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Electrochemical and textural characterization of iridium-doped polyaniline films for electrochemical capacitors
Materials Chemistry and Physics 65 (2000) 329–338
Electrochemical and textural characterization of iridium-doped polyaniline films for electrochemical capacitors Chi-Chang Hu∗ , Chien-How Chu Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan Received 10 September 1999; received in revised form 29 February 2000; accepted 2 March 2000
Abstract The electrochemical polymerization of aniline via cyclic voltammetry (CV) was performed in 1 M HCl and 0.2 M aniline solutions with various concentrations of IrCl3 ·xH2 O. The rate of polymerization strongly depends on the concentration of IrCl3 ·xH2 O and reaches a maximum at 0.2 mM IrCl3 ·xH2 O on the basis of the anodic peak current at ca. 280 mV (peak A1 ). The electrochemical properties of Ir-doped polyaniline (PAN) films for the application of electrochemical (EC) capacitors were characterized by their i–E and chronopotentiometric responses in 0.5 M H2 SO4 . The effect of iridium oxide on the chemical environments of PAN was demonstrated by X-ray photoelectron spectroscopy (XPS). Morphologies and crystalline structures of these Ir-doped PAN films are, respectively, examined by scanning electron microscopic (SEM) photographs and X-ray diffraction (XRD) patterns. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Iridium-doped polyaniline (PAN); Electrochemical (EC) capacitor; Cyclic voltammetry (CV); Chronopotentiometry
1. Introduction Considerable current attention has been focused on the subjects of certain conducting polymers, e.g. polyaniline (PAN) and polypyrrole, because of both fundamental interest and potential applications in the energy storage and conversion systems, electrochromic devices, sensors, anticorrosive coatings and electrocatalysts [1–7]. Since PAN is highly stable and becomes electronically conducting when it is oxidized via either chemical or electrochemical methods [8–10], this material is considered as promising for the application of energy storage devices [1,11,12]. The electrochemical (EC) capacitor is an important device in the energy storage and conversion systems. This device attracts significant attention due to its promising application in high pulse-power devices of energy-storage systems. The capacitance of an EC capacitor arises mainly from the redox reaction on/within the electrode materials [13–15] and these materials (e.g. conductive metal oxides and conducting polymers) usually have several oxidation states for the redox transition [13–16]. Due to the existence of several oxidation structures [16], the intrinsically chemical stability, and a low preparation cost, PAN is expected to be a potential material for the application of EC capacitors. ∗ Corresponding author. Fax: +886-52721206. E-mail address: [email protected] (C.-C. Hu)
The performance of an EC capacitor is strongly dependent on the electrochemical characteristics of the redox couples on/within the electroactive materials [13,15]. Thus, a fundamental understanding of the relationships between preparation methods and textural properties as well as between electrochemical characteristics and textural properties is very important in the application of this field. Although PAN has been widely investigated [8–10,16–18], its preparation and characterization by either electrochemical or chemical methods is still very attractive to many researchers. In addition, an increase in electroactive species of the redox transition on/within the electrode material is as important as an increase in electrochemical reversibility of these redox transitions. Since applications have usually preceded fundamental understanding, efforts in improving the electrochemical characteristics and the stability of PAN, and enhancing its propagation have to be carried out in order to improve the performance of this material in EC capacitors. Polyaniline exhibits redox transitions in a less positive potential range [8–10,16], while hydrous iridium oxide formed by potentiodynamic methods possesses redox transitions in a relatively positive potential region [19,20]. A composite film consisting of these two materials is thus expected to compensate their redox characteristics and possesses synergistic effects in EC capacitors. The purpose of this work is to investigate the effects of IrCl3 ·xH2 O on the rate of electrochemical polymerization of aniline. The electrochemical
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characteristics of Ir-doped PAN were also investigated for the application of EC capacitors. The relationship between textural properties and electrochemical characteristics of these polymer films were also examined in this work.
2. Experimental The 10 mm×10 mm×3 mm graphite supports (Nippon Carbon EG-NPL, N.C.K., Japan) were first abraded with ultrafine SiC paper, degreased with acetone and water, then etched in a 0.1-M HCl solution at room temperature (ca. 26◦ C) for 10 min, and finally degreased with water. The exposed geometric area of these pretreated graphite supports is equal to 1 cm2 while the other surface areas were insulated by a PTFE (polytetrafluorene ethylene) coating. These supports were directly coated with various Ir-doped PAN films by cyclic voltammetry in the potential region between −200 and 800 mV for 60 cycles in the solutions containing 1 M HCl (Merck GR), 0.2 M aniline (Merck GR), and variable concentrations of IrCl3 ·xH2 O (Johnson Matthey). After polymerization, the PTFE films were removed from the electrodes and these electrodes were doubly degreased with water and then dried in a vacuum oven at room temperature overnight. For the electrochemical studies, the electrode areas without the PAN coatings were doubly coated with epoxy resin and PTFE films while for textural analysis, the electrodes were employed without further treatments. Surface morphologies of these polymer films were examined by a scanning electron microscope (SEM JEOL JSM 35). X-ray diffraction patterns (XRD: Rigaku X-ray diffractometer using a Cu target) was employed to obtain crystalline information of these polymer films. X-ray photoelectron spectroscopic (XPS) measurements were performed with an ESCA 210 (VG Scientific) spectrometer. XPS spectra employed MgK␣ (h=1253.6 eV) irradiation as the photosource, with a primary voltage of 12 kV and an emission current of 17 mA. The analysis chamber pressure during scans was ≈10−10 m bar. Cyclic voltammetry for the electrochemical polymerization and characterization of the polymer films was performed by an electrochemical analyzer system, BAS 100W (Bioanalytic System, USA). Electrochemical research equipment, a model AFCBP1 Potentiostat (PINE Ins., USA), with an Y–t recorder was employed to perform the charging and discharging experiments using chronopotentiometry. All experiments were carried out in a three-compartment cell. An Ag/AgCl electrode (Argenthal, 3 M KCl, 207 mV vs. SHE at 25◦ C) was used as the reference, and a platinum wire with an exposed area equal to 2 cm2 was employed as the counter electrode. A Luggin capillary, whose tip was set at a distance of 1–2 mm from the surface of the working electrode, was used to minimize errors due to iR drop in the electrolytes. The scan rate of CV was kept at 50 mV s−1 for both growth and characterization of the polymer films.
All solutions used in this work were prepared with 18 M cm water produced by a reagent water system (MILLI-Q SP, Japan), and all reagents, not otherwise specified in this work, were Merck, Germany. In addition, the electrolyte, containing 0.5 M H2 SO4 , used to study the electrochemical behavior of various Ir-doped PAN films was degassed with purified nitrogen gas before voltammetric measurements while this gas flowed above this electrolyte during the experiments. The solution temperature was maintained at 25◦ C by means of a water thermostat (HAAKE DC3 and K20). 3. Results and discussion 3.1. Electrochemical synthesis of iridium-doped polyaniline films The electrochemical polymerization of aniline was induced by cyclic voltammetry. Typical results of the films polymerized from 1 M HCl and 0.2 M aniline solutions without, and with, 0.2 mM IrCl3 ·xH2 O are shown in Fig. 1a and b, respectively. Note that in Fig. 1a voltammetric currents gradually rise with the cycle number of CV, indicating the propagation of PAN. In addition, from the voltammetric responses in the investigated potential range, the following four processes have been proposed [9,16,21,22]. 1. The passive responses in the potential ranges respectively from −200 to 100 mV and from −100 to −200 mV on the positive and negative sweeps are due to the fact that leucoemeraldine form is in an insulated state. 2. Two redox peaks (labeled as C1 /A1 ) are clearly found at ca. 100 and 280 mV on the negative and positive sweeps, respectively. These peaks are attributed to the redox conversion of PAN from an insulated state (leucoemeraldine form) to a conducting state (polaronic emeraldine form) [9,16,21,22]. 3. Very large background currents, not attributable to the double-layer charging and discharging processes, are found in the potential range from 400 to 600 mV and from 750 to 300 mV on the positive and negative sweeps, respectively. 4. Anodic currents rise gradually with the positive shift in electrode potentials (labeled as A2 ) at potentials above 600 mV on the positive sweep while the corresponding reduction peak (labeled as C2 ) is not clear on the negative sweep. Peak A2 is attributed to the redox transition from the polaronic emeraldine form to the bipolaronic pernigraniline form. Note that additional peaks, usually appearing at ca. 500 mV due to the formation of other structures (e.g. the benzoquinone/hydroquinone couple) in PAN [10,23–25], are not found on these curves, although the upper potential limit of CV in this work is set at 800 mV. However, under similar polymerization conditions, the middle peak was observed
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Fig. 1. Growth behavior of PAN in a 1 M HCl+0.2 M aniline solution a) without, and b) with, 0.2 mM IrCl3 ·xH2 O at 25◦ C. Cyclic voltammetry was carried out at 50 mV s−1 in the −200 to 800 mV range for 60 cycles.
