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Enhanced Catalytic Activities of NiPt Truncated Octahedral Nanoparticles toward Ethylene Glycol Oxidation and Oxygen Reduction in Alkaline Electrolyte Tianyu Xia,†,‡ Jialong Liu,† Shouguo Wang,*,‡,§ Chao Wang,§ Young Sun,§ Lin Gu,§ and Rongming Wang*,†,‡ †

Department of Physics, Beihang University, Beijing 100191, China University of Science and Technology Beijing, Beijing 100083, China § Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡

S Supporting Information *

ABSTRACT: The high cost and poor durability of Pt nanoparticles (NPs) are great limits for the proton exchange membrane fuel cells (PEMFCs) from being scaled-up for commercial applications. Pt-based bimetallic NPs together with a uniform distribution can effectively reduce the usage of expensive Pt while increasing poison resistance of intermediates. In this work, a simple onepot method was used to successfully synthesize ultrafine (about 7.5 nm) uniform NiPt truncated octahedral nanoparticles (TONPs) in dimethylformamid (DMF) without any seeds or templates. The as-prepared NiPt TONPs with Pt-rich surfaces exhibit greatly improved catalytic activities together with good tolerance and better stability for ethylene glycol oxidation reaction (EGOR) and oxygen reduction reaction (ORR) in comparison with NiPt NPs and commercial Pt/C catalysts in alkaline electrolyte. For example, the value of mass and specific activities for EGOR are 23.2 and 17.6 times higher comparing with those of commercial Pt/C, respectively. Our results demonstrate that the dramatic enhancement is mainly attributed to Ptrich surface, larger specific surface area, together with coupling between Ni and Pt atoms. This developed method provides a promising pathway for simple preparation of highly efficient electrocatalysts for PEMFCs in the near future. KEYWORDS: NiPt truncated octahedral nanoparticles, EGOR, ORR, PEMFC, alkaline electrolyte



INTRODUCTION

be a promising method for the enhanced catalytic performance of Pt.12−16 This is ascribed to that 3d transition metals not only dramatically reduce the amount of valuable Pt used in electrodes, but also alter the surface electronic structure of Pt and modify its d-band center position.15,17,18 As a result, MPt alloys catalyst can improve the electrocatalytic activity to a large degree for both fuel oxidation reaction and ORR. However, MPt electrocatalysts are easily poisoned by chemisorbed COlike intermediates in acid media.19 Alternatively, the key issue will be greatly resolved in alkaline media. Meanwhile, the decreased competitive adsorption by nonreactive anions leads to much easier electrocatalytic steps in alkaline solutions than in acidic environments.20 Recently, a lot of focus have been given to control the composition, shape. and size of MPt nanostructures to improve catalytic activities toward fuel oxidation reaction and ORR. For example, the improved catalytic features were observed in CoPt

With the ever-increasing usage of fossil fuels and the limited supply of natural resources, proton exchange membrane fuel cells (PEMFCs) have been proven to be a promising candidate for energy conversion device for future energy applications.1−4 In analogy to other energy conversion devices, a typical PEMFC involves two processes, namely fuel oxidation reaction on the anodes and oxygen reduction reaction (ORR) on the cathodes.1,5 The good choice of fuels can be hydrogen, methanol, ethanol, formic acid, or ethylene glycol (EG), because of their unique properties such as high volumetric energy density, energy efficiency, capacity, and recyclability.1,6 Currently, EG is one of the most promising candidates because of its lower toxicity, low volatility, and high volumetric energy density.7,8 In PEMFCs, for both the fuel oxidation reaction and the ORR a catalyst is critical to lower their electrochemical overpotentials and to obtain high voltage outputs, Pt is considered the most effective catalyst for fuel cells with wide applications to date.6,9−11 In addition, introducing nonprecious 3d transition metals (M = Fe, Co, Ni, Mn, et al.) is proved to © 2016 American Chemical Society

Received: January 27, 2016 Accepted: April 11, 2016 Published: April 11, 2016 10841

DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Low-magnification TEM image and (b) the corresponding SAED pattern of NiPt TONPs. Inset of (a) corresponding size distribution histogram. (c) High-magnification TEM image. (d) HRTEM image of a single particle.



