Development of polymer–dopant interactions during electropolymerization, a key factor in determining the redox behaviour of conducting polymers Péter S. Tóth, Balázs Endrődi, Csaba Janáky & Csaba Visy
Journal of Solid State Electrochemistry Current Research and Development in Science and Technology ISSN 1432-8488 J Solid State Electrochem DOI 10.1007/s10008-015-2791-1
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Author's personal copy J Solid State Electrochem DOI 10.1007/s10008-015-2791-1
ORIGINAL PAPER
Development of polymer–dopant interactions during electropolymerization, a key factor in determining the redox behaviour of conducting polymers Péter S. Tóth & Balázs Endrődi & Csaba Janáky & Csaba Visy
Received: 9 January 2015 / Revised: 12 February 2015 / Accepted: 13 February 2015 # Springer-Verlag Berlin Heidelberg 2015
Abstract Investigation of ionic motion in connection with the redox transformation of conjugated polymers (CP) has always been at the leading edge of research. Motivated by recent proofs for the chemical bond formation between chlor i d e i o n an d α - p o s i t i on e d c a r b o n i n p ol y ( 3, 4– ethylenedioxythiophene) (PEDOT), comprehensive studies have been extended to another strongly electronegative halide (F−) and to another CP, polypyrrole (PPy). As the electrochemical quartz crystal nanobalance (EQCN) results proved, the movement of the bulky Bu4N+ cations has been exclusively experienced during the redox processes of both systems. Moreover, the decisive role of the anions being present in the polymerization solution in determining the redox capacity and, consequently, the maximum doping level of the films was evidenced. On the grounds of the systematic experiments, the strong and permanent chemical interaction of highly electronegative anions and the polymer has been demonstrated as a general phenomenon. Importantly, this observation requires the necessary reconsideration of specific polymer–dopant interactions and calls attention to the necessity of careful design of the polymerization procedure. Keywords PEDOT . Polypyrrole . EQCN . Electronegative anions . Polymer–dopant interactions This work is dedicated to Mikhail A. Vorotyntsev on the occasion of his 70th birthday in recognition of his valuable contribution to the exploration of the electrochemistry of redox-active films. P. S. Tóth : B. Endrődi : C. Janáky : C. Visy (*) Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Square 1. Sq. Aradi vt., Szeged 6720, Hungary e-mail:
[email protected] B. Endrődi : C. Janáky MTA-SZTE BLendület^ Photoelectrochemistry Research Group, Rerrich Square 1, Szeged 6720, Hungary
Introduction The interpretation of the redox transformation of conducting polymers (CPs) is the focus of interest since the discovery of this class of materials [1]. Charge transfer steps coupled with solvent and ion movements have received widespread attention since ion exchange properties may serve as basis for many applications [2, 3]. Electrochemical quartz crystal nanobalance (EQCN) technique is one of the most effectively applied methods for the investigation of the transport processes and to monitor the ionic and solvents movements at the polymer/solution interface [4]. In the simplest cases, when a CP is oxidized, anions migrate into the film, while during the reduction, anions move out from the film, so the CP behaves as an anion exchanger [5, 6]. Mixed ion exchanger layers are also well known, which behaviour depends mainly on the nature of the conducting electrolyte, employed during the synthesis [7, 8]. Moreover, bulky anions from an ionic surfactant (such as dodecyl-sulphate anion) or polyanions (such as polystyrene sulphonate) can be immobilized in the polymer matrix, leading to a CP acting as exclusively cation exchanger [9, 10]. Systematic studies showed that, depending on the potential applied during the electrodeposition cationic movements, they may also contribute to the charge compensation even in case of smaller anions [11, 12]. Such observations suggest that polymerization parameters can define the structure and the properties of the CPs. Moreover, it is generally accepted that the employed electrochemical synthesis method, the solvent and conducting salt—especially the size of the doping anion—are all important parameters affecting the morphology and the hydrophobic/hydrophilic character of polymer [13, 14]. For example, during the re-doping of poly (3,4– ethylenedioxythiophene) (PEDOT)—prepared in the presence of tetrabutylammonium tetrafluoroborate in acetonitrile—
Author's personal copy J Solid State Electrochem Table 1
The composition of the solutions and their notations in the text
Polymer
Electrolyte
Notation
Polypyrrole Poly(3,4-ethylenedioxythiophene) Poly(3,4-ethylenedioxythiophene) poly(3,4-ethylenedioxythiophene)
0.05 M Bu4NCl 0.05 M Bu4NF 0.1 M NaCl 0.1 M NaBF4
Ppy/Bu4NCl PEDOT/Bu4NF PEDOT/NaCl PEDOT/NaBF4
interfering with the chain-growth process in the solution phase [22, 23]. In this article, we further highlight the importance of nonorthodox ion-exchanging behaviour, which can be detected for both chloride ion-doped polypyrrole (PPy) and chloride/ fluoride ion-doped PEDOT; therefore, it is likely a general phenomenon to be considered with careful attention. Moreover, the effect of two synthesis parameters—such as the potential of the potentiostatic polymerization and the concentration of the conducting salt in the polymerization solution—on the redox behaviour is also demonstrated.
