Boosting electrocatalytic CO2-to-ethanol production via asymmetric C-C coupling

Electroreduction of CO2 (CO2RR) into value-added C1 and C2+ products enables storage of renewable electricity and mitigation of CO2 emissions. While many metals (Au, Ag, Sn, Pb) favor C1 products, Cu-based catalysts uniquely produce C2+ products via coupling of adsorbed CO intermediates. Ethanol (EtOH) is a key liquid fuel and chemical feedstock with high energy density and attractive storage/transport characteristics, but its selective production at high current density remains challenging. EtOH and ethylene (C2H4) share early intermediates (e.g., HCCOH), but EtOH’s more saturated structure makes subsequent intermediates harder to stabilize on pure Cu relative to C2H4, typically yielding EtOH at only one-half to one-third the rate of C2H4 on Cu. The research objective is to boost EtOH selectivity and production rate at commercially relevant high current densities by engineering Cu-based catalysts to favor reaction pathways and intermediates specific to EtOH formation.

Prior strategies to enhance EtOH on Cu-based catalysts include morphology and facet control, vacancy engineering, dopants/modifiers, and defect engineering. Bimetallic modification of Cu with other CO2-active metals is particularly promising. Reported Cu-Au and Cu-Ag systems can increase C–C coupling and stabilize oxygenated intermediates, sometimes through tandem mechanisms where CO produced on one site feeds Cu. Binding-site diversity and compressive strain in CuAg surface alloys have been linked to improved selectivity toward multicarbon oxygenates and EtOH. Nevertheless, EtOH formation generally remains lower than C2H4 because stabilizing EtOH-specific intermediates after the shared HCCOH is difficult on pure Cu. Therefore, designing Cu-based bimetallics that tune surface coordination and oxidation state to modulate CO adsorption (atop vs bridge) and facilitate asymmetric C–C coupling may unlock higher EtOH selectivity at high rates.

Catalyst synthesis: Cu2O/Ag nanocubes (NCs) were prepared via a one-pot seed-mediated route. Cu(OH)2 was formed by mixing NaOH and Cu(NO3)2 in water at room temperature, then reduced with ascorbic acid (AA) to generate Cu2O NCs. Subsequently, AgNO3 was added (with AA present), nucleating small metallic Ag nanoparticles on Cu2O NC surfaces. Ag content was controlled to 2.3% (and varied for series studies). Analogous procedures produced Cu2O and Cu2O/Au NCs (HAuCl4 as precursor). Characterization (pristine): TEM/HRTEM, SEM-EDS, XPS, XRD confirmed Cu2O cubes (~45 nm) decorated with metallic Ag NPs; interplanar spacings 0.214 nm (Cu2O (200)) and 0.236 nm (Ag (111)); surface electronic interaction (Cu2O to Ag electron transfer) indicated by Cu 2p shoulders and Cu LMM AES shifts. Activation and structural evolution: Catalysts were spray-coated on GDEs and electrochemically activated under CO2RR at 200 mA cm−2 in 1 M KOH for 30 min in a flow cell, yielding oxide-derived catalysts dCu2O, dCu2O/Ag, and dCu2O/Au. Post-activation HAADF-STEM/SEM showed ragged/hollow morphologies and loss of initial phase separation, with alloy formation indicated by XRD (dominant metallic Cu peaks; Cu(111) peak shift for Ag/Au-modified samples). In situ XAS (Cu K-edge) under activation revealed Cu valence ~0 to +1 (order: dCu2O < dCu2O/Ag < dCu2O/Au), similar Cu–Cu bond lengths (~6.7 Å−1 in WT-EXAFS), dominant Cu–Cu coordination at ~2.23 Å with residual Cu–O at ~1.35 Å for Ag/Au-modified samples. First-shell coordination numbers: 11.4 (dCu2O), 10.5 (dCu2O/Ag2.3%), 10.2 (dCu2O/Au2.3%). Ag 3d binding energies shifted higher after activation, evidencing Cu–Ag alloying and electron transfer. Electrochemical testing (flow cell): Microfluidic flow cell with anion-exchange membrane. Cathode: catalyst-coated GDE (~0.44 mg cm−2); reference: Ag/AgCl (4.0 M KCl); anode: Ni foam. Electrolyte: 1 M KOH, 15 mL min−1 circulation; CO2 gas: 30 mL min−1. Chronopotentiometry at fixed total currents (200–800 mA) for 1 h, no iR correction during operation. Potentials referenced to RHE with post hoc 85% iR correction. Gas products quantified by GC; liquid products by 1H NMR. Faradaic efficiencies (FE) and energy efficiencies (EE) computed using standard formulas; EtOH EE referenced to E0red = 0.08 V vs RHE and OER at 1.23 V. MEA testing: Two-electrode MEA with Sustainion AEM, Cu2O/Ag2.3%-GDE cathode (~0.44 mg cm−2) and Ni-foam anode; CO2 at 30 mL min−1; circulating anolyte (0.1 M). Operated at selected total currents/voltages; products analyzed similarly; stability assessed up to 12 h at 800 mA cm−2. Mechanistic probes: CORR (CO feed) measurements to test tandem-CO mechanism. In situ ATR-IRAS during CO2RR (Ge prism working electrode) to detect surface intermediates and CO adsorption configurations (atop vs bridge), and potential-dependent evolution. CO-TPD/CO2-TPD to assess adsorption strength. Additional analyses included ECSA by DLC and Pb UPD methods.

