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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 9: 1291 - 1303

DOI: https://doi.org/10.7536/PC240123

Review

Cu-Based Catalysts for Electrocatalytic Nitrate Reduction

  • Changzheng Lin 1 ,
  • Jinwei Zhu 2 ,
  • Weijia Li 1 ,
  • Hao Chen 1 ,
  • Jiangtao Feng , 1, * ,
  • Wei Yan 1
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  • 1 School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
  • 2 Shaanxi Electrical Apparatus Research Institute, Xi 'an 710025, China
*e-mail: (Jiangtao Feng)

Received date: 2024-01-26

  Revised date: 2024-03-04

  Online published: 2024-04-16

Supported by

New Energy Material Innovation Consortium Projects of Yunnan Province(202302AB080018)

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Abstract

in recent years,electrocatalytic nitrate reduction(ENitRR)has attracted considerable attention in the synthesis of ammonia at ambient conditions.Compared to the traditional Haber-Bosch process for ammonia synthesis,ENitRR offers lower energy consumption and milder reaction conditions.the design and optimization of ENitRR electrocatalysts are crucial for nitrate deoxygenation and hydrogenation.copper-based catalytic materials have been widely studied due to their unique structure,low cost,and excellent performance,making them highly promising electrocatalysts through various morphology and electronic structure modulation strategies.This article summarizes various effective design strategies using copper-based electrocatalysts as a typical example to enhance the ammonia production rate and conversion efficiency in ENitRR.It also introduces the reaction mechanism and the relationship between structural changes in Cu-based electrocatalysts and their performance.These strategies include morphology modulation,alloy engineering,lattice phase tuning,single-atom structures,as well as copper compound construction and composites with other materials.Finally,challenges faced by copper-based electrocatalysts are discussed along with future research directions that should be focused on in order to provide reference for researchers engaged in nitrate treatment in aqueous systems。

Contents

1 Introduction

2 Mechanism of ENitRR

3 Research status of Cu-based electrocatalysts

3.1 Metal Copper(Cu0 )

3.2 Cuprous based catalyst

3.3 Copper matrix composite

4 Conclusion and outlook

Key words： copper-based electrocatalysts; electrocatalyst; nitrate reduction; design strategies

Cite this article

Changzheng Lin , Jinwei Zhu , Weijia Li , Hao Chen , Jiangtao Feng , Wei Yan . Cu-Based Catalysts for Electrocatalytic Nitrate Reduction[J]. Progress in Chemistry, 2024 , 36(9) : 1291 -1303 . DOI: 10.7536/PC240123

1 Introduction

As one of the five elements in cell biomass,nitrogen plays a very important role in the life activities of organisms.With the continuous development and utilization of nitrogen resources,global environmental pollution and energy crisis have been caused[1,2]。 In particular,the increasing amount of fertilizer and industrial wastewater has released a large amount of nitrate into surface water and groundwater[3]。 Slow natural denitrification can not effectively remove excessive nitrate and its derivatives,which will cause certain harm to the environment and human health.First of all,when the concentration of nitrogen in the water body is too high,it will cause eutrophication of the water body,and then cause environmental pollution such as red tide[4,5]。 Secondly,excessive intake of nitrate can cause major diseases such as blue baby syndrome and seriously endanger human health[6]。 in addition,other derivatives produced in the nitrogen cycle also pose a great threat to human health.For example,nitrite is a carcinogenic pollutant,which can oxidize ferrous ions in hemoglobin to ferric ions,thus affecting the oxygen transport capacity of red blood cells[7]。
Excessive nitrate nitrogen pollution in water body has become an urgent problem to be solved.At present,traditional technologies involving chemical,photochemical,physical and biological treatment of nitrate have been widely used[8⇓~10]。 Although the operation of photocatalysis is simple,the recombination of photogenerated carriers greatly reduces the photocatalytic activity during the photocatalytic reaction[11]。 Biological denitrification technology can remove nitrogen from all kinds of sewage under suitable conditions.However,this technology also has some limitations,such as sludge production,easy spread of pathogenic bacteria and low efficiency[12]。 As for physical removal methods(such As reverse osmosis and ion exchange),the core of their role is nitrate transfer or concentration,not nitrate removal.As a result,secondary brine containing nitrate is produced during the treatment process,which increases the cost of subsequent treatment[10]。 In addition to the direct removal of excessive nitrogen pollution in water,converting it into ammonia for further use is also one of the current research priorities.Ammonia,as an important chemical raw material and energy carrier,is one of the important energy carriers due to its high energy density（3.5 kW·h·L-1,which is 1.45 times that of hydrogen),zero carbon emission,and easy storage and transportation(boiling point:−33.34°C )[13][14,15]。 However,in the past 100 years,the traditional Haber-Bosch(H-B)ammonia synthesis process has consumed 1%~2%of the world's energy and produced 1%of the world's CO2,mainly due to the high（941 kJ·mol−1）of nitrogen-nitrogen triple bond energy in the H-B ammonia synthesis process and the use of non-clean energy[16][17]。 Based on this,the method of using nitrate as nitrogen source and using clean energy to convert it into ammonia has become an important part of chemical energy storage。
Chemical nitrate reduction technology is a method to selectively convert nitrate into ideal products,and its driving force can be divided into heat,light-electricity and electricity[3,18]。 Among them,Electrochemical nitrate reduction reaction(ENitRR)is a promising technology for nitrate wastewater treatment due to its environmental compatibility,selectivity,energy efficiency and low cost.First,the reaction is driven entirely by electrical energy to achieve low-cost removal of nitrate[19,20]。 Secondly,in electrochemical reduction,nitrate can be efficiently and controllably reduced to nitrogen or other products with high added value,such as ammonia and hydroxylamine,by selecting different catalysts and experimental parameters,so as to meet different needs[3,21,22]。 Therefore,electrochemical nitrate reduction technology has become the frontier of 128 research hotspots[23]。
in this paper,the number of papers published in the field of ammonia synthesis by electrocatalytic nitrate reduction in the 10 years from 2014 to 2023 is counted,as shown in Figure 1.It shows that the number of published papers and citations in this field is increasing year by year.the number of published papers(372)in 2023 is 28.6 times that of 2014(13),and the number of citations(14,470)in 2023 is 24.4 times that of 2014(593).Among them,copper-based catalyst,as an excellent electrocatalytic catalyst for ammonia synthesis by nitrate reduction,accounts for about 31%.the rapid growth trend of the number of published papers and citations indicates that the research in the field of electrocatalytic nitrate reduction for ammonia synthesis is still hot;Since China put forward the Medium and Long-term Plan for the Development of hydrogen energy Industry(2021~2035)in 2021,ammonia,as an excellent Hydrogen carrier and clean Energy donor,has also been paid attention to,which indirectly promotes the research in the field of electrocatalytic nitrate reduction to synthesize ammonia。
图1 电催化硝酸盐还原合成氨领域的历年发文量和引用量

