Platinum Systems Electrodeposited in the Presence of Iron or

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Platinum Systems Electrodeposited in the Presence of Iron or Palladium on a Gold Surface Effectively Catalyze Oxygen Reduction Reaction Luna B Venarusso, Roseli H Sato, Pablo A Fiorito, and Gilberto Maia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp311343w • Publication Date (Web): 22 Mar 2013 Downloaded from http://pubs.acs.org on March 27, 2013

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The Journal of Physical Chemistry

Platinum Systems Electrodeposited in the Presence of Iron or Palladium on a Gold Surface Effectively Catalyze Oxygen Reduction Reaction Luna B. Venarussoa,§, Roseli H. Satob,#, Pablo A. Fioritob,#, Gilberto Maiaa,* a

Department of Chemistry, Universidade Federal de Mato Grosso do Sul, Caixa postal 549,

Campo Grande, MS 79070-900, Brazil b

Center for Natural and Human Sciences, Universidade Federal do ABC, Santo André, SP

09210-170, Brazil

ABSTRACT: In this study, Pt and Pd catalysts (in the absence and presence of Fe) and a PtPd alloy were generated on a polycrystalline Au surface by means of a direct electrodeposition approach. The electrochemical profile of these materials was investigated using cyclic voltammetry and microgravimetry (electrochemical quartz crystal microbalance). The electrocatalytic activity toward oxygen reduction reaction (ORR) was studied using hydrodynamic cyclic voltammetry. Physical characterization of samples was performed using scanning electron microscopy, energy dispersive X-ray microanalysis, and atomic force microscopy. Electrodeposited catalysts containing Pt exhibited high electrocatalytic activity toward ORR, stability in terms of specific activity, and repeatability of responses, even at low Pt concentrations in the electrodeposition solution, thus suggesting the possibility of applying these catalysts to acid fuel cells. The Au electrode modified with Pt(0.75)/Fe(0.25) exhibited a nanorounded, low-roughness structure, whereas the Au

electrode modified with

Pd(0.75)/Fe(0.25) showed poor electrocatalytic activity, a nanospaghetti-shaped structure, and enhanced roughness.

KEYWORDS: Tafel inclination, nanorounded shape, hydrogen peroxide, hydrodynamic cyclic voltammetry, electrochemical quartz crystal microbalance, d-band center

INTRODUCTION The 20th century was marked by rapid and significant global growth in industrial and transportation sectors, population levels, and consumption of energy. These developments have raised concerns over electricity supply, growing consumption of fossil fuels, and environmental pollution, increasing the need for efficient converters of energy for transport

ACS Paragon Plus Environment


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and stationary applications. Fuel cell technology is currently regarded as capable of fulfilling this demand.1 One barrier to the dissemination of fuel cells is the complexity and irreversibility of the oxygen reduction reaction (ORR) in aqueous media, which impose a series of practical limitations, in addition to difficulties obtaining information on the mechanism and kinetics of this phenomenon.2,3 ORR, the primary electrochemical reaction which takes place in cathode fuel cells, is central to the success of this technology for efficient, clean energy generation4 and remains the most formidable challenge to both fuel cell experimentalists and theorists, as is evident from the sizeable contribution that air cathode loss currently makes to overall voltage loss in any low-temperature fuel cell.5 Platinum is the most active material and the main component in catalysts employed in fuel cells for low temperature operation.3-6 Unfortunately, the scarcity of this metal is a key factor preventing production of fuel cells on a large commercial scale.3,4,6-10 In order to accelerate ORR and reduce the cost of catalysts, the preparation and characterization of platinum alloys with other noble or transition metals have been the focus of extensive research.3-15 Even with platinum catalysts, higher cathodic overpotentials in the range of 500600 mV are required in order to achieve useful current densities in fuel cells, revealing theslow kinetics of ORR in acid solutions.10 Electrodeposition is a practical method for generating a variety of Pt and Pt-based nanomaterials9 and has been applied to grow metal nanostructures and multilayer metal thin films.9,16 It has been reported as a method of choice, given its remarkable simplicity and low cost (low energy requirements and minimal material waste) for generating nanostructured materials.17 Metal electrodeposition on gold, platinum, or silver electrodes typically proceeds via a Stransky–Krastanove (SK) mechanism, in which initial metal deposition takes place via an atomic layer-by-layer process beginning with an underpotentially deposited (UPD) monolayer. At some critical thickness of the electrodeposited layer, islands that are more than one atomic layer in height begin to form atop this continuous metal multilayer.18 During deposition of two metals from one single bath, both are expected to deposit if the potential is suitably set for reduction of the less noble member, resulting in an alloy layer rather than one of pure metal.19 Adzic and co-workers produced Pt shells on a carbon-supported Pd and Pd3Co core using Cu UPD-mediated electrodeposition;20,21 obtained Pt monolayers on the surface of a Pd3Fe(111) single-crystal alloy using galvanic replacement of the Cu UPD adlayer by Pt;22


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and employed dissolved K2PtCl4 to build a Pd and Pd9Au1 alloy core/Pt monolayer shell produced by galvanic displacement of a Cu monolayer, obtained by UPD electrodeposition of a Cu monolayer on the reduced surfaces of the Pd nanoparticles.23 All these materials were examined for their catalytic activity toward ORR. Chen et al.24 synthesized Pt and PtPd nanotubes (50 nm in diameter, 5-20 µm long, 4-7 nm thick) performing a galvanic replacement reaction of silver nanowires and tested their suitability as catalysts for ORR. Kokkinidis and Sotiropoulos25,26 produced Pt- and Au-based catalysts by electrochemical deposition of M particles (M = Cu, Pb, Ni, Co, Fe) on carbon or Au supports, whose surface layers were subsequently replaced by Pt or Au. This replacement was achieved by spontaneous partial exchange of the non-precious metal deposits with Pt or Au upon immersion in aqueous acid solutions containing their metallic cations. The resulting materials were tested for their catalytic activity toward ORR. Tammeveski and co-workers,27-29 having studied electrochemical ORR on thin vacuum-evaporated Pt films (0.25-20 nm thick) on polycrystalline Au and glassy carbon substrates and Pd films (0.25-10 nm thick) on polycrystalline Au, concluded that the main product of O2 reduction is water and that only small amounts of H2O2 are produced. Also, the mechanism of O2 reduction was found to be the same both for the range of Pt or Pd film thicknesses studied and for bulk Pt or Pd. Xiao et al.30 performed electrochemical deposition of Pd on a Au rotating disk electrode (RDE) by tailoring the morphology of deposited Pd from nanoparticles to nanorods. The surface-specific activity toward ORR provided by Pd nanorods proved higher than that of Pd nanoparticles and comparable to that of Pt at operating potentials in fuel cell cathodes. A feature of Pd nanorod morphology is exposure of Pd(110) facets, which exhibit superior ORR activity. Bligaard and Nørskov31 addressed adsorption energies of the main intermediates in a surface-catalyzed reaction and determined their correlation with catalytic activity. Combined with kinetic models, these correlations, which involve activation barriers and reaction energies for a number of surface reactions, lead to volcano-shape relationships between catalytic activity and adsorption energies. Density functional theory (DFT) calculations provide a semi-quantitative description of surface adsorption phenomena. Bligaard and Nørskov showed that variations in adsorption energies from one transition metal to the next and from one geometrical surface structure to the next can be understood on the basis of a model describing the coupling between adsorbate states and transition metal d-states.



