L12 Atomic Ordered Substrate Enhanced Pt-Skin Cu3Pt Catalyst for

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L1 atomic ordered substrate enhanced Pt-skin CuPt catalyst for efficient oxygen reduction reaction Na Cheng, Ling Zhang, Shuying Mi, Hao Jiang, Yanjie Hu, Haibo Jiang, and Chunzhong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b11764 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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ACS Applied Materials & Interfaces

L12 Atomic Ordered Substrate Enhanced Pt-skin Cu3Pt Catalyst for Efficient Oxygen Reduction Reaction Na Cheng, Ling Zhang, Shuying Mi, Hao Jiang, Yanjie Hu, Haibo Jiang* and Chunzhong Li*

Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science & Technology, Shanghai 200237, China Keywords: oxygen reduction reaction; intermetallic; atomic ordering; Pt skin; oxygen adsorption energy ABSTRACT Constructing Pt skin on intermetallics has been confirmed as an efficient strategy to boost oxygen reduction reaction (ORR) kinetics. However, there still lacks a systematic study on revealing the influence of low-Pt-content intermetallic substrates (L12-PtM3). In this paper, Pt skin encapsulated low-Pt-mole-fraction L12 Cu3Pt has been constructed (denoted as Pt-oCu3Pt/C) and compared with its disordered analogue (denoted as Pt-d-Cu3Pt/C). The L12 substrate shows a contracted lattice structure and provides Pt-o-Cu3Pt/C with an excellent specific activity of 1.73 mAcm-2, which is 1.4- and 8.4-fold higher than that of Pt-d-Cu3Pt/C and commercial Pt/C, respectively. Density functional theory (DFT) calculations reveal that this superior performance is attributed to the more favorable oxygen adsorption energy (Eads(O)) of surface Pt atoms. Furthermore, the lower formation energy of L12 Cu3Pt combined with the enhanced antioxidant of Pt provide Pt-o-Cu3Pt/C with a superior durability, showing only a 12.5% loss in mass activity after 5000 potential cycles. Therefore, it is suggested that L12 1

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atomic ordered structure with a low Pt fraction is a promising substrate for building highperformance Pt-skin catalysts for ORR. 1. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) serving as a clean and efficient green energy device possess broad prospects.1-3 However, due to the sluggish cathodic oxygen reduction reaction (ORR) kinetics, noble metal Pt-based catalysts are needed, which makes it unaffordable for the large-scale commercialization of PEMFCs.4, 5 According to the 2017 target of the United States Department of Energy (DOE), the mass activity of Pt catalysts should be enhanced at least 4-fold compared to the state-of-the-art Pt/C to realize the commercialization of PEMFCs.6, 7 Alloying Pt with transition metals (Ni8, 9, Co10, Fe11 or Cu12) is acknowledged as the most successful method to lower the use of Pt in electrocatalysts and improve their catalytic properties simultaneously through synergistic effects, ligand effects and strain effects.13 Since the ORR kinetics is compositional and structural sensitive,14-16 tremendous effort has been made to adjust the elemental compositions, atomic ratios, bounded facets9,

17-19

and

architectures20-27 of Pt-based nanocrystals. Engineering Pt-skin structures has been confirmed as an ideal approach to improve the number of available Pt sites. Moreover, the catalytic efficiency of each Pt sites can be remarkably enhanced by selecting substrate with proper elemental compositions and architectures, such as PdAu28, Pd3Al (111)29, AgPt hollow particles30, PtFe nanowires31, etc..32, 33 Intermetallics (PtFe34-36, PtCo37, 38, PtCu39 and PtFeAu40) with definite compositions and 2


