S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient

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S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Electrocatalysts Jiajie Li, Yumin Zhang, Xinghong Zhang, Jinzhen Huang, Jiecai Han, Zhihua Zhang, Xijiang Han, Ping Xu, and Bo Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12547 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

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ACS Applied Materials & Interfaces

S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient Oxygen Reduction Reaction Electrocatalysts Jiajie Li,† Yumin Zhang,† Xinghong Zhang,† Jinzhen Huang,† Jiecai Han,*,† Zhihua Zhang,¶ Xijiang Han,Ж Ping Xu,*, Ж and Bo Song*,†,‡,∮ †

Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China. Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China. Ж School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150080, China. ‡ Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology, Harbin 150080, China. ¶

∮

Department of Physics, Harbin Institute of Technology, Harbin 150080, China.

ABSTRACT: Replacement of rare and precious metal catalysts with low-cost and earth−abundant ones is currently among the major goals of sustainable chemistry. Herein, we report the synthesis of S, N dual−doped graphene-like carbon nanosheets via a simple pyrolysis of a mixture of melamine and dibenzyl sulfide as efficient metal-free electrocatalysts for oxygen reduction reaction (ORR). The S, N dual−doped graphene-like carbon nanosheets show enhanced activity towards ORR as compared with mono−doped counterparts, and excellent durability in contrast to the conventional Pt/C electrocatalyst in both alkaline and acidic media. A high content of graphitic−N and pyridinic−N is necessary for ORR electrocatalysis in the graphene-like carbon nanosheets, but an appropriate amount of S atoms further contributes to the improvement of ORR activity. Superior ORR performance from the as−prepared S, N dual−doped graphene-like carbon nanosheets implies great promises in practical applications in energy devices. KEYWORDS: S, N−doped carbon, graphene-like, oxygen reduction reaction, electrocatalysis, metal-free

However, S, N dual-doped graphene materials are usually obtained via high-temperature annealing and/or hydrothermal treatment of graphene oxide (GO) with 2aminothiophenol/hydrazine monohydrate,35 melamine/benzyl disulfide,43 or other S, N containing precursors.44-47 However, these approaches often suffered from a poor control over the chemical homogeneity, reproducibility, and doping level,20, 48, 49 which hindered the understanding of the ORR catalytic nature and further applications of S, N dual-doped graphene materials. Therefore, it is urgent to develop a highly efficient as well as controllable route to synthesize S, N dual−doped graphene materials with high ORR catalytic activity. Herein, we report the synthesis of S, N dual−doped graphene-like carbon nanosheets via a simple and cost−effective approach by pyrolysis of a mixture of melamine and dibenzyl sulfide. The content of N and S atoms in dual doped carbon nanosheets can be well controlled by tuning the ratios of N and S sources. S, N dual−doped graphene-like carbon nanosheets exhibit an enhanced ORR activity as compared to mono−doped carbon nanosheets in both alkaline and acidic media. It is revealed that a high graphitic−N and pyridinic−N content is necessary for ORR activity in doped carbon materials, but an appropriate amount of S atoms can even improve the ORR performance. We believe our strategy in synthesizing dual-doped graphene-like carbon nanosheets with enhanced ORR activity may open up new avenues in the functionalization of carbon materials.

INTRODUCTION Electrochemical oxygen reduction is a critical process for many energy storage and conversion device such as fuel cells and metal–air batteries,1-4 while sluggish oxygen reduction reaction (ORR) kinetics at the cathode and high cost of Ptbased catalysts hinder their widespread commercialization.5-7 Recently, significant efforts have been devoted to exploring non-precious catalysts with promising ORR activity, including catalysts based on transition metals8-15 and doped carbon materials16-22. Among them, N−doped carbon materials have attracted increasing attention owing to their excellent electrocatalytic performance and low cost.19, 23-28 The origin of the ORR activity has been demonstrated to result from the doping effects of the heteroatoms, which changes the charge and spin densities of carbon atoms, and the doping atoms also serve as catalytic centers for the adsorption of oxygen molecules.20, 25, 29-31 Recent studies have found that superior catalytic capabilities of graphitic carbon materials can be achieved by dualdoping pathways using heteroatoms like N, S, P, and O.32-37 S atom, featuring p orbits in its outermost shell, possesses a close electronegativity to carbon, which can induce strain and stress in the graphene matrix.38-40 Consequently, in case of dual-doped carbon materials, S has received great interest because S atoms can easily induce the polarization of neighboring C and N atoms.33 Unique electron distribution can therefore be introduced, which then induces synergistic effects to create a higher density of active sites, rendering dual-doped carbon materials much more effective during ORR electrocatalysis.41, 42

