Study Of Preparation And Hydrogen Storage Properties Of Mg-Gd-(Y)-Zn Alloys

Hydrogen energy is a bridge for transition and conversion between traditional fossil energy and renewable energy,and it is an important choice to achieve carbon peak and carbon neutrality.However,the safe and efficient storage of hydrogen energy is the key to achieve practical application,because of its inflammable and explosive characteristics.Magnesium and its alloys are considered as an ideal solid hydrogen storage materials,because of their high theoretical hydrogen storage capacity and good reversible hydrogen de/absorption performance.However,it has some disadvantages such as high thermodynamic stability and slow dehydrogenation rate.Some rare earth magnesium alloys containing LPSO(long-period stacking ordered)Phases can in-situ form catalytic REHx hydride during hydrogen de/absorption process,which can effectively improve hydrogen storage performance of Mg alloys.However,there are few reports on hydrogen storage properties of Mg-Gd-(Y)-Zn alloys and effect of severe plastic deformation(such as equal diameter angular extrusion(ECAP))on hydrogen storage properties of alloy.In this paper,Mg-Gd-(Y)-Zn alloy was selected as research object,the microstructure and hydrogen storage properties of as-cast and ECAPed Mg-Gd-(Y)-Zn alloy were investigated,and the modification mechanism was explored by theoretical calculation.The specific research contents are as follows:(1)Three as-cast alloys Mg-4.4Gd-1.1Zn,Mg-2.8Gd-1.6Y-1.1Zn and Mg-4.4Y-1.1Zn were prepared by semi-continuous casting,and their microstructure and hydrogen storage properties were studied.The results show that the three as-cast alloys are all composed of a-Mg and LPSO phases.In contrast,the as-cast Mg-2.8Gd-1.6Y-1.1Zn alloy exhibits denser serrated surface morphology after filing,more microscopic cracks and fine particles after hydrogen de/absorption.Which are beneficial to shorten diffusion distance of H and improve hydrogen de/absorption kinetics.In addition,the LPSO phase is broken,decomposed and the(Gd/Y)Hx hydrides is in-situ formed during hydrogen de/absorption.These(Gd/Y)Hx hydrides can not only act as catalysts to provide activation sites for diffusion of H,but also act as pinning sites to inhibit growth of Mg/MgH2 grains.(2)The three as-cast alloys show excellent activation performance and hydrogen de/absorption kinetics after activation and isothermal hydrogen de/absorption.In contrast,the as-cast Mg-2.8Gd-1.6Y-1.1Zn alloy has the shortest "latency period" in the activation stage,and the hydrogen de/absorption capacity can be almost close to its theoretical hydrogen storage capacity(7.18wt%).In the isothermal hydrogen de/absorption stage,it can absorb about 5wt%of hydrogen in 500-1500s,and the maximum hydrogen absorption can reach 6.6wt%;meantime,it can desorption about 6.5wt%in 1000-3000 s,showing the most excellent hydrogen de/absorption.This may be caused by the synergistic modification effect of Gd and Y co-doping.(3)The as-cast Mg-2.8Gd-1.6Y-1.1Zn alloy with better hydrogen storage properties was selected,and its microstructure and hydrogen storage performance were further studied after ECAP processing.The results show that a large number of needle-like LPSO phases are precipitated in the α-Mg matrix of ECAPed Mg-2.8Gd-1.6Y-1.1Zn alloy,and the LPSO phases at the grain boundaries are twisted,bent and even broken.However,compared with as-cast alloys,they exhibit poorer activation and hydrogen de/absorption properties.The reasons may lie in two aspects:on the one hand,homogenization annealing makes RE/Zn atoms dissolved into Mg grains,which makes it more difficult for RE elements in grains to in-situ form REHx during hydrogen de/absorption,so that the content of REHx is lower than that of as-cast alloy,reducing its catalytic effect.On the other hand,high temperature activation may cause the growth of Mg/MgH2 grains,reducing its hydrogen storage performance.The first-principle calculations show that the GdH2 and YH2 hydrides can improve the hydrogen de/absorption dynamics of Mg alloy and weaken the bond energies of H-H and Mg-H bonds.In comparison,YH2 is more conducive to promote H2 dissociation and improve hydrogen adsorption kinetics of system,while GdH2 is more conducive to promote H desorption in MgH2 and improve the dehydrogenation kinetics effect of system.