Coupled cation–electron transfer at the Pt(111)/perfluoro-sulfonic acid ionomer interface and its impact on the oxygen reduction reaction kinetics

Electrochemical interfaces between polymer electrolytes and electrodes are central to electrochemical devices in the global transition towards renewable energy. Here we show that the adsorption and desorption of sulfonates in Nafion on Pt(111) involve distinct elementary steps, with the latter proceeding through a coupled cation–electron transfer. Adsorbed sulfonates not only block a fraction of surface Pt sites but, more importantly, generate two additional types of surface adsorbate, OHNafion and ONafion, which exhibit distinct kinetic properties from adsorbed OH and O on bare Pt(111), respectively. The impact of the adsorption of sulfonate groups in Nafion on the activity of the oxygen reduction reaction (ORR) on Pt cannot be rationalized by existing thermodynamic descriptors. The reduced ORR activity on the Nafion-covered Pt(111) is caused by the kinetically hindered *O→*OH conversion and *OH reduction on sites close to adsorbed sulfonates. The understanding of electrochemical interfaces between polymer electrolytes and metal electrodes, which is critical to many practical devices, remains limited. Now, the interaction between Nafion’s sulfonate groups and platinum and its impact on the oxygen reduction reaction is studied in detail, and a distinct coupled cation–electron transfer mechanism is identified.