Pt–RuO2 electrodes prepared by thermal decomposition of polymeric precursors as catalysts for direct methanol fuel cell applications

international journal of hydrogen energy 34 (2009) 2747–2757

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Pt–RuO2 electrodes prepared by thermal decomposition of polymeric precursors as catalysts for direct methanol fuel cell applications L.P.R. Profetia,*, D. Profetia, P. Olivib a

Departamento de Fı´sico-Quı´mica do Instituto de Quı´mica de Sa˜o Carlos, Universidade de Sa˜o Paulo, CEP 13560-970, Sa˜o Carlos, SP, Brazil b Departamento de Quı´mica da Faculdade de Filosofia, Cieˆncias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, CEP 14040-901, Ribeira˜o Preto, SP, Brazil

article info

abstract

Article history:

The electrocatalytic activity of Pt and RuO2 mixed electrodes of different compositions

Received 14 November 2008

towards methanol oxidation was investigated. The catalysts were prepared by thermal

Received in revised form

decomposition of polymeric precursors and characterized by energy dispersive X-ray,

6 January 2009

scanning electronic microscopy, X-ray diffraction and cyclic voltammetry. This prepara-

Accepted 7 January 2009

tion method allowed obtaining uniform films with controlled stoichiometry and high

Available online 11 February 2009

surface area. Cyclic voltammetry experiments in the presence of methanol showed that mixed electrodes decreased the potential peak of methanol oxidation by approximately

Keywords:

100 mV (RHE) when compared to the electrode containing only Pt. In addition, voltam-

Methanol electrooxidation

metric experiments indicated that the Pt0.6Ru0.4Oy electrode led to higher oxidation current

Platinum

densities at lower potentials. Chronoamperometry experiments confirmed the contribu-

Ruthenium oxide

tion of RuO2 to the catalytic activity as well as the better performance of the Pt0.6Ru0.4Oy

Polymeric precursors

electrode composition. Formic acid and CO2 were identified as being the reaction products formed in the electrolysis performed at 400 and 600 mV. The relative formation of CO2 was favored in the electrolysis performed at 400 mV (RHE) with the Pt0.6Ru0.4Oy electrode. The presence of RuO2 in Pt–Ru-based electrodes is important for improving the catalytic activity towards methanol electrooxidation. Moreover, the thermal decomposition of polymeric precursors seems to be a promising route for the production of catalysts applicable to DMFC. ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The electrochemical oxidation of small organic molecules has been extensively investigated due to the possible use of these molecules as fuel in energy conversion systems, e.g. fuel cells [1–3]. Among the studied electrode materials, the noble

metals, especially Pt, provide the best performance for the electrooxidation of organic molecules [4,5]. However, these metals undergo progressive loss of their catalytic activity caused by the strong adsorption of the reaction intermediate species on the electrode surface. These species are only desorbed at higher potentials, thus leading to a low performance

* Corresponding author. Tel./fax: þ55 16 33739952. E-mail address: [email protected] (L.P.R. Profeti). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.011

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of the fuel cell. The species that block the Pt surface have been identified as CO-like intermediates [6,7]. Materials capable of avoiding the formation of poisoning species or enabling the oxidation of intermediates have been developed with the aim of overcoming the limitations of using Pt as a catalyst. Such materials combine Pt with other metals (i.e. Ru, Sn, Mo, W) either by means of alloys or ad atoms [8–11]. Electronic-conductor metal oxides, such as RuO2, SnO2 and RhO2, are alternative electrode materials which may avoid the poisoning phenomenon. Indeed, metal oxides are expected to have varied metal oxidation states, which could lead to the adsorption of large amounts of OH species [12,13]. Among the metal oxides suitable for catalyzing methanol electrooxidation, RuO2 has some interesting properties that, combined with Pt, could decrease surface poisoning by CO species. Some of these properties include its electrochemical surface characteristics, which enables Ru oxidation/reduction through proton exchange with the electrolyte solution [14]: RuOx(OH)y þ de þ dHþ ! RuOxd þ d(OH)yþd

(1)

