C Nanosheets as Highly Efficient Electrocatalysts

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Sulfur-doped Fe/N/C nanosheets as highly-efficient electrocatalysts for oxygen reduction reaction Kui Hu, Li Tao, Dongdong Liu, Jia Huo, and Shuangyin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02078 • Publication Date (Web): 06 Jul 2016 Downloaded from http://pubs.acs.org on July 7, 2016

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INTRODUCTION The cathodic oxygen reduction reaction (ORR) in fuel cells and metal–air batteries plays a key role in energy efficiency.1 The common commercial catalysts used in fuel cells are Pt and Ptbased precious metal electrocatalysts. However, there are still a lot of drawbacks, such as fuel crossover, poor stability, and high cost of Pt. Thus, exploration of alternative to Pt with low-cost and high-performance catalysts for ORR has received great interest. Jasinski’s report portrayed the ability of macrocycles containing metal-nitrogen as oxygen reduction catalyst, which represented the dawn of a new era of nonprecious metal and metal-free catalyst.2,3 The hightemperature pyrolysis of iron salts and carbon-, nitrogen-containing macrocycles is also a good choice to manufacture a catalyst of better activity and stability. The Fe/N/C catalyst dominates a high ORR performance in both alkaline and acid media, which is considered to be one of the best alternatives of Pt/C.4 The nature of active sites is uncertainty for the Fe/N/C.5 Previous works have proposed two different hypotheses to describe the active sites of ORR in the transitionmetal-coordinated nitrogen-doped carbon (M/N/C) catalyst. One is the M−N species as active sites, and the other is the N atoms doping in the carbon matrix, as is the same in N-doped carbon materials.6 The existence of M-N active sites were verified by Mössbauer spectroscopy.7 It revealed Fe of high spin state is deemed to be active for ORR.8-10 Researchers have been committed to developing metal-free ORR electrocatalysts recently. Heteroatom-doped carbon materials as metal-free ORR electrocatalysts emerge prominently.1 The raised ORR performance of N-doped carbon was proposed by Dai group.11 Recently, our group have prepared B, N-doped carbon nanotube12 and graphene,1 and N, S-doped graphene for efficient ORR catalyst.13 Heteroatom doping is considered as an effective strategy to enhance ORR activity. However, there are few reports on heteroatom-doped M/N/C.4

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Scheme 1. Schematic representation of formation of S-Fe/N/C from thiourea and iron acetate.

Herein, we present an efficient and cost-effective approach for in-situ S-doping in Fe/N/C by pyrolyzing thiourea and iron acetate at 700 ºC under Ar atmosphere. Thiourea as a precursor can polymerize into S-doped carbon nitride (S-C3N4) at around 550 ºC.14 With the aid of Fe2+, the S-C3N4 was catalyzed to S-Fe/N/C at 700 ºC. S-Fe/N/C is of graphene-like structure with high electrochemically accessible surface area. The resulting catalyst exhibited advanced electrocatalytic activities for ORR under both alkaline and acid solutions, higher than the undoped Fe/N/C catalysts (prepared using starting materials melamine and iron acetate), which could be comparable to Pt/C. S-Fe/N/C has reliable stability and resistance to methanol. Therefore, the newly developed method can provide an efficient approach to produce non-noble metal electrocatalysts at low cost. RESULTS AND DISCUSSION S-doped Fe/N/C electrocatalyst was gained via a simple pyrolysis of thiourea and iron acetate mixture, in which thiourea acts as the source of N, S and C. Using single N, S and C pre-


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cusor makes synthetic procedure of S-Fe/N/C easier. Thiourea is promising for various synthetic fields. For example, we used it to prepare metal sulfides as electrocatalyts for hydrogen evolution

