts. In addition, it has excellent structural and electrochemical stability. This study of Fan et al. provides helpful guidance for expanding the scope of metallic oxide electrocatalysts for NO 3 À RR ( Table 3).

Co-Based electrocatalysts
As a strategic resource in the 21st century, metallic Co has the intrinsic catalytic ability for NO 3 À electroreduction, and much attention has been paid to Co-based catalysts. [107,108] For instance, Deng et al. [109] successfully prepared the metallic Co nanoarrays (Co-NAs) by electrochemical reduction of Co(OH) 2 -NAs, and were used for electrochemical NO 3 À RR production of NH 3 . Benefiting from the superior intrinsic NO 3 À RR activity of in situ generated Co 0 and abundant electrocatalytic active sites, the Co-NAs electrode produces a current density of À2.2 A cm À2 and an ammonia yield of 10.4 mmol h À1 cm À2 at À0.24 V versus RHE. Moreover, the NH 3 yield and FE are sustained for 10 h at À500 mA cm À2 (Figure 12a-c). Through theoretical and experimental screening of the influence of late transition metals (Fe, Ni, Co, Cu, and Zn) on the electrochemical reduction of NO 3 À to NH 3 , Kani et al. [110] found that Co had good activity and showed that oxide-derived Co was an efficient NO 3 À RR catalyst. Specifically, oxide-derived Co exhibited the highest specific activity and selectivity. Meanwhile, a maximum FE of 92.37 AE 6.7% and an NH 3 production current density of 565.26 mA cm À2 were exhibited at À0.8 V versus RHE. In addition, they integrated an oxide-derived Co catalyst into a photovoltaic electrolyzer, which uses solar cells to power the electrolysis, and achieved a staggering STF efficiency of 11% for NH 3 .
Cobalt phosphide (CoP) nanomaterials have broad application prospects in the conversion of NO 3 À to NH 3 and hydrogen production as their high activity, selectivity, and stability. [111,112] On account of the precipitation transformation and hyperthermy phosphidation method, Jia et al. [113] successfully prepared porous and amorphous structured CoP nanoshuttles (CoP PANSs) and further studied the NO 3 À RR performance in the neutral electrolytes. Benefiting from their shuttle-like morphology with porous structure, amorphous crystal structure, and high surface area, the CoP PANSs catalyst shows excellent electrocatalytic NO 3 À RR performance, achieving a high FE of 94.24 AE 2.8%   (Figure 13a-c). Hong et al. [114]  RHE. In addition, Ye et al. [115] reported CoP nanosheet arrays were grown on carbon fiber cloth (CoP NAs/CFC) for NO 3 À RR to produce ammonia with high Faradaic efficiency and selectivity (% 100%), as well as an excellent ammonia yield rate (9.56 mol h À1 m À2 ) and excellent stability. They found that Figure 11. a) Preparation of the Fe-cyano NPs and b) Fe-cyano-R NSs; the NH 3 yield rate and FE on the c) Fe-cyano-R NSs and d) Fe-cyano-R NSs; e) selectivity of Fe-cyano-R NSs. Reproducted with permission. [104] Copyright 2021, American Chemical Society. f ) Potential-dependent FE of NH 3 on Fe-MoS 2 , MoS 2 , Cu, Fe foil, the carbon support as a comparison; g) cycle test of the FE; h) scaling relationship between the energy barrier and onset potential of electrocatalytic NO 3 À RR. Reproducted with permission. [105] Copyright 2021, Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com phosphorus is essential for stabilizing the active phase and optimizing the energy barrier of NO 3 À RR. Furthermore, in situ XAS revealed for the first time that the electric field-induced 3d-4p electronic transition of Co is closely related to the nitrate reduction reaction (Figure 13d-g). Li et al. [116] reported that a Co-P alloy film electrodeposited on the Ti plate (Co-P/TP) is an electrocatalyst with high activity toward NO 3 À RR with excellent selectivity for NH 3 generation (Figure 13h-j). Noticeably, in the electrolyte of 200 ppm NO 3 À and 0.2 M Na 2 SO 4 , the as-prepared Co-P/TP catalyst provides a respectable NH 3 yield of 416.0 AE 7.2 μg h À1 cm À2 at À0.6 V versus RHE, and a high FE of 93.6 AE 3.3% at À0.3 V versus RHE.
