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Copyright © 2013 by American Scientific Publishers All rights reserved. Printed in the United States of America

Science of Advanced Materials Vol. 5, pp. 1–9, 2013 (www.aspbs.com/sam)

Surfactant-Free Synthesis of Cube-Like PtRu Alloy Nanoparticles with Enhanced Electrocatalytic Activity Toward Formic Acid Oxidation Yizhong Lu1, 2 , Yuanyuan Jiang1, 2 , Ruizhong Zhang1, 2 , and Wei Chen1 ∗ 1

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Jilin, China 2 University of Chinese Academy of Sciences, Beijing 100039, China

ABSTRACT

1. INTRODUCTION Due to the high energy conversion efficiency and zero or low environmental pollution, direct liquid fuel cells (DLFCs), including direct methanol fuel cells (DMFCs) and direct formic acid fuel cells (DFAFCs), have attracted enormous attention as an attractive power source in the past few decades.1–3 Compared to methanol fuel, formic acid is a nontoxic and nonflammable liquid fuel which can be easily handled and stored. On the other hand, although the energy density of formic acid is lower than that of methanol, the smaller crossover flux of formic acid through the electrolyte membranes can allow DFAFCs to work with higher concentrations of fuel and thinner membranes.4 In addition, formic acid itself can facilitate both proton and electron transport at the anode catalyst ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: xx Xxxx xxxx Accepted: xx Xxxx xxxx

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layer. Therefore, formic acid has received considerable attention over the past years as a potential fuel for DLFCs.5 6 However, despite extensive research progress, there are still scientific and technological difficulties hampering the widespread commercialization of fuel cells. For instance, the sluggish anode oxidation reaction kinetics has been a major bottleneck in the performance of direct liquid fuel cells. Although Pd displays excellent catalytic activity for formic acid oxidation, the high performance can not be sustained because of the easy dissolution of Pd in acidic solutions7 and its vulnerable property towards intermediate species.8 Nowadays, Pt has been widely used as anodic electrocatalyst because of its highest catalytic activity and stability among the anode metal catalysts for electro-oxidation of small organic fuels. Unfortunately, with platinum as an anode catalyst, the surface is usually heavily poisoned by the strong adsorption of CO intermediates produced during the oxidation of organic fuels, resulting in the blocking of the active sites and thus lowering their catalytic performance. In addition, the high cost and limited world supply of platinum has also hindered the

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Surfactant-free, monodispersed and cube-like PtRu bimetallic nanoparticles were successfully synthesized via a facile, one-step hydrothermal method. The morphology and the crystal structure of the as-prepared PtRu nanoparticles were characterized by TEM, XRD and XPS. The electrochemical studies indicated that the PtRu alloy nanocubes exhibit enhanced electrocatalytic activity toward formic acid oxidation with more negative onset potential, larger oxidation current density, and higher tolerance to CO poisoning effect compared to the commercial Pt/C catalyst in acidic media. The electron transfer kinetics of formic acid oxidation is elaborated with electrochemical impedance spectroscopic measurements. It was found that the diameter of the impedance arcs at the PtRu alloy nanocubes is at least one order of magnitude smaller than that at the commercial Pt/C catalyst, suggestive of lower charge-transfer resistance and thus, higher catalytic activity of the PtRu alloy nanoparticles for formic acid oxidation. Further XPS and CO stripping measurements demonstrated that the enhanced activity of the as-synthesized PtRu nanocrystals could be ascribed to the bifunctional mechanism and the ligand effect. The present work highlights the facile synthesis of surfactant-free PtRu bimetallic alloy nanocubes and their enhanced electrocatalytic performance which makes them potential candidates as anode catalysts in fuel cells. KEYWORDS: Surfactant-Free, PtRu Alloy Nanocubes, Electrocatalysts, Fuel Cell, Formic Acid Oxidation.