on the CV curves of a PAN film coated on an IrO2 /Ti electrode [10]. The above difference in voltammetric responses may be attributed to the oxidative activity of iridium oxide in a higher oxidation state (i.e. Ir(VI)), which is formed at highly positive potentials (>600 mV), on PAN and aniline, promoting the formation of the benzoquinone/hydroquinone couple. This inference was supported independently by the results of electrochemical polymerization of aniline on a RuO2 /Ti electrode [26]. From a comparison of Fig. 1a and b, the polymerization behavior of the solution containing 0.2 mM IrCl3 ·xH2 O is similar in shape to that from the pure aniline solution, indicating that the mechanism of aniline polymerization is not significantly affected by the presence of IrCl3 ·xH2 O. On the other hand, the peak current of A1 on the final cycle in Fig. 1b is approximately twice that in Fig. 1a. This result indi-
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Fig. 2. Voltammetric behavior of a carbon electrode in a 1 M HCl+0.2 M aniline solution (a) without, and (b) with, 0.2 mM IrCl3 ·xH2 O with a scan rate of 50 mV s−1 at 25◦ C.
cates that the rate of aniline polymerization is undoubtedly enhanced by the presence of IrCl3 ·xH2 O in the growth solution. Accordingly, the role of IrCl3 ·xH2 O playing in the electrochemical polymerization of aniline is worthy of being studied. In order to further understand the effect of IrCl3 ·xH2 O on the propagation of PAN, the initial two cycles of PAN synthesis in the solutions containing 1 M HCl and 0.2 M aniline without, and with, 0.2 mM IrCl3 ·xH2 O are shown in Fig. 2a and b, respectively. Note the positive sweep on curve 1 in Fig. 2a that no sensible current attributed to faradaic reactions is found at potentials negative to ca. 700 mV while a sharp increase in anodic currents commences at potentials positive than 700 mV. The latter result implies that the formation of aniline radicals on a carbon electrode commences at ca. 700 mV, initiating the polymerization of aniline. On
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the negative sweep of this curve, anodic currents are still observed at potentials positive than 450 mV and an obvious but relatively broad reduction peak is found in the potential region from 450 to −200 mV. On curve 2 in Fig. 2a, the redox transition between leucoemeraldine form and polaronic emeraldine form is obviously found although its peak potential difference is about 180 mV, indicating an irreversible characteristic. In Fig. 2b, there are visible background currents on the positive sweep of curve 1 at potentials positive than 150 mV meanwhile the sharp increase in anodic currents commencing at 600 mV indicates the initiation of aniline polymerization. The former result implies that some iridium species are oxidized at potentials positive than 150 mV. The latter result indicates that the onset potential for the initiation of aniline polymerization is negatively shifted by the presence of IrCl3 ·xH2 O, attributable to the oxidative properties of iridium species at a higher oxidation state (i.e. Ir(VI)). On the negative sweep of this curve, the cathodic currents begin at ca. 600 mV and a peak with its peak potential equal to 140 mV is attributed to the reduction of polaronic emeraldine form to leucoemeraldine form. Moreover, the redox transition between leucoemeraldine and polaronic emeraldine forms is obviously found on curve 2 meanwhile its peak potential difference is ca. 70 mV, indicating a reversible characteristic. The reversible behavior of the polaronic emeraldine/leucoemeraldine transition in Fig. 2b implies that the structure or properties of the initially polymerized aniline species are very close to a well-established polyaniline film. From a comparison between Fig. 2a and b, aniline polymerization is undoubtedly promoted by the presence of Ir species in a high oxidation state (i.e. Ir(VI)). A typical CV curve measured on a carbon electrode in 1 M HCl and 0.2 mM IrCl3 ·xH2 O (without aniline) is shown in Fig. 3. Note the pair of redox peaks located at 450 mV,
Fig. 3. Voltammetric behavior of a carbon electrode in 1 M HCl+0.2 mM IrCl3 ·xH2 O with a scan rate of 50 mV s−1 at 25◦ C.
Fig. 4. Dependence of ip of A1 on the growth cycle of CV obtained from 0.1 M HCl, 0.2 M aniline, and (1) 0; (2) 0.1; (3) 0.2; (4) 0.5; and (5) 1.0 mM IrCl3 ·xH2 O.
indicating the Ir(IV)/Ir(III) transition [28,29]. The Ir(IV) species are expected to be unable to oxidize aniline, since the electrochemical polymerization of aniline is not initiated at 450 mV on the positive sweep of curve 1 in Fig. 2b. On the other hand, the gradual increase in anodic currents at potentials positive to 600 mV indicates the fact that Ir(IV) is further oxidized to Ir(VI) [28,29]. This potential is actually equal to the onset potential for the initiation of aniline polymerization (see curve 1 in Fig. 2b). Therefore, the initiation of aniline polymerization is undoubtedly catalyzed by the presence of Ir(VI) species. Similar phenomena for the promotion of aniline polymerization via metal oxides on RuO2 had been independently reported in the literature [26]. Peak currents (ip ) of A1 on the cyclic voltammograms of aniline polymerization were recognized as an index indicating the relative loading of PAN [10]. In addition, the results and discussion of Figs. 1–3 indicate that the mechanism of aniline polymerization is, most likely, independent of the presence of IrCl3 ·xH2 O. Accordingly, the dependence of ip of peak A1 on the cycle number of CV is presumably employed to evaluate the effects of IrCl3 ·xH2 O on the rate of aniline polymerization in this work. Typical results are shown in Fig. 4. Note that on every curve, there exists an induced period during the initial 18–24 cycles since peak currents of A1 increase relatively slowly within these cycles. On the other hand, the slope of peak currents of A1 against the growth cycle becomes larger and maintains approximately constant after a significant amount of PAN is formed on the graphite substrate during the induced period. The above results are attributable to that the density of active centers (polaronic radicals) on the graphite substrate is relatively low during the initial potential cycling of aniline polymerization. On the other hand, the density
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of active centers is considered to be approximately constant during the propagation of PAN when a significant amount of this material has been formed on the substrate. Therefore, an induced period is found in the initial growth of PAN. Note that there is a linear region on every curve in Fig. 4 and the slope of the linear region initially increases, reaches the maximum (at 0.2 mM IrCl3 ·xH2 O), and then decreases with increasing the concentrations of IrCl3 ·xH2 O in the growth solutions. These results indicate that the rate of aniline polymerization reaches a maximum when the growth solution contains 0.2 mM IrCl3 ·xH2 O. This phenomenon is attributable to that at potentials above ca. 600 mV, the iridium species at the interface of PAN and electrolyte are oxidized to a high oxidation state (i.e. Ir(VI)), favoring the formation of aniline radicals and promoting the polymerization of aniline. On curves 2–5, however, a gradual decrease in slope is found at cycle number of CV greater than ca. 50, especially the forming solutions containing the maximal concentration of IrCl3 ·xH2 O. This is likely due to the fact that, at potentials above ca. 600 mV, oxyiridium species within the polymer matrix are also oxidized to the high oxidation state, which renders an irreversible oxidation of PAN (see Fig. 7), resulting in destroying the structure of PAN and decreasing the rate of PAN propagation. On the basis of the above results and discussion, the maximal rate of aniline polymerization occurring at the growth solution containing 0.2 mM IrCl3 ·xH2 O should be the result of a combination of the above two factors. 3.2. Pseudocapacitance evaluation Pseudocapacitance of an EC capacitor arises mainly from the redox transitions of the electroactive species within the electrode materials [13–15] and the performance of an EC capacitor is strongly dependent on the electrochemical characteristics of the redox transitions within these materials [13,15]. Therefore, voltammetric responses and charge (q∗ ), from a cyclic voltammogram measured in an inert electrolyte, are employed to evaluate the electrochemical characteristics of PAN. Furthermore, in our previous works [15,27], the pseudocapacitance of an electrode material can be calculated from its voltammetric charge on either positive or negative sweeps of a CV curve measured in an inert electrolyte. Accordingly, the cathodic voltammetric charge between −200 and 750 mV on the negative sweep of a CV diagram measured in a 0.5 M H2 SO4 solution is employed to indicate the relative amount of electroactive sites involving the redox transitions within the Ir-doped PAN films in the present work. Typical cyclic voltammograms measured at 50 mV s−1 in a 0.5 M H2 SO4 solution for the polymer films prepared from the growth solutions containing 0, 0.1, 0.2, 0.5, and 1 mM IrCl3 ·xH2 O are shown as curves 1–5 in Fig. 5. Note that the voltammetric behavior of pure PAN in an inert solution is very similar in shape to its polymerization
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Fig. 5. Voltammetric behavior of various Ir-doped PAN films in 0.5 M H2 SO4 at 25◦ C and 50 mV s−1 . These films were grown from the solutions containing 1 M HCl, 0.2 M aniline, and (1) 0; (2) 0.1; (3) 0.2; (4) 0.5; and (5) 1.0 mM IrCl3 ·xH2 O.
