Atomistic Simulations of Defect Structures in Solid Oxide Fuel Cell Electrolytes

Abstract

The defect structures in two promising intermediate temperature solid oxide fuel cell electrolytes, gadolinia-doped ceria (GDC) and scandia-doped zirconia (SDZ), were studied by atomistic computer simulations. In GDC, it was found that sub nano-scale defect clusters preferred a next-nearestneighbor, pyrochlore-type structure, and that they had a tendency to grow into larger clusters. For nano-scaled domains, however, the C-type rare earth structure, in which the dopants and vacancies are at nearest-neighbor sites, became more stable. It was suggested that nano-domains served as the precursor of phase separation and they could be easily formed during synthesis. Doping concentration limited the size of the nano-domains, and caused GDC to favor small pyrochlore-type clusters at lower concentrations, but C-type nano-domains at higher concentrations. As such, GDC was expected to show initially an increase in conductivity and then a decrease with increasing doping concentration. The lattice parameter of GDC should show the same trend and could be used as an indicator of the predominant defect structure. The cation mobility was another important factor limiting the size of defect clusters, and could be used to control the domain formations and thereby improve the electrolyte performance. It was also found that the defect structure in GDC could be modulated by strain through oxygen diffusing into or out of the nearest neighbor sites of dopants with an activation energy estimated to be 0.82 eV. Based on such a mechanism, an explanation of the “chemical strain/stress” phenomenon observed in gadolinia-doped ceria, as well as the benefit of zero or moderate tensile strain for electrolyte applications, was proposed. The phase system of SDZ, especially the cation ordering in the three rhombohedral phases, was studied with both classical empirical potentials and ab-initio methods utilizing the density functional theory. Characteristics of defect structures in SDZ at different concentrations were identified based on the structures in the phase system. The abilities of the two methods in simulating the structures and phase stability of the SDZ system were compared and evaluated.