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Surfactant-Free Synthesis of Cube-Like PtRu Alloy Nanoparticles with Enhanced Electrocatalytic Activity Toward Formic Acid Oxidation

commercialization of fuel cells. Thus, in order to reduce the cost of fuel cells and reduce the poisoning resistance, Pt-based bimetallic nanocrystals with controlled surface composition and structures have attracted increasing interest in recent years.9–14 Compared to the pure platinum electrocatalyst, Pt-based nanostructured materials exhibited higher performance and enhanced tolerance towards CO poisoning because of the so-called bifunctional mechanism or the ligand (or electronic) effect15 or a mixture of both.16 17 It should be noted that, among various Pt-based alloy nanostructured materials, PtRu alloys exhibit the best electrocatalytic performance in the practical applications.18 Based on the bifunctional mechanism, ruthenium is able to provide oxygen-containing species (Ru–OHad that could oxidize CO on adjacent platinum at more negative potentials than platinum. While from the ligand effect, the presence of ruthenium is reported to alter the electronic properties of Pt causing CO to adsorb less strongly on the catalyst surface.19 It is also generally accepted that both the catalytic efficiency, selectivity and reaction durability are highly dependent on the size, shape, composition, and structure of the catalyst nanoparticles.20–24 However, to realize the morphology- and composition-controlled synthesis of nanoparticles, organic ligands are usually used to stabilize the particles. Unfortunately, the presence of capping agents (surfactants) on the particle surface may severely reduce their catalytic activity. In the past several years, various post-treatment methods have been applied to remove the surfactants, including thermal annealing,25 acetic acid washing,26 plasma cleaning,27 UV-Ozone irradiation,28–30 butylamine replacing31 and CO stripping.32 While the cleaning process could cause the change in particle size, shape, composition, and thus change the intrinsic catalytic activity. Therefore, it is of great importance to prepare surfactant-free electrocatalysts for practical applications. Recently, substantial research efforts have been devoted to the surfactant-free synthesis of highly active catalysts. For instance, Peng et al.33 have developed a novel method to synthesize surfactant-free supported cubic platinum nanoparticles using CO and H2 as reducing agent. Carpenter and coworkers34 reported the solvothermal synthesis of Pt–Ni alloy using dimethylformamide (DMF) as both solvent and reductant. The as-prepared PtNi catalysts displayed ORR activities close to 3000 A/cm2Pt , which is almost 15 times that of a state-of-the-art Pt/C catalyst. Here we report a facile, one-pot hydrothermal method to synthesize monodispersed and cube-like PtRu alloy nanoparticles (PtRu NPs) in DMF without additional reducing/capping agents. The as-prepared PtRu NPs exhibited enhanced electrocatalytic activity towards formic acid oxidation. Compared to the commercial Pt/C catalyst, the oxidation of formic acid on PtRu NPs displayed more negative onset potential, larger oxidation current density, and higher tolerance to CO poisoning. 2

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2. EXPERIMENTAL DETAILS 2.1. Chemicals Platinum(II) 2,4-pentanedionate (Pt(acac)2 , 99%, Pt content: 49.6%), ruthenium(III) 2,4-pentanedionate (Ru(acac)2 , 99%), Nafion solution (5 wt%), and E-TEK Pt/C (nominally 40% by weight of 3.2 nm Pt nanoparticles on Vulcan XC-72 carbon support) were obtained from Alfa Aesar. Perchloric acid (HClO4 , A. R. grade) and acetone were purchased from Tianjin Chemical reagent. Formic acid (HCOOH, A. R. grade) was provided from Beijing Chemical Reagent. Water used in all experiments was supplied by a Water Purifier Nanopure water system (18.3 M cm). All reagents were used as received without further purification. Moreover, all glasswares were washed with Aqua Regia (HCl:HNO3 with volume ratio of 3:1), and rinsed with ethanol and ultrapure water. (Caution: Aqua Regia is a very corrosive oxidizing agent, which should be handled with great care.) 2.2. Synthesis of Surfactant-Free, Cube-Like PtRu Bimetallic Nanoparticles (PtRu NPs) In a typical synthesis,35 36 Pt(acac)2 (0.020 g) and Ru(acac)2 (0.020 g) were co-dissolved in 12 mL DMF to yield a homogenous solution under vigorous stirring. After 30 min stirring, the resulting dark red solution was transferred to a teflon-lined stainless steel autoclave. The sealed vessel was then heated at 200 C for 24 h before it was cooled to room temperature. The resulting dark solution was separated and purified by centrifugation and sonication with ethanol for three times. Finally, the obtained products were dried at 60 C under vacuum and redispersed in anhydrous hexane for further characterization. 2.3. Electrochemical Measurements Before each experiment, the glassy carbon (GC) electrode (3.0 mm in diameter) was first polished with alumina slurries (Al2 O3 , 0.05 m) power on a polishing cloth to obtain a mirror finish, followed by sonication in 0.1 M HNO3 , 0.1 M H2 SO4 and pure water for 10 min successively. To prepare a catalyst-coated working electrode, the catalyst was dispersed in a mixture of solvent containing water, isopropanol, and Nafion (5%) (v/v/v = 4:1:0.025) to form a 2 mg/mL suspension. The catalyst ink (10 L) was dropcasted on the polished GC electrode surface by a microlitre syringe followed by drying at ambient condition. Voltammetric measurements were carried out with a CHI 750D electrochemical workstation. The electrode prepared above was used as the working electrode. An Ag/AgCl (in 3 M NaCl, aq.) combination isolated in a double junction chamber and a Pt coil were used as the reference and counter electrodes, respectively. All electrode potentials in the present study were referred to this Sci. Adv. Mater., 5, 1–9, 2013


