Disulfide Electrode Studied by Electrochemistry

although anions are also mobile to some extent. This has important consequence for lithium ion batteries application where lithium cation transpor...
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J. Phys. Chem. 1996, 100, 15848-15855

A New Polypyrrole/Disulfide Electrode Studied by Electrochemistry and the Electrochemical Quartz Crystal Microbalance Siyu Ye and Daniel Be´ langer* De´ partement de Chimie, UniVersite´ du Que´ bec a` Montre´ al, Case Postale 8888, Succursale Centre-Ville, Montre´ al, Que´ bec, Canada H3C 3P8 ReceiVed: September 19, 1995; In Final Form: June 15, 1996X

A new polypyrrole/disulfide electrode has been prepared by electrochemical oxidation of an aqueous potassium 2,5-dimercapto-1,3,4-thiadiazole (K2DMcT) solution at an electropolymerized polypyrrole chloride, PPy/ Cl-, film electrode. The formation of this new electrode material was investigated by cyclic voltammetry and chronoamperometry in combination with the electrochemical quartz crystal microbalance (EQCM). Following cycling in a DMcT2- solution, the cyclic voltammogram of a PPy/Cl- electrode changed significantly as the common redox waves of the PPy/Cl- electrode were no longer observed. Instead, a new set of anodic and cathodic waves appeared at more negative potential in addition to an irreversible anodic wave at about 0.2 V. The mass of a Au/PPy/Cl- electrode, immersed in a 50 mM DMcT2-/0.1 M KCl solution, increases much more than that of a bare Au electrode when the potential was switched from 0 to 0.5 V whereas no mass variation was noticed for a Au/PPy/Cl- electrode when DMcT2- was not present in the solution. In contrast, the cyclic voltammogram and the mass of a poly(N-methylpyrrole) chloride electrode did not change at all when the experiments are carried out in the same conditions as for the PPy/Cl- electrode. These results suggest that acid/base chemistry may occur between PPy and DMcT2- although hydrogen bond formation between the NH moiety on the pyrrole ring and the sulfur of the monobasic form of DMcT cannot be ruled out completely. This PPy-DMcT electrode has also been characterized by cyclic voltammetry, EQCM, and in-situ conductivity measurements. This new material has unique properties compared to PPy/Cl- and is characterized by a redox potential of about -0.45 V vs Ag/AgCl, and the oxidized material has a conductivity of 7.5 × 10-2 S cm-1. The EQCM study shows that, unlike the PPy/Cl- electrode for which the anions are the mobile species, cation transport is significant for the PPy-DMcT electrode during potential cycling although anions are also mobile to some extent. This has important consequence for lithium ion batteries application where lithium cation transport is required. The cyclic voltammetry and EQCM data of the PPy-DMcT electrode are consistent with a mechanism that involves a polymerization/depolymerization reaction of the DMcT species entrapped in the electrode matrix and whose species interact chemically with PPy.

Introduction Recently, several electrocatalysts have been introduced to enhance the charge transfer kinetics of some organic disulfides which can be used as a new class of solid-state cathode materials for high-energy density battery.1-10 These organic disulfide electrode materials possess many highly desirable features, such as low cost, low toxicity, good cyclability, and high energy density. The charge-discharge process for most of these cathode materials is based on intercalation chemistry. That is, the formation of S-S bonds takes place when the organic disulfide is electrochemically oxidized to a disulfide polymer, and the scission of S-S bonds occurs when the polymer is electrochemically reduced to monomeric units. These processes are accompanied by the movement of cations (Li+) into (upon reduction) and out (upon oxidation) of the polymer matrix. The formation and scission of S-S bonds during the redox cycle were investigated by in-situ and ex-situ near-edge X-ray absorption fine structure (NEXAFS) spectra recently.11 However, the redox kinetics of these materials have been found to be rather slow, which can lead to significantly lower power density and rate capability for an electrochemical energy conversion system.2 In order to improve the battery performance, an appropriate electrocatalyst is required to enhance the

redox reversibility. Thus, transition metal phthalocyanines,2 transition metal thiolate salts,2 and conducting polyaniline7,8 have been tested as electrocatalyst for these organic disulfides. Furthermore, it is well-known that polypyrrole (PPy) film can improve the kinetics of the two electron-two proton redox of the hydroquinone-benzoquinone couple.12 Accordingly, the initial aim of this work was to improve the redox kinetics of organic disulfides, specifically 2,5-dimercapto-1,3,4-thiadiazole (DMcT2-), by using a PPy electrode. However, we have recently discovered that a new electrode material is produced by electrochemical oxidation of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) at an electropolymerized polypyrrole (PPy) film.13 This paper reports extensively on the cyclic voltammetry, in-situ conductivity measurements, and electrochemical quartz crystal microbalance (EQCM) studies of this conducting PPy-DMcT electrode. The EQCM has recently been developed as an extremely versatile technique for in-situ monitoring of gravimetric changes occuring at an electrode surface.14,15 It has been applied to study metal, semiconductor and metal oxide surface,16-19 redox polymers,20 and conducting polymers.21-27 Here, it is used to monitor the formation of the PPy-DMcT composite and to study the ionic transport during redox cycling of the PPy-DMcT electrode. Experimental Section

* To whom correspondence should be addressed. E-mail: BELANGER. [email protected]. X Abstract published in AdVance ACS Abstracts, September 1, 1996.