responses. On the other hand, from a comparison between curve 1 in Fig. 5 and the curve of the 60th cycle in Fig. 1a, two features should be notified. First, voltammetric currents of the final growth cycle (i.e. the 60th cycle in Fig. 1a) are much larger than that of the same electrode measured in an inert electrolyte (see curve 1 in Fig. 5). Second, a shoulder wave appears at ca. 730 mV on the negative sweep on curve 1 in Fig. 5 while this wave was not found on the growth diagrams of PAN. The former result reveals that the voltammetric currents of polymerization must be simultaneously contributed by the oxidation/reduction of aniline in the solution and the redox transitions of the already-deposited PAN. Therefore, the shape of CV curves for the PAN formation should be determined by a combination of the electrochemical characteristics of the already-deposited PAN and the kinetics of aniline polymerization in the forming bath. Accordingly, voltammetric responses for the redox transitions of PAN are very similar to its polymerization behavior. The latter result suggests that some iridium species were intercalated into the PAN film since the redox transitions of hydrous iridium oxides occurred in the highly positive potential region [29]. Note that from a comparison of all CV curves in Fig. 5, several features have to be mentioned. First, the maximal voltammetric currents occur at the PAN film formed from the growth solution containing 0.2 mM IrCl3 ·xH2 O. In addition, voltammetric charges integrated from the negative sweeps of these curves also reach the maximum at the same concentration of IrCl3 ·xH2 O (see Fig. 6). These results are consistent to the data in Figs. 1 and 4; results indicates the fact that ip of A1 is proportional to the loading of PAN on the graphite substrate and is reasonably indicative of its film thickness. Second, the potential window of various Ir-doped
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Fig. 6. Voltammetric charges of various Ir-doped PAN films in 0.5 M H2 SO4 against the concentration of IrCl3 ·xH2 O in the growth solution.
PAN films in the conducting state on the negative sweep of CV becomes wider with the concentration of IrCl3 ·xH2 O in the growth solution. The operative potential range of an Ir-doped PAN film for the EC capacitors is thus expected to be wider than that of a pure PAN film. Third, the voltammetric curve and the redox transition of peaks C1 /A1 , respectively, become smoother and unclear with increase in the concentration of IrCl3 ·xH2 O in the growth solution. Since the voltammetric responses of an ideal capacitor possess a rectangular cyclic voltammogram, the Ir-doped PAN films are considered as a more promising material in the application of EC capacitors. The charging and discharging behavior of various PAN Ir-doped films was examined by chronopotentiometry and typical results measured from 0 to 700 mV in 0.5 M H2 SO4 at 200 A cm−2 are shown in Fig. 7. Note that the slope of every chronopotentiometric curve is potential-dependent. Since the slope of a chronopotentiogram of an ideal capacitor should be potential-independent and maintains a constant value at a specified current density, the electrochemical properties of these polymer films must be improved to be more applicable in the EC capacitors. Also note that at potentials above 600 mV on curves 2–5, the E–t responses during the cathodic discharging process do not behave as a mirror-like curve of the anodic charging process while this phenomenon was not found for a pure PAN film. These results indicate that an irreversible oxidation occurs at potentials above 600 mV on the positive sweeps of chronopotentiograms for all Ir-doped PAN films. Accordingly, the electrochemical reversibility of the redox couple(s) in the high potential region (above 600 mV) within a pure PAN film is higher than that within the Ir-doped PAN films. The above irreversible oxidation is attributable to the formation of oxyiridium species in the high-oxidation state (i.e. Ir(VI))
Fig. 7. Chronopotentiomograms of various Ir-doped PAN films measured at 200 A cm−2 in 0.5 M H2 SO4 at 25◦ C. These films were grown from the solutions containing 1 M HCl, 0.2 M aniline, and (1) 0; (2) 0.1; (3) 0.2; (4) 0.5; and (5) 1.0 mM IrCl3 ·xH2 O.
[28,29] which oxidizes and destroys the structure of PAN during the anodic charging process. If this is the case, the Ir-doped PAN films should be less stable than a pure PAN film at potentials above ca. 600 mV although pseudocapacitance of the former films is much larger than that of the latter. In our previous work [15,27], the average pseudocapacitance of an electroactive film can be calculated from the
Fig. 8. Pseudocapacitance from chronopotentiomograms (Ccp ) against the pseudocapacitance from q∗ (Cq ∗ ) for various Ir-doped PAN films in 0.5 M H2 SO4 .
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following equation: Ccp =
i i ≈ dV /dt 1V /1t
In addition, from the chronopotentiograms of Ir-doped PAN films in Fig. 7, the charging responses at potentials above 600 mV is affected by the irreversible oxidation of PAN due to the presence of oxyiridium species in the high oxidation state (Ir(VI)). The mean pseudocapacitance of these PAN Ir-doped films was thus calculated from the results of the cathodic discharging curves (denoted as Ccp ). Note that data
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shown in Fig. 6 were also used to evaluate the mean pseudocapacitance of these polymer films (denoted as Cq ∗ ) on the basis of the following equation [15,27]: Cq ∗ =
|q ∗ | cathodic voltammetric charge ≡ potential range 1V
The relationship between Ccp and Cq ∗ for these polymer films was shown in Fig. 8. Note the linear relationship between Ccp and Cq ∗ , indicating that pseudocapacitance of the Ir-doped PAN films is minorly contributed by the oxyiridium species within the PAN matrix although the electrochemical
Fig. 9. SEM photographs of the Ir-doped PAN films grown from the solutions containing (a, d) 0; (b, e) 0.2; and (c, f) 1.0 mM IrCl3 ·xH2 O. The morphologies of photographs a–c and d–f were, respectively, enlarged under 3000 and 7000 times.