These characteristics can be related to the bifunctional mechanism of methanol oxidation on Pt–Ru catalysts proposed by Watanabe and Motoo [4], which the Ru atoms adsorb and dissociate H2O molecules, thus generating OH species at lower potentials than in the case of Pt. Consequently, oxygenated species formed on the Ru surface should react with the CO species adsorbed onto the Pt surface at lower potentials [15]. Based on the mechanism of methanol oxidation proposed in the literature, the various Ru oxidation states on RuO2 generated by the adsorption of H2O molecules are suggested to play an important role in the oxidation of CO species adsorbed onto the Pt catalysts. The study of Rolison et al. [15,16], who used commercial Pt– Ru alloys, confirmed the presence of oxygen not only on the surface but also in the bulk of these materials. This indicates that the Pt–Ru alloys contain oxides besides metallic Ru. In the same study, the methanol electrooxidation on hydrous Pt–Ru was investigated in order to compare the catalytic activity of this material with that of Pt–Ru alloy, and it was found that hydrous Pt–Ru catalysts are more efficient than Pt–Ru alloys, emphasizing the importance of Ru–OH species in the mechanism of methanol oxidation. In another study, Lasch et al. [17] investigated methanol oxidation on Pt–Ru catalysts synthesized by the Adams method at different calcination temperatures (400–600  C). The material prepared at 470  C consisted of either a metallic alloy or an amorphous phase corresponding to hydrous ruthenium oxide, and the best performance was achieved with this electrode. All the properties of the electrocatalysts are known to be hardly related with the synthesis method [18–20]. The coating composition and the component ratio of the electrode influence its morphological and structural characteristics, degree of interaction between different metals, electrochemical behavior and catalytic activity. The method most commonly employed in the preparation of oxide coatings is the thermal decomposition of metallic inorganic salts (chloride and nitrites). However, this method produces films with different final compositions. Comninellis and Vercesi [18,19] prepared many oxides by thermal decomposition of inorganic salts, and

it was verified, by ray-X fluorescence analysis, that RuO2, IrO2 and Ta2O5 are formed almost 100% yield, while PtOx and SnO2 are lost during the calcination process. These authors further observed that thermal decomposition of Pt precursors led to PtO2, PtO or Pt as final products, depending on the condition used during the preparation. Thus, the subscript ‘‘x’’ in PtOx denotes the various stoichiometries of the different Pt oxides. An alternative method for preparing electrodes consisting of metal oxide films is the calcination of polymeric precursors, also known as the Pechini method [21]. This method consists in dissolving the metal salts in the presence of a hydroxycarboxilic acid and a polyhydroxilic alcohol in order to obtain a solution with homogeneously distributed metals. The weak acid forms chelates with the metallic cations, which undergo a polysterification reaction under heating in the presence of the polyhydroxyalcohol. Thus, homogeneous oxide films may be formed by thermal decomposition of this polymeric network, and the loss of metals, such as Pt and Sn, may be avoided. The polymeric precursor method is a versatile preparation procedure which allows obtaining catalysts in different forms, such as oxide films deposited onto Ti plates [22], metallic oxide dispersed on carbon powder with high surface area [23], thin films deposited onto ITO electrode [24], etc., with the advantage of the easiness of the synthesis combined with the lower cost compared to the other methods (sol–gel based on metallic alkoxides) [25]. In light of the advantages of the polymeric precursor method, the current study aimed to investigate the catalytic activity towards methanol oxidation of Pt–RuO2 electrodes of different compositions supported on Ti plates, prepared by thermal decomposition of polymeric precursors. In a preliminary study [23], in which we investigated these catalysts supported on carbon powder, the fuel cell performance was found to be improved by changing the annealing temperature and optimizing the composition of the catalysts. In the current study, additional fundamental experiments were performed, including other catalyst compositions and annealing temperature, with the aim of providing new approaches for advance in the use of this method of catalyst preparation.

2.

Experimental

The electrodes were prepared as described elsewhere [26,27]. The oxide layers were prepared on Ti substrate by thermal decomposition of polymeric precursors. The Ti plate is a more suitable support than the carbon powder for fundamental studies, since it allows obtaining more reproducible and controlled catalyst layers when compared to those prepared on carbon powder [23]. Different PtxRu(1x)Oy/Ti molar compositions (x ¼ 0.3, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0) were prepared. The Pt and Ru precursor solutions were prepared by dissolving H2PtCl6 or RuCl3 (both in HCl 1:1 v/v) in a citric acid and ethylene glycol solution under constant stirring, at 90– 95  C. The metal:citric acid:ethylene glycol molar ratio employed in the preparation of these precursor solutions was 1:4:16. The concentration of the precursor solutions was determined by atomic absorption spectrophotometry. In order

