Fe-N/C catalysts synthesized by heat-treatment of iron triazine carboxylic acid derivative complex for oxygen reduction reaction

Article abstract of DOI:10.1039/c3ra44958k

4,4′,4″-s-Triazine-1,3,5-triyltri-p-aminobenzoic acid (H ₃TATAB) was used as a ligand to prepare an iron-TATAB (Fe-TATAB) complex for the development of an effective oxygen reduction reaction (ORR) catalyst. The activity of the catalyst depended on weight ratios between the Fe-TATAB complex and carbon black and heat-treated temperatures. The results showed that the Fe-N_70%/C-800 catalyst (the weight ratio of Fe complex to carbon black was 70:30 and the catalyst was pyrolyzed at 800 °C) had good catalytic activity toward ORR with the onset potential at 0.91 V vs. RHE and a kinetic current density of 4.3 mA cm^-2 at 0.6 V vs. RHE in alkaline medium. Moreover, the Fe-N_70%/C-800 catalyst had better tolerance to methanol crossover effect in comparison with commercial Pt/C (20%). The morphology and composition of the catalysts were characterized by high resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopic (XPS). The electrocatalytic activities were demonstrated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometric measurements and stability accelerated tests. According to rotating disk electrode (RDE) measurements and Koutecky-Levich analysis, the overall electron transfer number in the catalyzed ORR was found to be 3.7-3.9 and the ORR process was mainly a four-electron pathway. The results indicate that the Fe-N_70%/C-800 catalyst may be a promising cathode catalyst for ORR.

Full text of DOI:10.1039/c3ra44958k

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Fe–N/C catalysts synthesized by heat-treatment of
iron triazine carboxylic acid derivative complex for
oxygen reduction reaction
Cite this: RSC Adv., 2014, 4, 12168
Qian Cui,†a Shujun Chao,†ab Panhao Wang,a Zhengyu Bai,a Huiying Yan,a Kui Wanga
a
*
and Lin Yang
4,40,400-s-Triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB) was used as a ligand to prepare an iron–
TATAB (Fe–TATAB) complex for the development of an effective oxygen reduction reaction (ORR)
catalyst. The activity of the catalyst depended on weight ratios between the Fe–TATAB complex and
carbon black and heat-treated temperatures. The results showed that the Fe–N70%/C-800 catalyst (the
weight ratio of Fe complex to carbon black was 70 : 30 and the catalyst was pyrolyzed at 800 ꢀC) had
good catalytic activity toward ORR with the onset potential at 0.91 V vs. RHE and a kinetic current
density of 4.3 mA cmꢁ2 at 0.6 V vs. RHE in alkaline medium. Moreover, the Fe–N70%/C-800 catalyst had
better tolerance to methanol crossover effect in comparison with commercial Pt/C (20%). The
morphology and composition of the catalysts were characterized by high resolution transmission
electron microscopy (HRTEM), X-ray diffraction (XRD) as well as X-ray photoelectron spectroscopic
(XPS). The electrocatalytic activities were demonstrated by cyclic voltammetry (CV), linear sweep
voltammetry (LSV), chronoamperometric measurements and stability accelerated tests. According to
rotating disk electrode (RDE) measurements and Koutecky–Levich analysis, the overall electron transfer
number in the catalyzed ORR was found to be 3.7–3.9 and the ORR process was mainly a four-electron
pathway. The results indicate that the Fe–N70%/C-800 catalyst may be a promising cathode catalyst
for ORR.
