Electrocatalytic Activity of PtAu Nanoparticles Deposited on TiO2

Article pubs.acs.org/JPCC

Electrocatalytic Activity of PtAu Nanoparticles Deposited on TiO2 Nanotubes Shuai Chen, Monika Malig, Min Tian, and Aicheng Chen* Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

ABSTRACT: We report on a facile photoassisted approach to directly deposit PtAu nanoparticles (NPs) with controllable compositions on TiO2 nanotubes (TiO2NTs), which were synthesized by electrochemical anodization of Ti substrates. The fabricated TiO2NTs and TiO2-supported Pt and PtAu NPs were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). The electrocatalytic activity of the TiO2NT supported, variably composed PtAu NPs was investigated by using cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). Our studies have shown that TiO2NT-supported PtAu nanoparticles with the Au composition between 30 and 50% exhibit very high electrocatalytic activity toward formic acid oxidation and that the direct dehydrogenation is the dominant reaction pathway in the electrochemical oxidation process. to intermediate species during sustained current discharge.13 On the other hand, although platinum is the most active of metal catalysts for fuel cells, which can be used as both anode and cathode, it is well-known that, at room/moderate temperatures, Pt is readily poisoned by carbon monoxide, a byproduct of formic acid and methanol oxidation.4,14−18 Ptbased alloys (e.g., PtRu, PtAu) are considered as effective catalysts for the enhancement of activity and durability in the oxidation of HCOOH.5,14,16,19−23 Although Au has previously been deemed as catalytically less active than other noble metals, it has recently attracted much attention via the development of nanocrystalline Au-containing catalysts, which exhibit appealing catalytic properties and high stabilities.24−26 Therefore, the study of bimetallic PtAu nanomaterials as fuel cell electrocatalysts for formic acid oxidation is of great interest and importance. Although unsupported noble metal and noble alloy catalysts such as Pt and Pd have been investigated to elucidate the reaction mechanisms of electrocatalysis, the physical support of these electrocatalysts provides a practical means for achieving the optimal utilization of expensive noble metals in order to

1. INTRODUCTION Direct formic acid fuel cells have attracted considerable attention because of the advantages of formic acid over methanol as an electrolyte.1−8 First, formic acid is a nontoxic liquid fuel; it also avoids the risk of producing hazardous byproduct in the oxidation process, which, by contrast, poisonous formaldehyde is typically generated during the oxidation of methanol. In addition, formic acid possesses a lower penetration efficiency through Nafion membranes than methanol due to the repulsion of HCOO− from SO3− ions.8 Furthermore, although the volumetric energy density of formic acid (2086 Wh/L) is lower than that of methanol (4690 Wh/ L), highly concentrated formic acid may be utilized as fuel (e.g., 20 mol/L) to compensate for this shortfall.7 From an economic perspective, the electrooxidation of formic acid commences at a more negative potential compared with that of methanol, which would result in considerable cost reductions for large scale applications.8,9 Finally, as formic acid itself is an electrolyte, it can thus facilitate proton transport within the anode compartment.7 Recent studies show that the electrocatalytic activity of Pd for formic acid oxidation is very high. The CO poisoning effect may be overcome at Pd primarily through the direct pathway. 3,10−12 However, high performance cannot be maintained as Pd dissolves in acidic solutions and is vulnerable © 2012 American Chemical Society

Received: October 6, 2011 Revised: November 23, 2011 Published: January 10, 2012 3298

