Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three

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Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three-Dimensional Ordered N-Doped Porous Carbon as an Efficient Catalyst for Oxygen Reduction Reaction Hairong Xue, Jing Tang, Hao Gong, Hu Guo, Xiaoli Fan, Tao Wang, Jianping He, and Yusuke Yamauchi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05856 • Publication Date (Web): 21 Jul 2016 Downloaded from http://pubs.acs.org on July 22, 2016

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ACS Applied Materials & Interfaces

Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three-Dimensional Ordered N-Doped Porous Carbon as an Efficient Catalyst for Oxygen Reduction Reaction Hairong Xue,† Jing Tang,‡ Hao Gong,† Hu Guo,† Xiaoli Fan,† Tao Wang,† Jianping He,†* and Yusuke Yamauchi‡§* †

College of Materials Science and Technology, Jiangsu Key Laboratory of Materials

and Technology for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, PR China ‡

Mesoscale Materials Chemistry Laboratory, International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.

§

Institute for Superconducting and Electronic Materials, Australian Institute for Innovative Materials, University of Wollongong, North Wollongong, NSW 2500, Australia

* Corresponding authors: Prof. Jianping He, E-mail: [email protected] Prof. Yusuke Yamauchi, E-mail: [email protected]

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ABSTRACT PdCo bimetallic nanoparticles (NPs) anchored on three-dimensional (3D) ordered N-doped porous carbon (PdCo/NPC) were fabricated by an in situ synthesis. Within this composite, N-doped porous carbon (NPC) with an ordered mesoporous structure possesses a high surface area (659.6 m2 g-1), which can facilitate electrolyte infiltration. NPC also acts as a perfect 3D conductive network, guaranteeing fast electron transport. In addition, homogenously distributed PdCo alloy NPs (~15 nm) combined with the doping of the N element can significantly improve the electro-catalytic activity for the oxygen reduction reaction (ORR). Due to the structural and material superiority, although the weight percentage of PdCo NPs (~ 8 wt%) is much smaller than that of commercial Pt/C (20 wt%), the PdCo/NPC catalyst exhibits similar excellent electrocatalytic activity; however, its superior durability and methanol-tolerance ability of the ORR is as great as that of commercial Pt/C in alkaline media. Keywords: PdCo bimetallic nanoparticles, three-dimensional N-doped porous carbon, in situ synthesis, electrocatalyst, direct methanol fuel cell, oxygen reduction reaction,

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INTRODUCTION The direct methanol fuel cell (DMFC) is a promising choice for the development of a new-generation power source due to its high energy density, low cost, high efficiency, and environmental friendliness.1–2 As a matter of fact, properties of an energy conversion or storage system, including rate capacity, energy efficiency, lifetime, and cost, could be limited by the catalyst’s performance.1–2 Specifically, the catalyst has been a key component of DMFCs for large-scale commercialization, which must be efficient, durable, and inexpensive.3–4 Traditionally, porous carbon-supported Pt nanoparticles (NPs) have been served as typical catalysts in the commercial applications of fuel cells. However, high-cost, sluggish, cathodic oxygen reduction reactions (ORR) and anodic methanol oxidation, as well as carbon monoxide poisoning, have hindered the practical application of Pt-based catalysts.2 In consideration of the low cost, respectable ORR activity, and catalytic durability, more researchers have focused on Pd-based materials, especially Pd-transition metal alloy catalysts.5–7 Intensive research efforts have shown that Pd-based catalysts show promise as an alternative to Pt-based catalysts.8 As is well known, the Pd catalyst alloying with the transition metals (M), including Co, Fe, or Ni, not only reduces the Pd loadings but also significantly improves the ORR activity by changing the Pd-Pd interatomic distance, the charge distribution, and the surface species and forming more advantageous active sites for ORR.9 Pd-M alloys serving as high-performance ORR catalysts, particularly in the case of Co, have been researched in recent years. Bard et al. found that the O-O bonds 3



