Study Of Hydrogen Storage Properties Of Ball Milling Nanocrystalline/Amorphous RE-Mg-Ni Alloy

Magnesium-based hydrogen storage alloy is one of the most promising hydrogen storage materials because of its large hydrogen storage capacity and low price.However,it has the disadvantages of poor cycle stability,high hydrogen absorption and desorption temperature,etc.In this paper,the properties of alloys are improves by elemental method,mechanical ball milling and adding catalyst.La2-xRExMg16Ni(RE = Y,Sm,x = 0,0.1,0.2,0.3,0.4)hydrogen storage alloys were prepared by a medium frequency induction furnace under a helium atmosphere.Phase composition,microstructure,electrochemical hydrogen storage properties,gaseous hydrogen storage kinetics and thermodynamic properties of La2-xRExMg16Ni alloys were investigated.Subsequently,La1.8Y0.2Mg16Ni and La1.8gSm0.2Mg16Ni alloys are researched due to the better electrochemical hydrogen storage properties in the gained alloys.The effects of milling time on the properties with 150 wt.%Ni and the effect of Ni content on the properties of the two kinds of alloys for 20 hours ball milling were studied.The experimental results show that the optimum amount of Ni addition is 150wt.%,and the optimum milling time is 20 h.La2RExMg16Ni alloys are composed of La2Mg17,Mg2Ni and La2Ni3 phase.After hydrogen absorption,the alloys are made up of MgH2,Mg2NiH4 and LaH3 phase.Sm3H7 phase is existed in alloys of Sm substitution,while YH3 phase is in alloys of Y substitution.After hydrogen desorption,the alloys are composed of Mg,Mg2Ni and LaH3 phase.Sm3H7 phase appears in the alloys of Sm substitution,while YH2 phase appears in the alloys of Y substitution.The maximum discharge capacity and maximum hydrogen absorption capacity of the alloys are slightly influenced by increasing of Y and Sm substitution.However,both cyclic stability and electrochemical kinetics of the alloys are increased firstly and then decreased.When the substitution amount x is 0.2,the cyclic stability and electrochemical kinetics performance of the alloys are better.After partial replacement of Y or Sm elements,the gaseous hydrogen storage activation and gaseous hydrogen absorption and desorption properties of the alloys are enhanced.The main phase of La1.8RE0.2Mg16Ni + 150 wt.%Ni alloys milled for 10 h,20 h and 40 h is Ni phase.There are a small amount of La2Mg17 phase and nanocrystalline/amorphous structure after ball milling for 10 h and 20 h.The initial stage of ball milling time increased has a great influence on discharge specific capacity,and the following period of ball milling time has slightly affected on discharge specific capacity.The maximum discharge of Lal.8RE0.2Mg16Ni +150 wt.%Ni after ball milling for 40 h are 871.9 mAh/g and 873.6 mAh/g,respectively.With the increasing of ball milling time,the cyclic stability and electrochemical kinetics performance of hydrogen storage alloys are increased first and then decreased.The maximum capacity retention rates of La1.8Y0.2Mg16Ni + 150 wt.%Ni and La1.8Sm0.2Mg16Ni + 150 wt.%Ni alloys after ball milling for 20 h are S50=60.8%and S50=61.7%,the maximum hydrogen diffusion coefficients are 3.36×l0-11 cm2/s and 3.23×10-11-cm2/s,while the maximum current limiting density IL are 753.9 mA/g and 756.8 mA/g,respectively.The main phase of La1.8RE0.2Mg16Ni hydrogen storage alloys without Ni after being milled for 20 h are La2Mg17 phase and a small amount of Mg2Ni phase.When the amount of Ni addition is 100 wt.%and 150 wt.%,the main phase in the alloys is Ni,and a certain amount of La2Mg17 and Mg2Ni phase.When the amount of Ni addition is 200 wt.%,the main phase of alloys is Ni,La2Mg17 is transformed into amorphous.The milling for 20 h has slightly affected on the maximum discharge specific capacity of alloys without Ni addition.At the initial stage,with increasing Ni content,the discharge specific capacity increased significantly.In the late period,continues,the discharge capacity increased slightly with the addition of Ni.The maximum capacity retention rates of La1.8Y0.2Mg16Ni + 200 wt.%Ni and La1.8Sm0.2Mg16Ni + 200 wt.%Ni alloys are 788.8 mAh/g and 786.8 mAh/g,respectively.With the increase of Ni addition,the electrochemical cycling and electrochemical kinetics performance of the alloys are increased first and then decreased.The capacity retention,limiting current density and hydrogen diffusion coefficient exist maximum value when there are 150 wt.%Ni addition in the two kinds of alloys.