International audienceMicrobial 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

Numerous biocorrosion studies have stated that biofilms formed in aerobic seawater induce an efficient catalysis of the oxygen reduction on stainless steels. This property was implemented here for the first time in a marine microbial fuel cell (MFC). A prototype was designed with a stainless steel anode embedded in marine sediments coupled to a stainless steel cathode in the overlying seawater. Recording current/potential curves during the progress of the experiment confirmed that the cathode progressively acquired effective catalytic properties. The maximal power density produced of 4 mW m −2 was lower than those reported previously with marine MFC using graphite electrodes. Decoupling anode and cathode showed that the cathode suffered practical problems related to implementation in the sea, which may found easy technical solutions. A laboratory fuel cell based on the same principle demonstrated that the biofilm-covered stainless steel cathode was able to supply current density up to 140 mA m −2 at +0.05 V versus Ag/AgCl. The power density of 23 mW m −2 was in this case limited by the anode. These first tests presented the biofilm-covered stainless steel cathodes as very promising candidates to be implemented in marine MFC. The suitability of stainless steel as anode has to be further investigated.