Understanding the effects of anion interactions with ag electrodes on electrochemical  CO₂ reduction in choline halide electrolytes

Interactions of electrolyte ions at electrocatalyst surfaces influence the selectivity of electrochemical CO₂ reduction (CO₂R) to chemical feedstocks like CO. We investigated the effects of anion type in aqueous choline halide solutions (ChCl, ChBr, and ChI) on the selectivity of CO₂R to CO over an Ag foil cathode. Using an H-type cell, we observed that halide-specific adsorption at the Ag surface limits CO faradaic efficiency (FE_CO) at potentials more positive than −1.0 V vs. reversible hydrogen electrode (RHE). At these conditions, FE_CO increased from I⁻<Br⁻<Cl⁻, that is, in the opposite order to the strength of specific adsorption of the halide ions (Cl⁻<Br⁻<I⁻). At potentials of −1.0 to −1.3 V vs. RHE, restructuring of the Ag surface in ChI and ChCl via dissolution and re-electrodeposition led to more CO-selective Ag facets ([220], [311], and [222]) than in ChBr. This mechanism allowed very high faradaic efficiencies for CO of 97±2 % in ChI and 94±2 % in ChCl to be achieved simultaneously with high current densities at −1.3 V vs. RHE. We also demonstrate that high selectivity to CO (FE_CO>90 %) in ChCl (at −0.75±0.06 Vvs. RHE) and ChI (at −0.78±0.17 V vs. RHE) could be achieved at a current density of 150 mA cm⁻² in a continuous flow-cell electrolyser with Ag nanoparticles on a commercial gas diffusion electrode. This study provides new insights to understand the interactions of anions with catalysts and offers a new method to modify electrocatalyst surfaces.