Microporous and Mesoporous Materials 186 (2014) 30–36

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials

journal homepage: www.elsevier .com/locate /micromeso

Supercapacitors based on freeze dried MnO2 embedded PEDOT:PSS hybrid sponges

1387-1811/$ - see front matter � 2013 Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.micromeso.2013.11.035

⇑ Corresponding author. Tel./fax: +91 484 2802020.E-mail address: avinash.balakrishnan@gmail.com (A. Balakrishnan).

R. Ranjusha, K.M. Sajesh, S. Roshny, V. Lakshmi, P. Anjali, T.S. Sonia, A. Sreekumaran Nair,K.R.V. Subramanian, Shantikumar V. Nair, K.P. Chennazhi, A. Balakrishnan ⇑Nanosolar Division, Amrita Centre for Nanosciences and Molecular Medicine, Ponekkara, Kochi, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 April 2013Received in revised form 23 October 2013Accepted 21 November 2013Available online 28 November 2013

Keywords:SupercapacitorBatteryFreeze dryingMesoporousMnO2

The present study investigates in detail the synthesis and characterization of PEDOT: PSS/MnO2 hybridsponge electrodes for supercapacitor/battery applications. These hybrid sponges were prepared usingfreeze drying technique and showed hierarchical pores ranging from micron to nanometric size. Scanningelectron microscopy-energy dispersive X-ray showed uniform dispersion of MnO2 along the PEDOT: PSSmatrix. Thermo gravimetric-differential thermal analysis showed higher thermal stability for thesehybrid constructs compared to PEDOT: PSS sponges. From the electrochemical studies, an intrinsic cor-relation between overall specific capacitance, morphology and weight percentage of MnO2 in the PEDOT:PSS matrix has been defined and explained in KOH electrolyte system. High cyclic stability was observedat the end of 2000 cycles for these hybrid sponges with less than 5 % capacitance fading. These spongesexhibit mass specific capacitance values as high as 10688 F g�1 which was found to be 35% higher com-pare to PEDOT: PSS sponges as obtained from Weibull statistics. The application of these electrodes wasexplored in a fully functional asymmetric coin cell unit where an energy and power density of200 mWh kg�1 and 6.4 kW kg�1, was obtained, respectively.

� 2013 Elsevier Inc. All rights reserved.

1. Introduction

Manganese dioxide (MnO2) has been widely explored as anelectrode material for supercapacitors and batteries because ofits low-cost, abundance, non-toxicity and high theoretic specificcapacitance (1370 F g�1) [1–5]. However, its practical specificcapacitance is still low as compared with the high-cost RuO2

[6–10]. It is believed that the low energy density of MnO2 iscaused by its charge storage mechanism, which is based on ionadsorption or redox reactions on the surface layer [11,12]. In atypical charge–discharge process, only the first few nanometersfrom the surface of the electrode can be utilized while noreactions occur in the bulk, resulting in limited energy densities[13–15]. Therefore, improving the utilization of active materialsis the key to enhance energy densities of these electrodes.Recently, porous electrodes, which can facilitate electrolyte ionsto react with active materials more efficiently, have attracted alot of attention [16–18]. Previous studies have shown thatMnO2 based hybrid porous electrodes synthesized using CNT/conducting polymer have exhibited specific capacitanceof 427 F g�1 [19] while flexible electrodes using Zn2SnO4/MnO2/

carbon have exhibited mass specific capacitance of 642 F g�1

[20]. Studies by Lang et al. have shown that electrodes basedon Au/MnO2 can offer high performance with a mass specificcapacitance of 1145 F g�1 [21]. Although these studies haveshown promising results, all these processing methods are com-plicated which can limit its scalability. Hierarchical macroporousstructures like sponges are known for being strong absorbingmedia with significant internal surface area [22] giving it goodaccessibility and compatibility to aqueous solutions and electro-lytes. Therefore, it is anticipated that a hybrid material with suchsponge like structure exhibiting continuous conducting networkof material like PEDOT: PSS (poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)) (�1000 s cm�1) embedded with highcapacitive material like MnO2 could offer high storage perfor-mance. Besides, PEDOT: PSS polymers can also store charge;through electrical double layer mechanism and faradaic mecha-nism throughout its matrix. Thus, the present study providesthe first reports on the preparation of a hybrid material with a3-D hierarchical porous network system, comprising of a con-ducting polymer (PEDOT: PSS) as matrix embedded with MnO2

using a simple freeze drying process. The study also providesdetails on the physicochemical characterization of these hybridsponges and investigates into its application for supercapacitorelectrodes.

