C Catalyst Performance for Ethanol

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J. Phys. Chem. C 2008, 112, 6582-6587

Effect of Ni on PtRu/C Catalyst Performance for Ethanol Electrooxidation in Acidic Medium Zhen-Bo Wang,* Peng-Jian Zuo, Guang-Jin Wang, Chun-Yu Du, and Ge-Ping Yin Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China ReceiVed: January 11, 2008; In Final Form: February 5, 2008

This research is aimed to improve the utilization and activity of anodic catalysts for ethanol electrooxidation. The direct ethanol fuel cell anodic catalysts, PtRuNi/C and PtRu/C, were prepared by chemical reduction method. Their performances were tested by using a glassy carbon disk electrode through cyclic voltammetric curves, chronoamperometric curves, and amperometric i-t curves in a solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4. The particle size, lattice parameter, composition, micro-morphology, and surface states of metals in the PtRuNi and PtRu surface particles were determined by X-ray diffraction (XRD), energy dispersive analysis of X-ray (EDAX) analysis, transmission electron microscopy (TEM), and X-ray photoelectron spectrometry (XPS), respectively. XRD analysis showed that both of the catalysts exhibited face-centered cubic structures and had smaller lattice parameters than did Pt/C catalyst. Their sizes are relatively small, about 3.0 nm. Their size distribution of the metal nanoparticles is very homogeneous, without evident agglomeration. There are a lot of Ni oxides with different oxidation states in the PtRuNi/C catalyst from XPS results. No significant differences in the ethanol electrooxidation on both electrodes were found regarding the onset potential for ethanol electrooxidation by using cyclic voltammetry. Yet, the catalytic activity of the PtRuNi/C catalyst is much higher for ethanol electrooxidation than that of the PtRu/C catalyst. Also, its CO-tolerance is better than that of the PtRu/C catalyst. The hydrogen spillover effect of Ni hydroxides and electronic effect of metallic Ni play an important role in the catalytic performance of PtRuNi/C catalyst for ethanol electrooxidation.

1. Introduction The direct methanol fuel cell (DMFC) has been receiving increasing attention due to its advantages of easy transportation and storage of the fuel, reduced system weight, size and complexity, and high-energy efficiency.1-3 However, methanol has some particular disadvantages; for example, it is relatively toxic, inflammable, and it is not a primary fuel, nor a renewable fuel.4 Therefore, other alcohols begin to be considered as alternative fuels. Among organic small molecule alcohols, ethanol is the most attractive fuel for the electric vehicle, because it is a green fuel and can be easily produced in great quantity by the fermentation of sugar-containing raw materials for agriculture. Furthermore, ethanol is safer and more convenient, having more energy density (8.01 vs 6.09 kW‚h/kg) than methanol, and appears to fulfill most fuel requirements for lowtemperature fuel cells.5,6 However, ethanol electrooxidation is more difficult than that of methanol with the necessity of breaking the C-C bond for its complete oxidation at low temperatures. Much recent work was focused on the ethanol electrooxidation with Pt-based catalysts.7-14 A generally accepted ethanol oxidation mechanism has been proposed. The first step of ethanol oxidation in the mechanism is the cleavage of O-H bond, similar to methanol oxidation, forming ethoxy species. Its second step is to transform the ethoxy species into acetaldehyde, which then can be oxidized by various paths into numerous products, including acetate ion, acetone, crotonaldehyde, acetyl, methane, carbonate ion, CO, CO2, and other hydrocarbons, adsorbed on the Pt catalyst surface at lower * Corresponding author. Tel.: +86-451-86417853. Fax: +86-45186413707. E-mail: [email protected].

