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Solid State Sciences 13 (2011) 1978e1983

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Solid State Sciences

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Poly(3,4-ethylenedioxithiophene)/MnO2 composite electrodes for electrochemicalcapacitors

Emerson C. Riosa, Alessandra A. Correab, Fernando H. Cristovanc, Leandro A. Pocrifkaa,Adriane V. Rosarioa,*a Laboratório Interdisciplinar de Eletroquímica e Cerâmica, Departamento de Química, Universidade Federal de São Carlos, C. P. 676, 13565-905 São Carlos, SP, BrazilbCentro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC, R. Santa Adélia, 166, CEP 09210-170 Santo André, SP, Brazilc Instituto de Ciência e Tecnologia, Universidade Federal de São Paulo, R. Talim, 330, CEP 12231-280 São José dos Campos, SP, Brazil

a r t i c l e i n f o

Article history:Received 22 October 2010Received in revised form27 April 2011Accepted 24 August 2011Available online 7 September 2011

Keywords:Electrochemical capacitorsPseudocapacitanceManganese oxidePoly(3,4-ethylenedioxithiophene)Composite electrodes

* Corresponding author. Tel./fax: þ55 16 33 61 521E-mail address: vr.adriane@gmail.com (A.V. Rosar

1293-2558/$ e see front matter � 2011 Elsevier Masdoi:10.1016/j.solidstatesciences.2011.08.028

a b s t r a c t

Composite electrodes of poly(3,4-ethylenedioxithiophene) and manganese oxide (PEDOT/MnO2) havebeen prepared by electrodeposition of manganese oxide over PEDOT-modified titanium substrate. ThePEDOT layers are deposited on titanium by potentiostatic deposition at 1.4 V and at two differenttemperatures: 5 and 25 �C (named PEDOT(5) and PEDOT(25), respectively). The electrodes are charac-terized by field emission gun scanning electron microscopy (FEG-SEM) and their electrochemicalperformances are evaluated by using cyclic voltammetry (CV) in 1 molL�1 Na2SO4. The results show animprovement in the specific capacitance (Cs) of the oxide due to the presence of the polymer layer.Considering only the MnO2 mass, the Cs values of the electrodes Ti/MnO2, Ti/PEDOT(5)/MnO2 and Ti/PEDOT(25)/MnO2, estimated by the CV technique, are 151, 159 and 199 Fg�1 at 10 mVs�1 respectively. Themicrographies of electrodes show that the polymer layer leads to very significant changes in themorphology of the oxide layers, which in turn generates the improvement observed in the capacitiveproperty.

� 2011 Elsevier Masson SAS. All rights reserved.

1. Introduction

The interest in electrochemical capacitors has been stimulatedby their potential application as electrical storage devices, mainlyfor operation in hybrid systems for electric vehicles in parallel withbatteries [1,2]. The electrochemical capacitor could present highpower density to low energy density and fill the gap betweenconventional capacitors and batteries [3]. Electrochemical capaci-tors can be classified, according to the storage charge mechanism,as supercapacitors, when the capacitance occurs due to the double-layer electric charged or as pseudocapacitors, if the capacitanceoriginates in a charge-transfer faradaic process that takes place atthe electrode materials, producing so-called pseudocapacitance[3,4]. Conductor polymers, such as polyanilines [5,6], poly-thiophenes [7] and transition metal oxides, such as RuO2 [8e13],IrO2 [14], Co3O4 [15] and MnO2 [16e21], can be used as electrodematerials for electrochemical capacitors.

RuO2 is the material that presents the highest specific capaci-tance values (720 Fg�1) [12,13]. This is expensive, however, so

5.io).

son SAS. All rights reserved.

a practical application is, in part, unfavorable. By contrast, takinginto consideration the low cost, natural abundance and environ-mental safety, actually, the research and development of alternativeelectrode materials for electrochemical capacitors has focussed onMnO2. MnO2 electrodes have been prepared by different methods[16e21]. Using a factorial fractional design, Hu and Tsou [18]optimized the conditions of preparation for hydrous and amor-phous MnO2 deposited onto graphite substrate from MnSO4 solu-tion by anodic deposition. A specific capacitance of 220 Fg�1 wasobtained for an electrode produced at 3.7 mA, deposition charge of0.3 Ccm�2, in 0.16 molL�1 MnSO4 and pH 5.6. Prasad andMiura [21]deposited MnO2 on stainless steel by cyclic voltammetry in0.5 molL�1 H2SO4 and 0.5 molL�1 Na2SO4.5H2O at several scanrates. The highest specific capacitancewas of 482 Fg�1 in 0.1molL�1

