Cu‐Based Catalytic Materials for Electrochemical Carbon Dioxide Reduction: Recent Advances and Perspectives

Electrocatalytic CO2 reduction reaction (CRR) is a promising way to convert carbon dioxide (CO2) into value‐added hydrocarbons to alleviate the ever‐increasing environmental problem and accelerate the realization of carbon cycling. Cost‐effective and stable electrocatalytic materials with low overpotential, superior selectivity, excellent activity and great stability are critically important to achieve such a target. Cu‐based electrocatalysts are promising candidates for electrochemical CRR due to their versatile abilities of converting CO2 into various products. This review analyzes and summarizes the current progress in utilization of Cu‐based catalytic materials for electrochemical CRR. Monometallic, bimetallic, trimetallic, and multimetallic Cu‐based electrocatalysts with variable elemental compositions and tunable morphologies, including Cu nanowires, Cu nanocubes (NCs), Cu porous structures, Cu‐based alloys, Cu‐oxide/hydrogen oxide, Cu single atoms, and 2D substrate‐supported Cu electrocatalysts for CRR, are surveyed. Substantial advances in overcoming the existing bottlenecks of eletrocatalysts and effectively improving CRR performance of Cu‐based electrocatalysts for future applications are systematically discussed. Challenges and perspectives of Cu‐based electrocatalytic materials for CRR are also offered, which may shed light on further development of Cu‐based electrocatalysts with superior performance. It is anticipated that this review will provide a valuable insight into the rational design and synthesis of highly efficient Cu‐based electrocatalysts for large‐scale CRR utilization.


Introduction
[3][4][5] With the anticipated fossil fuel consumption doubling by 2050, [6] it is critically vital to address those abovementioned issues urgently.A potential strategy to mitigate or avoid CO 2 emissions is to develop and deploy clean, carbon-neutral, sustainable energy sources to replace the currently widely used fossil fuels.However, clean and renewable energy sources (like solar, wind, tide) only account for a very small stake of the current energy consumption due to their drawbacks such as poor energy conversion efficiency, expensive transport cost and complicated storage technologies.Another strategy that has attracted tremendous attention from governments and scientists worldwide is to recycle the "waste" CO 2 via chemical carbon dioxide reduction reaction (CRR) into valueadded fuels and useful materials.
Currently, CO 2 reduction can be realized through various routes, including biochemical, [7] electrochemical, [8] photochemical, [9] radiochemical [10] and thermochemical reactions. [11]mong the technologies available for CRR, electrochemical converting CO 2 into carbon monoxide (CO), formic acid (HCOOH), hydrocarbons, oxygenates, and other high value-added products using electrical energy is particularly interesting since it can be carried out under ambient conditions at room temperature with tunable products, free of emissions, and readily available for large-scale applications. [12,13]Nevertheless, the CRR performance and the formed products highly rely on the applied electrocatalysts due to the sluggish dynamics of the electrochemical CRR, as the carbon atoms in CO 2 are fully oxidized and very stable because of the line structure of CO 2 molecules.
Electrocatalytic CO 2 reduction reaction (CRR) is a promising way to convert carbon dioxide (CO 2 ) into value-added hydrocarbons to alleviate the everincreasing environmental problem and accelerate the realization of carbon cycling.Cost-effective and stable electrocatalytic materials with low overpotential, superior selectivity, excellent activity and great stability are critically important to achieve such a target.Cu-based electrocatalysts are promising candidates for electrochemical CRR due to their versatile abilities of converting CO 2 into various products.This review analyzes and summarizes the current progress in utilization of Cu-based catalytic materials for electrochemical CRR.Monometallic, bimetallic, trimetallic, and multimetallic Cu-based electrocatalysts with variable elemental compositions and tunable morphologies, including Cu nanowires, Cu nanocubes (NCs), Cu porous structures, Cu-based alloys, Cu-oxide/hydrogen oxide, Cu single atoms, and 2D substrate-supported Cu electrocatalysts for CRR, are surveyed.Substantial advances in overcoming the existing bottlenecks of eletrocatalysts and effectively improving CRR performance of Cu-based electrocatalysts for future applications are systematically discussed.Challenges and perspectives of Cu-based electrocatalytic materials for CRR are also offered, which may shed light on further development of Cu-based electrocatalysts with superior performance.It is anticipated that this review will provide a valuable insight into the rational design and synthesis of highly efficient Cu-based electrocatalysts for large-scale CRR utilization.
Most of the currently utilized CRR electrocatalysts in laboratory are metal-based heterogeneous catalysts. [14]Though a large number of metals can be utilized as CRR catalysts, the high cost and inferior capacity to yield various multihydrocarbon products greatly handicap their large-scale synthesis and practical utilization in CRR.Based on the reduction products generated due to the application of catalysts, the current electrocatalysts for CRR can be classified into four groups: 1) Sn, Pb, Bi and In-based catalysts that mainly result in the formation of formic acid (HCOOH) or formate (HCOO À ); [15][16][17][18] 2) Au, Ag and Znbased catalytic materials that lead to the formation of carbon monoxide (CO); [19][20][21] 3) Pt, Ti, Ni and Fe-based electrocatalysts that suppress CRR and largely favor hydrogen evolution reaction (HER); and 4) Cu and Cu-based compounds that result in CO and different hydrocarbons.Therefore, it is of both fundamental importance and practical desirability to rationally design and develop cost-effective, high-performance CRR electrocatalysts with good product selectivity.
[24] Although Cu-based catalytic materials have been extensively investigated as efficient CRR electrocatalysts, the sluggish kinetic dynamics, large overpotential, poor selectivity, and the coexisting side reactions (e.g., HER) during the CRR process inevitably handicap their utilization to achieve solo target product with high selectivity in large scale.Therefore, tremendous effort has been devoted to rationally tailoring the microstructures of Cu-based electrocatalysts in atomic scale to enhance their CRR performance. [25]his review summarizes the major advances and achievement of Cu-based heterogeneous electrocatalysts involved in CRR.Different strategies that remarkably improve CRR activities are analyzed to explore the key parameters in the development of advanced electrocatalysts for CRR.The potentially tunable chemical compositions of electrocatalysts, as well as controllable morphologies and topologies of the Cu-based catalysts, which can not only alleviate one or more of the existing drawbacks of the Cu-based electrocatalysts, but also improve the CRR performance, are discussed.Following the brief exploration of the reaction mechanisms of electrochemical CRR, Cu-based mono-, bi-, and trimetallic electrocatalysts that can overcome the typical problems of existing eletrocatalysts for CRR, including the relatively larger overpotential than the equilibrium thermodynamic potential, the inferior selectivity and the limited stabilities for long time under higher current density are highlighted.The outlooks and perspectives of future electrocatalysts with enhanced CRR performance to meet the potential large-scale demand are also provided.

