Recent Advance in Heterogenous Electrocatalysts for Highly Selective Nitrite Reduction to Ammonia Under Ambient Condition

Industrial ammonia production mainly relies on the conventional Haber–Bosch process accompanied by high energy consumption and plentiful carbon dioxide emissions, which triggered the recent interest to explore more energy‐efficient and environmentally benign alternatives. Very recently, electrochemical nitrite reduction in an aqueous medium promises new opportunities for advanced, energy‐efficient, and sustainable ammonia production at ambient conditions. The ammonia formation rate and Faradic efficiency are strongly associated with the adopted electrocatalysts; therefore, striving for high‐efficient electrocatalysts is the key to sustainable ammonia production via the electrochemical nitrite reduction reaction. Herein, a critical overview of recent advances in electrochemical nitrite reduction reaction to ammonia is presented, highlighting the latest innovative heterogenous electrocatalysts including noble metal catalysts, transition‐metal‐based catalysts, and their compounds. Meanwhile, the possible reaction pathway of nitrite electroreduction to ammonia, ammonia detection, and catalytic activity descriptor are briefly summarized. Finally, the perspective and research challenges of electrocatalysts that convert nitrite to ammonia are outlined, increasing their contributions in the route of realizing a neutral carbon footprint.


Introduction
Ammonia (NH 3 ) serves as an essential fundamental chemical to fabricate fertilizers that help support nearly half of the world's population.[3] The formation of NH 3 is a highly significant and captivating subject in the field of chemistry, which has led to the recognition of three Nobel Prizes in Chemistry (1918, 1931,  2007).In the biological system, NH 3 is synthesized by bacterial nitrogenase that are mainly composed of iron-molybdenum cofactor under benign conditions 3À where ATP and ADP represent the adenosine triphosphate and adenosine diphosphate, respectively), and this biological fixation process becomes the only possible approach for several billion years.Without any doubt, naturally synthesized NH 3 cannot meet the requirement for human survival and development. [4]Until the beginning of 20th century, the innovative invention of the Haber-Bosch process (HBP) by reacting nitrogen (N 2 ) with hydrogen under a high temperature (350-450 °C) and pressure (150-200 bar) altered this phenomenon fundamentally.And currently, large-scale NH 3 production heavily replies on the HBP.However, HBP consumes more energy and exhausts more carbon dioxides than any other processes associated with the large-volume chemicals manufactured worldwide.A decisive reason is hydrogen production, which is derived from the fossil fuels like reforming of coal gasification and steam methane.Besides, the harsh operating condition also needs substantial capital investments in the plants that implement HBP, making NH 3 production very centralized and difficult to couple with renewable sources of hydrogen from the water splitting. [5]In this regard, it is urgent to explore sustainable and green alternatives to replace the energy-intensive HBP for environmentally friendly NH 3 production.
Electrochemical N 2 reduction in aqueous solution has emerged as attractive alternative to the HBP for ambient NH 3 synthesis, which utilizes water as the hydrogen source.8][9][10] Nonetheless, such method suffers from bottlenecks of low Faradic efficiency (FE) and yield rate on account of the extremely stable N≡N bond and low solubility of nitrogen, as well as the competitive hydrogen evolution reaction (HER) owing to that its theoretical reaction potential (0.09 V versus reversed hydrogen electrode (RHE)) for the N 2 reduction reaction (NRR) is closed to that of HER.Although considerable recent efforts have been devoted toward the development of catalytic systems for the NRR, the practical NH 3 yield and FE still fail to meet the standard of industrial requirements.
[13][14] Such characteristics guarantee that the NO 2 À reduction reaction (NO 2 À RR) is both much thermodynamically and kinetically facile in contrast to NRR, anticipating to provide a practical route for NH 3 electrosynthesis with mild operating conditions.On another hand, NO 2 À is currently regarded as a contaminant widely existing in industrial wastewater and groundwaters owing to anthropogenic chemical production and crop fertilization, and its excessive emission and accumulation can destroy the ecological balance and result in harm human health.It is thus of great significance to environmental protection, public health, restoration of nitrogen cycle balance to electrochemically convert NO 2 À to NH 3 at room temperature from the point of "turn waste into wealth".Analogous to NRR, NO 2 À RR consumes water as the hydrogen source instead of fossil fuels and powered by sustainable energy (i.e., solar, wind, and wave), reducing the consumption of energy and emission of greenhouse gases.[17][18][19] In a word, NO 2 À RR represents one promising technology for NH 3 production and simultaneously NO 2 À removal, and unsurprisingly the extensive research attentions have been invested for this field.
In principle, the electroreduction reaction of NO 2 À to NH 3 usually involves six-electron pathway (Figure 1), which means the existence of multiple favored NO 2 À reduction products in thermodynamics and many of which possesses near reduction potentials, presenting a challenge to product selectivity.Additionally, intermediate generated during the reaction process of NO 2 À such as NH 3 OH þ can react with NO 2 À in solution to generate unwanted products such as N 2 O. [20] In this regard, it is imperative to exploit and design advanced electrocatalysts with high activity, selectivity, and excellent durability for selective NO 2 À to NH 3 conversion.Encouraged by the biological denitrification processes (microorganism), the homogenous electrocatalysts naturally have been explored to convert NO 2 À to NH 3 under ambient conditions.First, the homogenous catalysts are typical molecular complexes containing transition metals as active centers, where the metal sites can adsorb the reactants and store charges for converting reactants to products.Second, they can be dissolved uniformly in electrolytes, allowing the reaction systems free from reactant mass transfer limitations.Therefore, these homogenous electrocatalysts deliver high NH 3 selectivity and FE.[23] Therefore, exploring advanced heterogeneous electrocatalysts with excellent electrochemical activity for NO 2 À reduction is in urgent demand.Very recently, massive efforts have been denoted to design high-selective heterogenous electrocatalysts for NO 2 À RR to NH 3 conversion under mild operating conditions, making this field become the limelight.Consequently, an overview exclusively focused on rational design of heterogenous electrocatalysts with effective and facile strategies toward NO 2 À to NH 3 conversion is required and will hopefully promote further development within this field.Consequently, this review summarized the recent advancement in various electrocatalysts for NO 2 À RR to NH 3 , placing the emphasis on the catalytic activities and diverse designing strategies of catalysts.Meanwhile, we generalized the relationship between material structure and properties, as well as provided some insights into the development of electrocatalysts for NO 2 À RR to NH 3 .Finally, the remaining challenges and future direction in this emerging area are outlined.Thermodynamically, N 2 and NH 3 /NH 4 þ are the most stable products, and the latter is considered an optimal product from the perspective of energy.Thus, understanding the entire procedure of nitrite electroreduction is necessary for the electrocatalysts design to achieve the highly efficient ammonia production. [20,21]ccording to reported literatures, electrocatalytic nitrite reduction reaction to ammonia conversion over a heterogenous catalyst generally involves three basic processes: 1) adsorption of NO 2

