Synergistically Enhanced Electrocatalytic Activity of Sandwich-like N

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Synergistically Enhanced Electro-catalytic Activity of Sandwich-like N-Doped Graphene/Carbon Nanosheets Decorated by Fe and S for Oxygen Reduction Reaction Bao Men, Yanzhi Sun, Jia Liu, Yang Tang, Yongmei Chen, Pingyu Wan, and Junqing Pan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06329 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 14, 2016

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ACS Applied Materials & Interfaces

Synergistically Enhanced Electro-catalytic Activity of Sandwich-like N-Doped Graphene/Carbon Nanosheets Decorated by Fe and S for Oxygen Reduction Reaction Bao Men, Yanzhi Sun,* Jia Liu, Yang Tang, Yongmei Chen, Pingyu Wan,* Junqing Pan

National Fundamental Research Laboratory of New Hazardous Chemicals Assessment and Accident Analysis, Institute of Applied Electrochemistry, Beijing University of Chemical Technology, Beijing 100029, China.

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ABSTRACT: Although N-doped graphene-based electro-catalysts have shown good performance for oxygen reduction reaction (ORR), they still suffer from the single-type active site in the as-prepared catalyst, limited accessible active surface area due to easy aggregation of graphene, and harsh condition for preparation process of graphene. Therefore, further developing a novel type of graphene-based electro-catalyst by a facile and environmentally benign method is highly anticipated. Herein, we first fabricate a sandwich-like graphene/carbon hybrid using graphene oxide (GO) and nontoxic starch. Then the graphene/carbon hybrid undergoes post-processing with iron (III) chloride (FeCl3) and potassium sulfocyanide (KSCN) to acquire N-doped graphene/carbon nanosheets decorated by Fe and S. The resultant displays the features of interpenetrated three-dimensional hierarchical architecture composed of abundant sandwich-like graphene/carbon nanosheets and low graphene content in as-prepared sample. Remarkably, the obtained catalyst possesses favorable kinetic activity due to the unique structure and synergistic effect of N, S and Fe on ORR, showing high onset potential, low Tafel slope, and nearly four-electron pathway. Meanwhile, the catalyst exhibits strong methanol tolerance and excellent long-term durability. In view of the multiple active sites, unique hierarchical structure, low graphene content, and outstanding electrochemical activity of the as-prepared sample, this work could broaden the thinking to develop more highly efficient graphene/carbon electro-catalysts for ORR in fuel cells. KEYWORDS: graphene/carbon nanosheets, multiple active sites, synergistic effect, electro-catalytic activity, oxygen reduction reaction

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1. INTRODUCTION Oxygen reduction reaction (ORR) plays a pivotal role in various energy conversion and storage technologies, such as fuel cells, metal-air batteries, chlor-alkali industry, and sodium carbonate electrolysis.1,2 However, the inherent sluggish kinetics severely hampers its further practical applications.3 So, many extensive researches have been carried out to lower activation energy barrier of ORR as much as possible.4 Although Pt has been regarded as the best electro-catalyst for ORR all the time, its application still suffers from some bottlenecks, such as limited reserve, excessively high price, poor durability, and methanol crossover effect.5 It is imperative to exploit new catalysts with excellent performance as the alternatives of platinum-based catalysts. Previous studies have demonstrated that transition metal-based carbonaceous catalysts (e.g., oxides, chalcogenides, nitrides and carbides) show excellent ORR activity, especially for the kind of M-Nx/C (M=Fe, Co, Ni).6,7 In addition, metal-free carbon material with introduction of heteroatoms (e.g., B, N, S, P), as another substitute of Pt catalyst, also markedly improves the catalytic performance for ORR on account of the changes in the local charge density and asymmetry spin density of the carbon lattice adjacent to heteroatoms.4 Based on the investigation of a large number of literatures,7-9 we realize that the activity of ORR catalyst usually depends on three crucial aspects: (1) the intrinsic nature of the active site, governing whether or not the catalyst possesses activity; (2) the type of active site (single or multiple), determining whether the performance of catalyst is high or low; (3) the structure of catalyst material, governing the accessible active sites and mass transportation. 3



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Consequently, most carbon-based electro-catalysts, including transition metal-based and metal-free carbon catalysts, are prepared to obtain the hierarchical structure and large specific surface area by means of pyrolyzing the carbon precursors (e.g., polypyrrole,10 polyaniline,9,11 and so on). Though the catalytic activity can be partly improved, the carbon precursors are usually toxic and environment-hazardous organic polymers. Most of all, their performances are still likely to be retarded by the poor conductivity. Graphene, as a novel 2D carbon material with excellent electrical conductivity, has been intensively paid attention to and applied as ORR catalyst.12,13 However, the disadvantages of harsh preparation condition and high cost are obvious. To achieve good conductivity and reduce graphene content in the as-prepared catalyst, it will be a perfect strategy to fabricate a product with sandwich-like graphene/carbon structure followed by post-processing of graphene/carbon hybrid with active precursors. The resultant could have the hierarchical three-dimensional (3D) framework and enough electrochemical active sites, which are beneficial to boost ORR performance of the as-prepared catalysts. In the present paper, we propose a new type of ORR catalyst based on Fe and S decoration of N-doped graphene/carbon nanosheets. The whole synthesized procedure is facile and cost-effective, and contains only two basic steps. First, the graphene/carbon infrastructure is fabricated with GO and starch which is abundant in source and environment-friendly. Second, the graphene/carbon hybrid is pyrolyzed with FeCl3 and KSCN. The obtained sample not only decreases the graphene content and increases the conductivity due to existence of the unique sandwich-like structure,

