CoreâShell Nanostructured Au@NimPt2 Electrocatalysts with

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Core-shell Nanostructured Au@NimPt2 Electrocatalysts with Enhanced Activity and Durability for Oxygen Reduction Reaction Liu-Liu Shen, Gui-Rong Zhang, Shu Miao, Jingyue (Jimmy) Liu, and Bo-Qing Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02124 • Publication Date (Web): 26 Jan 2016 Downloaded from http://pubs.acs.org on January 27, 2016

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Core-shell Nanostructured Au@NimPt2 Electrocatalysts with Enhanced Activity and Durability for Oxygen Reduction Reaction Liu-Liu Shen1, Gui-Rong Zhang1, Shu Miao2, Jingyue (Jimmy) Liu2,3, and Bo-Qing Xu1,* 1

Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China.

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State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.

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Department of Physics, Arizona State University, Tempe, Arizona 85287, United States.

* Corresponding author: Prof. Bo-Qing Xu Department of Chemistry Tsinghua University Beijing, 100084, China Tel.: +86 10 6279 2122 Fax: +86 10 6277 1149 E-mail: [email protected]

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ABSTRACT: Fabricating Pt-alloy and core-shell nanostructures with Au NPs in the cores are considered as two general approaches to improve performance of Pt-based catalysts for cathodic oxygen reduction reaction (ORR) in acidic electrolyte. These two approaches are combined herein to develop a hetero-seed-mediated solvothermal method for synthesizing nearly monodisperse core-shell structured Au@NimPt2 nanoparticles (NPs) of 5.0−6.5 nm (with the atomic ratio of Ni/Pt/Au = m/2/1) as ORR catalysts. On controlling the amount and relative concentrations of the metal precursors in the starting solution, this method entitles not only facile manipulation of the shell composition and thickness, but also fine-tuning the core-shell interaction and surface electronic structures of the resultant Au@NimPt2 NPs, endowing the Au@NimPt2 NPs with improved Pt activity and durability for ORR. Subjecting the Au@NimPt2 NPs to a pretreatment in flowing 2%CO/Ar at 300 °C is shown to result in further improved Pt activity. Data are also presented to correlate the intrinsic Pt activity with experimentally determined CO adsorption property (CO-stripping peak potential) of Pt for the Au@NimPt2 samples, and to show the excellent electrochemical durability of the Au@NimPt2 NPs during 20,000 potential cycles between 0.6 and 1.1 V (vs. RHE) in O2-saturated 0.1 M HClO4. Compared to commercial E-TEK Pt/C catalyst, the most active Au@Ni2Pt2 NPs exhibit 3−4 and yet 4−6 folds higher Pt activity at 0.9 V before and after the 20,000 potential cycles, respectively. Factors relevant to the activity and durability control of the Au@NimPt2 catalysts for ORR are discussed. KEYWORDS: electrocatalyst, multimetallic nanoparticles, solvothermal synthesis, core-shell nanostructure, oxygen reduction reaction, catalyst design, electrochemical treatment

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1.

INTRODUCTION Growing concerns on the depletion of fossil fuel resources and their associated

environmental issues have made polymer electrolyte membrane fuel cells (PEMFCs) the demanding technology of the future, as they can operate with sustainably produced fuels with high efficiency and low emissions.1,2 Electrocatalysts are key components of PEMFCs and necessity to facilitate both reactions of anodic fuel oxidation and cathodic oxygen reduction reaction (ORR).3,4 Suitably dispersed platinum on carbon (Pt/C) has been considered as the most effective PEMFC catalyst and commercially available on large-scale, but high cost and low durability issues have to be overcome before broad market penetration.5 Over the past decade, intensive research efforts have been directed to improve the catalytic activity and durability of Pt catalysts by combining Pt with other metal(s) to form bi- or multi-metallic nanostructures.6−9 The most inspiring finding in this direction was made by Stamenkovic et al.6 who identified that the Pt-enriched surface derived from a well-defined extended crystalline Pt3Ni(111) surface was capable to offer for the cathodic ORR an intrinsic activity that exceeded those of the well-defined Pt(111) surface and commercial Pt/C catalyst by more than 10 and 90 folds, respectively. This surprisingly high activity of the Pt3Ni(111)-derived Pt surface was attributed to the weak interaction between the surface Pt atoms and unreactive oxygenated species, due to altered electronic properties of the topmost Pt atoms (Pt-skin) induced by the underneath Pt3Ni alloy.6 Attempts were also made later in different laboratories to imitate the Pt3Ni(111)-derived Pt surface in practically-relevant nanoscale electrocatalysts, by fabricating Pt3M (M = Ni, Co) nanooctahedra10 and truncated octahedral,11 icosahedra,12 and nanoframes.13 Investigations on the composition and size effects in alloyed PtxNi1-x NP catalysts were also conducted.14−16 Intrinsically most active catalyst was obtained from PtNi NPs in the work of Stamenkovic et al.14 but the work by Strasser et al.15,16 identified instead 3−10 nm Pt3Ni particles to be the most active in terms of Pt intrinsic and mass activity,15 and stability.16 Besides, methods of the catalyst synthesis, thermal pretreatment and dealloying were also among the keys to the performance of Pt-alloy catalyst.14,16−18,17,18 On the other hand, it was discovered by Adzic et al.19 that small Au clusters deposited on 3


