Visible-Light-Enhanced Electrocatalytic Activity of a Polypyrrole

Oct 26, 2010 - Visible-Light-Enhanced Electrocatalytic Activity of a Polypyrrole/Magnetite Hybrid. Electrode toward the Reduction of Dissolved Dioxyge...
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Visible-Light-Enhanced Electrocatalytic Activity of a Polypyrrole/Magnetite Hybrid Electrode toward the Reduction of Dissolved Dioxygen Csaba Jana´ky, Bala´zs Endro˝di, Otto´ Berkesi, and Csaba Visy* Department of Physical Chemistry and Materials Science, UniVersity of Szeged, Rerrich square 1, Szeged H-6720, Hungary ReceiVed: June 10, 2010; ReVised Manuscript ReceiVed: August 25, 2010

Conducting polymers are getting more and more interest as both supporting matrixes and electrocatalysts in the oxygen reduction reaction (ORR). A polypyrrole-magnetite nanocomposite layer has been synthesized by using potassium tetraoxalate as the conducting electrolyte. FT-IR measurements proved that chemical modification of the iron oxide by a reaction between the nanoparticles and the saltsleading to an iron oxalate layer on their surfacesendows a negative charge to the particles, which leads to their penetration into the polymeric film as a part of the charge compensation. The new hybrid material showed significant photoelectrocatalytic behavior in the ORR. The ratio observed between the stabilized stationary currents under and without illumination is 2.0 for this hybrid. Separate studies on the electrochemical decomposition of H2O2 also indicated an enhanced catalytic activity of the polypyrrole/magnetite hybrid compared with the neat polymer. The results may open new opportunities in the next generation of solar fuel cell applications. Introduction The combination of conducting polymers with magnetic iron oxides is in the focus of research and development because materials possessing both large magnetic susceptibility and high conductivity can be used in different applications, such as nonlinear optics, electrical and magnetic shielding, in electrochemical supercapacitators, and also in electrocatalysis.1,2 The electrocatalytic O2 reduction reaction (ORR) is one of the most intensively studied processes because it is generally used as the cathodic half-reactions in fuel cells, regardless of the fuel.3 The most widely applied catalysts in polymer electrolyte membrane (PEM) fuel cells are Pt and its alloys,4,5 which perform very well, but they are very expensive. Conjugated polymers have shown great potential in such applications in two aspects: (i) as a simple (conductive) polymeric support of catalytic nanoparticles, such as in the case polypyrrole/ polyaniline/polythiophene + Co/Pt composites,6-9 and (ii) they exhibit catalytic activity themselves as well, opening the opportunity to prepare inexpensive, environmentally friendly catalysts.10-12 Although polypyrrole thin layers are proved to be active in the electrocatalytic ORR alone,13 they are often combined with different ferrites to enhance the activity.14,15 Magnetite (Fe3O4) nanolayers are prominent candidates for electrocatalytic applications due to the redox switching between Fe3+/Fe2+. This behavior can be exploited in different processes, such as the O2 or H2O2 reduction.16,17 For electrocatalytic applications, the electropolymerization is the most appropriate route to synthesize such composite films, directly on the electrode surface. In the literature, only a few papers report on the electrochemical preparation of conjugated polymer-magnetic nanoparticle hybrids in aqueous18-22 and, especially, in nonaqueous solution.23 The successful immobilization is always related to an interaction between the nanoparticles and the monomer/doping ion. Bidan and his co-workers modi* To whom correspondence should be addressed. Phone: +36 62544667. Fax: +36 62544652. E-mail: [email protected].

