Engineered N/O-doped hierarchical porous carbons from coffee grounds for high-performance supercapacitors

This study presents a hydrothermal-activation strategy to convert coffee grounds into N/O co-doped hierarchical porous carbons (ACG-X) for high-performance supercapacitors. Hydrothermal carbonization transforms biomass into aromatic-rich carbon skeletons while converting inherent N/O heteroatoms into electrochemically active groups (CdbndO and N-5), and subsequent KOH activation constructs a 3D hierarchical porous network (2473.15 m²·g⁻¹ surface area). The optimized material achieves exceptional electrochemical performance: a specific capacitance of 546.08 F·g⁻¹ at 0.5 A·g⁻¹ (74.6 % retention at 10 A·g⁻¹) and symmetric supercapacitors delivering 16.48 Wh·kg⁻¹ energy density at 125.03 W·kg⁻¹ in 6 M KOH electrolyte, surpassing most reported biomass-derived carbons. Key innovations include (1) synergistic pore-heteroatom engineering via hydrothermal pretreatment and activation, (2) dual charge storage from electric double-layer capacitance (hierarchical pores) and pseudocapacitance (N/O functionalities), and (3) scalable waste-to-energy conversion with 81.19 % capacitance retention after 15,000 cycles. The work provides mechanistic insights into heteroatom-pore structure synergy during biomass conversion, establishing a paradigm for controllable synthesis of high-performance energy storage materials.