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J. Phys. Chem. B 2000, 104, 10522-10527
Hybrid Materials Polypyrrole/PW12O403-. 1. Electrochemical Synthesis, Kinetics and Specific Charges T. F. Otero,* S. A. Cheng,† and F. Huerta Laboratorio de Electroquı´mica, Facultad de Quı´micas, UniVersidad del Paı´s Vasco, P.O.Box 1072, E-20080 San Sebastia´ n, Spain ReceiVed: February 11, 2000; In Final Form: August 11, 2000
The potentiostatic synthesis and microgravimetric kinetics of hybrid materials polypyrrole/PW12O403- has been investigated. Every electrogenerated film was controlled by cyclic voltammetry. Charge consumed during polymerization, weight of the electrogenerated films, charge stored in the film, weight of the film after charge/ discharge cycles, productivity of the electropolymerization charge (mg mC-1) and specific charge (mA h g-1) stored in the material were obtained for every film. The influence of the variables potential of polymerization, pyrrole or polyoxometalate concentrations, temperature, and polymerization time was studied. Whatever the conditions of synthesis, a constant productivity of 1.9 × 10-3 mg mC-1 was obtained. Every electrogenerated film undergoes a weight loss up to 80% during the initial cathodic voltammetric control. The insoluble part of the films electrogenerated at increasing potentials stores an almost constant specific charge near 100 mA h g-1, showing that increasing rates of degradation processes, usually found in other electropolymerization processes, are not present in this system. The high reaction order of the kinetics related to the polyoxometalate concentration was attributed to the formation of adducts polyoxometalate-pyrrole, adducts being the electroactive species during electropolymerization. Results show that the mechanism of the electropolymerization process does not change in a significant way when the conditions of synthesis are changed.
1. Introduction Some transition metals in their respective highest oxidation states can form metal-oxygen cluster anions, usually known as polyoxometalates. Although the first polyoxometalates were synthesized in the past century, it has been in the past decades when considerable attention has been paid to them. These species are interesting not only for their molecular and structural diversity, but for their significance in different fields such as electrochemistry, catalysis, medicine or materials science. In particular, the catalytic redox activity of polyoxometalates is characterized by the high stability of their multiple oxidation states and by the possibility to modify their redox potentials through the substitution of the heteroatoms contained. Dissolved polyoxometalates or electrode surfaces modified with these macroanions seem suitable catalysts for several electrochemical reactions, such as nitrite,1 bromate,2 H2O2,3 and hydrogen reduction.4 In addition, polyoxometalates present significant photo-5 and electrochromism,6 which makes them attractive materials for some technological applications. The use of polyoxometalates in the field of electrically conductive polymers is, in particular, a very exciting subject. It is commonly accepted that most polyoxometalates can be easily incorporated as dopants inside polymer matrixes.7,8 Thus, by electropolymerization of the monomer unit (pyrrole, aniline, thiophene, etc.) in the presence of small amounts of the macroanion, a hybrid organicinorganic material can be synthesized. Polypyrrole itself is known to be an electrochromic material and is also able to store electric charges. Both properties have been investigated exten* Corresponding author. E-mail:
[email protected]. † Permanent address: Department of Chemistry, Zhejiang University, Hangzhou 310027, China.