factors such as the size, the structural complexity and the charge on the dopant had significant effects on the efficiency of the doping processes [15]. Other studies revealed that in the complex character of the mass change, transferred charge showed that also mixed ionic transport is rather general during the transformation of PEDOT in the presence of tetraalkyl ammonium tetrafluoroborate in both water [16] and acetonitrile [17, 18]. Recently, we reported an anomalous behaviour of the chloride ion-doped PEDOT [16]. As it was demonstrated, a strong interaction between the polymer and this anion leads to a film, which—in opposite with the expectations—exhibits perfect cation exchange property. Furthermore, we demonstrated the constant Cl content independently from the oxidation state of the polymer layer by energy dispersive X-ray (EDX) microanalysis and the covalent bond between the chlorine and the carbon of an EDOT unit by attenuated total reflectance (ATR) FT-IR spectroscopy. The question is whether this behaviour is unique for the PEDOT-chloride system or the phenomenon is much more general, occurring also with other polymers, synthesized in the presence of strongly electronegative anions. Such dilemma is closely linked to the matter of irreversible oxidation (also called overoxidation) of CPs. It is well known that in nucleophylic (aqueous) solutions, the polymerization is often hindered or even impossible. It is also probable that partial overoxidation of CPs may occur much easier and much more frequently than it is usually reported or assumed [19–21]. Finally, as it was demonstrated, the presence of Brönsted or Lewis bases inhibits the film formation by 60
Experimental Materials 3,4-Ethylenedioxythiophene (Bayer AG) and pyrrole (SigmaAldrich) monomers were freshly distilled; tetrabutylammonium chloride (Bu4NCl), tetrabutylammonium fluoride (Bu4NF) and sodium tetrafluoroborate (NaBF4) (Sigma-Aldrich) were dried in vacuum at 80 °C; and sodium chloride (NaCl) (VWR International) was used as received. The aqueous solutions were prepared with ultrapure-deionized water (Millipore). Methods All electrochemical measurements were performed on a PGSTAT 302 (Autolab) instrument in a classical threeelectrode electrochemical cell. The reference electrode was an Ag/AgCl/3 M NaCl electrode having a potential 0.200 V vs. the standard hydrogen electrode (SHE). PEDOT films were deposited potentiostatically at E=+ 1.1 V potential from 0.01 M monomer and 0.05 M Bu4NF or 0.1 M NaCl or 0.1 M NaBF4 containing solutions respectively. PPy films were prepared at E=+0.9 V potential from 0.1 M monomer and 0.05 M Bu4NCl containing solution. The effect of the deposition potentials and the electrolyte
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Fig. 1 a Cyclic voltammogram of PPy/Bu4NCl measured at a sweep rate of 100 mV s−1 in 0.05 M Bu4NCl in water, together with the mass change recorded by EQCN. b Mass change vs. the integrated charge from the cyclic voltammogram in Fig. a
Author's personal copy J Solid State Electrochem Table 2 The virtual molar masses of the moving species (Bu4N+) determined from EQCN data measured for the PPy/Bu4NCl system
measurements of the PPy/Bu4NCl is presented in Fig. 1a together with the recorded mass changes. The latter reflects that the mass decreases monotonously upon oxidation and increases during the reduction. This behaviour indicates that bulky Bu4N+ cations seem to be relatively more mobile here than Cl− anions, leading to the cationic exchanger pattern of this film, too. To quantify this behaviour, mass change vs. charge curve—obtained after the transformation of the data in Fig. 1a—is presented in Fig. 1b. The slope of the Δm vs. ΔQ curve gives the value of the virtual molar mass of the moving species related to the charge compensation during the transformation. The values calculated at four different sweep rates (Table 2.) are all close—within ±14 %—to the molar mass of Bu4N+ ion.
M(Bu4N+)=242.4 g mol−1 v/mV s−1
M/g mol−1
10
209.9
25 50 100
260.9 207.7 276.7
concentrations were also studied, and details are given in BEffect of the synthesis parameters^. For EQCN measurements, charge density was limited to 50 mC cm−2 to avoid viscoelastic effects [24]. Alteration of the doping level was investigated at a Pt working electrode (A=2 cm2). All data points, presented in Figs. 3 and 4, were obtained by averaging the results measured at three separate, but identically prepared films. Cyclic voltammetric (CV) curves were registered at four different sweep rates between 10 and 100 mV s−1 in aqueous solutions containing 0.05 M Bu4NF, 0.05 M Bu4NCl, 0.1 M NaBF4 and 0.1 M NaCl respectively. For clarity, the composition of the solutions and their notation are summarized in Table 1. EQCN measurements have been carried out by using an EQCM-Oscillator (Autolab module) (details are described elsewhere) [16].
