English

• INTRODUNTION

  Ammonia (NH₃) is considered as a promising energy carrier, which is expected to play a resilient and sustainable role in future energy scenarios.^[1] Meanwhile, NH₃ holds a main component in nitrogen fertilizers and the chemical industry.^[2-5] Currently, the industrial synthesis of ammonia still relies on the Haber-Bosch process operating at high temperature and pressure conditions, which results in a large amount of fossil fuel consumption and greenhouse gas emissions (Figure 1).^[6-9] Recently, electrocatalytic nitrogen reduction reaction (NRR) has been recognized as a promising way to produce ammonia, benefiting from the fact that the conversion of N₂ to NH₃ is achieved via using water as a hydrogen source and renewable electricity as energy source under ambient conditions.^[10-14] However, the low NH₃ yield rate and Faradaic Efficiency (FE), due to the low aqueous solubility of N₂, the strong N≡N bond energy (941 kJ mol^-1) of inert N₂ and the competitive hydrogen evolution reaction (HER), seriously hinder the development and application of the NRR technology.^[15-19] Therefore, it is attractive to explore more efficient, sustainable and scalable water-soluble nitrogen sources for the industrial-scale electrosynthesis of ammonia.

  Figure 1

  

  Figure 1.  Schematic illustration of sustainable ammonia synthesis via nitrate electrochemical reduction.

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  Among various nitrogen sources, nitrate (NO₃^-) exhibits extremely high solubility in aqueous electrolytes, which is beneficial for the adsorption and activation of nitrate on the electrode surface. Meanwhile, as the highest oxidized form of nitrogen, nitrate is widely distributed in the living environment.^[20-23] On one hand, the discharge of industrial wastewater, the overuse of nitrogen-containing agricultural fertilizers and the improper disposal of animal manure lead to high nitrate nitrogen levels in surface and groundwater, which threaten aquatic ecosystems and human health.^[24-26] Various nitrate treatment processes have been developed to eliminate the pollution of water resources caused by nitrate, such as reverse osmosis, ion exchange, biological denitrification and electrodialysis.^[27-31] However, these processes may have the risk of causing secondary pollution or inducing pathogenic bacteria, and only involve the collection and enrichment of nitrate.^[32-34] The electrochemical nitrate reduction reaction (NO₃RR) is an emerging technology for nitrate treatment, which could potentially produce high value-added product NH₃ by rationally controlling reaction conditions.^[35-38] On the other hand, nitrogen oxides (NO, NO₂, etc.) in exhaust gases from fossil fuel combustion (e.g., power plants, chemical plants, and motor vehicles) cause a range of environmental and health problems.^{[39, 40]} The traditional NO_x treatment process is to convert NO_x to harmless N₂ by selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR).^[41-42] However, these technologies face the disadvantages of high temperature, high operating costs and continuous supply of reducing agent. Notably, NO_x can be converted into nitrate solution by oxidation and leaching absorption.^{[43, 44]} The obtained nitrate solution could be further electrochemically reduced to high value-added ammonia. Moreover, thermodynamically, unlike the NRR process, the NO₃RR process requires lower energy to break the N=O bond (204 kJ mol^-1) in nitrate.^{[45, 46]} Therefore, the waste-to-energy technology, which utilizes NO₃^- in wastewater or NO_x in exhaust gas as nitrogen sources to produce NH₃ via the NO₃RR process, is meaningful.

  In the electrochemical reduction of nitrate to ammonia, the actual potential of NO₃^- to NH₃ is usually more negative than the HER potential (0 V versus reversible hydrogen electrode, RHE), thereby inevitably causing HER.^{[17, 47, 48]} The conversion from high-valence NO₃^- to low-valence NH₃ is a complex process involving an eight-electron transfer process (NO₃⁻ + 9H⁺ + 8e⁻ → NH₃ + 3H₂O).^{[49, 50]} During the process, various undesirable by-products (such as N₂, NO₂^-, N₂O, etc.) may be formed, and thus inhibit the formation of target product ammonia.^{[51, 52]} Therefore, the investigation of suitable electrocatalytic materials to improve the FE and selectivity of ammonia is an important research topic in the electrochemical ammonia synthesis from nitrate. In recent years, a considerable number of metal catalysts (including Ru, Ir, Pd, Pt, Ni, Co, Cu, Fe and so on) and their related compounds have been applied as electrocatalysts for the selective conversion of nitrate to ammonia.^{[17, 36, 46, 49, 53-57]}

  Among these potential catalysts, Cu-based materials are undoubtedly one of the most effective electrocatalysts for the conversion of NO₃^- to NH₃ (Figure 2). On one hand, from the perspective of economy, although some noble metal catalysts exhibit high activity and ammonia selectivity in the NO₃RR process, their practical application is hampered by the scarcity and high cost.^[58] The low-cost and earth-abundant Cu-based materials are more suitable for practical production applications.^[59] On the other hand, from the perspective of reaction dynamics, compared with other metal catalysts, Cu-based materials have stronger catalytic capacity for nitrate reduction to ammonia. Not only does Cu have a strong adsorption capacity for nitrate, but it exhibits fast reduction kinetics in the rate-determining step of the reduction of ^*NO₃ to ^*NO₂.^{[32, 60-62]} In fact, benefiting from these excellent properties, Cu-based catalysts have received extensive attention as electrocatalysts for nitrate reduction to ammonia. A large number of Cu-based catalysts have demonstrated remarkable catalytic performance.

  Figure 2

  

  Figure 2.  Schematic illustration of the advantages of Cu-based materials for nitrate reduction to ammonia.

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  The past decades have witnessed the vigorous development of electrochemical NO₃RR technology, and a large amount of scientific and technical knowledge has been accumulated and some review articles on electrocatalytic NO₃RR have been published. For example, Zhang et al. reviewed the latest development of non-noble metal electrocatalysts for nitrate reduction.^[32] Nirala Singh and his coworkers summarized the recent discoveries in the reaction mechanism of heterogeneous electrocatalytic NO₃RR.^[63] In view that many new breakthroughs in the study of properties and structures of NO₃RR electrocatalysts have been achieved and the applications are fruitful, our intention is to provide timely updates, with particular emphasis on Cu-based catalysts. In this review, we mainly focus on the latest development of Cu-based catalysts for electrochemical nitrate reduction to ammonia. Different types of Cu-based materials that can be used for electrocatalytic NO₃RR for ammonia synthesis, involving monometallic Cu catalysts, bimetallic Cu-based catalysts, Cu-based compounds, and Cu-based inorganic-organic hybrid materials and their derivatives, are discussed in detail. The structural characteristics and catalytic mechanism of various Cu-based catalyst systems are discussed, with special emphasis on several important and representative examples. In the end section, current issues of Cu-based catalysts are briefly discussed and a vision of future challenges and opportunities is provided.

  FUNDAMENTAL MECHANISM OF NO₃RR OVER COPPER-BASED ELECTROCATALYSTS

  The electrochemical reduction of nitrate to ammonia, involving a series of reaction intermediates and pathways, is a complex eight electron transfer and nine proton process.^{[64, 65]} Therefore, it is meaningful to investigate and discuss the electroreduction mechanism of nitrogen from high-valent nitrate to low-valent ammonia. The NO₃RR process includes two different reduction mechanisms. One is the adsorbed active atomic hydrogen-mediated (H_(ads)) reduction pathway, which is obtained from the reduction of protons by cathode electrons (Volmer process, Eq. (1)).^{[32, 66]} The other is performed by electron transfer and H⁺.^[67] As shown in Figure 3, both mechanisms begin with the adsorption of nitrate in solution on the catalyst surface via two oxygen atoms in a chelating O, O-bidentate configuration.^[63] The NO₃RR mechanism is different for various Cu-based catalysts. For composite catalysts are prepared by Cu and other metals with high affinity for adsorbing active atomic hydrogen (such as Pt, Ni, Rh, etc.), it is more likely to perform via the atomic hydrogen-mediated reduction mechanism (Pathway A).^{[67, 68]} In this pathway, atomic H_(ads), as a strong reducing agent, can be deoxidized step by step from nitrate adsorbed on the catalyst surface (NO_3(abs)^-) to form N_(abs) (Eq. (2)-(4)), and then the formed N_(abs) is hydrogenated and desorbed to obtain ammonia (Eq. (5)-(7)).^{[69, 70]} The reaction between the atomic H_(ads) and NO_3(abs)^- is considered as the rate-determining step in the atomic H_(ads) mediated NO₃RR process.^[30] Notably, the atomic H_(ads) reduction pathway may inhibit the occurrence of some side reactions since it is usually performed in a low overpotential range. Furthermore, the H_(ads) migration barrier of 0.10 eV is lower compared to the migration barrier of N_(ads) (0.75 eV), and the formation of N-H bond is kinetically easier than that of the N-N bond.^{[67, 71]} Thus, the enhanced affinity of the Cu-based catalyst surface for atomic H_(ads) facilitates the ammonia production.

  Figure 3

  

  Figure 3.  Schematic presentation of possible mechanism for electrochemical nitrate reduction to ammonia.

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  $ {\text{H}}_{\text{2}}\text{O +}{\text{e}}^{\text{-}}\text{}\text{→}{\text{H}}_{\text{(ads)}}\text{+}{\text{OH}}^{\text{-}} $

  (1)

  $ {\text{NO}}_{\text{3(ads)}}^{\text{-}}\text{+}{\text{2H}}_{\text{(ads)}}\text{→}{\text{NO}}_{\text{2(ads)}}^{\text{-}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (2)

  $ {\text{NO}}_{\text{2(ads)}}^{\text{-}}\text{+}{\text{H}}_{\text{(ads)}}\text{→}{\text{NO}}_{\text{(abs)}}\text{+}{\text{OH}}^{\text{-}} $

  (3)

  $ {\text{NO}}_{\text{(abs)}}\text{+}{\text{2H}}_{\text{(ads)}}\text{→}{\text{N}}_{\text{(abs)}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (4)

  $ {\text{N}}_{\text{(abs)}}\text{+}{\text{H}}_{\text{(ads)}}\text{→}{\text{NH}}_{\text{(abs)}}\text{} $

  (5)

  $ {\text{NH}}_{\text{(abs)}}\text{+}{\text{H}}_{\text{(ads)}}\text{→}{\text{NH}}_{\text{2(abs)}} $

  (6)

  $ {\text{NH}}_{\text{2(abs)}}\text{+}{\text{H}}_{\text{(ads)}}\text{→}{\text{NH}}_{\text{3(abs)}} $

  (7)

  As for the electron transfer pathway, it is more likely to occur on monometallic copper and Cu-based catalysts with low affinity for atomic H_(ads) due to their strong adsorption capacity for nitrate. In the electron transfer mechanism, the reduction of NO_3(abs)^- to NO_2(abs)^-, in which electrons are difficult to transfer into the π^* orbital due to the lowest unoccupied molecular π^* orbital (LUMO π*) of NO₃^-, has been noted as the rate-determining step.^{[70, 72]} For Cu-based catalysts, the d-orbital energy level is similar to the LUMO π^* of NO₃^-, which is beneficial for charge injection into the LUMO π^* of NO₃^- by promoting electron transfer.^{[73, 74]} The conversion from NO_3(abs)^- to NO_2(abs)^- is usually achieved via an electrochemical-chemical-electrochemical pathway. Specifically, NO_3(abs)^- is first electrochemically converted to short-lived NO_3(abs)^2- (Eq. (8)). Then, the NO_3(abs)^2- reacts chemically with 2 H⁺ to form NO_2(abs) (Eq. (9)). NO_2(abs)^- is finally obtained by a second electron transfer process (Eq. (10)). The electron transfer and hydrolysis reactions further occur on NO_2(abs)^- to form NO_(abs), as shown in Eq. (11).^{[66, 75, 76]} As the divergent center, the adsorption energies of NO_(abs) on the Cu-based catalysts surface and the reaction environment play a key role in determining the product selectivity. The various parallel reduction pathways of NO_(abs) will lead to a series of products, such as NH₃, N₂, NO₂ and so on.^{[71, 73]}

  $ {\text{NO}}_{\text{3(abs)}}^{\text{-}}\text{+}{\text{e}}^{\text{-}\text{}}\text{→}{\text{NO}}_{\text{3(abs)}}^{\text{2-}} $

  (8)

  $ {\text{NO}}_{\text{3(abs)}}^{2\text{-}}\text{+ 2}{\text{H}}^{\text{+}}\text{→}{\text{NO}}_{\text{2}\left(\text{ads}\right)}\text{}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (9)

  $ {\text{NO}}_{\text{2}\left(\text{ads}\right)}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NO}}_{\text{2(abs)}}^{\text{-}}\text{}\text{} $

  (10)

  $ {\text{NO}}_{\text{2(abs)}}^{\text{-}}\text{+ 2}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NO}}_{\text{(abs)}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (11)

  The electrochemical-electrochemical pathways are implemented for the conversion from NO_(abs) to NH₃, which can be divided into three possible pathways depending on the sequence of deoxygenation and hydrogenation. Regarding Pathway B, the NH₂OH_(abs) is first formed by the stepwise hydrogenation of NO_(abs) → NOH_(abs) → NH₂OH_(abs), and then NH₂OH_(abs) undergoes deoxygenation and hydrogenation reactions to produce NH_3(abs) (Eq. (12)-(16)).^{[17, 75, 77]} Notably, the NH₂OH_(abs) intermediates are easily desorbed from the catalyst surface to produce byproduct hydroxylamine. There are reports that the byproduct hydroxylamine is detected in the NO₃RR process catalyzed by various Cu-based catalysts, and these results confirm the existence of Pathway B.^[61] In Pathway C, NO_(abs) is directly deoxygenated by electrons and H⁺ to generate N_(ads), followed by step-by-step electrocatalytic hydrogenation to NH_3(abs) (Eq. (17)-(20)).^[78-80] Recently, Gua et al. reported a new electrochemical reduction pathway from nitrate to ammonia (Pathway D).^[81] Similar to Pathway B, NHOH_(abs) is obtained via a two-step hydrogenation reaction of NO_(abs). The difference is that NHOH_(abs) is directly deoxygenated to NH_(abs) (Eq. (21)) and then hydrogenated to NH_3(abs) along the same path as Pathway C. The density functional theory (DFT) calculations indicate that the conversion from NO_(abs) to NOH_(abs) requires a low activation energy of 0.08 eV on Cu (111), which is much smaller than that of NO_(abs) to N_(abs) (1.62 eV), although Pathway C is energetically more beneficial than Pathway B and D for thermodynamics. Furthermore, a lower activation energy is required from NHOH_(abs) to NH_(abs) (0.23 eV) compared to the high activation energy of NHOH_(abs) to NH₂OH_(abs) (1.36 eV). The kinetic energy barrier and free energy analysis results confirm that Pathway D is the optimal reaction path.

  $ {\text{NO}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NOH}}_{\text{(abs)}} $

  (12)

  $ {\text{NOH}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NHOH}}_{\text{(abs)}} $

  (13)

  $ {\text{NHOH}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{2}}{\text{OH}}_{\text{(abs)}} $

  (14)

  $ {\text{NH}}_{\text{2}}{\text{OH}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{2(abs)}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (15)

  $ {\text{NH}}_{\text{2(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{3(abs)}} $

  (16)

  $ {\text{NO}}_{\text{(abs)}}\text{+ 2}{\text{H}}^{\text{+}}\text{+}{\text{2e}}^{\text{-}}\text{→}{\text{N}}_{\text{(abs)}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (17)

  $ {\text{N}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{(abs)}} $

  (18)

  $ {\text{NH}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{2(abs)}} $

  (19)

  $ {\text{NH}}_{\text{2(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{3(abs)}} $

  (20)

  $ {\text{NHOH}}_{\text{(abs)}}\text{+}{\text{H}}^{\text{+}}\text{+}{\text{e}}^{\text{-}}\text{→}{\text{NH}}_{\text{(abs)}}\text{+}{\text{H}}_{\text{2}}\text{O} $

  (21)

  DEVELOPMENT OF COPPER-BASED ELECTROCATALYSTS FOR NO₃RR

  Cu-based materials show great advantages in promoting selective electroreduction of nitrate to ammonia in neutral, acidic and alkaline conditions, in terms of high ammonia yield rate and FE, as summarized in Tables 1-2. In this section, we will discuss four typical kinds of Cu-based materials based on differences in compositional and structural design, including monometallic Cu catalysts, bimetallic Cu-based catalysts, Cu-based compounds, and Cu-based inorganic-organic hybrid materials and their derivatives in detail, with emphasis on their structural and compositional features and functional mechanisms in promoting nitrate-to-ammonia conversion.

  Table 1

  

  Table 1.  Performance of Cu-based Electrocatalysts for the Reduction of Nitrate to Ammonia under Neutral and Acidic Conditions.

