Article pubs.acs.org/JACS
PtPb/PtNi Intermetallic Core/Atomic Layer Shell Octahedra for Efficient Oxygen Reduction Electrocatalysis Lingzheng Bu,† Qi Shao,† Bin E,† Jun Guo,‡ Jianlin Yao,† and Xiaoqing Huang*,† †
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123, China Testing & Analysis Center, Soochow University, Jiangsu 215123, China
‡
S Supporting Information *
ABSTRACT: Although explosive studies on pursuing high-performance Pt-based nanomaterials for fuel cell reactions have been carried out, the combined controls of surface composition, exposed facet, and interior structure of the catalyst remains a formidable challenge. We demonstrate herein a facile chemical approach to realize a new class of intermetallic Pt−Pb−Ni octahedra for the first time. Those nanostructures with unique intermetallic core, active surface composition, and the exposed facet enhance oxygen reduction electrocatalysis with the optimized PtPb1.12Ni0.14 octahedra exhibiting superior specific and mass activities (5.16 mA/cm2 and 1.92 A/mgPt) for oxygen reduction reaction (ORR) that are ∼20 and ∼11 times higher than the commercial Pt/C, respectively. Moreover, the PtPb1.12Ni0.14 octahedra can endure at least 15 000 cycles with negligible activity decay, showing a new class of Ptbased electrocatalysts with enhanced performance for fuel cells and beyond.
1. INTRODUCTION The current global energy status urgently requires the efficient and reliable conversion of fossil fuels or renewable sources to generate electric power on a large scale.1−5 In all different forms of electric energies, fuel cells are highly expected to realize widespread commercial applications in the fields of transportation, stationary, and portable power generations, due to their obvious advantages of promising efficiency, lightness, convenience, hardly pollution to the environment, etc.4−6 As for the cathodic oxygen reduction reaction (ORR) in fuel cells, Pt-based nanomaterials are considered to be the most efficient electrocatalysts.7−9 However, the high cost associated with Ptbased electrocatalysts largely obstructs their practical applications.10−12 So far, the widely used approach to deal with this significant challenge is to alloy low-cost transition metals with Pt.13−15 Indeed, the alloyed Pt-based nanocrystals (NCs), especially Pt−nickel (Pt−Ni) NCs with well controls in size, morphology, and composition, exhibit enhanced performance, where substantial work has been done.16−22 Another ingenious recipe for reducing the content of scarce Pt and improving its utilization is to design a core/shell nanostructure, because the core/shell structure with ultrathin Pt shell is beneficial to the enhancements of performance and lifetime for ORR.23−25 However, the reported core/shell structured PtM NCs with single-component core and/or shell have large limitations in terms of the ORR activity and durability enhancement,26−29 since the multicomponent PtM usually exhibits higher ORR activity than pure Pt.30,31 Considering the critical role of the © 2017 American Chemical Society
core in the enhancements of ORR performance, the PtM core for activating oxygen reduction catalysis should also be rationally selected.28 Pt-based intermetallic NCs, such as Pt− iron (Pt−Fe), Pt−copper (Pt−Cu), Pt−chromium (Pt−Cr), and Pt−lead (Pt−Pb) NCs, have attracted great research attention,32−36 mainly due to their better modulate over their geometric, structural, as well as electronic effects. The chemical and structural stability can contribute greatly to the durability of Pt-based intermetallic NCs in the process of electrocatalysis.32−36 The strong interaction between the active PtNi shell and the PtM core will greatly boost oxygen reduction catalysis. In this regard, the rational combination of PtM intermetallic NCs as the core and the active PtNi on the surface may create unique Pt-based electrocatalysts with simultaneously enhanced activity and durability for ORR, while it is still a formidable challenge. We show herein a facile chemical route to create unique intermetallic NCs, namely bimetallic Pt−Pb octahedra and trimetallic Pt−Pb−Ni octahedra. The successful creation of intermetallic Pt−Pb and Pt−Pb−Ni octahedra with welldefined morphology, composition, and size provides an ideal platform to study the structural effects, especially the surface composition and the exposed facets on ORR. It is shown that the Pt−Pb−Ni octahedra show outstanding activity and improved durability for ORR relative to the commercial Pt/ C, because of their ordered structure, well-defined morphology, Received: April 7, 2017 Published: June 28, 2017 9576
DOI: 10.1021/jacs.7b03510 J. Am. Chem. Soc. 2017, 139, 9576−9582
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relatively weak anisotropy can be attributed to the equivalent growth along the directions of three symmetry axes in the condition of relatively low reducing capacity (17.8 mg AA), as compared with the creation of hexagonal nanopaltes with high anisotropy in conditions of fast reducing rate (35.6 mg AA).36 Based on the relationship between shape percentage and amount of AA (Figure S8), we found the shape evolution from octahedra to nanoplates with the increased amounts of AA. Figures 1a,b and S9a-b show the representative TEM images of the Pt−Pb octahedra. TEM characterization shows that the
and alloy effect. To be specific, the intermetallic PtPb1.12Ni0.14 octahedra/C show 1.4, 1.9, 1.1, 2.7, and 11.4 times higher mass activity and 1.4, 1.9, 1.3, 2.9, and 19.6 times higher specific activity than those of PtPb1.07Ni0.10 octahedra/C, PtPb1.03Ni0.05 octahedra/C, Pt1.21Pb octahedra/C, PtPb octahedra/C, and Pt/ C, respectively. The intermetallic core structure and the unique core/shell structure also make the PtPb1.12Ni0.14 octahedra highly durable with negligible activity decay over 15 000 potential sweeps.
