1. No category

CARBON
5 7 ( 2 0 1 3 ) 4 4 3 –4 5 1
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Fe–N–carbon black for the oxygen reduction reaction
in sulfuric acid
Hui Xiao a,b, Zhi-Gang Shao
Baolian Yi a
a
b
a,*
,
Geng Zhang
a,b
, Yuan Gao
a,b
, Wangting Lu
a,b
,
Fuel Cell System and Engineering Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
Graduate School of Chinese Academy of Sciences, Beijing 100049, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A Fe–N–C catalyst was prepared by pyrolyzing carbon black supported Fe–imidazole com-
Received 30 August 2012
plexes. The effect of different pyrolysis temperatures on the catalysts’ oxygen reduction
Accepted 1 February 2013
reaction (ORR) activity was investigated by electrochemical rotating disk electrode tests
Available online 10 February 2013
in 0.5 M H2SO4. The Fe–N–C catalyst heat-treated at 700 °C was found to display the best
ORR activity with a highest half-wave potential of 0.656 V vs. normal hydrogen electrode
(NHE), which was only 64 mV lower than that of the commercial Pt–C (loading: 10 lgPt cm2),
and a lowest H2O2 yield (0.6% at 0.4 V vs. NHE). Furthermore, potential cycling tests showed
that the catalyst had a better electrochemical stability than Pt–C catalyst. X-ray photoelectron spectroscopy characterization of these catalysts was also conducted to identify the
active nitrogen species for ORR.
Ó 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cell (PEMFC) is considered as
one of the most promising energy conversion technologies
available today because of its high energy efficiency and low
environmental impact [1]. However, due to the high cost of
platinum and its limited reserves, the commonly used Ptbased catalysts have become a barrier to the commercialization of PEMFC [2]. Thus, to reduce the high cost of Pt-based
catalysts, intense research efforts have been made to develop
non-precious metal catalysts (NPMCs) to replace Pt-based catalysts for the oxygen reduction reaction (ORR) [3]. Me–N–C
(Me = Fe and/or Co) catalysts are among the most promising
NPMCs.
Since Jasinski discovered that cobalt phthalocyanine can
catalyze ORR in 1964 [4], Me–N–C catalysts have experienced
at least two breakthroughs, the introduction of heat-treatment
[5] and cheap nitrogen sources [6]. Nowadays, various studies
have demonstrated that Me–N–C materials are candidate
oxygen reduction catalysts. And there is a consensus that
Me–N–C catalysts can be prepared after heat-treatment at
500–1000 °C with simultaneous presence of metal ions, a
source of carbon and a source of nitrogen [2]. Recently, Wu
et al. prepared a Fe(Co)/polyaniline/carbon electrocatalyst by
pyrolyzing carbon-supported polyaniline-complexes of Fe
and/or Co that exhibited good activity and remarkable
stability of 700 h at 0.4 V [1]. Proietti et al. reported an iron-acetate/phenanthroline/zeolitic-imidazolate-framework-derived
electrocatalyst with increased volumetric activity of
230 A cm3 and an excellent power density of 0.75 W cm2 at
0.6 V [7], which is the best performance of NPMCs to date.
Although Me–N–C catalysts have shown great improvements
in ORR activity, the relative poor performance compared with
commercial Pt–C emphasizes the need for more research on
the development of new Me–N–C catalysts.
The ORR active sites of Me–N–C catalysts are still a subject
of immense debate. A number of active sites have been
proposed. They include Me–Nx moieties [8–10] such as
* Corresponding author: Fax: +86 411 84379185.
E-mail address: zhgshao@dicp.ac.cn (Z.-G. Shao).
0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.carbon.2013.02.017
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CARBON
5 7 ( 2 0 1 3 ) 4 4 3 –4 5 1
Fe–N2+2 (Fe coordinated with pyridinic N) and Fe–N4 (pyrrolic
N), and heterocyclic N groups such as pyridinic N at an edge
site [11,12] and quaternary N along the zigzag edges of graphite layers [13,14]. However, to improve ORR activity levels by
tailoring new catalyst structures, more investigations into
the nature of ORR active sites should be conducted.
