Shell Nanowires as Efficient Catalysts for

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Hierarchical Pt/PtxPb Core/Shell Nanowires as Efficient Catalysts for Electrooxidation of Liquid Fuels Nan Zhang, Shaojun Guo, Xing Zhu, Jun Guo, and Xiaoqing Huang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01642 • Publication Date (Web): 26 May 2016 Downloaded from http://pubs.acs.org on May 31, 2016

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Chemistry of Materials

Hierarchical Pt/PtxPb Core/Shell Nanowires as Efficient Catalysts for Electrooxidation of Liquid Fuels Nan Zhang,1 Shaojun Guo,2* Xing Zhu,3 Jun Guo,3 and Xiaoqing Huang1* 1

College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu, 215123, China.

2

Department of Materials Science & Engineering, College of Engineering, Peking University, Beijing, 100871, China. 3

Testing & Analysis Center, Soochow University, Jiangsu, 215123, China.

*

To whom correspondence should be addressed. E-mail: [email protected]; [email protected]

ABSTRACT: The development of highly efficient fuel cell devices is largely impeded by the limited electrocatalytic activity and stability of available Pt-based electrocatalysts. Herein we report a facile onepot strategy for the controlled synthesis of hierarchical Pt/PtxPb core/shell nanowires (NWs) with dendritic morphology. Different from the reported NWs, the present hierarchical core/shell NWs show the integrated features of one-dimensional (1D) structure, core/shell structure, alloy effect and high

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surface area. These important characteristics enable them be much more active and stable for methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) than Pt NWs, the Pt-Pb nanoparticles (NPs) and commercial Pt/C (20 wt%, Pt particle size: 2-5 nm, Johnson Matthey) catalyst. Particularly, the present PtPb0.27 NWs are very stable in the MOR and EOR conditions with much lower activity decay after 1000 potential cycles than those of Pt-Pb NPs and the commercial Pt/C. This work highlights the importance of the precise control over 3D hierarchical structure in enhancing electrocatalysis for liquid fuels oxidations.

INTRODUCTION Fuel cells are highly efficient renewable devices in converting chemical energy into electric power via the electrochemical process.1,2 The catalysts are the most important components of fuel cells.3-5 The rare and precious Pt is believed to be the most promising metal catalyst.6-10 To minimize the consumption and improve the utilization efficiency of Pt in fuel cells, reducing the size of the nanoparticles (NPs) and substituting Pt with much less expensive transition metal are the most widely used strategies.11-14 However, the major limitations of previous created Pt-based catalysts are still their high cost, moderate activity and low durability during the electrocatalytic process, largely impeding the large-scale commercialization of fuel cells.15-18 Recently, one-dimensional (1D) noble metal nanostructures such as nanowires (NWs) and nanorods have been highlighted as a new class of electrocatalysts for enhancing both the activity and the durability of different catalytic reactions because they have some obvious advantages such as inherent anisotropic structure, high flexibility, high surface area and high conductivity.19-23 Despite the great potentials, the biggest challenging issue of previous reported Pt-based NWs is they have smooth surface, leading to very limited electrocatalytic performance enhancements.24,25 In this regards, the realization of 3D


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hierarchical Pt-based NWs with precise surface and structure would be extremely beneficial for the creation of high-performance Pt-based catalysts towards liquid fuels electrooxidation. However, compared with the established Pd/Pt and Au/Pt bimetallic NWs with dendritic morphology, creating hierarchical PtPb-based NWs with high surface area shows limited success mainly due to the lack of suitable synthetic method in controlling the reduction process of Pb precursor.26-29 Herein, we report a facile strategy for preparing a new class of hierarchical Pt/PtxPb core/shell NWs as highly efficient electrocatalysts for liquid fuels electrooxidation. The most important feature of our new Pt/PtxPb core/shell NWs is that they integrate 1D structure, core/shell structure, alloy effect and high surface area. They exhibit the enhanced electrocatalytic activities towards methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) relative to the PtPb0.21 NPs and the commercial Pt/C (20 wt%, Pt particle size: 2-5 nm, Johnson Matthey) catalyst. As a consequence, the optimized PtPb0.27 NWs have the mass activity of 1.21 A/mgPt for MOR and 0.89 A/mgPt for EOR, which are 5.8 and 4.8 times higher than those of commercial Pt/C catalyst, respectively. They are also generally much more stable than the PtPb0.21 NPs, suggesting such hierarchical Pt-Pb NWs can be used as active and stable electrocatalysts for future practical fuel cell and beyond. EXPERIMENTAL SECTION Chemicals. Platinum(II) acetylacetonate (Pt(acac)2, 97%), lead(II) acetylacetonate (Pb(acac)2, 97%), and oleylamine (CH3(CH2)7CH=CH(CH2)7CH2NH2, 68-70%) were all purchased from Sigma-Aldrich. Hexadecyltrimethylammonium bromide (CH3(CH2)15N(Br)(CH3)3, CTAB, >97.0%) and glucose (C6H12O6, reagent grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). All the chemicals were used as received without further purification. The water (18 MΩ/cm) used in all experiments was prepared by passing through an ultra-pure purification system (Aqua Solutions).


