The Ni and Y co-doping effect on the structural stabilities and dehydrogenation properties of destabilized MgH2 was studied by first-principles calculations.

The properties of hydrogen storage on graphene with La decoration are investigated using a firstprinciples plane-wave pseudopotential method based on the density functional theory in this paper. The clustering problem of La decorated graphene is considered and B doping can solve it effectively in theory. We obtain the stable geometrical configuration of the modified system and the properties of hydrogen storage are excellent. It can absorb up to 6 H 2 molecules with an average adsorption energy range of −0.529 to −0.655 eV/H 2 , which meets the ideal range between the physisorbed and chemisorbed states for hydrogen storage. Furthermore, it is proved that the existence of La atom alters the charge distribution of H 2 molecules and graphene sheet based on the calculation and analysis about the electronic density of states and charge density difference of the modified system. La atom interacts with hydrogen molecules through Kubas interaction. Thereby, it improves the performance of graphene sheet for hydrogen storage. The modified system exhibits the excellent potential to become one of the most suitable candidates for hydrogen storage medium at near ambient conditions with molecule state.

The M-doping (M = Zr, Hf) effects on the electronic structures and thermoelectric performance of TiCoSb were studied by first-principles calculations. The band structure analysis shows that substituting Ti with M does not change the band structures of these systems significantly. Most of the M-doped systems have a lower band gap value than that of TiCoSb; especially Ti 0.5 Zr 0.5 CoSb has the lowest energy band gap value of 0.971 eV. Besides, the amplitudes of the density of states in the region of the valence bands for M-doped systems show a similar but slightly higher value than TiCoSb. Those suggest that these compounds could have better thermoelectric performance than TiCoSb. The phonon dispersion relations show that the larger mass of Zr/Hf with respect to Ti lowers the optical modes and induces mixing with the acoustic branches. Our calculations offer a valuable insight on how to characterize complicated crystal structures of thermoelectric materials and optimize the material composition.

First-principles calculations were used to study the energetics and electronic structures of Ni and Sc co-doped MgH 2 system. The preferential positions for dopants were determined by the minimal total electronic energy. The results of formation enthalpy indicate that Ni and Sc co-doped MgH 2 system is more stable than Ni single-doped system. The hydrogen desorption enthalpies of these two hydrides are investigated. Ni and Sc co-doping can improve the dehydrogenation properties of MgH 2 . The lowest hydrogen desorption enthalpy of 0.30 eV appears in co-doped system, which is significantly lower than that of Ni doping. The electronic structure analysis illustrates that the hybridization of dopants with Mg and H atom together weakens the Mg-H interaction. And the Mg-H bonds are more susceptible to dissociate by Ni and Sc co-doping because of the reduced magnitude of Mg-H hybridization peaks. These behaviors effectively improve the dehydrogenation properties of Ni and Sc co-doped cases. * Corresponding authors. G. L. Sun et al. 35some of the key factors for its practical applications [2]. It is desirable for the hydrogen "container" to have high gravimetric capacities, store and transport hydrogen economically and conveniently [3]- [5]. Storing hydrogen chemically in solid hydrides with atomic state has the potential to fulfill these requirements. Among solid hydrides, magnesium hydride (MgH 2 ) has been investigated extensively in the last two decades due to its high hydrogen storage capacity (7.6 wt%), good cyclability, cheap cost and lightweight of magnesium [5]- [7]. Unfortunately, a high thermodynamic stability and poor kinetics in the reaction of hydrogenation and dehydrogenation hampered practical applications of MgH 2 [6]- [8].Over the past several decades, many efforts have been made to overcome these disadvantages. Reducing the grain size [7]-[9], and introducing transition metals (TM) or transition metals oxides [10]- [14], can effectively improve the absorption/desorption kinetics of MgH 2 . Among these two methods, the introduction of transition metals into MgH 2 has played a vital role in the development of Mg-based hydrogen storage materials. The thermodynamic properties and dehydrogenation kinetics of MgH 2 can be tuned by doping it with different TM [10] [14]. Larsson et al. [14] carried out first-principles calculations based on DFT which showed that Ti, V, Fe, and Ni significantly lowered the H 2 desorption energies of MgH 2 nanoclusters and improved these dehydrogenation properties. Pozzo et al. [15] observed that Ni is the best possible choice of the transition metal doped on Mg(0001) surfaces due to low activation barriers for hydrogen dissociation and diffusion processes. More recently, many researchers reported that the hydrogen desorption kinetics of MgH 2 can further be tuned by multi-doping it with different TM [16]- [22]. A computational study carried out by Huang et al. [17] showed that the capacity retaining rate of Mg 1.5 Ti 0.5 Ni milled alloys is better than that of Mg 2 Ni milled ...

The first-principles plane-wave pseudopotential method within density functional theory formalism is used to investigate the effect of Y atom decoration of graphene on the properties for hydrogen storage. The clustering problem for the Y atoms decorated on graphene is considered, and substitutional boron doping is shown to effectively prevent the clustering of Y atoms on graphene. The geometrical configuration of the modified system is stable and the adsorption properties of H2 are excellent, which can adsorb up to 6 H2 molecules with an average adsorption energy range of-0.539 to-0.655 eV (per H2), as determined by theoretical analyses. This satisfies the theoretical ideal range for hydrogen storage. Moreover, based on the calculation and analysis of the Bader charge, the electronic density of states and the charge density difference of the H2/Y/B/graphene (G) system, it is proved that the Y atom exhibits bonding with graphene by charge transfer and interacts with hydrogen molecules through typical Kubas interactions. The existence of the Y atom alters the charge distribution of the H2 molecules and graphene sheet. Hence, the Y atom becomes a bridge linking the H2 molecules and graphene sheet. Thereby, the adsorption energies of the H2 molecule are adjusted to the reasonable region. The modified system exhibits excellent potential as one of the most suitable candidates for a hydrogen storage medium in the molecular state at near ambient conditions.