Fundamentals of Nitrite Reduction to Ammonia
À at the active sites on the surface of electrocatalysts; 2) hydrogenation and/or de-hydroxylation; and 3) desorption of NH 3 molecules.Specifically, the adsorbed NO 2 À is first reduced to nitric oxide (NO).Then, NO undergoes the cleavage of N═O bond and accomplishes the hydrogenation process to form HNO. Next, the hydrogenation process continues, and HNO is reduced to H 2 NO, as well as it acquires an electron to generate hydroxylamine.Finally, the adsorbed hydroxylamine is converted to ammonia.

Ammonia Detection
For the purpose of precisely accessing the electrochemical activities of NO 2 À RR catalysts, establishing a reliable approach is essential to quantitatively detect the as-produced NH 3 .The most commonly adopted detection technique is UV-vis spectroscopy.It is well known that ammonia itself has no absorption in UV-vis region (200-800 nm), and thus its detection requires the assistance of color reagent.Specifically, NH 3 reacts with indophenol blue to form a complex and can be identified at 653 nm via UV-vis spectroscopy in an acidic solution.While in alkaline medium, NH 3 is determined using Nessler's reagent as the color reagent at 420 nm.Finally, the yield of NH 3 in electrolyte can be quantified by making a calibration curve on basis of the Lambert-Beer law.In addition, gas chromatography and ion chromatography are also employed to detect ammonia yield.Moreover, the produced NH 3 can be quantitatively confirmed by an isotope-labeled tracer experiment by using 15 NO 2 À (99 atom%) as a nitrogen source.After electrocatalytic reduction, 15 NH 4 þ in the reaction solution was tested using 1 H nuclear magnetic resonance ( 1 H NMR, 600 MHz) spectroscopy.
The ammonia yield rate and FE are two critical descriptors of the catalytic activity of an NO 2 À RR catalysts.In terms of NH 3 formation rate, it means NH 3 yield per unit time and unit catalyst loading mass (or unit electrode surface area) and can be calculated by following equation or where c, V, t, S, and m represent the concentration of ammonia, volume of electrolyte in the cathode side, electrolysis time, area of the working electrode, and mass of electrocatalyst, respectively.while FE refers to the ratio of the charge consumed for NO 2 À reduction to the total charge passed through the circuit and can be calculated using equation where n, M, and Q are the number of transferred electrons, molar mass of ammonia, and recorded consuming charge, respectively.

RR to NH 3 Electrocatalysts
Exploring efficient electrocatalysts is critical to uphold an optimized conversion of NO 2 À to NH 3 .In terms of electrocatalytic NO 2 À RR, earlier studies mainly concentrated on the removal of NO 2 À in wastewater and polarographic measurements, which indicated that d-orbital electrons of metal were responsible for injecting charge into the lowest unoccupied molecular orbital of NO 2 À . [14,24][27][28][29] However, most studies have targeted selectivity of nitrite reduction to nitrogen rather than ammonia.
Only recently, with the development of various nanomaterials, some advancements on the electrocatalytic conversion process of nitrite to ammonia have been made, which was profit from the design of electrocatalysts.In this section, the investigation of the electrochemical active of NO 2 À RR catalysts associated with material design strategies and intrinsic properties of selected catalysts will be carefully reviewed.

