A freestanding co-doped Fe3O4/C electrode for high-energy-density solid-state sodium-ion asymmetric supercapacitors

Emerging high-power and energy supercapacitors (SCs) are a widely tracked goal for applications in transportation and energy storage systems. Herein, a well-synthesized Co-doped iron oxide carbon nanocomposite (Co@Fe 3 O 4 /C) thin film is deposited on nickel foam (NF) via a simple hydrothermal method and serves as a freestanding electrode for high-power and energy Na + ion asymmetric SCs (N-ASCs). The Co@Fe 3 O 4 /C framework, with a high specific surface area (41.03 m 2 /g), provides a three-dimensional, highly conductive network for fast charge transport and allows moderate loading of active materials (2 mg/cm 2 ). Moreover, porous Co@Fe 3 O 4 /C uniformly anchored on the NF current collector enables a rapid charge intercalation-deintercalation rate due to the increase in redox active sites of Co@Fe 3 O 4 /C and their good contact with conductive NF substrate as compared with pristine Fe 3 O 4 /C. As a result, the unique integrated electrode with a 3-D architecture exhibits a high specific capacitance of 1440 F/g at 2 A/g and an outstanding rate capability of 560 F/g at 12 A/g in 2 M NaOH electrolyte. Capacitance retention of 91.8% with Coulombic efficiency of 98.7% at 2 A/g over 5000 GCD cycles, and good cycling stability, suggesting great potential for high-performance Co@Fe 3 O 4 /C//AC N-ASCs. Moreover, the N-ASCs exhibits a high specific capacitance of 330 F/g at 2 A/g and an impressive specific energy density of 103.1 Wh/kg at a high-power density of 8437.5 W/kg. This investigation thus demonstrates its significant potential for developing novel approaches to energy storage devices.