C Nanotubes with Atomic Fe Sites: A Highly Active Cathode

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Fe/N/C Nanotubes with Atomic Fe Sites: A Highly Active Cathode Catalyst for Alkaline Polymer Electrolyte Fuel Cells Huan Ren, Ying Wang, Yao Yang, Xun Tang, Yanqiu Peng, Hanqing Peng, Li Xiao, Juntao Lu, Héctor D. Abruña, and Lin Zhuang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02340 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017

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Fe/N/C Nanotubes with Atomic Fe Sites: A Highly Active Cathode Catalyst for Alkaline Polymer Electrolyte Fuel Cells

Huan Ren1, Ying Wang1,2, Yao Yang3, Xun Tang1, Yanqiu Peng1, Hanqing Peng1, Li Xiao1,*, Juntao Lu1, Héctor D. Abruña3, and Lin Zhuang1,2,*

1

College of Chemistry and Molecular Sciences, 2Institute for Advanced Studies,

Wuhan University, Wuhan 430072, China; 3Department of Chemistry and Chemical Biology, Baker Lab, Cornell University, Ithaca, NY 14853-1301, U.S.A.

[email protected], [email protected]

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Abstract Fe-containing N-doped carbons (Fe/N/C) is a promising Pt-alternative catalyst for the

oxygen reduction reaction (ORR), and is believed to be more stable in alkaline media than in acids, thus particularly suitable to be applied as the cathode catalyst for alkaline polymer electrolyte fuel cells (APEFCs). However, there has hitherto been no successful report on high-performance APEFC based on Fe/N/C cathode, the reason of which is still not quite clear. Here we report a high-performance Fe/N/C catalyst and its application in APEFC. The catalyst precursor is adenosine, an environmentally benign N-rich biomolecule, which is polymerized via a solvothermal process and then carbonized through pyrolysis. The resulting Fe/N/C nanotubes are thoroughly characterized by a variety of microscopy and spectroscopy (SEM, TEM, XRD, XPS, Raman, Mössbauer, and STEM-EELS), which reveal a high surface N/C ratio (8 at%) and atomic Fe sites well dispersed at the wall of the nanotubes. The catalytic sites are identified to be Fe-N4. The volume-specific catalytic activity of the Fe/N/C catalyst toward the ORR is as good as that of the commercial 20 wt% Pt/C catalyst in alkaline solutions, and better in durability. The electronic conductivity of Fe/N/C turns out to be trivial in rotating-disk electrode experiments but key for fuel cell tests. The APEFC with Fe/N/C cathode (2 mg/cm2 in catalyst loading) exhibits a peak power density greater than 450 mW/cm2, the thus-far highest record in the literature for APEFC using nonprecious metal cathode. Our findings not only deepen the understanding of the structure-activity relationship of the Fe/N/C catalyst, but also mark a step toward its real application in APEFC.

Keywords Alkaline polymer electrolyte fuel cells, Oxygen reduction reaction, Nonprecious metal catalysts, Fe-containing N-doped carbons, Nanotubes, Atomic Fe sites, Structure-activity relationship

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Introduction The dependence of Pt-based catalysts is an inherent weakness of the proton exchange membrane fuel cells (PEMFCs), which has been precluding its widespread deployment.1,2 To avoid this obstacle, alkaline polymer electrolyte fuel cells (APEFCs) were proposed and intensively investigated in the recent decade.3 By switching from the acidic to alkaline media, it is, in principle, possible to employ nonprecious metal catalysts,3a,4 in particular for the cathode reaction, the oxygen reduction reaction (ORR), which demands a heavy use of Pt in acid. Several candidates, including metal oxides,5 metal carbides,6,7 and nitrogen-doped carbons (N/C),8 have been tested as the Pt-alternative ORR catalyst in alkaline solutions. Among them, the metal-containing N/C, especially Fe-containing N/C (Fe/N/C), seem to be highly promising due to the simple synthesis and the relatively high catalytic activity, as well as the stability in alkaline media.9 A realistic challenge for the Fe/N/C catalysts is that, although its catalytic activity toward the ORR looks comparable to that of Pt in rotating-disk-electrode (RDE) tests,9,10 the performance of APEFC using Fe/N/C cathode turns out to be still much lower than that using Pt cathode.11 Such a contradictory observation is related to the physical properties of the Fe/N/C material, such as the electronic conductivity, the active-site density, the porosity, and the hydrophilicity, in addition to the chemical property (i.e., the surface-specific catalytic activity).11f,12 Those physical properties can play important roles in a fuel-cell electrode, but may not have a major impact on the RDE results. Both the physical and chemical properties of Fe/N/C materials are strongly affected by the synthetic process.2,13 In order to gain sufficient electronic conductivity, high temperature graphitization processes are often applied,14 which, however, usually leads to a loss of the N element in the resulting catalyst. On the other hand, avoiding high-temperature treatment would result in hydrophilic amorphous structures, which will suffer from water flooding and high interfacial resistance during fuel cell operations.15 Another puzzle is about the role of Fe in Fe/N/C materials. Researchers believe that Fe can further activate N/C,16 but it is still unclear whether the Fe serves directly as the catalytic site (e.g., in a form of Fe-N4)17 or just as a promoter to tune the electronic property of N/C, even the Fe component is completely capped by carbon.18 In the present work, we report on a 3


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biomolecule-derived Fe/N/C nanotube catalyst with high N-doping level and well-dispersed atomic Fe sites. The catalytic sites are identified to be Fe-N4 through systematic characterizations. When applied in APEFC, the electronic conductivity of the Fe/N/C catalyst turns out to be a key factor, in addition to the catalytic activity. The APEFC using this Fe/N/C cathode has exhibited a power density greater than 0.45 W/cm2 at 60oC, which is thus far the highest record of APEFC using nonprecious metal catalysts.

