A Perspective on Cu‐Based Electrocatalysts for Nitrate Reduction for Ammonia Synthesis

Compared to the Haber–Bosch (H–B) process for ammonia synthesis with massive emission of greenhouse CO2 and necessary harsh reaction conditions, the electrocatalytic NO3− reduction (NO3RR) for ammonia synthesis under ambient temperature and pressure driven by renewable electricity has attracted great attention. NO3RR is also a promising route to construct nitrogen cycles utilizing NOx produced from industrial and agricultural operations. Cu‐based catalyst is one of the most promising platforms for this reaction. This perspective shows the latest understanding and research advances on the NO3RR by Cu‐based electrocatalysts. Through a deep analysis on the rate‐limiting step of NO3RR, the strategies of overcoming the deactivation and enhancing the performance of Cu‐based catalysts are discussed. The great significance of synergistic NOx and H activation in promoting the key kinetic step of electrocatalytic NO3RR by single‐atom, oxide, bimetallic of Cu on the catalytic reaction site construction with tunable adsorption of N‐containing intermediate and active H is stressed. Also, future challenges and perspectives toward coupling reaction of nitrate reduction catalyzed by Cu‐based catalysts are proposed. This perspective can move forward the future research on electrocatalytic NO3RR to ammonia synthesis over advanced Cu‐based electrocatalysts.

Compared to the Haber-Bosch (H-B) process for ammonia synthesis with massive emission of greenhouse CO 2 and necessary harsh reaction conditions, the electrocatalytic NO 3 À reduction (NO 3 RR) for ammonia synthesis under ambient temperature and pressure driven by renewable electricity has attracted great attention.NO 3 RR is also a promising route to construct nitrogen cycles utilizing NO x produced from industrial and agricultural operations.Cu-based catalyst is one of the most promising platforms for this reaction.This perspective shows the latest understanding and research advances on the NO 3 RR by Cu-based electrocatalysts.Through a deep analysis on the rate-limiting step of NO 3 RR, the strategies of overcoming the deactivation and enhancing the performance of Cu-based catalysts are discussed.The great significance of synergistic NO x and H activation in promoting the key kinetic step of electrocatalytic NO 3 RR by single-atom, oxide, bimetallic of Cu on the catalytic reaction site construction with tunable adsorption of N-containing intermediate and active H is stressed.Also, future challenges and perspectives toward coupling reaction of nitrate reduction catalyzed by Cu-based catalysts are proposed.This perspective can move forward the future research on electrocatalytic NO 3 RR to ammonia synthesis over advanced Cu-based electrocatalysts.
substantial dissociation energy of the N≡N bond (941 kJ mol À1 ) and the low solubility of N 2 in water under atmospheric pressure are formidable challenges in the electrocatalytic reduction of N 2 for ammonia synthesis.In this regard, nitrate (NO 3 À ) emerges as an exceptionally well-suited raw material due to its low dissociation energy (N═O bond energy of 204 kJ mol À1 ) and remarkably high solubility in water. [6]Moreover, the substantial release of NO x resulting from industrial and agricultural operations has inflicted severe damage upon the environment.Rectifying the imbalance in the natural nitrogen cycle and proficiently regulating nitrogen circulation in aquatic ecosystems have been designated as pivotal hurdles to overcome in the 21st century. [7]s a result, electrocatalytic reduction of NO 3 À (NO 3 RR) to NH 3 not only presents an economical and environmentally friendly technique for ammonia synthesis, but also offers a viable solution to rectify the imbalance in the natural nitrogen cycle.
