Influence of Carbon Precursors on the Structure, Composition, and

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Influence of Carbon Precursors on the Structure, Composition and Oxygen Reduction Reaction Performance of Nitrogen-Doped Carbon Materials Liang Chen, Zhongxue Chen, Zheng Huang, Zhongyuan Huang, Yingfei Wang, Huanxin Li, Haihui Zhou, and Yafei Kuang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10246 • Publication Date (Web): 15 Dec 2015 Downloaded from http://pubs.acs.org on December 16, 2015

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Influence of Carbon Precursors on the Structure, Composition and Oxygen Reduction Reaction Performance of Nitrogen-Doped Carbon Materials Liang Chena, Zhongxue Chena, Zheng Huanga, Zhongyuan Huanga, Yingfei Wanga, Huanxin Lia, Haihui Zhou*a and Yafei Kuang*a a

State Key Laboratory for Chemo/Biosensing and Chemometrics, College of Chemistry and

Chemical Engineering, Hunan University, Changsha, 410082, PR China

ABSTRACT: Graphene oxide (GO), oxidized unzipped carbon nanotubes (O-UCNTs) and GO/O-UCNTs composite were used as the carbon precursors to synthesize nitrogen-doped graphene (NG), nitrogen-doped unzipped carbon nanotubes (NUCNTs) and nitrogen-doped graphene/unzipped carbon nanotubes composite (NG-NUCNTs). The influence of these carbon precursors on the synthesis of their corresponding nitrogen-doped carbon materials was systematically investigated. The results show that the synthesis of nitrogen-doped carbon materials not only depends upon the structure and composition of their corresponding carbon precursors, but also the morphology and surface property. As for single carbon precursor, the carbon precursor with higher oxygen content tends to make the nitrogen-doped carbon material with 1

higher nitrogen content. Besides, we find that carbon precursor composite with 3D morphology and large specific surface areas contributes to fabricating nitrogen-doped carbon material with sufficient catalytic sites and high nitrogen content. It is the difference of structure, composition, morphology and surface property for the three

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carbon precursors that results in the as-prepared 3D NG-NUCNTs nework has the most catalytic sites and highest nitrogen content, which further makes the best ORR performance. Therefore, our results provide a good guidance on the systhesis of nitrogen-doped carbon materials with controllable structure and composition and advanced ORR performance.

1. Introduction The crucial oxygen reduction reaction (ORR) is one of the most important reactions not only in life process but also in artificial energy conversion and storage systems, such as fuel cells, metal-air batteries and so forth.1-2 Up to now, Pt and Pt-based alloys, have been widely investigated as the electrocatalysts for ORR due to their superior activity and durability. However, these kinds of electrocatalysts suffer greatly from high cost, low abundance, crossover effect and CO poisoning, which limits their widespread application.3-5 Therefore, the development of Pt-free ORR catalysts with high activity and durability is desperately needed. Under such circumstances, nitrogen-doped carbon materials as the promising candidates have been studied intensively in recent years, and they always show excellent ORR electrocatalytic activity because the nitrogen-doped active catalytic sites can facilitate the oxygen adsorption and decomposition of peroxide species.6-7 Generally, in order to synthesize nitrogen-doped carbon materials, two approaches are often adopted, in-situ doping and post doping.8 For the in-situ doping methods, such as arc-discharge, laser ablation and chemical vapor deposition (CVD),

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N atoms can be doped directly during the growth of carbon materials. Nonetheless, these doping processes are always performed under very strict conditions. Moreover, the extremely low yield and high cost limit their practical application to a large extent.9-11 While for the post doping methods, including thermal annealing and hydrothermal method, N atoms are doped into the pre-synthesized carbon precursors by using different kinds of N sources. Many compounds such as melamine, cyanamide, urea and the like, with high nitrogen content and low price, are quite suitable for the N sources.9,12,13,14 In contrast, the post doping methods can offer a more preferable way to produce the nitrogen-doped carbon materials in large quantities for practical application. When the post doping methods are adopted to dope N atoms into the structure of carbon materials, the structural defective areas on the surface of carbon materials are responsible for the successful N doping. Commonly, the structural defective areas exist in the form of oxygen-containing functional groups.15-17 Considering that the desperate shortage of defective areas on the surface of pristine carbon materials (like graphene or carbon nanotubes), pristine carbon materials need to undergo chemical oxidation to get lots of oxygen-containing functional groups for further N doping.15 In general, the chemically oxidized carbon materials (e.g. graphene oxide or oxidized carbon nanotubes) are regarded as the carbon precursors to fabricate the corresponding nitrogen-doped carbon materials.18-19 According to the previous studies,9,15,16,20,21 it is widely acknowledged that the synthesis of nitrogen-doped carbon materials mostly depends on the N doping conditions and the physicochemical

