Surface Segregation and Inter-Diffusion of Cations and Impurities in Microelectrodes for Solid Oxide Fuel Cells and Electrolyzers

The surface segregation and inter-diffusion of cations and impurities in La_0.6Sr_0.4Co_0.8Fe_0.2O_3-δ (LSCF) micro-patterned electrode structures on a single crystal Zr_0.84Y_0.16O_1.92(YSZ) electrolyte was investigated by high-resolution ion beam analysis (Time-of Flight Secondary Ion Mass Spectrometry and Low-Energy Ion Scattering). The outermost and near surface of these microelectrodes showed significant changes in the distribution of the cations and impurities after micro-patterning, annealing and electrochemical testing of the cells. As occurring for bulk LSCF ceramics, strong segregation of Sr towards the electrode surface is observed during annealing of the microelectrode, as well as due to the thermal treatments required for electrochemical measurements. This segregation might have a drastic effect on the surface oxygen exchange reaction as the electrochemically active transition metals (Fe, Co) are not directly exposed to the gas phase. Impurities, such as Si and Ca, are also observed at the annealed electrolyte surface. Furthermore, the microstructured electrode showed that impurities can also migrate between the different components (i.e. Na and Si impurities migrate from the electrolyte to the LSCF electrode, showing a “cleaner” surface after the thermal treatments).

This study has provided a further understanding of the migration and segregation of cations and impurities taking place in real devices, building upon previous studies performed on ceramics materials.