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Complex transition metal hydrides have been identified as being materials for multi-functional applications holding potential as thermal energy storage materials, hydrogen storage materials and optical sensors. Na2Mg2NiH6 (2Na+·2Mg2+·2H−·[NiH4]4−) is one such material. In this study, the decomposition pathway and thermodynamics have been explored for the first time, revealing that at 225 °C, hydrogen desorption commences with two major decomposition steps, with maximum H2 desorption rates at 278 and 350 °C as measured by differential scanning calorimetry. The first step of decomposition results in the formation of Mg2NiHx (x < 0.3) and NaH, before these compounds decompose into Mg2Ni and Na, respectively. PCI analysis of Na2Mg2NiH6 has determined the thermodynamics of decomposition for the first step to have a ΔHdes and ΔSdes of 83 kJ mol−1 H2 and 140 J K−1 mol−1 H2, respectively. Hydrogen cycling of the first step has been achieved for 10 cycles without any significant reduction in hydrogen capacity, with complete hydrogen desorption within 20 min at 395 °C. Despite the relatively high cost of Ni, the ability to effectively store hydrogen reversibly at operational temperatures of 318–568 °C should allow this material to be considered as a thermal energy storage material.

The Li-5(BH4)(3)NH complex hydride, obtained by ball milling LiBH4 and Li2NH in various molar ratios, has been investigated. Using X-ray powder diffraction analysis the crystalline phase has been indexed with an orthorhombic unit cell with lattice parameters a = 10.2031(3), b = 11.5005(2), and c = 7.0474(2) angstrom at 77 degrees C. The crystal structure of Lis(BH4)(3)NH has been solved in space group Pnma, and refined coupling density functional theory (DFT) and synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH(4):2Li(2)NH ball-milled and annealed sample. Solid-state nudear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the solved structure. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li+ cation is coordinated by three BH4- and one NH2- anion in a tetrahedral configuration. The room temperature ionic conductivity of the new orthorhombic compound is close to10(-6) S/cm at room temperature, with activation energy of 0.73 eV.

The dehydriding process and hydrogen–deuterium exchange (H–D exchange) of xLiBH4 + (1 − x)Mg2FeD6 (x = 0.25, 0.75) composites has been studied in detail. For the composition with x = 0.25, only one overlapping mass peak of all hydrogen and deuterium related species was observed in mass spectrometry. This implied the simultaneous dehydriding of LiBH4 and Mg2FeD6, despite an almost 190 °C difference in the dehydriding temperatures of the respective discrete complex hydrides. In situ infrared spectroscopy measurements indicated that H–D exchange between [BH4]− and [FeD6]4− had occurred during ball-milling and was promoted upon heating. The extent of H–D exchange was estimated from the areas of the relevant mass signals: immediately prior to the dehydriding, more than two H atoms in [BH4]− was replaced by D atoms. For x = 0.75, H–D exchange also occurred and about one to two H atoms in [BH4]− was replaced by D atoms immediately before the dehydriding. In contrast to the situation for x = 0.25, firstly LiBH4 and Mg2FeD6 dehydrided simultaneously with a special molar ratio = 1:1 at x = 0.75, and then the remaining LiBH4 reacted with the Mg and Fe derived from the dehydriding of Mg2FeD6.

Complex transition metal hydrides have potential technological application as hydrogen storage materials, smart windows and sensors. Recent exploration of these materials has revealed that the incorporation of anionic hydrogen into these systems expands the potential number of viable complexes, while varying the countercation allows for optimisation of their thermodynamic stability. In this study, the optimised synthesis of Na2Mg2TH8 (T = Fe, Ru) has been achieved and their thermal decomposition properties studied by ex situ Powder X-ray Diffraction, Gas Chromatography and Pressure-Composition Isotherm measurements. The temperature and pathway of decomposition of these isostructural compounds differs considerably, with Na2Mg2FeH8 proceeding via NaMgH3 in a three-step process, while Na2Mg2RuH8 decomposes via Mg2RuH4 in a two-step process. The first desorption maxima of Na2Mg2FeH8 occurs at ca. 400 °C, while Na2Mg2RuH8 has its first maxima at 420 °C. The enthalpy and entropy of desorption for Na2Mg2TH8 (T = Fe, Ru) has been established by PCI measurements, with the ΔHdes for Na2Mg2FeH8 being 94.5 kJ mol(-1) H2 and 125 kJ mol(-1) H2 for Na2Mg2RuH8.

