Journal of Electroanalytical Chemistry 652 (2011) 52–59

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Journal of Electroanalytical Chemistry

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Comparison of the electrochemical behaviour of buckypaperand polymer-intercalated buckypaper electrodes

Suriya Ounnunkad a, Andrew I. Minett a, Mark D. Imisides a,1, Noel W. Duffy b,2, Barry D. Fleming b,Chong-Yong Lee b, Alan M. Bond b,⇑, Gordon G. Wallace a,⇑a ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong Innovation Campus,AIIM Building, Squires Way, Fairy Meadow, NSW 2519, Australiab School of Chemistry, Monash University, Clayton, VIC 3800, Australia

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

Article history:Received 10 February 2010Received in revised form 9 August 2010Accepted 13 September 2010Available online 1 October 2010

Keywords:Polymer intercalationSingle-walled carbon nanotubeBuckypaperAC voltammetryPorous electrodes

1572-6657/$ - see front matter � 2010 Elsevier B.V. Adoi:10.1016/j.jelechem.2010.09.013

⇑ Corresponding authors. Address: ARC Centre of ExScience, Intelligent Polymer Research Institute, Univegong, NSW 2522, Australia. Tel.: +61 2 42213127 (G.G

E-mail addresses: Alan.Bond@sci.monash.edu.auedu.au (G.G. Wallace).

1 Present address: Rowe Scientific, 11 Challenge Bo6065, Australia.

2 Present address: CSIRO Energy Technology, BayvAustralia.

The performances of freestanding carbon nanotube (buckypaper) and polymer-intercalated buckypaperelectrodes in an electroanalytical chemistry context were evaluated via analysis of direct current andFourier Transform large-amplitude alternating current voltammograms derived from the ferrocenemono-carboxylic acid (FMCA0/+), ruthenium hexamine ([Ru(NH3)6]3+/2+) and ferricyanide ([Fe(CN)6]3�/4�) redoxcouples. The composite polymer-intercalated buckypaper electrodes exhibit substantially superior Fara-daic-to-capacitive background charging current ratios under both dc and ac conditions compared anddisplay close to ideal voltammetry for all three processes. A significant difference was detected in mid-point potentials determined by cyclic voltammetry at buckypaper and polymer-intercalated buckypaperelectrodes, commensurate with different mass transport mechanisms. It is proposed that the porosity ofthe buckypaper gives rise to a restricted diffusion model of mass transport within the pores and a largeelectrode over that generates a large capacitance current. Thus, polymer intercalation is required toachieve high quality electroanalytical performance. Simulations of voltammograms obtained at porouspolymer-intercalated buckypaper electrodes are consistent with the composite electrodes consisting ofa randomly arranged array of nano-/micro-electrode domains, implying that significant surface heteroge-neity is present. However, under slow scan rate conditions, when significant overlap of diffusion layersoccurs, voltammograms may be approximately interpreted in terms of a linear diffusion based masstransport model.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction in the paper having between 60% and 70% of its volume as free

Applications involving carbon nanotubes (CNTs) are many andvaried due to their interesting physical, chemical and mechanicalproperties [1–3]. One method to further enhance the usefulnessof CNTs in materials and electrochemical applications has beento prepare them into free-standing films, often referred to asbuckypaper [4]. However, compared to individual CNTs, themechanical and structural properties of buckypaper are relativelypoor. This is largely related to the weakly bound and randomlypacked bundles of CNTs formed during processing, which results

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cellence for Electromaterialsrsity of Wollongong, Wollon-. Wallace).

(A.M. Bond), gwallace@uow.

ulevard, Wangara, Perth, WA

iew Ave., Clayton, VIC 3169,

space [5]. The majority of the free space is typically made up ofinterbundle pores or small macropores, whose sizes range from10 to 1000 nm in diameter [5,6]. Intercalation of polymers intothe buckypaper free space, either in situ or post-production, hasproven to significantly improve its mechanical properties [5,7,8].The use of naturally occurring biopolymers, such as DNA andchitosan, has been shown to produce buckypaper that is suffi-ciently conductive and mechanically strong, and suitable for useas biocompatible electrodes [9].