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Variations in reactivity of a given metal are, to a large extent, governed by the local value of the average energy of d-states, when its surroundings are changed.31 Roughly, the calculated O-adsorption energy (Eads) and the d-band center energy (εd) are −2.8 and −3.56 eV for Au, −4.15 and −2.25 eV for Pt, and −4.2 and −1.83 eV for Pd, respectively, revealing that bonding between atomic oxygen and d-transition metal becomes stronger as Eads becomes more negative (the reverse direction applies to εd)—Au is very noble, with less bond energy per O atom than in O2 (1/2 O2(g) around −2.9 eV). The effect of alloying can be interpreted in terms of d-band shifts—e.g., placing Pt on top of Au can shift the d-band center by as much as −0.1 to 0.1 eV; Pd on top of Au, by as much as −0.4 to −0.1 eV; and Pt on top of Pd, by as much as −0.1 to 0.1 eV.31 As pointed above, direct electrodeposition is remarkably simple, has low cost, and involves less steps than galvanic displacement electrodeposition, constituting a practical method for generating a variety of Pt and Pt-based nanomaterials. Other important factors to be taken into account are the effects of alloying Pt with other metals and the geometrical surface structure of electrodeposits (variations in adsorption energies of adsorbate states of oxygen interpreted in terms of the d-band shifts in these alloys)—useful features for producing better catalysts for ORR. Thus, the purposes of the present study were to perform direct electrodeposition of Pt and Pd catalysts (in the absence and presence of Fe) and a Pt-Pd alloy on a polycrystalline Au surface; to investigate the electrochemical and mass change profiles of these catalysts using cyclic voltammetry (CV) and microgravimetry (with an electrochemical quartz crystal microbalance, EQCM); and to investigate electrocatalytic activity (important electrocatalytic activity was found for some of catalysts produced) and reaction mechanism in ORR using hydrodynamic cyclic voltammetry (HCV) with rotating ring-disk electrodes. Scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) microanalysis, and atomic-force microscopy (AFM) were employed to characterize the electrocatalysts.

EXPERIMENTAL SECTION Reagents and Instruments. Solutions containing Pd, Fe, and Pt were prepared in 0.1 M HClO4. The salts and precursor solutions employed were PdCl2, FeSO4⋅7H2O, and H2PtCl6⋅6H2O (all Vetec). Voltammetric measurements were carried out using a three-electrode glass cell with a working electrode consisting of a Teflon-embedded Au rotating ring-disk assembly (0.037


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and 0.164 cm2 in geometric area, respectively) (Pine Research Instrumentation), with a collection efficiency of N = 0.22. The counter-electrode was a Pt plate (Degussa). A reversible hydrogen electrode (RHE) was employed as the reference electrode. Electrode Preparation. The Au working electrode was sequentially polished with 2000- and 2500-grit emery paper and alumina slurries (1, 0.3, and 0.05 µm) and finally cleaned by sonication in Milli-Q water (Millipore), acetone (Vetec), and 0.1 M HClO4 (Tedia) solution for 5 min in each solvent. Subsequently, the electrode underwent 200 cycles at 900 mV s–1 in the potential range of 0.05 to 1.7 V (changing the solution three times whenever necessary to ensure a clean surface finish). Electrodeposition was then performed, which entailed subjecting the working electrode to five voltammetric cycles at 10 mV s–1 in the potential range of 1.2 to 0.05 V. After deposition, the modified electrode was thoroughly washed with Milli-Q water before being placed in an electrochemical cell containing 0.1 M HClO4. Electrodeposited electrodes are described according to the molar concentration ratios of the metals employed—e.g., the electrode denominated Pt(0.75)/Fe(0.25) was obtained using Pt and Fe molar concentrations of 0.75 and 0.25 mM, respectively, in 0.1 M HClO4, consistently resulting in a 1.00 mM concentration for the two-metal mixture (or pure metal, where applicable). The solutions were saturated with N2 (5.0 purity) or O2 (4.0 purity), both from Air Liquide. Apparatus and Measurements. CV and HCV, the electrochemical techniques employed, were run on an AFCBP1 bipotentiostat coupled to an MSRX speed controller, both from Pine Research Instrumentation. The microstructure of the electrodeposited nanoparticles was visually characterized by both SEM and AFM. SEM was performed on a JEOL JSM-6380LV field-emission scanning electron microscope equipped with an EDX system (Thermo Noran System Six) to microanalyze Pt and Pd ratios of the electrodeposited nanoparticles. An accelerating voltage of 30 kV was applied. The microscope used for the AFM study was a 5500 Agilent (N9410S) operating in contact mode. The tip was a highly doped single crystal silicon probe having a spring constant of 0.1 N m–1. Resolution was 512 px × 512 px for all images. An EQCM (Maxtek) was employed to measure resonance frequency shifts in situ. Microgravimetric studies were carried out in a GC-15 three-electrode glass cell that included a CHC-15 crystal holder, clamp, and stopper (Maxtek). A 5 MHz AT-cut quartz crystal (25.4 mm in diameter) vertically positioned in front of the counter-electrode served as the working



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electrode (polycrystalline Au), both sides of which were coated with Au sputtered on a Ti layer in a keyhole pattern (geometric area in contact with solution = 1.37 cm2) (Maxtek). The surface was thoroughly cleaned before electrodeposition by cycling 50 times in N2-saturated 0.1 M HClO4 between 0.05 and 1.7 V at 100 mV s−1, changing the solution three times where necessary. Deposition on the working electrode for EQCM measurements was carried out in 0.1 M HClO4 containing the same composition of metals as described above. After deposition, EQCM measurements were performed in 0.1 M HClO4. Assuming that the deposited layers are rigid and no viscoelastic changes occur at the electrode/solution interface, the Sauerbrey equation was applied: ∆m = –∆f/Cf,32 where ∆m is the mass change per unit area (g cm−2), ∆f is the resonance frequency shift (Hz), and Cf is the crystal sensitivity factor (Hz ng−1 cm2; 0.056 Hz ng−1 cm2 in the present case). Cyclic voltammograms were normalized to the electrochemically active surface area (ECSA). For electrodeposits involving Pt, this was done by integrating charges in the region of hydrogen desorption; when the electrodeposits involved only Pd or Pd/Fe, and in the case of bare Au, charge integration encompassed the region of oxide reduction. Charge density was taken as 210 µC cm–2 for desorption of a hydrogen monolayer on the Pt surface, and as 420 and 400 µC cm–2 for reduction of an oxygen monolayer covering the Pd and Au surfaces, respectively.33

