Solid State Energy Conversion Alliance (SECA)
A Primer on SOFC Technology
Cell, Stack, and Module Description
The material requirements for the cathode are: high electronic conductivity, chemical and dimensional stability in air up to the design temperature, thermal expansion compatibility with the other cell layers, and sufficient porosity for O2 diffusion. The material requirements for the electrolyte are: nonconductor of electrons (i.e., a dielectric), high O= conductivity, dimensional and chemical stability in both air and fuel up to the design temperature, thermal expansion compatibility, and impervious to molecular gas flow. The material requirements for the anode are: high electronic conductivity, chemical and dimensional stability in fuel up to the design temperature, thermal expansion compatibility, and sufficient porosity for H2 and CO diffusion, and for H2O and CO2 counter diffusion. A material set, which satisfies these requirements, is strontium-doped lanthanum manganite (LSM) for the cathode, yttrium-doped (or stabilized) zirconium oxide (YSZ) for the electrolyte, and a mixture of nickel metal and YSZ for the anode. The dopants are small amounts of other elements added to achieve the desired properties. Testing of a tubular SOFC at 875 C for almost eight years (68,000 hours) of continuous operation has demonstrated that this material set is very stable, with a voltage degradation rate of less than 0.1% per 1000 hours of operation. Other material sets are under development in order to improve cell performance at the lower end of the temperature range. Lanthanum-ferrite based cathodes (e.g., LSCF) are examples of mixed conductivity cathodes under development.
When two or more cells are connected in electrical series ([-] of one cell to the [+] of adjacent cell, or vice versa), an interconnect is required. As shown in the exploded-view cell-stack illustration of Figure 2, the interconnect plate electrically connects the anode (fuel side) of the lower cell to the cathode (air side) of the adjacent cell. It also forms the fuel channels for the lower cell and, in a cross flow arrangement, the air channels for the adjacent cell. The material requirements for the interconnect are: high electronic conductivity; chemical and dimensional stability in both air and fuel up to the design temperature; thermal expansion compatibility; and impervious to gas flow. Ferritic stainless steel is one example of a low cost interconnect material under development.
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| Figure 2 - Stacking Arrangement of Planar SOFC's |
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Figure 3 - Planar Fuel Cell Stack (92 Cells) |
The generated current flows normal to the plane of the SOFC. Cell current (Icell) and cell power (Pcell ) are directly proportional to the cell active area (i.e., electrode area) thru the relationships Icell = J*Acell and Pcell = Vcell*Icell , where J is the average current density (amps/cm2), Acell is the cell active area (cm2), and Vcell is the operating voltage (volts) across one cell (one cathode, electrolyte, anode combination). Increasing the cell active area raises the cell power output, reduces the number of cells required to produce a given plant/system power output, and thus can reduce the cost of electricity, as long as the cell manufacturing yield and cell reliability are not adversely affected. Planar cells of 550 cm2 active area have been successfully tested, and cells of 1000 cm2 active area are under development. Cell operating voltages are small (e.g., 0.7 to 0.9 volts). So multi-cell stacks (cells stacked upon one another in electrical series) are assembled to build up voltage. The equations governing planar cell stacks are Istack = Icell, Vstack = N*Vcell, and Pstack= Vstack*Istack where N is the number of cells in the stack. Figure 3 depicts a 92-cell stack with the following performance parameters: Acell=550 cm2, J=0.364 amps/cm2, Icell=200 amps, Vcell=0.85 volts, Pcell=170 watts, N=92, Istack=200 amps, Vstack=78.2 volts, and Pstack=15.6 kWe. Cell stack fabrication requires the use of seals to prevent leakage between air and fuel channels on the edges.
The module consists of an optimized number of cells and cell stacks, as determined by the fuel cell supplier, and includes inlet/outlet air manifolds, inlet/outlet fuel manifolds, electrical power take-offs, stack mechanical loading devices (if any), insulation, and outer container. It is a transportable building block, factory-manufactured, that would be replicated as needed for the power system application to achieve a specified power system rating. (Note that the outer container would be a pressure vessel in pressurized applications.) Thus, one or multiple modules can comprise an SOFC generator, and in this fashion, SOFC power systems can be configured with ratings as small as a few kWe, and as large as multi-100 MWe.
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