The electrochemistry at electrodes modified with CNTs hasbeen intensively investigated during the last two decades [10].The electrochemical reactivity of CNTs has been considered similarto that of highly-ordered pyrolytic graphite electrodes, composedof basal- and edge-plane sites possessing different reactivities,kinetics or electron transfer properties [11]. Imperfections are as-sumed to arise during the growth process or during sonication todisperse the CNTs or during the fabrication process for CNT elec-trodes. The CNTs may also contain impurities such as graphite par-ticles, carbon nanoparticles, other forms of carbon, and residualmetal based catalysts [5,12–14].

S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59 53

To date, there have been few reports on the electrochemicalcharacteristics of buckypaper electrodes [9,15]. In the presentinvestigation, we compare bare buckypaper and buckypaper thathas been intercalated with a series of non-conducting and conduct-ing polymers. We report on their electrochemical properties as re-vealed by dc and large-amplitude ac voltammetric studies of threestandard redox couples that encompass the three charged states ofneutral, negative and positive, viz. the oxidation of ferrocenemono-carboxylic acid (FMCA0/+), the reduction of ferricyanide ([Fe(CN)6]3�/4�), and the reduction of ruthenium hexamine ([Ru(NH3)6]3+/2+).

2. Experimental section

2.1. Chemicals

Ferrocenemonocarboxylic acid (FMCA) (Sigma–Aldrich, 97%purity), KCl (AR BDH), K4[Fe(CN)6]�3H2O (AR Ajax Finechem),[Ru(NH3)6]Cl3 (Sigma–Aldrich, 98% purity), phosphate buffer saline(PBS) tablets (Sigma–Aldrich), single-walled carbon nanotubes (CNInanotechnologies; lot P0317), Triton-X100 (AR. BDH), Ethanol (Uni-var, 95% purity), toluene (AR. Univar), poly(styrene-b-isobutylene-b-styrene) (MW = 160,362, a gift from Boston Scientific), polysty-rene (Mw = 45,000, Aldrich) and polyisobutylene (Mw = 500,000,Aldrich) all were used as received from the manufacturer. Poly(3-octyl pyrrole) (Fe-DBSA dopant) and poly((E)-4,400-didecoxy-30-sty-ryl[2,20:50,200]terthiophene) (an average polymer length of 11monomer units, �7.2 kDa) were chemically synthesized by theIntelligent Polymer Research Institute (Wollongong, Australia).Water derived from a Milli-Q purification system (resistivity18 MX cm) was used to prepare aqueous solutions. A PVDF mem-brane (Durapore� Membrane Filters, Millipore) with a pore size of0.22 lm was used for vacuum assisted filtration of the CNT disper-sion solution.

2.2. Preparation of buckypapers and intercalation

The preparation of single-walled carbon nanotube (SWCNT)buckypaper and polymer-intercalated buckypaper are describedin detail elsewhere [5,7]. SWCNT soot was used for fabrication ofbuckypaper without further purification. The dispersion solutionof 120 mg SWCNT in 240 mL of 1% v/v Triton-X100 aqueous solu-tion was prepared using horn sonication (pulse on for 1 s and offfor 1 s) for 3 h, followed by bath sonication for 2 h. The solution ob-tained was filtered through a 0.22 lm pore size hydrophobic PVDFmembrane linked to a vacuum line. The obtained buckypaper waswashed several times with water until the bubbles of the surfac-tant disappeared, indicating that the majority of the surfactanthad been removed. After washing with water, the buckypaperwas then rinsed with ethanol and then allowed to dry. The func-tionalising or adsorbed surfactant is present in the buckypapermaterials at a small level. The average thickness of resultantbuckypaper lies in the range of 50–70 lm. The dried buckypaperwas cut into rectangular strips (0.5 cm � 2.5 cm). The polymerwas intercalated into the buckypaper architecture via soaking thebuckypaper in a polymer solution for given periods of time. Thesolutions of poly(styrene-b-isobutylene-b-styrene), polystyreneand polyisobutylene were prepared at the concentration of 5% w/v in toluene while poly(3-octyl pyrrole) solution was of 0.125 %w/v in dichloromethane. Poly((E)-4,400-didecoxy-30-styryl[2,20:50,200]terthiophene) solution was used at a concentration of 0.027 %w/v in toluene. Finally, the polymer-intercalated buckypaper stripswere used as electrodes after carefully washing in toluene and dry-ing at room temperature for 24 h. The sample weights before andafter intercalation were measured using a microbalance (Sartorius)

