Nanoscale Analysis of LSM/YSZ Interfaces within Composite Cathodes for Commercial Solid Oxide Fuel Cells

Abstract

While performance degradation for solid oxide fuel cells (SOFCs) comes in many forms, one important contributor is cation transport during long-term operation. This transport alters the chemical composition at electrode/electrolyte interfaces. These interfaces are carefully engineered to provide the critical reaction sites for oxidation and reduction reactions that regulate SOFC electrical production. Therefore, fundamental understanding of the chemical evolution of these interfaces is critical. The current study coordinates scanning transmission electron microscopy (STEM) with atom probe tomography (APT) in order to probe composition at the nanoscale across cathode/electrolyte interfaces for anode-supported commercial SOFCs. These SOFCs contain a porous composite cathode layer, consisting of sintered (La0.8Sr0.2)0.95MnO3 cathode particles and yttria-stabilized zirconia (YSZ) electrolyte particles. SOFCs are operated up to 500 hours in duration at a current density of 0.75 A/cm2 (or at open circuit) and at an operation temperature of 800°C. Measured cell voltage increases over the first 100 hours of operation, followed by a steady and linear drop in cell voltage that translates to 5.35% performance loss per 1000 hours. STEM-based energy dispersive spectroscopy (EDS) indicates nanoscale Mn-oxide formation at LSM particle surfaces after 500 hours of operation. Compositional profiles acquired by APT across LSM/YSZ particle interfaces indicate as-sintered interfaces are chemically well-defined, but La and Mn penetrate up to 5 nm into YSZ particles over the course of 500 hours. Additionally, as-sintered LSM particles exhibit measurable Mn enrichment within 20 nm of an adjacent YSZ particle. After 100 hours of operation, A-site deficiency is restored for LSM particle surfaces adjacent to YSZ. Meanwhile, the YSZ composition adjacent to LSM exhibits depletion in Y content over the first 100 hours of operation. These results suggest that initial cell voltage increase within 100 hours of operation corresponds to both Mn and Y transport at LSM/YSZ interfaces. STEM-based electron energy loss spectroscopy (EELS) provides additional insight regarding local cation valence states in relation to measured composition variations for these LSM/YSZ interfaces.