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Preparation of hierarchical Pt-Pb NWs. In a typical preparation of hierarchical PtPb0.21 NWs, Pt(acac)2 (10.0 mg), CTAB (36.5 mg), glucose (60.0 mg) and 5 mL oleylamine were added into a glass vial (volume: 30 mL). After the vial had been capped, the mixture was ultrasonicated for 1.0 h. The resulting homogeneous mixture was then heated from room temperature to 160 °C, and maintained at that temperature for 0.5 h in an oil bath under magnetic stirring. After cooling to 60°C, Pb(acac)2 (2.0 mg) dissolved in 2 mL oleylamine was then added dropwise to the above mixture under magnetic stirring. The reaction was then increased to 160 °C and kept at this temperature for 4.0 h. The products were collected by centrifugation and washed three times with an ethanol/cyclohexane mixture. The preparations of PtPb0.16 NWs and PtPb0.27 NWs were achieved by changing the amount of Pt(acac)2 to 13.0 mg and 8.0 mg, respectively, while keeping the other parameters the same. Preparation of PtPb0.21 NPs. In a typical preparation of PtPb0.21 NPs, Pt(acac)2 (10.0 mg), Pb(acac)2 (2.0 mg), CTAB (36.5 mg), glucose (60.0 mg) and 5 mL oleylamine were added into a glass vial (volume: 30 mL). After the vial had been capped, the mixture was ultrasonicated for 1.0 h. The resulting homogeneous mixture was then heated from room temperature to 160 °C and kept at that temperature for 5.0 h. The products were collected by centrifugation and washed three times with an ethanol/cyclohexane mixture. Characterizations. The samples were prepared by dropping the cyclohexane dispersion of samples onto carbon-coated copper TEM grids using pipettes, and dried under ambient condition. Low-magnification transmission electron microscopy (TEM) was conducted on a HITACHI HT7700 transmission electron microscope at an acceleration voltage of 120 kV. High-magnification transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were conducted on an FEI Tecnai F20 transmission electron microscope at an acceleration voltage of 200 kV. Scanning electron microscopyenergy dispersive X-ray spectroscopy (SEM-EDS) spectra were taken with a HITACHI S-4700 cold field emission scanning electron microscope. PXRD patterns were collected on X’Pert-Pro MPD


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diffractometer (Netherlands PANalytical) with a Cu Kα X-ray source (λ=1.540598 Å). The concentration of catalysts was determined by the inductively coupled plasma atomic emission spectroscopy (710-ES, Varian, ICP-AES).

MOR and EOR measurements. The electrochemical workstation was CHI660E fabricated by Chen Hua (Shanghai, China). A three-electrode cell was used to do the electrochemical measurements. The working electrode was a glassy-carbon electrode (GCE) (diameter: 5 mm, area: 0.196 cm2) from Pine Instruments. The saturated calomel electrode (SCE) and Pt wire were used as reference and counter electrodes, respectively. To prepare a catalyst-coated working electrode, the catalyst was dispersed in a mixture of solvents containing isopropanol and Nafion (5%) to form a 0.20 mgPt/mL suspension. 10 µL of isopropanol dispersion of Pt-Pb NWs on C (0.20 mgPt/mL) was deposited on a glassy carbon electrode to obtain the working electrodes after the solvent was dried naturally. The electrochemical active surface area (ECSA) measurements were determined by integrating the hydrogen adsorption charge on the cyclic voltammetry (CV) at room temperature in 0.1 M HClO4 aqueous solution. The potential scan rate was 50 mVs-1 for the CV measurement. MOR was conducted in a 0.1 M HClO4 aqueous solution containing 0.15 M CH3OH. EOR was conducted in a 0.1 M HClO4 aqueous solution containing 0.15 M CH3CH2OH. The scan rates for MOR and EOR were 50 mVs-1. For the comparison, Pt-Pb NPs, Pt NWs and commercial Pt/C were used as the baseline catalysts, and the same procedures as described above were applied to conduct the electrochemical measurements. RESULTS AND DISCUSSION


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Figure 1. Morphology and structure characterizations of hierarchical PtPb0.21 NWs. Representative (a) lowmagnification HAADF-STEM image, (b) high-magnification HAADF-STEM image and (c, d) TEM images of an individual NW. (e) PXRD pattern, (f) HRTEM image and (g) EDS mapping images of PtPb0.21 NWs.