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catalytic Pt nanoparticles (NPs) would enhance the electrochemical stability during ORR of the Pt NPs. Presence of Au or Pd in various Pt-based multimetallic nanostructures was also found effective for improving the stability of Pt for ORR catalysis.20-24 Studies pioneering a combination of well-defined model surfaces and nanomaterial synthesis by Stamenkovic et al. further confirm the key role of subsurface Au-Pt (Au-Pt3Fe22 and Au-Pt3Ni/Au-PtNi25) interaction for significantly improving the electrochemical stability of topmost Pt atoms during ORR in nanostructured core@shell Au/FePt3 (Au@FePt3),22 Au@PtNi25 and Ni@Au@PtNi25 catalysts. These high-performance Au@FePt3 and Ni@Au@PtNi catalysts, the former was obtained by fabricating around 7 nm Au core particles with a 1.5 nm thick FePt3 shell and the latter by entrapping 3.1 nm Ni core with an Au interlayer and then with a PtNi shell of ca. 1 nm, showed much higher activity (3~8 folds) at 60 °C than similarly prepared reference Pt NP catalysts (5 nm),22,25 well combined the subsurface Pt3M-alloy (M = Fe, Ni) effect on enhancing the activity and the Au-Pt (Au-Pt3Fe or Au-PtNi) interaction effect on improving the electrochemical durability of the topmost Pt atoms. Similar effects were also witnessed by entrapping 5 nm Au NPs with a 1.0−1.5 nm thick shell of PtCu alloy to make trimetallic Au@CuPt catalyst.23 In these studies, however, the composition of the Pt-alloy shell entrapping the Au NPs (core) was kept either unchanged22,25 or little changed23. Besides, the Pt utilization efficiency26 or electrochemically active surface area of Pt (35−50 m2⋅g-1Pt)22-25 appeared significantly lower than conventional Pt/C (e.g., Pt/C E-TEK: 65−75 m2⋅g-1Pt). As the property including catalysis of the Pt-alloy and thus Au-Pt or Au-(Pt-alloy) interaction would vary according to the alloy composition, investigation on the composition effect14−16, on the performance of Au@Pt-alloy catalyst could help to optimize the Au-Pt and/or Au-(Pt-alloy) interaction and fine tune the electronic structure of the topmost Pt atoms and maximize the Pt utilization efficiency for ORR catalysis. Recent studies have indeed uncovered a key role of Pt-alloy composition to the electrocatalytic performance of bimetallic PtNi8,15,27 and trimetallic Au-containing PtAuNi,28 PtAuCo29 and PtIrCo29 NPs. By using an innovative hetero-seed-mediated solvothermal method to encapsulate 3.5 ± 0.4 nm Au NPs with a PtNi-alloy shell (ca. 1−1.5 nm thick), we have constructed in this work a series of nearly monodisperse core-shell structured Au@NimPt2 NPs with varied shell composition (as denoted by the atomic ratio Ni/Pt/Au = m/2/1) and investigated their 4


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electrocatalytic performance for ORR, attempting to integrate the alloy composition effect with the benefits of the Au core and PtNi-alloy shell. Our data feature that both the Au core and Ni are beneficial for enhancing the utilization efficiency of Pt. Varying the composition or Ni/Pt ratio in the shell layer appears effective in tuning the electronic property and intrinsic activity (IA) for ORR of the topmost Pt atoms (Pt-skin) on the Au@NimPt2 NPs, uncovering a volcano-shaped correlation of the IA with the shell Ni/Pt ratio. It is further shown that proper thermal treatment of these Au@NimPt2 NPs in CO-containing argon can significantly improve their catalytic performance. The best performing catalyst (Au@Ni2Pt2/C) features a long-term activity (after 20,000 electrochemical cycles between 0.6−1.1 V in O2-saturated 0.1 M HClO4) that is 4−5.8 folds higher than conventional Pt/C. These data may have broad implications for constructing various high-performance multimetallic nanostructures and for better utilizing precious metals in a plethora of catalytic systems. 2. EXPERIMENTAL SECTION 2.1. Materials. HAuCl43H2O (Acros), NaBH4 (Acros), K2PtCl6 (Alfa Aesar), Pt(acac)2 (Alfa Aesar), Ni(acac)2 (Alfa Aesar), polyvinylpyrrolidone (PVP, Mw=10000, Sigma Aldrich), benzyl alcohol (J&K), aniline (Alfa Aesar), ascorbic acid (Beijing modern original fine company), Nafion solution (5 wt. %, Alfa Aesar) and HClO4 (70%, Aldrich) were all analytical grade reagents and used as received. 2.2. Syntheses of Au and Pt0.13Âu NPs. Au NPs with average size around 3.5 nm were synthesized by adopting the seed-mediated growth method detailed in our previous work.3,30 A typical procedure consisted of the following steps: 555 mg PVP was added into 50 mL aqueous HAuCl4 solution (1.0 mM). An ice-cooled solution of NaBH4 was then quickly injected into the solution to enable formation of the primary Au seeds (1.9 ± 0.4 nm), followed by a seed-mediated growth procedure to synthesize Au NPs of 3.5 ± 0.4 nm.3,30 As will be seen later, we found after many trials and errors that the “pure” Au NPs had to be loaded with sub-nanometer Pt islands up to the levels of Pt/Au (atomic) ≥ 0.10 before 5


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they could be encapsulated with a full and uniform NiPt shell. This preloading of Pt was easily accomplished by reductive Pt deposition from PtCl62- on the Au NPs (colloids),3 which converted the Au NPs into their Pt-carrying counterpart particles (PtnÂu,26,31 n = Pt/Au ≥ 0.1). Specifically, to 20 mL of the as-prepared Au NP solution (1 mM) were added dropwise 0.5−2.0 ml aqueous K2PtCl6 (1.3 mM) and 3 ml aqueous asorbic acid (20 mM) under vigorous stirring. After stirring the mixture at 40 oC for 12 h, the color of the solution gradually changed from wine-red to dark-brown, indicating the formation of PtnÂu NPs. 2.3. Synthesis and carbon-immobilization of Au@NimPt NPs. A hetero-seed mediated solvothermal approach was developed to synthesize the Au@NimPt2 samples. PtnÂu NPs prepared as above were used as the hetero seeds, and the benzyl alcohol based solvothermal method disclosed by Li et al. for the synthesis of truncated octahedral PtNi nanocystals32 was applied to coat the PtnÂu NPs (hetero seeds) with a full and uniform NiPt-alloy shell to obtain the core-shell structured Au@NimPt2 samples. Specifically, to a mixture of 62 mL benzyl alcohol and 1.28 mL aniline were added the desired amounts of Pt(acac)2, Ni(acac)2 and PVP, followed by vigorous stirring at room temperature for 30 min. Then, to this mixture solution was added under stirring an enriched solution of the hetero seeds Pt0.13Âu NPs (21.3 mM Au, 0.6 mL). The final mixture was transferred into 100 mL Teflon-lined stainless-steel autoclave for solvothermal treatment at 150 oC for 12 h to produce the core-shell structured Au@NimPt2 NPs. Carbon-supported Au@NimPt2 samples (i.e., Au@NimPt2/C) were prepared by immobilization of Au@NimPt2 with a carbon black support (Vulcan XC-72).3 This was done by mixing the desired amounts of the carbon black with the colloidal solution of Au@NimPt2 for 10 min under vigorous stirring, followed by sonicating for 2 h. The solid was then separated by filtration, intensively washing with acetone/ethanol, and air-drying at 80 oC for 2 h to give Au@NimPt2/C. 2.4. Structural Analysis. Transmission electron microscopy (TEM) images were captured using a JEOL JEM-2010 microscope operated at 120 kV. High angle annular dark field scanning

