Influence of Oxide on the Oxygen Reduction Reaction of Carbon

Oct 19, 2009 - Carbon Segregation-Induced Highly Metallic Ni Nanoparticles for Electrocatalytic Oxidation of Hydrazine in Alkaline Media .... Here we ...
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Influence of Oxide on the Oxygen Reduction Reaction of Carbon-Supported Pt-Ni Alloy Nanoparticles Tae-Yeol Jeon, Sung Jong Yoo, Yong-Hun Cho, Kug-Seung Lee, Soon Hyung Kang, and Yung-Eun Sung* School of Chemical & Biological Engineering and Research Center for Energy ConVersion & Storage, Seoul National UniVersity, Seoul 151-744, South Korea ReceiVed: August 7, 2009; ReVised Manuscript ReceiVed: September 28, 2009

Pt-Ni alloy nanoparticles supported on carbon black (Pt:Ni ) 1:1) were prepared by the borohydride reduction method using acetate anions as a stabilizer in anhydrous ethanol solvent. Here, we surveyed the effect of oxide phases in Pt-Ni alloy nanoparticles on the electrocatalytic activity toward oxygen reduction reaction (ORR). As-prepared Pt1Ni1/C, which showed a relatively high degree of alloying, possessed the lower oxygen reduction reaction (ORR) activity as compared to pure Pt. However, following heat treatment in a flow of Ar at 300 °C for 3 h, Pt1Ni1/C showed oxygen reduction activity higher than that of commercial Pt/C (40 wt % Pt/C, Johnson-Matthey). The potential of zero total charge (PZTC) was calculated from cyclic voltammograms and the CO-displacement charge at dosing potentials at which anions are the main adsorbed species. The calculated value then shifted to a more positive potential after heat treatment. This indicates that the surface of the Pt-Ni nanoparticles became less oxophilic mainly due to the clustering of Pt. This anodic shift of the PZTC is consistent with the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure spectroscopy (XANES). Consequently, the observed catalytic enhancement by heat treatment is due to the increase of metallic Pt and NiO and the phase separation between metallic Pt and Ni oxides. 1. Introduction Although the cost of Pt has steadily increased, carbonsupported Pt catalysts are still widely used as electrocatalysts in the cathode of polymer electrolyte fuel cells (PEFCs).1 However, the large cathodic overpotential (0.3-0.4 V) resulting from slow ORR kinetics,1 along with the high cost of Pt, impedes the successful commercialization of PEFCs. Therefore, great effort has been concentrated on elucidating the kinetics and developing highly active electrocatalysts for ORR. The most popular strategy for the development of more active electrocatalysts for the ORR is alloying Pt with various 3d transition metals (Cr, Fe, Co, and Ni).2–13 It is generally accepted that carbon-supported Pt-Ni (or Co) alloy nanoparticles are the best alloy catalysts for the ORR due to the enhanced kinetics relative to pure Pt. However, carbon-supported Pt-Ni (or Co) alloy nanoparticles have a problem of stability in acidic media and show different activities with catalyst preparation and measurement conditions.3,7–10 Usually, carbon-supported Pt-Ni alloy nanoparticles were prepared by conventional borohydride reduction in aqueous solution.5,45 Yang et al. reported the preparation of nanosized Pt-Ni alloy electrocatalysts via the carbonyl complex route in methanol followed by H2 reduction at high temperature.7 Although several methods for the synthesis have been reported, it is difficult to find routes for the synthesis of Pt-M alloy nanoparticles having a high degree of alloying, high dispersion degree, large electrochemical active surface area (ECA),14 and narrow size distribution on carbon supports. To our best knowledge, Sun’s polyol method,15,16 which involves the alcoholmediated reduction of metal precursors in organic solvents contain* Corresponding author. Phone: +82-2-880-1889. Fax: +82-2-888-1604. E-mail: [email protected].