The hierarchical Pt/PtxPb core/shell NWs were prepared using Pt NWs-mediated PtxPb alloy shell growth strategy (See Experimental Section for details). The morphology and structure analyses for the hierarchical Pt/PtxPb core/shell NWs (Pt(acac)2/Pb(acac)2=1:0.2) are shown in Figure 1. The high-angle annular dark filed-scanning transmission electron microscopy (HAADF-STEM) (Figure 1a) and


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transmission electron microscopy (TEM) images (Figure 1c&1d) results clearly show the as-prepared NWs have the uniform diameter of 18-21 nm, and their length is from around 100 to 800 nm. If we take a closer look at the detailed structure, there are the dense core inside the NW and large amounts of the granular protuberances on the surface. The molar ratio of Pt to Pb in the Pt-Pb NWs is 1:0.21, determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES), being consistent with the molar ratio of the Pt to Pb precursors supplied in the synthesis (Table S1). X-ray powder diffraction (PXRD) was conducted to determine the phase of the PtPb0.21 NWs (Figure 1e), indicating the presence of both face-centered cubic (fcc) PtxPb phase and fcc Pt phase. High-resolution TEM (HRTEM) image of the NWs (Figure 1f) shows that the d-spacing of the adjacent fringes for the NWs is 0.143 nm, corresponding to (220) facet of fcc PtxPb. In order to further figure out whether the PtxPb and Pt phases are coexisted, the HAADF-STEM-energy dispersive X-ray spectroscopy (HAADFSTEM-EDS) elemental mapping of the PtPb0.21 NWs was analyzed (Figure 1g), showing Pt distributes throughout the entire NWs with higher distribution in the core, and Pb is observed within the whole NW, which indicates the formation of a core (Pt)/shell (PtxPb) structure. To elucidate the growth mechanism of the hierarchical PtPb0.21 NWs, the growth intermediates collected at different reaction durations were investigated by TEM (Figure 2&S2). As shown in Figure 2a, the Pt NWs with the diameter of 5-6 nm were obtained at the initial stage of the synthesis. As the reaction proceeded for another 0.5 h after the addition of the Pb(acac)2 into the synthesis mixture, the hierarchical NWs were produced with bigger diameter of 15-18 nm (Figure 2b). When the reaction time was further increased to 3.0 h, the hierarchical structure did not show much change, but the diameter of the PtPb NWs increased to 17-19 nm (Figure 2c). Hierarchical NWs with the diameter of 18-21 nm were obtained when the reaction time was 5.0 h (Figure 2d). The morphology and sizes of the hierarchical PtPb0.21 NWs were almost unchanged by further increasing the reaction time to 6.0 h


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(Figure 2e). The reaction intermediates were also analyzed by PXRD and ICP-AES, showing the product obtained at 0.5 h is comprised of pure Pt structure (Figure 2f&2g). Two distinct phases, assigned to Pt and PtxPb, were formed after the reaction had been proceeded for 1.0 h. A large percentage of Pb (The molar ratio of Pt/Pb is 0.23) was detected (Figure 2f). The molar ratio of Pb to Pt was detected to be 0.21 with no further change after 3.0 h mainly owing to the depletion or co-reduction of the precursors (Figure 2g). It should be noted that the Pt and PtxPb phases were maintained throughout the whole growth process after 0.5 h (Figure 2f), confirming the preparation of core/shell structured PtPb0.21 NWs. A series of control experiments were further used to explore the formation mechanism of hierarchical PtPb0.21 NWs. A one-pot synthesis by adding the Pt and Pb precursors simultaneously at the beginning with the synthetic conditions similar to the synthesis of hierarchical PtPb0.21 NWs yielded only PtPb0.21 NPs with irregular shapes (Figure S3). This indicates that the introduction of Pb precursor at the beginning can affect the growth kinetic of Pt NWs at the initial stage.30 This control experiment combined with time-dependent morphology and composition changes reveal that the formation of hierarchical PtPb0.21 NWs relies upon the initial formation of pure Pt NWs as the seeds and the sequential reduction/diffusion of Pb onto the preformed Pt NWs.31 The Pt NWs (Figure S4) were also prepared as the reference for the electrochemistry test as well as the PtPb0.21 NPs.