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  electrocatalysts	electrolytes	NH3 yield rate	FE (%)	potentials	ref.
  CuPc@MXene	0.5 M Na2SO4, 50 mg L-1 NO3--N	0.72 mg h-1·cm-2	—	-1.06 V vs. RHE	[62]
  Cu@CuHHTP	0.5 M Na2SO4, 500 mg L-1 NO3-	1.84 mg h-1 cm-2	67.55	-0.90 V vs. RHE	[82]
  La2Cu0.8Co0.2O4	0.5 M Na2SO4, 50 mg L-1 nitrate	0.0699 mmol h-1 mg-1	—	-0.68 V vs. RHE	[83]
  Cu@C-800	0.1 M Na2SO4, 500 mg L-1 NO3-	51.7 ± 0.6 mmol h-1 g-1	78 ± 0.9	-0.9 V vs. RHE	[84]
  1-Cu'	0.5 M Na2SO4, 5 mM NaNO3	53.43 mgNH3 h-1 mgCu-1	85.5	-0.9 V vs. RHE	[85]
  TiO2 NTs/CuOx	0.5 M Na2SO4, 100 mg L-1 KNO3-N	264.4 mmol g-1 h-1	83.51	-0.85 V vs. RHE	[86]
  SCF	0.5 M Na2SO4, 100 mg L-1 KNO3-N	0.31 mmol h-1 cm-2	70	-1.3 V vs. RHE	[87]
  Ni1Cu-SAA	0.5 M K2SO4, 200 mg L-1 NO3--N	326.7 μmol h-1 cm-2	~100	-0.55 V vs. RHE	[88]
  plasma treated Cu2O	0.5 M Na2SO4, 50 mg L-1 NaNO3-N	0.083 mmol h-1 mg-1	89.5	-0.58 V vs. RHE	[89]
  CuNi/NC-51	0.05M Na2SO4, 50 mg L-1 NO3-	—	79.6	-1.0 V vs. Ag/AgCl	[90]
  Rh@Cu-0.6%	0.1 M Na2SO4, 0.1 M KNO3	—	93	-0.2 V vs. RHE	[91]
  Ag/Cu2O	0.5 M Na2SO4, 100 mg L-1 nitrate-N	—	96.45	-0.80 V vs. RHE	[92]
  island-like Cu	0.5 M Na2SO4, 50 mM KNO3	—	98.28	-0.8 V vs. RHE	[93]
  Cu52Al48-LDO-r	50 mM Na2SO4, 22.5 mg L-1 NO3--N	661.6 mg-N m-2 h-1	70.4	-1.10 V vs. Ag/AgCl	[94]
  Cu NDs	0.01 M NaNO3	500 μg h-1cm-2	97	-0.3 V vs. RHE	[95]
  THH Cu NCs	100 mM K2SO4, 100 mM KNO3	9.13 mmol h-1 cm-2	98.3	-0.90 V vs. RHE	[96]
  FOSP-Cu-0.1	0.5 M Na2SO4, 0.1 M KNO3	101.4 μmol h-1 cm-2	93.91	-0.266 V vs. RHE	[97]
  PdCu/Cu2O	0.5 M Na2SO4, 100 mg L-1 NO3--N	0.190 mmol h-1 cm-2	94.32	-0.80 V vs. RHE	[44]
  Cu/Cu2O NWAs	0.5 M Na2SO4, 200 mg L-1 NO3--N	0.2449 mmol h-1 cm-2	95.8	-0.85 V vs. RHE	[98]
  Cu/Pd/CuOx	0.5 M K2SO4, 50 mg L-1 NO3--N	1510.33 µg h-1 mg-1	84.04	-1.3 V vs. SCE	[99]
  Cu@Cu2+1O NWs	0.5 M K2SO4, 50 mg L-1 NO3--N	576.53 µg h-1 mg-1	87.07	-1.2 V vs. SCE	[100]
  CuO@MnO2/CF	0.5 M K2SO4, 200 mg L-1 KNO3-N	0.240 mmol h-1 cm-2	94.92	-1.3 V vs. SCE	[101]
  CuPd/CN	0.5 M K2SO4, 200 mg L-1 KNO3-N	0.0904 mmol h-1 mgcat.-1	96.16	-1.1 V vs. SCE	[102]
  dr-Cu NPs	0.5 M K2SO4, 50 mg L-1 KNO3-N	781.25 µg h-1 mg-1	85.47	-1.3 V vs. SCE	[103]
  CuPd aerogels	0.5 M K2SO4, 50 mg L-1 KNO3-N	784.37µg h-1 mg-1	90.02	-0.46 V vs. RHE	[104]
  Cu3Fe	0.1 M Na2SO4 and 100 mg L-1 NO3-	—	74.2	-0.7 V vs. RHE	[105]
  PPy-Cu-E	0.4 M K2SO4, 0.2 M KNO3	0.588 mmol mgcat h-1	91.95 ± 1	-0.61 V vs. RHE	[106]
  Pd-Cu2O CEO	0.5 M K2SO4, 50 mg L-1 KNO3-N	925.11 µg h-1 mgcat.-1	96.56	-1.3 V vs. SCE	[107]
  Mo/H-CuW	0.5 M Na2SO4, 0.05 M NO3-	5.84 mg h-1 mgcat.-1	94.60	-0.7 V vs. RHE	[108]
  Cu-Co3O4	0.1 M Na2SO4, 500 mg L-1 NO3-	36.71 mmol h-1 g-1	86.5	-0.6 V vs. RHE	[109]
  CuCo2O4	0.1 M Na2SO4, 1.0 mM KNO3	48.8 μg h-1 cm-2	34.2	-0.85 V, -0.45 V vs. RHE	[110]
  Cu-N-C	0.05 M Na2SO4, 50 mg L-1 NO3-	37 μg h-1 cm-2	—	-1.3 V vs. SCE	[111]
  Cu-PTCDA	0.1 M PBS, 500 mg L-1 NO3-	436 ± 85 μg h-1 cm-2	85.9	-0.4 V vs. RHE	[47]
  Cu2.5Co	0.5 M Na2SO4, 0.1 M KNO3	164.23 mmol h-1 cm-2	95.43	-0.25 V vs. RHE	[112]
  CuO@CuFe2O4	0.1 M PBS, 0.1 M KNO3	9.21 mg h-1 mgcat.-1	91.08	-1.0 V vs. RHE	[113]
  Cu-N-C	0.5 M Na2SO4, 50 mg L-1 NO3--N	9.23 mg h-1 mgcat.-1	—	-1.5 V vs. SCE	[114]
  Cu/CuOx/CF	0.5 M K2SO4, 200 mg L-1 NO3--N	0.21 mmol h-1 cm-2	93.58	-1.3 V vs. SCE	[115]
  CuO@PANI/CF	0.5 M K2SO4, 200 mg L-1 KNO3-N	0.213 mmol h-1 cm-2	93.88	-1.3 V vs. SCE	[116]
  arCu/Ni	0.1 M Na2SO4, 45 mg L-1 KNO3	—	76 ± 3	-1.4 V vs. SCE	[117]
  Cu-Pt bimetallic	12.5 mM Na2SO4, 30 mg L-1 NO3-	194.4 ± 3.6 mg gcat.-1 L-1	22	-0.4 V vs. RHE	[118]
  CuxNiy/NC	0.5 M Na2SO4, 50 mg L-1 NO3-	133 μg h-1 cm-2	~50	-0.67 V vs. SCE	[119]
  CuFe	0.1 M K2SO4, 2 mM KNO3	0.23 mmol h-1 cm-2	94.5	-0.7 V vs. RHE	[120]
  Cu/rGO/GP	0.02 M NaCl, 0.02 M NaNO3	203 μg h-1 cm-2	—	-1.4 V vs. SCE	[121]
  pCuO-10	0.05 M H2SO4, 0.05 M KNO3	334 mmol cm-2 h-1	—	-0.7 V vs. RHE	[122]
  Cu (100), Cu (111)	0.1 M HClO4, 2 mM NaNO3	—	—	—	[61]
  BDD/Cu film	0.1 mol L-1 Na2SO4, 0.1 mol L-1 KNO3 (pH=2)	—	—	—	[123]

  Table 2

  

  Table 2.  Performance of Cu-Based Electrocatalysts for the Reduction of Nitrate to Ammonia under Alkaline Conditions.

  DownLoad: CSV

  electrocatalysts	electrolytes	NH3 yield rate	FE (%)	potentials (vs. RHE)	ref.
  Cu6Sn5-Sn NPs	1 M KOH, 0.1 M KNO3	1 mmol mg-1 h-1	90	0 V	[124]
  Ru & Cu/Cu2O	1 M KOH, 0.1 M KNO3	—	95	0.1 V	[125]
  CuCoSP on Cu foil	0.1 M KOH, 0.1 M NO3-	1.17 mmol h-1 cm-2	90.6	-0.175	[126]
  Cu2O/Cu	1 M KOH, 250 mg L-1 NO3-	2.17 mg cm-2 h-1	84.36	-0.25	[26]
  Cu10Ce10	1 M KOH, 1400 mg L-1 nitrate-N	0.99 mmol h-1 cm-2	98.43	-0.23 V	[52]
  CuPd nanocubes	1 M KOH, 1 M KNO3	6.25 mol/gcat*h	92.5	-0.5 V, -0.6 V	[127]
  Cu-N-C SAC	0.1M KOH, 0.1 M KNO3	12.5 molNH3 gCu-1 h-1	84.7	-1.00 V	[128]
  Co0.5Cu0.5	1 M KOH, 50 mM KNO3	—	95	-0.03 V	[129]
  Cu nanodisks	0.1 M KOH, 10 mM KNO3	2.16 mg mgcat-1 h-1	81.1	-0.5 V	[130]
  Au1Cu SAAs	0.1 M KOH, 7.14 mM NO3-	555 μg h-1 cm-2	98.7	-0.2 V	[131]
  Ru-CuNW	1 M KOH, 2, 000 mg L-1 NO3-	—	96	0.04 V	[132]
  BCN@Cu	0.1 M KOH, 100 mM NO3-	435.6 mmol h-1 mgcat.-1	89.3	-0.5 V	[133]
  BCN@Cu/CNT	0.1 M KOH, 100 mM KNO3	172, 226.5 μg h-1 mgcat.-1	95.32	-0.6 V	[134]
  Cu@NF	1 M KOH, 200 mg L-1 nitrate-N	0.252 mmol h-1 cm-2	96.6	0.23 V	[60]
  oxide-derived Cu (OD-Cu)	1 M KOH, 100 mM KNO3	1.1 mmol h-1 cm-2	92	-0.15 V	[65]
  HSCu-AGB@C	1 M KOH, 0.1 M NO3-	487.8 mmol gcat-1 h-1	94.2	-0.2 V	[135]
  CuOX	0.1 M KOH, 50 mg L-1 NO3-	449.41 ± 12.18 μg h-1 mgcat-1	74.18 ± 2.27	-0.25 V	[136]
  Cu@C	1 M KOH, 1 × 10-3 M NO3-	469.5 μg h-1 cm-2	72.0	-0.9 V	[137]
  Cu-NBs-100	1 M KOH, 0.1 M NO3-	650 mmol gcat-1 h-1	95.3	-0.15 V	[138]
  BCN-Cu	0.1 M KOH, 100 mM NO3-	1900.07 μg h-1 cm-2	98.23	-0.5 V	[139]
  Cu nanosheets + 0.1 M KOH	0.1 M KOH, 10 mM KNO3	390.1 μg mgCu-1 h-1	99.7	-0.15 V	[140]
  Cu50Ni50 alloy	1 M KOH, 0.1 M KNO3		99	-0.15 V	[55]

  Monometallic Cu Catalysts. Bulk metallic Cu shows potential for electrocatalytic nitrate reduction, however, suffers from several limitations including insufficient reactive sites on the surface and low intrinsic reactivity of reactive sites. Aiming to solve these mentioned limitations, more efforts have been devoted to finely tuning the morphology and structure of metallic Cu catalysts. Particularly, metallic Cu with various nanostructured morphologies have been developed and demonstrated enhanced electrocatalytic nitrate-to-ammonia capability. The synthesis of nanostructured metallic Cu often entails the reduction of Cu(I) or Cu(II) sources by means of synthetic techniques such as chemical treatment, electrochemical synthesis, photochemical techniques, sonochemical methods, and thermal treatment. The rational choice of synthetic technique is important to design nanostructured metallic Cu catalysts with desired shape, size, and morphology for various specific applications.

  Two-dimensional nanostructures (e.g., nanosheets, nanoplates, nanoflakes) show great application prospect in the field of catalysis due to their structural features and fascinating physicochemical properties. As an example, Kang and coworkers utilized Cu nanosheets to catalyze the electroreduction of nitrate to ammonia.^[140] In this study, {111}-exposing Cu nanosheets were prepared by solution-phase-synthesis, where ascorbic acid (AA) serving as reducing agent, and hexamethyl-enetetramine (HMTA) and tetradecyltrimethylammonium bromide (TTAB) as stabilizers (Figure 4a, b). Compared with Cu nanocubes and irregular Cu nanoparticles, competitive HER was suppressed to a greater extent on the {111}-exposing Cu nanosheets, and the reaction rate of the rate-determining step (NO₃⁻ + H₂O + 2e → NO₂⁻ + 2OH⁻) was largely enhanced. Under optimal conditions, the {111}-exposing Cu nanosheets could derive a NH₃ yield rate of 390.1 μg mg_Cu^-1 h^-1 with a NH₃ FE of 99.7% (Figure 4c).

  Figure 4

  

  Figure 4.  (a) Transmission electron microscopy (TEM) image of Cu nanosheets. (b) High resolution TEM (HRTEM) image of Cu nanosheets basal plane. (c) NH₃ FE of various Cu catalysts at different applied potentials. Reproduced with permission from Ref.^[140] Copyright 2020, Elsevier. (d) Schematic illustration of the synthetic process, (e) scanning electron microscopy (SEM) image, (f) high-angle annular dark-field scanning TEM (HAADF-STEM) image, and (g) NH₃ yield rate and FE at different applied potentials for the dr-Cu NPs. Reproduced with permission from Ref.^[103] Copyright 2021, Royal Society of Chemistry.

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  Surface defect engineering serves as an effective means to tune the electrocatalytic properties of 2D Cu nanostructures through tailoring the local surface microstructure and electronic structure. For instance, Xu et al. reported CuO nanoplates (CuO NPs) could be converted into defect-rich Cu nanoplates (dr-Cu NPs) by in situ electroreduction (Figure 4d-f).^[103] The enrichment of atomic defects in the converted dr-Cu NPs could promote the adsorption of nitrate ions and related intermediates on the electrocatalyst surface and inhibit the occurrence of side reactions, thus selectively promoting the NH₃ formation reaction pathway and achieving a significant NH₃ production rate of 781.25 µg h^-1 mg^-1 and NH₃ FE of 85.47% (Figure 4g). Another study by Liu group showed that Cu oxide catalysts could be reduced to metallic Cu with abundant stacking faults under the negative potential of NO₃RR.^[87] The oxide-derived Cu was demonstrated as the actual active species for NO₃RR electrolysis and tensile strain effect was caused due to the in situ generated stacking faults. The features made the oxide-derived Cu more favorable for nitrate adsorption and largely enhance its nitrate-to-ammonia capability.

  Dendritic nanostructures are another kind of promising and desirable electrode materials in electrocatalytic reactions. Compared with other nanostructures, dendritic nanostructures possess abundant catalytically active atoms at the edge and corner, which can function as catalytic sites and/or absorption sites for reactant molecules. In a study by Wang and co-workers, they constructed Cu nanodendrites (Cu NDs) grown along with the {200} facet by an electrodeposition method (Figure 5a).^[95] Calculations by DFT in this study revealed that the reduction and protonation of nitrate on all faces of Cu proceeded spontaneously, with the disintegration of ^*NO intermediate being the ratedetermining step (Figure 5b). From both experimental and theoretical results, it was demonstrated that electrocatalytic NO₃RR on Cu nanocatalysts was a facet-dependent process and Cu{200} offered lower activation energy for the dissociation of ^*NO intermediate (Figure 5c). The typical Cu NDs with {200} crystallographic planes exhibited outstanding electrochemical NO₃RR activity and achieved a high FE of 97%, using a ¹⁵NO₃^- isotope labeling method to identify the source of generated NH₃. Hierarchical metallic Cu hollow nanostructures composed of 1D or 2D building blocks are also reported as efficient electrode materials to catalyze the NO₃RR. High-energy surfaces in these 1D or 2D building blocks could bring about more exposed active sites. A two-step conversion strategy was developed by Li et al. to fabricate hierarchical Cu nanosheet-based nanotubes (Cu NTs) by using pre-synthesized Cu nanowires (Cu NWs) as the starting material (Figure 5d).^[141] Cu NWs were first chemically oxidized by H₂O₂ into CuO nanotubes (Figure 5e), which were then in-situ electrochemically reduced into Cu NTs (Figure 5f) under NO₃RR operating conditions. Cu NWs to Cu NTs conversion could expose more catalytically active sites and accelerate the charge and mass transfer in electrocatalysis, thus boosting electrocatalytic nitrate-to-ammonia capability. At -1.3 V (vs. saturated calomel electrode, SCE), Cu NTs could achieve 778.6 µg h^-1 mg^-1 of NH₃ yield, and 85.7% of FE NH₃ (Figure 5g).

