Hierarchical N-doped carbon nanosheets submicrospheres enable superior electrochemical properties for potassium ion capacitors

Hybrid ion capacitors show promise to bridge the gap between rechargeable batteries with high energy density and supercapacitors of high power. However, research efforts primarily focus on lithium-ion and/or sodium-ion based capacitors, potassium ion capacitor (KIC) is theoretically more sustainable and promising owing to the high abundance, low standard redox potential and low cost of the K sources. Herein, nitrogen-doped porous carbon submicrospheres derived from hierarchical micro-flowerlike polyimide superstructure are fabricated and investigated as novel anodes for excellent K ion storage. The effect of different electrolytes on potassium storage properties and the reaction kinetics are successfully revealed. A low cost nonaqueous KIC is assembled with commercial activated carbon as cathode and the polyimide derived carbon as anode. Benefiting from the unique structure characteristics of enriched active sites, high electronic conductivity, and the conspicuous pseudocapacitive effect of polyimide carbon, the as-fabricated device manifests the maximum energy density and power density of 90.1 Wh kg(-1) and 3000 W kg(-1), respectively. Even at 1540 W kg(-1), the energy density still remains at 43.5 Wh kg(-1). Moreover, the device achieves a near 100% Coulombic efficiency and favorable cycling stability over 5000 cycles at a current density 500 mA g(-1).