fied the surface of maghemite (γ-Fe2O3) nanoparticles with citrate and with other, bulky organic ions, acting as anionic complexing agents.18-20 Recently, Deslouis and his colleagues reported on an easy synthesis route leading to a polypyrrolemagnetite nanocomposite layer,21,22 by using potassium tetraoxalate as the conducting electrolyte. In this work, our aim is two-fold: (i) to elucidate the mechanism of the hybrid formation in the synthetic procedure applied in ref 21 and (ii) to study the photoelectrocatalytic properties of the PPy-magnetite hybrid in the ORR. The work is novel in multiple aspects: (i) we discovered the doping type inclusion mechanism of the magnetite nanoparticles into the conducting polymer matrix, (ii) we show the enhanced catalytic effect of the composite layers, and (iii) we demonstrate the extremely large photocatalytic activity of the hybrids and present data on its origin for both the neat polypyrrole and its magnetitecontaining composite. With respect to the exploitation of the solar energy, these results may open new opportunities in the next generation of solar fuel cell applications. Experimental Section Preparation of the Nanocomposites. Magnetite (Fe3O4) nanoparticles were synthesized by alkaline hydrolysis of iron(II) and iron(III) salts (FeCl2 · 4H2O and FeCl3 · 6H2O, SigmaAldrich).24 The synthesis resulted in magnetite nanoparticles with an average size around 12 nm. The characterization of the particles prepared in this way was reported elsewhere.25 Analytical-grade pyrrole (Py) monomer and potassium tetraoxalate (PTO) were purchased from Sigma-Aldrich. Pyrrole was freshly distilled before use. All polymerization solutions contained 0.1 M of the monomer pyrrole, and 0.05 M PTO in deionized water. The amount of magnetite particles was 10 g dm-3. Polypyrrole (PPy) and polypyrrole-magnetite (PPy/ Fe3O4) composite thin films were deposited galvanostatically at a current density of 3 mA cm-2 onto the working electrode. For further voltammetric studies, the solution was changed after the polymerization to a 0.5 M phosphate buffer (pH ) 7) in

10.1021/jp105338f  2010 American Chemical Society Published on Web 10/26/2010

A Polypyrrole/Magnetite Hybrid Electrode water. In the case of the measurements with an electrochemical quartz crystal microbalance (EQCM) and a transmission electron microscope (TEM), charge density was limited to 90 mC cm-2, in order to avoid viscoelastic effects;26 otherwise, it was 300 mC cm-2. Characterization Techniques. The interaction between the magnetic nanoparticles and the potassium tetraoxalate electrolyte has been investigated by FT-IR spectroscopy using a Bio-Rad Digilab Division FTS-65A/896 Fourier transform infrared spectrometer equipped with a Peltier cooled deuterated triglyceride-sulfate (DTGS) detector. The powder samples have been mixed with KBr, and their spectra were recorded in a Bio-Rad Universal Reflection Accessory in diffuse reflectance mode, between 4000 and 400 cm-1, at a resolution of 4 cm-1 by averaging 256 scans. The electrochemical measurements were performed on a PGSTAT 10 (Autolab) instrument, in a classical three-electrode electrochemical cell. The working electrode was a gold electrode with a size of A ) 0.0707 cm2, or A ) 2.00 cm2 for the measurements related to the reduction of H2O2. We used a Ag/ AgCl reference electrode, having a potential of 0.200 V vs SHE. All the potential values in the paper are given with respect to the silver/silver chloride electrode. The EQCM measurements were performed using a quartz crystal resonator and analyzer, EQCM type 5510 (Poland). In these measurements, we used a platinum-coated quartz crystal electrode (f0 ) 10 MHz, A ) 0.1964 cm2). Cyclic voltammograms of the thin films were registered in 0.5 M phosphate buffer solutions at five different sweep rates between 5 and 100 mV s-1. For the measurements in an oxygen atmosphere, oxygen gas was bubbled into the solutions for 30 min. A similar approach was followed for the experiments under a N2 atmosphere. For the photoelectrochemical measurements, the electrochemical cell was located in a dark box. The neat polymer and the hybrid electrode were illuminated by a Fiber-Lite A3000 (Dolan-Jenner) 150 W high-intensity light source (the UV component was filtered). The photon flux applied to the electrode surface was 300 mW/cm2. All experiments were performed at room temperature (25 ( 0.5 °C). Transmission electron microscopic investigation of the composite layersafter peeling it off from the electrodeswas performed using a Philips CM 10 type instrument, operating at an acceleration voltage of 100 kV. Results and Discussion Synthesis of the Polypyrrole/Magnetite Thin Composite Layers. The electrochemical polymerization was followed by the EQCM technique, and the mass of the neat polymer and the composite film, synthesized under exactly identical electrochemical conditions, were compared. In Figure 1, EQCM data registered during the galvanostatic polymerization are presented. One can see that the mass increase of the layers versus the transferred charge is linear for both the neat polymer and the hybrid. This good linearity can be related to the constant growth rate of the films, in which the particle incorporation is assumingly monotonous. At the same time, the difference in the slope of the curves is conspicuous, proving that magnetite nanoparticles are embedded during the polymerization. From the final mass values, the relative amount of the built-in iron oxide can be calculated, for which ∼26 m/m% magnetite was obtained. The inset shows the chronopotentiometric curves registered during the synthesis and their similarity; the reproduction within a 40 mV range gives evidence that the polymerization process is practically not influenced by the presence of the iron oxide sol.

