<i>Operando</i> Local pH Measurement within Gas Diffusion Electrodes Performing Electrochemical Carbon Dioxide Reduction

Abstract

The local pH near the surface of a CO2 reduction electrocatalyst strongly impacts catalytic selectivity and activity. Here, confocal fluorescence microscopy was used to map the electrolyte pH near a copper gas diffusion electrode during CO2 reduction with micron spatial resolution in three dimensions. We observed that the local pH increased from pH 6.8 to greater than pH 10 as the current density was increased from 0 to 28 mA/cm2 in a 100 mM KHCO3 electrolyte. Variations in the pH across the surface indicate areas of locally increased activity. Within deep trenches of the active layer, the local pH increases as trench width decreases. Computational models confirm these experimental results and also showed that the catalyst found within narrow trenches is more active than that found at the surface of the electrode. This study suggests that the overpotential required to perform selective CO2 reduction can be reduced by increasing the density of narrow trench regions in the microporous layer.