Electrochemical CO2 conversion to formic acid using engineered enzymatic catalysts in a batch reactor

Abstract

Formic acid is one of the most valuable fuel products for the capture and conversion of CO2 due to its unique usage in fuel cells and hydrogen storage. Electrochemically mediated conversion of CO2 to FA has its advantages over the traditional Kemira process, in that high temperatures and pressures are not required, cutting on manufacturing and operating costs. However, selectivity of the metal catalysts used in CO2 conversion to certain products remains limited. Here, an engineered enzymatic catalyst is employed to convert CO2 into formic acid (in the form of formate) in a batch reactor. This work seeks to maximize both formate production and Coulombic efficiency, which is achieved primarily through: (1) adjustment of the operating voltage, (2) implementation of an O2 scavenger to mitigate competition with dissolved O2, and (3) control of system pH to maintain a stable operating range for the catalyst. Peak formate production and peak efficiency achieved in long-term experiments (> 40 h) were 225 mM and 91 %, respectively, both of which show promise of strong metrics for CO2 conversion to formate. The optimal operating cathode voltage was shown to be below the baseline voltage of − 0.75 V vs. Ag/AgCl. In short-term experiments (≈ 1 h), operating below this range, at approximately − 0.85 V, could achieve efficiencies of 100 %. By implementing sodium thiosulfate as an O2 scavenger, the rate of formate production improved by over 4 ×. Long-term retention of the efficiency, however, remains an issue to be addressed due to the enzyme sensitivity to pH and increased current at higher values of acid used on the anode.