Effective Charge Transport in Poly(3,4-ethylenedioxythiophene) Based Hybrid Films Containing Polyoxometallate Redox Centers

Journal of The Electrochemical Society, 152 ~3! E98-E103~2005!0013-4651/2005/152~3!/E98/6/$7.00 © The Electrochemical Society, Inc.

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Effective Charge Transport in Poly„3,4-ethylenedioxythiophene…Based Hybrid Films Containing Polyoxometallate RedoxCentersLidia Adamczyk,a Pawel J. Kulesza,b,* ,z Krzysztof Miecznikowski,b

Barbara Palys,b Malgorzata Chojak,b and Dorota Krawczykb

aDepartment of Materials and Process Engineering and Applied Physics, Czestochowa University ofTechnology, PL-42-200 Czestochowa, PolandbDepartment of Chemistry, University of Warsaw, PL-02-093 Warsaw, Poland

Electrodeposition and electrochemical charging of hybrid organic/inorganic films composed of the poly~3,4-ethylenedioxythiophene!, PEDOT, conducting polymer matrix, and Keggin type polyoxometallate, phosphododecamolybdate(PMo12O40

32) or phosphododecatungstate (PW12O4032), redox centers, are described under conditions of aqueous solutions. The

systems are electropolymerized through potential cycling as thin and moderately thick~mm level! films on electrode surfaces.They are capable of fast charge propagation during redox reactions in strong acid medium~0.5 mol dm23 H2SO4). The highoverall physicochemical stability of PEDOT is explored to produce a robust, conductive, matrix for such polynuclear mixed-valence inorganic nanostructures as PMo12O40

32 and PW12O4032 . The composite~hybrid! materials are stabilized due to the existence

of electrostatic attraction between anionic phosphomolybdate or phosphotungstate units and positively charged conducting poly-mer ~oxidized!. Charge transport is facilitated by the fact that the reversible and fast redox reactions of polyoxometallate appearin the potential range where PEDOT is conductive. The effective diffusion coefficients are on the level 43 1028 cm2 s21. Thewhole concept may lead to the fabrication of composite~hybrid! films that are capable of effective accumulation and propagationof charge in redox capacitors.© 2005 The Electrochemical Society.@DOI: 10.1149/1.1859710# All rights reserved.

Manuscript submitted February 6, 2004; revised manuscript received August 26, 2004. Available electronically January 31, 2005.

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Over the last several years, there has been growing interconducting and redox polymer films on electrodes1-8 due to the prospects of their applications in various microtechnological systincluding charge storage devices, sensors, gas separatingbranes, molecular electronics, displays and light emitting dioand corrosion protection. Out of many conducting polympoly~3,4-ethylenedioxytiophene! ~PEDOT!has recently been of interest as a particularly stable, highly conductive, and electroaorganic polymer.8-18 For example, PEDOT has been used as anstatic coating,19 a conductive electrode in light emitting diodes20

and as a material for electrochromic devices.21 It is commonly accepted that PEDOT is very stable in its doped~oxidized!state,10,12-14

and it may reach conductivity as high as 200 S cm21.22 Although thelatter parameter is dependent on the film morphology, methopreparation, measurement, and experimental conditions, andbeen reported to be approximately an order of magnitude lunderin situ conditions,23 the overall conductivity of the polymerthe oxidized state is high.24 The exact nature of electrochemiprocesses occurring in PEDOT films is fairly complex,25 and thesystem is believed to undergo several overlapping fast redox ttions characterized by high diffusion coefficients for chapropagation.26 The above properties make PEDOT attractivepotential material for fabrication of composite matrices with imbilized metals, functionalized dopants, redox, and reacenters.27-31

PEDOT was synthesized both chemically and electrochemthrough oxidation of the corresponding monomer from mostlyganic nonaqueous and sometimes aqueous solutions. In thecase, application of water as a solvent was somewhat limited dthe low solubility of a thiophene monomer. More recently, thesibility of use of an aqueous anionic micellar medium contaisodium dodecyl sulfate to electrosynthesize robust and conduPEDOT films on inert electrode substrates have been explore32,33

The presence of anionic micelle has led to the improvement of stural properties of the resulting PEDOT films. The colloidal aquesolutions of PEDOT for the polymer deposition were obtachemically by oxidative polymerization~with potassium peroxysu

* Electrochemical Society Active Member.z E-mail: [email protected]

address. Redistribution subject to ECS terms155.97.178.73aded on 2014-10-16 to IP

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fate! of the monomer in the presence of polystyrenesulfonic ac34

It should also be remembered that aqueous solutions of phosacid35 and heteropolyacids36 were employed to perform electropomerization of thiophenes.

