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

The local pH variation near the surface of CO 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 CO reduction gas diffusion electrode under reaction conditions. At zero current, CO chemically reacts with the 1 M KOH electrolyte at the interface to form HCO and CO. The local pH on the cathode surface is 7.2, and the HCO 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 CO-OH neutralization reaction and the pH gradient layer still persist, albeit to a reduced extent, at CO reduction current densities up to 150 mA/cm.