Preparation and characterization of phosphonic acid functionalized siloxane/polyimide composite proton exchange membranes

• Phosphonic acid is anchored in the membranes effectively.
• There is no free phosphonic acid in the composite membranes.
• Polyimide improves the physical and chemical stability of membranes.
• Hydrogen bond is formed between phosphonic acid and polyimide.

High temperature proton exchange membranes based on phosphonic acid were prepared from epoxycyclohexylethyltrimethoxysilane (EHTMS), amino trimethylene phosphonic acid (ATMP) and polyimide (PI) by sol–gel process. Membranes were immersed in water and the free phosphonic acid was removed. These membranes show good hydrolytic stability and their IEC only has a small decrease after soaking in hot water for 24 h. The results of FT-IR reveal that the interaction between polyimide and ATMP mainly depends on hydrogen bonds, and the self-ionization of ATMP is limited when it was fixed in membrane. The results of scanning electron microscopy and optical microscope show that the microstructure of membranes was not destroyed in the process of removing free phosphonic acid. The proton conductivity of the prepared membranes strongly depends on temperature and the content of phosphonic acid. Hydrogen bond rather than protonation of phosphonic acid seems to play an important role when phosphonic acid groups are anchored in the membrane. The maximum power density of single fuel cell based on HM-15W membranes is 484 mW cm− 2 at 160 °C.