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Figure 1. EQCM data registered during the galvanostatic (j ) 3 mA cm-2) polymerization of pyrrole, for both the neat PPy and the PPy/ magnetite hybrid. The inset shows the chronopotentiometric curves.

Figure 2. Diffuse reflectance FT-IR spectra of pure magnetite and the surface-modified nanoparticles between 400 and 2000 cm-1.

To explore the mechanism of the nanoparticle incorporation into the conducting polymer layer during the electropolymerization, we performed separate FT-IR measurements. We compared the IR spectra of the neat magnetite nanoparticles with magnetite let in contact with the electrolyte, potassium tetraoxalate (0.05 M PTO in water, 10 g dm-3 magnetite), for 100 s, modeling the circumstances of the polymerization. A striking difference is visible in Figure 2. For the magnetite particles alone, the well-known, characteristic bands can be seen at 591 and 398 cm-1, which correspond to the Fe-O stretching modes of the magnetite lattice. The two shoulders at 629 and 437 cm-1 can be assigned to maghemite27 (the oxidized form of magnetite), probably present at the surface of the nanoparticles. In contrast to the neat material, in the sample obtained by the surface treatment with PTO, significant spectral modifications can be revealed. The newly detected bandssbeyond the evident appearance of the previously mentioned peakssare the clear consequence of a new compound, formed assumingly at the surface of the nanoparticles. On the basis of the overall shape of the spectrum, we can say that the newly formed species is iron oxalate, more precisely, a mixture of [Fe(II)ox2]2- and [Fe(III)ox2]-.28,29 This is not a surprise because magnetite is a mixed-valence iron oxide, containing both Fe(II) and Fe(III) (in detail, chelated oxalate groups generally show symmetric and asymmetric ν(CdO)

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Jana´ky et al.

SCHEME 1: Surface Modification of Magnetite Nanoparticles with the PTO Electrolyte

vibrations around 1710 cm-1 and β(O-CdO) vibrations at 798 cm-10.30 The peaks at 1396 and 1278 cm-1 are due to the ν(C-O) stretch.) These observations are in perfect agreement with previous studies, where the mechanism of complete dissolution (according to the equation mentioned below) of magnetite nanoparticles by acidic oxalates has been discussed.31,32 32Fe3O4 + 8HC2O4 f 2Fe(C2O4)3 + 2Fe(C2O4)2 + 4H2O

It is important to point out that the surface modification of the iron oxide endows a negative charge to the particles (Scheme 1), which leads to their penetration into the polymeric film as a part of the charge compensation,33 similar to the procedure applied in ref 18. Consequently, it is now clear why it is not possible to incorporate magnetite nanoparticles when only the generally used (inert) doping ions, such as dodecyl sulfate, are present. This dissolution reaction may raise the possibility of doping by iron oxalate complexes, which can be present in the solution.

Figure 3. TEM image of the nanocomposite layer taken at 92 000× magnification. The inset shows an enlarged area, taken at 130 000× magnification.

Figure 4. Cyclic voltammograms of the PPy/Fe3O4 hybrid at five different scan rates (5-100 mV s-1) in 0.5 M phosphate buffer (pH ) 7) under a N2 atmosphere. The inset shows the sweep rate dependence of the anodic and cathodic peak currents.