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of the same electrode measured by chronopotentiometry at a very low current density should be larger than that obtained by cyclic voltammetry. On the other hand, the fact that the increase in Ccp is about 1.5 times of that in Cq ∗ is not only attributed to the quasi-equilibrium charging/discharging processes but also attributed to the difference in potential windows between chronopotentiometry and cyclic voltammetry. Since the potential window for CV includes the conversion of PAN from the insulating state to the conducting state, pseudocapacitance of PAN in this potential region should be much lower than that of the other redox transitions in the conducting state. Accordingly, Cq ∗ calculated from voltammetric charges of CVs is much lower than Ccp evaluated from the slope of chronopotentiograms. 3.3. Textural characteristics
Fig. 10. XRD patterns of the PAN Ir-doped films grown from the solutions containing (a) 0 and (b) 1.0 mM IrCl3 ·xH2 O.
characteristics of PAN were indeed significantly changed by doping iridium species. Thus, pseudocapacitance of these Ir-doped PAN films should mainly come from the redox transitions of various oxidation structures of PAN and the amount of oxyiridium species within the polymer matrix should be very small. The latter inference is supported by the XPS data that the amount of iridium is <2 at.% (a/o) when PAN was formed from the growth solution with 1.0 mM IrCl3 ·xH2 O. Also note that the linear line in Fig. 8 does not pass the origin and the slope of this line is ca. 1.5. Since the DC current density employed in chronopotentiometry is very low (200 A cm−2 ), charging and discharging processes are believed to be performed under a condition close to the quasi-equilibrium state. Thus, the pseudocapacitance
The surface morphologies of all Ir-doped PAN films were examined by SEM. Typical images of the polymer films resulting from the growth solutions containing 0, 0.2, and 1.0 mM IrCl3 ·xH2 O are shown as Fig. 9a–f, respectively. Note that the surface morphology of the pure PAN film is relatively compact and its particle size is relatively large. On the other hand, the addition of IrCl3 ·xH2 O in the growth solution initially renders a rougher morphology of the polymer film (see Fig. 9b and e) with smaller particle sizes. However, a more compact morphology with larger particle sizes is obtained with the continuous addition of IrCl3 ·xH2 O into the growth solutions (see Fig. 9c and f). Moreover, PAN with the smallest particle size is obtained from the growth solution containing 0.2 mM IrCl3 ·xH2 O. Note that from the results and discussion of Figs. 2 and 3, Ir(IV) is further oxidized to Ir(VI) at potentials positive to 600 mV, rendering negative shift in onset potential for the formation of aniline radicals. In addition, in Fig. 4, the slope of iP of aniline polymerization against growth cycles reaches a maximum (i.e.
Fig. 11. An XPS spectrum of the Ir-doped PAN film grown from the solution containing 0.05 mM IrCl3 ·xH2 O.
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the highest density of active centers) when the growth solution contains 0.2 mM IrCl3 ·xH2 O, indicating the presence of Ir(VI) at the interface of PAN and electrolyte favoring the formation of aniline radicals (i.e. active centers) on the polymer. These phenomena are further supported by the SEM photographs that the Ir-doped PAN films with the smallest particle size results from the highest density of active centers on this polymer film. The crystalline information of all polymer films was examined by the X-ray diffraction (XRD) method. Typical XRD pattern of the polymer films grown from the solution containing 1.0 mM IrCl3 ·xH2 O is shown as curve b in Fig. 10. In addition, the XRD pattern of a graphite substrate without any coatings is also shown as curve a in Fig. 10 for comparison. Note that diffraction peaks corresponding to graphite (denoted as ×) are clearly found on these two spectra while only very small and unclear diffraction peaks corresponding to IrO2 are observed on curve b. The above results may be due to a combination of the following two reasons:
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1. The already-deposited iridium oxide on various Ir-doped PAN films has a microcrystalline and/or amorphous structure. 2. The amount of iridium oxide within the polymer matrix is too small to form a well-crystallined material. A typical XPS spectrum of an element survey for the polymer grown from a solution containing 0.05 mM IrCl3 ·xH2 O is shown in Fig. 11. Note the presence of Ir peaks, indicating that iridium species has been doped within the polymer film when PAN was grown from the solution containing the lowest concentration of IrCl3 ·xH2 O. Also note that the binding energies of C, N, O, and Ir were examined to elucidate the change in chemical environments of PAN due to the presence of Ir within the polymer matrix. Typical XPS spectra of C 1s, N 1s, O 1s, and Ir 4f7/2,5/2 for the polymer film formed from the growth solution containing 0.05 mM IrCl3 ·xH2 O are shown as curve 2 in Fig. 12a–d, respectively. Moreover, the XPS spectra of C 1s, N 1s, and O 1s for pure PAN are also shown as curve 1 in Fig. 12a–c, respectively, for comparison. Note that, in order to avoid the influence of
Fig. 12. XPS spectra of (a) C 1s; (b) N 1s; (c) O 1s; and (d) Ir 4f7/2,5/2 for Ir-doped PAN films grown from solutions containing (1) 0 and (2) 0.05 mM IrCl3 ·xH2 O.
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synthesis conditions on the oxidation state of PAN, the final oxidation state of both PAN and Ir-doped PAN films was surely remained at the insulated state (i.e. the leucoemeraldine form). This requirement was carried out by employing a constant end potential of CV (equal to −200 mV) during the electrochemical polymerization of aniline since the oxidation state of PAN is strongly dependent on the synthesis conditions (e.g. the end potential of CV in this work). In Fig. 12d, the binding energy of Ir 4f7/2 is centered at ca. 61.6 eV, indicating a positive shift in binding energy (1.0 eV) of Ir in comparison to its metallic state. Thus, the oxidation state of iridium within the polymer matrix should be above 0. In Fig. 12a, the binding energy of C 1s is not significantly affected by the presence of iridium within the polymer matrix while the binding energy of N 1s is skewed to the higher binding energy side (see Fig. 12b). Moreover, the binding energy of O 1s is not significantly shifted by the presence of Ir, while it rendered a change in the distribution of oxygen atoms in different binding energies, likely due to the formation of hydrous iridium oxide. From the above results and discussion, there should exist a bonding between N and Ir (although relatively weak) when iridium oxide is doped into PAN.
4. Conclusions The rate of aniline polymerization was determined by a combination of (i) the formation of aniline radicals promoted by the presence of Ir(VI) at the PAN-electrolyte interface and (ii) the degradation of PAN by oxy-Ir(VI) species within the polymer matrix from the results of voltammetric and chronopotentiometric studies. The rate of polyaniline propagation reached a maximum when it was prepared from the growth solution with 0.2 mM IrCl3 ·xH2 O. The linear relationship between Ccp and Cq ∗ revealed that pseudocapacitance of the Ir-doped PAN films mainly came from the redox transitions of PAN in different oxidation structures. From the results of voltammetric and SEM studies, the presence of suitable amount of Ir(VI) species at the PAN-electrolyte interface, i.e. the solution containing 0.2 mM IrCl3 ·xH2 O, rendered the highest density of electroactive aniline radicals on the PAN film, resulting in a rougher morphology of the Ir-doped PAN film with the smallest particle size. There should exist a bonding between N and Ir when iridium oxide was doped into PAN from the XPS results. X-ray diffraction patterns revealed a microcrystalline and/or amorphous structure of the doped Ir oxide within the polymer.
