Supercapacitor Electrodes of MnO₂ and MnO₂/Graphene Nanosheets Synthesized by Liquid Phase Exfoliation

Abstract

MnO 2 has been extensively investigated due to its high theoretical capacitance of 1100 to 1300 F.g -11 , environmental friendly nature and low cost 2 . The MnO 2 charge storage mechanism relies on the exchange of protons and/or cations with the electrolyte, redox activity involving a Mn(IV)/Mn(III) transition, and chemisorption of ions onto the MnO 2 surface 1,3,4 . As these are surface processes, it is paramount to design MnO 2 structures with an accessible high surface area. Recently, liquid phase exfoliation has become a powerful technique for the preparation of 2D nanosheets presenting a high surface area 5,6 . Therefore this technique can be used to produce nano layers of MnO 2 , which will present an enhanced utilization of the active material and a better electrochemical performance 7 . In the present work a high-surface area, porous MnO 2 powder was produced through the oxidation of Mn(NO3) 2 by KMnO 4 . A poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) triblock copolymer (PEG-PPO-PEG P123) was used as templating agent for the formation of a “flower-like” nanostructure (MOFN) with protruding 2D-nanostructures 8 . Subsequently, MOFN wére exfoliated in isopropanol at 37kHz for 3 hours, resulting in two types of materials: manganese oxide nanolayers (MOL) and a partially exfoliated material (PEMO). Following a novel approach, the MOFN were also exfoliated simultaneously with graphite resulting in a MnO 2 layers/Graphene hybrid (GMOH). The obtained dispersions were sprayed onto ITO electrodes and the electrochemical properties studied by cyclic voltammetry. By testing electrodes with different thicknesses it was found out that the electrochemical utilization is enhanced for GMOH (80.0 mF.cm -2 ) at a thickness of 9700 nm). A capacitance as high as 300 F.cm -3 was also achieved with GMOH thin electrodes followed by 225 F.cm -3 for MOL and 100 F.cm -3 for PEMO. (1) Wang, G.; Zhang, L.; Zhang, J. Chemical Society Reviews 2012 , 41 , 797. (2) Kang, J.; Hirata, A.; Kang, L.; Zhang, X.; Hou, Y.; Chen, L.; Li, C.; Fujita, T.; Akagi, K.; Chen, M. Angewandte Chemie International Edition 2013 , 52 , 1664. (3) Xu, C.; Kang, F.; Li, B.; Du, H. Journal of materials research 2010 , 25 , 1421. (4) Bélanger, D.; Brousse, L.; Long, J. W. The Electrochemical Society Interface 2008 , 17 , 49. (5) Coleman, J. N.; Lotya, M.; O’Neill, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.-Y.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Science 2011 , 331 , 568. (6) Nicolosi, V.; Chhowalla, M.; Kanatzidis, M. G.; Strano, M. S.; Coleman, J. N. Science 2013 , 340 . (7) Toupin, M.; Brousse, T.; Bélanger, D. Chemistry of Materials 2004 , 16 , 3184. (8) Jiang, H.; Sun, T.; Li, C.; Ma, J. Journal of Materials Chemistry 2012 , 22 , 2751.