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but also possesses desired features for ORR because of these multiple effective intrinsic active sites (i.e., N, S and Fe-Nx), hierarchical pore structure, and large specific surface area (749.5 m2 g-1). Undoubtedly, the proposed catalyst exhibits superior performance toward ORR, even better than commercial Pt/C in alkaline media. 2. EXPERIMENTAL SECTION 2.1 Reagents and Chemicals Natural flake graphite (325 mesh) was purchased from Aladdin Industrial Corporation (Shanghai, China). Soluble starch from potato was purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing). FeCl3 and KSCN were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing). The ultra-pure water (18.2 MΩ cm) was supplied from a Millipore system throughout the whole experiment. All other chemicals were analytical grade, purchased from Beijing Chemical Works, and used without further purification unless otherwise stated. 2.2 Preparation of N-Doped Graphene/Carbon Nanosheets Electro-catalyst Decorated by Fe and S GO was synthesized from natural flake graphite by the modified Hummers’ method.14,15 Then, GO was sonicated for 1 h and the concentration of obtained homogeneous solution was 5 mg mL-1. As illustrated in Scheme 1, there are two steps to synthesize Fe/S-decorated N-doped graphene/carbon (named as Fe,S/NGC) nanosheets. Nitrogen-doped Graphene/Carbon (NGC) hybrid, as the precursor of Fe,S/NGC, was prepared by hydrothermal reaction, similar to the description in our

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previous work.2 Typically, 6 g of starch was dissolved in water at 80 oC to form a translucent solution. Then, GO solution was dropped into the above translucent solution under vigorous magnetic stirring, followed by addition of urea. At last, the mixture was transferred to a sealed Teflon-lined stainless steel autoclave and kept at 190 oC for 12 h. After naturally cooling to room temperature, the resultant was washed several times until the filtrate became colorless and then dried in a vacuum oven at 70 oC for 8 h. Fe,S/NGC-900 was synthesized according to the following procedure. Briefly, the above as-prepared NGC powder was impregnated in an aqueous solution of FeCl3 (1 M, 2 mL) and KSCN (1 M, 6.2 mL). After completely removing the solvent at 80 oC, the acquired powder was fully grinded and heat-treated at 900 oC in high-purity Ar atmosphere for 1 h. To remove unstable and inactive phases, the pyrolysis product was subjected to pre-leaching in 3 M HCl at 80 oC for 8 h, and then filtrated and thoroughly washed with water. Finally, the pre-leached resultant was pyrolyzed again at 900 oC in high-purity Ar atmosphere for 3 h. Fe,S/NGC-900 electro-catalyst was obtained after extensive washing and drying. The control samples were prepared from only NGC (named as NGC-900) and from NGC treated with FeCl3 (denoted as Fe/NGC-900) or KSCN (named as S/NGC-900) under the same conditions. Besides, these products pyrolyzed at different controlled temperatures (700, 800, and 1000 oC) were synthesized and referred to as the Fe,S/NGC-T.

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(a)

(c)

(b) GO

starch

Fe3+ ＋－ SCN

＋

Hydrothermal reaction

Pyrolysis Acid leaching

CO(NH2)2 C

H

O

N

S

Scheme 1 Schematic illustration of synthetic procedure for the Fe,S/NGC-900 a

a

(a) The raw materials of synthesis of NGC nanosheets used as the precursor of

Fe,S/NGC-900. (b) Mixed aqueous solution of FeCl3 and KSCN (above) and NGC nanosheets prepared by hydrothermal reaction (below). (c) The as-obtained catalyst (above) and illustration of nitrogen and sulfur atoms in carbon skeleton (below) of Fe,S/NGC-900. 2.3 Physical Characterizations Scanning electron microscopy (SEM, Zeiss Supra 55) and high-resolution transmission electron microscopy (HRTEM, JEOL 2100) measurements were conducted to characterize the morphology and microstructure of the catalysts. The lacey carbon film supported on copper grids was used for HRTEM characterization. The surface elemental composition and valence of the as-prepared catalysts were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi) with a monochromated Al Kα source. The C 1s peak at 284.8 eV was used as the internal standard during the whole analysis process of XPS. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/max 2500 diffractometer with Cu Kα radiation 7