Huang et al. [117] developed a 3D flower-like zinc cobaltite (ZnCo 2 O 4 ) electrocatalyst to convert nitrate to ammonia at room temperature. The enhanced NO 3 À RR activity is contributed to the charge transfer from Co to Zn atoms generating the formation of electron-deficient Co active sites, which can lower the energy barrier for * NO 2 formation and suppress the HER process, thereby enhancing the yields of FE and ammonia. Specifically, in the electrolyte of 100 mM KNO 3 and 0.1 M KOH, the as-prepared ZnCo 2 O 4 catalyst exhibits NH 3 Faradaic efficiency as high as 95.4% at À0.4 V versus RHE and an outstanding NH 3 yield of 2100 μg h À1 mg À1 at À0.6 V versus RHE. It has excellent structural and morphological stability, surpassing most reported PGM-free electrocatalysts. This finding opens an avenue of bimetallic-based electrocatalysts for NO 3 À RR. To further improve electrocatalytic performance, Wang et al. [118] synthesized ultrathin CoO x nanosheets with a large amount of adsorbed oxygen by a simple one-pot method, which exhibited brilliant NO 3 À RR performance in converting nitrate to produce ammonia by electrochemical reduction. In detail, the catalyst can provide an ultrahigh ammonia production of 82.4 AE 4.8 mg h À1 mg cat À1 and a FE of 93.4 AE 3.8% at À0.3 V versus RHE. DFT calculations indicated that adsorbed oxygen was Figure 12. a) Calculated NH 3 production rate and TOF of Co-NAs; b) chronopotentiometry test of Co-NAs; c) reaction free energies for different intermediates on different surfaces toward NO 3 À RR. Reproducted with permission. [109] Copyright 2021, Wiley-VCH GmbH. The NH 3 yield rate is in the InGaP/GaAs/Ge triple-junction solar cell. critical to suppressing HER and enhancing the electrocatalytic NO 3 À RR activity of ultrathin CoO x nanosheets. This research demonstrates the feasibility of enhancing the electrocatalytic activity and ammonia selectivity of NO 3 À RR by introducing negatively charged species on the catalyst surface (Figure 14a-d).
Yu et al. [119] fabricated the heterostructured Co/CoO nanosheet arrays (Co/CoO NSAs) on nickel foam (NF) and used them as electrocatalysts for the NO 3 À RR. The experimental results show that at À1.3 V versus SCE, the heterostructured Co/CoO NSAs exhibit excellent FE (93.8%) and ammonia selectivity (91.2%), Figure 13. a) Synthetic procedures of CoP PANSs; b) SEM of CoP PANSs; c) Faradaic efficiency of NO 3 À RR and NH 3 yield at CoP PANSs at different potentials. Reproducted with permission. [113] Copyright 2021, American Chemical Society. Crystallographic structures of d) CoP and e) metallic Co; f ) histogram of FE for NH 3 , NO 2 À , and H 2 for CoP NAs/CFC and Co NAs/CFC at various potentials; g) histogram of yield rates for NH 3 of CoP NAs/CFC, and Co NAs/CFC at various potentials. Reproducted with permission. [115] Copyright 2022, Royal Society of Chemisty. h) Calculated FE and NH 3 production of Co-P/TP toward NO 3 À RR at different given potentials; i) calculated FE and NH 3 production of Co-P/TP and TP; j) NH 3 yield rates and FEs during the recycling tests. Reproducted with permission. [116] Copyright 2021, Royal Society of Chemisty.
www.advancedsciencenews.com www.small-structures.com which are significantly better than Co NSAs. Electrochemical in situ Fourier transform infrared spectroscopy, DEMS, and DFT indicate that the excellent NO 3 À RR performance originates from the Co/CoO heterostructure. Specifically, the electron transfer from Co to CoO at the interface can either repress the competitive HER or enhance the energy barrier to generate by-products, thereby improving the Faradaic efficiency and selectivity of NH 3 . This study offers a facile strategy to construct efficient electrocatalysts for NO 3 À RR ( Table 4).

