Efficient oxygen reduction reaction electrocatalysts synthesized from an iron-coordinated aromatic polymer framework

Article abstract of DOI:10.1039/c5ta08316h

Aromatic polymers with abundant nitrogen-coordinated iron (Fe-N_x) species provide a big opportunity to prepare iron-nitrogen co-doped carbon (Fe-N_x/C) catalysts with highly efficient oxygen reduction reaction (ORR) activity; however, such types of materials have not been reported so far. Herein, we synthesized a metal-coordinated aromatic polymer, Fe_(III)-poly(o-phenylene-diamine) (Fe-PPDA), with dense Fe-N_x species by a facile route. Fe-PPDA was obtained by polymerization of the o-phenylenediamine/ferric-ion complex in the presence of sodium bicarbonate, which was essential in retaining abundant Fe-N_x species and increasing the surface area of the polymer. Fe-PPDA polymer derived Fe-N_x/C catalysts demonstrated activity in ORRs, with onset and half-wave potentials (E_1/2) of 1.11V_RHE and 0.92V_RHE, respectively, which were superior to the values obtained for commercial Pt/C (E_1/2, 0.85V_RHE) and are the best values yet reported for the Fe-N_x/C family in alkaline medium. It also demonstrated comparable ORR activity to that of the commercial Pt/C catalyst in acidic medium (E_1/2, 0.76V_RHEvs. 0.78V_RHE). The excellent ORR activity was ascribed to the abundant Fe-N_x modulated carbon nanostructure and the high surface area.

Full text of DOI:10.1039/c5ta08316h

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Journal of Materials Chemistry A
ARTICLE
Efficient Oxygen Reduction Reaction Electrocatalysts Synthesized
from Iron-coordinated Aromatic Polymer Framework
Received 00th January 20xx,
Accepted 00th January 20xx
ab
b
b
b
Yong Zhao, Kazuhide Kamiya, Kazuhito Hashimoto*, Shuji Nakanishi*
DOI: 10.1039/x0xx00000x
Aromatic polymer with abundant nitrogen-coordinated iron (Fe-N
co-doped carbon (Fe-N /C) catalyst with highly efficient oxygen reduction reaction (ORR) activity, however, such type of
materials have not been reported so far. Herein, we synthesized a metal-coordinated aromatic polymer, Fe(III)-poly(o-
phenylene-diamine (Fe-PPDA), with dense Fe-N species by an facile route. Fe-PPDA was obtained by polymerization of o-
phenylenediamine/ferric-ion complex in the presence of sodium bicarbonate, which was essential in retaining abundant
Fe-N species and increasing surface area of polymer. Fe-PPDA polymer derived Fe-N /C catalysts demonstrated activity in
ORRs, with onset and half-wave potentials (E1/2) of 1.11 VRHE and 0.92 VRHE, respectively, which were superior to the values
obtained for commercial Pt/C (E1/2, 0.85 VRHE) and are the best values yet reported for the Fe-N /C family in alkaline
medium. It also demonstrated the comparable ORR activity to that of commercial Pt/C catalyst in acidic medium (E1/2, 0.76
RHE vs 0.78 VRHE). The excellent ORR activity was ascribed to the abundant Fe-N , moduated carbon nanostructure and the
high surface area.
x
) species provide a big chance to prepare iron-nitrogen
x
www.rsc.org/
x
x
x
x
V
x
Broader content
The preparation of highly-active and durable non-precious metal catalysts for oxygen reduction reaction (ORR) is highly important for
reducing the cost of fuel cells. Transition metal and nitrogen codoped carbon (Fe-N /C) materials are regarded as one of the most
promising catalysts for the cathode materials of fuel cells. The ORR activity of Fe-N /C materials is strongly dependent on the density of
catalytic active sites and the effective surface area. In this article, we report a facile protocol to prepare Fe-N /C materials with dense active
sites and high surface area by using an iron-coordinated aromatic polymer as the starting precursor, which has abundant N-coordinated-Fe
units in the metal-polymer framework. The results indicated that the ORR activity of Fe-N /C materials was substantially promoted by using
iron-coordinated polymer precursors compared with that synthesized from the physical mixture of iron salt and polymer in traditional
method. The newly-developed Fe-N /C catalysts exhibited much better and comparable ORR activity compared to that of commercial Pt/C
x
x
x
x
x
catalysts in the alkaline and acidic medium, respectively.
exploited as starting materials to prepare M-N /C catalysts.
x
Introduction
Among the family of N-contained polymers, aromatic polymer
(
for example, polyaniline and poly(phenylenediamine)) derived
Cost-effective oxygen reduction reaction (ORR) electro-
catalysts are essential for the industrial application of various
catalysts always showed the better activity and durability than
catalysts derived from aliphatic amine, implying that the type
1
-16
types of fuel cells and metal-air batteries.
