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 Mm0.7MgxNi2.58Co0.5Mn0.3Al0.12 (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 Mm0.7MgxNi2.58Co0.5Mn0.3Al0.12 (x = 0, 0.3) alloy electrodes depend on the alloy type and testing temperature. Mm0.7Mg0.3Ni2.58Co0.5Mn0.3Al0.12 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 Mm0.7Ni2.58Co0.5Mn0.3Al0.12 alloy. The electrochemical kinetics studies indicate that the activation energy of hydrogen diffusion and exchange current density (I0) of Mm0.7Mg0.3Ni2.58Co0.5Mn0.3Al0.12 alloy are lower than those of Mm0.7Ni2.58Co0.5Mn0.3Al0.12 alloy. When the temperature increases from 238 to 303 K, the capacity loss, high-rate dischargeability, exchange current density I0 and hydrogen diffusion coefficient (D/a2) 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.

► Both alloy electrodes exhibit high dischargeabilities, above 320 mAh g−1 even at 238 K.
► Outstanding low-temperature properties are related to low activation energy for hydrogen diffusion.
► Capacity degradation is ascribed to pulverization, other than the oxidation of active components.
► Rate-determining step is bulk hydrogen diffusion at 238 K, but is charge-transfer reaction above 273 K.