Modification of CO₂ Reduction Activity of Nanostructured Silver Electrocatalysts by Surface Halide Anions

This paper describes the effect of halide anions (X = Cl, Br, I) immobilized on the surface of nanostructured silver electrocatalysts on the efficiency and the mechanism of CO₂ reduction to CO in aqueous carbonate solutions. A simple oxidation–reduction cycle on Ag foil in the presence of halide anions produces high-surface-area nanostructured catalysts mainly composed of metallic Ag with a small amount of halide anions attached to the electrode surface (X–Ag) as demonstrated by XPS, XRD, and SEM studies. The activity of X–Ag electrocatalysts in 0.1 M NaHCO₃ at pH 6.8 is significantly higher than that of Ag foil or Ag nanoparticles with comparable surface area and morphology. The activity enhancement is attributed to the formation of active catalytic sites, presumably Cl^––Ag_n⁺ clusters on the surface of metallic Ag, as evidenced by XPS analysis. The activity of X–Ag catalysts is in the order Cl > Br > I, which is consistent with the proposed model of an active site. The Tafel analysis of electrochemical CO₂ reduction points to the sensitivity of the mechanism of electrocatalysis on the nature of X.