Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts

Article abstract of DOI:10.1002/anie.201400358

Nonprecious metal catalysts for the oxygen reduction reaction are the ultimate materials and the foremost subject for low-temperature fuel cells. A novel type of catalysts prepared by high-pressure pyrolysis is reported. The catalyst is featured by hollow spherical morphologies consisting of uniform iron carbide (Fe₃C) nanoparticles encased by graphitic layers, with little surface nitrogen or metallic functionalities. In acidic media the outer graphitic layers stabilize the carbide nanoparticles without depriving them of their catalytic activity towards the oxygen reduction reaction (ORR). As a result the catalyst is highly active and stable in both acid and alkaline electrolytes. The synthetic approach, the carbide-based catalyst, the structure of the catalysts, and the proposed mechanism open new avenues for the development of ORR catalysts. Oxygen reduction: Hollow spheres comprising uniform iron carbide (Fe₃C) nanoparticles encased by graphitic layers were synthesized (see picture). The spheres show excellent electrocatalytic activity and high stability in both acidic and alkaline media for the oxygen reduction reaction. A synergistic mechanism is proposed.

Full text of DOI:10.1002/anie.201400358

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DOI: 10.1002/anie.201400358
Electrocatalysis Very Important Paper
Hollow Spheres of Iron Carbide Nanoparticles Encased in Graphitic
Layers as Oxygen Reduction Catalysts**
Yang Hu, Jens Oluf Jensen, Wei Zhang, Lars N. Cleemann, Wei Xing,* Niels J. Bjerrum, and
Qingfeng Li*
À
Abstract: Nonprecious metal catalysts for the oxygen reduc-
tion reaction are the ultimate materials and the foremost
subject for low-temperature fuel cells. A novel type of catalysts
prepared by high-pressure pyrolysis is reported. The catalyst is
featured by hollow spherical morphologies consisting of
uniform iron carbide (Fe3C) nanoparticles encased by graph-
itic layers, with little surface nitrogen or metallic functionalities.
In acidic media the outer graphitic layers stabilize the carbide
nanoparticles without depriving them of their catalytic activity
towards the oxygen reduction reaction (ORR). As a result the
catalyst is highly active and stable in both acid and alkaline
electrolytes. The synthetic approach, the carbide-based catalyst,
the structure of the catalysts, and the proposed mechanism
open new avenues for the development of ORR catalysts.
tion-metal/nitrogen (M Nx/C, M = Co, Fe, Ni, etc.) com-
pounds,[5] metal-free nitrogen-doped carbon materials,[6] tran-
sition-metal chalcogenides.[7] Among the best is the M Nx/C
À
catalyst where the active sites are believed to involve surface
nitrogen coordinated with metals.[4]
Recently, a new type of NPMC based on Fe3C has been
reported to be active towards ORR. Wen et al.[8] synthesized
nitrogen-enriched Fe/Fe3C C nanorods as ORR catalysts in
À
neutral media while Lee et al.[9] found that the Fe/Fe3C-
functionalized melamine foam exhibited good ORR activities
in alkaline media. Though interesting, little information is
known about the role of the Fe3C phase in catalysis, probably
because of two main reasons: first, the prepared Fe3C-based
catalysts were found to contain a certain amount of surface
nitrogen,[8] indicating the possible presence of FexN or/and
NxC sites. Second, the reported catalysts were a mixture of
several phases, for example, carbon-coated Fe3C in different
scales, bare Fe3C, and metallic Fe particles, making it difficult
to conclude the role of the Fe3C phase in the ORR. More
critically, the possible use of the Fe3C-based catalysts in acidic
media is apparently out of question, as the carbide dissolves in
acid.[10]
H
ighly active and durable catalysts for the oxygen reduc-
tion reaction (ORR) are undoubtedly essential for the large-
scale application of fuel cells.[1] The state-of-the-art catalyst is
based on precious metals,[2] and the prohibitive costs, limited
resources, and insufficient durability preclude commerciali-
zation of the technology.[1,2b,3] Accordingly, the development
of cost-effective nonprecious metal catalysts (NPMCs) has
been a foremost subject of the field.[4] Owing to the harsh
ORR conditions especially in acidic media, only a few types of
materials have been found to be active and durable towards
the ORR, including, for example, carbon-supported transi-
À
On the other hand, the M Nx/C catalysts prepared by
pyrolysis are known to contain metal-rich particles encapsu-
lated in graphitic carbon shells.[5a] And it is assumed that the
encapsulation phases contribute little to the ORR activity.[11]
However, as reported by Bao and co-workers in a series of
studies,[12] when metal nanoparticles are confined inside
carbon nanotubes (CNTs), a unique host–guest electronic
interaction changes the local work function of the CNT walls.