Other Non-Noble Metal-Based electrocatalysts
Ni-based electrocatalysts have caused much concern for NO 3 À RR. Very recently, Yao et al. [120] prepared nickel phosphide (Ni 2 P) with (111) facet by an in situ growth approach on the Ni foam (NF) with a one-step phosphating process. Because of its unique superhydrophilic surface with abundant surface sites, metallic properties, and low impedance, Ni 2 P/NF is available for the adsorption of NO 3 À , the generation of * H, and the hydrogenation of NO 3 À to produce ammonia. The synthesized Ni 2 P/ NF has excellent NO 3 À RR performance as a self-supporting cathode with high NH 3 selectivity (89.1%) and excellent FE (99.2%). This study demonstrated Ni 2 P/NF with low energy consumption and excellent stability, which offers a facile and facile strategy for electrocatalytic NO 3 À reduction to synthesize ammonia (Figure 15a,b). Wei et al. [121] employed a single Ni 2 P phase as a novel PGM-free catalyst for the hydrogenation of nitrate to produce NH 3 . The introduction of P contributes to the Ni 2 P(001) facet possessing an H saturation density that is one-fifth to one-sixth lower than those of Ni (111) and Pd (111). This means that the phosphide phase could accommodate the otherwise better, very weakly coordinating NO 3 À and leads to 96% NH 3 selectivity. Jia et al. [122] designed and prepared the oxygenvacancy-enriched TiO 2 nanotube (TiO 2Àx ) catalysts for highly active and selective nitrate electroreduction. At À1.6 V versus SCE, it exhibits an ideal NH 3 production of 0.045 mmol h À1 mg À1 with a high FE of 85.0%, an excellent selectivity of 87.1%, and a superior nitrate conversion rate of 95.2%. In addition, the performance remains basically unchanged after 8 cycles of testing, proving that TiO 2Àx has excellent electrochemical stability. DEMS and DFT studies demonstrate the role of oxygen vacancies Reproducted with permission. [118] Copyright 2021, American Chemical Society. performance. This strategy provides guidance for designing rational nanostructured electrocatalysts for the electrocatalytic NO 3 À RR to produce ammonia with high activity and selectivity ( Figure 15c-e). Through DFT calculations, Niu et al. [123] comprehensively investigated the potential of SACs supported on graphitic carbonitride (Ti/g-CN and Zr/g-CN) as NO 3 (Figure 16a-d).
Indium (In) has been explored as a catalyst for NO 3 À RR. Lei et al. [124] reported the incorporation of In into sulfur-doped graphene for highly selective electroreduction from nitrate to ammonia. Benefiting from the unique structure endowed by the bond combination of In, S, and C, the catalyst possesses high active sites and a hybrid electronic structure. Thus, the catalyst can effectively catalyze nitrate reduction. Specifically, at the optimal potential of À0.5 V versus RHE, it can exhibit a total ammonia production of 220 mmol h À1 g cat À1 and a FE of 75%. This work provides an efficient strategy for developing In-based catalysts for efficient ammonia synthesis (Figure 17a-f ).
www.advancedsciencenews.com www.small-structures.com The above is a summary of the introduction of various PGM-free electrocatalysts in the NO 3 À RR. Compared with precious metals, we found that researchers are more inclined to study nonprecious metals with low cost and abundant reserves, which lays the foundation for the industrial production of ammonia through NO 3 À RR conversion in the future. The PGM-free electrocatalysts exhibited excellent activity, but the selectivity needs to be enhanced. Crystal face regulation, oxidation state regulation, defect engineering, heteroatom doping, and alloying engineering are effectivity strategies for leading NO 3 À RR research direction for future development. Meanwhile, more efforts should be made to study the mechanism of the NO 3 À RR. The relationship between reaction intermediates, products, and reaction pathways, as well as morphology and performance, needs further elucidation ( Table 5).

