ChemInform Abstract: Bonding and Stability of the Hydrogen Storage Material Mg2NiH4

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Distribution and dynamics of hydrogen in the low-temperature phase of Mg(2)NiH(4) studied by solid-state NMR

Distribution and dynamics of hydrogen atoms in the low-temperature phase of Mg(2)NiH(4) have been studied by means of (2)H and (1)H NMR for Mg(2)NiD(4) and Mg(2)NiH(4), respectively. (2)H NMR spectra have been measured in the temperature range between 200 and 340 K, and the line shapes were simulated. The temperature dependence of (2)H NMR spectra was quite well simulated assuming a distorted tetrahedral configuration and a pseudoisotropic rotation of the NiD(4) unit. The estimated jump frequency obeyed Arrhenius relation with a frequency factor of (0.8 +/- 0.6) x 10(13) Hz and an activation energy of 50.1 +/- 1.4 kJ/mol. (1)H NMR spectra were acquired from 240 to 360 K. The observed (1)H second moments were 202 kHz(2) in the rigid lattice (240 K) and 46.6 kHz(2) in a motional state (360 K). The value in the rigid lattice supported the tetrahedron model, and the value in a motional state indicated the isotropic rotation of the NiH(4) unit. Conclusively, the NiH(4) unit has the distorted tetrahedral configuration and undergoes the pseudoisotropic rotation.     

Superior hydrogen storage properties of LiBH4 catalyzed by Mg(AlH4)2

Mg(AlH(4))(2) is found to provide a synergistic effect on improving the de-/rehydrogenation properties of LiBH(4). The Mg(AlH(4))(2)-catalyzed LiBH(4) exhibits lower dehydrogenation temperature and faster de-/rehydrogenation kinetics than the individually MgH(2)- or Al-catalyzed LiBH(4).     

A new synthetic route to Mg(2)Na(2)NiH(6) where a [NiH(4)] complex is for the first time stabilized by alkali metal counterions

Mg2Na2NiH6 was synthesized by reacting NaH and Mg2NiH4 at 310 degrees C under hydrogen pressure. The novel structure type was refined from neutron-diffraction data in the orthorhombic space group Pnma (No. 62), with unit cell dimensions of a = 11.428(2), b = 8.442(2), and c = 5.4165(9) Angstrom and a unit cell volume = 523 Angstrom(3) (Z = 4). The structure can be described by (Mg2H2)(2+) layers intersected by (Na2NiH4)(2-) layers. The [NiH4](4-) complex is approximately tetrahedral, indicating formal zerovalent nickel. This is the first example of a solid-state hydride where a [NiH4](4-) complex is directly stabilized by alkali metal ions instead of the more polarizing Mg(2+) ions. A rather long nickel-hydrogen bond distance of 1.65 Angstrom indicates a weaker Ni-H bond as a result of the weaker support from the less polarizing alkali metal counterions.     

Mg2NiH4对高氯酸铵热分解过程的影响

采用置换-扩散法制备了储氢材料Mg2NiH4, 用XRD, ICP和DSC-TG方法对其结构进行了表征. 用热分析法(DSC)研究了Mg2NiH4对高氯酸铵(AP)热分解过程的影响. 研究结果表明, Mg2NiH4对AP热分解过程有较大影响. Mg2NiH4可以显著促进AP的低温热分解过程, 降低高温热分解温度, 使DSC表观分解热明显增大. 随着加入量的增加, Mg2NiH4对AP热分解的催化促进作用增强, 当Mg2NiH4加入的质量分数为30％时, DSC表观分解热最大. 吸氢量越大, 储氢材料对AP的催化促进作用越强. Mg2NiH4催化促进AP分解过程的作用机理为: Mg2NiH4分解释放的H2及Mg和Ni与AP分解产物发生反应.     

Ammine magnesium borohydride complex as a new material for hydrogen storage: structure and properties of Mg(BH4)2.2NH3

The ammonia complex of magnesium borohydride Mg(BH4)2.2NH3 (I), which contains 16.0 wt % hydrogen, is a potentially promising material for hydrogen storage. This complex was synthesized by thermal decomposition of a hexaaammine complex Mg(BH4)2.6NH3 (II), which crystallizes in the cubic space group Fm3 m with unit cell parameter a=10.82(1) A and is isostructural to Mg(NH3) 6Cl2. We solved the structure of I that crystallizes in the orthorhombic space group Pcab with unit cell parameters a=17.4872(4) A, b=9.4132(2) A, c=8.7304(2) A, and Z=8. This structure is built from individual pseudotetrahedral molecules Mg(BH4)2.2NH3 containing one bidentate BH4 group and one tridentate BH4 group that pack into a layered crystal structure mediated by N-H...H-B dihydrogen bonds. Complex I decomposes endothermically starting at 150 degrees C, with a maximum hydrogen release rate at 205 degrees C, which makes it competitive with ammonia borane BH 3NH3 as a hydrogen storage material.     

