First-Principles Investigation of Energetics and Electronic Structures of Ni and Sc Co-Doped MgH<sub>2</sub>

Abstract

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 ...

Catalytic Tuning of Sorption Kinetics of Lightweight Hydrides: A Review of the Materials and Mechanism

“…Since, the dissociation of H 2 molecules and diffusion of hydrogen atoms through the metal are two rate limiting steps for H 2 absorption in Mg, several theoretical studies have been conducted to analyze the hydrogen magnesium interaction in respect to the transition metal catalysts [38][39][40][41][42]. Kecik et al [38] used the first principle molecular dynamics method for the calculation of adsorption energies of Mo, Nb, Mn, Cr, Co, Fe, V, Pd, and Ni on a magnesium surface and suggested that the transition metals including and to the left of the VI-A column tend to be adsorbed on a substitutional site while other transition elements have a tendency to be adsorbed on a bridge site.…”