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.