ing strongly binding ligands, and its applications6,17,18 to carbonsupported Pt-based alloy nanoparticles demonstrated the homogeneous bimetallic phase and resultant high degree of alloying.5,19 Although these results present several methods for the synthesis of electrocatalysts in fuel cells, it is the conventional borohydride method that most effectively promotes mass production due to the simple synthesis process and low cost. Brown and Brown,20 by focusing on the hydrogenation reaction, first reported the formation of various monometallic nanoparticles such as Pt by borohydride. Thereafter, borohydrides have been widely used as strong reducing agents for the synthesis of small-sized, metal nanoparticles. We chose a novel chemical strategy for the preparation of carbon-supported Pt-Ni bimetallic nanoparticles (ca. 3 nm in diameter) by the NaBH4 method in anhydrous ethanol with sodium acetate as a stabilizer. Heat treatment for the synthesis of Pt-based bimetallic nanoparticles is usually performed by alloying Pt/C with impregnated second metal precursors on the surface of the Pt nanoparticles using either hydrogen or an inert gas.21,22 In the present work, post-thermal treatment was performed for the enhancement of the oxygen reduction activity of Pt-Ni alloy nanoparticles synthesized at low temperature. While the mechanisms of the heat treatment process are complicated and not fully understood, it is generally regarded as an important step in increasing alloying of the Pt-based binary nanoparticles, leading to the improvement of ORR activity. This is because the electrocatalytic activities are strongly dependent on the preparation procedure, the resulting mean particle size, the size distribution, and other factors such as impurities (e.g., residual surfactants and unwanted adsorbed ions, etc.).23 Despite this, experimental data on the effect of oxide phases on the ORR of Pt-based alloy nanoparticles were scarce. Heat treatment induces changes in the oxidation state and chemical composition of diverse

10.1021/jp9076273 CCC: $40.75  2009 American Chemical Society Published on Web 10/19/2009

Oxygen Reduction Reaction of Pt-Ni Alloy NPs oxide phases, depending on the atmosphere, temperature,5,24 time, and d-character25 of the other metal in the alloy system. In the case of Pt-Ni alloy nanoparticles, heat treatment can readily change the atomic ratios of metallic Pt to Pt oxides and Ni to Ni oxides. This results in the phase separation of metallic Pt and oxide phases such as NiO, thereby decreasing the degree of alloying. In other words, the increased amount of metallic Pt in the surface layer of nanoparticles contributes to the enhancement of ORR activity. However, these phenomena could decrease the stability of carbon-supported Pt-Ni bimetallic nanoparticles in the cathode of fuel cells, because 3d transition metal oxides are easily dissolved in the acidic media.26 The CO-displacement experiments were performed to measure the potential of zero total charge (PZTC),27 which is not dependent on surface area. The fundamental definitions of potential of zero free charge (PZFC) and PZTC were introduced by Frumkin and Petrii.28 The CO-displacement method for estimating PZTC was introduced in detail by Clavilier et al.29–31 Recently, the particle size effects of the high surface area Pt catalysts have been studied experimentally by Mayrhofer et al.,32–34 and theoretically by Greeley et al.35 A decrease in particle size results in an increase in oxophilicity as well as high overpotential during ORR.32 These results indicate that negative shifting of the PZTC makes the particle surface behave as the more cathodically charged, accompanied by a decrease in free sites for the adsorption of O2 or the dissociation of the O-O bond on the Pt surface. For the present research, we used the borohydride method to prepare 40 wt % Pt-Ni alloy nanoparticles in anhydrous ethanol containing dissolved sodium acetate and dispersed carbon black for the ORR catalysts in the PEFCs. The as-prepared sample was heated at 300 °C in Ar atmosphere. ORR activities of the as-prepared Pt-Ni alloys were lower than that of commercial Pt/C (40 wt %, Johnson-Matthey). However, following heat treatment in Ar atmosphere, Pt-Ni alloy had a more active ORR than did commercial Pt/C. This Article is therefore focused on elucidating why the ORR activity of heat-treated Pt1Ni1/C electrocatalyst is enhanced. In particular, we investigated the effect of oxides on the decrease of alloying in a Pt-Ni system, as well as the resulting enhancement of ORR activity. Asprepared and heat-treated samples were characterized by various physical and electrochemical measurements. The structural uniqueness and morphology of the carbon-supported alloy nanoparticles were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analysis were employed to reveal the behaviors of oxides and the fractional d-band occupancies of Pt and Ni. Electrocatalytic activities of the as-prepared and the heat-treated Pt-Ni alloy nanoparticles were determined by rotating disk electrode (RDE) measurement, whereas the influence of oxide phases on ORR kinetics was examined through the shift of the PZTC. 2. Experimental Section 2.1. Preparation and Characterization of Electrocatalysts. Carbon-supported Pt-Ni alloy nanoparticles were prepared via conventional borohydride reduction in anhydrous ethanol. All chemicals including sodium borohydride (NaBH4) were obtained from Aldrich and were of analytical grade. In short, 0.15 g of carbon black (Vulcan XC-72R) was dispersed in anhydrous ethanol (180 mL), and the mixture solution was stirred for 1 h, followed by sonication for 30 min. PtCl4 (0.1328 g), NiCl2 · 6H2O (0.0937 g), and sodium acetate were added to the