Noble Metal Catalysts for NO 2
À RR to NH 3 ][32][33] In contrast, only a handful of studies have focused on the ammonia electrosynthesis by nitrite conversion.For example, Pd-based catalysts exhibit excellent nitrite reduction activity and high selectivity to harmless N 2 under room conditions, while trace ammonia only exist as the contaminating in electrolyte.In terms of other noble-metals, there are almost no electrocatalytic activity for converting nitrite to N 2 or NH 3 .Surprisingly, Hörold et al. [34] found that the hydrogenation reaction of nitrite could be regulated by altering pH of electrolyte, affecting its nitrite conversion rate and ammonia selectivity of these noble metal catalysts.They demonstrated that hydroxyl anions were produced during reduction reaction process, and thus enhanced pH values and resulted in inferior activity.Consequently, Clark et al. [35] employed the alumina-supported Rh as a model catalyst and aluminasupported Pd as a benchmark, as well as investigated the effect of reaction condition of bulk solution pH on the catalytic chemistry.Experimental results demonstrated that Pd showed an excellent activity for conversion nitrite to N 2 under low pH and near inactivity at high pH, while Ru exhibited no catalytic activity at low pH, and showed a high NH 4 þ selectivity at high pH (Figure 2a-d).Density function theoretical (DFT) calculations discovered that Ru catalysts were prone to be poisoned easily by an essential intermediate *NO due to the quick dissociative adsorption of protonated nitrite (HNO 2 ) at acidic condition.As schemed in Figure 2e, the phenomena of *NO poisoning is relieved in alkaline condition, which is associated with that NO 2 À that does not dissociate in comparison to HNO 2 .As a result, Rh catalysts could achieve high electrocatalytic activities in terms of conversion of nitrite to NH 4 þ at high pH.This study has offered a new insight into the nitrite reduction process of other noble metal catalysts.Recently, Ag nanoparticles grown on NiO nanosheet arrays supported on carbon cloth (Figure 2f,g) were reported by our group and measured its electrocatalytic activity for converting NO 2 À to NH 3 under benign conditions.Specially, Ag nanoarray catalyst delivered a NH 3 formation rate of 57 510 μg h À1 mg À1 and an FE up to 97.7% at À0.7 and À0.4 V versus RHE when operated in alkaline electrolyte (Figure 2h,i), respectively.Meanwhile, such catalyst also delivered an outstanding stability with a nearly zero attenuation of current density after long-term electrolysis of 12 h (Figure 2j).Furthermore, we performed theoretical calculation to attain a deep insight into the catalytic mechanism for NO 2 À RR to NH 3 on Ag surface.As presented in Figure 2k, calculation results revealed that NO 2 À reduction reaction primarily occurred on Ag (110) facet, and the corresponding reaction pathway was *NO 2 !*NO 2 H ! *NO !*NOH !*N !*NH !*NH 2 !*NH 3 .Among them, the limiting step was the hydrogenation reaction of *NO 2 to generate *NO 2 H with an energy barrier of 0.58 eV. [33]espite noble metal catalysts deliver high electrochemical activities for conversion of nitrite to ammonia, the rare resource and expensive cost seriously blocked their large-scale application.In order to conquer the above-mentioned issues, alloying with non-noble metal is considered as one of the promising strategies.For instance, Li et al. [32] deposited Cu x Ir (100Àx) alloy nanoparticles on SiO 2 as an electrocatalyst for NO 2 À RR to NH 3 .They adjusted the stoichiometric ratios of Cu and Ir, and each catalyst delivered a NH 3 selectivity of nearly 100% in 0.1 M phosphate buffer solution (PBS).Moreover, the combination of noble metal and transitional metal compounds or carbon materials is another efficient approach to reduce the usage of noble metals, achieving the improvement of electrocatalytic activity related to their synergistic effects.In this regard, Liu et al. [36] designed Pd/CuO heterojunction for efficient NO 2 À RR, where Pd nanoparticles were uniformly anchored on porous olive-like CuO (Figure 3a-c).
In terms of Pd/CuO heterojunction, the abundant porous structure was beneficial to the sufficient penetrate of electrolyte and exposed more active sites, in addition to CuO acted as adsorption site of NO 2 À and Pd atoms are favorable for the adsorption of H þ within the electrolyte and further reacted with NO 2 À (Figure 3d).
Besides, a located electric field generated at the heterointerface between Pd and CuO owing to their difference of electronic structures promoted the rapid transport of charges, therefore modulating the adsorption energies of reactant and various reaction intermediates.As expected, the designed Pd/CuO catalyst showed a large NH 3 production rate of 906. 4 μg h À1 mg À1 , high FE of 91.8%, and NH 3 selectivity of 94.9% (Figure 3e) at -0.845 V versus RHE.In another study, Ke et al. [37] demonstrated that atomically dispersed Ru sites anchored on N-doped carbon matrix (Ru SA-NC) presented an attractive catalytic activity (average NH 3 generation rate of 1.1 mmol h À1 cm À2 ) and NH 3 selectivity of 97.8% for nitrite reduction in alkaline electrolyte (Figure 3f,g).As depicted in Figure 3h, the rate-limiting step on Ru SA-NC was the hydrogenation reaction of *NO (*NO !*NOH) with a small energy barrier of 0.495 eV, which was lower than that of the rate-limiting step (*N !*NH) over Ru nanoparticles (1.31 eV).This result also explained the origination of impressive catalytic activity of Ru SA-NC.

Transition Metal-Based Catalysts
In comparison with noble-metal-based catalysts, transition metal one features several superiorities with abundant resource, low cost, and desirable catalytic activity, which make them become alluring alternative electrocatalysts for highly efficient conversion of nitrite to ammonia under ambient conditions.So far, transition metal-based materials, including transition zero-metal, oxides, sulfides, phosphides, and their composites, have been investigated as electrocatalysts for NO 2 À RR to NH 3 .In this section, the emphasis would be focused on the zero-metal, oxides, and phosphides catalysts, owing to the limited report on this emerging area.

