Manganous nitrate -assisted potassium hydroxide activation of humic acid to prepare oxygen-rich hierarchical porous carbon as high-performance supercapacitor electrodes

Abstract The oxygen-rich hierarchical porous carbon (MOHPC) has been prepared through Mn(NO3)2-assisted KOH activation of humic acid with low-dose activating agent. We demonstrate that Mn(NO3)2 can promote the micropore formation, suppress the mesopore fabrication, and increase oxygen conent, thanks to the low-temperature pre-functionalization effect and synergistic activation effect with KOH at relatively low temperature of 700 °C. The MOHPC exhibits large specific surface area (1656 m2 g−1), high oxygen content (21.88 at.%), and optimized hierarchical porous structure possessing rich micropores, appropriate mesopore ratio with narrow mesopore size distribution, and interconnected macropores. Expectedly, MOHPC electrode delivers ultrahigh specific capacitances of 515 F g−1 and 402 F g−1 at 0.2 A g−1, 206 F g−1 and 233 F g−1 at 100 A g−1 in 1 M H2SO4 and 6 M KOH, respectively. Moreover, as symmetric supercapacitor electrode with commercial-level mass loading (~200 μm, ~11.30 mg cm−2), MOHPC also displays a high specific capacitance of 304 F g−1 at 0.057 A g−1, superior rate capability (246 F g−1 at 5.7 A g−1), and excellent cycling stability in 3 M KOH. The assembled supercapacitor device delivers a considerable energy density of 30 Wh kg−1 at the power density of 43.6 W kg−1 in organic electrolyte.