Experimental section Catalyst preparation. The synthetic procedure of Fe/N/C catalyst is illustrated in Figure 1. A solvothermal method was used to prepare the Fe/N/C catalyst. A certain amount of FeCl3·6H2O (1.08 g), ZnCl2 (1.09 g) and adenosine (2.14 g) were dispersed in a mixed solvent (containing 10 ml ethanol and 15 ml N,N-Dimethylformamide, DMF) by heating and thoroughly stirring. ZnCl2 is a catalyst for the polymerization of the adenosine, and in the subsequent pyrolysis, ZnCl2 can act as a pore-forming agent. DMF functions in dissolving adenosine, while ethanol promotes the dissolution of metal salts. The mixture was then transferred into a stainless-steel autoclave (50 ml in capacity), heated at 180oC for 12 h, and cooled to room temperature. The resulting brown precipitate was collected and sequentially centrifugal washed with ultrapure water and ethanol for several times, and further dried at 80oC. The obtained black powder (Figure 1b & S1A) was pyrolyzed at 800oC under flowing argon for 3 h and cooled to room temperature. The resulting Fe/N/C nanotubes (Figure 1c & S1B) was ball-milled into powder (Figure S1C) and stirred in 0.5 M H2SO4 at 80oC for 10 h to remove those unstable Fe components (Figure S1D), followed by repeated washing with water. A second pyrolysis step (800 oC under flowing argon for 3 h) was further applied to obtain the final product Fe/N/C (Figure 1d & S1E). Material characterizations. The powder X-ray diffraction patterns were performed on a Rigaku SmartLab 9 kW X-ray diffractometer at room temperature, in Bragg-Brentano geometry employing Cu Kα lines focused radiation (1.54059 Å, 1.54439 Å) at 9 kW (45 kV, 200 mA) power. A laser micro-Raman spectrometer (Reinshaw in Via, Renishaw, 532 nm excitation wavelength) was applied to characterize the feature of carbon. The Brunauer-Emmett-Teller

(BET)

surface

area

(SBET)

was

received

through

N2 4


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adsorption-desorption isotherms measured at 77 K on a Micromeritics ASAP-2020. Prior to measurements, the samples were out-gassed at 150oC under N2 ﬂow for 6 h in the pressure range of 10-6~10-5 Torr. Pore size distributions and average pore volume were calculated by BJH method. The X-ray photoelectron spectroscopy (XPS) measurements were recorded on a spectrometer (EscaLab 250 Xi, Thermo Fisher) using Al Kα radiation (h = 1486.6 eV). The quantitative analyses of N and Fe were based on the peak intensities of N 1s and Fe 2p signals, respectively. The inductively coupled plasma optical emission spectrometry ICP-OES (IRIS Intrepid II XSP, Thermo) was used to analyze the Fe content in the samples. Scanning electron microscope (SEM, MERLIN Compact, Zeiss) was used to characterize the morphology of the products in each synthetic step. High-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100) was applied to characterize the structure of the final product. The scanning transmission electron microscope (STEM) images and electron energy loss spectroscopy (EELS) maps were acquired on a fifth-order aberration-corrected STEM (Cornell Nion UltraSTEM) operated at 100kV and with a beam convergence semi-angle of 30 mrad. Subångström resolution is achievable under such operating conditions.19a The Fe/N/C samples were sonicated in ethanol before being dipped on a Cu TEM grid coated with lacey carbon film. Linear combination of Power Law (LCPL) method was used to subtract background and extract Fe elemental signals from Fe L-edge of original EELS spectra.19b An electrical resistivity tester (FT-300, Ningbo rooko) was applied to analyze the powder volume resistivity of samples, 200 mg of the powder were placed in a cylindrical mold (φ 10 mm) between two metal plungers.

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Fe Mössbauer spectroscopy (MS 500, Oxford Instruments)

was performed at room temperature, the corresponding spectra were recorded and fitted with Lorentzian lines. Electrochemical measurements. To prepare the working electrode, 5 mg sample was dispersed ultrasonically in 1 ml diluted Nafion alcohol solution (0.05 wt%), and the suspension was pipetted onto a rotating disk electrode (RDE) with a glassy carbon (GC) substrate (φ 4 mm) or a rotating ring-disk electrode (RRDE, φ 4.57 mm, Pine Inc.), which was buff-polished with an alumina suspension (φ 0.05 µm) prior to use. The coated electrode was dried under an infrared lamp, and the sample loading is 0.4 mg/cm2. For comparison, 20 wt% Pt/C (Johnson Matthey Co.) was used by the same electrode preparation method. The Pt 5


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loading on the RDE was 80 µg/cm2. A sheet of carbon paper was used as the counter electrode, and a Hg/HgO electrode in the same solution was used as the reference electrode. A reversible hydrogen electrode (RHE)20 was used to calibrate the Hg/HgO electrode. Electrode potentials throughout this paper are converted to be versus RHE: E (RHE) = E (Hg/HgO) + 0.0592 pH + 0.098 V (1M KOH, 25oC). The potentiostat was a CHI-600A electrochemical station. ORR evaluation was carried out in O2-saturated 1 M KOH solution at room temperature in a one-compartment glass cell (In these short-term tests, there is no observable difference in results between using a glass cell or a Teflon cell). The rotation rate was 900 rpm, and the ORR curves were recorded at a scan rate of 5 mV/s. The background capacitive current measured in Ar-saturated 1 M KOH solution has been subtracted to plot the polarization curves of ORR. To evaluate the H2O2 yield and the electron transfer number of the catalysts, the Pt ring potential was set to 1.3 V (vs. RHE) in the course of RRDE measurements. The H2O2 yield was calculated by the following equation: Hଶ Oଶ ሺ%ሻ = 200 ×

