Development of catalysts and reactor designs for CO₂ electroreduction towards C₂₊ products

Vacancy

Enhancing the performance of Cu-based electrocatalysts has largely centered on investigating the role of vacancies, the most prevalent type of defect. These studies aim to explore how vacancies can be leveraged to optimize catalytic activity toward C₂₊ products. Xue et al. designed Cu/CeO_2-x catalysts featuring different levels of oxygen vacancies, investigating the interaction between Ce³⁺ and Cu species and its impact on catalytic performance^[58]. The introduction of Cu into CeO_2-x generated a significant number of oxygen vacancies. Various Cu/CeO_2-x electrocatalysts were synthesized via the hydrothermal-reduction method [Figure 3A]. High-resolution transmission electron microscopy (HRTEM) analysis demonstrated the presence of structural defects in the Cu/CeO_2-x-4 catalyst, as depicted in Figure 3B. The pristine CeO₂ catalyst in Figure 3C only produces H₂ and CO, regardless of the applied potential. However, the Cu/CeO_2-x-4 catalyst, which has the highest oxygen vacancy concentration, showed improved selectivity for C₂H₄ and CH₄ [Figure 3D and E]. Fang et al. investigated the integration of Cu into CeO₂ (Cu/CeO₂) using an impregnation-calcination method for CO₂RR^[59]. The amount of Cu loading was controlled by adjusting the concentration of the copper precursor during the impregnation process. The electrocatalytic performance for CO₂RR was analyzed in an H-cell with 0.1 M CsI as the electrolyte solution. Due to the strong electronic interaction of the Ce⁴⁺-O^2--Cu⁺ structure of Cu/CeO₂ and the appearance of oxygen vacancies, the optimized catalyst featuring a Cu doping of 9.77 wt% achieved a C₂₊ Faradaic efficiency (FE) of 78.3% and a current density of 16.8 mA cm^-2 at -1.0V vs. reversible hydrogen electrode (RHE). Recently, Shen et al. enhanced the performance of Cu catalysts for C₂H₄ production by introducing oxygen vacancies through iodine doping in CuO (I-CuO)^[60]. Advanced characterization demonstrated that iodine doping facilitated the formation of Cu⁺ species in CuO and increased the density of oxygen vacancies. Additionally, the introduction of iodine significantly enhanced the hydrophobicity of the catalyst, as evidenced by a contact angle of 131.1°, compared to the 0° of bare CuO. Electrocatalytic CO₂ reduction was assessed in an H-type cell, and liquid products were analyzed using hydrogen nuclear magnetic resonance spectroscopy (¹H NMR). Among the catalysts, 1.0% I-CuO presented the highest FE for C₂H₄ and the lowest for H₂ across the broad potential range of -1.0 V to -1.4 V vs. RHE. Notably, the FE_C2H4 production reached up to 50.2%, which is 3.43 times greater than that of bare CuO. Additionally, the catalyst demonstrated stable performance over 15 h without any decline. Jiang et al. promoted ethylene production by optimizing the Sn doping and oxygen vacancies in CuO. The synergistic interaction between the Sn dopant and the oxygen vacancies significantly facilitated the CO₂ electroreduction to C₂H₄^[61]. Specifically, Sn-doped CuO (oxygen vacancies) achieved the highest FE_C2H4 of 48.5% ± 1.2%, accompanied by a j_C2H4 of 10.9 mA cm^-2, and demonstrated long-term durability over a period of 24 hours. Bie et al. structured atomically dispersed Cu atoms anchored on hydrogen-reduced UiO66-NH₂, characterized by abundant oxygen vacancies^[62]. This catalyst, referred to as Cu single atoms (SAs)/UIO-H₂, was constructed through a hydrothermal process followed by a H₂-spillover treatment [Figure 3F]. As illustrated in Figure 3G and H, the FE for C₂₊ products on Cu SAs/UIO reached a maximum of 27.71% at -1.06 V vs. RHE, which is significantly reduced compared to Cu SAs/UIO-H₂, highlighting the crucial role of oxygen vacancies in accelerating C-C coupling process. Furthermore, the Cu SAs/UIO-H₂ catalyst demonstrated a maximum FE of 46.28% for C₂H₅OH and 12.34% for acetic acid, with an overall FE for C₂₊ products reaching 58.62%. The catalyst also exhibited excellent stability over 12 hours, maintaining an ethanol FE of approximately 46% at -0.66 V vs. RHE [Figure 3I].

Figure 3. (A) Graphic representation of the Cu/CeO_2-x-4 catalyst synthesis; (B) HRTEM images of the Cu/CeO_2-x-4 catalyst; (C) FEs of bare CeO₂; (D) Cu/CeO_2-x-1; and (E) Cu/CeO_2-x-4. This figure is quoted with permission from Xue et al. Copyright (2024) Elsevier^[58]; (F) Schematic representation of the synthetic process for Cu SAs/UIO-H₂; (G) The corresponding FEs for the reduction products of Cu SAs/UIO and (H) Cu SAs/UIO-H₂; (I) Long-term electrolysis of Cu SAs/UIO-H₂ at -0.66 V vs. RHE. This figure is quoted with permission from Bie et al. Copyright (2024) Elsevier^[62]. FE: Faradaic efficiency; HRTEM: high-resolution transmission electron microscopy; SAs: single atoms; RHE: reversible hydrogen electrode.

Grain boundary

It is well established that grain boundaries (GBs) influence the selectivity of CO₂RR products^[63], and a quantitative correlation has been observed between the density of GBs in Cu-based electrocatalysts and CO₂-to-C₂₊ reduction^[64]. As a result, GBs are regarded as a crucial factor in enhanced selectivity for C₂₊ products of Cu-based catalysts. Enhancing GB density in Cu catalysts has been shown to significantly enhance C₂₊ production. Chen et al. demonstrated the Cu electrodeposition with polyvinylpyrrolidone (PVP) as an additive allows for controlled grain growth^[65]. The inert PVP adsorbed onto the Cu surface, accelerating the nucleation rate and reducing the size of crystal, resulting in a Cu electrode rich in GBs. The Cu electrode deposited without PVP is referred to as ED-Cu. When the GB-rich Cu electrode (GB-Cu) was used for eCO₂RR, it produced C₂H₄ and C₂H₆O across a broad potential range of -1.0 V to -1.3 V vs. RHE, achieving a high FE_C2+ of 73%. Notably, GB-Cu showed C₂H₄ selectivity of ~38% at -1.2 V vs. RHE and a C₂H₆O-to-C₂H₄ ratio of 0.85 [Figure 4A and B]. In situ infrared absorption spectroscopy revealed that the GBs in Cu were key in facilitating the adsorption of ^*CO intermediates, thereby promoting CO₂RR [Figure 4C]. Dendritic Cu catalysts with tunable GB were prepared by Zhang et al. using a pulsed electrochemical deposition method on the gas diffusion electrode (GDE)^[66]. The GB density was adjusted by varying the deposition times to 50 ms, 100 ms, 200 ms, and 500 ms, with a constant pulse duration of 250 ms, all under a constant current density of 100 mA cm^-2 [Figure 4D]. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image shown in Figure 4E revealed distinct high-contrast lines aligned parallel to the growth direction of the Cu dendrites, indicating a symmetrical lattice structure on either side, a typical feature of GBs. Notably, the selectivity for C₂₊ products increased linearly with the GB density across all applied potentials. The GB-Cu_29.6 catalyst exhibited the highest FE_C2+, reaching 46.2%, along with a j_C2+ of ~20 mA cm^-2 at a potential of -1.1 V vs. RHE [Figure 4F and G].Figure 4H shows activity mapping at a defined potential based on cyclic voltammetry, where GB-Cu_29.6 demonstrated distinct patterns of locally enhanced activity compared to Cu catalysts without GBs. This suggests that GBs facilitate electron transfer, thereby improving performance toward C₂₊ productions.