Mechanisms of CRR
CRR usually occurs on the electrode surface where a heterogeneous catalyst is utilized.This process normally includes the following three steps: 1) CO 2 molecule adsorbed on the surface of the catalysts is dissolved and the molecular structure of CO 2 is rearranged to form a curved structure; 2) the rearranged molecule couples with proton or transfered electrons under applied potential to break C─O bonds and form C─H bonds; and 3) the as-formed product desorbs from the active sites of the catalyst. [26]As CRR is a multistep process, the reaction products are highly dependent on the applied potential and the electrocatalysts.Depending on the applied potential, CRR can proceed via the 2e, 4e, 6e or 8e process to form different final products.
As summarized in Table 1, the equilibrium potentials of CO 2 reduction are largely comparable to the potential of HER, which matches well with the experimental observations where H 2 is the main byproduct almost in all the CRR processes with different products.The slight difference in equilibrium potential for various CRR products leads to low selectivity, which results in the simultaneous occurrence of more than two parallel reactions and the formation of more than one product.Moreover, due to the polarization effect in the liquid system, the actual CRR is usually carried out at more negative potential than the thermodynamic equilibrium potential. [27]The typical CRR pathways for the formation of different CRR products are proposed and presented in Figure 1. [28]The products formed from CRR include but are not limited to carbon monoxide (CO), formic acid (HCOOH) or formate (HCOO À ), oxalic acid (H 2 C 2 O 4 ) or oxalate (C 2 O 4 2À in basic solution), formaldehyde (CH 2 O), methanol (CH 3 OH), methane (CH 4 ), ethylene (CH 2 CH 2 ), and ethanol (CH 3 CH 2 OH). [29]t is well known that the surface structures, morphologies, and particle sizes of heterogeneous electrocatalysts can affect the CRR reaction mechanism via regulating the chemical adsorption models between CO 2 molecules and the unsaturated surface atoms and tune the followed reaction processes between the activated CO 2 , electrons (e À ), and protons (H þ ) on the surface of the catalysts, [30] which can remarkably change the CRR performance of the electrocatalysts.Unsurprisingly, previous research has discovered that Cu-based electrocatalysts in CRR highly rely on morphologies including the particle sizes and the surfaces due to the unique coordinate number of the surface Cu atoms and the density of the surface electronic states, which can largely influence the adsorption and activation of CO 2 molecules as well as the formed intermediates including *CO, *COOH, and COOH*, leading to variable reaction mechanisms and versatile final products. [31]It is believed that Cu catalyst is distinct from other metals since it has an intermediate binding strength to the formed CRR intermediate *CO, which can easily achieve a balance between the barrier for activation of CO 2 and the hydrogenation of *CO in CRR. [32]ble 1.Standard potential for CO 2 reduction in liquid solution (V versus standard hydrogen electrode (SHE)) at 1.0 atm and 25 °C.
To deeply understand the relationships between the chemical compositions, surface facets, morphologies, particle sizes, and their CRR performance, great attention has been paid to the rational design and synthesis of Cu-based electrocatalysts to generate targeted CCR products with superior selectivity and activity under relatively low overpotential.Cu-based electrocatalysts with tunable chemical compositions, various morphologies and controllable sizes can be readily synthesized using a variety of methods including thermal oxidation, solvothermal synthesis, electrodeposition and nitrogen doping. [33]n addition, besides the preparation method of the electrocatalysts, the applied potential, the cation in the electrolyte, the local pH value of the medium, and even the shape of the utilized reactor can impact the CRR reaction mechanisms and the catalytic behaviors. [34]For instance, Xie et al. revealed that the local pH value can effectively decrease the formation energy barrier of COCOH* on the surface of the Cu nanosheet, leading to a Faraday efficiency (FE) of 85.1% for the formation of C 2þ products. [35]

Monometallic Cu-Based Electrocatalysts for CRR
Since suitable electrocatalysts can effectively lower the energetic barriers for CRR by stabilizing reaction intermediates and transition states in the multistep reaction process, it is therefore important to regulate the Cu-based electrocatalyst to generate the desired reduction products, which can be readily achieved by controlling the specific morphology and electronic characteristics of the catalysts.Depending on the chemical compositions, Cu-based electrocatalysts used in CRR can be briefly classified into monometallic, bimetallic, trimetallic, and multimetallic Cu-based catalytic materials.
In this section, monometallic Cu-based electrocatalysts with various morphologies, including Cu nanowires, Cu nanocubes and Cu nanoparticles, Cu porous structures, dispersed 2D Cu materials, Cu single atoms, as well as Cu oxide and copper hydroxide, as catalysts in electrochemical CRR will be discussed.

Cu Nanowires
Cu nanowires (NWs) are effective electrocatalysts toward CRR.Featured with tunable length, diameter and surface area, as well as controllable density of uncoordinated atoms, Cu nanowires are capable of adsorption, activation, and converting CO 2 into various high value-added products.
[38][39][40] Hoshi et al. reported that the introduction of Cu (111) on the single Cu electrode leads to the Cu(s)-[n (100)x (111)] orientations with more active sites, which can effectively accelerate C 2 H 4 formation and suppress CH 4 generation. [41]Huang et al. also found that a Cu nanowire with rich surface is more thermodynamically favored than Cu (100), and C2 products are the main products due to the suppression of C1 products and HER, which lead to an FE of 77.40 AE 3.16% for C 2 H 4 formation in H cells up to %200 h running (Figure 2a). [42]Moreover, Smith et al. made use of a two-step method to synthesize Cu nanowire arrays via the electrochemical reduction of CuO nanowire arrays.Various Proposed CRR pathways for C1-C3 products on Cu surface.Reproduced with permission. [28]Copyright 2019, Amrican Chemical Society.
products including CO, HCOOH, C 2 H 4 , C 2 H 6 , C 2 H 5 OH, and npropanol can be formed by tuning the length and the density of the Cu nanowire and controlling the local pH of the reaction medium (Figure 2b). [43]rain boundaries in nanowires are effective to upgrade CRR performance.Huang et al. reported that Cu nanowires with grain boundaries are able to convert CO 2 into syngas (mixture containing CO and H 2 ) with a wide range of compounds under different operating potentials. [44]Luo et al. further confirmed that grain boundaries on the surface of the Cu nanowires can promote the FE for C2 products in CRR to 86% at À0.1 V with superior long stability throughout 28 h (Figure 2c,d). [45]Modification of Cu nanowires with amine groups can effectively tune the abilities of the electrocatalyst to capture and deliver protons to react with CO* intermediates, while phenolic hydroxyl groups can stabilize the CO* intermediates. [46]Wang et al. revealed that the amino acid modification of Cu electrode can stabilize the key intermediate CHO* on the surface of Cu nanowires, which finally leads to a remarkable enhancement in hydrocarbon generation. [47]cceleration of conversion efficiency of the reaction intermediate product CO can also improve the CRR activities.Sun's group discovered that the Cu nanowires with a diameter of about 25 nm exhibited better C2 (C 2 H 4 þ C 2 H 6 ) FE of 60% than that of the Cu nanowires with a diameter of about 50 nm, which clearly demonstrated that the diameter of Cu nanowires is a key factor for CO conversion. [48]Han et al also reported that the modified Cu nanowire with a high index facet of (311) via a simple squarewave potential treatment could adsorb and activate CO 2 , promote the stabilization of intermediate *COCOH, which finally led to an increment in FE up to nearly 60% for C 2 H 4 and suppresses hydrogen evolution to lower than 20%. [49]n addition, Cu nanowires with coordinately unsaturated (110) surface facets synthesized via reducing the pregrown CuO nanowires are capable of converting the primary CRR product CO into C 2 H 5 OH with an FE of 50% and an overpotential less than 0.5 V. [50] Zhang et al. developed a superaerophilic Cu electrode via the modification of Cu nanowires with polytetrafluoroethylene (PTFE) as a surface modifier, and the modified Cu nanowires exhibited optimized CRR activity and selectivity for CO and HCOOH under variable overpotentials.The enhancement in activity was ascribed to the accelerated gas and ionic transfer speed that inhibit the accumulative damage on the surface of the Cu NWs. [51]The electrocatalytic CRR performance (including main products, the FE and the overpotential) of different Cu nanowires in the reported literature are summarized in Table 2.
[54][55][56] Buonsanti et al. discovered that Cu NCs with a cube size of 44 nm exhibit the highest C 2 H 4 selectivity of 80% and FE of 41% among the NCs with different cube sizes of 24, 44, and 63 nm (Figure 3a). [57]Wang's group reported that Cu NCs with a length of %70 nm and (111) facets exhibited an FE of 60% for C 2 H 4 and the Cu NCs exhibited substantially enhanced catalytic activity and selectivity for CO 2 reduction compared to Cu nanospheres with similar particle sizes (Figure 3b). [58]Clémence Corminboeuf 's group claimed that the possible synergy effect between the enclosed dual facets of the Cu NCs was the dominant CRR mechanism instead of the facet selectivity observed in the NCs with a single facet (Figure 3c). [59]esides Cu NCs, Cu nanocrystals with high-index facets on the surface usually display superior CRR activities.This can be attributed to the synergistic effect between the different high-index facets of Cu catalysts. [60]Gong et al. reported Cu (100)-rich films produced via the deposition-etch-bombardment method without using the electrode assembly process, which benefited from the enhanced contact between the catalytic film and the substrate, exhibiting an FE of 86.5% for C 2þ products. [61]uang Cong et al. discovered that increasing the high-index facets of Cu catalysts led to a declined H 2 evolution reaction performance and inhibited the formation rate of C 1 products. [62]Liang Liang et al. found that Au-truncated tetragonal nanoprisms (Au DTPs) enclosed by 12 high-index facets-supported Cu surface yielded a CH 4 /CO ratio of 10:1 compared with a ratio of 1:1 for 7 nm Au@Cu nanoparticles (NPs). [63]Tang et al. demonstrated that entangled Cu nanowires with hierarchical pores in the presence of I À exhibited a C 2 FE of 80%, and the enhanced CRR activities could be derived from CO intermediate enrichment inside the hierarchical pores. [64]Hui Li et al. created a Cu nanocrystal with a highly exposed facet ratio of (100)/(111) via facet-selective atomic layer deposition technique to selectively cover the (111) surface of Cu with ultrathin Al 2 O 3 , resulting in a 22 times enhanced FE ratio of C 2 H 4 /CH 4 and a maximum FE of 60.4% for C 2 H 4. [65] Raffaella Buonsanti et al. discovered that Cu cubes are highly selective toward C 2 H 4 and Cu octahedra prefers to yield CH 4, while Cu spheres are of no selectivity to any specific products. [66]Later, they confirmed that accurate design and precise control over the morphologies, including the sizes and shapes, was the key to understand the relationship between the structure and the performance to govern the reduction products of CRR. [67]oreover, electrolyte flooding can also benefit the CRR performance.Seger et al. observed that Cu NCs supported gas diffusion electrodes (GDEs) exhibited higher FE for C 2 H 4 at elevated total current densities than the benchmark electrodes. [68]Zhang et al. discovered that Cu nanocrystal was unstable during the CRR process, which inevitably led to declined selectivity for hydrocarbons; however, the inferior stability of Cu NC catalysts in CRR could be improved by loading it on nitrogen-doped 2D graphene via the interaction between the Cu atom and the pyridinic nitrogen on the 2D graphene surface (Figure 3d). [69]n addition, Wang et al. discovered that the local pH on the electrode played a key role in altering chemical dynamics and molecular transport under the reaction conditions.High local pH favored the formation of multicarbon products but with limited transport of CO 2 molecules at large current densities. [70]The main products, the FE and the overpotential of monometallic Cu nanocubes as electrocatalysts toward CRR are also summarized in Table 2.

