Self-activation of carbons derived from bio-waste cabbage for a green supercapacitor based on seawater electrolyte

Carbonized carbons (CC) derived from cabbage were self-activated via a carbonization process at 500–900 °C in Ar. CC synthesized at 800 °C (CC-800), containing KCl, CaCO₃, Ca(ClO)₂, K₂SO₄, and Ca₅(PO₄)₃(OH), has the highest specific surface area (S_BET, 130.04 m² g⁻¹), a high specific capacitance (64.06 F g⁻¹), and an excellent rate capability (65.12 %). After washing the CC-800 powder in either deionized (DI) water (AC-DI) or hydrochloric acid (HCl) followed by DI water (AC-HCl-DI), S_BET values increased to 919.22 and 1146.51 m² g⁻¹, respectively. KCl, Ca(ClO)₂, and K₂SO₄ are removed from the AC-DI, whereas all compounds are washed from the AC-HCl-DI. Removing these compounds enlarges S_BET values, specific capacitance (114.47 F g⁻¹ at 0.5 A g⁻¹), and rate capability (68.95 %). A high capacitance retention of 97.56 % after 20,000 cycles was achieved from the AC-HCl-DI electrode with 6 M KOH. 0.6 M NaCl and seawater were applied as green electrolytes with the AC-HCl-DI electrode, resulting in the promising specific capacitance of 116.89 and 102.21 F g⁻¹ at 0.5 A g⁻¹, respectively. Symmetric AC-HCl-DI/seawater/AC-HCl-DI cell gives a high energy density of 2.32 Wh kg⁻¹ at a large power density of 0.25 kW kg⁻¹. The four serial AC-HCl-DI/seawater/AC-HCl-DI coin cells could light an LED over 60 s.