Hydrogen storage properties of Mg-Ni/Graphene composites

Graphene supported nickel catalysts (Ni/Graphene) were synthesized by physical method and chemical method, respectively. The as prepared Ni/Graphene was further mixed with Mg by ball milling pretreatment or ultrasonic dispersion. Then the well mixed powders were used to prepare Mg-Ni/Graphene composites by the combined process of hydriding combustion synthesis and mechanical milling (HCS+MM). The phase compositions, surface morphologies and hydriding/dehydriding properties of the samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas reaction controller (GRC). Results show that the Mg-Ni/Graphene composite which mixed Ni/Graphene synthesized by chemical method and Mg through ball milling pretreatment exhibits the best hydriding/dehydriding properties. It has the lowest onset hydrogen desorption temperature and faster dehydrogenation rate. About 6.21% hydrogen can be absorbed within 100 s at 373 K, and 6.05% hydrogen can be desorbed within 1800 s at 553 K. Ball milling pretreatment makes Ni/Grapheme and Mg contact homogeneous, thus the excellent thermal and electrical conductivity of Grapheme and the catalytic effect of Ni are enhanced. In addition, nano-crystalline Ni with smaller grain size synthesized by chemical method is helpful to the formation of nanosized Mg₂NiH₄ grains, leading to the improvement in hydrogen storage properties of the composite.