Nanoporous PtFe Nanoparticles Supported on N-Doped Porous

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Nanoporous PtFe nanoparticles Supported on N-doped Porous Carbon Sheets derived from MOFs for highefficiency and durability oxygen reduction Reaction Kang Yang, Peng Jiang, Jitang Chen, and Qianwang Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09428 • Publication Date (Web): 25 Aug 2017 Downloaded from http://pubs.acs.org on August 26, 2017

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ACS Applied Materials & Interfaces

Nanoporous PtFe nanoparticles Supported on N-doped Porous Carbon Sheets derived from MOFs for high-efficiency and durability oxygen reduction Reaction ，

Kang Yang1, Peng Jiang 1, Jitang Chen1, and Qianwang Chen1 2* 1

Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science &

Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, University of Science and Technology of China, Hefei 230026, China 2

High Magnetic Field Laboratory, Hefei Institutes of Physical Science, Chinese Academy of Sciences,

Hefei 230031, China

ABSTRACT Designing and exploring catalysts with high activity and stability for oxygen reduction reaction (ORR) at the cathode in acidic environment are imperative for the industrialization of proton exchange membrane fuel cells (PEMFCs). Theoretical calculations and experiments have demonstrated alloying Pt with a transition metal can not only cut down the usage of scarce Pt metal but also improve performance of mass activity compared with pure Pt. Herein, we exhibit the preparation of nanoporous PtFe nanoparticles (np-PtFe NPs) supported on N-doped nanoporous carbon sheets (NPCSs) via facile in-situ thermolysis of Pt modified Fe-based metal-organic framework. The np-PtFe/NPCSs exhibit a more positive half-wave potential (0.92 V) compared with commercial Pt/C catalyst (0.883V). The nanoporous structure allows our catalyst to possess a great mass activity, which is found to be 0.533 A mgPt-1 and 3.04 times better than that of Pt/C (0.175 A mgPt-1). Moreover, the structure conversion from porous to hollow of PtFe NPs can maintain the activity of electrocatalyst. Our strategy provides a facile design and synthesis process of noble-transition metal alloy electrocatalysts via noble metal modified MOFs as precursors. KEYWORDS: metal-organic framework, nanoporous , bimetallic, alloy, ORR

INTRODUCTION Polymer electrolyte membrane fuel cells (PEMFCs) have been of great concern as a potential energy supply source for equipments such as low-emission electric automobiles and various 1

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household electric generators1. However, there are still substantial impediments that need to be resolved before these devices can be designed as viable technologies. One of the most important problems is to overcome the sluggish kinetics of oxygen reduction reactions at the cathode2. Platinum has been researched comprehensively as an effective cathode catalyst to decrease undesired overpotentials in the oxygen reduction reactions due to its extraordinary electrocatalytic activities. However, the worldwide technological scalability of PEMFC is signiﬁcantly restricted by the high price and natural rarity of platinum3-5. Therefore, designing and exploring low-Pt even non-Pt ORR electrocatalysts with elevated catalytic activity and high stability are imperative for the industrialization of fuel cells. It’s worth noting that, either non-Pt or low-Pt ORR catalysts ought to satisfy the rigorous requirements for absolute activity and durability challenges6. It’s found that few electronically conductive non-noble catalysts are steady in the acidic (and fluorideion-containing) PEMFC environment, thereby decreasing the amount of Pt in the cathode catalysts is still the first choice for developing appropriate materials in the near term7. For the sake of widespread adoption of fuel cells, high Pt mass activity (the catalytic activity per given mass of Pt) must be obtained. Two factors determine the Pt mass activity, one is the specific activity (SA, normalized by surface area) and the other one is the electrochemically active surface area (ECSA, normalized by mass)5. Previous computational and experimental researches have revealed that the strong bonding energy between the oxygenated species and Pt atoms impedes the activity of pure Pt8. The high ECSA requires optimized geometric factors, thereby increasing utilization efficiency (UE) of platinum5. Therefore, Pt-transition metal alloys with unique structure, like caged, hollow and porous nanoparticles, draw more and more attention. On the one hand, the d-band center of Pt goes down when it is alloyed with first-row transition metals such as Fe, Co, and Ni, which leads to a reduction of the bonding strength between the oxygenated species and Pt9. On the other hand, compared with those solid nanoparticles, the hollow interior exposes more precious metal atoms, and their unique geometry provides a approach for adjusting physical and chemical properties10. Typically, these particular structures are made intentionally through following ways: treatment based on the Kirkendall effect, template-directed protocol relying on the removal of microbeads and nanobeads, the galvanic displacement reaction and the dealloying of Pt-transition metal alloys10-11. Yang’s group transformed crystalline PtNi3 polyhedra to Pt3Ni nanoframes through interior erosion, which led to the surfaces that could offer three-dimensional molecular 2