As a result, catalytic functionalities can be achieved on the
outside surface of the CNTs. One example is the metallic iron
nanoparticles confined within CNTs, which showed enhanced
ORR activity compared to that of the pristine CNTs.[13]
[*] M.Sc. Y. Hu, Dr. J. O. Jensen, Dr. L. N. Cleemann,
Prof. N. J. Bjerrum, Dr. Q. F. Li
Department of Energy Conversion and Storage
Technical University of Denmark
Kemitorvet 207, 2800 Lyngby (Denmark)
E-mail: qfli@dtu.dk
M.Sc. Y. Hu, Prof. W. Xing
Laboratory of Advanced Power Sources
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences, 130022, Changchun (China)
E-mail: xingwei@ciac.ac.cn
À
Herein, in an attempt to prepare the M Nx/C-type
catalyst under high pressure, we expected to promote the
formation of nitrogen functionalities. Instead we obtained
a novel type of ORR catalyst with a negligible amount of
nitrogen or metal on the surface. The catalyst shows well-
controlled structures of hollow spheres comprising uniform
Fe3C nanoparticles encased by uniform graphitic layers.
Because of the protection of the graphitic layers, the Fe3C
nanoparticles are chemically stable in hot acids. Towards the
ORR the catalyst (hereafter referred to as Fe3C/C) shows
excellent activities and high stabilities in both acidic and
alkaline media. More interestingly, the catalyst with the well-
controlled structure and the negligible amount of nitrogen or
iron on the surface provides a unique model material for
probing the ORR active sites of the catalysts.
Dr. W. Zhang
Department of Energy Conversion and Storage
Technical University of Denmark
Risø campus, Frederiksborgvej 399, 4000, Roskilde (Denmark)
[**] This work was supported by the Danish National Research
Foundation (Procon), Danish ForskEL (Catbooster), Danish Council
for Strategic Research (4M Centre), and National Natural Science
Foundation of China (grant number 21011130027). Dr. J. D. Zhang
and Q. J. Chi, Department of Chemistry DTU, are thanked for
assisting with the rotating ring disk electrode and helpful dis-
cussion.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400358.
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istic angle of 68.48, corresponding to the (À211) and (211)
planes of the Fe3C phase. The Fe3C/C-800 catalyst (obtained
from pyrolysis at 8008C) showed a structure similar to that of
Fe3C/C-700 (Figure S3a). For catalysts and other materials
prepared by pyrolysis, structure control is always a difficult
task.[4a,9,14] This is particularly true for preparation of Fe3C-
based materials by pyrolysis.[10a,15] To the best of our knowl-
edge, no Fe3C with uniform morphology and evenly dispersed
nanoparticles has been reported.
The bulk composition of the Fe3C/C-700 catalyst was
analyzed with energy dispersive X-ray spectroscopy (EDS),
and X-ray diffraction (XRD) and thermogravimetric analysis
(TGA). EDS (Figure 2a) shows that the catalyst contains C
Figure 1. a) Synthesis of Fe3C/C hollow spheres. b) SEM image of
Fe3C/C-700. c) TEM image of one typical hollow catalyst sphere of
Fe3C/C-700. d) TEM image of bamboo-like carbon nanotubes grown on
the surface of the hollow spheres. e) HRTEM image of a Fe3C nano-
particle in Fe3C/C-700 and the index crystal plane shown in the inset.
Figure 2. a) EDS spectrum of Fe3C/C-700. b) XRD patterns of Fe3C/C-
700 and Fe3C/C-800. c,d) High-resolution XPS N-1S (c) and Fe-2P (d)
spectra of Fe3C/C-700 and Fe3C/C-800.
As illustrated in Figure 1a, the Fe3C/C hollow spheres
were synthesized by one-step pyrolysis using ferrocene and
cyanamide as precursors (see the Supporting Information).
