Hydration Thermodynamics of High Temperature Proton Conductors

High temperature proton conducting oxides are materials that dissolve protons from water vapor up to 1000 °C. HydraThermPro project aims to derive correlations between the thermodynamics of water uptake and chemical and physical properties of various classes of proton conductors by means of different experimental and computational approaches. The project has undertaken studies of different series of proton conductors to investigate the effect of changes in the structure and/or basicity of the oxides on the hydration thermodynamics. Oxides with the perovskite structure have been emphasized both by means of experimental and computational approaches. The acceptor dopant has a significant effect on the hydration thermodynamics. DFT calculations reveal that this is may be due to trapping of protons and oxygen vacancies by the dopant. Moreover, the ionic character of the M-O bond is important for the hydration properties, and there are also data suggesting a link to the basicity of the oxide. The project has recently investigated the effect of larger defect clusters in some acceptor doped BaCeO3 where oxygen vacancies and acceptors may form stable complexes during synthesis. These complexes are found to be frozen-in after synthesis, which in turn means that specific heat treatments alter the defect structure, and consequently the hydration thermodynamics and transport properties. Further work is currently being pursued to fully understand the implications of such defect clusters by looking into other compositions. The project has more recently undertaken efforts to decouple the hydration reaction into the fundamental acid-base reactions, protonation of lattice oxygen ions, and hydroxylation of lattice oxygen vacancies, showing that the proton affinity of the oxides is always more favourable than the hydroxide affinity. The proton and hydroxide affinities correlate with the material?s electronic structure, indicating that the acid/base properties are fundamentally linked to the oxide?s bonding nature. Extensive efforts have been invested in elucidating the nature of the entropy of hydration, which is difficult to measure experimentally. A key effort in the project has therefore been to determine this property from first principles phonon calculations, allowing us to investigate both its fundamental nature, and its materials dependence. From calculations on BaZrO3, we show that the entropy of hydration to a large extent can be ascribed changes in the materials vibrational properties upon filling of the oxygen vacancy by an O2- ion: Filling of the vacancy leads both to outward relaxations of nearest-neighbor ions, and chemical expansion of the material. The project has also applied the methodology to a selection of different perovskites to determine both the compositional and structural dependence of the hydration entropy. The calculations for instance reveal that the entropy is significantly more negative (unfavourable) for the orthorhombic BaCeO3, than for the cubic BaZrO3, an effect which mainly is due to smaller entropy gain upon filling the oxygen vacancy in BaCeO3. With our recent knowledge linking chemical expansion and hydration thermodynamics, we expanded our approach to investigate the effects of ionic defects on the thermal expansion as well by accounting for a volume dependency of the phonon spectra. Our calculations revealed that both oxygen vacancies and protons lower the thermal expansion coefficient, which in turn only leads to small changes to the calculated hydration entropy and enthalpy if such volume dependencies were neglected. Lastly, the project has recently explored hydration of oxide surfaces computationally, focusing on BaZrO3 (where the bulk readily hydrates) and CeO2 (where the bulk oxide hardly hydrates). Interestingly, the calculations reveal that the surfaces of both oxides readily hydrate, and that the surfaces hydration thermodynamics does not correlate with the bulk hydration thermodynamics. For CeO2 we have explored hydration of various surface terminations, revealing that the surface hydration properties primarily depend on the coordination environment of the surface ions. Further investigations will pursued to explain this observation, which may be detrimental for understanding hydration of oxides in general.

HydraThermPro aims to derive correlations between the thermodynamics of proton dissolution and chemical and physical properties, primarily for oxides. This comprehension will be achieved by integrating novel experimental and computational approaches. Hydr ation thermodynamics will be determined directly by TG-DSC and compared to similar values calculated by DFT based modeling packages. Reliable correlations between the essential parameters of proton conduction and other materials properties will strengthen the predictive power in deriving new and improved high temperature proton conductors. HydraThermPro will, as such, contribute significantly to succeed in the development of technologies for an environment-friendly energy production.