Study On The LiNH₂-LiBH₄ Hydrogen Storage Composite Modified With Alkaline Earth Metal Cation

Nowadays,uncontrolled use of fossil fuels has triggered a series of environmental problems.In order to solve the global problems of environmental pollution and energy depletion,searching for green clean energy,which can replace traditional fossil fuels,has become the focus of researching all over the world.Hydrogen is one of the strongest contenders for its free pollution,high energy density and so on.In recent years,more and more attention has been paid to the safe and efficient hydrogen storage technology,and the light metal complex hydrides have attracted much attention due to their high hydrogen content.In this paper,the cationic substitution of LiBH₄-LiNH₂ composite system was adopted for further destabilization modification.The investigation includes the following two aspects:（1）The effect of metal ions M on the dehydrogenation of M（BH₄）_n-LiNH₂（M=Ca,Mg,Li）was studied.The samples were characterized by TG-DTA-MS,activation energy calculation and XRD.TG shows that Mg（BH₄）₂-4LiNH₂ and Ca（BH₄）₂-4LiNH₂ systems can effectively restrain the release of impurity gases.The DTA curves of Mg（BH₄）₂-4LiNH₂and Ca（BH₄）₂-4LiNH₂ samples show two distinct exothermic peaks from 250℃to400℃,while the LiBH₄-2LiNH₂ sample only shows one strong exothermic peak at359.29℃,indicating that the specific decomposition reactions of the three samples are different.MS shows that the main release gases for the three composites are hydrogen and ammonia.At 200-350℃,the activation energies of decomposition reactions of Ca（BH₄）₂-4LiNH₂ and Mg（BH₄）₂-4LiNH₂ samples were reduced by about 30%compared with LiBH₄-2LiNH₂ sample.Moreover,the kinetic mechanisms of the decomposition reaction of Ca（BH₄）₂-4LiNH₂ and Mg（BH₄）₂-4LiNH₂ samples are different from the LiBH₄-2LiNH₂ sample.The kinetic mechanism of M（BH₄）_n-2n LiNH₂ system is related to the metal element M.X-ray diffraction（XRD）shows that all the three systems appear obvious diffraction peak of Li-N-B-H complex at 2θ=28.88,31.25 and 48.10,indicating that the three composites can produce Li-N-B-H after ball milling.Although the main product of M（BH₄）_n-2n LiNH₂ system after ball milling is independent of the metal element M,the substitution of high electronegativity metal greatly improves the dehydrogenation performance of the M（BH₄）_n-2n LiNH₂ system.（2）Discussing the possibility of MgCl₂/CaCl₂-LiBH₄-LiNH₂ ternary system as a novel hydrogen storage material.The performance of composites with different reactant ratios was investigated by the TG-DTA-MS test.The optimum dosages of MgCl₂ and CaCl₂ are20 wt%and 10 wt%,respectively;moreover,the thermal decomposition process was also studied.Adding slight MgCl₂/CaCl₂,XRD shows Li-N-B-H complex.With the increase of MgCl₂/CaCl₂ content,the XRD patterns show strong diffraction peaks of Li Cl,indicating that Li metal can be replaced by Mg/Ca successfully.FT-IR shows that the structures of[BH₄^-]in LiBH₄ and[NH₂^-]in LiNH₂ remain in the novel material generated after ball milling.Moreover,with the increase of MgCl₂ content,the peaks of B-H stretching vibration at 2225,2292 and 2389 cm^-1 become weak,implying that the intercalation of high electronegativity metal weakens the B-H bond,which is conducive to the release of hydrogen.