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19733 ethanol solution containing dispersed carbon black, which for 4 h was stirred by constant mechanical stirring. The amount of sodium acetate added to the mixture was adjusted so that the sodium acetate:total metal molar ratio was ca. 22:1. After additional sonication for 5 min, 20 mL of anhydrous enthanol solution with NaBH4 was quickly added with vigorous stirring. The resultant solution was stirred for 4 h to complete the reduction of metal precursors, followed by filtration, washing, and drying in a vacuum oven at 60 °C. Heat treatment of the as-prepared Pt-Ni alloy nanoparticles was performed at 300 °C in Ar atmosphere for 3 h. The prepared electrocatalyst powder was examined by X-ray diffraction (XRD, Rigaku D/MAX 2500) with Cu KR radiation (40 kV, 200 mA). The possibility of Ni-loss was monitored by ICP-AES (Optima-4300 DV, Perkin-Elmer) and Element analyzer (EA1112, CE Instrument). The size distribution, particle shape, and degree of dispersion were confirmed by highresolution transmission electron microscopy (HR-TEM, JEOL 2010 at 200 kV). X-ray photoelectron spectra (XPS) were obtained from an Al KR source (ESCALAB 250 XPS spectrometer, VG Scientifics). Binding energies were calibrated with respect to C(1s) at 285 eV. Experimental data were curve-fitted using XPSPEAK4.1 software. Atomic ratios of different states were estimated from the area of the respective LorentzianGaussian peaks. Pt LIII edge and Ni K edge X-ray absorption spectroscopy (XAS) was recorded at the Pohang Light Source (PLS) using the 7C beamline with a ring current of 120-170 mA at 2.5 GeV. A Si(111) monochromator crystal was used with intensity detuned to 85% to eliminate high-order harmonics. Data were collected in fluorescence and transmission modes. Energy calibrations were performed for all measurements using either Pt foil or Ni foil placed in front of the third ion chamber. The University Washington data analysis program was the data analysis package used for X-ray absorption near-edge structure spectroscopy (XANES). The XANES spectra were first subjected to background removal by fitting pre-edge data to a Victoreen-type formula over a range of 200-40 eV below the edge, followed by extrapolation over the energy range of interest and subtraction from the data. After removal of all background, second derivatives calculated from inflection points of data from the reference channel were used to correct the spectra for edgeshifts. Normalization was performed by conventional procedures. The normalization value was chosen as the absorbance at the inflection point of one extended X-ray absorption fine structure (EXAFS) oscillation. The spectra were thus normalized by dividing each datum point by the normalization value. 2.2. Electrochemical Measurements. The catalyst ink slurry was prepared by mixing carbon-supported nanoparticles with 200 µL of DI water, 572 µL of 5 wt % Nafion solution (Aldrich Chem. Co) as a binding material, and 8 mL of 2-propanol per 0.1 g of electrocatalyst. Following mixing and ultrasonication, 7 µL of ink slurry was pipetted onto a glassy carbon substrate (0.283 cm2 geometric surface area), leading to a metal (Pt and Ni) loading of ca. 102 µgPt-Ni/cm2. The dried electrode was then transferred to the electrochemical cell, and a cyclic voltammogram was recorded in argon-saturated 0.1 M HClO4 (Aldrich). For the ORR, the 0.1 M HClO4 solution was purged with oxygen, and the cell temperature was fixed at 293 K. A rotating disk electrode (RDE, Ecochemi. Co.) was used as a working electrode (0.196 cm2 geometric surface area). All current densities were normalized to the geometric area of the rotating disk electrode, and the electrochemical measurements were performed at 293 K.

