Phosphoric acid-doped organic-inorganic cross-linked sulfonated poly(imide-benzimidazole) for high temperature proton exchange membrane fuel cells

• Organic-inorganic cross-linked membranes are synthesized.
• KH560 is used as a cross-linker and a PA-absorbing site.
• Cross-linked membranes are PA-doped.
• The resulted membranes have good mechanical strength and chemical stability.
• The resulted membranes have high proton conductivity.

Silane-cross-linked sulfonated poly(imide benzimidazole) (CSiSPIBI) membranes were prepared using γ-(2,3-epoxypropoxy) propyltrimethoxysilane (KH560) as a cross-linker, and the cross-linked structure was characterized using Fourier transform infrared spectroscopy and solubility test. The resulted cross-linked membranes were further doped with phosphoric acid (PA) by means of the acid–base interaction with the alkaline imidazole ring and the electrostatic interaction with siloxane. The results show that PA-doped CSiSPIBI membranes have high proton conductivity due to the formation of a new proton transport pathway between PA and sulfonic acid. Under high temperature and low humidity conditions, the proton conductivity of PA-doped sulfonated membranes is one to two orders of magnitude higher than that of non-PA-doped membranes and PA-doped non-sulfonated membranes. The silane-cross-linked membranes display improved chemical stability and mechanical strength, especially the oxidative stability. The complete dissolution time in Fenton's reagent increases from 510 min for the sulfonated polyimide/polybenzimidazole blend membrane to 1450 min for the silane-cross-linked membrane.