Nitrogen and Fluorine-Codoped Porous Carbons as Efficient Metal

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Nitrogen and Fluorine-codoped Porous Carbons as Efficient Metalfree Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells Yanlong Lv, Liu Yang, and Dapeng Cao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b11371 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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ACS Applied Materials & Interfaces

Nitrogen and Fluorine-codoped Porous Carbons as Efficient Metal-free Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells Yanlong Lv 1, Liu Yang 1 and Dapeng Cao *,1, 2 1

State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

2

Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China

Abstract The severe dependence of oxygen reduction reaction in fuel cells on platinum (Pt)-based catalysts greatly limits the process of their commercialization. Therefore, developing cost reasonable non-precious metal catalysts to replace Pt-based catalysts for ORR is an urgent task. Here, we use the composite of inexpensive polyaniline and superfine polytetrafluoroethylene powder as precursor to synthesize a metal-free N, F-codoped porous carbon catalyst (N, F-Carbon). Results indicate that the N, F-Carbon catalyst obtained at the optimized temperature 1000 ºC exhibits almost the same onset (0.97 V vs. RHE) and half-wave potential (0.84 V vs. RHE) and better durability and higher crossover resistance in alkaline medium compared to commercial 20% Pt/C, which is attributed to the well dispersion of fluorine and nitrogen atoms in the carbon matrix, high specific surface area and the synergistic effects of fluorine and nitrogen on the polarization of adjacent carbon atoms. This work provides a new strategy for in situ synthesis of N, F-codoped porous carbon as high efficient metal-free electrocatalyst for ORR in fuel cells. Keywords: Metal-free electrocatalysts; N, F-codoped porous carbon; Oxygen reduction reaction; Polytetrafluoroethylene; Polyaniline-based composite 1

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1. Introduction The fast growing demands of global economy and society accelerate development of clean and renewable energy devices such as metal-air batteries and fuel cells.1-5 However, their commercial applications have been limited because of the sluggish kinetics of oxygen reduction reaction (ORR) at the cathode.6-8 Currently, the ORR in fuel cells mainly depends on precious metal Pt catalysts.9-11 However, the Pt-based catalysts suffer from several severe defects as high cost, scarcity and poor long-term stability. Therefore, developing high efficient non-precious metal catalysts even metal-free electrocatalysts to replace Pt-based electrocatalysts in fuel cells is significantly important, and is still a great challenge. 12-19

Since Dai group reported that vertically aligned N-doped carbon nanotubes (VA-NCNTs) can act as efficient metal-free electrocatalysts for ORR in alkaline electrolytes with a four-electron pathway and free from methanol cross-over and CO-poisoning,6 a lot of efforts have been made to develop metal-free electrocatalysts, mainly including non-metal (N, S, P, F etc) doped carbon materials, such as carbon nanotubes,20,

21

graphene,15,

19, 22-25

mesoporous graphitic arrays

26-28

and metal organic

framework (MOF) -derived porous carbons.29-32 The non-metal doped carbon materials not only exhibit excellent durability and methanol tolerance but also are environmental friendly and price reasonable. Among these materials, nitrogen-doped carbons have been widely reported, because the electronegativity (3.04) of nitrogen can induce the charge redistribution of adjacent atoms in nitrogen-doped carbon surface, which will greatly enhance the ORR activity of carbon electrocatalysts.15, 33, 34 Xia group reported a facile 2


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thermal annealing approach for large-scale synthesis of nitrogen-doped graphene electrocatalyst, which opens up a possibility for the synthesis of N-doped graphene in gram-scale for electronic devices and cathodic materials for fuel cells.23 Shen group synthesized N-doped graphene by the pyrolysis of the composites of polyaniline/reduced graphene oxide and polypyrrole/reduced graphene oxide and proved the effects of different types of nitrogen on the ORR activity. 26 Recently, Cao group also used zeolite imidazolate framework to derive the nitrogen-doped carbon electrocatalysts for ORR, which shows comparable activity with commercial 20% Pt/C in alkaline media.30 Besides nitrogen, other non-metal atoms with different electronegativity, such as boron (B), sulfur (S), phosphorus (P) and fluorine (F), could also enhance the ORR activity of carbon catalysts.6 In particular, previous investigations indicate that doping carbon with two kinds of non-metal atoms or more (such as N and B, N and P, N and S, or N, B and P) could further enhance the ORR activity due to the synergic effects of different non-metal atoms on the ORR.