2. EXPERIMENTAL SECTION 2.1. Chemicals. Lead(II) acetylacetonate (Pb(acac)2, 99%), nickel(II) formate dihydrate (Ni(HCO2)2·2H2O), and oleic acid (C18H34O2, OA, >85%) were obtained from Sigma Aldrich. Lead(II) chloride (PbCl2, 99.99%), lead(II) acetate trihydrate (Pb(Ac)2·3H2O, >99%), and phloroglucinol (C6H6O3, ≥99%) were purchased from Aladdin. Diphenyl ether (C12H10O, DPE, analytical reagent, ≥99.0%) came from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals used in this work were described in our previous works.37−39 2.2. Synthesis of PtPb Octahedra, PtPb1.03Ni0.05 Octahedra, PtPb1.07Ni0.10 Octahedra, PtPb1.12Ni0.14 Octahedra, and Pt1.21Pb Octahedra. To synthesize the PtPb octahedra, 17.8 mg of AA, 8.0 mg of Pb(acac)2, 10.0 mg of Pt(acac)2, 2.5 mL of OAm, and 2.5 mL of ODE were mixed in a glass vial and heated at 160 °C for 5 h. The products were collected by centrifugation. All of the synthetic parameters for PtPb1.03Ni0.05 octahedra, PtPb1.07Ni0.10 octahedra, and PtPb1.12Ni0.14 octahedra are similar to those of PtPb octahedra but introducing additional Ni(HCO2)2·2H2O (0.4, 0.8, and 1.2 mg, respectively) into the synthesis. The Pt1.21Pb octahedra were obtained by adding 2.6 mg of Pt(acac)2, 17.8 mg of AA, 1.0 mL of OAm, and 1.0 mL of ODE into the PtPb octahedra mixture and further heated at 160 °C for 3 h. 2.3. Characterization. Fast Fourier transform (FFT) was collected with an FEI Tecnai F20 transmission electron microscope at 200 kV. All other characterization techniques in this work were described in our previous work.37−39 2.4. Electrochemical Measurements. The loading amounts of Pt for the PtPb octahedra/C, Pt1.21Pb octahedra/C, PtPb1.03Ni0.05 octahedra/C, PtPb1.07Ni0.10 octahedra/C, PtPb1.12Ni0.14 octahedra/C, and commercial Pt/C were all kept at 12.8 μg/cm2, as calculated by ICP-AES measurement. The equation for the calculation of electrochemically active surface area (ECSA) is shown as follows: ⎡ ⎤ Q H − adsorption(C) ⎥ × 105 ECSAPt,cat(m 2/g Pt) = ⎢ 2 2 2 ⎢⎣ 210μC/cm L Pt(mg Pt/cm )Ag (cm ) ⎥⎦
In this equation, QH‑adsorption(C) is the hydrogen adsorption charge calculated from the cyclic voltammograms (CVs). 210 μC/cm2 is used as the conversion factor. LPt (mgPt/cm2) is the working electrode Pt loading. Ag (cm2) is the geometric surface area of the glassy carbon electrode (i.e., 0.196 cm2). For the ORR polarization curve of each catalyst, the current value at 0.4 V (vs RHE) was defined as the diffusion limited current. All other electrochemical measurements were similar to our previous publication.37−39
Figure 1. Morphological and structural characterizations of PtPb octahedra. (a, b) TEM images, (c) PXRD pattern, (d) TEM-EDS, (e) HRTEM image and the FFT pattern, (f) HAADF-STEM image and elemental mappings, and (g) corresponding line-scans of the PtPb octahedra. The inset in panel a is the 3D model of the PtPb octahedra. The composition ratio of Pt/Pb is 50.9 ± 0.2/49.1 ± 0.2 as revealed by ICP-AES and 50.7 ± 0.5/49.3 ± 0.5 as confirmed by TEM-EDS.