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accessibility 10. Wei’s group synthesized hollow PtFe nanoparticles with Pt-Skin surface through structural evolution of solid Pt nanoparticles after treatment of space-confined pyrolysis and the nanoscale Kirkendall effect12. Although those methods have shown their advantages in the activity of the synthesized Pt-transition metal alloys electrocatalyst with particular structures, the synthetic procedures and complex contaminants removal greatly increase production costs, thereby resulting in economically unfeasible techniques. Lately, Metal–organic frameworks (MOFs) have exhibited tremendous advantages for using as templates to fabricate various carbon based composite materials via thermolysis13-17. The different kinds of organic ligands and the tunable metal ion centers (such as Fe, Co and Ni) produce a variety of precursors for synthesizing alloys consisted of different metal compositions18. During the calcination process, high annealing temperature provides enough Gibbs free energy for diffusion of different metal ions to form alloys; meanwhile, the organic ligands of MOF precursor will be catalyzed to form in situ carbon materials12, 19. It’s well-known that only supported on carbon materials or other bases with good conductivity can metal nanoparticles be used as efficient electrocatalysts. With this advantage, MOF calcination that fabricates electrocatalysis materials is undoubtedly a feasible way in the electrical catalysis field. Furthermore, different elements (N, P, S atoms) in changing organic ligands would make graphene layers with different dopants, which increasing the possibility of fabricating catalysts with excellent performance18. Our group had developed a process of preparing alloys that consist of transition metals and precious metals using MOF compounds as precursors and demonstrated those catalysts possess an outstanding activity in HER field20-22. For example, Su prepared RuCo@NC as an effective HER catalyst via adding RuCl3 solution into Co3[Co(CN)6]2 precursor. Chen also fabricated PdCo@NC through adding PdCl2 solution into Co3[Co(CN)6]2 precursor. After annealing in inert gases, they directly got the electrocatalysts without further operation. Inspired by this method, we design a facile way to fabricate nanoporous PtFe alloys supported on N-doped nanoporous carbon sheets via in-situ themolysis of Pt modified Fe-based metal-organic framework. Benefiting from the H2-assisted high-temperature pyrolysis of MOF, which provides high Gibbs free energy and large diffusion speed, np-PtFe NPs with uniform alloy composition are successfully synthesized and its electrocatalytic activity for the ORR in acidic electrolytes is much better than Pt/C.

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EXPERIMENTAL SECTION Synthesis of np-PtFe/NPCSs

Scheme 1 The synthetic process and model of nanoporous PtFe NPs on the N-doped porous carbon sheets. (a,b) Pt modified Fe-based metal-organic framework. (c) Annealing product of Pt/Fe-ZIF. (d) Enlarged model of PtFe NPs loaded on N-doped porous carbon sheets. (e ) Enlarged model of one nanoporous PtFe NPs.

The scheme 1 shows the synthetic route of preparing np-PtFe/NPCSs electrocatalysts. Firstly, 328mg 2-methylimidazole and 20mg PVP are dissolved in 25mL methanol to form solution A, 404mg ferric nitrate (Fe(NO3)3·9H2O,) is dissolved in 25mL methanol to form solution B. When the solution A is all dropped in solution B via a syringe (10 ml) drop by drop, a 1 mL of the synthesized solution containing Pt NPs was added and keep the solution stirring for 5 min. After keeping the reaction static for 24 h without any interruption at room temperature, we centrifuged the solution and washed the precipitation with methanol. The resulting precipitation was finally dried under vacuum drying oven overnight. The Figure S1 shows XRD pattern of the resulting Fe-ZIF precursor with two broad peaks which are similar to the reported ZIF, suggesting the successful synthesis of the targeted Fe-ZIF23. In order to obtain np-PtFe/NPCSs, the resulting Pt/Fe-ZIF composites were heated to 120 °C at 5 °C min-1 under ﬂowing 5%H2/95% Ar, held at 120 °C for 60 minutes, then heated at a rate of 5 °C min-1 to 700 °C and held at that temperature for 20 min. After slowly cooling the sample to room temperature, the pyrolyzed samples were etched with 3M HCl to remove the redundant Fe species .After washed by deionized water and 4


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alcohol for several times, the resulting products were then dried under vacuum drying oven for 24 h.

