Electrochemical performances of the Pd-added Ti-V-based hydrogen storage alloys

To improve the electrochemical kinetic performances of the Ti-V-based hydrogen storage alloys, the Pd element was introduced into the Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.0 alloy, and the structure and electrochemical properties of the Ti0.8Zr0.2V2.7Mn0.5Cr0.8Ni1.0Pdx(x=0–0.2)(x=0–0.2) alloys were systematically investigated by X-ray diffraction (XRD), galvanostatic charge/discharge, high rate discharge (HRD), electrochemical impedance spectroscopy (EIS), linear polarization and potentiostatic step discharge measurements. The structural investigations show that all of the alloys consist of a C14 Laves phase and a V-based solid solution (VSS) phase. The increase of Pd content reduces the abundance of the C14 Laves phase, accordingly increases that of the V-based solid solution phase. Electrochemical measurements indicate that the activation property of the alloy electrodes is obviously improved after adding Pd due to its good electrocatalytic activity. With increasing Pd content, the maximum discharge capacity of the alloy electrodes first increases from 331 mAh/g (x=0x=0) to 357 mAh/g (x=0.15x=0.15) and then decreases to 338 mAh/g (x=0.2x=0.2), meanwhile, the cycling stability of the alloy electrodes is markedly improved. The HRD results reveal that the proper addition of Pd can effectively improve the electrochemical kinetic performances of the alloy electrodes. Further kinetic parameter measurements show that the charge-transfer reaction resistance of the alloy surface first decreases when x increases from 0 to 0.1, and then increases with x   further increasing to 0.2, and the exchange current density and the hydrogen diffusion coefficient first increases and then decreases, indicating the charge-transfer reaction rate and the hydrogen diffusion rate were first accelerated and then slowed down due to the change of the lattice parameters, the crystallite size, the phase abundance and the electrocatalytic activity of the alloys with increasing Pd content. The optimum composition was found to be x=0.1x=0.1 in this work.