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NFCRC Tutorial: Molten Carbonate Fuel Cell (MCFC)

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The electrolyte typically consists of a combination of alkali (Na and K) carbonates retained in a ceramic matrix of LiAlO2. The cell operates at temperature of 1100 to 1300 deg F or 600 to 700 deg C in order to keep the alkali carbonates in a highly conductive molten salt form, the carbonate ions providing ionic conduction. The anode is made from Ni while the cathode is made from nickel oxide.

Three corporations actively pursuing the commercialization of MCFCs in the U. S. are Energy Research Corporation, International Fuel Cells Corporation, and MC Power Corporation, in Europe are Brandstofel Nederland (BCN), Deutsche Aerospace AG, Ansaldo (Italy), and in Japan are Hitachi, Ishikawajima Harima Heavy Industries, and Mitsubishi Electric Corporation.

The electrochemical reactions occurring in the cell are:
at the anode:
H2 + CO3= = H2O + CO2 + 2e-
at the cathode:
l/2O2 + CO2 + 2e- = CO3
with the overall cell reaction:
H2 + l/2O2 + CO2 (cathode) = H2O + CO2 (anode)

Note that CO is not directly used by the electrochemical oxidation, but produces additional H2 by the water gas shift reaction:
CO + H2O = H2 + CO2.

A fuel such as natural gas is either reformed externally or within the cell in the presence of a suitable catalyst to form H2 and CO by the reaction: CH4 + H2O = 3H2 + CO.
Any sulfur compounds present in the fuel have to be removed prior to use in the cell (upstream of the reformer) to a concentration of <0.1 ppmV. The fuel cell itself, however, can tolerate a maximum of 0.5 ppmV of sulfur compounds.

Typically the CO2 generated at the anode is recycled to the cathode where it is consumed. This requires additional equipment to either transfer the CO2 from the anode exit gas to the cathode inlet gas or produce CO2 by combustion of the anode exhaust gas and mixed with the cathode inlet gas.

One of the advantages of the high operating temperature of the MCFC is that the overall thermal efficiencies is high, with a potential of 50 to 60% conversion of the fuel (natural gas) LHV to electricity without recovery and conversion of the exhaust heat. Also, the exhaust heat from the MCFC is at relatively high temperatures (1200 deg F or 650 deg C) and may be recovered for the generation of steam which further increases the efficiency. Efficiencies >60% may potentially be achieved with the incorporation of a bottoming cycle. On the other hand, the higher operating temperature places severe demands on the corrosion stability and life of cell components.

The method for electrolyte management in an MCFC in order to establish a stable electrolyte/gas interface in the porous electrodes depends on a balance in capillary pressures to establish the electrolyte interfacial boundaries allowing the electrolyte matrix to remain completely filled with the molten carbonate, while the porous electrodes are partially filled, depending on their pore size distributions.

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