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

Abstract

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.

Electrochemical Reduction of Formic Acid on a Copper-Tin-Lead Cathode

“…The main products of the reduction were methanol, ethanol as well as small amounts of methane and ethane. The %CEs of methanol and ethanol displayed a common maximum of 30.3 and 37.6% respectively at -0.8 V. It is worth mentioning that a similar maximum was observed on tin cathode at -0.68 V, but in this case the only product was methanol [3]. As far as we know it is the first time that ethanol was found to be a main product of the reduction of HCOOH.…”

“…The %CEs of the products were calculated by assuming that the number of electrons is 4 for CH 3 OH, 8 for CH 3 CH 2 OH, 6 for CH 3 CHO and CH 4 and 10 for C 2 H 6 .…”

“…The electrochemical reduction of carbon dioxide on the high overpotential metals for the hydrogen evolution, such as Sn, In and Pb gives formic acid or formate by a %Current Efficiency (%CE) near 100% [1][2][3]. Formic acid has limited uses and thus its conversion to more valuable organic products is of crucial importance for the efficient recycling of carbon dioxide.…”

“…The maximum current density was 140 μA cm -2 , which is too low for commercial purposes. Russell and coworkers [3] showed that formic acid can be reduced to methanol in acidic solution in a narrow potential region between -0.68 and -0.72 V vs. SCE on Sn and between -0.9 and -1.0 V on Pb, where a %CE of 99% on Sn and 11.9% on Pb was achieved. The current density of the reduction was also quite low (less than 4 μΑ cm -2 ).…”

Transformation of Carbon Dioxide to Useable Products through Free Radical‐Induced Reactions

“…Electrochemical reduction of CO 2 leads to different products such as HCOOH, HCHO, CO, and CH 4 . The energy efficiency, ε, for HCOOH can be expressed [105] as: ε = ΔH × (percent faradaic efficiency) nF (E rev + ηc + ηa + ηIR) (2.9)…”