氧化物超高電容器之製備與分析

Synthesis and Characterization of Oxide Supercapacitors

郭信良(Shin-Liang Kuo)

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(2006年)

https://doi.org/10.6342/NTU.2006.02868

摘要

在本論文的第一部分，分別利用含浸法及循環伏安沈積法製備二氧化釕(RuO2)/二氧化錫(SnO2)複合超高電容器。針對含浸法所製備之複合電極，透過鍛燒溫度及RuO2負載之調整，該電極在1M KOH電解液中之電化學特性已被最佳化。結果顯示，當包含1.4% RuO2的複合電極經200oC鍛燒後，該電極可提供一最大RuO2貢獻比電容量約為710 F/g-RuO2。而在較高的RuO2負載時，推測其可能造成均勻成核而致使RuO2奈米微結晶總比表面積的縮減。另一方面，經最適溫度(150 oC)之熱處理程序，電鍍法製備而得的複合電極在1 M硫酸溶液中則可提供一RuO2貢獻比電容量高達930 F/g-RuO2，且在功率密度為1.5 kW/kg之操作條件下可得一能量密度約為0.5 Wh/kg。其他對照研究指出，此一複合電極相較於RuO2電鍍在平整之鈦片或多孔性導電碳黑時擁有更佳之電容表現。
另外，利用共沈法成功地製備MnFe2O4、Fe3O4、CoFe2O4以及NiFe2O4等鐵氧體材料(ferrites)，且該材料在NaCl水溶液中之電容行為亦被測試。本研究發現除了MnFe2O4具有偽電容量外，其他材料並無此一特性，且該偽電容行為僅發生在具微結晶相之MnFe2O4中，而在非晶相中則未被發現。MnFe2O4/CB複合電極在氯化鹽、硫酸鹽及亞硫酸鹽水溶液中皆表現出偽電容性質，其中在NaCl系統能提供最大之電容量。根據計算，MnFe2O4成分可提供一超過100 F/g-MnFe2O4之貢獻比電容量，以及一超過10 kW/kg之高功率輸出能力。針對氯化鹽水溶液系統，臨場X光近緣吸收光譜分析進一步證實該偽電容牽涉一在Mn和Fe的電荷轉移程序，其中主要是由佔據尖晶石結構之四面體位置的Mn所貢獻，且該電荷轉移伴隨著質子化反應或是電解液陽離子的嵌入反應。另外，複合電極在1.0 V的操作電壓下會有一約為10 mA/g的漏電流。而相較於非晶型的二氧化錳(MnO2)超高電容器，MnFe2O4可提供一較佳的循環穩定性以及一較緩慢的自放電速率。針對共沈法所製備的MnFe2O4/CB複合材料，在MnFe2O4和CB的重量比為7:3時可得到最佳電容值。
此外，MnO2·nH2O在LiCl、NaCl、KCl、CsCl以及CaCl2之鹽類水溶液中的偽電容儲電反應亦被進一步地研究。微結晶的二氧化錳薄膜及粉末經證實皆具有e-MnO2的晶相。臨場X光繞射分析顯示，在MnO2進行氧化/還原反應過程中，Mn離子的電荷轉移牽涉一陽離子在整體氧化物結構中的嵌入/遷出程序，並伴隨一可逆之晶格膨脹/收縮。另外，電化學石英晶體微天平及X光光電子光譜分析進一步指出，在所有系統中，H3O+在整體嵌入反應中扮演一主要的角色(> 60 %)，且陽離子嵌入量隨著離子大小的增加有先減少而後遞增的趨勢。該儲電反應可描述如下：
Mn(IV)O2-nH2O +δe- + δ(1-f)H3O+ δf M+-->(H3O)δ(1-f)Mδf[Mn(III)δMn(IV)1-δ]O2·nH2O, 其中M+為鹼金族陽離子。

關鍵字

超高電容器 ； 偽電容 ； 機制 ； MnFe2O4 ； MnO2

並列摘要

In the first part of the thesis, RuO2-SnO2 composite supercapacitors were synthesized via both the impregnation and cyclic voltammetric deposition. The RuO2-impregnated SnO2 xerogel was optimized for its electrochemical capacitance in aqueous 1 M KOH electrolyte by adjusting the calcination temperature and the RuO2 loading. A specific RuO2 capacitance of 710 F/g-RuO2, is obtained with a RuO2 loading of 1.4 wt. % and by calcination at 200 oC. Higher loadings presumably result in a homogeneous nucleation, causing severe reduction in the total surface area of the RuO2 crystallites. On the other hand, after the optimization of crystallization protocol (150 oC), the electroplated RuO2-SnO2 composite exhibited a specific RuO2 capacitance of 930 F/g-RuO2 in 1 M H2SO4 electrolyte and an overall specific energy of ~ 0.5 Wh/Kg at a specific power>1.5 kW/kg. Comparative studies demonstrated that this composite electrode exhibited a far superior performance than the electrodes having RuO2 similarly plated onto either smooth Ti or porous conductive carbon black.

In addition, ferrites including MFe2O4 where M = Mn, Fe, Co, or Ni have been synthesized by coprecipitation methods and tested for their capacitive behaviors in aqueous NaCl solution. MnFe2O4 has been found to exhibit pseudocapacitance, while the other ferrites do not. The results indicated the pseudocapacitance was observed only for crystalline, rather than amorphous, MnFe2O4 phase. The MnFe2O4/CB composite showed pseudocapacitance in solutions of chloride, sulfate and sulfite salts of alkali and alkaline cations, with NaCl solution giving the highest capacitance. It has exhibited specific MnFe2O4 contributed capacitances of >100 F/g-MnFe2O4 and high-power delivering capabilities of >10 kW/kg. For the chloride electrolytes, the pseudocapacitance has been identified, by in-situ X-ray absorption near edge spectroscopy study, to involve charge transfer at both the Mn and Fe sites, predominantly at the Mn ions at the tetrahedral sites of the spinel, balanced by insertion of cations from the electrolyte and protonation process. The composite electrode exhibits an operating potential window of 1.0 V with a maximum leakage current of 10 mA/g, and it exhibits superior cycling stability and reduced self-discharge rate than amorphous MnO2. The specific capacitance of the composite is a strong function of the CB content and the optimum capacitance occurs with the ferrite:CB weight ratio of 7:3.
Besides, pseudocapacitive charge-storage reaction of MnO2·nH2O in several aqueous alkali and alkaline salts solutions, including LiCl, NaCl, KCl, CsCl and CaCl2, has been studied on fine-grained MnO2·nH2O thin-films and particles, which possess the e-MnO2-type crystal structure. In-situ synchrotron X-ray diffraction analysis shows that charge-transfer at Mn sites upon reduction/oxidation of MnO2·nH2O is balanced by bulk insertion/extraction of the solution cations into/from the oxide structure, which causes reversible expansion and shrinkage in lattice spacing of the oxide during charging/discharging cycles. Electrochemical quartz-crystal microbalance and X-ray photoelectron spectroscopy data further indicate that H3O+ plays the predominant (> 60%) role in all cases, while the extent of participation of alkali cations first decreases and then increases with ionic size. The charge-storage reaction can be summarized as:
Mn(IV)O2-nH2O +δe- + δ(1-f)H3O+ δf M+-->(H3O)δ(1-f)Mδf[Mn(III)δMn(IV)1-δ]O2·nH2O,where M+ is alkali cation.

並列關鍵字

supercapacitor ； pseudocapacitance ； mechanism ； MnFe2O4 ； MnO2

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