Chemical and Structural Deviations at the Surfaces of Layered Oxides for Electrochemical Energy Conversion

Many of our predictions of the functional properties of layered perovskites and related oxide materials are based on the assumption that their chemical composition and structure is homogeneous. From these predictions and further experimental studies performed on model systems (i.e. epitaxial thin films, single crystals or dense ceramic materials), the mechanisms of the oxygen surface exchange and diffusion processes are investigated to understand their behaviour and improve their efficiency and durability as oxygen electrodes for solid oxide fuel cell (SOFCs) and solid oxide electrolysers (SOECs).

Although the bulk diffusion properties in these functional oxides are, in general, well known in terms of the bulk defect chemistry, the oxygen exchange between the surface and the gas phase is still not fully understood. For instance, the surface exchange process is likely to be strongly dependent on the chemical composition and microstructure of the outermost surface, where the adsorption and incorporation of oxygen takes place (or oxygen evolution in the case of the SOEC).

Many of the fundamental studies on the ionic transport properties of mixed-ionic electronic conductors (MIECs) with application as air electrodes in SOFCs/SOECs are performed on polycrystalline ceramic materials. Nevertheless, recent studies have shown that the surface and near-surface (e.g. 2-3 nm) chemistry of perovskite and perovskite-related polycrystalline materials can significantly differ from the bulk composition. These surface deviations are related to the segregation of matrix species or impurities present in the materials after being subjected to temperatures relevant for materials processing and cell operation (e.g. 400 to 1000°C).^{1, 2} Therefore, the local (defect) chemistry at the immediate and near-surface is likely to have a significant impact on oxygen exchange and diffusion mechanisms as a consequence of these atomic re-arrangements.

On the other hand, high quality crystal epitaxial thin films are very often used in order to deconvolute the influence of the crystal orientation (anisotropy) and the influence of strain on the ionic transport properties. Typical thin film growth techniques, such as vapour deposition techniques or pulsed laser deposition, also involve annealing steps at relatively high temperature (e.g. 400-800°C), and hence, these materials are susceptible to similar segregation processes as  occurring for ceramic materials³. For instance, PLD deposition at 850°C of a 24 nm-thick GdBaCo₂O_5+δ epitaxial thin film grown on (001)-SrTiO₃ led to an outermost surface which is predominantly BaO-terminated (Figure 1, solid black line) and a Co-enriched sub-surface (Figure 1, red dotted line) when compared with the bulk cation composition (Figure 1, blue dashed line), as analysed by Low-Energy Ion Scattering (LEIS).

In this work, we use high sensitivity LEIS analysis to understand the compositional deviation at the atomic level of the surface and near-surface in polycrystalline and epitaxial thin films of double perovskites (GdBaCo₂O_5+d) and how these systems are modified under the thermal treatments relevant for material processing or cells operation. These studies are essential in order to characterize the realistic surfaces that will play a role for the oxygen exchange or evolution at the electrode/gas phase interface.

1.            J. Druce, H. Téllez, M. Burriel, M. Sharp, L. Fawcett, S. N. Cook, D. McPhail, T. Ishihara, H. H. Brongersma and J. A. Kilner, Energ Environ Sci, 2014, 7, 3593-3599.

2.            H. Téllez, J. Druce, Y.-W. Ju, J. Kilner and T. Ishihara, Int J Hydrogen Energ, 2014, 39, 20856-20863.

3.            J. H. Lee, G. Luo, I. C. Tung, S. H. Chang, Z. Luo, M. Malshe, M. Gadre, A. Bhattacharya, S. M. Nakhmanson, J. A. Eastman, H. Hong, J. Jellinek, D. Morgan, D. D. Fong and J. W. Freeland, Nat Mater, 2014, 13, 879-883.