Network electrocatalytic films of conducting polymer-linked polyoxometallate-stabilized platinum nanoparticles

Electrochimica Acta 50 (2005) 5155–5162

Network electrocatalytic films of conducting polymer-linked polyoxometallate-stabilized platinum nanoparticles Pawel J. Kulesza a,∗ , Katarzyna Karnicka a , Krzysztof Miecznikowski a , Malgorzata Chojak a , Aneta Kolary a , Piotr J. Barczuk a , Galina Tsirlina b , Wojciech Czerwinski a a

Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland b Department of Electrochemistry, Moscow State University, Moscow 119992, Russia

Received 16 December 2004; received in revised form 23 March 2005; accepted 23 March 2005 Available online 1 July 2005

Abstract To fabricate electrocatalytic network films containing Pt nanoparticles, the ability of a Keggin-type polyoxometallate, phosphododecatungstate (PW12 O40 3− ), to form stable anionic monolayers on solid surfaces is explored. Three-dimensional assemblies on electrode (glassy carbon or platinum) surfaces are grown using the layer-by-layer method involving repeated alternate treatments in the solution of PW12 O40 3− (or in the colloidal suspension of polyoxometallate-protected Pt-nanoparticles) and in the solution of monomer (e.g., anilinium) cations. In the resulting structured (organic–inorganic) films, the layers of negatively charged polyoxometallate, or polyoxometallate-protected (stabilized) Pt-nanoparticles, interact electrostatically with the ultra-thin layers of such a positively charged conducting polymer as polyaniline. Consequently, physicochemical properties of organic conducting polymers, and reactivities of inorganic polyoxometallate and/or noble metal particles can be combined. The modification of Pt nanoparticles by adsorbing monolayers of phosphododecatungstate tends to activate them towards efficient electrocatalytic reduction of oxygen in acid medium. © 2005 Elsevier Ltd. All rights reserved. Keywords: Phosphotungstate; Monolayer; Stabilized platinum nanoparticles; Ultra-thin polyaniline; Layer-by-layer; Hybrid films; Oxygen reduction

1. Introduction During recent years, there has been tremendous interest in the self-assembly of ordered molecular arrays [1–3] that not only form spontaneously monolayers on solid surfaces but also permit controlled fabrication (e.g., through the sequential attraction) of any number of layers of different assemblies including polyelectrolytes and conjugated polymers [2–7]. For example, alkanothiols and their derivatives have been successfully employed to produce robust monomolecular (monolayer) and multilayered organized assemblies on gold [1–3]. Of particular interest to the advanced materials preparation is also synthesis and characterization of metal nanoparticles [8–16], their stabilization (e.g., through self-assembly), as well as organization into ∗

Corresponding author. Fax: +48 22 8225996. E-mail address: [email protected] (P.J. Kulesza).

0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.03.061

two-dimensional arrays and three-dimensional network films [13–19]. They can form nanosized materials with well-defined composition, structure and thickness. The interfaces can be also highly functionalized and exhibit specific catalytic or electrocatalytic reactivities [9–13], and unique electronic, optical or sensing properties [1,8]. Monolayers of alkanothiolates have been demonstrated to passivate gold nanoparticles and to produce the alkanothiolate-monolayer protected clusters of gold. A crucial function of the organic monolayer is to separate the metal nanoparticles and to prevent their agglomeration. An appealing alternative to alkanothiolates arises from the strong adsorption (physical or chemical) of inorganic monolayers on solid surfaces [20–24]. The concept has already been explored to the fabrication of two-dimensional arrays and three-dimensional (layer-by-layer) domains on electrodes [20–29]. Among inorganic systems, polyoxometallates, particularly Keggin-type heteropolyanions of molybdenum