Transition Zero-Metal Catalysts
In terms of transitional metal catalysts, their abundant d-orbital electrons and vacant orbitals favoring the activation of N═O bond have been extensively investigated as NO 2 À RR catalysts. [38,39]Up to now, the research emphasis of most investigations has been mainly placed on Co, Ni, and Cu.The consensus is that transitional metal nanoparticles are inclined to aggregate owing to the minimization of their total surface energies, decreasing the amount of exposed active sites when used them as electrocatalysts.Consequently, the deliberate selection of appropriate supporting substrates is essential to achieve the high dispersion of metal nanoparticles.Specifically, carbonaceous materials derived from biomass have been considered as fascinating supporting skeletons to anchor transitional metal nanoparticles, achieving high catalytic activities.For example, our group fabricated Co nanoparticles attached on the carbon matrix derived from Juncus biomass (Co@JDC) and investigated its electrocatalytic activity toward conversion of NO 2 À to NH 3 under the ambient condition.In this catalyst system, the unique plane, revealing from the reaction free-energy diagrams of NO 2 À RR (Figure 4e).In detail, the average charges of Co atoms were positive, while the charges of N atoms in NH x and O atoms in NO 2 À were negative, meaning that Co atoms would donate electrons to the bonded N and O atoms.Besides, the lower d-band centers of Co atoms could reduce adsorption energies for various intermediates, thereby bringing out superior catalytic activity and selectivity toward conversion of NO 2 À to NH 3 . [40]alogously, we also employed Juncus-derived carbon tube to Pd/CuO.Reproduced with permission. [36]Copyright 2022, Elsevier.f ) Atomic resolution HAADF-STEM image, g) FE and NH 4 þ yield rate of Ru SA-NC, and h) Gibbs Free energy diagrams of NO x À RR to NH 3 on Ru SA-NC.Reproduced with permission. [37]Copyright 2023, American Chemical Society.
support Cu nanoparticles (Cu@JDC) and used for efficient electrocatalytic NO 2 À RR to NH 3 .In alkaline media, Cu@JDC catalyst delivered an impressive NH 3 formation rate of 523.5 μmol h À1 mg À1 and an FE up to 93.2% at À0.7 and À0.6 V versus RHE, respectively. [41]In another work, Li et al. [42] embedded Ni nanoparticles into one-dimensional carbon nanotubes for converting NO 2 À into NH 3 at mild conditions, where a built-in electric field induced by Schottky barrier at the heterointerface between Ni and N-doped carbon can facilitate the catching and fixation of NO 2 À on the electrode surface, thereby contributing NH 3 production.As expected, such catalyst exhibited a large NH 3 yield rate up to 25.1 mg h À1 cm À2 and high FE of 99% in neutral solution at -0.5 V versus RHE.
In addition to carbon materials as supporting substrates, three-dimensional nanoarray grown on conductive substrate is another attractive one, which is associated with vertically aligned nanoarrays providing sufficient surface area and space to anchor the active nanoparticles, which allows electrolytes arrive to catalytic centers.In this respect, our group fabricated TiO 2 nanobelt array by facile hydrothermal method and adopted as a substrate to support Co, [43] Ni, [44] and Cu [45] nanoparticles (M@TiO 2 ) for direct conversion of NO 2 À /NO 3 À to NH 3 under ambient conditions.Taking Cu as an example, Cu nanoparticles were uniformly dispersed on the surface of TiO 2 nanobelt array (Figure 4f ).When used as an electrocatalyst, Cu@TiO 2 achieved an impressive catalytic activity for conversion of NO

