One-pot synthesis of Pt-Pd bimetallic nanodendrites with enhanced

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One-pot synthesis of Pt-Pd bimetallic nanodendrites with enhanced electrocatalytic activity for oxygen reduction reaction Rifeng Wu, Yanjie Li, Wenhao Gong, and Pei Kang Shen ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Sustainable Chemistry & Engineering

One-pot synthesis of Pt-Pd bimetallic nanodendrites with enhanced electrocatalytic activity for oxygen reduction reaction Rifeng Wu, † Yanjie Li, † Wenhao Gong, † and Pei Kang Shen †, * †Collaborative

Innovation Center of Sustainable Energy Materials, Guangxi Key

Laboratory of Electrochemical Energy Materials, State Key Laboratory of Processing for Non-ferrous Metal and Featured Materials, Guangxi University, Nanning, 530004, PR China. E-mail for R.W: [email protected] E-mail for Y.L: [email protected] E-mail for W.G: [email protected] *

E-mail for P.K.S: [email protected]

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ABSTRACT: One-pot synthesis Pt-Pd bimetallic nanostructure has become a promising way to get superior oxygen reduction reaction (ORR) performance and costeffective electrocatalyst for proton exchange membrane fuel cells (PEMFCs). In this work, we report a facile one-pot method by tuning the feed ratio of Pt and Pd (Pt3Pd1, Pt2Pd1, and Pt1Pd1) in ethylene glycol solution, at the effect of iodide ions and poly (vinylpyrrolidone) (PVP) to synthesize three different Pt-Pd bimetallic nanodendrites structure. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) revealed that the nanodendrites have a Pt-rich surface structure and numerous high-index facet at the branches surface, thus provided a great deal of catalytic sites. Carbon supported Pt-Pd nanodendrites catalysts was investigated for oxygen reduction reaction (ORR) performance and accelerated durability test (ADT). The Pt1Pd1/C catalyst shows the best activity and durability, and with a mass activity of 1.164 A mgPt-1 and a specific activity of 1.33 mA cm-2, far enhanced compared with commercial TKK-Pt/C catalysts (0.15 A mgPt-1 for mass activity, and 0.25 mA cm-2 for a specific activity). It also exhibits remarkable durability with almost negligible decay in performance after 10 000 cycles. These results provide us an attractive strategy for designing catalysts with a simple route, lower cost and remarkable catalytic activity and durability. KEYWORDS: One-pot method; Pt-Pd bimetallic catalysts; nanodendrites; Pt-rich surface; oxygen reduction reaction

2


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INTRODUCTION Environmental pollution and energy exhaustion seriously restrict the sustainable development of society, it is urgent to developing pollution-free and renewable energy. Fuel cells, a popular category of highly efficient energy conversion devices, not only reduce the use of fossil energy but also eliminate the emission of polluted gases 1. However, there are great scientific challenges to develop efficient catalysts for clean and renewable energy technologies 2. Especially, a high over-potential and complicated kinetic condition in the oxygen reduction reaction (ORR) severely blocked the energy conversion of fuel cells. Therefore, it is urgent to study the efficient cathode ORR catalyst. The catalytic activity of Pt-based noble metal nanostructure catalysts depends on its shape, particle size, and exposed crystal planes

3-4

. It have done great efforts

toward the synthesis of Pt-based nanostructures with specific morphology

5-10

, which

exposed abundant high-index facets and active sites on the surface, leading to a superior catalytic activity 11. Pt-M bimetallic catalysts can not only reduce the cost but also improves the catalytic activity and stability because of the synergistic effect between bimetals, when compared with monometallic Pt/C catalysts

12

. Among these strategies, many efforts

have been done to fabricate Pt-M (M=Ni, Co, Cu or Fe) alloy catalysts with transition metals 13-19, and they all show improved ORR activity. However, these transition metals have a low redox potential and dissolution during PEMFC operation, leading to a significant challenge for large scale application