Received 9th September 2013
Accepted 16th January 2014
DOI: 10.1039/c3ra44958k
www.rsc.org/advances
In 1964, Jasinski rstly reported the ORR catalytic activity of
cobalt phthalocyanine.13 Later studies indicated that the activity
1. Introduction
and stability of transition metal macrocycles could be enhanced
by pyrolysis.14 A signicant breakthrough was achieved years
later when Yeager and co-workers showed that the presence of
the macrocyclic structure was not essential to the ORR activity.15
Therefore, other kinds of NPMCs for ORR such as transition
metal chalcogenides, metal oxides/carbides/nitrides and
conductive polymer-based catalysts, have been developed in
recent years.16,17 Among the different kinds of NPMCs, pyrolyzed
carbon supported transition metal–nitrogen complexes (M–Nx/
C) are considered to be the most promising ORR catalysts for
their high activity and long-term stability.18–20 According to
previous literature, four necessary factors must coexist to obtain
a high performance catalyst for ORR, which are transition
metal, nitrogen precursor, carbon support and heat-treat-
ment.19,21 The metal (M) ions used in this type of catalyst are
mainly Fe and Co, which can give the most active metallic
centers.22,23 Recently, Wang et al. synthesized a Co-corrole/C
catalyst using corrole as a macrocyclic ligand and cobaltous
acetate as a metal precursor. Although the catalyst had a high
catalytic performance, the synthesis procedure was quite
complex.24 Moreover, it is generally believed that traditional
nitrogen-containing macrocyclic ligands (such as porphyrin
Fuel cells are quite promising as clean energy converting
devices to replace traditional energy devices.1,2 The cathode
oxygen reduction reaction (ORR) is a crucial step due to its
sluggish kinetics, and therefore, acceleration of the ORR is
necessary to enhance the performance of fuel cells.3,4 Platinum
(Pt) is the most active element for the ORR, so a high loading of
Pt or Pt-based alloys are usually used as the cathode catalysts in
order to accelerate the ORR.5–7 However, the high cost and
scarcity of Pt hinder the commercial implementation of fuel
cells.8,9 To reduce the cost, intensive research efforts have been
made to nd out a suitable substitute for Pt.10–12 Non-precious
metal catalysts (NPMCs) are probably the best option based on a
long-term and sustainable solution.
aSchool of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical
Media and Reactions, Ministry of Education, Collaborative Innovation Center of
Henan Province for Fine Chemicals Green Manufacturing, Henan Normal University,
Xinxiang 453007, P.R. China. E-mail: yanglin1819@163.com; Fax: +86-373-
3328507; Tel: +86-373-3325058
bDepartment of Chemistry, Xinxiang Medical University, Xinxiang 453007, P.R. China
† These authors contributed equally to this work and should be considered
co-rst authors.
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and phthalocyanine) are costly,25,26 hence, selecting a more cost-
effective ligand is crucial for ORR catalysts.
2.3 Characterization
The morphology of the catalysts was measured by a JEOL-100CX
HRTEM operated at 200 kV. The crystallographic phases and
purity information of the prepared samples were characterized
by XRD, and the corresponding XRD patterns were recorded on
a Bruker D & Advance X-ray diffractometer with a Cu Ka radia-
Studies have shown that the most commonly used coordi-
nation ligands as the nitrogen sources are bipyridine, poly-
pyrrole, polyaniline as well as phenanthroline, and so
on.21,23,27,28 It seems fairly certain that the activity of the M–Nx/C
catalyst is related to its nitrogen content and type of ligands.29
4,40,400-s-Triazine-1,3,5-triyltri-p-aminobenzoic acid (H3TATAB)
is a kind of nitrogen-rich triazine carboxylic acid ligand, and the
synthesis process is quite simple.30 Recently, H3TATAB has been
reported to bridge metal cations (Cu or Zn) in a well-dened
three-dimensional structure to form metal–organic frameworks
(MOFs).31 The facts have attracted intense interest in various
research areas such as heterogeneous catalysis, chemical
sensors, ion exchange and gas storage. However, there are only a
few researches that reported triazine carboxylic acid ligands
used for ORR. In view of the high nitrogen content and low cost
of H3TATAB, we synthesized an efficient Fe–N/C catalyst using
H3TATAB as a nitrogen-rich ligand for ORR. HRTEM, XRD as
well as XPS were used for the morphology and structural char-
acterization. The activities of the catalysts were then tested in
alkaline solution using CV, LSV, chronoamperometric
measurements and stability accelerated tests.
˚
tion source (l ¼ 1.5406 A). XPS measurements were performed
on an MULT1LAB2000 instrument using a monochromic Al
X-ray source.
2.4 Electrochemical measurement
The electrochemical measurements were carried out in a three-
electrode cell using a CHI660 D electrochemical workstation. A
glass carbon rotating disk electrode (RDE) coated with the
catalyst was used as the working electrode, a saturated calomel
electrode (SCE) as the reference electrode, and a Pt wire as the
counter electrode. All measured potentials were converted into
the values referring to a reversible hydrogen electrode (RHE).