sively during recent years due to the possibilities of application to several technologic fields (see refs 9-11, and references therein). Considering those evidences, specific hybrid materials formed by polypyrrole doped with polyoxometalates are expected to combine the redox properties inherent to any of the two independent constituents. In fact, these hybrids have been already employed in many electrocatalytic reactions1,12-14 and their possibilities in the field of charge storage devices have been also pointed out.15,16 Despite the great technological expectancies opened by those new materials, little attention has been paid to the kinetics of the production processes and how the conditions of synthesis act on the properties of the attained hybrid. Bidan et al.7 studied the electrosynthesis of the material on glassy carbon electrodes by anodic oxidation of the monomer under consecutive potential cycling, stating that the electrochemical behavior of the heteropolyanion inside the hybrid is different from that shown in bulk solutions. Shimidzu el al.17 focused on the electrochromic properties of the hybrid material. They electrogenerated the hybrid material on both ITO and platinum electrodes by anodic polarization at a constant potential. In both cases they observed the presence of the polyoxometalate in solution after cathodic reduction in neutral solution, which was attributed to an undoping process. This undoping does not occur in acidic media. Similar results were obtained by Dong et al.14 using electrogenerated (by cyclic voltammetry) phosphomolibdic anion doped polypyrrole. The growth of the hybrid material was attempt to be followed through the evolution of the current density on the maxima of the consecutive voltammograms, being the number of voltammograms or the concomitant consumed charge the only studied variable.13,15 Nowadays, it has been well stated that both
10.1021/jp000553w CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
Hybrid Materials Polypyrrole/PW12O403chemically or electrochemically initiated polymerization of basic conducting polymers (conducting polymers doped with small anions) are complex reactions where simultaneous processes coexist.11 The relative rates of the simultaneous processes change in a different way as a function of the chemical and electrochemical variables of synthesis. The final result is a mixed material, whose composition and properties are a function of the conditions of synthesis. In this context our aim is to study the influence of those variables of synthesis on the production of the hybrid material, followed by “ex situ” microgravimetric determination of the obtained dry material at different polymerization times. Moreover we will follow a property of each electrogenerated film, the charge stored per unit of mass of the material, which can be related to most of their physical or electrochemical properties. We are also interested in clarifying the origin of the polyoxometalate present in solution after electrochemical reduction of the material. Good understanding of the significance of the synthesis variables on the properties would allow the generation of tailored materials for specific applications. To check the influence of the conditions of synthesis on the properties of the material we have chosen, as a property of reference, the charge stored per unit of weight for every electrogenerated material. The whole work has been divided in two parts. In this first part we will deal with some questions about the electrochemical synthesis of hybrid materials polypyrrole/PW12O403-. The second part will be devoted to the characterization of the hybrid films and, in particular, we will discuss the film stability under potential cycling. 2. Experimental Section The polyoxometalate used in this work was H3PW12O40 supplied by Fluka. The monomer (pyrrole) was obtained from Jansen and was distilled at low pressure before use. The hybrid material polypyrrole/polyoxometalate was synthesized in freshly prepared acetonitrile (Panreac, p.a.) + 2% (v/v) ultrapure water solutions. The electrochemical behavior of the film was checked afterward in dry acetonitrile medium containing 0.1 M LiClO4 (Aldrich). All electrochemical experiments were performed in a classical nondivided cell. The working electrode was a platinum sheet with an area of 1 cm2. The counter electrode was a 3 cm2 stainless steel plate. Potentials were measured against the Ag/AgCl reference electrode and are presented in this scale. All the experiments were performed under nitrogen atmosphere. An M273 PAR potentiostat-galvanostat connected to a PS-5 IBM microcomputer and driven by means of the M270 software from EG&G was used for both the synthesis and electrochemical testing of the hybrid film. The film weight was evaluated with a 4504 MP8 Sartorius ultramicrobalance (10-7 g precision). 3. Results and Discussion 3.1. Electrochemical Polymerization. The electrochemical polymerization of pyrrole has been usually achieved either by potentiostatic oxidation or by cyclic scanning of the potential. Considering that a constant potential defines the rates of the possible simultaneous reactions, we have chosen the potentiostatic method to generate the films. Under these conditions, the electrochemical variable to take into account is the potential of synthesis. Figure 1 shows the effect of the potential on the weight of the formed material. The experiments were performed in solutions containing 0.1 M pyrrole and 5 mM PW12O403-. For potentials lower than 0.7 V, the polymerization rate is very
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10523
Figure 1. Dependence of the polymer weight on the potential of synthesis employed. [py] ) 0.1 M. [PW12O403-] ) 5 mM. T ) 22 °C. tpol ) 60 s.
