Well-Defined ZIF-Derived Fe–N Codoped Carbon Nanoframes as

Subscriber access provided by University of Newcastle, Australia

Article

Well-defined ZIF-derived Fe-N co-doped Carbon Nanoframes as Efficient Oxygen Reduction Catalysts Yijie Deng, Yuanyuan Dong, Guanghua Wang, Kailing Sun, Xiudong Shi, Long Zheng, Xiuhua Li, and Shijun Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b16851 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 4, 2017

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 free 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 accessible to all readers and 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.

ACS Applied Materials & Interfaces 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 28

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

ACS Applied Materials & Interfaces

Well-defined ZIF-derived Fe-N co-doped Carbon Nanoframes as Efficient Oxygen Reduction Catalysts Yijie Deng†, Yuanyuan Dong†, Guanghua Wang†, Kailing Sun†, Xiudong Shi†, Long Zheng†, Xiuhua Li†, Shijun Liao†,* †

The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key Laboratory

of New Energy, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China

ABSTRACT: A series of ZIF-derived Fe-N co-doped carbon materials with a welldefined morphology, high surface area, tunable sizes and porous nanoframe structure was successfully prepared by synthesizing Fe-doped ZIF-8 through the assembly of Zn2+ ions with 2-methylimidazole in the presence of iron(III) acetylacetonate, followed by pyrolysis at a high temperature and in an Ar atmosphere. The prepared optimum catalyst materials exhibited excellent activity for the oxygen reduction reaction (ORR) and outstanding durability in both acidic and alkaline solutions. We found that Fe doping during the ZIF-8 synthesis stage was crucial to achieve the materials’ well-defined morphology, tunable size, good particle dispersion, and high performance. XPS revealed that Fe doping greatly enhanced the fractions of graphiticN and pyridinic-N and decreased the fraction of oxidized-N. We suggest that the porosity and high surface area of the nanoframe structure originated from the metalorganic frameworks, the high dispersion of Fe in the nanoframe, and the enhanced proportions of active N species, all of which were responsible for the materials’ significantly enhanced ORR performance. KEYWORDS: carbon nanoframes, ZIF-8, high dispersion, oxygen reduction, fuel cell *

Corresponding author, e-mail: [email protected], fax +86 20 87113586.

1

ACS Paragon Plus Environment


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

Page 2 of 28

INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are considered as promising clean energy generation devices in many fields, including for mobile and stationary power supplies and for automobiles.1-5 Pt-based catalysts have long been regarded as the most effective electrocatalysts for boosting the sluggish oxygen reduction reaction (ORR) that occurs at the cathode.6-7 However, the scarcity, prohibitive cost, and poor stability of Pt have prevented the widespread utilization and commercialization of fuel cells.8-11 Considerable efforts have therefore been dedicated to exploring low-cost, earth-abundant,

high-performance,

durable

replacements for

Pt-based

ORR

electrocatalysts, such as transition metal-based materials (metal carbides, metal phosphides, metal oxides, metal chalcogenides, metal nitrides, etc.) and carbon-based catalysts.12-16 Among these, metal-nitrogen-carbon (M-N-C) systems are one of the most promising for the ORR on account of their unique electronic properties and structural features. In particular, Fe-N-C catalysts containing active C–N and Fe–N moieties on the surface have received a great deal of attention lately due to their excellent ORR activity in both acidic and alkaline solutions. 17-22 Recently, zeolitic imidazolate frameworks (ZIFs)––an extensive class of crystalline materials with permanent porosity, three-dimensional structure (3D), and a diverse array of metals and organic linkers––have been utilized as templates and precursors to fabricate porous carbon-based functional materials using direct heat treatment in an inert atmosphere and with subsequent acid leaching processes.23-28 A subclass of ZIFs, ZIF-8 nanocrystals, composed of imidazolate linkers containing carbon and nitrogen atoms and Zn2+ ions, have been identified as promising precursor for the preparation of nanoporous doped carbon electrocatalysts with high 2


Page 3 of 28

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


graphitization and a hierarchical porous structure, achieving a high mass transfer rate owing to the inherent presence of M, N, and C in the backbone, large surface areas, and large pore volumes.29-30 Although a number of ZIF-8-derived nano-carbon materials doped with transition metals via traditional impregnation method have been investigated as effective ORR electrocatalysts, most of them are not satisfactory in terms of retaining a good secondary microstructure, and they have insufficient electrochemical activity, especially in an acidic medium.31-33 Thus, developing a novel strategy to prepare ZIF-derived carbon materials with well-defined porous structures, special nanoframe particles, and high catalytic activity is highly desirable but remains challenging. 23, 34-35 Herein, we report a simple and effective approach for preparing well-defined, high-performance, ZIF-derived Fe-N co-doped carbon nanoframes by introducing Fe during ZIF-8 synthesis. We found that incorporating Fe preserved the well-defined 3D polyhedron morphology of the initial ZIF-8 particles, including their tunable particle size, and gave the final carbon nanoframes high surface area and a unique hierarchical porous structure. More importantly, the resulting Fe-N-doped 3D carbon nanoframes exhibited outstanding ORR activity and durability in both acidic and alkaline solutions. In an alkaline medium, the catalyst had a more positive half-wave potential and a current density at 0.70 V 1.26 times higher than that of a commercial Pt/C catalyst; in an acidic medium, the difference between the half-wave potential of our catalyst and that of commercial Pt/C was 80 mV(0.77V vs 0.85V), making it one of the best doped carbon catalysts in an acidic medium. We attribute the outstanding performance of our catalyst to its well-defined 3D nanoframe structure and the high dispersion of Fe moieties caused by the introduction of Fe in the ZIF synthesis stage.

3



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

EXPERIMENTAL SECTIONS Electrocatalyst Preparation. The synthesis of ZIF-8@iron(III) acetylacetonate nanocrystals was conducted using a slight modification of a previously reported method.36 In a typical synthesis, 1.75 g 2-methylimidazole was dissolved in 12.5 ml methanol, and 0.25 g Zn(NO3)2·6H2O and 0.25 g iron(III) acetylacetonate were dissolved in 6.25 ml methanol. Next, the second solution was added into the first solution under stirring, and the final mixture was stirred constantly for 24 h at room temperature to form ZIF-8. The product was collected by centrifugation, followed by washing with methanol and drying in a vacuum. ZIF-8 and FeZIF-Y (where Y = the molar ratio between iron(III) acetylacetonate and Zn(NO3)2·6H2O in the precursors) were prepared using a similar synthesis method. The prepared nanocrystals were pyrolyzed under an argon atmosphere in a tube furnace as follows. First, the temperature was increased from room temperature to 350°C at a rate of 5°C·min–1; After maintaining this temperature for 1 h, we increased it to 900°C at a rate of 5°C·min–1, then kept it at this temperature for 3 h; Second, the pyrolyzed product was treated with 0.5 M H2SO4 solution for 12 h, following by washing with deionized water three times and drying. Finally, the material was heated at 900°C for 1 h to achieve further pyrolysis and graphitization. Figure. 1 summarizes the procedure. We denote the as-prepared carbon catalyst as C-FeZIF-X-Y, where “X” indicates the pyrolysis temperature and “Y” indicates the molar ratio of iron(III) acetylacetonate /Zn(NO3)2·6H2O used in the preparation process. All of the ZIF-8 derived carbon catalysts described in this paper were prepared using the same procedures detailed above.