R. Ranjusha et al. / Microporous and Mesoporous Materials 186 (2014) 30–36 31

2. Experimental

2.1. Preparation of spongy composite electrode using freeze dryingmethod

All the chemicals used in the present study were of analyticalgrade. 5 wt.% PVA solution was mixed with polyethylene dioxythi-ophene-polystyrene sulfonate (PEDOT-PSS) (Alfa Aesar, India) for30 min. Different amounts of MnO2 (5, 10 and 20 wt.%) powder(Nice Chemicals, India, particle size �50 nm) were added to theabove mixture and stirred overnight. This stirred solution was agedat 20 �C for 24 h and then freeze dried (lyophilized) using a lyoph-ilizer (ALPHA 2-4 LD plus) for 24 h. Depending on the amount ofMnO2 embedded into the PEDOT: PSS as matrix material, thesespongy samples were designated as PPM-5, PPM-10 and PPM-20,where PP and M denotes PEDOT: PSS and MnO2, respectively.The number represents the amount of wt.% of MnO2 added to thePEDOT: PSS matrix.

2.2. Morphology and phase analysis of spongy systems

Morphological analysis was performed using scanning electronmicroscopy (SEM) at an accelerating voltage of 20 kV. The crystal-line phases of the samples were determined by X-ray powder dif-fraction method (XRD, X’Pert PRO Analytical) with Cu Ka radiationsgenerated at 40 kV and 20 mA. Thermal stability of the PPM systemwas evaluated using thermo gravimetric-differential thermal anal-ysis (TG-DTA) instrument (SII TG/DTA6200) in nitrogen atmo-sphere with a heating rate of 20 �C/min from room temperatureto 400 �C. Surface area measurements were performed using BETmethod (Micromeritics Instrumentation, USA). Current profiles ofPPM and PP samples were determined using scanning electro-chemical microscopy (SECM, Nanotec, Munich).

2.3. Electrochemical characterization of spongy electrode systems

Cyclic voltammetry (CV) and discharge studies (electrochemicalworkstation: Newport Model) were performed to evaluate thecapacitance, energy and power density of the prepared electrode.For these studies, a three electrode setup consisting of lyophilizedsponges, platinum and calomel electrode were used as a working,counter and reference electrodes respectively. KOH was used asan electrolyte.

2.4. Weibull analysis

The mass specific capacitance ‘C’ data was further analyzedwith the conventional ranking method [23] to yield the Weibull

Fig. 1. SEM image showing the hierarchical meso

parameters, knowledge of which would lead to complete charac-terization of the statistical properties of the measured ‘C’ data. Thiswas done by arranging the data in ascending order. The ith result inthe set of n = 20 data was assigned a cumulative probability ofoccurrence, Pi, which was calculated with:

Pi ¼i

nþ 1: ð1Þ

The measured ‘C’’ and Pi, were then analyzed, by using a simple,least-square regression method, according to the alternative formof the well-known two-parameter Weibull distribution equation

ln ln1

1� Pi

� �� �¼ m ln C �m ln C0 ð2Þ

where m and C0 are the Weibull modulus and the scale parameter,respectively.

3. Results and discussion

3.1. Effect of MnO2 content on the morphology of PPM electrodes

Fig. 1 shows the SEM image of PPM-10 samples under lyophi-lized condition. It was observed that the PPM samples exhibitedinterconnected macro-pores with sizes ranging from 100–150 lm(Fig. 1a–c). SEM images (Fig. 1d–f) of these PPM-10 scaffolds athigher magnifications revealed a highly roughened surface exhibit-ing mesoporous structure unlike PP samples where a smooth tex-ture (absence of mesopores, see SI-I-I) was observed. Both samplesshowed similar size macropores. The formation of these roughenedsurfaces seemed to increase with increase in wt.% of MnO2 inPEDOT: PSS (see SI-I-II and SI-I-III). However, as the amount ofMnO2 increased beyond 10 wt.% (i.e. for PPM-20 samples); thesamples seemed to lose the sponginess. SEM images (see SI-I-III)of PPM-20 showed deficit of macro-pores as compare to PPM-10samples.

