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  1. 1. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 10 Synthetic Metals xxx (2011) xxx–xxx Contents lists available at SciVerse ScienceDirect Synthetic Metals journal homepage: and characterization of conductive polyimide/carbon compositeswith Pt surface depositsJohn M. Kinyanjui a,∗ , David W. Hatchett a , Gina Castruita a , Asanga D. Ranasinghe a ,Lothar Weinhardt a,b , Timo Hofmann a , Clemens Heske aa Department of Chemistry, University of Nevada, Las Vegas, Las Vegas, NV 89154-4003, United Statesb Experimentelle Physik VII, Universität Würzburg, D-97074 Würzburg, Germanya r t i c l e i n f o a b s t r a c tArticle history: The preparation of free-standing polyimide/carbon (PI/Carbon) substrates and the electrochemical depo-Received 29 June 2011 sition of Pt to produce PI/Carbon/Pt electrodes are demonstrated to provide thermally stable andReceived in revised form 31 August 2011 conductive PI composites. The conductivity of polyimide (PI)/Carbon composites is evaluated as a func-Accepted 31 August 2011 tion of composition of a binary solvent involving DMSO (dimethyl sulfoxide) and highly volatile acetone,Available online xxx which enhances carbon dispersion (PI/Carbon) in the polymer precursor. The solution conditions have been optimized to provide the highest conductivity for the lowest relative carbon loading. The depo-Keywords: sition of Pt metal on PI/Carbon composite electrodes is demonstrated using cyclic voltammetry. TheCompositePolyimide conductivity of the PI/Carbon composite is sufficient that the metal precursor PtCl4 2− is fully reducedCarbon and deposited without the need for additional chemical reduction processes. Thermal gravimetric anal-Platinum ysis (TGA) shows that the thermal stability of PI is maintained with carbon incorporation and platinumMethanol oxidation deposition. Scanning electron microscopy (SEM) analysis shows that carbon aggregation at the PI surface is minimized and that Pt deposits are well dispersed. X-ray photoelectron spectroscopy (XPS) results indicate that the electrochemical reduction of PtCl4 2− produces metallic Pt deposits on the PI/Carbon composite. Four-point probe measurements are utilized to assess the conductivity of the materials and highlight 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 the catalytic oxidation of methanol in acidic solution. The electrochemical experiments demonstrate that the free-standing PI/Carbon composites are sufficiently conductive to observe the electrodeposition of Pt metal that is stable and reactive on the organic substrate. © 2011 Elsevier B.V. All rights reserved.1. Introduction back contact have been reported [6,7]. Furthermore, chemically modified PI with sulfonyl groups has been utilized for applications PMDA-ODA or poly(pyromellitic dianhydride-4,4 - including proton exchange membranes for fuel cells [8,9]. Theseoxydianiline)imide (referred to as PI in this work) has been examples highlight the diverse applications based on the inherentpredominantly used as a thermal and electrical insulator because properties of PI and new, emergent properties of modified PI andit is thermally robust, chemically resistant to degradation, and its interface with a large variety of different materials.possesses high tensile strength [1]. PI adhesive tapes are com- The thermal properties of PI also make it an attractive materialmercially available to provide thermal insulation based on these for applications that require high stability and electrical conduc-properties. The use of PI as a substrate for solar cells has also tivity of organic matrices at elevated temperatures. Therefore,been investigated, motivated by the polymer’s thermal stability modification of PI with secondary components that enhance theand mechanical flexibility [2–7]. However, the application of PI electrical and mechanical properties of the material has beensubstrates in solar cells is predicated on the ability to mechani- explored. For example, both single (SWNT) and multi-walled car-cally interface the material with cell components. For example, bon nanotubes (MWNT) have been successfully incorporated intoinsulating PI has been used as a substrate for flexible Cu(In,Ga)Se2 PI to improve tensile strength and minimize electrostatic chargethin film solar cells, but adhesion problems between the PI and Mo buildup [10–15]. The success of such materials is evident from the commercial availability of PI/Carbon nanotube composites such as AURUMTM® by Mitsui. However, the high cost of carbon nanotubes ∗ Corresponding author. Tel.: +1 702 328 2925; fax: +1 702 895 4072. is a drawback to the bulk production and application of such materi- E-mail address: (J.M. Kinyanjui). als, and carbon black materials have thus been considered as lower0379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.synthmet.2011.08.046 Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  2. 2. