Rescaled electrolyte charge density provides a unifying descriptor of electrocatalytic CO2 reduction in ionic liquid electrolytes.

Electric field-driven ion accumulation dictates the local environment for electrochemical reactions, often governing the rates and selectivity of electrocatalytic processes. Recent studies demonstrate that high salt concentrations can unlock new mechanisms to modulate reactivity, yet the influence of ion-ion correlations on electric double layer formation remains open to discussion. Here, we study ionic liquid-mediated electrochemical CO 2 reduction using 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIm BF 4 ) dissolved in acetonitrile, dimethyl sulfoxide, and propylene carbonate at concentrations ranging from dilute to neat ionic liquid. By bridging experiments with molecular dynamics simulations, we reveal alignment between electrocatalytic activity and bulk rescaled charge density, or the concentration of charges not neutralized in ion clusters. We also show ion aggregation behavior predicts interfacial screening lengths in quantitative agreement with surface forces measurements. Overall, we reveal that collective ion assembly plays a crucial role in electrocatalysis, opening frontiers in modulating reactivity beyond the active site reminiscent of secondary environment control in enzymatic catalysis.