to determine the weight percentage of polymer intercalation. Forconvenience, the unmodified and poly(styrene-b-isobutylene-b-styrene), polystyrene, polyisobutylene, poly(3-octyl pyrrole) andpoly((E)-4,400-didecoxy-30-styryl[2,20:50,200]terthiophene) interca-lated buckypaper electrodes will be referred to as BP, SIBS–BP,PS–BP, PIB–BP, POP–BP and PTP–BP, respectively.

2.3. Instrumentation and electrochemical procedures

The morphology of the BP architecture was investigated using aJEOL Scanning Electron Microscope. To obtain the cross-sectionalimages, the BP was frozen in liquid nitrogen and carefully snappedopen to expose the interior.

All electrochemical experiments were performed with instru-mentation capable of generating a sinusoidal ac waveform of fre-quency, f, and amplitude, DE, superimposed onto the triangulardc ramp which was swept over the potential range of interest ata given scan rate, v. Full details of the instrumentation are de-scribed elsewhere [16]. When large ac amplitudes are used, secondand higher order harmonics are generated. These can be resolvedby Fourier Transformation of data in the time domain to give thepower spectrum (frequency domain) followed by band selection(filtering) and then Inverse Fourier Transformation to give the indi-vidual harmonics (time domain) as also described in Ref. [16]. Fordc cyclic voltammetry, the amplitude of the applied sinusoidalwaveform was set to zero. Experiments were carried out using athree-electrode cell containing a BP type working electrode, a Ag/AgCl (3 M NaCl) reference electrode and a platinum wire auxiliaryelectrode. Voltammetric studies at an edge-plane graphite (EPG)working electrode was also undertaken to provide comparativeinformation. The BP electrode was connected to an alligator clip,suspended in the solution, with careful attention given to keepingthe connection well above the solution level. The area of the EPGelectrode was determined by comparisons of experimental datawith that obtained by simulations using DigiSim software [17].The diffusion coefficients, D, used in this and other simulationswere: 7.6 � 10�6, 6.3 � 10�6, 7.6 � 10�6, 7.8 � 10�6 cm2 s�1 for[Fe(CN)6]3�, [Fe(CN)6]4�, [Ru(NH3)6]3+ and [Ru(NH3)6]2+, respec-tively, and 6.0 � 10�6 cm2 s�1 for both FMCA0 and FMCA+. All vol-tammetric experiments were undertaken at room temperature(293 ± 2 K) in pH 7.4 phosphate buffer solutions containing 0.1 MKCl as the supporting electrolyte. High-purity nitrogen was bub-bled into the solution for at least 5 min to remove oxygen beforecommencing each experiment. The uncompensated resistance, Ru,for the cell was typically less than 100 O.

2.4. Simulations

Simulations of the dc cyclic voltammograms presented in thispaper refers to solution-phase, one-electron charge transfer pro-cesses, in which species Red is oxidised to Ox (or species Ox is re-duced to Red). Simulations were undertaken on the basis of theButler–Volmer model for electron transfer and linear or other des-ignated forms of diffusion. Thus, the oxidation process is summa-rised by following equation:

Red$kf

kb

Oxþ e� ð1Þ

where kf and kb are the forward and backward electron transfer rateconstants respectively.

In accordance with Butler–Volmer kinetics

kf ¼ k00 expanFRT

EðtÞ � E00� �� �

ð2Þ

kb ¼ k00 exp�ð1� aÞnF

RTEðtÞ � E00� �� �

ð3Þ

54 S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59

where k00 is the formal electron transfer rate constant, a is thecharge transfer coefficient (assumed to be 0.5), E00 is the formalreversible potential of the redox process, E(t) is the applied poten-tial, t is the time and n, R, T and F have their usual meanings.

