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

Abstract The reduction of CO 2 to CO from a bicarbonate feedstock offers an opportunity to directly use aqueous carbon capture solutions, while bypassing ex‐situ energy‐intensive gaseous CO 2 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 CO 2 formed through reaction of HCO 3 − (from the catholyte) with H + (sourced from the membrane); and (ii) the electroreduction of CO 2 into CO. Our results show that cation identity does not change the rate of in‐situ CO 2 formation, but it does enhance the rate of the CO 2 reduction reaction (CO2RR). Electrolysis experiments performed with a constant [HCO 3 − ] 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 ∼80 % during electrolysis of 1.5 M CsHCO 3 solutions at 100 mA cm −2 , while saturated LiHCO 3 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.