Synthesis of MOF-Derived Nonprecious Catalyst with High

Subscriber access provided by University of South Dakota

Kinetics, Catalysis, and Reaction Engineering

Synthesis of MOF-derived Non-precious Catalyst with the high electrocatalytic activity for Oxygen Reduction Reaction Yan Luo, Jie Zhang, Maryam Kiani, Yihan Chen, Jinwei Chen, Gang Wang, Siew Hwa Chan, and Ruilin Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02744 • Publication Date (Web): 14 Aug 2018 Downloaded from http://pubs.acs.org on August 18, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

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.

Page 1 of 34 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

Industrial & Engineering Chemistry Research

3

Synthesis of MOF-derived Non-precious Catalyst with the high electrocatalytic activity for Oxygen Reduction Reaction

4

Yan Luo1, Jie Zhang1, Maryam Kiani1, Yihan Chen1, Jinwei Chen1*, Gang Wang1, Siew

5

Hwa Chan2, Ruilin Wang1*

6

1

7

China

8

2

9

Singapore

1 2

College of Materials Science and Engineering, Sichuan University, Chengdu 610065,

School of Mechanical & Aerospace Engineering, Nanyang Technology University,

10

11

Abstract: Production of non-precious catalysts with high electrocatalytic activity is one

12

of the promising paths to drive fuel cells’ commercialization. Herein, we present

13

rational tuning to prepare Metal-Organic-Framework (MOF)-derived non-precious

14

catalysts. Nanoporous MOF-derived catalysts were hierarchically prepared by using

15

functional carbon black assembled with Material Institut Lavoisier (MIL)-101(Fe) as a

16

precursor, followed by nitrogen doping and carbonized by pyrolysis. The resulting

17

catalysts contained iron and iron carbide nanoparticles encapsulated in the nitrogen-

18

enriched mesoporous carbons, which were connected to functional carbon black

19

(Fe/Fe3C@NC). The hierarchical structure led to electron conductivity improved,

20

higher active sites exposed and stronger synergistic effects. The optimal catalyst *Corresponding authors. Tel: 028 8541-8018. E-mail address: [email protected] (J. Chen), [email protected] (R. Wang)

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

exhibited superior oxygen reduction reaction (ORR) activity with onset and half-wave

2

potentials of 0.85 and 0.70 V vs RHE, respectively. Furthermore, it showed much

3

higher stability and better methanol tolerance than those of the state-of-the-art Pt/C.

4

This could offer a practical approach to synthesizing non-precious catalysts with high

5

ORR activity for fuel cell applications.

6

Keywords: Oxygen reduction reaction, Non-precious catalyst, Iron-nitrogen doped

7

carbon, Metal-Organic Framework, Nanoporous

8

9

1. Introduction

10

Proton exchange membrane fuel cells (PEMFCs) are efficient and environmentally

11

friendly energy conversion devices, which can proficiently convert the chemical energy

12

of fuels into electrical energy through oxygen reduction reaction (ORR) at cathode and

13

hydrogen oxidation reaction (HOR) at anode1, 2. However, due to the sluggish ORR

14

kinetics, a large amount of Pt will be used at the cathode side to meet the requirement3,

15

4

16

commercialization of PEMFCs5-7. A lot of attentions have been paid to synthesis of non-

17

precious catalysts in recent years, such as transition metal oxides8, nitrides9, carbides10,

18

11

19

nitrogen-doped carbon nanostructures (NCNs)15, 16. Among them, M-N-C catalysts are

20

considered as one of the most promising candidates due to their low cost and excellent

21

ORR activity in alkaline solution. The results from both the experiment and DFT

. The high cost and limited resource of Pt significantly hinder the further

, transition metal-nitrogen-carbon (M-N-C) (M=Fe, Ni, Co, etc.) materials12-14 and

2


Page 2 of 34

Page 3 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

calculation have manifested that M-N-C catalysts consisted of both Fe/Fe3C

2

nanocrystals and Fe−Nx showed high electrocatalytic activity towards ORR17, 18. FeN4

3

embedded into porous carbon are likely to be of the active sites with favorable binding

4

energy for O2 adsorption and O=O bond dissociation. Nevertheless, the preparation of

5

M-N-C catalysts requires high temperature heat treatment, which could cause inevitable

6

agglomeration of metals and collapse of structure. The electrocatalytic activity is still

7

poorer than that of platinum catalyst19. To overcome those shortages, it is essential to

8

explore a new route to synthesize catalysts with a hierarchical porous structure.

9

Metal-Organic Frameworks (MOFs) are a novel type of sacrificial templates, which

10

coordinate polymers with metal centers and possess orderly porous structure with a high

11

specific surface area20. The carbonization of MOFs could yield highly porous carbon

12

with metal centers embedded in the carbon matrix. A number of studies show that

13

transition metal-nitrogen-carbon catalysts can be derived from MOFs such as zeolitic

14

imidazolate frameworks (ZIFs)21,

15

isoreticular metal–organic frameworks (IRMOFs)25 and so forth. However, most of the

16

MOF-derived carbon particles are isolated, leading to low conductivity which is not

17

conducive to ORR process. To overcome this drawback, some carbon materials were

18

introduced as structure agent to form continuous structures. Wei et al.26 developed a

19

graphene-directed assembly route to synthesize ZIF/rGO-700-AL, which showed

20

excellent ORR activity under both alkaline and acid condition. Tan et al.27 combined

21

carbon nanotubes (CNTs) with ZIF-8 through an in-situ pyrolysis process, such carbon-

22

carbon hybrids showed superior activity in ORR. Herein, to achieve catalyst

22

, MILs (Material Lavoisier Laboratory)23,

3


24

,


1

nanoparticles with good connections among them, functional carbon black was added

2

as structure-directing agent during the hydrothermal process. With the assistant of

3

sufficient functional groups on the carbon black surface, MIL-101(Fe) was grown

4

homogenously on carbon black, resulting a continuous precursor (MIL-101(Fe)/C).

5

During the pyrolysis, the functional carbon black acted as structure agent to connect the

6

discrete carbon nanoparticles derived from MIL, forming a continuous structure with

7

enhanced conductivity.

