acst

echn

Surfactant

en cicf anof Mtiondisct of

s andstudy

(ECs) ame altigh poquipmer sou

active material [9,10].The most widely used active electrode materials are carbon

(EDLCs) [1113], conducting polymers [14,15] and both nobleand transition- metal oxides (PC) [1,2,7,9,1619]. Until now, oneof the best electrode materials for electrochemical supercapacitoris ruthenium oxide [16,20,21]. However, it has the inherent disad-vantage of being both expensive and toxic, which has limited itscommercial use. Recently, the natural abundance and low cost of

mechanism of electrochemical processes [1,23]. These studies havebeen carried out using an inert electrode such as Pt. But Pt-groupmetals are highly expensive, for cost-effective applications Ni orStainless steel can be used. However, it was found that Ni under-goes corrosion and also oxidation in acidic electrolytes.

The aim of the present study is to electrochemically depositMnO2 in presence of suitable surfactant and to evaluate theelectrode for capacitor properties. Accordingly, MnO2 electrode-posited in presence of sodium lauryl sulphate (SLS) has shown thatthe specic capacitance, 314 F g1 against 252 F g1 obtained forMnO2 deposited in the absence of SLS.

Mobile: +91 974 3293223; fax: +91 824 2474033.

Journal of Electroanalytical Chemistry 690 (2013) 1318

Contents lists available at

n

lseE-mail address: suni@yahoo.co.inory, and purication of salt water [16].The supercapacitors have high power and energy density, very

long cycle life, short charge time, high safety and high efciency[5,7,8]. Based upon their charge-storage mechanisms, supercapaci-tors can be classied into two types of capacitors: (i) the non-faradicelectric double layer capacitors (EDLCs), in which the capacitancearises from the charge separation at an electrodeelectrolyte inter-face and (ii) the faradic pseudo-capacitors (PC), where the capaci-tance arises from the redox reactions of the electrochemically

manganese oxides, and thereby their pseudo-capacitive perfor-mance [22].

Generally, the physical and morphological properties of electro-deposited materials depend on the experimental conditions usedfor their preparation, which include the presence of foreign mole-cules in the electrolyte. A surface-active molecule possesses a polargroup attached to the end of a long hydrophobic tail. The adsorp-tion of these molecules at the electrode/electrolyte interface inu-ences the properties of the double layer and also the kinetics and1. Introduction

Growing environmental concernfuels have created great interest insources. Electrochemical capacitorspacitors or ultracapacitors, have becosystems for applications involving hshort duration such as camera ash eators, re/smoke alarms, backup pow1572-6657/$ - see front matter 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jelechem.2012.11.040fast depletion of fossiling alternative energylso known as superca-ernative energy storagewer requirements for aent, pulsed light gener-rce for computer mem-

manganese oxide, accompanied by its satisfactory capacitivebehaviour in mild electrolytes and environmental compatibility,have made it one of the most promising electrode materials of sup-ercapacitors. The preparation methods of manganese oxide forsupercapacitor applications include thermal decomposition, co-precipitation, solgel processes, physical vapour deposition, hydro-thermal synthesis and anodic/cathodic deposition. It has been con-rmed that the preparation methods and/or condition couldsignicantly affect the material characteristics of the obtainedCyclic voltammetryCharge/dischargeEffect of deposition method and the surfof electrochemically deposited MnO2 on

Suhasini Electrochemistry Research Laboratory, Department of Chemistry, National Institute of T

a r t i c l e i n f o

Article history:Received 17 July 2012Received in revised form 21 November 2012Accepted 26 November 2012Available online 14 December 2012

Keywords:SupercapacitorManganese dioxide

a b s t r a c t

Manganese dioxide has beorder to obtain a high spection of MnSO4 consisting oThe electrodeposited lmsgreater surface area in relaand galvanostatic chargeabout 22% due to the effec

Journal of Electroa

journal homepage: www.ell rights reserved.tant on high capacitanceainless steel substrate

ology Karnataka, Surathkal, Srinivasnagar 575 025, India

onsidered as a promising material for electrochemical supercapacitors. Incapacitance, MnO2 has been electrodeposited from an aqueous acidic solu-ionic surfactant, namely, sodium lauryl sulphate (SLS) on stainless steel.nO2 in the presence of the surfactant possess greater porosity and henceto the lms prepared in the absence of the surfactant. Cyclic voltammetryharge cycling experiments reveal that specic capacitance is higher bySLS.

2013 Elsevier B.V. All rights reserved.

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using several concentrations of SLS up to 5 g l1 in the electrolyte.

corded using JEOL JSM-6380LA Scanning Electron Microscopy

curves were used for the calculation of Cs (F) from the followingequation:

Cs I=dV=dtm 3where dV/dt (V/s) is the slope of the linear discharge curve, and I(mA cm2) is the current density.

