Cation-Induced Interphasial Viscosity Variations on Gold Electrocatalysts in Nanoconfined Aqueous Electrolytes.

The structure and viscosity of interfacial hydration layers critically govern charge-transfer, kinetics, and molecular diffusivity in electrocatalysis and energy conversion. Yet, despite their relevance particularly in confined catalysis, the viscosity variations across interfacial zones, of a finite width (interphases), remain largely unexplored. Here, we combine localized friction force analysis and molecular dynamics simulations, to reveal how alkali metal cations influence the hydrogen-bond network of gold-electrolyte interfaces under nanoconfinement. For potassium cations (K+) in aqueous perchlorate electrolyte, friction decreases linearly with increasing electrolyte concentration, evidencing a lubricating effect. Simulated density and viscosity profiles for chloride electrolytes show that chaotropic K+ ions weaken the hydrogen-bond network, similarly to cesium cations (Cs+). The interphasial layer exhibits a zone of reduced density and viscosity, followed by an adjacent zone where the viscosity clearly rises above the bulk level. These molecular-level insights are broadly relevant to understanding and quantitatively describing interphasial molecular mobility in catalysis and electrochemical sensing.