Effects of Pd substitution on the electrochemical properties of Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) hydrogen storage alloys

Amorphous Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) hydrogen storage alloys were prepared by mechanical alloying (MA). The effects of Pd substitution on the electrochemical properties of the Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) electrode alloys were studied by cyclic charge–discharge, linear polarization, anodic polarization, electrochemical impedance spectroscopy (EIS), and hydrogen diffusion coefficient experiments. It was found that the cyclic capacity retention rate C50/C1 of the quaternary alloys was greatly improved due to the substitution of Pd for Mg. Mg0.8Ti0.1Pd0.1Ni electrode alloy retained the discharge capacity above 200 mAh g−1 even after 80 charge–discharge cycles, possessing the longest cycle life in the studied quaternary alloys. The improvement of cycle life was ascribed to the formation of passive film on the surface of these electrode alloys. X-ray photoelectron spectroscopy (XPS) analysis proved that the passive film was composed of Mg(OH)2, TiO2, NiO, and PdO, which synergistically protected the alloy from further oxidation. The Auger Electron Spectroscopy (AES) study revealed that the thickness of passive film increased with augmentation of the Pd content. The electrochemical impedance study of electrode alloys after different cycles demonstrated that the passive film became thicker during cycles and its thickness also increased with Pd content augmentation. It was also found that the augmentation of Pd content resulted in the decrease of exchange current density I0 and the increase of the charge-transfer resistance Rct. With increasing the Pd amount in the Mg0.9−xTi0.1PdxNi (x = 0.04–0.1) electrode alloys, hydrogen diffusion coefficient D was gradually enhanced at first. Then, it decreased with augmentation of cycle due to the growth of passive film on the surface of the alloys.