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 102 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxxcost alternatives that can also enhance the electronic properties of the role of solvent composition in a co-solvent system of ace-PI. tone/DMSO for the formation of conductive PI/Carbon composite Traditionally, carbon materials have been dispersed into PI films is assessed. The electrodeposition of Pt at the PI/Carbonusing DMSO (dimethyl sulfoxide) and other common solvents to electrode interface using cyclic voltammetry is verified. The elec-obtain electronically conductive PI/Carbon composites [16–18]. trochemical properties of Pt, deposited at the PI/Carbon interface,The use of low-volatility solvents such as DMSO and NMP (n- are investigated using the ferricyanide redox couple and methanolmethylpyrrolidone) is problematic because the homogeneity of oxidation reaction. The electrochemical measurements confirm thethe PI/Carbon composite is influenced by dispersion of the sec- electrochemical activity of Pt metal deposited from the reductionondary component and trapped solvent in the resulting polymer of PtCl4 2− .film [17]. Uniform dispersion of the carbon black is also criticalto ensure that the mechanical integrity of PI is maintained, with 2. Experimentalhigh aggregation of the secondary component producing brittlematerials [19]. Therefore, a uniform dispersion of carbon and the 2.1. Chemicals and solutionsPI precursor [polyamic acid (PAA)] in a suitable solvent prior toimidization increases the homogeneity of the material [17,20–22]. Poly(pyromellitic dianhydride-co-4,4 -oxydianiline)amic acidMoreover, enhanced electrical conductivity has been previously (PAA) solution (Aldrich, 15.0 wt.% ±5 wt.% in NMP/aromatic hydro-attributed to the formation of small fibrous carbon black aggregates carbons, Cat.# 575828, trade name: Pyre-M.L.® RC-5057), dimethylwith chain-like structures within the polymer matrix [19]. Such sulfoxide, DMSO (Aldrich, ≥99.9%, Cat.# 154938), conductex® SCmicrostructures can be instrumental in the transport of electrons carbon black (Columbian Chemical company), potassium tetra-through insulating polymers and strongly influenced by solvent chloroplatinate, K2 PtCl4 (Strem, 98%, Cat.# 78-1970), perchloriccomposition. acid, HClO4 (J.T. Baker, 69–72%, Cat.# 9652-33), sulfuric acid, H2 SO4 The combination of PI with metals is not specifically tied to the (Mallinckrodt, 98%, Cat.# 2876), acetone (J.T. Baker, 99.5%, Cat.#formation of conductive PI using carbon. In fact, PI/metal com- 9006-7), potassium ferricyanide, K3 Fe(CN)6 , Methanol, CH3 OHposites have also been reported previously for microelectronic (VWR, 99.8%, EM-MX085-9). All materials were used as received.photoresist and biosensor applications using surface deposition orimpregnation of secondary metals on/into the insulating polymer 2.2. Preparation of PI/Carbon composites[23–25]. Surface modification of insulating PI has been achievedusing solution hydrolysis, photoirradiation, and chemical vapor The carbon/acetone mixture was prepared by combining 5.45 gdeposition [21] for a variety of metal species, including Ag [26,27], of conductex® SC carbon black with 100 ml acetone and stirringFe [28], Cu [27,29–31], Al [32,33], Au [27], Pt [34–36], and for 24 h. The solution was then sonicated for approximately 3 h.Pd [27,34,35]. The electrochemical deposition of metal on free- 54.5 g of polyamic acid (PAA) was dissolved in 20 ml of DMSO andstanding PI substrates has not been extensively studied due to the combined with the carbon/acetone solution to achieve a 10 wt.%low conductivity of the polymer substrate material. The electrode- PAA/Carbon composite.position of metal on thin PI films has thus been predominantly The PI/Carbon substrates were obtained by applying a uniformperformed on conductive substrates, including Au and glassy car- coating of the PAA/Carbon mixture on a glass substrate and allow-bon [23,37,38]. The method utilizes the inherent conductivity of the ing it to dry for 24 h. The glass substrate with the PAA/Carbon filmsubstrate rather than the polymer to facilitate the electrochemical was inserted into a vacuum oven at a temperature of 80 ◦ C for adeposition of metal. In addition, PI films are often grafted onto con- period of one week to ensure the removal of residual solvent. Ther-ductive substrates using methods such as photo-irradiation [39] mal imidization of PAA to PI was utilized to achieve polymerizationor by dipping [37] the substrates into the polymer precursor that [17]. A temperature range of 250–300 ◦ C is typically utilized for theforms thin films after polymerization. thermal imidization of PAA to form PI [22,42,43], and therefore a The electrodeposition of PAA in a solution containing metal salt final thermal imidization was conducted at 280 ◦ C for 30 min toat an electrode surface, thereby creating PAA/metal hybrids, has form the PI/Carbon composite. The typical film thickness for thealso been documented. For example, the incorporation of Au and delaminated PI/Carbon substrates obtained was 150 m.Ag nanoparticles into electrochemically deposited PAA has beenused to produce PAA/metal composites [40]. The method utilizesthe affinity of PAA to reduce the incorporated metal cations during 2.3. Electrochemical apparatus and conditionsthe polymerization process at the electrode surface. Specifically,when the carboxyl group of the PAA reacts with triethylamine, a Pt was electrochemically deposited on a free-standing PI/Carbonpolyamate salt is formed with a cation that readily exchanges for working electrode using cyclic voltammetry. Cyclic voltammetrythe metal cation. This allows the metal precursor to be dispersed was also used to probe the ferricyanide redox couple of the resultinginto the polymer matrix. The metal cation can be both thermally PI/Carbon/Pt composite films. All electrochemical measurementsand electrochemically reduced to metal during polymerization. were conducted using a CHI 760 potentiostat/galvanostat with The utilization of a PI/Carbon/Pt composite for fuel cell applica- included software. The experiments were performed in a one-tions has been recently explored for sulfonated PI as the polymer compartment, three-electrode cell. All potentials are referenced toelectrolyte film. A mixture of Pt in carbon black has been incorpo- a Ag/AgCl electrode (3 M KCl filling solution). As counter electrode,rated into sulfonated PI, cast onto an electrode, and used to examine we used a 0.5 mm-thick platinum sheet with an area exceeding thethe oxygen reduction reaction (ORR) [38]. Additionally, the elec- immersed area of the working electrode by a factor of two.trocatalytic properties of Pt-deposited polyimide/carbon nanotubefilms for methanol and nitrite oxidation have also been studied [41]. 