3. Results and discussion

3.1. Electrode morphology

Scanning electron microscopy (SEM) images of the surface and across-section of BP and PIB–PB electrodes are shown in Fig. 1. Forthe BP electrode a similar porous network of randomly orderedindividual and bundled CNTs is observed at the surface (Fig. 1a)and in the interior (Fig. 1b). For the PIB–BP electrode, pores are stillvisible at the surface (Fig. 1c), but no CNTs or porosity is observedat the fracture surface (Fig. 1d). This indicates that polymer massuptake is principally occurring in the interior of the BP, wherebythe PIB has intercalated into the free volume contained withinthe BP to fill up the ‘‘inter-bundle” pores formed by the SWCNTnetworks as well as the ‘‘inter-tube” (interstitial) channels [5]. Thiswas characteristic of all the polymer-intercalated BP electrodes.

3.2. Electrochemical characterisation

3.2.1. DC cyclic voltammetryThe dc cyclic voltammetric responses at bare BP and nanocom-

posite polymer–BP electrodes were obtained with three differentredox probes; ferrocenemonocarboxylic acid (FMCA0/+), rutheniumhexamine ([Ru(NH3)6]3+/2+) and ferricyanide ([Fe(CN)6]3�/4�). Foreach experiment the concentration of the redox probe was1.0 mM in a buffered phosphate solution (pH 7.4) containing0.10 M KCl as supporting electrolyte. DC (and large-amplitude ac)cyclic voltammograms were obtained at BP and polymer-interca-lated BP electrodes for each redox probe.

Fig. 2 provides a comparison of dc cyclic voltammograms ob-tained at BP and PS–BP electrodes at a scan rate of 59.60 mV s�1

for each of the redox probes. The BP electrode exhibits a very highbackground capacitive charging current with a much smaller Fara-daic current that is barely detected under the conditions of Fig. 2.

Fig. 1. SEM images showing top-down (a, b) and cross-sectional (c, d)

After intercalation with PS, the capacitive charging current of thePS–BP electrode is dramatically reduced. The Faradaic current, alsois diminished in magnitude, but to a much more limited extent.That is the faradaic to capacitance current ratio is markedly supe-rior at the PS–BP electrode so that the faradaic current is now read-ily detectable. The difference in the magnitude of the capacitancecurrent for the two forms of electrodes that have approximatelythe same apparent geometric area in contact with solution, isabout two orders of magnitude (note the different y-axis scalesfor the BP and PS–BP electrodes). The high porosity of the BP elec-trode allows solution to enter into this structure which results in alarge electroactive surface area that is reflected by the large capac-itance current. The decrease in the capacitive current in the PS–BPelectrode is consistent with the polymer filling up the pores in theBP and substantially reducing the electroactive surface area. At thelow scan rate of about 60 mV s�1, overlap of diffusion layers occursso that the faradaic current is now derived from a close to lineardiffusion model. The net effect is that the PS–BP electrode has asignificantly superior Faradaic-to-background current ratio thanthat obtained at bare and porous BP electrodes. Consequently,the faradaic response is much more accurately identifiable fromvoltammograms obtained at the PS–BP electrode and analyticaluse of the electrodes requires this format.

Table 1 provides a summary of the mid-point (Em) potentialdata (approximately reversible format potentials of E00 value) whenmass transport occurs predominantly by linear diffusion at poly-mer modified BP electrodes and also at the bare electrodes wherediffusion is restricted under conditions of cyclic voltammetry witha scan rate of 59.60 mV s�1. As also clearly revealed on examina-tion of Fig. 2, the mid-point potentials for the oxidation of FMCA0

and reduction of [Ru(NH3)6]3+ and [Fe(CN)6]3� differ significantlyin the two scenarios. In all three cases, the initial oxidation orreduction of the relevant species is harder to achieve at the bareBP electrode. This is attributable to the redox species being‘trapped’ inside the internal structure of the BP, and hence, diffu-sion is restricted so that the charge neutralisation process may be-come rate determining. Thus, the mass transport mechanismrelevant to each case differs significantly. The voltammetry offerrocyanide at a glassy carbon electrode modified with multiple

views of a bare BP (left) and PIB intercalated BP electrode (right).