  Figure 5

  

  Figure 5.  (a) SEM images of Cu NDs. (b) Potential energy diagram of NO₃RR on Cu{200}, Cu{220}, and Cu{111} surfaces. (c) Energy profile of NO dissociation on Cu{200}, Cu{220}, and Cu{111} surfaces. Reproduced with permission from Ref.^[95] Copyright 2021, America Chemical Society. (d) Schematic illustration of the synthetic process of Cu nanotubes. (e) SEM image of CuO nanotubes, (f) TEM image of Cu nanotubes. (g) NH₃ yield rate and FE at different applied potentials for Cu nanotubes. Reproduced with permission from Ref.^[141] Copyright 2022, Royal Society of Chemistry.

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  Cu-Based Bimetallic Catalysts. Cu-based bimetallic catalysts as a new set of catalysts are prepared by mixing Cu and another kind of metal component within a single catalyst. It is well recognized that the bimetallic effect endows Cu-based bimetallic catalysts with unique electronic and chemical properties, which makes the properties of Cu-based bimetallic catalysts significantly different from monometallic Cu counterpart and offers opportunities to construct new catalysts with enhanced performance towards specific reactions.

  One common strategy to construct Cu-based bimetallic catalysts is decorating preformed Cu nanostructures with another foreign metal component. The weak adsorption of H on Cu surface is recognized as the main limiting actor for ammonia synthesis from nitrate electroreduction on bare Cu surface. Aiming to enhance the hydrogenation capability of bare Cu, Luo and co-workers developed Cu nanowires (Cu NWs) coated with Rh single-atom (Rh@Cu) by galvanic replacement reaction between Cu and Rh³⁺ (Figure 6a, b).^[91] Synergistic catalytic cooperation between Rh and Cu sites in the Rh@Cu catalyst could effectively accelerate surface hydrogenation rate during electrocatalytic NO₃RR process, as evidenced by the series of investigations including electron paramagnetic resonance, in situ infrared spectroscopy, differential electrochemical mass spectrometry and DFT modeling. Specifically, the Cu site in the Rh@Cu catalyst preferentially stabilized nitrogen intermediate species, whereas the vicinal Rh site could generate the activated H species, which were then transferred from Rh sites to the ^*NO-adsorbed intermediate species located on Cu sites, thus promoting the hydrogenation step for ammonia synthesis (Figure 6c). The highest NH₃ yield rate over the Rh@Cu catalyst could reach a record value of 1.27 mmol h^-1cm^-2 (Figure 6d). By a cation exchange method, Wang and colleague synthesized Ru-dispersed Cu nanowire catalyst (Ru-Cu NW) (Figure 6e-k).^[132] Such a Ru-Cu NW catalyst could organically integrate the unique catalytic properties of Ru and Cu, that is, the highly dispersed Ru atoms could provide active nitrate reduction sites and the surrounding Cu sites could suppress the competitive HER. The combination of a suppressed HER, endothermic NO₂^- desorption and restricted movement of the N^* species on the surface leads to a high NH₃ FE for Ru-Cu NW, as supported by DFT calculations (Figure 6l, m). When functioning as electrocatalysts for NO₃RR at NO₃^- concentrations of typical industrial wastewater (2, 000 mg L^-1), the Ru-Cu NW could deliver an industrial-relevant NH₃ production rate of ~1 A cm^-2 while maintaining a high NH₃ FE of 96%. With an over 99% nitrate conversion efficiency catalyzed by Ru-Cu NW catalyst, the nitrate concentration could be reduced from an industrial wastewater level to a drinkable water level.

  Figure 6

  

  Figure 6.  (a) SEM image, (b) HAADF-STEM image, (c) schematic illustration of the electrocatalytic NO₃RR process, (d) NH₃ yield rate and FE at different applied potentials for the Rh@Cu catalyst. Reproduced with permission from Ref.^[91] Copyright 2022, WILEY-VCH. (e) Schematic illustration of the synthetic process of Ru-Cu NW. HAADF-STEM images of (f) Cu(OH)₂ NW, (g) Ru-CuO NW and (h) Ru-Cu NW. High-resolution HAADF-STEM images of (i) the Ru-CuNW surface structure and (j) its corresponding crystal structure and lattice spacing. (k) Energy-dispersive X-ray spectroscopy (EDS) mapping images of Ru-CuNW. (l) DFT-calculated minimum energy path for the NO₃RR. (m) Nitrate adsorption versus hydrogen evolution via an alkaline pathway on different catalytic surfaces. Reproduced with permission from Ref.^[132] Copyright 2022, Nature Publishing Group.

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  Alloying Cu with another metal component to form bimetallic catalysts is another avenue to regulate their electronic structure and thus boost the electrocatalytic performance. In a study by Wang et al., they demonstrated enhanced nitrate-to-ammonia activity on CuNi alloys.^[55] The composition of CuNi alloy system (e.g., Cu/Ni atomic ratio) had influence on the nitrate-to-ammonia activity. The optimal Cu₅₀Ni₅₀ alloy catalyst showed a 6-fold increase in nitrate-to-ammonia activity and a 0.2 V lower overpotential required to achieve the optimal NH₃ FE compared to pure Cu at 0 V vs. RHE (pH = 14). By investigating the electronic structure of catalysts using X-ray photoelectron spectroscopy (XPS), operando X-ray adsorption spectroscopy (XAS) and ultraviolet photoelectron spectroscopy (UPS), they found that the Cu d-band center upshifted toward the Fermi level in CuNi alloys and the adsorption energy of intermediates such as ^*NO₃^-, ^*NO₂ and ^*NH₂ could be regulated (Figure 7a). DFT simulations verified that alloying Cu with Ni moved the potential limiting step from NO₃^- adsorption to hydrogenation of ^*NH₂ intermediate due to the enhanced adsorption energy of NO₃^- on the CuNi surface and, thus lowering the overpotential (Figure 7b-d). In another example, mesoporous hollow sphere PdCu/ Cu₂O hybrids containing highly dispersed PdCu alloys and an ultrathin Cu₂O shell showed high selectivity (96.70%) and FE (94.32%) for ammonia synthesis from nitrate (Figure 7e-g).^[44] For this PdCu/Cu₂O hybrid system, Pd enabled electron transfer from Pd 3d orbitals to Cu 3d orbitals and caused the polarization of Cu 3d orbitals by forming partial PdCu alloys. This feature made Pd electron deficient but offered empty orbitals to adsorb NO₃^-. The electron polarization effect also made electron-rich Cu more conducive to the occurrence of NO₃^- reduction. The combination of online differential electrochemical mass spectrometry and DFT calculations demonstrated that PdCu alloys block the generation of ^*NOH intermediate and facilitate the formation of ^*N (Figure 7h, i). Another example reported the construction of Au₁Cu (111) single-atom alloys with surface Cu vacancies (V_Cu-Au₁Cu SAAs) via a facile galvanic replacement with a subsequent dealloying process.^[131] Single Au atoms and Cu vacancies worked synergistically on the neighboring Cu atoms, resulting in electron transfer from Cu site to Au site, which could promote the activation of H₂O to ^*H to boost NO₃^- hydrogenation kinetics and facilitate the desorption of ^*NH₃.

  Figure 7

  

  Figure 7.  (a) UPS spectra and d-band center positions of pure Cu catalysts and the CuNi alloys. (b) Reaction free energies for different intermediates on a CuNi surface. (c) Hydrogenation reaction of ^*NH₂ on a Cu₃₀Ni₇₀ surface. (d) The volcano-type relationship between experimental overpotentials of NO₃RR at 5 mA cm^-2 in 10 mM KNO₃ and adsorption energies of ^*NO₃^- on all CuNi alloys. Reproduced with permission from Ref.^[55] Copyright 2020, American Chemical Society. (e) Schematic illustration of the synthetic process of PdCu/Cu₂O hybrids. (f) Low-magnification HAADF-STEM image and (g) atomic-resolution HAADF-STEM image of PdCu/Cu₂O hybrids. (h, i) Reaction free energies for different intermediate during different NO₃RR paths. Reproduced with permission from Ref.^[44] Copyright 2021, Elsevier B.V.

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  Cu-Based Compounds. Differing from monometallic or bimetallic Cu-based materials that contain Cu(0) species, Cu-based compounds usually possess Cu sites with oxidation states of +I or +II. Diverse chemical composition, morphology and electronic structure make Cu-based compounds potentially promising for catalyzing NO₃RR. Cu oxides, as the commonest Cu-based compounds, have been utilized in different areas of electrocatalysis. Recent investigations have further demonstrated their potential for electrochemical nitrate reduction. Zhang group reported the preparation of Cu/Cu₂O nanowire arrays (Cu/Cu₂O NWAs) as highly active catalysts for the electrochemical nitrate reduction to ammonia.^[98] In this study, CuO NWAs were obtained by heat treatment of Cu(OH)₂ NWAs in an oxygen environment, which were then in situ converted to Cu/Cu₂O NWAs during the electrochemical process (Figure 8a). In-situ electrochemical reconstruction of the catalyst from CuO NWAs to Cu/Cu₂O NWAs was verified by in-situ electrochemical Raman spectroscopy, X-ray diffraction (XRD) patterns and auger electron spectroscopy (AES) spectroscopy. Electron transfer at the Cu/Cu₂O interface could regulate the electronic structure of converted Cu/Cu₂O NWAs and facilitate the formation of ^*NOH intermediate and suppress the competitive HER (Figure 8b, c), as confirmed by online differential electrochemical mass spectrometry (DEMS) and DFT calculation. Inspired by this work, Xu et al. developed a facile surface engineering strategy to fabricate Cu nanowires with concave-convex surface Cu₂₊₁O layers (Cu@Cu₂₊₁O NWs) (Figure 8d).^[100] The electronic interaction and interface effect between Cu/Cu₂₊₁O enabled tuning of the Cu d-band center and modulating the adsorption energies of intermediates, thus substantially enhancing the nitrate-to-ammonia capability (Figure 8e).

  Figure 8

  

  Figure 8.  (a) Schematic illustration of the conversion process of Cu/Cu₂O NWAs from CuO NWAs. (b) The reaction energies of H₂ formation over Cu/Cu₂O NWAs and Cu NWAs. (c) Free energy diagram for NO₃RR over Cu NWAs and Cu/Cu₂O NWAs. Reproduced with permission from Ref.^[98] Copyright 2020, WILEY-VCH. (d) HRTEM image of Cu@Cu₂₊₁O NWs. (e) Schematic illustration showing the electrocatalytic nitrate reduction mechanism over the Cu@Cu₂₊₁O NWs. Reproduced with permission from Ref.^[100] Copyright 2021, Elsevier B.V.

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  Incorporation of other appropriate metal with Cu oxides to form metal-Cu oxide composites can also enhance the performance of nitrate reduction to ammonia. Such metal-Cu oxide composites could function as dual-site or multi-site catalysts. The synergism of dual or multi active sites can potentially change the adsorption configuration of reactant molecules or intermediates, lower the energy barrier of key reaction steps and optimize the reaction path. Wang group constructed ultralow-content of Pd (2.93 at%) in-situ incorporated Cu₂O corner-etched octahedra composites (Pd-Cu₂O CEO) and demonstrated excellent nitrate conversion efficiency and ammonia yield.^[107] It was found that a large number of cavities and oxygen vacancies were formed in the Pd-Cu₂O CEO composite during Pd introduction process. This synergistic effect of cavities and oxygen vacancy defects played a significant role in promoting nitrate adsorption, weakening the N-O bond and inhibiting by-product formation. In addition, Pd sites provided active centers for the accumulation of hydride species, which facilitated the reaction pathway for NH₃ formation. In another example, depositing Ag nanoparticles on Cu₂O with assistance of oxygen vacancies formed Ag/Cu₂O hybrids.^[92] The formation of strong Ag-Cu bonds within Ag/Cu₂O hybrids could arouse strong interface interactions between Ag nanoparticles and Cu₂O. Moreover, the interface between Ag nanoparticles and Cu₂O was considered to function as new adsorption sites during NO₃RR electrocatalysis, which could efficiently boost the adsorption of NO₃^- and facilitate the formation of ^*N intermediates.

  Coupling Cu-based oxides with other metal compounds to form heterostructured catalysts represents another strategy directed toward enhancing these catalytic systems. The presence of multiple constituents institutes a synergistic effect that endows the catalyst with superior performance and appreciable potential in a diverse range of catalytic applications. He et al. developed a tandem catalyst system for nitrate-to-ammonia conversion by electrochemical transformation of Cu-Co binary sulfide precursors into core-shell Cu/CuO_x and Co/CoO phases (Figure 9a-d).^[126] It was found that the active Co²⁺-based phases of CuCoSP were stabilized by the Cu/CuO_x phases, both of which were combined to form a tandem system for cascade nitrate-to-ammonia conversion at low overpotentials (Figure 9e). When functioning as electrocatalysts for NO₃RR, the inner Cu/CuO_x component preferentially catalyzed NO₃^- reduction to NO₂^-, which was rapidly reduced to NH₃ at the Co/CoO shell nearby, reaching a high NH₃ yield rate of 1.17 mmol cm^-2 h^-1 and a half-cell energy efficiency of ~36%. By covering 1D CuO nanowire arrays with 2D MnO₂ nanosheets, Xu et al. constructed hierarchical CuO@MnO₂ core-shell nanoarrays (Figure 9f, g).^[101] Abundant exposed active sites and efficient mass transfer were obtained due to unique hierarchical core-shell nanoarray structures. More importantly, such a CuO/MnO₂ heterogeneous nanointerface could afford a well-designed built-in electric field at the interface region, which could trigger interfacial accumulation of nitrate ions and accelerate nitrate electroreduction kinetics by optimizing the chemisorption of nitrate ions or/and reaction intermediates (Figure 9h).

  Figure 9

  

  Figure 9.  (a) Schematic illustration of the preparation of a Cu/Co-based binary tandem catalyst. SEM images of the (b) ZIF-Co-R precursor, (c) CuCoSP_no and (d) CuCoSP on the Cu foil substrate. (e) A proposed reaction mechanism of CuCoSP tandem catalysis of NO₃RR at low overpotentials. Reproduced with permission from Ref.^[126] Copyright 2022, Nature Publishing Group. (f) TEM image and (g) HRTEM image of CuO@MnO₂ nanowires. (h) Schematic illustration showing schematic diagrams of the band structure of CuO and MnO₂, and the process of electrocatalytic nitrate reduction on the CuO@MnO₂/CF. Reproduced with permission from Ref.^[101] Copyright 2022, Royal Society of Chemistry.

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  Apart from Cu oxides, other Cu-based compounds can also find potential applications in NO₃RR catalysis. Li and his colleagues reported that Cu₃P/CF composites were obtained by a simple in situ phosphorylation process as a promising binderfree electrode material for electrochemical NO₃RR.^[142] They found that different phosphorylation temperatures had important effects on the catalytic activity and selectivity of the composites. As the phosphorylation temperature gradually increased, the nitrate conversion gradually decreased and reached the optimum conversion at 400 ℃. Although the loading rate of Cu continued to increase when the temperature rose continually, the electron transfer efficiency and electrochemical activity area decreased, leading to a decrease in nitrate conversion. At the same time, ammonia selectivity continued to rise with increasing the phosphorylation temperature. At the optimal phosphorylation temperature (400 ℃), the electrocatalytic nitrate reduction of the material achieved the best nitrate conversion (97.7%) with ammonia selectivity (~80%). It was observed that Cu₃P and Cu in copper foam play a dominant role in NO₃RR during the electrochemical process, rather than Cu²⁺ oxide species. These results suggested that suitable composition and crystallization, as well as high ECSA and interfacial electron transfer efficiency, were essential to ensure sufficient electrochemically active sites and superb electron transfer rates for electrocatalytic NO₃RR at the Cu₃P/CF composite electrode.

  Cu-Based Inorganic-Organic Hybrid Materials and Their Derivatives. Inorganic-organic hybrid materials are a kind of functional materials composed of molecular or nanoscale inorganic and organic building blocks. For inorganic-organic hybrid materials, their properties not only depend on the kind and proportion, but also on their dispersion of individual constituents. In this section, we mainly focus on two kinds of Cu-based inorganic-organic hybrid materials for electrocatalytic NO₃RR application, that is, molecular catalysts and metal-organic frameworks (MOFs).

