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Contents lists available at SciVerse ScienceDirect

Synthetic Metals

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ynthesis and characterization of conductive polyimide/carbon compositesith Pt surface deposits

ohn M. Kinyanjuia,∗, David W. Hatchetta, Gina Castruitaa, Asanga D. Ranasinghea,othar Weinhardta,b, Timo Hofmanna, Clemens Heskea

Department of Chemistry, University of Nevada, Las Vegas, Las Vegas, NV 89154-4003, United StatesExperimentelle Physik VII, Universität Würzburg, D-97074 Würzburg, Germany

r t i c l e i n f o

rticle history:eceived 29 June 2011eceived in revised form 31 August 2011ccepted 31 August 2011vailable online xxx

eywords:ompositeolyimidearbonlatinumethanol oxidation

a b s t r a c t

The preparation of free-standing polyimide/carbon (PI/Carbon) substrates and the electrochemical depo-sition of Pt to produce PI/Carbon/Pt electrodes are demonstrated to provide thermally stable andconductive PI composites. The conductivity of polyimide (PI)/Carbon composites is evaluated as a func-tion of composition of a binary solvent involving DMSO (dimethyl sulfoxide) and highly volatile acetone,which enhances carbon dispersion (PI/Carbon) in the polymer precursor. The solution conditions havebeen optimized to provide the highest conductivity for the lowest relative carbon loading. The depo-sition of Pt metal on PI/Carbon composite electrodes is demonstrated using cyclic voltammetry. Theconductivity of the PI/Carbon composite is sufficient that the metal precursor PtCl42− is fully reducedand deposited without the need for additional chemical reduction processes. Thermal gravimetric anal-ysis (TGA) shows that the thermal stability of PI is maintained with carbon incorporation and platinumdeposition. Scanning electron microscopy (SEM) analysis shows that carbon aggregation at the PI surfaceis minimized and that Pt deposits are well dispersed. X-ray photoelectron spectroscopy (XPS) resultsindicate that the electrochemical reduction of PtCl42− produces metallic Pt deposits on the PI/Carbon

composite. Four-point probe measurements are utilized to assess the conductivity of the materials andhighlight the influence of C and Pt on the electronic properties of modified PI. Finally, the electrochemi-cal reactivity of PI/Carbon/Pt composite is examined using the redox properties for ferricyanide and thecatalytic oxidation of methanol in acidic solution. The electrochemical experiments demonstrate thatthe free-standing PI/Carbon composites are sufficiently conductive to observe the electrodeposition ofPt metal that is stable and reactive on the organic substrate.

. Introduction

PMDA-ODA or poly(pyromellitic dianhydride-4,4′-xydianiline)imide (referred to as PI in this work) has beenredominantly used as a thermal and electrical insulator because

t is thermally robust, chemically resistant to degradation, andossesses high tensile strength [1]. PI adhesive tapes are com-ercially available to provide thermal insulation based on these

roperties. The use of PI as a substrate for solar cells has alsoeen investigated, motivated by the polymer’s thermal stabilitynd mechanical flexibility [2–7]. However, the application of PIubstrates in solar cells is predicated on the ability to mechani-

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ally interface the material with cell components. For example,nsulating PI has been used as a substrate for flexible Cu(In,Ga)Se2hin film solar cells, but adhesion problems between the PI and Mo

∗ Corresponding author. Tel.: +1 702 328 2925; fax: +1 702 895 4072.E-mail address: jmk@unlv.nevada.edu (J.M. Kinyanjui).

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.08.046

© 2011 Elsevier B.V. All rights reserved.

back contact have been reported [6,7]. Furthermore, chemicallymodified PI with sulfonyl groups has been utilized for applicationsincluding proton exchange membranes for fuel cells [8,9]. Theseexamples highlight the diverse applications based on the inherentproperties of PI and new, emergent properties of modified PI andits interface with a large variety of different materials.

The thermal properties of PI also make it an attractive materialfor applications that require high stability and electrical conduc-tivity of organic matrices at elevated temperatures. Therefore,modification of PI with secondary components that enhance theelectrical and mechanical properties of the material has beenexplored. For example, both single (SWNT) and multi-walled car-bon nanotubes (MWNT) have been successfully incorporated intoPI to improve tensile strength and minimize electrostatic chargebuildup [10–15]. The success of such materials is evident from the

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commercial availability of PI/Carbon nanotube composites such asAURUMTM® by Mitsui. However, the high cost of carbon nanotubesis a drawback to the bulk production and application of such materi-als, and carbon black materials have thus been considered as lower

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ost alternatives that can also enhance the electronic properties ofI.

Traditionally, carbon materials have been dispersed into PIsing DMSO (dimethyl sulfoxide) and other common solvents tobtain electronically conductive PI/Carbon composites [16–18].he use of low-volatility solvents such as DMSO and NMP (n-ethylpyrrolidone) is problematic because the homogeneity of

he PI/Carbon composite is influenced by dispersion of the sec-ndary component and trapped solvent in the resulting polymerlm [17]. Uniform dispersion of the carbon black is also criticalo ensure that the mechanical integrity of PI is maintained, withigh aggregation of the secondary component producing brittleaterials [19]. Therefore, a uniform dispersion of carbon and the

I precursor [polyamic acid (PAA)] in a suitable solvent prior tomidization increases the homogeneity of the material [17,20–22].

oreover, enhanced electrical conductivity has been previouslyttributed to the formation of small fibrous carbon black aggregatesith chain-like structures within the polymer matrix [19]. Suchicrostructures can be instrumental in the transport of electrons

hrough insulating polymers and strongly influenced by solventomposition.

