Characterization of graphite catalytic effect in reactively ball-milled MgH2–C and Mg–C composites

In the present work magnesium hydrogen sorption kinetics of different graphite nanocomposites prepared by low-energy ball milling is reported. A comparison is made between kinetic performance in a Sievert-type apparatus of pure Mg, Mg + xG (x = 5, 10, 20 wt.%, G = graphite) composites and a mixture obtained by adding 10 wt.% G to MgH2 already synthesized via reactive mechanical milling. Moreover, the influence of two types of graphite morphology (flakes and fine powder) is evaluated. In addition, thermal dehydriding properties are analyzed by differential scanning calorimetry (DSC) measurements and morphological and microstructural characteristics are determined by X-ray diffraction (XRD), laser granulometry, BET surface analysis as well as scanning and transmission electron microscopies (SEM and TEM) for every material under study.Graphite catalytic effect is independent of its morphology and is more pronounced when the additive is incorporated from the beginning of milling, as well as in a higher weight proportion. Furthermore, the additive preserves material microstructure during a few hydriding and dehydriding cycles, i.e. particle size distribution and crystallite enlargement, and as a result kinetic properties remain practically unchanged.

► Study of hydrogen sorption kinetics in MgH2 and Mg/graphite ball-milled composites.
► Graphite catalyses magnesium hydriding/dehydriding rates during volumetric cycles.
► With graphite proportion sorption rates increase while storage capacity decreases.
► Graphite catalytic effect is stronger when it is well distributed within the powder.
► Graphite preserves material microstructure during absorption/desorption cycles.