octahedra are the major products (nearly 100%). Figure S9c-d are the HAADF-STEM images, which further reveal the welldefined structure of the Pt−Pb octahedra, and the corresponding 3D model has been shown in the inset in Figure 1a. The Pt−Pb octahedra have narrow edge length distribution of around 18.0 nm (Figure S9e). As shown in Figure 1c, the distinct PXRD pattern of the Pt−Pb octahedra can match well with the reflections of the intermetallic Pt−Pb materials (Joint Committee on Powder Diffraction Standards, JCPDS, No. 060374). The composition ratio of Pt/Pb is 50.9 ± 0.2/49.1 ± 0.2, as revealed by the ICP-AES, which is consistent with the TEM-EDS (50.7 ± 0.5/49.3 ± 0.5) (Figure 1d) and SEM-EDS
3. RESULTS AND DISCUSSION We prepared the Pt−Pb and/or Pt−Pb−Ni NCs through a novel wet-chemical approach, in which Pt(acac)2, Pb(acac)2, and/or Ni(HCO2)2·2H2O were used as Pt, Pb, and/or Ni precursors, AA was chosen as reducing agent, and OAm and ODE were applied as mixed solvents and surfactants (see the Experimental Section for details). The synthetic parameters, such as the precursors, the species, the reducing agents, and the solvents, have been explored in detail (Supporting Information, Figures S1−S7). The formation of PtPb octahedra with 9577
DOI: 10.1021/jacs.7b03510 J. Am. Chem. Soc. 2017, 139, 9576−9582
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Journal of the American Chemical Society (50.4 ± 0.8/49.6 ± 0.8) results (Figure S9f). The interplannar spacing displays 0.304 nm, consistent with the (101) plane of hexagonal phase PtPb (P63/mmc) intermetallic structure (Figures 1e and S10a). The FFT pattern of an individual PtPb octahedron reveals the single-crystalline nature (inset in Figure 1e). The elements of Pt and Pb are homogeneously distributed throughout the whole octahedron (Figure 1f), as confirmed by the line-scans (Figures 1g and S10b), showing the alloyed structure of the PtPb octahedra. To uncover the growth mechanism, we have tracked the PtPb octahedra intermediates collected from different reaction times. At 0.5 h of the synthesis (Figure 2a), the product
intermediate of Pb3(CO3)2(OH)2 produced from the reaction time of 0.5 h was transformed into intermetallic PtPb after 3.0 h (Figures 2h and S11). To date, the most active ORR electrocatalysts are mainly Pt− Ni nanomaterials.40−42 To this end, we prepared Pt−Pb−Ni octahedra by introducing additional Ni precursor into the synthesis of PtPb octahedra while keeping other parameters unchanged. The optimized synthetic parameters of Pt−Pb−Ni octahedra, such as the precursors, the species, the reducing agents, and the solvents, have been also explored in detail (Figures S12−S19). We found that the introduction of Ni(HCO2)2·2H2O into the synthesis of PtPb hexagonal nanoplates cannot yield well-defined PtPb/PtNi nanoplates (Figure S20). As shown in Figures 3 and S21−S23, Pt−Pb−Ni
Figure 2. Growth mechanism of the PtPb octahedra. (a−e) TEM images of PtPb octahedra intermediates collected from (a) 0.5 h, (b) 1 h, (c) 3 h, (d) 5 h, and (e) 6 h. (f) SEM-EDS of PtPb octahedra intermediates collected from the reactions at different reaction times. (g) The changes on the composition ratio of Pb to Pt for the PtPb octahedra intermediates, as determined by ICP-AES measurements. (h) PXRD patterns of PtPb octahedra intermediates collected from the reactions at different reaction times.
Figure 3. Morphological and structural characterizations of PtPb1.03Ni0.05 octahedra, PtPb1.07Ni0.10 octahedra, and PtPb1.12Ni0.14 octahedra. (a) TEM image of PtPb1.03Ni0.05 octahedra. The inset in panel a is the HAADF-STEM image of an individual PtPb1.03Ni0.05 octahedron, and the elemental mappings are shown in panel b. (c) TEM image of PtPb1.07Ni0.10 octahedra. The inset in panel c is the HAADF-STEM image of an individual PtPb1.07Ni0.10 octahedron, and the elemental mappings are shown in panel d. (e) TEM image of PtPb1.12Ni0.14 octahedra. The inset in panel e is the HAADF-STEM image of an individual PtPb1.12Ni0.14 octahedron, and the elemental mappings are shown in panel f.
