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 (w_KOH:w_LC = 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.