Well-Coupled Nanohybrids Obtained by Component-Controlled

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 8155−8164

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Well-Coupled Nanohybrids Obtained by Component-Controlled Synthesis and in Situ Integration of MnxPdy Nanocrystals on Vulcan Carbon for Electrocatalytic Oxygen Reduction Yanan Lu,† Shulin Zhao,†,‡ Rui Yang,†,§ Dongdong Xu,† Jing Yang,† Yue Lin,*,‡ Nai-En Shi,§ Zhihui Dai,† Jianchun Bao,† and Min Han*,†,∥ †

Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, P. R. China ‡ Hefei National Laboratory for Physical Sciences at the Microscale, University of Science & Technology of China, Hefei 230026, China § Key Laboratory for Organic Electronics and Information Displays, Institute of Advanced Materials, Nanjing University of Posts & Telecommunications, Nanjing 210023, P. R. China ∥ State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: Development of cheap, highly active, and robust bimetallic nanocrystal (NC)-based nanohybrid (NH) electrocatalysts for oxygen reduction reaction (ORR) is helpful for advancing fuel cells or other renewable energy t echnologies. Here, four kinds of well-coupled MnxPdy(MnPd3, MnPd−Pd, Mn2Pd3, Mn2Pd3−Mn11Pd21)/C NHs have been synthesized by in situ integration of MnxPdy NCs with variable component ratios on pretreated Vulcan XC72 C using the solvothermal method accompanied with annealing under Ar/H2 atmosphere and used as electrocatalysts for ORR. Among them, the MnPd3/C NHs possess the unique “half-embedded and half-encapsulated” interfaces and exhibit the highest catalytic activity, which can compete with some currently reported non-Pt catalysts (e.g., Ag−Co nanoalloys, Pd2NiAg NCs, PdCo/N-doped porous C, G-Cu3Pd nanocomposites, etc.), and close to commercial Pt/C. Electrocatalytic dynamic measurements disclose that their ORR mechanism abides by the direct 4e− pathway. Moreover, their durability and methanol-tolerant capability are much higher than that of Pt/C. As revealed by spectroscopic and electrochemical analyses, the excellent catalytic performance of MnPd3/C NHs results from the proper component ratio of Mn and Pd and the strong interplay of their constituents, which not only facilitate to optimize the d-band center or the electronic structure of Pd but also induce the phase transformation of MnPd3 active components and enhance their conductivity or interfacial electron transfer dynamics. This work demonstrates that MnPd3/C NHs are promising methanol-tolerant cathode electrocatalysts that may be employed in fuel cells or other renewable energy option. KEYWORDS: nanohybrids, bimetallic nanocrystals, carbon nanostructures, electrocatalysis, oxygen reduction reaction metal−N−C material,15−18 non-Pt bimetallic or multimetallic nanocrystals (NCs),19−23 and so on, have been considered to be the promising alternatives for Pt-based ones. Among them, much attention have been paid on non-Pt bimetallic or multimetallic NCs owing to their good conductivity and efficient atomic ensemble and electronic coupling that facilitates to mediate the ORR activity and stability by varying component

1. INTRODUCTION The electrochemical oxygen reduction reaction (ORR) has aroused considerable interest because of its paramount applications in energy conversion and storage fields.1−3 Pt and Pt-based nanoalloys have been proven to be highly active electrocatalysts toward ORR.4−8 Nonetheless, the scarcity and high cost of Pt as well as the poor durability preclude their large-scale application.9 So, it is imperative but challenging to explore high-efficient and robust electrocatalysts for replacing Pt-based ones to apply in ORR. Recently, several nanostructured materials, such as heteroatoms-doped carbon materials,9−11 metal oxides or related composites,12−14 © 2018 American Chemical Society

Received: September 12, 2017 Accepted: January 31, 2018 Published: January 31, 2018 8155

DOI: 10.1021/acsami.7b13872 ACS Appl. Mater. Interfaces 2018, 10, 8155−8164

Research Article

ACS Applied Materials & Interfaces

catalytic activity, whose onset reduction potential and Tafel slope are 0.953 V (vs RHE) and 65 mV dec−1, respectively. Further electrocatalytic kinetic tests reveal that the ORR process of MnPd3/C NHs abides by the direct “4e−” pathway. Moreover, the catalytic stability of the MnPd3/C NHs is superior to Pt/C, which can continuously operate for 5000 cycles, nearly without loss of activity. Also, they exhibit better methanol tolerance ability than Pt/C. By combining microstructural, spectroscopic, and electrochemical analyses, the origin of their excellent catalytic performance has also been suggested.

and engineering phase structure, shapes, defects, and stress.20−24 As one of the important platinum-group metals, Pd is more abundant (200 times) and much cheaper than Pt, but its catalytic property is close to Pt. So, great efforts have been devoted to combining Pd and other cheap transition metals to develop alternative bimetallic or multimetallic catalysts for ORR. By far, many Pd-based bimetallic or multimetallic NCs, including Fe−Pd,24,25 Co−Pd,26 Ni−Pd,27 Cu−Pd,28,29 Pd− V,30 Pd−Pb,31 Pd−Cr,32 Ag(Au)@CuPd,22 Pd−Ni−Ag,23 and Au−Pd−Co,33 have been identified to be excellent electrocatalysts for ORR. It is reported that incorporation of a second metal into Pd lattice facilitates to improve the ORR activity of Pd by inhibiting the oxidation of Pd and stabilizing Pd nanoparticles.34 Being a typical 3d transition metal, Mn is expected to offer the same enhancement effect as that of other first-row transition metals (e.g., Fe, Co, and Ni) for Pd. Nonetheless, rational synthesis of MnxPdy NCs with a variable component ratio remains a challenge because of the low electronegativity or very negative electrode potential of Mn [φMn2+/Mn = −1.18 V (vs SHE)],35 and their electrocatalytic application for ORR has not been fully studied. To disperse and stabilize active metal NCs, a proper support is required. Usually, the highly conductive carbon materials with a large specific surface area, including activated carbon, Vulcan XC-72 carbon,30 carbon nanotubes,36 and graphene,37,38 are the ideal support for dispersion and integration of metal NCs. However, these pristine carbon materials cannot be directly used as the support because they lack specific functional groups or “landing sites” for capturing metal NCs, and only the weak physical interaction can be utilized. To enhance the interaction of metal NCs and carbon support and improve their catalytic performance, pretreating these carbon materials and introducing proper functional groups or “landing” sites for anchoring metal NCs to form strongly coupled nanohybrids (NHs) are considered to be an efficient avenue. For example, Wang and Dai found that strongly coupled inorganic nanoparticles and graphitic carbon NHs possess much higher electrocatalytic performance than the physically mixed inorganic nanoparticles and carbon materials.39 Inspired from Dai’s works and considering the research status of Pdbased bimetallic NCs, fabricating strongly coupled MnxPdy/C NHs and exploring their application for ORR attract our interest. Here, we report the synthesis of MnxPdy (MnPd3, MnPd− Pd, Mn2Pd3, and Mn2Pd3−Mn11Pd21)/C NHs and their electrocatalytic ORR performance. These MnxPdy/C NHs are fabricated by solvothermal treatment of Mn2(CO)10 and Pd(acac)2 precursors in the presence of carboxylic-functionalized Vulcan XC-72 C, polyvinylpyrrolidone (PVP), and citronellol and then annealing under Ar/H2 at 400 °C. In these MnxPdy/C NHs, the MnxPdy NCs with a variable composition ratio are dispersed or embedded into the C support to form well-coupled NHs. Particularly, for MnPd3/C NHs, the MnPd3 NCs are the ordered alloy with the tetragonal phase structure, which are half-embedded into the C support and halfencapsulated by thin C shells to form unique “all-inclusive” interfacial structures. Also, the C shells are found to be mesoporous with the average pore size of ∼5.4 nm, which is beneficial to electrocatalytic applications. Compared with pure Vulcan C and pure MnxPdy NCs, these MnxPdy/C NHs show greatly enhanced electrocatalytic ORR activity in alkaline solution. Among them, the MnPd3/C NHs exhibit the highest