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properties of their corresponding carbon precursors (such as the oxygen content, types of oxygen-containing functional groups, structural defect density and so on). Until now, most previous works about the synthesis of nitrogen-doped carbon materials were involved with one single carbon precursor (e.g. graphene oxide or oxidized carbon nanotubes),13,16,22,23 and they mainly focused on the influence of N doping conditions, especially for the N doping methods (in-situ doping or post doping) and the selection of N sources on the structure and ORR performance of the nitrogen-doped carbon materials.24-25 To the best of our knowledge, the synthesis of nitrogen-doped carbon materials has seldom been concerned with multiple carbon precursors, and it has never been systematically investigated from the perspective of carbon precursors. Nevertheless, the investigation on the influence of the carbon precursors (including structure, composition, morphology and surface property) on the synthesis of their corresponding nitrogen-doped carbon materials is highly crucial for fabricating the nitrogen-doped carbon materials with controllable structure and composition and advanced ORR performance. Supposing if one single carbon precursor is utilized to prepare the corresponding nitrogen-doped carbon material, how does the carbon precursor influence the synthesis of the corrsponding nitrogen-doped carbon material ? In the meantime, if two or more carbon precursors with different structure and composition are mixed together before N doping, what will happen on the resultant nitrogen-doped carbon composite ? Are the structure and composition of the resultant nitrogen-doped carbon composite the same as the direct adding of those of single nitrogen-doped carbon

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material ? In order to address these issues, in our work, graphene oxide (GO), oxidized unzipped carbon nanotubes (O-UCNTs) and GO/O-UCNTs (w/w=1/1) composite were used as the carbon precursors, and their corresponding nitrogen-doped carbon materials, nitrogen-doped graphene (NG), nitrogen-doped unzipped carbon nanotubes (NUCNTs) and nitrogen-doped graphene/unzipped carbon nanotubes (NG-NUCNTs) composite were successfully synthesized by a facile pyrolysis with urea. Meanwhile, the influence of the three carbon precursors on the structure, composition and ORR performance of their corresponding nitrogen-doped carbon materials was explored in depth.

2. Experimental section 2.1. Chemicals and materials All reagents (Sinopharm Chemical Reagent Co., Ltd., China) were of analytical grade without further treatment. Ultrapure water was used in this study. The pristine multi-walled carbon nanotubes (MWCNTs, purity>97%) with 5-15 μm in length and 60-100 nm in diameter were commercially supplied (L-MWNT-60100, Shenzhen Nanotech Port Co., Ltd.). 2.2. Synthesis of GO, O-UCNTs and GO/O-UCNTs composite In a typical synthesis, GO was prepared by chemical oxidation and exfoliation of natural graphite under acidic condition according to the Hummers’ method.26 O-UCNTs were synthesized from the pristine MWCNTs by using the modified Hummers’ method.19 Briefly, 1 g of MWCNTs and 0.5 g of sodium nitrate were

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dispersed in 46 mL of concentrated sulfuric acid (98%) in a 250 mL roundflask. The mixture was stirred in an ice bath for about 0.5 h. And then 5 g of potassium permanganate was slowly added into the suspension under vigorous stirring. After that, the resulting mixture was removed from the ice bath and then stirred at 40 °C for 2.5 h. Next, 50 mL of water was gradually added with vigorous agitation. After the solution cooled to room temperature, 5 mL of H2O2 (30 wt.%) and 100 mL of water were separately added. The final obtained mixture was centrifuged and washed with ethanol and deionized water for several times until the pH of the suspension was close to 7. The final products were dried at 60 °C under vacuum for 24 h as a result of O-UCNTs. The as-prepared GO and O-UCNTs were mixed in the mass ratio of 1/1 and subsequently treated by a ultrasonic cell disrupter for 0.5 h to make the GO/O-UCNTs (w/w=1/1) composite. 2.3. Synthesis of NG, NUCNTs and NG-NUCNTs composite 90 mg GO was dispersed in 90 mL distilled water in a beaker under ultrasonic treatment for 2 h. The mixture solution was stirred magnetically at room temperature for 30 min and 0.5 g urea was gradually added into the solution. The resulting dispersion was then dried at 60 °C in an oven and the obtained product was grinded into fine powder. Eventually, the powder was pyrolyzed at 800 °C for 1 h under a N2 atmosphere to prepare NG.12 Similarly, NUCNTs and NG-NUCNTs composite were prepared by the same pyrolysis method using O-UCNTs and GO/O-UCNTs composite as the carbon precursors, respectively. 2.4. Material characterizations

