J. Phys. Chem. B 1999, 103, 10143-10148
10143
Ion Transport in a Chemically Prepared Polypyrrole/Poly(styrene-4-sulfonate) Composite Guangchun Li and Peter G. Pickup* Department of Chemistry, Memorial UniVersity of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7 ReceiVed: May 5, 1999; In Final Form: September 20, 1999
The electrochemistry of a chemically prepared polypyrrole/poly(styrene-4-sulfonate) powder has been investigated by cyclic voltammetry and impedance spectroscopy in a variety of aqueous electrolytes. The facile p-doping/undoping of thick layers of the polymer composite is shown to be due to the high ionic conductivity imparted by electrolyte solution in pores. As a consequence, the ionic conductivities of the polymer layers are strongly dependent on the concentration and conductivity of the electrolyte solution used but relatively insensitive to the doping level of the polymer. In contrast, electrochemically prepared polypyrrole/ poly(styrene-4-sulfonate) films show much lower ionic conductivities with a pronounced dependence on doping level. It is concluded that layers of the chemically prepared material are approximately 10 times more porous than the electrochemically prepared films. High ionic conductivities previously obtained for polypyrrole/ poly(styrene-4-sulfonate) layers bonded to Nafion membranes have been shown to be due to HNO3 generated in pores by ion exchange of residual Fe3+ with H+ in the Nafion.
Introduction Conducting polymer/polyanion composites1,2 such as polypyrrole/poly(styrene-4-sulfonate)3,4 (PPY/PSS) have attracted much interest in recent years.5 Since PPY/PSS has high electronic3,6 and ionic7,8 conductivity, and particularly high proton conductivity,8,9 it has great potential in applications such as batteries,10-13 supercapacitors, and fuel cells.9,14 PPY/PSS has been prepared (eq 1) both electrochemically3,15,16 and chemically.2,6,17 The electrochemical method produces a PPY/PSS film on the electrode, which has the advantages of facile control of the sample’s thickness, morphology, and conductivity. In contrast, the chemical method produces a PPY/PSS powder. It is more suitable for mass production and has merits of low cost, speed, and simplicity. The composition of PPY/PSS is generally such that the charge on the fully oxidized polypyrrole (n ≈ 0.25 in eq 1), which is formed during synthesis, is fully compensated by PSS.
Reduction, or partial reduction of the polypyrrole,
leaves an excess of immobilized sulfonate ions in the polymer composite, which are compensated by mobile cations. Thus, * To whom correspondence should be addressed. E-mail: ppickup@ morgan.ucs.mun.ca.
PPY/PSS is a cation exchange material under most conditions, and its ion transport properties are dominated by cation transport.7,18-20 Ion transport in conducting polymer films has been the focus of intensive study in recent years.21,22 Fully understanding the ion transport properties of conducting polymers is crucial to understanding their electrochemistry and optimizing applications. Furthermore, the study of ionic conductivity can provide valuable insight into the structures of conducting polymer films.8,23 There have been a number of studies of ion transport in electrochemially prepared PPY/PSS films.7,8,18-20,24-32 Techniques employed include impedance spectroscopy,7,8,24,27,32 the quartz crystal microbalance,19,20,26,30,31 scanning electrochemical microscopy,29 and Rutherford backscattering.28 As expected from eq 2, cations have been found to dominate the ion transport properties of PPY/PSS, although anion transport has also been observed.20,26,30,32 Surprisingly, we have found that the ionic conductivities of packed layers of chemically prepared PPY/PSS (C-PPY/PSS) are much higher than those of electrochemically prepared PPY/ PSS (E-PPY/PSS) films.9 The C-PPY/PSS powder exhibited proton conductivities as high as 0.03 S cm-1 when pressed as a ca. 50 µm thick layer on a Nafion proton conducting membrane, which is 100 times higher than the proton conductivity of E-PPY/PSS immersed in 1 M H2SO4. The aims of the work reported here were to more fully characterize the ionic conductivity of C-PPY/PSS by using impedance spectroscopy and thereby to explain why it is so much more conductive than its electrochemically prepared counterpart. We have reported on the impedance and ion transport properties of E-PPY/PSS in detail elsewhere.7,8,32-34 These studies have revealed that the electrochemically prepared material has a two-phase structure consisting of permselective polymer aggregates and electrolyte-filled pores.8 The results reported here indicate that chemically prepared PPY/PSS layers have a similar structure but that they are significantly more porous than electrochemically prepared films.
