Uniform size and composition tuning of PtNi octahedra for systematic studies of oxygen reduction reactions

Journal of Catalysis 309 (2014) 343–350

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Uniform size and composition tuning of PtNi octahedra for systematic studies of oxygen reduction reactions Shang-Wei Chou a,b,1, Ying-Ren Lai a,c,1, Ya Yun Yang d, Chih-Yuan Tang d, Michitoshi Hayashi c, Hsieh-Chih Chen a,b, Hui-Lung Chen e,⇑, Pi-Tai Chou a,b,⇑ a

Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Center for Emerging Material and Advanced Devices, National Taiwan University, Taipei 10617, Taiwan Center for Condensed Matter Sciences, National Taiwan University, Taipei 10617, Taiwan d Instrumentation Center, National Taiwan University, Taipei 10617, Taiwan e Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan b c

a r t i c l e

i n f o

Article history: Received 4 June 2013 Revised 19 August 2013 Accepted 11 September 2013

Keywords: PtNi octahedra Shape control Oxygen reduction reaction Volcano-shaped plot DFT calculations Surface energy Adsorption energy

a b s t r a c t Uniform size and composition tuning of PtNi octahedra enclosed by 8 {1 1 1} facets were achieved by the control of specific crystal facet–surfactant bindings. In this chemical method, the optimal ratio of dioctyl amine to oleic acid is a key factor in the formation of PtNi octahedra with different alloying compositions, Pt:Ni = 18:6, 12:12, and 10:14. Careful electrochemical examinations show that the oxygen reduction reaction (ORR) mass activity and specific area activity of all PtNi octahedra outperform those of commercial Pt/C catalyst. The volcano-shaped trend of ORR enhancement proves that Pt12Ni12 octahedra have superior ORR activities. Further firm support is provided by the theoretical approach, which concludes that the adsorption energies of the H, O, O2, and OH species on a surface of Pt12Ni12(1 1 1) are much higher than those for Pt18Ni6(1 1 1) and Pt10Ni14(1 1 1) counterparts, whereas the molecularly adsorbed H2O product is calculated to be the lowest in adsorption energy on the Pt12Ni12(1 1 1) surface. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction The environmentally friendly power source fuel cells, including proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), have been intensively investigated during the past several decades [1–4]. Nevertheless, the slow oxygen reduction reaction (ORR) in the cathode and the high cost of Pt catalysts still limit the application of fuel cells in practical machines [4–6]. It is thus a challenging task to design more efficient and economical catalysts for ORR in fuel cells. One strategy for designing highly efficient and economical catalysts is to employ Pt-based alloys intermixing 3d transition metals, such as PtNi [7–10], PtCo [11–13], PtFe [14,15], and PtPd [16,17], which can achieve higher ORR activity than traditional carbon-supported Pt (Pt/C) catalysts. Many studies have pointed out that alloying of Pt catalysts offers

⇑ Corresponding authors at: Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan. (P.-T. Chou); Department of Chemistry and Institute of Applied Chemistry, Chinese Culture University, Taipei 111, Taiwan. (H.-L. Chen). Fax: +886 2 2369 5208. E-mail addresses: [email protected] (H.-L. Chen), [email protected] (P.-T. Chou). 1 These two authors contributed equally to this work. 0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2013.09.008

several unique features: (1) the shortening of Pt–Pt interatomic distances; (2) the more favorable chemisorption of the OH species onto the sites of transition metals; (3) the increase in d-band vacancies; and (4) the downshift of the d-band center [18–21]. The changes of interatomic structures and electronic states offer several advantages, such as enhancing dissociative absorption of O2, decreasing the adsorption strength of oxygenated species, and acceleration of the reaction rate in electroreduction, which are all beneficial for improving ORR activity. Among Pt-based alloys, it is noteworthy that the PtNi catalyst has superior enhancement in ORR activity. For example, Stamenkovic et al. experimentally demonstrated that the PtNi(1 1 1) surface prepared though UHV methods is 10-fold more active for the ORR than the corresponding Pt(1 1 1) surface and 90-fold more active than the current Pt/C catalysts [22]. They attribute this to the weak interaction between Pt atoms and nonreactive oxygenated species, and hence the increase in active sites for O2 adsorption. Via theoretical approaches, Matanovic´ et al. point out that the (1 1 1) surfaces of all models of PtxNi1x alloys enhance oxygen reduction activity in comparison with that of pure Pt(1 1 1) [23]. The calculated ORR overpotential of PtxNi1x alloy (1 1 1) surfaces is substantially lower than that of Pt(1 1 1) surfaces, manifesting ORR activity influenced by the alloy composition. The associated mechanism may lie in an unusual

