Structural and reaction pathway analyses of Mg(BH4)2·2NH3 for hydrogen storage : A first-principles study

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Abstract

Mg(BH4)2·2NH3 is a relatively new compound considered for hydrogen storage. The fundamental properties of the compound were comprehensively studied using first-principles calculations, such as crystal structure and electronic structure, reaction Gibbs free energy and possible reaction pathway. The calculated crystal structure is in good agreement with the experimental and other theoretical results. Results from electronic density of states (DOS) and electron localization function (ELF) show the covalent characteristics of the N–H and the B–H bonds, and the weak ionic interactions between the Mg atom and the NH3 ligands or the (BH4) ligands. The reaction Gibbs free energies of several possible decomposition reactions were calculated between 0 and 700 K. All the reactions are exothermic. The most likely reaction pathway of the dehydrogenation reaction was clarified to show five distinct steps.

Highlights

► Structural and electronic properties were studied. ► The most possible overall reaction was determined. ► Up to 500 °C, the dehydrogenation reaction could be divided to five steps.

Introduction

Hydrogen storage is a key issue in hydrogen fuel cell vehicular applications [1]. The U.S. Department of Energy sets the targets for the year 2015 for hydrogen storage systems as 5.5 wt % of gravimetric capacity and 0.055 kg/L of volumetric capacity [2]. If the ancillary components of the storage system (container, pumps, valves, etc.) are accounted for about half of the total weight and volume, the actual requirement for the capacity of the materials are much higher, at least 11 wt %. Only a few metal borohydrides possess such a capability. But many of these are not suitable for practical use because of high hydrogen release temperatures, high vapor pressure, or high toxicity [3], [4], [5], [6]. Another more important reason is that the dehydrogenation reaction is irreversible or only partly reversible. Ammonia borane NH3BH3, with a hydrogen storage capacity 19.6 wt %, is a promising hydrogen storage material. However, it gives only about 2 mol of H2 per mole of ammonia borane at practical temperatures (<500 °C) [7]. In the mean time, the dehydrogenation reaction of NH3BH3 is exothermic, which makes controlled hydrogen release difficult. In addition, decomposition of ammonia borane also produces ammonia or borazine; these products are harmful for proton exchange membrane (PEM) fuel cells. A very recent study shows that regeneration of ammonia borane from its spent fuel is possible [8], but it still does not fit for practical using.

To overcome some of the drawbacks of ammonia borane, many ammonia complexes of metal borohydrides M(BH4)n·mNH3 (M = Li, Al, Zn, Ca, Mg, Y, etc.) has been investigated [9], [10], [11], [12], [13], [14], [15], [16], [17]. In general, these new classes of the B–N–H hydrogen storage system have high hydrogen capacity and low decomposition temperature. For example, the H2 capacity reaches ∼9 wt % by 150 °C in LiBH4·NH3/AlCl3 mixture [17]. Mixing of LiBH4·NH3 with other compounds, particularly at the nano-scale can greatly enhance the dehydrogenation properties [9], [10]. If the ratio of NH3:LiBH4 is more than 1:1, an extremely high H2 capacity of 17.8 wt % can be obtained [11]. Among those metal borohydrides, Mg(BH4)2·2NH3 of 16 wt % hydrogen capability is very interesting because almost all the H atoms can be transformed to H2 [15]. The decomposition takes place in two distinct stages: the first is endothermic, starting at 150 °C and with a maximum hydrogen release rate at 205 °C; the second is exothermic, starting at about 220 °C and is almost complete at about 400 °C. The two steps release approximately equal amounts of hydrogen, with a total of ∼13.1 wt %, which is less than the theoretical value of 16.0 wt %. Increasing temperature to 500 °C improves the total to 14.9 wt % and continued heating at this temperature with periodic evacuation of the sample chamber raises the total to 15.9 wt %. In the final decomposition products, the only crystalline phase detected by XRD is BN. At the same time, neither MgH2 nor Mg was found. The possible overall decomposition reaction can be written as:3Mg(BH4)2·2NH3Mg3BN3+3BN+2B+21H2

Clearly, the decomposition temperature of the compound is too high for practical hydrogen storage application. In order to improve the hydrogen storage properties, a clearly understanding of the detailed structural and decomposition characteristics of the compound, such as the electronic structure and the dehydrogenation pathway, need to be clarified. Most recently, Chen and Yu have studied the electronic structure and initial dehydrogenation mechanism of Mg(BH4)2·2NH3 by first-principles method [18]. By calculating the removal energy of the hydrogen molecule, they show that the initial dehydrogenation via a combination of (N)H+⋯(B)H is energetically favorable. After the first H2 was released, the N–B distance is around 1.567 Å, indicating the formation of an N–B single bond. So a H2N–BH3 group was formed. Further study showed that the second H2 was also released via a combination of (N)H+⋯(B)H from the NH3 and the (BH4) group, not from the NH3 and (B)H in the adjacent H2N–BH3 group. Clearly, the above findings provide useful information on improving hydrogen storage property of this compound. However, there is still a lack of understanding on the thermodynamic property, possible reaction products, and reaction pathway. In order to clarify such issue, we present a comprehensive theoretical study, based on first-principles calculations of the structural and electronic properties of the Mg(BH4)2·2NH3 complex. We determined the reaction Gibbs free energies at different temperatures of possible decomposition reactions, and pointed out the most likely reaction pathway from the analyses.

Section snippets

Computational methods

The Vienna ab initio simulation package (VASP) [19], [20], [21] was used for calculations, which was based on the density functional theory [22]. The projector-augmented wave (PAW) method [23], [24] and the generalized gradient approximation (GGA) [25] were selected along with the Perdew–Burke–Ernzerhof (PBE) [26] exchange-correlation potential. Mg(BH4)2·2NH3 has an orthorhombic structure with space group Pcab (No. 61). The unit cell contains 152 atoms [15]. The equilibrium lattice parameters

Structural properties

The structure is shown in Fig. 1 (a), and the calculated atomic coordinates are shown in Table 1. Our result agreed with that in Ref. [15] well. In this structure, each Mg directly coordinates with two (BH4) groups and two NH3 groups, forming a tetrahedron (Fig. 1 (b)). Each (BH4) group or NH3 group is not shared by other tetrahedron. So this structure is essentially molecular and built from the tetrahedral molecule Mg(BH4)2·2NH3. The structure is different from such ionic structures as Mg(NH3

Conclusions

Theoretical study of newly developed hydrogen storage material Mg(BH4)2·2NH3 has been carried out to understand its structure and nature of bonding and decomposition. Structure analyses showed that the tetrahedral Mg(BH4)2·2NH3 molecule can be obtained in the crystal phase. The structures of NH3 and (BH4) group in the tetrahedron are similar to that in gas phase. But the tetrahedron has big changes with that in gas phase. Electronic density of states and electron localization function analyses

Acknowledgments

We thank Prof. Zhenyu Zhang for his helpful discussions. The work was supported partly by the NSF of China (Grant Nos: 10974182, 10874154), partly by the Program for Innovative Research Team of Science and Technology in University of Henan Province (Grant No: 2012IRTSTHN003), and partly by Program for Innovation Scientists and Technicians Troop Construction Projects of Henan Province (Grant No: 2009ISTTCPHN-021). The calculations were performed on the High Performance Clusters of Zhengzhou

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