RESULTS AND DISCUSSION

Figure 1 (a) The XRD pattern, (b) Raman spectrum, (c) nitrogen adsorption isotherm at 77 K for np-PtFe/NPCSs. The inset in (c) is the pore size distribution calculated using BJH equation from N2 adsorption. (d) XPS surveys of np-PtFe/NPCSs catalyst. (e) N1s XPS spectra of the np-PtFe/NPCSs catalyst. (f) Fe 2p XPS spectra of the np-PtFe/NPCSs catalyst.

The XRD pattern of synthesized np-PtFe/NPCSs is illustrated in Figure 1a. It exhibited three characteristic peaks, which were assigned to (111), (200), and (220) of Pt with fcc structure24. Compared with pure Pt, the higher angels of peaks’ positions indicate that Fe was successfully permeated into the Pt face centered cubic structure and lead to an alloy phase, resulting in the contracted lattice distance. Remarkably, we can identify no apparent diffraction peak between 20° -30°，indicating np-PtFe particles were in fact embedded on the graphene surface and suppressed the stacking of graphene layers25. Raman spectrum is a nondestructive characterization method to characterize disordered and ordered crystal structures of carbon materials26. A characteristic feature of the graphitic layers is the G band, which corresponds to the tangential vibration of the carbon atoms, while the disordered carbon or defective graphitic structures could be identified by the D band27. The ratios of the D band to G band integrated intensity (ID/IG) for the np-PtFe/NPCSs was 1.4(Figure 1b), suggesting that the generation of abundant defective edges 5



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arises from the pores of carbon sheets. Moreover, the broad and weak 2D band indicates that the carbon sheets consist of several layers graphene21. The specific surface area of np-PtFe/NPCSs is 34.7m2 g-1, demonstrated in the Brunauer–Emmett–Teller analysis of Figure 1c. The corresponding pore-size distribution analysis of np-PtFe/NPCSs (the inset of Figure 1c) clearly revealed that a great number of nanopores existed in the catalyst. Most pores are between 2 and 20 nanometers in diameter and large enough for free diffusion of O2 , which possesses a kinetic diameter of 0.346 nm12. The atomic percentage of the N element in np-PtFe/NPCSs was estimated to be 2.4% (Figure 1d). Meanwhile, the bands for the the graphitic N (ca. 401 eV), the pyrrolic N (ca. 399.8 eV), and pyridinic N (ca. 398.5 eV) could be distinctly resolved from the high resolution N 1s XPS spectrum (Figure 1e)

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, which indicates the formation of N-doped carbon after thermolysis of

Fe-ZIF. Figure 1f shows the Fe 2p XPS spectra of the np-PtFe/NPCSs catalyst. The Fe (0) of the PtFe alloy can be identified by the low energy peak at 707.7 eV, while the high energy peaks at 710.3 and 712.8 eV can be attributed to the Fe2+ and Fe3+ in the FeNx/C composites12. The weight contents of Pt and Fe for np-PtFe/NPCSs were determined to be 19.31% and 2.32% according to inductively coupled plasma-atomic emission spectrometer (ICP-AES) analysis. The porous nature of N-doped carbon sheets was further demonstrated by N2 adsorption–desorption experiment.

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Figure 2 (a-b) SEM images, (c)TEM image of np-PtFe/NPCSs, (d) HRTEM image of an individual np-PtFe,

(e-f) Elemental mapping of N and C. (g-h) Elemental mapping of Pt and Fe, respectively. (i) EDX line-scanning profile of an individual np-PtFe.