The combination of PI with metals is not specifically tied to theormation of conductive PI using carbon. In fact, PI/metal com-osites have also been reported previously for microelectronichotoresist and biosensor applications using surface deposition or

mpregnation of secondary metals on/into the insulating polymer23–25]. Surface modification of insulating PI has been achievedsing solution hydrolysis, photoirradiation, and chemical vaporeposition [21] for a variety of metal species, including Ag [26,27],e [28], Cu [27,29–31], Al [32,33], Au [27], Pt [34–36], andd [27,34,35]. The electrochemical deposition of metal on free-tanding PI substrates has not been extensively studied due to theow conductivity of the polymer substrate material. The electrode-osition of metal on thin PI films has thus been predominantlyerformed on conductive substrates, including Au and glassy car-on [23,37,38]. The method utilizes the inherent conductivity of theubstrate rather than the polymer to facilitate the electrochemicaleposition of metal. In addition, PI films are often grafted onto con-uctive substrates using methods such as photo-irradiation [39]r by dipping [37] the substrates into the polymer precursor thatorms thin films after polymerization.

The electrodeposition of PAA in a solution containing metal saltt an electrode surface, thereby creating PAA/metal hybrids, haslso been documented. For example, the incorporation of Au andg nanoparticles into electrochemically deposited PAA has beensed to produce PAA/metal composites [40]. The method utilizeshe affinity of PAA to reduce the incorporated metal cations duringhe polymerization process at the electrode surface. Specifically,hen the carboxyl group of the PAA reacts with triethylamine, aolyamate salt is formed with a cation that readily exchanges forhe metal cation. This allows the metal precursor to be dispersednto the polymer matrix. The metal cation can be both thermallynd electrochemically reduced to metal during polymerization.

The utilization of a PI/Carbon/Pt composite for fuel cell applica-ions has been recently explored for sulfonated PI as the polymerlectrolyte film. A mixture of Pt in carbon black has been incorpo-ated into sulfonated PI, cast onto an electrode, and used to examinehe oxygen reduction reaction (ORR) [38]. Additionally, the elec-rocatalytic properties of Pt-deposited polyimide/carbon nanotubelms for methanol and nitrite oxidation have also been studied [41].hese studies document the unique combination of metals, carbon,nd PI to produce synergistic properties for novel applications.

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In our study, the synthesis of a “free-standing” PI/Carbon/Ptomposite electrode is demonstrated. The dispersion of carbonnto PAA is examined and the conductivity of the composites pro-uced using different carbon loading is evaluated. Specifically,

PRESSetals xxx (2011) xxx– xxx

the role of solvent composition in a co-solvent system of ace-tone/DMSO for the formation of conductive PI/Carbon compositefilms is assessed. The electrodeposition of Pt at the PI/Carbonelectrode interface using cyclic voltammetry is verified. The elec-trochemical properties of Pt, deposited at the PI/Carbon interface,are investigated using the ferricyanide redox couple and methanoloxidation reaction. The electrochemical measurements confirm theelectrochemical activity of Pt metal deposited from the reductionof PtCl42−.

2. Experimental

2.1. Chemicals and solutions

Poly(pyromellitic dianhydride-co-4,4′-oxydianiline)amic acid(PAA) solution (Aldrich, 15.0 wt.% ±5 wt.% in NMP/aromatic hydro-carbons, Cat.# 575828, trade name: Pyre-M.L.® RC-5057), dimethylsulfoxide, DMSO (Aldrich, ≥99.9%, Cat.# 154938), conductex® SCcarbon black (Columbian Chemical company), potassium tetra-chloroplatinate, K2PtCl4 (Strem, 98%, Cat.# 78-1970), perchloricacid, HClO4 (J.T. Baker, 69–72%, Cat.# 9652-33), sulfuric acid, H2SO4(Mallinckrodt, 98%, Cat.# 2876), acetone (J.T. Baker, 99.5%, Cat.#9006-7), potassium ferricyanide, K3Fe(CN)6, Methanol, CH3OH(VWR, 99.8%, EM-MX085-9). All materials were used as received.

2.2. Preparation of PI/Carbon composites

The carbon/acetone mixture was prepared by combining 5.45 gof conductex® SC carbon black with 100 ml acetone and stirringfor 24 h. The solution was then sonicated for approximately 3 h.54.5 g of polyamic acid (PAA) was dissolved in 20 ml of DMSO andcombined with the carbon/acetone solution to achieve a 10 wt.%PAA/Carbon composite.

The PI/Carbon substrates were obtained by applying a uniformcoating of the PAA/Carbon mixture on a glass substrate and allow-ing it to dry for 24 h. The glass substrate with the PAA/Carbon filmwas inserted into a vacuum oven at a temperature of 80 ◦C for aperiod of one week to ensure the removal of residual solvent. Ther-mal imidization of PAA to PI was utilized to achieve polymerization[17]. A temperature range of 250–300 ◦C is typically utilized for thethermal imidization of PAA to form PI [22,42,43], and therefore afinal thermal imidization was conducted at 280 ◦C for 30 min toform the PI/Carbon composite. The typical film thickness for thedelaminated PI/Carbon substrates obtained was 150 �m.