To shift the competition between the negatively charged magnetite nanoparticles and the iron oxalate complexes in favor of the former, we applied a relatively large amount of iron oxide particles and a low concentration of PTO. Transmission electron microscopy was used (Figure 3) to prove the dominancy of the penetration of the nanoparticles over that of the complex ions. In Figure 3, the dark spots refer to magnetite particles embedded into the polypyrrole layer (gray). One can see either some self-standing particles (in the inset also), characterized by the initial particle size (12 nm), or some aggregates, similar to those pictures presented by Deslouis et al. for such hybrids.21 The presence of the built-in nanoparticles proves the mechanism proposed earlier and shown in Scheme 2. Electrochemical Behavior of the Hybrid Material in a Nitrogen Atmosphere. To gain information on the electrochemistry of the hybrids, we recorded a series of voltammograms in a N2 atmosphere, at five different sweep rates (5, 10, 25, 50, 100 mV s-1). If we have a look at the curves presented in Figure 4, the most important thing that should be emphasized is the quasi-reversible redox activity at all sweep rates. As a

SCHEME 2: Illustration of the Nanocomposite Formation during the Electrochemical Polymerization

A Polypyrrole/Magnetite Hybrid Electrode

Figure 5. Cyclic voltammograms of the hybrid layer in 0.5 M phosphate buffer (pH ) 7) under N2 and O2 atmospheres and in an O2 atmosphere under illumination, at a sweep rate of 10 mV s-1.

consequence, the anodic and cathodic charges are fairly equal during the cycles at each sweep rate. If we have a look at the two new redox peaks (compared to the neat polymer), which are related to the incorporation of the iron oxide nanoparticles, it is visible that the position of the anodic peak at ∼E ) -400 mV is independent of the sweep rate, which indicates a surface-related charge transfer. This conclusion is supported also by the change of the peak currents, which are directly proportional with the scan rate (as shown in the inset). As for the cathodic peak currents around E ) -0.5 Vswhich originates from the Fe3+/Fe2+ transformationsits linear sweep rate dependence also indicates a surface-controlled process.34 Effect of O2 and Illumination on the Electroactivity. In a recent paper it was shown that conjugated polymers (as organic semiconductors) exhibit photoelectrocatalytic activity toward the reduction of dissolved oxygen,35 although the origin of the photocurrents was not thoroughly investigated. On the basis of this experience, we studied the effect of the oxygen and the illumination on the electrochemistry of the composite layer. For comparison, we present cyclic voltammograms recorded in N2 and O2 atmospheres and in O2 under illumination at a scan rate of 10 mV s-1 (Figure 5). It is clearly visible that both O2 alone and the O2 under illumination change the shape of the curve drastically. We

J. Phys. Chem. C, Vol. 114, No. 45, 2010 19341 started our measurements from the anodic end of the potential window (+0.2 V). The three curves follow the same path until E ) 0.0 V only, but below this potential, a bifurcation can be observed related to O2. Later, at more negative potentials, a second current surplus evolves below E ) -0.15 V as a consequence of the illumination. This may suggest that the photocatalytic effect occurs only from this potential. We can get direct information on the above-mentioned effects if we transform these data. To visualize the cathodic currents originating from the presence of oxygen, we subtracted the negative going section of the voltammogram measured in N2 from that registered in O2. As for demonstrating the effect of the light, we subtracted the dark currents from those measured under illumination (both in O2). The curves shown in Figure 6A directly evidence the tendencies presumed before. Namely, the cathodic current related to the ORR appears at 0.0 V and increases rapidly until -0.4 V. Here, we reach the potential range in which the further increase of the overpotential does not result in the growth of the current because the process is not limited by the charge-transfer step any more. Although the rate-determining process may be both the diffusion and the adsorption of the oxygen molecules, further results (discussed later) indicated the relevance of both. However, this control does not result in a constant limiting current, but a maximum can be seen for both the neat PPy and the hybrid (at E ) -0.4 V and between E ) -0.3 and -0.8 V, respectively). These maxima may originate from the reduction of the films chemically reoxidized by the O2. In Figure 6A, the more or less peak-type current surplus can be connected to the electroactivity of the iron oxide content (shown in Figure 4). As for the illumination, we would like to point out that, in the case of the hybrid, the photoeffect is delayed (appears only below a potential of E ) -0.15 V), separated from the development of O2-related dark currents (from E ) 0.0 V). This fact indicates a difference in the mechanism of the photocatalytic process from the one in the dark. Oppositely, on the neat PPy, the two effects develop together, as it can be seen in Figure 6B, because the two current surpluses rise together between E ) 0.0 and -0.2 V. The effect of the illumination can be understood if we consider that the hybrid layer is immersed in aqueous electrolyte (phosphate buffer) containing dissolved oxygen as a potential photoelectron acceptor (liquid-junction arrangement). Illumination by a properly large photon energy creates excitons, which can decay by recombination or via charge transfer through the

