Redox deposition of manganese oxide on graphite for supercapacitors

Immersing a graphite disc electrode in the acidic solution of KMnO4 resulted in the deposition of a redox active coating on the electrode surface. Cyclic voltammograms of the coating in various neutral aqueous electrolytes (0.5 M LiCl, 0.5 M LiClO4, 0.5-2.0 M NaCl, 0.1-0.25 M Na2SO4 or 0.5-1.0 M KCl) exhibited typical pseudo-capacitive behaviour. In the potential window of 0-1.0 V (vs. Ag/AgCl), the capacitance of the coating in 0.5 M LiCl increased to 45 mF/cm(2) with the deposition time, up to 60 min, following very well the logarithm law. In addition, applying 1000 potential cycles on the coating in 0.5 M LiCl confirmed the high charge-discharge stability of the coating. The energy dispersive X-ray analysis revealed the atomic ratio of oxygen and manganese being greater than 2, indicating the coatings to be likely hydrous manganese oxide. The morphology of the coating before and after potential cycling was examined using scanning electron microscopy. The as-deposited coating appeared amorphous, however, after prolonged potential cycling, cross-linked and needle-like nanocrystallites were observed in the coating. The formation mechanism and capacitive behaviour of the coatings are discussed. It is anticipated that the application of this simple, fast and controllable "redox deposition" process to high surface area carbon materials may significantly improve their performance in supercapacitors. (C) 2004 Elsevier B.V. All rights reserved.