Microstructure and morphology changes in MgH2/expanded natural graphite pellets upon hydrogen cycling

► MgH2 was ball milled with Nb2O5 and expanded graphite to prepare pellets. ► Pellets were repeatedly cycled under hydrogen pressure to simulate tank exercise. ► H-absorption drives the formation of hollow MgO shells via Mg diffusion through MgO. ► Mg evaporation results in Mg particles completely depleted of Nb2O5 catalyst. ► A model which describes these microstructural modifications is presented.