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atomic ordered structures have attracted extensive attention and have been proposed as novel supports for Pt-skin catalysts toward ORR.41-44 For face-centered cubic (fcc) structured intermetallics, L10 and L12 are two representatives. Within L10, two elements alternate in the lattice with a ratio of 1:1; while in L12, one element is located at face center, and the other at vertices with a ratio of 3:1. So far, Pt skin encapsulated L10-PtCo45, L10-AuCu46, L12-Pt3Fe31, L10-PtFeCu47, 48, L12-Pt3Al49 and L12-(Pt1-xNix)3Al50 have been developed as substitutes for Pt/C. Beyond that, the catalytic differences between the intermetallic and atomic disordered substrate have also been studied. For instance, the Pt-skin encapsulated L12-Pt3Co was reported to exhibit an over two-fold enhancement in mass activity in comparison with the disordered one.51 Moreover, combined with density functional theory (DFT) calculations, the Sun group also suggested that the atomic ordering of the PtFe substrate can endow the Pt skin with superiority to boost ORR kinetics by regulating surface strain.46, 51 However, in terms of the Pt-skin-encapsulated low-Pt-mole-fraction intermetallics (L12-PtM3), few works have been carried out to investigate their structure-activity relationship. Additionally, the compositional and atomic configurational differences as well as the more severe lattice contraction of L12PtM3 compared with the previously studied L12-Pt3M and L10-PtM are speculated to generate a distinctly different ligand and stronger strain effect on Pt skin, which makes it difficult to identify whether the aforementioned tendency and influence mechanism are still applicable.52 Therefore, a systematic study is expected to reveal the influence of the atomic ordering of the substrate on the catalytic performance of PtM3 supported Pt skin. In this work, a combined experimental and theoretical study on tuning the catalytic efficiency

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of Pt-skin Cu3Pt by atomic ordered L12 substrate is reported. The atomic ordering transition of disordered Cu3Pt nanoparticles (NPs) were induced via a thermal annealing approach and a three-layer Pt skin was formed by a subsequent electrochemical dealloying. An obvious lattice contraction was observed for L12 Cu3Pt. Also, the ordered substrate showed a superior capability in enhancing activity compared to the disordered one. DFT calculations were conducted to shed light on the origin of the performance differences and suggested that the L12 substrate could weaken the Eads(O) by 0.22 eV, which was 0.02 eV closer to the theoretically predicted optimum value than the disordered Cu3Pt. Additionally, the lower formation energy of L12 Cu3Pt combined with the enhanced antioxidant of Pt provided the catalyst with a superior durability, which showed only a 12.5% loss in mass activity after 5000 potential cycles. 2. EXPERIMENT SECTION Materials: Platinum(II) acetylacetonate (Pt(acac)2, 98%, Pt>48%) was purchased from Energy Chemical (Shanghai, China), copper(II) acetylacetonate (Cu(acac)2, ≥99%) was purchased from DiBo Chemical Reagent Co. Ltd., Dodecyltrimethylammonium chloride (DTAC, 99%) was purchased from Aladdin-reagent Inc., oleylamine (68-70%) and Cabot Vulcan XC-72 were purchased from Macklin Biochemical Co. Ltd. (Shanghai, China), 5 wt% nafion was purchased from Sigma-Aldrich, ethanol and cyclohexane were purchased from Titan Scientific Co. Ltd., Pt/C (20 wt%) catalyst was purchased from HeSen electric Co. Ltd.. All chemicals were used without further purification. Synthesis of Cu3Pt Nanoparticles (Cu3Pt NPs). To synthesize low Pt mole fraction Cu3Pt NPs, 25.9 mg of DTAC was first dispersed 4


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uniformly in oleylamine (5.0 mL) under ultrasonic in a vial (volume: 30 mL), then Pt(acac)2 (10.0 mg) and Cu(acac)2 (6.6 mg) were added into the above suspension and mixed for 30 min with ultrasonic assistance. The final mixture was heated from room temperature to 180 °C for 4 h. The resulting product was centrifuged and washed three times with an ethanol/cyclohexane (5:1) mixture. Preparation of d-Cu3Pt/C and o-Cu3Pt/C. First, the as synthesized Cu3Pt NPs were loaded onto carbon supports. Typically, 2 mg of Cu3Pt NPs was added into a carbon-ethanol dispersion containing 8 mg of Vulcan XC-72 carbon. After being sonicated for 1 h, the mixture was collected by centrifugation and dried at 40 °C under vacuum condition to obtain Cu3Pt/C. Then, Cu3Pt/C was subjected to thermal annealing at 250 °C under flowing 5% H2 atmosphere (Ar balanced) for 2 h (heating rate: 5 °C/min) to remove organic surfactants and improve the crystallinity of Cu3Pt (denoted as dCu3Pt/C). Thermal annealing at 600 °C, with other conditions unchanged, was applied to induce the atomic ordering transition (denoted as o-Cu3Pt/C). Formation of Pt-skin Structure. The d-Cu3Pt/C and o-Cu3Pt/C were electrochemically activated under a potential scan between 0.06 and 1.30 V for 20 cycles to obtain Pt-skin Cu3Pt/C catalysts, the corresponding products were denoted as Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C, respectively. Characterizations: The morphologies of Cu3Pt NPs and all the catalysts were observed by transmission electron 5



