利用準分子雷射及氧電漿處理噴塗式奈米碳管薄膜 應用於超級電容之研究

Abstract

超級電容器(Supercapacitors, Ultracapacitors, Electrochemical Capacitors) 由於具有快速充放電、高功率密度、可靠性高以及循環壽命長的特性，近來越來越受到重視。此外，超級電容同時具備傳統電容器的高功率密度及電池的高能量密度，被認為是下個世代最有潛力的能量儲存裝置。在諸多的材料當中，由於奈米碳管(Carbon Nanotubes)擁有優異的高比表面積、豐富中孔結構、良好的導電性以及獨特的管狀結構因此可以使電荷和離子能在短時間內分布於材料界面並快速完成充放電。有鑑於此，本論文首先利用超音波噴塗技術製造均勻的奈米級三維堆疊的高品質奈米碳管薄膜作為良好多孔性及導電性之超級電容電極，進而分別以準分子雷射和氧氣電漿製程，兩種低溫、且不含化學溶劑及金屬汙染物的表面改質方式來達到元件比電容值(Specific Capacitance)特性的提升。 本研究首先探討了奈米碳管薄膜厚度對電容之影響，經由掃描式電子顯微鏡(SEM)驗證超音波噴塗的薄膜均勻性和厚度的控制性，並將不同厚度薄膜所測得之比電容值做比較，提出厚度與比電容值之關係。為了得到實際應用之比電容值及不造成材料的浪費，我們選定噴塗四十層的碳管薄膜作為後續表面改質的實驗條件。 第二部分中，為了驗證準分子雷射之熱退火處理對碳管薄膜之影響，分別利用掃描式電子顯微鏡(SEM)、X射線光電子能譜(XPS)以及拉曼光譜(Raman Spectroscopy)分析雷射處理後奈米碳管之結晶性和奈米結構之改變，並進一步量測不同雷射條件下元件之比電容值，提出準分子雷射改善其電容特性之機制。發現隨著雷射能量增加，雷射退火可使碳管再結晶化，增加其導電性。此外，部分的碳管會因受到高能量退火由管狀結構剝開形成石墨烯的片狀結構，此奈米碳管與石墨烯的混成結構可增加與電解液的反應面積並且維持碳管原始之三維結構。在雷射能量為400mJ/cm2時，其元件比電容值由原本的23.7 F/g提升至39.5 F/g，改善達1.7倍的超級電容電極。更高的雷射能量，則會造成濺蝕(Ablation)現象，而使比電容值反而下降。 最後，第三部分為了驗證氧電漿製程對碳管薄膜的效應，亦利用掃描式電子顯微鏡(SEM)、X射線光電子能譜(XPS)以及拉曼光譜(Raman Spectroscopy)分析氧電漿處理後之奈米碳管的表面形貌和官能基變化，並進一步量測不同電漿條件下元件之比電容值，提出氧電漿改善其比電容值之機制。氧電漿製程能使碳管表面有效鍵結上大量氧官能基，這些官能基會在電容充放電時扮演氧化還原反應的角色，使比電容值有效地提升。我們藉由調變電漿的處理時間，成功製備出高達49.3 F/g的奈米碳管薄膜，相對於改質前的23.7 F/g，改善高達2.1倍的超級電容電極，足見氧電漿製程對碳管薄膜特性有顯著的提升。

Supercapacitors have attracted more and more attention due to their excellent characteristics such as rapid charge and discharge rate, high power density, high reliability and long cycle life. Especially, supercapacitors have not only high power density of traditional capacitors but also high energy density of batteries; therefore, they are considered as the most promising energy storage device in the next generation. As the electrodes of supercapacitors, carbon nanotubes have high specific surface area, high porosity, excellent electrical conductivity, and unique tubular structure so that the charges and ions can be well distributed to the electrode/electrolyte interface and charge completely in very short time. In this thesis, the ultrasonic spraying system was utilized to build uniform, and nano-scale 3D porous carbon nanotubes thin films as the conductive electrodes of supercapacitors. In order to further increase the specific capacitance (Csp) performance of the CNTFs without chemical solvent and metal pollution under low temperature, the surface modification of KrF excimer laser treatment and oxygen plasma treatment were carried out, respectively. First, the influence of the thickness of CNTFs on the Csp was discussed. The uniformity and thickness controllability of the thin film with scanning ultrasonic spraying system were verified by scanning electron microscopy (SEM). By comparing different CNTFs thicknesses and corresponding CV results, the relation between thickness and Csp was proposed. In order to save materials and obtain practical Csp, 40 layers of CNTFs were selected for the following discussion. Secondly, in order to verify the effect of the excimer laser treatment on CNTFs, a series of material analyses including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to analyze the surface morphology and crystallinity of CNTs. By comparing the results of the material analyses with the Csp characteristics under different laser conditions for the supercapacitors, the effect of the excimer laser treatment on the CNTFs was proposed. With increasing laser energy, the crystallinity of the CNTFs would be improved and hence the better conductivity was achieved. Moreover, part of CNTs would be unzipped as graphene sheets with keeping the 3D structure and the graphenes could provide more surface area for charge storage after excimer laser irradiation with ISLED treatment. Under 400 mJ/cm2 excimer laser ISLED irradiation, the Csp would be promoted from 23.7 F/g to 39.5 F/g. The improvement ratio of Csp was 1.7x. However, too high energy densities, above 400 mJ/cm2, would cause the ablation of the CNTFs and result in the degradation of the Csp. In the end, to verify the effect of oxygen plasma treatment on CNTFs, a series of material analyses including SEM, XPS, and Raman were again used to analyze the surface morphology and functional groups of CNTs. By comparing the results of the material analyses with the Csp characteristics under different plasma conditions for the supercapacitors, the effect of the oxygen plasma treatment on CNTFs was proposed. With oxygen plasma treatment, the oxygen-containing functional groups would be effectively decorated on the surface of CNTFs. These oxygen-containing functional groups were verified to play a role in the redox reactions to produce extra capacitance during charging/discharging so that the better Csp could be achieved. By adjusting the plasma processing time, the Csp of supercapacitors could raise to 49.3 F/g with respect to 23.7 F/g for the as-sprayed CNTFs. The improvement ratio of Csp was 2.1x. Hence, this study exhibited that the CNTFs as the electrodes of supercapacitors are promising in the future developments for the energy-storage devices.