Carbon materials
1
7-21 of carbon nanostructure formed during pyrolysis might be one
co-doped with nitrogen and transition metals (M-N
x
/C)
2
of the critical factors for catalyst performance. Equally
1
have been widely studied as ORR catalysts because of their
tunable electronic and spin structure in outer orbital, and
advanced ORR activity and stability that resemble those of Pt-
important, incorporation of N-coordinated Fe (Fe-N ) species
x
into polymers can substantially improve the ORR activity of Fe-
2
1, 22
N
x
/C materials, demonstrating that Fe-N
x
species may
based precious metal catalysts.
firstly demonstrated as precursor to prepare M-N
Since polyacrylonitrile was
modulate the carbon nanostructure and/or be direct active
2
sites for ORR. It implied that synthesis of aromatic polymer
x
/C material
18, 24-28
4
2
as ORR catalyst, numerous N-contained polymers
3
were
with dense and uniformly-dispersed Fe-N species may have
x
the big opportunity to obtain the efficient ORR catalysts.
Almost all aromatic-polymer derived Fe-N /C catalysts were
x
a
Key Lab for Special Functional Materials of Ministry of Education, Collaborative
Innovation Center of Nano Functional Materials and Applications, Henan
University, Kaifeng, 475004, Henan Province, P. R. China.
synthesized from pyrolysis of the mechanically mixed iron salts
1
8, 24-26
and nano-sized polymers.
Fe-N species was only existed
x
b
Department of Applied Chemistry, The University of Tokyo 7-3-1 Hongo, Bunkyo-
on polymer surface, and it was difficult to incorporate Fe-Nx
species into the bulk of polymer with this conventional
ku, Tokyo 113-8656, Japan. E-mail: hashimoto@light.t.u-tokyo.ac.jp;
nakanishi@light.t.u-tokyo.ac.jp
Electronic Supplementary Information (ESI) available: additional XPS spectra,
XANES
DOI: 10.1039/x0xx00000x
2
1
method. Bulk of polymer is exposed to be surface after
pyrolysis treatment. The negligible amount of Fe-N species in
and
EXAFS
spectra,
and
electrochemical
tests.
See
x
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4 2 2 8 3
(NH ) S O (2.28 g) and 0.02 mol NaHCO (1.68 g) was poured
down to above suspension to polymerize the Fe-OPDA
complex to form Fe-PPDA polymer. The pH value was kept
constant at a value of 2.1 before and after polymerization of
Fe-OPDA. The suspension was centrifuged and washed with
distilled water three times to remove the soluble chemicals.
The resulted wet-powder was ultrasonicaly dispersed in 100 ml
o
distilled water, and then freeze-dried at -45 C for 24 h to
obtain the Fe-PPDA-1/C powder. The powder was pyrolyzed at
o
00 C for 2 h under a temperature program (0-to-800 C, 3 h;
o
8
8
o
00 C, 2h; 800-to-0 C, 3 h) in the tubular furnace. The
o
pyrolyzed sample was treated with concentrated HCl solution
for 8 h, followed by centrifugation and washing with distilled
water three times. Finally, the obtained powder was dried at
o
6
0 C.
Figure 1. a) Schematic illustration of the synthetic process for Fe-N
formation of Fe-OPDA complex; (2) polymerization of Fe-OPDA in the presence of
4 2 2 8
NaHCO and (NH ) S O ; (3) pyrolysis of the Fe-PPDA-1 polymer and treatment of Fe-PPDA-1 polymer for EXAFS and XANS
x
/C-1. (1)
3
the pyrolyzed samples with HCl acid.
