Electrochemical performance of activated carbon–thermally expanded graphite composites for supercapacitor applications

This study investigates the electrochemical performance of composite electrode materials for electric double-layer capacitors (EDLCs) based on thermally expanded graphite (TEG) and activated carbon (AC) derived from apricot kernel shells. Activated carbon was synthesized via carbonization of apricot shell biomass at temperatures ranging from 300 to 900°C, followed by chemical activation with KOH at a 1:1 weight ratio. Symmetrical supercapacitor electrodes containing 85 wt.% AC and 15 wt.% TEG were fabricated and tested in a 30 wt.% aqueous KOH electrolyte. The experimental results demonstrate that the carbonization temperature plays a decisive role in determining the structural and electrochemical properties of the composites. Specific capacitance values derived from galvanostatic charge–discharge (GCD) at 50 mA range from 155 to 172 F/g, showing excellent agreement with cyclic voltammetry (CV) data. The AC400–TEG composite delivers the peak specific capacitance of 172 F/g (GCD) and 171 F/g (CV) owing to rich surface chemistry, although it experiences a higher voltage drop due to incomplete graphitization. In contrast, the AC300–TEG composite exhibits promising cycling stability (retaining ∼97% Coulombic efficiency and ∼94% capacitance retention over 1000 cycles) and optimal gravimetric energy density (5.30 Wh/kg at 50 mA) due to a highly balanced pore structure and lower internal resistance. These findings demonstrate that optimizing the carbonization temperature of biomass-derived carbon and modifying it with a conductive TEG network provides a highly efficient, cost-effective, and environmentally friendly strategy for high-performance supercapacitor electrodes.