Correlation between Morphology, Water Uptake, and Proton Conductivity in Radiation Grafted Proton Exchange Membranes
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Correlation between Morphology, Water Uptake, and Proton Conductivity in Radiation Grafted Proton Exchange Membranes Sandor Balog1, Urs Gasser1, Kell Mortensen2, Lorenz Gubler3, Hicham Ben youcef3, and Guenther G. Scherer3 1
Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institut, 5232 Villigen PSI, Switzerland 2 Department of Natural Sciences, University of Copenhagen, 1871 Frederiksberg, Denmark 3 Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland ABSTRACT 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. INTRODUCTION Fuel cells are clean and efficient electrochemical energy conversion reactors. Among the various fuel cell types, the polymer electrolyte fuel cell (PEFC) is particularly attractive for applications with variable load profile and intermittent operation. In the PEFC, the electrolyte membrane, allowing transport of protons from anode to cathode, serves at the same time as a separator for electrons and reactant gases. Although perfluoroalkylsulfonic acid (PFSA) membranes, such as Nafion, are widely used owing their chemical stability and proton properties, the quest for more cost effective partially fluorinated or non-fluorinated materials is ongoing. The synthesis of block copolymers is a versatile strategy, which is pursued by a number of groups, whereby nonsulfonated blocks are combined with sulfonated blocks [1,2]. Graft copolymers represent yet another class of polymers in which two dissimilar polymer constituents may be combined [3]. We have extensively investigated the synthesis and characterization of radiation grafted membranes for application in PEFCs [4]. In this context, crosslinking has been a topic of high interest. Here, we report on the domain structure and morphology, as observed by small-angle neutron scattering (SANS), of radiation grafted and sulfonated membranes based on poly(ethylene-alt-tetrafluoroethylene) (ETFE) film. Our focus is on the effect of crosslinking on the relationship between the domain structure, water uptake, and proton conductivity.
EXPERIMENT ETFE (Tefzel 100LZ) films with 25µm thickness, purchased from DuPont (Circlevill
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