  Molecular catalysts are promising in achieving high activity and selectivity in electrocatalytic nitrate-to-ammonia conversion due to the precise synthetic control. Moreover, molecular catalysts are also considered as appropriate platforms for gaining mechanistic insights into NO₃RR catalysis. Inspired by nitrate reductase, Wang and colleagues synthesized a Cu-based molecular solid catalyst by the incorporation of Cu in an organic molecular solid (3, 4, 9, 10-perylenetetracarboxylic dianhydride, PTCDA) for eight-electron direct electroreduction of nitrate to ammonia.^[47] Mechanism of performance improvement in the O-Cu-PTCDA (PTCDA with an optimum content of Cu) molecular solid catalyst was explanted from two aspects, supported by both electrochemical experiments and DFT calculations (Figure 10a-d). On one hand, the unique 3d¹⁰ configuration of Cu on PTCDA enabled Cu active center capable of effectively adsorbing NO₃^-, suppressing HER and boosting the H-N combination. On the other hand, the PTCDA structure facilitated the conduction of protons and electrons to the active Cu sites. These advantages endowed the O-Cu-PTCDA molecular solid catalyst with impressive capability for direct reduction of NO₃^- into NH₃ with a high selectivity.

  Figure 10

  

  Figure 10.  (a, c) The adsorption energies of H (a) and NO₃^- (c) on the surface of the O-Cu-PTCDA molecular layer (103). (b, d) The adsorption energies of H (b) and NO₃^- (d) on the (111) plane of Cu. Reproduced with permission from Ref.^[47] Copyright 2020, Nature Publishing Group. (e) Preparation and evolution of the host-guest Cu catalyst through metal ion deposition and in situ clustering routes. Reproduced with permission from Ref.^[85] Copyright 2022, America Chemical Society.

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  MOF materials have great potential in electrocatalysis due to their structural specificity, uniformity, and designability. Unsaturated coordination environments with single metal sites have been shown to significantly enhance the catalytic activity of different reactions. MOF materials are promising carriers for the preparation of single-center catalysts. The Luo group modified crystalline Th-BPYDC (BPYDC = 2, 2′-bipyridine-5, 5′-dicarboxylic acid) by solid-liquid post-synthesis to synthesize robust Th-MOF-supported single-center Cu materials (Cu@Th-BPYDC) for nitrate electroreduction.^[143] Notably, the Cu sites exhibited a novel square unsaturated coordination structure determined by single-crystal X-ray diffraction. It was found that the combination of Cu with BPYDC greatly enhanced the electrical conductivity of the material. In addition, Cu in Cu@Th-BPYDC showed unsaturated coordination with organic ligands, which enhanced the electrocatalytic activity of the material. Meanwhile, the Cu site in electrocatalyst showed a planar tetra-ligand configuration as the effective active site for catalytic nitrate reduction, thus exhibiting high dispersion and density, which made it easier for the active center to bind NO₃^- in solution to promote the electroreduction process. As expected, Cu@Th-BPYDC exhibited excellent NO₃^- electroreduction performance, producing NH₃ in high yield (225.3 μmol h^-1 cm^-2) and FE (94.5%). Cao and co-workers reported an anticollapse MOF (Ce-UiO-66)-supported single-atom Cu precatalyst for electrocatalytic NO₃RR.^[85] In situ XAS revealed the association of the formation of real catalytic sites with the in situ clustering of single-atom Cu, leading to uniform ultrasmall Cu nanoclusters (ca. 4 nm) without excessive aggregation by the confinement effect of the host framework (Figure 10f). The DFT calculation further confirmed the size effect and unique host-guest interaction of the catalyst in facilitating the NO₃^- activation and reaction energy decrease.

  Beyond being directly employed as electrocatalysts for nitrate-to-ammonia conversion, MOF materials can also function as promising precatalysts or platforms for constructing advanced catalysts with desired compositions to achieve high electrocatalytic activities. Ren et al. reported the incorporation of Pd into HKUST-1 to form a kind of Pd-incorporated Cu-based MOF with p orous octahedral structure (CuPd-MOF) (Figure 11a).^[99] When serving as precatalysts for NO₃RR electrocatalysis, the CuPd-MOFs were in-situ reconstituted to Cu/Pd/CuO_x multi-phase heterostructure with porous nanonetwork structure under nitrate electroreduction operating conditions (Figure 11b), which acted as the actual catalytic active material for NO₃RR electrocatalysis. Electronic interaction between Cu, Pd, and CuO_x components within the Cu/Pd/CuO_x multi-phase heterostructure could regulate the electronic structure of Pd sites and Cu sites. Electron transfer from Pd to Cu was demonstrated and electron-deficient feature for Pd sites could promote the NO₃^- adsorption on Pd sites, which was further reduced on electron-rich Cu sites (Figure 11c).

  Figure 11

  

  Figure 11.  (a) SEM image of CuPd-MOF. (b) TEM image of Cu/Pd/CuO_x multi-phase heterostructure. (c) Schematic illustration showing the electrocatalytic nitrate-to-ammonia process over the Cu/Pd/CuO_x. Reproduced with permission from Ref.^[99] Copyright 2022, Elsevier. (d) SEM image and (e) TME image of Cu@C catalyst. (f) Maximum FE NH₃ of Cu@C and Cu with concentrations of NO₃^- ranging from 5 to 100 × 10^-3 M. (g) Simulated concentrations and distributions of local NO₃^- on the surface of Cu encapsulated with porous carbon. Reproduced with permission from Ref.^[137] Copyright 2022, WILEY-VCH.

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  MOFs can be employed as precursors to prepare other functional materials through various pyrolytic, chemical, and physical treatments, and in combination with other types of materials. Cations in MOF structures are ideal source materials for various metals, metal oxides, and metal compounds, while the organic linkers provide the source materials for various pure and doped carbons. For most MOF-derived materials, they could inherit several of the desirable structural properties from MOFs, such as large surface areas and high levels of porosity, and could also be tailored to exhibit many new functionalities, either internally on their surfaces or a combination of both. These features endow MOF-derived functional materials with outstanding electrocatalytic properties. For example, Cu-BTC MOF with octahedral morphology was used as precursors for synthesizing Cu@C catalysts.^[137] Conversion of Cu-BTC MOF into Cu@C material could be fulfilled by pyrolyzing the Cu-BTC MOF precursors at 700 ℃ in H₂ atmosphere, during which regular octahedral morphology was retained and Cu nanoparticles were in situ formed and encapsulated within the ligand-derived porous carbon framework (Figure 11d, e). One unique advantage in NO₃RR electrocatalysis for the Cu@C catalyst was its excellent nitrate-to-ammonia capability at ultralow NO₃^- concentrations. The enrichment effect of porous carbon framework in Cu@C structure was revealed to be the key factor that contributed to the enhancement in electrocatalytic performance as the presence of a porous carbon framework could effectively confine and concentrate NO₃^- on the surface of Cu@C catalyst and then facilitate the mass transfer of NO₃^- (Figure 11g). Compared with Cu nanoparticles (FE NH₃ of 19.9%), the Cu@C achieved a much higher FE NH₃ (72.0%) at -0.3 V vs. RHE in 1 × 10^-3 M NO₃^-. Moreover, a wide tolerance range for concentration was demonstrated over the Cu@C catalysts, as the FE NH₃ of Cu@C exceeded 85.6% when the concentration of NO₃^- varied between 5 and 100 × 10^-3 M (Figure 11f).

  CONCLUSION AND PERSPECTIVES

  Electrochemical nitrate-to-ammonia conversion is considered as a technology of turning waste into wealth, which could remove environmentally harmful nitrate from various types of wastewaters while simultaneously producing value-added ammonia. Cu-based materials show great advantages in promoting selective electroreduction of nitrate to ammonia in terms of high nitrate conversion efficiency, NH₃ selectivity and NH₃ FE. A variety of Cu-based materials including monometallic Cu catalysts, bimetallic Cu-based catalysts, Cu-based compounds, and Cu-based inorganic-organic hybrid materials and their derivatives have been widely studied, and their merits have been explored in detail. With these material platforms, various strategies have been developed to tailored electronic structures of reactive sites by optimizing the morphology, structure, and composition of catalysts. Each type of Cu-based materials has its own advantages and disadvantages. Monometallic Cu catalysts are featured with simple composition and a single type of active site, which is convenient for studying their structure-activity relationships, while the weak adsorption of H on pure Cu surface is the main limiting factor for hydrogenation reaction during NO₃RR process. Bimetallic Cu-based catalysts can realize the regulation of electronic structure and properties of each metal to promote the adsorption and activation of reactants on the catalyst surface. Noted that many bimetallic Cu-based catalysts, like monometallic Cu, still face passivation or the leaching of metal species during NO₃RR catalysis process. As for Cu-based compounds, they possess diverse chemical composition, morphology and electronic structure. One of the main disadvantages for Cu-based compounds as cathodic catalysts lies in that they often undergo potential-dependent phase evolution under NO₃RR operating conditions, which make it become more difficult to elucidate and underlie catalytic mechanism. In the case of Cu-based inorganic-organic hybrid materials, their unique advantages include but not limited to high surface-to-volume ratio, intrinsically uniform metal site distribution, and structural tailorability. For most of them, insufficient electrical conductivity and low chemical stability under NO₃RR operating conditions are essential shortcomings. Moreover, current methods of synthesizing inorganic-organic hybrid materials are often high cost and difficult to be scaled up.

  Although significant progress has been made, some problems and challenges need to be explored and resolved for Cu-based materials to electro-catalyze nitrate-to-ammonia conversion. First, inferior catalytic durability and stability are still the crucial issues that restrict the wider applications of Cu-based materials as most of them suffer from passivation (in alkaline conditions) or c orrosion and dissolution (in acidic environments). Second, the current ammonia yield rate and energy efficiency for nitrate-to-ammonia technology based on Cu-based materials are still far from the industrial requirements. Third, the separation and collection of ammonia product from aqueous-based electrocatalytic NO₃RR systems still face a double challenge from both technology and cost. To address these issues, great attention in the future research should be paid to the following aspects:

  Development of Novel Catalysts with Rationally Designed Electronic Structure and Local Chemical Microenvironment. Electrocatalysts play a dominant role in the nitrate-to-ammonia conversion process, as they largely affect the reaction efficiency. The activity and selectivity for electrocatalytic nitrate-to-ammonia conversion can be improved substantially through electronic structure regulation of catalysts to promote the adsorption and activation of nitrate and intermediates, and directionally change the reaction pathway. Current strategies for the design and construction of Cu-based catalysts mainly concentrate on modifying the electronic structure, increasing the active sites and enhancing the apparent or intrinsic activity of catalysts. Apart from the electronic structure of reactive sites of Cu-based catalysts, special attention should be paid to engineer the local chemical microenvironment of reactive sites including modulating the surface microenvironment of catalysts including their hydrophobicity, adsorbability, electronegativity and localization as these properties determine both the kinetic and thermodynamic catalytic processes (Figure 12a).

  Figure 12

  

  Figure 12.  Prospects for developing NO₃RR technology for sustainable ammonia synthesis based on Cu-based materials.

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  Innovation of Electrocatalytic NO₃RR System Based on Co-Electrolysis Mode. Energy conversion efficiency is one of the significant indicators of electrocatalytic NO₃RR system, as it is closely related to the final cost. For traditional aqueous-based electrocatalytic NO₃RR system, the major anodic reaction is the oxygen evolution reaction (OER). The sluggish kinetic of OER process generally results in a largely reduced overall electrolysis rate and energy conversion efficiency. Despite intensive efforts made to develop high-efficient OER catalysts for coupling with electrocatalytic NO₃RR, OER still suffers quite high oxidation potentials, leading to massive electric power consumption. Moreover, the product of anode OER is low-value oxygen. An innovative strategy is to rationally design anode reactions alternative to the traditional OER for the purpose of reducing power consumption and obtaining high value-added products (Figure 12b). In this regard, some recent advances have highlighted the promising potential to couple cathodic NO₃RR with the electrooxidation reaction of small organic molecules with value-added chemicals generated to lower the whole electrolysis voltage, elevate the NO₃RR performance and simultaneously generate value-added chemicals at anode. In addition to electrooxidation reaction of small organic molecules, more other anode reactions deserve more attention.

  Scale-Up of Electrocatalytic NO₃RR System for Treatment of Actual Industrial Wastewater. As an attractive avenue to converting waste nitrate into value-added ammonia, NO₃RR electrocatalysis has recently received increasing attention with many papers have been published. However, most studies have focused on developing methods and strategies for catalyst preparation and few have been done in the way of large-scale demonstrations. In many published studies, manually prepared electrolytes containing nitrate addition rather than actual industrial wastewater are used in a lab test. For some electrocatalysts, the fluctuation and complexity of the actual water composition may cause performance delay in selective nitrate-to-ammonia conversion. Correspondingly, good thermal and chemical stability in the harsh conditions of real wastewater should be taken into account when designing electrocatalysts for NO₃RR (Figure 12c).

  Integration of Electrocatalytic NO₃RR System with Ammonia Product Collection System. Another key obstacle for the development of nitrate-to-ammonia technology from basic re-search to practical applications lies in ammonia product collection. Current research in this field mainly concentrates on the nitrate-to-ammonia reaction, while isochronous NH₃ separation and recovery from the mother liquor are awfully neglected in state-of-the-art electrochemical NO₃RR systems. On one hand, ammonia product collection in forms of pure NH₃ or ammonia fertilizers (e.g., NH₄Cl, (NH₄)₂SO₄) is the prerequisite for its further utilization. On the other hand, the timely NH₃ separation can also promote the nitrate-to-ammonia conversion reaction due to the release of much more active sites. Gas stripping and membrane separation methods are potentially capable of ammonia recovery. Therefore, to promote the large-scale application of nitrate-to-ammonia technology, it is of great importance to electrochemically selective ammonia extraction from nitrate by integrating NO₃RR systems with NH₃ recovery system (Figure 12d).

  
  ACKNOWLEDGEMENTS: This work was financially supported by the National Natural Science Foundation of China (21701141). COMPETING INTERESTS
  The authors declare no competing interests.
  For submission: https://www.editorialmanager.com/cjschem
  ADDITIONAL INFORMATION
  Full paper can be accessed via http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0201
  
• 1. [1]

     Yao, Y.; Zhu, S.; Wang, H.; Li, H.; Shao, M. A Spectroscopic study of electrochemical nitrogen and nitrate reduction on rhodium surfaces. Angew. Chem., Int. Ed. 2020, 59, 10479-10483. doi: 10.1002/anie.202003071

  2. [2]

     Tang, C.; Qiao, S. Z. How to explore ambient electrocatalytic nitrogen reduction reliably and insightfully. Chem. Soc. Rev. 2019, 48, 3166-3180. doi: 10.1039/C9CS00280D

  3. [3]

     Qiu, W.; Xie, X. -Y.; Qiu, J.; Fang, W. -H.; Liang, R.; Ren, X.; Ji, X.; Cui, G.; Asiri, A. M.; Cui, G.; Tang, B.; Sun, X. High-performance artificial nitrogen fixation at ambient conditions using a metal-free electrocatalyst. Nat. Commun. 2018, 9, 3485. doi: 10.1038/s41467-018-05758-5

  4. [4]

     Qu, Y.; Dai, T.; Cui, Y.; Zhang, Y.; Wang, Z.; Jiang, Q. Tailoring electronic structure of copper nanosheets by silver doping toward highly efficient electrochemical reduction of nitrogen to ammonia. Chem. Eng. J. 2022, 433, 133752. doi: 10.1016/j.cej.2021.133752

  5. [5]

     Khalil, I. E.; Xue, C.; Liu, W.; Li, X.; Shen, Y.; Li, S.; Zhang, W.; Huo, F. The role of defects in metal-organic frameworks for nitrogen reduction reaction: when defects switch to features. Adv. Funct. Mater. 2021, 31, 2010052. doi: 10.1002/adfm.202010052

  6. [6]

     Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Molybdenum-catalysed ammonia production with samarium diiodide and alcohols or water. Nature 2019, 568, 536-540. doi: 10.1038/s41586-019-1134-2

  7. [7]

     Wang, Y.; Yu, Y.; Jia, R.; Zhang, C.; Zhang, B. Electrochemical synthesis of nitric acid from air and ammonia through waste utilization. Natl. Sci. Rev. 2019, 6, 730-738. doi: 10.1093/nsr/nwz019

  8. [8]

     Cui, X.; Tang, C.; Zhang, Q. A review of electrocatalytic reduction of dinitrogen to ammonia under ambient conditions. Adv. Energy Mater. 2018, 8, 1800369. doi: 10.1002/aenm.201800369

  9. [9]

     Utomo, W. P.; Leung, M. K. H.; Yin, Z.; Wu, H.; Ng, Y. H. Advancement of bismuth-based materials for electrocatalytic and photo(electro)catalytic ammonia synthesis. Adv. Funct. Mater. 2021, 32, 2106713.

  10. [10]

      Li, L.; Tang, C.; Yao, D.; Zheng, Y.; Qiao, S. -Z. Electrochemical nitrogen reduction: identification and elimination of contamination in electrolyte. ACS Energy Lett. 2019, 4, 2111-2116. doi: 10.1021/acsenergylett.9b01573

  11. [11]

      Liu, D.; Chen, M.; Du, X.; Ai, H.; Lo, K. H.; Wang, S.; Chen, S.; Xing, G.; Wang, X.; Pan, H. Development of electrocatalysts for efficient nitrogen reduction reaction under ambient condition. Adv. Funct. Mater. 2020, 31, 2008983.