19, 35-38

For example, Dai group reported that N,P-codoped vertically aligned

multiwalled carbon nanotube arrays exhibit a much better activity than the N-doped one. 19 Woo group also reported that B, P, N-doping could greatly enhance the ORR activity of graphite carbon.39 Antonietti group found that S, N-doping could improve the overall ORR electrocatalytic activity of the carbon material in both basic and acidic media.40 In fact, F atom has the largest electronegativity (4.0). Therefore, F-doping should be definitely beneficial for ORR activity. Moreover, it has been proved that N and F atoms could enhance ORR activity by a synergetic effect. Xing group reported that both F- and N, F-codoped carbon blacks are excellent ORR catalysts.41, 42 Kong group reported that N, F 3



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codoped ordered mesoporous carbon is a promising ORR catalyst.43 Li group found that N, F-codoped graphdiyne shows excellent ORR electrocatalytic activity.44 All the investigations indicate that F-doped or N, F-codoped porous carbons are excellent ORR electrocatalysts with comparable ORR activity with commercial Pt/C in alkaline media. However, doping fluorine in carbon materials always require a large amount of fluorine sources, because it is quite difficult to dope the fluorine into porous carbon matrix. Currently, NH4F is the most commonly used one. The facile decomposition property of NH4F increases the synthesis difficulty of F-doped porous carbons. Therefore, it is very significant to develop high efficient fluorine doping method. Herein, we present a facile method to synthesize N, F-codoped carbon electrocatalyst by using the polytetrafluoroethylene/polyaniline (PTFE/PANI) composite as a precursor. Then, the as-synthesized N, F-codped samples are characterized systematically, and the ORR activities of N, F-codoped porous carbons are further investigated. Finally, some discussion is also addressed. 2. Experimental Scheme 1 shows the synthesis illustrations of N, F-codoped porous carbons, in which PTFE was used as a core resource to mix with aniline to synthesize the PTFE/PANI composite by polymerization of aniline. Then, the PTFE/PANI composite was carbonized under Ar atmosphere to prepare N, F-codoped porous carbons. Compared to the commonly used fluorine source NH4F, PTFE particles not only possess higher content of fluorine but also could serve as anchors in the polymerization procedure of PANI. The attachment between PTFE particles and PANI, which was formed during polymerization, could impede 4


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fluorine loss and therefore cause high efficient doping of fluorine in porous carbons. The as-prepared N, F-codoped carbon electrocatalysts were marked as N, F-Carbon-T, where T stands for different temperatures of 700, 800, 900 and 1000 ºC. For comparison, we also carbonized pure PANI under 1000 ºC and marked as N-Carbon-1000. The detailed synthesis process is as below. 2.1 Preparation of PTFE/PANI composite PTFE/PANI composite was synthesized by an interfacial polymerization. Typically, superfine PTFE powder (1 g) was dispersed in 100 mL 1 M HCl solution. The solution was stirred for half an hour at least. Then, aniline (1 g) was added to the solution followed by another stirring of half an hour. Ammonium persulfate (APS, 1.25 g) was dissolved in 50 mL 1 M HCl solution. The solution of APS was added dropwise into the former under high speed stirring in 1 h and the stirring was kept overnight. The final product was filtered and washed with methanol for three times and then dried in an oven under 60 ºC for one day. 2.2 Preparation of N, F-Carbon catalysts 1 g PTFE@PANI composite was transferred into a quartz boat and placed in a furnace, and then carbonized under Ar atmosphere at different temperatures (here, T=700, 800, 900, 1000 ºC) for 2 hours with a heating rate of 3ºC min-1. The as-prepared sample was marked as N, F-Carbon-T (T=700, 800, 900 and 1000). For comparison, N-Carbon-1000 was prepared by the same procedure but without superfine PTFE powder. The yield of N, F-Carbon-1000 is about 20%. With the decrease of the carbonization temperature, the yield of N, F-Carbon-T sample has a slight increase. 2.3 Characterization 5



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All the as-prepared catalysts were systematically characterized by a series of analysis such as SEM, TEM, FT-IR, PXRD, Raman, N2 adsorption, and XPS, and the details were presented in Supporting Information. To evaluate ORR activity, all the electrochemical tests were performed in a standard three-electrode cell controlled by a CHI 760e electrochemistry workstation, using 0.1 M KOH as the electrolyte. Koutecky-Levich plots were used to calculate the electron transfer number (n). The RRDE test was applied to obtain the yield percentage of H2O2. The details on the electrochemical measurements were also presented in Supporting Information.

