Surface-Morphology-Dependent Electrolyte Effects on Gold-Catalyzed Electrochemical CO₂ Reduction

The electrocatalytic property of a flat or an oxide-derived nanostructure Au electrode was investigated using surface sensitive analysis methods such as impedance spectroscopy and Kelvin probe force microscopy (KPFM) when electrochemical conversion of carbon dioxide (CO₂) to carbon monoxide (CO) was performed with either KHCO₃- or NaHCO₃-based neutral electrolyte. A strong dependence on the cation of the electrolyte was exhibited on the flat Au electrode surface. CO selectivity and capacitance dispersion are significantly higher with the KHCO₃ electrolyte. On the other hand, the nanostructured Au electrodes, having much more improved activity and durability of CO₂ reduction, showed much less electrolyte-dependent catalytic activity. The difference in CO selectivity with KHCO₃ and NaHCO₃ electrolytes can be explained by the difference in hydration level and consequent adsorption strength of the cations on the flat Au metal electrodes, implying that ion-pairing interactions between the metal, cations, CO₂, and its intermediate play an important role in the reduction reaction. The local electric field fluctuation caused by the nanostructured rough Au surface can affect the electric double layer near the electrode surface and suppress the electrolyte-dependency of the reduction. Furthermore, according to X-ray spectroscopy analysis of the electrode after electrolysis, the nanostructured Au electrode is less prone to surface cation deposition. These results provide a basic understanding of the role of electrolyte cations in the CO₂ reduction reaction.