(Invited) Forming Multifunctional Ceramics with Different ZrO2 Polymorphs: Mixed Ion-Electron Conducting Properties of Zirconium Titanium Oxides

Abstract

Infiltrating porous electrodes with solution phase precursors is a strategy employed to improve performance, catalytic activity and longevity in high temperature electrochemical devices. For example, traditional Ni yttria-stabilized zirconia (Ni-YSZ) cermet anodes used in solid oxide fuel cells (SOFCs) are susceptible to carbon accumulation and to sulfur poisoning. To counter the effects of degradation, anodes have been infiltrated with secondary materials containing Sn, Ba, Al and a host of other constituents, typically at levels of 1-5% (by mass). Processing and preparing infiltrated electrodes, however, can lead to formation of new material phases that affect surface chemistry and electrode microstructure in unexpected ways.In this work, XRD and Raman spectroscopy are used to investigate the formation of functional zirconium titanate secondary phases in Ti/Zr oxides. Different zirconia polymorphs (either pure or stabilized with small amounts of yttria (YSZ)) mechanically mixed with TiO2 and heated above 1000 ˚C in stagnant air react to form several secondary phases. For sintering temperatures > 1100 ˚C, mixtures of TiO2 and m-ZrO2 form a ZrTiO4 secondary phase while mixtures of TiO2 and 3YSZ or 8YSZ form Zr5Ti7O24. The extent of secondary phase formation is enhanced with both higher sintering temperature and yttria content. The Zr5Ti7O24 secondary phase formed from TiO2/8YSZ samples sintered at 1400 ˚C exhibits unexpected mixed ionic and electronic conductivity at 800 ˚C. These findings suggest that secondary phases intentionally integrated into electrode architectures can improve the performance of high temperature electrocatalyst systems.