Abstract: Barium hafnate (BaHfO3) is one of the most promising proton-conducting perovskites. The material’s proton conductivity at intermediate temperature range (~500 Celsius) provides the possibility of its commercial use as a solid-state electrolyte in proton-conducting solid-oxide fuel cells (SOFCs). Although a considerable amount of computational and experimental research on similar perovskite systems (e.g., BaZrO3, BaCeO3, BaSnO3) have been carried out, there has been very limited research on BaHfO3. Here we take advantage of atomic-level simulation techniques to gain electronic and atomic level insight into A/B-site doped BaHfO3.
The proposed simulation technique is so-called “ab-initio thermodynamics,” coupling density functional theory based first-principles calculations with statistical thermodynamic theory. Specific theories or models include defect-formation energy calculation, finite-temperature vibrational energy calculation via phonon frequency evaluation under harmonic approximation, proton migration along minimum energy pathway via transition state theory, and configurational entropy contribution via cluster-expansion and Monte-Carlo method. A variety of relevant properties are investigated, including hydration percentage governed by hydration Gibbs free energy, proton diffusivity by proton migration barrier, microscopic defect-clustering between dopant-oxygen vacancy, and dopant-proton due to enthalpic and entropic balance. All these properties are sensitive to chemical dopants in the lattice (i.e., Li, Na, K, Rb, and Cs on A-site and Sc, Y, La, Gd, Lu, Al, Ga, and In on B-site). The goal is to understand the fundamental mechanism in BaHfO3 upon various chemical dopants, and achieve rational dopant selection and doping level optimization in BaHfO3. The computation workflow will be extremely useful in the design of proton-conducting solid-state electrolytes and have a broader impact on the community of solid-state ionics.