The PdHx system is a typical model to study the fundamentals of solute intercalation and phase transformations, relevant to a broad class of applications such as evolution and electro-oxidation of hydrogen (H2) in water electrolyzers and fuel cells respectively, as well as hydrogen (H) storage, its sensing and catalysis of many hydrogenation reactions. Pd absorbs hydrogen (applying a H2 pressure or an electrochemical polarization) leading to the formation of PdHx solid solution (α-phase) with slightly expanded lattice at low H content that fully transforms into a lattice-expanded β-phase non-stoichiometric hydride at higher H loading. It is well-documented that H atoms get progressively trapped in Pd under various sorption-desorption modes [1] [2], influencing its bulk and interfacial properties. This is of primary importance for understanding of structure–catalytic activity relationship on Pd based materials. Indeed, according to the d-band theory pioneered by Hammer and Norskov [3], trapping of H atoms into the Pd lattice drastically broadens its d-band, resulting in a negative shift of the d-band center, and a change in its catalytic properties. For instance, the lattice structure of PdH0.43 nanoparticles, stabilized in N,N-dimethylformamide, remains expanded over an extended period of time. These nanoparticles exhibit increased activity for the electrooxidation of methanol compared to their pure metal Pd counterparts [4]. Schmidt et al. have also recently shown that the formation of Pd subsurface hydride leads to weaker adsorption energy of adsorbed H, thereby enhancing the rate of the hydrogen evolution reaction in acidic medium [5]. Understanding these interesting effects requires Pd materials with well-defined H content and corresponding strain, which should remain unchanged throughout electrocatalytic experiments. In the context of the HERMES H2020 project, [6] we utilized a combination of electrochemistry and wide-angle X-ray scattering to demonstrate the progressive trapping of hydrogen atoms within the Pd lattice. This trapping process gradually expands the Pd lattice, thereby stabilizing the characteristic metal lattice of the PdHx-β phase. This phenomenon significantly influences the rates of hydrogen evolution and oxidation reactions, and is also evident across a range of acidic (H2SO4, HClO4) and alkaline (LiOH, KOH, NaOH, CsOH) electrolytes.