EXPERIMENTAL SECTION 1

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Material synthesis. S and N dual−doped graphene-like carbon nanosheets were synthesized by pyrolysis of a mixture of dibenzyl sulfide (99.99%, Alfa Aesar) and melamine (99.9%, Alfa Aesar) in a tube furnace at 700−1000 °C. In a typical synthesis, 1 mmol dibenzyl sulfide and 6 mmol melamine were mixed uniformly in an agate mortar and then transferred into a 5 cm long ceramic boat in a conventional tube furnace. Before the reaction, the tube furnace was degassed for three times and heated to 700–1000°C for 2 h with a ramp rate of 2°C/min under 80 sccm high−purity argon (99.999%). Then, S, N dual−doped graphene-like carbon nanosheets can be collected from the surface of the ceramic boat after the furnace was cooled down to room temperature (RT) naturally. For comparison, a series of dual-doped graphene-like carbon nanosheets samples with different ratios of dibenzyl sulfide to melamine were also prepared.

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of water, ethanol, and Nafion (5%) (Volume ratio = 1: 1: 0.2) to form an ink with 4 mg mL–1 catalyst. The ink was added dropwise on a rotating disk electrode (RDE) with a glassy carbon disk (3.0 mm in diameter). For comparison, a 4 mg mL–1 Pt/C suspension (20 wt. %, E−TEK) was also prepared following the same procedure as described above. The cyclic voltammograms (CVs) of different catalysts were performed in an Ar/O2−saturated KOH (0.1 M) or H2SO4 (0.5M) solution at a scan rate of 50 mV s– 1. For the RDE measurements, the working electrode was scanned cathodically at a scan rate of 10 mV s-1 with rotating speed varying from 225 to 2,025 rpm. The number of electrons transferred (n) at different electrode potentials was determined using the Koutecky−Levich plots (J −1 vs ω −1/2) according to Eq. (1): 1/J = 1/JL + 1/JK = 1/Bω1/2 + 1/JK (1) where J is the measured current density, JK and JL are the kinetic and diffusion-limiting current densities, ω is the rotating speed of the disk, B in Koutecky–Levich equations can be express as Eq.(2): B = 0.2nFC0D02/3v-1/6 (2) where n is the transferred electron number, F is the Faraday constant, C0 is the bulk concentration of O2, v is the kinematic viscosity of the electrolyte.

RESULTS AND DISCUSSION Scheme 1 depicts the simple protocol for the synthesis of S, N dual−doped graphene-like carbon nanosheets by pyrolysis of a mixture of dibenzyl sulfide and melamine. The resulting S, N dual−doped graphene-like carbon nanosheets samples are labeled as SxNyCT, where x and y represents the initial molar ratio of dibenzyl sulfide to melamine, and T is the pyrolysis temperature. Taking S1N6C900 for instance, the assynthesized product possessed typical sheet structures of graphene-like materials as shown in the SEM image (Figure 1a) and TEM images (Figures 1b and 1c). It was found that other SxNyCT samples also display similar sheet-like structures (Figure S1). SAED pattern (inset of Figure 1b) with the typical ring−like mode reveals the mostly disordered (amorphous) feature of S1N6C900. Further, high−resolution TEM (HRTEM) image (Figure 1c) clearly shows that S1N6C900is actually composed of few-layer graphene-like carbon nanosheets (3−6 layers). AFM image of S1N6C900 (Figure 1d) directly proves the layered structure with a layer thickness between 1.2 and 1.5 nm (Figure 1e), matching well with the thickness of few-layer graphene-like carbon nanosheets. XRD patterns of SxNyCT samples pyrolyzed at 700−1000°C display only one broad diffraction peak at ~ 24.7°, corresponding to the (002) planes of graphene(Figure S2a), indicating that all the precursors have completely decomposed above 700°C, which could be further confirmed by the Raman spectra (Figure S2b) with two typical peaks located at ~ 1352 and 1580 cm-1, assigned to the disordered sp3state (D band) and graphitic sp2state (G band) of carbon materials, respectively.50, 51 As the pyrolysis temperature increased from 700 to 1000°C, the relative intensity ratio of D to G band (ID/IG) increased slightly from 0.93 to 0.99, represents more disordered structure in graphitic carbon or increased graphitization degree of amorphous carbon in SxNyCT.52, 53 STEM (Figure 1f) elemental mapping of S1N6C900 displays a homogeneous distribution of N

Scheme 1. Schematic illustration of the synthesis procedure of S, N dual−doped graphene-like carbon nanosheets materials for the application in oxygen reduction electrocatalysis.

