MOF-Templated Assembly Approach for Fe3C Nanoparticles Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient Oxygen Reduction under Acidic and Basic Conditions

Article abstract of DOI:10.1002/chem.201701389

Developing high-performance non-precious metal catalysts (NPMCs) for the oxygen-reduction reaction (ORR) is of critical importance for sustainable energy conversion. We report a novel NPMC consisting of iron carbide (Fe₃C) nanoparticles encapsulated in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized by a new metal-organic framework (MOF)-templated assembly approach. The electrocatalyst exhibits excellent ORR activity in 0.1 m KOH (0.89 V at ?1 mA cm^?2) and in 0.5 m H₂SO₄ (0.73 V at ?1 mA cm^?2) with a hydrogen peroxide yield of below 1 % in both electrolytes. Due to encapsulation of the Fe₃C nanoparticles inside porous b-NCNTs, the reported NPMC retains its high ORR activity after around 70 hours in both alkaline and acidic media.

Full text of DOI:10.1002/chem.201701389

A Journal of
Accepted Article
Title: MOF-Templated Assembly Approach for Fe3C Nanoparticles
Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient
Oxygen Reduction under both Acidic and Basic Conditions
Authors: Arshad Aijaz, Justus Masa, Christoph Rösler, Hendrik Antoni,
Roland A Fischer, Wolfgang Schuhmann, and Martin Muhler
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To be cited as: Chem. Eur. J. 10.1002/chem.201701389
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MOF-Templated Assembly Approach for Fe3C Nanoparticles
Encapsulated in Bamboo-Like N-Doped CNTs: Highly Efficient
Oxygen Reduction under both Acidic and Basic Conditions
Arshad Aijaz, Justus Masa, Christoph Rösler, Hendrik Antoni, Roland A. Fischer, Wolfgang
Schuhmann,* and Martin Muhler*
Abstract: Developing high-performance non-precious metal cata-
lysts (NPMCs) for the oxygen reduction reaction (ORR) is of critical
importance for sustainable energy conversion. We report a novel
NPMC consisting of iron carbide (Fe3C) nanoparticles encapsulated
in N-doped bamboo-like carbon nanotubes (b-NCNTs), synthesized
by a new metal-organic framework (MOF)-templated assembly
approach. The electrocatalyst exhibits excellent ORR activity in 0.1
M KOH (0.89 V at -1 mA cm-2) and in 0.5 M H2SO4 (0.73 V at -1 mA
derived nanostructured materials for energy storage and conver-
sion applications.[7] MOF-derived NPMCs often exhibit large sur-
face area and hierarchical porous structures, which are essential
for high ORR performance.[8] Recently, NPMCs based on iron
carbide (Fe3C), encapsulated in carbon nanostructures, have
been reported to be very active towards the ORR.[3,5] For
example, Yu and co-workers synthesized Fe3C nanoparticles
encapsulated in porous Fe-N-doped carbon fibers exhibiting
cm-2) with a hydrogen peroxide yield of below 1% in both electrolytes. high ORR activity.[5a] Jia and Guo et al. found that bamboo-like
Due to encapsulation of the Fe3C nanoparticles inside porous b-
NCNTs, the reported NPMC retains its high ORR activity after ~70 h
in both alkaline and acidic media.
carbon nanotube (CNT)/Fe3C composites had excellent ORR
activity in both alkaline and acidic media.[5b] Li and coworkers
demonstrated very efficient ORR electrocatalysis using hollow
spheres of Fe3C nanoparticles encased in graphitic layers.[5c]
Encapsulating metal nanoparticles inside carbon structures
provides a unique synergistic effect that also activates the
adjacent atoms surrounding the nanoparticle, and the outer
surface carbon atoms for the ORR.[9] In addition, encapsulation
provides resistance against corrosion of the metal nanoparticles
especially in acidic electrolytes, which is a necessary prerequi-
site to ensure long-term performance.[10] Very few studies have
reported the synthesis of Fe3C-based catalysts from MOF pre-
cursors, probably because of two reasons. First, carbonization of
Fe-MOF mostly leads to a mixed phase product, for example,
Fe3C, iron oxide or metallic iron,[11] which complicates unambi-
guous identification of the true active sites for the ORR. Second,
the prepared catalysts do not have well-defined nanostruc-
tures.[12] Hence, new synthesis strategies are needed to explore
the potential of MOF-derived nanostructured Fe-electrocatalysts.
Herein, we report a new MOF-template-assembly strategy for
synthesis of a new NPMC based on Fe3C nanoparticles
encapsulated in porous bamboo-like N-doped carbon nanotubes.
