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 m(2) 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. (C) 2020 Elsevier Inc. All rights reserved.