Proton diffusion pathways and rates in Y-doped BaZrO3 solid oxide electrolyte from quantum mechanics

We carried out quantum mechanical calculations (Perdew-Becke-Ernzerhof flavor of density functional theory) on 12.5% Y-doped BaZrO3 (BYZ) periodic structures to obtain energy barriers for intraoctahedral and interoctahedral proton transfers. We find activation energy (Ea) values of 0.48 and 0.49 eV for the intraoctahedral proton transfers on O-O edges (2.58 and 2.59 Å) of ZrO6 and YO6 octahedra, respectively, and Ea =0.41 eV for the interoctahedral proton transfer at O-O separation of 2.54 Å. These results indicate that both the interoctahedral and intraoctahedral proton transfers are important in the BYZ electrolyte. Indeed, the calculated values bracket the experimental value of Ea =0.44 eV. Based on the results obtained, the atomic level proton diffusion mechanism and possible proton diffusion pathways have been proposed for the BYZ electrolyte. The thermal librations of BO6 octahedra and uncorrelated thermal vibrations of the two oxygen atoms participating in the hydrogen bond lead to a somewhat chaotic fluctuation in the distances between the O atoms involved in the hydrogen bonding. Such fluctuations affect the barriers and at certain O-O distances allow the hydrogen atoms to move within the hydrogen bonds from one potential minimum to the other and between the hydrogen bonds. Concertation of these intra- and inter-H-bond motions results in continuous proton diffusion pathways. Continuity of proton diffusion pathways is an essential condition for fast proton transport.