Electrochemical performances of Mm0.7MgxNi2.58Co0.5Mn0.3Al0.12 (x=0, 0.3) hydrogen storage alloys in the temperature range from 238 to 303 K

A series of experiments have been performed to investigate electrochemical properties of Mm(0.7)Mg(x)Ni(2.58)Co(0.5)Mn(0.3)Al(0.12) (x = 0, 0.3) alloy at various temperatures (238 K, 273 K and 303 K). The results indicate that both alloy electrodes exhibit high dischargeabilities after elemental substitution, above 320 mAh g(-1) even at 238 K. The capacity degradation of the two alloys are primarily ascribed to serious pulverization, other than the oxidation of active components at the initial stage. Moreover, the electrochemical performances of Mm(0.7)Mg(x)Ni(2.58)Co(0.5)Mn(0.3)Al(0.12) (x = 0, 0.3) alloy electrodes depend on the alloy type and testing temperature. Mm(0.7)Mg(0.3)Ni(2.58)Co(0.5)Mn(0.3)Al(0.12) alloy, consisting of LaNi5-phase and La2Ni7-phase, shows better properties of discharge capacity, cyclic stability, self-discharge and pulverization resistance at the three temperatures than those of single LaNi5-phase Mm(0.7)Ni(2.58)Co(0.5)Mn(0.3)Al(0.12) alloy. The electrochemical kinetics studies indicate that the activation energy of hydrogen diffusion and exchange current density (I-0) of Mm(0.7)Mg(0.3)Ni(2.58)Co(0.5)Mn(0.3)Al(0.12) alloy are lower than those of Mm(0.7)Ni(2.58)Co(0.5)Mn(0.3)Al(0.12) alloy. When the temperature increases from 238 to 303 K, the capacity loss, high-rate dischargeability, exchange current density I-0 and hydrogen diffusion coefficient (D/a(2)) of the two alloys increases, while capacity retention decreases. Further analysis of kinetics suggests that bulk hydrogen diffusion is the rate-determining step of the battery reaction at low temperature 238 K, and charge-transfer reaction on alloy surface is the rate-determining step when tested at 273 K and 303 K for both alloys. The perfect low temperature discharge capacities of the two alloys can mainly attribute to the decrease of activation energy for hydrogen diffusion after elemental substitution. (C) 2011 Elsevier Ltd. All rights reserved.