2.3. Electrochemical apparatus and conditions

Pt was electrochemically deposited on a free-standing PI/Carbonworking electrode using cyclic voltammetry. Cyclic voltammetrywas also used to probe the ferricyanide redox couple of the resultingPI/Carbon/Pt composite films. All electrochemical measurementswere conducted using a CHI 760 potentiostat/galvanostat withincluded software. The experiments were performed in a one-compartment, three-electrode cell. All potentials are referenced toa Ag/AgCl electrode (3 M KCl filling solution). As counter electrode,we used a 0.5 mm-thick platinum sheet with an area exceeding theimmersed area of the working electrode by a factor of two.

2.4. X-ray photoelectron spectroscopy (XPS)

XPS was employed to identify the oxidation state of platinum in

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the PI/Carbon/Pt composites. XPS measurements were performedusing a Specs PHOIBOS 150MCD electron analyzer and Mg K�

excitation. The base pressure was in the 10−10 mbar range for allmeasurements. The electron spectrometer was calibrated using

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PS and Auger line positions of different metals (Cu, Ag, and Au)44].

.5. Thermal analysis of PI/Carbon/Pt films

Simultaneous thermal analysis (STA) was conducted using Netzsch STA449 C thermal analyzer. Simultaneous differentialcanning calorimetry (DSC) and thermal gravimetric analysis (TGA)as performed on PAA, PI, PI/Carbon, and PI/Carbon/Pt under air

s both the purge (50 ml/min) and protective (20 ml/min) gas at aeating rate of 10 ◦C/min. Samples were placed in alumina pans andovered with an alumina lid with a centered pin-hole. The furnaceas evacuated to about 10−3 torr prior to introducing purge androtective gases.

Platinum loading in the PI/Carbon/Pt composites with a uniformarbon loading of 10% was estimated using TGA. The composite waseated to a temperature of about 900 ◦C, at which all organic com-onents of the PI/Carbon composite were decomposed. All mass

oss was assigned to the decomposition of the organic compo-ent. The remaining mass in the test crucible at the end of thexperiment was then used to estimate the metal content for theomposite material. For statistics, a minimum of three runs wasonducted.

.6. Scanning electron microscopy (SEM)

SEM images of PI/Carbon/Pt composites were obtained using JEOL 5600 electron microscope equipped with a backscatteredlectron (BSE) detector. The films were affixed to the sampleolder using carbon tape and measurements were performed atn acceleration voltage of 15 kV. Deposition of a metallic Au layero enhance conductivity was not required for SEM measurements.

.7. Electrical conductivity characterization

Electrical contacts to the PI/Carbon and PI/Carbon/Pt films wereade using a Cascade Microtech C4S-64/50 probe head with tung-

ten carbide electrodes. The four-point probe sheet resistance ofach film was then measured at locations across the surface ofhe substrate using an Agilent 34401A Digital Multimeter con-ected through a Cascade Microtech CPS-05 probe station. Constantressure for each measurement was maintained for the probeead contacting the substrate using a mechanical stop. Aver-ges for a minimum of five measurements at each location areresented, together with representative standard deviations andelative standard deviations. Electrical resistivity values collectedsing these methods were converted to electrical conductivity forlarity.

.8. Fourier transform infrared (FTIR) spectroscopy

All FTIR measurements were performed on pristine samplessing a Digilab FTS-7000 spectrometer and a photoacoustic detec-or (MTEC). Each sample was scanned 64 times with a resolutionetting of 4 cm−1, and scans were averaged to produce each spec-rum.

. Results and discussion

.1. Preparation of PI/Carbon

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The preparation of PI was accomplished using thermal imidiza-ion of PAA and carbon incorporated into the co-solvent systemf acetone and DMSO. The polymerization of PI using this mixtures evaluated using photoacoustic FTIR spectroscopy in Fig. 1 [21].

Fig. 1. Photoacoustic FTIR spectra of (a) PAA, (b) PAA/Carbon, (c) PI, and (d)PI/Carbon (600–1900 cm−1).

The FTIR spectra of PAA, PAA/Carbon, PI, and PI/Carbon in the spec-tral range between 600 and 1900 cm−1 are presented in Fig. 1a–d,respectively. The asymmetric C O band at 1780 cm−1 is observedfor both PI and PI/Carbon and appears as a shoulder on the broadC O symmetric stretch band centered at 1740 cm−1. In addition,a strong sharp band located at around 729 cm−1, assigned to theimide C O bending mode, appears in both PI and PI/Carbon. Theintensity decrease of the C–NH band for PAA at 1546 cm−1 and theincrease in band intensity for the C O imide functional group at730 cm−1 and 1778 cm−1 confirm the formation of PI after thermalimidization. Similarly, the spectra can be monitored in the rangebetween 1900 cm−1 and 3700 cm−1 (shown in the supporting doc-umentation) for changes in the carboxylic acid OH group of PAA. Inaddition, characteristic secondary amide NH stretch bands occur asmultiple bands in the same region between 3065 and 3300 cm−1

[45]. A broad decrease in band intensity for OH and NH is observedfor PAA after thermal imidization and the formation of PI/Carboncomposite.

