This study aims to enhance the direct carbon fuel cell (DCFC) efficiency by bubbling Ar gas in a carbon/carbonate slurry. The current discharge curve and CO and CO2 production rates were measured during discharge at 20 mA cm(-2). When carbon content in carbonates (W-C/carbonate) was 1.0 wt%, Ar bubbling decreased the CO/CO2 production ratio from 0.16 to 0.051, and increased the CO2 production rate, which approached the theoretical value for complete electrochemical oxidation of carbon. Moreover, bubbling increased the cell voltage by about 0.1 Vat steady state. At W-C/carbonate = 3.0 wt%, the CO/CO2 production ratio without bubbling increased significantly to 8.5, indicating that partial electrochemical oxidation became dominant. The C/O2- ratio is key for explaining the electrochemical oxidation of the carbon. When the C/O2- ratio was low, such as at W-C/carbonate = 1.0 wt%, complete electrochemical oxidation became dominant, and bubbling enhanced the complete electrochemical oxidation. When the C/O2- ratio was high, such as at W-C/carbonate = 3.0 wt%, partial electrochemical oxidation became dominant, and bubbling did not inhibit it. For W-C/carbonate = 1.0 Wt%, bubbling increased both coulombic and voltage efficiencies, resulting in an increase in total efficiency from 52% to 64%. (C) 2014 Elsevier B.V. All rights reserved.