It has been widely acknowledged that the reduction of initial NO 3 À to NO 2 À is the main obstacle to NO 3 RR (rate limiting step [RLS]) in most metal catalyst systems.Enhancing the reaction rate of this crucial step is the pivotal factor in developing efficient electrocatalysts. [8]Among various promising transition metalbased electrocatalysts for NO 3 RR, Cu is widely favored by researchers because of its low production cost and relatively high catalytic activity.It inhibits the occurrence of hydrogen evolution reaction and active sites that can form bimetallic synergistic *H with other metals.Especially, Cu exhibits immense potential due to their electronic structure that aligns well with the molecular orbital of NO 3 À .Thus, it could facilitate the reduction of the barrier in the RLS. [9]Different crystal faces of Cu have different catalytic activities.However, pure Cu catalysts often encounter rapid deactivation due to their tendency to strongly adsorb NO 3 RR intermediates (such as NO 2 À and NO). [10]Limited progress on improving the NO 3 À -to -NH 3 conversion was presented, which is the primary impediment to NO 3 RR.In recent years, breakthroughs have been made in adjusting the binding strength of NO 3 RR intermediates at the Cu atomic center and enhancing the hydrogenation process by synergistic *H and/ or e-transfer. [11]erein, we review the recent advances of the design of Cu-based catalysts as well as their electrocatalytic mechanisms toward NO 3 RR though a succinct overview of the latest studies.Perspective discussions addressing the strategies to mitigate the deactivation in Cu-based catalysts are offered in respect of tuning the adsorption of NO 3 RR intermediates and synergistic activation of N and H species for ammonia synthesis.The related discussions are also beneficial for this fascinating and rapidly changing field as well as the development of durable catalysts for the other catalytic processes involving NO 3 RR, such as C-N coupling reaction. [12] Deactivation of Cu-based Electrocatalysts toward NO 3 RR The electrocatalytic reduction of NO 3 À to NH 3 involves two components: NO 3 À prioritizing the generation of stable intermediate NO 2 À , followed by further reduction of NO 2 À to produce NH 3 after a tandem of hydrogenation steps.The electrochemical reduction of NO 3 À to NO 2 À , depicted in Figure 1a, is considered to be the RLS in the overall process of NO 3 À to NH 3 . [13]The slow kinetic process of NO 3 RR is attributed to the high energy level of the lowest unoccupied molecular π* orbital (LUMO π*) of NO 3 À , which greatly hinders the injection of charge into this orbital. [14]he high occupancy rate of Cu d-orbital is analogous to the energy level of the LUMO π* of NO 3 À , and the accessible open d-orbital shell of Cu facilitates electron transfer to the adsorbed NO 3 À , making it well-suited for NO 3 RR applications. [15]onetheless, pure Cu catalysts commonly encounter the issue of rapid deactivation, characterized by a gradual decline in current density during testing and the predominant formation of NO 2 À as the main product, as shown in red box of Figure 1b.In situ characterization data and density functional theory (DFT) calculation reveal that the primary reason of Cu catalyst Experimental conditions: scan rate 50 mV s À1 ; rotation rate 400 rpm.16a] Copyright 2019, Elsevier.c) Comparison of catalytic performance of different types of Cu-based electrocatalysts toward NO 3 RR.deterioration deactivation poisoning is the strong adsorption of the intermediate, notably NO, which hinders further hydrogenation to NH 3 and obstructs the active sites on the catalyst surface. [16]Substantial efforts have been made in alleviating the issues by regulating the adsorption strength of the intermediates at the Cu site or introducing *H formation active site to synergistically promote the NO hydrogenation and desorption steps, which can be achieved by forming different Cu crystal planes, copper oxides, or constructing Cu-based bimetallic catalysts with other transition metals (e.g., Fe, Co, and Ni) for synergistic N and H activation (Figure 1c).In the following sections, we will discuss the recent progress on the kinetic process of these Cu-based catalytic systems for NO 3 RR.

Regulating Intermediates' Adsorption on Single-Component Copper Catalysts
16a] To bolster the NO 3 RR activity of Cu catalysts, a clear approach is to directly control the adsorption strength of these intermediates.This can be achieved through various methods, such as Cu single-atom (Cu SACs) manipulation, crystal face engineering, and Cu-oxide optimization.These techniques effectively enhance the NO 3 RR activity by regulating the adsorption strength of intermediates, thus making copper-based catalysts more efficient in this reaction.
