N Electrocatalysts for Oxygen Reduction

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Efficient Bi-functional Fe/C/N Electrocatalysts for Oxygen Reduction and Evolution Reaction Yong Zhao, Kazuhide Kamiya, Kazuhito Hashimoto, and Shuji Nakanishi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511515q • Publication Date (Web): 09 Jan 2015 Downloaded from http://pubs.acs.org on January 10, 2015

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The Journal of Physical Chemistry

Efficient Bi-functional Fe/C/N Electrocatalysts for Oxygen Reduction and Evolution Reaction Yong Zhao, Kazuhide Kamiya, Kazuhito Hashimoto*, Shuji Nakanishi* Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Corresponding Authors: [email protected], [email protected] KEYWORDS nitrogen doped carbon, transition metal, iron, oxygen electrode ABSTRACT Efficient electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are critical components of various energy conversion devices such as regenerative

fuel

cells

and

metal-air

batteries.

Herein,

we

report

bi-functional

transition-metal-doped carbon/nitrogen (M/C/N) materials that simultaneously electrocatalyze the ORR and OER. The OER potential of the Fe/C/N catalyst at a current density of 10 mA cm-2 was 1.59 VRHE, and its ORR half-wave potential was 0.83 VRHE. Significantly, the Fe/C/N catalyst provided a potential gap of 0.76 V between the OER potential (at 10 mA cm-2) and the ORR half-wave potential; this is the highest activity reported to date for a non-precious metal catalyst. Two types of active center, the transition metal and a nitrogen atom, are likely responsible for the oxygen bi-functional activity. 1. Introduction

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Electrochemical conversion reactions between oxygen and water attract many researchers’ interests as these reactions are fundamental to energy conversion systems such as regenerative fuel cells.1 A regenerative fuel cell delivers an electric current when operating in the forward direction and acts as a water electrolyzer, producing hydrogen and oxygen to feed the fuel cell, when operating in the reverse direction.2-5 Therefore, effective fuel cells require bi-functional catalysts with low overpotentials for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).6-14 Although the electrocatalytic OER and ORR processes both involve water and oxygen through multi-electron transfer reactions, the OER and ORR processes utilize different reactions and mechanisms15-16; consequently, designing an effective bi-functional electrocatalyst for both processes is difficult. Precious metals such as platinum (Pt) and Pt alloys are well-known ORR electrocatalysts, but are poor OER electrocatalysts.17 In contrast, iridium and ruthenium oxide-based electrocatalysts have extraordinary OER activity, but poor ORR activity.18 Electrocatalyst hybrids comprising two precious metals, such as platinum and iridium or ruthenium, show moderate ORR and OER activities; however, the high cost and scarcity of these metals severely limits their wide use in energy conversion systems.17, 19-22

Recent efforts have thus focused on non-precious metal-based materials as bi-functional ORR and OER electrocatalysts.23 Perovskite,11, derivatives1,

26-28

24-25

cobalt oxide, manganese oxide, and their

are good OER electrocatalysts and have therefore been investigated as

bi-functional electrocatalysts. For example, manganese oxide catalyst exhibited an oxygen electrode activity of 1.04 V at pH 131, where the oxygen electrode activity1 was defined as the gap between the half-wave potential of the ORR and the OER potential required to oxidize water at a current density of 10 mA cm-2.6 Although cobalt and manganese oxides and perovskites can


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catalyze both the OER and ORR, their electrocatalytic ORR activities are unsatisfactory. It is possibly related with strong bond strength between the OOH*/OH* intermediates and the metal active sites.23 Recently, however, non-precious metal-based electrocatalysts such as nitrogenand metal-containing carbon materials (hereafter called M/C/N) have demonstrated high ORR activity under alkaline conditions similar to that of Pt/C catalysts in commercial alkaline fuel cells.29 Additionally, we recently reported that nitrogen-doped carbon materials catalyze OER, generating a current density of 10 mA cm-2 at an overpotential of 0.38 V in alkaline medium.30 This value is comparable to that of IrO2/C based catalysts. Emerging results suggested that nitrogen- (and metal-) containing carbons could serve as bi-functional materials catalyzing both ORR and OER. Here we report an optimized protocol for synthesizing an iron- and nitrogen-containing carbon-based material that provides an oxygen electrode activity of 0.76 V. This is the lowest oxygen electrode potential reported to date for a non-precious metal catalyst.

