Hydrothermal Synthesis of a New Kind of N-Doped Graphene Gel-like

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Hydrothermal Synthesis of a New Kind of N-doped Graphene Gel-like Hybrid as an Enhanced ORR electrocatalyst Qin Xiang, Yuping Liu, Xuefeng Zou, Bingbing Hu, Yujie Qiang, Danmei Yu, Wei Yin, and Changguo Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19122 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 17, 2018

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ACS Applied Materials & Interfaces

Hydrothermal Synthesis of a New Kind of N-doped Graphene Gel-like Hybrid as an Enhanced ORR electrocatalyst Qin Xiang, Yuping Liu, Xuefeng Zou, Bingbing Hu, Yujie Qiang, Danmei Yu, Wei Yin, and Changguo Chen* College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 401331, China.

ABSTRACT: In this work, g-C3N4@GO gel-like hybrid is obtained by assembling intentionally exfoliated g-C3N4 sheets on graphene oxide (GO) sheets under a hydrothermal condition. A specific N-doping process is firstly designed by heating the g-C3N4@GO interlaced hybrid in vacuum to form nitrogen-doped graphene nanosheets (NGS) with high level of pyridinic-N (56.0%) and edge-rich defect structure. The prepared NGS exhibited a great electrocatalysis for oxygen reduction reaction (ORR) in terms of the activity, durability, methanol tolerance, and the reaction kinetics. And the excellent electrocatalytic performance stems from the effective N-doped sites that the nitrogen atom is successfully doped at the defective edges of graphene, and the annealing temperature can play significant role of the doping pattern and location of N. The research provides a new insight into the enhancement of electrocatalysis for ORR based on nonmetal carbons by using the novel N-doping method.

KEYWORDS: oxygen reduction reaction, two-dimensional materials, metal-free carbon-based electrocatalyst, g-C3N4@GO gel-like hybrids, N-doped graphene nanosheets,

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1. Introduction Fuel cell, which is an ideal energy conversion device that offers the cleanest power generation possible, and changes the chemical energy directly into electrical energy through a chemical reaction with oxygen.1-3 However, the cathodic oxygen reduction reaction suffers from a complex electron transfer process, sluggish kinetics, and traditionally require the exclusive use of Pt-based catalysts. Unfortunately, that the prohibitive cost, CO poisoning and scarcity of noble-metal Pt hampered the widespread use of fuel cell. For that reason, the catalysts with a low-cost and competitive activity to noble-Pt must be developed for this reaction.4-7 As alternative, the metal-free carbons catalysts have been in the spotlight in recent years, due to their satisfactory cost, fuel tolerance, and prolonged durability. While, the electrocatalytic properties (e.g., onset potentials, overpotentials, and 4e- pathway selectivity) of these materials are still inferior to Pt/C catalysts.8-12 Generally, the most effective way used for enhancing the performances of metal-free carbon-based electrocatalysts is the optimization of their microstructures and/or electrical properties.38-40 Graphene, which has a good conductivity, excellent chemical stability, and can be functionalized in a controlled manner,13-15 is always used as conductive framework to fabricate various composites with a designed structure for superior performance. Additionally, intentional chemical doping into a graphene structure can effectively modulate its electrical and chemical properties.16-17 Currently, graphene doped by nitrogen,18-19 sulfur,20 phosphorus21 have triggered tremendous interest both in fundamental research and practical applications. In contrast to other heteroatoms, N 2


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atoms, with the comparable size of C atoms, play a significant role in modifying the carbon structure. And the five valence electrons presented in the N atoms can form the strong covalent bonds with C atoms and/or donate electrons to the carbon.10,

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Generally, the nitrogen-doped carbon-based catalysts can easily prepared by heating N-containing compounds such as melamine,23 urea,24

dopamine,25 pyrrole,26 or

aniline27 with various carbons at elevated temperatures and in an inert atmosphere. Actually, the structure of the N-doped precursor plays a significant role in tuning the microstructure and composition of ORR catalysts.28-30

Recently, a new attempt

by combining graphitic carbon nitride (g-C3N4) with graphene to pursue a higher catalytic activity and conductivity with a “van der Waals heterostructures” has aroused tremendous interest.31-34 g-C3N4 is consisted by sp2 hybridized nitrogen and carbon atoms and has a graphene-like framework, is one of the most nitrogen-rich materials, and importantly, the high level of pyridinic-N in g-C3N4 can provide rich electron lone pairs to active π electrons of carbon in graphene materials, which can great facilitate to the ORR.35-37 Despite great efforts in this field, the research of the electrocatalysts for ORR based on this hybrid is still in its infant stage because of the monotonous manufacturing method like one-pot chemical reduction or wet-chemical routes, including the combined impregnation-chemical reduction strategy. Thus, devising effective doping methods for optimization of their microstructures and/or electrical properties is critical to its application development. Here, we propose a new, facile strategy to synthesize the nitrogen-doped graphene nanosheets (NGS) by pyrolysis of g-C3N4@GO gel-like hybrid, which is prepared 3



through fabricating intentionally exfoliated g-C3N4 sheets on graphene oxide (GO) sheets forming an interlaced 3D hybrid film structure under a hydrothermal condition. The scheme of synthesis is illustrated in Figure 1 and the corresponding photographs are presented in Figure S1. Typically, the GO can be assembled into a volume-fluffy porous 3D structure during the thermal expansion process, which improved the accessibility to exfoliated g-C3N4 sheets and made the more efficient interaction between the two. The uniform distribution of N atom within graphene matrix is accompanied by the g-C3N4 decomposed and the reduction of GO with annealing of the g-C3N4@GO. Due to the special internal microenvironment of the prepared NGS, the catalyst displayed an outstanding ORR activity, which is better than the previous reported N-doped carbon catalysts, and can comparable to the commercial JM-Pt/C electrocatalyst. Moreover, the well-designed catalyst can be reproduced easily and is promising for a broad range of applications such as photocatalyst and lithium-air batteries, etc. 2. Experimental section 2.1. Synthesis of g-C3N4. 5 g melamine was placed in a muffle furnace using the ceramic crucible and kept at 550 °C for 4 h.41 After cooling down, the yellow bulk products were collected and ground into powder. 2.2. Synthesis of GO. Graphene oxide (GO) was obtained from a modified Hummers method.42 1 g of graphite flakes and 0.5 g of NaNO3 were added into a flask with 23 mL of 4