Cu Nanofoams and Nanoporous Cu Structures
Porous Cu structures such as Cu nanofoams possessing surface roughness and hierarchical porosity can readily confine or host reactant species in the pore channels and lead to the formation of CH 4 , C 2 H 4 , C 2 H 5 OH and (C 3 H 6 ) n , while the smooth Cu electrode is capable of producing HCOOH as the dominant product (Figure 4a). [71]The construction and reconstruction during the preparation process of porous Cu structures can introduce defects, special surface facets and grain boundaries into the resulting electrocatalysts, which may lead to alternative CRR mechanisms and changed product selectivity.Peng et al. revealed that due to the exposed specific crystalline orientations, Cu electrodes with nanoporous structures in CRR could result in less than 1% of CH 4 formation while a high level up to 35% of C 2 H 4 formation. [72]An annealed Cu foam with the reconstructed surface via air oxidation and electroreduction procedures exhibited a geometric current density of 9.4 mA cm À2 in CRR and an FE of 39% for CO and 23% for HCOOH at À0.45 V, which is superior to the pristine Cu foam (Figure 4b). [73]Yeo et al. reported that Cu mesocrystals prepared via the in situ reduction of a thin CuCl film demonstrated 18 times higher C 2 H 4 formation than that of CH 4 with an FE of 81% for total carbonaceous products.CO adsorption test suggested the remarkable C 2 H 4 selectivity was ascribed to the greater propensity of CO adsorption on Cu mesocrystals than on other types of Cu surfaces (Figure 4c). [74]ose Amal et al. developed a unique heterostructured Cu sandwich electrode via a simple two-pot treatment of commercially available Cu.Such Cu sandwich electrode in CRR yielded C 2 H 5 OH as the dominant product with FE of more than 50% at an overpotential of merely À0.3 V (versus RHE).The superior electrocatalytic activities could be ascribed to the greater exposure of Cu þ /Cu 2þ and a higher level of oxygen vacancies, while the long-term stabilities could be attributed to the sandwich heterostructures (Figure 4d,e). [75]Gewirth et al. fabricated a high-surface-area Cu film via electrodeposition of 3,5-diamino-1,2,4-triazole (DAT) on the surface of the Cu film.By tuning the pH and deposition current density, the CRR activities including the varied products and FE could be optimized.FE of 40% for C 2 H 4 % and 20% for C 2 H 5 OH at À0.5 V (versus RHE) could be readily achieved. [76]

Cu-MOFs for CRR
[79] Zhang et al. revealed that by introducing Cu 3 (BTC) 2 (Cu-MOF) into a carbon paper-based GDE, the CRR conversion efficiency and selectivity were all increased (Figure 5a). [80]Chen et al. decorated the classic paddlewheel Cu-coordinated HKUST-1 catalysts with zinc via the atomic layer infiltration (ALI) technique to form Zn-O-Zn sites linked to the neighboring Cu atoms in HKUST-1.Benefiting from the formed Zn-O-Zn structures, the adsorption enthalpy of CO 2 and the binding energy of COOH* intermediate could be well regulated, leading to an increased FE of CO from 20%-30% for pristine HKUST-1 to 70-80% for Zn-modified Cu-HKUST-1 (Figure 5b). [81]upp et al. introduced Cu NPs into a zirconium-MOF (NU-1000) via solvothermal deposition into MOF (SIM), followed by electrochemical reduction of Cu 2þ to metallic Cu.The resultant Cu-SIM exhibited a promising electrocatalytic activity for CO 2 reduction in an aqueous electrolyte, achieving FE of 31% for HCOOH at an overpotential of 0.82 V. [82] Making use of simulation calculation, Liu et al. discovered that embedded Cu nanocrystals in the oxidized matrix model showed increased CRR activity and selectivity via accelerating the kinetic dynamics of CO 2 activation and CO dimerization.Further investigation revealed that the acceleration of CRR on these Cu-embedded in the oxidized matrix could be derived from the synergy effect  [57] Copyright 2016, Wiley-VCH.b) Faradaic efficiency values measured for Cu NCs using 1 mol L À1 of KOH as the catholyte.Reproduced with permission. [58]Copyright 2019, American Chemical Society.c) 3D atomic model of a Cu NC.Reproduced with permission. [59]Copyright 2019, American Chemical Society.d) Diagram of the procedures utilized to increase the stability of Cu NCs.Reproduced with permission. [69]Copyright 2020, American Chemical Society.
between Cu þ on the surface and Cu 0 in the oxidized matrix model (Figure 6). [83]Wang et al. revealed that the modification of Cu surface with MOF layers could bring in negative-charged μ 3 -O and interact with the CO-covered Cu surface, leading to two times increased FE for CH 4 as compared to the bare Cu foil. [84]esides MOFs, covalent-organic frameworks (COFs) constructed via covalent bonds with tunable pore sizes and 2D structures were also explored as electrocatalysts in CRR.Chen et al. showed that a 2D Cu-metal-phthalocyanine-based COF with isolated active sites and high electron density could yield FE of 90.3% for CH 3 COOH at À0.8 V, higher than the FE of other catalysts with isolated active sites.The enhancement could be attributed to the upgraded C-C coupling rate of *CH 3 with CO 2 to produce acetate (Figure 7a). [85]Moreover, a strategy involved in a porphyrin-based COF nanosheet anchored on Cu-dispersed carbon nanotubes was developed by Zhang et al.The resulting electrocatalyst toward CRR yielded an FE of 71.2% for CH 4 in 1.0 M KOH in a flow cell (Figure 7b). [86]In addition, Lan et al. reported that COF-366-Cu, a porphyrin-based COF Cu compound, exhibited FE of 43% for CH 4 at an overpotential of 0.9 V. [87] Later, the same group developed another Cu-Tph-COF-Dct nanosheet via exfoliation and modification of bulk COFs using the immobilized functionalizing exfoliation agent.The obtained electrocatalyst in CRR yielded FE of 80% for CH 4 and the enhanced CRR activity was ascribed to the strengthened CO 2 absorption and activation abilities. [88]

