Vertically Aligned Carbon Nanotubes/Carbon Fiber Composites for Electrochemical Applications.

Eduardo Saito1, a, Vagner Eduardo Caetano1,b,Erica Freire Antunes1,c, Anderson Oliveira Lobo 2,dFernanda Roberto Marciano2,e,

Vladimir Jesus Trava-Airoldi1,f, Evaldo José Corat1,g 1Associated Laboratory of Sensors and Materials of the National Institute for Space Research, Av.

dos Astronautas 1758, São José dos Campos, 12227-010, SP, Brazil. 2Laboratory of Biomedical Nanotechnology / Institute of Research and Development at the

University of Vale do Paraíba, Av. Shishima Hifumi, 2911, São José dos Campos, SP, Brazil aesaito135@gmail.com, beps.eduardo@yahoo.com.br,

cericafa2009@hotmail.com,daolobo@univap.br, efrmarciano@univap.br, fvladimir@las.inpe.br,gcorat@las.inpe.br.

Keywords:Vertically Aligned Carbon nanotubes, Double layer supercapacitor, energy storage

Abstract: Carbon fibers have been studied for electrochemical applications. Recently, carbon

nanotubes present a wide potential uses in electric, mechanic, electrochemical and materials science

field. At present study, vertically aligned carbon nanotubes were produced over carbon fibers. The

process occurs catalytically by chemical vapor deposition (CVD) using mixture with camphor and

ferrocene. After that, the VACNT/CF composite are treated by oxygen plasma for oxygen

functionalization. Prior the electrochemical analysis, CNT/Carbon fibers are treated by hydrochloric

acid to remove residual catalyst. The electrodes were tested in a usual electrolyte (with H2SO4

0.5M) in a conventional electrochemical cell. The specific capacitance was tested in a separate

device. The configuration of carbon fibers and VACNT presents a high potential application for

electro analytical application and energy storage.

Introduction

Carbon fibers are a relevant material with a widespread structural application in several

industrial areas as aeronautics, prosthesis, etc.[1] Carbonaceous materials presents also a plethora of

uses in energy storage, water treatment [2,3,4,5],clean energy generation[6,7,8,9,10],

electrochemical detection[11], etc. The integration of nanotubes and carbon fibers is a promising

study with numerous applications e.g. health monitoring [12]. Vertically aligned carbon nanotubes

(VACNT) were grown on carbon fibers (CF) using a tubular furnace [13]. Previous the growth, the

CF array was recovered by a silicon interface fibers to avoid the catalyst diffusion into the CF [14].

The VACNT films were grown in a N2 atmosphere using canphor and ferrocene (84/16% wt) as a

carbon source and a catalyst, respectively. The as grown VACNT presents a high surface energy

which results in a hydrophobic surface [15]. For electrochemical applications, the VACNT/CF

composite is treated by oxygen plasma treatment during 1hour for high surface functionalization

[16].

This paper reports the potential application of VACNT/CF composite for electrochemical energy

storage as well as for electro-analytical applications.

Experimental procedure

Sample preparation

The Carbon fibers (Texiglas CS200) were placed in a chamber (0.14Torr, −700 V) and treated by

Silane (SiH4) plasma to perform the deposition of amorphous Si on their surface to avoid the

catalyst diffusion into CFs [14-15]. The VACNT was grown in a tubular furnace at 850°C with N2

atmosphere during 30 minutes. The samples (CF+VACNT) were further treated by oxygen plasma

for oxygen functionalization during 1 hour [17].

Materials Science Forum Vol. 802 (2014) pp 192-196 Submitted: 01.10.2014Online available since 2014/Dec/31 at www.scientific.net Accepted: 06.10.2014© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.802.192

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 200.136.177.120-19/01/15,22:01:56)

Characterization

The as grown samples were characterized by Scanning Electron Microscopy (JEOL-model JSM

5310) andRaman spectroscopy (Renishaw micro-Raman model 2000 with Ar gas, λ = 514.5 nm,

averaged on three spectra). In order to evaluate the electrochemical performance of the composite,

the CNT powder samples were tested in a classical three electrode cell with 1M H2SO4 (as support

electrolyte).The measurements were taken at room temperature using the Ag/AgCl (3M) electrode

as reference and pure platinum wire as a counter electrode. The charge-discharge curves were

performed in a Teflon three electrode cell. The measurements were performed by a computer

controlled potentiostat (Autolab 302N).

Results and Discussion

Scanning electron microscopy

Figure 1 presents the SEM images of the as received Carbon fiber samples and Figure 1(b)

presents the VACNT growth over CFs. The CFs presents 8µm average diameter and the VACNT

presents a 200-250µm height before 30min of growth. The VACNT array morphology is a

consequence of the reactive (Canphor/ferrocene) flow through the furnace. From figure 1(b) is

possible confirm the high density of VACNT grown over Carbon fiber.

Fig. 1(a) – Carbon fiber cross section image. (b)Vertically aligned carbon nanotubes growth on

carbon fibers.

Raman Spectroscopy

The Raman spectra of as received Carbon fibers and VACNT grown on CF are presented in

Figures 1(a) and 1(b).

