Electrodeposited manganese dioxide nanostructures on electro-etched carbon fibers: High performance materials for supercapacitor applications

Accepted Manuscript Title: Electrodeposited manganese dioxide nanostructures on electro-etched carbon fibers: High performance materials for supercapacitor applications Author: Sayed Habib Kazemi Mostafa Ghaem Maghami Mohammad Ali Kiani PII: DOI: Reference:

S0025-5408(14)00473-5 http://dx.doi.org/doi:10.1016/j.materresbull.2014.08.032 MRB 7612

To appear in:

MRB

Received date: Revised date: Accepted date:

31-5-2014 26-7-2014 20-8-2014

Please cite this article as: Sayed Habib Kazemi, Mostafa Ghaem Maghami, Mohammad Ali Kiani, Electrodeposited manganese dioxide nanostructures on electro-etched carbon fibers: High performance materials for supercapacitor applications, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2014.08.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrodeposited Manganese Dioxide Nanostructures on Electro-etched Carbon Fibers: High Performance Materials for Supercapacitor Applications Sayed Habib Kazemi a, b, *[email protected], Mostafa Ghaem Maghami a, Mohammad Ali Kiani c a

Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan

Center for Research in Climate Change and Global Warming (CRCC), Institute for Advanced

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Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran

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45137-66731, Iran

Chemistry & Chemical Engineering Research Center of Iran, P.O. Box 14335-186, Tehran-

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Iran

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* Corresponding author.

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Highlights

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Graphical abstract

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We report a facile method for fabrication of MnO2 nanostructures on electro-etched carbon fiber. MnO2-ECF electrode shows outstanding supercapacitive behavior even at high discharge rates.

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Exceptional cycle stability was achieved for MnO2-ECF electrode.

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The coulombic efficiency of MnO2-ECF electrode is nearly 100%.

Abstract

In this article we introduce a facile, low cost and additive/template free method to fabricate highrate electrochemical capacitors. Manganese oxide nanostructures were electrodeposited on

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electro-etched carbon fiber substrate by applying a constant anodic current. Nanostructured

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MnO2 on electro-etched carbon fiber was characterized by scanning electron microscopy, X-ray diffraction and energy dispersive X-ray analysis. The electrochemical behavior of MnO2 electro-

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etched carbon fiber electrode was investigated by electrochemical techniques including cyclic

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voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. A

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maximum specific capacitance of 728.5 F.g-1 was achieved at a scan rate of 5 mV s-1 for MnO2

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electro-etched carbon fiber electrode. Also, this electrode showed exceptional cycle stability,

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suggesting that it can be considered as a good candidate for supercapacitor electrodes.

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spectroscopy

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Keywords: Electrochemical properties; Nanostructures; Electron microscopy; Impedance

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1. Introduction

Electrochemical capacitors, also known as supercapacitors, are of the most promising energy

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strong devices that bridge the gap between batteries and conventional capacitors [1]. Delivering high energy and power densities make supercapacitors versatile devices with potential applications as power sources in a wide variety of applications. Carboneous materials, transition-

metal oxides and conducting polymers are the most employed materials for electrochemical supercapacitors [2-8]. Among different transition metal oxides studied so far; hydrous forms of ruthenium oxide are the most promising material for supercapacitor applications. They provide relatively high specific capacitance with remarkable cycleability [9, 10]. However, the practical application and commercialization of RuO2 has slowed down because of its high cost and toxicity [11]. Thus,

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research has been focused on developing low-cost transition metal oxides including MnO2,

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Co3O4, NiO, Fe3O4, VOx, and TiO2 [3, 12-17]. Amongst them, manganese oxide has attracted

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intense attention since it is an inexpensive and environmentally friendly material [18-24]. Manganese dioxide is one of the most stable manganese oxides with excellent physical and

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chemical properties under ambient conditions. A high capacitance (1370 F g-1) is expected for

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MnO2-based supercapacitor electrodes [25]. Such a high capacitance can be obtained by

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increasing the surface area and the material utilization. Direct deposition of manganese oxide on a carbon host, such as active carbon, graphite, carbon nanotubes and mesoporous carbon have

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been investigated to enhance the material utilization [26-32]. Electrochemical deposition is an exceptional technique developed to fabricate MnO2

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nanostructures, because of its opportunity to control the thickness and the structure of the

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deposited materials by changing several factors including electrolyte, electrodeposition current or voltage, and temperature [6, 13].