8

On the other hand, heteroatom (e.g., N, S, P, metals, etc.) doping can further improve

9

the catalytic performance28-32. In general, atomic doping could form catalytic active

10

sites, modify electronic structure, and increase structure defects. For instance, Tan et

11

al.33 prepared non-precious iron-nitrogen doped carbon catalyst (Fe-N/C) with a high

12

content of pyridinic nitrogen by using NH3 as an activation agent. The N content

13

dopants was a crucial factor for improving ORR performance, which is due to that

14

pyridinic N atoms induce irregular charge distribution whereby the strong electron

15

affinity of N suppresses the electron densities of adjacent C-atoms leading to positive

16

charge distribution on the C-atoms, thereby enhancing the O2 adsorption and reduction

17

processes34. However, controlling the amount of nitrogen doping in MOF-derived

18

catalysts is still a challenge, and it is necessary to develop a new synthesis method to

19

solve the above problem. Chemical vapor deposition approach yields nitrogen doped

20

graphene with nitrogen amount less than 10%2. Liquid-phase synthesis is more difficult

21

to operate. Using solid-phase synthesis could induce in-situ N-doping. In this paper,

22

MIL-101 was chosen as precursor to obtain the uniform pore distribution and stable 4


Page 4 of 34

Page 5 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

structure, and melamine was selected as a nitrogen dopant. By regulating the amount

2

of melamine, catalysts with different nitrogen contents can be synthesized.

3

To further enhance the ORR performance of MOF-derived non-precious catalysts,

4

orderly control had been taken to prepare Fe/Fe3C@NC in this report. With the facile

5

control of structure and chemical composition, the obtained Fe/Fe3C@NC materials

6

featured a hierarchical structure and exhibited outstanding ORR performance. This

7

could pave the way for design and development of cheaper and more durable

8

electrocatalyst for wider application.

9

2. Experimental

10

2.1. Materials

11

FeCl3·6H2O, Benzenedicarboxylic acid (BDC), melamine, N, N-dimethylformamide

12

(DMF), sulfuric acid, nitric acid, isopropanol and ethanol were purchased from

13

Sinopharm Chemical Reagent Co. Ltd, China. XC-72R was purchased from Cabot Co.

14

Boston, USA. Nafion (5 wt% in isopropanol) was purchased from Sigma-Aldrich. The

15

reference commercial catalysts, 20% Pt/C was obtained from Johnson-Matthey (UK).

16

All the reagents were used without further purification.

17

2.2. Preparation

18

2.2.1. Synthesis of functional carbon black

19

400 mg XC-72R was dispersed in the mixture of concentrated sulfuric acid and

20

concentrated nitric acid. After being ultrasonicated for 70 min under the temperature of 5



Page 6 of 34

1

50 °C, oxygen functional groups were introduced on the surface of XC-72R. The as-

2

prepared carbon black was washed with deionized water and dried overnight under

3

vacuum at 80 °C.

4

2.2.2. Synthesis of MIL-101 (Fe) and MIL-101(Fe)/C

5

MIL-101 (Fe) was prepared by the protocol presented earlier35. In a typical synthesis

6

procedure, 0.675 g FeCl3·6H2O and 0.206 g BDC were dispersed in 15 mL of N, N-

7

Dimethylformamide

8

ultrasonication. Subsequently, the solution was heated at 110 °C for 20 h in a Teflon

9

reactor. The resulting brown solids were washed with 60 °C ethanol for 3 times and

10

dried at 60 °C in air for half an hour to remove DMF. Finally, the products were dried

11

in a vacuum oven at 80 °C for 10 h. MIL-101(Fe)/C was prepared by the same

12

procedure for above MIL-101(Fe), apart from adding 100 mg functional carbon black

13

into the mixed solution and forming a homogenous solution before hydrothermal

14

process.

15

2.2.3. Synthesis of Fe/Fe3C@NC

16

100 mg MIL-101 (Fe)/C and melamine with different mass ratios were dissolved in 3

17

ml deionized water and stirred for 5 h at room temperature. The powders were ground

18

and annealed in Ar atmosphere at 700 °C for 3 h with the heating rate of 5 °C min-1.

19

After cooling to room temperature, the black powders were immersed into 1 M HNO3

20

for 10 h to remove unstable and inactive species. The etched samples were thoroughly

21

washed with deionized water. The mass ratios of MIL-101 (Fe)/C to melamine were

(DMF).

The

mixture

was

thoroughly

6


dissolved

with

Page 7 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2:1, 1:1 and 1:2, and the obtained samples were defined as Fe/Fe3C@NC-2:1,

2

Fe/Fe3C@NC and Fe/Fe3C@NC-1:2 respectively. Fe/Fe3C@C was prepared by

3

directly carbonizing MIL-101 (Fe)/C without melamine addition. For comparison,

4

Fe/Fe3C-C was obtained by directly carbonizing MIL-101 (Fe). 100 mg MIL-101 (Fe)

5

and 100 mg melamine were mixed and carbonized, and the obtained sample was

6

denoted as Fe/Fe3C-NC.

7

2.3. Structural characterizations

8

X-ray diffraction (XRD) patterns were obtained by using a powder diffractometer (DX-

9

2700, Dandong, China) using Cu-K radiation (l=1.54Å). SEM and TEM images were

10

obtained by a scanning electron microscope (JSM-5900LV, JEOL Co.) and an electron

11

transmission microscope (Carl Zeiss SMT, Libra 200FE). The specific surface area and

12

pore size were determined by nitrogen adsorption/desorption measurement with an

13

automated surface area and pore size analyzer (QUADRASORB SI, Quantachrome,

14

America). X-ray photoelectron spectroscope (XPS) was carried out on an ESCALAB

15

250Xi (Thermo Fisher Scientific, USA).

