Advanced, Flexible Ultracapacitor Electrodes on Carbon Fiber Cloth Using Nano-Architectured MnO2/CNT

The critical “energy-prosperity-environmental dilemma” has prompted academicians, policy makers, and industrial leaders worldwide to institute R&D programs and policies to meet the global energy challenge [1]. The transient nature of renewable energy sources like solar and wind cause detrimental fluctuations in power grids that lead to reliability problems. To mitigate these effects, energy storage systems are critically needed to store electricity and smooth out the abrupt changes in energy demand. Moreover, demand for EVs and HEVs as well as flexible and ‘wearable’ solutions to meet requirements of the military for providing power to the personnel, or unmanned aerial vehicles, space missions, etc., have further accelerated the search for next-generation energy storage devices, specifically batteries and supercapacitors, with high energy and power density, safety, low cost, and environmental friendliness. Though LIBs provide the highest energy density, they suffer from low specific power and poor safety [2-5]. Supercapacitors, on the other hand, have high power density, cyclability, and safety but suffer from low energy density and high production cost [2-5].

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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