20-21

. Therefore, to overcome this

problem, Pt and other precious metals form alloy catalysts, especially with Pd, have 3



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attracted tremendous attentions 22-26. Besides the improvement in catalytic activity, PtPd bimetallic catalysts also show a long-term stability 27. In order to make better use of Pt atoms, it should design the PtPd bimetallic ORR catalyst to have a specific morphology. We have successfully synthesis the nanodendrites with a Pt-rich surface, and large specific surface area, which provided abundant active sites on its surface, and more conducive to mass transfer 28. Yusuke Yamauchi and co-workers synthesized PtPd bimetallic nanodendrites by reduce Na2PtBr6 and K2PdBr4 with ascorbic acid (AA) and Pluronic F127 and etched the Pd core by concentrated nitric acid for five days to form a bimetallic nanocages

29

. Xia and co-workers adopt seed growth method: first,

Na2PdCl4 was reduced by L-ascorbic acid (AA) to form Pd seeds, and then K2PtCl4 was also reduced with AA to get Pt-Pd nanodendrites on the Pd seeds 30. We choose cheaper H2PtCl6·6H2O and Na2PdCl4 as precursors in a truly simple and quite different from the traditional synthetic route

28-33

and got superior oxygen

reduction reaction (ORR) activity and durability of Pt-Pd bimetallic nanodendrites. In our recipe, we adopt ethylene glycol as a solvent, PVP served as a surfactant and stabilizer and KI as a reducing agent 34. We modulate the reduction potentials between Pt and Pd by using halide ions, leading to Pd ions can be reduced before Pt ions 35. In our recipe, Pd precursors were reduced into Pd crystal preferentially, followed by galvanic replacement between the Pd crystal and Pt ion triggered by the iodine ion, leading to Pt atoms deposited on the Pd framework, and eventually achieved a Pt-rich surfaces structure

35-38

. The Pt-rich surface structure of nanodendrites is full of the

various high-index facet, not only provides abundant active sites but further increased 4


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the utilization of noble metal, thus achieving the high mass activity of the Pt-Pd catalyst. Owing to every factor affecting the progress of the reaction can’t be fully known for now, thus all these studies were obtained through trial and error to ensuring the accuracy of the result. As expected, carbon-supported Pt-Pd bimetallic catalysts have gradually become an efficient catalyst for cathodic ORR of PEMFC, and compared to commercial Pt/C, have an enhanced catalytic activity and better durability 39.

EXPERIMENTAL SECTION Chemical and Materials Chloroplatinic acid hexahydrate (H2PtCl6 ·6H2O, ≥37.0%), palladium chloride (PdCl2, ≥99%) were purchased from Changshu Changhong precious metal Co. Ltd (Jiangsu, China). Na2PdCl4 (19.8 mg/ml) solution is formed by dissolving PdCl2 in NaCl solution. Ethylene glycol (EG, AR, 98%) was purchased from Aladdin industrial corporation. Poly (vinylpyrrolidone) (PVP; AR, MW=58000), perchloric acid (HClO4, GR, 70.0-72%) and potassium iodide (KI, AR, 99.0%) were purchased from Macklin Regent. Acetone and ethanol were purchased from Tianjin Damao Chemical Reagent Factory. The commercial carbon black Vulcan XC-72R was supplied with Cabot Inc. The water used in all experiments was ultrapure (18.2 MΩ). All reagents were used as received without any further purification. Synthesis of Pt-Pd bimetallic nanodendrites In a standard synthesis, 160 mg PVP, 46 mg KI, and 10.0 ml ethylene glycol (EG) were mixed in a Teflon lined stainless-steel autoclave at room temperature. Subsequently, (0.85 mM, 1.25 mM, and 2.5 mM) Na2PdCl4, 2.5 mM H2PtCl6·6H2O was 5



