In this study, the capacitive performance of ultrathin-film manganese oxide (MnO2) electrodes for transparent supercapacitors is investigated. The highest transmittance reported so far, 99.1% at a 550 nm wavelength, is achieved with an areal capacitance of 1.6 mF/cm2. The thickness and resulting optical transparency of the MnO2 films can easily be controlled by changing the concentration of the precursor sols in the sol–gel-derived coating process. The thinnest electrode shows the highest transmittance and specific capacitance of 576 F/g. The relationship between the transmittance, optical conductivity and areal capacitance of the electrodes is also investigated. The largest capacitive figure of merits, FoMc, of 62.7 cm2/F is obtained for the thinnest electrode. Deconvolution of the capacitive elements indicates that the surface capacitive element is dominant for the ultrathin electrodes, whereas the contribution of the diffusion-controlled insertion element grows rapidly as the electrode thickness increases.

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