Oxygen vacancies altering the trapping in the proton conduction landscape of doped barium zirconate

Abstract

Acceptor-doped barium zirconate is one of the most promising proton conducting materials for stationary hydrogen fuel cells. Dopant-defect proton traps shape the proton conduction landscape. Inspired by findings that oxygen vacancies may decrease trapping near some dopant defects (Chem. Mater. 2018, 30, 4919−4925), the effect of oxygen vacancies on the proton conduction landscape of barium zirconate is explored at 12.5% doping with aluminum, scandium, and yttrium dopant at the zirconium site. Density functional theory (DFT) with the PBE functional in the Vienna ab initio simulation package (VASP) was used to find the electronic energy for barium zirconate configurations. The conjugate-gradient minimization method is used to find the lowest energy structures and the climbing nudged elastic band (cNEB) method is used to find transition states. As the dopant (D) ion radius increases, the lattice size expands from 4.24 to 4.29 Å and increases DO6 octahedral tilting in barium zirconate. Inclusion of an oxygen vacancy broadens ZrOD and ZrOZr angle distributions. While there are three distinct minima for oxygen vacancy locations, relative energies of minima and transition states show that only the dopant nearest neighbor oxygen vacancy is significant for aluminum- and scandium-doped barium zirconate. In contrast, the yttrium-doped system shows 67% and 33% probabilities at 800 K for the dopant nearest and second nearest neighbor oxygen vacancies, respectively. The dopant nearest oxygen vacancy leaves the dopant polyhedral center charge exposed. The small size of the aluminum dopant allows oxygen ions to shift and partially screen the dopant charge. This coupled with strong hydrogen bonds increases proton trapping. For the larger scandium and yttrium ions, there is no significant oxygen ion rearrangement around the dopant with a nearest neighbor oxygen vacancy. Instead, the positive dopant charge exposed by the vacancy raises the energy of several dopant nearest neighbor proton sites decreasing trapping locally.