Characterization. Scanning electron microscopy (SEM) images were recorded using a Hitachi SU8020 scanning electron microscope. X−ray diffraction (XRD) measurements were performed on a Rigaku D/max 2500 X−ray diffractometer (Cu Kα radiation). Transition electron microscopy images (TEM), selected-area electron diffraction (SAED) pattern, energy−dispersive EDX spectra, and elemental mapping were collected on a JEM−2100F. X−ray photoelectron spectra (XPS) were recorded on an ESCALAB MKII using Mg Kα as the excitation source. Raman spectra were collected on a confocal Renishaw in Via Raman spectrometer using a 633nm laser as the excitation source. Brunauer–Emmett–Teller (BET) specific surface areas were measured by nitrogen adsorption– desorption analyses using a Micromeritics ASAP 2020.

Electrochemical measurements. The electrochemical measurements were performed using a CHI 660D electrochemical testing system in a conventional three−electrode cell. Ag/AgCl (3 M KCl) and platinum wire were used as the reference and counter electrodes, respectively. All potentials in this study were corrected to that of reversible hydrogen electrode (RHE). The as−prepared catalysts were dispersed in a mixture

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Figure 1. Structural characterizations of S, N dual-doped graphene-like carbon nanosheets. (a) SEM and (b) TEM images of S1N6C900; the inset of (b) shows the SAED pattern of S1N6C900. (c) HRTEM image of S1N6C900. (d) AFM image of S1N6C900 and (e) corresponding height measurements. (f) Scanning TEM image of S1N6C900 and energy−dispersive X−ray spectroscopic elemental maps of C, N and S (g, h and i), respectively.

and S atoms in the sheet plane (Figure 1g−i). The surface area of S1N6C900 determined by BET measurements using N2 adsorption–desorption analysis is ~ 670 m2 g−1 (Figure S3 and Table S1). It needs to point out that without the existence of melamine, S1N0C900 presents an irregular morphology (Figure S4). Based on the above results, the formation of S, Ndoped graphene-like carbon nanosheets can be explained as below. Thermal condensation of melamine creates a layered carbon nitride template, and the carbon atoms from the decomposition of dibenzyl sulfide are intermediates by means of donor-acceptor interactions and confine their condensation in a cooperative process to the interlayer gaps of g-C3N4 at 600 o 28, 48 C. Since the g-C3N4 template undergoes complete thermolysis above 750 oC, the final graphene-like nanosheets are liberated at higher temperatures.50 To evaluate the doping effect on the electrocatalytic activity, we first carried out the tests in alkaline medium. CVs of S1N6CT materials toward ORR were collected in O2−saturated 0.1 M KOH solution at a constant active mass loading and a scan rate of 50 mV s−1 (Figure 2a). Notably, the cathodic ORR peaks and current densities of SxNyCT materials were significantly affected by the pyrolysis temperature, where S1N6C700 displays no activity and S1N6C800 has very limited catalytic activity. Interestingly, S1N6C900 exhibits the most positive cathodic ORR peaks of 0.798 V vs. RHE with a current density of −3.75 mA cm−2. However, an even higher pyrolysis temperature leads to deteriorated ORR activity in S1N6C1000. To further evaluate the role of pyrolysis temperature on electrocatalytic properties, the S1N6CT samples were analyzed by LSV measurement using RDE (Figure 2b) at 1,600 rpm in O2−saturated 0.1 M KOH aqueous solution. It was found that as the pyrolysis temperature increased, the limiting current

