Enhancing the performance of supercapacitor electrodes from corncob-derived 3D hierarchical porous carbon: Effects of N concentration

We report supercapacitors from hierarchical porous carbon nanostructures (HPCNs) by simple pyrolysis of corncob (CC), with urea doping providing performance comparable to the more common metal-doped electrodes. Via a systematic study of N doping, we establish for the first time that the types of nitrogen‑carbon species, which vary with the urea‑carbon ratio, affect electrode performance. The 1:3 ratio (CC 3 ) gave a higher specific capacitance of 335 F g −1 at 1 mV s −1 in a three-electrode system, and 287 F g −1 at 0.2 A g −1 in a symmetric device. We achieved solution and transfer resistances lower than reported for other materials: R s = 0.20 Ω and R ct = 0.78 Ω respectively. We attribute the better performance of CC 3 to its specific N-doping, dominated by graphitic-N plus oxidized nitrogen and C–OH groups; showing also a larger carbon (002) interplanar distance ( d (002) = 0.392) and high specific surface area (1149 m 2 g −1 ). The assembled prototype delivers a good energy density of 9.96 Wh kg −1 at 403.22 W kg −1 and retains ca. 76 % capacity after 10,000 cycles. This work shows that studying doping concentrations is essential to design and produce highly efficient biomass-based electrodes for energy storage applications, using simple synthesis methods and readily available reagents. • Metal-free activated carbon electrodes made from sustainable sources. • Urea doping yields performance comparable to co-doped materials. • Specific capacitances reached 335 and 287 F g −1 (2 and 3 electrode systems respectively). • Performance decreases if dopant agent exceeds an optimal ratio. • N-species produced depend on N-concentration, affecting performance.