Proton exchange membranes based on semi-interpenetrating polymer networks of polybenzimidazole and perfluorosulfonic acid polymer with hollow silica spheres as micro-reservoir

A new method to prepare reinforced proton exchange membranes (PEMs) for polymer electrolyte membrane fuel cells (PEMFCs), which can work either in humidified environment or at anhydrous state, is proposed via constructing semi-interpenetrating polymer network (semi-IPN) of polybenzimidazole (PBI) and perfluorosulfonic acid polymer, with poly(vinylbenzyl phosphonic acid) functionalized hollow silica spheres (HPSS) as the additive. Vinyl functionalized PBI, which is synthesized from poly[2,2′-(m-phenylene)-5,5′-bibenzimidazole] and p-vinylbenzyl chloride, is thermally crosslinked in the presence of Nafion® as well as HPSS during the membrane preparation. Thus semi-IPN structure is formed in the composite membranes. Compared with Nafion®, the composite membranes show higher thermal stability and water uptake, wider range of working temperature, improved mechanical properties, as well as better dimensional stability. Under humidified condition, the proton conductivity of the composite membranes is mainly provided by hydrated Nafion®, whereas PBI and the acidic moieties in the membranes contribute to the proton conductivity at higher temperature and anhydrous state. HPSS in the composite membranes behave like the micro-reservoir and help provide proton conductivity at the state between hydrated and anhydrous states.