氫氧化鎳/還原氧化石墨烯複合電極製備全固態超級電容器

Abstract

近年來，儲能裝置伴隨著再生能源而持續發展，其中超級電容器擁有高功率密度、高充放電效率，壽命高且高穩定性的優勢。提高超級電容器的能量密度，拓展其應用在攜帶式電子裝置、電動車和智慧電網是次世代儲能裝置的核心課題。 本研究藉由提高超級電容器電極的比電容數值與操作電位來提高能量密度，並使用膠體聚合物取代水溶液電解質製成全固態電容器以提升安全性及拓展應用範圍。石墨烯擁有高導電性，但其比電容會因為重組現象而降低，故另外配合高比電容數值但低導電性的氫氧化鎳製備複合電極。我們首先以XRD和FT-IR觀察水熱法合成電極的材料特性，再以電化學方法檢視以水熱法在180 反應三小時製成的複合電極，其在鹼性水溶液電解質擁有高達5404 F g-1的比電容值。 將此具高比電容值的複合電極，配合鋰離子膠體聚合物電解質組裝成全固態對稱超級電容器，再以循環伏安法分析電容器的反應電位，結果顯示若系統的操作電位控制在3 V內，能避免電解質裂解且具有可逆性。若以操作電位3 V進行長時間充放電，經過活化後電容器的比電容高達97 F g-1；推測是由氫氧化鎳氧化還原對貢獻並伴隨有機溶劑裂解。其性能展現類似電池的高能量密度121 Wh kg-1與高功率密度231 W kg-1，但庫倫效率僅54%。 為提高庫倫效率，將操作電位控制在2.5 V並同時提高電流密度，讓系統反應機制以電雙層電容為主；此時系統擁有的比電容數值6.8 F g-1，經過100次循環後，庫倫率效率都超過95%，能量密度為12 Wh kg-1，而功率密度高達6.8 kW kg-1。該系統經過5000次循環後，比電容數值還保有初始值的67%。 最後，經由電化學阻抗分析發現，對稱複合電極在鋰離子膠體聚合物電解質電化學特性會比在1 M KOH系統下，更接近理想電容器，代表全固態對稱超級電容器有更好的可逆性與穩定性。

Recently, energy storage devices continue to develop with the expanding of renewable energy sources. Among energy storage systems, supercapacitors are receiving increasing attention becaue of their high power density, high columbic efficiency, greater stability and long lifespan. The main challenge facing supercapaciors for further applications to portable electronics, electric vehicles, and smart grids is their low energy density. In this study, we raise the supercapacitor energy desnsity by incrasing the specific capacitance and widening the electrochemical operation window. In addition, we replace the aqueous electrolyte with a gel polymer to improve safety and widen applications of super capacitors. Serving as electrode materials in a supercapacitor, graphene possesses high electrical conductivity but the capacity it supplies could be low due to its re-staking phenomenon, while nickel hydroxide has excellent specific capacitance capability but low electrical conductivity. We aim to synthesize the Ni(OH)2/rGO composite electrode via the hydrothermal process to raise overall specific capacitance. First, material characteristics of the composite electrode were investigated by XRD and FT-IR. The electrochemical performance of the composite electrode in the alkaline aqueous electrolyte were examined. A composite electrode reacted for three hours at 180 °C via the hydrothermal method reaches the highest specific capacitance value of 5404 F g-1. To fabricate an all-solid-state symmetric supercapacitor, the optimized composite electrodes were combined with lithium-ion gel polymer electrolyte, which is to widen the operation window. Cyclic voltammetry was used to identify redox reactions occur within the supercapactior, and we found that to avoid the EC/PC decomposition the operation potential is limited to 3 V to preserve the reversibility of the system. The highest operation potential is thus set as 3 V for long-term galvanostatic charge and discharge. After activation, the specific capacitance of the all all-solid-state supercapacitor based on Ni(OH)2/rGO composite electrode is as high as 97 F g-1, which is attributed to the redox couple of nickel hydroxide and the oxidation of organic electrolyte. The supercapacitor exhibits a battery-like high energy density of 121 Wh kg-1and a high power density of 231W kg-1; however, its columbic efficicency is only of 54%. To raise columbic efficiency, the highest operation potential of the supercapacitor is controlled at 2.5 V. In turn, the specic capacitance of the system is 6.8 F g-1. After 100 cycles, the colmbic efficiency remains over 95% and achieves a better energy density of 12 Wh kg-1and high power density of 6.8 kW kg-1. The capacitance retention of the system is 67% after 5000 cycles. Finally, through electrochemical impedance analysis we found that the symmetric composite electrodes in lithium-ion gel polymer electrolyte behaves more similar to an ideal capacitor than in the 1 M KOH system. It also means the all-solid-state symmetric supercapacitor have better reversibility and stability than its 1 M KOH counter part.