4


Page 4 of 28

Page 5 of 28

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. 1. A schematic diagram showing the synthesis of Fe-doped ZIF-8 and the preparation of its derived doped carbon nanoframes.

Materials Characterization. The products were characterized using the below methods: transmission electron microscopy (TEM) on a JEM-2100 transmission electron microscope (JEOL, Japan) operating at 120 kV; X-ray diffraction (XRD) on a TD-3500 powder diffractometer (Tongda, China) operating at 30 kV and 20 mA, using Cu-Kα radiation sources; scanning electron microscopy (SEM) on a Merlin field emission SEM (Carl Zeiss); X-ray photoelectron spectroscopy (XPS) on a VG ESCALAB MK2 X-ray photoelectron spectrometer (VG corporation, UK), using an Al-Kα X-ray source; Brunauer–Emmett–Teller (BET) surface area and pore distribution employed on a Tristar 3020 gas adsorption analyzer at 77 K; and Raman analysis on a LabRAM Aramis Raman spectrometer (HJY, France). Electrocatalytic Testing. Electrocatalytic activity evaluations were conducted in a standard three-electrode system with a rotating disk electrode at room temperature (Ivium electrochemical workstation, Ivium, Netherlands). The three-electrode system contained a Ag/AgCl (3 M NaCl solution) for the acidic medium and a Hg/HgO(1 M KOH solution) for the alkaline medium respectively were used as reference electrode, a platinum wire was the counter electrode and glassy carbon electrode was used as the working electrode substrate (GC, 0.196 cm2). 5



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

The carbon catalyst-loaded electrodes were prepared using the following procedure. First, a catalyst ink was prepared by ultrasonically dispersing 5 mg catalyst in 1 mL Nafion/ethanol (0.25 wt% Nafion); the obtained 20 µL of catalyst ink was spread on a glassy carbon substrate electrode and dried. The catalyst loading was approximately 0.5 mg·cm–2. Cyclic voltammetry (CV) was performed at a scan rate of 10 mV·s−1 in O2- or N2saturated 0.1 M HClO4 and 0.1 M KOH electrolyte, and linear sweep voltammetry (LSV) was performed at various rotation rates, from 900 to 2500 rpm.The electron transfer number (n) per oxygen molecule during the ORR testing was calculated employing the Koutecky–Levich (K–L) equation: J −1 = J k−1 + ( 0.62nFCD 2/3γ −1/6ω1/2 )

−1

where J represents the measured disk current density, Jk represents the kinetic current density, n represents the electron transfer number, ω represents electrode rotation rate, F represents the Faraday constant, and C is the O2 saturated concentration in the electrolyte, D is the O2 diffusion coefficient in the electrolyte, and γ is the kinetic viscosity of the electrolyte. The chronoamperometric technique was employed to evaluate the catalyst’s durability. Durability testing was conducted at 0.62 V for 20,000 s with a rotation rate of 900 rpm in an O2-saturated 0.1 M HClO4 solution and an O2-saturated 0.1 M KOH solution. RESULTS AND DISCUSSION Figure. 2 shows SEM images of ZIF-8 and FeZIF-8 precursor samples with various molar ratios of Fe/Zn, as well as their derived carbon nanoframe materials. All of the nanocrystals showed well-defined polyhedrons, and the addition of Fe at the synthesis stage did not affect the their formation(see Figure. (S1-S3)). However, addition of iron(III) acetylacetonate did significantly affect the crystal sizes. ZIF-8 6


Page 6 of 28

Page 7 of 28

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


was 80–100 nm, whereas with the addition of iron(III) acetylacetonate, the crystals grew gradually: for the FeZIF-8 samples with Fe/Zn ratios of 0.42, 0.84, 2.53, and 3.54, the crystal sizes were 150–180, 190–240, 400–510, and 520–580 nm, respectively (Figure. 2 (a–e)). Clearly, the introduction of Fe resulted in a significant increase in particle size. What was the cause? According to the literature, the presence of iron(III) acetylacetonate in the preparation process of ZIF-8 can result in partial deprotonation of 2-methylimidazole linkers, as well as broader growth directions and larger crystals. 37-38 Figure. 2(f–j) show SEM images of the doped carbon nanoframe catalysts derived from ZIF-8 and FeZIF-8 samples with various Fe/Zn ratios. It is important that the carbon nanoframes derived from FeZIF-8 retained the polyhedral morphology of the ZIF-8 materials very well, while the surface of the nanoframes became much rougher due to dehydration and pyrolysis, exhibiting a well-defined 3D porous structure that will be highly favorable for fast diffusion of oxygen and electrolyte.39-41 Compared with the precursors, the derived materials had much smaller particle sizes, which is attributable to the contraction of the organic skeleton during pyrolysis. As the Fe content increased, the size of the derived nanoframes gradually increased, showing tunable size, which was consistent with the trend in their precursors. It is worth noting that when the Fe content reached 2.53 or 3.54 (Fe/Zn ratio), some carbon nanotubes (CNTs) could be clearly observed on the surface of the polyhedral nanoparticle, which is due to the addition of excess Fe may have played a catalytic role in the formation of CNTs. 42

7



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 SEM images of the precursors (a) ZIF-8, (b) FeZIF-0.42, (c) FeZIF-0.84, (d) FeZIF-2.53, (e) FeZIF-3.54; SEM images of (f) C-ZIF-8-900, (g) C-FeZIF-900-0.42, (h) C-FeZIF-900-0.84, (i) CFeZIF-900-2.53, (j) C-FeZIF-900-3.54

Figure. 3(a–e) show TEM images of all five samples, and it can be seen that all had a polyhedral shape and hierarchical porous structure. No Fe particles could be observed in the TEM images of C-FeZIF-900-0.42 or C-FeZIF-900-0.84, suggesting that the Fe may have been well dispersed and incorporated in the doped carbon framework.35,

43

This suggestion was furthermore supported by the EDX mapping

results (Figure. 3f), where the Fe was well distributed in the carbon nanoframework. However, for samples with high Fe content, some Fe species particles were observable (Figure. 3d and 3e), revealing that high iron addition will result in the formation of Fe species particles, except the incorporation in the carbon nanoframes. 44

As we will describe below, the formation of Fe species particles will block the

porous channel and cover the active surface of the materials, resulting in the decrease of the surface area and ORR performance.