Transition Metal Oxides
Transitional metal oxides are becoming the limelight in the field of NH 3 electrosynthesis because of their simple synthesis strategies and good stabilities.Owing to limited investigations, only several transition metal oxides have been experimentally demonstrated to exhibit excellent catalytic activities toward NH 3 electrosynthesis by converting NO 2 À under room condition.It is well known that transition metal oxides suffer from unsatisfactory electronic conductivity, thereby impeding the charge transportation during electrocatalysis.Defect engineering is a potential strategy for modulating the electronic structures of transitional metal oxides, where defect engineering involves introducing oxygen vacancies and doping heteroatoms.Particularly, the introduction of oxygen vacancies into metal oxides not only remarkably improves its electronic conductivity but also can regard as active sites for modulating the surface-adsorption properties toward reactants and various reaction intermediates and lowering the reaction barrier.Moreover, the existing oxygen defects would induce the charge distribution around vacancies, leading to the generation of local built-in electric field and promoting the rapid ionic/electronic transport. [47,48]Inspired by this, we conducted investigations on the NO 2 À RR to NH 3 performance of TiO 2Àx arrays with oxygen defects and combined theoretical calculations to offer mechanistic insight into the reaction process (Figure 5a,b).As illustrated in Figure 5c, such designed TiO 2Àx nanobelt arrays showed a large formation of 7898 μg h À1 mg À1 and a high FE of 92.7% in alkaline solution, where the applied potential was À0.7 V versus RHE.Additionally, this catalyst also presented an outstanding stability with the maintenance of its catalytic activity during successive eight recycling electrolysis (Figure 5d).DFT calculations revealed that oxygen vacancies on the surface of TiO 2 acted as adsorption sites for capturing NO 2 À and then occurred reduction reaction related to hydrogenation and dihydroxylation, where the limiting step is the hydrogeneration reaction of *NO 2 À .After that, three protonelectron pairs couple with N to generate NH 3 .During the reaction process, the oxygen defects obviously improved the electronic conductivity of TiO 2 and decreased the reaction energy barrier of limiting step, achieving high NH 3 formation rate and selectivity (Figure 5e). [49]Thenceforth, our group further investigated the electrocatalytic activity of oxygen vacancy-rich TiO 2Àx nanoarrays with different crystal structures.In alkaline electrolyte, anatase TiO 2Àx delivered a superior electrochemical performance for converting NO 2 À to NH 3 in comparison with rutile and mixed anatase/rutile phases, including a NH 3 generation rate of 12230.1 AE 406.9 μg h À1 cm À2 and an FE of 91.1 AE 5.5% at À0.8 V versus RHE.Interestingly, we demonstrated that a functional Zn-NO 2 À battery with anatase TiO 2Àx nanoarrays as cathode delivered a large-power density of 2.38 mW cm À2 and generated NH 3 production rate of 885 μg h À1 cm À2 under 20 mA cm À2 . [50]Recently, Qiu et al. [51] developed WO 2 nanoparticles with abundant oxygen vacancies toward NO 2 À RR to NH 3 conversion.An NH 3 yield rate of Reproduced with permission. [57]Copyright 2023, Royal Society of Chemistry.
14964.25 μg h À1 cm À2 and high FE of 94.32% (À0.9 V versus RHE) were attained on WO 2Àx nanoparticles when operating in 0.1 M NaOH with 0.1 M NO 2 À .Apart from oxygen defects, heteroatom doping is a conventional strategy for regulating intrinsic electronic structure and electrochemical activity of electrocatalysts.Namely, the incorporation of heteroatoms into original electrocatalysts could induce redistribution of charge around the doping sites, generating the oxygen vacancies and local electric field, which further arrives at the target of modulating the electronic structures and adsorption abilities for reactants or intermediates. [52,53]According to the diverse types of heteroatoms, it can be roughly divided into two categories, including metal atom and nonmetal doping.[56] First, nonmetal element P was employed as a dopant to improve intrinsic electronic structure of TiO 2 , which was related with the lone-pair electrons of 3p and vacant 3d orbitals of P element.Revealed from DFT calculation results, P doping could obviously enhance the electronic conductivity and induce the charge distribution around doping sites, which facilitated the ionic/electronic transport on the surface of electrode and lowered reaction barrier of potentialdetermining step, leading to fast reaction kinetics and high catalytic activity toward high selectivity of NH 3 synthesis.As expected, a large NH 3 yield rate of 560.8 μmol h À1 cm À2 and high FE of 90.6% (À0.6 V versus RHE) were obtained on the constructed P-doped TiO 2 nanoarray catalyst in 0.1 M Na 2 SO 4 with 0.1 M NO 2 À .Besides, P-doped TiO 2 nanoarray also displayed a good stability for thirteen successive recycling electrolysis (1 h for each cycle) at À0.6 V versus RHE. [56]Meanwhile, taking vanadium contained in nitrite reductase into consideration, we also developed V-doped TiO 2 nanoarray for NO 2 À RR to NH 3 in neutral media.On basis of the reactivity experiments, V-doped TiO 2 offered an attractive selectivity, a high FE of 93.2% at À0.6 V versus RHE, and a large NH 3 yield rate of 540.8 μmol h À1 cm À2 at À0.7 V versus RHE. [55]Combined with DFT calculations, except the improvement of electronic conductivity, the reaction energy barrier of limiting step (*NO 2 !*NO 2 H) over TiO 2 also decreased after V doping.In another work, Zhang et al. [57] fabricated a carbon-doped Co 3 O 4 hollow nanotubes as an electrocatalyst to develop a Zn-NO 2 À battery system for NH 3 synthesis under room condition (Figure 5f-h).Specifically, C-doped Co 3 O 4 hollow tubes assembled with nanoparticles provided a NH 3 formation rate up to 4.1 mg h À1 cm À2 and an ultrahigh FE of nearly 100% in neutral electrolyte under a potential range from À0.1 to 0.6 V versus RHE (Figure 5i).Meanwhile, assembled Zn-NO 2 À battery with C-doped Co 3 O 4 presented a large power density of 6.03 mW cm À2 with a 95.1% FE for NH 3 electrochemical production using 50 mM NO 2 À as electrolyte, as demonstrated in Figure 5j.Such outstanding electrocatalytic activity for NO 2 À RR to NH 3 might be attributed to C doping, which not only improved the electronic conductivity of Co 3 O 4 but also induced the formation of local electric field around doping sites and promoted the charge transport during electrochemical reaction process.In particular, DFT calculations also confirmed that the stronger catalytic activity stemmed from the lowered energy barrier of potential-determining step (*N !*HN) after C doping.
Iron, one of the most common elements among all transition metal elements found on Earth, is the active site of all nitrogenases and plays an important role in the enzyme-catalyzed electron transfer process.Inspired by this, we very recently demonstrated that FeOOH nanotube array showed an outstanding electrocatalytic activity for NO 2 À RR to NH 3 in neutral media, including a high FE of 94.7% and a large NH 3 yield rate of 11 937 μg h À1 cm À2 with good stability at À1.0 and À1.1 V versus RHE, respectively. [58]This result paved a fascinating route to develop Fe-based oxide catalysts for conversion of NO 2 À RR to NH 3 with the high activity and selectivity.

