Green synthesis of nitrogen and oxygen enriched porous carbon from lignite humate for high-performance supercapacitors

Heteroatom-doped porous carbons are widely utilized as electrode material for electric double layer capacitors (EDLCs) due to their excellent electrochemical properties. Nevertheless, the achievement of obtaining porous carbon materials with both abundant pores and appropriate levels of heteroatom doping consistently has always been a significant hurdle. In this work, a green and economical strategy was developed for the preparation of N- and O-enriched porous carbon using a simple two-step pyrolysis process. Lignite sodium humate was used as the raw material, urea as the N source and activation aids and a low-dose KOH as the activator, as well as the urea addition ratio and coactivation temperature were explored. The optimized sample of NHAC exhibits a high specific surface area (2323.2 m 2 /g), a significant microporous volume (0.97 cm 3 /g), appropriate pore size distribution and N/O content (N 2.5 at.%, O 17.31 at.%). Consequently, NHAC electrode demonstrates a high specific capacitance of 312.75 F/g @ 0.5 A/g and a satisfactory capacitance retention of 78.18 at.% 20 A/g. Furthermore, the assembled NHAC//NHAC symmetric supercapacitor has an excellent energy density of 13.34 Wh kg −1 at 150 W kg −1 and an outstanding cycling stability of 88 % (5 A/g) after 20,000 charge/discharge cycles. This study presents a promising application of lignite humate in energy storage.