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2.4. Material Characterization The morphology and size of the PtRu NPs were analyzed with a HITACHI H-600 Analytical Transimission Electron Microscope (TEM) with an accelerating voltage of 100 kV. High-resolution TEM (HRTEM) images were obtained on a JEM-2010 (HR) microscope operated at 200 kV. Power X-ray diffraction (XRD) was performed on a D/Max 2500 V/PC power diffractometer using CuK radiation with a Ni filter ( = 0154059 nm at 30 kV and 15 mA). X-ray photoelectron spectroscopy (XPS) measurements were performed by using a VG Thermo ESCALAB 250 spectrometer (VG Scientific) operated at 120 W. The binding energy was calibrated against the carbon 1 s line.

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Ag/AgCl reference. The working electrodes were first activated with cyclic voltammograms (CVs) (−0.3 to 1.0 V at 100 mV/s) in N2 -purged 0.1 M HClO4 solution until a steady CV was obtained. To measure the electro-oxidation of formic acid, the solution of 0.1 M HClO4 + 05 M HCOOH was purged with N2 gas before measurements were taken, and CVs were recorded in the potential range between −0.3 to 1.0 V at a scan rate of 50 mV/s. The amperometric current density–time (i–t) curves were measured at potentials of 0.4, 0.5, and 0.6 V for 1000 s, respectively, in 0.1 M HClO4 + 05 M HCOOH solution. The impedance spectra were recorded between 100 kHz and 50 mHz with the amplitude (rms value) of the ac signal of 5 mV. All experiments were conducted at room temperature. Oxidation of pre-adsorbed carbon monoxide (COad was measured by COad stripping voltammetry in 0.1 M HClO4 solution at a scan rate of 50 mV/s. Briefly, gaseous CO was purged into the electrolyte for 15 min to allow complete adsorption of CO onto the surface of electrocatalysts while maintaining a constant potential of 0.00 V. After N2 purging for another 15 min to eliminate any dissolved CO in the electrolyte, CO stripping was recorded at a scan rate of 50 mV/s. Two subsequent CV cycles were collected to verify the complete oxidation of the adsorbed CO.

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3. RESULTS AND DISCUSSION 3.1. Characterization of the Surfactant-Free Cube-Like PtRu Alloy Nanoparticles (PtRu NPs) The morphology of the as-synthesized nanoparticles was firstly characterized with TEM measurements. Figure 1 shows the representative TEM and HRTEM images of PtRu NPs at different magnifications. It can be seen that most of the nanoparticles are well-faceted crystals with a predominance of cubes and cubeoctahedra. One hundred nanoparticles were randomly selected to measure the particle size and shape distributions. From the particle size distribution histogram shown in Figure 1(B) Sci. Adv. Mater., 5, 1–9, 2013

Fig. 1. Representative TEM (A), (B) and HRTEM (C) images of the as-synthesized PtRu nanoparticles at different magnifications. The inset in (B) shows the particle size distribution histogram.

inset, the as-prepared nanoparticles exhibit an average size of 638 ± 036 nm with 75% cubes, 12% cubeoctahedra and 3% irregular shapes. The TEM measurements indicate that well-defined PtRu nanocrystals have been synthesized although no additional capping agents were added to direct the crystal growth during the reaction. Here, solvent 3

