Li-N-H System

The enthalpy change of the reaction (6.4) was deduced to be -44.5 kj (mol H2) from the database indicated in Chen s report [6]. However, Chen et al. [6] and Kojima and Kawai [10] have carefully measured the PC isothermal curve for the Li-N-H system at different temperatures and evaluated AH from the van t Hoff plot to be —66kJ (mol Hz). Recently, Isobe et al. evaluated AH for hydrogen desorption on the Li-N-H system by the direct measurement of DSC, indicating AH to be -67kJ(mol Hz) [11]. 

The detailed mechanism will be mentioned later in this chapter. 

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The above paper published in Nature by Chen et al. [127] triggered up an avalanche of research papers on various aspects of the hydrogen storage properties of Li-N-H system and its various modifications in the last 5 years. A nnmber of these papers published up to 2006 have already been reviewed [2-5, 7], Fnrther research [128-140] firmly established that the general reaction of (3.25) occnrs in a two-step path ... 

As mentioned in the previous section, the thermodynamic properties of the Li N H system are thought to be unsuitable for practical application, because the... 

Because their group investigated the reaction mechanism [14] of the Li-N-H system, they thought that the elementary step shown in reaction (6.5) needed to be improved. Therefore, Mg(NH2)2 was used instead of LiNH2 because Mg(NH2)2 was expected to be decomposed at a much lower temperature than IiNH2. 

Before that, Chen s group had already focused on the reaction (6.12) as a similar hydrogen storage system to the Li-N-H system [6, 35], which showed reversible hydrogen storage properties at a temperature of more than 500 °C, as shown in Figure 6.6. 

Janot ef al. [48] prepared lithium aluminum amide (LiAl(NH2)4) and used it instead of LiNH2 in the Li-N-H system. The mixture of LiAl(NH2)4 and 4LiH released more than 5 mass % hydrogen at 130 °G. Ho vever, the experimental result of the hydrogenation indicated that this combination vould have a poor reversibility because of the existence of AlN after the dehydrogenation. 

Hu and Ruckenstein [89] tried to improve the kinetics of the Li-N-H system by partial oxidation of Li3N and concluded that this oxidation is highly effective for hydrogen storage with fast kinetics and high reversible hydrogen capacity as well as... 

In order to improve the kinetics of the Li-N-H system, Xie et al. [96] prepared Li2NH hollow nanospheres by plasma metal reaction based on the Kirkendall effect. The special nanostructure showed significantly improved hydrogen storage kinetics compared to that of the Li2NH micrometer particles. The absorption temperature decreased markedly, and the absorption rate was enhanced dramatically because... 

An example of Li amide-imide pressure cycling data is presented, although there are detailed presentations in this book on thermodynamics and structure of these materials by other authors. Chandra et al. [148] determined the effect of long-term pressure cycling between (1, 56, 163, 501, and 1100 cycles) for the Li-N-H system using industrial hydrogen [hydrogen min% (v/v) 99%, water 32 ppm, O2 10 ppm, N2 400 ppm. Total hydrocarbons ... 

Many of the studies that followed the initial report of reversible storage in the Li-N-H system followed two key reaction steps (1) the desorption step where lithium amide combines with lithium hydride to release hydrogen and... 

Given the dual challenges of prohibitively slow uptake kinetics for nitride to imide conversion and the high desorption temperatures to return the amide to the imide for hydrogen release, strategies for the Li-N-H system turned to the modification of particle size and the introduction of additives (e.g. potential catalysts). These aspects are covered in the next section. 

Drawing on methods and observations from earlier hydride storage systems, many of the initial approaches to improving the desorption of hydrogen from the amide in the second stage of the Li-N-H cycle focused on two complementary approaches ball milling to reduce particle size and addition of potential catalysts to reduce desorption temperature. These approaches were first adopted in the Li-N-H system by Ichikawa et al. who used each to try to optimise desorption in the amide-imide reaction (Eqs 16.7 and 16.8)... 

By contrast to the 1 2 reaction, however, it was proposed that the equivalent 3 8 reaction was an ammonia-mediated one by analogy to the findings that had been made in the Li-N-H system [85, 97]. Hence, Mg(NH2)2 first decomposes to Mg3N2 -1- NH3 followed by the reaction of ammonia with LiH to produce Li(NH2) that reacts further with LiH to yield the imide (Eq. 16.28) ... 

Another promising multicomponent system is lithium alanate-lithium amide, this was originally investigated not as a destabilised multicomponent system, but as a means of utilising the lithium amide to destabilise the lithium hydride formed from the decomposition of lithium alanate (i.e. following the reactions for the Li-N-H system as reported by Chen et al. (2002)) and hence release all the hydrogen contained within the alanate (Lu and Fang, 2005) ... 

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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