Experimental Section Synthesis of Fe/C/N, Co/C/N and C/N The Fe/C/N sample was synthesized as reported previously.30-31 In brief, melamine (6.45 g) was reacted with formaldehyde (12 ml, 37%) in 200 ml alkaline solution (containing 1 g sodium hydroxide) at 80oC for 4 hours. Following hydroxylation, carbon particles (1 g, Ketjenblack EC-300J) were well-dispersed in the solution with an ultrasonic probe, then 20 ml hydrochloric acid (HCl) solution (0.1 M) and 5 g Fe(NO3)·9H2O were added. The pH of the reaction mix was quickly dropped to pH 2 by addition of concentrated HCl, followed by melamine-formaldehyde polymerization for 1 hour. NaOH solution (6M) was then added to raise the pH to 2-3, resulting in formation of a polymer gel paste. The gel paste was dried in an oven at 150oC for 5 hours to


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cross-link and solidify the melamine-formaldehyde resin, and this precursor resin was then ground into a powder and pyrolyzed in a tubular furnace in the presence of ammonia gas for 1.5 h. The pyrolyzed carbon mixture was ultrasonically leached into concentrated HCl for 8 h, and the leached material was then washed three times with distilled water. The Fe/C/N catalyst was collected by filtration and dried at 80°C. The Co/C/N samples were synthesized using a procedure identical to that used for the Fe/C/N catalysts, except that Co(NO3)·6H2O was used instead of Fe(NO3)·9H2O as the metal precursor. The synthesis of C/N materials was reported previously.30 Pt/C (20% Pt [w/w]) was purchased from Tanaka Kikinzoku, and IrO2/C was synthesized as reported previously.30, 32 Electrochemical measurements The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) were measured using a bipotentiostat (Pine Instrument Co.) equipped with a rotating ring-disk electrode (RRDE). A saturated calomel electrode (SCE) was used as the reference electrode in the RRDE test and was calibrated with respect to a reversible hydrogen electrode (RHE) (RHE=SCE+0.244 V+0.591*pH at 25°C). Cyclic voltammetry was conducted in a three-electrode electrochemical cell using a glassy carbon-disk (0.198 cm-2) as the working electrode, a Pt wire as the counter electrode, and a saturated Ag/AgCl electrode as the reference electrode. For RRDE measurements, the working electrode was prepared by loading catalyst ink on the glassy carbon electrode (0.2 mg cm-2) using a mixture of 2 mg of catalyst, 10 µl of Nafion® solution [5 wt%, DuPont], and 1 ml of ethanol. RRDE tests were conducted in an oxygen-saturated KOH solution (pH 13) at 25°C. All electrochemical measurements were conducted at a scan rate of 5 mV s-1. The rotation speed was set at 1500 rpm using a speed controller (Pine Instrument Co.). Prior to


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the RRDE experiments, the Pt ring was activated by cycling the potential between 0.1 and 1.4 V several times in 0.05 M H2SO4. Physical characterization Scanning electron microscopy was performed using a SU-8000 microscope (Hitachi High-Technologies Co.) and transmission electron microscopy was performed using a H-9000UHR microscope (300kV; Hitachi). X-ray diffraction and Raman spectra of the synthesized powders were analysed using an X-ray diffractometer (PW-1700, Spectris) and an integrated confocal Raman microscope (FV1000, Olympus), respectively, according to the manufacturers’ instructions. A Micromeritics Tristar 3000 gas adsorption analyzer (Shimadzu) was used to measure the Brunauer-Emmett-Teller surface area. Surface elemental composition was analyzed at 15 kV using a Kratos Ultra AXIS X-ray photoelectron spectrometer system equipped with a monochromatic Al-Kα source. The amount of evolved oxygen in KOH electrolyte (pH 13) was monitored simultaneously during positive potential scans using a needle-type oxygen micro-sensor (Microx TX3-trace, Presens).

Results and Discussion The ORR and OER activities of the M/C/N materials were assessed using the rotating ring-disk electrode (RRDE) system. We first pyrolyzed the polymer precursor with iron nitrate to find the optimum pyrolysis temperature providing the highest oxygen electrode activity (Figure S1). XPS measurements of samples pyrolyzed at different temperatures confirmed that increasing the pyrolysis temperature decreases the concentrations of nitrogen and metal species in the catalyst, with 700°C providing the optimized catalyst with properties similar to the C/N materials reported


