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Mar 13, 2015 - The electrochemical oxidation processes of CO and H2 have been used as probes to assess the surface states of carbon-supported Ni@Pt co...
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Small-Molecule (CO, H2) Electro-Oxidation as an Electrochemical Tool for Characterization of Ni@Pt/C with Different Pt Coverages Yumei Chen,*,†,‡ Jianchao Shi,† and Shengli Chen*,‡ †

Department of Chemistry, Henan Polytechnic University, Jiaozuo 454000, China College of Chemistry and molecular Science, Wuhan University, Wuhan 430072, China

‡

ABSTRACT: The electrochemical oxidation processes of CO and H2 have been used as probes to assess the surface states of carbon-supported Ni@Pt core-shell catalysts (Ni@ Pt/C) with different Pt coverages in electrochemical environments. The CO-stripping cyclic voltammograms (CVs) of Ni@Pt/C catalysts present split double peaks representing two different states of Pt atoms on Ni-core particles. The current peaks in the CO-stripping CVs of Ni@Pt/C catalysts under relatively more positive potentials are consistent with that of Pt/C (a single peak), which indicates that some of the Pt atoms on Ni-core particles behave like pure Pt as isolated Pt atoms. The shoulder peaks shift toward more negative potentials, which may be because these Pt atoms on Ni-core particles assemble Pt−Pt bonds, resulting in Pt lattice constriction. The more positive current peaks in the CO-stripping CVs of Ni@Pt/C catalysts are predominant with lower Pt coverage, and with increasing Pt surface coverage, the more negative current peaks become prominent, which illustrates the integrity of Pt− Pt ensembles with neighboring Pt atoms. This phenomenon can be demonstrated by the schematic diagram of changes in the morphologies of Ni@Pt/C catalysts with increasing Pt coverage. HOR measurements further exhibit clearly regular patterns; here, the exchange current density (j0) values of Ni@Pt/C catalysts decrease with increasing Pt coverage and correspond to that of Pt/C at high Pt coverage. This result can be explained by the existence of nickel hydroxides and hydrogen spillover.

1. INTRODUCTION Fundamental progress in the design of efficient cathodic electrocatalysts for the oxygen reduction reaction (ORR), which is considered to be a major obstacle barring commercialization of fuel cells, is an urgent necessity. Core− shell nanostructured catalysts with a nonprecious metal core and a thin (ideally monolayer) precious metal shell not only greatly reduce the amount of precious metal in the catalysts but also enhance their catalytic performance through properly tuned strain and ligand effects.1−6 Applying Pt monolayer on various substrates, such as on Ni,7,8 Co,9,10 Cu,11,12 Ru,13 Pd,14,15 Pd3Co,1 PdCu5;16 single-crystal surfaces17 of Pd, Au, Ru, and Ir, and so on; and adjustment of the thickness of Pt shell have been extensively studied in recent research. Besides metals and alloys, nonmetal materials, for example, WC,18 can also serve as a substrate for Pt monolayer electrocatalysts. The nature and composition of the substrates have been shown to exert a significant effect on the electronic structure of the Pt monolayer.5,17−19 The specific activity of surface Pt atoms could also be adjusted by changing the thickness of the Pt shell.1,8,19 A compact Pt surface layer with moderate lattice contraction is a common structural feature of these core−shell structure catalysts that enhances ORR activity.1,4,20 Density functional theory (DFT) calculations suggest that the binding strength of reaction intermediates decreases when the lattice contracts, which can lower or enhance the catalytic activity depending on whether adsorption or desorption of the intermediates limits the reaction rate.21−23 Studies on welldefined model systems have led to the determination of surface © XXXX American Chemical Society