Na2SO4 at 10 mVs�1 for an electrode deposited at 200 mVs�1.Conductor polymers are also promising materials for electro-

chemical capacitors. They possess a high electronic conductivitybecause of their conjugated bonds structure. The doping/undopingmechanisms and their ability to undergo bothn- andp- doping are themain advantages of this category of materials. Polythiophenes, inparticular, are interesting materials due to the ease of synthesis,reversibility and stability. Micaroni et al. [22,23] demonstrated a gran-ular morphology of high surface area to poly(3-methylthiophene)

Fig. 1. First cyclic voltammograms of the MnO2 deposition on electrodes: (_____) Ti,(...) Ti/PEDOT(5) and (———) Ti/PEDOT(25); 0.4 molL�1 MnSO4 at 25 �C andv ¼ 200 mVs�1.

Table 1Mass and specific capacitance of the electrodesa.

Electrodes Mass(mg)

Specificcapacitance(Fg�1)

PEDOT þ MnO2 Only MnO2 Csb Cs

c

Ti/MnO2 e 0.7 151 151Ti/PEDOT(5)/MnO2 2.7 2.3 136 159Ti/PEDOT(25)/MnO2 2.1 1.6 152 199

a Specific capacitance obtained by cyclic voltammograms at 10 mVs�1.b Based on total mass.c Based on MnO2 mass.

E.C. Rios et al. / Solid State Sciences 13 (2011) 1978e1983 1979

controlled by synthesis conditions. Despite exhibiting several suitablecharacteristics, reports of studies on the electrochemical capacitorproperties of PEDOT are rarely made [24,25]. Lisowska-Oleksiak et al.[25] have detailed the electrochemical characterization of PEDOT,doped with excess polystyrene-sulphonate (PSS). PEDOT-PSS filmswere electrochemically grown on gold electrodes and a capacitancevalue of 25 Fcm�3 obtained in a non-aqueous electrolytewas reported[26]. Assuming the density of the film to be equal to 1.45 gcm�3, thisvalue leads to a specific capacitance of about 17 Fg�1. These materialshave been used in their pure forms or with carbon and oxides inpseudocapacitive electrodes [24e30]. Conductive polymer/oxidecomposite electrodes have been used as electrochemical capacitors[31,32]. We [33] have recently studied the capacitive properties ofelectrodesbasedonMnO2preparedbyelectrodepositionofMnO2onTisubstrates modified with poly(3-methylthiophene). The specificcapacitance value was 218 Fg�1, at 20 mVs�1, for the sample preparedon poly(3-methylthiophene) synthesized with 250 mCcm�2, and thisvalue is three times higher if only themass of theMnO2 is considered.

In view of the effect of the substrate modification with poly(3-methylthiophene) on the capacitive properties of MnO2 layers,the purpose of the present work was to prepare PEDOT/MnO2

composite electrodes and to investigate the effect of the modifi-cation of the titanium substrate with conducting polymer depos-ited under different conditions. The choice of PEDOT was due to itsdistinct characteristics in comparing with other common con-ducting polymers, such as non-solubility in most of solvents, highconductivity and stability.

2. Experimental

2.1. Substrate previous treatment

Titanium plates with a working area of 1 cm2 and 99.7% purity(TiBrazil) were used as the substrate. The substrate was previouslytreated by sandblasting, and then chemically treated in hot 10% (w/w) oxalic acid solution for 15 min. Finally, the electrodes werewashed with Milli-Q water and dried at 100 �C.

2.2. Preparation of the PEDOT layers

PEDOT layers were synthesized by potentiostatic deposition ontitanium electrodes at 1.4 V (vs. Ag electrode) from a 0.1molL�13,4-ethylenedioxithiophene and 0.1 molL�1 LiClO4 in acetonitrilesolution. Synthesis was carried out at 5 �C (PEDOT(5)) and 25 �C(PEDOT(25)). Films were preparedwith the same deposition charges(600 mCcm�2). A one-compartment electrochemical cell and threeelectrodes were used. The Ti electrodes were used as workingelectrodes and a platinum plate and a silver wire were used ascounter electrode and pseudo-reference electrode respectively.After the PEDOT deposition, the Ti/PEDOT electrodes were washedwith acetonitrile and dried at 40 �C.

2.3. Preparation of MnO2 layers

The MnO2 was deposited on Ti and on Ti/PEDOT electrodes bycyclic voltammetry between the potential range of 0.0 and 1.5 V (vs.SCE), and the scan rate of 200 mVs�1. Three hundred cycles wereperformed in order to achieve the proper deposited thickness. A0.4 molL�1 MnSO4 solution was used as electrolyte. A platinumplate and a saturated calomel electrode (SCE) were used as counterelectrode and reference electrode respectively. Following thedeposition, these electrodes were washed in water and dried at40 �C. The mass of the electrodes was determined before and aftereach deposition to allow specific capacitance calculation.