• Ag-modified oxide-derived Cu (dCu2O/Ag2.3%) achieves high ethanol performance at commercially relevant rates:
  • Faradaic efficiency for EtOH: 40.8% at 800 mA cm−2.
  • Energy efficiency for EtOH: 22.3% (flow cell; 22.4% reported after iR correction under similar conditions).
  • Partial current density for EtOH: 326.4 mA cm−2 at −2.11 V vs RHE (no iR) or −0.89 V vs RHE (85% iR-corrected).
  • EtOH formation rate: 1014.9 µmol h−1 cm−2 at 800 mA cm−2.
• Total C2+ performance:
  • FE(C2+): up to 82.1% at 800 mA cm−2, with partial C2+ current density 656.8 mA cm−2 and formation rate 2042.2 µmol h−1 cm−2 at −2.11 V vs RHE (no iR correction).
  • FE(EtOH)/FE(C2H4) ratio reaches 1.17 at 800 mA (vs 0.51 for dCu2O and 0.71 for dCu2O/Au2.3%), indicating favored EtOH over C2H4.
• Structure and electronic changes upon activation:
  • Cu2O/Ag evolves to a Cu–Ag alloy with reduced Cu coordination (Cu–Cu coordination number 10.5 vs 11.4 for dCu2O) and residual Cu(I) fraction; Ag 3d shifts indicate alloying/electron transfer.
• Mechanistic insights from spectroscopy and probes:
  • In situ ATR-IRAS shows dCu2O/Ag2.3% supports both atop- and bridge-bound CO (*COatop at ~2044 cm−1; *CObridge at ~1923 cm−1), with significantly higher *CObridge/*COatop ratios than dCu2O and dCu2O/Au2.3%.
  • Evidence of *CHO formation increasing with potential on dCu2O/Ag2.3%, consistent with protonation of *CObridge and triggering asymmetric C–C coupling (*CO + *CHO) with lower barrier than *CO–*CO dimerization.
  • Signals assigned to *OCCOH and *OC2H5 indicate more stable EtOH-related intermediates on dCu2O/Ag2.3% (higher *OC2H5/*OCCOH ratio), supporting preferential EtOH pathway.
  • Absence of bicarbonate peak (~1547 cm−1) on dCu2O/Ag2.3% suggests higher local pH, favoring C–C coupling and suppressing HER.
  • CO-TPD/CO2-TPD show stronger binding (higher desorption temperatures) for CO2 and CO on dCu2O/Ag2.3%.
• Performance trends with Ag content:
  • Volcano relationship between Ag fraction and FE(EtOH); inverse trend for FE(CO); FE(C2H4) monotonically decreases with more Ag.
• CORR results refute a classic CO-tandem mechanism for this catalyst:
  • Under CO feed, dCu2O/Ag2.3% shows suppressed H2, enhanced C–C coupling, and higher C2+ partial current (696.0 mA cm−2 at −1.56 V vs RHE) than dCu2O/Au2.3% (~154.0 mA cm−2) and dCu2O (~188.0 mA cm−2).
• Stability:
  • Flow cell: ~6 h with no apparent activity decay; EtOH selectivity decreases by ~6%.
  • MEA: ~12 h at 800 mA cm−2 and −4.72 V cell voltage with ~3% decrease in FE(EtOH); morphology/structure largely retained post-test.

The work addresses the long-standing challenge of selectively producing ethanol at high current densities on Cu-based catalysts. Introducing Ag into Cu2O and activating under CO2RR forms a Cu–Ag alloy surface with moderated Cu coordination numbers and an optimal partial oxidation state, which tunes the *CO binding to a mixture of atop and bridge configurations. This mixed adsorption landscape facilitates protonation of *CObridge to *CHO and enables asymmetric C–C coupling between *CO and *CHO. The resulting unbalanced coordination environment preferentially stabilizes oxygenated, more saturated EtOH intermediates (e.g., *OC2H5) over less oxygenated C2H4 intermediates, steering selectivity toward EtOH. Spectroscopic evidence (ATR-IRAS) for elevated *CObridge/*COatop ratios, detection of *CHO, and increased *OC2H5/*OCCOH, together with higher local pH and stronger CO/CO2 binding, support this mechanism. Comparative results with Au-modified Cu2O indicate that Au predominantly boosts CO formation rather than C–C coupling, whereas Ag specifically tailors the Cu surface to enhance C–C coupling and EtOH selectivity. CORR experiments demonstrate behavior inconsistent with a simple CO-tandem mechanism, underscoring intrinsic changes to Cu active sites by Ag. Collectively, these effects produce the highest reported EtOH partial current density among Cu-based catalysts while maintaining high FE and competitive energy efficiency, highlighting the significance of asymmetric C–C coupling as a design principle for multi-electron CO2RR selectivity control.

A silver-modified oxide-derived copper catalyst (dCu2O/Ag2.3%) was developed that achieves high-rate, selective CO2-to-ethanol conversion. It delivers 40.8% FE for EtOH, up to 22.3–22.4% energy efficiency, and a record-high partial EtOH current density of 326.4 mA cm−2 at −0.89 V vs RHE (85% iR-corrected), with total C2+ FE up to 82.1% at 800 mA cm−2. In situ spectroscopy reveals that Ag induces moderated Cu coordination and optimal oxidation, promoting mixed atop/bridge CO adsorption and asymmetric C–C coupling that stabilizes EtOH intermediates. This mechanism contrasts with classic CO-tandem pathways and provides a clear strategy for catalyst design to favor specific multi-carbon products at commercially relevant current densities. Future work can extend this asymmetric coupling concept to other bimetal combinations and explore long-term durability and scale-up in industrially relevant electrolyzer architectures.