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electronic structure (d-band center position) and arrangement of surface atoms in the near-surface region. However, experimentally, a fair comparison to single out the composition effect for ORR remains difficult due to the composition-dependent changes of the crystal structure. To overcome this obstacle, herein, we report the rational synthesis of uniform PtNi octahedra enclosed by 8 {1 1 1} facets. Through fine tuning of the precursor ratio (Pt-to-Ni) in the presence of dioctyl amine (DiOL) and oleic acid (OAc), uniform PtNi octahedra with different alloy compositions were successfully prepared and well characterized via high-resolution transmission electron microscopy (HR-TEM) and powder X-ray diffraction (PXRD). In this chemical method, the optimal surfactant ratio for DiOL vs OAc plays a key role in the formation of PtNi octahedra with different alloy compositions, including Pt18Ni6, Pt12Ni12, and Pt10Ni14, due to the specific crystal facet–surfactant bindings on the growth seed [24–26]. We then prove that the ORR mass activity (MA) and specific area activity (SA) of all synthesized PtNi octahedra outperform those of commercial Pt/C catalysts in HClO4. Subsequently, a combination of chemical derivation and theoretical calculations is performed to gain in-depth insight into the ORR on PtxNi24x(1 1 1) surfaces. The results lead to understanding of adsorption behavior of ORR chemical species (H, O, O2, OH, and H2O) on PtxNi24x(1 1 1) surfaces and thus identify key factors of surface compositions that govern the reactivity. Details of results and discussion are elaborated as follows. 2. Materials and methods 2.1. Materials Platinum acetylacetonate (Pt(acac)2, ACROS, 97%), nickel acetylacetonate (Ni(acac)2, ACROS, 97%) 1,2-hexadecanediol (HDD, Aldrich, 90%), dioctyl amine (DiOL, ACROS, 98%), oleic acid (OAc, Aldrich, 90%), dibenzyl ether (ACROS, 99%), octyl ether (TCI, 95%), and Nafion solution (Aldrich, 5 wt% in mixture of lower aliphatic alcohols and water) were used. 2.2. Synthesis of PtNi octahedra with different alloy compositions A typical procedure for the synthesis of PtNi octahedra is as follows: first, Pt(acac)2 (78 mg, 0.199 mmole), Ni(acac)2 (37 mg, 0.144 mmole), and HDD (240 mg) were loaded into a three-neck flask and mixed with dibenzyl ether (3 mL). The reaction mixture was heated to 270 °C at a heating rate of 15 °C/min. During the heating process, a mixture of DiOL (600 lL), OAc (300 lL), and octyl ether (5 mL) was slowly injected into the above reaction solution at 170 °C. The color of the reaction solution gradually changed from light blue to light black. The reaction was maintained at a reflux temperature of 270 °C for 20 min before cooling to room temperature under N2. In the typical procedure of collection of the resultant particles, the black product was precipitated by adding an antisolvent (a mixture of toluene, ethanol, and ethyl acetate, volume ratio = 1:1:1) and then separated by centrifugation at 3500 rpm. After centrifugation, the light brown supernatant was removed for the further separation of small particles and the purification of PtNi octahedra (Fig. S1). The purification process was repeated 3–5 times. Finally, the black product was stored in hexane for the material characterization. All pertinent experimental parameters for the syntheses of PtNi octahedra with different alloying components and other PtNi nanocrystals are summarized in Table 1. 2.3. Characterization Low-resolution transmission electron microscopy (TEM) images and high-resolution transmission electron microscopy (HR-TEM)