3.2. Carbon loading and conductivity of PI/Carbon

The factors that influence the conductivity of polymer/carbonblack composites include the properties of the carbon speciesutilized, aggregation of the carbon within the polymer, and theheterogeneity and dispersion of the carbon in the polymer matrix.Variations in the density and particle size of the carbon play a crit-

1), doi:10.1016/j.synthmet.2011.08.046

ical role because they influence both aggregation and dispersion[17–19,46,47]. Increasing the density of the carbon filler allowsmore compact aggregation and packing, leading to enhanced elec-trical conductivity at lower percolation thresholds (i.e., the measure

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% Carboncontent

Resistivity(Ohm cm)

Conductivity(S/cm)

% Relative standarddeviation

2.5% 2.1 × 105 4.7 × 10−6 8.75.0% 27 0.037 0.95

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f the volume of a conducting phase required to change the mate-ial from insulating to conducting). For example, highly crystallizedraphite (HCG) exhibits a higher conductivity and a lower percola-ion threshold than carbon blacks because of its higher density. Inddition, smaller particle sizes enhance conductivity because theyllow higher aggregation and smaller gaps between the carbon cen-ers [17,19]. Significant changes in the conductivity are observed forI once a critical carbon content (percolation threshold) is reached19,48]. Networks form after carbon reaches a critical mass, lead-ng to coagulation and increased contact between the particles16,17,47]. The increase in conductivity is attributed to the forma-ion of fibrous chain-like carbon networks in the polymer matrixnd has been previously reported for PI/Carbon black composites16,17].

Previous studies of the influence of carbon loading on the elec-rical conductivity have focused on dispersion using low-volatilityolvents such as DMSO and NMP. These solvents are problematicecause they are often trapped in the polymer, leading to materi-ls that are less homogeneous. In our study we evaluate the use of

binary solvent, combining highly volatile acetone with DMSO toeduce the incorporation of solvent into the PI/Carbon composite.n addition, the solvent is used to homogeneously disperse car-on into the PAA prior to imidization to produce materials withercolation thresholds at carbon loadings consistent with knownI/Carbon materials.

The influence of the carbon content on the electrical conductiv-ty of PI is presented in Fig. 2 for the PI/Carbon materials producedsing the acetone/DMSO solvent system. The carbon content forhe composite is reported as the % carbon by weight in the polymerrecursor, PAA. An approximate ∼104 increase in the conductivity

s observed as the %Carbon is increased from 2.5% to 5%, as also

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isted in Table 1. The data suggests that the percolation threshold ischieved at carbon loadings between 2.5 and 5%. The conductivityf the materials continuously improves above 5% carbon content,

ig. 2. Conductivity of PI/Carbon as a function of the percent carbon by weight inAA.

7.5% 3.4 0.29 0.3210.0% 1.4 0.70 4.512.5% 0.80 1.2 0.10

but the mechanical stability of the films is compromised at car-bon loadings ≥12.5%, with the emergence of surface fractures anddecreased flexibility. Therefore, a carbon loading of 10% was utilizedfor all subsequent measurements in our studies. The conductiv-ity values between 8 and 16 wt.% carbon in PAA are significantlyhigher when compared to previously reported values for PI filmscontaining the same carbon filler (SC conductex) used in this study[17].

3.3. Pt deposition on PI/Carbon

Deposition of Pt metal on freestanding PI/Carbon workingelectrodes was achieved using cyclic voltammetry encompassingpotentials for the reduction of PtCl42− ion to Pt metal. The equationand potential for the reduction of PtCl42− is as follows [49]:

PtCl42− + 2e− → Pt(0) + 4Cl−, E◦ = 0.493 V vs. Ag/AgCl

The voltammetric response for a planar Pt reference and aPI/Carbon electrode in solution containing 1 M HClO4 are providedin Fig. 3a and b (bottom), respectively. The Pt electrode shows avoltammetric response at negative potentials, which is consistentwith proton adsorption at the metal surface. The response of thePI/Carbon electrode is strongly diminished due to the absence of Pt

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at the electrode surface. The electrochemical deposition of Pt fromsolutions containing 20 mM PtCl42− in 1 M HClO4 are provided forPt and PI/Carbon electrodes in the center of Fig. 3. The voltammetricresponse for both electrodes is very similar, with the characteristic

Fig. 3. (a) Cyclic voltammetry for a planar Pt electrode in 1 M HClO4 prior to depo-sition (bottom), during Pt deposition in a solution containing 2.0 × 10−2 M PtCl42−

in 1 M HClO4 (center), and the same electrode after Pt deposition in solution con-taining 1 M HClO4 (top). (b) Cyclic voltammetry for a PI/Carbon electrode in 1 MHClO4 prior to Pt deposition (bottom), during Pt deposition in a solution containing2.0 × 10−2 M PtCl42− in 1 M HClO4 (center), and the same electrode (PI/Carbon/Pt)in solution containing 1 M HClO4 (top). The scan rate for all measurements was0.01 V/s.

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lectrochemistry associated with the Pt surfaces clearly visible.pecifically, the formation and reduction of PtO is observed (A/A′).n contrast, Pt reduction is not evident for either electrode (smallhoulder at potentials more negative than 0.20 V). Furthermore,roton adsorption/desorption at the Pt metal surface is clearly vis-

ble (B/B′) for both electrodes during this metal deposition process.inally, the voltammetric response of the Pt planar electrode andhe PI/C/Pt composite electrode is evaluated in solution contain-ng 1 M HClO4 only (i.e., without the Pt precursor) to demonstratehat Pt deposition has been achieved at both electrodes. Variationsn the Pt surface morphology likely influence the small variationsbserved in the voltammetry for the PI/Carbon and bulk Pt surfaces.owever, the same voltammetric processes for the Pt surface are

esolved at each substrate. Significant changes in the voltammetryan be observed during Pt deposition for both electrodes. Specif-cally, each voltammetric cycle results in an increase in currentssociated with an increase in the surface area of deposited Pt. Theoltammetric waves associated with PtO (A/A′) [50,51] and Pt-HadsB/B′) [50,52–54] increase with each deposition cycle. The reduc-ion of Pt (small shoulder at potentials more negative than 0.20 V) isbserved for both electrodes during the Pt deposition and is absenthen both electrodes are immersed in solutions containing only

M HClO4. The voltammetry is consistent with the deposition oft through the reduction of the Pt metal precursor (PtCl42−) at theI/Carbon electrode surface.

