| The development of hydrogen storage materials with high volumetric and gravimetric hydrogen densities is essential for the on-board fuel cell vehicular applications. This has triggered an extensive effort to develop light-weight hydrogen storage materials, among which the complex hydride LiBH4 appears to be a potential candidate due to its exceptionally high hydrogen capacity on both a gravimetric and volumetric basis. However, high dehydrogenated temperature, sluggish de/rehydrogenation kinetics and extremely harsh rehydrogenation conditions block its commercial applications. Based on overview of the development and progress in LiBH4, this work has systematically improved its hydrogen storage performance by cation substitution, anion substitution, doping and reactant destabilization. Firstly, a cation substitution system LiMn(BH4)3/2LiCl was prepared by reactive ball-milling a mixture of LiBH4 and MnCl2, and its dehydrogenation performance and kinetic model were investigated. Various Ti-containing dopants were employed to further improve decomposition of the LiMn(BH4)3/2LiCl composite. Secondly, LiMn(BH4-xFx)3/2LiF composite by both cation and anion substitutions was prepared by ball-milling a mixture of LiBH4 and MnF2. The decomposition properties and kinetic model were investigated and compared with those of the LiMn(BH4)3/2LiCl composite, demonstrating the influence of the anion substitution of F-for H-. Thirdly, LiMn(BH4-xFx)3/2LiF composite was doped with a small amount of LiNH2 to suppresse the release of diborane and reduce the decomposition temperature. Finally, a novel strategy was proposed, that is destabilizing LiMn(BH4-xFx)3/2LiF composite by using MgH2 as a destabilizing additive. Upon dehydrogenation MgB2 was formed under argon atmosphere without obvious incubation period, which is quite different from previous observations. All the research contents and results are as follows:(1) By reactive ball-milling a mixture of LiBH4 and MnCl2, anion substitution LiMn(BH4)3/2LiCl composite was prepared, which consists of crystalline LiCl and amorphous LiMn(BH4)3. As compared to bulk LiBH4, dehydrogenation temperature of LiMn(BH4)3/2LiCl composite is decreased from 400℃to 135℃. It is found that the composite decomposes at 135-190℃with an emission of 7.0% gas, containing 93.2 mol% H2 and 6.8 mol% B2H6. It is the reason for why the mass loss of the LiMn(BH4)3/2LiCl composite is larger than that of theoretical hydrogen capacity of 6.3%. Moreover, the activation energy Ea for hydrogen release is 114 kJ/mol for LiMn(BH4)3/2LiCl composite, and kinetic modeling studies suggest that the hydrogen release from composite is governed by three-dimension diffusion mechanism. Furthermore, the influence of various Ti-containing dopants on the decomposition of the LiMn(BH4)3/2LiCl composite was studied. It is observed that among TiF3, TiC, TiN, and TiO2, only TiF3 achieved a reduction in decomposition temperature. Compared with the undoped LiMn(BH4)3/2LiCl composite, the onset decomposition temperature and the activation energy of the TiF3-doped composite are reduced to 125℃ and to 104 kJ/mol, respectively. These are attributed to the formation of Ti(BH4)3 in some local regions by the partial substitution of Ti for Li in LiMn(BH4)3·(2) LiMn(BH4-xFx)3/2LiF composite was prepared by ball-milling a mixture of LiBH4 and MnF2-Differing from the case for the LiMn(BH4)3/2LiCl composite where only cation substitution occurs, both the cation substitution of Mn2+ for Li+ and the anion substitution of F- for H- occur in the LiMn(BH4-xFx)3/2LiF composite. It is found that the LiMn(BH4-xFx)3/2LiF composite decomposes at lower temperature of 120℃ with an emission of 7.0% gas, containing 94.8 mol% H2 and 5.2 mol% B2H6. Moreover, the activation energy Ea for hydrogen release is 92 kJ/mol for LiMn(BH4-xFx)3/2LiF composite, which is remarkably reduced as compared to that of LiMn(BH4)3/2LiCl composite. Furthermore, kinetic modeling studies suggest that the hydrogen release from composite is governed by One-dimension nucleation and growth mechanism. These results suggest that MnF2 has superior destabilization effects over MnCl2 in the decomposition of LiBH4, which can be attributed to the reduced stability of B-H bonds by partial anion substitution of the F-for the H-(3) The evolution of diborane accompanying H2 release during the decomposition of LiMn(BH4-xFx)3/2LiF composite reduces the purity of evolved hydrogen and results in capacity loss during cycling. To solve the problem, a small amount of LiNH2 is doped into a LiMn(BH4-xFx)3/2LiF composite. Doping LiNH2 facilitates the cation exchange between MnF2 and LiBH4, forming LiF and amorphous LiMn(BH4-xFx)3. As compared to undoped LiMn(BH4-xFx)3/2LiF composites, the onset decomposition temperature was reduced to 101℃,96℃,94℃,78℃ and 77℃, and the weight loss is 6.8 wt.%,5.0 wt.%,4.8 wt.%,6.5 wt.% and 9.3 wt.% for LiMn(BH4-xFx)3/2LiF composites doped with 2.5 wt.%,5 wt.%,7.5 wt.%,10 wt.% and 15 wt.% LiNH2, respectively. MS results indicted that when the LiNH2 content is more than 5 wt.%, the formation of B2H6 is completely suppressed with the existence of NH3 emission. A pure hydrogen release is achieved only in 5 wt.% LiNH2-doped LiMn(BH4-xFx)3/2LiF composites, which released 5 wt.% hydrogen between 100～40℃ with decomposition activation energy Ea of 91 kJ/mol. These improvements in the decomposition performance are mainly attributed to the prevention of the formation of B-H-B bonds for B2H6 and the destabilization of B-H bonds in borohydrides by the interaction of BH4- and NH2-.(4) Based on conception of hydride destabilization, MgH2 was added into LiMn(BH4-xFx)3/2LiF composites to further improve de-/re-hydrogenation performance. It is indicated that MgH2-LiMn(BH4-xFx)3/2LiF composites release hydrogen in two steps:first, LiMn(BH4-xFx)3/2LiF composites decomposes at 120～160℃ with an emission of 4.9% gas; then MgH2 and remaining LiBH4 decompose at 350～500℃ with an emission of 2.1% gas. MgB2 is formed under inert gas atmospheres without obvious incubation periods upon dehydrogenation of MgH2-LiMn(BH4-xFx)3/2LiF composites. As compared to that of MgH2-LiBH4 composites, MgB2 can be generated only under 0.3～0.5 MPa hydrogen back pressure, with long-term incubation periods. These improvements on formation of MgB2 are attributed to the change in reaction pathway and effect of transition metal Mn. MgH2-LiBH4 composites fulfills two-step dehydrogenation reaction:MgH2 first decomposes completely to form Mg, and then a subsequent reaction between Mg and LiBH4 produces MgB2. MgH2-LiMn(BH4-xFx)3/2LiF composites fulfills two-step dehydrogenation reaction:LiMn(BH4-xFx)3/2LiF composites first decomposes completely to form Boron, then Mg atoms are readily released from MgH2 with much less energy and thus are more available to react with Boron once the dehydrogenation of MgH2 occurs. In addition, it is demonstrated that favorable effect of Mn is due to its strong ability to improve the diffusion rate of Mg in MgB2 crystal and thus facilitate the nucleation of MgB2. |
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