2. EXPERIMENTAL SECTION 2.1. Synthesis of MnPd3/C NHs. Before the experiments, the commercial Vulcan XC-72 C was pretreated in HNO3 solution to graft carboxylic groups on it (Supporting Information). For each synthesis, 200 mg of PVP, 4 mL of citronellol, 50 mg of pre-treated Vulcan XC72 C, 0.05 mmol of Mn2(CO)10, and 0.3 mmol of Pd(acac)2 were added into a Teflon-lined autoclave at room temperature (RT) in turn. By violent magnetic stirring for 1 h, the black suspension was formed. Then, the autoclave was sealed and transferred to an oven, which was heated from RT to 150 °C and kept at 150 °C for 6 h. After separated by centrifugation and washed with acetone/absolute ethanol several times, the black solid was obtained, which was dried in vacuum and then placed in the central region of a horizontal furnace for annealing treatment under Ar and H2 atmosphere. The heating rate was 2 °C min−1, and the annealing temperature and time were 400 °C and 1 h, respectively. 2.2. Synthesis of Other MnxPdy/C NHs. The synthesis procedures for other MnxPdy/C NHs were similar to those for MnPd3/C NHs. Letting other conditions unchanged and solely varying the initial mole ratio of Mn and Pd precursors from 1:3 to 1:5, 1:1, and 3:1, other MnxPdy (MnPd−Pd, Mn2Pd3 and Mn2Pd3− Mn11Pd21)/C NHs could also be obtained. 2.3. Synthesis of Pd/C NHs. The Pd/C NHs were also synthesized using the method similar to that for MnPd3/C NHs. In this case, only Pd(acac)2 was employed as the metal precursors. 2.4. Materials Characterization. The X-ray energy-dispersive spectra (EDS) and induction couple plasma (ICP) tests were performed on a JSM-5610LV-VANTAGE energy spectrometer and a Jarrel-Ash 1100 + 2000 quantometer, respectively. The powder X-ray diffraction (XRD) patterns were recorded on a SmartLab (9 kW) Xray diffractometer (Rigaku Corp.) equipped with the rotating anode Cu target (λKα = 1.54060 Å) and a D/Tex Ultra 250 high-speed detector. For XRD tests, the used working voltage was 45 kV, and the corresponding current was 200 mA. The Raman spectra were acquired on a JY HR 800 (France) instrument that equipped an optical multichannel spectrometer Microdil 28 (Dilor) along with a microscope. To focus the excitation light (Ar+ laser, 488 nm) and collect the scattered light, an objective with 100× magnification was employed. The related transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images as well as the high-angle annular dark field scanning TEM (HAADF-STEM) images and elemental mapping analysis were performed on JEOL-2100F and probe aberration-corrected JEM ARM 200F apparatuses, respectively. The X-ray photoelectron spectra (XPS) were collected by using a scanning X-ray microprobe (PHI 5000 Versa, ULACPHI, Inc.) apparatus, and the alternating current impedance spectra were recorded on the CHI 660E electrochemical workstation in O2saturated 0.1 M KOH at 25 °C by applying a 10 mV potential modulation at −0.4 V (vs Ag/AgCl). The related frequency for impedance tests was set to be 10−1 to 106 Hz. 2.5. Electrocatalytic Tests. The electrocatalytic properties were evaluated on a CHI 700E workstation that equipped a Gamry’s rotating disk or ring-disk electrode [(rotating disk voltammetry (RDV) or rotating ring-disk voltammetry (RRDV)] with a diameter of 5 mm. For all the tests, a standard three-electrode system was adopted. The MnxPdy/C NHs- or other control catalyst-modified RDV or RRDV, Pt 8156


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Figure 1. (A,B) XRD pattern (A) and Raman spectrum (B) of MnPd3/C NHs. (C−F) Core-level XPS spectra for C 1s (C), O 1s (D), Mn 2p (E), and Pd 3d (F) peaks.

efficient bonding or “landing” sites for the nucleation and growth of MnxPdy NCs. By solvothermal treatment of Mn2(CO)10 and Pd(acac)2 with different molar ratios in the presence of carboxylic-functionalized Vulcan XC-72 C, PVP, and citronellol, and then annealing under Ar and H 2 atmosphere at 400 °C, the desired MnxPdy/C NHs can be obtained. The component, crystallinity, and phase structure of the typical MnPd3/C NHs were characterized by EDS, ICP, XRD and Raman and XPS. Besides the small O signal from the adsorbed air or residual oxygen-containing groups, only Mn, Pd, and C elements are found in the EDS pattern (Figure S2). By integral calculation, the Mn and Pd molar ratio is approximately 1:3, which is consistent with the ICP analysis and implies that the MnPd3 NC component may be present in the NHs. Further XRD analysis can support this viewpoint. As shown in Figure 1A, eight obvious diffraction peaks that marked with the red hearts are observed at the 2θ of 23.2°, 32.9°, 40.1°, 46.7°, 53.2°, 58.7°, 68.1°, and 82.1°, which can be indexed to the (101), (110), (114), (200), (211), (109), (220), and (314) planes of the MnPd3 with the tetragonal phase structure (JCPDS-65-5549). Among them, the four small and featured diffraction peaks appeared at 23.2°, 32.9°, 53.2°, and 58.7° reveal that the MnPd3 NCs in the NHs may be the ordered alloy or the intermetallic phase with the Cu3Au structure type.40,41 Except for these eight diffraction peaks, another broad and shoulder peak centered at 25° that is marked with the black

plate, and Ag/AgCl electrode were used as the working electrode, counter electrode, and reference electrode, respectively. As for the working electrode, it was fabricated according to a previous report.28 The loading amount of various catalysts was 0.204 mg cm−2. For ORR tests, the electrolyte (0.1 M KOH) was first bubbled with pure O2 (>99.99%) for 0.5 h, and then the solution was blanketed with O2 gas in the whole experiment. The linear sweep voltammetry plots were recorded at −0.8 to 0.2 V (vs Ag/AgCl) under different electrode rotating rates. The related scan rate was 5 mV s−1. As for calculating the number (n) of transferred electrons, the detailed procedures are given in the Supporting Information section. The methanol tolerance tests were executed in the O2-saturated electrolyte that contains 0.1 M KOH and 1 M methanol by using a chrono-amperometric method. To compare conveniently, all the potentials were transformed to reversible hydrogen electrode (RHE) values through the formula E (RHE) = E (Ag/AgCl) + 0.197 + 0.0591pH. All the current densities were calculated based on the electrode apparent area.

