Probing the Catalyst-Ionomer Interface during Electrochemical CO 2 Reduction

The catalyst-ionomer interface can significantly impact the selectivity of the electrochemical CO 2 reduction (CO 2 R) reaction. Ionomers such as Nafion are commonly used in conjunction with nanostructured CO 2 R catalysts in flow cells and zero-gap devices, with their presumed function being to act as a binder for the catalyst. However, recent studies have reported that ionomers such as Nafion and Sustainion can affect the reaction rate and selectivity of CO 2 R, and the roles of ionomer in controlling mass transfer and local pH have been hypothesized. We employed in situ Raman spectroscopy to examine CO 2 R intermediate and local pH on a sputtered copper (sCu) surface with and without an ionomer overlayer. We observed a difference in the potential-dependent formation of surface CO in the presence of Nafion compared to clean sCu. Using a combination of pH-sensitive probe molecules, we also determined the pH at the catalyst-ionomer interface.