Preparation And The Hydrogen Storage Properties Of High Entropy Alloys Containing Magnesium

Since traditional fossil energy sources are not renewable and will eventually be depleted,and the burning of large amounts of fossil energy causes many environmental problems,hydrogen energy could be an extremely anticipated new energy supply as an alternate.It is important to study hydrogen storage alloys with excellent hydrogen storage performance.High entropy alloys(HEAs)have unique characteristics such as structural simplification and high entropy effect,which are very suitable materials for hydrogen storage.And Magnesium is a cheap and abundant element with high theoretical hydrogen storage capacity.In this paper,the alloy powders of the HEAs were prepared by high energy ball milling technique,and X-ray diffraction(XRD)tests were performed to determine the crystal structure and grain size of the samples after ball milling.The morphological changes of the microscopic particles were tested by scanning electron microscopy(SEM),and the elemental distribution and atomic percentages of the samples were tested by energy spectrometry(EDS).The hydrogen storage properties were tested mainly by Sievert's tester for kinetic and P-C-T properties of the alloys,and the thermodynamic properties were calculated and analyzed.Behavior of hydrogen absorption and desorption of the MgVAlCrMn high-entropy alloy with a single BCC phase and a lattice constant of a=3.4586(?)was analyzed.The hydrogen absorption kinetics were tested at temperatures from 523 K to 623 K and hydrogen pressure from 6 to 8 MPa,and it was concluded that the maximum hydrogen absorption of 2.3wt%could be achieved at 623 K,8 MPa,but the hydrogen absorption rate was slow.On the other hand,when the temperature was decreased from 623 K to 523 K,the hydrogen absorption rate was increased while the hydrogen absorption was decreased.The enthalpy changes?H and entropy change?S of the alloy of hydride are-13.39 k J·mol^-1 and-31.72 J·mol^-1K^-1,respectively.Moreover,the Mg content can affect the hydrogen storage performance of Mg_xVAl Cr Mn(x=0,0.5,1,1.5,2)alloys.At the conditions of 423K and 6 MPa,the maximum hydrogen absorption capacity was continuously increased with the increase of x value from 0to 1,and the maximum hydrogen absorption capacity of MgVAlCrMn alloy was 1.3wt%.Furtherly,when the x value was increased to 2,the maximum hydrogen absorption capacity of Mg₂VAl Cr Mn alloy was decreased to 0.41wt%.New HEA systems such as Mg_xTiVNiAlCr(x=0,0.5,1),Mg TiVNiAlCo and Mg TiVNiAlMn were synthesized.Mg_xTiVNiAlCr(x=0,0.5,1)HEAs are all composed by BCC and C14 Laves phases.With the increase of Mg content,the maximum hydrogen absorption is increased,hydrogen absorption activation is reduced,and the formation enthalpy of hydride is also reduced.The maximum hydrogen absorption capacity,hydrogenation enthalpy and entropy of Mg TiVNiAlCr alloy are 1.3wt%,-22.101 k J·mol^-1 and-43.876J·mol^-1K^-1,respectively.Besides,the hydrogen storage properties of the three groups of alloys Mg TiVNiAlM(M=Cr,Co,Mn)were compared.The alloy containing Cr has the better hydrogen absorption kinetics,and 0.7wt%hydrogen was absorbed at 473 K,5 MPa hydrogen pressure for 300s.The alloy containing Cr has a lower formation enthalpy of-22.101 k J·mol^-1,better hydrogenation stability and the largest hydrogen absorption capacity.