Ex-situ nitrogen-doped porous carbons as electrode materials for high performance supercapacitor

Nitrogen (N) doping of porous carbon materials is an effective strategy for enhancing the electrochemical performance of electrode materials. Herein, we report on ex-situ (post) nitrogen-doped porous carbons prepared using a biomass waste, peanut shell (PS) as a carbon source and melamine as the nitrogen source. The synthesis method involved a two-step mechanism, initial chemical activation of the PS using KOH and post N-doping of the activated carbon. The effect of the activating agent/precursor ratio and the ex-situ N-doping on the structural, textural, electrochemical properties of the porous carbons was studied. The ex-situ N-doped porous carbon with an optimum amount of KOH to PS exhibited the best capacitance performance with a specific surface area (SSA) of 1442 m² g^-1 and an enriched nitrogen content (3.2 at %). The fabricated symmetric device exhibited a 251.2 F g^-1 specific capacitance per electrode at a gravimetric current of 1 A g^-1 in aqueous electrolyte (2.5 M KNO₃) at a wide cell voltage of 2.0 V. A specific energy of 35 Wh kg^-1 with a corresponding specific power of 1 kW kg^-1 at 1 A g^-1 was delivered with the device still retaining up to 22 Wh kg^-1 and a 20 kW kg^-1 specific power even at 20 A g^-1. Moreover, long term device stability was exhibited with an 83.2% capacity retention over 20 000 charge/discharge cycles and also a good rate capability after 180 h of floating at 5 A g^-1. This great performance of the symmetric supercapacitor can be correlated to the surface porosity and post nitrogen-doping effect which increased the electrochemically-active sites resulting in a remarkable charge storage capability.