Nafion Nanocomposite Membranes for the Direct Methanol Fuel Cell

Direct methanol fuel cells are seen as an attractive potential replacement for lithium ion batteries in small portable electronic devices. Crucial to the operation of direct methanol fuel cells is the proton exchange membrane, which conducts protons from the anode to the cathode, while acting as an insulator to electrons. The proton exchange membrane should also act as a physical barrier to the methanol and water at the anode, and the air or oxygen at the cathode, however the most commonly used proton exchange membrane, Nafion, suffers from significant methanol permeation from the anode to the cathode during operation. This results in markedly reduced performance, as the methanol reacts with oxygen at the cathode leading to a mixed potential, and the overall cell efficiency is reduced because the methanol does not react at the anode. Consequently, there is a significant effort in a number of laboratories worldwide to develop new proton exchange materials with high proton conductivity and low methanol permeability, or to modify existing materials such as Nafion to enhance these properties. This thesis examined the effect of modifying Nafion membranes through in situ sol gel synthesis of silicon oxide nanoparticles with varying surface chemistry and microstructure. Detailed structural characterisation of the composite structure was undertaken using a wide variety of techniques including small angle X-ray and neutron scattering, wide-angle X-ray scattering, thermal analysis techniques and positron annihilation lifetime spectroscopy. Transport properties of the composite membranes were evaluated by impedance spectroscopy and pervaporation techniques. A structural model was proposed to explain the structure of the composite membranes formed using mono-, di- and tri-functional silicon alkoxide precursors. Nafion composite membranes synthesised using (3-mercaptopropyl)trimethoxysilane as the silicon alkoxide precursor were found to have highly favourable transport properties. In particular, their selectivity for transport of protons over methanol molecules was found to more than three times that of Nafion 117. The performance of the composite membranes in a 5 cm2 direct methanol fuel cell was not completely consistent with the transport properties, which was attributed to factors in the preparation of the membrane electrode assemblies