3. RESULTS AND DISCUSSION Owing to its low cost, high specific surface area, good conductivity, and easy functionalization, the commercial Vulcan XC-72 C is chosen as the support. To efficiently capture Mn and Pd precursors to obtain MnxPdy NCs, the Vulcan XC-72 C is first refluxed in HNO3 solution to introduce carboxylic groups. In contrast to pristine Vulcan XC-72 C (Figure S1A), there is nearly no shape variation for the sample after acidic treatment (Figure S1B). The related Raman and FT-IR spectra (Figure S1C,D) confirm that the carboxylic groups have been introduced into the C support after acidic treatment, providing 8157


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ACS Applied Materials & Interfaces rhomb can be ascribed to the (002) plane of the hexagonal graphite structure, indicating the presence of carbon-based nanostructures in the NHs. Further evidence comes from the Raman spectra, as shown in Figure 1B. Two featured Raman peaks for MnPd3/C NHs are observed at about 1349 and 1587 cm−1, which can be assigned to the D-band (sp3-hybridization) and G-band (sp2-hybridization) of carbon-based nanostructures,42 respectively. Moreover, both the Raman peaks of MnPd3/C NHs are positively shifted by ∼7 cm−1 relative to pure Vulcan XC-72 C (Figure S3), indicating that there is a strong coupling of MnPd3 NCs and C nanostructures in the NHs.43 For further identifying surface components and chemical states of various elements, XPS tests are also performed. The survey XPS spectrum (Figure S4) confirms the existence of C, O, Mn, and Pd elements, in accordance with EDS analysis. The related C 1s core-level XPS spectrum (Figure 1C) is slightly asymmetric and can be convoluted into one strong peak at 284. 6 eV as well as two small peaks at 285.9 and 289.8 eV, corresponding to form −CC−, −C−O−, and OC−O− bonds, respectively.13 Compared with that of the −CC− bond, the very small integration areas for −C−O− and OC−O− peaks indicate that the O content is low. Usually, the trace amount of the O element is believed to arise from the residual oxygen containing groups in initially functionalized carbon nanostructures or from the adsorbed air, whereas in our NHs, the O 1s core-level XPS spectrum (Figure 1D) is also asymmetric and can be deconvoluted into four peaks that centered at 533.8, 532.7, 531.6, and 530.4 eV, ascribing to form −CO/M−O−C−, −C−O−/H2O, −OH, and M−O bonds, respectively.44 So, the small amount of the O element is mainly attributed to the residual oxygen-containing groups in initially carboxylic-functionalized Vulcan C support. As for the Mn 2p core-level XPS spectrum (Figure 1E), the two peaks centered at 641.3 and 652.5 eV are the characteristic 2p3/2 and 2p1/2 peaks for Mn2+ ions.13,45 The small satellite peak for Mn2+ ions is observed at 646.2 eV. The other two peaks at 642.9 and 654.2 eV are the featured 2p3/2 and 2p1/2 peaks for Mn3+ ions.13 The presence of high-valenced Mn signals may be attributed to the following two reasons: (1) a part of metallic Mn may be oxidized to form amorphous oxides or coordinate with residual oxygen-containing groups to form the complex, as inferred from the O 1s spectrum; (2) the Mn atoms may donate electrons to Pd, letting them possess the positive charge in the form. Further evidence comes from the Pd 3d core-level spectrum. As shown in Figure 1F, two peaks observed at 335.8 and 341.1 eV are the featured 3d5/2 and 3d3/2 peaks for metallic Pd(0), respectively.46 Relative to pure Pd/C NHs (Figure S5), both Pd 3d5/2 and 3d3/2 peaks of MnPd3/C NHs are slightly shifted to the low binding energy (BE) direction, implying that the interatomic polarization between Mn and Pd actually occurs and the incorporated Mn atoms donate electrons to Pd that make the Pd possess the negative charge in form.47 Such a negative charge can weaken the adsorption of intermediate species on Pd and expose more available active sites, facilitating to enhance ORR activity. Additionally, on the basis of the integral areas of Mn 2p and Pd 3d peaks, the molar ratio of Mn and Pd is about 1:3, further confirming the formation of the MnPd3 component in the NHs. The shape and the microstructure of the MnPd3/C NHs were further studied by TEM and HRTEM. From the low magnification TEM image (Figure 2A), many quasi-spherical black dots can be observed, which are decorated on the edges

Figure 2. (A,B) Low- (A) and high- (B) magnification TEM images for MnPd3/C NHs. The inset of (A) shows the histogram for addressing the size distribution of MnPd3 NCs in the NHs. (C) Representative HRTEM image for an individual MnPd3 NC on the edge region of the C support. (D) Related HAADF-STEM image and elemental mapping figures for an individual MnPd3 NC in the NHs.

or planes of the shallow disks or sheets. According to their difference on contrast, the black dots are the formed MnPd3 NCs, while the shallow disks or sheets are the Vulcan C support. With respect to the average size of the MnPd3 NCs, it is estimated to be ∼14 nm on the basis of statistical analysis (inset of Figure 2A). The related high magnification TEM image (Figure 2B) reveals that most of the MnPd3 NCs are anchored or embedded into the planes or edges of C support to form well-coupled NHs. To clearly identify the interface between MnPd3 NCs and C support, 100 MnPd3 NCs on the edges or planes of the C supports have been selected to make the HRTEM analysis. On the basis of the statistical analysis, the interface between the MnPd3 NC and C support is found to be the “half-embedded and half-encapsulated” type, i.e., one-half of the MnPd3 NC is embedded into the C support, and the another-half of the MnPd 3 NC is extended out and encapsulated by a thin layer of the C shell. Figure 2C shows the representative HRTEM image for an individual MnPd3 NC on the edge region of one C nanosheet support, from which the unique “half-embedded and half-encapsulated” interface structure can be seen. Such an “all-inclusive” interface structure favors electrocatalytic application, especially for enhancing electrocatalytic stability of the NHs according to previous reports.48−50 Moreover, the MnPd3 NC exhibits the clear lattice fringes, and the lattice spacing is ∼2.30 Å, corresponding to the (114) plane of tetragonal phase MnPd3 (JCPDS-65-5549). As for the C support, it also shows the clear lattice fringes, and the lattice spacing is ∼3.40 Å, assigning to the (002) plane of the hexagonal graphite structure. Further HAADF-STEM and elemental mapping analyses (Figure 2D) for an individual MnPd3 NC in the NHs reveal that the Mn and Pd elements are homogeneously distributed along the whole NC, confirming the generation of the Mn−Pd nanoalloy structure. Additionally, the corresponding N2 adsorption−desorption tests are carried 8158


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Figure 3. (A) Cyclic voltammetry curves of the MnPd3/C NHs in the N2- (a) or O2- (b) saturated 0.1 M KOH electrolyte. (B) Rotating ratedependent ORR polarization plots for MnPd3/C NHs. The inset exhibits the related Koutecky−Levich curve at 0.5 V (vs RHE). (C) Yield of peroxide species (black lines) and the number of transferred electrons (n) (blue line) at different potentials for MnPd3/C NHs obtained from RRDV data. (D) Durability test for MnPd3/C NHs.

