Phosphoric acid doped polymer electrolyte membrane based on radiation grafted poly(1-vinylimidazole-co-1-vinyl-2-pyrrolidone)-g-poly(ethylene/tetrafluoroethylene) copolymer and investigation of grafting kinetics

Abstract

A novel composite membrane containing Phosphoric Acid (PA) for possible application in high temperature Proton Exchange Membrane Fuel Cell (HT-PEMFC) was prepared by radiation-induced copolymerization of 1-vinylimidazole (1-Vlm) and 1-vinyl-2-pyrrolidone (1-V-2-P) onto poly (ethylene-alt-tetrafluoroethylene), ETFE films (ETFE-g-P(1-Vlm-co-1-V-2-P)) followed by protonation through PA doping. The preparation procedure involved three steps: i) Irradiation of ETFE films by an electron beam (EB) accelerator, ii) copolymerization of 1-Vlm-co-1-V-2-P onto the EB-preirradiated ETFE films under selected conditions and iii) acid doping of the grafted ETFE films with PA. The physiochemical properties of the resulted membranes were analysed in terms of degree of grafting (DG), grafting compositions, ionic conductivity, thermal properties and thermal stability using Fourier transform infrared spectroscopy (FTIR) fitted with attenuated total reflectance (ATR) and X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and Differential scanning calorimetry (DSC), respectively. The results showed that the physiochemical properties of the membranes are comparable to Nafion 117 especially their thermal stability. At 120��C and 0% relative humidity the membrane remained stable at 76% DG and 7.6�mmol repeat polymer unit−1 with ionic conductivity of 53�mS�cm−1. Overall the characterization tests indicated that the membrane displayed impressive thermos-chemical and physical properties with less water dependency. At 200��C the membrane remained thermally stable which enhances the membrane's potential application in high temperature proton exchange membrane fuel cell (HT-PEMFC) operating at 100 �C and above. Grafting kinetics of nitrogenous and heterocyclic 1-Vim-co-1-V-2-P onto EB-preirradiated ETFE films were also investigated in conjunction with reaction parameters namely: monomer concentration (M), reaction temperature (RT), and absorbed dose (D). This was achieved by the determination of three kinetic parameters namely: the initial polymerization rate (rp0), characteristics radical recombination rate (γ) and delay time (t0) respectively. The variation of rp0 with D and M allows the determination of the order of dependence of grafting rate (Rg) on D and M which are 2.23 and 3.39 respectively. Activation energy (Ea) was also determined followed by temperature effect investigation in the range of 50–70��C.