dx.doi.org/10.1021/jp209630e | J. Phys. Chem. C 2012, 116, 3298−3304

The Journal of Physical Chemistry C

Article

prolong catalyst longevity.27−29 Considerable efforts have been invested in the synthesis of nanoscale noble metal materials on stable and inexpensive substrates. Recent studies have shown that TiO2 substrates have great advantages due to their low cost and high stability.30−32 Specifically, TiO2 nanostructured materials such as nanotubes, nanowires, quantum dots, and nanofibers have become the focus of considerable interest, as they possess unique properties that are relevant in photocatalytic applications.33−35 Among them, nanomaterials with inherent tubular structures have been considered as the most suitable means for achieving larger surface areas without associated increases in geometric area. The electrochemical anodization of titanium in fluorinated electrolytes was employed in this study, which is a relatively simple process for the fabrication of highly ordered TiO2 nanotube arrays.36 Several techniques have been reported as relates to the preparation of binary alloy NPs deposited on TiO2 nanotubes (TiO2NTs), such as under potential deposition (UPD), thermal treatment, spontaneous formation, and successive reduction.37 The PtAu system has a positive heat of formation, and the equilibrium phase diagram displays a large immiscibility gap below 1260 °C (critical temperature).26 As a result, PtAu nanomaterials with expected compositions are difficult to prepare as homogeneous alloyed catalysts. Thus, there is great interest in the development of a simple and straightforward approach for the synthesis of PtAu nanostructures onto the surfaces of TiO2 nanotubes. The photoassisted deposition (PAD) method is a promising strategy for controlling the mean sizes of the alloy particulates, and for the synthesis of homogeneous alloy catalysts.38,39 In this study, highly ordered TiO2NTs, fabricated via anodization, were employed as a support for Pt and PtAu NPs. For the first time, a photoassisted deposition method was employed in the preparation of PtAu NPs, which were directly deposited on TiO2NTs at room temperature. The compositions of the as-synthesized face centered cubic (fcc) PtAu alloys were easily controlled utilizing this facile method. The physicochemical properties of the PtAu electrodes were evaluated by scanning electron microscopy (SEM), X-ray energy dispersive spectrometry (EDS), and X-ray diffraction (XRD). The electrocatalytic properties of the variably composed PtAu NPs in the electrooxidation of formic acid were investigated, showing that the PtAu NPs with a ratio of Pt:Au between 2:1 and 1:1 exhibit considerably higher activity compared to the Pt NPs.

To fabricate the PtAu NPs, 0.13 M of each inorganic precursor was made by dissolving 0.6732 g of H2PtCl6·6H2O in 5 mL of ultrapure water and 0.5137 g of AuCl3×xH2O in 5 mL of ultrapure water, respectively. The desired compositions were prepared by combining the appropriate volume ratio of each precursor, and the total volume for the Pt and Au precursors was 40 μL. The mixed solution of H2PtCl6·6H2O and AuCl3·xH2O and 5 mL of 50% (v/v) methanol was initially deaerated for 20 min with ultrapure argon gas prior to UV irradiation to eliminate any dissolved oxygen. It was then irradiated by ultraviolet light for 30 min. Finally, the electrode was removed from the light, rinsed with ultrapure water, and dried in a vacuum oven at 40 °C. A series of TiO2NT supported Pt and PtAu NPs with different compositions (Pt:Au = 3:1, 2:1, 1:1, and 1:2) was prepared; they were denoted as Pt, PtAu25%, PtAu33%, PtAu50%, and PtAu67%. The mass of the Pt and PtAu NPs loaded on the TiO2NTs was measured using a high-precision (0.01 mg) balance (Mettler Toledo). 2.2. Electrode Characterization. The surface morphology and composition of the TiO2NT supported Pt and PtAu NPs were characterized using JEOL 5900LV scanning electron microscopy and X-ray energy dispersive spectrometry. The Xray diffraction patterns were recorded on a PW1050-3710 diffractometer using a Cu Kα (λ = 1.5405 Å) radiation source. X-ray photoelectron spectra were collected using a Thermo Scientific K-α XPS spectrometer. All of the samples were run at a takeoff angle (relative to the surface) of 90°. A monochromatic Al Kα X-ray source was utilized, with a spot area of 400 μm. Charge compensation was provided, and the position of the energy scale was adjusted to place the main C 1s feature (C−C) at 284.6 eV. All data processing was performed using XPS peak software. 2.3. Catalytic Activity. The catalytic activity and electrochemical performance of all the TiO2NT supported Pt and PtAu NPs were evaluated using a Voltalab Potentiostat PGZ301. A three-electrode cell system was employed in all cases. The counter electrode was a polycrystalline Pt coil, which was flame annealed prior to the experiments. The reference electrode was a saturated calomel electrode (SCE) which was connected to the cell through a salt bridge. All the fabricated TiO2NT supported Pt and PtAu NPs were used as the working electrodes. An electrolyte comprised of 0.5 M H2SO4 was used to examine the hydrogen sorption and desorption behaviors, whereas an electrolyte of 0.1 M HCOOH + 0.5 M H2SO4 was utilized to investigate the formic acid oxidation activity of the TiO2NT supported Pt and PtAu NPs. The electrolyte solutions were deaerated via the continuous passage of ultrapure Ar gas into the electrolyte prior to electrochemical measurements, and an Ar flow was dispensed above the electrolyte during the electrochemical measurements. The electrochemical methods used in this work included cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and electrochemical impedance spectroscopy (EIS). The chronoamperometric measurements were carried out by initially holding the potential at 0 mV for 30 s, with subsequent stepping to various potentials for 600 s. The amplitude of modulation potentials for EIS measurements was 10 mV, and the frequency was changed from 40 kHz to 40 mHz.