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to achieve the desired electrode composition, the final solutions were obtained by mixing the precursor solutions in appropriate proportions. The solutions were carefully brushed on Ti substrate, the solvent was evaporated at low temperature (80–90  C), and the material was then calcined at 400 or 500  C for 5 min under a 5 L min1 O2 flow in a preheated oven. These steps were repeated until a thickness of 2 mm was reached. The electrodes were finally calcined at 400 or 500  C under O2 flow for 1 h. Scanning electronic microscopy (SEM) and energy dispersive X-ray (EDX) analyses were performed using a Zeiss DSM 940 microscope linked to a Link Analytical QX 2000 microanalyzer. The X-ray diffraction (XRD) data for the Pt–RuO2 films were obtained on SIEMENS D5005 diffractometer equipped with a grazing incident angle attachment, using a Cu Ka radiation source. The apparent size of platinum particles was calculated by the Scherrer formula, L ¼ 0.9l/b2q cos qmax, ˚ for the Cu Ka in which l is the X-ray wavelength (1.54056 A radiation), b2q is the width of the Pt diffraction peak at halfheight, and qmax is the Bragg angle at the peak position. The catalytic activities towards methanol oxidation were evaluated in half-cell experiments by cyclic voltammetry and chronoamperommetry at room temperature in 0.5 mol L1 H2SO4 (Merck) nitrogen saturated solution. A conventional electrochemical cell was used with a Pt platinized wire as counter-electrode and a reversible hydrogen reference electrode (RHE). All electrochemical measurements were performed in an Ecochemie Autolab PGSTAT30 potenciostat/galvanostat. High performance liquid chromatography (HPLC) was used for the analysis of the reaction products. The chromatograph (Shimadzu) was equipped with a central controller system (SCL-10Avp), an isocratic pump (LC-10Atvp) coupled to a flow controller valve (FCV-10ALvp), and an HPX-87H column (Bio Rad). The products were detected with both a UV–visible detector (SPD-10Avp) working at l ¼ 210 nm and a differential refractometer (RID-10A). The chromatograms were recorded and integrated with a Class-vp software. The eluent was a 3.33 mmol L1 H2SO4 solution. The product reactions were identified by comparing their retention times with those of pure reference products under the same conditions.

thickness of 4 mm with micropores distributed on the coating surface. Since the nominal thickness is 2 mm, the pore volume represents a substantial part of the film, thus contributing to the formation of a rough surface. This is consistent with previous observations on electrodes containing PtOy prepared from inorganic precursors [28–32]. The film compositions were investigated by EDX, which was carried out by either global or cross-section analysis. Information about the average layer composition and punctual compositions was obtained by selecting regions of the film cross-section (i.e. P1, region on the surface layer; P2, central region of the layer; P3, region nearest to the Ti support; P4 and P5, other points in the central regions of the layer as shown in Fig. 2a). The atomic percentages as a function of the composition obtained for the different regions of the samples are reported in Table 1. The atomic percentages obtained for all the surface regions are close to the nominal concentration, indicating a homogeneous distribution of the film components. This result is different from those observed in previous studies, since Ptsurface enrichment on PtOy-based electrodes and material losses are frequently reported in the literature [19,32]. The Pt segregation results in a Pt concentration on the electrode surface that is higher than the nominal composition. This implies that it is difficult to establish a direct relationship between composition and catalytic activity. In the current study, the oxide layers prepared by thermal decomposition of polymeric precursors enabled the formation of a homogeneous film. In the preparation method used in this work, a polymeric chain was formed and the Pt was arrested inside it during the calcination process, avoiding Pt diffusion to the outer oxide layer and preventing the loss of Pt. Other evidence confirming the homogeneity of the oxide film is shown in the cross-section micrographs obtained by SEM in the backscattering electron (BSE) acquisition mode (Fig. 2a). This mode allows the identification of the elements present in the layer throughout the color change. Fig. 2a shows that the color of the film is uniform, indicating a homogeneous distribution on the layer.

3.2.

3.

Results and discussion

3.1.

