High temperature proton exchange membranes based on polybenzimidazoles for fuel cells

Abstract

Abstract To achieve high temperature operation of proton exchange membrane fuel cells (PEMFC), preferably under ambient pressure, acid–base polymer membranes represent an effective approach. The phosphoric acid-doped polybenzimidazole membrane seems so far the most successful system in the field. It has in recent years motivated extensive research activities with great progress. This treatise is devoted to updating the development, covering polymer synthesis, membrane casting, physicochemical characterizations and fuel cell technologies. To optimize the membrane properties, high molecular weight polymers with synthetically modified or N-substituted structures have been synthesized. Techniques for membrane casting from organic solutions and directly from acid solutions have been developed. Ionic and covalent cross-linking as well as inorganic–organic composites has been explored. Membrane characterizations have been made including spectroscopy, water uptake and acid doping, thermal and oxidative stability, conductivity, electro-osmotic water drag, methanol crossover, solubility and permeability of gases, and oxygen reduction kinetics. Related fuel cell technologies such as electrode and MEA fabrication have been developed and high temperature PEMFC has been successfully demonstrated at temperatures of up to 200 °C under ambient pressure. No gas humidification is mandatory, which enables the elimination of the complicated humidification system, compared with Nafion cells. Other operating features of the PBI cell include easy control of air flow rate, cell temperature and cooling. The PBI cell operating at above 150 °C can tolerate up to 1% CO and 10 ppm SO 2 in the fuel stream, allowing for simplification of the fuel processing system and possible integration of the fuel cell stack with fuel processing units. Long-term durability with a degradation rate of 5 μV h −1 has been achieved under continuous operation with hydrogen and air at 150–160 °C. With load or thermal cycling, a performance loss of 300 μV per cycle or 40 μV h −1 per operating hour was observed. Further improvement should be done by, e.g. optimizing the thermal and chemical stability of the polymer, acid–base interaction and acid management, activity and stability of catalyst and more importantly the catalyst support, as well as the integral interface between electrode and membrane.