contains the small-size NCs. The SEM-EDS result shows that the Pt/Pb in the initial intermediate is 11.6/88.4, revealing high content of Pb in the initial intermediate. They are the mixtures of Pt (JCPDS No.04-0802), Pb3(CO3)2(OH)2 (JCPDS No.130131), and PtPb (JCPDS No.06-0374), as revealed by the PXRD analysis (Figures 2h and S11). At the reaction time of 1.0 h, the NCs became faceted (Figure 2b) and the composition of Pt increased from 11.6% to 32.8% (Figure 2g). After the 3.0 h reaction, the size and the Pt content (42.7%) of the intermediates increased associated with the presence of regular octahedra (Figure 2c,f,g). When the reaction time was 5.0 h, the intermetallic PtPb octahedra with the Pt/Pb ratio of about 1/1 (50.6/49.4) were obtained (Figure 2d,f−h). When the reaction time was 6 h, there were hardly size, morphology, composition, and phase changes of octahedra (Figure 2e−h). During the reaction time from 0.5 to 1.0 and to 3.0 h, there are obvious changes in both the PXRD pattern and the structure of the intermediates. The dominant
NCs with monodisperse features have similar sizes and morphologies as the PtPb octahedra. The PtPb1.03Ni0.05 octahedra (Figures 3a,b and S21), PtPb1.07Ni0.10 octahedra (Figures 3c,d and S22), and PtPb1.12Ni0.14 octahedra (Figures 3e,f and S23) were further characterized by a series of techniques, such as TEM, STEM, EDS, and elemental mappings. It is worth mentioning that the Ni mapping is slightly larger than Pb mapping in PtPb1.03Ni0.05 octahedra (Figure 3b), PtPb1.07Ni0.10 octahedra (Figure 3d), and 9578
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(9/1) to remove the excess surfactant. Such treatments allowed the NCs to uniformly distribute on C (Figures S27−S30). The resulting electrocatalysts, namely, PtPb octahedra/C (Figure S27a−c), PtPb 1.03 Ni 0.05 octahedra/C (Figure S28a−c), PtPb1.07Ni0.10 octahedra/C (Figure S29a−c), and PtPb1.12Ni0.14 octahedra/C (Figure S30a−c), were selected as ORR electrocatalysts and compared with the Pt/C (Figure S31a,b). CVs of different electrocatalysts are shown in Figure 5a. According to
PtPb1.12Ni0.14 octahedra (Figure 3f), indicating that the Ni precursor was reduced after the formation of PtPb octahedra. The PXRD for Pt−Pb octahedra and Pt−Pb−Ni octahedra was also carried out (Figure S24). There is hardly a difference among the PXRD patterns for Pt−Pb and Pt−Pb−Ni octahedra (Figure S24a), whereas the enlarged patterns reveal that the half-peak width such as the (110) facet for Pt−Pb−Ni octahedra reduces with increasing the Ni composition (Figure S24b). To further analyze the Pt−Pb−Ni nanostructure, HRTEM, line-scan, and XPS tests have been carried out (Figures 4, S25, and S26). We can see that the PtPb1.12Ni0.14
Figure 5. ORR performances of PtPb 1.12 Ni0.14 octahedra/C, PtPb1.07Ni0.10 octahedra/C, PtPb1.03Ni0.05 octahedra/C, PtPb octahedra/C, and Pt/C. (a) CVs of different electrocatalysts in 0.1 M HClO4 solution. (b) Histogram of ECSAs of different electrocatalysts. (c) ORR polarization curves of different electrocatalysts. (d) Histogram of mass and specific activities of different electrocatalysts. The activities and standard deviations were calculated based on five parallel measurements after Ohmic drop correction. The current densities in panel c were normalized to the geometric area of a rotating disk electrode (RDE, 0.196 cm2).
Figure 4. Structural analyses of PtPb1.03Ni0.05 octahedra, PtPb1.07Ni0.10 octahedra, and PtPb1.12Ni0.14 octahedra. (a, d, g) TEM images, (b, e, h) HRTEM images, and (c, f, i) high-magnification HAADF-STEM images and the corresponding line-scans of (a−c) PtPb1.03Ni0.05 octahedra, (d−f) PtPb1.07Ni0.10 octahedra, and (g−i) PtPb1.12Ni0.14 octahedra.
