Engineered Nanocomposites for Solid Oxide Fuel Cells By Colloidal Crystal Templating
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Engineered Nanocomposites for Solid Oxide Fuel Cells By Colloidal Crystal Templating Martyn A McLachlan, An Ying, John A Kilner, David W McComb, and Stephen J Skinner Department of Materials and London Centre for Nanotechnology, Imperial College London, Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, United Kingdom, London, SW7 2AZ, United Kingdom INTRODUCTION Colloidal crystals exhibit three-dimensionally (3D) periodic and porous structures that are increasingly finding use as templates for the formation of 3D ordered macroporous (3DOM) solids [1, 2]. The use of 3DOM materials as photonic band gap materials has attracted considerable interest over the past decade [3-6]. More recently their large surface to volume ratios have been exploited for new applications including chemical sensing, photocatalysis, chromatography and renewable energy [7-10]. 3DOM solids can be described as two interpenetrating networks; usually a solid material and air, in the approximate ratio of 26:74% if the original template was close packed. The dimensions of the minor and to some extent the major phase can be controlled by altering the sphere diameter used in template formation contain two interpenetrating networks. Thus 3DOM solids are ideal hosts or the formation of ordered composites for applications in which interfaces play an important role in device performance. In our current work the use of 3DOM solids to enhance the performance of electrodes in solid oxide fuel cells (SOFCs) is under investigation. SOFCs are electrochemical devices that convert chemical energy to electrical energy at high efficiencies with, depending on the choice of fuel, low (hydrocarbons) or zero (hydrogen) CO2 emissions. Considerable effort has been devoted to the development of materials for SOFCs and there are many excellent review articles detailing the latest developments [11-19]. In general an SOFC consists of an oxide ion-conducting electrolyte, typically yttria stabilized zirconia (YSZ), a fuel electrode (Ni-YSZ composite) and an air electrode (La1-xSrxMO3-d, M = Co, Mn, Fe). In order to enhance SOFC performance several parameters require optimization. For the cathode, the electrochemical processes occurring have to be considered, these are: the catalytic dissociation of O2 to O2-; the transport of the ionic species to the electrolyte surface and the incorporation of the ionic species into the electrolyte (charge transfer). A further function of the cathode is that it acts as a current collector for the cell, and hence is required to have high electronic conductivity. For cells based on YSZ, the operating temperature of the device is greater than 800 oC and due to this temperature requirement the selection of materials available as a cathode is limited. Typically La1-xSrxMnO3-d (LSM) materials are used. Unfortunately these are poor ionic conductors and hence a solution that uses composite YSZ-LSM cathodes with LSM current collectors has been developed [20-26]. The processes occurring at the cathode would ideally tak
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