Enhanced Electrochemical Carbon Dioxide Methanation via Inhibition of Proton Transport in the Outer Helmholtz Plane

The structured water network within the electric double layer (EDL) plays a pivotal role in governing the interfacial reactivity and selectivity in electrocatalysis. Here, we present a rational electrolyte design strategy to modulate proton transport across the EDL water network, particular within the outer Helmholtz plane (OHP), thereby enhancing carbon-based fuel production via electrochemical CO2 reduction. Trace amounts of ethylenediaminetetraacetic acid (EDTA) disrupt the hydrogen-bond network in the OHP, achieving a methane (CH4) Faradaic efficiency of 61.2% and 4-fold suppression of the hydrogen evolution reaction (HER), while maintaining stable operation for more than 35 h. Mechanistic investigations reveal strong EDTA–water interactions that inhibit interfacial proton transport and elevate local pH, whereas electron transfer between EDTA and adsorbed *CO contributes to the stabilization of reaction intermediates. These synergistic effects suppress HER and enhance CH4 selectivity, even under strongly acidic conditions. EDL modulation via chelating agents demonstrates broad applicability, and the hydrogen-bond acceptor density per unit mass serves as a predictive descriptor of product selectivity. This framework provides a versatile strategy for targeted OHP engineering, thereby advancing carbon-neutral fuel electrosynthesis.