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transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopic (EDS) elemental mapping measurements were performed on a state-of-the-art JEOL JEM-ARM200F microscope operated at 200 kV equipped with a CEOS probe corrector at a guaranteed resolution of 0.08 nm and an EDS X-ray detector. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI 5300 ESCA1610 SAM instrument using Mg Kα radiation (1253.6 eV). The binding energies (BE) were calibrated using the adventitious C 1s line at 284.8 eV. The overall metal loadings and composition by Pt, Ni, and Au atoms in the Au@NimPt2/C samples were determined by using inductively coupled plasma atomic emission spectrometry (ICP-AES) on an IRIS Intrepid II XSP (ThermoFisher) instrument. 2.5. Electrochemical measurements. Electrochemical measurements were performed on a CHI750E potentiostat using a conventional three electrode cell in 0.1 M HClO4 solution at 25 oC. A Pt wire (diameter: 0.5 mm) and an Hg/Hg2SO4 electrode (0.5 M H2SO4, -0.69 V vs. RHE) were used as the counter and reference electrode, respectively. To prevent a possible SO42- cross-contaminating of the electrolyte in the working cell, the reference electrode was placed in a separate compartment. All the potentials reported here are given with respect to reversible hydrogen electrode (RHE). Prior to the preparation of the working electrode, glass carbon rotating disk electrode (GC-RDE, 0.196 cm2, Pine Research Instrument) was polished to a mirror finish using 0.05 µm alumina powder. To prepare the working electrode, a catalyst ink was prepared by sonicating a catalyst suspension in a solution containing 80 vol% water, 15 vol% isopropanol and 5 vol% nafion solution (5 wt.%). A calibrated amount of the suspension was carefully deposited onto the pre-wetted GC-RDE and then slowly dried in air. The Pt loading on the working electrode normalized to the geometric area of GC-RDE was 20 µg cm-2. Cyclic voltammetry (CV) was carried out from 0.05 to 1.55 V with a scanning rate of 50 mV s-1 in N2-saturated 0.1 M HClO4 solution. Measurements of the catalysts for ORR were conducted in O2-saturated 0.1 M HClO4 solution using GC-RDE at a rotation rate of 1600 rpm. The ORR polarization curves were collected with a scanning rate of 10 mV s-1 from 0.05 to 1.2 V. For CO-stripping measurements, the catalysts surface was first saturated with 7


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high purity CO by bubbling CO (99.9%) into the electrolyte at open circuit potential for 25 min. The remained CO in the electrolyte was then removed by flowing high purity N2 for 15 min. The CO-stripping curves were recorded with a scanning rate of 50 mV s-1 from 0.05 to 1.55 V. Durability test was carried out up to 20,000 repeated potential cycles between 0.6 and 1.1 V in O2-saturated 0.1 M HClO4 solution at 25 oC. 3.

RESULTS AND DISCUSSION

3.1. Syntheses and characterization of Au@NimPt2 structures Nearly monodisperse Au NPs (3.5 ± 0.4 nm, Figure S2) can be facilely synthesized according to our previously developed seed-mediated method.3,30 However, attempts to directly coat the Au NPs with a PtNi shell for constructing core-shell nanostructured Au@NimPt2 particles always ended up with the observations of prevailing self-nucleation of Pt and Ni, leading to mixtures of separated Au, Pt and Ni NPs. The afore mentioned solvothermal method, documented for the synthesis of truncated octahedral PtxNi1-x nanocystals,2732 was then tested to obtain the desired core-shell Au@NimPt2 samples. This was done by adding, before the solvothermal treatment at 150°C, a certain amount of Au core particles into the benzyl alcohol solution of Pt(acac)2) and Ni(acac)2.2732 This kind of solvothermal treatment could avoid the self-nucleation of Pt and Ni, but the product Au@NimPt2 particles were produced as severely aggregated NPs and always showed broad size distribution (Figure S3), probably due to the poor thermal stability of the Au NPs. Motivated by our previous discovery that the stability of small Au NPs could be substantially improved by deposition of a small amount of sub-nanometer Pt islands,3,26 we converted the Au NP seeds to their Pt-carrying hetero seeds (i.e., PtÂu), which proven to be crucial for the formation of nearly monodisperse (uniform) core-shell nanostructured Au@NimPt2 particles. To feature this key role of the hetero-seed PtÂu particles, the present synthesis of uniform core-shell Au@NimPt2 particles is referred to as a hetero-seed-mediated solvothermal approach for core-shell multimetallic nanostructures, as depicted Scheme 1.

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Scheme 1. Schematic hetero-seed-mediated solvothermal synthesis of uniform core-shell Au@NimPt2 nanoparticles.

Interestingly, we found that Au@NimPt2 structures can be fabricated as long as the atomic Pt/Au ratio (n) in the PtnÂu seeds was no lower than 0.10. Determination of the suitable n for the PtnÂu seeds can be found in Supporting Information. It was documented earlier that the small amount of Pt (Pt/Au = 0.05) deposited on Au nanorods (average length 62 nm, average width 12 nm) could catalyze the reduction of Ni2+ to form core-shell like Au@Ni nanorods.33 Although the pre-loaded Pt was on the tips of the Au nanorods, the coating of Ni led to formation of a uniform shell on the Au nanorods.33 Analogously, the pre-deposited Pt on the Au NPs in the present work may not just improve the thermal stability of the underlying Au, but may also catalyze the later reduction of Pt and Ni precursors. Nevertheless, a higher Pt/Au ratio is needed herein, probably because of the much higher reactivity or higher surface areas of the present small Au NP seeds (3.5 nm). The catalytic reduction may stem from the relatively small curvature radius of pre-deposited Pt clusters, making themselves preferential nucleation sites for the simultaneous deposition of Pt and Ni during the solvothermal processing.34 It should be noted that Ni played an important role in maintaining the spherical shape and uniform sizes of the final core-shell structured Au@NimPt2 NPs. In the absence of Ni, Pt atoms had a strong tendency to nucleate directly at the pre-deposited Pt entities, instead of the Au surface, according the Volmer–Weber mechanism,2 which finally led to particles with irregular shapes (Figure S4).

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Figure 1. TEM images of carbon-supported Au@NimPt2 samples: (a) [email protected], (b) [email protected], (c) Au@Ni1Pt2, (d) Au@Ni2Pt2, (e) Au@Ni4Pt2, and (f) Au@Ni6Pt2.

Figure 1 a−f shows the representative TEM images and their corresponding size distribution of the as-prepared carbon-supported Au@NimPt2 samples. These images demonstrate that nearly monodisperse NPs with approximately spherical shape were uniformly distributed on the carbon support without agglomeration. The statistical analyses (more than 200 particles) reveal that the average sizes of the Au@NimPt2 NPs increased slightly with the increase in m, from 5.0 nm at m = 0.2 to 6.5 nm at m = 6. More TEM images of higher magnifications for the carbon-supported Au@NimPt2 samples are given in Figure S6.

