Proton conducting Y‐doped BaZrO3, BaCeO3 and their solid solutions are receiving increasing attention due to their promising application as electrolytes in ceramic fuel cells and electrolysis cells. However, the literature indicates a clear tendency that the performance of the cells increases with increasing Ce content in the electrolyte. In this work, to elucidate this phenomenon, a systematic work is performed on investigating the phase, hydration, and transport behaviors of BaZr0.8−xCexY0.2O3−δ (BZCY20). The results reveal that in the temperature range between 500 and 700 °C, with increasing Ce content, the dehydration temperature elevates and the proton concentration increases, showing that the Ce component favors the stabilization of protons. Furthermore, the transport number of hole conduction decreases, whereas the transport number of ionic conduction increases with the increasing Ce content. By further separating the contribution of oxide ions and protons, it is found that the oxide ion conductivity increases with the increasing Ce content at higher temperatures of 600 and 700 °C. Such decreased hole conductivity and increased oxide ion conductivity result in the enhancement of the ionic conduction of BZCY20 with increasing Ce content, and therefore improve the performance of fuel cells and electrolysis cells.

The impact of phonons on the hydration and defect thermodynamics of undoped and acceptor (Sc, In, Y and Gd) doped BaZrO 3 is addressed by means of first principles supercell calculations. In contrast to previous, similar investigations, we evaluate contributions from all phonon modes, and also pressure/volume effects on the phonon properties. The calculations are performed at the GGA-level with the PBE and RPBE functionals, both of which predict for BaZrO 3 a stable cubic perovskite structure. For all dopants, the vibrational formation entropy of the doubly positively charged oxygen vacancy, which therefore also is the dominant contribution to the entropy of hydration, in addition to loss of H 2 O(g). The large, negative contribution from phonons to the formation entropy of V cc O stems both from large local structural relaxations, and contraction of the entire supercell and corresponding blue-shift of the phonon spectrum. Neglect of the phonon contribution to the vacancy free formation energy leads to a considerable error (100 kJ mol À1 at 1000 K). For the acceptor doped systems, the calculated hydration entropy becomes more negative in the order Y < Gd < In < Sc, which stems from that both the formation volume and vibrational formation entropy of OH c O and V cc O become more negative with increasing dopant ion size. Both functionals also give similar defect entropies for undoped, and Scand In-doped BaZrO 3 . However, for the larger Y-and Gd-ions, PBE underestimates (too negative) the vibrational formation entropies due to a poor representation of low frequency modes at the R point of the Brillouin zone. The calculated hydration entropies are in good agreement with experimental results, lending support to the applicability of the adopted method.

In the current contribution, we study the defect chemistry of bulk rutile TiO 2 through first principles defect calculations in combination with thermodynamic modeling. We apply three levels of theory, PBE, PBE+U, and the HSE hybrid functional. The defect calculations reveal that the included isolated defects, v Ti q , v O q , Ti i q , OH O q , and H O q , display shallow thermodynamic charge transition levels, whereas oxygen interstitials, O i q , display a deep (0/-2) transition level. The formation energies of the complexes between v Ti 4/ , v O •• , and OH O • are as low or even lower than those of the isolated defects, and all included complexes display exothermic binding enthalpies, indicating their possible presence under thermodynamic equilibrium. Through thermodynamic modeling, we show that the finite temperature defect structure of rutile TiO 2 largely is dominated by v Ti 4/ , OH O • , and their mutual complexes under wet oxidizing conditions and lower temperatures and by v Ti 4/ , v O•• , and their complexes at higher temperatures. Hence, we indicate that TiO 2 cooled from high temperatures or grown at low temperatures will display defect structures very different from previously known, which together with previous results for defects in surfaces gives a new basis for understanding for instance photocatalytic properties of TiO 2 .