Figure 3A. Under both cases, the observed four pairs of redox peaks can be assigned to the Hupd adsorption−desorption peaks, Mn redox peaks, and Pd redox peaks. The detailed data and assignments are given in the Supporting Information, Figure S10. Moreover, relative to that in the N2-saturated condition, the occurrence of a strong reduction current peak in O2-saturated electrolyte indicates that the MnPd3/C NHs can efficiently catalyze ORR. By magnifying the CVs (Figure S11), the onset reduction potential of the MnPd3/C NHs is identified to be 0.953 V (vs RHE), which is slightly lower than Pt/C and comparable or superior to recently reported Ag−Co nanoalloys,20 Pd2NiAg NCs,23 G-Cu3Pd nanocomposites (NCPs),28 G-FePd3 NCPs,38 PdCo/N-doped porous C NHs,51 PdNiCu/ NG and PdNiSn/NG,52 Ag4Sn/C NHs,53 AuCu/C NHs,54 PdHPW-CMK,55 Pd-g-C3N4,56 Co@Co3O4@PDD NCPs,57 Co− N/CNFs or CNTs,58,59 and so on. The detailed comparison of various catalytic parameters is summarized in Table S1. To better understand the catalytic behavior, the RDV tests are performed. Figure 3B exhibits the rotating rate-dependent ORR polarization plots, from which the number of transferred electrons (n) during the ORR process can be computed using the Koutecky−Levich (K−L) equation (please see the details in the Supporting Information section). The K−L plot (inset of Figure 3B) at 0.5 V (vs RHE) displays good linearity, indicating that the ORR on MnPd3/C NHs complies with the first-order kinetics. On the basis of the slope of the K−L plot, the n value of MnPd3/C NHs is calculated to be about 4.0, revealing that the ORR process of MnPd3/C NHs may follow the direct “4e−” avenue. To further confirm the ORR mechanism and identify the yield of intermediate peroxide species, the RRDV tests are further carried out. According to the RRDV data, the generated peroxide species during the ORR process of MnPd3/C NHs is lower than 12% (Figure 3C) at 0.30−0.70 V (vs RHE). Such a low yield of peroxide species implies that the ORR efficiency is high, favoring the direct “4e−” reaction mechanism. Computed from the ring current (IR) and disk current (ID) data, the n values at 0.30−0.70 V (vs RHE) are found to be 3.78−3.95,

out yet, as shown in Figure S6. The results demonstrate that the MnPd3/C NHs possess mesoporous structures with a BET surface area of ∼65.3 m2 g−1 and a average pore size of ∼5.4 nm. Combined with HRTEM analysis (the MnPd3 NCs are solid, not hollow or porous), the mesopores may result from the carbon shell layers that covered on MnPd3 NCs, which could be helpful for the reactants to contact the inner MnPd3 cores and trigger or promote the desired catalytic reaction. Moreover, except for MnPd3/C NHs, other MnxPdy/C ones with a variable component ratio and phase structure also can be obtained by adjusting metallic precursors’ mole ratio, while keeping other conditions unchanged. For instance, when the molar ratio of Mn and Pd precursors is decreased to 1:5 (Figure S7), the Mn−Pd bimetallic NCs in the finally obtained NHs are the mixed phase product composed of cubic-phase MnPd (JCPDS-65-4083) nanoalloy, and cubic-phase Pd (JCPDS-87637). To describe simply, the product obtained under this case is labeled as MnPd−Pd/C NHs. As the molar ratio of Mn and Pd precursors is increased to 1:1 (Figure S8), the Mn2Pd3/C NHs will be obtained, and the Mn2Pd3 NC active component exhibits the tetragonal phase structure (JCPDs-18-807). Further increasing the molar ratio of Mn and Pd precursors to be 3:1 (Figure S9), the Mn−Pd bimetallic NCs in the finally obtained NHs are identified to be the mixed-phase product made up of tetragonal phase Mn2Pd3 (JCPDS-18-807) and Mn11Pd21 (JCPDS-65-9676). Thus, the product generated under this circumstance is marked as Mn2Pd3−Mn11Pd21/C NHs. Besides the component and phase structure, the shapes of the MnxPdy NCs in the finally obtained NHs are also changed with the variation of Mn and Pd precursor molar ratios. On the whole, by increasing the amount of Mn precursors, the MnxPdy NCs in the NHs will progressively change from a quasi-sphere to a thin sheet-like structure. The electrocatalytic properties of MnPd3/C NHs were examined in the N2- and O2-saturated 0.1 M KOH electrolyte at the potential ranging from 0.17 to 1.17 V (vs RHE). The corresponding cyclic voltammogram (CV) curves are given in 8159


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Figure 4. (A) ORR polarization plots of MnPd3/C (a), MnPd−Pd/C (b), Mn2Pd3/C (c), and Mn2Pd3−Mn11Pd21/C (d) NHs by fixing the electrode rotating rate at 1600 rpm. (B) Related Tafel plots for those MnxPdy/C NHs. (C,D) Kinetic current densities (C) and mass specific activities (D) of those MnxPdy/C NHs at distinct potential.

Figure 5. (A) BEs of Pd 3d5/2 peaks and the Jm of MnxPdy/C NHs as a function of Pd atomic percentage in MnxPdy active components. (B) BEs of Pd 3d3/2 peaks and the Jm of MnxPdy/C NHs as a function of Pd atomic percentage in MnxPdy active components.

Pt/C (∼50.5%) under the identical conditions. This result indicates that the MnPd3/C NHs possess better methanol tolerance ability than Pt/C yet, showing great promise as a methanol-tolerant cathode catalyst to apply in alkaline fuel cells. Furthermore, the electrocatalytic performance of the MnPd3/ C NHs is also compared with that of other MnxPdy/C ones. Their ORR polarization plots are placed together and shown in Figure 4A. Relative to MnPd−Pd/C, Mn2Pd3/C, and Mn2Pd3− Mn11Pd21/C NHs, the MnPd3/C NHs exhibit the most positive onset reduction potential and half-wave potential, indicating that their electrocatalytic activities are much higher than other MnxPdy/C ones. The related Tafel plots (Figure 4B) will provide further evidence for this. Deduced from the Tafel plots, the Tafel slope of MnPd3/C NHs is 65 mV dec−1, which is smaller than those of MnPd−Pd/C (88 mV dec−1), Mn2Pd3/C (68 mV dec−1), and Mn2Pd3−Mn11Pd21/C (85 mV dec−1) ones and the recently reported Pd3Fe/C (71.4 mV dec−1),25 GCu3Pd NCPs (68.9 mV dec−1),28 and Pd-HPW-CMK (76.9 mV dec−1).55 The smallest Tafel slope observed on MnPd3/C NHs means that they possess the highest catalytic activity. To confirm this, the related kinetic current densities (Jk) at different potentials are calculated. As presented in Figure 4C,

which is close to 4.0, confirming that the ORR on MnPd3/C NHs abides by the direct “4e−” pathway that is consistent with the K−L plot analysis from RDV. Besides the activity and reaction mechanism, another key parameter for affecting the application of a desired catalytic material is its long-term stability or durability. Figure 3D shows the durability test plots for MnPd3/C NHs. After continuously working for 5000 cycles, they can preserve 98.5% of their initial current density. Additionally, the difference of their half-wave potentials (ΔE1/2) before and after the cycling test is very low, only 4.0 mV, whereas for commercial Pt/C (Figure S12), it only can reserve 92.1% of its initial current density after the same cycling test. Also, its ΔE1/2 value (28 mV) is much larger than that of MnPd3/C NHs. Similar phenomenon has been observed on core/shell Ag(Au)/CuPd nanoparticles by Sun and co-authors.22 These results reveal that the catalytic stability of the MnPd3/C NHs is much higher than Pt/C. Moreover, in O2-saturated 0.1 M KOH +1 M CH3OH electrolyte, the methanol tolerance capability of the MnPd3/C NHs is further evaluated by using a chronoamperometric method at 0.7 V (vs RHE). As illustrated in Figure S13, the current density on MnPd3/C NHs is only reduced by 6.2%, far less than that on 8160


