Nonprecious-metal Fe/N/C Catalysts Prepared from π-Expanded Fe Salen Precursors toward an Efficient Oxygen Reduction Reaction

Article abstract of DOI:10.1002/cctc.201701629

Non-precious metal electrocatalysts are under investigation as alternatives to platinum-group metal electrocatalysts for the oxygen reduction reaction (ORR), which is required for cathode materials in fuel cells. Herein, we describe a new method for the synthesis of metal and nitrogen-containing carbon (M/N/C) catalysts with high ORR activity using π-expanded Fe(Salen) precursors. The Fe/N/C ORR catalysts were obtained by pyrolysis of a mixture of carbon support (Vulcan XC-72R) and the iron complex precursors. The Fe/N/C catalyst prepared from N,N'-bis(2-hydroxy-1-naphthylidene)-1,2-phenylenediamino-iron(III) chloride has an onset potential of 860 mV, which is positively shifted by 60 mV from that of the catalyst prepared from the simple Fe(Salen) complex. The catalyst promotes efficient four-electron reduction in the ORR. XAFS and XPS studies reveal that the Fe/N/C catalysts have atomically-dispersed iron active sites and that the activity depends on the high content of pyridinic nitrogen. This new methodology facilitates the design of non-precious metal-carbon catalysts with excellent ORR activity.

Full text of DOI:10.1002/cctc.201701629

Accepted Article
Title: Nonprecious-metal Fe/N/C Catalysts Prepared from π-Expanded
Fe Salen Precursors toward an Efficient Oxygen Reduction
Reaction
Authors: Akira Onoda, Yuta Tanaka, Shin-ichi Okuoka, Tomoyuki
Kitano, Koki Matsumoto, Takao Sakata, Hidehiro Yasuda, and
Takashi Hayashi
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10.1002/cctc.201701629
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Nonprecious-metal Fe/N/C Catalysts Prepared from -Expanded
Fe Salen Precursors toward an Efficient Oxygen Reduction
Reaction
Yuta Tanaka,[a] Akira Onoda,*[a] Shin-ichi Okuoka,[b] Tomoyuki Kitano,[c] Koki Matsumoto,[a] Takao
Sakata,[d] Hidehiro Yasuda,[d] Takashi Hayashi*[a]
Abstract: Nonprecious metal electrocatalysts are being explored as
alternatives to platinum-group metal electrocatalysts for the oxygen
reduction reaction (ORR) which is required for cathode materials in
fuel cells. Herein, we describe a new method for preparing metal and
nitrogen-containing carbon (M/N/C) catalysts with high ORR activity
using -expanded Fe(Salen) precursors. The Fe/N/C ORR catalysts
were obtained by pyrolysis of a mixture of carbon support (Vulcan XC-
72R) and the iron complex precursors. The Fe/N/C catalyst prepared
has stimulated extensive investigations for development of
alternative low-cost non-precious metal catalysts (NPMCs).[7−9]
Carbon catalysts containing a first row transition metal, such as
Fe or Co, and nitrogen (which are known as M/N/C catalysts),
have been generally considered to represent promising
alternatives to the PGM catalysts, because the M/N/C catalysts
have been shown to promote high levels of ORR activity while
having suitable durability for use in PEFCs.[10−15] Studies on
M/N/C catalysts were initiated after the discovery of the ORR
activity of cobalt phthalocyanine under alkaline conditions.[16] The
pyrolyzed transition metal (Fe or Co) macrocycles adsorbed on
carbon supports have been proven to improve ORR activity and
stability under acidic conditions, which is a requirement for
PEFCs.[17,18] A breakthrough study by Yeager revealed that the
often-expensive macrocycles can be replaced with individual
nitrogen and metal precursors.[19] Therefore, a series of low-cost
nitrogen precursors such as phenanthroline,[10] polypyrrole,[11] and
polyaniline[12] have been utilized in the preparation of M/N/C
catalysts. The exact chemical structures of the metal-containing
active sites in M/N/C catalysts prepared by pyrolysis is still under
debate,[20,21] although a general M−Nx site structure has been
proposed on the basis of various spectroscopic techniques.[22−24]
It is believed that metals coordinated by the N-groups play an
important role in enhancing the ORR activity of M/N/C
catalysts.[10,25]
It has been demonstrated that the chemical structure of metal
complex precursors is critical for the graphitization process that
incorporates the metal and nitrogen atoms into carbon materials
to form the M/N/C active sites. The optimization of nitrogen
sources,[26] transition metal species,[27] and carbon supports,[7]
has been investigated to control the M/N/C active sites and
enhance the ORR activity. We began focusing on a new method
by systematically tuning an aromatic framework of Fe complex
precursors to improve the ORR activity of M/N/C catalysts. An
Fe(Salen) complex is a low-cost precursor that enables simple
diversification of the chemical structure of the N-containing
ligands. We designed Fe(Salen) derivatives attached to various
aromatic moieties, which are expected to have higher thermal
stability to optimize the annulation process of the complexes
during pyrolysis. In this work, we describe preparation and
characterization of the Fe/N/C catalysts via pyrolysis of the -
expanded Fe(Salen) complexes (Fe(Xsal)) and their ORR activity.