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(λ=0.15418 nm). Raman spectra were acquired on a Micro-Raman spectrometer (Renishaw, 514 nm laser excitation). Nitrogen adsorption/desorption isotherm was collected on a Quantachrome Instrument at 77 K. Prior to measurement, the sample was degassed in vacuum at 200 oC for 12 h. The specific surface area (SSA) and pore size distribution stemmed from the Brunaer-Emmett-Teller (BET) method. Thermogravimetric analysis (TGA) of the as-obtained sample was performed on the STA 449 F3 Jupiter (NETZSCH) thermal analyzer under N2 atmosphere from ambient temperature to 900 oC at a rate of 5 oC min-1. 2.4 Electrochemical Measurements The electrochemical measurements were carried out on a Bipotentiostat model AFCBP1 electrochemical workstation (Pine Instrument Company, USA) in a standard three-electrode cell. A platinum wire and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. Unless otherwise specified, all the potentials in this paper were calibrated relative to the reversible hydrogen electrode (RHE) as reported in the literature.17,18 2.4.1 Rotating Disk Electrode (RDE) Measurements The working electrode was composed of a glassy carbon electrode (GCE, 5.61 mm in diameter) and the catalyst ink. Before test, the GCE was polished with alumina powder, followed by thoroughly washing with ethanol and water in turn. The preparation method of catalyst ink was as follows: 5.0 mg of catalyst powders were ultrasonically dispersed in 1 mL solution containing 970 µL of dimethyl formamide (DMF) and 30 µL of Nafion solution (5 wt%, Du Pont) for at least 40 min. Then, 10

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µL of the homogeneous suspension was evenly dropped onto the surface of the clean GCE and completely dried at room temperature, corresponding to a catalyst loading amount of 0.2 mg cm-2. For comparison, the commercial Pt/C catalyst (20 wt%, Johnson Matthey Company, HiSPEC 3000) was also prepared with the same method and the loading amount was 20 µgPt cm-2. ORR performance of the as-prepared catalysts was tested via

cyclic

voltammograms (CVs) and linear sweep voltammograms (LSVs) in ultrahigh pure Ar versus O2 saturated electrolyte for 30 min. The scan rate of CV was 50 mV s-1 while that of LSV was 10 mV s-1. To evaluate the electron transfer number per O2 molecule during the process of ORR, the data of LSVs were collected at the electrode rotated from 400 to 2500 rpm. The slopes derived from the Koutecky-Levich plots at various electrode potentials, were used to calculate the electron transfer number (n) on the basis of the following Koutecky-Levich equation.16,17

1 1 1 1 1 = + = + J J K J L J K Bω1 2

(1)

B = 0.62nFDo2/3 ν −1/6 C o

(2)

J K = nFkC o

(3)

Where J is the measured current density, JK and JL are the kinetic current density and diffusion-limited current density, respectively; n is the number of electrons transferred per O2; ω is the rotating speed of the electrode; F is the Faraday constant (F = 96485 C mol-1); Do is the diffusion coefficient of O2 in electrolyte (Do = 1.9×10-5 cm2 s-1 in 0.1 M KOH; 1.93×10-5 cm2 s-1 in 0.1 M HClO4); Co is the bulk concentration of O2 in the electrolyte (Co = 1.2×10-6 mol cm-3 both in 0.1 M KOH and HClO4); υ is the 9



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kinematic viscosity (υ = 1.0×10-2 cm2 s-1 both in 0.1 M KOH and HClO4), and k is the electron-transfer rate constant. 2.4.2 Rotating Ring-Disk Electrode (RRDE) Measurements RRDE measurements were used to evaluate the percentage of peroxide species — (HO2 ) and the electron transfer number. Catalyst inks and electrodes were prepared

as the same as the method described above. RRDE measurement was scanned at a rate of 10 mV s-1. The constant ring potential was controlled at 1.3 V in O2-saturated alkaline solution. The H2O2 yields and electron transfer numbers could be calculated as H 2 O 2 (%) = 200 ×

n=

I R /N (I R /N) + I D

4I D (I R /N) + I D

(4)

(5)

where IR and ID are the ring and disk current, respectively. N is current collection efficiency of the Pt ring, of which the value is 0.4 obtained from the reduction of K3Fe[CN]6.16,17 The stability of as-prepared catalyst was recorded by chronoamperometric curves at 0.75 V in O2-saturated 0.1 M KOH solution with a rotation speed of 1600 rpm. Methanol crossover test was also performed by chronoamperometric curves in the corresponding O2-saturated electrolyte with 3 M methanol.

3. RESULTS AND DISCUSSION The typical microstructure morphology of the as-obtained hybrid graphene/carbon nanosheets was first characterized by SEM in Fig. 1. As shown in Fig. 1a, NGC

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composites exhibit the main structure with randomly isolated two-dimensional nanosheets. A majority of nanosheets have the lateral thickness of approximately 34 nm in Fig. 1b. After the carbonaceous hybrid plus iron and sulfur precursors undergoes high-temperature heat treatment, the framework of the resultant (Fe,S/NGC-900) is quite different from that of NGC in Fig. 1a. According to Fig. 1c, it can be seen clearly that Fe,S/NGC-900 possesses a highly interpenetrated porous architecture of three-dimension (3D). The structure of Fe,S/NGC-900 is composed of numerous interconnected thin nanosheets with a lateral thickness of about 14 nm (Fig. 1d). The corresponding elemental mapping images prove that N, S, and Fe have been successfully doped and homogenously dispersed in Fe,S/NGC-900 (Fig. S1). Compared with the sample prepared with pure graphene (Fe,S/NG-900), Fe,S/NGC-900 successfully avoids aggregation of graphene (Fig. S2). It is noteworthy that the unique hierarchical frameworks can afford a large specific surface area with pore sizes ranging from mesopore to macropore. Thereby, it would not only contribute to transmission of oxygen and electrolyte, but also provide enough active sites during the ORR process.7,18