RESULTS AND DISCUSSION Electrodeposition, Electrochemical Profile and Mass-Change Characterization of Modified Au Electrodes. Figure 1 shows first-cycle voltammetric responses for the electrodeposition of different metals, combined or alone, on a Au surface in 0.1 M HClO4. No faradaic currents were detected for the Au surface in the absence of metals in the potential range studied (see curve — (black line), Figure 1A). When the Au surface was put in contact with the Pt solution (curve — (red line), Figure 1A), a peak current density of around 60 µA cm–2 was detected at around 0.61 V, followed by a second peak current density at around 0.46 V in the negative-going scan (for attribution of these peaks, see topic S1 in the Supporting Information (SI)). Expected responses for UPD hydrogen adsorption/desorption on the Pt surface were subsequently detected in the potential region spanning from 0.31 to 0.05 then to 0.33 V.34,35 Because they are not centered at zero, these current densities of hydrogen adsorption/desorption on the Pt surface provide evidence of both Pt deposition and normal responses from the Pt surface in


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the presence of HClO4. In the positive-direction potential scan, formation of Pt oxide was more evident above 0.9 V.34,35

Figure 1. Cyclic voltammetric curves for bare Au and Au in the presence of a 1.00 mM concentration of metal salt—alone (A) or combined (B)—in N2-saturated 0.1 M HClO4. ν = 10 mV s–1. Scans start at 1.2 V.

A similar behavior was observed for a Au surface in the presence of Pd solution (curve — (green line), Figure 1A), this time with a peak current density of around 34 µA cm–2 (about half the current observed for a Pt solution), which occurs at around 0.87 V—a more catalytic reduction (more positive potential) than for Pt on the Au surface—followed by a second peak current density at around 0.80 V, detected in the negative-going scan (see topic S1 in SI for attribution of these peaks). UPD hydrogen adsorption/desorption on a Pd surface occurs in the same potential region as on Pt. In the positive-going scan, formation of Pd oxide was more evident than that of Pt oxide above 0.83 V (compare curves — (green line) and — (red line), Figure 1A). For the Au surface placed in contact with the Fe solution (curve — (blue line), Figure 1A), a redox couple centered at 0.81 V was detected, attributed to a Fe2+/Fe3+ redox couple in the vicinity of the standard electrode potential (0.771 V vs. NHE).36 This couple was attributed to the reaction Fe3+ + e–

Fe2+, exhibiting reversible features (peak current

densities of around 20 µA cm–2—i.e., around half the current observed for a Pd solution—and difference in peak potentials of around 68 mV). This means that no Fe deposition was observed on the Au surface in the potential region depicted in Figure 1A, since some Fe



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deposition is expected at potentials close to the standard electrode potential of –0.44 V vs. NHE36 in the Fe2+ + 2e–

Fe reaction.

Contact of the Au surface with a solution containing two metals revealed peak current densities corresponding to individual metal responses for Pt(0.75)/Fe(0.25), at around 0.80, 0.63, and 0.40 V (see curve — (cyan line), Figure 1B); for Pt(0.75)/Pd(0.25), at about 0.82, 0.62, and 0.47 V (see curve — (magenta line), Figure 1B); and for Pd(0.75)/Fe(0.25), at approximately 0.82 V (see curve — (purple line), Figure 1B). This means that, in the presence of Fe, Pt is deposited even when employing a concentration ratio of Pt(0.25)/Fe(0.75) (see Figure S1A, whereas Fe is not deposited as an alloy with Pt (see Fe2+/Fe3+ redox couple, well defined even at a ratio of Pt(0.50)/Fe(0.50), in Figure S1A)). Pt and Pd are most probably deposited as an alloy (see cyclic voltammetric behavior transition from Pd to Pt when Pt concentration is increased in relation to Pd—Figures S1B and 1). Pd and Fe follow similar behavior to Pt and Fe—i.e., Pd is deposited when in the presence of Fe, even at a concentration ratio of Pd(0.25)/Fe(0.75) (note small currents related to hydrogen adsorption/desorption, and an oxidation peak that appears to enclose two peaks—the first of them related to Fe2+ oxidation—in the positive-direction potential scan, Figure S1C). Figure S2 shows cyclic voltammetric responses for the fifth deposition cycle (adopted as a suitable indicator of metal deposition on a Au surface) for the same curves depicted in Figure 1 (Fe and bare Au responses are not shown, as they do not differ from first to fifth cycle). A noteworthy finding was that every curve shown in Figure S2 remained unchanged from the second to the fifth cycle, the only difference being an overall increase in current densities with each subsequent cycle, suggesting that metal deposition persists after each cycle. Also, the cyclic voltammetric behavior observed from the second cycle onwards is characteristic

of

Pt

(better

defined

UPD

hydrogen

adsorption/desorption

and

formation/reduction of Pt oxide (see e.g. curve — (red line), Figure S2). A reduction peak corresponding to Pt oxide reduction, however, appears at around 0.97 V, followed by a reduction peak for Pt4+ → Pt0 at around 0.83 V. More clearly evident, in the case of Pd, was a reduction peak corresponding to reduction of Pd oxide and Pd2+ → Pd0 deposition at around 0.78 V (see curve — (green line), Figure S2). Other cases depicted in Figure S2 follow a similar behavior. Figure S3 shows current density and mass density changes for a Au surface placed in contact with a solution containing Pt(0.75)/Fe(0.25), revealing peak current densities at around 0.78, 0.64, and 0.49 V for the first half-cycle, as previously described (see also Figure