8


Page 8 of 28

Page 9 of 28

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


As shown in Figure. S4, the ZIF-8 and FeZIF-0.84 precursors displayed typical Type I behavior, and the adsorption quantity sharply increased at a low relative pressure, demonstrating that they were mainly microporous materials. Similar isotherms for ZIF-8 have been reported in the literature.3, 24, 45 It is notable that the surface area (1253 m2×g–1) of FeZIF-8 (Fe/Zn = 0.84) was significantly lower than that of ZIF-8 (1434m2×g–1), implying that the addition of iron(III) acetylacetonate in the ZIF synthesis stage may have caused structural changes in ZIF-8.38

Figure. 3 TEM images of (a) C-ZIF-8-900, (b) C-FeZIF-900-0.42, (c) C-FeZIF-900-0.84, (d) CFeZIF-900-2.53, (e) C-FeZIF-900-3.54; (f) HAADF-STEM images and elemental mapping of C-FeZIF900-0.84

Figure. 4 shows the nitrogen adsorption/desorption curves for all five carbon nanoframe samples. Samples C-ZIF-8 and C-FeZIF-900-0.42 (Figure. 4a and 4b) displayed Type I behavior curves, almost the same as those of their precursors (see Figure. S4). However, as the Fe content increased, the nitrogen adsorption/desorption isotherms exhibited typical Type IV isotherm characteristics, with hysteresis loops in the medium pressure region, which is generally attributed to a mesoporous 9



structure.46-47 Clearly, the added Fe played a crucial role for the formation of mesoporous

structure,

we

believe

that

it

may

catalyze

the

decomposition/dehydrogenation of organic ligand of ZIF-8, and induce the formation of mesoporous structure, as it worked in the formation of carbon nanotube.48 When enough Fe compound was added to the ZIF-8, the resulting C-FeZIF nanoframes displayed a hierarchical micro-mesoporous structure, which would be beneficial for the efficient mass transport of the electrolyte and oxygen and for exposure of the

600

a

500

400

b

2.0

C-FeZIF-900-0.42 C-FeZIF-900-0.84 C-FeZIF-900-2.53 C-FeZIF-900-3.54 C-ZIF-8-900

dV/dlog(D) (cm-3g-1nm-1)

3 -1

Quantity Adsorbed (cm g STP)

active sites. 49

300

200

C-ZIF-8-900 C-FeZIF-900-0.42 C-FeZIF-900-0.84 C-FeZIF-900-2.53 C-FeZIF-900-3.54

1.5

1.0

0.5

0.0 100 0.0

0.2

0.4

0.6

0.8

1.0

0

40

120

160

200

240

2.4

c

d

C-FeZIF-700-0.84 C-FeZIF-800-0.84 C-FeZIF-900-0.84 C-FeZIF-1000-0.84

400

2.0

dV/dlog(D) (cm-3g-1nm-1)

500

80

Pore Diameter (nm)

Relative Pressure (P/Po)

Quantity Adsorbed (cm3g-1 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

Page 10 of 28

300

200

100

1.6

C-FeZIF-700-0.84 C-FeZIF-800-0.84 C-FeZIF-900-0.84 C-FeZIF-1000-0.84

1.2 0.8 0.4 0.0

0.0

0.2

0.4

0.6

0.8

1.0

0

Relative Pressure (P/Po)

40

80

120

160

200

Pore Diameter (nm)

Figure. 4 N2 adsorption and desorption isotherms and pore-size distributions of C-FeZIF-900 with different molar ratios of iron(III) acetylacetonate/Zn(NO3)2·6H2O (a and b, respectively) and C-FeZIF0.84 at different temperatures (c and d, respectively)

We found that all of the carbon nanoframe samples had lower surface areas than those of their precursors, which may have been caused by the destruction of the microporous structure of the ZIF materials. The surface area was also affected by the addition of Fe; as the amount of Fe increased, the surface area significantly decreased 10


Page 11 of 28

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


(See Table 1). When the Fe content increased to 2.53 or 3.54 (Fe/Zn ratio), the surface area markedly reduced to 654 and 505 m2×g–1, respectively. Table 1. BET surface areas and current densities at 0.70 V of the as-prepared samples Current density at 0.70 V Samples

BET surface area (m2·g–1) 0.1 M KOH

0.1 M HClO4

C-ZIF-8

1045

-2.93

-0.04

C-FeZIF-900-0.42

952

-5.40

-3.31

C-FeZIF-900-0.84

926

-6.48

-5.51

C-FeZIF-900-2.53

654

-5.65

-4.16

C-FeZIF-900-3.54

505

-4.73

-3.18

C-FeZIF-700-0.84

477

-3.57

-0.57

C-FeZIF-800-0.84

700

-5.04

-2.82

C-FeZIF-1000-0.84

843

-5.54

-4.46

XRD patterns of all the samples before and after acid leaching are shown in Figure. 5b and 5a, respectively. For the samples with less added Fe, no peaks for iron oxides were observable, but these peaks did appear for samples with more Fe: C-FeZIF-9002.53 and C-FeZIF-900-3.54. This information is quite important, as it may indicate that in samples with Fe/Zn ratios equal to or less than 0.84, all the Fe atoms were well distributed in the framework of the doped carbon materials at the atomic level or as very fine particles, and excess iron addition could lead to the formation of Fe species particles which may filled the pores and block the porous channel and a lower specific surface area(see Table 1).35,

44, 50-51

Furthermore, it can be seen that the peak for

graphitic carbon positively shifted and become very sharp, indicating that the addition of Fe resulted in a higher degree of graphitization, which is generally beneficial for 11



enhancing a catalyst’s ORR performance.52 After acid leaching, the peaks belonging to the Fe oxides were noticeably smaller, indicating that some of these compounds may have been dissolved in the acid solution. No obvious changes were observable in the carbon peaks.

a

b

∗ Fe2O3 ∗

∗

C(101)

∗ ∗

∗

∗

∗

∗ Fe2O3

C(101)

C(002)

C-FeZIF-900-3.54

C-FeZIF-900-2.53 C-FeZIF-900-0.84

C-FeZIF-900-3.54

∗

∗

Intensity/ a.u.

C(002)

Intensity/ a.u.

∗

∗

∗

C-FeZIF-900-2.53

C-FeZIF-900-0.84 C-FeZIF-900-0.42

C-FeZIF-900-0.42

C-FeZIF-900

10

20

30

40

50

60

70

10

20

30

40

50

60

70

2θ/ degree

2θ/ degree

c

C-ZIF-8-900 C-FeZIF-900-0.84 D

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

Page 12 of 28

G

ID/IG=1.19

ID/IG=1.13 800

1000

1200

1400

1600

1800

2000

Raman Shift / cm-1

Figure. 5 XRD patterns of C-ZIF-8-900 and C-FeZIF-900 with different molar ratios of iron(III) acetylacetonate/(Zn(NO3)2·6H2O) after acid leaching (a) and before acid leaching (b); and (c)Raman spectra of C-ZIF-8-900 and C-FeZIF-900-0.84

The Raman spectra of C-ZIF-8-900 and C-FeZIF-900-0.84 are displayed in Figure. 5c. The ID/IG ratio could provide the demonstration for amount of structural defects induced by heterogeneous atom incorporation into the carbon layers.53 The ID/IG ratios of C-ZIF-8-900 and C-FeZIF-900-0.84 were 1.13 and 1.19, respectively, demonstrating that C-FeZIF-900-0.84 may have more structural defects forming catalytic sites, which could have been due to the Fe incorporation, and enhanced N incorporation in the carbon framework in the existence of Fe.54 Furthermore, this 12


