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 CO32-. The local pH on the cathode surface is 7.2, and the HCO3- concentration profile extends a distance of 120 mu m 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/cm(2).