Metal-Free electrocatalysts for the NO 3 À RR
Recently, carbon-based materials attracted interest as electrocatalysts for NO 3 À RR. [61] To increase the NH 3 selectivity, Li et al. [126] reported a charcoal electrode made by carbonizing natural wood for the electrocatalytic NO 3 À RR to produce ammonia. Benefiting from its high HER overpotential, oxygen-containing surface groups, and moderate sp 3 C structure, the charcoal electrode exhibits excellent NO 3 À RR activity and ammonia selectivity under ambient conditions. Specifically, at À3.6 V versus Hg/Hg 2 SO 4 , the nitrate conversion rate, NH 3 selectivity, and NH 3 production can reach 91.2%, 96.0%, and 0.325 mmol L À1 h À1 cm À2 , respectively (Figure 18a-d). Li et al. [127] prepared a fluorine-doped carbon (FC) by pyrolysis of the discarded cigarette filter saturated with polytetrafluoroethylene solution. They found that the as-prepared FC electrocatalyst Figure 16. a,b) Free energy diagrams of NO 3 À RR reaction pathways; c,d) corresponding structures of electrocatalytic NO 3 À RR intermediates. Reproducted with permission. [123] Copyright 2020, Wiley-VCH GmbH.
www.advancedsciencenews.com www.small-structures.com exhibited excellent NO 3 À RR performance with an NH 3 yield of 23.8 mmol h À1 g cat À1 and a FE of 20%, which was 4 and 2-fold higher than that of the bare carbon electrocatalyst. In addition, the FC electrocatalyst also achieves excellent electrochemical stability. DFT calculation results show that F doping effectively decrease the energy barrier for NO 3 À hydrogenation and suppresses the HER. This work will promote the constructing of high-performance metal-free electrocatalysts for NO 3 À RR to produce ammonia (Figure 18e,f ).
Huang et al. [128] successfully prepared polymeric graphitic carbon nitride (g-C 3 N 4 ) with a controllable number of nitrogen vacancies (NVs) for NO 3 À RR. The g-C 3 N 4 with optimized NV concentration exhibited high FE (89.96%), NH 3 selectivity (69.78%), and excellent stability. DFT calculations show that NVs can introduce new electronic states at the Fermi level to facilitate nitrates' adsorption, subsequent activation, and dissociation. Furthermore, appropriate NV content is beneficial to adjust the adsorption energy of the reaction intermediates ( * NO, * NOH, * NH 2 , etc.), and helps improve NH 3 selectivity and FE (Figure 19a-d).
Although metal-free catalysts are rarely reported in electrocatalytic NO 3 À RR synthesis of ammonia, they have been widely used in other fields. In our view, metal-free materials as the carrier and non-noble metal as the active center are an effective strategy for improving the activity, selectivity, and stability of Reproducted with permission. [124] Copyright 2020, Elsevier. it is crucial to develop more metal-free catalysts for electrochemical NO 3 À RR to synthesize ammonia in the future. This is also a critical step in converting them into practical applications ( Table 6).

Conclusion and Future Outlook
The same as electrochemical NRR, electrochemical NO 3 À RR is considered to be a promising strategy in the field of electrocatalytic ammonia synthesis. It has a low reaction energy barrier, Figure 18. a) Schematic diagram of reactor; b) concentration of NO 3 À and ammonium; c, d) conversion rate of NO 3 À and selectivity of NH 3 .
