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Composite of hierarchically porous N-doped carbon/carbon nanotube with greatly improved catalytic performance for oxygen reduction reaction Jinhui Tong, Wenyan Li, Lili Bo, Jiangping Ma, Tao Li, Yuliang Li, Qi Zhang, and Haiyan Fan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00463 • Publication Date (Web): 06 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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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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Composite

of

hierarchically

porous

N-doped

carbon/carbon nanotube with greatly improved catalytic performance for oxygen reduction reaction Jinhui Tong a,*, Wenyan Li a, Lili Bo b, Jiangping Ma a, Tao Li a, Yuliang Li a, Qi Zhang a, Haiyan Fan c [a]

Key Laboratory of Polymer Materials of Gansu Province; Key Laboratory of Eco-Environment-Related Polymer Materials Ministry of Education; College of Chemistry and Chemical Engineering, Northwest Normal University, 967 Anning East Road, Lanzhou 730070, Gansu, P. R. China.

[b]

College of Science, Gansu Agricultural University, No. 1 Yingmen village, Anning District, Lanzhou 730070, Gansu, P. R. China.

[c]

Department of Chemistry, School of Science and Technology, Nazarbayev University, 53 Kabanbay Batyr Avenue, Astana 010000, Kazakhstan.

Corresponding authors: [email protected] (Jinhui Tong) ;

Abstract In this work, a series of catalysts were synthesized by pyrolysis of in-situ formed hybrids of metal organic framework MIL-101(Fe) and polypyrrole (PPy) nanotube followed by acid etching. The obtained catalysts exhibit composite structure of hierarchically porous carbon and carbon nanotube, which endowed the catalysts high surface areas, plenty of active sites and high conductivity, and thus high electrocatalytic performance on oxygen reduction reaction (ORR). The optimum onset and half-wave potential of 17 mV and -116 mV (vs. Ag/AgCl) have been obtained, respectively, which is even 4 mV and 12 mV positive than commercial Pt/C (20%) in alkaline. The catalyst also exhibits superior long-term stability and durability against methanol. Kinetic investigations have shown that ORR on the catalyst tended to a more effective 4e- dominant transfer process, which makes it a promising non-precious metal ORR catalyst for fuel cell. Keywords: Hierarchically porous carbon; Carbon nanotube; Composite structures; Noble-metal free catalyst; Oxygen reduction reaction.

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Introduction As a kind of clean energy conversion device, proton exchange membrane fuel cells (PEMFCs) have attracted more and more attention over recent decades because of the high power density, high energy-conversion efficiency, and zero or low emission of pollutants.1-2 Up to now, at the cathode, the sluggish reaction kinetics for oxygen reduction reaction (ORR) is the major hurdle for PEMFCs. Pt/C is deemed as the best ORR electrocatalyst due to its high onset potential and preferred four-electron pathway.3 However, the limited reserve in nature and the resulting high-cost limits the wide commercialization of Pt based catalysts.4 In addition, Pt catalyst has shown vulnerability in tolerance for CO and methanol in the alkaline electrode.5 Consequently, much effort has been devoted to developing cheap and effective electrocatalysts for ORR in the past few decades.6-7 Metal-organic frameworks (MOFs), a porous crystalline hybrid materials constructed by organic linkers and metal clusters, have attracted great attention in a wide range of application such as gas storage,8 separation9 and catalysis,10 due to their tunability in compositions, structures and properties. Moreover, heteroatoms, usually acting as reaction activity sites, can be easily introduced by addition of heteroatom-containing organic molecules.11-13 Such outstanding features make MOFs emerge as promising candidates for ORR.14-15 However, the performance offered by MOFs only is still unsatisfactory. The hugest challenge that stops the ORR catalysts developed from the MOFs precursors being effective is likely the intrinsically poor electroconductive on the carbon skeletons. On the other hand, heteroatoms, especially nitrogen doped porous carbon materials have attracted great interests as supports or catalysts for ORR because of their high specific surface areas and good electrical conductivities.16-18 Therefore, recently, MOFs have emerged as a new prospect in synthesizing porous carbon, metal/carbon and metal oxide/carbon applied in electrocatalysis due to the tunable characteristics of porosity and surface area.19-20 Nevertheless, the nanocarbon materials derived from MOFs mainly involve micropores which are disadvantageous for diffusion of electrolytes and reactants.9 Heteroatoms, especially nitrogen doped carbon nanotubes (CNTs) and carbon nanofibers have been used as either efficient catalysts or the supports for ORR catalysts due to their high electrical conductivities.21-25 ORR catalysts synthesized with metal-free nitrogen-doped CNTs were discovered to exhibit fairly good 2

