Pd-on-Au Supra-nanostructures Decorated Graphene Oxide: An

Aug 2, 2016 - We report a very easy and effective approach for synthesizing unique palladium-on-gold supra-nanostructure (Au@Pd-SprNS)-decorated graph...
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Pd-on-Au Supra-nanostructures Decorated Graphene Oxide: An Advanced Electrocatalyst for Fuel Cell Application Yingzhou Tao,∥,†,‡ Anirban Dandapat,∥,§ Liming Chen,†,‡ Youju Huang,*,‡ Yoel Sasson,§ Zhenyu Lin,† Jiawei Zhang,‡ Longhua Guo,*,† and Tao Chen*,‡ †

Ministry of Education Key Laboratory of Analysis and Detection for Food Safety, Fujian Provincial Key Laboratory of Analysis and Detection for Food Safety, Fuzhou University, Fuzhou, Fujian 350116, China ‡ Division of Polymer and Composite Materials, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo 315201, People’s Republic of China § Casali Center of Applied Chemistry, Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: We report a very easy and effective approach for synthesizing unique palladium-on-gold supra-nanostructure (Au@Pd-SprNS)-decorated graphene oxide (GO) nanosheets. The SprNSs comprising Au nanorods as core and a unique close-packed assembly of tiny anisotropic Pd nanoparticles (NPs) as shell were homogeneously distributed on the GO surface via electrostatic self-assembly. Compared with the traditional one-pot method for synthesis of metal NPs on GO sheets, the size and shape of core−shell Au@Pd SprNSs can be finely controlled and uniformly distributed on the GO carrier. Interestingly, this Au@Pd-SprNSs/GO nanocomposite displayed high electrocatalytic activities toward the oxidation of methanol, ethanol, and formic acid, which can be attributed to the abundance of intrinsic active sites including high density of atomic steps, ledges and kinks, Au−Pd heterojunctions and cooperative action of the two metals of the SprNSs. Additionally, uniform dispersion of the SprNSs over the GO nanosheets prevent agglomeration between the SprNSs, which is of great significance to enhance the long-term stability of catalyst. This work will introduce a highly efficient Pd-based nanoelectrocatalyst to be used in fuel cell application.

1. INTRODUCTION

Pd-based nanomaterials have emerged as an attractive replacement for Pt in direct alcohol fuel cells (DAFCs), exhibiting improved steady-state behavior than Pt over a large assortment of substrates in alkaline media.8,9 Although the activity of pure Pd in comparison to platinum is slightly lower for alcohol oxidation, however, its activity can be enhanced by several ways:10,11 (i) Coupling with other suitable metal (s) to form bimetallic (or multimetallic) nanostructures can enhance the activities due to synergistic interaction of the different metal centers.12−14 For example, Song et al. found that porous bimetallic palladium−silver (Pd63Ag37) alloy nanocorals exhibited significantly higher activity for glycerol electrooxidation.15 Ho et al. established PdCo and PdCu nanoparticles (NPs) to show better catalytic activities toward electrooxidation of formic acid.16 (ii) Appropriate morphology of the nanocatalyst to provide very high surface area is also very important to show superior activities.17 In addition, that unique

Current interest in fuel cell technology continues to flourish in recent decades because of their remarkable advantages including very high energy-conversion efficiency and low toxicity to diminish the ever-increasing energy demands, fossil fuel depletions, and environmental pollution issues throughout the world.1,2 In this direction, particularly in portable electronic devices and fuel-cell vehicles, liquid fuels (e.g., methanol, ethanol, formic acid etc.) have attracted enormous interest owing to their easy handling, storage, and transportation, while higher energy density compared to existing technologies involving gaseous fuels such as hydrogen. To establish a high performance fuel cell, development of highly efficient electrocatalyst is very important.3,4 Currently, Pt-based electrocatalysts are predominant,5,6 however high cost, limited supply, CO adsorbate poisoning and slow kinetics in alcohol oxidation of Pt-based electrocatalyst create certain obstacles for successful applications of Pt-based fuel cells for commercial use,7 which demands to put notable efforts toward the development of new electrocatalysts. © XXXX American Chemical Society

