Structure, interactions and functionality in novel electrolyte materials for fuel cell applications

Doctoral thesis, 2007

It is today well known that fossil fuels will sooner or later be depleted, and the need to find alternative energy sources is now accepted. Among the diverse technologies developed to produce energy, considerable interest is currently directed to the hydrogen fuel cell, a device that converts chemical into electrical energy, is versatile and has a low environmental impact. However, in order to meet the requirements for mass-commercialization, several technical aspects must be improved. For instance, the performance of the polymer electrolyte membrane, which is a central part of the fuel cell, should be enhanced with respect to the operational temperature window and the cost of production. In this thesis, several approaches are explored with the aim of finding new polymer electrolyte materials with high proton conductivities in addition to thermal, mechanical and chemical stability. The main idea is to confine a liquid electrolyte in a polymer based membrane, where the function of the latter is mainly to act as a solid container. We investigate different electrolyte systems, such as aqueous solutions of acids, solid acid hydrates and the new generation of ionic liquids, which are incorporated in membranes based on the polymer poly(vinylidene fluoride) (PVDF). PVDF is only partially fluorinated hence relatively cheap, is both chemically and thermally resistant, and can be used as a starting material to form self-standing porous membranes. The key-issue in this thesis is to understand the microscopic properties that relate to the functionality of the membrane. The investigations were made using vibrational spectroscopy (Raman and infrared), a powerful technique to probe the local coordination of atoms in materials. The results show that PVDF adopts different crystalline forms, depending on the thermal treatment as well as on the type of electrolyte introduced into the pores. In addition, the properties of the liquid electrolyte, such as acid dissociation and thermal dependence of conductivity, have been addressed. We find that these properties are affected upon confinement into a solid matrix. We also add valuable information for a better understanding of the structure--property relations in bis(trifluoromethanesulfonyl)imide (TFSI-) based ionic liquids. The configuration and the conformation of the constituting ions is determined, and the nature of ion--ion interaction is addressed. Keywords: Polymer electrolyte membrane (PEM), fuel cell, vibrational spectroscopy, ionic liquid, proton conductivity