SOFC - A playground for solid state chemistry

High processing and operation temperatures, as well as the presence of non-equilibrium phases turn solid oxide fuel cells into a playground for solid state reactivity. Formation of reaction products, diffusion, segregation, electronic structure modifications at electrolyte-electrode interfaces and at- surfaces affect the oxygen exchange rates and impact the resulting SOFC performance. In the present work, various types of solid state reactivity in solid oxide fuel cells and particularly in systems with zirconia electrolyte and oxide cathodes are addressed. Examples of solid state reactivity induced modifications of bulk, interface and surface chemistry in the SOFC cathode are discussed and correlated with its electrochemical performance. The formation of pyrochlore is considered as example of solid state reactions that occur in zirconia-manganite and zirconia-cobaltite cathodes. Possible reaction mechanisms and their contributions are discussed for various perovskites and different geometries. The local interface chemistry and electronic structure in zirconia-manganite cathodes are studied by ELNES. Differences between annealed and operated cathodes reveal that upon SOFC operation (cathode polarization) a highly defective structure with high concentrations of partially reduced transition metal develops at the interfaces. Based on the results, the impact of interface chemistry on cathode performance is evaluated. The influence of cathode and electrolyte surface chemistry on SOFC performance is illustrated in several examples that include electrochemical promotion, cathode poisoning by silicates and by chromium oxide. In situ spectro-microscopy is used to investigate the surface chemistry of operating model cathodes and electrolyte. The study reveals not only drastic differences between surface and bulk chemistry, it also shows that the cathode and electrolyte surface compositions change with cathode polarization. Oxygen incorporation in the SOFC cathodes with its different reaction steps is explored in terms of the local chemical environment of potential oxygen exchange sites and local conductivities. The concepts are then applied to explain the electrochemical performance of the cathodes. (c) 2007 Elsevier Masson SAS. All rights reserved.