High and rapid alkali cation storage in ultramicroporous carbonaceous materials

• Ultramicroporous carbonaceous materials (UCMs) were fabricated by a simple method.
• Alkali cations in ultramicropores delivered high capacitances even in high current rates.
• UCM-based symmetric supercapacitors showed notable electrochemical performances.

To achieve better supercapacitor performance, efforts have focused on increasing the specific surface area of electrode materials to obtain higher energy and power density. The control of pores in these materials is one of the most effective ways to increase the surface area. However, when the size of pores decreases to a sub-nanometer regime, it becomes difficult to apply the conventional parallel-plate capacitor model because the charge separation distance (d-value) of the electrical double layer has a similar length scale. In this study, ultramicroporous carbonaceous materials (UCMs) containing sub-nanometer-scale pores are fabricated using a simple in situ carbonization/activation of cellulose-based compounds containing potassium. The results show that alkali cations act as charge carriers in the ultramicropores (<0.7 nm), and these materials can deliver high capacitances of ∼300 F g−1 at 0.5 A g−1 and 130 F g−1, even at a high current rate of 65 A g−1 in an aqueous medium. In addition, the UCM-based symmetric supercapacitors are stable over 10,000 cycles and have a high energy and power densities of 8.4 Wh kg−1 and 15,000 W kg−1, respectively. This study provides a better understanding of the effects of ultramicropores in alkali cation storage.