The Cu SACs can make full use of the Cu active site and unique electronic property to enhance the catalytic activity. [17]ts catalytic performance is much higher than that of the conventional-phase Cu.The d-band center of Cu is moved near the Fermi level, contributing to the enhanced adsorption of reaction intermediates and atoms *H.Thus it could boost the conversion of *NO to *NOH during the NO 3 RR process. [18]The unique electronic property of Cu-N x structure in Cu-N-C SACs has controllability for intermediate N. NO 2 À is more prone to be adsorbed onto Cu-N 2 (Figure 2a).The unit point property of Cu-N 4 is conducive to the further transformation of NO 2 À (Figure 2b 1 -2b 2 ) and suppressed the formation of doublenitrogen products. [19]Consequently, Cu SACs catalysts are effective in promoting NO 3 RR at appreciably higher rates.Different crystal faces can directly affect the adsorption strength of the intermediate.Currently, research has shown that the conversion of NO 3 À to NO 2 À is more likely to occur on Cu (100).The Faraday efficiency (FE) is as high as 95% and conversion rate of NO 3 À is 98% for NO 2 À formation, making it difficult to perform subsequent steps at Cu (100). [20]Therefore, it is chosen to promote the adsorption of intermediate on the surface of Cu (111) to complete the subsequent ammonia synthesis.The generation of NH 3 could be realized through the coordinated tandem catalysis of Cu ( 100) and ( 111).It has been reported that Cu nanosheet catalysts were designed and prepared with different crystal planes to tandem catalyze NO 3 À RR.The NO 2 À generated on the Cu (100) facets is subsequently hydrogenated on the Cu (111) facets (Figure 2c).The tandem catalysis promotes the crucial hydrogenation of intermediate products, thus promoting the production of NH 3 .19a] Copyright 2020, Wiley-VCH Verlag.b 1 ) Fourier transformation of the extended X-ray absorption fine structure spectra of Cu-N-C, Cu foil, and Cu 2 O at the R space.b 2 ) N concentration in formed NO 2 À for different catalysts at À1.5 V versus SCE.19b] Copyright 2022, Elsevier B.V. c) Schematic illustration for enhancing electrochemical NO 3 À RR over Cu nanosheets via facet tandem catalysis.Tandem interaction of Cu (100) and Cu (111) facets.Reproduced with permission. [21]Copyright 2023, John Wiley and Sons Ltd.
reduced ammonia synthesis for 700 h.At 365 mA cm À2 , FE is 88%, achieving efficient ammonia synthesis. [21]The active center of the NO 3 RR process is reduced by CuO nanosheets to metal Cu, and the presence of cupric oxide provides the possibility of high stability for the reaction process, which is much higher than that of pure Cu. [22] Moreover, the interfacial interaction between the oxide metal and Cu makes the catalyst show long-term stability during the reaction. [23]he electron-deficient nature of the Cu site in Cu oxide facilitates the adsorption of the Lewis base NO 3 À . [24]Further, it has been proved that due to the nature of electron deficiency of Cu oxide, it has the ability to inhibit hydrogen evolution and promote the production of more *H (Figure 3a 1 ), while pure Cu is far from reaching the level (Figure 3a 2 ). [25]Oxygen vacancies are designed to efficiently promote nitrate reduction.A one-step room-temperature Ar plasma strategy has been developed to regulate the surface oxygen species of Cu 2 O electrocatalysts, resulting in the formation of abundant oxygen vacancies on Cu 2 O surface after plasma treatment.The valence state of Cu decreased after oxygen vacancy formation, to emerge electron-deficient nature (Figure 3b).Additionally, the higher concentration of oxygen vacancies on the Cu x O (111) facets can enhance the dissociation of water molecules on the surface, forming hydroxyl groups that inhibit hydrogen evolution reaction (HER).Therefore, oxygen vacancy and hydroxyl group catalyze NO 3 RR synergistically on the Cu x O (111) facets. [26]The synergistic catalysis of oxygen vacancy and hydroxyl group provides a new strategy for NO 3 RR, but the energy cannot be fully utilized due to the involvement of heterogeneous catalysis, and the FE of NH 3 in this catalytic mechanism is less than 95%.Another example is constructing Reproduced with permission. [25]Copyright 2022, Elsevier.The polarization charge density b 1 ) before and b 2 ) after oxygen vacancy formation obtained by Bader charge analysis.26d] Copyright 2022, Elsevier.