The synthesis of Fe-PPDA-1 polymer was similar to that of Fe-
PPDA-1/C materials, while no carbon particle was added.
the bulk means that the modulation of carbon nanostructure
by Fe-N
favored on the catalyst surface. Aromatic polymer with
abundant Fe-N species both in the bulk and on the surface can
provide an opportunity to flexibly modulate carbon
nanostructure by Fe-N and achieve highly dense Fe-N species
on catalyst surface for enhancement of ORR activity.
x x
species and formation of dense Fe-N species are not
The preparation of Fe-N /C-2
x
The synthesis procedure of Fe-N /C-2 material was similar to
x
x
that of Fe-N /C-1, while no NaHCO was added during the
x
3
polymerization of Fe-OPDA. The calcinations procedure was
x
x
the same as that of Fe-N /C-1.
x
In this work, we show an unique synthesis protocol to obtain
iron-coordinated aromatic polymer, (Fe(III)-poly(o-phenylene-
The preparation of Fe-N /C-3
x
0
.01 mol OPDA dissolved in water was poured into the well-
dispersed carbon suspension (0.2 g). A solution of 0.01 mol
NH ) S O (2.28 g) was added into above suspension to
4
diamine), Fe-PPDA), with high content of Fe-N species. Fe-
PPDA polymer was successfully synthesized by polymerization
of o-phenylenediamine/ferric-ion complex in the presence of
sodium bicarbonate, which was essential in retaining high
(
4 2
2 8
polymerize OPDA to form PPDA/C composite. The PPDA/C
material was centrifuged and washed with water for three
times, following with addition of 0.0025 mol FeCl3 and
content of Fe-N
polymer. Experimental characterization revealed that Fe-N
materials possessed dense and mono-dispersed Fe-N species,
4
species and increasing surface area of
x
/C
o
ultrasonically dispersion. The PPDA/Fe-salt was dried at -45 C
x
to get the PPDA/Fe-salt hybrid powder. The calcinations
high ratio of quaternary-N to pyridinic-N, and big effective
surface area, which were considered as the determined factors
for their efficient ORR activity in both alkaline and acidic
medium.
procedure was the same as that of Fe-N /C-1.
x
Pt/C
Commercial Pt/C (20 wt%) was purchased from Tanaka
Kinzoku.
Experimental
Instrumentations
Materials
X-ray absorption near edge structure (XANES) and extended X-
ray absorption fine structure (EXAFS) measurements were
performed at the hard X-ray beam line BL01B01 at SPring-8,
Japan. Transmission-yield spectra were acquired using a
double-crystal Si (111) monochromator. X-ray photoelectron
spectroscopy (XPS) was conducted using a Kratos Ultra AXIS
Spectrometer system equipped with a monochromatic Al-Kα
ortho-phenylenediamine (OPDA), ferric chloride (FeCl3),
sodium bicarbonate (NaHCO3), ammonium persulfate
((NH ) S O ), sodium sulfate (Na SO ) were purchased from
4 2 2 8 2 4
Wako Company, Japan. Ketjen carbon black (EC300J) was a gift
from Mistubishi chemical company. O2 and Ar gas was
purchased from Taiyo Nippon Sanso Corporation.
2
source. N adsorption/desorption isotherm was tested by a
The preparation of Fe-N /C-1
x
Micrometitics ASAP 2020 system (USA). The specific surface
area was determined through Brunauer-Emmett-Teller (BET)
method.
The carbon particles (0.2 g) were ultrasonically dispersed in
water solution with ultrasonic probe system. 0.01 mol OPDA
(1.08 g) dissolved in distilled water was poured into above
suspension and stirred with one hour. And then, 0.005 mol
FeCl ·6H O (1.35 g) was added into above suspension, and
Electrochemical characterization
3
2
Electrochemical measurements were performed in a bi-
potentiostat (Pine Instrument Co.) equipped with a rotating
stirred for several minutes. A hybrid solution of 0.01 mol
2
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(a)
(a)
(b)
x x
Figure 3. SEM images of a) Fe-N /C-1 and b) Fe-N /C-2 catalysts.
