N/P/O/S Heteroatom-Doped Porous Carbon Nanofiber Mats Derived from a Polyacrylonitrile/l-Cysteine/P₂O₅ Precursor for Flexible Electrochemical Supercapacitors

It is still a great challenge to achieve high volumetric capacitance without losing gravimetric capacitance and cycling performances for carbon-based electrode materials to fulfill the demands for next-generation supercapacitors. In this work, we have fabricated carbon nanofiber (CNF) mats doped with tetraheteroatoms (N, P, O, and S) via a two-step process: electrospinning and carbonization. The surface porosity, mass density, and doping density of the as-prepared doped CNFs were optimized by varying the l-cysteine-to-potassium pentoxide mass ratio in the polyacrylonitrile (PAN) precursor solution. The structural evaluation of different heteroatom species in the carbon lattice of the as-prepared NPOS-CNFs was systematically studied by various spectroscopic techniques. The optimized NPOS-CNF 12 nanofibers, which were prepared from the PAN precursor containing an l-cysteine/P2O5 mass ratio of 1:2, have a high specific surface area (502.5 m2/g), high mesopore volume (1.561 cm3/g), high mass density (1.07 g/cm3), and highest content (10.34 atom %) of active heteroatom species such as pyrrolic N, pyridinic N, C═O, C–S, and C3–PO species. Due to these unique features, the NPOS-CNF 12 exhibited an ultrahigh volumetric capacitance of 625.8 F/cm3 at a current density of 0.2 A/g and also exhibited high gravimetric capacitance (584.8 F/g) and good cycling stability (93.5% capacitance retention after 10 000 cycles) in a 6 M KOH electrolyte. As assembled, the NPOS-CNF 12//NPOS-CNF 12 supercapacitor in a 1 M Na2SO4 electrolyte delivered a high gravimetric energy density of 102.6 Wh/kg at a power density of 105 W/kg with a wide potential window of 0–1.8 V.