Potentiometric Rotating Ring Disk Electrode Study of Interfacial pH during CO₂ Reduction and H₂ Generation in Neutral and Weakly Acidic Media

Abstract

Electrocatalytic reduction of CO2 to CO in aqueous media offers an important path to more sustainable carbon utilization. Optimization of this process relies on a detailed understanding of the dynamics of boundary layer chemistry, e.g., pH, adjacent to the electrode, which is difficult to quantify without perturbing the system. In this work, a versatile IrOx pH-sensing thin film is fabricated on the ring of a rotating ring disk electrode (RRDE) to enable potentiometric boundary layer pH measurement under well-defined mass flow conditions. An analytical reaction–convection–diffusion model is used to translate ring measurements to the pH at the disk surface under reaction conditions. The measurements and model effectively capture the drastic pH shear on a Au electrode during hydrogen evolution reaction in a weak acid electrolyte. The pH trends are measured during CO2 reduction reaction in a 0.5 mol L–1 bicarbonate electrolyte, where the impact of buffering action is clearly demonstrated as a function of the rotation rate and RRDE geometry. The model and measurement provide a useful platform for further exploration of electrode kinetics influenced by reaction-induced pH change.