2.4. X-ray photoelectron spectroscopy (XPS)These studies document the unique combination of metals, carbon,and PI to produce synergistic properties for novel applications. XPS was employed to identify the oxidation state of platinum in In our study, the synthesis of a “free-standing” PI/Carbon/Pt the PI/Carbon/Pt composites. XPS measurements were performedcomposite electrode is demonstrated. The dispersion of carbon using a Specs PHOIBOS 150MCD electron analyzer and Mg Kinto PAA is examined and the conductivity of the composites pro- excitation. The base pressure was in the 10−10 mbar range for allduced using different carbon loading is evaluated. Specifically, measurements. The electron spectrometer was calibrated using Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  3. 3. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 10 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxx 3XPS and Auger line positions of different metals (Cu, Ag, and Au)[44].2.5. Thermal analysis of PI/Carbon/Pt films Simultaneous thermal analysis (STA) was conducted usinga Netzsch STA449 C thermal analyzer. Simultaneous differentialscanning calorimetry (DSC) and thermal gravimetric analysis (TGA)was performed on PAA, PI, PI/Carbon, and PI/Carbon/Pt under airas both the purge (50 ml/min) and protective (20 ml/min) gas at aheating rate of 10 ◦ C/min. Samples were placed in alumina pans andcovered with an alumina lid with a centered pin-hole. The furnacewas evacuated to about 10−3 torr prior to introducing purge andprotective gases. Platinum loading in the PI/Carbon/Pt composites with a uniformcarbon loading of 10% was estimated using TGA. The composite washeated to a temperature of about 900 ◦ C, at which all organic com-ponents of the PI/Carbon composite were decomposed. All massloss was assigned to the decomposition of the organic compo-nent. The remaining mass in the test crucible at the end of theexperiment was then used to estimate the metal content for thecomposite material. For statistics, a minimum of three runs wasconducted.2.6. Scanning electron microscopy (SEM) SEM images of PI/Carbon/Pt composites were obtained usinga JEOL 5600 electron microscope equipped with a backscatteredelectron (BSE) detector. The films were affixed to the sampleholder using carbon tape and measurements were performed atan acceleration voltage of 15 kV. Deposition of a metallic Au layerto enhance conductivity was not required for SEM measurements. Fig. 1. Photoacoustic FTIR spectra of (a) PAA, (b) PAA/Carbon, (c) PI, and (d)2.7. Electrical conductivity characterization PI/Carbon (600–1900 cm−1 ). Electrical contacts to the PI/Carbon and PI/Carbon/Pt films were The FTIR spectra of PAA, PAA/Carbon, PI, and PI/Carbon in the spec-made using a Cascade Microtech C4S-64/50 probe head with tung- tral range between 600 and 1900 cm−1 are presented in Fig. 1a–d,sten carbide electrodes. The four-point probe sheet resistance of respectively. The asymmetric C O band at 1780 cm−1 is observedeach film was then measured at locations across the surface of for both PI and PI/Carbon and appears as a shoulder on the broadthe substrate using an Agilent 34401A Digital Multimeter con- C O symmetric stretch band centered at 1740 cm−1 . In addition,nected through a Cascade Microtech CPS-05 probe station. Constant a strong sharp band located at around 729 cm−1 , assigned to thepressure for each measurement was maintained for the probe imide C O bending mode, appears in both PI and PI/Carbon. Thehead contacting the substrate using a mechanical stop. Aver- intensity decrease of the C–NH band for PAA at 1546 cm−1 and theages for a minimum of five measurements at each location are increase in band intensity for the C O imide functional group atpresented, together with representative standard deviations and 730 cm−1 and 1778 cm−1 confirm the formation of PI after thermalrelative standard deviations. Electrical resistivity values collected imidization. Similarly, the spectra can be monitored in the rangeusing these methods were converted to electrical conductivity for between 1900 cm−1 and 3700 cm−1 (shown in the supporting doc-clarity. umentation) for changes in the carboxylic acid OH group of PAA. In addition, characteristic secondary amide NH stretch bands occur as2.8. Fourier transform infrared (FTIR) spectroscopy multiple bands in the same region between 3065 and 3300 cm−1 [45]. A broad decrease in band intensity for OH and NH is observed All FTIR measurements were performed on pristine samples for PAA after thermal imidization and the formation of PI/Carbonusing a Digilab FTS-7000 spectrometer and a photoacoustic detec- composite.tor (MTEC). Each sample was scanned 64 times with a resolutionsetting of 4 cm−1 , and scans were averaged to produce each spec- 3.2. Carbon loading and conductivity of PI/Carbontrum. The factors that influence the conductivity of polymer/carbon3. Results and discussion black composites include the properties of the carbon species utilized, aggregation of the carbon within the polymer, and the3.1. Preparation of PI/Carbon heterogeneity and dispersion of the carbon in the polymer matrix. Variations in the density and particle size of the carbon play a crit- The preparation of PI was accomplished using thermal imidiza- ical role because they influence both aggregation and dispersiontion of PAA and carbon incorporated into the co-solvent system [17–19,46,47]. Increasing the density of the carbon filler allowsof acetone and DMSO. The polymerization of PI using this mixture more compact aggregation and packing, leading to enhanced elec-is evaluated using photoacoustic FTIR spectroscopy in Fig. 1 [21]. trical conductivity at lower percolation thresholds (i.e., the measure Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  4. 4. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 104 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxxof the volume of a conducting phase required to change the mate- Table 1 Electronic properties of PI/Carbon.rial from insulating to conducting). For example, highly crystallizedgraphite (HCG) exhibits a higher conductivity and a lower percola- % Carbon Resistivity Conductivity % Relative standardtion threshold than carbon blacks because of its higher density. In content (Ohm cm) (S/cm) deviationaddition, smaller particle sizes enhance conductivity because they 2.5% 2.1 × 105 4.7 × 10−6 8.7allow higher aggregation and smaller gaps between the carbon cen- 5.0% 27 0.037 0.95ters [17,19]. Significant changes in the conductivity are observed for 7.5% 3.4 0.29 0.32 10.0% 1.4 0.70 4.5PI once a critical carbon content (percolation threshold) is reached 12.5% 0.80 1.2 0.10[19,48]. Networks form after carbon reaches a critical mass, lead-ing to coagulation and increased contact between the particles[16,17,47]. The increase in conductivity is attributed to the forma- but the mechanical stability of the films is compromised at car-tion of fibrous chain-like carbon networks in the polymer matrix bon loadings ≥12.5%, with the emergence of surface fractures andand has been previously reported for PI/Carbon black composites decreased flexibility. Therefore, a carbon loading of 10% was utilized[16,17]. for all subsequent measurements in our studies. The conductiv- Previous studies of the influence of carbon loading on the elec- ity values between 8 and 16 wt.% carbon in PAA are significantlytrical conductivity have focused on dispersion using low-volatility higher when compared to previously reported values for PI filmssolvents such as DMSO and NMP. These solvents are problematic containing the same carbon filler (SC conductex) used in this studybecause they are often trapped in the polymer, leading to materi- [17].als that are less homogeneous. In our study we evaluate the use ofa binary solvent, combining highly volatile acetone with DMSO to 3.3. Pt deposition on PI/Carbonreduce the incorporation of solvent into the PI/Carbon composite.In addition, the solvent is used to homogeneously disperse car- Deposition of Pt metal on freestanding PI/Carbon workingbon into the PAA prior to imidization to produce materials with electrodes was achieved using cyclic voltammetry encompassingpercolation thresholds at carbon loadings consistent with known potentials for the reduction of PtCl4 2− ion to Pt metal. The equationPI/Carbon materials. and potential for the reduction of PtCl4 2− is as follows [49]: The influence of the carbon content on the electrical conductiv-ity of PI is presented in Fig. 2 for the PI/Carbon materials produced PtCl4 2− + 2e− → Pt(0) + 4Cl− , E ◦ = 0.493 V vs. Ag/AgClusing the acetone/DMSO solvent system. The carbon content for The voltammetric response for a planar Pt reference and athe composite is reported as the % carbon by weight in the polymer PI/Carbon electrode in solution containing 1 M HClO4 are providedprecursor, PAA. An approximate ∼104 increase in the conductivity in Fig. 3a and b (bottom), respectively. The Pt electrode shows ais observed as the %Carbon is increased from 2.5% to 5%, as also voltammetric response at negative potentials, which is consistentlisted in Table 1. The data suggests that the percolation threshold is with proton adsorption at the metal surface. The response of theachieved at carbon loadings between 2.5 and 5%. The conductivity PI/Carbon electrode is strongly diminished due to the absence of Ptof the materials continuously improves above 5% carbon content, at the electrode surface. The electrochemical deposition of Pt from solutions containing 20 mM PtCl4 2− in 1 M HClO4 are provided for Pt and PI/Carbon electrodes in the center of Fig. 3. The voltammetric response 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 PtCl4 2− 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 M HClO4 prior to Pt deposition (bottom), during Pt deposition in a solution containing 2.0 × 10−2 M PtCl4 2− in 1 M HClO4 (center), and the same electrode (PI/Carbon/Pt)Fig. 2. Conductivity of PI/Carbon as a function of the percent carbon by weight in in solution containing 1 M HClO4 (top). The scan rate for all measurements wasPAA. 0.01 V/s. Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  5. 5. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 10 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxx 5electrochemistry associated with the Pt surfaces clearly visible. In contrast, the voltammetry associated with H adsorp-Specifically, the formation and reduction of PtO is observed (A/A ). tion/desorption at the Pt metal deposited on each electrode isIn contrast, Pt reduction is not evident for either electrode (small significantly different. The reaction for the process is:shoulder at potentials more negative than 0.20 V). Furthermore,proton adsorption/desorption at the Pt metal surface is clearly vis- H+ + e− → H → Hads.ible (B/B ) for both electrodes during this metal deposition process.Finally, the voltammetric response of the Pt planar electrode and The potentials for the proton adsorption/desorption (B/B )the PI/C/Pt composite electrode is evaluated in solution contain- for the Pt planar and PI/Carbon/Pt electrodes are observed ating 1 M HClO4 only (i.e., without the Pt precursor) to demonstrate approximately −0.07/−0.12 V and −0.02/−0.16 V, respectively,that Pt deposition has been achieved at both electrodes. Variations with modest shifts in potential as a function of the number of cyclesin the Pt surface morphology likely influence the small variations employed. The peak-splitting for the Pt electrode is on the order ofobserved in the voltammetry for the PI/Carbon and bulk Pt surfaces. 50 mV, while a value of 140 mV is observed for the PI/Carbon/PtHowever, the same voltammetric processes for the Pt surface are composite. In addition, the voltammetric waves associated with Ptresolved at each substrate. Significant changes in the voltammetry deposition are broader relative to the planar Pt electrode. Protoncan be observed during Pt deposition for both electrodes. Specif- adsorption at a polycrystalline platinum electrode is a two-stepically, each voltammetric cycle results in an increase in current process, involving weak adsorption followed by strong adsorp-associated with an increase in the surface area of deposited Pt. The tion of hydrogen atoms at the Pt surface [54]. This process isvoltammetric waves associated with PtO (A/A ) [50,51] and Pt-Hads highly dependent on the crystal surface structure of Pt, with the(B/B ) [50,52–54] increase with each deposition cycle. The reduc- voltammetric response determined by the number and orientationtion of Pt (small shoulder at potentials more negative than 0.20 V) is of crystal planes of Pt that are exposed [53,55]. The presence ofobserved for both electrodes during the Pt deposition and is absent defects and impurities can lead to disorder in the crystal structure ofwhen both electrodes are immersed in solutions containing only the electrode, further reducing the distinct adsorption/desorption1 M HClO4 . The voltammetry is consistent with the deposition of voltammetry characteristics of different crystal planes of Pt. ThePt through the reduction of the Pt metal precursor (PtCl4 2− ) at the data suggests that differences in Pt crystallinity may translate intoPI/Carbon electrode surface. different electrochemical responses for the oxidation/reduction of More in-depth analysis of the voltammetry associated with PtO solution species.(A/A ) provides a measure of the Pt surface chemistry and theinvolved electrochemical processes. Specifically, the formation and 3.4. Scanning electron microscopy (SEM)reduction of PtO is observed at ∼0.73 V and ∼0.60 V for both elec-trodes. The voltammetry is consistent with the following reaction SEM is used to investigate the morphology of the PI/Carbon/Ptwhere shifts in potential are common based on the electrode com- composite and the distribution of Pt particles on the surface. Theposition [53]: SEM images of PI/Carbon/Pt electrodes with Pt loading of 6 and 30% are shown in Fig. 4. The images show the immersed interface withPtO(s) + 2e− + 2H+ Pt(s) + H2 O, E = 0.78 V vs. Ag/AgCl Pt deposits (left and/or top) and the electrode material that was notFig. 4. SEM images of (a) PI/Carbon/Pt composite with low Pt loading (6%, ×2000), (b) PI/Carbon/Pt composite with high Pt loading (31%, ×2000), (c) the latter at highermagnification (×5000), and (d) PI/Carbon/Pt with high Pt loading at low magnification (×35). Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  6. 6. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 106 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxximmersed (i.e., no Pt deposited) for clarity. The Pt metal deposits In contrast, the observed asymmetric lineshape further indicates aat 6% (Fig. 4a, ×2000) loading appear as evenly spaced, discrete metallic environment of the Pt atoms—it is well known that metallicclusters with diameters of ∼400 nm on the PI/Carbon substrate. In systems allow a continuum of low-energy excitations in the finalcontrast, the Pt deposits at 30% Pt loading in Fig. 4b (×2000) show state of the photoemission process, giving rise to a characteris-a coalesced deposit of metal with no discernible clusters on the tic asymmetric shape (commonly described by a Doniach-Sunjic ˇ ´PI/Carbon substrate. The Pt deposits (6% loading) suggest that car- profile). It is not necessarily correct to assume that several initial-bon dispersion in PI is uniform for the acetone/DMSO co-solvent state species, described by symmetric Gaussian or Voigt functionssystem. In fact, carbon aggregation at the PI surface is not discern- (including a metallic species), are needed to describe the spectrumable on the substrates at higher magnification (×8000) in Fig. 4c. properly, as has been done in earlier publications of related sys- Previous studies in literature that have used SEM to evaluate the tems [57]. Furthermore, local differences in final-state screeningmorphology of PI/Carbon films have shown large (approximately of the created photo-hole might lead to small shifts of core level60 m diameter) surface defects when prepared with carbon black. lines as well, which might be responsible for the small differenceThe defects were attributed