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1B). In addition, a linear increase in mass density (120 ng cm−2, from 0.70 to 0.59 V) can be observed during the first half-cycle at potentials more negative than 0.70 V—a feature that coincides with increased current densities, culminating in a peak at 0.64 V during the first CV half-scan. This linear increase in mass density (410 ng cm–2, from 0.59 to 0.05 V) slightly decreases the slope, relative to potentials more positive than 0.59 V, suggesting the occurrence of fast deposition of Pt4+ as Pt0 from 0.70 to 0.59 V, followed by a second deposition regime below 0.59 V on the bare Au and/or Pt-deposited surface. At the end of the first potential half-cycle, the increase observed in mass density was 530 ng cm−2. Completion of the first cycle yielded an increase in mass density, albeit under a different deposition regime—i.e., the mass density vs. potential slope was lower than that observed in the negative-direction potential scan—up to 0.72 V. Above 0.72 V, mass density remained constant until the end of the first cycle massogram. A mass density increase of 820 ng cm−2 was detected at the end of the first potential cycle. A similar behavior was observed on subsequent cycle massograms (gradual mass density increase upon further potential cycling), with mass density increases of around 2600 ng cm−2 from the first to the fifth scan. The behavior of subsequent cyclic voltammograms was similar to that previously described in Figure S2. Faradaic efficiency of 6% was calculated from the slope of the ∆m vs. ∆q plot (not shown) for the first electrodeposition half-scan (data from Figure S3). Assuming the density of Pt atoms on the polycrystalline Pt surface to be 1.31 × 1015 cm–2,37 the corresponding mass density expected for a monolayer of Pt atoms should be 424 ng cm−2. A Pt monolayer of around 0.30 (120/424 ng cm−2) was calculated up to 0.59 V along the first potential scan half-cycle, assuming that the entire mass density change can be attributed to the production of a uniform Pt monolayer. By the end of the first potential halfcycle, a 1.25 Pt monolayer was deposited (530/424 ng cm−2); after five cycles, this increased to 8.0 Pt monolayers (3420/424 ng cm−2). After deposition, the modified Au electrodes were transferred to a N2-saturated 0.1 M HClO4 solution and electrochemical profiles were obtained (Figure 2). Figures 2 and S4 (see curves — (black lines)) depict the production of Au oxide at a bare Au electrode above 1.2 V in the positive-going scan, with a corresponding reduction of Au oxide (reduction peak centered at 1.15 V) visible in the negative-going scan. Peaks which were not well-defined for production/reduction of Au oxide were observed for Au electrodes upon contact with the Fe solution (see curve — (red line), Figure S4), suggesting merely a



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decrease in the number of active sites on the Au surface as a result of adsorption of impurities from the Fe solution. With modified Au electrodes, a number of features—namely, welldefined current densities (peaks) for UPD hydrogen desorption/adsorption (0.05-0.4 V), current densities related to double layer charging/discharging (0.4-0.7 V, positive-going scan), oxide formation (0.70-1.2 V, positive-direction scan), and oxide reduction (peak current density at around 0.75 V, negative-going scan) on Pt and Pd surfaces, with current densities centered at zero—collaborate to enhance the responses of electrodeposited Pt and/or Pd on the Au surface, whereas for bare Au no faradaic responses were detected in this potential region (0.05-1.2 V). These voltammetric profiles of Pt and Pd are consistent with published findings.34,35,38

Figure 2. Cyclic voltammograms for bare and modified Au electrodes in N2-saturated 0.1 M HClO4. Bare Au () and Pt(0.75)/Pd(0.25)* (—) curves start at 0.05 V (ν = 100 mV s–1). Scans for other modified electrodes start at 1.2 V (ν = 50 mV s–1).

A noteworthy feature is that Au electrodes modified with Pd (see curves — (green line) and — (purple line), Figure 2) exhibited higher current densities than did Pt-modified Au electrodes, as a result of the smaller ECSA values of those modified with Pd. A similar behavior was observed for other compositions (see Figure S4).


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In the Au electrode modified with Pt(0.75)/Pd(0.25)*, for instance, the scanning started at 0.05 V (curve — (dark cyan line), Figure 2) revealed large current densities on average, in comparison with the same electrode scanned from 1.2 V (see curve — (magenta line), Figure 2). These large current densities are explained by differences in scan rates (see, for example, the curve for bare Au at 50 mV s–1 in Figure S4, indicating lower current densities for production/reduction of Au oxide, relative to CV measurements at 100 mV s–1 in Figure 2). Nonetheless, large current densities related to Pt and Pd oxide production were detected from 0.7 to 1.55 V in the positive-going potential scan, while large current densities related to oxygen production can be seen above 1.55 V. More importantly, there is an absence of a pronounced reduction peak at around 1.15 V, characteristic of Au oxide reduction (see curve — (black line), Figure 2), and the presence of a well-defined reduction peak at around 0.64 V, characteristic of a peak for Pt oxide and Pd oxide reduction in the negative-direction scan. These responses demonstrate complete coverage of the Au surface during the metal deposition step in this experiment. Figure S5 shows current density and mass density changes vs. potential for a Au electrode modified with Pt(0.75)/Fe(0.25) in 0.1 M HClO4. Three regions can be identified, as defined elsewhere.34,35 Briefly, region I is related to UPD hydrogen adsorption/desorption; region II, to double-layer charging/discharging of the Pt surface; and region III, to Pt surface oxidation to form a Pt oxide. In region I, mass density change approaches 16 ng cm–2 at 0.35 V (positive-going scan) and 19 ng cm–2 at 0.30 V (negative-going scan). In region II, mass density change approaches 14 ng cm–2 (mass difference between 0.75 and 0.35 V potentials, positive-direction scan) and −10 ng cm–2 (mass difference between 0.30 and 0.60 V potentials, negative-going scan). In region III, change approaches 23 ng cm–2 from 0.75 to 1.2 V (positive-going scan) (Figure S5A). As in previous studies,34,35 quantitative identification of adsorbed species onto the Pt surface was carried out, whereby M (Table 1) was determined from the linear relation ∆m vs. ∆q in regions I-III depicted in Figure S5A. In the positive-going scan (Figure S5A), M values for region I were calculated as 6.1 g mol–1, which corresponds to a 0.34 monolayer of water (18 g mol–1), and 6.5 g mol–1, corresponding to a 0.36 monolayer of water (Figure S5B). Adsorption of ClO4− ions in region II is assumed to be associated with mass change increases on a Pt electrode.34,35 The results reported in Table 1 correspond to values of 0.30 ClO4− (M = 29.4 g mol–1) and 0.14 ClO4− (M = 13.6 g mol–1) monolayers (Figures S5A and S5B, respectively). These ions, however, are not being assumed as solvated, because their M values



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were smaller than that of ClO4− (99.45 g mol–1). The M values of 13.8 g mol–1 (Table 1) and 10.7 g mol–1 (n = 2) (Figures S5A and S5B) obtained for Pt in region III can be attributed to PtOxads + 2H+ + 2e−,34,35 resulting in 0.86 and 0.67

oxidation of Pt as Pt + H2O

monolayers of oxygen (8 g mol–1) on the Pt surface, respectively. (See topic S2 in SI for a discussion related with negative-direction potential scan shown in Figures S5A and S5B and Table 1.) Table 1. Interfacial molar mass changes in regions I-III, calculated from current density and mass density changes vs. potential in Figures S5A and S5B. Figure

(∆m/∆q)*(Fn) = M (g mol–1), positive-direction

∆m/∆q*(Fn) = M (g mol–1), negative-direction

potential scan

potential scan

Region I

Region II

Region III

Region I

Region II

Region III (loss

(substitution of

(adsorption

(adsorption of

(substitution of

(loss of

of oxygen after

adsorbed UPD

of ClO4

hydrogen by

ions)