3
affinity to protons than to Fe , Fe is released during
+
3+
2
polymerization. Therefore, we added NaHCO
9
3
at step-2 as a
in
neutralizer to ensure that the density of coordinated Fe-N
x
(b)
Fe-PPDA was higher, which was an essential aspect of this
work. As the oxidation of 1 mol OPDA released 2 mol protons,
the mole ratios of OPDA and NaHCO
was confirmed that the pH values was unchanged before and
after Fe-PPDA synthesis in the presence of NaHCO , indicating
that the generated protons had been removed via the
3
were controlled at 1:2. It
3
-
+
following reaction: HCO
should also be noted that the evolved CO
mechanically disrupt the aggregated Fe-PPDA polymers and
3
+ H
ꢀ
CO
2
+ H
2
O, as expected. It
2
gas bubbles can
3
0
decrease the particle size of Fe-PPDA, which was effective for
increasing the surface area of Fe-PPDA polymer and final
pyrolyzed materials (Figure 2, S1). Hereafter, Fe-PPDA
polymers synthesized in the presence and absence of NaHCO
3
addition are denoted Fe-PPDA-1 and Fe-PPDA-2, respectively.
The resulting product was washed with distilled water three
Figure 2. (a) N
2
adsorption/desorption isotherm curves of Fe-Nx/C materials
derived from different precursors; (b) The measured CV diagrams of the Fe-Nx/C
catalysts (0.4 mg cm-2) on glassy carbon electrode (capacitor, 1: 2: 3=1.9: 1.6: 1). times. The last step was pyrolysis of Fe-PPDA to form Fe-N
/C
step 3), which was treated with concentrated HCl solution to
remove impurities before characterization. The pyrolyzed Fe-
x
(
ring-disk electrode (RRDE). A saturated calomel electrode (SCE)
was used as a reference electrode and calibrated with respect
to a reversible hydrogen electrode (RHE) (RHE=SCE*+0.244+
PPDA-1 and Fe-PPDA-2 were denoted Fe-N
respectively.
x x
/C-1 and Fe-N /C-2,
0.591×pH at 25 °C). A glassy-carbon disk electrode loaded with
catalyst/nafion was used as the working electrode and Pt wire
was used as the counter electrode. The working electrode was
Physicochemical characterization
-2
x x
Their morphologies Fe-N /C-1 and Fe-N /C-2 materials are
prepared by loading catalyst ink (0.4 mg cm ; catalyst to
nafion: 4:1) on a glassy carbon electrode. RRDE tests were
shown in Figure 3 with observation of scanning electron
microscopy (SEM). It was shown that the particle sizes of Fe-
2 4
conducted in an oxygen-gas bubbled KOH and K SO (0.05 M)
-1
N
x
/C-1 (Figure 2a) were smaller than that of Fe-N
b), indicating the surface area of Fe-N /C-1 was bigger than
that of Fe-N /C-2. Low-temperature N absorption/ desorption
measurements confirmed the bigger Brunauer-Emmett-Teller
x
/C-2 (Figure
solution (pH 13) at 25 °C. A scan rate of 5 mV s and rotational
speed of 1500 rpm were used for the ORR activity test. The
poised potential of the Pt ring was 1 V vs. RHE. The Pt counter
electrode was replaced with a carbon stick to exclude the
possible contribution of the dissolved Pt species originating
from the Pt counter electrode to ORR activity during long-term
operation.
2
x
x
2
2
-1
x
(BET) surface area (Figure 3a) of Fe-N /C-1 (621 m g )
2
-1
compared with that of Fe-N
addition of NaHCO played the roles not only for proton
neutralization but also for generating CO gas bubbles to break
the aggregated polymer, resulting in the higher surface area of
Fe-N /C sample. The big difference of BET surface area
between Fe-PPDA-1 polymer (13.5 m g ) and Fe-N
x
/C-2 (512 m g ). It meant that
3
2
x
Results and discussion
2
-1
x
/C-1 (621
Synthesis of Fe-N
Figure 1 shows the synthetic route for the Fe-N
First, FeCl solution was added to a mixture of o-phenylene-
x
/C catalysts
2
-1
m g ) indicated that partial elements were removed from
polymer during pyrolysis and acidic treatment, and the
pyrolyzed sample had highly porous structure. In another
x
/C material.
