This work focuses on the absorption of hydrogen in palladium (Pd), which is a model system for hydrogen absorption in metals. The disparate behavior of hydrogen absorption in nanoparticles and bulk materials, is a crucial topic for the development of functional hydrogen storage systems. The work presented here and carried out during this PhD aims to unite. The palladium-hydrogen system and the experimental techniques used to study it draw from both physics and chemistry. The combined knowledge and tools from these fields are used in concert to deepen the understanding of hydrogen absorption in palladium nanoparticles. Electrochemical techniques are a powerful tool to both drive and study the absorption of hydrogen in Pd nanoparticles. They are used in this work to observe the thermodynamics and kinetics of hydrogen absorption in ensembles of Pd nanoparticles. Bragg Coherent Diffraction Imaging (BCDI) has emerged as a robust X-ray technique that can reconstruct the three-dimensional morphology, lattice parameter, and strain of individual nanoparticles of Pd. The evolution of these structural properties is studied in in situ electrochemical and gas absorption environments. The rate of hydrogen absorption in Pd nanoparticles is an important characteristic for practical solid-state hydrogen storage. Studies have primarily focused on the rate of the hydriding phase transition, however, knowledge of the kinetics inside of the hydrogen-poor α phase is lacking. Using time-resolved X-ray nano-diffraction, the rate of absorption at the single Pd nanoparticle level is observed to be slower in the electrochemical driven absorption than in the gas phase. The adverse effects of radiolysis from the interaction of the X-ray beam with the electrolyte are explored and mitigated. Chronoamperometry measurements observed the transient reduction current attributed to hydrogen absorption to follow the same time dependence as that of the lattice parameter for small steps of the cell potential. Deuterium absorption is observed to be slower than that of hydrogen in the electrochemical system. Furthermore, Sievert’s law of solubility of gases in metals was observed to hold for individual Pd nanoparticles in the α phase. In the framework of this thesis, a high-pressure gas reactor was developed. It is compatible with X-ray nanobeam and coherent imaging techniques, enabling for the first time the study of single-particle evolution under high-pressure gas environments. BCDI studies of Pd nanoparticles show the existence of a hydrogen rich subsurface in the α phase. The evolution of the strain state in Pd nanoparticles indicates that this subsurface layer is stable at temperatures up to 200 ◦ C with no external H 2 pressure. Above 400 ◦ C the strain state is modified indicating a possible partial desorption of this subsurface layer. A Pd nanoparticle was followed in the supercritical phase at 310 ◦ C, and at pressures up to 45 bar. The strain and lattice parameter of the nanoparticle change very little up to 21 bar of H2 . A high strain and following relaxing of the nanoparticle are observed in time at 23 bar, corresponding to a large change of the lattice parameter from the uptake of hydrogen. A second large uptake is observed at 38 bar where a large strain develops in the nanoparticle. This previously unobserved structure of the isotherm in the supercritical phase raise questions about the thermodynamics in these nanoparticles. This research has sought to bring new insights into the well known PdH system by using the advanced characterization tools of synchrotrons, and by applying them to individual nanoparticles and ensembles of nanoparticles. The combined approach of electrochemistry and X-ray studies are a promising research method to extend the understanding of absorption in metals for better hydrogen storage materials.