S0022-3654(95)02750-X CCC: $12.00

Chemicals and Electrodes. Pyrrole and N-methylpyrrole (Aldrich) were freshly distilled before use. The dipotassium © 1996 American Chemical Society

A New Polypyrrole/Disulfide Composite Electrode salt of DMcT (Aldrich) and potassium chloride (Fisher) were used as received. Solutions were prepared for twice distilled water. Platinum disk (area ) 0.007 85 cm2), glassy carbon (area ) 0.0707 cm2), and tin oxide-coated glass (area ) 2 × 2.4 cm2) were used as working electrodes and were pretreated according to the procedure described before.28 The auxiliary electrode was a platinum flag, and the reference electrode was Ag/AgCl. All potentials hereafter are given relative to this reference electrode. Equipment. Electrochemical measurements were performed by using EG&G PAR Model 273 potentistat/galvanostat or a Pine Model RDE-4 bipotentiostat equipped with a Kipp and Zonen BD91 x-y-y′ recorder. All electrochemical measurements were carried out in a conventional one-compartment cell. All solutions used were purged with nitrogen prior to electropolymerization or electrochemical analysis. The electrochemical quartz crystal microbalance (EQCM) system consists of the Model EQCN-500 Electrochemical Quartz Crystal Nanobalance (ELCHEMA, Potsdam, NY) and a Model RDE-4 bipotentiostat (Pine Instruments Inc., Grove City, PA) and a Kipp and Zonen BD91 x-y-y′ recorder. Procedure. Polypyrrole chloride, PPy/Cl-, and poly(Nmethylpyrrole) chloride, PNMPy/Cl-, films were prepared potentiostatically or galvanostatically by anodic electrochemical deposition at 0.7 V or 1 mA cm-2, respectively, from an aqueous solution containing 0.1 M of the monomer and 0.1 M KCl. The thickness of the film was controlled by the charge passed during the electropolymerization. All films used were thoroughly washed and distilled water before being transferred to another electrolyte. The PPy-DMcT composite was generated by either potential scanning between -0.9 and 0.5 V or potentiostatically at 0.5 V in a 50 mM DMcT2-/0.1 M KCl solution. In-situ conductivity measurements were made with a double-band electrode assembly.29-32 Briefly, two electrodes (3.3 mm long, 25 µm wide, and separated by a Mylar spacer of 3.5 µm), called source and drain, are connected with polymer film, and a small fixed potential, Ed (10-20 mV), is maintained between them. At the same time, their potentials versus a reference electrode SCE, Vg, is varied, thus changing the state of charge of polymer and therefore its conductivity. When the polymer is in a conducting state, a significant drain current Id flows between the source and the drain, but when the polymer is insulating, the drain current is negligible. Id is directly proportional to the conductivity of polymer. Thus, a plot of Id vs Vg gives the relative conductivity as a function of potential. The EQCM system was used to simultaneously monitor the current and mass change occurring on the working electrode during the polymerization and the redox cycling. A gold-coated 10 MHz AT-cut quartz crystal (International Crystal Mfg. Co. Inc., Oklahoma City) was used as working electrode. It was sealed to a 20 mL glass cell by Tub&Tile Silicone Sealant (Canadian Tire Corp., Toronto, Canada). The geometric area of this working electrode was about 0.25 cm2, and it has an absolute mass sensitivity of 1.1 ng Hz-1. In order to apply Sauerbrey equation33 to the studied polymer film, the film must behave as a rigid, perfect elastic overlayer. If the polymer overlayer thickness is small compared to the thickness of the crystal and if the frequency change resulted from the overall mass loading is much smaller than the resonant frequency of the unloaded crystal, this rigid approximation is valid.21 In order to meet this rigid film requirement, the film thickness was limited to small values in the EQCM experiments. Results and Discussion Electrochemcial Preparation of the PPy-DMcT Composite Film. The electrochemical behavior of DMcT (in the

J. Phys. Chem., Vol. 100, No. 39, 1996 15849

Figure 1. Cyclic voltammograms for a bare platinum electrode (- - -) and a PPy-coated platinum electrode, first cycle (-‚-) and stable voltammogram (s) in a 50 mM DMcT2-/0.1 M KCl solution. Scan rate ) 10 mV/s.

protonated form) has been previously investigated at a gold electrode in nonaqueous electrolyte (0.1 M LiClO4/propylene carbonate).34 EQCM experiments have shown that electrochemical oxidation of monomeric DMcT yielded soluble dimeric species when the potential scanning was restricted to potential lower than 0.8 V vs Ag/Ag+ and that cycling to more positive potential led to the precipitation of a polymer on the electrode surface. Thus, it is expected that the redox processes involved when cycling a conducting polymer electrode such as polypyrrole in a DMcT2- solution are more complicated than when a simple redox couple such as Fe(CN)63-/4- is present in the electrolyte. Indeed, we have recently shown that a polypyrrole electrode does not act as an electrocatalytic electrode for DMcT2- and have proposed that instead a new composite electrode material was formed.13 The cyclic voltammogram of a 50 mM aqueous DMcT2-/ 0.1 M KCl (pH ) 9.3) solution at a bare platinum electrode shown in Figure 1 (- - -) is characterized by a potential peak separation, ∆Ep, between the anodic and cathodic peak potentials, of 830 mV at a scan rate of 10 mV/s. This large peak separation indicates rather slow electrode reaction kinetics. In a recent study,35 the cyclic voltammetry of another thiazole, 1-methyl-5-thiotetrazole in aqueous media, showed a smaller potential peak separation at the same scan rate, indicating faster electron transfer kinetics. Nonetheless, the electron transfer kinetics of such organosulfur compounds are relatively slow.1,2,13,35 A platinum electrode was then coated with a polypyrrole film according to the procedure described in the Experimental Section. A typical cyclic voltammogram for a PPy/Cl--coated electrode shows a sharp anodic peak at -0.15 V and a broad cathodic wave at -0.18 V (vide infra).13 Following a thorough rinse with water, the polypyrrole-coated electrode was transferred to the 50 mM DMcT2 solution. The cyclic voltammogram recorded during the first scan in the direction of negative potentials and shown in Figure 1 (-‚-) is characterized by a broad cathodic peak. Upon scan reversal, the anodic peak corresponding to the oxidation of polypyrrole is observed as well as another anodic peak at ca. 0.4 V which is attributed to the oxidation of DMcT2- at the polypyrrole electrode. During the second scan (not shown for clarity), the initial cathodic wave of PPy disappeared, and a set of cathodic and anodic waves

