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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Hydrogen sorption mechanisms in lithium amide and metal hydride reactive systems

Author: YAO, JInhan

Year: 2007

Supervisor(s): Xiao Guo

Considerable effort has been devoted to the Li-N-H system for solid-state hydrogen storage. The desorption characteristics of LiNH2 and a mixture of (LiNH2 + LiH) were firstly comparatively studied for further understanding of H2 desorption in the (LiNH2 + LiH) system. Mass spectrometry and thermal analysis of (LiNH2 + LiH) mixtures indicate that approximately 5 mass% of H2 is released at 180°C after four hours of milling without any apparent release of NH3, whereas insufficient mixing of the two compounds cannot stop the escaping of NH3 from the mixture. Non-uniform mixing can lead to the escape of NH3 from the mixture. The evidence further support the notion that NH3 intermediated reaction is a possible reaction path within the thermal desorption of the (LiNH2 + LiH) mixture. BN additive, among selected nitrides shows the best effect on desorption from (LiNH2 + LiH). (LiNH2 + MgH2) materials with different molar ratios (4:3,4:2 and 4:1) were also studied on their sorption properties and mechanisms. Results show that more than 6 mass% H2 is desorbed from 150°C for the (4LiNH2 + 3MgH2) mixture, with two H2 peaks at 200 and 320°C. Meanwhile, there is only ~ 5 mass% for (4LiNH2 + 2MgH2) mixture with one H2 peak at 200°C. Reversibility measurements suggest that LiNH2 and MgH2 cannot be recovered after absorption; instead, Li2NH and Mg(NH2)2 (or MgNH) take over to perform the H2 storage functions. The (4LiNH2 + 3MgH2) mixture possesses a greater H2 capacity in first desorption, but shows less than 2 mass% reversible capacity in subsequent cycles. However, there is only about 1mass% capacity loss during the reversibility measurement for the (4LiNH2 + 2MgH2) mixture. Other M-N-H systems, mainly NaH, KH, AlH3 and CaH2, were also investigated, and only CaH2 shows the capability of reacting with LiNH2 to produce H2 among these candidates.