pH Dependence of Cu Surface Speciation in the Electrochemical CO Reduction Reaction

Cu surfaces are known for their ability to electrochemically convert CO2 and CO into multicarbon (C2+) products. Although the impact of the electrolyte alkalinity on the electrochemical CO2 and CO reduction reactions (CO2RR and CORR, respectively) has been extensively investigated on Cu surfaces, systematic investigation of the pH dependence of the surface Cu speciation has been lacking. Herein, we demonstrate for the first time the pH dependence of Cu surface speciation and CO adsorption configuration in weakly to strongly alkaline electrolytes using in situ surface enhanced Raman spectroscopy. Oxygen-containing species, e.g., CuOx/(OH)y, are absent in near-neutral electrolyte of KHCO3 (pH = 8.9), and their onset potential becomes more positive with increasing the alkalinity of electrolytes. Concurrently, adsorbed CO transitions from atop-bonded (COatop) to bridge-bonded configuration (CObridge) as the pH of the electrolyte increases. CO is proposed to adsorb in the atop configuration only on Cu sites without neighboring oxygen-containing species, while the appearance of the CObridge band correlates with the presence of the oxygen-containing species. Surface speciation on Cu microparticles (Cu MPs) and oxide-derived Cu (OD-Cu) are similar in all the electrolytes investigated, while the CO adsorption bands show subtle differences on these two surfaces, reflecting variations in the microenvironment of CO adsorbed in the same configuration. Selectivity for C2+ products in the CORR on Cu MPs increases with the electrolyte alkalinity at −0.4 VRHE, which correlates with the presence of the oxygen-containing Cu species. However, the presence of the oxygen-containing Cu species alone is shown to be an unreliable predictor of the CORR performance on Cu MPs and OD-Cu, which could be attributed to the difference in the microenvironment of the surface Cu sites. Results reported in this work highlight that the assumption that the surface Cu speciation under reducing potentials is independent of the electrolyte pH must be treated with caution.