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ACS Applied Materials & Interfaces the Jk of MnPd3/C NHs can reach 31.3 mA cm−2 at the potential of 0.65 V (vs RHE), which is superior to MnPd−Pd/ C NHs, Mn2Pd3/C NHs, and Mn2Pd3−Mn11Pd21/C NHs. On the basis of the Jk plots of those MnxPdy/C NHs at various potential, their mass specific activities (Jm) can be computed using the formula: Jm = [Jk × Rf (roughness factor)]/Lcatalyst (the loading mass of catalyst),60 by supposing that the Rf is equal to 1.0. As can be seen from Figure 4D, the Jm of the MnPd3/C NHs can approach 163.4, 153.2, 96.1, and 51.1 mA/ mgNHs at the potential of 0.60, 0.65, 0.70, and 0.75 V (vs RHE), respectively, which are much higher than those of MnPd−Pd/ C, Mn2Pd3/C, and Mn2Pd3−Mn11Pd21/C ones. It should be mentioned that the Jm of MnPd3/C NHs will surpass 200 mA/ mgPd by using pure Pd’s mass for calculation. For instance, the Jm of MnPd3/C NHs at 0.65 V (vs RHE) can reach 696.3 mA/ mgPd, outperforming the recently reported G-Cu3Pd NCPs (∼430 mA/mgPd).28 All the results reveal that the MnPd3/C NHs possess the highest catalytic activity among the obtained four MnxPdy/C NHs. To tentatively understand why the MnPd3/C NHs possess the best catalytic activity among these MnxPdy/C NHs, the dband center or the electronic structure of Pd in MnxPdy alloy components is studied with the aim to correlate their ORR activities. Previous research demonstrates that the variations of surface core-level shifts can reflect the similar changes of the dband center of desired catalytic materials.61 Figure 5 summarizes the evolution of core-level BEs of Pd 3d peaks and Jm of MnxPdy/C NHs with the Pd contents in alloy NCs. Here, the Pd contents for those MnxPdy NCs are obtained from ICP tests. With the increase of Mn content, the distorted “W”type or inverse double-volcano curves are observed for both BEs of Pd 3d5/2 and 3d3/2 peaks, disclosing that the insertion of Mn into the Pd lattice can efficiently modulate the d-band center or electronic structures of Pd in MnxPdy NCs, whereas for the Jm of MnxPdy/C NHs, it only shows a normal single volcano plot with the variation of the BEs of both Pd 3d5/2 and 3d3/2 peaks, implying that the ORR activities of the MnxPdy/C NHs are not consistent with the evolution of the d-band center or the electronic structure of Pd in MnxPdy alloy NCs. This discrepancy can be explained by the classic Hammer−Nørskov model,62 i.e., the variation of the d-band center has two opposite effects toward the chemisorption of O2 molecules and intermediates (OHads), which determines the ORR performance. To maximize the ORR activity, there should be an optimized d-band center or Pd 3d BEs for MnxPdy/C NHs. Among them, the MnPd3/C NHs exhibit a middle BEs value for both Pd 3d5/2 and 3d3/2 peaks, which may establish a good balance between the chemical activation of O2 molecules and adsorption−desorption of reaction intermediates (OHads). So, the MnPd3/C NHs are located at the vertex of the activity single-volcano plot. That is to say, the MnPd3/C NHs possess the highest ORR activity. On the basis of abovementioned analysis, the best catalytic activity of MnPd3/C NHs relative to other MnxPdy/C ones may originate from the proper component ratio of Mn and Pd and their efficient “atomic ensemble” or electronic coupling, which can optimize the dband center or the electronic structure of Pd that facilitates to balance the adsorption−desorption of O2 molecules and reaction intermediates. Besides the component effect that mediates alloy NCs’ coordination environment and electronic structure, the size effect should be further considered. As disclosed from microstructural analyses, the size of the MnPd3 NCs is not the smallest among these MnxPdy/C NHs. So, the

size effect for the superior ORR activity of MnPd3/C NHs can be excluded. In addition, to further understand the superior ORR performance of MnPd3/C NHs, the electrocatalytic properties of pure Vulcan XC-72 C and pure MnPd3 NCs (Figure S14) are examined by RDV under identical conditions. The corresponding ORR polarization curves are provided in Figure S15. It can be seen that both the onset reduction potential and half-wave potential of pure Vulcan XC-72 C and pure MnPd3 NCs are more negative than that of MnPd 3/C NHs. Additionally, the apparent current densities of pure Vulcan XC-72 C and pure MnPd3 NCs are much smaller than that of MnPd3/C NHs. That is to say, the electrocatalytic performance of MnPd3/C NHs is significantly enhanced relative to pure Vulcan XC-72 C and pure MnPd3 NCs. As revealed from the microstructural analyses, the size and shape of the MnPd3 NCs active phase in the NHs are nearly identical to that of pure MnPd3 NCs. Why the MnPd3/C NHs exhibit enhanced electrocatalytic ORR performance? Does the Vulcan XC-72 C offer a significant contribution? To answer these questions, the XPS spectra of MnPd3/C NHs and pure MnPd3 NCs are first compared. As illustrated in Figure S16A, the C 1s core-level XPS spectrum of the MnPd3/C NHs is slightly shifted to the low BE direction relative to pure MnPd3 NCs, whereas the Pd 3d and Mn 2p core-level XPS spectra of the MnPd3/C NHs (Figure S16B,C) are obviously shifted to high BE direction compared with that of pure MnPd3 NCs. These results imply that there is a strong interplay or electronic coupling between Vulcan C support and MnPd3 NCs in the NHs, and the electrons may penetrate or transfer from the MnPd3 NCs to C support. Further evidence comes from the electrochemical impedance spectra tests. The related Nyquist plots are shown in Figure S16D. After fitting the Nyquist plots, the interfacial electron transfer resistance (Rct) of the MnPd3/C NHs is identified to be 36.9 Ω, which is much smaller than that of pure MnPd3 NCs (Rct = 84.4 Ω). The smaller Rct value observed on MnPd3/C NHs means that they possess much faster interfacial electron transfer kinetics relative to pure MnPd3 NCs. Besides enhancing the conductivity of MnPd3 NCs and improving their interfacial electron transfer kinetic, the presence of the C support and its strong interaction with the MnPd3 NCs may induce the structure transformation or break the crystal symmetry of MnPd3 NCs. As evidenced by the XRD analyses, pure MnPd3 NCs possess a highly symmetric face-centered cubic phase (fcc) structure, whereas the MnPd3 NCs in the NHs are transformed into a low symmetric tetragonal phase structure. The reduced symmetry of MnPd3 NCs in the NHs could offer another positive contribution for ORR. A similar phenomenon has been observed on face-centered tetragonalphase FePt nanoparticles, which show enhanced ORR activity compared with that of fcc FePt ones.6 On the basis of the abovementioned analyses, the enhanced catalytic property of MnPd3/C NHs may be ascribed to the presence of Vulcan XC72 C support and its strong interplay with MnPd3 NCs, which will induce structure transformation of MnPd3 NCs and enhance their electrical conductivity or interfacial electron transfer kinetics.