3. Results and discussion Fig. 1a shows the typical SEM image of the as-prepared PTFE/PANI composite precursor, which proves the successful polymerization of PANI on PTFE particles. The SEM images of the N, F-Carbon catalysts carbonized under 700-1000 ºC were shown in Fig. 1b-e, respectively. It is found that the PTFE particles vanish after pyrolysis, while the morphology of PANI was preserved partially. Generally, with the increase of pyrolysis temperature, the loss of fluorine atoms becomes easier. Without loss of generality, we mainly explored the doping situation of fluorine atoms in the sample obtained at high temperature of T=1000 ºC. The TEM image and elemental mapping analysis of N, F-Carbon-1000 were shown in Fig. 1f and 1g. A uniform distribution of N and F in N, F-Carbon-1000 sample was observed, indicating the successful doping of N and F in the carbon matrix. The successful F-doping can also be confirmed by FT-IR tests. As shown in Fig. 2a, several small peaks corresponding to C-F bond appear in the range of 1000-1100 6


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cm-1 in the IR spectra for all the four N, F-carbon catalysts, while no obvious peak appears in the corresponding range for N-Carbon-1000. PXRD analyses were also performed to validate the graphitic nature of N, F-Carbon catalysts (Fig. 2b). Four N, F-Carbon catalyst samples possess similar graphitic structure, in which two broad peaks at around 23º and 43º are indexed to the carbon (002) and (101) diffractions, indicating a certain degree of graphitization. Fig. 2c shows two peaks around 1335 cm-1 and 1588 cm-1, corresponding to D and G bands, respectively. The D and G bonds represent the disordered and ordered carbon structures in a carbon material, respectively. The ratio of ID:IG is similar for four N, F-carbon catalysts and N-Carbon-1000 samples. The adsorption–desorption isotherms of N2 at 77 K were measured to investigate the pore properties of the resulting samples (Fig. 2d). Compared to PANI precursor, the porosity of N-Carbon-1000 sample gets an obvious improvement. The isotherms of all the carbon catalysts belong to typical type-IV type with a very small hysteresis loop, indicating the existence of both micropores and small amount of mesopores. Particularly, the BET surface areas of N-Carbon-1000 and N, F-Carbon-1000 are 989 m2 g-1 and 838 m2 g-1, respectively, which are much higher than that of PANI precursor (44 m2 g-1) and the other three N, F-Carbon-700, -800 and -900 catalysts (453, 523 and 436 m2 g-1). Thus, a proper pyrolysis temperature is critical to obtain large BET surface area. The pore size distributions (PSDs) of the five samples were obtained by non-local density functional theory and shown in Figure S1. It can be observed that the PSDs do not show significant difference from each other. A larger BET surface area and the existence of micropores and mesopores are always beneficial for ORR activity. The porous properties of all the materials 7



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prepared here were summarized in Table 1. We also performed XPS measurements to investigate the chemical composition and the contents of heteroatoms N and F in the resulting catalysts (Fig. 3a). The XPS spectra of five samples distinctly show the presence of N1s, C1s and O1s peaks. The surface content of F in the N, F-Carbon-700, -800, -900 and -1000 catalysts are 0.34%, 0.37%, 0.27% and 0.22% and the surface content of N are 3.09%, 3.92%, 3.36% and 1.74%, respectively. Generally, the relative surface contents of N and F decrease with the increase of temperature when the carbonization temperature is greater than 800 ºC. However, for all samples, no significant peak corresponding to F appears in the XPS spectra. The possible reason is that XPS is a surface detection technique with an effective depth of no more than 10 nm beneath the material surface, where the F atoms are likely to flow away during pyrolysis. We also tried to increase the ratio of F/N by increasing the loading of superfine PTFE powder. However, the low PANI coverage on PTFE often leads to the loss of F in the carbonization process, and is not beneficial for synthesis of N,F-Carbon with the relatively high F content. All the four N, F-Carbon catalysts show visible N peaks in XPS spectra, and the relative N species could be analyzed through high resolution N1s spectra. Each high resolution XPS N1s spectrum of four N, F-Carbon catalysts can be deconvoluted into three peaks centered at 398.4 ± 0.2 eV, 400 ± 0.2 eV and 401 ± 0.2 eV, corresponding to pyridinic N, pyrrolic N and graphitic N, respectively (Fig. 3b-e). It can be observed that the relative contents of pyridinic N in the four catalysts decrease while the relative contents of graphitic N increase with the increase of pyrolysis temperature (Fig. 3f). For all samples, the relative content of pyrrolic N is quite low. In particular, almost no pyrrolic N (~0.01%) was left in 8


