A scalable method for preparing Cu electrocatalysts that convert CO₂ into C₂⁺ products

The study targets the challenge of designing scalable Cu electrocatalysts that selectively convert CO2 into valuable multi-carbon (C2+) products via electrochemical CO2 reduction (CO2RR). While copper uniquely catalyzes CO2 to hydrocarbons and alcohols, practical deployment requires catalysts that minimize two competing pathways: hydrogen evolution reaction (HER) and formation of C1 products (e.g., CH4, HCOOH) that reduce the surface coverage of carbon intermediates needed for C–C coupling. The authors argue that minimizing local pH rises (to suppress HER) necessitates low surface roughness, whereas maximizing C–C coupling requires a high density of defect sites to increase the surface coverage of reactive C1 intermediates (e.g., *CO, *CHO, *COH, *CH2, *CHOH). Simulations and prior mechanistic studies indicate lower onset for *CO formation on stepped facets like Cu(211), lower barriers for CO dimerization on Cu(100), and second-order kinetics for C2 formation with respect to *CO coverage; defect sites and possibly Cu+ and subsurface oxygen can promote adsorption and coupling. The research question is whether a controlled, scalable electrochemical preparation can deliver Cu surfaces that optimally balance high defect density with low roughness to achieve high FE for C2+ products while suppressing HER and C1 products.

Previous strategies to improve Cu-based CO2RR include porous Cu foams, nanoparticle ensembles, oxide-derived Cu, plasma-treated Cu, nanocubes, nanowires, wet-oxidation CuCl-derived Cu, and single-crystal Cu. Reports of cubic Cu microstructures (often enriched in (100) facets) via electrochemical cycling in halide electrolytes improved ethylene selectivity vs methane but typically achieved FE for C2H4 of ~15–45%, likely due to complexities of cycling protocols. Mechanistic and DFT studies suggest: (i) lowest onset for *CO formation on Cu(211) vs (111)/(100); (ii) lowest barrier for C–C coupling (*OCCO) formation on Cu(100); (iii) CO dimerization barrier decreases with increasing *CO coverage; (iv) C2 formation is second order in *CO; (v) under-coordinated sites (steps, grain boundaries, vacancies) enhance C–C coupling; (vi) roles of Cu+ and subsurface O remain debated but may promote CO2 adsorption and coupling. High surface roughness in high-current systems drives local pH up, decreasing [CO2]/[H+] and potentially reducing CO2RR efficiency relative to HER. This context motivates a method that independently controls halide chemistry, potentials, pH exposure, and roughness to generate defect-rich yet not overly rough Cu surfaces.

