PFeW11-doped polymer film modified electrodes and their electrocatalytic activity for H2O2 reduction

Analytica Chimica Acta 385 (1999) 111±117

PFeW11-doped polymer ®lm modi®ed electrodes and their electrocatalytic activity for H2O2 reduction Szilveszter Gaspar, Liana Muresan, Adrian Patrut, Ionel Catalin Popescu* Faculty of Chemistry and Chemical Engineering, Babes-Bolyai University, Str. Arany J. No. 11, 3400 Cluj-Napoca, Romania Received 27 May 1998; received in revised form 27 October 1998; accepted 1 November 1998

Abstract A study of electrochemical and electrocatalytic properties, toward H2O2 reduction, of the [PFeW11O39]ÿ4 polyoxoanion in solution as well as immobilized in partially oxidized polypyrrole (pPy) and in polyvinyl alcohol bearing styrylpyridinium groups (PVA) ®lms is reported. Using cyclic voltammetry at different pH values and scanning rates it was observed that: (i) the two bielectronic waves corresponding to the tungstate-oxo cage are pH-dependent; (ii) the Fe center of the polyoxoanion has an excellent electrocatalytic effect on H2O2 reduction, giving a catalytic ef®ciency and an electrode sensitivity, both higher when dissolved than when immobilized onto electrode surface; and (iii) the one-step method used to obtain the pPy-doped ®lm assures a greater amount of immobilized polyoxoanion than the ionic-exchange method used for PVA ®lms. From rotating-disk electrode (RDE) measurements, the polyoxoanion diffusion coef®cient was estimated at 4.110ÿ6 cm2 sÿ1, which agrees to that obtained by cyclic voltammetry, 3.910ÿ6 cm2 sÿ1, and the heterogeneous rate constant was found to vary between 3.510ÿ3 and 1.910ÿ3 cm sÿ1 at a variation of the electrode potential from 30 to ÿ30 mV vs. SCE. The best amperometric response observed for the PFeW11-doped pPy membrane, showed a linear response to H2O2 additions up to 9 mM and a sensitivity of 4.8 mA Mÿ1 H2O2 in the ®rst day of utilization. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Heteropolytungstate modi®ed electrode; Hydrogen peroxide amperometric sensor; Electrocatalysis; Polypyrrole

1. Introduction Keggin-type transition metal substituted heteropolyoxoanions have some useful properties that make them very attractive as redox catalysts (mediators) for indirect electrochemical processes, such as 1. high stability in acidic media and inertness toward reactive intermediates [1];

*Corresponding author. Tel.: +40-64-19-38-33 ext. 25; fax: +4064-19-08-18; e-mail: [email protected]

2. possibility to modify the formal potential of the transition metal which is incorporated into the polymetalate structure by changing the nature of the central heteroatom [2]; 3. presence of highly oxidized metallic centers, stabilized by the negatively charged polyoxo ligand, and very useful in oxidation processes [3]; and 4. occurrence of an inner sphere electron transfer mechanism through the free co-ordination orbital of the transition metal [1]. As the attachment of the redox catalysts to the electrode surfaces is of major interest, several methods

