Intermediate temperature proton exchange membrane fuel cells: a systematic review of recently electrochemically evaluated materials

The integration of proton exchange membrane fuel cells (PEMFCs) in heavy-duty vehicles and other demanding applications, such as aviation, would be simplified if the stacks could operate above 100 °C instead of the traditional low temperature (LT, up to 80 °C), thereby allowing a reduction in cooling system in size and power. This review offers a comprehensive compilation of experimental studies reported in the literature on PEMFCs operated in the intermediate temperature (IT)-range, here defined as above 80 °C and up to 120 °C, which represented the targeted upper temperature for PEMFCs. Membranes, electrodes and gas diffusion layers for IT-PEMFCs are discussed. Particular attention is paid to polymers in membranes and catalyst layer ionomers. Results from current state-of-the-art perfluorosulfonic acids and alternatives, including hydrocarbon polymers, are evaluated considering their properties and limitations. Further, system benefits and drawbacks of IT- compared to the traditional LT-operation are discussed, such as the interplay between vapour and oxygen pressure, hydrogen crossover and water management. We report on the lack of consistency between ex-situ and in-situ studies and underline the importance of in-situ tests, proposing guidelines to evaluate novel materials. For IT-operation, the development of stable polymers, which are the weakest components of the PEMFCs, is the most urgent challenge. As degradation happens faster at higher temperatures, further long-term tests are needed above 80 °C and accelerated stress tests should be specifically designed for IT-operation according to the polymer chemistries. We conclude that, compared to LT-, IT-operation requires improved materials and additional research.