Dehydrogenated state

For the case of Li/Mg = 8/3 , Nakamura et al. determined the phase of Li2.6MgN2Di 4 as a dehydrogenated state (in vacuum conditions, at 200 °C) by using synchrotron radiation (SR)-X-ray and neutron diffraction [109]. For Li/Mg = 6/3, 8/3, 12/3 systems, Aoki et al. performed PC-isotherm measurements only for the dehydrogenation at 250 °C and observed the complex imide Li2Mg(NH)2 as a dehydrogented state by SR-XRD [110]. Considering all these experimental results, the reaction steps of the Li-Mg-N-H system can be stated as below,... 

As shown above, one possible strategy to lower the absolute value of reaction enthalpies in lightweight hydrides is to look for elements, which exhibit negative heats of mixing with the hydride-forming elements/compounds and thus stabilize the dehydrogenated state. 

Lightweight Hydrides Forming Stable Compounds in the Dehydrogenated State... 

As discussed above, due to the absence of any stable compound in the dehydrogenated state and the formation of a rather stable hydride the desorption enthalpy is not decreased compared to the original single alkali composed hydrides. Fossdal et al. [58] measured pressure-composition isotherms for TiF3-enhanced material in the temperature range 170-250 °C. From the van t Floff plot they determined a dissociation enthalpy of 56.4 0.4kJ (mol Fl2) and a corresponding entropy of 137.9 0.7 (mol H2). The respective value of the reaction enthalpy of reaction (7.4) is 47 kj (mol H2) [19]. 

Other combinations of hydrides with amides have been considered. Nakamori et al 80 83 others have looked at mixtures of Li amide with Li alanate and with Li borohydride. DFT calculations suggest that these systems will behave in a fashion similar to that of the destabilized borohydrides described above that is, the reaction products in the dehydrogenated state should have lower enthalpies than the borohydride or the alanate alone. The potential yields are higher than with LiH. For the borohydride case, the expected reaction pathway is... 

The composites of Mg(NH2)2 and LiH are independently reported by Luo et al. [102], Leng et al. [128], Xiong et al. [103], and Nakamori et al. [120] almost at the same time as the developed Li-N-H system, which are synthesized with different molar ratios of components. Although the hydrogen desorption properties of each composites are almost the same, the reversible hydrogen capacities and dehydrogenated states are different depending on the ratios as follows ... 

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