A Novel High Specific Surface Area Conducting Paper Material

Sep 6, 2008 - A Novel High Specific Surface Area Conducting Paper Material Composed of Polypyrrole and Cladophora Cellulose. Albert Mihranyan,*,† Le...
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J. Phys. Chem. B 2008, 112, 12249–12255

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A Novel High Specific Surface Area Conducting Paper Material Composed of Polypyrrole and Cladophora Cellulose Albert Mihranyan,*,† Leif Nyholm,‡ Alfonso E. Garcia Bennett,† and Maria Strømme† Nanotechnology and Functional Materials, Department of Engineering Sciences, The Ångstro¨m Laboratory, Box 534, 75121 Uppsala, Sweden, and Department of Materials Chemistry, The Ångstro¨m Laboratory, Box 538, 75121 Uppsala, Sweden ReceiVed: March 7, 2008

We present a novel conducting polypyrrole-based composite material, obtained by polymerization of pyrrole in the presence of iron(III) chloride on a cellulose substrate derived from the environmentally polluting Cladophora sp. algae. The material, which was doped with chloride ions, was molded into paper sheets and characterized using scanning and transmission electron microscopy, N2 gas adsorption analysis, cyclic voltammetry, chronoamperometry and conductivity measurements at varying relative humidities. The specific surface area of the composite was found to be 57 m2/g and the fibrous structure of the Cladophora cellulose remained intact even after a 50 nm thick layer of polypyrrole had been coated on the cellulose fibers. The composite could be repeatedly used for electrochemically controlled extraction and desorption of chloride and an ion exchanging capacity of 370 C per g of composite was obtained as a result of the high surface area of the cellulose substrate. The influence of the oxidation and reduction potentials on the chloride ion exchange capacity and the nucleation of delocalized positive charges, forming conductive paths in the polypyrrole film, was also investigated. The creation of conductive paths during oxidation followed an effective medium rather than a percolative behavior, indicating that some conduction paths survive the polymer reduction steps. The present high surface area material should be well-suited for use in, e.g., electrochemically controlled ion exchange or separation devices, as well as sensors based on the fact that the material is compact, light, mechanically stable, and moldable into paper sheets. Introduction Since the electronically conductive properties of acetylene were described in the late 1950s,1 this and similar materials have attracted much attention. The conductive properties of polyacetylene resulted in the 2000 Nobel Prize in Chemistry and various analogues of it have been investigated throughout the years including polyphenylene, polyphenylene sulfide, polypheylene vinylene, polypyrrole, polythiophene, and polyaniline. Polypyrrole (PPy) and polyaniline (PAni) are probably the most promising of currently known conductive polymers in a number of applications due to their reasonably high conductivity, good stability of the oxidized state, and ease of processing. Of the two, PPy is often preferred due to its superior electrical conductivity and facile synthesis both in aqueous and organic media. During the polymerization of pyrrole, Figure 1, anions in the electrolyte solution are incorporated in the polymer film to maintain charge balance. The presence of these so-called dopant ions greatly influences the properties of the film. It is generally conceived that both anions and cations as well as accompanying water can enter the polymer film upon oxidation and reduction.2-5 If a small anion with high mobility is incorporated into a polymer film as a dopant during polymerization, it can be expelled when the polymer is reduced. By doping the film with large anions, which have mobilities low enough to confine them inside the polymer, one can reversibly absorb and desorb cations * Corresponding author. E-mail: [email protected]. † Nanotechnology and Functional Materials, Department of Engineering Sciences, The Ångstro¨m Laboratory. ‡ Department of Materials Chemistry, The Ångstro ¨ m Laboratory.

which will be attracted to the large anions to maintain charge neutrality. Various biologically active entities such as enzymes,6,7 antibodies for immunosensors8 or metal complexing entities9 can be incorporated in PPy films for highly specific molecular and/or ionic recognition and separation. This phenomenon lays the foundation for various applications of PPy films in ion exchange membranes and separation,10-13 wherein the adsorption and desorption of valuable entities is achieved merely by varying an electrochemical potential. The general trend in contemporary science and technology is miniaturization of devices. When employing electronically conducting polymers as electrochemically controlled ion exchangers, miniaturization, however, results in decreasing ion exchange capacities of the conductive polymers. Previous designs of electrochemically controlled ion exchange and solidphase microextraction devices have been based on deposition of a PPy film on electrodes wherein the theoretical capacity of the film generally was controlled by the thickness of the film.14,15 Although it is well-known that these conductive polymer films can easily be synthesized electrochemically on the surface of an electrode, such deposition is generally not suitable for industrial scaling-up. Alternatively, polymerization can be performed via chemical processing as is described in this work. On the other hand, when depositing PPy films on electrodes using electro-polymerization, thick PPy coatings can easily be manufactured. The capacities of these films are, however, generally only marginally increased by an increase in the thickness as the absorption and desorption of the ions are, at least partly, diffusion limited processes and the transport of ions and polar species therefore is restricted to the outermost layer of the film. This effect is particularly pronounced for large ions

