Recently, the microbial rechargeable battery (MRB) has been proposed as a potentially sustainable and low-cost electrical energy storage technology. In the MRB, bioelectrochemical CO 2 reduction and subsequent product oxidation has successfully been combined in one integrated system. However, finding a suitable counter electrode is hindering its further development. In this work, we have tested two alternative counter electrodes in duplicate-namely, i) oxygen/water and ii) a capacitive electrode-for use in the MRB platform. During daily charge/ discharge cycling over periods of 11 to 15 days, experimentally obtained energy efficiencies of 25 and 3.7 % were reported when using the capacitive and the oxygen/water electrodes, respectively. Large overpotentials, resulting in a voltage efficiency of 15 % and oxygen crossover leading to coulombic efficiencies of 25 % caused the considerably lower efficiency for the oxygen/water systems, despite the theoretical higher voltage efficiency. Although the capacitive electrode equipped systems performed better, energy density is limited by the operational potential window within which capacitive systems can operate reliably. Microbial community analysis revealed dominant presence of Geobacter in the bioanode and Selenomonadales in the biocathode. These results do not necessarily bring practical application of the MRB closer, but they do provide new insights in the working principle of this new technology.[a] Dr.

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