3
diamine (OPDA) and carbon particles to form an Fe-OPDA-
complex/C suspension (step 1). Then, polymerization of Fe-
words, bulk of polymer was evolved to be surface of Fe-N
material after pyrolysis treatment. The BET surface area of Fe-
/C-1 was over 20 times higher than that of Fe-N /C-3 (22.8
m g ), indicating the effective BET surface area of Fe-N /C was
x
/C
OPDA to Fe-PPDA was carried out by adding (NH
4 2 2 8
) S O (step
N
x
2
x
2). Notably, the oxidative polymerization process decreases
-1
x
the local pH. As the amine-ligand in OPDA and PPDA has higher
highly promoted by using the iron-coordinated polymer
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difference of iron-species concentration was also clearly
demonstrated from the XPS spectra (Figure S2). The valence
states of Fe and Fe-N coordination structure in the bulk of Fe-
PPDA-1 polymer were confirmed by X-ray absorption near
edge structure (XANES) and extended X-ray absorption fine
structure (EXAFS) measurement as shown in Figures 4. In the
normalized XANES spectra (Figure 4a), the Fe K-edge position
of Fe-PPDA-1 agreed well with those of Fe(III)
(a)
2 3
tetraphenylporphyrin chloride (FeIIITPPCl) and Fe O , indicating
3
that the Fe valence state was dominant in Fe-PPDA-1. The
+
3
Fourier transform of K -weighted EXAFS oscillations for Fe-
PPDA-1 is shown in Figure 4b. Two peaks were dominant in the
reference Fe O /FeO, and the corresponding peak at around
2
3
1.5 Å/1.65 Å was assigned to the first Fe-O bond, while the
peak between 2 and 3.5 Å was attributed to the Fe-Fe bond. In
contrast to the spectra of iron oxide, only one peak at 1.58 Å
(b)
was dominant, demonstrating
a very low content of
1
3
neighboring Fe-Fe species in Fe-PPDA-1. Combination with
the above results, as well as the similarity to the spectrum of
Fe TPPCl (radius 1.64 Å), the peak at 1.58 Å was assigned to
III
the Fe-N bond in the Fe-PPDA-1 polymer. The minor difference
in the radius (1.64 Å vs 1.58 Å) was possibly ascribed to the
distorted/flexible square-structure of the Fe-N bond in the
polymer. The peak intensity of the Fe-N bond (7.8) in Fe-PPDA-
1
was close to that of Fe TPPCl (8.2), which indicated the
III
(
a)
Figure 4 (a) Normalized Fe K-edge XANES spectra and (b) radial distribution
functions of Fourier-transform k3-weighted Fe K-edge EXAFS for Fe-PPDA-1
2 3
polymer. FeO, Fe O and Fe(III) tetraphenylporphyrin chloride (FeIIITPPCl) were
used as references.
precursor with porous structure compared to that of physical
mixture of iron/polymer precursors.
We investigated whether the Fe-PPDA-1 precursor prepared
3
in the presence of NaHCO possessed the desired structure.
The Fe concentration in the Fe-PPDA polymer was determined
by surface elemental analysis using X-ray photoelectron
spectroscopy (XPS). As shown in Table 1, the Fe concentration
in Fe-PPDA-1 was 3.1 mol%, which was much greater than that
(
b)
3
of Fe-PPDA-2 (0.2 mol%) prepared without NaHCO . The big
Table 1 Surface elemental contents of Fe-Nx/C materials derived from different
x
Figure 5 XPS (a) N 1s and (b) Fe 2p3/2 spectra of different Fe-N /C materials.
precursors.
4
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species in Fe-PPDA-1, the Fe content in Fe-N
was much greater than in Fe-N /C-2 (0.1 mol%).
The Fe coordination structure and valence state in Fe-N
were characterized by XANES and EXAFS measurements
Figure 6). In the normalized Fe K-edge XANES spectra (Figure
a), the Fe K-edge position of Fe-N /C-1 was between those of
x
/C-1 (0.6 mol%)
(a)
x
x
/C-1
(
6
x
3
+
2+
2 3
Fe (references: Fe O and FeIIITPPCl) and Fe (reference:
3
+
2+
FeO), indicating that both Fe and Fe valence states existed.
