Predicting ionic conductivity of solid oxide fuel cell electrolyte from first principles

Abstract

First-principles quantum simulations complemented with kinetic Monte Carlo calculations were performed to gain insight into the oxygen vacancy diffusion mechanism and to explain the effect of dopant composition on ionic conductivity in yttria-stabilized zirconia (YSZ). Density-functional theory (DFT) within the local-density approximation with gradient correction was used to calculate a set of energy barriers that oxygen ions encounter during migration in YSZ by a vacancy mechanism. Kinetic Monte Carlo simulations were then performed using Boltzmann probabilities based on the calculated DFT barriers to determine the dopant concentration dependence of the oxygen self-diffusion coefficient in (Y2O3)x(ZrO2)(1−2x) with x increasing from 6% to 15%. The results from the simulations suggest that the maximum conductivity occurs at 7–9mol% Y2O3 at 600–1500K and that the effective activation energy increases at higher Y doping concentrations in good agreement with previously reported literature data. The increase in the effective activation energy for migration arises from the higher-energy barrier for oxygen vacancy diffusion across an Y–Y common edge relative to diffusion across one with a Zr–Y common edge of two adjacent tetrahedra. The binding energies between oxygen vacancies and dopants were extracted up to the fourth nearest-neighbor interaction. Our results reveal that the binding energy is the strongest when the vacancy is in the second nearest-neighbor position relative to the Y dopant atom. The methodology was also applied to scandium-doped zirconia (SDZ). Preliminary results from quantum simulations of SDZ suggest that the effective activation energy for vacancy diffusion in SDZ is lower than that of YSZ, in agreement with experimental observations. The agreement with experimental studies on the two systems analyzed in this paper supports the use of this technique as a predictive tool on electrolyte systems not yet characterized experimentally.