Figure 4. (A) FEs on GB-Cu; (B) FE_C2+ of GB-Cu; (C) In situ ATR-SEIRAS spectra of GB-Cu. This figure is quoted with permission from Chen et al. Copyright (2020) American Chemical Society^[65]; (D) Electrochemical deposition procedure for different Cu catalysts; (E) FFT-refined aberration-corrected HAADF-STEM image of the GB-Cu_29.6; (F) FEs for C₂₊ and (G) ECSA-normalized j_C2+ of Cu with different GB densities; (H) Spatially resolved electrochemical map derived from the individual linear sweep voltammetry (LSV) curves. This figure is quoted with permission from Zhang et al. Copyright (2024) American Chemical Society^[66]. FE: Faradaic efficiency; HAADF-STEM: high-angle annular dark-field scanning transmission electron microscopy; GBs: grain boundaries; ATR-SEIRAS: attenuated total reflectance-surface-enhanced infrared absorption spectroscopy; FFT: fast Fourier transform; ECSA: electrochemically active surface area.

Ding et al. introduced an innovative and straightforward salt-assisted annealing technique that induced unconventional grain fragmentation, significantly increasing the density of GBs^[67]. The optimized electrocatalysts attained a maximum FE_C2+ of 70.0% for the H-type cell and 68.2% for the flow cell, achieving a j_C2+ of 0.768 mA cm^-2 at -1.28 V vs. RHE, surpassing the performance of most previously reported counterparts. To investigate the correlation between GBs and the selectivity of the catalysts for C₂₊ products, in situ Raman spectroscopy and Density functional theory (DFT) calculations were conducted. The results indicated that a greater GB density generates additional active sites for CO₂RR, resulting in a higher concentration of ^*CO intermediate. This, in turn, promotes C-C coupling and ultimately increases C₂₊ production. Kong et al. prepared small Cu nanoparticles (NPs) with enhanced small GBs (RGBs-Cu) using spatial confinement and in situ electroreduction^[68]. Figure 5A illustrates the schematic of the synthesis process for RGBs-Cu electrocatalysts, while HRTEM images reveal the numerous GBs present in the catalysts. CO₂RR electrolysis using the RGBs-Cu electrode was conducted using flow cells with different concentrations of KOH. Notably, elevating the KOH concentration to 2 and 3 M resulted in an additional enhancement of the FE for C₂₊ products, achieving 77.3% at a current density of 400 mA cm^-2 in 2 M KOH [Figure 5B]. Additionally, RGBs-Cu reached a maximum C₂₊-to-C₁ (C₂₊/C₁) ratio of 2.57 at 400 mA cm^-2, which notably exceeded the control sample’s ratio of 0.75 [Figure 5C]. Moreover, RGBs-Cu demonstrated exceptional stability, maintaining consistent C₂₊ selectivity of 60% over 134 h at a total current density of 500 mA cm^-2 [Figure 5D].

Figure 5. (A) Overview of the fabrication process of RGBs-Cu sample; (B) FE_C2+ of RGBs-Cu with different electrolyte concentrations; (C) C₂₊/C₁ product selectivity of RGBs-Cu and PGBs-Cu; (D) Evaluation of the long-term stability of the RGBs-Cu electrode at 500 mA cm^-2 in a flow cell. This figure is quoted with permission from Kong et al. Copyright (2024) American Chemical Society^[68]; (E) Schematic representation of Cu₂O HoMSs fabrication process; (F) Distribution of CO₂RR products on Cu₂O HoMSs catalysts 1-3 shell; (G) C₂₊/C₁ product selectivity on HoMSs; (H) Long-term durability of 3-shell HoMSs for 8 h. This figure is quoted with permission from Liu et al. Copyright (2022) John Wiley and Sons^[76]. FE: Faradaic efficiency; CO₂RR: CO₂ reduction reaction; HoMSs: hollow multi-shell structures; RGBs-Cu: Cu nanoparticle catalyst comprising abundant grain boundaries.

