resulting from Grain growth and coarsening, the actual creep behavior of SOFCs is currently not completely understood Target DOE target requirements are 40,000 hours of service for stationary fuel cell applications and greater than 5,000 hours for transportation systems (fuel cell vehicles) at a factory cost of $40/kW for a 10 kW coal-based system without additional requirements. Lifetime effects (phase stability, thermal expansion compatibility, element migration, conductivity and aging) must be addressed. The Solid State Energy Conversion Alliance 2008 (interim) target for overall degradation per 1,000 hours is 4.0%. Research Research is going now in the direction of lower-temperature SOFCs (600 °C). Low temperature systems can reduce costs by reducing insulation, materials, start-up and degradation-related costs. With higher operating temperatures, the temperature gradient increases the severity of thermal stresses, which affects materials cost and life of the system. An intermediate temperature system (650-800 °C) would enable the use of cheaper metallic materials with better mechanical properties and thermal conductivity. New developments in nano-scale electrolyte structures have been shown to bring down operating temperatures to around 350 °C, which would enable the use of even cheaper steel and elastomeric/polymeric components. Lowering operating temperatures has the added benefit of increased efficiency. Theoretical fuel cell efficiency increases with decreasing temperature. For example, the efficiency of a SOFC using CO as fuel increases from 63% to 81% when decreasing the system temperature from 900 °C to 350 °C. Research is also under way to improve the fuel flexibility of SOFCs. While stable operation has been achieved on a variety of hydrocarbon fuels, these cells typically rely on external fuel processing. In the case of natural gas, the fuel is either externally or internally reformed and the sulfur compounds are removed. These processes add to the cost and complexity of SOFC systems. Work is under way at a number of institutions to improve the stability of anode materials for hydrocarbon oxidation and, therefore, relax the requirements for fuel processing and decrease SOFC balance of plant costs. Research is also going on in reducing start-up time to be able to implement SOFCs in mobile applications. This can be partially achieved by lowering operating temperatures, which is the case for proton exchange membrane fuel cells (PEMFCs). Due to their fuel flexibility, they may run on partially reformed diesel, and this makes SOFCs interesting as auxiliary power units (APU) in refrigerated trucks. Specifically, Delphi Automotive Systems are developing