Early Gas Evolution during Electrode Polarization in Water-in-Salt Electrolyte

High salt concentration water-in-salt electrolytes (WiSEs) have proven effective at modulating water activity at polarized electrode interfaces, to the benefit of battery and electrocatalytic systems. To fully leverage the unique properties of WiSE for electrochemical systems, the interfacial structure and dynamics of water under polarization must be systematically investigated to establish how electrolyte composition dictates reactivity. Here we combine cyclic voltammetry (CV), quantitative electrochemical mass spectrometry (ECMS), and surface-sensitive shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) to unravel the interfacial pathways of water reduction and oxidation in WiSE on polycrystalline Pt electrodes. These conditions result in clear decoupling of H2 evolving from proton reduction and water reduction and reveal an unexpected anodic wave with coincident finite O2 detection via ECMS, prior to the onset of continuous O2 evolution from water oxidation. SHINERS pinpoints the molecular origin of these phenomena, capturing field-induced water reorientation, vibrational stiffening of interfacial O-H modes, and the emergence of Pt-OH, Pt-O2-, and Pt-Ox species that remain trapped within the inner Helmholtz plane under static hydrodynamic conditions, which later oxidize to O2. These results establish that interfacial water electrolysis in acidic WiSE uniquely exhibits suppressed proton mobility and stabilization of intermediate hydroxyl species within a constrained microenvironment. This work thus provides the first integrated molecular-level picture of how interfacial water structures drive nonclassical gas evolution pathways in WiSEs, which represent critical considerations for aqueous batteries and catalytic devices.