Electrochemical Characterization of Ethylenetetrafluoroethylene-g-polystyrenesulfonic Acid Solid Polymer Electrolytes

Abstract

Proton-exchange-membrane fuel cells (PEMFCs) have received considerable interest as a reliable power source due to their ability to attain high power densities with high energy efficiency. 1 A vital component of the PEMFC is the proton exchange membrane (PEM), which provides the ionic path between the anode and the cathode while separating the two reactant gases. The PEM material most frequently used for this type of application is Nafion ® due to its chemical and mechanical stability and its commercial availability. Since Nafion is relatively costly, much research is being directed toward developing less expensive membrane materials. 2 One such alternative class of membrane derives from studies of radiation-grafted polymerization of styrene monomer into matrices such as fluorinated ethylene propylene (FEP), ethylenetetrafluoroethylene (ETFE), and poly(vinylidene fluoride) (PVDF) with subsequent sulfonation of the polystyrene. 3-6 The grafting and sulfonation processes allow the introduction of ionic conductivity while maintaining the mechanical characteristics of the base polymer. The process uses prefabricated commercial films and thus circumvents difficulties in obtaining thin films of uniform thickness. Parameters such as cross-linking density and membrane thickness can also be controlled. 3,7 The first step in the radiation-grafting technique is irradiation of the base polymer membrane usually with a gamma-source or electron-beam to form radicals on the polymer backbone. This is followed by introduction of the monomer, subsequent copolymerization, and concurrent attachment to the base polymer as a graft chain. Finally, the grafted chains are sulfonated. 3 Scherer's group has studied the radiation grafting approach extensively. They have explored the influence of synthetic conditions on the degree of grafting, structure, and physicochemical properties of FEP-g-polystyrenesulfonic acid (PSSA) films and evaluated their performance, stability, and degradation in PEMFCs. 3,4,7-9 From these studies they have been able to identify important membrane properties, for example: optimum thickness, cross-linking density, and specific resistance required for fuel-cell applications. 3 The performance of PEMFCs is dependent on many factors, but three are prominent and involve the PEM: (i) the ohmic overpotential due to membrane resistance, (ii) the activation overpotential due to slow kinetics of the oxygen reduction reaction (ORR) at the electrode/membrane interface, and (iii) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surface. 10 In order to understand fully the applicability of PEMs in PEMFCs, it is important that the materials be systematically investigated to establish correlations between membrane composition and electrochemical properties. A better understanding of these properties at a fundamental level should lead to further advances in membrane development for applications in PEMFCs.Lehtinen et al. have studied the electrochemical properties of PVDF-g-PSSA membrane...

Alkaline anion-exchange radiation-grafted membranes for possible electrochemical application in fuel cells

“…New bands were observed; these were present in the Raman spectrum of poly(vinylbenzyl chloride). The most significant of these were strong bands at 1613, 1268, 1003 cm -1 , the latter of which was the most intense and was also present as the 12 band in the spectrum of poly(styrene). 22 There was evidence of a small amount of residual VBC monomer in the material (a band at 1632 cm -1 in the Raman spectrum of F1 -see thermogravimetry of F1 later); this feature corresponds to the strong C=C band located in the Raman spectrum of VBC, which does not occur in the spectrum of poly(vinylbenzyl chloride).…”

“…21 The new bands observed were all present in the Raman spectrum of poly(vinylbenzyl chloride). The most significant of these were strong bands at 1613, 1269, 1003 cm -1 , the latter of which was the most intense and was also present as the 12 band in the Raman spectrum of poly(styrene). 22 Bands at 1633 and 1408 cm -1 in the spectrum of VBC associated with the C=C str.…”

“…In fuel cell membrane research, the effort has concentrated on PEMs with lower methanol permeabilities 8 and lower cost than Nafion ® (> US$ 780 m -2 ) 9 (note, it has been recently reported that new solution cast Nafion ® membranes (NR-111) would allow the cost to drop to US$ 50 m -2 with volume production of 2 million m 2 per annum). 10 A proportion of this effort has examined the radiation-grafting (using -rays and e-beams) of styrene onto non-fluorinated films 11 such as poly(ethylene), partially fluorinated films 11,12 such as poly(ethylene-cotetrafluoroethylene) ( 14 There is, however, some concern about the lack of stability of these membranes in a fuel cell environment, 15 with peroxyradical attack of the -hydrogen of the styrenic backbone being principally responsible (especially at the anode); ESR studies, 15 however, suggest that the OH · radicals are only damaging at pH as a shift reference) spectra were recorded using cross polarisation (with a flip-back pulse after acquisition) and spinning rates of 3 -5 kHz. Measurements were carried out at ambient temperature and pressures.…”

Water Soluble Polymers as Proton Exchange Membranes for Fuel Cells

“…For this reason, DAIS membranes are being promoted for the low temperature (<60 °C) portable fuel cell power market. In the grafting strategy, thermally and chemically stable fluorinated polymers (fluorinated ethylene propylene (FEP) [95,96]) or partially fluorinated polymers [ethylene tetrafluoroethylene (ETFE) [97,98] or polyvinylidene fluoride (PVDF) [99]] were chosen as the base polymers and shown to produce materials with some improved performance. It has been reported that the most stable polystyrene analogue is a vinyl perfluorosulfonic system developed by Ballard Power [100].…”

The radiation-grafting of vinylbenzyl chloride onto poly(hexafluoropropylene-co-tetrafluoroethylene) films with subsequent conversion to alkaline anion-exchange membranes: optimisation of the experimental conditions and characterisation

“…A significant proportion of previous fuel cell membrane research has examined the radiation-grafting of styrene onto polyethylene films, partially fluorinated films such as poly(vinylidene fluoride) (PVDF) and poly(ethylene-co-tetrafluoroethylene) (ETFE), and fully fluorinated poly(tetrafluoroethene-co-hexafluoropropylene) (FEP) films with subsequent sulfonation to yield cation-exchange sites [8][9][10][11][12][13][14][15], the properties and compositions of the final materials being easily controlled [8,12]. Such protonexchange membranes have been tested in DMFC mode by Scott et al [16].…”