Investigation of the Interfacial pH at Catalyst-Ionomer Interface Using In-Situ Surface-Enhanced Raman Spectroscopy

Renewable energy-powered electrochemical CO₂ reduction reaction (CO₂RR) offers a potentially viable method to close the carbon cycle and produce high-value multicarbon(C<i>₂₊</i>) products. Ionomers such as Nafion, commonly used as a binder in nanostructured CO_<i>2</i>RR catalytic systems, can also affect the selectivity of CO₂RR by modulating the local pH and alkali cation transport to the catalyst surface. However, literature reports have shown either a decrease, increase, or no effect of Nafion on C<i>₂₊</i> selectivity, demanding a better understanding of the catalyst-ionomer interface. In this study, we utilized in-situ Raman spectroscopy to monitor changes in local pH on a plasmonic rough gold surface, both with and without ionomer coating, using a pH-sensitive probe. We used the Hydrogen evolution reaction (HER) as a model reaction because, similar to CO₂RR, it can also increase the local pH by consuming protons or producing hydroxide ions. Our results on bare gold surface show no change in the local pH with applied potential when tests are conducted in strongly acidic medium. However, in mildly acidic or near-neutral bulk conditions, the local pH quickly changes to alkaline post-HER onset. Upon conducting these tests with Nafion-coated gold electrodes, we observed both lower local pH and current at the same applied potential compared to bare electrode. However, when similar current densities are applied, the difference in local pH compared to bare electrode is not significant, indicating that improvement in C₂₊ selectivity, as proposed by earlier reports, might not be due to the local pH increase from the OH- trapping effect of Nafion.