SEM/EDX analysis

The surface features of the PtxRu(1x)Oy electrodes calcined at 400 and 500  C were analyzed by SEM. Fig. 1 shows representative SEM micrographs of oxide layers calcined at 400  C. The Pt0Ru1Oy electrode shows the well-known cracked-mud morphology (Fig. 1a). This morphology is characteristic of electrodes obtained by thermal decomposition [20]. However, SEM results reveal a substantial change in surface upon the introduction of Pt, since a compacted microstructure was produced with some cracked-mud regions (Fig. 1b–d). A surface recovered by small grains is obtained in Pt1Ru0Oy film (Fig. 1e). A more compacted and less porous structure was observed by Silva et al. [28,29] when introducing Pt into IrO2 þ TiO2 electrodes. Fig. 2 shows representative cross-section SEM micrographs of the PtxRu(1x)Oy system, which has an average layer

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X-ray diffraction

The structure of the oxide films deposited on the Ti support was analyzed by XRD. Fig. 3 shows the pattern of the PtxRu(1x)Oy films calcined at 400  C. The presence of the RuO2 phase with a tetragonal structure was observed in the absence of Pt. The peak at 2q ¼ 35 indicates the presence of the crystalline material with rutile phase, probably RuO2 (JCPDS PDF#40-1290). Peaks at 2q ¼ 40 , 47 , and 67 (JCPDS PDF#040802), which are associated with the Pt metallic structure, were observed in the Pt1Ru0Oy film. The XRD data obtained for the material with intermediate composition provides evidence of the presence of both phases, as well as the change on peak intensities according to the Pt and Ru content in the electrode composition. No peaks due to Pt oxide species were observed. Comninellis and Vercesi [19] showed that calcination temperatures above 200  C promote the formation of Pt with a high degree of crystallinity, instead of its oxide. This fact can be noted in the diffractograms through the presence of high narrow Pt peaks. The average crystal particle size of

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Fig. 1 – SEM micrographs of electrodes calcined at 400 8C. (a) RuO2, (b) Pt0.7Ru0.3Oy, (c) Pt0.5Ru0.5Oy, (d) Pt0.3Ru0.7Oy and (e) PtOy, amplification: 20003.

platinum was calculated from X-ray diffractograms by the Scherrer’s equation using the Pt diffraction peak at 2q ¼ 47 , where no influence of the ruthenium phases was observed. The values were observed to vary between 30 nm for Pt1Ru0Oy and 14–18 nm for the bimetallic catalysts. The intensities of the Ru peaks are lower than those of the Pt peaks. This result may be attributed to either the low Ru content in the oxide composition or the formation of a solid solution, although no evidence for the latter has been found yet. Indeed, the formation of a mixture of platinum with high degree of crystallinity dispersed in the RuO2 matrix probably occurs. According to thermogravimetric analysis and X-ray diffraction performed by Lasch et al. [17], the calcination

temperature used here (400  C) leads to the formation of an amorphous and hydrous RuO2 phase, and only temperatures up to 500  C results in phases with a high degree of crystallinity. In this study, the method used for preparing the Pt–RuO2 films allowed obtaining Pt in its metallic state, with no need of a further step for Pt reduction in the films by means of a reducing agent (H2 or sodium borohydride).

3.3.

Electrochemical characteristics

3.3.1.

Cyclovoltammetric behavior

The surface features of the electrodes were characterized by cyclic voltammetry at 20 mV s1 in 0.5 mol L1 H2SO4 solution,

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Fig. 2 – Cross-section SEM micrographs of electrodes calcined at 400 8C. (a) Pt0.3Ru0.7Oy (amplification: 50003 in BSE mode) (b) Pt0.3Ru0.7Oy (amplification: 100003).

covering the 0.05–1.4 V (RHE) potential region. Prior to the voltammetric experiments, the electrodes were submitted to 20 successive cycles at 20 mV s1 in order to yield reproducible results. The stabilization of the voltammetric pattern may be attributed to the hydration of the internal sites, since a small increase in the current values could be observed during the previous voltammetric experiments. Fig. 4 compares the voltammetric curves of the Pt0.5Ru0.5Oy electrode and the pure materials, calcined at 500  C. The voltammetric response of the RuO2-coated electrode prepared by thermal decomposition of polymeric precursors is similar to those observed for electrodes prepared by thermal decomposition of inorganic salts in previous research [33]. Fig. 4 shows that the RuO2 cyclic voltammogram has features of high background currents and broad peaks in both positive and negative sweeps. This profile is attributed to the ruthenium surface redox transitions. The first pair of peaks at 0.7 V (RHE) is due to the Ru(III)/Ru(IV) solid-state surface transition, while the second, located at 1.2 V (RHE), is due to ruthenium transitions to higher oxidation states [33]. An increase in the charge is observed in the cyclic voltammograms during the initial cycles. This behavior has been reported in the literature [20,33] and is related to the proton exchange between the surface OH groups and the solution, according to the reaction (1). The voltammetric curve of Pt1Ru0Oy closely resembles the one obtained with platinum in acid medium with hydrogen adsorption/desorption (0–0.4 V), double-layer charging region