densities were increased except for S1N6C1000. Among all the S1N6CT samples, S1N6C900 exhibits the highest limiting current densities of about −4.9 mA cm−2 (at 0.2V vs. RHE). Electrochemical impedance spectra (EIS, Figure S5) reveal that the electrical conductivity was improved with increase in the pyrolysis temperature. However, no obvious improvement in ORR activity was found for S1N6C1000 as compared to S1N6C900, probably due to the significantly decreased of S, N doping level even with the highest electrical conductivity of S1N6C1000. Above results clearly reveal that catalytic activity of the as-prepared graphene-like carbon materials is highly dependent on the pyrolysis technique in a way to affect the heteroatom content and configuration in the carbon matrix. To gain insight into the effect of content and type of S, N atoms on ORR performance, electrochemical measurements for SxNyC900 (x:y= 0:1, 1:0, 1:1, 1:2, 1:4, etc.) were carried out. Obviously, all the SxNyC900 samples showed a distinct cathodic ORR peak in O2−saturated electrolyte (Figure S6 and S7), where shift of cathodic ORR peak towards the anodic direction can be observed with the introduction of N atoms in SxNyC900 samples except for S1N8C900 and S0N1C900. Again, S1N6C900shows the best ORR electrocatalytic performances among this series of samples. Next, the ORR activity of SxNyC900 was further investigated by LSV measurement. As revealed in Figure 2c, in contrast to S1N0C900, the onset potential (Eonset) and half−wave potentials (E1/2) of S1N(y≥1)C900 became more positive, and the current density was increased significantly, indicating that introduction of N atoms into the matrix of the S doped graphene (S1N1C900, S1N2C900, etc.) contributed greatly to the improved catalytic activity.

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Figure 2. (a) CV curves of S1N6CT (T=700, 800, 900 and 1000) in O2-saturated 0.1 M KOH aqueous solution. (b) LSV of the S1N6CT and Pt/C (20 wt. %, E–TEK) at 1,600rpm. (c) LSV of the as–prepared SxNyC900 samples and Pt/C catalyst (20 wt. %, E–TEK) at 1,600 rpm. (d) LSV curves of S1N6C900 at different rotating rates from 225 to 2,025 rpm. (e) The corresponding Koutecky–Levich plots of S1N6C900 at different potentials. (f) JK and n based on the RDE data on various samples (at 0.365V vs. RHE).

S1N4C900, S1N6C900 and S1N8C900provide higher current densities probably due to the dual doping of S and N as well as appropriate ratios of S/N in contrast to mono-N doped graphene (S0N1C900) and Pt/C (20 wt. %, E-TEK). In addition, the Eonset and E1/2 of SxNyC900 gradually shift to more positive values (Figure 2c) except for S1N8C900 and S0N1C900. Notably, S1N6C900 gives the highest electrocatalytic activity toward ORR with the most positive Eonset (~ 0.95 V vs. RHE), E1/2 (~ 0.83 V vs. RHE) and largest current density (~4.86 mA cm−2), comparable to those of Pt/C (~ 0.944 V vs. RHE and~ 0.85 V vs. RHE for Eonset and E1/2, respectively). Further studies on SxNyC900 materials were performed by using LSV measurements at different rotating speeds from 225 to 2,025 rpm. Take S1N6C900 for example, as shown in Figure 2d, the current density of S1N6C900 increases evenly as the rotating speed increases. Koutecky–Levich (K–L) plots (Figure 2e) with a good linear relationship of S1N6C900 can be obtained from the LSV curves (Figure 2d). The kinetic parameters including kinetic current density (JK) and electron transfer number (n) were analyzed on the basis of the RDE measurements and K–L equations (Figure 2c, d). As shown in Figure 2f, S1N0C900 mainly exhibits a two−electron pathway for ORR (n = 2.3, at 0.6 V vs. RHE) and a low JK (1.5 mA cm−2). Remarkably, the S1N(y≥1)C900 materials follow a nearly four-electron process for ORR (n = 3.6 – 3.9, at 0.6 V vs. RHE) and a higher JK (15.4 – 29.2 mA cm−2), where S1N6C900 has the highest JK (29.2 mA cm−2) among all samples. To further investigate the ORR performance for the SxNyC900, their electrocatalytic activity in O2-saturated 0.5M H2SO4 were also measured (Figure 3). Compared to S1N0C900, the introduction of N atoms presents better ORR performance in terms of Eonset, E1/2 and current density. The Eonset and E1/2 of S1N1C900 become more positive, and the current density was increased significantly, indicating that introduction of N atoms into the matrix of the S doped graphene (S1N1C900,