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and tungsten [24–31], are attractive since they can not only adsorb strongly on solid surfaces but also exhibit reversible stepwise multielectron transfer reactions of importance to electrocatalysis, electrochromism, molecular electronics and sensing. The layer-by-layer formation of bilayer and multilayer hybrid (organic–inorganic) assemblies composed sequentially from monolayers of phosphododecamolybdate anions and ultra-thin films of positively charged conducting polymers (such as polyaniline, polypyrrole or poly(3,4ethylenedioxythiophene)/PEDOT), has been recently described [7,32,33]. By repeated and alternate treatments in the appropriate (polyoxometallate or monomer) solutions, the amount of material on the electrode surface has been increased in a controlled manner. We have also demonstrated the ability of phosphomolybdate to form rigid monolayers on platinum nanoparticles of ca. 7 nm diameter [33,34]. The concept of stabilization of nanostructured Pt goes back to the previous reports describing the formation of alkanothiol-protected Pt [35], Au/Pt alloy [36], or alkyl isocyanide-derivatized [37] and mercaptoaniline-functionalized [38] platinum nanoparticles. The electrocatalytic properties of the latter systems are, however, rather limited with respect to the potential fuel cell applications. On the other hand, the catalytic reactivity of colloidal platinum nanoparticles is enhanced when they are supported by mixed metal oxides [39]. In the present work, we pursue an idea of the protecting of Pt particles with polyoxometallate monolayers by coating them with phosphododecatungstate (PW12 ), the well-defined oxygen-bridged metal clusters related parent tungsten oxides. PW12 has been reported to undergo strong adsorption on platinum [40,41] in the broad potential range, including hydrogen and oxygen adsorption regions. Further tungsten oxides and polytungstates are known to enhance catalytic reactivity of Pt centers towards electroreduction of oxygen [42,43]. Since the application of polymer matrices [44–46] have been found to improve such physical properties as dispersion, size and morphology of dispersed metals, we link PW12 covered platinum nanoparticles together through ultra thin-conducting polymer (polyaniline) bridges to form three-dimensional network films. Our results are consistent with the view that the nanosized PW12 stabilized platinum layers produce interfaces with the promising electrocatalytic properties towards reduction oxygen.

2. Experimental Phosphododecatungstic acid, H3 PW12 O40 (PW12 ) and aniline were obtained from Fluka. Platinum black clusters (surface area, 20 m2 g−1 ) were obtained from Johnson & Matthew. All other chemicals were reagent grade purity, and they were used as received. Solutions were prepared using doubly distilled and subsequently deionized (Millipore MilliQ) water. Ultra high purity argon gas was used to deaerate investigated solutions. Experiments were carried out at room temperature (20 ± 2 ◦ C).