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can be easily broken on the site constituted by the M of Pd-M alloys, and the metal particle size can be decreased by the addition of M.10 What’s more, Suo et al. developed a main concept for choosing Pd-Co alloy NP catalysts with high performance, suggesting that the bimetallic materials with Pd as sell and alloy as core deliver the extraordinary ORR activity.11 Liu et al. prepared Pd4Co nanoalloys supported on carbon using a modified polyol reduction method that showed improved catalytic activity for ORR, which is capable of Pd/C and Pt/C in PEMFC, benefiting from the enhanced degree of alloying.7 Therefore, multicomponent Pd-Co alloy materials are considered in this study to be promising ORR catalysts with enhanced electrocatalytic performance. As supporting materials for Pd-based catalysts, porous carbon (PC) materials, especially carbon black, have been widely employed in low-temperature fuel cells due to their high chemical stability, good electrical conductivity, and large surface area.12 Recently, plenty of novel carbon materials have attracted tremendous attention, such as carbon nanotubes, graphene, and graphite nanofibers.13–15 Of these carbon materials, ordered mesoporous carbon has been considered to be a promising catalyst support because of the large surface area, adjustable and uniform pore size, and chemical inertness.6 In addition, the lastest researches in N-doped carbon materials have revealed that N-doping can not only provide high electron mobility but also enhance the interaction between catalyst and support, resulting in the improved stability and activity of the catalysts.16–17 Especially, many researchers have shown that N-doped porous carbon (NPC) materials exhibit good electrocatalytic activity for 4


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ORR, which even can be used as a metal-free catalyst.18 Thus, the preparation of NPC-supported

Pd-based

catalysts

is

an

effective

method

of

obtaining

high-performance ORR catalysts for practical application in fuel cells. Generally, for commercial Pd-based catalysts, Pd NPs are loosely absorbed on carbon supports.19 Because of the fairly weak catalyst-support interaction, the catalyst NPs may easily detach, aggregate, and even dissolve during the cyclic process, thus leading to degraded performance. Normally, in order to achieve good catalytic performance, NPC must be surface functionalized, which is beneficial for anchoring catalyst NPs on the carbon supports. However, surface functionalization, such as a strong oxidizing acid treatment, may decrease the electrical conductivity and damage the porous structure.20 Consequently, it is necessary to develop an efficient and facile strategy to design and synthesis the ordered NPC-supported Pd-based catalysts, which possess excellent ORR catalytic performance. Hence, we have proposed a facile in situ evaporation-induced self-assembly (EISA) strategy to obtain a novel PdCo/NPC composite (Pd:Co=3:1). As a result, the successfully doped N element reveals the main existence of pyridine-like N atoms in the NPC matrix. Furthermore, benefiting from the confinement effect of the porous carbon framework structure, PdCo nanoparticles uniformly disperse in the NPC matrix. Combining NPC supports with enhanced surface properties and PdCo bimetallic nanoparticles with high catalytic activity can provide catalyst with advantageous properties for fuel cell applications, and the obtained PdCo/NPC catalyst is specifically expected to have excellent ORR catalytic activity. 5



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RESULTS AND DISCUSSION

Scheme 1. Schematics showing the preparation of a PdCo/NPC composite

Scheme 1 shows the in situ fabrication of a PdCo/NPC composite. It is synthesized by an EISA method combined with a high-heat treatment process that uses resol and urea as the C and N precursors, PdCl2 and CoCl2 as the Pd-Co precursor, and F127 as the template agent. The resol precursor, metal salt precursor, and F127 are uniformly dissolved into the ethanol solvent in the EISA process, which is beneficial in forming a rod-like micelle after evaporating ethanol solvent.16–17 Afterward, pyrolysis of the soft template (F127) at 350 ºC can result in an ordered mesoporous structure. Meanwhile, a 3D framework is formed by the thermal polymerization of the resol precursor. Finally, the carbonization of an ordered porous carbon and the formation of PdCo NPs can be obtained by a post-annealing process at 900 ºC. As a result, PdCo bimetallic NPs are anchored in situ on a 3D ordered N-doped porous carbon matrix after EISA process and thermal treatment.

