Rational design of viologen redox additives for high-performance supercapacitors with organic electrolytes

Introducing redox species into the electrolytes of traditional electric double layer capacitors (EDLCs) is an efficient strategy to enhance their energy density owing to Faradic reactions. However, few studies have elucidated the effect of the molecular structures of organic redox species on the performance of relative supercapacitors, which is important in the development of redox additives for super-capacitors. In this context, we synthesized several viologens and used them as new organic redox additives for super-capacitors with organic electrolytes. The detailed experimental analysis and theoretical calculation results show that the electrochemical performance of viologens relies heavily on their side chains and conjugated cores. Specifically, the side chains of the viologens affect their electronic structures and are consistent with behaviours between the molecules and the electrode pores due to the size effect, thus influencing their specific capacities. In addition, a larger conjugated aromatic core endows viologens with a smaller band gap and a higher degree of electron delocalization, resulting in better rate performance and cycling stability. Consequently, a pi-conjugated viologen derivative is selected as a favourable additive and enables an EDLC-type supercapacitor to exhibit a high energy density (34.0 W h kg(-1)at 856 W kg(-1)) and good cycling performance.