Effects of the compaction pressure and of the cycling process on kinetics and microstructure of compacted MgH2-based mixtures

Materials to be used in tanks for hydrogen storage should have suitable thermal properties otherwise their performances in term of kinetics and hydrogen capacity would rapidly decrease during exercise. For these reason in this work ball milled powders of MgH2 with Nb2O5 as catalyst and Expanded Natural Graphite (ENG) have been compacted at different pressures in the range between 100 and 600 MPa. The pellets were tested until 50 cycles after which the material was able to absorb and desorb hydrogen without significant reduction in maximum capacity. The mechanical stability of these compacted systems during repeated cycling under hydrogen pressure is discussed together with the modifications of the microstructure. The compaction pressure revealed to affect largely the distribution of ENG in the pellets and their mechanical stability upon cycling while this parameter have only a slight effect on the sorption kinetics. Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and volumetric Sievert's type apparatus have been used to characterize the samples.