It is found that tuning the ligand structure of the Fe(Xsal) precursor
leads to a remarkable positive shift (+60 mV) of onset potentials
and an increase in efficiency of the four-electron reduction
process in the ORR activity of pyrolyzed Fe/N/C catalysts.
from
N,N´-bis(2-hydroxy-1-naphthylidene)-1,2-phenylenediamino-
iron(III) chloride is found to have an onset potential of 860 mV, which
is positively shifted by 60 mV from that of the catalyst prepared from
the simple Fe(Salen) complex. The catalyst promotes efficient four-
electron reduction in the ORR. XAFS and XPS studies reveal that the
Fe/N/C catalysts have atomically-dispersed iron active sites and that
the activity depends on high content of pyridinic nitrogen. This new
methodology facilitates design of the nonprecious metal-carbon
catalysts with excellent ORR activity.
Introduction
Electrochemical reduction of O2 is a key reaction for improving
the performance of energy conversion devices such as fuel cells,
metal-air batteries, and electrolyzers.[1-3] In particular, polymer
electrolyte fuel cells (PEFCs) have been recognized as efficient
energy converters enabling low emissions and low environmental
impacts. Precious metal group (PMG) catalysts have been the
most widely used catalysts for the cathodic oxygen reduction
reaction (ORR) in PEFCs.[4−6] However, the high cost and low
abundance of precious metals have been the main obstacles
facing widespread commercialization of PEFCs. This situation
[a]
Y. Tanaka, Dr. A. Onoda, K. Matsumoto and Prof. Dr. T. Hayashi
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
2-1 Yamadaoka, Suita 565-0871 (Japan)
E-mail: onoda@chem.eng.osaka-u.ac.jp,
thayashi@chem.eng.osaka-u.ac.jp
[b]
[c]
[d]
S. Okuoka
Research Center, Nippon Shokubai Co., Ltd.
5-8 Nishi Otabi-cho, Suita 564-0034 (Japan)
Dr. T. Kitano
Analysis Technology Center, Nippon Shokubai Co., Ltd.
5-8 Nishi Otabi-cho, Suita 564-0034 (Japan)
Dr. T. Sakata, Prof. Dr. Y. Yasuda
Research Center for Ultra-High Voltage Electron Microscopy,
Osaka University
Ibaraki, 567-0047 (Japan)
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. Structures of Fe(Xsal) complexes as precursors and the preparation of Fe/N/C catalysts. (a) Structures of Fe(Salen)Cl, Fe(Saloph)Cl, Fe(1NAED)Cl,
Fe(1NAPD)Cl, Fe(2NAED)Cl, and Fe(2NAPD)Cl. (b) (A) The Fe(Xsal) complex (orange) is mixed with carbon materials, (B) pyrolyzed under N2 atmosphere, and
(C) treated with H2SO4aq to remove iron impurities (blue) and to prepare Fe/N/C catalysts containing the iron active sites (red).