images were obtained with a JEOL 1230 transmission electron microscope (at 100 kV) and a Philips/FEI Tecnai 20 G2 S-Twin transmission electron microscope (at 200 kV), respectively. The compositional analysis was performed on an X-ray energy-dispersive spectrometer, attached to a Philips/FEI Tecnai 20 G2 S-Twin transmission electron microscope. The powder X-ray diffraction scan (PXRD) was recorded by a PANalytical X’Pert PRO diffractometer. The procedure was carried out with Cu Ka radiation (k = 1.54178 Å). 2.4. Electrochemical measurements The electrochemical measurements were performed in a threeelectrode cell under a rotating disc electrode setup and with a Metrohm Autolab PGSTAT 100 electrochemical workstation. A saturated calomel electrode was the reference electrode and a platinum electrode (CH Instruments) was used as the counter electrode. The working electrode was a glassy carbon rotating disk electrode (RDE, 0.196 cm2 in area). Before electromeasurements, the RDE was polished with Al2O3 powder (CH Instruments, 0.05 lm) and rinsed with deionized water (DI water). A solution of 0.1 M HClO4 was used as the supporting electrolyte. All nanocatalysts were exposed under UV and ozone overnight. The capping ligands on the crystal surface were removed by this UV–ozone treatment. The UV–ozone treatment was performed on a homemade UVO cleaner assembled fro, a UV lamp (wavelength = 183 and 254 nm, output = 110 mW/cm) and an ozone generator (flow rate = 500 mg/h). For the preparation of the working electrode, preweighted PtNi octahedra were dispersed in 2 mL of the mixture through sonication. The mixture contained ethanol, isopropanol, and nafion in a volume ratio of 4:1:0.05. Then a few microliters of PtNi octahedra were drop cast on the RDE. The rate of RDE was set at 1600 rpm for all measurements. All the potentials in this report are given vs a normal hydrogen electrode (NHE). Cyclic voltammetry (CV) was carried out in the supporting electrolyte under an N2 purge for 30 cycles to further clean the nanocatalyst surface. The potential was swept between 0 and 1.2 V (vs NHE) at a rate of 100 mV/s1. The active surface area was determined by a hydrogen underpotential deposited (Hupd) region of 0.03–0.4 V and assuming 210 lC/cm2 for a monolayer of adsorbed hydrogen on the Pt surface. The measurements of the ORR were performed in an O2-saturated 0.1 M HClO4 and O2 gas flow. The potential was swept from 1.2 to 0 V at a sweep rate of 5 mV/s. The rate of the RDE was set at 1600 rpm for the ORR measurements. 2.5. Theoretical study The periodic density functional theory (DFT) calculations were performed by using the Vienna Ab initio Simulation Package (VASP) code [27]. For the exchange and correlation energy, we used the spin-polarized generalized gradient approximation with the Perdew–Wang 91 function [28]. Ultrasoft pseudopotentials [29] and a kinetic energy cutoff of 400 eV were used to expand the electron wave functions in a plane-wave basis. The Brillouin zone was sampled with the Monkhorst–Pack grid, and the calculations were performed with 5  5  1 Monkhorst–Pack mesh k-points [30]. The convergence criterion for the electronic self-consistent iteration was set to 104 eV. The PtNi alloys were modeled with a six-layered slab using a 2  2 supercell. A vacuum gap of about 15 Å in the z-direction was included to separate periodic slabs. The atoms in the top four layers were allowed to relax, while the atoms in the remaining two layers were fixed at their ideal bulk positions. The binding energies of possible ORR chemical species at different adsorption sites were fully examined at 0.25 monolayer (ML) coverage, which are defined as the following equation:

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S.-W. Chou et al. / Journal of Catalysis 309 (2014) 343–350 Table 1 The experimental parameters of PtNi octahedra with various alloying composition and PtNi nanocrystals.

a

PtNi nanocrystals

DiOL/OAca

Pt(acac)2/Ni(acac)2/HDD

Pt18Ni6 octahedra Pt12Ni12 octahedra Pt10Ni14 octahedra PtNi heterostructure PtNi polyhedra PtNi irregular particles PtNi netlike nanostructure PtNi netlike nanostructure