Reproducted with permission. [126] Copyright 2021, Elsevier. e) FE and NH 3 production; f ) FE and nitrite yield rate of bare C and FC electrocatalysts. Reproducted with permission. [127] Copyright 2021, Elsevier. which can not only solve the problem of chemical energy but also save energy and protect the safety of the water environment. This review summarizes the latest research results of electrocatalytic NO 3 À RR to ammonia production in detail from the perspective of catalysts. Although the research on this topic has made significant progress in recent years, there are still a series of fundamental challenges, such as generally high overpotential, low FE, poor stability, and competitive hydrogen evolution reaction that need to be solved urgently: 1) Electrocatalytic nitrate reduction is an immature emerging technology. There is still a lack of discussion on the durability of electrocatalysts in the long-term electrolysis process, and the system of electrocatalyst development has not been perfected; 2) Given the electrocatalytic NO 3 À reduction to NH 3 , many by-products such as N 2 , NO, NH 2 OH, and H 2 may appear. Therefore, it is crucial to elucidate the actual catalytic process, the leading role of the active catalytic site, and the catalytic mechanism, and researchers need to explore further. For the nine protons and eight electrons chemical reaction, high activity, selectivity, and stability catalysts urgently promote ammonia production and reduce the formation of by-products; 3) The use of advanced operational characterization techniques can provide a comprehensive understanding of the complex reactions carried out on different catalysts, which can help to deepen the knowledge of the kinetic mechanism and catalyst synthesis, which can further promote the development of electrocatalytic nitrates; 4) At present, the research in this field mainly focuses on the exploration of metal materials, and there is less research on supported or metal-free catalysts. In line with the green chemistry concept of energysaving and emission reduction, we vigorously develop , and ammonia over PCNV-600; d) conversion rate, selectivity, FE, and NH 3 production over different samples. Reproducted with permission. [128] Copyright 2021, American Chemical Society. www.advancedsciencenews.com www.small-structures.com economical and green carbon-based materials, which are used as catalysts for ammonia production by electrochemical reduction of nitrates, laying the foundation for its future large-scale industrial applications; 5) Focus on the development of data reproducibility and accuracy. The surrounding environment quickly affects ammonia detection, and a complete and accurate product detection system is conducive to better developing electrocatalytic ammonia production; 6) Pay attention to the combination of theoretical calculation and experiment. Theoretical calculations should be regarded as a powerful tool for catalyst screening, which can quickly select suitable catalysts, significantly reducing the experimental cost to achieve high NO 3 À RR catalytic performance and guiding further development in this field.
Electrocatalytic NO 3 À RR, with drinking water and industrial wastewater conversion, can simultaneously decrease nitrogen pollution and enhance ammonia manufacturing. Electrocatalytic NO 3 À RR to obtain NH 3 directly from non-N 2 sources is a promising strategy for producing ammonia under ambient conditions. The comparison and combination of NO 3 À RR with NRR are strongly encouraged, which can significantly accelerate the research and development of related technologies to overcome the existing shortcomings. In order to achieve the practical application of NO 3 À RR, designing a high activity, selectivity, and stability catalyst is necessary.
Overall, the NO 3 À RR electrocatalysts summarized in this review are driving this technology's development. This progress will provide a profound understanding and inspiration for the rational design of electrocatalysts that can be applied on a large scale in the future. Furthermore, researchers were encouraged to study the NO 3 À RR according to the above six points and expect to offer some inspiration for NRR to promote the progress of ammonia synthesis. We hope this review will be helpful to researchers in the field of electrocatalytic NO 3 À RR to ammonia production.
Haiding Zhu received his bachelor's degree from Dalian University of Technology in 2020. Currently, he is pursuing his master's degree at Dalian University of Technology, supervised by Dr. Anmin Liu. His research focuses on the electrocatalytic NH 3 synthesis at ambient conditions using MXene-based materials.
Xiaoxuan Yang was a visiting Ph.D. student at University at Buffalo, the State University of New York (SUNY). She obtained her Ph.D. at Northeast Normal University in 2021. Currently, she is a postdoc at Zhejiang University. Her research interests mainly focus on the design, synthesis, and characterization of functional nanomaterials for electrochemical energy storage and conversion. He joined UB in 2014 as an assistant professor and was promoted to tenured associate professor in 2018 and a full professor in 2020. His research interests are electrochemical energy science and technology, with an emphasis on advanced electrocatalysis for fuel cells, water electrolzyers, CO 2 reduction, and chemical electrosynthesis. He is a highly cited researcher ranked by Thomson Reuters, Clarivate Analytics since 2018.