reaction mechanism and identification of physical parameters that govern activity.2,4,17,21−24 Although many investigations on electrochemical Pt monolayer surface on various substrates have been performed for model systems with single-crystal cores, polycrystal cores, and carbon-supported nanoparticle cores, fundamental issues, such as the voltage-dependent structure, the atom state within the electrochemical environment, and the origin of the mechanism for enhanced ORR activity remain unclear. Moreover, most structural and compositional information is gleaned from physical detection methods, such as transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis.1−16 As such, the atom state within the electrochemical process can not be obtained. Because carbon monoxide (CO) is a potentially poisoning intermediate of the surface reaction, CO-stripping has been used as a molecular surface probe to check the interfacial charges at ordered platinum electrodes or Pt-based electrocatalysts.25−29 For example, even trace amounts of CO present in the reformate H2/CO mixtures tend to poison the catalyst surface and substantially increase the overpotential for this reaction.30,31 CO is also a reaction intermediate produced during the methanol oxidation reaction (MOR), and the majority of kinetic studies reveal that the CO oxidation is the rate-determining step for MOR.32,33 The CO-poisoning of Pt anode catalysts arises from active surface sites bonding with CO Received: December 10, 2014 Revised: March 13, 2015

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The Journal of Physical Chemistry C strongly.30−33 Adsorbed CO exhibits a surface-sensitive behavior that has been observed by electro-oxidation of CO on Pt single crystals with regular (100), (110), and (111) surfaces in acidic electrolytes, for example, see refs 25, 27, and 34. Thus, most experiments and theoretical studies on bimetallic surfaces use CO or small organic molecules as a probe to detect changes in surface reactivity and/or catalytic activity.30,33 Use of electro-oxidation of adsorbed CO to probe the surface details of Pt−M alloys can provide an abundance of information by which to analyze the surface structure of bimetallic particles. Such results show that bimetallic particles of the same size, shape, and composition present significant differences in activity when configured into different architectures, such as alloy or core−shell. However, there are limited data on the CO electro-oxidation of core−shell bimetallic catalysts to provide surface information.28,35 Therefore, studying CO-stripping on Ni@Pt/C electrocatalysts in the given electrochemical atmosphere is not only important for obtaining the surface information on core−shell bimetallic nanoparticles but also vital in designing efficient catalysts for CO tolerance. Besides CO-stripping measurements, the electrochemical reactions of other small molecules, such as H2 or O2, may also be used to probe the surface information on electrocatalysts.36−38 In our work, the Ni@Pt/C catalysts with different Pt coverages were mainly measured by the CO electro-oxidation technique to probe the surface information in the electrochemical process combining hydrogen oxidation reaction (HOR).

Figure 1. Representative TEM images (left-hand panels) and the corresponding histograms of the particle size distribution (right-hand panels) of the prepared Ni@Pt/C catalysts with Pt/Ni ratios of (a) 2:10 and (b) 4:10.

than that required for a monolayer shell formation (Pt/Ni = 3:10) suggest that the Pt forms a thin shell or an extended layer on Ni-core particles without formation of aggregates or clusters, and there is a continuous lattice expansion of the Ni@Pt/C catalysts upon deposition of Pt on Ni-core particles.8 After forming a Pt monolayer, the XRD peak position becomes almost independent of the Pt/Ni ratio.8 This invariance of the XRD peak position can not provide information on the nearsurface structure of the Ni@Pt/C catalysts. X-ray photoelectron spectroscopy (XPS) responses of Ni@ Pt nanoparticles (Pt/Ni = 2:10) obtained using a Kratos Ltd. XSAM-800 spectrometer with Cu Kα radiation show that the Ni states of the exposed Ni sublayers consist of Ni metal (Ni0) as well as Ni oxide and hydroxides (NiO, Ni(OH)2, and NiOOH) by fitting the Ni 2p3/2 peaks using the software XPSPEAK41 (Shirley function as baseline, Gauss−Lorentzian linearity fitting, Figure 3). After the shakeup peak is taken into account, the Ni 2p3/2 XPS peaks at binding energies of 852.7, 854.4, 855.8, and 857.4 eV correspond to Ni0, NiO, Ni(OH)2, and NiOOH, respectively. The major component of the Ni sublayer is nickel hydroxides, suggesting that a majority of Ni at the exposed sublayer has been oxidized during the postprocessing. The presence of the nickel hydroxides on the catalysts has some favorable properties, such as proton and electronic conductivity.39 Angle-resolved XPS responses of the Ni@Pt/C catalysts also suggest that the Pt atoms mostly were deposited on the particle surface forming a Ni@Pt core−shell structure and the electronic structure of surface Pt is modified by the underlying Ni.8 2.2. Electrochemical Measurements. Electrochemical measurements were performed with a three-electrode configuration. The working electrodes were made by casting the carbon supported nanoparticles (Ni@Pt/C or Pt/C) as a thin film onto a glass carbon (GC) rotating disk electrode (geometric area, 0.196 cm2) with Nafion as binding agent. The counter electrode is a Pt foil, and the reference electrode is a saturated calomel electrode (SCE) which is separated from