Page 13 of 28

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


suggestion is supported by the XPS results(shown in Figure. 6a–d and Table 2), indicating that C-FeZIF-900-0.84 contained far higher N content––7.74 at%, compared with 6.37 at% for C-ZIF-8-900. The high-resolution XPS spectra of Fe 2p in C-FeZIF-900-0.84 (Figure. 6d) could be deconvoluted into five peaks, including the Fe(III) 2p1/2 peak at 725.6 eV, the Fe(II) 2p1/2 peak at 723.4 eV, the Fe(III) 2p3/2 peak at 714.6 eV, the Fe(III) 2p3/2 peak at 710.7 eV, and the satellite peak at 719.6 eV, indicating that Fe existed as Fe(II) and Fe(III) in the C-FeZIF-900-0.84. The highresolution N1s XPS spectra of C-ZIF-8-900 and C-FeZIF-900-0.84, along with the deconvolution results of each spectrum, are presented in Figure. 6b and 6c and can be deconvoluted to four peaks: pyridinic-N (N1, 398.4 ± 0.1 eV), pyrrolic-N (N2, 399.8 ± 0.1 eV), graphitic-N (N3, 400.9 ± 0.1 eV), and oxidized-N (N4, 402.4 ±0.3 eV). The amounts of each N type in the two as-prepared catalysts, obtained by integrating each deconvolution peak, are shown in Table 2. It has been reported that pyridinic-N and graphitic-N play a pivotal role in catalyzing the ORR by providing active sites.55 The pyridinic-N and graphitic-N content in C-FeZIF-900-0.84 (N1 35.53% and N3 38.44%) was much higher than in C-ZIF-8-900 (N1 29.7% and N3 26.23%), revealing the favorable effect of the Fe upon catalysis.9 It is reasonable to expect that the ORR performance of C-FeZIF-900-0.84 would be much higher than that of C-ZIF-8-900. Table 2. Fraction of the different N species present in C-ZIF-8-900 and C-FeZIF-900-0.84 Oxidized Samples

N

Graphitic N

Pyrrolic N

Pyridinic N

(%)

(%)

(%)

(%)

C-ZIF-8-900

15.91

26.23

28.16

29.70

C-FeZIF-900-0.84

7.16

38.44

18.88

35.53

13



a

b

C1s

O1s

N1s

Fe2p

800

700

600

500

400

Graphitic N

Intensity/a.u.

Intensity/a.u

C-FeZIF-900-0.84 C-ZIF-8-900

300

200

Pyrrolic N

408

100

Pyridinic N

Oxidized N

406

404

402

400

398

396

394

Binding energy / eV

Binding Energy/eV

c

d Fe2p Graphitic N

Pyridinic N

Oxidized N

Pyrrolic N

723.4eV 2+ Fe 2p1/2

725.6eV 3+ Fe 2p1/2

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

Page 14 of 28

714.6eV 3+ Fe 2p3/2

710.7eV 2+ Fe 2p3/2

719.6eV Satellite Peak 408

406

404

402

400

398

396

394

740

Binding Energy /eV

735

730

725

720

715

710

705

700

Binding energy / eV

Figure. 6 (a) Survey XPS spectra, high-resolution; (b) N1s XPS spectra of C-ZIF-8-900, (c) N1s XPS spectra of C-FeZIF-900-0.84, and (d) XPS spectra of Fe2p

The electrocatalytic activity of the Fe-N-doped carbon nanoframes toward the ORR was evaluated in both acidic and alkaline electrolytes. In the N2-saturated electrolyte, featureless voltammetric curves with no obvious redox peak were observed. However, when the electrolyte was saturated with O2, a well-defined oxygen reduction peak appeared at 0.86 V in 0.1 M KOH (see Figure. 7a) and at 0.77 V in 0.1 M HClO4 (see Figure. 7b), suggesting that the as-prepared C-FeZIF-900-0.84 had pronounced ORR electrocatalytic activity in both electrolytes. We also investigated the effects of the Fe/Zn ratio on performance. In an alkaline medium, all of the catalysts with Fe showed much higher ORR activity than the catalyst without Fe (C-ZIF-8-900). C-FeZIF-900-0.84, with a Fe/Zn ratio of 0.84, exhibited the highest ORR performance; its current density at 0.7 V (vs. RHE) reached -6.48 mA×cm–2 (Figure. 7c), which is almost 1.26 times that of a commercial Pt/C catalyst (JM 20% Pt/C). Almost the same trend was observed in an acidic 14


Page 15 of 28

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


medium (Figure. 7d). All of the catalysts containing Fe showed greatly enhanced ORR performance in an acidic medium, with C-FeZIF-900-0.84 presenting the best ORR performance. The half-wave potential of C-FeZIF-900-0.84 was only 80 mV lower than that of a Pt/C catalyst. For Fe-N-C system, a lot of researchers suggested that the appropriate content of iron is crucial to acquire good performance of electrocatalyst.44, 50 For our catalyst, the lower ORR activity of samples with lower Fe addition could attribute to their low density of active sites and without formation of mesoporous structure. Why higher Fe content also results in the inferior ORR activity? We intend to attribute it to following factors: firstly, the larger particle size caused by the excess addition of Fe, and the increase of particle size will result in the decrease of surface area and activity, as demonstrated by our experimental results; as we known, performance of oxygen reduction reaction is closely related with the appropriate particle size and specific surface area of catalyst.31,

43, 56

The second factor is the

formation of Fe species particles, as described above, it may fill up the pores, block the pore channel, and cover the active surface of the catalyst which results in lower the active site density; Finally, the changes of porous structure and specific surface area caused by the excess addition of Fe may affect the ORR activity of the catalysts, as shown in Figure 4a and Table 1, the samples with higher Fe addition exhibit obviously different features, compared with those of the catalyst with lower Fe addition: more meso or macro pores and lower specific surface area existed in the higher Fe added samples, and more micro pores existed in the lower Fe added samples. In summary, the outstanding electrocatalytic performance of our C-FeZIF-900-0.84 catalyst could be attributed to the well-defined 3D porous structure, the uniform distribution of doped Fe and N in the carbon nanoframe, the high surface area and hierarchical mirco-meso porous structure caused by the addition of appropriate added 15



amount of Fe. To the best of our knowledge, the ORR performance of C-FeZIF-9000.84 is one of the best for doped carbon catalysts currently used for the ORR (see Tables S1 and S2). 3

a

O2-saturated N2-saturated

b

Current Density(mA⋅cm )

2

O2-saturated N2-saturated

-2

-2


2

1

0

-1

1

0

-1

-2

-2 0.0

0.2

0.4

0.6

0.8

1.0

-3 0.0

1.2

0.2

0.4

-1

-3 -4 -5 -6

-2 -3 -4 -5 C-ZIF-8-900 C-FeZIF-900-0.42 C-FeZIF-900-0.84 C-FeZIF-900-2.53 C-FeZIF-900-3.54 Pt/C

-6 -7

-7

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-8 0.2

1.1

0.3

0.4

100

80

90

Relative current(%)

100

60

40

C-FeZIF-900-0.84 Pt/C

20

e 10000

0.6

0.7

0.8

0.9

1.0

15000

80

70

C-FeZIF-900-0.84 Pt/C

60

f

0 5000

0.5

Potential(V vs RHE)

Potential(V vs RHE)

0

1.2

-2


-2

-8 0.2

1.0

d

-1

C-ZIF-8-900 C-FeZIF-900-0.42 C-FeZIF-900-0.84 C-FeZIF-900-2.53 C-FeZIF-900-3.54 Pt/C

-2


0.8

0

c

0

0.6

Potential(V vs RHE)

Potential(V vs RHE)

Relative Current (%)

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

Page 16 of 28

20000

time (s)

50 0

5000

10000

15000

20000

time/s

Figure. 7 CV curves of C-FeZIF-900-0.84 in N2–and O2–saturated 0.1 M KOH (a) and HClO4 (b); LSV ORR curves of C-ZIF-8-900, C-FeZIF-900-0.42, C-FeZIF-900-0.84, C-FeZIF-900-2.53, C-FeZIF-9003.54, and 20 wt% Pt/C in O2–saturated 0.1 M KOH (c) and HClO4 (d) (rotation rate: 1600 rpm); durability evaluation of C-FeZIF-900-0.84 and Pt/C for 20,000 s in O2–saturated 0.1 M KOH (e) and HClO4 (f)