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electrocatalytic performance and impressive long term stability, with a transferred electron number (n) of 3.53-3.92 at the potential range from -0.1 to 0.2 V.26 Among various forms of nitrogen, such as pyridinic N, pyrrolic N, graphitic N and quaternary N, the pyridinic N was found the active sites in the ORR catalysts developed from the nitrogen-doped carbon nanomaterials.27-30 A series of hybrids of nitrogen-doped graphitic porous carbon and carbon nanotubes (NGPC/NANTs) prepared using MOF-5 and urea by Hong et al. exhibited superior catalytic activity, durability and methanol tolerance.31 Zhu et al. prepared an ORR electrocatalyst using in situ growth of metal-organic frameworks on carbon nanotubes, and attributed its excellent performance to the formation of 3D structured porous and N doped carbon/carbon nanotubular composites.32 While pyridinic N have planer structures, the quaternary N have a 3D structure. It is generally believed that the 3D structure brought by quaternary N atoms would interrupt the π˗π conjugation of N-doped nanostructure. In the present work, we intend to increase the content of pyridinic N and try to avoid the quaternary N in order to achieve a higher ORR activity.33 On the other hand, to improve the catalytic activities of nitrogen-doped carbon catalysts, active sites density in the catalysts must be effectively increased. In this respect, the most desirable nitrogen-doped carbon material should be endowed high accessible surface area, plenty of active sites, facile reactant and product transport, and high electron conductivity, which strongly depend on the pore structure and graphitization-degree of the catalysts, as well as the type of precursors used.22 Based on the above consideration, a series of carbon materials with composite structure of hierarchically pores and nanotube were synthesized by pyrolyzing hybrids of MIL-101(Fe) in situ growth on PPy nanotubes instead of commercial carbon tube. The composites of hierarchically porous N-doped carbon/carbon nanotube were prepared and the electrochemistry performances of the as-prepared catalysts were investigated for ORR in detail. The catalyst displayed a desirable combination of high active sites density, high electric conductivity and large accessible surface area, resulting in much positive onset and half-wave potentials, excellent electrochemistry stabilities and great tolerances to methanol. Experimental section Chemicals and reagents 3

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Pt/C (20 wt% Pt on carbon black) and Nafion (5 wt%) were brought from Alfa Aesar. All other chemicals and reagents are of analytical grade and were used as received. Synthesis of catalysts The polypyrrole nanotube was synthesized according to the previous report.34 Briefly, methyl orange (0.784 g) and FeCl3·6H2O (3.888 g) were dissolved in 480 mL water, and then pyrrole was dropped slowly into the solution under stirring. After maintaining above mixture agitated for 24h at room temperature, the black product was filtrated. The solid product was washed with water and ethanol for several times and dried in vacuum at 80 oC overnight. The precursor of MIL-101(Fe)/PPy nanotube was prepared by in situ growth of MIL-101(Fe) on PPy nanotube. Briefly, certain amount of polypyrrole nanotube was added to DMF containing FeCl3·6H2O and terephthalic acid (H2BDC) (molar ratio is 2:1). The mixture was then hydrothermally treated at 110 °C for 20h. After that, the solid was filtrated with Buchner funnel and washed with water and ethanol for several times, and then dried in vacuum at 80 oC overnight. Three precursors with different content of MIL-101(Fe) were prepared and denoted as 10%-MIL/PPy, 30%-MIL/PPy and 50%-MIL/PPy, respectively. The catalysts were obtained by pyrolysis of above precursors in N2 atmosphere at different temperature (700, 800 and 900 oC) for 3h with a ramp of 3 oC/min, and then treated with 1M HCl for 6 hours three times to remove iron species at 80 oC. Finally, the samples were brought to neutral pH by washing with water and ethanol and dried in vacuum at 80 oC overnight. Five catalysts were obtained and designated as 10%MIL/PPy-800, 30%MIL/PPy-700, 30%MIL/PPy-800, 30%MIL/PPy-900 and 50%MIL/PPy-800, respectively. The pyrolyzed product of PPy nanotube and MIL-101(Fe) at 800 oC was denoted as PPy-800 and MIL-800, respectively. Physical characterization of the catalysts Scan electron microscopy (SEM) images were obtained using a field-emission scanning electron microscope (JSM-6701F, FEOL) with an accelerating voltage of 5 4

