From pollutant to high-performance supercapacitor: Semi-coking wastewater derived N–O–S self-doped porous carbon

Heteroatom-doped porous carbon has attracted enormous attention in fabricating high-performance supercapacitors. However, designing heteroatoms containing carbon materials through an eco-friendly and cost-effective route remains challenging. In this work, N-O-S self-doped porous carbon materials are prepared from semi-coking wastewater-based phenolic resin (SWPR) via a simple Na2CO3 activation strategy. The abundant phenols, NH4+, and SO42- in semi-coking wastewater endows the versatile N, O, and S doping in porous carbon material, remarkably enhancing its conductivity. The optimal SWPR-700–2.5 porous carbon material is obtained at an activation temperature of 700 °C and a mass ratio of 2.5:1 for SWPR: Na2CO3. The symmetrical supercapacitor is assembled with SWPR-700–2.5, exhibiting a high specific capacitance of 170.5 F g−1 at 0.5 A g−1, with a high energy density of 5.92 Wh kg−1 at 124.9 W kg−1. After 10,000 cycles, the Coulombic efficiency and capacity retention are 100% and 99.4%, respectively, at a current density of 20 A g−1. The excellent performance of SWPR-700–2.5 based capacitor is attributed to the porous structure with a large specific surface area (718 m2 g−1) and abundant N-O-S self-doping (2.739 at% N, 8.627 at% O and 1.378 at% S). This study provides a simple route to prepare N-O-S self-doped porous carbon for supercapacitors and pushes forward the advancement of semi-coking wastewater treatment.