Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) technology, which offers many advantages over traditional energy conversion systems including low emission and high efficiency, has become increasingly attractive to the utility, automotive, and defense industries (as shown in Figure 1). As an all solid-state energy conversion device, the SOFC operates at high temperatures (700-1,000 C) and produces electricity by electrochemically combining the fuel and oxidant gases across an ionically conducting oxide membrane. To build up a useful voltage, a number of cells or PENs (Positive cathode-Electrolyte-Negative anode) are electrically connected in series in a stack through bi-polar plates, also known as interconnects. Shown in Figure 2 (a) is a schematic of the repeat unit for a planar stack, which is expected to be a mechanically robust, high power-density and cost-effective design. In the stack (refer to Figure 2 (b)), the interconnect is simultaneously exposed to both an oxidizing (air) environment on the cathode side and a reducing (fuels such as hydrogen or natural gas) environment on the anode side for thousands of hours at elevated temperatures (700-1,000 C). Other challenges include the fact that water vapor is likely to be present in both of these environments, and the fuel is likely to containmore » sulfide impurities. Also, the interconnect must be stable towards any sealing materials with which it is in contact, under numerous thermal cycles. Furthermore, the interconnect must also be stable towards electrical contact materials that are employed to minimize interfacial contact resistance, and/or the electrode materials. Considering these service environments, the interconnect materials should possess the following properties: (1) Good surface stability (resistance to oxidation and corrosion) in both cathodic (oxidizing) and anodic (reducing) atmospheres. (2) Thermal expansion matching to the ceramic PEN and other adjacent components, all of which typically have a coefficient of thermal expansion (CTE) in the range of 10.5-12.0 x 10{sup -6} K{sup -1}. (3) High electrical conductivity through both the bulk material and in-situ formed oxide scales. (4) Satisfactory bulk and interfacial mechanical/thermomechanical reliability and durability at the SOFC operating temperatures. (5) Good compatibility with other materials in contact with interconnects such as seals and electrical contact materials. Until recently, the leading candidate material for the interconnect was doped lanthanum chromite (LaCrO3), which is a ceramic material which can easily withstand the traditional 1000 C operating temperature. However, the high cost of raw materials and fabrication, difficulties in obtaining high-density chromite parts at reasonable sintering temperatures, and the tendency of the chromite interconnect to partially reduce at the fuel gas/interconnect interface, causing the component to warp and the peripheral seal to break, have plagued the commercialization of planar SOFCs for years. The recent trend in developing lower temperature, more cost-effective cells which utilize anode-supported, several micron-thin electrolytes and/or new electrolytes with improved conductivity make it feasible for lanthanum chromite to be supplanted by metals or alloys as the interconnect materials. Compared to doped lanthanum chromite, metals or alloys offer significantly lower raw material and fabrication costs.« less