10.1021/jp805123w CCC: $40.75  2008 American Chemical Society Published on Web 09/06/2008

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Figure 1. Electrochemical polymerization of pyrrole (Py).

Figure 2. The green Cladophora algae polluting coastal areas of the Baltic Sea.

and molecules. Increasing the surface area of the electrode, while keeping the film thickness constant, is consequently an alternative obvious way to improve the capacity of the electrode. However, this is not easy to achieve as this requires access to inexpensive electrodes with very high surface areas or chemical polymerization of an electronically conducting polymer on a high surface area material. If porous materials are used care must be taken to avoid clogging of the pores to prevent the material from becoming reduced to an essentially nonporous low surface area material. One way to increase the surface area of an electronically conducting polymer film is to deposit the polymer on a powdered material consisting of small particles.16 Such materials must, however, be packed into columns or coated on a suitable electrode (with a high surface area) prior to use in applications involving changes in the electrochemical potential of the polymer. Powdery ion exchangers are therefore less convenient in batch-wise processing where the polymer is put directly into the sample to be processed. In electrochemically controlled ion exchange, the application of a potential across a packed column will generally result in potential gradients within the column which may give rise to problems. This risk should clearly be much smaller if the conducting polymer could be deposited directly on a solid carrier with a very large surface area. It infers from the above description that a composite material consisting of an electronically conducting polymer coated on a solid substrate of high surface area and serving as an electrode in an electrochemically controlled separation technique would be highly desirable. However, polymerizing a continuous thin polymer film on a suitable porous, high surface area solid substrate, while preserving both the large surface area of the substrate and the functionality of the film, is challenging.

Functionalization with PPy of various natural and artificial polymers has previously been performed including wool17 and textiles.18 It was reported that cellulose has a high affinity for, e.g., PPy and PAni and that cellulose fibers hence can be coated with these polymers without impairing the integrity of the cellulose fibers.19 Further, the inclusion of small amounts of microcrystalline cellulose (MCC) was found to be advantageous as it significantly improved the mechanical properties of conductive polymers which are otherwise brittle.20 We have previously reported on a cellulose powder with a specific surface area comparable in magnitude to that of industrial adsorbents. This cellulose is extracted from green Cladophora sp. algae and features additional distinct properties not seen in cellulose derived from land plants. Compared to ordinary microcrystalline cellulose, Cladophora cellulose has a higher degree of crystallinity (about 95% vs 80% as measured with X-ray diffraction), larger surface area (60-90 m2/g vs 1 m2/g as measured by N2 gas adsorption), and larger crystallites (20-30 nm vs 4-5 nm).21 The Cladophora cellulose material was found to be useful in a number of applications involving a tabletting agent,22 drug carrier for liquid drugs,23 or a suspending aid to improve the stability and texture of other dispersive systems.24 It should be noted that the source of the abovementioned cellulose material is filamentous green algae Cladophora which is known for polluting coastal areas due to excessive growth,22 see Figure 2. The eutrophication of Cladophora algae is a serious global environmental problem, and there is a strong social demand to find industrial application for the removed algae. The latter would create a commercial interest to address environmental issues which otherwise constitute heavy financial burdens for the local communities to cope with.

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Figure 3. Paper sheet produced by polymerization of polypyrrole on Cladophora cellulose (a). The sheet can be foiled without impairing the integrity of paper. SEM picture of polypyrrole functionalized Cladophora cellulose fibers (b). TEM image of a typical PPy coated cellulose fiber showing the formation of a spine-like structure with PPy strands of ∼6 nm thickness running perpendicular to the fiber c-axis direction. Lighter contrast seen in the center of the fiber is associated with a single cellulose fibril, estimated to be ∼20 nm in diameter, consistent with a PPy coating of ∼50 nm in thickness (c).