Fig. 1 The number of publications and citations in the field of electrocatalytic reduction of nitrate to ammonia over the years

The design of efficient and highly selective electrode materials is the key to the ENitRR reaction.Currently,a variety of working electrodes have been reported,including noble metals(platinum,palladium,iridium,ruthenium,rhodium),transition metals(copper,tin,iron,cobalt,nickel,titanium,etc.),and bimetallic alloys（Cu2O-Cu@Ti,CuPd,CuCo,CuNi,etc. )[24⇓~26][16,27⇓⇓⇓~31][32⇓⇓~35]。 However,the large-scale application of noble metals such as Pt,Pb,and Au in the ENitRR reaction is hindered by high cost,susceptibility to poisoning,and limited lifetime[36,37]。
transition metal(such as lead(Pb),nickel(Ni),tin(Sn),zinc(Zn),copper(Cu)and so on)catalytic materials have certain nitrate catalytic activity.Among them,copper has become an important research object of ENitRR catalyst because of its special catalytic performance for nitrate.Compared with other Transition metal catalysts,copper-based catalysts have the following advantages:first,the hydrogen evolution activity of copper-based catalysts is relatively low,which can effectively inhibit the side reaction of hydrogen evolution[35,38⇓~40]; Second,the copper metal catalyst has a good binding ability for NO3−due to the good energy level matching between the d-electron orbital of copper and the LUMO p*of nitrate[41]; Third,the good conductivity of copper metal catalyst can promote electron transfer and efficiently catalyze the conversion of ENitRR rate-determining step（NO3−to NO2− )[42,43]; Fourthly,the copper-based catalyst can drive the catalytic reduction of nitrate at a lower potential,so as to achieve the purpose of energy saving and consumption reduction[44,45]。 Copper-based catalysts have attracted great attention from scholars at home and abroad because of their good selectivity and high efficiency for nitrate reduction[46⇓⇓~49]。
Although the application of copper-based catalysts In nitrate reduction has achieved good results,there are still some problems to be solved.For example,in the electrolyte,the stability of the catalyst is the key to determine whether it can be put into practice.in addition,one of the urgent problems to be solved is to reduce the formation of by-products by regulating the selectivity of nitrate reduction reaction[50]。 as shown in Fig.2,the electrochemical reduction of nitrate to ammonia is an 8-electron transfer reaction,and its reduction reaction potential(0.69 V vs RHE)is higher than the 6-electron transfer process(0.09 V vs RHE)required for the reduction of nitrogen to ammonia.Therefore,the electrocatalytic reduction of nitrate to ammonia with nitrate as the nitrogen source has more advantages than the synthesis process with nitrogen as the nitrogen source.Although the theoretical electrode potential of electrocatalytic nitrate reduction for ammonia synthesis is higher than the electrode potential of hydrogen evolution reaction(hydrogen evolution reactions,HER)(0 V vs RHE),the current reduction potential of nitrate to ammonia is generally lower than the HER potential,which leads to the side reaction of Hydrogen evolution and the consumption of electron donors,thus reducing the Faradaic efficiency of ENitRR reaction process.How to realize the conversion of nitrate to ammonia with high efficiency and selectivity,and inhibit the formation of N≡N bond and Hydrogen evolution side reaction is an urgent problem to be solved[51]。 In view of the above problems,this paper comprehensively reveals the electrochemical reduction mechanism of copper-based electrode materials for nitrate by summarizing the existing literature.Effective ways to improve the performance of copper-based catalysts by increasing the specific surface area of the electrode,adjusting the coordination state,regulating the crystal plane,alloying,changing the metal valence state and compounding with other materials are summarized,which provides a theoretical basis for the design of new and efficient copper-based nitrate reduction electrode materials(Figure 3)。
图2 析氢反应、氮还原反应(NRR)和ENitRR的电化学还原电极电势值

Fig. 2 Electrochemical reduction electrode potential values for hydrogen evolution reaction, nitrogen reduction reaction (NRR) and ENitRR

图3 通过对比表面积、晶面、配位环境、合金化、价态调控和与复合材料提升铜基电催化剂的ENitRR性能

Fig. 3 The electrocatalytic performance of Cu-based catalysts was improved by optimizing factors such as surface area, crystal plane orientation, coordination environment, alloying strategies, valence regulation, and composite materials

2 Mechanism of ENitRR

The efficient conversion of nitrate into ammonia with high added value by electrocatalysis can effectively promote the nitrogen cycle in nature and improve the effective utilization of nitrogen resources.ENitRR reaction is a process involving multiple proton-coupled electron transfer.Generally,when the concentration of nitrate in water is lower than the 1 mol·L−1,the electrocatalytic nitrate reaction mechanism is a direct reaction mechanism.The direct reaction mechanism is further divided into two different pathways,i.e.,electron transfer reduction and proton-electron coupling reduction.In the nitrate reduction pathway mediated by electron transfer,a large number of intermediates may be produced,Such as NO2,NO2−,NO,N2O,N2,NH2OH and NH3.In the proton-electron coupled catalytic nitrate reduction pathway,the electron acts as a reducing agent and the proton acts as a hydrogenating agent,and the reaction process is described in Equation 1.In the proton-electron coupled reduction pathway,the reactive species*NO is the key intermediate controlling the selectivity of nitrate reduction[52]。 If*NO is hydrogenated to*HNO,NH3is the major product,whereas other reaction pathways would lead to the formation of N2(Equation 2 and Figure 4 )[53]。
图4 电催化硝酸盐还原的主要反应途径（*表示该物质被吸附或靠近电极）