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previously.30-31 Specifically, following our previous work,30 pyrolysis at 700°C was confirmed to be the optimal synthesis condition both for the conductivity and the concentration of active species, resulting in high ORR and OER activity. The optimized M/C/N catalysts (M/C/N pyrolyzed at 700oC) were used unless otherwise specified. Figure 1 shows the OER and ORR activities of Fe/C/N, Co/C/N, C/N, IrO2/C and commercial Pt/C catalysts in KOH electrolyte (pH 13). Table 1 summarizes several parameters obtained from Figure 1, including the ORR half-wave potential, OER potential at a current density of 10 mA cm-2, and oxygen electrode activity for each of the catalysts. Fe/C/N, Co/C/N and C/N catalysts have an oxygen electrode activity between 0.76-0.80 V. Fe/C/N shows the smallest potential gap, 0.76 V; this is smaller than the potential gaps obtained using IrO2/C materials (0.87 V) or commercial Pt/C catalysts (0.9 V). Furthermore, the Fe/C/N catalyst shows better activity for the oxygen electrode activity than the various bi-functional materials reported to date, including Mn3O4 (~1.04 V)1, Co3O4/N-graphene (~0.81 V)6 and perovskite (~1.0 V).24-25 The OER activities of the Fe/C/N, Co/C/N, C/N and IrO2/C catalysts were examined in detail. All the M/C/N catalysts afforded high OER activity at small overpotentials, as shown in Figure 1 and Table 1. Fe/C/N, Co/C/N, and C/N catalysts achieved a current density of 10 mA cm-2 at a potential of 1.59±0.01 VRHE, 1.61±0.01 VRHE, and 1.60±0.01 VRHE, respectively, similar to that of IrO2/C (10 mA cm-2, 1.60±0.01 VRHE). The amount of evolved oxygen was monitored at the Fe/C/N electrode simultaneously with current density (j) versus potential (U) measurements using a needle-type oxygen microsensor. Evolved O2 was detected at an onset potential of 1.52±0.02 VRHE (Figure S2), clearly showing that the Fe/C/N catalyst has OER activity. The OER activity (i.e., the potential at a current density of 10 mA cm-2) of the Fe/C/N, Co/C/N and C/N catalysts are slightly higher than those of other reported OER catalysts such as


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Co3O4/N-graphene (1.63 VRHE, pH14)6, Mn3O4/CoSe2 hybrids (1.7 VRHE, pH13)33 and IrO2 nanoparticles (1.7 VRHE, pH13)34. No prominent difference in OER overpotential was observed among the M/C/N catalysts, indicating that the metal species has little influence on OER activity. Details regarding the ORR activity of Fe/C/N, Co/C/N and C/N catalysts were obtained from RRDE experiments; these are presented in Figure 2, which shows an expansion of the relevant part of Figure 1. Figure 2a shows disk polarization curves representing the ORR activities of the M/C/N and Pt/C catalysts. The ORR onset potential (extrapolated value using the Tafel plot, Figure S3) and half-wave potential of Fe/C/N catalyst were 0.94±0.02 VRHE and 0.83±0.01 VRHE, respectively; these values are slightly lower than those of Pt/C catalyst (0.98±0.02 VRHE, 0.86±0.01 VRHE). The corresponding ORR onset and half-wave potentials are 0.89±0.02 VRHE and 0.80±0.01 V for the Co/C/N catalyst and 0.87±0.02 VRHE and 0.78±0.02 VRHE for the C/N catalyst. The ORR half-wave potentials of other reported non-precious metal catalysts (NPMCs) are 0.82 VRHE for Co3O4/N-graphene (pH 13)6, 0.84 VRHE for an N-doped carbon nanotube array (pH 13)35, and 0.72 VRHE for N-doped graphene (pH 13)36; of these, the ORR half-wave potential value for Fe/C/N is the highest. The amperometric responses of the Pt ring electrodes in the RRDE system were also monitored for detecting peroxide species formed at the disc electrode (Figure 2b). The number of electrons transferred per oxygen molecule (n) during ORR is calculated using the equation n = 4ID/(ID + IR/N), where ID is the kinetic current from the disk electrode, IR is the kinetic current from the ring electrode, and N is the collection efficiency of hydroperoxide (0.19).36-37 The n values for Fe/C/N, Co/C/N and C/N catalysts were calculated from the data in Figure 2 to be over 3.90 at 0.20-0.85 VRHE (Figure S3), suggesting that the M/C/N catalysts favour four-electron reduction. The electrochemically active surface areas (EASA) were determined from measurements of electrochemical capacitors constructed from