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concentrated H2SO4. After stirring for 30 min in an ice bath, 3 g of KMnO4 was added gradually under stirring, and keep the reaction temperature below 20 °C. Sequentially, the mixture was kept at 35 ℃ for 30 min, and at 98 ℃ for 15 min after added 50 mL of water slowly. Finally, 140 mL of water and 10 mL of 30% H2O2 solution were added into the obtained solution, and it turned into luminous yellow. The product was centrifuged and washed with HCl solution and water, respectively. Then the product was dried at 50 ℃ for 24 h. 2.3. Synthesis of g-C3N4. 5 g melamine was placed in a muffle furnace using the ceramic crucible and kept at 550 °C for 4 h.42 After cooling down, the yellow bulk products were collected and ground into powder. 2.4. Synthesis of g-C3N4@GO gel-like hybrids and hydrothermal GO. A certain amount of g-C3N4 was added into 60 ml of GO (0.5mg ml-1) aqueous solution with mass ratio of 4:1 of g-C3N4 to GO under ultrasonication for 4h to promote exfoliation and interaction of the both layered materials. Then, the prepared suspension was added into a Teflon autoclave, and kept at 160 °C for 24 h in an oven to synthesis of volume-fluffy g-C3N4@GO4 hydrogel. Finally, the product was filtrated and dried at 60 ℃ for overnight. Other g-C3N4@GO hydrogel (g-C3N4@GO2, g-C3N4@GO6) were synthesized in the same manner, but varying the mass ratio of g-C3N4 to GO (mg-C3N4: mGO = 2, and 6, respectively). As a control experiment, the hydrothermal GO can be obtained under the same conditions without g-C3N4. 2.5. Synthesis of NGS-t. 5



A series of the nitrogen-doped graphene nanosheets (NGS) electrocatalysts were prepared by varying the annealing temperature of the obtained g-C3N4@GO layered composite from 700 to 1000 °C in a vacuum for 1 h, and the prepared materials are denoted as NGS-700, 800, 900, and1000, respectively. 2.6. Characterization. The morphology and chemical composition of samples were investigated by TEM (FEI Tecnai G2F20), FE-SEM (JSM-7800F), XPS (ESCALAB250Xi), FT-IR (Nicolet 550II) and TGA (Shimadzu DTG-60H). The phase composition and crystallinity of samples were investigated by XRD (Shimadzu XRD6000) and Raman (LabRAM HR Evolution). Nitrogen sorption isotherms were measured at 77 K on a Micromeritics Analyzer (ASAP 2020). 3. Results and discussion Morphology, Composition and Structure of the Catalysts. The self-assembled g-C3N4@GO gel-like hybrid was synthesized by exfoliating g-C3N4 with a facile liquid sonication in the presence of GO to introduce the g-C3N4 into GO sheets. In this procedure, water is utilized to exfoliate g-C3N4 due to the similar surface energy of g-C3N4 (115 mJ m-2) and water (102 mJ m-2), as well as the hydrogen bonds between g-C3N4 and water can readily form to facilitate the exfoliation of g-C3N4.43 The obtained suspension was kept in a Teflon autoclave to synthesis of g-C3N4@GO gel-like at a higher hydrothermal temperature. In this temperature, the interaction within g-C3N4 was further destroyed with the enhanced solvation, facilitating g-C3N4 nanosheets stripping, and the exfoliated g-C3N4 6


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nanosheets can be attached to the surface of hydrothermal GO sheets due to the π-π stacking between the sp2 lattice of g-C3N4 and graphene, and the hydrogen-bond between the N-containing species in g-C3N4 and the O-containing species of hydrothermal GO also contribute to the formation of [email protected] The pristine GO and g-C3N4 show a stacked sheet-like structure with wrinkles, fringes and porosity as revealed by SEM (Figure 2a,b), after thermal expansion by hydrothermal treatment, the GO was assembled into a volume-fluffy porous 3D structure (Figure S2), and the obtained g-C3N4@GO exhibited a more compact structure (Figure 2c) in comparison to the hydrothermal GO with the addition of g-C3N4, indicating that the g-C3N4 sheet is encapsulated within graphene layers, forming an edge-rich and well-dispersed sheet-on-sheet structure, which is in good consistent with the image displayed by TEM (Figure 2e and Figure S3). After the 900 °C treatment, the sheets of NGS become thinner and transparent with many edge sites (Figure 2d,f), this may because the g-C3N4 was almost decomposed (Figure S4), could only leaving some specific N-doping species within graphene matrix. As measured by N2 adsorption and desorption isotherms (Figure 3a), the BET specific surface areas (SBET) determined for g-C3N4, g-C3N4@GO, and NGS4-900 are 8, 26, and 137 m2 g-1, respectively. And their corresponding cumulative pore volumes are 0.02, 0.07, and 0.31 cm3 g-1, respectively. Compared to the other two, the NGS4-900 has a larger hysteresis loop in the high P/P0 area, indicating that NGS synthesized by the thermal annealing g-C3N4@GO has more porous than the pristine g-C3N4 and that gel-like hybrid film, which further suggests that the g-C3N4 and 7