to the formation of water vapor pock- between the here-observed value and the literature reference forets produced during imidization [17]. Alternatively, these defects bulk metallic Pt.could also be created by the fast expulsion of residual low-volatilitysolvents as the temperature is increased to initiate imidization. The 3.6. Thermal gravimetric analysis (TGA) of PI/Carbon/Pt filmslack of large-scale defects observed in the PI/Carbon substrates pro-duced in these studies at much lower magnification (×35) in Fig. 4d The thermal properties of PI (a), PI/Carbon (b), and twosuggests that the temperature program was effective in expelling PI/Carbon/Pt composites (c and d) were evaluated using ther-water vapor and/or the binary solvent prior to imidization. Fur- mal gravimetric analysis (TGA), as shown in Fig. 6. The thermalthermore, the absence of defects in the PI/Carbon substrates may properties of PI, PI/Carbon, and PI/Carbon/Pt can be evaluated atbe attributed to the minimization of low-volatility solvents in the temperatures that result in the thermal degradation and loss ofpreparation. The results demonstrate that Pt deposition at PI/C mass associated with the polymer. The residual mass at 900 ◦ C pro-composite electrodes can be controlled electrochemically to pro- vides an estimate of residual carbon in the PI/Carbon sample andduce deposits with variable thickness and morphology. metal loading in the PI/Carbon/Pt composites. For example, the final mass of PI with no carbon reaches a value of zero, consistent with3.5. X-ray photoelectron spectroscopy of PI/Carbon/Pt the complete loss of polymer during the thermal processes shown in Fig. 6a (note that the origin of the ordinate was offset for better X-ray photoelectron spectroscopy (XPS) is used to examine the viewing). In contrast, the TGA of PI/Carbon shows a residual massoxidation state of the Pt metal deposited on PI. The XPS survey of 2% relative to PI (Fig. 6b). Similarly, Pt loadings of 4% and 26% arespectrum of a representative PI/Carbon/Pt film is shown in Fig. 5. obtained for the PI/Carbon/Pt composites when the residual carbonIt confirms the presence of Pt, C, O, and N at the surface, in addi- content of 2% is subtracted from the residual mass shown in Figs. 6ction to some minor Cl, S, and Zn impurities. While the Cl and S and d, respectively.impurities are likely due to the precursor chemicals, solvent, and/orelectrolyte, the Zn impurities might have been adsorbed duringtransfer in the XPS ultra-high vacuum system. A detail scan of thePt 4f core levels is shown in the inset of Fig. 5. The 4f7/2 peak maxi-mum is found at 71.35 eV (±0.05), which is in good agreement withreference values for bulk Pt metal (71.1 or 71.2 eV) [56] and sug-gests that the Pt atoms are indeed in a metallic environment. Mostrelevant Pt-containing compounds (with non-zero oxidation state)exhibit binding energies of 72.5 eV and above. In particular, PtO andPtO2 are expected to be found at 74.2 eV and 75.0 eV, respectively[56], and would also be expected to exhibit a symmetric lineshape.Fig. 5. Mg K XPS survey spectrum of PI/Carbon/Pt composite (inset: Pt 4f detail Fig. 6. TGA curves of (a) PI, (b) PI/Carbon, (c) PI/Carbon/Pt (6% Pt loading), and (d)spectrum). PI/Carbon/Pt (31% Pt loading), using a heating rate of 10 K/min. Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  7. 7. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 10 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxx 7 The thermal analysis of the materials also provides an important Table 2 Electronic properties of PI/Carbon/Pt.measure of the thermal stability of PI, PI/Carbon, and PI/Carbon/Ptmaterials. In this case, the thermal properties of all four samples %Pt content Resistivity Conductivity %Relative standardare very similar, with onset temperatures of 576 and 570 ◦ C for PI (Ohm cm) (S/cm) deviationand PI/Carbon, respectively, and ∼560 ◦ C for the PI/Carbon/Pt com- 0.0% 7.0 0.14 3.6posites. A small mass change attributed to the loss of low-volatile 4.6% 4.2 0.24 0.029NMP and DMSO solvents can be observed for all materials prior to 12% 0.93 1.1 5.0 22% 0.15 6.7 3.5the onset temperature, corresponding to a mass decrease of ∼2–4%. 30% 0.091 11 0.58For comparison, the operating temperature range for Kapton® HN 36% 0.051 20 0.69film over long periods of use (as reported by the manufacturer) is−269–400 ◦ C [1], which is well below the temperatures reportedin our study. This is due to the fact that PI has a glass transi- is recorded up to a value of 36%, as shown in Fig. 7 and Table 2. Thetion reported at ∼400 ◦ C, where Kapton® HN becomes rigid [58]. data suggests that Pt loading should be increased to maximize con-The onset temperatures for the initial mass loss of the materials ductivity; however, costs associated with the metal deposition alsoproduced in this study suggest that they can be utilized at high need to be considered and a reasonable value of Pt loading must betemperatures (up to 550 ◦ C) without severe thermal degradation. defined based on the conductivity required. Thus the conductiv-However, it may be prudent to work at temperatures below the ity requirements of the specific application will dictate a suitableglass transition to ensure that the materials remain mechanically “compromise” for the Pt loading.stable. 