nanoflowers21 with porous three-dimensionally interconnected structures, ultrathin NiPt hollow nanospheres8 with shell thickness of about 2 nm, MnPt nanocubes22 as a potential candidate for both cathode and anode catalyst in fuel cells. In particular, a facile approach to prepare truncated-octahedral (TO) NiPt3 catalysts with dominant exposure of {111} facets was developed by Wu et al. The mass activity of those TONPs is 4 times larger than that of the commercial Pt catalyst.23 In 2012, the use of N, N-dimethylformamide (DMF) as both solvent and reductant in the solvothermal synthesis was reported to produce NiPt NPs, where ORR specific activities are 3−5 times greater than that of Pt standard catalyst.24 More recently, Ir with electrochemically stable in acid media has been innovatively introduced into NiPt TONPs to synthesize ternary alloys in order to improve the stability of Pt-based catalysts by Yang et al.25 In general, the catalytic reaction usually takes place on the catalyst surface, therefore it is of great importance to fabricate a catalyst with enlarged surface area and more active sites. It is critical to find a way to fabricate ultrafine catalysts with great dispersity and Pt-rich surfaces. In this paper, a facile one-pot approach is innovated to successively prepare uniformly dispersed NiPt TONPs with an average size of 7.5 nm. Pt-rich surfaces were identified by both X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectrometer (ICP-OES). A large mass activity up to 9.1 A/mgPt and specific activity up to 10.3 mA/cm2pt have been obtained for NiPt TONPs, where the value is 0.4 A/mgPt, 0.6 mA/cm2pt for commercial Pt/C in EGOR, respectively. The enhanced activities can be mainly attributed to Pt-rich surface, larger specific surface area, together with coupling between Ni and Pt atoms.

EXPERIMENTAL DETAILS

Chemicals and Materials. Nickel(II) acetylacetonate [Ni(acac)2], platinum(II) acetylacetonate [Pt(acac)2], commercial Pt/C catalyst were purchased from Alfa Aesar company. DMF, poly(vinylpyrrolidone) (PVP) (K-30, AR), potassium hydroxide (KOH), ethylene glycol (C2H6O2, EG), and sulfuric acid (H2SO4) were purchased from Chinese reagent companies. All reagents were analytic grade and used as received. Synthesis of NiPt TONPs. In a 25 mL Teflon-lined stainless-steel autoclave, 14 mg of Ni(acac)2, 28.5 mg Pt(acac)2 and different mass of PVP were dissolved in 20 mL of DMF. The sealed vessel was then heated to 120 °C and kept for 45 h before it was cooled down to room temperature (RT). The black nanoparticles (NPs) were collected by centrifugation, and further cleaned by anhydrous ethanol and acetone several times. Characterization. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were respectively performed on JEOL-2100F and probe-corrected JEOL JEMARM200F at 200 kV, where TEM and STEM samples were prepared by dispersing the powder in ethanol with ultrasonic treatment, then a small amount of solution was dropped on a Cu grid coated with porous carbon film. X-ray diffraction (XRD) patterns were detected using Rigaku D/max-2400 X-ray diffractometer with a Cu Ka X-ray source (λ = 1.5405 Å). The chemical compositions of as-prepared products were characterized via Hitachi S-4800 scanning electron microscopy (SEM) with an energy dispersive X-ray spectroscope (EDS), and PerkinElmer Optima 7000DV ICP-OES. XPS was performed on an ESCALAB 250Xi system, and binding energies were calibrated by C 1s peak. M−H loops and M−T curves were carried on superconducting quantum interference device (SQUID, Quantum Design) magnetometer. Electrochemical Measurements. For EGOR, the electrode potential was controlled in three electrode configuration by a CH Instruments 660D electrochemical workstation at RT. The working electrode was a glassy carbon electrode (the geometric area is 0.071 10842

DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

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ACS Applied Materials & Interfaces

