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Influence of supporting electrolyte on the pseudocapacitive properties of MnO2/carbon nanotubes

The aim of this study was to prepare MnO2 from a MnCl2 solution by electrode polarisation at a constant potential. MnO2 was deposited at a platinum electrode or platinum modified by carbon nanotubes (CNTs) with a varying degree of oxidation. The mass of the electrodeposited MnO2 was calculated from the current transient and the result was corrected by the factor that considers the presence of the equivalent amount of water within the MnO2 layer. An electrochemical quartz crystal microbalance (EQCM) enabled determination of the correction factor. The obtained electrodes (MnO2, MnO2/CNT, MnO2/CNT-EO1 and MnO2/CNT-EO2) were tested in NaCl, LiCl and MnCl2 solutions by cyclic voltammetry and it was proven that the best pseudocapacitive properties were registered in the MnCl2 solution. The factors influencing the pseudocapacitive properties were clarified by the EQCM method. It was shown that the electrolyte cation size, the cation valence and the cation hydration shell all influence the MnO2 pseudocapacitive redox reaction. The pseudocapacitive properties of MnO2 were improved by MnO2 deposition onto CNTs and oxidised CNT electrodes. The oxidation of CNTs was accomplished by a simple electrochemical procedure in a Na2SO4 solution (CNT-EO1 and CNT- EO2 electrodes).

supercapacitors ; MnO2 ; CNT ; cyclic voltammetry ; electrochemical quartz crystal microbalance

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