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High Performance Polymers 2013 vol. 25 no. 2 198-204.
Mohamed Mahmoud Nasef, Amgad Ahmed Ali, Hamdani Saidi.
Universiti Teknologi Malaysia, Institute of Hydrogen Economy, Energy Research Alliance, Jalan Semarak, Kuala Lumpur, Malaysia
Mohamed Mahmoud Nasef, Universiti Teknologi Malaysia, Institute of Hydrogen Economy, Energy Research Alliance, Jalan Semarak, Kuala Lumpur, 54100, Malaysia. Email: [email protected]
Abstract
Composite membrane containing phosphoric acid (PA) for possible use in a fuel cell was prepared by radiation-induced grafting of 1-vinylimidazole (1-VIm) onto poly(ethylene-co-tetrafluoroethylene) (ETFE) films followed by protonation with PA doping. The preparation procedure involved three steps: (i) irradiation of ETFE films by an electron beam, (ii) grafting of 1-VIm onto the irradiated films under selected conditions and (iii) doping the grafted film with PA. The membrane composition, thermal properties and thermal stability were evaluated using Fourier-transformed infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry, respectively. The obtained membrane was found to have a degree of grafting of 54% and 6.6 mmol PA per poly-VIm repeating unit with ionic conductivity of 140 mS cm−1 at 120°C and ∼20% relative humidity. The overall results suggest that the membrane has a promising combination of physicochemical properties appealing for possible application in polymer electrolyte membrane fuel cell above 100°C.
Additional Information
High temperature polymer electrolyte membrane (HT-PEM) fuel cell has been proposed for replacing its counterpart operating at low temperature to bring about improvement of electrode kinetics, reduction or elimination of humidification, increment of tolerance to reformed fuel impurities, enhancement of ionic conductivities and increment of values of excess heat by cogeneration. Currently, Nafion and its analogous membranes are deemed to be unsuitable for HT-PEM fuel cell application due to deterioration of their proton conductivity gas retention properties. The former is based on proton flow in the aqueous phase of the membrane, which is hindered by dryness whereas the latter is caused by the variation in the membrane viscoelastic properties. Therefore, the search for alternative proton conducting membranes that can sustain high temperature operation in PEM fuel cell is fast growing.
Composite membranes composed of basic substrates doped with inorganic proton donors have recently attracted much attention due to their high proton conductivity, chemical and electrochemical stability at high temperature in addition to facile processing procedure.Phosphoric acid (PA) is one of the most attractive inorganic proton donors have been found to maintain high conductivity and reasonable stability at elevated temperature. A typical example is PA doped polybenzimadazole (PBI) membranes which have been subjected to frequent investigations and showed reasonable performance in PEM fuel cell at temperatures up to 190 °C without an additional humidification. However, such membranes are vulnerable to degradation partially caused by electrolyte leaching.
Alternatively, PA membranes prepared by doping of precursor films obtained by radiation induced grafting of heterocyclic monomers such as 4-vinylpyridine (4-VP) and 1-vinylimidazole (VIm) onto poly(ethylene-co-tetraflouroethylene) (ETFE) were reported in literature and found to be an attractive materials for HT-PEM fuel cell. Compared to conventional fuel cell membranes preparation procedure, the use of radiation induced grafting simplifies the procedure, allows composition and properties control and provides solution for film formation as reaction starts form pre-existing sheets.
In this work, 1-vinylimidazole (1-VIm) was grafted onto poly(ethylene-co-tetrafluorethylene) (ETFE) films using radiation induced grafting method under controlled conditions. The obtained poly(1-vinylimidazole) (P(1-VIm)) grafted films provided a matrix containing basic nitrogen capable of forming of acid/base complex upon doping with PA under controlled conditions in a post grafting reaction.