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The PZTC was calculated from the voltammetric contribution of specifically adsorbed anions that were displaced by CO as a neutral probe, such as chloride and bisulfate ions. The COdisplacement method described by Orts et al.31 was employed. Briefly, CO was introduced into the Ar-purged electrolyte at a constant potential at which anions were the dominant species adsorbed on the electrode surface. Therefore, the resulting displacement current was recorded versus time, and then calculated into charge by integration. All electrochemical measurements were conducted in a standard three-compartment electrochemical cell using a glassy carbon electrode, Pt wire, and saturated calomel electrode (SCE) as working, counter, and reference electrodes, respectively. All measurements were recorded and reported versus normal hydrogen electrode (NHE). 3. Results and Discussion 3.1. Preparation and Characterization of the CarbonSupported Pt-Ni Alloy Nanoparticles. Anhydrous ethanol was chosen as the solvent for synthesis of the Pt-Ni bimetallic nanoparticles, which possess a high degree of alloying and low oxide content. To minimize the large gap in redox priority between Pt and Ni, PtCl4 and NiCl6 · 6H2O as precursors were chosen because water molecules in starting materials can help promote reduction in anhydrous solvent. Even so, Ni-loss can usually occur due to its low redox potential as compared to Pt. Assuming that Pt is completely reduced by NaBH4, the alloy composition and the metal loading were confirmed by ICP-AES and Element analyzer. The reduced amounts relative to nominal value of Ni with different conditions were summarized in Table S1 (see the Supporting Information). It was found that 40 wt % of total metal loading was confirmed by Element analyzer, whereas the complete reduction of Ni precursor was achieved by the use of acetate ions and an increase of NiCl6 · 6H2O concentration. TEM images of carbon-supported alloy nanoparticles prepared in water and ethanol solvents are shown in Figure S1 (see the Supporting Information). It is noted that ethanol as a solvent improves the dispersion of metal nanoparticles on a carbon support relative to that of water. This apparent difference in the dispersion between water and ethanol is not completely understood. A possible explanation could be the high dispersion of carbon black due to higher donor number of alcohol36 as compared to water and the extremely low content of water in anhydrous ethanol solvent. As-prepared and heat-treated Pt1Ni1/C were characterized by HR-TEM as shown in Figure 1a and b, respectively. It is shown that the Pt-Ni alloy nanoparticles are well-dispersed on the surface of the carbon support and that the average measured particle diameter was approximately 3.4 nm regardless of heat treatment. It should be mentioned here that no marked changes in either shape or particle size of the nanoparticles after heating were observed throughout the HR-TEM analysis. This indicates that a temperature of 300 °C is not sufficient for the sintering of carbon-supported Pt-Ni alloy nanoparticles. Figure 2 shows XRD powder scans of as-prepared and heattreated Pt-Ni electrocatalysts with different duration of heat treatment. Peaks of as-prepared Pt1Ni1/C shifted to higher angles as compared to those of pure Pt (dashed lines, Pt(111) and (220)), thereby indicating a contraction of the lattice upon substitution of Pt with Ni. This is supported by the fact that Ni atom is 11% smaller in size relative to Pt, and that the substitutional solid solution usually occurs when the size of the smaller atom is at least 87% of the larger. Because the crystal structures of both Pt and Ni are face-centered cubic, the lattice

Figure 1. TEM images of carbon-supported Pt-Ni electrocatalysts: (a) as-prepared sample and (b) sample heated for 3 h in Ar atmosphere.

constant of the solid solution of Pt and Ni should adhere to Vegard’s law (linear decrease of the lattice constant upon increase of Ni in Pt matrix). The (220) peak position of asprepared Pt1Ni1/C is lower than 71.5°, as calculated from Vegard’s law. This peak position was used to evaluate the degree of alloying, as given in Table 1. The degree of alloying was decreased as heating duration increased, as evidenced by a sudden decrease after 1 h of heating. Upon increasing the heating time from 1 to 3 h, the magnitude of the decrease becomes nearly constant but smaller. The degree of alloying in asprepared Pt1Ni1/C is decreased from 69% to 41% after 1 h of heating and 35% after 3 h of heating. Variations in the degree of alloying versus heating time were shown in detail in Figure S2 (see the Supporting Information). The 69% degree of alloying is relatively high as compared to previously reported results.7,8,37 However, the unalloyed portion (31%) originates from imper-

Oxygen Reduction Reaction of Pt-Ni Alloy NPs

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Figure 2. Powder XRD patterns of the as-prepared and heat-treated samples for various periods of time (1-3 h). The dotted lines at 39.76° and 67.45° are the Pt(111) and Pt(220) peak positions for pure Pt, respectively.

TABLE 1: Structural Characterization by XRD Analysis of As-Prepared and Heat-Treated Pt-Ni/C catalyst

lattice parameter (nm)

particle size (nm)

degree of alloying (%)

