Direct Electroreduction of Carbonate to Formate.

A cause of losses in energy and carbon conversion efficiencies during the electrochemical CO 2 reduction reaction (eCO 2 RR) can be attributed to the formation of carbonates (CO 3 2- ), which is generally considered to be an electrochemically inert species. Herein, using in situ Raman spectroscopy, liquid chromatography, 1 H nuclear magnetic resonance spectroscopy, 13 C and deuterium isotope labeling, and density functional theory simulations, we show that carbonate intermediates are adsorbed on a copper electrode during eCO 2 RR in KHCO 3 electrolyte from 0.2 to -1.0 V RHE . These intermediates can be reduced to formate at -0.4 V RHE and more negative potentials. This finding is supported by our observation of formate from the reduction of Cu 2 (CO 3 )(OH) 2 . Pulse electrolysis on a copper electrode immersed in a N 2 -purged K 2 CO 3 electrolyte was also performed. We found that the carbonate anions therein could be first adsorbed at -0.05 V RHE and then directly reduced to formate at -0.5 V RHE (overpotential of 0.28 V) with a Faradaic efficiency of 0.61%. The nature of the active sites generating the adsorbed carbonate species and the mechanism for the pulse-enabled reduction of carbonate to formate were elucidated. Our findings reveal how carbonates are directly reduced to a high-value product such as formate and open a potential pathway to mitigate carbonate formation during eCO 2 RR.