The fact, that application of anionic surfactants in aqueous~in-cluding acid!media facilitates electrosynthesis and improves phcochemical properties of PEDOT, has prompted us to considertrodeposition of PEDOT in the presence of polyan~polyoxometallates!. The well-defined structure, reversible muelectron electrochemical reactions, and unique photoelectrocheproperties of these nanosized model oxide particles37,38 make themattractive for many applications as functional materials,e.g., as electrode materials for energy storage devices.39-41 In this context, conducting polymers are also promising,3,42-47 although their application is often limited due to low capacity to store charge in ssystems. It has been established that incorporation of polyoxomlates to such conducting polymers as polyanilinepolypyrrole36,39-41,47-50 produces composite~hybrid! materials inwhich the polymeric structure is retained whereas the inorganicters are largely responsible for the system integrity and overalltroactivity. Recent reports51-53 are consistent with the view that PDOT shows promise for the design of novel composite matewith improved stability and charge propagation dynamics.

Here, we consider and characterize robust and redox facilbrid films composed of the Keggin type polytungstatepolymolybdate47-50,54-59 inorganic centers immobilized within tPEDOT matrix. The concept goes back to the earlier pionereports7,36 describing immobilization of heteroplyanions in varioconducting [email protected]., polyaniline, polypyrrole, or poly~3-methylthiophene!# to produce redox polymers of potential utilityelectrocatalysis. In the present work, we have utilized a sligdifferent thiophene derivative to generate~under aqueous condtions! a very stable polymer~PEDOT!8-18 matrix for polyoxometallate redox centers. The resulting hybrid materials are capable ofast charge transport in strong acid medium~0.5 mol dm23 H2SO4).In comparison to typical redox polymers,5,6,8 our systems are chaacterized by the pretty high effective diffusion coefficientscharge propagation~on the level of 43 1028 cm2 s21!. The resultdescribed here are consistent with the view that such orginorganic composites are promising materials for effective acclation of charge in high-density charge storage redox capacito

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Journal of The Electrochemical Society, 152 ~3! E98-E103~2005! E99

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Experimental

All chemicals were of analytical grade purity. 3dioxyetylenethiophene~EDOT! monomer was kindly donatedBayer. Heteropolyacids, H3PMo12O40 (PMo12) and H3PW12O40

(PW12) were from Fluka. They were recrystalized beforeDeionized water was used a solvent to prepare solutions for mfication and electrolytes. Ultrahigh purity argon gas was usedeaerate investigated solutions. Experiments were carried oroom temperature (201 2°C).

Electrochemical measurements were performed using CH Inments~model CHI 660!workstation~Austin, USA!. A glassy carbodisk ~geometric area, 0.2 cm2! from Mineral ~Warsaw, Polandserved as a working electrode. Its surface was activated by polion a piece of cloth with alumina of decreasing grain size, from0.5 mm. Pt-flag and saturated calomel electrode~SCE!were used acounter and reference electrodes, respectively. Unless othestated, the PMo12-containing PEDOT (PMo12-PEDOT! films wereobtained on glassy carbon by potential cycling for 1200 s at 500s21 from 20.1 to 1.0 V in the solution for modification that wobtained by dissolving 0.5 g PMo12 and 0.1 cm3 EDOT in 10 cm3

H2O. The PW12-containing PEDOT (PW12-PEDOT!films were obtained in an analogous manner except that potential cyclingdone in the range from20.8 to 0.8 V.

Film thickness was determined using profilometry~Talysurf 50,Rank Taylor Hobson!. Infrared ~IR! spectra were measured wShimadzu 8400 Fourier transform infrared~FTIR! spectrometer. Thinfrared reflectance absorption spectra~IRRAS! were recorded usina Specular Reflectance Accessory model 500 produced by STech. The beam incidence angle was equal to 80° with respectsurface normal. Typically, 500 scans were averaged for a sreflectance spectrum. Scanning electron microscopy~SEM! imageswere obtained using JOEL Model JSM-5400. The microscopeequipped with EDX analyzer.