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Figure 2. Current-time and mass-time transients recorded following a potential step from 0.0 to 0.5 V for a Au/PPy/Cl- film (s) electrode in a 50 mM DMcT2-/0.1 M KCl solution, a Au/PPy/Cl- electrode in 0.1 M KCl in the absence of DMcT2- (- - -), and current-time (2) and mass-time (4) transients for a bare Au electrode in a 50 mM DMcT2-/0.1 M KCl solution.

appeared at -0.46 and -0.42 V, respectively. Also during the second scan, the anodic wave of PPy (seen at -0.15 V in the first scan) is shifted to more positive potential. In the subsequent scans, in addition to the redox waves centered at -0.44 V a persistent anodic wave is observed at 0.15 V, and a positive shift of the oxidation peak of DMcT2- is observed upon continuous cycling; eventually this peak appeared as a shoulder at ∼1 V on the increasing current (not shown). Furthermore, the redox peaks at -0.46 and -0.42 V and the anodic wave at 0.15 V were still observed when the electrode was transferred to a 0.1 M KCl solution, suggesting that a new PPy-DMcT electrode material was formed. The set of redox peaks centered at -0.44 V shows a peak separation of only 40 mV at a scan rate of 10 mV/s. The electrochemical behavior of the resulting film will be discussed in detail below. The EQCM technique was also used to study the redox reaction of DMcT2- at a Au/PPy/Cl- film electrode during a potential step experiment. The frequency and charge vs time responses for the constant potential polymerization of 0.1 M pyrrole in an aqueous 0.1 M KCl solution at the Au electrode (not shown) are similar to those reported previously for the polymerization of 0.05 M pyrrole in 0.1 M tetraethylammonium tosylate/CH3CN solution21 and of 0.1 M pyrrole in 0.1 M NaCl solution.22 Following the electropolymerization, the cell was washed thoroughly with distilled water to remove excess pyrrole monomer, and the cell was filled with an aqueous solution containing 50 mM K2DMcT and 0.1 M KCl. The PPy/Clelectrode potential was then step from 0 to 0.5 V. Note that at 0.5 V DMcT2- can be oxidized and PPy cannot be overoxidized.36 The current and mass vs time responses were recorded and shown are Figure 2. For comparison, the current and mass vs time responses for a naked Au electrode in the same solution containing DMcT2- and for a Au/PPy/Cl- film in 0.1 M KCl solution in the absence of DMcT2- are also shown in Figure 2. For the latter, a very small current following double-layer charging current is observed, and the mass remains fairly constant in the time period investigated. It should be noted that the PPy/Cl- film must be prepared as described above; i.e., the electropolymerization must be stopped by going from potential control to open circuit to avoid electrochemical reduction of the as-prepared PPy/Cl- film. Without this precaution, a mass change associated to the redox reaction of the polypyrrole layer would be observed (vide infra). By contrast, in the presence of DMcT2- in the electrolyte, the initial

Ye and Be´langer

Figure 3. Charge vs mass curve (from the data of Figure 2) associated with the electrochemical reaction of 50 mM DMcT2- at a PPy/Cl- film electrode. Electrolyte ) 0.1 M KCl.

curren spike is followed by a rising transient current similar to those seen for an electrochemical process involving a nucleation and growth mechanism.37 This is accompanied by a slow mass increase in the initial stage followed by a linear increase after about 60 s. The cyclic voltammogram of the resulting electrode in 0.1 M KCl solution shows features similar to the stable cyclic voltammogram obtained after cycling a PPy/Cl- electrode in the DMcT2- solution and shown in Figure 1 (s). By integration of the current during the electrolysis of the DMcT2- solution, the charge consumed during this process can obtained. The corresponding charge (which reaches 90 mC after 150 s) increases linearly with time (not shown), indicating that the charge builds up at a constant rate. From the charge vs time data the theoretical amount of material which should be deposited into the PPy film can be calculated from a modified version of Faraday’s law,21

Q ) nF∆m/M

(1)

where n represents the number of moles of electrons required to deposit 1 mol of monomer units and thus can be viewed as an electropolymerization efficiency,21 ∆m is the mass change during the electropolymerization, and M is the molar mass of DMcT2-. A plot of charge vs mass is shown in Figure 3 and is nonlinear for low masses and becomes linear for mass higher than 10 µg. The linear relationship observed in Figure 3 for the larger masses confirms the rigid layer approximation in these experimental conditions. This behavior indicates a variation in the number of electrons involved in the deposition of DMcT2unit, or the electropolymerization efficiency, during the deposition. In the earlier stage of the electrolysis (t < 40 s), the n value is nearly infinity because the charge consumed by the oxidation of the DMcT2- monomer to its radical and dimer does not lead to the deposition of DMcT2- oxidation products into the PPy film. This result may also indicate that in this earlier stage polymerization of DMcT2- does not occur as poly(DMcT) is insoluble in water. In another experiment, a 50 mM aqueous DMcT2- solution was electrolyzed at a PPy/Cl- film electrode for less than 40 s and then transferred to a 0.1 M KCl solution after thoroughly washing with water. In this case, the cyclic voltammogram of this electrode is similar to that of the PPy/ Cl- following electrolysis in the free DMcT2- solution, indicating that no DMcT2- or DMcT2- oxidation products are deposited into the PPy film. In the second stage of the electrolysis (see Figure 3 for 40 e t < 60 s), a n value of 6 can be computed, indicating a slow incorporation of DMcT2oxidation products into the PPy film, but most of the charge is