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microscopy (TEM; JEOL-2100 transmission electron microscope with LaB6-cathode (200kV)). Energy-dispersive X-ray spectroscopy (EDX) connected with aberration-corrected high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) were conducted on Grand ARM 300F. The X-ray diffraction (XRD) was performed on D8 Advance X-Ray Polycrystalline Diffractometer (Germany) in a scan range of 10-90°, with a step size of 0.02° and a scan rate of 0.36 s per step to determine the crystal structure. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Agilent 725ES) was used to measure the chemical compositions of the catalysts. X-ray photoelectron spectroscopy (XPS) were conducted on a Thermo Scientific ESCALAB 250 XI X-ray photoelectron spectrometer to investigate the chemical state and composition of surface elements. Electrochemical Measurements: Before electrochemical tests, 2.5 mg of the as-prepared catalysts were dispersed in a mixture containing 480 μL isopropanol and 20 μL Nafion (5%) under ultrasonication for 30 min to obtain a homogeneous catalyst ink. The concentration of Pt was controlled at 0.50 mgPt/mL based on the ICP-AES measurement. Then, 8 μL ink was placed onto a precleaned glassy carbon rotating disk electrode (RDE, Pine Research Instrumentation, geometric area: 0.196 cm2) to prepare the working electrode, with the loading amount of Pt being 4 μg. In terms of the Pt/C catalyst (20 wt% Pt loading amount), 2.5 mg of Pt/C was dispersed in 480 μL isopropanol and 20 μL Nafion (5%) and sonicated for 30 min. 4 μL of the ink was deposited onto the glass carbon RDE and dried under ambient conditions. Electrochemical measurements were carried out in a three-electrode cell connected to a CHI 6


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760E electrochemical workstation (CHI Instruments, Shanghai, Chenhua Co., Ltd.). Glass carbon RDE, Pt wire and saturated calomel electrode (SCE) electrode were used as working electrode, counter electrode, and reference electrode, respectively. All potentials were converted with reference to a reversible hydrogen electrode (RHE). The cyclic voltammograms (CVs) measurements were conducted in a N2-saturated 0.1 M HClO4 solution with a scanning potential range from 0.06 to 1.3 VRHE at a scan rate of 100 mV/s. From the CV curve, the electrochemical active surface area (ECSA) was calculated by the equation below: ECSA =

Q m×0.21

(1)

Where Q represents the charge obtained by integration of H-desorption region, m is the Pt loading amount on the glass carbon electrode and 0.21 is the charge for oxidizing a monolayer of H2 on clean Pt. The ORR polarization curves of the catalysts were measured in an O2-saturated 0.1M HClO4 solution with a scanning potential range from 0.08 to 1.1 VRHE at a scan rate of 10 mV/s (rotating rate of 1600 rpm). The diffusion limiting current density (Id) and the experimentally measured current density (I) can be read from the curve. Thus, the kinetic current (Ik) can be obtained by Koutecky-Levich equation (at 0.90 VRHE): 1 I

1

1

Ik

Id

= +

(2)

Hence, based on the data calculated above, the mass activity (MA) and specific activity (SA) can be derived from the following equations: 7



MA = SA =

Ik m Ik

ECSA×m

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(3) (4)