(0.4–0.8 V), oxide film formation/removal (0.8–1.4 V). However, this voltammetric profile is formed after the second cycle covering the 0.05–1.4 V (RHE) potential region, reaching maximum current values in the fifth cycle, which remains stable until at least the 500th cycle. This behavior is attributed to the fact that platinum is easily oxidized and reduced in the first four cycles. During oxide reduction, the place–exchange reaction increases the population of the Pt ad atoms on the electrode surface, thus providing a rougher surface [31]. The increase in the current values of the cyclic voltammograms can also be explained in terms of the oxide surface hydration phenomenon [20]. In the case of the bimetallic materials, there are some significant changes in the profile of the i–E voltammetric features of the electrodes. First, an apparent current increase is observed for the intermediate composition. In the doublelayer charging region; a transition anodic broad peak of Ru(III)/ Ru(IV) at 0.7 V (RHE) and a transition anodic peak of higher ruthenium oxidation states at 1.2 V (RHE) can also be noted. For comparative purposes, the electrodes were also prepared and calcined at 400 and 500  C. The voltammetric profiles of the electrodes calcined at 400  C are similar of those calcined at 500  C (Fig. 4), but the charge values of the former are higher. This may be a consequence of the decrease in the effective surface area due to the crystallization and sintering phenomena taking place at higher temperatures. These phenomena lead to crystal growth, thus decreasing the surface area and the number of active sites on the surface [34].

Table 1 – Compositions by EDX for Pt–RuO2 electrodes calcined at 400 8C. Nominal

% Atomics experimental Global

Regions P1

Pt 0.3 0.5 0.7

Ru 0.7 0.5 0.3

P2

P3

P4

P5

Pt

Ru

Pt

Ru

Pt

Ru

Pt

Ru

Pt

Ru

Pt

Ru

0.34 0.57 0.73

0.65 0.42 0.26

0.31 0.49 0.70

0.69 0.51 0.29

0.32 0.47 0.75

0.68 0.52 0.24

0.27 0.43 0.72

0.73 0.47 0.27

0.33 0.51 0.70

0.67 0.49 0.29

0.31 0.51 0.74

0.69 0.49 0.25

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Fig. 5 – Surface area obtained from the CO stripping charge as a function of electrode composition at both calcination temperatures. Fig. 3 – X-ray diffractograms for Pt–RuO2 deposited on a Ti support, calcined at 400 8C.

3.3.2.

Determination of platinum area by CO stripping charge

The Pt-surface area was obtained by stripping of a monolayer of CO and considering a charge of 420 mC cm2 [35]. For this purpose, the CO saturation coverage on the catalysts was achieved by bubbling CO for 10 min at 50 mV followed by bubbling Ar for 20 min in order to eliminate dissolved CO. Fig. 5 shows the behavior of the area as a function of Pt content at both calcination temperatures. As expected, the areas obtained for the electrodes calcined at 400  C are larger than those of the electrodes calcined at 500  C. Electrodes calcined at temperatures higher than 400  C undergo crystallization through a sintering phenomenon. This results in

surfaces with active sites less accessible to adsorption of CO, consequently decreasing the area available for this process. Additionally, the surface area increases as a function of the Pt loading until a maximum is reached for Pt0.6Ru0.4Oy (400  C) and Pt0.8Ru0.2Oy (500  C) electrodes, and subsequently decreases for higher Pt concentrations. A similar behavior was found by Hu et al. [36] for Pt–RuO2 electrodes, with a maximum of area obtained for electrodes containing 60% Pt, followed by a decrease for higher Pt amounts. The authors attributed the occurrence of a maximum to the degree of Pt dispersion within these coatings, the variation in surface roughness and the difference in the electronic structure of the deposits. Da Silva et al. [28,29] also observed this feature and attributed to the presence of less porous grains at high Pt concentrations.