added dropwise to the mixture with a micropipet, and the mixture was kept under stirring for another 10 min. The mixed solution was then heated in an oil bath at 160 ℃ with magnetic stirring for 1 h before it cooling down to room temperature. The reactant was centrifuged at 8000 rpm and washed four times with ethanol and acetone mixture to remove residual PVP. After supported with commercial Vulcan-72 carbon, we obtain Pt-Pd nanodendrites catalysts. Details are described in Supporting Information (SI). Electrochemical measurements Glassy carbon (GC) disk electrode (0.196 cm-2 in a geometric area) served as the substrate for the support, and it was polished using aqueous alumina suspension prior to use. To prepare the working electrode, weigh the dried catalyst 2 mg dispersed in 2 ml 0. 5 wt. % Nafion and ethanol solution followed by ultrasonication for 35 min, and then 10 ul catalytic suspension was pipetted using a micropipette onto the pre-cleaned glassy carbon rotating disk electrode (RDE) and spin dry at 500 rpm to form a uniform thin film that was further characterized in an electrochemical cell. The Pt loading for Pt1Pd1/C, Pt2Pd1/C and Pt3Pd1/C nanocrystal electrocatalysts and the commercial Pt/C catalyst (TKK, 46.7wt.% Pt, Japan) was determined at 8.3, 9.5, 12.6 and 24.3 μg cm−2, respectively, it is based on the geometric electrode area of 0.196 cm 2 and determined by ICP-MS measurements. The electrochemical measurements were conducted in a three-compartment electrochemical cell with a Pine rotational disk electrode setup connected with a bipotentiostat (AFCBP1E, Pine Instrument Co., USA), GC covered with catalyst acts as a working electrode, Pt mesh as a counter electrode, and a reversible hydrogen electrode (RHE) as a reference electrode. The CV curve was 6


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recorded at 25 ℃ in an N2-saturated 0.1 M HClO4 solution in the potential range of 0.05−1.1 V RHE at a scanning rate of 50 mV s−1. ORR measurements were conducted in O2-saturated 0.1 M HClO4 solution, and the ORR polarization curves were collected at 1600 rpm with a scan rate of 10 mV s−1. The accelerated durability test (ADTs) were performed at 25℃ in O2-saturated 0.1 M HClO4 solutions by applying cyclic potential sweeps between 0.6 and 1.1 V versus RHE at a sweep rate of 100 m V s−1 for 10 000 cycles. The ORR data were corrected by ohmic iR drops compensation.

RESULTS AND DISCUSSION A facile stirring-assisted solvothermal method directly prepared the Pt-Pd anodendrites. We conduct a series of comparative experiments via tuning feed ratio of precursor and concentration of PVP and KI to further explore the morphology evolution and formation of Pt-Pd nanodendrites. When 0.85 mM Na2PdCl4 and 2.5 mM H2PtCl6·6H2O (Pt: Pd=3:1, denoted as Pt3Pd1) was added in our recipe and adopting the standard synthesis procedure uniform Pt3Pd1 spherical porous nanodendrites with an average particle size of 44.9±2.6 nm was got (Figure 1a and Figure S1a). Figure 1a, b shows a low-magnification bright-field and high angle annular dark field scanning transmission electron microscope (HAADF-STEM) images, which clearly showed the as-prepared PtPd nanostructure had a well-dispersed and abundant nano-porous structure. The EDS elemental mapping and line scanning profile showing Pt-rich exteriors and Pd-rich interiors of dendritic structure (Figure 1c, d). As shown in Figure 1e-h, the individual Pt3Pd1 nanodendrites and the local HR-TEM image shows the Pt3Pd1 nanodendrites with numerous under-coordinated atomic steps at the branches 7



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surface of the nanodendrites. Both the HR-TEM image and the corresponding fast Fourier transform (FFT) pattern show that the nanocrystal was a piece of single crystal. It can assign the lattice of 0.23 nm and 0.19 nm to the {111} and {200} planes of facecentered cubic (fcc) Pt, respectively 30. However, only those Pt atoms on the surface of a catalyst were directly involved in ORR, this inherent structural feature of the Pt−Pd nanodendrites is highly valuable for high electrocatalytic performance

