Electrochemistry Communications 13 (2011) 869–871

Contents lists available at ScienceDirect

Electrochemistry Communications

j ourna l homepage: www.e lsev ie r.com/ locate /e lecom

Cycling stability and self-protective properties of a paper-based polypyrrole energystorage device

Henrik Olssona, Gustav Nyströma, Maria Strømmea, Martin Sjödina, Leif Nyholmb,⁎a Nanotechnology and Functional Materials, The Ångström Laboratory, Uppsala University, Box 534, 751 21 Uppsala, Swedenb Department of Materials Chemistry, The Ångström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden

⁎ Corresponding author. Tel.: +46 18 4713742; fax:E-mail address: Leif.Nyholm@mkem.uu.se (L. Nyholm

1388-2481/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.elecom.2011.05.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 May 2011Received in revised form 18 May 2011Accepted 18 May 2011Available online 26 May 2011

Keywords:Conducting polymerPolypyrroleCycling stabilityCompositeCellulose

Acomposite consisting of polypyrrole and cellulose fromthe Cladophora sp. greenalgae is shown toexhibit excellentcycling stabilitywhen used as the electrodes in an aqueous symmetric supercapacitor device. The capacitance of thedevice,whichwas32.4 F g−1, onlydecreasedby0.7%during4000galvanostatic cycles employinga currentof10 mAand potential cut-off limits of 0 and 0.8 V. No change in the electrode material's morphology could be seen whencomparing cycled and pristine materials with scanning electron microscopy. Furthermore, no significant loss incapacitancewasobservedevenwhencharging thedevice to1.8 V.Measurementsof theelectrodepotentialsversusacommon reference show that this effect was due to a device intrinsic self-protective mechanism which preventeddegradation of the polypyrrole.

+46 18 513548.).

l rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Polypyrrole (PPy) is one of the most investigated conductivepolymers for energy storage applications [1–3] as it has shownpromise in terms of easy manufacturing, high conductivity and acharge storage capacity of ~116 mAhg−1 for a doping level of 33%.Problems with PPy (and conducting polymers in general) include arelatively low cycle stability, high self-discharge rates and limitedshelf-life [3]. Although much work has been done to investigate thecycling stability of conducting polymers, and significant progress hasbeen made [4–11], results vary with reports between 9% [12] and 50%[13] loss in capacitance during 1000 cycles in aqueous electrolytes.This can be explained by the use of different synthesis methods,substratematerials and stability evaluationmethods [3]. It is thereforeclear that further research is needed to fully understand the processesdetermining the PPy cycling stability.

The stability of PPy depends on the employed potential window[14] and pH [15]. Different cycling stability may also be expected fordifferent electrochemical techniques [3]. As improved stabilities havebeen reported for conductive polymer composites, for instancecontaining carbon nanotubes [16,17], Nafion [18] and cellulosenanocrystals [19], it is interesting to extend the cycling stabilitystudies to other types of PPy composites.

We have reported on an all-polymer paper-based battery withcharge capacities of 33 mAhg−1 [20]. This paper material is based on a

high surface area cellulose from the Cladophora sp. algae which offershighly crystalline nano fibres [21] onwhich PPy adheres well [22]. Theelectrochemical behaviour of the material has been studied usingcyclic voltammetry and chronoamperometry [22]. The aim of thepresent work is to investigate the galvanostatic cycling stability of thiscomposite when used as the electrodes in a symmetric supercapacitordevice as well as in a three-electrode electrochemical cell.

2. Experimental

Cellulose from the Cladophora sp. algae was extracted as previouslydescribed [21]. Pyrrole (N97%), iron (III) chloride hexahydrate (N99%),Tween 80 and sodium chloride (N99.5%) were purchased from VWRinternational and used as received. Deionised water was usedthroughout the synthesis. A dispersion of cellulose was prepared bysonicating 300 mg cellulose and 50 mL of water for 8 min. 100 mL of0.42 M pyrrole containing one drop of Tween 80 was then added to thecellulose dispersion and sonicated for one additional minute. Topolymerise the pyrrole, 100 mL of 0.30 M FeCl3 was added to themixture. The polymerisation was allowed to continue for 10 min afterwhich the products were washed thoroughly with water and suckedinto a cake using a Büchner funnel.