potentials together with the reactant species. The species of metal-OH, which is generated from dissociative adsorption of H2O on the catalyst surface, enables the ethanol electrooxidation at low potentials, but inhibits it at high potentials due to the strong association between -OH group and the catalyst surface. At present, to increase the electroactivity of ethanol is crucial, and together with its complete oxidation into carbon dioxide it is still a challenge.15 Although there has been tremendous progress in the last decades, the most important fundamental research efforts worldwide are currently based on the development of effective anode catalysts.7,8,16 So far, the best catalyst for ethanol electrooxidation in acid medium has been found to be platinum.9 Platinum itself is known to be rapidly poisoned at its surface by strongly adsorbed species coming from the dissociation of ethanol molecules. So, the anodic current from ethanol electrooxidation on Pt is very sluggish, especially at low temperatures. There is a need to improve the activity of catalysts for ethanol electrooxidation.17,18 The only possible way is to modify the electrode surface so that, at low potentials, its coverage of oxygenated species (e.g., adsorbed OH) from the dissociation of water will be increased. These OH species are necessary to oxidize the species from the dissociation of ethanol completely to carbon dioxide. It is well known that the addition of transition metal to Pt-based catalysts lowers the overpotential for ethanol electrooxidation reaction significantly through a socalled bifunctional mechanism.19,20 So far, PtRu alloys are still considered to be the best catalysts, because of their CO-tolerance, and are widely used in DMFC.17 Yet the performance of binary PtRu alloy catalyst for ethanol electrooxidation needs to be improved.18 Furthermore, platinum

10.1021/jp800249q CCC: $40.75 © 2008 American Chemical Society Published on Web 04/03/2008

Effect of Ni on PtRu/C Catalyst Performance and ruthenium are noble metals. The resource of platinum is limited, and that of ruthenium is pinching. So, it is worth studying that a base metal may be used as an additional one in the anodic catalyst for DEFC. This may lower its cost and accelerate its industrialization. Theoretical calculations have shown that the segregation processes that generally lead to Pt surface enrichment are unlikely to occur in the PtNi system.21 Furthermore, in the potential range at which the methanol electrooxidation proceeds, Ni from the PtNi alloy would not dissolve in the electrolyte. The resistance to the dissolution has been attributed to a nickelhydroxide passivated surface and the enhanced stability of Ni in Pt lattice.22 Despite these apparent advantages, the carbonsupported PtNi systems as DMFC anodes were studied only a little.16 Methanol electrooxidation on nickel-based (PtNi) thin film and alloy (PtRuNi) nanoparticles as anodic catalysts has been reported.22,23 The use of PtNi systems as DMFC cathodes has been much investigated.24-28 The carbon-supported PtRuNi used as anodic catalysts for ethanol electrooxidation was published little. The effect of the Pt/Ru/Ni composition on performance of PtRuNi/C was investigated in detail in our previous work.29 Rojas et al.30 prepared PtRuNi/C via the colloidal method with H2PtCl6, RuCl3, and NiCl2 as precursors and studied the effect of Ni on performance of PtRu/C for CO stripping and methanol electrooxidation. They thought that the oxidation state of Ni played a key role in the catalytic performance of PtRuNi/C catalyst, particularly in the CO oxidation due to the development of Pt-Ni electric interaction. Moreno et al.31 prepared PtRuNi/C catalysts by combustion synthesis from a stoichometric mixture of organic Pt, Ru, and Ni compounds, and CO(NH2)2 as precursors with sucrose and urea as an alternative fuel, respectively, for hydrogen/oxygen fuel cells. Yang et al.32 prepared PtRuNi/C via a carbonyl route with Na2PtCl6‚6H2O, RuCl3, and NiCl2‚6H2O as precursors. Their investigation displayed that the as-prepared PtRuNi catalyst with an atomic ratio of 60:30:10 exhibited significantly enhanced electrocatalytic activity and good stability for methanol oxidation in comparison to commercial PtRu catalyst available from Johnson-Matthey. In this article, we prepared carbonsupported PtRuNi catalyst from co-deposition method with Pt(NH3)2(NO2)2, Ru(NO3)3, and Ni(NO)2 as precursors. In the process, the Ni effects on performance of PtRu/C catalyst were characterized for ethanol electrooxidation in acidic medium by electrochemical methods, XRD, EDAX, TEM, and XPS. 2. Experimental Details 2.1. Preparation of Catalysts. The carbon black powder (Vulcan XC-72, Cabot) was used as the support for the catalyst. The samples contained 20% metals in weight of the catalysts dispersed in the powder. The PtRuNi (with an atomic ratio of 6:3:1)/C or PtRu (with an atomic ratio of 1:1)/C catalyst, 0.25 g, was obtained by chemical reduction33,34 with sodium borohydride as reducing agent at 80 °C. Pt(NH3)2(NO2)2, Ru(NO3)3, and Ni(NO)2 were used as precursors, respectively. The carbon black was ultrasonically dispersed in a mixture of ultrapure water and isopropyl alcohol for 20 min. The precursors were added to the ink and then mixed thoroughly for 15 min. The pH value of the ink was adjusted by NaOH solution to 8, and then its temperature was raised to 80 °C. 15 mL of 0.2 mol/L solution of sodium borohydride was added into the ink drop by drop, and the bath was stirred for 1 h. The mixture was cooled, dried, and washed repeatedly with ultrapure water (18.2 MΩ cm) until no Na+ and B(OH)-4 ions existed. The catalyst powder was dried for 3 h at 120 °C and then stored in a vacuum vessel. All chemicals used were of analytical grade.