40-41

. When the

Pt-Pd ratio was changed from 3:1 to 2:1 (Pt: Pd=2:1, denoted as Pt2Pd1), we got a nanodendrites structure with smaller particle size and coarser branch. From the low magnification TEM images and EDS elemental mapping and line scanning profile, the Pt2Pd1 nanodendrites have a uniform distribution with an average particle size of 34.3±2.1 nm (Figure 2a-d and Figure S2a). The individual Pt2Pd1 nanodendrites and the local HR-TEM image shows a great number of terrace and step atoms are exposed on the surface of the dendrites structure (Figure 2e-h). As shown in the pictures, the stepped surface is enclosed by high-index facets with (111) terraces and (110) steps, such as {221} and {331}, which usually act as highly active sites 42. In the same recipe, when the feed ratio of Pt/Pd are 1/1 (denoted as Pt1Pd1), highly branched and faceted Pt-Pd nanocrystal comprising numerous interconnected arms in a quasi-cubic shape were produced in high yield (Figure 3a-d). The Pt1Pd1 nanodendrites has an average particle size of 25.1±1.8 nm (Figure S3a). Figure 3e-f shows the individual Pt1Pd1 nanodendrites and the local HR-TEM image, we can also observe abundant high-index facet ({221} and {331}) are exposed on the dendrites surface. As shown in Figure 3h, it shows the cross section of the atomic model of {221} and {331} high-index facets. 8


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From Figure S1b, S2b and S3b EDS spectrum, we can find Pt3Pd1, Pt2Pd1 and Pt1Pd1 nanodendrites have a Pt/Pd atom ratio of 78.2/21.8, 64.3/35.7 and 58.6/41.4, respectively. It is slightly higher than the feed ratio of Pt/Pd, which also confirmed the Pt-rich surface structure. The wide-angle X-ray diffraction (XRD) patterns of all catalysts are for randomly oriented fcc metallic crystals are presented in Figure 4a, it is not clear to distinguishing Pt and Pd in the XRD pattern, owing to the Pt/Pd lattice match ratio is 99.23%. The positions of diffraction peak can be clearly indexed to {111}, {200}, {220}, {311} and {222} diffractions of face-centered-cubic (fcc) structure type (Pt PDF#87-0647 and Pd PDF#87-0645). The XRD patterns of the three kinds of Pt-Pd nanodendrites all shifted to higher 2θ values compared to the commercial TKK-Pt/C catalyst. It confirmed the formation of Pt-Pd alloys because of incorporating Pd atoms into the Pt fcc lattice, which could lower their d-band centers43-45. XPS as a surface analysis technology, it can determine the surface chemical state and elemental composition of the nanostructure. Figure 4b, c, d and Figure S4 show XPS spectra of three kinds of nanodendrites and TKK-Pt/C. The Pt 4f spectrum contains two peaks of 4f 7/2 and 4f 5/2, and each one can deconvoluted into two peaks. According to Pt 1Pd1 nanodendeites, the peaks at 71.55 (4f7/2) and 74.90 eV (4f5/2) assign to Pt0, and those at 72.78 and 76.20 eV assign to Pt2+. The XPS results of different catalysts are summarized in the Table 1. The Pt 4f in Pt/C with the binding energies (BEs) of 71.35 and 74.70 eV, which correspond to Pt 4f 7/2 and Pt 4f 5/2 of Pt0. Clearly, the BEs of Pt0 have a positive shift of three kinds nanodebdrites catalysts when compared with Pt/C. In addition, the Pt2+ 4f7/2 has a shift about 0.6 eV as compared to Pt0 4f7/2 of all catalysts. The positive 9