As for the accelerated durability test, CVs and linear sweep voltammetry were implemented after 5000 potential cycles between 0.6 and 1.1 VRHE at a scan rate of 0.1V/s. DFT study All DFT calculations were done in the Dmol3 code, except the calculations of the differential density of electrons being conducted by CASTEP. The calculations of the electronic structure and the exchange correlation potential was based on the generalized gradient approximation and Perdew-Burke-Emzerhof (GGA-PBE) method. The valence electrons were expanded by double numerical basis set and polarization function (DNP). DFT semi nuclear pseudopotential was used to freeze the inner layer electrons of atoms. A 2◊2 unit cell was applied to build a four-layer Cu slab and was repeated in a supercell. In order to prevent the interaction between periodic structures, a vacuum region was set as 10 Å. To construct atomic ordered Cu3Pt, the vertices of the Cu slab were replaced by Pt atoms. For the disordered Cu3Pt, the replacement was random while maintaining the atomic ratio to be 3:1. Then the models went through geometric optimization. The top three layers of the models were substituted by pure Pt and subjected to geometric optimization to simulate Pt-skin Cu3Pt. The adsorption energies of O (Eads(O)) were calculated via the following equation: Eads(O) = Etotal – EO – E(111)

(5)

Etotal, EO and E(111) represent the total energy of adsorption system, the energy of oxygen atom 8


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and the energy of (111) metal slab, respectively. In order to evaluate the stability of both the atomic ordered and disordered Cu3Pt, the formation energies were calculated via the following equation: ΔECu3Pt = [ECu3Pt – n*EPt – m*ECu]/(n+m)

(6)

where ECu3Pt, EPt and ECu represented the total energy of Cu3Pt, pure Pt and pure Cu. n and m represented the atom number of Pt and Cu in Cu3Pt models, respectively. 3. RESULTS AND DISCUSSION Characterization of d-Cu3Pt/C and o-Cu3Pt/C. The initial Pt-Cu NPs were synthesized via a facile solvothermal method as described in the experiment section, in which the chloride ions present in DTAC were found to play a critical role. As revealed in Figure S1, when Cl- was absent or replaced by Br-, monodisperse Cu-Pt NPs could not be obtained. The small particles present were speculated to be pure Pt resulting from the homogeneous nucleation of Pt. Previous studies have shown that chloride ions preferentially bind on Pt ions, and the amine groups can promote the reduction of Cu. As a result, the reduction rate of Pt was slowed down and low Pt mole fraction Cu3Pt NPs were finally formed (determined by ICP-AES).53, 54 These as-prepared Cu3Pt NPs presented a spherical-like morphology and a uniform size distribution with a mean diameter around 6.78 nm (Figure S2). Recently, the carbon coating-thermal annealing strategy was prevalent to prevent the ripening and growth of PtFe nanoparticles during high-temperature thermal treatment.34,

45

However, considering that the coating

thickness has a non-negligible influence on catalytic performance, facile thermal annealing at

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600 °C under 5% H2/Ar atmosphere was conducted to induce the atomic ordering transition in this work. As shown in Figure S2, after deposited on carbon with a low mass loading (20 wt%), the nanoparticles could withstand high temperatures with little sign of aggregation. The corresponding products are denoted as o-Cu3Pt/C. Figure 1a and b show the typical TEM image of d-Cu3Pt/C and o-Cu3Pt/C catalysts. It was observed that heat treatment at 250 °C generated almost no morphological change and a negligible increase in particle size (Figure S2). Moreover, according to the FTIR spectrum, the surfactants were efficiently removed from the catalyst (Figure S3). As for o-Cu3Pt/C, owing to the sparse and homogeneous dispersion of Cu3Pt NPs on carbon support, only a few particles underwent slight ripening and growth (Figure S2). The crystal structure of the as-prepared Cu3Pt was first determined by XRD. An alloy phase was confirmed since no peak separation was observed and four diffraction peaks corresponding to (111), (200), (220) and (300) facets lay between those of pure Pt and Cu, which were consistent with those of Cu3Pt with an fcc structure (Figure S4). The fcc structure was retained for d-Cu3Pt/C. Moreover, the crystallinity of the nanoparticles was improved after low-temperature annealing as revealed by the contracted half peak width and the increased peak intensity (Figure S4). For o-Cu3Pt/C, two additional peaks of (100) and (110) were observed (Figure 2), indicating that the crystal structure transformed from fcc to L12.56-58 A distinct narrowing and a 0.29° positive shift of the diffraction peaks were observed for o-Cu3Pt/C as well (Figure 2, inset). To gain further insight into the crystal structure changes caused by the atomic ordering transition, XRD Rietveld refinements were conducted (Figure S5 and Table S1). As revealed in the fitting results, o-