Received: April 11, 2016 Revised: July 28, 2016

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and thus to reduce the cost.4 Precious metal alone as a catalyst has the disadvantages of high cost and easy agglomeration in solution, which might decrease the activity of the catalyst.28 Second, GO-supported nanoelectrocatalysts are easily interacted with electrodes, offering a great opportunity to achieve enhanced catalytic stability. Third, compared with the traditional one-pot method for synthesis of metal NPs on GO sheets, the size and shape of core−shell Au@Pd SprNSs can be finely controlled and uniformly distributed on the GO carrier. Via electrostatic self-assembly, Au@Pd-SprNSs evenly distributed on GO keep away from agglomeration between the SprNSs, and enhance the persistent stability of catalyst. The electrocatalytic activity of the developed nanocomposites (Au@ Pd-SprNSs/GO) was studied using electrooxidation of ethanol, methanol, and formic acid as the model systems. Results demonstrated that the Au@Pd-SprNSs/GO have very high electrochemical active surface areas (ECSAs) and showed significantly high electrocatalytic activities toward all the reactions. The present work promises an interesting strategy to prepare Pd-based catalysts for direct fuel cell applications.

morphology could also offer additional active sites due to the atomic steps, edges, corner atoms and rough surfaces, which will enhance activity of the electrocatalyst. (iii) In order to further maximize the activity and stability of the electrocatalysts, it is necessary to load the nanostructures uniformly on the surface of any supporting materials with high surface area, and good mechanical strength, which not only maximize the availability of nanosized electrocatalyst surface area for electron transfer but also provide long-term stability of better mass transport of reactants to the electrocatalyst. Moreover, an excellent supports can prevent agglomeration between the NPs, which is of great significance to the catalytic activities and stability as well.18,19 The recent emergence of graphene oxide (GO) nanosheets has opened a new avenue for utilizing 2D new carbon material as a support because of its high mechanical strength, large surface area, abundance of functional groups on the surface for anchoring metal nanostructures and potential low manufacturing cost. For example, Wang and his co-workers prepared a nanoflower shaped gold−palladium alloy on graphene oxide nanosheets with exceptional activity for electrochemical oxidation of ethanol.20 In addition, GO possess good mechanical strength and long-term stability, which can offer a great opportunity to achieve enhanced catalytic stability.21 Inspired by the above advantages, we aimed to combine all those characteristics in one approach to develop Au−Pd bimetallic nanostructures loaded GO as highly efficient electrocatalyst for fuel cell applications. Although previous studies have effectively enhanced the catalytic activity of Pdbased electrocatalysts,22,23 it should be noted that to obtain lager specific surface area, most of those methods are concentrated on small Pd-based NPs as nanoelectrocatalysts. Larger Pd-based NPs have been investigated little. Furthermore, because of the synchronous reduction of GO and noble metal ions in the synthesis of graphene oxide-supported Pdbased nanoelectrocatalysts, it is hard to obtain uniform NP distribution on the GO. Nevertheless, relatively larger size (to generate highly active sites e.g. sharp edges, atomic steps, edges, corner atoms and rough surfaces) and better uniformity of the NPs to gain superior activity and selectivity toward different reactions are scarcely reported in GO composites, and the preparation of proper core−shell structure on the GO still remains a challenge. Herein, we report a simple and facile approach for preparation of high-quality three-dimensional (3D) core−shell palladium-on-gold supra-nanostructures (Au@ Pd-SprNSs) decorated graphene oxide (GO) nanosheets. Au− Pd bimetallic nanostructures have excellent synergistic effect.24 The charge transfer between different metals can improve the catalytic efficiency.25 Additionally, the unique close-packed assembly of tiny anisotropic Pd NPs on Au nanorods endows the supra-nanostructures with large specific surface area.26 A larger surface area can provide more catalytically active sites.12 However, only those SprNSs were not very active in fuel cell applications, rather use of only noble metal catalyst will cost very high. To overcome those challenges/difficulties, the SprNSs were uniformly decorated onto graphene oxide nanosheets. The composite was further used as a highly efficient fuel cell catalyst. Several innovation point of this work are inspiring: First, compared with other materials, graphene oxide surface has a large number of functional groups that are advantageous to load the metal nanoparticles.27 Graphene oxide as a carrier can effectively improve the dispersion degree of precious metal catalysts to reduce the dosage of noble metals

