Synergistic Interfacial and Defect Engineering of Fe–Cu Phosphate/Heteroatom-Doped g-C3N4 Composites for High-Performance Supercapacitive Energy Storage

Abstract The ever-increasing need for energy storage technologies that provide both high energy density and long operational lifespans has sparked significant interest in developing carbon-based nanostructures coupled with transition-metal phosphates, representing a promising pathway for advanced supercapacitors (SCs). In this work, we introduce a two-step chemical methodology for constructing iron–copper (Fe–Cu) phosphate nano/microarchitectures chemically bridged with (phosphorus (P) and/or sulfur (S) doped) graphitic carbon nitride (g-C3N4) frameworks (collectively referred to as the FCP-CN series). The detailed structural and morphological investigations verified the effective fabrication of these Fe–Cu phosphate/(P,S-doped) g-C3N4 composites, which possess hierarchically structured mesoporous architectures composed of interconnected nano/microsheets and particles, a morphology that promotes superior electrical conductivity, robust interfacial coupling, and swift ion/electron transport dynamics. Among the prepared materials, the optimized FCP-CN-5 formulation demonstrated a notable specific capacitance (Csp) of 1283 F/g at 5.0 A/g, along with exceptional cyclability, with 96% retention over 10,000 consecutive charge–discharge operations. This outstanding electrochemical efficiency originates from the synergistic combination of redox-active Fe–Cu phosphate and nitrogen-rich, heteroatom-doped g-C3N4, which together provide plentiful electroactive sites, well-developed mesoporous diffusion pathways, and significantly accelerated charge-transfer kinetics. To assess the real-world feasibility of this material, we assembled a solid-state asymmetric supercapacitor (SASC) incorporating FCP-CN-5 (cathode) and reduced graphene oxide (rGO) (anode), using a PVA–KOH electrolyte. This SASC operated stably within a +1.6 V window, delivering a notable Csp of 92 F/g at 1.0 A/g and retaining 91% of its capacitance after 10,000 cycles. Significantly, the constructed SASC exhibited an energy density of 32.9 Wh/kg at a power density of 800 W/kg and illuminated commercial light-emitting diodes (LEDs), revealing its enormous potential for functional applications. Collectively, these results confirm that the hierarchically mesoporous Fe–Cu phosphate/P-doped g-C3N4 hybrid composite is a highly efficient and attractive electrode material for advanced SC technologies.