How Cation Size Modulates the Anion Effect in CO ₂ Electroreduction: Insights from Multiscale Modeling of Electrochemical Interfaces

The interplay of cations and anions within the electric double layer (EDL) under an applied potential is crucial for the activity and selectivity of CO2 electroreduction (eCO2RR). Yet, first-principles level modeling of the EDL’s complex structure on large spatiotemporal scales remains challenge. Here, we combine grand canonical ensemble density functional theory with classical molecular dynamics to investigate ion effects under constant potential. Our simulation revealed a critical yet subtle link between cation and anion effects, uncovering an unexpected mechanism for the known size-dependent cation effects. We found that cation modulation of near-surface anion distribution, rather than direct intermediate stabilization of a *COO– intermediate, is the dominant factor. Larger cations, such as Cs+, more effectively shield anions from the cathode and thereby reduce their inhibition of CO2 adsorption. Our operando-mimicking simulations not only reveal the multiple roles of alkali metal cations in eCO2RR through their hydration dynamics and anion shielding effects but also provide insight into their size dependence, guiding the precise modulation of EDL for enhanced eCO2RR performance.