In this study, a Fe–N–carbon black catalyst with high ORR
activity and low H2O2 yield was reported. Specifically, imidazole was used as nitrogen precursor, since it has rich nitrogen
content and may form a structure like Fe–porphyrin [11,15],
which may contribute to the formation of active sites for
ORR. Meanwhile, the possible ORR active sites were explored.
2.
Experimental
2.1.
Preparation of Fe–N–C electrocatalysts
The Fe–N–C catalyst was synthesized following a procedure in
the literature [15]. Imidazole, iron chloride and Ketjen black
EC300J (EC300) were used as the metal, nitrogen and carbon
precursor, respectively. For example, the catalyst with a nominal iron content of 10 wt.% is prepared as follows, 194 mg FeCl3Æ6H2O, 160 mg imidazole and 200 mg EC300 were dispersed
uniformly in 50 mL ethanol by agitating vigorously for 6 h.
Then the mixture was dried under vacuum at 60 °C for 8 h. Finally, the dry samples were heat-treated at various temperatures ranging from 600 to 900 °C for 2 h under nitrogen flow.
These catalysts are denoted as Fe–N–C-UHT (unheat-treated),
Fe–N–C-600, Fe–N–C-700, Fe–N–C-800 and Fe–N–C-900, accordingly. In order to explore the role of nitrogen and iron in ORR
catalysis, N–C-700 catalyst was synthesized by heat-treating
the same amount of imidazole and EC300 at 700 °C, and C700 catalyst was prepared by heat-treating EC300 at 700 °C.
2.2.
Characterization of Fe–N–C electrocatalysts
The surface area of the catalyst was measured using BET
method on a QuadraSorb SI Automated Surface Area & Pore
Size analyzer. The sample was degassed by heating at 300 °C
for 2 h under vacuum before measuring the surface area.
The surface morphology of Fe–N–C samples was observed
by high resolution transmission electron microscopy (HRTEM)
using a JEOL JEM-2010F microscope operated at 200 kV. The
Fe–N–C crystalline structures were characterized by X-ray diffraction (XRD) analysis performed on a PANalytical X’Pert Pro
diffractometer using Cu Ka radiation. The 2h angular region
extended from 20° to 80° at a scan rate of 5° min1 with a step
of 0.02°. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer, using Al-Ka sources.
Electrochemical measurements were conducted in a
three-electrode electrochemical cell using CHI Electrochemical Station (Model 730D) with 0.5 M H2SO4 electrolyte at
room temperature. All potentials in this work are referred to
normal hydrogen electrode (NHE). The working electrode
was a rotating ring disk electrode (RRDE, PINE AFE7R9GCPT)
with a glassy carbon disk (5.61 mm diameter) and a ring made
of platinum (6.25 mm inner diameter and 7.92 mm outer
diameter). Platinum foil and a saturated calomel electrode
(0.241 V vs. NHE) were used as the counter and reference electrode, respectively. The catalyst ink was prepared by mixing
5 mg catalyst, 1 mL isopropanol and 50 lL Nafion (5 wt.%, Du
Pont Corp.). The ink (20 lL) was dropped onto the glassy
carbon disk, which was then left to dry in air at room temperature, to yield a catalyst loading of ca. 385 lg cm2.
In rotating disk electrode tests, the background capacitive
currents were recorded in a potential range from 1.0 to 0.05 V
in nitrogen-saturated electrolyte at a scan rate of 5 mV s1.
Then linear sweep voltammograms in oxygen-saturated electrolyte were measured at an electrode rotation speed of
900 rpm. The oxygen reduction current was corrected by the
background current. In RRDE tests, the ring potential was
set to 1.15 V to oxidize the hydrogen peroxide produced during oxygen reduction on the disk electrode. Especially, in order to clean the surface of the catalysts, 30 cycles of cyclic
voltammetry were performed by scanning the potential between 0 and 1.2 V at a scan rate of 100 mV s1 in nitrogen-saturated electrolyte.
