Hydrogen Storage Properties And Electrochemical Performance Of La_0.75Mg_0.25Ni_2.85Co_0.45-x（AlSn）_x（x=0-0.3）Alloys

La0.75Mg0.25Ni2.85Co0.45-x(AlSn)x(x=0-0.3) based alloy ingots were prepared by magnetic induction-melting furnance under Argon atmosphere. The structures of alloys were studied by X-ray diffraction. Gaseous hydrogen storage and electrochemical properties of alloys were investigated by PCT and electrochemical instrument. In this paper, we discussed the influence of elements substitution, annealing treatment and dopants addition on the overall properties of Rare-Mg-Ni-Co based alloys.For the La0.75Mg0.25Ni2.85Co0.45-x(AlSn)x(x=0,0.1,0.2,0.3) alloys, LaNi5,La2Ni7,LaNi3were the main phases as Ni3Sn4and NiSn were the vice phases. With the amount of Al, Sn added, the discharge capacity of compounds first increased then decreased. The high rate discharge performance was relatively poor. The La0.75Mg0.25Ni2.85Co0.35(AlSn)0.1alloy electrode, had the maximum discharge capacity retention rate S40, best cyclic stability and superior electrochemical kinetic performance.The effect of annealing temperature on the hydrogen storage and electrochemical properties of La0.75Mg0.25Ni2.85Co0.35(AlSn)0.1alloy was investigated. The alloys possessed multi-Phases, which included La(Ni,Al,Sn)5,LaNi4.27Sno.24,La2Ni7and LaNi3phase. Heating the alloys at800℃for5h resulted in the improvement of the hydrogen absorption and cyclic stability. However after annealing treatment, the electrochemical kinetic of alloy electrodes performed lower than that of as-cast alloy.In order to further improve the electrode performance, the annealing treatment was performed at800℃for5h,10h,15h and20h on the as-cast Lao.75Mgo.25Ni2.85Coo.35(AlSn)0.1alloys. The alloy phases mainly composed of La(Ni,Al,Sn)5,LaNi4.27Sn0.24and La2Ni7phase. The maximum hydrogen storage capacity was1.30wt.%(as-cast),1.46wt.%(5h),1.38wt.%(10h),1.32wt.%(15h),1.44wt.%(20h), respectively. The discharge capacity retention rate S45increased from73.9%(as-cast) to74.8%(10h),75.0%(15h),75.3%(20h).After annealing treatment, the kinetic parameters were poor than the as-cast alloy.For Lao.75Mg0.25Ni2.85Co0.35(AlSn)0.1-Ti1.5Zr5.5V2.5Ni7.5alloy, the addition of Ti1.5Zr5.5V2.5Ni7.5alloy effectively lowered the hydrogen storage capacity and improved the hydrogen absorption/desorption kinetics. The discharge capacities decreased from363.76mAh/g(x=0) to363.76mAh/g(x=0). With the increase of x value, the HRD of the composite electrode first increased then decreased when the discharge current was1200mAh/g. The reaction kinetics of the electrode not only was controlled by the H diffusion rate from the interior to surface of the composite particles, but by the charge-transfer reaction at the composite/electrolyte interface.The influence of annealing temperature on the La0.75Mg0.25Ni2.85Coo.35(AlSn)0.1-3wt.%Ti1.5Zr5.5V2.5Ni7.5alloy electrode was investigated and reported.For the samples annealed at750℃, the hydrogen absorption/desorption kinetics of the alloy was better. The electrochemical properties of the annealed/as-cast alloy was reported, which revealed that the discharge capacity retention S70(S70=C70/Cmax×100%) increased from60.5%(as-cast)to61.3%(750℃),62.1%(850℃),63.2%(950℃) with increasing annealed temperature. The anodic peak current density, the limiting current density and the exchange current density first increased then decreased. The value of the diffusion coefficient D remained almost unchanged, so the electrochemical kinetic performance was controlled by the charge-transfer reaction of hydrogen at the composite/electrolyte interface.