Local Distortions and Dynamics in Hydrated Y-Doped BaZrO3.

Amangeldi Torayev,Clare P Grey,Maria A Gomez,Luke Sperrin,Céline Merlet,John A Kattirtzi

doi:10.1021/acs.jpcc.0c04594

Abstract

Y-doped BaZrO3 is a promising proton conductor for intermediate temperature solid oxide fuel cells. In this work, a combination of static DFT calculations and DFT based molecular dynamics (DFT-MD) was used to study proton conduction in this material. Geometry optimizations of 100 structures with a 12.5% dopant concentration allowed us to identify a clear correlation between the bending of the metal–oxygen–metal angle and the energies of the simulated cells. Depending on the type of bending, two configurations, designated as inward bending and outward bending, were defined. The results demonstrate that a larger bending decreases the energy and that the lowest energies are observed for structures combining inward bending with protons being close to the dopant atoms. These lowest energy structures are the ones with the strongest hydrogen bonds. DFT-MD simulations in cells with different yttrium distributions provide complementary microscopic information on proton diffusion as they capture the dynamic distortions of the lattice caused by thermal motion. A careful analysis of the proton jumps between different environments confirmed that the inward and outward bending states are relevant for the understanding of proton diffusion. Indeed, intra-octahedral jumps were shown to only occur starting from an outward configuration while the inward configuration seems to favor rotations around the oxygen. On average, in the DFT-MD simulations, the hydrogen bond lengths are shorter for the outward configuration which facilitates the intra-octahedral jumps. Diffusion coefficients and activation energies were also determined and compared to previous theoretical and experimental data, showing a good agreement with previous data measuring local proton motion.

Highlights

Perovskites have emerged as attractive electrolytes for intermediate-temperature solid oxide fuel cells (SOFCs) owing to their good ionic transport properties as well as adequate mechanical and chemical stabilities.[1]
Y atoms on the energy of the cells, we focus on the 100 structures with a 12.5% dopant concentration that contain only one Y and proton per supercell. (In the 25% doped structures, there are two protons and two yttrium atoms, and it is difficult to separate the effects of the two H−Y configurations; the relative energies of the different Y−Y configurations would need to be accounted for.) To characterize the local distortions shown in Figure 5a, we used a signed “B−OH−B bending distance” which is the nonzero component of the vector from the center of the solid black line connecting the two transition metals (Zr or Y) to the oxygen covalently bonded to the proton
While the proximity of protons, oxygen, and yttrium atoms can lead to complex hydrogen bonding situations, which can influence both the local structure and proton dynamics,[12,39,40] here we focus on the characterization of these hydrogen bonds to determine how important this is in determining the calculated configurational energies

Summary

Introduction

Perovskites have emerged as attractive electrolytes for intermediate-temperature solid oxide fuel cells (SOFCs) owing to their good ionic transport properties as well as adequate mechanical and chemical stabilities.[1] In particular, Ydoped BaZrO3 has been widely studied as this material shows very high protonic conductivities of up to 1 × 10−2 S cm−1 at 450 °C when exposed to water vapor.[2,3] When BaZrO3 is doped with yttrium, the charge imbalance between the yttrium and zirconium ions is accommodated by the generation of oxygen vacancies leading to the composition BaZr1−xYxO3−x/2. The high ionic conductivities achieved in wet conditions are promising for applications of this material in intermediate-temperature SOFCs.[3]

Methods

Results

Conclusion