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Figure 1. (a) XRD patterns of Pd/NPC and PdCo/NPC samples (green: Pd/NPC, orange: PdCo/NPC); (b) X-ray photoelectron spectrum, (c) TEM and HRTEM, and (d) elemental mapping images of the PdCo/NPC sample

The crystalline structures of Pd/NPC and PdCo/NPC catalysts are investigated by wide-angle XRD. Pd/NPC shows one broad diffraction peak located at around 2θ of 23° (Figure S1), which is indexed to the (002) diffraction of amorphous carbon. As shown in Figure 1(a), intensity diffraction peaks at around 2θ of 40°, 46°, 68°, and 83° in the XRD patterns of Pd/NPC are attributed to crystalline Pd (111), (200), (220), and (311) facets, respectively, suggesting that the face-centered cubic (fcc) phase of Pd/NPC is in accord with the standard Pd card (JCPDS no. 05-0681). However, as compared with the diffraction peaks in the Pd/NPC, these four corresponding diffraction peaks of PdCo/NPC catalysts shift slightly to higher 2θ values, indicating the lattice contraction caused by the incorporation of Co into the Pd fcc structure. This result confirms the formation of a Pd-Co alloy.9 Moreover, the XRD pattern

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shows no characteristic peak of metallic cobalt or cobalt oxides, suggesting that PdCo/NPC exists in the form of an alloy. Figure 1(b) shows the XPS spectrum of the PdCo/NPC to further confirm its detailed composition and elemental valence. As shown in Figure 1(b), two intense pair peaks (Pdo) of Pd/NPC are detected at 336, 341.4 eV in the Pd3d spectrum, which can be assigned to highly crystalline metallic Pd.21 However, for the PdCo/NPC catalyst, a gradual shift of the two corresponding peaks (Pdo) to higher bonding energy can be observed, resulting from the chemical interaction between Pd and Co elements; this is in good accordace with the Pd-Co alloy reported in literature.9,22 The content of N element in PdCo/NPC is estimated to be 1.05 at% by XPS analysis. The N1s spectrum is deconvoluted into three peaks and found at 398.5, 400.1, and 401 eV, which are attributed to pyridine-N, pyrrolic-N, and quaternary-N, respectively.17,23–24 The main type of N element is pyridinic-N, which makes up most of the N element. As shown in Figure S2(a), the C1s spectrum is fitted into three peaks centered at around 285, 285.7, and 287 eV, which can be assigned to pure graphitic sites, the sp3 C-C bond, and the C=N bond, respectively.25–26 Therein, pure graphitic sites exhibit much higher percentages, indicating a high degree of graphitization. The Co2p spectrum (Figure S2(b)) is fitted into peaks at 781.7, 780.5, and 777.8 eV. The existence of metallic Co is evidenced by the peak of 778.5 eV, which corresponds to that of Coo. However, the two peaks at 781.7 and 780.5 eV are much higher than the peak of Coo; these peaks are attributed to the Co2+ and Co3+ states. 27–29 This result is in accord with the literature regarding the PdCo alloy.9,17,30 The high-resolution TEM 8


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(HRTEM) image of PdCo/NPC is shown in Figure 1(c). Obvious d-spacings (~0.22 nm) can be found in the lattice fringes of a PdCo nanocrystal, close to the fcc Pd (111) facets (0.225 nm), indicating high crystallization. It should be noted that there are many defects in the lattice fringes (insert), including vacancy, boundary, and dislocation defects. These defects can act as catalytic sites, effectively improving the catalytic activity.31–32 In addition, the even distribution of Pd and Co elements in PdCo NPs can be observed clearly by elemental mapping analysis, which unambiguously confirms the formation of a PdCo bimetallic alloy through the PdCo/NPC composite (Figure 1(d)). Moreover, the N element is uniformly dispersed in the whole porous carbon framework, suggesting the successful doping of the N element.

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Figure 2. (a) N2 adsorption-desorption isotherms, (b) pore-size distribution, and (c–f) TEM and HRTEM images of Pd/NPC and PdCo/NPC samples; (c,d) Pd/NPC, (e,f) PdCo/NPC

N2 adsorption-desorption isotherms of Pd/NPC and PdCo/NPC composites are shown in Figure 2(a), and Figure 2(b) illustrates their pore-size distribution curves. As can be seen in Figure 2(a), the N2 adsorption-desorption isotherms of all samples exhibit type-IV curves with the H1-type hysteresis loop, indicating that all samples possess a mesoporous structure.33–37 The noteworthy nitrogen uptake at the P/Po of 0.40–0.70 is typical of the capillary condensation of N2 in the mesoporous materials. The curves in Figure 2(b) demonstrate that Pd/NPC and PdCo/NPC possess similar

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narrow

pore-size

distributions

centered

at

about

3

nm.