Confinement effect

Confinement structures in Cu-based electrocatalysts can enhance C₂₊ selectivity by increasing the retention time of intermediates such as ^*CO/^*COH by facilitating C-C coupling reactions^[71,72]. Consequently, significant research has focused on tailoring the Cu surface with specialized confinement morphologies to optimize both the activity and selectivity for C₂₊ product formation. Fan et al. fabricated three-dimensional (3D) ordered porous cuprous oxide cuboctahedra (3DOP Cu₂O-CO) through a hard templating method^[73]. This method successfully created a structure characterized by ordered macropores that are interconnected with mesoporous channels throughout the Cu₂O cuboctahedra. The 3DOP-Cu₂O-CO configuration offers the benefits of uniformly distributed and interconnected pore channels. Consequently, these electrocatalysts demonstrated superior performance in CO₂RR compared to control catalysts lacking ordered porous architectures. Notably, 3DOP-Cu₂O-CO achieved a significant electrochemical double-layer capacitance of 2.9 mF cm^-2, with a maximum C₂₊ FE of 73.4% at -1.4 V vs. RHE in an H-cell, and 81.7% at -1.0 A cm^-2 using a flow-type cell. Finite element method (FEM) simulations demonstrated that the structured pore architecture of 3DOP Cu₂O-CO capably confines and holds adequate ^*CO for adsorption during the CO₂ reduction process, thereby facilitating the strong dimerization required for C₂₊ product formation. Liu et al. designed distinctive Cu-based catalysts with a cavity structure, achieved through in situ electrochemical reduction of Cu₂O cavities^[74]. Both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) analyses confirmed the presence of cavity architecture. FEM simulations also confirmed that the cavity structure significantly enhances the concentration of CO intermediates at the reaction site, thereby promoting the C-C coupling reaction. These Cu cavity catalysts demonstrated excellent C₂₊ FE of 75.6 ± 1.8% and achieved j_C2+ of 605 ± 14 mA cm^-2 in a microfluidic flow cell. Moreover, the experimental C₂₊/C₁ of Cu cavity catalysts reached the maximum of 5.4 ± 0.6. Pan et al. investigated the use of hollow mesoporous carbon spheres (HMCS) to explore the influence of confinement on the production of C₂₊ products^[75]. They prepared a series of Cu-incorporated HMCS with copper loadings of 10%, 20%, and 30%, designated as Cu/HMCS5-10%, Cu/HMCS5-20%, and Cu/HMCS5-30%, respectively. The results revealed that Cu/HMCS₅-20% achieved the greatest FE_C2+ of 88.7%, outperforming 69.3% and 80.1% FE_C2+ recorded for Cu/HMCS₅-10% and Cu/HMCS₅-30%, respectively, at a potential of -1.0 V vs. RHE in a 1.0 M KOH solution. Notably, Cu/HMCS₅-20% exhibited remarkable stability, maintaining its performance over 20 h at -1.0 V vs. RHE without significant degradation. The exceptional C₂₊ selectivity of Cu/HMCS₅-20% was attributed to the unique confinement effects provided by HMCS, along with the synergistic interactions among Cu atoms with varying oxidation states. Liu et al. described the development of Cu₂O hollow multi-shell structures (HoMSs) with adjustable shell numbers^[76]. The synthesis of Cu₂O HoMSs was accomplished using the Ostwald ripening method, as depicted in Figure 5E. Electrocatalytic CO₂ reduction was conducted in a flow reactor using 0.5 M KHCO₃. Prior to testing, Cu₂O HoMSs underwent electroreduction at -0.82 V vs. RHE for ten minutes. Notably, the Cu₂O structure with three shells reached a peak C₂₊ FE of 77.0 ± 0.3%, outperforming the 1-shell (40.3 ± 1.0%) and 2-shell (62.2 ± 0.3%) counterparts at -0.88 V vs. RHE [Figure 5F]. The increase in shell number clearly enhanced C₂₊ production [Figure 5G]. Furthermore, the 3-shell HoMS catalysts exhibited excellent long-term stability, maintaining performance for 8 h at -300 mA cm^-2, underscoring the advantages of the nanoconfinement effect in converting CO₂ to C₂₊ products [Figure 5H].

Morphology/facet effect

Aside from confinement engineering, the morphology and crystal facets of Cu-based electrocatalysts have a fundamental role in influencing CO₂ electroreduction activity and product selectivity. For example, Wu et al. revealed a correlation between the structural dimensions of oxide-derived Cu (OD-Cu) electrocatalysts and their ability to selectively produce C₂₊ products^[77]. Their study involved the synthesis of three distinct nanostructures: one-dimensional nanowires (NWs), two-dimensional nanosheets (NSs), and 3D nanoflowers (NFs), all of which displayed notable resistance to structural degradation [Figure 6A]. Initial evaluations of the CO₂RR activity and the electrochemical characteristics of the OD-Cu catalysts were conducted using H-cell systems. As demonstrated in Figure 6B, the product distribution from CO₂RR over the OD-Cu catalysts was evaluated across a potential range from -0.6 V to -1.4 V vs. RHE. Of the three morphologies, OD-Cu NWs achieved the highest preference for C₂₊ products, reaching a maximum FE of 50.2% at -1.4 V vs. RHE, which notably surpassed the performance of OD-Cu NSs (35.6%) and NFs (13.3%). Additionally, the performance of OD-Cu catalysts was additionally evaluated with flow cells. Notably, OD-Cu NWs, which have the maximum surface Cu-O coordination, exhibited the most favorable CO₂RR kinetics for C₂₊ production, achieving 77.7% of FE_C2+, 233.2 mA cm^-2 for j_C2+, 701.8 μmol h^-1 cm^-2 of C₂₊ yield, and 51.1% C₂₊ energy efficiency [Figure 6C]. Additionally, OD-Cu NWs exhibited remarkable stability, maintaining consistent CO₂RR performance at 300 mA cm^-2 for 72 h.

Figure 6. (A) Diagram illustrating the preparation of OD-Cu catalysts with different morphologies; (B) FE_C2+ of OD-Cu NWs using H-cell; (C) FE_C2+ of OD-Cu NWs using flow cells. This figure is quoted with permission from Wu et al. Copyright (2024) Elsevier^[77]; (D) Schematic illustration for the synthesis of various Cu₂O; (E) LSV curves of all three catalysts; (F) FEs of various liquid products on f-Cu₂O; (G) j_C2H6O on f-Cu₂O. This figure is quoted with permission from Yang et al. Copyright (2024) Elsevier^[80]. OD-Cu: Oxide-derived Cu; NWs: nanowires; LSV: linear sweep voltammetry; FE: faradaic efficiency.

Moreover, Cu-based catalysts with a two-dimensional NS morphology are appealing due to their plentiful low-coordination edge sites, which are frequently regarded as essential for improving catalytic performance. Xie et al. synthesized Cu NSs derived from CuO during CO₂RR, having an average size of approximately 30 nm and thickness of about 20 nm^[78]. With an abundant concentration of low-coordination edge sites, these Cu NSs enabled efficient C-C coupling reactions, reaching a peak FE_C2H4 of 56.2% and a j_C2H4 of 171.0 mA cm^-2 at -0.871 V vs. RHE in a flow cell. Wang et al. developed thin-layered Cu₂O NSs having thickness of 0.9 nm (Cu₂O-NS) via coprecipitation method^[79]. For comparison, other ultrathin catalysts, Cu-NS and CuO-NS, were also fabricated. The Cu₂O-NS demonstrated a total current density of -187.6 mA cm^-2 and a FE of 81% for C₂₊ products, with FE_C2H4 reaching 40%, and stability of ten hours. The remarkable CO₂RR performance is attributed to coordination defects and oxygen vacancies, which enhanced ^*CO adsorption and improved C₂₊ product selectivity by stabilizing the surface oxidation state of Cu.

