Protons in In-doped BaZrO3: incorporation, distribution and local environments

Licentiate thesis, 2016

Acceptor doped proton conducting oxides are currently of large interest for their potential application as electrolyte materials in the future generation of solid oxide fuel cells. However, their proton conductivities are still lower than the target conductivity required for practical applications. The development of new materials with better performance depends on the advance of our understanding of the atomic-scale structure and dynamics, and their coupling with macroscopic proton conductivity, in the currently available materials. The aim of this thesis is to obtain new insights into the structure and dynamics in In-doped BaZrO3, one of the most investigated and promising classes of proton conducting oxides. To achieve this aim, the thesis is divided into two, complementary, research themes. The first research theme is dedicated to the analysis of the O-H vibrational spectra and a detailed characterization of the different proton sites present. Of specific interest is the nature of the hydrogen bond between the protons and neighboring oxygen atoms, O-H···O, which relates to the proton conducting properties. The second research theme focuses on studies of the proton concentration depth profile and proton conductivity in thin films of the same material. The investigations were performed using a combination of inelastic neutron scattering, infrared spectroscopy, molecular dynamics simulations, X-ray diffraction, Rutherford backscattering, X-ray and neutron reflectivity, nuclear reaction analysis and impedance spectroscopy. The results show that for low In-doping levels the protons are predominantly found close to the dopant atoms. These proton sites are characterized by a relatively weak hydrogen bonding and act as traps for the protons. For higher In-doping levels the results reveal the presence of a growing population of proton sites associated with stronger hydrogen bonds, and the trapping effect around the dopant atoms is now less pronounced, or even removed. In the studies of thin film samples, a key result is the observation of the presence of a thin proton-enriched near-surface region, characterized by relatively weak hydrogen bonding and lower proton mobility compared to the bulk of the material.