Figure 2. Electron microscopic analysis of the as-prepared Au@Ni2Pt2 NPs. (a-1, a-2) HAADF-STEM 10


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images of Au@Ni2Pt2 particles. (b-e) EDS elemental maps for (b) Au, (c) Ni, (d) Pt and (e) the RGB composite map of Ni, Au and Pt.

To unambiguously gain spatially resolved compositional information of these as-prepared Au@NimPt2 structures, we employed HAADF-STEM in conjunction with STEM-EDX analysis to determine the elemental distribution of these trimetallic nanostructures. Figure 2a-1 shows the HAADF-STEM image of a representative single Au@Ni2Pt2 particle. The image features a bright core with a lattice spacing of 0.23 nm and a less bright shell with a lattice spacing of 0.21 nm, which are close to the {111} lattice spacing of Au and the alloyed PtNi crystals, respectively. The STEM-EDX elemental mapping of the Au@Ni2Pt2 sample was also collected for the area of Figure 2a-2 to better probe the distribution of each elemental in the NPs. Presented in Figure 2 b−e are the elemental maps of the Au, Ni, Pt and their RGB composite image, respectively, which clearly confirm the core-shell nature of the nanostructured Au@Ni2Pt2 particles; with Au NP in the core and PtNi alloy in the shell layer. These results confirm that the present hetero-seed-mediated solvothermal method is highly efficient for constructing the desirable small (< 10 nm) core-shell nanostructured Au@NimPt2 NPs.

Figure 3. XPS spectra for the as-prepared Au@NimPt2/C samples. The dashed vertical lines mark, respectively, the binding energies for the reference Pt0, Au0, Ni0 and Ni2+.

The surface composition and electronic properties of the Au@NimPt2/C samples were probed by X-ray photoelectron spectroscopy (XPS). Figure 3 displays the XPS signals for Pt 4f, Au 4f and Ni 2p, respectively, where the dashed vertical lines mark the binding energies 11


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(BEs) for their corresponding references. For all of the Au@NimPt2/C samples, the BEs of both Pt and Au shifted positively by ca. 0.3 eV relative to pure metal references. The Ni 2p3/2 signals featured two peaks centering at 853.0 and 856.1 eV, which can be assigned to Ni0 and Ni2+, respectively, and were also positively shifted relative to the references. The “unreduced” Ni2+ species would probably be arisen from air oxidation of surface Ni atoms due to air exposure after the sample preparation. With the increase of the Ni content, the particle surfaces were gradually enriched with Ni species, as evidenced by the surface atomic ratio of Ni/Pt/Au of 0.01:2.4:1 for [email protected], 5.3:2.7:1 for Au@Ni2Pt2, and 12.5:3:1 for Au@Ni6Pt2. And, at the same time, the attenuated Au signals demonstrate that the PtNi shell encapsulating the Au cores gradually thickened on increasing Ni content. These results corroborate the conclusion from the STEM-EDS results that the Au@NimPt2 NPs were formed as core-shell nanostructures, with Au in the core and alloyed PtNi in the shell. To be mentioned, the XPS results showed no signature for any oxidized Pt species in all the Au@NimPt2 samples, even [email protected] that had the lowest Ni content. This fact suggests that the presence of even a low concentration of Ni (Ni/Pt = 0.1) could prevent the co-existing surface Pt from air-oxidation due to the larger oxygen affinity of Ni as compared to Pt.17

Figure 4. Voltammetric characteristics during the electrochemical pretreatment (dealloying) of the as-prepared Au@Ni2Pt2/C catalyst by potential cycling between 0.05 V to 1.55 V with a scanning rate of 50 mV s-1 in N2-saturated 0.1 M HClO4. The inset presents a schematic showing of the enrichment of Pt atoms at the outermost layers after the electrochemical pretreatment.

Every catalyst was subject to an electrochemical pretreatment by repeated potential 12


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cycling between 0.05 V to 1.55 V with a scanning rate of 50 mV s-1 in N2-saturated 0.1 M HClO4, prior to measuring any electrochemical property of the catalyst. The repetition was done for at least up to the 10th cycle, when the voltammograms became stabilized. Figure 4 presents the representative CV curves for the Au@Ni4Pt2/C catalyst in such electrochemical pretreatment process, during which the change in voltammetric characteristics due to electrochemical dealloying was seen to be complete after the 8th cycle. The curve for the 1st cycle features clearly suppressed H desorption signals (0.05−0.4 V), being consistent with a surface enriched with Ni (or Ni oxide) of the as-prepared catalyst. Sweeping anodically, the oxidation peak associated with the dissolution of Ni species (e.g., NiO18) appeared at ca. 1.0 V and stretched up to 1.55 V. With increasing the cycling number, this oxidative dissolution peak of Ni gradually declined, while the H desorption signals became more pronounced, indicating that the dissolution of Ni from the alloy layer leads to more exposure of Pt. The signals associated with Ni dissolution disappeared while those with the H desorption remained invariant after ca. 8−9 cycles, featuring a selective dissolution of Ni to attain stable “pure” Pt surface (Pt-skin) during this repeated potential cycling process. The dissolution of Ni has been widely reported in literature during the electrochemical pretreatment in acidic electrolyte.1,11,14,15,35 Note that all the CV curves show no signature for any exposed Au atoms, which should give a reduction peak at ca. 1.15 V22 during this repeated potential cycling. It is proposed that the selective Ni dissolution would also lead to surface roughening and restructuring, enriching Pt atoms at the outermost layers of the PtNi-alloy shell, as illustrated schematically in the inset of Figure 4.

Figure 5. (a) HAADF-STEM image of the electrochemically pretreated (dealloyed) Au@Ni2Pt2 particles. 13


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(b) Nanoprobe EDS analyses conducted, respectively, on areas 1, 2 and 3 marked in (a). STEM-EDX elemental maps of Au@Ni2Pt2 particles after the electrochemical (dealloy) pretreatment, showing the spatial distributions of (c) Au, (d) Ni, (e) Pt and (f) the RGB composite map of Ni, Au and Pt.