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M/C/N materials. It was confirmed that the Fe/C/N, Co/C/N and C/N exhibited similar EASA values (17.8 cm2 for C/N, 18.1 cm2 for Co/C/N, and 18.1 cm2 for Fe/C/N; electrode geometric surface area, 0.2 cm2). The physicochemical properties of the M/C/N materials were characterized using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and tunnelling electron microscopy (TEM). Figure 3a shows XRD spectra of the Fe/C/N, Co/C/N and C/N samples. The peaks ranked at 2θ = 26.4o and 2θ = 44o correspond to the (002) plane of the graphite structure and the (101) plane of disordered amorphous carbon, respectively. The degree of graphitization can be estimated by comparing the relative intensities of the (002) plane and (101) plane reflections according to the empirical formula provided by Leis et al.38 The degree of graphitization of the M/C/N samples (I002/I100, 4.5 for Fe/C/N, 4.9 for Co/C/N and 4.4 for C/N) determined by XRD is similar. This similarity was confirmed by Raman spectroscopy (Figure 3b). The integrated intensity ratio ID/IG for the D band (1300-1400 cm-1) and G band (1580-1650 cm-1) is widely used to quantify the defects in graphitic materials.39 As shown in Fig. 3b, the ratio of ID/IG was similar for the three M/C/N catalysts (ID/IG, 1.1 for Fe/C/N, 1.1 for Co/C/N and 1.2 for C/N). The scanning electron microscopy image of the Fe/C/N catalyst provided in Figure 4a shows that the diameter of the catalyst particles is ca. 30-40 nm; the Co/C/N and C/N nanoparticles are also approximately this size (Figure 4b, 4c). TEM images of the Fe/C/N catalyst verified the size of the catalyst particles (Figure 4d). High-resolution TEM images of the Fe/C/N sample (Figure 4e) indicated the presence of metal particles in the sample, whereas no metal particles were observed in the C/N samples (Figure 4f).37 Nitrogen absorption/desorption isotherm analyses


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showed that the Fe/C/N catalyst contains meso-pores centred at approximately 4 nm in diameter (Figure S4). The surface area of the Fe/C/N catalyst was 570 m2 g-1. The nitrogen species in the M/C/N materials were analyzed from the deconvoluted XPS N 1s spectra. The N1s spectra of the Fe/C/N catalysts deconvoluted into four peaks (pyridinic-N, pyrrolic-N, quaternary-N, pyridinic-N+-O-) according to their binding energies (Figure 5),3, 6, 31, 37

with pyridinic-N (52%) and quaternary-N (37%) being the dominant species (Table S1).

Additionally, the XPS Fe 2p spectra of the Fe/C/N catalyst (Figure 5d) showed that the Fe 2p peaks at 711 eV is due to the nitrogen-coordinated iron species, previously shown to be the active species for ORR activity.31 The corresponding nitrogen species are also dominant for the Co/C/N and C/N catalysts. (Table S2) XPS analysis also showed that the nitrogen and iron concentration gradually decreased as the pyrolysis temperature increased (Table S3). The data provide insights into the mechanisms controlling the OER and ORR active sites in these bi-functional electrocatalysts. We recently reported that nitrogen/carbon materials can serve as active catalysts for the OER.30 In addition, as described above, the low concentration of metal species in our M/C/N catalysts did not influence OER activity. Thus, the OER activity of the M/C/N catalysts is likely mainly due to the nitrogen-containing active sites. In contrast, the catalytic mechanism underlying the ORR active sites can be explained by one of two models. In the first model, the N-binding metal species is the active moiety; in the second model, the carbon atom next to the nitrogen, or the nitrogen itself, in conjunction with the transition metal act as graphitization catalysts.31, 40 According to the two models, the ORR active sites of the M/C/N catalyst are possibly a combination of nitrogen/carbon and transition metal. Two lines of evidence support the assumption. First, the reported nitrogen/carbon materials, lacking any metals, showed good ORR activity (around 0.9 VRHE or lower) in alkaline medium;36 second, the