constant N2 gas flow. The carbon materials were immediately
pyrolyzed at 1000 °C for 2 h under constant N2 gas flow for
carbonization.[31] The pyrolyzed powder was ground and leached
Results and Discussion
in an acidic solution (0.5 M H2SO4) to remove inactive species
such as iron oxide and unincorporated iron species. After washing
twice with an excess of water and drying, the Fe/N/C catalysts,
Preparation of Fe/N/C catalysts. A series of precursors,
Fe(Xsal) complexes with expanded aromatic ligand frameworks
were synthesized according to the literature; Fe(Salen)Cl,
Fe/Salen@VC,
Fe/Saloph@VC,
Fe/1NAED@VC,
Fe(saloph)Cl,
diaminoiron(III) chloride (Fe(1NAED)Cl), N,N´-bis(1-hydroxy-2-
naphthylidene)-1,2-phenylenediaminoiron(III) chloride
N,N´-bis(1-hydroxy-2-naphthylidene)ethylene
Fe/1NAPD@VC, Fe/2NAED@VC and Fe/2NAPD@VC, were
used in further experiments.
(Fe(1NAPD)Cl), N,N´-bis(2-hydroxy-1-naphthylidene)ethylenedi-
aminoiron(III) chloride Fe(2NAED)Cl, and N,N´-bis(2-hydroxy-1-
Characterization of the Fe/N/C catalysts. The carbon materials
in the Fe/N/C catalysts were first analyzed by Raman
spectroscopy (Figure 2a). It is known that highly ordered graphite
has a band between 1100 and 1700 cm−1, whereas a disordered
carbon structure has significantly different spectral characteristics
with a D (disorder) band in the vicinity of 1355 cm−1. In addition,
graphitized carbons give rise to a G (graphite) band in the vicinity
of 1580 cm-1, which is assignable to in-plane displacement of the
carbon strongly coupled in the hexagonal sheets.[32,33] The Raman
spectra of the Fe/N/C catalysts presenting typical D- and G-bands
indicate that the disordered carbonaceous structure is included in
the Fe/N/C catalysts. The integrated intensity ratio ID/IG for the D-
band and G-band is widely used to quantify the defects in
graphitic materials. The ID/IG ratio for the six Fe/N/C catalysts is
approximately 1.1, suggesting that all Fe/N/C catalysts contain
similar content of graphitic and disordered structure in the
carbonaceous materials. The nitrogen-doped graphene has been
known to show D’ peak near 1600 cm−1.[34−36] The G band peaks
for the Fe/N/C catalysts exhibit near 1600 cm−1, which shift from
the peak at 1580 cm−1 for VC without nitrogen atom. This result
indicates that nitrogen atoms are doped in the carbon materials.
naphthylidene)-1,2-phenylenediaminoiron(III)
chloride
(Fe(2NAPD)Cl) (Figure 1a).[28−30] The thermal decomposition
temperatures (TD) values of Fe(1NAED)Cl, and Fe(2NAED)Cl
with the expanded aromatic ring framework are shifted more than
30 °C above the TD value of Fe(Salen)Cl. In addition, the TD of
Fe(Xsal) complexes with the o-phenylenediamine bridge
(Fe(saloph)Cl, Fe(1NAPD)Cl, and Fe(2NAPD)Cl) range between
375 − 417 °C, which are approximately 40 °C higher relative to
the Fe(Xsal) complexes with the ethylenediamine bridge
(Fe(Salen)Cl, Fe(1NAED)Cl, and Fe(2NAED)Cl) (Figure S1). The
thermogravimetric analysis of the precursor indicates that the
aromatic rings in the ligand frameworks significantly enhance the
thermal stability of Fe(Xsal) complexes during pyrolysis.
The Fe/N/C electrocatalysts were prepared by pyrolysis of the
mixture of the Fe(Xsal) complexes and carbon black (VC; Vulcan
XC-72R, Cabot) (Figure 1b). The Fe(Xsal) complex dissolved in
CH2Cl2 was vigorously mixed with VC and the solvent was
removed to afford the precursor. The obtained precursor was
preheated to 300 °C for 1 h and then incubated for 2 h under
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Figure 2. Physical characterization of the Fe/N/C catalysts prepared from Fe(Xsal) precursors. (a) Raman spectra and (b) XRD patterns of the Fe/N/C catalysts.
