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Typical microbial fuel cells (MFCs) rely on precious metals for reduction of oxygen at the cathode, but recently MFCs have been developed that use biocathodes for power generation with alternate electron acceptors. It is shown here that with illumination it is possible to develop a biocathode that uses dissolved carbon dioxide (bicarbonate) as the acceptor. During acclimation, the cathode was set at a potential of 0.242 V (vs.SHE) using a potentiostat. After approximately one month of acclimation, a current of 1 mA was sustained. Bicarbonate was reduced in stoichiometric agreement with current generation, with 0.28 ± 0.02 moles of bicarbonate reduced per mole of electrons. When this biocathode was used in a two-bottle MFC, a power density of 750 mW m−2 was produced. These results demonstrate that MFCs can be used to fix carbon dioxide as well as produce electricity.