4. CONCLUSIONS In summary, four kinds of MnxPdy (MnPd3, MnPd−Pd, Mn2Pd3, and Mn2Pd3−Mn11Pd21)/C NHs have been synthesized by in situ integration of MnxPdy NCs on carboxylicfunctionalized Vulcan XC-72 C support using the solvothermal 8161


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Natural Science Foundation (1708085MA06), and the opening research foundations of State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Solid State Microstructures, Nanjing University.

method along with annealing under Ar/H2 atmosphere at 400 °C. In these MnxPdy/C NHs, the MnxPdy NCs are dispersed or embedded into the C support to form the well-coupled NHs. Especially for the MnPd3/C NHs, the MnPd3 NC active components are found to be the ordered alloy with a tetragonal phase structure, which are half-embedded into the C support and half-encapsulated by thin mesoporous C shells to form a unique interface structure that is helpful for electrocatalytic applications. Compared with pure Vulcan XC-72 C and pure MnxPdy NCs, these MnxPdy/C NHs show greatly enhanced electrocatalytic ORR activity under basic electrolytes. Among them, the MnPd3/C NHs exhibit the highest catalytic activity, which is comparable or superior to some recently reported nonPt catalysts (e.g., Ag−Co nanoalloys, Pd2NiAg NCs, G-Cu3Pd NCPs, PdCo/N-doped porous C, etc.) and close to commercial Pt/C. Further electrocatalytic kinetic tests reveal that their ORR mechanism complies with the direct “4e−” pathway. Most importantly, their durability and methanol-tolerant capability outperform Pt/C. Deduced from spectroscopic and electrochemical analyses, the excellent catalytic property of MnPd3/C NHs originates from the proper component ratio of Mn and Pd and the strong interplay of their components, which can optimize the d-band center or the phase structure of MnPd3 active components and improve their conductivity or interfacial electron transfer dynamics. This work not only offers a facile avenue to prepare strongly-coupled Pd-based bimetallic NCs/C NHs but also screens out a high-efficient and robust non-Pt electrocatalyst that may be utilized in fuel cells or other renewable energy devices.

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(1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. Design of Electrocatalysts for Oxygen- and Hydrogen-Involving Energy Conversion Reactions. Chem. Soc. Rev. 2015, 44, 2060−2086. (2) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. EarthAbundant Nanomaterials for Oxygen Reduction. Angew. Chem., Int. Ed. 2016, 55, 2650−2676. (3) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, No. eaad4998. (4) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; Zhu, E.; Yu, T.; Jia, Q.; Guo, J.; Zhang, L.; Goddard, W. A., III; Huang, Y.; Duan, X. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354, 1414−1419. (5) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally Ordered Intermetallic Platinum-Cobalt Core-Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12, 81−87. (6) Li, Q.; Wu, L.; Wu, G.; Su, D.; Lv, H.; Zhang, S.; Zhu, W.; Casimir, A.; Zhu, H.; Mendoza-Garcia, A.; Sun, S. New Approach to Fully Ordered fct-FePt Nanoparticles for Much Enhanced Electrocatalysis in Acid. Nano Lett. 2015, 15, 2468−2473. (7) Luo, S.; Tang, M.; Shen, P. K.; Ye, S. Atomic-Scale Preparation of Octopod Nano-frames with High-Index Facets as Highly Active and Stable Catalysts. Adv. Mater. 2017, 29, 1601687. (8) Jiang, K.; Shao, Q.; Zhao, D.; Bu, L.; Guo, J.; Huang, X. Phase and Composition Tuning of 1D Platinum-Nickel Nanostructures for Highly Efficient Electrocatalysis. Adv. Funct. Mater. 2017, 27, 1700830. (9) Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, 444−452. (10) Ito, Y.; Qiu, H.-J.; Fujita, T.; Tanabe, Y.; Tanigaki, K.; Chen, M. Bicontinuous Nano-porous N-Doped Graphene for the Oxygen Reduction Reaction. Adv. Mater. 2014, 26, 4145−4150. (11) Van Tam, T.; Kang, S. G.; Babu, K. F.; Oh, E.-S.; Lee, S. G.; Choi, W. M. Synthesis of B-doped Graphene Quantum Dots as a Metal-free Electrocatalyst for the Oxygen Reduction Reaction. J. Mater. Chem. A 2017, 5, 10537−10543. (12) Yan, X.; Jia, Y.; Chen, J.; Zhu, Z.; Yao, X. Defective-ActivatedCarbon Supported Mn-Co Nanoparticles as a Highly Efficient Electrocatalyst for Oxygen Reduction. Adv. Mater. 2016, 28, 8771− 8778. (13) Du, J.; Chen, C.; Cheng, F.; Chen, J. Rapid Synthesis and Efficient Electrocatalytic Oxygen Reduction/Evolution Reaction of CoMn2O4 Nanodots Supported on Graphene. Inorg. Chem. 2015, 54, 5467−5474. (14) Wu, G.; Wang, J.; Ding, W.; Nie, Y.; Li, L.; Qi, X.; Chen, S.; Wei, Z. A Strategy to Promote the Electrocatalytic Activity of Spinels for Oxygen Reduction by Structure Reversal. Angew. Chem., Int. Ed. 2016, 55, 1340−1344. (15) Lin, L.; Zhu, Q.; Xu, A.-W. Noble-Metal-Free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136, 11027−11033. (16) Shang, L.; Yu, H.; Huang, X.; Bian, T.; Shi, R.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. WellDispersed ZIF-Derived Co,N-Co-doped Carbon Nanoframes through Mesoporous-Silica-Protected Calcination as Efficient Oxygen Reduction Electro-catalysts. Adv. Mater. 2016, 28, 1668−1674. (17) Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.; Wei, Z.; Wan, L.-J. Understanding the High

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13872. Additional experiments and component and microstructural characterization as well as additional electrochemical data for MnPd3/C NHs and other control samples (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *Phone: 86-25-85891051. Fax: +86-25-85891051. E-mail: [email protected] (M.H.). ORCID

Dongdong Xu: 0000-0003-1403-8088 Yue Lin: 0000-0001-5333-511X Zhihui Dai: 0000-0001-7049-7217 Min Han: 0000-0003-1659-845X Author Contributions