10.1021/jp9914727 CCC: $18.00 © 1999 American Chemical Society Published on Web 10/30/1999
10144 J. Phys. Chem. B, Vol. 103, No. 46, 1999
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Figure 1. Schematic diagram of the cell used in this work. The separators were filter paper saturated with electrolyte solution or hydrated Nafion 117 membranes.
Experimental Section Chemicals and Materials. Sodium poly(styrene-4-sulfonate) (Aldrich, Mw ∼70 000), Fe(NO3)3‚9H2O (BDH), and carbon fiber paper (CFP (Toray TGPH90) + 10% PTFE) were used as received. Poly(tetrafluoroethylene) (PTFE, 60% suspension in water, Dupont) was diluted to 15% with deionized water. Pyrrole (Aldrich) was purified by passage through a dry SiO2 (230-400 mesh) column immediately before use. All other chemicals were reagent grade or better and used as received. Preparation and Characterization of the PPY/PSS Powder. The PPY/PSS composite was prepared as previously reported6 by chemical oxidation of 0.13 M pyrrole containing 0.04 M sodium poly(styrene-4-sulfonate) by using 0.6 M Fe(NO3)3 in 0.06 M HNO3 as an oxidant. After 80 min the black precipitate was collected by filtration, washed copiously with water, and dried at room temperature under vacuum. The S/N ratio from elemental analysis (52.28% C, 4.09% H, 11.91% N, 18.82% O, 5.13% S; Canadian Microanalytical Services) gives a PPS/PPY ratio in the composite of 0.19, which is essentially the same as the ratio obtained in a previous synthesis under similar conditions.6 However, the total analysis summed to only 92.23%. Microprobe analysis (X-ray emission spectroscopy, calibrated with FePSS3 35) indicated the additional presence of Fe (ca. 1.2% by mass based on the Fe/S ratio and the analyzed %S). This Fe is presumably in the form of Fe3+ counterions associated with PSS in the composite. A charge balance and global fitting of the analytical results indicated that there is also NO3- in the composite. A composition of PPY(PSS)0.25Fe0.05(NO3)0.15‚0.4H2O, corresponding to a PPY doping level of 0.25, fits the analytical data reasonably well (calculated: 55% C; 4% H; 12% N; 20% O; 6% S; 2% Fe; the use of only one or two significant figures reflects the uncertainty caused by the low total analysis; the Fe analysis is assumed to be the least accurate). The Fe3+ and NO3- cannot be washed from the film with water presumably because the Fe3+ is strongly associated as a counterion with the PSS and the NO3- is required to balance the excess charge on the Fe3+. In addition, some of the Fe3+ may be the counterion in PSS-rich regions of the composite while NO3- is the counterion in PPY-rich regions.32 The Fe3+ can be removed by ion exchange with HNO3.
