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The electrolysis of (bi)carbonate solutions can upgrade captured carbon dioxide (CO2) into chemicals and fuels. This style of electrolysis relies on a pH swing to release the CO2 from the sorbent. Here, we report a bicarbonate electrolyzer equipped with an operando Raman spectroscopic probe that can measure the pH, at or near the cathode surface, as a function of current density. We correlate this cathode surface pH to the selectivity of the CO2 reduction reaction products when electrolyzing (bi)carbonate, a CO2 capture solution, set at different pH values. Our results show that C2 products (C2H5OH) are favored over C1 products (CO, CH4, HCOO–) when the pH at the cathode surface is high. This trend is consistent with the literature. However, at higher current densities and where the surface pH is highest, C2 product formation is actually less favored. This outcome is because less CO2 reactant is available for C−C coupling events. Consequently, the highest Faradaic efficiency for C2 product formation (22%) occurred at the lowest current density measured (100 mA cm−2).
Ma et al. (Mon,) studied this question.