
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.