The as-synthesized catalyst possesses the desired features for
efficient ORR including intrinsic active sites (Fe3C@carbon or
Fe-Nx/C or N-doped carbon), high surface area, nanoporous
structure, as well as a highly graphitic bamboo-like carbon
framework. As a result, the electrocatalyst showed excellent
ORR activity and stability in both alkaline and acidic media.
The oxygen reduction reaction (ORR) is a key process in fuel
cells and metal-air batteries.[1] The state-of-the-art catalysts for
this sluggish cathodic reaction are based on platinum and its
alloys.[2] However, the limited availability of platinum, high cost,
and insufficient durability preclude its long-term wide scale use.
In recent years, non-precious metal catalysts (NPMCs),
especially metal-nitrogen-carbon (M-N-C, M = Co, Fe, Mn), have
drawn much attention owing to their low cost and high catalytic
ORR activity.[3,4] They are very promising catalysts in harsh ORR
conditions but need further improvement in terms of synthetic
approach and durability. Very few NPMCs have been found to
have sufficient ORR activity and stability in the more challenging
acidic media.[5,6] Therefore, platinum is still the most used ORR
catalyst.
The use of metal-organic frameworks (MOF) as self-sacrifi-
cial templates for porous carbon was first reported in 2008 by Xu
et al.,[7d] resulting in an enormous upsurge of interest in MOF-
[*]
Dr. A. Aijaz,a M. Sc. H. Antoni,a Prof. Dr. M. Muhlera,b
aLaboratory of Industrial Chemistry
Ruhr-University Bochum
44780 Bochum (Germany)
E-mail: muhler@techem.rub.de
bMax Planck Institute for Chemical Energy Conversion, 45470
Mülheim an der Ruhr (Germany)
o
In a typical synthesis, 1.0 g of ZIF-8 (pre-activated at 200 C
under vacuum) and 1.0 g of pyrrole were thoroughly mixed,
followed by drop-wise addition of 6.0 mL of an aqueous solution
of FeCl3.6H2O (0.1 M) over a period of ~3 h. This led to
polymerization of pyrrole residing inside and outside of the ZIF-8
pores, and coordination of pyrrole nitrogen to Fe3+. ZIF-8 was
deliberately chosen as this N-containing Zn-MOF is known for its
high thermal/chemical stability and large surface area with 1.2
nm cavity size. The resulting homogeneous mixture was left to
dry overnight at room temperature. The obtained powder was
pyrolyzed at a rate of 5 oC/min to 800 oC in a N2 atmosphere and
kept at this temperature for 2 h. The resulting catalyst is denoted
as Fe3C/b-NCNT, where b-NCNT refers to bamboo-like nitrogen-
doped carbon nanotube.
Dr. J. Masa, Prof. Dr. W. Schuhmann
Analytical Chemistry - Center for Electrochemical Sciences (CES)
Ruhr-University Bochum, 44780 Bochum (Germany)
E-mail: wolfgang.schuhmann@rub.de
M. Sc. C. Rösler
Anorganische Chemie II, Organometallics & Materials, Ruhr-
University Bochum, 44780 Bochum (Germany)
Prof. Dr. R. A. Fischer
Department of Chemistry, Technical University Munich,
Lichtenbergstraße 4, 85748 Garching, Germany, and Catalysis
Research Centre, Technical University Munich, Ernst-Otto-Fischer
Strasse 1, 85748 Garching (Germany)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201701389
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in Fe3C/b-NCNT. e) HAADF-STEM image of Fe3C/b-NCNT, and it′s (f-h) EDS
elemental mappings.
The powder X-ray diffraction (PXRD) pattern of Fe3C/b-
NCNT displayed a strong peak at ~26.5o assigned to the (002)
reflection of graphitic carbon, in agreement with the structure of
multi-walled CNTs (Figure 1a). The other peaks in Figure 1a
located at 37.8, 43.1, 43.1, 43.9, 44.9, 46.0, 48.7 and 49.3o
correspond to the Fe3C phase. No diffraction pattern corres-
ponding to zinc was observed due to formation of amorphous
zinc metal followed by its vaporization,[8g] although residual 2.8
wt% zinc was found by inductively coupled plasma mass
spectrometry (ICP-MS) analysis. Scanning electron microscopy
(SEM) and transmission electron microscopy (TEM) analysis of
Fe3C/b-NCNT revealed typical features of b-NCNTs with a
diameter of 50-100 nm (Figures 1b, 1c, and S1-S2). Small Fe3C
nanoparticles, visible as black dots in Figure 1c, were distributed
mainly within b-NCNTs. High-angle annular dark-field scanning
TEM (HAADF-STEM) further identified the encapsulated black
particles in b-NCNTs as iron carbide with a lattice distance of
0.21 nm, corresponding to the (211) crystal planes of Fe3C
(Figure 1d). Selected area electron diffraction (SAED) patterns
also demonstrated the presence of crystalline Fe3C (Figure S3).
