High Performance Materials for Electric Double Layer Capacitors -- Nanostructured Carbon Electrodes and Gel Electrolytes

• Main Authors: Cheng-WeiHuang, 黃正瑋
• Other Authors: Hsi-Sheng Teng
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/94921279281718890770

博士 === 國立成功大學 === 化學工程學系碩博士班 === 100 === This dissertation includes three parts: (1) Mesoporous carbon spheres grafted with carbon nanofibers for high-rate electric double layer capacitors. (2) Electric double layer capacitors based on a composite electrode of activated mesophase pitch and carbon nanotubes. (3) Gel electrolyte derived from poly(ethylene glycol) blending poly(acrylonitrile) applicable to roll-to-roll assembly of electric double layer capacitors. In the first part, carbon nanofibers were grafted onto mesoporous carbon spheres to produce “sea urchin-like” mesoporous carbon with a nanofiber content of 25 wt%. Because of its combined features of high electronic conductivity and efficient electrolyte transport, the sea urchin-like mesoporous carbon assembled in electric double layer capacitors shows outstanding high-rate performance with a voltammetric scan rate as high as 3000 mV s-1. Ac impedance analysis shows that this method of carbon nanofiber grafting promotes electronic percolation and ionic transportation in the carbon electrode, reducing the capacitive relaxation time to less than one fourth of its original value. Electrochemical oxidation in sea urchin-like mesoporous carbon produces a capacitance increase of ca. 200% while retaining high electronic and ionic conductivities in the electrode. The second part reports on a novel composite of KOH activated mesophase pitch (aMP) and carbon nanotubes (CNTs) that shows outstanding performance as electrodes for electric double-layer formation in 2 M H2SO4. The aMP powder is highly porous and the KOH activation may produce pores that are populated with graphitic edges. The resulting aMP electrode has a capacitance value of 295 F g-1 at 0.125 A g-1 discharge and decreases to 180 F g-1 at 100 A g-1. With particle milling, the pore diffusion resistance of the aMP electrode decreases significantly because of the elimination of a hindered diffusion mode for the particle interior. CNT addition provides inter-particle spacing and bridging media for the milled aMP and reduces the Warburg diffusion and electrical resistances. The composite of milled aMP and CNTs have capacitance values of 305 F g-1 at 0.125 A g-1 and 214 F g-1 at 100 A g-1. With a small potential widow of 1 V, the resulting symmetric cells can deliver an energy level of 8.2 Wh kg-1 at a high power of 10,000 W kg-1. These cells show superior stability, with no decay of specific capacitance after 10,000 cycles of galvanostatic charge and discharge. Third part reports the synthesis of a gelled polymer electrolyte (GPE) using poly(ethylene glycol) blending poly(acrylonitrile) (i.e., PAN-b-PEG-b-PAN) as a host, dimethyl formamide (DMF) as a plasticizer, and LiClO4 as an electrolytic salt for electric double layer capacitors (EDLCs). The PAN-b-PEG-b-PAN copolymer in the GPE has a linear configuration for high ionic conductivity and excellent compatibility with carbon electrodes. When assembling the GPE in a carbon-based symmetric EDLC, the copolymer network facilitates ion motion by reducing the equivalent series resistance and Warburg resistance of the capacitor. This symmetric cell has a capacitance value of 101 F g-1 at 0.125 A g-1 and can deliver an energy level of 11.5 Wh kg-1 at a high power of 10,000 W kg-1 over a voltage window of 2.1 V. This cell shows superior stability, with little decay of specific capacitance after 30,000 galvanostatic charge-discharge cycles. The distinctive merit of the GPE film is its adjustable mechanical integrity, which makes the roll-to-roll assembly of GPE-based EDLCs readily scalable to industrial levels.