shift of Pt0 indicate lowering of the d-band centers, due to the lattice strain and charge transfer occurred between Pt and Pd atoms 46-47. According to the deconvolution peak area of XPS, Pt and Pd mainly exist in the form of zero-valent metal. In order to get closer to understanding the formation mechanism of nanodendrites, we set up a series of comparative experiments with the Pt/Pd ratio of 3/1. First, we studied the role of KI in the synthesis. In the present system, the presence of iodine ions was found to be critical to the formation of PtPd nanodendrites. When the synthesis was performed in the absence of I- ions, and keep other conditions unchanged, we got PtPd nanoparticles consist of nanostructure polyhedral have a uniform distribution with an average particle size of 4.2±1.8 nm (Figure 5a). To further explore the effects of iodide ions, we experimented with transparent glass vials that can clearly see the changes of solution color. There is no color change was observed without the addition of I- ions, however, when 46 mg KI was added, the color immediately become much darker (Figure S5). That's because Pt4+ ions and Pd2+ ions form [PtI6]2- and [PdI4]2- ligands with I- ions, and they are both colored much darker than [PtCl6]2- and [PdCl4] 35, 48. The presence of iodide ions also significantly alters the dominating forms of the metal precursors and reduce the reduction kinetics of metal precursors, thus change the reduction sequence of Pt and Pd

35

. When the amount of KI added with 23 mg, each

nanoparticle is surrounded by many finer nanodendrites (Figure 5b). It may because the low concentration of I- ions can’t form enough [PtI6]2- and [PdI4]2- ligands so that Pt and Pd cannot be completely nucleation and growth. When the amount of KI was increased to 92 mg, we obtained sphere nanodendrites with slightly larger particle size 10


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and thicker branch (Figure 5c). However, the morphology will change into quasi-cubic shape was composed of many interconnected thick branches when added 138 mg KI (Figure 5d). We also explored the effects of other halides, when the I- ions were substituted with other equal concentration halides, such as Cl- and Br-, there was no PtPd nanodendrites observed, and we also obtained nanoparticles with a uniform distribution and average particle size of 5.4±1.9 nm, 4.6±0.6 nm, respectively (Figure 5e, f). These results indicate that the presence and an appropriate amount of I- is vital to the shape and size of Pt-Pd nanodendrites. Thus, introducing KI is critical to the formation of Pt-Pd nanodendrites in the present synthesis system. We also found the role of PVP in our recipe to be critical to the formation of nanodendrites. PVP used here served as a surfactant and stabilizer, when the synthesis was conducted in the absence of PVP, aggregated and irregular nanostructure was obtained (Figure S6a). When 160 mg of PVP was added, the three kinds of Pt-Pd ratio nanostructure showed a very uniform distribution. These results confirmed that the introduction of PVP plays a significant role in the formation of Pt-Pd nanodendrites with a homogeneous distribution and particles size. While there is a slight change of morphology and size distribution of Pt-Pd nanodendrites with increasing the amounts of PVP from 160 mg to 240 mg (Figure S6b). By further increasing the amount of PVP to 480 mg, the sphere nanodendrites changed into blocky at the outside surface (Figure S6c). In addition, we can see from the Figure S6c, the picture looks blurry; it is because a high concentration of PVP was hard to clean thoroughly by conventional methods, which was harmful to the activity of a catalyst. Hence, it’s easy to find, appropriate 11



amounts of PVP is essential to disperse the nanoparticles, it is in good agreement with previous works

49-51

. We also investigate the influence of reaction time, but we find

there is a slight change in the morphology when the reaction time is 10 min, 30 min and 60 min (Figure S7a-c), which indicate a fast reaction rate. As we further prolong the reaction time to 6h, the nanodendrites with a coarser branch will obtain (Figure S7d). In conclusion, the presence of iodide ions can modify the reduction potentials between Pt and Pd, make the Pd ions can be reduced before Pt ions. Owing to the effect of surfactant and the fast growth rate, the adsorption rate of adsorption atoms does not depend on the crystal surface where the adsorption takes place, and the diffusion restricted aggregation or oriented attachment mechanism leads to growth occurs toward no preferred crystallization