Figure 1. SEM (a) and TEM image (b) of S-Fe/N/C. HRTEM image in the inset. reaction.15 We successfully prepared N, S-graphene by annealing graphene oxide with thiourea.13 The formation of S-Fe/N/C is schematically illustrated in Scheme 1. The mixture of thiourea and iron acetate was subjected to annealing at 500 ºC to obtain Fe2+-functionalized, S-doped graphitic carbon nitride (Fe2+-S-g-C3N4). S-Fe/N/C is achieved by the thermal treatment of Fe2+S-g-C3N4 at 700 ºC in Ar atmosphere.16 S-Fe/N/C possesses a graphene-like structure from Scanning electron microscopy (SEM) image (Figure 1a) and transmission electron microscopy (TEM) image (Figure 1b), which was also observed on undoped Fe/N/C (Figure S1). Temperature affects the morphology (Figure S2) of the S-Fe/N/C catalysts. At 650 ºC, no obvious carbon structures were observed. Starting from 700 to 800 ºC, carbon shell aggregates were realized for all the samples. The HRTEM image (inset of Figure 1b) indicates the presence of some carbon-encapsulated Fe nanoparticles. The presence of Fe nanoparticles is also confirmed via powder X-ray diffraction (XRD). In Figure 2, Fe nanoparticles in S-Fe/N/C are not just metallic iron, but of five types: Fe, FeS, FeN, FeC, and Fe3O4. Fe-N as the centre of catalytic activity is acknowledged.17 The presence of Fe-S in the sample may be due to the sulfidation of Fe species during the pyrolysis. XRD analysis also gives a clear evidence Fe3O4.


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According to a recent report, Fe3O4 encapsulated within the carbon layers also enhances the ORR activity.18 The prominent peak of Fe-O-Fe in Fe3O4 at around 680 cm-1 in Raman spectra (Figure S3) is absent in S-Fe/N/C.19 This may indicate the species are well encapsulated within the carbon shells.

Figure 2. XRD patterns of Fe/N/C and S-Fe/N/C. Cyclic voltammetry (CV) measurements were carried out in 0.1 M KOH (Figure 3a and 3c) and 0.1 M HClO4 (Figure 3b and 3d) solution. S-Fe/N/C shows a quasi-rectangular double-layer capacitance current in N2-saturated electrolyte is obvious. The ORR peak current in alkaline and acid solution for S-Fe/N/C is much larger than Fe/N/C, reaching to 2.81 and 1.81 mA cm-2, while that of Fe/N/C is only 1.25 and 1.35 mA cm-2, respectively. The higher capacitance currents of SFe/N/C in CV suggest higher electrochemical accessible surface area in S-Fe/N/C, which may promote O2 transport within the catalyst layers.20 A comparison of ORR performance of SFe/N/C, Fe/N/C, and Pt/C in alkaline media is presented in Figure 3e. Loading of the three above species was 0.16 mg cm-2. S-Fe/N/C shows more positive half-wave potential (E1/2) (0.799 vs. 0.507 V) and onset potential (0.911 vs. 0.841 V) than Fe/N/C. It evidences that S-Fe/N/C is of


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higher ORR performance compared to Fe/N/C apparently.4 The E1/2 and onset potential of SFe/N/C is approaching to 0.839 and 0.939 V of Pt/C, respectively. Catalytic activity of them in acid electrolyte is showed in Figure 3f. The E1/2 of S-Fe/N/C in acidic condition is 0.825 V,

Figure 3. CVs in 0.1 M KOH of S-Fe/N/C (a) and Fe/N/C (c) and 0.1 M HClO4 of S-Fe/N/C (b) and Fe/N/C (d). RDE voltammograms of Pt/C, Fe/N/C and S-Fe/N/C in (e) 0.1 M KOH and (f) 0.1 M HClO4 at 1600 rpm.