10


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Cu3Pt/C showed a decrease in both lattice parameter and cell volume by 0.066 and 2.77, respectively, in contrast with that of d-Cu3Pt/C. These results were in accordance with the HRTEM images and FFT patterns which showed that the d-Cu3Pt/C nanocrystals displayed a lattice distance of 2.14 Å, 0.02 Å larger than that of o-Cu3Pt/C (Figure 1c-f). The same tendency was further confirmed by DFT calculations which suggested that the lattice parameters of Cu3Pt NPs shrank from 3.95 nm to 3.83 nm after the atomic ordering transition. Therefore, it was concluded that the atomic ordering transition from d-Cu3Pt/C and o-Cu3Pt/C caused not only a regular atom occupation, but also a contracted lattice distance. Characterization of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C. The extended potential cycling process has been widely applied in activating catalysts. However, the inevitable structural changes during this process are often neglected.59,

60

Taking advantage of this electrochemically

activated process, Pt-skin structure NPs, namely Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C, were prepared. During potential cycles, surface Cu atoms were oxidized, dissolved and leached out, which resulted in the segregation and rearrangement of Pt atoms on the surface. The dealloying process was recorded by cyclic voltammograms (CVs). As shown in Figure S6, the ECSA calculated from the hydrogen desorption peaks (0.072 V < E < 0.44 V) increased gradually with cycle numbers and reached a plateau after 20 cycles for both catalysts. The morphologies of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C showed no change (Figure S7). The lattice distances measured from the inner parts of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C maintained that of Cu3Pt (Figure 3a, b). Moreover, the well-organized lattice structure obtained by IFFT (inverse fast fourier transform), seen in the red dotted line area in Figure 3b, demonstrated that the L12

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structure was retained for Pt-o-Cu3Pt/C (Figure 3c). The Pt-skin structure was confirmed by STEM-EDX mapping and the corresponding line profiles showed that a Pt skin of 0.67 nm and 0.62 nm (~3 atomic layers) were formed for Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C, respectively (Figure 3d-g). Electrochemical properties towards ORR. To study the ORR electrocatalytic activities of the above catalysts, the electrochemical properties of the d-Cu3Pt/C and o-Cu3Pt/C catalysts were first evaluated and benchmarked against commercial Pt/C catalyst. ORR polarization curves showed that the half-wave potentials of d-Cu3Pt/C, o-Cu3Pt/C and Pt/C were 0.872 V, 0.850 V and 0.872 V, respectively (Figure S8). The mass activity of d-Cu3Pt/C (0.137 A mg-1pt) was just comparable to that of Pt/C (0.120 A mg-1pt), and for o-Cu3Pt/C (0.093 A mg-1pt), it was even poorer than Pt/C. These results indicated that the exposed Pt-Cu surfaces for both ordered and disordered Cu3Pt were inactive for enhancing ORR. After encapsulated with a three-layer Pt skin via electrochemical dealloying, the CVs of Ptd-Cu3Pt/C and Pt-o-Cu3Pt/C were recorded and shown in Figure 4a. The ECSA of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C were calculated to be 43.16 m2/gpt and 37.01 m2/gpt respectively, both lower than that of commercial Pt/C (58.86 m2/gpt), due to the larger size (7.22 and 7.37 vs. 3.5 nm). The ORR polarization curves are shown in Figure 4b. Specific activities and mass activities were calculated from the corresponding kinetic current densities and presented in Figure 4c, d and Table S2. It was obvious that the formation of Pt skin prompted the catalytic activities of Cu3Pt catalysts greatly. At 0.9 VRHE, the specific activity of Pt-o-Cu3Pt/C catalyst (1.73 mAcm2