2. EXPERIMENTAL SECTION 2.1. Material Preparation and Characterization. Ni@ Pt/C core−shell catalysts were synthesized through a two-step reduction procedure in the “modified polyol process” described elsewhere.7,8 In brief, Ni/C was first synthesized at 138 °C in 1.2-propanediol by KBH4 as a reducing agent and oleic acid as a protected agent. After the formation of Ni/C, the deposition of Pt atoms on Ni-core particles was achieved with 1.2propanediol as a weak reducing agent inhibiting the homogeneous nucleation of Pt. The reacting solution is stirred continuously for 2 h and then cooled to room temperature. The reaction process was carried out under N2 atmosphere to prevent Ni particle oxidation. Finally, the black powders were isolated by centrifugation and washed 3−4 times by ethanol. The thus-prepared catalysts were dried at 40 °C in a vacuum oven. The Ni@Pt/C catalysts with different coverages were synthesized by adjusting the Pt/Ni atomic ratios in this work (Pt/Ni = 1:10−7:10). Transmission electron microscopy images were obtained on a JEOL JEM-2100FEF instrument, and the corresponding histograms of the particle size distribution show that the prepared Ni@Pt nanoparticles are relatively uniformly dispersed on the carbon supports and the size of catalyst particles increase with the increasing Pt/Ni ratios (Figure 1; also see ref 8). Figure 2 gives the high-resolution TEM (HRTEM) images of the selective catalyst particles with Pt/Ni ratios of 2:10, 3:10, and 4:10, which show that lattice distortion occurs with the deposition of Pt on Ni-core particles. The Pt shell can not be distinguished in these HRTEM images (Figure 2; also see ref 8), but the average distances between lattice planes (d) measured in different regions imply a core−shell structure and the lattice contraction of Pt−Pt ensembles.7,8 The XRD responses of Ni@Pt/C catalysts with Pt/Ni ratios not higher B

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Figure 2. Representative HRTEM images of the catalyst particles with Pt/Ni ratios of (a) 2:10, (b) 3:10, and (c) 4:10.

0.125 V (vs RHE) for 30 min to ensure that the surface was saturated. CO in the bulk solution was removed by purging with Ar while maintaining the potential for 30 min. Therefore, the CO-stripping CVs were carried out under the scanning rate 20 mV/s, and CO electro-oxidation behavior did not occur in the second cycles. The steady-state polarization curves for the HOR were recorded by scanning the potential with a rate of 5 mV/s and an electrode rotation rate of 4800 rpm. Prior to the HOR experiments, the electrolyte was saturated with pure H2 for 30 min. The working disk electrode surface was cleaned and electrochemically stabilized by potential cycling until a steadystate cyclic voltammogram was obtained. The measurements were carried out at room temperature (27 ± 1 °C), and the load of catalysts for Ni@Pt/C is 2 ug and Pt/C (JM) is 1 ug.

Figure 3. XPS spectra for Ni@Pt particles with Pt/Ni = 2:10. The black dots are the measured XPS responses. The black solid line is the background determined with the Shirley function. The red solid line is the superposition of the deconvoluted component spectra of Ni 2p3/2 with different valence states.