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N, F-Carbon-1000. Definitely, N, F-Carbon-1000 contains almost 100% content of the graphitic and pyridinic N species, which may be beneficial for ORR performance, because previous investigations indicate that both graphitic

26, 45, 46

and pyridinic N species

47-49

are

the dominant active sites for ORR. The linear sweep voltammetry (LSV) tests on a rotating disk electrode (RDE) were performed to explore the electrocatalytic activity of the as-prepared carbon catalysts in O2-saturated 0.1 M KOH electrolyte at a scanning rate of 5 mV s-1 and the rotation rate of 1600 rpm. As expected, Fig. 4a shows that N, F-Carbon-1000 catalyst possesses better electrochemical activity than N, F-Carbon-700, 800 and 900 catalysts. Compared with N, F-Carbon-700, 800 and 900 catalysts, N, F-Carbon-1000 not only shows more positive onset potential and half-wave potential but also shows visible higher limit current density (see Table 2). The enhanced ORR activity of N, F-Carbon-1000 indicates the importance of pyrolysis conditions and it is probably caused by the enlarged BET surface area and almost 100% content of pyridinic and graphitic N. Although the BET surface area of N-Carbon-1000 sample is larger than that of N, F-Carbon-1000, the synergistic effects of fluorine and nitrogen on the polarization of adjacent carbon atoms make N, F-Carbon-1000 have better electrocatalytic performance. Interestingly, the N, F-Carbon-1000 exhibits the same onset potential (0.97 V vs. RHE) and half-wave potential (0.84 V vs. RHE) with commercial 20% Pt/C and a slightly lower limiting current density (5.2 mA cm-2). To our best knowledge, the N, F-Carbon-1000 sample as a metal-free catalyst exhibits better electrocatalytic properties in alkaline media, compared to the previously reported metal-free electrocatalysts 6, 50-54. For a comprehensive comparison, we also summarized in Table 2 the 9



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representative results from the previously reported excellent carbon ORR catalysts containing the N-doped carbon, N,S-codoped carbon, N, P-codoped carbon and N, F-codoped carbon catalysts. Apparently, the onset potential of N, F-Carbon-1000 is almost the same as commercial 20% Pt/C, VACNT, N, P-GCNS and WHC-700, and it is higher than most other electrocatalysts in Table 2. Moreover, the half-wave potential of N, F-Carbon-1000 is also among the highest level of the metal-free catalysts, and is obviously higher than the N-doped counterparts. This observation indicates the successful codoping of N and F atoms in porous carbon matrix is greatly beneficial for obtaining the high efficient metal-free ORR electrocatalysts, due to the synergic effects of N and F atoms with different electronegativities. Compared to N-Carbon-1000 sample, after N, F codoping, the onset potential of the catalyst shows a positive shift of 110 mV, and the half-wave potential shows a positive shift of 180 mV as well as the limiting current density increases by 1.2 mA cm-2. Obviously, N, F codoping is a critical factor for N, F-Carbon-1000 catalyst to gain a higher ORR activity. Therefore, in next text, we paid more attention on N, F-Carbon-1000 catalyst. The cyclic voltammetry was further tested in 0.1 M KOH aqueous solution saturated with N2 and O2 gas at room temperature to evaluate the activity of N, F-Carbon-1000 electrocatalysts (Fig. 4b). Only featureless curve within the range of from 0 to 1.0 V (vs. RHE) was observed in N2 saturated solution, while an oxygen reduction peak as high as 0.83 V emerges in the curve of O2 saturated solution, which is more positive than the most metal-free catalysts.13, 14, 51, 55 The LSV curves of N, F-Carbon-1000 at different rotation rates from 400 to 2025 rpm were shown in Fig. 4c. The current increases with the rotation rate because of the shortened diffusion layer. We have also explored the ORR performance 10