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graphene sheets do not stack in the composite, but form hierarchical porous structure as evidenced by SEM. Additionally, the pore size distributions displayed that NGS4-900 has many defined micropore and mesopore (Figure 3b),

which is

facilitate to the infiltration of electrolyte and O2, and are generally desired for electron and mass transportation in the ORR.45 X-ray diffraction (XRD) analysis was employed to investigate the microstructure of the samples. As shown in Figure 4a, a strong peak located at 10.2° of GO indicates the introduction of various oxygen-containing groups, this is also reﬂected in the TGA curve of GO in Figure S5. And the typical XRD pattern of g-C3N4 shows two broad peaks located at 13.0° and 27.4°, which ascribed to (100) and (002) crystal planes. After hydrothermal treatment, there is a negligible peak of GO, and no diffraction peak related to (100) plane of g-C3N4 is observed in g-C3N4@GO, suggesting the regular stacking of GO and the in-plane structural packing of g-C3N4 was destroyed, indicating there is an interaction between g-C3N4 and GO, and g-C3N4 sheets could be well separated by GO sheets in g-C3N4@GO composite as revealed by SEM and TEM.14, 44-45 The XRD peak of NGS4-900 shifted negatively to 26.6° via annealing the resultant g-C3N4@GO, which can be assigned to the (002) plane of graphite, demonstrating that annealing of the gel-like hybrid film will partially restore the graphitic crystal structure owing to the reduction effect of high temperature and nitrogen doping.16, 19 Moreover, the fact that the peak shifted negatively from 27.4° to 26.6° also confirmed the g-C3N4 completely decomposed. Meanwhile, the XRD of other NGS with different mass ratio of g-C3N4 to GO is presented in Figure S6, 8


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which is almost the same as that of NGS4-900, indicating the quantity of nitrogen precursor has a little effect on the microstructure of the catalysts. The Raman spectra (Figure S7) of pristine GO, hydrothermal GO, and NGS4-900 all display the D band (1340 cm-1) and G band (1584 cm-1), and the corresponding ID/IG intensity ratios are 1.11, 1.14 and 1.09, suggesting the presence of numerous functional groups or defects, disorders within the samples,46-48 which could greatly benefit ORR catalysis. The elemental composition as well as functional group of the samples can be further evidenced by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy later. As shown in Figure 4b, the overall XPS of GO present the strong C signal with O about 27.9 atom %, without any other impurities, g-C3N4@GO and NGS4-900 suggest C, N and O as their dominant components, and the elemental quantification analysis of the samples are shown in Table 1. The high-resolution C1s XPS of the NGS4-900 in comparison to that of GO and g-C3N4@GO (Figure 4c) reveals that all samples have a C1s peak at around 284.6 eV, which assigned to graphite-like sp2 C, (the C1s XPS were normalized with respect to the graphite-like sp2 C peak.) but for g-C3N4@GO, another main peak at higher binding energy of 288.2 eV is attributed to N=C-N2 coordination that may origin from g-C3N4,49 or because the interactions between g-C3N4 and hydrothermal GO.17 For GO, a weak signal at 286.8 eV is ascribed to the O-containing bands (C-OH).50-51 No obvious signal at the higher binding energy of C1s for NGS4-900 suggests that the GO has been almost completely reduced into graphene and the g-C3N4 has decomposed during the thermal 9



annealing process, which is also evidenced by the gradual narrowing of sp2 C peaks compared to that of g-C3N4@GO and GO,48 which further indicates a great degree graphitization of carbon in the NGS4-900. A deconvoluted N1s XPS of g-C3N4@GO and NGS4-900 are shown in Figure 4d, and the corresponding structures of nitrogen atom in catalysts was shown in Figure S8. For g-C3N4@GO, there are three types of N species including 52.2% pyridinic-N (398.3 eV), 14.4% pyrrolic-N (400.0 eV), and 33.4% sp2-hybridized nitrogen (C=N-C) (398.7 eV).17, 52 After thermal annealing, the different N species in the NGS4-900 are 56% pyridinic-N, 28.3% pyrrolic-N, and a new peak of 15.7% graphitic-N (401.2 eV) is emerged. This result demonstrates that pyridinic-N is the main component in the as-prepared catalyst, owing to possessing a lone pair of electrons, pyridinic-N can contribute positively to the adsorption of O2 and tend to be catalytic active site in N-doped carbon catalyst.28, 52-53 The functional groups of the samples was also determined by FT-IR spectra as reflected in Figure S9. Obviously, the skeletal vibration of graphene at 1558 cm-1 is appeared,54 and both the characteristic peaks of GO and g-C3N4 cannot find in the NGS4-900, suggesting that GO was completely reduced into graphene and g-C3N4 may be converted to some N incorporation into the sp2 network of graphene in NGS4-900. ORR Activity, Kinetics and Durability The ORR performance was tested by RDE technique (supporting information). The linear sweep voltammetric (LSV) curves were displayed in Figure 5a. It can be observed that pristine GO has hardly exhibited the ORR activity because of its poor 10


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conductivity, while the catalytic activity has risen when it compounded with g-C3N4 under hydrothermal conditions according to the enhanced onset potential (Eonset = 0.791V), half-wave potential (E1/2 = 0.6 V) and the limiting current density (JL = 1.12 mA cm-2). It is also superior to the pristine g-C3N4 sample (Eonset = 0.757 V, E1/2 = 0.588 V, and JL = 0.78 mA cm-2). Surprisingly, the pure hydrothermal GO has presented a relatively higher catalytic activity with more positive Eonset (0.829 V), E1/2 (0.666 V), and JL (2.93 mA cm-2) than that of g-C3N4@GO, this may due to the GO was reduced into graphene partly after the hydrothermal treatment, which possesses a higher conductivity and some ORR activity, but when hydrothermal GO coupled with the g-C3N4, which has a low conductivity, the ORR catalytic activity of the product could be reduced. However, when pyrolysis of g-C3N4@GO, there are significantly improve of the ORR performance owing to the doped of N atoms and the forming of abundant edge-rich defect structure in graphene skeleton, as well as the greater degree of carbon graphitization in the graphene at high temperature. The NGS4-900 exhibited the best catalytic activity for ORR, with more positive Eonset (0.984 V), E1/2 (0.859 V) as well as a larger JL (5.98 mA cm-2) compared to the Pt/C catalyst (0.971 V, 0.848 V, and 5.41 mA cm-2, respectively). The excellent electrocatalytic activity of NGS4-900 was also reflected in the cyclic voltammetric (CV) curves as shown in Figure 5b, and there is no cathodic peak when performed the ORR in N2-saturated KOH solution. Figure 5c shows LSV at different rotation rates in 0.1 M KOH solution, and the Koutecky-Levich plots were derived from the curves (Figure 5d). The average 11