as-prepared heat-treated, 1 h heat-treated, 2 h heat-treated, 3 h

0.3789 0.3844 0.3850 0.3855

2.1 2.0 2.1 2.2

69 41 38 35

fections in the solid solution of Ni and Pt. Because the high degree of alloying implies homogeneous alloying of Pt and Ni without Ni-loss or oxide phases, it therefore can lead to the strong modification of the electronic structure responsible for the low overpotential in the ORR, that is, the downshift of the Pt d-band center.13,38,39 As heat treatment progresses, the shape of Pt nanoparticles can be changed even at 300 °C.40 At the same time, the segregation of Pt atoms on the surface is activated by thermal energy. However, the large decrease in the degree of alloying following heat treatment cannot be attributed to the surface segregation of Pt, due to the principle of X-ray diffraction and the relatively low segregation energy of Pt-Ni alloy system.41 The ready oxidation of Ni by oxygen sources such as air and water and additives such as sodium hydroxide, which is often used in various chemical reduction methods (e.g., polyol process), encourages the formation of various Pt or Ni oxides and can be the main reason for the loss of the degree of alloying. Identifying why the as-prepared sample has a low degree of alloying that is further decreased by heat treatment can provide new insights for the design of high surface area Pt-Ni alloy nanoparticles. To thoroughly investigate this large decrease in alloying, experiments involving rotating disk electrode (RDE), COdisplacement for calculation of the PZTC, and synchrotron XANES as well as XPS were performed. Both samples (asprepared and 3 h heat-treated samples) were compared because all other samples had a degree of alloying similar to that of the sample heated for 3 h as determined by XRD analysis. 3.2. XPS/XANES Characterization. The reactivity of any given metal (e.g., Pt) can be changed substantially by alloying. For Pt-based alloy catalysts used for the ORR, Pt is usually alloyed with 3d transition metals so as to downshift the d-band center of surface Pt atoms.39 The position of the d-band center relative to the Fermi energy can be directly related to the binding energies of reaction intermediates including oxygen. Shifting

Figure 3. XPS spectra of Pt 4f region for the (a) as-prepared and (b) heat-treated samples for 3 h. (c) Pt 4f peak shift before and after heat treatment. Dotted lines are centers of the Pt 4f7/2 peaks before and after heat treatment.

the d-band center for surface Pt, whose d-band is more than half-full, is well-known to be accompanied by a change in the surface core-level shift (SCLS) in the same direction.25,42 X-ray photoelectron spectroscopy (XPS) was performed with the as-prepared and heat-treated Pt1Ni1/C samples. Figure 3a and b shows the Pt 4f core-level peaks of as-prepared and heattreated Pt1Ni1/C. From curve-fitting, it was noticed that Pt exists in various states such as 71.75 eV of Pt0 (metallic Pt), 72.8 eV of Pt(II) (PtO), and 74.42 eV of Pt(IV) (PtO2) and that the ratio among these states was significantly changed after heat treatment. For the heat-treated sample, the metallic Pt phase was intensified due to the reduction of Pt(II) and Pt(IV) resulting from the increased oxidation of metallic Ni, which is highly

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TABLE 2: Chemical States and Area Ratios of Pt-Ni Alloy Nanoparticles by XPS XPS area ratio (%) heat treatment metallic Pt PtO PtO2 metallic Ni NiO Ni(OH)2 as-prepared heat-treated, 1 h heat-treated, 2 h heat-treated, 3 h

48.6 48.3 52.0 56.5

23.9 23.0 30.7 31.1

27.4 28.7 17.3 12.4

19.8 10.4 8.7 8.5

54.0 89.6 91.3 91.5

26.2

oxophilic and has lower cohesive energy as compared to Pt.43 The quantitative area ratios of each element are summarized in Table 2. Figure 3c compared the Pt 4f core-level peaks of the as-prepared and heat-treated samples. The core-level peak of the heat-treated sample was shifted to a lower binding energy more so than the as-prepared sample. Because the XPS technique is not surface-sensitive at the monolayer level, this large shift (0.6 eV in BE) must be attributed to changes in the nearest neighbors surrounding the Pt atom. Moreover, refs 25 and 42 suggest that variation in the d-band center for metal overlayers is accompanied by a similar variation in the SCLS. In addition, according to the effective medium theory,44 the downshift of BE in this alloy system can be caused by an increased number of neighboring Pt atoms around a single Pt atom during heat treatment in a noble gas environment. Figure 4 shows Ni 2p3/2 core-level peaks for as-prepared and heat-treated Pt1Ni1/C, respectively. In general, the Ni 2p spectrum displays intense and broad shakeup peaks, which are attributed to multielectron excitation.45 Here, Ni states were comprised of Ni oxide (854.6 eV of NiO), Ni hydroxide (855.8 eV of Ni(OH)2), and metallic Ni (852.4 eV of Ni0). By fitting the Ni 2p spectra, it is clear that the Ni(OH)2 phase disappeared and that metallic Ni0 decreased following the heat treatment. Note that after heat treatment NiO became the dominant phase, occupying 91.5% of total Ni. This means that Ni atoms in the as-prepared alloy nanoparticles are bound by oxygen and hydroxide, leading to subsequent changes in the oxide phase by thermal heating. As given in Table 2, decrease of the Ni0 fraction by heat treatment may further decrease the degree of alloying, which may be closely related to the phase separation between Pt and Ni based on the XPS data. Therefore, it is clear that Pt becomes more metallic after removal of oxygen. In the case of Ni, the NiO phase is increased to ca. 92% of the total Ni. The role of oxygen was further verified by X-ray absorption near-edge structure (XANES) analysis to remove ambiguities in the XPS analysis curve-fitting. Figure 5 represents the Pt LIII edge and Ni K edge of the Pt1Ni1/C. The XANES region of Pt LIII and Ni K, which depends on heat treatment, can be used for assessing qualitative changes in the fractional d-band occupancies of Pt and Ni, because the XANES region of the XAS spectrum is dependent mainly on multiple scattering and multiphoton absorptions.46 From white line analysis, it can be determined whether Pt and Ni oxides increase or decrease. However, with heat treatment the oxidation of Pt and Ni shows the opposite trend; that is, the intensity of the Pt LIII edge decreased, while that of the Ni K edge increased. Therefore, changes in the white line intensity can be a consequence of increased metallic Pt and Ni oxides by thermal heating. These XANES data are in very good agreement with the XPS results in Figures 3 and 4. The Pt and Ni atoms have higher energy states on the surface than in the interior of nanoparticles due to the narrowing of surface d-bands and their upward shift to preserve the degree