3. Results and discussion

3.1. Electrodeposition of MnO2

From the kinetic study by cyclic voltammetric method, based onthe results of analysis of original data [24], the accepted, more sub-stantiated scheme of the mechanism of Mn2+ ions electrooxidationin the acidic medium with subsequent disproportionation of theproduct of irreversible electrode reaction and hydrolysis, is asfollows:

2Mn2 2e! 2Mn3 4

ytical Chemistry 690 (2013) 1318(SEM). Infrared spectra were recorded with a model 330 NicoletAVATAR FT-IR spectrophotometer. Thermo gravimetric analysis(TGA) and differential thermal analysis (DTA) were recorded inthe temperature range from ambient to 900 C in air at a heatingrate of 10 C per min using PerkinElmer thermal analyzer modelEXSTAR-6000 TG/DTA.

2.3. Electrochemical characterization

The electrochemical measurements including cyclic voltamme-try (CV), charge/discharge and electrochemical impedance spec-troscopy (EIS) were measured by a VersaSTAT3 Potentiostat/Galvanostat (Princeton Applied Research) in an electrochemicalcell. For MnO2 electrochemical supercapacitors, the electrochemi-cal cell contained samples on a SS as working electrode, a platinumfoil as counter electrode and a SCE as reference electrode, and0.1 M Na2SO4 solution as the electrolyte. The discharge capacity/capacitance of the MnO2 in the positive electrode was based onthe amount of active material (MnO2) excluding the weight ofthe SS.

Cs Q=mDV 2The specic capacitance (Cs, in Farad, F) was calculated using

half the integrated area of the CV curves to obtain the charge (Q,in Coulomb, C), and subsequently dividing the charge by the massof the deposit (m, in grams, g) and the width of the potential win-As the effective concentration of SLS was found to be 5 g l1, amajority of the studies were carried out using this concentration.The deposition of MnO2 on SS was carried out by galvanostatic,potentiostatic, potentiodynamic and cyclic voltammetric tech-niques. Subsequent to the deposition, the electrode was separatedfrom the cell, rinsed with distilled water, dried and weighed.

2.2. Instrumental analysis

A Mettler Toledo balance of model AL54 CEN315 with 0.1 mgsensitivity was used for weighing the electrodes. The powder X-ray diffraction (XRD) patterns of the samples were recorded usingJEOL JDX-8P diffractometer using Cu Ka (k = 1.5418 ) as a source.The surface morphology of electrodeposited MnO2 images was re-2. Experimental

2.1. Nanostructured MnO2 electrodeposition

The amorphous MnO2 nanostructured thin lms were electro-deposited under room temperature with three electrode system.Prior to electrodeposition, the stainless steel (SS, grade 304) work-ing electrode, was polished using polishing wheel, degreased withtrichloro ethylene, washed with distilled water and then dried un-der an air stream. A saturated calomel electrode (SCE) was used asreference electrode and SS strip as counter electrode. The electro-lyte was aqueous acidic 1 M MnSO4 prepared using distilled water.The electrochemical reaction of the manganese oxide formationoccurs as in the following equation [24]:

Mn2 2H2O 2e!MnO2 4H 1For the deposition of MnO2 in presence of SLS, required amount

of SLS was added followed by a thorough stirring of the electrolytetill a clear solution was obtained. Experiments were carried out

14 Suhasini / Journal of Electroanaldow (DV, in Volts, V), using Eq. (2)Galvanostatic charge/discharge cycling was carried out at con-

stant current density 1 mA cm2. Obtained charge/discharge2Mn3 !Mn2 Mn4 5Mn4 2H2O!MnO2 4H 6

Mn2 2H2O 2e!MnO2 4H

In the acidic medium, the last stage, namely, the formation ofmanganese dioxide is the slowest stage. MnO2 thus formed usuallydeposits as a lm on the anode [1,23,30,31]. A large variety of stud-ies on electrodeposition of MnO2 from acidic electrolytes has beenreported [9,29]. These studies have been carried out using an inertelectrode such as Pt. However it is imperative to employ an inex-pensive substrate instead of an expensive Pt group metal for acost-effective application. For this reason, in the present study, SSwas used as the substrate for the deposition of manganese oxide.Ni can be also used as substrate [23]. The advantage of using SSover Ni substrate is that, in acidic medium Ni undergoes oxidationand hence undergoes corrosion. But there is no much difference inthe specic capacitance value, for MnO2 on Ni substrate as re-ported gives 246.8 F g1 [23] and on SS gives 252 F g1.