‫ܫ‬ோ /ܰ଴ ‫ܫ‬ோ /ܰ଴ + ‫ܫ‬஽

where IR and ID are the ring and disk currents, respectively, and N0=0.22 is the collection efficiency of Pt ring. Accelerated durability tests were performed to assess the stability of catalysts in alkaline media by cycling the electrode potential between 0.6 V and 1.0 V at 100 mV/s for 10000 cycles in Ar-saturated electrolytes. The ORR polarization curves were obtained before and after 10,000 cycles, the electrolyte was changed to a fresh batch before each RDE test. Fuel cell tests. Single cells of APEFC were assembled and tested under a galvanic mode (850e Multi range, Scribner Associate Co.) using fully humidified H2 and O2 gasses flowing at a rate of 200 ml/min with a back pressure 0.1 MPa. The alkaline polymer electrolyte (APE) was aQAPS-Sx produced in our lab,3c,21 where x = 8 for the membrane (50±5 µm in thickness) and x = 14 for the ionomer. The weight percentage of aQAPS-S14 ionomer in the anode and the cathode was controlled to be 20 wt%. PtRu/C catalyst (60 wt%, Johnson Matthey) was used in the anode with a metal loading 0.4 mg/cm2. Fe/N/C catalyst was tested in the cathode with different loadings: 1 mg/cm2, 2 mg/cm2 and 4 mg/cm2. The catalyst-coated membrane

6


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(CCM) was pressed between two pieces of carbon paper (AvCard GDS3250) to make the membrane electrode assembly (MEA)，the active area of the single cell is 4 cm2. The fuel cell was activated by gradually increasing the current density under fully humidified conditions at 60oC. APEFC with Pt/C (20 wt%) cathode was also tested for comparison under the same conditions. The Pt loading in the cathode was 0.4 mg/cm2.

Results and discussion Structural analyses of the Fe/N/C catalyst. Before electrochemical and fuel-cell tests, in-depth characterizations were carried out to investigate the structure and the active site of the Fe/N/C catalyst. The structures of intermediates involved in each synthetic step and the final product were first monitored by XRD (Figure 2a). While there is no diffraction peak in the solvothermal product, Fe and Fe3C phases appear after the first pyrolysis step and remain unchanged after the ball-milling step. After the acid treatment and the second pyrolysis steps, the diffraction lines of the Fe3C phase become weak but still recognizable. Meanwhile, the diffraction line of the graphitic carbon component become very clear after the second pyrolysis step. The formation of the graphitic structure is also supported by Raman spectroscopy results (Figure 2b); the decreasing intensity ratio between the D band and the G band (ID/IG) indicates that the graphitization degree increases along with the preparation process. The surface atomic ratio (at%) and the bulk mass ratio (wt%) of samples at each step were estimated based on elemental analysis and XPS measurements (Table S1 & S2). While the elemental analysis indicates the bulk N content in the final Fe/N/C catalyst to be 6.75 wt%, the XPS measurements monitor the change in surface N content during the synthetic procedure: a decrease from the initial level of 26.17 at% (21.37 wt%) to the final level of 6.92 at% (7.58 wt%), corresponding to a final surface N/C ratio of 8.04 at%. This is a fairly high N-doping level for Fe/N/C materials reported in the literature.22 The XPS N-1s spectra of the final product reveals six binding energies: 398.3, 399.1, 399.9, 400.8, 401.8 and 403.4 eV (Figure 2c), corresponding to the pyridinic N, Fe-Nx, pyrrolic N, quaternary N or hydrogenated N,23d,23e graphitic N, and oxidized N, respectively. The high percentage of pyridinic N (25 at% in the total nitrogen) and Fe-Nx (20 at%) are believed to be favorable for 7


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the electrocatalysis of ORR.8f,23,24 The surface and bulk Fe content was detected by XPS and ICP, respectively (Table S1 & S2). The bulk Fe content of each sample turns out to be much more than the surface one, indicating that most of the Fe contents are effectively coated by carbon. To further reveal the chemical environments of Fe in the catalyst, 57Fe Mössbauer spectroscopy was employed to probe various Fe-sites in different oxidation and/or spin states. Figure 2d shows the Mössbauer spectra of the final sample, which are fitted with two doublets and two sextets. The isomer shift (IS), quadrupole splitting (QS), and hyperfine splitting are listed in Table S3. The doublet 1 can be assigned to the in-plane low-spin FeIIN4/C center, which is believed to be mainly responsible for the catalytic activity.2,24 The doublet 2 is attributed to an FeIIN4 site with Fe in low-spin state,24d distinguished from the first doublet by an Fe atom out of the N4 plane. The sextet 1 is corresponding to Fe3C, while the sextet 2 was assigned to α-Fe phase,24d accounting for 42% of all the Fe components according to the peak area. Figure 3 details the morphology and structure of the final Fe/N/C catalyst. The nanotube was broken by ball-milling, and most of the Fe component inside the nanotube was leached out by acid washing, but parts of them still remained. The crystalline lattice spacing of 0.37 nm and 0.26 nm, corresponding to the (011) and (102) planes of the Fe3C phase, can be clearly observed. The graphitic structure of the wall of the nanotubes is obvious, which, however, cannot be seen in samples prepared under the same conditions but without FeCl3 precursor (Figure S2). This indicates that Fe is necessary for the graphitization at 800oC. Coated by thick carbon nanotubes, the Fe3C particles should not be the responsible component for the electrocatalysis. To locate the Fe-N4 sites suggested by XPS and Mössbauer spectroscopy, high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) equipped with electron energy-loss spectroscopy (EELS) was employed to make further examinations. It can be clearly seen that even at the wall of those hollow Fe/N/C nanotubes (Figure 3d), there exist single Fe atoms (Figure 3e). EELS analyses further confirms that the element of the bright spots in STEM images is indeed Fe (Figure 3f & S3). Since single Fe atoms in metallic state are extremely active and should not be stable under electrochemical conditions, it is reasonable to conclude that these atomic Fe sites are coordinated by N, such as Fe-N4, as unraveled by XPS and Mössbauer spectroscopy. 8


