Tri-metallic Au@PtPd Mesoporous Nanorods as Efficient

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Tri-metallic Au@PtPd Mesoporous Nanorods as Efficient Electrocatalysts for Oxygen Reduction Reaction Yaoyao Deng, Hairong Xue, Shuanglong Lu, Yujie Song, Xueqin Cao, Liang Wang, Hongjing Wang, Youliang Zhao, and Hongwei Gu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00920 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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Tri-metallic Au@PtPd Mesoporous Nanorods as Efficient Electrocatalysts for Oxygen Reduction Reaction Yaoyao Deng,†, § Hairong Xue,‡, § Shuanglong Lu,† Yujie Song,† Xueqin Cao,† Liang Wang,‡ Hongjing Wang,*,‡ Youliang Zhao,*,† Hongwei Gu*,† †

Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China. ‡ College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, P. R. China. § Yaoyao Deng and Hairong Xue equally contributed to the work. ABSTRACT: Porous Pt-based nanostructures are highly promising electrocatalysts for fuel cells, because of their high catalytic surface area and enough catalytically active sites. Herein we adopt a facile method in aqueous solution to synthesize Au@PtPd mesoporous nanorod (Au@PtPd MNR) in which Au nanorod (NR) serves as core and mesoporous PtPd acts as shell. Owing to the mesoporous core-shell structure and composition effect, Au@PtPd MNR exhibits superior catalytic activity and durability for oxygen reduction reaction (ORR), as compared with PtPd mesoporous sphere (MS), Au@Pt MNR and commercial Pt/C catalyst. This synthetic strategy is appropriate for preparation of other mesoporous core-shell structures for various promising applications. KEYWORDS: core-shell structure; mesoporous nanoshell, tri-metallic Au@PtPd nanorods, electrocatalyst, oxygen reduction reaction

INTRODUCTION In recent years, fuel cells have received extensive attention, owing to the environment-friendly and renewable energy they can provide without using fossil fuels. The oxygen reduction reaction (ORR) has gained paramount importance, because of its significant role in fuel cell applications.1 The strong O=O bond makes ORR difficult due to its sluggish reduction kinetics.2-3 Various nanomaterials have been adopted as catalysts to enhance the ORR performance.4-12 It is well known that Pt is one of the most active electrocatalyst for ORR.13-14 However, it is necessary to increase the specific surface area and atomic utilization rate of Pt to enhance catalytic activity.1522 Besides, the high cost and scarcity of Pt are key obstacles for its broad application in fuel cells. To reduce the cost and enhance activity and stability of catalysts, Pt-based nanocrystals with various morphologies and compositions have been widely developed. Among them, core-shell nanomaterials are one of the highly active and promising catalysts for ORR in fuel cells.23-26 Core-shell nanomaterials have many advantages in terms of their stability and synergistic effect between compositions.27-29 Their complex electron interaction can change the surface electronic properties of core-shell nanomaterials. Therefore, core-shell nanomaterials usually exhibit high catalytic activity and stability, compared with mixtures of monometallic nanomaterials or alloyed counterparts. For instance, Pd@Pt core-shell concave decahedra shows enhanced catalytic activity and durability for ORR, profiting from the stress effect caused by lattice mismatch of Pt and Pd.30 Furthermore,

Au@Pt core-shell star-shaped decahedra exhibits excellent catalytic performance toward ORR, because lattice-matched interface between Au and Pt may cause strong electron interaction.31 It is noteworthy that the shell regions many previously reported are dense structures, which may be difficult for guest species to access the inner cores and devalue the advantage of core-shell structure. Considering the above issue, nanoporous shells are designed onto the metal-based cores to enhance the electrocatalytic performance of core-shell nanomaterials, which can provide sufficient accessible active sites.32-36 The lyotropic liquid crystalline method and templating method are two common methods for the synthesis of metal mesoporous nanomaterials.37-38 Yamauchi group has successfully synthesized a series of mesoporous bimetallic core-shell nanoparticles for methanol oxidation reaction, such as mesoporous Pd@Pt nanoparticles, Ag@Pt nanoparticles.39-41 Although bimetallic mesoporous nanoparticles have been extensively reported, tri-metallic mesoporous nanoparticles are relatively scarce and likely to become effective catalysts with enhanced activity and stability.42 Inspired by the description above, we propose a facile method for synthesis of tri-metallic Au@PtPd mesoporous nanorod (Au@PtPd MNR) at a high yield in aqueous solution, which is composed of Pt-rich shell and Au core (Scheme 1). The structure and composition of the nanorods can be readily tuned via simply controlling the metallic sources amounts. The newly designed Au@PtPd MNR exhibits superior activity and durability for ORR, owing to its mesoporous core-shell structure combined with its tri-metallic compositions.