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Transmission electron microscope (TEM) images were recorded on a JEM-2100F with an EDX analytical system. Scanning electron microscope (SEM) was carried out with a field emission scanning electron microanalyzer (Hitachi S-4800, Japan). X-ray photoelectron spectroscopy (XPS) analysis was conducted on ESCALAB250 XPS spectrometer with an Mg Kα X-ray source (1350 eV). Specific surface area and pore volume were calculated based on the nitrogen physical sorption (Beckman Coulter SA-3100, USA). Raman spectroscopy was performed on a Raman spectrometer (Labram-010, France) from 1000 cm−1 to 2000 cm−1. 2.5. Electrochemical measurements 5 mg of the sample was dispersed in the distilled water (5 mL) and then ultrasonicated for 1 h. 20 μL of this suspension was dropped and adhered on the pre-polished glassy carbon electrode (5 mm diameter, 0.196 cm2 geometric area) by using Nafion solution (5 μL, 0.05 wt%). The electrochemical measurements were carried out in a conventional three-electrode cell on the CHI 660C electrochemical workstation. A platinum foil electrode and a Ag/AgCl electrode filled with saturated KCl aqueous solution were used as the auxiliary and reference electrodes, respectively. The electrocatalytic activity towards ORR was measured in O2-saturated 0.1 M KOH solution by using a rotating disk electrode (Pine Instrument, MSR analytical rotator). The kinetic parameters can be analyzed by the Koutechy-Levich (K-L) equation in the following: i-1=ik-1+iL-1=(nFAkC0)-1+(0.62nFAD2/3C0ν-1/6ω1/2)-1

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Where, i is the measured current, ik and iL are the kinetic- and limiting- currents, respectively. n is the electron transfer number in the ORR process, F is the Faraday constant (96485 C/mol), A is the geometric area of the rotating disk electrode (RDE), D is the diffusion coefficient of O2 in the KOH electrolyte (1.9×10-5 cm2/s), C0 is the bulk concentration of O2 (1.2×10-3 M), ν is the kinetic viscosity of the electrolyte (0.01 cm2/s), ω is the angular velocity of the disk (ω=2πN, N is the linear rotation speed) and k is the electron transfer rate constant. Cyclic voltammetry (CV) tests were measured within a voltage range from 0.2 V to -1.0 V (vs. Ag/AgCl electrode) at a scan rate of 50 mV/s. Linear sweep voltammetry (LSV) at a scan rate of 10 mV/s was performed at various rotation speeds from 400 to 1600 rpm.

3. Results and discussion Usually, GO and O-UCNTs are categorized as two different sorts of oxidized carbon materials. GO, as one-atom-thick layers of sp2-hybridized carbon atoms contains a lot of oxygen-containing functional groups, such as hydroxyls, carboxyls etc. on the carbon planes,27-28 while O-UCNTs prepared by the similar chemical oxidation method have many oxygen-containing functional groups on both sides of the unzipped edges.29-31 Compared with GO, O-UCNTs with the analogous curly graphene nanoribbon structure always show various structure and composition. When GO and O-UCNTs are used as the carbon precursors, it can be speculated that structure and composition of the corresponding nitrogen-doped carbon materials, NG and NUCNTs will be different, but how GO and O-UCNTs influence the synthesis of

