Enhanced Hydrogen Storage Performance Of LiBH₄-Based Composites

As a potential hydrogen storage material, LiBH4 possesses an extremely high hydrogen capacity of 18.4wt%. However, it suffers from unfavorable thermal stability, inferior kinetics and poor reversibility. Compared with pure LiBH4, the 2LiBH4/MgH2 composite can release and uptake hydrogen under milder conditions, but its hydrogen desorption kineticss is still poor. In the present work, we introduced some metal borides into the LiBH4-based materials to improve their hydrogen storage properties. Three kinds of composites, LiBH4+MB2(M=Mg, Ti, Zr),2LiH/MgB2+0.2MBx(M=Ni, Co, Fe) and 2LiBH4/MgH2+0.1AlF3+0.04TiF3 were investigated.First, the as-received metal borides MgB2, TiB2, ZrB2 were introduced as dopants into LiBH4. Non-isothermal desorption measurements showed that all of these metal borides can significantly reduce the hydrogen desorption temperature of LiBH4. H2 curves of mass spectrometry demonstrated that the peak desorption temperature of LiBH4 with dopants of MgB2, TiB2, ZrB2 was decreased by about 64℃,81℃ and 139℃ respectively, compared with that of pure LiBH4. The 11B NMR analysis indicated that an intermediate product Li12B12H12 was formed at 420℃.Second, NiBx, CoBx, Fe with nano sizes were prepared by wet chemical method and then introduced into the 2LiH/MgB2 composite. Experimental results showed that all of these nanosized catalysts improved the hydrogen absorption kinetics of the 2LiH/MgB2 composite, while only the NiBx and CoBx enhanced the hydrogen desorption kineticss of the hydrogenated 2LiH/MgB2 composite. Further studies showed that it was the in situ formed MgNi3B2 and CoB phases that acted as the catalysts for improvement of hydrogen storage properties of the 2LiH/MgB2 composite.Finally, hydrogen storage properties of the 2LiBH4/MgH2 composites with co-addition of AlF3 and TiF3 were studied. It was found that a simple addition of AlF3 hindered the reversibility of the 2LiBH4/MgH2 composites due to the formation of Mg(Al) solid solution instead of AlB2 upon dehydrogenation. However, with the co-addition of AlF3 and TiF3, the hydrogen storage properties, especially the reversibility, of the 2LiBH4/MgH2 composites were significantly improved. It is supposed that the improvement of hydrogen storage performance may be due to the formation of Ti-Al alloy or Al(Ti)B2 phase, which might act as nucleation centers of MgB2.