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DMF actually also acts as a weak reducing agent because of the produce of CO and dimethylamine at or beyond its boiling point (153 C).35 36 In the previous studies, it was found that the endogenous CO could serve both as a reducing agent and an efficient protecting agent during the formation of metal nanoparticles.37–39 Thus, the well dispersity of the as-synthesized PtRu NPs could be attributed to the protecting effects of CO and dimethylamine produced from DMF. The crystal structure of the as-prepared PtRu NPs was further studied with powder X-ray diffraction (XRD) measurements. Figure 2 shows the XRD pattern of the PtRu NPs. For comparison, bulk Pt (NO. 65-2868) from the Joint Committee on Powder Diffraction Standards (JCPDS) is also included. It can be seen that the strong diffraction peaks from the PtRu NPs have the same facecentered cubic (fcc) crystal structure as Pt, but with a slight shift toward higher angles. This suggests that the random distribution of Ru atoms in the Pt lattice leads to the reduced Pt Pt bond lengths in the formed nanoparticles,40 which has also been observed with other Pt–Ru alloys reported in the previous studies.18 41 42 Such observation together with the absence of diffraction peaks from single Pt or Ru crystal phases indicates the formation of alloy structure within the PtRu NPs. The average size of the PtRu NPs can also be calculated based on the broadening of the (111) diffraction peak according to Scherer’s equation: d = 09/B2 cos , where represents the wavelength of the X-ray, is the angle of the peak, and B2 is the width of the peak at half height. The average size of PtRu NPs was calculated to be approximately 6.5 nm, which is in good agreement with the TEM data. The oxidation states of Pt and Ru were investigated by XPS measurements. Figures 3(A) and (B) show the Pt 4f and Ru 3p region of the PtRu NPs, respectively. Note that, since the Ru 3d peaks overlap with the C 1s peak, the Ru 3p was chosen for the analysis of the Ru oxidation states. It can

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Fig. 3. XPS patterns of the PtRu NPs: (A) Pt 4f region; (B) Ru 3p3/2 region.

be seen that the peaks at 71.2 and 74.4 eV in the Pt 4f photoelectron spectrum could be assigned to the binding energies of Pt 4f7/2 and Pt 4f5/2 of metallic Pt0 , respectively. The peak at 462.0 eV in the Ru XPS spectrum is assigned to Ru 3p3/2 . This binding energy is comparable to that of the zerovalent state of Ru metal. The XPS results further verified the formation of bimetallic Pt–Ru nanocrystals. 3.2. Formic Acid Oxidation with PtRu NPs

Fig. 2. XRD patterns of the as-synthesized PtRu NPs. For comparison, bulk Pt (No. 65-2868) from the JCPDS is also included.

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The electrocatalytic activity of the as-synthesized PtRu NPs towards formic acid oxidation was evaluated by cyclic voltammetry and benchmarked against that of the commercial Pt nanoparticles on carbon (Pt/C, Pt wt% = 40%) catalyst. Figure 4(A) shows the typical cyclic voltammograms of PtRu NPs and the commercial Pt/C catalyst modified glass carbon (GC) electrodes with the same loading in N2 -saturated 0.1 M HClO4 solution at a scan rate of 50 mV/s. It can be seen that the typical electrochemical features of platinum could be observed obviously Sci. Adv. Mater., 5, 1–9, 2013

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at the PtRu NPs modified electrode. For instance, the obvious hydrogen adsorption–desorption current peaks on the Pt surface between − 0.25 and 0.0 V; the formation of platinum oxides in the positive potential scan and a subsequently reduction peak of platinum oxides at 0.6 V during the reverse scan. These characteristics of Pt indicate that the as-prepared PtRu NPs are electrochemically active. Figure 4(B) displays the steady CVs of formic acid oxidation at the PtRu NPs (black curve) and Pt/C (red curve) in 0.1 M HClO4 + 05 M HCOOH at a potential scan rate of 50 mV/s. Note that the voltammetric currents have been normalized to the electrochemically active surface area (ECSA), which was calculated by measuring the charge collected in the Hupd adsorption/desorption region after double-layer correction and assuming a value of 0.21 mC/cm2 for the adsorption of a hydrogen monolayer.18 26 ECSA = QH /[metal] ∗ 021

(1)

where [metal] represents the loading of Pt. The ECSA of the PtRu NPs and commercial Pt/C catalysts are calculated to be 37.9 and 22.7 m2 /mgPt , respectively. The larger Sci. Adv. Mater., 5, 1–9, 2013

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Fig. 4. Cyclic voltammograms of the PtRu NPs and Pt/C modified electrodes in 0.1 M HClO4 solution (A) and in 0.1 M HClO4 + 0.5 M HCOOH solution (B). Potential scan rate 50 mV/s.