3. RESULTS AND DISCUSSION 3.1. CO-Stripping Measurement. CO-stripping measurement is well-known as a surface-sensitive reaction that is commonly used to probe surface properties. In this process, the surface of solid catalysts is first saturated with a CO adlayer at weak adsorption potential. Because of a strong Pt−COads interaction, CO molecules stay adsorbed on Pt surface atoms while the electrolyte is purged with Ar to drive away the free CO molecules. CV measurement is then applied to electrooxidize the COads. The CO electro-oxidation behaviors of Ni@ Pt/C catalysts with different Pt coverages and Pt/C (JM) are shown in Figure 4a. For clarity and simplicity, magnified COstripping current peaks during the first stripping cycles are shown in Figure 4b and the inset presents the onset potentials for CO stripping between 0.5 and 0.7 V versus the RHE. Figure

the working electrode by a Luggin capillary. However, the potentials reported throughout this paper are quoted against the reversible hydrogen electrode (RHE). The electrochemical measurements were conducted using CHI660A. The rotatingdisk electrode (RDE) measurements were accomplished on an AFMSRCE RDE unit from Pine Research Instrumentation. CO electro-oxidation measurement is a common method for testing the surface state of the electrocatalysts. Prior to the COstripping experiments, the electrode (load of catalysts, 50 ug) was activated by cyclic voltammograms (CVs) in argonsaturated 0.5 M H2SO4 under a scanning rate of 500 mV/s. Then CO was bubbled through the solution at the potential of

Figure 4. (a) CO-stripping CVs of the Ni@Pt/C catalysts with different Pt coverages and the Pt/C (JM) in Ar-saturated 0.5 M H2SO4; (b) enlarged current peaks in CO-stripping CVs. Scanning rates: 20 mV/s. Inset of panel b: the onset potentials of the CO-stripping curves. C

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coverage on Ni substrate (atom diameters: rPt = 0.129 nm and rNi = 0.124 nm), the Pt−Pt lattice contraction with forming a Pt−Pt ensemble results in a weak Pt−CO bond strength compared to that of Pt/C. In our experiments, the more positive current peaks in the CO-stripping CVs of Ni@Pt/C catalysts are predominant at lower Pt/Ni ratios (Pt/Ni = 1:10 and 2:10), indicating the surface Pt atoms are mainly isolated and behave like the pure Pt. With increasing Pt coverage, the number of Pt−Pt ensembles with high coordination of Pt atoms gradually increases and the shortened Pt−Pt interatomic distance (evidenced by HRTEM and XRD analyses) induces weaker Pt−CO bond strength, which explains why the more negative CO-stripping peaks become prominent. The nonuniform deposition of the Pt shell on Ni-core particles and the multilayer structure could explain the coexistence of split CO electro-oxidation peaks in all of the Ni@Pt/C catalysts. The corresponding schematic diagram is shown in Figure 5; here,