2. EXPERIMENTAL SECTION 2.1. Electrode Preparation. A series of PtAu NPs with different compositions were synthesized and directly deposited onto TiO2NT substrates via the photoassisted reduction process. Briefly, Ti plates (99.2%, 1.25 cm × 0.80 cm × 0.5 mm) were cleaned by sonication in acetone, followed by pure water (18.2 MΩ cm), then etched in an 18% HCl solution at 85 °C for 15 min, and finally rinsed with the pure water. The TiO2NTs were fabricated in a dual-electrode electrochemical cell. An etched Ti plate served as the working electrode, whereas the counter electrode was a Pt plate, which was cleaned prior to each experiment by flame-annealing. Anodization took place in a solution that contained dimethyl sulfoxide (DMSO) with 2% (wt) HF at 40 V for 8 h. Finally, the sample was annealed at 450 °C for 3 h to form the anatase structured TiO2NTs.

3. RESULTS AND DISCUSSION 3.1. Surface Characterization of the TiO2NT Supported Pt and PtAu NPs. The surface morphology and composition of the synthesized TiO2NT substrates and the Pt 3299



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and PtAu NPs deposited on the TiO2NTs were examined by SEM and EDS. Figure 1A and B presents the typical SEM

Figure 1. The representative SEM images of the TiO2NT supported Pt (A) and PtAu33% (B).

images of the TiO2NT supported Pt and PtAu33% samples, respectively. The SEM image of the TiO2NTs (not shown here) was very similar to Figure 1A, showing that high-density, uniform TiO2NTs were grown directly on the titanium substrate by electrochemical anodic oxidation. The diameters of these nanotubes were ∼80 nm, with wall thicknesses of ∼10 nm and lengths of ∼1−2 μm estimated from the SEM image of the side view of the sample. The formed Pt NPs were very small and invisible (Figure 1A); in contrast, larger PtAu NPs were formed on the TiO2NTs (Figure 1B). This is because Pt has the tendency to form single crystal seeds due to its chemical nature,40 while PtAu NPs are inclined to congregate. The atomic ratios of Pt to Au, estimated from EDS spectra (not shown) are 74:26, 66:34, 51:49, and 34:66 for the four Pt Au electrodes, which are very close to the ratio of Pt to Au in their precursor concentrations of 3:1, 2:1, 1:1, and 1:2, respectively. These results indicate that PtAu NPs have been successfully synthesized and the composition of Pt and Au can be effectively controlled using the photoassisted deposition method proposed in this study. XPS was performed to analyze the surface composition as well as the electronic interaction of the PtAu NPs deposited on the TiO2NTs. Figure 2A displays the Pt (4f) regions of XPS spectra of the TiO2NT supported Pt, PtAu33%, and PtAu50% NPs. The binding energies of the Pt (4f7/2) and Pt (4f5/2) peaks were shifted negatively by ca. 0.18 eV for PtAu33% and ca. 0.40 eV for PtAu50% compared with the Pt NPs. The negative shift of Pt (4f) peaks is indicative of a lowering of the Pt binding energy due to the addition of Au, and implies that partial electron transfer occurs from Au to Pt, which is consistent with the theoretical studies based on the TiO2/Au2−Pt2 model cluster using the density functional theory (DFT).41 Therefore,

Figure 2. (A) XPS spectra of Pt(4f) of Pt (red line), PtAu33% (blue line), and PtAu50% (green line). XPS spectra of the Pt(4f) (B) and Au(4f) (C) regions for the PtAu33% sample (green circular symbols, raw data; black dashed line, baseline; black solid line, total fit; red line, metallic state fit; blue line, oxidation state fit).

