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, V ₂O₅, PbO₂, Co₃O₄), Ni(OH)₂, intercalation compounds (LiCoO₂, LiMn ₂O₄, Li[NiCoMn]_1/3O₂, NaMnO ₂ 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.