This was consistence with the results observed in XPS Fe 2p3/2
3
spectra. The Fourier transform of the K -weighted EXAFS
x
oscillations for Fe-N /C-1 is shown in Figure 6b. In contrast to
the spectrum of iron oxide, only one peak at 1.63 Å was
dominant, which demonstrated that the content of
neighboring Fe-Fe species was very low and that no Fe oxide or
metal species existed in Fe-N /C-1. It indicated that
x
(b)
mononuclear Fe species were well dispersed on the surface
and bulk of Fe-N /C-1, which was possibly important for
x
efficient ORR catalysts. The spectra were similar to those of
Fe TPPCl (radius 1.64 Å), and thus, the peak at 1.63 Å can
III
3
1
possibly be assigned to the Fe-N bond. Although the
possibility of an O-Fe-N structure cannot be ruled out, the
dominant Fe species was assumed to be the N-coordinated Fe
units. This was due to the fact that only Fe-N units exist in the
Fe-PPDA-1 precursor, as well as the high nitrogen content
(N=4.8%) of Fe-N /C-1 based on XPS analysis. The calculated
x
coordination number, based on the peak intensity of Fe-N /C-1
x
(
6.3) was approx. 3.3 with respect to the reference compound,
3
Fe TPPCl (intensity, 7.7 Å). Taken together, these results
1
III
demonstrated that highly concentrated and mononuclear Fe-
N units existed in the Fe-N /C-1 sample.
x
x
Figure 6 (a) The normalized XANES spectra at Fe K-edge and (b) Radial
3
distribution functions of Fourier-transformed k -weighted Fe K-edge EXAFS for
Electrochemical characterization
x 2 3
Fe-N /C-1 materials. FeO, Fe O , Fe(III) tetraphenylporphyrin chloride (FeIIITPPCl)
were taken as references.
Next, the ORR activity of Fe-N /C-1 was examined. Figure 7a
x
3
+
31
shows the ORR polarization curves of Fe-N
Pt/C (20 wt%), measured on a rotating ring-disk electrode
RRDE) at 1500 rpm in a medium of O saturated KOH (pH13).
x x
/C-1, Fe-N /C-2, and
quasi-four/four N coordinated Fe in Fe-PPDA-1.
Next, we characterized the elemental compositions and
contents of Fe-N /C-1 and Fe-N /C-2, obtained by pyrolyzing
Fe-PPDA-1 and Fe-PPDA-2, respectively. Figure 5 shows the
XPS N 1s and Fe 2p3/2 spectra of Fe-N /C-1 and Fe-N /C-2. The
(
2
x
x
A material prepared from a pyrolyzed hybrid, PPDA/Fe-salt
Fe-N /C-3), was also evaluated as a reference sample. The
(
x
x
x
background capacitive current was corrected (Figure S3). As
shown in Figure 7a, the ORR onset potential (Eonset) and half-
N 1s peak positions corresponding to pyridinic-N (398.5 eV),
pyrrolic-N (399.5 eV), quaternary-N (400.8 eV) and quaternary-
+
N -O (403.1 eV) were close, indicating the similarity in the
-
wave potential (E1/2) of Fe-N
much greater than the corresponding values of Fe-N
catalyst (1.00 V and 0.83 V) and commercial Pt/C (20 wt%, 0.08
x
/C-1 (1.08 V and 0.92 V) were
1
8
x
/C-2
type of N-species. A big difference was detected in the ratios
of N-species calculated from the deconvoluted N 1s spectra
-2
mg cm (Pt), 1.02 V and 0.85 V). Additionally, Fe-N /C-1 with a
x
(
Figure 5a), in which the ratio of quaternary-N species (>60 %
in total N species) in Fe-N /C-1 was much greater than that in
Fe-N /C-2 (<40%). Quaternary-N was the result of in-plane
doping, and Fe-N species acted as graphitization catalysts to
-2
loading amount of 1 mg cm gave Eonset and E1/2 of 1.11 V and
.92 V, respectively. The ORR activity of Fe-N /C-1 (E1/2, 0.92 V)
was much better than those of the recently-reported excellent
x
0
x
x
x
1
7
Fe-N
x
/C catalysts under the similar condition , such as N-Fe-
induce N-atoms preferentially doping into carbon plane rather
than at the edges of graphene layer. The higher Fe content in
Fe-PPDA-1 resulted in higher ratio of quaternary-N to pyridinc-
1
CNT/CNP materials (E1/2, 0.87 V), FePhen@MOF-ArNH
3
.86 V), Fe/ANT/C (E1/2, 0.82 V), and so on. Its’ ORR
7
3
(E1/2,
2
33
34
0
2
1
activity was also better than those of the excellent metal-free
carbon materials, such as nitrogen-doped carbon nanotube
N compared with that of Fe-PPDA-2. In the XPS Fe 2p3/2
2
spectra (Figure 5b), both Fe (710.9 eV) and Fe (712.9 eV)
+
3+
6
array , N-doped graphene , B,N-co-doped carbon nanotube
7
1
8
were detected in the Fe-N
x
/C samples. Table 1 also
/C materials.
