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

Electrochemical CO 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 CO mass transfer rate to the catalyst layer. In addition to CO 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 CO 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 than at -10 mA cm, implying that bubble-induced advection outweighs the additional OH flux at these current densities. We also prove that the pH buffering effect of CO from the gas feed and dissolved CO in the catholyte prevents the gas diffusion electrode from becoming strongly alkaline. Our findings suggest that gas-fed CO 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.