Facet engineering is essential in defining the catalyst's activity toward desired products. This can be achieved through various methods. Yang et al. explored the impact of morphology on the performance of Cu₂O catalysts for the production of liquid products^[80]. Cu₂O electrocatalysts with various morphologies were synthesized using PVP as a capping agent. A schematic representation of the synthesized catalysts, including cube-shaped Cu₂O (c-Cu₂O), tetrakaidecahedron-shaped Cu₂O (t-Cu₂O), and flower-like Cu₂O (f-Cu₂O), is provided in Figure 6D. Surface coordination-unsaturated Cu atoms on the (111) facet were observed to improve CO₂ reduction by facilitating the adsorption of CO₂ molecules. Oxygen vacancies were confirmed using electron paramagnetic resonance spectroscopy and ultraviolet-visible diffuse reflection spectroscopy. Figure 6E presents the linear sweep voltammetry (LSV) curves of the Cu₂O catalysts in an H-cell, showing that the f-Cu₂O with exposed (111) facets exhibited higher current density and a minimized onset potential. The selectivity toward liquid products was further assessed using ¹H NMR. At -0.8 V vs. RHE, the f-Cu₂O catalyst achieved an ultimate C₂H₆O FE of 52.6% and a j_C2H6O of 9.1 mA cm^-2 at -0.9 V vs. RHE, attributed to the abundance of oxygen vacancies on its surface [Figure 6F and G]. Furthermore, the ethanol formation rate on f-Cu₂O rose markedly from 0.43 μmol h^-1 cm^-2 at -0.5 V to 28.56 μmol h^-1cm^-2 at -1.1 Vvs. RHE. Significantly, the ethanol production rate on the f-Cu₂O catalyst was 3.4 times higher than that of c-Cu₂O and 1.6 times greater than that of t-Cu₂O. Gregorio et al. demonstrated facet-dependent selectivity on the Cu electrode, achieved through morphology adjustment via colloidal chemistry^[81]. Aligned with prior findings, Cu nanocubes enclosed by (100) facets show superior selectivity for C₂H₄, while Cu electrocatalysts with (111) facets demonstrate the highest selectivity toward CH₄. Fu et al. revealed that Cu catalysts with exposed (100) facets exhibit significantly higher selectivity for C₂₊ products compared to those with (111) and (110) facets^[82]. They developed three types of catalysts with distinct morphologies-NWs, NPs, and NSs-each exposing different crystal facets through pre-reduction and reconstruction methods. Among them, the NS-D Cu catalyst, characterized by its (100) facet, was particularly effective at enhancing ^*CO adsorption, increasing surface ^*CO coverage, and facilitating CO dimerization, thereby boosting its activity for C₂₊ product formation. This catalyst achieved a FE of 67.5% towards C₂₊ products, a j_C2+ of 25.1 mA cm^-2, and maintained stable performance for ten hours. Additionally, the NS-D Cu showed a maximum C₂₊/C₁ ratio of 6.2, surpassing NW-D Cu and NP-D Cu, with ratios of 1.7 and 1.9, respectively.

Dong et al. proposed a novel mechanism elucidating the link between the crystal facets of Cu₂O and the formation of distinct reduction products^[83]. Through a combination of theoretical calculations and experimental validation, they supported their findings. The exposed Cu atoms on the (100) facet displayed a dense Cu₄O₁ surface coordination, which enhanced the adsorption of ^*CO intermediates. Consequently, Cu₂O with the (100) facet showed significantly higher selectivity for C₂H₄ in 0.1 M KHCO₃ solution, achieving a FE for C₂H₄ of 61.3% at 1.2 V vs. RHE. Compared to other Cu-based catalysts reported in recent studies, their Cu₂O with (100) facets demonstrated superior selectivity toward C₂H₄. Luo et al. synthesized six distinct morphologies of Cu₂O NPs using NH₂OH·HCl as a reductant, each exposing different crystal facets, and then applied a ZIF-8 shell to improve their stability [Figure 7A]^[84]. The NPs, labeled A-Cu₂O through F-Cu₂O, were obtained by varying the amount of NH₂OH·HCl. Figure 7B and C illustrates the electrochemical performance of these Cu₂O catalysts in 0.1 M KHCO₃ over a potential range of -0.7 V to -1.3 Vvs. RHE. Among them, F-Cu₂O, which primarily exposed the (332) facet, showed the highest FE_C2H4 production, reaching 74.1% at -1.2 V vs. RHE. It was also observed that increasing the proportion of (111) facets significantly enhanced both the selectivity and activity of as synthesized electrocatalysts. DFT calculations further confirmed that the (332) facet, with its step-like structure, substantially lowered the Gibbs free energy for ^*CHO intermediate coupling, leading to improved electrocatalytic performance.

Figure 7. (A) Preparation of Cu₂O nanoparticles and Cu₂O@ZIF-8 composites; (B) FE_C2H4 of Cu₂O nanoparticles with different facets; (C) FE of products on F-Cu₂O at different potentials. This figure is quoted with permission from Luo et al. Copyright (2022) John Wiley and Sons^[84]; (D) Schematic illustration for the synthesis of various Cu₂O; (E) C₂₊ and C₁ faradaic efficiencies; (F) j_C2+ on Cu-D and Cu-P; (G) Durability analysis for the Cu-D and Cu-P electrodes at 300 mA cm^-2.This figure is quoted with permission from Niu et al. Copyright (2021) American Chemical Society^[97]. FE: Faradaic efficiency.

Size effect

Another approach to develop electrocatalysts is controlling the size of electrocatalysts. Reducing the size increases the surface-to-volume ratio, thereby enhancing the utilization of metal atoms. Additionally, the number of undercoordinated sites increases, leading to a perturbed electronic structure and enhanced activity.

Merino-Garcia et al. investigated the selectivity and activity of the catalyst based on the size of the Cu particles, which were synthesized with sizes ranging from 25 nm to 80 nm^[85]. Cu particles of different sizes were mixed with a binder and isopropanol to prepare a GDE by depositing the mixture onto carbon paper. The electrode with 25 nm Cu particles exhibited significantly better performance compared to those with Cu particle sizes of 40-60 nm and 60-80 nm, achieving a remarkably high ethylene FE of 92.8% and a production rate of 1,148 μmol m^-2 s^-1. This enhanced performance is attributed to an increase in the fraction of under-coordinated sites as the particle size decreases. Furthermore, Rong et al. explored the size effects of Cu catalysts, ranging from SAs to nanoclusters, using a facile acetylenic-bond-directed site-trapping approach^[86]. Three different catalysts of varying sizes were synthesized: single-atoms (~0.1 nm), subnanometric clusters (0.5~1 nm) and nanoclusters (1~1.5 nm). They demonstrated that increasing the size of Cu nanoclusters enhanced both catalytic activity and selectivity toward C₂₊ products. The electrocatalytic performance was assessed in a three-compartment flow cell. Among the synthesized catalysts, the optimized Cu-based nanoclusters with a size of 1-1.5 nm exhibited the best performance, achieving a FE_C2+ of 91.2%, a partial current density of 312 mA cm^-2, and moderate stability lasting approximately 22 h. Nam et al. synthesized Cu clusters using copper benzene-1,3,5-tricarboxylate (HKUST-1) as a precursor^[87]. The calcination time was controlled to remove benzene tricarboxylate moieties, resulting in Cu dimer distortion and the generation of undercoordinated Cu sites within the Cu clusters. HKUST-1 calcined at 250 °C for 3 h exhibited the lowest Cu-Cu Coordination number of 9.5 ± 0.9 among the synthesized Cu clusters. The electrocatalytic performance of the catalysts was evaluated in a flow cell with 1 M KOH. Among the samples, HKUST-1 calcined at 250 °C for three hours showed the highest selectivity for C₂H₄, achieving a FE_C2H4 of 45%, significantly outperforming the as-prepared HKUST-1 (FE_C2H4=10%). Su et al. synthesized electrocatalysts consisting of CuO clusters anchored on N-doped carbon NSs (Cu/N_0.14C)^[88]. Cu/N_xC with varying nitrogen contents was prepared in two steps: (1) simple mixing of copper phthalocyanine and dicyandiamide followed by drying, and (2) pyrolysis of the dried mixture at different temperatures. The optimized Cu/N_0.14C catalyst exhibited a superior FE_C2+ of 73%, with a selectivity towards ethanol of 51% and a partial current density of 14.4 mA cm^-2 at -1.1 V vs. RHE. Moreover, it demonstrated excellent long-term durability, maintaining stable FE_C2+ and partial current density for over ten hours.