  12. [12]

      Wan, Y.; Zhou, H.; Zheng, M.; Huang, Z. H.; Kang, F.; Li, J.; Lv, R. Oxidation state modulation of bismuth for efficient electrocatalytic nitrogen reduction to ammonia. Adv. Funct. Mater. 2021, 31, 2100300. doi: 10.1002/adfm.202100300

  13. [13]

      Tan, Y.; Yan, L.; Huang, C.; Zhang, W.; Qi, H.; Kang, L.; Pan, X.; Zhong, Y.; Hu, Y.; Ding, Y. Fabrication of an Au25-Cys-Mo electrocatalyst for efficient nitrogen reduction to ammonia under ambient conditions. Small 2021, 17, 2100372. doi: 10.1002/smll.202100372

  14. [14]

      Hong, Q.; Li, T.; Zheng, S.; Chen, H.; Chu, H.; Xu, K.; Li, S.; Mei, Z.; Zhao, Q.; Ren, W. Tuning double layer structure of WO₃ nanobelt for promoting the electrochemical nitrogen reduction reaction in water. Chin. J. Struct. Chem. 2021, 40, 519-526.

  15. [15]

      Wang, G.; Shen, P.; Luo, Y.; Li, X.; Li, X.; Chu, K. A vacancy engineered MnO_2-x electrocatalyst promotes nitrate electroreduction to ammonia. Dalton Trans. 2022, 51, 9206-9212. doi: 10.1039/D2DT01431A

  16. [16]

      Xie, L.; Liu, Q.; Sun, S.; Hu, L.; Zhang, L.; Zhao, D.; Liu, Q.; Chen, J.; Li, J.; Ouyang, L.; Alshehri, A. A.; Hamdy, M. S.; Kong, Q.; Sun, X. High-efficiency electrosynthesis of ammonia with selective reduction of nitrate in neutral media enabled by self-supported Mn₂CoO₄ nanoarray. ACS Appl. Mater. Interfaces 2022, 33242-33247.

  17. [17]

      Niu, H.; Zhang, Z.; Wang, X.; Wan, X.; Shao, C.; Guo, Y. Theoretical insights into the mechanism of selective nitrate-to-ammonia electroreduction on single-atom catalysts. Adv. Funct. Mater. 2020, 31, 2008533.

  18. [18]

      Yan, L.; Xu, Z.; Liu, X.; Mahmood, S.; Shen, J.; Ning, J.; Li, S.; Zhong, Y.; Hu, Y. Integrating trifunctional Co@NC-CNTs@NiFe-LDH electrocatalysts with arrays of porous triangle carbon plates for high-power-density rechargeable Zn-air batteries and self-powered water splitting. Chem. Eng. J. 2022, 446, 137049. doi: 10.1016/j.cej.2022.137049

  19. [19]

      Wang, S.; Wang, H.; Huang, C.; Ye, P.; Luo, X.; Ning, J.; Zhong, Y.; Hu, Y. Trifunctional electrocatalyst of N-doped graphitic carbon nanosheets encapsulated with CoFe alloy nanocrystals: the key roles of bimetal components and high-content graphitic-N. Appl. Catal. B Environ. 2021, 298, 120512. doi: 10.1016/j.apcatb.2021.120512

  20. [20]

      Wu, Z. Y.; Karamad, M.; Yong, X.; Huang, Q.; Cullen, D. A.; Zhu, P.; Xia, C.; Xiao, Q.; Shakouri, M.; Chen, F. Y.; Kim, J. Y. T.; Xia, Y.; Heck, K.; Hu, Y.; Wong, M. S.; Li, Q.; Gates, I.; Siahrostami, S.; Wang, H. Electrochemical ammonia synthesis via nitrate reduction on Fe single atom catalyst. Nat. Commun. 2021, 12, 2870. doi: 10.1038/s41467-021-23115-x

  21. [21]

      Chauhan, R.; Srivastava, V. C. Electrochemical denitrification of highly contaminated actual nitrate wastewater by Ti/RuO₂ anode and iron cathode. Chem. Eng. J. 2020, 386, 122065. doi: 10.1016/j.cej.2019.122065

  22. [22]

      Li, J.; Zhang, Y.; Liu, C.; Zheng, L.; Petit, E.; Qi, K.; Zhang, Y.; Wu, H.; Wang, W.; Tiberj, A.; Wang, X.; Chhowalla, M.; Lajaunie, L.; Yu, R.; Voiry, D. 3.4% solar-to-ammonia efficiency from nitrate using Fe single atomic catalyst supported on MoS₂ nanosheets. Adv. Funct. Mater. 2021, 32, 2108316.

  23. [23]

      Lv, X.; Mou, T.; Li, J.; Kou, L.; Frauenheim, T. Tunable surface chemistry in heterogeneous bilayer single-atom catalysts for electrocatalytic NO_x reduction to ammonia. Adv. Funct. Mater. 2022, 32, 2201262. doi: 10.1002/adfm.202201262

  24. [24]

      Su, L.; Han, D.; Zhu, G.; Xu, H.; Luo, W.; Wang, L.; Jiang, W.; Dong, A.; Yang, J. Tailoring the assembly of iron nanoparticles in carbon microspheres toward high-performance electrocatalytic denitrification. Nano Lett. 2019, 19, 5423-5430. doi: 10.1021/acs.nanolett.9b01925

  25. [25]

      Yu, H.; Qu, S.; Chen, P. R.; Ou, K. Q.; Lin, J. Y.; Guo, Z. H.; Zheng, L.; Li, J. K.; Huang, S.; Teng, Y.; Zou, L.; Song, J. L. CO₂ bubble-assisted in-situ construction of mesoporous Co-doped Cu₂(OH)₂CO₃ nanosheets as advanced electrodes towards fast and highly efficient electrochemical reduction of nitrate to N₂ in wastewater. J. Hazard. Mater. 2022, 430, 128351. doi: 10.1016/j.jhazmat.2022.128351

  26. [26]

      Fu, W.; Hu, Z.; Zheng, Y.; Su, P.; Zhang, Q.; Jiao, Y.; Zhou, M. Tuning mobility of intermediate and electron transfer to enhance electrochemical reduction of nitrate to ammonia on Cu₂O/Cu interface. Chem. Eng. J. 2022, 433, 133680. doi: 10.1016/j.cej.2021.133680

  27. [27]

      Banasiak, L. J.; Schäfer, A. I. Removal of boron, fluoride and nitrate by electrodialysis in the presence of organic matter. J. Membr. Sci. 2009, 334, 101-109. doi: 10.1016/j.memsci.2009.02.020

  28. [28]

      Samatya, S.; Kabay, N.; Yüksel, Ü.; Arda, M.; Yüksel, M. Removal of nitrate from aqueous solution by nitrate selective ion exchange resins. React. Funct. Polym. 2006, 66, 1206-1214. doi: 10.1016/j.reactfunctpolym.2006.03.009

  29. [29]

      Bae, B. -U.; Jung, Y. -H.; Han, W. -W.; Shin, H. -S. Improved brine recycling during nitrate removal using ion exchange. Water Res. 2002, 36, 3330-3340. doi: 10.1016/S0043-1354(02)00012-X

  30. [30]

      Xu, D.; Li, Y.; Yin, L.; Ji, Y.; Niu, J.; Yu, Y. Electrochemical removal of nitrate in industrial wastewater. Front. Environ. Sci. Eng. 2018, 12, 9.

  31. [31]

      Zhang, R.; Shuai, D.; Guy, K. A.; Shapley, J. R.; Strathmann, T. J.; Werth, C. J. Elucidation of nitrate reduction mechanisms on a Pd-In bimetallic catalyst using isotope labeled nitrogen species. ChemCatChem 2013, 5, 313-321. doi: 10.1002/cctc.201200457

  32. [32]

      Zhang, X.; Wang, Y.; Liu, C.; Yu, Y.; Lu, S.; Zhang, B. Recent advances in non-noble metal electrocatalysts for nitrate reduction. Chem. Eng. J. 2021, 403, 126269. doi: 10.1016/j.cej.2020.126269

  33. [33]

      Clark, C. A.; Reddy, C. P.; Xu, H.; Heck, K. N.; Luo, G.; Senftle, T. P.; Wong, M. S. Mechanistic insights into pH-controlled nitrite reduction to ammonia and hydrazine over rhodium. ACS Catal. 2019, 10, 494-509.

  34. [34]

      Wei, L.; Liu, D. -J.; Rosales, B. A.; Evans, J. W.; Vela, J. Mild and selective hydrogenation of nitrate to ammonia in the absence of noble metals. ACS Catal. 2020, 10, 3618-3628. doi: 10.1021/acscatal.9b05338

  35. [35]

      Sun, W. J.; Ji, H. Q.; Li, L. X.; Zhang, H. Y.; Wang, Z. K.; He, J. H.; Lu, J. M. Built-in electric field triggered interfacial accumulation effect for efficient nitrate removal at ultra-low concentration and electroreduction to ammonia. Angew. Chem., Int. Ed. 2021, 60, 22933-22939. doi: 10.1002/anie.202109785

  36. [36]

      Jia, R.; Wang, Y.; Wang, C.; Ling, Y.; Yu, Y.; Zhang, B. Boosting selective nitrate electroreduction to ammonium by constructing oxygen vacancies in TiO₂. ACS Catal. 2020, 10, 3533-3540. doi: 10.1021/acscatal.9b05260

  37. [37]

      Yu, Y.; Wang, C.; Yu, Y.; Wang, Y.; Zhang, B. Promoting selective electroreduction of nitrates to ammonia over electron-deficient Co modulated by rectifying Schottky contacts. Sci. China: Chem. 2020, 63, 1469-1476. doi: 10.1007/s11426-020-9795-x

  38. [38]

      Li, Y.; Xiao, S.; Li, X.; Chang, C.; Xie, M.; Xu, J.; Yang, Z. A robust metal-free electrocatalyst for nitrate reduction reaction to synthesize ammonia. Mater. Today Phys. 2021, 19, 100431. doi: 10.1016/j.mtphys.2021.100431

  39. [39]

      Kwon, Y. -I.; Kim, S. K.; Kim, Y. B.; Son, S. J.; Nam, G. D.; Park, H. J.; Cho, W. -C.; Yoon, H. C.; Joo, J. H. Nitric oxide utilization for ammonia production using solid electrolysis cell at atmospheric pressure. ACS Energy Lett. 2021, 6, 4165-4172. doi: 10.1021/acsenergylett.1c01972

  40. [40]

      Liang, J.; Liu, P.; Li, Q.; Li, T.; Yue, L.; Luo, Y.; Liu, Q.; Li, N.; Tang, B.; Alshehri, A. A.; Shakir, I.; Agboola, P. O.; Sun, C.; Sun, X. Amorphous boron carbide on titanium dioxide nanobelt arrays for high-efficiency electrocatalytic NO reduction to NH₃. Angew. Chem., Int. Ed. 2022, 61, e202202087.

  41. [41]

      Zhang, L.; Liang, J.; Wang, Y.; Mou, T.; Lin, Y.; Yue, L.; Li, T.; Liu, Q.; Luo, Y.; Li, N.; Tang, B.; Liu, Y.; Gao, S.; Alshehri, A. A.; Guo, X.; Ma, D.; Sun, X. High-performance electrochemical NO reduction into NH₃ by MoS₂ nanosheet. Angew. Chem., Int. Ed. 2021, 60, 25263-25268. doi: 10.1002/anie.202110879

  42. [42]

      Ko, B. H.; Hasa, B.; Shin, H.; Zhao, Y.; Jiao, F. Electrochemical reduction of gaseous nitrogen oxides on transition metals at ambient conditions. J. Am. Chem. Soc. 2022, 144, 1258-1266. doi: 10.1021/jacs.1c10535

  43. [43]

      Wang, Y.; Shu, S.; Peng, M.; Hu, L.; Lv, X.; Shen, Y.; Gong, H.; Jiang, G. Dual-site electrocatalytic nitrate reduction to ammonia on oxygen vacancy-enriched and Pd-decorated MnO₂ nanosheets. Nanoscale 2021, 13, 17504-17511. doi: 10.1039/D1NR04962C

  44. [44]

      Yin, H.; Chen, Z.; Xiong, S.; Chen, J.; Wang, C.; Wang, R.; Kuwahara, Y.; Luo, J.; Yamashita, H.; Peng, Y.; Li, J. Alloying effectinduced electron polarization drives nitrate electroreduction to ammonia. Chem. Catal. 2021, 1, 1088-1103. doi: 10.1016/j.checat.2021.08.014

  45. [45]

      Chen, Q.; Liang, J.; Yue, L.; Luo, Y.; Liu, Q.; Li, N.; Alshehri, A. A.; Li, T.; Guo, H.; Sun, X. CoO nanoparticle decorated N-doped carbon nanotubes: a high-efficiency catalyst for nitrate reduction to ammonia. Chem. Commun. 2022, 58, 5901-5904. doi: 10.1039/D2CC00997H

  46. [46]

      Fan, X.; Xie, L.; Liang, J.; Ren, Y.; Zhang, L.; Yue, L.; Li, T.; Luo, Y.; Li, N.; Tang, B.; Liu, Y.; Gao, S.; Alshehri, A. A.; Liu, Q.; Kong, Q.; Sun, X. In situ grown Fe₃O₄ particle on stainless steel: a highly efficient electrocatalyst for nitrate reduction to ammonia. Nano Res. 2021, 15, 3050-3055.

  47. [47]

      Chen, G. -F.; Yuan, Y.; Jiang, H.; Ren, S. -Y.; Ding, L. -X.; Ma, L.; Wu, T.; Lu, J.; Wang, H. Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper-molecular solid catalyst. Nat. Energy 2020, 5, 605-613. doi: 10.1038/s41560-020-0654-1

  48. [48]

      Crawford, J.; Yin, H.; Du, A.; O'Mullane, A. P. Nitrate-to-ammonia conversion at an InSn-enriched liquid-metal electrode. Angew. Chem., Int. Ed. 2022, 61, e202201604.

  49. [49]

      Deng, X.; Yang, Y.; Wang, L.; Fu, X. Z.; Luo, J. L. Metallic Co nanoarray catalyzes selective NH₃ production from electrochemical nitrate reduction at current densities exceeding 2 A cm^-2. Adv. Sci. 2021, 8, 2004523. doi: 10.1002/advs.202004523

  50. [50]

      McEnaney, J. M.; Blair, S. J.; Nielander, A. C.; Schwalbe, J. A.; Koshy, D. M.; Cargnello, M.; Jaramillo, T. F. Electrolyte engineering for efficient electrochemical nitrate reduction to ammonia on a titanium electrode. ACS Sustainable Chem. Eng. 2020, 8, 2672-2681. doi: 10.1021/acssuschemeng.9b05983

  51. [51]

      Li, Z.; Liang, J.; Liu, Q.; Xie, L.; Zhang, L.; Ren, Y.; Yue, L.; Li, N.; Tang, B.; Alshehri, A. A.; Hamdy, M. S.; Luo, Y.; Kong, Q.; Sun, X. High-efficiency ammonia electrosynthesis via selective reduction of nitrate on ZnCo₂O₄ nanosheet array. Mater. Today Phys. 2022, 23, 100619. doi: 10.1016/j.mtphys.2022.100619

  52. [52]

      Yang, L.; Li, J.; Du, F.; Gao, J.; Liu, H.; Huang, S.; Zhang, H.; Li, C.; Guo, C. Interface engineering cerium-doped copper nanocrystal for efficient electrochemical nitrate-to-ammonia production. Electrochim. Acta 2022, 411, 140095. doi: 10.1016/j.electacta.2022.140095

  53. [53]

      Li, J.; Zhan, G.; Yang, J.; Quan, F.; Mao, C.; Liu, Y.; Wang, B.; Lei, F.; Li, L.; Chan, A. W. M.; Xu, L.; Shi, Y.; Du, Y.; Hao, W.; Wong, P. K.; Wang, J.; Dou, S. X.; Zhang, L.; Yu, J. C. Efficient ammonia electrosynthesis from nitrate on strained ruthenium nanoclusters. J. Am. Chem. Soc. 2020, 142, 7036-7046. doi: 10.1021/jacs.0c00418

  54. [54]

      Lim, J.; Liu, C. -Y.; Park, J.; Liu, Y. -H.; Senftle, T. P.; Lee, S. W.; Hatzell, M. C. Structure sensitivity of Pd facets for enhanced electrochemical nitrate reduction to ammonia. ACS Catal. 2021, 11, 7568-7577. doi: 10.1021/acscatal.1c01413

  55. [55]

      Wang, Y.; Xu, A.; Wang, Z.; Huang, L.; Li, J.; Li, F.; Wicks, J.; Luo, M.; Nam, D. H.; Tan, C. S.; Ding, Y.; Wu, J.; Lum, Y.; Dinh, C. T.; Sinton, D.; Zheng, G.; Sargent, E. H. Enhanced nitrate-to-ammonia activity on copper-nickel alloys via tuning of intermediate adsorption. J. Am. Chem. Soc. 2020, 142, 5702-5708. doi: 10.1021/jacs.9b13347