–

–

oxygen, forming

water molecules

ClO4

PtOx)

by adsorbed

ions)

and loss of ClO4– ions)

UPD hydrogen)

water

PtOx reduction

molecules) S5A

6.1

29.4

13.8

6.9

19.0

18.4

S5B

6.5

13.6

10.7

7.9

5.7

23.2

Evaluation of the Electrocatalytic Activity of Modified Au Electrodes toward ORR. Figure 3A shows the hydrodynamic voltammetric curves for bare and modified Au electrodes in an O2-saturated 0.1 M HClO4 solution without subtraction of background hydrodynamic voltammetric curves for bare and modified Au electrodes in an N2-saturated 0.1 M HClO4 solution (see topic S3 and Figure S6 in SI for justification of this information). Current densities were calculated from the geometric area of the bare Au electrode. For bare Au, the kinetic current densities in the positive-going scan start far from the diffusion-limiting current densities (jd)at a far less positive potential than for, e.g., a Pdmodified Au electrode (compare curves — (black line) and — (green line), Figure 3A)—and reach zero at 0.57 V, characterizing a surface that provides poor electrocatalytic responses to ORR. A similar behavior is observed for a Au electrode initially cycled in Fe solution (see curve — (black line), Figure S7), yet with lower electrocatalytic responses to ORR, thus suggesting merely a decrease in the number of active sites on the Au surface as a result of adsorption of impurities from the Fe solution.


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Figure 3. (A) Hydrodynamic voltammetric curves for bare and modified Au disk electrodes in O2saturated 0.1 M HClO4. ω = 1600 rpm; ν = 10 mV s–1. Scans start at 0.05 V. (B) Plot for specific activity (SA) and electrochemically active surface area (ECSA) vs. modified Au electrodes in 0.1 M HClO4 solution saturated with O2 (SA plot) and N2 (ECSA plot).

At the Pt-modified Au electrode (curve — (red line), Figure 3A) (see topic S4 in SI for a more detailed discussion), the positive-going scan revealed that O2 reduction starts very close to the maximum current density—the region of jd, with a very small increase in current densities (in module) up to around 0.35 V, and responsible for producing small amounts (0.8%) of H2O2 (note the small currents decreasing until near 0.35 V in the — curve (black line), Figure S8; see also Table S1 and SI for explanation and eqs S1 and S2, used to calculate the percentage of H2O2 formation,

). From 0.35 V to 0.75 V O2 reduction reaches its

maximum—the region of jd and negligible production of H2O2 takes place above 0.35 V (0.1%) (see Figure S8 and Table S1, including negative-going potential scan). Above 0.75 V current densities were considerably decreased. Persisting in the positive direction, a potential region was found which lacks O2 reduction (null current densities) close to 1.0 V, most probably the potential where the Pt surface is completely covered with PtOx. When the potential scan direction is reversed, current densities related to O2 reduction begin to increase as the potential reaches around 0.98 V, most likely the potential where PtOx starts to be reduced. Proceeding toward more negative potentials, current densities increase throughout the kinetic region and reach the potential region where jd prevails. For this modified electrode, E1/2 (the potential corresponding to half the value of jd) was found to be around 0.9 V in the positive-going scan and around 0.84 V in the negative direction (see curve — (red line), Figure 3A). This 60 mV difference in E1/2 between forward and back scans lends credence to



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the assumption that PtOx absence or presence influences the catalysis of ORR taking place on this modified electrode as reported in references.39-41 E1/2 was higher for the Au electrode modified with Pt(0.75)/Fe(0.25) and smaller for the Au electrode modified with Pt(0.75)/Pd(0.25)the latter with high production of H2O2 (1% at 0.05 V in the positive-going scan; see Table S1 and Figure S8)than for the Au electrode modified with Pt(1.00) (compare curves — (cyan line) and — (magenta line) with — (red line), respectively, in Figure 3A). Other modified Au electrodes containing Pt (Figure S7) exhibit E1/2 values smaller than the Au electrode modified with Pt(0.75)/Pd(0.25). E1/2 values for the other Pt-modified electrodes, however, are higher than for modified Au electrodes lacking Pt. The highest E1/2 value found in the positive-direction scan for Au electrodes modified with Pd or Pd/Fe was obtained for a Pd-modified Au electrode (see curve — (green line), Figure 3A). When Pd concentration was decreased and Fe concentration increased during deposition, E1/2 values were considerably decreased (Figures 3A and S7). H2O2 production was high at the Au electrode modified with Pd(0.75)/Fe(0.25) at 0.05 and 0.65 V (Table S1 and Figure S8). The negative-going scan yielded a peak at around 0.65 V (see curve — (green line), Figure 3A) and revealed a 25 mV difference in E1/2 values between forward and back scans. This difference disappeared when the Pd concentration was decreased and Fe concentration was increased during deposition (see Figures 3A and S7). This behavior can be attributed to structural rearrangement at the Pd surface during Pd oxide reduction, a rearrangement that does not occur at the Pt surface. H2O2 production was high at 0.55 V at the Au electrode modified with Pd(0.75)/Fe(0.25) (see Table S1 and Figure S8). Pd oxide production/reduction must involve a hydrogen atom, since H2O2 production is most effective in the potential region of UPD desorption of hydrogen, at 0.05 V, and in the potential region of Pd oxide reduction (see Table S1 and Figures S8 and 2). In addition, Figure 3B shows calculated values for SA (see eq S3) at potentials close to E1/2 values and calculated values for ECSA for modified Au electrodes (Figure 3B) (see topic S5 in SI for a comment on ECSA provided in terms of surface area per unit mass of Pt being used (specific ECSA)). The highest SA—e.g., at 0.9 V (close to the E1/2 value)—and ECSA values were found for Pt(0.25)/Fe(0.75) (0.56 mA cm–2) and Pt(0.75)/Fe(0.25) (1.84 cm2), respectively. The fact that Fe was not deposited during the deposition step lends support to our hypothesis that Fe contributes as a modifier of the Pt structure during deposition, improving (or at least maintaining at a high level) electrocatalytic activity (high positive E1/2, SA, and ECSA values) toward ORR at electrodes modified with Pt in the presence of Fe, even