600 lL/300 lL 600 lL/300 lL 600 lL/300 lL 600 lL/300 lL 300 lL/600 lL 2000 lL/300 lL 600 lL/300 lL 600 lL/300 lL

78 mg 55 mg 55 mg 55 mg 78 mg 78 mg 78 mg 78 mg

(0.199 mmole)/37 mg (0.140 mmole)/37 mg (0.140 mmole)/43 mg (0.140 mmole)/55 mg (0.199 mmole)/37 mg (0.199 mmole)/37 mg (0.199 mmole)/37 mg (0.199 mmole)/37 mg

(0.144 mmole)/240 mg (0.144 mmole)/240 mg (0.167 mmole)/240 mg (0.214 mmole)/240 mg (0.144 mmole)/240 mg (0.144 mmole)/240 mg (0.144 mmole)/240 mg (0.144 mmole)/240 mg

Reaction temperature (°C)

Reaction time (min)

270 270 270 270 270 270 295 270

20 20 20 20 20 20 20 120

DiOL and OAc were loaded into octyl ether of 5 mL for the injection.

DEads ¼ E½surface þ adsorbate  ðE½surface þ E½adsorbateÞ; where E[surface + adsorbate], E[surface], and E[adsorbate] are the calculated electronic energies of adsorbed species on PtxNi24x(1 1 1) surfaces, the clean PtxNi24x(1 1 1) surfaces, and the gas-phase atom/molecule, respectively. In addition, the surface energy calculation is based on the equation:

ESurf ¼ ðESlab  nEBulk Þ=2A; where ESurf is the surface energy, ESlab the total energy per repeated slab supercell, EBulk the energy per unit cell in the bulk, n the number of unit cells, and A the total surface area per repeated unit. 3. Results and discussion 3.1. Characterization and shape control of PtNi octahedra The PtNi nanocrystals are prepared with different alloy compositions in the presence of DiOL and OA. Under TEM observation, shown in Fig. 1, the as-prepared PtNi nanocrystals reveal a rhombic projection on the facet, and therefore their octahedral structure is unambiguous [31]. The alloy compositions of these samples are confirmed by X-ray energy-dispersive spectrometer (EDS), shown in Fig. S2, showing that the Pt:Ni ratios are 75:25, 46:54, and 36:64 for samples in Fig. 1a–c, respectively. For convenience in further discussion, these PtNi octahedra with different compositions

are abbreviated as Pt18Ni6, Pt12Ni12, and Pt10Ni14. As a result, TEM images (Fig. 1a–c) exhibit monodisperse Pt18Ni6, Pt12Ni12, and Pt10Ni14 octahedra, respectively. As shown in Fig. S3, the average lengths of PtNi octahedra with different alloy compositions are 13.28 ± 0.93 nm for Pt18Ni6, 13.67 ± 1.48 nm for Pt12Ni12, and 12.73 ± 1.04 nm for Pt10Ni14. Also, HRTEM images (Fig. 1d–f) clearly show that all samples are crystalline and there is no amorphous phase present in a particle [25]. The lattice fringes correspond to a spacing distance of 0.21–0.22 nm, which is close to the lattice distance of the (1 1 1) facet of PtNi alloys. Further careful examination indicates that the lattice distance of PtNi octahedra increases as the Pt ratio increases in the alloy composition, being on the order of 0.211, 0.215, and 0.219 nm for Pt10Ni14, Pt12Ni12, and Pt18Ni6 octahedra, respectively (Fig. 1d–f; also see Fig. S4 for the analytical results) [32]. The scans of powder X-ray diffraction (PXRD), as shown in Fig. 2, display the characteristic peaks for the (1 1 1) and (2 0 0) facets. The associated patterns reveal that PtNi octahedra, being independent of the alloy compositions, are in a face-centered cubic (FCC) structure. The combinations of XRD scans and HRTEM images therefore demonstrate that Ni and Pt atoms are distributed randomly within the alloy as a solid solution [25,33]. We then examine the pattern positions of Pt carefully and notice the high-angle shift of the PtNi octahedra with different alloy compositions. As indicated by gray dashed lines in Fig. 2, the (1 1 1) peak positions of the three PtNi octahedra are 40.4° for Pt18Ni6, 40.6° for Pt12Ni12,

Fig. 1. (a–c) TEM and (d–f) HRTEM images of (a, d) Pt18Ni6; (b, e) Pt12Ni12; and (c, f) Pt10Ni14 octahedra.