Cu Nanoclusters for CRR
Nanocluster is an important type of nanostructure with lowcoordinated atoms and it bridges links between atoms and NPs.Liu et al. discovered that the CRR activities of Cu 8 clusters were highly dependent on their structures, and the Cu 8 cluster with a tetrahedron shape exhibited an FE of 92% for HCOOH, Reproduced with permission. [71]opyright 2014, American Chemical Society.b) FEs for H 2 , CO, and HCOOH for CO 2 reduction electrolysis over pristine Cu foam and annealed Cu foam electrodes at À0.45 V versus RHE in CO 2 -saturated 0.1 M KHCO 3 .Reproduced with permission. [73]Copyright 2016 Elsevier.c) FE ratios of ethylene to methane on different electrodes from À1.2 to À1.5 V (vs.RHE).Reproduced with permission. [74]Copyright 2015, The Royal Society of Chemistry.d) SEM images of Cu sandwich.Reproduced with permission. [75]Copyright 2018, John Wiley and Sons.e) FE for formate, EtOH, and MeOH of Cu sandwich electrode.
which was higher than the Cu 8 cluster with a cube shape.The enhancement in CRR performance can be ascribed to the superior abilities to suppress HER and lower adsorption energies of HCOO* (Figure 8a-f ). [89]A carbon-supported Cu cluster prepared by Xu et al. can achieve an FE of 91% for CH 3 CH 2 OH at an overpotential of 0.7 V. Further investigation confirmed that the FE of the Cu cluster decreased quickly as it was reassembled into larger particles (Figure 8g). [90]Du  [80] Copyright 2018, American Chemical Society.b) Schematic illustration of the synthesis process for HKUST-1-nC-Zn.Reproduced with permission. [81]Copyright 2022, American Chemical Society.and CH 3 OH can be well regulated. [91]A Cu cluster prepared by He et al. exhibited FE of 81.7% for CH 4 .The superior activities can be ascribed to two main reasons.One is the enhanced adsorption energy for the intermediate *H and *CO and the other is the optimized electronic structure of the Cu clusters. [92]ang et al. used hollow mesoporous carbon spheres to confine Cu clusters.The produced material was utilized as electrocatalysts in CRR and it achieved a selectivity of 88.7% for C2 products, due to the accelerated conversion of *CO to *CHO and the C─C coupling by the confined structure. [93]The electrocatalytic CRR performance (including main products, the FE, and the overpotential) of different porous Cu structures in the reported literature is presented in Table 3.

Supported 2D Cu-Based Materials
2D materials usually possess a higher surface-to-volume ratio, more low-coordination atoms, and faster electronic transfer speed than bulk materials with other morphologies and grain boundaries.These unique characteristics of 2D nanostructures possess a large number of vacancy defects and numerous disorders formed during synthesis, which can promote their ability to adsorb, activate and react with CO 2 molecules, and consequently enhance the CRR performance. [94,95]Moreover, the smooth surface of 2D materials makes it a promising substrate for the introduction of a trace level of metal or nonmetal NPs to dramatically alter their electronic state and therefore their CRR performance. [96]revious research has demonstrated that carbon materialssupported Cu NPs display improved FE in CRR, probably due  [85] Copyright 2016, Wiley-VCH.b) Schematic diagram of the synthetic process of the MWCNT-Por-COF-M (M: Co, Ni, Fe, Cu) composite materials.Reproduced with permission. [86]Copyright 2022, Elsevier.
to the increased fraction of undercoordinated atoms at the corners and edges, and the defects of support. [97]A pyridinic-N graphene (p-NG)-supported 7 nm Cu NPs developed by Sun.et al. exhibited FE of 19% for C 2 H 4 and 79% for hydrocarbon, as the p-NG functions as a CO 2 and proton absorber, accelerating the hydrogenation and C-C coupling on Cu atoms. [98]y tuning the grain boundaries between Cu/Cu x O and the reduced graphene, Wen et al. successfully synthesized reduced graphene-supported Cu/Cu x O nanohybrids as electrocatalysts in CRR and it could generate CO at an overpotential of 0.19 V. [99] Besides the reduced graphene, MoS 2 is another ideal platform to support Cu NP catalysts, which can promote CRR performance and increase the formation of CH 4 up to 7-fold in comparison with bare MoS 2 , due to the reduced transfer resistance and higher active surface area. [100]

