Magnesium hydride, MgH2, is one of the most promising candidates for hydrogen storage due to its high reversible hydrogen storage capacity (theoretically up to 7.6 wt. %) and low cost ($3/kg). However, the relatively slow kinetics and high operating temperature limit its commercial application. In this research, the lightest transition metal, scandium (Sc), was melted together with magnesium to form a bulk MgO.6SSC0.3S alloy which was phase separated to a nanostructured MgO.6SSCO.3S-H system during its first hydrogenation. The hydrogen storage properties of the MgO.6SSC0.3s-H system were thoroughly evaluated. The results showed a high reversible hydrogen storage capacity (4.2 ± 0.1 wt.%), fast kinetics with an activation energy of 82 ± 5 klmor' and good cycling stability. The enthalpy and entropy values were 77.4 ± 0.9 klmor' (H2) and 141 ± 2 J.mor'.K' (H2) respectively. The nanostructure comprised a Sc-rich hydride (MgO.07SCO.93H2.32) nano-cluster distributed within the MgH2 phase with grain sizes ranging from 40 to 100 nm. The in-situ powder neutron diffraction showed that the reaction pathway, phase separation, formed a ScDx-rich phase and an intermediate phase. It is hypothesised that the nano-sized Sc-rich hydride, which has a fluOl'ite structure and higher mobility of hydrogen, provides a fast diffusion pathway to the magnesium core via the grain boundary. In addition, the nano-structure formed after phase separation was stable and remained during cycling. The Sc-rich hydride nano-clusters also function as a grain refiner and reduce the grain growth rate of Mg/MgH2 during cycling. Abstract A xMgH2/(l-x)ScH2 system was also investigated to explore the effect of the ball milling duration and the ScH2 catalyst content on the kinetics of MgH2 dehydrogenation. It was found that the optimal content of the catalyst ScH2 was ca. 12 at. %, which was achieved by the 0.90MgH2/O.lOScH2-40h ball milled sample with an activation energy value of 62 ± 5 kJ.mor i and the reversible hydrogen storage capacity of 5.7 ± 0.1 wt.%. Alternative Sc-based alloys (ScSi and ScAh) were investigated as candidates to reduce the enthalpy of dehydrogenation of MgH2. The samples were prepared by ball milling Mg or MgH2 together with the synthesised ScSi or ScAh alloy at different composition ratios. It was found that the ScSi phase separated to SCH2 and Mg2Si, a process that was irreversible during the hydrogenation cycles of the 3Mg:ScSi-lOh sample. In contrast, the ScAl3 remained unchanged over three cycles but achieved fast kinetics; the decomposition reaction reached its equilibrium within 35 min at 354 QC. 11
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:605586 |
| Date | January 2013 |
| Creators | Luo, Xuanli |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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