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Molten Carbonate Fuel Cells

The Molten Carbonate Fuel Cell (MCFC) evolved from work in the 1960's aimed at producing a fuel cell which would operate directly on coal. While direct operation on coal seems less likely today, operation on coal-derived fuel gases or natural gas is viable.

MCFC Design and Operation

The MCFC uses a molten carbonate salt mixture as its electrolyte. The composition of the electrolyte varies, but usually consists of lithium carbonate and potassium carbonate. At the operating temperature of about 1200°F (650°C), the salt mixture is liquid and a good ionic conductor. The electrolyte is suspended in a porous, insulating and chemically inert ceramic (LiA102) matrix.

The MCFC reactions that occur are:

The anode process involves a reaction between hydrogen and carbonate ions (CO3=) from the electrolyte which produces water and carbon dioxide (CO2) while releasing electrons to the anode. The cathode process combines oxygen and CO2 from the oxidant stream with electrons from the cathode to produce carbonate ions which enter the electrolyte. The need for CO2 in the oxidant stream requires a system for collecting CO2 from the anode exhaust and mixing it with the cathode feed stream.

As the operating temperature increases, the theoretical operating voltage for a fuel cell decreases and with it the maximum theoretical fuel efficiency. On the other hand, increasing the operating temperature increases the rate of the electrochemical reaction and thus the current which can be obtained at a given voltage. The net effect for the MCFC is that the real operating voltage is higher than the operating voltage for the PAFC at the same current density.

The higher operating voltage of the MCFC means that more power is available at a higher fuel efficiency from a MCFC than from a PAFC of the same electrode area. As size and cost scale roughly with electrode area, this suggests that a MCFC should be smaller and less expensive than a "comparable" PAFC.

The MCFC also produces excess heat at a temperature which is high enough to yield high pressure steam which may be fed to a turbine to generate additional electricity. In combined cycle operation, electrical efficiencies in excess of 60% (HHV) have been suggested for mature MCFC systems.

The MCFC operates at between 1110°F (600°C) and 1200°F (650°C) which is necessary to achieve sufficient conductivity of the electrolyte. To maintain this operating temperature, a higher volume of air is passed through the cathode for cooling purposes.

As mentioned above, the high operating temperature of the MCFC offers the possibility that it could operate directly on gaseous hydrocarbon fuels such as natural gas. The natural gas would be reformed to produce hydrogen within the fuel cell itself.

The need for CO2 in the oxidant stream requires that CO2 from the spent anode gas be collected and mixed with the incoming air stream. Before this can be done, any residual hydrogen in the spent fuel stream must be burned. Future systems may incorporate membrane separators to remove the hydrogen for recirculation back to the fuel stream.

At cell operating temperatures of 1200°F (650°C) noble metal catalysts are not required. The anode is a highly porous sintered nickel powder, alloyed with chromium to prevent agglomeration and creep at operating temperatures. The cathode is a porous nickel oxide material doped with lithium. Significant technology has been developed to provide electrode structures which position the electrolyte with respect to the electrodes and maintain that position while allowing for some electrolyte boil-off during operation. The electrolyte boil-off has an insignificant impact on cell stack life. A more significant factor of life expectancy has to do with corrosion of the cathode.

The MCFC operating temperature is about 1200°F (650°C). At this temperature the salt mixture is liquid and is a good conductor. The cell performance is sensitive to operating temperature. A change in cell temperature from 1200°F (650°C) to 1110°F (600°C) results in a drop in cell voltage of almost 15%. The reduction in cell voltage is due to increased ionic and electrical resistance and a reduction in electrode kinetics.