Herein we describe a novel high specific surface area conductive PPy cellulose composite material potentially useful in various electrochemically controlled ion exchange and separation processes. The functionality of the composite is demonstrated by extracting Cl- ions from saturated sodium chloride solutions and the material is characterized using a number of different techniques including e.g. scanning electron microscopy (SEM), transmission electron microscopy (TEM), and chronoamperometry. Experimental Section Materials. Cladophora algae were collected from the Baltic Sea, Figure 2. The cellulose was extracted from Cladophora algae as described previously.24 Pyrrole (Py), FeCl3, and HCl were used as supplied by VWR, Sweden. Fresh pyrrole was used. Materials Preparation. A 200 mg sample of Cladophora powder was dispersed in 50 mL of water using high energyultrasonic treatment (VibraCell 750W, Sonics, USA) for 8 min and the dispersion was collected on a filter paper. Three mL of Py was put in a volumetric flask and the total volume was brought to 100 mL. The collected cellulose cake was mixed with Py solution and dispersed using ultrasonicator for 1 min. The dispersion was allowed to stand for 30 min and then collected on filter paper. Then 8 g of FeCl3 was dissolved in 100 g of water and run through the filter cake to induce polymerization (the reaction was allowed to continue for 10 min prior to filtration). A fluffy sponge like cake was formed. Then, 100 mL of 0.1 M HCl was run through the cake. The product was then thoroughly washed with water and dried (the cake was redispersed using an ultrasonicator to form a homogeneous layer). Electron Microscopy. SEM images were taken using a Leo Gemini 1550 FEG SEM, UK operated at 2-3 kV with no gold coating. The samples were mounted on aluminum stubs using a double-sided adhesive tape and sputtered with Au-Pt prior to microscopy. TEM images were recorded using a JEOL-3010 microscope, operating at 300 kV (Cs 0.6 mm, resolution 1.7 Å) with a CCD camera (model Keen View, SIS analysis, size

1024 × 1024, pixel size 23.5 × 23.5 µm) at 25 000-150 000× magnification using low-dose conditions. As-prepared samples were crushed using a mortar and transferred to a holey-carbon TEM grid using ethanol. N2 Gas Adsorption/Desorption. The N2 gas adsorption and desorption isotherms were obtained with ASAP 2020, Micromertitics, USA. The specific surface area was measured according to BET method.25 I-V Measurements. The sample resistance was measured using a semiconductor device analyzer (B1500A, Agilent Technologies, USA). The PPy/cellulose samples were dried in a desiccator over P2O5 over a period of 10 days prior to analysis. The samples were then transferred to another desiccator with controlled relative humidity and stored for at least 4 days. The relative humidity was controlled by using saturated salt solutions of LiCl, K2CO3, NaI, NaCl corresponding 11, 37, 54, and 75%. All measurements were performed at room temperature. Electrochemical Measurements. Cyclic voltammetry and chronoamperometry (potential step) measurements were performed in a standard 3-electrode electrochemical cell employing an Autolab/GPES interface (ECO Chemie, The Netherlands) with the sample as the working electrode, a Pt wire as counterelectrode and an Ag/AgCl electrode as the reference. The measurements were carried out in saturated NaCl solutions at room temperature. Results and Discussion Figure 3a shows a typical PPy/cellulose paper sheet sample consisting of Cladophora cellulose fibers coated with PPy. It should be mentioned that these sheets possess significant mechanical strength and elasticity as they may be bent, twisted or foiled without breaking. This is an important advantage in electrochemically controlled ion exchange and separation applications as the manufactured composite can be directly immersed into the solution to be processed. Electron Microscopy. From the SEM micrograph in Figure 3b, fine fibers with a diameter of ∼ 100 nm in width are clearly seen. The fibers are intertwined and form a three-dimensional network. No islands of PPy on cellulose fibers are seen in

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Figure 4. TEM images recorded on typical PPy coated cellulose fibers showing the following: (a) the formation of a spine-like structure with PPy strands of ∼6 nm thickness running perpendicular to the fiber c-axis direction; (b) open ended fiber where the cellulose fibril can be seen to protrude from the center of the PPy coating; (c) the presence of PPy units of ∼80 nm in length.

