Thermodynamics permits the carbon fuel cell, which generates electrical power via the electrochemical combustion of its carbon fuel, to realize a theoretical efficiency of 100%. A recent paper [Nunoura et al. Ind. Eng. Chem; Res. 2007, 46, 734-744] reported promising results that were obtained from a moderate-temperature, aqueous-alkaline biocarbon fuel cell. In view of the fact that aqueous-alkaline hydrogen fuel cells have been used to power an Austin car and a commercial Black Cab in London, these recent results suggest the potential use of aqueous-alkaline carbon fuel cells for vehicular transportation. Usually, the practicality of an aqueous-alkaline carbon fuel cell is discounted, because the carbon dioxide product of carbon oxidation reacts with and consumes hydroxyl ions in the aqueous-alkaline electrolyte, thereby forming carbonate ions. As a result of this reaction, the performance of an aqueous-alkaline carbon fuel cell is expected to deteriorate over time. Contrary to this expectation, in this paper, we show that the aqueous-carbonate ion can be as effective as the hydroxyl ion as a charge carrier when the temperature of the cell approaches 300 degrees C. Thermodynamic estimates of the Gibbs free energy of formation (Delta(f)G degrees) of the aqueous-carbonate ion indicate that the change in Gibbs free energy of the relevant anodic carbon oxidation reaction by carbonate ion equals that of carbon oxidation by hydroxyl ion at temperatures that approach 300 degrees C. Also, consideration of the temperature dependence of the standard hydrogen electrode reveals that aqueous-hydroxyl ion production on the cathode should be favored at temperatures as high as 300 degrees C. These findings are a cause for optimism, concerning the performance of an aqueous alkaline/carbonate biocarbon fuel cell designed to operate at 300 degrees C, and they should encourage further work at temperatures that approach 300 degrees C.