Microstructural evolution of NbF5-doped MgH2 exhibiting fast hydrogen sorption kinetics

The microstructure of MgH2 with 1 mol% NbF5, prepared by high-energy ball milling (HEBM), was studied using high resolution transmission electron microscopy (HR-TEM) with an X-ray energy dispersive spectrometer (EDS) before and after hydrogen sorption cycles. The TEM samples were prepared without any air exposure by a novel, focused ion beam (FIB) system specially designed for highly air sensitive materials. During HEBM, the doping agent, NbF5, was distributed as an extremely thin, film-like, amorphous phase along the grain boundaries of the nanocrystalline MgH2. After 10 sorption cycles, amorphous Nb-F phase was transformed into crystalline Nb hydrides. It is believed that the Nb hydride played a decisive role in improving the sorption kinetics of MgH2.