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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. References: Jefferson W. Tester, et. al: Chapter 1, p. 41, MIT Press, Cambridge, Massachusetts, 2005 ISBN 0-262-20153-4. B. Scrosati, Nature 372 (1995) 557. G. Ceder, Y.M. Chiang, D.R. Sadoway, M.K. Aydinal, Y.I. Jang, B. Huang, Nature 392 (1998) 694. S. Wei, W. P. Kang, J. L. Davidson, B. R. Rogers, and J. H. Huang, ECS Transactions 28 (2010) 97. P. Simon and Y. Gogotsi, Nature (materials) 7 (2008), 845-854.

Geo-political concerns, global warming, damage to ecosystem and biodiversity have resulted in greater focus on research and development of alternative energy sources. Energy storage became a dominant factor in economic development with the global energy consumption forecast to grow by 56% between 2010- 2040 with renewable energy expected to grow at 2.5% per year [1]. Solar and wind energy are very consistent from year to year but have significant variation over shorter time frames. Batteries and ultracapacitors have been deployed to better utilize these energy sources. However, by developing advanced ultracapacitors which provide greater energy density while retaining high power density we can revolutionize energy storage solutions for both military and civilian applications. Supercapacitors are rechargeable electrochemical energy storage devices which can provide high capacitance, suitable for high power applications [2]. Based on the charge storage mechanism, supercapacitors can be divided into three typical classes: (i) electrochemical double layer capacitors (EDLCs), (ii) pseudocapacitors, and (iii) hybrid-supercapacitors. By developing hybrid-supercapacitors which integrate high specific surface area CNTs and thin-film of pseudocapacitive MnO2 material, we can achieve high performance at low cost. In addition to the double layer capacitance from the high surface area, we get additional capacitance from the pseudocapacitive behavior involving rapid, reversible faradaic reactions where the oxidation state of Mn varies between +3 and +4 in conjunction with the intercalation and deintercalation of the electrolyte cation, as represented by the following equation [2,3]: MnO2 + X+ + e- ↔ MnOOX (X= H, Li, Na, K). In this paper, we present fabrication and characterization of advanced 3D micropatterned MnO2/CNT electrodes on SiO2/Si substrates and nanostructured ultracapacitor electrodes using MnO2/CNT on graphite foil substrate for integration into an ultracapacitor prototype cell. Conventional silicon microfabrication and hot-filament CVD processes were combined to fabricate CNT microelectrode arrays which were then coated with MnO2 using direct electrochemical deposition. The SEM micrographs in figure 1 show typical structures that have been fabricated including an interdigitated design. Low-cost, thermal CVD process in a tube furnace was used to synthesize CNTs, which act as the current conductors, on flexible, conducting graphite foil current collector. Thin-film MnO2 deposition, directly on the CNT was achieved by electrochemical technique which provides excellent bonding between the two for enhanced stability. Such a structure provides a 3-D surface allowing for better MnO2 incorporation, shorter solid state diffusion of cations, facile electron transfer, and over all improved charge storage performance. The SEM images for the MnO2 coated CNTs on graphite substrate can be seen in figure 2, where the thin-film coating on individual CNT is visible as well as the porous microstructure of the MnO2can be distinguished. Electrochemical characterization was performed using cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy. In a three electrode configuration with Ag/AgCl reference, the microelectrode array delivered a high value of 1.8F/cm2 or 240F/cm3. The CVs seen in figure 3 show the increase in current with increasing MnO2 thickness, showing the direct effect of MnO2 on the electrode performance. The flexible electrode MnO2/CNT/Graphite ultracapacitor delivered 2.8F capacitance which is equal to 235mF/cm2 or 250F/g at 2mV/s. This translates into a high specific energy of 34.7Wh/kg and specific power of 7.6kW/kg. Cyclic voltammograms recorded at different scan rates 2mV/s-100mV/s can be seen in figure 4. These results are based on aqueous electrolyte. Detailed fabrication and characterization of the novel ultracapacitors will be presented. References: http://www.eia.gov/forecasts/ieo/ S. Wei, W. P. Kang, J. L. Davidson, B. R. Rogers, and J. H. Huang, ECS Transactions, 28 (8) 97-103 (2010). Wei Chen, Zhongli Fan, Lin Gu, Xinhe Bao and Chunlei Wang, Chem. Commun., 2010, 46, 3905–3907.

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