Recent Progresses on Complex- and Perovskite-Hydrides for Hydrogen Storage
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0927-EE07-06
Recent Progresses on Complex- and Perovskite-Hydrides for Hydrogen Storage Shin-ichi Orimo, Yuko Nakamori, Kazutaka Ikeda, and Hai-Wen Li Institute for Materials Research, Tohoku University, Sendai, Miyagi, 980-8577, Japan ABSTRACT Recent progresses on two series of light-element hydrides are briefly explained from the viewpoint of 1) prevention/suppression of gas-phase ammonia from mixtures of Mg(NH2)2 and LiH, and also 2) occurrences of reversible dehydriding and rehydriding reactions in NaMgH3. INTRODUCTION Hydrogen storage materials with high volumetric and gravimetric hydrogen densities and also with low dehydriding temperatures are being extensively developed [1]. In this paper, fundamental properties of two series of the light-element hydrides, that is, complex- and perovskite-hydrides, are presented as potential candidates for advanced solid-state hydrogen storage materials. On the complex hydrides (M-B-H and M-N-H systems), we have proposed two developing methods, “substitution of M elements [2]” and “preparation of appropriate mixtures [3]”, with the primary aim of destabilizing the complex hydrides for promoting their dehydriding reactions. In the former method, first principles calculations have been applied to LiBH4 and LiNH2 for characterizing the electronic structures [4]. Based on the theoretical studies, it is found that an effective method for the destabilization is to partially substitute Li with other elements that have larger electronegativity, such as Mg. Experimental results on the dehydriding reactions of LiBH4 and LiNH2 with/without Mg substitutions prove that the dehydriding temperatures of the samples become lower with increasing Mg concentrations. In the latter method, we have also studied mixtures of amide and hydride. The mixture of Mg(NH2)2 with a low decomposition temperature and LiH showing a rapid reaction to ammonia formed probably at the surface of Mg(NH2)2 was selected as one of the best composition ratios for promoting the dehydriding reactions. A problem to be solved was to prevent/suppress the release of gas-phase ammonia from the mixture. On the perovskite hydride, we have reported the formation criteria governed by geometric restrictions of the composed ions using Goldschmidt’s tolerance factor [5]. According to the criteria, a single phase hydride, NaMgH3, was synthesized by mechanical milling of the binary hydrides, NaH and MgH2, at ambient temperature. It was of great importance to obtain experimental results on the reversible dehydriding and rehydriding reactions in the perovskite hydrides. In this paper, therefore, we briefly review 1) the method to prevent/suppress the release of gas-phase ammonia from the mixtures of Mg(NH2)2 and LiH [6], and also 2) the occurrence of the reversible dehydriding and rehydriding reactions in NaMgH3 [7].
EXPERIMENTAL DETAILS On the complex hydrides, the mixture of Mg(NH2)2 and LiH was prepared by the following two methods. Method-1: The mixture of elemental nitrides, Mg3N2 and Li3N, was mechanically milled under argon and
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