Local Structure and Mixed Proton-Hole Conduction in Y and Al-Doped BaZrO3

Abstract

A mixed proton-hole conductor is expected to be an excellent cathode material for proton-conducting solid oxide fuel cells (PCFCs). Accepter-doped BaZrO3 has oxygen vacancies that can be either proton defects in wet or holes in oxidizing conditions. When both proton and hole concentrations are high enough, mixed proton-hole conduction is achieved. Our group has investigated the relationship between local structural distortion and hydration energy of accepter-doped BaZrO3. It was suggested that large Y3+ prefers the compensation by protonic defects; on the other hand, small Al3+ does holes or oxygen vacancies. Thus, by doping Y and Al into BaZrO3 simultaneously, high mixed proton-hole conduction is expected without transition metals. This study focuses on the local structure around dopants, especially around Al, which can be observed by nuclear magnetic resonance (NMR) spectroscopy. The coordination number of oxygen around Al can be converted to the number of oxygen vacancies. Based on the local structural analysis, the defect equilibrium and mixed conductivity of Y and Al co-doped BaZrO3 can be discussed. BaZr0.8Y0.2-xAlxO3-δ (BZYA) was prepared by solid-state reaction method. Proton concentration was evaluated by thermogravimetric analysis at 350 ºC. AC impedance measurement was performed to evaluate the electrical conductivity of BAYA in dry or wet Ar. The proton transport number was measured at 600 ºC using a water-vapor concentration cell. 27Al Magic-Angle Spinning (MAS) NMR was performed to investigate the configuration around Al. The proton concentration of BZY10A10 and BZY15A5 was 0.6 mol%, 1.8 mol%, respectively. These proton concentrations were much smaller than that of BZY20, 18.6 mol%. This indicates that Al substitution led to a decrease in proton concentration. Meanwhile, those BZYAs showed a relatively high proton transport number, 0.84. This was found to be due to the unique oxygen configuration around Al. The local structure around Al and its effect on proton concentration and mixed conduction will be discussed in the presentation.