Figure 6. Effect of O2 and illumination during the cathodic part of the cycles recorded in 0.5 M phosphate buffer (pH ) 7) at 10 mV s-1, for the PPy/magnetite hybrid (A, PPy/Fe3O4; B, PPy).

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Figure 7. Scan rate dependence of the cathodic charge surplus during the voltammetric cycles, with and without illumination. The inset shows the ratio of the surpluses at each sweep rate.

electrode. If an electron acceptor is present in the solution, the excited electron can be transferred by the scavenger, generating a cathodic photocurrent. This explanation of the origin of photocurrents is also supported by the fact that almost no photocurrent was measured in the N2 atmosphere. However, the phenomenon is not that simple because the photoeffect is not the direct consequence of exciton generation under illumination but is related to it strongly because it is coupled to the formation of H2O2 under illumination (as a result of the interaction between the generated e- and O2). It is known from the literature36 that photoreduction of O2 on semiconductor catalysts results in H2O2. Thus, the origin of the photocurrent is the electroreduction of the such formed H2O2. It is important to point out that, although the possible photodecomposition of iron oxalates is known from the literature,37 we did not observe any degradation of the films; their charge capacity remained unchanged during the subsequent cycles (even in the course of long-term measurements). The changes related to the presence of O2 and illumination are more and more expressed at lower sweep rates, when the CVs gradually loose their original character, leading to a shape where the ORR dominates over the self-electroactivity of the

Jana´ky et al. hybrids. The effect of the scan rate on the asymmetry of the cyclic voltammograms can be visualized if we compare the anodic and cathodic charges at various sweep rates. Because the oxidation charges are equal in O2 and N2 atmospheres and we obtained the same charge for the reduction in the N2 atmosphere, the effect of O2 can be quantified by the cathodic charge surpluses. In Figure 7, we present data calculated form a typical set of voltammograms. As it can be seen, the smaller the sweep rate, the more significant effect of the oxygen is visible, proving that the process behind the extra charge (O2 reduction) is much slower than the redox transformation of the polymer. This is coherent with the previous assumption of the rate-controlling role of the diffusion and adsorption below a certain potential. Moreover, what is more interesting in Figure 7 is that the charge coupled to the photoeffect in the ORR seems to be exactly the double of those obtained without illumination at each scan rate. To control the reality of this “mysterious” coincidence, we performed stationary, chronoamperometric measurements at a potential of E ) -0.7 V, where the self-electroactivity of the hybrid is negligible; thus, the O2 reduction reaction can be studied separately from the redox transformation of the hybrid. In Figure 8, we show the photocatalytic effect in two subsequent illumination steps. In Figure 8, one can see at first glance that both electrodes exhibit photocatalytic behavior. When we compare PPy and PPy/ Fe3O4, it is clearly visible that both dark and photocurrents are higher in the case of the hybrid. As it was expected, the ratio between the stabilized stationary currents under and without illumination is really 2.05 for the hybrid, although also the neat polymer shows photocatalytic activity with a ratio of ∼1.75. As for the role of the embedded nanoparticles, although also the dark current is definitely larger at this electrode, the significant effect of the presence of magnetite appears under illumination. If we have a look at the evolution and the cease of the photocurrent in Figure 8, one can see a slow development and cease of the photoeffect. This observation on the shape of the curves is in good agreement with the above-proposed origin of the photocurrent, namely, its connection to H2O2 reduction. This way, the long time needed to reach the stationary current can be easily understood: when switching off the light, the formation of H2O2 stops; however, the photocurrent diminishes slowly due to the reduction of H2O2 still present in the solution, until it is completely removed. To check the reproducibility of

Figure 8. Chronoamperometric curves registered at a potential of E ) -0.7 V in 0.5 M phosphate buffer, under an O2 atmosphere upon switching on and off the illumination. In panel B, the average dark and photocurrents are compared for both the neat PPy and the hybrid.

