非對稱超高電容器之製備與分析

Synthesis and Characterization of Asymmetric Supercapacitors

林彥伯(Yen-Po Lin)

指導教授 ： 吳乃立

國立臺灣大學/工學院/化學工程學研究所/博士(2011年)

https://doi.org/10.6342/NTU.2011.00533

摘要

本論文首先利用由MnFe2O4偽電容材料做為陽極和LiMn2O4電池材料做為陰極，以及9M硝酸鋰(LiNO3)水溶液做為電解液所組成一新型水系非對稱超高電容器，其合成與鑑定將在本論文中討論。奈米結晶的MnFe2O4負極材料具有約99 F/g的比電容量，而LiMn2O4正極在10~100 C-rate提供了將近128~100 mAh/g的比電量。整個非對稱型超高電容具有約1.3 V的最大電位操作範圍，該範圍是被MnFe2O4的還原電位所限制。隨著增加負極對正極的重量比(A/C)，而非對稱型電容的功率密度和能量密度也跟著增加，且在A/C~4.0時達到飽和，此時基於兩個電極重量的能量密度和功率密度分別為10和5.5 Wh/kg在0.3和1.8 kW/kg下。此非對稱型超高電容表現出良好的循環壽命，其在經過5,000次的充放電後，只有損失5%以內的電容量，以及相較於MnFe2O4對稱型電容及其它非對稱電容有較低的自放電速度。
另外，經由化學共沉法將MnO2沉積在碳黑(CB)和多壁奈米碳管(CNT)形成兩種MnO2@C的複合體，它們的物理性質和在1M NaCl(aq)電解液中的電化學性質將在此被討論。從XRD的數據分析顯示此兩種氧化物的分別為spinel-type MnO2@CNT(S-MnO2@CNT)和birnessite-type MnO2@CB(B-MnO2@CB)。SEM和TEM的觀察顯示MnO2以不同型態分散在CB和CNT上，MnO2均勻地分散在CNT上呈奈米薄片狀，而MnO2在CB上的型態為奈米粒狀。B-MnO2@CB和S-MnO2@CNT 的BET比表面積分別為138和156 m2/g。S-MnO2@CNT在2 mV/s和200 mV/s下表現出309 F/g-MnO2和247 F/g-MnO2，比B-MnO2@CB(在2 mV/s和200 mV/s下表現出229 F/g-MnO2和132 F/g-MnO2)有較好的效能，主要因為高的比表面積提供了較多與電解液接觸面積、較均勻的氧化物分佈，以及高導電的多壁奈米碳管基材也增加了電極的效能。S-MnO2@CNT所組成的對稱電容也展現了相當不錯的循環壽命，其在經過50 mV/s和100 mV/s下各循環5000次，經過循環10,000次後，依然保有96%電容量，而B-MnO2@CB的對稱電容在50 mV/s下循環充放10,000次後，只剩下76.7%的電容量。在自放電的測試中，S-MnO2@CNT電極的自放電速度較B-MnO2@CB和amorphous MnO2@CB複合電極來得慢許多，顯示出其應用在超高電容的潛能。
此外，使用MnO2@CB and MnO2@CNT做為Mn的前驅物，經由一步驟的水熱方式可以成功地合成LiMn2O4@CB和LiMn2O4@CNT複合材料。從XRD的鑑定顯示出此兩種複合體含有spinel LiMn2O4和類石墨碳的結構，其LiMn2O4晶粒大小也分別由Debye-Sherrer方程式計算得到。熱重分析的結果顯示碳含量在LiMn2O4@CB和LiMn2O4@CNT複合材料分別為37.6 wt%和19.2 wt%。LiMn2O4@CB在含有鋰的水系和有機系電解液中表現出128 mAh/g的可逆電容量。此外，LiMn2O4@CB在25 oC和55 oC下進行循環壽命的測試，在5 C-rate下經過了660次充放電後，其電量仍有97%。LiMn2O4@CNT複合材料在0.5 C-rate下的9M LiNO3(aq)電解液中展現出約130 mAh/g的可逆電量，且在500 C的充放電速度下表現出111 mAh/g的高功率輸出。

