Project
Inorganic-polymer-composites from molecular precursors: preparation, characterization, proton conductivity

Prof. Dr. J. Caro, PD Dr. M. Wark
Institute for Physical Chemistry and Electrochemistry
University of Hannover, Germany

Aim of the proposed project is the development of membranes for the use in polymer electrolyte membrane fuel cells (PEMFC). In comparison to commercially available membranes the new membranes shall offer an improved proton conductivity especially in the absence of water. Up to now in fuel cells of the PEMFC type mostly membranes made from Nafion and modified with sulfonic acid groups are used. These membranes must be kept moist all the time since water molecules are needed as vehicles for the proton transport which occurs according to a Grotthus mechanism. The necessary humidification is costly and shall be avoided. Moreover, water-free membranes allow on the one hand a higher working temperatures above 100°C and with that a higher efficiency, and on the other hand the tolerance against traces of CO or hydrocarbons in the used fuel gases (H₂, CH₃OH) will be increased.

We plan to synthesize composite materials consisting of polysiloxanes and porous silica or metal oxide frameworks. Both, the porous inorganic frameworks as well as the polysiloxanes shall be modified with organic functionalities, which enable a efficient cross-linking and effective proton transport (e.g. via sulfonic acid or imidazol groups). The reason for the use of metal oxides, which contain nanochannels, lies in the general observation that at surfaces a higher proton mobility is possible. Furthermore, the mechanical stability of the membranes will be enhanced and an unintentional swelling in the presence of water shall be avoided.

The porous metal oxide films or layers can be formed in the presence of structure-directing agents (e.g. surfactants or block-co-polymers) either in a sol-gel process or by electrochemical deposition (Meⁿ⁺ + ne^- + ½ O₂ → MeO). The polysiloxanes will be synthesized and functionalized according the recipes developed by collaboration partners at the University of Bremen.

In order to obtain self-supporting composite membranes suitable already functionalized polysiloxane precursors (monomers or oligomers) shall be directly added to the gels from which the porous SiO₂ or metal oxide frameworks are formed. Via in-situ preparation the coupling and cross-linking shall be achieved.

Besides characterization with the typical methods (XRD, determination of the porosity by gas adsorption, IR- and UV/Vis spectroscopy, NMR) the obtained composites will be mainly analyzed by analytical electron microscopye (REM and TEM with EDX and EELS). The proton conductivity will be determined by impedance spectroscopy and the inhibition of the diffusion of the fuels H₂, CH₃OH and O₂ via the membrane will be checked by testing the gas permeation.