Boosting Oxygen Reduction Catalysis with N-doped Carbon Coated

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Energy, Environmental, and Catalysis Applications

Boosting Oxygen Reduction Catalysis with N-doped Carbon Coated CoS Microtubes 9

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Zexing Wu, Jie Wang, Min Song, Guangming Zhao, Ye Zhu, Gengtao Fu, and Xien Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07207 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 6, 2018

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ACS Applied Materials & Interfaces

Boosting Oxygen Reduction Catalysis with N-doped Carbon Coated Co9S8 Microtubes Zexing Wu,†,# Jie Wang,‡,# Min Song,† Guangming Zhao,‡ Ye Zhu,*,‡ Gengtao Fu,*,§ Xien Liu*,† †

State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and

Molecular Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, 266042, Qingdao P. R. China. ‡

Department of Applied Physics, The Hong Kong Polytechnic University, Hung Horn,

Kowloon, Hong Kong. §

School of Chemical and Biomedical Engineering, Nanyang Technological University,

Singapore 637459, Singapore #

Zexing Wu and Jie Wang contributed equally to this work.

E-mail: [email protected]; [email protected]; [email protected]

KEYWORDS: Co9S8, microtubes, exposed (022) lattice sites, N-doped carbon layer, coordination effect, oxygen reduction reaction

ABSTRACT: Herein, nitrogen-doped carbon coated hollow Co9S8 microtubes (Co9S8@N-C microtubes) are prepared through a facile solvothermal procedure, followed by dopamine polymerization process together with a post-pyrolysis which present excellent electrocatalytic activity for oxygen reduction reaction (ORR). The Co9S8 within the hollow Co9S8@N-C microtubes presents a well-defined single-crystal structure with dominated (022) plane. To obtain desired electrocatalyst, 1

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the annealing temperature and the thickness of carbon layer tuned by change the dopamine concentration are optimize systematically. The electrochemical results demonstrate that the coordination of N-doped carbon layer, exposed (022) plane and hollow architecture of Co9S8 microtubes calcined at 700 oC affords outstanding ORR performance to Co9S8@N-C microtubes. The moderate thickness of carbon layer is crucial for improving ORR activity of Co9S8@N-C microtubes, while increase or decrease the thickness would result in the activity decrease. More importantly, the N-doped carbon layer can protect inner Co9S8 from undergoing aggregation and dissolution effectively during the ORR, resulting in excellent electrocatalytic stability.

Introduction The increasing global environmental pollution and the depletion of fossil fuels have stimulated researchers to seek eco-friendly and sustainable resources and devices. Fuel cells, as an environmental friendly and high energy conversion technique, have attracted tremendous attentions.1-4 However, the sluggish kinetics of ORR greatly hindered the commercial and scalable applications of fuel cells.5, 6 To resolve this problem, it is indispensable to develop highly efficient and durable catalysts. Although Pt-based nanomterials present outstanding electrocatalytic activities for ORR,7,

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their scarcity, high-cost and poor methanol crossover resistance

tremendously limited the commercially large-scale applications of fuel cells.9-12 Therefore, it is urgent to investigate Pt-free electrocatalyst with low-cost, abundant-reserves, high activity, excellent durability and outstanding methanol tolerance as alternatives for ORR.13-15 2


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Emerging as a class of potential alternative, transition metal Co-based catalysts such as CoOx, 16, 17 CoPx18, 19 and CoSx20-22 have attracted tremendous attentions as ORR electrocatalysts, due to its low-cost, outstanding catalytic property and excellent resistance to methanol crossover. Dai’s group initially demonstrated the potentiality of this new type of materials, mainly focus on Co3O4. 23 Relative to CoOx and CoPx materials, CoSx species have gained special attention on account of their prominent structure advantages. Taking Co9S8 as an example, S2- ions offer adsorption sites for O, which can facilitate the bond scission of O-O. Meanwhile, (202) plane partially covered by OH– of Co9S8 is benefit for O2 reduction with Pt-like overpotential. Several recently published works have well demonstrated these advantages with both theoretical calculations and experimental methods. 24-28 Despite significant progresses have been made in exploring Co9S8 catalysts, they have not satisfied the practical usages of metal-air batteries or fuel cells owing to intrinsic low conductivity and easy aggregation. One of the effective strategies for solving this problem is the combination of active component with various carbon supports, such as graphene, carbon tubes and especially for heteroatoms doped carbon. 29-31 Heteroatoms doping is important to optimize the catalytic performance by tune the local electronic distribution of carbon atoms and changed the absorbed style of oxygen which is benefiting for ORR.