52-53

. This results in the growth of randomly oriented

branches in any radial direction to produce a highly branched nanodendrites structure 54-55

. The electrocatalytic properties of Pt-Pd bimetallic catalysts for ORR were

evaluated and compared to commercial Pt/C (46.7% Pt, TKK Japan) catalyst using the commonly used test protocol. The performance of Pt-based electrocatalyst is highly associated with the amounts of catalytically active sites exposed on the surface, the high electrochemically active surface area (ECSA) is significant to improving the utilization of precious metals. Figure 6a shows cyclic voltammograms (CV) of the Pt3Pd1, Pt2Pd1, and Pt1Pd1 supported on carbon and the commercial TKK-Pt/C catalyst, which recorded at 25℃ in N2-purged 0.1 M HClO4 solutions with a sweep rate of 50 mV/s. The ECSA was calculated using CV curves by measuring the charge collected in the H(upd) 12


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adsorption region and then normalized to the loading amount of metal in order to obtain the specific ECSA 56-57. The PtPd nanodendrites catalysts shows a high specific ECSA, and in the following sequence of Pt3Pd1/C (46.2 m2 g -1 Pt) < Pt2Pd1/C (49.3 m2 g -1 Pt) < Pt1Pd1/C (53.8 m2 g

-1

Pt),

but they are all slightly smaller than Pt/C (56.2 m2 g

-1

Pt)

(Figure S8 and Table 2). It may associate the high ECSA value of Pt-Pd nanodendrites with their unique structure of the dendritic surface, which will provide abundant active sites on the inside and outside. The ORR measurements were performed at 25℃ in O2saturated 0.1 M HClO4 solutions using a glassy carbon rotating disk electrode (RDE) with a sweep rate of 10 mV/s and a rotation rate of 1600 rpm, and the ORR polarization curves for different electrocatalysts are shown in Figure 6b. The half-wave potentials of these ORR catalysts in the order of commercial TKK-Pt/C (863 mV) < Pt3Pd1/C (884 mV) < Pt2Pd1/C (899 mV) < Pt1Pd1/C (916 mV) versus a reversible hydrogen electrode (RHE). Beside the enhanced activity, the Pt-Pd nanodendrites catalysts also exhibited remarkable durability in an accelerated durability test (ADT) between 0.6 and 1.1 V for 10 000 cycles (Figure 7a, b, c and d) (at sweep rates of 100 mV s−1 in O2-saturated 0.1 M HClO4 electrolyte). After 10 000 potential cycles, the half-wave potential of Pt1Pd1/C, Pt2Pd1/C, Pt3Pd1/C and Pt/C catalysts are reduced by only 13.5, 35.5, 27.0 and 52.6 mV, respectively. Figure 6c shows the ORR catalysts Tafel plots, all catalysts exhibit comparable Tafel slopes at a fixed potential, suggesting that they may the same reaction mechanism over the majority of the potential range 58. To achieve a better understanding of mass and surface effects, the kinetic currents of a polarization curve were calculated by following Koutecky−Levich equation and then normalized against the Pt mass and 13