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Scheme 1. The formation of an octahedron and a polyhedron through specific surfactant–facet bindings at different ratios of DiOL/OAc. In route (A), the octahedron is obtained at the optimal ratio of DiOL/OAc (600 lL/300 lL) due to the higher adsorption of surfactant molecules on {1 1 1} facets. In route (B), the polyhedron is obtained at a low ratio of DiOL/OAc (300 lL/600 lL) due to the similar adsorption of surfactant molecules on {1 1 1} and {1 0 0} facets. Note: G denotes the growth rate of crystal facets.

Fig. 2. (a) PXRD scans of Pt18N6, Pt12Ni12, and Pt10Ni14 octahedra. Note: The gray lines point out the shift of patterns of PtNi octahedra due to their alloy components. (b) The (1 1 1)/(2 0 0) peak intensity ratios of Pt18N6, Pt12Ni12, and Pt10Ni14 octahedra.

and 40.9° for Pt10Ni14. The alloy-composition-dependent high-angle shift can be well rationalized by Vegard’s law, which states that the diffraction peak shifts monotonically from low to high angle for Ni-rich PtNi octahedra [32]. Further analysis of the diffraction peaks gives similar full width at half maximum (FWHM), indicating that their average lengths are comparable, consistent with those measured by TEM [34]. The intensity ratio for the (1 1 1) vs the (2 0 0) peak, in a qualitative manner, could provide evidence of the preferred orientation of the PtNi octahedra [31,32]. As a result, the intensity ratios of all three samples are measured to be 2.1–2.6, higher than that (1.66) of the bulk Pt alloys, reconfirming the octahedral morphology enclosed by {1 1 1} planes for all as-prepared PtNi alloys. Different surfactant ratios may play a key role in the shape control of bimetallic systems [24–26]. On one hand, when the ratio of DiOL/OAc was altered from 600 lL/300 lL to 300 lL/600 lL, PtNi polyhedra were obtained, as shown in Fig. S5a. On the other hand, the employment of a higher DiOL/OAc ratio (e.g., 2000 lL/300 lL) resulted in the formation of PtNi particles with bipod-like and other irregular structures (Fig. S5b). Generally, specific interaction between surfactant molecules and various crystal facets, dubbed as specific surfactant–facet binding, is considered to be a main factor in shape control [24,25]. In our case, the optimal DiOL/OAc ratio can result in more adsorption of surfactant molecules and effectively decrease the surface energy of {1 1 1} facets on the growth seeds, as shown in the (A) route of Scheme 1. Afterward, the growth rate of the {1 1 1} facet (G{1 1 1}) is lower than that of the {1 0 0} facet (G{1 0 0}), and eventually, PtNi octahedra enclosed by 8 {1 1 1} facets are obtained, as shown in Scheme 1 [26,35]. Alternatively, the lower DiOL/OAc ratio results in similar adsorption of surfactant molecules on the {1 1 1} and {1 0 0} facets, as shown in the (B) route of Scheme 1. Thus, G{1 1 1} is comparable to G{1 0 0}, and consequently the formation of PtNi polyhedra is expected and experimentally observed. Temperature effects were also carefully examined. When temperature was raised to 295 °C, PtNi netlike nanostructures were

