In this study, self-doped porous activated biochar derived from Venice lagoon’s Sargassum brown macroalgae (ABS) has been successfully prepared through thermochemical carbonization (pyrolysis) followed by CO2 physical activation and used as electrodes for supercapacitor (SC) applications. The ABS exhibits a remarkable specific surface area of 821 m2 g-1 and heteroatoms (N, O, and S) doping, both key features to attain high-performance carbon-based SC electrodes. The electrochemical performances of ABS-based SCs were assessed in three different electrolytes. Two are aqueous (i.e., 1 M H2SO4 and 8 M NaNO3), while the third one is the prototypical organic, namely 1 M TEABF4 in acetonitrile. In these three electrolytes, the ABS-based electrodes exhibited specific capacitance values (Cg) of 109.5, 79.0, and 64.3 F g-1, respectively, at a current density of 0.1 A g-1. The capacitive performance resulted in SC energy densities of 3.45 Whkg-1 at 22.5 Wkg-1, 6.35 Whkg-1 at 36.1Wkg-1, and 12.4 Whkg-1 at 57.4 Wkg-1 and maximum power densities of 147, 222, and 378 kWkg-1 in the acidic, quasi-neutral aqueous electrolyte and organic electrolyte, respectively. The ABS electrodes were used to realize a flexible solid-state SC based on the sulfonated polyether ether ketone (SPEEK):functionalized niobium disulfide flakes (f-NbS2) composite membrane. The flexible solid-state SC displayed a remarkable 97% Cg retention even under various mechanical stresses, including bending up to 1000 times and folding angles up to 180º, while keeping a Coulombic efficiency above 98%. This study reveals algae-derived ABS as a promising sustainable source of active materials for