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The slow kinetics of the electrochemical oxygen reduction reaction (ORR) is a crucial bottleneck in the development of microbial fuel cells (MFCs). This article firstly gives an overview of the particular constraints imposed on ORR by MFC operating conditions: neutral pH, slow oxygen mass transfer, sensitivity to reactive oxygen species, fouling and biofouling. A review of the literature is then proposed to assess how microbial catalysis could afford suitable solutions. Actually, microbial catalysis of ORR occurs spontaneously on the surface of metallic materials and is an effective motor of microbial corrosion. In this framework, several mechanisms have been proposed, which are reviewed in the second part of the article. The last part describes the efforts made in the domain of MFCs to determine the microbial ecology of electroactive biofilms and define efficient protocols for the formation of microbial oxygen-reducing cathodes. Although no clear mechanism has been established yet, several promising solutions have been recently proposed.

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

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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.

Stainless steel and graphite electrodes were individually addressed and polarized at −0.60 V vs. Ag/AgCl in reactors filled with a growth medium that contained 25 mM fumarate as the electron acceptor and no electron donor, in order to force the microbial cells to use the electrode as electron source. When the reactor was inoculated with Geobacter sulfurreducens, the current increased and stabilized at average values around 0.75 A m −2 for graphite and 20.5 A m −2 for stainless steel. Cyclic voltammetry performed at the end of the experiment indicated that the reduction started at around −0.30 V vs. Ag/AgCl on stainless steel. Removing the biofilm formed on the electrode surface made the current totally disappear, confirming that the G.sulfurreducens biofilm was fully responsible for the electrocatalysis of fumarate reduction. Similar current densities were recorded when the electrodes were polarized after being kept in open circuit for several days. The reasons for the bacteria presence and survival on non-connected stainless steel coupons were discussed. Chronoamperometry experiments performed at different potential values suggested that the biofilm-driven catalysis was controlled by electrochemical kinetics. The high current density obtained, quite close to the redox potential of the fumarate/succinate couple, presents stainless steel as a remarkable material to support biocathodes.

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