| Summary: | 碩士 === 國立成功大學 === 材料科學及工程學系 === 89 === The effects of alloying element and electrolyte composition on the electrochemical characteristics of the AB5 type metal hydride electrodes were investigated.
Two series of hydrogen storage alloys included LaNi3.6∼4.2M1.4~0.8 (M=3Co+1Mn+1Al) and LaNi3.8(Co+Mn)0.96Al0.24 were prepared by arc melting. The experimental results showed that all the LaNi3.6∼4.2M1.4~0.8 electrodes had the capacity as high as 330mAh/g. For LaNi3.8(Co+Mn)0.96Al0.24 electrodes with the Co/Mn ratio higher than 1/1, the capacity progressively decreased with increasing Mn content. A high Ni content or a high Co/Mn ratio in the alloys could improve the discharge voltage of the full battery.
X-ray diffraction analysis showed that the unit cell volumes were expanded with decreasing Ni content in the LaNi3.6∼4.2M1.4~0.8 alloys. Similarly, unit cell volume expansion was found to increase with decreasing Co/Mn ratio in the LaNi3.8(Co+Mn)0.96Al0.24 alloys. The expansion of the lattice might enhance the hydrogen diffusion in the bulk material that improved the hydrogen-storing rate of the alloys. The subsequent electrochemical tests confirmed that the alloys with a larger unit cell volume exhibited a better high rate charging performance.
The high rate discharging performance of the LaNi3.6∼4.2M1.4~0.8 electrodes could be improved when the Ni content in the alloys was raised from 3.6 to 4.0, that may related to the improved of charge transfer reaction at the electrode surface. But when the Ni content in the alloy was further raised to 4.2, the dischargeability at high current density was lessened. It suggested that even though the hydrogen-releasing rate in the electrolyte was mainly determined by the charge transfer reaction at the electrode surface but it was limited by hydrogen diffusion in the bulk material when the unit cell volume was contracted substantially (Ni=4.2). The dischargeability was not obviously influenced by the Co/Mn content ratio adjustment in the LaNi3.8(Co+Mn)0.96Al0.24 electrodes.
When the cyclic stability of the metal hydride electrodes was concerned, it was found that decreasing the Ni content in the LaNi3.6~4.2M1.4~0.8 alloys and increasing Co/Mn ratio in the LaNi3.8(Co+Mn)0.96Al0.24 alloys could extend the cycle life effectively.
The effects of electrolyte composition on the discharge characteristics, cyclic stability, and corrosion behavior of the metal hydride electrodes were also investigated. The results showed that the discharge voltages of the electrodes were moved to more positive potentials when the temperature was lowered or the LiOH concentration was raised. On the other hand, the addition of LiOH into the 7m-KOH solution could increase the discharge capacity of the metal hydride electrodes at high temperature and low charge/discharge current density. But when the temperature was lowered and the charge/discharge current density was increased, the beneficial effect of LiOH addition on increasing the capacity was gradually diminished.
According the experiment results obtained in this study, the appropriate addition of LiOH (about 10g/l) in the electrolyte could improve the durability of the metal hydride electrode. The solution analysis using inductively coupled plasma (ICP) revealed that the dissolution of La and Ce elements from the metal hydride electrode were depressed when LiOH was added into the electrolyte. On the other hand, the scanning electron micrograph (SEM) for the surface morphology showed that addition of LiOH could retard the pulverization of the hydrogen storage alloy powder in the KOH electrolyte.
總目錄……………………………………………………………..……V
表目錄…………………………………………………..……….…...VIII
圖目錄…………………………………..………………………….... .IX
第一章 前言…………………………………..……………...………1
第二章 研究背景與相關理論…………….…………………...….5
2.1研究背景…………………….………………...…………………..5
2.2鎳氫電池簡介……….…………………….………………………8
2.3儲氫合金材料簡介……….…………………….………………..10
2.4電解質水溶液……….…………………….……………………..15
2.5鎳氫電池發展的趨勢與目標……….…………………….……..17
第三章 研究方法與步驟…………………………………………20
3.1探討合金元素對LaNi5型儲氫合金材料電化學性質的影響….20
3.1.1合金的製備………. ……….…………………….………...21
3.1.2 X-ray粉末繞射分析……….…………………….………...22
3.1.3儲氫合金電極的製備……….…………………….……….22
3.1.4組成全電池……….…………………….………………….22
3.1.5電池的活化……….…………………….………………….23
3.1.6最大電容量、放電曲線與充放電效率量測……………….24
3.1.7探討不同充電電流密度對合金放電容量的影響………...25
3.1.8探討不同放電電流密度對合金放電容量的影響………...25
3.1.9評估儲氫合金電極經循環充放電後之電容量衰退速率...26
3.2探討電解液成份對儲氫合金電極於各種溫度下之電化學性質
影響……….…………………….………………….…………..27
3.2.1量測不同成份電解液的溶液導電度……….……………..27
3.2.2儲氫合金電極於不同溫度的各成份電解液中之電化學性質量測……….…………………….………………………27
3.2.3探討於電解液中添加氫氧化鋰對儲氫合金電極循環充放電穩定性的影響……….…………………….……………28
第四章 結果與討論……….…………………….………………...30
4.1鎳含量的多寡對儲氫合金電極電化學性質的影響……………30
4.1.1 XRD分析……….…………………….……………………31
4.1.2儲氫合金電極之最大電容量、放電曲線與充放電效率….31
4.1.3各成份合金電極於不同充電速度下的放電容量………...33
4.1.4不同放電速度下的放電電容量…………………………...35
4.1.5合金循環穩定性……….…………………….…………….36
4.2 鈷與錳的相對含量比例對儲氫合金電極電化學性質的影響..39
4.2.1 XRD分析……….…………………….……………………39
4.2.2儲氫合金電極之最大電容量、放電曲線與充放電效率….40
4.2.3各成份合金電極於不同充電速度下的放電容量………...41
4.2.4不同放電速度下的放電電容量……….…………………..43
4.2.5合金循環穩定性……….…………………….…………….44
4.3 電解液成份對儲氫合金電極於各種溫度下之電化學性質
影響. ……….…………………….………………….…………46
4.3.1電解液中氫氧化鋰添加量對溶液導電度的影響………...46
4.3.2電解液中添加氫氧化鋰後對儲氫合金電極放電曲線的
影響……….…………………….………………….……...46
4.3.3電解液溫度對儲氫合金電極放電曲線的影響…………...47
4.3.4電解液中添加氫氧化鋰對儲氫合金電極於不同溫度及充放電速度下放電電容量的影響……….………………….48
4.3.5電解液中氫氧化鋰的添加對儲氫合金電極循環充放電穩定性的影響……….…………………….…………………52
第五章 結論……….…………………….…………………….……55
參考文獻……….…………………….…………………….………….59
誌謝………………………………………..………………………….128
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