Dehydrogenation Properties and Mechanisms of MgH2-NiCl2 and MgH2-NiCl2-Graphene Hydrogen Storage Composites

The four hydrogen storage systems including pure MgH2, MgH2-5 wt% NiCl2, MgH2-10 wt% NiCl2 and MgH2-10 wt% NiCl2-10 wt% graphene were prepared by ball-milling in this work. Using experimental X-ray diffraction, scanning electron microscopy and differential scanning calorimetry testing methods in combination with first-principle calculations, the dehydrogenation properties and mechanisms of NiCl2 single-doped and NiCl2-graphene co-doped MgH2 composites were systematically investigated. Experimental results show that the NiCl2 single-doping is conductive to decreasing the size of MgH2 grains and particles. The co-doping of NiCl2 and graphene not only reduces the size of MgH2 grains and particles, but also contributes to the uniformity of MgH2 particles. As compared with milled pure MgH2, the dehydrogenation peak temperatures are decreased by 24 degrees C and 47 degrees C for the 10 wt% NiCl2 single-doped and 10 wt% NiCl2-10 wt% graphene co-doped MgH2 systems, respectively. It is demonstrated that the co-doping of NiCl2 and graphene exhibits the synergistic effects of confinement and catalysis on improving the dehydrogenation properties of MgH2. The first-principle calculations indicate that the codoping of NiCl2 and graphene leads to the distortion of MgH2 atomic-configuration and results in the charge transfer between the dopants and MgH2, which induce the weakened structural stability and decreased dehydrogenation enthalpy of MgH2.