A review of proton migration and interaction energies in doped barium zirconate

Abstract

Barium zirconate is a promising material for application as solid electrolyte in ceramic fuel cells due to its high protonic conductivity when doped with trivalent oxides such as yttria. On the atomic level, the transport behaviour can be elucidated using density functional theory and energy parameters controlling the mobility have been calculated by several researchers in the last years. In this work, we give a summarizing overview of the available literature data. We consider the interaction energies between protons and dopant ions and the migration barriers of proton motion for individual jumps in bulk BaZrO3 with the focus on Y-doping. Generally, there is a large spread of energy values, which is related to the variations in the calculation parameters and variable distortions in the ionic lattice introduced by defects. Especially the cell size has a paramount influence on the results. In comparison to other dopants, yttrium exhibits a similar trapping of protons in both nearest and second nearest neighbour positions. This seems to be beneficial for conductivity as it leads to overlapping trapping zones and probably moderate barriers for the detrapping jump to the third nearest neighbour position.Based on the collected data, we estimated the protonic conductivity using kinetic Monte Carlo simulations. We applied energy models with different radii of interactions and further varied the energy parameters within the range of literature values. The results show that the interaction radius has only a limited effect while the impact due to the variable energy range is more pronounced. Nevertheless, similar trends are found for all models, especially a minimum in the mobility at low dopant fractions.