Electrochemical and physicochemical characterization of radiation-grafted membranes for fuel cells
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Doctoral thesis (article-based)
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en
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53, [49]
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Abstract
This thesis considers alternative proton conducting membrane materials for polymer electrolyte fuel cells (PEFC). The membrane is a key component of the PEFC accounting for the separation of the reactants and allowing the transport of hydrogen ions produced by the anode reaction to the cathode for the cathode reaction while enforcing the electrons to move through the external circuit so that the electrical energy can be utilized. Here the applicability of radiation-grafted membranes in the PEFC with hydrogen or methanol as a fuel has been considered. In particular, the influence of the matrix material of a radiation-grafted membrane on its the behaviour is clarified. The experimental membranes studied were prepared from fluoropolymer films by irradiating with an electron beam, subsequently grafting with styrene and finally sulfonating. Poly(vinylidene fluoride) PVDF, poly(vinylidene fluoride-co-hexafluoropropylene), PVDF-co-HFP, poly(ethylene-alt-tetrafluoroethylene), ETFE and poly(tetrafluoroethylene-co-hexafluopropylene), FEP, were chosen as matrix fluoropolymers. In addition the effect of thickness of the matrix was examined with three PVDF films. Scanning electrochemical microscopy was used as a new tool to investigate proton transport and distribution in the ionically conducting membranes. Such essential membrane properties as conductivity, oxygen permeability, water drag coefficients, methanol permeability, and the actual performance under the fuel cell conditions were found to depend on the crystallinity, or, more precisely, on the water uptake of the membrane, which was higher in membranes with lower crystallinity. The degradation of the side chains ensuring the protonic conductivity was one of the problems restricting the lifetime of the radiation-grafted membranes in the fuel cell with hydrogen as a fuel. However, also the ability of the matrix material itself to withstand this aggressive environment by sustaining the pristine structural arrangement appeared to affect the membrane durability. In general, it appeared that crystallites and a greater matrix thickness brought more strength, provided the matrix did not suffer from phase changes. Using a bis(vinyl phenyl)ethane crosslinker for the PVDF based membranes was detected to protect the side of the membrane facing the anode from chemical degradation. However, no significant improvement in the membrane lifetime was attained due to the degradation of the cathode side of the membrane, with a resulting loss of protonic conductivity. When using methanol as a fuel it was observed that similar performances to commercial materials could be achieved with the radiation-grafted membranes despite of their lower conductivities. This was attributed to the lower methanol permeability of the radiation-grafted membranes due to slower methanol diffusion through the membranes as a consequence of differences in the structures of the membranes.Description
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Parts
- Walsby N., Sundholm F., Kallio T. and Sundholm G., 2001. Radiation-grafted ion-exchange membranes: influence of the initial matrix on the synthesis, structure and non-transport properties. Journal of Polymer Science Part A: Polymer Chemistry 39, pages 3008-3017.
- Ericson H., Kallio T., Lehtinen T., Walsby N., Sundholm G., Sundholm F. and Jacobsson P., 2002. Confocal raman spectroscopic investigations of fuel cell tested sulfonated styrene grafted PVDF membranes. Journal of The Electrochemical Society 149, pages A206-A211.
- Walsby N., Hietala S., Maunu S. L., Sundholm F., Kallio T. and Sundholm G., 2002. Water in different polystyrene sulfonic acid grafted fluoropolymers. Journal of Applied Polymer Science 86, pages 33-42.
- Kallio T., Lundström M., Sundholm G., Walsby N. and Sundholm F., 2002. Electrochemical characterization of radiation-grafted ion-exchange membranes based on different matrix polymers. J. Appl. Electrochem. 32, pages 11-18.
- Kallio T., Jokela K., Serimaa R., Ericson H., Sundholm G., Jacobsson P. and Sundholm F., 2003. The effect of fuel cell test on the structure of radiation-grafted ion-exchange membranes based on different fluoropolymers. J. Appl. Electrochem. 33, pages 505-514.
- Kallio T., Slevin C., Sundholm G., Holmlund P. and Kontturi K., 2003. Proton transport in radiation-grafted membranes for fuel cells as detected by SECM. Electrochemistry Communications 5, pages 561-565.