1064
Langmuir 2008, 24, 1064-1069
Porous Conducting Polymer/Heteropolyoxometalate Hybrid Material for Electrochemical Supercapacitor Applications Graeme M. Suppes, Bhavana A. Deore, and Michael S. Freund* Department of Chemistry, UniVersity of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada ReceiVed September 12, 2007. In Final Form: October 24, 2007 A porous conducting polymer/heteropolyoxometalate hybrid material that displays high specific capacitance and low ionic resistance has been prepared for electrochemical supercapacitor applications. Polypyrrole/phosphomolybdate composite films were chemically synthesized in tetrahydrofuran in the presence of sodium sulfate, which acts as a porogen. While the phosphomolydic acid could be removed from the film upon rinsing with pure tetrahydrofuran or acetone, rinsing with water or methanol resulted in retention of the heteropolyoxometalate at a level high enough to easily observe its electrochemistry. The retained phosphomolybdate exhibits fast and reversible redox behavior, adding a significant amount of pseudocapacitance to the polymer. Porous films were obtained by leaching out the sodium sulfate porogen from the films using water. The morphology obtained using this method is altered by varying the monomer-to-porogen ratio. Increasing the porosity increases the rate at which the hybrid material can be charged/ discharged (i.e., oxidized/reduced) by increasing the ionic conductivity and in turn lowering the resistor-capacitor time constant of the material. The ability to tune the porosity of the material allows the optimization of performance characteristics for use in supercapacitor applications. Impedance measurements indicate that the ionic conductivity of these porous structures can be increased more than an order of magnitude over that observed for standard conducting polymer films and that the hybrid material displays peak specific capacitance of around 700 F/g as well as excellent reversibility and cyclability.
Introduction The interest in supercapacitors comes from their possible use in devices that require transient and high power input. For example, there is interest in combining supercapacitors with fuel cells for boosting acceleration in electric vehicles.1,2 Electrochemical supercapacitors provide higher energy density than conventional parallel-plate or double-layer capacitors. Highsurface-area carbons3-6 and metal oxides7-10 are widely used as electrode materials in electrochemical capacitors. With highsurface-area carbons the charge storage is electrostatic, where an electrochemical double layer is formed at a porous carbon electrode interface with the electrolyte.11 Metal oxides store charge in the form of ions via a redox pseudocapacitive charge storage mechanism.11 Conducting polymers present another option and have been extensively studied as promising materials for rechargeable batteries and electrochemical capacitors due to their redox chemistry and high conductivity in the doped state.12-15 * Corresponding author. E-mail:
[email protected]. (1) Mastragostino, M.; Arbizzani, C.; Paraventi, R.; Zanelli, A. J. Electrochem. Soc. 2000, 147, 407. (2) Conway, B. E.; Pell, W. G. J. Solid State Electrochem. 2003, 7, 637. (3) Zheng, J. P.; Huang, J.; Jow, T. R. J. Electrochem. Soc. 1997, 144, 2026. (4) Frackowiak, E. Carbon 2001, 39, 937. (5) Niu, C.; Sickel, E. K.; Hoch, R.; Moy, D.; Tennent, H. Appl. Phys. Lett. 1997, 70, 1480. (6) Diederich, L.; Barborini, E.; Piseri, P.; Podesta, A.; Milani, P.; Schneuwly, A.; Gallay, R. Appl. Phys. Lett. 1999, 75, 2662. (7) Zheng, J. J. Electrochem. Soc. 1995, 142, L6. (8) Khudo, T.; Ikeda, Y.; Watanabe, T.; Hibino, M.; Miyayama, M.; Abe, H.; Kajita, K. Solid State Ionics 2002, 152, 833. (9) Long, J. W.; Swider, K. E.; Merzbacher, C. I.; Rolison, D. R. Langmuir 1999, 15, 780. (10) Hu, Y.; Guo, Y.; Sigle, W.; Hore, S.; Balaya, P.; Maier, J. Nat. Mater. 2006, 5, 713. (11) Conway, B. E. Electrochemical Capacitor, Scientific Fundamentals and Technological Application; Plenum Press: New York, 1999, p 105. (12) Novak, P.; Mueller, K.; Santhanam, K. S. V.; Haas, O. Chem. ReV. 1997, 97, 207. (13) Oyama, N.; Tatsuma, Y.; Sato, T.; Sotomura, T. Nature 1995, 373, 598. (14) Rudge, A. D.; John, R. I.; Gottesfeld, S.; Ferraris, J. P. J. Power Source 1994, 47, 89.