Cu Single Atoms
Since the first report of single-atom catalysts (SACs) by Zhang  et al. in 2011, it has attracted tremendous attention due to the relatively high utilization efficiency of active atoms together with reduced material costs compared with bulk materials. [101]lectrocatalytic SACs with reduced particle sizes that can effectively enhance the catalytical performance of the materials have been widely explored and the range of available materials involved in catalytic reactions has been expanded.A two-step method to prepare Cu SACs from copper (II) acetate monohydrate and 1,10-phenanthroline monohydrate was reported by Zhang et al. and the resulting Cu SACs showed an FE of merely 15% for CO (Figure 9a). [102]Zheng et al. demonstrated that Cu SACs dispersed at single-atomic level on CeO 2 nanorods was a direct association of three oxygen vacancies with each Cu atom on CeO 2 surface.Such an Cu SACs/CeO 2 electrocatalyst displayed an FE of 58% for CH 4 (Figure 9b-d). [103]Moreover, Li et al. discovered that Lewis acid sites in metal oxides could promote CRR performance, achieving FE up to 62% for CH 4 via effectively regulating the electronic structure of Cu atoms to optimize the absorption of reaction intermediates. [104]ifferent from the utilization of Cu SACs as active sites, Edward H. Sargent et al. discovered that Cu-supported Fe single atoms could act as highly efficient electrocatalysts to realize hydrogenation of CO 2 and exhibited FE of 64% for CH 4 . [105]e et al. demonstrated a large-scale synthesis of Cu SACs-decorated through-hole carbon nanofibers.The unique nanostructure enables Cu atoms with a relatively higher binding energy for *CO intermediate, leading to further reaction of CH 4 with FE up to 44%. [106]A similar work on nitrogen-doped graphene Reproduced with permission. [89]Copyright 2022, Wiley-VCH; b) Cu 8 -2; and c) Cu 8 -3.The coordination modes of the thiocarboxylate group in d) Cu 8 -1; [89] e) Cu 8 -2; and Cu 8 -3, and the thiolate groups in f ) Cu 8 -2 and Cu 8 -3.Color codes: Cu (brown), S (yellow), C (gray), O (red), N (blue).Hydrogen atoms are omitted for clarity.g) Step-by-step preparation of the carbon-supported Cu SA catalyst using an amalgamated Cu-Li method.Reproduced with permission. [90]Copyright 2020, Spring Nature.matrix-supported Cu single atoms (Cu-N 4 -NC) was reported by Qiao et al.Benefited from the optimized CO 2 activation due to the Cu-N 4 active site and water dissociation facilitated by graphene, the Cu-N 4 -NC SACs exhibited FE of 80.6% for CO. [107] Another similar research further confirmed that the Cu-N 4 active sites could effectively upgrade the selectivity and efficiency of CO formation. [108]eng et al. discovered that the hydrogen bond could promote the binding strength between the CRR reaction intermediates and Cu single atoms decorated on carbon materials, resulting in an increase in FE up to 98% for CO. [109] The preparation of single Cu atoms supported on MXene (Ti 3 C 2 T x ) nanosheets was developed by Sun et al.The fabricated Cu-SAC/Ti 3 C 2 T x exhibited enhanced CRR performance and achieved selectivity of 98% for multicarbon products and FE of 71% for C 2 H 4 at an overpotential of 0.7 V (vs.RHE), which was ascribed to the highly dispersed Cu-O 3 sites that favored C─C coupling of carbon monoxide to generate intermediate *CO-CHO. [110]ifferent from most of the current CRRs, Zheng et al. reported the use of nitrogen-coordinated single-atomic Cu sites doped on carbon framework as an efficient electrocatalyst toward electrocarboxylation reaction, which could readily convert CO 2 and styrene into phenylsuccinic acid with FE of 92%. [111]6.Cu Oxide and Cu Hydroxide Oxidation and/or hydroxidation of the surface of Cu spices were proved to be a feasible process to moderate CRR performance of Cu-based electrocatalysts.[112] For instance, Broekmann et al. revealed that the coexistence of Cu and Cu oxide on the activated Cu mesh could alter CRR mechanism and avoid the accumulation of poisoning surface species; consequently, it led to FE of 13% for C 2 H 5 OH and FE of 11.8% for n-PrOH (Figure 10a).[113] It was also reported by Hwang et al. that the presence of Cu(OH) 2 on the surface of Cu electrode could effectively regulate the electrochemical reduction environment in CRR, which was critical in the formation of C 2 H 4 as CRR product.[114] The formation of Cu 2 O species on Cu-based electrocatalyst is particularly interesting and widely explored.For example, Figure 9. a) Diagram of two-step method to prepare SACs.Reproduced with permission. [102]Copyright 2019, Spring Nature.b) TEM images of CeO 2 nanorods.Reproduced with permission.[103] Copyright 2018, American Chemical Society.c) HRTEM and d) high-angle annular dark field (HAADF)scanning transmission electron microscopy (STEM) images of Cu-CeO 2 -4% nanorods.
Kanan et al. prepared thin Cu 2 O layers by annealing of Cu electrode at 130 °C, which led to an increased FE for CO and HCOOH at an exceptionally low overpotential of less than 0.4 V with a geometric current density higher than 1 mA cm À2 . [115]Moreover, Cu 2 O NPs decorated with hexagonal boron nitride (h-BN) nanosheets could effectively stabilize Cu þ via the electronic interactions between BN and Cu þ , leading to the fact that the CRR product ratio of C 2 H 4 to CO was 1.6 times higher than that of bare Cu 2 O. [116] Zeng et al. developed a series of multishell Cu 2 O hollow structures with tunable shell numbers as catalysts for CRR and the resulting structures exhibited a maximum FE of 77% with a conversion rate of 513.7 mA cm À2 for C 2þ products.Wu et al. also reported the synthesis of a series of Cu 2 O NCs decorated with imidazolyl groups connected to the surface of ionic liquid-functionalized graphite sheets as catalyst for CRR.When the concentration of the Cu precursor was tuned to 100 mmol L À1 , a maximum FE of C 2 H 4 up to 31.1% was recorded, which was ascribed to a larger electrochemically active surface area. [117]merous efforts were devoted to improving the C 2 H 4 selectivity in CRR products for CuO-based electrocatalyst.Kang et al. demonstrated that amino groups-modified carbon dotsdecorated Cu/CuO electrodes could enhance the selectivity of C 2 H 4 in CRR by 1.2 times compared with pristine Cu/CuO electrodes.It was believed that the reduced transfer resistance between Cu/CuO particles and the increased CO 2 adsorption capacities by NH 2 -functionized catalysts contributed to the improved C 2 H 4 selectivity.(Figure 10b-d). [118]Sn-decorated Cu oxide nanowire was reported to exhibit FE of 90% and current density of up to 4.5 mA cm À2 for CO at overpotential of 690 mV. [119]Moreover, Sinton et al. created a Cu oxidederived catalyst with enhanced C 2 H 4 partial current of 35.6 mA cm À2 , which is a 3.4-fold increase over the flat comparison.Further investigation suggests the enhancement can be ascribed to the coupled effect of the applied potential on current density and the local pH. [120]Furthermore, tuning the interface of Cu oxide can also upgrade the CRR activities.Sun et al. reported that H f O 2 -decorated Cu oxide catalyst exhibited FE of 62.6% AE 1.3% for C 2 H 4 at a current density of 300 mA cm À2 , while the pure Cu oxide showed FE of just 11.6% for C 2 H 4 .It was found that the CuO x -HfO 2 interface greatly strengthened the CO 2 adsorption and the binding of *CO for further C-C coupling to form C 2 H 4 . [121]Gao et al. found that Cu oxide enclosed by ( 100) and ( 111) facets displayed a high FE of 74.9% for multicarbon products at current density of 300 mA cm À2 in 1 M KHCO 3 media, which was due to the favorable local electronic structure, the enhanced *CO adsorption and the decreased C-C coupling activation energy contributed by the Cu (100)/ Cu (111) interface. [122]In addition, Mistry et al. revealed that oxygen plasma was a highly efficient technique to create oxidized Cu electrode with uncoordinated Cu þ and porous nanostructures on the electrode surface at ambient conditions, which resulted in increased CRR performance with low overpotential and record selectivity up to 60% for C 2 H 4 . [123]he effect of particle sizes of the Cu oxide-based electrocatalyst on their CRR performance was also explored.Kalyani Gupta et al. reported the synthesis of an ultrafine Cu oxide via a continuous hydrothermal flow method.The obtained ultrafine Cu oxide exhibited FE of more than 60% for HCOOH due to the existence of (111) planes in the catalyst. [124]Hwang et al. discovered the electrochemical fragmentation of 20 nm Cu 2 O-based NP into 2 to 4 nm small particles showed increasing CRR activity with doubled FE of C 2 H 4 , which accounts for 87% of the total CRR products.It was claimed that the unique morphology and small NPs stacked upon one another contribute to the promotion of C─C coupling reaction selectivity. [125]The main products, the FE and the overpotential of supported 2D Cu structures, Cu single atoms and Cu-oxide compounds toward CRR are summarized in Table 4.

