Shell Nanoplates Enable Efficient

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Intermetallic hcp-PtBi/fcc-Pt Core/Shell Nanoplates Enable Efficient Bifunctional Oxygen Reduction and Methanol Oxidation Electrocatalysis Yingnan Qin, Mingchuan Luo, Yingjun Sun, Chunji Li, Bolong Huang, Yong Yang, Yingjie Li, Lei Wang, and Shaojun Guo ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04406 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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ACS Catalysis

Intermetallic hcp-PtBi/fcc-Pt Core/Shell Nanoplates Enable Efficient Bifunctional Oxygen Reduction and Methanol Oxidation Electrocatalysis Yingnan Qina,b#, Mingchuan Luoa#, Yingjun Suna,b, Chunji Lia, Bolong Huangc*, Yong Yanga, Yingjie Lia, Lei Wangb and Shaojun Guoa,d,e* a

Department of Materials Science & Engineering, College of Engineering, Peking University,

Beijing, 100871, China. b

College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology,

Qingdao 266042, China. c

Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University,

Hung Hom, Kowloon, 999077, Hong Kong SAR. d

BIC-ESAT, College of Engineering, Peking University, Beijing, 100871, China.

e

Department of Energy and Resources Engineering, College of Engineering, Peking University,

Beijing, 100871, China. #

These authors contributed equally.

Corresponding author: [email protected]; [email protected]

Abstract Two dimensional (2D), ordered intermetallic and core/shell architectures are highly desirable structural features for promoting electrocatalysis on Pt-based nanocrystals in terms of activity, durability and cost. However, it is an extreme challenge to achieve all these features in single catalytic nanostructure currently. Herein, we report a new class of 2D nanoplate catalyst composed of intermetallic hcp-PtBi core and ultrathin fcc-Pt shell synthesized by a facile one-pot wet-chemical approach. The unique structural features of PtBi/Pt core/shell nanoplates make them exhibit the highest oxygen reduction reaction (ORR) activity in all the reported PtBi-based catalytic system, and 5 times more active than commercial Pt/C catalyst for ORR. The combination of cyclic voltammograms, x-ray photoelectron spectroscopy and density functional theory calculations reveals that an optimal oxygen adsorption energy and efficient reduction on both edge surface and interface regions between Pt-shell and PtBi-core from hcp-PtBi/fcc-Pt core/shell nanoplates relative to that on 1 / 26

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commercial Pt, deriving from the Bi-p empty band suppression at the core/shell interface, is the key in greatly boosting ORR activity of PtBi nanocatalysts system. The PtBi-Pt interface performs a relative lower overpotential compared to the edge surface because of excellent further reduction from OH to the H2O. Thanks to the intermetallic phase and core/shell architecture, hcp-PtBi/fcc-Pt core/shell nanoplates show little loss in electrochemically active surface area and ORR activity during the accelerated durability test. They also show enhanced catalytic performance for the electro-oxidation of liquid fuels in both acid and alkaline electrolytes. This work sheds light on the rational design of new 2D core/shell nanostructured catalysts for enhancing fuel cells electrocatalysis.

Keywords: oxygen reduction reaction; PtBi; nanoplates; intermetallic; core/shell

1. Introduction Polymer membrane electrolyte fuel cell (PEMFC) is an attractive energy conversion technology due to its high efficiency, high energy density and low environmental impact, which can potentially reduce our reliance on fossil fuels.1-5 However, the practical adoption of this technology is greatly limited by the high cost and low efficiency of the state-of-the-art platinum-based electrocatalysts that are used to compensate the high overpotential mainly from the sluggish oxygen reduction reaction (ORR) at the cathode.6 Although a vast amount of efforts have been devoted to addressing this challenge,7-10 a large gap regarding the electrocatalysis remains currently. To bridge this gap, a more efficient Pt-based electrocatalyst is highly desirable, and is expected to possess the following advanced properties: high Pt utilization, enhanced activity and satisfied durability.11-12 In the past decade, dramatic enhancement in electrocatalytic performance has been realized by tuning the morphology and architecture of PtM (M represents other transition metals) alloyed nanocrystals.13-17 On one hand, the morphology of a given nanocrystal directly impacts the exposed facets, mass transport and the interaction with carbon support, and thus governs the catalytic performance.17 Compared with the conventional nanoparticulate catalyst, anisotropic structures are more interesting for electrocatalysis.18-19 In particular, 2 dimensional (2D) nanosheets or nanoplates have drawn extensive attention due to their high ratio of high-coordination surface atoms that are 2 / 26


