Surface Coating Process <sub> 3 </ Sub> Type Hydrogen Storage Alloy Electrochemical Properties Of Rare Earth Based Ab

AB₃-type hydrogen storage alloys are promising candidates for reversible hydrogen storage because of their high discharge capacity , easy activation and good hydrogen pressure, but AB3-type hydrogen storage alloys deteriorate faster during cycling. In this paper, the influence of Ni-P and Ni-Co-P coatings on the electrochemical properties of AB3-type hydrogen storage alloy electrode was studied.For further improving the performances of the negative electrodes, microencapsulation of the alloy particles was done . In this paper the technique of Ni-P coating by chemical plating was systematically studied and optimized initially. The results of SEM showed that some global grain was covered on the surface of alloys. The electrochemical results showed that, after Ni-P coating, the cyclic stability of the alloy electrodes was improved, while the maximum discharge capacity was reduced. The discharge capacity retaining rate of the MH electrodes coated with Ni-P was about 71 percent after 100 cycles, whereas the discharge capacity retaining rate of bare electrode was 60 percent. The kinetic properties of alloy electrode were evaluated by using the potentiodynamic method. The Ni-P-coated powder electrode showed improved performance, i.e., a higher exchange density Io, higher limiting current density I₁, lager high rate dischargeability and hydrogen diffusion coefficient D. It was found that the surface microencapsulation was ineffective in prohibiting disintegration of the La_0.67Mg(0.33)Ni_2.5Co_0.5 alloy powder, main reason for which is probably ascribed to larger expansion of the La_0.67Mg_0.33Ni_2.5Co_0.5 hydrogen storage alloy hydride.Moreover, Ni-Co-P coating on the alloy surface was done. The results of SEM showed that microencapsulation was close-grained, the effect of coating was preferable. The discharge capacity retaining rate of the MH electrodes coated with Ni-Co-P was about 88 percent after 100 cycles, whereas the discharge capacity retaining rate of bare electrode was 73 percent. Optic micrographs of alloy particles which were charged and discharged about 50 times showed that the effect of the microencapsulation is preferable, the layer of the coating existed, and the results of XRD showed that the oxidation of the surface of the alloy electrode was controlled, there was no La(OH)₃. The kinetic properties of alloy electrode were evaluated by using the potentiodynamic method. The kinetic properties of alloy electrode were improved greatly.