Phosphoric acid-doped cross-linked sulfonated poly(imide-benzimidazole) for proton exchange membrane fuel cell applications

• Phosphoric acid-doped sulfonated poly(imide benzimidazole) are synthesized.
• Benzimidazole ring is used as both a cross-linking site and a basic site.
• The resulting membranes have good mechanical strength and chemical stability.
• The resulting membranes have a high proton conductivity in high temperature.

A series of covalently cross-linked sulfonated poly(imide benzimidazole)s (CBrSPIBIs) were synthesized using 4,4′-bibromomethenyl diphenyl ether as a cross-linker. However, the cross-linked structure results in a reduction of ion conductivity due to the blockage of the hydrophilic channel. Thus, the alkaline imidazole ring was used as a cross-linking site and a basic site to absorb phosphoric acid (PA) molecules. Even at a low humidity and PA-doping level, the proton conductivity of the resulting PA-doped CBrSPIBI (PA-CBrSPIBI) membranes is one to two orders of magnitude higher than that of CBrSPIBI and Nafion due to the formation of a new proton transport pathway between PA and sulfonic acid groups. In addition, these membranes display excellent mechanical strength and chemical stability because of the cross-linked structure. The tensile strength of PA-doped cross-linked membranes ranges from 47.2 to 55.5 MPa, and they can maintain their integrity and mechanical strength in water for more than two months, which is superior to the corresponding sulfonated polyimide and polybenzimidazole blend membranes.