Hydrogen in oxides: a density-functional study of thermodynamic and kinetic aspects

Doktorsavhandling, 2007

Many solid oxides, especially those possessing a perovskite structure, exhibit significant proton conductivity when being acceptor doped and placed in hydrogen-containing atmospheres. Consequently, they have attracted attention for use as solid membranes in electrochemical devices such as gas sensors, steam electrolyzers and fuel cells. In particular, there has been a recent interest in BaZrO$_3$, which when properly synthesized combines high proton conductivity with high chemical stability. However, for the extensively investigated Y-doped BaZrO$_3$ the reported bulk conductivities vary several orders of magnitude at a given temperature. A similar variation is also found when different dopant atoms are tested. This example illustrates that tailor-making of highly conductive oxides requires good control over the reactions taking place during sample preparation as well as detailed understanding of the protonation and proton transport mechanisms. In this work density-functional calculations have been used to extract structural and electronic properties of dopant atoms, oxygen vacancies and hydrogen interstitials in their relevant charge states in BaZrO$_3$. By combining the first-principles results with thermodynamic theory, defect formation energies at finite temperatures and pressures have been obtained and compared with experiments. Small trivalent dopants such as Ga, In and Sc are found to preferentially substitute for Zr atoms in the lattice. At low temperatures this modifies the electronic structure of the oxide so that holes are formed at the top of the valence band, corresponding to p-type electrical conductivity. At high temperatures and low oxygen partial pressures, dopants are instead charge compensated by formation of oxygen vacancies. Upon subsequent exposure to a humid atmosphere, protons are introduced via dissociative absorption of water into the vacancies. The calculated enthalpy for this hydration reaction is in acceptable agreement with experimental values. In addition, a study of preferred proton positions in pure and doped La$_2$Zr$_2$O$_7$ is also included. Moreover, the proton vibrational and diffusional motion in doped BaZrO$_3$ has been investigated, also experimentally by infrared spectroscopy and neutron spin-echo measurements. Important parameters, such as proton binding energies in different local surroundings, migration pathways, and the corresponding energy barriers, have been obtained directly from first-principles and used as input to a jump-diffusion model describing the long-range transport of protonic defects. Finally, the calculations predict strong, attractive dopant-proton and dopant-oxygen vacancy interactions. When these effects are taken into account the experimentally observed trends in both hydration enthalpy and proton migration enthalpy among the differently doped phases are reproduced.