Coaxial carbon nanofibers/MnO₂ nanocomposites as freestanding electrodes for high-performance electrochemical capacitors

Carbon nanofibers (CNFs)/MnO₂ nanocomposites were prepared as freestanding electrodes using in situ redox deposition and electrospinning. The electrospun CNFs substrates with porosity and interconnectivity enabled the uniform incorporation of birnessite-type MnO₂ deposits on each fiber, thus showing unique and conformal coaxial nanostructure. CNFs not only provided considerable specific surface area for high mass loading of MnO₂ but also offered reliable electrical conductivity to ensure the full utilization of MnO₂ coatings. The effect of MnO₂ loading on the electrochemical performances was investigated by cyclic voltammetry (CV), impedance measurements and Galvonostatic charging/discharging technique. The results showed that an ultrathin MnO₂ deposits were indispensable to achieve better electrochemical performance. The maximum specific capacitance (based on pristine MnO₂) attained to 557 F/g at a current density of 1 A/g in 0.1 M Na₂SO₄ electrolyte when the mass loading reached 0.33 mg/cm². This freestanding electrode also exhibited good rate capability (power density of 13.5 kW/kg and energy density of 20.9 Wh/kg at 30 A/g) and long-term cycling stability (retaining 94% of its initial capacitance after 1500 cycles). These characteristics suggested that such freestanding CNFs/MnO₂ nanocomposites are promising for high-performance supercapacitors.