Figure 2. (a) XRD pattern indexed to NiPt TONPs. (b) EDS analysis with Ni/Pt ratio of 44:56. High-resolution XPS spectra of (c) Ni 2p and (d) Pt 4f for NiPt TONPs, respectively. cm2). The counter electrode and reference electrode were Pt wire and saturated calomel electrode (SCE), respectively. For ORR, the electrochemical experiments were performed in three electrode configuration using CH Instruments 700E electrochemical workstation at RT. The working electrode was a glassy carbon rotating disk electrode (RDE) with the geometric area of 0.196 cm2. The counter and reference electrode were Pt foil (the geometric area is 1 cm2) and a KCl-saturated Ag/AgCl electrode, respectively. Before EGOR and ORR measurements, the working electrodes were mechanically polished with 0.3 and 0.05 μm alumina powder successively until a mirror-finish surface was obtained. The polished electrode was then rinsed with ultrapure water and sonicated in ethanol and water, respectively. To compare the catalytic behavior, we prepared three samples (NiPt TONPs, NiPt NPs, and commercial Pt/C). Four milligrams of NiPt TONPs and 6 mg of Vulcan XC-72 carbon were dissolved in 4 mL of ethanol and sonicated for 20 min. Then 5 μL of the suspension and 5 μL of 0.1% nafion solution, in turn, were dropped onto the work electrode surface and naturally dried in air. Similarly, samples with NiPt NPs and commercial Pt/C (not mixed with carbon) were prepared following the same process as NiPt TONPs.

selected area electron diffraction (SAED) patterns in Figure 1b. The obvious concentric rings can be indexed to the (111), (200) and (220) planes with the corresponding d-spacing of 2.18, 1.89, and 1.34 Å, respectively, in good agreement with XRD results shown in Figure 2a. The crystal structures were checked by powder XRD technique. The XRD pattern shown in Figure 2a confirms that no isolated Pt and Ni peaks are observed, suggesting that the products are alloys without sole metallic particles. Three peaks of 41.4, 48.0, and 70.2° are recognized as (111), (200), and (220) planes, respectively. This is a typical pattern of fcc lattice, in agreement with previous results.26 A slightly enlarged peak together with a delicate shift can be attributed to smallsize effects and alloying of Ni with Pt. The composition was investigated by SEM-EDS spectrum shown in Figure 2b, where only Ni and Pt characteristic peaks were observed. It is reasonable to conclude that the as-synthesized products contain only Ni and Pt elements. The Ni/Pt ratio was estimated by ICP-OES to be Ni:Pt = 44:56, consistent with that from SEMEDS in Figure 2b. Moreover, the magnetic behavior of NiPt TONPs such as M-H loops and M-T curves were shown in Figure S1. The XPS spectra of Ni (Figure 2c) and Pt (Figure 2d) are calibrated by C 1s peak with the binding energy of 284.6 eV. The peaks located at 869.3 and 851.9 eV in Figure 2c correspond to Ni 2p1/2 and Ni 2p3/2, respectively, characteristic Ni 2p peaks. Compared with pure Ni, a small negative shift of 0.4 eV for Ni 2p peaks was observed, which could be attributed to the Ni and Pt alloying.27 Except for the main Ni 2p peaks, two peaks at higher binding energy were clearly shown, locating at 873.3 and 855.4 eV, respectively. According to XPS handbook, they can be assigned to oxidized Ni.28 Similarly, for Pt 4f spectra given in Figure 2d, the peaks located at 74.03 and 70.72 eV corresponds to Pt 4f5/2 and Pt 4f7/2, respectively. The peaks can be divided into two pairs of doublets: metallic Pt peaks (74.08 and 70.73 eV), and Pt2+ peaks (74.73 and 71.48 eV), respectively. The existence of Pt2+ peaks is an evidence for the binding of Pt and O.26,29 In addition, Ni-scarce surface and



