From lignite to supercapacitor carbon: tracking the role of ammonolysis atmosphere in stepwise carbonization and KOH activation

Ammonolysis atmosphere dictates N configuration beyond total N content. • N 2 atmosphere enriches active N-6/N-5 species while suppressing inert N-X species. • Stepwise synergy of N-engineering and KOH etching unlocks hierarchical pores. • Hierarchical porosity and active N species synergistically boost capacitance. Transforming low-grade lignite into functional carbon materials for energy storage is challenging because it is difficult to simultaneously achieve high nitrogen doping efficiency, well-developed porosity, and favorable nitrogen configurations. Here, we investigate the atmosphere-dependent mechanism of ammonolysis under N 2 , air, and O 2 and its synergistic coupling with subsequent carbonization and KOH activation for the preparation of nitrogen-doped porous carbon from Zhaotong lignite. Although O2 and air atmospheres produce higher total nitrogen contents, N 2 ammonolysis preferentially preserves electrochemically active N-6 and N-5 species, with relative contents of 19.64% and 20.64%, respectively, while suppressing the formation of electrochemically inactive N-X species to 6.22%. This chemical preconditioning is retained during carbonization and amplified by KOH activation, which exposes nitrogen sites, removes mineral impurities, and generates a hierarchical pore network containing ultra-micropores, micropores, mesopores, and macropores. The resulting KCZR N exhibits a high defect density ( I D /I G  = 1.0094), a specific surface area of 1003.8 m 2 /g, and abundant N-6/N-5 active sites. Consequently, the KCZR N electrode delivers a specific capacitance of 340.5 F/g at 0.5 A/g in a three-electrode system, with a rate capability of 55.3% retention and a cycling stability of 85.9% after 10,000 cycles. A symmetric supercapacitor assembled with KCZR N achieves an energy density of 12.1 Wh/kg at 249.9 W/kg. This work demonstrates the coupled effect between ammonolysis atmosphere and KOH activation, and provides a strategy for upgrading low-grade lignite into carbon electrodes for energy storage applications. Download: Download high-res image (673KB) Download: Download full-size image