2. EXPERIMENTAL SECTION 2.1. Materials and Instrumentation. Hexadecyltrimethylammonium bromide (CTAB), Sodium borohydride (NaBH4), Nafion, Pd/ C, and PdCl2 were obtained from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). 5-Bromosalicylic acid (5-BrSA) was purchased from Aladdin Company in Shanghai. L-Ascorbic Acid (AA) was obtained from Energy Chemical in Shanghai. Chloroauricacid (HAuCl4·3H2O, 99.9%), Silver nitrate (AgNO3), Methanol, Ethanol and Formic acid were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai). All other reagents were of analytical grade and used as received. Ultrapure water (Millipore System, 18.2 MΩ cm) was used as solvents. A 10 mM H2PdCl4 aqueous solution was prepared by complete dissolution of 44.5 mg of PdCl2 in 25 mL of 20 mM HCl in a boiling water bath. Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100F instrument and operated at 200 kV. Scanning electronic microscopy (SEM) measurements were carried out by a JEOL JMS-6700F scanning microscope. The surface compositions and chemical states of the samples were analyzed by X-ray photoelectron spectroscopy (XPS) (AXIS ULTRA DLD, Kratos Analytical Ltd., Manchester, UK). X-ray diffraction (XRD) patterns of the catalysts were measured on a Bruker AXS D8 Discover diffractometer using Cu Kαas the radiation source (λ= 1.54056 Å). The actual loading and composition of Au and Pd in the as-prepared Au@Pd/GO catalysts were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (PerkinElmer, Optima 2100Dv). 2.2. Synthesis of Graphite Oxide. Graphene oxide (GO) sheets were synthesized by a modified Hummers’ method and exfoliation of graphite oxide was achieved by a strong ultrasonication method.29 The obtained brown dispersion was then washed and centrifuged to remove any unexfoliated graphite oxide. Graphene oxide powders were suspended in water by sonication for 4 h, giving a 0.01 mg mL−1 homogeneous dispersion of GO. 2.3. Synthesis of Gold Nanorods (AuNRs). AuNRs were prepared according to our previous reports.26,30,31 First, the seed solution was prepared by the addition of freshly prepared aqueous NaBH4 solution (0.6 mL of 0.01 M NaBH4 was diluted to 1.0 mL with water) into an aqueous mixture composed of HAuCl4 (0.01 M, 0.25 mL) and CTAB (0.1 M, 9.75 mL). The resultant solution was mixed by rapid inversion for 2 min and was then kept at room temperature for at least 30 min before use. To prepare the growth solution, 4.5 g of CTAB together with 0.55 g of 5-BrSA were dissolved in 125 mL of water at 70 °C. The solution was then allowed to cool to 30 °C, and 12 mL of 4 mM AgNO3 solution was added. Then, 125 mL of 1 mM HAuCl4 solution was added. After 15 min of slow stirring (400 rpm), 1.0 mL of 0.064 M AA was added, and the solution was vigorously B