  56. [56]

      Wang, Z.; Young, S. D.; Goldsmith, B. R.; Singh, N. Increasing electrocatalytic nitrate reduction activity by controlling adsorption through PtRu alloying. J. Catal. 2021, 395, 143-154. doi: 10.1016/j.jcat.2020.12.031

  57. [57]

      Zhu, J. Y.; Xue, Q.; Xue, Y. Y.; Ding, Y.; Li, F. M.; Jin, P.; Chen, P.; Chen, Y. Iridium nanotubes as bifunctional electrocatalysts for oxygen evolution and nitrate reduction reactions. ACS Appl. Mater. Interfaces 2020, 12, 14064-14070. doi: 10.1021/acsami.0c01937

  58. [58]

      Yao, Q.; Chen, J.; Xiao, S.; Zhang, Y.; Zhou, X. Selective electrocatalytic reduction of nitrate to ammonia with nickel phosphide. ACS Appl. Mater. Interfaces 2021, 13, 30458-30467. doi: 10.1021/acsami.0c22338

  59. [59]

      Wang, X.; Zhu, M.; Zeng, G.; Liu, X.; Fang, C.; Li, C. A three-dimensional Cu nanobelt cathode for highly efficient electrocatalytic nitrate reduction. Nanoscale 2020, 12, 9385-9391. doi: 10.1039/C9NR10743F

  60. [60]

      Li, J.; Gao, J.; Feng, T.; Zhang, H.; Liu, D.; Zhang, C.; Huang, S.; Wang, C.; Du, F.; Li, C.; Guo, C. Effect of supporting matrixes on performance of copper catalysts in electrochemical nitrate reduction to ammonia. J. Power Sources 2021, 511, 230463. doi: 10.1016/j.jpowsour.2021.230463

  61. [61]

      Pérez-Gallent, E.; Figueiredo, M. C.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic reduction of nitrate on copper single crystals in acidic and alkaline solutions. Electrochim. Acta 2017, 227, 77-84. doi: 10.1016/j.electacta.2016.12.147

  62. [62]

      Li, L. -X.; Sun, W. -J.; Zhang, H. -Y.; Wei, J. -L.; Wang, S. -X.; He, J. -H.; Li, N. -J.; Xu, Q. -F.; Chen, D. -Y.; Li, H.; Lu, J. -M. Highly efficient and selective nitrate electroreduction to ammonia catalyzed by molecular copper catalyst@Ti₃C₂T_x MXene. J. Mater. Chem. A 2021, 9, 21771-21778. doi: 10.1039/D1TA06664A

  63. [63]

      Wang, Z.; Richards, D.; Singh, N. Recent discoveries in the reaction mechanism of heterogeneous electrocatalytic nitrate reduction. Catal. Sci. Technol. 2021, 11, 705-725. doi: 10.1039/D0CY02025G

  64. [64]

      Bae, S. -E.; Stewart, K. L.; Gewirth, A. A. Nitrate adsorption and reduction on Cu(100) in acidic solution. J. Am. Chem. Soc. 2007, 129, 10171-10180. doi: 10.1021/ja071330n

  65. [65]

      Yuan, J.; Xing, Z.; Tang, Y.; Liu, C. Tuning the oxidation state of Cu electrodes for selective electrosynthesis of ammonia from nitrate. ACS Appl. Mater. Interfaces 2021, 52469-52478.

  66. [66]

      Gao, J.; Jiang, B.; Ni, C.; Qi, Y.; Zhang, Y.; Oturan, N.; Oturan, M. A. Non-precious Co₃O₄-TiO₂/Ti cathode based electrocatalytic nitrate reduction: preparation, performance and mechanism. Appl. Catal. B Environ. 2019, 254, 391-402. doi: 10.1016/j.apcatb.2019.05.016

  67. [67]

      Gao, J.; Jiang, B.; Ni, C.; Qi, Y.; Bi, X. Enhanced reduction of nitrate by noble metal-free electrocatalysis on P doped three-dimensional Co₃O₄ cathode: mechanism exploration from both experimental and DFT studies. Chem. Eng. J. 2020, 382, 123034. doi: 10.1016/j.cej.2019.123034

  68. [68]

      Liu, R.; Zhao, H.; Zhao, X.; He, Z.; Lai, Y.; Shan, W.; Bekana, D.; Li, G.; Liu, J. Defect sites in ultrathin Pd nanowires facilitate the highly efficient electrochemical hydrodechlorination of pollutants by H*ads. Environ. Sci. Technol. 2018, 52, 9992-10002. doi: 10.1021/acs.est.8b02740

  69. [69]

      Martínez, J.; Ortiz, A.; Ortiz, I. State-of-the-art and perspectives of the catalytic and electrocatalytic reduction of aqueous nitrates. Appl. Catal. B Environ. 2017, 207, 42-59. doi: 10.1016/j.apcatb.2017.02.016

  70. [70]

      de Vooys, A. C. A.; van Santen, R. A.; van Veen, J. A. R. Electrocatalytic reduction of NO₃^- on palladium/copper electrodes. J. Mol. Catal. A: Chem. 2000, 154, 203-215. doi: 10.1016/S1381-1169(99)00375-1

  71. [71]

      Wang, Y.; Wang, C.; Li, M.; Yu, Y.; Zhang, B. Nitrate electroreduction: mechanism insight, in situ characterization, performance evaluation, and challenges. Chem. Soc. Rev. 2021, 50, 6720-6733. doi: 10.1039/D1CS00116G

  72. [72]

      Garcia-Segura, S.; Lanzarini-Lopes, M.; Hristovski, K.; Westerhoff, P. Electrocatalytic reduction of nitrate: fundamentals to full-scale water treatment applications. Appl. Catal. B Environ. 2018, 236, 546-568. doi: 10.1016/j.apcatb.2018.05.041

  73. [73]

      Wang, J.; Feng, T.; Chen, J.; Ramalingam, V.; Li, Z.; Kabtamu, D. M.; He, J. -H.; Fang, X. Electrocatalytic nitrate/nitrite reduction to ammonia synthesis using metal nanocatalysts and bio-inspired metalloenzymes. Nano Energy 2021, 86, 106088. doi: 10.1016/j.nanoen.2021.106088

  74. [74]

      Lu, X.; Song, H.; Cai, J.; Lu, S. Recent development of electrochemical nitrate reduction to ammonia: a mini review. Electrochem. Commun. 2021, 129, 107094. doi: 10.1016/j.elecom.2021.107094

  75. [75]

      Zeng, Y.; Priest, C.; Wang, G.; Wu, G. Restoring the nitrogen cycle by electrochemical reduction of nitrate: progress and prospects. Small Methods 2020, 4, 2000672. doi: 10.1002/smtd.202000672

  76. [76]

      Duca, M.; Koper, M. T. Powering denitrification: the perspectives of electrocatalytic nitrate reduction. Energy Environ. Sci. 2012, 5, 9726-9742. doi: 10.1039/c2ee23062c

  77. [77]

      Li, P.; Jin, Z.; Fang, Z.; Yu, G. A single-site iron catalyst with preoccupied active centers that achieves selective ammonia electrosynthesis from nitrate. Energy Environ. Sci. 2021, 14, 3522-3531. doi: 10.1039/D1EE00545F

  78. [78]

      Liu, J. -X.; Richards, D.; Singh, N.; Goldsmith, B. R. Activity and selectivity trends in electrocatalytic nitrate reduction on transition metals. ACS Catal. 2019, 9, 7052-7064. doi: 10.1021/acscatal.9b02179

  79. [79]

      Crawford, J.; Yin, H.; Du, A.; O'Mullane, A. P. Nitrate-to-ammonia conversion at an insn-enriched liquid-metal electrode. Angew. Chem. 2022, e202201604.

  80. [80]

      Shih, Y. -J.; Wu, Z. -L.; Huang, Y. -H.; Huang, C. -P. Electrochemical nitrate reduction as affected by the crystal morphology and facet of copper nanoparticles supported on nickel foam electrodes (Cu/Ni). Chem. Eng. J. 2020, 383, 123157. doi: 10.1016/j.cej.2019.123157

  81. [81]

      Hu, T.; Wang, C.; Wang, M.; Li, C. M.; Guo, C. Theoretical insights into superior nitrate reduction to ammonia performance of copper catalysts. ACS Catal. 2021, 11, 14417-14427. doi: 10.1021/acscatal.1c03666

  82. [82]

      Zhu, X.; Huang, H.; Zhang, H.; Zhang, Y.; Shi, P.; Qu, K.; Cheng, S. -B.; Wang, A. -L.; Lu, Q. Filling mesopores of conductive metal-organic frameworks with Cu clusters for selective nitrate reduction to ammonia. ACS Appl. Mater. Interfaces 2022, 14, 32176-32182. doi: 10.1021/acsami.2c09241

  83. [83]

      Gong, Z.; Zhong, W.; He, Z.; Jia, C.; Zhou, D.; Zhang, N.; Kang, X.; Chen, Y. Improving electrochemical nitrate reduction activity of layered perovskite oxide La₂CuO₄ via B-site doping. Catal. Today 2022, 402, 259-265. doi: 10.1016/j.cattod.2022.04.019

  84. [84]

      Niu, Z.; Fan, S.; Li, X.; Wang, P.; Tadé, M. O.; Liu, S. Optimizing oxidation state of octahedral copper for boosting electroreduction nitrate to ammonia. ACS Appl. Energy Mater. 2022, 5, 3339-3345. doi: 10.1021/acsaem.1c03969

  85. [85]

      Xu, Y. -T.; Xie, M. -Y.; Zhong, H.; Cao, Y. In situ clustering of singleatom copper precatalysts in a metal-organic framework for efficient electrocatalytic nitrate-to-ammonia reduction. ACS Catal. 2022, 12, 8698-8706. doi: 10.1021/acscatal.2c02033

  86. [86]

      Qiu, W.; Chen, X.; Liu, Y.; Xiao, D.; Wang, P.; Li, R.; Liu, K.; Jin, Z.; Li, P. Confining intermediates within a catalytic nanoreactor facilitates nitrate-to-ammonia electrosynthesis. Appl. Catal. B Environ. 2022, 315, 121548. doi: 10.1016/j.apcatb.2022.121548

  87. [87]

      Fang, L.; Wang, S.; Song, C.; Lu, S.; Yang, X.; Qi, X.; Liu, H. Boosting nitrate electroreduction to ammonia via in situ generated stacking faults in oxide-derived copper. Chem. Eng. J. 2022, 446, 137341. doi: 10.1016/j.cej.2022.137341

  88. [88]

      Cai, J.; Wei, Y.; Cao, A.; Huang, J.; Jiang, Z.; Lu, S.; Zang, S. -Q. Electrocatalytic nitrate-to-ammonia conversion with ~100% faradaic efficiency via single-atom alloying. Appl. Catal. B Environ. 2022, 316, 121683. doi: 10.1016/j.apcatb.2022.121683

  89. [89]

      Gong, Z.; Zhong, W.; He, Z.; Liu, Q.; Chen, H.; Zhou, D.; Zhang, N.; Kang, X.; Chen, Y. Regulating surface oxygen species on copper(I) oxides via plasma treatment for effective reduction of nitrate to ammonia. Appl. Catal. B Environ. 2022, 305, 121021. doi: 10.1016/j.apcatb.2021.121021

  90. [90]

      Liu, Y.; Deng, B.; Li, K.; Wang, H.; Sun, Y.; Dong, F. Metal-organic framework derived carbon-supported bimetallic copper-nickel alloy electrocatalysts for highly selective nitrate reduction to ammonia. J. Colloid Interface Sci. 2022, 614, 405-414. doi: 10.1016/j.jcis.2022.01.127

  91. [91]

      Liu, H.; Lang, X.; Zhu, C.; Timoshenko, J.; Ruscher, M.; Bai, L.; Guijarro, N.; Yin, H.; Peng, Y.; Li, J.; Liu, Z.; Wang, W.; Cuenya, B. R.; Luo, J. Efficient electrochemical nitrate reduction to ammonia with copper-supported rhodium cluster and single-atom catalysts. Angew. Chem., Int. Ed. 2022, 61, e202202556.

  92. [92]

      Yin, H.; Zhao, X.; Xiong, S.; Peng, Y.; Chen, Z.; Wang, R.; Wen, M.; Luo, J.; Yamashita, H.; Li, J. New insight on electroreduction of nitrate to ammonia driven by oxygen vacancies-induced strong interface interactions. J. Catal. 2022, 406, 39-47. doi: 10.1016/j.jcat.2021.12.031

  93. [93]

      Wang, C.; Ye, F.; Shen, J.; Xue, K. H.; Zhu, Y.; Li, C. In situ loading of Cu₂O active sites on island-like copper for efficient electrochemical reduction of nitrate to ammonia. ACS Appl. Mater. Interfaces 2022, 14, 6680-6688. doi: 10.1021/acsami.1c21691

  94. [94]

      Jiang, G.; Peng, M.; Hu, L.; Ouyang, J.; Lv, X.; Yang, Z.; Liang, X.; Liu, Y.; Liu, H. Electron-deficient Cu^δ+ stabilized by interfacial Cu-O-Al bonding for accelerating electrocatalytic nitrate conversion. Chem. Eng. J. 2022, 435, 134853. doi: 10.1016/j.cej.2022.134853

  95. [95]

      Patil, S. B.; Liu, T. R.; Chou, H. L.; Huang, Y. B.; Chang, C. C.; Chen, Y. C.; Lin, Y. S.; Li, H.; Lee, Y. C.; Chang, Y. J.; Lai, Y. H.; Wen, C. Y.; Wang, D. Y. Electrocatalytic Reduction of NO₃^- to ultrapure ammonia on {200} facet dominant Cu nanodendrites with high conversion faradaic efficiency. J. Phys. Chem. Lett. 2021, 12, 8121-8128. doi: 10.1021/acs.jpclett.1c02236

  96. [96]

      Chen, L. -F.; Xie, A. -Y.; Lou, Y. -Y.; Tian, N.; Zhou, Z. -Y.; Sun, S. -G. Electrochemical synthesis of tetrahexahedral Cu nanocrystals with high-index facets for efficient nitrate electroreduction. J. Electroanal. Chem. 2022, 907, 116022. doi: 10.1016/j.jelechem.2022.116022

  97. [97]

      Zhao, Y.; Liu, Y.; Zhang, Z.; Mo, Z.; Wang, C.; Gao, S. Flower-like open-structured polycrystalline copper with synergistic multi-crystal plane for efficient electrocatalytic reduction of nitrate to ammonia. Nano Energy 2022, 97, 107124. doi: 10.1016/j.nanoen.2022.107124

  98. [98]

      Wang, Y.; Zhou, W.; Jia, R.; Yu, Y.; Zhang, B. Unveiling the activity origin of a copper-based electrocatalyst for selective nitrate reduction to ammonia. Angew. Chem., Int. Ed. 2020, 59, 5350-5354. doi: 10.1002/anie.201915992

  99. [99]

      Ren, T.; Yu, Z.; Yu, H.; Deng, K.; Wang, Z.; Li, X.; Wang, H.; Wang, L.; Xu, Y. Interfacial polarization in metal-organic framework reconstructed Cu/Pd/CuO_x multi-phase heterostructures for electrocatalytic nitrate reduction to ammonia. Appl. Catal. B Environ. 2022, 318, 121805. doi: 10.1016/j.apcatb.2022.121805

  100. [100]

       Ren, T.; Ren, K.; Wang, M.; Liu, M.; Wang, Z.; Wang, H.; Li, X.; Wang, L.; Xu, Y. Concave-convex surface oxide layers over copper nanowires boost electrochemical nitrate-to-ammonia conversion. Chem. Eng. J. 2021, 426, 130759. doi: 10.1016/j.cej.2021.130759

  101. [101]

       Xu, Y.; Sheng, Y.; Wang, M.; Ren, T.; Shi, K.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Interface coupling induced built-in electric fields boost electrochemical nitrate reduction to ammonia over CuO@MnO₂ core-shell hierarchical nanoarrays. J. Mater. Chem. A 2022, 10, 16883-16890. doi: 10.1039/D2TA02006H

  102. [102]

       Xu, Y.; Shi, K.; Ren, T.; Yu, H.; Deng, K.; Wang, X.; Wang, Z.; Wang, H.; Wang, L. Electronic metal-support interaction triggering interfacial charge polarization over CuPd/N-Doped-C nanohybrids drives selectively electrocatalytic conversion of nitrate to ammonia. Small 2022, 2203335.

