Nanoparticle on CoO Nanosheets ... - ACS Publications

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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Platinum Cluster/Nanoparticle on CoO Nanosheets with Coupled Atomic Structure and High Electrocatalytic Durability H.-J. Qiu,*,†,‡,⊥ J. J. Gao,†,⊥ Y. R. Wen,*,§ B. Shang,† J. Q. Wang,*,∥ Xi Lin,*,‡ and Y. Wang*,† †

School of Chemistry and Chemical Engineering, Congqing University, Chongqing 400044, China School of Materials Science and Engineering, Harbin Institute of Technology, Shenzhen 518055, China § Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ∥ Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China ‡

S Supporting Information *

ABSTRACT: Seeking electrocatalysts with a low Pt content, high activity, and durability in oxygen reduction reaction (ORR) remains one major challenge for fuel cell applications. Herein, by chemical dealloying of AlCoPt ternary alloys, CoO nanosheets with evenly doped ultrasmall Pt clusters/nanoparticles have been obtained. Thanks to the coherent atomic structure between CoO and Pt, the Ptdoped CoO (Pt/CoO) can work stably for ORR in 0.1 M HClO4 after tens of thousands of electrochemical cycles and exhibits a 13.5-fold enhancement in the electrochemically active surface area normalized ORR activity over that of commercial Pt/C catalyst. KEYWORDS: Pt clusters, dealloying, epitaxial growth, fuel cells, oxygen reduction reaction

A

solutions since metal oxides are generally unstable in acidic solutions.22 Therefore, developing a low-Pt Pt/metal oxide catalyst which can work stably in acidic solutions with protonexchange membranes is very challenging. The relatively low stability of metal oxide supported Pt should be due to the fact that only a small part of Pt contacts with the substrate, and their interaction is mainly based on physical or electronic adsorption. Thus, building chemical bonds between Pt and the substrate and increasing the contact surface should remarkably enhance the stability of Pt. In this work, we introduce a self-growing process to prepare Pt clusters/NPs on Co oxide (Pt/CoO) nanosheets by dealloying a designed AlCoPt alloy in alkaline solution. The as-prepared Pt/CoO demonstrates much enhanced ORR catalytic activity and selectivity and excellent long-term stability compared with state-of-the-art Pt/C catalyst. More importantly, due to the strong Pt−CoO interaction, Pt/CoO can perform stably in both acid and alkaline solutions. First, we examine the precursor alloys by energy-dispersive X-ray spectrometry (EDS), which shows that the atomic ratio of three precursors is in good agreement with the feed ratio of each element (Figure S1 in the Supporting Information). XRD patterns of the three precursor alloys are nearly identical, indicating that the presence of a small percentage of Pt has little effect on the phases of these precursor alloys. Most of these peaks can be ascribed to fcc Al and AlxCo phases (Figure S2a).

lthough it has been well recognized as an environmentally friendly, sustainable, and efficient energy source, the proton-exchange membrane fuel cells (PEMFC) suffer greatly from sluggish oxygen reduction reaction (ORR) on the cathode side.1−6 Traditionally, the Pt-based heterogeneous catalysts have been widely used for promoting ORR despite their prohibitive price and poor stability.7 As an alternative to Ptbased catalysts, nonprecious transition metal oxides, phosphide, and N-doped carbon have also demonstrated reasonable ORR activities and attractive resistance toward methanol oxidation.8,9 However, these non-Pt catalysts were unstable in acidic solutions, as required in the ORR in PEMFC. These technical pros and cons have motivated new strategies to lower the effective Pt contents in the ORR, such as fabricating single layer Pt on a different nanoscrystal surface by Cu underpotential deposition followed by galvanic reaction with Pt,10 alloying Pt with transition metals including Fe,11 Co,12 Ni,13 and Cu,14 by electrodeposition, and solvothermal synthesis, etc. Recently, it has been demonstrated that the existence of strong metal support interactions between the Pt nanoparticles (NPs) and metal oxide surfaces were responsible for high electrocatalytic activity15 and that further decreasing of the size of Pt NPs down to Pt clusters or single atoms can enhance the utilization efficiency and catalytic activity of noble metals.16−18 One problem occurring frequently in these Pt and metal oxide composite systems was that the Pt nanoparticles tended to aggregate in repeated electrocatalytic cycles and eventually detached from the substrates of CeO2,19 Ta2O5,20 ZrO2,21 CoO,22 and WO3,23 etc. On the other hand, some of these Pt and metal oxide composites can only work in alkaline or neutral © XXXX American Chemical Society

