**2.3 Active materials**

Depending on the charging mechanism, active materials can usually be divided into four categories. (1) Carbonaceous materials include CNTs, graphene, and its allotrope, where graphene is mainly based on double-layer capacitance, which utilizes only the surface of the electrode material; however, the capacitance of bare carbonaceous will be relatively small, limiting the applications of supercapacitors. (2) Metal oxides, especially Ni oxide, Co oxide, Mn oxide, and Co oxide, include MnO2 [12], NiO, Co3O4, and Co(OH)2 [13], and these metals have many energy levels [41], which can easily absorb and emit some electrons. Furthermore, the power density of these metal oxides is the largest among all active materials; however, they have shown poor high cycle and charge abilities, as well as environmental pollution. (3) Conductive polymers include polypyrrole (PPY) [24], polyaniline (PANI) [11], polythiophenes (PYH), polyphenylenevinylene (PPA), and polyacetylene (PA). These conductive polymers have shown good mechanical properties, easy fabrication, power density, and environmental friendliness. (4) Composites of the aforementioned active materials have shown better performance in all of these parameters, especially a higher power density, better mechanical properties, and charge and cycle abilities. These materials include graphene@ MnO2, PPY@NiO, Graphene@PPY film, C@ TiN [3], and ZIF-Li(Zn)@TisC2Tx [32]. Comparing the performances of the single active materials, the association of the different active materials shows the promising candidates in power storages.