Overview
Cu-based catalytic materials with various morphologies and particle sizes in nanoscale can be readily produced via different methods and utilized as efficient CRR electrocatalysts.The structures, morphologies, compositions as well as surface chemistry of the Cu-based electrocatalysts can largely influence the adsorption and activation energy of the reactant carbon dioxide, the unstable intermediates, and the mass transfer and electronic transfer efficiency in CRR, which finally lead to tunable carbon dioxide reduction reaction mechanisms and CRR performance.Currently, tremendous researches are mainly focused on the formation of C1-C3 products in CRR, and little attention is paid to high value-added multicarbon compounds.It is therefore of utmost importance to develop Cu-based catalysts to promote the formation of multicarbon products with excellent selectivity via C─C coupling reaction in CRR.

Cu-Based Bimetallic Catalysts
Zheng et al. prepared an ultrathin Cu/Ni(OH) 2 nanosheet that could convert CO 2 and H 2 O into tunable syngas with an FE of 92% for CO at an overpotential of merely 0.39 V and a current density of 4.3 mA cm À2 , due to the presence of an atomically thick Cu(0)-enriched surface on the Ni(OH) 2 nanosheet. [129]orales-Guio et al. reported that an Au/Cu bimetallic nanostructure was highly active to electrocatalytically reduce CO 2 to alcohols rather than hydrocarbons via the formation of CO as the intermediate reaction product on Cu surface under local basic conditions due to synergistic effect (Figure 11a). [130]A second metal element was introduced into N-heterocyclic carbeneligated Cu complexes to form analogous Cu-based bimetallic nanostructures including Cu-Fe, Cu-W, and Cu-Mo materials.These bimetallic mechanisms exhibited a tunable CRR reaction mechanism and product selectivity, with products ranging from solo HCOOH to varied mixtures of CO and HCOOH.It was revealed that the selectivity of CO versus HCOOH was highly dependent on the electronic nature of the Cu/M pairing, and the high selectivity for CO could be achieved by Cu-Mo catalyst. [131]Moreover, Xiong et al. reported that Cu-Pt alloy NCs with tunable compositions displayed compositiondependent electrocatalytic CO 2 reduction activity (Figure 11b). [132]Later, Wang et al. developed planar CuAg alloys with controlled surface composition and roughness via galvanically exchange of Ag from AgNO 3 into Cu surface.The resulting catalyst exhibited FE of 70% for CO reduction (COR) to CH 3 CHO.Further investigation indicated that the enhancement might be attributed to the weaker binding energy of the key intermediates acetaldehyde of CH 3 CH 2 O*, which inhibited its further reduction to ethanol under reaction conditions, due to the electronic effect from Ag incorporation in the Cu catalyst (Figure 11c,d). [133] Cu 68 Ag 32 bimetallic nanocrystal was reported by Chen et al. exhibiting an extremely high FE of up to 60% for CH 4 in CRR, which was three times that of pure Cu nanowires.Further investigation revealed that the reoxidation/reduction-driven atomic interdiffusion between Cu and Ag greatly facilitated CH 4 formation.[134] Limtrakul et al. carried out a systematical simulation investigation on the electrochemical CRR to form CH 4 and CH 3 OH on Cu-based bimetallic alloys.It was discovered that the formation of HCOOH could be efficiently catalyzed on Cu 3 Pt, Cu 3 Ni, Cu 3 Co and Cu 3 Rh surfaces, while CH 3 OH formation was favored on Cu 3 Pd and Cu 3 Pt surfaces.[135] Moreover, Bao reported that CRR performance of PdCu alloys on carbon could be optimized to achieve a maximum FE of 86% for CO on a Pd 85 Cu 15 /C catalyst.It was suggested that the presence of an optimum ratio of Cu element and low-coordination of Pd led to high selectivity toward CO production with the catalyst (Figure 12a).[136] The production of uniform Au-Cu bimetallic NPs with varied compositions as CRR catalyst was reported by Yang.It was claimed that the electronic and the geometric effect of the bimetallic as well as the intermediate binding could be well regulated via tuning the compositions, therefore further moderating the CRR performance (Figure 12b).[137] Cu-supported In nanostructure exhibited FE of 93% for CO in electrochemical CRR.DFT calculations revealed that the u-In interface improved *COOH adsorption strength while reduced the adsorption energy of *H, which led to enhanced CO production and suppressed H 2 evolution.[138] In addition, a series of bimetallic catalysts with atomically dispersed dual-metal sites such as Ni-Fe, Fe-Co and Ni-Co comprised two N atoms, shared NiN 4 and FeN 4 (2N-bridge (Fe-Ni)N 6 ) were developed, which exhibited improved CRR performance, due to the optimal *COOH adsorption energy and *CO desorption energy originated from the synergistic effect between the shared dual-metal sites.[139] CuNi 3 @CuNNi 3 electrocatalyst produced by the introduction of N into the body of CuNi 3 to form a thin CuNNi 3 nanoshell coated on CuNi 3 alloy core delivered a high CO FE of 96% compared with a value of 70% for the bulky counterpart, which was ascribed to the formation of abundant Lewis basic sites with various strengths and the favored strong adsorption of the critical species derived from N doping.It was noticed that significant nanosizing effect and interfacial interaction effects between the CuNNi 3 nanoshell and the CuNi 3 alloy core also played important roles in the much improved CRR performance (Figure 12c).[140] Interestingly, Wei et al., discovered that abundant Cu-O-Mo interface active sites could be formed on the surface of Cu NCs when modified by polyoxometalate.The obtained catalyst could effectively decrease the energy barrier for the formation of *CH 3 and successive coupling with CO 2 insertion to achieve FE of 48.68% for CH 3 COOH.[141] It was revealed that increasing the coverage of the intermediate *CO on the surface of the coupled CoPc-Cu GDE could lead to FE of 82% for C 2þ products, which could be ascribed to the decreased formation energy of *OCCO. [142]Regulation of the local pH on the surface of the Cu electrode can be an effective strategy to decrease the energy barrier of C 2 H 4 intermediate formation.[143] Kim demonstrated that a thin Au layer on the surface of Cu via galvanic displacement approach could remarkably improve the CRR performance and CO FE via the increase in electron population in the S-band and the upshift of the d-band center position of Au in Au/Cu bimetallic structures.It stated that the geometric effect also played a substantial role in the performance enhancement.[144] Huang et al. found that the copper oxide/hollow tin dioxide heterostructure catalyst highly relied on the abundance of copper/tin dioxide interfaces involved in the CRR process, which could effectively decrease the formation energy of COOH*(Figure 12d).[145]

