Composite Electrodes for Electrochemical Supercapacitors
© The Author(s) 2010
Received: 30 July 2009
Accepted: 17 December 2009
Published: 7 January 2010
Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 4–6 nm were prepared by a chemical precipitation method. Composite electrodes for electrochemical supercapacitors were fabricated by impregnation of the manganese dioxide nanofibers and multiwalled carbon nanotubes (MWCNT) into porous Ni plaque current collectors. Obtained composite electrodes, containing 85% of manganese dioxide and 15 mass% of MWCNT, as a conductive additive, with total mass loading of 7–15 mg cm−2, showed a capacitive behavior in 0.5-M Na2SO4 solutions. The decrease in stirring time during precipitation of the nanofibers resulted in reduced agglomeration and higher specific capacitance (SC). The highest SC of 185 F g−1 was obtained at a scan rate of 2 mV s−1 for mass loading of 7 mg cm−2. The SC decreased with increasing scan rate and increasing electrode mass.
KeywordsManganese dioxide Carbon nanotube Nickel plaque Supercapacitor Impregnation Composite
Porous Ni materials, such as plaques  and foams , are widely used in industry for the fabrication of electrodes for rechargeable batteries. Nanostructured active materials are impregnated chemically or electrochemically into the porous Ni structures, which are used as current collectors . Ni plaques are current collectors of choice for battery applications demanding high power and reliability, along with long cycle life, such as batteries for aerospace and railway applications, power tools and some portable electronics . The pore size of Ni plaques is smaller compared to that of foams. The smaller pore size decreases the distance for electrons to travel from the current collector into the active material during cell discharge and consequently improves the discharge rate characteristics for high power applications . Significant advances in the development of Ni plaques were achieved by the use of filamentary Ni particles with high surface area, which improved contact with active materials .
A new wave of interest in the application of porous Ni materials is related to the development of electrochemical supercapacitors (ES) . ES can complement or replace batteries in electrical energy storage applications when high-power delivery is required . Manganese dioxides with various crystalline structures are important materials for electrodes of ES . The interest in the application of manganese dioxide in ES is related to low cost of this material, which exhibits high SC in environmentally friendly aqueous electrolytes .
where A+ = Li+, Na+, K+, H+. Equation 1 indicates that high electronic and ionic conductivity of the electrode material are important in order to utilize the high theoretical capacitance (1,370 F g−1) of manganese dioxide . Composite  and thin film electrodes  were developed and investigated. The properties of MnO2 are influenced by crystalline structure, particle size, porosity and surface area . A complicating factor in the application of MnO2 in ES is low electronic conductivity of this material. This problem can be addressed by the use of advanced current collectors, such as Ni plaques.
In a previous investigation , manganese dioxide nanofibers were prepared by a chemical precipitation method and utilized for the fabrication of composite manganese dioxide—MWCNT films by electrophoretic deposition. The composite films deposited on stainless steel foils showed high SC in a voltage window of 1.0 V. However, the SC decreased significantly with increasing film thickness. The results presented below indicated that relatively high SC can be achieved at high active material loading using Ni plaques as current collectors. In this approach, the high surface area and porous structure of the Ni plaques provided improved electrical contact of the current collector with active material and enabled good electrolyte access to the active material. We presented experimental results on the fabrication of composite electrodes, investigation into electrode microstructure and electrochemical behavior.
MWCNT were provided by Arkema company. The average diameter of MWCNT was ~15 nm and length ~0.5 μm. Ni plaques with mass of 0.1 mg cm−2 were provided by Inco company and impregnated with a manganese dioxide slurry containing 15 mass% of MWCNT.
XRD studies were performed with a diffractometer (Nicolet I2) using monochromatic Cu Kα radiation at a scanning speed of 0.5 deg min−1. TGA and DTA of the manganese dioxide nanofibers were carried out in air at a heating rate of 5 °C/min using a thermoanalyzer (Netzsch STA-409). Electron microscopy investigations were performed using a JEOL 2010F transmission electron microscope and a JEOL JSM-7000F scanning electron microscope equipped with energy-dispersive spectroscopy.
Impedance spectroscopy investigations were performed in the frequency range of 0.1 Hz–100 kHz at amplitude voltage of 5 mV. Simulations of the impedance behavior were performed on the basis of the equivalent-circuit models using ZsimpWin 3.10 commercial software.
Results and Discussion
In the previous investigation , it was found that manganese dioxide nanofibers and MWCNT formed a porous fibrous network, which was beneficial for the electrolyte access to the active material. However, the SC of the films deposited on metal foil substrates decreased significantly with increasing film mass from 50 to 300 μg cm−2. In contrast, the results presented below showed that relatively high SC can be obtained for active material loading of 7–15 mg cm−2 using Ni plaque current collectors. In this approach, porous Ni current collectors  provided improved contact with active material, whereas MWCNT was used as a conductive additive.
Manganese dioxide nanofibers with length ranged from 0.1 to 1 μm and a diameter of about 4–6 nm were prepared by a chemical precipitation method. Composite electrodes for ES, containing two different fibrous materials, were fabricated by impregnation of slurries of the manganese dioxide nanofibers and MWCNT, as a conductive additive, into porous Ni plaque current collectors. The composite electrodes with total mass loading of 7–15 mg cm−2 showed good capacitive behavior in the 0.5 M Na2SO4 solutions. The reduction in stirring time of the precipitated nanofibers resulted in lower agglomeration and higher SC. The highest SC of 185 F g−1 was obtained at a scan rate of 2 mV s−1 for materials loading of 7 mg cm−2. Testing results indicated that Ni plaques are promising current collector materials for application in ES.
The authors gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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