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 degrees C in Ar. CC synthesized at 800 degrees C (CC-800), containing KCl, CaCO3, 3 , Ca(ClO)2, 2 , K2SO4, 2 SO 4 , and Ca5(PO4)3(OH), 5 (PO 4 ) 3 (OH), has the highest specific surface area (SBET, S BET , 130.04 m 2 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 m 2 g- 1 , respectively. KCl, Ca(ClO)2, 2 , and K2SO4 2 SO 4 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 ACHCl-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.