Nanostructured materials for the construction of asymmetrical supercapacitors

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.