Investigation On Effects Of Si To The Phase Structure And Electrochemical Performances Of La-Mg-Ni System A₂b₇Type Hydrogen Storage Alloys

The most promising new type hydrogen storage electrode materials with highcapacity are the La-Mg-Ni system alloys in view of their higher capacity and excellenthydrogen absorption/desorption kinetics characteristic, however, their poorelectrochemical cyclic stability limit their commercial utilization. Therefore, it is thekey question of utilization researches that how to improve the cyclic stabilities of thealloy electrodes. The La-Mg-Ni system A₂B₇type hydrogen storage La_0.75Mg_0.25Ni_3.1-xCo_0.3Al_0.1Si_x（x=0,0.05,0.1,0.15,0.2） alloys were prepared by vacuum induction meltingand annealing technology.The phase component and phase structure were analyzed byX-ray diffractometer （XRD）.Electrochemical properties and dynamic characteristics ofalloys were measured by electrochemical measurement system. Cast and annealedalloys of gaseous absorption hydrogen curve were measured with the PCT curve tester.The phase structure and electrochemical performance of La-Mg-Ni system A₂B₇type alloy with Si element substituting Ni were investigated. The results of the analyzedstructure show that the alloys are composed of multi-phase structures containing LaNi₅,（La,Mg）3Ni main phases and a small amount of LaNi₂and Al₃Ni phase.（La,Mg）₃Niphase becomes less and LaNi₅phase becomes more with the Si substituting Ni,and thecell volume of LaNi₅phase increases first then decreases.The alloy has excellentactivation characteristic. The platform pressure of absorbing and diffusing hydrogenfalls after the Si substituting Ni,which results that the discharge capacity of the alloysdecreases.The cyclic stabilities of the alloys get improved with the optimum quantity ofSi.The charge transfer resistance,limit current density and hydrogen diffusioncoefficient of alloy electrode increase first and then decrease with the Si contentincreasing,which supports the change rule of the alloys’ high-rate dischargeabilities,thus the alloy electrode’s dynamic characteristics raises first and then falls withthe Si content increasing.The effects of annealing technology on the microstructure and electrochemicalperformances of the alloys were studied. Annealing treatment results in thedisappearance of the （La,Mg）Ni₃phase and the increase of LaNi₅phase,The annealedalloy also has excellent activation characteristic.The discharge capacity ofLa_0.75Mg_0.25Ni_3.1-xCo_0.3Al_0.1Si_x（x=0⁰.15） alloy increases after annealed with the well temperature,but the discharge capacity of alloy La_0.75Mg_0.25Ni_3.1-xCo_0.3Al_0.1Si_x（x=0.2）increases after annealed,and the cyclic stabilities of the alloys get improved afterannealed.The high-rate discharge abilities of alloy increase after annealed when the Sicontent x are0,0.15and0.2,but decrease after annealed when the Si content x are0.05and0.1.The kinetics properties of gaseous hydrogen absorption capacity of alloyLa_0.75Mg_0.25Ni_3.1-xCo_0.3Al_0.1Si_x（x=0,0.1） was systematically investigated.There is wellincipient hydrogen absorption speed of alloy.The hydrogen absorption capacityincreases at high temperature.Si substituting Ni worsens the hydrogen absorptionbecause the alloy cell volume decreased.Alloy’s hydrogen absorption capacity wouldincrease after annealed with well temperature,but the increasing capacity is small whenthe Si substitutes Ni.