Acknowledgements The financial support of this work, by the National Science Council of the Republic of China under contract No. NSC 89-2214-E-194-007, is gratefully acknowledged. References [1] T. Sotomura, H. Uemachi, K. Takeyama, K. Naoi, N. Oyama, Electrochim. Acta 37 (1992) 1851. [2] J.M. Pope, N. Oyama, J. Electrochem. Soc. 145 (1998) 1893. [3] F. Leroux, G. Goward, W.P. Power, L.F. Nazar, J. Electrochem. Soc. 144 (1997) 3886. [4] M. Hepel, J. Electrochem. Soc. 145 (1998) 124. [5] C. Barbero, M.C. Miras, O. Haas, R. Kotz, J. Electrochem. Soc. 144 (1997) 4170. [6] T.F. Otero, I. Cantero, H. Grande, Electrochim. Acta 44 (1999) 2053. [7] B. Wessling, J. Posdorfer, Electrochim. Acta 44 (1999) 2139. [8] K.L. Tan, B.T.G. Tan, S.H. Khor, K.G. Neoh, E.T. Kang, J. Phys. Chem. Solids 52 (1992) 673. [9] W.W. Focke, G.E. Wnek, Y. Wei, J. Phys. Chem. 91 (1987) 5813. [10] C.-H. Yang, T.-C. Wen, J. Appl. Electrochem. 24 (1994) 166 and references cited therein. [11] E.M. Genies, S. Picart, Synth. Met. 69 (1995) 165. [12] K. Naoi, K.-I. Kawase, M. Mori, M. Komiyama, J. Electrochem. Soc. 144 (1997) L173. [13] B.E. Conway, J. Electrochem. Soc. 138 (1991) 1539. [14] J.P. Zheng, P.-J. Cygon, T.R. Jow, J. Electrochem. Soc. 142 (1995) 2699. [15] C.-C. Hu, Y.-H. Huang, J. Electrochem. Soc. 146 (1999) 2465. [16] E.T. Kang, K.G. Neoh, K.L. Tan, in: H.S. Nalwa (Ed.), Handbook of Organic Conductive Molecules and Polymers, Vol. 3, Wiley, 1997, 121 pp. [17] S.H. Glarum, J.H. Marshall, J. Electrochem. Soc. 134 (1987) 2160. [18] M. Gholamian, A.Q. Contractor, J. Electroanal. Chem. 252 (1988) 291. [19] S.H. Glarum, J.H. Marshall, J. Electrochem. Soc. 127 (1980) 1467. [20] L.D. Burke, M.E.G. Lyons, in: R.E. White, J.O’M. Bockris, B.E. Conway (Eds.), Modern Aspects of Electrochemistry, Plenum Press, New York, 1986, p. 109. [21] W.-S. Huang, B.D. Humphrey, A.G. MacDiarmid, J. Chem. Soc. Faraday Trans. 1 82 (1986) 2385. [22] E.M. Genies, M. Lapkowski, J. Electroanal. Chem. 220 (1987) 67. [23] E.M. Genies, M. Lapkowski, J.F. Penneau, J. Electroanal. Chem. 249 (1988) 97. [24] E.M. Genies, M. Lapkowski, J. Electroanal. Chem. 236 (1987) 199. [25] E.M. Genies, C. Tsintavis, J. Electroanal. Chem. 195 (1985) 109. [26] S.-M. Lin, T.-C. Wen, Electrochim. Acta 39 (1994) 393. [27] C.-C. Hu, K.-H. Chang, Electrochim. Acta, accepted. [28] T.-C. Wen, C.-C. Hu, J. Electrochem. Soc. 139 (1992) 2158. [29] H. Elzanowska, J. Segal, V.I. Birss, Electrochim. Acta 44 (1999) 4515.
Electrochemical and textural characterization of iridium-doped polyaniline films for electrochemical capacitors Chi-Chang Hu∗ , Chien-How Chu Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan Received 10 September 1999; received in revised form 29 February 2000; accepted 2 March 2000
Abstract The electrochemical polymerization of aniline via cyclic voltammetry (CV) was performed in 1 M HCl and 0.2 M aniline solutions with various concentrations of IrCl3 ·xH2 O. The rate of polymerization strongly depends on the concentration of IrCl3 ·xH2 O and reaches a maximum at 0.2 mM IrCl3 ·xH2 O on the basis of the anodic peak current at ca. 280 mV (peak A1 ). The electrochemical properties of Ir-doped polyaniline (PAN) films for the application of electrochemical (EC) capacitors were characterized by their i–E and chronopotentiometric responses in 0.5 M H2 SO4 . The effect of iridium oxide on the chemical environments of PAN was demonstrated by X-ray photoelectron spectroscopy (XPS). Morphologies and crystalline structures of these Ir-doped PAN films are, respectively, examined by scanning electron microscopic (SEM) photographs and X-ray diffraction (XRD) patterns. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Iridium-doped polyaniline (PAN); Electrochemical (EC) capacitor; Cyclic voltammetry (CV); Chronopotentiometry
1. Introduction Considerable current attention has been focused on the subjects of certain conducting polymers, e.g. polyaniline (PAN) and polypyrrole, because of both fundamental interest and potential applications in the energy storage and conversion systems, electrochromic devices, sensors, anticorrosive coatings and electrocatalysts [1–7]. Since PAN is highly stable and becomes electronically conducting when it is oxidized via either chemical or electrochemical methods [8–10], this material is considered as promising for the application of energy storage devices [1,11,12]. The electrochemical (EC) capacitor is an important device in the energy storage and conversion systems. This device attracts significant attention due to its promising application in high pulse-power devices of energy-storage systems. The capacitance of an EC capacitor arises mainly from the redox reaction on/within the electrode materials [13–15] and these materials (e.g. conductive metal oxides and conducting polymers) usually have several oxidation states for the redox transition [13–16]. Due to the existence of several oxidation structures [16], the intrinsically chemical stability, and a low preparation cost, PAN is expected to be a potential material for the application of EC capacitors. ∗ Corresponding author. Fax: +886-52721206. E-mail address: [email protected] (C.-C. Hu)
The performance of an EC capacitor is strongly dependent on the electrochemical characteristics of the redox couples on/within the electroactive materials [13,15]. Thus, a fundamental understanding of the relationships between preparation methods and textural properties as well as between electrochemical characteristics and textural properties is very important in the application of this field. Although PAN has been widely investigated [8–10,16–18], its preparation and characterization by either electrochemical or chemical methods is still very attractive to many researchers. In addition, an increase in electroactive species of the redox transition on/within the electrode material is as important as an increase in electrochemical reversibility of these redox transitions. Since applications have usually preceded fundamental understanding, efforts in improving the electrochemical characteristics and the stability of PAN, and enhancing its propagation have to be carried out in order to improve the performance of this material in EC capacitors. Polyaniline exhibits redox transitions in a less positive potential range [8–10,16], while hydrous iridium oxide formed by potentiodynamic methods possesses redox transitions in a relatively positive potential region [19,20]. A composite film consisting of these two materials is thus expected to compensate their redox characteristics and possesses synergistic effects in EC capacitors. The purpose of this work is to investigate the effects of IrCl3 ·xH2 O on the rate of electrochemical polymerization of aniline. The electrochemical
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characteristics of Ir-doped PAN were also investigated for the application of EC capacitors. The relationship between textural properties and electrochemical characteristics of these polymer films were also examined in this work.
2. Experimental The 10 mm×10 mm×3 mm graphite supports (Nippon Carbon EG-NPL, N.C.K., Japan) were first abraded with ultrafine SiC paper, degreased with acetone and water, then etched in a 0.1-M HCl solution at room temperature (ca. 26◦ C) for 10 min, and finally degreased with water. The exposed geometric area of these pretreated graphite supports is equal to 1 cm2 while the other surface areas were insulated by a PTFE (polytetrafluorene ethylene) coating. These supports were directly coated with various Ir-doped PAN films by cyclic voltammetry in the potential region between −200 and 800 mV for 60 cycles in the solutions containing 1 M HCl (Merck GR), 0.2 M aniline (Merck GR), and variable concentrations of IrCl3 ·xH2 O (Johnson Matthey). After polymerization, the PTFE films were removed from the electrodes and these electrodes were doubly degreased with water and then dried in a vacuum oven at room temperature overnight. For the electrochemical studies, the electrode areas without the PAN coatings were doubly coated with epoxy resin and PTFE films while for textural analysis, the electrodes were employed without further treatments. Surface morphologies of these polymer films were examined by a scanning electron microscope (SEM JEOL JSM 35). X-ray diffraction patterns (XRD: Rigaku X-ray diffractometer using a Cu target) was employed to obtain crystalline information of these polymer films. X-ray photoelectron spectroscopic (XPS) measurements were performed with an ESCA 210 (VG Scientific) spectrometer. XPS spectra employed MgK␣ (h=1253.6 eV) irradiation as the photosource, with a primary voltage of 12 kV and an emission current of 17 mA. The analysis chamber pressure during scans was ≈10−10 m bar. Cyclic voltammetry for the electrochemical polymerization and characterization of the polymer films was performed by an electrochemical analyzer system, BAS 100W (Bioanalytic System, USA). Electrochemical research equipment, a model AFCBP1 Potentiostat (PINE Ins., USA), with an Y–t recorder was employed to perform the charging and discharging experiments using chronopotentiometry. All experiments were carried out in a three-compartment cell. An Ag/AgCl electrode (Argenthal, 3 M KCl, 207 mV vs. SHE at 25◦ C) was used as the reference, and a platinum wire with an exposed area equal to 2 cm2 was employed as the counter electrode. A Luggin capillary, whose tip was set at a distance of 1–2 mm from the surface of the working electrode, was used to minimize errors due to iR drop in the electrolytes. The scan rate of CV was kept at 50 mV s−1 for both growth and characterization of the polymer films.