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Scheme 1. Schematic illustration of the synthesis of Au@PtPd MNR.

EXPERIMENTAL SECTION Materials and chemicals. Hexachloroplatinic acid hexahydrate (H2PtCl6•6H2O), potassium tetrachloroplatinate (K2PtCl4), silver nitrate (AgNO3), chloroauric acid tetrahydrate (HAuCl4•4H2O), hexadecyl trimethyl ammonium bromide (CTAB), ethanol, and hydrochloric acid (HCl, 35%-36%) are purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium tetrachloropalladate (Na2PdCl4, 98%) and sodium oleate (NaOL) are purchased from Tokyo Chemical Industry Co., Ltd. Pluronic F127 (PEO100PPO65PEO100) is purchased from Shanghai Macklin Biochemical Co., Ltd. L-ascorbic acid (AA, 99%) is purchased from Alfa Aesar. Sodium borohydride (NaBH4) is purchased from J&K Scientific Ltd. Commercial Pt/C catalyst (20 wt% Pt) is ordered from Alfa Aesar. Ultrafiltered water used in all the reactions is obtained by using a Millipore purification system. All the chemicals are of analytical grade and used as received without further purification. Synthesis of gold nanorods (Au NRs). The synthesis of Au NRs is based on previously reported procedures with some modifications.43 Firstly, the seeds solution was prepared by adding HAuCl4 solution (5 mL, 0.5 mM) into CTAB solution (5 mL, 0.2 M), followed by the addition of ice-cold, freshly prepared NaBH4 (0.01 M, 0.60 mL) under vigorous shaking. The resulting seed solution was undisturbed at room temperature for at least 2 h before use. The growth solution was prepared by dissolving CTAB (7 g) and NaOL (1.234 g) into warm water (250 mL). Then AgNO3 solution (18 mL, 4 mM) was added. After keeping undisturbed for 15 min, HAuCl4 solution (250 mL, 1 mM) was added. After 90-min stirring, the colour of solution became colourless and HCl solution (2.1 mL) was added. After 15-min stirring, AA (1.25 mL, 64 mM) was added and the solution was vigorously stirred for 30 s. Finally, 0.8 mL of seed solution was added into the growth solution. The reaction solution was mixed by gentle inversion for 10 s and then left undisturbed overnight at room temperature. Au NRs were purified using centrifugation to remove CTAB and NaOL with water several times. Au NRs were collected and dispersed into water for further experiments. Synthesis of Au@PtPd MNRs. 0.6 mL of Na2PdCl4 solution (20.0 mM), 0.72 mL of K2PtCl4 solution (20.0 mM), 1.08 mL of H2PtCl6 solution (20.0 mM), 60.0 µL HCl solution (6.0 M), and 60.0 mg of Pluronic F127 were mixed together. After

F127 was completely dissolved, the as-prepared Au NRs and 3.0 mL AA solution (0.1 M) was added to the above solution. Then, the mixed solution was continuously sonicated in a water bath for 4 h. Finally, the sample was collected by centrifugation at 7,000 rpm for 7 min three times with ethanol and water. Materials Characterization. The morphologies of the Au@PtPd MNRs were examined through scanning electron microscopy (SEM) (Hitachi S-4700), transmission electron microscopy (TEM) (TecnaiG20, FEI, American) equipped with a Gatan CCD794 camera operated at 200 kV. The highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), elemental mapping, highresolution TEM (HRTEM) and energy dispersive X-ray (EDX) of a single nanorod were carried out on a Tecani F20 instrument at an accelerating voltage of 200 kV. Wide angle powder X-ray diffraction (XRD) patterns were tested on an X'Pert-Pro MPD diffractometer (Netherlands PANalytical). ICP-OES (Inductively coupled plasma optical emission spectrometry) was analyzed by using a Thermo Scientific iCAP6300 (Thermo Fisher Scientific, US). Electrochemical investigations. A 5 µL of ink (2 mg mL-1) of catalyst was droped on the glassy carbon (GC) electrode surface and then dried. The formed catalyst film was further dropped with 5 µL of Nafion (0.5 %). After drying, the catalyst electrode was obtained. Cyclic voltammetry (CV), rotating disk electrode (RDE), rotating ring-disk electrode (RRDE) and chronoamperometry measurements were performed on a CHI 760D electrochemical analyzer under a three-electrode system, in which the as-prepared catalyst electrode as the working electrode, Ag/AgCl electrode as a reference electrode, and Pt wire electrode as a counter electrode. CV measurement was tested by using a scan rate of 50 mV s-1 in the N2- and O2-saturated 0.1 M HClO4 electrolyte. RDE and RRDE measurements for linear sweep voltammograms (LSVs) were performed in an O2-saturated electrolyte at a scan rate of 5 mV s-1.

