A study on the structural and electrochemical properties of La0.7-xNdxMg0.3Ni2.45Co0.75Mn0.1Al0.2 (x=0.0-3.0) hydrogen storage alloys

The effects of substitution of Nd for La on the structural and electrochemical properties of the La0.7-xNdxMg0.3Ni2.45Co0.75Mn0.1Al0.2 (x=0.0-0.3) hydrogen storage alloys had been studied systematically. X-ray powder diffraction (XRD) investigation showed that all the alloys were mainly composed of the (La,Mg)Ni-3 phase and the LaNi5 phase, and the lattice parameters and cell volumes of the component phases gradually decreased with increasing Nd content. The electrochemical studies revealed that the maximum discharge capacity decreased when x increases from 0.0 to 0.3. However, the cycling stability of the alloy electrodes was slightly improved. After 100 charge/discharge cycles, the capacity retention (C-100/C-max) of the alloy electrode was increased from 78.2 to 81.3% with x increasing from 0.00 to 0.30. The high rate dischargeability of the alloy electrodes were first increased from 61.8% (x = 0.00) to 70. 1 % (x = 0. 10) and then decreased to 53.7% (x = 0.30) at the discharge current density I-d = 1000 mA/g. Meanwhile, the results obtained from the electrochemical impedance spectroscopy, linear polarization, Tafel polarization, and hydrogen diffusion coefficient measurements indicated that the exchange current density I-0, the limiting current density I-L and the hydrogen diffusion coefficient D of the alloy electrodes also first increased and then decreased with increasing Nd content, which implies that the electrochemical kinetics of the testing alloys could be effectively improved by partial substitution of Nd for La. Considering the overall effects of substitution of Nd for La on the La(0.7-)xNd(x)Mg(0.3)Ni(2.45)Co(0.75)Mn(0.1)Al(0.2) (x = 0.00-0.30) alloy electrodes, the optimum composition was found to be x = 0. 10. (C) 2007 Published by Elsevier B.V.