Investigation On Effects Of Si To The Phase Structure And Electrochemical Performances Of La-Mg-Ni System A₂B₇ Type Hydrogen Storage Alloys

The most promising new type hydrogen storage electrode materials with high capacity are the La-Mg-Ni system alloys in view of their higher capacity and excellent hydrogen absorption/desorption kinetics characteristic, however, their poor electrochemical cyclic stability limit their commercial utilization. Therefore, it is the key question of utilization researches that how to improve the cyclic stabilities of the alloy electrodes. The La-Mg-Ni system A₂B₇ type hydrogen storage La0.75Mg0.25Ni3.1-xCo0.3Al0.1Six(x=0,0.05,0.1,0.15,0.2) alloys were prepared by vacuum induction melting and annealing technology .The phase component and phase structure were analyzed by X-ray diffractometer (XRD).Electrochemical properties and dynamic characteristics of alloys were measured by electrochemical measurement system. Cast and annealed alloys of gaseous absorption hydrogen curve were measured with the PCT curve tester.The phase structure and electrochemical performance of La-Mg-Ni system A₂B₇ type alloy with Si element substituting Ni were investigated. The results of the analyzed structure show that the alloys are composed of multi-phase structures containing LaNi5, (La,Mg)3Ni main phases and a small amount of LaNi2 and Al3Ni phase. (La,Mg)3Ni phase becomes less and LaNi5 phase becomes more with the Si substituting Ni,and the cell volume of LaNi5 phase increases first then decreases.The alloy has excellent activation characteristic. The platform pressure of absorbing and diffusing hydrogen falls after the Si substituting Ni,which results that the discharge capacity of the alloys decreases.The cyclic stabilities of the alloys get improved with the optimum quantity of Si.The charge transfer resistance,limit current density and hydrogen diffusion coefficient of alloy electrode increase first and then decrease with the Si content increasing ,which supports the change rule of the alloys' high-rate discharge abilities,thus the alloy electrode's dynamic characteristics raises first and then falls with the Si content increasing.The effects of annealing technology on the microstructure and electrochemical performances of the alloys were studied. Annealing treatment results in the disappearance of the (La,Mg)Ni3 phase and the increase of LaNi5 phase,The annealed alloy also has excellent activation characteristic.The discharge capacity of La_0.75Mg_0.25Ni_3.1-xCo_0.30Al_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_0.30Al_0.1Si_x(x=0.2) increases after annealed,and the cyclic stabilities of the alloys get improved after annealed.The high-rate discharge abilities of alloy increase after annealed when the Si content x are 0,0.15 and 0.2,but decrease after annealed when the Si content x are 0.05 and 0.1.The kinetics properties of gaseous hydrogen absorption capacity of alloy La_0.75Mg_0.25Ni_3.1-xCo_0.30Al_0.1Si_x(x=0,0.1) was systematically investigated.There is well incipient hydrogen absorption speed of alloy.The hydrogen absorption capacity increases at high temperature.Si substituting Ni worsens the hydrogen absorption because the alloy cell volume decreased.Alloy's hydrogen absorption capacity would increase after annealed with well temperature,but the increasing capacity is small when the Si substitutes Ni.