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slightly negative than 0.855 V of Pt/C. Activity differences of the two catalysts are presented in Tafel plots clearly. For instance, the kinetic current density of S-Fe/N/C is around 5.05 mA cm-2 in 0.1 M KOH (Figure S4a) at 0.80 V and 4.71 mA cm-2 in 0.1 M HClO4 (Figure S4b) at 0.65 V, significantly higher than Fe/N/C.21 S-Fe/N/C, with using thiourea not melamine, shows very outstanding ORR performance. Previous studies demonstrated the pyrolysis temperature affecting ORR performance of Fe/N/C.22 The reason is N and Fe can easily incorporate into the carbon matrix at a low temperature of calcination, and a high pyrolytic temperature may improve electrical conductivity of the carbon material.23 To find an optimum condition for our S-Fe/N/C catalysts, we conducted a series of attempts. The results (Figure S5) tell that 700 ºC and ratio 1:0.2 is the optimal. The S-Fe/N/C is still better than Fe/N/C pyrolyzed at 900 ºC and 1:0.3 showing the best performance in all Fe/N/C in Figure S5b. To further quantitatively characterize S-Fe/N/C, Fe/N/C and Pt/C, we calculate the electron transfer number (n) of ORR for each of them. Generally, the 4e process is believed to be more efficient than the 2e process that produces H2O2. It can be observed that the peroxide species output in 0.1M KOH is at a low level for S-Fe/N/C, below 3% from 1.0 to -0.2 V (Figure S6). The n is 3.95 and increase nearly to 4 as the potential moves towards more negative value. As for in an acid electrolyte, Figure S6d shows that the value of n is almost constant at 3.97 from 0.5 to 0.2 V and general average of n is 3.96.24 Thus, S-Fe/N/C for ORR follows a direct 4e pathway where O2 was directly transformed into water (or OH- in alkaline solutions), close to an ideal pathway. Clearly, Pt/C (Figure S7) catalyzes ORR process in 4e pathway, while Fe/N/C (Figure S8) does not. S-Fe/N/C shows higher ORR performance than Fe/N/C. To investigate the reason, we performed series of physical characterizations. X-ray photoelectron spectroscopy (XPS) spectra


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of them are in Figure 4.4 Figure 4a presents a Fe 2p peak, a C 1s peak, an O 1s peak, a N 1s peak, and two S peaks, which confirms the presence of elemental C, O, N, and S in S-Fe/N/C.25 There is a similar elemental Fe, C, O, N peak in Fe/N/C, yet the S peak is very weak. As listed in Table

Figure 4. XPS of S-Fe-N/C (a), high-resolution spectra of C 1s (b), N 1s (c), and S 2p (d), respectively. S1, the atom contents of Fe, C, N, O, and S were 1.38 %, 68.57 %, 15.09 %, 10.21 %, and 4.76% for S-Fe/N/C, while 1.56 %, 78.43 %, 10.93 %, 8.22 % and 0.86 % for Fe/N/C, separately. From above data, S-Fe/N/C containing Fe and O is the almost same but less C in comparison with FeN/C. It needs to point out here that S-Fe/N/C contains significantly more N and S than Fe/N/C. The peak corresponding to ~ 285.8 eV is a good evidence for nitrogen connecting with carbon matrix for the two catalysts in the fitted C 1s spectrum (Figure 4b and S9c). C–S bonds partially contribute to the peak at 285.86 eV in Figure 4d.26 The N 1s (Figure 4c) spectrum can be further deconvoluted into five N species based on the literature’s method. The N1 to N5 species are pyridinic-N, Fe-N, pyrrolic-N, graphitic-N and N-“O”.27 Atomic percentage of the different


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nitrogen content is given in Table S2. According to calculation, the atom percentage of the Fe coordinated nitrogen of Fe-N is 18.84 % in S-Fe/N/C less than 20 % in Fe/N/C. Meanwhile, the pyridinic nitrogen of Fe-S/N/C (45.14 %) is lower than Fe/N/C (45.48 %), which is very close. However, if we take the content of N into consideration, 1.16% is only a slight difference. It is interesting to observe that the graphitic nitrogen in S-Fe/N/C (13.96 %) is more than that in Fe/N/C (12.33 %). Recent literatures demonstrate the crucial role of the graphitic nitrogen in promoting ORR activity.28-29 Besides, the lone-pair electrons of pyridinic N and pyrrolic N can serve as metal coordination sites.30 It is not hard to find that these three kinds of active nitrogen are of high proportion in S-Fe/N/C. With no doubt, uniform distribution of Fe and N species obtained from Figure 5 is good for ORR process.24 These could synergistically lead to a high ORR performance of S-Fe/N/C. Obviously, S is a decent choice for doping in the metal-free electrocatalysts where S can change electric structure, 31 which benefits ORR.32-33 High-resolution S 2p spectrum (Figure 4d) of S-Fe/N/C can be deconvoluted into S 2p3/2 and S 2p1/2 centered at 163.66 and 164.87 eV respectively. It reveals the presence of C-S-C that is characteristic of thiophene-S. SOx groups are minor peaks at 168.08 and 169.47eV.25 Apart from S, the peaks corresponding to the iron moiety also give an evidence for the surface iron species (Figure S9). Similar peaks are observed in S-Fe/N/C and Fe/N/C. Interestingly, ratio of Fe0 and FeOx to iron moiety decreased in SFe/N/C attribute to sulfidation of Fe species.34 In view of the process of synthesis of S-Fe/N/C, the S is from thiourea that is polymerized into S-C3N4 at 500 ºC. Uniform distribution of S species is observed from the elemental mapping conclusion (Figure 5). Importantly, the synergistic effect N and S contributing to ORR was demonstrated by previous works.35 Some of