3.2. Mechanism of formation of pores in PP and PPM electrodes

The schematic representation of pore formation in PP and PPMsponges is shown in Fig. 2. The formation of macro-pores in PPsamples can be attributed to sublimation of water from the PP ma-trix during the freeze drying (Fig 2a). MnO2 particles being hydro-philic in nature ensures nano-islands of entrapped water in PPmatrix being sublimated during freeze drying resulting inmesopores (Fig. 2b). However as the amount of MnO2 increasesin the PP matrix the concentration of these nano-islands alsoincreases preventing the formation of any macro-sized water en-

porous structure in PPM-10 hybrid sponges.

Fig. 2. Schematic illustration showing the mechanism of formation of hierarchical pores in (a) PP (b) PPM-10 samples and (c) PPM-20.

Fig. 4. TG analysis of PPM and PP samples.

32 R. Ranjusha et al. / Microporous and Mesoporous Materials 186 (2014) 30–36

claves as seen in case of PP electrodes. This can create deficits ofmacropores as seen in case of PPM-20 composites (Fig. 2c).

3.3. Elemental and phase analysis of PP and PPM electrodes

Fig. 3 shows the characteristic XRD spectrum of spongy com-posite electrode (PPM) where the diffraction peaks of MnO2 corre-sponded to tetragonal phases (JCPDS 72–1984). Since it wasdifficult to phase contrast MnO2 in PPM samples, the dispersionof MnO2 in the spongy PPM was confirmed by elemental analysisof Mn using EDAX (see SI-II-I and SI-II-II). The EDAX analysis indi-cated uniform dispersion of MnO2 along the polymer matrix. BETanalysis of the PPM samples showed that addition of MnO2 intothe PP matrix seems to increase the surface area (see SI-III).

3.4. Thermal stability analyses

Thermo gravimetric analysis performed from room temperatureto 400 �C under nitrogen atmosphere illustrate the alteration inthermal stability of PPM sample caused by the addition of MnO2

(Fig. 4). In PP, onset of thermal degradation occurs at around30 �C. Elevated degradation rate and pronounced increase inslope of thermal degradation were found in PP system whereas,

Fig. 3. Typical XRD spectrum of PPM sponges.

incorporation of MnO2 seemed to augment the thermal stabilityof the composite. The PPM material showed a reduced degradationprofile and better thermal stability. PPM material lost �30 wt.%when compared to 70 wt.% loss in PP system. Thermal stability inPPM can be attributed to the presence of MnO2 which acts as heatsink material creating a thermal resistance against heat transfer.Since both polymer and metal oxide exhibit different thermal con-ductivity, the thermal energy that is transferred from a localizedsite (in present case MnO2) to a larger area (polymer matrix) doesnot distribute uniformly along the matrix. This thermal resistancecauses a large temperature gradient between the matrix and theheat sink resulting in lower temperatures at the surrounding area.It is anticipated that such thermal gradient can increase the ther-mal stability [24].

3.5. The role of MnO2 on the electrochemical performance of the PPMelectrode

In order to determine the optimal wt.% of MnO2 in the compositefor electrode applications, cyclic voltammetry (CV) tests at a scanrate of 10 mV s�1 were performed using standard calomel electrode(SCE) and platinum as reference and counter electrode respectively

Fig. 5. (a) CV curves PPM-5, PPM-10 and PPM-20 electrodes at 10 mV s�1 scan rate and (b) plot showing the relation between specific capacitance and wt.% of MnO2 in PPM.

R. Ranjusha et al. / Microporous and Mesoporous Materials 186 (2014) 30–36 33

in KOH electrolyte. The working electrodes comprised of PP, PPM-5,PPM-10 and PPM-20 electrode systems. Traditionally reports haveshown that electrolytes like LiClO4 are more suitable for PEDOT:PSS based supercapacitor system. But in the present study, PPMsamples in KOH electrolyte showed better results (see SI-IV). Thiscould be primarily attributed to the presence of MnO2 on thesurface of PP and its electrochemical affinity towards KOH asshown in earlier literature [25]. It was found that forconcentration >1 M the dissolution of the electrode in KOH wasobserved. At lower concentrations <0.1 M the current responseobtained from CV studies were lower compare to 0.1 M KOH. Oncomparing the CV performance of stand alone MnO2 and PP(SI-V); the current response at a given potential of MnO2 in KOHwas found to be higher as compare to PP. This is indicative of thefact that MnO2 electrodes in KOH shows combined effects of highmass transfer rates and low depletion rate of the K+ from the elec-trolyte in the diffusion layer as compare to PP. Further the lowhydrated radius of K+ (0.25 nm) can have enhanced ionic mobilityand better interaction with MnO2 as compared to Li+ (0.42 nm)[25] resulting in better CV performance. However CV performanceof PPM-20 was found to be lower as compare to PPM-10 sampleswhich could be attributed to lower electrolyte percolation resultingfrom macropore deficits (see Fig. 5a and b). Unlike PPM-20, PPM-5showed macropores but the reduction in capacitance in this casecould be due to the absence of mesopores (see SI-I-II). The bestCV performance was observed for PPM-10 electrodes. Thus in order,to complement best of the electrical, electrochemical and mechan-ical properties of these porous structures, the further characteriza-tions was performed on PPM-10 electrodes in 0.1 M KOH. SECManalysis showed that the current profile at the surface of PPM-10

