The Influence of the Interfacial pH on the Electrochemical CO2 Reduction Towards Formate on Cu Electrodes

Abstract

The electrochemical reduction of CO2 (CO2RR) using renewable electricity is considered a promising way to replace fossil fuels as feedstock. In aqueous media, the activity and selectivity for the CO2RR is strongly influenced by electrolyte composition at the solid-electrolyte interface. Due to competing H+ reduction, the interfacial pH is strongly affected by the applied potential which in turn determines the balance between CO2, HCO3 -, and CO3 2-. The Bicarbonate equilibrium and cation-hydroxide interactions are directly connected to the CO2RR and HER rate.[1]To learn more about the impact of the electrolyte composition within the double layer on the CO2RR mechanism we deploy time-resolved (sub-second) FT-IR reflection-absorption spectro-electrochemistry at grazing angle using polarized light. This allows us to differentiate conversions at the electrode-electrolyte interface (i.e. CO2RR) from the diffuse double layer (i.e. pH effects). Herein, we elucidate the electrochemical CO2RR towards formate on polycrystalline Cu in aqueous bicarbonate electrolyte. We compare the rate of CH formation to the interfacial pH changes determined by the ratio between the intensity of the CO2, HCO3 -, and CO3 2- bands. We discuss how potential steps and sweeps relate to interfacial pH and CO2RR.By applying a ‘pre’potential more positive from the open circuit potential (OCP: ca. +0.5 V vs. RHE) before stepping to -0.9 V vs. RHE, the interfacial pH decreases due to the oxidation of Cu. As a result, the rate of C-H formation is initially hindered for several seconds before setting off. Likewise, an applied ‘pre’potential more negative from the OCP results into poor C-H formation kinetics.Reference:[1] Liu, X.; Monteiro, M.C.O.; Koper, M. T. M. Phys. Chem. Chem. Phys. 2023, advance article. Figure 1