Ag-based

Ag is known for its high activity in CO production and is one of the most commonly used metals as a component in tandem catalysts. Although its CO selectivity is lower than that of Au, it is suitable for use in tandem catalysts from a cost perspective.

Recently, Duan et al. fabricated Cu@Ag/C via in-situ electrochemical reconstruction of Cu₂CO₃(OH)₂/AgCl/C composite^[105]. Cu₂CO₃₍OH)₂/AgCl/C was first synthesized by precipitating different amounts of AgCl, followed by electroreduction using an H-type reactor. The performance of the Cu@Ag/C catalyst was assessed in an H-type cell with 0.1 M KCl. As depicted in Figure 8A, the FE_C2H4 achieved 50.41% at -1.1 V vs. RHE, which is 1.7 times superior to bare Cu/C catalysts (30.41%). We further compared the FE_CO of Cu@Ag/C and Cu/C to emphasize the role of Ag in promoting CO production [Figure 8B]. The incorporation of Ag significantly enhanced CO formation, increased ^*CO coverage, and consequently promoted C-C coupling, contributing to the formation of C₂H₄. Moreover, electrocatalytic performance was evaluated using flow-type cells to improve CO₂ mass transfer. Cu@Ag/C demonstrated a maximum FE_C2H4 of 58.03% at -0.85 V vs. RHE, as shown in Figure 8C. Jeon et al. prepared bimetallic catalysts with varied Cu/Ag ratios by ultrasonic spray pyrolysis process^[106]. The electrochemical performance of Cu_XAg_100-X catalysts was evaluated in a single cell across a voltage range of 1.8 to 2.3 V. As expected, only CO and H₂ were produced with Ag₁₀₀, while C₂H₄, CH₄, CO, and H₂ were produced with the Cu-containing catalyst. Moreover, Cu-containing catalysts showed a decrease in FE_CO and an increase in FE_C2H4 at potentials that favor C₂H₄ production. This is because a crucial step in C₂H₄ formation involves the C-C coupling reaction, which consumes CO as a reactant. The optimized Cu_XAg_100-X with a ratio of 9:1 demonstrated the highest FE_C2H4 of 33% at a cell voltage of 2.2 V, which is significantly higher than that of bare Cu (22%). Recently, Liu et al. constructed Cu-Ag tandem catalysts utilizing ethylenediaminetetraacetic acid (EDTA) as a ligand, leveraging ligand modification in CO₂RR^[107]. Here, EDTA played a dual role in modulating both the electrode structure and the local pH around the surface Cu atoms during the electrochemical process. Cu-Ag tandem catalysts modified with EDTA (CuAg₄/EDTA) exhibited an excellent FE_CO at low potentials, and achieved an FE_C2+ of 86.56%, compared to Cu (44.13%), CuAg₄ (33.32%) and Cu/EDTA (52.15%). Luan et al. developed a tandem catalyst designed for C₂H₆O production. Several electrode configurations were fabricated, including layered Cu/Ag, layered Ag/Cu, and mixed CuAg structures [Figure 8D]^[108]. The layered Cu/Ag catalyst exhibited outstanding electrochemical performance for CO₂-to-C₂H₆O conversion compared to other types of electrodes and bare Cu catalysts. Remarkably, the best-performed layered Cu/Ag electrode achieved a FE of 56.5 ± 2.6% for C₂H₆O and a current density of 356.7 ± 9.5 mA cm^-2 at -1.1 V vs. RHE [Figure 8E and F]. This elevated performance, compared to the inverse layered Ag/Cu structure, can be credited to the efficient mass transport of CO gas produced in the Ag catalyst layer (CL). Huang et al. constructed Ag-Cu nanodimers (NDs) by establishing Cu domains onto pre-existing Ag NPs, resulting in an adjustable interface between the two distinct metals^[109]. This arrangement strengthened CO adsorption on the surface, promoting the conversion of CO into C₂H₄, resulting from electron depletion in Cu due to electron migration from Cu to the Ag domains. Consequently, the Ag-Cu NDs with an Ag and Cu in a 1:1.1 mass ratio showed a notable increase in both the FE for C₂H₄ and the j_C2H4 compared to bare Cu NPs. Similarly, Ma et al. described the preparation of Janus-like Ag-Cu nanostructures, where Cu having exposed (100) facets was restricted to one of the six equivalent faces of Ag nanocubes^[110]. Compared to Cu nanocubes, the resulting Ag₆₅-Cu₃₅ Janus nanostructures with (100) facets (Ag₆₅-Cu₃₅ JNS-100) demonstrated a substantially advanced FE for C₄H₄ production, reaching 54%, with a total C₂₊ product efficiency of 72%. Additionally, the selectivity of the optimized Ag₆₅-Cu₃₅ JNS-100 toward C₂H₄ surpassed that for CH₃CH₂OH and CH₃COOH, with FEs of 1.7% and 4.2%, respectively. These enhanced results were due to CO induced on the Ag domains, which could migrate to the Cu (100) facets, promoting dimerization on the Cu surface owing to electron flow between Ag and Cu.