low and any significant formation of polymer was not detected. Polymerization potentials above 0.7 V give rise to constant polymerization current densities, so much higher as increasing potentials of electrogeneration were used. It can be observed that the weight of the dried films increases linearly with the potential of synthesis. Smooth, uniform and adherent hybrid films were always obtained. The color of the film depends strongly on its thickness and varies from light blue for thin films to dark blue for thick (heavy) ones. The density of the just synthesized films was 2.95 g cm-3 and the film thickness can be varied between 1 and 30 µm (or more), either by increasing the potential of polymerization or the reaction time. By integration of the chronoamperogram recorded during the polymerization, the charge consumed to generate the material was obtained. The quotient between the polymer weight and the polymerization charge gives the productivity of the consumed charge as mg/mC. All the hybrids pPy/PW12O403synthesized at different potentials always show a similar productivity of 1.9 × 10-3 mg mC-1. This result points to a similar mechanism of polymerization in the studied potential range. This hypothesis has to be checked later by the constancy of a property of the attained material (e.g., the specific charge stored in each electrogenerated material). From the results presented in Figure 1, it could be deduced that high potentials of synthesis are appropriate to obtain thick films at identical times of polymerization. However, it is known that high potentials may cause the appearance of some side reactions, namely degradations, that can affect strongly the final properties of the material.11 Thus, it is desirable to reach a compromise that allows the synthesis of thick films at moderate times of reaction with little degradation (as low as possible). There are some methods to follow the degradation of conducting polymers, such as the measurement of a physical property directly related to the “quality” of the film. Particularly, some properties such as electrical conductivity, charge storage capacity (specific charge) or magnetic characteristics seem adequate for this purpose. 3.2. Electrochemical Characterization. To select the most appropriate potential of synthesis for the hybrid material, we will follow the evolution of the specific charge (from here referred to as ) stored in films as a function of the potential of synthesis. This was obtained by dividing the electric charge transferred during a complete charge (or discharge) process by the weight of the material. To measure this charge, we have integrated the current of a cyclic voltammogram in the potential
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Figure 3. Variation of the specific charge stored in the materials as a function of the potential of synthesis. Same conditions as in Figure 1. Figure 2. Cyclic voltammograms recorded for a hybrid film at 20 mV s-1 in 0.1 M LiClO4 acetonitrile solutions. First (f) and second (s) cycles. Potential of synthesis: 1.2 V.
range where the charge (or discharge) process takes place. Figure 2 shows cyclic voltammograms recorded in 0.1 M LiClO4 acetonitrile solution for a freshly synthesized hybrid material. The electric contact was done at 0.4 V and the potential was then swept to -0.8 V to achieve its whole reduction. During the first negative-going cycle, two partially overlapped cathodic peaks appear in the voltammogram at -0.28 and -0.48 V, respectively. Just when the current reaches the minimum at -0.48 V, a blue cloud falls down from around the electrode to the bottom of the electrochemical cell. During the reverse scan from -0.8 to +0.4 V, only one anodic peak appears. This peak is closely related with the cathodic one observed at -0.39 V during the second cathodic potential excursion down to -0.8 V, thus evidencing a reversible charge-discharge process undergone by the hybrid material. The charge involved during the first reduction scan in Figure 2 was 22.5 mC. Such a value contrasts with those measured for the reversible redox process in the second scan (Qox ) 16.5 mC and Qred ) 17 mC). So, charge excess close to 5.5 mC was obtained for the first reduction process. Moreover, the film weight decreased from 0.2122 mg before cycling to 0.0509 mg, 76% down after the two cycles. From these results, it can be stated that the hybrid undergoes a potential-induced loss of mass during the first cathodic polarization. The nature of this process will be studied in the subsequent paper. The quotient between the reversible redox charge related to the insoluble fraction of the material and its weight (obtained by ex situ ultramicrogravimetry) gives the specific charge stored in the stable (insoluble) fraction of the material. Figure 3 shows the plot of (measured for each of the films synthesized in Figure 1) against the potential of synthesis. Polymerization potentials above 0.8 V give rise to increasing specific charges. This tendency is maintained for potentials as high as 1.2 V, but remains almost constant from this value. Such observation points clearly to 1.2 V as the limit potential from which the degradation rate and the polymerization rate equalize keeping a constant mechanism of polymerization under the studied experimental conditions. This is an unexpected result because most of the electropolymerization reactions studied using smallsize counterions18 show maximum values at the lowest potential of synthesis, and decreasing exponential-like values at increasing potentials. This fact was explained in terms of the presence of increasing rates of parallel degradation processes
Figure 4. Evolutions of a hybrid material weight with the polymerization time from different polyoxometalate concentrations. E ) 1.2 V. [py] ) 0.1 M, T ) 22 °C.
