Abstract
AB2-type Laves-phase alloys as negative electrodes of nickel/metal hydride (Ni/MH) batteries have been studied intensively since they generally have higher discharge capacity than AB5-type alloys. However, AB2-type alloy is inherently rather difficult to activate in conventional means. In this study, the electrochemical performance of AB2-type (Ti0.35Zr0.65Ni1.2V0.6Mn0.2Cr0.2) alloys modified by hot KOH etching and electroless nickel coating has been investigated. It is found that the alloy modified with hot KOH solution shows quick activation but at the expense of cycle-life stability. The alloy coated with nickel was effectively improved in both cycle-life stability and discharge capacity. A duplex surface modified alloy has been developed, viz., alloy first treated with hot KOH solution and then coated with nickel, which performs satisfactorily with respect to both quick activation and long cycle life. In addition, the high-rate discharge capability of the electrode with duplex surface modification is superior to that of electrode solely treated with KOH etching or Ni plating. For practical uses, pulse activation process was characterized by several advantages over the surface treatments. Therefore, we have used both the pulse-potential and pulse-current processes to activate the hydrogen-storage alloy electrode. In the pulse-potential process, electrode shows fairly good activation and high discharge capacity when hydriding and dehydriding potentials were set at -1.2V and -0.8V, respectively, with respect to Hg/HgO electrode. Similar results were found in the pulse-current process, where the applicable hydriding and dehydriding currents were 400 mAg-1 and 200 mAg-1, respectively. On the other hand, an increase in total pulse time was not only useful for activation but also beneficial to the discharge capacity. The time interval scarcely affects electrode's capacity but significantly influences activation. A longer duty cycle favors electrode's maximum capacity but slows down activation. A recommended duty cycle for pulse potential was 0.5, and for pulse current was 0.83. In addition, the cycle-life stability and high-rate discharge capability of pulsed electrodes were superior to that of untreated electrodes. In general, high temperature is detrimental to battery's performances. The Ni/MH battery is particularly sensitive to temperature. We have employed two mathematical models, i.e. one-dimensional and two-dimensional, to predict the thermal behavior of Ni/MH batteries. The temperature rise of battery can reach to around 23oC during 15A charging under natural convection. In the discharging case, the temperature rise of battery is around only 6oC under adiabatic condition. Batteries attached with aluminum fins exhibit an effective heat dissipation behavior. In a closely packed module, natural and forced convection can not reduce the temperature increase in the center part of a module. Finally, we have employed a two-dimensional transient thermal model to analyze the effect of attaching cold plates into nickel/metal hydride batteries on its heat dissipation. The influences of configuration parameters and operation conditions on the thermal performance of cold plates are also explored. Since cold plates with lower thermal conductivity reduce the heat dissipation, materials with higher thermal conductivity were selected for the thermal management of batteries. The heat dissipated away from the top and bottom surfaces by forced convection constitutes only a very small portion of the heat generated by batteries. In addition, the average surface temperature rise during charging of a packed module without cold plates can reach to 21oC, whereas with cold plates, the temperature rise decreased to 4oC. During discharging, the temperature rises of modules with and without cold plates can reach to about 1oC and 3oC, respectively.