Direct Coal Oxidation in Modified Solid Oxide Fuel Cells

Solid oxide fuel cells are fuel flexible in the sense that they can convert a variety of fuels into electricity because of the use of an oxygen ion conducting electrolyte. Traditional solid oxide fuel cells utilize gaseous reactants, such as hydrogen and carbon monoxide, derived from hydrocarbon based fuels, but also ammonia has been reported. A challenge is still the application of this type of cell for the conversion of solid, carbon containing fuels such as coal or biomass derived carbon in terms of pyrolized wood , e.g. charcoal (direct carbon fuel cell (DCFC)). However, several high power densities based on carbon as fuel have been reported for so called hybrid solid oxide fuel cells.

Hybrid direct carbon fuel cells may employ a classical solid oxide fuel cell e.g. using Ni-cermet anodes together with carbon dispersed in a carbonate melt on the anode side. In a European project, the utilization of various coals has been investigated with and without addition of an oxidation catalyst to the carbon-carbonate slurry or anode layer.

The nature of the coal and the catalyst composition affect both open circuit voltage and power output. Highest open circuit potentials and power densities were observed for bituminous coal and by adding manganese oxide or praseodymium-doped ceria to the carbon/carbonate mixture. Comparing the performance of an anode supported (315 micron thick anodes) and cathode supported cells (15 micron thick cathode) indicates a superior performance of the latter cell type when using carbon black as carbon source. Using biomass derived carbon (charcoal) as fuel without an additional catalyst resulted in an open circuit voltage of 984 mV and a maximum power density of 121 mW/cm², similar to the power output of bituminous coal and using ceria or manganese based catalysts.

Further outlook on the kinetics of the carbon oxidation processes occurring in the vicinity of the anode of these cells will be provided.