RESULTS AND DISCUSSION The NPs were synthesized by one-pot reduction of Ni(acac)2 and Pt(acac)2 in the presence of PVP and DMF. First, the microstructure was characterized by TEM with typical images at different magnifications shown in Figure 1. Figure 1a clearly demonstrates that the shape of as-prepared products is truncated octahedron with a yield nearly 100%, which is further confirmed by Figure 1c. As shown in Figure 1a, c, the particle size shows a high uniformity on a large scale, and the average value is estimated to be about 7.5 nm (Figure 1a, inset). A well-defined crystal structure of a single TONP is characterized by high resolution TEM (HRTEM) image presented in Figure 1d, and the steps are marked with blue arrows at the edges. The detected d-spacings for the adjacent fringes correspond to the (111) and (200) planes of Pt facecentered cubic (fcc) structure. The lattice fringes coherently extend over the whole particle, indicating a single-crystal structure. To further confirm the crystallization, we show 10843

DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

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Figure 3. (a) Representative HAADF-STEM image of NiPt TONPs. (b) EDS line scans along the arrow in a. Corresponding EDS elemental mappings of (c) Ni and (d) Pt, respectively. The scale bar is 2 nm.

directions presented in Figure S2c, d. With the continuous deposition of Ni atoms, the Ni-rich phase at the concave (111) planes (Figure S2e) is formed, and the formation of the truncated octahedrons is completed. For PVP effects, two TEM images for samples without PVP and with 70 mg of PVP are shown in Figure S3. It is easy to find that the PVP effect is mainly beneficial to dispersivity of the NPs, in good agreement with previous works.32,33 Furthermore, from Figure 4, it clearly shows that the composition in the bulk can be changed by adding PVP. For example, Ni is only 26% for samples without PVP, but increased to 43.7% with 70 mg PVP, respectively. However, the

the composition were verified by XPS because it only detects the elemental signals from the top several nanometers of the samples.14,25,30 The Ni/Pt composition estimated from XPS spectra is Ni:Pt = 20:80. However, the average ratio of Ni:Pt in the bulk is nearly 44:56 obtained from EDS and ICP examinations, indicating a Ni-scarce surface (Figure 3). The Ni-scarce/Pt-rich surface was further characterized by high angle annular dark field (HAADF) STEM. Figure 3a showed a representative HAADF-STEM image of a single NiPt TONP, and the EDS line scan along this red dotted arrow was shown in Figure 3b. It clearly indicates that the Ni is mainly distributed in the center for this single NiPt TONPs. The Ni/ Pt distributions were demonstrated in Figure 3c, d, respectively. The Ni content on the surface was significantly lower than that in the center, in good agreement with XPS results. Moreover, there is no any isolated Ni or Pt on the EDS mappings, indicating the alloyed structure. The mechanism for the formation of NiPt TONPs and the corresponding TEM images were summarized in Figure S2. The similar mechanism of the DMF-(acac)− system but without PVP was discussed in previous reports.30,31 During the initial reaction stage, a faster reduction of Pt ions relative to Ni ions takes place, because the standard reduction potential of Ni2+/Ni (E0 = −0.25 eV vs SHE) is lower than that of the Pt2+/ Pt (E0 = 1.18 eV vs SHE).27,32 The DMF-(acac)− ligand has stronger adsorption on (111) planes to form Pt-rich cuboctahedrons shown in Figure S2b. After that, Pt-rich NPs rapidly grow into hexapods/concave octahedrons along

Figure 4. Surface and bulk composition of Pt atoms in NiPt NPs with different PVP masses. 10844

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Figure 5. Cyclic voltammograms (CV) measured of NiPt TONPs (red), NiPt NPs (blue), and commercial Pt/C (black) as electrocatalytic in (a) Arpurged 0.5 M H2SO4 solution and (b) 1 M KOH solution with 1 M C2H6O2. (c) Corresponding mass and specific activities calculated from the peak currents in the positive scans. (d) Chronoamperometric curves of three catalysts at 0.08 V in 1 M KOH solution with 1 M C2H6O2.