Fig. 2. Comparison of dc cyclic voltammograms at bare BP (red) and PS–BP (blue) for the oxidation of 1.0 mM FMCA (a), reduction of 1.0 mM [Ru(NH3)6]3+ (b) and reduction of1.0 mM [Fe(CN)6]3� (c), respectively. All voltammograms were measured in phosphate buffer solution (pH 7.4) containing 0.1 M KCl at a scan rate 59.60 mV s�1 andtemperature of 293 K. The current scale for the bare BP is on the left y-axis and the current scale for the PS–BP is on the right y-axis, as shown via the arrow. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Table 1The mid-point potential, Em, and minimum peak separation, DEp, values observed forthe FMCA0/+, [Ru(NH3)6]3+/2+ and [Fe(CN)6]3�/4� redox couples at each of the BPelectrode surfaces.

Polymer FMCA0/+ Ru(NH3)3+/2+ Fe(CN6)3�/4�

Em/mV DEp/mV Em/mV DEp/mV Em/mV DEp/mV

BP 431 280 �258 277 230 184SIBS 320 72(e) �168 69(a) 219 136(a)

PS 309 69(b) �151 77(b) 218 74(e)

PIB 310 83(e) �177 73(b) 220 130(e)

POP 307 100(c) �152 66(c) 226 393(c)

PTP 308 82(c) �164 77(d) 229 119(c)

EPG 316 61 �169 59 222 62

Minimum DT values are given and then observed after (a) 72 h, (b)30 min, (c)192 h,(d)120 h, (e)1 h and were obtained under conditions of cyclic voltammetry with ascan rate of 59.60 mV s�1.

S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59 55

layers of SWCNTs was also shown to be influenced by the electro-active species being trapped within pockets of solvent located inbetween the nanotubes [18]. Importantly, with all polymer–BPelectrodes the Em values for the redox couples are close to thoseobserved at an edge-plane graphite (EPG) electrode, as predictedif mass transport conforms closely to linear diffusion theory. Inthe case of the bare BP, restricted diffusion theory is more relevantand gives rise to a different Em – value to that expected when masstransport occurs by planar diffusion.

Table 1 also provides a summary of the minimum peak-to-peak separation, DEp, values obtained from dc cyclic voltammo-grams for each electrode configuration under designated condi-tions. The DEp values at the polymer intercalated electrodes

provide a qualitative indication of the electron transfer kinetics;the smaller the DEp value the faster the rate of electron transfer(larger k00-value) when mass transport occurs by linear diffusion.For the inner-sphere electron transfer processes, FMCA0/+ and[Ru(NH3)6]3+/2+, the DEp values at the polymer–BP electrodes ap-proach those found at the EPG electrode. The [Fe(CN)6]3�/4� redoxcouple is a quasi-reversible (slower) outer-sphere electron trans-fer process with a k00-value that is sensitive to the electrode com-position, the electrode surface and any surface pre-treatment thattogether contribute to the generally observed larger values of DEp

associated with the slower rate of electron transfer. Minimal DEp

values presented in Table 1 required different polymer contacttimes, reflecting the different rates of intercalation of polymerinto the BP electrode. For example, for the oxidation of FMCA0,the minimum DEp value at a PS–BP electrode of 69 mV was ob-tained within 30 min of intercalation time, whereas for thePOP–BP electrode 192 h were required to achieve the minimumvalue of 100 mV.

Notably, interpretation of differences in DEp values obtained atbare and polymer-intercalated BP electrodes in terms of electrontransfer kinetics is inappropriate given that the mass transportmechanisms differ. However, an implication may be drawn thatthe overall process is sluggish in the bare BP case and faradaic-to-capacitance rates at the porous electrode are too poor to provideanalytically useful data.

3.2.2. FT ac voltammetryThe FT ac voltammetric methodology provides additional

understanding of electrode mechanisms [16]. In particular, exami-nation of the harmonic content under large amplitude conditions

56 S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59

provides unique insights into the capacitance and faradaic currentcontributions [19]. Typically, a combination of faradaic and capac-itance current is detected in the fundamental harmonic, whereasonly purely faradaic current is present in the higher orderharmonics.

