Oil tea shell derived porous carbon with an extremely large specific surface area and modification with MnO₂ for high-performance supercapacitor electrodes

Activated carbon (AC) with an extremely large specific surface area (2851 m²/g) and large pore volume (2.68 cm³/g) was derived from bio-waste oil tea shells by using ZnCl₂ as the activation agent. The porous carbon had a high amount of CO₂ adsorption (3.61 mmol/g) at ambient conditions (25 °C, 1 bar). Comprehensive characterizations including XRD, Raman, SEM, TEM, and TGA proved graphite existed in the AC samples. However, the capacitance (146 F/g @ 0.5 A/g) was in the normal range of carbon materials. By coating with a thin layer of MnO₂, the capacitance of MnO₂/AC was enhanced significantly (1126 F/g @ 0.5 A/g) without sacrificing the rate capability and cycle stability, even though the surface area was reduced to 23 m²/g and pore volume reduced to 0.05 cm³/g. A two-electrode (MnO₂/AC//AC) supercapacitor cell was set up, the energy density reached 24 Wh/kg with a power density of 275 W/kg.