Before the catalyst was loaded to the glassy carbon disk, the
electrode was polished mechanically with g-aluminite powder
under an abrasive paper to obtain a mirror-like surface, washed
with DD water and absolute ethanol and allowed to dry at room
temperature. The working electrode was prepared as follows: 2
mg catalyst was added into 1 mL ethanol and 40 mL 5 wt%
peruorosulfonic acid, which was dispersed by ultrasonication
for approximately 10 min to obtain a homogeneous suspension.
Next, 18 mL of the dispersion was uniformly dispersed on a
freshly polished glassy carbon electrode (4.0 mm in diameter)
and dried under the lamp. The catalyst loading on the glassy
carbon electrode was 0.27 mg cmꢁ2. By using the same electrode
conguration, the same loading of commercial Pt/C (20%)
modied electrode was prepared in the same way for compar-
ison. Current densities were calculated by the ratio of current
and the geometric surface area of working electrode.
2. Experimental sections
2.1 Materials
Ferrous chloride tetrahydrate (FeCl2$4H2O), N,N-dime-
thylformamide (DMF) and absolute ethanol were purchased
from China National Pharmaceutical Group Corp. Commercial
20% Pt/C catalyst was obtained from Alfa Aesar. Vulcan XC-72
(carbon black) was obtained from Cabot and used as a catalyst
support. All chemical reagents used in this study were of
analytical grade and were used as received without further
purication. Double distilled water (DD water) was used
throughout the experiments.
The electrochemical experiments were carried out in O2
saturated 0.1 M KOH electrolyte. The cyclic voltammograms
were recorded with a scan rate of 50 mV sꢁ1 in the potential
range 0 to 1.2 V vs. RHE. RDE measurements were conducted at
different rotating speeds from 400 to 2500 rpm with a scan rate
of 10 mV sꢁ1. All electrochemical experiments were performed
2.2 Catalyst preparation
H3TATAB was synthesized according to previous literature.30
0.486 g obtained H3TATAB was added to 40 mL DMF to form a
clear solution and stirred for 30 min at room temperature,
which was a deprotonation step. Aerward, 0.099 g FeCl2$4H2O
was dispersed in the above solution by ultrasonic treatment for
30 min. The product was stirred for 4 h at 30 ꢀC and 1 mL 30%
ꢀ
at 25 ꢂ 1 C.
3. Results and discussion
H2O2 was added to end the reaction. This suspension was stir- Details of the formation mechanism of the Fe–N/C catalysts are
red for another 10 min and dried with a rotary vacuum evapo- shown in Fig. 1. As illustrated by Fig. 1, the Fe–N/C catalysts
rator at 90 ꢀC for 1 h. Then, 0.07 g resulting product and 0.03 g were prepared by a three-step strategy. Firstly, the Fe–TATAB
carbon black were dispersed in 40 mL absolute ethanol with complex was synthesized via Fe2+ coordinating with H3TATAB.
ultrasonic treatment for 30 min and constant stirring for 10 h at Secondly, the Fe–TATAB complex was mixed with carbon black
room temperature. The resulting black mixture was ltered and with different weight ratios. At last, the Fe–N/C catalysts were
rinsed with DD water and absolute ethanol several times, and acquired by the pyrolysis of carbon supported Fe–TATAB
dried in a vacuum at 40 ꢀC overnight. The resulting powder was complex in owing nitrogen. In order to nd the most appro-
ground in a mortar and placed in a tube furꢁn1ace, and pyrolyzed priate proportion of Fe–TATAB to carbon black and the pyrolysis
ꢀ
ꢀ
at 800 C for 120 min at a rate of 3 C min under a nitrogen temperature, different Fe complex to carbon weight ratios
atmosphere. The obtained catalyst was denoted as Fe–N70%
C-800.
/
(20 : 80, 40 : 60, 60 : 40, 70 : 30 and 80 : 20) were synthesized
and pyrolyzed at 600 C, 800 C and 1000 C under a nitrogen
ꢀ
ꢀ
ꢀ
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Fig. 3 XRD patterns of (a) Fe–TATAB complex, carbon black and Fe–
N70%/C samples before and after heat-treatment at different
temperatures; (b) the Fe–N/C-800 samples synthesized with different
Fe complex to carbon black mass ratios.