Cyclic voltammograms recorded during preparation of MnO2and MnO2(s) on SS are shown in Fig. 1. Although the voltammo-grams in the absence (curve i) and in the presence of SLS (curveii) are similar in shape, the current is higher in the latter case.The anodic peaks at about 1.281.32 V correspond to the oxidationof Mn2+ to MnO2 and the cathodic peaks at about 0.80.6 VFig. 1. Cyclic voltammograms for deposition of (i) MnO2 and (ii) MnO2(s) on SS in1 M MnSO4 solution at potential scan rate of 50 mV s1.

corresponding to the reduction of MnO2 to MnOOH. The effect ofSLS in the electrolyte is clearly reected in the voltammograms.

The kinetics of oxidation of Mn2+ to MnO2 depends on the elec-trochemical method, namely, galvanostatic, potentiostatic, poten-tiodynamic and cyclic voltammetric method employed; it wasintended to study the inuence of these techniques on the capaci-tance of MnO2 and MnO2(s). Several electrodes were prepared byvarying current density (galvanostatic current density, cd = 18mA cm2), potential (potentiostatic, potential = 1.11.5 V), sweeprate (potentiodynamic, m = 120 mV s1 and cyclic voltammetric,m = 5400 mV s1) and their capacitances were evaluated usingcyclic voltammetry and galvanostatic charge/discharge cycling asdescribed later. From these experiments, it was clear from theTable 1 that, the MnO2 and MnO2(s) prepared by the cyclic voltam-metric method in the potential range between 0 and 1.6 V at a

1

acidic electrolytes, which is generally called the EMD, has eitherc structure or amorphous in nature [23,25]. The X-ray diffractionstudies reveal that, the MnO2 prepared by different electrochemi-cal methods namely, galvanostatic, potentiostatic, potentiodynam-ic and cyclic voltammetric method were found to be amorphous innature. In order to examine the effect of the surfactant in the solu-tion, the XRD patterns of the bare SS, MnO2 and MnO2(s) were re-corded (which are not shown here). The data suggest that theMnO2 andMnO2(s) both are amorphous in nature. From SEMmicro-graphs (provides morphological effect of surfactant on surface)Fig. 2, it is seen that the surface of MnO2(s) (Fig. 2b) appears rough,suggesting high surface area than the smoother surface of MnO2(Fig. 2a) surface obtained by cyclic voltammetric method. Similarlyfor surfaces obtained by galvanostatic method as in gure Fig. 2c(MnO2) and Fig. 2d (MnO2(s)). The important aspect seen in theseimages is the cracked mud appearance of the lms as clearlyobserved in Fig. 2a and b. The cracking is the important factor,which enhances the cycle lives by facilitating the expansion/con-traction process [26].

Fig. 3 shows the IR spectra of the obtained deposit. The absorp-tion band at 437.7 cm1 can be assigned to the mMnO vibrations,those in the 1074 cm1 vibration of hydrated water moleculesand 1616 cm1 range to the bending vibrations of the adsorbedwater molecules. According to the literature, the absorption bandstrongly depends on the particle size and polymorphism of the me-

Table 1Effect of deposition method on specic capacitance.

Method of deposition Cs of MnO2 (F g1) Cs of MnO2(s) (F g1)

Galvanostatic 175.4 202.0Potentiostatic 137.0 151.1Potentiodynamic 160.9 179.9Cyclic voltammetric 252.0 314.0

Suhasini / Journal of Electroanalytical Chemistry 690 (2013) 1318 15sweep rate 50 mV s gave maximum capacitance values. The rea-son could be the partial reduction of MnO2 [or MnO2(s)], whichstarts occurring at about 1.1 V during cathodic half-cycle. Duringrepeated cycling, the partial reduction and then further oxidationor deposition of the fresh oxide layer on the previous layer couldhave resulted in a high porosity of the oxides. Accordingly, theelectrodes prepared using the cyclic voltammetric method be-tween 1 and 1.6 V at 50 mV s1 were employed for the rest ofthe studies.