2.4. Characterization of the electrodes

Electrochemical characterization of the deposited layers wasperformed in 1.0 molL�1 Na2SO4 using an Autolab potentiostat/galvanostat model PGSTAT 30. The Ti/MnO2 and Ti/PEDOT/MnO2were used as working electrodes, and a platinum plate and an SCEwere used as counter electrode and reference electrode respec-tively. Surface morphologies were examined using a Zeiss modelSupra 35 field emission gun scanning electron microscope (FEG-SEM). FEG-SEM images were analyzed using ImageJ� software.

3. Results and discussion

Fig. 1 shows the first cyclic voltammetry of MnO2 deposition onTi and Ti/PEDOT electrodes. A similar profile was observed for allthree electrodes. In the first potential cycle, the oxidation of MnSO4began around 0.7 V and the anodic current increases with theincreasing potential. In the reverse scan, a cathodic peak wasobserved at 0.30, 0.41 and 0.47 V toTi, Ti/PEDOT(5) and Ti/PEDOT(25)substrates respectively. An additional anodic peak at 0.5 V wasobserved in the samples deposited on the polymer layer. In theinitial cycles, the anodic and cathodic charges increased and for thesubsequent cycles the charges decreased and became virtuallystable until the last cycle. The MnO2 deposition charges on Ti andon Ti/PEDOT(5) and Ti/PEDOT(25) were different; however, the massof MnO2 deposited on Ti substrate was two times lower than the Ti/PEDOT(25) and three times lower than the Ti/PEDOT(5) (see Table 1).

E.C. Rios et al. / Solid State Sciences 13 (2011) 1978e19831980

In order to investigate the electrochemical performance of thecomposite electrodes, cyclic voltammetry measurements weretaken in 1.0 molL�1 Na2SO4 solution. Cyclic Voltammogramsrecorded for the Ti/MnO2 and the composite electrodes at 10mVs�1

are shown in Fig. 2. No redox peaks were observed and the shape ofthe curves is almost rectangular, which is characteristic of pseu-docapacitive oxides. The voltammetric profile of MnO2 depositedon PEDOT is the same as for the oxide deposited directly on Ti;however, the Ti/PEDOT(25)/MnO2 electrodes show higher currentdensities in comparison to the other electrodes.

The specific capacitance (Cs) was determined using the cathodicor anodic charges integrated from the cyclic voltammogramsaccording to Eq. (1).

Cs ¼ QDE$m

(1)

where Cs is given in Fg�1, Q is the voltammetric charge (C), DE is thepotential window (V) and m is the mass of material (g).

The mass and specific capacitance values of the electrodes, at10 mVs�1, are shown in Table 1. The Cs of the composite electrodeswas estimated considering the total mass (PEDOT þMnO2) and theMnO2 mass alone. It is clear from this table that the Ti/PEDOT(25)/MnO2 has a storage charge capacity higher than the other elec-trodes. When the total mass is considered, the specific capacitance(C�

s ) values are practically the same as those of the Ti/PEDOT(25)substrate, and lower than those of the Ti/PEDOT(5) substrate incomparison to the oxide deposited on Ti. On the other hand, if onlythe MnO2 mass is considered, the specific capacitance (C��

s ) valuesare higher for the polymer-modifying electrodes than for the oxidedeposited on the Ti substrate. For the best composite electrode, thepresence of PEDOT leads to an increase of 32% on the Cs of theMnO2layer. According to Patra and Munichandraiah [24], when a rela-tively thicker film of the MnO2 is used as an electrode, the charge-transfer reaction kinetic is limited due to the poor electronicconductivity and the penetration of the electrolyte into the bulk ofthe oxide material is made it difficult; this is due to the densemorphology, which has been considered to be a significant cause ofsubstantial reduction of the specific capacitance of MnO2. Anextremely thin film of this material resulted in a higher Cs valuethan for the thicker film, as the bulk film does not take part in thecharge storage process; only the film surface takes part in thisprocess.

Fig. 2. Cyclic voltammograms of the electrodes: (_____) Ti/MnO2, (...) Ti/PEDOT(5)/MnO2 and (———) Ti/PEDOT(25)/MnO2; 1.0 molL�1 Na2SO4 at 25 �C and v ¼ 10 mVs�1.