In order to better understand the surface structure evolution of the catalysts after they were subjected to the electrochemical pretreatment, HAADF-STEM and STEM-EDS nanoprobe analyses were conducted on the electrochemically pretreated samples. The dealloyed Au@NimPt2 NPs appeared as nonporous solid particles in the STEM images (Figure 5a, some representative high-resolution STEM images are shown in Figure S7), whose sizes and shapes changed little from their as-prepared counterparts (Figure 2). The dealloy treatment therefore exerted insignificant effects on the overall size, morphology and “solidity” of the Au@NimPt2 NPs though the selective dissolution of Ni could result in rough particle surfaces, as demonstrated earlier by Strasser et al. for their PtNi3 particles.16 This is probably due to that the dealloy treatment dissolved only a small fraction of Ni in the sample (measured by ICP-AES as 15% for Au@NimPt2) and the small sizes of the Au@NimPt2 NPs (3~10 nm), for which the surface diffusion rate of the noble Pt atoms could match the dissolution rate of the less noble Ni atoms.16 The composition and elemental distribution of the dealloyed NPs were assessed by conducing the spatially-resolved nanoprobe EDS measurements. As shown in Figure 5b, the EDS probe detected no Ni in the surface and near surface regions of the NPs, and it is conceivable that the surface of the dealloyed NPs would predominantly terminated by a rough Pt-skin, which would delay or prevent leaching of the underneath Ni. Figure 5c-e shows the EDS elemental mapping images of Au, Ni, and Pt, respectively. The adduct image (Figure 5f) also reveals that the outermost layers of the electrochemically (dealloyed) treated Au@Ni2Pt2 NPs were dominated with Pt atoms.

Figure 6. (a) HAADF-STEM image showing in the left an electrochemically pretreated (dealloyed) Au@Ni2Pt2 particle hanging in vacuum from the carbon support. (b) Nanoprobe EELS analysis conducted, respectively, on area 1, 2 and 3 marked in (a). 14


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As the EDX signals cannot avoid the interference due to electron scattering from elements in the unintended regions, it remains an argument that EDX would not be that reliable to probe the very local composition for NPs. Electron energy loss spectroscopy (EELS) was then employed to further probe the distribution of Ni in different regions of the dealloyed Au@Ni2Pt2 NPs. We were fortunate to find a few Au@Ni2Pt2 NPs that were hanging in vacuum from the edge of the carbon support for the EELS measurement (e.g., Figure 6); otherwise it was impossible to observe clear EELS peaks as the carbon support always showed a huge EELS peak that can overshadow any other small EELS peaks. As shown in Figure 6 (the raw EELS results are provided in Figure S8), the surface and near surface region (Position 3) of the dealloyed Au@Ni2Pt2 NP was clearly free of Ni. The distinct Ni EELS peaks in Position 2 confirm that the inner part of the shell remained as a PtNi alloy while the hardly visible Ni peaks in Position 1 could be caused by the lowering of the EELS signals due to increased sample thickness or involvement of the carbon support. The amount of Ni removed (dissolved) from the Au@Ni2Pt2 NPs during the electrochemical pretreatment (dealloy) was also determined by ICP-AES analysis of the electrolyte. Quantitatively, the Ni dissolved into the electrolyte during the dealloy treatment of this Au@Ni2Pt2/C sample corresponded only to ca. 15% of its total Ni. Therefore, all these results are consistent in demonstrating that the dealloy treatment leads to Pt enrichment via selective Ni dissolution at the outermost layers of the Au@NixPt2 (x < m) NPs on the carbon support, making the topmost surface a Pt-skin free of Ni atoms. However, the inner part of the shells for these Au@NixPt2 NPs maintained the feature for a PtNi-alloy. 3.2. Electrocatalytic properties of Au@NimPt2 catalysts.

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Figure 7. Electrochemical measurements of the Au@NimPt2/C catalysts: (a) CV curves in N2-saturated 0.1 M HClO4 solution, (b) EAS measured according to the hydrogen desorpition peak, (c) ORR polarization curves in O2-saturated 0.1 M HClO4 solution, (d) IA and MSA data at 0.9 V.

Figure 7a shows the CV curves of three selected Au@NimPt2/C catalysts (m = 0.2, 2 and 6) after the electrochemical (dealloy) pretreatment. To be emphasized, these CV curves are characteristic of “pure” Pt surface since neither Ni nor Au could be sensed on these CV curves, further confirming that the surfaces of the delloyed Au@NimPt2 NPs are free of Ni or Ni atoms at the surface of the as-prepared Au@NimPt2 NPs were completely removed during the electrochemical pretreatment. No signal characteristic of exposed Au (e.g., any exposed Au would feature a clear electrochemical reduction peak at ca. 1.15 V22) in the CV curves should also be taken as the electrochemical evidence for no presence of any unencapsulated Au NPs in the samples. Therefore, the stable CV curves for surface Pt after the electrochemical pretreatment entitle us to measure the electrochemically active surface areas (EAS) of the catalysts by quantifying the charges in the H adsorption/desorption region after double-layer correction, which are given in Figure 7b. It was shown that for Pt-skin surface formed by acid leaching and annealing (400 °C) treatment of PtNi-alloy NPs, the Hupd charge may underestimate the EAS for the Pt-skin surface owing to suppression of the H adsorption by the underlying alloy.36 To check whether such suppression of the H adsorption occurred to 16


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the Pt-skin surface of the present dealloyed Au@NimPt2 NPs, we also measured the CO-stripping charges for these samples (see SI for the details). The EAS numbers obtained according to these CO stripping and Hupd charges were found close for each Au@NimPt2/C sample (Table S1). This is not surprising as Strasser et al. also found earlier that the Hupd charge was as valid as the CO-stripping charge for measuring the EAS for the their electrochemically dealloyed PtxNi1−x NPs.15,16 Even in ref 36, the EAS measured from the Hupd charge (EASHupd) was close to that from the CO-stripping charge (EASCO) for as-prepared PtNi/C catalyst (EASCO/EASHupd = 1.05) though the ratio was EASCO/EASHupd = 1.42 for the annealed (400 °C) PtNi/C catalyst. All the Au@NimPt2/C catalysts showed higher EAS (75−91 m2⋅gPt-1) than the reference Pt/C (71 m2⋅gPt-1), indicating a higher Pt utilization efficiency.26,27,31 The Au@Ni6Pt2/C catalyst exhibited the highest EAS of 91 m2⋅gPt-1, which may arise from its highest initial Ni content that would lead to heavier Ni dissolution, rougher Pt-skin surface and more exposure of Pt atoms. To be mentioned, the EAS numbers of the present Au@NimPt2/C catalysts appear significantly higher than those (30−50 m2⋅gPt-1) of the earlier Au@FePt3/C,22 Au@CuPt/C,23 Au@PtNi/C25 and Ni@Au@PtNi/C,25 and most bimetallic PtxNi1−x/C8−12,15,16,27 catalysts. Polarization curves for ORR recorded in anodic direction are shown in Figure 7c. Clear diffusion limiting current is obtained in the low potential region (< 0.7 V) for all the catalysts, followed by a mixed kinetic-diffusion control region between 0.8 and 1.0 V. The corresponding Tafel plots for the two representatives Au@NimPt2 catalysts (m = 0.2 and 2) are shown in Figure S9 by comparison with that for commercial E-TEK Pt/C catalyst. Apparently, the present Au@NimPt2/C catalysts show superior ORR activity compared with Pt/C, in terms of both the onset and half-wave potentials, demonstrating faster ORR kinetics on these Au@NimPt2/C catalysts. The IA and mass activity (MSA) of Pt in these catalysts were measured at 0.9 V to quantitatively compare the ORR activity, which are shown in Figure 7d. The IA numbers show a volcano-shaped dependence on the atomic Ni/Pt ratio from 0.1 to 3 (m = 0.2−6) with the maximum at Ni/Pt = 1 (m = 2). The highest MSA (445 mA⋅mgPt-1) was obtained on the Au@Ni6Pt2/C catalyst due to its higher utilization of Pt; the MSA numbers for Au@Ni4Pt2/C and Au@Ni2Pt2/C (ca. 420 mA⋅mgPt-1) were slightly lower but still 2 times higher than the commercial Pt/C catalyst (191 mA⋅mgPt-1). When the mass 17