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ACS Catalysis

beneficial to both the catalytic activity and long-term stability.20 On the other hand, rationally arranging the atomic position of Pt and M enables further promotion in catalytic performance.21-22 For example, constructing core/shell structure with a pure Pt shell and a M or PtM core has achieved good electrocatalytic activity for ORR in the past decade normally due to the ligand and/or strain effects.23-25 Meanwhile, the presence of Pt shell can protect the inner M from dissolution during the electrochemical operation, thus contributing to the long-term stability.26 Previous studies also show the stability can be further promoted by constructing an intermetallic PtM core, in which the mobility of M is greatly reduced.27-31 Together, it is highly desired to develop intermetallic Pt-based nanocatalysts with a 2D morphology and a core/shell architecture. The well-established studies on PtBi alloy catalysts mainly focused on applications of methanol-tolerance or methanol/formic acid oxidation because the addition of Bi enabled high tolerance to CO poisoning.32-35 Nevertheless, no report has shown significantly improved ORR activity on PtBi alloy catalysts over benchmark Pt/C, probably due to the lack of deliberate control/tuning in composition, morphology and architecture. Herein, we report the direct synthesis of a novel 2D nanoplate with a core/shell structure composed of an intermetallic hcp-PtBi core and an ultrathin fcc-Pt shell for fuel cells related electrocatalysis. The PtBi nanoplates with a thickness of around 4.5 nm were obtained from a facile wet-chemical approach. Strikingly, as-prepared PtBi nanoplates showed the highest ORR activity in all the reported PtBi-based catalyst system, which is over 5 times higher than the commercial Pt/C catalyst. DFT calculations suggest the reason for ORR activity enhancement of hcp-PtBi/fcc-Pt core/shell nanoplates is caused by that the presence of inner Bi-p empty band reduces the over-binding of Pt-O bond at the interface between Pt-shell and PtBi-core from the hcp-PtBi/fcc-Pt core/shell nanoplates. Intermetallic phase and core/shell architecture also enable the PtBi nanoplates exhibit high durability. Furthermore, the PtBi/Pt core/shell nanoplates exhibit much enhanced catalytic performance towards the methanol oxidation reaction (MOR) relative to commercial Pt/C in both acid and alkaline electrolytes. This work emphasizes the importance of rational design and construction of promising electrocatalytic nanostructures for clean and renewable energy innovations.

2. Experimental section 3 / 26


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2.1. Synthesis of PtBi nanoplates In a typical synthesis of PtBi nanoplates, Pt(acac)2, (10 mg), Bi(Ac)3, (9.6 mg), L-ascorbic acid (35.6 mg), NH4Br, (50 mg), and a mixture of 1-octadecene and oleylamine (5 mL, v:v = 1:1) were added into a 25 mL flask. Afterwards, the flask was capped and sonicated for 1 h to obtain a clear homogeneous solution. Then, the flask was heated to 160℃ in 30 min with a heating mantle, and kept at 160 ℃ for 5 h. After cooling to room temperature, the colloidal products were collected by centrifugation, washed with cyclohexane for three times and dispersed in cyclohexane for further use. 2.2. Characterization The TEM samples were prepared by dropping cyclohexane dispersed product onto carbon-coated copper transmission electron microscopy (TEM) grids using pipettes and dried under ambient condition. Low-magnification TEM was conducted on a FEI Tecnai-G2 T20. High-magnification transmission electron microscope (HRTEM) was conducted on a FEI Tecnai-G2 F30 at an accelerating voltage of 300 KV. XRD was conducted on an X’Pert-Pro X-ray powder diffractometer equipped with a Cu radiation source (λ＝0.15406 nm). The valence state of Pt and Bi element was conducted by an X-ray photoelectron spectroscopy (XPS) (AXIS supra/ultra Kratos Analytical Ltd.). The elemental composition and concentration of PtBi/C catalyst were determined by the inductively coupled plasma mass spectrometer (Agilent SureCycler 8800 Gradient Cycler). 2.3. Electrochemical measurements Before electrochemical tests, the as-prepared PtBi nanoplates were deposited onto carbon black (EC-300) to obtain PtBi nanoplates/C catalyst. The catalyst powder was then dispersed and sonicated in a mixture containing isopropanol, water and Nafion solution (v: v: v = 1: 1: 0.016) to form a homogeneous catalyst ink, which was then quantitatively loaded onto a rotating disk glassy carbon electrode (RDE, diameter: 5 mm, area: 0.196 cm2) to prepare the thin film work electrode. The Pt loading for each catalyst was around 10 µg/cm2geo. All electrochemical tests were conducted in a three-electrode system, with the catalyst coated RDE as the working electrode, Ag/AgCl electrode as the reference electrode and Pt foil as the counter electrode, respectively. Cyclic voltammograms (CVs) were conducted in N2-saturated 0.1 M HClO4 solution at a rate of 50 mV/s. The ORR polarization curves were recorded in O2-saturated 0.1 M HClO4 at a scan rate and a rotation rate of 20 mV/s and 1600 rpm, respectively. The current densities in the ORR polarization curve were normalized in reference to the geometric area of the glassy carbon RDE (0.196 cm2). The stability tests were performed at O2-saturated 0.1 M HClO4 solution at room temperature by applying the cyclic potential sweep between 0.6 and 1.0 V versus RHE at a sweep rate of 100 mV/s for 5000 4 / 26