However, one of the main problems related to their application is a relatively low capacity to store charge in such devices. The capacity to store charge in pseudocapacitive conducting polymer electrode materials is limited by many factors, such as the percentage of dopant, doping mechanism, redox switching, and redox stability. For example, the polymer is typically formed by oxidative polymerization with the incorporation of doping anions, and the reduction of these materials takes place with deintercalation of anions rather than with insertion of cations. The exchange of anions between polymer and electrolyte usually negatively impacts the electric charge density together with switching speed and cycleability. In order to overcome this problem, new approaches combining faradaic (metal oxide and conducting polymers) and nonfaradaic (carbon materials) capacitive systems into a higher performance hybrid material have been pursued.16-20 In the past few years, the combination of conducting polymers and electroactive molecular clusters or extended inorganic species to form hybrid materials has provided an opportunity to create materials with improved stability and charge propagation dynamics and enhanced storage capacity.21-23 Polyoxometalates are an attractive option as an oxidative agent and dopant anion. As pseudocapacitive materials themselves, polyoxometalates, such as phosphomolybdic acid (PMA), show excellent redox chemistry in acidic (15) Chen, W. C.; Wen, T. C. J. Power Sources 2003, 117, 273. (16) Reddy, A. L. M; Ramaprabhu, S. J. Phys. Chem. 2007, 111, 7727. (17) Wang, Q.; Wen, Z.; Li, J. AdV. Funct. Mater. 2006, 16, 2146. (18) Hughes, M.; Chen, G. Z.; Shaffer, M. S. P.; Fray, D. J.; Windle, A. H. Chem. Mater. 2002, 14, 1610. (19) Snook, G. A.; Chen, G. Z.; Fray, D. J.; Hughes, M.; Shaffer, M. J. Electroanal. Chem. 2004, 568, 135. (20) Frackowiak, E.; Khomenko, V.; Jurewicz, K.; Lota, K.; Beguin, F. J. Power Sources 2006, 153, 413. (21) Go´mez-Romero, P. AdV. Mater. 2001, 13, 163. (22) Cuentas-Gallegos, K.; Lira-Cantu´, M, Casan˜-Pastor, N.; Go´mez-Romero, P. AdV. Funct. Mater. 2005, 15, 1125. (23) Vaillant, J.; Lira-Cantu´, M.; Cuentas-Gallegos, K.; Casan˜-pastor, N.; Go´mez-Romero, P. Prog. Solid State Chem. 2006, 34, 147.
10.1021/la702837j CCC: $40.75 © 2008 American Chemical Society Published on Web 12/28/2007
Polymer/Heteropolyoxometalate Hybrid Material
solution.24 However polyoxometalates are typically soluble and in turn need to be covered or otherwise immobilized. PMA is sufficiently oxidative to chemically polymerize pyrrole,25 thiophene,26 and aniline27 and has been shown to be retained in the film after reduction.28 As the films are reduced, cations enter the film to balance the charge of the negative PMA ion that is entrapped in the polymer matrix. The present work focuses on the creation of open, porous morphologies of conducting polymer/heteropolyoxometalate hybrid materials for use as supercapacitor electrodes. Previously, polyoxometalate-doped conducting polymer electrodes prepared by vapor-grown and electrochemical polymerization have been used for supercapacitor application.29,30 The chemical polymerization method reported above for generating polypyrrole (PPy)25 and polythiophene26 provides an opportunity to manipulate morphology and porosity by introducing porogen during the polymerization process. Removal of the porogen should leave behind a porous form of the polymer. In principle, by controlling the structure of the polymer, it should be possible to optimize the electronic and ionic conductivity to minimize the total resistance of the system and thereby maximize the performance characteristics of this material for use in supercapacitor applications. In the present report, synthesis of porous PPy/PMA composite films and the role of porosity on the electronic and ionic conductivity of PPy using electrochemical techniques and in turn its implication for performance as a supercapacitor material are reported.
