Effect of Dry Oxidation on the Performance of Carbon Nanotube Arrays Electrochemical Capacitors
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Effect of Dry Oxidation on the Performance of Carbon Nanotube Arrays Electrochemical Capacitors Adrianus I. Aria1 and Morteza Gharib1 1 Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125, U.S.A. ABSTRACT In this study, the effect of dry oxidation on the electrochemical properties of carbon nanotube arrays is investigated. Oxygenated surface functional groups were introduced to the arrays by oxygen plasma treatment, where their surface concentrations were varied by controlling the exposure time. The finding presented herein shows an augmentation of nearly thirty times in term of specific capacitance when the arrays are oxidized. Similar behavior is also observed in the non-aqueous electrolytes where the specific capacitance of the oxidized carbon nanotube arrays is measured more than three times higher than that of the pristine ones. However, overexposure to oxygen plasma treatment reverses this effect. At such high oxidation level, the damage to the graphitic structure becomes more pronounced such that the capacitive behavior of the arrays is overshadowed by their resistive behavior. These findings are important for further development of carbon nanotube based electrochemical capacitors. INTRODUCTION In recent years, the performance of electrochemical double-layer capacitors (EDLC) has been increased significantly since the introduction of carbon nanotube (CNT) arrays as their electrodes that replace the standard activated carbon electrodes. Such advancement was possibly achieved because of the inherent physical and electrical properties of the CNT arrays: they have a very large surface area and a very high electrical conductivity [1-3]. Although a lot of works have been done to increase the performance of CNT based EDLC in term of power and energy density using various electrolytes [4-6], the effect of surface chemistry to such performance is yet to be investigated. It was thought that the surface chemistry of CNT electrodes would play a very important role in their electrochemical behavior and performance. We hypothesize that the specific capacitance of pristine CNT electrodes will be much less than that of oxidized ones due to the inherent incompatibility between the non-polar CNT and the polar electrolyte (figure 1). Current methods for introducing oxygen adsorbates to the CNT originate mainly from wetchemical methods, including the well-established Hummer’s method [7]. These methods usually use harsh chemicals, such as strong acids or oxidizing agents, to introduce oxygenated functional groups to the CNT. In contrast, no harsh chemicals are involved in the dry oxidation methods, allowing them to be used safely and conveniently. In this study, the dry oxidation process was done using an oxygen plasma treatment. The concentration of oxygenated functional groups attached to the CNT can be varied by controlling the plasma exposure dose [8].
Figure 1. Schematic drawing of a CNT based EDLC. Typical configuration of such capacitor consists of two symmetric current collectors a
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