Preparation And Study Of Phosphonic Acid Functionalized Siloxane/polyimide High Temperature Proton Exchange Membranes

Proton exchange membrane fuel cell is an environmental-friendly and highly efficientpower generation system,which can convert chemical energy into electrical energy by hydrogen and oxidizing agents.Currently,researches in PEMFCs are focusing on elevating its working temperature.Elevating the working temperature not only can reduce the use of precious metal electrodes,but also simplify the water thermal management system.It can decrease the cost of PEMFCs.However,the most widely used membrane in PEMFC,fluorine sulfonic acid membrane?Nafion?,which is very dependent on water in the process of proton conduction.It is difficult to acquire high proton conductivity under high temperature and low humidity.Phosphonic acid is one kind of amphoteric acid?acidic or alkaline?,which can make proton conducted between phosphonic acid molecules through the formation and fracture of dynamic hydrogen bond network,realizing the anhydrous proton conductive.So,phosphonic acid based proton exchange membranes have been widely studied.In this paper,ATMP was chemically bound to EHTMS by epoxy ring opening.Polyimide was mixed with ATMP functionalized EHTMS to prepare the HM membranes.The bonding condition of ATMP was analyzed.The HMW membranes exclusive of free phosphonic acid were prepared by immersing HM membranes in water.Furthermore,Chemical structure,physical and chemical stability,mechanical properties,surface morphology,proton conductivity and proton conduction mechanism of HM and HMW membranes were investigated.Morphology and mechanical property study showed that cracks appear on the surface of membranes after the process of removing free phosphonic acid,which result in the decline of mechanical property.But the microstructure of membranes was not destroyed.The results of FT-IR revealed that phosphonic acid had been successfully bonded to siloxanes by ring-opening reaction,and siloxane was hydrolyzed and formed the Si-O-Si network structure successfully.Besides,the interaction between polyimide and ATMP mainly depends on hydrogen bonds,and the self-ionization of ATMP is limited.When ATMP was anchored in membrane,it could not produce HPO₄^2-and H₂PO₄^-.The physical and chemical stability of membranes is increased with the addition of polyimide.HM-X and HM-XW membranes showed good oxidative and thermal stability,Which can be stable under 250?and not broken after immersing in Fenton for 24h.The HM-XW membranes which had removing free phosphonic acid have good Hydrolytic stability.Even though it was immersed in water under 80?,its IEC only has little decrease.The proton conductivity of the prepared membranes strongly depends on temperature and the content of phosphonic acid.HM-XW membranes have high activation energy.Hydrogen bond rather than protonation of phosphonic acid seems to play an important role when phosphonic acid groups are anchored in the membrane.The proton conductivity can achieve 10^-3 S·cm^-1under anhydrous conditions at 160?,which has the value for practical application.