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of N, F-Carbon-1000 in acidic media, and the LSV curve was shown in Figure S2. However, the ORR activity of N, F-Carbon in acidic media is basically the same as the previously reported metal-free electrocatalysts,56-58 and still needs further improvement. The RDE data were analyzed using K-L equation. The K-L plots of N, F-Carbon-1000 catalyst at different potentials of 0.4, 0.5 and 0.6 V (vs. RHE) were shown in Fig. 4d. The well-defined linear relationship of K-L plot suggests first-order reaction kinetics related to the oxygen concentration.59 The exact electron transfer numbers (n) calculated from K-L equation are 3.94, 3.98 and 3.92 at three different potentials, respectively, suggesting a four-electron dominated pathway of ORR on N, F-Carbon-1000 with H2O as the main product. As shown in Fig. 4e, the Tafel slope (129 mV dec-1) of N, F-Caron-1000 sample is a little higher than that of commercial 20% Pt/C (118 mV dec-1), suggesting that the intrinsic ORR activity and kinetics of N, F-Carbon-1000 is close to the 20% Pt/C. We also performed rotating ring-disk electrode tests to calculate the percentage of peroxide formation. The H2O2 yield relative to the total products for N, F-Carbon-1000 is below 10% in the potential range from 0 to 0.7 V vs. RHE (Fig. 4f), which is beneficial for the good performance of the sample because relatively high yield of H2O2 will accelerate the degradation of the membrane electrode assemblies and further reduce the cell performance60, 61. High durability is another important feature for high-performance electrocatalysts. Continuous potential cycling tests in O2-saturated 0.1 M KOH solution were performed to evaluate the capacitance stability of N, F-Carbon-1000 sample. As shown in Fig. 5a, there is no significant current decrease in the CV curve after 10,000 circles. The half-wave 11



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potential shows a negative shift of ~10 mv compared to the initial one (Fig. 5b). We also performed 18,000 s chronoamperometry tests for both N, F-Carbon and commercial 20% Pt/C toward ORR by a constant voltage of 0.6 V (vs. RHE) at the rotation rate of 1000 rpm. As shown in Fig. 5c, the current densities of both catalysts decrease with time, but N, F-Carbon-1000 catalyst still retains a high relative current of 91% while only 75% of the initial current density is maintained for commercial 20% Pt/C catalyst. In addition, in real application, the crossover effect of ORR electrocatalyst should be considered. Thus, we add methanol into O2-saturated solution at 200 s to measure the ORR selectivity of N, F-Carbon-1000 and commercial 20% Pt/C against the electrooxidation of methanol (Fig. 5d). An obvious drop of the current density was observed for commercial 20% Pt/C, while only little change happens for N, F-Carbon-1000 catalyst. Thus, N, F-Carbon catalyst possesses not only better durability but also higher crossover resistance than commercial 20% Pt/C, which indicates that N, F-Carbon-1000 may be an excellent metal-free electrocatalyst for ORR in fuel cells. 4. Conclusions In summary, we have proposed a facile and effective approach for synthesis of N, F-codoped porous carbon catalysts. The as-prepared N, F-Carbon-1000 sample exhibits not only a similar ORR activity but also a better durability and higher crossover resistance compared to commercial 20% Pt/C catalyst. The high electrocatalytic activity of N, F-Carbon-1000 catalyst could be ascribed to the synergistic effect of N and F, high BET surface area and hierarchical pore structure with the existence of both micropore and mesopore. We believe that this work provides a new strategy for in situ synthesis of N, 12


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F-codoped porous carbon as high efficient metal-free electrocatalyst for ORR in fuel cells. ADDITIONAL INFORMATION Supporting Information Detailed structure characterization and electrochemical measurements of as-synthesized porous carbons, Pore size distribution of as-synthesized samples and the LSV curves in acidic medium. Supplementary information is available free of charge on the ACS publications website http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by National Science Fund for Distinguished Young Scholars (No. 21625601) and the Major Project of NSF of China (No. 91334203) and Outstanding Talent Fund from BUCT.

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Table 1 Summary of porosity parameters of N-Carbon-1000, N, F-Carbon-700, N, F-Carbon-800, N,F-Carbon-900 and N, F-Carbon-1000 samples

SBETa (m2g-1)

Pore volumeb (cm3g-1)

N-Carbon-1000

989 453 523 436 838

0.691 0.378 0.375 0.277 0.581

N, F-Carbon-700 N, F-Carbon-800 N, F-Carbon-900 N, F-Carbon-1000 a

Pore size distribiution (nm)

0.060-0.119; 14-25 0.058-0.15 0.058-0.127 0.054;0.108;0.128 0.081-0.17;0.82-1.32

The specific surface area was calculated by the Brunauer-Emmett-Teller (SBET) method. SBET calculated in the region of

P/P0 =0.05 to 0.3. b Pore volume represents the total pore volume, determined at P/P0=0.9997.