at higher potentials. From these results it is deduced that the presence of the polyoxometalate avoids those degradation processes at increasing potentials of polymerization. The compromise mentioned above is reached at around 1.2 V. So, the following experiments will be performed with hybrid materials synthesized at this potential. 3.3. Analysis of the Polymerization Rate. Among the different variables of synthesis, apart from the polymerization potential, three of them were found to affect significantly the polymerization rate (rp) of the hybrid material: the monomer concentration, the polyoxometalate concentration and the temperature. The rate of polymerization can be evaluated through the mass increment produced per time unit when a given variable is changed. The experimental procedure followed to analyze the influence of the most significant variables of synthesis on rp involves several steps that have been described elsewhere.11 As an example, we will show the procedure employed to calculate the dependence of rp on the concentration of polyoxometalate (Figure 4). The weights of films synthesized at different concentrations of PW12O403- and different polymerization times were evaluated through individual experiments. Each experiment was performed with a just prepared solution and a clean platinum electrode. The electrode was polarized at 1.2 V for each specific polymerization time and the response current/time (chronoamperogram) was recorded. After integration, the charge consumed during polymerization was obtained. The coated electrode is extracted, rinsed, dried and weighed.
Hybrid Materials Polypyrrole/PW12O403-
Figure 5. (a) Logarithmic plot of the rate of polymerization against the concentration of pyrrole (b) and polyoxometalate ([) employed in the synthesis. (b) Dependence of log(rp) on 1/T.
As can be seen in Figure 4, the polymer weight follows straight lines along the polymerization time for every concentration of polyoxometalate employed. The slope of each line represents the polymerization rate, rp, expressed in mg s-1 at the concentration studied. From this figure, we can draw a double logarithmic plot of rp against the concentration of polyoxometalate and eventually obtain the reaction order related to PW12O403- from the slope of the resulting straight line. The procedure is repeated for the other significant variables (concentration of pyrrole and temperature). The influence of the mentioned three kinetic variables can be readily compared in Figure 5, where logarithmic plots of the polymerization rate versus the logarithms of pyrrole and polyoxometalate concentration (Figure 5a) and against 1/T (Figure 5b) have been represented. The plot in Figure 5a shows that the rate of polymerization follows a linear dependence on the concentrations of both monomer and polyoxometalate. The positive character of the slopes reveals that higher polymerization rates can be obtained by using high concentrations of both species, even though the significant differences in the slopes clearly shows that the effect of polyoxometalate on rp is much higher than that of pyrrole. On the other hand, the plot in Figure 5b shows an opposite dependence of the rate of polymerization with 1/T. Finally, from the results presented it can be deduced that one should operate with high concentrations of polyoxometalate and high temperatures in order to obtain high polymerization rates. Changes in pyrrole concentration have not significant influence on rp, particularly if we take into account the narrow concentration range where such concentration can be varied due to the low pyrrole solubility.
