Relationship Between Electrical Conductivity and Supercapacitor Properties of Co3O4 Nanofibers Fabricated by Electrospinning

Co3O4 with a spinel structure has been utilized as supercapacitor materials due to their active surface sites, strong absorption capacity, excellent electrochemical activity, and stability. In this study, we tried to explore the optimized electrospinning conditions, including heat-treatment temperature for Co3O4 nanofiber fabrication for supercapacitor applications. The X-ray diffraction patterns of Co3O4 nanofibers annealed at 600 and 800 degrees C showed a cubic spinel crystal structure without a secondary phase, but CoO was found in the specimens annealed at 400 degrees C. From the XPS curve fitting, Co3+ increased in the Co3+/Co2+ ratio with increasing heat-treatment temperature. The electrical conductivity of the Co3O4 nanofibers heated at 400, 600, and 800 degrees C is 7.53 x 10(-3), 1.12 x 10(-2), and 6.26 x 10(-3) ohm(-1) cm(-1), respectively. The Co3O4 nanofibers heat treated at 600 degrees C showed the highest conductivity value, and the conduction mechanism was polaron hopping between Co3+ and Co2+. The supercapacitor properties of Co3O4 nanofibers are evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance measurement using a three-electrode system in a 3 M KOH electrolyte. The GCD tests showed that the Co3O4 nanofibers heated at 600 degrees C had the highest specific capacitance of 579.66 F/g. From the electrochemical impedance measurements, the charge transfer resistance (R-ct) of calcined Co3O4 nanofibers at 600 degrees C showed the lowest value of 1.27 ohm. Also, the Co3O4 nanofiber exhibits excellent cycle stability with capacitance retention over 99% until 1000 cycles at a current density of 2 A/g. Therefore, the excellent supercapacitor performance of Co3O4 nanofibers annealed at 600 degrees C is due to its nanofiber structure without a secondary phase providing a larger surface area and charge transfer.