Cathode Surface pH Modulates Multicarbon Product Selectivity during the Electrochemical Conversion of CO<sub>2</sub> Capture Solutions
Siwei Ma, Yongwook Kim, Zishuai Zhang, Shaoxuan Ren, Chaitanya Donde, Luke Melo, Aubry S. R. Williams, Monika Stolar, Edward R. Grant, Curtis P. Berlinguette
Abstract
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 ).