The synthesis was facile and, from more than 15 batches of the
synthesis, a high reproducibility in terms of structure and
performance was achieved. The obtained products were
porous and magnetic powders along with a high-pressure
gas phase. The structure of the typical catalyst Fe3C/C-700 (as
obtained from pyrolysis at 7008C) is shown in Figure 1 as well
as Figure S1–2 in the Supporting Information. The character-
istic morphology is dimensionally uniform spheres with
a diameter of 400–500 nm. As revealed by transmission
electron microscopy (TEM) images (Figure S2a), a large
proportion of these spheres are hollow spheres with a shell
thickness of about 60 nm. A closer look at these spheres
reveals that Fe3C nanoparticles (about 10 nm in diameter)
uniformly dispersed in the sphere structure. Around these
spheres some Fe3C nanoparticle-filled carbon nanotubes
(CNTs) are observed, as shown in Figure 1d ad Figure S2.
Focusing on the inner part of these hollow spheres, one can
also distinguish some tube structures (Figure S2b). In the shell
region of these spheres, high-resolution TEM (HRTEM)
images (Figure S2d) show that the Fe3C nanoparticles are
coated by four to nine graphitic layers. These Fe3C nano-
particles were checked by HRTEM analysis. As shown in
Figure 1e for a typical nanoparticle, the spacing of crystalline
lattices in two directions was both 0.21 nm with a character-
(ca. 82.1 wt%) and Fe (ca. 15.0 wt%) with a small amount of
O (ca. 2.9 wt%). TGA measurement (Figure S4a) reveals an
iron content of 15.8 wt% in Fe3C/C-700, in good agreement
with the EDS result. The XRD spectra confirm the presence
of graphitic carbon atoms and a Fe3C phase in Fe3C/C-700. As
shown in Figure 2b the broad peak of the XRD pattern at
about 26.58 is indexed to the (002) planes of the graphitic
carbon, while the other diffraction peaks were characteristic
of Fe3C (JCPDS 35-0772). Beside these two phases, the Fe3C/
C-800 sample contained traces of metallic iron (JCPDS 06-
0696) (Figure 2b). The surface composition is of particular
interest and the surface contents of N and Fe were examined
by high-resolution X-ray photoelectron spectroscopy (XPS).
As shown in Figure 2c, a trivial N-1s signal was observed for
Fe3C/C-700 (mainly from pyridinic N), signifying a negligible
share of nitrogen atoms (ꢀ 0.50 at%) on the catalyst surface.
As to the Fe3C/C-800 sample, the nitrogen content (ꢀ
0.20 at%) was even smaller. Likewise, the Fe-2p spectra
(Figure 2d) revealed a negligible surface iron content on the
Fe3C/C-700 (ꢀ 0.08 at%) and the Fe3C/C-800 (below detec-
tion limit). We have noticed the fact that XPS signals were
noisy and tried to increase the number of scans, which
however did not lead to any improvement unless for those
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samples with higher elemental contents. Such low nitrogen
and iron surface contents imply that the Fe3C nanoparticles
are exclusively encased by carbon layers in the catalysts. This
structure was further verified by leaching the Fe3C/C-700
sample in 0.5m H2SO4 solution at 858C for 9 h. The resulting
sample was named as Fe3C/C-700L. Since the pure Fe3C phase
is soluble in acid, only protected by carbon layers can it
survive in the leaching process. As shown in Figure 5a and
Figure S3b, the corresponding XRD and TEM analyses for
the Fe3C/C-700L show little difference from the Fe3C/C-700,
revealing the well-preserved Fe3C nanoparticles after the
leaching process. Considering the use of a nitrogen-containing
precursor, the surface nitrogen content for the obtained
catalyst is surprisingly low, different from all previously
reported catalysts prepared by pyrolysis under ambient
pressure.[16] It seems the high pressure of pyrolysis is making
the difference. The TGA (Figure S4b) demonstrates that both
precursors are volatile and/or decomposed completely during
pyrolysis, thus creating high pressure in the autoclave, which
was estimated to be about 600 bar (see Supporting Informa-
tion).
Figure 3. a) Linear sweep voltammograms (LSVs) of Fe3C/C-700, Fe3C/
The ORR activities of Fe3C/C catalysts were assessed in
both acidic and alkaline solutions with the rotating disk
electrode (RDE) and rotating ring disk electrode (RRDE)
measurements (Figure 3). The results were compared with
that of a commercial Pt/C catalyst. In 0.1m HClO4, the ORR
polarization curve of Fe3C/C-700 exhibited a high onset (ca.
0.90 V) and a half-wave potential (ca. 0.73 V), both about
0.1 V lower than that of Pt/C. The mass specific activity at
0.8 V was about 0.5 AgÀ1, showing little dependence on the
catalyst loadings on the RDE (see Figure 3e). This activity is
comparable with the best MNx/C catalysts reported.[4a,17] The
H2O2 yield was found to be less than 8% at all potentials and
decreased to about 6% at potentials below 0.6 V (Figure 3c),
indicating a four-electron pathway dominated ORR process.