the CVs curves, the ECSAs of these electrocatalysts were calculated to be 37.2, 38.0, 37.6, 39.5, and 63.8 m2/g for the PtPb 1.12 Ni 0.14 octahedra/C, PtPb 1.07 Ni 0.10 octahedra/C, PtPb1.03Ni0.05 octahedra/C, PtPb octahedra/C, and Pt/C (Figure 5b). The PtPb1.12Ni0.14 octahedra/C has similar ECSA to the PtPb 1.07 Ni 0.10 octahedra/C, PtPb 1.03 Ni 0.05 octahedra/C, and PtPb octahedra/C, due to their similar size and morphology. Figure 5c shows the ORR polarization curves of various electrocatalysts (O2-saturated 0.1 M HClO4, 10 mV/s, and 1600 rpm at room temperature). The mass activity and specific activity for the different electrocatalysts, based on Figure 5c, are presented in Figures 5d and S32 and Table S1 (at 0.90 V vs RHE). In general, the PtPb1.12Ni0.14 octahedra/C has the largest highest mass activity of 1.92 A/mgPt among these five catalysts, which is 1.4, 1.9, 2.7, and 11.4 times larger than those for the PtPb 1.07Ni 0.10 octahedra/C (1.40 A/mgPt), PtPb1.03Ni 0.05 octahedra/C (1.02 A/mgPt), PtPb octahedra/C (0.71 A/ mgPt), and the commercial Pt/C (0.17 A/mgPt), respectively, and about 4.4 times that of the 2020 U.S. Department of Energy target (0.44 A/mgPt).43 The ORR specific activity of the PtPb1.12Ni0.14 octahedra/C reaches to 5.16 mA/cm2, which is 1.4, 1.9, 2.9, and 19.6 times greater than those of the PtPb1.07Ni0.10 octahedra/C, PtPb1.03Ni0.05 octahedra/C, PtPb octahedra/C, and Pt/C, respectively, all of which shows that the PtPb1.12Ni0.14 octahedra/C is one of the best ORR electrocatalysts reported to date (Table S2). We also compared the ORR activities of the PtPb1.12Ni0.14 octahedra/C before and
octahedra possess a clearer core/shell structure than the PtPb1.07Ni0.10 octahedra and PtPb1.03Ni0.05 octahedra (Figure 4a,b,d,e,g,h), because the former contains more Ni to form a thicker PtNi shell. The PtNi shell thicknesses were calculated to be around 0.3 nm for the PtPb1.03Ni0.05 octahedra, around 0.5 nm for the PtPb1.07Ni0.10 octahedra, and around 0.8 nm for the PtPb1.12Ni0.14 octahedra. It is evident that the PtNi shell locates outside the octahedra and distributes homogeneously on the surface, revealed by the line scans and the XPS spectra (Figures 4c,f,i and S26). As determined by XPS (Figure S26), the surface molar ratio of Ni to Pb to Pt is 17.0/40.5/42.5, 18.3/38.3/43.4, and 23.6/34.1/42.3 for PtPb1.03Ni0.05 octahedra, PtPb1.07Ni0.10 octahedra, and PtPb1.12Ni0.14 octahedra, respectively, higher than the overall Ni/Pb/Pt compositions of different Pt−Pb−Ni octahedra, indicating the Ni-rich surface of different Pt−Pb−Ni octahedra. The integration of strong tensile strain of PtPb to the Pt (110) facet, the ordered intermetallic phase core,36 and the active PtNi shell40−42 are highly beneficial for the enhancement of ORR performance. To investigate the ORR properties of different nanostructures, the obtained PtPb octahedra, PtPb1.03Ni0.05 octahedra, PtPb1.07Ni0.10 octahedra, and PtPb1.12Ni0.14 octahedra were loaded onto carbon black (C, Vulcan XC-72R) and then extensively washed with the mixture of ethanol/cyclohexane 9579
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Journal of the American Chemical Society after washing treatment (Figure S33). The PtPb1.12Ni0.14 octahedra/C without washing treatment shows much lower mass activity and specific activity (0.81 A/mgPt and 2.17 mA/ cm2 at 0.90 V vs RHE) than those of the catalyst after treatment, revealing the necessity of severe washing treatment for the preparation of active ORR catalysts. Additionally, the shape evolution of PtPb NCs from nanoplates to octahedra as well as their corresponding electrocatalytic performances have also been checked (Figures S4 and S34). We found that the Pt−Pb NCs/C-22.3 mg AA exhibits the mass activity of 1.04 A/ mgPt and the specific activity of 2.53 mA/cm2 at 0.90 V vs RHE, whereas the Pt−Pb NCs/C-26.8 mg AA (2.02 A/mgPt and 4.59 mA/cm2) and Pt−Pb NCs/C-31.2 mg AA (2.57 A/mgPt and 5.19 mA/cm2) show higher ORR activities than those of the Pt−Pb NCs/C-22.3 mg AA, which is likely due to their higher yield of nanoplates in the products in the presence of more AA. The durability is another important criterion for the electrocatalyst evaluation.17,39,44 An electrocatalyst with better durability can exhibit a much longer lifetime, which is the key for practical applications. To determine ORR durability of the catalysts, the ORR activities of different catalysts before and after different cycles of accelerated durability tests (ADTs, 100 mV/s) were measured. As shown in Figures 6a−d and S35− S36, after 15 000 cycles of ADTs, the ECSA of PtPb1.12Ni0.14 octahedra/C changed from 37.2 m2/g to 34.2 m2/g with only 17.2% mass activity and 9.9% specific activity losses, respectively (Figures 6a,b and S35a,b). While the PtPb1.07Ni0.10