16

2.4. Electrochemical measurements

17

The electrochemical measurements were carried out on an Autolab electrochemical

18

analyzer using a standard three-electrode system. A graphite plate was used as the

19

counter electrode and a Hg/HgO electrode was used as the reference electrode. All

20

electrode potentials in this study were calibrated versus reversible hydrogen electrode

21

(RHE). To prepare the working electrode, 5 mg catalyst was suspended in the solution 7



1

of 0.5 ml of ethanol and 0.5 ml of deionized water with 35 L Nafion solution in an

2

ultrasonic bath for 30 min to form a homogeneous ink. Then, 12 L of electrocatalyst

3

ink was cast onto the surface of the clean rotating disk working electrode (RDE)

4

(diameter = 5 mm) and dried. Cyclic voltammtry (CV) and Linear sweep voltammetry

5

(LSV) tests were performed in 0.1 M KOH solution. Before measurements, the

6

solutions were first purged with Ar (for CV tests) or O2 (for LSV tests) for about 30

7

minutes. The CV curves were conducted at a scan rate of 50 mV s-1. The LSV curves

8

were obtained on a Pine electrochemical system at different rotating rates varying from

9

400 rpm to 2025 rpm with a scan rate of 10 mV s −1. The electron transfer number (n)

10

was calculated according to the Koutechy-Levich equation36: 1

11

𝐽

1

1

1

𝑘

𝐿

𝑘

1

= 𝐽 + 𝐽 = 𝐽 + 𝐵ω0.5

B = 0.62nF(𝐷𝑂2 )2/3 𝑣 −1/6 𝐶𝑂2

12

13

where, J represents the apparent current density, while JL and Jk are the diffusion and

14

kinetic limiting current densities, respectively. ω is the angular velocity of the disk, and

15

n is electron transfer number. F is the Faraday constant (96485 C mol-1), 𝐶𝑂2 is the bulk

16

concentration of O2 dissolved (1.2×10

17

diffusion coefficient in the electrolyte (1.9×10-5 cm 2 s -1) and 𝑣 is the kinetic viscosity

18

of 0.1 mol L

19

performed on a CHI electrochemical workstation with an AC amplitude of 5 mV in 5

20

mM Fe(CN)63-/4- containing 0.5 M KCl. The frequency was in the range of 0.1 Hz to

21

100 kHz.

-1

-6

mol cm

-6

for 0.1 M KOH), 𝐷𝑂2 is the O2

KOH (0.01 cm2 s-1). The electron impedance spectra (EIS) were

8


Page 8 of 34

Page 9 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

3. Results and discussion

2

3.1. Structural characterizations of the catalysts

3

A one-pot method was adopted to synthesize the functional carbon black supported

4

MOFs. XRD pattern of MIL-101(Fe)/C showed a typical MIL-101 phase without

5

impurity (Figure 1A). Figure 1B showed the XRD pattern of Fe/Fe3C@NC. The peaks

6

located at 37.9, 42.8°, 43.3°, 44.9, 46.0 and 49.3 were attributed to Fe3C (PDF#89-

7

2867). The diffraction peaks at 44.8°and 65.1°were indexed to (110) and (200) planes

8

of Fe (JCPDS, No. 06-0696). While a typical strong peak located at 26°corresponded

9

to the (002) facet of graphite carbon. Compared with Figure 1A, the peak was sharper,

10

which might attribute to the higher graphitic degree after carbonization.

11 12

Figure 1 (A) XRD patterns of synthesized MIL-101(Fe)/C and simulated MIL-

13

101(Cr); (B) XRD pattern of Fe/Fe3C@NC.

14

SEM images were revealed in Figure 2 for the catalysts of MIL-101 (Fe), MIL-101

15

(Fe)/C, Fe/Fe3C-NC and Fe/Fe3C@NC. The images in both Figure 2A and 2B showed

16

that MOFs had well-defined octahedral structures (the same as in the insets of Figure 9



1

2A and 2B) which revealed the successful preparation of MIL-101(Fe). The particle

2

sizes of MIL-101(Fe) showed in Figure 2A were of uneven distribution from 1 to 10

3

m. However, with the assistant of functional carbon black, MIL-101(Fe)/C exhibited

4

homogeneous growth with a uniform size distribution of ~10 m. After mixed with

5

melamine and pyrolysis, the composite Fe/Fe3C-NC (Figure 2C) aggregated seriously

6

and exposed few active sites. By contrast, the composite Fe/Fe3C@NC (Figure 2D)

7

showed a hybrid structure of functional carbon black and MOF-derived particles. The

8

individual MOF-derived particle with a diameter of about 500 nm was surrounded by

9

functional carbon black (insert of Figure 2D). The strong interaction between functional

10

carbon black and particles could hinder the aggregation of grains.

11 12

Figure 2 Different magnification SEM images of (A) MIL-101(Fe), (B) MIL-

13

101(Fe)/C, (C) Fe/Fe3C-NC, and (D) Fe/Fe3C@NC

14

From the TEM images of Fe/Fe3C@NC sample, a homogenous hybrid was observed in

15

Figure 3. Fe-based nanoparticles were encapsulated into the carbon with a diameter

16

range of 10 to 50 nm. The red arrow in Figure 3C referred to the carbon shell which got 10


Page 10 of 34

Page 11 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

rid of unstable components after acid leaching. High-resolution TEM (HRTEM) images

2

(Figure 3D) showed that Fe/Fe3C nanoparticles were wrapped in a 5.0 nm thick

3

graphitic carbon layer. The fringe spacing of about 0.20 nm was corresponded to the

4

spacing between planes (031) of Fe3C. Meanwhile, the graphitic carbon layers

5

efficiently hampered Fe/Fe3C species aggregation.

6 7

Figure 3 (A-D) TEM images of Fe/Fe3C@NC.

8

The N2 sorption isotherm of Fe/Fe3C@NC sample showed a typical curve with

9

hysteresis loop in Figure 4A, and the BET surface area was calculated to be 107 m2 g-1.

10

Figure 4B showed that Fe/Fe3C@NC has mesoporous and macroporous structures with

11

the pore size distribution ranging from 2 to 100 nm based on the BJH model. The large

12

surface and pore volume might result from the porous structure of MOF precursor,

13

which could expose more active sites and enhance the transportation of ORR-relevant

14

species24.

11



1 2

Figure 4 N2 adsorption-desorption isotherm (A) and the corresponding pore-size

3

distribution curve (B) of Fe/Fe3C@NC.

