Catalytic And Nanosizinq Modification Of 4LiBH₄-MgH₂-Al Hydrogen Storage System

As one of the most promising candidates for hydrogen storage,lithium borohydride(LiBH4)shows enormous potential due to its high theoretical hydrogen storage capacity of 18.5 wt.%.However,the stable thermodynamics,slow kinetics and poor reversibility of LiBH4 largely limit its practical use.Utilizing reactive hydride composites(RHC)on LiBH4 could destabilize the thermodynamics and improve dehydrogenation behaviors,such as LiBH4-MgH2 and LiBH4-Al systems.In this work,the de/rehydrogenation performance and capacity decay mechanism of LiBH4-MgH2-Al have been studied,and the influence of catalytic doping and nanosizing modification also have been investigated.In the study of 4LiBH4-MgH2-Al doped with/without NbF5 additive,it can be found that the dehydrogenation of 4LiBH4-MgH2-Al can release about 9.0 wt.%H2 quickly without incubation period.In particular,it is the first time to achieve completely reversible hydrogen desorption property of 4LiBH4-MgH2-Al by doping with NbFs and dehydrogenating under 4 bar hydrogen back-pressure in experiment.Microstructure Analysis shows that the formation of Mg-Al alloys could result in the formation of Li2B12H12 and subsequently lead to the cycling capacity degradation in 4LiBH4-MgH2-Al composite system.With the additive NbF5,it shows a totally different pathway and a significant inhibition effect on the alloying between Mg and Al,leading to an improved de/rehydrogenation behavior without the by-product Li2B12H12.Meanwhile,niobium compounds formed in cycles could facilitate the products MgAlB4 and LiH to be fully rehydrogenated into LiBH4,MgH2 and Al,which contributes to the complete reversibility of 4LiBH4-MgH2-Al.The research on 4LiBH4-MgH2-Al system reverse-hydrogenated by 4LiH-MgB2-AIB2 has been conducted.Reverse in-situ hydrogenation is a good strategy for 2LiH-MgB2 system to prepare 2LiBH4-MgH2,diminishing the incubation period of MgB2 and improving its kinetic properties.However,it is not ideal to utilize this reverse-hydrogenating method into 4LiH-MgB2-AlB2 system.Due to the unique properties of close combination between atoms resulted from in-situ hydrogenation of 4LiH-MgB2-A1B2,the increase of kinetics will sacrifice the reversibility of composite system.The close combination between Mg and A1 will cause B existing as amorphous elementary substance,resulting in decease in dehydrogenation capacity.Even though NbFs additive could partially improve the stability,the influence of sacrificial effect will not be completely eliminated and the system could not completely de/rehydrogenate during cycles.Furthermore,it is reported a new method to fabricate nano-system,named Liquid-phase ball milling(LBM),in which nanoscale hydrides can be directly formed during loading on nano-carrier.Compared with the method of Liquid-phase loading(LL),nano-particles prepared by Liquid-phase ball milling(LBM)show better dispersible uniformity and connection with nano-carrier.In the study of 4LiBH4-MgH2-Al@GR nano-system,samples show a different reaction pathway.The decomposition of LiBH4 in relatively low-temperature will release B2H6,leading to B loss and causing reduction of capacity.Thanks to NbF5 additive,the dehydrogenation capacity will become stable after 3 cycles without appearance of side reactions.Thus,the reversibility of system can be improved.The above research is helpful to deeply understand the capacity decay mechanism of 4LiBH4-MgH2-Al system and the effect of catalytic doping and nanosizing modification,which will provide theoretical and experimental basis for the subsequent explorations on this type of hydrogen storage materials.