Direct Imaging of Local pH Reveals Bubble-Induced Mixing in a CO₂ Electrolyzer

Electrochemical CO₂ reduction poses a promising pathway to produce hydrocarbon chemicals and fuels without relying on fossil fuels. Gas diffusion electrodes allow high selectivity for desired carbon products at high current density by ensuring a sufficient CO₂ mass transfer rate to the catalyst layer. In addition to CO₂ mass transfer, the product selectivity also strongly depends on the local pH at the catalyst surface. In this work, we directly visualize for the first time the two-dimensional (2D) pH profile in the catholyte channel of a gas-fed CO₂ electrolyzer equipped with a bipolar membrane. The pH profile is imaged with operando fluorescence lifetime imaging microscopy (FLIM) using a pH-sensitive quinolinium-based dye. We demonstrate that bubble-induced mixing plays an important role in the Faradaic efficiency. Our concentration measurements show that the pH at the catalyst remains lower at -100 mA cm^-2 than at -10 mA cm^-2, implying that bubble-induced advection outweighs the additional OH^- flux at these current densities. We also prove that the pH buffering effect of CO₂ from the gas feed and dissolved CO₂ in the catholyte prevents the gas diffusion electrode from becoming strongly alkaline. Our findings suggest that gas-fed CO₂ electrolyzers with a bipolar membrane and a flowing catholyte are promising designs for scale-up and high-current-density operation because they are able to avoid extreme pH values in the catalyst layer.