4

In order to investigate the bonding state of iron and nitrogen, (XPS) spectra of

5

Fe/Fe3C@NC-2:1, Fe/Fe3C@NC, Fe/Fe3C@NC-1:2 were tested. In Figure 5A, the

6

survey spectra demonstrated the presence of C, N, O and Fe in all samples. By the

7

calculation, Fe/Fe3C@ NC processed the highest nitrogen content (5.40%), while the

8

nitrogen content of Fe/Fe3C@NC-2:1, Fe/Fe3C@NC-1:2 were 3.56% and 3.05%,

9

respectively. It suggested that nitrogen element had been efficiently doped in

10

Fe/Fe3C@NC. High nitrogen content tended to form more active species. The high-

11

resolution N1s spectra of Fe/Fe3C@NC-2:1, Fe/Fe3C@NC and Fe/Fe3C@NC-1:2

12

showed five peaks in Figure 5B, C and D, which correspond to pyridinic N (398.4 eV),

13

Fe–N (399.4 eV), pyrrolic N (400.4 eV), graphitic N (401.3 eV) and N–O (403.4 eV),

14

respectively37-39. The graphitic N plays a crucial role for ORR40, besides, pyridinic N

15

and Fe-NX take an important role in reducing oxygen to water41, 42. The concentrations

16

in the different types of the samples were demonstrated in Figure 5E. Fe/Fe3C@NC

17

possessed the highest nitrogen content with the highest pyridinic N content, while the

18

of Fe/Fe3C@NC-2:1 possessed the lowest nitrogen content. Further, the Fe 2p region 12


Page 12 of 34

Page 13 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

of Fe/Fe3C@NC was showed in Figure 5F. The binding energy at 706.8 eV and 710.3

2

eV could be assigned to metallic Fe and Fe-Nx43.

3 4

Figure 5 (A) XPS spectra of Fe/Fe3C@NC-2:1, Fe/Fe3C@NC, Fe/Fe3C@NC-1:2,

5

respectively. (B) High-resolution N1s XPS spectra of Fe/Fe3C@NC-2:1. (C)

6

Fe/Fe3C@NC. (D) Fe/Fe3C@NC-1:2; (E) The content of pyridinic N, Fe-N, pyrrolic N,

7

graphitic N and N-O of Fe/Fe3C@NC-2:1, Fe/Fe3C@NC, Fe/Fe3C@NC-1:2,

8

respectively. (F) High-resolution Fe 2p of Fe/Fe3C@NC. 13



1

3.2. Electrochemical measurements

2

The ORR electrocatalyst activity of Fe/Fe3C@NC was first evaluated by CV

3

measurements in 0.1 M KOH with a scan rate of 50 mV s-1. CV curves were showed in

4

Figure 6A. Compared with no observed current in the deoxygenation electrolyte, a

5

distinct oxygen reduction peak emerged in the O2-saturated electrolyte. Then, RDE

6

polarization curves were recorded by LSV measurements to further demonstrate the

7

ORR activities. To identify the effect of functional carbon black, LSV curves of

8

different samples were displayed in the Figure 6B. Fe/Fe3C-C showed inferior ORR

9

activity, indicating that it is essential to tuning the properties of MOF-derived materials

10

for improving ORR activity. Fe/Fe3C@NC exhibited better ORR activity as compare

11

to Fe/Fe3C-NC, with a more positive onset potential of 0.85 V and a lager half-wave

12

potential of 0.70 V, which suggested the positive effect of functional carbon black in

13

the catalyst. Further, in order to study how the nitrogen doping effected on the ORR

14

properties, samples prepared with different amounts of melamine were evaluated by

15

LSV measurements and the results were showed in Figure 6C. Fe/Fe3C@C exhibited

16

the worst ORR catalytic activity, due to no nitrogen compounds promoting the ORR

17

progress. Besides, Fe/Fe3C@NC had a more positive half-wave potential and higher

18

limiting diffusion current density than those of Fe/Fe3C@NC-1:2 and Fe/Fe3C@NC-

19

2:1. This could be attributed to that the low content of N element might not form

20

sufficient catalytic sites and the high content of N element could decrease the

21

carbonization yield. Moreover, both onset potential and half-wave potential of

22

Fe/Fe3C@NC were similar to those of commercial Pt/C (Figure 6F). But the limiting 14


Page 14 of 34

Page 15 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

diffusion current density of Fe/Fe3C@NC was 4.8 mA cm-2, much higher than that of

2

Pt/C. The ORR performance was comparable with other non-precious catalysts such as

3

Co-N-C composites14 and Co@O-NPC-700 composites20. The onset potential was also

4

higher than those of other nanomaterial, like that of NC90021, PDI-900/GC36.

5

The reaction kinetics of the Fe/Fe3C@NC catalysts during ORR was evaluated by

6

polarization measurements at various rotation rates in the range of 400−2025 rpm. As

7

shown in Figure 6E, the limiting diffusion current density of Fe/Fe3C@NC catalysts

8

increased with an increased rotating rate, due to the shortened O2 diffusion distance in

9

the electrolyte at high rotation rates, revealing that ORR on the Fe/Fe3C@NC catalysts

10

was a kinetics-controlled process. The corresponding Koutecky–Levich (K–L) plots at

11

different potentials showed good liner relationship, implying first-order reaction

12

kinetics during the progress. As shown in Figure 6F, the calculated electron-transfer

13

numbers were 3.88-4.05 at 0.3-0.6 V, indicating that Fe/Fe3C@NC exhibited a

14

dominant 4-electron oxygen reduction process.

15 15



Page 16 of 34

1

Figure 6 (A) CVs of Fe/Fe3C@NC in Ar and O2-saturated 0.1 M KOH solution at 50

2

mV s−1. (B-D) RDE polarization curves on the different as-prepared samples and Pt/C

3

in O2-saturated 0.1 M KOH solution at 1600 rpm. (E) RDE polarization curves of

4

Fe/Fe3C@NC at different rotation rates for ORR at a scan rate of 10 mV s

5

plots and electron transfer numbers at various potentials for the Fe/Fe3C@NC electrode.