More in-depth analysis of the voltammetry associated with PtOA/A′) provides a measure of the Pt surface chemistry and thenvolved electrochemical processes. Specifically, the formation andeduction of PtO is observed at ∼0.73 V and ∼0.60 V for both elec-rodes. The voltammetry is consistent with the following reaction

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here shifts in potential are common based on the electrode com-osition [53]:

tO(s) + 2e− + 2H+ � Pt(s) + H2O, E = 0.78 V vs. Ag/AgCl

ig. 4. SEM images of (a) PI/Carbon/Pt composite with low Pt loading (6%, ×2000), (b) Pagnification (×5000), and (d) PI/Carbon/Pt with high Pt loading at low magnification (×

PRESSetals xxx (2011) xxx– xxx 5

In contrast, the voltammetry associated with H adsorp-tion/desorption at the Pt metal deposited on each electrode issignificantly different. The reaction for the process is:

H+ + e− → H → Hads.

The potentials for the proton adsorption/desorption (B/B′)for the Pt planar and PI/Carbon/Pt electrodes are observed atapproximately −0.07/−0.12 V and −0.02/−0.16 V, respectively,with modest shifts in potential as a function of the number of cyclesemployed. The peak-splitting for the Pt electrode is on the order of50 mV, while a value of 140 mV is observed for the PI/Carbon/Ptcomposite. In addition, the voltammetric waves associated with Ptdeposition are broader relative to the planar Pt electrode. Protonadsorption at a polycrystalline platinum electrode is a two-stepprocess, involving weak adsorption followed by strong adsorp-tion of hydrogen atoms at the Pt surface [54]. This process ishighly dependent on the crystal surface structure of Pt, with thevoltammetric response determined by the number and orientationof crystal planes of Pt that are exposed [53,55]. The presence ofdefects and impurities can lead to disorder in the crystal structure ofthe electrode, further reducing the distinct adsorption/desorptionvoltammetry characteristics of different crystal planes of Pt. Thedata suggests that differences in Pt crystallinity may translate intodifferent electrochemical responses for the oxidation/reduction ofsolution species.

3.4. Scanning electron microscopy (SEM)

SEM is used to investigate the morphology of the PI/Carbon/Pt

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composite and the distribution of Pt particles on the surface. TheSEM images of PI/Carbon/Pt electrodes with Pt loading of 6 and 30%are shown in Fig. 4. The images show the immersed interface withPt deposits (left and/or top) and the electrode material that was not

I/Carbon/Pt composite with high Pt loading (31%, ×2000), (c) the latter at higher35).

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mmersed (i.e., no Pt deposited) for clarity. The Pt metal depositst 6% (Fig. 4a, ×2000) loading appear as evenly spaced, discretelusters with diameters of ∼400 nm on the PI/Carbon substrate. Inontrast, the Pt deposits at 30% Pt loading in Fig. 4b (×2000) show

coalesced deposit of metal with no discernible clusters on theI/Carbon substrate. The Pt deposits (6% loading) suggest that car-on dispersion in PI is uniform for the acetone/DMSO co-solventystem. In fact, carbon aggregation at the PI surface is not discern-ble on the substrates at higher magnification (×8000) in Fig. 4c.

Previous studies in literature that have used SEM to evaluate theorphology of PI/Carbon films have shown large (approximately

0 �m diameter) surface defects when prepared with carbon black.he defects were attributed to the formation of water vapor pock-ts produced during imidization [17]. Alternatively, these defectsould also be created by the fast expulsion of residual low-volatilityolvents as the temperature is increased to initiate imidization. Theack of large-scale defects observed in the PI/Carbon substrates pro-uced in these studies at much lower magnification (×35) in Fig. 4duggests that the temperature program was effective in expellingater vapor and/or the binary solvent prior to imidization. Fur-

hermore, the absence of defects in the PI/Carbon substrates maye attributed to the minimization of low-volatility solvents in thereparation. The results demonstrate that Pt deposition at PI/Composite electrodes can be controlled electrochemically to pro-uce deposits with variable thickness and morphology.

.5. X-ray photoelectron spectroscopy of PI/Carbon/Pt

X-ray photoelectron spectroscopy (XPS) is used to examine thexidation state of the Pt metal deposited on PI. The XPS surveypectrum of a representative PI/Carbon/Pt film is shown in Fig. 5.t confirms the presence of Pt, C, O, and N at the surface, in addi-ion to some minor Cl, S, and Zn impurities. While the Cl and Smpurities are likely due to the precursor chemicals, solvent, and/orlectrolyte, the Zn impurities might have been adsorbed duringransfer in the XPS ultra-high vacuum system. A detail scan of thet 4f core levels is shown in the inset of Fig. 5. The 4f7/2 peak maxi-um is found at 71.35 eV (±0.05), which is in good agreement with

eference values for bulk Pt metal (71.1 or 71.2 eV) [56] and sug-ests that the Pt atoms are indeed in a metallic environment. Most

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elevant Pt-containing compounds (with non-zero oxidation state)xhibit binding energies of 72.5 eV and above. In particular, PtO andtO2 are expected to be found at 74.2 eV and 75.0 eV, respectively56], and would also be expected to exhibit a symmetric lineshape.

ig. 5. Mg K� XPS survey spectrum of PI/Carbon/Pt composite (inset: Pt 4f detailpectrum).