Fig. 3 shows a comparison of the power spectra (frequencydomain) and the 1st to 5th harmonic (time domain) ac componentsderived from the oxidation of 1 mM FMCA at bare BP and SIBS–BPelectrodes when a sinusoidal waveform (f = 35.02 Hz, DE = 80 mV)is superimposed upon a dc ramp (v = 59.60 mV s�1). The powerspectrum in the SIBS–BP case shows that a larger number of har-monics are detectable (up to 8th harmonic at intervals of 35 Hzin this case) compared with the BP electrode (up to 3rd harmoniconly detected). The large number of harmonics obtained in theSIBS–BP case is indicative of significant levels faradaic current de-rived from fast electron transfer kinetics. Applying the inverse FTprocedures allows the individual harmonics to be resolved.

Fig. 3. Comparison of the power spectra (a), fundamental (b), second (c), third (d), fourthbare BP (red) and SIBS–BP (blue) electrodes when measured in phosphate buffer soluv = 59.60 mV s�1, Estart = 0 mV, and Eswitch = 800 mV, T = 293 K. The power spectra ininterpretation of the references to colour in this figure legend, the reader is referred to

Significant Faradaic current is observed for the second and high-er order harmonic compounds associated with the SIBS–BP elec-trode process with only a minor contribution from capacitancecurrent in the fundamental harmonic case. For the 1st to 5th har-monic components (Fig. 3b–f) the peak maxima (1st, 3rd, 5th har-monic) and minima (second and fourth) harmonic currents centredat 5.2 s and 22 s are due to the FMCA0/+ oxidation and FMCA+/0

reduction processes, respectively. These times, when convertedto potential, are the same (0.319 V). Theoretically, if the processis reversible (large k00-value) with mass transport by linear diffu-sion these potentials should equate closely to the reversible formal(E00) potential for the FMCA0/+ process (approximated by Em values)derived from dc cyclic voltammetry at SIBS–BP (0.320 V) and EPG(0.316 V) electrodes (see Table 1). This close equivalence of theseac and dc based potentials was also found in studies under theFMCA0/+ process undertaken with the other non-conducting poly-mer–BP electrodes.

(e) and fifth (f) harmonic components obtained for the oxidation of 1.0 mM FMCA attion (pH 7.4) containing 0.1 M KCl. Conditions employed: f = 35 Hz, DE = 80 mV,

(a) have been offset by an arbitrary amount to allow ease of comparison. (Forthe web version of this article.)

S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59 57

Interestingly, the higher harmonic components observed in thepower spectrum for the raw BP electrode are predominantly re-lated to the non-linearity of background capacitance and not theFMCA0/+ Faradaic process. In this case, only the 1st harmonicexhibits evidence of Faradaic current being present for the bareBP electrode and even this is swamped by the large backgroundcapacitance current. Again, these electrodes are not useful in theanalytical sense even under ac conditions.

The fundamental harmonic ac voltammograms provide a usefulway of comparing the Faradaic-to-background ratios for the differ-ent electrodes. Even if the electrode area varies between experi-ments, a comparison of the Faradaic current to backgroundcurrent provides a useful normalised form of comparison. Forexample, in the fundamental harmonics shown in Fig. 3b the Fara-daic-to-background ratio of the BP electrode is 0.03, whilst for theSIBS–BP electrode it is 4.5. That is, over two orders of magnitudedifference in the Faradaic-to-background ratio are encountered. Asimilarly high Faradaic-to-background ratio was consistently ob-served for BP electrodes intercalated with the non-conductingpolymers, SIBS, PIB and PS. The same analysis performed with anEPG electrode revealed a Faradaic-to-background ratio of around1.5. Thus, on average, the BP electrodes intercalated with non-con-ducting polymer had a Faradaic-to-background ratio of at leasttwice that of an EPG electrode. The fast electron transfer rate cou-pled with a significantly improved Faradaic-to-background capac-itive current ratio suggests that these polymer-intercalated BPelectrodes can offer significant advantages in electroanalyticaldevices.

