A direct carbon fuel cell

The key economic driving forces behind research into fuel cell technology are the world's dwindling oil reserves and the ever-growing global economy's search for viable power sources for the future. Cheap fuel may not always be available, therefore it is important to find high energy density power sources to fuel our requirements. Fuel cells offer high energy efficiency and scalability, and a range of different electrolyte materials (e.g. alkali metal salt, phosphoric acid, molten carbonate or solid oxide), with hydrocarbons or hydrogen as fuel source, have received intensive investigations. However, inherent problems associated with cost, handling or chemical sensitivity remain for these designs. In direct carbon-air fuel cells (DCFCs), carbon is the fuel which is oxidised electrochemically. Elemental carbon has an extremely high energy density per unit mass and can easily be stored. The major advantage to this form of electrochemical cell over other variants lies in the theoretical energy efficiency for the overall oxidation of carbon to carbon dioxide, which slightly exceeds 100% as a result of the entropy increase associated with the reaction. Moreover, the energy per unit volume released during the oxidation reaction (20.0 kW h/l) significantly exceeds, for example, that of methane (4.2 kW h/l). hydrogen (2.4 kW h/l) or magnesium (1 1.8 k W h/l). Further operational advantages displayed by DCFCs over other types include the use of a fully renewable solid fuel source and the opportunity for scale-up. A carbonate-based DCFC makes use of a molten mixed alkali metal carbonate as the electrolyte. The reactions occurring in the cell are: Cathode: O2 + 2 CO2 + 4 c- -> 2(CO3)2- Anode: C + 2 (CO3)2- -> 3 CO2 + 4 e- Overall: C + O2 -> CO2 Efforts to develop an electrochemical power source based on the direct oxidation of carbon date back over one hundred years, with the first patent for a hydroxide electrolyte based direct carbon power source filed by W. W. Jacques in 1896. Early attempts failed to produce a practical device, due to the poor power densities obtained and the build-up in the electrolyte of nonvolatile impurities from the carbon, poisoning it. Today, we have the option of fuelling with pure carbon obtained from the decomposition of hydrocarbons, and thus the technology has the potential to contribute to a sustainable energy scheme. Research into DCFC technology remains in its infancy. A direct carbon fuel cell is currently being developed in St Andrews, which utilises carbon particles wetted in a eutectic molten alkali metal carbonate mixture as its fuel and a solid electrolyte. Operating in the temperature range between 550-750 oC, the cell exhibits good current-voltage ratios. We report here the electrochemical data of our studies into DCFC systems.