Fast-pyrolysis lignin-biochar as an excellent precursor for high-performance capacitors

Lignin-based activated carbons (LAC) were produced using thermochemical pretreatment and chemical activation with KOH. Fast pyrolysis at 550 °C in a CO2/N2 atmosphere resulted in lignin char (LC) with a more developed porous structure than slow pyrolysis. The effect of activation conditions (KOH usage, temperature, and duration) on surface/physicochemical properties and electrochemical characteristics of the resulting LAC was fully studied. Using fast pyrolysis lignin char as a precursor and optimized activation conditions (wKOH:wLC = 2, 800 °C, and 2 h), the resulting LAC featured a large surface area of 2149.5 m2 g−1, a total pore volume of 0.88 m3 g−1, and high capacitance of 300 F g−1 at 0.5 A g−1 in a 6 mol L−1 KOH electrolyte. The LAC-based symmetric supercapacitor could offer superior energy density (19.15 W h kg−1 at 250 W kg−1 power density) and a stable lifetime (98.2% of original capacity after 10,000 charge-discharge cycles). The excellent capacitor performance of LAC was attributed to its microporous-mesoporous structure developed from fast pyrolysis and subsequent chemical activation.