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.