A Polypyrrole/Magnetite Hybrid Electrode

Figure 9. Hydrodynamic voltammograms for the ORR on the PPy/ Fe3O4 electrocatalyst in O2-saturated 0.5 M phosphate buffer (pH ) 7.0) solution, at V ) 25 mV/s. The inset shows the Koutecky-Levich plot for the data obtained at E ) -0.7 V.

these measurements and the drawn tendencies, we present the average values of the stationary current densities obtained from three steps, measured at 5-5 different, separate electrodes, synthesized under identical conditions. Looking at the values of the dark currents, they are smaller than the diffusion limitation would allow at the actual oxygen concentration, according to our calculations (not shown here) based on the Cottrell equation.34 To determine the kinetic parameters of the ORR, we performed independent rotating disk electrode (RDE) measurements. In Figure 9, linear voltammograms, registered at seven rotation rates (250, 500, 1000, 1500, 2000, 2500, and 3000 rpm) are presented. One can see the obtained gradually increasing limiting currents in the series of voltammetric curves. The overall shape of the curves suggests the transformation from the kinetic control to a mixed control38,39 of the limiting current densities by the diffusion of molecular oxygen and the adsorption of dioxygen on the surface of the modified electrode, leading to the rate-controlling regime of the latter in the more negative potential range. To estimate the number of transferred electrons in the ORR without illumination, we applied the Koutecky-Levich (K-L) plots34 for the data presented in Figure 9. Consequently, at selected potentials, j-1 was plotted versus ω-1/2, and the best linearity could have been obtained only for those at more negative potentials (-0.6 to -0.8 V). In the inset, we present data observed at a potential of E ) -0.7 V, which clearly shows the linear dependence. It is well known that the ORR can follow two different catalytic pathways, one with 2 (+2) e- transfer and the other directly with 4 e-.39,40 Whereas in the first reaction, H2O2 formation takes place, in the latter case, the final product is H2O. In our case, from the slope of the K-L plot, n ) 3.75 (the details of the calculation are shown in the Supporting Information) was obtained for the number of transferred electrons, which indicates the dominancy of the 4 e- mechanism. That was observed earlier for both PPy14 and Fe3O4.16,41 The dominancy of the 4 e- process and, as a consequence, the only small rate of H2O2 formation is very favorable from the lifetime perspective of the catalyst, since degradation, caused by the presence of hydrogen peroxide, is a crucial point in this respect.42 To gain further evidence for the proposed origin of the photocurrent, we carried out RDE measurements under illumination. We found that, in this case, illumination had no effect on the measured current densities. This is consistent with

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Figure 10. Chronoamperometric measurements recorded at E ) -0.3 V in the presence and absence of 0.05 mM H2O2 in 0.5 M phosphate buffer (pH ) 7), under a N2 atmosphere.