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Langmuir stirred for 30 s until it became colorless, followed by the addition of the seed solution (1.0 mL). The above mixture was subjected to gentle inversion for 10 s and then left undisturbed for at least 6 h. The asgrown AuNR solution was first centrifuged at 8000 rpm for 10 min and then redispersed in water for further use. 2.4. Preparation of Gold−Palladium Suprananostructures. Au@Pd-SprNSs were synthesized according to our previous work.26 Briefly, 1.0 mL of AuNR solution was added into 5 mL of 0.001 M H2PdCl4 aqueous solution. The solution was sonicated for 1 min, followed by rapid injection of 1 mL of 0.01 M AA aqueous solution. The above mixture was subjected to rapid inversion for 10 s and then left undisturbed for at least 6 h. Finally, the mixture was centrifuged and washed with water repeatedly to remove the excess surfactants, and then the obtained precipitate was also dispersed into 0.2 mL deionized water for later use. 2.5. Preparation of Au@Pd/GO Nanocomposites. The asprepared Au@Pd-SprNSs solution and GO solution were mixtured in distilled water, stirred for 6 h. The above mixture was centrifuged and washed with water twice. The obtained nanocomposites were stored in common temperature for further characterizations and applications. 2.6. Electrochemical Characterization. All electrochemical measurements were carried out in a three-electrode cell using a CHI 660E electrochemical workstation.32 A conventional three-electrode system was used with a glassy carbon electrode (GCE, diameter: 3.0 mm) as the working electrode, a platinum wire as the counter electrode, and a Ag/AgCl electrode filled with saturated KCl aqueous solution as the reference electrode, respectively. The glassy carbon electrode was pretreated using the following process. First, the surface of the glassy carbon electrode was polished with 1.0, 0.3, and 0.05 mm α-alumina powders in sequence, rinsed thoroughly with twice-distilled water and placed in a water-filled ultrasonic bath for a 2 min period. Subsequently 20 μL of the Au@Pd-SprNSs/GO nanocomposites was loaded on the surface of a glassy carbon electrode (3 mm in diameter, 0.07 cm2). After drying, 5 μL of Nafion solution (2.5 wt %) was covered on the modified electrode surface, yielding the working electrode. For comparison, the electrocatalytic performance of commercial 10 wt % Pd/C catalyst purchased from Sigma-Aldrich was also investigated under the same conditions.

Figure 1. SEM images of (A) Au@Pd-SprNSs and (B,C) Au@PdSprNSs/GO composites; (D) TEM images of Au@Pd-SprNSs/GO composites.

length and width of 130 ± 6 and 80 ± 5 nm, respectively. Highresolution TEM (HRTEM) images in Supporting Information Figure S1 further reveal that the metallic NPs are crystalline with clear lattice distance of 0.224 nm, corresponding to the (111) lattice plane of the face-centered cubic (fcc) structure of Pd.17 Interestingly, unlike conventional continuous core−shell nanostructures, Au@Pd-SprNSs possess a very unique discontinuous Pd shell, which will allow the electrolytes to be diffused toward the Au-core and get the influences of both Auand Pd- sites to show synergistic effects enabling enhanced electrocatalytic activities. Some theoretical work has indicated that Pd (111) is not the ideal crystal face for the dissociation of ethanol molecules with low energy barrier.33,34 Therefore, the synergistic effect between Au and Pd play crucial role in enhancement of catalytic activities for fuel cells. Figure 1B,C shows the SEM images of the Au@Pd-SprNSs/GO composites. As can be observed, most of the Au@Pd-SprNSs are uniformly distributed on the GO sheet and keep away from the aggregation, which will greatly improve their catalytic properties. TEM images (Figure 1D) of Au@Pd-SprNSs/GO composites also reveals the uniform distribution of the SprNSs on the GO surface, which proves the strong attachment between the Au@Pd-SprNSs and GO surface to prevent aggregation of the NPs. Similarly, the different size and shape of core−shell Au@Pd SprNSs could be uniformly decorated on the GO surface (Figure S2). The surface composition and chemical oxidation states of the prepared GO and Au@Pd-SprNSs/GO were characterized by XPS (Figure 2). Four peaks centered at 284.5, 286, 287, and 288.5 eV, are obtained in C 1s XPS spectra of GO (Figure 2A) and correspond to C−C in aromatic rings, C−O (epoxy and alkoxy), CO, and OC−OH groups, respectively. The C 1s spectrum of the Au@Pd-SprNSs/GO as shown in Figure 2B clearly indicates that the GO had no significant changed after stirring the mixture of Au@Pd-SprNSs and GO dispersions. The Pd 3d XPS spectra of Pd/GO and Au@Pd/GO are showed in Figure S3A. For the Pd/GO, the two obvious peaks located at binding energies of 339.8 and 334.5 eV, respectively, correspond to the Pd 3d3/2 and Pd 3d5/2 of metallic Pd,35 while the weaker couple corresponds to Pd2+. The sample mainly consists of Pd0 with a very small amount of +2 oxidation state