Fe/Salen@VC (black), Fe/Saloph@VC (blue), Fe/1NAED@VC (green), Fe/1NAPD@VC (orange), Fe/2NAED@VC (brown), Fe/2NAPD@VC (red), and pyrolyzed
VC (gray). The XRD peaks are assigned to Fe3O4 (JCPDS 00-001-1111) (closed circle), Fe2O3 (JCPDS 00-016-0895) (closed triangle), Fe3C (JCPDS 01-089-3689)
(open circle), Fe3C (JCPDS 00-034-0001) (open triangle), and Fe5C2 (JCPDS 03-065-6169) (open square). TEM images of (c) Fe/Salen@VC, (d) Fe/Saloph@VC,
(e) Fe/1NAED@VC, (f) Fe/1NAPD@VC, (g) Fe/2NAED@VC, and (h) Fe/2NAPD@VC.
Figure 3. RDE and RRDE. (a) ORR polarization curves, (b) the percentage of generated H2O2, and (c) the number of electrons transferred during O2 reduction in
O2-saturated 0.1 M HClO4 solution at 5 mV·s−1 with 2000 rpm of Fe/N/C catalysts. Fe/Salen@VC (black), Fe/Saloph@VC (blue), Fe/1NAED@VC (green),
Fe/1NAPD@VC (orange), Fe/2NAED@VC (brown) and Fe/2NAPD@VC (red). Catalyst loading, 2.0 mg/cm2.
X-ray diffraction (XRD) patterns also confirm that each
carbonaceous structure of the Fe/N/C catalysts prepared from the
Fe(Xsal) complexes is very similar (Figure 2b). In general, powder
samples of carbons in an amorphous structure provide diffraction
peaks for (002) (2 = ca. 26.0°), and (101) (2 = ca. 44.0°).[37,38]
The carbon catalysts were found to exhibit a strong and broad
peak at ca. 26.0° and a weak and broad peak at 44.0°. The degree
of graphitization of the Fe/N/C catalysts can be estimated on the
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Figure 4. Comparison of Fe K-edge XAFS spectra. (a) Normalized XANES spectra of Fe/N/C catalysts: Fe/Salen@VC (black), Fe/Saloph@VC (blue),
Fe/1NAED@VC (green), Fe/1NAPD@VC (orange), Fe/2NAED@VC (brown) and Fe/2NAPD@VC (red). (b) Normalized XANES spectra of reference samples,
Fe2O3, FeO, Fe-foil, Fe3C and Fe/2NAPD@VC (red). (c) FT-EXAFS of Fe/N/C catalysts: Fe/Salen@VC (black), Fe/Saloph@VC (blue), Fe/1NAPD@VC (orange),
and Fe/2NAPD@VC (red).
basis of the relative intensities of reflections from the (002) and
(101) planes according to the empirical formula. The degree of
summarized in Table 1. Although each carbonaceous structure in
the series of Fe/N/C catalysts is very similar, the N and Fe content
of each of Fe/Saloph@VC, Fe/1NAPD@VC and Fe/2NAPD@VC
(which were prepared from phenylene-bridged precursors) is
higher than the N and Fe content of ethylene-bridged precursors.
These results suggest that the aromatic rings in the Xsal ligand
frameworks enhance incorporation of N and Fe atoms into carbon
materials during pyrolysis, thereby increasing the content of both
N and Fe in the Fe/N/C catalysts.
graphitization of the Fe/N/C catalysts (I002/I101
:
3.9 for
Fe/Salen@VC, 3.8 for Fe/Saloph@VC, 3.9 for Fe/1NAED@VC,
4.0 for Fe/1NAPD@VC, 3.5 for Fe/2NAED@VC, and 3.8 for
Fe/2NAPD@VC) were determined from XRD. This also indicates
that the carbonaceous structures of the Fe/N/C catalysts
containing the iron active sites are similar. The peaks close to
44.0º indicate the presence of Fe3C and Fe5C2 in the Fe/N/C
catalysts (Figure 2b).[39,40] The peaks at 35.4° and 36.4° also
indicate the presence of Fe3O4 and Fe2O3.[41,42] Judging from the
values of reference intensity ratio (RIR) factor, I/Icor (Icor, integrated
intensity of the corundum peak used for the RIR) (I/Icor(Fe3O4) =
141.3, I/Icor(Fe3C) = 6.93, I/Icor(Fe5C2) = 1.98), the contents of
Fe3O4 species are less than 1.8 % relative to that of the iron
carbide species involved in the Fe/N/C catalysts.
