Self-Nitrogen-Guided Activation of Algae Biomass into Hierarchical Porous Carbon Electrodes for Aqueous Supercapacitors

Biomass-derived porous carbons are promising supercapacitor electrodes, but their electrochemical performance is often limited by the trade-off between activation-induced pore formation and heteroatom retention. In this work, algae biomass was used as an intrinsic N/O/S-containing precursor to prepare self-nitrogen-doped hierarchical porous carbon by pre-carbonization followed by controlled KOH activation. A temperature-, dosage- and time-dependent sample library was constructed to correlate activation conditions with textural properties, nitrogen configuration, wettability, charge-transfer resistance and electrochemical behavior. The optimized AHPC-850 sample exhibits a BET surface area of 1486 m2 g−1, a total pore volume of 0.96 cm3 g−1, a retained surface nitrogen content of 2.91 at.%, and a charge-transfer resistance of 0.41Ω. In a three-electrode configuration, AHPC-850 delivers 386 F g−1 at 1 A g−1 and retains 62.4% of its capacitance at 20 A g−1. During 10,000 cycles at 10 A g−1, the electrode maintains 96.3% capacitance retention with a stable coulombic efficiency above 98.8%. A symmetric aqueous device based on AHPC-850 achieves an energy density of 24.8 Wh kg−1 at 250 W kg−1. These results indicate that algae-derived carbon can be improved by balancing pore accessibility, nitrogen retention and transport resistance rather than by maximizing surface area alone.