3. RESULTS AND DISCUSSION 3.1. Synthesis of Au@Pd/GO nanocomposites. First, bimetallic Au@Pd-SprNSs were synthesized in a seed-mediated growth method via controlled Pd growth over Au NR seeds with the help of a binary mixture of CTAB and 5-BrSA as structure directing agents. In a separate procedure, exfoliated graphene oxide (GO) nanosheets were prepared by the modified Hummers’ method. Then we coupled these two dispersions in one pot to obtain Au@Pd-SprNSs/GO nanocomposites, as shown in Scheme 1. Scheme 1. Schematic Illustration for the Synthesis of Au@ Pd/GO Composites

3.2. Morphological, Compositional, and Structural Characterization. The morphology and structural features of the Au@Pd-SprNSs and Au@Pd-SprNSs/GO composites were elucidated by scanning electron microscopic (SEM) and transmission electron microscopic (TEM) analyzes. It can be observed from the SEM images (Figure 1A) that the Au@Pd SprNSs are nearly rod in shape with an average longitudinal C

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Figure 2. XPS profiles of (A) carbon 1s of GO and (B) carbon 1s of Au@Pd-SprNSs/GO.

which may be caused by surface oxidation during the preparation and characterization process. For the Au@Pd SprNSs/GO catalysts, the Pd 3d peak slightly shifts toward higher values compared with the XPS spectrum of monometallic Pd. The Au 4f spectra of Pd/GO and Au@Pd/GO are showed in Figure S3B. For the Au/GO, the obvious peaks located at 87.81 and 84.13 eV correspond to Au 4f5/2 and Au 4f7/2 respectively.24 Meanwhile, it can be clearly seen that the binding energies of Au 4f for the binary Au@Pd-SprNS/GO catalysts shift to a lower binding energy. Together, the shifting in the binding energies for Pd and Au in Au@Pd-SprNS/GO catalysts could be due to the electronic interactions between Pd and Au. The XRD of the Au@Pd-SprNS/GO along with the Au/GO and Pd/GO are shown in the Figure S4. All of the catalysts display a typical fcc structure. For the monometallic Au, the diffraction peaks located at the 2θ values of 38.18, 44.52, and 64.74° are assigned to Au (111), (200), and (220) lattice planes, respectively (JCPDS-04-0784).24 In the case of monometallic Pd, the peaks at about 40.14, 46.57, and 68.09° are attributed to the (111), (200) and (220) lattice planes of the fcc crystalline structure of Pd, respectively (JCPDS 461043).36 The XRD patterns of the Au@SprNS/GO clearly reveal the presence of pure Pd; however, the peak for Au is very weak, which is justified by the fact that small amounts of Au were present in the core of the Au@PdSprNS. 3.3. Electrocatalytic Performance of Au@Pd-SprNSs/ GO for Ethanol, Methanol, and Formic Acid Oxidation. Inspired by the above attractive characteristics (e.g., monodisperse bimetallic Pd-on-Au core−shell nanostructures, discontinuous Pd shell for easy accessibility of electrolytes to both Au and Pd, and uniform distribution of the SprNSs on GO surface), Au@Pd-SprNSs/GO nanocomposites were employed as the electrocatalysts for the oxidations of ethanol, methanol, and formic acid keeping in view their successful applications in direct fuel cell. Before electrocatalytic tests, the electrochemically active surface areas (ECSAs) of both Au@Pd SprNSs/GO and a commercial Pd/C catalyst were estimated by a calculation of the hydrogen desorption area from cyclic voltammograms (CVs) in 0.5 M H2SO4 solution (Figure 3). The oxidation currents were normalized to the ECSAs (0.364 cm2 for Au@Pd-SprNSs/GO and 0.343 cm2 for Pd/C) for further comparison of the activities of the developed catalysts with commercial Pd/C catalyst. Due to the high theoretical energy density and low toxicity of ethanol, ethanol-fueled direct alcohol fuel cells have attracted much attention. The ethanol oxidation on Pd in alkaline media was proposed to have the following reaction mechanism:7

Figure 3. CV of different catalysts in nitrogen saturated aqueous solution of 0.5 M H2SO4 at a scan rate of 50 mV s−1.