Potassium silanide (KSiH3): a reversible hydrogen storage material

KSi silicide can absorb hydrogen to directly form the ternary KSiH₃ hydride. The full structure of α‐KSiD₃, which has been solved by using neutron powder diffraction (NPD), shows an unusually short Si? D lengths of 1.47 Å. Through a combination of density functional theory (DFT) calculations and experimental methods, the thermodynamic and structural properties of the KSi/α‐KSiH₃ system are determined. This system is able to store 4.3 wt % of hydrogen reversibly within a good P–T window; a 0.1 M Pa hydrogen equilibrium pressure can be obtained at around 414 K. The DFT calculations and the measurements of hydrogen equilibrium pressures at different temperatures give similar values for the dehydrogenation enthalpy (≈23 kJ mol^?1 H₂) and entropy (≈54 J K^?1 mol^?1 H₂). Owing to its relatively high hydrogen storage capacity and its good thermodynamic values, this KSi/α‐KSiH₃ system is a promising candidate for reversible hydrogen storage.     

Functions of MgH(2) in hydrogen storage reactions of the 6LiBH(4)-CaH(2) reactive hydride composite

A significant improvement of hydrogen storage properties was achieved by introducing MgH(2) into the 6LiBH(4)-CaH(2) system. It was found that ～8.0 wt% of hydrogen could be reversibly stored in a 6LiBH(4)-CaH(2)-3MgH(2) composite below 400 °C and 100 bar of hydrogen pressure with a stepwise reaction, which is superior to the pristine 6LiBH(4)-CaH(2) and LiBH(4) samples. Upon dehydriding, MgH(2) first decomposed to convert to Mg and liberate hydrogen with an on-set temperature of ～290 °C. Subsequently, LiBH(4) reacted with CaH(2) to form CaB(6) and LiH in addition to further hydrogen release. Hydrogen desorption from the 6LiBH(4)-CaH(2)-3MgH(2) composite finished at ～430 °C in non-isothermal model, a 160 °C reduction relative to the 6LiBH(4)-CaH(2) sample. JMA analyses revealed that hydrogen desorption was a diffusion-controlled reaction rather than an interface reaction-controlled process. The newly produced Mg of the first-step dehydrogenation possibly acts as the heterogeneous nucleation center of the resultant products of the second-step dehydrogenation, which diminishes the energy barrier and facilitates nucleation and growth, consequently reducing the operating temperature and improving the kinetics of hydrogen storage.     

Mg7(AsO4)2(HAsO4)4: a new magnesium arsenate with a very strong hydrogen bond

A new compound, heptamagnesium bis­(arsenate) tetrakis(hydrogenarsenate), Mg₇(AsO₄)₂(HAsO₄)₄, was synthesized by a hydro­thermal method. The structure is based on a three‐dimensional framework of edge‐ and corner‐sharing MgO₆, MgO₄(OH)₂, MgO₅, AsO₃(OH) and AsO₄ polyhedra. Average Mg—O and As—O bond lengths are in the ranges 2.056–2.154 and 1.680–1.688 Å, respectively. Each of the two non‐equivalent OH groups is bonded to both an Mg and an As atom. One OH group is involved in a very short hydrogen bond [O⋯O = 2.468 (3) Å]. The formula unit is centrosymmetric, with all atoms in general positions except for one Mg atom, which has site symmetry . The compound is isotypic with Mn₇(AsO₄)₂(HAsO₄)₄ and M₇(PO₄)₂(HPO₄)₄, where M is Fe, Co or Mn.     

Efficient hydrogen storage with the combination of lightweight Mg/MgH2 and nanostructures

Efficient hydrogen storage plays a key role in realizing the incoming hydrogen economy. However, it still remains a great challenge to develop hydrogen storage media with high capacity, favourable thermodynamics, fast kinetics, controllable reversibility, long cycle life, low cost and high safety. To achieve this goal, the combination of lightweight materials and nanostructures should offer great opportunities. In this article, we review recent advances in the field of chemical hydrogen storage that couples lightweight materials and nanostructures, focusing on Mg/MgH(2)-based systems. Selective theoretical and experimental studies on Mg/MgH(2) nanostructures are overviewed, with the emphasis on illustrating the influences of nanostructures on the hydrogenation/dehydrogenation mechanisms and hydrogen storage properties such as capacity, thermodynamics and kinetics. In particular, theoretical studies have shown that the thermodynamics of Mg/MgH(2) clusters below 2 nm change more prominently as particle size decreases.     

(Nd(1.5)Mg(0.5))Ni7-based compounds: structural and hydrogen storage properties

The structural and hydrogen storage properties of (Nd(1.5)Mg(0.5))Ni(7)-based alloys (i.e., A(2)B(7)-type) with a coexistence of two structures (hexagonal 2H and rhombohedral 3R) are investigated in this study. In both 2H- and 3R-type A(2)B(7) structures, Mg atoms occupy Nd sites of Laves-type AB(2) subunits rather than those of AB(5) subunits because Mg substitution for Nd in the AB(2) subunits more significantly strengthens the ionic bond in the system. An increase in the A-atomic radius or the B-atomic radius stabilizes the 2H structure, but a decrease in the A-atomic radius or the B-atomic radius is favorable for formation of the 3R structure. The 2H-A(2)B(7) and 3R-A(2)B(7) phases in each alloy have quite similar equilibrium pressures upon hydrogen absorption and desorption, which show a linear relationship with the average subunit volume. The hydriding enthalpy for the (Nd(1.5)Mg(0.5))Ni(7) compound is about -29.4 kJ/mol H(2) and becomes more negative with partial substitution of La for Nd and Co/Cu for Ni but less negative with partial substitution of Y for Nd.     

