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.20183049194925), 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.

Revised Centrality Measures Tell a Robust Story of Ion Conduction in Solids

“…Distinct oxygen-ion types are labeled I, II, or III based on whether they are the nearest, second nearest, or third nearest to the yttrium-ion dopant. Further, the distinct types of protons are labeled with a subscript based on the type of oxygen ion to which they are bonded and a superscript indicating whether the oxygen ion opposite the hydroxide is close or far . The H I Far proton site occupied in all of the minima is highlighted in yellow.…”

Dopant-induced changes of local structures for adjusting the hydration ability of proton-conducting lanthanum scandates

“…The minimum energy of an oxygen vacancy depends on the dopant ionic radius due to the competition of octahedral tilts with stretching of chemical bonds, which are energetically less favorable. 9 In the Ca-Sr-Ba series of dopants in La 0.95 M 0.05 -ScO 3−d , the octahedral tilting decreases, as well as the spread in inter-oxygen distances, which leads to less stability of oxygen vacancies and protons. The depth of local donor and acceptor levels associated with defects in the band gap determines the defect formation energy.…”

“…8 Or it can be considered through the affinity for the hydroxyl group, which can be characterized as "acidity". The energies of the formation of protons and oxygen vacancies are interrelated 9 as well as the energies of their trapping by an acceptor dopant. 10,11 Thus, the energy levels of the acceptor dopant in the band gap affect the valence band maximum state, which affects the thermodynamics of hydration.…”

The Impact of Nanoscale Percolation in Yttrium‐Doped BaZrO3 on the Oxygen Ion and Proton Conductivities: A Density Functional Theory and Kinetic Monte Carlo Study

“…There are only few studies that investigated oxygen vacancies [ 10,11 ] or even the oxygen ion conductivity in Y‐doped BaZrO 3 . [ 2,11–14 ] They reported activation energies of the oxygen ion conductivity between 0.93 and 1.2 eV, depending on the dopant fraction. In comparison, the activation energies of the proton conductivity in Y‐doped BaZrO 3 are typically around 0.4 eV.…”

“…There are only few studies that investigated oxygen vacancies [10,11] or even the oxygen ion conductivity in Y-doped BaZrO 3 . [2,[11][12][13][14] They reported activation energies of the oxygen ion conductivity between 0.93 and 1.2 eV, depending on the dopant fraction. In comparison, the activation energies of the proton conductivity in Y-doped BaZrO 3 are typically around 0.4 eV.…”