Transition Metal Phosphides
Transition metal phosphides feature several attractive advantages with high electronic conductivity and modulate hydrogen evolution activity, holding great promise in the field of NH 3 electrosynthesis by converting NO 2 À .In this regard, our group experimentally constructed a group of transition metal phosphides (Co, Ni, and Cu) nanoarray as electrocatalysts for NO 2 À RR to NH 3 under ambient condition.As illustrated in Figure 6a,b, Cu 3 P nanowire array in situ grown on Cu foam was fabricated and first utilized as an electrocatalyst toward NO 2 À RR.In neutral electrolyte, such catalyst gained an NH 4 þ formation rate of 1626.6 AE 36.1 μg h À1 cm À2 , a large selectivity of 88.1 AE 1.6%, and a high FE of 91.2 AE 2.5% at À0.5 V versus RHE (Figure 6c).Additionally, Cu 3 P catalyst also displayed an outstanding stability with ignorable changes for both selectivity and FE during the consecutive five recycling electrolysis (2 h for each cycle) (Figure 6d).The electroreduction mechanism of NO 2 À RR over Cu 3 P catalyst was revealed by DFT, as illustrated in Figure 6e.On the basis of calculation result, Cu 3 P exhibited strong adsorption and activation for NO 2 À by donating electrons into NO 2 À .Meanwhile, the reaction energy barrier of potentialdetermining step (*NO !*NOH) on Cu 3 P catalyst was only 0.22 eV, indicating its fast reaction kinetics.Accordingly, the superior NO 2 À RR catalytic activity for Cu 3 P could be attributed to two aspects: 1) Cu 3 P itself enjoyed a high intrinsic catalytic activity toward NH 3 electrosynthesis by reducing NO 2 À , such as stronger adsorption ability for NO 2 À and lowered reaction energy barrier and 2) three-dimensional nanoarray could expose extensive activity sites and ensure the contact between active sites and electrolyte. [59]Analogously, CoP and Ni 2 P nanoarrays were fabricated to have impressive electrocatalytic activity including large NH 3 yield rate and high selectivity toward electrochemical reduction of NO 2 À to NH 3 in neutral media. [60,61]On basis of the above-mentioned results, Yuan et al. [62] recently employed FeP nanoarray as studied model electrocatalyst and investigated the effect of applied potential, NO 2 À concentration, pH, and reaction temperature for NO 2 À RR performance.Revealed from the experimental results that the applied potential played an essential role in NH 3 selectivity by balancing the reduction reaction of NO 2 À and HER, while NH 3 selectivity declined with the increment of NO 2 À concentration.Although pH and operating temperature have little influence toward electrocatalytic activity of FeP nanoarray, the optimized electrochemical performance for NH 3 synthesis was obtained in neutral media.

Other Electrocatalysts
Recently, other transition metal compounds have also been investigated for electrochemical NO 2 À RR to NH 3 conversion, such as metal sulfides and metal borides.For instance, our group employed TiO 2 nanobelt arrays as the supporter to anchor NiS 2 nanoparticles (NiS 2 @TiO 2 ) and measured its catalytic activity of NO 2 À RR.In alkaline electrolyte, NiS 2 @TiO 2 provided an impressive catalytic activity with a large NH 3 production rate of 591.9 μmol h À1 cm À2 and high FE of 92.1% at À0.6 and À0.5 V versus RHE, respectively. [63]In another work, Yi et al. [64] developed MoS 2 nanosheets electrocatalyst to have an NH 3 yield rate of 8.99 mg cm À2 h À1 (at À0.9 V versus RHE) with a high FE of 93.52% (at À0.5 V versus RHE) in neutral medium.
Transition metal borides as an important inorganic material feature the unique physical and chemical properties with high electronic conductivity and structural strength and have versatile applications in the fields of electrocatalysis and energy storage.Especially, the amorphous metal borides have exotic structure characteristic with rich defects and dangling bonds, endowing abundant catalytic sites when used as electrocatalysts. [65,66]onsequently, our group designed amorphous CoB nanoarray for electrocatalytic reduction of NO 2 À to NH 3 under room temperature (Figure 6f,g).When operating in neutral electrolyte, an NH 3 yield rate of 233.1 μmol h À1 cm À2 and FE of 95.2% were gained over CoB catalyst at À0.7 V versus RHE (Figure 6h,i).Besides, it also presented an outstanding durability that both NH 3 yield rate and FE had no obviously fluctuation during the successive 12 recycling electrolysis (Figure 6j). [67]