A New Polypyrrole/Disulfide Composite Electrode still consumed by the oxidation of DMcT2- to yield soluble products. In the third stage (t g 60 s), the charge increases linearly as the mass increases, and the n value is 2.7. Since the polymerization of DMcT2- involves two electrons,7b,34 an apparent n value higher than 2 indicates a relatively low electropolymerization efficiency for DMcT2- at the PPy film electrode. It should be noted that in the later calculation it is assumed that the oxidation of DMcT2- yields an insoluble polymer and that the polymer is not solvated. Indeed, solvation of the polymer (PPy or/and poly(DMcT)) would increase the mass deposited as a function of charge and give a high electropolymerization efficiency. On the other hand, the generation of soluble oligomers would lead to a decrease of the electropolymerization efficiency as these oligomers diffuse into the bulk solution. Moreover, the time delay before a significant mass change is noticeable on Figure 2 suggests that the reduction peak on the cyclic voltammogram of DMcT2- at a bare platinum electrode (see Figure 1) is associated with the reduction of dimers and other soluble products and not the polymer. The potential step experiment at a bare Au electrode in the DMcT2- solution (Figure 2) reveals that the mass change for an electrolysis of 150 s is significantly smaller (0.8 µg) than that found at the Au/PPy electrode (40 µg). This demonstrates that the deposition of a polymer derived form DMcT2- is not effective in these conditions. This difference between the Au and the Au/PPy electrode might be explained by the higher surface area of the PPy electrode. A substantial electric charge must be consumed before a critical concentration of oxidized DMcT2- species is reached and a significant mass change occurred at the Au/PPy electrode (see Figure 2). Presumably, this critical concentration cannot be reached at a bare Au electrode. However, we believe that another explanation ought to be considered as well. When DMcT2- is oxidized at the Au/PPy electrode, it will not only form dimers and oligomers, as in the case at the bare Au electrode, but some specific interactions (see below) between PPy and the oxidized form of DMcT will occur, and those will lead to the material that is characterized by the set of redox waves centered at about -0.45 V. The mass increases significantly because PPy is gradually transformed into a new PPy-DMcT electrode material. Further evidence that supports the formation of a new polypyrroleDMcT material have been obtained by UV-vis spectroscopy.13a In this earlier communication, the modification of the spectra upon electrolysis of a DMcT2- solution was attributed to the formation of a new PPy-DMcT composite whose electronic state and chemical structure differ from those of a PPy/Cl- film. That this kind of change is generated by the electrochemical oxidation of DMcT2- in PPy/Cl- film was further evidenced by the fact that the absorption spectrum and the electrochemical behavior of a PPy/Cl- film remained unchanged following immersion in a DMcT2- solution under open circuit conditions. Furthermore, the enhancement of redox kinetics for a soluble redox couple at electrode surface is usually accompanied by a decrease of the separation of anodic and cathodic peak potentials, ∆Ep. For example, ∆Ep for the benzoquinone/ hydroquinone redox couple decreases from 450 to 60 mV by coating a Pt electrode with a PPy layer.12a Clearly, a similar phenomenon is not occurring with the Pt/PPy-DMcT electrode system because the cyclic voltammogram shown by the full line in Figure 1 is also recorded in a pure 0.1 M KCl solution. Consequently, a new PPy-DMcT electrode material is likely formed. The shape and the shift to more negative potential (in comparison to a PPy/Cl- film electrode) of the cyclic voltammogram is reminiscent of PPy doped with immobile dopants.28,38 However, the immobile dopant hypothesis does not explain why

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Figure 4. Cyclic voltammograms of a platinum electrode in a 5 mM DMcT2-/0.1 M KCl solution in the absence (- - -) and presence of 0.5 M pyrrole (s, first cycle; -‚‚‚-, second cycle; -‚-, fifth cycle). Scan rate ) 10 mV/s.