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RESULTS AND DISCUSSION The proposed synthetic method is performed by a seeded growth strategy. The Au NRs are firstly synthesized at a high yield and with uniform shape and size (Figure 1a) because of using a binary surfactant mixture composed of CTAB and NaOL.43-44 The diameter and length of the Au NRs are about 20 nm and 82 nm, respectively (Figure 1b). Then, the Au NRs are used as seeds to synthesize the Au@PtPd MNRs in the subsequent step. The yield of the as-prepared Au@PtPd MNRs is nearly 100%, and both of their shape and particle size (with an average diameter of 106 nm) are very uniform (Figure 2a). It can be distinctly observed that spatially and locally separated mesopores distribute over entire outer surface of NRs, whose pore size is about 10 nm (Figure 2b). Based on the N2 adsorption-desorption isotherms and diagram of pore size distribution, the BET surface area of the Au@PtPd MNRs is about 13.4 m2 g-1 (Figure S1a), and their average mesoporous size is around 7 nm (Figure S1b). As observed, the thickness of mesoporous layer within the Au@PtPd MNRs is around 40 nm, in which the mesoporous walls are composed of the abundant connected nanoparticles (Figure 2c). Selected-area electron diffraction (SAED) pattern of NRs show several intense spots, which are indexed to a typical metallic face-centered cubic (fcc) structure, indicating polycrystalline structure (Figure 2d). It is noted that the rugged NRs surface can offer enough active sites and enhance the catalytic activity. As shown in Figure 2e, the obvious lattice fringes suggest a high crystallinity of PtPd nanoparticles. In the Fourier filtered image, the measured lattice distance is 0.22 nm, which is attributable to the fcc (111) plane of a PtPd alloy (Figure 2f).

mono-metallic Au and bi-metallic PtPd alloy (Figure S2a). Furthermore, the XPS results confirm the metallic states of Au, Pd and Pt in Au@PtPd MNRs (Figure S2b-d).

Figure 2. (a and b) SEM, (c) TEM, (d) SAED and (e) HRTEM images of the Au@PtPd MNRs. (f) The lattice fringes in the square area in (e). The inset in (f) displays the corresponding FFT pattern.

Figure 1. (a) Typical TEM image of Au NRs, and (b) highly magnified TEM image of a single Au NR.

In order to explore the distribution of Pd and Pt, element mapping was used to investigate the individual Au@PtPd MNR. The HAADF-STEM and corresponding element mapping images indicate that most of Pd coats on Au NR surface, whereas Pt is deposited on the outside of the Au@PtPd MNR (Figure 3a-e), suggesting that Pd precursor is more preferentially reduced than Pt precursor.45 Moreover, the line-scanning profile of the Au@PtPd MNR further reveals the elemental composition of Au@PtPd MNR, which is in agreement with the elemental mapping results (Figure 3f). The mass ratio of Au:Pt:Pd is 24:40:36, confirmed by ICP-OES analysis (Table S1). These results indicate that the mesoporous tri-metallic Au@PtPd MNR is successfully fabricated, whose Pt-rich surface is hopeful to be an effective catalyst. The XRD pattern of the as-prepared Au@PtPd MNRs shows two sets of diffraction peaks, which is assigned to fcc crystal structure of

Figure 3. (a) HAADF-STEM image, (b-e) EDX elemental mapping images, and (f) compositional line profiles of a single Au@PtPd MNR.