ECSA of the as-prepared PtRu NPs is favorable for their application in electrocatalysis. As can be seen by comparing the two CVs in Figure 4(B) that the Ru incorporation (PtRu NPs) could enhance the HCOOH oxidation reaction, where formic acid can be oxidized at lower potentials. However, the oxidation current is strongly inhibited on the commercial Pt/C catalysts. At potentials below 0.0 V, the current from formic acid oxidation is negligible in both voltammograms because the surface active sites are poisoned by strongly adsorbed carbon monoxide (COads , an intermediate from dehydration of HCOOH (HCOOH → COads + H2 O). With potential increasing, formic acid oxidation current takes off rapidly on the synthesized PtRu NPs, indicating that significant formic acid oxidation occurs. In contrast, on the commercial Pt/C, along with increasing electrode potential, a weak and broad oxidation peak appears at around + 0.44 V, followed by a sharp peak at + 0.76 V in the forward scan. Earlier studies have proposed that the former oxidation peak is attributed to the direct oxidation of formic acid to CO2 on surface catalytically active sites that were not poisoned by COads , while the latter is assigned to the oxidation of COads and formic acid.29 43 Onset potential and peak current density have been recognized as two major parameters to evaluate the catalytic activity of an electrocatalysts. From Figure 4(B), the PtRu NPs exhibit much more negative onset potential (−0.034 V) of formic acid oxidation compared to the commercial Pt/C catalyst (0.007 V). On the other hand, at 0.44 V the HCOOH oxidation current density obtained from PtRu NPs is nearly 2.78 times larger than that from the commercial Pt/C catalyst. Moreover, when the oxidation current is normalized by noble metal loading, the current density at 0.44 V obtained from PtRu NPs (163.4 mA/mg) is 4.33 times higher than that of commercial Pt/C (37.7 mA/mg). In order to further investigate the electrochemical activity and stability of the synthesized PtRu NPs and commercial Pt/C catalyst for formic acid oxidation, chronoamperometric measurements were performed at different initial potentials. Figures 5(A)–(C) shows the J –t curves at + 0.4, + 0.5, and + 0.6 V in 0.1 M HClO4 + 0.5 M HCOOH. It can be seen that, although the current continues to decay gradually, the PtRu NPs electrocatalyst shows a much lower deterioration rate compared to the commercial Pt/C catalyst over the entire time period examined, demonstrating its enhanced stability. On the other hand, maximum initial (0.78 mA/cm2 vs. 0.42 mA/cm2 at 0.5 V) and steady-state oxidation current densities after 1000 s (0.15 mA/cm2 vs. 0.058 mA/cm2 obtained from the present PtRu NPs are larger than those from the commercial Pt/C. The chronoamperometric measurements indicate again that PtRu NPs have better activity and stability for formic acid oxidation in comparison with the commercial Pt/C catalyst, in good agreement with the voltammetric results.


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t (s) Fig. 5. Chronoamperometric curves of the PtRu NPs (black curves) and Pt/C (red curves) modified electrodes in 0.1 M HClO4 + 0.5 M HCOOH solution at different potentials: (A) +0.4 V, (B) +0.5 V, and (C) +0.6 V.

Electrochemical impedance spectroscopy (EIS) is a powerful method to probe the kinetics of electrochemical reactions and has recently been utilized to understand the anodic processes for CH3 OH oxidation44 45 and HCOOH oxidation.29 46 47 Here, the electrooxidation dynamics of formic acid catalyzed by the as-synthesized PtRu NPs was then examined by EIS measurements. Figure 6 depicts the representative complex-plane (Nyquist) impedance plots of the PtRu NPs (A) and commercial Pt/C (B) modified electrodes, respectively, in 0.1 M HClO4 + 05 M HCOOH with varied electrode potentials. For the PtRu NPs, at E = −02 V, the impedance spectrum shows a large arc in the first quadrant, which indicates the presence of resistive and capacitive components and a slow electron-transfer rate of formic acid oxidation. However, the diameter of the arcs decreases with increasing potential from − 0.2 V to + 0.2 V, suggesting the faster electron-transfer rate of formic acid oxidation at this potential region. In accordance with the impedance, the HCOOH oxidation current 6

