Direct Electroreduction of Carbonate to Formate

A cause of losses in energy and carbon conversion efficiencies during the electrochemical CO2 reduction reaction (eCO(2)RR) can be attributed to the formation of carbonates (CO32-), which is generally considered to be an electrochemically inert species. Herein, using in situ Raman spectroscopy, liquid chromatography, H-1 nuclear magnetic resonance spectroscopy, C-13 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 KHCO3 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(CO3)(OH)(2). Pulse electrolysis on a copper electrode immersed in a N-2-purged K2CO3 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.