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kV. Transmission electron microscopy (TEM) images were taken on a JEM-2010 transmission electron microscope operated at an accelerating voltage of 200 kV. The X-ray power diffraction (XRD) patterns of the samples were collected on the Shimadzu XRD-6000 diffractometer with Cu Kα radiation (λ=1.54178Å) at 40 kV and 200 mA of the operation voltage and current, respectively. Nitrogen adsorption experiments were performed with ASAP 2020 Micromeritics Instrument at 77 K, each sample was degassed at 350 oC for 4 h prior to the measurement. The specific surface areas of the catalysts were analyzed from the adsorption data using the Brunauer–Emmett–Teller (BET) method and the pore size distributions were calculated

from

the

desorption

branch

isotherms

using

the

BJH

(Barrett–Joyner–Halenda) method. The X-ray photoelectron spectroscopy (XPS) was obtained at a Thermo Fisher Scientific’s K-Alpha X-ray photoelectron spectrometer. Electrochemical tests of the catalysts For electrochemical testing, a CHI electrochemical workstation equipped with the software Versa Studio was used. A three-electrode cell assembly was used including 3 mm GC electrode or 3 mm GC rotating disk electrode (RDE) as working electrode, Ag/AgCl (in 3M KCl solution) as reference electrode, and graphite rod as counter electrode for cyclic voltammetry (CV) and linear sweep voltammetry (LSV) tests. The ink for test were prepared as following: 3.0 mg of the catalyst was dispersed in 0.5 mL of ethanol containing 10 µL of 5 wt% Nafion solution, and then the mixture was ultrasonically treated for more than 30 min. Afterwards, 3 µL of the above solution was dropped onto the surface of the well polished working electrode. The electrode was dried at room temperature and put for CV and LSV tests in the potential range of 0.2 V to -0.8 V versus Ag/AgCl electrode with a scan rate of 10 mV/s in oxygen-saturated 0.1 M KOH aqueous solution at room temperature. The electron transfer number (n) and kinetic current density (Jk) were calculated according to the well-known Koutecky-Levich equation: 1/J= 1/JL+1/JK=1/(Bω1/2)+1/Jk

(1)

B=0.62nFC0D02/3v-1/6

(2)

where J is the measured current density, JK and JL are the kinetic and diffusion-limiting current densities, respectively, ω is the angular velocity, n is transferred electron number, F is the Faraday constant (96485 C/mol), C0 is the bulk 5

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concentration of O2 (1.2×10-6 mol/cm3), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10-5 cm2/s), and ν is the kinematic viscosity of the electrolyte (0.01 cm2/ s). The electron transferred number (n) as well as the HO2- intermediate production percentage (HO2-%) can also be obtained from the rotating ring disk LSV based on the following equations: n=

4I D I ID + R

(3)

N IR

2

% HO = 200 × ID +

N IR

(4)

N

Where ID, IR and N (= 0.424) are the disk current, ring current and collection efficiency, respectively for the employed RRDE.

Results and discussion Catalyst characterization The XRD patterns of the as-prepared catalysts were shown in Fig. 1. The broad diffraction peaks at 2θ around 26° and 43° can be attributed to the (002) and (100) planes of graphitic carbon,31 respectively, which confirms that the samples were graphitized in the process of pyrolysis.

MIL-800

Intensity(a.u.)

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PPy-800 10%MIL/PPy-800 30%MIL/PPy-700 30%MIL/PPy-800 30%MIL/PPy-900 50%MIL/PPy-800

10

20

30

40

50

60

70

80

90

2θ (degree) Fig. 1 XRD patterns of the as-prepared catalysts. The Raman spectra of the catalysts were shown in Figs. 2. The D band, which presents the disordered structure or structure defects on the graphitic plane in carbon 6

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materials, appeared around 1360 cm-1. The G band, which presents the degree of graphitization, appeared around 1590 cm-1. The ID/IG ratio is 0.837, 0.839, 0.851, 0.915, 0.947, 0.930 and 0.874 for the sample MIL-800, PPy-800, 10%MIL/PPy-800, 30%MIL/PPy-700, 30%MIL/PPy-800, 30%MIL/PPy-900 and 50%MIL/PPy-800, respectively. All the ratios are in the range of 0.8−1.0, which confirms the presence of crystalline graphitic carbon in the samples. The highest ID/IG ratio of 0.947 was obtained on the sample 30%MIL/PPy-800, which indicates more structure defects existing in the sample. Generally, more structure defects can bring more exposed active sites and thus endow the catalyst higher catalytic performance.

600

MIL-800 PPy-800 10%MIL/PPy-800

a Intenstity (a.u.)

Intenstity (a.u.)