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Such Co9S8/carbon composite can provide interfacial

chemical interactions and/or electrical coupling as a means of synergistic effects between two components, leading to the enhanced catalytic performance and durability.34 Another efficient approach to enhance electrocatalytic activity of 3



catalysts is tune the nanostructure of catalysts to expose more catalytic sites and offer high accessibility. Particularly, hollow structure is an interesting and attracting morphology which possesses high surface area and also can provide rich active sits for catalytic process.35,

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However, the traditional synthetic protocols to prepare

hollow structures including templates or surfactants strategies and the tedious and time-consuming drawbacks limits their large-scale production. Therefore, exploring simple and efficient strategy to synthesize carbon-based CoSx catalysts with high conductivity and abundant active sits is highly desirable, yet remains a great diffculty. In this contribution, an efficient and facile synthetic strategy was developed to prepare N-doped carbon coated Co9S8 microtubes (Co9S8@N-C microtubes) as highly-active and stable ORR catalysts. The CoS1.097 microwires were first prepared using a facile solvothermal synthesis with the assistance of L-cysteine. The subsequent dopamine polymerization process together with a post-pyrolysis resulted in the N-doping and carbon coated on Co9S8 simultaneously. It is identified that Co9S8 microtubes reveal unique single-crystal structure with more exposed (022) plane, which can work as active ORR center. Meanwhile, the moderate thickness of N-doped carbon as a conductive layer can contribute significantly to improve the electron conductivity of Co9S8 active centers. Thanks to the cooperation effects between Co9S8 microtubes and N-doped carbon layer, Co9S8@N-C microtubes present remarkable ORR activity close to Pt/C, and excellent durability and outstanding methanol resistance ability.

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Results and discussions Figure 1a schematically illustrates the preparation process of Co9S8@N-C microtubes, which involvs three main steps: (i) the simple synthesis of CoS1.097 microwires under the solvothermal condition with the assistance of L-cysteine; (ii) in-situ polymerization of CoS1.097 microwires in dopamine aqueous solution forming CoS1.097@polydopamine; (iii) thermal-pyrolysis of CoS1.097@polydopamine in Ar atmosphere to promote the formation of N-doped carbon coated Co9S8 phase. The experimental details were provided in experimental section. The formation of CoS1.097 phase was verified via XRD, as shown in Figure 1b. It can be seen clearly that the diffraction peaks located at 2θ of 30.8o, 32o, 35.5o, 47o, 54.6o and 74.7o, can be indexed to (204), (213), (220) (306) and (330) (606) reflections of CoS1.097 (PDF#19-0366). After thermal pyrolysis of CoS1.097@dopamine, N-doped carbon coated Co9S8 microtubes were successfully synthesized, as also proved by XRD. As indicated in Figure 1c, all the diffraction peaks located at 25.6o, 30.1o, 31.4o, 36.3o, 39.7o, 47.5o, 52.2o, 61.3o, 62.2o, 73.3o and 76.8o are corresponding to the (220), (311), (222), (400), (331), (511), (440), (533), (622), (731) and (800) reflections, respectively, in consistent with the cubic Co9S8 phase (PDF#65-6801). It should be noted that by annealing the CoS1.097 microtubes, carbon-free Co9S8 microtubes also can be obtained (Figure S1a). Besides, the amount of dopamine would not influence the formation of Co9S8 phase (Figure S1b), and the pyrolysis of dopamine only results in the carbon phase with near amorphous structure (Figure S1c).