3.3.3.

Double-layer capacitance determination (Cdl)

The cyclic voltammograms of PtxRu(1x)Oy electrodes has a higher currents in the double-layer region compared to that of Pt1.0Ru0Oy electrode. This increase in the capacitive current was obtained for the electrodes with higher RuO2 loading, except for RuO2 alone. This indicates that the combination of two compounds leads to the formation of a porous and less

Table 2 – Double-layer capacitance for each electrode composition at both calcination temperatures. Cdl/mF

Catalyst 

Fig. 4 – Representative voltammetric curves at 20 mV sL1 for Pt–RuO2 electrodes (calcined at 400 8C) in 0.5 mol LL1 H2SO4 solution.

Pt0Ru1Oy Pt0.3Ru0.7Oy Pt0.5Ru0.5Oy Pt0.6Ru0.4Oy Pt0.7Ru0.3Oy Pt0.8Ru0.2Oy Pt0.9Ru0.1Oy

T ¼ 500 C

T ¼ 400  C

12.40 28.40 32.26 33.66 24.21 26.84 14.02

44.64 77.90 61.80 50.13 46.40 33.53 17.63

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the electrodes calcined at 400  C are also higher than the Cdl of the electrodes calcined at 500  C. These results are in agreement with the surface area calculated by CO stripping, since the mixed oxide films form a surface with sites more exposed to species adsorption, and thus increasing the surface area.

Fig. 6 – Cyclic voltammetry for the Pt–RuO2 electrodes (calcined at 400 8C) at 20 mV sL1 in 0.5 mol LL1 H2SO4 D 0.1 mol LL1 methanol solution.

compacted surface, with more exposed RuO2 sites. This high current in the double-layer region was ascribed to the adsorption of oxygen-containing species (probably OH) on the Ru atoms [37]. In order to confirm the RuO2 contribution, the Cdl was determined by measuring the current values from the cyclic voltammogram at 0.4 V (RHE) as a function of the scan rate. Although the amount of Pt varied within the coating, a linear dependence starting from the origin and with a correlation coefficient close to 1 was obtained. This linearity may be a result of the fact that an increase in Pt loading probably leads to a small increase in the double-layer charging current [38]. Gojkovic et al. [37] and Lasch et al. [39] observed the same feature for Pt and Ru catalysts and confirmed the ability of RuO2 to adsorb OH species. Table 2 shows the Cdl values for electrodes calcined at both temperatures. The Cdl obtained for mixed oxides are higher than those of the pure oxides. Furthermore, the Cdl values of

3.4.

Catalytic activity

3.4.1.

Cyclic voltammetry experiments

In order to hydrate the surface sites before the electrooxidation experiments, the electrodes were cycled between 0.05 and 1.45 V (RHE) in 0.5 mol L1 H2SO4 solution until a stable electrode state was reached. Fig. 6 shows a typical anodic scan obtained in the presence of 0.1 mol L1 methanol solution for PtxRu(1x)Oy electrodes at different compositions compared to that of the Pt1.0Ru0Oy electrode. The voltammetric profiles are similar to those observed for Pt and Pt–Ru metallic electrodes. For mixed electrodes, the methanol oxidation was initiated at approximately 400 mV and a maximum peak current density was achieved at about 700 mV. The first methanol oxidation peak was observed around 400 mV for the mixed catalysts may be attributed to the slow process of methanol dehydrogenation at lower potentials [40]. In contrast, no oxidation peaks were observed at lower potentials for Pt1.0Ru0Oy electrode. Moreover, in this electrode the methanol oxidation started at 600 mV and reached a maximum at around 850 mV. Although the general voltammetric behavior is similar for all electrodes, the electrode composition plays a significant influence on methanol oxidation current densities, as shown in Fig. 7. In this figure, the methanol oxidation current densities at lower potential values are plotted as a function of Pt content. In Fig. 7a, all methanol oxidation current are normalized by the geometric area (2 cm2), providing a global catalytic activity, that is related to the electrode roughness. However, when these values are normalized by the area calculated from the CO stripping charge, the intrinsic catalytic activity is obtained; i.e., the catalytic activity per Pt site (Fig. 7b).

Fig. 7 – Methanol oxidation current densities normalized by geometric area (a) and CO stripping area (b) at lower potential values as a function of Pt content. The electrodes were calcined at 400 8C.