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To better understand the possible fabrication process of Au@PtPd MNRs, the time-dependent growth process is monitored by the TEM (Figure 4). During the initial 15 min, some grain nanoparticles with an average diameter of about 6 nm deposit on the surface of Au NRs (Figure 4a). Then, more grain nanoparticles attach to the outer surface of NRs within 30 min (Figure 4b). As the reaction time increased to 60 min, more and more nanoparticles assemble on the NRs (Figure 4c). Finally, the Au@PtPd MNRs are formed after 120 min (Figure 4d). Based on the above results and element mapping analysis, the formation process of Au@PtPd MNRs can be interpreted as follows. Pd precursor is preferentially reduced to grain nanoparticles deposited on the surface of Au NRs, and then Pt precursor is reduced to nanoparticles attached on the surface of nanorods. The presence of HCl slows down the reduction reaction process because HCl can decrease reduction capacity of AA.46-47 When the reaction was carried out in the absence of HCl, many dissociative nanoparticles were obtained, owing to the fast reduction kinetics (Figure S3). During the formation process, the F127 with a high concentration can form micelles in the reaction mixture. The exteriors of these F127 micelles with hydrophilic ethylene oxide group readily adsorb metalaqua complexes, which are formed by the metal ion (Pt and Pd) species and water. As a template, the F127 micelle efficiently results in the formation of the mesoporous PtPd layer on the surface of Au NRs.45 When the reaction is carried out in the absence of F127, the structure of Au@PtPd MNR is not uniform that nonporous bulk aggregations are formed (Figure S4). These results demonstrate that F127 plays important roles in the preparation of Au@PtPd MNR.

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precursor (Pt:Pd = 5:1), the mesoporous Au@PtPd NRs similar to typical Au@PtPd MNRs are formed, as evidenced from SEM and TEM images (Figure S5a and Figure S5b). However, we can observe the presence of mesoporous spheres. The mass ratio of Au:Pt:Pd is determined to be 14:66:20 by the ICP-OES (Table S1), named Au14@Pt66Pd20 MNRs. With equimolar usage of Pt precursor and Pd precursor (Pt:Pd = 1:1), the mass ratio of Au:Pt:Pd is 27:30:43 (Table S1), named Au27@Pt30Pd43 MNRs. The average diameter is about 43 nm and the length is about 108 nm (Figure S5c and Figure S5d), nevertheless, their mesoporous structure is not distinct. Hence, the optimized metallic precursor amounts in the typical synthesis are suitable for high-quality preparation of the Au@PtPd MNRs. Pt-based bimetallic nanocrystals are prepared under the identical synthetic procedure. The synthesis of mesoporous PtPd spheres is consistent with the synthesis process of Au@PtPd MNR except for Au NRs. All the PtPd spheres (~185 nm in diameter) possess mesoporous structure over the entire spheres surface (Figures 5a and Figure 5b). Similarly, mesoporous Au@Pt MNR is synthesized without Na2PdCl4. For as-synthesized Au@Pt MNRs, their mesoporous structure can be observed over the entire area of the nanorods surface (Figure 5c). The average diameter and length of the Au@Pt MNRs are about 65 nm and 115 nm, respectively (Figure 5d).

Figure 5. (a) SEM and (b) TEM images of the PdPt MS. (c) SEM and (d) TEM images of Au@Pt MNRs.

Figure 4. TEM images of samples at different stages of the synthetic process of Au@PtPd MNRs: (a) 15 min, (b) 30 min, (c) 60 min, (d) 120 min.

Several control experiments are performed to optimize the synthetic parameters of the Au@PtPd MNRs. The composition of the Au@PtPd NRs is tuned by changing the amounts of K2PtCl4 and H2PtCl6. With high usage of Pt

In general, many core-shell nanoparticles are extensively studied and show excellent electrocatalytic activity, in which nanoshells are mostly dense structure and harsh reaction conditions are highly required. For instance, core-shell Au@Pd nanoparticles were synthesized via a three-step thermal treatment at 150 oC in oleylamine.28 Monodisperse Cu@PtCu nanocrystals were prepared at 170 oC for 3 h.48 In contrast, we adopt a facile method with mild condition to synthesize core-shell nanorods with mesoporous shells and high yield, which can provide high surface area and enough


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ACS Applied Energy Materials catalytically active sites. Meanwhile, the Au@PtPd MNR with well-defined interface may exhibit excellent catalytic properties induced by complex electron interaction as well as the lattice strain effects.49 Besides, the introduction of Au into noble-metal nanocrystals can enhance the electrocatalytic stability of catalysts.50-51 The Au NRs also possess enough catalytically active sites on the highly curved nanorods surfaces, because they are enclosed by multifaceted surfaces composed of various types of high-index facets.52-53 Hence, Au@PtPd MNR with tri-metallic composition and mesoporous core-shell structure is hopeful to become an effective electrocatalyst.