3.
Results and discussion
3.1.
ORR activity and selectivity characterization
RRDE tests were conducted for all samples to determine their
ORR activity and selectivity. For comparison, the performance
of commercial Pt–C (20 wt.%, Johnson Matthey) was also
investigated. The lower part of Fig. 1a plots disk current (Id)
against applied potential and the upper part plots the ring
current (Ir) as a function of applied potential. From these results, it is clear that a heat-treatment step is crucial, where
Fe–N–C-UHT demonstrated negligible ORR activity. The ORR
kinetic currents, which were derived from Koutecky–Levich
analysis in the mixed kinetic/diffusion-limited region of the
ORR curves [16], were compared to elucidate the effect of
the heat-treated temperature (Fig. 2a). It shows that the ORR
activity, evaluated by the kinetic current, decreases in the order of Fe–N–C-700 > Fe–N–C-800 > Fe–N–C-600 > Fe–N–C-900.
With the increase of heat-treated temperature from 600 to
900 °C, the ORR kinetic currents firstly increases to a maximum at 700 °C and then decreases with further increase of
temperature.
Fe–N–C-700, which displays the optimal activity, demonstrates an onset potential of 0.828 V and a half-wave potential
of 0.656 V. The disparity in the precious and non-precious metal catalyst loading notwithstanding, the Fe–N–C-700 in this
work has shown a comparable ORR activity with the commercial Pt–C, whose half-wave potential is only 64 mV lower than
that of Pt–C (ca. 0.720 V) at a loading of 10 lgPt cm2. Moreover, compared with the reported Co–N–C catalysts [15] using
the same nitrogen resource of imidazole, Fe–N–C-700 shows
obvious diffusion-limited current density, which attests to
the high number and uniform distribution of ORR active sites
on the surface of catalysts [17].
The total electron-transfer number (n) and the hydrogen
peroxide yield (%H2O2) in the ORR is calculated as follows:
%H2 O2 ¼
2Ir
100%
NjId j þ Ir
CARBON
5 7 (2 0 1 3) 4 4 3–45 1
Fig. 1 – (a) ORR curves, (b) n and %H2O2 of Fe–N–C catalysts
heat-treated at different temperatures and Pt–C catalysts,
obtained at 900 rpm. The Pt loading is 10 lg cm2.
n¼42
%H2 O2
100
where N is the RRDE collection efficiency, which was determined to be 0.37 herein. Fig. 1b shows the n values and
%H 2 O 2 of Fe–N–C catalysts heat-treated at different
temperatures. The n values at 0.4 V decreases in the order of
Fe–N–C-700 > Fe–N–C-800 > Fe–N–C-900 > Fe–N–C-600, which
is consistent with ORR activity results. Specially, Fe–N–C-700
shows the highest n (>3.95) and the lowest %H2O2 (<2.5%),
which exhibits a preference for a direct four-electron reduction pathway. Moreover, the H2O2 yield is significantly lower
than that of the reported Co–N–C catalyst (11.8–33.4%) [15],
which shows the Fe-based catalysts can acquire a reduced
H2O2 yield. As shown in Fig. 1b, the %H2O2 of Pt–C is almost
independent of potential, which is calculated to below 2%.
At the low potential (<0.6 V), the Fe–N–C-700 catalyst even
has a lower %H2O2 than Pt–C catalyst.