Cu-Based Core-Shell Structures
The formation of bimetallic Cu-based core-shell structures can introduce a synergistic effect in the catalysts, which is capable of altering the electronic density of the catalyst surface environment, increasing surface area and tuning the adsorption energy of CO 2 and the reaction intermediates during the CRR process, [146] consequently promoting CRR performance.
Bimetallic Ag@Cu core-shell nanostructures were prepared under varied reaction times and they exhibited CRR activity with variable products including CO, CH 4 , and C 2 H 4 ; the tunable CRR performance was attributed to the geometric effect instead of the dilution effect between Ag and Cu. [147]Similar work was reported by Chen et al. who coated a thin layer of Au on the surface of Cu nanowires, and the prepared Cu@Au core-shell  [130] Copyright 2018, Spring Nature.b) Growth mechanisms for the Cu x Pt 100Àx NCs prepared in the presence or absence of 1,2-tetradecanediol (TDD).Reproduced with permission. [132]Copyright 2015, The Royal Society of Chemistry.c) FEs and geometric current densities for COR on CuAg electrode.Reproduced under the terms of the CC-BY license. [133]Copyright 2020, The Authors, published by PNAS.d) FEs and geometric current densities for COR on Cu electrode.
nanostructures exhibited FE of 30% for CO in CRR. [148]owever, Rodriguez et al. discovered that HCOOH was the only liquid product on Au@Cu core-shell structures with varied layers, while gas products highly relied on the number of layers of Cu atoms.While Au cube structure coated with 7-8 layers of Cu atoms favored the formation of hydrogen and ethylene as gas products, more than 14 layers of Cu atoms led to hydrogen and methane as dominated gas products. [149]urface etching is a very important material processing method to prepare core-shell heterostructures toward CRR applications.Yin et al. successfully synthesized a series of Pd@Cu core-shell structures via a controlled chemical etching procedure.The produced bimetallic Pd@Cu core-shell structures with enriched (110) facets exhibited higher selectivity toward CH 4 , C 2 H 4 , C 2 H 6 , and C 3 H 8 , while the Pd@Cu structures with enriched (100) facet displayed the highest CRR activities among the Pd@Cu structures (Figure 13a,b). [150]Interestingly, a unique Ag@porous-Cu-shell nanostructure with two distinct catalytic active sites was developed by Schuhmann.When used as catalyst in CRR, while the first step occured on the surface of the Ag core, a subsequent reaction took place on the Cu shell to form n-propanol and propionaldehyde at an overpotential of merely 0.6 V, better than that of the monometallic Ag or Cu.The enhancement was ascribed to the similar feature as the enzymes that are capable of performing complex multistep cascade reaction by the multiple distinct catalytic sites in the substrate channeling. [151]Moreover, Shao and co-workers demonstrated that Cu/Pb core/shell nanocrystals as electrocatalysts in CRR yielded a boosted FE of up to 81.6% for the C 2þ liquid products.DFT calculations indicated that the reduced formation energy barriers of *COOH and *OCCOH due to the synergetic effect between Cu and Pd contributed to the significant increase in the selectivity of C 2þ liquid products (Figure 13c). [152]In addition, Bao et al. reported that a SnO x shell in Sn 2.7 Cu catalyst with a hierarchical Sn-Cu core could be reconstructed in situ at a Sn/SnO x interface, which exhibited a stable C 1 FE of 99.0 AE 0.5% for 40 h and a high current density of 406.7 mA cm À2 (Figure 13d). [153]nstead of using the synergistic effect between the core and the shell of the core-shell structures, the novel Ag@C@Cu core-shell catalysts with a thin carbon interlayer inhibited the direct interaction between Ag and Cu and isolated tandem effect, leading to a FE of 31.5% for C 2 H 5 OH, higher than the monometallic Cu catalyst. [154]Hu et al. discovered that Ti atoms on the subsurface of the unique CuTi@Cu catalyst with coordinatively unsaturated Cu sites on amorphous CuTi alloy could achieve a total FE of 49% for multicarbon products at an overpotential of À0.8 V due to the reduction of the energy barriers required for *CO dimerization and trimerization, which was resulted from the increased electron density of surface Cu sites together with the enhanced adsorption of *CO intermediate. [155]gure 12. a) FE of CO on PdCu/C and Pd/C catalysts in CO 2 -saturated 0.1 M KHCO 3 solution.Reproduced with permission. [136]Copyright 2016, Elsevier.b) TEM image of AuCu 3 NPs with average size 11.20 AE 1.65 nm (scale bar, 100 nm).Reproduced with permission. [137]Copyright 2014, Spring Nature.c) Flowchart of synthesis of the assemblage of core-shell-structured CuNi 3 @CuNNi 3 /C nanocomposites.Reproduced with permission. [140]Copyright 2022, American Chemical Society.d) TEM image and atom model of CuSn NPs.Reproduced with permission. [145]Copyright 2018, Spring Nature.

Overview
The formation of Cu-based bimetallic nanostructures with tunable morphologies and coordinate atoms have been utilized to design and synthesize new catalysts showing different CRR behaviors when compared with their monometallic counterparts.Due to the different electronegativity, Cu-based bimetallic nanostructures exhibite variable CO 2 adsorption performance, including tunable adsorpted atoms and tunable adsorption strength, leading to different subsequent reactions with e À and H þ .Consequently, the final products can be well regulated.Clearly, tuning the compositions of the electrocatalyst during the synthesis is an effective way to enhance the CRR performance, control the products, and improve the lifespan of the catalysts.

Tri-and Multimetallic Cu-Based Electrocatalysts for CRR
The introduction of third or multiple metal elements into Cubased catalytic materials can further moderate the geometry and electronic states of the materials, therefore affecting their electrocatalytic performance in CRR.In particular, since AgAuCuPdPt high-entropy alloys (HEA) were first investigated as CRR electrocatalysts by Nellaiappa et al, [156] the rational design and construction of Cu-based HEAs including tri-and multimetallic Cu-based electrocatalysts have been considered as a feasible and effective method to regulate CRR behaviors via the formation of a larger surface area, tips, and corners, [157][158][159] due to the fact that HEAs can not only regulate the electronic and geometric structures to a large degree, but also serve as a platform to construct catalysts with desired performance.
Hu et al. reported an effective method to regulate AuAgCu heterogeneous catalysts using tandem catalysis and electronic effect, which led to FE of 37.5% for ethanol at a potential of À0.8 V over asymmetric Au 1 Ag 1 Cu 5 heterostructures with segregated domains of three constituent metals. [160]Berlinguette et al. evaluated ternary alloy of Cu-Zn-Sn as catalyst in CRR and it exhibited FE of more than 80% for CO and HCOOH formation with a current density of 3 mA cm À2 at an overpotential of merely 200 mV. [161]ased on DFT simulation and supervised machine learning, Rossmeisl and co-workers discovered that the composition of the HEAs could be regulated to favor the formation of CO via improving CO adsorption energy and suppress the formation of molecular hydrogen via weakening hydrogen adsorption (Figure 14a-c). [162]Pathek et al. demonstrated the use of machine learning for high-throughput screening of Cu-based HEAs catalysts including different elements, compositions, and surface microstructural features for the formation of methanol via CO 2 hydrogenation.Successful prediction of adsorption energies of the adsorbates using CuCoNiZnMg-based training data was Figure 13.a) Schematic illustration of nanocrystal syntheses through two different ways: growth and etching.Reproduced with permission. [150]Copyright 2016, American Chemical Society.b) FE between Cu NCs and Cu RDs toward CO 2 electroreduction.c) Schematic illustration of CO 2 RR over Cu/Pb core/ shell nanocrystals.Reproduced with permission. [152]Copyright 2021, American Chemical Society.d) Potential-dependent FE for the formate, CO, and H 2 on the surface of Sn, Sn 2.7 Cu, SnCu 1.1 , and SnCu 4.0 .Reproduced with permission. [153]Copyright 2020, Wiley-VCH.
achieved.The result indicated that a series of high-entropy-based catalysts could be utilized as promising candidates for methanol synthesis (Figure 14d). [163]The reported electrocatalytic CRR performance (including main the product, FE and overpotential) of bimetallic, trimetallic, and multimetallic Cu-based electrocatalysts is summarized in Table 5.

Conclusions and Perspectives
Electrochemical conversion of CO 2 into vale-added chemicals and fuels on Cu-based catalysts with various nanostructures using sustainable electricity provides a feasible strategy to realize carbon neutrality for our society.The highly tunable reduction products and the economic feasibility of electrochemical CRR offer a potential route to store renewable energy in chemical bonds.In this review, the current progress in the design and synthesis of Cu-based heterogeneous electrocatalysts with various chemical compositions, structures, morphologies, and topologies toward CRR is analyzed and summarized.The effect of electrocatalysts on the formation of different high-value-added CRR products including CO, HCOOH, CH 4 , C 2 H 4 and CH 3 CH 2 OH is discussed.Currently, Cu-based electrocatalysts in CRR are far from ideal with some drawbacks and cannot meet the potential large-scale demands.It is anticipated that in the future, Cu-based electrocatalysts for CRR will be further developed in the following directions.