Electrochemical measurements were done with CH Instruments (Austin, USA) Model 750 workstation. Rotating disk electrode voltammetry experiments were performed using Pine Instruments/Princeton Applied Research (USA) Model 636 system. A standard three-electrode cell was used for the preparation of films and for other electrochemical measurements. The working electrode was a glassy carbon disk (3 mm diameter) supplied by Bioanalytical Systems (West Lafayette, IN, USA) or a platinum disk (2 mm diameter) from Mineral, Poland. Before modification, a glassy carbon substrate was subjected to polishing (on a cloth) with successively finer grade aqueous alumina slurries (grain size, 5–0.5 ␮m) whereas a platinum electrodes was also activated by potential cycling from −0.4 to 1.4 V in 0.5 mol dm−3 H2 SO4 for 1–2 h. The counter electrode was made from Pt wire. All potentials were expressed versus the saturated (KCl) Ag/AgCl electrode. To prepare glassy carbon electrodes modified with PW12 free Pt nanoparticles, 12–40 ␮L of the suspension formed by sonication of Pt black clusters (0.33 g) in water (10 cm3 ) was introduced onto the electrode substrate followed by drying and rinsing with water. Modification of glassy carbon or platinum with a PW12 monolayer (adsorbate) was achieved by dipping the electrode substrate for 30 min in an aqueous solution of 10 mmol dm−3 H3 PW12 O40 . PW12 -protected platinum nanoparticles (PW12 -Pt) were produced as follows. A suspension of a known amount (0.33 g) of Pt black was formed in 10 mmol dm−3 aqueous PW12 solution (10 cm3 ). The suspension was sonicated for 2 h, left overnight and then centrifuged 3–4 times. Each supernatant solution was removed and replaced with a fresh PW12 solution. The final centrifuging step was done with water. The resulting Pt-PW12 colloidal suspension (pH ≈ 3) was stable for at least a month. Preparation of multilayer films of PW12 and polyaniline (PANI) on platinum was achieved via the alternate immersion scheme analogous to that described by us earlier for phosphomolybdate on glassy carbon [7]. Anilinium (monomer) ions were introduced into PW12 monolayer by exposing the PW12 -modified Pt electrode to a 0.07 mol dm−3 solution of aniline in 0.5 mol dm−3 H2 SO4 for 5 min. Ultra-thin PANI was electrochemically polymerized by potential cycling (at 50 mV s−1 ) during positive scans from −0.1 to 0.85 V in 0.5 mol dm−3 H2 SO4 . After each treatment, the electrode was thoroughly rinsed with water. Film loadings, expressed as PW12 surface coverages (in mol cm−2 ), were estimated (upon consideration of the extrapolated baseline) from charges under the system’s reduction voltammetric peak recorded at ca 0.0 V at a slow scan rate, 5 mV s−1 . The network films containing Pt nanoparticles were assembled through the alternate immersion (layer-by-layer) scheme analogous to that described above. The glassy carbon electrode was initially exposed to the solution (colloidal suspension) of PW12 -protected platinum (Pt-PW12 ) particles for 30 min, followed by rinsing with water. In the next step, the layer containing Pt-PW12 nanoparticles was exposed to 0.07 mol dm−3 solution of aniline in 0.5 mol dm−3 H2 SO4

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for 10 min. Consequently, the protonated anilinium ions were electrostatically attracted and introduced into the anionic PW12 monolayers on Pt nanopartices. Electropolymerization of ultra-thin PANI was achieved as before [7,34] by potential cycling (at 50 mV s−1 ) from −0.1 to 0.85 V in 0.5 mol dm−3 H2 SO4 . By performing additional alternate immersions in the respective solutions, three-dimensional hybrid (organic–inorganic) network films containing PW12 Pt nanoparticles can be produced. The morphology of platinum electrocatalytic particles was monitored in a manner described earlier [34] using a Philips CM 10 scanning transmission microscope (TEM) operating at 100 kV. Optically transparent indium tin oxide (ITO) covered glass electrodes for spectrophotometry were from Delta Technologies, Stillwater, USA.

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where n is equal to 1 or 2. Although the second (more negative) set of peaks seems to be already affected by the proton discharge reaction (Fig. 1b), the charges under two oxidation peaks appearing at potentials ca. 0 and 0.3 V are

approximately equal. The full-width at half-maximum of the peak for the reduction appearing at about 0 V is only slightly exceeding the theoretical value of 90 mV expected for an ideal one-electron surface type voltammetric peak. The peak heights are directly proportional to scan rates up to ca. 5 V s−1 . Moreover, formal potentials of the two sets peaks in Fig. 1 are largely independent of scan rate. For the best defined redox transition at about 0 V, the respective ratios of oxidation-to-reduction peak currents are close to unity. Based on the data for the cathodic peak at 0 V (Fig. 1b), we have estimated the apparent surface coverage of PW12 to be ca. 6 × 10−10 mol cm−2 . Although the roughness factor of glass carbon is unknown, it is reasonable to interpret the latter value in terms of coverage equivalent to, or somewhat exceeding, the monolayer (PW12 ) type coverage. Fig. 2 shows cyclic voltammetric responses of (a) bare platinum disk electrode and (b) its surface subsequently modified with a monolayer of PW12 . The fact that hydrogen adsorption peaks appear on bare Pt (Curve a) is consistent with the existence of a clean (activated) platinum surface [48]. The data of Fig. 2 are consistent with recent findings showing the spontaneous adsorption of polytungstate on polycrystalline platinum [40,41]. This phenomenon has been interpreted in terms of partial charge transfer from Pt to tungstate. It is apparent from Fig. 2 that the PW12 voltammetric peaks appear in the potential range where hydrogen adsorption peaks exist on bare Pt. It has been postulated that polytungstates adsorbed on Pt interact with hydrogen adatoms to form mixed hydrogen/tungstate adlayers that are characterized by reversible redox behavior interpreted in terms of spillover effect [40,41]. Our present results also support this view. Fig. 3 illustrates the layer-by-layer growth of a multilayer hybrid film (on Pt) consisting of PW12 and PANI. An increase of voltammetric peak currents occurs following alternate treatments in PW12 and monomer solutions (the latter step is coupled with interfacial electropolymerization of PANI) as

