The Electrochemical Reduction of Carbon Dioxide, Formic Acid, and Formaldehyde

The prospects for the electrochemical reduction of carbon dioxide to methanol were examined by investigating the intermediate reactions. The reduction of carbon dioxide was carried out in a neutral electrolyte at a mercury electrode. The high overvoltage observed for carbon dioxide reduction to the formate anion reflects a low value for the efficiency of electric energy utilization for this process. Formic acid can be reduced to methanol in a perchloric acid electrolyte (at a lead electrode) or in a buffered formic acid electrolyte (at a tin electrode). The faradaic efficiency for methanol formation is close to 100% at the tin electrode in a narrow potential region corresponding to a low current density. The potential dependence of formic acid reduction to methanol suggests that the adsorption of formic acid on the electrode, near the pzc, may be the rate‐controlling step in the over‐all reaction. The reduction of formaldehyde to methanol occurs with a faradaic efficiency exceeding 90% in a basic solution. The Tafel slope decreases when either the formaldehyde concentration is increased (at constant pH) or when thepH of the solution is increased (at constant concentration). The polyoxymethylene glycols present as impurities in formaldehyde solutions may influence the mechanism of the electrode process through interaction with formaldehyde molecules and/or other adsorbed species resulting in small changes of the Tafel slope.