Langmuir, Vol. 24, No. 3, 2008 1065
Figure 1. Cyclic voltammograms of PPy/PMA composite film in (a) 0.5 M H2SO4 and (b) acetonitrile with 0.1 M tetrabutylammonium perchlorate on a GC electrode (0.07 cm2) at a scan rate of 100 mV/s. The films were prepared using 5 mg/mL pyrrole and 68.4 mg/mL PMA in THF. Scheme 1. Proposed Method for the Preparation of Porous PPy/PMA Composite Films: (A) Solvent Evaporation and Polymerization and (B) Removal of Unreacted PMA and Porogen in Methanol and Water
Experimental Section Materials. Phosphomolybdic acid hydrate (H3PMo12O40·xH2O), pyrrole, tetrahydrofuran (THF), and sodium sulfate were purchased from Aldrich and used without any further purification. Indiumdoped tin oxide (ITO, 6 ( 2 Ω/square) glass slides were purchased from Delta Technologies, Limited. Bulk distilled water was filtered then ion exchanged to yield 18.2 MΩ cm quality water using MilliQ-Academic A10 (Millipore Corp.). Preparation of Porous PPy/PMA Composites. The PPy/PMA composite films were prepared by mixing equal volumes of pyrrole and PMA (2:1 concentration ratio) in THF.25,26 The films were chemically synthesized by spin-coating the metastable reaction mixture onto glass and ITO-coated glass slides at 2000 rpm for 10 s. Upon completion of the spin-coating process, films were then left to dry at room temperature overnight before rinsing with methanol to remove reduced PMA and unreacted material. The PMA could be removed from the film upon rinsing with pure tetrahydrofuran or acetone. However, rinsing with water or methanol31 resulted in retention of the heteropolyoxometalate at a level high enough to observe its electrochemistry. The films were subsequently left to dry before characterization. The films obtained after the rinsing process were blue-gray and four-point-probe measurements of films on insulating glass demonstrated that these films were in the oxidized form (conducting state). The PPy/PMA composite films were prepared on glassy carbon (GC) electrodes by dipping in a reaction mixture. The porous PPy/PMA composite films were prepared on glass, ITO-coated glass slides, and GC electrodes in a similar manner by varying the weight percent of sodium sulfate (ground powder, insoluble in THF) in the polymerization mixture, followed by dissolution of the salt in the polymer structure by exposing (24) Sadakane, M.; Steckhan, E. Chem. ReV. 1998, 98, 219. (25) Freund, M. S.; Karp, C.; Lewis, N. S. Inorg. Chim. Acta 1995, 240, 447. (26) Bravo-Grimaldo, E.; Hachey, S.; Cameron, C. G.; Freund, M. S. Macromolecule 2007, 40, 7166. (27) Go´mez-Romero, P.; Casan˜-Pastor, N.; Lira-Cantu´, M. Solid State Ionics 1997, 101, 875. (28) Lira-Cantu´, M.; Go´mez-Romero, P. Chem. Mater. 1998, 10, 698. (29) Vaillant, J.; Lira-Cantu´, M.; Cuentas-Gallegos, K.; Casan˜-Pastor, N.; Go´mez-Romero, P. Prog. Solid State Chem. 2006, 34, 147. (30) White, A. M.; Slade, R. T. C. Electrochim. Acta 2003, 48, 2583. (31) Cheng, C. H. W.; Lonergan, M. C. J. Am. Chem. Soc. 2004, 126, 10536.