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were used to immobilize polyoxometalates: adsorption [4±6], precipitation with large cations [7±10], electrochemical deposition and codeposition [11,12] and entrapping into polymeric membranes (mostly electroconducting polymers) [8,13±24]. The last method exhibits several advantages such as: a stronger retention of the redox active polyanion on the electrode surface, due to its electrostatic interaction with the positivelly charged polymeric matrix, for instance polypyrrole [8,14,16±18,20,21, 23,24] and polyvinylpyridine [8,13,19,21]; a direct possibility to electrochemically control the mediator concentration onto the electrode surface [8,14,16±18,20,21,24]; and a simple way to obtain modified electrodes (MEs) with electrocatalytic activity toward the reduction of O2 [17,21,22], H2O2 [8,23], ClOÿ 3 [21,22,24], ÿ BrOÿ [22,24], NO [19,20,22] and catecholamines 3 2 [18] (for more details see the recent review of Sadakane and Steckhan [25]). Few investigations on the electrochemical behavior of free or immobilized PFeW11 [2] as well as of the electrocatalytical activity of this compound toward H2O2 [6] and NOÿ 2 [29] reduction, were reported. Digital simulation of the voltammetric response proved that the heteropolytungstate anion, adsorbed at Hg electrode, exhibited a higher electrocatalytical activity than in homogeneous solution [6]. In this paper, a detailed study of electrochemical and electrocatalytic properties, toward H2O2 reduction, of the [PFeW11O39]ÿ4 polyoxoanion in solution as well as immobilized in partially oxidized polypyrrole (pPy) and in polyvinyl alcohol bearing styrylpyridinium group (PVA) ®lms is presented. The diffusion coef®cient of the dissolved polyoxoanion was determined using two different methods: cyclic voltammetry and the rotating-disk electrode. The heterogeneous rate constant for the redox process, involving the Fe center of polyoxoanion, was calculated. For the immobilized PFeW11, a known method of immobilization in polypyrrole (pPy) [20] was compared with its immobilization in an electrochemically inactive polymer matrix (PVA). Amperometry was used to determine the H2O2 concentration by electrocatalytic reduction and a comparison concerning the response times and the sensitivity of the electrodes obtained with the two types of membranes, was carried out.

2. Experimental 2.1. Reagents K4[PFeW11O39] was obtained, beginning with the preparation of the corresponding de®cient heteropolyoxoanion as described by Tourne et al. [26], by a method presented by PaÆtrut, et al. [27]. Pyrrole (99%) and polyvinylalcohol bearing styrylpyridinium groups (polymerization degree 1700, 10±11% in aqueous solution) were provided by Aldrich and Toyo Gosei (Japan), respectively. The buffer solutions (pH 2±4) were prepared from Na2SO4 (Reactivul, Bucharest, Romania) and H2SO4 (95±97%) (Merck). In all the experiments, bidistilled water was used. 2.2. Preparation of CME 2.2.1. PFeW11-doped pPy-modified electrodes (G/PPy/PFeW11) After wet polishing on emery paper (grit 400 and 600, Buehler, Lake Bluff, IL), the doped pPy ®lm was deposited onto the surface of a graphite disk (Spektralkohlen, Ringsdorff Werke GmbH, Bonn-Bad Godesberg, Germany, S0.057 cm2) or a glassy carbon disk (Type 16, Hochtemperature-Werkstoffe GmbH, Thierhaupten, Germany, S0.096 cm2) electrode by electropolymerization from a fresh aqueous solution containing 0.1 M pyrrole (Py) and 0.01 M PFeW11 at a constant potential of 0.7 V vs. saturated calomel electrode (SCE). The polymerization process was interrupted after 90 s. No background electrolyte was used in order to prevent the incorporation of anions, more mobile than polyoxoanion, into the polymeric matrix. 2.2.2. PFeW11-doped PVA-modified electrodes (G/PVA/PFeW11) A drop of monomer (used as received) was deposited on a graphite electrode similar to that described above, followed by photopolymerization as described by Marty et al. [28]. Then, the electrode, thus modi®ed, was soaked for 48 h into a 0.5 M Na2SO4 solution (pH 2) containing 0.05 M PFeW11. Between experiments, all the modi®ed electrodes (MEs) were stored in an aqueous 0.5 M Na2SO4 solution of pH 2.

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2.3. Electrochemical measurements All measurements were performed in a one-compartment electrochemical cell containing the working electrode, an SCE as reference electrode and a Pt foil as counter electrode. The supporting electrolyte was an aqueous solution of 0.5 M Na2SO4 adjusted at the required pH value with H2SO4. For cyclic voltammetry experiments, the set-up consisted of a potentiostat (LP 7E, Laboratorni, Prague) controlled by an IBM Olivetti AT 486 DX computer, through a National Instruments AT M10 16F acquisition board interface. The rotating-disk electrode (RDE) experiments were performed using a modulated speed rotator (model AMSFRX, Pine, Grove City, PA) and a computer controlled potentiostat (EF 451 Elektro¯ex, Hungary). The amperometric measurements were performed with an amperometric detector (641 VADetector Methrom, Switzerland) connected to a strip chart recorder (W‡W Recorder 1100, Scienti®c Instruments, Switzerland).

