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 ͑YSZ͒. Density-functional theory ͑DFT͒ 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 ͑Y 2 O 3 ͒ x ͑ZrO 2 ͒ ͑1−2x͒ with x increasing from 6% to 15%. The results from the simulations suggest that the maximum conductivity occurs at 7-9 mol % Y 2 O 3 at 600-1500 K 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 ͑SDZ͒. 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.

First-principles study and modeling of strain-dependent ionic migration in ZrO2

“…Such a sensitivity has been reported before. 21,22 Secondly, it turns out that the migration barrier maximum does not vanish if ions are prevented from relaxing, but it rather shifts to stronger compression values and at the same time increases the migration barrier height at all strains.…”

Communication: Different behavior of Young's modulus and fracture strength of CeO2: Density functional theory calculations

“…[12][13][14][15][16] We have also used first-principles calculations to investigate metal-oxide systems such as ZnO, 17 the stress-strain curve for defective aluminum. 28 Kohyama studied cubic SiC to explain the grain boundary effects.…”

Revisiting the Temperature Dependent Ionic Conductivity of Yttria Stabilized Zirconia (YSZ)

“…13,15,28,29,34,35 and ab initio metadynamic simulations revealed surprising effects due to complex (multi-)defectdefect interaction such as locally unstable sites for vacancies (several ten percent of the sites) and concerted vacancy jumps. 36 Interestingly, only few numerical simulation studies dealt with the temperature dependent activation energy of YSZ, e.g., 13,14,26,27,37,38 An activation energy lowering at high temperatures was reported in Refs. 27,37, even though calculated changes were smaller than the experimentally measured ones.…”

“…Interestingly, only few numerical simulation studies dealt with the temperature dependent activation energy of YSZ, e.g., 13,14,26,27,37,38 An activation energy lowering at high temperatures was reported in Refs. 27,37, even though calculated changes were smaller than the experimentally measured ones.…”

“…[10][11][12][13][14][15][16][17]19,[23][24][25][26]28 and is often discussed in terms of spatially varying migration barriers. For example, the vacancy migration barrier strongly depends on the two cations of the tetrahedral edge, which is crossed by an oxide ion during a jump to a nearest neighbor site of the anion sub-lattice: [13][14][15]20,23,26 Refs., 13,14,26 report barriers between 0.3 and 0.67 eV for a Zr-Zr edge and 0.85-1.29 eV for a Y-Zr edge. An increased number of such high barrier edges lowers the effective mobility for increasing doping concentrations.…”