Interfacial Engineering of Zn–Ag Bimetallic Catalysts to Enable Efficient CO2-to-CO Conversion in Both Acidic and Alkaline Media

Abstract

Electrochemical CO2 reduction (CO2RR) offers a promising strategy for converting greenhouse gases into value-added chemicals; however, maintaining high selectivity across diverse pH environments remains challenging. Acidic electrolytes enhance carbon efficiency by suppressing carbonate formation but simultaneously favor the hydrogen evolution reaction, whereas alkaline media provide high activity while causing CO2 loss through carbonate/bicarbonate formation. Here, we report a nanoporous-Zn–Ag catalyst with engineered interfacial sites that enables efficient CO2-to-CO conversion in both acidic and alkaline environments. The catalyst was synthesized by CTAB-assisted electrodeposition followed by controlled galvanic replacement, generating Zn–Ag interfaces that optimize CO2RR activity. In a flow-cell configuration, the optimized Zn–Ag catalyst prepared with 0.02 mM AgNO3 (denoted ZnAg-1) achieved a Faradaic efficiency (FE) exceeding 90% for CO2 conversion into CO and formic acid production at –100 mA cm–2 in both 1.0 M KOH (pH 14) and 0.05 M H2SO4 + 1.0 M KCl (pH 1), with stable operation long-term electrolysis. Interestingly, the product distribution depended strongly on electrolyte pH:CO selectivity reached 87% in alkaline media but decreased to ∼71% under acidic conditions, while formate/formic acid contribution increased from 3 to 20%. Density functional theory free-energy calculations using representative Zn–Ag interfacial models suggest that local Zn–Ag coordination can lower the thermodynamic requirement for *COOH formation and facilitate CO desorption; the potential-limiting PCET step is *COOH formation from *CO2 (ΔG = 0.34 eV), corresponding to UL = –0.34 V vs RHE. The experimentally observed increase in formate/formic acid under acidic conditions is tentatively attributed to the competing *OCHO pathway, which is not explicitly evaluated in the present DFT calculations. These results demonstrate that interfacial engineering in bimetallic catalysts enables efficient CO2RR across a wide pH range, providing a viable pathway toward industrial implementation.