it to methanol and water (Scheme 1). Sodium sulfate crystals were ground using mortar and pestle, and the fine powder was added in the polymerization mixture. For purposes of comparison, PPy films were grown electrochemically in 0.5 M H2SO4. Characterization. The films were characterized by cyclic voltammetry, impedance spectroscopy, and scanning electron microscopy. All cyclic voltammetric measurements were performed using a CH Instrument CHI-660 workstation controlled by a PC. A three-electrode configuration was used including a platinum wire (0.2 mm diameter) counter electrode, Ag/AgCl reference electrode, and a GC or ITO working electrode. All measurements were performed using 0.5 M H2SO4. Impedance measurements were made using a Solartron 1287 electrochemical interface and 1255B frequency response analyzer. A three-electrode configuration was used including a platinum wire (0.2 mm diameter) counter electrode, Ag/AgCl reference electrode, and a GC working electrode. Impedance measurements were conducted using ZPLOT software with a perturbation amplitude of 5 mV over a frequency range of 100 kHz to 0.1 Hz (10 points per decade). The micrographs of the samples were taken by a JEOL 5900 IVAN-LV scanning electron microscope (SEM).
Results and Discussion Figure 1 shows the cyclic voltammograms of a typical PPy/ PMA composite film in aqueous acid and nonaqueous solutions. In aqueous acid solution, the composite film exhibits facile redox chemistry associated with PMA that is in good electronic communication with the PPy (Figure 1a). The three characteristic sets of peaks are attributed to the phosphomolybdate anion being progressively reduced/oxidized.24 The redox peaks of PPy are not visible due to the dominance of PMA peaks, similar to a
1066 Langmuir, Vol. 24, No. 3, 2008
Suppes et al.
Figure 2. SEM of PPy/PMA films on glass slides without and with porogen. Films were prepared using 50 mg/mL pyrrole and 684 mg/mL PMA in THF with (a) 0 mg/mL, (b) 10 mg/mL, (c) 20 mg/mL, and (d) 40 mg/mL porogen.
previous report.32 The potential window for cyclic voltammograms in aqueous acid solution was limited from -0.1 to 0.6 V, since more-negative potentials lead to the irreversible decomposition of PMA under these conditions.33 Under identical conditions, in nonaqueous solution (i.e., acetonitrile and methanol) containing lithium perchlorate or tetrabutylammonium perchlorate as electrolyte, the composite film does not exhibit the welldefined redox behavior typically associated with PMA (Figure 1b). These results suggest that the PMA does not remain in the form of a Keggin structure within the polymer. The amount of PPy as well as the amount of PMA trapped in a composite film can be estimated from the charge observed under both of these conditions. The amount of PMA can be determined from the difference in charge (determined by integration of the forward scan in cyclic voltammograms within the same potential window and dividing by scan rate) in the aqueous acid solution (Figure 1a) and in nonaqueous solution (Figure 1b), respectively. Since this corresponds to the charge associated with PMA, it is possible to use Faraday’s law, Q ) nzF, where Q is charge, n the number of moles, z is the number of electrons transferred (6 in the case of PMA) in the redox reaction, and F is Faraday’s constant, to determine the number of moles of active material. The amount of pyrrole deposited must be estimated using a different approach. Generally, the moles of pyrrole within a deposited film can be calculated assuming 2.25 e- per pyrrole molecule during the electropolymerization reaction, in which the oxidized form of the polymer is produced.34 Since this cannot be measured directly during the chemical polymerization, the moles of pyrrole in the polymer film was assumed to be equal to that calculated for an electrochemically deposited PPy/ClO4- film35 exhibiting the same (32) Wang, P.; Li. Y.; J. Electroanal. Chem. 1996, 408, 77; Gormez-Romero, P.; Lira-Cantu, M. AdV. Mater. 1997, 7, 144. (33) Wang, B.; Dong, S. J. Electroanal. Chem. 1992, 245, 328. (34) Smyrl, W. H.; Lien, M. In Applications of ElectroactiVe Polymers; Scrosati, B., Ed.; Chapman and Hall: London, 1993; Chapter 2, p 29. (35) Ren, X.; Pickup, P. G. J. Phys. Chem. 1993, 97, 5356.