Catalyst preparation (three-step, scalable): (i) Anodic halogenation of electropolished Cu foils in 0.1 M KX (X=Cl, Br, I) using a three-electrode setup (Cu working, Pt gauze counter, Ag/AgCl reference). Measured OCPs: −0.115 V (KCl), −0.134 V (KBr), −0.315 V (KI) vs Ag/AgCl. Applied potentials: 1.1 V (KCl), 0.18 V (KBr), −0.2 V (KI) vs Ag/AgCl; effective anodic potentials defined by Veff = Vapp − Voc to drive halogenation. Typical halogenation times ranged 1–300 s; representative 50 s produced CuX layers of ~1.25 µm (CuCl), 1.11 µm (CuBr), 0.61 µm (CuI). (ii) Oxide formation by immersion in air-saturated 0.1 M KHCO3 (pH ~9.0) for 10 min to convert CuX partially to Cu2O via 2CuX + OH− → Cu2O + 2X− + H+. Extent/rate follow halide coordination affinity and solubility (CuCl < CuBr < CuI stability). (iii) Electroreduction by linear sweep voltammetry (LSV) in CO2-saturated 0.1 M KHCO3 (pH ~6.8), scanning from OCP to −1.8 V vs Ag/AgCl at 5 mV/s, extracting halide and reducing Cu(I) species to Cu0, yielding reconstructed, defect-rich Cu with modest roughness. Characterization: GI-XRD (Bruker D8, 2° incidence, Cu Kα) to identify surface phases (CuCl/CuBr/CuI after halogenation; Cu2O after base exposure; metallic Cu after reduction). SEM (LEO 1530 VP) for morphology; EDS (Oxford Inca X-sight) for elemental composition converted to Cu, Cu2O, CuX fractions (assuming only these species per GI-XRD). XPS for residual halide/oxidation states. Double-layer capacitance by CV (−0.35 to 0.5 V vs Ag/AgCl) to estimate roughness factor relative to electropolished Cu. Electrochemical CO2RR testing: Two-compartment H-cell with Nafion 117 separator; 8.2 mL 0.1 M KHCO3 per compartment; Cu working and Ag/AgCl reference in catholyte; Pt gauze anode; CO2 flow 20 mL/min; stirring ~1500 rpm. Chronoamperometry for 40 min at constant potential; potentials reported vs Ag/AgCl and iR-corrected to RHE using Vcomp(Ag/AgCl)=Vapp+iR and Vcomp(RHE)=Vcomp(Ag/AgCl)+0.197+0.059·pH. Product analysis: GC (methanizer+FID for CO/hydrocarbons; TCD for H2; N2 carrier) after 10 and 38 min; liquid products by 1H NMR with internal standards (phenol, DMSO) and WET water suppression. Controls and variables: Control oxidation in 0.05 M K2SO4 at 1.1 V produced Cu2O (no halide). Halogenation time systematically varied to tune coverage, defect density, and roughness; roughness–HER correlations quantified; local speciation of carbonate system calculated vs pH to relate [CO2]/[H+] to HER/CO2RR competition.

- Phase formation by GI-XRD: Control oxidation in sulfate yields Cu2O; anodic halogenation in KCl/KBr/KI yields CuCl/CuBr/CuI, respectively. After base immersion, partial conversion to Cu2O occurs (extent Cl > Br >> I); after LSV reduction, patterns resemble metallic Cu, indicating halide extraction and reduction. - Morphology evolution (SEM): 50 s halogenation yields CuX layers of ~1.25 µm (CuCl), 1.11 µm (CuBr), 0.61 µm (CuI). CuCl/CuBr show surface wrinkling from volume expansion; CuI shows triangular pyramids. After base immersion (pH 9 KHCO3): CuKCl rapidly forms Cu2O cubes; CuKBr shrinks/wrinkles modestly; CuKI shows minimal change. After LSV reduction: CuKCl forms smaller, uniform cubes; CuKBr shrinks further with cracks; CuKI undergoes dramatic restructuring due to rapid CuI reduction (CuI + e− → Cu + I−). - Surface composition (EDS converted): As-prepared CuKCl ~4.06% Cu2O, 67.3% CuCl; after immersion: 46.2% Cu2O, 12.0% CuCl. As-prepared CuKBr ~4.13% Cu2O, 66.6% CuBr; after immersion: 63.7% Cu2O, 22.4% CuBr. CuKI changes slightly: 16.1% Cu2O/83.9% CuI → 21.6% Cu2O/78.3% CuI after immersion. After LSV, trace halides remain (e.g., ~0.33% CuBr, 0.12% CuI); CuKI shows higher Cu2O (~22.7%) due to rapid air re-oxidation indicating high defect density/susceptibility. - CO2RR selectivity and activity: Major product on halide-derived Cu is ethylene; peak FE(C2H4) at ~−1.15 V vs RHE of 45.1% (CuKCl), 49.5% (CuKBr), 44.5% (CuKI). Electropolished Cu predominantly yields CH4 with FE up to 54.0% at similar potentials. Halide-derived catalysts produce CO with FE 23–28% at ~−0.68 V vs RHE vs 0.5% on electropolished Cu, indicating higher *CO coverage. - Optimized halogenation times balance defect density and roughness, maximizing C2+: CuKCl (60 s) FE(C2H4)=50.2%, FE(C2+)=70.7%; CuKBr (90 s) FE(C2H4)=50.9%, FE(C2+)=71.5%; CuKI (10 s) FE(C2H4)=50.0%, FE(C2+)=72.6%. Excessive halogenation time (up to 300 s) increases roughness and HER (e.g., FE(H2) ~15–20%). - Roughness–HER correlation: Roughness factor (from double-layer capacitance) increases with halogenation time and correlates with higher FE(H2). Modeling of carbonate speciation shows [CO2]/[H+] ratio maximized near pH ~8.3 and drops sharply above pH ~9.9; high roughness drives local pH up, lowering [CO2]/[H+], favoring HER over CO2RR. - Mechanistic insight: High density of defect/under-coordinated sites boosts *CO coverage and C–C coupling; surfaces stabilize Cu+ and subsurface oxygen at defect sites under basic conditions. Incidence-angle dependent GI-XRD indicates decreased surface crystal ordering with reconstruction (e.g., CuKBr), consistent with defect-rich surfaces. Ethane appears only at very high roughness (roughness factor >30; FE(C2H6) ~1.2%), implying concurrent high surface *C2H4 and *H, further evidencing roughness–HER linkage. - Scalability and control: Anodic halogenation durations at fixed potentials tune defect density and roughness reproducibly; iodide reacts rapidly allowing short treatments (e.g., 10 s) to achieve optimal performance.