Hydrogenation of Nitrate on Synergistic Cu-Based Bimetallic Catalysts
In bimetallic catalysts, the introduction of a second metal can be used as HER site to produce *H.The active *H promotes *NO to *NHO to improve NH 4 þ kinetic process.Moreover, it can also regulate the adsorption of Cu sites to N-containing intermediates, leading to improved NO 3 RR activity.NO 3 RR involves a proton-coupled electron transfer mechanism, making the introduction of *H on Cu-based catalyst advantageous for improving the efficiency of the NO 3 RR.Although Cu-based catalysts have been reported to exhibit excellent catalytic performance in nitrate reduction, the intermediates produced by NO 3 À reduction (*NO 2 , *NO, *N, and so on) cannot be hydrogenated in a timely manner due to the poor adsorption of Cu by *H, resulting in limited efficiency of ammonia synthesis.The introduction of metals conducive to HER can effectively promote the synthesis of NH 3 by NO 3 RR.Usually, the reaction pathway of alkaline HER is the Volmer Heyrovsky or Volmer Tafel step, and it is generally believed that the Volmer step (H 2 O þ e À !H ads þ OH À ) is the step that determines the entire HER rate. [27]The generation and consumption of *H need to reach a dynamic equilibrium state in order to achieve the highest FE and NH 3 yields of NO 3 RR, which has aroused researcher's interest in hydrogen radicals (Figure 4a). [28]*H is usually formed in large quantities on the surface of metals (Pt, Pd, Ni, etc.) with strong HER ability. [29]hen *H spills transfer to the heterojunction interface, and hydrogenation reaction occurs with intermediate nitrogen oxides at the metal heterojunction interface to finally deoxidize into NH 3 (Figure 4b). [30]The electron spin resonance (ESR) technique is used to prove the existence of *H (Figure 4c 1 ).The mode in which hydronium ions participate reduces the TS energies of HÀNÀO !H 2 ÀNÀO and NÀH 2 !NÀH 3 on the strain surface to different degrees respectively (Figure 4c 2 -4c 3 ), suggesting the promotion effect of *H on hydrogenating intermediates.Therefore, effective dissociation of H 2 O to generate *H while inhibiting the dimerization of *H to H 2 is the key to enhancing the hydrogenation kinetics of NO 3 RR. [31]A Ni 3 Fe-CO 3 LDH/Cu foam catalyst could be prepared using layered double hydroxide (LDH)-modified foam copper electrode, which could promote the generation of *H.Ni 3 Fe-CO 3 LDH/Cu foam exhibits 8.5-fold higher productivity compared to pure Cu foam.Radical trapped Reproduced with permission. [28]Copyright 2022, Springer Nature.b) The active *H-involved interfacial Pt-Ni coreduction toward alloy nanoshells.Reproduced with permission. [30]Copyright 2023, Springer Nature.c 1 ) ESR spectra of the solutions obtained after 10 min of NO 3 RR using Ru-ST-12, Ru-ST-5, and Ru-ST-0.6 in 1 M KOH under argon using DMPO as the •H-trapping reagent.c 2 -c 3 ) Energy diagram of the reaction steps from HNO to H 2 NO and from NH 2 to NH 3 over the strained Ru surface.31a] Copyright 2020, American Chemical Society.d) ESR spectra of the solutions obtained by the Ni 3 Fe-CO 3 LDH/Cu foam and Cu foam electrodes in the presence of 50 mM DMPO in 1 M KOH.The electrochemical reaction was conducted by chronoamperometric technique for an hour to trap the hydrogen radicals.Reproduced with permission. [32]Copyright 2023, Royal Society of Chemistry.
