Cross-linked polybenzimidazole membranes for fuel cells

The phosphoric-acid-doped polybenzimidazole (PBI) membranes, which exhibit high proton conductivity under low water contents for high-temperature operation of proton-exchange membrane fuel cells, were discussed. The operating features of the PBI-based fuel cells included little humidification, high carbon monoxide tolerance, better heat utilization, and possible integration with fuel processing units. It was found that the conductivity of the acid-base polymer membranes depends on the protonation equilibrium of the acid. Covalent cross-linking of PBI can be achieved by thermal treatment or an amide-type linkage through imidazole groups of the polymer. The linear PBI membrane was doped with 15.5 mol phosphoric acid per PBI repeat unit, which resulted in a volume swelling of the membrane by 160%. The results show that membranes based on perfluorinated sulfonic acid such as Nafion exhibit better chemical stability than membranes based on polyaromatic hydrocarbons.