obtained, shown in Fig. S6a. Similar PtNi netlike nanostructures were obtained when the reaction time was increased from 20 to 120 min (Fig. S6b). These results can be rationalized by the thermodynamic control of interparticle interactions, together with heat-induced interparticle fusion in the nucleation and ripening process with continuous/excess energy supply, resulting in thermodynamically stable netlike nanostructures [36–38]. The precursor Pt:Ni ratio turned out to be a crucial factor as well for obtaining homogeneous bimetallic octahedra. In an experiment where we intentionally decreased the precursor Pt:Ni (in molar ratio) to <0.70, the resultant nanocrystals, shown by the TEM image in Fig. S7, revealed irregular morphologies and heterostructure. A further XRD scan (Fig. S7b) gave characteristic signals at 41.1°, 44.3°, 48.1°, and 51.5°, corresponding to the XRD patterns of PtNi (marked by a black spot) and Ni (marked by a gray spot). The result clearly indicates the formation of a heterostructure in the excessamount-of-Ni precursor. Upon optimization of temperature, surfactants, metal precursors, and their respective ratios, homogeneous and uniform bimetallic octahedra are thus obtained in this study (vide supra). 3.2. Electrochemical measurements of PtNi octahedra Fig. 3 shows the electrochemical measurements of the three PtNi octahedra and a commercial Pt catalyst (Pt E-TEK) in 0.1 M HClO4 at room temperature. The electrochemical surface areas (ECSAs) are determined by the CV curves as shown in Fig. 3a within the integrated area of Hupd region of 0.03–0.4 V. The ECSAs of all the samples as summarized in Table S1 are 80.95 ± 3.57 m2/gPt for Pt E-TEK, 46.91 ± 2.08 m2/gPt for Pt18Ni6, 38.88 ± 1.36 m2/gPt for Pt12Ni12, and 35.25 ± 1.04 m2/gPt for Pt10Ni14, respectively. Obviously, the ECSAs of PtNi octahedra are positive relative to the Pt content of the alloying composition. Also, the correlation reflects that the bulk alloy composition could influence the surface composition. Fig. 3b exhibits ORR polarization curves obtained in the O2-saturated 0.1 M HClO4 at a scan rate of 5 mV/s. Further Tafel plots shown in Fig. 3c and d indicate that all samples have ORR enhancement in kinetic currents compared with the Pt E-TEK benchmark at 0.9 V. The insets of Fig. 3c and d reveal the kinetic current densities at 0.9 V normalized to the effective Pt mass and ESCAs, respectively. In detail, the ORR MA for all samples increases

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Fig. 3. (a) Cyclic voltammograms and (b) polarization curves of Pt18Ni6, Pt12Ni12, and Pt10Ni14 octahedra. Tafel plots and comparison of (c) mass activities and (d) specific area activities of all PtNi octahedra. Note: The mass activity and specific area activity of commercial Pt/C catalyst (Pt E-TEK) served as the reference.

in the order of Pt E-TEK (1.78 ± 0.01 mA/mgPt) < Pt18Ni6 (3.02 ± 0.07 mA/mgPt) < Pt10Ni14 (3.18 ± 0.04 mA/mgPt) < Pt12Ni12 (3.68 ± 0.05 mA/mgPt). Also, SA for all samples is in the sequence Pt E-TEK ð2:19  0:01 lA=cm2Pt Þ < Pt18 Ni6 ð6:44 0:15 lA=cm2Pt Þ < Pt10 Ni14 ð9:02  0:12 lA=cm2Pt Þ < Pt12 Ni12 ð9:46  0:14 lA=cm2Pt Þ. Clearly, the ORR activities of all as-prepared PtNi octahedra outperform those of Pt E-TEK. As elaborated in an early section, the catalytic effect of bimetallic Pt-based alloy in ORR incorporates the intermixing of 3d transition metals, which are enabled to change both Pt–Pt interatomic structure and electronic state [14,18–21]. In this study, because the ORR activity can be compared fairly among PtxNi24x octahedra with uniform shape and size, its dependence on the Pt:Ni molar ratio can be singled out. As a result, the trend of ORR enhancements of PtxNi24x octahedra directly related to their alloying composition can be fairly made. On this basis, the Pt12Ni12 octahedra have the highest MA and SA, while any higher or the lower Pt content in PtNi octahedra results in a decrease of the ORR activities. As a result, the ORR activities as a function of the alloying composition of PtNi octahedra reveal a volcano-shaped plot shown in Fig. 3c and d [14,23,39,40]. 3.3. Theoretical study With the aim of further portraying the scenario of volcanoshaped ORR enhancement and the adsorption behaviors of oxygenated species, we have employed an ab initio method using periodic density functional theory (DFT) to predict the interactions between ORR chemical species and PtxNi24x(1 1 1) surfaces. To ascertain the computational approach used in this study, we first calculated lattice parameters of bulk PtNi alloys with different Pt:Ni ratios and compared the results with our experimental data. A FCC bulk model (PtxNi12x) of suitable size was chosen and all possible

