In Situ N, O-Dually Doped Nanoporous Biochar Derived from Waste Eutrophic Spirulina for High-Performance Supercapacitors

Sustainable and high-performance energy storage materials are crucial to address global energy and environmental challenges. In this study, Spirulina platensis was used as the carbon and nitrogen source, and Spirulina-based nanoporous biochar (SNPB) was synthesized through chemical activation using KOH as the activating agent in N(2) atmosphere. SNPB-800-4 was characterized by N(2) adsorption–desorption and XPS, showing a high specific surface area (2923.7 m(2) g(−1)) and abundant heteroatomic oxygen (13.78%) and nitrogen (2.55%). SNPB-800-4 demonstrated an exceptional capacitance of 348 F g(−1) at a current density of 1 A g(−1) and a remarkable capacitance retention of 94.14% after 10,000 cycles at a current density of 10 A g(−1) 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(−1) and 162.48 W kg(−1), respectively, at a current density of 0.5 A g(−1), and still maintained 2.66 Wh kg(−1) when the power density was increased to 9685.08 W kg(−1) at a current density of 30 A g(−1). 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.