Controllable synthesis of nitrogen-doped porous carbon from metal-polluted miscanthus waste boosting for supercapacitors

High-value reclamation of metal-polluted plants involved in phytoremediation is a big challenge. In this study, nitrogen-doped nanoporous carbon with large specific area of 2359.1 m² g^-1 is facilely fabricated from metal-polluted miscanthus waste for efficient energy storage. The synergistic effect of KOH, urea and ammonia solution greatly improve the nitrogen quantity and surface area of the synthesized carbon. Electrodes fabricated with this carbon exhibit the excellent capacitance performance of 340.2 F g^-1 at 0.5 A g^-1 and a low combined resistance of 0.116 Ω, which are competitive with most of previously reported carbon-based electrodes. In addition, the as-obtained carbon electrode shows a high specific capacitance retention of over 99.6% even after 5000 cycles. Furthermore, the symmetric supercapacitor fabricated using the synthesized carbon achieves a superior energy density of 25.3 Wh kg^-1 (at 400 W kg^-1) in 1 mol L^-1 Na₂SO₄ aqueous solution. This work provides an efficient route to upcycle metal-polluted plant waste for supercapacitor applications.