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Fig.1 SEM images of (a, b) NGC and (c, d) Fe,S/NGC-900. The 3D hierarchical architecture of Fe,S/NGC-900 was further revealed according to TEM images shown in Fig. 2. As illustrated in Fig. 2a, the as-prepared sample consists of abundant semi-transparency nanosheets with wrinkled edges. Significantly, the thin nanosheets have the structure of ternary carbon layers, where both sides of the inner graphene layer are coated by amorphous carbon layers (Fig. 2b). This structure will play a key role in electronic transmission.2 Besides, it is beneficial to keep the 3D structure of nanosheets from collapsing due to the inherent excellent mechanical strength of graphene. TGA declares that Fe,S/NGC-900 has the obvious weight loss (about 5.9%) below 100 oC, which is most likely attributed to the evaporation of adsorbed water (Fig. S3). When the temperature is over 100 oC, the TGA curve basically keeps unchanged, suggesting Fe,S/NGC-900 is thermostable.19 Fig. 2a also 12


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shows many pores on Fe,S/NGC-900, the diameters of which are in the range from tens to hundreds of nanometers. This result is in good agreement with that of SEM. The detailed texture features of the product were indicated by the high-resolution TEM measurement. Fig. 2c clearly depicts that there exist large quantities of dense pores containing micropores and mesopores on the surface of Fe,S/NGC-900, which are also revealed by the result of pore size distribution (inset in Fig. 2d).18 As expected, the large specific surface area was demonstrated by nitrogen adsorption/desorption test in Fig. 2d. It is no wonder that Fe,S/NGC-900 possesses large specific surface area (749.5 m2 g-1) on account of the hierarchical architecture. Taken together, the advantages of the unique framework are conspicuous for improving electrochemical kinetics of ORR. For one thing, the hierarchical porous structure is quite favorable to expose the multiple active sites as much as possible. For another, the graphene in ternary carbon layers of nanosheet could promote electron transport.2,8 As a result, Fe,S/NGC-900 is very hopeful to act as an efficient electro-catalyst in industry application due to the novel hierarchical structure and low graphene amount.

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(a)

(b)

(c)

500 450

(d)

400 350 300

0.8

0.6 dV/dlog(D)

Quantity Adsorbed /cm3 g-1STP

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0.4

0.2 0

10

20 30 Pore Size / nm

40

50

250 200 0.0

0.2 0.4 0.6 0.8 Relative Pressure / p/p0

1.0

Fig. 2 (a) TEM image, (b, c) HRTEM images and (d) Nitrogen adsorption/desorption isotherm of Fe,S/NGC-900.

The analyses of XRD patterns and Raman spectra were conducted to reveal the phase composition and crystal structure of the as-obtained samples (Fig. 3). For Fe,S/NGC-900 and NGC-900, there is a broad peak at 2θ ≈ 23.8°, assigned to the (002) planes of graphitic carbon in Fig. 3a. While the (002) peak intensity of Fe,S/NGC-900 is much stronger than that of NGC-900, suggesting the higher degree of graphitization with Fe precursor.20 Except that, the other difference between them is that there are two groups of characteristic peaks for Fe,S/NGC-900, arising from Fe2O3 (JCPDS No. 39-1346) and Fe3N (JCPDS No. 49-1662), respectively. Combined with the very low peak intensity, it is indicated that Fe2O3, as the inactive phase for 14


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ORR, has almost been removed after leaching by acid, although there are still a tiny amount (marked by red dashed circle in Fig. 2a) existing in the gray carbon matrix. This can be further demonstrated by XPS survey. The Fe3N existing in Fe,S/NGC-900 may be the active centre and can improve electrochemical performance during ORR process. Moreover, it is interesting that a rapid increase of intensity at the low-angle scatter is observed for both samples. It clarifies that the as-prepared material has abundant pores.21 Raman spectra were carried out to further assess graphitic structure of the samples, as shown in Fig. 3b and Fig. S4. The G band at 1594 cm-1 reflects graphite in-plane vibrations, whereas the D bond at 1358 cm-1 represents the disordered degree of carbon framework. The relative intensity of D and G band (ID/IG) is usually used to evaluate the graphitization degree of carbonaceous materials.2 Compared with that of NGC-900 (0.95), the value of Fe,S/NGC-900 is just 0.92. This result displays that Fe,S/NGC-900 has a higher graphitization degree than NGC-900, in agreement with the result of XRD.

(a)

(b)

Fe, S/NGC-900 NGC-900 # Fe2O3

(002)

♣ Fe3N # #

♣

Intensity / a.u.

Intensity / a.u.

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


NGC-900

# #

Fe,S/NGC-900

15

30

45 60 2θ / degree

75

90 500

1000 1500 2000 -1 Raman shift / cm

2500

Fig. 3 (a) XRD patterns and (b) Raman spectra of NGC-900 and Fe,S/NGC-900.