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cycles.

3. Results and Discussion The PtBi/Pt core/shell nanoplates were directly synthesized by heating a mixture of platinum (II) acetylacetone (Pt(acac)2), bismuth (III) acetate (BiAc3), ammonium bromide (NH4Br), ascorbic acid, oleylamine and 1-octadecene in a flask at a temperature of 160 °C for 5 h. The resultant black colloid product was collected by centrifugation and washed with the mixture of cyclohexane/ethanol for several times before being dispersed in cyclohexane for further use. Powder X-ray diffraction (PXRD) was used to characterize the crystalline phase of as-obtained PtBi/Pt core/shell nanoplates. As shown in Figure 1a, the diffraction peaks at 29°, 41° and 42° can be indexed to the (101), (102), (110) planes of intermetallic hcp-PtBi, indicating a NiAs-type structure (P63/mmc). The morphology and composition of the as-prepared PtBi/Pt core/shell nanoplates were characterized by the means of transmission electron microscope (TEM) (Figure 1b,c), high-angle annular dark-field scanning TEM (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDX). TEM image shows the majority of products are 2D nanoplates with either hexagonal or triangle shape. The EDX result reveals the Pt/Bi ratio of intermetallic PtBi/Pt core/shell nanoplate is around 66/34 (Figure 1d), in accordance with the result from Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The average thickness of PtBi/Pt core/shell nanoplates is 4.6 nm, determined from the statistic of 20 stacks that lies vertically on the TEM grids (Figure 1e,f and Figure S1). STEM-EDX elemental mapping analysis demonstrates the homogeneous distribution of both Pt and Bi throughout the nanoplate (Figure 1g), further confirming the intermetallic structure. X-ray photoelectron spectroscopy (XPS) confirms the presence of both Pt and Bi in the obtained nanoplate (Figure 1h and Figure S2). The majority of surface Pt is in metallic state as observed from the Pt 4f XPS spectra, while the majority of Bi takes the oxidation state. Furthermore, the Pt 4f peak of PtBi/Pt core/shell nanoplates shifts to the high binding energy by around 0.2 eV, indicating the electron transfer between Pt and Bi.36 High resolution TEM (HRTEM) image further reveals the crystalline nature of the product. As shown in Figure 2a,b and Figure S3, a grain boundary can be readily identified along the edge, confirming the core/shell structure of as-obtained PtBi nanoplates. The thickness of the shell is around 1.0 nm, corresponding to nearly 4 atomic layers. The lattice spacing of the inner region is around 0.35 nm, being close to that of the (101) plane of hcp-PtBi, and in agreement with the XRD results.[13a] However, the interplanar spacing of edge region is determined to be 0.22 nm, suggesting 5 / 26