magnitude of charging current density (between -0.2 and +0.2 V, in Figure 1b). On the basis of this approach, the molar ratio of PPy to PMA is approximately 10:1. This value indicates that each phosphomolybdate anion with a charge of 3 balances the positive charge on 9 pyrrole units in the oxidized PPy/PMA (0.1-0.6 V) composite film. These results suggest that there is no significant excess PMA within the polymer films. Our previous work demonstrated that smooth thin films of PPy can be produced by spin-coating metastable mixtures of pyrrole and PMA in THF on insulating substrates.25,26 Since polymerization occurs following the application of the mixture, it is relatively straightforward to incorporate other components such as soluble insulating polymers36 or, as in this case, insoluble salt particles. Once polymerization is complete, the particles (acting as a porogen) can be dissolved in a different solvent. Figure 2 shows SEM images of PPy/PMA composite films without and with the addition of a porogen (following its removal). The PPy/PMA film without porogen is smooth and pinhole-free, as shown in Figure 2a. The SEM images of PPy/PMA composite films with porogen exhibit a smooth surface with circular shaped pores (Figure 2c,d). The increase in pore size and number of pores is observed with the porogen content. These results suggest that the approach presented here provides a relatively straightforward way of varying the porosity within the film on a micron to submicron scale. In this particular case, pore sizes range from 100 nm to 3 µm in diameter. It is clear that a number of approaches can be used to produce porogen with smaller and better-defined particle sizes and shapes by using different materials and processing approaches. In order to study the influence of porosity on the redox switching of PPy/PMA films, we systematically altered the level of porogen in films of the conducting polymer hybrid material. Our working hypothesis was that the presence of solvent-containing channels within the bulk of the polymer would facilitate rapid movement (36) Freund, M. S.; Lewis, N. S. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2652.
Polymer/Heteropolyoxometalate Hybrid Material
Langmuir, Vol. 24, No. 3, 2008 1067
Figure 3. Normalized peak current of chemically prepared PPy/ PMA films with varying porosity as a function of scan rate on a GC electrode (0.07 cm2) in 0.5 M H2SO4. Films prepared using 5 mg/ mL pyrrole and 68.4 mg/mL PMA without porogen (4), with 1 mg/mL (]), 2 mg/mL (O), and 4 mg/mL (0) porogen in THF. The peak current of the second redox couple was normalized with respect to low scan rate (10 mV/s) peak current, where RC effects are not a factor.
of counterions while at the same time not reduce the electronic conductivity to a point where it degrades response characteristics. Figure 3 shows the normalized peak current of PPy/PMA composite films with varying porosity as a function of scan rate. In this particular study, the films were deposited in such a way that the amount of polymer was approximately the same with varying amount of porogen, as indicated by the peak currents at lower scan rate. This way, deviations in behavior cannot simply be due to differences in the absolute magnitude of the current flowing through the film. In order to compensate for small changes in the absolute amount of polymer deposited, the peak current of the second redox couple was normalized at each scan rate with respect to the peak current at the low scan rate (10 mV/s). As can be seen in Figure 3, as the porosity increases (by increasing the porogen content in the polymerization mixture), deviations from ideal behavior (i.e., linear peak current versus scan rate) are not observed until scan rates of 20 V/s, where most studies on conducting polymer supercapacitors only go up to 500 mV/s or less.37-39 This trend is consistent with more efficient ion mobility within the structure with increasing porosity. It should be noted, however, that with a further increase in porosity (4 mg/mL porogen content in the polymerization mixture), a deviation from ideal behavior is observed at high scan rates similar to nonporous film. This is likely due to the increase of the electronic resistance of the highly porous PPy/PMA composite film. Figure 4 shows that the time required for charging of the film upon switching the potential at 0.6 V is significantly reduced with increasing porosity, as seen in the gray boxes. Specifically, the gradual sloping decrease in current in the return cycle [characteristic of a large resistor-capacitor (RC) time constant] is replaced by a sharp transition with increased porosity. These results point toward a significant reduction of the RC time constant for the charging of the material with increased porosity. In order to support the cyclic voltammetric results, impedance measurements of PPy/PMA composite films, with and without pores, were carried out on GC electrodes in 0.5 M H2SO4. The thickness of the PPy/PMA composites films was estimated from (37) Bhat, D. K.; Kumar, M. S.; J. Mater. Sci. 2007, 42, 8158. (38) Chen, W.-C.; Wen, T.-C. J. Power Sources 2003, 117, 273. (39) Fan, L.-Z.; Maier, J. Electrochem. Commun. 2006, 8, 937.
