Cathode Surface pH Modulates Multicarbon Product Selectivity during the Electrochemical Conversion of CO₂ Capture Solutions

The electrolysis of (bi)carbonate solutions can upgrade captured carbon dioxide (CO 2 ) into chemicals and fuels. This style of electrolysis relies on a pH swing to release the CO 2 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 CO 2 reduction reaction products when electrolyzing (bi)carbonate, a CO 2 capture solution, set at different pH values. Our results show that C 2 products (C 2 H 5 OH) are favored over C 1 products (CO, CH 4, 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, C 2 product formation is actually less favored. This outcome is because less CO 2 reactant is available for C−C coupling events. Consequently, the highest Faradaic efficiency for C 2 product formation (22%) occurred at the lowest current density measured (100 mA cm −2 ).