PRESSetals xxx (2011) xxx– xxx

In contrast, the observed asymmetric lineshape further indicates ametallic environment of the Pt atoms—it is well known that metallicsystems allow a continuum of low-energy excitations in the finalstate of the photoemission process, giving rise to a characteris-tic asymmetric shape (commonly described by a Doniach-Sunjicprofile). It is not necessarily correct to assume that several initial-state species, described by symmetric Gaussian or Voigt functions(including a metallic species), are needed to describe the spectrumproperly, as has been done in earlier publications of related sys-tems [57]. Furthermore, local differences in final-state screeningof the created photo-hole might lead to small shifts of core levellines as well, which might be responsible for the small differencebetween the here-observed value and the literature reference forbulk metallic Pt.

3.6. Thermal gravimetric analysis (TGA) of PI/Carbon/Pt films

The thermal properties of PI (a), PI/Carbon (b), and twoPI/Carbon/Pt composites (c and d) were evaluated using ther-mal gravimetric analysis (TGA), as shown in Fig. 6. The thermalproperties of PI, PI/Carbon, and PI/Carbon/Pt can be evaluated attemperatures that result in the thermal degradation and loss ofmass associated with the polymer. The residual mass at 900 ◦C pro-vides an estimate of residual carbon in the PI/Carbon sample andmetal loading in the PI/Carbon/Pt composites. For example, the finalmass of PI with no carbon reaches a value of zero, consistent withthe complete loss of polymer during the thermal processes shownin Fig. 6a (note that the origin of the ordinate was offset for betterviewing). In contrast, the TGA of PI/Carbon shows a residual massof 2% relative to PI (Fig. 6b). Similarly, Pt loadings of 4% and 26% are

1), doi:10.1016/j.synthmet.2011.08.046

Fig. 6. TGA curves of (a) PI, (b) PI/Carbon, (c) PI/Carbon/Pt (6% Pt loading), and (d)PI/Carbon/Pt (31% Pt loading), using a heating rate of 10 K/min.

IN PRESSG Model

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hetic Metals xxx (2011) xxx– xxx 7

mmaapNtFfi−itTptHgs

3

tcbPtP1rFpit

Table 2Electronic properties of PI/Carbon/Pt.

%Pt content Resistivity(Ohm cm)

Conductivity(S/cm)

%Relative standarddeviation

0.0% 7.0 0.14 3.64.6% 4.2 0.24 0.02912% 0.93 1.1 5.022% 0.15 6.7 3.5

The cyclic voltammetric response of a PI/Carbon electrode with-out Pt, in solution containing 0.1 M K3Fe(CN)6 in 1 M KNO3, ispresented in Fig. 8a. The PI/Carbon film shows minimal elec-

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The thermal analysis of the materials also provides an importanteasure of the thermal stability of PI, PI/Carbon, and PI/Carbon/Ptaterials. In this case, the thermal properties of all four samples

re very similar, with onset temperatures of 576 and 570 ◦C for PInd PI/Carbon, respectively, and ∼560 ◦C for the PI/Carbon/Pt com-osites. A small mass change attributed to the loss of low-volatileMP and DMSO solvents can be observed for all materials prior to

he onset temperature, corresponding to a mass decrease of ∼2–4%.or comparison, the operating temperature range for Kapton® HNlm over long periods of use (as reported by the manufacturer) is269–400 ◦C [1], which is well below the temperatures reported

n our study. This is due to the fact that PI has a glass transi-ion reported at ∼400 ◦C, where Kapton® HN becomes rigid [58].he onset temperatures for the initial mass loss of the materialsroduced in this study suggest that they can be utilized at highemperatures (up to 550 ◦C) without severe thermal degradation.owever, it may be prudent to work at temperatures below thelass transition to ensure that the materials remain mechanicallytable.

.7. Conductivity of PI/Carbon/Pt composites

The electrical conductivity of PI/Carbon/Pt composites as a func-ion of platinum loading is shown in Fig. 7. The values should beompared to the conductivity of the PI/Carbon material at 10% car-on loading (see Fig. 2, and first data point in Fig. 7). The percentt loading was determined using TGA. As expected, the conduc-ivity of the composite increases as the platinum loading on theI/Carbon surface is increased. The conductivity for PI/Carbon with0% carbon content is 0.140 ± 0.005 S/cm, which is lower than thateported for PI/Carbon (0.700 ± 0.003 S/cm) for the same material in

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ig. 2, suggesting that batch-to-batch variations exist for the com-osite materials. However, both values are consistent with a ∼104

ncrease in conductivity when the carbon content in PI is increasedo 10%. As a function of Pt loading, a steady increase in conductivity

Fig. 7. Conductivity of PI/Carbon/Pt composite as a function of Pt loading.

30% 0.091 11 0.5836% 0.051 20 0.69

is recorded up to a value of 36%, as shown in Fig. 7 and Table 2. Thedata suggests that Pt loading should be increased to maximize con-ductivity; however, costs associated with the metal deposition alsoneed to be considered and a reasonable value of Pt loading must bedefined based on the conductivity required. Thus the conductiv-ity requirements of the specific application will dictate a suitable“compromise” for the Pt loading.