All electrochemical experiments were carried out in 2 M NaCl, ifnot stated otherwise, using an Autolab PGSTAT302N potentiostat(Ecochemie, the Netherlands). In the three-electrode setup, a coiledPt-electrode (in a fritted separate compartment) was used as theauxiliary electrode. A 3 M NaCl Ag/AgCl reference electrode was usedas the reference electrode. For the two-electrode measurements,symmetric supercapacitor devices were assembled by placing two

870 H. Olsson et al. / Electrochemistry Communications 13 (2011) 869–871

PPy/cellulose electrodes, separated by a filter paper, between twomicroscope glass slides (the distance between the slides was about2 mm). Each electrode contained ~10 mg of the composite and wascontacted with a Pt-foil. The assembled device was immersed in anelectrolyte filled crystallisation dish. For the low pH measurements, a0.01 M HCl+2M NaCl solution (pH~2) was prepared using 37% (wt.)HCl (VWR International).

In the two-electrode measurements, the potentials of theindividual electrodes in the supercapacitor device were recordedversus a 3 MNaCl Ag/AgCl reference electrode (placed ~1 cm from theelectrodes) by connecting the electrodes and reference electrode to a34401A multimeter (Agilent Technologies, USA).

Scanning electron microscopy (SEM) micrographs were obtainedwith a LEO1550 field emission SEM instrument (Zeiss, Germany).Micrographs of both electrodes were recorded after galvanostaticcycling in a two-electrode device at 10 mA (with potential cut-offlimits of 0 and 0.8 V) for more than 4000 cycles, as well as for a non-cycled material.

3. Results and discussion

Galvanostatic two-electrode measurements, with a symmetricsupercapacitor device, were carried out using a current of 10 mA andpotential cut-off limits of 0 and 0.8 V. The five typical charge–dischargecycles shown in Fig. 1a clearly show that the device functioned as anelectrochemical capacitor. The cell specific capacitance was calculatedfrom the slope of the linear region of the discharge curves, using thefollowing equation:

C =i

ΔV =Δtð Þ⋅m; ð1Þ

where idenotes the applied current andm is the total dryweight of bothelectrodes (20.8 mg). Amaximumspecific cell capacitance of 32.4 F g−1

was obtained, corresponding to a specific electrode capacitance of129.6 F g−1[16].

Fig. 1. Characterization of the PPy/Cellulose composite. (a) Five typical cycles for a symmnumber. (c) SEM micrograph of the positive electrode from the two-electrode device aftegalvanostatic measurements in 2.0 M NaCl (triangles) and 2.0 M NaCl+0.01 M HCl (square

In Fig. 1b, the specific cell capacitance is shown as a function of thenumber of charge–discharge cycles. The two-electrode cell showedexcellent cycling stability with only 0.7% loss in the capacitance during4000 cycles with potential cut-off limits of 0 and 0.8 V. This result is incontrast with previous work by Khomenko et al. [14], which indicatedthat re-doping of the negative electrode did not occurwhen discharginga symmetric cell, unless the cell was charged to very low cell voltages.

Fig. 1c shows a SEM micrograph of the positive electrode aftermore than 4000 cycles. In contrast to results in the literature [9,23], nochange in morphology with respect to that of the non-cycled materialwas observed for any of the cycled electrodes. This indicates excellentadhesion of the polymer to the cellulose substrate, and that nofragments were lost during cycling due to the repeated swelling andcontraction of the PPy layer.

In galvanostatic three-electrode measurements, a rapid decrease inthe capacitance was, on the other hand, observed during cycling (seeFig. 1d). The triangles depict the electrode capacitance (calculated usingEq. (1), where m in this case is the mass of the single electrode) as afunction of the cycle number, for a current of 5 mA and potential cut-offlimits of 0 and 0.6 V vs. Ag/AgCl. Based on previously published results[24] it is reasonable to assume that this rapid degradation stems from anucleophilic attack of OH− (present in the neutral electrolyte) on PPy.This hypothesis is supported by the fact that the stability was improveddramaticallywhen the experimentswere repeated in an electrolyte alsocontaining 0.01 M HCl (pH~2), see Fig. 1d.