Several papers have presented capacitance specific values in theorder of 400e1380 Fg�1 [16,21,34]. However, the capacitance of199 F/g obtained with the Ti/PEDOT(25)/MnO2 electrode is similar orbetter than other Cs values to MnO2 reported in the literature[18,19,35e37]. Hu and Tsou [18] reported an optimized Cs value of220 Fg�1 for MnOx.nH2O prepared by anodic deposition and char-acterized in 0.1 M Na2SO4. Broughton and Brett [19] investigatedthe effect of some variables on the electrochemical performance ofelectrodeposited MnO2. The authors observed some morphologicalchanges in function of the acetate ions concentration in the elec-trodeposition solution. Moreover, an important decrease in Cs from260 to 50 Fg�1 as material thickness increases was produced.Toupin et al. [38] obtained 166 Fg�1 in 0.1 M Na2SO4 at 2 mVs�1 forMnO2 synthesized by coprecipitation method. Recently, Lu et al.[39] reported a synthesis of polythiophene/MnO2 nanocompositewith submicron-sphere/nanosheet hierarchical structures, whichCs was of 257 Fg�1 in 1 M Na2SO4 at 10 mVs�1. This large range ofspecific capacitance values is a consequence of the complex nature,characteristic of nonestoichiometric oxides as MnO2. In fact, thecapacitive characteristics of MnO2 were found to be dependent ofmorphology, particle size, physisorbed and structural water,structural vacancies or change of the oxidation state of Mn ions[40,41]. High specific capacitances are normally obtained for elec-trodes prepared with very thin film or with the MnO2 dispersed incarbon powder to improve the electronic conductivity (MnO2 hasa poor conductivity: 10�5e10�6 Scm�1). This strategy favors therapid transport of electrolyte to the MnO2 dispersed phase,resulting in higher electrochemical performance.

Fig. 3 shows C��s as a function of the scan rate. As expected, the

specific capacitance decreases when the scan rate increases. Thisoccurs because the charge is dependent not only on the potential,but also on the scan rate, as the adsorption and/or insertion processis limited by the cation diffusion to active sites of the electrode. Thecapacitances of all samples decrease around 60%when the scan rateis increased from 10 to 100 mVs�1.

Despite the fact that PEDOT is also a capacitive material, its Csvalues are much smaller than those of the MnO2 deposits in MnSO4aqueous solution. The electrochemical characterization of thePEDOT layers deposited on titanium shows a lower currentresponse. It is known that the polythiophenes, in general, performbetter in non-aqueous solvents, such as acetonitrile [42]. Patra andMunichandraiah [24] have detailed electrochemical capacitorstudies of PEDOT on stainless steel electrodes in aqueous media. Cs

Fig. 3. Specific capacitance of MnO2 as a function of the scan rate.

Fig. 4. Cyclic voltammograms of the electrodes: (_____) Ti/PEDOT(5)/MnO2, and (...) Ti/PEDOT(5); 1.0 molL�1 Na2SO4 at 25 �C and v ¼ 10 mVs�1.

E.C. Rios et al. / Solid State Sciences 13 (2011) 1978e1983 1981

values higher than 250 Fg�1 in oxalic acid were obtained; however,a rapid decrease in Cs values was observed with the cycling. Honget al. [27] studied electrodesmade by deposition of RuOx on PEDOT;they concluded that both PEDOT and RuOx contributed to thehighest specific capacitance of the composite electrode incomparison to the RuOx electrode based on electrochemicalimpedance spectroscopy studies. Nevertheless, we believe that thepolymer does not have an important electrochemical contributionto the Cs of the composite electrode due to the fact that the polymerlayer is covered with a dense MnO2 layer and thus probably notaccessible to the solution. Moreover, as cited previously, the

Fig. 5. FEG-SEM images of the electrodes: a) and

pseudocapacitive behavior of PEDOT in aqueous electrolyte is notexpressive. As can see in comparison of CV curves between Ti/PEDOT and Ti/PEDOT/MnO2 electrodes (Fig. 4), the Ti/PEDOT(5)/MnO2 composite electrode exhibits much higher currentdensity than the Ti/PEDOT(5) electrode prepared on the samecondition. Similar difference was observed by Huang et al. [43] forRuO2.xH2O dispersed in PEDOT-PSS matrix. According to theauthors, the rough and porous features of the polymer increases theeffective active area of RuO2.xH2O. On the other hand, it is evidentthat the polymer layer modifies the substrate surface, as shown themorphological analysis.

