Study on the properties of Mm based hydrogen stroage alloy electrodes in different electolytes and surface treatments

• Main Authors: Peng, Hui-Ling, 彭惠玲
• Other Authors: S. L. I Chan
• Format: Others
• Language: zh-TW
• Published: 2001
• Online Access: http://ndltd.ncl.edu.tw/handle/42793635357443432404

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 89 === Abstract The objectives of this study have been to improve the cycle life, high rate capability and self-discharge characteristics of a nickel/metal hydride battery. In this system an AB5 based storage metal hydride alloy was used as the negative electrode. Two treatments have been adopted for this purpose. Electrochemical impedance spectrometry (EIS) was used to determine the kinetics of hydrogen adsorption. SEM, EDS, XRD, and BET analyses were used to identify the crystal lattice expansion, nature of products and micro-cracking on the electrode surface, and degree of pulverization of the hydrogen storage alloy. In the first treatment, aluminum powder was added in the electrolyte until it was saturated with Al. This inhibited the A1 in the AB5 alloy to dissolve from the alloy during repeated charge/discharge cycling. By this way, the discharge capacity, cycle life, high rate capacity and self-discharge were largely improved. From EIS analysis results, it was found that the resistance of the alloy electrode after a 150-cycle testing was smaller than that of the original alloy. This suggests that by simply changing the composition of the electrolyte, the degradation of the alloy electrode caused by oxidation or dissolution of the alloy components can be avoided, thereby prolonging the cycle life of the battery. The decrease in the charge transfer resistance as compared to the original alloy also demonstrated that the treated electrode has a better high rate capability. From SEM analyses results, we observed different corrision products on the alloy surface. The second treatment involved a surface treatment of the alloy powder with boiling alkaline solution at 80℃ for 5 hours. Two alkaline solutions, one with 30wt.﹪KOH and the other with 30wt.﹪KOH＋2wt.﹪Al, have been used. It was found that SEM analyses show that different forms of rare earth metal hydroxide were produced under different treatment conditions. Some needle particles were produced after 30wt.﹪KOH treatment. Many silk-like particles produced after treating with the 30wt.﹪KOH＋2wt.﹪Al. Using the boiling alkaline treatment, the discharge capacity was increased and cycle life was prolonged. The best performance was found for the batteries treated with the 30wt.﹪KOH＋2wt.﹪Al with a very impressive rate capability. Both treatments could reduce the irreversible self-discharge, and the 30wt.﹪KOH treated alloy electrode inhibited hydrogen diffusion in the alloy at 25℃. However at 60℃, both treatments failed to improve reversible self-discharge because of the formation of micro-cracks at the alloy surface. These surface micro-cracks provided short diffusion paths for hydrogen from the alloy. In summary, by treating with both boiling alkaline solution, the discharge capacity was increased and cycle life was prolonged. Micro-cracks on the alloy surface is of benefit to the high rate capability and due to fail to improve self-discharge.