Tafel slope was also determined to evaluate the ORR kinetic
character of Fe–N–C-700 and Pt–C. Tafel plots of Fe–N–C-700
and Pt–C (Fig. 2b) show two well-defined linear regions with
similar Tafel slope values, which suggest that Fe–N–C-700
and Pt–C may have a similar ORR mechanism. The Tafel slope
445
Fig. 2 – Tafel plots for ORR at (a) Fe–N–C catalysts heattreated at different temperatures and (b) Pt–C catalyst,
deduced from the ORR curves in Fig. 1a.
values of 58 and 63 mV dec1 for Fe–N–C-700 and Pt–C at high
potentials suggest that ORR rate may be determined by migration of adsorbed oxygen intermediates [1,18]. Furthermore, at
low potentials, the Tafel slope of 123 and 122 mV dec1 for Fe–
N–C-700 and Pt–C could be ascribed to the transfer of the first
electron as a rate-determining step [18].
The ORR behaviors of Fe–N–C-700, N–C-700 and C-700 were
also determined using the RRDE approach (Fig. 3). In the case
of the heat-treated metal-free N–C-700 sample, the introduction of nitrogen with imidazole leads to an obvious improvement in the ORR activity, relative to the heat-treated nitrogenfree C-700 sample. Meanwhile, the %H2O2 decreases from 74%
to 30% at 0.20 V. However, the N–C-700 catalyst appears unable to effectively catalyze ORR yet. A substantial improvement in the ORR activity and four-electron selectivity was
only achieved after an addition of Fe. For Fe–N–C-700, diffusion-limited current is reached and the onset potential increases to 0.828 V. The addition of Fe not only significantly
improves ORR activity, but also reduces H2O2 yields to 0.6%
at 0.4 V. The high ORR activity and four-electron selectivity
of Fe–N–C-700 catalyst show that both introduction of nitrogen and Fe (especially Fe) are critical for the ORR activity.
446
CARBON
5 7 ( 2 0 1 3 ) 4 4 3 –4 5 1
shows a pair of well-defined redox peaks at ca. 0.65 V. The significant redox peaks are attributed to the Fe(II)/Fe(III) redox
couple [1]. The oxidation state of the transition metal, the redox potential of M(II)/M(III) and the ligand effect has been
proved to dominate the ORR activity of Fe-based and Co-based
macrocyclic compounds [19]. In this case, the radical enhancement of Fe to ORR activity and selectivity shows that Fe
species may participate in the oxygen reduction process of
Fe–N–C-700, or even can be the dominant active species in
Fe–N–C-700.
It has shown that the addition of Fe is critical for the high
ORR activity of the catalysts, and the optimal Fe content must
be determined for the Fe–N–C catalysts, since it has been
found that the optimum transition metal content in non-precious metal catalysts depends on the nitrogen precursor and
preparation conditions [17]. The nominal Fe content in the
initial mixture was varied from 1 to 20 wt.% while following
the synthesis procedure described in Section 2.1. Fig. 5 shows
the effect of nominal Fe content on the ORR activity of Fe–N–C
catalysts. The ORR activity increases as the iron content increases from 1 to 10 wt.%, but the addition of more iron results in no significant changes to the activity. This
phenomenon is also confirmed by many other reports
[17,18,20]. In this report, the optimal nominal Fe content of
10 wt.% was used. What’s more, because of a positive correlation between the ORR activity and the nominal Fe amount,
there is also a possibility that Fe metal species could be directly related to active sites.
3.2.
Fig. 3 – (a) ORR curves, (b) n and %H2O2 of C-700, N–C-700 and
Fe–N–C-700 catalysts, obtained at 900 rpm.
Physical characterization
The BET surface area of Fe–N–C-700 catalyst is 444.6 m2 g1.
Compared with the surface area of Ketjen black EC300J
(860 m2 g1) [20], the lower surface area of Fe–N–C-700 suggests that the pore structures of the carbon support were
blocked by the decomposition products of Fe–imidazole complex. However, the BET surface area of Fe–N–C-700 is much
higher than that of reported catalysts using the same carbon
supports, such as 300 m2 g1 of Fe–N–C catalyst prepared by
Fig. 4 – CV curves of Fe–N–C-700 and N–C-700 catalysts in
N2-saturated 0.5 M H2SO4. Scan rate, 100 mV s1.
