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

Compacted mixtures based on ball-milled magnesium hydride (MgH2) have gained significant research interest as suitable materials for hydrogen storage tanks. The issue related to their stability during practical service conditions is one of paramount relevance. In this work, we investigate the microstructure and morphology of pellets obtained by the compaction of ball-milled MgH2/Nb2O5 powders mixed with expanded natural graphite. The pellets are subjected to repeated hydrogen sorption cycles to measure hydrogen storage properties and its stability with cycling. Moreover, the effect of air-exposure on the hydrogen sorption behavior is studied. Electron microscopy observations of as-prepared and cycled pellets point to a dramatic modification of the materialʹs microstructure upon repeated hydrogen cycling. In particular, the appearance of MgH2 particles depleted of the Nb2O5 catalyst and the formation of hollow MgO shells are highlighted. These findings are discussed by a simple model which takes into account the basic mechanisms intervening during the metal-hydride transformation in the pellet.