Preferential Oxygen Transport in Nanophase Mesoporous Ceramic Ion Conducting Membranes
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Preferential Oxygen Transport in Nanophase Mesoporous Ceramic Ion Conducting Membranes C. Guizard, C. Levy, L. Dalmazio and A. Julbe Institut Européen des Membranes (CNRS UMR 5635) UM II - CC 047, Place Eugène Bataillon , 34095 Montpellier cedex 5, France. ABSTRACT Recent data from the literature dealing with the influence on oxygen transport of porous nanophase ion conducting ceramics are reviewed, and then transposed to the design of mesoporous nanophase ceria-based membranes. Mesoporous CeO2/Al2O3 and Gd doped CeO2 membranes containing Pd and Pt nanoparticles were prepared using the sol-gel process. Permeation of N2 and O2 single gases was studied in a temperature range 20-500°C. Permeation measurements indicate an activated oxygen transport in agreement with the literature data. A synergetic effect of the noble metal nanoparticles on oxygen transport has been evidenced, in relation with the triple phase boundary concept. However, these membranes do not perform totally the preferential oxygen transport predicted by the theory. Several directions are proposed for membrane improvement, in particular concerning pore and grain optimal sizes.
INTRODUCTION Ceramic ion conducting membranes (CICM) with oxygen transport property are applied in a number of high temperature electrochemical and separation devices, such as solid oxide fuel cells, ceramic oxygen generators and catalytic membrane reactors. Normally, ion conducting materials involved in these devices must exhibit a dense structure in order to provide a selective transport of oxygen ions and to prevent diffusion of other gas molecules. Starting from recent advances in nanophase materials, an interesting aspect is the role of nanophase structures and porosity in solidstate ionic materials. Due to the large fraction of grain boundaries in nanostructured materials, surface properties tend to determine bulk properties. For example the space charge phenomena and quantum confinement regime in nanograins have been shown to strongly influence electrical conductivities of ZrO2 or CeO2 based solid electrolytes. In other respects a number of works from the literature have predicted an enhancement of oxygen transport from the gas phase in mixed ionic electronic conducting mesoporous electrodes. This is possible from the large contact surface (several hundred m2/g) between the bulk and the gas phase, and the combined surface diffusion and bulk conduction on and in nanograins. As a matter of fact, the field of nanophase solid oxide conductors offers new opportunities for the design of porous membranes exhibiting preferential oxygen transport. This is the case for ceria based ceramics, able to form oxygen vacancies in oxygen poor atmospheres and, conversely, to fill these vacancies in oxygen rich atmospheres. These oxides are of great interest for applications in various technological fields. However, partial ionic and electronic conductivities are key properties determining the use of these materials. Solid electrolytes, oxygen pumps and electrochemical sensors shoul
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