This study examines the performance of a tubular direct carbon fuel cell (DCFC) with varying gas atmosphere. DCFC performance is higher under a CO2 atmosphere when compared with a N-2 atmosphere, where mass transport limitations in the anode compartment and the additional two-electron oxidation pathway from CO contribute. This hypothesis is verified by comparing the slurry arrangement to a solid working anode where mass transport is not required. In order to maximize the carbon utilization efficiency, operating below the thermodynamic temperature limit for reverse Boudouard gasification (700 degrees C) is recommended with agitation in slurry-based systems. For a maximum power output, operating under CO2 at higher temperatures (>800 degrees C) and passing the CO containing flue gas over an oxygen reduction cathode achieve optimal results.