Selective CO2 and CO electroreduction to C2–C7 hydrocarbons on Pd–Au electrodes: Role of interfacial impedance and CO adsorption modulation

Abstract

Electrochemical reduction of CO₂ and CO was systematically studied using bare Pd and Au-modified Pd electrodes in KHCO₃ and phosphate electrolytes to explore the impact of electrode composition and electrolyte environment on product selectivity and reaction kinetics. CO and H₂ were the dominant gaseous products, with minor CH₄ and C₂₊ hydrocarbons detected. Formate and methanol were observed only under CO₂ conditions. Au incorporation into Pd significantly enhanced hydrocarbon formation—by more than 10 times—particularly in phosphate electrolyte, by weakening CO binding and promoting intermediate desorption. The use of phosphate electrolyte increased the C₂₊ Faradaic efficiency by over 8-fold compared to KHCO₃. Anderson–Schulz–Flory analysis confirmed Fischer–Tropsch-like behavior up to C₇ hydrocarbons, with alkenes showing higher chain growth probability than alkanes. Electrochemical impedance spectroscopy revealed that KHCO₃ induces higher charge transfer and interfacial resistance, likely due to strong CO adsorption, whereas phosphate promotes more favorable interfacial kinetics. Au deposition further reduced interfacial resistance and modulated double-layer capacitance, depending on electrolyte pH and applied potential. Spectroscopic characterization confirmed that Au modified the electronic structure of Pd, stabilizing the active surface under reductive conditions. These findings highlight the synergistic effects of Au alloying and phosphate electrolytes in optimizing CO₂/CO electroreduction toward long-chain hydrocarbon production.