ADVANCED MATERIALS AND METHODS FOR THE FABRICATION OF ELECTROCHEMICAL SUPERCAPACITORS

Abstract

<p>Nanostructured manganese oxides in amorphous or various crystalline forms have been found to be promising electrode materials for electrochemical supercapacitors (ES). Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 3-10 nm were prepared by a chemical precipitation method. Electrophoretic deposition (EPD) method and mpregnation techniques have been developed to fabricate thin film and composite electrodes for ES. As-prepared nanofibers and electrodes were studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The capacitive behavior of electrodes was investigated by cyclic voltammetry (CV) and chronopotentiometry method using a three-electrode cell in the mild Na₂SO₄.</p> <p>The composite electrodes fabricated by impregnation of manganese dioxide nanofibers and multi-walled carbon nanotubes (MWCNTs) into porous nickel foam and nickel plaque current collectors showed excellent capacitive performance with large material loading of 7-40 mg cm^-2 in 0.1-0.5 M Na₂SO₄. MnO₂ nanofibers and MWCNTs can form a porous fibrous network, which is beneficial for the electrolyte access to the active materials. In addition, MWCNTs formed a secondary conductivity network within the porous nickel structures. The highest specific capacitance (SC) of 185 F g^-1 was obtained at a scan rate of 2 mV S^-1 in the 0.5 M Na₂SO₄ solutions. The effect of the electrolyte concentration, scan rate and active material composition on the capacitive behavior was discussed.</p> <p>Obtained thin film and composite electrodes by EPD showed a capacitive behavior in the 0.1 M Na₂SO₄ aqueous solutions with a potential range of 0-1.0 V. The highest SC of 412 F g^-1 was obtained for the thin film electrodes at a scan rate 2 mV S^-1 in the 0.1 M Na₂SO₄. The SC decreased with increasing deposit mass and scan rate. It was found that the addition of MWCNTs can improve the capacitive performance of manganese dioxide electrodes with smaller equivalent series resistance (ESR). The mechanisms and kinetics of all the deposition methods were discussed.</p>