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800 1000 1200 1400 1600 1800 -1 Raman Shift (cm )

600

30%MIL/PPy-700 30%MIL/PPy-800 30%MIL/PPy-900 50%MIL/PPy-800

b

800 1000 1200 1400 1600 1800 Raman Shift (cm-1)

Fig. 2 Raman spectra of the as-prepared samples.

The SEM (Fig. 3a) and TEM (Fig. 3c) images of PPy-800 show that the sample is composed of nanotubes with the diameter from 80 nm to 120 nm, around 10 nm in wall tickness and several micrometer in length, respectively. The SEM of MIL-800 (Fig. 3b) shows a kind of tousy and hierarchically porous material. The SEM image of the sample 30%MIL/PPy (Fig. 3d) shows a mace like composite with MIL-101(Fe) particles growing on PPy nanotubes. The TEM images of the sample 30%MIL/PPy-800 shows a composite structure of hierarchically porous MIL-800 wrapped PPY-800 carbon nanotubes (Fig. 3e-f).

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a

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b

c

d

e

f

Fig. 3 (a) SEM image of PPy-800; (b) SEM image of MIL-800; (c) TEM image of PPy-800; (d) SEM image of 30%MIL/PPy; (e) TEM image of 30%MIL/PPy-800 and (f) Two times magnification image of (e).

To

investigate

the

specific

surface

area

and

pore

distribution,

N2

adsorption-desorption experiments were performed for the as-prepared catalysts. The isotherm for 30%MIL/PPy-800 was shown in Fig. 4 and those for other hybrid catalysts were shown in Fig. S1. The BET surface areas and pore volumes of the samples were summarized in Table 1.

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800 3.7 nm

0.6

3

dV/dD (cm /nm/g)

0.8

700

3

Adsorbed N2Volume (cm /g STP)

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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600 500

0.4 1.4 nm

0.2 0.0 0

20

400

40 60 80 Pore diameter (nm)

100

300 200

30% MIL-PPy-800

100 0.0

0.2

0.4

0.6

0.8

1.0

(Relative pressure P/Po)

Fig. 4 N2 adsorption-desorption isotherms and corresponding pore size distribution plots (inset) of 30%MIL/PPy-800. As depicted in Fig. 4 and Fig. S1, all the samples exhibited type-II isotherms, which indicates the wide distribution in pore sizes including micropores with diameter of around 1.4 nm, mesopores with diameter of around 3.7 nm and macropores with diameter above 60 nm. The appearance of hysteresis between adsorption and desorption branches demonstrates the existence of mesopores, especially tubular. The data in Table 1 shows that all the samples have high BET surface area and pore volume. It also suggests a decrease in BET surface area and pore volume with increase of MIL-101(Fe) loading and pyrolysis temperature caused by degradation and agglomeration of the moieties, especially MIL-101(Fe) at high temperature.35-36 The hierarchically porous and tubular composite structures render the carbon materials to be a useful electrocatalyst and facilitates the diffusion of molecular oxygen during the process of ORR.35 Table 1 The specific surface areas and pore volumes of the catalysts Catalyst BET Surface area ( m²/g) Pore Volume (cm3/g)

10%MIL-

30%MIL-

30%MIL-

30%MIL-

50%MIL-

PPy-800

PPy-700

PPy-800

PPy-900

PPy-800

1241

921

892

722

584

1.8

1.3

1.2

1.0

0.7

9

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XPS analyses were performed to determine the atomic contents and chemical identities of C and N. The XPS survey spectrum of 30%MIL/PPy-700, 30%MIL/PPy-800 and 30%MIL/PPy-900 were listed in Fig. S2. The high-resolution C1s and N1s spectrum of the sample 30%MIL/PPy-800 were shown in Fig. 5, and those of the samples 30%MIL/PPy-700 and 30%MIL/PPy-900 were shown in Fig. S3-S4. The element content and atomic content of various surface C and N of the samples were shown in Table 2-4. Graphitic C

Pyridinic-N

a

C in C−N C in C−O

282

284

286

288

290

292

Graphitic-N

Intensity (a.u.)