Fig. 6. SECM spatial analysis of (

did not alter much after the addition of MnO2 (see Fig. 6a andb).This meant that PP can act as an effective current collector insuch hybrid electrode systems.

The mass specific capacitance was calculated using the follow-ing equation:

C ¼ 1M � DV � s

Z V2

V1

IdV ð3Þ

where C is the mass specific capacitance (F g�1), M is the mass ofthe active electrode, DV is the voltage window, s is the scan rateand I represent the current.

The Faradaic reactions occurring on the surface of both PP andMnO2 are the major charge storage contributors resulting fromthe intercalation/de-intercalation process of K+ at the electrode/KOH interfaces. Inorder to quantitate the scattering in the dataweibull statistics was used. Both the samples showed similar scat-tering in ‘C’ data i.e. the weibull modulus value ‘m’ was found to be14 and 18 for PP and PPM-10 samples, respectively (Fig. 7). Similarscattering was indication that MnO2 was getting uniformly dis-persed in the PP matrix. The mean specific capacitance C0 valuefor PPM-10 and PP electrodes at 1 mV s�1 scan rate was found tobe 1068 and 687 F g�1, respectively. For different scan rates, itwas observed that the specific capacitance value for these compos-ites decreased from 1118 to 206 F g�1, as the scan rate increasedfrom 1 to 100 mV s�1 (see Fig. 8). The specific capacitance valueof PP system was found to be 772–97 F g�1, as the scan rateincreased from 1 to 100 mV s�1 (see SI-VI). The reason for this low-er specific capacitance could be a) the less number of active siteson PP electrode as compared to PPM-10 electrode due to its com-paratively lower surface area (see SI-IV). This meant that the

a) PPM and (b) PP systems.

Fig. 7. Weibull plot of mass specific capacitance PEDOT: PSS and PPM-10 obtainedfrom CV at 1 mV s�1.

Fig. 8. Typical CV pattern of PPM-10 at different scan rates.

34 R. Ranjusha et al. / Microporous and Mesoporous Materials 186 (2014) 30–36

increased surface area can indeed increase K+ intercalation/dein-tercalation mechanism in the electrode, contributing more in thetotal capacitance of the composite electrode, b) the low reactivityof PP in KOH electrolyte as compared to PPM-10 samples (see SI-V).

3.6. Cycling and constant current discharge behavior of PPM-10electrode

The cycling stability was determined using CV tests for bothPPM-10 and PP electrodes for 1000 cycles at a scan rate of100 mV s�1 (see Fig. 9a). Large capacitance fading (67%) was

Fig. 9. (a) Cycling stability studies of PPM-10 and PP spongy electrodes and (b

observed in the case of PP electrode system at the end of 1000thcycle (Fig. 9a). But for PPM-10 electrodes there was no such capac-itance fading observed even at the end of 1000th cycle. The largefading could be attributed to the formation of C: PSS as shown inthe scheme

PEDOT : PSSþMþ þ e� $ PEDOTþM : PSS ð4Þ

where M+ denotes the positively charged ion and e- representsthe electron. Oxidation of PP films promotes expulsion of M+ to-wards the solution and film shrinking. The reduction promotesinsertion of M+ from the solution; it leads the swelling of film.But in the case of PPM-10 system the more active sites are contrib-uted by MnO2 that was uniformly coated on the PP matrix.

