Surface Composition and Oxygen Transport Properties of LSCF: From Bulk Ceramics to Devices

The exchange of oxygen between the gas phase and a mixed conducting ceramic, and its subsequent solid state diffusion, is key to the operation of electrochemical energy conversion devices such as Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysers (SOECs), as well as dense membranes for oxygen separation. Indeed, it is often the surface reaction that determines the overall performance in such applications. However, whilst the mechanisms of oxygen diffusion in the bulk of these materials are generally well understood in terms of the defect chemistry and crystal structure, such an atomistic description of the surface exchange reaction still eludes us.

Whilst ¹⁸O isotopic tracer diffusion experiments coupled with SIMS analysis have been used for more than 30 years to study the kinetics of oxygen exchange and transport in candidate mixed conductors, relatively little attention has been paid to the characterization of the realistic surface composition of such materials; despite the great attention which is paid to the bulk chemistry, it is often assumed that the surface is a simple termination of the bulk. More recently, it is becoming apparent that the surface and near-surface composition after high temperature treatment is often radically different from the bulk.

Some of this recent work has been facilitated by the availability of new tools to probe the surface composition. Our group in particular has been applying Low Energy Ion Scattering (LEIS) spectroscopy, which is uniquely capable of determining the elemental composition of the single outer atomic layer of a material (i.e. the very same atomic surface that interacts with oxygen in the gas phase).

In this contribution we will use the commonly used perovksite La_0.6Sr_0.4Co_0.2Fe_0.8O_3-d (LSCF) to illustrate how LEIS and SIMS can contribute to our understanding of the relevant surface composition, complementing the existing understanding of the bulk properties of this commonly used electrode material. Starting from studies of surface segregation and restructuring in dense ceramic pellets after heat treatments similar to sintering conditions, we will then show how compositional changes can also occur at targeted device operating temperatures (around 500 ºC). Finally, we will show how laterally resolved LEIS analysis can be applied to understand compositional changes and interdiffusion occurring in the very outer atomic surfaces of LSCF microelectrode – YSZ electrolyte electrochemical half cells.