We investigated the influence of the substrate on the thermodynamic properties of metal hydride thin films by hydrogenography, using PdHx as a model system. After appropriate hydrogen cycling, reproducible hydrogenation properties are found at the same equilibrium pressure for all substrates studied. Comparing these thin films with free-standing films-measured both by hydrogenography and by Sievert's method-we find a very similar behavior. Hence, thin films can be used to study the hydrogenation behavior of the corresponding bulk materials.

Hydrogenography is a new optical thin film combinatorial method that follows hydrogenation and determines its associated thermodynamic properties. Due to clamping to the substrate, stresses generated in thin films are larger than in bulk. This must be taken into account for a comparison between these two types of systems. In this article, we follow the microstructure, surface morphology and in-plane stress changes of thin polycrystalline PdHx films upon several hydrogen ab/desorption cycles and correlate them to the evolution in shape and hysteresis of pressure-optical transmission isotherms (PTIs) recorded by hydrogenography. The in-plane stress in the first instance is relaxed inhomogeneously by buckling, and a more complete, homogeneous relaxation is only reached after the creation of a buckle-and-crack network that is the two-dimensional analogue of bulk decrepitated grains. This sequence of changes is clearly visible in the PTIs, demonstrating another useful facet of hydrogenography for characterizing metal-hydrogen systems.