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Fig. 8 – Current–time curves for the Pt–RuO2 electrodes (calcined at 500 8C), at 600 mV for 10 h in 0.5 mol LL1 H2SO4 D 1.0 mol LL1 methanol solution.

A similar behavior was observed for both global and intrinsic catalytic activities. The Pt0.6Ru0.4Oy and Pt0.8Ru0.2Oy electrodes exhibited the best catalytic activities. It is worthy of note that the Pt0.8Ru0.2Oy electrode posses lower surface area than that of Pt0.7Ru0.3Oy electrode. However, the catalytic activity of the Pt0.7Ru0.3Oy electrode is lower than expected. The better catalytic activity obtained for Pt0.6Ru0.4Oy electrode may be explained in terms of the roughness or higher surface area available for occurrence of the adsorption process. On the other hand, the catalytic activity of the Pt0.8Ru0.2Oy electrodes undergoes strong influence of the intrinsic activity. This suggests that a possible loss in the catalytic activity caused by the decrease of the surface area was compensated by the increase of the activity per Pt site. Probably, the distribution of Ru in adjacent sites of Pt particles in Pt0.8Ru0.2Oy electrodes facilitated the bifunctional mechanism and/or enhanced the electronic effects.

3.4.2.

Chronoamperommetric experiments

The methanol oxidation performed by cyclic voltammetry showed that the best performance was achieved with the

Fig. 10 – Cyclic voltammetry for the Pt0.6Ru0.4Oy electrodes calcined at 400 and 500 8C. At 20 mV sL1 in 0.5 mol LL1 H2SO4 D 0.1 mol LL1 methanol solution.

Pt0.6Ru0.4Oy and Pt0.8Ru0.2Oy electrode compositions. In order to confirm this behavior, chronoaperommetric oxidation experiments were carried out in a 0.5 mol L1 H2SO4 and 1.0 mol L1 methanol solution at room temperature, at 400 and 600 mV, for 10 h, which is a period long enough to monitor the current decrease until a steady state is achieved. The oxidation current drop close to 0 is attributed to the CO adsorbed species accumulated on the electrode surface [41]. Fig. 8 shows representative chronoamperograms obtained at 600 mV, normalized by the geometric area. A similar behavior was obtained for all electrode compositions. For the bimetallic electrodes, the current falls to about 90% of the initial value after 30 min, and remains approximately constant for the following 10 h. Similarly to Pt1.0Ru0Oy electrode, the current falls close to 0, indicating a very pronounced deactivation for this electrode. The results presented here showed that the enhancement of the catalytic activity towards methanol oxidation depends on the RuO2 addition. Furthermore, the Pt0.6Ru0.4Oy was

Fig. 9 – Methanol oxidation current densities obtained from chronoamperommetric experiments at (a) 400 mV and (b) 600 mV after 5 min, normalized by geometric area and CO stripping area as a function of Pt content. The electrodes were calcined at 400 8C.

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Fig. 11 – Dependence of methanol, CO2 (a) and formic acid (b) concentration on electrolysis time at 400 mV from a 0.5 mol LL1 H2SO4 D 5 mmol LL1 methanol solution for Pt0.6Ru0.4Oy electrodes calcined at 400 8C.

shown to lead to the best catalytic activity, as also observed in Fig. 9. In this figure, the oxidation current densities after 5 min of electrolysis are plotted as a function of the Pt content for the experiments performed at 400 (Fig. 9a) and 600 mV (Fig. 9b), normalized by the geometric area and the CO stripping charge. A maximum activity was obtained for the Pt0.6 Ru0.4Oy electrode, corroborating the data obtained by cyclic voltammetry. However, the Pt1.0Ru0Oy electrode displayed high intrinsic catalytic activity at 600 mV. It is noteworthy that large amounts of OH species, required for oxidation of the intermediates, are adsorbed onto the Pt surface at higher potential values. Consequently, the effect of the Ru addition is minimized due to the fact that the presence of a second metal for OH adsorption is unnecessary. Additionally, the potentials higher than 600 mV are too elevated for practical usage.

