研究生: |
鄭慧雯 Hui-Wen Cheng |
---|---|
論文名稱: |
鋅空氣電池之放電特性與鋅陽極回收研究 Discharge Characterization and Zinc Anode Recovery of Zinc-Air Battery |
指導教授: |
徐 統
Tung Hsu 陳瑞凱 Swe-Kai Chen |
口試委員: | |
學位類別: |
碩士 Master |
系所名稱: |
工學院 - 材料科學工程學系 Materials Science and Engineering |
論文出版年: | 2001 |
畢業學年度: | 89 |
語文別: | 中文 |
論文頁數: | 130 |
中文關鍵詞: | 一次鋅空氣電池 、定電流放電法 、循環伏特安培法 、半電位量測法 、導電度量測法 、混合漿料法 、鋅陽極 、空氣陰極 |
外文關鍵詞: | primary zinc-air cell/battery, constant-current discharging measurement, cyclic voltammetry, half-cell potential measurement, conductivity measurement, slurry, zinc anode, air cathode |
相關次數: | 點閱:2 下載:0 |
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本研究係利用四種常用的電化學方法,包括定電流放電法、循環伏特安培法、半電位量測法以及導電度量測法,針對影響一次鋅空氣電池的配方、電解液與製程的不同因素,經過一系列系統性實驗,得到適合一次鋅空氣電池的最佳電極配方、電解液濃度與製程條件。
在本實驗中,電極係以混合漿料法製作的。此法所製作的多孔性電極結構內,活性物質反應的表面積大,故能降低電極表面的極化現象,同時也可提高質傳速率。本實驗所得最佳之陽極和陰極的製程分別簡述如下:鋅陽極是將漿料直接塗覆在不□鋼網上,陽極成分為4 wt % PTFE、94 wt % Zn和2 wt %的Bi2O3。空氣極則是將擴散層與催化層疊壓在不□鋼網上。擴散層能防水並保持氣體通道,而催化層可提供電化學反應位置。擴散層成分由60%碳黑與40% PTFE組成;催化層成分則含30%碳黑、30%石墨、30% MnO2和10% PTFE。兩電極均以350℃熱壓10 min,可使電極內的PTFE擴散,形成一完整的PTFE骨架,將活性物質黏結起來,而得一穩定的電極形貌。電解液為8 M的KOH,導電度此時達最大值,為55 S.m-1。
將上述條件所作成的鋅極和空氣極分別進行電位量測,結果顯示鋅極電位能很快達到熱力平衡;但空氣極的電位不易達熱力平衡,而受動力學控制,以本實驗所加入的催化劑MnO2為例,空氣極電位與熱力平衡電位相差達0.3 V。以循環伏特安培法研究各種鋅陽極的電化學反應,顯示當電解液濃度由6 M增大到8 M時,尖峰電壓值提高0.07 V。實驗也顯示Bi2O3添加劑有在電極表面還原成鉍的現象,並形成較高的過電位,故有抑制鋅自放電的效應。組裝成電池後,進行定電流放電,結果顯示當放電電流從0.2 A下降到0.05 A時,放電深度從34%增加到59%。
鋅是容易電解還原的物質,其回收效率和回收鋅的微結構均受電極片種類和電流密度的影響。本研究使用的電極有鉑片、銀片及鎳片。所得的鋅均為疏鬆的樹枝狀結構,這也是電池充電後電極產生形變而降低循環壽命的主因。在回收效率方面,當電流密度定為0.05 A/cm2時,鉑最好,為89%;銀次之,為35%;鎳最差,為22%。若電極固定為銀片,將電流密度從0.025 A/cm2升高到0.1 A/cm2,回收效率也從14%增加到87%,所得的樹枝晶結構也較細密。若攪拌電解液,可提高回收效率約10%。
The purpose of this study is to investigate the effects of electrode chemistry, electrolyte concentration, and manufacturing process on the performance of the primary zinc-air cell. The investigation is via the utilization of familiar electrochemical methods including the constant-current discharging measurement, the cyclic voltammetry, the half-cell potential measurement, and conductivity measurement. Systematic experiments have been made by varying all important factors in manufacturing the primary zinc-air cells in this study.
The porous electrodes are made of slurry with various kinds of compositions. With a high specific surface area of the reactive materials in the electrodes, the polarization of porous electrodes is low and the mass transportation in the cells is high. Manufacturing processes and optimal conditions for both the zinc anode and the air cathode are described as follows. Anodic slurry composed of 94 wt % Zn, 2 wt % Bi2O3, and 4 wt % PTFE powders is coated on a stainless wire net which acts as the anode current collector. Air cathode is a dual diffusion-catalytic layer structure. The diffusion layer, a water-proof and air diffusion layer, is comprised of 60 wt % carbon black and 40 wt % PTFE powders. The catalytic layer, an electrochemical reaction supply layer, is made of 30 wt % carbon black, 30 wt % graphite, 30 wt % MnO2, and 10 wt % PTFE powders. The diffusion layer and the catalytic layer are pressed together with a stainless wire net to form the air cathode. Both the zinc anode and the air cathode are then hot-pressed at 350oC for 10 min. Thus the PTFE powders in both electrodes bind the reactive materials together to form stable electrodes as they are submerged in KOH electrolyte. The maximal conductivity is 55 S.m-1 at 8 M KOH.
The half-cell potential measurements show fast and low reaction rates for the zinc anode and the air cathode, respectively. Experiments in this study manifest that the MnO2 air cathode has a potential difference of 0.3 V for the measured and the thermodynamic values, while the zinc anode has no difference in this aspect. The cyclic voltammetry indicates that as the electrolyte concentration increases from 6 to 8 M, the peak potential has an increase of 0.07 V. The cyclic voltammetry also indicates that for an effective self-discharge inhibitor such as Bi2O3 there is a Bi reduction reaction on the zinc anode. However, there is none for ineffective additives. The reduction effect of the inhibitor increases the overvoltage of zinc oxidation and inhibits the self-discharge of the cell. The constant-current discharging measurement of a typical cell assembly in this study shows that the depth of discharging of a cell increases from 34% to 59% as the discharged current decreases from 0.2 to 0.05 A.
The efficiency of zinc reduction study is performed with various kinds of electrode metals and different amounts of charged current density. The obtained results indicate that for all cases the reduction structure of zinc is a loose dendritic one in shape. This explains both the drastic shape change of zinc anode after each discharge-charge cycle of a cell and the short cycle life of zinc-air battery. When Ag sheets are used as electrodes, the typical zinc reduction efficiency increases from 14 to 87% as the charged current density increases from 0.025 to 0.1 A/cm2. At 0.05 A/cm2, the reduction efficiency in this experiment is 89, 35, and 22% for Pt, Ag, and Ni electrodes, respectively.
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