Received: February 21, 2018 Accepted: May 1, 2018 Published: May 1, 2018 A

DOI: 10.1021/acsaem.8b00263 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

Figure 1. SEM (low magnification (a) and high magnification (b)), HRTEM (c), and HAADF-STEM (d) images of the dealloyed Pt/CoO from Al85Co13Pt0.5; (e) corresponding FFT image. Simulated diffraction patterns of CoO (f) and Pt (g). HAADF-STEM image and the corresponding EDS elemental mappings of the dealloyed nanoporous Pt/CoO (h). Atomic models of CoO and Pt (i). HAADF-STEM images of electrochemical activated Pt/CoO (j) and Pt/CoO after stability test (k and l).

The dealloying would selectively remove Al, while Co were oxidized into oxides or hydroxides due to the active property of Co. The inert Pt were left uniformly in or on the Co oxide due to low diffusivity of Pt. As shown in Figure S2b, the disappearance of previous diffraction peaks demonstrates that the precursor alloys have been completely dealloyed. The formed weak diffraction peak around 45° is assigned to (200) of CoO. The peak is weak and broad, indicating the formation of nanoscale metal oxides. No peaks from metal Pt phase are observed, suggesting that the small amount of Pt is uniformly distributed in the dealloyed sample without any large Pt crystals formed, namely, the inert Pt left in/on the CoO lattice mainly in the form of clusters or isolated single atoms. The doping of Pt atoms also leads to a slight shift of the diffraction peak to the left since Pt atom has a bigger size than Co. Scanning electronic microscope (SEM) images in Figure 1a,b indicate that the dealloyed sample shows a hierarchical morphology composed of both big pores/channels and small nanopores. The formation of big pores should be due to the removal of pure Al phases. The small nanopores are formed

due to the self-growth of interconnected CoO thin nanosheets (Figure 1b). As shown in the high-resolution transmission electron microscopy (HRTEM) image (Figure 1c), since elements with high atomic numbers normally have a sharp contrast, darker Pt cluster/NPs with a size of ∼1−2 nm are uniformly distributed in the formed metal oxide. The parallel diffraction pattern in the HRTEM image reveals the coherent relationship between the Pt clusters and Co oxides. A highangle annular dark field (HAADF) STEM image further shows that bright Pt cluster/NPs with coherent lattice distribute on the metal oxides (Figure 1d and Figure S3) and some single bright Pt atoms can also be frequently observed (Figure S3a). The intensity profiles of Pt lattice show almost uniform contrast, indicating the Pt are island-like structures not spherical particles (Figure S3b). The formation of a Pt island structure is probably via a manner of epitaxial growth during the co-growth of Pt and CoO nanosheets. The measured lattice distance can be ascribed to the (200) plane of CoO and the (111) plane of Pt (Figure 1d). Theoretical simulation was used to further illustrate the atomic structure of the Co oxide and Pt. B


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Figure 2. XPS spectra of scan (a), Co 2p (b), Pt 4f (c), and O 1s (d) for just-dealloyed Pt/CoO and Pt/CoO after stability test.