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Figure 1. (a) Schematic illustration for the preparation of Co9S8@N-C microtubes; XRD patterns of (b) CoS1.097 microwires and (c) 0.5-Co9S8@N-C microtubes. The microstructure and physical characterization of the catalysts were investigated via SEM, TEM and scanning TEM (STEM) equipped with electron energy loss spectrum (EELS). SEM image of CoS1.097 shows a hierarchically porous wire-like structure with numerous nanosheets as the building blocks (Figure S2a). After high-temperature annealing of dopamine-free CoS1.097, the evident morphology changes of product (i.e., carbon-free Co9S8) were observed (Figure 2a and Figure S2b), in which the outside thin nanosheets disappeared and a porous-like structure on the surface formed. TEM image in Figure 2b shows that carbon-free Co9S8 presents obviously hollow tube-like structure. Figure 2c illustrates the high-resolution TEM 6


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(HRTEM) picture of carbon-free Co9S8. An interplanar spacing of 0.35 nm (Figure 2c) should be indexed to (022) plane of Co9S8 phase. The corresponding selected area electron diffraction (SAED) pattern along zone axes demonstrate the single-crystal structure of Co9S8 with exposed (111) plane (Figure 2d). The {022} plane was proved to the active sites towards ORR via recent calculating results.24 For comparison, the Co9S8-free N-doped carbon was also prepared, which exhibits sphere-like morphology with a average particle size of ~ 100 nm (Figure S2c). As for Co9S8@N-C electrocatalyst, the porous-like surface structure (Figure S2d) disappeared in comparison with carbon-free Co9S8, implying that the Co9S8 microtube was covered by carbon layer. TEM image in Figure 2e shows that the Co9S8@N-C also exhibit the microtube structure with internal diameter of ~ 70 nm. Benefiting from the hollow structure, 0.5-Co9S8@N-C exhibits large Brunauer−Emmett−Teller (BET) surface area of about 110.8 m2 g-1, as confirmed by N2 adsorption–desorption experiment (Figure S3). STEM image in Figure 2f proves the carbon layer covered structure. Raman measurement was further conducted to demonstrate the existence of carbon layer (Figure S4). Both typical D band and G band peaks were observed at around 1350 cm-1 and 1580 cm-1 with low ID/IG, indicating the existence of graphitized carbon in the as-prepared catalyst. The existence of carbon layer can efficiently enhance the electronic-conductivity of Co9S8, which is benefit to increase the electron transfer rate during electrocatalysis. To investigate the surface crystal structure of carbon coated Co9S8, the HRTEM image (Figure 2g) and the corresponding SAED pattern (Figure 2h) were further conducted. Similar to 7



carbon-free Co9S8, the Co9S8 within the Co9S8@N-C microtubes also present the single-crystal nature with dominated (111) plane exposed, indicating that the introduction of carbon has no influence on surface crystal structure of Co9S8. Besides, the lattice fringe shown from the HRTEM are indexed to be (022) plane, which is expected to coordinate with the carbon layer outside the microtube to synergistically enhance the ORR performance.

Figure 2. (a) SEM image of carbon-free Co9S8 microtubes; (b) TEM image of Co9S8 microtubes. (c) HRTEM image and (d) the corresponding SAED pattern along zone. (e) TEM image and (f) STEM image of the Co9S8@N-C. (g) HRTEM image and (h) the corresponding SAED pattern along zone. The thickness of the carbon layer for the Co9S8@N-C microtubes depends strongly on the amount of dopamine. The structure and thickness change of the 8


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products obtained by calcination of the CoS1.097@dopamine at different dopamine concentration (0.1, 0.5 and 1 mg mL-1) were investigated by STEM and EELS mappings (Figure 3). All the EELS mappings recorded from STEM images (red box region) shows uniform distribution of Co and S elements within the Co9S8@N-C microtubes; meanwhile, a thin carbon layer can be detected in the outside of the microtubes. The result demonstrates that the amount of dopamine has no influence on Co9S8 structure (Figure S5). The thickness of the carbon layer was calculated ca. 20, 30 and 50 nm for 0.1-Co9S8@N-C, 0.5-Co9S8@N-C, 1-Co9S8@N-C, respectively, as the promotion of the dopamine concentration.

