The cell and stack level is an intermediate stage between the component-based analysis and the system level. On the way to the durability targets of 5,000 h for automotive applications, as formulated by the US DOE for 2010/2015, or the Japanese NEDO's lifetime targets for stationary applications of 40,000 and 90,000 h in 2010 and 2015, respectively, investigations on a technical cell level are required.
On the cell and stack level, therefore, more complex boundary and operating conditions need to be investigated than on the component level. Combinations and coupling of the degradation of different components complicate the analysis. New boundary conditions on the cell and stack level include the effects of contamination from different sources or the effect of freezing. Effects of contamination from the ambient air at the cathode or from a fuel reforming process on the anode side are investigated. Also the effect of ambient temperature, here mainly the exposure to subfreezing conditions, is a challenge for reaching the durability targets. On the level of technical cells and stacks, the complexity and interaction of the processes is also higher than in small single cells used for component analysis owing to lateral gradients in the cells, i.e., in water vapor pressure. These effects are important and may be lifetime-limiting. On the technical level, the influence of parameters such as flow-field design, cell or stack compression, and interaction between cells in the stack can be of decisive importance for the rate of degradation and, therefore, for the lifetime of the stack.
In Part II, the most important degradation phenomena on the cell and stack level are analyzed and discussed by highly rated experts in the field from industry and academia.