Metal hydrides are essential for electrochemical energy storage. They are promising to safely store hydrogen in solid form and are used as active materials in Nickel-metal hydride rechargeable batteries. To improve the hydrogen absorption efficiency, to speed up the (de)hydrogenation kinetics and to increase the materials cyclability, two strategies are now considered: alloying and miniaturization. This however makes active materials investigations much more challenging due to the presence of nanostructured interfaces, heterogeneities among particles and nanoscopic changes in materials composition. The objective of the ECHoS project is then to develop advanced analytical strategies capable of probing operando the electrochemical insertion and release of hydrogen in solid materials from the nanoscale to the microscale. To achieve these challenging tasks and because hydrogen is the lightest chemical element, ECHoS relies on a highly sensitive optical interferometric scattering microscope (iSCAT) coupled to electrochemistry to visualize and quantify instantaneously the formation of metal hydride in different systems with high spatial and high temporal resolution. First employed at the single nanoparticle level, from palladium nanoparticles to more complex metal nano-alloys synthesized with novel nanoscale-confined electrodeposition methods, the nanoscale imaging technique extends to encompass ABy-type (2