The findings demonstrate that electrochemically prepared Cu catalysts with a high density of defect sites combined with relatively low surface roughness are crucial for selectively converting CO2 into C2+ products. The three-step protocol—anodic halogenation, base-induced oxide formation, and electroreduction—drives controlled surface reconstruction that produces under-coordinated Cu sites, stabilizes Cu+ and subsurface oxygen at basic pH, and enhances *CO coverage, thereby lowering barriers for C–C coupling and increasing second-order C2 formation rates. Simultaneously, limiting surface roughness mitigates local pH rises that reduce [CO2]/[H+], suppressing HER and avoiding excessive conversion of dissolved CO2 to bicarbonate/carbonate in the interfacial region. The strong roughness–HER correlation, supported by double-layer capacitance measurements and carbonate speciation calculations, rationalizes why optimizing halogenation time is essential. Compared to single-crystal or complex wet-chemistry routes, the anodic halogenation approach is simple, consistent, and scalable while achieving FE(C2+) up to ~72.6%, advancing beyond prior cuprous-halide-derived methods (~58% FE(C2+)). These results underscore that defect engineering and interfacial pH management, rather than solely favoring specific low-index facets, govern high selectivity toward multi-carbon products on Cu.

This work introduces a scalable, controllable electrochemical method to prepare Cu(I)-halide-derived Cu electrocatalysts that deliver high FE for C2+ products (up to 72.6%) in aqueous CO2RR. The key is balancing a high density of under-coordinated/defect sites (enhancing *CO coverage and C–C coupling) with low surface roughness (suppressing HER via mitigation of local pH rise). GI-XRD, SEM/EDS, and electrochemical analyses elucidate phase evolution, morphology, composition, and the roughness–HER relationship. The approach is simple, reproducible, regenerative, and scalable, offering a practical route for carbon utilization technologies. Future research could explore long-term stability and durability under continuous operation and higher current densities, operando probes of active sites and oxidation states (Cu+/subsurface O) during CO2RR, translation to gas-diffusion electrode architectures, optimization across electrolytes and pH to further control [CO2]/[H+], and extension to alloyed or modified Cu surfaces to tailor defect chemistry.

- Durability and long-term stability are not extensively evaluated; electrolysis periods are 40 min, and susceptibility to rapid re-oxidation (notably for reduced CuKI) indicates potential stability concerns upon air exposure and possibly during operation. - Measurements are conducted in an H-cell at modest current densities; performance under commercially relevant conditions (e.g., gas-diffusion electrodes, higher current) is not assessed. - Residual halides and surface oxides, although low post-reduction, may influence activity/selectivity; precise identification and stability of Cu+ and subsurface oxygen during CO2RR remain indeterminate. - Optimal performance depends on halogenation time/potential and specific KHCO3 electrolyte conditions; generalizability to other electrolytes/pH regimes requires further validation.