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

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294

296

298

b

N-oxide

394

396

398

400

402

404

406

408

410

Binding Energy (eV)

Binding Energy (eV)

Fig. 5 High-resolution XPS spectra of C1s (a) and N 1s (b) of the sample 30% MIL/PPy-800. It can be seen from Table 2 that with increase of pyrolysis temperature, the C content increased while the N and O contents decreased due to cleavage of C-N and C-O bonds at high temperature. The samples exhibited high N content, especially for the samples 30%MIL/PPy-700 and 30%MIL/PPy-800, more than 5% of N was detected. The high-resolution C1s spectra of the samples can be deconvoluted to three peaks ascribed to graphitic C (ca. 284.6 eV), C in C-N bonds (ca. 285.4 eV) and C in C=O bonds (ca. 287.5 eV), respectively.37-38

Table 2 Element content of the samples determined by XPS analysis. C (content

N (content

O (content

at%)

at%)

at%)

30%MIL/PPy-700

86.43

6.95

6.62

30%MIL/PPy-800

89.66

5.04

5.30

30%MIL/PPy-900

95.73

1.14

3.13

Sample

10

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Table 3 shows that the three samples contained more than 60% of graphitic C, which indicates that the samples have high electron conductivities. The high-resolution N 1s spectra of the samples can be deconvoluted to four peaks ascribed to pyridinic N (ca. 398.3 eV), graphitic N (ca. 400.7 eV), Quaternary N (ca. 401.7 eV) and N in N-oxide (ca. 403.2 eV), respectively.27-29 As can be seen from Table 4, the pyrolysis temperature greatly influences the content and identity of N in the samples. As s result, the samples 30%MIL/PPy-700 and 30%MIL/PPy-800 contained higher than 40% of pyridinic N and graphitic N without quaternary N while the sample 30%MIL/PPy-900 contained 53.1% of quaternary N without N-oxide. The sample 30%MIL/PPy-800 contained the highest 46.1% of pyridinic N which is considered generally the active sites in the ORR catalysts developed from the nitrogen-doped carbon materials.

Table 3 Relative integrated intensities (%) various surface carbon based on XPS spectra C in graphitic C-C

C in C-N

C in C=O

(ca. 284.6 eV)

(ca. 285.4 eV )

(ca. 287.5 eV )

30%MIL/PPy-700

66.3

23.5

10.2

30%MIL/PPy-800

60.6

26.9

12.5

30%MIL/PPy-900

66.0

22.6

11.4

Sample

Table 4 Relative integrated intensities (%) of various surface nitrogen based on XPS spectra Pyridinic N

Graphitic N

Quaternary N

N in N-oxide

(ca. 398.3 eV)

(ca. 400.7 eV)

(ca. 401.7 eV)

(ca. 403.2eV)

30%MIL/PPy-700

40.0

40.8



19.2

30%MIL/PPy-800

46.1

42.0



11.9

30%MIL/PPy-900

13.9

33.0

53.1



Sample

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Electrocatalytic activity for ORR

To investigate the electrocatalytic activities of the as-fabricated catalysts, LSV measurements were performed in 0.1M KOH with a scan rate of 10 mV/s (Fig. 6a-c). To get more detailed information of reaction kinetics, the mass transport corrected Tafel plots were obtained by plotting potential versus logjk39 (Fig. 6d) and the Tafel slopes were obtained by linear fitting of the corresponding Tafel plots. The corresponding data characteristic of the electrocatalytic activity were shown in Table 5. It can be seen clearly that all the hybrid materials showed more positive onset potential, half-wave potential and limiting current density than PPy-800 and MIL-800 (Fig. 6a-b). That is, composite of hierarchically porous with nano-tubular structure can greatly facilitate the improvement of the catalytic performances of the catalysts. The pyrolysis temperature and proportion of MIL-101(Fe)/PPy were also optimized and the results were shown in Fig.6b. Based on Fig. 6b, the 30%MIL/PPy-800 exhibits the most positive onset potential and half-wave potential as well as the highest limiting current density compared to the rest of hybrids, indicating that 30% is the optimal primary weight ratio for MIL/PPy and 800 oC is the best pyrolysis temperature. LSV measurement was used to compare the activity of 30%MIL/PPy-800 with commercial Pt/C (20%) (Fig. 6c). The onset and half-wave potential for 30%MIL/PPy-800 reach 17 mV and -116 mV, and are 4 mV and 12 mV more positive than those of Pt/C at the scan rate of 10 mV/s, although the Pt/C catalyst harvests the larger limiting current density of 4.7 mA cm-2. To the best of our knowledge, this ORR activity of 30%MIL/PPy-800 is among the highest reported activities of various types of carbon materials including carbon nanotubes based non-precious metal catalysts (Table S1). The Tafel slopes for the as-prepared samples ranged from -72 mV/decade to -130 mV/decade (Table 5). The Tafel slope on the 30%MIL/PPy-700 sample is -72 mV/decade, which is close to the “typical” value of -60 mV/decade indicating the transfer of the first electron as a rate-determining step and Temkin conditions of intermediate adsorption.39 However, the Tafel slopes on the other samples are close to the “typical” value of -120 mV/decade indicating the first discharge step as rate-controlling step under Langmuir conditions of adsorption.39 The exchange current 12

