Coupled Cation-Electron Transfer at Pt(111)/Perfluoro-Sulfonic Acid Ionomer Interface and Kinetic impact on the Oxygen Reduction Reaction

Electrochemical interfaces between polymer electrolytes and metal electrodes are central to many key electrochemical devices in the global sustainable energy transition, however, much of the electrochemical insights were gained at the liquid/solid electrochemical interfaces. In this work, we analyze the adsorption and desorption processes of sulfonate groups in Nafion, the most widely used polymer electrolyte and binder, on Pt(111), and demonstrate that they are distinct from specific adsorption of common anions in the following aspects: 1) The main thermodynamic driving force for the specific adsorption of sulfonate groups is not the chemical bonds formed between the anion and the surface. 2) The adsorption and desorption of sulfonate in Nafion on Pt(111) involve distinct elementary steps, with the latter proceeding through a coupled cation-electron transfer. 3) Adsorbed sulfonate groups in Nafion not only block a fraction of surface Pt sites, but more importantly, generate two new types of surface adsorbates, i.e., OHNafion and ONafion, which exhibit distinct kinetic properties from adsorbed OH and O on bare Pt(111), respectively. The impact of adsorbed Nafion on the activity of the oxygen reduction reaction (ORR) cannot be rationalized by existing thermodynamic descriptors, e.g., OH binding energy. The reduced ORR activity on Nafion-covered Pt(111) is caused by the kinetically slower *O → *OH conversion and *OH reduction on the Pt sites in the vicinity of adsorbed sulfonate groups. Our findings highlight that the kinetic properties of key surface intermediates must be taken into account in the design of ORR catalysts.