Polymer Ionic Materials: A Study of new Materials for Solid Polymer Fuel Cells and Rechargeable Polymer Batteries

Doctoral thesis, 1998

Ion-containing polymeric materials are interesting for use in a wide variety of applications, where the flexible, yet mechanically stable, polymer matrix is essential. Intensive research is presently going on worldwide for applications as electrolytes in solid polymer batteries and fuel cells, where the ion-conducting polymer also serves as an electrode separator. The research is triggered by the fact that polymer-based batteries and fuel cells provide environmentally friendly alternatives for combustion engines and conventional batteries. The activities are mainly focused on the material properties in order to develop the optimal structure for stability and ion conduction. The work in this thesis concerns issues of importance for understanding the microstructure of polymer ionic materials. Raman spectroscopy has been the main tool in the investigations, although neutron diffraction and luminescence spectroscopy have in some cases also been used. A series of poly(vinylidene fluoride)(PVDF) poly(styrene sulfonic acid) graft copolymer membranes, developed for fuel cell applications, were investigated in order to correlate variations of physical properties with membrane preparation. Depth resolution was achieved by using confocal micro-Raman spectroscopy and the graft distribution, the sulfonation efficiency and the influence of treatment on the PVDF host matrix structure were determined. A Raman study of a membrane subjected to a crude fuel cell test was performed. The PVDF-based membranes show promising characteristics for fuel cell applications, although the material properties need to be improved concerning homogeneity and stability. Ion-ion and ion-polymer interactions in salt-doped polyethers and sulfonated polyether derivatives were studied. In the salt-doped polyethers, recognized for their excellent ion-conducting properties, the cations were found to be distributed over many sites and to interact with both the anions, the ether oxygens and the polar end groups. In the polyether sulfonate derivatives, on the other hand, no ether oxygen-cation interactions were found and the cations were located at the sulfonate sites only. The findings imply that despite the fact that the salt-doped polyethers are excellent ionic conductors, even in their dry state, their sulfonated counterparts, the sulfonated polyether derivatives, can hardly perform as electrolytes, at least not without the addition of water or other solvents.