We therefore attribute this performance to the following factors: (1) a welldefined 3D porous structure that facilitates the fast diffusion of O2 and electrolyte; (2) uniform distribution of Fe and N in the carbon nanoframe, resulting in good dispersion and favourable exposure of the highly active sites; and (3) appropriate

16


Page 17 of 28

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


particle size. All of these are due to the addition of an appropriate amount of Fe (or the Fe/Zn ratio). We also investigated the effects of pyrolysis temperature (700, 800, 900, and 1000°C) on the activity of the catalysts with a Fe/Zn ratio of 0.84 (see Table 1 and Figure. S5). The catalyst pyrolyzed at 900oC, C-FeZIF-900-0.84, exhibited the highest ORR activity, so the optimal pyrolysis temperature seems to be around 900oC. To further understand the effects of adding Fe during the synthesis process of ZIF8, we prepared a Fe-N-C catalyst, for comparison, by impregnating ZIF-8 with iron(III) acetylacetonate solution, followed by pyrolysis at 900oC. XRD detected a considerable amount of iron oxides (Figure. S6(b)). Furthermore, the ORR performance of the catalyst prepared using this impregnation method was far inferior to that of C-FeZIF-900-0.84 in both alkaline and acidic media, demonstrating the superiority of our catalyst, which had Fe added during the synthesis stage of ZIF-8 (Figure. S8). The ORR kinetics in alkaline and acidic media were further investigated by gathering LSV curves at different electrode rotation rates (see Figure. 8(a, b)) and were analyzed using the Koutecky–Levich (K–L) equation to calculate the exact electron transfer numbers. The K–L plots (j−1 vs. ω−1/2) of C-FeZIF-900-0.84 with good linearity at different electrode potentials are presented in Figure. 8(c, d). According to the K–L equation, in 0.1 M KOH, the average n value of C-FeZIF-9000.84 was close to 4 (n = 3.8), demonstrating that the ORR on C-FeZIF-900-0.84 mainly followed a four-electron process; in 0.1 M HClO4, the average apparent electron transfer number was about 3.4, indicating that with C-FeZIF-900-0.84 as the catalyst, a four-electron reaction again dominated.

17



0

0

900r/min 1225r/min 1600r/min 2025r/min 2500r/min

-4

-2

-2

-2

-1



a

-6

-8

b

900 r/min 1225 r/min 1600 r/min 2025 r/min 2500 r/min

-2 -3 -4 -5 -6 -7 -8 -9

-10 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-10 0.2

1.1

0.3

0.4

0.22

0.5

0.6

0.7

0.8

0.9

Potential(V vs RHE)

Potential(V vs RHE)

0.24

c

0.22

0.20

d

0.18

J (mA cm )

0.14

0.12

0.18

0.288V 0.338V 0.388V 0.438V 0.488V 0.538V 0.588V 0.638V

-1

2

0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70

0.16

-1

-1

-1

2

J (mA cm )

0.20

0.16 0.14 0.12 0.10

0.10 0.06

0.07

0.08 -1/2

ω

0.09

0.10

0.11

0.06

0.07

0.08 -1/2

-1/2

(rps )

ω

0.09

0.10

0.11

-1/2

(rps )

4

Transfer electron number

4

Transfer electron number

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

Page 18 of 28

3

2

1

e

3

2

1

f 0

0 0.4

0.5

0.6

0.7

0.40

0.8

0.45

0.50

0.55

0.60

0.65

Potential(V vs RHE)

Potential(V vs RHE)

Figure. 8 ORR curves of C-FeZIF-900-0.84 obtained at different rotation rates in O2-saturated 0.1 M KOH (a) and HClO4 (b); K–L plots of j−1 versus ω–1/2 on C-FeZIF-900-0.84 in O2-saturated 0.1 M KOH (c) and HClO4 (d); transferred electron numbers at different potentials in 0.1 M KOH (e) and HClO4 (f)

As shown in Figure. 7e and 7f, after 20,000 s of continuous chronoamperometric measurements at 0.62 V in 0.1 M KOH, commercial Pt/C lost almost 20% of its initial current density, whereas C-FeZIF-900-0.84 maintained more than 95% of its initial ORR performance; in 0.1 M HClO4, C-FeZIF-900-0.84 still retained 93% of its initial current density after 20,000 s, while commercial Pt/C exhibited a current loss of about 17%. The C-FeZIF-900-0.84 exhibited not only excellent ORR activity but also excellent stability in both alkaline and acidic media, which makes it a promising substitutes towards oxygen reduction electrocatalysts.

18


Page 19 of 28

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


CONCLUSIONS A high-performance doped carbon nanoframe catalyst was successfully prepared by adding Fe during the synthesis of ZIF-8, followed by pyrolysis in an Ar flow. The addition of Fe greatly enhanced the catalyst’s ORR performance, and we investigated how the catalyst’s structure and performance were affected by the amount of Fe added and the pyrolysis temperature. The catalyst exhibited outstanding ORR performance in alkaline and acidic media: our optimal catalyst, C-FeZIF-900-0.84, yielded a current density at 0.70 V 1.26 times higher than that of a commercial Pt/C catalyst in an alkaline medium, while in an acidic medium, the half-wave potential of C-FeZIF900-0.84 was only 80 mV lower than that of the Pt/C catalyst. The catalyst also exhibited excellent stability in both acidic and alkaline media. We attribute its outstanding ORR performance to the addition of Fe in the synthesis stage of the ZIF-8 precursor. The addition of Fe fabricated a catalyst with a well-retained ZIF morphology, well-defined porous structure, high surface area, large proportions of active N species, and high active site density. The excellent ORR performance of our catalyst makes it one of the best doped carbon catalysts reported to date, and our work may provide a new pathway for developing and designing other high-performance doped carbon ORR catalysts. ASSOCIATED CONTENT Supporting Information is about XRD, SEM,TEM, LSVs and FTIR of samples. AUTHOR INFORMATION Corresponding Authors *

Fax: +86 20 87113586. E-mail: [email protected]. 19



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

Page 20 of 28

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (NSFC Project Nos. 21276098, 21476088, 51302091, U1301245), China Postdoctoral Science Foundation (Project No. 2016M592492), the Department of Science and Technology

of

Guangdong

Province

(Project

Nos.

2014A010105041,

2015B010106012, 2016A010103028), the Natural Science Foundation of Guangdong Province (Project No. 2015A030312007), and the Educational Commission of Guangdong Province (Project No. 2013CXZDA003). REFERENCES (1) Ding, W.; Li, L.; Xiong, K.; Wang, Y.; Li, W.; Nie, Y.; Chen, S.; Qi, X.; Wei, Z., Shape Fixing via Salt Recrystallization: A Morphology-Controlled Approach to Convert Nanostructured Polymer to Carbon Nanomaterial as a Highly Active Catalyst for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2015, 137, 5414-5420. (2) Nie, Y.; Li, L.; Wei, Z., Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 44, 2168-2201. (3) Thomas, M.; Illathvalappil, R.; Kurungot, S.; Nair, B. N.; Mohamed, A. A. P.; Anilkumar, G. M.; Yamaguchi, T.; Hareesh, U. S., Graphene Oxide Sheathed ZIF-8 Microcrystals: Engineered Precursors of Nitrogen-Doped Porous Carbon for Efficient Oxygen Reduction Reaction (ORR) Electrocatalysis. ACS Appl. Mater. Interfaces 2016, 8, 29373-29382. (4) Zhu, C.; Li, H.; Fu, S.; Du, D.; Lin, Y., Highly Efficient Nonprecious Metal Catalysts Towards

Oxygen

Reduction

Reaction

Based

on

Three-Dimensional

Nanostructures. Chem. Soc. Rev. 2016, 45, 517-531.

