Interface solid-state reactions investigated by X-ray microspectroscopy using synchrotron radiation: electrode-electrolyte couples for Solid Oxide Fuel Cells

The structural and chemical compatibility between electrodes and electrolytes for use in Solid Oxide Fuel Cells (SOFCs) is a critical issue: the materials should display similar thermal expansion coefficients, and most importantly the interdiffusion of chemical species at the interface has to be minimised. Chemical compatibility is commonly investigated by means of X-ray diffraction on powder mixtures and/or scanning electron microscopy/energy-dispersive spectroscopy on cross-sections of the bilayer assembly after prolonged annealing at high temperatures, to simulate both the long-term operation of a fuel cell and the thermal treatments necessary for processing. This work reports on the application of X-ray microspectroscopy to evaluate the chemical and local structural fate of cations interdiffusing across several electrolyte/cathode bilayers. X-ray microspectroscopy combines an X-ray microprobe providing information on the spatial distribution of cations (X-ray fluorescence (XRF) maps) with chemical and structural information about the environment of atomic species, i.e. chemical and coordination state of cations, using space-resolved micro-X-ray absorption near-edge structure (microXANES). The interfaces were studied using the focused submicrometre-sized beam available at the SXM-II endstation on the ID21 beamline of ESRF. Different electrode-electrolyte couples, annealed at 1150 C for either 12 h or 72 h, were analysed with this technique. Both proton conductors such as BaCe0.9Y0.1O3 (BCY) and La0.98Ca0.02NbO4 (LNC), [1] and an oxide-ion conductor, Ce0.8Sm0.2O2 (SDC), [2] were studied in combination with common cathode materials. The effect of prolonged thermal treatment on cation diffusion, accumulation of cations at the interface and the eventual formation of secondary phases in some of the bilayers will be shown. The present results represent the first application of X-ray absorption spectroscopy to the study of materials compatibility for ceramics, specifically in materials for SOFCs. This approach can be extended to other materials or complete SOFCs: this can give insight on the mechanisms governing electrolyte-electrode compatibility and electrochemical performance in SOFCs. References. [1] F. Giannici et al., Chem. Mater. 2015, 27, 2763 [2] F. Giannici et al., ACS Appl. Mater. Interfaces 2017, 9, 44466 Acknowledgements. This work was funded through the MIUR project FIRB2012 INCYPIT, Ref. RBFR12CQP5. We acknowledge the ESRF for provision of beamtime.