Hybrid Supercapacitor Based on Coaxially Coated Manganese Oxide on Vertically Aligned Carbon Nanofiber Arrays

Hybrid supercapacitor electrodes with remarkable specific capacitance have been fabricated by coaxially coating manganese oxide thin films on a vertically aligned carbon nanofiber array. Ultrathin manganese oxide layers are uniformly coated around each carbon nanofiber via cathodic electrochemical deposition, likely based on water electrolysis initiated electrochemical oxidation. This results in a unique core-shell nanostructure which uses the three-dimensional brush-like vertical carbon nanofiber array as the highly conductive and robust core to support a large effective surface area and provide reliable electrical connection to a thin redox active manganese oxide shell. The pseudo-capacitance of 313 F/g in addition to the electrical double layer capacitance of 36 F/g is achieved by cyclic voltammetry at a scan rate of 50 mV/s and maintains at this level as the scan rate is increased up to 2000 mV/s. A maximum specific capacitance of 365 F/g has been achieved with chronopotentiometry in 0.10 M Na2SO4 aqueous solution with ∼7.5 nm thick manganese oxide. This hybrid core-shell nanostructure demonstrates high performance in maximum specific energy (32.5 Wh/kg), specific power (6.216 kW/kg), and cycle stability (∼11% drop after 500 cycles), which are derived from cyclic voltammetry and galvanostatic charge−discharge measurements. This new architecture can be potentially developed as multifunctional electrical energy storage devices.