(Invited) Effects of Microstructure on Surface Segregation: Role of Grain Boundaries
Authors: Téllez Helena , Druce John , Ishihara Tatsumi , Kilner John A. 
Published: February 2020

Cation segregation is a common phenomenon ocurring at high and intermediate temperatures in the perovskite materials often used as oxygen electrodes in solid oxide fuel cells and electrolyzers (SOFC/SOEC). Most of these perovskite oxides, with the general formula ABO3, are doped either in the A-site or B-site lattice positions in order to tailor their stability, electronic conductivity and ionic transport properties. For instance, LaCoO3perovskites are typically doped with Sr cations at the A-site in order to increase the oxygen vacancy concentration, and hence improve the catalytic activity of the material for oxygen reduction/evolution reactions, as well as the ionic conductivity.

Nevertheless, at the typical processing and operation temperatures for SOFC/SOEC applications, aliovalent doping of (3,3) perovskites results in surface segregation due to elastic and electrostatic interactions within the host lattice. This leads to a compositional gradient at the near- and outer surface of the oxide electrode, showing a full SrO-perovskite surface termination and, in the longer term, to the precipitation of secondary phases (Figure 1) [1].

   Recent studies based on low-energy ion scattering (LEIS) spectroscopy have shown that this segregation occurs in a short time-scale and at relatively low temperatures. For instance, Sr segregation occurs even during the fabrication of dense polycrystalline thin films of La0.6Sr0.4CoO3-
d(LSC) by pulsed laser deposition of [2]. The Sr cations which segregate during the deposition of the film and its subsequent cooling can be removed by rinsing in deionized water - indicating that some water-soluble Sr secondary phases can be formed. However, when the LSC thin films are subjected to further annealing in oxygen at temperatures higher than 500°C, segregation of the divalent dopant takes place again, indicating this is the equilibrium surface termination at the high temperature.

The kinetics of the surface segregation is, however, highly dependent on the microstructure of the oxide material. As observed in Figure 2, the size of the grains (i.e. the grain boundary density) will have a strong impact effect on the kinetics of the Sr segregation. In the case of the LSC thin films (with an average grain size of about 200 nm), annealing at 600°C for 1 h results in a significant Sr surface coverage, whereas the Sr segregation is less evident for a polycrystalline ceramic sample annealed in the same conditions (grain size of around 40 mm). These results are in agreement with a previous study by Kubicek et al. [3], in which the diffusion of Sr in LSC thin films at the temperatures of interest was estimated to be three orders of magnitude faster through the grain boundaries compared to the bulk.

Given the detrimental effect of Sr segregation on the electrochemical performance of the electrode (e.g. by increasing the electrode surface resistance), the impact of faster cation diffusion through the grain boundaries should be considered when optimizing the microstructure of the electrode material. Although  decreasing grain size may improve performance, it could lead to faster segregation. On the other hand, decreasing the grain boundary density at the surface while reducing the working temperature (T < 500°C) might sufficiently impede the segregation kinetics and enhance durability. On the other hand, nanostructured thin film electrodes with high grain boundary density would be expected to show very rapid degradation.

References:

1.         J. Druce, H. Téllez, M. Burriel, M. D. Sharp, L. J. Fawcett, S. N. Cook, D. S. McPhail, T. Ishihara, H. H. Brongersma and J. A. Kilner, Energy & Environmental Science
2014, 7, 3593.

2.         G. M. Rupp, H. Téllez, J. Druce, A. Limbeck, T. Ishihara, J. Kilner and J. Fleig, J. Mater. Chem. A
2015, 3, 22759.

3.         M. Kubicek, G. M. Rupp, S. Huber, A. Penn, A. K. Opitz, J. Bernardi, M. Stoger-Pollach, H. Hutter and J. Fleig, Physical Chemistry Chemical Physics
2014, 16, 2715.

Figure 1