Beyond Surface Catalysis: Dynamic Cation-Regulated Interfacial Water Gating for Aqueous Polysulfide Electrochemistry

Aqueous sulfur electrochemistry holds great promise for high-energy density storage but is currently constrained by sluggish redox kinetics and severe polarization. Despite extensive efforts in catalyst design to enhance polysulfide adsorption, kinetic improvements have largely plateaued, suggesting that localized surface-active sites are no longer the primary bottleneck. By transcending the conventional focus on surface-active sites, we identify a fundamental kinetic limitation governed by the electrical double layer (EDL) structure, which has been largely overlooked as a passive background. We propose a generalizable framework of interfacial water gating, wherein electrolyte cations dynamically modulate the hydrogen-bond network and electrostatic shielding within the EDL to regulate polysulfide accessibility. By integrating operando UV-visible spectroscopy and in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), we reveal a cation-hydration-mediated, volcano-type dependence of redox kinetics on cation concentration, dictated by the competition between electrostatic cation-shielding and steric interfacial congestion. Our findings redefine interfacial water as a dynamic tunable medium and a participatory component of the redox reaction. This work shifts the focus from localized surface catalysis to global EDL regulation, providing molecular-level guidance for overcoming kinetic limits in multivalent aqueous electrochemical systems.