| Summary: | Sustainable and high-performance energy storage materials are crucial to address global energy and environmental challenges. In this study, <i>Spirulina platensis</i> was used as the carbon and nitrogen source, and <i>Spirulina</i>-based nanoporous biochar (SNPB) was synthesized through chemical activation using KOH as the activating agent in N<sub>2</sub> atmosphere. SNPB-800-4 was characterized by N<sub>2</sub> adsorption–desorption and XPS, showing a high specific surface area (2923.7 m<sup>2</sup> g<sup>−1</sup>) and abundant heteroatomic oxygen (13.78%) and nitrogen (2.55%). SNPB-800-4 demonstrated an exceptional capacitance of 348 F g<sup>−1</sup> at a current density of 1 A g<sup>−1</sup> and a remarkable capacitance retention of 94.14% after 10,000 cycles at a current density of 10 A g<sup>−1</sup> in 6 M KOH. Notably, symmetric supercapacitors SNPB-800-4//SNPB-800-4 achieved the maximum energy and power densities of 17.99 Wh kg<sup>−1</sup> and 162.48 W kg<sup>−1</sup>, respectively, at a current density of 0.5 A g<sup>−1</sup>, and still maintained 2.66 Wh kg<sup>−1</sup> when the power density was increased to 9685.08 W kg<sup>−1</sup> at a current density of 30 A g<sup>−1</sup>. This work provides an easily scalable and straightforward way to convert waste algae biomass into in situ N, O-dually doped biochar for ultra-high-power supercapacitors.
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