氧化錳奈米纖維電極結合離子液體電解液之超高電容器應用

Abstract

本論文研究主旨在於如何利用多孔導電基材以及特定離子液體電解液增進氧化錳奈米纖維之電容值並擴展反應電位窗的方法，希望藉此開發出具有良好能量密度的超高電容器。實驗規劃為利用電化學陽極沉積法可將氧化錳的奈米纖維製備於具有高表面積的鎳泡棉基材上，並且利用循環伏安法及臨場的X光吸收光譜(In-situ x-ray absorption spectroscopy, in-situ XAS)探討其電位改變對材料價態改變以及晶體結構的影響。 結果顯示奈米纖維氧化錳沉積於多孔性鎳泡棉基材並搭配特定離子液體電解液，具有理想的擬電容特性與電化學可逆性，其比電容值可達350Fg-1，另外由於電化學之反應電位窗從0.9V提升至2.5V，使得能量密度亦提高到304 Wh kg-1。並透過臨場X光吸收光譜發現在充放電的過程中，其氧化錳的價數變化差異達到0.91，也凸顯了擬電容電容器的特色。 此外加入鎳片基材及碳纖維布基材進行比較，分別在水溶液及離子液體中進行電化學測試。測試後發現，以鎳片當基材的電化學表現結果不如其餘兩者，另外以碳纖維布為基材的擬電容特性雖不如鎳泡棉優越，但在大多數的離子液體中，電位窗都能超過2.5V，在Urea-LiClO4中更擴大到3.5V，同時也具有優異的能量密度。 上述研究結果各有其特色，分別提供了優異的擬電容特性及寬廣的反應電位窗，皆可使能量密度有明顯的提升，提供做為超高電容器材料的不同選擇。

This research is aim to develop a supercapacitor with high enery density. We design a simple method to prepare nano-fiber manganese oxide on different porous substrates by electrochemical anodic deposition method. Cyclic voltammetry (CV) measurements in ionic liquids demonstrated that the MnO2 on porous substrates exhibited excellent specific capacitance and great cycling stability . We used ionic liquid to replace traditional aqueous as electrolyte , which enhanced the capacitance value up to 350Fg-1.This also led the potential window of the reaction increasing from 0.9V to 2.5V, therefore enhancing the energy density to a value of 304 Wh kg-1. The variation of the Mn oxidation state during charge and discharge cycles were elucidated with in situ X-ray absorption near-edge structure (XANES) spectra, where the valence change was 0.91electron per Mn. This demonstrated the significant characteristics of pseudo capacitors. Finally, comparing with flat nickel and carbon fiber substrate in ionic liquids, we found that the capacitance characteristics of carbon fiber as the substrate were poorer than those of the nickel foam. However, the potential window can be expanded from 2.5V to 3.5V in Urea-LiClO4 ionic liquid, which provided a different option to reach the goal of increasing the energy density.