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metal chalcogenides (e.g. CoSex, CoSx, FeSx, etc.) also serve as effective ORR catalysts.36 However, the S content is very low in Fe/N/C, which is from originated from bisulfite group of

Figure 5. Representative (a) SEM image and (b−f) elemental mapping of S-Fe/N/C in a scale bar of 1 µm. the commercial melamine (Figure S10).37 So, it is easy to get that higher catalytic performance of S-Fe/N/C may also attribute to more S doping.4 Generally, high surface area is necessary to improve active site density, which also facilitates efficiently reactant transport. As shown in Figure S11, the Brunauer-Emmet-Teller (BET) surface area is 273.38 m2 g-1 for S-Fe/N/C, slightly larger than 272.66 m2 g-1 for Fe/N/C. The two catalysts also show similar pore volume (0.4929 vs. 0.4831 cm3 g-1). However, S-Fe/N/C shows higher capacitance currents in CV (Figure 3), indicating higher


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electrochemically accessible surface area. 20 This result demonstrates that S doping improved the electrochemical accessibility of Fe/N/C, which is very significant to any electrochemical process.

Figure. 6 Electrochemical durability tests of Fe-S/N/C and Pt/C electrodes in (a) 0.1 M KOH at 0.75 V and (b) 0.1 M HClO4 at E1/2, 400 rpm. Besides, a high conductivity contributes to high activity for ORR.38 We performed four-probe conductivity measurements and the conductivity of S-Fe/N/C was 11.19 S cm-1 preceding that of Fe/N/C (7.46 S cm-1). The tolerance to fuel crossover and stability of the catalysts are the major concerns of commercial fuel cells. Figure 6 depicts the chronoamperometric measurements of SFe/N/C and Pt/C. From Figure 6a, it is very clear that S-Fe/N/C show a reliable stability in alkaline media, and the current decay rate is just 3.33% far below 26.56% of Pt/C. In Figure 6b, there is infinitely superior stability of S-Fe/N/C in acid solution. The response toward methanol was examined by CV measurements. A small decrease of the ORR peak in S-Fe/N/C is presented in Figure S12. However, it is a sharp contrast that the peak in Pt/C disappears, meanwhile, methanol oxidation arises (Figure S13). It clearly indicates that the S-Fe/N/C shows better


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tolerance toward methanol than Pt/C. Thus, it is easy to find that S-Fe/N/C precedes Pt/C for ORR in direct methanol fuel cells. CONCLUSION In summary, we have developed a simple but effective approach for in-situ S-doping in Fe/N/C by direct high-temperature pyrolysis of thiourea and iron acetate. The optimal catalyst pyrolyzed at 700 ºC is of excellent ORR performance comparable to Pt/C, far better than undoped Fe/N/C. Moreover, S-Fe/N/C exhibits better stability and higher tolerance toward methanol compared with Pt/C. More notably, ORR on the S-Fe/N/C in alkaline and acid solution follows the efficient 4e pathway. Taken together, our developed sulfur-doped Fe/N/C catalyst as a substitute for commercial Pt/C could provide great possibility in fuel cells and other applications with superior ORR performance and high stability. EXPERIMENTAL SECTION Materials preparation. The synthesis of S-Fe/N/C was performed on directly pyrolyzing the mixture of thiourea and iron acetate. 2 g thiourea and 0.4 g iron acetate were mixed in ultrapure water with vigorous mechanical agitation to form a homogeneous suspension. After dried, the precursor mixture was grounded. Then the mixed powder was calcined at 500 ºC for 2 h and then at 700 ºC (HT1) under Ar atmosphere to acquire black product. The pyrolyzed product was transferred to 0.5 M H2SO4 and stirred at 80 ºC lasting 12 h. The leached sample was washed to neutral with water and ethanol. The sample then was dried in vacuum drying oven at 60 ºC for 12 h. Finally, the obtained powder was thermally treated for the second time (HT2) at the same temperature with HT1 for 1 h. The final product was labeled as S-Fe/N/C. We also prepared a variety of S-Fe/N/C samples at various temperatures from 650 to 800 ºC, marked as S-Fe/N/C-


