Rational Design of a Redox-Active Nonaqueous Electrolyte for a High-Energy-Density Supercapacitor Based on Carbon Nanotubes

A redox-active electrolyte supercapacitor (RAES) is a promising system that increases the energy density of a supercapacitor by providing additional pseudocapacitance, which arises from an inserted redox couple in an electrolyte. However, the energy density of RAESs is still considerably low for practical applications, and enhancing the operating voltage window as well as the capacitance of RAESs is very necessary. In this study, we rationally designed a nonaqueous redox-active electrolyte mainly on the basis of the relative position of the electrochemical stability window of a supporting electrolyte and the redox potential of a redox couple. Therefore, the judicious combination of a redox pair, decamethylcobaltocene/decamethylcobaltocenium, and a supporting electrolyte, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide dissolved in adiponitrile, not only provides additional capacitance from a faradaic redox reaction (21.3 -> 57.1 F g(-1)) but also extends the operating voltage window to almost the entire electrochemical stability window of the supporting electrolyte (3.1 V). A carbon-nanotube-based RAES exhibits outstanding performance, including improved energy density (75.6 Wh kg(-1)), compared to previously reported nonaqueous RAESs. Our systematic approach might play a crucial role in the further development of high-performance nonaqueous RAESs.