A reality check on using NaAlH4 as a hydrogen storage material

Sodium aluminum hydride or sodium alanate (NaAlH₄) has been considered as a potential material for hydrogen storage. Although its theoretical hydrogen storage capacity is 5.5 wt.% at 250 °C, the material still has its drawback in the regeneration issue. With the use of certain catalysts, the regeneration problem can somewhat be alleviated with added benefits in the decrease in the hydrogen decomposition temperature and the increase in the decomposition rate. This work summarizes what we have learned from the decomposition of NaAlH₄ with/without catalysts and co-dopants. The decomposition was carried out using a thermovolumetric apparatus. For the tested catalysts—HfCl₄, VCl₃, TiO₂, TiCl₃, and Ti—the decomposition temperature of the hydride decreases; however, they affect the temperature in the subsequent cycles differently and TiO₂ appears to have the most positive effect on the temperature. Sample segregation and the morphological change are postulated to hinder the reversibility of the hydride. To prevent the problems, co-dopants—activated carbon, graphite, and MCM-41—were loaded. Results show that the hydrogen reabsorption capacity of HfCl₄- and TiO₂-doped NaAlH₄ added with the co-dopants increases 10–50% compared with that without a co-dopant, and graphite is the best co-dopant in terms of reabsorption capacity. In addition, the decomposition temperature in the subsequent cycles of the co-dopant doped samples decreases about 10–15 °C as compared to the sample without a co-dopant. Porosity and large surface area of the co-dopant may decrease the segregation of bulk aluminum after the desorption and improve hydrogen diffusion in/out bulk of desorbed/reabsorbed samples.     

Modified lithium borohydrides for reversible hydrogen storage (2)

This paper reports the results of the effort to destabilize lithium borohydride for reversible hydrogen storage. Various metals, metal hydrides, and metal chlorides were selected and evaluated as destabilization agents for reducing dehydriding temperatures and improving dehydriding/rehydriding reversibility. The most effective material was LiBH4 + 0.2MgCl2 + 0.1TiCl3 which starts desorbing 5 wt % of hydrogen at 60 degrees C and can be rehydrogenated to 4.5 wt % at 600 degrees C and 70 bar. X-ray diffraction and Raman spectroscopic analysis show the interaction of LiBH4 with additives and the unusual change of B-H stretching.     

Mg5TiO4(BO3)2

Single crystals of pentamagnesium titanium(IV) tetraoxide bis(borate), Mg₅TiO₄(BO₃)₂, were prepared by slow cooling of the melt from 1623 K in air. The crystal is isostructural with the mineral ludwigite (Mg₂FeO₂BO₃). The Mg and Ti atoms are coordinated by six O atoms and the B atom is coordinated by three O atoms. There are three Mg sites and one mixed site statistically occupied by Mg and Ti atoms. Atoms are at the following special positions: 2a (0, 0, 0) and 2d (0, , ) for two Mg atoms, 4g (x, y, 0) for the mixed Ti/Mg site and the BO₃ group, and 4h (x, y, ) for a third Mg and two oxide O atoms. MgO₆ and (Ti/Mg)O₆ octahedra are connected by sharing of edges to form zigzag folding layers along the c axis. Triangular prismatic tunnels are formed between the folding layers by sharing apical O atoms of the MgO₆ and (Ti/Mg)O₆ octahedra.     

Syntheses and Structures of Na2Mg3(OH)2(SO4)3·4H2O and K2Mg3(OH)2(SO4)3·2H2O

The crystal structures of Na₂Mg₃(OH)₂(SO₄)₃ · 4H₂O and K₂Mg₃(OH)₂(SO₄)₃ · 2H₂O, were determined from conventional laboratory X‐ray powder diffraction data. Synthesis and crystal growth were made by mixing alkali metal sulfate, magnesium sulfate hydrate, and magnesium oxide with small amounts of water followed by heating at 150 °C. The compounds crystallize in space group Cmc2₁ (No. 36) with lattice parameters of a = 19.7351(3), b = 7.2228(2), c = 10.0285(2) Å for the sodium and a = 17.9427(2), b = 7.5184(1), c = 9.7945(1) Å for the potassium sample. The crystal structure consists of a linked MgO₆–SO₄ layered network, where the space between the layers is filled with either potassium (K⁺) or Na⁺‐2H₂O units. The potassium‐bearing structure is isostructural to K₂Co₃(OH)₂(SO₄)₃ · 2(H₂O). The sodium compound has a similar crystal structure, where the bigger potassium ion is replaced by sodium ions and twice as many water molecules. Geometry optimization of the hydrogen positions were made with an empirical energy code.