associated with the complete EGOR and with the removal of incompletely oxidized carbonaceous species, respectively.8 It is amazing to find that the value of highest forward current density for NiPt TONPs reaches as high as 129.8 mA/cm2, whereas it is only 53.7 and 6.9 mA/cm2 for NiPt NPs and commercial Pt/C. The value for NiPt TONPs is 2.4 and 18.6 times larger than that of NiPt NPs and commercial Pt/C, respectively. The peak current of different catalysts was normalized by Pt weight and ECSA value to calculate the mass and specific activities, respectively. The values were shown in Figure 5c and Table S1. The mass activity of NiPt TONPs, NiPt NPs and commercial Pt/C is 9.1, 3.4, and 0.4 A/mgPt, respectively. Therefore, the mass activity is significantly enhanced for NiPt TONPs, about 2.7 and 23.2 times higher than that of NiPt NPs and commercial Pt/C. Similarly, NiPt TONPs exhibits a greatly increased specific activity of 10.3 mA/cm2pt, which is 1.9 and 17.6 times higher than that of NiPt NPs (5.4 mA/cm2pt) and commercial Pt/C (0.6 mA/cm2pt), respectively. It is well-known that the catalyst stability is considerably critical for its industrial application. In order to have a further evaluation about the stability, chronoamperometry test was recorded for 200 s at a fixed potential of 0.08 V in 1 M KOH and 1 M C2H6O2. For above three samples. The results are presented in Figure 5d, shown by red, blue, and black lines for NiPt TONPs, NiPt NPs, and commercial Pt/C, respectively. For the whole test duration, the oxidation current density of NiPt TONPs is always higher than that of NiPt NPs and commercial Pt/C. Apparently, the initial current density for three samples shows a rapid decay with time due to the formation of intermediate species during oxidation reaction. However, the decay for NiPt TONPs shown by red line is much slower in comparison with NiPt NPs and commercial Pt/ C. For example, when the test time is 100 s, only 28.4% of initial current density is kept for commercial Pt/C, but 51.6%

composition in the bulk is not changed with further increasing PVP. It is of great interest to find that the surface composition does not show any obvious variation with and without PVP, shown in Figure 4. The effect of PVP as a surfactant on the dispersivity is much clearer, but on the composition is not identified completely. The electrocatalytic activities for three samples were tested, namely NiPt TONPs, NiPt NPs synthesized without PVP under the same experimental conditions, and commercial Pt/C. The electrochemical surface areas (ECSA) is not only associated with the number of active sites available, but also includes access of a conductive path to transfer the electrons to and from the electrode surface.8 Hydrogen adsorption/ desorption peaks are typically used to evaluate the ECSA of catalysts. Figure 5a shows cyclic voltammograms (CV) of NiPt TONPs, NiPt NPs, and commercial Pt/C in Ar-saturated 0.5 M H2SO4 at a rate of 50 mV/s. In the potential region of from −0.25 to 1 V versus SCE, typical hydrogen adsorption and desorption peaks from Pt were observed. It was estimated to be 88.1 m2/gPt on the NiPt TONPs, much larger than that of commercial Pt/C with a value of 66.7 m2/gPt and the value of 61.7 m2/gPt for NiPt NPs. On the basis of the above results, it is reasonable to conclude that the electrocatalytic performance for NiPt TONPs is greatly improved, originating from the unique structures and good dispersion. The performance of above three samples in the EGOR were further investigated in 1 M KOH and 1 M C2H6O2 between −0.8 and 0.2 V at a sweep rate of 50 mV/s. The results were presented in Figure 5b. The characteristic behavior of EGOR is shown by the well separated anodic peaks in the positive and negative scans. As shown in Figure 5b, for the positive scan, the current increases first with increasing potential and then decreases with further increasing, resulting in a peak at about −0.08 V. Meanwhile, the other peak at −0.25 V for the negative sweep was also found. The positive and negative peak is 10845

DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

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Figure 6. (a) ORR polarization curves in O2-saturated 0.1 M KOH with a rotation rate of 1600 rpm and a sweep rate of 10 mV/s. (b) Corresponding mass and specific activities at 0.9 V versus RHE.

Figure 7. Polarization curves of ORR on (a) NiPt TONPs, (b) NiPt NPs, and (c) commercial Pt/C at different rotation rates (top to bottom: 225, 400, 625, 900, 1225, 1600, and 2025 rpm). (d) Corresponding K−L plots at 0.7 V.