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XPS was performed to confirm the element composition and bonding configurations of Fe,S/NGC-900. Fig. 4a exhibits the survey spectrum of the as-prepared catalyst, which reveals the presence of C, O, N, S, and Fe. Among these elements, the content of N and S, usually regarded as the active centres in N, S-doped carbon materials, is 2.22 and 0.56 at%, respectively.18 Notably, the negligible signal intensity of Fe implies that the content of iron in Fe,S/NGC-900 is very low. The detailed information about bonding configuration of each element was given by high-resolution XPS spectra. In the light of the results from high-resolution XPS spectra, the schematic of different types of nitrogen and sulfur is shown in Fig. 4b. For C 1s, there are five peaks at 283.9, 284.6, 285.6, 287.1, and 289.1 eV, corresponding to C-S, C-C, C-O/C=N, C=O/C-N, and O-C=O, respectively (Fig. S5),8,22 demonstrating that N and S have been successfully introduced into the framework of the as-obtained carbon material. Meanwhile, the N 1s spectrum is fitted into three peaks as shown in Fig. 4c, namely, pyridinic N (398.3 eV, 20.72 at%), pyrrolic N (399.4 eV, 4.8 at%), and graphitic N (400.8 eV, 74.48 at%).2 Remarkably, the total percentage content of pyridinic N and graphitic N is as high as 95.2 at% among the three kinds of nitrogen in Fe,S/NGC-900. Fig. 4d displays that the S 2p spectrum is also divided into three peaks located at 163.7, 164.6, and 168.0 eV, respectively.23 The former two main peaks are assigned to the binding sulfur in -C-Sand conjugated -C=S- bonds, while the third minor peak is attributed to the oxidized sulfur (-SOx). Compared with oxidized sulfur, the former two sulfur species are presumed to more effectively facilitate ORR in alkaline solution.23 So, additional

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doped S further creates more active centres during the ORR process catalyzed by Fe,S/NGC-900. Note that there should be a peak at about 398.3 eV from Fe-Nx, because pyridinic N and pyrrolic N owning lone-pair electrons can serve as metalcoordination sites.6 Nonetheless, it could not substantially distinguish the peak of pyridinic N from that of Fe-Nx due to only a small difference of binding energy between them.9 Previously, plentiful studies indicated that the pyridinic N and graphitic N among doped nitrogen, the doped sulfur or the Fe-Nx moiety was well recognized as the active sites of ORR.2,6,9,18,22-24 Based on the aforementioned analysis, one can absolutely draw a conclusion that Fe,S/NGC-900 would possess the outstanding electro-catalytic performance for ORR because of the synergistic effect of so many electro-active sites on the 3D interconnected graphene/carbon hybrid.

(a)

(b)

C 1s Fe 2p

N S

Intensity / a.u.

O 1s

SO2

N

S 2p 700

710

720

N

730

N H

A

N 1s

N

160 164 168 172 176

S

N

N N

0

200

(c)

400 600 800 Binding Energy / eV

1000

S

(d)

Graphitic N Pyridinic N Pyrrolic N

S 2p 3/2 S 2p 1/2 SOx-

Intensity / a.u.

Intensity / a.u.

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


396


404 160

162

164 166 168 170 Binding Energy / eV

172

Fig. 4 (a) XPS survey spectrum of Fe,S/NGC-900, (b) schematic of different types of 17



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nitrogen and sulfur in Fe,S/NGC-900, (c) and (d) high-resolution spectra of N 1s and S 2p for Fe,S/NGC-900, respectively.

The ORR catalytic activities of Fe,S/NGC-900 as well as the control samples and commercial Pt/C were first evaluated by cyclic voltammetry, rotating disk electrode, and rotation ring-disk electrode measurements. As depicted in Fig. 5a and Fig. S6, compared with featureless CV plots in Ar-saturated 0.1 M KOH solution, the pronounced enhanced reduction peaks appear for each as-prepared catalyst in O2-saturated electrolyte. This implies that the four kinds of carbon material possess the favorable catalytic performance for ORR in alkaline solution. Interestingly, no matter whether the peak potential or the peak current is referred, Fe,S/NGC-900 obviously outperforms the other three samples. Evidently, the ORR activity improvement of Fe,S/NGC-900 originates from the introduction of additional active sites and the synergistic effect of various active sites apart from the unique 3D graphene/carbon architecture. Fig. 5b shows that there occurs the similar behavior in LSV plots for these samples. For NGC-900, it displays the most inferior onset potential (0.90 V) and half-wave potential (0.76 V) as well as the lowest diffusion-limited current density. When iron or sulfur was solely introduced into graphene/carbon composites, the electrochemical performance was improved to a certain extent. However, the ORR activity was enhanced substantially when introducing Fe and S simultaneously. So an outstanding catalytic performance with more positive onset potential of 0.95 V and half-wave potential of 0.83 V is observed for Fe,S/NGC-900. These parameters even surpass the onset potential (0.93 V) and 18