Fig. 4 The main reaction pathway of the electrocatalytic nitrate reduction (* indicates that the substance is adsorbed or close to the electrode)

NO 3 − +6H 2 O+8e − → NH 3 +9OH − E o =-0.12V vs SHE
NO 3 − +3H 2 O+5e − → 1 / 2 N 2 +6OH − E o =0.26V vs SHE
In the electron transfer mediated nitrate reduction process,NO3−is initially adsorbed on the catalyst surface to obtain the adsorbed state*NO3（Equation 3).The adsorbed*NO3is reduced to*NO2（(Equation 4)by combining with*H in the liquid phase to give electrons.This reaction is usually the rate-determining step because it requires a high activation energy and exhibits slow reaction kinetics[54,55]。 The reaction intermediate*NO2has high activity on the catalyst surface and can further generate adsorbed*NO,which is generally considered as the selectivity determining step(Equation 5 )[55]。 On the one hand,*NO may be transformed into NO in solution(aq)to leave the catalyst surface(Equation 6).Weakly adsorbed NO dimers may be converted into*N2O（(Equation 7),followed by desorption and release of N2O gas from the system(Equation 8 )[52,56]。 In addition,the intermediate species*N2O may also be converted to N2（(Eq.9 )[57,58]。
NO 3 − ⇌ *NO 3
*NO3 + 2H+ + 2e− → *NO2 + H2O
*NO2 + 2H+ + e− → *NO + H2O
*NO → NO$
*NO + NO(aq) + 2H+ + 2e−→*N2O + H2O
*N2O → N2O
*N2O + 2H+ + 2e− → N2 + H2O
In addition,nitrate reduction can also be achieved by a proton-electron coupled reduction pathway,and active hydrogen can be produced by water reduction via the Volmer process(Equation 10)[59,60]。 As a strong reducing agent,（E°（H+/H）=−2.31 V vs SHE）,active hydrogen can reduce adsorbed NO3−and intermediate species of NO2−and NO(formulas 11–13 )[61]。 Since the formation of the N—H bond mediated by*H is kinetically more favorable than the formation of the N≡N bond,the main end product in this process is ammonia(Equations 14 to 16)[60,62]。
H2O + e− → *H + OH− (Volmer)
*NO3 + 2*H → *NO2 + H2O
*NO2 + *H→ *NO + OH−
*NO + 2*H → *N + H2O
*N + *H → *NH
*NH* + *H → *NH2
*NH2 + *H → *NH3
the hydrogen evolution reaction(hydrogen Evolution reaction,HER)is an inevitable side reaction in the ENitRR process because of the similar thermodynamic barrier between ENitRR and HER,and the HER competition Reaction is more intense at high current density.In order to improve the Faradaic efficiency of ENitRR,it is necessary to consider the suppression of HER side reactions as much as possible at the beginning of catalyst design.In the HER process,the electrochemical adsorption and desorption of active hydrogen(*H)on the electrocatalyst surface are competitive reactions in nature.the adsorption of*H on the optimal HER catalyst should not be Too strong or Too weak.too strong*H adsorption will lead to the accumulation of*H on the surface of the catalyst,which is not conducive to the desorption of hydrogen.too weak*H adsorption will lead to insufficient hydrogen atoms and poor hydrogen Evolution performance.Therefore,when designing an electrocatalytic nitrate reduction electrode,the"volcano curve"of the Gibbs free energy of the metal for the adsorption of hydrogen atoms should be taken as a reference(Fig.5),and an electrode with suitable adsorption performance for*H should be used to suppress the HER side reaction[65]。 However,in the actual reaction process,the continuous hydrogenation reaction involving*H is of great significance to the production of NH3.Insufficient supply of*H will slow down the production rate of NH3,while excessive*H will lead to the combination of hydrogen atoms to produce H2.Therefore,metals with less strong adsorption capacity for*H are generally used to maintain the dynamic balance between the supply and consumption of*H[63,64]。
图5 交换电流密度和吉布斯自由能(∆G*H)的火山曲线[65]

Fig. 5 Volcano curves of the exchange current density and Gibbs free energy ∆G*H. Copyright 2010, American Chemical Society

3 Research Status of Copper-based Electrocatalyst

as a widely used metal catalyst for carbon dioxide reduction,copper can effectively promote carbon-carbon coupling reactions to produce value-added multi-carbon products(such As ethanol and ethylene)[66,67]。 in addition,copper also plays a vital role In ENitRR electrocatalyst[68,69]。 It was found that the conversion of nitrate to nitrite is the rate-determining step of ENitRR,and the copper catalyst has a significant effect on this process[34,70]。 based on the molecular orbital theory,compared with other metal elements,copper has a higher highest occupied molecular orbital(HOMO,d-orbital)and lowest unoccupied molecular orbital(LUMO),which is theoretically similar to nitrate(π-orbital),which means that copper can easily inject electrons into the N—O bond of nitrate and activate the ENitRR process.This characteristic endows the copper-based catalyst with excellent catalytic activity for nitrate reduction.In the reduction of nitrate to nitrite,Cu-based catalysts can exhibit high Faradaic efficiency even at relatively low overpotentials(e.g.,0 V vs RHE)。

3.1 Metallic copper （Cu0）.

copper is considered to be one of the most promising catalysts for the conversion of ENitRR to ammonia because of its excellent catalytic performance for nitrate and its intermediates in the reduction process,as well as its weak hydrogen evolution performance.the research on copper catalysts mainly focuses on the morphology,crystal form,single atom and alloying of copper。

3.1.1 Morphology control

Nanostructured zero-valent copper has been shown to be able to enhance the electrocatalytic reduction performance toward nitrate.copper nanostructures with different morphologies were synthesized by chemical reduction,photochemical synthesis and thermal reduction using cuprous ion and copper ion as copper sources[71,72][73][74]。 Nanostructures in different dimensions(zero-dimensional(0 D),one-dimensional(1 D),two-dimensional(2D),and three-dimensional(3D))have different structural features and physicochemical properties,and exhibit different electrocatalytic nitrate reduction properties(Fig.6)[75,77,78,80]。 Table 1 summarizes the electrocatalytic nitrate reduction performance of four elemental copper catalysts with different dimensional morphologies。
图6 （a）铜纳米颗粒（0D）[75]、（b）铜纳米片（1D）[80]、（c）铜纳米线（2D）[77]和（d）多孔铜结构（3D）[78]的SEM图