TONPs have a higher positive onset potential, compared with that of the NiPt NPs and commercial Pt/C. Furthermore, the half-wave potential of NiPt TONPs is about 921 mV, a slight enhancement with 25 mV and 15 mV compared with that of NiPt NPs and commercial Pt/C, respectively. It is well-known that the LSV curve can be divided into two parts, that is, diffusion-limit and mixed kinetic-diffusion.19,34 For our results shown in Figure 6a, the diffusion-limit region is below 0.6 V and mixed kinetic-diffusion takes place between 0.6 and 1.0 V, respectively. The diffusion-limit current density (jd) can be experimentally obtained from polarization curves, and the kinetic current density (jk) is calculated according to the Koutecky−Levich (K−L) equation34

for NiPt TONPs. Accelerated durability test (ADT) was performed to examine the durability of NiPt TONPs. After 1000 cycles, the variation of ECSA for three catalysts was summarized in Figure S5. The value of ECSA for NiPt TONPs can maintain 72.8% of initial one (Figure S5a) after ADT, whereas only 69.8 and 40.7% for NiPt NPs (Figure S5b) and commercial Pt/C (Figure S5(c)) are kept, respectively. The results for stability together with CV measurements confirm that the electrocatalytic activity and stability of NiPt TONPs are greatly enhanced compared with NiPt NPs and commercial Pt/C samples. The improved catalytic behavior for the EGOR is mainly attributed to additional active sites due to Pt-rich surface for NiPt TONPs. Electrocatalytic ORR performance of NiPt TONPs, NiPt NPs and commercial Pt/C was investigated by linear sweep voltammetry (LSV) measurements in O2-saturated 0.1 M KOH solution with a sweep rate of 10 mV/s and a rotation rate of 1600 rpm by RDE, respectively. The ORR polarization curves were shown in Figure 6a. The results show that the NiPt

1 1 1 = + j jk jd

(1)

In general, the value of jk at 0.9 V is used to compare the ORR activities of the catalysts. Based on this model, the mass and 10846

DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

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Pt/C. For ORR, the mass activity of NiPt TONPs is 1.8 times larger than that of commercial Pt/C. The mechanism of enhanced electrocatalytic activities mainly includes the following factors: (i) Enlarged ECSA comes from more active sites due to Pt-rich surface together with some steps at the edge of TONPs shown in Figure 1(d). (ii) The synergistic effect between Ni and Pt atoms, where Ni can adjust the electronic structures of Pt or it acts as electron donation to Pt in NiPt alloy, resulting in a decrease of Pt 4f binding energy. This decrease can be obviously observed from XPS results shown in Figure 2(d), which consequently reduces the bond strength of Pt-intermediate species.27 (iii) The role of PVP mainly acts as a dispersing agent for the products to prevent the agglomeration, leading to enlarged surface areas.33 Furthermore, the ultrafine size down to 7.5 nm can enlarge surface areas, playing a positive effect on electron transfer during eletrocatalytic action. The above reasons should be well-balanced in order to enhance the eletrocatalytic performance for NPs. Our work could shed some light on the optimization of synthesis methods to produce PEMFC in the near future.

specific activities of three catalysts for ORR were summarized in Figure 6b and Table S1, which were normalized by ECSA, Ptloading weight, and the K−L equation. The value of mass activity is shown by te left scale. For NiPt TONPs it reaches up to 0.44 A/mgPt, 1.6 and 1.8 times larger than that of NiPt NPs (0.27 A/mg Pt ) and commercial Pt/C (0.25 A/mg Pt ), respectively. The value of the specific activity is indicated by the right scale in Figure 6b. For NiPt TONPs, its value is 0.51 mA/cm2pt, much higher than that of NiPt NPs and commercial Pt/C (0.43 and 0.38 mA/cm2pt, respectively). On the basis of the data in Figure 6, the values of mass and specific activities are found to be greatly increased for NiPt TONPS. The improved ORR electrocatalysis for NiPt TONPs can be mainly attributed to the inhibition of Pt−OHad formation via adjusting the electronic structures of Pt with Ni and their unique truncated octahedral structures.21 More detailed investigation on the kinetic behavior for ORR were performed on NiPt TONPs, NiPt NPs and commercial Pt/C catalysts modified electrodes by increasing the rotation speed from 225 to 2025 rpm. The results were presented in Figure 7a−c. With increasing rotation rate, the value of jd increases for three samples. To have a better understanding, the K−L equation can be described as19,35 1 1 1 = + j jk Bω1/2