All solutions used in this work were prepared with 18 M cm water produced by a reagent water system (MILLI-Q SP, Japan), and all reagents, not otherwise specified in this work, were Merck, Germany. In addition, the electrolyte, containing 0.5 M H2 SO4 , used to study the electrochemical behavior of various Ir-doped PAN films was degassed with purified nitrogen gas before voltammetric measurements while this gas flowed above this electrolyte during the experiments. The solution temperature was maintained at 25◦ C by means of a water thermostat (HAAKE DC3 and K20). 3. Results and discussion 3.1. Electrochemical synthesis of iridium-doped polyaniline films The electrochemical polymerization of aniline was induced by cyclic voltammetry. Typical results of the films polymerized from 1 M HCl and 0.2 M aniline solutions without, and with, 0.2 mM IrCl3 ·xH2 O are shown in Fig. 1a and b, respectively. Note that in Fig. 1a voltammetric currents gradually rise with the cycle number of CV, indicating the propagation of PAN. In addition, from the voltammetric responses in the investigated potential range, the following four processes have been proposed [9,16,21,22]. 1. The passive responses in the potential ranges respectively from −200 to 100 mV and from −100 to −200 mV on the positive and negative sweeps are due to the fact that leucoemeraldine form is in an insulated state. 2. Two redox peaks (labeled as C1 /A1 ) are clearly found at ca. 100 and 280 mV on the negative and positive sweeps, respectively. These peaks are attributed to the redox conversion of PAN from an insulated state (leucoemeraldine form) to a conducting state (polaronic emeraldine form) [9,16,21,22]. 3. Very large background currents, not attributable to the double-layer charging and discharging processes, are found in the potential range from 400 to 600 mV and from 750 to 300 mV on the positive and negative sweeps, respectively. 4. Anodic currents rise gradually with the positive shift in electrode potentials (labeled as A2 ) at potentials above 600 mV on the positive sweep while the corresponding reduction peak (labeled as C2 ) is not clear on the negative sweep. Peak A2 is attributed to the redox transition from the polaronic emeraldine form to the bipolaronic pernigraniline form. Note that additional peaks, usually appearing at ca. 500 mV due to the formation of other structures (e.g. the benzoquinone/hydroquinone couple) in PAN [10,23–25], are not found on these curves, although the upper potential limit of CV in this work is set at 800 mV. However, under similar polymerization conditions, the middle peak was observed
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Fig. 1. Growth behavior of PAN in a 1 M HCl+0.2 M aniline solution a) without, and b) with, 0.2 mM IrCl3 ·xH2 O at 25◦ C. Cyclic voltammetry was carried out at 50 mV s−1 in the −200 to 800 mV range for 60 cycles.
on the CV curves of a PAN film coated on an IrO2 /Ti electrode [10]. The above difference in voltammetric responses may be attributed to the oxidative activity of iridium oxide in a higher oxidation state (i.e. Ir(VI)), which is formed at highly positive potentials (>600 mV), on PAN and aniline, promoting the formation of the benzoquinone/hydroquinone couple. This inference was supported independently by the results of electrochemical polymerization of aniline on a RuO2 /Ti electrode [26]. From a comparison of Fig. 1a and b, the polymerization behavior of the solution containing 0.2 mM IrCl3 ·xH2 O is similar in shape to that from the pure aniline solution, indicating that the mechanism of aniline polymerization is not significantly affected by the presence of IrCl3 ·xH2 O. On the other hand, the peak current of A1 on the final cycle in Fig. 1b is approximately twice that in Fig. 1a. This result indi-
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Fig. 2. Voltammetric behavior of a carbon electrode in a 1 M HCl+0.2 M aniline solution (a) without, and (b) with, 0.2 mM IrCl3 ·xH2 O with a scan rate of 50 mV s−1 at 25◦ C.
cates that the rate of aniline polymerization is undoubtedly enhanced by the presence of IrCl3 ·xH2 O in the growth solution. Accordingly, the role of IrCl3 ·xH2 O playing in the electrochemical polymerization of aniline is worthy of being studied. In order to further understand the effect of IrCl3 ·xH2 O on the propagation of PAN, the initial two cycles of PAN synthesis in the solutions containing 1 M HCl and 0.2 M aniline without, and with, 0.2 mM IrCl3 ·xH2 O are shown in Fig. 2a and b, respectively. Note the positive sweep on curve 1 in Fig. 2a that no sensible current attributed to faradaic reactions is found at potentials negative to ca. 700 mV while a sharp increase in anodic currents commences at potentials positive than 700 mV. The latter result implies that the formation of aniline radicals on a carbon electrode commences at ca. 700 mV, initiating the polymerization of aniline. On
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the negative sweep of this curve, anodic currents are still observed at potentials positive than 450 mV and an obvious but relatively broad reduction peak is found in the potential region from 450 to −200 mV. On curve 2 in Fig. 2a, the redox transition between leucoemeraldine form and polaronic emeraldine form is obviously found although its peak potential difference is about 180 mV, indicating an irreversible characteristic. In Fig. 2b, there are visible background currents on the positive sweep of curve 1 at potentials positive than 150 mV meanwhile the sharp increase in anodic currents commencing at 600 mV indicates the initiation of aniline polymerization. The former result implies that some iridium species are oxidized at potentials positive than 150 mV. The latter result indicates that the onset potential for the initiation of aniline polymerization is negatively shifted by the presence of IrCl3 ·xH2 O, attributable to the oxidative properties of iridium species at a higher oxidation state (i.e. Ir(VI)). On the negative sweep of this curve, the cathodic currents begin at ca. 600 mV and a peak with its peak potential equal to 140 mV is attributed to the reduction of polaronic emeraldine form to leucoemeraldine form. Moreover, the redox transition between leucoemeraldine and polaronic emeraldine forms is obviously found on curve 2 meanwhile its peak potential difference is ca. 70 mV, indicating a reversible characteristic. The reversible behavior of the polaronic emeraldine/leucoemeraldine transition in Fig. 2b implies that the structure or properties of the initially polymerized aniline species are very close to a well-established polyaniline film. From a comparison between Fig. 2a and b, aniline polymerization is undoubtedly promoted by the presence of Ir species in a high oxidation state (i.e. Ir(VI)). A typical CV curve measured on a carbon electrode in 1 M HCl and 0.2 mM IrCl3 ·xH2 O (without aniline) is shown in Fig. 3. Note the pair of redox peaks located at 450 mV,
Fig. 3. Voltammetric behavior of a carbon electrode in 1 M HCl+0.2 mM IrCl3 ·xH2 O with a scan rate of 50 mV s−1 at 25◦ C.
Fig. 4. Dependence of ip of A1 on the growth cycle of CV obtained from 0.1 M HCl, 0.2 M aniline, and (1) 0; (2) 0.1; (3) 0.2; (4) 0.5; and (5) 1.0 mM IrCl3 ·xH2 O.