By

using

the

Barrett-Joyner-Halenda (BJH) model and Brunauer-Emmett-Teller (BET) method, the PdCo/NPC composite shows a larger pore sizes, higher specific surface area, and more desirable pore volume than does the Pd/NPC composite (Table 1). Figure S3 shows the typical small-angle XRD patterns of the Pd/NPC and PdCo/NPC composites. As shown in Figure S3, an intense peak (100) can be observed around low-range 2θ ≈ 1° in the patterns of the samples, which indicates an ordered two-dimensional hexagonal mesoporous structure. The intensity of the (100) peak gradually decreases after adding cobalt salt, suggesting a deterioration of the long-range ordered mesoporous structure. This result reveals that the addition of cobalt salt has some influence on the stability and distribution of the F127 micelles in the self-assembly process; however, the ordered mesoporous structure can be still maintained. In addition, the peak (100) of the PdCo/NPC composite shifts gradually to the lower 2θ value by adding cobalt salt, suggesting the expansion of the unit-cell size and d-spacing of its carbon support. This can be seen clearly in Figure 2(c,e), where TEM images show the good dispersibility of the Pd or PdCo NPs in the NPC. The ordered parallel mesoporous channel shown in Figure 2(d,f) is estimated to be ∼3 nm, which is consistent with the pore size based on the BJH analysis. The insets in Figure 2(d,f) show that Pd and PdCo NPs have good crystallinity. The clear lattice of Pd and PdCo NPs are calculated to be ∼0.19 and 0.22 nm, respectively, which correspond to the interplanar distances of Pd (200) and PdCo (111) facets. Furthermore, Figure S4 shows the size distribution histograms of the Pd and PdCo 11



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NPs in Pd/NPC and PbCo/NPC catalysts, revealing average particle sizes of 18.8 nm for Pd NPs and 15.5 nm for PdCo NPs. We infer that the PdCo/NPC composite with these advantages is preferred for the sufficient penetration and the saturated lodging of electrolytes, which is beneficial to the facile ion transportation. Table 1. Pore-structure parameters of PdCo/NPC and Pd/NPC composites Sample

SBET/m2·g

Vtotal/cm3·g

D/nm

PdCo/NPC Pd/NPC

659.6 480.4

0.37 0.27

2.3 2.0

Figure 3. (a) CV (20 mV s-1; dash line: N2; solid line: O2) and (b) LSV (5 mV s-1 with 1600 rpm) polarization curves of the Pd/NPC, PdCo/NPC, and commercial Pt/C catalysts; (c) magnified LSV polarization curves of the PdCo/NPC and Pt/C catalysts; (d) K–L plots fitted from the RDE data at 12


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-0.6V vs. SCE, (e) histogram of the electron transfer numbers under different potentials, (f) chronoamperometric responses at -0.3 V vs. SCE of the Pd/NPC, PdCo/NPC, and commercial Pt/C catalysts.

Figure 3a shows cyclic voltammetry (CV) curves of Pd/NPC, PdCo/NPC, and commercial Pt/C (20%) catalysts in N2-saturated and O2-saturated 0.1 M KOH solution. All of the catalysts exhibited a pronounced cathodic peak in the presence of O2, confirming a substantial ORR process. As observed, the PdCo/NPC catalyst shows a more positive onset potential (Eonset) and reduction peak potential (Epeak) for the ORR than does the Pd/NPC catalyst, which is similar to that of the Pt/C (20%) catalyst. The linear sweep voltammetry (LSV) curves of the three catalysts show increased limiting current density (JL) and more positive Eonset and half-wave potential (E1/2) of the PdCo/NPC catalyst as compared with that of the Pd/NPC catalyst (Figure 3b). Especially, as shown in Figure 3c, the Eonset (-0.082 V) and E1/2 (-0.146 V) of the PdCo/NPC catalyst are only 10 and 21 mV negatively shifted, respectively, as compared to the Pt/C (20%) catalyst (Eonset=0.072 V, E1/2=-0.167V). In addition, the JL of the PdCo/NPC catalyst reaches about -5.1 mA cm-2, which is similar to that of the Pt/C (20%) catalyst (-5.0 mA cm-2). These results suggest the high ORR catalytic activity of the PdCo/NPC catalyst in alkaline conditions. To further characterize the ORR kinetics of the catalysts, rotating disk electrode (RDE) measurements were performed at different rotating speeds (400–1600 rpm) (Figure S5). Using the Koutecky-Levich equation, the electron transfer numbers of the Pd/NPC, PdCo/NPC, and commercial Pt/C (20%) catalysts are calculated to be 3.5, 3.87, and 3.91, respectively, based on the slopes of the Koutecky-Levich plots at 0.6