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density (J0) was obtained by extrapolating the linear fitted Tafel plot to 0.26 V vs. Ag/AgCl (1.23 V vs. RHE), which is the theoretical equilibrium potential for ORR. 40 The 30%MIL/PPy-800 and 30%MIL/PPy-900 obtained the highest J0 of 1.46×10-3 and 1.40 ×10-3 mA/cm2, which is 1.4 times as high as that of Pt/C (1.04×10-3 mA/cm2), much higher than those for the other composite catalysts and even 1-2 order of magnitude higher than that for PPy-800 (8.03×10−6 mA/cm2) and MIL-800 (1.54×10−6 mA/cm2), reflecting greatly enhanced intrinsic ORR rate on the 30%MIL/PPy-800

0

Current density (mA/cm2)

Current density (mA/cm2)

and 30%MIL/PPy-900 catalysts .41

a

-1 -2 -3 -4 -5 -0.8

30% MIL/PPy-800 MIL-800 PPy-800

-0.6 -0.4 -0.2 0.0 0.2 Potential (V vs. Ag/AgCl)

c

-1

12 mV

-2 -3 -4 -5 -0.8

30% MIL/PPy-800 Pt/C

-0.6 -0.4 -0.2 0.0 Potential (V vs. Ag/AgCl)

0.2

Potential (V vs Ag/AgCl)

0

1

b

10% MIL/PPy-800 30% MIL/PPy-800

0

50% MIL/PPy-800

-1

30% MIL/PPy-700 30% MIL/PPy-900

-2 -3 -4 -5 -0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs. Ag/AgCl)

1 Current density (mA/cm2)

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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0.15 30% MIL/PPy-800 MIL-800 0.10 50% MIL/PPy-800 PPy-800 30% MIL/PPy-700 10% MIL/PPy-800 0.05 30% MIL/PPy-900 0.00 Pt/C (20%) -0.05 -0.10 -0.15 -0.20 -0.25 -0.30 -0.35 -0.40 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 2 Log [jk(mA/cm )]

d

Fig. 6 (a-c): LSV curves of the as-prepared catalysts and commercial Pt/C (20%) in O2-saturated 0.1M KOH at 1600 rpm with a scan rate of 10mV/s (The background of double layer charging/discharging currents have been corrected); (d) Tafel plots based on the LSV curves.

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Table 5 The characteristic data for electrocatalytic activity of the samples Onset

Half-wave

Exchange

potential

potential

Tafel slope

current

(mV vs

(mV vs

(mV/decade)

density

Ag/AgCl)

Ag/AgCl)

MIL-800

-213

-516

-123

1.54×10-6

PPy-800

-124

-316

-122

8.03×10-6

10%MIL/PPy-800

-63

-203

-113

9.95×10-4

30%MIL/PPy-800

17

-116

-105

1.46×10-3

50%MIL/PPy-800

-95

-211

-113

8.23×10-4

30%MIL/PPy-700

-59

-155

-72

2.10×10-4

30%MIL/PPy-900

-90

-293

-130

1.40×10-3

Pt/C (20%)

13

-128

-108

1.04×10-3

electrocatalytic

activities

Sample

Based

on correlation

between

(mA/cm2)

and

physical

characterizations of the catalysts, it can be concluded that composite structure of hierarchically

porous/nanotube,

which

can

be

contributed

to

the

higher

electrochemically accessible surface area and facile reactant and product transport, is of key importance for improvement of the catalytic activities based on the fact that all the hybrid materials exhibited much higher catalytic activities for ORR than the two moieties of MIL-800 and PPy-800. High content of N, especially pyridinic N which can supply high density of active sites, is another most important factor for high catalytic activity. As can be seen, higher content of pyridinic N endowed the sample 30%MIL/PPy-800 much higher catalytic activity than 30%MIL/PPy-700 although the two samples are comparative in other physical properties. Besides, high specific area and degree of graphitization are generally considered important for exposure of active sites and high conductivity. Eventually, the excellent electrocatalytic activity was achieved by the 30%MIL/PPy-800 with a synergistic contribution of above factors. In 14