Cyclic voltammetry (CV) curves of Fe–N–C-700 and N–C-700
in N2-saturated 0.5 M H2SO4 were also recorded (Fig. 4). While
CV of N–C-700 is virtually featureless, the CV of Fe–N–C-700
Fig. 5 – Effect of the nominal Fe content on the ORR activity of
Fe–N–C catalysts.
CARBON
5 7 (2 0 1 3) 4 4 3–45 1
447
Wu et al. [18] and 315 m2 g1 of polyaniline–Fe–C catalyst prepared by Wu et al. [20].
Fig. 6 shows the XRD patterns of Fe–N–C catalysts heattreated at different temperatures. All the samples showed a
very broad carbon (0 0 2) peaks at 2h = 25° which can be attributed to two different forms of carbon, referred to turbostratic
carbon (carbon black) and graphene carbon (graphitic structure) [17]. The Fe–N–C-UHT without heat-treatment shows
some quite strong diffraction peaks (2h = 21.0°, 23.8°, 25.8°)
due to the existence of Fe–imidazole complexes. In contrast,
all the heat-treated samples give strong peaks at 44.6° and
65.0°. These peaks can be attributed to the bulk of carbidicstate Fe and/or the formation of a-Fe [21]. It seems that the
content of a-Fe and/or carbidic-state Fe increases with the increase of heat-treatment temperature, since the intensity of
peaks at 44.6° and 65.0° increase significantly. Even though
the carbidic-state Fe and/or a-Fe have also been demonstrated
to have some ORR activity under acidic conditions [21], no
correlation between ORR activity and the Fe species content
was found, we speculate carbidic-state Fe and a-Fe should
not be the main origin of ORR activity.
HRTEM images of Fe–N–C-700 catalyst are shown in Fig. 7.
There are some dark spots in the low magnification images
(Fig. 7a and b), which shows a possible existence of carbidicstate Fe and/or a-Fe. However, we cannot find a crystallite
structure which can be attributed to Fe species in the high magnification images (Fig. 7c and d). It seems that dark spots in the
low magnification TEM images are not formed by the different
optical contrast between carbon and Fe species, but the piling
of carbon spherical particles. The difficulty in finding the crystallite carbidic-state Fe and/or a-Fe in HRTEM images may be
attributed to the low intensity of related peaks in the XRD pattern of Fe–N–C-700. The majority of iron species in Fe–N–C-700
were likely present in an amorphous form, possibly coordinated with other species that survived the pyrolysis [17].
XPS analysis was used to study the nature of nitrogen surface groups on the carbon support. As shown in Fig. 8a, for the
Fe–N–C-UHT catalyst without heat-treatment, there are two
peaks for N1s at 399.3 and 400.9 eV. It is well known that imidazole has two kinds of nitrogen, one is imine-like nitrogen
Fig. 7 – HRTEM images for Fe–N–C-700 catalyst.
Fig. 6 – XRD patterns of Fe–N–C catalysts.
(–N@) and the other is amine-like nitrogen (–NH–). The two
peaks can be attributed to the coordination between Fe(III)
and the two different nitrogen species.
448
CARBON
5 7 ( 2 0 1 3 ) 4 4 3 –4 5 1
Fig. 8 – XPS N1s spectra of (a) Fe–N–C-UHT, (b) Fe–N–C-600, (c) Fe–N–C-700, (d) Fe–N–C-800 and (e) Fe–N–C-900.