Pd + CH3CH 2OH → Pd−CH3COads + 3H 2O + 3e− (1) −

Pd + OH ← → Pd−OHads + e

−

(2)

Pd−CH3COads + Pd−OHads → Pd−CH3COOH + Pd (3)

The electrocatalytic activities of the Au@Pd-SprNSs/GO toward ethanol oxidation were evaluated by cyclic voltammetry in a 1.0 M NaOH + 0.5 M ethanol solution at a scan rate of 50 mV s−1. The activity of commercially available Pd/C catalyst was also investigated under identical conditions for comparison. To compare the catalytic activity of different catalysts, the current density was normalized to the ECSAs. Additionally, the current density toward ethanol oxidation normalized on the basis of the mass of the Pd is displayed in Figure S5. Two obvious oxidation peaks can be observed in Figure 4A: the anodic peak at around −0.26 V is assigned to the oxidation of newly chemisorbed species coming from ethanol adsorption during the ethanol electro-oxidation in the forward scan, while the cathodic peak at the potential of −0.34 V is ascribed to oxidation currents for incompletely oxidized in the forward scan. The oxidation peak during the forward sweep is usually used to evaluate the catalytic activity of the catalyst.37 It was observed that the Au@Pd-SprNSs/GO catalyst shows higher catalytic activity toward ethanol oxidation than commercial Pd/ C catalyst. In particular, the forward peak current density of the Au@Pd-SprNSs/GO catalyst reaches a value of 2.24 mA cm−2, which is about 2.3 times that of commercial Pd/C catalyst (0.97 mA cm−2). Moreover, It can be seen that the ethanol oxidation on Au@Pd-SprNSs/GO started at about −0.56 V, which is lower than the onset voltage for commercial Pd/C catalyst (−0.46 V). This result also implies Au@Pd-SprNSs/GO possesses better catalytic activity than commercial Pd/C catalyst in the oxidation of ethanol. The high electrocatalytic D

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Figure 4. (A) CV of different catalysts in a nitrogen saturated aqueous solution of in 1.0 M NaOH + 0.5 M ethanol solution at a scan rate of 50 mV s−1. (B) Chronoamperometry curves of different catalysts in 1.0 M NaOH + 0.5 M ethanol solution. (C,D) Long-term stability test of the different catalysts in a 1.0 M NaOH + 0.5 M ethanol solution at a scan rate of 50 mV s−1.

Figure 5. (A) CV of different catalysts in a nitrogen saturated aqueous solution of in a 1.0 M NaOH + 1 M methanol solution at a scan rate of 50 mV s−1. (B) Chronoamperometry curves of different catalysts in 1.0 M NaOH + 1 M methanol solution. (C,D) Long-term stability test of the different catalysts in 1.0 M NaOH + 1 M methanol solution at a scan rate of 50 mV s−1.

CA) were performed (Figure S7). The forward peak current density of the Au@Pd-SprNSs/GO (2.24 mA cm−2) is about 1.18-fold than that of Au@Pd-SprNSs (1.89 mA cm−2) and about 1.16-fold than that of Au@Pd-SprNSs/CA (1.93 mA cm−2), respectively, which can be attributed to the uniform dispersion of the SprNSs over the GO nanosheets. Nyquist plots of Au@Pd-SprNSs and Au@Pd-SprNSs/GO is shown in Figure S8, which suggests that the conductivities of the Au@ Pd-SprNSs does not affect much after loading on a small number of GO. In order to highlight the better electrocatalytic

activity of Au@Pd-SprNSs/GO can be attributed to the effective electronic conduction through the highly uniform and interconnected networks of Au@Pd-SprNS-loaded GO, the electronic states of the surface atoms because of the unique closely packed Pd shell morphology, and the exposure of their intrinsic active sites including high density of atomic steps, ledges and kinks, and Au−Pd heterojunctions to show synergistic effects.38 To illustrate the benefits from graphene oxide, control experiments using bare Au@Pd-SprNSs (Figure S6) and the SprNSs loaded on active carbon (Au@Pd-SprNSs/ E

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Figure 6. (A) CV of different catalysts in a nitrogen-saturated aqueous solution of in 0.5 M H2SO4 + 0.5 M HCOOH solution at a scan rate of 50 mV s−1. (B) Chronoamperometry curves of different catalysts in 0.5 M H2SO4 + 0.5 M HCOOH solution. (C,D) Long-term stability test of the different catalysts in in 0.5 M H2SO4 + 0.5 M HCOOH solution at a scan rate of 50 mV s−1.