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ORR onset potential of pyrolyzed M/C/N catalyst differed depending on the metal species (0.88, 0.90, and 0.94 VRHE for C/N, Co/C/N and Fe/C/N, respectively, Figure 1, Table S2). The OER stability of Fe/C/N electrode was evaluated with cyclic voltammetry (CV) scan from 1.00 V to 1.63 V.41 As shown in Figure 6, the OER catalytic current of Fe/C/N at the 10th sweep was essentially identical to the first sweep, but decreased thereafter. Currently, the reason for the decrease in OER activity is unknown, since a large amount of oxygen was evolved during the OER process and the catalyst layer peeled off from the glassy carbon electrodes. We also checked the ORR stability of Fe/C/N electrode by continuous CV scanning from 0.20 V to 1.10V with a continuous supply of O2. The ORR catalytic current of Fe/C/N after the 3000th sweep was essentially identical to the first sweep, demonstrating the high ORR stability of the Fe/C/N catalysts. We next examined the toxic effect of carbon monoxide on the ORR activity of the Fe/C/N catalyst (Figure S5) by measuring the chronoamperometric response of the catalyst while continuously bubbling O2 and CO gas at 30 mL min-1 through the electrolyte. No poisoning of the Fe/C/N electrode by CO was observed, indicating that the Fe/C/N active sites are tolerant to CO. This tolerance is advantageous for stable ORR catalysis in electrochemical devices.

Conclusion M/C/N electrocatalysts with two types of active sites (ORR and OER) provided small overpotentials for oxygen evolution and reduction in alkaline media. This is the first report of iron/nitrogen-doped carbon materials used as bi-functional oxygen catalysts. The characteristics of these bi-functional oxygen catalysts are among the most promising reported to date and demonstrate the feasibility of bi-functional catalysts based on two types of active sites within one electrocatalyst.


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ASSOCIATED CONTENT Supporting Information. Optimized condition for Fe/C/N, the evolved oxygen by sensor, tafel plots, isotherms, elemental contents and CO poisonous test. These materials are available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

(K.H.) E-mail: [email protected], (S.N.): [email protected]

ACKNOWLEDGMENTS We greatly thank Dr. R. Nakamura and Mr. S. Matsuda for their valuable discussion and help about the SEM and TEM measurements.


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16. Duan, Z. Y.; Wang, G. F. A first principles study of oxygen reduction reaction on a Pt(111) surface modified by a subsurface transition metal M (M = Ni, Co, or Fe). Phys. Chem. Chem. Phys. 2011, 13, 20178-20187. 17. Kong, F. D.; Zhang, S.; Yin, G. P.; Zhang, N.; Wang, Z. B.; Du, C. Y. Pt/porous-IrO2 nanocomposite as promising electrocatalyst for unitized regenerative fuel cell. Electrochem. Commun. 2012, 14, 63-66. 18. Shao, Y. Y.; Park, S.; Xiao, J.; Zhang, J. G.; Wang, Y.; Liu, J. Electrocatalysts for Nonaqueous Lithium-Air Batteries: Status, Challenges, and Perspective. Acs Catalysis 2012, 2, 844-857. 19. Zhang, Y. N.; Zhang, H. M.; Ma, Y. W.; Cheng, J. B.; Zhong, H. X.; Song, S. D.; Ma, H. P. A novel bifunctional electrocatalyst for unitized regenerative fuel cell. J. Power Sources 2010, 195, 142-145. 20. Chen, G. Y.; Bare, S. R.; Mallouk, T. E. Development of supported bifunctional electrocatalysts for unitized regenerative fuel cells. J. Electrochem. Soc. 2002, 149, A1092-A1099. 21. Huang, S. Y.; Ganesan, P.; Jung, H. Y.; Popov, B. N. Development of supported bifunctional oxygen electrocatalysts and corrosion-resistant gas diffusion layer for unitized regenerative fuel cell applications. J. Power Sources 2012, 198, 23-29. 22. Chen, G. Y.; Delafuente, D. A.; Sarangapani, S.; Mallouk, T. E. Combinatorial discovery of bifunctional oxygen reduction - water oxidation electrocatalysts for regenerative fuel cells. Catal. Today 2001, 67, 341-355. 23. Calle-Vallejo, F.; Koper, M. T. M.; Bandarenka, A. S. Tailoring the catalytic activity of electrodes with monolayer amounts of foreign metals. Chem. Soc. Rev. 2013, 42, 5210-5230. 24. Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383-1385. 25. Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y. Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal-air batteries (vol 3, pg 546, 2011). Nat. Chem. 2011, 3, 647-647. 26. Pickrahn, K. L.; Park, S. W.; Gorlin, Y.; Lee, H. B. R.; Jaramillo, T. F.; Bent, S. F. Active MnOx Electrocatalysts Prepared by Atomic Layer Deposition for Oxygen Evolution and Oxygen Reduction Reactions. Adv. Energy Mater. 2012, 2, 1269-1277. 27. Wu, X.; Scott, K. A non-precious metal bifunctional oxygen electrode for alkaline anion exchange membrane cells. J. Power Sources 2012, 206, 14-19. 28. Gorlin, Y.; Jaramillo, T. F. Investigation of Surface Oxidation Processes on Manganese Oxide Electrocatalysts Using Electrochemical Methods and Ex Situ X-ray Photoelectron Spectroscopy. J. Electrochem. Soc. 2012, 159, H782-H786. 29. Chung, H. T.; Won, J. H.; Zelenay, P. Active and stable carbon nanotube/nanoparticle composite electrocatalyst for oxygen reduction. Nat. Commun. 2013, 4, 1922. 30. Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-doped carbon nanomaterials as non-metal electrocatalysts for water oxidation. Nat. Commun. 2013, 4, 2390 31. Zhao, Y.; Watanabe, K.; Hashimoto, K. Efficient oxygen reduction by a Fe/Co/C/N nano-porous catalyst in neutral media. J. Mater. Chem. A 2013, 1, 1450-1456. 32. Hara, M.; Waraksa, C. C.; Lean, J. T.; Lewis, B. A.; Mallouk, T. E. Photocatalytic water oxidation in a buffered tris(2,2 '-bipyridyl)ruthenium complex-colloidal IrO2 system. J. Phys. Chem. A 2000, 104, 5275-5280.