1000 1500 2000 2500 3000

arb

rita

ry u

nits

Raman Shift (cm-1)

CF(a)

1000 1500 2000 2500 3000

arb

rita

ry u

nits

Raman Shift (cm-1)

VACNT(b)

Fig. 2(a)Raman spectrum of as receiver Carbon fibers (before VACNT growth). (b)Raman

spectrum of VACNT grown on farbon fibers (sealed by conductive amorphous Si).

Materials Science Forum Vol. 802 193

Both spectra presents the tipical raman peaks from carbonaceous materials. These kind of materials

presents four specific signnals obtained with Ar Laser (514,5 nm):D (∼1352cm−1

), G (∼1582cm−1

),

D’(∼1600cm−1

) and G’(∼2700cm−1

).The G`band with relative high intensity compared with G band

can be attributed to ordenated nanographite or spaced interlayer separation between graphene planes

from turbostratic structure [15,16,17].

Electrochemical characterization

Cyclic Voltammetry

Figures 3 shows the voltammogram recorded in 0.5M H2SO4 aqueous electrolyte with a sweep

rate of 50mV.s-1

of the carbon fibers with and without the VACNT (same mass of both samples).

As presented, the current-voltage curve of composite presents high capacitive behavior between the

potentials of 0.0 -1.0Vvs Ag/AgCl(3M). The oxidation-reduction peaks at peaks at 0.53V and

0.34V vs. Ag/AgCl(3M), respectively is attributed to reactions of oxygen terminations of

functionalized CNT.

Fig. 3- Cyclic Voltammetry plots of the Carbon fiber with and without VACNT.

The functionalization of VACNT grown over carbon fibers allows the occurrence of potential

dependent faradaic reactions [18] described by equations (1),(2) and (3):

-COOH ↔ -COO + H+ + e

- (1)

R-C-OH ↔ R-C=O + H+ + e

- (2)

R-C=O + e- ↔ R-C-O

- (3)

Those faradaic reactions are responsible for increment of energy storage of functionalized

carbonaceous double layer capacitors and support the pseudocapacitive response of this material

[19].

Figure 4 shows the Cyclic voltammetry of functionalized VACNT/Carbon fibers composite in

different sweep rate As we can see,the voltammogram presents near ideal rectancular capacitive

shape. This response allied with increment of capacitance suggests the potential application of this

composite as electrochemical double layer capacitors.

194 Advanced Powder Technology IX

Fig. 4 Cyclic voltammetry of the CF/VACNT composite with different scans rate (25-150mV.s

-1).

Galvanostatic Charge-Discharge curve

The more practical experiment to evaluate the specific capacitance of composite is in a

galvanostatic charge-discharge measurement. The composite treated with plasma were inserted in a

Teflon three electrode cell with a cellulose separator and a miniaturized reference electrode [20]

between the poles. The system has two graphite rod as a current collector with negligible

capacitance (0.01F/g). Figure 5(a) shows the Teflon system photograph used in these

measurements.

In this system is possible to increase the mass of the sample and consequently enhance the

precision of specific capacitance. The composite loaded in this work were 0.01g. Previous all

measurement the composite were washed with HCl(conc) during 1 hour to remove residual catalyst in

composites. Figure 5(b) presents the galvanostatic charge-discharge curve obtained from CF/CNT

composite. The curve presents the pseudocapacitive response on charging-discharging plot

generated by redox surface reactions.

0 100 200 300 400 500

0.0

0.2

0.4

0.6

0.8

1.0

E(v

s.A

g/A

gC

l(3

M))

T ime (s)

0.65A/g(b)

Fig. 5 – (a)Teflon system used to evaluate the specific capacitance of plasma treated carbon

fiber/nanotubes composite. (b) Charge-Discharge plots of the samples.

The composite evaluated by this system presented the specific capacitance of 28.08F.g-1

measured at 0.65A.g-1

.Besides other works presents higher values of specific capacitance of carbon

nanotubes [20], we attribute this value as a consequence of the limited access of oxygen reactives

species created by plasma to inner volume of VACNT forest. This capacitance increment and

superhidropfobic surface also supports the electrochemical detection applications [21]

There is any negative impact of conductive silicon interface between VACNT and Carbon fibers

and this response allows to associate the electrical properties of this composite to others

application(e.g. structural).

Conclusions

In the present work, vertically aligned carbon nanotubes grown on carbon fibers and treated by

oxygen plasma were evaluated for electrochemical double layer capacitors. From Cyclic

voltammetry was possible confirm the capacitance increment of VACNT in comparison to Carbon

Materials Science Forum Vol. 802 195

fibers. The charge discharge curve performed quantified the specific capacitance of 28.08F.g-1

.The

limited access of reactive plasma to inner VACNT films explains this capacitance. There is any

negative impact of conductive silicon interface between VACNT and Carbon fibers. This response

allows to associate the electrical properties of this composite to others application.

Acknowledgments

The authors gratefully acknowledge the financial support and fellowships from CNPq [processo

160856/2011-6]) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP-Process

2011/17877-7and 2011/20345-7).

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