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Faraday Discussions Accepted Manuscript

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DOI: 10.1039/C4FD00123K

Graphene-supported iron-based nanoparticles encapsulated in nitrogen-doped carbon as a synergistic catalyst for hydrogen

Published on 03 July 2014. Downloaded by Université Laval on 09/07/2014 07:42:00.

Jing Wang,[a, b] Guoxiong Wang,*[a] Shu Miao,[c] Jiayuan Li,[a, b] and Xinhe Bao*[a, c] a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of

Sciences, 457 Zhongshan Road, Dalian, 116023, China. b

University of Chinese Academy of Sciences, Beijing, 100039, China.

c

Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian, 116023, China

Abstract Electrolyzers and fuel cells have been extensively investigated as a promising solution for renewable energy storage and conversion. Hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR) are important electrocatalytic processes in electrolyzers and fuel cells. Exploring efficient non-precious metal catalysts for HER and ORR in acidic medium remains a great challenge. Herein, we report graphene-supported iron-based nanoparticles encapsulated in nitrogen-doped carbon (Fe@N-C) hybrid material acts as an efficient HER and ORR catalyst. The hybrid material was synthesized by pyrolysis of graphene oxide and ammonia ferric citrate followed by acid-leaching. During the pyrolysis, nitrogen was doped into graphene lattice, and the carbon nanoshell grown on graphene effectively suppressed the stacking of graphene sheet, exposing more active sites to reactants. The hybrid 1

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evolution and oxygen reduction reactions

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material showed higher electrocatalytic activities than graphene sheet or Fe@N-C alone, which was probably attributed to the synergetic role of nitrogen-doped

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*Corresponding authors. Tel: +86-411-84686637. Email: [email protected] (Guoxiong Wang); [email protected] (Xinhe Bao)

Introduction Renewable electricity from solar and wind energy has drawn much attention in recent years due to the increasing scarcity of fossil fuels and environmental deterioration.1-3 A major challenge for the renewable electricity is directly merged into the electricity grid because of its intermittent and localized nature. Therefore, it is essential to develop efficient energy storage and conversion technologies towards the rapid development of renewable electricity.4-6 Electrochemical technologies such as rechargeable batteries, supercapacitors, electrolyzers and fuel cells have been extensively investigated as promising solutions.1,7, 8 The combination of electrolyzers and fuel cells is a particularly interesting candidate. Hydrogen and oxygen are produced by water splitting in electrolyzers using the renewable electricity, and then fed into the anode and cathode of proton exchange membrane fuel cell (PEMFC) for generating

electricity

as

portable

and

transportation

power

sources.4,9,10

Electrocatalysts are key materials in electrolyzers and PEMFCs for catalyzing hydrogen evolution/oxidation reactions and oxygen evolution/reduction reactions. Developing low-cost, efficient electrocatalysts for the above reactions is one of the 2

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graphene and Fe@N-C towards the electrocatalytic reactions.

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most important challenges in the research and development of electrolyzers and PEMFCs.11, 12

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support to disperse metal or metal oxide nanoparticles on it.13,14 Graphene, a two-dimensional plane structure with a thickness of single-layer carbon atom, has attracted great interest in the field of energy storage and conversion due to its high electronic conductivity, large surface area, and excellent chemical stability.15-17 In PEMFC, electrochemical corrosion of carbon materials as catalyst supports leads to aggregation of Pt-based nanoparticles, and thus decreases the electrochemical surface area and eventually the long-term durability of PEMFC.18,19 Traditional carbon supports widely used in PEMFC are carbon black (CB), carbon nanotube (CNT) and carbon nanofiber.20,21 Because of the above unique properties, graphene has been investigated as promising supporting materials in carbon-supported Pt-based electrocatalysts.22,23 Previous studies show that graphene-supported Pt-based electrocatalysts demonstrate enhanced catalytic activity.24-26 However, the surface area of graphene sheets will be decreased during the electrochemical cycling due to the stacking of graphene sheets resulted from van der Waals force and π-π interaction.27,28 CB or CNT was often inserted between graphene sheets as nanospacers to suppress this stacking behavior.29,30 The Graphene-CB and graphene-CNT hybrid support materials provide a three-dimensional (3D) porous architecture for the deposition of Pt-based nanoparticles, expanding the electrochemical three-phase interface and thus enhancing the electrocatalytic activity and durability. 3

