Microbial fuel cells can directly convert chemical energy into electrical energy, but significant energy losses result from the use of O2 as the cathode. Microbial batteries (MBs) replace conventional O2 cathodes with solid-state cathodes that can be re-oxidized under favorable conditions, enabling more efficient energy recovery. Previously, we demonstrated proof-of-concept for MBs with a silver oxide cathode. The high cost of silver and energy required for re-oxidation make this material impractical. In this work, we evaluate Prussian Blue (PB), a hexacyano-Fe complex material, as a low-cost solid-state cathode in a single-chamber, membrane-free MB. Microbial oxidation of organic matter at the anode drives the reduction of PB. Reduced PB is re-oxidized by exposure to air. MBs equipped with a PB electrode efficiently transfer charge to PB (85% charge transfer), achieving net energy recovery efficiencies of 18–33%, with no loss of capacity over 20 cycles of operation. The operating potential for the PB electrode creates a trade-off: increasing the PB electrode potential improves the efficiency of energy recovery, but more material is required to maintain power output and ensure efficient oxidation of organic matter. Operation is otherwise straightforward: energy is only required to move reduced PB into the air then back to the MB. We conclude that PB is a promising cathode material for single-chamber, membrane-free MBs.