Fig. 6 SEM images of (a) copper nanoparticles (0D) [75], (b) copper nanosheets (1D) [80], (c) copper nanowires (2D) [77], and (d) porous copper structures (3D) [78]. Copyright ©, John Wiley & Sons, Ltd; Copyright ©, American Chemical Society

表1 Performance Summary of Copper Nanostructures with Different Dimensions for Electrochemical Nitrate Reduction to Ammonia

Table 1 A comprehensive review on electrochemical nitrate reduction for ammonia synthesis using copper nanostructures of varying dimensions

Dimension Cathode Performance ref
0D Copper nanoparticles 73%~74% NO3− conversion rate;
97% NH3 selectivity		 79
1D Copper nanowire NO2− and NH3 with current efficiencies of 91.5% and 100% 77
2D Copper nanosheet 390.1 μg·mg−1 Cu h−1 ammonia rate; 99.7% Faradaic efficiency 76
3D 3D foam Cu-catalyst Partial current densities beyond 1 A·cm-2 78
Wen et al.Prepared three kinds of metallic copper nanostructures(copper nanoparticles(0D),copper nanocubes(0D)and copper nanosheets(2D))by chemical reduction method,and studied the electrocatalytic reduction of nitrate to ammonia[75]。 Compared with Cu nanocubes and irregular Cu nanoparticles,Cu nanosheets effectively suppress the hydrogen evolution side reaction rate and increase the reaction rate of the rate-determining step（NO3−→NO2−）.Fu et al.Synthesized copper nanosheets by chemical precipitation method for nitrate reduction experiments,and the results showed that the stability of the catalyst could be increased by morphology control[76]。 Under the optimal experimental conditions,the ammonia yield of Cu nanosheets with exposed(111)facet was 390.1μg·mg−1Cu h−1,and the faradaic efficiency of NH3was 99.7%.Jiang et al.Successfully prepared 1D copper nanowire structure on the surface of copper foam by chemical oxidation,which is conducive to the rapid transport and diffusion of reactants,intermediates and products on the electrode surface[77]。 the 1D structure of copper nanowires has more catalytic active sites,so it has efficient reaction kinetics and excellent catalytic efficiency.Wang et al.Electrodeposited a copper catalyst with 3D porous structure on The surface of copper foam material by dynamic bubble template method[78]。 nitrate transfer from the electrolyte to the catalyst surface was effectively promoted,and the rate-determining step of Nitrate reduction to ammonia was significantly increased。
the above results show that the 0 D copper nanostructure can increase the active area of the catalyst.However,it is nanoscale in three dimensions,and the number of catalytic active sites is limited.the 1D copper nanowire structure can effectively increase the number of active sites,but there is a risk of breakage.2D copper nanosheets not only increase the number of active sites but also improve the stability,but the vertical or parallel distribution of copper nanosheets on the substrate will significantly affect the number of active sites.the 3D porous copper structure ensures the stability of the catalyst while increasing the electrocatalytic active sites.the nanostructure of copper-based materials can effectively promote the mass transfer process in the electrocatalytic nitrate reduction process and significantly improve the reaction rate。

3.1.2 Crystal face control

Facet control is a technique to optimize The catalytic performance by adjusting the phase structure of the catalyst and related factors.the transformation of crystal plane structure is usually affected by several factors such as dynamic stability,strain,spin and surface energy,which can be precisely controlled by parameters such as temperature,pressure and solution composition。
Gewirth et al.Studied the effect of different crystal faces of copper on ENitRR activity in acidic medium[81⇓~83]。 The results show that the electrocatalytic reduction of nitrate by Cu(100)and(111)is similar,and the onset potential of Cu(111)is lower than that of Cu(100).Koper et al.Studied the effect of different crystal planes of copper,Cu(100)and Cu(111),on the electrocatalytic reduction of nitrate in alkaline medium,and the results showed that the catalytic activity was similar to that in acidic medium,and Cu(111)showed a lower reduction potential of NO3−than Cu(100 )[43]。 As the potential decreases,Cu(100)exhibits higher catalytic activity because of the higher catalytic activity of Cu(100)toward the subsequent deep reduction products of NO2−(i.e.,hydroxylamine,ammonia )[84]。 Based on the excellent catalytic activity of Cu(111)and Cu(100)for nitrate and nitrite,respectively,Fu et al.Synthesized Cu nanosheets(with both Cu(111)and Cu(100)facets)by solvothermal method for the electrocatalytic reduction of nitrate to ammonia,and the Cu nanosheet catalyst showed excellent catalytic performance at−0.59 V vs RHE with a NH3partial current density of V vs RHE and a NH3yield of 1.41 mmol·h-1·cm-2 )[85]。 the mechanistic study shows that the high performance of copper nanosheets can be attributed to the synergistic catalysis between Cu(111)and Cu(100)facets(Fig.7).the electrocatalytic nitrate reduction performance can be effectively improved by controlling the parameters in the crystal face synthesis process and utilizing the synergistic catalytic performance between crystal faces。
图7 Cu(100)和Cu(111)之间的协同催化（NO3−在Cu(100)生成NO2−，NO2−在Cu(111)上被氢化为NH3）[85]

Fig. 7 Synergistic catalysis of Cu(100) and Cu(111) (NO3− forms NO2− on Cu(100), which is hydrogenated to NH3 on Cu(111))[85]. Copyright ©2023, John Wiley & Sons, Ltd