3
5
17, 32-34
array, and so on.
Although the electrochemically active surface area (EASA) of
Fe-N /C-1 was increased (1.2 times) compared with that of Fe-
summarizes the elemental contents of the Fe-N
x
Similar to the higher concentration of N-coordinated Fe
x
N
x
/C-2 due to the enhancement of BET surface area (Figure
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ARTICLE
Journal Name
a)
b)
x x x
Figure 8 ORR polarization curves from (1) Fe-N /C-1, (2) Fe-N /C-2, (3) Fe-N /C-3,
(
4) commercial Pt/C catalysts at pH 1 medium (H
2
4
SO ).
x
The methanol tolerance of Fe-N /C-1 catalyst in alkaline
medium was also investigated under ORR condition (Figure
S5a), and no methanol oxidation current was observed at the
scanned potentials. An accelerated stability test of Fe-N
was conducted in the RRDE system with continuous O
bubbling. Eonset and E1/2 were negatively shifted only approx.
0-20 mV after 1000 cycles (Figure S5b), indicating good
x
/C-1
2
1
stability in the alkaline medium. The activity of the catalyst for
ORR in an acidic medium was also examined in the RRDE
system in a medium at pH 1 (H
/C-1 catalyst were 0.88 V and 0.76 V, respectively (Figure 8),
much better than those of Fe-N /C-2 and Fe-N /C-3, which
highlighted the effect of high content of Fe-N species on ORR
2 4
SO ). The Eonset and E1/2 of Fe-
N
x
x
x
Figure 7 (a) ORR polarization curves and (b) detected H
from (1) Fe-N /C-1, (2) Fe-N /C-2, (3) Fe-N /C-3, (4) commercial Pt/C catalysts
with a loading amount of 0.4 mg cm , (5) Fe-N /C-1 with the loading amount of
2 2
O production currents
x
x
x
x
-2
x
activity under different pH condition. The ORR E1/2 in pH1
medium was also comparable to that of commercial Pt/C (20
wt Pt%) catalysts (0.78 V), indicating the advanced ORR activity
-
2
1
mg cm (pH 13).
2
b), the difference in the EASA value was not the only reason
for the higher activity of Fe-N /C-1. RRDE experiments
revealed that H production by Fe-N /C-1 was lower than
that of Fe-N /C-2 (Fig. 7b). The electron transfer numbers (n)
Figure S4) were calculated based on the ratio of disk and ring
current in RRDE system according to the equation:
n=I /(I +I /N), where I and I are the kinetic currents from disk
and ring, respectively, and N is the collection efficiency (0.19).
The generated H oxidative current of Fe-N /C-1 was very
x x
of Fe-N /C-1 materials. In addition, the ORR stability of Fe-N /C
x
in acidic media was checked and it showed slightly decreased
after cycling 500 times (Figure S6). One of the possible reason
2
O
2
x
x
for the decreased activity was ascribed the loss of Fe-N
x
active
(
sites in acidic environment, which was discussed in previous
2
reports.
1, 34
d
d
r
d
r
ORR active site
Fe-N species in polymer have two functions for the formation
of Fe-N /C materials as ORR catalysts: the one is the
2
O
2
x
x
low from 1.0 V to 0.6 V, which was similar to that of
commercial Pt/C. The calculated electron transfer number at
x
graphitization catalyst to form graphene structure, which is
effective to modulate the ratio of quaternary-N to pyridinic-N
0
.7 V for Fe-N
oxidative currents from Fe-N
of Fe-N /C-1, and the calculated electron transfer number at
.7 V was 3.95. Compared with those of Fe-N /C-2, the higher
diffusion current density, Eonset and E1/2 values should be
ascribed to higher EASA and active site density of Fe-N /C-1.