the cyclic voltammogram of a poly(N-methylpyrrole) chloride, PNMPy/Cl-, is unchanged following potential cycling in a DMcT solution.13 These observations with the PPy/Cl- and the PNMPy/Cl- electrodes suggest that the chemical interaction between DMcT and PPy occurs via the nitrogen atom of PPy similarly to polyaniline and DMcT.8 Since the pKa’s of DMcT are 2.5 and 7.5,7c and that of oxidized PPy has been reported to be around 3,39a acid/base chemistry is likely to occur at the PPyDMcT electrode. On the other hand, the pKa of reduced PPy is expected to be higher, and consequently it will be protonated in the 0.1 M KCl (pH 6) solution used for this work. When PPy is oxidized electrochemically in a DMcT2-/0.1 M KCl (pH ) 9.3) solution, proton transfer from the pyrrole to DMcT2can occur. This process is followed by the oxidation of the monobasic form to the radical which can then couple to form the dimer.7c,34 This mechanism can also explain very well the small mass deposited at a naked Au electrode (Figure 3, 4) in the DMcT2- solution since in this instance the aforementioned acid/base chemistry cannot occur. This explanation would also suggest that the dimerization of dibasic DMcT2- (and further polymerization) is unlikely in our experimental conditions. Furthermore, the validity of the above mechanism is demonstrated by the lack of interaction between poly(N-methylpyrrole) and DMcT2-. Finally, hydrogen bond formation39b,40 can also occur between pyrrole and DMcT. A definite explanation of our results will require the use of the other analytical technique to determine the chemical structure of the species in the electrode material. A shift of the redox potentials of the anodic and cathodic waves of polypyrrole in comparison to Nsubstituted polypyrrole was explained in terms of hydrogen bonding.39b The latter mechanism is not operative with poly(N-methylpyrrole) because the methyl group prevents hydrogen bonding and the chemical interaction with DMcT. Thus, the redox peaks centered at -0.44 V correspond to the redox reaction of a new polypyrrole-based electrode material. The results described above demonstrate clearly that a new material is generated when a polypyrrole film electrode is cycled in an aqueous DMcT2- solution. In light of these observations, we have attempted to prepare a PPy-DMcT film in one step from a deposition solution containing both pyrrole and DMcT2-. Figure 4 shows the cyclic voltammograms for pyrrole and DMcT2- in 0.1 M KCl solution between -1 and 0.8 V. The first cycle (s) gave an anodic peak at 0.38 V, which corresponds to the oxidation of DMcT2- by comparison with the cyclic voltammogram of 5 mM DMcT2- in 0.1 M KCl (Figure 4,

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Figure 5. Cyclic voltammograms of PPy (2 µm)-DMcT (1 C/cm2) composite electrode in 0.1 M KCl solution. The scan rate is indicated.

- - -). A shoulder is also clearly noticeable at about 0.55 V. The intensity of the current decreased, and the anodic peak potential shifted positively with continuous cycling. There is no evidence of film formation even after several cycles in this potential range. In addition, the current-time transient resulting from a potential step between 0 and 0.8 V at a Pt electrode in a solution containing pyrrole and DMcT2- in 0.1 M KCl solution (not shown) is characterized by a steady decrease of current after the initial double-layer charging current, and no film was formed either. These results indicate that the polymerization of pyrrole is inhibited by the presence of DMcT2-. The inhibition of the polymerization of pyrrole by pyridine was previously observed41-44 and was related to the fact that the radical cation formed by the oxidation of pyrrole is intercepted by a nucleophilic pyridine molecule before it can couple with another pyrrole molecule or radical cation.41,45 In our case, the oxidation of DMcT2- which occurs before that of the pyrrole monomer yields a radical which may combine with another DMcT2- radical to form soluble dimeric species. The formation of a polymer from only DMcT2- is not possible unless a more positive potential is applied.34 On the other hand, when the potential becomes sufficiently positive, some pyrrole radical cations are generated, but the DMcT2- species will remove the proton from the nitrogen atom of the pyrrole radical cation before dimerization occurs and thus prevent further polymerization. Since direct electropolymerization of a PPy-DMcT composite cannot be achieved, the PPy-DMcT electrodes for which data are presented in the remainder of this work were prepared by electrochemical deposition of (i) polypyrrole at 0.7 V and (ii) DMcT at 0.5 V by using deposition solutions for whose the compositions are given in the Experimental Section. Electrochemical Behavior of PPY-DMcT Composite Electrode. Figure 5 shows the scan rate dependence of the cyclic voltammetric behavior of PPy (2 µm)-DMcT (1 C/cm2) composite electrode in aqueous 0.1 M KCl. Well-defined anodic and cathodic peaks are observed, and for this electrode the anodic and cathodic peak potentials are -0.495 and -0.505 V, respectively, with ∆Ep ) 10 mV at a scan rate of 20 mV/s. An increase of ∆Ep was observed upon increasing the scan rate. Figure 5 also shows that the ipa/ipc ratio is significantly smaller than 1. However, when the positive limit of the sweep is set at a less positive value, of about 0.1 V, the intensity of anodic and cathodic peaks is identical and ipa/ipc ) 1. The shape and the redox potential shift of the cyclic voltammograms for PPyDMcT composite electrode are similar to those for PPy doped with immobile dopants like MoS42-,28,38 dodecyl sulfate,46 and

Ye and Be´langer

Figure 6. Id-Ed curve for the PPy-DMcT composite film-coated dualmicroband electrode in 0.1 M KCl solution. Ed ) 10 mV and ν ) 10 mV/s.

Nafion.32,47 In order to evaluate the mechanism that controls the charge transport in the PPy-DMcT composite film, the peak current (ip) data of Figure 5 were plotted as a function of the scan rate, ν, in a logarithm format (not shown). The values of the slope for this PPy (2 µm)-DMcT (1 C/cm2) composite film are 0.78 for both anodic and cathodic peak current data, suggesting that the redox reaction has the mixed characteristics of a diffusion-controlled process and a surface wave process. Indeed, when the data below 100 mV/s were used, the ip vs ν plot shows a linear relationship, characterizing a surface wave process. On the other hand, when the data higher than 100 mV/s are used, the ip vs ν1/2 plot shows a linear relationship, characterizing a diffusion-controlled process. A similar analysis was carried out for another PPy-DMcT composite film which has an equal amount of charge consumed during DMcT deposition, but the thickness of PPy layer is twice as that shown in Figure 5. For ν ) 10-500 mV s-1, the slope of log ip vs log ν plot is 0.5, suggesting that the redox reaction was diffusion-controlled for a thicker PPy layer. These results indicate that the mechanism which controls the charge transport in PPy-DMcT composite film is film thickness dependent. In-situ conductivity measurements were performed on the PPy-DMcT composite film, and Figure 6 shows the Id-Vg characteristics for the PPy-DMcT-coated dual microband electrodes in 0.1 M KCl solution. From -1.0 to -0.2 V Id is very small and almost constant, indicating that the composite film is insulating in this potential window. For potential positive of -0.2 V, the abrupt increase of Id is characteristic of an increase of conductivity and Id reaches a maximum value at 0.2 V. It can be noted from Figure 6 that the Id at a given potential differs for the forward (from negative to positive potentials) and reverse scans. The Id values are higher on the reverse scan than on the forward scan. Similar behavior has been reported for polypyrrole32 and poly(3-methylthiophene).48 This hysteresis (Figure 6) is independent of scan rate and has been explained in terms of potential-dependent changes in the polymer structure.49,50 Assuming that the potential difference across the polymer film, Ed, is small enough to provide an approximately constant gradient, the conductivity of the film, σ(E), at any potential E can be obtained from the steady state current, Id(E), at either electrode using