3.1.3 Monoatomic structure

the reduction of metal nanoparticles to individual atoms is an effective strategy to develop highly selective,efficient,and stable electrocatalysts.Single-atom catalysts(Single Atom Catalyst,SACs)have made new progress in the field of electrocatalysis and have received increasing attention for their ability to achieve maximum Atom utilization efficiency.However,due to the high surface energy of isolated individual atoms,it is a great challenge to uniformly disperse and anchor individual atoms on the substrate to prevent migration and aggregation.Typical strategies for constructing SACs include methods such as introducing defects on the substrate,designing restricted spatial positions,and fabricating coordination sites(e.g.,N,O,and S form coordination with lone pairs of electrons)to achieve anchoring and stabilization of individual atoms。
Zhu et al.Prepared a single-atom copper catalyst by pyrolysis,and introduced polyethyleneimine to synthesize a pyridine-rich nitrogen-type copper single-atom catalyst PR-CuNC[46]。 Compared with the catalyst without polyethyleneimine,the pyridine nitrogen content of PR-CuNC was significantly increased.The results of the electrocatalytic nitrate reduction test showed that PR-CuNC had excellent ENitRR performance,with the highest faradic efficiency of 94.61%and the yield of NH3of 130.71 mg NH3mg−1Cu h−1.Theoretical studies reveal that different coordinated nitrogens can cause changes in the electronic structure of the Cu-N4site,and the Cu-pyridinic-N4site exhibits more favorable NO3−adsorption capacity and lower energy barrier for the*NO hydrogenation step during the ENitRR reaction(Figure 8 A).In order to further clarify the active site of the single-atom catalyst,Yang et al.Supported single-atom copper on carbon nitride(Cu-N-C)by chemical oxidation and impregnation methods for electrocatalytic reduction of nitrate to ammonia.During the experiment,as the potential gradually decreased,Cu2+was reduced to Cu+and Cu0,followed by the formation of∼5 nm-sized nanoparticles.The yield of NH3also increased continuously when Cu atoms aggregated into nanoparticles,where the maximum yield of ammonia reached 4.5 mg·cm−2·h−1and the faradaic efficiency reached 84.7%.After the end of catalysis,the aggregated Cu nanoparticles were reversibly dispersed into single atoms and re-restored to the Cu-N4structure.The Cu single-atom catalyst aggregates into Cu nanoparticles during catalysis,and the Cu nanoparticles are the real active sites for the reduction of nitrate to ammonia(Figure 8 B )[86]。
图8 （a）NO3−和NO2−在Cu(111)、Cu-N4和Cu-N2表面上的自由能[46]；（b）Cu 单原子应用电位驱动的聚集机制和氧化环境中的再分散[86]

Fig. 8 (a) Free energies of NO3− and NO2− on Cu(111), Cu-N4 and Cu-N2 surfaces; (b) Potential driven aggregation mechanisms of Cu SAC application and redispersion in an oxidative environment[86]. Copyright ©2022, John Wiley & Sons, Ltd, Copyright ©2022, American Chemical Society

single-atom copper catalysts have the highest atom utilization efficiency and good catalytic performance,which can enhance the selectivity of products by regulating the adsorption and catalysis of nitrogen intermediates through the regulation of active atoms and low-coordinated metals.the stability can be effectively increased by regulating the coordination atoms around the single atom.However,there are few studies on the electrocatalytic nitrate reduction of single-atom copper catalysts,and the mechanism of the real catalytic effect in the single-atom catalytic process is still unclear,so more theories and experiments are needed to fully study the catalytic process。

3.1.4 Alloying

Although pure copper catalysts have good catalytic activity,they have been proved to be prone to spontaneous oxidative dissolution and surface deactivation,thus reducing their own activity[83][76,87]。 More importantly,as the concentration of nitrate decreases,the concentration of ammonia in the solution increases,resulting in the inevitable reaction of copper and ammonia to form copper-ammonia complex,which is fatal to copper catalysts.alloy engineering can achieve tuning of the electronic structure and properties of each metal to promote the activity and stability of the reactants on the catalyst surface.Conventional preparation methods of bimetallic Alloy materials mainly include co-reduction,simultaneous pyrolysis and electrochemical co-deposition.in addition,many special strategies such as pulsed laser ablation andγ-ray irradiation have been developed to overcome the inherent incompatibility between the two metals in bimetallic materials.Table 2 summarizes the performance of copper catalysts with different noble metal and non-noble metal alloying structures for electrocatalytic nitrate reduction。
表2 Summary of Ammonia Synthesis by Electrochemical Nitrate Reduction with Different Metals and Copper Alloying Structures

Table 2 Summary of electrochemical nitrate reduction for ammonia synthesis using various metal and copper alloy structures

metal Cathode Performance ref
Precious metals Ru@Cu 162 mA·cm−2 for NH3 production；Faradaic efficiency of 93% 88
Ru-Cu NW Faradaic efficiency of 93%; Industrial-relevant nitrate reduction current of 1 A·cm-2 39
PA-RhCu cNCs Faradaic efficiency of 93.7%; NH3 production yield of 2.40 mg·h−1 mg·cat−1 45
RuxCuy/
rGO		 Faradic efficiency of 98%; Ammonia formation rate of 0.38 mmol·cm−2·h−1 89
PdCu MSs Faradaic efficiency of 85%; yield rate of 3058 µg·h−1·mg−1 90
Cu-Pt yielding 194.4 mg NH3-N L−1·gcat−1, SNH3 of 84% 91
Non-precious metals Cu50Ni50 0.12 V upshift in the half-wave potential; 6-fold increase in activity 35
Ni1Cu-
SAA		 Faradaic efficiency of ~100%; NH3 yield rate of 326.7 μmol·h−1·cm−2 50
CuCo nanosheet Faradaic efficiency of 100 %± 1%; 4.8 mmol·cm−2·h−1 of NH3 production rate 64
Cu49Fe1 Faradaic efficiency up to 94.5 %; Selectivity of 86.8 %. 92
Fe/Cu Faradaic efficiency of 92.51%; NH3 yield rate of 1.08 mmol·h−1·mg−1 93
Zn/Cu Faradaic efficiency of 98.4%; NH3 yield rate of 5.8 mol·g−1·h−1 94
the electrocatalytic nitrate reduction activity on the surface of elemental copper is low,mainly due to the relatively weak hydrogen adsorption activity.At present,many studies have confirmed that noble metals(Pd,Pt,Ru)have excellent hydrogen adsorption characteristics[95][96][39,89,97]。 in order to overcome the poor performance of elemental copper catalysts In hydrogenation reactions,researchers have introduced noble metal doping to improve their active hydrogen adsorption capacity.Liu et al.Successfully prepared rhodium nanoclusters and rhodium single atom-coated copper nanowires(Rh@Cu)by chemical replacement reaction[88]。 the results of electrochemical nitrate reduction experiments show that the introduction of Rh can significantly improve the catalytic performance of copper nanowires.the experimental and DFT mechanistic results show that the main reason for the performance improvement is the synergistic catalytic effect between Rh and Cu sites,specifically,on Rh@Cu catalysts,Cu sites preferentially adsorb nitrogen intermediates.the adjacent Rh site can generate active H species and transfer from the Rh site to the*NO intermediate adsorbed on the Cu surface,thus promoting the hydrogenation step in the ammonia synthesis process(Fig.9a).Chen et al.Synthesized a ruthenium dispersed copper nanowire catalyst(Ru-CuNW)by cation exchange method,which makes full use of the unique catalytic properties of both Ru and Cu,in which the highly dispersed Ru atoms can provide highly active nitrate reduction sites,while the surrounding Cu sites can inhibit competitive HER,resulting in excellent electrocatalytic nitrate reduction performance of the catalyst[39]。 In a typical industrial wastewater with a NO3-concentration of 2000 mg·L-1,Ru-Cu NWs as electrocatalytic nitrate reduction catalyst can reach a current density of about 1 A·cm-2while maintaining a high faradaic efficiency of 96%.It is worth noting that the Ru-Cu NW catalyst has a nitrate conversion rate of more than 99%,which can treat 2000 mg·L-1of nitrate industrial wastewater to drinking water level of（<50 mg·L-1）and still maintain a Faradaic efficiency of more than 90%,and has great application prospects 。
图9 （a）Cu和Rh@Cu表面上的自由能[88]；（b）纯Cu催化剂和CuNi合金的UPS光谱和d带中心位置[35]

