Alkaline Metal Borohydride-metal Hydride Compisites For Hydrogen Storage

Alkaline metal borohydride LiBH4 and NaBH4 are potential hydrogen storage materials due to their high hydrogen capacities of 18.4wt% and 10.4wt%. respectively. But their thermodynamics of dehydrogenation are very stable and the reaction kinetics are also slow, which restrain their application. Currently, there are four ways to improve the re/dehydrogenation properties of alkaline metal borohydrides, including catalyst modification, nanoporus confinement, electro-negativity modification and reactive composite formation. LiBH4/MHx hydride composites have relatively low reaction enthalpy and stable hydrogen ab/desorption properties, which have attracted considerable attention as potential hydrogen storage materials. Studies have shown that the formations of metal borides were affected by the hydrogen back pressure in the dehydrogenation processes. To further explore the effects of hydrogen back pressure on hydrogen storage properties of the alkaline metal borohydrides based materials, we studied hydrogen storage performance of 2LiBH4+MgH2 6LiBH4+La/Ce and 2NaBH4+MgH2 composites with emphasis on their reaction kinetics and pathway.In this work, the dehydrogenation kinetics of the 2LiBH4+MgH2 composite under different hydrogen back pressures was studied. The applied hydrogen back pressure significantly was found to influence the hydrogen release rate of the uncatalyzed composite. Higher hydrogen pressures enhanced the nucleation of MgB2, resulting in better dehydrogenation kinetics and cycle stability. The composite with a CuCl2 catalyst demonstrated significantly improved dehydrogenation kinetics because the nucleation of MgB2 was promoted by heterogeneous nuclei. However, similar effects of hydrogen back pressure on dehydrogenation kinetics were also observed for the CuCl2-catalyzed composite. The extraordinary results suggest that hydrogen back pressure plays an indispensable role in the formation of MgB2, which determines not only the reaction pathway but also the kinetics of dehydrogenation for the 2LiBH4 + MgH2 composite.Dehydrogenation properties of 6LiBH4+La/Ce composites were found to be strongly dependent on the applied hydrogen back pressure. La or Ce hydride (LaHx/CeHx 2≤x≤3)was formed during the ball milling process. Under static vacuum, hydrogen was desorbed primarily through the independent decomposition of LiBH4. With the increase of hydrogen back pressure, the self decomposition of LiBH4 was gradually suppressed, and the reaction between LiBH4 and LaHx/CeHx was enhanced. However, only under a pressure as high as 13 bar did the dehydrogenation reach its highest capacity. Catalytic additives such as TiF3 improved the kinetics of hydrogen release but did not alter the reaction pathway. Further experiments such as SEM observations revealed that the reaction between LaHx/CeHx and LiBH4 was blocked halfway under an intermediate pressure such as 3 bar. The formation of the intermediate Li2B12H12 on the surface of LaHx/CeHx particles supposedly accounted for the observed phenomenon.Compared with the 2LiBH4+MgH2 system, the 2NaBH4+MgH2 system demonstrated higher dehydrogenation temperature. There was MgB2 generated when 2NaBH4+MgH2 was dehydrogenated under vacuum. As the hydrogen back pressure increased, the dehydrogenation of 2NaBH4+MgH2 was suppressed. When the hydrogen pressure was up to 10bar, there almost no hydrogen released. Compared with the 2LiBH4+MgH2 system, the LiBH4+NaBH4+MgH2 demonstrated better cycle performance. In the dehydrogenation process of LiBH4+NaBH4+MgH2 in vacuum. LiBH4 decomposed into LiH, B and H2, and partial NaBH4 and MgH2 reacted to form MgB2. During the rehydrogenation process, LiH react with MgB2 to form LiBH4. However, because NaBH4 was not reproduced in the rehydrogenation process, the reversibility of the system decreased gradually.