Figure 4. XPS spectra of Ni 2p3/2 region for the (a) as-prepared and (b) heat-treated samples for 3 h.

of d-filling. For this reason, structural change may start from the surface upon thermal heating in Ar atmosphere. In the following section, electrochemical measurements are performed to detect changes in surface structure as well as ORR activity. 3.3. Electrochemical Characterization. ORR measurements were conducted in O2-saturated 0.1 M HClO4 solution using the rotating disk electrode (RDE) technique. Figure 6 compares the rates of ORR for the as-prepared and heat-treated Pt1Ni1/C. For comparison, commercial Pt/C (Pt-loading of 40 wt %, Johnson-Matthey) was also measured. For all polarization curves, the ORR started in the positive direction from 0.15 V vs NHE. In this figure, it was shown that the ORR activity was increased in the following order: as-prepared Pt-Ni < commercial Pt < heat-treated Pt-Ni. In other words, the activity of the heat-treated Pt-Ni is higher than those of commercial Pt and as-prepared Pt-Ni. Further supporting evidence is given by the half-wave potential, in that the half-wave potential of the heat-treated sample is shifted positively to about 30 mV. Stamenkovic et al. found that the ORR of Pt-skin Pt3Ni(111) alloy was enhanced by a factor of 10 as compared to Pt(111), due to the modification of the d-character of Pt.13 In electrocatalysts with high surface area, nanosized Pt-based alloy particles reveal various oxide phases due to increased oxophilicity as compared to their bulk state.47 Therefore, increased ORR activity by heat treatment is possibly due to the behavior

Oxygen Reduction Reaction of Pt-Ni Alloy NPs

Figure 5. XANES spectra of (a) Pt LIII edge and (b) Ni K edge for the as-prepared and heat-treated for 3 h samples.

Figure 6. Comparison of polarization curves for the ORR of 40 wt % Pt/C, as-prepared Pt1Ni1/C, and heat-treated Pt1Ni1/C. The electrode geometric area is 0.196 cm2.

of various Pt and Ni oxides in accordance with the XPS and XANES results. The cyclic voltammograms of the as-prepared and heat-treated samples are presented in Figure S3 (see the Supporting Information). However, changes in the physical properties of the surface before and after heat treatment are unclear. Therefore, CO-displacement was performed to calculate the PZTC values. Because the potential of zero charge (EPZC) can provide insights into designing electrocatalysts with higher catalytic activities, it is possible to understand the double-layer properties by calculating EPZC of transition metals-electrolyte interfaces. The EPZC can be considered as the PZFC and the PZTC. The PZFC is the potential at which the electronic excess charge density of the metal surface equals zero, and moreover will equal the PZTC when no interfacial charge transfer occurs. Therefore, the PZFC is directly related to the work function of

