Preparation and characterization of flexible all solid state CNT/MnO2 Supercapacitors

Abstract

In this work, CNTs/MnO2 hybrid supercapacitors were prepared using RF magnetron sputtering as deposition technique. The as-prepared composite materials were characterized by different morphology and spectroscopy techniques, CNTs/MnO2 composites were then tested as electrode materials in symmetric supercapacitors in solution and in solid state. The surface morphology, structural and mechanical properties of the CNTs and CNTs/MnO2 electrodes were characterized by Raman spectroscopy, scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). XRD, Raman spectroscopy and XPS measurements confirmed the formation of MnO2 films. While SEM images demonstrated, the uniform distribution of MnO2 layers. Furthermore, tensile tests demonstrated the flexibility of the prepared CNTs/MnO2 composite electrodes. The electrochemical behavior of three CNTs/MnO2 composites with 350, 700, and 1000 nm thick MnO2 was investigated in 1 M Na2SO4 aqueous solution via cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge (GCD). The CV and GCD tests showed that all electrodes with different MnO2 thicknesses exhibit excellent pseudocapacitive behavior as compared with CNTs sheet in the potential window of 0 -1.0 V for CV and 0 - 0.8 V for GCD.

At small scan rates (5 mV/s), the electrode of 1000 nm MnO2 thickness displayed specific capacitance of CV (1676 F/g), which is much higher than that of the other electrodes (609, and 441 F/g) for the electrodes with 700 nm and 350 nm, respectively. The specific capacitance measured using charge/discharge process showed similar thickness dependence as in CV measurements, where the specific capacitance increases with increasing the MnO2 thickness. Additionally, the electrode with 1000 nm MnO2 thickness exhibited a cyclic stability with capacitance retention of 85.8% of its peak value after 400 cycles meanwhile, the other electrodes exhibited almost the same cyclic stability with capacitance retention of 59% after 400 cycles. The all solid-state flexible supercapacitors consisting of two-electrode system with a PVA/H3PO4/CNTs/MnO2 symmetric capacitor with 1000 nm MnO2 thickness showed the highest specific capacitance in comparison with the other devices with 350 and 700 nm thick MnO2. The device with 700 nm thick MnO2 exhibited the best cyclic stability with capacitance retention of 67% of its initial capacitance after 400 cycles compared with the other devices.