# Application of the Supercapacitor for Energy Storage in China: Role and Strategy
## 2 Technology Characterization
### Faraday Pseudocapacitor  
The pseudocapacitors display relatively higher energy density and capacities, owing to the fast and reversible faradic redox reactions of electroactive materials of the electrodes in the range of certain potential on the electrode surface or near the surface. It causes capacitance by fast and reversible surface or near-surface reactions rather than the electrostatic force. The transition metal oxides are being widely studied because of their abundance, easy preparation, and environmentally friendly quality. For example, NiO, MnO\({}_{2}\), Co\({}_{3}\)O\({}_{4}\) and TiO\({}_{2}\) exhibit good electrochemical performance, owing to their reversible multi-electron redox Faradaic reaction. However, an electrode using a single metal oxide has drawbacks, such as low conductivity and poor cycling stability. Coupled metal cations can contribute to improving the metallic conductivity and electrochemical activity, which are indispensable for pseudocapacitor applications. Binary metal oxides, such as NiMn\({}_{2}\)O\({}_{4}\), NiCo\({}_{2}\)O\({}_{4}\), MnCoO\({}_{3}\), NiMoO\({}_{4}\), and CuCoO\({}_{4}\), are widely studied. For example, [PERSON] et al. reported that NiCo\({}_{2}\)O\({}_{4}\)/N-rGO nanocomposites exhibited excellent electrochemical performance for pseudocapacitors and these materials can be derived via the solvothermal and hydrothermal methods [10]. NiCo\({}_{2}\)O\({}_{4}\)/N-rGO nanocomposites have high activity, large specific capacitance, and good rate performance as electrode materials for supercapacitors because NiCo\({}_{2}\)O\({}_{4}\)/N-rGO shows a high specific capacitance of 2090 Fg\({}^{-1}\) at a current density of 1 Ag\({}^{-1}\). When the current density reaches 10 Ag\({}^{-1}\), the electrode material still maintains the specific capacitance of 1257.5 Fg\({}^{-1}\) and the capacitance retention rate is 96.19% after 5000 cycles. [PERSON] et al. [11] found that a polycrystalline nanoflake NiMoO\({}_{4}\) thin file could be used to prepare electrochromic supercapacitor (smart supercapacitor), which exhibits high specific capacitance up to 1853 F/g at a current rate of 1 A/g and display remarkable specific capacitance retention of 65% after 2500 cycles. [PERSON] et al. conducted a large number of systematic studies on bimetallic or polymetallic oxide supercapacitors, including Mn-Ce [12], Mn-Ni [13], Ce-Ni, Ni-Fe-Sr [14], etc. Bimetallic oxides are generally superior to monometal oxides as pseudocapacitance electrodes. Figure 2 shows various bimetallic or polymetallic oxides electrodes and typical pseudocapacitances. Figure 2a describes the main preparation process of pseudocapacitance electrode materials. Take Ni-Fe electrode prepared from nickel-based zeolite capsule complex with Fe\({}_{3}\)O\({}_{4}\) as an example, this fabrication process is easy to operate and popularize. Ni-Fe pseudocapacitance performs good specific capacitance of 739.8 Fg\({}^{-1}\) at the current density of 1 Ag\({}^{-1}\) in a 6 M KOH solution [15]. Figure 2b compares the specific capacitance properties of various pseudocapacitance electrode materials. It can be seen that the average specific capacitance performance of nickel-based bimetallic materials is generally better, which is the main reason why nickel-based pseudocapacitors have attracted much research attention in recent years. However, the performance of polymetallic electrode is not better than that of bimetallic electrode. For example, the specific capacitance of Ni-Fe-Sr trimetal composite electrode is less than half of that of Ni-Fe bimetal composite electrode, and similar results also appear in other metal combinations. Even more noteworthy, strontium-based bimetal oxides (Sr\({}_{x}\)M\({}_{y}\)O\({}_{z}\), where M is any metal) possessing excellent metallic properties and show immense potential for pseudocapacitances. [PERSON] et al. systematically studied the properties of strontium-bismuth bimetallic oxides as pseudocapacitances, theoretically and experimentally.