In situ catalyzed and nanoconfined magnesium hydride nanocrystals in a Ni-MOF scaffold for hydrogen storage

Abstract

Nano-confinement (NC) of Mg/MgH₂ in suitable host materials is a promising strategy for simultaneously tuning the thermodynamics and kinetics of Mg-based hydride materials. In the present study, the fabrication of multifunctional host materials for NC of MgH₂ was firstly attempted with the aim of improving the hydrogen sorption properties of MgH₂. The NC of MgH₂ in the mesoporous structure of a Ni-based metal–organic-framework (denoted as MgH₂@Ni-MOF) was successfully conducted via a combination of solvothermal method and wet impregnation method followed by hydrogenation at relatively lower temperatures. Structural inspections through X-ray diffraction and high resolution transmission electron microscopy demonstrate the in situ formation of Mg₂NiH₄ around the MgH₂ nanocrystals (∼3 nm), due to the reaction between Ni ions from the Ni-MOF and Mg ions from the dibutyl-magnesium (MgBu₂) precursor in a hydrogen atmosphere. Hydrogen sorption tests reveal that the thermodynamics (−65.7 ± 2.1 and 69.7 ± 2.7 kJ mol⁻¹ H₂ for ab-/desorption, respectively) and kinetics (41.5 ± 3.7 and 144.7 ± 7.8 kJ mol⁻¹ H₂ for ab-/desorption, respectively) of Mg/MgH₂ in the MgH₂@Ni-MOF composite have been significantly improved. Meanwhile, the Ni-MOF scaffold acts as an “aggregation blocker” to impede the growth and agglomeration of Mg/MgH₂ nanocrystals during hydriding/dehydriding cycles, leading to outstanding hydrogen cycling stability. The improvement in thermodynamics and kinetics of MgH₂@Ni-MOF has been attributed to the synergistic “nanosize effect” of nanoconfined Mg/MgH₂ and catalytic effects from in situ formed Mg₂Ni/Mg₂NiH₄.