the addition of Au modifies the electronic structure of Pt and an electronic effect is generated. Figure 2B and C exhibit the high resolution XPS spectra for Pt (4f) and Au (4f), respectively, of the PtAu33% NPs. The spectrum of Pt (4f) shows a doublet peak located at a low binding energy (4f5/2) at 74.35 eV and at a high binding energy (4f7/2) at 71.02 eV (Figure 2B). The binding energy peaks of the Au (4f5/2) and Au (4f7/2) (Figure 2C) were measured at 87.76 and 84.09 eV, respectively. The percentages of the metallic state of Pt and Au 3300



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3.2. Electrochemistry of the TiO2NT Supported Pt and PtAu NPs. The electrochemical characterization of assynthesized nanocatalysts was carried out by running cyclic voltammetry of the Pt and PtAu electrodes in 0.5 M H2SO4 at a scan rate of 20 mV/s. Figure 4A presents the CVs of the Pt and

were calculated on the basis of the integration of their individual metallic components and the total components. It was found that 91.2% of Pt is in its metallic state and almost 100.0% of Au is in its metallic state, verifying that the photoassisted deposition method can effectively reduce the Pt and Au precursors simultaneously. The actual atomic composition of Au was also estimated on the basis of the area under the peaks to be 31.5%, which is very close to the 33.3% ratio, estimated from the EDS analysis. XRD was utilized to characterize the internal crystalline geometry of the nanostructured samples. The XRD patterns of Pt and PtAu and Au deposited on TiO2NTs are shown in Figure 3. The peaks marked with stars are derived from the Ti

Figure 3. XRD patterns of pure Pt and pure Au and PtAu with the ratio of 67:33 and 33:67 deposited on the TiO2NTs. The peaks marked as stars are derived from the Ti substrate and the anatase phase of the TiO2NTs.

substrate and the anatase phase of theTiO2 NTs. The Pt NPs exhibit diffraction peaks at 46.4, 67.3, and 82.0°, corresponding to the characteristic (200), (220), and (311) reflections of a Pt face-centered cubic (fcc) structure (JCPDS file no. 4-0802). No distinct Pt and Au peaks were shown in the Pt−Au33% and Pt−Au67%, confirming the formation of PtAu alloys without crystalline metal phase separation. The (200) peaks of the PtAu NPs shifting to a lower 2θ angle compared with the pure Pt NPs indicates the increase in lattice constant of Pt due to the incorporation of Au atoms. The average particle dimensions for the Pt and PtAu NPs were calculated from the (200) peak using the Scherrer equation:

L=

0.94λ β cos θ

Figure 4. Cyclic voltammograms of different ratios of the PtAu electrodes recorded in short (A) and long (B) potential ranges in 0.5 M H2SO4 at a scan rate of 20 mV/s.

PtAu electrodes recorded in the range of −225 to 600 mV vs SCE. The Pt NPs show a typical hydrogen adsorption and desorption behavior. Similar hydrogen adsorption and desorption features are observed with Au compositions of 25 and 33% on the PtAu alloys. With a further increase of Au in the alloy compositions (e.g., 50 and 67%), the two pairs of hydrogen adsorption and desorption peaks become obscure. Increasing the percentage of Au in the alloys results in the diminishment of the peak current of hydrogen adsorption and desorption, which may be attributed to the fact that Au does not adsorb hydrogen.23 Figure 4B shows CVs of the TiO2NT supported Pt, PtAu33%, and PtAu50% NPs recorded in 0.5 M H2SO4 solution in the potential range from −225 to 1350 mV vs SCE. The reduction peaks at 900 mV vs SCE observed for the PtAu33% and PtAu50% electrodes confirm the presence of Au, while the peaks at 440 mV for the three electrodes verify the existence of Pt. The reduction current of the Au oxide at ca.

(1)

where L is the average crystallite size, 0.94 is a constant for small spherical particles of similar size and distribution, λ is the wavelength of X-ray radiation (Cu Kα = 0.15405 nm), β is the full width at half-maximum (fwhm) in radians, and θ is the location of the (200) peak. Herein, the rationale for the selection of the (200) peak is that interference is imparted by the substrate and the peak is relatively strong. The average crystallite sizes of Pt, Au, PtAu33%, and PtAu67% NPs formed on the Ti/TiO2NT substrates are 22.3, 38.7, 15.7, and 19.4 nm, respectively. 3301