octahedra/C have 20.9% mass activity and 13.3% specific activity losses, the PtPb1.03Ni0.05 octahedra/C show 21.6% and 13.3% losses in mass and specific activities, respectively (Figures S35−S36). Other Pt−Pb−Ni octahedra/C also show higher ECSA losses than that of the PtPb1.12Ni0.14 octahedra/C (Figure S35a−f). Moreover, the PtPb octahedra/C exhibits larger ORR activity loss, about 27.3% in mass activity and 12.9% in specific activity, and also the larger ECSA loss (Figures 6c,d and S35g,h). By contrast, the commercial Pt/C shows as large as 39.9% loss in mass activity, but 26.6% enhancement in specific activity along with the 52.4% ECSA loss (Figures S35i,j and S36e,f). All of these electrocatalysts after 15 000 cycles of ADTs were also investigated by a series of characterizations, such as TEM, HRTEM, EDS, and line scans. It reveals that there were limited changes in the shapes, sizes, compositions, and structures of PtPb1.03Ni0.05 octahedra/C (Figure S28d−f), PtPb1.07Ni0.10 octahedra/C (Figure S29d−f), and PtPb1.12Ni0.14 octahedra/C (Figures S30d−f , S37, and 6e,f) after 15 000 potential cycles. For the PtPb octahedra/C, after durability tests, there was observable composition change, and the PtPb octahedra changed into porous NCs (Figure S27d−f). On the sharp contrast, the nanoparticles on Pt/C became aggregated and larger after ADTs (Figure S31c,d), confirming the improved stability of the Pt−Pb−Ni octahedra/C. The highlight in our work is the improved ORR activities of Pt−Pb−Ni octahedra compared with those of the PtPb octahedra.18,40−42 To understand the origin of enhanced performance of Pt−Pb−Ni octahedra with intermetallic core/ atomic layer shell nanostructures, we further carried out additional study to illustrate the advantage of PtNi versus Pt shells on ORR catalysis (Figures S38−S40). We can see that the Pt1.21Pb octahedra/C with PtPb/Pt core/shell nanostructure exhibits the specific activity of 4.11 mA/cm2 and the mass activity of 1.72 A/mgPt, which is lower than that of the PtPb1.12Ni0.14 octahedra/C, as well as the higher ECSA and activity losses after the durability tests (12.2% ECSA loss, 19.8% mass activity loss, and 8.3% specific activity loss), further revealing the obvious advantage of the active PtNi shell versus the Pt shell (Figures 5, 6, and S40). Therefore, the superior ORR performance of PtPb1.12Ni0.14 octahedra likely arises from their intimate interaction between the intermetallic PtPb core and atomic layer PtNi shell, exposed active facets, and the synergistic effect between different components. Thanks to the addition of Ni, trimetallic PtPb/ PtNi nanostructures with atomic layers shell on the surface have been created, resulting in the maximized Pt utilization, higher than those of PtPb octahedra. Consequently, the PtPb1.12Ni0.14 octahedra exhibit more activity than Pt/C. Furthermore, due to the stable PtPb intermetallic core and the atomic PtNi layers, the intermetallic PtPb 1.12 Ni 0.14 octahedra exhibit outstanding durability, compared with those for the commercial Pt/C and the PtPb octahedra. All of these superiorities ensure the PtPb1.12Ni0.14 octahedra have high ORR performance, even better than many PtNi-based ORR electrocatalysts (Table S2).
Figure 6. ORR durability of PtPb1.12Ni0.14 octahedra/C and PtPb octahedra/C. (a) ORR polarization curves and (b) the corresponding histogram of mass and specific activities of the PtPb1.12Ni0.14 octahedra/C before and after different potential cycles. (c) ORR polarization curves and (d) the corresponding histogram of mass and specific activities for the PtPb octahedra/C before and after different potential cycles. (e) HAADF-STEM image and (f) the corresponding elemental mappings of the PtPb1.12Ni0.14 octahedra/C after 15 000 cycles. The current densities in panels a and c were normalized to the geometric area of a RDE (0.196 cm2).