Figure 4. Cyclic voltammograms of chemically prepared PPy/ PMA films with varying porosity as a function of scan rate (10 mV/s-20 V/s) on a GC electrode (0.07 cm2) in 0.5 M H2SO4. Films were prepared using 5 mg/mL pyrrole and 68.4 mg/mL PMA with (a) 0 mg/mL, (b) 20 mg/mL, and (c) 40 mg/mL porogen in THF.
the charge density used for deposition of PPy/ClO4- film previously. Specifically, it is reported that the deposition of 2 µm of PPy/ClO4- requires a charge density of 0.48 C/cm2.35 On the basis of the integration of cyclic voltammograms, the charge exhibited by a PPy/PMA film is nominally the same as that observed for an electrochemically grown 2 µm thick PPy/ClO4film. Figure 5 shows the complex plane impedance plots of PPy/ PMA composites films with and without pores at various potentials. The plots show two well-separated patterns: one at high-frequency exhibiting an ∼45°-inclined Warburg-type line and a region at low-frequency exhibiting capacitive behavior.40-42 For all data shown in Figure 5a,b, the real axis intercept at high frequency is independent of potential and coincides with the (40) Albery, W. J.; Elliott, C. M.; Mount, A. R. J. Electroanal. Chem. 1990, 288, 15. (41) Duffitt, G. L.; Pickup, P. G. J. Chem. Soc., Faraday Trans. 1992, 88, 1417. (42) Pickup, P. G. J. Chem. Soc., Faraday Trans. 1990, 86, 3631.
1068 Langmuir, Vol. 24, No. 3, 2008
Suppes et al.
Figure 6. Specific capacitance of chemically prepared PPy/PMA film (a) without and (b) with porogen (20 mg/mL) on an ITO electrode (0.7 cm2) in 0.5 M H2SO4 at a scan rate of 10 mV/s. Films were prepared using 16.7 mg/mL pyrrole and 212.7 mg/mL PMA in THF.
Figure 5. Complex plane impedance (Nyquist) plots (100 kHz0.1 Hz, perturbation amplitude 5 mV) of PPy/PMA composite films prepared (a) without and (b) with 2 mg/mL porogen in 0.5 M H2SO4 on a GC electrode (0.07 cm2) at (]) -0.1 V, (0) 0.2 V, (4) 0.4 V, (×) 0.6 V, and (GC) for a bare electrode at open-circuit potential. Frequencies of ([) 1 Hz, (2) 0.5 Hz, and (b) 0.1 Hz are identified. Films were prepared using 5 mg/mL pyrrole and 68.4 mg/mL of PMA in THF. Table 1. Ionic Conductivities from Impedance Spectroscopy of PPy/PMA Composite Films (2 µm) Prepared with Different Amounts of Porogen at Various Potentials in 0.5 M H2SO4 ionic conductivities (µS/cm) PPy/PMA film
-0.1 V
0.2 V
0.4 V
0.6 V
without porogens 1 mg/mL 2 mg/mL 4 mg/mL
0.9 55.8 77.7 73.0
5.0 49.2 115.1 97.2
4.2 48.7 121.6 88.6
3.4 10.7 10.8 9.2
uncompensated resistance of solution, Rs. The Rs calculated using a bare GC electrode is 7 Ω. Initially, the small ac perturbations to the system are at such a high-frequency that redox reactions occur only at the polymer/solution interface. As the frequency is decreased the redox reactions occur deeper within the entire polymer film. A transmission line model used by Albery40 and Pickup et al.41,42 to fit impedance data for PPy suggests that there are two separate distributed resistances to represent the transport of electrons and counterions within the polymer. Also, an assumption is made in which one resistance rail (RE or RI) is considered negligible compared to the solution resistance Rs.43 Pickup et al. have demonstrated that the electronic resistance of PPy is negligible based on its relatively high electronic conductivity and could in turn use a simplified model as mentioned above.35 Using this model, the ionic resistances Rion of the PPy/ PMA composites can be calculated from Rion ) 3(Rlow - Rs). The ionic conductivity of the film is calculated from σion ) (43) Ren, X.; Pickup, P. G. J. Chem. Soc., Faraday Trans. 1993, 89, 321.