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NG and NUCNTs is unclear. Meanwhile, if GO and O-UCNTs are mixed in the mass ratio of 1:1, and the resultant GO/O-UCNTs composite is used to prepare the corresponding nitrogen-doped carbon material, NG-NUCNTs composite, what will take place on the as-synthesized NG-NUCNTs composite is of great interest to us. To study the real impact of the carbon precursors on the synthesis of their corresponding nitrogen-doped carbon materials, the structure, composition, morphology and surface property of the carbon precursors (GO, O-UCNTs and GO/O-UCNTs composite) and those of the corresponding nitrogen-doped carbon materials (NG, NUCNTs and NG-NUCNTs composite) are investigated in detail. To study the structure, composition, morphology and surface property of the carbon precursors, the Raman, XPS, SEM and BET specific surface areas are measured, respectively. As seen from Figure 1, the Raman spectra of GO, O-UCNTs and GO/O-UCNTs composite are presented. It is clear that all samples exhibit a D-band and a G-band. Generally, the structural defect nature can be reflected from the value of ID/IG.32-33 It can be seen from Figure 1 that GO and O-UCNTs display various defect density, and GO (ID/IG=1.28) shows higher defect density when compared with O-UCNTs (ID/IG=1.18). Compared with GO and O-UCNTs, the value of ID/IG for GO/O-UCNTs composite is 1.24, which demonstrates the structural defect density of GO/O-UCNTs composite locates in between.

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Figure 1 Raman spectra of GO, O-UCNTs and GO/O-UCNTs composite

Figure 2 shows the XPS spectra of GO, O-UCNTs and GO/O-UCNTs composite, and the specific XPS results are presented in Table 1. It is obvious that all the samples exhibit different oxygen content and various oxygen-containing functional groups. The oxygen content of GO (37.2%) is clearly higher than that of O-UCNTs (28.1%). The higher O content indicates more oxygen-containing functional groups on the surface of GO, and it means GO can provide more defective areas for N doping, thus this result is in accordance with the Raman result for GO. Besides, we can also find that the absolute percentage of C=O (the percentage ratio of C=O to C-O + C=O) in O-UCNTs (14.4%) is much larger than that in GO (1.9%). As for the GO/O-UCNTs composite, the O content (31.5%) is between those of GO and O-UCNTs, and the absolute percentage of C=O (5.75%) also locates between those of GO and O-UCNTs. Based on the Raman and XPS results, it can be concluded that the structure and composition of the three carbon precursors are different. The defect density and O content of GO are clearly larger than those of O-UCNTs, and the defect density and O content of GO/O-UCNTs composite are almost the direct adding of those of GO and O-UCNTs.

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Figure 2 XPS spectra of GO, O-UCNTs and GO/O-UCNTs composite (a), high resolution C 1s spectra of GO (b), O-UCNTs (c) and GO/O-UCNTs composite (d) Table 1 XPS results of GO, O-UCNTs and GO/O-UCNTs composite on the atomic percentages of C, O and the C distributions C distribution (%) Sample

C(%)

O(%)

C=C (284.7 eV)

C-O (286.0 eV)

C=O (288.3 eV)

GO

62.8

37.2

37.3

61.5

1.2

O-UCNTs

71.9

28.1

41.5

50.1

8.4

GO/O-UCNTs composite

68.5

31.5

39.1

57.4

3.5

Figure 3 provides the SEM image of the GO/O-UCNTs composite. For comparison, the SEM images of GO and O-UCNTs are also shown in Figure S1 (see Supporting information). It can be seen that the morphology of the three carbon precursors are of great difference. GO with two-dimensional (2D) nanosheets morphology are stacked seriously, while O-UCNTs with quasi-1D analogous curly graphene nanoribbon morphology are also agglomerated in a degree. As for the GO/O-UCNTs composite, the quasi-1D O-UCNTs (see the arrow areas of Figure 3) insert uniformly into the 2D GO nanosheets, forming the 3D network. 11



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Figure 3 SEM image of the 3D GO/O-UCNTs network

Figure 4 presents the N2 adsorption/desorption isotherms of the three carbon precursors, and the related BET results are shown in Table 2. According to Figure 4, all of the isotherms are the type IV isotherms with a clear capillary condensation step, confirming the porosity of the as-prepared materials.34 On the basis of the BET results, it can be found that the BET specific surface areas of the 3D GO/O-UCNTs composite (83.1 m2/g) are at least twice larger than those of GO (14.4 m2/g) and O-UCNTs (31.8 m2/g). At the same time, the pore volume of the GO/O-UCNTs composite (0.251 cm3/g) is also much larger than those of GO (0.0606 cm3/g) and O-UCNTs (0.195 cm3/g). Compared with GO and O-UCNTs, the GO/O-UCNTs composite has the largest specific surface areas and pore volume, and this result can be ascribed to that the quasi-1D O-UCNTs can effectively impede the stacking of the 2D GO nanosheets and increase the basal spacing, which further constitutes the 3D network.34-35 In the light of the SEM and BET results, it is not hard to find that the morphology and surface property of the three carbon precursors are distinct, and the GO/O-UCNTs composite shows a typical 3D morphology and has the largest specific surface areas and pore volume among them. 12


