Due to its high hydrogen storage capacity of 18.5 wt.%, LiBH4 has gained much attention as a potential onboard hydrogen storage medium for automotive applications. Unfortunately, LiBH4 only decomposes fully above 600 QC, and hydrogenation does not occur below 600 QC and requires hydrogen pressures of at least 350 bar. However, these conditions can be significantly improved by thermodynamic tuning. In this study, LiBH4 was augmented with two different AI-sources (metallic Al and from the decomposition of LiAlH4) and intermetallic alloys FeTi and CaNis. The effectiveness of the various additives on the dehydrogenationlhydrogenation behaviour was investigated along with the efficacy of using a catalyst. For 2LiBH4:LiAlH4 longer ball-milling times (4 h) in the presence of TiCb resulted in higher H2 release than reported in the literature. In addition, a lower dehydrogenation temperature and improved reversibility (under 85 bar H2 and at 350 QC) were achieved for the LiAlH4-containing samples than in the case of metallic AI. The TiCh was found to catalyse the dehydrogenation of LiAlH4 during ball milling, resulting in highly dispersed Al through the LiB~. This proved to be a more effective route to deliver the Al destabilisation agent, leading to higher capacities and improved reversibility of the system. Pre-milling the individual components together prior to the addition of the Ti-catalyst was found to be detrimental to the system, resulting in higher dehydrogenation temperatures than achieved by co-milling all the reagents. The enthalpy of dehydrogenation was found to be 38.2 kJ mor' (H2) 1 Abstract and the temperature for a 1 bar equilibrium hydrogen pressure was calculated to be in the range 240 - 300 QC. Addition of the intermetallic FeTi showed no evidence of lowering the dehydrogenation temperature of LiBfu. The presence of Pd in the 2LiBH4:FeTi(Pd) 4 h milled sample showed the dehydrogenation temperature to occur at 313 QC, which is 59 QC lower compared to the corresponding uncatalysed sample. From the XRD measurements on the decomposed material, no evidence for the formation of titanium- or iron borides was found, as only diffraction peaks for the FeTi alloy were identified. This would suggest that the observed lowering of the dehydrogenation temperature is most likely a catalytic effect than a thermodynamic effect. Furthermore, the system proved to be irreversible under the investigated conditions of 2 h at 350 QC under 85 bar of H2, and is most likely due to the stability of the intermetallic FeTi alloy. Finally, the addition of CaNis as an alternative nickel source to metallic Ni in order to tune thermodynamically the dehydrogenation temperature of LiBH4 was found to start decomposing at 11 OQC. This was identified through in-situ neutron diffraction measurement by an increase in deuteride pressure and formation of LiD, which was completed by 200 QC showing all of the LiBH4 had decomposed in the solid state (i.e. < 270QC). In-situ neutron diffraction also identified the formation of the nickel borides NbB and NhB at temperatures above 250 QC. Attempts to deuteride these end products with 11 Abstract isothermal experiments at 200 DC' were unsuccessful, however; isothermal experiments at or below 175 DC proved to be successful (> 1.5 wt.%). III
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:605573 |
| Date | January 2012 |
| Creators | Meggouh, Mariem |
| Publisher | Nottingham Trent University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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