Supercapacitors Based on Iron Decorated Carbon Nanofibers with Excellent Cyclability

Abstract

Supercapacitors are attractive energy storage devices due to their high specific power and excellent cyclability. The supercapacitor field is currently dominated by porous carbon-based electric double layer capacitors (EDLC) that lack energy density. The energy density of porous carbon materials can be effectively improved by fabricating composites of carbon (for EDLC) with metal oxides/hydroxides (for pseudocapacitance). Among the metal oxides, iron oxide shows great promise owing to its high theoretical capacitance, negligible environmental impact and natural abundance. In this work, a facile methodology for obtaining freestanding, binder-free, iron decorated porous carbon nanofibers (Fe-CNF) for supercapacitor application is outlined. Briefly, blend of iron precursor and polyacrylonitrile (PAN) was electrospun into non-woven, freestanding nanofiber mats and subsequently subjected to heat treatment at 1000 degree Celsius in an inert atmosphere. Besides this thermal treatment, no additional chemical or physical activation process was conducted. The morphology of the nanofiber mat was characterized using scanning electron microscopy. The fibers exhibit diameters in the range of 150-200 nm. Electrochemical activity of the electrode was studied using cyclic voltammetry and galvanostatic charge-discharge techniques. Electrochemical measurements showed a large specific gravimetric capacitance of ~260 F/g at 20 mV/s in 6 M KOH aqueous electrolyte between 0 to -1.3 V range. The resulted synergistic effect between iron nanoparticles (pseudocapacitance) and carbon nanofibers (EDLC and conductivity) leads to such high capacitance value. The electrodes retain ~82% capacitance after 6500 cycles indicating excellent cyclic stability of these electrodes. Further, Fe-CNF’s electrodes were extensively characterized at various stages of charge-discharge using X-ray diffraction, high resolution transmission electron microscopy and X-ray photoelectron spectroscopy to understand the redox reactions of the fabricated electrode material. Figure 1