Advanced, Flexible Ultracapacitor Electrodes on Carbon Fiber Cloth Using Nano-Architectured MnO2/CNT
Ultracapacitors can be classified into electrical double layer capacitors (EDLCs), pseudocapacitors, and hybrid supercapacitors, based on the electrode design and charge storage mechanism. Recently, nanostructured electrode materials have played a key role in the advancement of supercapacitor technologies; these nano-scaled materials have higher capacities and better response rates than traditional bulk materials. In this paper, we present the innovative approach of using 3D nano-architectured double-sided MnO2/CNT electrodes on a free-standing conductive carbon fiber (CF) cloth which eliminates the need for a metal substrate conductor layer, thereby reducing cost and weight, and allows for multilayer stacking of electrodes to further enhance areal capacitance. The use of CF cloth also allows flexibility in cell design. The 3D nano-structured electrodes allow shorter ion diffusion distance, lower contact resistance, faster electron transfer kinetics, and high specific surface area for optimum loading of the MnO2nanoparticles to boost energy and power density.
Hot-filament CVD process was used to synthesize CNT on a piece of woven carbon fiber cloth. Electrochemical deposition was performed for thin-film coating of MnO2 on the CNTs. SEM micrographs in figure 1 show the section of the CF cloth, with and without the vertically aligned CNTs and an overview of the CF cloth post CNT synthesis. The SEM images of the electrode after MnO2 deposition can be seen in figure 2. The porous structure of the MnO2 deposited uniformly on the CNTs is evident. Electrochemical characterization was performed in a modified cell in 3 electrode configuration in 0.1M KCl electrolyte and also after assembling the electrodes in a symmetric configuration in a pouch cell prototype with 1M TEATFB in acetonitrile. Excellent values of the areal and specific capacitances of 455 mF/cm2 and 1035 F/g, respectively, were obtained at a scan rate of 5mV/s. The electrode also retains more than 67% of its capacity at a higher scan rate (the areal and specific capacitance drop to 305 mF/cm2and 694 F/g when the scan rate increases from 5 to 100mV/s). Detailed fabrication and material analysis of the advanced, flexible electrodes and the prototype cell will be presented. Results from electrochemical characterization will also be discussed.
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