Figure 7. Cyclic voltammograms of chemically prepared porous PPy/PMA film on a GC electrode (0.07 cm2) in 0.5 M H2SO4 at a scan rate of 100 mV/s. Films were prepared using 5 mg/mL pyrrole and 68.4 mg/mL PMA with 2 mg/mL porogen in THF. The cyclic voltammograms were taken up to 4000 cycles; the figure shows the first and last five cycles.
d/RionA, where A is the geometric area of the electrode and d the dry film thickness.35 The projected length of the high-frequency Warburg-type line on the real axis characterizes the slow ion migration process from solution into the pores and the bulk of polymer film. The complex plane impedance plots of a nonporous film (Figure 5a) indicate the higher barrier to ionic transport compared to that of a porous PPy/PMA composite film (Figure 5b). The porous PPy/PMA composite films show superior ionic transport due to the increased fraction of volume occupied by solvent in the porous composite structure. For example, the ionic conductivity of 0.5 M H2SO4 is approximately 0.1 S/cm, whereas the polymer by itself is 4 × 10-6 S/cm (see Table 1). Therefore, the higher ionic conductivities observed for porous films are attributed to the faster ionic transport in the porous composite structure. The ionic conductivity of both the nonporous and porous PPy/PMA composite films changes as a function of potential, decreasing with increasing potential (see Table 1). The ionic conductivity is high at lower potentials (0.2 and 0.4 V), presumably due to the large concentration of ionic charge carriers (H+) in the polymer at more negative potentials as opposed to the more positive potentials, where the polymer is oxidized and only the negative PMA ion is present. Similar trends are reported by Pickup et al.35 for PPy/polystyrenesulfonate, where the
Polymer/Heteropolyoxometalate Hybrid Material
Langmuir, Vol. 24, No. 3, 2008 1069
negatively charged sulfonate group acted as an immobile dopant but without the redox chemistry of PMA. In order to have a sufficient mass of material to determine the specific capacitance of the hybrid polymer, thicker films were prepared by spin-coating higher concentrations of pyrrole and PMA (same ratio). Figure 6 shows that the specific capacitance, determined from cyclic voltammetry, is independent of porosity. The specific capacitance of PPy/PMA composite film has a maximum for the second redox peak around 700 F/g and a total integrated specific capacitance of 210 F/g at a scan rate of 10 mV/s. The previously described conducting polymer/polyoxometalate hybrids were reported to have maximum specific capacitance values of 240 F/g for the second redox peak and a total integrated specific capacitance of 130 F/g (at 1 mV/s) for a PPy/PMA film.30 The stability of porous PPy/PMA composite films was examined by means of a cycle-life test performed for a large number of cycles. The porous PPy/PMA composite film was cycled 4000 times in 0.5 M H2SO4 at scan rate of 100 mV/s. As shown in Figure 7, a decrease of less than 10% of the original current is observed after 4000 cycles, where the same loss is seen in other systems at about 1000-2000 cycles,44 suggesting higher stability of porous PPy/PMA composite film.
Conclusion
(44) Go´mez-Romero, P.; Malgorzata, M.; Cuentas-Gallegos, K.; Asensio, J. A.; Kulesza, P. J.; Casan˜-Pastor, N.; Lira-Cantu´, M. Electrochem. Commun. 2003, 5, 149.
The present work demonstrates that our synthetic strategy is an effective method for controlling the porosity of conducting polymers during chemical polymerization. In addition, the porous heteropolyoxometalate/conducting polymer hybrid material exhibits excellent specific capacitance, extremely fast switching speeds, and impressive stability. Both cyclic voltammetry and impedance results indicate that the ionic conductivity of porous structures are more than an order of magnitude larger than that observed for standard conducting polymer structures. These results indicate that this material represents a significant step forward in the implementation of this for supercapacitor applications. Acknowledgment. This work was supported by the Defense Research and Development Canada and the Natural Sciences and Engineering Research Council (NSERC) of Canada, the Canada Foundation for Innovation (CFI), the Manitoba Research and Innovation Fund, and the University of Manitoba. This research was undertaken, in part, thanks to funding from the Canada Research Chairs Program. LA702837J