The electronic conductivity of the dry PPY/PSS powder, measured by using a four-point probe in which the polymer is pressed into a pellet in situ,6 was 0.21 S cm-1. Electrochemical Cell Design. As shown in our previous work on the electrochemistry of PPY/PSS powders,9 excellent results can be obtained by spreading the polymer on a carbon fiber paper (CFP) suppport with a PTFE suspension as a binder. However, the cell used in that work was designed for fuel cell experiments and suffered from large uncompensated resistances when used with nonaqueous and dilute electrolyte solutions. To minimize the effects of the uncompensated solution resistance, a small sandwich configuration cell has been designed (Figure 1). The working electrode is a 0.2 cm2 CFP disk that is evenly coated on one side with a layer of PPY/PSS containing PTFE as a binder. The separator consists of two pieces of Whatman no. 541 filter paper saturated with electrolyte solution and sandwiching a PPY/PSS coated Pt wire reference electrode (see below). The counter electrode is a 1 cm2 CFP disk coated with 5 mg of PPY/PSS containing 2 mg of PTFE. This sandwich cell minimizes the distance between working electrode and reference electrode and therefore minimizes the uncompensated solution resistance. It produces excellent electrochemical results even in high-resistance electrolyte solutions. Preparation of the PPY/PSS Coated Pt Wire Reference Electrode. Commercial reference electrodes, such as the SCE and Ag/AgCl, are too big to be fitted into our cell design. In addition, they have the problem of leakage of KCl into the cell. A small, stable, noncontaminating reference electrode based on a PPY/PSS coated fine Pt wire was therefore designed. PPY/PSS was electrochemically deposited onto a 0.127 mm diameter Pt wire (Aldrich) from an aqueous solution of 0.5 M pyrrole and 0.1 M NaPSS as supporting electrolyte by using a constant anodic current of 3 mA cm-2 for 20 min. The electrode potential during polymerization remained steady at ca. 0.5 V vs SCE. For calibration, and to check its stability, the potential of this PPY/PSS coated electrode was measured against a calomel electrode (SSCE) in various stirred 0.5 M aqueous electrolyte solutions and in various concentrations of H2SO4(aq). Its potential (ca. +0.3 V vs SSCE) was stable to within ca. 5 mV for a period of hours after the first hour, and no significant
Polypyrrole/Poly(styrene-4-sulfonate)
J. Phys. Chem. B, Vol. 103, No. 46, 1999 10145
Figure 2. Cyclic voltammograms (10 mV s-1) of a C-PPY/PSS layer (2.5 mg cm-2) containing 5% polyvinylferrocene in contact with 0.5 M LiCl(aq).
Figure 4. Anodic current at 0 V (vs Pt/PPY/PSS) vs scan rate for cyclic voltammograms (-0.4 to +0.3 V) of a C-PPY/PSS layer (2.5 mg cm-2) in contact with 0.1 M Et4NCl(aq).
TABLE 1: Voltammetric Charges and Specific Capacitances in 0.5 M H2SO4(aq) for Various Loadings of C-PPY/PSS on a 0.2 cm2 Electrode PPY/PSS loading (mg) voltammetric chargea (mC) specific capacitance (F/g) a
Figure 3. Cyclic voltammograms (20 mV s-1) of a C-PPY/PSS layer (2.5 mg cm-2) in contact with 0.1 M H2SO4(aq).
concentration dependence (less than (2 mV) was observed in H2SO4(aq). These characteristics are adequate for the present study where control and knowledge of the reference potential to better that 10 mV are not important. Calibration of the PPY/ PSS reference electrode can be accomplished in situ by adding ca. 5 mass % of polyvinylferrocene (Polysciences) to the PPY/ PSS working electrode (e.g., Figure 2), although the limited potential dependence of the results reported here made this unnecessary. Electrochemistry. All of the electrochemical experiments were conducted with the cell shown schematically in Figure 1. Electrodes were prepared by mixing PPY/PSS powder with a PTFE aqueous suspension in an ultrasonic bath for 5 min and spreading the resulting paste evenly over 1 cm2 of CFP. The 0.2 cm2 disks were cut for use as working electrodes. The PTFE content of the PPY/PSS layer was kept constant at ca. 30% by mass. Impedance measurements were conducted with a perturbation amplitude of 5 mV rms over a frequency range 65-0.1 kHz with a Solartron frequency response analyzer (model 1250) coupled to a Solartron electrochemical interface (model 1286). All data were collected and analyzed on a PC with commercial ZPLOT software (Scribner Associates Inc.). All potentials are quoted with respect to a PPY/PSS coated Pt wire reference electrode. Results and Discussion Cyclic Voltammetry. Figures 2 and 3 show cyclic voltammograms of PPY/PSS coated CFP electrodes in aqueous 0.5 M
0.5 55 170
1 163 250
1.5 224 253
2 268 230
For an anodic scan from -0.4 to +0.3 V at 10 mV s-1.
