Ultracapacitors Based on Graphene/MWNT Composite Films
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Ultracapacitors Based on Graphene/MWNT Composite Films Wei Wang1, Shirui Guo2, Jiebin Zhong3, Jian Lin3, Mihrimah Ozkan1, 4, Cengiz Ozkan1, 3 Materials Science & Engineering, University of California, Riverside, Riverside, CA, U. S. A. 2 Chemistry, University of California, Riverside, CA, U. S. A 3 Mechanical Engineering, University of California, Riverside, CA, U. S. A 4 Electrical Engineering, University of California, Riverside, CA, U. S. A 1
ABSTRACT
Ultracapacitors are promising candidate for alternative energy storage applications since they can store and deliver energy at relatively high rates. In this work, we integrated large area CVD graphene with multi-walled carbon nanotubes (MWNTs) to fabricate highly conductive, large surface-area composite thin films used as electrodes in ultracapacitors. Uniform, large area graphene layers were produced by CVD on copper foils and were chemically modified. Chemically shortened MWNTs, ranging in length of 200~500 nm, were deposited by dropping on graphene layers. Graphene/MWNT composite films with different thicknesses were obtained. The surface morphology was investigated by SEM. The results demonstrated relatively dense and homogeneous net nanostructure. The measurements of cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy (EIS) are conducted to determine its performance of graphene/MWNT film structures.
INTRODUCTION
Ultracapacitor is a promising alternative energy storage system due to its relatively fast rate of energy storage and delivery. The high energy and power density make the ultracapacitors suitable for a wide variety of applications [1]. The excellent chemical and physical properties of carbon materials such as high conductivity and surface-area, good corrosion resistance, excellent temperature stability, and relatively low cost make it a valuable candidate for the electrode of ultracapacitor [2]. Recently, intensive research has been conducted on carbon based supercapacitors, especially 1-D Carbon Nanotube (CNT) and 2-D graphene [3, 4]. The CNT-graphene hybrid structure is also a well studied area especially the patterned growth of vertically aligned CNT on graphene by chemical vapor deposition and also the solution processing of graphene-CNT hybrid materials [5, 6]. However, connecting CNT with CVD graphene without destroying their own advantages, i.e. high conductivity for graphene and large surface area for CNT is still a challenge. In this work, we reported a novel way to connect and stack chemically shortened multi-walled carbon nanotubes (MWNTs) to single layer CVD
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graphene (SLG) while keeping their own advantage for ultracapacitors. Electrochemical measurements show high energy density and specific capacitance of the as obtained SLG/MWNTs hybrid composite structure which demonstrates its promise as the electrode of ultracapacitors.
EXPERIMENT
The preparation of the electrode Graphene films were grown on a 25
thick copper foil (Alfa Aesar, item No.
13382) by CVD method using a mixture of methane and hy
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