Fig. 1. Cyclic voltammograms of PFeW11 on graphite electrode without (a) and with (b) 0.88 mM H2O2. Experimental conditions: 1 mM PFeW11; supporting electrolyte, pH 2; scan rate, 20 mV/s.

peak potentials (Table 1) are further apart than should be the case for a reversible process, the fact that they do not change signi®cantly with the potential scan rate (v) suggests that these redox processes are reversible. As expected [2], the insertion of the Fe atom in the tungsten±oxo cage shifts the Fe(III/II) formal standard potential (Table 1) toward negative potentials. This shift is smaller than in similar compounds with Si or Ge in the place of P [2]. Moreover, for the Fe(III/II) couple, the peak currents (Iap and Icp) depend linearly on v1/2 (Fig. 2), suggesting a diffusion-controlled electrode process and the absence of a notable adsorption of PFeW11 on the electrode surface. From the slope of the Icp vs. v1/2 dependence, a diffusion coef®cient of (3.90.36)10ÿ6 cm2 sÿ1 was estimated (average value of three determinations). For peaks II and III (Fig. 1), the variation of the pH (between 2 and 4, because of polyoxoanion instability at higher pH values) shows a negative shift of the formal standard potentials (E00 , estimated as the aver-

3. Results and discussion 3.1. Electrochemical characterization of the dissolved PFeW11 Fig. 1 presents the cyclic voltammogram of the PFeW11 polyoxoanion in solution. Its voltammetric response exhibits three pairs of waves attributed [2,29] to two two-electron redox process corresponding to the tungsten-oxo cage (couples II and III) and to a oneelectron redox process involving the Fe atom (couple I). Although the corresponding anodic and cathodic

Table 1 Formal standard potentials and separation of the anodic and cathodic peak potential for the redox couples corresponding to dissolved and immobilized PFeW11 Electrode

I

II 00

a

G/PFeW11 (aq) G/pPy/PFeW11 b G/PVA/PFeW11 b a b

00

III

E (mV)


Ep (mV)

E (mV)


Ep (mV)

E00 (mV)


Ep (mV)

84 61 4

69 79 109

ÿ516 ÿ528 ÿ566

54 100 111

ÿ678 ÿ708 ÿ701

58 79 121

Experimental conditions: supporting electrolyte (pH 2) with 1 mM PFeW11; scan rate 10 mV/s. Experimental conditions: supporting electrolyte (pH 2) without 1 mM PFeW11; scan rate 10 mV/s.

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Fig. 2. Dependence of the peak current on the potential scan rate for Fe(III/II) couple on G/PFeW11(aq) electrode (A) and GC/pPy/PFeW11 electrode (B) Experimental conditions: (A) as in Fig. 1(a); (B) supporting electrolyte, pH 2; scan rate 10 mV/s.

Table 2 pH Dependence of the formal standard potential for the voltammetric waves exhibited by the dissolved PFeW11 (Fig. 1(a)). Experimental conditions: 1 mM PFeW11, supporting electrolyte adjusted to different pHs with H2SO4. pH

E00 I

E00 II

E00 III

2.00 2.35 2.80

84 81 79

ÿ516 ÿ552 ÿ596

ÿ678 ÿ704 ÿ729

age of anodic and cathodic peak potentials) with increasing values of pH (see Table 2). The slopes of E00 vs. pH are linear with a slope of 86 mV/
pH for peak II and 59.5 mV/
pH for peak III, corresponding to redox processes involving three and two protons, respectively, in good agreement with the literature [2]. The small variation observed in the case of the Fe(III)/ Fe(II) couple may be due to a competition between protonation and coupling with other cations of the negatively charged polyoxoanion [2]. At pH>4, the shape of the voltammograms changes and only a single pair of peaks is exhibited by the tungstate± oxo cage. In order to estimate the rate constant of the heterogeneous electron transfer at G/PFeW11(aq) electrode, the reciprocal value of the stationary current intensity, obtained in RDE measurements, was depicted as a function of the square root of the electrode rotation speed (!, rad/s), at different overpotentials (Fig. 3). From the intercept of the Koutecky±Levich plot [30], the rate constant (ks, cm/s) was estimated (Table 3), while the slope lead to the coef®cient of diffusion of

