A power source for an unmanned undersea vehicle with increased energy density is described that employs a self-contained fuel system to address carbon dioxide evolution. A solid oxide fuel cell serves as the power source in the self-contained fuel system. In combination with the solid oxide fuel cell, the system comprises a chemical composite that is combined with water to create both a hydrocarbon fuel for the solid oxide fuel cell and a water-soluble byproduct. The byproduct is then combined with the carbon dioxide gas generated by the fuel cell to create a storable solid precipitate.