Fig. 1 Schematic diagram of the synthesis of Fe–N/C samples.
of the Fe–TATAB complex disappear due to decomposition of
the Fe–TATAB complex. Comparing samples with and without
heat-treatment, several characteristic diffraction peaks are
observed aer heat-treatment. Fig. 3b shows the XRD patterns
of the Fe–N/C samples synthesized at different mass ratios aer
pyrolysis at 800 ꢀC. In all the XRD patterns, the wide peak
located at about 2q of 25ꢀ can be assigned to the carbon
support.32 The diffraction peaks at the 2q values of 30.24ꢀ,
35.63ꢀ, 43.28ꢀ, 57.27ꢀ and 62.93ꢀ are attributed to maghemite
Fe2O3 (220), (311), (400), (511) and (440) crystalline facet,
respectively (JCPDS Card no. 39-1346). These results indicate
that part of the carbon supported Fe–TATAB complex has
decomposed into iron oxide during the heat-treatment process.
In addition, the magnitude of crystalline peaks of Fe2O3
increases with increasing Fe–TATAB loading from 20 to 70%.
With a further increase of the complex loading (up to 80%), the
shape of these peaks becomes weak and narrow, which indi-
atmosphere, respectively. The following electrochemical
measurements indicated that Fe–N70%/C-800 was the most
active ORR catalyst among all the synthesized samples. There-
fore, the Fe–N70%/C-800 catalyst was mainly discussed in the
present work. According to the HRTEM, XRD and XPS analysis
results, the carbon supported Fe–TATAB complex was decom-
posed into Fe2O3 nanoparticles, pyridinic N (26.9 at% in total
doped nitrogen), metal–N (15.9 at%), graphitic N (23.7 at%) and
pyrrolic N (33.6 at%). In order to study the role of Fe2O3 in the
ORR, the Fe–N70%/C-800 catalyst was pre-leached in 4 M
hydrochloric acid at 80 ꢀC for
nanoparticles.
6 h to remove Fe2O3
3.1 Physical characterization
cates the presence of a high amount of Fe2O3 in the Fe–N70%
C-800 sample.
/
Fig. 2a shows the typical TEM image of the Fe–N70%/C catalyst
pyrolyzed at 800 C. These nanoparticles exhibit a d spacing of
ꢀ
In order to obtain information about the surface atomic
composition of the samples, XPS analysis was performed for the
unpyrolyzed sample and the sample heat-treated at 800 ꢀC. Due to
the fact that the most abundant elements on the sample surface
were Fe, N, C and O, XPS analysis in the binding energy region of
each element was carried out. The relative atomic percentages of
Fe, N, C and O are 0.43, 6.28, 80.98 and 12.31 in the unpyrolyzed
sample, meanwhile, the Fe–N70%/C-800 catalyst shows the relative
atomic percentages are 0.53, 1.2, 91.88 and 6.41, respectively.
Fig. 4a and b present the Fe 2p spectral region for the unpyrolyzed
sample and the sample heat-treated at 800 ꢀC, respectively. Two
bands can be observed, which correspond to lower energy (Fe 2p3/
2) and higher energy (Fe 2p1/2) asymmetric bands originated from
the spin-orbital splitting. The peaks at 711.2 and 724.9 eV from
Fig. 4a and b can be attributed to Fe 2p3/2 and Fe 2p1/2, respec-
tively. According to the literature, the standard Fe 2p3/2/Fe 2p1/2
signals corresponding to metallic iron, ferrous and ferric states
are located at 706.7/719.8, 709.2/722.8 and 711.2/724.8 eV,
respectively.33 It can be concluded that the ferric state is the
dominant species on the two samples. The presence of Fe(III)
should be due to the conversion of Fe(II) irons coming from the
starting material used in the synthesis. These Fe3+ species can be
bonded with oxygen or nitrogen to form Fe(III)–Nx or Fe(III)
oxides,3,34 which agree well with the XRD results.
0.2516 nm, corresponding to the (311) plane of Fe2O3, as shown
by the HRTEM image in Fig. 2b. Because the content of Fe2O3 is
low, twelve TEM images were taken to perform the histogram of
particle size distribution. The average size of Fe2O3 nano-
particles was calculated to be approximately 31 nm in Fig. 2c.