σ(E) ) Id(E)d/EdA

(2)

where d and A are the interband spacing and the area of the

A New Polypyrrole/Disulfide Composite Electrode

Figure 7. Cyclic voltammogram (- - -) and mass-potential curve (s) for a (A) PPy/Cl- electrode and (B) PPy-DMcT composite electrode in 0.1 M KCl. The scan rate is 10 mV/s. The charge used to grow the PPy/Cl- film is 59 mC.

polymer film (taken approximately as the product of the film thickness and the length of the electrode), respectively. For instance, at 0.2 V, a σ value of 7.5 × 10-2 S cm-1 can obtained from the data of Figure 6. This value is comparable with that of a PPy-MoS42-MoS3 film,28,51 but 3 orders of magnitude lower than a PPy film.32,52 The EQCM was used to monitor the mass change during the redox cycling of PPy/Cl- and PPy-DMcT electrodes. A typical cyclic voltammogram for a PPy/Cl--coated electrode13 in aqueous KCl is shown in Figure 7A (- - -) and is characterized by a large capacitive envelope at the more positive potentials, a sharp anodic peak at -0.15 V, and a broad cathodic wave at -0.18 V. Figure 7A also shows that the mass remains constant between -1 and -0.7 V and then decreases (ca. 0.16 µg) between -0.7 and -0.2 V. Further scanning results in a substantial mass increase (ca. 0.8 µg). This behavior represents two different types of dominating ion transport, specifically cationic and anionic. The initial mass decrease in the film is due to the expulsion of cations (K+) and possibly solvent from the film, while the mass increase thereafter is related to the insertion of anion (Cl-) into the film. This potential-dependent dual ion transport behavior in PPy film has been previously noticed.22,23,54-57 The anion transport is quite common for a polymer doped with small anion such as Cl-; however, the cation transport is seldom noticed.25,55-57 A recent EQCM study22 on PPy/Cl- film electrode in aqueous solutions showed an apparent insensitivity of the mass uptake to the cationic composition of the electrolytes, but a quantitative study was not undertaken. In contrast, Rajeshwar et al.58 have recently demonstrated that cations are also mobile for PPy/Cl- films, although anion transport remains the dominant component as determined by a combination of EQCM, pH, and ion-selective electrochemical measurements. The possible reason for the cation transport in the PPy/Cl- film is that there may exist two types of Cl- ions in this oxidized polymer film. One is immobilized in the film, the so-called “deeply trapped” ions;59,60 another is highly mobile and responsible for the anion flux in the redox cycle. Since the cation and anion transport correspond to mass changes of only ca. 0.16 and 0.8 µg, respectively, the amount of the immobile (or “deeply trapped”) ions is much lower than that of mobile ion. It represents only 17% of the

J. Phys. Chem., Vol. 100, No. 39, 1996 15853

Figure 8. Plot of the mass as a function of the charge involved in the redox cycling of a (A) PPy/Cl- electrode (B) PPy-DMcT electrode in 0.1 M KCl (from the data of Figure 7).

total amount of ion existing in the polymer film if the solvent transport is not taken into account. The relative content of the two types of ions in the polymer film is certainly dependent on the electropolymerization conditions. Further quantitative analysis of the data can be done to obtain the molar masses of the species involved in the redox processes. This is accomplished by plotting the variation of mass as a function of charge, and Figure 8A presents such plot that can be constructed from the data of Figure 7A. The initial mass decrease in the first portion of the plot and corresponding to a potential range of -0.7 and -0.35 V has a slope of -36 g/mol, which is consistent with the expulsion of K+ cation (39 g/mol). This is followed by a region of mixed ionic transport, and finally for charge between 3 and 6 mC/cm2 (between 0.1 and 0.5 V), a slope of 29 g/mol can be computed. This is not exactly the mass anticipated if chloride anions were the only species involved (35.45 g/mol), but a lower value indicates that solvent molecules might be expelled from the polymer while the anionic species are incorporated. Finally, Figure 7A also shows that on the cathodic scan the ionic transport is reversed; a mass decrease (anions are expelled) is seen between 0.5 and -0.5 V and a mass increase (cations uptake) between -0.5 and 0.7 V. Figure 7B shows the cyclic voltammogram and masspotential curve for the PPy-DMcT electrode upon cycling in a DMcT2- free solution (e.g., 0.1 M KCl). Figure 7B differs from Figure 7A in that a continuous decrease of mass occurs when scanning anodically from -1.0 to 0.3 V (ca. 0.6 µg). At the end of the anodic scan a small mass increase is also clearly observed. When the direction of the scan is reversed, the mass increases slightly between 0.5 and 0.2 V, increases more abrupty between 0.2 and -0.7 V, and decreases at the negative end of the scan. The overall mass decrease observed upon electrochemical oxidation is most likely related to the expulsion of K+ cations from the polymer coating. This is confirmed by the data of Figure 8B that yields a slope of -42 g/mol for the data that encompass the anodic wave, between -0.55 and -0.15 V. It should be noted that the slope for the data in the earlier stage of the scan (e.g., between -1 and -0.6 V and when the current is still cathodic) is higher with a value of -79 g/mol. This mass decrease could be tentatively attributed to the expulsion of anions (see below) and is the followup of the decrease seen at the end of the cathodic scan. Conversely, the