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V (vs. SCE) (Figure 3d). In addition, Figure 3e shows the similar electron transfer numbers of the three catalysts at different potentials (-0.5, -0.6, -0.7, and -0.8 V (vs. SCE)). The PdCo/NPC catalyst possesses a higher electron transfer number of the ORR than does the Pd/NPC catalyst, suggesting the one-step and four-electron pathway reaction kinetics for the ORR. The durability of the ORR catalytic activity was tested at -0.3 V (vs. SCE) by chronoamperometric measurement. It can be observed in Figure 3f, the initial activity decay of the Pt/C catalyst is ∼14.6% after 3600 s of testing, while the PdCo/NPC catalyst exhibits only a loss of 4.6% of its initial activity after 3600 s, indicating much higher stability. In addition, chronoamperometry measurement was used to evaluate the methanol-tolerance ability of ORR catalysts. As shown in Figure S6, after adding 3 M methanol, the obvious current decay of the commercial Pt/C (20%) catalyst can be observed, resulting from the methanol oxidation reaction. In contrast, the PdCo/NPC catalyst exhibits no noticeable current decay, suggesting a better tolerance of the methanol crossover than that of the commercial Pt/C (20%) catalyst. Furthermore, we also prepared other proportional PdCo alloy NPs (Pd:Co=5:1, 1:1, labeled as 5-PdCo/NPC, 1-PdCo/NPC, respectively). As shown in Figure S7(a), XRD confirms the formation of different proportional PdCo alloy NPs. Moreover, Figure S7(b–d) shows TEM images of the three samples, which exhibit the good dispersity of the PdCo alloy NPs and the ordered parallel mesoporous channel. The result of LSV testing, shown in Figure S8, indicates that the PdCo/NPC (Pd:Co=3:1, labeled as 3-PdCo/NPC) catalyst possesses the highest ORR catalytic activity in alkaline conditions. This superior catalytic 14


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activity for the ORR of the PdCo/NPC catalyst mainly results in material and structural superiorities.

Figure 4. Schematic structure and the catalytic mechanism for ORR of the PdCo/NPC catalyst

Figure 4 shows the schematic structure and the catalytic mechanism for ORR of the PdCo/NPC catalyst. As has been reported, if one metal catalyst (such as Pd, Au) with fully occupied d-orbitals alloys with another metal (such as Co, Ni) with a low occupancy of d-orbitals, the Gibbs free energy of the electronic transmission for the ORR process can be significantly decreased due to the d-orbital coupling effect between the two metals, thus leading to the improved ORR kinetics.38 Furthermore, the modification of Pd catalysts with more active metals, for example Co, could facilitate the dissociation of the adsorptive O2 to form dissociated O atoms, and these dissociated O atoms could move from the Co to Pd site, which is beneficial to achieve less polarization of electroreduction.9 In addition, a net positive charge can be created by N atoms on the adjacent C atoms, which is conducive to easily attracting electrons derived from the anode due to its electron-accepting ability, leading to improvement of the catalytic performance and facilitation of the ORR. Moreover, the porous structure facilitates the infiltration of electrolytes, and the 3D carbon framework 15



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serves as a conductive network that ensures fast electronic transmission. Finally, this PdCo/NPC catalyst achieved a remarkable catalytic performance for the ORR and superior durability.