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Carbon materials are usually the essential component in electrocatalysts as a

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Although Pt-based electrocatalysts have demonstrated high electrocatalytic activities, the high cost and limited resources hinder a wide application of Pt-based

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hydrogen evolution reaction.31-33 Meanwhile, the Pt-based electrocatalysts are vulnerable to poisonous species such as SO2 and methanol, etc.31-33 Many efforts have been devoted to develop non-precious metal catalysts for electrocatalytic reactions. Heteroatom-doped carbon materials have been considered as a new class of electrocatalysts. Due to the intrinsically chemical inertness of graphene, nitrogen is usually doped into graphene lattice to tailor its electronic structure and possess the electrocatalytic activity.17, 34, 35 Various methods have been developed to synthesize nitrogen-doped graphene, such as thermally annealing of graphene or graphene oxide (GO) with NH3, chemical vapor deposition, nitrogen plasma treatment of graphene and arc discharge of graphite with pyridine/NH3.36, 37 However, these approaches have some limitations, such as high cost, inability to scale up, etc. In addition, the stacking behavior of nitrogen-doped graphene is even more serious than that of graphene-supported Pt-based nanoparticles, and the accessible active sites in nitrogen-doped graphene are only exposed limitedly. It has been shown that some non-noble metal oxide nanoparticles, such as Co3O4 and MnOx, supported on graphene sheet can suppress the stacking behavior,29, 38 however, the majority of the metal oxide/graphene hybrid materials can only work in alkaline medium because of the acid-leaching of metal oxide. Iron-based nanoparticles encapsulated in nitrogen-doped carbon (Fe@N-C) have 4

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electrocatalysts in electrocatalytic reactions such as oxygen reduction reaction and

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been synthesized using ammonia ferric citrate as single-source molecular precursor in our group.39 The carbon phase with a hollow nanoshell structure is formed by

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stacking behavior of graphene sheet. Inspired by this, ammonia ferric citrate was mixed with GO uniformly and pyrolyzed in Ar atmosphere. The stacking of graphene sheets was effectively suppressed by growing hollow carbon nanoshell on the graphene surface, meanwhile, GO was doped with nitrogen and reduced to graphene by the decomposition species from ammonia ferric citrate during the pyrolysis. The encapsulation structure prevents direct contacts of the iron-based nanoparticles with oxygen and acidic medium. The graphene-supported Fe@N-C hybrid material demonstrated higher electrocatalytic activities towards hydrogen evolution and oxygen reduction reactions than Fe@N-C or graphene sheet alone. Experimental Material synthesis

GO was prepared from natural graphite flakes using a modified Hummers method.40 The hybrid material was prepared by freeze-drying of aqueous dispersion of ammonia ferric citrate and GO, followed by pyrolysis in Ar atmosphere and acid-leaching. In a typical experiment, 15 g ammonium ferric citrate (C6H11FeNO7, J&K Chemical Ltd.) and 1.5 g GO were added to 300 mL of de-ionized water. After 10 min sonication, the stable suspension was freeze-dried. The mixture was placed in a quartz tube of a horizontal furnace and pyrolyzed in Ar atmosphere at a flow rate of 50 mL min-1 for 2 h at 600 oC. The product was leached in 0.5 M HClO4 solution at 80 oC for 10 h to 5

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pyrolysis and acid-leaching, which would be an ideal separator to suppress the

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remove unstable Fe species, and washed with de-ionized water thoroughly. Finally, the sample was dried at 60 oC in an oven, which is denoted as Fe@N-C/RGO. For

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which are denoted as Fe@N-C and RGO, respectively. Dicyandiamide was added into the precursor mixture as an additional nitrogen source, and the mixture was pyrolyzed at 700 oC and followed by acid-leaching, which is denoted as H-Fe@N-C/RGO.