Figure 2 shows the transmission electron microscope (TEM) and scanning electron microscope (SEM) images of as-prepared np-PtFe/NPCSs as well as the corresponding Energy Disperse Spectroscopy (EDS) element mapping. The SEM image (Figure 2a) of np-PtFe/NPCSs in the low magnification displays the sheets morphology of carbon bases, and the higher magnification image (Figure 2b) clearly exhibits the rough surface of the carbon sheets because of themolysis and washing with 3M HCl solution29. The TEM image (Figure 2c) further confirms the sheets-like feature of the carbon bases. Remarkably, as shown in Figure S2 and Figure 2c, the carbon sheets consist of several graphene layers, which are corresponding to the Raman results. Moreover, the np-PtFe particles uniformly dispersed on the NPCSs. At the same time, we can also notice that 7



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some bright dots also disperse uniformly on the carbon bases, which demonstrates that the carbon bases are porous again. Figure 2d shows the HRTEM image of an individual PtFe nanoparticle with about 15 nm diameter which has a spongy multi-hollow structure. The HAADF-STEM image of np-PtFe/NPCSs (Figure S3) clearly shows a bright contrast at the edge and some dark contrast dots in the inner of PtFe particles, which further confirms the formation of multi-hollow structure. The HAADF cross-sectional compositional line profiles (Figure 2i and Figure S4) also exhibit nanoporous characteristic of the PtFe nanoparticles. With regard to the line-scanning, the signal of Pt rises significantly on the side edge of the individual particle and reaches its peak at 9, 15, and 20 nm and two minimum Pt signals can be observed at 11 and 17.5 nm. Meanwhile, the Fe/Pt atomic ratio (Figure S5) shows that whether in the inner or at the edge of the multi-hollow PtFe NP, the Pt and Fe atoms are disperse uniformly. The distance of adjacent lattice fringes is 0.22 nm, which matched well with that of the (111) planes of fcc PtFe alloy, suggesting that the PtFe NPs have been successfully synthesized through the annealing of Pt modified Fe-ZIF. Figure 2g–h display the HAADF-STEM micrographs and EDX maps of Pt and Fe elements, which also investigating Pt and Fe elements are evenly dispersed in the whole nanoparticle, revealing a uniform alloy composition. Furthermore, the dark areas observed in the element maps also manifest that the nanoparticles are nanoporous. Figure 2e–f are the HAADF-STEM micrographs and EDX maps of C and N elements. For the same reason, the dark area indicates the porous structure of carbon sheets.

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Figure 3 (a) Electrochemical CV curves of np-PtFe/NPCSs and Pt/C catalysts recorded at a scan rate of 50 mV

s-1 in N2-saturated 0.1 M HClO4 solution. (b) Electrochemical polarization curves of np-PtFe/NPCSs and Pt/C catalysts recorded with a sweep rate of 10 mV s-1 and a rotation rate of 1600 rotations min-1 in O2- purged 0.1 M HClO4 solution at room temperature. (c) Specific activity and (d) mass activity at 0.9 V versus reversible hydrogen electrode (RHE) for np-PtFe/NPCSs and Pt/C catalysts.

We carried out both cyclic voltammetry (CV) and linear sweep voltammetry tests to evaluate the electrocatalytic performances of the np-PtFe/NPCSs and chose commercial 20% Pt/C catalysts as reference. Figure 3a shows the CV curves of the np-PtFe/NPCSs catalyst and commercial 20% Pt/C in N2-purged 0.1M HClO4 solution and the scan rate is 50mV s-1. After measuring the Coulombic charge for hydrogen adsorption/desorption, we can obtain the value of electrochemical surface area (ECSA). The specific values of the ECSA normalized to the Pt mass for np-PtFe/NPCSs is estimated to be 111.58 m2g-1, while the value of the commercial 20% Pt/C catalyst is only 90 m2 g-1. The high specific value of ECSA of np-PtFe/NPCSs maybe ascribe to two merits of np-PtFe/NPCSs. Firstly, the np-PtFe NPs are loading on the porous carbon sheets and no carbon layers is around the nanoparticles. Moreover, the porous structure of PtFe NPs exposes more Pt atoms on the surface of NPs which can adsorb the H+ and be used for oxygen reduction reaction. Figure 3b shows the electrochemical polarization curves of np-PtFe/NPCSs and Pt/C catalysts operated with a rotation rate of 1600 rotations min-1 in O2-purged 0.1 M HClO4 9