Fig. 3. Koutecky±Levich plot for G/PFeW11(aq) electrode at different electrode potentials. Experimental conditions: scan rate 5 mV/s; other parameters as in Fig. 1(a). Table 3 Rate constant for the heterogeneous electron transfer at G/ PFeW11(aq) electrode, estimated from RDE measurements, performed at different electrode potentials. Experimental conditions: as in Fig. 3. E (mV)

ks (103 cm/s)

30 15 0 ÿ15 ÿ30

3.5 3.5 4.1 4.1 4.9

the polyoxoanion ((4.10.27)10ÿ6 cm2 sÿ1). Within the experimental error the value of the coef®cient of diffusion is in agreement with that obtained by cyclic voltammetry. It is interesting to note that, in the potential domain investigated, the calculated rate constants are independent of the electrode potential,

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suggesting that the electron transfer between Fe centers and the graphite electrode is not the rate limiting step. The PFeW11 polyoxoanion behaves as an active electrocatalyst for H2O2 reduction at a potential corresponding to reduction of Fe(III) to Fe(II) (Fig. 1). Taking into account that direct electroreduction was observed on bare graphite at ca. ÿ500 mV vs. SCE, the PFeW11 presence induced a potential shift of >500 mV. This potential shift is greater than that obtained with other polyanions like [P2W17O61 (Fe.OH2)]ÿ8 (mediated electroreduction takes place at ÿ100 mV vs. Ag/AgCl/sat KCl at a glassy carbon electrode [31]) and [H2OFeSiW11O39]ÿ5 (ÿ250 mV vs. SCE at glassy carbon electrode [1]). This high electrocatalytic effect is the result of an inner sphere reduction mechanism, favored by the Fe free coordination center [1,6,29]. The catalytic current, Icat, (obtained as the difference observed between the cathodic peak current in the presence, and absence, of H2O2) depends linearly on H2O2 concentration (Fig. 4) up to 6.5 mM. The electrode sensitivity (the slope of Icat vs. H2O2 concentration) and the catalytic ef®ciency [32] are presented in Table 4. 3.2. Electrochemical characterization of PFeW11doped polymer film modified electrodes The voltammograms corresponding to the PFeW11 doped pPy and PVA modi®ed electrodes (Fig. 5(A and B), curves a) exhibit the three characteristic waves of the polyoxoanion. The short-time stability of PFeW11-

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Table 4 Catalytic efficiency and electrode sensitivity for different mediating schemes for the H2O2 electrocatalytic reduction, based on PFeW11 Electrode

Catalytic efficiency (%)

Sensitivity (mA/M)

G/PFeW11 (aq) a G/PVA/PFeW11 b G/PPy/PFeW11 b

346.4 17.4 5.5

9.7 1.2 5.5

a Experimental conditions: supporting electrolyte (pH 2) with 1 mM PFeW11; scan rate 20 mV/s. b Experimental conditions: supporting electrolyte (pH 2) without 1 mM PFeW11; scan rate 20 mV/s.

modi®ed electrodes was suf®ciently high and, consequently, no differences were observed between consecutive cyclic voltammograms. As a consequence of the immobilization, a negative shift of the peak potentials and an increase of the separation of the peak potentials are observed (see Table 1). As was reported for similar cases [33,34], these phenomena can be attributed to the interactions between the negatively charged redox couple and the oppositely charged polymer matrix. By comparing the peak currents for the FeIII/FeII couple (Fig. 5(A and B), curves a) it appears that the pPy ®lm represents a better immobilization matrix than the PVA one. For the PFeW11-doped pPy modi®ed electrode, the reduction of the PFeW11 waves located in the negative-potentials domain could be explained by the low conductivity of polypyrrole in its reduced form [14].

Fig. 4. Dependence of the catalytic current on the H2O2 concentration. (A) Cyclic voltammetry measurements: G/PFeW11(aq) electrode, 1 mM PFeW11 (*); G/pPy/PFeW11 electrode (&); and G/PVA/PFeW11 electrode (^). Experimental conditions: supporting electrolyte pH 2; scan rate, 20 mV/s. (B) Steady-state amperometric measurements: G/pPy/PFeW11 electrode (&); G/PVA/PFeW11 electrode (^). Experimental conditions: applied potential, ÿ40 mV vs. SCE; stirred solution of supporting electrolyte pH 2.

