Highly stable Megalopolis lignite based N and S self-doped hierarchically porous activated carbons for high performance supercapacitors and ash content effects on performance

• Activated carbon with significantly high surface area was produced from a lignite. • Effects of N and S self-doped on electrochemical performances were investigated. • Demineralisation process increased the specific capacitance almost 10 times. • The specific capacitance of the cell measured at 1 A g -1 was 260.47 F g -1 . • Highly stable energy density values obtained between 1 A g -1 and 100 A g -1 . This study proposes that high value-added activated carbon (AC) is produced from lignite (low rank coal) and used as electrode material in supercapacitors. In addition, the effects of the ash content of the activated carbon on supercapacitor performance are determined. The ACs at a temperature of 800 °C are produced in three different impregnation ratios (IR) (IR: 1, 2 and 4: KOH/precursor) using KOH as the activation agent. While the surface area of ​​the AC produced at IR4 is 2531.4 m 2 g −1 , it is determined as 1321.3 (IR1) and 1989.0 (IR2) m 2 g −1 for the others. The pore size analysis of the samples shows that the mesopores are dominant in the IR1 (52.73%) and IR4 (88.74%) samples, while it is observed that the micropores are dominant in the IR2 (69.03%) sample. The IR4 sample gives excellent results. The cell specific capacitances of this sample are determined as 260.47 F g −1 (at 1 A g −1 ) and 150.3 F g −1 (at 50 A g −1 ). The amount of ash in the AC samples dramatically reduces the supercapacitor performances and significantly increases the resistances.