關鍵字

非對稱超高電容 ； 錳鐵氧尖晶石材料 ； 鋰錳氧尖晶石材料 ； 二氧化錳 ； 水系電解液

並列摘要

A new type of aqueous asymmetric supercapacitor that contains MnFe2O4 pseudocapacitive anode and LiMn2O4 battery cathode with 9M LiNO3(aq) as electrolyte has been synthesized and characterized. The anode and cathode electrodes were characterized separately in 1M and 9M LiNO3(aq). Both electrodes showed superior performance in high concentration electrolyte and high temperature. The nanocrystalline MnFe2O4 anode material have a specific capacitance of ca. 99 F/g and the LiMn2O4 cathode a specific capacity of ca. 128~100 mAh/g under 10~100 C-rate. The cell has a maximum operating voltage window of ca. 1.3 V, limited by irreversible reaction of MnFe2O4 toward reducing potential. The specific power and specific energy of the full cell were found to increase with increasing anode-to-cathode mass ratio (A/C) and saturate at A/C~4.0, which gives specific cell energies, based on total mass of two electrodes, of 10 and 5.5 Wh/kg at 0.3 and 1.8 kW/kg, respectively. The 4-1 cell shows good cycling stability that only within 5% capacitance loss after 5000 cycles, and exhibits significantly slower self-discharge rate than the MnFe2O4 symmetric cell and other asymmetric capacitors.
In addition, the physical and electrochemical properties of two kinds of MnO2@C composite materials in 1M NaCl aqueous solution were determined and discussed. MnO2 was deposited onto muti-walled carbon nanotubes (CNT) and carbon black (CB) by chemical co-deposition to form composite materials. From X-ray powder diffraction characterizations, composites are spinel-type MnO2@CNT (abbreviated as S-MnO2@CNT) and birnessite-type MnO2@CB (denoted as B-MnO2@CB). SEM and TEM observations reveal that S-MnO2 was well-dispersed onto MWCNT with nano-flake structure, and the morphology of the B-MnO2 of the other composite was nano-particulate, and the BET surface area of B-MnO2@CB and S-MnO2@CNT are 138 and 156 m2/g. The S-MnO2@CNT-based electrode delivered 309 F/g-MnO2 and 247 F/g-MnO2 at 2 mV/s and 200 mV/s, respectively, which showed superior performance than that of B-MnO2@CB (229 F/g-MnO2 and 132 F/g-MnO2 at 2 mV/s and 200 mV/s), and therefore, the excellent performance is attribute to a larger contact area with the electrolyte, more homogeneous dispersion of oxide and the highly conductive substrate (CNT) also helped to enhance the performance of the electrode. A long-term stability test of the S-MnO2@CNT-based symmetric cell was carried out at 50 mV/s and 100 mV/s, and each sweep rate involved 5000 cycles. After 10000 cycles, the capacitance of the S-MnO2@CNT-based symmetric cell remained above 96%, but the B-MnO2@CB-based symmetric cell only retained 76.7% capacitance after 10000 cycles at 50 mV/s. Self-discharge tests show that S-MnO2@CNT could store charge longer than the B-MnO2@CB composite electrode or the amorphous MnO2@CB composite electrode, indicating that S-MnO2@CNT has superior performance for the application of supercapacitors.
Besides, LiMn2O4@CB and LiMn2O4@CNT composite materials were synthesized successfully through a one-step hydrothermal process that employed B-MnO2@CB and S-MnO2@CNT as Mn precursors. The XRD characterizations show the features of spinel LiMn2O4 structure and graphitic structure of carbons, and the crystallite size of LiMn2O4 were calculated by using Debye-Sherrer equation individually. The carbon content among LiMn2O4@CB and LiMn2O4@CNT composite were analyzed using TGA, which are 37.6 wt% and 19.2 wt%, respectively. The electrochemical performance of LiMn2O4@CB composite was characterized in aqueous and organic electrolyte contain with Li ion, and the LiMn2O4@CB electrode exhibited 128 mAh/g reversible capacity in both electrolytes. Moreover, LiMn2O4@CB showed long-term stability while charging/discharging in a half cell at 25 oC and 55 oC, which remained 97% capacity after 660 cycles. The LiMn2O4@CNT electrode showed a reversible capacity of 130 mAh/g at 0.5 C-rate and presented an extremely high power density in 9M LiNO3 aqueous electrolyte, which delivered 111 mAh/g at 500 C charging/discharging rate.

並列關鍵字

Asymmetric Supercapacitors ； MnFe2O4 ； LiMn2O4 ； MnO2 ； Aqueous Electrolyte

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