Studies On Surface Modification Of La-Mg-Ni System A₂B₇-type Hydrogen Storage Electrode Alloys

A₂B₇-type hydrogen storage alloy of La-Mg-Ni system is a kind of new type high capacity electrode alloy with better overall electrochemical properties which was found in recent years,however,the cyclic life of alloy electrodes is poor and constrains further development and application of the alloy,then it is the key question of presently researches that how to improve the cyclic life of alloy electrodes.In this paper,the influence of nickel coatings and copper coatings by chemical plating,nickel coating by milling on the electrochemical properties of A₂B₇-type hydrogen storage alloy electrode through different method was studied.Firstly,in this paper the nickel coating by chemical plating was systematically studied.The results of SEM showed that some global grain was covered on the surface of alloys.The section optic microscope picture of powder alloy showed that the nickel coating was very symmetrical.The electrochemical results showed that,after nickel coating,the cyclic stability of the alloy electrodes was some improved,while the maximum discharge capacity was reduced.The results of linear polarization and impedance spectroscopy show that hydrogen storage alloys coated could increase exchange current density I₀,reduce electrochemical reaction resistance,so the high-rate discharge performance of the coated electrode is successfully improved.Then,the effect of copper coatings by chemical plating and annealing on the electrochemical properties of La_0.75Mg_0.25Ni_3.44Al_0.06hydrogen storage alloy electrode was systematically studied.The result of study showed that annealing on the electrode performance of the activation was not obvious,but is conducive to increase cycling stability of alloy electrode;Make the alloy electrode of surface coating by copper before annealing could obvious improve the performance of the alloy and the reason is complicated;otherwise the dynamic performance of annealing-plated alloy electrode was not improve.The results of research for the surface coating by chemical plating showed that this method can improve the stability of the cycle of alloy electrodes,but the effect was not obvious improvement.In order to find the reasons for it,simulation electrochemical cycle of alloy electrode in lye,and rule out the possibility of corrosion impact on A₂B₇-type hydrogen storage alloy,made the powder of copper coating absorb and discharge gaseous hydrogen cycle under certain conditions.The results showed that,in the course of gaseous hydrogen filling cycle,alloy powder was pulverized;coating was crack or even falling completely from the surface of alloy. Coating does not inhibit the alloy powder and corrosion and surface coating can not effectively improve the stability of the cycle of La-Mg-Ni system A₂B₇ type hydrogen storage alloy.Finally,in order to improve the cyclic stability of A₂B₇-type alloy electrodes,the nickel coating by milling and annealing on the electrochemical properties of La_0.75Mg_0.25Ni_3.44Al_0.06hydrogen storage alloy electrode was systematically studied. The results showed that milling-coated 10%nickel in different time could quicken the activation of alloy electrode and improve cycling stability of alloy electrode.The test of electrode dynamic performance of results showed that milling-coated electrode treatment could increase the exchange of current density,reduce the contact resistance and surface charge transfer resistance,raise the limit current density and speed up the proliferation rate of hydrogen in the alloy,thus improved the high-rate discharge performance of alloy electrode;Comparing to coating alloys,when the coating alloys was treated at different time,whose the activation of electrode were deteriorated and the largest discharge capacity were reduced,but after eight cycles the discharge capacity of the electrode higher than the untreated alloy electrode.