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10525 The quotient between the polymer weight of an electrogenerated film and the charge consumed during the polymerization process gives the productivity of the charge (expressed as mg/ mC). Hybrids pPy/PW12O403- synthesized under different experimental conditions always show similar productivities of 1.9 × 10-3 mg mC-1. This constancy points to the existence of the same polymerization mechanism whatever the studied conditions of synthesis: potential of polymerization, polyoxometalate or pyrrole concentration, or temperature. 3.4. Charge Storage Capacity. As mentioned above, the charge/discharge process for hybrid films can be easily followed by cyclic voltammetry. For applications in batteries, electrochromic devices or any other device where the material is submitted to consecutive flow of anodic and cathodic currents, it is required a material having stable behavior under consecutive charge/discharge processes. So, the weight of the film (after the first cycle of control, once an important fraction of the mass was lost) and the reversible redox charge involved in the concomitant voltammogram were employed to obtain the specific charge. This procedure was also followed in Figure 3. The effect of the three variables of synthesis on the specific charge is depicted in Figure 6. The dependence of on the concentration of pyrrole and polyoxometalate (Figures 6a and 6b) is very little. One can only observe a slight decrease in the specific charge at low concentrations of both species (below 0.1 M for pyrrole and 5 mM for PW12O403-). But no significant change of the specific charge is observed whatever the concentrations employed above these values. The temperature of synthesis has a more important effect on . The plot in Figure 6c shows a continuous rise of the specific charge as the temperature increases. Values near 100 mA h g-1 were obtained for materials synthesized at 35 °C. Finally, in Figure 7 we present the evolution of the specific charge with the film’s weight (after cycling) for all the films studied in this work. Although the hybrid materials were obtained in different conditions, it should be noted that there is a general tendency for . The modification of the synthesis conditions give increasing charge storage capacities for initial growing films. However, as the weight increases, the films range in narrower values of and, finally, it is observed that heavy films always present values close to 100 mA h g-1. This result seems interesting for the future technological applications of these hybrids, because of the possibility of synthesizing large quantities of material without any significant loss of its charge storage capability during growing, as was usually observed in other polypyrrole-based conducting polymers.11 It should be mentioned that similar values of the specific charge displayed by thick films of other polypyrrole-based materials could only be attained after optimization of the conditions of synthesis by means of a statistical design of experiments.19 The little influence of the conditions of synthesis on either, the productivity of the consumed charges (a characteristic of the polymerization process) and the specific charge stored in the obtained material (a property of the material) is another unexpected result. These facts support a practically unchanged polymerization mechanism under the studied conditions of synthesis. It is worth mentioning that during the synthesis of polypyrrole-based materials, the electrochemical polymerization coexists with parallel chemical polymerization, cross-linking and degradation processes (see refs 11 and 19, and references therein). There, any change in the variables of synthesis affect in a different way the relative rates of the parallel reactions, thus modifying productivities and/or specific charges.
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Figure 7. Plot of the specific charge obtained for all the materials synthesized against the weight (measured after some charge/discharge cycles).
electrochemical oxidation initiates the polymerization process. In this way, the concentration of the polyoxometalate has three main effects during the polymerization. (i) Influences strongly the formation of the electroactive species, (ii) acts as the supporting electrolyte and (iii) is the counterion required to keep the electroneutrality inside the formed polymeric film. These facts can explain the high reaction order related to the polyoxometalate concentration when compared to that of the monomer (slopes in Figure 5). The high and constant polymerization rates, the high specific charges stored in the stable fraction of the material and the possible presence of a soluble fraction in the electrogenerated hybrid, which will be characterized in a subsequent paper, open new technological possibilities for these polymeric materials. Synthesis of very thick films (several mm) until now unattainable for other polypyrrole-based materials, applications in batteries, or possible soluble materials of hybrids allowing applications by casting, spinning, etc on non conducting substrates can be now envisaged. 4. Conclusions
Figure 6. Influence of the variables of synthesis on the specific charge of the produced material: (a) pyrrole concentration; (b) polyoxometalate concentration; (c) temperature. Experimental conditions: (a) [polyoxometalate] ) 5 mM, T ) 22 °C; (b) [pyrrole] ) 0.1 M, T ) 22 °C; (c) [polyoxometalate] ) 5 mM, [pyrrole] ) 0.1 M.
This constant polymerization mechanism seems related to the strong influence of the polyoxometalate on the polymerization rate (Figure 5a). When the monomer was added to the solution containing the polyoxometalate, a softness change in color was observed. This change was earlier observed by Bidan et al.7 by means of optical spectroscopy and attributed to a reaction between the polyoxometalate and pyrrole to form adducts.