20


Porous

Carbon

Page 21 of 28

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


(5) Debe, M. K., Electrocatalyst Approaches and Challenges for Automotive Fuel Cells. Nature 2012, 486, 43-51. (6) Khan, I. A.; Qian, Y.; Badshah, A.; Nadeem, M. A.; Zhao, D., Highly Porous Carbon Derived from MOF-5 as a Support of ORR Electrocatalysts for Fuel Cells. ACS Appl. Mater. Interfaces 2016, 8, 17268-17275. (7) Wang, C.; Hu, F.; Yang, H.; Zhang, Y.; Lu, H.; Wang, Q., 1.82 Wt.% Pt/N, P Co-Doped Carbon Overwhelms 20 Wt.% Pt/C as a High-Efficiency Electrocatalyst for Hydrogen Evolution Reaction. Nano Res. 2017, 10, 238-246. (8) Li, W.; Ding, W.; Wu, G.; Liao, J.; Yao, N.; Qi, X.; Li, L.; Chen, S.; Wei, Z., Cobalt Modified Two-Dimensional Polypyrrole Synthesized in a Flat Nanoreactor for the Catalysis of Oxygen Reduction. Chem. Eng. Sci. 2015, 135, 45-51. (9) Wu, R.; Chen, S.; Zhang, Y.; Wang, Y.; Nie, Y.; Ding, W.; Qi, X.; Wei, Z., Controlled Synthesis of Hollow Micro/Meso-Pore Nitrogen-Doped Carbon with Tunable Wall Thickness and Specific Surface Area as Efficient Electrocatalysts for Oxygen Reduction Reaction. J. Mater. Chem. A 2016, 4, 2433-2437. (10)

Yan, Z.; He, G.; Cai, M.; Meng, H.; Shen, P. K., Formation of Tungsten Carbide

Nanoparticles on Graphitized Carbon to Facilitate the Oxygen Reduction Reaction. J. Power Sources 2013, 242, 817-823. (11) You, C.; Zheng, R.; Shu, T.; Liu, L.; Liao, S., High Porosity and Surface Area Self-Doped Carbon Derived from Polyacrylonitrile as Efficient Electrocatalyst towards Oxygen Reduction. J. Power Sources 2016, 324, 134-141. (12)

Liu, Y.; Kelly, T. G.; Chen, J. G.; Mustain, W. E., Metal Carbides as Alternative

Electrocatalyst Supports. ACS Catal. 2013, 3, 1184-1194. (13) Shang, L.; Yu, H.; Huang, X.; Bian, T.; Shi, R.; Zhao, Y.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C. H.; Zhang, T., Well-Dispersed ZIF-Derived Co,N-co-doped Carbon Nanoframes through Mesoporous-Silica-Protected Calcination as Efficient Oxygen Reduction Electrocatalysts. Adv. Mater. 2016, 28, 1668-1674.

21



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

(14) Tian, X.; Luo, J.; Nan, H.; Fu, Z.; Zeng, J.; Liao, S., Binary Transition Metal Nitrides with Enhanced Activity and Durability for the Oxygen Reduction Reaction. J. Mater. Chem. A 2015, 3, 16801-16809. (15) Tian, X.; Luo, J.; Nan, H.; Zou, H.; Chen, R.; Shu, T.; Li, X.; Li, Y.; Song, H.; Liao, S.; Adzic, R. R., Transition Metal Nitride Coated with Atomic Layers of Pt as a Low-Cost, Highly Stable Electrocatalyst for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2016, 138, 15751583. (16) Wang, S.; Li, X.; Jiang, Q.; Dou, S.; Deng, L.; Huo, J., ZIF-67-Derived Co-NC@CoP-NC Nanopolyhedrals as Efficient Bifunctional Oxygen Electrocatalysts. J. Mater. Chem. A 2016, 4, 15836-15840. (17) Fu, X.; Zamani, P.; Choi, J.-Y.; Hassan, F. M.; Jiang, G.; Higgins, D. C.; Zhang, Y.; Hoque, M. A.; Chen, Z., In Situ Polymer Graphenization Ingrained with Nanoporosity in a Nitrogenous Electrocatalyst Boosting the Performance of Polymer-Electrolyte-Membrane Fuel Cells. Adv. Mater. 2017, DOI: 10.1002/adma.201604456 (18) Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.; Wei, Z.; Wan, L.-J., Understanding the High Activity of Fe–N–C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe–Nx. J. Am. Chem. Soc. 2016, 138, 3570-3578. (19) Lee, J.-S.; Park, G. S.; Kim, S. T.; Liu, M.; Cho, J., A Highly Efficient Electrocatalyst for the Oxygen Reduction Reaction: N-Doped Ketjenblack Incorporated into Fe/Fe3C-Functionalized Melamine Foam. Angew. Chem. 2013, 125, 1060-1064. (20) Niu, Y.; Huang, X.; Hu, W., Fe3C Nanoparticle Decorated Fe/N Doped Graphene for Efficient Oxygen Reduction Reaction Electrocatalysis. J. Power Sources 2016, 332, 305-311. (21) Zheng, R.; Liao, S.; Hou, S.; Qiao, X.; Wang, G.; Liu, L.; Shu, T.; Du, L., A Hollow Spherical Doped Carbon Catalyst Derived from Zeolitic Imidazolate Framework Nanocrystals Impregnated/Covered with Iron Phthalocyanines. J. Mater. Chem. A 2016, 4, 7859-7868. (22) Wu, Z. Y.; Xu, X. X.; Hu, B. C.; Liang, H. W.; Lin, Y.; Chen, L. F.; Yu, S. H., Iron Carbide Nanoparticles Encapsulated in Mesoporous Fe-N-Doped Carbon Nanofibers for Efficient Electrocatalysis. Angew. Chem. Int. Ed. 2015, 54, 8179-8183. 22


Page 22 of 28

Page 23 of 28

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


(23)

