It is experimentally known that electrochemical CO₂ reduction (eCO₂R) does not take place on Cu, Ag and Au without a cation near the electrode surface, and that both organic and inorganic cations modulate eCO₂R activity and selectivity. The rational optimization of microenvironments in eCO₂R requires understanding of the intricate chemical and transport phenomena taking place across length scales.

We modeled CO₂ reduction to CO on Ag via a multiscale approach, accounting for the role of the cation at all scales. The transport is modeled by generalized modified Poisson-Nernst-Planck equations and a microkinetic model is used to calculate the current densities of CO₂ reduction to CO and the competing reduction of water (H₂OR) and H⁺ to H₂ based on the local conditions. Kinetic rate constants are obtained from atomic-scale calculations (DFT and AIMD). In particular, the number of active sites (the microenvironments) depends on the local cation concentration.

We considered different buffers, including alkali (Li⁺, K⁺ and Cs⁺), alkali-earth (Mg²⁺ and Ba²⁺) and organic (tetramethylammonium - TMA⁺) cations, and we evaluated different concentrations (from 1 mM to 2 M) over a large potential window (-0.4 to -1.6 V vs RHE). We observed consistent behaviors across cations of different nature: at slightly negative applied potentials, higher cation concentration leads to improved activity, despite the lower CO₂ solubility in higher ionic strength buffers. At strongly negative applied potentials, cation accumulation due to double layer charging leads to transport limitations and the CO current density is lower at higher cation concentrations. Cs⁺ leads to higher CO current density and high CO selectivity for a combination of favorable chemical and transport properties. We applied the same framework to a TMA⁺ - based anion exchange membrane (AEM) in the vicinity of an Ag electrode and evaluated the effect of a fixed background charge at different hydration levels on eCO₂R activity and selectivity. We showed that, when the AEM is in contact with an electrolyte, the inorganic cation is still present at the AEM-electrolyte interface and contributes to the CO₂R and H₂OR microenvironments.

The model consistently reproduces experimental trends and helps to elucidate optimal local condition for eCO₂R on Ag in aqueous electrolytes and ionomers. Our work paves the way for a coupled multiscale electrolyte and ionomer design for electrochemical interfaces.