PW12O40(H2O)m3- + nPy S PW12O40(H2O)m-n (Py)n3- + nH2O This new adducts being the electroactive species whose
The electrogeneration of hybrid materials polypyrrole/ PW12O403- has been studied following the influence of the conditions of synthesis on the productivity of the charge consumed during polymerization and on the specific charge stored in each electrogenerated film. Under all the studied conditions, films thick enough to have industrial interest were synthesized. Three variables have been found to affect significantly the electropolymerization rate: on one hand, pyrrole and polyoxometalate concentrations and, on the other, the synthesis temperature. The most appropriate potential for the electrochemical synthesis of the hybrid seems 1.2 V. Lower potentials produce not fully active films for charge storage purposes. Materials synthesized between 1.2 and 1.6 V show a constant specific charge indicating the inhibition of the water discharge with the subsequent polymeric degradation and drop of . The polymerization rate, rp, was strongly affected by the concentration of the polyoxometalate due to the formation of adducts between the macroanion and the pyrrole. The adduct seems the electroactive species for the initiation of the polymerization. It was found that the specific charge exhibited by the material is a function of the variables of synthesis only for light films. Heavy films show values near 100 mA h g-1, whatever the conditions of synthesis employed. It has also been observed a substantial loss of weight during the first reduction process. The just synthesized film loses about 76% mass in one cycle, but
Hybrid Materials Polypyrrole/PW12O403this does not affect the capability to store electric charge in the remaining material. In the second part of the work we will try to gain some insight on the observed weight loss. Acknowledgment. The government of the Basque Country and CEGASA supported this work. References and Notes (1) Xi, X.; Dong, S. J. Mol. Catal. A 1996, 114, 257. (2) Cheng, L.; Zhang, X.; Xi, X.; Liu, B.; Dong, S. J. Electroanal. Chem. 1996, 407, 97. (3) Toth, J. E.; Melton, J. D.; Cabelli, D.; Bielski, B. H. J.; Anson, F. C. Inorg. Chem. 1990, 29,1952. (4) Keita, B.; Nadjo, L. J. Electroanal. Chem. 1988, 243, 87. (5) Yamase, T.; Ikawa, T.; Kokado, H.; Inoue, E. Chem. Lett. 1973, 615. (6) Tell, B.; Waguer, S. Appl. Phys. Lett. 1978, 33, 873. (7) Bidan, G.; Genies, E. M.; Lapkowski, M. J. Electroanal. Chem. 1988, 251, 297.
J. Phys. Chem. B, Vol. 104, No. 45, 2000 10527 (8) Keita, B.; Essaadi, K.; Nadjo, L. J. Electroanal. Chem. 1989, 259,127. (9) ElectroactiVe Polymer Electrochemistry; Lyons, M. E. G., Ed.; Plenum Press: New York, 1996; Part 2. (10) Handbook of Organic ConductiVe Molecules and Polymers; Nalwa, H. S., Ed.; Wiley & Sons: Chichester, U.K., 1999; Vol. 4. (11) Otero, T. F. In Modern Aspects of Electrochemistry; White, R. E., et al., Eds.; Kluwer Academic/Plenum Publishers: New York, 1999; Vol. 33. (12) Wang, P.; Li, Y. J. Electroanal. Chem. 1996, 408, 77. (13) Dong, S.; Liu, M. Electrochim. Acta 1994, 39, 947. (14) Dong, S.; Jin, W. J. Electroanal. Chem. 1993, 354, 87. (15) Sung, H.; So, H.; Paik, W. Electrochim. Acta 1994, 39, 645. (16) Go´mez-Romero, P.; Lira-Cantu´, M. AdV. Mater. 1997, 9,144. (17) Shimidzu, T.; Ohtani, A.; Aiba, M.; Honda, K. J. Chem. Soc., Faraday Trans. 1988, 84, 3941. (18) Otero, T. F.; Cantero, I. J. Power. Sourc. 1999, 81-82, 838. (19) Otero, T. F.; Cantero, I. J. Electrochem. Soc. 1999, 146, 4118.