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at room temperature. It can be seen that np-PtFe/NPCSs exhibit a more positive half-wave potential (0.92 V) compared with commercial Pt/C catalyst (0.883 V). In the meantime, for the sake of the fact that the ORR catalytic activity originates from the np-PtFe rather than the Fe-Nx/C structure, we test the sample annealed from Fe-ZIF without adding Pt NPs (Figure S6). From the LSV result, we can get the point that the sample annealed from Fe-ZIF without adding Pt NPs possesses inferior catalytic activity to ORR in acid environment. The Tafel plots in Figure S7 show slopes of 40 and 48 mV dec-1 for np-PtFe/NPCSs and Pt/C, respectively. The np-PtFe/NPCSs possess a smaller slope, which demonstrate an improved kinetics for ORR5. The specific activity is calculated by normalizing kinetic currents to both ECSA and the loading amount of metal Pt. The value of specific activity of np-PtFe/NPCSs and Pt/C and help us further investigate different catalysts for better catalytic activity. As shown in Figure 3c, the specific activity of the np-PtFe/NPCSs catalyst is 0.477 mA cm-2 at 0.9 V, which is 2.44 times greater than that of Pt/C (0.195 mA cm-2). After normalizing kinetic currents to the loading amount of Pt, the mass activity of the np-PtFe/NPCSs (Figure 3d) catalyst is found to be 0.533 A mgPt-1, which is 3.04 times greater than that of Pt/C (0.175 A mgPt-1) and surpass the U.S. Department of Energy’s 2017 target (0.440 A mgPt-1) 30. We also applied np-PtFe/NPCSs in the PEMFC (Figure S10) and the result also demonstrated that the np-PtFe/NPCSs has a better eletrocatalytic activity than 20% Pt/C. We compared our material with some disordered PtFe catalysts from the previous literatures in Table S111-12, 31-33. From the table, it was found that we obtained a catalyst with promising catalytic activity via a facile synthesis process, which makes this technique economically feasible. The greatly enhanced performance of np-PtFe/NPCSs catalyst primarily ascribe to following two reasons. Firstly, previous calculations have investigated that the charge transfer from Fe to Pt in the PtFe alloy plays an important role in modulating electronic structure over the strain effect, which results in a significantly decrease in surface average charge and thereby the decline of oxygenated species’ adsorption energy12, 34. Secondly, nanoporous architectures not only expose more active sites for ORR and increase the utilization efficiency (UE) of platinum but also provide the integral molecular transport paths33.

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Fig. 4 ORR polarization curves of np-PtFe/NPCSs (a) and 20% Pt/C (b) catalysts before and after 2000

potential cycles between 0.6 and 1.1 V versus RHE. The insets in panels (a) and (b) show the CV curves of np-PtFe/NPCSs and 20% Pt/C before and after 2000 CV cycles.(c) TEM image and particle-size distribution of aged np-PtFe/NPCSs catalyst after 2000 CV cycles. (d) The changes of mass activities of np-PtFe/NPCSs and 20% Pt/C before and after 2000 potential cycles.

In addition to the catalytic activity, stability is another important evaluation criterion that determines whether the electrocatalyst can be employed in practical application. So, we further performed the accelerated durability test (ADT) between 0.6 and 1.1 V in O2-saturated 0.1 M HClO4 to evaluate the electrochemical durability of the catalyst. The 20% commercial Pt/C catalyst was also used as a baseline catalyst for comparison. In the Figure 4a, np-PtFe/NPCSs catalyst shows negligible attenuation of diffusion limited current density (Jd) and half-wave potential (E1/2) after 2000 CV cycles, while those of Pt/C showed apparent decay (Figure 4b). The mass activity of the aged np-PtFe/NPCSs catalyst was still as high as 0.444 A mgPt-1 (Figure 4d), showed only 16.7% decreases from the initial mass activity, while the aged Pt/C (0.047 A mgPt-1) retained only 26.9% of the initial mass activity (0.175 A mgPt-1). In order to understand the reason for the superior stability, we utilize the TEM (Figure S8 and Figure 4c) to characterize the morphologies of np-PtFe/NPCSs sample before and after the 2000 CV cycles. After 2000 CV cycling, the diameter of the PtFe NPs increases from 5-14nm to 12-24 nm and exhibits a broad 11



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size distribution. However, we can also clearly identify the np-PtFe NPs have converted into hollow PtFe NPs with a Pt-skin. This unique alloy structure with a nearly pure Pt-skin results from the reconstruction of neighboring surface Pt atoms because of gradual erosion of Fe atoms on the surface during the CV cycling33. A previous study have demonstrated that during the PEMFC operation condition, Pt3Co nanoparticles are inclined to convert into hollow Pt nanoparticles due to depletion of transition metal in the acid environment35. Remarkably, this hollow structure with Pt-skin surface can effectively decrease the effect of low coordination sites where the binding strength between oxygenated species and Pt atoms is very strong and thereby weaken the adsorption of nonreactive oxygenated species on the surface of Pt, retaining a high electrocatalytic activity 12, 35.

