Exploring Novel Systems And Their Mechanisms For Hydrogen Storage Based On The Interaction Of LiBH₄ With Nitride

Lithium borohydride?LiBH₄?,a light-element complex hydride,have been paid great attentions in use in hydrogen storage,because of its high theoretical hydrogen storage capacities(18.4 wt.%and 121 kg H₂/m³ of gravimetric density and volumetric density,respectively.However,the dehydrogenation and rehydrogenation of LiBH₄ require harsh conditions of high temperature and high pressure.In the previous research,adding halides,borides or compounds with H^?+?containing O-H or N-H bond?has been explored for improving the hydrogen storage performance of LiBH₄ to some extent.Even though,it is still a challenge to improve the comprehensive hydrogen storage performance of LiBH₄ in de/rehydrogenation cycling and hydrogen storage capacity synchronously.In this dissertation,we design the novel LiBH₄-based hydrogen storage systems with high hydrogen capacity and cycle stability by constructing LiBH₄-nitride hybrid systems.The main research contents include the hydrogen diffusion mechanism of LiBH₄-MgH₂ composite during dehydrogenation;the preparation of nano-structured layered materials by melting,inserting,hydrolyzing of LiBH₄;and the designing and constructing of LiBH₄ and light-element nitrides composites for the high hydrogen-capacity system.Two hydrogen diffusion mechanisms,i.e.,“joint-dehydrogenation machanism”and“hydrogen substitution mechanism”have been revealed by isotope tracing method in the dehydrogenation of LiBH₄-MgD₂ system.HD?hydrogen-deuterium?molecule releases from the system by a“joint-dehydrogenation”mechanism.H atom in borohydride and D atom in metal deuterium are combined at the interface of borohydride and metal deuterium.The dehydrogenation of metal deuterium improves the dehydrogenation of borohydride.The“hydrogen substitution”mechanism occurs in molten LiBH₄ and metal deuterium compounds.Benefiting from the rapid diffusion of hydrogen atoms or molecules in molten LiBH₄,the hydrogen atoms combine at the outer surface of LiBH₄ rather than at the surface of metal deuterium,which results in the substitution of D atoms in metal deuterium for H atoms in LiBH₄.During the“hydrogen substitution”mechanism,the diffusion of hydrogen atoms is accelerated and the dehydrogenation kinetics of metal hydrides is improved consequentially.Two-dimensional materials of MoS₂ and h-BN are prepared by mechanical-assisted exfoliation and LiBH₄ embedded-expansion exfoliation.Both of isopropanol and ethanol-water solution can be used as effective solvents for exfoliating the thinned layered materials.The ball-milling method can be applied to introduce energy for exfoliation more effectively than the ultrasonic method.By planetary ball-milling,ultra-thin h-BN nanosheets with only 3 nm thickness are prepared in 55 vol.%ethanol solution.However,the mechanical force by ultrasonic or ball milling could not exfoliate the layered materials efficiently neither uniformly,and the yield is too low to be used.Two-demensional materials of nano-structured MoS₂ and h-BN can be prepared by LiBH₄ embedded-expansion and hydrolyzing-exfoliation.MoS₂ nano-sheets with a width of² um and a thickness of⁴ nm and MoS₂ nano-flakes with a thickness of only 1-2 atomic layers are prepared as well.For h-BN,h-BN nano-sheets with a thickness of¹0 nm;h-BN nano-flakes with a thickness of 1 nm;as well as spongy-like nano-porous h-BN materials with a pore size of 30-40 nm are prepared.The nanocrystalline h-BN and above-prepared nano-porous h-BN are composited with LiBH₄ by ball milling,respectively,which are applied to design a high-capacity reversible hydrogen storage systems.Composited with nanocrystalline h-BN at a molar ratio of 1:3,LiBH₄+3BN composite can dehydrogenate 3.1 wt%hydrogen at 400oC,which is close to its theoretical value.The hydrogen capacity of the second and third cycles can still be maintained at 3.1 wt%after rehydrogenation under 400oC and 10 MPa hydrogen.The first rapid dehydrogenation is due to the new B-H bond formed on the surface of h-BN after ball-milling.The novel B-H bond can destabilize the B-H bond of LiBH₄ and make LiBH₄ dehydrogenate at a lower temperature.The hydrogen storage performance of LiBH₄ can be further catalyzed by nano-porous BN.LiBH₄+0.3NP-BN can release 10.4 wt%hydrogen rapidly at 400oC.The reversible capacity of the rehydrogenated composite under 400oC and 10 MPa hydrogen is stable at 5.7 wt.%after four cycles.The amorphous state,which have been formed after ball-milling,can be maintained after cyclic hydrogen absorption.During such cycle,dehydrogenation-state is the structure of nanocrystalline Li_xBN around the amorphous product.This structure not only avoids the growth of LiH but also limits the region of dehydrogenation products,thus improved the cyclic hydrogen storage performance of the system.Li_x BN is formed by the reaction between h-BN and LiBH₄ or LiH,which can release more hydrogen from LiBH₄,thus promoted the dehydrogenation capacity of the system.Nano-structured AlN with N-H,O-H and C-H groups on the surface are prepared by alcoholysis of LiBH₄.When LiBH₄ is combined with oven-dried AlN at a mass ratio of 3:2,the composite can release 6.8 wt.%hydrogen at 400oC for the first time,but its reversible capacity is relatively low.When LiBH₄ is compounded with freeze-dried AlN at a mass ratio of 2:1,the composite can release 11.8 wt.%hydrogen at 400oC for the first time.During the following cycle,the dehydrogenation capacity of the system can also reach 7.5,6.6 and 6.1wt.%in the 2-4^th cycle after rehydrogenated under 400oC and 10 MPa hydrogen.It still has more than 6 wt.%capacity after four cycles.It is suggested that AlN provide diffusion channels for H or Li atoms,which makes LiBH₄ or LiH dehydrogenate at a lower temperature.The doped oxygen elements can help LiBH₄ dehydrogenation avoid the formation of intermediate products.The O-H and C-H groups on the surface of AlN can provide H^?+to combine with H^?-in LiBH₄ so that the system has considerable high hydrogen storage capacity.High capacity hydrogen storage systems are prepared by the composition of graphite-like carbon nitride?g-C₃N₄?or melamine?C₃H₆N₆?with LiBH₄ or LiH.When LiBH₄ and g-C₃N₄are composited with ratio of 6:1,the composite can release 8.1 wt.%hydrogen rapidly at 400oC.This is due to the product of LiBH₄ and g-C₃N₄ reaction by ratio of 2:1,which plays a catalytic role in the subsequent dehydrogenation process.However,the reversible hydrogen storage capacity of this system is poor.A Li-rich system of LiH+C₃N₄-10:1 can stably store2.0 wt%hydrogen reversibly.This is due to the reaction between LiH and Li_xH_2-x?NCN?.The combination of H^?-in LiH and H^?+in N-H makes the system dehydrogenate at a low temperature,but the unstable N-H bonds lead to the dehydrogenation of ammonia.Li₂?NCN?can be prepared by the reaction between LiBH₄ or LiH and C₃H₆N₆.When Li₂?NCN?and LiBH₄ are composited with 1:1 ratio,the system can reversibly store 5.3 wt.%hydrogen at400oC and 10 MPa hydrogen.This is due to the introduction of a small number of B atoms would stabilize N atoms by forming h-BN,and h-BN can store and release Li atoms in the system,thus improves the cyclic hydrogen storage performance of the system.