The Development and Stability of Palladium-based Thin Films for Hydrogen-related Energy Applications

Hydrogen has been contemplated as a desirable energy source of the future with enormous possibilities to create a carbon-neutral society. Since palladium (Pd) readily absorbs hydrogen even at low pressure and room temperature, Pd and its alloys are suitable for hydrogen production, purification, storage, gas sensors, and fuel cell catalyst. However, primary requirements for industrial applications are not always satisfied, such as usability under operation conditions, minimum capital cost, and sustained hydrogen embrittlement. Therefore, developing stable Pd-based thin films to investigate correlations between microstructural features and mechanical properties of a material is of great importance for many hydrogen-related technologies.
In this thesis particular interest has been focused on the stability of a series of magnetron sputtered Pd thin films of different nanostructures i.e., non-voided compact and nano-voided open columnar morphology. The X-ray diffraction (XRD) analysis methods are advanced, utilizing the tailored microstructures of the Pd films suitable to investigate the interplay between microstructure and hydrogenation properties of Pd-based thin films. Interpretation of the stress state and microstructural changes during hydrogen cycling are studied utilizing XRD line-profile analysis and the deformation mechanisms are systematically discussed. The change in dislocation density by the generation and annihilation of dislocations at interfaces reflects the difference in film-substrate interaction. The insertion of an intermediate layer between the Pd film and a rigid substrate can prevent buckle-delamination that is caused by the large volume expansion due to hydrogen absorption but it also changes the hydrogen absorption performance. The different effects on the absorption properties in the case of compliant (polyimide) and rigid (titanium) intermediate layers are illustrated. The results of this work showed that the strong clamping usually suppresses or reduces hydrogen absorption, whereas, the flexible layer enhances the lifetime of Pd thin films when exposed to prolonged hydrogen during cycling. The research in this thesis deepens the understanding about an appropriate combination of film microstructure and choice of the intermediate layer to strengthen Pd-based thin films.