The crystallographic phases and purity information of the
prepared samples were characterized by XRD. Fig. 3a presents
the XRD patterns of the Fe–TATAB complex, carbon black and
Fe–N70%/C samples before and aer heat-treatment at different
temperatures. In Fig. 3a, the peaks at 16.8ꢀ and 26.2ꢀ are
attributed to the Fe–TATAB complex, while the peak at 24.8ꢀ is
assigned to carbon black. The result shows the crystallographic
phase of carbon black doesn't change with and without heat-
treatment. Aer pyrolysis, the characteristic diffraction peaks
Fig. 2 (a) TEM image, (b) HRTEM image and (c) histogram of particle
size distribution of the Fe–N70%/C-800 catalyst.
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Fig. 5 XPS spectra of C 1s for the Fe–N70%/C sample before (a) and
after (b) heat-treatment at 800 ꢀC, O 1s for the Fe–N70%/C sample
before (c) and after (d) heat-treatment at 800 ꢀC.
Fig. 4 XPS spectra of Fe 2p for the Fe–N70%/C sample before (a) and
after (b) heat-treatment at 800 ꢀC, N 1s for the Fe–N70%/C sample
before (c) and after (d) heat-treatment at 800 ꢀC.
The results of the O 1s spectral region are presented in
Fig. 5c and d for both the unpyrolyzed and pyrolyzed samples.
The corresponding spectra deconvoluted into three contribu-
tions are noted as OI, OII, and OIII, respectively. The OI band at
the lower binding energy of 530 eV could be assigned to oxygen
which is bonded to Fe to form Fe2O3 or other oxides.33,38 The OII
band at the binding energy of 531.6 eV could be related to
oxygen in C]O bonds. The OIII band at 533.5 eV may be
assigned to C–OH or C–O–C groups in carboxylic, epoxy or
phenol groups on the carbon surface. Compared with Fig. 5c
and d, the appearance of OI is probable due to the formation of
small amounts of Fe2O3 nanoparticles aer the pyrolysis, which
is consistent with previous HRTEM and XRD results.
For the unpyrolyzed sample in Fig. 4c, the N 1s band can be
separated into three contributions (NI, NII and NIII) with
binding energies of 398.6, 399.7 and 400.6 eV, respectively. The
lower energy band of NI can be attributed to nitrogen atoms in
the triazine rings and the energy band of NIII can be attributed
to the nitrogen atoms bonded to the edges of the rings in the
form of 2C–NH.35,36 The NII band at 399.7 eV should be attrib-
uted to metal–N bonds.33,37 For the sample pyrolyzed at 800 C
ꢀ
in Fig. 4d, the N1s band can be converted into four peaks (NI,
NII, NIII and NIV) with binding energies of 398.7, 399.7, 400.5
and 401.6 eV, which can be attributed to pyridinic N, metal–N,
pyrrolic N and graphitic N, respectively. It is found that 26.9 at%
N from pyridinic N, 15.9 at% from metal–N, 23.7 at% from
graphitic N and 33.6 at% from pyrrolic N. Studies show that
pyridinic N, graphitic N and metal–Nx are possible active sites,
which are conducive to improve the ORR activity and
stability.33,37
In comparison with Fig. 5a and b, it is found that the
relative content of C–N single bonds is obviously increased
aer pyrolysis. In all cases, ve bands can be observed, which
are marked as CI, CII, CIII, CIV and CV, respectively. The
dominant contributions are the CI signal at 284.6 eV for the C
sp2 bonds in the graphitic domains or the aromatic struc-
ture.24,27 The peak of CII at 285.4 eV for the C sp3 bonds could
be assigned to that in the amorphous carbon substrate or in
the C–N single bonds formed during the pyrolysis. Obviously,
the CII band of the catalyst pyrolyzed at 800 ꢀC is sharper and
broader than that of the unpyrolyzed sample, which is
ascribed to additional C–N single bonds formed during the
pyrolysis. The CIII band at 286.4 eV and the CIV band at
288.0 eV could be assigned to the C]N bonds and the C–O
single bonds which exist on the carbon black surface,
respectively. Finally, the broader CV band at the binding
energy region of 290.7 eV could be assigned to the C]O bonds
of different materials such as carbonyl and carboxyl, which
reside on the carbon substrate.33
3.2 Electrochemical characterization
To evaluate the electrocatalytic activity of the Fe–N/C catalysts,
CV experiments choosing Fe–N70%/C-800 were tested through a
conventional three-electrode method in O2 or N2 saturated 0.1 M
KOH aqueous solutions (Fig. 6a). Despite the capacitive back-
ground, a characteristic ORR peak appears at ca. 0.77 V vs. RHE