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Carbon fiber (CF) substrate offers advantages include high conductivity, chemical stability and three dimensional structures, which made it as an excellent substrate for supercapacitor electrode [2]. In this report, we report the fabrication of MnO2 nanostructures on electro-etched carbon

fiber surface (MnO2-ECF). It is noteworthy that the use of electro-etched carbon fibers is proposed because of the porous three-dimensional structure with large amount of interior channels, which improves the diffusibility of electrolyte ions [2].

2. Experimental

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The CF paper was purchased from Toray Carbon Fibers Inc. (America); with the average

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diameter of 8 μm for each carbon fiber. All chemicals were of analytical grade and were

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purchased from Sigma-Aldrich Company. CF substrate was electrochemically etched (ECF) by applying a constant potential of 2 V (Using Autolab potentiostat-galvanostat 101, Eco Chemie,

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Bv Netherlands) for 10 min in a 1 M of H2SO4 electrolyte solution [2].

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Prior to anodic deposition, the electro-etched carbon fiber paper was washed with acetone, and

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then rinsed with deionized water. Manganese oxide nanostructures were deposited on ECF substrate by applying an anodic current of 0.5 mA cm-2 in a solution of 0.1 M manganese acetate

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and 0.1 M sodium sulfate for 30 s at room temperature. After electrodeposition, the MnO2-ECF

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electrode was rinsed several times by deionized water and annealed at 300 °C for 1 h in air.

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Electrochemical measurements were carried out in a three electrode cell comprised of an

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Ag/AgCl reference electrode, platinum wire counter electrode and MnO2-ECF working electrode, in a 1M solution of Na2SO4. The surface morphology of the electrodeposited

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manganese oxide was studied by field emission scanning electron microscope (FESEM, Carl Zeiss ΣIGMA) equipped with EDX analyzer. The crystal structure of MnO2-coated electrode was

examined by a Bruker Advance D8 spectrometer with a Cu target (Cu-Kα line). Diffraction data were collected over 2θ, ranging from 10° to 90°.

Galvanostatic charge/discharge (CD), Cyclic voltametery(CV), and cycle-life stability experiments were performed in a potential range of 0 to 1 V vs. Ag/AgCl at room temperature. Also, the electrochemical characteristics of MnO2-ECF electrodes were further studied by acimpedance measurements (EIS). AC-impedance measurements were performed under opencircuit condition by a Zahner/Zennium potentiostat-galvanostat (Zahner, Germany). AC perturbation amplitude of 10 mV was imposed on the open-circuit potential in the frequency

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range of 100 kHz to 0.01 Hz.

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3. Results and Discussion

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3.1. Surface characterization of MnO2-ECF

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Firstly, ECF substrate was obtained by electro-etching in H2SO4. Scanning electron microscopy

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images (SEM), before and after electro-etching, revealed a densely packed and randomly

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oriented structure for CF and ECF substrates (Figure S-1 and S-2, supporting information). As

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seen in Figure S-1 and S-2, this randomly oriented structure provides three-dimensional porous

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structures. The porous structure of CF is expected to enhance the accessibility of electrolyte ions

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to this structure [2]. Figure S-2 shows that after electro-etching, new channels are formed on the

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fiber surface, which results in a considerable increase in substrate surface area. Figure 1-a and 1b present SEM images of electrodeposited MnO2 nanostructures on ECF surface (different

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magnification). The average thickness of electrodeposited MnO2 nanostructures was estimated to be 20 nm. Additionally, a low magnification SEM image of MnO2-ECF is provided as

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supporting information to show the even distribution of MnO2 nanostructures on the carbon fibers (Figure S-3). It can be seen that MnO2 nanostructures created 3D and microscopically open nanostructure, through which electrolyte ions can easily be transported. Also, Figure 1-c

shows the EDX elemental analysis of MnO2-ECF, confirming the presence of Mn, O and carbon (Carbon substrate). As seen in Figure 1-c, the weight ratio of Mn to oxygen content is nearly in agreement with formation of manganese dioxide. In the next step, as-deposited MnO2 nanostructures were annealed at 300 °C. It should be noted that the specific capacitance of manganese oxide depends on its crystal structure, which is

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affected by annealing temperature. The as-prepared MnO2 nanostructures usually include water molecules trapped inside the structure, enhancing the transportation of ionic species. Reddy and

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co-workers reported that if MnO2 is dried at 400 °C, all of the water content will be removed

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from the structure, resulting in a decreased capacitance [10, 33]. Hence, they have concluded that higher capacitance for as-prepared MnO2 could be originated from chemically adsorbed water

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molecules instead of physically adsorbed water. Pang et al. have proved that the electrodeposited

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manganese oxide dried in air at 300 °C shows a higher specific capacitance than that dried at

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room temperature [34].

