Morphological effect on the electrochemical behavior of electric double-layer capacitors
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M.S. Dresselhaus Department of Physics and Department of Electrical Engineering, and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 14 May 2001; accepted 6 September 2001)
Activated milled mesophase carbon fibers (AC-mMPCF, MP-series) show a higher specific capacitance in spite of a smaller specific surface area than those of powder-type activated carbons (AC-series). This phenomenon can be interpreted to mean that it is difficult to predict the capacitance of an electric double-layer capacitor (EDLC) knowing just the surface area and the pore size. More information is needed about other inherent characteristics of the samples, for example, the equivalent series resistance (ESR), shape of the pores, etc. We investigate here other characteristics of the samples. Consequently, it was deduced that the MP-series of EDLCs have a slit pore shape, which affects the accessibility of the electrolyte ion onto the electrode surface. Moreover, the MP-series of materials have suitable ESR values, and these material properties themselves should be considered as factors that affects the deterioration of the specific capacitance of EDLCs.
I. INTRODUCTION
The electrochemical double layer capacitor (EDLC) is a new type of capacitor, offering new features, with characteristics between those of a battery and a simple capacitor.1,2 In general, positive and negative electrical charges are arrayed in opposite positions with an extremely short distance between two different phases (e.g., the solid electrode and the liquid). This charge distribution layer is called the electric double layer. There are various explanations for this interfacial charge distribution.3–9 EDLCs typically consist of a pair of polarizable electrodes, a separator, and an aqueous or organic electrolyte.10,11 Although there are plenty of parameters to optimize and variations to be tried, the most important source of variation of the capacitance may be the electrode material itself. The capacitive storage comes from the electrochemical double layer formed at the electrode/ electrolyte interface. Therefore, the specific capacitance of EDLC electrodes are linked to the physical area, the pore size, the ionic accessibility of the electrode material, and the electrolyte properties. Recently, activated carbon (AC) and activated carbon fiber (ACF) research has
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J. Mater. Res., Vol. 16, No. 12, Dec 2001 Downloaded: 18 Mar 2015
stimulated interest in these materials as electrodes of EDLC, because of their rich pore structures, which form within the electric double layers, and because of the relatively high conductivity of carbon. The samples chosen as precursors in this study are coconut shell and milled mesophase pitch. Both materials have quite different morphologies and both are used generally. Coconut shell is a well-known precursor for the preparation of high-quality activated carbons.12 However
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