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formic acid oxidation through the reduction of poison formation due to the “third body effect”, which is based on the role of a second metal that obstructs surface sites against side reactions, which generate poisonous species, or by blocking the adsorption of inhibiting species that require multiple surface sites for adsorption.9,42 Another plausible explanation might be the electronic modification of Pt, which results in the potent interaction of HCOOH with Pt. Subsequent to the incorporation of Au atoms, the electronic surface properties of Pt atoms may be altered, which influence the reaction kinetics and thus reduce the tendency for poisoning. Therefore, PtAu alloy NPs might facilitate the suppression of adsorbed poisonous species and the modification of surface adsorption strength.9,43 Chronoamperometry was used to further investigate the activity and stability of the electrodes. The electrode potential was held at 0 mV for 30 s and then stepped up to 100 mV (Figure 6A) and 360 mV (Figure 6B), respectively. As shown in Figure 6A, there was an initial sharp current drop, followed by a slow decay for all TiO2NT-supported electrodes. The reason for the sluggish decay might be ascribed to poisoning and structural changes in the PtAu NPs as a result of a perturbation of the potentials during the reaction, especially in the presence of the intermediate produced during the oxidation of HCOOH.44 The current density at 600 s on the PtAu50% is found to be 5.19 mA cm−2 mg−1, over 8 times higher than that on the Pt (0.6 mA cm−2 mg−1), which indicates that the introduction of a certain amount of Au into the nanocomposites endows the catalyst with a much higher steady-state current density. As depicted in Figure 6B, the steady state currents of all the electrodes are reached in 300 s. In the initial 70 s, the current densities of the electrodes follow the order of PtAu33% > PtAu50%, PtAu25% > Pt > PtAu67%, which is in agreement with the results of the aforementioned CV. At the same time, the PtAu33% maintains the highest current density among all of the electrodes. This is due mainly to the more facilitative direct oxidation of formic acid on the electrode. It is worth noting that the current densities of the Pt and PtAu25% electrodes were continually increasing rather than slowly decaying, which is in contrast to PtAu when dispersed on other substrates such as carbon.23,24,45 At the end of the test, the oxidation current values on the Pt, PtAu25%, PtAu50%, and PtAu67% were 22.8, 19.4, 24.1, 14.3, and 7.9 mA cm−2 mg−1, respectively. Figure 6C shows the chronoamperometric curves on the PtAu33% electrode at different potentials. The steady state current density reaches the highest value at the potential of 360 mV. The decrease of the steady-state current at the higher electrode potentials could be attributed to the formation of Pt oxide and Au oxide. This result, combined with the cyclic voltammetry (CV) measurements above, further confirms the superior catalytic activity and stability of PtAu33% in the oxidation of HCOOH. In order to further investigate the electrochemical activity of prepared electrodes in formic acid oxidation, electrochemical impedance measurements were carried out. Figure 7 illustrates Nyquist plots of the Pt and PtAu electrodes obtained at a potential of 100 mV vs SCE in the 0.1 M HCOOH and 0.5 M H2SO4 solution, where Zr and Zi represent the real and imaginary components of the impedance, respectively. The equivalent circuit shown in the inset was used to fit the experimental data. Rs represents the uncompensated solution resistance, Rct denotes the charge transfer resistance, and CPE, defined as CPE_T and CPE_P, represents the constant phase

900 mV decreases, while the reduction current of the Pt oxide at 440 mV increases as the amount of Pt increases, providing direct electrochemical evidence of a modified composition of the formed PtAu alloy. 3.3. Electro-oxidation of Formic Acid on the TiO2NT Supported Pt and PtAu NPs. The electrocatalytic activity of the Pt and PtAu NPs was investigated in the oxidation of formic acid. Figure 5 presents linear sweep voltammograms of the Pt

Figure 5. Anodic sweeps of cyclic voltammograms of different ratios of the PtAu electrodes recorded in 0.5 M H2SO4 + 0.1 M HCOOH at a scan rate of 20 mV/s.

and PtAu electrodes recorded in a 0.1 M HCOOH + 0.5 M H2SO4 solution at a potential scan rate of 20 mV/s. The overall current density is normalized by the total mass of the Pt and Au loaded on the TiO2NTs. It can be observed that there are two overlapping peaks that emanate for each electrode: at ca. 370 mV (peak 1) and at ca. 630 mV (peak 2). Dual pathways exist for formic acid oxidation: a dehydrogenation pathway (direct pathway) and a dehydration pathway (indirect pathway):2,4

Direct dehydrogenation producing CO2:

HCOOH → HCOOads + H+ + e−

(2)

