Study On The Light Metal Hydrogen Storage Materials Modified By Functional Graphite (Graphene) And Their Corresponding Mechanisms

Due to the high hydrogen storage capacity, light metal borohydrides and magnesium hydride have gained enormous attention during the past decades for hydrogen storage system. For example, the gravimetric hydrogen capacity of L1BH4, NaBH4 and Mg(BH4)2 is 18.5 wt%,10.6 wt% and 14.9 wt%, respectively. However, the practical applications of these borohydrides are restricted by unfavorable high thermal stability, sluggish de/rehydrogenation kinetics and bad reversibility, etc. MgH2 also has very high theoretical gravimetric hydrogen capacity (7.6 wt%) and good reversibility, but hydrogen release from MgH2 is difficult as the bond between Mg and H is too strong. Based on a comprehensive overview of the research and development of light metal borohydrides and MgH2 for hydrogen storage, strategies such as nanoconfinenment, catalytic doping and compositing with hydrides are used along or synergistically to improve their hydrogen storage properties. The microstructure evolution of materials and corresponding catalytic mechanisms are also investigated systematically in this dissertation.Firstly, FGi, which has functional F ion was adopted to modify the hydrogen storage property of NaBH4. The results show that the dehydrogenation temperature and kinetics of NaBH4 modified system could be significantly improved:The onset dehydrogenation temperature of ball-milled 55NaBH4-45FGi composite can be decreased to 125℃ and about 4.8 wt% hydrogen could be released at 130℃in 45 seconds. NaBH4 would react with FGi to produce hydrogen during ball milling and the onset dehydrogenation temperature as well as the hydrogen capacity decreases with the increasing ball milling time. The use of FGi has successfully decreased the dehydrogenation temperature and improved the dehydrogenation kinetics and may shed light on future study on searching for new strategies to improve both the thermodynamics and kinetics of light-metal hydrogen storage materials.Secondly, nano-sized LiBH4 modified with FGi system was easily prepared via ball milling LiBH4 and FGi. The F ion on the surface of FGi could react with LiBH4 to reduce the size of LiBH4 to 90 nm, thus greatly improve its hydrogen storage property. The 50LiBH4-50FGi composite starts to release hydrogen without impurity gas at around 180 ℃, and obtains a hydrogen desorption capacity of 7.2 wt% below 200℃in 30 seconds. The dramatically improved dehydrogenation thermodynamics and kinetics is largely attributed to the nano-modifying effect and the exothermic reaction between LiBH4 and FGi during the dehydrogenation process.β-Mg(BH4)2 is successfully synthesized by ball milling MgCl2 and NaBH4 in the solution of diethyl ether via a wet-chemical method and the size ofγP-Mg(BH4)2 is among the range of 0.1-1 μm. After ball milling Mg(BH4)2 with FGi, Mg(BH4)2 particles become flaky and covers the surfaces of FGi. The desorption of 60Mg(BH4)2-40FGi composite could be started as early as 169.8℃ and completed in 100 seconds, which is largely improved compared with as-synthesized Mg(BH4)2. However, MS measurement reveals that B2H6 and HF are also released with H2, which not only reduce the actual dehydrogenation capacity but also poison the PEMFCs. By modifing the eutectic Mg(BH4)2-LiBH4 composite with FGi, it has been demonstrated that the desorption temperature further decrease to 125.7℃and more importantly, HF is totally suppressed while only a negligible trace of H2B6 can be found in the released gas.Fluorographene (FG), which inherits the properties of graphene and fluorographite (FGi), was successfully fabricated through a simple sonochemical exfoliation route in N-methyl-2-pyrrolidone (NMP) and then was used to modify the hydrogen storage property of LiBH4. The 50LiBH4-50FG composite starts to release hydrogen at 148.1 ℃, which is 46.9℃lower than that of 50LiBH4-50FGi composite. Also, a hydrogen desorption capacity of 8.2 wt% is obtained in 20 seconds. The onset dehydrogenation temperature decreases and the dehydrogenation capacity increases with the increasing amount of FG (from 30 wt% to 50 wt%) below 200℃.In order to avoid the formation of metal florides during de/rehydrogenation process, the as-synthesized FG was adopted to improve the hydrogen storage property of MgH2. DSC measurement shows that the desorption temperature of MgH2-FG composite is 65℃ lower than that of as-received MgH2.The MgH2-FG composite can uptake 6.0 wt% H2 in 5 min and release 5.9 wt% H2 within 50 min at 300℃, while the as-received MgH2 uptakes only 2.0 wt% H2 in 60 min and hardly releases hydrogen at the same condition. TEM observations show that MgH2 particles were embedded in FG layers during ball milling, and the FG at the surface can inhibit the sintering and agglomeration of MgH2 particle, thus it improves the cycling dehydrogenation and rehydrogenation of MgH2-FG composite. The dehydrogenation apparent activation energy for the MgH2 is reduced from 186.3 kJ·mol01 (as-received MgHb) to 156.2 kJ·mol-1 (MgH2-FG composite). The catalytic mechanism has been proposed that F atoms in FG serve as charge-transfer sites and accelerate the rate of hydrogen incorporation and dissociation, consequently enhance the dehydrogenation and rehydrogenation properties of MgH2-FG composite.