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half-wave potential (0.81 V) of the state of the art Pt/C catalyst, not to mention that of Fe/NGC-900 and S/NGC-900. Concurrently, the cathodic current density of Fe,S/NGC-900 is also superior to that of Pt/C in the whole potential range. The above results indicate that the introduction of Fe and S creates more active sites indeed and these active sites are also fully utilized by the reactants on the basis of the perfect hierarchically interpenetrated structure, synergistically boosting the electrochemical activity of Fe,S/NGC-900.25 For insight into the ORR kinetics and reactive mechanism, the kinetic parameters containing electron transfer number (n) and kinetic current density (JK) were analyzed by Koutecky-Levich (K-L) equation using the data of RDE measurement at different rotating speeds. Fig. 5c shows that the diffusion-limited current density of Fe,S/NGC-900 rises as rotating speed increases because the diffusion distance is shortened at high speeds.8,26 The corresponding K-L plots, standing for the relation between j-1 and ω-1/2, keep quite good linearity and nearly overlap each other, suggesting first-order reaction kinetics toward the concentration of dissolved oxygen and similar electron transfer number for ORR at different potentials.16,27 In addition, the calculated results show the electron transfer number for Fe,S/NGC-900 is 3.98-4.00 in the potential range from 0.2 V to 0.6 V as compared with 3.47-3.87 for NGC-900, 3.76-3.97 for Fe/NGC-900, and 3.82-3.92 for S/NGC-900 in the same potential range (Fig. S7). It demonstrates Fe,S/NGC-900 favors a nearly four-electron pathway for ORR in alkaline media, similar to that of Pt/C. Moreover, the kinetic current density (Jk) of Fe,S/NGC-900 calculated from the K-L plots is the highest

19



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value of 54.85 mA cm-2 at 0.6 V among the four samples, slightly higher than that for Pt/C (52.71 mA cm-2). Meanwhile, the value is approximately 1.25, 1.61, and 3.54 times as high as those for S/NGC-900 (43.88 mA cm-2), Fe/NGC-900 (34.05 mA cm-2), and NGC-900 (15.50 mA cm-2), respectively (Fig. 5d). The highest kinetic current density further demonstrates the most excellent electrochemical activity for Fe,S/NGC-900. To further appraise the kinetic characteristic of ORR, the Tafel slopes which describe the overall resistance during the ORR process were obtained from the LSVs at 1600 rpm for different electro-catalysts.28 Typically, the Tafel plots of all samples exhibit two-stage linear regions at low and high overpotential. The overall ORR speed is controlled by the surface reaction rate on the catalyst at low overpotential, whereas it is dominated by the mass transfer inside the material at high overpotential (Fig. S8).17 The corresponding values in each region were shown in Fig. 5e. As for Fe,S/NGC-900, the Tafel slope of 67 mV dec-1 is the lowest in the low overpotential region, likely attributed to the faster electron transfer provoked by the incorporated graphene in the thin graphene/carbon nanosheets and more electro-active centres (N, S, and Fe-Nx specie) exposed on the surface of the as-prepared catalyst with large surface area.29 The situation is same to the high overpotential region, that is to say the value of Fe,S/NGC-900 is also the lowest (108 mV dec-1). This is −

responsible for more efficient mass (such as O2 and OH ) diffusion facilitated by the interconnected hierarchical porous framework.29 As a result, one can draw a conclusion that Fe,S/NGC-900 is a high-performance ORR catalyst in accordance with the results discussed above. This conclusion would also be verified by RRDE

20


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tests which were used to systematically assess the ORR process. As shown in Fig. S9, when Fe,S/NGC-900 is used as the ORR catalyst in alkaline solution, the ring current density is fairly suppressed as compared with other control samples. This signifies the direct 4e pathway is dominant for Fe,S/NGC-900 during the ORR process. According −

to these data calculated from the disk and ring currents in Fig. 5f, the HO2 yield of Fe,S/NGC-900 is below 2% over a wide potential range and the average electron transfer number is 3.96 as we expected. The n value is close to that of Pt/C and in good agreement with the results from the K-L plots. Likewise, Fe,S/NGC-900 also displays an outstanding ORR activity in acidic solution. As shown in Fig. S10a, Fe,S/NGC-900 exhibits an onset potential of 0.83 V and diffusion-limited current density of 4.95 mA cm-2 at 0.1 V in O2-saturated 0.1 M HClO4. Although the onset potential is about 70 mV negative to that for Pt/C catalyst, the diffusion-limited current density is very close to that of Pt/C (Fig. S11). Moreover, the average number of electron transfer for Fe,S/NGC-900 is calculated to be 3.98 from the data of LSVs in Fig. S10b. This proves that Fe,S/NGC-900 also possesses a selectivity of 4e pathway for ORR in acidic media. It is worthy to be mentioned that Fe,S/NGC-900 shows its own merit as compared with most of other similar catalysts reported by recent papers in term of electrochemical performance (Table S1). Combined with the structure features and electrochemical activity of Fe,S/NGC-900, the constructional design of sandwich-like graphene/carbon nanosheets, the morphology of 3D hierarchical porous configuration, introduction of N, S, and Fe dopants and the simple green synthesis approach make it become an extraordinary ORR catalyst and

21



affordable for large-scale commercial application. 4

(a)