Physicochemical characterizations The morphologies of the samples were investigated by an FEI Tecnai G2 microscope at 120 kV, a JEM-2100 microscope at 200 kV and a QUANTA 200 FEG scanning electron microscope (SEM) at 20 kV. X-ray diffraction (XRD) was performed on a Rigaku D/Max-2500 diffractometer with a Cu Kα radiation source (λ=1.5418 Å) at 40 kV and 200 mA at a scan rate of 5° min-1. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific spectrometer with an Escalab 250 Xi X-ray as excitation source. Thermogravimetry (TG) measurements were performed on a TGA-DSC analyzer (NETZSCH STA 449 F3 Jupiter®). Dry air provided by a pressured tank with a flow rate of 50 mL min−1 was used as the carrier gas. The experiments were carried out from room temperature to 900 °C with a heating rate of 10 °C min−1. Nitrogen adsorption/desorption was carried out using a Quantachrome QUADRASORB SI system at 77 K, and specific surface areas of the samples were calculated by the Brunauer–Emmet–Teller (BET) equation.

Electrochemical measurements 6

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comparison, ammonia ferric citrate and GO were prepared under the same conditions,

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Electrochemical measurements were carried out in a 150 mL three-electrode cell (AKCELL3, Pine Research Instrumentation) at 25 oC. A commercial glassy carbon

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Instrumentation) covered by the sample with Nafion ionomer as a binder and Pt-wire electrode (Pine Research Instrumentation) were used as the working electrode and counter electrode. Ag/AgCl and Hg/HgO electrodes (Shanghai Yueci Electronic Technology Co. Ltd.) are used as the reference electrode in acidic and alkaline electrolyte solutions, respectively. The rotation rate and the potential of the working electrode were controlled by a MSR Electrode Rotator (Pine Research Instrumentation) and an Autolab potentiostat/galvanostat (PGSTAT 302N). The Ag/AgCl reference electrode was calibrated vs. reversible hydrogen electrode (RHE) in 0.5 M HClO4 and 0.1 M HClO4 solution, while the Hg/HgO reference electrode was calibrated vs. RHE in 0.1 M KOH solution. All potential values in this paper are referred to the RHE. Five mg of the catalyst was dispersed in a mixture of 2 mL ethanol and 50 µL Nafion solution (5 wt%, Aldrich) with ultrasonic stirring to form a homogenous ink. The catalyst layer was prepared by dropping 25 µL of the ink onto a GC disk electrode by a micropipettes and drying at room temperature. The HER activities of the catalysts were evaluated by rotating disc electrode (RDE) measurement in Ar-saturated 0.5 M HClO4 solution with a scan rate of 2 mV s-1. The ORR activities of the catalysts were evaluated by rotating disc electrode (RDE) measurement in O2-saturated 0.1M HClO4 solution or 0.1 M KOH solution with a scan rate of 10 mV s-1. Current-voltage data was recorded at a rotation rate of 2500 7

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(GC) electrode (AFE2M050GC, 5 mm in diameter, 0.196 cm2, Pine Research

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rpm. Rotating ring disk electrode (RRDE) measurements of the samples in 0.1 M

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polycrystalline Pt biased at 1.06 V. The following equations are used for calculation of H2O2 (η) formation yield and the electron transfer number (n) per oxygen molecule: ∗



ƞ =  ∗ n=

∗

  /

(Eq. 1)