The results from the three-electrode measurements show that thestability of the PPy is strongly dependent on the electrolyte pH inagreement with the findings of Li and Qian [15]. In view of the highobserved stability in the two-electrode measurements, even in theabsence of a pH-adjustment, the rapid degradation in the three-electrode setup may seem contradictory. The stable responseobserved for the two-electrode device can, however, be explainedbased on a partial degradation of the PPy coating resulting in aliberation of protons and degradation compounds including malei-mide [25]. The latter effect would result in a local decrease in the pHwithin the two-electrode device as this was constructed in such a waythat the glass microscope slides covered the entire electrode surface

etric supercapacitor device (i=10 mA). (b) Normalized cell capacitance versus cycler 4000 cycles. (d) Normalized capacitance versus cycle number from three-electrodes).

871H. Olsson et al. / Electrochemistry Communications 13 (2011) 869–871

and hence limited the diffusion to and from the cell. It is thereforereasonable to assume that a local acidic environment was createdinside the two-electrode device and that this protected the polymeragainst further degradation. A decrease in the pH of the electrolytewas also found after the experiment.

In further investigations of the stability of the PPy composite,galvanostaticmeasurementswere carriedout ona two-electrodedevicethat had been pre-cycled employing 10 mA and potential cut-off limitsof 0 and 1.2 V. During the pre-cycling, the potential of each electrodeequilibrated at 0 V vs. Ag/AgClwhen the cell potentialwas 0 V, (i.e. bothelectrodes were partially oxidised). The cell was then cycled galvanos-tatically with an applied current of 10 mA, but with potential cut-offlimits of 0 and 1.8 V, for 20 h. Even with these potential limits, the cellshowed no significant degradation over 600 cycles, and the average cellcapacitancewas found to be33.5 F g−1. This result is in contrastwith thebehaviour of a previously reported PPy/CNT composite [5], for which arapid loss in the capacitance was seen for two-electrode devices cycledto cell voltages higher than 0.4 V. It should be noted that despitecharging the cell to a higher potential, the available cell capacitywas notsignificantly higher than when the cell was charged to 0.8 V since theextra charge needed to reach 1.8 V was very small.

To better understand this behaviour, the potential of each electrodewas logged externally vs. Ag/AgCl (see Fig. 2). From these experiments itis clear that the potential of the oxidised electrode did not reach morethan +0.6 V vs. Ag/AgCl and thus never attained potentials wheresignificant degradation is expected [26]. The potential of the reducedelectrode, on the contrary, dropped as far as to−1.2 V vs. Ag/AgCl whenthe device was charged to a cell voltage of 1.8 V. During the reduction,the initially partially oxidised PPy was reduced to uncharged (non-conducting) PPy. In Fig. 2b, a rapid drop in the potential of the reducedelectrode at ~−0.45 V vs. Ag/AgCl is observed which indicates asignificant increase in resistance of the reduced PPy electrode. The iRdrop, evaluated from the instantaneous potential change upon polarityinversion at the cut-off points in Fig. 2b, was 85 mV at 0 V cell potentialand 490 mV at 1.8 V cell potential. This corresponds to resistances of8.5Ω and 49Ω, respectively (for i=10 mA). It should be noted that forthe cell charged to 0.8 V, the corresponding resistances were 7.2Ω and8.5Ω at 0 Vand0.8 V cell potential, respectively (evaluated fromFig. 1a).

Fig. 2. Individual electrode potentials versus time for a symmetric supercapacitordevice during galvanostatic cycling with cut-off limits of 0 V and 1.8 V for (a) the fullduration of the experiment and (b) a few typical cycles.

The increased resistance of the reduced electrode explains the dramaticshift in the potential as a continuously increasing negative potential wasneeded to enable further reduction of the PPy. This resistive feature ofthe system, which is based on the limiting capacity of the negativeelectrode, thus works as an inherent self-protection mechanism againstoveroxidation of the positive electrode. In contrast, with conductivepolymers on conductive high capacitance substrates, e.g. carbonnanotubes [10], a less rapid change in the potential would be expected,since the capacitance of the substrate would be significant even whenthe polymer is fully reduced. The use of non-conductive substrates, insymmetric cells with conductive polymer composites, may thus beadvantageous compared to conductive high capacitance substrates.

4. Conclusions

Symmetric supercapacitors based on electrodes composed ofpolypyrrole and cellulose composites can be cycled for 4000 cycles inaqueous 2 M NaCl electrolytes with only 0.7% drop in the capacitance.One reason for the excellent cycling stability was found to be the localdecrease in pHwithin the device, which decreased the degradation rateof the polymer. The supercapacitor has also been shown to possess anintrinsic protection against overoxidation at high potentials. This wasexplained by an increasing resistance of the negative electrode duringreduction, which led to a rapid potential drop of the reduced electrodepreventing the oxidised electrode from reaching potentials wheredegradation occurs. It is concluded that these effects can increase thecycling stability of polypyrrole based supercapacitors significantly andthat conventional three-electrode cycling stability tests therefore maybe misleading.