J. Phys. Chem. C, Vol. 112, No. 16, 2008 6583 2.2. Preparation of Working Electrode and Its Electrochemical Measurements. Preparation of Working Electrode. Glassy carbon disk electrodes, 3 mm in diameter (electrode area 0.0706 cm2), polished with 0.05 µm alumina to a mirror-finish before each experiment, were used as substrates for the carbonsupported catalysts. For the electrode preparation, 5 µL of an ultrasonically redispersed catalyst suspension was pipetted onto the glassy carbon substrate. After the solvent evaporation, the deposited catalyst (28 µgmetal/cm2) was covered with 5 µL of a dilute aqueous Nafion solution (5 wt %). The resulting Nafion film with a thickness of e0.2 µm had sufficient strength to attach the catalyst particles permanently to the glassy carbon electrode without producing significant film diffusion resistances.35,36 Electrochemical Measurement. Electrochemical measurements were carried out with a conventional three-electrode electrochemical cell at 25 °C. The glassy carbon electrode as the working electrode (electrode area 0.0706 cm2) was covered with the catalyst powder. A piece of Pt foil of 1 cm2 area was used as the counter one. The reversible hydrogen electrode (RHE) was used as the reference with its solution connected to the working electrode by a Luggin capillary whose tip was placed appropriately close to the working electrode. All potential values are versus RHE. All chemicals used were of analytical grade. All of the solutions were prepared with ultrapure water (MilliQ, Millipore, 18.2 MΩ cm). A solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4 was stirred constantly and purged with ultrapure argon gas. Cyclic voltammograms (CV) were plotted within a potential range from 0.05 to 1.2 V with a scanning rate of 0.02 V/s. The chronoamperometric and i-t experiments were carried out by using a CHI630A electrochemical analysis instrument controlled by an IBM PC. The potential jumped from 0.1 to 0.5 V. Because of a slight contamination from the Nafion film, the working electrodes were electrochemically cleaned by continuous cycling at 0.05 V/s until a stable response was obtained before the measurement curves were recorded. 2.3. Physical Measurements. X-ray Diffraction (XRD). XRD patterns reveal the bulk structure of the catalyst and its support. XRD analysis was carried out for the catalysts with a D/maxrB (Japan) diffractometer using a Cu KR X-ray source operating at 45 kV and 100 mA. The XRD patterns were obtained at a scanning rate of 4° min-1 with an angular resolution of 0.05° of the 2θ scan. Energy DispersiVe Analysis of X-ray (EDAX). Chemical composition analysis by EDAX was performed with an EDAX Hitachi-S-4700 analyzer associated with a scanning electron microscope (SEM, Hitachi Ltd. S-4700). Incident electron beam energies from 3 to 30 keV had been used. In all cases, the beam was at normal incidence to the sample surface, and the measurement time was 100 s. All of the EDAX spectra were corrected by using the ZAF correction, which takes into account the influence of the matrix material on the obtained spectra. Transmission Electron Microscopy (TEM). After the catalyst samples were finely ground and ultrasonically dispersed in isopropanol, a drop of the resulting dispersion was deposited and then dried on the standard copper grid coated with polymer film. Transmission electron micrographs were taken with a JEOLJEM-1200EX microscope (made in Japan), at the applied voltage of 100 kV, with a magnification of 200 000 and the spatial resolution of 1 nm. X-ray Photoelectron Spectrometry (XPS). The surface composition XPS analysis was performed with the VG ESCALAB MKIIX-ray photoelectron spectrometer, with the Al KR X-ray source (1486.6 eV). The XPS spectra were recorded at a 45°