Generally, it is believed that the nitrogen content and
nitrogen species proportion in the carbon materials play a
key role for the improved activity [22]. To illustrate the possible relationship between ORR activity and nitrogen species,
the N1s spectra of various samples (Fig. 8b–e) are deconvoluted into four peaks representing pyridinic (397–399.5 eV),
pyrrolic (400.2–400.9 eV), quaternary (401–403 eV), and
oxidized (402–405 eV) nitrogen functionalities, respectively
[23]. Furthermore, the peak at a binding energy of 397–
399.5 eV may also include a contribution from nitrogen bound
to the metal [20]. The relative amount of pyridinic nitrogen for
Fe–N–C-600, Fe–N–C-700, Fe–N–C-800 and Fe–N–C-900 are
42.62%, 45.36%, 33.36% and 21.92%, respectively. The relative
amount of pyridinic nitrogen increases and reaches a
CARBON
5 7 (2 0 1 3) 4 4 3–45 1
maximum value at the pyrolysis temperature of 700 °C, and
then decreases when temperature continues to rise, which
correlates with the ORR activity. However, since the total
nitrogen content decreases with the increasing pyrolysis temperature from 600 to 900 °C (Fig. 9), the absolute content of
pyridinic nitrogen shows a monotone decreasing trend
(Fig. 10). Thus, there is not a convincible relationship between
the amount of pyridinic nitrogen and the ORR activity.
For comparison, the absolute content of different types of
nitrogen is shown in Fig. 10. Just like the absolute content of
pyridinic nitrogen, the content of pyrrolic nitrogen drops significantly at elevated temperature. However, the absolute
content of quaternary nitrogen reaches maximum at 700 °C,
and decreases by further increasing the pyrolysis temperature. This phenomenon is consistent with the aforementioned ORR activity. Similar correlation between quaternary
nitrogen content and the ORR activity is also consistent with
the computational studies by Terakura et al. [14], and by X-ray
absorption analysis reported by Oshima et al. [13], where the
ORR active site was proposed to be on carbon atoms neighboring quaternary nitrogen atoms along the zigzag edges of
graphite layers. Besides, correlation between quaternary
nitrogen content and ORR activity was also founded in the
polyaniline–iron–carbon oxygen reduction catalyst [24]. The
good correlation suggests that the quaternary nitrogen could
be part of the ORR active sites. For Fe–N–C-700, the incorporation of electron-accepting nitrogen atoms in the conjugated
carbon plane appears to impart a relatively high positive
charge density on adjacent carbon atoms, where oxygen
molecular is adsorbed to continue the ORR [25].
As mentioned earlier, the nature of the active ORR sites in
Me–N–C catalysts is still a controversial subject. Some research groups have suggested that rather than participating
directly in the ORR active sites, transition metals only act as
a catalyst to facilitate the incorporation of nitrogen groups
into the carbon during pyrolysis. They proposed pyridinic or
graphitic nitrogen on the carbon surface are catalytically active for oxygen reduction [26,27].
However, based on time of flight secondary ion mass spectrometry studies of Fe–N–C catalysts, Dodelet’s group has
Fig. 10 – Atomic concentration of different types of N relative
to C for Fe–N–C catalysts, calculated from the data of Figs. 7
and 8
proposed two possible active site configurations, that is Fe–
N2+2/C and Fe–N4/C, claiming that the majority of active site
structures consists of an Fe–N2+2/C configuration bridging
two adjacent graphitic crystallites [28]. The Fe–Nx centers,
where Fe ions coordinate with pyridinic or pyrrolic nitrogen,
were founded in catalysts prepared by adsorbing an iron porphyrin on carbon black and heat-treating the assembly in inert gas [29,30]. Analyzed by 57Fe Mo¨ssbauer spectroscopy and/
or extended X-ray absorption fine structure, some Fe–Nx centers are also found to correlated well with the ORR activity
and correspond to majority of the active sites [24,29–31].
Considering the signal of Fe(II)/Fe(III) peaks in the CV
curves (Fig. 4) and the correlation between the dissolution
of Fe centers and the degradation of Fe–N–C catalysts during
potential cycling (Figs. 11 and 12, discussed in Section 3.3),
we cannot solely assign the ORR activity of Fe–N–C catalysts
to quaternary nitrogen-doped carbon structures. Thus, we
speculate both Fe–Nx centers and quaternary nitrogen incorporated into carbon matrix are the possible ORR active sites
in Fe–N–C catalysts. While the Fe–Nx active sites where Fe
ions coordinate with pyrrolic or pyridinic species appear
likely, a further confirmation of their presence with highly advanced surface analysis technique is necessary.
