Oxygen-enriched porous carbon derived from acid washed and oxidized lignite via H3PO4 hydrothermal for high-performance supercapacitors

Lignite-based porous carbon with clustered pore structure, large specific surface area (SSA) and high oxygen content is prepared by H3PO4 hydrothermal followed by KOH activation, which uses Shengli lignite modified by acid washing followed with oxidization (OAWSL) as carbon precursor. A part of ash in lignite is removed and abundant oxygen-containing functional groups are introduced through HCl washing and H2O2 oxidation. H3PO4 introduces phosphorus into OAWSL and promotes the formation of pores, then plentiful micropores are further formed by KOH. The POPC-2, which shows the best performance, has a high SSA (2852 m2 g- 1) and an oxygen content of 15.73 wt%, providing more charge capacity and active sites. In addition, the POPC-2 as an electrode for supercapacitors exhibits a high specific capacitance of 320 F g- 1 at 1 A g- 1 and 272 F g- 1 at 30 A g- 1 in a three-electrode system in 6 M KOH electrolyte. Futhermore, the symmetric supercapacitors based on POPC-2 also shows high specific capacitance of 390 F g- 1 at 0.05 A g- 1, an energy density of 10.75 Wh kg-1 and outstanding cycling stability with 88% of the initial capacitance retention after 25,000 cycles at 2 A g- 1.