In this work, a free-standing, flexible and highly conductive vanadium-carbon nanofiber composite has been fabricated as electrode for Supercapacitors. Vanadium monoxide (VO) coupled with amorphous vanadium covalent bonds (VOx) are successfully incorporated into carbon nanofibers (VO/VOx/CNF) by electrospinning and heat treatment. A theoretical explanation is proposed for the formation of VO/VOx in CNF composites. The VO and VOx are respectively reduced and formed from vanadium precursors of vanadium dioxide (VO2) and vanadyl (IV) acetylacetonate (VOA) by means of self-reduction method, in which carbon precursors (polyacrylonitrile and polyvinylpyrrolidone), small evolved gas molecules (CO, H-2, HCN) and graphitized carbon act as self-reductants. No additional reductants is needed before or after heat treatment, avoiding the secondary contamination. The VO/VOx/CNF electrode has a specific capacitance of 325.7 F g(-1) at a current density of 1 A g(-1) and is capable of reserving 92% of its initial capacitance after 5000 cycles operating at a current density of 4 A g(-1) in a symmetric two-electrode capacitor using 6 M KOH as an electrolyte. The superior electrochemical performance of VO/VOx/CNF may be attributed to two advantages. The first is the enhanced conductivity brought upon the incorporation of quasi-metallic VO (similar to 10(2) Omega(-1)cm(-1)) and the network of nanowire, and the second is the rapid ion transfer rate caused by the rich vanadium redox couples VO/VOx and the well-developed pore structure. Notably, this work has also provided a facile method to obtain varaible low valence states from vanadium oxides through self-reduction, which may also be applied to synthesize other metal oxidescarbon nanofiber composites. (C) 2016 Elsevier Ltd. All rights reserved.