Effects of surface coating with Cu-Pd on electrochemical properties of A₂B₇- type hydrogen storage alloy

La_0.75Mg_0.25Ni_3.2Co_0.2Al_0.1 hydrogen storage alloy, the nickel-metal hydride (MH/Ni) secondary battery negative electrode, was modified by CuSO₄ solution (3 wt% in Cu in contrast with alloy weight) and PdCl₂ solution varied from 1 wt% to 4 wt% in Pd in contrast with alloy weight with a simplified pollution-free replacement plating method, aiming at improving its comprehensive electrochemical properties. The XRD analysis and SEM images combined with EDS results reveal that Cu and Pd nanoparticles are uniformly plated on the pristine alloy surface. The relative amount of Pd on the Cu-Pd coated alloy surface increases notably as the PdCl₂ concentration increases in the plating solution. Electrochemical tests indicate that alloy electrodes modified by Cu-Pd composite coating show perfect activation performance, which achieve the maximum discharge capacity at the first charge-discharge cycle. Moreover, alloy electrodes coated with Cu-Pd perform dramatically enhanced high rate dischargeability (HRD). The enhancement increases firstly and then decreases as the content of Pd increases in the Cu-Pd coating. Meanwhile, the cycle life of modified alloys is also improved significantly. Among all the samples, the Cu-Pd coated alloy with 3 wt% Pd content in the PdCl₂ solution reinforces the comprehensive electrochemical properties most sufficiently, with dischargeability of 86.4% under 1500 mA/g and remaining capacity of 82.7% after 100 cycles.