(623g) Investigation of Interfacial pH and Surface Species at the Catalyst-Ionomer Interface

Interactions at the catalyst-ionomer interface are important in controlling the selectivity of electrochemical CO2 reduction (CO2R). Ionomers such as Nafion are commonly used as binders for nanostructured CO2R catalysts in various types of cells, including flow cells and zero-gap devices, primarily to hold the catalyst together. However, recent research has indicated that ionomers, including Nafion and Sustainion, can modify the rate and selectivity of the CO2R reaction. These studies suggest that ionomers may play significant roles in managing mass transfer and adjusting local pH levels. To investigate these effects further, we utilized in situ Raman spectroscopy to monitor local pH changes on a sputtered gold surface, both with and without an ionomer coating, using pH-sensitive probe molecules. For a sputtered gold electrode, the interfacial pH remains unchanged in highly acidic electrolytes with increasing cathodic potential. However, the interfacial pH quickly becomes alkaline under cathodic potential when the starting electrolyte pH is closer to neutral. The presence of a Nafion overlayer affects the rate of pH increase and potential-dependent trends in pH. Our in situ Raman results also revealed a difference in the potential-dependent trends of CO formation when Nafion is present compared to a bare sputtered copper surface.