Au@Pt MNR (5.0 mA cm-2), respectively, and comparable to that of Pt/C (5.5 mA cm-2). The above results suggest the high catalytic activity for ORR of the Au@PtPd MNR. For comparsion, the Au@PtPd MNR shows the better Eonset and E1/2 of (1.01 and 0.92 V) in comparison to those of previously reported Pt-based catalysts (Table S2). Moreover, the high specific activity of the Au@PtPd MNR (0.78 mA cm-2) can be observed, which is higher than those of the PtPd MS (0.39 mA cm-2) and Au@Pt MNR (0.43 mA cm-2), and Pt/C (0.21 mA cm-2), respectively (Figure S7). Tafel plots are carried out to analyze the ORR kinetics of the samples. The Tafel slopes for the Au@PtPd MNR and Au@Pt MNR are similar to that for Pt/C (64 mV dec-1), respectively, and are smaller than that of PtPd MS (Figure 6d). The lower Tafel slope indicates that the rate determining step for ORR is the first electron transfer. For the low Tafel slope, the LSV curve of the sample shows the slowly increased ORR overpotential with the increasing current density (Figure 6b), which implys the easily adsorbed and activated oxygen on the sample surface.

Figure 6. (a) CVs, (b) ORR polarization curves, (c) the comparisons of the Eonset and E1/2, and (d) Tafel slopes of different catalysts. All catalysts measured in an O2-saturated 0.1 M HClO4 electrolyte.

Owing to its unique mesoporous core-shell structure and trimetallic compositions, the Au@PtPd MNR is tested as a catalyst for ORR. As comparison, PtPd MS, Au@Pt MNR and commercial Pt/C catalyst are used as benchmark catalysts. According to the CV curves of the samples in N2-saturated 0.1 M HClO4 solution, the Au@PtPd MNR shows the highest electrochemical active surface area (ECSA) of 31.4 m2 g-1, which is 1.6 and 1.3 times of those of the PtPd MS (19.2 m2 g1 ) and Au@Pt MNR(24.3 m2 g-1), and is close to commercial Pt/C catalyst (43.1 m2 g-1) (Figure S6). As a reference, the ECSA of Au@PtPd MNR (31.4 m2 g-1) is also higher than previously reported PtPd nanoflowers (18.56 m2 g-1), Pt nanoparticles (27 m2 g-1), PtCuCo ternary nanoalloys (22.99 m2 g-1), and PtCo nanomyriapods (24.49 m2 g-1). After injecting O2 into the electrolyte solution, all of the samples possess the distinct cathodic peaks in the CV curves, indicating a substantially electrocatalytic ORR process on the samples (Figure 6a). The LSV testing results demonstrate that the Au@PtPd MNR achieves the most positive onset potential (Eonset) (1.01 V) and half wave potential (E1/2) (0.92 V) for ORR process, as compared with those of the PtPd MS (0.97 and 0.89 V, respectively), Au@Pt MNR (0.98 and 0.90 V, respectively) and Pt/C (0.96 and 0.88 V, respectively) (Figure 6b and Figure 6c). It can be seen that the Au@PtPd MNR shows a limiting current density of 5.4 mA cm-2, which is much higher than those of the the PtPd MS (4.6 mA cm-2) and

Figure 7. (a) ORR polarization curves with different RDE rotation rates, and the insets show the electron transfer numbers at different potentials. (b) The electron transfer numbers of the Au@PtPd MNR and commercial Pt/C. (c) RRDE test of ORR. (d) Peroxide percentage and electron transfer numbers obtained by RRDE. (e) LSVs before and after durability test and (f) i-t response. All catalysts measured in an O2-saturated 0.1 M HClO4 electrolyte.

