Influence of temperature on the electrochemical characteristics of MmNi3.03Si0.85Co0.60Mn0.31Al0.08 hydrogen storage alloys

The heat of hydride formation is a crucial parameter in characterizing a hydrogen storage alloy for battery applications. Novel AB(5)-type, non-stoichiometric, lanthanum-rich MmNi(3.03)Si(0.85)Co(0.60)Mn(0.31)Al(0.08) (Mm: Misch metal) hydrogen storage metal hydride alloy electrodes are prepared. Electrochemical hydrogen absorption/desorption and electrochemical impedance measurements are carried out at various temperatures in conjunction with sintered nickel hydroxide positive electrodes. The specific capacity of the prepared metal hydride electrodes decreases from 283 mAh g(-1) at 303 K to 213 mAh g(-1) at 328 K. Electrochemical pressure-composition-temperature (PCF) isotherms are constructed from galvanostatic discharge curves and the change in enthalpy (Delta H-e degrees) and the change of entropy (Delta S-e degrees) of the metal hydride alloy electrodes are evaluated as -41.74 kJ mol(-1) and 146.28 J mol(-1) K-1, respectively. Kinetic parameters are obtained by fitting the electrochemical impedance spectrum performed at different temperatures. The charge-transfer resistance decreases with temperature, whereas exchange current density and diffusion coefficient parameters increase with temperature. It is concluded that the deterioration in capacity is due to enhanced surface activity at higher temperatures. (C) 2008 Elsevier B.V. All rights reserved.