Transmission electron microscope (TEM) images of the
Fe/N/C catalysts with elemental mapping reveal that the iron
species are well-dispersed within the carbon catalysts (Figures
2c−h, S2, and S3). The Fe/N/C catalysts have a spherical form
with diameters ranging from 40 to 70 nm, which are retained in
the structure of VC. Iron nanoparticles were not observed in TEM
images, suggesting that Fe active sites are atomically dispersed
within the carbon structure.
ORR activity. The ORR activities of the Fe/N/C catalysts were
determined on a rotating ring-disk electrode (RRDE) (Figure 3, S4,
and S5). The measurements were performed using RRDE rotated
at 2000 rpm in a medium of O2 saturated 0.1 M HClO4 at pH 1. All
Fe/N/C catalysts prepared from the Fe(Xsal) precursors show
significant cathodic current during O2 reduction, indicating high
levels of ORR activity. The Fe/N/C catalyst prepared from the
Fe(Salen) complex provides a positively-shifted onset potential
relative to the catalyst prepared from the mixture of Fe source and
the Salen ligand (Figure S4a). This clearly indicates that the Salen
ligand binding to the Fe ion affects the process of chemical
transformation of the Fe active site during pyrolysis. In addition,
the onset potential of O2 reduction with the Fe/N/C catalysts shift
positively when an Fe(Xsal) complex with the -expanded ligand
is used as a precursor. In particular, Fe/2NAED@VC (0.85 V) and
The content of N and Fe atoms in the Fe/N/C catalysts was
determined by elemental analysis with inductively coupled plasma
atomic emission spectrometry (ICP-AES) measurements. The
elemental compositions of the Fe/N/C catalysts prepared from
Fe(Xsal) precursors with different ligand frameworks are
Fe/2NAPD@VC (0.86 V) have significantly shifted
onset
potentials, suggesting that the introduction of aromatic rings to the
ligand framework of the precursor positively shifts the onset
potential by 60 mV in Fe/2NAPD@VC relative to Fe/Salen@VC.
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Figure 5. The distribution of pyridinic N, pyrrolic N, graphitic N and N-oxide in Fe/N/C catalysts. (a) XPS spectra of Fe/N/C catalysts in the N1s region with fitted
peaks of pyridinic-N (red), pyrrolic-N (yellow), graphitic-N (blue) and N-oxide (green). (b) The proportion of four nitrogen species in the Fe/N/C catalysts.
Table 1. Characterization and electrochemical activity of Fe/N/C catalysts.
Elemental analysis
XPS (N1s)
TD
Eonset
Fe/N/C catalyst[a]
n[d]
(V)[c]
Fe[b]
(wt%)
N
(wt%)
Pyridinic
(%)
Pyrrolic
Graphitic
(%)
N-oxide
(%)
(°C) [a]
(%)
23.4
21.4
27.7
17.3
25.1
13.4
Fe/Salen@VC
Fe/Saloph@VC
Fe/1NAED@VC
Fe/1NAPD@VC
Fe/2NAED@VC
Fe/2NAPD@VC
333
375
362
411
375
417
0.7
1.3
0.6
1.0
0.4
0.9
0.5
0.8
0.5
0.8
0.6
0.8
11.8
12.5
20.4
18.9
22.1
24.9
47.5
52.0
44.7
54.2
47.3
50.2
17.4
14.1
7.2
0.80
0.81
0.83
0.84
0.85
0.86
3.9
3.8
4.0
3.8
4.0
4.0
9.6
5.5
11.5
[a] Decomposition temperature of Fe(Xsal) precursors determined by TG-DTA. [b] Determined by ICP-AES. [c] The potential at I = 0.05 mA·cm−2 in RDE. [d]
The number of electrons transferred during O2 reduction at 0.6 V in RRDE.
