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Currently there has been very little reliability or end-of-life analysis conducted for proton exchange membrane fuel cell (PEM FC) systems, and detailed s of such systems are still in a rapid evolutionary stage. Voltage degradation as a fuel cell ages is a widely observed phenomenon, but little systematic information has been reported. As the development of fuel cell stacks approaches full commercialization, more interest will be placed on the reliability and the life of the systems. For commercial applications the end-of-life performance is usually specified, and therefore estimates for reliability, availability and a performance degradation rate (durability) need to be developed.
The operating history of the stack will influence fuel cell stack degradation and performance characteristics at the end-of-life. The development of systematic reliability analysis and a better understanding of the degradation mechanisms, especially as it relates to the materials in the electrochemical system, will also be of interest for the development of control systems.
With respect to fuel cell stacks, the definitions for reliability of the stack will include failure modes that can lead to catastrophic failure (i.e. a mean time to failure can be developed) and performance degradation below an acceptable level. Durability or irreversible degradation of the stack is defined as the ability of an membrane electrode assembly (MEA) to resist permanent change , in performance over time; that is degradation that does not lead to catastrophic failure but simply a decrease in performance that is not recoverable or reversible. Stability or reversible degradation, is the recoverable function, of voltage or current density decay.
Failure causes of PEM fuel cell failure are likely low level contamination, corrosion of the plates leading to increased contact resistance, thermal or hydration cycling leading to mechanical stress, catalyst particle ripening, swelling of polymer materials in the active catalyst layer change water removal characteristics, compaction of the gas diffusion layer due to mechanical stresses and surface chemistry changes in gas diffusion layer . Reliability analysis is a useful tool for product development of both MEAs and stacks as a whole, and a reliability block diagram for a PEM fuel cell stack can be developed. (reference)