in the O2 saturated electrolyte, while no signicant response is
observed in the oxygen-absent medium, indicating the effective
electrochemical reduction of oxygen initiated on the Fe–N70%/C-
800 catalyst. To further investigate the ORR performance, we
carried out the LSV measurements on a RDE for each of the
electrode materials in O2 saturated 0.1 M KOH at a scan rate of
10 mV sꢁ1 and a rotation rate of 1600 rpm. It is worth
mentioning that onset potential and kinetic current density are
two essential parameters that represent the kinetic and ther-
modynamic characteristics of the catalytic reaction. Fig. 6b
shows the ORR polarization curves of the Fe–N70%/C samples
synthesized at different heat-treatment temperatures and
commercial Pt/C (20%). The onset potential increases from 0.87
to 0.91 V vs. RHE with increasing heat-treatment temperature,
indicating that the Fe–TATAB complex might form several bonds
such as Fe–Nx and C–N with the carbon support during the heat
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current density decreases slightly, which suggests that Fe2O3
doesn't play a critical role in the ORR. However, according to the
previous work, an appropriate amount of iron oxide could
promote the enhancement of N-doping during the synthesis
procedure.39,40 In our work, the Fe–N70%/C-800 with a high
amount of Fe2O3 was the most active ORR catalyst among all the
synthesized samples. Therefore, the Fe2O3 obtained during the
pyrolysis process may facilitate N-doping in carbon and further
increase the catalytic activity for ORR.
Fig. 6e shows the RDE polarization curves for ORR on the Fe–
N
70%/C-800 catalyst modied electrode with various rotation
rates from 400 to 2500 rpm. ORR occurs under mixed kinetic-
diffusion control at high potential regions, followed by a
plateau of the diffusion limiting current, which increases with
the rotation rate due to the increase of the oxygen diffusion
through the electrode surface. The transferred electron number
per oxygen molecule involved in the ORR process was evaluated
using the below Koutecky–Levich (K–L) equation:41
1/j ¼ 1/jk + 1/(Bu1/2
)
Fig. 6 Catalytic activity towards electrochemical reduction of oxygen
in 0.1 M KOH at 25 ꢂ 1 ꢀC. (a) CVs of the Fe–N70%/C-800 catalyst in O2
and N2-saturated 0.1 M KOH solution at a scan rate of 50 mV sꢁ1; (b)
LSVs of the Fe–N70%/C samples synthesized at different heat-treat-
ment temperatures and commercial Pt/C (20%) with a scan rate of 10
mV sꢁ1 and a rotation rate of 1600 rpm; (c) LSVs of Fe–N/C-800
samples with different mass ratios; (d) LSVs of the Fe–N70%/C-800
catalyst with and without acid treatment. (e) LSVs of the Fe–N70%/C-
800 catalyst at various rotation rates; (f) Koutecky–Levich plots for the
Fe–N70%/C-800 catalyst obtained from LSVs in (e) at different
potentials; inset showing the distribution graph of transferred electron
number.
where j represents the measured current density, jk is the kinetic
current density, and u is the angular velocity of the electrode. B
could be calculated from the slope of K–L plots based on the
Levich equation as follows:
B ¼ 0.62nFCD2/3 yꢁ1/6
where n is the number of electrons transferred, F is the Faraday
constant (F ¼ 96485 C molꢁ1), C is the concentration of O2 gas
in 0.1 M KOH solution (C ¼ 1.2 ꢃ 10ꢁ6 mol cmꢁ3), D is the
diffusion coefficient of O2 gas (D ¼ 1.9 ꢃ 10ꢁ5 cm2 sꢁ1), and y is
treatment, facilitating the electron transfer from the carbon the kinematic viscosity of 0.1 M KOH solution (y ¼ 0.01 cm2
support to O2.23 The same trend could also be observed in the sꢁ1). Fig. 6f depicts the K–L plots of the Fe–N70%/C-800 catalyst
kinetic current density, which increases from 3.7 to 4.1 mA cmꢁ2 at various potentials from 0.3 to 0.7 V vs. RHE. There is a good
at 0.6 V vs. RHE. With a further increase in temperature up to linearity between jꢁ1 and uꢁ1/2 over the examined potential
1000 ꢀC, the onset potential remains the same while the kinetic range. The numbers of electrons transferred per O2 molecule (n)
current density declines slightly. These observations indicate are calculated to be 3.7–3.9 from the slope of the K–L plots (inset
that the optimal pyrolysis temperature is around 800 ꢀC. The LSV in Fig. 6f). The results suggest that the ORR is mainly a four-
of commercial Pt/C (20%) gives the onset potential at 0.95 V vs. electron and one-step process to generate H2O.
