Potential Dependence of the Local pH in a CO₂ Reduction Electrolyzer

Quantifying the local pH of a gas diffusion electrode undergoing CO2 reduction is a complicated problem owing to a multitude of competing processes, both electrochemical- and transport-related, possibly affecting the pH at the surface. Here, we present surface-enhanced Raman spectroscopy (SERS) and electrochemical data evaluating the local pH of Cu in an alkaline flow electrolyzer for CO2 reduction. The local pH is evaluated by using the ratio of the SERS signals for HCO3- and CO32-. We find that the local pH is both substantially lower than expected from the bulk electrolyte pH and exhibits dependence on applied potential. Analysis of SERS data reveals that the decrease in pH is associated with the formation of malachite [Cu2(OH)2CO3, malachite] due to the presence of soluble Cu(II) species from the initially oxidized electrode surface. After this initial layer of malachite is depleted, the local pH maintains a value >11 even at currents exceeding -20 mA/cm2.