奈米碳基/錳氧化物複合電極材料於超高電容器之應用研究

Abstract

超高電容器(Supercapacitors, ultracapacitors, or electrochemical capacitors) 屬電化學構裝電容式儲能元件為現階段受到重視的儲能元件之一，具備可靠性高、循環壽命長、急速充放電等優異的特徵，逐漸受到國際的重視。可惜國內產業在超高電容器領域，長期欠缺自有技術的建立。與傳統電池相比，雖能提供比電池更高的瞬間功率但其能量密度相顯遜色，因此，高能量密度超高電容器之研發具有迫切性的需求。為了獲得具備優異之比電容值及高掃描速率下仍維持良好的電化學穩定性之高性能超高電容器，電極活性材料應滿足三個要求，具備良好電子導電性、高比表面積的電化學離子通道、高的離子質傳效率，不幸的，直至目前為止很難由單一電活性材料達到此需求。本論文採用奈米複合材料作為超高電容器電極材料之選擇，希望擷取各類電極材料的優點來搭配互相結合，並加以改良。於研究過程中導入新製程方式製備碳基/錳氧化物奈米複合材料，目的以不同維度下之碳奈米結構(例如：奈米碳管、石墨烯奈米片…等)作為主要的載體，利用材料本身具有較佳的電導性、化學穩定性及高表面積配合，與錳氧化物所造成之法拉第電容效應互相結合，形成奈米混合網狀結構電極材料，提升超高電容器內的離子擴散傳輸效率，同時降低元件內部等效電阻，使超高電容器之比電容向上提升，進而提升超高電容器的功率密度及能量密度。以下將簡要地討論各章所包含的內容。第一部份利用氧化還原滴定法於室溫下合成奈米碳管/錳氧化物(MnO2/CNTs)奈米複合材料，後續以電泳沉積技術製備超高電容器電極。結果發現，電極經過熱處理，其比電容會有增加的趨勢，經過6,000次充/放電循環，仍保有86 % 的起始電容值，相較於未經熱處理試片保有起始80% 的比電容值來得穩定，有效改善法拉第電容在高循環次數下嚴重衰減的情形。另外，本研究藉由氫離子取代有機高分子來作為表面活性劑，使得MnO2/CNTs複合材料的表面帶有正電荷，利用電泳的方式直接於鎳基材上沉積MnO2/CNTs複合材料，過程中少了高分子及黏著劑的影響，使得超高電容器在長時間使用下來仍具備有優異之電化學特性表現；第二部分則是藉由簡單化學剝離法製備石墨烯薄片，以石墨烯/CNTs混合碳結構為載體，後續利用氧化還原法合成出石墨烯/CNTs/MnO2奈米複合材料，接著以電泳法完成超高電容器電極製備，藉由氧化還原方式所製備出來的複合式電極，其比電容約略為481 F g-1 (5 mV s-1)，隨掃描速度增加至300 mV s-1，比電容值約衰減23%，定電流(3 A g-1)下進行充/放電循環15,000次後，最終得到之比電容值衰減幅度約略為17%，證明所合成出之石墨烯複合式電極材料表現出優異的電容特性和循環壽命；第三部分則是發展出一套高效率且環保之石墨烯剝離技術，搭配氧化還原法合成出奈米複合物(石墨烯/CNTs/MnO2)，利用電泳的方法完成超高電容器陰極材料之製備，單體電極仍具備優異之比電容值約略為964 F/g (1 A g-1)，高掃描速度下500 mV s-1，其比電容值約為529 F g-1，後續非對稱型超高電容器(石墨烯/CNTs // 石墨烯/CNTs/MnO2+CNTs)組裝上搭配水系電解液(Na2SO4)，具有優異之電化學特性表現(Energy density: 304 Wh kg-1; Power density: 226.5 kW kg-1)，在定電流(0.55 A g-1)下進行充/放電循環10,000次後，最終得到之比電容值衰減幅度約略為11%。證明改良式石墨烯複合式電極材料應用於非對稱型超高電容器明顯表現出優異的電容特性和循環壽命。

The development of high-performance energy storage systems has sparked interest because of the environmental issues and the decreasing availability of fossil fuels. Supercapacitors (SCs), also named electrochemical capacitors, are supposed to be a promising candidate for alternative energy storage devices due to their high rate capability, fast charging/discharging rate, excellent cycle stability, and long cycle life. However, a major drawback of SCs is that they have limited energy density. To obtain high-performance (high specific capacity, good rate performance, excellent cycling stability, high energy density and high power density) SCs, the electroactive materials as electrode must meet the following requirements, i.e., good electron conductivity, high effective surface area and fast ions transportation. Herein, we develop one-step electrophoretic deposition approach to fabricate the binder-free porous network CNTs/MnO2 nanocomposites SCs electrode by optimized heat treatment and it exhibited high specific capacitance (469 F g-1 at 100 mV s-1), fast rate capability, and high stability (∼86% capacitance retention after 6,000 charge/discharge cycles). Furthermore, we present a modified chemical exfoliation route for the large scale synthesis of few layered graphene sheets, and designed hybrid (graphene/CNTs/MnO2 composites + CNTs) nanocomposites provided a synergistic improvement on electrochemical performance (481 F g-1 at 5 mV s-1) and good stability (~ 83.3% capacitance retention over 15,000 cycles) for SCs. Here, CNTs play a critical role in enhancing electrochemical properties, which acts as a spacer to prevent the re-stacking of individual graphene/CNTs/MnO2 nanocomposites sheets. Lastly, we discuss a green route to prepare graphene and the hybrid (graphene/CNTs/MnO2 composites + CNTs) nanocomposites for the construction of asymmetrical SCs (graphene/CNTs/MnO2 composites + CNTs // graphene/CNTs composites), enhancing the maximum voltage of SCs and providing unprecedented energy density. The single electrode based on designed graphene/CNTs/MnO2 composites + CNTs nanocomposites displays excellent electrochemical performance with maximal specific capacitance of 964 F g-1 (at the current density of 1 A g-1), remarkable rate capability (with the residual capacitance of 529 F g-1 at the high scan rate of 500 mV s-1), and fast Na+ diffusion (intercalation value: 6.34 × 10-7 cm2 s-1, deintercalation value: 8.86 × 10-7 cm2 s-1). The optimized asymmetric SCs possesses superior performance with a maximum energy density of record high 304 Wh kg-1 and retaining 56.2% of its initial specific energy density at the power density up to 226.5 kW kg-1. It also shows excellent cycling stability with 89% specific capacitance maintained after 10,000 cycles. These results indicate that our asymmetric pseudocapacitors are promising for practical applications.