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when Pt-to-Fe concentration ratios during the deposition procedure are low. One of the highest Pt loading values, of 3.4 µg cm−2, obtained from EQCM experiments for an Au electrode modified with Pt(0.75)/Fe(0.25), resulting in a specific ECSA value of 103 m2 g–1 and a mass activity value at 0.9 V of 0.47 mA

even better values than those reported

42

in reference suggest a very good electrocatalytic activity towards other electrodeposited electrodes, since these electrodes had lower Pt loadings than this modified electrode. (See topic S5 in SI for an explanation on why other Au modified electrodes have not been discussed in terms of specific ECSA and mass activity.) No similar behavior was observed for Pd in the presence of Fe during deposition—better electrocatalysis was observed for Au modified with Pd(1.00) than when the Fe concentration was increased in relation to Pd (in the latter situation, SA and E1/2 were substantially decreased, as shown in Figure 3B), most probably because Fe does not substantially influence the structure of deposited Pd. However, Pt deposited with Pd, even at small Pt concentrations, exhibited good electrocatalytic behavior in ORR, despite its lower electrocatalytic activity (low positive E1/2, SA, and ECSA values) relative to pure Pt. In summary, the good electrocatalytic behavior in ORR found for electrocatalysts containing Pt and Pt electrodeposited in the presence of Pd may be explained by the presence of Pt on top of Au and Pt on top of Pd sufficiently shifting the d-band center so as to diminish the likelihood of strong bonding taking place between atomic oxygen and d-transition metal.31 Electrocatalysts containing Pd on top of Au, however, may not shift the d-band center sufficiently,31 therefore not exhibiting good electrocatalytic behavior in ORR. ORR Mechanism at Modified Au Electrode Surfaces. In order to determine the main mechanism involved in ORR at the modified Au electrodes, hydrodynamic voltammetric curves were obtained at different rotation rates (Figure 4A), as well as responses from the bare Au ring electrode, to detect the presence of any H2O2 (Figure S8 and Table S1). The linear behavior and parallelism of I–1 vs. ω–1/2 curves (Koutecký–Levich plots, Figure 4B) for different values of potential indicate that eq S5 is satisfied36 (see SI for a description of this equation), in which case the intercepts correspond to Ik (kinetic current) and the slopes allow n (number of electrons involved in ORR) to be calculated (Table S2). The average n value was 4.0 electrons, suggesting that the mechanisms involved during ORR using the catalysts investigated here follow a direct pathway involving four electrons (O2 + 4H+ + 4e– → 2H2O). This is corroborated by the low production of H2O2 observed. The highest H2O2



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percentage (2.8% at 0.05 V) was detected for the Au disk electrode modified with Pd(0.75)/Fe(0.25) (Table S1). Tafel curves43,44 (E vs. log (I/(Id – I), Figure 4C) were applied to facilitate understanding of the ORR mechanism. Two principal regions of linear behavior can be identified in the Tafel curves depicted in Figure 4C. Values of −65 mV dec–1 and −122 mV dec–1 on average were found for these linear regions (see Table S3 for slope values) with low and high currents, respectively, using modified Au disk electrodes—values considerably close to those of −60 mV dec–1 and −120 mV dec–1 expected for low- and high-current regions reported to occur during ORR on bulk Pt43 and Pd.44 Transition of ORR kinetics from low- to high-current regions may be interpreted in terms of the first charge-transfer step as a ratedetermining factor in both current regions. This first charge-transfer step, however, involves adsorption of reaction intermediates under Temkin conditions in the low current regions and Langmuirian conditions in the high current regions.43

Figure 4. (A) Hydrodynamic linear potential scan curves for modified Au disk electrodes in O2saturated 0.1 M HClO4 at different ω values. ν = 10 mV s–1. Scans start at 0.05 V. (B) Koutecký– Levich curves obtained from data in Figure 4A. (C) Tafel curves obtained from data in Figure 4A.


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Electrocatalyst Stability Test. In order to have practical applications, a catalyst must ensure stable catalytic activity in the long term. Drawing on published reports,42,45-48 potential continuous cycling was selected as a stability test for catalytic activity in the potential range of 0.60 to 1.0 V in O2-saturated 0.1 M HClO4 at 50 mV s–1 (a protocol recommended by the U.S. automotive industry47). After 10 000 cycles of potential sweep (Figure 5), one of the tested catalysts found to have superior catalytic activity—the Au disk electrode modified with Pt(0.75)/Fe(0.25)— exhibited small differences between cyclic voltammograms before and after 10 000 cycles of potential sweep for the Au disk electrode modified with Pt(0.75)/Fe(0.25) (Figure S9), better demonstrated by displacement of the Pt-oxide reduction peak potential to more a positive potential after 10 000 cycles of potential sweep. After 10 000 cycles of potential sweep, however, ECSA was decreased to 1.15 cm2 (a 37% loss, from 1.84 cm2). SA correction for an ECSA value of 1.15 cm2 resulted in 2.11, 0.58, and 0.11 mA cm–2 at 0.85, 0.90, and 0.95 V, respectively—slightly higher values than before 10 000 cycles of potential sweep (see Figure 3B). This was accompanied by a mere 12 mV degradation at E1/2 in the positive-direction potential scan, suggesting that SA is well conserved even after the loss in ECSA—a desirable feature for possible application of this electrocatalyst to a cathode in acidic fuel cells.

Figure 5. Hydrodynamic voltammetric curves for an Au disk electrode modified with Pt(0.75)/Fe(0.25), before and after stability test (10 000 cycles between 0.6 and 1.0 V at 50 mV s–1) in O2-saturated 0.1 M HClO4. ω = 1600 rpm. ν = 10 mV s–1. Scans start at 0.05 V.



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This conservation of SA even with loss of ECSA may be explained by the phenomenon of Ostwald ripening4,49,50—i.e., Pt dissolution and redeposition.4,49 We are assuming that the phenomenon of Pt dissolution and redeposition does not have deleterious effects on the SA of the deposited catalysts here investigated, given the high number of deposited Pt monolayers (see first section of the Results and Discussion), a feature that allows Pt to dissolve and redeposit without affecting SA. Used as the cathode in fuel cells, this type of electrodeposited catalyst can alleviate their stability problems,49 at least in terms of SA. In the present study, ECSA loss was lower than that reported by Lim et al.42 for Pd-Pt nanodendrites after 10 000 cycles in accelerated durability tests, but higher than that reported by Zhang et al.48 for an Au/Pt/C catalyst. In terms of SA, our values were high if compared with the absence of changes in Pt SA of an Au/Pt/C catalyst attributed to no recordable loss of the Pt surface area.48 In addition, our degradation in half-wave potential over the cycling period after the accelerated stability test was close (12 mV) to that of 5 mV reported in reference48. Physical Characterization of Catalysts. Figure S10 shows SEM images acquired ex situ for bare and modified Au electrodes. Bare Au surfaces (Figure S10A) are flatter (i.e., less rough) than their counterparts on modified Au electrodes. The image corresponding to the Au surface modified with Pt(0.75)/Fe(0.25) (Figure S10B) suggests the presence of a rough, thick porous film. Au modified with Pt(0.75)/Fe(0.25) also appears as a rough, yet less porous, film after the stability test (10 000 cycles) (Figure 10C). Modified in the presence of Pt(0.75)/Pd(0.25), the Au surface undergoes marked changes, becoming considerably rougher, with small clusters resembling “nanorocks” ranging from 30 nm to 100 nm in diameter (Figure S10D). These clusters are also visible on the Au electrode modified with Pd(0.75)/Fe(0.25) (Figure S10E)—the electrode with lowest E1/2, SA, and ECSA values among the modified electrodes represented in Figure 3B. Table S4 presents calculated and EDX-derived values of atomic percentage for modified Au electrodes, confirming the absence of Fe in electrodeposits grown in the presence of this metal. When electrodeposition was conducted in the presence of Pt and Pd, both metals were deposited, with changes in Pt:Pd composition from an initial ratio of 83.8:16.2 to a ratio of 63.0:37.0 after electrodeposition, meaning that Pd is more effectively electrodeposited than Pt, since the reduction and deposition potentials for Pd electroreduction are more positive than those for Pt (see Figure 1 and related discussion).