(Eq. 2)

where Ir and Id are ring and disk currents, respectively, and N is collection efficiency (0.38). Results and Discussion Scheme 1 illustrates the typical procedures to synthesize Fe@N-C/RGO hybrid material. Firstly, GO and ammonia ferric citrate are mixed into an aqueous solution. The presence of many oxygen-containing groups on the surface of the GO sheets makes GO uniformly dispersed in the ammonia ferric citrate aqueous solution. Subsequently, the mixture is freeze-dried and then pyrolyzed in Ar atmosphere. Then ammonia ferric citrate is decomposed to form Fe@N-C, meanwhile, GO is reduced to nitrogen-doped graphene sheet. After acid-leaching, incompletely encapsulated iron-based nanoparticles are removed and hollow carbon nanoshells are left behind between graphene sheets. Some entirely encapsulated iron-based nanoparticles are supported on the graphene sheet. SEM images of Figs. 1 a and b display the obvious differences between RGO and Fe@N-C/RGO. Fig. 1a shows very smooth surface of the typical graphene sheet. In sharp contrast, carbon nanoshells are uniformly 8

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HClO4 solution were conducted on a glassy carbon disk (Ø 5.7 mm) with a

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decorated on the graphene sheet in Fe@N-C/RGO. Freeze-drying process can facilitate uniform distribution of ammonia ferric citrate powder among GO sheets.41

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hybrid structure is considered to be much more stable than graphene/CNT and graphene/CB hybrid structures produced by mechanical mixing. Thanks to the unique encapsulation structure, the iron-based nanoparticles are prevented from contact with oxygen, electrolyte and contaminants, and remain stable in a wide pH range. Therefore, Fe@N-C/RGO hybrid material can be used as electrocatlaysts under harsh reaction conditions. TEM images in Figs. 1 c and d confirm the obvious differences between RGO and Fe@N-C/RGO. Fig. 1c illustrates typical stacked graphene sheets, while hollow carbon nanoshells in Fe@N-C/RGO are supported on graphene sheet with a much smaller size compared with that in [email protected] The presence of GO separates continuous distribution of ammonia ferric citrate, resulting in formation of small carbon nanoshells on the graphene sheet during the pyrolysis. And some nanoparticles are also observed to be entirely encapsulated within carbon nanoshell. As shown in Fig. 1e, one nanoparticle with a size of about 15 nm is completely wrapped by carbon nanoshell with 3-4 graphene layers. Some hollow graphitic pores with a size of ~20 nm are also observed, which was generated by removing incompletely encapsulated iron-based nanoparticles during acid-leaching. The present strategy for the synthesis of this unique Fe@N-C/graphene hybrid architecture has several advantages. Firstly, the preparation procedures are quite simple, consisting of freeze-drying, pyrolysis and acid-leaching using non-expensive equipments. Secondly, 9

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During the pyrolysis, carbon nanoshells grow on the surface of graphene sheet. This

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GO is reduced and doped with nitrogen uniformly by the decomposition of ammonia ferric citrate without the need of any other reductive or nitrogen-containing gas.

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carbon nanoshells on its surface, constructing an open 3D architecture, facilitating transport of reactants to active sites. Thus, this synthesis process exactly meets the requirements to scale up for mass production. Fig. 2 shows XRD patterns of Fe@N-C, RGO and Fe@N-C/RGO. The broad peaks at 26.1o and 43o are assigned to diffractions from the (002) and (10) planes of the hexagonal structure of graphite, indicative of a high graphitic structure in the three samples.42 The thickness of the graphitic domains (Lc) in different samples is calculated according to the Scherrer equation using the full width at half maximum (FWHM) values of (002).43 The calculated value of the thickness of graphitic domains (Lc) can be used to quantify the average thickness of carbon nanoshell and graphene sheet. For Fe@N-C and RGO, the average thickness of the graphitic domains is 5.8 nm and 3.0 nm. It should be noted that the average value of Lc in Fe@N-C/RGO is only 1.5 nm, much smaller than those of Fe@N-C and RGO. This result indicates that pyrolysis of the mixture of GO and ammonia ferric citrate plays a synergistic role in suppressing the aggregation of graphene sheet and carbon nanoshell. The continuous distribution of GO and ammonia ferric citrate in the mixture is interrupted, and thus the thickness of graphitic domains is decreased. For Fe@N-C, the weak peaks at around 44.5o and 82.3o are assigned to Fe44 and the other strong peaks are assigned to Fe3C38. In contrast, diffraction peaks associated with Fe and Fe3C are not observed for 10

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Thirdly, the stacking of graphene sheets is suppressed effectively by the growth of

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the hybrid material, probably due to overlapping with the diffraction peaks of graphene.