4a demonstrates that the current peaks of Hupd desorption can not be seen on the first CV curves, which show the CO prior to absorbing on the surface of catalysts poisoning the Pt catalytic activity to H2.29,30,36 The second scans show that nearly the entire CO overlayer on the surface was oxidized in the first sweep. Also it can be seen that the CO-stripping CV of Pt/C (JM) presents a single peak under these conditions with overpotential of 0.785 V versus RHE, but a split double peak appears for all Ni@Pt/C catalysts. Notably, the current peaks in the CO-stripping CVs of Ni@Pt/C catalysts under relatively more positive potentials are consistent with that of Pt/C, and the shoulder peaks of CO-stripping shift toward more negative potentials. The CO-stripping curves of Ni@Pt/C catalysts clearly exhibit regular patterns (Figure 4a,b). When the CO electro-oxidation peak potentials and the coverages of Pt shell on Ni-core particles of the Ni@Pt/C catalysts are compared, for catalysts with Pt/Ni ratios lower than that forming a Pt monolayer (the Pt monolayer formed on Ni-core particles at about Pt:Ni = 3:10 was discussed in detail in our previous paper8), the more positive current peaks are predominant; as Pt coverage increases, the more negative shoulder peaks are gradually enhanced. Furthermore, the CO-stripping current peaks shift toward more negative potentials, as seen from the X and Y dividing lines in Figure 4b, and the onset potentials gradually shift negatively toward lower values with increasing Pt coverage, as seen in the inset of Figure 4b. To understand the CO-stripping behavior of our synthesized Ni@Pt/C catalysts, we must consider the process by which the Pt shells form on Ni-core particles. In the experiment, the twostep “modified polyol process” with 1.2-propanediol as a weak reducing agent was used to achieve a coating of Pt atoms on Nicore particles considering the reducing temperature between the homogeneous and heterogeneous nucleation. However, because of the presence of XC-72 support and the fast kinetics of chemical reduction methods, the ideal monolayer Pt or Pt atom distribution on Ni-core particles could not be obtained. The Pt shell may possibly be replaced by Pt metal clusters or small islands, which can not be detected by XRD and TEM before forming the Pt phase singly. We assume that different states of Pt atoms exist on the Ni@ Pt/C catalysts. The more positive current peaks in the COstripping CVs of Ni@Pt/C catalysts corresponding to Pt/C mean that partial Pt atoms within the Pt shell are more likely to behave like pure Pt, indicating a stronger Pt−CO bond strength close to that of Pt/C. The shoulder peaks shifting toward more negative potentials are mainly due to the lattice contraction of the Pt atoms on Ni-core particles, which means the Pt−CO bond strength becomes weaker. This phenomenon may be explained by the results reported by Wang and co-workers35 and Du et al.40 For example, Wang and co-workers, through CO-stripping experiments carried out on a series of submonolayer decorated Pt@Au/C electrocatalysts with different Pt surface coverages, found that whether the surface Pt atoms could form a Pt−Pt ensemble with another neighboring Pt surface atom is a critical parameter determining CO adsorption and oxidation behaviors on the electrocatalyst. In Wang’s experiment, increasing the coverage of Pt atoms (e.g., 70% and 100% Pt surface coverage) shifted the CO-stripping peak potentials positively toward higher values, which indicates a stronger Pt−CO bond strength compared with that of Pt/C. This change is mainly due to Pt−Pt lattice expansion of Pt atoms on the Au substrate (atom diameters: rPt = 0.129 nm and rAu = 0.134 nm). We can infer, on the contrary, applying Pt

Figure 5. Schematic diagram of morphologies of Ni@Pt/C catalysts with respect to the surface coverage of Pt on Ni-core particles: (a) an ideal Pt monolayer; (b) Pt/Ni ratio lower than that forming a Pt monolayer; (c) almost a Pt monolayer forming on Ni-core particles; (d) Pt/Ni ratio higher than that forming a Pt monolayer.

the Pt atoms indicated by red circle may behave like pure Pt, and the Pt lattice contracts when surface Pt−Pt ensembles are formed. The model can rationally explain the results of CO electro-oxidation of Ni@Pt/C catalysts with different Pt coverages. Peak areas in the CO-stripping CVs of Ni@Pt/C catalysts under more negative potentials were singly split by Gaussian− Lorentz fitting methods with the second CV curve as the baseline forming a closed curve with the first CV curve. The representative fitting results (Pt:Ni = 3:10 and 4:10) are shown in Figure 6. The corresponding data of peak potentials and area ratios are shown in Table 1. The predominant CO-stripping peaks show a sudden change between Ni/Pt = 3:10 and Ni/Pt = 4:10, which suggests that the surface Pt−Pt ensembles on Nicore particles suffer a dramatic change. This change means a more compacted Pt monolayer may be formed at Pt/Ni between 3:10 and 4:10 considering the nonuniformity of Pt atoms on Ni-core particles. Ni@Pt/C catalysts with Pt/Ni = 3:10 exhibited optimal ORR activity (discussed in ref 8). This seeming contradiction would predict that further optimization can be obtained by finely adjusting the surface Pt states. As for the surface composition, in Table 2, the charges of Hupd and CO electro-oxidation are compared for the Ni@Pt/C catalysts with different Pt coverages by calculation of the area of Hupd desorption (AHupd) and CO electro-oxidation (ACO). Theoretically, the ratio between the charges of CO-stripping D