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the typical (111) plane of fcc-Pt in the shell. The core/shell structure was further verified by EDX spectroscopy line-scan profiles of single PtBi nanoplate (Figure 2c,d). Based on the above observation, we conclude the as-obtained nanoplate is composed of an intermetallic hcp-PtBi core and a fcc-Pt shell. To shed light on the growth mechanism for 2D PtBi/Pt core/shell nanoplates, we traced the morphological evolution by characterizing the product collected at different stages. Figure 3a shows the TEM image of products collected at the moment when the reaction solution just turned black, representing a typical phenomenon of burst nucleation stage of nanocrystals.37 The majority of products were nanoclusters with an average size of 2.5 nm, which served as the nuclei for further growth. After the temperature reached 160 °C for a period of 5 min, a mixture of ill-defined nanoplates and nanoparticles were obtained. During this period, the average size of product rapidly increased to 9.5 nm (Figure 3b). For the next 5 h, the nanoparticles gradually diminished, giving rise to highly pure and uniform nanoplates with a hexagonal shape and a lateral size of 29 nm (Figure 3c). However, if the reaction time was prolonged to 10 h, the lateral size of products further increased to 35.3 nm with a much wider size distribution relative to that of products at 5 h, even though the 2D morphology could be well maintained (Figure 3d). This phenomenon is probably due to the Ostwald ripening, where the smaller nanocrystals gradually dissolved into the solution and then re-deposited onto the surface of larger nanocrystals to lower the surface free energy.38 The overall structural evolution of PtBi/Pt core/shell nanoplates is illustrated in Figure 3e. The wet-chemical synthesis of anisotropic nanocrystal with high aspect ratio is very challenging due to the inherently thermodynamic instability of anisotropic morphologies, such as 1D nanowires or nanotubes, 2D nanoplates or nanosheets.39 It is well established that the resultant morphology of nanocrystal is governed by the kinetics of nucleation and growth, which experimentally depends on the choice and concentration of reducing agents, the solvent, the capping agents and other structure-directing agents.40 To investigate the key experimental parameters for the formation of 2D PtBi/Pt core/shell nanoplates, a series of control experiments have been carried out. We found the bromide ions were essential to the yield of uniform nanoplates, because only irregular products could be obtained in the absence of NH4Br (Figure S4). Additionally, PtBi nanoplates could also be obtained by replacing NH4Br with other bromide ions suppliers, such as CTAB (Figure S5). It has 6 / 26


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ACS Catalysis

been reported that the halide ions could selectively adsorb on distinct surfaces of nanocrystals and guide the anisotropic growth, thus resulting in different morphologies.41 We then replaced Br− with Cl− and I− to investigate if other halide ions had the same structural-directing effects. However, even though 2D nanoplates could also be obtained in these cases, the products were severely aggregated with non-uniform size and morphology (Figure S6). These control experiments highlight the essential role of bromide ion in the synthesis of 2D nanoplates in our system. In addition to bromide ion, CO molecules were also widely used as capping agents to guide the growth of 2D metallic nanocrystals.42. However, when Mo(CO)6 was used as the CO supplier in our synthetic system, only PtBi nanocubes could be obtained (Figure S7). Besides, the morphology of products also depended on the choice of solvent, as no nanoplates could be obtained if pure oleylamine was used as the solvent (Figure S8). We also investigated the influence of reducing agents on the final morphology of products, and found: (1) a lower or higher concentration of AA than the optimized value could lead to nanoplates with inhomogeneous size distribution (Figure S9); (2) the use of glucose as reducing agent could also produce nanoplates (Figure S10), whereas the use of a weak reducing agent, such as phenylamine, could only yield irregular products (Figure S11). As shown in Figure S12, the feeding ratio of Pt/Bi precursors played the dominant role in determining the final products. Increasing the Pt/Bi ratio from 1/1 to 2/1 resulted in an obvious increase in the shell thickness, while decreasing the Pt/Bi ratio to 1/2 resulted in a homogeneous nanoplate in the absence of core/shell structure, further implying that the shell was made of pure Pt. To study the electrocatalytic properties of PtBi/Pt core/shell nanoplates, we first deposited the freestanding nanoplates onto commercial carbon black (C, EC-300) by sonication to obtain carbon supported catalyst (denoted as PtBi nanoplates/C). The as-obtained PtBi nanoplates/C was then subjecting to annealing at 220 for 1 h to remove the excess OAm before being used to prepare the catalysts ink. TEM images show that the PtBi nanoplates were uniformly dispersed on the carbon support without changes in their original morphology after this treatment (Figure S13c). For a comparison, the commercial Pt/C (Jonhson-Matthey Corp.) (Figure S13a) was used as benchmark. Cyclic voltammetries (CVs) were first conducted to reveal the surface property of as-obtained catalyst. As shown in Figure 4a, the PtBi nanoplates/C displayed typical Pt signals: well-defined hydrogen adsorption/desorption region (0.05 to 0.4 V versus RHE) and redox Pt oxidation/reduction 7 / 26