3.2. Microstructure of MnO2 lm

It has been known that MnO2 exists in several crystallographicforms, and an oxide prepared by electrochemical oxidation inFig. 2. SEM micrographs of the: (a) MnO2 and (b) MnO2(s) electrode deposited by cyctal oxide. As particle size decreases, bands shift towards higher fre-quency region [27]. TGA and DTA thermograms of amorphousMnO2 are shown in Fig. 4i and ii. The TGA curve shows that thelarge and quick weight loss up to 100 C owing to desorption ofphysically adsorbed water, this may lead to the cracking of theelectrode surface and thereafter the weight loss in the temperaturerange of 100500 C is due to dehydration of crystalline water,with very broad exothermic peaks shown in the DTA curve. At500 C the amorphous MnO2 transformed into crystalline Mn2O3[9] and another 3% weight loss at 750 C corresponds to phase tran-sition from Mn2O3 to Mn3O4. In the present case, the weight lossdue to adsorbed water was 20% as compared to 39% hydrationof MnO2. In the present case, the deposited manganese oxide canlic voltammetric method and (c) MnO2 and (d) MnO2(s) by galvanostatic method.

ytical Chemistry 690 (2013) 131816 Suhasini / Journal of Electroanalbe expressed as the following chemical formula: MnO2H2O. AlsoChun et al. reported that the hydrated MnO2 will possess greaterpseudocapacitance than dehydrated MnO2 [28].

3.3. Capacitive characterization of the MnO2 lm

The charge storage mechanism in MnO2, as has been reported[17], is due to a redox reaction, which involves insertion of cationsfrom the electrolytes into MnO2 lattice. The kinetics of reversibleinsertion/extraction of the cation varies with the electrolytes; asolution of 0.1 M Na2SO4 was the better electrolyte for all charac-terisation studies [23]. MnO2(s) electrodes prepared from 1 MMnSO4.H2O + 0.15 M H2SO4 + 5 g l1 SLS electrolyte showed maxi-mum capacitance and was used for characterization studies. Theresults of cyclic voltammetric studies were also conrmed bygalvanostatic charge/discharge studies.

Cyclic voltammograms of MnO2 and MnO2(s) electrodes and alsobare SS electrode in 0.1 M Na2SO4 are as shown in Fig. 5a, forcomparison. There is no signicant charge density on the bare SS

Fig. 3. FTIR spectra of MnO2(s) deposited by cyclic voltammetric method.

Fig. 4. TG/DTA curves of MnO2(s) deposited by cyclic voltammetric method heatedat the rate of 10 C min1.

Fig. 5. (a) CV curves of the: (ii) MnO2, (iii) MnO2(s) and (i) SS in 0.1 M Na2SO4solution at 10 mV s1 scan rate, (b) Chargedischarge curves of the (i) MnO2, (ii)MnO2(s) at 0.5 mA cm2.

electrode, thus suggesting that the contribution of the substrate tothe measured capacitance of MnO2 or MnO2(s) is negligibly small.Cyclic voltammograms of MnO2 and MnO2(s) are nearly rectangularin shape. Rectangular shape of the voltammogram is a ngerprintfor capacitive behaviour [8]. In addition to existence of doublelayer capacitance, MnO2 and MnO2(s) possess pseudocapacitancedue to Mn4+/Mn3+ reversible redox reaction. This process isaccompanied by reversible insertion/de-insertion of alkali cation(Na+) or protons (H3O+) present in the electrolyte [1]. The speciccapacitance (Cs) for MnO2 and MnO2(s) are 252 and 314 F g1

respectively, at the scan rate of 5 mV s1. Thus there is an increaseof Cs by about 22% for MnO2(s) in comparison with MnO2.

The electrodes were subjected to galvanostatic charge/dis-charge cycling at current density of 1 mA cm2 is as shown inFig. 5b. The linear variation of potential during both charging anddischarging process is another criterion for capacitance behaviourof material in addition to exhibiting rectangular voltammograms[8]. There is a linear variation of the potential during charge anddischarge regions for both MnO2 and MnO2(s). The Cs valuesobtained from the discharge data for MnO2 and MnO2(s) are 201.0and 259.0 F g1 respectively.

Both MnO2 and MnO2(s) electrodes were subjected to an ex-tended cycle-life test with CV at 25 mV s1 scan rate and the cyclicvoltammogram of rst 10 cycles are as shown in Fig. 6a. It is seenthat the specic capacitance of MnO2 is 202.5 F g1 during theinitial stages of cycling. There is a decrease in capacitance on

cycling and for MnO2(s), a Cs of 261.0 F g1 is obtained during theinitial cycles, it decreases to 242.3 F g1 at 1500th cycle. The vari-ation of Cs with cycle number is as shown in Fig. 6b.