Explicitly, the theoretical diffraction data from CoO (Fm3m ̅ ) matches well with the experimental FFT data, and the calculated L/H value of 1.413 aligned closely with the experimental L/H value of 1.404 (Figure 1e,f). The simulated diffraction pattern and crystal lattice of Pt are quite similar to those of CoO as shown in Figure 1f,g,i. They are both facecentered cubic structures. The lattice constant of Pt (3.92 A) is slightly smaller than that of CoO (4.24 A), which may cause residual strain effects between the Pt and the CoO matrix. The STEM-EDS elemental mapping analysis shows that the material is mainly composed of Co, O, and Pt (Figure 1h). Co and O are uniformly disturbed across the entire sample. The Pt islands locating in the CoO matrix are also confirmed. The N2 adsorption−desorption isotherms suggest that there are mesopores in the Pt/CoO (Figure S4), and the Brunauer− Emmett−Teller (BET) surface area of the dealloyed sample is ∼43.4 m2 g−1. Since nanopores cannot be clearly observed on the as-prepared Pt/CoO nanosheets by TEM, the detected nanopores by BET method may be pores between these nanosheets. It is worth mentioning that the dealloyed samples from the three precursors show similar morphology. In Figure 2a, the X-ray photoelectron spectroscopy (XPS) spectrum indicates the as-prepared Pt/CoO contains cobalt, oxygen, and platinum (Figure 2a). The high-resolution Co 2p XPS spectra were plotted in Figure 2b. For the Pt/CoO samples, the binding energies of 780.8 eV for 2p3/2 and 796.8 eV for 2p1/2 with satellite peaks at 786.1 and 802.7 eV indicate the formation of CoO.22 For the CoO sample, the satellite peaks almost disappear, and the peak of 2p1/2 shifts to smaller binding energy, suggesting the transformation of CoO into Co3O424 when Pt is absent. This result also suggests that the incorporation of Pt may stabilize CoO to some extent. Two Pt 4f peaks in Figure 2c can be assigned to Pt 4f7/2 and Pt 4f5/2 of metallic Pt0, respectively. Compared with those of Pt/C and Pt3.3/CoO, the Pt binding energy peaks of Pt13.3/CoO shift more negatively, probably indicating a stronger residual strain effect in these Pt islands due to the slightly mismatched CoO

substrate lattice, a stronger electron transfer effect from CoO to Pt,22 or both. The O 1s spectrum in Figure 2d can mainly be ascribed to metal−OH (531.3 eV), metal−O (529.8 eV).25,26 The peak density shows that metal−OH dominates in the sample of the CoO/Co(OH)2 matrix. The Pt/CoO nanosheets with two different Pt doping amounts (Pt3.3/CoO and Pt13.3/CoO) were then evaluated as electrocatalysts for ORR in acidic aqueous solutions. The Pt/ CoO samples were first activated in 0.1 M HClO4 electrolyte (Figure S5). Then, the characteristic cyclic voltammograms (CV) curve of Pt is clearly observed (Figure 3a). Based on the hydrogen adsorption/desorption process, the calculated electrochemically active surface areas (ECSAs) are 61.5 m2 g−1 for Pt13.3/CoO and 80.4 m2 g−1 for Pt3.3/CoO. In Figure 3b, the onset/half-wave potentials of Pt13.3/CoO and Pt3.3/CoO exhibit a clearly positive shift compared with that of Pt/C, indicating the enhanced ORR activities. To further illustrate their catalytic activities, the specific surface and mass activities were shown in Figure 3c. The specific surface activities of Pt13.3/CoO and Pt3.3/CoO reach 3.24 and 2.85 mA cm−2 at 0.90 V, 1 order of magnitude higher than that of Pt/C catalyst of 0.24 mA cm−2. Similarly, owing to the low Pt doping content, Pt13.3/CoO and Pt3.3/CoO have much higher mass activities, 377.2 and 588.3 mA mgPt−1, respectively. Figure 3d shows a group of polarization curves of Pt13.3/CoO for ORR at different rotation rates. The corresponding Koutecky−Levich plots were plotted in Figure 3e. The good linearity and parallelism of these plots indicate the underlying first-order kinetics with respect to O2 in the ORR. From the slope of the Koutecky−Levich plot, the transferred electron number (n) is calculated to be 3.93 for Pt13.3/CoO at 0.65− 0.85 V, suggesting an apparent 4e− oxygen reduction pathway. To evaluate the methanol tolerance level, the change of oxygen reduction current at a constant potential of 0.5 V with respect to the addition of 5 mL of methanol was recorded (Figure 3f). It is observed that the current drops sharply by half in the Pt/C case. In contrast, there were little reduction current variations C