LiCl and 0.1 M H2SO4 solutions, respectively. The potential scan range was restricted to (0.5 V vs the PPY/PSS reference electrode to avoid degradation of the polymer. For the experiment depicted in Figure 2, 5% by mass of polyvinylferrocene was added to the PPY/PSS layer as an internal reference. Its electrochemistry appears at a formal potential of 0.12 V, with insignificant drift during the nine scans at 10 mV s-1 between the two voltammograms shown in the figure. This confirms the operational stability of the PPY/PSS reference electrode. Voltammograms of C-PPY/PSS were similar in all aqueous electrolytes tested (H2SO4, HCl, LiCl, LiClO4, NaClO4, NaCl, Na2SO4, and Et4NCl), that is, featureless with large capacitancelike currents characteristic of conducting polymers. The current peaks normally seen in voltammograms of polypyrrole are beyond the lower potential limit used in this work.9 Voltammograms showed only very small changes through multiple cycles over the (0.5 V potential range (e.g., Figure 3), demonstrating good stability. Currents increased almost linearly with increasing scan speed, even with the bulky Et4N+ counterion (Figure 4), indicating that both ion and electron transport in PPY/PSS are fast. Currents also increased with increasing loading of PPY/PSS on the electrode (i.e., increasing layer thickness). This is illustrated in Table 1, which lists charges and specific faradaic pseudocapacitances from voltammograms at 10 mV s-1 in 0.5 M H2SO4(aq) as a function of PPY/PSS loading. The capacitances were calculated from the averages of the anodic and cathodic currents at 0 V in the 10 mV s-1 voltammograms by using the following equation.
specific capacitance )
average current (3) scan rate × mass of polymer
At the lowest loading used (2.5 mg cm-2) the voltammetric charge and capacitance are both low relative to the higher loadings (Table 1). This is presumably due to the loss of some
10146 J. Phys. Chem. B, Vol. 103, No. 46, 1999
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TABLE 2: Specific Faradaic Pseudocapacitances (F g-1) at +0.1 V (vs PPY/PSS), from 10 mV s-1 Cyclic Voltammetry and Impedance Spectroscopy on 2.5 mg cm-2 C-PPY/PSS Layers in Various 0.5 M Electrolyte Solutions electrolyte voltammetry impedance
H2SO4 170 135
HCl 150 115
LiClO4 165 145
LiCl 140 90
Figure 5. Complex plane impedance plots for a C-PPY/PSS layer (2.5 mg cm-2) in contact with 0.05 (2), 0.1 (4), 0.25 (b), or 0.5 (O) M H2SO4(aq) at 0.1 V vs Pt/PPY/PSS.