Figure 8. (A) FE distribution of C₂H₄ on Cu/C and Cu@Ag/C catalysts; (B) FE_CO on Cu/C and Cu@Ag/C catalysts; (C) FE of various products on Cu@Ag/C at various potentials. This figure is quoted with permission from Duan et al. Copyright (2024) Elsevier^[105]; (D) Schematic illustration of three types of tandem catalysts; (E) FEs of products on layered Cu/Ag; (F) Partial current density of ethanol on Cu, mixed CuAg, layered Ag/Cu, and layered Cu/Ag catalysts. This figure is quoted with permission from Luan et al. Copyright (2024) American Chemical Society^[108]; (G) Schematic diagrams of Cu needle-Ag tandem catalysts; (H) FEs for H₂, C₂H₄, CH₄, HCOO^-, C₂H₅OH, CO, and propanol on Cu needle-Ag catalysts; (I) FEs for different products of the Cu needle-Ag tested in a flow cell. This figure is quoted with permission from Wei et al. Copyright (2023) John Wiley and Sons^[111]. FE: Faradaic efficiency.

In a tandem catalyst system, two main pathways are involved: (1) CO₂ is first reduced to CO by a metal with high CO selectivity, and (2) the generated CO then migrates to neighboring Cu sites, where it undergoes C-C coupling and hydrogenation to form C₂₊ products. However, if the initial CO formation occurs too rapidly, C-C coupling may be limited, leading to higher selectivity for CO over C₂₊ products. Extending the residence time of CO on Cu active sites increases the likelihood of CO absorption and further reduction, thereby decreasing the FE_CO and enhancing the FE_C2+. Thus, controlling CO residence time on Cu sites is a key factor in optimizing tandem catalyst systems. Wei et al. fabricated a 3D tandem catalyst by depositing Ag NPs at the base of Cu nanoneedle arrays to control the transport distance of CO [Figure 8G]^[111]. The electrocatalytic performance of the obtained tandem catalyst was initially measured in an H-type cell. Compared to a similar structure lacking Ag NPs at the base, the Cu needle-Ag achieved a selectivity of 45.6% for C₂H₄ at -1.0 V vs. RHE [Figure 8H]. Additionally, Cu needle-Ag catalysts were evaluated in a flow cell to achieve commercially relevant current densities. As depicted in Figure 8I, the Cu needle-Ag catalyst demonstrated exceptional selectivity towards C₂₊ products across applied current densities between 200 mA cm^-2 to 350 mA cm^-2. Specifically, the FE for C₂₊ products rose from 58.2% at 100 mA cm^-2 to 70% at 350 mA cm^-2. The densely packed Cu nanoneedles create significant diffusion resistance for CO as it moves from the Ag NPs, forcing CO to travel a longer path. This extended diffusion increased the likelihood of CO adsorption, promoting dimerization into C₂₊ products and thereby contributing to the enhanced CO₂RR performance.

Au-based

Similar to Ag, Au is particularly noted for its high selectivity towards CO, as it efficiently weakens CO₂ bonds and promotes the formation of CO intermediates. Bimetallic Au-Cu tandem catalysts have shown improved catalytic performance, with Au facilitating efficient CO production and Cu driving the subsequent C-C coupling, which is necessary to produce higher-value C₂₊ products. For example, Morales-Guioet al. employed electron-beam evaporation to arrange Au NPs onto polycrystalline copper foil (Au/Cu), creating tandem electrocatalysts^[112]. These Au/Cu catalysts displayed superior CO₂ reduction activity towards C₂₊ alcohols compared to recently developed Cu-based bimetallic catalysts. The existence of Au NPs on the copper foil enhances the local concentration of CO on adjacent copper surfaces, which subsequently facilitates its reduction to C₂₊. Notably, the Au/Cu catalysts demonstrated a selectivity for C₂₊ formation that was 100 times higher than that for CH₄ or CH₃OH at low overpotentials. Recently, Wang et al. used Au NPs of varying sizes and densities as a CO-producing component to boost CO₂ electroreduction into high-value chemicals^[113]. They developed gold-copper oxide (Au@Cu₂O) tandem catalysts through a galvanic replacement reaction. Through modification of the Au NP size and the concentration of Au precursors, they explored the relationship between size and performance. The Au@Cu₂O samples were designated as S-0.25, 0.5, and 2.0-Au@Cu₂O for smaller Au NPs and L-0.25, 0.5, and 2.0-Au@Cu₂O for larger Au NPs, reflecting the different sizes and concentrations of the Au precursors. The CO₂ reduction performance of L-0.5-Au@OD-Cu, S-0.5-Au@OD-Cu, S-2.0-Au@OD-Cu, and OD-Cu was assessed in a 1 M KHCO₃ using a flow cell. Our results showed that catalysts with smaller Au NPs exhibited more efficient CO₂ reduction reactions at a reduced overpotential compared to those with larger Au NPs and OD-Cu. Additionally, elevating the loading amount of Au NPs in Au@Cu₂O catalysts led to a decline in the selectivity of C₂₊ products such as C₂H₄ and C₂H₆O, while enhancing the FE of n-propanol. Cao et al. synthesized tandem catalysts consisting of Au and cubic Cu₂O through a galvanic replacement reaction followed by pre-reduction using the LSV method^[114]. Unlike bare Cu₂O, the Au_xCu₂O catalysts exhibited a shift in selectivity, with CO formation becoming more prominent. A significant increase in C₂H₄ selectivity was observed at potentials beyond -1.1 V vs. RHE. Among the catalysts, Au_0.02Cu₂O achieved the highest FE for C₂H₄, reaching 24.4% with a total current density of approximately 5.7 mA cm^-2 at -1.3 V vs. RHE, which was 2 to 2.5 times higher than that of the other Au_xCu₂O catalysts and five times greater than bare Cu₂O. This enhanced selectivity toward C₂H₄ was attributed to the optimal CO availability provided by Au atoms and the improved C-C coupling facilitated by the synergistic effects of Cu⁺ and Cu⁰. Zhu et al. introduced an effective strategy for designing an epitaxial Au-Cu heterostructure aimed at enhancing the CO₂-to-C₂₊^[115]. The electrochemical CO₂ reduction was conducted in an H-cell using 0.1 M KHCO₃ as the electrolyte. The Au-Cu heterostructure demonstrated notable C₂₊ alcohols production, with an onset potential approximately 150 mV more positive than that of pure Cu and a conversion rate for CO₂ to C₂₊ alcohols that was about 2.5 times higher at -1.0 V vs. RHE [Figure 9A]. As illustrated in Figure 9B and C, the Au-Cu heterostructure not only produced more alcohol compared to Cu but also effectively suppressed hydrocarbon formation, achieving a superior ratio of C₂₊ alcohols/(CO + > 2e^- products). Furthermore, a dynamic restructuring was observed, where Au-Cu bimetals with phase separation transitioned into alloy-supported Au@Cu core-shell nanoclusters during electrocatalysis, forming Au@Cu-AuCu structures [Figure 9D]. A unique tandem mechanism was introduced, highlighting the accumulation of ^*CO as a key factor for maintaining the durability of C₂₊ alcohol production. Wei et al. applied a homo-nucleation method to decorate Au NPs onto the surface of Cu NWs^[116]. Cu NWs are well-recognized for their significant aspect ratio with dense GBs, both of which have been shown to enhance electron transport and provide abundant active sites. They synthesized three tandem catalysts with varying Cu-to-Au mass ratios, labeled Cu_99.8Au_0.2, Cu_99.3Au_0.7, and Cu_96.7Au_3.3. Of these, Cu_99.3Au_0.7 demonstrated the best catalytic performance, achieving a FE_C2+ of 65.3% at -1.25 V vs. RHE, which is a notable advancement over the pristine Cu NWs (39.7%). Among the C₂₊ products, selectivity was highest for C₂H₄ at 35%, followed by C₂H₆O at 19.1%. Zheng et al. developed tandem electrocatalysts based on Janus NPs to overcome the limitations posed by the spatial arrangement of different constituent components^[117]. The Janus structure ensures optimal accessibility of the two linked metals, allowing the NPs to serve as dual-functional agents. Au-Cu Janus nanostructures (Au-Cu Janus NSs) were synthesized with N-oleyl-1,3-propanediamine (OPDA) as a capping agent. OPDA, with its amine group, is essential in inhibiting the surface oxidation of Cu NPs. To highlight the significance of OPDA in the formation of the Janus nanostructures, they also synthesized Au-Cu tandem catalysts using octadecylamine and oleylamine. When OPDA was replaced with other capping agents, the electrocatalysts did not display the Janus structure. The optimized Au-Cu Janus NSs exhibited selective CO₂ reduction towards C₂₊ products, achieving the highest total current density of nearly 1 Acm^-2, FE_C2+ of 67% and a j_C2+ of -0.29 A cm^-2 at -0.75 V vs. RHE, outperforming both Au@Cu core-shell nanostructures and Cu NPs [Figure 9E-H]. The distinctive structure of the Au-Cu Janus NSs promoted CO spillovers from the Au sites to neighboring Cu, enhancing CO coverage and facilitating C-C coupling.