4. CONCLUSIONS To summarize, we have demonstrated a wet-chemical strategy to successfully synthesize PtPb, Pt1.21Pb, PtPb1.03Ni0.05, PtPb1.07Ni0.10, and PtPb1.12Ni0.14 intermetallic NCs with welldefined shape, size, and composition. The PtPb1.12Ni0.14 octahedra with an intermetallic PtPb core and active PtNi shell exhibit superior activity for ORR with ∼11 and ∼20 times 9580
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(17) Wang, D. L.; Xin, H. L. L.; Hovden, R.; Wang, H. S.; Yu, Y. C.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Nat. Mater. 2012, 12, 81− 87. (18) Choi, S.-I.; Xie, S. F.; Shao, M. H.; Odell, J. H.; Lu, N.; Peng, H. C.; Protsailo, L.; Guerrero, S.; Park, J.; Xia, X. H.; Wang, J. G.; Kim, M. J.; Xia, Y. N. Nano Lett. 2013, 13, 3420−3425. (19) Saleem, F.; Zhang, Z. C.; Xu, B.; Xu, X. B.; He, P. L.; Wang, X. J. Am. Chem. Soc. 2013, 135, 18304−18307. (20) Xu, X. L.; Zhang, X.; Sun, H.; Yang, Y.; Dai, X. P.; Gao, J. S.; Li, X. Y.; Zhang, P. F.; Wang, H. H.; Yu, N. F.; Sun, S. G. Angew. Chem., Int. Ed. 2014, 53, 12522−12527. (21) Bian, T.; Zhang, H.; Jiang, Y. Y.; Jin, C. H.; Wu, J. B.; Yang, H.; Yang, D. R. Nano Lett. 2015, 15, 7808−7815. (22) Zhang, L.; Roling, L. T.; Wang, X.; Vara, M.; Chi, M. F.; Liu, J. Y.; Choi, S.-I.; Park, J.; Herron, J. A.; Xie, Z. X.; Mavrikakis, M.; Xia, Y. N. Science 2015, 349, 412−416. (23) Wang, J. X.; Inada, H.; Wu, L. J.; Zhu, Y. M.; Choi, Y. M.; Liu, P.; Zhou, W.-P.; Adzic, R. R. J. Am. Chem. Soc. 2009, 131, 17298− 17302. (24) Zhang, L.; Iyyamperumal, R.; Yancey, D. F.; Crooks, R. M.; Henkelman, G. ACS Nano 2013, 7, 9168−9172. (25) Wang, X. M.; Orikasa, Y.; Takesue, Y.; Inoue, H.; Nakamura, M.; Minato, T.; Hoshi, N.; Uchimoto, Y. J. Am. Chem. Soc. 2013, 135, 5938−5941. (26) Oh, A.; Baik, H.; Choi, D. S.; Cheon, J. Y.; Kim, B.; Kim, H.; Kwon, S. J.; Joo, S. H.; Jung, Y.; Lee, K. ACS Nano 2015, 9, 2856− 2867. (27) He, D. S.; He, D. P.; Wang, J.; Lin, X.; Yin, P. Q.; Hong, X.; Wu, Y. E.; Li, Y. D. J. Am. Chem. Soc. 2016, 138, 1494−1497. (28) Zhang, S.; Hao, Y. Z.; Su, D.; Doan-Nguyen, V. V. T.; Wu, Y. T.; Li, J.; Sun, S. H.; Murray, C. B. J. Am. Chem. Soc. 2014, 136, 15921− 15924. (29) Zhao, X.; Chen, S.; Fang, Z. C.; Ding, J.; Sang, W.; Wang, Y. C.; Zhao, J.; Peng, Z. M.; Zeng, J. J. Am. Chem. Soc. 2015, 137, 2804− 2807. (30) Zhang, J.; Yang, H. Z.; Fang, J. Y.; Zou, S. Z. Nano Lett. 2010, 10, 638−644. (31) van der Vliet, D. F.; Wang, C.; Tripkovic, D.; Strmcnik, D.; Zhang, X. F.; Debe, M. K.; Atanasoski, R. T.; Markovic, N. M.; Stamenkovic, V. R. Nat. Mater. 2012, 11, 1051−1058. (32) Prabhudev, S.; Bugnet, M.; Bock, C.; Botton, G. A. ACS Nano 2013, 7, 6103−6110. (33) Dhavale, V. M.; Kurungot, S. ACS Catal. 2015, 5, 1445−1452. (34) Wang, D. L.; Yu, Y. C.; Zhu, J.; Liu, S. F.; Muller, D. A.; Abruña, H. D. Nano Lett. 2015, 15, 1343−1348. (35) Cui, Z. M.; Chen, H.; Zhou, W. D.; Zhao, M. T.; DiSalvo, F. J. Chem. Mater. 2015, 27, 7538−7545. (36) Bu, L. Z.; Zhang, N.; Guo, S. J.; Zhang, X.; Li, J.; Yao, J. L.; Wu, T.; Lu, G.; Ma, J. Y.; Su, D.; Huang, X. Q. Science 2016, 354, 1410− 1414. (37) Bu, L. Z.; Ding, J. B.; Guo, S. J.; Zhang, X.; Su, D.; Zhu, X.; Yao, J. L.; Guo, J.; Lu, G.; Huang, X. Q. Adv. Mater. 2015, 27, 7204−7212. (38) Bu, L. Z.; Feng, Y. G.; Yao, J. L.; Guo, S. J.; Guo, J.; Huang, X. Q. Nano Res. 2016, 9, 2811−2821. (39) Bu, L. Z.; Guo, S. J.; Zhang, X.; Shen, X.; Su, D.; Lu, G.; Zhu, X.; Yao, J. L.; Guo, J.; Huang, X. Q. Nat. Commun. 2016, 7, 11850. (40) Cui, C. H.; Gan, L.; Heggen, M.; Rudi, S.; Strasser, P. Nat. Mater. 