We have investigated the hydrogen storage properties of a ball-milled mixture of 3Mg(NH 2 ) 2 and 8LiH after first synthesizing Mg(NH 2 ) 2 by ball milling MgH 2 under an atmosphere of NH 3 gas at room temperature. The thermal desorption mass spectra of the mixture without any catalysts indicated that a large amount of hydrogen (∼7 wt %) was desorbed from 140 °C, and the desorption peaked at ∼190 °C under a heating rate of 5 °C/min with almost no ammonia emission. Moreover, the reversibility of the hydrogen absorption/desorption reactions was confirmed to be complete. The above results indicate that this system is one of the promising metal-N-H systems for hydrogen storage.

Kinetics of hydrogen absorption and desorption reactions was investigated on the MgH 2 composite doped with 1 mol% Nb 2 O 5 as a catalyst by ballmilling. The composite after dehydrogenation at 200 • C absorbed gaseous hydrogen of ∼4.5 mass% even at room temperature under lower pressure than 1 MPa within 15 s and finally its capacity reached more than 5 mass%. On the other hand, the catalyzed MgH 2 after rehydrogenation desorbed ∼6 mass% hydrogen at 160 • C under purified He flow, which followed the first order reaction. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be ∼71 kJ/mol H 2 , indicating the product was significantly activated due to the catalytic effect of Nb 2 O 5 .

After some metal amides M(NH 2) x such as LiNH 2 , NaNH 2 , Mg(NH 2) 2 and Ca(NH 2) 2 were synthesized by ball milling the corresponding metal hydrides MH x under ammonia atmosphere at room temperature, their thermal decomposition properties were examined, which play important roles for designing a new family of novel Metal-N-H systems. The results indicate that the kinetics of their synthesizing reactions are faster in the order of Na amide > Li amide > Ca amide > Mg amide, while both Mg(NH 2) 2 and Ca(NH 2) 2 decompose and emit NH 3 at lower temperature than LiNH 2 .

The Li-Mg-N-H system composed of 3 Mg(NH2)2 and 8 LiH reversibly desorbs/absorbs approximately 7 wt % of H2 at 120-200 degrees C and transforms into 4 Li2NH and Mg3N2 after dehydrogenation. In this work, the mechanism of the hydrogenation reaction from 4 Li2NH and Mg3N2 to 8 LiH and 3 Mg(NH2)2 was investigated in detail. Experimental results indicate that 4 Li2NH is first hydrogenated into 4 LiH and 4 LiNH2. At the next step, 4 LiNH2 decomposes into 2 Li2NH and 2 NH3, and the emitted 2 NH3 reacts with (1/2) Mg3N2 and produces the (3/2) Mg(NH2)2 phase, while the produced 2 Li2NH is hydrogenated into 2 LiH and 2 LiNH2 again. Such successive steps continue until all 4 Li2NH and Mg3N2 completely transform into 8 LiH and 3 Mg(NH2)2 by hydrogenation.

Hydrogen desorption and absorption properties of magnesium borohydride (Mg(BH4)2) were studied for three cycles. Effect of cobalt additives and their local structure upon cycling were investigated in detail.

The decomposition and rehydrogenation of γ-Mg (BH 4 )

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a Hydrogen-fluorine exchange in the NaBH 4 -NaBF 4 system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF 2 H 2 in the temperature range T = 200 to 215 1C. Combined use of solid-state 19 F MAS NMR, FT-IR and DFT calculations supports the formation of a BF 2 H 2 complex ion, reproducing the observation of a 19 F chemical shift at 144.2 ppm, which is different from that of NaBF 4 at 159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at B310 1C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF 3 or amorphous products such as closo-boranes, e.g. Na 2 B 12 H 12 . The NaBH 4 -NaBF 4 composite decomposes at lower temperatures (300 1C) compared to NaBH 4 (476 1C), as observed by thermogravimetric analysis. NaBH 4 -NaBF 4 (1 : 0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH 4 as measured using Sievert's method under identical conditions, but more than 50% using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na 2 B 12 H 12 . On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH 4 -NaF.