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Hydrogen Storage Properties of 3Mg(NH2)2–2Li3AlH6

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journal contribution
posted on 12.01.2012, 00:00 by Dongan Liu, Andrea Sudik, Jun Yang, Patrick Ferro, Chris Wolverton
Recent efforts have been made to develop high-capacity complex hydride composites by combining alanates and amides. The hydrogen storage mechanisms in those composites are not unambiguously clarified because of chemical reactions during the sample preparation process. In this Article, we have studied the effects of sample preparation conditions on the phase stability of a mixture of 3Mg(NH2)2–2Li3AlH6 and identified that unlike high-energy ball-milling light mixing generates a physical mixture of the reactants without decomposition. Subsequently, the hydrogen storage properties, the desorption pathway, and the reversible reaction mechanism of the composite were investigated through a combination of kinetic measurements and phase and microstructure analyses. The results reveal that the first step of hydrogen release (initiated at 170 °C) involves decomposition of Li3AlH6 to LiH and Al. The second step of hydrogen release occurs as the temperature increases (to 230 °C) when Mg(NH2)2 reacts with LiH to form Li2Mg(NH)2. If desorption of the 3Mg(NH2)2–2Li3AlH6 mixture is limited to a temperature of 400 °C, then the reversible reaction takes place between Li2Mg(NH)2 (plus H2) and LiH and Mg(NH2)2. We find that the Al generated from the first hydrogen release step does not participate in the reversible hydrogen storage process and instead inhibits mass transfer that results in higher desorption temperatures (and low desorption rates) and lowers the overall reversible capacity (2.7 wt %) as compared with the neat reaction (i.e., ∼4.3 wt % without the presence of Al).