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Figure 4 Nitrogen adsorption/desorption isotherms of GO, O-UCNTs and the 3D GO/O-UCNTs network Table 2 BET results of GO, O-UCNTs and GO/O-UCNTs composite Sample

Specific surface area (m2/g)

Pore volume (cm3/g)

GO

14.4

0.0606

O-UCNTs

31.8

0.195

GO/O-UCNTs composite

83.1

0.251

When taking GO, O-UCNTs and the 3D GO/O-UCNTs network as the carbon precursors, their corresponding nitrogen-doped carbon materials, NG, NUCNTs and NG-NUCNTs composite can be successfully synthesized by the pyrolysis method, as is schematically shown in Figure 5.

Figure 5 Schematic diagram of the synthesis of NG, NUCNTs and 3D NG-NUCNTs network

Likewise, the structure, composition, morphology and surface property of the three nitrogen-doped carbon materials are also investigated. As shown in Figure 6, the 13



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Raman spectra of the three samples are provided. Apparently, the values of ID/IG for NG and NUCNTs are 1.35 and 1.23, respectively. Compared with GO and O-UCNTs, we find that NG and NUCNTs exhibit higher defect density, which indicates the structural defect density of the carbon precursors are not reduced after the process of N doping. Out of our expectation, NG-NUCNTs composite (ID/IG=1.40) has the highest structural defect density, which signifies that the structural defect density of NG-NUCNTs composite are not the direct adding of those of NG and NUCNTs. The highest defect density for NG-NUCNTs composite means the best N doping effect and most active catalytic sites for ORR.15,36

Figure 6 Raman spectra of NG, NUCNTs and NG-NUCNTs composite

As presented in Figure 7, the XPS spectra of NG, NUCNTs and NG-NUCNTs composite are investigated, and the specific XPS results are displayed in Table 3. In accordance with the previous reports, N atoms are expected to replace C atoms, forming four types of N-configurations: pyridinic N (N-1), pyrolic N (N-2), graphitic N (N-3) and oxygenated N (N-4).37-38 It can be seen from Table 3 that the N percentage of NUCNTs (4.0%) is clearly smaller than that of NG (7.3%), but the sum of the percentages of N-1 and N-3 (N-1 and N-3 are demonstrated very useful to improve the ORR performance)39-41 for NUCNTs (73.8%) is much larger than that of 14


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NG (51.3%), which can be attributed to the difference of oxygen composition in GO and O-UCNTs. Combining the XPS results of GO and O-UCNTs, we find that the O content in GO is much higher than that in O-UCNTs, indicating that GO contains more defective areas for N doping, and this further results in NG has higher N content when compared with NUCNTs. Meanwhile, we can also observe that the O atoms in the form of C=O (14.4% for O-UCNTs and 1.9% for GO) is more likely to form N-1 and N-3 (73.8% for NUCNTs and 51.3% for NG). When compared with NG and NUCNTs, we unexpectedly find that NG-NUCNTs composite has the largest N content (8.0%), and the sum of the percentages of N-1 and N-3 for NG-NUCNTs composite can reach 62.3%, so it can be calculated that the absolute N-1 and N-3 content for NG-NUCNTs composite (4.98%) is obviously larger than those of NG (3.74%) and NUCNTs (2.95%). The highest structural defect density and N content (including total N content and the N content for N-1 and N-3) for NG-NUCNTs composite can be explained as follows. Although the structural defect density and oxygen content for the GO/O-UCNTs composite locate between those of GO and O-UCNTs, the special 3D morphology and largest specific surface areas for GO/O-UCNTs composite make more defective areas exposed to the reaction atmosphere and offer more effective reactive sites for N doping, which further results in the GO/O-UCNTs composite are easier to achieve N doping when compared with GO and O-UCNTs. As a result, the obtained NG-NUCNTs composite shows the highest structural defect density and N content.