electron numbers (n) is of 3.9 for NGS4-900, which is obtained from the slopes of K-L plots. It is in good agreement with an efficient 4 e- process for ORR, just as the Pt/C catalyst (Figure S10), which indicates that the barriers on NGS4-900 were comparable to that of Pt (111).47 For the pure hydrothermal GO product, however, the value of n was calculated to be 2.3 only (Figure S11). The result indicates that the ORR catalytic activity could be improved greatly when introduced the nitrogen atom in the graphene skeleton. The other electrochemical parameters of hydrothermal GO, NGS4-900 and Pt/C are concluded in Figure 6a, the result indicates that the performance of NGS4-900 can be comparable to those of excellent metal-free carbon-based electrocatalysts in alkaline medium.7, 55-56 Furthermore, the catalyst shows extraordinary stability (Figure 5e). After running 5000 cycles of CV, there is 9 mV positive shift of E1/2 for NGS4-900, and after running 10000 cycles of CV, only 4 mV negative shift of E1/2 for NGS4-900, with a sharp contrast, that a 13 mV negative shift of E1/2 after running 4000 cycles of CV for Pt/C. Besides, the NGS4-900 catalyst presents an excellent tolerance to methanol poisoning effect compared to the noble-Pt catalyst as shown in Figure 5f. The prolonged durability and the immune of methanol cross-over effects may attribute to the nonmetal composition of NGS4-900.47 Meanwhile, the Tafel plots of the catalysts were investigated to assess the unique structural and compositional advantages of NGS4-900 (Figure 6b). A lower Tafel slope value corresponds to a large decrease in overpotential with the current density, which facilitates the enhancement of the ORR electrocatalytic activity.57-58 The Tafel slope in the potential range on NGS4-900 is 72 12


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mV decade-1, is smaller than that (136 mV decade-1) on the hydrothermal GO surface, and is same as that (72 mV decade-1) on the Pt/C surface, confirming that NGS4-900 has a good ORR kinetic process as same as the Pt/C catalyst. Effect of quantity of g-C3N4 and the heating temperature on the ORR Activity In attempt to research the effect of N-doped percent on the as-prepared catalysts, the electrochemical performance of NGS with different initial quantity of g-C3N4 was studied. However, the electrochemical results showed that there were no significant changes among different g-C3N4 to GO ratio for NGS (Figure 7a), and the morphology of the samples is almost the same (Figure S12). In this work, the initial mass ratio of 4:1 of g-C3N4 to GO was kept to explore other optimized condition. The corresponding CV curves, Koutecky-Levich plots and the electron transfer numbers were shown in Figure S13-14. And the electrochemical parameters were further concluded in Table S1. Besides, we further varied the heating temperature from 700 ℃ to 1000 ℃ to explore the optimal annealing temperature for the catalysts. Figure 7b shows the effect of heating temperature on ORR activity of the catalyst. Results suggest that the catalyst shows the best activity with an annealing temperature of 900 °C with the superior JL (5.98 mA cm-2), Eonset (0.984 V), and E1/2 (0.859 V) compared with the others. Results at 700℃ ,800℃ and 1000℃ are presented in Figure S15-17. The Eonset of ORR are 0.905, 0.919 and 0.973 V, respectively, and the same trend is seen in the half-potentials, they are 0.758, 0.756 and 0.817V, respectively. And the limiting 13



current density are 4.12, 5.98, and 5.59 mA cm-2, respectively. Furthermore, the morphology of samples at different temperatures varies greatly (Figure S18). Generally, as the annealing temperature increases, the sheets of NGS becomes thinner and transparent with more edge sites, which is more benefit of the ORR.47 These results confirmed that it is the annealing temperature not the gross doping content can be the key factor to determine the catalytic activity for ORR. At a lower temperature, there are some non-electroactive species would remain in the materials, whereas, when the annealing temperature is too high, the active species would be lost, for example, some pyridinic N could be converted to graphite N at the higher temperature.48 In this experiments, 900°C is the most suitable temperature, which leads to an enhanced ORR activity, attributing to the formation of numerous pyridinic N active sites (Table 1) or sites with higher intrinsic activity at this temperature. The result is in good agreement with previous reports,48,59 in which the electrocatalytic activity of ORR is not directly related to the N content, but to the doping pattern and location of N in carbon. 4. Conclusions In summary, we have developed a different approach from conventional methods for preparation of NGS served for the ORR electrocatalyst by heating a g-C3N4@GO gel-like hybrid in vacuum. The gel-like hybrid was obtained through a simple hydrothermal process, in which the g-C3N4 was immobilized on the surface of GO, forming a “van der Waals heterostructures” with abundant functional groups and 14


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surficial defect structure. Subsequent heating treatment made the uniform distribution of N atom within the graphene matrix, especially the N-doping at defective edges of graphene that giving rise to the NGS catalyst an outstanding ORR performance with glorious Eonset of 0.984 V, E1/2 of 0.859 V and JL of 5.98 mA cm-2, which is superior to the Pt/C catalyst (0.971V, 0.848 V, and 5.41 mA cm-2) and better than most of the nonmetal doped carbon catalyst, as well as the prolonged durability and methanol tolerance. Moreover, it is also found that the appropriate doping configuration and location are more critical than the gross doping content, and based on the XPS results, the pyridinic N can be responsible for the enhanced ORR activity except the edge-rich defect structure in graphene. A further optimize the doping method of ORR electrocatalysts is absolutely important for the practical application of the advanced carbon-based catalysts.

