Osmanthus fragrans-derived hierarchically porous carbons for high-performance supercapacitors

The sustainable conversion of biomass wastes into high-performance carbon materials for supercapacitors continues to present a critical challenge. Herein, a green and facile strategy was proposed to synthesize N/S co-doped porous carbons using discarded Osmanthus fragrans flowers as the precursor. By employing a NaCl/ZnCl 2 molten-salt system as a synergistic template/activator and urea as the heteroatom source, hierarchical porous architectures with interconnected networks were constructed. Systematic investigations revealed that activation temperature critically governs pore structure evolution, surface chemistry, and electrochemical performance. The optimized O-PC-700 °C sample exhibited a high specific surface area (938.6 m 2 g −1 ), a balanced micro-/meso-/macroporosity distribution, and abundant N/S/O functional groups. As a supercapacitor electrode, O-PC-700 °C delivered a remarkable specific capacitance of 230 F g −1 at 1 A g −1 in a three-electrode system. The symmetric device achieved an exceptional energy density of 68.88 Wh kg −1 at 500 W kg −1 and outstanding cycling stability (retaining almost 100 % capacitance after 10,000 cycles). The dual activation mechanism of molten salts and heteroatom doping synergistically enhances ion accessibility, pseudo-capacitance, and conductivity. This work not only advances the sustainable valorization of floral biomass but also provides a versatile strategy for designing hierarchical carbon materials for high-energy-density energy storage systems.