Role of Carbon Nanomaterials on Enhancing the Supercapacitive Performance of Manganese Oxide-Based Composite Electrodes

MnO2 is an attractive material owing to its high specific capacitance, excellent electrochemical activity, thermal and chemical stability, environmental benignity and broad potential window. However, its poor electrical conductivity limits its performance in supercapacitor applications. The electrical conductivity of MnO2 can be enhanced by making its composites with carbon nanomaterials as these offer superior electrical conductivity and high specific surface area. The present study is a comparative study on the effect of various types of carbon nanomaterials such as carbon nanotubes (CNTs), graphene oxide, thermally reduced graphene oxide, activated carbon and carbon nanofibers on the supercapacitive performance MnO2 electrodes by making composite electrodes. MnO2 was synthesized by a facile chemical reduction method, and calcination was performed at 200 °C to obtain amorphous state. MnO2-based composite electrodes were prepared using 10 wt.% of various carbon nanomaterials. Characterization of different carbon materials was carried out by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and atomic force microscopy. Composite electrodes were prepared by coating mixture, consisting of MnO2, carbon nanomaterials, carbon black and polyvinylidene fluoride on the surface of highly porous Ni foam using a high-speed vacuum planetary centrifugal mixer. Electrochemical characterization of the prepared electrodes was performed by cyclic voltammetry, galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS). GCD revealed that MnO2-CNTs composite electrodes showed maximum specific capacitance (SC) of 330 F/g at 1 A/g current density and areal capacitance of 3.16 F/cm2 with 5–7 mg mass loading of active material in 3 M KOH as an aqueous electrolyte. This SC was ca. 79% higher than pure MnO2 and also higher than previously reported MnO2/CNTs composite electrodes. MnO2-CNTs symmetric supercapacitor device showed excellent SC of 177 F/g at 1 A/g current density with 94% charge retention after 1000 GCD cycles. EIS analysis showed that MnO2-CNTs composite electrode had the lowest charge transfer resistance compared to other electrodes.