Neutron scattering investigations of proton dynamics in acceptor doped barium zirconates

Licentiate thesis, 2016

Hydrogen fuel cells represent one of the most promising sustainable technologies for energy conversion. The advantages of the combination of a solid state electrolyte and low operational temperatures (< 750 C), not yet achievable, might be enabled by the use of proton conducting solid electrolytes, bringing these devices into everyday life. However, even for the best solid electrolytes available today, the proton conductivities at the desired temperatures remain too low. In this context, a deeper understanding of the proton conduction mechanism in currently available materials is crucial for the development of new materials combining sufficiently high conductivities with good chemical stability. This thesis presents an investigation of the proton dynamics in barium zirconates, a well-known promising class of proton conducting oxides. It has been shown that the macroscopic proton conductivities differ order of magnitudes depending on the chemical composition of the materials. The aim of the present study is therefore to obtain a detailed description of the atomic scale proton dynamics in the two hydrated proton conducting oxides BaZr0.9M0.1O2.95 (M= Y and Sc), and correlate this with the macroscopic proton conductivity of the materials. For this reason, the combination of different neutron spectrometers was exploited to map an extensive dynamical region, and the degree of hydration of the samples was also carefully monitored. In the picosecond time region, we observed localised proton dynamics with low energy barriers, no strong dependence on the type of dopant atoms (Y or Sc), and a spatial geometry compatible with proton jumping between two neighbouring oxygens or reorientation of the hydroxyl group. Studies over a wide time scale suggest a complex pattern of several dynamics, most likely related to the presence of different proton sites.