Figure 3. (a) STEM image of 0.1-Co9S8@N-C and the corresponding EELS mapping of Co, S, C, Co vs. S and the overlay of Co, S, C; (b) STEM image of 0.5-Co9S8@N-C and the corresponding EELS mapping of Co, S, C, Co vs. S and the overlay of Co, S, C; (c) STEM image of 1-Co9S8@N-C and the corresponding EELS 9



mapping of Co, S, C, Co vs. S and the overlay of Co, S, C. The composition and chemical valence of 0.5-Co9S8@N-C microtubes were investigated with XPS. XPS survey-scan spectrum (Figure S6) shows the Co, S, N and C elements are coexistent in 0.5-Co9S8@N-C microtubes. For C 1s in Figure 4a, the spectrum could be divided into five kinds of peaks including 284.5 eV (C-C), 285 eV (C-OH), 285.7 eV (C=O), 287.2 eV (C-N) and 283.9 eV (C-S), which demonstrates N and S doped into carbon shell. The atomic content of N was estimated to be 3.45%. Three kinds of N can be observed in the obtained product at 400.6 eV, 398.6 eV and 397.9 eV which are corresponding to graphitic N, pyrrolic N and pyridinic N, respectively (Figure 4b). From the S 2p spectrum (Figure 4c), the peak located at 161.7 eV can be attributed to the interaction of Co-S, and other peaks are attributed to C-S-C and SOx. 28 As shown in Figure 4d for Co 2p spectrum, the peaks at 781.8 and 783.7 eV are corresponding to Co3+ and Co2+ in Co9S8. 37-39 Thus, the carbon shell was co-doped by N and S elements and interactions between metal phase and heteroatoms are benefit for catalytic process. 40

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Figure 4. High-resolution XPS spectra of C 1s (a), N 1s (b), S 2p (c) and Co 2p (d) in the prepared 0.5-Co9S8@N-C composite. The ORR activities of Co9S8@N-C with different carbon content were first evaluated with RDE. For comparison, the performances of pure N-doped carbon (N-C), Co9S8 and Pt/C were evaluated, in which Pt/C presents the best catalytic activity. Although pure N-C catalyst has abundant N content (8.47%, Figure S7), it present poorer ORR activity relative to three types of Co9S8@N-C catalysts, which indicates the benefit of the Co9S8-doping for the ORR. It is noteworthy that Co9S8 alone cannot explain good ORR activities of Co9S8@N-C catalysts, because of its poor electrical conductivity. As shown in Figure 5a, Co9S8@N-C catalysts exhibit superior ORR activities compared with that of pure Co9S8 catalysts, demonstrating the 11



importance of the synergistic integration of N-doped carbon layer and Co9S8. 41, 42 The electrocatalytic performance of Co9S8@N-C was affected by the different thickness of carbon layer. Evidently, 0.5-Co9S8@N-C presents the lowest half-wave potential (E1/2, 0.83 V), which is 70 and 24 mV higher relative to 0.1-Co9S8@N-C and 1-Co9S8@N-C, respectively. As shown in Figure 5b, both the treatment temperature and dopamine contents could affect the catalytic performance for ORR, 700 oC and 0.5 g of dopamine are the best parameters. In order to research the ORR kinetics of the prepared Co9S8@N-C, LSV were performed at various rotation rates from 400-2000 rpm (Figure 5c). Electron transfer numbers (n) of the prepared 0.5-Co9S8@N-C were calculated from Koutecky-Levich (K-L) equation (see supporting information) at the potentials of 0.65, 0.7, 0.75 and 0.8 V (Figure 5d) and the reaction electron numbers is around 4, which confirms a 4 e- transfer pathway for OH- production in basic electrolyte of 0.5-Co9S8@N-C. The excellent catalytic performance of the prepared catalyst can be attributed to several factors: the hollow structured microtube can offer abundant catalytic sites for ORR and benefit the electrolyte transfer; the carbon shell not only in favor of electrical conductivity but also provide cooperation effect with the metal core; reasonable thickness of carbon shell can’t hinder the metal core for ORR; the exposed (022) crystal facet exhibits outstanding catalyticactivity.