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addition, although no detectable Fe content as confirmed by XPS analysis, trace Fe species may still be retained in the obtained samples even after treated with HCl. As reported previously, the presence of Fe species during carbonization helps increasing the catalytically active N–C sites such as pyridinic-N and pyrrolic-N species, which can more efficiently facilitate the ORR activity. Fe can also increases the graphitic phase of prepared carbon, which improves electrical conductivity of the carbon framework.42 It also suggested that the Fe–N related species are responsible for the high ORR activities of the Fe-contained catalysts.43 The above conclusions are also supported by poor activity of the Fe-free PPy-800 and the N-free MIL-800 in our work. The cathode material for ORR is also significant in the reaction kinetics as indicated by Density Function Theory (DFT) modeling that pyridinic N enhances O2 adsorption on the neighboring carbon atoms, thus promotes a four electron process of ORR.44 In order to further study the ORR kinetics, the polarization curves of the optimum catalyst 30%MIL/PPy-800 was measured on rotating disk electrode (RDE) at different rotation speeds from 400 rpm to 2500 rpm with a scan rate of 10 mV/s. Fig. 7a shows the current density increases with the increase in the rotating speed from 400 to 2500 rpm, due to the enhanced oxygen flux to the electrode surface at high rotating speed.45 The curves also show relatively wide plateaus below -0.2 V at all rotating speeds. The corresponding Koutecky-Levich plots show good linearity at various potentials indicating a first-order reaction kinetic with regard to the concentration of dissolved oxygen46 (Fig. 7b). Fig. 7c presents the number of transferred

electron

(n)

under

different

potential

calculated

using

the

Koutecky-Levich (K-L) equations. The average n is 3.86 and the largest value is 3.99 at the potential of 0.5 V, indicating an approximate 4e- ORR pathway. Generally, the 4e- process is considered to be more efficient for ORR.12 To quantify the ORR electron transfer pathway, a RRDE was employed, with which the amount of HO2- generated at the disk electrode could be accurately determined.42 Fig. 8a shows the LSV of RRDE over 30%MIL/PPy-800 indicating the production of only a minimal amount of peroxide species during oxygen reduction. Fig. 8b shows hydrogen peroxide yields (%) and the number of electrons (n) 15

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transferred during ORR. The 30%MIL/PPy-800 sample shows an electron transfer number of 3.7–3.9 with its peroxide yield of below 15% in a wide potential range of -0.1 to -0.8 V vs. Ag/AgCl. This is consistent with the result obtained from the Koutecky-Levich plots based on RDE measurements, suggesting the ORR catalyzed by our 30%MIL/PPy-800 is mainly by 4e- reduction. Current density (mA/cm2)

0 -1

a

-2 -3 400 900 1600 2500

-4 -5 -0.8

-0.6 -0.4 -0.2 0.0 Potential (V vs.Ag/AgCl)

0.50

b

0.40 0.35

0.2

4.2

-0.30 V -0.35 V 0.30 -0.40 V 0.25 -0.45 V -0.50 V 0.20 0.020 0.025 0.030 0.035 0.040 0.045 0.050 ω-1/2(rpm-1/2)

Transferred electron (n)

0.45 -1 -1 2 J ( mA cm )

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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c

3.96

4.0

3.8

3.77

3.78

3.99

3.79

3.6 0.30

0.35 0.40 0.45 0.50 Potential (V vs.Ag/AgCl)

Fig. 7 (a):LSV polarization plots of 30%MIL/PPy-800 in O2-saturated 0.1M KOH at different RDE rotation rates with a scan rate of 10mV/s; (b): Koutecky-Levich plots of 30%MIL/PPy-800 based on (a); (c): The transferred electron number (n) based on (b).

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Transferred electron (n)

0.012

a

0.009

Current(mA)

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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0.006

Ring

0.003 0.000 0.0 -0.1

Disk

-0.2

b

3.9

-0.6

-0.4

-0.2

0.0

Potential (V vs. Ag/AgCl)

0.2

14

3.8

12

3.7

10

3.6

8

3.5

Transferred electron Peroxide percentage

3.4

6 4

3.3

-0.3 -0.8

16

4.0

Peroxide percentage

Page 17 of 24

2 -0.8

-0.6 -0.4 -0.2 Potential (V vs.Ag/AgCl)