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Figure 6. Representative fitting patterns of the CO-stripping curves of the Ni@Pt/C catalysts by Gaussian−Lorentz methods.

further with rational design and strict control of their nanoscale architecture. 3.2. HOR Measurement. The steady-state current− potential profiles for HOR of the Ni@Pt/C catalysts with different Pt coverages were tested by the poor thin-film RDE technique36,44 in H2-saturated 0.5 M H2SO4 solution at 27 °C and compared with that of Pt/C (JM) catalysts. The relevant results are shown in Figure 7. According to Chen and

Table 1. Data of Peak Potentials and the Area Values of the Split Double Peaks Ni@Pt/C Pt:Ni Pt:Ni Pt:Ni Pt:Ni Pt:Ni Pt:Ni

= = = = = =

1:10 2:10 3:10 4:10 5:10 7:10

peak1 low E

A1 (×10−6)

peak2 high E

A2 (×10−6)

A1/A2

0.7535 0.7501 0.7322 0.7217 0.7156 0.7113

3.41 4.35 6.53 8.12 10.25 12.74

0.7981 0.8054 0.7853 0.7935 0.7914 0.7775

3.99 6.45 6.17 3.48 3.95 6.16

0.855 0.674 1.058 2.333 2.595 2.068

Table 2. Area of CO-Stripping (ACO) and Hupd desorption (AHupd) on Pt/C and Ni@Pt/C Ni@Pt/C Pt:Ni Pt:Ni Pt:Ni Pt:Ni Pt:Ni Pt:Ni Pt/C

= = = = = =

1:10 2:10 3:10 4:10 5:10 7:10

ACO (×10−6)

AH (×10−6)

ACO/2AHupd

7.4 10.8 12.7 11.6 14.2 18.9 25.5

2.44 4.39 6.59 6.94 8.42 11.86 12.16

1.51 1.23 0.963 0.84 0.84 0.82 1.04

and Hupd desorption on Pt atoms is approximatively 2:1, applying 420 uC/cm2 is required to oxidize a monolayer of CO and 210 uC/cm2 of Hupd.40 However, for the Ni@Pt/C catalysts with a Pt/Ni ratio lower than that forming a Pt monolayer, the ACO/AHupd ratios are higher than 2:1 (e.g., samples 1 and 2). As Pt coverage increase, the ACO/AHupd ratios gradually decrease to values less than 2:1. By contrast, the charge ratio of Pt/C (JM) agrees with the theoretical value. This finding contrasts previous experiment results.41−43 For example, suppression of Hupd adsorption has been reported on the skin-type Pt surfaces of Pt3Ni(111), PtNi/C, and core− shell structure. Such skin-type Pt surfaces demonstrated ideal assembly of Pt monolayer or multilayer on substrates consisting only of Pt after a particular treatment, such as thermal treatment or acid treatment/annealing. For the Ni@Pt/C catalysts in our experiment, deviation of the charge ratios of CO electro-oxidation and Hupd desorption may be explained by the nonuniformity of the Pt shell on Ni-core particles shown by the model in Figure 5. Delicate structure−function correlation in bimetallic electrocatalysts with core−shell nanostructure has been demonstrated by CO electro-oxidation as a probe. Although the Ni@Pt/C catalysts with Pt monolayer (Pt/Ni = 3:10) showed optimum activity for ORR with 3−4 times Pt/C (JM) in specific activity, core−shell Ni@Pt/C electrocatalysts for ORR can be enhanced

Figure 7. Steady-state polarization curves in H2-saturated 0.5 M H2SO4 for the prepared Ni@Pt/C catalysts with different Pt coverages and Pt/C (JM) catalysts. Scan rate: 5 mV/s. Inset: the polarization curves near equilibrium potential.