For the conducting polymers, a longer intercalation periodwas needed to achieve analytically optimal electrode perfor-mance. This was evidenced by noting data provided for DEp val-ues (Table 1), the increase in the number of ac harmonicresponses in the relevant power spectra (Fig. 3a), and as high-lighted in Fig. 4, closer overlap of the peak potentials for the oxi-dative and reductive components of processes observed infundamental harmonic voltammograms. For a reversible electrontransfer process with mass transport by planar diffusion, thepeak or null current positions (as relevant) for the relevant har-monics should coincide with the E00 values found at the EPG elec-trode. Fig. 4 shows the fundamental harmonic voltammogramsobtained for the [Ru(NH3)6]2+/3+ redox process at POP–BP andPIB–BP electrodes at designated intercalation times. The positionsof the reductive and oxidative peak potentials become close tocoincident for the POP–BP and PIB–BP electrodes after 30 minand 192 h, respectively. In general, a longer intercalation time(>1 day) was required for the conducting polymers, POP andPTP, to achieve this condition whilst for the insulating polymers,less than 1 h was required. This was the case for all harmonics

Fig. 4. Comparison of 1st harmonic voltammograms obtained for the reduction of 1.0 mMand (b) PIB. Conditions employed: as for Fig. 3. The harmonics within each plot have be

generated at the SIBS–BP electrode (Fig. 3), as also applies forPS and PIB electrodes (data not shown).

3.3. Mechanistic insight gained by experiment-simulation comparisons

The wave shape observed in dc cyclic voltammograms is indic-ative of the nature of the electron transfer and mass transport pro-cesses. For example, a sigmoidal waveshape is characteristic of asteady-state process with radial diffusion at a microdisk electrodewhereas a peak-shape is associated with a transient process andplanar diffusion derived from a macrodisk electrode. For the poly-mer–BP electrodes, sigmoidal and peak–shape characteristics wereapparent, with the peak shaped one being dominant. In principle,this variability could be related to the heterogeneous nature ofthe BP electrode and the impact of the intercalation process. Exper-iment-simulation comparisons of dc cyclic voltammograms assistin providing information on the mass transport properties relevantto these composite electrodes.

Fig. 5 shows a comparison of the dc experimental and simulatedvoltammograms for reduction of [Ru(NH3)6]3+ at PIB–BP and PS–BPelectrodes obtained at scan rates of 14.9, 119.2 and 954 mV s�1.The simulation data for the PIB–BP case is based on use of thehemispherical diffusion approximation model, with the assump-tion that the voltammograms are derived from the summation of1000 micro-electrodes each with a radius of 0.0045 cm. The simu-lation parameters utilised are provided in the caption to Fig. 5.Noteworthy, the distribution of the arrays of micro-electrodes inthe experimental case may lead to variability with respect to thesize and spacing. Therefore, as expected, this simplified simulationmodel is unable to quantitatively match experimental data but itdoes predict some of the features. To obtain a better fit, a moresophisticated model such as diffusion domain approximation ap-proach that was developed by Compton’s group may be required[17,20]. However, judging by the parallel change of shape of thesimulated and experimental voltammograms as a function of scanrate, the PIB–BP electrode exhibits micro-electrode array typebehaviour.

On the other hand, simulations based on purely planar diffusionprovide a good model for voltammograms obtained at a PS–BPelectrode at all scan rates examined. In this case, the micro-elec-trodes in the array may be more closely spaced which allowsextensive overlap of the diffusion layers. As a consequence, theelectrode behaviour mimics that predicted for a planar electrode.For some other electrodes (results not shown), the scan rate depen-dence is indicative of a combination of both linear and radial diffu-sion. In fact, the observed behaviour for the polymer-intercalatedBP electrodes encompasses many of the possible scenarios pre-dicted for an array of micro-electrodes [17].

[Ru(NH3)6]3+ at BP electrodes intercalated for different periods of time with (a) POPen offset by an arbitrary amount to allow ease of comparison of the peak positions.