RHE and the kinetic current density of 4.9 mA cmꢁ2 at 0.6 V vs.
As an ORR electrocatalyst for fuel cells, a high catalytic
RHE. The results indicate that the catalytic activity of Fe–N70%/C- selectivity for cathode reactions against fuel oxidation is
800 is very close to commercial Pt/C (20%). The Fe–N70%/C important. The Fe–N70%/C-800 catalyst has a better stability and
catalyst before heat-treatment indicates some activity. The tolerance to methanol crossover effect than commercial Pt/C
activity may be attributed to Fe–Nx bond and carbon support, (20%) by the chronoamperometric measurements. In Fig. 7a,
because triazine has no activity towards ORR. Fig. 6c further the Fe–N70%/C-800 catalyst exhibits a strong and stable amper-
investigates the effect of Fe–TATAB content on the activity of the ometric response aer the introduction of 10 vol% methanol,
Fe–N/C-800 samples. With the increasing of Fe–TATAB complex suggesting a remarkably good tolerance to methanol crossover
loading from 20 to 70 wt%, the onset potential of the Fe–N/C-800 effect. For Pt/C (20%), however, the current instantaneously
samples positively shied and the kinetic current density for the jumped to positive values once adding methanol to the elec-
ORR increases. Further increasing the complex loading up to trolyte at 4000 s, indicating a poor tolerance to methanol
80 wt%, the activity of the Fe–N80%/C-800 drops dramatically. It crossover effect. The results imply that the Fe–N70%/C-800
is indisputable that the Fe–N70%/C-800 catalyst has a better ORR catalyst modied electrode has a good selectivity for ORR when
catalytic activity than other samples. Fig. 6d shows LSVs of the methanol coexists. The durability of the Fe–N70%/C-800 and Pt/
Fe–N70%/C-800 catalyst with and without acid treatment. C (20%) catalysts are also measured through current–time
Compared with the Fe–N70%/C-800 catalyst without acid treat- measurements at 0.7 V vs. RHE (Fig. 7b). Only ca. 4% current
ment, the onset potential has a little negative shi and the reduction is observed on the Fe–N70%/C-800 catalyst aer
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in 0.1 M KOH solution. The catalyst exhibited a high electro-
chemical activity, long-term stability and tolerance to methanol
crossover effect. The performance of the Fe–N70%/C-800 catalyst
is close to that of commercial Pt/C (20%), which makes it a very
promising electrocatalyst for fuel cells.
Fig. 7 (a) Current–time chronoamperometric response of the Fe–
N70%/C-800 catalyst and commercial Pt/C (20%). The arrow indicates
the introduction of 10 vol% methanol; (b) the chronoamperometric
measurements for the ORR on the Fe–N70%/C-800 catalyst and
commercial Pt/C (20%) at 0.7 V vs. RHE. The measurements were
performed in O2 saturated 0.1 M KOH solution; (c) polarization curves
on Fe–N70%/C-800 and Pt/C catalysts at 1600 rpm in O2-saturated
0.1 M KOH electrolyte before and after 50 000 potential cycles with a
scan rate of 10 mV sꢁ1. The potential cycles were from 0.6 to 1.05 V vs.
RHE in O2-saturated 0.1 M KOH solution at room temperature with a
Acknowledgements
This work was nancially supported by the National Natural
Science Foundation of China (grant no. 21301051) and Inno-
vative Research Team in University (grant no. IRT 1061).
Notes and references
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