Defect Engineered Multi-Walled Carbon Nanotube arrays as Electrochemical Double Layer Capacitors
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Defect Engineered Multi-Walled Carbon Nanotube arrays as Electrochemical Double Layer Capacitors
Rajaram Narayanan1, Mark Hoefer2 and Prabhakar R. Bandaru 1,2 1 Department of Nanoengineering,2 Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92037-15, U.S.A. ABSTRACT The efficacy of vertically aligned defect engineered multi-walled carbon nanotube (MWCNT) arrays as electrochemical double layer capacitors (EDLCs) was investigated using standard electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). We report a ~ 200% improvement in specific double layer capacitance of MWCNT arrays by extrinsically introducing defects using argon plasma irradiation. The capacitance-voltage characteristics of argon irradiated MWCNTs provide insights into the nature of the defects and their influence on the specific capacitance (capacitance/area). INTRODUCTION In EDLCs, energy is stored by means of charge separation at the electrode/electrolyte interface in response to an applied potential. The ability of such devices to store charge is the double layer capacitance Cdl is proportional to A/d; where A is the specific surface area (area per unit mass) of the electrode and 'd' is the thickness of the double layer formed by ions of opposite charge at the electrode where potential is applied. In EDLCs, 'd' is typically at the sub nm scale resulting in very large ' Cdl '. In a given electrode/ electrolyte system where charge can be stored over a potential range (V), the energy stored W= Cdl V2/2 and with a power delivery capability P= V2/4R, where R is the electrical resistance of the capacitor. Thus, an ideal EDLC electrode material must possess (i) high specific surface area and (ii) low electrical resistance. Carbon Nanotubes (CNTs) have been proposed for EDLC electrode materials because they possess very high specific surface area (~300m2/ g) as well as high electrical conductivity . Consequently, CNTs could constitute desirable electrode materials for energy storage applications [1-3]. In this proceeding, we focus on measuring Cdl of vertically aligned CNTs and suggest ways to increase Cdl by controlled introduction of charged defects [4-6] on the CNTs through argon irradiation. Standard electrochemical characterization techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) have been used to measure Cdl . The morphology of the CNT constituted electrodes have been characterized using scanning electron microscopy (SEM). EXPERIMENT Vertically aligned MWCNTs were grown on 5nm thick Iron (Fe) catalyst on Silicon substrates . Growth was conducted under atmospheric pressure and at a temperature of 6750 C. Ethylene, Hydrogen and Argon were used in a 1:2:4 (flow rate) ratio as feed gases. The performance of the MWCNTs as electrodes was then characterized through CV and EIS experiments using a PCI4– 300 potentiostat (Gamry Instruments) where the CNTs were used as the working electrode, with Platinum (Pt)
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