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Figure 7 XPS spectra of NG, NUCNTs and NG-NUCNTs composite (a), high resolution N 1s spectra of NG (b), NUCNTs (c) and NG-NUCNTs composite (d)

Table 3 XPS results of NG and NUCNTs on the atomic percentages of C, O, N and the N distributions

Sample

C(%)

O(%)

N distribution (%)

N(%) N-1

N-2

N-3

N-4

NG

88.2

4.5

7.3

33.9

39.1

17.4

9.6

NUCNTs

86.0

10.0

4.0

44.8

18.0

29.0

8.2

NG-NUCNTs composite

87.1

4.9

8.0

43.1

23.3

19.2

14.4

According to the analysis of the results above, we can reach the conclusion that different carbon precursors can make different nitrogen-doped carbon materials, and the synthesis of nitrogen-doped carbon materials is not only correlated with the structure and composition of the corresponding carbon precursors, but also the morphology and surface property. Specifically, as for single carbon precursor (GO or O-UCNTs), the carbon precursor with higher O content tends to make the corresponding nitrogen-doped carbon material with higher N content. With respect to 16


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the carbon precursor composite (GO/O-UCNTs composite), the carbon precursor composite with 3D morphology and large surface areas is helpful for preparing nitrogen-doped carbon material with sufficient catalytic sites and high N content. In order to investigate the morphology and surface property of the as-prepared NG-NUCNTs composite, the SEM, TEM and BET specific surface areas are measured, respectively. As shown in Figure 8, the SEM and TEM images of the NG-NUCNTs composite are provided. For comparison, the SEM and TEM images of NG and NUCNTs are also presented in Figure S2 (see Supporting information). It is clear that the 3D morphology of the NG-NUCNTs network can be discerned from Figure 8a. Compared with GO/O-UCNTs composite, the 3D NG-NUCNTs network is highly crumpled, and there exists lots of “nanoscroll” in the structure of 3D NG-NUCNTs network (see Figure 8b), which can be ascribed to the chemical reaction happening on the defective areas of the carbon precursors under the condition of pyrolysis. As shown in Figure S3 (see Supporting information), the magnified TEM and HRTEM images of NG-NUCNTs network are presented. It is found that the location of the “nanoscroll” shows an obvious lattice fringe, which indicates good conductivity.42 Figure S4 (see Supporting information) provides the elemental analysis images of the NG and NUCNTs in a typical 3D NG-NUCNTs network, respectively. According to the elemental analysis results, the simultaneous N doping on the graphene and unzipped carbon nanotubes can be well demonstrated. Figure S5 (see Supporting information) shows the N2 adsorption/desorption isotherms of NG, NUCNTs and NG-NUCNTs network, and the related BET results are presented in

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Table S1 (see Supporting information). Based on the BET results, it can be found that the 3D NG-NUCNTs nework has the largest specific surface areas (276 m2/g) and pore volume (0.793 cm3/g) when compared with those of NG and NUCNTs, and this results are consistent with the related BET results for their corresponding carbon precursors, which further demonstrates the 3D morphology makes for the increased specific surface areas.

Figure 8 SEM (a) and TEM (b) images of the 3D NG-NUCNTs network

On the basis of the researches above, it is found that the three different carbon precursors have different structure, composition, morphology and surface property, and it leads to much difference of structure and composition existing in their corresponding nitrogen-doped carbon materials, which further has an influence on the performance towards ORR. Compared with NG and NUCNTs, the 3D NG-NUCNTs network shows the most catalytic sites and highest N content (especially for N-1 and N-3), signifying the 3D NG-NUCNTs network has the best electrocatalytic performance for ORR.15,19,21,43 Herein, the RDE measurement is performed to investigate the ORR activity of NG-NUCNTs electrode. In contrast, the ORR activities of NG, NUCNTs (see Supporting information, Figure S6) and commercial Pt/C (De Nora Elettrodi Co. Ltd., 20 wt.% platinum on carbon black) are also