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19737 the electrode surface. Specific adsorption in an electrolyte solution leads to deviation of the PZTC from the PZFC, which cannot be measured. The PZTC is the only experimentally accessible parameter depending on interfacial charge transfer, that is, anion adsorption. Pt and Pd normally show relatively high work function as compared to other transition metals, and therefore the highest anodic values were usually revealed for them. Consequently, the PZTC depends on surface properties such as the work function and surface geometry of the electrode, along with the concentration and adsorbability of solution anions. As previously mentioned, electrochemically important processes such as the adsorption and desorption of hydrogen, anions, and oxygenated species were investigated through COdisplacement. Several reports have shown that specific anions (e.g., bisulfate, chlorine, perchlorate, and acetate ions) belonging to electrolytes strongly adsorb to the Pt surface. This specific adsorption correlates to the various surface reactions, such as ORR,47 CO adsorption and oxidation,48 and Hupd.31 It is wellknown that the PZTC can be calculated by CO-displacement, which involves replacement of the preadsorbed species by CO molecules.27–31 This occurs at a potential ranging from 0.05 to 0.1 V for Pt, where the dominant adsorbate is hydrogen. To calculate the PZTC, the so-called Hupd charge is subtracted by the CO-displacement charge. However, as hydrogen evolution can change the Hupd and CO-displacement charge, CO molecules were displaced at a potential in which anions were the dominant species adsorbed on the electrode surface. In addition, COdisplacement at various potentials in 0.1 M HClO4 solution revealed that carbon-supported Pt-Ni alloy nanoparticles possess a PZTC of ca. 0.142 V, a value ca. 0.14 V lower than pure Pt/C (ca. 0.28 V).32 Pt-Ni alloy nanoparticles with such low PZTC imply that the oxidation potential of Pt1Ni1/C has been decreased. Therefore, the start potential of the cyclic voltammograms should also be lowered to exactly calculate the Hupd charge. For this reason, the negative electric charge from the transient was used to determine the nature of the species displaced by CO, which acted as a neutral probe. The transient current created by CO-displacement was integrated and subtracted after calculating the charge from the CO dosing potential to the start potential in the cyclic voltammograms. As a result, the potential at which the calculated charge is zero is identified as the PZTC. This value is equal to the potential at which the displaced charge is zero. Figure 7 shows the PZTC of Pt1Ni1/C before and after heat treatment in different electrolytes. Curves denoted by a gray line in Figure 7a and b denote the voltammetric profiles of the as-prepared sample, while black curves represent the heat-treated sample. The two different electrolytes resulted in different PZTC values due to the variations in the strength of the specific adsorption of anions, as shown in Figure 7a and b. It is natural that the PZTC of Pt-Ni alloy nanoparticles is lower than that of pure Pt/C, irrespective of heat treatment. This is because surface Ni atoms of Pt-Ni alloy nanoparticles, which have the lower work function as compared to Pt, contribute to the lower PZTC values relative to pure Pt, which is the transition metal with the largest work function except for Au. Therefore, the low PZTC value is attributed to the alloying of Pt with Ni. Alloying Pt (the stable d-character toward the adsorption of oxygenated species) with Ni (the lower work function of Ni as compared to Pt) results in a cathodic shift of the equilibrium oxidation potential of the surface. It was noted that PZTC shifted toward a higher potential following heat treatment. As was already mentioned, Stamenkovic et al.13 reported that “Pt-skin”

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Figure 7. Voltammetric profiles of the as-prepared and heat-treated Pt1Ni1/C electrodes in (a) 0.1 M HClO4 + 0.02 M NaCl and (b) 0.5 M H2SO4 solutions. Inset: Plots of current-time transient recorded during CO adsorption at 0.18 V in 0.1 M HClO4 + 0.02 M NaCl (upper curve) and at 0.22 V in 0.5 M H2SO4 (lower curve). The arrows point to positions of the potentials of zero total charge.

