| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 88 === The objective of this research has been to study the self—discharge properties of the Mm based and Ti-Zr based hydrogen storage alloy electrodes as negative electrodes of Ni-MH batteries. Another purpose of this study has been to study the effect of electro less nickel coating on the Mm based electrodes. The alloy powder in this study was prepared by arc melting, and pulverized by mechanical crushing. The self -discharge testing this study was divided into two parts, namely continuous model and step model. The variation of capacity retention with storage time and storage temperature was measured in continuous self-discharge test. The correlation between reversible self-discharge, irreversible self-discharge and used time was investigated in step self- discharge test. It is believed that the reversible self-discharge loss was caused by the difference between the plateau pressure of negative electrodes and hydrogen partial pressure in the cell. The oxidation of negative electrodes and leaching of some elements from the hydrogen storage alloy increased the irreversible self-discharge loss.
The capacity retention of the original AB5 alloy electrodes storage for 16 days at 25℃ decayed to 55% and to 22% for 7 days storage at 60℃. However, the capacity retention of the Ni coated AB5 alloy electrodes storage for 16 days at 25℃ improved to 63% and to 31% for 7 days storage at 60℃. The raise of capacity retention of Ni -MHX battery was attributed to the Ni coating layer served as a diffusion barrier for the hydrogen atom.
In step self-discharge test, the reversible capacity loss of the original AB5 alloy electrodes increased from 40mAh/g to 75mAh/g after 15 charging discharging cycles at 25℃. However, the one of the original AB2 alloy electrodes maintained at 30mAh/g after 15 charging discharging cycles at 25℃. It’s revealed that the mechanism of the reversible self—discharge between the Mm based and Ti-Zr based alloy electrodes is different. The reversible capacity loss can be kept to 25 mAh /g after 11 charging discharging cycles at 25℃ by adding 2wt.% Al in electrolyte. The same results can be obtained for the Ni coated alloy electrodes without Al additives. It’s suggested that the Al can be retained in the alloy powder by nickel coating treatment and adding Al in electrolyte, thus the reversible capacity loss can be kept to a constant value in our experiment.
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