Microbial fuel cells (MFCs) have been studied for over a decade for possible applications of generating electrical power from wastewater treatment. The power densities achieved so far – although increased by orders of magnitude since the first studies – are still well below those required to compete with other anaerobic treatment processes.

We are exploring the use of MFCs for the production of hydrogen peroxide (H₂O₂). H₂O₂ is a powerful oxidant and disinfectant that can be used onsite at a treatment plant. Its production in MFCs is possible by allowing the 2-e- oxygen reduction reaction (ORR) to occur at the cathode. We have studied several aspects of the selection of cathode catalyst, design of the MFCs, and their operation, to optimize the rates and concentrations of H₂O₂ that can be achieved in a continuous-flow catholyte scheme.

First, we determined the thickness of the Vulcan carbon catalyst necessary for the cathode to promote the 2-e- vs. the 4-e- ORR, as higher thicknesses tend to lead to the consumption of H₂O₂ within the catalyst layer. Next, we studied various anion exchange membranes (AEMs) for their compatibility with H₂O₂. Although high-conductivity AEMs are desirable to decrease Ohmic overpotentials, we had to select an AEM that has a relatively low conductivity, because of its higher compatibility with H₂O₂ that would allow long-term operation. We also optimized the catholyte composition and hydraulic retention time (HRT) within the MFCs to allow for H₂O₂ production at a high cathodic Coulombic efficiency, by avoiding its disintegration within the cathode chamber.

Through these advances, we have been able to show consistent H₂O₂ production in MFCs fed with acetate as the electron donor on the anode to concentrations of >0.3%. These concentrations are significantly higher than those needed for many of the possible onsite applications of H₂O₂, for e.g. disinfection. Our results should pave the way for further development and scale-up of MFCs for the production of H₂O₂.