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Figure 3. CV curves in N2-purged 0.1 M HClO4 solution (a) and ORR polarization curves in O2-saturated 0.1 M HClO4 solution (b) for the Pt13.3/ CoO, Pt3.3/CoO, and commercial Pt/C catalysts. Specific activity and mass activity of Pt13.3/CoO, Pt3.3/CoO, and commercial Pt/C catalysts at 0.90 V (c). Polarization curves for ORR on the Pt13.3/CoO catalyst at various rotation rates (d), and Koutecky−Levich plots at different potentials: 0.65, 0.70, 0.75, 0.80, and 0.85 V (e). Chronoamperometric response of Pt13.3/CoO and commercial Pt/C catalyst upon addition of 5.0 M methanol at 0.50 V (f).

may be due to some slight structure change between Pt and CoO (explained later by TEM and STEM). The O 1s spectrum shows the increase of Co−O bond (529.6 eV) and the decrease of Co−OH bond (531.5), suggesting the loss of water and transformation from Co(OH)2 to Co oxides. The peaks at 530.2 and 530.7 eV can be ascribed to highly oxidative oxygen species (O22−/O−) and CO from the added carbon, respectively.27 During the transformation from Co(OH)2 to CoO/Co3O4, nanopores form in the oxide matrix (Figure S7d,e), which would compensate for the lattice vacancy created by the loss of water molecules. The formed nanoporous CoO/ Co3O4 matrix by electrochemical cycling (activation) is also confirmed by STEM characterization (Figure 1j), and it is quite stable after even 20000 cycles (Figures 1k and S7e). Although significant structure change occurred on the CoO matrix, the Pt islands still connected well with the matrix and, more importantly, no obvious Pt coarsening can be observed (Figure 1k). Some small Pt clusters can still be oberved (Figures 1l and S7f). In sharp contrast, electrochemical cycling caused Pt NPs coarsening has been observed from both commercial Pt/C and nanoporous Pt. On the other hand, no Co ions were detected in the electrolyte after the durability test, indicating the

for the Pt13.3/CoO electrode, a clear sign of excellent methanol tolerance of the Pt/CoO catalyst. We then evaluated the stabilities of Pt/CoO by continuous CV cycling from 0.6 to 1.1 V in O2 saturated 0.1 M HClO4. As shown in Figure S6, the specific surface and mass activities of Pt13.3/CoO dropped by only ∼8.9% and ∼12.0%, respectively (Figure S6b). For the Pt3.3/CoO, the polarization curve exhibits a slight degradation of 18 mV in its half-wave potential (Figure S6c) and ∼18.0% loss in the mass activity (Figure S6d). Under the same condition, however, the commercial Pt/C showed a clear negative shift in ORR polarization curves (Figure S6e) with 60.7% loss of its mass activity (Figure S6f). These results demonstrate the enhanced stability of the Pt/CoO catalyst. To probe the structure change after electrochemical cycling, the sample was analyzed by XPS, TEM, and STEM. After electrochemical stability test, XPS scan in Figure 2a shows two new peaks of C and F, which are due to the addition of carbon powder and Nafion for the catalyst ink preparation. The evidently decreased satellite peak intensity at ∼787 eV and the shift of 2p1/2 peak to smaller binding energy suggest some Co2+ change to Co3+ in the form of CoOOH or Co3O4 (Figure 2b).24 Pt 4f peaks shift slightly to the right (Figure 2c), which D


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Figure 4. ORR polarization curves of CoO, Pt13.3/CoO, Pt3.3/CoO, and commercial Pt/C catalyst in O2-saturated 0.1 M KOH solution (a). ORR polarization curves of Pt13.3/CoO (b), Pt3.3/CoO (c), and Pt/C catalysts (d) before and after different potential cycles.

