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Most methods for improving supercapacitor performance are based on developments of electrode materials to optimally exploit their storage mechanisms, namely electrical double layer capacitance and pseudocapacitance. In such cases, the electrolyte is supposed to be electrochemically as inert as possible so that a wide potential window can be achieved. Interestingly, in recent years, there has been a growing interest in the investigation of supercapacitors with an electrolyte that can offer redox activity. Such redox electrolytes have been shown to offer increased charge storage capacity, and possibly other benefits. There are however some confusions, for example, on the nature of contributions of the redox electrolyte to the increased storage capacity in comparison with pseudocapacitance, or by expression of the overall increased charge storage capacity as capacitance. This report intends to provide a brief but critical review on the pros and cons of the application of such redox electrolytes in supercapacitors, and to advocate development of the relevant research into a new electrochemical energy storage device in parallel with, but not the same as that of supercapacitors.

Three-dimensional electrodes offer great advantages, such as enhanced ion and electron transport, increased material loading per unit substrate area, and improved mechanical stability upon repeated charge-discharge. The origin of these advantages is discussed and the criteria for ideal 3D electrode structure are outlined. One of the common features of ideal 3D electrodes is the use of a 3D carbon- or metal-based porous framework as the structural backbone and current collector. The synthesis methods of these 3D frameworks and their composites with redox-active materials are summarized, including transition metal oxides and conducting polymers. The structural characteristics and electrochemical performances are also reviewed. Synthesis of composite 3D electrodes is divided into two types - template-assisted and template-free methods - depending on whether a pre-made template is required. The advantages and drawbacks of both strategies are discussed.

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A recently claimed ultra high specific capacitance of 3407 F g(-1) for aligned polyaniline nanorods by the titled communication is shown to be contradictory to both the mechanism of charge storage in conducting polymers, and the experimental findings in other nanofibrils of polyaniline, and may thus stimulate debate.

Asymmetrical supercapacitors with aqueous electrolytes were fabricated from carbon nanotubes (CNTs) individually coated with SnO2 (CNTs/SnO2) and MnO2 (CNTs/MnO2) as the negative and positive electrodes, respectively. The CNTs/SnO2 nanocomposite is used as the negative electrode material in an asymmetrical supercapacitor. The physicochemical properties of the CNTs/SnO2 and CNTs/MnO2 nanocomposites were examined by X-ray diffraction, scanning and transmission electron microscopy, cyclic voltammetry, and galvanostatic charge–discharge. Individually, the supercapacitors were tested for charge and discharge to a cell voltage of 1.70 V in 2.0 M KCl without noticeable water decomposition. The asymmetrical cell could reach the specific energy of 20.3 Wh/kg, which is comparable to that obtained from electric double-layer supercapacitors using organic electrolytes (17–18Wh/kg) . The maximum specific power of the cell, 143.7 kW/kg, is perhaps the highest among all reported aqueous asymmetrical supercapacitors. It also shows an exceptional stability of over 1000 cycles, with the capacity loss being less than 8%. A 10 V stack was also constructed with nine individual supercapacitors connected through bipolar electrodes of the nanocomposites and porous separators containing 1.0 M Na2SO4 . The stack exhibited remarkable capacitive behavior resulting from the individual cells.

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