Conclusions and Outlook
In this review, recent progresses in NO 2 À RR to NH 3 electrocatalysts, including noble-metal-based materials, transition zerometal-based materials, and transition metal compounds-based materials, are systemically summarized and their electrocatalytic activity are compared as represented in Figure 7 and Table 1.Meanwhile, various strategies for enhancing NH 3 yield, selectivity, and FE of NO 2 À RR electrocatalysts such as morphology regulation, defect engineering (heteroatom doping and vacancy modulation), and heterojunction engineering are underlined.In particular, the former is beneficial to increase the number of active sites and rise the utilization of electrocatalysts, with the purpose of enhancing the apparent activity, while the latter two approaches aim at improving the intrinsic activities of NO 2 À RR electrocatalysts by tailoring electronic structures, increasing active sites, and inducing the formation of built-in electric field.Although significant progresses have been made in the electrocatalyst material design toward conversion of NO 2 À to NH 3 under ambient conditions, great efforts needing to be made before the technology can be commercially applied for sustainable artificial ammonia production: 1) In terms of NO 2 À RR electrocatalysts, noble metal materials usually display RR to NH 3 on Cu 3 P. Reproduced with permission. [59]Copyright 2023, Royal Society of Chemistry.f ) SEM and g) TEM images of CoB nanoarray.h) Line sweep voltammetry, i) NH 3 formation rate, and FE and j) stability of CoB nanoarray.Reproduced with permission. [67]opyright 2023, Royal Society of Chemistry.
the high NH 3 yield rate, selectivity, and FE by altering operating conditions (e.g., pH and temperature), but the limited resources and expensive cost severely impeded their practical applications.Transitional metal-based materials, such as zero-metal, metal oxides, and metal phosphides, have been employed as electrocatalysts for NO 2 À RR, exhibiting good NH 3 yield, selectivity, and FE.However, most of reported literatures focused on Co, Ni, and Cu-based materials.In comparison with other electrochemical reactions, the variety of electrocatalysts investigated for NO 2 À RR to NH 3 is rather few.Therefore, exploring new fascinating electrocatalysts such as Fe-, Bi-, Sn-based materials and single-atom catalysts continues to be an interesting area; 2) For improving the electrocatalytic NO 2 À RR to NH 3 , various strategies for material design including morphology and defect engineering have been employed for boosting the apparent or intrinsic activity of catalysts.In particular, regulation of morphology is beneficial to increase the number of active sites for capturing NO 2 À and key intermediates, as well as rise the utilization of electrocatalysts with the purpose of enhancing the apparent activity.The defect engineering (heteroatom doping and vacancy modulation) aims at improving the intrinsic activities of NO 2 À RR electrocatalysts by tailoring electronic structures, increasing active sites, and inducing the formation of built-in electric field.For instance, oxygen vacancies in metal oxides are benefit for the cleavages of N═O bond, since oxygen vacancies can be filled with oxygen in nitrite to weaken the oxygen-nitrogen bond.However, rationally designed strategies with crystal facet regulation, strain engineering, and constructing heterostructure widely applied in other electrochemical processes have not been used in the field of NO 2 À RR to NH 3 .
The former can expose more catalytic sites, while the latter two can modulate the electronic structure by regulation of surface atomic spacing generated by strain engineering and charge redistribution induced by coupling different compounds.Thus, the above-mentioned strategies might deserve to be further employed to promote the electrochemical NO 2 À RR to NH 3 .In addition, a combination of different design strategies also affords one to jointly optimize the binding energies of the reactant and key intermediates, facilitating preferential NO 2 À RR toward NH 3 conversion; 3) Identification of catalytic mechanism and the Ni@JBC-800 0  detailed reaction pathway play an essential role in design of highefficient electrocatalysts toward NO 2 À to NH 3 conversion.At present, the reaction process of nitrite electroreduction to ammonia is alone uncovered via theoretical investigations based on the pristine catalyst model; nonetheless, this catalyst materials may undergo the surface structural evolution and element valence change owing to different operating conditions (acid, neutral, and alkaline media), which are difficult to be accurately predicted using theoretical studies.In this respect, advanced in situ characterizations should be performed to reveal the reaction pathway and identify the real-catalytic sites of electrocatalysts, constructing the structure-performance relationships and further providing a guide for the design of electrocatalyst; 4) Based on the above-mentioned discussion, NH 3 yield rate and FE are usually regarded as descriptors to access the catalytic activity of NO 2 À RR electrocatalyst.From the viewpoint of practical application, the stability of electrocatalyst is an extremely important parameter.At present, the reported stability tests in the literatures are very commonly dozens of hours, which is a too short for industrial implementation where thousands of hours of stable operation under large densities over 100 mA h À1 are expected.Therefore, it is recommended that sufficient attention should be paid to the durability of electrocatalysts for long-term consecutive electrolysis.Besides, overpotential is another critical descriptor for the conversion of NO 2 À RR to NH 3 , deciding the efficiency of energy conversion.The normally investigated electrocatalysts usually require an overpotential over 400 mV to achieve NO 2 À RR, leading to excessive energy consumption.Consequently, much more research efforts are required to reduce the overpotential of NO 2 À RR, and future protocol for catalyst needs to target much low overpotential; and 5) In order to accurately assess the electrochemical performance of NO 2 À RR catalysts, the testing conditions including nitrite concentration, pH value, and type of the supporting electrolyte should be unified.The disunity of these testing conditions not only makes it impossible to compare the electrochemical performance of various catalysts but also affects the NH 3 yield rate, selectivity, overpotential, and even reaction pathway.For instance, when operating in neutral electrolyte consisting of Na 2 SO 4 , in situ NH 3 will continually increase the local pH value during the electrolysis owing to the base property of NH 3 , which might alter the surface structure of catalysts, reduction potential and reaction pathway, and further misleads the real-activity sites and mechanism study.Therefore, uniform of testing conditions holds the key to promoting the design of highly efficient electrocatalysts for NO 2 À RR conversion and further accelerating the development of electrochemical NH 3 synthesis.Overall, electrochemical NO 2 À RR affords a promising route for NH 3 production under ambient condition.This review summarizes the recent advances and bottlenecks of electrocatalysts and proposes some advice regarding with rationally designed electrocatalysts for NO 2 À RR to NH 3 conversion.To realize the practical applications of electrochemical NO 2 À RR to NH 3 formation, it is highly desired to seek and design advanced electrocatalysts with high catalytic activity, small overpotential, high FE (selectivity), large current density arriving industrial level, and long-term stability.