Fig. 1. Cyclic voltammetric responses of (a) bare glassy carbon substrate and (b) PW12 monolayer adsorbed on glassy carbon. Electrolyte, 0.5 mol dm−3 H2 SO4 . Scan rate, 50 mV s−1 .

Fig. 2. Cyclic voltammetric responses of platinum electrode substrates (a) bare and (b) modified with PW12 monolayer. Electrolyte, 0.5 mol dm−3 H2 SO4 (saturated with argon). Scan rate, 50 mV s−1 .

3. Results and discussion Fig. 1 shows a typical cyclic voltammetric (CV) response of a glassy carbon electrode modified with a monolayer of PW12 (Curve b). For comparison, a response of bare glassy carbon is provided (Curve a). To avoid interference from hydrogen evolution reaction at negative potentials, we have restricted our considerations to the two most positive sets of voltammetric peaks. Despite some mechanistic differences regarding the actual involvement of proton, the redox reactions can be described in terms of two consecutive one-electron processes [24,25,47]: PWVI 12 O40 3− + ne− + nH+ ⇔ Hn PWV n WVI 12 − n O40 3− (1)

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Fig. 3. Cyclic voltammetric responses of platinum electrode (a) modified with PW12 -PANI bilayer and (b–d) after processing through additional 1–3 cycles of alternate treatments in 2 mmol dm−3 PW12 and 0.07 mol dm−3 aniline (in 0.5 mol dm−3 H2 SO4 ). Electrolyte, 0.5 mol dm−3 H2 SO4 . Scan rate, 50 mV s−1 .

described in Section 2. In the procedure, the monomer cations are initially electrostatically attracted by the anionic PW12 monolayer. The polymerization step of the surface confined monomer units is completed electrochemically upon medium transfer to acid electrolyte. It seems plausible to attribute the set of peaks appearing in the multilayer film at about 0.0–0.2 V to the electroactivity of PANI, namely to the redox behavior of emeraldine/leucoemeraldine system [49–51]. The voltammetric data of Fig. 3 seem to be dominated by the response characteristic of PANI layers whereas typical peaks of PW12 are practically invisible. It is possible that aniline monomers enter the gallery space of the inorganic structure [52], and they undergo intercalative polymerization. The fact that PW12 peaks are barely visible in Fig. 3 may also be due to the low conductivity of PANI interlayers at potentials

lower than 0.1 V. By plotting the film loading (calculated from the charge under the PANI component oxidation peak), an approximately linear dependence (with effectively zero intercept) versus a number of treatments in PANI solutions (slope; 5 × 10−10 mol cm−2 per immersion cycle) has been obtained. Here, it has been assumed (according to the commonly accepted PANI redox mechanism involving the first pair of peaks) that only one-fourth of the total number of monomers are involved in the reaction at about 0–0.2 V. It is reasonable to expect that PW12 undergoes similar adsorption on nanosized Pt (platinum black) as on a bulk platinum surface (Fig. 2). The procedure of modification of Pt nanoparticles with monolayers of PW12 anions involves a few exposures of the Pt clusters (composed of the agglomerated Pt nanoparticles) to PW12 solutions followed by centrifuging and decanting (as described in Section 2). At the end, a colloidal suspension of nanosized platinum in water is formed. The existence of electrostatic repulsive interactions between the negatively charged PW12 monolayers are presumably responsible for the splitting of Pt clusters and stabilizing the colloidal solution of Pt nanoparticles. Fig. 4A illustrates a representative TEM image of such PW12 -covered Pt particles (embedded into the carbon film). Most of particles have diameters ranging from 5 to 10 nm. Contrary to PW12 free platinum particles, where neither dilution nor extensive sonication of the original suspension have led to significant changes in the nanoparticle size distribution, prolonged sonication of PW12 -covered Pt nanostructures tends to diminish sizes of nanoparticles. When a conductive glass (ITO) substrate was dipped into the solution of Pt nanoparticles, they assembled to form an ultra thin film on its surface. A representative spectrum (in the visible range) of nanostructured PW12 -Pt with the film’s characteristic maximum absorbance at about 440 nm is illustrated in Fig. 4B.

