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Summary 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.

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