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 CO 2 reduction (CO 2 R) 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 CO 2 R 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 − 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.