Figure 9. (A) Rate of C₂₊ production on Au-Cu and Cu electrodes; (B) Dependence of the molar ratio of C₂₊ alcohols to hydrocarbons and of (C) alcohols to (CO + > 2e^- products) on Au-Cu and Cu as a function of potential; (D) Diagram depicting phase transformation and structural reconstruction of Au-Cu during the CO₂RR process. This figure is quoted with permission from Zhu et al. Copyright (2022) Elsevier^[115]; (E) Total current density of Cu, Au@Cu core-shell, and Au-Cu Janus electrocatalysts; (F) FE_C2+ of three different catalysts. (G) Comparison of optimal FE_C2+ obtained by using Au@Cu core-shell, Au-Cu Janus, and Cu NPs; (H) j_C2+ of Au@Cu core-shell, Au-Cu Janus, and Cu NPs. This figure is quoted with permission from Zheng et al. Copyright (2022) John Wiley and Sons^[117]. FE: Faradaic efficiency.

Molecular catalysts and single-atom catalysts based

Bimetallic systems integrating Cu with other CO-producing metals, including Au and Ag, have been extensively studied as electrocatalysts for C₂₊ production, as mentioned above^[118-120]. However, a significant challenge with these catalysts is the potential for changes in composition and morphology during operation, which can alter both their physical and electronic properties. These variations complicate the insights into catalytic behavior and hinder the development of effective design principles to enhance catalytic performance. Consequently, substantial studies have concentrated on developing tandem catalysts that employ CO-producing molecular catalysts and single-atom catalysts (SACs). Liu et al. developed high-performance tandem catalysts composed of cobalt phthalocyanines (CoPc), acetylene black, and Cu₂O nanocrystals (Cu₂O NCs-C-CoPc)^[121]. The electrocatalytic performance for C₂₊ products was evaluated using both H-cell and flow-cell systems. In the H-cell, Cu₂O NCs-C-Copc achieved a maximum FE for C₂H₄ of 58.4%, with a j_C2H4 of -29.74 mA cm^-2 and durability of ten hours. When tested in the flow cell, the selectivity for C₂H₄ increased significantly to 70.3%, with a corresponding j_C2H4 of -226.18 mA cm^-2. Similarly, Kong et al. developed a tandem catalyst by integrating CoPc with a Cu GDE^[122]. Initially, Cu was sputtered onto a polytetrafluoroethylene (PTFE) substrate, followed by spraying a CoPc-methanol solution onto the Cu GDE, creating the Cu-CoPc GDE. The optimized Cu-CoPc GDE exhibited excellent performance for CO₂ conversion to C₂₊ products, resulting from the high ^*CO coverage facilitated by the CO-producing CoPc. Specifically, the Cu-CoPc GDE achieved a FE of 82% for C₂₊ products at an applied current density of 480 mA cm^-2, which is 1.8 times higher than that of the Cu GDE alone. Min et al. utilized metal porphyrins, recognized for their efficiency in CO₂ adsorption and ^*CO intermediate formation^[123]. Using a liquid-phase approach, they developed self-supporting tandem catalysts by modifying a cuprous oxide NW array supported on copper mesh decorated with cobalt(II) tetraphenylporphyrin (CoTPP) molecules, resulting in the CoTPP-Cu₂O@CM catalyst [Figure 10A]. The optimized CoTPP-Cu₂O@CM catalyst demonstrated enhanced C₂₊ selectivity in an H-cell, achieving a FE of 73.2% for C₂₊ products and a current density of -52.9 mA cm^-2 at -1.1 V vs. RHE, surpassing the performance of recently reported Cu-based catalysts [Figure 10B and C]. Furthermore, CoTPP-Cu₂O@CM catalyst exhibited long-term durability of about 14 h without significant change in FE_C2H4 and current density [Figure 10D].

Figure 10. (A) Fabrication of CoTPP-Cu₂O@CM NWs array; (B) FE_C2+ of CoTPP-Cu₂O@CM, Cu₂O@CM, CoTPP-CM, and Cu mesh; (C) j_C2+ of CoTPP-Cu₂O@CM, Cu₂O@CM, CoTPP-CM, and Cu mesh; (D) Stability test of CoTPP-Cu₂O@CM for 15 h. This figure is quoted with permission from Min et al. Copyright (2024) John Wiley and Sons^[123]; (E) Graphic representation of PTF(Ni)/Cu; (F) Total current densities on PTF(Ni)/Cu and PTF/Cu catalysts; (G) FEs for the production of C₂H₄ and CH₄ on PTF(Ni)/Cu and PTF/Cu catalysts at different potentials; (H) j_C2H4 of PTF(Ni)/Cu. This figure is quoted with permission from Meng et al. Copyright (2021) John Wiley and Sons^[128]. CoTPP: cobalt(II) tetraphenylporphyrin; PTF: porphyritic triazine framework.