agreement with previous findings.19 The SEM micrograph suggests the presence of significant porosity and the PPy coating does not seem to impair the fine pore network typical for the Cladophora cellulose.26,27 TEM observations of the PPy coated cellulose fibers reveal the formation of monodispersed fibers of lengths in the micrometer scale with an average fiber thickness of ∼100 nm. The cellulose fibril diameter can be measured to be ∼20 nm from open ended PPy coated fibers, which is consistent with previously reported values.28 Only a single cellulose core fibril per PPy coated cellulose fibers can be discerned from the TEM studies conducted on open ended fibers, where the cellulose fibril core can be observed to protrude from the center of the PPy coating, see Figure 4. The large majority of fibers were, however, capped at both ends with coated PPy. This coating was found to be ∼50 nm in thickness and possess a rough surface, owing to the formation of PPy segments of some 80 nm in length. Interestingly, as shown in Figure 3c, the formation of a spinelike structure with PPy joins of ∼6 nm thickness running perpendicular to the fiber c-axis direction is clearly observed in all fibers, Figure 4. The formation of such distinct features may be indicative of a preferential deposition/ polymerization mechanism, where the orientation of crystalline fibrils may play an important role in the growth of PPy from deposited droplets of the monomer solution. N2 Gas Adsorption/Desorption. The N2 gas adsorption and desorption isotherm of the composite, Figure 5, is a typical type II curve and the majority of the pores in the mesopore range have a diameter between 200 and 600 Å. The measured specific surface area was found to be 57 m2/g and the total pore volume was 0.18 cm3/g (single point adsorption volume of pores less than 1370 Å with p/p0 ) 0.986). The results of the gas adsorption analysis are in compliance with the SEM micrographs showing that the large surface area and pore volume characteristic for Cladophora cellulose were not impaired by polymerization of Py on the fibers. I-V Measurements. A typical I-V sweep curve of a dry composite sample as well as the resistivity as a function of the relative humidity (RH) curve can be found in Figure 6. The conductivity of dry PPy cellulose composites followed Ohm’s

Figure 5. N2 gas adsorption and desorption isotherms (a) and differential pore volume distribution (b) of the polypyrrole/Cladophora cellulose composite.

Figure 6. I-V sweep performed on a dry composite sample with a cross section area of 30 mm × 1 mm and a length of 17 mm (a) and the resistivity of composites recorded at various relative humidities (b). The error bars in panel b correspond to absolute deviations over three measurements.

law and the resistivity of the PPy/cellulose composite decreased linearly from ∼ 0.45 to 0.1 Ωcm, when the RH was increased indicating that the new composite material under study may be useful in moisture sensor applications. Electrochemical Measurements. The ion exchange capacity of the produced composite material was found to be so high that miniature samples of only about 10-20 mg in weight (having macroscopic dimensions of about 10 mm × 5 mm × 1 mm) were sufficient for the electrochemical characterizations. Using considerably larger specimens (yielding larger currents) for electrochemical analysis was not feasible as the conductivity

Conducting Paper Material

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Figure 7. Cyclic voltammogram recorded at the displayed voltage sweep rate in saturated NaCl solution at room temperature using a standard three electrode setup with the PPy cellulose composite as working electrode, a Pt wire as counter electrode and an Ag/AgCl electrode as reference. The size of the composite used in the measurement presented was 9 mm × 5 mm × 1 mm and the sample weight was 26 mg.

in the three-electrode setup then was limited by the resistivity of the electrolyte solution. The importance of film thickness with respect to the ion exchange capacity has previously been discussed in the literature.29 Whereas smaller ions may be able to penetrate into the polymer bulk, large ions are likely to be confined to the outermost layer of the polymer adjacent to the liquid interface. It has, for instance, been shown that the fraction of a ptoluenesulfonic acid (PTSA) doped PPy coating actively involved in extraction of dansyltryptophan was as small as 5%.30 This indicates that it is generally difficult to increase the real ion exchange capacity of the film by merely increasing the thickness of the polymer film. This is especially true when working with large ions which only are capable of traveling short distances into the bulk of the polymer film. Relatively thin polymer films distributed over a large surface area are therefore preferable to improve the efficiencies and specific sorption capacities of this type of devices. To further characterize the polymer, cyclic voltammograms were recorded using a scan rate of 2.5 mV/s. A typical voltammogram is shown in Figure 7. The oxidation and reduction peaks, corresponding to chloride uptake and release, were found to be well resolved provided that care was taken to minimize the size of the uncompensated ohmic drop by utilizing a sufficiently small sample size. Chronoamperometric measurements in which the potential was stepped between -0.5 and +0.7 V and held at each potential for 300 s to test the functionality of the composite over a number of oxidation and reduction cycles (n ) 60) are displayed in Figure 8a. During the reduction step (-0.5 V) Clions are desorbed from the film. When the oxidation potential (+0.7 V) is applied the Cl- ions are driven into the film to maintain electroneutrality. Figure 8b displays the number of chloride ions absorbed and desorbed during the oxidation and reduction steps, respectively. According to this figure, the ion exchange capacity gradually decreases during the cycling, most likely due to an incomplete reduction of the polymer during the reduction step. However after 60 cycles, corresponding to almost 10 h of continuous use, the film was still found to retain about 85% of its original Cl- incorporation ability. The number of Cl- ions adsorbed/desorbed in the composite under study was typically around 2.3 × 1021 (which corresponds to a charge of about 370 C) per g sample. This is explained by the high surface area of ∼ 57 m2/g). According to ref 31 and references therein, a 20 nm thick PPy layer has an approximate ion

