Oxygen Transport of Lscrf-Scsz Based Mixed Conducting Ceramic Composites

Abstract

Materials for membrane reactors must possess both high ionic and electronic conductivity as well as good stability over a wide range of oxygen partial pressure. In order to improve the oxygen diffusion, substituting elements is an approach commonly adopted to increase vacancy concentration, but it may also reduce the chemical stability of the membrane [1]. Single phase materials, e.g. La1-x Sr x Co1-y Fe y O3-δ (LSCF), have been shown to possess excellent oxygen permeation properties but suffer from dimensional and chemical instabilities under operation. To improve the long-term stability manganese, chromium etc. can substitute for cobalt but this will sacrifice the permeability of a membrane [2]. Alternatively, focus has been shifted onto dual phase materials which consist of one phase as a good ionic conductor and the other phase as a good electronic conductor. Among all the essential criteria, transport properties of the dense composite layer can profoundly affect the membrane performance. Previous transport measurements work have been carried out on La0.8Sr0.2MnO3-δ (LSM)-Yttria-Stabilised Zirconia (YSZ) [3], LSCF-Ce0.9Gd0.1O2-δ [4] etc. dual phase systems, whilst in this work the La1-x Sr x Cr1-y Fe y O3-δ (LSCrF)-Scandia-Stabilised Zirconia (ScSZ) based dual phase composite system has been investigated. Isotopic Exchange Depth Profiling (IEDP)[5] combined with Secondary Ion Mass Spectrometry (SIMS) was performed in this present work to investigate the mass transport in the composite materials. In order to achieve improved performance optimisation strategies have been applied, which include changing doping level and doping elements in the component LSCrF and ScSZ, and using different volume fractions (LSCrF:ScSZ) in the dual phase composites and optimising the microstructures of the dual phase composites. Additionally, the effects of a humidified environment on the diffusion behaviours of both the single phase and dual phase samples have been studied by using pure labelled H2 18O to reflect operating atmospheres. The diffusion coefficient (D*) at 800 ̊C of LSCrF was found to increase dramatically by 3 orders of magnitude compared to a similar experiment under dry conditions indicating an increase of oxygen vacancy concentration whilst ScSZ remains a good ionic conductor in a pure water atmosphere, and moreover the surface exchange coefficient (k) of ScSZ has been improved to 1.54x10-6 cm/s at 800 ̊C while in dry O2 conditions almost no 18O has been exchanged into the sample. An explanation for this behaviour is that electronic species are not required for the surface exchange process in pure water atmosphere [6] so the rate-limiting steps of surface exchange in ScSZ shifts from dissociation and oxygen reduction in the case of pure oxygen to adsorption steps for water. To obtain further understanding of how the surface has changed during the annealing, X-ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS) have been performed in order to observe the surface and sub-surface chemistry and compositional information.