Extension to other small halide anion/bulky cation system: PEDOT/Bu4NF Beyond previous results having evidenced the irreversible incorporation of the Cl− ions into the PEDOT layers [16], in the previous subsection, we have demonstrated that very similar pattern can be observed for a substantially different conducting polymer (PPy). Consequently, the next step of our investigation was to study whether this behaviour is a specific feature of Cl− ions or it is a more general phenomenon. Thus, analogue experiments were performed with a PEDOT film polymerized in Bu4NF solution. Cyclic voltammogram and the coupled mass change curve for this system are presented in Fig. 2, where a very similar shape, demonstrating the dominancy of cationic motion, is clearly visible. The transformation of the EQCN data confirms that—similarly to Cl − —also F − ions became partially immobilized (Table 3.) As one of the reviewers pointed out, the hydrophilic/ hydrophobic nature of the cations may also play a role in their contribution to ion exchange. Cation movements were detected also in the case of hydrophilic cations, e.g. in NaCl
Results and discussion Electrochemical behaviour of the PPy/Bu4NCl system As it was demonstrated, a strong interaction between the PEDOT polymer and chloride ion led to a film, which—in opposite of the expectations—exhibited perfect cation exchange property. In order to see whether this anomaly occurs also with other polymers, the PPy/Bu4NCl system was studied by cyclic voltammetry. A typical CV obtained during EQCN 30
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Fig. 2 a Cyclic voltammogram of PEDOT recorded at 100 mV s−1 sweep rate in 0.05 M Bu4NF in water, together with the mass change recorded by EQCN. b Mass change vs. the charge derived from a
Author's personal copy J Solid State Electrochem 0.21
v/mV s−1
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solution. Figure 3 presents results for a PEDOT film, prepared in aqueous NaCl solution, then cycled first in monomer-free NaCl, followed by a solution change and oxidized/reduced several times in cetyltrimethylammonium chloride solution. The experiments revealed that the charge capacity drastically diminished after the exchange of solution, proving that Na+ ions were exchanged for cetyltrimethylammonium ions. As an explanation, we assume that—due to the memory effect—these bulky cations are not able to penetrate into the film to the same extent. Effect of the synthesis parameters Irreversible interaction between the oxidized polymeric chain and the dopant halide ion may lead to irreversible changes and, consequently, to an inferior electroactivity of the polymer. To shed further light on the polymer backbone, halide interactions, assumed on the grounds of the above described results, a systematic study on the effect of two polymerization parameters was performed. As an extension, PEDOT films were electrogenerated from both NaCl and NaBF4 containing solutions and the electrochemical polymerization was executed (i) at five different potential values and (ii) at five different concentrations of the actual supporting electrolyte. Doping levels were calculated from polymerization charge and redox charges during voltammetric transformations [4, 600 400
0.18
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0.06 1.00
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Epol / V Fig. 4 Comparison of doping levels for PEDOT samples studied in NaCl and in NaBF4 (PEDOT/NaCl and PEDOT/NaBF4 systems). Doping level values are plotted as a function of the polymerization potential (all polymers were electrogenerated from aqueous solution containing 0.1 M supporting electrolyte)
19]. Comparison of their values as a function of the polymerization potential and the concentration of the conducting electrolyte is depicted in Figs. 4 and 5, respectively. The differences of the two kinds of doping levels (polymerized in NaCl and NaBF4) are about 0.06–0.08 in every case, which means that the charge capacity (Qtot) of the halide containing film is half of its analogue generated in BF4− containing solution. Since all polymers were electrogenerated identically from aqueous solution containing 0.1 M supporting electrolyte, the difference—in agreement with our expectations—reflects the inferior electroactivity of the PEDOT/NaCl samples. This pronounced difference is not surprising in light of the interaction leading to chemically bonded chlorine, resulting in a diminishing electroactivity and a smaller maximum oxidation level of the actual film. Note that even in the case of BF4−, the
NaCl Cetyl-1 Cetyl-2
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M(Bu4N+)=242.4 g mol−1
y (2*Qox/Qpol-Qox)
Table 3 The virtual molar masses of the moving species determined from EQCN data of the PEDOT/ Bu4NF system
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E/V Fig. 3 Cyclic voltammograms for a PEDOT film prepared in aqueous 0.1 M NaCl solution (black), then cycled first in monomer-free NaCl, followed by a solution change and oxidized/reduced several times in 0.1 M cetyltrimethylammonium chloride solution (red and green)
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Fig. 5 Doping level values plotted as a function the logarithm of the conducting salt concentration in the polymerization solution (all polymers were electrogenerated potentiostatically at E=1.1 V)
Author's personal copy J Solid State Electrochem 3