SACs are a unique type of catalyst with well-defined coordination and distinct electronic structures^[124]. Among the SACs, Ni SACs demonstrate excellent conversion of CO₂ to CO, making them particularly appealing for use in tandem catalysts. For instance, Zhang et al. developed a highly C₂H₄ selectivity tandem catalyst by simply mixing colloidal Cu NPs and Ni SACs (Cu NPs/Ni-N-C)^[125]. The colloidal Cu NPs were first synthesized using Tetradecylphosphonic acid (TDPA) as a capping agent, allowing precise control over the size and shape of the NPs. The optimized Cu NPs/Ni-N-C catalyst exhibited excellent performance for C₂H₄ production, achieving a FE of 14.52%, a j_C2H4 of -1.67 mA cm^-2, and a C₂H₄/CH₄ ratio of 20.1 at -0.9 V vs. RHE. Zhang et al. developed CuO/Ni tandem catalysts, composed of CuO and Ni SAs, aimed at enhancing C₂₊ production^[126]. They investigated three strategies for fabricating tandem catalytic electrodes: layer-by-layer spraying, adjacent nanostructures, and physical mixing. Among these, the adjacent nanostructure electrode demonstrated superior efficiency by promoting more effective CO generation at the Ni sites, thereby increasing the local CO concentration near the Cu sites and enhancing C₂₊ product formation. Consequently, the CuO/Ni SA tandem catalyst attained an impressive j_C2+ of 1,220.8 mA cm^-2, along with a high FE for C₂₊ products at 81.4%. Among the FE_C2+, FE towards C₂H₄ resulted in 54.1% and 28.8% for C₂H₆O. Additionally, Liu et al. developed tandem electrocatalysts featuring nickel SAs, which exhibited a wide potential window for CO generation when combined with copper (Ni SAC + Cu-R)^[127]. To assess the function of Ni SACs, DFT calculations were conducted to compare the activation barriers for ^*OCCO formation, using Ag NPs and Nickel phthalocyanines as control references. To support their theoretical results, Ni SACs were synthesized through electrostatic self-assembly, while Cu catalysts were produced using a one-step wet chemical reduction method. The Ni SAC + Cu-R tandem catalysts were then prepared by thoroughly mixing Ni SAC and Cu-R. The CO₂ electroreduction performance was evaluated in a flow cell with 1M KHCO₃ solution. The Ni SAC + Cu-R catalysts showed significantly improved performance compared to individual Ni SAC and Cu-R catalysts, achieving a j_C2H4 of ~370 mA cm^-2 for C₂H₄ with a FE of ~62% and a stability of approximately 14 h at a current density of 500 mA cm^-2. Meng et al. developed a facile approach to construct a tandem catalyst composed of Cu NPs on a porphyritic triazine framework anchored with atomically isolated nickel-nitrogen sites [PTF(Ni)]^[128]. PTF(Ni), with abundant porosity, was fabricated, and Cu NPs were embedded through a reduction reaction step, resulting in PTF(Ni)/Cu [Figure 10E]. H-type cells with the mixed electrolyte of 0.1 M KHCO₃ and 0.1 M KCl saturated with CO₂ were utilized to evaluate the catalytic performance of the catalysts. Compared to PTF/Cu, PTF(Ni)/Cu demonstrated outstanding performance, achieving a peak FE_C2H4 of 57.3% and a j_C2H4 of 3.1 mA cm^-2 at -1.1 V vs. RHE [Figure 10F-H]. This suggests that the atomically dispersed Ni sites in PTF(Ni)/Cu serve a vital function in increasing the selectivity for C₂H₄ by facilitating the generation of CO, causing an increased accumulation of *CO intermediate. Similarly, Chen et al. encapsulated Cu NPs in mesoporous carbon (CMK-8) with doped atomic Ni-N₄ moieties, resulting in a-Ni/Cu-NP@CMK^[129]. Electrochemical tests indicate that a-Ni/Cu-NP@CMK exhibits remarkable selectivity for C₂H₄, achieving a FE_C2H4 of 72.3% at a notable current density of 406.1 mA cm^-2 when operated in a flow cell under neutral conditions. Furthermore, when measured using a membrane electrode assembly (MEA) cell, a-Ni/Cu-NP@CMK demonstrated excellent stability of over 30 h under the 200 mA cm^-2 and a FE_C2H4 of 63% at -2.8 V vs. RHE. Furthermore, the optimized a-Ni/Cu-NP@CMK exhibited superior energy efficiency for C₂H₄ production of about 28.3%. The authors suggested that the abundance of CO molecules around the active sites and the hydrophobic environments contributed to the high performance of a-Ni/Cu-NP@CMK in C₂H₄ production.

Non-Cu-based catalysts

Cu is the primary metal known to produce C₂₊ products; however, recent studies indicate that several non-Cu-based catalysts can also generate C₂₊ products with careful design. Paris et al. fabricated a novel Ni₃Al deposited glassy carbon electrode by drop-casting and furnace reduction process^[130]. The as-synthesized electrode successfully converted CO₂ into C₂₊ and C₃₊ products for the first time, with the maximum FEs for ethane, ethylene, and 1-propanol reaching 1.7%, 0.4%, and 1.9 ± 0.3%, respectively. Torelli et al. synthesized several Ni-Ga alloys with distinct phases (NiGa, Ni₃Ga, and Ni₅Ga₃), which can convert CO₂ to C₂₊ at low overpotentials^[131]. The Ni₅Ga₃ alloy achieved a FE of 1.3% for C₂H₆ at -0.48 V vs RHE, whereas Cu-based catalysts only produced CH₄ at the same low overpotential. Recently, Ding et al. reported Sn-based electrocatalysts with superior catalytic performance for CO₂ electroreduction to ethanol^[132]. Sn-based electrocatalyst is composed of SnS₂ NSs and Sn SACs (SnS₂/Sn₁-O₃G). The electrocatalytic performance for CO₂RR was evaluated in an H-cell with 0.5 M KHCO₃ as the electrolyte. SnS₂/Sn₁-O₃G exhibited excellent selectivity toward ethanol, with a FE above 70% over a wide potential window, reaching a maximum FE of 82.5% for ethanol at -0.9 V vs. RHE. Furthermore, it demonstrated exceptional long-term stability, maintaining 97% of its initial activity after 100 h. This outstanding performance is primarily attributed to the dual active centers of Sn and O atoms on Sn₁-O₃G, which facilitate the formyl-bicarbonate coupling pathway, thereby enhancing selectivity toward ethanol.