Effect of Co content on the structural and electrochemical properties of the La0.7Mg0.3Ni3.4-xMn0.1Cox hydride alloys II.: Electrochemical properties

Electrochemical studies were carried out on the La0.7Mg0.3Ni3.4-xMn0.1Cox (x = 0.00, 0.30, 0.60, 0.75, 0.90, 1.05, 1.15, 1.30, 1.45, 1.60) hydrogen storage alloys. Firstly, it was found that partial substitution of Ni by Co could improve effectively the cyclic stability of the La-Mg-Ni-Mn-Co system hydrogen storage alloys with little loss of the maximum discharge capacity, which can be ascribed to a relative lower cell volume change during hydriding/dehydriding cycling and an increase in the surface passivation. Secondly, the high rate dischargeability (HRD) measurement shows that the electrochemical kinetics of the alloy electrodes first increases owing to the concentrations of Co and Ni on the surface of the alloy particles and the subsequent formation of the Raney Ni-Co film with higher electrocatalytic activity after moderate Co substitution for Ni, and then decreases with the increase in Co content because of the reduction of surface electrocatalytic activity due to the diminution of the effective surface area and the increase of the attractive interaction between absorbed hydrogen atoms. Thirdly, the electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization, potential step, and cyclic voltammetry measurements also confirm the change of the electrochemical kinetics of these type of alloy electrodes. Above all, the optimal content of Co in La0.7Mg0.3Ni3.4-xMn0.1 Co-x alloys for negative electrodes in alkaline rechargeable secondary batteries lies in the range 0.75 less than or equal to x less than or equal to 1.15. (C) 2004 Elsevier B.V. All rights reserved.