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EDX analysis corroborated the view that presence of Fe or Pd in electrodeposition solutions containing Pt contributes to structural arrangement of Pt nanoparticles, improving their electrocatalytic activity toward ORR or at least approaching the electrocatalytic activity of pure Pt, in comparison with solutions deprived of Fe and Pd—a noteworthy finding that could allow a decrease in the Pt content in catalysts used in ORR. Figure 6 shows AFM images acquired ex situ for bare and modified Au electrodes, accompanied by their respective height profiles obtained from more uniform regions so as to avoid valleys that are likely to appear on bare Au in the polishing process. It should be stressed that the discussion involving the height profiles depicted in Figure 6 is based on the maximum and minimum marks shown in each image. Also, these height profiles are not to be interpreted as indicators of film thickness, since they have been selected from more uniform regions in each Au modified electrode.

Figure 6. AFM images for bare Au electrode (A) and Au electrodes modified with Pt(0.75)/Fe(0.25) before stability test (B), with Pt(0.75)/Fe(0.25) after stability test (10 000 cycles) (C), with Pt(0.75)/Pd(0.25) (D), and with Pd(0.75)/Fe(0.25) (E). Each image, covering 1 µm × 1 µm and accompanied by its respective height profile, is representative of five different regions from each sample.

Bare Au surfaces (Figure 6A) are smoother than in modified Au electrodes, despite the evident presence of irregularities (scratches) resulting from polishing (note the relatively



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smooth height profile in Figure 6A). The Au surface modified with Pt(0.75)/Fe(0.25) (Figure 6B) contains nanoparticles of rounded shape ranging from 30 to 100 nm in diameter, a feature reflected in the pronounced hills in its height profile. After 10 000 cycles in the stability test (Figure 6C), these nanoparticles decreased in size, acquiring a rounded shape no higher than about 50 nm in diameter, while the distance between them also decreased, leading to a “packing” effect. Overall, the height profile (Figure 6C) exhibited pronounced peaks, in contrast with the visible hills observed for Au modified with Pt(0.75)/Fe(0.25) before the stability test (Figure 6B). We attributed the decrease in nanoparticle size on Au modified with Pt(0.75)/Fe(0.25)

after

the

redepositionOstwald ripening.

stability 4,49,50

test

(Figure

6C)

to

Pt

dissolution

and

Even these decreases in nanoparticle size and ECSA

value (see previous section), resulting in approximately constant values of, for instance, SA at 0.9 V and only 12 mV degradation in E1/2 observed for this electrode, suggest good prospects in the use of these electrodeposited catalysts in the cathode of fuel cells, since their degradation is not deleterious in terms of SA after stability test (a very important experiment to identify and explain these changes). Similarly to our findings, Perez-Alonzo et al.51 observed size decrease and disappearance and sintering of some nanoparticles after cycling conducted to simulate the start-up and shut-down of a fuel cell, viewing dissolution phenomena as the principal cause of degradation in Pt/C electrocatalysts under ORR conditions. In contrast with our observations, Wang et al.50 detected no significant losses in SA, either in size or shape, in Au/FePt3/C after 60 000 potential cycles. Nonetheless, they found that Pt and FePt3 nanoparticle size changed substantially after cycling, resulting in large particles being formed, of over 20 nm in diameter—a phenomenon attributed to Ostwald ripening.4,49,50 The Au surface modified with Pt(0.75)/Pd(0.25) (Figure 6D) exhibits nanoparticles with rounded shape and dimensions similar to those found on Au modified with Pt(0.75)/Fe(0.25) (Figure 6B). The hills in the corresponding height profile (Figure 6D), however, are far more pronounced than those on the Au surface modified with Pt(0.75)/Fe(0.25) (Figure 6B). This effect can be attributed to the presence of Pd electrodeposited in conjunction with Pt on the former electrode. A remarkable difference in shape was found in the nanoparticles formed on Au modified with Pd(0.75)/Fe(0.25) (see Figure 6E)—hence the name “nanospaghetti” for this type of structurein sharp contrast with the rounded shapes of electrodeposited Pt nanoparticles. These nanospaghetti may be longer than 100 nm and wider than 15 nm (Figure 6E); their height profile (Figure 6E) is characteristic of a rougher surface, relative to an Au


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surface modified with Pt(0.75)/Pd(0.25) (Figure 6D)—a feature caused by the large amount of electrodeposited Pd. The shape of nanospaghetti structures does not improve the catalytic activity of Au modified with Pd(0.75)/Fe(0.25), given their low SA, ECSA, and E1/2values. (E1/2 = 0.76 V— i.e., 140 mV more negative than 0.90 V, the value obtained for the best catalysts investigated in this study, and 40 mV more negative than 0.80 V, the value obtained for the best catalysts involving Au modified with Pd(1.00), or even lower for Au modified with Pd in the presence of an increased concentration of Fe during deposition (see Figure S7).) Xiao et al.30 reported that, relative to Pd nanoparticles, Pd nanorods provide high surface-specific activity toward ORR, comparable to that of Pt at operating potentials of fuel cell cathodes. The Pd nanospaghetti structures obtained in the present study are considerably inferior to, and therefore not comparable with, Pt nanorounded particles in terms of SA, ECSA, and E1/2 toward ORR. Based on different expansions of AFM images, calculated root mean square (RMS) values were plotted against two-dimensional image areas, yielding the graph shown in Figure 7. Roughness corresponds to the RMS value at which it tends to remain constant even as the AFM image is further expanded—i.e., 10.6 for a Au electrode modified with Pt(0.75)/Fe(0.25) before the stability test, 13.5 for a Au electrode modified with Pt(0.75)/Fe(0.25) after 10 000 cycles, and 17.4 for Au electrodes modified with Pt(0.75)/Pd (0.25) and Pt(0.75)/Fe(0.25).

Figure 7. RMS vs. scanned area of modified Au surfaces.