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region (0.7 to 1.1 V versus RHE), again suggesting the exposed surface of PtBi nanoplates are pure Pt.43 Notably, the on-set potential for oxidation peak, corresponding to the commencement of hydroxyl adsorption, shifted positively by more than 100 mV in comparison with that of commercial Pt/C, indicating a weaker oxygen binding energy on the surface of PtBi nanoplates.44. According to the Sabatier principle, slightly weakening the Pt-O strength could shift the oxygen binding energy of Pt to the optimized value (the peak position of volcano plot), thus leading to enhanced ORR activity. The hydrogen desorption peak in the recorded CV was applied to calculate the electrochemically active surface area (ECSA) of each catalyst. As listed in Table S1, the ECSA of PtBi nanoplates/C was 33.9 m2/g, which is smaller than that of commercial Pt/C owing to a larger dimensional size. Figure 4b compares the ORR polarization curves of various catalysts in O2-saturated 0.1 M HClO4 solution. In comparison with Pt/C, a positive shift in the half-wave potential (E1/2) can be readily identified on PtBi nanoplates/C, indicating its fast kinetics for the electro-reduction of oxygen. To provide a quantitative and fundamental comparison on intrinsic activity, it is routinely to obtain specific activity (SA) at 0.9 V versus RHE by calculating the kinetic current (ik) via the Koutechy-Levich equation and then normalizing ik to the specific surface area. Similarly, mass activity (MA) was obtained by normalizing ik to the Pt mass. As shown in Figure 4c, the SA of PtBi nanoplates/C reaches 1.04 mA/cmPt-2, which is 5.2 times higher than that of Pt/C. Meanwhile, even with a smaller ECSA, the mass activity of PtBi nanoplates/C was more than 2 times higher than that of Pt/C. Impressively, the PtBi nanoplates/C also exhibits enhanced durability relative to Pt/C. As compared in Figure 4d, after a course of 5,000 potential cycles, the PtBi nanoplates/C still maintains more than 90 % of the initial ECSA, much higher than that of Pt/C. The loss of ECSA for Pt/C is mainly caused by the growth and aggregation of active components, which can be verified from the TEM of catalysts before and after durability tests (Figure S13b). TEM-EDX of PtBi nanoplates/C after durability test shows that a small amount of Bi has been lost during cycling, which might be responsible for the slight decrease in specific activity (Figure 4d and Figure S13e). Both the intermetallic phase and the Pt shell can minimize the mobility and dissolution of non-stable Bi atoms, thus contributing to the enhanced long-term durability. It is well-known that the optimum oxygen adsorption energy (OAE) is essential to the ORR activity, which can be reflected by the position of d-band centre.45-46 We then carried out density 8 / 26


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ACS Catalysis

functional theory (DFT) calculations to shed light on the impressive ORR activity of PtBi nanoplates/C. The flat slab interface structural model has been built between hcp-PtBi (P63/mmc) and fcc-Pt with four-layered thickness. The final relaxed structure shows the interface has been well formed by the flexibly arranged local Pt-Bi bonding structure. The local lattice of the fcc-Pt shows a slight distortion. The electronic bonding (filled-states) and anti-bonding (empty states) orbitals have both been shown (Figure 5a). An edge model has been also built to describe and compare the electronic activities of the PtBi-nanoplates. The interfacial bonding also shows the thin Pt shell layer strongly bonded with surface Bi and Pt sites of the PtBi part. Some jagged local morphological feature has presented potentially provide highly active areas for locating oxygen and anchoring the intermediates for reactions. On the final relaxed edge structural model, the 3D local orbital contour plots show the top layer of the Pt sites on the edge surface provides electron-rich area where the valley regions have antibonding character. These characters indicate that only the top surface Pt sites are highly active while the second layer Pt sites may contribute a good desorption efficiency (Figure 5b). We move further to the discussions on electronic properties for supporting the high reactivity by both interface and edge models. There are three main peaks for the broadened Pt-5d band near Fermi level (EF), which indicates three different activities bonding and charge transfer found in this interface system. The Bi-p bands cross the EF denotes potentially high electronic activities to overlap similarly high-lying s-bands (Figure 5c-up). The variations of the d-band center for the Pt-5d orbital within different regions of interface have been illustrated. The introduction of electronically active Bi-p orbital suppresses the Pt-5d band centers (Figure 5c-down). From the fcc-Pt bulk to the surface Pt-site from hcp-PtBi alloy, the band center has been downshifted about 1.8 eV. The over-binding effect of Pt site has shown to be ameliorated. However, too weak bonding does not possess a favorable initial adsorption. We further notice that the Pt-site at the interface has moderately suppressed the band center with about 0.9 eV shifting. This indicates the energy evolution pathway can be favorably driven by different local Pt contributions. The variations of 5d-bands are illustrated by the projected partial density of states (PDOS) for the Pt-sites at the different regions from the PtBi-nanoplates edge model. The side position of the edge-site shifts toward higher positions closer to the EF compared to the Pt-site from the PtBi bulk region with a displacement of 0.85 eV for the 5d-band center. The surface electronic activities in the topmost (centered at -1.53 eV) edge area turns 9 / 26