  103. [103]

       Xu, Y.; Wang, M.; Ren, K.; Ren, T.; Liu, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Atomic defects in pothole-rich two-dimensional copper nanoplates triggering enhanced electrocatalytic selective nitrate-to-ammonia transformation. J. Mater. Chem. A 2021, 9, 16411-16417. doi: 10.1039/D1TA04743D

  104. [104]

       Xu, Y.; Ren, K.; Ren, T.; Wang, M.; Liu, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Cooperativity of Cu and Pd active sites in CuPd aerogels enhances nitrate electroreduction to ammonia. Chem. Commun. 2021, 57, 7525-7528. doi: 10.1039/D1CC02105B

  105. [105]

       Tang, Z.; Bai, Z.; Li, X.; Ding, L.; Zhang, B.; Chang, X. Chloride-derived bimetallic Cu-Fe nanoparticles for high-selective nitrate-to-ammonia electrochemical catalysis. Processes 2022, 10, 751. doi: 10.3390/pr10040751

  106. [106]

       Li, Z.; Wang, L.; Cai, Y.; Zhang, J. -R.; Zhu, W. Electrochemically reconstructed copper-polypyrrole nanofiber network for remediating nitrate-containing water at neutral pH. J. Hazard. Mater. 2022, 440, 129828. doi: 10.1016/j.jhazmat.2022.129828

  107. [107]

       Xu, Y.; Ren, K.; Ren, T.; Wang, M.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Ultralow-content Pd in-situ incorporation mediated hierarchical defects in corner-etched Cu₂O octahedra for enhanced electrocatalytic nitrate reduction to ammonia. Appl. Catal. B Environ. 2022, 306, 121094. doi: 10.1016/j.apcatb.2022.121094

  108. [108]

       Chen, D.; Zhang, S.; Bu, X.; Zhang, R.; Quan, Q.; Lai, Z.; Wang, W.; Meng, Y.; Yin, D.; Yip, S.; Liu, C.; Zhi, C.; Ho, J. C. Synergistic modulation of local environment for electrochemical nitrate reduction via asymmetric vacancies and adjacent ion clusters. Nano Energy 2022, 98, 107338. doi: 10.1016/j.nanoen.2022.107338

  109. [109]

       Niu, Z.; Fan, S.; Li, X.; Liu, Z.; Wang, J.; Duan, J.; Tade, M. O.; Liu, S. Facile tailoring of the electronic structure and the d-band center of copper-doped cobaltate for efficient nitrate electrochemical hydrogenation. ACS Appl. Mater. Interfaces 2022, 14, 35477-35484. doi: 10.1021/acsami.2c04789

  110. [110]

       Wang, J.; Zhang, S.; Wang, C.; Li, K.; Zha, Y.; Liu, M.; Zhang, H.; Shi, T. Ambient ammonia production via electrocatalytic nitrate reduction catalyzed by a flower-like CuCo₂O₄ electrocatalyst. Inorg. Chem. Front. 2022, 9, 2374-2378. doi: 10.1039/D1QI01656C

  111. [111]

       Zhu, T.; Chen, Q.; Liao, P.; Duan, W.; Liang, S.; Yan, Z.; Feng, C. Single-atom Cu catalysts for enhanced electrocatalytic nitrate reduction with significant alleviation of nitrite production. Small 2020, 16, 2004526. doi: 10.1002/smll.202004526

  112. [112]

       Chen, Y.; Zhao, Y.; Zhao, Z.; Liu, Y. Highly dispersed face-centered cubic copper-cobalt alloys constructed by ultrafast carbothermal shock for efficient electrocatalytic nitrate-to-ammonia conversion. Mater. Today Energy 2022, 29, 101112. doi: 10.1016/j.mtener.2022.101112

  113. [113]

       Zhu, H.; Dong, S.; Du, X.; Du, H.; Xia, J.; Liu, Q.; Luo, Y.; Guo, H.; Li, T. Defective CuO-rich CuFe₂O₄ nanofibers enable the efficient synergistic electrochemical reduction of nitrate to ammonia. Catal. Sci. Technol. 2022, 12, 4998-5002. doi: 10.1039/D2CY00910B

  114. [114]

       Chen, H.; Zhang, C.; Sheng, L.; Wang, M.; Fu, W.; Gao, S.; Zhang, Z.; Chen, S.; Si, R.; Wang, L.; Yang, B. Copper single-atom catalyst as a high-performance electrocatalyst for nitrate-ammonium conversion. J. Hazard. Mater. 2022, 434, 128892. doi: 10.1016/j.jhazmat.2022.128892

  115. [115]

       Wang, H.; Guo, Y.; Li, C.; Yu, H.; Deng, K.; Wang, Z.; Li, X.; Xu, Y.; Wang, L. Cu/CuO_x in-plane heterostructured nanosheet arrays with rich oxygen vacancies enhance nitrate electroreduction to ammonia. ACS Appl. Mater. Interfaces 2022, 14, 34761-34769. doi: 10.1021/acsami.2c08534

  116. [116]

       Xu, Y.; Wen, Y.; Ren, T.; Yu, H.; Deng, K.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Engineering the surface chemical microenvironment over CuO nanowire arrays by polyaniline modification for efficient ammonia electrosynthesis from nitrate. Appl. Catal. B Environ. 2023, 320, 121981. doi: 10.1016/j.apcatb.2022.121981

  117. [117]

       Hou, M.; Pu, Y.; Qi, W. -K.; Tang, Y.; Wan, P.; Yang, X. J.; Song, P.; Fisher, A. Enhanced electrocatalytic reduction of aqueous nitrate by modified copper catalyst through electrochemical deposition and annealing treatment. Chem. Eng. Commun. 2018, 205, 706-715. doi: 10.1080/00986445.2017.1413357

  118. [118]

       Cerrón-Calle, G. A.; Fajardo, A. S.; Sánchez-Sánchez, C. M.; Garcia-Segura, S. Highly reactive Cu-Pt bimetallic 3D-electrocatalyst for selective nitrate reduction to ammonia. Appl. Catal. B Environ. 2022, 302, 120844. doi: 10.1016/j.apcatb.2021.120844

  119. [119]

       Fang, L.; Wang, S.; Song, C.; Yang, X.; Li, Y.; Liu, H. Enhanced nitrate reduction reaction via efficient intermediate nitrite conversion on tunable Cu_xNi_y/NC electrocatalysts. J. Hazard. Mater. 2022, 421, 126628. doi: 10.1016/j.jhazmat.2021.126628

  120. [120]

       Wang, C.; Liu, Z.; Hu, T.; Li, J.; Dong, L.; Du, F.; Li, C.; Guo, C. Metasequoia-like nanocrystal of iron-doped copper for efficient electrocatalytic nitrate reduction into ammonia in neutral media. ChemSusChem 2021, 14, 1825-1829. doi: 10.1002/cssc.202100127

  121. [121]

       Yin, D.; Liu, Y.; Song, P.; Chen, P.; Liu, X.; Cai, L.; Zhang, L. In situ growth of copper/reduced graphene oxide on graphite surfaces for the electrocatalytic reduction of nitrate. Electrochim. Acta 2019, 324, 134846. doi: 10.1016/j.electacta.2019.134846

  122. [122]

       Daiyan, R.; Tran-Phu, T.; Kumar, P.; Iputera, K.; Tong, Z.; Leverett, J.; Khan, M. H. A.; Asghar Esmailpour, A.; Jalili, A.; Lim, M.; Tricoli, A.; Liu, R. -S.; Lu, X.; Lovell, E.; Amal, R. Nitrate reduction to ammonium: from CuO defect engineering to waste NO_x-to-NH₃ economic feasibility. Energy Environ. Sci. 2021, 14, 3588-3598. doi: 10.1039/D1EE00594D

  123. [123]

       Couto, A. B.; Santos, L. C. D.; Matsushima, J. T.; Baldan, M. R.; Ferreira, N. G. Hydrogen and oxygen plasma enhancement in the Cu electrodeposition and consolidation processes on BDD electrode applied to nitrate reduction. Appl. Surf. Sci. 2011, 257, 10141-10146. doi: 10.1016/j.apsusc.2011.07.006

  124. [124]

       Yu, J.; Kolln, A. F.; Jing, D.; Oh, J.; Liu, H.; Qi, Z.; Zhou, L.; Li, W.; Huang, W. Precisely controlled synthesis of hybrid intermetallic-metal nanoparticles for nitrate electroreduction. ACS Appl. Mater. Interfaces 2021, 13, 52073-52081. doi: 10.1021/acsami.1c09301

  125. [125]

       Cai, J.; Qin, S.; Akram, M. A.; Hou, X.; Jin, P.; Wang, F.; Zhu, B.; Li, X.; Feng, L. In situ reconstruction enhanced dual-site catalysis towards nitrate electroreduction to ammonia. J. Mater. Chem. A 2022, 10, 12669-12678. doi: 10.1039/D2TA01772E

  126. [126]

       He, W.; Zhang, J.; Dieckhofer, S.; Varhade, S.; Brix, A. C.; Lielpetere, A.; Seisel, S.; Junqueira, J. R. C.; Schuhmann, W. Splicing the active phases of copper/cobalt-based catalysts achieves high-rate tandem electroreduction of nitrate to ammonia. Nat. Commun. 2022, 13, 1129. doi: 10.1038/s41467-022-28728-4

  127. [127]

       Gao, Q.; Pillai, H. S.; Huang, Y.; Liu, S.; Mu, Q.; Han, X.; Yan, Z.; Zhou, H.; He, Q.; Xin, H.; Zhu, H. Breaking adsorption-energy scaling limitations of electrocatalytic nitrate reduction on intermetallic CuPd nanocubes by machine-learned insights. Nat. Commun. 2022, 13, 2338. doi: 10.1038/s41467-022-29926-w

  128. [128]

       Yang, J.; Qi, H.; Li, A.; Liu, X.; Yang, X.; Zhang, S.; Zhao, Q.; Jiang, Q.; Su, Y.; Zhang, L.; Li, J. F.; Tian, Z. Q.; Liu, W.; Wang, A.; Zhang, T. Potential-driven restructuring of Cu single atoms to nanoparticles for boosting the electrochemical reduction of nitrate to ammonia. J. Am. Chem. Soc. 2022, 144, 12062-12071. doi: 10.1021/jacs.2c02262

  129. [129]

       Jeon, T. H.; Wu, Z. -Y.; Chen, F. -Y.; Choi, W.; Alvarez, P. J. J.; Wang, H. Cobalt-copper nanoparticles on three-dimensional substrate for efficient ammonia synthesis via electrocatalytic nitrate reduction. J. Phys. Chem. C 2022, 126, 6982-6989. doi: 10.1021/acs.jpcc.1c10781

  130. [130]

       Wu, K.; Sun, C.; Wang, Z.; Song, Q.; Bai, X.; Yu, X.; Li, Q.; Wang, Z.; Zhang, H.; Zhang, J.; Tong, X.; Liang, Y.; Khosla, A.; Zhao, Z. Surface reconstruction on uniform Cu nanodisks boosted electrochemical nitrate reduction to ammonia. ACS Mater. Lett. 2022, 4, 650-656. doi: 10.1021/acsmaterialslett.2c00149

  131. [131]

       Zhang, Y.; Chen, X.; Wang, W.; Yin, L.; Crittenden, J. C. Electrocatalytic nitrate reduction to ammonia on defective Au1Cu (111) single-atom alloys. Appl. Catal. B Environ. 2022, 310, 121346. doi: 10.1016/j.apcatb.2022.121346

  132. [132]

       Chen, F. Y.; Wu, Z. Y.; Gupta, S.; Rivera, D. J.; Lambeets, S. V.; Pecaut, S.; Kim, J. Y. T.; Zhu, P.; Finfrock, Y. Z.; Meira, D. M.; King, G.; Gao, G.; Xu, W.; Cullen, D. A.; Zhou, H.; Han, Y.; Perea, D. E.; Muhich, C. L.; Wang, H. Efficient conversion of low-concentration nitrate sources into ammonia on a Ru-dispersed Cu nanowire electrocatalyst. Nat. Nanotechnol. 2022, 17, 759-767. doi: 10.1038/s41565-022-01121-4

  133. [133]

       Zhao, X.; Hu, G.; Tan, F.; Zhang, S.; Wang, X.; Hu, X.; Kuklin, A. V.; Baryshnikov, G. V.; Ågren, H.; Zhou, X.; Zhang, H. Copper confined in vesicle-like BCN cavities promotes electrochemical reduction of nitrate to ammonia in water. J. Mater. Chem. A 2021, 9, 23675-23686. doi: 10.1039/D1TA05718A

  134. [134]

       Zhao, X.; Li, X.; Zhang, H.; Chen, X.; Xu, J.; Yang, J.; Zhang, H.; Hu, G. Atomic-dispersed copper simultaneously achieve high-efficiency removal and high-value-added conversion to ammonia of nitrate in sewage. J. Hazard. Mater. 2022, 424 (Pt A), 127319.

  135. [135]

       Hu, Q.; Qin, Y.; Wang, X.; Zheng, H.; Gao, K.; Yang, H.; Zhang, P.; Shao, M.; He, C. Grain boundaries engineering of hollow copper nanoparticles enables highly efficient ammonia electrosynthesis from nitrate. CCS Chem. 2022, 4, 2053-2064. doi: 10.31635/ccschem.021.202101042

  136. [136]

       Geng, J.; Ji, S.; Xu, H.; Zhao, C.; Zhang, S.; Zhang, H. Electrochemical reduction of nitrate to ammonia in a fluidized electrocatalysis system with oxygen vacancy-rich CuO_x nanoparticles. Inorg. Chem. Front. 2021, 8, 5209-5213. doi: 10.1039/D1QI01062J

  137. [137]

       Song, Z.; Liu, Y.; Zhong, Y.; Guo, Q.; Zeng, J.; Geng, Z. Efficient electroreduction of nitrate into ammonia at ultralow concentrations via an enrichment effect. Adv. Mater. 2022, 34, 2204306. doi: 10.1002/adma.202204306

  138. [138]

       Hu, Q.; Qin, Y.; Wang, X.; Wang, Z.; Huang, X.; Zheng, H.; Gao, K.; Yang, H.; Zhang, P.; Shao, M.; He, C. Reaction intermediate-mediated electrocatalyst synthesis favors specified facet and defect exposure for efficient nitrate-ammonia conversion. Energy Environ. Sci. 2021, 14, 4989-4997. doi: 10.1039/D1EE01731D

  139. [139]

       Zhao, X.; Jia, X.; He, Y.; Zhang, H.; Zhou, X.; Zhang, H.; Zhang, S.; Dong, Y.; Hu, X.; Kuklin, A. V.; Baryshnikov, G. V.; Ågren, H.; Hu, G. Two-dimensional BCN matrix inlaid with single-atom-Cu driven electrochemical nitrate reduction reaction to achieve sustainable industrial-grade production of ammonia. Appl. Mater. Today 2021, 25, 101206. doi: 10.1016/j.apmt.2021.101206

  140. [140]

       Fu, X.; Zhao, X.; Hu, X.; He, K.; Yu, Y.; Li, T.; Tu, Q.; Qian, X.; Yue, Q.; Wasielewski, M. R.; Kang, Y. Alternative route for electrochemical ammonia synthesis by reduction of nitrate on copper nanosheets. Appl. Mater. Today 2020, 19, 100620. doi: 10.1016/j.apmt.2020.100620

  141. [141]

       Li, C.; Liu, S.; Xu, Y.; Ren, T.; Guo, Y.; Wang, Z.; Li, X.; Wang, L.; Wang, H. Controllable reconstruction of copper nanowires into nanotubes for efficient electrocatalytic nitrate conversion into ammonia. Nanoscale 2022, 14, 12332-12338. doi: 10.1039/D2NR03767J

  142. [142]

       Yao, F.; Jia, M.; Yang, Q.; Chen, F.; Zhong, Y.; Chen, S.; He, L.; Pi, Z.; Hou, K.; Wang, D.; Li, X. Highly selective electrochemical nitrate reduction using copper phosphide self-supported copper foam electrode: performance, mechanism, and application. Water Res. 2021, 193, 116881. doi: 10.1016/j.watres.2021.116881

  143. [143]

       Gao, Z.; Lai, Y.; Tao, Y.; Xiao, L.; Zhang, L.; Luo, F. Constructing well-defined and robust Th-MOF-supported single-site copper for production and storage of ammonia from electroreduction of nitrate. ACS Cent. Sci. 2021, 7, 1066-1072. doi: 10.1021/acscentsci.1c00370

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  Figure 1  Schematic illustration of sustainable ammonia synthesis via nitrate electrochemical reduction.

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  Figure 2  Schematic illustration of the advantages of Cu-based materials for nitrate reduction to ammonia.

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  Figure 3  Schematic presentation of possible mechanism for electrochemical nitrate reduction to ammonia.

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  Figure 4  (a) Transmission electron microscopy (TEM) image of Cu nanosheets. (b) High resolution TEM (HRTEM) image of Cu nanosheets basal plane. (c) NH₃ FE of various Cu catalysts at different applied potentials. Reproduced with permission from Ref.^[140] Copyright 2020, Elsevier. (d) Schematic illustration of the synthetic process, (e) scanning electron microscopy (SEM) image, (f) high-angle annular dark-field scanning TEM (HAADF-STEM) image, and (g) NH₃ yield rate and FE at different applied potentials for the dr-Cu NPs. Reproduced with permission from Ref.^[103] Copyright 2021, Royal Society of Chemistry.

