The Electrochemical Properties Studies On Microencapsulated La-Mg-Ni-based 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, this kind of alloy possesses an important prospect of development and utilization. However, the cyclic life of alloy electrodes is poor, then it is the key question of unilization researches that how to improve the cyclic life of alloy electrodes. So in this paper, the influence of Ni-P, Co-P and Cu coatings on the electrochemical properties of A₂B₇-type hydrogen storage alloy electrode was studied.Firstly, in this paper the Ni-P 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 Ni-P coatings was very symmetrical.The electrochemical results showed that, after Ni-P coating, the cyclic stability of the alloy electrodes was some improved, while the maximum discharge capacity was reduced. The kinetic properties of surface microencapsulation alloy electrode were improved by testing kinetic parameter i.e., a higher exchange density I₀ and lager high rate dischargeability.For further improving the performances of the A₂B₇-type alloy electrodes, The effect of Co-P coatings and annealing on the electrochemical properties of La_1.5Mg_0.5Ni₇ hydrogen storage alloy electrode was systematically studied. The results of SEM showed that many small cobalt global grains were covered on the surface of alloys. Heat treatment make cobalt grains grow and some pore appear on the surface. XRD show that electroless plated Co-P alloys coating is crystalline,being a solid solution of phosphors in cobalt matrix with finer grains as plated .when heated, grains growth stress relaxation occur in the coating.With the heating temperature increasing, coating crystal structure was changed,metallic compounds CoP₃ and β— Co phases deposition and allotropic transformation from α—Co, at the same time, LaNi₅ phases increase. The metamorphic temperature is 773K. The electrochemical results showed: material has a higher capacity and has two discharge voltage compared to the bare alloy due to the faradaic reaction of cobalt during discharge, the discharge capacity of coating electrodes could be up to 433.1mAh/g, but the cycle life of coated material was not improved compared to the bare alloy, comparing to coating alloys,When the coating alloys was treated at the low heating temperature ,whose capacity approximated to coating alloys and cycling stability was improved ,with the heating temperature increasing, the capacity and cycling stability were all deteriorated.The Co-P coated powder electrode is better than bare relectrode in exchange current density I₀ limiting current density I₀. The kinetic properties of alloy electrode change rarely when the coating alloys were annealed at the low temperature, butwith the heating temperature increasing, the kinetic properties were deteriorated.Finally, in order to improve the cyclic stability of A₂B₇-type alloy electrodes, the Cu coating by chemical plating was systematically investigated.We deal with alloy make use of two different methods, at the same time, the cover Ni-P and Co-P on the Cu coating alloy electrodes were also studied.The picture of SEM showed the copper aggradation surface with hypophosphite reducer was covered with very close-grained grains,the surface layer forms of alloys by erode plating were associated with the chroma of vitriol .The electrochemical results showed the cycle life of the materials coated by Cu and Cu-Co were improved .Cu coating could obviously improve overall electrochemical properties of alloy electrodes. After 10% Cu coating the capacity retention rate after 140 charge/discharge cycles could reach73.2%, whereas the discharge capacity retaining rate of bare electrode was 66.1%. The cycle life of Cu coating alloys by erode plating all were improved obviously, but their discharge capacity minished evidently. The kinetic properties testing show: The Cu, Cu-Co, Cu-Ni surface microencapsulation all obviously improved the kinetic properties of alloy electrodes.