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Roughness values are closely related to the height profile depicted in Figure 6—i.e., roughness is greater with a larger number of hills. In addition, a direct relationship between roughness and height profile can be attributed to the presence of Pd in the electrodeposition solution—i.e., greater amounts of Pd in the electrodeposition solution will exacerbate roughness and the height profile of the modified Au surface. In the present case, the least rough surface exhibited high electrocatalytic activity. In the catalysts subjected to stability testing and in the composition containing Pd, a “packing” effect was observed (smaller distance between particles), which increases surface roughness. For the catalyst subjected to stability testing, a small effect of SA increase was detected, caused by ECSA decrease, in addition to 12 mV degradation occurring at E1/2 in the positivedirection potential scan, leading to unbalances favoring a decrease in electrocatalytic activity—our definition of electrocatalytic activity encompasses high positive E1/2, SA, and ECSA values, and two of these values were decreased as a result of increased surface roughness. In the catalysts containing Pd, an increase in surface roughness led to a decrease in electrocatalytic activity toward ORR. In contrast with our observations, Brussel et al.41 claimed that the relatively rough surface of a Pt/(Cu60s)/Au modified electrode probably explains, at least partly, the increased catalytic activity toward ORR. Greeley52,53 combined thermodynamic formalism with periodic DFT calculations to analyze two effects in metal deposition/dissolution processes involved in the stability of electrocatalysts: the effect that variations in the elemental identity of admetals and substrates have on deposition/dissolution potentials and the effect that changes in local geometric structure—for isolated adatoms, dimers, and more extended kink structures—have on these potentials. Greeley’s studies revealed that shifts in deposition/dissolution potentials for kinks of solute adstructures on close-packed metal substrates had the values−0.17, −0.10, and −0.02 V for Pt in Au, Pd in Au, and Pt in Pd, respectively. These values lend support to the assumption that deposition/dissolution processes may take place with the catalysts investigated in the present study, as shown by changes in roughness and electrocatalytic activity, after the stability test. In summary, we may have at least three factors responsible for the good electrocatalytic activity of our electrodeposited catalysts: (a) variations in adsorption energies from one transition metal to the next and (b) from one geometrical surface structure to the next—both of which can be understood on the basis of a model describing a coupling between adsorbate states and transition metal d-states31—and (c) metal deposition/dissolution


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processes involved in the stability of electrocatalysts—which include variations in the elemental identity of admetals and substrates and changes in local geometric structure.52,53 Therefore, electrocatalysts containing Pt on top of Au and exhibiting low roughness are capable of shifting the d-band center sufficiently to decrease the likelihood of strong bonding taking place between atomic oxygen and d-transition metal,31 resulting in very good electrocatalytic activity. After the stability test, these electrocatalysts which underwent metal deposition/dissolution and increased roughness conserved good SA. Electrocatalysts containing Pt electrodeposited in the presence of Pd (Pt on top of Pd), which exhibit high roughness, even shifting the d-band center, were not sufficiently benefited so as to decrease the likelihood of strong bonding between atomic oxygen and d-transition metal,31 resulting in electrocatalytic activity slightly lower than that of Pt electrocatalysts. However, electrocatalysts containing Pd on top of Au and exhibiting high roughness did not sufficiently shift the d-band center,31 thus not demonstrating good electrocatalytic behavior in ORR.

CONCLUSIONS A rapid, straightforward method was proposed for electrodeposition of Pt and Pd nanoparticles—both of which can be structurally modified by the presence of Fe in the deposition solution—and Pt/Pd alloys on a Au surface. The results showed that the Ptcontaining electrodeposited catalysts investigated exhibit high electrocatalytic activity toward ORR (in terms of SA and ECSA, and E1/2 ≈ 0.9V), in addition to stability in terms of SA and repeatability of responses. The Au electrode modified with Pt(0.75)/Fe(0.25), which demonstrated highest electrocatalytic activity, has nanorounded structure and exhibits low roughness. Although its roughness was increased after 10 000 potential cycles (stability test), SA was slightly increased—a feature attributed to the small number of layers in the electrodeposited film, allowing Pt to dissolve and redeposit without affecting SA, despite losses in ECSA. Au electrodes modified with Pt(0.25)/Fe(0.75) or Pt(0.50)/Fe(0.50) also exhibited very good electrocatalytic activity, indicating their utility for application in acid fuel cells. The presence of Pd increases roughness in deposited electrocatalysts—e.g., Au electrodes modified with Pt(0.75)/Pd(0.25) have good electrocatalytic activity, nanorounded structure, and pronounced roughness, whereas Au electrodes modified with Pd(0.75)/Fe(0.25) exhibit poor electrocatalytic activity (relative to Pt- and Pd-electrodeposited electrodes), nanospaghetti-shaped structure, and pronounced roughness, possibly related to the fact that the presence of Pd on top of Au may not shift the d-band center sufficiently to decrease the occurrence of strong bonding between atomic oxygen and d-transition metal, which, however,



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can be rendered less likely by the presence of Pt on top of Au (low-roughness film) and Pt on top of Pd (pronounced roughness). Tafel inclination values (close to −65 mV dec–1 and −122 mV dec–1 for regions with low and high currents, respectively), number of transferred electrons (4.0), and low percentage of H2O2 formation during ORR are characteristic features of direct reduction to water.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel.: +55 67 3345-3551, fax: +55 67 3345-3552. Author Contributions §

L.B.V. conducted all the experimental part of the study and prepared the first draft of the

manuscript. #R.S.H. and P.A.F. performed the AFM measurements.

ACKNOWLEDGMENTS The authors wish to thank PROPP-UFMS and CNPq (grants 471569/2010-0 and 301403/2011-2) for their financial support. L.B.V. thanks CAPES for the fellowship.

ASSOCIATED CONTENT Supporting Information Available: The Supporting Information contains text, figures, tables, and SEM images concerning supplementary results and discussion, in addition to equations and references. This information is available free of charge via the Internet at http://pubs.acs.org.

REFERENCES [1] Song, C. Fuel Processing for Low-Temperature and High-Temperature Fuel Cells. Challenges, and Opportunities for Sustainable Development in the 21st Century. Catal. Today 2002, 77, 17-49. [2] Tarasevich, M. R.; Sadkowski, A.; Yeager, E. In Comprehensive Treatise of Electrochemistry; Conway, B. E., Bockris, J. O´M., Yearger, E., Khan, S. U. M., White, R. E., Eds.; Plenum Press: New York, 1983; Vol. 7; Chapter 6, pp 301-398. [3] Keith, J. A.; Jacob, T. In Theory and Experiment in Electrocatalysis, Modern Aspects of Electrochemistry; Balbuena, P. B, Subramanian, V. R., Eds.; Springer: New York, 2010; Vol. 50; Chapter 3, pp 89-132.


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Platinum Systems Electrodeposited in the Presence of Iron or

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