dissolution of CoOx in the Pt/CoOx was negligible and the Pt/ Co ratio should be stable. Last, we evaluated the ORR activities of Pt/CoO in O2saturated alkaline solutions. Figure 4a shows the polarization curves of four catalysts. Although CoO is also active for ORR,28,29 it cannot compete with the Pt-containing catalysts. The half-wave potential of Pt13.3/CoO was 0.908 V, which is clearly higher than that of Pt3.3/CoO (0.875 V) and that of Pt/ C catalyst (0.860 V), suggesting the higher ORR activity of the Pt13.3/CoO. Even with only 3.3 at. % Pt doping, its half-wave potential is still higher than Pt/C, suggesting the greatly enhanced catalytic activity of Pt/CoO. Panels b−d of Figure 4 show that the half-wave potentials of Pt13.3/CoO and Pt3.3/ CoO after 20000 cycles exhibit only 12 and 20 mV negative shifts, respectively. As a sharp contrast, the ORR polarization curve of Pt/C shows a 35 mV negative shift. This result indicates that the CoO matrix greatly stabilizes the Pt catalyst during the repeated testing conditions. According to first-principles calculations,30 the d-band center moves toward the Fermi level with decreasing Pt cluster size and the activity also decreases. This conclusion is in accordance with our experimental results that Pt13.3/CoO with more Pt exhibits a higher catalytic activity than Pt3.3/CoO. DFT calculation was used to calculate the d-band centers of singly dispersed Pt atom/CoO and Pt cluster/CoO (Figure S8). It shows that compared with the extreme condition (single Pt atom/CoO), the Pt cluster/CoO has a negatively shifted dband center, which is in good agreement with the result in ref 30 and also explains the higher catalytic activity of Pt13.3/CoO. In summary, ultrasmall Pt island-like clusters/NPs doped thin CoO nanosheets (Pt/CoO) with adjustable Pt content have been prepared by a dealloying strategy. The Pt islands show coherent atomic structure with the CoO matrix, and the strong interaction between them makes Pt/CoO quite stable. When used as a low-cost catalyst in both acidic and alkaline solutions, the Pt/CoO catalyst exhibits a much enhanced

performance for ORR compared with commercial Pt/C catalyst.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.8b00263. Experimental details, EDS, XRD, BET, and TEM, etc. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(H.-J.Q.) E-mail: [email protected]. *(Y.R.W.) E-mail: [email protected]. *(J.Q.W.) E-mail: [email protected]. *(X.L.) E-mail: [email protected]. *(Y.W.) E-mail: [email protected]. ORCID

H.-J. Qiu: 0000-0003-0396-1942 Y. Wang: 0000-0003-2883-1087 Author Contributions ⊥

H.-J.Q. and J.J.G. are co-first authors.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was sponsored by the National Natural Science Foundation of China (Grant Nos. 51702031, 51501085, 21373280, and 21403019), the Chongqing Basic and Frontier Research Project (Grant No. cstc2015jcyjA50026), the Thousand Young Talents Program of the Chinese Central Government (Grant No. 0220002102003), the Hundred Talents Program at Chongqing University (Grant No. E