Developing Novel Preparation Methods for Catalysts with Special Structures
Microstructure is the key parameter to boost CRR performance of Cu-based catalysts.An ideal surface structure of catalysts should have more active sites where CO 2 can be adsorbed and activated from a line structure into a curve structure.Different from bulk materials, nanostructured Cu catalysts with a large amount of low-coordinated and uncoordinated atoms on the surface such as high-index-facet (e.g., (111), (731), (211))  [162] Copyright 2020, American Chemical Society.b) Plot of the CO 2 RR/CORR selectivity and CORR activity achievable by CoCuGaNiZn.c) Plot of the CO 2 RR/CORR selectivity and CORR activity achievable by AgAuCuPdPt.d) Diagram of machine learning-assisted exploration of HEA catalysts for CRR to methanol.Reproduced with permission. [163]opyright 2022, American Chemical Society.active sites are intrinsically electrochemical active in CRR.A milestone work of Cu compound, cuprous cyanamide (Cu 2 NCN) nanocrystal with isolated Cu(I) conjugated NCN 2À displayed FE of up to 70% for CH 3 OH and superior stability, due to the promoted release of *OCH 3 (to form CH 3 OH) by Cu atoms. [164]Such a novel nanostructure not only improves the stability of Cu þ , but also maintains superior CRR activities.However, most of the current catalyst preparation methods can hardly balance the stability and activity of the catalysts simultaneously.In addition, many catalyst preparation procedures consume a large amount of energy, which may not be an issue for fundamental research, but it will be a huge concern if the catalyst is to be produced in large-scale.Therefore, developing low-cost, facile, and novel preparation methods for catalysts with special structures and desirable CRR performance is highly sought after.

Improving Catalyst Stability under High Current Density
To be utilized as industrial-scale catalysts in the future, the prerequisites for CRR electrocatalysts are cheap, able to maintain a high current density of more than 200 mA cm À2 , and operate stably with long running time up to 100 h.However, most of the current studied electrocatalysts for CRR are far beyond the industrial demand due to the high overpotential and poor stability of less than 100 h.Although noble metals such as Pt, Au, Pd and Ag can be used as CRR catalysts, their high cost prevents them from practical applications.The development of novel catalysts with superior electron transfer speed can avoid surface reconstruction due to electron accumulation during the reduction reaction process.This could be an effective way to upgrade the CRR performance with remarkably enhanced stability and lifespan that may potentially meet the industrial application requirements.

Design of Novel Electrochemical Reactors
Under ambient and room-temperature conditions, CO 2 solubility in a liquid electrolyte is low, which results in only a trace amount of CO 2 involved in CRR procedures even though CO 2 is bubbled into the reactor at a high speed, due to a limited mass transfer speed when the reaction is carried out in H-type cells.To overcome this bottleneck, flow cell containing a gas diffusion layer (GDL) with porous structure can enable CO 2 to pass through directly and react on the surface of the catalyst.However, CO 2 will react with alkaline electrolyte that leads to a decrement in pH, increased resistance and limited energy.When CRR is carried out in a new membrane-electrode assembled (MEA) reactor, a GDE in the MEA reactor is directly pressed on an ion-exchange membrane free of high ohmic losses and the accompanied neutral flow cells.CRR and water oxidation reaction can be carried out in two separate components, which will benefit separation and purification of the different reaction products.
In addition, MEAs are cost-effective and also feasible to be utilized in large scale.
Yongde Xia earned his Ph.D. from Fudan University, China.After postdoctoral experience in Korea and France, he moved to UK and worked as a research fellow at the University of Nottingham.Followed by joining Exeter in 2010, he is now a senior lecturer in Functional Materials in the Department of Engineering, Faculty of Environment, Science and Economy at University of Exeter.His main research interests include novel porous materials for energy, electrochemical energy storage and conversion, photocatalysis, and adsorption.He has contributed to over 160 peer-reviewed journal publications.

Figure 3 .
Figure 3. a) FEs for each product on Cu NC cubes with different sizes and in the Cu foil at À1.1 V versus RHE.Reproduced with permission.[57]Copyright 2016, Wiley-VCH.b) Faradaic efficiency values measured for Cu NCs using 1 mol L À1 of KOH as the catholyte.Reproduced with permission.[58]Copyright 2019, American Chemical Society.c) 3D atomic model of a Cu NC.Reproduced with permission.[59]Copyright 2019, American Chemical Society.d) Diagram of the procedures utilized to increase the stability of Cu NCs.Reproduced with permission.[69]Copyright 2020, American Chemical Society.

Figure 4 .
Figure 4. a) Product distribution as a function of applied potential during the electrochemical reduction of CO 2 .Reproduced with permission.[71]Copyright 2014, American Chemical Society.b) FEs for H 2 , CO, and HCOOH for CO 2 reduction electrolysis over pristine Cu foam and annealed Cu foam electrodes at À0.45 V versus RHE in CO 2 -saturated 0.1 M KHCO 3 .Reproduced with permission.[73]Copyright 2016 Elsevier.c) FE ratios of ethylene to methane on different electrodes from À1.2 to À1.5 V (vs.RHE).Reproduced with permission.[74]Copyright 2015, The Royal Society of Chemistry.d) SEM images of Cu sandwich.Reproduced with permission.[75]Copyright 2018, John Wiley and Sons.e) FE for formate, EtOH, and MeOH of Cu sandwich electrode.

Figure 6 .
Figure 6.Top and side views of the three surface models.Reproduced under the terms of the CC-BY license. [83]Copyright 2017, The Authors, published by PNAS.A) A 4 Â 4 Cu (111) surface, the model for a metallic matrix (MM).B) A2 Â 2 Cu 2 O (111) surface, the model for fully oxidized matrix (FOM).(C) Metal embedded in an oxidized matrix (MEOM) derived by reducing one-quarter of a 2 Â 2 Cu 2 O (111) surface.Here Cu is dark blue, where the active (Cu þ ) is marked light blue, and red is O. Purple dashes mark the border between Cu 0 and Cu þ regions.

Figure 11 .
Figure 11.a) Potential dependence of the molar ratio of C 2þ to C 1 products for Cu and Au/Cu bimetallic nanostructure.Reproduced with permission.[130]Copyright 2018, Spring Nature.b) Growth mechanisms for the Cu x Pt 100Àx NCs prepared in the presence or absence of 1,2-tetradecanediol (TDD).Reproduced with permission.[132]Copyright 2015, The Royal Society of Chemistry.c) FEs and geometric current densities for COR on CuAg electrode.Reproduced under the terms of the CC-BY license.[133]Copyright 2020, The Authors, published by PNAS.d) FEs and geometric current densities for COR on Cu electrode.

Figure 14 .
Figure 14.a) Diagram of high-entropy AgAuPdCu alloys.Reproduced with permission.[162]Copyright 2020, American Chemical Society.b) Plot of the CO 2 RR/CORR selectivity and CORR activity achievable by CoCuGaNiZn.c) Plot of the CO 2 RR/CORR selectivity and CORR activity achievable by AgAuCuPdPt.d) Diagram of machine learning-assisted exploration of HEA catalysts for CRR to methanol.Reproduced with permission.[163]Copyright 2022, American Chemical Society.

Table 2 .
CRR performance of Cu-based nanowires and nanocubes.

Table 3 .
Electrocatalytical CRR performance of various porous Cu structures.

Table 4 .
CRR performance of supported 2D Cu structures, Cu single atoms, and Cu oxide compounds.