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Evaluation of the ORR activity. RDE and RRDE tests were applied to evaluate the ORR activity of the Fe/N/C catalyst. To make a meaningful and practical comparison with the commercial Pt/C catalyst, we controlled the mass of carbon in Fe/N/C to be the same as that in Pt/C, such that the thickness of the catalyst layer on the electrode surface was the same (assuming the packing densities of carbon are equal). In this work, the carbon content of Fe/N/C turned out to be 82 wt%, close to that in 20 wt% Pt/C. Therefore, we can directly compare the ORR polarization curves between Fe/N/C and 20 wt% Pt/C with the same catalyst loading. As displayed in Figure 4a, the Fe/N/C exhibits a relatively negative onset potential but an identical half-wave potential of 0.93 V (vs. RHE) compared with Pt/C, and the Tafel slopes are also close for Fe/N/C and Pt/C (Figure S4). With lower catalyst loading, the Fe/N/C catalyst can perform even slightly better than does the Pt/C (Figure S5). The H2O2 yield and the electron transfer number, revealed by RRDE measurements, confirm that the ORR occurring on Fe/N/C is mostly a 4-electron reaction (Figure 4b). Accelerated durability tests were performed to assess the stability of catalysts in alkaline media by cycling the electrode potential between 0.6 V and 1.0 V for 10000 cycles. As shown in Figure 4c & 4d, while the ORR performance of Pt/C experienced a decay by ca. 40 mV negative shift in half-wave potential, the change of Fe/N/C under the same condition is only 15 mV, indicating the durability of the Fe/N/C catalyst in alkaline media is superior over Pt/C. APEFC performance. The Fe/N/C catalyst was finally applied to a real APEFC, the single cell performance is presented in Figure 5a. When the catalyst loading was increased from 1 mg/cm2 to 2 mg/cm2, the peak power density increased from 287 mW/cm2 to 450 mW/cm2. Further increase in catalyst loading did not bring an obvious enhancement in the power density, a peak power density of 473 mW/cm2 was obtained when the catalyst loading was 4 mg/cm2. This is understandable since greater catalyst loading results in thicker catalyst layer and greater resistance to the charge transfer and mass transport. To the best of our knowledge, the here-reported cell performance of APEFC with Fe/N/C cathode is the highest record in the literature (Table S5),11,25 yet it is still lower than that with Pt/C cathode26 (Figure S6). This is most probably because of the greater electrical resistance of the Fe/N/C electrode, which can be judged from the larger slope of the polarization curve of Fe/N/C at moderate current densities (0.2~0.8 A/cm2 in Figure S6), in comparison to the 9


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Pt/C cathode. To further enhance the conductivity, Fe/N/C catalysts were prepared with higher pyrolysis temperature (900oC & 1000oC, respectively), whose electronic conductivity is nearly one order greater than that prepared at 800oC (Figure S7a). Figure 5b shows that the peak power density of APEFC with the Fe/N/C catalyst prepared at 1000oC can be further increased to 485 mW/cm2 at a moderate catalyst loading (2 mg/cm2). Although the N-doping level of the 900oC and 1000oC treated samples actually decline to some degree (Table S2), there is no obvious change in the BET surface area (Table S4) and the ORR activity (Figure S7b), which, once again, indicates that the electronic conductivity of the Fe/N/C catalyst is not sensitive to RDE test but important to the cell performance.

Conclusion In the present work, a high-performance Fe/N/C catalyst was prepared by using a novel precursor, adenosine, which is a naturally abundant, N-rich biomolecule. The structure of the Fe/N/C catalyst is carbon nanotubes with high N-doping level (8 at% of surface N/C ratio). Atomically dispersed Fe-N4 sites are found at the wall of the nanotubes, which are the source of the observed high activity. The Fe/N/C catalyst exhibited a Pt-comparable ORR activity and a better stability in alkaline media. The power density of APEFC with the Fe/N/C cathode is over 450 mW/cm2 at a moderate catalyst loading (2 mg/cm2). In addition to the intrinsic catalytic activity, the electronic conductivity of the Fe/N/C catalyst turns out to be a key factor for the cell performance. These findings have not only furthered out understanding of the structure-activity relationship of Fe/N/C, but also made an effective progress in the development of APEFC technology with nonprecious metal catalysts.

Supporting Information: Table S1~S5 and Figure S1~S7.

Acknowledgments This work was financially supported by the National Basic Research Program (2012CB932800, 2012CB215503), the National Natural Science Foundation (21125312, 91545205), the 111 project (111-2-10), and the Fundamental Research Funds for the Central Universities (2014203020207). The STEM characterization was carried out under financial 10


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support of the energy materials center at Cornell (emc2) an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences, under award no. DESC0001086. This work made use of TEM facilities of the Cornell Center for Materials Research (CCMR) which are supported through the National Science Foundation Materials Research Science and Engineering Center (NSF MRSEC) program (DMR-1120296). We are grateful to Malcolm (Mick) Thomas at CCMR for the help of Nion UltraSTEM. We appreciate the assistance about EELS mapping from Prof. David Muller in School of Applied & Engineering Physics at Cornell University.