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density increases with potential increasing as shown in the CV measurements (Fig. 4(B)). Interestingly, with potential further increasing, for instance, at 0.3 V, the impedance arcs appears in the second quadrant and negative impedance was observed. Such negative impedance has also been found previously in formic acid oxidation on the nanostructured electrocatalysts, and was ascribed to the formation of chemisorbed hydroxyl species at electrode surface.46–48 At potentials more positive than 0.9 V, the impedance curves return to the normal behavior in the first quadrant. In contrast, although the similar trend is found for the commercial Pt/C catalyst, the diameters of the impedance arcs of the PtRu NPs are significantly larger than those obtained from the PtRu NPs, indicating substantially higher charge-transfer resistance for formic acid oxidation on the commercial Pt/C. That is, the catalytic activity of Pt/C catalysts is much lower than that of our synthesized PtRu NPs catalysts. Based on the voltammetric and impedance results, the equivalent circuits shown in Figure 7 insets were used to fit the impedance data obtained from PtRu NPs and Pt/C. The equivalent circuits in inset (a) and (b) are for the normal and negative impedance, respectively. Here, RS represents the solution resistance, CPE (constant-phase element) is the double Sci. Adv. Mater., 5, 1–9, 2013

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(0.506 V). Moreover, the CO stripping on PtRu NPs produced a peak potential at 0.356 V, which is 0.187 V more negative than that on commercial Pt/C catalyst. These results clearly show that the as-synthesized PtRu NPs are more electrocatalytically active for CO removal, which is very likely due to the incorporation of Ru composite. The presence of ruthenium is able to provide oxygencontaining species (Ru–OHad that could oxidize CO on adjacent platinum at more negative potentials than platinum, thus, enhanced formic acid oxidation was achieved at low potential. To probe the possible perturbation on Pt electronic structure induced by incorporation of Ru content, XPS analysis of Pt 4f was performed. The result was displayed in Figure 3. It can be seen that the incorporation of Ru content did markedly alter the electronic structure of Pt. For example, the binding energies of Pt 4f7/2 and Pt 4f5/2 electrons of the as-synthesized PtRu NPs are 71.22 and 74.53 eV, respectively, which are both down-shifted compared to the commercial Pt/C catalysts. This down-shift of the Pt binding energy, which reflects a shift towards lower binding energy of the Pt d-band center, could cause CO adsorb less strongly on the catalyst surface, thus, enhance the catalytic activity.49 50 On the other hand, the binding energy of Ru 3p3/2 (∼ 462.1 eV) is found up-shifted compared to the early report (∼ 461.0 eV),51 which further 7

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layer capacitance, RCT is the charge transfer resistance, C0 and R0 represent the capacitance and resistance of the electrooxidation of adsorbed CO intermediates, respectively. The representative fits (solid black lines) are shown in each of the Nyquist plots in Figure 6. From the fitting, the variations of the charge-transfer resistance (RCT with the potentials on the electrodes are shown in Figure 7. It can be seen that within the potential range of − 0.2 to 1.0 V, the RCT at the PtRu NPs is indeed markedly smaller than that at the commercial Pt/C catalyst. This result indicates again that the electron-transfer kinetics for formic acid oxidation at the PtRu NPs is much better facilitated than that at the Pt/C catalyst. We then attempted to understand the mechanism of enhanced catalytic activity of the as-synthesized PtRu NPs for formic acid oxidation. It is generally accepted that electrochemical oxidation of formic acid takes place on Pt-based catalysts through two parallel paths: (1) direct oxidation to produce CO2 , and (2) indirect pathway, in which CO intermediate produced via a dehydration step, followed by the oxidation of CO to CO2 at high potentials. As previously reported, a frequently invoked explanation of enhanced formic acid oxidation activity and stability on PtRu NPs versus Pt/C is likely caused by the wellestablished bifunctional mechanisms or the ligand effect or both of them. Here, CO stripping and XPS analysis are performed. Figures 8(A) and (B) show the CO stripping voltammetric curves on PtRu NPs and Pt/C catalysts. It can be seen that on both catalysts, the hydrogen desorption peaks are fully suppressed in the first positivegoing scan due to the blocking of the active surface sites by the pre-adsorbed CO. As potential increasing, a sharp oxidation current peak appears, signifying the oxidation of pre-adsorbed CO. For the PtRu NPs, the onset oxidation potential at 0.290 V is more negative than that of Pt/C


indicates the interaction between Pt and Ru atoms in the as-synthesized PtRu NPs. Therefore, the enhanced catalytic activity for formic acid oxidation on PtRu NPs could be explained in terms of the bifunctional mechanism and the ligand effect.