Fig. 5. Comparison of experimental (–) and simulated (s) dc cyclic voltammograms for the reduction of 1.0 mM [Ru(NH3)6]3+ measured at PIB–BP (a1–a3) and PS–BP (b1–b3)electrodes with scan rates of 14.90, 119.21 and 953.67 mV s�1, respectively. All voltammograms were measured in phosphate buffer solution (pH 7.4) containing 0.1 M KCl.For a1–a3, the simulated data are based on a hemispherical diffusion model with the assumption that the voltammograms are derived from the summation of array of 1000micro-electrodes each with radius of 0.0045 cm and double-layer capacitance of 0.01 lF cm�2. For b1–b3, the simulated data are based on a planar diffusion model using asingle electrode with a total area of 0.50 cm2 and double-layer capacitance of 60 lF cm�2. Others parameters used in simulation are: process assumed to be reversible,diffusion coefficients of [Ru(NH3)6]3+ = 7.6 � 10�6 cm2 s�1 and [Ru(NH3)6]2+ = 7.8 � 10�6 cm2 s�1 respectively, concentration of [Ru(NH3)6]3+ = 1.0 mM, uncompensatedresistance = 100 X and temperature = 293 K.

58 S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59

The micro-electrode domains are likely to consist of individualcarbon nanotubes or bundles of carbon nanotubes that are exposedat the surface of the electrode. There are some similarities betweenthe polymer–BP electrodes and carbon paste electrodes (CPEs).Both electrodes consist of a conducting carbon-based material dis-persed in an insulating matrix. The conductivity of CPEs is relianton intimate physical contact between the carbon particles, whichin the presence of the insulating material, typically paraffin oil,can be restricted. The BP consists of an inter-woven array of CNTsthat are inherently conductive. It was found that the intercalatedinsulating polymer does not affect the bulk conductivity of the

electrode, but merely fills the pore spaces left vacant within therandom CNT framework. This reduces the effective surface areaof the electrode. More importantly, it may leave some individualor bundled CNTs as isolated nano- or micro-electrode domains(that are still electrically connected to the electrode bulk). The areaof these exposed domains will vary across the entire electrode sur-face as will the spacing between them. In addition, the CNTs havedistinct edge- and basal-plane domains which offer different sur-face energies and electrochemical reactivity. Therefore, the totalvoltammetric response is a complex composite of contributionsfrom each of the separate domains.

S. Ounnunkad et al. / Journal of Electroanalytical Chemistry 652 (2011) 52–59 59

4. Conclusions

Buckypaper electrodes intercalated with insulating polymersgive rise to voltammograms that exhibit high Faradaic-to-back-ground ratios and hence are well suited to electroanalytical appli-cations. Low capacitance that generates small backgroundcurrents is needed in amperometric and voltammetric studies toachieve low limits of detection. Devices made from this interca-lated buckypaper should offer fast signal generation as well ashigh analytical sensitivity. In addition, the use of biopolymers willenhance their biocompatibility, and hence prospects for use inbio-sensing, as biofuel cell electrodes, and as platforms for cellcultures. Nevertheless, the voltammetric characteristics detectedalso imply the presence of substantial inhomogeneity in the poly-mer–BP nanocomposite surface which needs to be taken into ac-count. Clearly, base BP electrodes are not suitable forelectroanalytical studies, but may be useful in devices where highcapacitance is needed.

Acknowledgements

We thank Dr. Darrell Elton, Dr Gareth Kennedy, Dr. AnastassijaKonash and Peter Sherrell for helpful discussions, Dr. DamyantiSharma for SEM measurements, Dr. Syed A. Ashraf for providingPOP and Prof. David L. Officer, Dr. Pawel Wagner, and Dr. AmyM. Ballantyne for provision of PTP. The financial support of theAustralian Research Council is gratefully acknowledged. S.O. alsoacknowledges the award of a PhD Scholarship in Nanoscienceand Nanotechnology (Nanodevices) by Higher Educational Strate-gic Scholarships for Frontier Research Network from the Commis-sion on Higher Education, Ministry of Education, Royal ThaiGovernment. A.I.M. acknowledges a QEII Research Fellowship from

the ARC. A.M.B and G.G.W gratefully acknowledge the support ofthe ARC Federation Fellowship scheme.

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