Optimization of mixed conducting perovskites for the application in dense ceramic oxygen separation membranes, used in partial oxidation of natural gas to synthesis gas

The aim in this project was the optimization of LaFeO3-based perovskite materials for oxygen separation membranes (OSM). The work was carried out by the Laboratory for High Performance Ceramics, Empa in collaboration with the Laboratory for Industrial Energy Systems, LENI, EPFL and the Ceramics Laboratory, LC, EPFL. The ceramic material of interest is a mixed conductor for oxygen ions and electrons at high temperature (800°-900°C) and therefore can separate oxygen from nitrogen with 100% selectivity when exposed to an oxygen partial pressure gradient (e.g. air vs. natural gas). In this capacity, such materials are attractive for the application of partial oxidation (POX) of natural gas (NG) to synthesis gas in an important and growing business. The crucial point in this developing technology is the chemical and mechanical stability of the membrane under the operating conditions (high temperature, PO2 gradient). Because of thermal and chemical expansion, large stress can be induced in the material, which can lead to fracturing. Additional destabilizing processes such as kinetic demixing (elemental segregation under influence of the oxygen gradient) or slow phase transformation can lead to decreased performance with time. The overall goal in this Ph.D. thesis was to address these stability problems from a combined experimental and theoretical point of view in order to develop a more detailed understanding and strategy for improving the materials properties. Experimentally, the ceramic membrane materials were synthesized with different B-site substitutions with the aim of decreasing chemical expansion. The focus lied on LaFeO3 materials substituted with Sr on A-site. Detailed characterizations of the most promising compositions were done including structural, transport, and mechanical properties.