3.8. Electrochemistry at PI/Carbon/Pt composite electrodes

The conductivity of PI/Carbon and, in particular, of thePI/Carbon/Pt composites suggests that the materials can be uti-lized as freestanding electrodes to probe electrochemical reactions.Specifically, the oxidation/reduction reaction involving the ferri-cyanide couple can be examined [49]:

Fe(CN)63− + e− � Fe(CN)6

4− E◦ = 0.161 V vs. Ag/AgCl

1), doi:10.1016/j.synthmet.2011.08.046

trochemical activity for the oxidation/reduction processes of

Fig. 8. Cyclic voltammetry of (a) PI/Carbon, (b) PI/Carbon/Pt (125 Pt depositioncycles), and (c) a planar Pt electrode in solution containing 0.1 M K3Fe(CN)6 in 1 MKNO3. The scan rate for all measurements was 0.01 V/s. Crosshair marks the zeropotential and zero current for each voltammogram.

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8 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx– xxx

Table 3Oxidation/reduction potentials (Epc and Epa), peak splitting (�Ep), and number ofelectrons transferred (n) for the oxidation and reduction [Fe(CN)6

4−/Fe(CN)63−] of

ferricyanide at PI/Carbon, PI/Carbon/Pt, and a planar Pt electrode (∼1 cm2).

Electrode (1 cm2) Epc (V) Epa (V) �Ep (mV) n

PI – – – –

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P

P

TwCpCia

Fig. 9. Cyclic voltammetry for the oxidation of methanol in solution containing 1 MCH OH in 1 M H SO for (a) PI/Carbon, (b) PI/Carbon/Pt (2 Pt deposition cycles), (c)

TOr

PI/Carbon/Pt 0.157 0.414 257 0.230Planar Pt electrode 0.183 0.308 125 0.472

erricyanide. In contrast, the electrochemical response for theI/Carbon/Pt electrode (Fig. 8b) is clearly enhanced. For comparisonnd reference, a planar platinum sheet electrode was also used tovaluate the oxidation/reduction processes of ferricyanide, Fig. 8c,hich are clearly observed. The voltammetry is significantly more

esolved for ferricyanide at the planar Pt electrode when comparedo the same electrochemical reaction at the PI/Carbon/Pt electrode.

The peak separation �Ep = Epa − Epc (Epa and Epc are the anodicnd cathodic peak potentials, respectively) is an indicator forlectron transfer kinetics during the oxidation/reduction of fer-icyanide at each electrode. For kinetically fast electrochemicalrocesses, Nernstian kinetics are expected, where �Ep is equalo (59/n) mV, n being the number of electrons transferred (n = 1or ferricyanide). The peak potentials (Epc and Epa) and the peakplitting (�Ep) are obtained from the voltammetric response oferricyanide at both PI/Carbon/Pt and Pt planar electrodes (andisted in Table 3). The voltammetry for the PI/Carbon/Pt electrodehows non-nerstian peak splitting of 257 mV (i.e., with a value sig-ificantly greater than 59 mV). The data suggests that the electronransfer is kinetically limited at the PI/Carbon/Pt electrode relativeo the planar Pt electrode. The values obtained for the Pt planarlectrode (peak splitting of 125 mV) also suggest that the electronransfer is kinetically limited. However, the smaller peak splittinguggests that the electron transfer at the bulk Pt surface is enhancedelative to the PI/Carbon/Pt electrode. Although factors such asolution concentrations and non-linear diffusion are important,urface inhomogeneity, including defects on the electrode surface,ikely play a larger role in the more sluggish electron transfer kinet-cs for the PI/Carbon/Pt composite relative to the planar Pt reference59].

The catalytic oxidation of methanol on Pt was also probedsing the freestanding PI/Carbon/Pt composites—the correspond-

ng voltammograms are shown in Fig. 9. The oxidation of methanols strongly influenced by the Pt surface morphology and can be useds a measure of the chemical reactivity of the PI/Carbon/Pt com-osite. The oxidation of methanol occurs via two reaction paths, asresented below [60,61]:

t-CH3OHads + H2O → Pt + CO2 + 6H+ + 6e−

t-CH3OHads → Pt-COads + 4H+ + 4e−

he first path involves the direct oxidation of methanol to CO2,hile the second one involves the formation of a strongly adsorbedO intermediate prior to oxidation. CO adsorption causes surface

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oisoning by blocking Pt sites necessary for further oxidation ofH3OH. Additionally, methanol oxidation during the cathodic scan

s hindered by the formation of PtO at the electrochemical interfacend does not occur prior to the reduction of the surface oxide.

able 4xidation/reduction potentials (Epf and Epr), peak splitting (�Ep), current ratios (ipf/ipr),

espectively), associated with methanol oxidation at a PI/Carbon/Pt electrode (∼1 cm2).

Electrode (1 cm2) Epf (V) Epr (V)

PI/Carbon/Pt (2 cycles) 0.705 0.570

PI/Carbon/Pt (10 cycles) 0.955 0.640

3 2 4

PI/Carbon/Pt (12 Pt deposition cycles) and (d) PI/Carbon/Pt (2 Pt deposition cyclesand 15 methanol oxidation cycles) The scan rate for all measurements was 0.01 V/s.