arrangements replacing Pt atoms with Ni atoms were then calculated in three bulk models (Pt9Ni3, Pt6Ni6, and Pt5Ni7), which were used to represent three different ratio of PtNi alloys according to the experimental condition. As a result, the most stable calculated bulk models of Pt9Ni3, Pt6Ni6, and Pt5Ni7 are illustrated in Fig. S8. In addition, a comparison of calculated and experimental lattice parameters has been made and the pertinent data are listed in Table S2. The lattice parameters of bulk PtNi alloys with different Pt:Ni ratios (Pt:Ni = 0.75:0.25, 0.50:0.50, and 0.42:0.58) predicted with consideration of the spin polarization at the GGA level are within the range 3.74–3.89 Å, in good agreement with our experimental values (3.65–3.79 Å). As this agreement confirms the chosen method to be suitable, we are thus confident in the reliability of the following calculations. As shown in Fig. 4, we establish three slab models denoted as (a) Pt18Ni6(1 1 1), (b) Pt12Ni12(1 1 1), and (c) Pt10Ni14(1 1 1) by wellmixed bulk PtNi alloy to examine the effect of Ni-doped behavior on ORR activity. If we further consider the different kinds of surface termination on the Pt10Ni14(1 1 1) surface (c), three more surface models of (c1), (c2), and (c3) are thus established. We then calculate the surface energies of these models. The sequence of instability of the corresponding surfaces is in the order (c3; 1.298 J/m2) < (c1; 1.465 J/m2) < (c2; 1.472 J/m2), indicating that the presence of more Pt atoms both in the first layers of the top and bottom surfaces gives a lower surface energy. As a consequence, it is reasonable for us to chose model c3 for the Pt10Ni14(1 1 1) surface to perform subsequent computational studies of ORR activity. In theory, the increased d-band intensity proximal to the Fermi level produces increased interaction between the metal and the adsorbed molecule [41]. To gain in-depth insight into the alloying effect, we then calculated the local density of states (LDOS) of PtxNi24x(1 1 1) surfaces as a function of composition x, shown in

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0.11 ML) are similar (1.34 eV vs 1.36 eV). Accordingly, the surface size of 0.25 ML for PtxNi24x(1 1 1) shown in Fig. 6 should be sufficient for characterizing the oxygen reduction reaction. Among five adsorption species adsorbed on different ratios of PtxNi24x(1 1 1) surfaces (as shown in Table 2), the calculated adsorption energies of the H, O, O2, and OH species onto the surface of Pt12Ni12(1 1 1) are much larger than those of Pt18Ni6(1 1 1) and Pt10Ni14(1 1 1) counterparts. Obviously, the middle Pt content of PtxNi24x(1 1 1) surface has better adsorption for any ORR chemical species. This computational approach therefore reveals a trend of ORR enhancement having a volcano-shaped plot for different PtNi alloy compositions, firmly supporting the experimental observations. In addition, the molecularly adsorbed H2O on the Pt12Ni12(1 1 1) surface, which could be deemed a final state for desorption, was calculated to be at the lowest energy (0.25 eV) among the alloys, indicating that the Pt12Ni12(1 1 1) surface also exhibits the highest activity for desorption of final products. Therefore, obviously, the binding strength with ORR reactant/intermediate chemicals is maximized at Pt:Ni = 12:12. Moreover, as for the binding strength with the ORR product such as H2O, it also reveals a downward volcano-shaped tendency with minimum at Pt:Ni = 12:12. The combination of these two gives the highest ORR activity at Pt:Ni = 12:12, consistent with our experimental observations.