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Figure Captions Figure 1 a) The OER and ORR activities of pyrolyzed C/N, Co/C/N and Fe/C/N electrodes in KOH electrolyte (pH 13). b) Comparison of the OER activities of Fe/C/N, commercial Pt/C (20 wt%) and IrO2/C (20 wt%) electrodes. (Catalyst loading: 0.2 mg cm-1; rotation speed, 1500 rpm). Figure 2 RRDE data showing ORR activities of C/N, Co/C/N and Fe/C/N materials loaded on disks at 0.2 mg cm-1 in KOH medium (pH 13). a) disk currents; b) ring currents (rotation speed, 1500 rpm) The Fe/C/N catalyst shows an ORR onset potential and half-wave potential of 0.95 VRHE and 0.83 VRHE, respectively. Table 1 The OER, ORR and oxygen electrode activities ( (OER-ORR)) of different catalysts on a rotating disk electrode. Figure 3 a) XRD spectra and b) Raman spectra of the Fe/C/N, Co/C/N and C/N materials. Figure 4 SEM images of a) Fe/C/N, b) Co/C/N and c) C/N samples and TEM images of d), e) Fe/C/N and f) C/N samples. Figure 5 High resolution XPS N 1s spectra of a) Fe/C/N, b) Co/C/N, c) C/N and d) Fe 2p spectrum of Fe/C/N. Figure 6 Stability tests results for a) ORR activity and b) OER activity of the Fe/C/N catalyst in the RRDE system. Catalyst loading, 0.2 mg cm-2; electrolyte, oxygen saturated KOH solution (pH 13); scan rate 5 mV s-1; rotation speed, 1500 rpm).


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Figures Figure 1

a)

b)

Figure 1 a) The OER and ORR activities of the pyrolyzed Fe/C/N, Co/C/N and C/N electrodes in KOH electrolyte (pH13). b) Comparison of the OER activities of Fe/C/N, commercial Pt/C (20 wt%) and IrO2/C (20 wt%) electrodes. (Loading catalyst: 0.2 mg cm-1; rotation speed, 1500rpm)


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Table 1

Table 1 The OER, ORR and oxygen electrode activities ( (OER-ORR)) of different catalysts on a rotating disk electrode.


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Figure 2

a)

b)

Figure 2 RRDE data showing ORR activities of C/N, Co/C/N and Fe/C/N materials loaded on disks at 0.2 mg cm-1 at KOH medium (pH13). a) disk currents; b) ring currents. (rotation speed, 1500rpm) The Fe/C/N catalyst shows the ORR onset potential and half-wave potential as 0.95 V and 0.83 V vs RHE, respectively.


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Figure 3

a)

b)

Figure 3 a) XRD spectrum and b) Raman spectrum of (1) Fe/C/N, (2) Co/C/N and (3) C/N materials.


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Figure 5

a)

c)

b)

d)

Figure 5 The high resolution XPS N 1s spectrum of a) Fe/C/N, b) Co/C/N and c) C/N.


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Figure 6

a)

b)

Figure 6 Stability tests of a) ORR activities and b) OER activities with Fe/C/N catalyst in RRDE system. (catalyst loading, 0.2 mg cm-2; electrolyte, KOH solution (pH13); scan rate of 5 mV s-1; rotation speed, 1500 rpm).


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N Electrocatalysts for Oxygen Reduction

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