Methanol oxidation is not observed for PI/Carbon electrodeswithout Pt in solutions containing 1 M H2SO4 and 1 M methanol,as shown in Fig. 9a. However, methanol oxidation is observed inthe same solution using a PI/Carbon free-standing electrode aftertwo electrochemical reduction cycles using PtCl42−, Fig. 9b. Thevoltammetry confirms the formation of a PI/Carbon/Pt compositeelectrode with an electrochemically active Pt surface. The surfacearea and Pt content of the composite electrode can be varied bychanging the number of Pt deposition cycles. For comparison, thevoltammetric response of a PI/Carbon electrode after twelve Ptreduction cycles is presented in Fig. 9c. The oxidation of methanolis observed for both, the forward and reverse scans. In addition, theoxidation of methanol on the forward (positive) and the reverse(negative) scan increases with increasing Pt content.

For PI/Carbon/Pt after two Pt deposition cycles, the oxidation ofmethanol is characterized by two voltammetric waves centered at0.70 V for the forward scan and 0.57 V for the reverse scan (Fig. 9b).After twelve Pt deposition cycles, the oxidation of methanol isobserved at 0.95 V for the forward scan and 0.64 V for the reversescan. The waves for both electrodes are more positive than thevalues reported for PI/CNT/Pt [41] and for a conductive polymer(polyaniline)/Pt composite [62]. The data suggests that oxidationof methanol may be thermodynamically more favorable at thePI/Carbon/Pt electrodes produced in this study.

Further analysis of the peak currents, charge passed, and peaksplitting provides an overview of the oxidation of methanol usingPI/Carbon/Pt electrodes (given in Table 4). For example, both theratio of the peak currents (ipf/ipr = 1.01) and the charge-ratio asso-ciated with methanol oxidation (Qf/Qr = 1.09) obtained for thePI/Carbon/Pt after two Pt reduction cycles are close to unity, indicat-ing that the processes are chemically reversible. The peak splittingis on the order of 135 mV and can be used as an indicator for sur-face poisoning at the metal surface. The peak splitting associated

1), doi:10.1016/j.synthmet.2011.08.046

with the oxidation of methanol for conductive polymer polyaniline(PANI)/Pt composites was measured to be 45 mV [62]. Although theresults confirm that PI/Carbon substrates are sufficiently conduc-tive to allow the deposition of Pt metal to produce freestanding,

and charge ratios (Qf/Qr) for the forward and reverse scans (after 2 and 10 cycles,

�Ep (mV) ipf/ipr Qf/Qr

135 1.01 8.18 mC/7.50 mC = 1.09315 0.89 141 mC/46 mC = 3.09

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lectrochemically active PI/Carbon/Pt surfaces, the polymer doesot provide enhanced protection against poisoning at the Pt surfaceelative to PANI/Pt composites [62]. For example, the charge ratioor the oxidation of methanol between the forward and reversecans (Qf/Qr = 3.09) indicates that poisoning is more prominent forhis electrode. The peak splitting associated with the forward andeverse process is also significantly larger (315 mV) in comparisono the PI/Carbon/Pt electrode obtained from two Pt reduction cyclesnd PANI/Pt [58]. The data indicates that the surface contaminantsust be fully reduced and desorbed from the Pt to observe the

xidation of methanol on the reverse scan. Furthermore, the peakurrents decrease with each subsequent cycle in methanol for theI/Carbon/Pt electrode obtained from two Pt reduction cycles ashown in Fig. 9d. This signifies that surface contaminants the cat-lyst efficiency is reduced with successive cycling due to surfaceoisoning. The data suggests that the deposition of Pt at conduc-ive PI/Carbon electrodes should be explored more thoroughly toully understand how the number of Pt deposition cycles influenceshe Pt morphology and efficiency of methanol oxidation. In addi-ion, the co-deposition of Pt-Ru in a 1:1 composition ratio shoulde investigated. This ratio has been found optimal in the decreas-

ng surface poisoning by removal of CO species [63]. However,he results confirm that Pt can be electrochemically deposited atonductive PI/Carbon substrates to metallize the surfaces and torovide chemically reactive and thermally stable PI/C/Pt compos-

tes.

. Conclusions

The studies demonstrate that conductive PI/Carbon films can berepared through the thermal imidization of PAA, premixed witharbon black from a binary (5:1 acetone:DMSO) solvent system.he solvent system minimizes the use of low volatility solvents thatre difficult to expel during the curing and imidization process androvides high carbon dispersion. In addition, the electrochemicaleposition of Pt was demonstrated at free-standing PI/Carbon elec-rodes to form conductive PI/Carbon/Pt composite films. Thermalravimetric analysis of PI/Carbon and PI/Carbon/Pt demonstrateshat the thermal stability of PI is not compromised with either car-on incorporation or Pt deposition. Electrochemical measurementssing ferricyanide show that the PI/Carbon/Pt film is electroac-ive. However, the electron transfer for ferricyanide at PI/Carbon/Ptas kinetically limited relative to a bulk Pt electrode. The sur-

ace reactivity of Pt for the composite materials was evaluated as function of the number of Pt reduction cycles and increasing Ptontent. The oxidation of methanol provides a measure of com-lex chemical reactions at Pt surfaces electrochemically depositedn the conductive PI/Carbon composite. The results confirm thathe electrochemical formation of PI/Carbon/Pt composites withlectrochemically active Pt metal surfaces has been achieved. Thelectrochemical measurements suggest that the Pt surface reac-ivity may be influenced by factors including deposit morphology,hickness, and overall Pt loading. These parameters can be con-rolled electrochemically, and thus it is possible to optimize theroperties of conductive PI/Carbon/metal composites for a varietyf applications.

cknowledgements

This work was support by the U.S. Department of Energy throughhe UNLV FCAST program under Grant No. DE-FG36-05GO85028.

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