Y.L. and S.Z. contributed equally. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China for the project (nos. 21671106, 21271105, 11404314, 21471082, and 21501095), research fund from the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Anhui Provincial 8162


Research Article

ACS Applied Materials & Interfaces Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx. J. Am. Chem. Soc. 2016, 138, 3570−3578. (18) Yin, P.; Yao, T.; Wu, Y.; Zheng, L.; Lin, Y.; Liu, W.; Ju, H.; Zhu, J.; Hong, X.; Deng, Z.; Zhou, G.; Wei, S.; Li, Y. Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts. Angew. Chem., Int. Ed. 2016, 55, 10800−10805. (19) Wu, X.; Chen, F.; Zhang, N.; Lei, Y.; Jin, Y.; Qaseem, A.; Johnston, R. L. Activity Trends of Binary Silver Alloy Nanocatalysts for Oxygen Reduction Reaction in Alkaline Media. Small 2017, 13, 1603387. (20) Slanac, D. A.; Hardin, W. G.; Johnston, K. P.; Stevenson, K. J. Atomic Ensemble and Electronic Effects in Ag-rich AgPd Nanoalloy Catalysts for Oxygen Reduction in Alkaline Media. J. Am. Chem. Soc. 2012, 134, 9812−9819. (21) Holewinski, A.; Idrobo, J.-C.; Linic, S. High-Performance Ag-Co Alloy Catalysts for Electrochemical Oxygen Reduction. Nat. Chem. 2014, 6, 828−834. (22) Guo, S.; Zhang, X.; Zhu, W.; He, K.; Su, D.; Mendoza-Garcia, A.; Ho, S. F.; Lu, G.; Sun, S. Nanocatalyst Superior to Pt for Oxygen Reduction Reactions: the Case of Core/Shell Ag(Au)/CuPd Nanoparticles. J. Am. Chem. Soc. 2014, 136, 15026−15033. (23) Liu, S.; Zhang, Q.; Li, Y.; Han, M.; Gu, L.; Nan, C.; Bao, J.; Dai, Z. Five-Fold Twinned Pd2NiAg Nanocrystals with Increased Surface Ni Site Availability to Improve Oxygen Reduction Activity. J. Am. Chem. Soc. 2015, 137, 2820−2823. (24) Jiang, G.; Zhu, H.; Zhang, X.; Shen, B.; Wu, L.; Zhang, S.; Lu, G.; Wu, Z.; Sun, S. Core/Shell Face-Centered Tetragonal FePd/Pd Nanoparticles as an Efficient Non-Pt Catalyst for the Oxygen Reduction Reaction. ACS Nano 2015, 9, 11014−11022. (25) Cui, Z.; Li, L.; Manthiram, A.; Goodenough, J. B. Enhanced Cycling Stability of Hybrid Li-Air Batteries Enabled by Ordered Pd3Fe Intermetallic Electrocatalyst. J. Am. Chem. Soc. 2015, 137, 7278−7281. (26) Wang, D.; Xin, H. L.; Wang, H.; Yu, Y.; Rus, E.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Facile Synthesis of Carbon-Supported PdCo Core-Shell Nanoparticles as Oxygen Reduction Electrocatalysts and Their Enhanced Activity and Stability with Monolayer Pt Decoration. Chem. Mater. 2012, 24, 2274−2281. (27) Chen, L.; Guo, H.; Fujita, T.; Hirata, A.; Zhang, W.; Inoue, A.; Chen, M. Nanoporous PdNi Bimetallic Catalyst with Enhanced Electrocatalytic Performances for Electro-oxidation and Oxygen Reduction Reactions. Adv. Funct. Mater. 2011, 21, 4364−4370. (28) Zheng, Y.; Zhao, S.; Liu, S.; Yin, H.; Chen, Y.-Y.; Bao, J.; Han, M.; Dai, Z. Component-Controlled Synthesis and Assembly of Cu-Pd Nanocrystals on Graphene for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2015, 7, 5347−5357. (29) Jiang, K.; Wang, P.; Guo, S.; Zhang, X.; Shen, X.; Lu, G.; Su, D.; Huang, X. Ordered PdCu-Based Nanoparticles as Bifunctional Oxygen-Reduction and Ethanol-Oxidation Electro-catalysts. Angew. Chem., Int. Ed. 2016, 55, 9030−9035. (30) Liu, S.; Han, L.; Zhu, J.; Xiao, W.; Wang, J.; Liu, H.; Xin, H.; Wang, D. Enhanced Electrocatalytic Activity and Stability of Pd3V/C Nanoparticles with a Trace Amount of Pt Decoration for the Oxygen Reduction Reaction. J. Mater. Chem. A 2015, 3, 20966−20972. (31) Cui, Z.; Chen, H.; Zhao, M.; DiSalvo, F. J. High-Performance Pd3Pb Intermetallic Catalyst for Electrochemical Oxygen Reduction. Nano Lett. 2016, 16, 2560−2566. (32) Duan, H.; Xu, C. Nanoporous PdCr Alloys as Highly Active Electrocatalysts for Oxygen Reduction Reaction. Phys. Chem. Chem. Phys. 2016, 18, 4166−4173. (33) Kuttiyiel, K. A.; Sasaki, K.; Su, D.; Wu, L.; Zhu, Y.; Adzic, R. R. Gold-Promoted Structurally Ordered Intermetallic Palladium Cobalt Nanoparticles for the Oxygen Reduction Reaction. Nat. Commun. 2014, 5, 5185. (34) Tarasevich, M. R.; Zhutaeva, G. V.; Bogdanovskaya, V. A.; Radina, M. V.; Ehrenburg, M. R.; Chalykh, A. E. Oxygen Kinetics and Mechanism at Electrocatalysts on the Base of Palladium-Iron System. Electrochim. Acta 2007, 52, 5108−5118.