6584 J. Phys. Chem. C, Vol. 112, No. 16, 2008

Figure 1. Cyclic voltammograms of ethanol electrooxidation on the PtRuNi/C and PtRu/C electrodes in an Ar-saturated solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4 at 25 °C. Scan rate: 0.02 V/s.

takeoff angle, while the chamber pressure was held below 5 × 10-9 Pa. The C 1s electron binding energy was referenced at 284.6 eV, and a nonlinear least-squares curve-fitting program was employed with a Gaussian-Lorentzian production function.22,37 The deconvolution of the XPS spectra was carried out according to the reported methods.38-41 High-resolution spectral envelopes were obtained by curve fitting synthetic peak components using the software XPS peak. The raw data were used with no preliminary smoothing.

Wang et al.

Figure 2. Chronoamperometric curves of ethanol electrooxidation on the PtRuNi/C and PtRu/C electrodes in an Ar-saturated solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4 at 25 °C. Potential jumps from 0.1 to 0.5 V.

3. Results and Discussion 3.1. Evaluation of Electrocatalytic Activity. CV curves of ethanol electrooxidation on homemade PtRuNi/C and PtRu/C catalysts are shown in Figure 1. The performance of PtRu/C catalyst presented here is the best available result obtained from various samples prepared by the same method and from the same precursors. The performance of PtRuNi/C with an atomic ratio of 6:3:1 is also the best available result obtained from various samples prepared by the same method and from the same precursors.29 Although there is essentially the same onset potential at about 0.55 V for both PtRuNi/C and PtRu/C catalysts, the peak current potential located at 0.929 V (vs RHE) with a peak current density of 33.1 mA/cm2 during positive potential scanning process on PtRuNi/C catalyst compares sharply to that on PtRu/C catalyst (0.935 V and 20.1 mA/cm2). The negative scanning process results in the similar peak locations, 0.794 V and 21.9 mA/cm2 for PtRuNi/C as compared to 0.785 V and 13.2 mA/cm2 for PtRu/C. In brief, the peak potential on PtRuNi/C catalyst is about 6 mV lower than that on PtRu/C, while its peak current density is 13.0 mA/cm2 higher than that on the latter. The performance of PtRuNi/C catalyst for ethanol electrooxidation is obviously better than that of PtRu/C catalyst. The CO-tolerance of the anodic catalysts can be evaluated with the steady-state current densities on the chronoamperometric curves of ethanol electrooxidation as shown in Figure 2, which are the result of measurement in the Ar-saturated solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4 on a constant potential jump from 0.1 to 0.5 V at 25 °C. The first high current density mainly is due to the double-layer fast charging, which then decays with time in the parabolic style and reaches an apparent steady-state within 600 s. The higher current density (1.66 mA/cm2 at 1000 s) on the PtRuNi/C catalyst, as compared to that (1.24 mA/cm2 at 1000 s) on the PtRu/C catalyst at the same potentials, indicates the superior CO-tolerance of the PtRuNi/C, which is similar to the result of cyclic voltammetry. For practical applications of DEFC, one of the requirements is the electrocatalyst stability in the acidic medium, which can

Figure 3. Amperometric i-t curves of ethanol electrooxidation on the PtRuNi/C and PtRu/C electrodes in an Ar-saturated solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4 at 25 °C at a fixed potential of 0.5 V.

Figure 4. XRD patterns of the Pt/C(E-Tek), PtRu/C, and PtRuNi/C catalysts.

be judged by the stable current densities on the current-time (i-t) curves. Figure 3 displays the result obtained on the PtRuNi/C and PtRu/C in an Ar-saturated solution of 0.5 mol/L CH3CH2OH and 0.5 mol/L H2SO4 at 25 °C at a fixed potential of 0.5 V (vs RHE). After the current gradually decreases and finally stabilizes in about 60 min, it can be seen clearly that the current density on the PtRuNi/C catalyst (0.84 mA/cm2 at 10 000 s) is relatively higher than that on the PtRu/C catalyst (0.48 mA/cm2 at 10 000 s). Together with those of chronoamperometric curves and CV measurement, this result further confirms the superiority of the PtRuNi/C over PtRu/C with respect to the catalytic activity, CO-tolerance, and stability. 3.2. XRD Characterization of PtRuNi/C Catalyst. Figure 4 shows the XRD patterns of PtRuNi/C, PtRu/C, and Pt/C(ETek) catalysts, which reveal the structural information for the bulk of catalyst nanoparticles together with its carbon support. The first broad peak located at the 2θ value of about 24.8° in the XRD pattern is attributed to the carbon support, whereas