) displayed a 1.4-fold enhancement over the Pt-d-Cu3Pt/C catalyst and an 8.2-fold 12


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enhancement over Pt/C, which suggested that the catalytical efficiency of each activity site had been improved. The mass activity of Pt-o-Cu3Pt/C catalyst (0.64 Amg-1Pt) was slightly higher than that of Pt-d-Cu3Pt/C (0.57 Amg-1Pt) and 5.2 times better than that of Pt/C catalyst (0.12 Amg-1Pt). The enhancement factors for both Pt-skin Cu3Pt/C catalysts with respect to commercial Pt/C were remarkably high among the Pt-Cu bimetallic catalysts towards ORR (Table S3). Previous theoretical studies suggested that for Pt catalysts, at around 0.90 V, the hydrogenation of adsorbed OH becomes the rate determining step for ORR, that is, the surface is likely to be saturated with adsorbed O or OH and becomes unreactive61-63. Since the stability of adsorbed OH is in proportion to adsorbed O, Eads(O) was accepted as a criterion to measure ORR kinetics. Herein, DFT simulations were performed to calculate Eads(O) on different surfaces to understand the superior activity caused by low-Pt-mole-fraction L12 Cu3Pt as well as the evident activity enhancement after forming Pt skin. As illustrated in Figure 5a, (111) surfaces of d-Cu3Pt, o-Cu3Pt, Pt-d-Cu3Pt and Pt-o-Cu3Pt were constructed. Based on the structure characterization, Pt skins of Pt-d-Cu3Pt and Pt-oCu3Pt were set as three layers (Figure 3e, g). The -Eads(O) on pure Pt (111) was calculated to be 3.75 eV, which was consistent with experiment values (3.47-3.73 eV) and the calculation values reported in other works.61 It has been acknowledged that the adsorption energy of oxygen on Pt bulk was too strong, which was 0.2 eV stronger than the optimal value (3.55 eV in this work).64 Thus, in this work, -Eads(O) was expected to be located between 3.35 eV and 3.75 eV for catalysts more active than pure Pt. Figure 5b and c present the -Eads(O) values and 13



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partial density of states of surface Pt atoms, respectively. In terms of d-Cu3Pt and o-Cu3Pt, the -Eads(O) values were 4.13 eV and 4.31 eV, respectively, which were extremely high and even exceeded that of pure Pt. This can be attributed to the following reasons: (1) the surfaces were Cu-rich, therefore oxygen atoms inevitably bind with Cu atoms, which presents a strong adsorption energy for oxygen (4.56 eV);65 (2) The d-band center of surface Pt atoms on d-Cu3Pt (-2.87 eV) showed only a 0.04 eV downshift when compared with that of pure Pt (-2.83 eV). and for that on o-Cu3Pt (-2.78 eV), the d-band center moved even closer to Fermi level, which indicated a strong coupling between O 2p states and Pt d states when bonding with oxygen atoms. Although the electron transfer from Cu to Pt, as confirmed by the downshift of binding energy peaks compared with Pt/C in XPS (Figure S9), should have caused a downshift of Pt dband center, the positive shift for Pt atoms on o-Cu3Pt surface remained reasonable when taking the complexity and particularity of top surface into account. Moreover, this upshift was confirmed by Hammer et al., as well.66 After encapsulated with Pt skin, the lattice mismatch between Cu3Pt and Pt generated compression strain on Pt skin, which modified the electronic structure of surface Pt atoms and sufficiently altered the adsorption capability of oxygenated intermediates. As illustrated in Figure 5c, the d-band centers of Pt atoms on Pt-d -Cu3Pt and Pt-o-Cu3Pt surfaces shifted 0.33 eV and 0.39 eV away from the Fermi level compared to pure Pt, respectively. The -Eads(O) values of both Pt-d-Cu3Pt and Pt-o-Cu3Pt dropped sharply, falling into the range of 3.35 eV3.75 eV. The calculated -Eads(O) value of Pt-o-Cu3Pt (3.53 eV) was 0.06 eV weaker than that of Pt-d-Cu3Pt (3.59 eV) and 0.02eV closer to the optimal value. This lower adsorption energy

