Microbial fuel cell operation with continuous biological ferrous iron oxidation of the catholyte.

The oxygen reduction rate at the cathode is a limiting factor in microbial fuel cell (MFC) performance. In our previous study, we showed the performance of an MFC with ferric iron (Fe3+) reduction at the cathode. Instead of oxygen, ferric iron was reduced to ferrous iron (Fe2+) at the cathode with a bipolar membrane between the anode and cathode compartment. This resulted in a higher cathode potential than is usually obtained with oxygen on metal-based chemical catalysts in MFCs. In this study, we investigated the operation of the same MFC with ferric iron reduction at the cathode and simultaneous biological ferrous iron oxidation of the catholyte. We show that the immobilized microorganism Acidithiobacillus ferrooxidans is capable of oxidizing ferrous iron to ferric iron at a rate high enough to ensure an MFC power output of 1.2 W/m2 and a current of 4.4 A/m2. This power output was 38% higher than in our previous study at a similar current density without ferrous iron oxidation. The bipolar membrane is shown to split water into 65-76% of the needed protons and hydroxides. The other part of the protons was supplied as H2SO4 to the cathode compartment. The remaining charge was transported by K+ and HSO4-/SO4(2-) from the one compartment to the other. This resulted in increased salt concentrations in the cathode. The increased salt concentrations reduced the ohmic losses and enabled the improved MFC power output. Iron could be reversibly removed from the bipolar membrane by exchange with protons.