Figure 8. Chronoamperometric potential step response of a composite during a 60 oxidation and reduction step experiment (a) and number of Cl- ions absorbed and desorbed during each step (b). The potential was stepped between -0.5 V and +0.7 V and the potential was kept constant for 300 s after each step. The current response shows that Cl- ions move in (oxidation, positive current) and out (reduction, negative current) of the composite. An enlargement of cycle 11-15 is also displayed. The composite size used in this particular measurement was 5 mm × 3 mm × 1 mm and the sample weight was 8.8 mg.

incorporation capacity of ∼ 80 C/m2. It infers from this value that the estimated PPy layer penetrated by the Cl- ions in the experiment discussed above was only about 16 Å. This effective coating thickness (which depends on the time domain of the experiments) of 16 Å is clearly small compared to the total thickness of the coating, i.e. 50 nm obtained from the TEM images. It is, hence, clear that only a small portion of the polymer layer is electrochemically active even for this relatively thin layer of PPy on the cellulose fibers. It is important to elucidate how the functionality of the composite changes as a function of applied oxidation and reduction potentials. Therefore, a set of experiments was done wherein the oxidation and reduction potentials were varied and the nucleation behavior, i.e., the build-up of conducting paths in the composite, was studied. In Figure 9a, the reduction potential was kept constant (-0.8 V) during pretreatment for 500 s, whereas the oxidation potential was varied between +0.2 and +1 V. It is seen that the larger the difference between the potentials in the reduction and oxidation steps, the higher is the current and the faster the conduction paths are built as indicated by the peak heights and positions for the different oxidation potentials. The build-up of the conducting paths in the film is accompanied by significant transformations in the polymer chains which result in the formation of delocalized positive charges (or “holes”) which are capable of incorporating anions to maintain electroneutrality. In the initial part of the curve, the number of conductive paths increases rapidly as is manifested by the steep increase in the current. As the number of holes in the film is progressively increased, anions are driven into the film to maintain the electroneutrality.

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Figure 9. Chronoamperometric potential step responses of a PPy cellulose composite during oxidation at the displayed potentials (a) following a reduction process in which the potential was kept at -0.8 V for 500 s. The oxidation potential was applied for 300 s. The same sample was used throughout the entire potential step process and the oxidation steps were applied to consecutively larger absolute values. The size of the composite used in the measurement presented was 5.5 mm × 3 mm × 1 mm and the sample weight was 9.8 mg. Chronoamperometric potential step responses of the composite (b), current vs absorbed charge (c), and absorbed charge vs square root of time (d) during oxidation at +0.7 V following a reduction process in which the potential was kept at the displayed potentials for 500 s. The oxidation potential was applied for 300 s. The same sample was used throughout the entire potential step process and the reduction steps were applied to consecutively larger absolute values. The size of the composite used in the measurement presented was 5 mm × 5 mm × 1 mm and the sample weight was 14.7 mg.