Shen, K.; Chen, X.; Chen, J.; Li, Y., Development of MOF-Derived Carbon-Based

Nanomaterials for Efficient Catalysis. ACS Catal. 2016, 6, 5887-5903. (24) Wang, Q.; Xia, W.; Guo, W.; An, L.; Xia, D.; Zou, R., Functional Zeolitic-ImidazolateFramework-Templated Porous Carbon Materials for CO2 Capture and Enhanced Capacitors. Chem.-Asian J. 2013, 8, 1879-1885. (25) Lee, H. J.; Choi, S.; Oh, M., Well-Dispersed Hollow Porous Carbon Spheres Synthesized by Direct Pyrolysis of Core-Shell Type Metal-Organic Frameworks and Their Sorption Properties. Chem. Commun. 2014, 50, 4492-4495. (26) Zhong, H. X.; Wang, J.; Zhang, Y. W.; Xu, W. L.; Xing, W.; Xu, D.; Zhang, Y. F.; Zhang, X. B., ZIF-8 Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angew. Chem. Int. Ed. 2014, 53, 14235-14239. (27) Jiang, M.; Cao, X. P.; Zhu, D. D.; Duan, Y. X.; Zhang, J. M., Hierarchically Porous NDoped Carbon Derived from ZIF-8 Nanocomposites for Electrochemical Applications. Electrochim. Acta 2016, 196, 699-707. (28) Jiang, M.; Cao, X. P.; Liu, P.; Zhang, T. P.; Zhang, J. M., ZIF-8@Polyvinylpyrrolidone Nanocomposites Based N-Doped Porous Carbon for Highly Efficient Oxygen Reduction Reaction in Alkaline Solution. J. Electrochem. Soc. 2016, 163, H459-H464. (29) Chaikittisilp, W.; Ariga, K.; Yamauchi, Y., A New Family of Carbon Materials: Synthesis of MOF-Derived Nanoporous Carbons and Their Promising Applications. J. Mater. Chem. A 2013, 1, 14-19. (30) Zhao, D.; Shui, J. L.; Grabstanowicz, L. R.; Chen, C.; Commet, S. M.; Xu, T.; Lu, J.; Liu, D.J., Highly Efficient Non-Precious Metal Electrocatalysts Prepared from One-Pot Synthesized Zeolitic Imidazolate Frameworks. Adv. Mater. 2014, 26, 1093-1097. (31) Morozan, A.; Sougrati, M. T.; Goellner, V.; Jones, D.; Stievano, L.; Jaouen, F., Effect of Furfuryl Alcohol on Metal Organic Framework-Based Fe/N/C Electrocatalysts for Polymer Electrolyte Membrane Fuel Cells. Electrochim. Acta 2014, 119, 192-205. (32) Palaniselvam, T.; Biswal, B. P.; Banerjee, R.; Kurungot, S., Zeolitic Imidazolate Framework ZIF-Derived, Hollow-Core, Nitrogen-Doped Carbon Nanostructures for Oxygen-Reduction Reactions in PEFCS. Chem.-Eur. J. 2013, 19, 9335-9342. 23



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

(33) Su, P.; Xiao, H.; Zhao, J.; Yao, Y.; Shao, Z.; Li, C.; Yang, Q., Nitrogen-Doped Carbon Nanotubes Derived from Zn-Fe-ZIF Nanospheres and Their Application as Efficient Oxygen Reduction Electrocatalysts with in Situ Generated Iron Species. Chem. Sci. 2013, 4, 2941-2946. (34) Zhang, P.; Sun, F.; Xiang, Z.; Shen, Z.; Yun, J.; Cao, D., ZIF-Derived in Situ NitrogenDoped Porous Carbons as Efficient Metal-Free Electrocatalysts for Oxygen Reduction Reaction. Energy Environ. Sci. 2014, 7, 442-450. (35) Wang, X. J.; Zhang, H. G.; Lin, H. H.; Gupta, S.; Wang, C.; Tao, Z. X.; Fu, H.; Wang, T.; Zheng, J.; Wu, G.; Li, X. G., Directly Converting Fe-Doped Metal Organic Frameworks into Highly Active and Stable Fe-N-C Catalysts for Oxygen Reduction in Acid. Nano Energy 2016, 25, 110-119. (36) Cravillon, J.; Nayuk, R.; Springer, S.; Feldhoff, A.; Huber, K.; Wiebcke, M., Controlling Zeolitic Imidazolate Framework Nano and Microcrystal Formation: Insight into Crystal Growth by Time-Resolved in Situ Static Light Scattering. Chem. Mater. 2011, 23, 2130-2141. (37) He, M.; Yao, J.; Liu, Q.; Wang, K.; Chen, F.; Wang, H., Facile Synthesis of Zeolitic Imidazolate Framework-8 from a Concentrated Aqueous Solution. Microporous Mesoporous Mater. 2014, 184, 55-60. (38) Wu, Y.; Zhou, M.; Li, S.; Li, Z.; Li, J.; Wu, B.; Li, G.; Li, F.; Guan, X., Magnetic Metal– Organic Frameworks: α-Fe2O3@MOFs via Confined in Situ Pyrolysis Method for Drug Delivery. Small 2014, 10, 2927-2936. (39) Xiao, M.; Zhu, J.; Feng, L.; Liu, C.; Xing, W., Meso/Macroporous Nitrogen-Doped Carbon Architectures with Iron Carbide Encapsulated in Graphitic Layers as an Efficient and Robust Catalyst for the Oxygen Reduction Reaction in Both Acidic and Alkaline Solutions. Adv. Mater. 2015, 27, 2521-2527. (40) He, W.; Jiang, C.; Wang, J.; Lu, L., High-Rate Oxygen Electroreduction over Graphitic-N Species Exposed on 3D Hierarchically Porous Nitrogen-Doped Carbons. Angew. Chem. Int. Ed. 2014, 53, 9503-9507. (41) Song, P.; Wang, Y.; Pan, J.; Xu, W.; Zhuang, L., Structure-Activity Relationship in HighPerformance Iron-Based Electrocatalysts for Oxygen Reduction Reaction. J. Power Sources 2015, 300, 279-284. 24


Page 24 of 28

Page 25 of 28

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


(42) Li, J. S.; Li, S. L.; Tang, Y. J.; Han, M.; Dai, Z. H.; Bao, J. C.; Lan, Y. Q., Nitrogen-Doped Fe/Fe3C@Graphitic Layer/Carbon Nanotube Hybrids Derived from MOFs: Efficient Bifunctional Electrocatalysts for ORR and OER. Chem. Commun. 2015, 51, 2710-2713. (43) Liu, T.; Zhao, P.; Hua, X.; Luo, W.; Chen, S.; Cheng, G., An Fe-N-C Hybrid Electrocatalyst Derived from a Bimetal-Organic Framework for Efficient Oxygen Reduction. J. Mater. Chem. A 2016, 4, 11357-11364. (44) Cui, X.; Yang, S.; Yan, X.; Leng, J.; Shuang, S.; Ajayan, P. M.; Zhang, Z., PyridinicNitrogen-Dominated Graphene Aerogels with Fe–N–C Coordination for Highly Efficient Oxygen Reduction Reaction. Adv. Funct. Mater. 2016, 26, 5708-5717. (45) Torad, N. L.; Hu, M.; Kamachi, Y.; Takai, K.; Imura, M.; Naito, M.; Yamauchi, Y., Facile Synthesis of Nanoporous Carbons with Controlled Particle Sizes by Direct Carbonization of Monodispersed ZIF-8 Crystals. Chem. Commun. 2013, 49, 2521-2523. (46) Liang, J.; Zhou, R. F.; Chen, X. M.; Tang, Y. H.; Qiao, S. Z., Fe–N Decorated Hybrids of CNTs Grown on Hierarchically Porous Carbon for High-Performance Oxygen Reduction. Adv. Mater. 2014, 26, 6074-6079. (47) Lai, Q.; Zhao, Y.; Liang, Y.; He, J.; Chen, J., In Situ Confinement Pyrolysis Transformation of ZIF-8 to Nitrogen-Enriched Meso-Microporous Carbon Frameworks for Oxygen Reduction. Adv. Funct. Mater. 2016, 26, 8334-8344. (48) Wang, X.; Li, Q.; Pan, H.; Lin, Y.; Ke, Y.; Sheng, H.; Swihart, M. T.; Wu, G., SizeControlled Large-Diameter and Few-Walled Carbon Nanotube Catalysts for Oxygen Reduction. Nanoscale 2015, 7, 20290-20298. (49)