Supporting Information Supporting Information Available: electrochemical testing method; photographs for preparation of catalysts; characterization of the morphology, structure, and composition of as-prepared materials including precursor, hydrothermal products, and the resulting catalysts; ORR performance testing of as-prepared materials and commercial Pt/C.

AUTHOR INFORMATION Corresponding Author E-mail address: [email protected] (C.G. Chen) Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No.21273292, No.21406021) and Large Instrument Open Fund of Chongqing University (Project No.201506150027).

References 1.

Shen, H.; Gracia-Espino, E.; Ma, J.; Zang, K.; Luo, J.; Wang, L.; Gao, S.; Mamat, X.; Hu, G.;

Wagberg, T.; Guo, S., Synergistic Effects between Atomically Dispersed Fe−N−C and C−S−C for the Oxygen Reduction Reaction in Acidic Media. Angew. Chem. 2017, 56, 13800-13804. 2.

Scofield, M. E.; Liu, H.; Wong, S. S., A concise guide to sustainable PEMFCs: recent advances in

improving both oxygen reduction catalysts and proton exchange membranes. Chem. Soc. Rev. 2015, 44, 5836-5860. 3.

Yao, Z.; Nie, H.; Yang, Z.; Zhou, X.; Liu, Z.; Huang, S., Catalyst-free synthesis of iodine-doped

graphene via a facile thermal annealing process and its use for electrocatalytic oxygen reduction in an alkaline medium. Chem. Commun. 2012, 48, 1027-1029. 4.

Debe, M. K., Electrocatalyst approaches and challenges for automotive fuel cells. Nature 2012,

486, 43-51. 5.

Wang, Y.-J.; Zhao, N.; Fang, B.; Li, H.; Bi, X. T.; Wang, H., Carbon-supported Pt-based alloy

electrocatalysts for the oxygen reduction reaction in polymer electrolyte membrane fuel cells: particle size, shape, and composition manipulation and their impact to activity. Chem. Rev. 2015, 115, 3433-3467. 6.

Tao, L.; Wang, Q.; Dou, S.; Ma, Z.; Huo, J.; Wang, S.; Dai, L., Edge-rich and dopant-free

graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction. Chem. Commun. 2016, 52, 2764-2767. 7.

Yu, H.; Shang, L.; Bian, T.; Shi, R.; Waterhouse, G. I.; Zhao, Y.; Zhou, C.; Wu, L. Z.; Tung, C. H.;

Zhang, T., Nitrogen℃Doped Porous Carbon Nanosheets Templated from g℃C3N4 as Metal℃Free Electrocatalysts for Efficient Oxygen Reduction Reaction. Adv. Mater. 2016, 28, 5080-5086. 8.

Xia, W.; Zou, R.; An, L.; Xia, D.; Guo, S., A metal–organic framework route to in situ

encapsulation of Co@ Co3O4@ C core@ bishell nanoparticles into a highly ordered porous carbon matrix for oxygen reduction. Energ. Environ. Sci. 2015, 8, 568-576. 9.

Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S., Bamboo-like carbon nanotube/Fe3C nanoparticle

hybrids and their highly efficient catalysis for oxygen reduction. J. Am. Chem. Soc. 2015, 137, 1436-1439. 10. Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P., High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011, 332, 443-447. 11. Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z., Origin of the electrocatalytic oxygen reduction activity of graphene-based catalysts: a roadmap to achieve the best performance. J. Am. Chem. Soc. 2014, 136, 4394-4403. 12. Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z., Origin of the Electrocatalytic Oxygen Reduction Activity of Graphene-Based Catalysts: A Roadmap to Achieve the Best Performance. J. Am. Chem. Soc. 2014, 136, 4394-4403. 13. Wang, G.; Yang, J.; Park, J.; Gou, X.; Wang, B.; Liu, H.; Yao, J., Facile synthesis and characterization of graphene nanosheets. J. Phys. Chem. C 2008, 112, 8192-8195. 16