Furthermore, these findings demonstrate that the ORR catalyst
obtained from the Fe(Xsal) precursor with the higher thermal
stability promotes excellent ORR activity.
than 10% for the other carbon catalysts. The average number of
electrons transferred in the O2 reduction reaction was calculated
from the percentage of hydrogen peroxide generated. This
average number of electrons transferred exceeds 3.95 over the
range from 0.0 V to 0.8 V for the Fe(2NAPD)@VC. This value is
higher than the 3.8 value measured for the other carbon catalysts.
These results indicate that the carbon catalysts prepared from the
Fe(Xsal) complexes promote four-electron reduction of O2.
RRDE measurements were also performed to quantify
hydrogen peroxide as a byproduct during electrocatalysis. The
percentage of peroxide with respect to the total O2 reduction
products and the number of electrons transferred during O2
reduction (n) were calculated from RRDE curves recorded at 2000
rpm in a 0.1 M HClO4 solution for each carbon catalyst (Figure
3b−c). The yield of hydrogen peroxide was less than 3% over the
measurement range for the Fe(2NAPD)@VC and generally lower
XANES and FT-EXAFS measurements. The structures of the
iron species highly-dispersed in the carbon catalysts were further
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analyzed by X-ray absorption fine structure (XAFS) Conclusions
measurements. Fe K-edge X-ray absorption near-edge
spectroscopy (XANES) was employed to determine the chemical
state of the iron atoms in the catalysts (Figure 4a). The XANES
spectra of FeO, Fe2O3, Fe-foil, and Fe3C[42,43] were compared as
standard samples (Figure 4b). The pre-edge and edge features of
the carbon catalysts are clearly different from those of FeO, Fe2O3,
and Fe-foil, whereas they match well with those of Fe3C. In
addition, the shape of the white line in the XANES spectrum of
each of the Fe/N/C catalysts is similar to that of Fe3C. This implies
that the majority of the iron species among the Fe/N/C catalysts
has electronic and chemical properties similar to those of Fe3C.
Next, to evaluate the environment of the iron atoms in the Fe/N/C
catalysts, FT-EXAFS analysis was carried out as shown in Figure
4c. The Fe−Fe bond distances in Fe3C, which is one of the
candidates of the main component for the iron species, are known
to be ca. 2.5−2.7 Å in the first coordination shell and ca. 4.0 Å in
the second coordination shell.[39,42] In contrast, the Fe/N/C
catalysts provide a sharply different profile as shown in Figure 4c.
The presence of a strong peak near 2.1 Å and the absence of
peaks near 4.0 Å provide support for our proposal that the iron
atoms are located in the first coordination shell and that formation
of general Fe3C should be ruled out. This is consistent with the
TEM results that show the absence of clear iron particles within
the nanometer size range. A weak peak near 4.0 Å is found in the
case of Fe/Salen@VC. In contrast, Fe/N/C catalysts prepared
from Fe(Xsal) precursors do not show the peak. Furthremore, the
small peak identified in the range below 2.1 Å should be assigned
to Fe−C and N bonds connecting the iron sites and the nitrogen-
containing carbon support.[44−46] The peak intensity of this region
is greater for Fe/1NAPD@VC and Fe/2NAPD@VC relative to that
for Fe/Salen@VC. The EXAFS result indicates that the -
expanded structure of the precursor results in higher dispersion
of the iron sites and the formation of Fe−Nx structure.
