Direct Electroreduction of Carbonate to Formate.

A cause of losses in energy and carbon conversion efficiencies during the electrochemical CO reduction reaction (eCORR) can be attributed to the formation of carbonates (CO), which is generally considered to be an electrochemically inert species. Herein, using Raman spectroscopy, liquid chromatography, H nuclear magnetic resonance spectroscopy, C and deuterium isotope labeling, and density functional theory simulations, we show that carbonate intermediates are adsorbed on a copper electrode during eCORR in KHCO electrolyte from 0.2 to -1.0 V. These intermediates can be reduced to formate at -0.4 V and more negative potentials. This finding is supported by our observation of formate from the reduction of Cu(CO)(OH). Pulse electrolysis on a copper electrode immersed in a N-purged KCO electrolyte was also performed. We found that the carbonate anions therein could be first adsorbed at -0.05 V and then directly reduced to formate at -0.5 V (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 eCORR.