Effect of Ni on PtRu/C Catalyst Performance

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TABLE 1: The Lattice Parameters, Particle Sizes, and Specific Areas of the Pt/C(E-Tek), PtRu/C, and PtRuNi/C Catalysts

catalysts

2θ (deg)

d-value (nm)

lattice parameter (nm)

particle size (nm)

specific area (m2/g)

Pt/C PtRu/C PtRuNi/C

67.64 68.65 69.34

0.13839 0.13634 0.13541

0.3914 0.3856 0.3830

2.5 2.7 3.0

112.1 123.5 104.7

Figure 6. TEM micrograph of the PtRu/C (A) and PtRuNi/C (B) catalysts.

Figure 5. EDAX pattern of the PtRu/C (A) and PtRuNi/C (B) catalysts.

TABLE 2: The Atomic Compositions of the PtRu/C and PtRuNi/C Catalysts (atom %) nominal content

determined by EDAX

catalysts

Pt

Ru

Ni

Pt

Ru

Ni

PtRu/C PtRuNi/C

50 60

50 30

10

55.9 66.0

44.1 27.7

6.3

the four other peaks, characteristic of the face centered cubic (fcc) crystalline Pt, correspond to the planes (111), (200), (220), and (311), respectively. This displays the dominant feature of the disordered single-phase structures (i.e., solid solutions) for the alloy catalyst. As compared to the diffraction peaks of Pt/C (E-Tek) catalyst, those for PtRuNi/C and PtRu/C catalysts are shifted to the evidently higher 2θ values. It is worthwhile to note the lack of diffraction peaks characteristic of Ru, Ni, or their oxides/hydroxides, which suggests that Ru and Ni atoms either form an alloy with Pt or exist as oxides in amorphous phases. The data in Table 1 based on the Pt (220) crystal face reflect the formation of a solid solution. Among three catalysts, the lattice parameter for PtRuNi/C is the smallest, while that for Pt/C(E-Tek) is the biggest. Actually, the progressive decrease in lattice parameters of the alloy catalysts reflects the progressive increase in the incorporation of Ru and Ni into the alloy state. No significant difference between PtRuNi/C and PtRu/C catalysts has been found in Table 1 with respect to the average particle sizes and specific surface areas, which are estimated from full width at half-maximum (fwhm) according to the Debye-Scherrer formula.42,43 3.3. EDAX Analysis. Figure 5 shows the EDAX patterns of the PtRu/C and PtRuNi/C catalysts. Their actual chemical compositions, determined by EDAX detections, which were carried out on multiple regions with the similar result, are close to the theoretical values with the metal atomic ratios of 66.0: 27.7:6.3 and 55.9:44.1 as shown in Table 2. 3.4. TEM Manifestation. TEM images were obtained on the PtRu/C and PtRuNi/C nanoparticles, respectively, to test the effect of mixing Ni into PtRu system on the particle morphology and the particle size. Figure 6A and B shows the typical bright field TEM micrographs of PtRu/C and PtRuNi/C nanoparticles, respectively, with metal grains in black and carbon support in gray. The metal particles in both catalysts exist homogeneously on carbon support grains.

Figure 7. Size distribution of the metal nanoparticles of the PtRu/C and PtRuNi/C catalysts.

Figure 8. XPS core level spectra for the Pt 4f and Ru 3p photoemission from the PtRu/C catalyst.

In the histograms of the particle sizes (see Figure 7A for PtRu/C and 7B for PtRuNi/C), it is easy to pinpoint the peak particle sizes at about 3 nm, similar for both catalysts. From the particle distribution, from 1 to 7 nm for PtRu/C nanoparticles and from 1 to 6 nm for PtRuNi/C, the more detailed estimation results in the average particle size of 3.1 nm for PtRu/C nanoparticles and 3.3 nm for the PtRuNi/C, which were computed from TEM measurement of a series of distinct particle diameters, coded individually with numbers, with the established equation44,45 as follows: n

dhn )

di ∑ i)1 n

(1)

where dhn is the averaged diameter of metal particles in nanometer, n is the total number of the used codes, and di is the ith coded diameter. The similarity of particle size distribution is of the implication that the size factor has nothing to do with the significant difference between the two catalysts regarding their catalytic activity for ethanol electrooxidation.

