(Invited) Mixed Ionic Electronic Conductors (MIECs): The Importance of Fast and Slow Processes

Abstract

MIECs are ideal materials for use as electrodes in solid state ionic devices. They have found application in SOFCs, SOECs, and solid state battery devices. In this contribution we will focus on MIEC used in the SOFC/SOEC where oxygen is the mobile ion. Most of these materials are complex perovskite materials, for example La1-xSrxCoyFe1-yO3-δ (LSCF). The focus of experimental observation in these materials has been on the fast process of transport of the highly mobile oxygen ion and much is known about the effect of non-stoichiometry in determining the level of oxygen ion transport. Most of these materials are very good mixed conductors at temperatures above 600°C but the lattice oxygen transport drops off markedly at lower temperatures. Recently there have been attempts to improve the lower temperature behaviour by producing nanomaterials, usually in thin film form [1-3], with a high density of grain boundaries. These have been shown to introduce fast diffusion paths along the grain boundaries and have been shown to improve the electrochemical performance. What has been neglected in the study of these materials is the evolution of the cation distribution in these complex oxides. The cation distribution has a marked effect upon the performance of an MIEC electrode, for example, the segregation of Sr to the surface which is detrimental to the oxygen exchange [4, 5], the development of defect clusters which can trap the oxygen vacancies, and the local chemistry of the grain boundaries [6], affecting transport in polycrystalline materials. Cation transport is a slow process but has importance during the short times and high temperatures during processing, and even more relevantly for the low temperatures but very long times associated with operation. This has implications for the durability of these devices. The contrast between these fast and slow processes will be discussed in this contribution and their relevance to different processing regimes and operational durability.