forms in the surface of Pt3Ni(111) following heat treatment. Therefore, the resulting oscillatory segregation of Pt greatly enhanced catalytic activity of the ORR, due to an increased number of free Pt sites on the surface that were uncovered with oxygen-containing species after the downshift of the d-band center. If similar segregation had occurred in our electrocatalysts, then accepting that the PZTC of the heat-treated Pt1Ni1/C would experience an anodic shift would be difficult because the Ptrich surface has a reduced Hupd charge. One possible reason for the increased PZTC might be an increase in crystallite ordering. The PZTC of the carbon-supported metal nanoparticles prepared by the chemical reduction method at low temperature is predicted to shift toward positive potentials after heat treatment. This is because the metal nanoparticles reduced at low temperature are disordered in the structure initially and then become part of a higher ordered structure after heat treatment. The various changes between the Pt and Ni phases caused by heat treatment in the XPS and XANES studies involve the corelevel binding energy of Pt being shifted to a lower value and the d-band vacancy of Pt being decreased by heat treatment. Therefore, the ordering of crystallites may not be suitable. The reason for the anodic shift in the PZTC is possibly due to the increased NiO phase that already exists in the as-prepared alloy nanoparticles. In the same manner, metallic Ni and two kinds of Ni oxides may be transformed to NiO as the main phase, as discussed in the previous section. The number of Pt atoms surrounding another neighboring Pt atom may be increased beyond that in the as-prepared sample, due to an increased amount of NiO. Therefore, the work function of Pt in the heattreated Pt-Ni alloy particles increased, as evidenced by the positive shift of PZTC. For the Pt-Ni alloy system, the anodic shift of PZTC is closely connected to the existence of oxygen in the alloy nanoparticles. Consequently, enhancement of the ORR activity can be explained by the upshift of the Pt d-band center as compared to its as-prepared state. 4. Conclusions The effect of oxides on the ORR activity of carbon-supported Pt-Ni alloy nanoparticles has been studied. The developed reduction method was confirmed to be efficient in synthesizing Pt-Ni alloy nanoparticles with a high degree of alloying. The choice of acetate anions as stabilizer, and anhydrous ethanol as

Jeon et al. solvent, aimed to allow the synthesis of highly pure Pt and Ni without oxides. However, chemical analysis by XPS led us to conclude that the formation of Ni oxides cannot be avoided due to the existence of various oxygen sources, that is, water in the Ni precursor, air, etc. The XPS and XANES data indicated that the amount of metallic Pt in the as-prepared Pt-Ni nanoparticles was increased, whereas metallic Ni was decreased after heating at 300 °C in Ar atmosphere. These results are in excellent agreement with the observed decrease in alloying in the Pt-Ni bimetallic system. The PZTC was measured to reveal the effect of oxide on electrochemical activity. The PZTC underwent a positive shift without particle growth after heating, as shown in the TEM and the electrochemical measurements. This upshift is in excellent agreement with the increase of metallic Pt and NiO. Further corroboration is provided by the XPS and XANES spectra, which found a decrease of d-band vacancies in Pt and an increase of vacancies in Ni. The enhanced oxygen reduction activity after heat treatment is attributed to an increased number of Pt atoms surrounding a neighboring Pt atom on the surface. This clustering of Pt atoms most likely originated from the increase of NiO phase and the decrease of metallic Ni. That is, the disappearance of Ni(OH)2 is followed by the oxidation of metallic Ni to NiO, and so the clustering of Pt atoms is induced. Therefore, the enhanced ORR activity of Pt1Ni1/C heated for 3 h is directly connected to the increase of NiO and the decrease of the degree of alloying to 35%. This means that the ratio of Pt to metallic Ni in the surface largely increased due to the increase of inactive NiO. All of these results clearly indicate that for Pt-Ni electrocatalysts with large surface area, oxide can act as a reaction inhibitor by diminishing the electronic modification of alloying with 3d transition metals. Furthermore, these results can be extended to electrocatalysts with higher ORR activities. These require careful control of metal oxides during the preparation process in carbon-supported Pt-based bimetallic catalysts. Acknowledgment. This work was supported by the Research Center for Energy Conversion & Storage and the WCU (World Class University) program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science, and Technology (400-2008-0230). Supporting Information Available: Chemical analysis of the amount of reduced Ni; TEM images of Pt1Ni1/C electrocatalysts prepared in water and anhydrous ethanol in the presence of sodium acetate as a stabilizer; the calculated degrees of alloying with various heating periods; and the cyclic voltammograms of commercial 40 wt % Pt/C, as-prepared, and heat-treated Pt1Ni1/C. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Gottesfeld, S.; Zawodzinski, T. A. Polymer Electrolyte Fuel Cells. In AdVances in Electrochemical Science and Engineering, 1st ed.; Alkire, R. C., Gerischer, H., Kolb, D. M., Tobias, C. W., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 5; p 195. (2) Antolini, E.; Passos, R. R.; Ticianelli, E. A. Electrochim. Acta 2002, 48, 263. (3) Paulus, U. A.; Wokaun, A.; Scherer, G. G.; Schmidt, T. J.; Stamenkovic, V.; Radmilovic, V.; Markovic, N. M.; Ross, P. N. J. Phys. Chem. B 2002, 106, 4181. (4) Drillet, J. F.; Ee, A.; Friedemann, J.; Kotz, R.; Schnyder, B.; Schmidt, V. M. Electrochim. Acta 2002, 47, 1983. (5) Salgado, J. R. C.; Antolini, E.; Gonzalez, E. R. J. Phys. Chem. B 2004, 108, 17767.

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