References 1. a) Berger, D. J., Science 1999, 286, 49-49; b) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B: Environ 2005, 56, 9-35; c) Wang, Y. J.; Wilkinson, D. P.; Zhang, J. Chem. Rev. 2011, 111, 7625-7651; d) Debe, M. K. Nature 2012, 486, 43-51; e) Rabis, A.; Rodriguez, P.; Schmidt, T. J. ACS Catal. 2012, 2, 864-890. 2. Chen, Z.; Dodelet, J. P.; Zhang, J. Non-noble Metal Fuel Cell Catalysts, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, 2014. 3. a) Lu, S.; Pan, J.; Huang, A.; Zhuang, L.; Lu, J. Proceedings of the National Academy of Sciences 2008, 105, 20611-20614; b) Varcoe, J. R.; Slade, R. C. Fuel Cells 2005, 5, 187-200; c) Pan, J.; Chen, C.; Zhuang, L.; Lu, J. Acc. Chem. Res. 2012, 45, 473-481; d) Asazawa, K.; Yamada, K.; Tanaka, H.; Oka, A.; Taniguchi, M.; Kobayashi, T. Angew. Chem. Int. Ed. 2007, 46, 8024-8027. 4. a) Spendelow, J. S.; Wieckowski, A. Phys. Chem. Chem. Phys. 2007, 9, 2654-2675; b) Yang, C.; Huang, B.; Xiao, L.; Ren, Z.; Liu, Z.; Lu, J.; Zhuang, L. Chem. Commun. 2013, 49, 11023-11025; c) Sheng, W.; Bivens, A. P.; Myint, M.; Zhuang, Z.; Forest, R. V.; Fang, Q.; Chen, J. G.; Yan, Y. Energy Environ. Sci. 2014, 7, 1719-1724; d) Li, Q.; Cao, R.; Cho, J.; Wu, G. Adv. Energy Mater. 2014, 4, 1301415; e) Ge, X.; Sumboja, A.; Wuu, D.; An, T.; Li, B.; Goh, F. T.; Hor, T. A.; Zong, Y.; Liu, Z. ACS Catal. 2015, 5, 4643-4667. 5. a) Cheng, F.; Shen, J.; Peng, B.; Pan, Y.; Tao, Z.; Chen, J. Nat. Chem. 2011, 3, 79-84; b) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T., Dai, H. Nat. Mater. 2011, 10, 780-786; c) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; 11


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Shaohorn, Y. Nat. Chem. 2011, 3, 546-550; d) Xu, J.; Gao, P., Zhao, T. Energy Environ. Sci. 2012, 5, 5333-5339. 6. a) Zhou, X.; Qiu, Y.; Yu, J.; Yin, J.; Gao, S. Int. J. Hydrogen Energy 2011, 36, 7398-7404; b) Zhong, G.; Wang, H.; Yu, H.; Peng, F. Fuel Cells 2013, 13, 387-391. 7. Hu, Y.; Jensen, J. O.; Zhang, W.; Cleemann, L. N.; Xing, W.; Bjerrum, N. J.; Li, Q. Angew. Chem. Int. Ed. 2014, 53, 3675-3679. 8. a) Gong, K.; Du, F.; Xia, Z.; Durstock, M. F.; Dai, L. Science 2009, 323, 760-764; b) Qu, L.; Liu, Y.; Baek, J.; Dai, L. ACS Nano 2010, 4, 1321-1326; c) Liang, H. W.; Zhuang, X.; Brüller, S.; Feng, X.; Müllen, K. Nat. Commun. 2014, 5, 4973; d) Yang, W.; Fellinger, T. P., Antonietti, M. J. Am. Chem. Soc. 2010, 133, 206-209; e) Zhao, Y.; Yang, L.; Chen, S.; Wang, X.; Ma, Y.; Wu, Q.; Jiang, Y.; Qian, W.; Hu, Z. J. Am. Chem. Soc. 2013, 135, 1201-1204; f) Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Science 2016, 351, 361-365. 9. a) Chung, H. T.; Won, J. H.; Zelenay, P. Nat Commun. 2013, 4, 1922; b) Liu, J.; Sun, X.; Song, P.; Zhang, Y.; Xing, W.; Xu, W. Adv. Mater. 2013, 25, 6879-6883; c) Li, Y.; Zhou, W.; Wang, H.; Xie, L.; Liang, Y.; Wei, F.; Idrobo, J. C.; Pennycook, S. J.; Dai, H. Nat. Nanotechnol. 2012, 7, 394-400; d) Lin, L.; Zhu, Q.; Xu, A. W. J. Am. Chem. Soc. 2014, 136, 11027-11033; e) Niu, W.; Li, L.; Liu, X.; Wang, N.; Liu, J.; Zhou, W.; Tang, Z.; Chen, S. J. Am. Chem. Soc. 2015, 137, 5555-5562; f) Kim, B. J.; Lee, D. U.; Wu, J.; Higgins, D.; Yu, A.; Chen, Z. J. Phys. Chem. C 2013, 117, 26501-26508; g) Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M. T.; Jaouen, F.; Mukerjee, S. Nat Commun. 2015, 6, 7343. 10. a) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443-447; b) Proietti, E.; Jaouen, F.; Lefèvre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J. P. Nat Commun. 2011, 2, 416; b) Wang, Y. C.; Lai, Y. J.; Song, L.; Zhou, Z. Y.; Liu, J. G.; Wang, Q.; Yang, X. D.; Chen, C.; Shi, W.; Zheng, Y. P.; Rauf, M.; Sun, S. G. Angew. Chem. Int. Ed. 2015, 54, 9907-9910; c) N. Ramaswamy, U. Tylus, Q. Jia, S. Mukerjee, J. Am. Chem. Soc. 2013, 135, 15443-15449; d) U. I. Kramm, I. Herrmann-Geppert, J. Behrends, K. Lips, S. Fiechter, P. Bogdanoff, J. Am. Chem. Soc. 2016, 138, 635-640. 11. a) Rao, C. V., Ishikawa, Y., J. Phys. Chem. C 2012, 116, 4340-4346; b) Guo, J.; He, H.; Chu, 12