Kucernak,45 the current−potential relation near the equilibrium potential can be expressed by 1 2F η 1 = · − i0 RT i idL (1) where i0 and idL are the exchange current and the limiting diffusion current for HOR on the catalyst-loaded RDE, respectively, and F, R, and T have their usual meanings. Thus, the exchange current density (j0) for HOR of the interested catalyst can be estimated according to slope of the relatively linear current−potential dependence (i ∼ η) near the equilibrium potential (inset of Figure 7) and the electrochemical active area (ESA) of Pt given by the catalyst, according to the formula j0 = i0/ESA. The ESA can be estimated with the UPD H charges in the CVs for various catalysts (Figure 4). Figure 8 shows the relationship between the HOR j0 values and the atom ratios of Pt/Ni. The HOR j0 values of Ni@Pt/C catalysts decrease with increasing Pt E

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gradually correspond to that of Pt/C. This result can be explained by the existence of nickel hydroxides and hydrogen spillover. Electrochemical processes of small molecules, such as CO stripping and HOR, can be used as a probe to assess the surface states of electrocatalysts in an electrochemical environment. The electrochemical data obtained provide a new route for identification of M@Pt core−shell structures.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: (+86) 391-3987811. E-mail: [email protected]. cn.

Figure 8. Dependence of HOR j0 values and Pt/Ni ratios of the Ni@ Pt/C catalysts.

Notes

The authors declare no competing financial interest.

coverage and gradually approach that of Pt/C. The maximum HOR j0 (67.8 mA/cm2) measured from Ni@Pt/C catalysts with Pt/Ni = 1:10 is about 6 times higher than that determined for Pt/C (JM) (11.59 mA/cm2) and other Ni@Pt/C samples (11.5 mA/cm2 for Pt/Ni = 5:10 and 10.68 mA/cm2 for Pt/Ni = 7:10). The excellent performance of HOR on Ni@Pt/C catalysts with lower Pt coverages may be explained by the high electronic and protonic conductivity of the nickel hydroxides (Figure 3) and hydrogen spillover on the catalysts, as proposed in the literature.46,47 The reaction scheme (as follows and as proposed in ref 46) involved in the hydrogen spillover may illustrate the patterns of the HOR performance of Ni@Pt/C catalysts.

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ACKNOWLEDGMENTS This work is supported by the Ministry of Science and Technology (Grants 2012CB932800 and 2013AA110201) and the Educational Commission of Henan Province of China (14A150036).

