|Abstract: ||在本研究製作一種結合多孔性的自組裝石墨烯電極於製作可撓式超級電容。在製作上，使用電化學剝離石墨烯並透過凍乾技術製備出三維石墨烯電極。本實驗探討在不同的石墨烯濃度下此大型多孔性結構對電容性能的影響，而在此裝置中使用的是膠態電解液(1M H2SO4-PVA)。實驗發現孔洞的大小與溶液濃度有關，孔洞大小4.88μm、 1.19μm、1.02μm、0.39μm 分別對應到溶液濃度 10 mg/mL、 15 mg/mL、 20 mg/mL跟 25 mg/mL。|
結果顯示低石墨烯濃度(10mg/mL)製備的電極在水系電解液(6M KOH)跟膠態電解液(1M H2SO4-PVA)的比電容值分別是45.40 F/g跟23.89 F/g比起在高濃度(25 mg/mL)所製備的電極呈現較高的電容值(水系電解液:31.85 F/g 跟膠態電解液: 10.43 F/g )。可以明確的發現孔洞的擴大有效提供升的電容值，增加離子的遷移性。
此外，由實驗中可以發現膠態電解質的預浸入多孔電極之方法，可以用於提升可撓式超級電容的性能。而非預浸入式所製作的電容，其比電容值為17.23 F/g (比預浸入式的低27.91%)，造成此現象的主要原因是電解液可以有效浸入電極內部，提升離子擴散性。本實驗比較電化學剝離石墨烯與還原氧化石墨烯電容的性能，發現還原氧化石墨烯由於其較低的導電性(rGO片電阻: 7.07x10-3Ω/sq；EC-graphene片電阻: 6.60 x10-3Ω/sq)與較少的含氧官能基團，使得電化學剝離石墨烯電容性能比較高。可撓式電容在彎曲測試中呈現高度的穩定性，在彎折180o後可保有94%的電容值，且在經過50次彎折仍有90%的電容性能保存。本實驗提出一個具有可撓性、環保且具量產性的高效能石墨烯超電容。;In this study, we report a flexible supercapacitor including a unique macroporosity self-assembly of graphene as electrode. The graphene supercapacitor made by three-dimensional graphene electrodes were prepared by electrochemical exfoliated graphene (EC-graphene) follow by freeze-dried and annealed process. Here we study the effects of macroporous structured electrodes on capacitor performance by altering the graphene concentration. Moreover, flexible supercapacitor devices made by such unique electrodes were demonstrated by using gel electrolyte (1M H2SO4 /PVA).
We found out the pore size of 3D graphene electrodes correlated to the solution concentration of initial graphene suspension, where the pore size were 4.88μm, 1.19μm, 1.02μm, 0.39μm corresponding to 10 mg/mL, 15 mg/mL, 20 mg/mL, and 25 mg/mL, respectively.
The result shows that graphene electrode exhibit superior high specific capacitance of 45.40 F/g in liquid electrolyte (6M KOH) and 23.89 F/g in gel electrolyte (1M H2SO4/PVA) at low concentration of graphene (10mg/mL), which was higher than that (31.85 F/g in liquid electrolyte and 10.43 F/g in gel electrolyte) of samples prepared at high concentration (25 mg/mL). The device shows excellent rate performance and cycling stability. It was clearly seen the larger pore sized electrodes, result in higher capacitance which was due to few-layered stacked graphene creating more accessible surface area and better kinetic process of ion diffusion.
In addition, it can be seen that the dipping method can be used to gain higher performance of flexible supercapacitor. The specific capacitance of EC-graphene without dip is 17.23 F/g (27.91% lower than dipping method). This can be caused by higher efficient ion diffusion in sample dips into dilute PVA-H2SO4 electrolyte due to more time electrolyte immersed into electrode. Moreover, it was found out that EC-graphene shows higher performance than conventional used reduced graphene oxide(rGO) (43.45 F/g in 6M KOH electrolyte), which was attributed to its poor electrical conductivity (sheet resistance of rGO 7.07x10-3 Ω/sq while EC-Graphene 6.60 x10-3 Ω/sq) and lower oxygen functional groups.
The flexible supercapacitor, exhibit superior working stability during the bending testing, where 94% of capacitance was preserved for bending angle up to 180o and 90% after 50 times bending cycles. This work introduces a new concept of flexible, environment friendly, large scale production, and high performance graphene-based supercapacitors that could have a way for practical applications in energy devices.