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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.

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.

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The enormous technical developments and rapid changes in life patterns made in the recent decades have largely been attributed to the exploitation of contemporary forms of energy sources, i.e. fossil fuels. However, their finite availability and significantly high environmental impacts have aroused concerns and spurred research to find alternatives and more efficient ways to store energy. In particular, recent developments of batteries and fuel cells as energy storage devices have been proven to be very promising, but their poor power characteristics and cyclic stability hinder their wider applications. Conversely, conventional capacitors display a great outputting pulsed power, but disappointing energy characteristics. Electrochemical capacitors (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. The main focus of this review is to outline the latest developments of the ECs and determine their current status in terms of energy and power characteristics. In particular, recent developments in materials including new synthesis methods, structural studies and advanced configurations of ECs are discussed. Moreover, several technical challenges to further development are identified. Based on the latest results, the potential of developing supercapatteries, whose performance is in between batteries and contemporary supercapacitors, are also discussed in this review.

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