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to be even higher than the Pt-site deep in the Pt-shell layer with 2.01 eV shifting towards EF, which is also the closest position of the Pt-5d band next to the EF and even higher than the Pt-bulk 5d-band behavior (Figure 5d-up). This active 5d-band implies the edge of the PtBi@Pt ultrathin core@shell nanoplates have a strong adsorption capability for both oxygen and hydrogen species and rather easy for further molecule bond-cleavage or splitting. While as our analysis, it can also potentially introduce over-binding effect for these reaction intermediates. We also lined-up the Pt-5d bands from different Pt-sites from different layers of the Pt-shell part for comparing the electronic activities. The Pt-sites at the contacting part and top surface of the edge area exhibit the highest contrast with about 1.62 eV shifting of d-band center, where the 5d-band of the Pt-site at topmost edge surface is centered at -1.53 eV with 0.76 eV higher than the optimal one at the interface model. Meanwhile, the Pt-sites within the middle area of the Pt-shell-layer show almost similar electronic characters (Figure 5d-down). We have evidently seen that surface electronic activity and property of the PtBi nanoplate catalyst can be tuned by coating such ultrathin Pt-shell layer. Thus, as we compared in physical trend, the contrast confirms the edge area can also promote the ORR activity with a stronger adsorption. The free energetic pathways for acidic four-electron ORR based on the systems of PtBi-Pt-interface, PtBi-Pt-edge, PtBi surface, and Pt surface have been shown and compared, respectively (Figure 6a). We dominantly compared these four-electron ORR pathways in both electrode potential U=0 and U=1.23 V for discussing different up- and down-hill behaviors. Overall at the U=0 environment, similar to the behavior of the Pt (111), both PtBi-Pt interface and edge models present downhill processes. Especially, the PtBi-Pt interface has nearly the same first step adsorption energy as the one of Pt (111), while the PtBi-Pt edge gives even more intense adsorption. In the following process until the last reduction from OH to H2O, the PtBi-Pt edge system maintains a slightly lower energy level than the interface system. While the last reduction shows the energy at the edge (0.95 eV) is about 0.06 eV higher than the interface (0.89 eV). Compared to the above two systems, the superiority for the energy downhill of the Pt presents at the second step where it clearly gains the greatest energy for the reduction of adsorbed O2 into the OOH. The following steps for the Pt (111) remain nearly a constant level of 0.85 eV towards the last step, with a small energy gain of 0.21 eV. For the plain PtBi surface, the higher index surface (211) show similar downhill with weaker adsorption of [O2+4H] than the others, and the last reduction step of transforming the OH 10 / 26


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ACS Catalysis

into H2O shows a barrier of 0.43 eV.

The PtBi (110) present a barrier of 0.19 eV from [*OOH+3H]

into the [*O+2H+H2O], and it possesses relative higher energy level compared to the other systems. In an average view, the PtBi-Pt nanoplates are more energetic favorable than PtBi to generate 2H2O via ORR and already benchmarked through the initial energetic preferred adsorption of *O2 and 4H*. Moreover, for PtBi system, the inefficiency may arise because of the too strong in-plane Bi-(p) and O-(p) or H-(s) overlapping, which leads to over-binding effect of *O=O-H and adsorbing H+. This yields an energetic barrier to form *O+H2O+2(H++e-). Another barrier is found from *OH+(H++e-) to H2O at the higher index PtBi surface, which arises because of strong overlapping between Bi-(p) and H-(s). Finally, a clear contrast is shown for the product H2O formation. Based on the overall energetic evolution behaviors, we now can estimate the electrode overpotential for the ORR among these five systems via comparison of the last reduction step. Although the absolute values may be varied with real experimental measurement, the difference relative to the Pt system can still follow the physicochemical trend. In particular to the interface and edge systems, we find their overpotentials are nearly the same and estimated to be 0.89 V and 0.95 V respectively. The differences are 40 mV and 60 mV with reference to our calculated overpotential of 0.85 V for Pt (111) system, where the differences are consistent in trend and also in a good agreement with our experimentally measured difference of ~40 mV. It is consistent that our DFT estimated overpotential U for Pt (111) shows with 0.07 V slightly higher than the value calculated by Nøskov et al45 based on the two-electron ORR pathway. On the other hand for the PtBi plain system, it either performs with too high overpotential (i.e. 2.07 V for (211) surface) or negative potential (i.e. -0.43 V for (110) surface), which is too diversified. In summary, free energetic analysis shows that ORR along the four-electron pathway is facilitated at the nearly the same potential to the Pt catalyst and determined at the last reduction step for OH into H2O. Look through the four-electron ORR pathway with high potential U=1.23 V, all of the model systems illustrate the energetic uphill process. Especially to the final reduction from OH to H2O, only the PtBi (110) performs a local downhill process, the others are all with barriers. Therefore, the as synthesized PtBi-nanoplate system could contribute an efficient catalytic ORR at relatively lower potentials. As above discussion, we know and confirm such four-electron favored pathway. Recapping the ORR process, it majorly rests on the four typical steps, of which the reduction overall can be seen as 11 / 26


