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

The electrochemical CO2 reductionreaction dependsstrongly on the local pH at the cathode gas diffusion electrode. Weuse a 2D imaging technique to show that gas bubble evolution playsa critical role in the local pH profile and CO2 mass transfer. Electrochemical CO2 reduction poses a promisingpathwayto produce hydrocarbon chemicals and fuels without relying on fossilfuels. Gas diffusion electrodes allow high selectivity for desiredcarbon products at high current density by ensuring a sufficient CO2 mass transfer rate to the catalyst layer. In addition toCO(2) mass transfer, the product selectivity also stronglydepends on the local pH at the catalyst surface. In this work, wedirectly visualize for the first time the two-dimensional (2D) pHprofile in the catholyte channel of a gas-fed CO2 electrolyzerequipped with a bipolar membrane. The pH profile is imaged with operandofluorescence lifetime imaging microscopy (FLIM) using a pH-sensitivequinolinium-based dye. We demonstrate that bubble-induced mixing playsan important role in the Faradaic efficiency. Our concentration measurementsshow that the pH at the catalyst remains lower at -100 mA cm(-2) than at -10 mA cm(-2), implyingthat bubble-induced advection outweighs the additional OH- flux at these current densities. We also prove that the pH bufferingeffect of CO2 from the gas feed and dissolved CO2 in the catholyte prevents the gas diffusion electrode from becomingstrongly alkaline. Our findings suggest that gas-fed CO2 electrolyzers with a bipolar membrane and a flowing catholyte arepromising designs for scale-up and high-current-density operationbecause they are able to avoid extreme pH values in the catalyst layer.