At the initial stages of oxidation, the transport of ions is only limited by the number of holes forming conductive paths in the film. However, after a certain period of time (which varies with the applied potential), a diffusion layer adjacent to the surface of the polymer is formed which limits the rate of the incorporation of additional ions. The latter is manifested by the gradual decrease in current with time. It should, however, be mentioned that if a too large positive potential is applied, the film can be overoxidized and its functionality will be irreparably hampered. It has been shown that conductive paths in electronically conducting polymers can be broken during the reduction of the polymers.32,33 To investigate this mechanism the following set of experiments was performed. The as synthesized samples, which were in the doped (or oxidized) state, were pretreated at different reduction potentials (between -0.2 and -1 V) for 500 s, followed by a potential step to a constant potential (+0.7 V). The uptake of ions during the oxidation step was monitored and is summarized in Figure 9b. From this graph it is evident that the oxidation current following reduction at -0.2 and -0.4 V attains it maximum value already at time zero, followed by a gradual decrease due to formation of a diffusion layer. This shows that these reduction potentials are insufficient to break the conduction paths; which means that the film is saturated with conduction paths prior to the oxidation. At reduction potentials of -0.6, -0.8, and -1.0 V, many conducting paths are broken and need to be built up again during the oxidation

to reach the level of maximum film conductivity as is evident from the initial increase of the oxidation current following these reduction steps. Clearly there exists a threshold value (between -0.6 and -0.4 V) below or above which the conductive paths are either being broken or maintained intact. Are all conduction paths broken during some of these reduction cycles carried out at potentials larger than or equal to -0.6 V? To answer this question, the current during the initial part of the oxidation process was plotted vs the charge, Figure 9c. When the hole concentration in the film is in the vicinity of the value necessary for creation of the first few conduction paths, the current vs charge curve is expected to follow a percolation theory behavior34 for which the current increases with increasing amount of charge in a power law manner with an exponent close to 2. Figure 9c, however, shows that the current increases more or less linearly with the amount of charge. The latter is in accordance with an effective medium (EM) behavior34,35 and not with a percolative behavior, which indicates that a significant number of conduction paths remains intact at all reduction potentials studied. It should also be noted that the different reduction potentials all gave rise to about the same oxidation peak value. One implication of this result is that the film cannot be over-reduced. For diffusion limited processes, the movement of ions between the composite and the surrounding solution follows a square-root-of-time behavior according to the Cottrell equation.36 Figure 9d shows the charge vs square root of time plot for a

Conducting Paper Material PPy cellulose film during the above-described reduction steps. From this graph it is clear that the insertion of charge is exclusively diffusion-limited throughout the entire oxidation process when the reduction potentials are small (-0.2 and -0.4 V), i.e., when the conductive paths remain intact. However, below the threshold value, the charge insertion is not diffusion limited during the initial part of the oxidation process. This is in accordance with the nucleation mechanism described above. Conclusion Summarizing, a new type of high surface area conductive PPy algal cellulose composite material potentially useful in various electrochemically controlled ion exchange and separation processes, was presented and characterized. It was shown that the high surface area and mesopore range porosity of the Cladophora cellulose was preserved upon the chemical polymerization of a 50 nm thin layer of polypyrrole onto the cellulose fibers creating a mechanically stable and solid high surface area conducting paper sheet. It was further demonstrated that this solid high surface area composite material exhibited excellent ion-exchange capacity and cycling stability when used as a working electrode in a chloride containing solution. Particularly, the effect of “hole” nucleation forming conductive paths in the composite as a function of reduction and oxidation potential was studied. It was concluded that the creation of conductive paths followed an effective medium behavior rather than a percolative behavior, indicating that the potentials applied to reduce the composite were not able to break all conduction paths in the polymer. It was also found that a threshold value is located between -0.6 and -0.4 V vs Ag/AgCl below which the conductive paths present in the as-synthesized material (in the oxidized state) begin to break and above which the conductive paths remain intact. When anions were intercalated into the composite during the build-up of conduction paths the current was found to be limited by the hole nucleation process (EM behavior) whereas the current was found to be diffusion limited after the conduction paths were in place. The presented composite material and its analogues functionalized with other anions than chloride, may be useful as electrodes in high efficiency miniaturized separation and exchange devices. The material is compact, lightweight, mechanically stable, and easily moldable into paper sheets. Such materials should be very useful whenever batch-wise separations are of concern wherein the material can be directly immersed in the electrolyte solution to be processed. Acknowledgment. The Swedish Foundation for Strategic Research (SSF), the Swedish Research Council (VR), and the Knut and Alice Wallenberg Foundation (KAW) are acknowledged for their financial support.

