Thermodynamics of hydration and oxidation in the proton conductor Gd-doped barium cerate from density functional theory calculations

Abstract

Hydration and oxidation of gadolinium-doped barium cerate, a system with highly promising properties when used as electrolyte for protonic ceramic fuel cells, are investigated by means of density functional calculations. The energy landscape of oxygen vacancies and interstitial protons in this strongly distorted orthorhombic perovskite is computed. Although the most stable sites for protons are found in the close vicinity of the dopant, the picture of a very complex energy landscape emerges, in which some sites far away from Gd are found more stable than other ones in its close vicinity, due to the highly distorted geometry of the host materials. The fully hydrated phase can be approximated by a structure with 16 local minima. Both hydration (water incorporation) and oxidation (oxygen incorporation) are found to be exothermic processes with reaction enthalpies of −1.34 eV/H 2 O molecule and −0.70 eV/O atom, respectively. The hole polaron resulting from the exothermic incorporation of oxygen is found localized on oxygens around the dopant (small polaron) and carries a spin magnetic moment. Finally, the competition between hydration and oxidation is studied and discussed as a function of oxygen and water partial pressures.

The pivotal role of the dopant choice on the thermodynamics of hydration and associations in proton conducting BaCe0.9X0.1O3−δ (X = Sc, Ga, Y, In, Gd and Er)

“…Hermet et al 35 . The relative stability between the isolated point defects and their associated forms; ሺX େୣ OH ) × and ሺX େୣ v ) • , is given by their respective association enthalpies.…”

DFT+ U study of self-trapping, trapping, and mobility of oxygen-type hole polarons in barium stannate

“…Theoretically, previous studies have revealed that advanced electronic structure calculations are needed to correctly describe this oxidation phenomenon. Indeed, the generalized gradient approximation (GGA) finds it exothermic in several perovskite oxides [10][11][12], while hybrid functionals (PBE0) provide an endothermic picture [12]. This is related to the band-gap problem, more precisely to the incorrect position of the valence band maximum, formed by oxygen-2p states, obtained with GGA.…”

Thermal evolution of the crystal structure of proton conducting BaCe0.8Y0.2O3−δ from high-resolution neutron diffraction in dry and humid atmosphere

“…[39,40]. It may, however, be more accurate to consider the structural relaxation around dopant-vacancy pairs, or even extended Y-V O -Y defects, remembering that two dopants ions are required per oxygen vacancy and that a strong associative attraction between the positively charged vacancies and the acceptor dopants is expected from computational studies, see for example refs.…”