Page 16 of 25

Page 17 of 25 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


14. Xiang, Q.; Yu, J.; Jaroniec, M., Preparation and enhanced visible-light photocatalytic H2-production activity of graphene/C3N4 composites. The J. Phys. Chem. C 2011, 115, 7355-7363. 15. Zhou, X.; Qiao, J.; Yang, L.; Zhang, J., A Review of Graphene℃Based Nanostructural Materials for Both Catalyst Supports and Metal℃Free Catalysts in PEM Fuel Cell Oxygen Reduction Reactions. Adv. Energy Mater. 2014, 4, 1289-1295. 16. Wang, C.; Su, K.; Wan, W.; Guo, H.; Zhou, H.; Chen, J.; Zhang, X.; Huang, Y., High sulfur loading composite wrapped by 3D nitrogen-doped graphene as a cathode material for lithium–sulfur batteries. J. Mater. Chem. A 2014, 2, 5018-5023. 17. Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H., Simultaneous nitrogen-doping and reduction of graphene oxide. J. Am. Chem. Soc. 2009, 131, 15939–15944. 18. Tang, C.; Zhang, Q., Nanocarbon for Oxygen Reduction Electrocatalysis: Dopants, Edges, and Defects. Adv. Mater. 2017, 29, 1604103. 19. Sheng, Z.-H.; Shao, L.; Chen, J.-J.; Bao, W.-J.; Wang, F.-B.; Xia, X.-H., Catalyst-free synthesis of nitrogen-doped graphene via thermal annealing graphite oxide with melamine and its excellent electrocatalysis. ACS Nano 2011, 5, 4350-4358. 20. Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X. a.; Huang, S., Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction. ACS Nano 2011, 6, 205-211. 21. Zhang, C.; Mahmood, N.; Yin, H.; Liu, F.; Hou, Y., Synthesis of phosphorus℃doped graphene and its multifunctional applications for oxygen reduction reaction and lithium ion batteries. Adv. Mater. 2013, 25, 4932-4937. 22. Parvez, K.; Yang, S. B.; Hernandez, Y.; Winter, A.; Turchanin, A.; Feng, X.; Müllen, K., Nitrogen-doped graphene and its iron-based composite as efficient electrocatalysts for oxygen reduction reaction. ACS Nano 2012, 6, 9541-9550. 23. Liu, Z.; Zhang, G.; Lu, Z.; Jin, X.; Chang, Z.; Sun, X., One-step scalable preparation of N-doped nanoporous carbon as a high-performance electrocatalyst for the oxygen reduction reaction. Nano Research 2013, 6, 293-301. 24. Jin, J.; Pan, F.; Jiang, L.; Fu, X.; Liang, A.; Wei, Z.; Zhang, J.; Sun, G., Catalyst-Free Synthesis of Crumpled Boron and Nitrogen Co-Doped Graphite Layers with Tunable Bond Structure for Oxygen Reduction Reaction. ACS Nano 2014, 8, 3313-3321. 25. Qu, K.; Zheng, Y.; Dai, S.; Qiao, S. Z., Graphene oxide-polydopamine derived N, S-codoped carbon nanosheets as superior bifunctional electrocatalysts for oxygen reduction and evolution. Nano Energy 2016, 19, 373-381. 26. Wu, Z. Y.; Xu, X. X.; Hu, B. C.; Liang, H. W.; Lin, Y.; Chen, L. F.; Yu, S. H., Iron Carbide Nanoparticles Encapsulated in Mesoporous Fe℃N℃Doped Carbon Nanofibers for Efficient Electrocatalysis. Angew. Chem. 2015, 127, 8297-8301. 27. Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L., A metal-free bifunctional electrocatalyst for oxygen reduction and oxygen evolution reactions. Nat. Nanotechnol. 2015, 10, 444-452. 28. Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S., Exploration of the active center structure of nitrogen-doped graphene-based catalysts for oxygen reduction reaction. Energ. Environ. Sci. 2012, 5, 7936-7942. 29. Li, Q.; Cao, R.; Cho, J.; Wu, G., Nanocarbon electrocatalysts for oxygen reduction in alkaline media for advanced energy conversion and storage. Adv. Energy Mater. 2014, 4: 1301415. doi: 10.1002/aenm.201301415. 17



30. Zhu, C.; Fu, S.; Shi, Q.; Du, D.; Lin, Y., Single-Atom Electrocatalysts. Angew. Chem. 2017, 56, 13944-13960. 31. Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z., Porous C3N4 Nanolayers@N-Graphene Films as Catalyst Electrodes for Highly Efficient Hydrogen Evolution. ACS Nano 2015, 9, 931-940. 32. Xiang, Q.; Yu, J.; Jaroniec, M., Preparation and Enhanced Visible-Light Photocatalytic H2 -Production Activity of Graphene/C3N4 Composites. J. Phys. Chem. C 2011, 115, 7355-7363. 33. Tang, L.; Jia, C. T.; Xue, Y. C.; Li, L.; Wang, A. Q.; Xu, G.; Liu, N.; Wu, M. H., Fabrication of Compressible and Recyclable Macroscopic g-C3N4 /GO Gel-like Hybrids for Visible-light Harvesting: A Promising Strategy for Water Remediation. Appl. Catal. B Environ. 2017, 219, 241-248. 34. Wang, C.; Su, K.; Wan, W.; Guo, H.; Zhou, H.; Chen, J.; Zhang, X.; Huang, Y., High sulfur loading composite wrapped by 3D nitrogen-doped graphene as a cathode material for lithium-sulfur batteries. J. Mater. Chem. A 2014, 2, 5018-5023. 35. Zheng, Y.; Jiao, Y.; Zhu, Y.; Cai, Q.; Vasileff, A.; Li, L. H.; Han, Y.; Chen, Y.; Qiao, S. Z., Molecule-level g-C3N4 Coordinated Transition Metals as a New Class of Electrocatalysts for Oxygen Electrode Reactions. J. Am. Chem. Soc. 2017, 139, 3336-3339. 36. Yu, H.; Shang, L.; Bian, T.; Shi, R.; Waterhouse, G. I.; Zhao, Y.; Zhou, C.; Wu, L. Z.; Tung, C. H.; Zhang, T., Carbon Nanosheets: Nitrogen-Doped Porous Carbon Nanosheets Templated from g-C3N4 as Metal-Free Electrocatalysts for Efficient Oxygen Reduction Reaction. Adv. Mater. 2016, 28, 5080. 37. Jiang, Y.; Yang, L.; Sun, T.; Zhao, J.; Lyu, Z.; Zhuo, O.; Wang, X.; Wu, Q.; Ma, J.; Hu, Z., Significant Contribution of Intrinsic Carbon Defects to Oxygen Reduction Activity. ACS Catalysis 2015, 5, 151005110901002. 38. Li, X.; Wang, H.; Robinson, J. T.; Sanchez, H.; Diankov, G.; Dai, H., Simultaneous Nitrogen Doping and Reduction of Graphene Oxide. J. Am. Chem. Soc. 2009, 131, 15939-15944. 39. Wang, X.; Li, X.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H.; Guo, J.; Dai, H., N-doping of graphene through electrothermal reactions with ammonia. Science 2009, 324, 768-771. 40. Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G., Synthesis of N-doped graphene by chemical vapor deposition and its electrical properties. Nano Lett. 2009, 9, 1752-1758. 41. Liu, J.; Liu, Y.; Liu, N.; Han, Y.; Zhang, X.; Huang, H.; Lifshitz, Y.; Lee, S.-T.; Zhong, J.; Kang, Z., Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway. Science 2015, 347, 970-974. 42. Zhang, W.; Zou, X.; Zhao, J., Preparation and performance of a novel graphene oxide sheets modified rare-earth luminescence material. J. Mater. Chem. C 2015, 3, 1294-1300. 43. Schwinghammer, K.; Mesch, M. B.; Duppel, V.; Ziegler, C.; Senker, J. r.; Lotsch, B. V., Crystalline carbon nitride nanosheets for improved visible-light hydrogen evolution. J. Am. Chem. Soc. 2014, 136, 1730-1733. 44. Wang, G.; Zhang, J.; Kuang, S.; Zhang, W., Enhanced Electrocatalytic Performance of a Porous g℃C3N4/Graphene Composite as a Counter Electrode for Dye℃Sensitized Solar Cells. Chemistry-A European Journal 2016, 22, 11763-11769. 45. Tang, L.; Jia, C.; Xue, Y.; Li, L.; Wang, A..; Xu, G.; Liu, N.; Wu, M., Fabrication of compressible and recyclable macroscopic g-C3N4/GO gel-like hybrids for visible-light harvesting: A promising strategy for water remediation. Appl. Catal B: Environ. 2017, 219, 241-248. 46. Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Z., Porous C3N4 nanolayers@ N-graphene films as catalyst electrodes for highly efficient hydrogen evolution. ACS Nano 2015, 9, 931-940. 47. Xiang, Q.; Yin, W.; Liu, Y.; Yu, D.; Wang, X.; Li, S.; Chen, C., A study of defect-rich carbon 18