3.8. Electrochemistry at PI/Carbon/Pt composite electrodes3.7. Conductivity of PI/Carbon/Pt composites The conductivity of PI/Carbon and, in particular, of the The electrical conductivity of PI/Carbon/Pt composites as a func- PI/Carbon/Pt composites suggests that the materials can be uti-tion of platinum loading is shown in Fig. 7. The values should be lized as freestanding electrodes to probe electrochemical reactions.compared to the conductivity of the PI/Carbon material at 10% car- Specifically, the oxidation/reduction reaction involving the ferri-bon loading (see Fig. 2, and first data point in Fig. 7). The percent cyanide couple can be examined [49]:Pt loading was determined using TGA. As expected, the conduc-tivity of the composite increases as the platinum loading on the Fe(CN)6 3− + e− Fe(CN)6 4− E ◦ = 0.161 V vs. Ag/AgClPI/Carbon surface is increased. The conductivity for PI/Carbon with The cyclic voltammetric response of a PI/Carbon electrode with-10% carbon content is 0.140 ± 0.005 S/cm, which is lower than that out Pt, in solution containing 0.1 M K3 Fe(CN)6 in 1 M KNO3 , isreported for PI/Carbon (0.700 ± 0.003 S/cm) for the same material in presented in Fig. 8a. The PI/Carbon film shows minimal elec-Fig. 2, suggesting that batch-to-batch variations exist for the com- trochemical activity for the oxidation/reduction processes ofposite materials. However, both values are consistent with a ∼104increase in conductivity when the carbon content in PI is increasedto 10%. As a function of Pt loading, a steady increase in conductivity Fig. 8. Cyclic voltammetry of (a) PI/Carbon, (b) PI/Carbon/Pt (125 Pt deposition cycles), and (c) a planar Pt electrode in solution containing 0.1 M K3 Fe(CN)6 in 1 M KNO3 . The scan rate for all measurements was 0.01 V/s. Crosshair marks the zero Fig. 7. Conductivity of PI/Carbon/Pt composite as a function of Pt loading. potential and zero current for each voltammogram. Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  8. 8. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 108 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxxTable 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)6 3− ] offerricyanide at PI/Carbon, PI/Carbon/Pt, and a planar Pt electrode (∼1 cm2 ). Electrode (1 cm2 ) Epc (V) Epa (V) Ep (mV) n PI – – – – PI/Carbon/Pt 0.157 0.414 257 0.230 Planar Pt electrode 0.183 0.308 125 0.472ferricyanide. In contrast, the electrochemical response for thePI/Carbon/Pt electrode (Fig. 8b) is clearly enhanced. For comparisonand reference, a planar platinum sheet electrode was also used toevaluate the oxidation/reduction processes of ferricyanide, Fig. 8c,which are clearly observed. The voltammetry is significantly moreresolved for ferricyanide at the planar Pt electrode when comparedto the same electrochemical reaction at the PI/Carbon/Pt electrode. The peak separation Ep = Epa − Epc (Epa and Epc are the anodic Fig. 9. Cyclic voltammetry for the oxidation of methanol in solution containing 1 M CH3 OH in 1 M H2 SO4 for (a) PI/Carbon, (b) PI/Carbon/Pt (2 Pt deposition cycles), (c)and cathodic peak potentials, respectively) is an indicator for PI/Carbon/Pt (12 Pt deposition cycles) and (d) PI/Carbon/Pt (2 Pt deposition cycleselectron transfer kinetics during the oxidation/reduction of fer- and 15 methanol oxidation cycles) The scan rate for all measurements was 0.01 V/s.ricyanide at each electrode. For kinetically fast electrochemicalprocesses, Nernstian kinetics are expected, where Ep is equalto (59/n) mV, n being the number of electrons transferred (n = 1 Methanol oxidation is not observed for PI/Carbon electrodesfor ferricyanide). The peak potentials (Epc and Epa ) and the peak without Pt in solutions containing 1 M H2 SO4 and 1 M methanol,splitting ( Ep ) are obtained from the voltammetric response of as shown in Fig. 9a. However, methanol oxidation is observed inferricyanide at both PI/Carbon/Pt and Pt planar electrodes (and the same solution using a PI/Carbon free-standing electrode afterlisted in Table 3). The voltammetry for the PI/Carbon/Pt electrode two electrochemical reduction cycles using PtCl4 2− , Fig. 9b. Theshows non-nerstian peak splitting of 257 mV (i.e., with a value sig- voltammetry confirms the formation of a PI/Carbon/Pt compositenificantly greater than 59 mV). The data suggests that the electron electrode with an electrochemically active Pt surface. The surfacetransfer is kinetically limited at the PI/Carbon/Pt electrode relative area and Pt content of the composite electrode can be varied byto the planar Pt electrode. The values obtained for the Pt planar changing the number of Pt deposition cycles. For comparison, theelectrode (peak splitting of 125 mV) also suggest that the electron voltammetric response of a PI/Carbon electrode after twelve Pttransfer is kinetically limited. However, the smaller peak splitting reduction cycles is presented in Fig. 9c. The oxidation of methanolsuggests that the electron transfer at the bulk Pt surface is enhanced is observed for both, the forward and reverse scans. In addition, therelative to the PI/Carbon/Pt electrode. Although factors such as oxidation of methanol on the forward (positive) and the reversesolution concentrations and non-linear diffusion are important, (negative) scan increases with increasing Pt content.surface inhomogeneity, including defects on the electrode surface, For PI/Carbon/Pt after two Pt deposition cycles, the oxidation oflikely play a larger role in the more sluggish electron transfer kinet- methanol is characterized by two voltammetric waves centered atics for the PI/Carbon/Pt composite relative to the planar Pt reference 0.70 V for the forward scan and 0.57 V for the reverse scan (Fig. 9b).