The perovskite BaZrO3 has attracted considerable attention in the recent decade due to its high temperature proton conducting properties, and possible application as electrolyte in intermediate temperature fuel cells and electrolyzers. In this contribution, we performed, for the first time, first-principles calculations of the phonon contribution to the defect thermodynamics of the ZrO2 terminated (001) surface of BaZrO3. The approach allows us to determine both the segregation enthalpy and entropy of defects, which we apply to two fundamental defects in BaZrO3; fully charged oxygen vacancies (vO ••) and protonic defects (OHO •). The calculations show that both defects exhibit favorable segregation enthalpies of −65 and −125 kJ/mol, respectively. Further, the vibrational formation entropy of the surface vO •• is significantly higher than that of the bulk vO ••, due to smaller local structural relaxations of the surface defect, leading to a finite surface segregation entropy of 53 J/mol K. OHO •, on the other hand, displays nearly identical vibrational spectra at the surface and in the bulk, and the segregation entropy is therefore negligible. Hence, phonons not only stabilize the surface vO •• compared to the bulk defect thermodynamically at high temperatures, but also affect the relative stability of vO •• and OHO • at the surface. Finally, we apply a simplified space charge model to the (001) surface, and show that neglect of phonons results in strongly underestimated surface concentrations of vO ••.
In the present contribution, we assess the efficiency of the alkali metals (Na, K, Rb and Cs) as A-site acceptor dopants in proton conducting BaZrO 3 by first principles calculations. The calculated acceptor-proton complexes become weaker with increasing dopant size, with binding energies ranging from À0.33 eV for Na to À0.10 eV for Cs, which is in the range of, or even lower than, those found for B-site doped BaZrO 3 . By mapping out all relevant minimum energy pathways for the proton, we reveal that the highest migration energy barrier for most of the alkali metals is comparable or even lower than that of Y.Further, all A-site dopants display more exothermic hydration enthalpies compared to that of Y-doped BaZrO 3 , ranging from À131 kJ mol À1 to À83 kJ mol À1 for Na and Cs, respectively. The calculated dopant solubility increases in the order Na < Cs < Rb < K, with the predicted solubilities of the two latter being in the range of that of e.g. Y. Although Cs would lead to the highest proton mobility, the higher solubility of K and Rb renders them more attractive A-site dopants for BaZrO 3 . Overall, our results suggest that alkali metals as A-site dopants may enhance the bulk proton conductivity of BaZrO 3 , compared to Y-doped BaZrO 3 .
Heterogeneous doping through nano-structuring has been demonstrated to greatly enhance the transport and mass storage properties of halides and Li-ion conductors by decoupling of chargecompensating species. In this contribution, we explore the potential of heterogeneous doping of ionically conducting oxides through first principles and thermodynamic calculations of a model system consisting of acceptor-doped SrTiO3 and BaZrO3. We show that the two dominating defects, v O •• and OH O • , are thermodynamically favored in BaZrO3 compared to SrTiO3 by 0.64 and 0.56 eV, respectivelyin turn leading to an electrostatic potential barrier as large as 0.64 V (relative to BaZrO3) at 800 K. v O •• and OH O • , and electron holes as such accumulate on the BaZrO3side of the interface, leading to a charge-depletion layer in SrTiO3, which thus acts as a heterogeneous acceptor for BaZrO3. These interface effects may as such by used to drastically enhance the charge-carrier concentration of weakly doped ionic systems which in turn may lead to novel nanostructured composite materials with enhanced transport properties or for instance kinetics for oxygen reduction or water splitting.
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