Electrode properties and microstructures of MnO2 nanosheet thin films as cathodes for electrochemical capacitors

The electrochemical properties of MnO2 nanosheet thin films were examined to determine the intrinsic electrochemical properties of MnO2 nanosheets and the influence of a laminated structure on the electrode properties as cathodes for electrochemical capacitors in a mild alkaline system. The MnO2 nanosheet thin films were prepared on an ITO substrate using electrophoretic deposition. The XRD patterns and SEM and STEM observations showed that films composed of nanosheets with large lateral sizes of approximately 1 μm (large-NS) had a homogeneous microstructure like a bulk material. Thin films composed of nanosheets with small lateral sizes of 50–100 nm (small-NS) had a heterogeneous microstructure with numerous pores. The small-NS film exhibited a large capacitance of 1200 F g− 1 at a small mass density of 0.57 μg cm− 2 and a low sweep rate of 3 mV s− 1. The small-NS film maintained a large capacitance of 450 F g− 1 even at a high sweep rate of 600 mV s− 1. The small-NS film exhibited a better rate capability than the large-NS film because of the fast ion diffusion through the heterogeneous microstructure with numerous pores. This heterogeneous microstructure with numerous pores was found to contribute to the good electrode properties of the nanosheet electrodes.