Flexible supercapacitor of high areal performance with vanadium/cobalt oxides on carbon nanofibers as a binder-free membrane electrode

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For flexible capacitive energy storage units in wearable devices, areal metrics, such as areal capacitance and energy density, are exceptionally important, because of limitations on device volume and weight. One-dimensional carbon nanofibers composited with active metal oxides, such as cobalt monoxide (CoO), possess extraordinary electrochemical properties. However, low areal metrics and the polarity in the flexible asymmetric supercapacitor device are two crucial challenges. Herein, vanadium/cobalt oxides on carbon nanofibers (VCO/CNFs) are purposely designed and fabricated by electrospinning and controllable post-calcination, which can be used as a binder-free non-polarity membrane electrode for the assembly of high-performance flexible supercapacitor (FSC). Importantly, the rational incorporation of vanadium with varying valence states helps promote the formation of CoO phase, enriches the reaction sites and enhances the capability of local electron transport. As expected, the single electrode of VCO/CNFs yields an extraordinarily high areal capacitance of 1.83 F cm−2 at the current density of 8 mA cm−2. The as-obtained FSC with VCO/CNFs as both anode and cathode delivers an excellent areal energy density of 44.2 μWh cm−2 at a power density of 2.8 mW cm−2, a long-term cycling stability (retention rate of 95.2% over 10,000 cycles), and a good flexibility under even various bending conditions. To our best knowledge, these areal metrics are among the superior values reported.