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Styrene radiation grafted ETFE based proton conducting membranes are subject to degradation under fuel cell operating conditions and show a poor stability. Lifetimes exceeding 250 h can only be achieved with crosslinked membranes. In this study, a novel approach based on the increase of the intrinsic oxidative stability of uncrosslinked membranes is reported. Hence, the co-grafting of styrene with methacrylonitrile (MAN), which possesses a protected α-position and strong dipolar pendant nitrile group, onto 25 μm ETFE base film was investigated. Styrene/MAN co-grafted membranes were compared to a styrene based membrane in durability tests in single H2/O2 fuel cells. It is shown that the incorporation of MAN considerably improves the chemical stability, yielding fuel cell lifetimes exceeding 1000 h. The membrane preparation based on the co-grafting of styrene and MAN offers the prospect of tuning the MAN content and introduction of a crosslinker to enhance the oxidative stability of the resulting fuel cell membranes. Keywords: Polymer electrolyte fuel cell, Proton exchange membrane, Radiation grafting, Styrene, Methacrylonitrile, Membrane electrode assembly

Selective exposures of poly(ethylene-alt-tetrafluoroethylene) (ETFE) films with hard X-rays through high aspect ratio Ni-masks were performed at the LIGA3 beamline of the “Angstrom Quelle Karlsruhe” (ANKA) to create patterns of radicals used as initiators for the grafting of styrene into the bulk of the ETFE films. Grafted films were then sulfonated to obtain proton-conducting membranes. The structure definition, as investigated by scanning electron microscopy (SEM), showed a perfect discrimination between exposed and shaded areas through all the film thickness. Structuring results in a more homogeneous appearance of the membrane without affecting the degree of grafting and proton conductivity in the grafted areas. In fuel cell tests the structured membranes showed slightly lower performance due to 10% lower active area, but had a significantly higher lifetime.

Proton-exchange membranes are required to exhibit chemical, mechanical, and thermal stability for fuel cell applications. The present investigation has been carried out to explore the thermal behavior of poly(ethyl- ene-alt-tetrafluoroethylene) (ETFE)-based proton-conduct- ing membranes, both uncrosslinked and crosslinked, pre- pared by radiation grafting and subsequent sulfonation. The influence of preparation steps (irradiation, grafting, sulfonation, crosslinking) on the thermal degradation, crystallinity, and melting behavior of membranes with varying degree of grafting was examined. ETFE base film and grafted films were studied as the reference materials. Furthermore, poly(tetrafluoroethylene-co-hexafluoropro- pylene)-based grafted films and membranes were investi- gated as well for comparison. Membrane preparation steps, degree of grafting, crosslinking, type of base poly- mer have considerable inf luence on the thermal pro- perties of membranes. The crystallinity of the films decreases slightly by grafting, while a significant decrease was observed after sulfonation. For instance, crystallinity decreased from 37% (pristine ETFE) to 36% (uncrosslinked grafted film) and 23% (uncrosslinked ETFE-based membrane). On the other hand, the melting temperature of the base polymer was almost unaffected by irradiation and grafting. The crosslinked ETFE-based membranes exhibit a slightly higher melting temperature (262.58C) than their corresponding grafted films (261.38C) and the base film (260.68C). 2008 Wiley Periodicals, Inc. J Appl Polym Sci 108: 3577-3585, 2008

This study concerns the investigation of the effect of reaction conditions on the radiation-induced grafting of styrene into poly(ethylene-alt-tetrafluoroethylene) base film. Grafting was obtained by the pre-irradiation method, using styrene in isopropanol/water mixture. The influence of irradiation and reaction conditions, such as irradiation dose, monomer concentration, water content of the grafting solution and reaction temperature on the grafting yield was investigated. The grafting kinetics in terms of initial polymerization rate (rp0), radical recombination rate (γ) and delay time (t0) was evaluated. It was found that the rate and the extent of grafting were strongly dependent on the grafting conditions. The order of dependence of the rate of grafting was found to be 0.46, 1.5 and 0.62 for pre-irradiation dose, monomer concentration and water content in the grafting mixture, respectively. The activation energy was determined to be 98.4 kJ/mol for the temperature range of 50–80 °C. The use of kinetic equations for the polymerization gives a useful description of the grafting process. It was determined that the initial polymerization rate and the radical recombination rate vary significantly with reaction conditions. The delay time is dependent mainly on irradiation dose and to some extent on monomer concentration.

