X-ray microspectroscopy study of electrolyte-cathode interface in Solid Oxide Fuel Cells

Solid-oxide fuel cells require materials that are mechanically and chemically resistant, such as doped cerium oxide or zirconium oxide1, and perovskite mixed conductors. The high working temperatures, and the intimate contact between electrolyte and electrode can bring about cation diffusion and solid-state reactions between the cell components. To investigate the local and chemical structure of the species at the interfaces, we recently employed X-ray microspectroscopy on electrode-cathode bilayers2,3. We investigated the interface between several different electrolytes, either proton conducting or oxide-ion conducting, and perovskite electrodes, after annealing at high temperatures (1100 °C for 12-72 h) to simulate the fabrication processes and prolonged operation. The interfaces were studied with space-resolved X-ray absorption spectroscopy using the focused submicrometer-sized beam available at the ID21 beamline of ESRF. We used XRF to obtain concentration maps of all elements, and collected microXANES spectra at Ca, Mn, Fe, Co and Ce absorption edges. We found that the rate of interdiffusion of the cations across the interface, and the formation of secondary phases, depends critically on the specific electrolyte/electrode pair. Moreover, microXANES spectra indicate that some cations show space-resolved changes in their oxidation and coordination state during diffusion. [1] J.W. Fergus, J. Power Sources 162 (2006), 30-40. [2] F. Giannici et al., Chem. Mater. (2015) 27 (8), 2763-2766. [3] F. Giannici, et al., ACS Appl. Mater. Interfaces 9.51 (2017), 44466-44477.