Importance of the hydrogen route in up-scaling electrosynthesis for microbial CO2 reduction

Abstract

Microbial electrochemical reduction of CO₂ was carried out under two different applied potentials, −0.36 V and −0.66 V vs. SHE, using a biological sludge as the inoculum. Both potentials were thermodynamically appropriate for converting CO₂ to acetate but only −0.66 V enabled hydrogen evolution. No acetate production was observed at −0.36 V, while up to 244 ± 20 mg L⁻¹ acetate was produced at −0.66 V vs. SHE. The same microbial inoculum implemented in gas–liquid contactors with H₂ and CO₂ gas supply led to acetate production of 2500 mg L⁻¹. When a salt marsh sediment was used as the inoculum, no reduction was observed in the electrochemical reactors, while supplying H₂ + CO₂ gas led to formate and then acetate production. Finally, pure cultures of Sporomusa ovata grown under H₂ and CO₂ gas feeding showed acetate production of up to 2904 mg L⁻¹, higher than those reported so far in the literature for S. ovata implemented in bioelectrochemical processes. Unexpected ethanol production of up to 1411 mg L⁻¹ was also observed. All these experimental data confirm that hydrogen produced on the cathode by water electrolysis is an essential mediator in the microbial electrochemical reduction of CO₂. Implementing homoacetogenic microbial species in purposely designed gas–liquid biocontactors should now be considered as a relevant strategy for developing CO₂ conversion.