Page 13 of 22

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

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D.; Chen, R., Electrocatal. 2012, 3, 252-264; c) Song, P., Wang, Y., Pan, J., Xu, W., Zhuang, L., J. Power Sources 2015, 300, 279-284; d) Chen, C.; Yang, X. D.; Zhou, Z. Y.; Lai, Y. J.; Rauf, M.; Wang, Y.; Pan, J.; Zhuang, L.; Wang, Q.; Wang, Y. C., Chem. Commun. 2015, 51, 17092-17095; e) Kim, O.; Cho, Y.; Chung, D. Y.; Kim, M. J.; Yoo, J. M.; Park, J. E.; Choe, H.; Sung, Y., Sci. Rep. 2015, 5, 8376; f) Li, X.; Popov, B. N.; Kawahara, T.; Yanagi, H., J. Power Sources 2011, 196, 1717-1722; g) Mamlouk, M.; Kumar, S. M. S.; Gouerec, P.; Scott, K., J. Power Sources 2011, 196, 7594-7600; h) Sa, Y. J.; Park, C.; Jeong, H. Y.; Park, S.; Lee, Z.; Kim, K. T.; Park, G.; Joo, S. H., Angew. Chem. Int. Ed. 2014, 53, 4102-4106; i) Sa, Y. J.; Seo, D.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T. J.; Shin, H. S., J. Am. Chem. Soc. 2016, 138, 15046-15056; j) Du, R.; Zhang, N.; Zhu, J.; Wang, Y.; Xu, C.; Hu, Y.; Mao, N.; Xu, H.; Duan, W.; Zhuang, L., Small 2015, 11, 3903-3908; k) S. Lee, M. Choun, Y. Ye, J. Lee, Y. Mun, E. Kang, J. Hwang, Y. H. Lee, C. H. Shin, S. H. Moon, S. K. Kim, E. Lee, J. Lee, Angew. Chem. Int. Ed. 2015, 54, 9230. 12. a) Charreteur, F.; Jaouen, F.; Ruggeri, S.; Dodelet, J. P. Electrochim. Acta 2008, 53, 2925-2938; b) Morozan, A.; Sougrati, M. T.; Goellner, V.; Jones, D. J.; Stievano, L.; Jaouen, F. Electrochim. Acta 2014, 119, 192-205; c) Borup, R. L.; Meyers, J. P.; Pivovar, B. S.; Kim, Y. S.; Mukundan, R.; Garland, N.; Myers, D. J.; Wilson, M. S.; Garzon, F. H.; Wood, D. Chem. Rev. 2007, 107, 3904-3951; d) F. Charreteur, S. Ruggeri, F. Jaouen, J.P. Dodelet, Electrochim. Acta 2008, 53, 6881; e) Antolini, E. Appl. Catal. B: Environ 2009, 88, 1-24; f) He, C.; Desai, S.; Brown, G.; Bollepalli, S. Interface-Electrochemical Society 2005, 14, 41; g) Shanahan, P. V.; Xu, L.; Liang, C.; Waje, M.; Dai, S.; Yan, Y. J. Power Sources 2008, 185, 423-427; h) Li, A.; Han, M.; Chan, S. H.; Nguyen, N. T. Electrochim. Acta 2010, 55, 2706-2711; i) Jaouen, F.; Dodelet, J. P. J. Phys. Chem. C 2007, 111, 5963-5970; j) Jaouen, F.; Serventi, A. M.; Lefèvre, M.; Dodelet, J. P.; Bertrand, P. J. Phys. Chem. C 2007, 111, 5971-5976; k) Zhang, Z.; Veith, G. M.; Brown, G. M.; Fulvio, P. F.; Hillesheim, P. C.; Dai, S.; Overbury, S. H. Chem. Commun. 2014, 50, 1469-1471; l) Zhou, M.; Yang, C.; Chan, K.Y. Adv. Energy Mater. 2014, 4, 1400840. m) Fu, X.; Zamani, P.; Choi, J.; Hassan, F. M.; Jiang, G.; Higgins, D. C.; Zhang, Y.; Hoque, M. A.; Chen, Z. Adv. Mater. 2017, 29, 1604456. 13. a) Bezerra, C. W. B.; Zhang, L.; Liu, H. C.; Lee, K.; Marques, A. L. B.; Marques, E. P.; Wang, H.; Zhang, J. J. Power Sources 2007, 173, 891-908; b) Othman, R.; Dicks, A. L.; Zhu, 13


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Z. Int. J. Hydrogen Energy 2012, 37, 357-372; c) Dombrovskis, J. K.; Palmqvist, A. E. C. Fuel Cells 2016, 16, 4-22; d) Wu, G.; Zelenay, P. Acc. Chem. Res. 2013, 46, 1878-1889; e) Jaouen, F.; Herranz, J.; Lefevre, M.; Dodelet, J. P.; Kramm, U. I.; Herrmann, I.; Bogdanoff, P.; Maruyama, J.; Nagaoka, T.; Garsuch, A. ACS Appl. Mater. Interfaces 2009, 1, 1623-1639; e) Jaouen, F.; Proietti, E.; Lefèvre, M.; Chenitz, R.; Dodelet, J. P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P. Energy Environ. Sci. 2011, 4, 114-130; f) Chen, Z.; Higgins, D.; Yu, A.; Zhang, L.; Zhang, J. Energy Environ. Sci. 2011, 4, 3167-3192; g) Nie, Y.; Li, L.; Wei, Z. Chem. Soc. Rev. 2015, 44, 2168-2201; h) Noto, V. D.; Negro, E.; Polizzi, S.; Vezzu, K.; Toniolo, L.; Cavinato, G. Int. J. Hydrogen Energy, 2014, 39, 2812-2827; i) Noto, V. D.; Negro, E.; Vezzu, K.; Bertasi, F.; Nawn, G. Electrochem. Soc. Interface, 2015, 54, 59-64. 14. a) Ding, W.; Wei, Z.; Chen, S.; Qi, X.; Yang, T.; Hu, J.; Wang, D.; Wan, L. J.; Alvi, S. F.; Li, L. Angew. Chem. Int. Ed. 2013, 52, 11755-11759; b) K Singh, K. P.; Song, M. Y.; Yu, J. J. Mater. Chem. A 2014, 2, 18115-18124; c) Yang, D.; Chaudhari, S.; Rajesh, K. P.; Yu, J. ChemCatChem 2014, 6, 1236-1244; d) Xu, Z.; Shen, L.; Wu, Q.; Sun. T.; Xu. Y.; Li, D.; Du, L.; Yang, L.; Wang. X.; Hu, Z.; Acta Chim. Sinica 2015, 73, 793-798. 15. a) Yang, L.; Larouche, N.; Chenitz, R.; Zhang, G.; Lefevre, M.; Dodelet, J. P. Electrochim. Acta 2015, 159, 184-197; b) Kitahara, T.; Nakajima, H.; Okamura, K., J. Power Sources 2015, 283, 115-124; c) Joo, J.; Choun, M.; Kim, K.; Uhm, S.; Kim, Y. D.; Lee, J. Curr. Appl. Phys. 2014, 14, 1374-1379; d) Choi, J. Y.; Yang, L.; Kishimoto, T., Fu, X.; Ye, S.; Z. Chen, Z. Energy Environ. Sci. 2016, 10, 296-305. 16. a) Lei, Z.; Lee, K.; Bezerra, C. W. B.; Zhang, J.; Zhang, J. Electrochim. Acta 2009, 54, 6631; b) Wu, G.; Johnston, C. M.; Mack, N. H.; Artyushkova, K.; Ferrandon, M.; Nelson, M.; Lezamapacheco, J. S.; Conradson, S. D.; More, K. L.; Myers, D. J. J. Mater. Chem. 2011, 21, 11392-11405; c) Oh, H. S.; Kim, H. J. Power Sources 2012, 212, 220-225; d) Masa, J.; Zhao, A.; Xia, W.; Sun, Z.; Mei, B.; Muhler, M.; Schuhmann, W. Electrochem. Commun. 2013, 34, 113-116; e) Wang, L.; Ambrosi, A. Angew. Chem. Int. Ed. 2013, 52, 13818-13821; f) Masa, J.; Zhao, A.; Xia, W.; Muhler, M.; Schuhmann, W. Electrochim. Acta 2014, 128, 271-278; g) Peng, H.; Liu, F.; Liu, X.; Liao, S.; You, C.; Tian, X.; Nan, H.; Luo, F.; Song, H.; Fu, Z. ACS Catal. 2014, 4, 3797-3805; h) Qiao, X.; Peng, H.; You, C.; Liu, F.; Zheng, R.; Xu, D.; Li, X.; Liao, S. J. Power Sources 2015, 288, 253-260. 14