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I / mA
Fig. 6 Cyclic voltammograms (recorded at 100 mV s−1 sweep rate in aqueous 0.05 M NaBF4) of identically prepared PPy/BF4 films, electrochemically Baged^ in NaBF4 (a) and NaCl (b) solutions at E=1.1 V for 100 s
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absolute values of the doping levels show themselves smaller than those usually obtained for CPs. This is probably the consequence of the fact that polymerization was performed in aqueous solutions because electrodeposition of PEDOT is more favourable either in organic media or in aqueous micellar solutions [25, 26]. More importantly, one can clearly realize the trend that the higher the polymerization potential, the smaller is the doping level. This tendency can be rationalized by the more expressed overoxidation at higher potentials. Similarly to the above described experiments, the effect of the electrolyte concentration on the charge capacities was compared for the films deposited in the presence of either tetrafluoroborate or chloride ions (Fig. 5). Doping levels, as a function of the logarithm of the concentration of the electrolyte in the polymerization solution, are plotted in Fig. 5. Just as in Fig. 4, large differences can be revealed between Cl− and BF4− ion-doped films, with the largest difference obtained at the concentration of 0.1 M. At the same time, it is worth looking on the trends of these values (double Y-axis is used for better visibility). In the case of BF4−, the doping level varies only within the standard deviation range, and no definite trend can be identified. Oppositely, for the Cl− containing polymer, a monotonous decrease of the doping level can be observed with increasing electrolyte concentration. This tendency corroborates our previous conclusions, namely that the decrease of the electroactivity is rooted in the irreversible interaction between the halide ion and the oxidized polymer. Such an effect is more pronounced at higher concentrations, as there is a higher probability of the interaction. The differences in the trends may also suggest that in case of the PEDOT/BF4− system, the overoxidation observed at higher potentials (Fig. 4) is more likely the result of the interaction of the oxidized polymer and water molecules, and not BF4− anions. Irreversible interactions after polymerization In order to confirm the general pattern of the above described anomalies and to identify the possibility of polymer-dopant interactions after the polymerization, identically prepared PPy/BF4 films were electrochemically Baged^ in NaBF4 and
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PPy Bu4NCl treated PPy
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NaCl solutions. Two parallel experiments were performed when two films were aged at a constant 1.1 V potential either in 0.1 M NaBF4 or 0.1 M NaCl solutions. The charge capacity of the films was measured by cyclic voltammetry both before and after the ageing experiment (Fig. 6). As parallel and independent data sets prove, the oxidative treatment at 1.1 V slightly decreases the total charge even in NaBF4 solution (see Fig. 6a). However, this effect is almost three times larger in the chloride-containing solution (5 vs. 15 %). These data indicate that certain anions (halogenides in this study) support the overoxidation of the polymer even after the polymerization, although the extent of the phenomenon is much smaller than that observed during polymerization.
Conclusions The observations gained by EQCN prove that the previously described exclusive cationic movements during the redox transformations is not a unique feature of the PEDOT/Cl system, but the phenomenon has been evidenced also in the case of a different polymer (PPy) and anion (F−). Furthermore, both halide concentration and the bias potential value applied during the electropolymerization have a substantial effect on the redox capacity of the synthesized polymer. This effect, however, is more pronounced in the presence of halide anions (compared to BF4− containing counterparts), indicating the fundamental role of the chemical interaction between the in situ forming oxidized polymer and the strongly electronegative doping ions. All these findings mean that two types of halogen exist permanently in the film. One is the covalently bond form, neutral chlorine. This bonding process—being irreversible—diminishes the redox capacity of the polymer; in other words, it suffers partial overoxidation. The second type of halogens, although electrostatically fixed, preserves its charge and insures electroactivity, although at a minor extent, which is accompanied by the movement of mobile cations. Importantly, this interaction between in situ forming polymer and strongly electronegative anions seems to be a general phenomenon, where its extent depends on the applied parameter values such as the halide concentration or the
Author's personal copy J Solid State Electrochem
deposition potential. This fact necessitates the reconsideration of the polymer–dopant interaction since it is a key factor in determining the redox behaviour of conducting polymers. The results call the attention also to the need for careful design of the experimental circumstances of the electropolymerization processes. Acknowledgments This work has been sponsored by the National Development Agency (NFÜ) under contract no. TÁMOP-4.2.2.A-11/1/ KONV-2012-0047.
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