The inhibition of the proton donor ability of bicarbonate promotes the electrochemical conversion of CO2 in bicarbonate solutions

Gaseous CO2 solutions are widely used for CO2 electrochemical conversion to various valuable products. However, capture, liberation and storage of gaseous CO2 prior to reduction is cumbersome and costly. CO2 electrocatalytic reduction from captured CO2 (in the form of concentrated bicarbonate solution) offers an option to optimize CO2 reduction processes. Using concentrated bicarbonate solutions from captured CO2 as large carbon feedstock and reducing CO2 directly from bicarbonate electrolyte remain a challenge. Indeed, current efficiency is too low (i.e. low selectivity and/or activity) as a consequence of the strong competition with the hydrogen evolution reaction, which bicarbonate promotes to a great extent. In this study, up to 2 M bicarbonate solution is used as electrolyte (mimicking a captured CO2 solution from an alkaline media) for a CO2 electrochemical reduction system, which potentially pretends to be a 2 M carbon source, compared to 0.033 M present in a saturated gaseous CO2 solution. In order to improve the Faradaic efficiency of the process and thereby making the CO2 electrocatalytic reduction from a bicarbonate feedstock an efficient system and attractive alternative for the reduction starting from gaseous CO2, a reaction mechanism where bicarbonate acts as a CO2 donor instead of as a proton donor is proposed. To achieve such property, we inhibit the proton donor ability of bicarbonate and water by covering the surface of the electrode with cationic surfactants, allowing non-polar molecules, like the CO2 derived from the equilibrium with bicarbonate, to diffuse to the surface of the electrode while avoiding bicarbonate and water to promote the hydrogen evolution reaction. To the best of our knowledge, the Faradaic Efficiency to formate obtained in this study (>70 %) sets a new benchmark in systems involving unsaturated and saturated bicarbonate solutions without previously purging CO2.

Automated summary

Gaseous CO2 solutions are commonly used for electrochemical conversion, but the process of capturing, liberating, and storing CO2 is costly and cumbersome. This study proposes a mechanism where bicarbonate acts as a CO2 donor to improve the Faradaic efficiency of the CO2 electrocatalytic reduction. The research sets a new benchmark with a Faradaic Efficiency to formate of over 70%.