Impact of Alkali Cation Identity on the Conversion of HCO3- to CO in Bicarbonate Electrolyzers

The reduction of CO2 to CO from a bicarbonate feedstock offers an opportunity to directly use aqueous carbon capture solutions, while bypassing ex-situ energy-intensive gaseous CO2 regeneration. In this study, we resolved how the electrolyte cation identity (Li+, Na+, K+, Cs+) affects the two reactions that make bicarbonate electrolysis possible: (i) the production of in-situ CO2 formed through reaction of HCO3- (from the catholyte) with H+ (sourced from the membrane); and (ii) the electroreduction of CO2 into CO. Our results show that cation identity does not change the rate of in-situ CO2 formation, but it does enhance the rate of the CO2 reduction reaction (CO2RR). Electrolysis experiments performed with a constant [HCO3-] showed that CO selectivities progressively increased for the series Li+, Na+, K+, and Cs+, respectively. Optimization of the electrolyte composition yielded a CO selectivity of similar to 80 % during electrolysis of 1.5 M CsHCO3 solutions at 100 mA cm(-2), while saturated LiHCO3 solutions (0.84 M) yielded CO selectivities values of merely 30 % at the same current density. This study demonstrates a quantitative relationship between CO product selectivity and the cation radius, which provides a pathway to integrate bicarbonate electrolysis to carbon capture.