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100% mass loss at the end of TG experiment. In contrast, Fe@N-C and Fe@N-C/RGO have similar residual mass of about 8%. During the heating process, two types of oxidation reactions occurred.39 One is the oxidation of iron-based species to Fe2O3, and the other is the oxidation of carbon to CO2. CO2 is released from the samples, and the residual material after TG experiments in air atmosphere is Fe2O3. The complete mass loss of RGO suggests that it contains negligible metal or metal oxide, and the similar mass loss of Fe@N-C and Fe@N-C/RGO confirms that there are nearly identical contents of encapsulated iron species in both samples. The mass loss of RGO mainly occurs in a temperature range of 410 oC and 590 oC, while that of Fe@N-C occurs in a temperature range of 300 oC and 750 oC. The mass loss of RGO is very fast, indicating that RGO has a uniform composition distribution to activate oxygen in the air simultaneously and thus is oxidized to CO2 in a narrow temperature window. Although Fe@N-C has a higher graphitization extent than RGO considering its high value of Lc, the onset oxidation temperature of Fe@N-C is much lower than that of RGO, suggesting the encapsulation structure of Fe@N-C can activate oxygen favorably. The mass loss at high oxidation temperature is probably ascribed to the oxidation of hollow carbon nanoshells without encapsulated iron-based nanoparticles. Interestingly, Fe@N-C/RGO hybrid material shows the lowest onset oxidation temperature and a narrow temperature range of carbon oxidation, suggesting an 11

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TG curves of different samples in air atmosphere are shown in Fig. 3. RGO has a

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improved oxygen-activating ability compared with RGO and Fe@N-C. The total nitrogen content in Fe@N-C/RGO is measured to be 4.2 wt% by XPS,

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citrate was decomposed and the nitrogen species was conveniently doped into the graphene due to an intimate contact between GO and ammonia ferric citrate in the mixture after freeze-drying. Therefore, the doped nitrogen content in Fe@N-C/RGO is greatly improved. The high resolution N 1s peaks in the XPS spectra are fitted into four components originating from pyridinic N (398.5 ± 0.2 eV), pyrrolic N (400.5 ±0.2 eV), graphitic N (401.4 ±0.2 eV), oxidized N (402.6 ±0.2 eV),39 as shown in Fig. 4. The percentage of different nitrogen species is listed in Table 1, compared with Fe@N-C, the proportion of nitrogen species in Fe@N-C/RGO is different especially for the oxidized N. Both of pyridinic-N and graphitic-N are reported to be responsible for the electrocatalytic activity.45,46 The percentage of pyridinic-N and graphitic-N in Fe@N-C/RGO is 52.6% of the total nitrogen content, exceeding that of 44.7% in Fe@N-C, suggesting that a high content of nitrogen functional groups in Fe@N-C/RGO will contribute to the electrocatalytic activity. The N2 adsorption-desorption curves of RGO and Fe@N-C/RGO belong to the isotherms of type IV (based on IUPAC classification) with a hysteresis loop, indicative of the existence of mesopores, as shown in Fig. 5. The specific surface area and pore volume of RGO are 302 m2 g-1 and 0.88 cm3 g-1, while those of Fe@N-C/RGO are 453 m2 g-1 and 0.80 cm3 g-1. Compared with RGO, the specific surface area of Fe@N-C/RGO is greatly improved, indicative of a more open 12