indicating the Ir(IV)/Ir(III) transition [28,29]. The Ir(IV) species are expected to be unable to oxidize aniline, since the electrochemical polymerization of aniline is not initiated at 450 mV on the positive sweep of curve 1 in Fig. 2b. On the other hand, the gradual increase in anodic currents at potentials positive to 600 mV indicates the fact that Ir(IV) is further oxidized to Ir(VI) [28,29]. This potential is actually equal to the onset potential for the initiation of aniline polymerization (see curve 1 in Fig. 2b). Therefore, the initiation of aniline polymerization is undoubtedly catalyzed by the presence of Ir(VI) species. Similar phenomena for the promotion of aniline polymerization via metal oxides on RuO2 had been independently reported in the literature [26]. Peak currents (ip ) of A1 on the cyclic voltammograms of aniline polymerization were recognized as an index indicating the relative loading of PAN [10]. In addition, the results and discussion of Figs. 1–3 indicate that the mechanism of aniline polymerization is, most likely, independent of the presence of IrCl3 ·xH2 O. Accordingly, the dependence of ip of peak A1 on the cycle number of CV is presumably employed to evaluate the effects of IrCl3 ·xH2 O on the rate of aniline polymerization in this work. Typical results are shown in Fig. 4. Note that on every curve, there exists an induced period during the initial 18–24 cycles since peak currents of A1 increase relatively slowly within these cycles. On the other hand, the slope of peak currents of A1 against the growth cycle becomes larger and maintains approximately constant after a significant amount of PAN is formed on the graphite substrate during the induced period. The above results are attributable to that the density of active centers (polaronic radicals) on the graphite substrate is relatively low during the initial potential cycling of aniline polymerization. On the other hand, the density
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of active centers is considered to be approximately constant during the propagation of PAN when a significant amount of this material has been formed on the substrate. Therefore, an induced period is found in the initial growth of PAN. Note that there is a linear region on every curve in Fig. 4 and the slope of the linear region initially increases, reaches the maximum (at 0.2 mM IrCl3 ·xH2 O), and then decreases with increasing the concentrations of IrCl3 ·xH2 O in the growth solutions. These results indicate that the rate of aniline polymerization reaches a maximum when the growth solution contains 0.2 mM IrCl3 ·xH2 O. This phenomenon is attributable to that at potentials above ca. 600 mV, the iridium species at the interface of PAN and electrolyte are oxidized to a high oxidation state (i.e. Ir(VI)), favoring the formation of aniline radicals and promoting the polymerization of aniline. On curves 2–5, however, a gradual decrease in slope is found at cycle number of CV greater than ca. 50, especially the forming solutions containing the maximal concentration of IrCl3 ·xH2 O. This is likely due to the fact that, at potentials above ca. 600 mV, oxyiridium species within the polymer matrix are also oxidized to the high oxidation state, which renders an irreversible oxidation of PAN (see Fig. 7), resulting in destroying the structure of PAN and decreasing the rate of PAN propagation. On the basis of the above results and discussion, the maximal rate of aniline polymerization occurring at the growth solution containing 0.2 mM IrCl3 ·xH2 O should be the result of a combination of the above two factors. 3.2. Pseudocapacitance evaluation Pseudocapacitance of an EC capacitor arises mainly from the redox transitions of the electroactive species within the electrode materials [13–15] and the performance of an EC capacitor is strongly dependent on the electrochemical characteristics of the redox transitions within these materials [13,15]. Therefore, voltammetric responses and charge (q∗ ), from a cyclic voltammogram measured in an inert electrolyte, are employed to evaluate the electrochemical characteristics of PAN. Furthermore, in our previous works [15,27], the pseudocapacitance of an electrode material can be calculated from its voltammetric charge on either positive or negative sweeps of a CV curve measured in an inert electrolyte. Accordingly, the cathodic voltammetric charge between −200 and 750 mV on the negative sweep of a CV diagram measured in a 0.5 M H2 SO4 solution is employed to indicate the relative amount of electroactive sites involving the redox transitions within the Ir-doped PAN films in the present work. Typical cyclic voltammograms measured at 50 mV s−1 in a 0.5 M H2 SO4 solution for the polymer films prepared from the growth solutions containing 0, 0.1, 0.2, 0.5, and 1 mM IrCl3 ·xH2 O are shown as curves 1–5 in Fig. 5. Note that the voltammetric behavior of pure PAN in an inert solution is very similar in shape to its polymerization
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Fig. 5. Voltammetric behavior of various Ir-doped PAN films in 0.5 M H2 SO4 at 25◦ C and 50 mV s−1 . These films were grown from the solutions containing 1 M HCl, 0.2 M aniline, and (1) 0; (2) 0.1; (3) 0.2; (4) 0.5; and (5) 1.0 mM IrCl3 ·xH2 O.
responses. On the other hand, from a comparison between curve 1 in Fig. 5 and the curve of the 60th cycle in Fig. 1a, two features should be notified. First, voltammetric currents of the final growth cycle (i.e. the 60th cycle in Fig. 1a) are much larger than that of the same electrode measured in an inert electrolyte (see curve 1 in Fig. 5). Second, a shoulder wave appears at ca. 730 mV on the negative sweep on curve 1 in Fig. 5 while this wave was not found on the growth diagrams of PAN. The former result reveals that the voltammetric currents of polymerization must be simultaneously contributed by the oxidation/reduction of aniline in the solution and the redox transitions of the already-deposited PAN. Therefore, the shape of CV curves for the PAN formation should be determined by a combination of the electrochemical characteristics of the already-deposited PAN and the kinetics of aniline polymerization in the forming bath. Accordingly, voltammetric responses for the redox transitions of PAN are very similar to its polymerization behavior. The latter result suggests that some iridium species were intercalated into the PAN film since the redox transitions of hydrous iridium oxides occurred in the highly positive potential region [29]. Note that from a comparison of all CV curves in Fig. 5, several features have to be mentioned. First, the maximal voltammetric currents occur at the PAN film formed from the growth solution containing 0.2 mM IrCl3 ·xH2 O. In addition, voltammetric charges integrated from the negative sweeps of these curves also reach the maximum at the same concentration of IrCl3 ·xH2 O (see Fig. 6). These results are consistent to the data in Figs. 1 and 4; results indicates the fact that ip of A1 is proportional to the loading of PAN on the graphite substrate and is reasonably indicative of its film thickness. Second, the potential window of various Ir-doped
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Fig. 6. Voltammetric charges of various Ir-doped PAN films in 0.5 M H2 SO4 against the concentration of IrCl3 ·xH2 O in the growth solution.
PAN films in the conducting state on the negative sweep of CV becomes wider with the concentration of IrCl3 ·xH2 O in the growth solution. The operative potential range of an Ir-doped PAN film for the EC capacitors is thus expected to be wider than that of a pure PAN film. Third, the voltammetric curve and the redox transition of peaks C1 /A1 , respectively, become smoother and unclear with increase in the concentration of IrCl3 ·xH2 O in the growth solution. Since the voltammetric responses of an ideal capacitor possess a rectangular cyclic voltammogram, the Ir-doped PAN films are considered as a more promising material in the application of EC capacitors. The charging and discharging behavior of various PAN Ir-doped films was examined by chronopotentiometry and typical results measured from 0 to 700 mV in 0.5 M H2 SO4 at 200 A cm−2 are shown in Fig. 7. Note that the slope of every chronopotentiometric curve is potential-dependent. Since the slope of a chronopotentiogram of an ideal capacitor should be potential-independent and maintains a constant value at a specified current density, the electrochemical properties of these polymer films must be improved to be more applicable in the EC capacitors. Also note that at potentials above 600 mV on curves 2–5, the E–t responses during the cathodic discharging process do not behave as a mirror-like curve of the anodic charging process while this phenomenon was not found for a pure PAN film. These results indicate that an irreversible oxidation occurs at potentials above 600 mV on the positive sweeps of chronopotentiograms for all Ir-doped PAN films. Accordingly, the electrochemical reversibility of the redox couple(s) in the high potential region (above 600 mV) within a pure PAN film is higher than that within the Ir-doped PAN films. The above irreversible oxidation is attributable to the formation of oxyiridium species in the high-oxidation state (i.e. Ir(VI))
Fig. 7. Chronopotentiomograms of various Ir-doped PAN films measured at 200 A cm−2 in 0.5 M H2 SO4 at 25◦ C. These films were grown from the solutions containing 1 M HCl, 0.2 M aniline, and (1) 0; (2) 0.1; (3) 0.2; (4) 0.5; and (5) 1.0 mM IrCl3 ·xH2 O.
[28,29] which oxidizes and destroys the structure of PAN during the anodic charging process. If this is the case, the Ir-doped PAN films should be less stable than a pure PAN film at potentials above ca. 600 mV although pseudocapacitance of the former films is much larger than that of the latter. In our previous work [15,27], the average pseudocapacitance of an electroactive film can be calculated from the
Fig. 8. Pseudocapacitance from chronopotentiomograms (Ccp ) against the pseudocapacitance from q∗ (Cq ∗ ) for various Ir-doped PAN films in 0.5 M H2 SO4 .