DMPO-mediated ESR spectra of Cu foam and Ni 3 Fe-CO 3 LDH/Cu foam were carried out to comparatively study the synergistic effect of Ni 3 Fe-CO 3 LDH/Cu foam, which confirms that polymetallic active site is conducive to promoting the generation/ transfer of hydrogen free radicals in the electrochemical NO 3 RR process (Figure 4d). [32]y introducing additional metal atoms sites, it becomes feasible to finely tune the adsorption strength of the Cu site toward these intermediates. [33]Besides, the new active site can also regulate the local chemical environment and surface properties. [34]urthermore, the strong interactions and synergistic effects between the interfacial bimetallic sites can help to reduce the barrier of the rate-determining step, [35] to help stabilize the reaction species and improve NO 3 RR performance.The Fe and Cu atoms are combined with two nitrogen atoms.The presence of the dualatom heterostructure leads to the weakening of N─O bonds.It is observed that the strong coupling between NO 3 À and the d-orbitals of the dual-metal atoms contributes to lowering energy barrier for NO 3 RR.The synergistic effect of dual-site results in high NH 3 yield and selectivity (Figure 5a). [36]By forming Cu-Ni alloys, it becomes possible to adjust the Cu d-band center to a positive shift of the Cu 3d band toward the Fermi level, thereby influencing the adsorption energies of intermediates like *NO 3 À , *NO 2 , and *NH 2 (Figure 5b). [37]The correlation between the adsorption energy of intermediates and the d-band center position of catalysts is significant.This adjustment results in a decrease in antibonding occupation and a stronger bond formation with the adsorbates.Thus, the RLS barriers are effectively decreased for the improved f NO  RR.Reproduced with permission. [36]Copyright 2022, Springer Nature.b) Ultraviolet photoelectron spectroscopy spectra and d-band center positions of pure Cu catalysts and the CuNi alloys.37a] Copyright 2020, American Chemical Society.*.38a] Copyright 2023, American Chemical Society.d) Proposed mechanism of NO 3 À reduction reaction over the PdCu-P 4 /CS catalyst.The black, orange, and purple balls refer to Pd, Cu, and P atoms, respectively.
Reproduced with permission. [39]Copyright 2022, John Wiley and Sons Ltd.
regulate the electron transfer of NO 3 À , lowering the reaction energy barrier and inhibiting the formation of N-N bonds.Finally, the conversion rate of more than 98% was obtained, and the conversion rate of NH 3 synthesis by pure Cu catalyst was optimized.The synergistic interaction of Pd-Cu diatoms with adjacent P atoms forms covalent bonds, which reduce the adsorption energy of the reactants.This could promote the breaking of N-O and accelerate the conversion of NO 2 À with shorter bond lengths between the two atoms, ultimately resulting in synergistic catalysis and in efficient production of NH 3 (Figure 5d). [39]he concept of "tandem catalysis" involves coupling intermediate phases of different transition metals.That is to say, different active sites complete the catalytic reaction in sequence, jointly promoting the occurrence of the reaction, in an oxidationreduction reaction.Due to the ability of synergistic active sites to achieve cascade conversion from NO 3 À to NH 3 , in turn avoiding the scaling relations, the use of Tandem catalysts can play a crucial role in the nitrate reduction process.First, NO 3 À is reduced to NO 2 À at the Cu site, and the new metal site promotes the production of *H.Then, NO 2 À undergoes a hydrogenation reaction with *H through migration, ultimately reducing to synthesize NH 3 . [40]The working principle of Cu-Ni phase catalyst in series catalysis is shown in Figure 6a. [41]Pristine Cu-Ni alloys are transformed into the active Cu/Cu 2 O-Ni/Ni(OH) 2 complexes under pulse conditions, where the Cu/Cu 2 O structure preferentially catalyzes the generation of NO 3 À to NO 2 À , and the reaction Reproduced with permission. [42]Copyright 2023, John Wiley and Sons Ltd. b) Schematic illustration of the preparation of a Cu/Co-based binary 'tandem catalyst.Reproduced with permission. [47]Copyright 2022, John Wiley and Sons Ltd.Reproduced with permission. [48]Copyright 2023, John Wiley and Sons Ltd.