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Fig. 5. Cyclic voltammograms of the G/pPy/PFeW11 (A) and G/PVA/PFeW11 (B) modified electrodes: (a) immediately after the preparation; (b) after three days of use. Experimental conditions: supporting electrolyte pH 2; scan rate, 10 mV/s.

The effect of potential scan rate on the FeII/FeIII peak current reveals a signi®cant difference between the PVA and pPy behaviors as immobilization matrix. Whereas for the PVA ®lm the diffusion appeared as the rate determining step of the electrode processes (linear dependence between peak current and v1/2, results not shown), for the pPy ®lm a linear dependence of peak currents on potential scan rate was observed (Fig. 2(B)), suggesting adsorption of the electroactive species. Cyclic voltammetry measurements performed in the presence of H2O2 proved that for both the immobilization matrices the polyoxoanion retains its electrocatalytic properties toward H2O2 reduction (Fig. 4(A)), but the corresponding modi®ed electrodes (MEs) showed a smaller electrocatalytic ef®ciency and H2O2 sensitivity than those corresponding to the dissolved polyoxoanion (Table 4). Among the MEs, the G/pPy/PFeW11 electrode exhibited the highest sensitivity, but, unexpectedly, the smallest electrocatalyitic ef®ciency. The ME amperometric response to successive additions of H2O2, at an applied potential of ÿ40 mV vs. SCE, can be seen in Fig. 6, while the catalytic current dependence on the H2O2 concentration is presented in Fig. 4(B). The response time corresponding to an H2O2 addition (t90%) lies between 9±11 s (depending on H2O2 concentration) for G/pPy/PFeW11 and between 40±70 s for G/PVA/PFeW11. The sensitivity of the MEs varied signi®cantly with time. Thus, for the G/pPy/PFeW11 electrode, it decreased from 4.8 to 0.61 mA/M in six days, and for the G/PVA/PFeW11 electrode from 0.74 to

Fig. 6. Steady-state current intensity response of the G/pPy/ PFeW11 on the addition of 1% H2O2. Experimental conditions: applied potential, ÿ40 mV vs. SCE; stirred solution of 10 ml supporting electrolyte, pH 2.

0.06 mA/M in two days, indicating a pronounced instability of both the systems. It was suggested that this instability may be due to a chemical transformation of PFeW11, involving the extraction of the Fe ions from the Fe centers followed by their exclusion by the repulsive electrostatic forces existing in a polycationic ®lm [8]. Consequently, the de®cient polyoxoanion changes into the parent compound, as can be observed from the cyclic voltammograms recorded after some

S. Gaspar et al. / Analytica Chimica Acta 385 (1999) 111±117

time of use of the electrodes, and presented in Fig. 5(A) (curve b) and (B) (curve b). In addition, for both these ®lms the contribution of the ionexchange process to the polyoxoanion loss, induced by the high ionic content of the supporting electrolyte should be considered as well as the exclusion of the polyoxoanion from the pPy matrix when the applied potential brings pPy to its reduced form, and the chemical degradation of the membrane by the H2O2 attack [8]. 4. Conclusions The performed investigation on the electrochemical behavior of the dissolved PFeW11 polyanion characterizes it as an excellent electrocatalyst for the H2O2 reduction. The polyoxoanion, immobilized in a polymeric inert (PVA) or an electroconductive (pPy) membrane, qualitatively retained its electrochemical behavior exhibited in solution, but for both the immobilization matrices the electrocatalytic ef®ciency and the sensitivity toward H2O2 reduction were lower. Moreover, the loss of stability, showed by the investigated polyoxoanion when immobilized, limits the utilization of these types of modi®ed electrodes and directed our efforts toward softer immobilization techniques. Acknowledgements Financial support from CNCSU (grant No. 7010/ 1997) is gratefully acknowledged. Szilveszter Gaspar acknowledges the scholarship from Babes-Bolyai University, Cluj-Napoca, Romania. References [1] J.E. Toth, D.J. Melton, D. Cabelli, B.H.J. Bielski, F.C. Anson, Inorg. Chem. 29 (1990) 1952.

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