CONCLUSIONS In conclusion, we designed a facile synthesis route to fabricate a double nanoporous structure comprised nanoporous PtFe nanoparticles supported on N-doped porous carbon sheets through the pyrolysis of Pt modified Fe-based metal-organic framework. The synergistic effect between the open framework structure of the nanoporous PtFe nanoparticles and porous carbon sheets not only creat more active sites, but also is beneficial to the mass and charge transport, which facilitates the electrochemical catalysis process. Specially, the porous structure of PtFe NPs immensely enhanced the utilization efficiency (UE) of Pt, thereby we are able to use equivalent Pt content of commercial 20% Pt/C to obtain higher activity. Furthermore, the structure conversion from porous to hollow of PtFe NPs can maintain the activity of electrocatalyst. We envision that high performance and economically feasible catalysts for polymer electrolyte membrane fuel cells could be achieved via the above-mentioned facile approach.

ASSOCIATED CONTENT Supporting Information Additional XRD, TEM, HRTEM, XPS, LSV curves, EDX spectra

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] 12


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (NSFC, 21271163, U1232211), CAS/SAFEA International Partnership Program for Creative Research Teams and CAS Hefei Science Center (2016 HSC-IU011). The authors thank Yanyan Gao of Dalian Institute of Chemical Physics, Chinese Academy of Sciences for her help on the PEMFC test.

REFERENCES (1) Yang, D.-S.; Kim, M.-S.; Song, M. Y.; Yu, J.-S. Highly Efficient Supported PtFe Cathode Electrocatalysts Prepared by Homogeneous Deposition for Proton Exchange Membrane Fuel Cell. Int. J. Hydrogen Energy 2012, 37 (18), 13681-13688. (2) Vinayan, B. P.; Ramaprabhu, S. Platinum-Tm (Tm = Fe, Co) Alloy Nanoparticles Dispersed Nitrogen Doped (Reduced Graphene Oxide-Multiwalled Carbon Nanotube) Hybrid Structure Cathode Electrocatalysts for High Performance PEMFC Applications. Nanoscale 2013, 5 (11), 5109-5118. (3) Bu, L. Z.; Zhang, N.; Guo, S. J.; Zhang, X.; Li, J.; Yao, J. L.; Wu, T.; Lu, G.; Ma, J. Y.; Su, D.; Huang, X. Q. Biaxially Strained PtPb/Pt Core/Shell Nanoplate Boosts Oxygen Reduction Catalysis. Science 2016, 354 (6318), 1410-1414. (4) He, D.; Zhang, L.; He, D.; Zhou, G.; Lin, Y.; Deng, Z.; Hong, X.; Wu, Y.; Chen, C.; Li, Y. Amorphous Nickel Boride Membrane on a Platinum-Nickel Alloy Surface for Enhanced Oxygen Reduction Reaction. Nat. Commun. 2016, 7, 12362. (5) Li, M. F.; Zhao, Z. P.; Cheng, T.; Fortunelli, A.; Chen, C. Y.; Yu, R.; Zhang, Q. H.; Gu, L.; Merinov, B. V.; Lin, Z. Y.; Zhu, E. B.; Yu, T.; Jia, Q. Y.; Guo, J. H.; Zhang, L.; Goddard, W. A.; Huang, Y.; Duan, X. F. Ultrafine Jagged Platinum Nanowires Enable Ultrahigh Mass Activity for the Oxygen Reduction Reaction. Science 2016, 354 (6318), 1414-1419. (6) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. Earth‐Abundant Nanomaterials for Oxygen Reduction. Angew. Chem. Int. Ed. 2016, 55 (8), 2650-2676. (7) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and Requirements for Pt, Pt-Alloy, and Non-Pt Oxygen Reduction Catalysts for Pemfcs. Appl. Catal., B: Environ. 2005, 56 (1-2), 9-35. (8) Stamenkovic, V. R.; Mun, B. S.; Arenz, M.; Mayrhofer, K. J.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Trends in Electrocatalysis on Extended and Nanoscale Pt-Bimetallic Alloy Surfaces. Nat. Mater. 2007, 6 (3), 241-247. (9) Guo, S.; Li, D.; Zhu, H.; Zhang, S.; Markovic, N. M.; Stamenkovic, V. R.; Sun, S. FePt and CoPt Nanowires as Efficient Catalysts for the Oxygen Reduction Reaction. Angew Chem Int Ed Engl 2013, 52 (12), 3465-3468. (10) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343 (6177), 1339-1343. (11) Shui, J.-l.; Chen, C.; Li, J. C. M. Evolution of Nanoporous Pt-Fe Alloy Nanowires by Dealloying and 13



