A Metal-Free Organic Aqueous Capacitor Using Quinonic Compounds Couple

Redox-capacitors are electrochemical energy storage devices using the electric double layer capacity and pseudo-capacity derived from redox reactions. Because of its higher energy density than conventional electric double layer capacitors, higher power density than batteries and long cycle life, redox-capacitor is a promising candidate for grid-scale stationary energy storage device.

We demonstrated the aqueous redox-capacitors using inexpensive quinonic and hydroquinonic organic compounds as redox active materials, which facilitate charge/discharge by proton insertion/extraction in aqueous electrolyte. These quinonic/hydroquinonic compounds are supported in nanoporous carbon. This capacitor exhibited high energy density (＞20 Wh kg^-1) and high power density (＞5 A g^-1) without the degradation after 10000 charge/discharge cycles [Sci. Rep. 4, 3591 (2014).].

In this study, we investigated the dependence of loading rate of quinonic compound to carbon materials for further enhancement of energy density. It was found that quinonic compound, which is highly loaded in carbon (>70 wt%), can be utilized by adding a slight amount of carbon with high electrical conductivity. Finally, the high energy density of approximately 30 Wh kg^-1 was achieved in the metal-free organic aqueous capacitor using quinonic compounds couple.