15854 J. Phys. Chem., Vol. 100, No. 39, 1996 slight mass increase at the end of the anodic scan is attributed to the uptake of anions. It should be noted from Figure 8A,B that the mass value at a given potential differs for the positive and negative scans. A similar behavior has been noticed by other research groups.25,55-58 It was explained in terms of asymmetry of the anions and cations transport components in the two scan directions.58 This kind of hysteresis was also observed in the in-situ conductivity measurements of PPY film32 and PPy-DMcT composite film (vide supra). The electrochemical processes for polypyrrole films doped with small anions is commonly dominated by the movement of anions,61-63 but dominant cations transport has also been observed for polypyrrole films doped with large anionic dopants such as poly(styrenesulfonate),23,61 toluenesulfonate,63 and selfdoped polypyrrole.62 However, as mentioned above, the DMcT2- species do not act as immobile dopants because there is no reason to expect that DMcT2- does not play the same role with poly(N-methylpyrrole). Thus, we propose that, upon electrochemical oxidation of the polypyrrole units, the DMcTbased species (oligomers and polymers) that interact chemically to the pyrrole units (vide supra) undergo a polymerization process in such a way that cationic species are expulsed from the polymer matrix to maintain the electrical neutrality of the polymer film electrode according to the following overall reaction:

PPy/xCl-/nDMcT2-/(x + 2n)K+ f PPy+/{(DMcT)n}2-/K+ + (x + 2n - 1)K+ + xCl- + (2n - 1)e- (3) In addition, in a recent study it was suggested that the presence of a NH moiety slows down the movement of anionic species in the polymer matrix, thus favoring uptake of cations.40 The EQCM data also indicate that the DMcT species (monomer, dimer, or oligomer) are not expelled from the composite upon cycling since the mass-potential curve of Figure 8B is stable, even during prolonged potential cycling. Since the DMcT species are retained in the polymer matrix, the mass decrease of 79 g/mol seen in the early stage of the anodic scan is attributed to the expulsion of chloride anions and water molecules. Concluding Remarks Polypyrrole disulfide composite can be prepared by electrochemical oxidation of DMcT2- at an electropolymerized PPy/ Cl- film. The results presented here strongly suggest that a new material is formed when a PPy/Cl- film is cycled in solution containing DMcT2-, and the behavior of this electrode material is retained when it is cycled in a solution that does not contain DMcT2-. In this new material, chemical interactions, by acid/base chemistry and/or hydrogen bonding between polypyrrole and DMcT, might explain the difference in behavior of a naked Au and a Au/PPy electrode upon electrochemical cycling in a DMcT2- solution. However, many aspects of the electrochemical processes of DMcT-based electrode material7-10 are complicated and not currently well understood. Therefore, further characterization of the PPy/DMcT electrode material by spectroscopic techniques is required to confirm the chemical nature of the species being formed. This conducting PPyDMcT electrode shows a fast redox reaction rate. Its conductivity reaches a value of 7.5 × 10-2 S/cm. The EQCM study shows that although anions are also mobile, cation transport is significant for this composite electrode during potential cycling. Acknowledgment. This work was supported by grants from the Natural Sciences and Engineering Research Council,

