Electrode–Electrolyte Compatibility in Solid-Oxide Fuel Cells: Investigation of the LSM–LNC Interface with X-ray Microspectroscopy

Ca:LaNbO4 (LNC) constitutes the last real breakthrough in high-temperature proton conductors, with better chemical and mechanical stability with respect to cerate and zirconate perovskites. However, the low amount of bivalent dopant that can be hosted in the LaNbO4 matrix poses a limit to the proton concentration in the electrolyte. Using synchrotron X-ray microspectroscopy, we investigated the compatibility of annealed LNC/LSM electrolyte/cathode bilayers for proton-conducting SOFCs. The element maps are complemented by microEXAFS and microXANES, giving information on the fate of different cations after diffusion. The X-ray microspectroscopy approach described here is applied for the first time to the study of materials for energy, and it is proposed as a useful structural tool, complementary to electrochemical characterization, for the investigation of the compatibility between materials for SOFCs. We demonstrate that an impressive calcium drift towards the LSM cathode takes place: the dopant is depleted throughout a region of LNC several micrometers wide, causing a decrease of charge carriers in the electrolyte and eventually impairing its conductivity. This poses a significant challenge for evaluating electrolyte/electrode couples in proton-conducting SOFCs based on LNC.