Results and Discussion

Fabrication of polyoxometallate/PEDOT films.—Controlledelectrodeposition of composite~hybrid! films of PEDOT with Keggin type heteropolyanions (PMo12 and PW12) was achieved by votammetric potential cycling in the respective mixtures for modifition as described in Experimental section. Figure 1A illustrategrowth of PMo12-PEDOT film, as evidenced from the increasethe peak currents, during the first 272 voltammetric cycles. Fugrowth of the film during potential cycling was much slower. Tanalogous pattern applies to the fabrication of PW12-PEDOT film~Fig. 1B!. In all cases, it was observed that the most pronoufilm growth occurred during the first cycles in the solution for mofication. The main difference between the data of Fig. 1A anconcerns the range of potentials applied. Due to the possibilistructural reorganization of PMo12, negative potential excursiowere limited to20.1 V in the case of Fig. 1A.

The result is consistent with the view that organic polymer~PE-DOT! layers are generated on the electrode surface during popotential scans,1-8 whereas polynuclear PMo12 or PW12 nanostructures are simultaneously attracted during potential cyclinggrowth of the film. The microstructures of polymer and polynucinorganic compound are expected to interact electrostaticallyeach other because oxidized PEDOT is positively chargedpoloxometallate is anionic. The latter characteristics lead to thcreased stability of the composite film. Regardless of whetheDOT is electrodeposited in dispersed granular form or as trueappearing in a composite film, the fact that all voltammetric pcurrents~characteristic of both components! increase regularly iFig. 1 implies fairly homogeneous distribution of a polyanion witthe film.

The choice of experimental parameters~time of electrodepostion, scan rate, and potential limits! has a profound effect on thdynamics of preparation of PMo12-PEDOT and PW12-PEDOT films.For simplicity, we concentrate here on PMo-PEDOT but the com
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ments described below apply to PW12-PEDOT as well. An increasof the electrodeposition time from 200 to 1200 s results in themation of PMo12-PEDOT films of higher loadings,i.e., characterized by larger volatmmetric peak currents recorded in 0.5 mol d23

H2SO4 ~Fig. 2!. Another important issue was the choice of potelimits during electrodeposition by potential cycling. Figure 3 illtrates cyclic voltammetric~CV! responses~in 0.5 mol dm23 H2SO4)of PMo12-PEDOT films obtained by potential cycling within diffeent potential limits. The efficiency of the film growth, expresseterms of the height of peak currents, was the highest when thetrode was cycled in the solution for modification from20.1 to 1.0 V~at 500 mV s21! in the solution for modification mentioned aboIn conclusion, the preparative conditions of Fig. 1 can be vieweoptimum.

Figure 1. Voltammetric generation of~A! PMo12-PEDOT and ~B!PW12-PEDOT films from the solutions for modification. Scan rates~A! 0.5and ~B! 0.1 V s21.



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Physicochemical identity of hybrid films.—When the compositPMo12-PEDOT film ~Fig. 4a! was scanned in the potential ranfrom 20.2 and 0.8 V in the supporting electrolyte only, the voltametric response was characterized by three sets of well depeaks. For comparison, we provide a response of a simple PEfilm ~Fig. 4b! that was generated as the composite film~Fig. 4a!except that no PMo12 but the equivalent amount~in moles! ofH2SO4 was added to the solution for modification. The data of4a and b are consistent with relatively low contribution from

Figure 2. Voltammetric responses of PMo12-PEDOT films generated at eletrodeposition times of~a! 200,~b! 600,~c! 1000, and~d! 1200 s. Electrolyte0.5 mol dm23 H2SO4 . Scan rate: 50 mV s21.

Figure 3. CV responses of PMo12-PEDOT films recorded following thegeneration~as for Fig. 1A!except that potential limits were varied at~a!20.4 to 1.2,~b! 20.2 to 1.2,~c! 20.1 to 1.2,~d! 0.1 to 0.8, and~e! 0.1 to 1.0Electrolyte: 0.5 mol dm23 H SO . Scan rate: 50 mV s21.
2 4

DOT to the overall electrochemical charging of the PMo12-PEDOTfilm. In other words, voltammetric behavior of the hybrid filmdominated by the redox characteristics of a polyoxometallateavoid interference from hydrogen evolution reaction at negativetentials, we have restricted our considerations to the three mostive sets of voltammetric peaks. In view of the literature,34-58 theabove redox reactions shall be interpreted in terms of three contive two-electron processes that can be described as

PMo12VIO40

23 1 ne2 1 nH1 ⇔ HnPMonVMo122n

VI O4023 @1#

wheren is equal to 2, 4, or 6. Note that by analogy to the prevreports describing the electrochemical behavior of PMo12 monolayeon glassy carbon50 or PMo12 attached to the self-assembled molayer of 4-amonotiophenol on gold,57 the full-width at half-maximum of the peak for the second oxidation or reduction~appearing at about 0.2 V in Fig. 4a! is very close to the theoreticvalue of 45 mV expected for an ideal two-electron surfacevoltammetric peak. The peak heights are directly proportionscan rates up to at least 0.6 V s21 ~Fig. 5A!. Moreover, the formapotentials of the three sets peaks~Fig. 5A! are largely independeof scan rate. Despite the fact that the film has been fairly thick~ca.200 nm!, the above observations are consistent with the surfacbehavior of the system and good dynamics of charge propagatthe composite system.