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an incrementally hydrogenation and O-O bond cleavage processes. The adsorbed *O2 is initially hydrogenated as *OOH, then moves on an O-O dissociation to *O and H2O*, where the *O will further be hydrogenated as *OH towards final product of H2O. Our calculated free energy levels show that the elementary steps of the ORR along a four-electron pathway are downhill at an electrode potential is rather close to the one of Pt (~0.85 V vs. standard hydrogen electrode). The rate-determining step appears at the last reduction step, [*OH+H]→H2O*, with a reaction barrier of 0.8~0.9 eV. Although the overpotential Pt (111) may be varied to the absolute experimental reference level due to the error of the GGA-PBE functional framework within DFT, the relative difference of the overpotentials compared to the Pt can be further estimated in consistent with experimental measurement. From the local structures (Figure 6b), the O-related intermediate molecules are simultaneously bonding with Bi and Pt sites at the interface, where has higher catalytic activities. Therefore, in the PtBi-Pt interface system, the Bi suppresses the Pt-(H, O) overbinding effect, and the efficiency of adsorption/desorption steps have been increased with barrier free. By discussing the trends of electronics and energetics, we further confirm that the interface and edge areas both have high electronic activities and activating the high ORR energetic performance simultaneously. The electrocatalytic properties of PtBi nanoplates/C towards the MOR was evaluated by conducting CVs in 0.1 M HClO4 containing 0.1 M methanol. Obviously, the recorded peak current corresponding to the oxidation of methanol of PtBi nanoplates/C was far higher than that of commercial Pt/C (Figure 7a,b). The specific and mass activities were obtained by normalizing the peak current to the specific area and mass of Pt, respectively. As shown in Figure 7c, the specific and mass activities of PtBi nanoplates/C reach 3.18 mA/cm2 and 1.1 A/mg, 7.4 and 3.7 times higher than those of Pt/C, respectively. The electrochemical stability was assessed by recording i-t curves at a constant potential of 0.67 V versus RHE. After a course of 4000 s, the MOR current of PtBi nanoplates/C was still higher than that of Pt/C, indicating its good durability (Figure 7d). We also evaluated the catalytic performance of MOR in alkaline electrolyte, conducted in identical conditions with that of acidic electrolyte except changing the electrolyte to 0.1 M KOH containing 0.1 M MeOH. As expected, the PtBi nanoplates/C also shows higher activity and better durability than commercial Pt/C (Figure S14). 4. Conclusions 12 / 26


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In conclusion, we demonstrate a new class of intermetallic hcp-PtBi/fcc-Pt core/shell nanoplates for promoting fuel cells related electrocatalysis. The PtBi nanoplate shows bi-functionalities and outperforms commercial Pt/C not only in the electro-reduction of oxygen, but also in the electro-oxidation of liquid fuels. DFT calculations reveals that the enhanced ORR electrocatalysis stems from the fact the presence of Bi-p empty bands can reduce the over-binding effect of Pt-O on Pt shell of hcp-PtBi/fcc-Pt core/shell nanoplates. The hcp-PtBi/fcc-Pt core/shell nanoplates are stable for ORR by reflecting little loss in electrochemically active surface area and activity after the accelerated durability test due to their interesting intermetallic phase and core/shell architecture. Even higher mass activity of this catalytic system can be envisioned by further decreasing the thickness of nanoplates and the fcc-Pt shell. This study presents a new catalytic structure with excellent activity and durability, and also emphasizes the importance of rational structural design and construction of electrocatalysts with excellent performance.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. * E-mail: [email protected]. Notes The authors declare no competing financial interest.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of the experiments and Tables S1−S3 and Figures S1−S14 as described in the text. (PDF) Acknowledgements This work was financially supported by the National Natural Science Foundation of China (NSFC) (No. 51671003), National Basic Research Program of China (No. 2016YFB0100201 and 2017YFA0206701), the China Postdoctoral Science Foundation (No. 2017M610022), Open Project Foundation of State Key Laboratory of Chemical Resource Engineering and the start-up supports from Peking University and Young Thousand Talented Program. 13 / 26


