Studied On Synthesis And Hydrogen Storage Performance Of LiBH₄@CMK-8 Composites Constructed By A 3-D Mesoporous KIT-6

The increasing requirements of environmental protection make obtaining clean fuels become very important. Hydrogen as a complete clean energy, due to environment friendly properties, has triggered considerable attentions. Solid hydrogen storage materials storing hydrogen into solid materials by chemical reaction or physical adsorption methods, attributed to its high energy density and good safety, are considered as most promising hydrogen storage methods.. In this paper, LiBH₄ possessing high hydrogen capacity was selected as a research object, and explore the effect of the nanoconfinement on the hydrogen storage properties.Based on the previous work of our laboratory, A mesoporous KIT-6 molecular sieves was synthesized and its synthesis route was optimized. In this synthesis system,tetraethyl orthosilicate（TEOS） acted as silicon source, P123 was used as a template agent. Obtained optimum synthesis conditions of KIT-6 molecular sieves: H+/SiO₂=1.93,BuOH/SiO₂ =1.31, P123/SiO₂=1.732, H₂O/SiO₂=195, crystallization time of 40 h and stirring rate was 600 rpm. Under the condition, the synthesis sample was provided with well degree. Its specific surface area may achieve 733.3 m2/g, and pore size was 6.3 nm,pore volume was 0.86 cm3/g. Mesoporous carbon materials（CMK-8 and CMK-3） were further prepared by a reversable replication method using the KIT-6 and SBA-15 as hard templates, respectively. Thus, the mesoporous carbon materials possessed the same dimensions and pore structures deriving from mesoporous molecular sieves. The mesoporous carbon with well-order degree had the specific surface area of 926.0 m2/g,the pore size of 5.3 nm, and pore volume of 1.34 cm3/g. Finally, LiBH₄ was nanoconfined into a mesoporous carbon by a melt-impregnation strategy. The nanosizing of LiBH₄ particle size was fulfilled. DSC results presented that the activation energy of the nanoconfined LiBH₄ appeared the significant decreasing of 189 to 141 kJ/mol.Correspondingly, its kinetic and thermodynamic performances also were improved. The object of rapid releasing hydrogen was carried on at low temperature. Hydrogen releasing amount of the nanoconfined LiBH₄ were 2.4 wt.% at 350 ℃ in 3000 seconds,respectively and the amount was increased to 3.5 wt.% at 400 ℃ at the same time.However, pristine LiBH₄ only recieved 1.0 wt.% and 1.5 wt.% under the same conditions,respectively. Similarly, the reversible hydrogen absorption was increased to 4.2 wt.%,while the pristine LiBH₄ barely absorbed less 1.0 wt.%. Undoubtedly, the nanoconfinement really had an effect on dehydrogenation and hydrogen adsorption in moderation condition due to the particle nanosizing, the coeffct between pore channels and hydrogen storage materials and the improvements of the kinetic and thermodynamic performances.A porous carbon nitride（MeC₃N₄） material was prepared by the reversible replication method using melamine replacing sucrose and realized the carbon nitride doping. The MeC₃N₄ was used as the supporter of the LiBH₄ nanoconfinemnt. The nanoconfined hydrogen storage material was prepared by the same melt-impregnation method with the mass ratio of 10:1（LiBH₄:MeC₃N₄）. Finally, the results exhibited that the carbon nitride brought about effects on the properties of LiBH₄ hydrogen storage.The most prominent improvement was that the releasing hydrogen temperature dramatically decreased.