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Figure 5. (a) LSV curves of 0.1-Co9S8@N-C, 0.5-Co9S8@N-C, 1-Co9S8@N-C, N-C, Co9S8 and Pt/C at a rotation speed of 1600 rpm under O2 saturated 0.1 M KOH with a scan rate of 5 mV s-1. (b) The relationship between E1/2 and treatment temperatures and content of dopamine. (c) LSV curves of 0.5-Co9S8@N-C at 400 rpm, 600 rpm, 900 rpm, 1200 rpm, 1600 rpm and 2000 rpm in O2 saturated 0.1 M KOH with a scan rate of 5 mV s-1. (d) K-L plots of 0.5-Co9S8@N-C at different potentials of 0.65, 0.7, 0.75 and 0.8 V. Apart from the influence of the thickness for ORR, the annealing temperature are also investigated towards ORR in order to optimize the preparation conditions. The detailed characterizations of samples are presented in Figure S8 and Figure S9, XRD and SEM images show almost no difference for the catalysts annealing at temperature of 600 oC, 700 oC and 800 oC. Therefore, the STEM images and the corresponding 13



EELS were also performed for further characterization. It can be seen in Figure S10 that the carbon layer was not coated on the microtube tightly, indicating an insufficient carbonization of dopamine at temperature of 600 oC. In comparison, the catalysts calcined at 800 oC showed obvious discontinuous of carbon layers, which may be ascribed to the damage of the carbon materials at higher calcine temperature (Figure S11). Thus, the insufficient carbonization or the damage of carbon materials may both play negative behavior to the ORR performance. The electrocatalytic ORR activities of the prepared Co9S8@N-C at different treatment temperatures were first measured by cyclic voltametry (CV) in N2 (dot line) and O2 (solid line) saturated 0.1 M KOH electrolyte (Figure 6a). Relative to CV curves in N2 with no evident peaks, evident oxygen reduction reaction peaks were detected in all the samples obtained at various temperatures indicating the oxygen reduction ability of different electrodes. Relative to Co9S8@N-C treated at 600 oC and 800 oC, Co9S8@N-C exhibits more positive oxygen reduction peaks around 0.79 V, which suggests Co9S8@N-C prepared at 700 oC exhibits the best catalytic performance. LSV tests were conducted in O2 saturated 0.1 M KOH to determine the electrocatalytic performance (Figure 6b). An excellent catalyst for ORR should have positive onset-potential and E1/2. As shown in Figure 4b, Co9S8@N-C prepared at 700 oC exhibits the highest onset-potential (0.89 V) and E1/2 (0.83 V) relative to 600 oC (0.82, 0.75 V) and 800 oC (0.84, 0.76 V). It can be concluded that the processing temperature was a significant route to tune the catalytic activity. On the one hand, Co9S8@N-C treated at 700 oC presents uniform hollow structure and continuous carbon layer, relative to 600 oC and 800 oC, which 14


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can offer sufficient catalytic sites and excellent conductivity, benefiting electron and electrolyte transfer. On the other hand, high temperature will induce less N-doping in carbon, as verified by XPS survey-scan spectrum (Figure S6). The N atomic content of Co9S8@N-C was calculated to be 4.17%, 3.45% and 2.04%, respectively, at 600 oC, 700 oC and 800 oC. This may be another major reason leading to a significant decrease in ORR performance for Co9S8@N-C treated at 800 oC.

