Neutron scattering for sustainable energy materials: investigations of proton dynamics in acceptor doped barium zirconates

Doctoral thesis, 2018

Proton conducting oxides are currently receiving considerable attention for their present or potential use as electrolytes in technological devices such as sensors and electrolysers and, in particular, solid oxide fuel cells, which are among the most promising apparatuses for energy conversion. One of the main challenges for these latter devices is to combine the advantages of a solid electrolyte with those of operational temperatures below 750 °C, which is currently hampered by insufficient conductivities in the targeted temperature range. The development of new electrolytes meeting the requirements for applications depends on a better understanding of the physico-chemical processes underlying ionic conductivity in these materials. Towards this aim, this thesis reports on investigations of key properties in hydrated samples of the perovskites BaZr0.9M0.1O2.95 with M=Y and Sc and BaZr1-xInxO3-x/2 with x=0.1—0.275, well-known and promising proton conducting oxides. Of specific concern in this thesis is the study of the effect of the type (M) and concentration (x) of dopant atoms on the atomic-scale proton dynamics over a wide time-range, from picoseconds to nanoseconds, using different state-of-the-art neutron scattering techniques at the neutron scattering facilities Institut Laue-Langevin in Grenoble, France, and Forschungs-Neutronenquelle Heinz Maier-Leibnitz in Garching, Germany.
The results show a complex dynamics, arising from a distribution of different proton sites, a consequence of a disordered structure of the materials. Analysis of the short time scale dynamics discloses localized dynamics interpretable as proton jumps and reorientations of the hydroxyl groups. Faster local motions are observed in more distorted structures associated with higher doping levels, whereas no substantial differences are observed for different dopant ions. Analysis of the long time scale dynamics reveals long-range diffusion of protons, which can be described as a jump-diffusion process. Higher dopant concentrations lead to higher activation energies, still well below those for macroscopic proton conductivities, but larger fractions of mobile protons.
This new insight adds to the previous knowledge of proton dynamics in perovskite materials and can be useful to develop strategies for the design of improved proton conductors for technological applications