14


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can be due to the stronger compressive strain on the surface of Pt-o-Cu3Pt, caused by the smaller lattice distance of o-Cu3Pt, as discussed in previous section. In order to directly characterize the electron transfer between oxygen and surface atoms as well as the oxygen adsorption capacity, electron density differences were calculated. Figure S10 shows that through the formation of Pt skin, the electron cloud density of adsorbed O evidently reduced, further validating the weakening of oxygen binding energy. The oxygen adsorption capacity of the Pt-Cu catalysts was confirmed by XPS as well. As shown in Figure S9 and Table S4, the Pt0/Pt2+ ratios of d-Cu3Pt/C (1.23) and o-Cu3Pt/C (1.11) were slightly lower than Pt/C (1.54). While, the Pt0/Pt2+ ratios of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C were 2.03 and 2.45, respectively, indicating that the Pt-skin structure along with the L12 substrate could further weaken the oxygen affinity, making Pt-o-Cu3Pt more insusceptible to be poisoned by oxygenated intermediates. Although the theoretical calculations were on the basis of ideal surfaces and may not accurately correspond to the real surface structure, they indeed provided a reasonable and sufficient explanation for the enhanced catalytic activity of Pt-o-Cu3Pt/C. The long-term durability of the catalysts was evaluated via accelerated durability tests (ADT) by operating potential cycles between 0.6 and 1.1 VRHE. As shown in Figure 6 and Table S5, after 5000 cycles of ADTs, the commercial Pt/C exhibited a dramatic decrease of 29.7% in mass activity, but an increase of 70.2% in specific activity as a result of a 58.7% ECSA loss. As for the Pt-d-Cu3Pt/C catalyst, the ECSA decreased from 43.16 m2/gpt to 30.85 m2/gpt with mass activity loss of 28.3%. However, for the Pt-o-Cu3Pt/C catalyst, the ECSA showed a negligible loss of 3.7% with only 12.5% mass activity and 9.2% specific activity loss. TEM

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images revealed that after 5000 cycles, both Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C catalysts displayed negligible morphology change and aggregation. In contrast, commercial Pt/C suffered from severe aggregation (Figure S11). Therefore, it was speculated that the performance degradation of Pt based bimetallic compounds was mainly caused by the oxidation and leaching of Cu atoms under the rigorous durability testing conditions, which was reported to be associated with the formation energy of the compound.67, 68 The formation energy calculated by DFT of d-Cu3Pt and o-Cu3Pt was -0.114 eV/atom and -0.157 eV/atom, respectively, indicating the stronger bonding nature of Pt-Cu and stabilization of Cu within the L12 structure. The prohibited leaching of Cu in the ordered substrate was also verified via ICP-AES results, since the Pt-to-Cu atom ratios of Pt-o-Cu3Pt/C and Pt-o-Cu3Pt/C were 0.85:1 and 0.44:1, respectively, after 5000 potential cycles. Moreover, as shown in Figure 4a, the Pt oxidation potential of Pto-Cu3Pt/C was 0.92 VRHE, beyond that of Pt/C (0.87 VRHE) and Pt-d-Cu3Pt/C (0.89 VRHE), which suggested that Pt-o-Cu3Pt/C had a more antioxidant nature. 4. CONCLUSION In summary, we have constructed Pt-skin-encapsulated low-Pt-mole-fraction Cu3Pt NPs and studied the effects of substrate’s atomic ordering on catalytic performance. The results indicated that the L12 Cu3Pt showed a further lattice contraction compared with its disordered analogue. In comparison to Pt-d-Cu3Pt/C, Pt-o-Cu3Pt/C displayed enhanced catalytic activities, of which the mass and specific activity are 5.2 and 8.2 times greater than those of commercial Pt/C. Furthermore, Pt-o-Cu3Pt/C possessed superior durability as well, showing only a 12.5% loss in mass activity. The mechanistic origin for the superiority brought by the atomic ordering 16


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difference and the Pt-skin structure was studied through DFT calculations. It proved that the formation of Pt skin could reduce the -Eads(O) sharply. In contrast to the disordered substrate, the L12 ordered substrate can further downshift the d-band center of Pt-skin structure, thereby weaken the coupling between O 2p electrons and Pt d electrons. The resulting -Eads(O) of Pto-Cu3Pt/C was 0.06 eV lower than that of Pt-d-Cu3Pt/C, which was more favorable to boost ORR kinetics. Furthermore, due to the lower formation energy, the L12 Cu3Pt could better stabilize the Cu atoms, which combined with the enhanced resistance to oxidation of Pt atoms provided Pt-o-Cu3Pt/C with reinforced durability.

