Effect of KOH char ratio on the properties of lignin-derived activated carbons for electric double layer capacitors

Activated carbons (ACs) were prepared from lignin as the char precursor by means of the KOH activation method. The influence of KOH char mass ratio (K/C) on the pore properties and morphology of ACs, which play an important role on the electrochemical behavior of AC electrodes, were investigated by scanning electron microscope (SEM) and nitrogen adsorption technique. The results showed that K/C has a great effect on the pore properties including the specific surface area (S_BET), total pore volume (V_t) and micro pore volume (V_mic). The S_BET and V_t of ACs reached a maximum (1506 m²/g and 0.795 cm³/g) as the K/C was equal to 3. With the aim of studying, the influence of K/C on the electrochemical properties of AC electrodes was investigated by cyclic voltammetry, constant current charge-discharge and electrochemical impedance spectroscopy. The specific capacitance of AC electrodes reached a maximum (267.8 F/g measured at a current density of 50 mA/g) as K/C was equal to 3, even the specific capacitance remained the capacitance retention ratio of 83.9% after 2000 cycles, which was attributed to high specific surface area, appropriate pore distribution and low electronic resistance of ACs.