Hydrogen Storage Properties And Favorable Co-doping Mechanism Of LiBH₄ Co-doped With H-BN Supported Nb-based Additions

Hydrogen energy has attracted great attention all over the world as a kind of ideal clean engergy with great development potential. However, the technique of safe, economic and efficicent hydrogen storage is the key barrier that prevent hydrogen energy from widespread utilization.LIBH4 is regarded as one of the most prosmising hydrogen storage materials, due to its high gravimetric and volumetric hydrogen capacity of 18.5 wt% and 121 kg H2/m3, respectively. However, its high thermostability, sluggish kinetics and unfavorable rehydrogenation conditions limit its practical application. Based on an overall review of the reseach and the development of LiBH4. h-BN, NbCl5/h-BN and NbH@h-BN were doped into LiBH4 to improve the dehydrogenation and rehydrogenaiton properties of LiBH4, and the cooresponding mechanisms are also evaluated.The remarkable hydrogen de/absorption properties of lithium borohydride are achieved by mechanically milling LiBH4 with hexagonal boron nitride (h-BN). It is found that the dehydrogenation performance was increased with additional amount of h-BN, such as doped with 5 mol%,15 mol%,30 mol% and 50 mol% h-BN. And the results show that 30 mol% h-BN doped LiBH4 sample behaved the best hydrogen storage performances:The 30 mol% h-BN doped LiBH4 composite starts to release hydrogen from just 180℃, which is 100℃lower than the onset dehydrogenation temperature of ball milled LiBH4. The apparent activation energy (Ea) of hydrogen desorption had been reduced from 198.31 kJ/mol for ball milled LiBH4 to 155.8 kJ/mol for 30 mol% h-BN doped LiBH4. Moreover, the 30 mol% h-BN doped LiBH4 composite can release 12.6 wt% hydrogen in 120 min at 400℃, while ball milled LiBH4 released the same amount of hydrogen in the same conditions will take about 6000 min. In addition, the rehydrogenation of the composite is achieved under 400℃ and 10 MPa of H2 with an improved cycle stability, the composite also could release 6 wt% during the 3rd dehydrogenation. Based on the analyses of the microstructure of the 30 mol% h-BN doped LiBH4 composite indicates that these remarkable improved hyhydrogen storage properties are largely attributed to the lone pair electrons of nitrogen induced destabilization of LiBH4 and LiBH4 was prefer to decomposite on the surface of h-BN, and its production play the role of heterogeneous nucleation to reduce the dehydrogenation temperature of LiBH4.In order to further improve the hydrogen storage properties of LiBH4, NbCl5/h-BN was co-doped into LiBH4. It is found that the addition of NbCl5/h-BN co-dopant can significantly enhance the dehydrogenation kinetics of LiBH4, and the catalytic effect of co-dopant is better than that of NbCl5 or h-BN dopant separately. After co-doping with 1 mol% NbCl5 and 30 mol% h-BN, LiBH4 can release 12.19 wt% within 20 min, which is 17.41 and 2.61 times more than that of 1 mol% NbCl5 or 30 mol% h-BN separately doped LiBH4 samples in same conditions. The major dehydrogenation temperature of NbCl5/h-BN co-doped LiBH4 is reduced to 377℃, much lower than that of ball-milled LIBH4 (464℃). The apparent activation energy (Ea) of hydrogen desorption is reduced from 195.81 kJ/mol of LiBH4 to 122.75 kJ/mol of NbCl5/h-BN co-doped LiBH4. The microstructural results reveal that the catalytic effect of NbCl5/h-BN co-dopant on improving the dehydrogenation kinetics of LIBH4 could be ascribed to the in situ formed nano NbH@h-BN, which plays the important role of enhancement of hydrogen storage properties of LiBH4.Finally, the NbH@h-BN catalyst of h-BN supported 5-20 nm NbH particles were prepared by mechanical ball milling method. And it was introduced into LiBH4 to improve the hydrogen storage properties of LiBH4. After doped with 1 mol% 3NbH@h-BN, The major dehydrogenation temperature of NbH@h-BN doped LiBH4 is reduced to 380℃. And the time for LiBH4 dehydrogenates completely was shortened from 6000 min for ball milled LiBH4 to 30 min. In addition, the rehydrogenation of the composite is achieved under 400℃ and 10 MPa of H2 with an improved cycle stability, the composite also could release 8 wt% during the 3rd dehydrogenation. The microstructural results reveal that the extraordinarily catalytic effect of NbH@h-BN serves as the heterogeneous nucleation site to reduce the decomposition temperature of LiBH4. And nano NbH particles are dispersing on h-BN surface, which could provide sufficient nucleation sites, and shorten the distance of the solid-liquid phase boundary movement of LiBH4 decomposition to highly improve the dehydrogenation kinetics of LiBH4.