of Ni/Ni(OH) 2 provides binding site of *H, enabling further hydrogenation.Tandem catalysis can balance the reaction rates of NO 3 À -to-NO 2 À and NO 2 À -to-NH 3 , thereby significantly enhancing above conversion at relatively low overpotentials. [42]he same dual-site tandem catalytic mechanism is also applicable to various other catalyst combinations.In this process, one active site activates the NO 3 À intermediate, while another enhances the supply of *H and facilitates the hydrogenation of the intermediate.The efficient catalysis of NO 3 À to NH 3 is promoted through tandem work. [43]The introduction of a second metal site can not only adsorb *H, intermediates can also be strongly adsorbed.The Cu phase preferentially catalyzes the reduction of NO 3 À to NO 2 À and the rapid reduction of NO 2 À to NH 3 in other nearby metal phases (Co, [44] Ru, [45] Rh, [46] and so on).
Co atom was introduced to combine with Cu atom for tandem catalysis of the intermediate.The schematic illustration depicts the synthesis of tandem catalysts based on Cu/Co.(Figure 6b). [47]he catalytic mechanism of Cu followed by the conversion of NO 2 À to NH 3 at the close-by located Co 3 O 4 particles.It was confirmed that the effect of twocomponent tandem catalysis was much better than that of single-component catalysis, and the details of series tandem catalysis mechanism were revealed. [48]n addition, the introduction of foreign metals and the development of tandem work with Cu atoms have shown dependence Cu@C [15] 1 mM NO Fe/Cu [32] 1 M KOH þ0.1 M KNO Ru 1 Cu 10 /rGo [41] 1 M KOH þ1 M KNO 3 -98 À0.05 0.38 mmol cm À2 h À1 Rh@Cu-0.6% [42] 0 Cu 1 Co 1 HHTP [46] 0.5 M Na 2 SO 4 þ0.5 M NaNO 3 -96.4À0.6 299.9 μmol h À1 cm À2 on the pH of the electrolyte or the potential of electrochemical reactions.NO and NH 4 þ could be formed when an acidic electrolyte is used. [49]16a] The FE of NH 3 is linearly related to the reaction potential. [50]Consequently, catalysts composed of the same elements but combined in different forms exhibit varying catalytic performances.For comparison, various Cu-based electrocatalysts for NO 3 RR are summarized in Table 1.The comparison shows that Cu-based bimetallic catalysts show excellent performance at lower potential in NO 3 RR.