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650, S-Fe/N/C-800 etc. If unspecified, S-Fe/N/C represents the sample obtained at 700 ºC. If the mass ratio thiourea and iron acetate is not indicated, it is 1:0.2. For comparison, we also prepared the undoped Fe/N/C with melamine and iron acetate as precursors with a fixed mass ratio of melamine and iron acetate (1:0.2) from 700 to 1050 ºC. When the ratio is 1:0.3, the pyrolytic temperature was set to 800, 900 and 1000 ºC. If there are no special instructions, the mass ratio melamine and iron acetate is 1:0.2 and the temperature is 700 ºC. Physical characterizations. XRD (Siemens, D500 diffractometer), SEM (Hitachi, S-4800) equipped with EDX (Oxford, Inca), TEM (Tecnai, G2 F20) and HRTEM (GIF, Quantum 963), XPS (ESCALAB, 250Xi) with a monochromic Al X-ray source, BET surface area (NOVA, 1000e), Raman spectrometer (Labram-010) using a 632 nm laser, conductivity (4 probes tech, RTS-4). Electrochemical measurements. We performed all electrochemical measurements in a standard three-electrode cell (Pt wire and SCE). The ORR performance of S-Fe/N/C was collected from an electrochemical workstation (CHI 760E) with RRDE-3A (ALS). 4.0 mg electrocatalyst, 1 mL ethanol and 0.05 mL Nafion solution (5 wt %, Sigma Aldrich) were mixed and ultrasonicated for 1 h to obtain homogeneous slurry. Then 5 microlitres of the slurry was dropped onto pre-polished glassy carbon electrode (4 mm in diameter). The electrolyte was bubbled with O2 or N2 for at least 30 min before each experiment.


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ASSOCIATED CONTENT Supporting information Tables for element analysis by XPS of Fe/N/C and S-Fe/N/C. SEM and Raman of the others. High-resolution spectra XPS for Fe/N/C. Fe 2p XPS of S-Fe/N/C. Electrochemical characterizations such as RDE voltammograms, peroxide yield, number of electron transfer, and tolerance toward methanol. EDX mapping of Fe/N/C. BET surface and diameter distribution. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant No.: 51402100, 21573066). REFERENCES (1) Wang, S.; Zhang, L.; Xia, Z.; Roy, A.; Chang, D.; Baek, J.; Dai, L. BCN Graphene as Efficient Metal-free Electrocatalyst for the Oxygen Reduction Reaction. Angew. Chem. Int. Ed. Engl. 2012, 51 (17), 4209-4212. (2) Jasinski, R. A New Fuel Cell Cathode Catalyst. Nature 1964, 201, 1212-1213. (3) Ferrandon, M.; Kropf, A. J.; Myers, D. J.; Artyushkova, K.; Kramm, U.; Bogdanof, P.; Wu, G.; Johnston, C. M.; Zelenay, P. Multitechnique Characterization of a Polyaniline–Iron– Carbon Oxygen Reduction Catalyst. J. Phys. Chem. C 2012, 116 (30), 16001-16013.


