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NFCRC Tutorial: Solid Oxide Fuel Cell (SOFC)

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The electrolyte consists of a solid, nonporous metal oxide, typically Y2O3 stabilized ZrO2 with the anode made from CoZrO2 or NiZrO2 cermet, while the cathode is made from Sr doped LaMnO3.. The cell operates at 1200 to 1830 deg F or 650 to 1000 deg C such that conduction by oxygen ions through the electrolyte may occur.

Advantages of the SOFC are that there is no liquid electrolyte with its associated corrosion and electrolyte management problems, and with an operating temperature greater than 1100 deg F or 600 deg C internal reforming can be achieved. Overall thermal efficiencies are high, typically in the 45 to 50% range for conversion of the fuel (natural gas) bound energy to electricity on an LHV basis. Also, the exhaust heat from the SOFC is at very high temperatures (1832 deg F or 1000 deg C) and may be used in a bottoming cycle or recovered for the generation of steam for cogeneration purposes which further increases the efficiency. The bottoming cycle may consist of an expander (may be fired) in the case of an SOFC operating at high pressure (200 psi). With the addition of a bottoming cycle, the efficiency for converting the fuel bound energy to electricity may be as high as >60% on an LHV basis. The high temperature is also conducive to fast reaction kinetics without requiring any precious materials, and producing high quality exhaust heat for cogeneration or for use in a bottoming cycle. The high temperature of the SOFC, however, places stringent requirements on the materials of construction.

The SOFC with its solid state components may in principle be constructed in any configuration. Cells are being developed in several configurations such as:

The electrochemical reactions occurring within the cell are:
at the anode:
1/2 O2 + 2e- = O
at the cathode:
H2 + 1/2O= = H2O + 2e-
with the overall cell reaction: l/2O2 + H2 = H20

CO and hydrocarbons such as CH4 can also be used as fuels in an SOFC. At the high temperatures within the cell, it is feasible for the water gas shift reaction:
CO + H2O = H2 + CO2
and the steam reforming reaction:
CH4 + H2O = 3H2 + CO (in the case of natural gas)
to take place to produce H2 that is easily oxidized at the anode. The direct oxidation of CO in fuel cells is well established while the direct oxidation of CH4 has not been thoroughly investigated. Any sulfur compounds present in the fuel have to be removed prior to use in the cell to a concentration of <0.1 ppmV.

The material selected for use in the SOFC are constrained by the chemical stability in oxidizing and/or reducing conditions, the conductivity and the thermomechanical compatibility in high temperatures. Another restriction placed on the cell components is that they must be capable of withstanding thermal cycling.

Current designs consist of the electrode, electrolyte, and interconnect material deposited in layers and sintered together to form a cell structure, the fabrication techniques differ, however, according to the type of cell configuration and developer.

The three major configurations for stacking the cells together to increase the voltage and power are: the tubular (as developed by Westinghouse and Mitsubishi Heavy Industries), flat plate (as developed by Ceramatec and Mitsubishi Heavy Industries), and monolithic (as developed by Allied Signal).

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