the previous explanation because, due to the convection (caused by the rotation of the electrode), the photogenerated H2O2 is removed from the close vicinity of the electrode before it could be reduced. H2O2 Decomposition Reaction. To explain the differences between the catalytic behavior of the two electrodes, we studied the electroreduction of H2O2 separately. In Figure 10, we show the results of the performed chronoamperometric measurements, and it is clearly visible that, whereas the background currents (in a N2 atmosphere, without H2O2) are equal in the two cases, the difference in the currents related to H2O2 is striking: although PPy also shows some catalytic activity, the decomposition reactionscalculated by subtracting the background currents in both casesstakes place 7.5 times faster at the hybrid electrode. This difference can explain the detection of a larger dark current in the ORR for the hybrid, whereas in the case of the neat polymer, only some partial decomposition of H2O2 (formed during the 2-electron ORR) can contribute to the overall current. The catalytic activity of the hybrid toward the H2O2 reduction can explain the enhanced photocatalytic activity of the hybrid in the ORR as well, taking into account that the photoeffect in the ORR involves the H2O2 reduction step. Moreover, it is also important to emphasize that illumination had absolutely no effect on the direct hydrogen peroxide reduction in these experiments. This proves that the current related to H2O2 originates from its direct electrochemical decomposition and not from the reduction of O2, formed by the chemical decomposition of H2O2 on the magnetite nanoparticles (as the latter would be influenced by the illumination). Conclusions In this paper, we investigated the mechanism of the incorporation of magnetite nanoparticles into polypyrrole, based on the procedure available in the literature. We evidenced the iron oxalate formation on the nanoparticles’ surfacesas a consequence of the interaction between the nanoparticles and the potassium tetraoxalate electrolytesby performing diffuse reflectance FT-IR measurements. This surface reaction results in a negative surface charge, enabling the nanoparticles to incorporate into the polymeric film as part of the charge compensation during the polymerization. Electrochemical quartz crystal microbalance data indicated the incorporation of 26 m/m% magnetite. The electrochemical studies showed that the voltammograms of the polypyrrole/magnetite layers became asymmetric in the

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presence of oxygen. We proved that the cathodic charge surplus is larger at lower sweep rates, and it is related to O2 reduction. The RDE studies indicated the dominancy of the 4 e- reduction of O2. We have shown that, under illumination, significantly higher currents can be detected, due to the reduction of H2O2, formed in the reaction between the photogenerated electron and oxygen. Moreover, we demonstrated that, under chronoamperometric circumstances, the photocurrent is the double of the dark current. The electrocatalytic decomposition of H2O2 (as the intermediate of the ORR) was also studied, and the composite layers showed 7.5 times larger catalytic activity in this reaction compared with the neat polypyrrole. We strongly believe that the presented photocatalytic effect can be exploited in the future in solar fuel cell applications. To contribute to the achievement of this goal, further studies on the photoeffect and on its mechanism are in progress. Acknowledgment. This work has been sponsored by the Hungarian National Research Fund (OTKA) and the National Development Agency (NFÜ) under Contract No. OTKA K72989, and TÁMOP-4.2.1/B-09/1/KONV-2010-0005. We thank the two anonymous reviewers for insightful and positive comments on the initial manuscript. Supporting Information Available: The calculation of the kinetic parameters of the ORR is shown. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Pyun, J. Polym. ReV. 2007, 47, 231. (2) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (3) Carrette, L.; Friedrich, K. A.; Stimming, U. ChemPhysChem 2000, 1, 162. (4) Gewirth, A. A.; Thorum, M. S. Inorg. Chem. 2010, 49, 3557. (5) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974. (6) Bose, C. S. C.; Rajeshwar, K. J. Electroanal. Chem. 1992, 333, 235. (7) Olson, T. S.; Pylypenko, S.; Atanassov, P.; Asazawa, K.; Yamada, K.; Tanaka, H. J. Phys. Chem. C 2010, 114, 5049. (8) Yuan, X. X.; Zeng, X.; Zhang, H. J.; Ma, Z. F.; Wang, C. Y. J. Am. Chem. Soc. 2010, 132, 1754. (9) Kingsborough, R. P.; Swager, T. M. Chem. Mater. 2000, 12, 872. (10) Wang, P.; Li, Y. F. J. Electroanal. Chem. 1996, 408, 77. (11) Kumar, S. A.; Chen, S. M. J Mol. Catal. A 2007, 278, 244. (12) Winther-Jensen, B.; Winther-Jensen, O.; Forsyth, M.; MacFarlane, D. R. Science 2008, 321, 671. (13) Khomenko, V. G.; Barsukov, V. Z.; Katashinskii, A. S. Electrochim. Acta 2005, 50, 1675. (14) Nguyen-Cong, H.; Guadarrama, V. D.; Gautier, J. L.; Chartier, P. Electrochim. Acta 2003, 48, 2389.

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