4. Conclusions

Fig. 4. Relaxed configurations of (a) Pt18Ni6(1 1 1) surface, (b) Pt12Ni12(1 1 1) surface, and (c) Pt10Ni14(1 1 1) surfaces. For Pt10Ni14(1 1 1) surfaces, different kinds of surface termination are considered. Gray and blue balls represent Pt and Ni atoms, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5, and the energy of the d-band centers, listed in Table S3. Careful examination reveals the gradual increase and congregation of right-shifted d-band centers near the Fermi energy as Ni composition increases from Pt18Ni6(1 1 1), Pt12Ni12(1 1 1) to Pt10Ni14(1 1 1). The result could not rationalize the volcano-shape reactivity experimentally observed. This is not surprising. For alloys, limited by the current computation level and functional, it is not quite possible to extract meaningful d-band center data for the use of rationalizing the ORR activity tendency. Alternatively, in this study, the simulated PtNi alloy data are focused on the binding energy. We then investigated the adsorption of all ORR chemical species (H, O, O2, OH, and H2O) on different ratios of PtxNi24x(1 1 1) surfaces at a coverage of 0.25 monolayers (ML). This was done by placing an assigned molecule at each predominant site, as depicted in Fig. 6. It should be noted that the adsorption energies per O2 molecule on the two different surface coverage models of the Pt12Ni12(1 1 1) surface (0.25 ML vs

In summary, uniform PtNi octahedra with different alloy compositions have been successfully prepared through the control of the specific crystal facet–surfactant bindings on the growth seed. Careful electrochemical measurements show that the MA of Pt18Ni6, Pt10Ni14, and Pt12Ni12 octahedra are 1.7 folds, 1.8 folds and 2.1 folds, respectively, higher than that of the Pt/C catalyst. The SA of Pt18Ni6, Pt10Ni14, and Pt12Ni12 octahedra are 2.9 folds, 4.1 folds, and 4.3 folds, respectively, higher than that of the Pt/C catalyst as well. The plot of ORR enhancements as a function of the alloy composition of PtNi octahedra reveals a volcanoshaped plot, being maximized at equal molar ratio. The results are firmly supported by theoretical works, concluding that the Pt12Ni12(1 1 1) surface provides larger adsorption energies for H, O, O2, and OH species than those of Pt18Ni6(1 1 1) and Pt10Ni14(1 1 1) counterparts. The calculation also shows that the middle Pt content of PtxNi24x(1 1 1) surface exhibits the highest activity for the desorption of final product. The combination of experimental and theoretical works thus provides fundamental understanding of the adsorption behavior on the surface of uniform PtxNi24x(1 1 1) octahedra of the same sizes, for which the Pt:Ni molar ratio alone plays a key role. The knowledge developed and gained herein on correlations among surface alloying composition, adsorption energies of oxygenated species, and catalytic activities thus provides valuable information for future strategic design of high-performance nanocatalysts in fuel cells.

Fig. 5. Local density of states (LDOS) of top layers of Pt18Ni6(1 1 1), Pt12Ni12(1 1 1), and Pt10Ni14(1 1 1) (d-orbital) models.

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Fig. 6. Atomic structure of the lowest-energy configurations for various adsorbates on PtxNi24x(1 1 1) surfaces. (1) H atom adsorbed at an FCC site, (2) O atom adsorbed at an FCC site, (3) O2 molecule adsorbed at a top-FCC-bridge(t-f-b) site, (4) OH molecule adsorbed at a bridge site, (5) H2O molecule adsorbed at a top site. Letters are used to classify the three surfaces with different composition ratios: (3a) Pt18Ni6(1 1 1) surface, (3b) Pt12Ni12(1 1 1) surface, and (3c) Pt10Ni14(1 1 1) surface. In the figure, gray balls represent Pt atoms, blue balls represent Ni atoms, red balls represent O atoms, and white balls represent H atoms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Appendix A. Supplementary material Table 2 Binding energies of ORR chemical species on different ratio of PtxNi24x(1 1 1) Species

H O O2 OH H2O

Site

FCC FCC t-f-b Bridge Top

Binding energies (eV) Pt18Ni6

Pt12Ni12

Pt10Ni14

2.74 4.86 1.08 2.64 0.38

2.77 5.23 1.34 2.97 0.25

2.74 5.15 1.27 2.90 0.30

Acknowledgment This work was supported by the National Science Council of the Republic of China under Grant NSC 101-2628-M-002-008.

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