of the polymer into cavities in the hydrophobic CFP support, where it is isolated from the solution and therefore not electrochemically active. At intermediate loadings (5-7.5 mg cm-2) the voltammetric charge scales linearly with loading and the specific capacitance is constant. However, at higher loadings (10 mg cm-2) the voltammetric charge begins to level off and the specific capacitance falls as the time required to charge and discharge the layer becomes longer than the cycle time. Voltammetric charges and specific faradaic pseudocapacitances showed little dependence on the electrolyte employed (Table 2). The 7.5 mg cm-2 loading corresponds to a polymer layer thickness of ca. 0.16 mm, and it is surprising that such a thick layer is fully electrochemically active at a scan rate of 10 mV s-1. Since electroactivity of the polymer layer requires facile ion transport as well as good electronic conductivity, this result illustrates the excellent ion transport characteristics of the PPY/ PSS composite layer. In combination with the very high specific capacitance obtained at this loading (253 F g-1) this makes C-PPY/PSS very attractive as a charge storage material for supercapacitors.36 Impedance Spectroscopy. Impedance spectroscopy is a powerful and simple technique to measure the ionic resistance of an electronically conducting film or layer on an electrode, without having to assume any particular physical model.37 The ionic resistance of the polymer layer (RI) is given by
RI ) 3[Z′low - Z′high]
(4)
where Z′low is the low-frequency limiting resistance and Z′high is the high-frequency intercept with the real impedance axis.38,39 We have investigated the impedance of PPY/PSS layers in a variety of aqueous electrolyte solutions (H2SO4, HCl, LiClO4, LiCl, NaClO4, NaCl, Na2SO4, and Et4NCl) as a function of both concentration and applied potential. This type of broad study provides insight into both the nature of the ion transport processes in a polymer layer and the structure of the layer.8 An illustrative set of impedance data at different electrolyte concentrations is shown in Figure 5. The general shape of these complex impedance plots approximates the behavior of a finite transmission line, as predicted theoretically.39 The deviations from ideality have been shown to be relatively unimportant in
NaClO4 155 125
NaCl 150 100
Na2S4 150 110
Et4NCl 150 110
the determination of ionic resistances,37,40 which is the objective in this work. The curves shift along the real impedance (Z′) axis with decreasing electrolyte concentration because of the increasing uncompensated solution resistance. The increasing length of the approximately 45° region (increasing Z′low - Z′high) reflects an increasing ionic resistance in the polymer layer (eq 4). Ionic resistances were obtained from impedance data by using eq 4 and then converted to ionic conductivities (σI ) d/(RIA)). The thickness (d) of a 2.5 mg cm-2 dry PPY/PSS layer was measured as 55 µm with a micrometer. Resistances increased with increasing loading to 7.5 mg cm-2 (satisfactory impedance plots were not obtained at 10 mg cm-2), with conductivities of 17, 28, and 18 mS cm-1, respectively, being obtained for 2.5, 5.0, and 7.5 mg cm-2 loadings in 0.5 M H2SO4(aq). All subsequent experiments were conducted with polymer loadings of 2.5 mg cm-2. In the following analysis and discussion we assume that C-PPY/PSS layers have a two-phase structure consisting of permselective polymer aggregates and electrolyte-filled pores, as proposed previously for E-PPY/PSS.8 Evidence for the permselectivity of the polymer phase comes from the potential dependence of the ionic conductivity at low electrolyte concentrations (see below), as discussed in ref 8. It should be noted that since the PPY/PSS particles are ion-exchange polymers,5 ions can therefore penetrate them and they are electroactive throughout their volume. The ionic conductivity of PPY/PSS layers was found to be virtually independent of potential in most electrolyte solutions studied (except Et4NCl and LiClO4), indicating that it is dominated by electrolyte solution in pores.8 However, in the low-concentration solutions of H2SO4, there was a slight dependence consistent with eq 2. That is, the ionic conductivity of the layer increases as the layer is reduced and an increasing number of mobile countercations are inserted into the layer. Thus, at low electrolyte concentrations, the ionic conductivity of the polymer phase begins to become significant. The opposite trend with potential was observed in Et4NCl and LiClO4 solutions, even at high concentrations, suggesting that in the presence of such large cations, anion transport can dominate in the polymer phase. Figure 6 shows the ionic conductivity of the polymer as a function of electrolyte concentration in H2SO4(aq). Data for two different electrodes are shown, together with a linear leastsquares fit of all the data. The strong dependence of the polymer layer’s ion conductivity on the electrolyte concentration again shows that it is dominated by electrolytes in pores. The y intercept of the plot is close to zero (slightly negative), indicating that the intrinsic ionic conductivity of the polymer (i.e., in the absence of electrolyte in pores) is too small to accurately assess from these data (i.e.,