J. Phys. Chem. B, Vol. 112, No. 39, 2008 12255 References and Notes (1) Natta, G.; Mazzanti, G.; Corradini, P. Atti Acad. Naz. Lincei Cl. Sci. Fis. Mat. Nat. Rend. 1958, 25, 3. (2) Saidman, S. B. Electrochim. Acta 2003, 48, 1719. (3) Silk, T.; Tamm, J. Electrochim. Acta 1996, 41, 1883. (4) Yang, H.; Kwak, J. J. Phys. Chem. B 1997, 101, 774. (5) Yongfang, L.; Renyuan, Q. J. Electroanal. Chem. 1993, 362, 267. (6) Foulds, N. C.; Lowe, C. R. Anal. Chem. 1988, 60, 2473. (7) Rajesh, S.; Pandey, S. S.; Takashima, W.; Kaneto, K. Curr. Appl. Phys. 2005, 5, 184. (8) Xiao, Y.; Li, C. M.; Liu, Y. Biosensors Bioelectron. 2007, 22, 3161. (9) Fabre, B.; Simonet, J. Coord. Chem. ReV. 1998, 178-180, 1211. (10) Collins, G. E.; Buckley, L. J. Synth. Met. 1996, 78, 93. (11) Kaner, R. B., Knobler, C. M., Guo, H. Chiral recognition polymer and its use to separate enantiomers. United States Patent 6,265,615, 2001; Vol. 6265615. (12) Pich, A.; Lu, Y.; Adler, H. J. P. Polymer 2006, 47, 6536. (13) van de Leur, R. H. M.; van der Waal, A. Synth. Met. 1999, 102, 1330. (14) Guo, F.; Gorecki, T.; Irish, D.; Pawliszyn, J. Anal. Commun. 1996, 33, 361. (15) Wu, J. C.; Pawliszyn, J. J. Chromatogr. A 2001, 909, 37. (16) Flandin, L.; Bidan, G.; Brechet, Y.; Cavaille, J. Y. Polym. Composites 2000, 21, 165. (17) Johnston, J. H.; Kelly, F. M.; Moraes, J.; Borrmann, T.; Flynn, D. Curr. Appl. Phys. 2006, 6, 587. (18) Wu, J.; Zhou, D.; Too, C. O.; Wallace, G. G. Synth. Met. 2005, 155, 698. (19) Johnston, J. H.; Moraes, J.; Borrmann, T. Synth. Met. 2005, 153, 65. (20) van den Berg, O.; Schroeter, M.; Capadona, J. R.; Weder, C. J. Mater. Chem. 2007, 17, 2746. (21) Mihranyan, A.; Llagostera, A. P.; Karmhag, R.; Strømme, M.; Ek, R. Int. J. Pharm. 2004, 269, 433. (22) Strømme, M.; Mihranyan, A.; Ek, R. Mater. Lett. 2002, 57, 569. (23) Mihranyan, A.; Andersson, S. B.; Ek, R. Eur. J. Pharm. Sci. 2004, 22, 279. (24) Mihranyan, A.; Edsman, K.; Strømme, M. Food Hydrocolloids 2007, 21, 267. (25) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (26) Mihranyan, A.; Strømme, M. Chem. Phys. Lett. 2004, 393, 389. (27) Strømme, M.; Mihranyan, A.; Ek, R.; Niklasson, G. A. J. Phys. Chem. B 2003, 107, 14378. (28) Ek, R.; Gustafsson, C.; Nutt, A.; Iversen, T.; Nystro¨m, C. J. Mol. Recognition 1998, 11, 263. (29) Liljegren, G.; Pettersson, J.; Markides, K. E.; Nyholm, L. Analyst 2002, 127, 591. (30) Deinhammer, R. S.; Porter, M. D.; Shimazu, K. J. Electroanal. Chem. 1995, 387, 35. (31) Shimidzu, T.; Ohtani, A.; Iyoda, T.; Honda, K. J. Electroanal. Chem. 1987, 224, 123. (32) Kalaji, M.; Nyholm, L.; Peter, L. M. J. Electroanal. Chem. 1991, 313, 271. (33) Kalaji, M.; Nyholm, L.; Peter, L. M. J. Electroanal. Chem. 1992, 325, 269. (34) Sahimi, M. Applications of percolation theory; Taylor & Francis: London, 1994. (35) Kirkpatrick, S. ReV. Mod. Phys. 1973, 45, 574. (36) Bard, A. J., Faulkner, L. R. Electrochemical methods: Fundamentals and Applications; Wiley: Chichester, U.K., 2001.

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