Effects of Outer Helmholtz Plane-Located Anions on the Oxygenate Adsorption and Oxygen Reduction Catalysis on Pt(111).

The efficiency of electrocatalysis, a key factor in integrating renewable electricity into energy systems, is fundamentally influenced by the distribution of ions within the electric double layer. While the effects of specifically adsorbed ions (SAIs) at the inner Helmholtz plane (IHP) and nonspecifically adsorbed ions at the outer Helmholtz plane (OHP) are well-established, the role of OHP-located SAIs has remains unexplored. In this study, we probe the unknown by constructing a model Pt(111) interface where sulfate anions are exclusively confined to the OHP within the oxygen reduction reaction (ORR) potential region, achieved through their distinct pH dependencies. Strikingly, these OHP-located sulfates exert an asymmetric influence on the *O/*OH redox, which impedes the reduction of *O to *OH but not its reverse oxidation. This asymmetry is explained via a place-exchange model for the *O/*OH redox, in which the OHP-located sulfates primarily influence the electron-transfer-free process. The established OHP-anion behaviors are further correlated with ORR rates on Pt(111) in various electrolytes. Our study elucidates the effects of OHP-residing SAIs, expanding the knowledge boundary of ion effects on electrocatalysis.