Outlook
In this work, we stressed the great significance of synergistic NO x and H activation in promoting the key kinetic step of electrocatalytic NO 3 RR over Cu-based catalysts.At present, catalysts that alloyed Cu atoms with other precious metals such as Ag, Ru, Rh, and Pd have been developed for efficient ammonia synthesis.The higher performance for ammonia synthesis over Cu-based bimetallic demonstrates the significance to introduce nonprecious metals (Ni, Co, etc.) as H activation sites to promote the hydrogenation process.At the same time, this also implies that other catalytic active components such molybdenum sulfide with H activation ability can be also introduced for the design of Cu-based catalysts to promote the reaction synergistically for efficient ammonia synthesis. [51]n recent years, new optimization concepts have emerged in the field of electrocatalysis.One such concept is the coupling reaction, which involves combining a cathodic reduction reaction with an anodic oxidation reaction.This coupling reaction system offers several benefits. [52]It promotes the generation of desired products and enhances product selectivity.Additionally, it helps reduce the adsorption of intermediate product NO 2 À and facilitates the generation and selectivity of ideal anode products. [53]his approach allows for the optimization of both economic benefits and yield for electrochemical synthesis.This new ideas could be provided for sustainable development and green economy, [54] by constructing a coupling reaction system between nitrate reduction and other oxidation reactions (reduction of carbon dioxide, [55] glycerol oxidation, [56] formaldehyde oxidation, [57] propane anaerobic oxidation, [58] urea electro-oxidation [59] etc.).By designing and preparing a dual-functional electrocatalyst, it is possible to achieve simultaneous cathodic and anodic reactions at ultralow voltage and to obtain products with high selectivity and high FE.Moreover, this coupling reaction system accelerates the overall reaction rate, promotes the adsorption of intermediate products, and avoids catalyst poisoning caused by the aggregation of intermediate products and low adsorption of *H.Introducing nonprecious metals/compounds with H activation ability into Cu-based catalysts to design bifunctional catalysts and construct a coupled reaction system is a very meaningful work.

Figure 1 .
Figure 1.Study on poisoning mechanism of nitrate reduction and the key RLS.a) The mechanistic route of NO 3 RR to NH 3 .b) Cu (100) and Cu (111) electrodes in 0.1 M NaOH in the presence of 2 mM NaNO 3 as a function of the cycle number.First cycle starts at 0.35 V versus reversible hydrogen electrode (RHE).Experimental conditions: scan rate 50 mV s À1; rotation rate 400 rpm.Reproduced with permission.[16a]Copyright 2019, Elsevier.c) Comparison of catalytic performance of different types of Cu-based electrocatalysts toward NO 3 RR.

Figure 2 .
Figure 2. Calculated free energies for a 1 ) NO 2 À adsorption on Cu (111), Cu-N 4 , and Cu-N 2 surfaces, respectively.The brown, gray, blue, and red balls represent C, N, Cu, and O atoms, respectively.Reproduced with permission.[19a]Copyright 2020, Wiley-VCH Verlag.b 1 ) Fourier transformation of the extended X-ray absorption fine structure spectra of Cu-N-C, Cu foil, and Cu 2 O at the R space.b 2 ) N concentration in formed NO 2 À for different catalysts

Figure 4 .
Figure 4. Representative Cu-based catalysts for promoting *H adsorption.a) The proposed NO 3 RR mechanism on cobalt phosphide.Color code: Co light blue, P pink, H white, O red, N dark blue.Source data are provided as a Source Data file.Reproduced with permission.[28]Copyright 2022, Springer Nature.b) The active *H-involved interfacial Pt-Ni coreduction toward alloy nanoshells.Reproduced with permission.[30]Copyright 2023, Springer Nature.c 1 ) ESR spectra of the solutions obtained after 10 min of NO 3 RR using Ru-ST-12, Ru-ST-5, and Ru-ST-0.6 in 1 M KOH under argon using DMPO as the •H-trapping reagent.c 2 -c 3 ) Energy diagram of the reaction steps from HNO to H 2 NO and from NH 2 to NH 3 over the strained Ru surface.Reproduced with permission.[31a]Copyright 2020, American Chemical Society.d) ESR spectra of the solutions obtained by the Ni 3 Fe-CO 3 LDH/Cu foam and Cu foam electrodes in the presence of 50 mM DMPO in 1 M KOH.The electrochemical reaction was conducted by chronoamperometric technique for an hour to trap the hydrogen radicals.Reproduced with permission.[32]Copyright 2023, Royal Society of Chemistry.