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much higher than that in Fe@N-C (1.8 wt%). During the pyrolysis, ammonia ferric

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structure. However, the pore volume of Fe@N-C/RGO is slightly decreased, probably because that the majority of pore volume is contributed from the interspace of

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interspace volume. The high specific surface area of Fe@N-C/RGO is expected to facilitate exposing more active sites to reactants with enhanced mass-transport properties during the electrocatalytic process.39 The electrocatalytic activities of RGO, Fe@N-C and Fe@N-C/RGO towards HER are evaluated in Ar-saturated 0.5 M HClO4 solution. As shown in Fig. 6a, RGO has a negligible current density, indicating that pure carbon material is almost inert to HER. Fe@N-C shows an inferior activity for HER. In contrast, Fe@N-C/RGO demonstrates a greatly improved HER activity. The Tafel plots are shown in Fig. 6b. The slopes were calculated to be 100 mV dec-1 for Fe@N-C/RGO, 232 mV dec-1 for Fe@N-C and 356 mV dec-1 for RGO. In acidic medium, HER is composed of two out of three microscopic steps (Tafel/Volmer or Heyrovsky/Volmer):47, 48 Volmer step: H++e-=Had Heyrovsky step: Had+H++e-=H2 Tafel step: 2Had=H2 The Tafel curve of Fe@N-C/RGO clearly demonstrates that the Volmer step is the rate-determining step. The Volmer reaction involves the adsorption of proton and transfer of electron to form adsorbed H species. The comparatively lower onset potential and smaller Tafel slope of Fe@N-C/RGO than those of Fe@N-C and RGO alone denote that the presence of more nitrogen species favors proton adsorption 13

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graphene sheets, and the carbon nanoshells supported on graphene sheet decrease the

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kinetics. For HER in acidic medium, Pt/C has demonstrated the best activity.49 Development of non-previous metal catalysts for HER has been one of the major

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sulfide (such as MoS2, WS2, etc.), metal nitride (such as Ni-Mo-N) and metal phosphides (such as Ni2P) have demonstrated high HER activity in acidic medium.50-52 However, the durability of these catalysts is not satisfactory because the transition metal species are directly contacted with acidic electrolyte solution and prone to dissolution. Carbon materials are very stable in acidic medium, but there are only few reports on carbon materials for application in HER. Herein, Fe@N-C/RGO has served as a promising alternative for HER in acidic medium. Further functionalization of Fe@N-C/RGO hybrid material is essential to improve the HER activity. The electrocatalytic activities of Fe@N-C, RGO and Fe@N-C/RGO towards ORR are also assessed in O2-saturated 0.1 M HClO4 solution. Fig. 7a shows linear sweep voltammetries of different samples. RGO and Fe@N-C have similar ORR activities. In contrast, Fe@N-C/RGO displays an obvious positive shift on the onset potential at 0.7 V and high cathodic current density. The ORR activity on Fe@N-C/RGO is much higher than that of RGO or Fe@N-C alone, confirming a synergetic effect of nitrogen-doped graphene and Fe@N-C for catalyzing ORR. The ORR pathway is further confirmed by RRDE, which is fit to monitor the number of transferred electron and the formation of peroxide species (HO2-) (Fig. 7b). The measured n is between 3.7 and 3.9 in the potential range of -0.15 V~0.6 V, and the 14

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challenges in renewable energy storage and conversion in recent years.50-52 Metal

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H2O2 yields between 14% and 6% within the same potential window. Dicyandiamide has been used as a suitable nitrogen source to increase the