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Figure 2 represents the XRD patterns of MnO2-ECF annealed at 300 °C for 1 h in air. The

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pattern of MnO2-ECF can be compared with the patterns of α-MnO2 (JCPDS 44-0141) [35]. Although, the main phase can be attributed to α-MnO2, some other phases, such as γ-MnO2 may

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also be present. The peaks appeared at 2θ= 20-30° are due to the presence of graphite structure of

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carbon fibers. The carbon fiber peaks are more intense compared to peaks of MnO2 phase. This implies incomplete crystalline structure and a small grain size of manganese oxide nanostructure

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after annealing at 300 °C. The average grain size of the manganese oxide deposited on CF was estimated to be 10 nm using Scherrer’s equation:

where λ is the X-ray wavelength, W is the full width at half-maximum (FWHM), and θ is the Bragg angle. 3.2. Electrochemical studies of MnO2-ECF Figure 3 shows the electrochemical behavior of CF electrode before (Fig. 3A) and after (Fig. 3B) electro-etching (ECF). While the CF substrate shows negligible capacitive current in

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voltammograms, considerable capacitive current can be observed for ECF substrate. This indicates that electro-etching significantly increases the accessible surface area of CF substrate.

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Also, the voltammograms recorded for the MnO2-ECF electrode is provided in Figure 3. As seen

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from Figure 3C, deposition of MnO2 significantly enhances the capacitive current compared to

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ECF substrate. This indicates that MnO2 nanostructures significantly contribute to the total

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capacitance through both the double layer and pseudocapacitive charge storage process. The

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shape of recorded voltammograms for MnO2-ECF electrode is almost rectangular, which is an

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important characteristic of an ideal capacitor [36]. It should be noted that the improved

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hydrophilicity of the ECF might partially affect the capacitance of MnO2-ECF electrode [2].

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Effects of the scan rate on the voltammetric behavior of MnO2-ECF electrode was also

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investigated in the potential range of 0 to 1 V vs. Ag/AgCl. Figure 4 shows recorded voltammograms for MnO2-ECF electrode in a 1.0 M of Na2SO4 solution at various scan rates. To

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evaluate the supercapacitive performance of MnO2-ECF electrode; specific capacitances against

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scan rate were calculated. Specific capacitance for MnO2-ECF electrode could be calculated from the CV curves according to the following equation:

where Q is the charge corresponding to the selected voltammograms, ΔE is the value of potential window, and m is the mass of the active material (electrodeposited MnO2 on ECF). The approximate specific capacitances of the electrode in 1.0 M of Na2SO4 solution were calculated as 728.5, 700.0, 633.3, 612.5 and 512.5 F g-1 at scan rates of 5, 25, 50, 100 and 200

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mV s-1, respectively (Figure 5). This excellent specific capacitance is mainly attributed to the

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even distribution of MnO2 nanostructures on ECF substrate. Furthermore, the α-MnO2 phase of

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MnO2-ECF is an important factor allowing facile electron/ion insertion reaction, including hydrated cations. The whole charge/discharge process may include cation transport in the

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electrolyte; adsorption/desorption of cations at the surface sites of MnO2-ECF electrode, and

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cation extraction/insertion into solid MnO2 matrix [36]. Therefore, the high effective surface area

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of MnO2 nanostructures is a crucial factor contributing to the high specific capacitance observed in this work.

proposed

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The decrease in the specific capacitance at high scan rates can be explained using the typically mechanism

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storage

process

in

MnO2

nanostructures.

The

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intercalation/extraction of proton or alkali cations (usually Li+, Na+ and K+) into the bulk of

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MnO2 particles with accompanying redox reaction of Mn ions is responsible for charge storage

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[34].

MnO2 + M+ + e-

(M+ = Li+, Na+, K+, or H3O+ )

MnOOM

At slow scan rates, almost all of the pores within the structure are accessible for electrolyte ions, leading to complete insertion/extraction reactions. However, at higher scan rates, the effective diffusion of ions into the electrode structure is reduced, thus resulting in a reduced specific capacitance [29, 32, 36, 37]. Herein, for the MnO2-ECF electrode, the diffusion of Na+ ions into the pores of nanostructured MnO2 was decreased at higher scan rates, thus the major part of capacitance is originated from the charging of outer surface of the electrode material. This infers

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that the interior of MnO2 nanostructures and deep pores of the electrode material does not

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actively contribute to the capacitance at higher scan rates.

