Nanocomposites of manganese oxides and carbon nanotubes for aqueous supercapacitor stacks

Symmetrical supercapacitors and their serially connected two-cell stacks via a bipolar electrode were constructed with nanocomposites of manganese oxides and carbon nanotubes (MnOx/CNTs) as the electrode materials. Nanocomposites with different contents of MnOx were synthesised through the redox reaction between KMnO4 and CNTs in aqueous solutions. The nanocomposites were characterised by scanning and transmission electron microscopy, BET nitrogen adsorption and X-ray diffraction before being examined in a three-electrode cell with a novel trenched graphite disc electrode by electrochemical means, including cyclic voltammetry, galvanostatic charging–discharging, and electrochemical impedance spectroscopy. The nanocomposites demonstrated capacitive behaviour in the potential range of 0–0.85 V (vs Ag/AgCl) in aqueous KCl electrolytes with less than 9% capacitance decrease after 9000 charging–discharging cycles. Symmetrical supercapacitors of identical positive and negative MnOx/CNTs electrodes showed capacitive performance in good agreement with the individual electrodes (e.g. 0.90 V, 0.53 F, 1.3 cm2). The bipolarly connected two-cell stacks of the symmetrical cells exhibited characteristics in accordance with expectation, including a doubled stack voltage and reduced internal resistance per cell.