Investigation On The Synthesis And Hydrogen Storage Performance Of Li-Ca-B-H Complex Systems

Owing to the high gravimetric and volumetric hydrogen storage densities (18.5 wt.%and 121 kg H2/m3), LiBH4 has become the present key research in the high capacity hydrogen storage materials. Forming the dual cation borohydrides by metal diversification of LiBH4 can be an effective method for decreasing the thermodynamic stability of LiBH4, and improving the de/rehydrogenation properties of LiBH4. Based on the comprehensive review of the research progress in dual cation borohydrides research all over the world, Li. Ca dual cation borohydride was selected as the investigated subject in this paper. Above all, the mechanical ball-milling and wet chemical milling methods were adopted to explore the synthesis of Li. Ca dual cation borohydride. And X-Ray Diffraction (XRD), Fourier Infrared Spectrum (FTIR), Raman Spectrum, synchronized Differential Scanning Calorimetry/ Thermogravimetry/Mass Spectrum (DSC/TG/MS), and Temperature Programmed Desorption (TPD) were employed to characterize the phase compositions, microstructure and de/rehydrogenation properties of the ball-milling products, and investigate the de/rehydrogenation mechanisms. In addition, the de/rehydrogenation behaviors of 6LiBH4+CaHCl and 6LiBH4+CaH2 systems were also investigated.The investigation on the synthesis of Li、Ca dual cation borohydride by mechanical ball-milling the mixture of 3LiBH4+CaCl2 shows that:the mixture of 3LiBH4+CaCl2 at the pre-milling process reacts to Ca(BH4)2 and LiCl by the displacement reaction, but Ca(BH4)2 and LiCl will continuously convert to Ca(BH4)2-xClx and LiBH4 because they can not be co-existent, and finally all Ca(BH4)2 and LiCl convert to LiBH4 and CaCl2. If the mechanical ball-milling proceeds, it will cyclically repeat the phenomenon above. In order to avoid the displacement reaction between Ca(BH4)2 and LiCl, the wet chemical milling is adopted, and the mixture of 3LiBH4+CaCl2 is added into THF organic solvent, then Ca(BH4)2 produced by ball-milling process dissolves into THF and generates Ca(BH4)2-2THF, and finally Ca(BH4)2-2THF reacts with the remaining LiBH4 obtaining the new dual cation borohydride of LiBH4·Ca(BH4)2·2THF.The investigation on the hydrogen storage properties of LiBH4·Ca(BH4)2·2THF demonstrates that:the decomposition temperature of LiBH4·Ca(BH4)2·2THF is much lower than that of the pristine LiBH4 and Ca(BH4)2, its decomposition process involves four steps:firstly, LiBH4·Ca(BH4)2·2THF decomposes into high temperature phase of LiBH4. CaB6 and an intermediate phase Ca-B-H-Cl hydride, and synchronously releases a large number of H2 and THF gas together with less B2H6; secondly, the eutectic melting of LiBH4 and Ca-B-H-Cl hydride; thirdly, Ca-B-H-Cl hydride decomposes into CaHCl; fourthly, LiBH4 reacts with CaHCl. The activation energies of the first, third and fourth dehydrogenation steps are 291.95 kJ/mol,132.06 kJ/mol and 117.13 kJ/mol, respectively. Moreover, LiBH4-Ca(BH4)2-2THF has a certain de/rehydrogenation reversibility, especially its rehydrogenation products has a very stable cyclic dehydrogenation behavior. From the results above, it can be inferred that the new hydrogen storage system of LiBH4+xCaHCl may have a very good cyclic de/rehydrogenation performance.The investigation on the de/rehydrogenation behaviors of 6LiBH4+CaHCl and 6LiBH4+CaH2 systems shows that:LiBH4 doping with CaHCl can decrease its thermodynamic stability and improve its dehydrogenation kinetics at low temperature greater than CaH2 do. The activation energy of LiBH4 dehydrogenation decreases from 160.11 kJ/mol in 6LiBH4+ CaH2 system to 112.39 kJ/mol in 6LiBH4+CaHCl system. The dehydrogenation scheme of 6LiBH4+CaHCl system is different from 6LiBH4+CaH2 system, and its dehydrogenation reaction is 6LiBH4+CaHCl�'5LiH+LiCl+CaB6+10H2 T↑However, due to stable LiCl formed after dehydrogenation, the rehydrogenation route of 6LiBH4+CaHCl system is the same as 6LiBH4+CaH2 system to regenerate LiBH4 and CaH2 (in molar ratio of 6:1), which indicates that 6LiBH4+CaHCl system can not be fully rehydrogenated. However,6LiBH4+CaHCl system has more stable cyclic dehydrogenation property than 6LiBH4+CaH2 system. Finally, by comparing the de/rehydrogenation reactions of 6LiBH4+CaHCl and LiBH4+xCaHCl system, it can be obtained that the requirement for CaHCl can be reversibly formed during the rehydrogenation process of LiBH4+xCaHCl system is the presence of CaH2 after its dehydrogenation, i.e.x must is greater than 1/6.