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.

Supercapacitors with aqueous electrolytes and nanostructured composite electrodes are attractive because of their high charging-discharging speed, long cycle life, low environmental impact and wide commercial affordability. However, the energy capacity of aqueous supercapacitors is limited by the electrochemical window of water. In this paper, a recently reported engineering strategy is further developed and demonstrated to correlate the maximum charging voltage of a supercapacitor with the capacitive potential ranges and the capacitance ratio of the two electrodes. Beyond the maximum charging voltage, a supercapacitor may still operate, but at the expense of a reduced cycle life. In addition, it is shown that the supercapacitor performance is strongly affected by the initial and zero charge potentials of the electrodes. Further, the differences are highlighted and elaborated between freshly prepared, aged under open circuit conditions, and cycled electrodes of composites of conducting polymers and carbon nanotubes. The first voltammetric charging-discharging cycle has an electrode conditioning effect to change the electrodes from their initial potentials to the potential of zero voltage, and reduce the irreversibility.

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Mechanically robust power devices of high energy efficiency are one of the keys towards overcoming the challenges from the daunting climate change and the depletion of fossil fuels on the earth. The importance of mechanical engineering has been long recognized in physical type power devices, but less so in those based on electrochemical processes, such as batteries, fuel cells, and electrochemical capacitors (ECs). Particularly, ECs, which are also known as supercapacitors, bridge the crucial performance disparity between fuel cells or batteries with high energy capacities and the traditional capacitors capable of outputting pulsed high power. New materials and advanced configurations are the two essential elements for ECs to cope with mechanical engineering issues at both macro and micro levels. This review describes the design and characteristics of ECs and the emerging asymmetrical construction utilizing nanostructured composites that enable energy storage through both ion adsorptions (interfacial capacitance) and fast and reversible redox reactions (pseudo-capacitance). It is specially intended to rouse interest towards newly reported high-energy and high-power aqueous ECs with nanocomposites of transition metal oxides, nitrides or conducting polymers, and carbon nanotubes or activated carbons. Current collector materials and structures are also examined as important mechanical engineering elements in ECs. The chemical, material, and mechanical issues reviewed here call for more joined efforts among scientists, engineers, and industries to further advance ECs as a promising new energy technology.

An asymmetrical supercapacitor was fabricated with a CNTs/SnO2 negative electrode and a CNTs/MnO2 positive electrode. It was tested in charge and discharge to 1.7 V in aqueous electrolyte without showing noticeable current resulting from water decomposition. Physicochemical properties of the CNTs/SnO2 and CNTs/MnO2 nanocomposites were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectrometry (EIS). At a current density of 0.25A g-1, the energy density of the hybrid supercapacitor was found to be 20.3 Whkg-1 which is comparable to that obtained from electric double layer supercapacitors using organic electrolytes (17~18Wh kg-1). The asymmetrical supercapacitor also shows exceptional stability over 1000 cycles with the capacity loss being less than 8%.

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