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Figure 2. TEM images of the intermediates obtained after the reaction had been proceeded for (a) 0.5, (b) 1.0, (c) 3.0, (d) 5.0 and (e) 6.0 h. (f) PXRD patterns of the intermediates. (g) The changes in the molar ratio of Pb to Pt in the intermediates, determined by ICP-AES analysis. All the scale bars in (a)-(e) are 20 nm.

The key feature of our synthesis is that we can readily tune the compositions of Pt/PtxPb core/shell NWs by varying the amount of Pt(acac)2 while keeping the concentration of Pb(acac)2 unchanged. As shown in Figure 3, all the other Pt-Pb NWs exhibit the hierarchical structure. Along with the increased molar ratio of Pb to Pt precursors, the diameter of Pt-Pb NWs is changed from 17 nm to 24 nm (Figure 3a-b&3f-g). According to the ICP-AES data (Table S1), the PtPb0.16 and PtPb0.27 NWs were obtained when the amounts of Pt(acac)2 supplied were 13.0 mg and 8.0 mg, respectively. The PXRD patterns of PtPb0.16 and PtPb0.27 NWs are similar to that of PtPb0.21 NWs (Figure 3c&3h&Figure S1), where two set of peaks, indexed to fcc Pt and fcc PtxPb, are observed. The HRTEM images of the two Pt-Pb NWs (Figure 3d&3i) show the lattice distances of 0.202 nm and 0.143 nm, attributed to the facets of (200) and (220), respectively. The hierarchical core/shell structure of both Pt-Pb NWs was further confirmed by the HAADF-STEM-EDS elemental mapping (Figure 3e&3j).


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Figure 3. Morphology and structure characterizations for hierarchical PtPb0.16 NWs and PtPb0.27 NWs. Representative (a, f) HAADF-STEM images, (b, g) TEM images of individual NWs, (c, h) PXRD patterns, (d, i) HRTEM images, (e, j) HAADF-STEM images and EDS mapping images of PtPb0.16 NWs (left column) and PtPb0.27 NWs (right column).

We chose MOR and EOR to examine the electrocatalytic properties of hierarchical Pt-Pb NWs.32-35 The NWs were mixed with a carbon support (Vulcan XC-72R carbon, C) via sonicating the mixture of carbon and NWs in solution (Figure S5). Excess surfactant was removed by treatment with acetic acid twice at room temperature. Figure 4a shows the cyclic voltammograms (CVs) of PtPb0.27 NWs, PtPb0.21


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NWs, PtPb0.16 NWs, PtPb0.21 NPs (Figure S6), Pt NWs and commercial Pt/C (Figure S7) in 0.1 M HClO4 aqueous solution. The electrochemical activity surface area (ECSA) was calculated by measuring the charge collected in hydrogen adsorption region assuming an adsorbed hydrogen monolayer on metal surface is 210 µC/cm2. The ECSAs were calculated to be 50.5 m2g-1 for PtPb0.27 NWs, 37.8 m2g-1 for PtPb0.21 NWs, 31.7 m2g-1 for PtPb0.16 NWs, 20.0 m2g-1 for PtPb0.21 NPs, 35.0 m2g-1 for Pt NWs and 54.4 m2g-1 for commercial Pt/C.

Figure 4. Electrochemistry and electrocatalysis for MOR of PtPb0.27 NWs, PtPb0.21NWs, PtPb0.16 NWs, PtPb0.21 NPs, Pt NWs and the commercial Pt/C. (a) CVs of different catalysts recorded at room temperature in 0.1 M HClO4 aqueous solution at the scan rate of 50 mVs-1. (b) MOR curves recorded at room temperature in a 0.1 M HClO4 + 0.15 M CH3OH aqueous solution at the scan rate of 50 mVs-1. (c) Specific and mass activities of different catalysts. (d) The comparison of the durability among PtPb0.27 NWs, PtPb0.21 NPs, Pt NWs and the commercial Pt/C. Potential was continuously scanned at 50 mVs-1 in 0.1 M HClO4 + 0.15 M CH3OH aqueous solution at the scan rate of 50 mVs-1.