Page 18 of 25

Page 19 of 25 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


spheres as a metal-free electrocatalyst for an efficient oxygen reduction reaction. J. Mater. Chem. A 2017, 5, 24314-24320. 48. Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D., Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation. J. Am. Chem. Soc. 2017, 139, 14143–14149. 49. Khabashesku, V. N.; Zimmerman, J. L.; Margrave, J. L., Powder synthesis and characterization of amorphous carbon nitride. Chem. Mater. 2000, 12, 3264-3270. 50. Sun, Y.; Li, C.; Xu, Y.; Bai, H.; Yao, Z.; Shi, G., Chemically converted graphene as substrate for immobilizing and enhancing the activity of a polymeric catalyst. Chem. Commun. 2010, 46, 4740-4742. 51. Liu, R.; Liu, H.; Li, Y.; Yi, Y.; Shang, X.; Zhang, S.; Yu, X.; Zhang, S.; Cao, H.; Zhang, G., Nitrogen-doped graphdiyne as a metal-free catalyst for high-performance oxygen reduction reactions. Nanoscale 2014, 6, 11336-11343. 52. Singh, D. K.; Jenjeti, R. N.; Sampath, S.; Eswaramoorthy, M., Two in one: N-doped tubular carbon nanostructure as an efficient metal-free dual electrocatalyst for hydrogen evolution and oxygen reduction reactions. J. Mater. Chem. A 2017, 5, 6025-6031. 53. Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J., Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016, 351, 361-365. 54. Nethravathi, C.; Rajamathi, M., Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide. Carbon 2008, 46, 1994-1998. 55. Gao, S.; Li, L.; Geng, K.; Wei, X.; Zhang, S., Recycling the biowaste to produce nitrogen and sulfur self-doped porous carbon as an efficient catalyst for oxygen reduction reaction. Nano Energy 2015, 16, 408-418. 56. Jeon, I. Y.; Zhang, S.; Zhang, L.; Choi, H. J.; Seo, J. M.; Xia, Z.; Dai, L.; Baek, J. B., Edge℃selectively sulfurized graphene nanoplatelets as efficient metal℃free electrocatalysts for oxygen reduction reaction: the electron spin effect. Adv. Mater. 2013, 25, 6138-6145. 57. Li, Y.; Guo, C.; Li, J.; Liao, W.; Li, Z.; Zhang, J.; Chen, C., Pyrolysis-induced synthesis of iron and nitrogen-containing carbon nanolayers modified graphdiyne nanostructure as a promising core-shell electrocatalyst for oxygen reduction reaction. Carbon 2017, 119, 201-210. 58. Liu, J.; Sun, X.; Song, P.; Zhang, Y.; Xing, W.; Xu, W., High℃Performance Oxygen Reduction Electrocatalysts based on Cheap Carbon Black, Nitrogen, and Trace Iron. Adv. Mater. 2013, 25, 6879-6883. 59. Wu, G.; Mack, N. H.; Gao, W.; Ma, S.; Zhong, R.; Han, J.; Baldwin, J. K.; Zelenay, P., Nitrogen-doped graphene-rich catalysts derived from heteroatom polymers for oxygen reduction in nonaqueous lithium-O2 battery cathodes. ACS Nano 2012, 6, 9764-9776.

19



Graphical Abstract

The g-C3N4 nanosheets was immobilized on the surface of graphene nanosheets to form a 3D gel-like hybrid with many functional groups and edge-rich defect structure under a hydrothermal condition. After heating of the hybrid, the specific N-doping species were introduced into graphene matrix, especially at the defective edges of graphene, giving rise to a favorable microstructure for the NGS served for an enhanced oxygen reduction electrocatalyst.

20


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Graphical Abstract

The g-C3N4 nanosheets was immobilized on the surface of graphene nanosheets to form a 3D gel-like hybrid with many functional groups and edge-rich defect structure under a hydrothermal condition. After heating of the hybrid, the specific N-doping species were introduced into graphene matrix, especially at the defective edges of graphene, giving rise to a favorable microstructure for the NGS served for an enhanced oxygen reduction electrocatalyst.



Figure 1. Schematic illustration of the preparation process of NGS4-900 film.