number. Nonetheless, normalized current density values were significantly higher on the Au@Pd-SprNSs/GO than that on Pd/C in all the respective cycles. Again, the activity loss of Pd/ C at the 200th cycle was approximately 60% compared to the first cycle, whereas the Au@Pd-SprNSs/GO catalyst revealed 30% activity loss from the initial activity. All these results confirmed the Au@Pd-SprNSs/GO composite to be a much more stable and superior catalyst toward ethanol oxidation than commercial Pd/C. Figure 5A depicts the CVs of methanol oxidation on Au@ Pd-SprNSs/GO and commercial Pd/C using 1.0 M NaOH + 1 M methanol solution. The potential was scanned from −0.6 to 0.2 V vs Ag/AgCl at a scan rate of 50 mV s−1. The electrocatalytic properties of the catalysts toward methanol oxidation are generally analyzed by the oxidation peak potential and specific oxidation peak current density in the forward scan. The oxidation peak potential of the Au@Pd-SprNSs/GO and Pd/C is nearly the same, but the oxidation peak potential of the Au@Pd/GO is relatively broader than that of Pd/C, which indicates the greater activity range of Au@Pd-SprNSs/GO than Pd/C catalyst. Meanwhile, the peak current density on Au@PdSprNSs/GO is 1.74 mA cm−2, which is nearly 1.25 times larger than Pd/C (1.39 mA cm−2). Chronoamperometric (CA) measurements at −0.10 V were then performed to study the catalytic stabilities of the electrocatalysts. As can be seen from Figure 5B, after 1000 s, the current density from the Au@PdSprNSs/GO is still higher than Pd/C, demonstrating the higher electrocatalytic durability of Au@Pd-SprNSs/GO than Pd/C catalysts. To further examine the stability of the electrocatalysts, CV scans of 200 cycles were performed on both the Au@PdSprNSs/GO and Pd/C catalyst in a 1.0 M NaOH + 1 M methanol solution at 50 mV s−1. As shown in Figure 5C and Figure 5D, with an increase of cycle scan number, the peak current density of methanol oxidation reaction on Au@PdSprNSs/GO decreases at a slower rate than Pd/C. After 200 cycles, the Au@Pd-SprNSs/GO experienced a 25% loss in its

activity of the bimetallic Au@Pd-SprNS/GO nanostructures relative to their monometallic nanostructure, a control experiment using Pd/GO as catalyst was also performed (Figure S9). The forward peak current density of the Au@PdSprNSs/GO (2.24 mA cm−2) is about 1.94 times that of Pd/ GO (1.15 mA cm−2). This can be attributed to the synergistic interaction of the two metal centers. The antipoisoning abilities of the Au@Pd-SprNSs/GO and Pd/C catalyst for ethanol oxidation were evaluated by chronoamperometric measurements at a potential of −0.25 V for 1000 s in 1.0 M NaOH + 0.5 M ethanol solution, and the results are shown in Figure 4B. An initially rapid current decay for all catalysts was observed, probably because of the accumulations of poisonous carbonaceous intermediates (such as COads, CH3CHOads, etc.) on the catalyst surface during the ethanol oxidation reaction.10 However, it is interesting to note that the current density of ethanol oxidation on Au@PdSprNSs/GO is higher throughout the entire time span, and the current density decay is significantly slower than those on the Pd/C catalysts, demonstrating that the Au@Pd SprNSs/GO catalyst has relatively higher catalytic activity and better stability for ethanol oxidation as compared to Pd/C, which is consistent with the CV results displayed in Figure 4A. The long-term stability of the electrocatalyst is of great importance for practical applications. With this aim in mind, CV scans of 200 cycles were performed on the Au@PdSprNSs/GO and Pd/C catalysts in a 1.0 M NaOH + 0.5 M ethanol solution at 50 mV s−1. Figure 4C shows the forward peak current density as a function of cycle scan number. For both catalysts, it can be observed that the forward oxidation peak current density decreases gradually as the number of scans increases. However, the forward peak current density of Au@ Pd-SprNSs/GO catalyst decreases in a lower extent than that of Pd/C catalyst. Figure 4D shows a gradual decrease of normalized current density (forward peak current density divided by the first current density) as a function of cycle scan F

