Effects Of New-type Carbon Materials On Hydrogen Storage Properties Of Li-Mg-B-H System

Compared with the traditional alloy hydrogen storage materials,metal coordination hydrides hydrogen storage materials have been widely researched due to its excellent quality density and volume density of hydrogen storage.However,at this stage,most of the metal coordination hydrides have become the main bottleneck restricting the utilization of hydrogen energy due to the high hydrogen releasing temperature,poor hydrogen-releasing kinetics and harsh reversible hydrogen absorbing conditions.In this paper,carbonized starch?CS?with two-dimensional lamellar structure was prepared by carbonization of soluble starch,and two types of zero-dimensional carbon spheres material?HCS/CA?were prepared by two-step carbonization and inverse emulsion polymerization,respectively.Then,MgH₂@CS with two-dimensional and the MgH₂@HCS/MgH₂@CA with shell-core structure were prepared by hydrogenation combustion and ball milling method,directly using the metal magnesium powder and carbonaceous material as the raw materials,and then introduced LiBH₄ to build a Li–Mg–B–H system by ball milling.The thermodynamics and kinetics of hydrogen absorption and desorption on Mg-based composites were studied systematically,and the effects of carbon materials addition on the synthesis of MgH₂ and on the adsorption of Li–Mg–B–H composite by means of XRD,SEM/EDAX and TEM.The results show that the initial hydrogen evolution temperature of the prepared MgH₂@HCS and MgH₂@CA composites are about 100°C and 300°C lower than that of the pure MgH₂ system,respectively,and they could release about 2.5 wt%and 5.0 wt%at150°C,and after several cycles of hydrogen absorption and desorption process,the capacity retention rate of the composites can still reach 98.0%and 99.0%,respectively,showing the excellent hydrogen storage stability.The hydrogen storage performance of2LiBH₄–MgH₂@HCS/2LiBH₄–MgH₂@CA composites have also been greatly improved after the introduction of LiBH₄.The initial hydrogen evolution temperature is about 150°C lower than pure 2LiBH₄–MgH₂ system,and the kinetics of hydrogen absorption and desorption are also enhanced.It was found that during the hydrogenation procedure of carbon material and magnesium powder,the in-situ generated MgH₂ nanoparticles can be encapsulated into the pores of the carbon material,and uniformly dispersed on the surface of carbon material due to the defects and the mesoporous structure of the carbon materials,after further compounding with LiBH₄,the resulting MgB₂ phase changes the hydrogen desorption path of LiBH₄ and increases the hydrogen absorption and desorption kinetics of the composite system.In addition,the presence of mesopores in the carbon material structure provides a fast channel for the diffusion of hydrogen to further enhance the kinetics of hydrogen absorption and desorption,and no significant agglomeration of the composite material occurs after several cycles of hydrogen absorption and desorption,indicating carbon materials can effectively inhibit the agglomeration of magnesium-based composite materials in the process of hydrogen absorption and discharge appear.At the same time,the use of metal magnesium powder instead of MgH₂ as the initial material for preparing Mg-based hydrogen storage material,reduceing the cost of Mg-based hydrogen storage material.