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Wang et al.

TABLE 3: Surface Contents of Pt, Ru, and Ni Metal and Their Oxides in the PtRuNi/C and PtRu/C Catalysts materials compositions (%) Pt/oxide

Ru/oxide

Ni/oxide

metal/compounds

Pt

PtO

PtO2

Ru

RuO2

Ni

NiO

Ni(OH)2

NiOOH

PtRu/C PtRuNi/C

70.7 68.1

16.8 26.3

13.5 5.6

85.7 70.5

14.3 29.5

50.6

18.2

14.9

16.3

3.5. XPS Revelation. Because the particle morphology and particle size have been confirmed not to be the major factors to differentiate the catalytic performances between the two catalysts, the PtRuNi/C and PtRu/C, attention is naturally drawn to the XPS analysis for determination of their metal oxidation states on the surface. Fortunately, the averaged particle sizes of both nanoparticles meet the requirement for XPS detection. Figure 8A and B displays the XPS spectra of the Pt 4f and Ru 3p regions of PtRu/C catalyst, respectively. The minor Ru 3p region was chosen to do analysis because the major Ru 5d region overlaps the carbon 1s region. The two most intense peaks in Figure 8A, located at the binding energies of 70.98 eV (Pt 4f7/2) and 74.33 eV (Pt 4f5/2), maintain an area ratio near 4:3 as expected theoretically for pure Pt (71.20 eV of Pt 4f7/2 and 74.53 eV of Pt 4f5/2), and although their positions have been shifted lower.37 Hence, they are originated from metallic Pt0 without doubt. The peaks at 71.85 and 75.40 eV can be attributed to Pt2+ in the form of PtO or Pt(OH)2, which can be electrochemically reduced,37,39,46 while the peak at 74.56 eV arises from Pt4+, possibly in PtO2. The oxidation states of Pt, Ru and their relative amounts are listed in Table 3. The Pt 4f XPS spectrum of PtRu/C catalyst manifests that Pt contains 70.7% of metallic Pt0, 16.8% of Pt2+ (as PtO or Pt(OH)2), and 13.5% of Pt2+ (as PtO2) as shown in Table 3. The Ru 3p3/2 spectrum was deconvoluted into two peaks located at 462.56 and 464.62 eV, which are characteristic of Ru0 and RuO2, respectively.21,37,41 The calculation shows that the large fraction of Ru (about 85.7%) exists as the metallic state Ru0, while a much smaller fraction exists as RuO2 (about 14.3%).

The core level spectra of Pt 4f, Ru 3p, Ni 2p, and O 1s for the PtRuNi/C catalyst are depicted in Figure 9. The oxidation states of Pt, Ru, Ni, and their relative amount are also listed in Table 3. Like those of the PtRu/C catalyst, the two most intense peaks located at the binding energies of 70.95 eV (Pt 4f7/2) and 74.30 eV (Pt 4f5/2). As compared to that of PtRu/C, the PtRuNi/C displays a big XPS shift, which is consistent with the bigger difference in the electronegativity between Pt and Ni versus that between Pt and Ru. It indicates that the shift of d electron density from Ni to Pt would lower DOS on the Fermi level and reduce the bond energy between Pt and COads from ethanol electrooxidation. The result of deconvolution indicates a phase composition that contains 68.1% of Pt in metallic Pt0, 26.3% in Pt2+ (as PtO or Pt (OH)2), and 5.6% in Pt4+ as PtO2.46,47 The calculation shows that the Ru 3p spectrum demonstrates that Ru is composed of 70.5% of metallic Ru and 29.5% of RuO2 as shown in Table 3. The characteristic Ni 2p peak in Figure 9C, which is associated with the spin-orbit coupling effect, needs a very cautious treatment, because Ni exists in various oxidation states with their overlapping 2p region.22,30,31 By deconvolution of Ni 2p spectra, the resulted peaks are associated as follows: the metallic Ni peak at 852.7 eV, the NiO peak at 853.8 eV, the Ni(OH)2 peak at 855.60 eV, and the NiOOH peak at 857.30 eV.31 Their contents are 50.6%, 18.2%, 14.9%, and 16.3%, respectively, as shown in Table 3. That a great deal of metallic Ni can insert into the lattice of Pt as shown in XRD results due to a large fraction of metallic Ni on PtRuNi/C catalyst, which lowers the Pt-CO bond energy to improve the activity of

Figure 9. XPS core level spectra for the Pt 4f, Ru 3p, Ni 2p, and O 1s photoemission from the PtRuNi/C catalyst.