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activity was normalized according to the total mass of Pt and Au (both precious metals in the catalysts), these three Au@NimPt2 catalysts (m = 6, 4 and 2) would show still overall MSAPt+Pt of 280−300 mA⋅mg(Au+Pt)-1, which is ca. 1.5 times higher than the reference Pt/C catalyst.

Figure 8. (a) CO-stripping curves on the Au@NimPt2/C catalysts. (b) Intrinsic activity for ORR and CO-stripping peak potential as a function of atomic Ni/Pt ration on the Au@NimPt2/C catalysts. The inset shows the dependence of the intrinsic activity on Pt b-band center using the CO-stripping peak potential as the scaler.

The volcano-shaped dependence of the Pt IA on the initial atomic Ni/Pt ratio of the Au@NimPt2/C catalysts would be related with the electronic interaction of Pt with Ni and possibly Au, also. According to the theoretic d-band model as well as experiment observations, the catalysis of Pt surface for ORR is limited by the strongly adsorbed oxygenated species.37,38 It is predicted that a downshift of the Pt d-band center for the topmost Pt-skin layers by ca. 0.2 eV, through interaction with a transition metal (M), could generate the most active PtM-alloy catalyst.37 As a direct measurement of the Pt d-band center energy for Pt-alloy electrocatalysts under conditions relevant to ORR is not viable in most catalysis research laboratories including ours. We employ instead the electrochemical CO-stripping method to probe the surface electronic structures for the present Au@NimPt2/C catalysts, as the CO-stripping peak potential of Pt catalysts was found to correlate directly with the Pt d-band center energy.3,39,40 The CO-stripping peak potential recorded under the conditions relevant to ORR catalysis could thus be recommended as a practical rough measure of the relative Pt d-band center (or surface electronic structure) for nanostructured Pt 18


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catalysts.3 Shown in Figure 8a are the electrochemical CO-stripping curves for the Au@NimPt2/C catalysts. Increasing in m of the Au@NimPt2/C catalysts apparently made the CO-stripping peak shift towards higher potentials, however, the peak recorded for the reference Pt/C showed the highest potential (0.92 V). These results demonstrate irrespective of m that the Pt d-band center for Au@NimPt2/C was lower than that for the Pt/C, which explain why all Au@NimPt2/C appeared more active than Pt/C (Figure 7d). Figure 8b correlates the Pt IA for ORR and the CO stripping peak potential with the initial atomic Ni/Pt ratio for the Au@NimPt2/C catalysts. The volcano-shaped dependence of the Pt IA with the Ni/Pt ratio would be an expression of the Sabatier principle applying to the present Au@NimPt2/C catalysts for the ORR; the fundamental insight behind this dependence behavior is shown by the inset that correlates the Pt IA with the CO-stripping peak potential (Pt d-band center energy or electronic structure of the Pt surface). The optimum initial Ni content in the present Au@NimPt2/C sample would thus be around Ni/Pt = 1.0 (m = 2) for producing the intrinsically most active Pt-skin catalyst, too little Ni (Ni/Pt < ca. 0.5) would over-downshift the Pt d-band center but too much Ni (Ni/Pt > 2) would made the d-band center too close to that of pure Pt/C. Interestingly, the volcano-shaped correlation point to neither an initial Pt3Ni6,11 nor a PtNi315,16,24 shell for producing the maximum IA, which may hint that Au NPs in the cores would play a role. In the search for more active and stable dealloyed PtNi catalysts from bimetallic PtxNi1-x NPs with various compositions, Strasser et al. disclosed that Ni distribution at the inner shell (down to ca. 10 atomic layers from the surface) was a key to the control of catalytic activity for ORR of the topmost Pt (Pt-skin); their dealloyed PtNi3 showed the highest activity both by IA and MSA.15 In the work of Stamenkovic et al., however, the Pt shell thickness was highlighted to played a crucial role in controlling the ORR activity; higher initial Ni content in PtxNi1-x NPs with composition varying from Pt3Ni to PtNi3 resulted in more severe Ni dissolution and thereby thicker Pt shell and lower ORR activity.14 It is likely that the Au cores in the Au@NimPt2/C samples also affected the radial Ni distribution and then Pt-skin thickness after the deallory treatment. Our preliminary characterization using synchrotron radiation photoemission spectroscopy (SRPES) to probe the radial element distribution in the delloyed and as-prepared Au@NimPt2/C samples 19


