Article pubs.acs.org/JPCC
Gel Polymer Electrolyte Based Electrical Double Layer Capacitors: Comparative Study with Multiwalled Carbon Nanotubes and Activated Carbon Electrodes Yogesh Kumar, G. P. Pandey,† and S. A. Hashmi* Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India
ABSTRACT: We report the comparative studies on gel polymer electrolyte based flexible electrical double layer capacitors (EDLCs) fabricated with two different electrodes, namely multiwalled carbon nanotubes and activated carbon (charcoal, AC). The gel electrolyte comprising lithium trifluoromethanesulfonate (Li-triflate or LiTf)/ionic liquid, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITf)/poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) exhibits ionic conductivity on the order of 10−3 S cm−1 at room temperature, which is suitable enough to be used in EDLCs. A mixture of ethylene carbonate (EC) and propylene carbonate (PC) has also been incorporated to enhance the flexibility of the electrolytes. The comparative studies on EDLCs have been performed from the electrode and electrolyte points of views using impedance analysis, cyclic voltammetry and galvanostatic charge−discharge tests. The MWCNT-based EDLC shows the specific capacitance value of 32 F g−1, whereas the activated charcoal offers the substantially higher value of 157 F g−1. These result in the higher specific energy of AC-based EDLCs as compared to the devices with MWCNTs electrodes. However, the MWCNTs electrodes, due to their mesoporous nature, exhibit high rate capability and hence high specific (pulse or continuous) power relative to the predominantly microporous AC electrodes. The charge−discharge cyclic performance of MWCNT-based EDLC is also excellent (for >5 × 104 cycles) with respect to that for AC-based device, limited to 4000−5000 cycles.
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INTRODUCTION Supercapacitors are electrical energy storage devices being developed for a wide range of applications including daily use electronic equipment, medical utilities, transportation, and defense related systems.1−4 To gratify the growing performance demands, various parameters associated with the supercapacitors, namely the specific energy/power, charge−discharge cycle life, and safety are needed to be considerably improved.1−6 The device referred to as “electrical double layer capacitor (EDLC)” is a class of supercapacitors that specifically use carbon, in its various forms, as electrodes (namely, activated carbon, carbon xerogels, carbon nanotubes, graphene, etc.) possessing large surface area and porosity.1−10 The double layer formation, i.e., the separation of translational charges in the micro- and/or meso-porous structure of carbons is responsible for high specific capacitance of EDLCs.4,5,8,9 Another class of supercapacitors is referred as “redox or pseudo-capacitors” in which charge storage is owing to the fastfaradaic reactions at the electrode−electrolyte interfaces.1,3 © 2012 American Chemical Society
Electroactive oxides (e.g., RuO2, MnO2, NiO, etc.) or conducting polymers (e.g., polypyrrole, polythiophene, etc.) are primarily used as electrodes in such a class of supercapacitors.1,3,11−17 The activated carbons (charcoals, ACs) are prominent electrode materials for commercial EDLCs, which offer high capacitance values because of their large microporosity (pore dimension
