Design and Development of 3D Nanostructured MnO2/CNT Electrodes for Supercapacitor Applications

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:

1. http://www.eia.gov/forecasts/ieo/
2. S. Wei, W. P. Kang, J. L. Davidson, B. R. Rogers, and J. H. Huang, ECS Transactions, 28 (8) 97-103 (2010).
3. Wei Chen, Zhongli Fan, Lin Gu, Xinhe Bao and Chunlei Wang, Chem. Commun., 2010, 46, 3905–3907.