Figure 4b shows the MOR of different catalysts in 0.1 M HClO4 aqueous solution containing 0.15 M CH3OH at the scan rate of 50 mVs-1. Figure 4c displays the specific and mass activities for MOR of PtPb0.27 NWs, PtPb0.21 NWs, PtPb0.16 NWs, PtPb0.21 NPs, Pt NWs and the commercial Pt/C. For specific


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activities, the MOR current density of PtPb0.27 NWs (2.41 mAcm-2), PtPb0.21 NWs (2.14 mAcm-2), PtPb0.16 NWs (1.42 mAcm-2), PtPb0.21 NPs (1.19 mAcm-2), Pt NWs (1.07 mAcm-2) are 6.2, 5.5, 3.6, 3.1, 2.7 times higher than that of commercial Pt/C catalyst (0.39 mAcm-2). The PtPb0.27 NWs also show the highest mass activity in all the investigated NWs. The mass activity of PtPb0.27 NWs is 1.21 A/mgPt, 5.8 times higher than that of the commercial Pt/C (0.21 A/mgPt), 4.9 times higher than that of the PtPb0.21 NPs (0.25 A/mgPt) and 3.2 times higher than that of the pure Pt NWs (0.38 A/mgPt). According to the HRTEM of PtPb0.27 NWs (Figure S8), the steps are frequently present on the surface of PtPb0.27 NWs, which will be beneficial for their enhanced catalysis. To evaluate the MOR durability, the electrocatalysts were continually scanned in 0.1 M HClO4 aqueous solution containing 0.15 M CH3OH for 1000 cycles (Figure S9a-d). The result shows that the mass activity of the PtPb0.27 NWs is much higher than those of PtPb0.21 NPs and the commercial Pt/C after 1000 sweeping cycles. The mass activity of PtPb0.27 NWs is only decayed by 41.6%, better than those of the Pt NWs (56.4%), PtPb0.21 NPs (65.9%) and Pt/C catalyst (79.9%), respectively (Figure 4d).

Figure 5. Electrocatalysis for EOR of PtPb0.27 NWs, PtPb0.21 NWs, PtPb0.16 NWs, PtPb0.21 NPs, Pt NWs and the commercial Pt/C. (a) EOR curves recorded at room temperature in 0.1 M HClO4 solution containing 0.15 M


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CH3CH2OH aqueous solution at the scan rate of 50 mVs-1. (b) Specific and mass activities of different catalysts. (c) The durability comparison of PtPb0.27 NWs, PtPb0.21 NPs, Pt NWs and the commercial Pt/C in 0.1 M HClO4 solution containing 0.15 M CH3CH2OH at the scan rate of 50 mVs-1.

Figure 5a shows the EOR of six different catalysts in a 0.1 M HClO4 aqueous solution containing 0.15 M CH3CH2OH. Both the EOR specific and mass activities of PtPb0.27 NWs are the highest among six different catalysts (Figure 5b). The mass activity of the PtPb0.27 NWs is 4.8, 3.9 and 2.1 times higher than those of the commercial Pt/C, PtPb0.21 NPs and Pt NWs. Furthermore, after 1000 sweeping cycles, the mass activity of PtPb0.27 NWs can maintain 74.9% of its initial value, better than those of Pt NWs (67.2%), PtPb0.21 NPs (31.6%) and Pt/C catalyst(23.3%), respectively, showing their enhanced durability for EOR (Figure 5c&Figure S9e-h). Moreover, after the durability test, there are almost no changes in morphology and composition of PtPb0.27 NWs (Figure S5 and Table S2), while the PtPb0.21 NPs and commercial Pt/C happen the severe structure and composition changes after the durability test (Figure S6&S7), further highlighting the excellent durability of the hierarchical Pt-Pb NWs. CONCLUSIONS To summarize, we demonstrate the efficient synthesis of a new class of hierarchical Pt/PtxPb NWs as highly efficient nanocatalysts for boosting the liquid fuels electrooxidation. The sequential introduction of Pt and Pb precursors was proved to be the key step to realize these unique NWs with large surface area. The composition of hierarchical Pt-Pb NWs can be readily controlled by tuning the amount of Pt(acac)2. These newly-generated Pt-Pb NWs are a new class of high-performance electrocatalysts towards liquid fuels electrooxidations. The mass activity of Pt/PtxPb NWs with the optimized composition is 1.21 A/mgPt for MOR and 0.89 A/mgPt for EOR, which is 5.8 and 4.8 times higher than those of commercial Pt/C, respectively, and also more active and stable than the PtPb0.21 NPs and pure Pt


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NWs, presenting a promising candidate as advanced electrocatalyst for liquid fuel electrooxidations with excellent activity and durability. ASSOCIATED CONTENT Supporting Information. Figure S1-9 and Table S1-2. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected]; [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the start-up fundings from Soochow University and Peking University, Young Thousand Talented Program, the National Natural Science Foundation of China (21571135), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD)

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