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Their Catalytic Property for Oxygen Reduction Reaction. Adv. Funct. Mater. 2011, 21 (17), 3357-3362. (12) Wang, Q.; Chen, S.; Shi, F.; Chen, K.; Nie, Y.; Wang, Y.; Wu, R.; Li, J.; Zhang, Y.; Ding, W.; Li, Y.; Li, L.; Wei, Z. Structural Evolution of Solid Pt Nanoparticles to a Hollow PtFe Alloy with a Pt-Skin Surface Via Space-Confined Pyrolysis and the Nanoscale Kirkendall Effect. Adv. Mater. 2016, 28 (48), 10673-10678. (13) Liu, B.; Shioyama, H.; Akita, T.; Xu, Q. Metal-Organic Framework as a Template for Porous Carbon Synthesis. J. Am. Chem. Soc. 2008, 130 (16), 5390-5391. (14) Zhang, L.; Wu, H. B.; Lou, X. W. Metal–Organic-Frameworks-Derived General Formation of Hollow Structures with High Complexity. J. Am. Chem. Soc. 2013, 135 (29), 10664-10672. (15) Zheng, F.; Xia, G.; Yang, Y.; Chen, Q. MOF-Derived Ultrafine MnO Nanocrystals Embedded in a Porous Carbon Matrix as High-Performance Anodes for Lithium-Ion Batteries. Nanoscale 2015, 7 (21), 9637-9645. (16) Hu, L.; Yan, N.; Chen, Q.; Zhang, P.; Zhong, H.; Zheng, X.; Li, Y.; Hu, X. Fabrication Based on the Kirkendall Effect of Co3O4 Porous Nanocages with Extraordinarily High Capacity for Lithium Storage. Chem. Eur. J. 2012, 18 (29), 8971-8977. (17) Hu, L.; Zhang, R.; Wei, L.; Zhang, F.; Chen, Q. Synthesis of FeCo Nanocrystals Encapsulated in Nitrogen-Doped Graphene Layers for Use as Highly Efficient Catalysts for Reduction Reactions. Nanoscale 2015, 7 (2), 450-454. (18) Yang, Y.; Lun, Z.; Xia, G.; Zheng, F.; He, M.; Chen, Q. Non-Precious Alloy Encapsulated in Nitrogen-Doped Graphene Layers Derived from MOFs as an Active and Durable Hydrogen Evolution Reaction Catalyst. Energy Environ. Sci. 2015, 8 (12), 3563-3571. (19) Yang, Y.; Lin, Z.; Gao, S.; Su, J.; Lun, Z.; Xia, G.; Chen, J.; Zhang, R.; Chen, Q. Tuning Electronic Structures of Nonprecious Ternary Alloys Encapsulated in Graphene Layers for Optimizing Overall Water Splitting Activity. ACS Catal. 2017, 7 (1), 469-479. (20) Jiang, P.; Yang, Y.; Shi, R.; Xia, G.; Chen, J.; Su, J.; Chen, Q. Pt-Like Electrocatalytic Behavior of Ru–Moo2 Nanocomposites for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5 (11), 5475-5485. (21) Su, J.; Yang, Y.; Xia, G.; Chen, J.; Jiang, P.; Chen, Q. Ruthenium-Cobalt Nanoalloys Encapsulated in Nitrogen-Doped Graphene as Active Electrocatalysts for Producing Hydrogen in Alkaline Media. Nat. Commun. 2017, 8, 14969. (22) Chen, J.; Xia, G.; Jiang, P.; Yang, Y.; Li, R.; Shi, R.; Su, J.; Chen, Q. Active and Durable Hydrogen Evolution Reaction Catalyst Derived from Pd-Doped Metal-Organic Frameworks. ACS Appl. Mater. Interfaces 2016, 8 (21), 13378-13383. (23) Zheng, F.; He, M.; Yang, Y.; Chen, Q. Nano Electrochemical Reactors of Fe2O3 Nanoparticles Embedded in Shells of Nitrogen-Doped Hollow Carbon Spheres as High-Performance Anodes for Lithium-Ion Batteries. Nanoscale 2015, 7 (8), 3410-3417. (24) Wang, D.; Xin, H. L.; Hovden, R.; Wang, H.; Yu, Y.; Muller, D. A.; DiSalvo, F. J.; Abruña, H. D. Structurally Ordered Intermetallic Platinum–Cobalt Core–Shell Nanoparticles with Enhanced Activity and Stability as Oxygen Reduction Electrocatalysts. Nat. Mater. 2013, 12 (1), 81-87. (25) Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Mullen, K. 3d Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134 (22), 9082-9085. (26) Zhong, X.; Wang, L.; Zhuang, Z.; Chen, X.; Zheng, J.; Zhou, Y.; Zhuang, G.; Li, X.; Wang, J. Double Nanoporous Structure with Nanoporous PtFe Embedded in Graphene Nanopores: Highly Efficient Bifunctional Electrocatalysts for Hydrogen Evolution and Oxygen Reduction. Adv. Mater.Interfaces 14