Fig. 4. (A) Transmission electron micrograph (TEM) of the PW12 -stabilized platinum nanoparticles. (B) Spectrum of PW12 -covered Pt nanoparticles (in visible range) that have been deposited on transparent ITO electrode.

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Fig. 5. (A) Cyclic voltammetric responses of an ultra thin film of PW12 protected Pt-nanoparticles deposited on glassy carbon. (B) For comparison, voltammetric characteristics of PW12-free Pt nanoparticles immobilized on glassy carbon is provided. Other conditions as for Fig. 1.

Fig. 5A shows a cyclic voltammetric response (in deoxygenated electrolyte) of a glassy carbon electrode modified with an ultra thin of PW12 -covered Pt nanoparticles. They have been spontaneously deposited by simple dipping of the electrode substrate in the PW12 -stabilized colloidal suspension. For comparison, the cyclic voltammogram of PW12 -free platinum particles introduced onto the glassy carbon surface is also provided (Fig. 5B). The voltammetric pattern of Fig. 5A is characterized by mixed (overlapping) heteropolytungstate and the adsorbed hydrogen responses in the potential range from 0.1 to −0.15 V. It is interesting to note that the proton discharge current in Fig. 5A tends to appear at potentials less negative than the analogous current observed on bare Pt nanoparticles (Fig. 5B) or at the bulk Pt electrode (Fig. 2a). Apparently, the presence of PW12 monolayer has some activating effect on the Pt-catalyzed proton reduction. The growth of the multilayer network film consisting of PtPW12 and PANI is evident from the increase of voltammetric peak currents (Fig. 6) recorded in the electrolyte following alternate immersions in Pt-PW12 colloidal suspension and the anilinium solution (the latter step was combined with the interfacial electropolymerization of PANI). Approximately, 1.5 × 10−9 mol cm−2 of PANI is added per each immersion cycle. It is plausible to expect that PW12 (that is attached to platinum) is rigid and retains its anionic properties necessary for the electrostatic attraction to positively charged PANI. Most probably the protons compensating charge of the closely located PW12 anions are replaced by organic cations. To address the electrocatalytic reactivity of platinum nanoparticles dispersed within network film towards reduction of dioxygen, we have performed a series of voltammetric experiments in 0.5 mol dm−3 H2 SO4 solutions saturated with oxygen (Fig. 7). While Curve a shows the response obtained at Pt-PW12 monolayer type coverage on glassy carbon, Curves b and c refer to the oxygen reduction at the hybrid (network) organic–inorganic films prepared by alternate

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Fig. 6. Cyclic voltammetric responses of a network hybrid PW12 -Pt/PANI film (on glassy carbon) recorded after processing through (a) one, (b) two and (c) four cycles of alternate treatments in the solutions of PW12 -protected Pt and 0.07 mol dm−3 aniline (in 0.5 mol dm−3 H2 SO4 ). Other conditions as for Fig. 3.

