Nitrogen and Sulfur Self-Doped Activated Carbon Directly Derived from Elm Flower for High-Performance Supercapacitors

N,S-Doped activated carbon was directly prepared via a facile and cost-efficient hydrothermal reaction, followed by alkali activation of elm flower (EL)-derived biomass. The EL-derived activated carbon (ELAC) had N and S contents of 2.21 and 6.06 atom %, respectively, in addition to a high Brunauer–Emmett–Teller (BET) surface area of 2048.6 m2 g–1 and moderate pore volume of 0.88 cm3 g–1. Owing to its high BET surface area and N/S functional groups, ELAC achieved a specific capacitance of 275 F g–1 at a current density of 1 A g–1 and retained a capacitance of 216 F g–1 at 20 A g–1. In addition, a symmetric supercapacitor based on N,S-self-doped ELAC electrode provided a capacitance of 62 F g–1 at a current density of 10 A g–1, with maximum energy and power densities of 16.8 Wh kg–1 and 600 W kg–1, respectively. The capacitance retention was also high, at 87.2%, at 4 A g–1 after 5000 cycles.