Kinetic currents derived from the mass transport correction
of the ORR currents (Figure 3 f) show a Tafel slope of 59 mV
per decade at low over-potentials, close to 57 mV per decade
for the Pt/C catalyst. Therefore, similar to the ORR processes
on Pt/C catalyst in acidic media, transfer of the first electron is
probably the rate-determining step of the ORR for the Fe3C/
C catalysts.[18] To verify the chemical stability of the Fe3C/C
catalyst in acid for the ORR, the activity of the acid-leached
Fe3C/C-700L sample was also tested. It shows little change in
the ORR activity compared with that for Fe3C/C-700,
indicating the catalytically active phase remained intact
during the acid leaching process. The Fe3C/C-800 catalyst
showed a lower ORR activity than that of Fe3C/C-700 in 0.1m
HClO4 but a higher activity in 0.1m KOH, just as good as the
Pt/C catalyst (Figure 3b and Figure S5). The onset potentials
and half-wave potentials were 1.05 and 0.83 V, respectively,
for both Fe3C/C-800 and Pt/C catalysts. The ORR of Fe3C/C-
800 in KOH involved a trivial H2O2 yield at low over-
potentials but a high H2O2 yield (ca. 18%) in the diffusion-
limiting current region (Figure 3d), indicating a different
ORR mechanism in alkaline solution from that in acid. The
ORR pathways in acidic or alkaline media were also assessed
by an approach using the Koutecky–Levich equation, which
showed similar results as those determined by RRDE
C-700L and Pt/C (20 wt% Pt on Vulcan XC-72, Johnson Matthey) at
900 rpm in O2-saturated 0.1m HClO4 before and after 4500 potential
cycles. Potential cycling was carried out between 0.6 and 1.0 V in N2-
saturated 0.1m HClO4. b) LSVs of Fe3C/C-800 and Pt/C at 900 rpm in
O2-saturated 0.1m KOH before and after 4500 potential cycles.
Potential cycling was carried out between 0.6 and 1.0 V in N2-saturated
0.1m KOH. c) Peroxide yield of the Fe3C/C-700 and Pt/C catalysts for
the ORR in 0.1m HClO4. d) Peroxide yield of the Fe3C/C-800 and Pt/C
catalysts for the ORR in 0.1m KOH. e) Mass-specific LSVs of Fe3C/C-
700 with different catalyst loadings on RDE at 900 rpm in O2-saturated
0.1m HClO4. f) Tafel plots obtained from the RDE measurements on
Fe3C/C-700 and Pt/C at 900 rpm. The catalyst loading was 0.6 mgcmÀ2
for Fe3C/C catalyst and 0.25 mgcmÀ2 (50 mg(Pt) cmÀ2) for Pt/C catalyst.
The scan rate was 10 mVSÀ1
.
measurements (Figure S6). Alongside the high ORR activity,
Fe3C/C catalysts showed also excellent methanol and H3PO4
tolerance for the ORR (Figure S7).
The ORR stability of the Fe3C/C catalyst in acidic and
alkaline media was evaluated using an accelerated durability
test protocol.[19] As shown in Figure 3a and b, after 4500
potential cycles, the half-wave potentials for Fe3C/C-700L in
HClO4 and Fe3C/C-800 in KOH both shifted negatively by
about 22 mV, similar to those for the Pt/C catalyst (16 mV in
HClO4 and 20 mV in KOH, Figure 3a and Figure S8a).
Further extension of the stability test to 31500 cycles
(140 h) in HClO4 showed a small additional shift of the half-
wave potential by 40 mV (Figure S8b). Chronoamperometric
measurements were then carried out to confirm the stability,
which showed even better stability of Fe3C/C-800 in compar-
ison with Pt/C (Figure S8c).
Concerning the nature of the ORR active sites, on one
hand, the negligible surface nitrogen and iron content on the
Fe3C/C catalyst implies the fact that another mechanism
À
instead of or in parallel to the well-recognized Fe Nx and/or
À
À
C Nx active sites for the M Nx/C-type catalysts might prevail
for the ORR activity. Of particular note is a recent report by
Ferrandon et al.[20] who showed that the presence of an Fe3C
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phase prohibited the Fe-N4 active site formation during the
pyrolysis. On the other hand, as discussed before and shown in
Figure 4, the Fe3C nanoparticles, which were encased by
graphitic layers, are not in direct contact with the electrolyte
or O2, and hence not the direct ORR active sites.