3.3.
Fig. 9 – Atomic concentration of N and Fe relative to C for Fe–
N–C catalysts, obtained from the XPS N1s, Fe3p3 and C1s
spectra.
449
ORR stability characterization
Another major concern about electrocatalysts is their durability in PEMFC. Herein, the stability of Fe–N–C-700 catalysts was
evaluated by the ORR activity before and after 1000 potential
cycles, scanning between 0 and 1.2 V with a scan rate of
50 mV s1 in nitrogen-saturated 0.5 M H2SO4. Fig. 11 shows
the ORR curves of Fe–N–C-700 and Pt–C before and after
1000 cycles. For Fe–N–C-700, the measured degradation of
the half-wave potential after 1000 cycles is 64 mV. In contrast,
the corresponding change for the commercial Pt–C
(10 lg cm2) amounts to 117 mV. The lower activity degradation of Fe–N–C-700 suggests that it is more stable than the
commercial Pt–C under the same test conditions.
It has been reported that the Pt nanoparticles’ growth, coalescence, dissolution and/or detachment from carbon support
450
CARBON
5 7 ( 2 0 1 3 ) 4 4 3 –4 5 1
attributed to the nature of its active sites. CV curves of Fe–
N–C-700 are collected before and after potential cycling
(Fig. 12). After potential cycling, the intensity of the Fe(II)/
Fe(III) redox peaks at ca. 0.65 V decays, which shows the Fe
centers dissolve during potential cycling. We speculate that
the weakening of the peaks is attributed to the destruction
of Fe–Nx active sites, initiated by the breaking of the iron
nitrogen bondings [30], further contribute to the degradation
of Fe–N–C-700. As for the possible active sites of carbon atoms
neighboring quaternary nitrogen atoms, we believe they are
more stable due to the strong covalent N–C bond [33]. The relative strong strength of covalent N–C bond and Fe–N4 coordination linkage, compared with the adsorption forces exerted
between Pt catalysts and their supports, may account for
the high stability of Fe–N–C catalysts.
4.
Fig. 11 – ORR curves of (a) Fe–N–C-700 catalyst and (b) Pt–C
catalyst before and after 1000 cycles. The potential cycles
were from 0 to 1.2 V in an N2-saturated 0.5 M H2SO4 solution
at room temperature. Scan rate, 50 mV s1.
Summary
We prepared a serious of Fe–N–C catalysts by pyrolyzing carbon black supported Fe–imidazole complexes at different
temperatures. Among them, Fe–N–C-700 pyrolyzed at 700 °C
demonstrates the best ORR activity and a preference for a direct four-electron reduction pathway. The pyrolysis temperature influences the surface atomic composition and
chemistry of Fe–N–C catalysts, such as the iron content, nitrogen content and nitrogen species proportion, which results in
different ORR activities. Based on the XPS and CV results, we
speculate that both Fe–Nx centers (where Fe ions coordinate
with pyridinic or pyrrolic nitrogen) and quaternary nitrogen
incorporated into carbon matrix are the possible ORR active
sites in the Fe–N–C catalysts.
Furthermore, Fe–N–C-700 displays a comparable ORR
activity with Pt–C and a better electrochemical stability than
Pt–C, which shows that it could be a promising candidate
for ORR catalysts in the future.
Acknowledgements
Financial support from the National High Technology Research and Development Program of China (No.
2011AA050701, 2011AA11A273) and the National Natural Science Foundations of China (Nos. 20936008, 21076208) is gratefully acknowledged.
R E F E R E N C E S
Fig. 12 – CV curves of Fe–N–C-700 before and after 1000
cycles. Scan rate, 100 mV s1.
are the possible reasons for the activity degradation of Pt–C
[32]. As for Fe–N–C-700, the higher stability could be
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