Page 15 of 22

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17. a) Yeager, E. Electrochim. Acta 1984, 29, 1527-1537; b) Wiesener, K. Electrochim. Acta 1986, 31, 1073-1078; c) Lefèvre, M.; Dodelet, J.; Bertrand, P. J. Phys. Chem. B 2002, 106, 8705-8713; d) Koslowski, U. I.; Abswurmbach, I.; Fiechter, S.; Bogdanoff, P. J. Phys. Chem. C 2008, 112, 15356-15366; e) Anderson, A. B.; Sidik, R. A. J. Phys. Chem. B 2004, 108, 5031-5035; f) Kramm, U. I.; Herranz, J.; Larouche, N.; Arruda, T. M.; Lefevre, M.; Jaouen, F.; Bogdanoff, P.; Fiechter, S.; Abswurmbach, I.; Mukerjee, S. Phys. Chem. Chem. Phys. 2012, 14, 11673-11688; g) Ferrandon, M.; Kropf, A. J.; Myers, D. J.; Artyushkova, K.; Kramm, U. I.; Bogdanoff, P.; Wu, G.; Johnston, C. M.; Zelenay, P. J. Phys. Chem. C 2012, 116, 16001-16013; h) Sahraie, N. R.; Kramm, U. I.; Steinberg, J.; Zhang, Y.; Thomas, A.; Reier, T.; Paraknowitsch, J. P.; Strasser, P. Nat. Commun. 2015, 6; 8618; i) Deng, D.; Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Deng, J.; Sun, G.; Bao, X. Angew. Chem. Int. Ed. 2013, 52, 371-375; j) Wang, Q.; Zhou, Z. Y.; Lai, Y. J.; You, Y.; Liu, J. G.; Wu, X. L.; Terefe, E.; Chen, C.; Song, L.; Rauf, M. J. Am. Chem. Soc. 2014, 136, 10882-10885; k) Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M. T.; Jaouen, F.; Mukerjee, S. Nat. Commun. 2015, 6, 7343. 18. a) Wen, Z.; Ci, S.; Zhang, F.; Feng, X.; Cui, S.; Mao, S.; Luo, S.; He, Z.; Chen, J. Adv. Mater. 2012, 24, 1399-1404; b) Lee, J.; Park, G. S.; Kim, S. T.; Liu, M.; Cho, J. Angew. Chem. Int. Ed. 2013, 125, 1060-1064; c) Hou, Y.; Huang, T.; Wen, Z.; Mao, S.; Cui, S.; Chen, J. Adv. Energy Mater. 2014, 4, 1400337; d) Wang, J.; Wang, G.; Miao, S.; Jiang, X.; Li, J.; Bao, X. Carbon 2014, 75, 381-389; e) Wu, Z.; Xu, X.; Hu, B.; Liang, H.; Lin, Y.; Chen, L.; Yu, S. Angew. Chem. Int. Ed. 2015, 54, 8179-8183; f) Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S. J. Am. Chem. Soc. 2015, 137, 1436-1439. 19. a) Huang, P. Y.; Ruizvargas, C. S.; Der Zande, A. M. V.; Whitney, W. S.; Levendorf, M. P.; Kevek, J. W.; Garg, S.; Alden, J. S.; Hustedt, C. J.; Zhu, Y. Nature 2011, 469, 389-392; b) Cueva, P.; Hovden, R.; Mundy, J. A.; Xin, H. L.; Muller, D. A. Microscopy and Microanalysis 2012, 18, 667-675. 20. Gong, S.; Lu, J.; Yan, H. J. Electroanal. Chem. 1997, 436, 291-293. 21. Pan, J.; Chen, C.; Li, Y.; Wang, L.; Tan, L.; Li, G.; Tang, X.; Xiao, L.; Lu, J.; Zhuang, L. Energy Environ. Sci. 2014, 7, 354-360. 22. a) Serov, A.; Artyushkova, K.; Atanassov, P. Adv. Energy Mater. 2014, 4,1301735; b) Liu, 15