The low ORR activity of Fe-N /C-3 was possibly ascribed to the
low EASA and concentration of active site species, which was
consistent to reported results that hybrid of FeCl /PPDA was
used as the precursor. The above results indicate that Fe-
/C-1 possessed superior ORR activity and selectivity over
those of Fe-N /C-2 and Fe-N /C-2 catalysts, even better than
commercial Pt/C in an alkaline medium.
x
/C-1 was 3.99, similar to that of Pt/C (3.98). The
H
2
O
2
x
/C-2 were greater than that
2
1, 25
in carbon matrix;
the other is the precursor to form Fe-N
It was well recognized the effect and importance
of N-species and Fe-N species on the ORR activity of Fe-N /C
and both were proposed as the possible
candidates as catalytic active centers. Comparing the samples
of Fe-N /C-1 and Fe-N /C-2, their large difference of ORR
x
x
2
4, 37
species.
0
x
x
x
6
, 7, 21
materials,
x
x
x
x
activities and selectivity should correlate with N-coordinated
Fe concentration (0.6 mol% vs 0.1 mol%) and the ratio of
quaternary-N to pyridinic-N. Although we cannot conclude
which one played the dominant role, the mono-dispersed Fe-
3
3
6
N
x
x
x
N
4
with high concentration in the polymer precursor was
revealed to be essentially important to obtain the Fe-N /C with
x
6
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1
3
both dense Fe-N
x
species and high ratio of quaternary-N to
13 F. Calle-Vallejo, M. T. M. Koper and A. S. Bandarenka,
1
pyridinic-N. Synthesis of Fe-coordinated polymer in present Chem. Soc. Rev., 2013, 42, 5210-5230.
4
1
study is the key to obtain the extraordinary ORR activity of Fe-
/C materials.
14 W. Cheng and R. G. Compton, Angew. Chem., Int. Ed.,
1
N
x
2015, 54, 7082-7085.
1
5
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5 Z. L. Li, G. L. Li, L. H. Jiang, J. L. Li, G. Q. Sun, C. G. Xia and
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F. W. Li, Angew. Chem., Int. Ed., 2015, 54, 1494-1498.
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Conclusions
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6 M. S. Faber and S. Jin, Energy Environ. Sci., 2014,
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7, 3519-
3
1
542.
7
In summary, we synthesized iron-coordinated aromatic
polymer, Fe-PPDA-1, by polymerization of Fe-OPDA complex in
the presence sodium carbonate, which was taken as the key
factor in retaining the Fe-N coordination units in the Fe-PPDA-
1
7 H. T. Chung, J. H. Won and P. Zelenay, Nat. Commun.,
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4:1922,
2
1
013,
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8 Y. Zhao, K. Watanabe and K. Hashimoto, J. Am. Chem.
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1
1
2
polymer, and increasing surface area by evolved CO gas
9
1
9 I. Hijazi, T. Bourgeteau, R. Cornut, A. Morozan, A.
breaking the aggregated polymer. The dense and mono-
dispersed Fe-N species, modulated carbon structure, and high
surface area were thought to be the reasons for the efficient
Filoramo, J. Leroy, V. Derycke, B. Jousselme and S. Campidelli, J.
1
Am. Chem. Soc., 2014, 136, 6348-6354.
2
0
2
0 J. Liu, X. J. Sun, P. Song, Y. W. Zhang, W. Xing and W. L. Xu,
1
ORR activity of Fe-N
new insights of bottom-up synthesis method to obtain efficient
M-N /C catalysts from rational design of catalyst precursors.
x
/C catalyst. The present study provides
Adv. Mater., 2013, 25, 6879-6883.
2
1
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1 G. Wu and P. Zelenay, Acc. Chem. Res., 2013, 46, 1878-
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x
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The table of contents
Iron-coordinated polymer (Fe(III)-poly(o-phenylenediamine))
produced iron/nitrogen co-doped carbon electrocatalyst
demonstrates highly efficient oxygen reduction reaction activity
in both alkaline and acidic media.
8
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