CONCLUSION In summary, the PdCo alloy nanoparticle anchored on an ordered N-doped porous carbon (PdCo/NPC) has been successfully fabricated through a synthesis process in situ. Its porous structure with its high specific surface area can facilitate electrolyte infiltration; the perfect conductive network of 3D carbon framework ensures the fast electronic transmission. The doping of the N element and homogenously distributed PdCo alloy nanoparticles can significantly improve the ORR catalytic activity. Thus, as compared with the commercial Pt/C catalyst, this PdCo/NPC catalyst exhibits similar excellent electrocatalytic performance but superior durability of the ORR. Due to its superior performance, it can serve as a promising efficient catalyst for energy applications.

EXPERIMENTAL METHODS Preparation of PdCo bimetallic nanoparticles anchored on three-dimensional N-doped porous carbon (PdCo/NPC): Firstly, F127 template agent (1.0 g) was dissolved in ethanol (14 mL) to form a homogeneous transparent solution. Simultaneously, a certain amount of urea, cobalt chloride, and palladium chloride were added to ethanol solution (6 mL) at 40 °C under stirring for 30 min to obtain 16


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primrose yellow solution. Then, the resol-ethanol solution (20 wt%, 5.0 g) was added dropwise into the above primrose yellow solution. After stirring for 10 min, the obtained solution was slowly added into the F127 ethanol solution under constant stirring for 1 h. Subsequently, the precursor solution was transferred to the evaporating dish, followed by the evaporation of ethanol in the air for 12 h; it was then put into a vacuum-drying oven to thermo-polymerize at 100 °C for 24 h. The resultant pale yellow composite film collected from the evaporating dish was calcined under a flow of nitrogen at 350 °C for 3 h, and then 900 °C for 2 h, using heating rates of 1 °C min-1 and 5 °C min-1, respectively. When cooling at room temperature, the ordered N-doped porous carbon-PdCo nanoalloy composites were formed, named PdCo/NPC. The atom ratio for palladium and cobalt is 3:1. The theoretical weight percentage of PdCo NPs is about 8 wt%. Structural and morphological characterization: The chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe). X-ray diffraction (XRD, Bruker D8 Advance) was used to identify the crystalline phases of samples. The diameter and distribution of Pd or PdCo alloy particles within the N-doped ordered mesoporous carbon framework were observed by transmission electron microscopy (TEM, JEM-2100, 200 kV). A N2 adsorption isotherm (Micromeritics ASAP 2010, 77 K) was used to measure the porous structures of the samples. Pore size distributions, specific surface areas, and pore volumes were estimated and calculated by using the Barrett-Joyner-Halenda (BJH) model and the Brunauer-Emmett-Teller (BET) method. 17



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Electrochemical measurements: The preparation of catalyst electrode was as follows: the as-prepared PdCo alloy catalyst or commercial Pt/C (20%) catalyst (5 mg) was mixed with 0.05 wt % Nafion ethanol solution (1 mL); the mixture was then sonicated for 30 min to form a well-dispersed ink solution. Subsequently, the ink solution (25 µL) was dropped on a glassy carbon (GC) electrode surface with a 4-mm diameter. Finally, after drying under an infrared lamp, a thin layer of 0.1256 cm2 was obtained as the working electrode. Rotating disk electrode (RDE), cyclic voltammetry (CV), and chronoamperometry measurements were tested by using a three-electrode cell on a CHI660C electrochemical workstation. A saturated calomel electrode (SCE) was served as the reference electrode, and a platinum sheet was served as the counter electrode. CVs were carried out between -0.8 and 0.1 V in the O2-saturated KOH solution (0.1 mol L-1). RDE experiments were also tested in the O2-saturated KOH solution (0.1 mol L-1). The scan rates of CV and RDE experiments are 20 mV s-1and 5 mV s-1, respectively.

ASSOCIATED CONTENT Supporting Information XRD patterns, TEM image, XPS spectra, the size distribution histograms, chronoamperometric responses, LSV polarization curves, K–L plots at different potentials, and the histogram of transfer electron number (n) at different potentials (inset).

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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (J. P. He) * E-mail: [email protected] (Y. Yamauchi)

ACKOWLEDGMENTS This research was supported by grants from the National Natural Science Foundation of China (Nos. 51372115 and 11575084) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors would like to acknowledge this financial support.

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Fabrication of PdCo Bimetallic Nanoparticles Anchored on Three

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