The dehydriding process and hydrogen–deuterium exchange (H–D exchange) of x LiBH 4 + (1 − x )Mg 2 FeD 6 ( x = 0.25, 0.75) composites has been studied in detail. For the composition with x = 0.25, only one overlapping mass peak of all hydrogen and deuterium related species was observed in mass spectrometry. This implied the simultaneous dehydriding of LiBH 4 and Mg 2 FeD 6 , despite an almost 190 °C difference in the dehydriding temperatures of the respective discrete complex hydrides. In situ infrared spectroscopy measurements indicated that H–D exchange between [BH 4 ] − and [FeD 6 ] 4− had occurred during ball-milling and was promoted upon heating. The extent of H–D exchange was estimated from the areas of the relevant mass signals: immediately prior to the dehydriding, more than two H atoms in [BH 4 ] − was replaced by D atoms. For x = 0.75, H–D exchange also occurred and about one to two H atoms in [BH 4 ] − was replaced by D atoms immediately before the dehydriding. In contrast to the situation for x = 0.25, firstly LiBH 4 and Mg 2 FeD 6 dehydrided simultaneously with a special molar ratio = 1:1 at x = 0.75, and then the remaining LiBH 4 reacted with the Mg and Fe derived from the dehydriding of Mg 2 FeD 6 .

In previous studies, complex hydrides LiBH4 and Mg2FeH6 have been reported to undergo simultaneous dehydrogenation when ball-milled as composite materials (1 − x)LiBH4 + xMg2FeH6. The simultaneous hydrogen release led to a decrease of the dehydrogenation temperature by as much as 150 K when compared to that of LiBH4. It also led to the modified dehydrogenation properties of Mg2FeH6. The simultaneous dehydrogenation behavior between stoichiometric ratios of LiBH4 and Mg2FeH6 is not yet understood. Therefore, in the present work, we used the molar ratio x = 0.25, 0.5, and 0.75, and studied the isothermal dehydrogenation processes via pressure–composition–isothermal (PCT) measurements. The results indicated that the same stoichiometric reaction occurred in all of these composite materials, and x = 0.5 was the molar ratio between LiBH4 and Mg2FeH6 in the reaction. Due to the optimal composition ratio, the composite material exhibited enhanced rehydrogenation and reversibility properties: the temperature and pressure of 673 K and 20 MPa of H2, respectively, for the full rehydrogenation of x = 0.5 composite, were much lower than those required for the partial rehydrogenation of LiBH4. Moreover, the x = 0.5 composite could be reversibly hydrogenated for more than four cycles without degradation of its H2 capacity.

The dehydriding property of xNaBH4 + (1 1 x) Mg2FeH6 (x = 0.1­0.75) is measured to investigate the effect of combining with complex hydride Mg2FeH6 on reducing dehydriding temperature of metal borohydrides, which has lately been found to be effective for LiBH4. When x = 0.1 and 0.125, a single-step dehydriding reaction is observed, while in the composition range x 2 0.25, a multi-step dehydriding reaction is observed. Despite the different dehydriding process, X-ray diffraction measurements confirmed that Mg2FeH6 and NaBH4 began releasing hydrogen simultaneously over the entire composition range. The results also indicate that the dehydriding temperature of NaBH4 is reduced by at least 150K when combining with Mg2FeH6. [doi:10.2320/matertrans.MG201404]

Combinations of complex metal borohydrides ball milled with the transition metal complex hydride, Mg2FeH6, are analysed and compared. Initially, the Reactive Hydride Composite (RHC) of Mg2+ cation mixtures of Mg2FeH6 and γ-Mg(BH4)2 is combined in a range of molar ratios and heated to a maximum of 450 °C. For the molar ratio of 6 Mg2FeH6 + Mg(BH4)2, simultaneous desorption of the two hydrides occurred, which resulted in a single event of hydrogen release. This single step desorption occurred at temperatures between those of Mg2FeH6 and γ-Mg(BH4)2. Keeping this anionic ratio constant, the desorption behavior of four other borohydrides, Li-, Na-, K-, and Ca-borohydrides was studied by using materials ball milled with Mg2FeH6 applying the same milling parameters. The mixtures containing Mg-, Li-, and Ca-borohydrides also released hydrogen in a single event. The Mass Spectrometry (MS) results show a double step reaction within a narrow temperature range for both the Na- and K-borohydride mixtures. This phenomenon, observed for the RHC systems at the same anionic ratio with all five light metal borohydride mixtures, can be described as simultaneous hydrogen desorption within a narrow temperature range centered around 300 °C.