Study On The Absorption And Release Kinetics Of Mg-Al Matrix Composite Hydrogen Storage Materials

Magnesium based hydrogen storage alloy is considered to be the most promising hydrogen storage alloy material due to its advantages such as large hydrogen storage capacity,abundant reserves,low price and transportation safety,etc.However,its application is limited by its shortcomings such as high temperature of hydrogen absorption and desorption,easy oxidation,difficult activation and poor hydrogen absorption and desorption kinetics.Simple nanocrystalization or alloying,surface catalysis,only a certain structure and a certain microscopic process of the alloy can be touched,and the whole hydrogen absorption and desorption is composed of multiple steps of internal and surface reactions,so a single means of regulation cannot achieve the purpose of excellent comprehensive performance.Therefore,to obtain excellent hydrogen storage performance,it is necessary to improve both surface and internal characteristics of the material.The research object of this paper is Mg-Al-based hydrogen storage alloy.The research content mainly focuses on the effects of changing the mass ratio of Mg-Al,adding transition metal Ni and catalyst on the micromorphology and phase composition of Mg-Al-based hydrogen storage alloy before and after hydrogen absorption.The activation properties,kinetics and thermodynamic properties of Mg-Al-based hydrogen storage alloys and Mg-Al-Ni-based composite hydrogen storage alloys were analyzed.The phase transformation and microstructure evolution of the alloys were demonstrated during the process of hydrogen absorption and deionization.The mechanism of hydrogen absorption and deoxidation of the alloys was discussed.In this paper,Mg-Al hydrogen storage alloys with mass ratio of Mg-Al 9:1,4:1,7:3 and 3:2 were prepared by mechanical chemical reaction method.The phase composition,microstructure and phase composition of the alloys were studied by means of SEM,XRD and EDS.During the preparation process,the phase analysis found that Mg₁₇Al₁₂ phase was the main peak when the mass ratio of Mg-Al was 3:2 and the preparation time was 16 h.There was a small amount of Mg₂Al₃ phase,MgH₂ phase,Mg phase and Al phase.The generated Mg-Al intermetallic compounds were beneficial to improve the thermodynamic and kinetic properties of Mg-based hydrogen storage alloy.Before the mechanical chemical reaction,magnesium and aluminum particles are coarse,which is not conducive to hydrogen absorption and desorption.After the mechanical chemical reaction for 16 h,the particles become smaller,increasing the diffusion channel of H absorption and desorption.It was found that with the increase of Al content,the maximum hydrogen absorption capacity of the sample was reduced,but the addition of A1 improved the thermodynamic performance of the hydrogen storage alloy.It was found that at 250℃,the hydrogen absorption capacity of the samples with Mg-Al ratio of 9:1 and 3:2 was 6.0 w.%and 4.3 w.%within 60 min under the condition of low temperature.Secondly,on the basis of magnesium aluminum ratio of 9:1 and 3:2,transition metal Ni is introduced to complete alloying,refine the microstructure,and increase the phase/grain boundary.By using the control variable method,the content of Ni added and the existence form of Ni,namely the mechanical chemical reaction time,were changed respectively.It was found that with the increase of Ni content,the hydrogen absorption capacity of the samples of mechanical chemical reaction for 8 h and mechanical chemical reaction for 16 h decreased with the increase of Ni content,but the hydrogen storage properties of the alloys were improved.Especially the sample containing 5 wt.%Ni.For example,when the mass ratio of magnesium to aluminum is 3:2 and 5 wt.%Ni is added,the hydrogen absorption capacity increases from 4.2 wt.%to 6.03 wt.%within 60 minutes without Ni;when the mass ratio of magnesium to aluminum is 9:1 and 5 wt.%Ni is added,the hydrogen absorption capacity increases from the previous one without Ni.Hydrogen absorption in 60 min increased from 4.59wt.%to 6.4 wt.%.The boundary and crystal interface of these phases can be used as the adhesion site and diffusion channel of hydrogen atoms in the alloy,and the diffusion distance of hydrogen atoms in the magnesium hydride layer can be shortened when the elemental and compound Mg₂Ni coexist in the mechanical chemical reaction for 16 h.Finally,carbon nanotubes（CNT）and niobium pentoxide（Nb₂O₅）catalysts were added on the basis of Mg-Al binary and Mg-Al-Ni terra-alloy.The introduced CNT and Nb₂O₅ were evenly distributed on the surface particles by high-energy ball milling in a short time.High active sites were introduced into the matrix,and the structure was preserved and the surface properties were improved.Complete the overall control of Mg-Al alloy internal refinement and surface activation.The effects of single and compound addition of catalyst on hydrogen storage properties were studied.Compared with Mg-Al alloy,the hydrogen emission activation energy of Mg-Al system decreased from 88.02 kJ/mol to 59.33 kJ/mol when CNT and Nb₂O₅ were added.Compared with Mg-Al-Ni alloy,after Nb₂O₅ was added,the dehydrogenation activation energy of Mg-Al-Ni system decreases from 78.94 kJ/mol to 77.41 kJ/mol,indicating that the dehydrogenation activation energy of Mg-Al system decreases due to the addition of catalyst,which improves the hydrogen release performance of the alloy.The results show that the excellent hydrogen absorption and desorption kinetics can be obtained after the addition of CNT and Nb₂O₅,which depends on the synergistic catalysis among the auxiliary diffusion of H₂ by CNT,the adsorption and dissociation of H₂ by Nb₂O₅ and the heterogeneous nucleation of MgH₂.