Fig. 8 (a): RRDE LSV recorded on 30%MIL/PPy-800 in O2-saturated 0.1M KOH at 1,600 rpm. The disk potential was scanned at 10 mV/s and the ring potential was constant at 0.44 V vs. Ag/AgCl (1.40 V vs. RHE). (b): Percentage of peroxide and the electron transfer number (n) of 30%MIL/PPy-800 at various potentials based on the corresponding RRDE data in (a). An effective ORR catalyst is expected to be directly applied to the methanol fuel cells with significant stability. The stability and tolerance toward the methanol was tested on 30%MIL/PPy-800 and Pt/C. The long-term stability of the catalysts was tested by continuous CV with a scan rate of 100 mV/s. Fig. 9a shows that 30%MIL/PPy-800 remains stable in O2-saturated 0.1M KOH even after 10000 CV cycles. As can be seen that only slight decrease in limiting current density was observed without apparent negative shift in onset potential and half-wave potential. However, the oneset potential, halfwave potential and limiting current density dramatically dropped from 13 mV to -107 mV, -128 mV to -174 mV and -4.6 mA/cm2 to -1.7 mA/cm2 in the case of Pt/C, respectively (Fig. 9b). The results show that the 30%MIL/PPy-800 sample is much more stable than Pt/C in alkaline conditions. When methanol was added, the ORR peak for 30%MIL/PPy-800 negatively shifted from -82 mV to -102 mV (Fig. 9c). While in the case of Pt/C, the cathodic peak for ORR has vanished and the methanol oxidation peak at -101.9 mV appeared instead (Fig. 9d). The test convinces that 30%MIL/PPy-800 can be potentially applied to the methanol fuel cells.

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2

Current density (mA/cm

Initial After 10000 cycles

-1 -2 -3 -4

0.3 0.0 -0.3 -0.6 N2-saturated 0.1M KOH

-1.2

O2-saturated 0.1M KOH

-1.5

b

-2

Initial After 10000 cycles

-3 -4

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs. Ag/AgCl)

c

-0.9

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-1

-5 -0.8

-0.6 -0.4 -0.2 0.0 0.2 Potential (V vs. Ag/AgCl)

Current density (mA/cm2)

0.6

0

)

a

0

-5 -0.8

Current density (mA/cm2)

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

Current density (mA/cm2)

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N2-saturated 0.1M KOH

6

O2-saturated 0.1M KOH

d

O2-saturated 0.1M KOH+

4

3M CH3OH

2 0

O2-saturated 0.1M KOH+3M CH3OH

-0.8

-0.6

-0.4

-0.2

0.0

0.2

Potential (V vs. Ag/AgCl)

0.4

-0.8

-0.6 -0.4 -0.2 0.0 E / V(vs. Ag/AgCl)

0.2

Fig. 9 LSV curves of the first and the one after 10000 CV cycles of 30%MIL/PPy-800 (a) and Pt/C (20%) (b) in O2-saturated 0.1M KOH at 10 mV/s; CV measurements of 30%MIL/PPy-800 (c) and Pt/C (20%) (d) for tolerance to CH3OH at a scan rate of 10 mV/s. Conclusions In summary, we have developed a simple but highly effective electrocatalyst for ORR using hybrid of MIL-101(Fe)/PPy as precursor. The composite structure of hierarchically porous and nanotube endowed the catalyst high accessible surface area and facile diffusion of electrolytes and reactants. The high content of N, especially up to 46.1% of pyridinic N supplied high density of active sites for the catalyst. The synergistic contribution of all the above factors endued the sample superior catalytic activity compared with commercial Pt/C (20%) and robust cycling durability and high tolerance toward the methanol crossover. More notably, the oxygen reduction reaction on the catalyst in 0.1M KOH follows the closed 4e- pathway indicating a complete reduction of oxygen into water. Thus, this hybrid is a promising non-precious metal ORR electrocatalyst that can be potentially applied to the fuel cells. 18

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Supporting Information N2 adsorption-desorption isotherms and corresponding pore size distribution plots (inset)

of

10%MIL/PPy-800,

30%MIL/PPy-700,

30%MIL/PPy-900

and

50%MIL/PPy-800; XPS survey spectrum of 30%MIL/PPy-700, 30%MIL/PPy-800 and 30%MIL/PPy-900; High-resolution spectra of C1s and N1s for 30%MIL/PPy-700 and 30%MIL/PPy-900; Comparison the catalytic performances of 30% MIL/PPy-800 with other analogous catalysts reported in 0.1M KOH. Acknowledgment The authors are grateful to the National Natural Science Foundation of China (21363021, 51302222), the Program for Students Academic Innovative Research of Northwest Normal University and Program for Changjiang Scholars and Innovative Research Team in University (IRT15R56) for financial support.

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(For Table of Contents Use Only) Composite catalysts of hierarchically porous N-doped carbon/carbon nanotube were facilely synthesized and the optimized catalyst exhibits high electrocatalytic performance on oxygen reduction reaction.

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