Study Of The Modification And Mechanisms Of LiBH₄-based Materials For Hydrogen Storage Of Based On CaB₆ Structure Products

Developing safe,high-efficiency and reversible solid-state hydrogen storage technologies is critical to promote the practical utilization of hydrogen energy.LiBH4,with a theoretical hydrogen storage capacity of 18.5 wt%,has attracted intense interest as a high-density hydrogen storage material.Based on the domestic and foreign studies,the hydrogen storage composites with CaB6 cubic crystal structure products are fully researched.Several methods,such as incorporating metal hydrides,catalytic doping and nanoconfinement,are selected to modify LiBH4-based composite system.Then,the structure and composition during the de-/rehydrogenation and their corresponding mechanisms are also investigated systematically.The hydrogen storage performance of 6LiBH4-SrH2 system significantly improved by combining with MgH2.6LiBH4-SrH2-3MgH2 was test to be an optimized combination,with the terminate desorption temperature reducted by 85?compared with the pristine sample.Hydrogen desorption of 6LiBH4-SrH2-3MgH2 was a two-step reaction,the self-decomposition of MgH2 firstly and then the reaction between LiBH4 and SrH2 to form SrB6,CaB6 crystal structure product,and LiH.The hydrogenation products were LiBH4 and Sr2Mg3H10.Isothermal de-hydrogenation rate were 17.7 times and 7.4 times faster than that of the pristine sample.After 5 Cycles,with a 6.8 wt%hydrogen storage capacity.The apparent activation energy of the two-step reaction lowered by 23.8%and 20%than those of the pristine sample,respectively.C10H10Cl2Ti was selected as an additive catalyst to improve the hydrogen storage properties of the 6LiBH4-CaH2-3MgH2 sample,with optimum hydrogen storage performances by 5wt%additional.The onset and terminate temperature for the dehydrogenation of the sample was 30? and 25 ? lower than those of the pristine sample and its reversible hydrogen storage capacity was about 8.1 wt%.The release rate of first isothermal dehydrogenation increased 178%than that of the pristine sample and that of the secondary dehydrogenation had still improved 40%.The apparent activation energy of the two-step reaction reduced by 18.6%and 15.8%in comparison with the pristine sample,respectively.Further XPS analyses indicated that the variable valence state Ti compounds,generated during thermal decomposition of C10H10Cl2Ti,played an important role to improve hydrogen storage properties of the sample.The hydrogen storage performance of 6LiBH4-CaH2 system significantly improved by confined into the nanoporous carbon(NPC).The(6LiBH4-CaH2)@NPC(5:5)sample was test to be the optimum hydrogen storage performances.The onset temperature of nanoconfined sample was 80? lower than that of pristine sample,95.2%of hydrogen storage capacity was released in 300 min at 310 ?,and the reversible hydrogen storage capacity after 5 cycles is more than 60%of the initial desorption capacity.The isothermal dehydrogenation release rate of nanoconfined sample was 29 times than that of the pristine sample,and the apparent activation energy lowered by 24.6%.The sample is nanoconfined in a pore size of-3.3 nm by NPC,which prevents agglomeration and growth of the product during the circulation,increases the specific area and shortens the distance of atom diffusion,which then result in the improvement of hydrogen absorption and desorption kinetics.