Activated corn stalk biochar as a porous carbon electrode for supercapacitors

Corn stalk (CS), a renewable and sustainable agricultural waste, was used to produce electrode materials for supercapacitors. CS was first pre-carbonized at 600°C under nitrogen and then activated with potassium hydroxide (KOH) at biochar:KOH mass ratios of 1:1‒1:4 and temperatures of 600-900°C. The activated biochars were characterized by Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller surface area (BET), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), X-Ray diffraction (XRD), and Raman spectroscopy. The materials exhibited micropore-rich hierarchical porous structures with specific surface areas ranging from 1178‒2998 m² g^-1 and I_D/I_G ratios ranged in 0.77 to 1.11. Eight different electrodes were prepared using the biochars, carbon black, and polyvinylidene fluoride (PVDF) at a weight ratio of 8:1:1. Electrochemical performance was evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectrometry (EIS) techniques in three- and two-electrode systems (reference electrode: Ag/AgCl, counter electrode:Pt. electrolyte: 6 M KOH in three-electrode configuration). CS-800-K2, activated at 800°C with biochar:KOH mass ratio of 1:2, exhibited the best electrochemical performance, with a specific capacitance of 220F g^-1 at 0.5 A g^-1 in three-electrode system. The electrode retained 105% of its capacitance after 7500 cycles. In a symmetric two-electrode configuration, CS-800-K2 delivered 124F g^-1at 0.5 A g^-1, with a maximum energy density of 4 Wh kg⁻¹ and a maximum power density of 553 W kg⁻¹. These results demonstrate the potential of corn-stalk-derived activated biochar as a low-cost, sustainable electrode material for supercapacitors.