Assessing the influence of various imidazolium groups on the properties of poly(vinyl chloride) based high temperature proton exchange membranes

Abstract

Low cost poly(vinyl chloride) (PVC) is functionalized by eight different imidazole compounds to fabricate high temperature polymer electrolyte membranes. Due to the presence of chloride naturally in the backbone, PVC reacts with various imidazoles via the SN2 nucleophilic reaction avoiding the chloromethylation reaction. Different from previous imidazolium groups grafted chloromethyl polymers, PVC membranes functionalized with mono-imidazoles (such as methylimidazole, dimethylimidazole, 1-butyl-2-methyl-imidazole and 1-decyl-2-methyl imidazole) exhibit quite low phosphoric acid doping contents. Bis-imidazole compounds (including 1,4-bis(imidazolyl)butane (7#) and 1,4-bis(2-methylimidazol-1-yl)butane) are synthesized to be used as the imidazolium functionalized reagents. PVC membranes grafted by bis-imidazoles with the long and flexible linkage display superior acid doping capability and high conductivities. Meanwhile, porous poly(tetrafluoroethylene) (PTFE) is employed as the enhanced matrix to improve the mechanical strength. The resultant PVC-42%7#/PTFE membrane with a phosphoric acid doping content of 100.7% shows a volume swelling of 64%, a conductivity of 41.8 mS·cm−1 at 160 °C without humidification, and a tensile strength of 14.2 MPa at room temperature, whose fuel cell performance is determined to demonstrate the technical feasibility for fuel cells.