Suppression of Hydrogen Evolution in Acidic Electrolytes by Electrochemical CO₂ Reduction

In this article we investigate the electrochemical reduction of CO₂ at gold electrodes under mildly acidic conditions. Differential electrochemical mass spectroscopy (DEMS) is used to quantify the amounts of formed hydrogen and carbon monoxide as well as the consumed amount of CO₂. We investigate how the Faradaic efficiency of CO formation is affected by the CO₂ partial pressure (0.1-0.5 bar) and the proton concentration (1-0.25 mM). Increasing the former enhances the rate of CO₂ reduction and suppresses hydrogen evolution from proton reduction, leading to Faradaic efficiencies close to 100%. Hydrogen evolution is suppressed by CO₂ reduction as all protons at the electrode surfaces are used to support the formation of water (CO₂ + 2H⁺ + 2e^- → CO + H₂O). Under conditions of slow mass transport, this leaves no protons to support hydrogen evolution. On the basis of our results, we derive a general design principle for acid CO₂ electrolyzers to suppress hydrogen evolution from proton reduction: the rate of CO/OH^- formation must be high enough to match/compensate the mass transfer of protons to the electrode surface.