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measured. As clearly shown in Figure 9a, for NG, NG-NUCNTs, NUCNTs and Pt/C, the values of onset potential are -0.12 V, -0.05 V, -0.14 V and -0.03 V, respectively. Obviously, the three different nitrogen-doped carbon materials exhibit different ORR performance. NG-NUCNTs electrode shows the most positive onset potential and the largest current among the three different kinds of nitrogen-doped carbon materials, which indicates the best ORR activity. Furthermore, it can be observed that the ORR activity of NU-NUCNTs could be comparable with that of commercial Pt/C. Figure 9b shows the RDE voltammograms for ORR on the NG-NUCNTs electrode at different rotation speeds. The kinetic parameters can be analyzed with the K-L equations. Figure 9c shows the corresponding K-L plots (i-1 vs. ω-1/2) and it can be seen that the data display good linearity. The electron transfer number n for the NG-NUCNTs electrode is calculated as 3.71–3.94 at the potentials ranging from -0.5 V to -0.8 V, which demonstrates that NG-NUCNTs electrode exhibits a close four-electron pathway for ORR. Figure 9d shows the K-L plots at the potential of -0.8 V for NG, NG-NUCNTs and NUCNTs. The plots of all samples exhibit good linearity. The electron transfer number (n) of NG, NG-NUCNTs and NUCNTs is calculated to be 2.38, 3.94 and 2.34, respectively. It is worth noting that NG-NUCNTs has the largest electron transfer number among the three different kinds of nitrogen-doped carbon materials, which further proves that the 3D NG-NUCNTs network has the best activity towards ORR.

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Figure 9 (a) RDE voltammograms in O2 saturated 0.1 M KOH electrolyte (rotation speed 1600 rpm, scan rate 10 mV/s) for the NG, NG-NUCNTs, NUCNTs and Pt/C; (b) RDE voltammograms for ORR on the NG-NUCNTs electrode at different rotation speeds; (c) K-L plots (i-1 vs. ω-1/2) at different potentials; (d) K-L plots (i-1 vs. ω-1/2) at the potential of -0.8 V for the NG, NG-NUCNTs and NUCNTs

To further study the ORR performance of NG-NUCNTs electrode, the CV and chronoamperometry curves are also measured. Figure S7a (see Supporting information) exhibits the CV curves on the NG-NUCNTs electrode in N2 and O2 saturated 0.1 M KOH electrolyte. In N2 saturated KOH electrolyte, a featureless capacitive current background is observed. After introducing oxygen, a large cathodic current with an obvious peak at -0.26 V, evidences the catalytic activity of NG-NUCNTs towards ORR. Figure S7b (see Supporting information) shows the CV curves on the NG-NUCNTs electrode in oxygen saturated 0.1 M KOH electrolyte before and after 10000 cycles. Clearly, after 10000 consecutive CV cycles, NG-NUCNTs shows no obvious degradation, which manifests the excellent stability 20


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towards ORR. As can be seen from Figure S7c (see Supporting information), the chronoamperometry curves of NG-NUCNTs and commercial Pt/C are presented. Clearly, Pt/C catalyst suffers a sharp decrease (about 29% loss) in current after the addition of 3 M methanol, whereas the value of current for NG-NUCNTs is slightly changed (only 1% loss). The result shows that NG-NUCNTs possesses more outstanding immunity towards methanol than Pt/C catalyst. Hence, as a new type of electrocatalyst for ORR, the 3D NG-NUCNTs framework exhibits superior catalytic activity, long-term stability and high methanol tolerance, which provides a promising candidate for the application in the fields of fuel cells and metal-air batteries.44-45

4. Conclusions Different carbon precursors can make different nitrogen-doped carbon materials, and the synthesis of nitrogen-doped carbon materials not only relies on the structure and composition of the corresponding carbon precursor, but also the morphology and surface property. Specifically, as for single carbon precursor, the carbon precursor with higher O content tends to make the corresponding nitrogen-doped carbon material with higher N content. Besides, the carbon precursor composite with 3D morphology and large surface areas is helpful for preparing nitrogen-doped carbon materials with sufficient catalytic sites and high N content, which makes the nitrogen-doped carbon material shows excellent electrocatalytic performance towards ORR. Therefore, our results provides a good guidance on the systhesis of nitrogen-doped carbon materials with controllable structure and composition and

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advanced ORR performance.

ASSOCIATED CONTENT Supporting Information This section includes some supporting figures mentioned in this article. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 73188821603, Fax: +86 731 88713642; *E-mail: [email protected](Yafei Kuang), [email protected](Haihui Zhou). Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by National Natural Science Foundation of China (Grant No.51071067, 21271069, J1210040, 51238002, J1103312), Science and Technology Program of Hunan Province (No.2015JC3049) and Hunan Provincial Innovation Foundation For Postgraduate (No.CX2015B083).

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Table of Contents

The synthesis of 3D NG-NUCNTs network is not only correlated with the structure and composition of the GO/O-UCNTs composite, but also the morphology and surface property.

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Influence of Carbon Precursors on the Structure, Composition, and

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