3 À(
3 RR activity.Bader charge and the charge density differences in the N-coordinated Cu-Ni dual-SAC (Cu/Ni-NC) indicate that different electronegativity induces strong electron transfer from Ni atoms to Cu atoms (Figure 5c 1 ).The charge density differences are calculated based on the electron localization function (ELF), indicating a stronger interaction between Cu-Ni dual-single-atom and *NO Figure5c2 ).This indicates that compared to Cu-NC and Ni-NC, it has higher selectivity for FEs and NH 3 synthesis in NO 3 RR.[38]Cu-Ni bimetallic catalyst is formed by introducing Ni metal to Cu metal.Ni alloying can be used to regulate the d-band center of Cu atom, which in turn modulates the adsorption of N-containing intermediates.The strong electronic interactions between the Cu-Ni bimetals can also be utilized to jointly

Figure 5 .
Figure 5. Representative Cu alloy-based catalysts for optimizing intermediate adsorption.a 1 -a 2 ) Theoretical and experimental analyses of N─O bond activation.a 1 ) Crystal orbital Hamilton population (-COHP) and its integrated value (ICOHP) of NO* adsorption on different metal sites.a 2 ) DEMS analyses of hydrogenation intermediates after the *NO adsorption step during the NO 3 À c 1 -c 2 ) Charge density difference and corresponding charge transfer on c 1 ) Cu/Ni-NC, and c 2 ) Cu/Ni-NC-NO 3 À *, Cu-NC-NO 3 À *, and Ni-NC-NO 3 À

Figure 6 .
Figure6.a) Tandem reaction scheme for Cu 50 Ni 50 under pulsed NO 3 RR at pH = 12.Reproduced with permission.[42]Copyright 2023, John Wiley and Sons Ltd. b) Schematic illustration of the preparation of a Cu/Co-based binary 'tandem catalyst.Reproduced with permission.[47]Copyright 2022, John Wiley and Sons Ltd. c 1 -c 3 ) In situ Raman analysis of c 1 ) Cu 2 O, c 2 ) Co 3 O 4 , and c 3 ) Cu 2 O þ Co 3 O 4 at different applied potentials in electrolytes containing 0.1 mol L À1 NO 3À , 0.045 mol L À1 Na 2 SO 4 , and 0.01 mol L À1 NaOH.Reproduced with permission.[48]Copyright 2023, John Wiley and Sons Ltd.
Figure6.a) Tandem reaction scheme for Cu 50 Ni 50 under pulsed NO 3 RR at pH = 12.Reproduced with permission.[42]Copyright 2023, John Wiley and Sons Ltd. b) Schematic illustration of the preparation of a Cu/Co-based binary 'tandem catalyst.Reproduced with permission.[47]Copyright 2022, John Wiley and Sons Ltd. c 1 -c 3 ) In situ Raman analysis of c 1 ) Cu 2 O, c 2 ) Co 3 O 4 , and c 3 ) Cu 2 O þ Co 3 O 4 at different applied potentials in electrolytes containing 0.1 mol L À1 NO 3À , 0.045 mol L À1 Na 2 SO 4 , and 0.01 mol L À1 NaOH.Reproduced with permission.[48]Copyright 2023, John Wiley and Sons Ltd.
2 O þ Co 3 O 4 tandem catalyst was analyzed by in situ Raman spectroelectrochemistry. Raman peaks of Cu 2 O þ Co 3 O 4 (Figure 6c 3 ) show similar phase evolution characteristics of both Cu 2 O (Figure 6c 1 ) and Co 3 O 4 (Figure 6c 2 ) alone.NO 3 RR on Cu 2 O þ Co 3 O 4 can be divided into two sequential steps of the reduction of NO 3 À to NO 2 À on Cu 2 O

Table 1 .
Activity comparison of Cu-based electrocatalysts for NO 3 RR.