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following equation: Ccp =
i i ≈ dV /dt 1V /1t
In addition, from the chronopotentiograms of Ir-doped PAN films in Fig. 7, the charging responses at potentials above 600 mV is affected by the irreversible oxidation of PAN due to the presence of oxyiridium species in the high oxidation state (Ir(VI)). The mean pseudocapacitance of these PAN Ir-doped films was thus calculated from the results of the cathodic discharging curves (denoted as Ccp ). Note that data
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shown in Fig. 6 were also used to evaluate the mean pseudocapacitance of these polymer films (denoted as Cq ∗ ) on the basis of the following equation [15,27]: Cq ∗ =
|q ∗ | cathodic voltammetric charge ≡ potential range 1V
The relationship between Ccp and Cq ∗ for these polymer films was shown in Fig. 8. Note the linear relationship between Ccp and Cq ∗ , indicating that pseudocapacitance of the Ir-doped PAN films is minorly contributed by the oxyiridium species within the PAN matrix although the electrochemical
Fig. 9. SEM photographs of the Ir-doped PAN films grown from the solutions containing (a, d) 0; (b, e) 0.2; and (c, f) 1.0 mM IrCl3 ·xH2 O. The morphologies of photographs a–c and d–f were, respectively, enlarged under 3000 and 7000 times.
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of the same electrode measured by chronopotentiometry at a very low current density should be larger than that obtained by cyclic voltammetry. On the other hand, the fact that the increase in Ccp is about 1.5 times of that in Cq ∗ is not only attributed to the quasi-equilibrium charging/discharging processes but also attributed to the difference in potential windows between chronopotentiometry and cyclic voltammetry. Since the potential window for CV includes the conversion of PAN from the insulating state to the conducting state, pseudocapacitance of PAN in this potential region should be much lower than that of the other redox transitions in the conducting state. Accordingly, Cq ∗ calculated from voltammetric charges of CVs is much lower than Ccp evaluated from the slope of chronopotentiograms. 3.3. Textural characteristics
Fig. 10. XRD patterns of the PAN Ir-doped films grown from the solutions containing (a) 0 and (b) 1.0 mM IrCl3 ·xH2 O.
characteristics of PAN were indeed significantly changed by doping iridium species. Thus, pseudocapacitance of these Ir-doped PAN films should mainly come from the redox transitions of various oxidation structures of PAN and the amount of oxyiridium species within the polymer matrix should be very small. The latter inference is supported by the XPS data that the amount of iridium is <2 at.% (a/o) when PAN was formed from the growth solution with 1.0 mM IrCl3 ·xH2 O. Also note that the linear line in Fig. 8 does not pass the origin and the slope of this line is ca. 1.5. Since the DC current density employed in chronopotentiometry is very low (200 A cm−2 ), charging and discharging processes are believed to be performed under a condition close to the quasi-equilibrium state. Thus, the pseudocapacitance
The surface morphologies of all Ir-doped PAN films were examined by SEM. Typical images of the polymer films resulting from the growth solutions containing 0, 0.2, and 1.0 mM IrCl3 ·xH2 O are shown as Fig. 9a–f, respectively. Note that the surface morphology of the pure PAN film is relatively compact and its particle size is relatively large. On the other hand, the addition of IrCl3 ·xH2 O in the growth solution initially renders a rougher morphology of the polymer film (see Fig. 9b and e) with smaller particle sizes. However, a more compact morphology with larger particle sizes is obtained with the continuous addition of IrCl3 ·xH2 O into the growth solutions (see Fig. 9c and f). Moreover, PAN with the smallest particle size is obtained from the growth solution containing 0.2 mM IrCl3 ·xH2 O. Note that from the results and discussion of Figs. 2 and 3, Ir(IV) is further oxidized to Ir(VI) at potentials positive to 600 mV, rendering negative shift in onset potential for the formation of aniline radicals. In addition, in Fig. 4, the slope of iP of aniline polymerization against growth cycles reaches a maximum (i.e.
Fig. 11. An XPS spectrum of the Ir-doped PAN film grown from the solution containing 0.05 mM IrCl3 ·xH2 O.
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the highest density of active centers) when the growth solution contains 0.2 mM IrCl3 ·xH2 O, indicating the presence of Ir(VI) at the interface of PAN and electrolyte favoring the formation of aniline radicals (i.e. active centers) on the polymer. These phenomena are further supported by the SEM photographs that the Ir-doped PAN films with the smallest particle size results from the highest density of active centers on this polymer film. The crystalline information of all polymer films was examined by the X-ray diffraction (XRD) method. Typical XRD pattern of the polymer films grown from the solution containing 1.0 mM IrCl3 ·xH2 O is shown as curve b in Fig. 10. In addition, the XRD pattern of a graphite substrate without any coatings is also shown as curve a in Fig. 10 for comparison. Note that diffraction peaks corresponding to graphite (denoted as ×) are clearly found on these two spectra while only very small and unclear diffraction peaks corresponding to IrO2 are observed on curve b. The above results may be due to a combination of the following two reasons:
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1. The already-deposited iridium oxide on various Ir-doped PAN films has a microcrystalline and/or amorphous structure. 2. The amount of iridium oxide within the polymer matrix is too small to form a well-crystallined material. A typical XPS spectrum of an element survey for the polymer grown from a solution containing 0.05 mM IrCl3 ·xH2 O is shown in Fig. 11. Note the presence of Ir peaks, indicating that iridium species has been doped within the polymer film when PAN was grown from the solution containing the lowest concentration of IrCl3 ·xH2 O. Also note that the binding energies of C, N, O, and Ir were examined to elucidate the change in chemical environments of PAN due to the presence of Ir within the polymer matrix. Typical XPS spectra of C 1s, N 1s, O 1s, and Ir 4f7/2,5/2 for the polymer film formed from the growth solution containing 0.05 mM IrCl3 ·xH2 O are shown as curve 2 in Fig. 12a–d, respectively. Moreover, the XPS spectra of C 1s, N 1s, and O 1s for pure PAN are also shown as curve 1 in Fig. 12a–c, respectively, for comparison. Note that, in order to avoid the influence of
Fig. 12. XPS spectra of (a) C 1s; (b) N 1s; (c) O 1s; and (d) Ir 4f7/2,5/2 for Ir-doped PAN films grown from solutions containing (1) 0 and (2) 0.05 mM IrCl3 ·xH2 O.
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synthesis conditions on the oxidation state of PAN, the final oxidation state of both PAN and Ir-doped PAN films was surely remained at the insulated state (i.e. the leucoemeraldine form). This requirement was carried out by employing a constant end potential of CV (equal to −200 mV) during the electrochemical polymerization of aniline since the oxidation state of PAN is strongly dependent on the synthesis conditions (e.g. the end potential of CV in this work). In Fig. 12d, the binding energy of Ir 4f7/2 is centered at ca. 61.6 eV, indicating a positive shift in binding energy (1.0 eV) of Ir in comparison to its metallic state. Thus, the oxidation state of iridium within the polymer matrix should be above 0. In Fig. 12a, the binding energy of C 1s is not significantly affected by the presence of iridium within the polymer matrix while the binding energy of N 1s is skewed to the higher binding energy side (see Fig. 12b). Moreover, the binding energy of O 1s is not significantly shifted by the presence of Ir, while it rendered a change in the distribution of oxygen atoms in different binding energies, likely due to the formation of hydrous iridium oxide. From the above results and discussion, there should exist a bonding between N and Ir (although relatively weak) when iridium oxide is doped into PAN.
4. Conclusions The rate of aniline polymerization was determined by a combination of (i) the formation of aniline radicals promoted by the presence of Ir(VI) at the PAN-electrolyte interface and (ii) the degradation of PAN by oxy-Ir(VI) species within the polymer matrix from the results of voltammetric and chronopotentiometric studies. The rate of polyaniline propagation reached a maximum when it was prepared from the growth solution with 0.2 mM IrCl3 ·xH2 O. The linear relationship between Ccp and Cq ∗ revealed that pseudocapacitance of the Ir-doped PAN films mainly came from the redox transitions of PAN in different oxidation structures. From the results of voltammetric and SEM studies, the presence of suitable amount of Ir(VI) species at the PAN-electrolyte interface, i.e. the solution containing 0.2 mM IrCl3 ·xH2 O, rendered the highest density of electroactive aniline radicals on the PAN film, resulting in a rougher morphology of the Ir-doped PAN film with the smallest particle size. There should exist a bonding between N and Ir when iridium oxide was doped into PAN from the XPS results. X-ray diffraction patterns revealed a microcrystalline and/or amorphous structure of the doped Ir oxide within the polymer.
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