These reactions can be shown as

MnO2 þ yMþ þ e� $ yMnOOMþ ð1� yÞMnO2 þ e� ð5Þ

Here y represents the total number of active sites present on thesurface of the MnO2 structure. During redox reaction these meso-porous metal oxide film acts as active electrode layer and activelyparticipate in the intercalation/deintercalation mechanism. Thisprotects the PP layer by reducing the swelling and shrinkage.Another reason for capacitance reduction in the case of PP elec-trode could be attributed to the dissolution of PP material intothe electrolyte, due to the dimensional or structural variationswithin an electrode during the cycling [26,27]. The constant cur-rent discharge curves of lyophilized PPM-10 composite electrodeat different discharge currents are shown in Fig. 9b. From these dis-charge curves, the energy density, power density and capacitancevalues were derived. The capacitance values were obtained usingthe following equation:

C ¼ I � DtM � DV

ð6Þ

where C is the specific capacitance, I is the discharge current, DVis the potential window, Dt is the discharge time, m is the mass ofthe active material.

The energy and the power density was calculated using the fol-lowing Eqs. (7) and (8):

Pmax ¼V2

4R� 1

Mð7Þ

E ¼ 12

CV2

Mð8Þ

where R = equivalent series resistance (ESR).V = maximum charged voltage (V).Pmax = maximum specific power density (W/kg).E = energy density (Wh/kg).

) discharge profile of PPM-10 electrode at different discharging currents.

Fig. 10. (a) PPM sponges used as cathode in a coin cell, (b) fabricated coin cell and (c) discharge curves at 10 and 40 mA.

R. Ranjusha et al. / Microporous and Mesoporous Materials 186 (2014) 30–36 35

C = capacitance (F).M = mass of the active material (kg).The value of ESR was calculated from the charge discharge

curve.The obtained mass specific capacitance values were 333 and

212 F g�1 at discharge currents of 5 and 10 mA, respectively.The capacitance reduction at higher discharging currents can beassigned to many factors like large voltage drop, pore resistancedistribution and redox reactions on the electrode surface [28].The non-linear behavior of the discharge curve indicates the redoxnature of the material. This variation could be attributed to eitherof the following factors or its combinations i.e. (a) change in theequivalent series resistance, (b) the redistribution of charges with-in the pores of the material structure during the discharging cyclesand (c) pseudocapacitive behavior of the material [29–31]. Themaximum energy density of �221 Wh kg�1 at 5 mA and powerdensity of �9 kW kg�1 at 10 mA were achieved.

As a preliminary study, the application of these PPM-10 elec-trodes for supercapacitor application was explored in a fully func-tional asymmetric coin cell unit (Fig. 10a and b). Fig. 10c shows thedischarge curve at 10 and 40 mA in 1 M KOH electrolyte. Theobtained energy and power density for these coin cells were foundto be 200 mWh kg�1 and 6.4 kW kg�1, respectively. However, themain conclusion of this investigation can only be validated oncethe performances of these electrodes can be evaluated at bothhigher and lower temperatures for different discharging currents.Also the role of substrate needs to be investigated when it comesto determining the final performance of the device. This work isbeen currently pursued by our group.

4. Conclusions

In summary, the present study investigates the performance ofPPM hierarchical spongy composite electrodes for supercapacitorelectrode applications. The addition of MnO2 to PP have shown

to increase the surface area. This hybrid porous morphology showsadvantages especially from points of better surface area and massspecific capacitance. The mean mass specific capacitance value wasfound to be 1068 F g�1 from CV studies. These electrodes exhibitedenergy and power density values of 221 Wh kg�1 and 9 kW kg�1,respectively. As a preliminary study, the application of thesePPM-10 electrodes for energy storage was explored in a fully func-tional asymmetric coin cell unit where an energy and power den-sity of 200 mWh kg�1 and 6.4 kW kg�1, was obtained , respectively.The superior performance of these hierarchical spongy PPM-10composite electrode as compared to PP electrode makes thempromising systems for rechargeable energy units for battery/sup-ercapacitor applications.

Acknowledgment

Council of Scientific and Industrial Research (CSIR), Governmentof India is gratefully acknowledged for their financial supportunder Project No. 22(0565)/12/EMR-II.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.micromeso.2013.11.035.