immersions and containing one and two PANI interlayers, respectively. The results are consistent with the view that PW12 -protected Pt nanoparticles catalyze reduction of oxygen in the potential range characteristic of metallic Pt (Fig. 7a). On the other hand, the voltammetric peaks for the oxygen reduction at the network film (in which Pt-PW12 nanoparticles are linked by ultra-thin PANI layers) appear at somewhat less positive potential (Fig. 7, Curves b and c) in comparison to the data of Curve a. Our preliminary rotating disk measurements indicate that, while the oxygen reduction was effectively four-electron diffusion-controlled process in case of the system of Fig. 7 (Curve a), the interfacial electron-transfer kinetic control became apparent (starting

Fig. 7. Electrocatalytic reduction of oxygen at (a) PW12 -Pt ultra-thin film deposited on glassy carbon, (b) the three-layer network hybrid film of the type PW12 -Pt/PANI/PW12 -Pt and (c) the five-layer network hybrid film of the type PW12 -Pt/PANI/PW12 -Pt/PANI/PW12 -Pt. Voltammetric responses were recorded in 0.5 mol dm−3 H2 SO4 saturated with dioxygen. Scan rate, 50 mV s−1 .

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from 1600 rpm) upon introduction of PANI layers. It is unclear whether this inhibition effect is caused by interfacial electropolymerization of aniline monomer on the surface of PW12 -stabilized Pt nanoparticles or by the passivating effect of the monomer itself. It should be remembered, however, that conducting polymers (including PANI) can produce attractive matrices for dispersed platinum [53]. An important issue may be that PANI must be first polymerized before Pt particles are introduced into the film [34]. Further research is necessary to clarify the role of the organic linking component in our electrocatalytic network films. At larger amounts of PW12 -Pt nanoparticles (i.e., when the 40 ␮L aliquot of the PW12 -Pt solution was introduced onto the electrode surface, dried and rinsed with water) attached to glassy carbon, fairly large (background subtracted) voltammetric current (76 ␮A) and even more positive potential (0.57 V) for the oxygen reduction has been observed (Fig. 8A). The fact that PW12 is interfacially present on nanostructured Pt is apparent from the existence of the

overlapping peaks originating from PW12 itself and electroactivity of the adsorbed hydrogen atoms. An additional important feature of the data of Fig. 8A is that, presumably due to capability of PW12 polyanions to link Pt nanoparticles by adsorbing on their surfaces, as well as to interact with a glassy carbon substrate, fairly thick and stable coatings with the loading of platinum on the level of ca. 100 ␮g cm−2 can be obtained. For comparison, when the analogous amount PW12 -free Pt nanoparticles was introduced onto the glassy carbon substrate, dried and rinsed with water (as before for Fig. 8A), the recorded background currents (Fig. 8B, dotted line) were more than order of magnitude lower. Contrary to PW12 -stabilized and linked Pt nanoparticles (Fig. 8A), most of PW12 -free platinum particles (Fig. 8B) were presumably lost during rinsing. Consequently, the loading of Pt nanoparticles was in the latter case lower. Thus, for the voltammetric reduction of oxygen (Fig. 8B), the peak potential and the net peak current were less positive and lower, respectively, when compared to the analogous behavior investigated under the analogous conditions but using the PW12 modified Pt nanoparticles (Fig. 8A). In order to obtain more reliable assessment on the oxygen reduction activity, we have performed the rotating disk electrode (RDE) voltammetric measurements. A typical wellbehaved RDE current potential curve (at 3000 rpm rotation rate) for the electroreduction of oxygen on the glassy carbon electrode modified with PW12 -Pt nanoparticles (as for Fig. 8A except that the loading of Pt was somewhat lower, 30 ␮g cm−2 ) is shown in Fig. 9. The dependence of the respective RDE limiting currents versus the square root of rotation rate (Inset A) shows deviation from linearity (dotted line), i.e., from the ideal behavior characteristic of a system

Fig. 8. (A) Voltammetric oxygen reduction (solid line) at the higher loading of PW12 -Pt nanoparticles immobilized on glassy carbon. (B) For comparison, oxygen reduction has been also recorded at PW12 -free Pt nanoparticles immobilized on glassy carbon. Electrolyte: 0.5 mol dm−3 H2 SO4 saturated with dioxygen. Scan rate: 50 mV s−1 . Dotted curves refer to the respective background responses in the oxygen-free (argon-saturated) electrolyte.

