Phosphoric acid doped fuel cell membranes by radiation grafting of 4-Vinylpyridine/Comonomers mixtures onto poly(ethylene-co-tetrafluoroethylene) films

Abstract

Proton exchange membrane fuel cell (PEMFC) is one of the most promising green technologies for providing clean and efficient energy and operating above 100 °C is highly desired to enhance the electrodes kinetics and increase tolerance to carbon monoxide impurities from reformed hydrogen. However, the commercially available membranes for fuel cell such as Nafion® are expensive and have limited operational temperature (< 80 °C). This work aims to develop alternative phosphoric acid (PA) doped membranes using basic radiation grafted precursor films for PEMFC operating at temperatures 120 °C. Particularly, the main objective of this study was to develop three PA doped membranes by radiation induced grafting of mixture of 4-vinylpyridine (4-VP) with glycidyl methacrylate (GMA), 1-vinylimidazole (1-VIm) or triallyl cyanurate (TAC) onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by doping with PA. A membrane obtained by grafting of 4-VP alone onto ETFE film and acid doping was used as a reference. The degree of grafting (DG) was controlled by optimization of the reaction parameters such as absorbed dose, composition of monomer mixture, temperature and reaction time whereas the acid doping level (DL) was manipulated by variation of PA concentration, reaction temperature and time. The properties of the PA doped membranes denoted as ETFE-g-P(4-VP)/PA, ETFE-g-P(4-VP/GMA)/PA, ETFE-g-P(4-VP/1-VIm)/PA, ETFE-g-P(4-VP/TAC)/PA together with the corresponding grafted and pristine ETFE films were evaluated in correlation with type and concentration of second monomer added to 4-VP (comonomer) using Fourier transform infrared, field emission scanning electron microscope, thermal gravimetric analysis and x-ray diffraction. The membranes were also subjected to elemental as well as mechanical analysis and their proton conductivity together with fuel cell test were investigated at 120 °C. The DG was found to be strongly dependent upon grafting parameters. The obtained membranes attained high DL which reached 97 %, 115 %, 119 % and 113 % for membranes grafted with 4-VP, 4-VP/GMA, 4-VP/1-VIm and 4-VP/TAC, respectively. All the membranes displayed well-defined structures, good thermal stability, reasonable mechanical strength and high proton conductivity in the range of 33-44 mS/cm (at 120 °C and 0 % RH). The mechanical properties of ETFE-g-P(4-VP/TAC)/PA membrane was significantly improved by introducing TAC as a comonomer during grafting, which crosslinked the PA doped grafted chains compared to the other two membranes. ETFE-g-P(4-VP/1-VIm)/PA membrane showed the best fuel cell performance (226 mW/cm2) at 120 °C and 20 % RH conditions compared to the other two membranes and this is due to the increase of number of protonated pyridine and imidazole rings that could host more PA. The sequence of the membranes’ performance in PEMFC represented by power density was ETFE-g-P(4-VP/TAC)/PA (84 mW/cm2) > ETFE-g-P(4-VP/GMA)/PA (76 mW/cm2) > ETFE-g-P(4-VP/1-VIm)/PA (70 mW/cm2) > ETFE-g-P(4-VP)/PA (53 mW/cm2) under dry conditions. Thus, it can be concluded that grafting of comonomers is an effective method to enhance the conductivity of PA doped membranes in way making them more suitable for fuel cell operation above 100 °C.