Effects of size of nickel powder additive on the low-temperature electrochemical performances and kinetics parameters of AB5-type hydrogen storage alloy for negative electrode in Ni/MH battery

The purpose of this study is to systematically investigate the effects of size of nickel powder additive on the low-temperature electrochemical performances and kinetics of MmNi(0.38)Co(0.7)Mn(0.3)Al(0.2) hydrogen storage alloy under various assigned low temperatures, including 273 K, 253 K and 233 K. The results demonstrate that replacement of nano-nickel powder with average particle size of 40 nm (N40) for T255 nickel powder enhances the low-temperature electrochemical performances more dramatically than those of nano-nickel powder with average particle size of 100 nm (N100). The discharge capacity of N40 electrode at 233 K and 0.2 C rate is 216.8 mA h g(-1) and low-temperature dischargeability (LTD) even reaches 67.6%, both of which are more higher than those of T255 electrode (150.4 mA h g(-1), 51.3%, respectively). The kinetics parameters measured by linear polarization, electrochemical impedance spectrum (EIS) and potentiostatic discharge technology expound that, both the electrochemical charge-transfer process at the alloy-electrolyte interface and the hydrogen diffusion process in the bulk of alloy particles become increasingly limited with the decrease of the temperatures from 293 K down to 233 K, which correspondingly results in poor electrochemical performances of the electrodes at low temperatures. Further, the substitution of N40 for T255 nickel powder reduces the charge-transfer resistances and increases both the exchange current densities and the hydrogen diffusion coefficients of the electrodes more significantly than those of N100 at low temperatures, which were attributed to the excellent electro-catalytic activity of N40, explaining the evident differences on the low-temperature electrochemical performances of T255, N100 and N40 electrodes. (C) 2015 Elsevier B.V. All rights reserved.