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––Agn+ 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.