Fig. 9 (a) the free energies on the surface of Cu and Rh@Cu [88], Copyright © 2022, John Wiley and Sons; (b) UPS spectrum and D-band center position of pure Cu catalyst and CuNi alloy [35], Copyright @2020, American Chemical society

Noble metals have excellent hydrogen adsorption performance,and copper has good catalytic performance for nitrogen and intermediates.The combination of noble metals and copper can effectively improve the catalytic activity of catalysts for nitrate,but the price and abundance of noble metals limit their large-scale application.By combining Cu with non-noble metals,researchers can not only reduce the cost of the electrode,but also enhance the catalytic activity and stability of the electrode.In recent years,copper-nickel alloys have attracted much attention because of their excellent properties and corrosion resistance.Wang et al.Successfully prepared a series of CuNi alloys by electrodeposition,and found that the Cu50Ni50alloy exhibited the best electrocatalytic nitrate reduction performance[35]。 Under the conditions of 0 V vs RHE and pH=14,the activity of V vs RHE alloy catalyst for nitrate reduction was increased by 6 times compared with that of pure copper material.At the same time,the DFT calculation results show that after the introduction of Ni atom,the d-band center of Cu moves up to the Fermi level,and the antibonding orbital is weakened,which significantly improves the adsorption capacity of nitrogen intermediates,so that the rate-determining step is transferred from the adsorption of nitrate to the hydrogenation process of*NH2intermediates,thus reducing the overpotential(Fig.9b )[35]。 Iron is an important element of nitrogenase.In nature,microorganisms use nitrogenase to reduce nitrogen to ammonia.Inspired by this,Wang et al.Doped Fe uniformly into Cu by electrodeposition to obtain Cu49Fe1catalyst.In neutral medium,the faradaic efficiency of ammonia synthesis by reduction of Cu49Fe1nitrate is as high as 94.5%,and the ammonia selectivity is as good as 86.8%[92]。 the mechanism study shows that Fe doping adjusts the electronic structure of Cu 3D band and redistributes it to a deeper level,and Fe doping effectively adjusts the adsorption energy of the reaction intermediate and greatly enhances the electrocatalytic nitrate reduction activity。
Copper alloy catalysts can improve the catalytic reduction efficiency and stability of nitrate,and also open up a new way for the design of high-performance alloy electrocatalysts by exploring the relationship between the electron arrangement between two or more metals and the electrocatalytic activity。

3.2 Cuprous catalyst

Copper metal oxides have different surface and crystal compositions and play an important role in the field of catalysis.In the electrocatalytic process,copper metal will undergo phase transformation and reduction to form copper metal oxides with different crystal phases and compositions,and their catalytic properties are closely related to their compositions.In fact,due to the negative working voltage of the electrocatalytic nitrate reduction reaction,compared to the Cu+（Cu+→Custandard electrode potential:−0.360 V),Cu2+is more easily reduced to Cu+or Cu（Cu2+→Cu standard electrode potential:−0.222 V),so it is difficult for Cu2+to exist stably during nitrate reduction,and the real catalytic active site is generally considered to be Cu+ 。
Wang et al.Converted CuO nanowire arrays(CuO NWAs)into Cu/Cu2O NWAs by in situ electrochemical reduction,thereby exhibiting high ammonia selectivity(81.2%)and excellent faradaic efficiency(95.8%)in ENitRR[38]。 The results show that the Cu/Cu2O is the main active site,and the electron transfer to Cu at the Cu2O interface inhibits HER and promotes the formation of*NOH intermediate.Ren et al.Successfully prepared Cu nanowire（Cu@Cu2+1O NWs）with rough surface Cu2+1O coating by surface engineering strategy[98]。 The catalyst exhibited excellent electrocatalytic nitrate reduction performance(nitrate conversion of 78.57%,ammonia yield of V vs SCE,and faradic efficiency of 87.07%)at−1.2 V vs SCE for 2 H.The results show that the binding energy of Cu 2p3/2in Cu@Cu2+1O NWs shifts negatively compared with that of standard metal Cu,which indicates that the electronic structure and d-band center of Cu atom in Cu@Cu2+1O NWs have changed.As the electrolysis time increases,the concentration of nitrate decreases while the concentration of ammonia increases,and the concentration of nitrite remains at a low level.Compared with V vs SCE,Cu@Cu2+1O NWs/CF showed better performance at−1.2 V vs SCE,including nitrate conversion and ammonia selectivity.The results show that the inner metal Cu nanowire has efficient electron transport ability,while the outer concave-convex Cu2+1O layer is the catalytic active site(Fig.10a )[98]。 Wang et al.Deposited island-like copper particles on the surface of nickel foam to reduce CuO to Cu2O by electrochemical nitrate reduction[99]。 Compared with copper foam and nickel foam,the island copper exhibits higher current density and Faradaic efficiency,reaching 98.28%at-0.8 V vs RHE.In the initial stage of nitrate reduction,most of the nitrate is converted into ammonia,and a part of the by-product nitrite is also converted into ammonia in the later stage of the reaction.The DFT calculation results show that the energy barrier for the formation of H2on Cu2O is much higher than that on Cu and the interface(Fig.10 B),which indicates that the active sites on Cu2O inhibit the hydrogen evolution reaction.Moreover,in terms of nitrate adsorption and*NO3formation,Cu2O has a faster rate and requires less energy in its hydrogenation to generate*NH2O(Fig.10 C).The surface of Cu2O is more likely to generate*NH2OH,and the presence of oxygen atoms effectively inhibits the generation of hydrogen and reduces the formation of by-products 。
图10 （a）Cu@Cu2+1O NWs的电催化硝酸盐还原机制示意图；（b）氢气吸附模型和生成氢气的反应能；（c）反应自由能[99]