Nyquist plots of ac impedance spectra of MnO2 prepared by var-ious deposition methods are presented in Fig. 7a and b. The Ny-quist plot consists of a high-frequency intercept on the real axiscorresponding to the electrolyte resistance (RX), a semicircle corre-sponding to a parallel combination of charge-transfer resistance(Rct) and double-layer capacitance (Cdl), nally a linear region atlow frequency range. In general, the low frequency linear plot ofimpedance should make 45 to the real axis if the process is underdiffusion control in the low-frequency region or it should make 90if the behaviour of the system is purely capacitive in nature [1].However linear portion of the plot exhibits an angle between 45and 90. The impedance data were analyzed using the electricalequivalent circuit shown in Fig. 7b (iii). The circuit elements are ex-plained as follows: RX represents the electrolyte resistance, Rct ischarge-transfer resistance, Q1 is constant phase element (CPE) usedin place of double-layer capacitance (Cdl), W is Warburg arisingfrom a diffusion controlled process at low-frequency and Q2 isCPE used in place of pseudo-capacitance of the material. Q1 wasused instead of Cdl because the semicircle is depressed and Q2was used because the low-frequency behaviour is not ideal capac-itive. The non-ideal nature of capacitances arises due to non-homogeneous nature of the electrode. The impedance data were

Suhasini / Journal of Electroanalytical Chemistry 690 (2013) 1318 17Fig. 6. (a) CV for MnO2(s) at 25 mV s1 in 0.1 M Na2SO4 for cycle life, (b) Cycle lifedata for (i) MnO2 and (ii) MnO2(s) at 25 mV s1 scan rate in 0.1 M Na2SO4.Fig. 7. (a) Nyquist plots of MnO2 prepared by: (i) galvanostatic (ii) potentiostatic(iii) potentiodynamic and (iv) cyclic voltammetric method. (b) Electrical equivalentcircuit for MnO2(s) deposited by cyclic voltammetric method.

subjected to ZSimpWin tting program and the impedance param-eters were obtained. It was clear from the Table 2 that, there iseffect of deposition method on Rct value.

4. Conclusion

In order to improve the specic capacitance, amorphous MnO2has been electrodeposited from an acidic aqueous solution consist-ing of SLS as the surfactant. The electrodeposited lms of MnO2 inthe presence of the surfactant possess greater porosity, and hencesurface area, in relation to the lms prepared in the absence of thesurfactant. From TG/DTA analysis it is conformed that there is for-mation of hydrated MnO2. Cyclic voltammetry and galvanostaticchargedischarge cycling experiments reveal that the Cs is higherby about 22% due to the effect of SLS. It has been found that5 g l1 SLS in 1 M MnSO4H2O solution is the optimum concentra-tion for obtaining maximum Cs.

[7] Y. Zhao, Y.Y. Wang, Q.Y. Lai, L.M. Chen, Y.J. Hao, X.Y. Ji, Synth. Met. 159 (2009)331337.

[8] B.E. Conway, Electrochemical Supercapacitors, Kluwer, New York, 1999.[9] Takuya Shinomiya, Vinay Gypta, Norio Miura, Electrochim. Acta 51 (2006)

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(2002) 493498.[15] S.K. Tripathi, Ashok Kumar, S.A. Hashmi, Solid State Ionics 177 (2006) 2979

2985.[16] Yong-Qing Zhao, Guo-Qing Zhang, Hu-Lin Li, Solid State Ionics 177 (2006)

13351339.[17] Mathieu Toupin, Thierry Brousse, Daniel Blanger, Chem. Mater. 16 (2004)

31843190.[18] Qinghua Huang, Xianyou Wang, Jun Li, Electrochim. Acta 52 (2006) 1758

1762.[19] N. Nagarajan, I. Zhitomirsky, J. Appl. Electrochem. 36 (2006) 13991405.[20] Bong-Ok Park, C.D. Lokhande, Hyung-Sang Park, Kwang-Deog Jung, Oh-Shim

Joo, J. Power Sources 134 (2004) 148152.[21] V.D. Patake, C.D. Lokhande, Oh-Shim Joo, Appl. Surf. Sci. 255 (2009) 4192

Table 2Effect of deposition method on Rct.

Method Galvanostatic Potentiostatic Potentiodymic Cyclic voltammetric Cyclic voltammetric (with SLS)

Rct (X) 105.5 660.4 160.9 50.33 77.65

18 Suhasini / Journal of Electroanalytical Chemistry 690 (2013) 1318Acknowledgement

Suhasini is grateful to NITK, Surathkal, for providing the Insti-tute Fellowship that allowed carrying out this work.

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Effect of deposition method and the surfactant on high capacitance of electrochemically deposited MnO2 on stainless steel substrate1 Introduction2 Experimental2.1 Nanostructured MnO2 electrodeposition2.2 Instrumental analysis2.3 Electrochemical characterization

3 Results and discussion3.1 Electrodeposition of MnO23.2 Microstructure of MnO2 film3.3 Capacitive characterization of the MnO2 film

4 ConclusionAcknowledgementReferences