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Langmuir activity compared to the first cycle; by contrast, commercial Pd/C catalyst loses 30% of its initial activity in the same time. These results also support the superior electrocatalytic activity and stability of our developed Au@Pd-SprNSs/GO composite. To generalize and successfully apply our developed materials in fuel cell application, electro-oxidation of formic acid was also studied on Au@Pd-SprNSs/GO composite. Figure 6A represents the CVs of Au@Pd-SprNSs/GO and commercial Pd/C in the solution containing 0.5 M H2SO4 and 0.5 M HCOOH at a scan rate of 50 mV s−1. In the positive going potential scan, the peak current density on the Au@PdSprNSs/GO was 3.31 mA cm−2, ∼2-fold higher than that on commercial Pd/C (1.59 mA cm−2), which implies that the Au@ Pd-SprNSs/GO composite possesses better catalytic activity than Pd/C in the electro-oxidation of formic acid. To evaluate the stability and long-term performance of our catalysts, chronoamperometric curves (Figure 6B) were recorded at 0.1 V for 1000 s. Both of the catalysts showed a decrease in current density because of surface poisoning with the intermediate species formed from formic acid oxidation. To gain deep insight into the catalytic durability of Au@Pd-SprNSs/GO, the CVs were recorded in 0.5 M H2SO4 containing 0.5 M HCOOH for 50 cycles. As shown in Figure 6C,D, current density on both Au@Pd-SprNSs/GO and Pd/C catalysts gradually decreases, which is consistent with the previous reports.16 Nevertheless, Au@Pd-SprNSs/GO composite showed higher current density than those on Pd/C over the entire time range, describing the higher efficiency and stabilities of the Au@Pd-SprNSs/GO. Therefore, it has been confirmed that our developed Au@PdSprNSs/GO composite has some unique properties that enable it to exhibit much higher catalytic activity and stability than commercial Pd/C catalyst toward all the above electrochemical reactions used in fuel cells.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ∥

These authors contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Chinese Academy of Science for Hundred Talents Program, the Chinese Central Government for Thousand Young Talents Program, the Natural Science Foundation of China (21404110, 51473179, 51303195, 21304105, 21277025), the Excellent Youth Foundation of Zhejiang Province of China (LR14B040001), the Ningbo Science and Technology Bureau (Grants 2014B82010 and 2015C110031), the Youth Innovation Promotion Association of Chinese Academy of Sciences (2016268), the Technology Foundation for Selected Overseas Chinese Scholar, Ministry of Personnel of China (2015), and the Foundation of Fujian Educational Committee for Distinguished Young Talents in Universities (JA13024).

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REFERENCES

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4. CONCLUSIONS In this report, we have demonstrated a facile, efficient, and controllable route to develop Pd-on-Au SprNS-grafted graphene oxide nanosheets for high-performance electrochemical activities. The Au@Pd-sprNPs comprises a very unique close-packed arrangement of tiny anisotropic Pd NPs to offer the abundance of intrinsic active sites including high density of atomic steps, ledges and kinks, and Au−Pd heterojunctions to show enhanced catalytic/electrocatalytic activities. Moreover, uniform distribution of the SprNSs on GO surface, which proves the strong attachment between the Au@ Pd-SprNSs and GO surface to prevent aggregation of the NPs, further enhances the electrocatalytic stability of the composites. The activities of the commercial Pd/C were also studied for comparison. Our developed Au@Pd-SprNSs/GO showed larger electrochemically active surface area, and thus showed very high electrocatalytic activity, greater stability, and enhanced antipoisoning capacity in comparison with the commercial Pd/C catalyst toward the electro-oxidations of methanol, ethanol, and formic acid. These results indicate that this Au@Pd-SprNSs/GO nanocomposite will be a promising Pd-based catalyst for fuel cell applications.

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DOI: 10.1021/acs.langmuir.6b01382 Langmuir XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.langmuir.6b01382 Langmuir XXXX, XXX, XXX−XXX