Chen, Y.-Y.; Zhang, Y.; Jiang, W.-J.; Zhang, X.; Dai, Z.; Wan, L.-J.; Hu, J.-S.,

Pomegranate-Like N,P-Doped Mo2C@C Nanospheres as Highly Active Electrocatalysts for Alkaline Hydrogen Evolution. ACS Nano 2016, 10, 8851-8860. (50) Wu, G.; Johnston, C. M.; Mack, N. H.; Artyushkova, K.; Ferrandon, M.; Nelson, M.; Lezama-Pacheco, J. S.; Conradson, S. D.; More, K. L.; Myers, D. J.; Zelenay, P., SynthesisStructure-Performance Correlation for Polyaniline-Me-C Non-Precious Metal Cathode Catalysts for Oxygen Reduction in Fuel Cells. J. Mater. Chem. 2011, 21, 11392-11405.

25



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

(51) Chen, P.; Zhou, T.; Xing, L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L.; Yan, W.; Chu, W.; Wu, C.; Xie, Y., Atomically Dispersed Iron–Nitrogen Species as Electrocatalysts for Bifunctional Oxygen Evolution and Reduction Reactions. Angew. Chem. Int. Ed. 2017, 56, 610-614. (52) Peng, H.; Mo, Z.; Liao, S.; Liang, H.; Yang, L.; Luo, F.; Song, H.; Zhong, Y.; Zhang, B., High Performance Fe- and N- Doped Carbon Catalyst with Graphene Structure for Oxygen Reduction. Sci. Rep. 2013, 3, 1765. (53) Geng, D.; Chen, Y.; Chen, Y.; Li, Y.; Li, R.; Sun, X.; Ye, S.; Knights, S., High OxygenReduction Activity and Durability of Nitrogen-Doped Graphene. Energy Environ. Sci. 2011, 4, 760-764. (54) Lin, L.; Zhu, Q.; Xu, A.-W., Noble-Metal-Free Fe–N/C Catalyst for Highly Efficient Oxygen Reduction Reaction under Both Alkaline and Acidic Conditions. J. Am. Chem. Soc. 2014, 136, 11027-11033. (55) Lai, L.; Potts, J. R.; Zhan, D.; Wang, L.; Poh, C. K.; Tang, C.; Gong, H.; Shen, Z.; Lin, J.; Ruoff, R. S., Exploration of the Active Center Structure of Nitrogen-Doped Graphene-Based Catalysts for Oxygen Reduction Reaction. Energy Environ. Sci. 2012, 5, 7936-7942. (56) Armel, V.; Hindocha, S.; Salles, F.; Bennett, S.; Jones, D.; Jaouen, F., Structural Descriptors of Zeolitic–Imidazolate Frameworks Are Keys to the Activity of Fe–N–C Catalysts. J. Am. Chem. Soc. 2017, 139, 453-464.

26


Page 26 of 28

Page 27 of 28

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 CAPTIONS AND TABLE CAPTIONS: Figure 1. A schematic diagram showing the synthesis of Fe-doped ZIF-8 and the preparation of its derived doped carbon nanoframes. Figure. 2 SEM images of the precursors (a) ZIF-8, (b) FeZIF-0.42, (c) FeZIF-0.84, (d) FeZIF-2.53, (e) FeZIF-3.54; SEM images of (f) C-ZIF-8-900, (g) C-FeZIF-900-0.42, (h) C-FeZIF-900-0.84, (i) C-FeZIF900-2.53, (j) C-FeZIF-900-3.54 Figure. 3 TEM images of (a) C-ZIF-8-900, (b) C-FeZIF-900-0.42, (c) C-FeZIF-900-0.84, (d) C-FeZIF900-2.53, (e) C-FeZIF-900-3.54; (f) HAADF-STEM images and elemental mapping of C-FeZIF-900-0.84 Figure. 4 N2 adsorption and desorption isotherms and pore-size distributions of C-FeZIF-900 with different molar ratios of iron(III) acetylacetonate/Zn(NO3)2·6H2O (a and b, respectively) and C-FeZIF0.84 at different temperatures (c and d, respectively) Figure. 5 XRD patterns of C-ZIF-8-900 and C-FeZIF-900 with different molar ratios of iron(III) acetylacetonate/(Zn(NO3)2·6H2Oafter acid leaching (a) and before acid leaching (b); and (c)Raman spectra of C-ZIF-8-900 and C-FeZIF-900-0.84

Figure. 6 (a) Survey XPS spectra, high-resolution; (b) N1s XPS spectra of C-ZIF-8-900, (c) N1s XPS spectra of C-FeZIF-900-0.84, and (d) XPS spectra of Fe2p Figure. 7 CV curves of C-FeZIF-900-0.84 in N2–and O2–saturated 0.1 M KOH (a) and HClO4 (b); LSV ORR curves of C-ZIF-8-900, C-FeZIF-900-0.42, C-FeZIF-900-0.84, C-FeZIF-900-2.53, C-FeZIF-9003.54, and 20 wt% Pt/C in O2–saturated 0.1 M KOH (c) and HClO4 (d) (rotation rate: 1600 rpm); durability evaluation of C-FeZIF-900-0.84 and Pt/C for 20,000 s in O2–saturated 0.1 M KOH (e) and HClO4 (f) Figure. 8 ORR curves of C-FeZIF-900-0.84 obtained at different rotation rates in O2-saturated 0.1 M KOH (a) and HClO4 (b); K–L plots of j−1 versus ω–1/2 on C-FeZIF-900-0.84 in O2-saturated 0.1 M KOH (c) and HClO4 (d); transferred electron numbers at different potentials in 0.1 M KOH (e) and HClO4 (f) Table 1. BET surface areas and current densities at 0.70 V of the as-prepared samples Table 2. Fraction of the different N species present in C-ZIF-8-900 and C-FeZIF-900-0.84

27



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

For Table of Contents Only:

Well-defined ZIF-derived Fe-N co-doped Carbon Nanoframes as Efficient Oxygen Reduction Catalysts Yijie Deng†, Yuanyuan Dong†, Guanghua Wang†, Kailing Sun†, Xiudong Shi†, Long Zheng†, Xiuhua Li†, Shijun Liao*,†

A series of ZIF-derived Fe-N co-doped carbon materials with a well-defined morphology, high surface area, and porous nanoframe structure was successfully prepared by synthesizing Fe-doped ZIF-8 through the assembly of Zn2+ ions with 2-methylimidazole in the presence of iron(III) acetylacetonate, followed by pyrolysis at a high temperature and in an Ar atmosphere. Our optimal catalyst, C-FeZIF-900-0.84, yielded a current density at 0.70 V 1.26 times higher than that of a commercial Pt/C catalyst in an alkaline medium, meanwhile in an acidic medium, the half-wave potential of C-FeZIF-900-0.84 was only 80 mV (0.77V vs 0.85V) lower than that of the Pt/C catalyst, making it one of the best doped carbon catalysts in an acidic medium.

28


Page 28 of 28

Well-Defined ZIF-Derived Fe–N Codoped Carbon Nanoframes as

Recommend Documents