Ionic conductivity and phase stability of yttria stabilized zirconia doped with monovalent and pentavalent cations for solid oxide fuel cell electrolyte applications

This work is focused on the ionic conductivity and thermal stability of monovalent and pentavalent cation doped 4 mol% and 8 mol% Yttria Stabilized Zirconia (4YSZ and 8YSZ) with an emphasis on the grain boundary structure. Li2O was selected for monovalent doping because of the size similarity of Li+ and Zr4+ extra vacancies that would be generated, and due to the large grains characteristic of Li2O doped YSZ. Based on theoretical reports of enhanced conductivity for pentavalent doping at grain boundaries, Ta2O5 was used for experiments on pentavalent doping. The structure and conductivity of the bulk and at grain boundaries were analyzed using various characterization tools including Impedance Spectroscopy (IS), Transmission Electron Microscopy (TEM), and Kelvin Probe Force Microscopy (KPFM). An improvement in ionic conductivity was observed only for 0.1 mol% Li2O doped 4YSZ. Excess amounts of Li2O in 4YSZ destabilized the crystal structure and induced a tetragonal to monoclinc phase transformation that that negatively impacts ionic conductivity. For 8YSZ, Li2O additions increased grain growth, most likely due to the formation of a liquid phase during the sintering. In spite of the large grain growth and lower density of grain boundaries, Li 2O additions did not improve the ionic conductivity. Defect association and oxygen vacancy depletion regions associated with the grain boundary are proposed explanations for this phenomenon. 0.1 mol% Ta2O5 doping slightly deteriorated the initial ionic conductivity but improved the long term conductivity of 8YSZ at 1000°C. Therefore, a small amount of Ta2O5 doping is a potential promising way for improving thermal stability without a significant sacrifice of ionic conductivity. This research discusses how various factors such as doping concentration, grain size, and heat treatment influences the ionic conductivity and Schottky barrier height.