Fig. 9. Representative RDE voltammogram recorded in the oxygensaturated solution at the moderate rotation rate, 3000 rpm. Scan rate, 10 mV s−1 . Electrolyte, 0.5 mol dm−3 H2 SO4 . Insets (A and B) show the Levich (limiting current, ilim , vs. square root of rotation rate, ω) dependence and the Koutecky-Levich reciprocal plot, respectively. Limiting currents were measured at 0.2 V.

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limited solely by convective diffusion of oxygen in solution, at rotation rates higher than ca. 2500 rpm. This result implies that only at faster rotation rates, the electrocatalytic reaction was too small to allow the convective-diffusional control to be operative. The analysis, performed by means of so-called Koutecky-Levich reciprocal plots [44] (Inset B) yielded a fairly small positive intercept equal to about 0.04 mA−1 cm2 . Using the approach described earlier [44], the kinetic parameter, namely the intrinsic rate constant of heterogeneous charge-transfer, has been found to be ca. 2–3 × 10−2 cm s−1 for the catalytic electroreduction (in 0.5 mol dm−3 H2 SO4 ) of oxygen at 0.2 V (Fig. 9) using the glassy carbon electrode modified with PW12 -Pt nanoparticles. For the electrode modified with PW12 -free Pt nanoparticles (prepared as for Fig. 8B), the analogous kinetic analysis has yielded the intrinsic rate constant closer to 1 × 10−2 cm s−1 . Because the systems differed in the morphology and loading of Pt nanoparticles, unequivocal comparison of kinetic results is not possible here. Certainly, if not activating, the act of stabilizing and linking of Pt nanoparticles with PW12 does not inhibit the electroreduction of dioxygen. What is probably even more important, no ring current (at 1.2 V) was detected, i.e., no hydrogen peroxide intermediate was formed during the RDE measurements with use of PW12 -Pt nanoparticles. The latter result implies the effectively four-electron electroreduction of oxygen (to water) under conditions of Fig. 9 experiment.

4. Conclusions PW12 undergoes strong spontaneous chemisorption on solid surfaces (e.g., glassy carbon, Pt). We also demonstrate the usefulness of the layer-by-layer approach to the fabrication of hybrid films composed of the heteropolyanion (e.g., PW12 ) monolayers and ultra-thin layers of conducting polymers (e.g., PANI). Although a concept of the preparation of composite films of polyoxometallates and conducting polymers is not new, our approach is based on electropolymerization of surface-confined monomer ions that are electrostatically attracted to a negatively charged polyoxometallate (PW12 ) monolayer. An important issue is the feasibility of stabilization of platinum nanoparticles by protecting them through chemisorption of PW12 monolayers on their surfaces. Modification of platinum nanoparticles with PW12 has been demonstrated to produce highly reactive platinum centers (e.g., towards reduction of oxygen). Having in mind the chemical analogies between PW12 and its parent oxide (WO3 ), PW12 has been found to provide similar environment to tungsten oxide/hydrogen bronze [44] matrices. Indeed, no hydrogen peroxide intermediate has been detected during the ring-disk voltammetric measurements. Finally, the PW12 -Pt nanoparticles can not only be deposited on electrode surfaces but also linked by via ultra-thin polymer (PANI) layers to form electrocatalytic network films.

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Acknowledgements This work was supported by Ministry of Science (Poland) under the State Committee for Scientific Research (KBN) grants 3 T09A 05426 and 4 T09A 12225 (doctoral). K.M. was supported in part by the University of Warsaw under BW-1637/9/04. M.Ch. acknowledges the stipend from Foundation for Polish Science, FNP. Technical help of A. Piranska is appreciated. G.Ts. thanks the Russian Science Support Foundation. The support from the Network Efficient Oxygen Reduction for Electrochemical Energy Conversion (coordinated by ZSW, Ulm, Germany) is highly appreciated.

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