Polyoxometalate-Promoted Electrocatalytic CO₂ Reduction at Nanostructured Silver in Dimethylformamide

Electrochemical reduction of CO₂ is a promising method to convert CO₂ into fuels or useful chemicals, such as carbon monoxide (CO), hydrocarbons, and alcohols. In this study, nanostructured Ag was obtained by electrodeposition of Ag in the presence of a Keggin type polyoxometalate, [PMo₁₂O₄₀]^3- (PMo). Metallic Ag is formed upon reduction of Ag⁺. Adsorption of PMo on the surface of the newly formed Ag lowers its surface energy thus stabilizes the nanostructure. The electrocatalytic performance of this Ag-PMo nanocomposite for CO₂ reduction was evaluated in a CO₂ saturated dimethylformamide medium containing 0.1 M [ n-Bu₄N]PF₆ and 0.5% (v/v) added H₂O. The results show that this Ag-PMo nanocomposite can catalyze the reduction of CO₂ to CO with an onset potential of -1.70 V versus Fc^0/+, which is only 0.29 V more negative than the estimated reversible potential (-1.41 V) for this process and 0.70 V more positive than that on bulk Ag metal. High faradaic efficiencies of about 90% were obtained over a wide range of applied potentials. A Tafel slope of 60 mV dec^-1 suggests that rapid formation of *CO₂^•- is followed by the rate-determining protonation step. This is consistent with the voltammetric data which suggest that the reduced PMo interacts strongly with CO₂ (and presumably CO₂^•-) and hence promotes the formation of CO₂^•-.