Electrolyte Structure Governs Formate Oxidation in Water-in-Salt Systems

High-concentration “water-in-salt” electrolytes remain underexplored in electrocatalysis, although their altered structure could offer benefits for catalytic processes. Understanding how these structural changes influence electrocatalytic performance is essential for advancing electrolyte design and leveraging salt concentration as a tunable parameter. Here, we investigate formate oxidation as a model reaction using two structurally distinct high-concentration electrolyte series. By combination of rotating disk electrode studies with molecular dynamics and in-situ surface enhanced FTIR spectroscopy, we elucidate structure–performance relationships on polycrystalline Pt catalysts. We show that an increasing concentration of sodium formate enhances formate availability and thus increases the direct oxidation current. However, entering the water-in-salt regime induces kosmotropic sodium-formate clustering, impeding interfacial transport and limiting current density. Introducing chaotropic perchlorate ions disrupts this clustering, further increasing current but simultaneously altering interfacial pH, slowing proton transport, and enhancing CO poisoning when increasing the perchlorate-to-formate ratio. This leads to shifts in formate oxidation onset potentials and modified current responses. Our findings establish the structural tunability of high-concentration electrolytes as a promising strategy for optimizing electrocatalytic performance.