The charge under two reduction peaks appearing at potentiaca.0.35 and 0.2 V~Fig. 4a!are about equal within 10%. Also, for theredox waves, the respective ratios of oxidation-to-reductioncurrents are close to unity. The set of peaks at more negative ptials seems to be somewhat affected by to proton discharge reaBased on the data for second best-defined cathodic peak at abV, we have estimated the apparent surface coverage of PMo12 to beon the level ofca. 4 3 1028 mol cm22.

The stability of composite PMo12-PEDOT film was diagnosed bsubjecting the system to the long-term voltammetric potentiacling in 1 mol dm23 H2SO4 electrolyte within the selected potenlimits. For example, the following potential cycling for 1 h withthe potential range20.2 to 0.8 V in 1 mol dm23 H2SO4 electrolyteto the decrease of voltammetric peak currents for PMocomponen

Figure 4. ~a! CV of the optimum PMo12-PEDOT film. For comparison, thresponse of PEDOT film~b! is provided. Electrolyte: 0.5 mol dm23 H2SO4 .Scan rate: 50 mV s21.

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did not exceed 5%. It is reasonable to expect that the abovecurrent decreases originate from surface dissolution~in contact withliquid supporting electrolyte! rather than structural degradationthe composite material. On the whole, the result is consistentgood stability of the composite system. The stability will be furimproved when a separator in a form of solid electrolyte memb~e.g., polybenzimidazole40! is used.

The presence of PMo12 within the composite film was also evdent from theex situFTIR ~Fig. 6! examination~by reflectance!ofthe glassy carbon electrode surfaces modified with thin films o~a!PEDOT and~b! PMo12-PEDOT. Comparison was also made tospectrum characteristic of PMo12 in KBr ~Fig. 6c!. It is reasonableexpect that bands at 975, 832, and 780 cm21 originated from PMo1~Fig. 6b!. Because they were somewhat shifted in comparisondata of PMo12 in KBr ~namely, to bands in Fig. 6c at 962, 870, a787 cm21 characteristic of Mo5 O~terminal), Mo-O~cornesh!-Mo and Mo-O~edge sh!-Mo stretching vibrations39!, strong in-teraction between polyanions and polymer backbone60 can be postulated. Further, the PEDOT~Fig. 6a!and PMo12-PEDOT~Fig. 6b!samples differ in the oxidation state. Indeed, the latter speccontains so-called doping induced bands at 1295, 1128, ancm21.13

We have also considered a composite PW12-PEDOT film ~Fig.7!. The system is also characterized by fast and reversiblereactions~involving protons! that are usually described as

PW12VIO40

23 1 ne2 1 nH1 ⇔ HnPWnVW122n

VI O4023 @2#

where n is typically equal to 1, 2, or 4.59 Figure 7A shows CVresponses of PW12-PEDOT film recorded at different scan ra~from 0.02 to 1.0 V s21!. Although the first two sets of PW12 volta-mmetric peaks tend to overlap each other, their positions are n~asbefore in the case of PMo12-PEDOT!largely dependent on scan r~Fig. 7B!. When the peak currents have been plottedvs. scan ratethe dependence is practically linear~effectively with zero intercep!up to about at least 0.5 V s21. Despite the fact that the film thickne~ca. 1 mm! much exceeds the monolayer level, the system stil

Figure 5. ~A! CV responses of the optimum PMo12-PEDOT film recorded ascan rates of~a! 0.6, ~b! 0.4, ~c! 0.2, ~d! 0.1, ~e! 0.05, and~f! 0.01 V s21. ~B!Inset: the dependence of the reduction peak current~at ca. 0.2 V! on scanrate. Other conditions as for Fig. 4.


k

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hibits the surface type characteristics. This behavior impliesdynamics of charge propagation in the composite system.

