Proton Conduction in Nonstoichiometric ∑3 BaZrO3 (210)[001] Tilt Grain Boundary Using Density Functional Theory

Abstract

We investigate proton conduction in a nonstoichiometric ∑3 BaZrO 3 (210)[001] tilt grain boundary using density functional theory (DFT). We employ the space charge layer (SCL) and structural disorder (SD) models with the introduction of protons and oxygen vacancies into the system. The segregation energies of proton and oxygen vacancy are determined as -0.70 and -0.54 eV, respectively. Based on this data, we obtain a Schottky barrier height of 0.52 V and defect concentrations at 600K, in agreement with the reported experimental values. We calculate the energy barrier for proton migration across the grain boundary core as 0.61 eV, from which we derive proton mobility. We also obtain the proton conductivity from the knowledge of proton concentration and mobility. We find that the calculated conductivity of the nonstoichiometric grain boundary is similar to those of the stoichiometric ones in the literature.

Recent progress in the experimental and theoretical studies on the barium zirconate proton conductors: A review

“…This study strengthens previously suggested mechanisms of improving proton transport by decreasing dopant-lattice interaction [173,215]. For speci c insight on the tilt grain boundary effect, Kim et al [216], utilize space charge layer and structural disorder to incorporate proton and oxygen vacancies into the system. Based on segregation energies, the energy barriers for proton transport are calculated.…”

Comparison of Space-Charge Formation at Grain Boundaries in Proton-Conducting BaZrO3 and BaCeO3

“…These regions form because of segregation of positively charged defects (protons and oxygen vacancies) to the GB cores and become depleted of protons, which reduces the conductivity across the GB. Many experimental − and theoretical − studies of BZO have been conducted where space-charge potentials between 0.3 and 0.9 V have been found depending on doping and sintering conditions. While the space-charge model also has been used to explain GB effects in BCO, fewer in-depth studies of this phenomenon exist for this material.…”

Defect segregation to grain boundaries in BaZrO3 from first-principles free energy calculations

“…This phenomenon has been investigated intensively and it is generally accepted that the high GB resistance is due to positively charged GB cores, resulting in space-charge regions surrounding the cores which are effectively depleted of protonic charge carriers. [9][10][11] Theoretical studies of a variety of different GBs, [12][13][14][15][16][17][18][19][20] and also a recent experiment using atom-probe tomography, 21 have shown that the charged GBs stem from accumulation of both doubly charged oxygen vacancies ðv O Þ and protons ðOH O Þ. None of these theoretical studies, however, consider free energies of defect segregation, but rather the energy at zero Kelvin thus neglecting the temperature dependence and zero-point vibrational contribution to defect segregation energies.…”