Increasing Energy Storage in Activated Carbon based Electrical Double Layer Capacitors through Plasma Processing
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Increasing Energy Storage in Activated Carbon based Electrical Double Layer Capacitors through Plasma Processing Marcelis L. Muriel1, Rajaram Narayanan2, Prabhakar R. Bandaru1,2,3 1 Program in Materials Science, 2Department of Nanoengineering, 3 Department of Mechanical & Aerospace Engineering, University of California, San Diego, La Jolla, CA 920393, U.S.A. ABSTRACT We present a methodology to enhance the electrical capacitance of activated carbon (AC) electrodes based on the introduction of electrically charged defects through argon plasma processing. Extensive characterization using electrochemical techniques incorporating cyclic voltammetry, constant current charge/discharge, and electrical impedance spectroscopy indicated a close to seven-fold increase in capacitance with respect to untreated AC electrodes, not subject to plasma processing. INTRODUCTION The wide usage of portable electronics and the necessity for energy storage provides substantial motivation for the study of electrochemical energy storage devices, such as batteries and capacitors. While the energy density of batteries is high, they are inefficient for quick charging (e.g., portable electronics) or quick discharge (e.g., power spurts in automobile acceleration). Alternately, the attribute of large power density is inherent to electrochemical capacitors (ECs), as most of the energy is stored on the surface for quick release. However, the energy density of the ECs is presently low. In this paper, we consider one possible idea and related methodology for enhancing EC energy density. In this context, a prototypical material for EC electrodes is activated carbon (AC), due to the material’s relatively low cost, high surface area/mass ratio (>1000 m2/g), and ease of manufacture [1,2]. However, the net obtainable capacitance obtainable from AC based electrodes is often hampered by inadequate electrolyte accessibility within the pores of the AC, in addition to space charge capacitance (Csc), in series with the expected double-layer capacitance (Cdl). Previous work from our group showed that Argon irradiation can contribute to charge transfer and current density through atomic scale carbon removal at random sites. Ar plasma was chosen for its inertness as opposed to other plasma (e.g. Oxygen, Cl, etc) which may introduce functional groups on the surface of the carbon that contribute more to “redox capacitance” than simple double layer capacitance. [3]. Structural changes, as evidenced through Raman spectroscopy as well as transmission electron microscopy have been indicated in our previous work related to carbon nanotubes (CNTs) [4] and graphene [5]. The creation of edge-plane defects, comprising electron rich dangling bonds, which have intrinsically higher density of states (DOS) and enhance the effective charge (and capacity), is involved in the charged defects, and similar effects are feasible in the plasma processed AC as well. However, the efficacy of the plasma-based method to bulk structures has not yet been shown. In this work, we indicate that a
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