Polybenzimidazole and Phosphonic Acid Groups-Functionalized Polyhedral Oligomeric Silsesquioxane Composite Electrolyte for High Temperature Proton Exchange Membrane

Here, we report composite membrane consisting of poly[2,2′-(m-phenylene)-5,5′-(bibenzimidazole)] (PBI) and polyhedral oligomeric silsesquioxane functionalized with phosphonic acid groups (PO(OH)₂-POSS) for high temperature proton exchange membrane. 7 phosphonic acid groups are incorporated into the phenyl rings of POSS via bromination in a high yield (93%), followed by substitution of the bromine elements by phosphonate ester groups via a Pd(0) catalyzed P-C coupling reaction. Phosphonic acid groups are formed by the hydrolysis of the phosphonate ester groups in hydrobromic acid solution. At a 50 wt% of PA content in the membranes, PBI/PO(OH)₂-POSS composite membrane shows larger proton conductivity of 3.2 × 10^-3 S cm^-1 than 2.8 × 10^-3 S cm^-1 of PBI membrane at 150°C and anhydrous conditions, owing to the multiple phosphonic acid groups of PO(OH)₂-POSS that can function as proton transport medium at high temperature and low humidity conditions.