Ye and Be´langer NSERC, of Canada, through a Research Grant and an Equipement Grant (EQCM) and a postdoctoral fellowship to S.Y. from la Fondation de l’Universite´ du Que´bec a` Montre´al. We also thank Walter Bowyer and Mary Elizabeth Clark from Hobart and William Smith Colleges for providing the double-band electrode required for the in-situ conductivity measurements. References and Notes (1) Visco, S. J.; Mailhe, C. C.; De Jonghe, L. C.; Armand, M. B. J. Electrochem. Soc. 1989, 136, 661. (2) Liu, M.; Visco, S. J.; De Jonghe, L. C. J. Electrochem. Soc. 1990, 137, 750. (3) Liu, M.; Visco, S. J.; De Jonghe, L. C. J. Electrochem. Soc. 1991, 138, 1891, 1896. (4) Doeff, M. M.; Visco, S. J.; De Jonghe, L. C. J. Electrochem. Soc. 1992, 139, 1808. (5) Doeff, M. M.; Lerner, M. M.; Visco, S. J.; De Jonghe, L. C. J. Electrochem. Soc. 1992, 139, 2077. (6) Visco, S. J.; Liu, M.; Doeff, M. M.; Ma, Y. P.; Lampert, C.; De Jonghe, L. C. Solid State Ionics 1993, 60, 175. (7) (a) Naoi, K.; Menda, M.; Ooike, H.; Oyama, N. J. Electroanal. Chem. 1991, 318, 395. (b) Sotomura, T.; Uemachi, H.; Takeyama, K.; Naoi, K.; Oyama, N. Electrochim. Acta 1992, 37, 1851. (c) Genie`s, E. M.; Picart, S. Synth. Met. 1995, 69, 165. (8) Oyama, N.; Tatsuma, T.; Sato, T.; Sotomura, T. Nature 1995, 373, 598. (9) Kaminaga, A.; Tatsuma, T.; Sotomura, T.; Oyama, N. J. Electrochem. Soc. 1995, 142, L47. (10) Tatsuma, T.; Sotomura, T.; Sato, T.; Buttry, D. A.; Oyama, N. J. Electrochem. Soc. 1995, 142, L182. (11) Yang, X. Q.; Xue, K. H.; Lee, H. S.; Guo, Y. H.; McBreen, J.; Skotheim, T. A.; Okamoto, Y.; Lu, F. J. Electrochem. Soc. 1993, 140, 943. (12) (a) Haimerl, A.; Merz, A. J. Electroanal. Chem. 1987, 220, 55. (b) Provencher, F.; Be´langer, D. Manuscript in preparation. (13) (a) Ye, S.; Be´langer, S. J. Electrochem. Soc. 1994, 141, L49. (b) Be´langer, D.; Ye, S. ACS Polym. Prepr. 1994, 35, 225. (14) Buttry, D. A. Electroanal. Chem. 1991, 17, 1. (15) Buttry, D. A. In Electrochemical Interface: Modern Techniques for in Situ Interface Characterization; Abruna, H. D. Ed.; VCH: Ne York, 1991; p 529. (16) Schumacher, R.; Borges, G.; Kanazawa, K. K. Surf. Sci. 1985, 163, L621. (17) Grabbe, E. S.; Buck, R. P.; Melroy, O. R. J. Electroanal. Chem. 1987, 216, 127; 1987, 223, 67. (18) Mori, E.; Baker, C. K.; Reynolds, J. R.; Rajeshwar, K. J. Electroanal. Chem. 1988, 252, 441. (19) Deakin, M. R.; Melroy, O. R. J. Electrochem. Soc. 1989, 136, 349. (20) Bruckenstein, S.; Wilde, C. P.; Shay, M.; Hillman, A. R.; Loveday, D. C. J. Electroanal. Chem. 1989, 288, 457. (21) Baker, C. K.; Reynolds, J. R. J. Electroanal. Chem. 1988, 251, 307. (22) Qiu, Y.-J.; Reynolds, J. R. J. Polym. Sci.: Polym. Chem. 1992, 30, 1315. (23) Lien, M.; Smyrl, W. H.; Morita, M. J. Electroanal. Chem. 1991, 309, 333. (24) Reynolds, J. R.; Pyo, M.; Qiu, Y.-J. Synth. Met. 1993, 55-57, 1388. (25) Kaufman, J. H.; Kanazawa, K. K.; Street, G. B. Phys. ReV. Lett. 1984, 53, 2461. (26) Orata, D.; Buttry, D. A. J. Am. Chem. Soc. 1987, 109, 3574. (27) Orata, D.; Buttry, D. A. J. Electroanal. Chem. 1988, 257, 71. (28) Girard, F.; Ye, S.; Laperrie`re, G.; Be´langer, D. J. Electroanal. Chem. 1992, 334, 35. (29) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. Soc. 1984, 106, 7389. (30) Paul, E. W.; Ricco, A. J.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 1441. (31) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 5133. (32) Ye, S.; Be´langer, D. J. Electroanal. Chem. 1993, 344, 395. (33) Sauerbrey, G. Z. Phys. 1959, 155, 206. (34) Naoi, K.; Oura, Y.; Iwamizu, Y.; Oyama, N. J. Electrochem. Soc. 1995, 142, 354. (35) (a) Malservisi, M. Me´moire de Maıˆtrise, Universite´ du Que´bec a` Montre´al 1995. (b) Shi, J.; Malservisi, M.; Marsan, B. Electrochim. Acta 1995, 40, 2425. (36) Beck, R.; Braun, P.; Oberst, M. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 967. (37) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 245. (38) Ye, S.; Girard, F.; Be´langer, D. J. Phys. Chem. 1993, 97, 12373.

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J. Phys. Chem., Vol. 100, No. 39, 1996 15855 (53) The mass change (increase) during the electropolymerization of the PPy/Cl- film is ca. 18.8 µg. If a value of 0.25 Cl- anion per pyrrole in PPy film is used, the molar mass unit per pyrrole can be calculated to be 73.86 g mol-1 by the sum of the molar mass of pyrrole (65 g mol-1) and the contribution from the Cl- anions (35.45 g mol-1 × 0.25). Therefore this mass value (ca. 18.8 µg) corresponds to 0.25 µmol of pyrrole for this PPy/Cl- film. (54) Zhong, C.; Doblhofer, K. Electrochim. Acta 1990, 35, 1971. (55) Miller, L. L.; Zinger, B.; Zhou, Q.-X. J. Am. Chem. Soc. 1987, 109, 2267. (56) Tsai, E. W.; Jang, G.-W.; Rajeshwar, K. J. Chem. Soc., Chem. Commun. 1987, 1776. (57) Duffitt, G. L.; Pickup, P. G. J. Phys. Chem. 1991, 95, 9634. (58) Bose, C. S. C.; Basak, S.; Rajeshwar, K. J. Phys. Chem. 1992, 96, 9899. (59) Mermilliod, N.; Tanguy, J. J. Electrochem. Soc. 1986, 133, 1073. (60) Tanguy, J.; Mermilliod, N.; Hoclet, M. J. Electrochem. Soc. 1987, 134, 795. (61) Baker, C. K.; Qiu, Y.-J.; Reynolds, J. R. J. Phys. Chem. 1991, 95, 4446. (62) Basak, S.; Bose, C. S. C.; Rajeshwar, K. Anal. Chem. 1992, 64, 1813. (63) Schmidt, V.; Heitbaum, J. Electrochim. Acta 1993, 38, 349.

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