ECSA to get the mass and specific activities, respectively (Figure 6d). The Pt loading for Pt1Pd1/C, Pt2Pd1/C and Pt3Pd1/C electrocatalysts and the commercial Pt/C catalyst (TKK, 46.7wt.% Pt, Japan) are determined at 8.3, 9.5, 12.6 and 24.3 μg cm−2, respectively. In addition, the Pt+Pd loading for Pt1Pd1/C, Pt2Pd1/C and Pt3Pd1/C electrocatalysts are 11.4, 12.3 and 14.4 μg cm−2, respectively. It is based on the geometric electrode area of 0.196 cm2 and determined by ICP-MS measurements. Especially, the Pt1Pd1/C nanodendrite exhibited the maximum mass activity of 1.16 A mg-1Pt, and specific activity of 1.33 mA cm-2, which are 7.76- and 5.32-fold enhancements compared with the commercial Pt/C catalyst (0.15 A mg-1Pt and 0.25 mA cm-2), respectively. It may ascribe to the larger specific surface with more active sites on the Pt-rich surface than two others PtPd nanodendrites catalysts. The Pt2Pd1/C and Pt3Pd1/C catalysts present 0.79 A mg-1Pt and 0.68 A mg-1Pt of mass activity, and 0.93 mA cm-2 and 0.89 mA cm-2 of specific activity, respectively. The mass activity by Pt+Pd loading of three kinds of Pt-Pd nanodendrites catalysts are in the following sequence: Pt3Pd1/C (0.59 A mg-1Pt+Pd) < Pt2Pd1/C (0.61 A mg-1Pt+Pd) < Pt1Pd1/C (0.85 A mg-1Pt+Pd) (Figure 7e). The ORR performance of four kinds of catalysts are shows in the Table 2. All the Pt-Pd nanodendrites catalysts showed better mass and specific activity than commercial TKK Pt/C catalyst, and superior durability after ADT test (Figure 7f). The enhanced performance is ascribed to the strain arising from the mismatch in lattice constant between Pt and Pd and large specific surface with abundant active sites on the Pt-rich surface, and the high density of under-coordinated surface atoms at their branches

40-41, 59-61

. Moreover, the synergy between Pt and Pd is also essential to 14


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improve the activity of the catalyst.

CONCLUSIONS In summary, we have developed a very simple and cost-effective one-pot solvothermal method for the direct and high-yield synthesis of Pt-Pd bimetallic nanodendrites ORR catalysts with designed Pt and Pd feed ratios. It is a truly simple process and quite different from the traditional seed-mediated growth strategy with the two-step method. we got Pt3Pd1 porous sphere nanodendrites, Pt2Pd1 nanodendrites with coarser branches and Pt1Pd1 nanodendrites with quasi-cubic shape branches, respectively, by only regulated the feed ratio. Through the analysis of TEM, XRD and XPS, we have proved that Pt-Pd bimetallic alloys structure and nanodendrites have Ptrich surface and contain abundant high-index facets, which is essential to enhance the activity of the catalysts. From the electrochemical test, all of PtPd/C nanodendrites catalysts showed better mass and specific activity than commercial TKK Pt/C catalyst. In addition, the durability of PtPd/C nanodendrites catalysts is much better than commercial TKK-Pt/C catalyst. The route for synthesis Pt-Pd bimetal nanostructure with nanodendrites morphology and superior ORR performance provide us a simple and workable method for catalysts synthesis.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Physical characterizations, electrochemical characterization of the materials, and supplementary tables. 15



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ACKNOWLEDGEMENTS This work was supported by the Guangxi Science and Technology Project (AA17204083, AB16380030), National Basic Research Program of China (2015CB932304), the link project of the National Natural Science Foundation of China and Fujian Province (U1705252), the Natural Science Foundation of Guangdong Province (2015A030312007) and the Danish project of Initiative toward Non-precious Metal Polymer Fuel Cells (4106-000012B).

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Figure 1. (a) Bright field TEM image and (b) HAADF-STEM image of Pt3Pd1 nanodendrites. (c) HAADF-STEM image and EDX elements mapping and (d) linescanning profiles. (e) The individual Pt3Pd1 nanodendrites and (f), (g), (h) the corresponding local HR-TEM image. The inset in (f), (g), (h) shows the corresponding fast Fourier transition (FFT) pattern.