The morphologies of the polymer layers are shown in Fig. 5. Thetemperature changes in the PEDOT synthesis leads to importantmorphologic differences. The surfaces of both PEDOT depositspresent granular agglomerates; however, the PEDOT prepared at5 �C has a tendency to form a fiber structure, while the PEDOTprepared at 25 �C has a more compact granular structure. More-over, the grain size of the PEDOT(25) is higher than that of thePEDOT(5). This variation on the morphology of the substrate hasproved to be a determining factor in the subsequent morphology oftheMnO2 deposited (as seen in Fig. 6), and, as a consequence, in thecapacitive properties of the composite electrodes. Fig. 6 shows thesurface morphologies of the MnO2 deposited on titanium and onthe polymer-modified titanium. Ti/MnO2, just as Ti/PEDOT(5)/MnO2and Ti/PEDOT(25)/MnO2, shows cracks, which are characteristic ofthis oxide category [19] (smaller amplification), and a fine granularstructure (higher amplification). The FEG-SEM images show thatthe Ti/PEDOT(25)/MnO2 electrode has more cracks distributed onthe surface, and that the islands are much smaller than those of theTi/PEDOT(5)/MnO2 electrode. The Table 2 list the island size andarea of cracks of the electrodes obtained of the FEG-SEM image(seen inset Fig. 6). The Ti/PEDOT(25)/MnO2 electrode present 2.7% ofcracks and an island area twelve times larger than Ti/PEDOT(5)/MnO2. It is known that the capacitance of oxides is directly related

c) Ti/PEDOT(5); and b) and d) Ti/PEDOT(25).

Fig. 6. FEG-SEM images of the electrodes: (a) and (b) Ti/MnO2; (c) and (d) Ti/PEDOT(5)/MnO2; and (e) and (f) Ti/PEDOT(25)/MnO2. Inset shows the FEG-SEM treated images of theelectrodes.

E.C. Rios et al. / Solid State Sciences 13 (2011) 1978e19831982

to the active area of the material; the sample with more cracksshould therefore have higher capacitance. On the other hand, theanalysis of the higher amplification images shows that the MnO2deposited directly on the Ti has an open network granular struc-ture. This fact also leads to more active area, and, as a consequence,higher capacitance; nevertheless, this is not observed. The finegranular structures of the oxides deposited on polymer-modified Tiare also varied. It appears that the Ti/PEDOT(25)/MnO2 electrode hasa more compact structure than the Ti/PEDOT(5)/MnO2 electrode,which suggests smaller capacitance values for the Ti/PEDOT(25)/MnO2 electrode; this, however, is not true. In this case, the highercapacitance values of the Ti/PEDOT(25)/MnO2 electrode should be

Table 2Island size and area of cracks of the electrodes.

Islands (cm2) Area of cracks (%)

Ti/MnO2 1431 1.8Ti/PEDOT(5)/MnO2 387 2.5Ti/PEDOT(25)/MnO2 4543 2.7 Fig. 7. Cycle life of the prepared Ti/PEDOT(25)/MnO2 composite electrode. The potential

cycling was performed in 0.1 molL�1 Na2SO4 at sweep rate of 10 mVs�1.

E.C. Rios et al. / Solid State Sciences 13 (2011) 1978e1983 1983

associated with the higher density of cracks in its surface, as theelectrolyte can penetrate cracks in contact with inferior layers ofthe composite, as seen in the comparison of Fig. 6 a, c and e.

The long-term stability of the Ti/PEDOT(25)/MnO2 compositeelectrode was investigated by the subjecting electrode to a largenumber of cyclic voltammetric scans and the variation of specificcapacitance over 600 cycles is depicted in Fig. 7. A gradual decreasein the specific capacitance value was observed for the first 100cycles but, Cs remains almost constant thereafter. About 92% of theinitial Cs is still kept after this 600-cycle test. This preliminary resultindicates the promising performance of this composite electrode inelectrochemical capacitors.

4. Conclusion

The PEDOT layer does not change the profile of the electrodepo-sition and electrochemical characterization of the MnO2. However,the results show that the composite electrodesdisplayhigher specificcapacitance values than theMnO2 deposited directly on the titaniumsubstrate. The electrochemical properties of theMnO2 are dependenton the substrate characteristics. The Cs values of the Ti/MnO2, Ti/PEDOT(5)/MnO2 and Ti/PEDOT(25)/MnO2, estimated by using the CVtechnique in 1.0 molL�1 Na2SO4 solution, were 151, 159 and 199 Fg�1

at 10 mVs�1 respectively. The higher Cs value of the Ti/PEDOT(25)/MnO2 electrode in comparison to the other electrodes is believed tobe due to the higher active area. These results demonstrate thepossibility ofmaximizing the electrochemical properties of theMnO2by using a polymer-modified electrode.

Acknowledgments

The authors would like to thank FAPESP, CNPq and CAPES forfinancial support.

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