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Figure 1 (a, b) TEM images of d-Cu3Pt/C and o-Cu3Pt/C, respectively. (c,e) HRTEM images and insets are Fast Fourier transform (FFT) images of o-Cu3Pt/C and d-Cu3Pt/C, respectively. (d, f) The measured lattice distances over the blue boxes in c and e.

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Figure 2 Schematic diagram of atomic ordering transition and XRD patterns of d-Cu3Pt/C and o-Cu3Pt/C. Inset is the enlarged patterns in the range of 35 and 55 degrees, the diffraction peaks of o-Cu3Pt/C positively shift 0.29°.

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Figure 3 (a, b) HRTEM images of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C, (c) IFFT (inverse fast fourier transform) image of the red dotted line area in b and corresponding atomic model (the blue and orange atoms represent Pt and Cu, respectively), (d, f) EDX mapping of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C, (e, g) EDX line profiles recorded along the yellow arrows in d, f. (The endpoints are determined as where the intensity of Pt or Cu starts to become stable.)

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Figure 4 Comparison of electrocatalytic activities among the Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C and Pt/C. (a) CVs curves recorded in a N2-saturated 0.1 M HClO4 solution at a scan rate of 100 mV/s. The Pt oxidation potentials were circled by a gray dashed line. (b) corresponding ORR polarization curves recorded in an O2-saturated 0.1M HClO4 solution at a scan rate of 10 mV/s (rotating rate of 1600 rpm), (c, d) specific activity and mass activity Tafel plots and corresponding histograms summarized at 0.9 VRHE

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Figure 5 DFT calculations for the further understanding the structure-activity relationship (a) calculation models of d-Cu3Pt/C, o-Cu3Pt/C, Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C (111) surfaces (from left to right) viewed from top (up) and side (down), (b) calculated E ads(O) on different (111) surfaces, (c) calculated projected d-density of states of surface Pt atoms

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Figure 6 (a, c, e) ORR polarization curves of different catalysts before and after 5000 potential cycles between 0.6 and 1.1 VRHE. Insets are the corresponding CVs curves. (b, d, f) Changes on mass and specific activity of different catalysts after 5000 potential cycles.

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AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (A/Prof. H. B. Jiang), Tel: +86 021 64250996 E-mail: [email protected] (Prof. C. Z. Li), Tel: +86 021 64250949. Notes The authors declare no competing financial interest. ASSOCIATED CONTENT Supporting Information. TEM images of Cu3Pt NPs synthesized with different capping agent, d-Cu3Pt/C and oCu3Pt/C; The FTIR spectra and XRD spectra of Cu3Pt NPs and d-Cu3Pt/C; Rietveld refinements for d-Cu3Pt/C and o-Cu3Pt/C; Electrochemical dealloying process recorded by cyclic voltammograms; TEM images of Pt-d-Cu3Pt/C and Pt-o-Cu3Pt/C; ORR polarization curves of d-Cu3Pt/C and o-Cu3Pt/C; XPS of d-Cu3Pt, o-Cu3Pt, Pt-d-Cu3Pt, Pt-o-Cu3Pt and Pt/C; Calculated electron density difference of O; TEM images of the catalysts after 5000 potential cycles. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21776092, 91534202, 91534122), the Social Development Program of Shanghai (17DZ1200900), Innovation Program of Shanghai Municipal Education Commission, the Fundamental Research 24


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Table of Contents Graphic

Briefs: Weakened oxygen adsorption energy and enhanced catalytic efficiency of Pt-skin Cu3Pt by atomic ordered L12 substrate.

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L12 Atomic Ordered Substrate Enhanced Pt-Skin Cu3Pt Catalyst for

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