In Situ Observation of the pH Gradient near the Gas Diffusion Electrode of CO₂ Reduction in Alkaline Electrolyte

Abstract

The local pH variation near the surface of CO₂ reduction electrodes is important but hard to study. We develop a continuous-flow Raman electrochemical cell that enables the first experimental study of the local pH near a CO₂ reduction gas diffusion electrode under reaction conditions. At zero current, CO₂ chemically reacts with the 1 M KOH electrolyte at the interface to form HCO₃^- and CO₃^2-. The local pH on the cathode surface is 7.2, and the HCO₃^- concentration profile extends a distance of 120 μm into the electrolyte, which verifies that the nominal overpotential reduction from using alkaline electrolyte originates from the Nernst potential of the pH gradient layer at the cathode/electrolyte interface. The CO₂-OH^- neutralization reaction and the pH gradient layer still persist, albeit to a reduced extent, at CO₂ reduction current densities up to 150 mA/cm².