Electrode materials for aqueous asymmetric supercapacitors
An asymmetric supercapacitor (ASC) is a supercapacitor based on two different electrode materials. One electrode is based on redox (Faradic) reactions with or without non-faradic reactions, and the other one is mostly based on electric double-layer (non-Faradic or electrostatic) absorption/desportion. Aqueous electrolytes have higher ionic conductivity, larger capacitance and better safety than the organic electrolytes. Herein, some key electrode materials for aqueous ASCs are primarily reviewed, which provide a new direction for power sources to have higher energy density at high power densities, compared with traditional capacitors. Their negative electrode materials include carbonaceous materials (porous carbons, carbon nanotubes and graphene), oxides (V2O5 and MoO3) and their composites, and their positive electrode materials include carbonaceous materials, oxides (RuO2, MnO2, MoO3, V2O5, PbO2, Co3O4), Ni(OH)2, intercalation compounds (LiCoO2, LiMn2O4, Li[NiCoMn]1/3O2, NaMnO2 and KMnO2). We describe the latest work on these electrode materials, and a particular focus is directed to the fabrication and electrochemical performance of various nanostructured electrode materials and some assembled ASCs. Finally, the future trends and prospects on advanced energy storage materials are suggested.