Sequential anode–cathode configuration improves cathodic oxygen reduction and effluent quality of microbial fuel cells

The reduction of oxygen at the cathode and the diffusion of protons from the anode to the cathode are currently perceived as two major bottlenecks of microbial fuel cells (MFCs). To address these issues, we have designed an MFC configuration in which the effluent of an acetate-fed anode was used as a feed for an aerated, biocatalysed cathode. The development of a cathodic biofilm achieved a four-fold increase of the current output compared with the non-catalysed graphite cathode, while the pH variation in the cathode compartment was reduced due to the additional transfer of protons via the liquid stream. The sequential anode–cathode configuration also provided for chemical oxygen demand (COD) polishing at the cathode by heterotrophic bacteria, with overall acetate removal consistently greater than 99%. The anode achieved an organic substrate removal of up to 2.45 kg COD/m3 of anode liquid volume per day, at Coulombic efficiencies of 65–95%. Electron balances at the cathode revealed that the main cathodic process was oxygen reduction to water with no significant Coulombic losses. The maximal power output during polarization was 110 W/m3 cathode liquid volume. The process could be operated in a stable way during more than 9 months of continuous operation. Excessive organic loading to the cathode should be avoided as it can reduce the long-term performance through the growth of heterotrophic bacteria.