Improved hydrogen desorption properties of MgH2 by graphite and NiF2 addition: experimental and first-principles investigations

The high dehydrogenation temperature is still the impediment for the practical application of magnesium-based hydride (MgH2) as a potential hydrogen storage medium. In order to improve the hydrogen desorption properties of MgH2, the graphite and NiF2 are selected as additives, and the MgH2–graphite and MgH2–graphite–NiF2 composites are prepared by high-energy ball milling. Using experimental X-ray diffraction, scanning electron microscopy, differential scanning calorimetry characterizations and first-principle calculations, the effects and mechanisms of graphite and NiF2 addition on the hydrogen desorption properties of MgH2 are systematically investigated. Experimental results show that the single addition of graphite is beneficial to the refinement of MgH2 grains and particles. The size of MgH2 particles can be further decreased after the co-addition of graphite and NiF2. Either the single addition of graphite or the co-addition of graphite and NiF2 reduces the dehydrogenation temperature of MgH2. As compared with pure milled MgH2, the dehydrogenation peak temperatures are decreased by 31 and 63 °C for MgH2–graphite and MgH2–graphite–NiF2 composites, respectively. Apparently, the co-addition of graphite and NiF2 exhibits the synergistic catalytic effects in improving the hydrogen desorption properties of MgH2. The first-principle calculations reveal that the co-addition of graphite and NiF2 leads to the structural distortion of MgH2 and results in the charge transfer between the additives and MgH2, which induce the weakened structural stability and decreased dehydrogenation enthalpy of MgH2. These may be the underlying reasons for the reduced dehydrogenation temperature of MgH2 with graphite and NiF2 addition.