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indicates that the electrochemical dealloy treatment induced some Au atoms diffused from the Au cores to the inner layers of the PtNi-alloy shell, up to 0.4−0.5 nm from the surface. This preliminary observation is now extending to an extensive SRPES investigation of Pt, Ni and Au distributions in the dealloyed Au@NimPt2 NPs, the results of which will be presented in a future article. It has been documented that properly controlled thermal pretreatment in suitable atmosphere of Pt-based bi- or multimetallic nanostructures could lead to more stable structure and more or less element segregation to the surface,16−17,41−43 offering new opportunity for catalytic performance improvement. The Au@Ni1Pt2/C and Au@Ni2Pt2/C samples were then subjected to a pretreatment at 300 oC for 4 h in flowing Ar containing 2% CO; the low CO concentration (2% CO) and moderate pretreatment temperature (300 oC) were chosen to avoid the formation of Ni(CO)x and thus severe Ni enrichment at the particle surfaces.17,44 The thermally pretreated samples, denoted respectively as Au@Ni1Pt2-CO/C and Au@Ni2Pt2-CO/C, featured several changes in structure and catalytic properties compared to their as-prepared counterparts: 1) their EAS numbers (72.4 m2⋅gPt-1 for Au@Ni1Pt2-CO/C, and 69.3 m2⋅gPt-1 for Au@Ni2Pt2-CO/C, Figure 9a) were slightly lowered; 2) after the electrochemical dealloy treatment, they showed a co-presence of Au atoms at the surfaces, as indicated by the Au-O feature on the CV curves shown in Figure S10; 3) their catalytic activity for ORR were ca. 20−40% higher (Figure 9c). The lowering in EAS would be explained by assuming a surface smoothing due to element redistribution or reconstruction in the NiPt alloy shell during the thermal pretreatment, which would result in reduced surface roughness and thus less exposure of Pt atoms. The electrochemically detected exposure of Au atoms on the thermally pretreated samples can arise from a diffusion of Au atoms to the shell layers form the Au cores during such pretreatment. On careful examination of the initial CV curves during the dealloy treatment, we are assured that the CV signature for the exposed Au atoms were developed during the electrochemical dealloy process. The exposure of a small number of Au atoms at the surface is also consistent with the observed slightly lower EAS of the thermally pretreated samples (Figure 9a). The catalytic activity enhancement could seemingly arise from the altered surface arrangement and then electronic structures of Pt. However, the CO-stripping peaks for Au@Ni1Pt2-CO/C and Au@Ni2Pt2-CO/C were 20


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essentially indistinguishable from those for their as-prepared counterparts (Figure S10), indicating little electronic structure change in the topmost Pt. The exposed Au atoms at the surface are therefore assumed to play a role in promoting intrinsic Pt activity for ORR. There is a debate in literature about the impact of surface Au atoms on the Pt activity for ORR.19,25 The deposition of small Au clusters on Pt NPs by Adzic et al.19 imposed little effect on the intrinsic activity of the Pt NPs for ORR while those Au entities intentionally placed to partly cover (10%−50%) the surfaces of PtNi-alloy NPs and extended Pt3Ni by Stamenkovic et al.25 led to significantly reduction in the intrinsic Pt activity. Thus, this present work reports an unexpected observation that a co-presence of Au atoms at surfaces of the Au@Ni1Pt2-CO/C and Au@Ni2Pt2-CO/C catalysts can significantly enhance the intrinsic activity of their Pt surface. The discrepancy between this and the previous observations25 might arise from a low surface concentration of Au at the surface, as evidenced by the small Au-O feature (Figure S10) and the insignificant drop of the Pt EAS (Table S1) and for the thermally pretreated samples relative to their as-prepared counterparts. Unlike those Au clusters on “pure” Pt NPs,19 intentionally placed Au entities (probably monolayer islands) on PtNi NPs25 as well as those surface Au atoms in annealed (400 °C) Ni@Au@PtNi NPs,25 these few Au atoms would serve as atomic dopants and activity promoter to the catalytic Pt-skin by occupying or prevent the formation of the high energy sites (kinks, edges and vertices/corners) that would be easily poisoned by surface intermediates (OH, OOH and O) during ORR.38,45

Figure 9. (a) EAS, (b) IA and (C) MSA change during durability test. Pt/C E-TEK: , Au@Ni2Pt2/C: ; Au@Ni1Pt2-CO/C: ; Au@Ni2Pt2-CO/C: .

, Au@Ni1Pt2/C:

The durability of Au@Ni1Pt2/C and Au@Ni2Pt2/C catalysts was evaluated by repeated cycling for 20,000 potential cycles between 0.6 V and 1.1 V in O2-saturated 0.1 M HClO4 at 25 oC. Figure 9 reports the results of these durability tests in comparison with that for the 21


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commercial Pt/C catalyst, by plotting respectively the EAS, IA and MSA at 0.9 V as a function of the potential cycle number. It can be seen that the EAS values of all investigated catalysts decreased on increasing the cycle number. The Pt/C catalyst retained less than 55% of its initial EAS after the 20,000 cycles while in contrast the two Au@NimPt2/C (m =1 and 2) catalysts preserved 60%−80% of their initial EAS. The two Au@NimPt2/C catalysts also showed comparable IA numbers, which decreased by less than 20% during the stability test; however, the commercial Pt/C catalyst exhibited 34% loss in IA during the test. The two Au@Ni1Pt2-CO/C and Au@Ni2Pt2-CO/C were also included in the durability tests (Figure 9). Regardless of their lower Pt EAS numbers at the beginning of the durability tests, the Pt EAS numbers for both Au@NimPt2−CO/C (m =1 and 2) catalysts were found higher than for their as-prepared Au@NimPt2/C counterparts at the end of the durability tests (after the 20,000 cycles). These results strongly indicate that the thermal pretreatment in the CO-containing Ar significantly improved the electrochemical stability of the catalyst, which is especially evident for Au@Ni1Pt2−CO/C (Figure 9a). The IA and MSA dada, especially those measured between 10,000 and 20,000 cycles (Figure 9 b and c), also clearly show that the superior durability of the Au@NimPt2−CO/C catalysts. The highest activity (IA: 0.80 mA⋅cmPt-2; MSA: 560 mA⋅mgPt-1) of Pt at the beginning of the durability test was registered for the Au@Ni2Pt2-CO catalyst, being about 1.5 times that for Au@Ni2Pt2 and 3−4 times that for commercial Pt/C catalyst (Figure 9b). The activity decay by loss percentage in either IA or MSA of Pt in the two Au@NimPt2-CO (m =1 and 2) catalysts was also much slower than the Pt/C catalyst (Figure 9 b and c). Though on further increasing the potential cycling number up to 20,000 cycles the MSA numbers decayed to 380−403 mA⋅mgPt-1 (Figure 9c), the enhancement relative to the Pt/C catalyst in the same test (69 mA⋅mgPt-1) reached 5−6 folds. Normalizing the activity to the total mass of Pt and Au gives an overall MSAPt+Pt of 253−269 mA⋅mgPt-1 after the 20,000 cycles for the Au@NimPt2 (m = 1 and 2) catalysts, which is still 3.7−3.9 times more active than the Pt/C catalyst, demonstrating a great advantage of the present core-shell nanostructured Au@NimPt2-CO catalysts for saving precious metals in catalytic electrodes. It is noticed that the most active Au@Ni2Pt2-C (Ni/Pt = 1) catalyst was, however, less active than those state-of-art nonporous solid Ni3Pt (Ni/Pt = 3, 30−10 nm) catalyst developed by Strasser et al.15,16 However, the activity loss for both 22