To further explore the ORR kinetics of the Au@PtPd MNR, its LSV curves were tested at different rotation rates (Figure 7a). With the increase of the rotation rate, the rapidly increased limiting current densities of the Au@PtPd MNR can be found. Based on Koutecky-Levich equation, the K-L plots for the Au@PtPd MNR show the good linear relation at the various potentials (0.5, 0.6, 0.7, and 0.8 V), indicating the first order reaction kinetics for ORR. At 0.5, 0.6, 0.7, and 0.8 V,



the transferred electron number (n) of the Au@PtPd MNR are calculated to be 3.91, 3.94, 3.95 and 3.98, respectively, which are better than those of the Pt/C (3.92, 3.93, 3.93 and 3.92) at these potentials, suggesting a one-step four-electron pathway for ORR (Figure 7b and Figure S8). The four-electron selectivity of the Au@PtPd MNR was further revealed by using the RRDE measurement. It is noted that the Au@PtPd MNR has a negligible ring current (IR) (the oxidation of H2O2), as compared to its disc current (ID), indicating its greatly restrained H2O2 evolution during the ORR process (Figure 7c). Using the values of IR and ID, the H2O2 yield and n of the Au@PtPd MNR are calculated to be under 1.1% and above 3.96 from 0.6 to 0.1 V (Figure 7d), which are much lower and higher than those of 3.9% and 3.92 of the Pt/C, respectively. The above results further confirm the high four-electron selectivity of the Au@PtPd MNR, evidenced by its low H2O2 yield together with near optimal four-electron pathway. The stability is an important evaluation criterion for the ORR electrocatalysts, and an accelerated durability test is used to evaluate the stability. After testing for 5000 cycles, the Eonset of the Au@PtPd MNR shows a very minor change, and its limiting current density also has a negligible loss on the LSV curve, as compared with the initial LSV curve (Figure 7e). Notably, the Au@PtPd MNR shows a much lower degradation for E1/2 (12 mV) relative to that of the Pt/C (28 mV), indicating its excellent stability during the ORR process (Figure S9). We also carried out chronoamperometric measurement to further test the durability of the Au@PtPd MNR. After testing for 5 h at 0.6 V, the decay of the initial catalytic current density for the Au@PtPd MNR is 7.9%, which is much smaller than that for Pt/C (39.8%), showing its superior long-term durability (Figure 7f).

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catalysts, the Gibbs free energy for ORR can be decreased by the alloying contribution between Pt and Pd with unsaturated and fully occupied d-orbitals, respectively, leading to the enhanced ORR kinetics because of the coupling effect between the d-orbitals of Pt and Pd. Moreover, the composition effect can observably facilitate the dissociation of adsorbed O2, and thus reducing the polarization for the electroreduction of the dissociated O atoms. Besides, a combination between Au core and bi-metallic PtPd shell can change the electronic structures through their strong coupling, obtaining an attractive ORR catalyst. Therefore, the Au@PtPd MNR shows the excellent catalytic performance for ORR, benefiting from the above superiorities.

CONCLUSIONS In conclusion, we successfully synthesize Au@PtPd MNR in which Au nanorod serves as core and mesoporous PtPd acts as shell. The size and morphology of nanorods can be tuned by adjusting reactant concentrations and reaction conditions. Benefiting from its mesoporous core-shell structure with trimetallic compositions, the Au@PtPd MNR shows superior catalytic activity and stability for ORR. The proposed method is feasible for synthesis of Pt-based catalysts with mesoporous core-shell structures for various catalytic applications.

ASSOCIATED CONTENT Supporting Information. Additional characterization data and electrocatalytic performance data. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *[email protected]; [email protected]; [email protected]

ORCID Hongjing Wang: 0000-0003-0641-3909 Youliang Zhao: 0000-0002-4362-6244 Hongwei Gu: 0000-0001-9962-4662

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Figure 8. Schematic catalytic process for ORR on the Au@PtPd MNR.

The Au@PtPd MNR has the enhanced ORR performance, which can be mainly ascribed to its the Au core, Pt-rich shell and mesoporous structure together with bi-metallic compositions (Figure 8). Due to the presence of the Au core, the PtPd shell can be firmly grown on the Au core, which can reduce the potentially unfavorable degradation and dissolution of Pt-based electrocatalysts, resulting in excellent long-term stability. The mesoporous structure with high specific surface area offers sufficiently assessible catalytic sites and then achieve a high ORR catalytic activity. In addition, the selfsupported porous structure is beneficial to a high-efficiency transport and diffusion of reactants (i.e. H+ and O2) relative to the supported non-porous catalysts. For the bi-metallic ORR

This work was supported by National Natural Science Foundation of China (No. 21373006, 21601154, 21776255) and the Science and Technology Program of Suzhou (SYG201732). The project was funded by the Priority Program Development of Jiangsu Higher Education Institutions (PAPD). We cordially thank Jun Guo (Analysis and Testing Centre, Soochow University) for the assistance of TEM measurements.

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Tri-metallic Au@PtPd Mesoporous Nanorods as Efficient

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