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

The aim of this study was to prepare MnO 2 from a MnCl 2 solution by electrode polarisation at a constant potential. MnO 2 was deposited at platinum and platinum modified by carbon nanotubes (CNTs) with a varying degree of oxidation. The mass of the electrodeposited MnO 2 was calculated from the current transient. An electrochemical quartz crystal microbalance (EQCM) enabled the determination of the correction factor which considers the presence of the equivalent amount of water within the MnO 2 layer. The obtained electrodes (MnO 2 , MnO 2 /CNT, MnO 2 /CNT-EO1 and MnO 2 /CNT-EO2) were tested in NaCl, LiCl and MnCl 2 electrolytes by cyclic voltammetry, demonstrating the best pseudocapacitive properties in the MnCl 2 solution. It was proven that the electrolyte cation size, its valence and the hydration shell significantly influence the MnO 2 pseudocapacitive redox reaction. The pseudocapacitive properties of MnO 2 were improved by MnO 2 deposition onto CNTs and oxidised CNT electrodes. The oxidation of CNTs was accomplished by a simple electrochemical procedure in a Na 2 SO 4 solution (CNT-EO1 and CNT-EO2 electrodes). Electrochemical impedance spectroscopy has shown that charge transfer resistance has increased by oxidation process. However, it did not influence the overall pseudocapacitive properties significantly.