Study On Annealing Activation And Cycling Stability Of Mg-Based Hydrogen Storage Materials

Hydrogen energy is one of the main energy sources in the 21st century.It has the advantages of wide source,no pollution and high efficiency.In the development of hydrogen energy technology,hydrogen storage is a key link.Magnesium-based hydrogen storage materials are considered as the most promising solid hydrogen storage materials because of their high hydrogen storage capacity,low cost and low density.However,Mg is very active and its surface is easily passivated when stored in air.The MgO layer formed will prevent H2 from adsorbing and dissociating on the surface of particles.It is important to destroy the oxide layer on the surface of Mg particles before hydrogenation to improve the kinetics of hydrogen absorption.The cycle stability of Mg-based hydrogen storage materials is a necessary condition for the application.Particle agglomeration and grain growth occur after hydrogen absorption and desorption cycles.The kinetics of hydrogen absorption and desorption is slowed down,which reduces the hydrogen storage performance of the material.The activation mechanism of Mg-based hydrogen storage materials was studied by in situ high temperature annealing.The phase and morphology of the material were characterized by XRD,SEM and HRTEM.High pressure gas adsorption instrument and differential scanning calorimeter were used to test the absorption and desorption properties of the samples.The experimental results show that,due to the different thermal expansion coefficients of magnesium and magnesium oxide,the magnesium oxide film on the surface of magnesium particles will crack at high temperature,and the magnesium metal will be exposed.In addition,the crystallinity of magnesium oxide crystal on the surface of raw magnesium is poor,and high temperature conditions can also lead to the agglomeration and growth of magnesium oxide.The diffraction peaks of MgO appear on the surface of activated magnesium particles after annealing treatment.The hydrogen absorption properties of materials before and after activation were tested by high pressure gas sorbent.The unactivated raw material magnesium does not absorb hydrogen at 300? and 2.0 MPa hydrogen pressure.The samples annealed at 380? for 0 h and 0.5 h did not undergo hydrogen absorption reaction under the same conditions.The hydrogen absorption kinetics of the materials annealed for more than 1 h has been significantly improved.The hydrogen uptake of samples annealed for 1h,2 h and 4 h was 3.54 wt.%,5.47 wt.%and 5.61 wt.%in 10 h,respectively.The cycle stability of Mg-based hydrogen storage materials with different Ni content was studied under the conditions of hydrogen absorption pressure 2.0 MPa and 15 min,hydrogen release pressure 0.05 MPa and 30 min at 300?,respectively.Under this condition,the reversible hydrogen capacity of pure Mg is very low and unstable,and the maximum hydrogen uptake is only 0.56 wt.%.The hydrogen absorption and desorption cycle performance of the material has been greatly improved by adding Ni.When the content of Ni is 5%,the hydrogen absorption is 4.57 wt.%in the fifth cycle and 3.90 wt.%in the thirtieth cycle.The average hydrogen uptake per cycle decreased by 0.027 wt.%.When the content of Ni is 10%and 20%,the hydrogen absorption of the material is relatively stable during the cycling process.The hydrogen absorption capacity of materials with 20%Ni content is almost independent of the number of cycles.Magnesium-based hydrogen storage materials with 5%,10%and 20%Ni content decreased the peak temperature of dehydrogenation by 15-20?,2-3?,11-13? and the activation energy was reduced by 28.62 kJ/mol,5.59 kJ/mol and 11.56 kJ/mol,respectively.Ni exists in the form of elementary substance in the material after ball milling.After 30 cycles,it is completely transformed into Mg2Ni alloy.There is only one endothermic peak in the DSC curve of pure Mg,which corresponds to the peak of hydrogen evolution of MgH2.Three endothermic peaks appeared in DSC curves of Mg-based hydrogen storage materials with Ni addition.The endothermic peaks of about 250? correspond to the dehydrogenation peaks of Mg2NiH4,and there are two dehydrogenation peaks of MgH2 according to different dehydrogenation paths.