HCOOads → CO2 + H+ + e−

(3)

Dehydration generating CO (poisoning intermediate): HCOOH → CO + H2O (4)

CO + H2O → CO2 + 2H+ + 2e−

(5)

Peak 1 corresponds to formic acid oxidation via the direct pathway, while peak 2 may be attributed primarily to formic acid oxidation via the indirect pathway.4 For the Pt electrode, the current density of peak 2 is higher than that of peak 1, which indicates that formic acid oxidation at this electrode is chiefly through the indirect pathway, and thus produces the undesirable CO intermediate. In contrast, the current densities of peak 1 of the PtAu NPs with over 30% Au are higher than those of peak 2, which indicates that the direct pathway dominates the overall oxidation process. In addition, the onset potential of HCOOH oxidation on the Pt catalyst is ∼50 mV vs SCE, while those on the PtAu33% and PtAu50% catalyst are negatively shifted to ∼ −50 mV vs SCE. Au-based alloys have been suggested as promising materials for the enhancement of 3302



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Figure 7. Nyquist plots (dotted line) of Pt and PtAu electrodes recorded at a potential of 100 mV in 0.5 M H2SO4 + 0.1 M HCOOH and corresponding fitted curves (solid line). The amplitude of modulation potential was 10 mV. The frequency was changed from 40 kHz to 40 mHz. The inset is the corresponding equivalent electric circuit.

Table 1. Values of the Elements in Equivalent Electric Circuit Fitted in the Nyquist Plots of Figure 7 samples

Rs (Ω cm2)

Rct (Ω cm2)

CPE1-T

CPE1-P

Pt PtAu25% PtAu33% PtAu50% PtAu67%

0.99 0.87 0.91 0.75 1.47

171 43 25 18 53

0.0077 0.0084 0.0095 0.0069 0.0080

0.95 0.94 0.93 0.92 0.91

results further demonstrate that the synthesized PtAu NPs exhibit much higher activity in comparison to the Pt NPs.

4. CONCLUSION In summary, novel TiO2NT supported PtAu NPs have been successfully synthesized by a facile photoassisted deposition approach. This synthesis method has allowed for the easy and relatively precise control of the compositions of the fabricated PtAu NPs. Our XRD result shows that PtAu bimetallic alloyed structures have been formed and small average sizes of the particulates in the PtAu alloys have been achieved. In comparison to the Pt NPs, the fabricated PtAu NPs with the Au composition between 30 and 50% exhibit not only a more negative onset potential but also a much higher current density for formic acid oxidation. The results of impedance measurements of these two electrodes (PtAu33% and PtAu50%) show very small charge transfer resistances and a much higher activity for the oxidation of formic acid. The large surface area, ease of preparation, eco-friendliness, high chemical inertness, and cost effectiveness make TiO2 nanotubes attractive noncarbon support materials. The facile and effective approach proposed in this study opens opportunities to develop highly active PtAu catalysts for the pressing energy and environmental applications.

Figure 6. Chronoamperometric curves of HCOOH electro-oxidation on the Pt and PtAu catalysts in 0.1 M HCOOH + 0.5 M H2SO4 at a potential of 100 and 360 mV vs SCE shown in parts A and B, respectively. (C) Chronoamperometric curves (i−t) of HCOOH electro-oxidation on the PtAu33% catalysts in 0.1 M HCOOH + 0.5 M H2SO4 at different potentials.

element, which takes into account formic acid adsorption and oxidation. As shown in Figure 7, the proposed model fits the EIS data very well. The corresponding data for each element is listed in Table 1. The resistance of the supporting electrolyte (Rs) is in the range of ca. 0.75−1.47 Ω cm2 at different electrodes. The PtAu33% and PtAu50% electrodes exhibited very low charge transfer resistances, at approximately 18 Ω cm2, which is ∼10 times smaller than that of Pt (171 Ω cm2). These

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*Phone: +1 807 343 8245. Fax: +1 807 343 7775. E-mail: [email protected]. 3303



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REFERENCES

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This work was supported by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada (NSERC). A.C. acknowledges NSERC and the Canada Foundation of Innovation (CFI) for the Canada Research Chair Award in Material and Environmental Chemistry. We also thank the Surface Interface Ontario/Chemical Engineering & Applied Chemistry at the University of Toronto for carrying out the XPS analysis of the PtAu samples.

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