6

To evaluate the electron transfer resistance, the electronic impedance spectroscopy (EIS)

7

spectra of Fe/Fe3C@NC and Fe/Fe3C-NC were measured. As shown in Figure 7, the

8

arc in the Nyquist plot in the high-frequency (HF) region was due to limitations on ion

9

(H+, K+, OH−) transport44. The impedance spectra were fitted using an equivalent circuit

10

consisting of the resistance (electrolyte (Rs), charge transfer (Rct), and constant phase

11

element (Cd)45. The lower Rct suggested the superior electron transfer ability during the

12

ORR, which can facilitate the ORR process. As shown in Figure 8A, EIS was recorded

13

in 5 mM Fe (CN)63-/4- containing 0.5 M KCl. The Fe/Fe3C@NC catalyst exhibited the

14

smaller radius, which clearly demonstrated that the electrode with Fe/Fe3C@NC

15

possessed a smaller Rct and showed much smaller charge transfer resistance and thus

16

allowed a much faster transport of electrons during ORR. Figure 7B showed Nyquist

17

plots obtained in O2-saturated 0.1 M KOH. As compared to the diameter of the

18

semicircle, Rct for Fe/Fe3C@NC was smaller than that for Fe/Fe3C-NC, indicating that

19

the enhanced the electron transfer and more efficiently ORR kinetics. Therefore, we

20

can conclude that using functional carbon black as a structure agent to form continuous

21

structures would enhance the conductivity.

16


−1

. (F) K-L

Page 17 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 3-/4-

2

Figure 7 EIS of Fe/Fe3C@NC and Fe/Fe3C-NC conducted in (A) 5 mM Fe(CN)6

3

containing 0.5 M KCl and (B) in O2-saturated 0.1 M KOH.

4

Additionally, tolerance of Fe/Fe3C@NC and Pt/C to methanol crossover effects were

5

tested. As shown in Figure 8A, when 3 M methanol was added into 0.1 M KOH solution

6

at the points of 300 s during the current-time test, the current of Fe/Fe3C@NC electrode

7

showed a slight change, while the current on Pt/C electrode decreased quickly, which

8

suggesting that an obvious methanol oxidation reaction occurred on Pt/C electrode

9

surface. To study the stability of Fe/Fe3C@NC, chronoamperometric measurements

10

were evaluated in Figure 8B. After 20000 s, the relative current of the Fe/Fe3C@NC

11

catalyst remained 85%, which was much higher than that of Pt/C (73.8%). Based on the

12

above results, Fe/Fe3C@NC exhibited excellent ORR performance with high stability.

13 17



1

Figure 8 (A) Chronoamperometric response of Pt/C and Fe/Fe3C@NC in O2-saturated

2

0.1 M KOH solution (125 ml) with the introduction of 12.5 ml of methanol at the time

3

of 300 s. The arrow indicates the introduction of 3 M methanol. (B) Current-time

4

chronoamperometric response of Pt/C and Fe/Fe3C@NC in O2-saturated 0.1 M KOH

5

solution for 20000 s in O2-saturated 0.1 M KOH solution at a rotating rate of 1600 rpm.

6

The possible reason for enhancement the ORR performance of Fe/Fe3C@NC could be

7

summarized as follows: 1) Using MOF as precursor and functional carbon black as

8

structure agent could obtained hierarchical structure. Functional carbon black acted as

9

structure-agent decentralizing the activity sites and improving the conductivity of

10

catalysts which can be verified by the TEM imagines and EIS spectra. 2) Nitrogen

11

doping can greatly improve the electrochemical activities in ORR owing to their role in

12

modulating structural and electronic changes28. The efficient nitrogen dopant formed

13

more activity sites such as graphitic N, pyridinic N and Fe-NX, which can attribute to

14

oxygen reduction in alkaline electrolyte. 3) Oriented control of the formation of

15

graphitic carbon shell embedded Fe/Fe3C would provide efficient active sites. The

16

catalysts with both Fe/Fe3C nanocrystals and Fe−Nx exhibited the high activity17. Due

17

to the synergetic effects between metal cores and N-doped carbon shells46,

18

Fe/Fe3C@NC showed high performance towards ORR.

19

4. Conclusions

20

With the controllable tuning of MOF-derived material, a novel Fe/Fe3C@NC catalyst

21

was successfully prepared and applied for ORR. In this work, a continuous and

22

homogenous MIL-101(Fe)/C precursors were synthesized by adopting a simple one18


Page 18 of 34

Page 19 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

step solvothermal method. This precursor can maintain the continuous structure with

2

high electronic conductivity after pyrolysis. And then, systematically study of nitrogen

3

doping in MOF-derived material was conducted. The high nitrogen concentration,

4

especially high content of graphitic N, pyridinic N and Fe-NX can attribute to the

5

reduction of oxygen into water in alkaline electrolyte. By proper tuning both structure

6

and chemical composition of the MOF-derived material, Fe/Fe3C@NC showed

7

superior ORR performance with a larger onset potential of 0.85 V and half-wave

8

potential of 0.70 V, meanwhile exhibited high stability and methanol tolerance.

9

Acknowledgement

10

This work was supported by the Key Research and Development Projects in Sichuan

11

Province (2017GZ0397, 2017CC0017), the Science and Technology Project of

12

Chengdu (2015-HM01-00531-SF).

13

References

14

(1)Shao, M.; Chang, Q.; Dodelet, J. P.; Chenitz, R. Recent Advances in Electrocatalysts for

15

Oxygen Reduction Reaction. Chem. Rev. 2016, 116, (6), 3594-657.

16

(2)Xia, Z.; An, L.; Chen, P.; Xia, D. Non-Pt Nanostructured Catalysts for Oxygen Reduction

17

Reaction: Synthesis, Catalytic Activity and its Key Factors. Adv. Energy Mater. 2016, 6,

18

(17), 1600458.