Fig. 10 (a) Schematic illustration showing the electrocatalytic nitrate reduction mechanism over the Cu@Cu2+1O NWs. (b) Hydrogen adsorption model and reaction energy of hydrogen generation. (c) free energy of reaction[99]. Copyright@2022, American Chemical Society

The above results show that cuprous ion can effectively enhance the electrocatalytic activity of nitrate reduction,but how to improve the stability of cuprous ion has become the focus of research.Li et al.Obtained binderless Cu3P/CF composite on copper foam substrate(CF)by a simple in situ phosphating method[100]。 It was found that the electrocatalytic activity of the prepared electrode was closely related to its composition and crystallinity,which were affected by the phosphating temperature.With the increase of phosphating temperature,the conversion rate of nitrate decreased gradually,and reached the highest conversion rate(97.7%)at 400℃.In addition,cyclic voltammetry and electrochemical impedance spectroscopy showed that Cu0mainly reduced the adsorbed nitrate by reacting with it.Meanwhile,Cu3P can not only act as an electron mediator or bridge to accelerate the electron transfer during nitrate reduction,but also act as a donor of active hydrogen to reduce nitrite to ammonia.These results indicate that the excellent catalytic activity of Cu3P/CF for electrochemical reduction of nitrate is mainly attributed to the inherent catalytic properties of Cu3P and the close contact with the highly conductive copper foam.Therefore,the problems of leaching,corrosion and passivation in the long-term operation of copper-based electrocatalysts can be effectively solved by the combination of nitrogen,sulfur and phosphorus 。

3.3 Copper matrix composite

Although copper and its compounds have excellent catalytic performance and weak hydrogen evolution performance for nitrate and its intermediates in the reduction process,copper-based catalysts have the problems of weak nitrate adsorption capacity and insufficient active intermediates in the ENitRR process.by compounding with metal compounds,nonmetals and other materials,the electrocatalytic nitrate reduction performance of the copper-based catalyst can be significantly improved By utilizing the excellent active hydrogen supply capacity and adsorption performance of the copper-based catalyst。

3.3.1 Cu-based catalyst-metal compound composite material

Electrocatalytic ammonia synthesis by nitrate reduction involves a proton-coupled electron transfer mechanism.Copper-based catalysts have excellent catalytic performance for nitrogen intermediates,but due to their weak hydrogen adsorption ability,the intermediates produced by NO3−reduction,such as（*NO2,*NO,*N,etc.,cannot be hydrogenated in time,thus limiting the ammonia synthesis efficiency.The introduction of metals favorable for HER can effectively promote the synthesis of NH3by ENitRR.In general,the reaction pathway of alkaline HER is the Volmer Heyrovsky or Volmer Tafel step,and it is generally believed that the Volmer step（H2O+e−→*H+OH−）is the rate-limiting step determining the overall HER rate.In order to achieve the highest FE and NH3yield of ENitRR,the formation and consumption of*H need to reach a dynamic equilibrium state,which has aroused the interest of researchers in active hydrogen.Metal compounds are widely used as hydrogen evolution catalysts because of their low cost,high intrinsic activity and good stability.By introducing a metal compound as a site for active hydrogen generation,the presence of active hydrogen promotes the reduction of the nitrogen intermediate,promoting the ENitRR performance of the copper-based catalyst 。
Kim et al.Prepared a Ni3Fe-CO3LDH/Cu foam（CF）catalyst using a layered bimetallic modified copper foam electrode[101]。 Compared with pure copper foam,the productivity of Ni3Fe-CO3LDH/CF is increased by 8.5 times.The synergistic effect of Ni3Fe-CO3LDH/CF was investigated by comparing the ESR spectra of CF and Ni3Fe-CO3LDH/CF mediated by radical trapping DMPO,which confirmed that the multi-metal active sites are beneficial to promote the generation/transfer of active hydrogen in the electrochemical ENitRR process[101]。 Li et al.Obtained an electrocatalytic nitrate reduction catalyst on a Cu-supported amorphous CeOx（Cu/a-CeOx）,in which Cu has a strong adsorption ability for NO3−and an ability to convert NO3−into NO2−[102]; On the other hand,the introduced amorphous CeOxcan effectively adjust the electronic structure of Cu,which is due to the large number of defect sites caused by the disordered arrangement of atoms,which improves the ability of ENitRR intermediate to provide*H,optimizes the adsorption energy of ENitRR intermediate,and improves the ENitRR performance 。
Copper-based catalysts suffer from insufficient supply of active hydrogen,and the electrocatalytic hydrogen evolution process can be effectively promoted by combining with metal compounds with excellent active hydrogen supply ability。

