Carbon nanofibers decorated with FeO_x nanoparticles as a flexible electrode material for symmetric supercapacitors

We have produced flexible, freestanding, and light weight mats of FeO_x-decorated carbon nanofibers (CNFs) and demonstrated their use in supercapacitors with high energy and power density and excellent long term capacitance retention. Highly flexible carbon-iron oxide nanofibers were synthesized by electrospinning a solution of polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), and iron acetylacetonate (FeAcAc), followed by annealing to carbonize the PAN, pyrolyze the PMMA to produce pores, and convert FeAcAc to FeO nanoparticles. The morphology of the FeO_x/CNF composite was determined by scanning and transmission electron microscopies, which showed that the embedded FeO_x nanoparticles were well distributed in the CNF electrode. We employed cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy to evaluate the electrochemical performance of symmetric supercapacitors prepared from the FeO_x/CNF composite. The supercapacitors exhibited high specific capacitance (427 F·g⁻¹ at 10 mV·s⁻¹ and 436 F·g⁻¹ at 1 A·g⁻¹ in the optimal case) and good stability, retaining 89% of their initial capacitance after 5000 cycles at a current density of 1 A·g⁻¹. The optimal device achieved an energy density of 167 Wh·kg⁻¹ at a power density of 0.75 kW·kg⁻¹, and an energy density of 66 Wh·kg⁻¹ at a power density of 7.5 kW·kg⁻¹. These combinations of energy and power densities can meet the needs of many emerging supercapacitor applications.