19

(3)Xu, G.-R.; Wang, B.; Zhu, J.-Y.; Liu, F.-Y.; Chen, Y.; Zeng, J.-H.; Jiang, J.-X.; Liu, Z.-H.;

20

Tang, Y.-W.; Lee, J.-M. Morphological and Interfacial Control of Platinum Nanostructures

21

for Electrocatalytic Oxygen Reduction. ACS Catal. 2016, 6, (8), 5260-5267.

22

(4)Liu, H.; Liu, X.; Li, Y.; Jia, Y.; Tang, Y.; Chen, Y. Hollow PtNi alloy nanospheres with 19



1

enhanced activity and methanol tolerance for the oxygen reduction reaction. NANO Res.

2

2016, 9, (11), 3494-3503.

3

(5)Zhang, Y.; Lin, Y.; Jiang, H.; Wu, C.; Liu, H.; Wang, C.; Chen, S.; Duan, T.; Song, L. Well-

4

Defined Cobalt Catalyst with N-Doped Carbon Layers Enwrapping: The Correlation

5

between Surface Atomic Structure and Electrocatalytic Property. Small 2018, 14, (6),

6

1702074.

7

(6)Yang, W.; Zhang, Y.; Liu, X.; Chen, L.; Jia, J. In situ formed Fe-N doped metal organic

8

framework@carbon nanotubes/graphene hybrids for a rechargeable Zn-air battery.

9

Chem. Commun. (Camb.) 2017, 53, (96), 12934-12937.

10

(7)Proietti, E.; Jaouen, F.; Lefevre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J. P. Iron-

11

based cathode catalyst with enhanced power density in polymer electrolyte membrane

12

fuel cells. Nat. Commun. 2011, 2, 416.

13

(8)Liu, X.; Liu, W.; Ko, M.; Park, M.; Kim, M. G.; Oh, P.; Chae, S.; Park, S.; Casimir, A.; Wu, G.;

14

Cho, J. Metal (Ni, Co)-Metal Oxides/Graphene Nanocomposites as Multifunctional

15

Electrocatalysts. Adv. Funct. Mater. 2015, 25, (36), 5799-5808.

16

(9)Pan, Z.; Xiao, Y.; Fu, Z.; Zhan, G.; Wu, S.; Xiao, C.; Hu, G.; Wei, Z. Hollow and porous

17

titanium nitride nanotubes as high-performance catalyst supports for oxygen reduction

18

reaction. J. Mater. Chem. A 2014, 2, (34), 13966.

19

(10)Zhang, J.; Chen, J.; Jiang, Y.; Zhou, F.; Wang, G.; Wang, R. Tungsten carbide

20

encapsulated in nitrogen-doped carbon with iron/cobalt carbides electrocatalyst for

21

oxygen reduction reaction. Appl. Surf. Sci. 2016, 389, 157-164.

22

(11)Li, J. S.; Li, S. L.; Tang, Y. J.; Han, M.; Dai, Z. H.; Bao, J. C.; Lan, Y. Q. Nitrogen-doped

23

Fe/Fe3C@graphitic layer/carbon nanotube hybrids derived from MOFs: efficient

24

bifunctional electrocatalysts for ORR and OER. Chem. Commun. (Camb.) 2015, 51, (13),

25

2710-3.

26

(12)Huang, X.; Yang, Z.; Dong, B.; Wang, Y.; Tang, T.; Hou, Y. In situ Fe2N@N-doped

27

porous carbon hybrids as superior catalysts for oxygen reduction reaction. Nanoscale 20


Page 20 of 34

Page 21 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

2017, 9, (24), 8102-8106.

2

(13)Chen, Y.; Gokhale, R.; Serov, A.; Artyushkova, K.; Atanassov, P. Novel highly active and

3

selective Fe-N-C oxygen reduction electrocatalysts derived from in-situ polymerization

4

pyrolysis. Nano Energy 2017, 38, 201-209.

5

(14)Gao, J.; Ma, N.; Zhai, J.; Li, T.; Qin, W.; Zhang, T.; Yin, Z. Polymerizable Ionic Liquid as

6

Nitrogen-Doping Precursor for Co–N–C Catalyst with Enhanced Oxygen Reduction

7

Activity. Ind. Eng. Chem. Res. 2015, 54, (32), 7984-7989.

8

(15)Xue, Q.; Ding, Y.; Xue, Y.; Li, F.; Chen, P.; Chen, Y. 3D nitrogen-doped graphene

9

aerogels as efficient electrocatalyst for the oxygen reduction reaction. Carbon 2018, 139,

10

137-144.

11

(16)Tan, A.-d.; Wang, Y.-f.; Fu, Z.-y.; Tsiakaras, P.; Liang, Z.-x. Highly effective oxygen

12

reduction reaction electrocatalysis: Nitrogen-doped hierarchically mesoporous carbon

13

derived from interpenetrated nonporous metal-organic frameworks. Appl. Catal. B:

14

Environ 2017, 218, 260-266.

15

(17)Jiang, W. J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L. J.; Wang, J. Q.; Hu, J. S.; Wei,

16

Z.; Wan, L. J. Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen

17

Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-N(x). J. Am. Chem. Soc. 2016,

18

138, (10), 3570-8.

19

(18)Liang, W.; Chen, J.; Liu, Y.; Chen, S. Density-Functional-Theory Calculation Analysis of

20

Active Sites for Four-Electron Reduction of O2 on Fe/N-Doped Graphene. ACS Catal.

21

2014, 4, (11), 4170-4177.

22

(19)Amiinu, I. S.; Pu, Z.; Liu, X.; Owusu, K. A.; Monestel, H. G. R.; Boakye, F. O.; Zhang, H.;

23

Mu, S. Multifunctional Mo-N/C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn-Air

24

Batteries. Adv. Funct. Mater. 2017, 27, (44), 1702300.

25

(20) Zhou, X.; Gao, Y.-j.; Deng, S.-w.; Cheng, S.; Zhang, S.-h.; Hu, H.; Zhuang, G.-l.; Zhong,

26

X.; Wang, J.-g. Improved Oxygen Reduction Reaction Performance of Co Confined in

27

Ordered N-Doped Porous Carbon Derived from ZIF-67@PILs. Ind. Eng. Chem. Res. 2017, 21



1

56, (39), 11100-11110.