3.3.2 Copper-based catalyst-other compound complex

Although copper-based catalysts have excellent catalytic performance for nitrogen and its intermediates,due to the repulsion of negative charges in the electrochemical reduction process and the low concentration of nitrate,it is difficult for nitrate in the solution to transfer to the electrode surface,which becomes one of the key obstacles in the electrocatalytic reduction of nitrate to ammonia.By combining with a substance with a porous structure and a certain adsorption capacity for nitrate,the mass transfer of nitrate between the liquid-solid two phases can be effectively promoted,and finally the efficient electrocatalytic nitrate reduction reaction is realized。
Song et al.Prepared copper nanoparticles with porous carbon framework(Cu@C)by in situ encapsulation for efficient electroreduction of NO3−to NH3at ultralow concentration[103]。 Due to the enrichment of porous carbon skeleton in Cu@C,NO3−can be concentrated to promote the mass transfer of NO3−,thus achieving efficient electroreduction to NH3at ultra-low concentration.Xu et al.Synthesized polyaniline(PANI)-modified copper foam supported CuO nanowire arrays(CuO@PANI/CF)as a free-standing electrocatalyst for selective nitrate electroreduction to ammonia[104]。 Reasonable surface modification of CuO nanowire arrays by polyaniline can not only preserve the structure of nanowire arrays,but also adjust the surface chemical microenvironment of the catalyst.The lone electron on the polyaniline N atom in the constructed CuO@PANI hybrid structure can capture protons from the hydrated ions and form a positively charged surface,which is beneficial for the enrichment and immobilization of NO3-anions on the catalyst surface.The protonated amine group in the polyaniline layer has a high positive charge density and is easily electroreduced to form adsorbed*H,which is the key hydrogenation substance for the conversion of nitrate to ammonia.The hierarchical structure of CuO@PANI nanowire arrays is beneficial to the mass transport/diffusion during nitrate reduction electrocatalysis.Chen et al.Synthesized three dual-ligand Cu-based MOFs electrocatalysts for efficient ENitRR reaction[105]。 The reaction rate and reaction path of the three electrocatalysts were investigated,and in situ spectroscopy measurements were carried out,which showed that the ligand and coordination of Cu@MOFs would affect the electronic structure of Cu and the adsorption performance for NO3−,resulting in the difference in the performance of electrocatalytic nitrate reduction.At the same time,the double-ligand structure of Cu@MOFs can better protect the Cu(II)center from corrosion and leaching 。
By combining carbon materials with large specific surface area,conductive polymers,MOFs and other compounds with copper-based catalysts,low-concentration nitrate in the solution can be efficiently adsorbed,which has a certain enrichment effect on nitrate and a certain protection effect on copper-based materials。

4 Conclusion and outlook

in this paper,the research progress of copper-based catalytic materials In electrocatalytic nitrate reduction is reviewed.the relationship between the structure and the selectivity and catalytic activity was analyzed from the point of view of the electrocatalytic reduction of nitrate by elemental copper,cuprous matrix composites and copper matrix composites。
(1)Elemental copper:the kinetics of electrochemical nitrate reduction can be improved by controlling the nanostructure of copper.Cu(100)and Cu(111)with different exposed crystal faces have specific catalytic properties for nitrate reduction and nitrite reduction,and the combination of the two crystal faces can achieve efficient nitrate reduction.Single-atom copper electrode materials exhibit high activity and selectivity.However,copper is easy to be corroded and has poor catalytic stability,and the stability of the catalyst can be significantly improved by introducing other metal elements to form an alloy,and the multi-metal electrode has better corrosion resistance and higher catalytic activity due to the synergistic effect.the combination of non-noble metals and copper can not only increase the catalytic activity,but also reduce the cost of electrode preparation。
(2)Cu-based catalyst:In electrocatalytic nitrate reduction,copper ions（Cu2+and Cu+）also play an important role,and elements such as P,N and S can be introduced to increase the relative stability of copper ions.Due to its low cost,relatively high faradaic efficiency and activity,Cu is considered to be a promising catalyst for efficient catalytic nitrate reduction either alone or in conjunction with other P,N,and S dopants 。
(3)copper-based composite materials:in order to make up for the shortcomings of copper-based catalysts,coupling them with other compounds can also improve their electrocatalytic nitrate reduction performance.Metal compounds have excellent active hydrogen supply energy,which can make up for the problem of insufficient active hydrogen supply of copper-based catalysts.Carbon materials,conductive polymers,MOF and other compounds have a certain protective effect on copper-based catalysts,while enriching nitrate In the solution。
Although copper-based catalysts have made some progress,there are still some problems and challenges.We should focus on solving the following problems。
(1)long-term stable operation,especially in alkaline system,copper cathode is easy to passivate and deactivate,which inhibits its Long-term electrocatalytic performance。
(2)Although the ENitRR reaction pathway on the surface of Cu catalyst has been extensively studied and DFT calculations have been performed,the actual ENitRR reaction is much more complex,with many potential side reactions,resulting in the production of a variety of by-products,such as nitrogen and nitrogen or hydrogen and hydrogen coupling to form N2or H2.Meanwhile,the formation mechanism of NH3is still unclear and needs further exploration 。
(3)There is still a big gap between the laboratory-scale electrocatalytic system and the practical application.the large-scale industrial application of electrocatalytic nitrate reduction for ammonia synthesis needs to balance and optimize the faradic efficiency,total energy consumption and economic benefits.in order to achieve good economic and environmental benefits,the application of copper-based catalysts In practical environment needs to consider various parameters such as working voltage,electrolyte type and temperature。
(4)Due to the excellent catalytic performance of copper-based catalysts for carbon dioxide and the strategic goal of carbon peak and carbon neutralization proposed by China,the chemical conversion of carbon dioxide into useful chemical raw materials or products is one of the most potential carbon dioxide conversion technologies[106]。 Although many studies have shown that using CO2and H2O as raw materials,C1or C2+products can be generated,the potential of further expanding the product range under this system is limited[107,108][109,110]。 Electrocatalytic CO2reduction combined with N atom can synthesize more organic nitrogen compounds with wide application value,such as urea and methylamine[111,112][113,114]。 Urea synthesis by electrocatalytic C—N coupling is a strategy to efficiently utilize CO2to obtain high value-added products.The synthesis process is energy-saving and environmentally friendly,which improves the utilization rate of carbon resources and reduces the emission of CO2.It is expected to replace the traditional industrial urea production process 。

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