石墨烯材料合成及其電化學電容行為探討

Preparation and Electrochemical Capacitive Behavior of Graphene Materials

許舒閔(Shu-Min Hsu)

指導教授 ： 謝建德

元智大學/工程學院/化學工程與材料科學學系/碩士(2011年)

https://doi.org/10.6838/YZU.2011.00158

摘要

本研究利用循環伏安法、定電流充電/放電循環測試以及電化學阻抗分析，探討以石墨烯為主的電容器之電化學特性。利用modified Hummers法及氣體還原能夠製備出石墨烯奈米片。以石墨烯(Graphene, GN)及氧化石墨烯(Graphene oxide, GO)作為碳極材料，在1M硫酸下作電容器之電化學測試。研究結果顯示，氧化石墨烯電容器能提供一個較穩定的電容量(1000圈後，電容量大約為61.5 F/g)及較低的等效電路電阻(1000圈後，電阻大約為7.6 Ω)。由於存在於材料表面缺陷之氧化物能有效提升電雙層的親水性覆蓋率(電雙層電容)及提供氧化還原反應的活性位置(偽電容)，故總觀結構(奈米片)及化學(表面氧化物)兩因素，氧化石墨烯應用於電化學電容器或其他儲能裝置上為具有潛力的電極材料。
本研究更進一步以GO作為電極材料，探討於不同電解質下，如：硫酸鋰及硫酸鈉，其電化學行為。其在1M硫酸鋰及硫酸鈉電解質中，及0.5 mA/cm2之電流密度下，電容量各別為238.0及98.8 F/g。並且，計算出對於鋰離子及鈉離子而言，電化學活性面積各別為452.8及219.3 m2/g。顯示出水合後的鋰離子傾向於雙層堆疊，而水合後的鈉離子則為單層吸附於氧化奈米片上。由Randles圖譜中，得知鋰離子擴散速率為3.1×10-15 cm2/s，其為鈉離子於氧化石墨烯電極中之擴散速率的三倍。相對於硫酸鈉電解質，氧化石墨烯電極於硫酸鋰電解質中，展現出高電容量、高擴散能力及低內阻之表現，其歸因於較小的離子及較低的水合數，因此，有利於電容之效能。

關鍵字

石墨烯奈米片 ； 氧化石墨烯 ； 電雙層電容 ； 電化學電容器 ； 表面氧化物 ； 電解質 ； 擴散係數

並列摘要

We investigate the electrochemical properties of graphene-based capacitors using cyclic voltammetry, constant charge/discharge cycling, and electrochemical impedance spectroscopy combined with equivalent circuit. A facile route incorporated with modified Hummers method and gas-fed reduction is capable of preparing graphene nanosheets (GNs). Two types of capacitors fabricated with GN and graphene oxide (GO) powders are examined in 1 M H2SO4 within a potential of 0 V and 0.8 V vs. Ag/AgCl. The GO-based capacitor presents not only a better stable capacitance (ca. 61.5 F/g after 1000 cycles) but also a lower equivalent series resistance (ca. 7.6 Ω after 1000 cycles), compared with the GN-based capacitor. The presence of surface oxides, attached to the edges or defects of basal planes, imparts hydrophilic coverage for formation of double-layer (double-layer capacitance) and active sites for reversibly redox reaction (pseudocapacitance). Incorporating with structural (nanosheets) and chemical (surface oxides) factors, the GO powder serves as a promising electrode material for electrochemical capacitors or other energy storage devices.
GO based electrochemical capacitors have been fabricated and investigated in 1 M Li2SO4 and Na2SO4 aqueous electrolytes. The GO sheets were derived from natural graphite powders and subsequently coated over carbon paper, forming a composite electrode. The GO sheets have highly oxidized planes and edges, occupied by oxygen functionalities including hydroxyl, carboxyl, and carbonyl groups. The GO-based capacitor displays specific capacitances of 238.0 and 98.8 F/g at 0.5 mA/cm2 in Li2SO4 and Na2SO4 electrolytes, respectively. The electrochemically active areas for Li and Na ions are calculated to be 452.8 and 219.3 m2/g at the first discharge cycle, respectively. The staking layer of the hydrated Li molecules forms dual layers, whereas the hydrated Na molecules tend toward a monolayer adsorption on the oxidized sheets. Based on the Randles plot, the apparent diffusion coefficient of Li+ is calculated to be 3.1×10-15 cm2/s, which is about three times higher than that of Na+ in the GO-based electrodes. Compared with the Na2SO4 electrolyte, the GO-based capacitor in Li2SO4 exhibits high stable capacitance, low inner resistance, and high diffusivity. This originates from the smaller ionic size and the lower hydration number, thus facilitating the performance of the capacitor.

並列關鍵字

Electric double-layer capacitance ； Electrochemical capacitors ； Surface oxides ； Electrolyte ； Diffusion coefficient ； Graphene nanosheet ； Graphene oxides

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