Synthesis and NMR Studies of Electron and Proton Conducting Mesoporous Nb, Ta and Ti Oxide Composites for Alternative Energy Applications

Lead Research Organisation: University of Warwick
Department Name: Physics

Abstract

The synthesis of materials with periodic porosity on the mesoscale (ie 20-500 Angstrom range) is one of the most active areas of current research. By varying the composition of the structure of the walls between the mesopores it is possible to tailor in an enormous variety of physical properties for various applications such as selective gas sorption or catalysis or unusual electronic, optical or magnetic, properties. By impregnating the pore structure of these materials with secondary guest phases that modify the properties of the mesoporous composite, it is possible to create cooperative effects not observed in the pure mesoporous solid. One of the most fundamental properties of any material is its charge transport behavior because moving electrons and protons through a solid is a key step in myriad catalytic or electronic applications. While countless examples of inorganic materials with a periodic mesostructure exist, very few of them possess high electron conductivity. This is because the walls of the mesostructure are often two thin to possess any degree of regular structure, which leads to localization of the electrons in traps, or that the synthesis conditions to fabricate the conducting mesostructure are not compatible with the conditions that lead to conducting properties in the inorganic phase. This is problematic because a high surface area porous material with high conductivity and variable oxidation states would be ideal for use in a Li battery or as a catalyst support material for reactions requiring electron mobility. Proton conductivity through a solid is also important, especially for fuel cell membranes which regulate the flow of charge from one side of the electrochemical cell to the other, but Nafion, a fluorinated polymer with proton conducting sulfonate groups that is the most commonly used fuel cell membrane materials, possess low stability to dehydration, which leads to a decrease in proton conductivity at temperatures over 120 C. Nafion is also vulnerable to alcohol diffusion through the structure, which is problematic since many fuel cells are designed to use alcohol as a feedstock. For this reason many researchers have turned to the exploration of mesoporous material/polymer composite membranes, which promise to overcome the problems with Nafion. The research in this proposal sets out to modify the structure of mesoporous Nb, Ta and Ti oxides, materials possessing variable oxidation states and high acidities/proton conductivity with various agents to create composites with high electronic conductivity or metallic behavior in the one instance, and high proton conductivity in the other, depending on the agent used. Hence, mesoporous Nb, Ta and Ti oxides will be treated with reagents to replace the surface oxides with S or Se, hence creating a conducting sulfide or selenide phase on the inner surface of the material which will allow electron transport to the metal oxide under layer. Impregnation of the pores with electron conducting polymer nanowires of polyaniline or polythiophene will also be carried out to try to exploit these single strand polymers as electron pathways to the metal oxide centers on the walls of the pore channels. These materials will then be tested for use as cathode materials in Li batteries, where it is anticipated that electron mobility through the pores will lead to superior charge and discharge kinetics, an area of performance that is often a problem with Li batteries. The second part of this proposal involves the impregnation of mesoporous Nb, Ta and Ti oxides with sulfonated polymers with high proton conductivity. Here it is anticipated that the mesoporous channels will help overcome dehydration problems of Nafion by holding onto moisture at performance temperatures, while also limiting alcohol diffusion through the membrane. Once synthesized these materials will be tested as fuel cell membranes in operating conditions.

Publications

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Description The outcomes of this project have clearly demonstrated that improved membrane materials for proton conduction fuel cells can be made (with better stability and improved conductivity properties) in comparison to Nafion which is the industry standard. Mesoporous Ti, Ta and Nb composite materials based around the mesporous structured oxidesTiO2, TaO2 or Nb2O5 with NSF (naphthalene sulphonic acid) doped into channels has produced excellent characteristics for this application.
Exploitation Route These findings are a basis to move forward and find a replacement for Nafion in proton conduction fuel cells which exhibits a limited lifetime and which has a high fluorine content.
Sectors Agriculture, Food and Drink,Chemicals,Energy,Environment