Figure 2. FE-SEM images of GO(a), g-C3N4(b), g-C3N4@GO(c), and NGS4-900 (d). TEM images of g-C3N4@GO(e), and NGS4-900(f).

300

b

g-C3N4

dV/dlog(r) Pore Volume (cm3 g-1 A-1)

a Quantity Adsorbed (cm3 g-1)

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

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。

g-C3N4@GO

250

NGS4-900 200

150

100

50

0 0.0

0.2

0.4

0.6

0.8

1.0

1.4

g-C3N4 1.2

g-C3N4@GO NGS4-900

1.0 0.8 0.6 0.4 0.2 0.0 10

Relative Pressure (P/P0)

100

。 Pore Radius (A)

Figure 3. N2 adsorption-desorption isotherms (a) and pore distribution curve (b) of g-C3N4, g-C3N4@GO and NGS4-900.


Page 23 of 25

a

C1s

Intensity （a.u.）

Intensity （a.u.）

b GO

g-C3N4 g-C3N4@GO

O1s

NGS4-900

N1s

g-C3N4@GO

GO

NGS4-900 10

20

30

40

2 Theta （degree）

c

50

Finally

2.0

1200

1000

800

600

400

200

Binding Energy (eV)

d

GO g-C3N4@GO

C1s

N1s

NGS4-900

1.5

NGS4-900

Intensity (a.u.)

Norm. 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


1.0

0.5

g-C3N4@GO

0.0 292

290

288

286

284

282

408

406

404

Binding Energy (eV)

402

400

398

396

394

Binding energy (eV)

Figure 4. XRD patterns (a), XPS surveys (b), C1s XPS peak (c) and N1s XPS peak(d) of GO, g-C3N4@GO and NGS4-900.


392


a

2

b

0

1

-4

J (mA cm-2)

J (mA cm-2)

-2

GO g-C3N4 g-C3N4@GO

0.2

0.4

0.6

0.8

1.0

0

-1

Eonset=0.984V -2

hydrothermal GO Pt/C NGS4-900

-6

N2 O2

-3

1.2

0.2

0.4

Potential (V vs. RHE)

c

0.6

0.8

1.0

1.2


d

0

4.2

0.35

4.1

n=3.9

4.0

n

0.30

-2

3.9

-4

400rpm 625rpm 900rpm 1225rpm 1600rpm 2025rpm 2500rpm

-6

-8

J-1(mA-1 cm2)

J (mA cm-2)

3.8 3.7

0.25

0.3

0.4

0.7

0.7V 0.65V 0.6V 0.55V 0.5V 0.45V 0.4V 0.35V

0.15

0.4

0.6

0.8

0.020

1.0

0.025

0 -1 J (mA cm-2)

-1

-3 -4

-2

0.030

0.035

0.040

0.045

f

Pt/C △E1/2=13mV

100

NGS4-900

90

0.848V

0.835V -3

80

-4

After 4000 cycles

Initial

-5 -6

0.4

0.6

0.8

1.0

70 60

Potential （V vs. RHE）

-5

Initial After 5000 cycles After 10000 cycles

-6 0.2

0.050

ω-0.5 （rpm-0.5）

I/I0 (%)

0

-2

0.6

0.20


e

0.5

Potential （V vs. RHE）

0.10

0.2

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

Page 24 of 25

0.4

0.6

0.8

1.0

Pt/C

50 40 0

200


400

600

800

1000

Time （s）

Figure 5. (a) LSV curves of the prepared catalysts and commercial 20 wt.% JM-Pt/C obtained on 1600 rpm RDE in O2 saturated 0.1 M KOH with 10 mV s-1; (b) CV curves of NGS4-900 in N2 and O2 saturated 0.1 M KOH at 50 mV s-1; (c) LSV curves of NGS4-900 on different rotating rates RDE; (d) K-L plots at different potentials; (e) LSV curves of NGS4-900 before and after cycling for 5000 and 10000 cycles with 1600 rpm RDE (Inset, LSV curves of Pt/C before and after cycling for 4000 cycles); (f)The durability test of NGS4-900 and Pt/C for methanol.


a

10

Eonset

JL

1.0

b

0.9

0.8

72 mV dec-1 72 mV dec-1

JL (mA cm-2)

E1/2

0.9

6 0.8

4

0.7


8

hydrothermal GO NGS4-900 Pt/C

0.7

136 mV dec-1

0.6 0.4

2

0.6

0.6 Hydrothermal GO

NGS4-900

0.8

1.0

log (|Jk| (mA cm-2))

Pt/C

Figure 6. ORR parameters (a) and Tafel plots with 1600 rpm RDE (b) of hydrothermal GO, NGS4-900 and Pt/C

a

0

-1

b

NGS2-900 NGS4-900 NGS6-900

0

700℃ 800℃ 900℃ 1000℃

-2

J (mA cm-2)

-2

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



Page 25 of 25

-3

-4

-4

-5 -6

-6 0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0



Figure 7. (a) LSV curves of NGS synthesized with different mass ratio (2:1, 4:1, 6:1) of g-C3N4 and GO at 900 ℃; (b) LSV curves of different annealing temperature at 700 ℃, 800 ℃, 900 ℃, 1000 ℃ with mass ratio of 4:1 of g-C3N4 to GO in O2-saturated 0.1 M KOH solution.

Table 1. The element compositions and the ratios of various nitrogen-functionality in GO, g-C3N4@GO, and NGS4-900. Samples GO g-C3N4@GO NGS4-900

C content [at%] 72.1 49.1 95.4

O content [at%] 27.9 12.6 2.7

N content [at%] -38.3 2.0

pyridinic-N ratio -52.2% 56.0%

pyrrolic-N ratio -14.4% 28.3%


graphitic-N ratio --15.7%

C-N=C -33.4% --

Hydrothermal Synthesis of a New Kind of N-Doped Graphene Gel-like

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