S1N2C900, etc.) contributed greatly to the improved catalytic activity. Again, S1N6C900 has the best ORR performance with high overpotential (Eonset=0.785 V vs. RHE, E1/2=0.47 V vs. RHE) as compared to the mono-doped samples of S0N1C900 and S1N0C900. This further confirms the critical role of S, N dual doping in the improvement of ORR performance. Furthermore, S1N4C900 and S1N6C900 provide higher current densities, more positive Eonset and E1/2 as compared to S1N1C900 and S1N2C900, which means lower S/N ratios are beneficial for the improvement of ORR performance in acidic media. However, when the S/N ratio is further decreased as in S1N8C900, decreased Eonset and E1/2 values are obtained with respect to S1N6C900. LSV measurements were further performed for S1N6C900. As shown in Figure 3b, as the rotation speed increased from 225 to 2,500 rpm, larger current density can be detected and Koutecky–Levich (K–L) plots of S1N6C900 (Figure 3c) obtained from LSV curves exhibit a good linear relationship. Further, the selectivity of catalyst towards ORR was conducted by the addition of 3M methanol in acidic medium for 8,000s (Figure 3d). The original cathodic current density of S1N6C900 remained almost unchanged after the addition of 3 M methanol as compared to Pt/C catalyst (20 wt. %, E–TEK) which has a tremendous change in current density. This suggests that the S1N6C900 electrocatalyst exhibits superior selectivity for ORR as compared to the commercial Pt/C catalyst (20 wt. %, E–TEK) in acidic medium. Based on the above results in both alkaline and acidic media, it can be concluded that the improved ORR performance from S1N6C900 sample has probably come from the dual doping of S and N as well as appropriate ratios of S/N in contrast to mono N doped graphene-like carbon nanosheets (S1N0C900 and S0N1C900). It is well accepted that for doped carbon materials, the doping level, chemical structure between dopant and the graphitic

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Figure 3. (a) LSV curves of SxNyC900 samples in O2-saturated 0.5 M H2SO4 aqueous solution. (b) LSV curves of S1N6C900 at different rates from 225 to 2500 rpm. (c) The corresponding Koutecky–Levich plots of S1N6C900 at different potentials. (d) I–t chronoamperometric response of the S1N6C900 and Pt/C catalyst (20 wt. %, E–TEK) upon the addition of 3 M methanol in oxygen-saturated 0.5 M H2SO4 solution, the arrow indicates the addition of methanol.

carbon atoms, and the specific surface area can greatly affect the electrocatalytic activity. However, there is not much difference in the BET surface area of different S1N(y≥1)C900 samples (Table S1), and thus the nitrogen and sulfur doping might play an important role in determining the ORR performance. To evaluate the influence of chemical structure on ORR performance, the resulting SxNyCT materials were characterized by XPS (Figure 4, Figures S8 and S9). For S1N6C900 (Figure 4a), high resolution S2p peaks can be resolved into three different peaks at 163.9 and 165.2 eV, 168.5eV assigned to −C−S−C, −C=S and –C-SOx-C bonds, respectively,54-56 a strong evidence of the covalent modification of carbon frameworks by S atoms. The N1s spectrum of S1N6C900 (Figure 4b) could be well fitted into three peaks at the binding energies of ~ 398.6, 400.5, and 401.2, corresponding to the pyridinic−N, pyrrolic−N and graphitic−N, respectively.57, 58 For S1N6CT samples, as the pyrolysis temperature increased from 700 to 1000 °C (Figure 4c and Table S1), it was found that the N/C atomic ratio was decreased significantly from ~ 0.621 to 0.052, and S/C atomic ratio followed a similar decrease trend (from ~ 0.014 to 0.008). However, the S/N atomic ratio was increased from ~ 0.023 to 0.131. For the N species (Figure 4d), pyrrolic−N has no obvious change, but pyridinic−N was decreased and graphitic−N was increased significantly with increased pyrolysis temperature. Therefore, an optimized pyrolysis temperature (900°C) with well-tuned S/N atomic ratios can provide superior ORR activity. To investigate the influence of S/N atomic ratio on ORR, the molar ratio of dibenzyl sulfide to melamine was carefully controlled during the preparation of SxNyC900 samples. According to XPS results (Figure 4e and Table S1), the atomic ratio of N/C was increased, but the atomic ratios of S/C and S/N were decreased, matching well with the molar ratios of

