Scalable fabrication of nitrogen-doped graphitic layer-encapsulated Fe₃O₄/graphene-decorated hollow carbon nanofibers for binder-free supercapacitor electrodes

The growing demand for efficient renewable energy technologies has intensified the need for high-performance supercapacitors (SCs) with long-term operational stability. However, simultaneously achieving high capacitance, durability, and scalable manufacturability in electrode materials remains a critical issue, particularly due to structural complexity and binder-related limitations. In this study, a straightforward and scalable fabrication approach was developed to produce robust hollow carbon nanofiber (H–CNF)-based binder-free hybrid electrodes. The electrode was engineered by anchoring iron oxide (Fe₃O₄) nanoparticles and reduced graphene (G) onto the H–CNF surface, followed by the formation of a conformal nitrogen-doped graphitic carbon (NG) encapsulation layer. This NG encapsulation layer markedly enhanced the electrical conductivity, charge carrier density, structural stability, and hierarchical pore characteristics of the electrode. Consequently, the NG@Fe₃O₄–G/H–CNF electrode exhibited a 3.7-fold higher specific capacitance than bare H–CNF electrode with outstanding cycling stability, achieving 95.5% capacitance retention after 100,000 charge–discharge cycles. Furthermore, symmetric SCs assembled using identical NG@Fe₃O₄–G/H–CNF electrodes achieved a high energy density of 49.17 Wh·kg⁻¹ at a power density of 3 kW·kg⁻¹, while maintaining 94.6% capacitance retention after 10,000 charge–discharge cycles. High Coulombic efficiency and self-discharge behavior further verify stable and reversible charge-storage characteristics at the device level. These results demonstrate that the NG@Fe₃O₄–G/H–CNF is a highly promising, mechanically resilient, and scalable binder-free electrode material for high-performance SC applications.