0

O2

-4

Ar

0.4 0.6 0.8 E / V vs. RHE

j / mA cm-2

-1

0.0 -1.5

1.0

0.25 0.20

0.08

0.10

0.12

0.14

ω -1/2 / rad-1/2 s1/2

-3.0

0.16

2500 2025 1600 1225 900 625 400

-6.0 0.2

0.4 0.6 E / V vs. RHE

180

-1

0.4 0.6 E / V vs. RHE

(e) 159 150

0.8

110

108 77 67

n=4.00

Jk at 0.6 V vs. RHE

n=3.89

30 20

n=3.47

10 0

100

0 00 00 00 -90 C C-9 C-9 C-9 GC Pt/ NG /NG NG S S/N , Fe / Fe

(f)

68

60

3

NGC-900 Fe/NGC-900 S/NGC-900 Fe,S/NGC-900

60 40

2

HO2 %

1

20

30 0 0 00 00 - 90 - 90 t /C C- 9 C- 9 P GC GC NG NG S /N S /N , F e/ e F

0 0.0

4

n

80

132

104 88

n=3.99

1.0

40

1.0

Low overpotential High overpotential 139

0.8

n=3.82

-4.5

0

0.2

(d) 50

0.2 V 0.3 V 0.4 V 0.5 V 0.6 V 0.06

90

-4

60

0.30

0.15

0.0

-3

-6 0.0

1.2

0.35

-1

(c)

j / mA cm2

0.2

-2

-5

Jk / mA cm-2

-6 0.0

Tafel slope / mV dec

j / mA cm-2

-2

120

NGC-900 Fe/NGC-900 S/NGC-900 Fe,S/NGC-900 Pt/C

-1

0

1.5

(b)

HO2- %

j / mA cm-2

2

n

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

Page 22 of 33

0.2

0.4 0.6 E / V vs. RHE

0.8

0 1.0

Fig. 5 (a) CV curves in Ar versus O2-saturated electrolyte of Fe,S/NGC-900. (b) LSV curves at the rotating speed of 1600 rpm of the as-prepared samples and Pt/C catalyst in O2-saturated electrolyte. (c) LSV curves of Fe,S/NGC-900 at various rotating speeds in O2-saturated electrolyte. (Inset) Corresponding K-L plots at different potentials. (d) Kinetic current density of the as-prepared samples and Pt/C catalyst at 0.6 V. (e) The corresponding values of Tafel slope at low and high overpotential

22


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regions of above samples. (f) Peroxide yields (below) and corresponding electron transfer number (above) of the obtained samples during the process of ORR. Electrolyte is 0.1 M KOH solution; For CV, the scan rate is 50 mV s-1, while for LSV, the scan rate is 10 mV s-1, and the rotating speed is 1600 rpm.

Why does Fe,S/NGC-900 display so excellent electrochemical activity toward ORR? To the best of our knowledge, the reason is still ambiguous because there have been few reports about doping multiple-heteroatoms (N, S, and Fe) in carbon material so far. However, combined with the experimental results, the most likely reasons are hypothesized as follows. (1) when iron and sulfur precursors are added to N-doped graphene/carbon composites separately, the Fe-Nx and S may act as the major active sites to conspicuously improve the ORR performance of Fe/NGC-900 and S/NGC-900, respectively.25,30 When Fe and S are added simultaneously, the multiple active sites enhance the electrochemical property of Fe,S/NGC-900; (2) the unique 3D hierarchical porous architecture guarantees efficient mass transportation during reaction process; (3) the more the active sites on the surface of the as-prepared catalysts, the better the ORR performance; and (4) the inner graphene in the very thin sandwich-like graphene/carbon nanosheets could promote electron transfer. Of course, the pyrolysis temperature is also an important factor for improving ORR activity. As is well-known, pyrolysis is a directly effective approach to achieve the doping of heteroatoms into carbon materials. In general, the content and bonding configuration of heteroatoms are susceptible to the pyrolysis temperature, further influencing the electro-catalytic activity toward ORR. In view of this, the different 23



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temperatures (700, 800, 900, and 1000 oC) were discussed to obtain optimum condition of heat treatment in this paper. According to Fig. 6a, it can be seen that the content of O, N, and S decreases with increasing pyrolysis temperature (Table S2), signifying the improvement of graphitization for the prepared samples. The high-resolution N 1s spectra indicate that as temperature increases, the relative percentage content of pyrrolic N reduces gradually, whereas that of pyridinic N and graphitic N increases shown in Fig. 6b. The result arises from thermolability of pyrrolic N which is prone to transfer into the other two types of nitrogen.2,6 When the pyrolysis temperature is 900 oC, the total relative content of pyridinic N and graphitic N is the highest among the four samples prepared at different temperatures (Fig. 6c). So, by means of the previous studies,7,8 it is reasonable that the high percentage content of pyridinic N and graphitic N is conducive to enhance the catalytic activity and electrochemical kinetics of Fe,S/NGC-900. Subsequently, electrochemical properties were tested and shown in Fig. 6d. The results prove that Fe,S/NGC-900 leaps out as the best ORR electro-catalyst among the series in terms of the lowest overpotential and largest diffusion-limited current density.

24


Intensity / a.u.