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REFERENCES

(1) Wang, J. X.; Inada, H.; Wu, L. J.; Zhu, Y. M.; Choi, Y. M.; Liu, P.; Zhou, W. P.; Adzic, R. R. Oxygen Reduction on Well-Defined CoreShell Nanocatalysis: Particle Size, Facet, and Pt Shell Thickness Effects. J. Am. Chem. Soc. 2009, 131, 17298−17302. (2) Zhou, W. P.; Yang, X.; Vukmirovic, M. B.; Koel, B. E.; Jiao, J.; Peng, G.; Mavrikakis, M.; Adzic, R. R. Improving Electrocatalysts for O2 Reduction by Fine-Tuning the Pt−Support Interaction: Pt Monolayer on the Surfaces of a Pd3Fe(111) Single-Crystal Alloy. J. Am. Chem. Soc. 2009, 131, 12755−12762. (3) Shao, M.; Shoemaker, K.; Peles, A.; Kaneko, K.; Protsailo, L. Pt Monolayer on Porous Pd−Cu Alloys as Oxygen Reduction Electrocatalysts. J. Am. Chem. Soc. 2010, 132, 9253−9255. (4) Wang, X.; Orikasa, Y.; Takesue, Y.; Inoue, H.; Makamura, M.; Minato, T.; Hoshi, N.; Uchimoto, Y. Quantitating the Lattice Strain Dependence of Monolayer Pt Shell Activity toward Oxygen Reduction. J. Am. Chem. Soc. 2013, 135, 5938−5941. (5) Xing, Y.; Cai, Y.; Vukmirovic, M. B.; Zhou, W. P.; Karan, H.; Wang, J. X.; Adzic, R. R. Enhancing Oxygen Reduction Reaction Activity via Pd−Au Alloy Sublayer Mediation of Pt Monolayer Electrocatalysts. J. Phys. Chem. Lett. 2010, 1, 3238−3242. (6) Chol, S.; Shao, M.; Lu, N.; Ruditskiy, A.; Peng, H. C.; Park, J.; Guerrero, S.; Wang, J.; Kim, M. J.; Xia, Y. Synthesis and Characterization of Pd@Pt-Ni Core-Shell Octahedra with High Activity toward Oxygen Reduction. ACS Nano 2014, 8, 10363−10371. (7) Chen, Y.; Yang, F.; Dai, Y.; Chen, S. Ni@Pt Core-Shell Nanoparticles: Synthesis, Structural and Electrochemical Properties. J. Phys. Chem. C 2008, 112, 1645−1649. (8) Chen, Y.; Liang, Z.; Yang, F.; Liu, Y.; Chen, S. Ni-Pt Core-Shell Nanoparticles as Oxygen Reduction Electrocatalysts: Effect of Pt Shell Coverage. J. Phys. Chem. C 2011, 115, 24073−24079. (9) Lin, R.; Cao, C. H.; Zhao, T. T.; Huang, Z.; Li, B.; Wieckowski, A.; Ma, J. X. Synthesis and Application of Core-Shell Co@Pt/C Electrocatalysts for Proton Exchange Membrane Fuel Cells. J. Power Sources 2013, 223, 190−198. (10) Li, Z. S.; He, C. Y.; Cai, M.; Kang, S.; Shen, P. K. A Strategy for Easy Synthesis of Carbon Supported Co@Pt Core-Shell Configuration as Highly Active Catalyst for Oxygen Reduction Reaction. Int. J. Hydrogen Energy 2012, 37, 14152−14160. (11) Henry, J. B.; Maljusch, A.; Huang, M.; Schuhmann, W.; Bondarenko, A. S. Thin-Film Cu−Pt(111) Near-Surface Alloys: Active Electrocatalysts for the Oxygen Reduction Reaction. ACS Catal. 2012, 2, 1457−1460.

NiOOH + Pt−H → Ni(OH)2 + Pt Ni(OH)2 → NiOOH + H+ + e−

The presence of nickel hydroxides promotes the hydrogen spillover from the platinum sites to the neighboring nickel hydroxides, which facilitates the oxidation of hydrogen. The performance of HOR on the Ni@Pt/C catalysts further verifies the integrity of Pt coverage on Ni-core particles.

4. CONCLUSION CO electro-oxidation and HOR may be applied as electrochemical tools for characterizing a series of Ni@Pt/C core− shell catalysts with different Pt coverages. Information available in the elctrochemical environment can help us better understand the behavior of core−shell bimetallic catalysts and design more effective catalytic structures. The CO-stripping CVs of the Ni@Pt/C catalysts present split double peaks, which indicate two different states of the Pt atoms on Ni-core particles. The current peaks in the CO-stripping CVs of Ni@ Pt/C catalysts under relatively more positive potentials are consistent with that of Pt/C, which indicates that some Pt atoms behave like pure Pt as isolated Pt atoms. The shoulder peaks shift toward more negative potentials, thus revealing that the Pt atoms on Ni-core particles form Pt−Pt bonds resulting in Pt lattice contraction. The more positive current peaks in CO-stripping CVs of Ni@Pt/C catalysts are predominant with lower Pt coverages, and with increasing Pt coverage the more negative current peaks are enhanced, which illustrate the integrity of Pt−Pt ensembles with neighboring Pt atoms. This phenomenon can be illustrated by the simple schematic diagram of changes in the morphologies of Ni@Pt/C catalysts with different Pt coverages. HOR measurements of Ni@Pt/C catalysts also show Pt coverage dependent behaviors; here, the HOR j0 values decrease with increasing Pt coverage and F

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DOI: 10.1021/jp512283c J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/jp512283c J. Phys. Chem. C XXXX, XXX, XXX−XXX