Crosslinking of styrene grafted and sulfonated fuel cell membranes is essential to obtaining well-performing and durable membranes. The properties and fuel cell performance of radiation grafted membranes based on ETFE (25 micron) with different extent of crosslinking, using divinylbenzene (DVB) as co-monomer to styrene, was investigated. At high degrees of crosslinking, fuel cell performance was limited by the high ohmic resistance of the membrane, whereas at low degrees of crosslinking the membrane-electrode interface was of poor quality. Optimum performance was obtained at a degree of crosslinking corresponding to a styrene:DVB monomer ratio (v/v) in the grafting solution of 95:5.

Crosslinked proton-exchange membranes were prepared by radiation-induced grafting of styrene into poly(ethylene- alt -tetrafluoroethylene) (ETFE) and subsequent sulfonation. The influence of the crosslinker, divinylbenzene (DVB), on various film and membrane properties was investigated in detail. A series of grafted films and membranes were prepared at varying DVB concentrations at a fixed graft level (∼25%) for this purpose. Grafting kinetics and kinetic parameters, including initial polymerization rate ( r po ), radical recombination rate ( γ ) and delay time ( t 0 ) at different DVB concentrations were evaluated. Moreover, resultant grafted films and membranes have been characterized with respect to their dimensional stability, chemical composition, thermal stability, crystallinity and fuel cell relevant properties. The concentration of crosslinker in the initial grafting solution was found to influence crucially the grafting reaction and graft levels. Dimensional increase, considerable especially after sulfonation, can be controlled by the introduction of DVB. Furthermore, the FTIR-ATR measurements exhibited that the grafted films were highly crosslinked within the surface near as compared to the entire bulk region. The influence of DVB on the crystallinity and thermal stability was not significant. On the contrary, the water uptake and proton conductivity of the membranes decreased substantially, while the IEC decreased slightly with increasing DVB concentration.

In this study, the effect of cross-linker content on the chemical stability of polyethylene-alt-tetrafluoroethyleneETFE-based radiation-grafted and sulfonated membranes was investigated. An ex situ degradation test in H2O2 solution showed a strong increase in stability of cross-linked membranes compared to uncross-linked ones. Excessive cross-linking, however, is detrimental to the chemical and mechanical properties. Furthermore, the stability of grafted membranes based on ETFE was superior to those based on polytetrafluoroethylene-co-hexafluoropropyleneFEP. An in situ long-term test in a H2/O2 single cell over 2180 h, using an ETFE-based grafted membrane optimized with respect to cross-linker content, with a graft level of 25% was carried out. The performance of the membrane electrode assembly exhibited a voltage decay rate of 13 Vh 1 over the testing time at a current density of 500 mA cm 2 and a cell temperature of 80°C, while the hydrogen permeation showed a steady increase over time. This indicates that, to some extent, changes in the membrane morphology occur over the operating period. Local postmortem analysis of the tested membrane reveals that high degradation was observed in areas adjacent to the O2 inlet and in other areas nearby.

Cross-linking of styrene grafted and sulfonated fuel cell membranes is essential to obtain well-performing and durable membranes. In this study, the properties and fuel cell performance of radiation grafted membranes based on poly(ethylene-alt-hexafluoropropylene) (ETFE) (25 μm) with different extent of cross-linking, using divinylbenzene (DVB) as a comonomer to styrene, were investigated. The water uptake and proton conductivity decrease as a function of increasing the extent of cross-linking. Also, the membranes become more brittle. Fuel cell performance is limited by the high ohmic resistance of the membrane at high degrees of cross-linking, whereas at low degrees of cross-linking the membrane-electrode interface is of poor quality. Optimum performance is obtained at an extent of cross-linking corresponding to a styrene:DVB monomer ratio (v/v) of 95:5 in the grafting solution.

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A small-angle neutron scattering (SANS) investigation of saturated aqueous proton exchange membranes is presented. Our membranes were synthesized by radiation-induced grafting of poly(ethylene-alt-tetrafluoroethylene) (ETFE) with styrene in the presence of crosslinker (divinylbenzene, DVB) and the polystyrene was sulfonated subsequently. The contrast variation method was used to understand the relationship between morphology, water uptake, and proton conductivity. We find that the membranes are separated into two phases, mostly following the morphology already defined in the semi-crystalline ETFE base film. The amorphous phase hosts the water and swells upon hydration, swelling being inversely proportional to the degree of crosslinking. Proton conductivity and volumetric fraction of water are related by a power law, indicating a percolated and most likely random network of finely dispersed aqueous pores in the hydrophilic domains.