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2017, 4 (5), 1601029. (27) Li, Z.; Xu, Z.; Tan, X.; Wang, H.; Holt, C. M. B.; Stephenson, T.; Olsen, B. C.; Mitlin, D. Mesoporous Nitrogen-Rich Carbons Derived from Protein for Ultra-High Capacity Battery Anodes and Supercapacitors. Energy Environ. Sci. 2013, 6 (3), 871. (28) Wang, M.-Q.; Yang, W.-H.; Wang, H.-H.; Chen, C.; Zhou, Z.-Y.; Sun, S.-G. Pyrolyzed Fe–N–C Composite as an Efficient Non-Precious Metal Catalyst for Oxygen Reduction Reaction in Acidic Medium. ACS Catal. 2014, 4 (11), 3928-3936. (29) Zheng, F.; Yang, Y.; Chen, Q. High Lithium Anodic Performance of Highly Nitrogen-Doped Porous Carbon Prepared from a Metal-Organic Framework. Nat. Commun. 2014, 5, 5261. (30) Agro, S.; DeCarmine, T.; DeFelice, S.; Thoma, L. Annual Progress Report for the Doe Hydrogen Program, 2005. US Department of Energy (DOE) website: http://www. hydrogen. energy. gov 2005, 790. (31) Carpenter, M. K.; Moylan, T. E.; Kukreja, R. S.; Atwan, M. H.; Tessema, M. M. Solvothermal Synthesis of Platinum Alloy Nanoparticles for Oxygen Reduction Electrocatalysis. J. Am. Chem. Soc. 2012, 134 (20), 8535-8542. (32) Zheng, F.-S.; Liu, S.-H.; Kuo, C.-W. Ultralow Pt Amount of Pt–Fe Alloys Supported on Ordered Mesoporous Carbons with Excellent Methanol Tolerance During Oxygen Reduction Reaction. Int. J. Hydrogen Energy 2016, 41 (4), 2487-2497. (33) Duan, H.; Hao, Q.; Xu, C. Nanoporous PtFe Alloys as Highly Active and Durable Electrocatalysts for Oxygen Reduction Reaction. J. Power Sources 2014, 269, 589-596. (34) Wang, J. X.; Ma, C.; Choi, Y.; Su, D.; Zhu, Y.; Liu, P.; Si, R.; Vukmirovic, M. B.; Zhang, Y.; Adzic, R. R. Kirkendall Effect and Lattice Contraction in Nanocatalysts: A New Strategy to Enhance Sustainable Activity. J. Am. Chem. Soc. 2011, 133 (34), 13551-13557. (35) Dubau, L.; Durst, J.; Maillard, F.; Guétaz, L.; Chatenet, M.; André, J.; Rossinot, E. Further Insights into the Durability of Pt3Co/C Electrocatalysts: Formation of “Hollow” Pt Nanoparticles Induced by the Kirkendall Effect. Electrochim. Acta 2011, 56 (28), 10658-10667.

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Nanoporous PtFe Nanoparticles Supported on N-Doped Porous

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