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Page 16 of 22

G.; Li, X.; Ganesan, P.; Popov, B. N. Appl. Catal. B: Environ 2009, 93, 156-165; c) Ganesan, S.; Leonard, N.; Barton, S. C. Phys. Chem. Chem. Phys. 2014, 16, 4576-4585; d) Parvez, K.; Yang, S.; Hernandez, Y.; Winter, A.; Turchanin, A.; Feng, X.; Müllen, K. ACS Nano 2012, 6, 9541-9550; e) Ding, W.; Li, L.; Xiong, K.; Wang, Y.; Li, W.; Nie, Y.; Chen, S.; Qi, X.; Wei, Z. J. Am. Chem. Soc. 2015, 137, 5414-5420. 23. a) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S. Energy Environ. Sci. 2012, 5, 7936-7942; b) Yang, S.; Feng, X.; Wang, X.; Müllen, K. Angew. Chem. Int. Ed. 2011, 50, 5339-5343; c) Sharifi, T.; Hu, G.; Jia, X.; Wågberg, T. ACS Nano 2012, 6, 8904-8912; d) Artyushkova, K.; Serov, A.; Rojascarbonell, S.; Atanassov, P. J. Phys. Chem. C 2015, 119, 25917-25928; e) Matanovic, I.; Artyushkova, K.; Strand, M. B.; Dzara, M. J.; Pylypenko, S.; Atanassov, P. J. Phys. Chem. C 2016, 120, 29225-29232. 24. a) Kramm, U. I.; Herrmanngeppert, I.; Bogdanoff, P.; Fiechter, S. J. Phys. Chem. C 2011, 115, 23417-23427; b) Kramm, U. I.; Larouche, N.; Schmeisser, D.; Dodelet, J. P. J. Am. Chem. Soc. 2014, 136, 978-985; c) Bouwkampwijnoltz, A. L.; Visscher, W.; Van Veen, J. A. R.; Boellaard, E.; Der Kraan, A. M. V.; Tang, S. C. J. Phys. Chem. B 2002, 106, 12993-13001; d) Szakacs, C. E.; Lefèvre, M.; Kramm, U. I.; Dodelet, J. P.; Vidal, F., Phys. Chem. Chem. Phys. 2014, 16, 13654-13661. 25. a) Alesker, M.; Page, M.; Shviro, M.; Paska, Y.; Gershinsky, G.; Dekel, D. R.; Zitoun, D. J. Power Sources 2016, 304, 332-339; b) Li, X.; Popov, B. N.; Kawahara, T.; Yanagi, H. J. Power Sources 2011, 196, 1717-1722; c) He, Q.; Li, Q.; Khene, S.; Ren, X.; López-Suárez, F. E.; Lozano-Castelló, D.; Bueno-López, A.; Wu, G. J. Phys. Chem. C 2013, 117, 8697-8707. 26. a) Wang, Y.; Wang, G.; Li, G.; Huang, B.; Pan, J.; Liu, Q.; Han, J.; Xiao, L.; Lu, J.; Zhuang, L. Energy Environ. Sci. 2015, 8, 177-181; b) Kaspar, R. B.; Letterio, M. P.; Wittkopf, J. A.; Gong, K.; Gu, S.; Yan, Y. J. Electrochem. Soc. 2015, 162, F483- F488; c) Li, G.; Pan, J.; Han, J.; Chen, C.; Lu, J.; Zhuang, L. J. Mater. Chem. A 2013, 1, 12497-12502; d) Mamlouk, M.; Horsfall, J. A.; Williams, C.; Scott, K. Int. J. Hydrogen Energy 2012, 37, 11912-11920.

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Figure 1. The synthetic procedure of Fe/N/C nanotubes derived from adenosine. Adenosine (a) was used as the precursor and polymerized through a solvothermal method, the thus-prepared sample (b) was pyrolyzed at 800oC to form Fe/N/C nanotubes (c). After ballmilling, acid washing, and second pyrolysis step, the Fe/N/C nanotubes were broken (d), parts of which still contain Fe3C particles (e). Scale bars = 200 nm (b, c, d); 50 nm (e).

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Figure 2. Structural analyses of the Fe/N/C nanotubes. (a) XRD patterns of samples obtained after each synthetic step (as indicated in the figure). (b) Raman spectra of samples of each step. (c) XPS N 1s spectrum of the final sample. (d)

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Fe Mössbauer spectrum of the final

sample.

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Figure 3. Identification of atomic Fe sites at the wall of Fe/N/C nanotubes. TEM images (a~c) reveal the Fe/N/C nanotubes after second pyrolysis are broken, and parts of them still contain Fe3C particles. STEM observations find that at the wall of those nanotubes without Fe3C (d), atomic Fe sites can be identified (e). EELS analyses confirm the existence of Fe at the wall of the nanotube (f).

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Figure 4. The electrocatalytic behavior of Fe/N/C, compared with Pt/C, toward the ORR in KOH solution. (a) RDE data recorded at rotation rate of 900 rpm and scan rate of 5 mV/s. (b) H2O2 yield and electron transfer number obtained from RRDE measurements. (c) ORR activity of Fe/N/C before and after durability tests. (d) ORR activity of Pt/C before and after durability tests. The carbon loadings for both Fe/N/C and Pt/C were controlled to be the same (0.32 mg/cm2) in all these tests.

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Figure 5. Single-cell performance of APEFC using Fe/N/C cathode. (a) Cell performance with different Fe/N/C loadings. (b) Cell performance with Fe/N/C catalysts prepared at different temperatures. 60 wt% PtRu/C catalyst was used in the anode with a metal loading 0.4 mg/cm2. The APE membrane was aQAPS-S8 and the APE ionomer in electrodes was aQAPS-S14 (20 wt% in the catalyst layer). Fuel cell operating conditions: temperature = 60 °C, backpressure of gas = 0.1 MPa at each side of the cell. Fully humidified H2 and O2 were fed at a flow rate 200 ml/min.

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TOC Graph

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C Nanotubes with Atomic Fe Sites: A Highly Active Cathode

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