ETFE grafted membranes using 1-vinylimidazole, 1-vinyl-2-pyrrolidone, triallyl-cyanurate doped with phosphoric acid for high temperature applications

Abstract

Novel phosphoric acid (PA) doped proton exchange membranes were synthesized by radiation induced graft copolymerization of 1-vinylimidazole (1-VIm) and triallyl cyanurate (TAC), and 1-VIm-co-1-vinyl-2-pyrrolidone (1-VIm-co-1-V-2- P) onto poly (ethylene-alt-tetrafluoroethylene) (ETFE) films followed by protonation by PA doping. The ETFE base films were pre-irradiated by an electron beam (EB) accelerator prior to grafting and PA doping. They were denoted as ETFE-g-P(1-VImco- TAC) PA and ETFE-g-P(1-VIm-co-1-V-2-P) PA doped membranes. The main focus of this work is to synthesize membranes that have desirable properties and to investigate kinetics of 1-VIm and TAC onto ETFE base film and 1-VIm-co-1-V-2-P onto ETFE base films. From the optimization study using Box-Behnken design module of the response surface methodology available in the “Minitab®” software, degree of grafting (DG) was found to depend strongly on grafting parameters such as, crosslinker concentration and reaction temperature. Proton conductivity of the membranes was measured using four-probe conductivity cell and conductivity increased with an increase in doping level. ETFE-g-P(1-VIm-co-TAC) PA doped membranes achieved maximum DG (%) of 53%, proton conductivity of 33 mS cm–1 at 120 oC and 0% relative humidity condition. ETFE-g-P(1-VIm-co-1-V-2-P) PA doped membranes achieved maximum DG (%) of 76%, of 53 mS cm–1 at 120 oC and 0% relative humidity condition suggesting less water dependent conductivity. Properties of the developed membranes were investigated using different equipment such as, Fourier transform infrared spectrometer, thermal gravimetric analyzer and differential scanning calorimeter. Kinetic modeling of radiation induced graft copolymerization of 1-VIm-co-1-V-2-P was also attempted. It can be concluded that the synthesized membranes possessed desirable properties including mechanical and thermal stablility. This makes them appealing for possible application in high temperature proton exchange membrane fuel cell operated above 100 oC