(35) Holade, Y.; da Silva, R. G.; Servat, K.; Napporn, T. W.; Canaff, C.; de Andrade, A. R.; Kokoh, K. B. Facile Synthesis of Highly Active and Durable PdM/C (M = Fe, Mn) Nano-catalysts for the Oxygen Reduction Reaction in an Alkaline Medium. J. Mater. Chem. A 2016, 4, 8337−8349. (36) Shi, J.; Lei, K.; Sun, W.; Li, F.; Cheng, F.; Chen, J. Synthesis of Size-Controlled CoMn2O4 Quantum Dots Supported on Carbon Nanotubes for Electrocatalytic Oxygen Reduction/Evolution. Nano Res. 2017, 10, 3836−3847. (37) Guo, S.; Sun, S. FePt Nanoparticles Assembled on Graphene as Enhanced Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 2492−2495. (38) Yin, H.; Liu, S.; Zhang, C.; Bao, J.; Zheng, Y.; Han, M.; Dai, Z. Well-Coupled Graphene and Pd-Based Bimetallic Nanocrystals Nanocomposites for Electrocatalytic Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2014, 6, 2086−2094. (39) Wang, H.; Dai, H. Strongly Coupled Inorganic-Nano-Carbon Hybrid Materials for Energy Storage. Chem. Soc. Rev. 2013, 42, 3088− 3113. (40) Gunji, T.; Noh, S. H.; Tanabe, T.; Han, B.; Nien, C. Y.; Ohsaka, T.; Matsumoto, F. Enhanced Electrocatalytic Activity of CarbonSupported Ordered Intermetallic Palladium-Lead (Pd3Pb) Nanoparticles toward Electrooxidation of Formic Acid. Chem. Mater. 2017, 29, 2906−2913. (41) Du, X. X.; He, Y.; Wang, X. X.; Wang, J. N. Fine-Grained and Fully Ordered Intermetallic PtFe Catalysts with Largely Enhanced Catalytic Activity and Durability. Energy Environ. Sci. 2016, 9, 2623− 2632. (42) Sheng, Z.-H.; Shao, L.; Chen, J.-J.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H. Catalyst-Free Synthesis of Nitrogen-Doped Graphene via Thermal Annealing Graphite Oxide with Melamine and its Excellent Electrocatalysis. ACS Nano 2011, 5, 4350−4358. (43) Giovanni, M.; Poh, H. L.; Ambrosi, A.; Zhao, G.; Sofer, Z.; Šaněk, F.; Khezri, B.; Webster, R. D.; Pumera, M. Noble Metal (Pd, Ru, Rh, Pt, Au, Ag) Doped Graphene Hybrids for Electrocatalysis. Nanoscale 2012, 4, 5002−5008. (44) McCalla, E.; Abakumov, A. M.; Saubanere, M.; Foix, D.; Berg, E. J.; Rousse, G.; Doublet, M.-L.; Gonbeau, D.; Novak, P.; Van Tendeloo, G.; Dominko, R.; Tarascon, J.-M. Visualization of O-O Peroxo-Like Dimers in High-Capacity Layered Oxides for Li-Ion Batteries. Science 2015, 350, 1516−1521. (45) Tang, T.; Jiang, W.-J.; Niu, S.; Liu, N.; Luo, H.; Chen, Y.-Y.; Jin, S.-F.; Gao, F.; Wan, L.-J.; Hu, J.-S. Electronic and Morphological Dual Modulation of Cobalt Carbonate Hydroxides by Mn Doping Toward Highly Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. J. Am. Chem. Soc. 2017, 139, 8320−8328. (46) Cui, X.; Hutt, D. A.; Scurr, D. J.; Conway, P. P. The Evolution of Pd/Sn Catalytic Surfaces in Electroless Copper Deposition. J. Electrochem. Soc. 2011, 158, D172−D177. (47) Liu, D.; Xie, M.; Wang, C.; Liao, L.; Qiu, L.; Ma, J.; Huang, H.; Long, R.; Jiang, J.; Xiong, Y. Pd-Ag Alloy Hollow Nanostructures with Interatomic Charge Polarization for Enhanced Electrocatalytic Formic Acid Oxidation. Nano Res. 2016, 9, 1590−1599. (48) Wang, J.; Wu, H.; Gao, D.; Miao, S.; Wang, G.; Bao, X. HighDensity Iron Nanoparticles Encapsulated within Nitrogen-Doped Carbon Nanoshell as Efficient Oxygen Electrocatalyst for Zinc-Air Battery. Nano Energy 2015, 13, 387−396. (49) Cui, X.; Ren, P.; Deng, D.; Deng, J.; Bao, X. Single Layer Graphene Encapsulating Non-precious Metals as High-Performance Electrocatalysts for Water Oxidation. Energy Environ. Sci. 2016, 9, 123−129. (50) Li, J.; Wang, Y.; Zhou, T.; Zhang, H.; Sun, X.; Tang, J.; Zhang, L.; Al-Enizi, A. M.; Yang, Z.; Zheng, G. Nanoparticle Superlattices as Efficient Bifunctional Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14305−14312. (51) Xue, H.; Tang, J.; Gong, H.; Guo, H.; Fan, X.; Wang, T.; He, J.; Yamauchi, Y. Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three-Dimensional Ordered N-Doped Porous Carbon as an 8163


Research Article

ACS Applied Materials & Interfaces Efficient Catalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2016, 8, 20766−20771. (52) Sun, L.; Liao, B.; Ren, X.; Li, Y.; Zhang, P.; Deng, L.; Gao, Y. Ternary PdNi-based Nanocrystals Supported on Nitrogen-Doped Reduced Graphene Oxide as Highly Active Electrocatalysts for the Oxygen Reduction Reaction. Electrochim. Acta 2017, 235, 543−552. (53) Lu, Y.; Zhang, N.; An, L.; Li, X.; Xia, D. Synthesis of Highly Dispersed Intermetallic Ag4Sn/C and Its Enhanced Oxygen Reduction Reaction Activity. J. Power Sources 2013, 240, 606−611. (54) Wang, G.; Xiao, L.; Huang, B.; Ren, Z.; Tang, X.; Zhuang, L.; Lu, J. AuCu Intermetallic Nanoparticles: Surfactant-Free Synthesis and Novel Electrochemistry. J. Mater. Chem. 2012, 22, 15769−15774. (55) Liu, H.; Zheng, Y.; Wang, G.; Qiao, S. Z. A Three-Component Nanocomposite with Synergistic Reactivity for Oxygen Reduction Reaction in Alkaline Solution. Adv. Energy Mater. 2015, 5, 1401186. (56) Konda, S. K.; Amiri, M.; Chen, A. Photoassisted Deposition of Palladium Nano-particles on Carbon Nitride for Efficient Oxygen Reduction. J. Phys. Chem. C 2016, 120, 14467−14473. (57) Wang, Z.; Li, B.; Ge, X.; Goh, F. W. T.; Zhang, X.; Du, G.; Wuu, D.; Liu, Z.; Hor, T. S. A.; Zhang, H.; Zong, Y. Co@Co3O4@PPD Core@bishell Nanoparticle-Based Composite as an Efficient Electrocatalyst for Oxygen Reduction Reaction. Small 2016, 12, 2580−2587. (58) Cheng, Q.; Yang, L.; Zou, L.; Zou, Z.; Chen, C.; Hu, Z.; Yang, H. Single Cobalt Atom and N Codoped Carbon Nanofibers as Highly Durable Electrocatalyst for Oxygen Reduction Reaction. ACS Catal. 2017, 7, 6864−6871. (59) Kim, S.; Jang, D.; Lim, J.; Oh, J.; Kim, S. O.; Park, S. CobaltBased Active Species Molecularly Immobilized on Carbon Nanotubes for the Oxygen Reduction Reaction. ChemSusChem 2017, 10, 3473− 3481. (60) Mayrhofer, K. J. J.; Strmcnik, D.; Blizanac, B. B.; Stamenkovic, V.; Arenz, M.; Markovic, N. M. Measurement of Oxygen Reduction Activities via the Rotating Disc Electrode Method: From Pt Model Surfaces to Carbon-Supported High Surface Area Catalysts. Electrochim. Acta 2008, 53, 3181−3188. (61) Zhao, Y.; Liu, J.; Zhao, Y.; Wang, F. Composition-Controlled Synthesis of Carbon-Supported Pt-Co Alloy Nanoparticles and the Origin of their ORR Activity Enhancement. Phys. Chem. Chem. Phys. 2014, 16, 19298−19306. (62) Hammer, B.; Nørskov, J. K. Theoretical Surface Science and Catalysis-Calculations and Concepts. Adv. Catal. 2000, 45, 71−129.

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