Design and fabrication of PVC-modified coal tar pitch-based N/O co-doped porous carbon nanosheets for high-performance zinc ion hybrid capacitors

Abstract

Carbon nanosheets have emerged as promising cathode materials for aqueous zinc-ion hybrid capacitors (ZICs) owing to their high surface area, abundant exposed active sites, and short ion diffusion pathways. Coal tar pitch, a low-cost byproduct of coke production, is an attractive precursor for preparing such carbons because of its high carbon yield and aromatic-rich molecular structure. However, its inherent tendency to form large polyaromatic domains during pyrolysis often leads to dense stacking and limited pore accessibility, resulting in uncontrolled morphology and suboptimal electrochemical performance. This study aims to overcome the structural limitations of coal tar pitch-derived carbons by introducing a molecular engineering strategy that enables precise control over morphology and pore structure. Specifically, nitrogen and oxygen co-doped porous carbon nanosheets (NPCS) were synthesized via polyvinyl chloride modification of coal tar pitch, followed by high-temperature co-pyrolysis with potassium acetate and melamine. The introduction of aliphatic structures effectively improves the pyrolysis behavior of the pitch molecules, suppressing the excessive fusion of aromatic domains by weakening the π-π interactions and promoting the formation of ultrathin nanosheets with hierarchical porosity and enlarged micropores. The optimized material (NPCS-P0.5) exhibits a high specific surface area (1358.7 m² g⁻¹), a mean pore size of 2.31 nm, abundant N/O heteroatom doping, and a wrinkled-layer morphology. These structural and compositional features synergistically offer a high density of electrochemically active sites, facilitate efficient Zn²⁺ transport, and enhance interfacial charge transfer kinetics. Electrochemical measurements demonstrate that NPCS-P0.5 delivers a high specific capacitance of 278.4 F g⁻¹ at 0.5 A g⁻¹ and retains 229.8 F g⁻¹ at 20 A g⁻¹ in a three-electrode configuration. When assembled into a ZIC, the Zn//NPCS-P0.5 device achieves a high energy density of 109.1 Wh kg⁻¹ at 160.9 W kg⁻¹ and maintains 101.16 % of its initial capacitance over 12,000 cycles. This work demonstrates a scalable, structure-tunable route for converting coal-derived feedstocks into high-performance carbon nanosheet materials, highlighting a new molecular-level design concept for advanced ZIC electrodes.