(a)

C 1s O 1s

N 1s

(b)

Fe,S/NGC-700 Fe,S/NGC-800 Fe,S/NGC-900 Fe,S/NGC-1000

Fe,S/NGC-1000

Fe 2p

S 2p

Fe,S/NGC-900 Fe,S/NGC-800

Fe,S/NGC-700

0 100 Relative N content / at. %

200

(c)

80

400 600 800 Binding Energy / eV Pyridinic N Pyrrolic N Graphitic N

60 40

396

1000 0


(d)

404

Fe,S/NGC-700 Fe,S/NGC-800 Fe,S/NGC-900 Fe,S/NGC-1000

-1

j / mA cm-2

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


Intensity / a.u.

Page 25 of 33

-2 -3 -4

20

-5

0 700

800 900o Temperature / C

1000

-6 0.0

0.2

0.4 0.6 E / V vs. RHE

0.8

1.0

Fig. 6 (a) XPS survey spectra, (b) high-resolution N 1s spectra, (c) relative percentage contents of different nitrogen kinds and (d) corresponding electrochemical performance of samples prepared at different temperatures (T=700, 800, 900, and 1000 oC).

Except for electrochemical activity, the durability is another important standard to assess the performance of catalysts. As shown in Fig. 7a, the long-term stability test was first performed by chronoamperometry. After continuously testing for 20000 s, Fe,S/NGC-900 does not show apparent activity attenuation. But, in sharp contrast, the commercial Pt/C catalyst shows an inferior durability with only about 70% performance retention under the same condition because of particle aggregating and falling off.5 In addition, the tolerance against methanol crossover was also

25



investigated by injecting 3 M methanol into O2-saturated 0.1 M KOH solution. Fig. 7b displays no conspicuous current decay for Fe,S/NGC-900 as compared with Pt/C. The aforementioned results manifest Fe,S/NGC-900 possesses better long-term stability and stronger methanol tolerance than Pt/C catalyst. This just highlights the merits of Fe,S/NGC-900 over other similar ORR catalysts reported by the literatures. 100

100

80

(a)

60 40

Fe,S/NGC-900 Pt/C

20

Relative current / %

Relative current / %

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

Page 26 of 33

0 0

5000

10000 Time / s

15000

20000

80

(b)

60 40

Fe,S/NGC-900 Pt/C

20 0 0

200

400

600 800 Time / s

1000 1200

Fig. 7 (a) The i-t chronoamperometric responses of Fe,S/NGC-900 and Pt/C at 0.75 V in O2-saturated 0.1 M KOH solution. (b) The methanol tolerance tests of Fe,S/NGC-900 and Pt/C by injecting 3.0 M CH3OH into O2-saturated 0.1 M KOH solution at about 450 s. The rotation speed of above tests is 1600 rpm.

4. CONCLUSIONS In summary, Fe,S/NGC-900 was successfully fabricated by a facile and simple synthetic procedure. The resultant which exhibits the interpenetrated hierarchical porous architecture is composed of unique sandwich-like graphene/carbon nanosheets. Remarkably, due to the novel structure with large specific surface area and multiple-heteroatoms (N, S, and Fe) introduction, Fe,S/NGC-900 possesses outstanding ORR performance within the context of onset potential, half-wave

26


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potential, electron transfer number, long-term stability, and methanol tolerance. In consideration of low graphene content in hybrid carbon material and excellent electrochemical activity toward ORR, Fe,S/NGC-900 is cost-effective and promising to be a substitute of Pt/C catalyst.

ASSOCIATED CONTENT Supporting Information The elemental mapping images and TGA curve of Fe,S/NGC-900; SEM images of Fe,S/NC-900 and Fe,S/NG-900; Raman spectra of NGC-900, S/NGC-900, Fe/NGC-900, and Fe,S/NGC-900; The high resolution spectra of C 1s in Fe,S/NGC-900; CV curves and LSV curves at various rotation speeds and corresponding Kouteck-Levich plots of NGC-900, Fe/NGC-900, and S/NGC-900 in 0.1 M KOH solution; Tafel plots of NGC-900, Fe/NGC-900, S/NGC-900, Fe,S/NGC-900, and Pt/C catalyst; RRDE voltammograms at the rotating speed of 1600 rpm of as-prepared catalysts in 0.1 M KOH solution; LSV curves at different rotation rates and corresponding Kouteck-Levich plots for Fe,S/NGC-900 and Pt/C in 0.1 M HClO4 solution; Comparison of ORR performance from this work and literature reported relevant catalysts in 0.1 M KOH solution; The element content of samples prepared at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mails: [email protected]; [email protected]

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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No. 51374016,21506010 & 21476022), BUCT Fund for Disciplines Construction and Development (No. XK1531), the State Key Program of National Natural Science of China (No. 21236003), the Fundamental Research Funds for the Central Universities (No. JD1515 & YS1406).

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TOC 0

(O2)

NGC-900 Fe/NGC-900 S/NGC-900 Fe,S/NGC-900 Pt/C

-1 j / mA cm -2

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


-2 -3

Fe

S

-4 -5 -6 0.0

(OH-)

1 µm

0.2

0.4 0.6 E / V vs. RHE

0.8

N

1.0

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Synergistically Enhanced Electrocatalytic Activity of Sandwich-like N

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