Sputtered manganese oxide thin film on carbon nanotubes sheet as a flexible and binder-free electrode for supercapacitors

In this work, flexible carbon nanotubes (CNTs)/manganese oxide (MnO2) composite electrode was fabricated by direct deposition of MnO2 nanoparticles on CNTs sheet by RF magnetron sputtering. The surface morphology and microstructure of the CNTs and CNTs/MnO2 composite electrodes were characterized by X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was found that 1-μm thick MnO2 film covered the surface of CNTs sheet with MnO2 mass loading of 0.125 mg/cm2. CNTs/MnO2 composite was tested as electrode materials for supercapacitors in sulfate media (1-M H2SO4 and Na2SO4) by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD). The results obtained showed that CNTs/MnO2 composite electrode displayed good electrochemical performance in 1-M Na2SO4, while the chemical stability of MnO2 film was highly affected due its dissolution in acidic medium. A specific capacitance of 940 F/g was retained (with a capacitance retention of about 80%) after 3000 GCD cycles. CNTs/MnO2 all-solid symmetric supercapacitor using PVA/H3PO4 gel electrolyte exhibited an initial specific capacitance 80 F/g and decreased by 25% after 3000 cycles.