melamine to dibenzyl sulfide. It was revealed that the doped S atoms could introduce stress and defects in carbon matrix which always induce a two−electron pathway for ORR.20 However, the N atoms in graphene can transform the ORR pathway to the four−electron behavior.46 This can explain the restricted ORR activity of S1N0C900 and significantly improved ORR activity of S1N(y≥1)C900 after the introduction of N atoms. Compared to S1N0C900, S1N1C900 has a more positive onset potential, E1/2 and current density (Figure 2c). The ORR activity of SxNyC900 was further improved with decreased S/N ratio except for the S1N8C900, which means an appropriate ratio of S/N is essential for the even enhanced ORR process. Furthermore, S0N1C900 has an ORR activity just a little inferior to S1N6C900 (Figure 2c), an indication that S atom is necessary for further improvement of ORR activity in mono−N doped graphene. As reported, S atoms in the carbon materials can easily induce the polarization of neighboring C and N atoms, resulting in strain and defects in the carbon framework which eventually facilitate oxygen chemisorptions.20, 43 Besides, the chemical nature of the doped N atoms can also influence the ORR activity. It was reported that graphitic−N and pyridinic−N are the most active N species to promote the ORR performance.41, 59 Compared to S1NyC900 (y≦6), S1N6C900 has a higher content of graphitic−N and pyridinic−N species, which is beneficial to the ORR performance. Though S1N8C900 has the highest content of graphitic−N and pyridinic−N species (Figure 4e, 4f and S9) among the S1NyC900series, its too limited S content cannot promise better ORR performance. Therefore, appropriate S/N ratio and N contents can create more active sites and thus the catalytic activity of S1N6C900 is better than that of other SxNyC900 samples.

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Figure 4. (a) High−resolution S 2p and (b) high−resolution N 1s XPS spectra of S1N6C900. (c) N/C, S/C and S/N atomic ratios and (d) the percentages of the graphitic−N, pyridinic−N and pyrrolic−N as a function of the pyrolysis temperature. (e) N/C, S/C and S/N atomic ratios and (f) the percentages of the graphitic−N, pyridinic−N and pyrrolic−N of SxNyC900 samples.

The stability of S1N6C900 in ORR was evaluated by chronoamperometric measurements at a constant voltage of 0.664 V vs. RHE with a RDE rotating speed at 1000 rpm. As shown in Figure S10, after 35,000 s of reaction, 82% of the current density towards ORR can be maintained for S1N6C900, which is much higher than that of the commercial Pt/C catalyst (68% after 35,000s).

Corresponding Author *[email protected]; [email protected]; [email protected].

CONCLUSION

This work was supported financially by the National Natural Science Foundation of China (Grant Nos. 51372056, 51672057, 21471039, 21671047), Science Fund for Creative Research Groups of the National Natural Science Foundation of China (Grant No. 10821201), Fundamental Research Funds for the Central University (Grant Nos. HIT.BRETIII.201220, HIT.NSRIF.2012045, HIT.ICRST.2010008, PIRS of HIT A201502 and HIT. BRETIII. 201223), International Science & Technology Cooperation Program of China (2012DFR50020) and the Program for New Century Excellent Talents in University (NCET-13-0174).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

In conclusion, S, N dual−doped graphene-like carbon nanosheets were synthesized via a simple and cost−effective pyrolysis of a mixture of melamine and dibenzyl sulfide. With carful control of the S and N content, the S, N dual−doped graphene-like carbon nanosheets can show enhanced electrocatalytic activity towards ORR as compared with mono−doped counterparts, as well as excellent durability and stability in contrast to the commercial Pt/C electrocatalyst in both alkaline and acidic media. The results reveal that the doping effect for ORR is heavily dependent on the synergistic effect of S and N atoms in S, N dual−doped carbon materials. This simple method to design well controlled dual-doped graphene-like carbon nanosheets can be further extended to the development of other dual-doped carbon nanosheets as functional materials for electrochemical applications related to ORR such as fuel cells and metal-air batteries.

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ACCOCIATED CONTENT Supporting Information. Figure S1-S10 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION 6


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SYNOPSIS TOC: Herein, we report the synthesis of S, N dual−doped graphene-like carbon nanosheets via a simple and cost−effective approach by pyrolysis of a mixture of melamine and dibenzyl sulfide. The content of N and S atoms in dual-doped graphene-like carbon nanosheets can be well controlled by tuning the ratios of N and S sources. S, N dual−doped graphene-like carbon nanosheets exhibits an enhanced ORR activity as compared to mono−doped carbon nanosheets in both alkaline and acidic media which can be used for electrochemical applications such as alkaline fuel cells and metal-air batteries.

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S, N Dual-Doped Graphene-like Carbon Nanosheets as Efficient

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