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Au@NimPt2−CO/C (m =1 and 2) was within 10% after 10,000 cycles and less than 20% after 20,000 cycles (Figure 9), being close to those for the earlier Ni3Pt16 and Ni@Au@PtNi/C25 catalysts. The superior durability of the present Au@NimPt2 catalysts in ORR would arise from the electronic modification effect of the Au cores,3, 21−23,25 the altered element arrangements in the shell layers14−17 and then electronic structure of Pt.37 The electrochemical dealloy treatment conducted prior to the catalytic activity test caused not only the dissolution and atomic rearrangements of Ni but also rearrangement of Pt and Au atoms. The fact that the PtNi-alloy shell layers of Au@NimPt2 became doped with some Au atoms after this dealloy treatment adds a piece of evidence to support the earlier claim that Au NPs in the cores can lead to efficient stabilization of its carrying bimetallic Pt-alloy shells,25 probably through the well-known hindered place-exchange mechanism.22−25,44,46 The fundamentally new insight generated here in this work is that such stabilization effect would be realized through the diffusion of Au atoms to the inner shells from Au NPs in the cores. Thus, the Au NP cores also served as the source for such mobile Au atoms, whose mobility could reach across several (up to 6+) atomic layers. The co-presence of Au atoms in the inner shells would modify the radial atomic arrangements of Ni and Pt throughout the shell layers, leading to a more complex but more stable trimetallic Au-Ni-Pt shell in the “real” catalyst. The stability of Au@NimPt2 catalysts can be further improved to some extent by the thermal pretreatment in flowing 2%CO/Ar (Figure 9). Recalling that after the electrochemical dealloy treatment the surfaces of Au@NimPt2-CO became doped with some Au atoms but that of Au@NimPt2 (m = 1 and 2) were totally free of Au (e.g. Figure S10), it is suggested that the increased stability of the Au@NimPt2-CO catalysts came as a consequence of the co-presence of a small number of Au atoms at the surface of the former catalysts. Such surface Au atoms would occupy, reduce or prevent formation of the unstable high energy Pt sites in the Au@NimPt2-CO catalysts, making the “Au-doped” Pt-skin more stable under ORR relevant conditions. The diversified observations on effect of the surface Au atoms on the Pt activity and stability, the positive effect of the surface Au atoms on both the activity and stability observed for both Au@NimPt2-CO catalysts in this study, the positive effect on the stability19−23 and 23


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neutral19 or negative23,25 effect on the activity in literature, thus points to that the number, location and aggregation state of the Au atoms at the surface of bi- or multimettalic Pt-alloy catalyst are sensitive to the method of Au placement. Further studies are needed to find out how such sensitivity could be related with the initial Au NP sizes in the cores, PtNi-alloy composition, radial element distribution in the shells as well as Pt-skin thickness, which will uncover the viability dimension of such surface Au atoms for promoting the Pt activity and stability in multimetallic catalytic nanostructures. 4. CONCLUSIONS A hetero-seed-mediated solvothermal method was disclosed for synthesizing a series of nearly monodisperse core-shell structured Au@NimPt2 NPs with small Au NPs (ca. 3.5 nm) as the cores and NiPt alloy in shell (thickness: 0.75 ~ 1.5 nm). After immobilization with Vulcan XC-72 carbon and electrochemical (dealloy) pretreatment, these Au@NimPt2 NPs offered up to 2-fold higher activity and catalytic durability than commercial Pt/C catalyst for the cathode ORR in acidic electrolyte, due to alloy effect, core-shell interactions and increased Pt utilization. The Pt activity by IA numbers for these Au@NimPt2 NP/C catalysts was shown to correlate with its property for CO (and oxygen) chemisorption. Thermal pretreatment of the Au@NimPt2/C samples in flowing 2%CO/Ar further boosted their Pt activity; for instance, the Au@Ni2Pt2/C sample after such pretreatment showed 3-fold higher IA (0.8 mA⋅cmPt-2) and MSA (576 mA⋅mgPt-1) at 0.9 V vs. RHE than commercial E-TEK Pt/C catalyst. The thermally pretreated Au@NimPt2/C catalysts also showed much higher catalytic stability; after 20,000 potential cycles between 0.6-1.1 V in O2-saturated 0.1 M HClO4, the pretreatment Au@Ni2Pt2/C catalyst became 4−6 folds more active (IA: 0.72 mA⋅cm-2, MSA: 403 mA⋅mgPt-1) than the commercial Pt/C catalyst (IA: 0.18 mA⋅cmPt-2, MSA: 69 mA⋅mgPt-1). Besides enhancing Pt utilization efficiency for catalysis, Au NPs in the cores also generated during the thermal pretreatment mobile Au atoms that can diffuse to the shell layers, probably up to the subsurface; some Au atoms even appeared as atomic dopant of the Pt-skin after electrochemical dealloying. These mobile Au atoms were shown to play an important role in enhancing the activity and stability of the Au@NimPt2/C catalysts for ORR. It is 24


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anticipated that fine-control of the number and placement of such mobile Au atoms within the PtNi-alloy shells would lead to more better nanostructured catalysts for ORR and other reactions.

ASSOCIATED CONTENT Supporting Information Available. Methods for the determination of electrochemically active surface (EAS), intrinsic activity (IA) and mass specific activity (MSA); More data for Au and Pt0.13Âu NPs (UV-Vis spectra and TEM images); More information on synthesis of core-shell nanostructured Au@NimPt2 NPs; Other TEM images of carbon-supported Au@NimPt2 NPs, high-resolution bright and dark field HAAF-STEM images of the electrochemically dealloy treated Au@Ni2Pt2 NPs, raw EELS of Ni for the electrochemical dealloyed Au@Ni2Pt2 NP, and Tafel plots for the ORR on Au@NimPt2 (m = 0.2 and 2) and Pt/C catalysts; CO-stripping curves for the as-prepared Au@Ni1Pt2/C and thermally pretreated Au@Ni1Pt2-CO/C catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (grant: 2013CB933103), the National Natural Science Foundation of China (grant: 21221062) and Tsinghua University (grant: 20131089311). JL is supported by the start-up fund of the College of Liberal Arts and Sciences of Arizona State University. The authors acknowledge the use of the John M. Cowley Center for High Resolution Electron Microscopy facilities at Arizona State University.

References (1) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; 25


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