This work demonstrates a new method for improving the ORR
activity of Fe/N/C catalysts by introducing aromatic moieties into
the structures of Fe(Xsal) precursors. The Fe/N/C catalysts were
prepared by pyrolysis of the mixture of carbon support and the
Fe(Xsal) precursors. The catalysts prepared using the Fe(Xsal)
complexes with -expanded ligand frameworks such as the
Fe(2NAPD) complex exhibit a remarkable positive shift (+ 60 mV)
in the onset potential for ORR relative to catalysts prepared from
the simple Fe(Salen) complex. XRD and XAFS analyses clarify
that the main species of Fe exist as iron carbide in the Fe/N/C
catalysts. As Fe nanoparticles were not observed in TEM images
with elemental mapping, the iron species are dispersed within the
carbonaceous structure in these catalysts. In particular, the
Fe/N/C catalysts prepared from -expanded Fe(Salen)
complexes have iron species with an atomically dispersed
structures as indicated by the EXAFS results. Furthermore, we
clarified that the thermal stability of the ligand structures of the
precursors increases the content of pyridinic nitrogen which forms
the Fe−Nx structure, resulting in improvement of the ORR activity
although the initial structure of the Fe(Salen) complexes is not
retained after pyrolysis. These findings prove that rational design
of aromatic ligand frameworks for metal precursor complexes
contributes to enhancement of ORR activity of non-precious
M/N/C catalysts which are appropriate for incorporation into fuel
cells and metal-air battery applications.
Experimental Section
Catalyst preparation. Carbon black Vulcan XC-72R (VC, Cabot, USA)
was used as a carbon support. Electrocatalysts were prepared from
Fe(Xsal) complexes and VC using a pyrolysis in N2 gas flow. The
electrocatalyst, which abbreviated to Fe/Xsal@VC, was prepared as
follows: Fe(salen)Cl (30 mg, 84 mol) was dissolved in a minimum volume
of CH2Cl2 and mixed with a powder of VC (30 mg). The suspension was
vigorously vortexed and sonicated for 30 min. The solvent was removed
and residue was used as a precursor for an electrocatalyst. The precursor
was placed on an alumina boat (length: 80 mm, width: 16 mm, height: 10
mm) which was then placed in a quartz tube (diameter 50 mm, length 800
mm). The quartz tube was installed in a hinge split tube furnace (Koyo
Thermo Systems Co. Ltd., KTF045N1). The precursor was preheated from
ambient temperature to 300 °C for 1 h under N2 flow (0.2 L·min−1), and
incubated for 2 h. The precursor was then heated to 1000 °C for 1 h
immediately, and incubated for 2 h. The temperature of the sample inside
the furnace was recorded with a thermocouple equipped with a data logger
(CHINO Corporation, MC3000). After cooling, the pyrolyzed catalyst was
ground and further treated in a 0.5 M H2SO4 solution at 80 °C for 3 h to
leach out impurities such as iron oxide, and then washed with excess
volumes of deionized water twice. The dried carbon catalyst was used for
XPS measurement. The chemical structures of the nitrogen
atoms in the Fe/N/C catalyst were determined by X-ray
photoelectron spectroscopy (XPS) measurements (Figure 5a).
Four types of nitrogen species are confirmed by the N1s peaks in
the range between 398.0 and 404.0 eV.[47,48] The N1s spectra
were deconvoluted into four different nitrogen species including
pyridinic N (398.0-398.9 eV), pyrrolic N (399.5−400.4 eV),
graphitic N (400.5−402.0), and N-oxide groups (N+–O–) at binding
energies higher than 402.0 eV. Interestingly, the content of
pyridinic N of the catalyst prepared from the Fe(Xsal) precursor
with the expanded aromatic rings is increased: 11.8% for
Fe/Salen@VC, 12.5% for Fe/Saloph@VC, 20.4% for
Fe/1NAED@VC, 18.9% for Fe/1NAPD@VC, 22.1% for
Fe/2NAED@VC, and 24.9% for Fe/2NAPD@VC, although the
proportions of the other nitrogen species vary for each of Fe/N/C
catalysts prepared from different precursors (Figure 5b). Taken
together, these results indicate that higher content of pyridinic N
provides higher ORR activity (Table 1). Therefore, the ligand
frameworks of the precursors remarkably affect the content of
pyridinic N in the Fe/N/C catalysts.
the
experiments.
Other
carbon
catalysts
(Fe/Saloph@VC,
Fe/1NAED@VC, Fe/1NAPD@VC, Fe/2NAED@VC and Fe/2NAPD@VC)
were obtained using the same protocol with each indicated iron complex.