Acknowledgements

The Swedish Foundation for Strategic Research (SSF) (grant RMA08-0025), the Swedish Science Council (VR), the Bo Rydin Foundation andthe Nordic Innovation Centre (contract number 10014) are gratefullyacknowledged for financial support.

References

[1] J. Heinze, B.A. Frontana-Uribe, S. Ludwigs, Chem. Rev. 110 (2010) 4724.[2] G.A. Snook, P. Kao, A.S. Best, J. Power Sources 196 (2011) 1.[3] L.Nyholm,G.Nyström,A.Mihranyan,M.Strømme,TowardFlexiblePolymerandPaper-

Based Energy Storage Devices. Adv. Mater. (in press), doi:10.1002/adma.201004134.[4] L.L. Zhang, S. Li, J.T. Zhang, P.Z. Guo, J.T. Zheng, X.S. Zhao, Chem.Mater. 22 (2010) 1195.[5] E. Frackowiak, V. Khomenko, K. Jurewicz, K. Lota, F. Beguin, J. Power Sources 153

(2006) 413.[6] B.C.Kim, J.S. Kwon, J.M. Ko, J.H. Park, C.O. Too, G.G.Wallace, Synth.Met. 160 (2010) 94.[7] K.S. Ryu, S.K. Jeong, J. Joo, K.M. Kim, J. Phys. Chem. B 111 (2007) 731.[8] W.C. Chen, T.C. Wen, J. Power Sources 117 (2003) 273.[9] B.C. Kim, J.M. Ko, G.G. Wallace, J. Power Sources 177 (2008) 665.

[10] E. Frackowiak, K. Jurewicz, S. Delpeux, V. Bertagna, S. Bonnamy, F. Beguin, Mol.Cryst. Liq. Cryst. 387 (2002) 297.

[11] C. Peng, J. Jin, G.Z. Chen, Electrochim. Acta 53 (2007) 525.[12] L.Z. Fan, J. Maier, Electrochem. Commun. 8 (2006) 937.[13] R.K. Sharma, A.C. Rastogi, S.B. Desu, Electrochim. Acta 53 (2008) 7690.[14] V. Khomenko, E. Raymundo-Pinero, E. Frackowiak, F. Beguin, Appl. Phys. A: Mater.

Sci. Process. 82 (2006) 567.[15] Y.F. Li, R.Y. Qian, Electrochim. Acta 45 (2000) 1727.[16] V. Khomenko, E. Frackowiak, F. Beguin, Electrochim. Acta 50 (2005) 2499.[17] S.R. Sivakkumar, W.J. Kim, J.A. Choi, D.R. MacFarlane, M. Forsyth, D.W. Kim, J.

Power Sources 171 (2007) 1062.[18] R.Y. Song, J.H. Park, S.R. Sivakkumar, S.H. Kim, J.M. Ko, D.Y. Park, S.M. Jo, D.Y. Kim, J.

Power Sources 166 (2007) 297.[19] S.Y. Liew, W. Thielemans, D.A. Walsh, J. Phys. Chem. C 114 (2010) 17926.[20] G.Nyström,A. Razaq,M. Strømme, L. Nyholm,A.Mihranyan,NanoLett. 9 (2009) 3635.[21] A. Mihranyan, A.P. Llagostera, R. Karmhag, M. Strømme, R. Ek, Int. J. Pharm. 269

(2004) 433.[22] A. Mihranyan, L. Nyholm, A.E.G. Bennett, M. Strømme, J. Phys. Chem. B 112 (2008)

12249.[23] C.Y. Wang, G. Tsekouras, P. Wagner, S. Gambhir, C.O. Too, D. Officer, G.G. Wallace,

Synth. Met. 160 (2010) 76.[24] F. Beck, R. Michaelis, Mater. Corros. 42 (1991) 341.[25] D.S. Park, Y.B. Shim, S.M. Park, J. Electrochem. Soc. 140 (1993) 609.[26] T.W. Lewis, G.G. Wallace, C.Y. Kim, D.Y. Kim, Synth. Met. 84 (1997) 403.