Introduction
Hydrogen is the fuel with high energy density and clean resource, which
may be a promising energy carrier to replace hydrocarbons. However, the
difficulty in the efficient storage of hydrogen is considered to be a
key challenge in the application of hydrogen resource [1, 2]. One of
the most viable and effective possible solutions is to store hydrogen in
metal hydrides [3-5]. Among lightweight chemical hydrides, magnesium
hydride (MgH2) has been widely studied due to its low
cost and high hydrogen capacity of up to 7.6 wt%. However, bulk
MgH2 has high desorption energy (75 kJ/mol) and sluggish
reaction kinetics, leading to a high temperature (573 K) for hydrogen
release [6, 7]. Considerable approaches have been conducted on
improving the thermodynamics and kinetics of MgH2, such
as additive-addition (adding metal oxides, metal halides, or carbon
adding etc.) [8, 9] and alloying [10, 11]. Although these
approaches can effectively reduce the operating temperature, the
additional weight of additive leads to a lower hydrogen storage capacity
in comparison to the bulk MgH2 [12, 13]. Recently,
nanosizing magnesium-based hydrides have been proposed as an alternative
method for improving hydrogen storage capacity [14, 15]. Since the
large specific surface area of nanostructure increases the ability of
hydrogen adsorption, the Mg nanoparticles have superior hydrogen storage
property in comparison with bulk Mg [16-18]. Moreover, the large
specific surface area can shorten the diffusion path of adsorbed
hydrogen atoms, accelerating hydrogen release.
Many experiments and theoretical calculations have confirmed that
nanosizing Mg particles can effectively improve the hydrogen storage
properties of MgH2 [4, 16, 19-21]. Xia et al.
synthesized monodisperse MgH2 nanoparticles with an
average size of 4.7−6.0 nm under the structure-directing role of
graphene. These MgH2 nanoparticles could release 5.4
wt% hydrogen at 250°C within 30 min and the formation enthalpy of
MgH2 is reduced to 62.1 kJ/mol, in comparison to 75
kJ/mol for bulk MgH2 [22]. Zhang et al. developed
ultrafine MgH2 hydrides of 4-5 nm without involving any
scaffold or protection agent. The hydrogen release enthalpy is decreased
to 59.5 kJ/mol [23]. Konarova M. et al. loaded MgH2into CMK3 mesoporous scaffolds with a pore size of only 3.5 nm. The
dissociation enthalpy of the MgH2/CMK3 composite is
52.38 kJ/mol H2, and the initial dissociation
temperature 253°C, which is much lower than the bulk
MgH2 (300-400°C) [24].
Theoretically, Li et al. have performed the DFT calculations for the
effect of size of nanowires on the thermodynamic stability of
MgH2 nanowires. They found the desorption enthalpies of
φ0.85 nm (MgH2.33) and φ1.24 nm
(MgH2.17) nanowires are reduced to 34.54 kJ/mol and
68.22 kJ/mol [25-27] respectively. Although
Mg-H nanowires improve hydrogen
storage capacity, the structures are unstable and will collapse into
nanoparticles after a few cycles [21]. For nanoclusters, the
first-principle calculation by Wagemans et al. showed that the hydrogen
desorption enthalpy of Mg9H18 cluster is
63 kJ/mol, corresponding to the hydrogen release temperature of 200℃
[28]. H. Chen et al. carried out the density functional theory (DFT)
calculations for hydrogen dissociation reactions of MgH2nanoclusters doped by a Sc atom, and found that MgScH15cluster has a high hydrogen storage capacity of 17.8 wt% [29].
Aditya Kumar et al. found Mg2B6 cluster
has a maximum H2 adsorption of 8.10 wt% at ambient
temperature and 1 bar pressure [30]. Although the first-principle
calculations show that the doped elements can improve the hydrogen
storage capacity, the high weight and cost of dopant limit its
application in hydrogen storage using magnesium hydride.
While many works have focused on the thermodynamics and kinetics of
saturated MgmHn (n = 2m) nanoclusters,
to our best knowledge, no literature reports the
MgmHn nanoclusters with the
stoichiometric composition of n:m > 2. In the present
study, we find four hydrogen-enriched
MgmHn (n:m>2:1)
nanoclusters, Mg3H7,
Mg4H9,
Mg5H11,
Mg6H13, in which the hydrogen capacities
are higher than 8.3 wt%. The ab initio molecular dynamics
simulations show that the hydrogen dissociation reactions of
hydrogen-enriched nanoclusters occur at a very short time (<
200fs) at room temperature, which may be promising for the hydrogen
release at ambient temperature and pressure. This work deepens the
understanding of the kinetic mechanism of hydrogen dissociation reaction
for MgmHn (n≥2m) and provides new
insights into the hydrogen storage of nano-magnesium materials.