Preparation And Properties Of Ion Exchange Membranes With Excellent Long-term Stabilitybased On Polybenzimidazole

Low temperature proton exchange membrane fuel cells（LT-PEMFCs）were applied all over the world.However,low operating temperature and the reliance of precious metal prevented the further development of LT-PEMFCs.In this situation,high temperature proton exchange membrane fuel cells（HT-PEMFCs）and anion exchange membrane fuel cells（AEMFCs）attracted much attention in recent years.The most common electrolyte used for HT-PEMFCs was phosphoric acid（PA）doped polybenzimidazole（PBI）membranes.The proton conductivity of PBI/PA membranes was positive correlation of the amount of PA absorption.In order to improve the proton conductivity of PBI/PA membranes,generally,the researchers introduced the ionic liquid（IL）that could absorb PA into the PBI matrix.However,the IL is“free”in the matrix and will escape during the long-term operation.Thus,we prepared the PBI/IL composite membranes with cage-like cross-linked structure based on PBI containing hydroxyl groups,3-（triethoxysilyl）propyl isocyanate and 1-butyl-3-methylimidazolium dihydrogen phosphate（BMI-DHPH）.The basic characteristic of imidazole rings in BMI-DHPH enhanced the PA doping ability of obtained cPBI-IL X membranes,thus,the proton conductivity of which was elevated.The cage-like cross-linked structure was formed under acidic conditions,and such structure can not only strengthen the mechanical properties of cPBI-IL X membranes but also prevent the leakage of IL to some extent.After being placed in 80℃,40%relative humidity conditions for 144 h,the residual IL weight percentage of cPBI-IL 10 membrane was 71.6wt.%,while that of PBI-IL 10 membrane without cage-like cross-linked structure was only2 wt.%.The cPBI-IL 8 membrane exhibited the highest proton conductivity of 133 mS cm^-1 at 160℃.Anion exchange membranes（AEMs）also met the challenge of long-term stability especially when the membranes contain ethyl bonds.OH^-attacked cationic groups and ethyl bonds under alkaline conditions,displayed as the electrical loss of AEMFCs directly.Regarding the issues mentioned above,we prepared a series of AEMs based on partial N-quaternization polybenzimidazole which prepared by polybenzimidazole,sodium hydride and 1-iodopentane.The N-quaternization degree was quantitatively controlled by adjusting the feed ratio of 1-iodopentane to PBI repeating units.The partial N-quaternization characteristics guaranteed the membranes excellent mechanical properties and the tensile strength was in the range of 52.9 to 62.8 MPa.There were no aryl ether bonds locating on backbone and the benzene rings connecting with C₂ positon of imidazole ring endowed excellent alkaline resistance to the membranes.For example,the PIL-1.47 membrane displayed the highest ionic conductivity of 40 mS cm^-1 under 90℃ and 100%relative humidity conditions and the highest ionic conductivity of it was still remained 86.23%after being immersed in 3.5 M NaOH for 1200 h under ambient atmosphere.