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reduction catalyzed by gold nanoclusters supported on carbon nanosheets. Nanoscale 2016, 8, 6629−6635. (18) Tang, Z. H.; Wu, W.; Wang, K. Oxygen Reduction Reaction Catalyzed by Noble Metal Clusters. Catalysts 2018, 8 (2), 65. (19) Vayssilov, G. N.; Lykhach, Y.; Migani, A.; Staudt, T.; Petrova, G. P.; Tsud, N.; Skala, T.; Bruix, A.; Illas, F.; Prince, K. C.; Matolin, V.; Neyman, K. M.; Libuda, J. Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticles. Nat. Mater. 2011, 10, 310−315. (20) Sakamoto, H.; Ohara, T.; Yasumoto, N.; Shiraishi, Y.; Ichikawa, S.; Tanaka, S.; Hirai, T. Hot-Electron-Induced Highly Efficient O2 Activation by Pt Nanoparticles Supported on Ta2O5 Driven by Visible Light. J. Am. Chem. Soc. 2015, 137, 9324−9332. (21) Cheng, N. C.; Banis, M. N.; Liu, J.; Riese, A.; Li, X.; Li, R. Y.; Ye, S. Y.; Knights, S.; Sun, X. L. Extremely Stable Platinum Nanoparticles Encapsulated in a Zirconia Nanocage by Area-Selective Atomic Layer Deposition for the Oxygen Reduction Reaction. Adv. Mater. 2015, 27, 277−281. (22) Meng, C.; Ling, T.; Ma, T. Y.; Wang, H.; Hu, Z. P.; Zhou, Y.; Mao, J.; Du, X. W.; Jaroniec, M.; Qiao, S. Z. Atomically and Electronically Coupled Pt and CoO Hybrid Nanocatalysts for Enhanced Electrocatalytic Performance. Adv. Mater. 2017, 29, 1604607. (23) Liu, Y.; Shrestha, S.; Mustain, W. E. Synthesis of Nanosize Tungsten Oxide and Its Evaluation as an Electrocatalyst Support for Oxygen Reduction in Acid Media. ACS Catal. 2012, 2, 456−463. (24) Kosmala, T.; Calvillo, L.; Agnoli, S.; Granozzi, G. Enhancing the Oxygen Electroreduction Activity through Electron Tunnelling: CoOx Ultrathin Films on Pd(100). ACS Catal. 2018, 8, 2343−2352. (25) Si, C. H.; Wang, Y.; Zhang, J.; Gao, H.; Lv, L. F.; Han, L. L.; Zhang, Z. H. Highly Electrocatalytic Activity and Excellent Methanol Tolerance of Hexagonal Spinel-Type Mn2A1O4 Nanosheets towards Oxygen Reduction Reaction: Experiment and Density Functional Theory Calculation. Nano Energy 2016, 23, 105−113. (26) Li, C.; Han, X. P.; Cheng, F. Y.; Hu, Y. X.; Chen, C. C.; Chen, J. Phase and Composition Controllable Synthesis of Cobalt Manganese Spinel Nanoparticles towards Efficient Oxygen Electrocatalysis. Nat. Commun. 2015, 6, 7345. (27) Si, C. H.; Zhang, J.; Wang, Y.; Ma, W. S.; Gao, H.; Lv, L. F.; Zhang, Z. H. Nanoporous Platinum/(Mn,AI)3O4 Nanosheet Nanocomposites with Synergistically Enhanced Ultrahigh Oxygen Reduction Activity and Excellent Methanol Tolerance. ACS Appl. Mater. Interfaces 2017, 9, 2485−2494. (28) He, Q.; Li, Q.; Khene, S.; Ren, X.; López-Suárez, F. E.; LozanoCastelló, D.; Bueno-López, A.; Wu, G. High-Loading Cobalt Oxide Coupled with Nitrogen-Doped Graphene for Oxygen Reduction in Anion-Exchange-Membrane Alkaline Fuel Cells. J. Phys. Chem. C 2013, 117, 8697−8707. (29) Liang, Y.; Wang, H.; Diao, P.; Chang, W.; Hong, G.; Li, Y.; Gong, M.; Xie, L.; Zhou, J.; Wang, J.; Regier, T. Z.; Wei, F.; Dai, H. Oxygen Reduction Electrocatalyst Based on Strongly Coupled Cobalt Oxide Nanocrystals and Carbon Nanotubes. J. Am. Chem. Soc. 2012, 134, 15849−15857. (30) Toyoda, E.; Jinnouchi, R.; Hatanaka, T.; Morimoto, Y.; Mitsuhara, K.; Visikovskiy, A.; Kido, Y. The d-Band Structure of Pt Nanoclusters Correlated with the Catalytic Activity for an Oxygen Reduction Reaction. J. Phys. Chem. C 2011, 115, 21236−21240.

0903005203205), and The State Key Laboratory of Mechanical Transmissions Project (SKLMT-ZZKT-2017M11).

