Proton conducting membrane by radiation-induced grafting of 1-vinylimidazole onto poly(ethylene-co-tetrafluoro ethylene) film and phosphoric acid doping

Abstract

Phosphoric acid doped proton conducting membranes denoted as ETFE-g- P(1-VIm) for possible use in high temperature polymer electrolyte membrane fuel cell (PEMFC) were prepared by radiation induced graft polymerization of 1- vinylimidazole (1-VIm) onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by doping with phosphoric acid (PA). The ETFE films were irradiated by electron beam (EB) accelerator prior to grafting. The effect of the grafting parameters such as monomer concentration, absorbed dose, reaction time and medium temperature onto the degree of grafting (G%) were studied. The G% was found to be strongly dependent upon the investigated grafting parameters, which were optimized using response surface method (RSM) through the Box-Behnken design expert software. This led to the development of a quadratic model capable of predicting the degree of grafting. The validity of the statistical model was supported by the small deviation between the predicted (G = 61%) and experimental (G = 57%) values. The optimum conditions for achieving maximum G% were determined at: monomer concentration of 55 vol%, absorbed dose of 100 kGy, reaction time in the range of 14-20 h and medium temperature of 61°C. The effect of phosphoric acid doping parameters on the doping behaviour of the grafted ETFE films was also optimized using Taguchi method through implementing a Taguchi L9 (34) orthogonal array. The optimum parameters for achieving a maximum acid doping level (7.45 mmol/repeat polymer unit) were: G of 54%, acid concentration of 65%, temperature of 100oC and time of 5 days. The predicted doping value was deviated by 4.9% from the experimental one suggesting the validity of the model in prediction and optimization of acid doping reaction. The kinetics of phosphoric acid doping reaction was also investigated and two rate constants of 0.46 and 0.16 for PA doping reaction were graphically obtained suggesting a zeroth order reaction. The proton conductivity of the membranes was investigated using 4-probe conductivity cell attached to a direct current source meter in correlation with temperature and relative humidity. The proton conductivity was found to increase with the increase in doping level at constant temperature and relative humidity. Proton conductivity of 143 mS/cm at 20% relative humidity was achieved in the membranes having G of 38 and 54% suggesting a less water dependant conductivity. It can be concluded that the obtained membranes have very good combinations of physico-chemical and material properties suitable for possible application in PEMFC operating above 100 oC.