To investigate the situation a destructive test of the Fe3C/
C catalyst was conducted by ball-milling to destroy the
protective carbon shells around the carbide nanoparticles.
The resulting sample was named as Fe3C/C-700BL. After the
milling, the Fe3C nanoparticles were exposed and therefore
soluble in hot acid during the following leaching. As revealed
by XRD, SEM and TEM (Figure 5a and Figure S9), Fe3C
nanoparticles were largely removed after the milling-leaching
process. Accordingly, it lost the magnetic property as well as
the catalytic activity towards ORR. It seems clear that,
despite the fact that the encased Fe3C nanoparticles are not in
direct contact with the electrolyte, they still play a key role in
the catalysis. This role is most likely defined by the synergetic
interaction between the carbide and protective graphitic
layers. In other words, the encased carbide nanoparticles
activate the surrounding graphitic layers, making the outer
surface of the carbon layer active towards the ORR, as
schematically illustrated in Figure 4.
Figure 5. a) XRD patterns of Fe3C/C-700, Fe3C/C-700L, and Fe3C/C-
700BL. b) The corresponding ORR LSVs at 900 rpm in O2-saturated
0.1m HClO4. The scan rate was 10 mVSÀ1. c) XRD patterns of
samples S1, S2-700, and S2-800 (see the Supporting Information for
experimental details). d) ORR LSVs of Vulcan XC-72R, S1, S2-700, and
S2-800 at 900 rpm in O2-saturated 0.1m HClO4. The catalyst loadings
were 0.6 mgcmÀ2 for all electrodes. e,f) TEM images of the samples S1
and S2-700. The arrows indicate the encapsulated Fe3C particles.
tubes filled with Fe3C particles (ca. 5 wt% of Fe by EDS,
Figure 5 and Figure S10), similar to the previously described
Fe3C/C samples but not clustered in porous spheres. The
S2 samples exhibited significant ORR activities in terms of
onset potentials and reduction currents, though lower than
those for the Fe3C/C-700 catalyst, probably because of the
different morphology and lower iron contents. This result
clearly confirmed the ORR activity of the nitrogen-free
catalyst and hence the assumed synergistic mechanism.
In summary, a novel ORR catalyst in form of hollow
spheres comprising uniform Fe3C nanoparticles encased by
graphitic layers was synthesized by a one-step high-pressure
pyrolysis process. The Fe3C/C catalyst exhibited excellent
ORR activity and stability in both acidic and alkaline media.
With negligible surface nitrogen or iron functionalities, the
catalyst is a model system for understanding the ORR active
sites of this type of encapsulation catalysts. The outer
protective graphitic layers stabilize the carbide nanoparticles
in hot acids while the inner Fe3C nanoparticles, though not in
direct contact with the electrolyte, are proposed to play
a synergetic role in activating the outer surface of the
graphitic layers towards the ORR. The findings of the
synthetic approach, the synthesized catalyst of the unique
structure, and the proposed mechanism paves a way to further
development of active and durable ORR catalysts for fuel cell
applications.
Figure 4. Oxygen reduction process on Fe3C/C-700 (scale
bar=5.00 nm).
From previous work it was understood that nitrogen is
a prerequisite for the active site formation of Fe-based ORR
catalysts.[4a,21] To exclude the possible effect of nitrogen and its
À
À
associated Fe Nx or/and C Nx species we attempted to
synthesize the catalyst using nitrogen-free precursors. By
doing so, any activity would be attributed to the catalytic
nature of the carbon-surrounded Fe3C particles. In the first
synthesis, ferrocene was used as a single precursor (see the
Supporting Information). As shown by XRD and TEM
(Figure 5), the obtained sample (referred to as S1) was iron
particles surrounded by thick carbon shells, and did not show
more ORR activity than the Vulcan carbon black. This
observation is in agreement with the previous report[21] that
nitrogen-free precursors did not generate an ORR-active
catalyst. However, this is not the case when a mixture of
ferrocene and durene (C10H10) was used as nitrogen-free
precursors. Thus obtained samples (referred to as S2-700 and
S2-800 for samples prepared at 700 and 8008C, respectively)
possessed morphologies primarily consisting of carbon nano-
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D. A. Stevens, J. R. Dahn, J. Electrochem. Soc. 2008, 155, B547 –
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Keywords: electrocatalysis · fuel cells · iron carbides ·
.
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