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Electrochemical impedance spectroscopy (EIS) is an important and informative method to evaluate the electrochemical behavior of electrode material [4, 6, 38, 39]. Figure 6 shows the

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complex plane plot recorded for MnO2-ECF electrode in Na2SO4 solution at open circuit

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potential. In the low-frequency region, the impedance plot increases sharply and appears as

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nearly vertical lines (nearly 82°), revealing the characteristics of an ideal capacitor. At the high to medium frequency regions, a very small semicircle was observed indicating insignificant

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charge transfer resistance at the MnO2-ECF electrode-electrolyte interface. Such a low charge

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power sources.

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transfer resistance makes the MnO2-ECF electrode material suitable alternative for the high-rate

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3.2. Galvanostatic charge/discharge characteristics of MnO2-ECF electrode Figure 7 represents the galvanostatic charge/discharge curves for MnO2-ECF at a current density

of 5 A g-1. Charge/discharge curves are almost linear, indicating very good capacitive behavior of electrode material [40]. The linear dependency of charge/discharge curves (potential against

time) may be due to the dominant role of double layer charge storage mechanism in total specific capacitance. Thus, symmetrical charge/discharge curves are expected for MnO2-ECF supercapacitor. In the present work, the coulombic efficiency of ECF electrode is more than 86% using the following equation, which is another characteristics of an ideal capacitor [2]:

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where td and tc are discharge and charge times, respectively.

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The cycle life stability is another important factor affecting the consideration of a material for practical applications. The cycle life stability of MnO2-ECF electrode was examined by

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galvanostatic charge/discharge experiment at a current density of 20 A g-1. Figure 8 illustrates

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the relationship between capacitance retention and successive charge/discharge cycles for the

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MnO2-ECF electrode. As seen in Figure 8, specific capacitance decreased slightly during 1000

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cycles. A small decrease in specific capacitance after 1000 successive cycles of charge/discharge

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possibly originates from MnO2 flaking off. Nevertheless, MnO2-ECF electrode has greatly

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retained more than 95 % of its initial specific capacitance after 1000 charge/discharge cycles,

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even at a high charge/discharge current of 20 A g -1. This good cycle life stability can be attributed to apt contact between MnO2 nanostructures and ECF substrate. The charge/discharge

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characteristics of MnO2-ECF electrode suggest it as a good candidate for high-performance

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supercapacitor purposes. 4. Conclusions In conclusion, a facile, low cost and additive/template free method was introduced to fabricate electrode materials for high-rate electrochemical capacitors. Electrochemical deposition was

used to fabricate manganese dioxide nanostructures on electro-etched carbon fiber. MnO2-ECF electrode with shortened electron and ion transport pathway showed outstanding supercapacitive behavior even at high scan rates in addition to good cycle life stability. These characteristics may be originated from pertinent contact between MnO2 nanostructures and ECF substrate. The charge/discharge characteristics of MnO2-ECF electrode suggest it as a good candidate for

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fabrication of high-rate charge/discharge supercapacitors. Acknowledgement:

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Financial supports of the work by Institute for Advanced Studies in Basic Sciences and Iranian

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National Sciences Foundations (INSF-Grant number of 90003039) are acknowledged. Authors

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are grateful to Dr Ehsan Salamifar and Anita Zaitouna from the University of Nebraska, Lincoln

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for English editing the manuscript.

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Figure caption

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Figure 1. SEM images of a) and b) MnO2-ECF at two different magnification, and c) EDX

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analysis of MnO2-ECF

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Figure 2. X-ray diffraction pattern recorded for the nano MnO2-ECF

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Figure 3. CV curves recorded for CF before (A) and after (B) electro-etching (ECF), and MnO2-

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ECF (C) at a scan rate of 25 mV s-1 in Na2SO4 solution.

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solution.

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Figure 4. Cyclic voltammograms of MnO2-ECF electrode at various scan rates in Na2SO4

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Figure 5. Specific capacitance of MnO2-ECF electrode against the scan rate Figure 6. Typical Nyquist plot recorded for MnO2-ECF electrode in Na2SO4 solution at open circuit potential

Figure 7. Galvanostatic charge/discharge curve of MnO2-ECF electrode at a current density of 5 A g-1 Figure 8. Long-life cycle stability of MnO2-ECF electrode during successive charge/discharge

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