Effect Of Ca And Catalyst Addition On Microstructures Evolution And Hydrogen Storage Properties Of Mg-based Hydrogen Storage Alloys

Hydrogen is an ideal energy carrier,which is the key to solve the dilemma of carbon cycle and realize the transformation of energy structure.However,safe,efficient and low-cost hydrogen storage and transportation is the bottleneck restricting the development of hydrogen energy.Magnesium materials have attracted much attention in the field of solid hydrogen storage due to their advantages of high hydrogen storage capacity,low cost and high safety.However,due to the high thermodynamic stability and slow kinetic behavior of magnesium reversible hydrogen absorption and desorption reaction,its wide application is limited.To solve the above problems,microalloying and catalyst addition were used to optimize the thermal/kinetic properties of magnesium alloy.In this study,Ca element microalloying and TiO2 and Nb2O5 catalyst addition were used to optimize the thermodynamic and kinetic properties of magnesium alloy.The effects of Mg80-xNi20Cax(x=1,2,4,6)alloy on microstructure evolution and hydrogen absorption and desorption properties were deeply explored.The main work and conclusions are as follows:Mg80-xNi20Cax(x=1,2,4,6)alloy was prepared by induction melting method.The phase structure and microstructure of Mg80-xNi20Cax(x=1,2,4,6)alloy were characterized by XRD,SEM and EDS.Hydrogen storage properties of Mg80-xNi20Cax alloy were tested at 250℃～350℃ and 0-4MPa.It was found that the addition of Ca(1wt.%～4wt.%)was beneficial to the refining of α-Mg matrix and the precipitation of Mg2Ni.With the increase of Ca content,Ca precipitates from the solid solution and forms Ca-Ni and Mg-Ca intermediate phases.The hydrogen storage capacity of the material increased first and then decreased(3.98→5.53→4.59→3.97,wt.%),and the hydrogen absorption rate decreased first and then increased(0.46→0.23→0.07→1.08,wt.%/min),indicating that Ca had a significant promoting effect on the hydrogen absorption and desorption rate.Mg78Ni2oCa2 alloy was selected and Mg78Ni20Ca2+5wt.%TiO2/Nb2O5 was prepared by ball milling method.The phase structure and microstructure of Mg78Ni20Ca2+5wt.%TiO2/Nb2O5 were characterized,and the hydrogen storage performance was tested.The results showed that a few catalysts have an obvious effect on the hydrogen absorption and desorption properties of Mg-Ni alloy.TiO2 and Nb2O5 didn’t change the phase structure of Mg-based hydrogen storage material matrix,but slightly widen the tristrong peak of Mg matrix and improve the intensity of its diffraction peak.The hydrogen absorption and desorption test results showed that TiO2 and Nb2O5 significantly improved the hydrogen absorption/desorption kinetics(hydrogen absorption:0.23→0.76→0.83,wt.%/min;dehydrogenation:0.46→0.64→0.79,wt.%/min),and hydrogen storage also increased(5.53→5.92～5.93,wt.%).The JMAK model of material hydrogen absorption/desorption kinetics was constructed by in-depth study of Mg80-xNi20Cax hydrogen absorption/desorption kinetics.The results showed that the growth mode of hydride after nucleation could be changed by the increase of Ca content and the addition of a few catalysts.The growth mode of hydride after nucleation changed from one-dimensional growth model to three-dimensional growth model,which was also the essential reason for the increase of hydrogen absorption/desorption rate.