Fe₃O₄/Nitrogen‐Doped Carbon Electrodes from Tailored Thermal Expansion toward Flexible Solid‐State Asymmetric Supercapacitors

Abstract Transition metal oxide and heteroatoms doped carbon are promising in high performance energy storage upon complete redox reaction. Herein, nanoparticle‐embedded carbon nanosheets are fabricated by a one‐pot tailored thermally expansion of viscous precursors. The obtained biomass carbon exhibits ideal surface area, crystal structures, and heteroatom doping. Benefiting from the synergistic effect of the constituent components, this composite electrode reaches a high potential window of 1.1 V and delivers a good specific capacitance of 522.7 F g⁻¹ in aqueous electrolyte. After combining with a capacitive carbon nanotubes (CNTs) film cathode, the assembled flexible solid‐state supercapacitors deliver a high energy density of 18.1 Wh kg⁻¹ at a power density of 125 W kg⁻¹. Moreover, in cases of frequent bending and elevated temperature (up to 80 °C), the hybrid device maintains superior electrochemical attributes. This facile and scalable process, as well as their high reversible capacity, are promising for next‐generation energy‐storage devices.