2

(21)Aijaz, A.; Fujiwara, N.; Xu, Q. From metal-organic framework to nitrogen-decorated

3

nanoporous carbons: high CO2 uptake and efficient catalytic oxygen reduction. J. Am.

4

Chem. Soc. 2014, 136, (19), 6790-3.

5

(22)Liang, Y.; Wei, J.; Hu, Y. X.; Chen, X. F.; Zhang, J.; Zhang, X. Y.; Jiang, S. P.; Tao, S. W.;

6

Wang, H. T. Metal-polydopamine frameworks and their transformation to hollow

7

metal/N-doped carbon particles. Nanoscale 2017, 9, (16), 5323-5328.

8

(23)Yang, L.; Bai, Y.; Zhang, H.; Geng, J.; Shao, Z.; Yi, B. Nitrogen-doped porous carbon

9

derived from Fe-MIL nanocrystals as an electrocatalyst for efficient oxygen reduction. RSC

10

Adv. 2017, 7, (36), 22610-22618.

11

(24)Guan, B. Y.; Yu, L.; Lou, X. W. A dual-metal–organic-framework derived electrocatalyst

12

for oxygen reduction. Energy Environ. Sci. 2016, 9, (10), 3092-3096.

13

(25)Wu, Y.; Zhao, S.; Zhao, K.; Tu, T.; Zheng, J.; Chen, J.; Zhou, H.; Chen, D.; Li, S. Porous

14

Fe-Nx/C hybrid derived from bi-metal organic frameworks as high efficient electrocatalyst

15

for oxygen reduction reaction. J. Power Sources 2016, 311, 137-143.

16

(26)Wei, J.; Hu, Y.; Wu, Z.; Liang, Y.; Leong, S.; Kong, B.; Zhang, X.; Zhao, D.; Simon, G. P.;

17

Wang, H. A graphene-directed assembly route to hierarchically porous Co–Nx/C catalysts

18

for high-performance oxygen reduction. J. Mater. Chem. A 2015, 3, (32), 16867-16873.

19

(27)Tan, B.; Wu, Z.-F.; Xie, Z.-L. Fine decoration of carbon nanotubes with metal organic

20

frameworks for enhanced performance in supercapacitance and oxygen reduction

21

reaction. Sci. Bull 2017, 62, (16), 1132-1141.

22

(28)Wang, R.; Dong, X. Y.; Du, J.; Zhao, J. Y.; Zang, S. Q. MOF-Derived Bifunctional Cu3P

23

Nanoparticles Coated by a N,P-Codoped Carbon Shell for Hydrogen Evolution and

24

Oxygen Reduction. Adv. Mater. 2018, 30, (6).

25

(29)Wang, J.; Huang, Z.; Liu, W.; Chang, C.; Tang, H.; Li, Z.; Chen, W.; Jia, C.; Yao, T.; Wei,

26

S.; Wu, Y.; Li, Y. Design of N-Coordinated Dual-Metal Sites: A Stable and Active Pt-Free 22


Page 22 of 34

Page 23 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Catalyst for Acidic Oxygen Reduction Reaction. J. Am. Chem. Soc. 2017, 139, (48), 17281-

2

17284.

3

(30)Su, P.; Xiao, H.; Zhao, J.; Yao, Y.; Shao, Z.; Li, C.; Yang, Q. Nitrogen-doped carbon

4

nanotubes derived from Zn–Fe-ZIF nanospheres and their application as efficient oxygen

5

reduction electrocatalysts with in situ generated iron species. Chem. Sci. 2013, 4, (7), 2941.

6

(31)Liu, T.; Yang, F.; Cheng, G.; Luo, W. Reduced Graphene Oxide-Wrapped Co9-xFexS8

7

/Co,Fe-N-C Composite as Bifunctional Electrocatalyst for Oxygen Reduction and

8

Evolution. Small 2018, 14, (10).

9

(32)Ishii, T.; Maie, T.; Hamano, M.; Kishimoto, T.; Mizushiri, M.; Imashiro, Y.; Ozaki, J.-i.

10

Synergistically enhanced oxygen reduction activity of iron-based nanoshell carbons by

11

copper incorporation. Carbon 2017, 116, 591-598.

12

(33)Tan, H.; Li, Y.; Jiang, X.; Tang, J.; Wang, Z.; Qian, H.; Mei, P.; Malgras, V.; Bando, Y.;

13

Yamauchi, Y. Perfectly ordered mesoporous iron-nitrogen doped carbon as highly

14

efficient catalyst for oxygen reduction reaction in both alkaline and acidic electrolytes.

15

Nano Energy 2017, 36, 286-294.

16

(34)Amiinu, I. S.; Liu, X.; Pu, Z.; Li, W.; Li, Q.; Zhang, J.; Tang, H.; Zhang, H.; Mu, S. From 3D

17

ZIF Nanocrystals to Co-Nx/C Nanorod Array Electrocatalysts for ORR, OER, and Zn-Air

18

Batteries. Adv. Funct. Mater. 2018, 28, (5), 1704638.

19

(35)Maksimchuk, N. V.; Kovalenko, K. A.; Fedin, V. P.; Kholdeeva, O. A. Cyclohexane

20

selective oxidation over metal-organic frameworks of MIL-101 family: superior catalytic

21

activity and selectivity. Chem. Commun. 2012, 48, (54), 6812-6814.

22

(36)Liu, R.; Wu, D.; Feng, X.; Mullen, K. Nitrogen-doped ordered mesoporous graphitic

23

arrays with high electrocatalytic activity for oxygen reduction. Angew. Chem. Int. Ed. Engl.

24

2010, 49, (14), 2565-9.

25

(37)Li, Z.; Sun, H.; Wei, L.; Jiang, W.-J.; Wu, M.; Hu, J.-S. Lamellar Metal Organic

26

Framework-Derived Fe–N–C Non-Noble Electrocatalysts with Bimodal Porosity for

27

Efficient Oxygen Reduction. ACS Appl. Mater. Interfaces 2017, 9, (6), 5272-5278. 23



1

(38)Hu, F.; Yang, H.; Wang, C.; Zhang, Y.; Lu, H.; Wang, Q. Co-N-Doped Mesoporous

2

Carbon Hollow Spheres as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction.