Effect of Ni on PtRu/C Catalyst Performance PtRuNi/C for ethanol electrooxidation. Previous studies have established that the nickel hydroxides (Ni(OH)2 and NiOOH) have a high electron and proton conductivity and also exhibit high catalytic activity as heterogeneous catalysts.23 In addition, the hydroxide layer can protect from corrosion under ethanol electrooxidation conditions.22 Simultaneously, Ni hydroxides can also play an important role in the shift of Hads on Pt surface, for example, the so-called hydrogen spillover effect.22,39 4. Conclusions Electrocatalytic activity of the PtRuNi/C catalyst, prepared by reduction with NaBH4 of the inorganic precursor salts without Cl- ions, was investigated with respect to the electrooxidation of ethanol in H2SO4 solution. The experimental data reported in this article indicate that the particle morphology, particle size, and particle structure have been confirmed not to be the major factors to differentiate the catalytic performance between the two catalysts, the PtRuNi/C and PtRu/C. The performance of the PtRuNi/C catalyst for ethanol electrooxidation is better than that of the PtRu/C catalyst due to the promoting function of Ni. The tolerance performance to CO from intermediates of ethanol electrooxidation on the PtRuNi/C is also better than that on PtRu/C. The effect of Ni in the PtRuNi/C catalyst can be illustrated by the hydrogen spillover effect of Ni hydroxides and electron effect of metallic Ni. The enhanced activity of the PtRuNi/C catalyst is surely throwing some light on the research and development of effective DEFC catalysts. To further confirm the role of Ni, more experiments are ongoing with catalysts dispersed in the membrane electrode assembly (MEA) for tests in the DEFC single cell strand, together with the long-term life testing process.48 Acknowledgment. This research is financially supported by the National Natural Science Foundation of China (Grant No. 20606007), Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province of China (LBH-Q07044), Harbin Innovation Science Foundation for Youths (2007RFQXG042), and Scientific Research Foundation for Returned Scholars of Ministry of Education of China. References and Notes (1) Sarma, L. S.; Chen, C.-H.; Wang, G.-R.; Hsueh, K.-L.; Huang, C.-P.; Sheu, H.-S.; Liu, D.-G.; Lee, J.-F.; Hwang, B.-J. J. Power Sources 2007, 167, 358. (2) Lee, C. H.; Lee, C. W.; Kim, D. I.; Jung, D. H.; Kim, C. S.; Shin, D. R. J. Power Sources 2000, 86, 478. (3) Chu, D.; Jiang, R. Z. Electrochim. Acta 2006, 51, 5829. (4) Lamy, C.; Lima, A.; LeRhun, V.; Delime, F.; Coutanceau, C.; Leger, J. M. J. Power Sources 2002, 105, 283. (5) Zhou, W. J.; Song, S. Q.; Li, W. Z.; Sun, G. Q.; Xin, Q.; Kontou, S.; Poulianitis, K.; Tsiakaras, P. Solid State Ionics 2004, 175, 797. (6) Xu, C. W.; Shen, P. K. J. Power Sources 2005, 142, 27. (7) Fujiwara, N.; Friedrich, K. A.; Stimming, U. J. Electroanal. Chem. 1999, 472, 120. (8) Camara, G. A.; de Lima, R. B.; Iwasita, T. Electrochem. Commun. 2004, 6, 812. (9) Delime, F.; Leger, J.-M.; Lamy, C. J. Appl. Electrochem. 1999, 29, 1249. (10) Kirillov, S. A.; Tsiakaras, P. E.; Romanova, I. V. J. Mol. Struct. 2003, 651-653, 365. (11) Tripkovic, A. V.; Popovic, K. D.; Lovic, J. D. Electrochim. Acta 2001, 46, 3163.

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