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4. CONCLUSIONS In summary, surfactant-free cube-like PtRu alloy nanoparticles have been successfully synthesized via a facile and versatile hydrothermal method. TEM measurements indicated that the as-synthesized PtRu nanoparticles have an average diameter of 638 ± 036 nm with 75% cubes, 12% cubeoctahedra and 3% irregular shapes. Compared to the commercial Pt/C catalysts, the PtRu alloy nanoparticles (PtRu NPs) exhibited much enhanced electrocatalytic activity for formic acid oxidation with larger oxidation current density, higher CO tolerance, more negative onset and peak potentials, and higher stability. The EIS results showed that the charge transfer resistance obtained on the as-synthesized PtRu NPs is much smaller than that from the commercial Pt/C catalyst, indicating the highly facilitated electron-transfer kinetics for formic acid oxidation at the PtRu NPs. In the CO stripping tests, the PtRu NPs exhibit much more negative onset and peak potentials of CO oxidation compared to the Pt/C catalyst. XPS studies displayed that the binding energy of Pt 4f and Ru 3p experienced down-shift and up-shift, respectively, compared to the pure Pt and Ru catalysts. Therefore, the enhanced electrocatalytic activity of the as-synthesized PtRu NPs could be ascribed to the following aspects: (1) the surfactantfree surface, which provides more active sites available for formic acid oxidation reaction; (2) the bifunctional mechanism reflected by the CO stripping test; (3) the ligand effect, shown in the XPS measurements. Acknowledgments: This work was supported by the National Natural Science Foundation of China (Nos. 21275136, 21043013) and the Natural Science Foundation of Jilin province, China (No. 201215090).

References and Notes 1. C. Lamy, A. Lima, V. LeRhun, F. Delime, C. Coutanceau, and J. M. Leger, J. Power Sources 105, 283 (2002). 2. E. Antolini, J. Power Sources 170, 1 (2007). 3. M. Winter and R. J. Brodd, Chem. Rev. 104, 4245 (2004). 4. J. Willsau and J. Heitbaum, Electrochim. Acta 31, 943 (1986). 5. C. Rice, R. I. Ha, R. I. Masel, P. Waszczuk, A. Wieckowski, and T. Barnard, J. Power Sources 111, 83 (2002). 6. Y. M. Zhu, S. Y. Ha, and R. I. Masel, J. Power Sources 130, 8 (2004). 7. J. Solla-Gullon, V. Montiel, A. Aldaz, and J. Clavilier, Electrochem. Commun. 4, 716 (2002). 8. P. K. Babu, H. S. Kim, J. H. Chung, E. Oldfield, and A. Wieckowski, J. Phys. Chem. B 108, 20228 (2004). 9. B. Lim, M. J. Jiang, P. H. C. Camargo, E. C. Cho, J. Tao, X. M. Lu, Y. M. Zhu, and Y. N. Xia, Science 324, 1302 (2009).

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Lu et al.