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(4) Wang, Y.; Lai, Y.; Song, L.; Zhou, Z.; Liu, J.; Wang, Q.; Yang, X.; Chen, C.; Shi, W.; Zheng, Y.; Rauf, M.; Sun, S. S-Doping of an Fe/N/C ORR Catalyst for Polymer Electrolyte Membrane Fuel Cells with High Power Density. Angew. Chem. Int. Ed. Engl. 2015, 54 (34), 9907-9910. (5) Wang, Q.; Zhou, Z.; Lai, Y.; You, Y.; Liu, J.; Wu, X.; Terefe, E.; Chen, C.; Song, L.; Rauf, M.; Tian, N.; Sun, S. Phenylenediamine-based FeN(x)/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and its Active-site Probing. J. Am. Chem. Soc. 2014, 136 (31), 10882-10885. (6) Lin, L.; Zhu, Q.; Xu, A.-W. Noble-metal-free Fe-N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136 (31), 11027-11033. (7) Singh, D.; Mamtani, K.; Bruening, C.; Miller, J.; Ozkan, U. Use of H2S to Probe the Active Sites in FeNC Catalysts for the Oxygen Reduction Reaction (ORR) in Acidic Media. ACS Catal. 2014, 4 (10), 3454-3462. (8) Kramm, U. I.; Herranz, J.; Larouche, N.; Arruda, T. M.; Lefe`vre, M.; Jaouen, F.; Bogdanoff, P.; Fiechter, S.; Irmgard, A.-W.; Mukerjeec, S.; Dodelet, J. Structure of the Catalytic Sites in Fe/N/C-catalysts for O2-reduction in PEM Fuel Cells. Phys. Chem. Chem. Phys. 2012, 14 (33), 11673-11688. (9) Kramm, U. I.; Irmgard, A.-W.; I., H.; Radnik, J.; Fiechter, S.; Bogdanoff, P. Influence of the Electron-Density of FeN[sub 4]-Centers Towards the Catalytic Activity of Pyrolyzed FeTMPPCl-Based ORR-Electrocatalysts. J. Electrochem. Soc. 2011, 158 (1), B69. (10) Müller, K.; Richter, M.; Friedrich, D.; Paloumpa, I.; Kramm, U.; Schmeißer, D. Spectroscopic Characterization of Cobalt–Phthalocyanine Electrocatalysts for Fuel Cell Applications. Solid State Ionics 2012, 216, 78-82. (11) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760-764. (12) Wang, S.; Iyyamperumal, E.; Roy, A.; Xue, Y.; Yu, D.; Dai, L. Vertically Aligned BCN Nanotubes as Efficient Metal-free Electrocatalysts for the Oxygen Reduction Reaction: A Synergetic Effect by Co-doping with Boron and Nitrogen. Angew. Chem. Int. Ed. Engl. 2011, 50 (49), 11756-11760. (13) Wang, X.; Wang, J.; Wang, D.; Dou, S.; Ma, Z.; Wu, J.; Tao, L.; Shen, A.; Ouyang, C.; Liu, Q.; Wang, S. One-pot Synthesis of Nitrogen and Sulfur Co-doped Graphene as Efficient Metal-free Electrocatalysts for the Oxygen Reduction Reaction. Chem. Commun. 2014, 50 (37), 4839-4842. (14) Hong, J.; Xia, X.; Wang, Y.; Xu, R. Mesoporous Carbon Nitride with in Situ Sulfur Doping for Enhanced Photocatalytic Hydrogen Evolution from Water under Visible Light. J. Mater. Chem. 2012, 22, 15006-15012. (15) Dou, S.; Wu, J.; Tao, L.; Shen, A.; Huo, J.; Wang, S. Carbon-coated MoS2 Nanosheets as Highly Efficient Electrocatalysts for the Hydrogen Evolution Reaction. Nanotechnology 2016, 27 (4), 045402. (16) Zou, X.; Huang, X.; Goswami, A.; Silva, R.; Sathe, B. R.; Mikmekova, E.; Asefa, T. Cobalt-embedded Nitrogen-rich Carbon Nanotubes Efficiently Catalyze Hydrogen Evolution Reaction at All pH Values. Angew. Chem. Int. Ed. Engl. 2014, 126 (17), 4461-4465. (17) Thorum, M. S.; Hankett, J. M.; Gewirth, A. A. Poisoning the Oxygen Reduction Reaction on Carbon-Supported Fe and Cu Electrocatalysts: Evidence for Metal-Centered Activity. J. Phys. Chem. Lett. 2011, 2, 295–298.


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C Nanosheets as Highly Efficient Electrocatalysts

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