How Cation Size Modulates Anion Effect in CO2 Electroreduction: Insights From Operando-Mimicking Modeling

The interplay of cations and anions within electric double layer (EDL) under applied potential are crucial for activity and selectivity of CO2 electroreduction (eCO2RR). Yet, first-principle level modeling the EDL’s complex structure in 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 *COO- intermediate, is the dominant factor. Larger cations, like 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 new insight into their size dependence, guiding the precise modulation of EDL for enhanced eCO2RR performance.