2013, 12, 765−771. (41) Chen, C.; Kang, Y. J.; Huo, Z. Y.; Zhu, Z. W.; Huang, W. Y.; Xin, H. L. L.; Snyder, J. D.; Li, D. G.; Herron, J. A.; Mavrikakis, M.; Chi, M. F.; More, K. L.; Li, Y. D.; Markovic, N. M.; Somorjai, G. A.; Yang, P. D.; Stamenkovic, V. R. Science 2014, 343, 1339−1343. (42) Huang, X. Q.; Zhao, Z. P.; Cao, L.; Chen, Y.; Zhu, E. B.; Lin, Z. Y.; Li, M. F.; Yan, A. M.; Zettl, A.; Wang, Y. M.; Duan, X. F.; Mueller, T.; Huang, Y. Science 2015, 348, 1230−1234. (43) U.S. Department of Energy, Technical Plan: Fuel Cells, 2016, (http://www.energy.gov/sites/prod/files/2016/06/f32/fcto_myrdd_ fuel_cells_0.pdf).
enhancement on mass activity and specific activity than the Pt/ C, respectively, as well as better than those of the PtPb octahedra, even better than those of the PtPb/Pt core/shell octahedra, making it among the most promising ORR electrocatalysts up to now. Those PtPb1.12Ni0.14 octahedra are also very stable after long-term durability investigation. We anticipate the present work will inspire the rational design of high-performance Pt-based NCs with well-controlled structure, phase, and surface composition for heterogeneous reactions, fuel cells, and beyond.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03510. Experimental details and data. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Xiaoqing Huang: 0000-0003-3219-4316 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology (2016YFA0204100), the National Natural Science Foundation of China (21571135), Young Thousand Talented Program, the start-up supports from Soochow University, and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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REFERENCES
(1) Bashyam, R.; Zelenay, P. Nature 2006, 443, 63−66. (2) Wang, Y. J.; Wilkinson, D. P.; Zhang, J. J. Chem. Rev. 2011, 111, 7625−7651. (3) Guo, S. J.; Wang, E. K. Nano Today 2011, 6, 240−264. (4) Wang, Y. J.; Zhao, N. N.; Fang, B. Z.; Li, H.; Bi, X. T. T.; Wang, H. J. Chem. Rev. 2015, 115, 3433−3467. (5) Chen, D. J.; Chen, C.; Baiyee, Z. M.; Shao, Z. P.; Ciucci, F. Chem. Rev. 2015, 115, 9869−9921. (6) Shao, M. H.; Chang, Q. W.; Dodelet, J.-P.; Chenitz, R. Chem. Rev. 2016, 116, 3594−3657. (7) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J. J.; Lucas, C. A.; Wang, G. F.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241−247. (8) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G. F.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493−497. (9) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T. F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Nat. Chem. 2009, 1, 552−556. (10) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal., B 2005, 56, 9−35. (11) Peng, Z. M.; Yang, H. Nano Today 2009, 4, 143−164. (12) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443−447. (13) Zhang, H.; Jin, M. S.; Xia, Y. N. Chem. Soc. Rev. 2012, 41, 8035− 8049. (14) Porter, N. S.; Wu, H.; Quan, Z. W.; Fang, J. Y. Acc. Chem. Res. 2013, 46, 1867−1877. (15) Wu, J. B.; Yang, H. Acc. Chem. Res. 2013, 46, 1848−1857. (16) Kim, J.; Lee, Y. M.; Sun, S. H. J. Am. Chem. Soc. 2010, 132, 4996−4997. 9581
DOI: 10.1021/jacs.7b03510 J. Am. Chem. Soc. 2017, 139, 9576−9582
Article
Journal of the American Chemical Society (44) Xia, B. Y.; Wu, H. B.; Li, N.; Yan, Y.; Lou, X. W.; Wang, X. Angew. Chem., Int. Ed. 2015, 54, 3797−3801.
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DOI: 10.1021/jacs.7b03510 J. Am. Chem. Soc. 2017, 139, 9576−9582