[59]. After twelve Pt deposition cycles, the oxidation of methanol is The catalytic oxidation of methanol on Pt was also probed observed at 0.95 V for the forward scan and 0.64 V for the reverseusing the freestanding PI/Carbon/Pt composites—the correspond- scan. The waves for both electrodes are more positive than theing voltammograms are shown in Fig. 9. The oxidation of methanol values reported for PI/CNT/Pt [41] and for a conductive polymeris strongly influenced by the Pt surface morphology and can be used (polyaniline)/Pt composite [62]. The data suggests that oxidationas a measure of the chemical reactivity of the PI/Carbon/Pt com- of methanol may be thermodynamically more favorable at theposite. The oxidation of methanol occurs via two reaction paths, as PI/Carbon/Pt electrodes produced in this study.presented below [60,61]: Further analysis of the peak currents, charge passed, and peak splitting provides an overview of the oxidation of methanol usingPt-CH3 OHads + H2 O → Pt + CO2 + 6H+ + 6e− PI/Carbon/Pt electrodes (given in Table 4). For example, both the ratio of the peak currents (ipf /ipr = 1.01) and the charge-ratio asso-Pt-CH3 OHads → Pt-COads + 4H+ + 4e− ciated with methanol oxidation (Qf /Qr = 1.09) obtained for the PI/Carbon/Pt after two Pt reduction cycles are close to unity, indicat-The first path involves the direct oxidation of methanol to CO2 , ing that the processes are chemically reversible. The peak splittingwhile the second one involves the formation of a strongly adsorbed is on the order of 135 mV and can be used as an indicator for sur-CO intermediate prior to oxidation. CO adsorption causes surface face poisoning at the metal surface. The peak splitting associatedpoisoning by blocking Pt sites necessary for further oxidation of with the oxidation of methanol for conductive polymer polyanilineCH3 OH. Additionally, methanol oxidation during the cathodic scan (PANI)/Pt composites was measured to be 45 mV [62]. Although theis hindered by the formation of PtO at the electrochemical interface results confirm that PI/Carbon substrates are sufficiently conduc-and does not occur prior to the reduction of the surface oxide. tive to allow the deposition of Pt metal to produce freestanding,Table 4Oxidation/reduction potentials (Epf and Epr ), peak splitting ( Ep ), current ratios (ipf /ipr ), and charge ratios (Qf /Qr ) for the forward and reverse scans (after 2 and 10 cycles,respectively), associated with methanol oxidation at a PI/Carbon/Pt electrode (∼1 cm2 ). Electrode (1 cm2 ) Epf (V) Epr (V) Ep (mV) ipf /ipr Qf /Qr PI/Carbon/Pt (2 cycles) 0.705 0.570 135 1.01 8.18 mC/7.50 mC = 1.09 PI/Carbon/Pt (10 cycles) 0.955 0.640 315 0.89 141 mC/46 mC = 3.09 Please cite this article in press as: J.M. Kinyanjui, et al., Synthetic Met. (2011), doi:10.1016/j.synthmet.2011.08.046
  9. 9. ARTICLE IN PRESSG ModelSYNMET-13632; No. of Pages 10 J.M. Kinyanjui et al. / Synthetic Metals xxx (2011) xxx–xxx 9electrochemically active PI/Carbon/Pt surfaces, the polymer does [2] C.A. Kaufmann, A. Neisser, R. Klenk, R. Scheer, Thin Solid Films 480 (2005)not provide enhanced protection against poisoning at the Pt surface 515–519. [3] F. Kessler, D. Rudmann, Solar Energy 77 (2004) 685–695.relative to PANI/Pt composites [62]. For example, the charge ratio [4] R.E. Southward, D.S. Thompson, D.W. Thompson, M.L. Caplan, A.K. Stclair,for the oxidation of methanol between the forward and reverse Chemistry of Materials 7 (1995) 2171–2180.scans (Qf /Qr = 3.09) indicates that poisoning is more prominent for [5] A.N. Tiwari, A. Romeo, D. Baetzner, H. Zogg, Progress in Photovoltaics 9 (2001) 211–215.this electrode. The peak splitting associated with the forward and [6] A.C. Vasko, X. Liu, A.D. Compaan, 34th IEEE Photovoltaic Specialists Conferencereverse process is also significantly larger (315 mV) in comparison (PVSC) (2009) 001552– the PI/Carbon/Pt electrode obtained from two Pt reduction cycles [7] Z. Li, H. Qing, J. Wei-Long, L. Chang-Jian, S. Yun, Chinese Physics Letters 25 (2008) 734.and PANI/Pt [58]. The data indicates that the surface contaminants [8] G. Gebel, G. Meyer, L. Gonon, P. Capron, D. Marscaq, C. Marestin, R. Mercier,must be fully reduced and desorbed from the Pt to observe the Journal of Power Sources 157 (2006) 293–301.oxidation of methanol on the reverse scan. Furthermore, the peak [9] K. Okamoto, Journal of Photopolymer Science and Technology 16 (2003) 247–254.currents decrease with each subsequent cycle in methanol for the [10] D. Hill, Y. Lin, L.W. Qu, A. Kitaygorodskiy, J.W. Connell, L.F. Allard, Y.P. Sun,PI/Carbon/Pt electrode obtained from two Pt reduction cycles as Macromolecules 38 (2005) 7670–7675.shown in Fig. 9d. This signifies that surface contaminants the cat- [11] X.W. Jiang, Y.Z. Bin, M. Matsuo, Polymer 46 (2005) 7418–7424.alyst efficiency is reduced with successive cycling due to surface [12] M. Shigeta, M. Komatsu, N. Nakashima, Chemical Physics Letters 418 (2006) 115–118.poisoning. The data suggests that the deposition of Pt at conduc- [13] K.E. Wise, C. Park, E.J. Siochi, J.S. Harrison, Chemical Physics Letters 391 (2004)tive PI/Carbon electrodes should be explored more thoroughly to 207–211.fully understand how the number of Pt deposition cycles influences [14] B.K. Zhu, S.H. Xie, Z.K. Xu, Y.Y. Xu, Composites Science and Technology 66 (2006) 548–554.the Pt morphology and efficiency of methanol oxidation. In addi- [15] M. Lebron-Colon, M.A. Meador, J.R. Gaier, F. Sola, D.A. Scheiman, L.S. McCorkle,tion, the co-deposition of Pt-Ru in a 1:1 composition ratio should ACS Applied Material Interfaces 2 (2010) 669– investigated. This ratio has been found optimal in the decreas- [16] Y. 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