References

[1] E. Frackowiak, F. Beguin, Carbon 40 (2002) 1775–1787.[2] R. Ranjusha, V. Prathibha, S. Ramakrishna, A. Sreekumaran, Scripta Mater. 68

(2013) 881–884.[3] L. Xue, H. Hao, Z. Wei, T. Huang, A. Yu, J. Solid State Electrochem. 15 (2011)

485–497.[4] M.S. Wu, P.J. Chiang, J.T. Lee, J.C. Lin, J. Phys. Chem. B 109 (2005) 23279–23284.[5] M.N. Patel, X.Q. Wang, B. Wilson, D.A. Ferrer, S. Dai, K.J. Stevenson, K.P.

Johnston, J. Mater. Chem. 20 (2010) 390–398.[6] J.P. Zheng, J. Electrochem. Soc. 152 (2005) A1864–1869.[7] C.Q. Bian, A.S. Yu, H.Q. Wu, Electrochem. Commun. 11 (2009) 266–269.

36 R. Ranjusha et al. / Microporous and Mesoporous Materials 186 (2014) 30–36

[8] F. Fusalba, P. Gouerec, D. Villers, D. Belanger, J. Electrochem. Soc. 148 (2001)A1–A6.

[9] M. Mastragostino, C. Arbizzani, F. Soavi, J. Power Sources 812 (2001) 97–98.[10] S.C. Pang, M.A. Anderson, T.W. Chapman, J. Electrochem. Soc. 147 (2000) 444–

450.[11] M. Toupin, T. Brousse, D. Bélanger, Chem. Mater. 16 (2004) 3184–3190.[12] S.F. Chin, S.C. Pang, M.A. Anderson, Mater. Lett. 64 (2010) 2670–2672.[13] C.Z. Yuan, B. Gao, L.H. Su, X.G. Zhang, J. Colloid Interface Sci. 322 (2008) 545–

550.[14] P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845–854.[15] S. Devaraj, N. Munichandraiah, J. Electrochem. Soc. 154 (2007) A901–909.[16] L. Cao, F. Xu, Y.Y. Liang, H.L. Li, Adv. Mater. 16 (2004) 1853–1857.[17] R. Ranjusha, S. Ramakrishna, A. Sreekumaran Nair, P. Anjali, S. Vineeth, T.S.

Sonia, N. Sivakumar, K.R.V. Subramanian, Shantikumar V. Nair, A.Balakrishnan, RSC Adv. 3 (2013) 17492–17499.

[18] F. Beguin, K. Szostak, G. Lota, E. Frackowiak, Adv. Mater. 17 (2005) 2380–2384.[19] Y. Hou, Y.W. Cheng, T. Hobson, J. Liu, Nano Lett. 10 (2010) 2727–2733.[20] L.H. Bao, J.F. Zang, X.D. Li, Nano Lett. 11 (2011) 1215–1220.[21] X.Y. Lang, A. Hirata, T. Fujita, M.W. Chen, Nat. Nanotechnol. 6 (2011) 232–236.

[22] W. Chen, R.B. Rakhi, L. Hu, X. Xie, Y. Cui, H.N. Alshareef, Nano Lett. 11 (2011)5165–5172.

[23] S.K. Padmanabhan, A. Balakrishnan, M.C. Chu, T.N. Kim, S.J. Cho, Dent. Mater.26 (2010) 100–105.

[24] T.R. Anthony, W.F. Banholzer, J.F. Fleischer, L. Wei, P.K. Kuo, R.L. Thomas, R.W.Pryor, Phys. Rev. B 42 (1989) 1104–1111.

[25] R. Ranjusha, A.S. Nair, S. Ramakrishna, P. Anjali, K. Sujith, K.R.V. Subramanian,N. Sivakumar, T.N. Kim, S.V. Nair, A. Balakrishnan, J. Mater. Chem. 22 (2012)20465–20471.

[26] A. Leela, M. Reddy, M.M. Shaijumon, S.R. Gowda, P.M. Ajayan, J. Phys. Chem. C114 (2010) 658–663.

[27] H. Xia, Y.S. Meng, X. Li, G. Yuan, C. Cui, J. Mater. Chem. 21 (2011) 15521–15526.[28] S.K. Meher, P. Justin, G.R. Rao, Electrochim. Acta 55 (2010) 8388–8396.[29] S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano 4 (2010) 2822–2830.[30] G.D. Sulka, V. Moshchalkov, G. Borghs, J.P. Celis, J. Appl. Electrochem. 37 (2007)

789–797.[31] J. Chen, N. Xia, T. Zhou, S. Tan, F. Jiang, D. Yuan, Int. J. Electrochem. Sci. 4 (2009)

1063–1073.