title: Novel solid state materials for chemical hydrogen storage
Creators: name: Liu, Zhe
Publisher: name: University of Glasgow
Year: 2017
Subjects: subject: hydrogen storage, metal hydrides, complex hydrides, crystallorgraphy.

description: This work investigates the hydrogen storage potential of a variety of solid state materials. The work has showed their synthesis, structure, morphology and hydrogen storage properties comprehensively. An MgH2 nanocomposite composed of 80% tetragonal α-MgH2 and 18% orthorhombic γ-MgH2 has been prepared for the first time without recourse to high pressure or temperature. By optimizing the ball milling conditions, addition of LiCl and use of THF solvent, the α-/γ-MgH2 nanocomposite so-produced is capable of releasing 6.6 wt% H2 with rapid kinetics, from ca. 260 °C without the use of a catalyst. Moreover, Ti-catalyzed MgH2 offering a capacity of 5.5 wt. % of H2 and superior hydrogen desorption kinetics has been successfully prepared by a novel wet chemical route. The MgH2 material, containing approximately 2~3 wt. % of Ti-additive exhibits hydrogen desorption at a temperature approximately 220 °C lower than pristine MgH2 where pure hydrogen evolution starts at ca. 420 °C via a synergetic effect of mechanochemical treatment and additives. Neutron scattering was employed to study the structure of activated MgD2 and for the first time local disorder in activated MgD2 has been verified using total neutron scattering (PDF fitting). Small angle neutron scattering (SANS) analysis indicates a surface fractal geometry, i.e high degree of surface roughness for activated MgD2 particles, in accordance with SEM analysis suggesting the morphological alteration introduced by mechanochemical treatment. A novel PANI-LiBH4 composite has been successfully fabricated through simple mixing. It is found that PANI-LiBH4 composites dehydrogenates from ca. 200 °C with over 10 wt.% H2 released by 400 °C, significantly outperforming pristine LiBH4. Importantly, rehydrogenation can be achieved under conditions unprecedented for LiBH4 in isolation (200 °C; 100 bar H2 or 330 °C, 20 bar H2 vs. 600 °C, 350 bar H2). Moreover, the PANI-LiBH4 composite can be readily cycled and a new endothermic uptake event at 140 °C, a remarkably low temperature for LiBH4-based systems, suggests that the polymer thermodynamically alters the hydrogenation mechanism. PANI-NaBH4 and PANI-LiH also exhibit vastly improved dehydrogenation properties compared with the respective hydride materials alone. The structures of some first row transition metal halide hydrazinates, TMX2·2N2H4 (TM= Mn, Fe, Co, Ni, Cu and Zn; X= Cl and Br), have been revisited and detailed structural information of three typical complexes, MnCl2·2N2H4, ZnCl2·2N2H4 and MnBr2·2N2H4 have been accurately determined by using a combination techniques of PXD, FTIR and PND. It is also found that TMX2·2N2H4 decomposes at relatively high temperature (&gt; 250 °C) with massive weight loss due to the dissociation and decomposition of the N2H4 ligand. However the major gas evolution has been determined to be N2 and NH3 with only a minor amount of H2 (and undesired impurity N2H2) released, which makes TMX2·2N2H4 unsuitable for hydrogen storage. Our strategy to combine TMCl2·2N2H4 with LiBH4 to fabricate novel transition metal borohydride hydrazinates has been proven to be successful. Two novel complexes, Mn(BH4)2·2N2H4 and Zn(BH4)2·2N2H4 have been successfully prepared via a facile mechanochemical route with careful manipulation over the milling parameters. The crystal structure of Mn(BH4)2·2N2H4 has been determined using SR-PXD to be isostructural with its parent material MnCl2·2N2H4. The phase evolution behaviour of Zn(BH4)2·2N2H4 has been probed with evidence of various intermediate phases during preparation when various milling conditions were employed. The dehydrogenation properties of both complexes have been studied using DTA-TGA coupled with MS. Mn(BH4)2·2N2H4 and Zn(BH4)2·2N2H4 are very promising materials for off-board hydrogen storage due to their high hydrogen content and useful dehydrogenation properties.