Physicochemical characterization. The decomposition temperatures of
Fe(Xsal) complexes were determined by themogravimetry-differential
thermal analysis (TG-DTA) using a Mac Science TG-DTA TMA DSC with
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10.1002/cctc.201701629
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FULL PAPER
a heating rate of 10 °C min−1 in an N2 stream and platinum pan. Fe content
of each of the Fe/N/C catalysts was characterized by inductively coupled
plasma atomic emission spectroscopy (ICP-AES) using a SHIMAZDU
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number
25708031 in a Grant-in-Aid for Young Scientists (A), JP15H03857,
JP17H05370 in Innovative Areas “Coordination Asymmetry” to
A.O., JSPS KAKENHI Grant Number JP15H05804 in Innovative
Areas “Precisely Designed Catalysts with Customized Scaffolding”
to T.H. Y.T. expresses his special thanks to the support from the
Interactive Material Science Cadet program (IMSC) and a JSPS
Research Fellowship (16J00939). We acknowledge Dr. Shinji
Tamura and Prof. Nobuhito Imanaka at Department of Applied
Chemistry, Graduate School of Engineering, Osaka University for
Raman measurements, and the staff for their excellent support
during the data collection on BL11 at SAGA-LS.
ICPS-7510 system. Specific surface areas were obtained using
a
MicrotracBEL BELSORP-miniII system. The Raman spectra were
obtained using a JASCO NRS-3100 instrument with a 532 nm laser. X-ray
diffraction (XRD) patterns of the samples were obtained using an X-ray
diffractometer (Rigaku, SmartLab) equipped with a Cu K source. The
contents of iron oxide for the catalysts were estimated by the peak intensity
with the correction using the values of reference intensity ratio (RIR); 0.9%
(Fe/Salen@VC), 1.3% (Fe/Saloph@VC), 1.8% (Fe/1NAPD@VC), 1.1%
(Fe/2NAED@VC), and 1.2% (Fe/2NAPD@VC).Transmission electron
microscopy (TEM) images were obtained using a HITACHI H-7650
microscope operated at 100 kV of accelerating voltage. High-resolution
transmission electron microscopy (HRTEM) observations and the
chemical composition analyses were carried out using a HITACHI HF-
2000 field emission TEM operated at an accelerating voltage of 200 kV.
The chemical composition of each samples was analyzed by EDS made
of NORAN Instruments. The analyses were carried out using an electron
probe of approximately 25 nm in diameter. The characteristic X-ray of iron
was collected with an ultra-thin window X-ray detector at a high take-off
angle of 68 degrees. X-ray absorption spectroscopy (XAS) was performed
at the BL11 beamline of SAGA Light Source (SAGA-LS) in Kyusyu, Japan.
The Fe K-edge spectra were analyzed by Athena program.[49] X-ray
photoelectron spectroscopy (XPS) measurements were performed on a
KRATOS AXIS-165x (SHIMADZU) system, equipped with a Mg K X-ray
source. Individual chemical components of the N 1s binding energy region
were fitted to the spectra after a Tougaard-type background subtraction.
Electrochemical measurement. A rotating ring-disk electrode (RRDE)
with a glassy carbon disk electrode ( =5 mm) and platinum ring was used
for evaluation of the carbon catalysts. Electrode rotation rates were
controlled using a Pine Instruments AFMSRCE rotator with a Pine MSRX
motor controller. The electrode was polished to mirror flat with alumina
powder (50 nm) before use. The catalyst ink was prepared with 4.0 mg of
Keywords: Fe/N/C catalyst • -expanded Fe(salen) complexes •
oxygen reduction reaction • cathode catalyst • fuel cell
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Entry for the Table of Contents (Please choose one layout)
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Y.Tanaka, A. Onoda, S. Okuoka, T.
Kitano, and T. Hayashi*.
Page No. – Page No.
Nonprecious-metal Fe/N/C Catalysts
Prepared from -Expanded Fe Salen
Precursors toword an Efficient
Oxygen Reduction Reaction
Iron and nitrogen-containing carbon (Fe/N/C) catalysts were prepared by pyrolysis
of a mixture of carbon support and -expanded Fe(salen) precursors. The Fe/N/C
catalysts have atomically-dispersed iron active sites with high content of pyridinic
nitrogen and promotes efficient four-electron reduction of dioxygen.
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