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Figure 2. (a) Bright field TEM image and (b) HAADF-STEM image of Pt2Pd1 nanodendrites. (c) HAADF-STEM image and EDX elements mapping and (d) linescanning profiles. (e) The individual Pt2Pd1 nanodendrites and (f), (g), (h) the corresponding local HR-TEM image. The inset in (f), (g), (h) shows the corresponding fast Fourier transition (FFT) pattern.

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Figure 3. (a) Bright field TEM image and (b) HAADF-STEM image of Pt1Pd1 nanodendrites. (c) HAADF-STEM image and EDX elements mapping and (d) linescanning profiles. (e) The individual Pt1Pd1 nanodendrites and (f), (g) the corresponding local HR-TEM image. (h) cross-section of the atomic model of {221} and {331} high-index facets. The inset in (f), (g) shows the corresponding fast Fourier transition (FFT) pattern.

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Figure 4. (a) XRD spectra of three kinds of Pt-Pd/C and commercial TKK-Pt/C catalysts obtained at different feed ratios, at a scan rate of 5°min-1. XPS spectra of (b) Pt 4f, (c) Pd 3d and (d) wide survey spectrum of three kinds of Pt-Pd nanodendrites catalyst.

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Figure 5. Typical TEM images of Pt-Pd nanodendrites prepared at Pt/Pd feed ratio of 3/1 by using the standard procedure, (a) in the absence of KI, (b) with 23 mg KI, (c) with 92 mg KI, (d) with 138 mg KI; replace KI with same molar (e) NaCl and (f) NaBr, respectively.

29



Figure 6. (a) Cyclic voltammograms recorded at room temperature in a N2-purged 0.1 M HClO4 solution with a sweep rate of 50 mV s−1; (b) ORR polarization curves recorded in an O2-saturated 0.1 M HClO4 solution with a sweep rate of 10 mV s−1 and a rotation rate of 1600 rpm; (c) Tafel plots of four kinds of catalysts, and (d) Histogram of specific and mass activities at 0.9 V versus RHE for four catalysts.

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Figure 7. ORR polarization curves of (a) Pt1Pd1/C, (b) Pt2Pd1/C, (c) Pt3Pd1/C and (d) Pt/C catalyst before and after 10 000 potential cycles, obtained between 0.6 and 1.1 V (vs RHE) at sweep rates of 100 mV s−1 in O2-saturated 0.1 M HClO4 electrolyte. (e) Histogram of metal (Pt+Pd) mass activities at 0.9 V versus RHE for Pt-Pd nanodendrites catalysts. (f) Histogram of mass activities at 0.9 V versus RHE for these Pt-Pd bimetallic catalysts after 10 000 cycles accelerated durability test.

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Table 1. Summary of XPS result of different catalysts Catalyst Pt/C Pt1Pd1/C Pt2Pd1/C Pt3Pd1/C

Pt0 4f 7/2 (eV) 71.35 71.55 71.41 71.44

Pt2+ 4f 7/2 (eV)

Pt0 4f 5/2 (eV)

Pt2+ 4f 5/2 (eV)

71.91 72.14 71.92 71.99

74.70 74.90 74.79 74.76

75.76 76.20 75.43 75.81

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Table 2. ORR performances of four kinds catalysts Catalyst Pt/C Pt3Pd1/C Pt2Pd1/C Pt1Pd1/C

ECSA (m2 g-1Pt) 56.2 46.2 49.3 53.8

E1/2 (mV)

MA 0.9 V (A mg-1Pt)

MA 0.9 V (Amg-1Pt+Pd)

SA 0.9 V (mA cm-2Pt)

863 884 899 916

0.15 0.68 0.79 1.16

/ 0.59 0.61 0.85

0.25 0.89 0.93 1.33

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For Table of Contents Use Only

Pt-Pd nanodendrites prepared with simple and cost-effective method by tune feed ratio exhibit excellent ORR performance, shows great sustainability for PEMFC development.

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One-pot synthesis of Pt-Pd bimetallic nanodendrites with enhanced

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