Ni/NiOx heterostructured materials, comprising metallic Ni sites and Ni (hydr)oxide sites (NiOx), are considered among the most promising non-noble metal catalysts for the hydrogen evolution reaction (HER) in alkaline media. The effect of chemical composition on their activity (i.e., the need of an optimum Ni/NiOx surface ratio for an optimal H adsorption strength) is well documented. However, crystallography-to-activity correlations are yet to be established. In this proposal, we aim to unveil operando, in the same electrochemical cell (i) how the Ni nanoparticles (NPs) form during electrodeposition from a crystallographic standpoint; (ii) how their crystallographic properties evolve when producing Ni/NiOx heterostructures and (iii) how their crystallography is further altered during the HER (e.g., formation of nickel hydrides) and associated aging test, thus gathering insights over the entire lifespan of the electrocatalyst by wide and small angle X-ray scattering.