10. D. Xu, Z. P. Liu, H. Z. Yang, Q. S. Liu, J. Zhang, J. Y. Fang, S. Z. Zou, and K. Sun, Angew. Chem. Int. Ed. 48, 4217 (2009). 11. W. Chen, L. P. Xu, and S. W. Chen, J. Electroanal. Chem. 631, 36 (2009). 12. A. C. Chen and P. Holt-Hindle, Chem. Rev. 110, 3767 (2010). 13. Y. J. Kang and C. B. Murray, J. Am. Chem. Soc. 132, 7568 (2010). 14. W. Chen, J. M. Kim, S. H. Sun, and S. W. Chen, Langmuir 23, 11303 (2007). 15. C. Lu, C. Rice, R. I. Masel, P. K. Babu, P. Waszczuk, H. S. Kim, E. Oldfield, and A. Wieckowski, J. Phys. Chem. B 106, 9581 (2002). 16. P. Waszczuk, A. Wieckowski, P. Zelenay, S. Gottesfeld, C. Coutanceau, J.-M. Léger, and C. Lamy, J. Electroanal. Chem. 511, 55 (2001). 17. P. Waszczuk, G.-Q. Lu, A. Wieckowski, C. Lu, C. Rice, and R. I. Masel, Electrochim. Acta 47, 3637 (2002). 18. Y. Lu and W. Chen, Chem. Commun. 47, 2541 (2011). 19. C. Roth, A. J. Papworth, I. Hussain, R. J. Nichols, and D. J. Schiffrin, J. Electroanal. Chem. 581, 79 (2005). 20. S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai, and P. Yang, Nat. Mater. 6, 692 (2007). 21. N. Tian, Z. Y. Zhou, S. G. Sun, Y. Ding, and Z. L. Wang, Science 316, 732 (2007). 22. Y. Z. Lu and W. Chen, J. Power Sources 197, 107 (2012). 23. W. Chen and S. W. Chen, Angew. Chem. Int. Ed. 48, 43836 (2009). 24. H. B. Wu and W. Chen, J. Am. Chem. Soc. 133, 15236 (2011). 25. Z. Liu, M. Shamsuzzoha, E. T. Ada, W. M. Reichert, and D. E. Nikles, J. Power Sources 164, 472 (2007). 26. V. Mazumder and S. Sun, J. Am. Chem. Soc. 131, 4588 (2009). 27. H. Yang, J. Zhang, K. Sun, S. Zou, and J. Fang, Angew. Chem. Int. Ed. 49, 6848 (2010). 28. W. Chen, J. Kim, L. P. Xu, S. Sun, and S. Chen, J. Phys. Chem. C 111, 13452 (2007). 29. W. Chen, J. Kim, S. Sun, and S. Chen, Phys. Chem. Chem. Phys. 8, 2779 (2006). 30. W. Chen, J. M. Kim, S. H. Sun, and S. W. Chen, J. Phys. Chem. C 112, 3891 (2008). 31. J. Wu, J. Zhang, Z. Peng, S. Yang, F. T. Wagner, and H. Yang, J. Am. Chem. Soc. 132, 4984 (2010). 32. Y. Z. Lu, Y. Y. Jiang, H. B. Wu, and W. Chen, J. Phys. Chem. C 117, 2926 (2013). 33. Z. Peng, C. Kisielowski, and A. T. Bell, Chem. Commun. 48, 1854 (2012). 34. M. K. Carpenter, T. E. Moylan, R. S. Kukreja, M. H. Atwan, and M. M. Tessema, J. Am. Chem. Soc. 134 (2012). 35. M. K. Carpenter, T. E. Moylan, R. S. Kukreja, M. H. Atwan, and M. M. Tessema, J. Am. Chem. Soc. 134, 8535 (2012). 36. C. Cui, L. Gan, H.-H. Li, S.-H. Yu, M. Heggen, and P. Strasser, Nano Lett. 12, 5885 (2012). 37. Y. Dai, X. L. Mu, Y. M. Tan, K. Q. Lin, Z. L. Yang, N. F. Zheng, and G. Fu, J. Am. Chem. Soc. 134, 7073 (2012). 38. Y. J. Kang, X. C. Ye, and C. B. Murray, Angew. Chem. Int. Ed. 49, 6156 (2010). 39. X. Q. Huang, S. H. Tang, X. L. Mu, Y. Dai, G. X. Chen, Z. Y. Zhou, F. X. Ruan, Z. L. Yang, and N. F. Zheng, Nat. Nanotechnol. 6, 28 (2011). 40. C. Wang, M. Chi, G. Wang, D. Van der Vliet, D. Li, K. More, H.-H. Wang, J. A. Schlueter, N. M. Markovic, and V. R. Stamenkovic, Adv. Funct. Mater. 21, 147 (2011). 41. A. A. Dameron, T. S. Olson, S. T. Christensen, J. E. Leisch, K. E. Hurst, S. Pylypenko, J. B. Bult, D. S. Ginley, R. P. O’Hayre, H. N. Dinh, and T. Gennett, ACS Catal. 1, 1307 (2011). 42. W. Vogel, P. Britz, H. Bonnemann, J. Rothe, and J. Hormes, J. Phys. Chem. B 101, 11029 (1997). Sci. Adv. Mater., 5, 1–9, 2013

Lu et al.


43. A. Capon and R. Parsons, J. Electroanal. Chem. 45, 205 (1973). 44. W. Sugimoto, K. Aoyama, T. Kawaguchi, Y. Murakami, and Y. Takasu, J. Electroanal. Chem. 576, 215 (2005). 45. Y.-C. Liu, X.-P. Qiu, W.-T. Zhu, and G.-S. Wu, J. Power Sources 114, 10 (2003). 46. Y. Lu and W. Chen, J. Phys. Chem. C 114, 21190 (2010). 47. Y. Lu and W. Chen, ACS Catal. 2, 84 (2012).

48. W. Chen and S. W. Chen, J. Mater. Chem. 21, 9169 (2011). 49. A. Ramstad, F. Strisland, T. Ramsvik, and A. Borg, Surf. Sci. 458, 135 (2000). 50. T. E. Shubina and M. T. M. Koper, Electrochim. Acta 47, 3621 (2002). 51. K.-S. Lee, Y.-H. Cho, T.-Y. Jeon, S. J. Yoo, H.-Y. Park, J. H. Jang, and Y.-E. Sung, ACS Catal. 2, 739 (2012).

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