To address surface distribution of system, we have examinecompared morphologies of a single component, PEDOT~Fig. 8A!,and the composite~Fig. 8B! films using scanning electron microcopy ~SEM!. Although the structure is granular in all cases,actual size and distribution of grains is different for PEDOT in cparison to PMo12-PEDOT. The morphology of the latter microstr

Figure 6. FTIR spectra of~a! PEDOT and~b! PMo12-PEDOT films deposited on glassy carbon. Curve~c! stands for the spectrum of H3PMo12O40 inKBr.

Figure 7. ~A! Voltammetric behavior of PW12-PEDOT film at scan rates~a! 1, ~b! 0.5 ~c! 0.1, and~d! 0.02 V s21. Electrolyte, 0.5 mol dm23 H2SO4 .~B! Inset shows the dependence of the reduction peak current~at ca.20.4 V!on scan rate.



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ture is granular but seems to be denser. Similar microscopic~for simplicity not shown here! has been obtained for PW12-PEDOTfilms. Therefore, it is likely that electronic communication betwinorganic~polyoxometallate! redox centers occurs via PEDOT cotact. Further research is in progress aiming at elucidating sumorphology of films underin situ conditions using nanoscopprobe techniques~STM!.

Dynamics of charge transport.—We also performed a serieschronoculometric~potential step!experiments in which effectiv~apparent!diffusion coefficients ()Deff) were estimated by usumeans,i.e. from the slopes of dependencies of charge~Q! vs.squareroot of time (t1/2) and using an integrated Cottrell equation

Q/t1/2 5 2nFp1/2r 2Deff1/2Co @3#

wherer stands for the electrode~disk! radius and other parametehave their usual meaning. As it comes from the chronoculomplot for PMo12-PEDOT film ~Fig. 9!, the linear portion of the plotwell-defined, and determination of theQ/t1/2 slope is feasible. Thconcentration of PMo12 redox (Co) centers was estimated toequal to 0.8 mol dm23 upon consideration of such parameters asloading of PMo12 (1.6 3 1028 mol cm22! and the film thicknes~0.2 mm!.

Although, there is some uncertainty in the estimation ofCo , it isimportant to note that the value ofDeff 5 4 3 1028 cm2 s21 ob-tained for PMo12-PEDOT film is fairly high. The fact, that a fairthick ~0.2 mm! film exhibited a pretty high diffusion coefficient,consistent with the view that ohmic limitations which are typic

Figure 8. SEM images of~A! PEDOT and~B! PMo12-PEDOT films ofglassy carbon that were prepared as for Fig. 4b and a, respectively.


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complicating redox behavior of thick polymeric films on electroare practically negligible in our composite structure. Similar consions~for simplicity, the respective chronocoulometric data areshown here!can be made about charge transport in PW12-PEDOTfilms.

Conclusions

We have demonstrated the feasibility of the formation~underaqueous conditions! of hybrid PMo12-PEDOT and PW12-PEDOTfilms on carbon substrates. PEDOT serves as stable and condmatrix for the fast outer-sphere electron-transfer polyoxometaredox sites immobilized within. The polyoxometallate clustervides an ultimate degree of dispersion for an oxide phase becatwelve MO6 ~where M is Mo or W! moieties are at the surface ofcluster.40,41 An important issue is that these nanometric inorgclusters can be introduced into the conducting polymer at thehigh concentration~ca. 0.8 mol dm23! level. Thus the distance btween heteropolymolybdate or tungstate mixed-valence redoxters is small enough to assure the feasibility of fast electron hobetween them. The composite films are also expected to be penough to permit the unimpeded flux of protons. The experimhave successfully been performed in aqueous acid medium bePEDOT is stable, and the counterion (H1) flux and the concomitaprotonation of polyoxometallate clusters facilitates the systemversible redox chemistry. Consequently, our PEDOT-based comite films are characterized by fast dynamics of charge transpor~dif-fusion coefficients at least on the 1028 cm2 s21 level! in 0.5 moldm23 H2SO4 . Because the formal potentials of PMo12-PEDOT andPW12-PEDOT redox processes are different, and both films careadily investigated in the same acid electrolyte, the materials cconsidered as electrodes for asymmetric redox~faradaiccapacitors.61 Further research is along this line.

Acknowledgments

This work was supported by Ministry of Science and Informaunder KBN projects 4T08C06525 and 7 T09A 05426. A giftEDOT monomer from Bayer is highly appreciated.

The University of Warsaw assisted in meeting the publication costs oarticle.

Figure 9. Double potential step chronoculometric plots for PMo12-PEDOTfilm deposited on glassy carbon. An initial potential step was from 00.05 V ~Fig. 4a!. Electrolyte, 0.5 mol dm23 H2SO4 .



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