Table 2 Summary of the electrochemical properties of catalysts in O2-saturated 0.1M KOH electrolyte. (All data in this work were taken from the LSV tests at a rotational speed of 1600 rpm with a catalyst loading of 255µg cm-2 and a potential window of 1.0 V)

Electrocatalyst

Onset Potential

Limiting current

Half-wave potential

[V] (vs. RHE)

Density [mAcm-2]

[V] (vs. RHE)

reference

N-Carbon-1000 N,F-Carbon-700

0.89 0.81

4.0 4.2

0.68 0.62

this work this work

N,F-Carbon-800

0.77

3.5

0.61

this work

N,F-Carbon-900

0.85

4.7

0.71

this work

N,F-Carbon-1000

0.97

5.2

0.84

this work this work

20% Pt/C

0.97

5.5

0.84

NCNC700/900

0.87

-

0.78

51

N,S,O-OMC

0.85

4.0

0.73

37

NGSH

0.88

4.8

0.71

28

Carbon-L

0.86

4.6

0.70

30

N-CNF aerogel

0.91

5.4

0.80

62

NG-800

0.97

3.8

0.78

63

WHC-700

0.98

4.0

0.85

64

NPMC-1000

0.94

4.2

0.85

19

N,P-GCNS

1.01

5.8

0.86

65

C-COP-4

0.93

5.4

0.78

15

VA-NCNT

0.97

-

0.83

6

N,S-CN

0.90

4.3

0.75

38

P-G

0.91

4

0.74

66

BP2000-NF

0.96

-

0.88

42

FN-C-1000

0.91

6.1

0.81

43

GO-PANi31-FP

0.93

-

0.81

54

NF-MG3

-

5.4

0.83

53

18


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Scheme 1. The synthesis illustration of N, F codoped porous carbon as ORR electrocatalyst

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Fig. 1 SEM images of samples. (a) PTFE/PANI, (b) N, F-Carbon-700, (c) N, F-Carbon-800, (d) N, F-Carbon-900, (e) N, F-Carbon-1000. (f) TEM image of N, F-Carbon-1000, (g) the distribution mapping of C, N and F of N, F-Carbon-1000. The scale bars in (a)~ (g) are 2 µm, 1µm, 200 nm and 100 nm, respectively.

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Fig. 2 (a) FT-IR spectra, (b) PXRD graph, (c) Raman spectra, (d) N2-adsorption isotherms of five samples.

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Fig. 3 (a) XPS graphs of five samples. High resolution N1s XPS spectra of (b) N, F-Carbon-700 and (c) N, F-Carbon-800, (d) N, F-Carbon-900 and (e) N, F-Carbon-1000. (f) The relative content of three types of nitrogen in four N, F-Carbon samples.

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Fig. 4 (a) Rotating-disk voltammograms of four N,F-Carbon catalysts, N-Carbon and 20% Pt/C (the catalyst loading was about 0.25 mg cm-2 for all samples) in O2-saturated 0.1 M KOH with a sweep rate of 5 mV s-1 at the rotating rate of 1600 rpm. b) CV curves of N, F-Carbon-1000 in O2-saturated (solid line) and N2-saturated (dashed line) in 0.1 M KOH at a sweep rate of 50 mV s-1.c) Rotating-disk voltammograms of N, F-Carbon-1000 under same conditions b) but at different rotating rates. d) The Koutecky–Levich (K–L) plots at different potentials (0.4, 0.5 and 0.6 V vs. RHE). e) The Tafel slopes of 20% Pt/C and N, F-Carbon-1000. f) Percentage of peroxide yield of N, F-Carbon-1000.

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Fig. 5 (a) CV curves of the N, F-Carbon-1000 electrode in oxygen-saturated 0.1 M KOH before and after a continuous potentiodynamic sweep for 10,000 circles at room temperature with a scan rate of 50 mv s-1; b) Linear-sweep voltammograms of N, F-Carbon-1000 in 0.1 M KOH under oxygen bubbling at a scan rate of 5 mV s-1 and electrode-rotation speed of 1600 rpm before and after a continuous potentiodynamic sweep for 10,000 circles; c) current-time (i-t) chronoamperometric response of N,F-Carbon-1000 and commercial 20% Pt/C electrodes at 0.4 V (vs. RHE) in O2 saturated 0.1 M KOH at a rotation rate of 1000 rpm; d) current-time (i-t) chronoamperometric response of N,F-Carbon-1000 and 20% Pt/C electrodes by adding methanol at about 200s.

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Table of Contents Graphics

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Nitrogen and Fluorine-Codoped Porous Carbons as Efficient Metal

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