Nanostructured Ti-catalyzed MgH₂for hydrogen storage

Abstract

Nanocrystalline Ti-catalyzed MgH(2) can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH(2) sample. The catalyzed MgH(2) nanocrystalline sample consists of two MgH(2) phases-a tetrahedral β-MgH(2) phase and an orthorhombic high-pressure modification γ-MgH(2). Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N(2) adsorption measurement shows a BET surface area of 108 m(2) g(-1) of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130 °C lower compared to commercial MgH(2). After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH(2) at 300 °C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH(2) nanocrystalline sample are 77.7 kJ mol(-1) H(2) and 138.3 J K(-1) mol(-1) H(2), respectively. Thermodynamic properties do not change with the nanostructure.

MgH2 Dehydrogenation Thermodynamics: Nanostructuring and Transition Metal Doping

“…Indeed, (MgH 2 ) 6 has a strong resemblance to (MgH 2 ) 5 + MgH 2 bound together, but with additional distortion, and (MgH 2 ) 7 resembles (MgH 2 ) 6 + MgH 2 with additional distortion. The (MgH 2 ) 8 cluster has a very high degree of symmetry; we therefore expect this cluster to be more stable towards dehydrogenation than other clusters of a similar size. Both (MgH 2 ) 9 and (MgH 2 ) 10 are more compact clusters with no short H-H distances.…”

“…Indeed, for n = 5, 6, 8, and 9 the energy cost is trivial, with the (MgH 2 ) 8 cluster unstable with respect to Ni-doping. E F (eV) Bulk Rutile MgH 2 1.64 (MgH 2 ) 2 2.01 (MgH 2 ) 3 0.60 (MgH 2 ) 4 0.22 (MgH 2 ) 5 0.01 (MgH 2 ) 6 0.00 (MgH 2 ) 7 0.21 (MgH 2 ) 8 -0.08 (MgH 2 ) 9 0.00 (MgH 2 ) 10 0.15 As Ni dopants have a significant effect on the geometry and electronic structure of the MgH 2 cluster, we also calculated the effects on the dehydrogenation thermodynamics.…”

“…E F (eV) Bulk Rutile MgH 2 1.64 (MgH 2 ) 2 2.01 (MgH 2 ) 3 0.60 (MgH 2 ) 4 0.22 (MgH 2 ) 5 0.01 (MgH 2 ) 6 0.00 (MgH 2 ) 7 0.21 (MgH 2 ) 8 -0.08 (MgH 2 ) 9 0.00 (MgH 2 ) 10 0.15 As Ni dopants have a significant effect on the geometry and electronic structure of the MgH 2 cluster, we also calculated the effects on the dehydrogenation thermodynamics. In particular we calculate the removal energy for first H 2 removal from both the undoped and Ni-doped MgH 2 clusters, see We calculated the charge deformation density in order to understand the effects of Ni dopants on the electronic structure of the MgH 2 clusters and thus the mechanism behind the reduced removal energy, see Figure 5.…”

“…2,3,4,5,6,7 A favoured material for hydrogen storage is magnesium hydride (MgH 2 ), as it has a high gravimetric hydrogen storage capacity of 7.6 wt% and is cheap and plentiful. However, unmodified MgH 2 is not suitable for technological applications due to a high enthalpy for dehydrogenation of 77 kJ/mol, 8 low plateau pressure of 10 Pa at ambient temperature and pressure, 9 and relatively slow absorption and desorption kinetics. 10 Nanostructuring and doping are popular methods of modifying properties of materials, as is the case for MgH 2 .…”

Facile preparation of β-/γ-MgH2 nanocomposites under mild conditions and pathways to rapid dehydrogenation

“…Recent theoretical calculations and experimental results show that the high-pressure meta-stable γ-MgH 2 phase exhibits improved dehydrogenation kinetics compared to that of the room temperature/pressure phase, β-MgH 2 . [20][21][22][23] However, it should be noted that full transformation of β-MgH 2 to γ-MgH 2 has not been successfully achieved previously. γ-MgH 2 is usually produced using extreme processing conditions requiring high pressure (2.5-8 GPa) and high temperature (250-900 °C) or via ultra-high energy reactive ball milling under ca.…”