Figure 1 .
Figure 1.Schematic illustration and the possible reaction pathway of NO 2

2. 1 .
Mechanism Insight of Electrocatalytic Nitrite to AmmoniaElectrocatalytic nitrite reduction is a complicated proton-coupled electron transfer reaction, involving the formation of various by-products like NO, N 2 O, N 2 , and NH 3 /NH 4 þ .

Figure 2 .
Figure 2. NH 3 yield of a) Pd and b) Rh electrocatalysts at different pH values.c) NH 3 selectivity and d) formation rate of Pd and Rh catalysts under different pH values.e) Schematic illustration of NH 3 electrosynthesis on Rh catalysts under low and high pH.Reproduced with permission.[35]Copyright 2019, American Chemical Society.f ) Scanning electron microscopy (SEM) and g) transmission electron microscopy (TEM) images of Ag nanoarray.h) FE, i) NH 3 yield rate, and j) stability of Ag nanoarray.k) Free energy diagrams for NO 2

Figure 3 .
Figure 3. a-c) TEM and HRTEM images, and d) NH 3 formation rate and FE of Pd/CuO.e) Schematic illustration of reaction mechanism for NO 2 À RR on

Figure 4 .
Figure 4. a,b) SEM images of Co@JDC.c) NH 3 generation rate and d) FE of Co@JDC under applied potentials.e) Free energy diagrams of NO 2 À RR to

Figure 5 .
Figure 5. a) SEM and b) TEM images of TiO 2Àx nanoarray.c) NH 3 yield rate and FE and d) stability of TiO 2Àx nanoarray.e) Free energy diagrams of NO 2À RR to NH 3 on TiO 2Àx .Reproduced with permission.[49]Copyright 2023, Royal Society of Chemistry.f,g) TEM and corresponding mapping images of C-doped Co 3 O 4 .h) Schematic illustration of C-doped Co 3 O 4 -based Zn-NO 2 À battery.i) NH 3 yield rate and FE of C-doped Co 3 O 4 under different potentials.j) NH 3 yield rate and FE of Zn-NO 2 À battery assembled with C-doped Co 3 O 4 cathode.k) Free energy diagrams of NO 2 À RR to NH 3 on C-doped Co 3 O 4 .Reproduced with permission.[57]Copyright 2023, Royal Society of Chemistry.

Figure 6 .
Figure 6.a,b) SEM and corresponding element mapping images of Cu 3 P nanoarray.c) NH 4 þ yield rate and FE and d) durability of Cu 3 P nanoarray.e) Free energy diagrams of NO 2 À

Figure 7 .
Figure 7. Electrochemical activity map of different catalysts for NH 3 yield rate and FE: a) alkaline electrolyte; b) neutral electrolyte.

Luchao
Yue received his Ph.D. degree in 2022 from Sichuan University and then joined North University of China as a lecturer.His research focuses on the design of nanomaterials for sustainable electrochemical energy storage and conversion, such as metal-ion batteries and electrochemical nitrate/ nitrite reduction.Wei Song is currently a lecturer at North University of China.She received her Ph.D. degree in 2020 from Taiyuan University of Technology.Her research focuses on the development of advanced anode materials for metal-ion batteries.Qian Liu received her Ph.D. degree in 2018 from Southwest University.She worked as a postdoctor in Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences from 2019 to 2021 and then joined Chengdu University as a distinguished research fellow.Her research mainly focuses on the design and regulation of nanomaterials for energy storage and conversion, such as nitrogen reduction and nitrate/nitrite reduction.Xuping Sun received his Ph.D. degree from Changchun Institute of Applied Chemistry (CIAC), Chinese Academy of Sciences in 2006.He started his independent research career as a full professor at CIAC in 2010, moved to Sichuan University in 2015, and joined the University of Electronic Science and Technology of China in 2018.He was recognized as a highly cited researcher in both areas of chemistry and materials science by Clarivate Analytics.His research mainly focuses on rational design of nanocatalysts toward applications in electrosynthesis of green hydrogen and ammonia as well as electrochemical denitration of vehicle exhausts and industrial wastewater.
2 À to NH 3 in neutral electrolyte, including a large NH 3 formation rate of 237.7 μmol h À1 mg À1 at À0.8 V versus RHE and a high FE of 95.3% at À0.6 V versus RHE (Figure4g).Additionally, Cu@TiO 2 also showed an outstanding electrochemical durability after eight consecutive electrolysis (1 h for each cycle), where both NH 3 formation rate and FE only have minor fluctuations [46].In particular, the origination of Ni-NSA-V Ni with excellent electrocatalytic activity was revealed by DFT calculations.Namely, the introduction of Ni vacancy on Ni nanosheet decreased the surface electron cloud density, contributing to the improvement of adsorption and activation energies toward NO 2 À , as well as leading to an enhanced NH 4 þ selectivity and FE.[46]

Table 1 .
Electrochemical activities of NO 2 À RR electrocatalysts under ambient conditions.