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(44) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Improved Oxygen Reduction Activity on Pt3Ni(111) via Increased Surface Site Availability. Science 2007, 315, 493-497. (45) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jónsson, H. Origin of the Overpotential for Oxygen Reduction at A Fuel-Cell Cathode. J. Phys. Chem. B 2004, 108, 17886-17892. (46) Stamenkovic, V.; Mun, B. S.; Mayrhofer, K. J.; Ross, P. N.; Markovic, N. M.; Rossmeisl, J.; Greeley, J.; Norskov, J. K. Changing the Activity of Electrocatalysts for Oxygen Reduction by Tuning the Surface Electronic Structure. Angew. Chem. Int. Ed. 2006, 45, 2897-2901.

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Figure 1. Morphology and structure characterization of PtBi/Pt core/shell nanoplates. (a) PXRD pattern, (b, c) TEM image, and (d) EDX of PtBi/Pt core/shell nanoplates. (e, f) TEM of PtBi/Pt core/shell stacks that lie vertically on the TEM grid and the corresponding thickness estimation. (g) STEM elemental mappings of single PtBi/Pt core/shell nanoplate. (h) Pt /4f XPS spectra of PtBi/Pt core/shell nanoplates and commercial Pt/C.

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Figure 2. (a, b) Representative HRTEM image of as-prepared PtBi/Pt core/shell nanoplate, (c, d) EDX elemental line scan across single PtBi/Pt core/shell nanoplate.

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Figure 3. Structural evolution of PtBi/Pt core/shell nanoplates: (a) Representative TEM image and corresponding size distribution of products collected at 1 min, (b) 30 min, (c) 5 h, (d) 10 h. (e) Schematic illustrations of the structural evolution process of PtBi/Pt core/shell nanoplates.

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Figure 4. ORR performance. (a) CVs of PtBi/C and Pt/C catalysts. (b) ORR polarization curves of commercial Pt/C and PtBi/C catalysts. (c) Comparison on the specific activities and mass activities. (d) Comparison of ECSA and mass activity before and after 5,000 cycles.

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ACS Catalysis

Figure 5. DFT calculations. (a) The local structure of PtBi-Pt interface model system (top- and side-view) with bonding (blue) and anti-bonding (green) orbital contour plots (down). (b) The local structure of PtBi-Pt edge model system (side-view) with bonding (blue) and anti-bonding (green) orbital contour plots (down). (c) The projected density of states (PDOS) of the PtBi-Pt interface model system (up). The related variation of the PDOS for the Pt-5d band is also shown from different regions (down). (d) The Pt-5d PDOS of the PtBi-Pt edge model system from different regions (up). The related variation the Pt-5d band has been also shown from different layers near the edge region (down).

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Figure 6. (a) The free energetic pathways for acidic four-electron ORR based on the systems of PtBi-Pt-interface, PtBi-Pt-edge, PtBi (211) surface, PtBi (110) surface, and Pt (111) surface have been shown and compared, respectively. The free energy diagrams of ORR in both zero electrode potential (U=0) and equilibrium potential (U=1.23 V) for discussing different up- and down-hill behaviors have been illustrated and compared. (b) The evolutions of the local structural configurations for the simulated ORR process from the PtBi-Pt interface model system.

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Figure 7. MOR Performance. (a) ECSA- and (b) Pt mass-normalized CVs of PtBi nanoplate/C and commercial Pt/C recorded in 0.1 M HClO4 solution containing 0.1 M MeOH. (c) Histogram of specific activity and mass activity. (d) i-t test of PtBi nanoplate/C and commercial Pt/C in 0.1 M HClO4 containing 0.1 M MeOH, recorded at a constant potential of 0.67 V vs. RHE.

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Shell Nanoplates Enable Efficient

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