Figure 6. (a) CV curves of 0.5-Co9S8@N-C at different temperatures of 600 oC, 700 o

C and 800 oC under N2 (dot line) and O2 (solid line) saturated 0.1 M KOH electrolyte

with a scan rate of 10 mV s-1. (b) LSV curves of 0.5-Co9S8@N-C at different temperatures in O2 saturated 0.1 M KOH with 1600 rpm at scan rate of 5 mV s-1. RRDE measurement was performed on Co9S8@N-C electrode to further investigate the ORR procedure. In Figure 7a, the ring current is negligible relative to the disk current, demonstrating little H2O2 produced during ORR. The yield of H2O2 and n were calculated through the equations of 3 and 4 (see the experimental section). As shown in Figure 7b, the H2O2 yield of the prepared Co9S8@N-C is below 20% in 0.2-0.85 V and n is around 4 which is in accordance with the RDE results, demonstrating the obtained nanomaterial was mainly through an ideal 4 e- process for ORR. The durability of methanol tolerance is also an important parameter for an 15



excellent catalyst for ORR, due to methanol molecules can permeate the membrane and causing serious performance degradation of the catalyst. In Figure 7c, the LSV of Co9S8@N-C remained almost unchanged, relative to Pt/C, before and after methanol added into 0.1 M KOH electrolyte, demonstrating the prepared catalyst possesses outstanding methanol resistance. For practical applications, the cycling durable ability is a significant assessment for the prepared catalyst and the stability of the catalyst was evaluated by current time (i-t) chronoamperometric strategy at 0.7 V with a rotation rate of 1600 rpm (Figure 7d). The prepared Co9S8@N-C presents about 15% loss after 20000 s which is lower than Pt/C with around 40% current loss. Therefore, the obtained Co9S8@N-C catalyst possesses an ideal 4 e- pathway for ORR, excellent tolerance for methanol crossover and outstanding durability in basic solution.

Figure 7. (a) Ring and disk currents of 0.5-Co9S8@N-C electrode with 1600 rpm in 16


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0.1 M KOH. (b) H2O2 production and n of 0.5-Co9S8@N-C. (c) LSV curves of 0.5-Co9S8@N-C and Pt/C in O2 saturated 0.1 M KOH electrolyte at the presence of 1 M methanol. (d) Durability tests of 0.5-Co9S8@N-C and Pt/C at 0.7 V with a rotation rate of 1600 rpm in O2 saturated 0.1 M KOH electrolyte.

Conclusion To summarize, we developed an effective ORR electrocatalyst consisting of N-doped carbon layer coated hollow Co9S8 microtubes (Co9S8@N-C microtubes) using novel CoS1.097 microwires as the precursor. The uniform and controlled coating of N-doped carbon layer on Co9S8 surface was easily obtained through simple immersion of CoS1.097 microwires in dopamine solution and followed by thermal pyrolysis at different temperatures. The Co9S8 microtubes exhibits single crystal nature with dominated (022) plane, as evidenced by TEM equipment with electron energy loss spectrum. Benefiting from hollow architecture and exposed (022) plane of Co9S8 microtubes as well as strong synergetic effect between N-doped carbon layer and Co9S8 microtubes, the newly developed catalyst presents excellent ORR performances with high E1/2, outstanding durability and methanol tolerance. The results indicate that Co9S8@N-C microtubes used as the cathode electrocatalyst possess a promising application in fuel cells.

ASSOCIATED CONTENT Supporting Information 17



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Detailed additional SEM and TEM images, XRD patterns, electrochemical performance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Email:

[email protected];

[email protected];

[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This study was supported by Doctoral Found of QUST (010022873 and 0100229001), Natural Science Foundation of Shandong Province of China (ZR2017MB054). S/TEM and EELS mapping work was carried out at the Department of applied physics, The Hong Kong Polytechnic University, which are supported by the Hong Kong Research Grants Council through the Early Career Scheme (Project No. 25301617) and the Hong Kong Polytechnic University grant (Project No. 1-ZE6G). J. W., G. Z. and Y. Z. thanks Dr. Lu Wei for optimizing the JEOL JEM-2100F microscope. REFERENCES (1) Debe, M. K. Electrocatalyst approaches and challenges for automotive fuel cells.

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Boosting Oxygen Reduction Catalysis with N-doped Carbon Coated

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