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  Figure 5  (a) SEM images of Cu NDs. (b) Potential energy diagram of NO₃RR on Cu{200}, Cu{220}, and Cu{111} surfaces. (c) Energy profile of NO dissociation on Cu{200}, Cu{220}, and Cu{111} surfaces. Reproduced with permission from Ref.^[95] Copyright 2021, America Chemical Society. (d) Schematic illustration of the synthetic process of Cu nanotubes. (e) SEM image of CuO nanotubes, (f) TEM image of Cu nanotubes. (g) NH₃ yield rate and FE at different applied potentials for Cu nanotubes. Reproduced with permission from Ref.^[141] Copyright 2022, Royal Society of Chemistry.

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  Figure 6  (a) SEM image, (b) HAADF-STEM image, (c) schematic illustration of the electrocatalytic NO₃RR process, (d) NH₃ yield rate and FE at different applied potentials for the Rh@Cu catalyst. Reproduced with permission from Ref.^[91] Copyright 2022, WILEY-VCH. (e) Schematic illustration of the synthetic process of Ru-Cu NW. HAADF-STEM images of (f) Cu(OH)₂ NW, (g) Ru-CuO NW and (h) Ru-Cu NW. High-resolution HAADF-STEM images of (i) the Ru-CuNW surface structure and (j) its corresponding crystal structure and lattice spacing. (k) Energy-dispersive X-ray spectroscopy (EDS) mapping images of Ru-CuNW. (l) DFT-calculated minimum energy path for the NO₃RR. (m) Nitrate adsorption versus hydrogen evolution via an alkaline pathway on different catalytic surfaces. Reproduced with permission from Ref.^[132] Copyright 2022, Nature Publishing Group.

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  Figure 7  (a) UPS spectra and d-band center positions of pure Cu catalysts and the CuNi alloys. (b) Reaction free energies for different intermediates on a CuNi surface. (c) Hydrogenation reaction of ^*NH₂ on a Cu₃₀Ni₇₀ surface. (d) The volcano-type relationship between experimental overpotentials of NO₃RR at 5 mA cm^-2 in 10 mM KNO₃ and adsorption energies of ^*NO₃^- on all CuNi alloys. Reproduced with permission from Ref.^[55] Copyright 2020, American Chemical Society. (e) Schematic illustration of the synthetic process of PdCu/Cu₂O hybrids. (f) Low-magnification HAADF-STEM image and (g) atomic-resolution HAADF-STEM image of PdCu/Cu₂O hybrids. (h, i) Reaction free energies for different intermediate during different NO₃RR paths. Reproduced with permission from Ref.^[44] Copyright 2021, Elsevier B.V.

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  Figure 8  (a) Schematic illustration of the conversion process of Cu/Cu₂O NWAs from CuO NWAs. (b) The reaction energies of H₂ formation over Cu/Cu₂O NWAs and Cu NWAs. (c) Free energy diagram for NO₃RR over Cu NWAs and Cu/Cu₂O NWAs. Reproduced with permission from Ref.^[98] Copyright 2020, WILEY-VCH. (d) HRTEM image of Cu@Cu₂₊₁O NWs. (e) Schematic illustration showing the electrocatalytic nitrate reduction mechanism over the Cu@Cu₂₊₁O NWs. Reproduced with permission from Ref.^[100] Copyright 2021, Elsevier B.V.

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  Figure 9  (a) Schematic illustration of the preparation of a Cu/Co-based binary tandem catalyst. SEM images of the (b) ZIF-Co-R precursor, (c) CuCoSP_no and (d) CuCoSP on the Cu foil substrate. (e) A proposed reaction mechanism of CuCoSP tandem catalysis of NO₃RR at low overpotentials. Reproduced with permission from Ref.^[126] Copyright 2022, Nature Publishing Group. (f) TEM image and (g) HRTEM image of CuO@MnO₂ nanowires. (h) Schematic illustration showing schematic diagrams of the band structure of CuO and MnO₂, and the process of electrocatalytic nitrate reduction on the CuO@MnO₂/CF. Reproduced with permission from Ref.^[101] Copyright 2022, Royal Society of Chemistry.

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  Figure 10  (a, c) The adsorption energies of H (a) and NO₃^- (c) on the surface of the O-Cu-PTCDA molecular layer (103). (b, d) The adsorption energies of H (b) and NO₃^- (d) on the (111) plane of Cu. Reproduced with permission from Ref.^[47] Copyright 2020, Nature Publishing Group. (e) Preparation and evolution of the host-guest Cu catalyst through metal ion deposition and in situ clustering routes. Reproduced with permission from Ref.^[85] Copyright 2022, America Chemical Society.

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  Figure 11  (a) SEM image of CuPd-MOF. (b) TEM image of Cu/Pd/CuO_x multi-phase heterostructure. (c) Schematic illustration showing the electrocatalytic nitrate-to-ammonia process over the Cu/Pd/CuO_x. Reproduced with permission from Ref.^[99] Copyright 2022, Elsevier. (d) SEM image and (e) TME image of Cu@C catalyst. (f) Maximum FE NH₃ of Cu@C and Cu with concentrations of NO₃^- ranging from 5 to 100 × 10^-3 M. (g) Simulated concentrations and distributions of local NO₃^- on the surface of Cu encapsulated with porous carbon. Reproduced with permission from Ref.^[137] Copyright 2022, WILEY-VCH.

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  Figure 12  Prospects for developing NO₃RR technology for sustainable ammonia synthesis based on Cu-based materials.

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  Table 1.  Performance of Cu-based Electrocatalysts for the Reduction of Nitrate to Ammonia under Neutral and Acidic Conditions.

  electrocatalysts	electrolytes	NH3 yield rate	FE (%)	potentials	ref.
  CuPc@MXene	0.5 M Na2SO4, 50 mg L-1 NO3--N	0.72 mg h-1·cm-2	—	-1.06 V vs. RHE	[62]
  Cu@CuHHTP	0.5 M Na2SO4, 500 mg L-1 NO3-	1.84 mg h-1 cm-2	67.55	-0.90 V vs. RHE	[82]
  La2Cu0.8Co0.2O4	0.5 M Na2SO4, 50 mg L-1 nitrate	0.0699 mmol h-1 mg-1	—	-0.68 V vs. RHE	[83]
  Cu@C-800	0.1 M Na2SO4, 500 mg L-1 NO3-	51.7 ± 0.6 mmol h-1 g-1	78 ± 0.9	-0.9 V vs. RHE	[84]
  1-Cu'	0.5 M Na2SO4, 5 mM NaNO3	53.43 mgNH3 h-1 mgCu-1	85.5	-0.9 V vs. RHE	[85]
  TiO2 NTs/CuOx	0.5 M Na2SO4, 100 mg L-1 KNO3-N	264.4 mmol g-1 h-1	83.51	-0.85 V vs. RHE	[86]
  SCF	0.5 M Na2SO4, 100 mg L-1 KNO3-N	0.31 mmol h-1 cm-2	70	-1.3 V vs. RHE	[87]
  Ni1Cu-SAA	0.5 M K2SO4, 200 mg L-1 NO3--N	326.7 μmol h-1 cm-2	~100	-0.55 V vs. RHE	[88]
  plasma treated Cu2O	0.5 M Na2SO4, 50 mg L-1 NaNO3-N	0.083 mmol h-1 mg-1	89.5	-0.58 V vs. RHE	[89]
  CuNi/NC-51	0.05M Na2SO4, 50 mg L-1 NO3-	—	79.6	-1.0 V vs. Ag/AgCl	[90]
  Rh@Cu-0.6%	0.1 M Na2SO4, 0.1 M KNO3	—	93	-0.2 V vs. RHE	[91]
  Ag/Cu2O	0.5 M Na2SO4, 100 mg L-1 nitrate-N	—	96.45	-0.80 V vs. RHE	[92]
  island-like Cu	0.5 M Na2SO4, 50 mM KNO3	—	98.28	-0.8 V vs. RHE	[93]
  Cu52Al48-LDO-r	50 mM Na2SO4, 22.5 mg L-1 NO3--N	661.6 mg-N m-2 h-1	70.4	-1.10 V vs. Ag/AgCl	[94]
  Cu NDs	0.01 M NaNO3	500 μg h-1cm-2	97	-0.3 V vs. RHE	[95]
  THH Cu NCs	100 mM K2SO4, 100 mM KNO3	9.13 mmol h-1 cm-2	98.3	-0.90 V vs. RHE	[96]
  FOSP-Cu-0.1	0.5 M Na2SO4, 0.1 M KNO3	101.4 μmol h-1 cm-2	93.91	-0.266 V vs. RHE	[97]
  PdCu/Cu2O	0.5 M Na2SO4, 100 mg L-1 NO3--N	0.190 mmol h-1 cm-2	94.32	-0.80 V vs. RHE	[44]
  Cu/Cu2O NWAs	0.5 M Na2SO4, 200 mg L-1 NO3--N	0.2449 mmol h-1 cm-2	95.8	-0.85 V vs. RHE	[98]
  Cu/Pd/CuOx	0.5 M K2SO4, 50 mg L-1 NO3--N	1510.33 µg h-1 mg-1	84.04	-1.3 V vs. SCE	[99]
  Cu@Cu2+1O NWs	0.5 M K2SO4, 50 mg L-1 NO3--N	576.53 µg h-1 mg-1	87.07	-1.2 V vs. SCE	[100]
  CuO@MnO2/CF	0.5 M K2SO4, 200 mg L-1 KNO3-N	0.240 mmol h-1 cm-2	94.92	-1.3 V vs. SCE	[101]
  CuPd/CN	0.5 M K2SO4, 200 mg L-1 KNO3-N	0.0904 mmol h-1 mgcat.-1	96.16	-1.1 V vs. SCE	[102]
  dr-Cu NPs	0.5 M K2SO4, 50 mg L-1 KNO3-N	781.25 µg h-1 mg-1	85.47	-1.3 V vs. SCE	[103]
  CuPd aerogels	0.5 M K2SO4, 50 mg L-1 KNO3-N	784.37µg h-1 mg-1	90.02	-0.46 V vs. RHE	[104]
  Cu3Fe	0.1 M Na2SO4 and 100 mg L-1 NO3-	—	74.2	-0.7 V vs. RHE	[105]
  PPy-Cu-E	0.4 M K2SO4, 0.2 M KNO3	0.588 mmol mgcat h-1	91.95 ± 1	-0.61 V vs. RHE	[106]
  Pd-Cu2O CEO	0.5 M K2SO4, 50 mg L-1 KNO3-N	925.11 µg h-1 mgcat.-1	96.56	-1.3 V vs. SCE	[107]
  Mo/H-CuW	0.5 M Na2SO4, 0.05 M NO3-	5.84 mg h-1 mgcat.-1	94.60	-0.7 V vs. RHE	[108]
  Cu-Co3O4	0.1 M Na2SO4, 500 mg L-1 NO3-	36.71 mmol h-1 g-1	86.5	-0.6 V vs. RHE	[109]
  CuCo2O4	0.1 M Na2SO4, 1.0 mM KNO3	48.8 μg h-1 cm-2	34.2	-0.85 V, -0.45 V vs. RHE	[110]
  Cu-N-C	0.05 M Na2SO4, 50 mg L-1 NO3-	37 μg h-1 cm-2	—	-1.3 V vs. SCE	[111]
  Cu-PTCDA	0.1 M PBS, 500 mg L-1 NO3-	436 ± 85 μg h-1 cm-2	85.9	-0.4 V vs. RHE	[47]
  Cu2.5Co	0.5 M Na2SO4, 0.1 M KNO3	164.23 mmol h-1 cm-2	95.43	-0.25 V vs. RHE	[112]
  CuO@CuFe2O4	0.1 M PBS, 0.1 M KNO3	9.21 mg h-1 mgcat.-1	91.08	-1.0 V vs. RHE	[113]
  Cu-N-C	0.5 M Na2SO4, 50 mg L-1 NO3--N	9.23 mg h-1 mgcat.-1	—	-1.5 V vs. SCE	[114]
  Cu/CuOx/CF	0.5 M K2SO4, 200 mg L-1 NO3--N	0.21 mmol h-1 cm-2	93.58	-1.3 V vs. SCE	[115]
  CuO@PANI/CF	0.5 M K2SO4, 200 mg L-1 KNO3-N	0.213 mmol h-1 cm-2	93.88	-1.3 V vs. SCE	[116]
  arCu/Ni	0.1 M Na2SO4, 45 mg L-1 KNO3	—	76 ± 3	-1.4 V vs. SCE	[117]
  Cu-Pt bimetallic	12.5 mM Na2SO4, 30 mg L-1 NO3-	194.4 ± 3.6 mg gcat.-1 L-1	22	-0.4 V vs. RHE	[118]
  CuxNiy/NC	0.5 M Na2SO4, 50 mg L-1 NO3-	133 μg h-1 cm-2	~50	-0.67 V vs. SCE	[119]
  CuFe	0.1 M K2SO4, 2 mM KNO3	0.23 mmol h-1 cm-2	94.5	-0.7 V vs. RHE	[120]
  Cu/rGO/GP	0.02 M NaCl, 0.02 M NaNO3	203 μg h-1 cm-2	—	-1.4 V vs. SCE	[121]
  pCuO-10	0.05 M H2SO4, 0.05 M KNO3	334 mmol cm-2 h-1	—	-0.7 V vs. RHE	[122]
  Cu (100), Cu (111)	0.1 M HClO4, 2 mM NaNO3	—	—	—	[61]
  BDD/Cu film	0.1 mol L-1 Na2SO4, 0.1 mol L-1 KNO3 (pH=2)	—	—	—	[123]

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  Table 2.  Performance of Cu-Based Electrocatalysts for the Reduction of Nitrate to Ammonia under Alkaline Conditions.

  electrocatalysts	electrolytes	NH3 yield rate	FE (%)	potentials (vs. RHE)	ref.
  Cu6Sn5-Sn NPs	1 M KOH, 0.1 M KNO3	1 mmol mg-1 h-1	90	0 V	[124]
  Ru & Cu/Cu2O	1 M KOH, 0.1 M KNO3	—	95	0.1 V	[125]
  CuCoSP on Cu foil	0.1 M KOH, 0.1 M NO3-	1.17 mmol h-1 cm-2	90.6	-0.175	[126]
  Cu2O/Cu	1 M KOH, 250 mg L-1 NO3-	2.17 mg cm-2 h-1	84.36	-0.25	[26]
  Cu10Ce10	1 M KOH, 1400 mg L-1 nitrate-N	0.99 mmol h-1 cm-2	98.43	-0.23 V	[52]
  CuPd nanocubes	1 M KOH, 1 M KNO3	6.25 mol/gcat*h	92.5	-0.5 V, -0.6 V	[127]
  Cu-N-C SAC	0.1M KOH, 0.1 M KNO3	12.5 molNH3 gCu-1 h-1	84.7	-1.00 V	[128]
  Co0.5Cu0.5	1 M KOH, 50 mM KNO3	—	95	-0.03 V	[129]
  Cu nanodisks	0.1 M KOH, 10 mM KNO3	2.16 mg mgcat-1 h-1	81.1	-0.5 V	[130]
  Au1Cu SAAs	0.1 M KOH, 7.14 mM NO3-	555 μg h-1 cm-2	98.7	-0.2 V	[131]
  Ru-CuNW	1 M KOH, 2, 000 mg L-1 NO3-	—	96	0.04 V	[132]
  BCN@Cu	0.1 M KOH, 100 mM NO3-	435.6 mmol h-1 mgcat.-1	89.3	-0.5 V	[133]
  BCN@Cu/CNT	0.1 M KOH, 100 mM KNO3	172, 226.5 μg h-1 mgcat.-1	95.32	-0.6 V	[134]
  Cu@NF	1 M KOH, 200 mg L-1 nitrate-N	0.252 mmol h-1 cm-2	96.6	0.23 V	[60]
  oxide-derived Cu (OD-Cu)	1 M KOH, 100 mM KNO3	1.1 mmol h-1 cm-2	92	-0.15 V	[65]
  HSCu-AGB@C	1 M KOH, 0.1 M NO3-	487.8 mmol gcat-1 h-1	94.2	-0.2 V	[135]
  CuOX	0.1 M KOH, 50 mg L-1 NO3-	449.41 ± 12.18 μg h-1 mgcat-1	74.18 ± 2.27	-0.25 V	[136]
  Cu@C	1 M KOH, 1 × 10-3 M NO3-	469.5 μg h-1 cm-2	72.0	-0.9 V	[137]
  Cu-NBs-100	1 M KOH, 0.1 M NO3-	650 mmol gcat-1 h-1	95.3	-0.15 V	[138]
  BCN-Cu	0.1 M KOH, 100 mM NO3-	1900.07 μg h-1 cm-2	98.23	-0.5 V	[139]
  Cu nanosheets + 0.1 M KOH	0.1 M KOH, 10 mM KNO3	390.1 μg mgCu-1 h-1	99.7	-0.15 V	[140]
  Cu50Ni50 alloy	1 M KOH, 0.1 M KNO3		99	-0.15 V	[55]

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