Operando resolution of CO2 electrolysis interfaces at application-relevant current density reveals water gating

Design of CO2 electroreduction (CO2R) interfaces requires understanding of the dynamic interplay among reactants, surface species and ions. Simplified ideal frameworks, which are conventionally employed to identify these dependencies, provide limited insights under relevant conditions. Here we resolve species interplay in membrane electrode assemblies up to high current densities (1 A cm−2), combining operando Raman spectroscopy and ab initio molecular dynamics. We identify a key role of water ordering in regulation of access to the interface along different operation regimes. Initially, double-layer water exhibits a dominant four-hydrogen-bond (4-HB) coordination with a random configuration. With increasing current density, water rearranges to a three-hydrogen-bond coordination and sits H-down close to the surface, acting as the primary proton source for CO2R and generating adsorbed hydroxides. Eventually, this results in carbonate build-up that increases the concentration of H-down 4-HB water, restricting interface access to COx reactants and promoting hydrogen evolution. These results increase our understanding of CO2R interfaces up to application-relevant current densities. Understanding the dynamic interplay between all components in complex electrocatalytic systems is crucial to advancing their performance. Here operando Raman spectroscopy is used in combination with molecular dynamics simulations to establish the role of water ordering in the mechanism of electrocatalytic CO2 reduction over a range of current densities and under practical conditions.