The electrolyses were performed using the Pt0.6Ru0.4Oy electrode at constant potential (400 and 600 mV) for 3 h, and solutions samples were withdrawn at several times during the experiment. By the chromatography results it was possible to observe the variation in methanol concentration, as well as

3.4.3. The annealing temperature effect on methanol oxidation As mentioned in some studies [15–17,39,42–44], a RuO2 or RuOxHy hydrated and amorphous structure is desirable if a high activity for methanol oxidation is to be achieved. In order to investigate the annealing temperature effect on methanol electrooxidation, the electrodes were calcined at 500  C. The electrochemical experiments were carried out in the same way as described for the electrodes calcined at 400  C. Fig. 10 shows that catalytic activity was significantly decreased when the electrodes were annealed at 500  C. Higher synthesis temperatures cause the sintering and crystallization of the layer oxide, thus leading to a decrease in surface area and loss of the hydration of the catalytic layer. In summary, the RuO2 amorphous phase seems to be really necessary for catalyzing the reaction, and RuO2 plays a fundamental role in the adsorption of OH species. Moreover, the lower catalytic activity for the electrodes annealed at 500  C may be attributed to poor proton conductivity, which decreases at higher calcination temperatures [15,16].

3.4.4.

Electrolysis experiments

The HPLC technique was used to determine methanol consumption and byproduct formation as a function of electrolysis time. Methanol, formic acid and Na2CO3 solutions were used as standards for the chromatograms.

Fig. 12 – Dependence of methanol, CO2 (a) and formic acid (b) concentration on electrolysis time at 600 mV from a 0.5 mol LL1 H2SO4 D 5 mmol LL1 methanol solution for Pt0.6Ru0.4Oy electrodes calcined at 400 8C.

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the formation of formic acid and CO2. The formation of formaldehyde was not observed. The CO2 formed in the reaction was determined through the concentration of the carbonate formed in the trap containing a sodium hydroxide solution, which was placed in the outlet of the cell anodic compartment. Fig. 11 shows the methanol consumption and the formation of formic acid and CO2 as a function of the electrolysis time at 400 mV. In this figure, it is possible to see that the concentration of methanol decreases ca. 50%, while the concentration of formic acid increases and reaches a plateau. After 2 h of electrolysis, the concentration of formic acid starts to decrease. Moreover, CO2 production increases with methanol consumption, as observed in Fig. 11b. Fig. 12 shows the electrolysis experiments performed at 600 mV. Total methanol consumption and an increase in formic acid production are observed, and formic acid is then further partly consumed after 2 h. The concentration of CO2 increases and reaches a plateau in the end of the electrolysis. It was difficult to determine the mass balance for all electrolyses due to some CO2 loss in the system tubing, but it is evident that the formation of a large amount of CO2 occurs at lower potentials (400 mV). However, in comparing the molar ratio of the products formed in the middle of the reaction time at different potentials, the CO2/formic acid ratio was observed to be around 95 and 32 at 400 and 600 mV, respectively. Accordingly, the relative formation of CO2 appears to be favored at lower potentials, despite the lower consumption of methanol.

4.

Conclusion

The PtxRu(1x)Oy electrodes were prepared at different compositions by thermal decomposition of the polymeric precursors. The materials were characterized and their catalytic activity towards methanol oxidation was investigated by cyclic voltammetry and chronoamperommetry. The products of the reaction were identified by HPLC. The preparation method used in the present study was suitable for the synthesis of Pt–RuO2 films with good chemical and physical stability, and uniform composition throughout the whole coating. Additionally, the method used for preparing the catalysts allowed obtaining Pt in its metallic state, with no use of any reducing agent. The enhancing effect of the PtxRu(1x)Oy electrodes on methanol oxidation seems to be associated with the presence of amorphous and hydrated RuO2, as well as with its ability to donate OH species that promote the oxidation of CO to CO2. The products of the reaction were identified as being formic acid and CO2. The relative formation of CO2 was favored in the electrolysis performed at 400 mV (RHE). The electrodes prepared at 400  C are better catalysts than those annealed at 500  C, probably because of the structural and chemical properties favored during the calcination process. The lower annealing temperature led to the formation of layers with good protonic conductivity, which seems to be fundamental for a better catalyst performance. These results showed that the hydrous RuO2 plays an important role in the improvement of methanol

electrooxidation. Moreover, thermal decomposition of polymeric precursors is a suitable route for the production of catalysts applicable to DMFC.

Acknowledgments The authors wish to thank the Brazilian Research Funding Institutions CAPES and FAPESP.

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