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mixture, which was then pyrolyzed and acid-leached, and the obtained material is denoted as H-Fe@N-C/RGO. Fig. 8 shows TEM image of H-Fe@N-C/RGO. Compared with Fe@N-C, the density of encapsulated iron-based nanoparticles is obviously increased with the addition of dicyandiamide, and the graphene sheet can also be clearly observed, indicating that the encapsulated iron nanoparticles are supported on graphene sheet. Several carbon encapsulated iron, iron-cobalt and iron carbide nanoparticles have been synthesized for catalyzing ORR in acidic medium.54,55 It is proposed that the electronic interaction between inner iron-based nanoparticle and outer carbon nanoshell promotes oxygen adsorption and association on the surface of the carbon nanoshell.54,55 Therefore, high-density Fe@N-C would be expected to increase ORR activity. The ORR activities on H-Fe@N-C/RGO and Pt/C in acidic and alkaline mediums are shown in Fig. 9. The onset potential and half-wave potential (E1/2) are 0.89 V and 0.67 V in 0.1 M HClO4 solution, only slightly smaller than that of Pt/C catalyst. Interestingly, H-Fe@N-C/RGO shows even higher ORR activity in 0.1 M KOH solution. We have compared the ORR activities of the reported non-previous metal catalysts, as listed in Table 2. H-Fe@N-C/RGO shows high ORR activity both in acidic and alkaline mediums amongst the reported non-precious metal catalysts

with

encapsulation

structure.38,44,54-57

In

the

alkaline

medium,

H-Fe@N-C/RGO exhibits an ORR activity similar to the high-performance cobalt 15

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content of doped nitrogen.53 We also incorporate dicyandiamide into the precursor

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oxide/nitrogen-doped carbon hybrid materials58, 59 and a much higher ORR activity than metal-free nitrogen-doped graphene/carbon nanotube hybrid material.60

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promising applications in fuel cells.

Conclusions Graphene-supported Fe@N-C hybrid material was obtained by pyrolysis of graphene oxide and ammonium ferric citrate in Ar atmosphere followed by acid-leaching. Hollow carbon nanoshells grown on graphene sheet by pyrolysis of ammonium ferric citrate suppressed the stacking of graphene sheet effectively, meanwhile, the decomposition species reduced the graphene oxide and doped nitrogen into the graphene lattice. The hybrid material showed higher electrocatalytic activities for HER and ORR in acidic medium than RGO or Fe@N-C alone, suggesting a synergistic role of nitrogen-doped graphene and Fe@N-C towards the electrocatalytic reactions. By incorporating dicyandiamide to the precursor mixture, the hybrid material exhibited high ORR activities both in acidic and alkaline mediums, which is amongst the best reported non-previous metal catalysts. Acknowledgements We gratefully acknowledge the financial support from the Ministry of Science and Technology of China (Grants 2012CB215500 and 2013CB933100), the National Natural Science Foundation of China (Grants 21103178 and 21033009). References 1. B. C. H. Steele and A. Heinzel, Nature, 2001, 414, 345-352. 16

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Therefore, H-Fe@N-C/RGO has shown high ORR activities in a wide pH range and

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2. J. Zhang, K. Sasaki, E. Sutter and R. R. Adzic, Science, 2007, 315, 220-222. 3. A. Hagfeldt, G. Boschloo, L. C. Sun, L. Kloo and H. Pettersson, Chem. Rev., 2010,

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15. Y. Zhao, C. G. Hu, Y. Hu, H. H. Cheng, G. Q. Shi and L. T. Qu, Angew. Chem. Int. Ed., 2012, 51, 11371-11375.

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16. C. H. Zhang, L. Fu, N. Liu, M. H. Liu, Y. Y. Wang and Z. F. Liu, Adv. Mater., 2011,

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27. Y. Q. Sun, Q. O. Wu and G. Q. Shi, Energ. Environ. Sci., 2011, 4, 1113-1132.

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39. J. Wang, G. X.Wang, S. Miao, X.L. Jiang, J.Y. Li, X.H. Bao, Carbon, 2014, 75, 381-389.

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40. W. S. Hummers and R. E. Offeman, J. Am. Chem. Soc., 1958, 80, 1339-1339.

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53. A. Thomas, A. Fischer, F. Goettmann, M. Antonietti, J. O. Muller, R. Schlogl and J.