CONCLUSIONS In summary, small and uniformly dispersed NiPt TONPs were successfully synthesized by a one-pot and facile method. The as-prepared NiPt TONPs exhibit the enhanced catalytic activities together with a highly better stability for EGOR and ORR in alkaline electrolyte, compared with NiPt NPs and commercial Pt/C. The remarkable performance originates from a Pt-rich surface with ultrafine particles (7.5 nm) and uniform dispersion. This work might provide a great potential candidate for both cathode and anode catalysts in fuel cells.

(2)

Where ω is the electrode rotating rate. The parameter B can be written as following19,35 B = 0.2nF(DO2)2/3 ν−1/6CO2

(3)



Where n represents the number of electrons transferred per O2, F is the Faraday constant (96485 C/mol), DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10−5 cm2/s), v is the kinetic viscosity (0.01 cm 2 /s), and C O 2 is the bulk concentration of O2 (1.2 × 10−6 mol/cm3), respectively. The constant 0.2 is adopted when the rotation speed is expressed in rpm. Experimentally, the value of B can be obtained from the slope of the K−L plots (j−1 vs ω−1/2), which was shown in Figure 7d for three samples. Therefore, the transferred electron numbers per O2 (n) can be calculated by eq 3 with the experimental value of B. As illustrated in Figure 7d, a good linear relationship between j−1 and ω−1/2 were observed at 0.7 V for three samples, which gives the value of B. Here, the value of n is 4.09, 3.91, and 4.04 for NiPt TONPs, NiPt NPs, and commercial Pt/C, respectively. Therefore, based on above discussion, it is reasonable to conclude that the efficient reduction of O2 to H2O for samples investigated here takes place by the four-electron reaction. As pointed out in the introduction, it is much easier for electrocatalysts in nanometer size to be poisoned by chemisorbed CO-like intermediates in acid media. Therefore, our focus is given to their behavior in alkaline electrolytes, where the key issue can be greatly resolved. The electrocatalytic performance, such as EGOR and ORR, for the ultrafine and uniformly dispersed NiPt TONPs were tested in alkaline electrolytes. The analysis of both CV of EGOR and LSV of ORR clearly indicate that the as-prepared NiPt TONPs exhibit considerably enhanced catalytic activities compared with NiPt NPs and commercial Pt/C. For example, the value of highest forward current density for NiPt TONPs reaches up to 129.8 mA/cm2 for EGOR, 18.6 times higher than that of commercial

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01115. Formula to calculate ECSA and a simple explanation, M−H loops and M−T curves of NiPt TONPs, schematic illustration for the formation of NiPt TONPs, TEM images of NiPt TONPs and NiPt NPs under the same method without PVP, CV curves of NiPt TONPs, NiPt NPs and commercial Pt/C in 1 M KOH+1 M C2H6O2, CV curves of NiPt TONPs, NiPt NPs, and commercial Pt/C before and after ADT in Ar-saturated 0.5 M H2SO4, mass and specific activities of different electrocatalytics in EGOR and ORR (PDF)



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the National Basic Research program of China (2015CB921401), the National Instrument Program of China (2012YQ120048), the Natural Science Foundation of China (51371015, 51331002, 51431009, 51471183, and 11274371), the Instrument Development Program of Chinese Academy of Sciences (YZ201345), and 10847

DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849

Research Article

ACS Applied Materials & Interfaces (35) Wang, S.; Yu, D.; Dai, L.; Chang, D. W.; Baek, J.-B. Polyelectrolyte-Functionalized Graphene as Metal-Free Electrocatalysts for Oxygen Reduction. ACS Nano 2011, 5, 6202−6209.

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DOI: 10.1021/acsami.6b01115 ACS Appl. Mater. Interfaces 2016, 8, 10841−10849