3

Small 2017, 13, (3), 1602507.

4

(39)Chunhui, X.; Xu, C.; Yuhai, T. Surface-rough Fe-N/C composite wrapped on carbon

5

nanotubes as efficient electrocatalyst for oxygen reduction reaction. Nanotechnology

6

2017, 28, (22), 225401.

7

(40)Jia, Q.; Ramaswamy, N.; Tylus, U.; Strickland, K.; Li, J.; Serov, A.; Artyushkova, K.;

8

Atanassov, P.; Anibal, J.; Gumeci, C.; Barton, S. C.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.;

9

Mukerjee, S. Spectroscopic insights into the nature of active sites in iron–nitrogen–carbon

10

electrocatalysts for oxygen reduction in acid. Nano Energy 2016, 29, 65-82.

11

(41)Li, J.; Jia, Q.; Ghoshal, S.; Liang, W.; Mukerjee, S. Highly Active and Stable Fe–N–C

12

Catalyst for Oxygen Depolarized Cathode Applications. Langmuir 2017, 33, (37), 9246-

13

9253.

14

(42)Wang, J.; Xia, H.; Peng, Z.; Lv, C.; Jin, L.; Zhao, Y.; Huang, Z.; Zhang, C. Graphene Porous

15

Foam Loaded with Molybdenum Carbide Nanoparticulate Electrocatalyst for Effective

16

Hydrogen Generation. ChemSusChem 2016, 9, (8), 855-862.

17

(43)Xing, J.; Li, H.; Ming-Cheng Cheng, M.; Geyer, S. M.; Ng, K. Y. S. Electro-synthesis of

18

3D porous hierarchical Ni-Fe phosphate film/Ni foam as a high-efficiency bifunctional

19

electrocatalyst for overall water splitting. J. Mater. Chem. A 2016, 4, (36), 13866-13873.

20

(44)Bera, B.; Kar, T.; Chakraborty, A.; Neergat, M. Influence of nitrogen-doping in carbon

21

on equivalent distributed resistance and capacitance – Implications to electrocatalysis of

22

oxygen reduction reaction. J. Electroanal. Chem. 2017, 805, 184-192.

23

(45)Li, L.; Dai, P.; Gu, X.; Wang, Y.; Yan, L.; Zhao, X. High oxygen reduction activity on a

24

metal–organic framework derived carbon combined with high degree of graphitization

25

and pyridinic-N dopants. J. Mater. Chem. A 2017, 5, (2), 789-795.

26

(46)Kuang, M.; Wang, Q.; Han, P.; Zheng, G. Cu, Co-Embedded N-Enriched Mesoporous 24


Page 24 of 34

Page 25 of 34 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Carbon for Efficient Oxygen Reduction and Hydrogen Evolution Reactions. Adv. Energy

2

Mater. 2017, 7, (17), 1700193.

3

25



Figure 1 (A) XRD patterns of synthesized MIL-101(Fe)/C and simulated MIL-101(Cr); (B) XRD pattern of Fe/Fe3C@NC. 199x88mm (300 x 300 DPI)


Page 26 of 34

Page 27 of 34 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


Figure 2 Different magnification SEM images of (A) MIL-101(Fe), (B) MIL-101(Fe)/C, (C) Fe/Fe3C-NC, and (D) Fe/Fe3C@NC 199x152mm (300 x 300 DPI)



Figure 3 (A-D) TEM images of Fe/Fe3C@NC. 209x140mm (300 x 300 DPI)


Page 28 of 34

Page 29 of 34 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


Figure 4 N2 adsorption-desorption isotherm (A) and the corresponding pore-size distribution curve (B) of Fe/Fe3C@NC. 489x189mm (300 x 300 DPI)



Figure 5 (A) XPS spectra of Fe/Fe3C@NC-2:1, Fe/Fe3C@NC, Fe/Fe3C@NC-1:2, respectively. (B) Highresolution N1s XPS spectra of Fe/Fe3C@NC-2:1. (C) Fe/Fe3C@NC. (D) Fe/Fe3C@NC-1:2; (E) The content of pyridinic N, Fe-N, pyrrolic N, graphitic N and N-O of Fe/Fe3C@NC-2:1, Fe/Fe3C@NC, Fe/Fe3C@NC-1:2, respectively. (F) High-resolution Fe2p of Fe/Fe3C@NC. 184x219mm (300 x 300 DPI)


Page 30 of 34

Page 31 of 34 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


Figure 6 (A) CVs of Fe/Fe3C@NC in Ar and O2-saturated 0.1 M KOH solution at 50 mV s−1. (B-D) RDE polarization curves on the different as-prepared samples and Pt/C in O2-saturated 0.1 M KOH solution at 1600 rpm. (E) RDE polarization curves of Fe/Fe3C@NC at different rotation rates for ORR at a scan rate of 10 mV s −1. (F) K-L plots and electron transfer numbers at various potentials for the Fe/Fe3C@NC electrode. 250x130mm (300 x 300 DPI)



Figure 7 EIS of Fe/Fe3C@NC and Fe/Fe3C-NC conducted in (A) 5 mM Fe(CN)6 (B) in O2-saturated 0.1 M KOH. 473x180mm (300 x 300 DPI)


Page 32 of 34

3-/4-

containing 0.5 M KCl and

Page 33 of 34 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


Figure 8 (A) Chronoamperometric response of Pt/C and Fe/Fe3C@NC in O2-saturated 0.1 M KOH solution (125 ml) with the introduction of 12.5 ml of methanol at the time of 300 s. The arrow indicates the introduction of 3 M methanol. (B) Current-time chronoamperometric response of Pt/C and Fe/Fe3C@NC in O2-saturated 0.1 M KOH solution for 20000 s in O2-saturated 0.1 M KOH solution at a rotating rate of 1600 rpm. 250x93mm (300 x 300 DPI)



graphical abstract 320x200mm (96 x 96 DPI)


Page 34 of 34

Synthesis of MOF-Derived Nonprecious Catalyst with High

Recommend Documents