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Table 1 The total nitrogen content and percentage of different nitrogen species in the samples. Total nitrogen content

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(wt%)

Percentage of different nitrogen species (%) Pyridinic Pyrrolic Quaternary

Oxidized

Fe@N-C

1.8

33.4

43.0

11.3

12.3

Fe@N-C/RGO

4.2

37.9

45.1

14.7

2.3

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Samples

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Table 2 The ORR activities on the non-precious catalysts in acidic and alkaline mediums. All the potential values from references were converted to vs. RHE for

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Sample

Activity (vs. RHE)

H-Fe@N-C/RGO

FeCo in pod-like carbon nanotube

Reference

acidic

alkaline

Eonset=0.89 V

Eonset= 1.02 V

E1/2=0.67 V

E1/2= 0.86 V

Eonset=0.74 V

not measured

54

not measured

56

Eonset=0.74 V

Eonset=0.85 V

57

E1/2=0.64 V

E1/2=0.79 V

not measured

Eonset=0.93 V

This work

E1/2=0.47 V FeCo in thin pod-like carbon nanotubes

Eonset=0.78 V E1/2=0.59 V

Co in nitrogen-doped carbon nanoshells

Fe/Fe3C in nitrogen-doped carbon

44

E1/2=0.76 V Fe3C in graphitic layers

Eonset=0.90 V

Eonset=1.05 V

55

E1/2=0.73 V

E1/2=0.83 V

Fe3O4 in 3D nitrogen-doped graphene aerogel

not measured

Eonset=0.78 V

38

CoxO/carbon nanotube

not measured

Eonset=0.93 V

58

E1/2=0.85 V Co/N-doped nanoporous carbon

not measured

Eonset=0.88 V

59

E1/2=0.79 V Nitrogen-doped graphene/carbon nanotube

not measured

Eonset=0.87 V E1/2=0.69 V

23

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comparison.

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Figure Captions Scheme 1 The schematic preparation procedures of Fe@N-C/RGO.

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Figure 2 XRD patterns of RGO, Fe@N-C and Fe@N-C/RGO. Figure 3 TG curves of RGO, Fe@N-C and Fe@N-C/RGO in air atmosphere. Figure 4 The N 1s XPS spectra of Fe@N-C and Fe@N-C/RGO. Figure 5 Nitrogen adsorption-desorption isotherm of Fe@N-C and Fe@N-C/RGO. Figure 6 Linear sweep voltammetries (a) and Tafel slopes (b) of RGO, Fe@N-C and Fe@N-C/RGO for hydrogen evolution reaction in Ar-saturated 0.1 M HClO4 at 25 oC. Figure 7 Linear sweep voltammetries (a), electron transfer number and H2O2 yield (b) of RGO, Fe@N-C and Fe@N-C/RGO in O2-saturated 0.1 M HClO4 at 25 oC. Figure 8 TEM image of H-Fe@N-C/RGO. Figure 9 Linear sweep voltammetries of H-Fe@N-C/RGO and Pt/C in O2-saturated 0.1 M HClO4 (a) and O2-saturated 0.1 M KOH (b) solutions at 25 oC.

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Figure 1 SEM and TEM images of RGO (a), (c) and Fe@N-C/RGO (b), (d), (e), (f).

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Scheme 1

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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100 nm Figure 8

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Figure 9

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ToC Graphic and text Graphene-supported iron-based nanoparticles encapsulated in nitrogen-doped carbon

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towards hydrogen evolution and oxygen reduction reactions in acidic medium by exposing more active sites to reactants.

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(Fe@N-C) demonstrate a synergetic role of nitrogen-doped graphene and Fe@N-C

Graphene-supported iron-based nanoparticles encapsulated in nitrogen-doped carbon as a synergistic catalyst for hydrogen evolution and oxygen reduction reactions.

Electrolyzers and fuel cells have been extensively investigated as promising solutions for renewable energy storage and conversion. Hydrogen evolution...
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