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REFERENCES

(1) Kuroki, H.; Tamaki, T.; Matsumoto, M.; Arao, M.; Takahashi, Y.; Imai, H.; Kitamoto, Y.; Yamaguchi, T. Refined Structural Analysis of Connected Platinum−Iron Nanoparticle Catalysts with Enhanced Oxygen Reduction Activity. ACS Appl. Energy Mater. 2018, 1, 324− 330. (2) Jia, Q. Y.; Li, J. K.; Caldwell, K.; Ramaker, D. E.; Ziegelbauer, J. M.; Kukreja, R. S.; Kongkanand, A.; Mukerjee, S. Circumventing Metal Dissolution Induced Degradation of Pt-Alloy Catalysts in Proton Exchange Membrane Fuel Cells: Revealing the Asymmetric Volcano Nature of Redox Catalysis. ACS Catal. 2016, 6, 928−938. (3) Dai, L. M.; Xue, Y. H.; Qu, L. T.; Choi, H. J.; Baek, J. B. MetalFree Catalysts for Oxygen Reduction Reaction. Chem. Rev. 2015, 115, 4823−4892. (4) Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction Reaction. Acc. Chem. Res. 2013, 46, 1878−1889. (5) Shao, M. H.; Chang, Q. W.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594−3657. (6) Sui, S.; Wang, X. Y.; Zhou, X. T.; Su, Y. H.; Riffat, S.; Liu, C. J. A Comprehensive Review of Pt Electrocatalysts for the Oxygen Reduction Reaction: Nanostructure, Activity, Mechanism and Carbon Support in PEM Fuel Cells. J. Mater. Chem. A 2017, 5, 1808−1825. (7) Zadick, A.; Dubau, L.; Sergent, N.; Berthome, G.; Chatenet, M. Huge Instability of Pt/C Catalysts in Alkaline Medium. ACS Catal. 2015, 5, 4819−4824. (8) Gebremariam, T. T.; Chen, F.; Wang, Q.; Wang, J.; Liu, Y.; Wang, X.; Qaseem, A. Bimetallic Mn−Co Oxide Nanoparticles Anchored on Carbon Nanofibers Wrapped in Nitrogen-Doped Carbon for Application in Zn−Air Batteries and Supercapacitors. ACS Appl. Energy Mater. 2018, 1, 1612−1625. (9) Li, L.; Yang, J.; Yang, H.; Zhang, L.; Shao, J.; Huang, W.; Liu, B.; Dong, X. Anchoring Mn3O4 Nanoparticles on Oxygen Functionalized Carbon Nanotubes as Bifunctional Catalyst for Rechargeable Zinc-Air Battery. ACS Appl. Energy Mater. 2018, 1, 963−969. (10) Koenigsmann, C.; Santulli, A. C.; Gong, K.; Vukmirovic, M. B.; Zhou, W.; Sutter, E.; Wong, S. S.; Adzic, R. R. Enhanced Electrocatalytic Performance of Processed, Ultrathin, Supported Pd− Pt Core−Shell Nanowire Catalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2011, 133, 9783−9795. (11) Guo, S.; Zhang, S.; Su, D.; Sun, S. Seed-Mediated Synthesis of Core/Shell FePtM/FePt (M = Pd, Au) Nanowires and Their Electrocatalysis for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2013, 135, 13879−13884. (12) Xia, B. Y.; Wu, H. B.; Li, N.; Yan, Y.; Lou, X. W.; Wang, X. OnePot Synthesis of Pt-Co Alloy Nanowire Assemblies with Tunable Composition and Enhanced Electrocatalytic Properties. Angew. Chem., Int. Ed. 2015, 54, 3797−3801. (13) Luo, L. X.; Zhu, F. J.; Tian, R. X.; Li, L.; Shen, S. Y.; Yan, X. H.; Zhang, J. L. Composition-Graded PdxNi1‑x Nanospheres with Pt Monolayer Shells as High-Performance Electrocatalysts for Oxygen Reduction Reaction. ACS Catal. 2017, 7, 5420−5430. (14) Qiu, H. J.; Shen, X.; Wang, J. Q.; Hirata, A.; Fujita, T.; Wang, Y.; Chen, M. W. Aligned Nanoporous Pt-Cu Bimetallic Microwires with High Catalytic Activity toward Methanol Electrooxidation. ACS Catal. 2015, 5, 3779−3785. (15) Kim, J. H.; Chang, S.; Kim, Y. T. Compressive Strain as the Main Origin of Enhanced Oxygen Reduction Reaction Activity for Pt Electrocatalysts on Chromium-doped Titania Support. Appl. Catal., B 2014, 158-159, 112−118. (16) Wang, L. K.; Tang, Z. H.; Yan, W.; Yang, H. Y.; Wang, Q. N.; Chen, S. W. Porous Carbon-Supported Gold Nanoparticles for Oxygen Reduction Reaction: Effects of Nanoparticle Size. ACS Appl. Mater. Interfaces 2016, 8, 20635−20641. (17) Wang, Q. A.; Wang, L. K.; Tang, Z. H.; Wang, F. C.; Yan, W.; Yang, H. Y.; Zhou, W. J.; Li, L. G.; Kang, X. W.; Chen, S. W. Oxygen F


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