Careful evaluation of the HAADF-STEM images revealed the
porous and hollow nature of the b-NCNTs (Figure 1e). We
further investigated the elemental distribution in Fe3C/b-NCNT
using HAADF-STEM and energy-dispersive X-ray spectroscopy
(EDS) techniques, which indicated the presence of C, N, and Fe
(Figures 1f-1h). Notably, except for the nanoparticle regions, all
elements were homogeneously distributed in the Fe3C/b-NCNT
sample. High uniformity of the Fe species in the b-NCNT matrix
was observed at the atomic level, and the Fe and N intensities
coincided to a significant degree with each other, suggesting
that there are probably Fe species stabilized by N coordination,
so-called Fe-Nx sites, which are commonly assumed to be active
sites for the ORR.[5a] In the nanoparticle region, the strong Fe
signals and weak C signals were observed simultaneously,
which possibly indicate the iron carbide phase. Indeed, PXRD
measurement confirmed that the nanoparticles in Fe3C/b-NCNT
are pure Fe3C.
The porous nature of Fe3C/b-NCNT was further assessed by
N2 sorption analysis. Figure 2a shows typical adsorption-desorp-
tion isotherms for samples with both micro- and mesopores. The
Brunauer-Emmett-Teller (BET) surface area and pore volume
were 185 m2/g and 0.35 cm3/g, respectively, with a pore size
distribution mainly centered at 3.7 nm (Figure S4). The elemen-
tal composition and graphitic nature of Fe3C/b-NCNT were
evaluated by ICP-MS analysis and Raman spectroscopy
respectively. The amount of Fe and N in Fe3C/b-NCNT was 7.1
and 8.6 wt%, respectively. The D (1360 cm–1) and G (1590 cm–1)
bands in the Raman spectrum provide information on the defect
density and crystallinity of sp2 carbon. Fe3C/b-NCNT showed a
ID/IG value of 1.09 indicating a high degree of graphitization in
agreement with the PXRD results (Figure 2b). The porous and
graphitic nature of Fe3C/b-NCNT is favorable for rapid diffusion
and enhanced electrical conductivity, respectively.
a)
b)
Fe3C/b-NCNT
Fe3C
300 nm
c)
d)
e)
Figure 2. a) N2 sorption isotherms, b) Raman spectrum, c) N 1s XPS
spectrum, and d) Fe 2p XPS spectrum for Fe3C/b-NCNT.
0.21 nm
High-resolution X-ray photoelectron spectroscopy (XPS)
measurements were conducted to elucidate the surface
properties of Fe3C/b-NCNT. The deconvoluted N 1s spectrum
showed 4 peaks, assigned to pyridinic N (398.5 eV), pyrrolic N
(400.4 eV), graphitic N (401.7 eV), and oxidized N (404.2 eV)
(Figure 2c). The peak at a binding energy of 398.5 eV also
includes some contribution from N bound to iron (N-Fe),
because there is only a small difference between the binding
energies of pyridinic N and N-Fe.[5a] The amount of pyridinic N
2 nm
50 nm
h)
f)
g)
N
C
Fe
50 nm
50 nm
50 nm
Figure 1. a) PXRD pattern, b) SEM image, and c) TEM image of Fe3C/b-
NCNT. d) High-resolution HAADF-STEM image of a single Fe3C nanoparticle
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was determined to be about 59%. The Fe 2p3/2 and Fe 2p1/2
signals were centered at 710.7 eV and 723.4 eV, respectively
(Figure 2d). Deconvolution of the Fe 2p3/2 region suggests the
presence of Fe3C (or metallic Fe) (707.8 eV).[3b] The main peak
at 710.7 eV strongly supports the EDS elemental analysis that
Fe is present as N-coordinated Fe in the Fe3C/b-NCNT
sample.[3b,13] It has to be noted that the peak at 710.7 eV
assigned to Fe-Nx in Fe3C/b-NCNT shows a 0.7 eV higher
binding energy as compared to the standard values (~710
eV).[3b,13] This shift suggests a lower electron density on the Fe
atoms, which could originate from the interaction of Fe-Nx and
Fe3C nanoparticles.[3b] Recent reports have demonstrated that a
high proportion of pyridinic N can increase the spin density and
the density of π states of the carbon atoms near the Fermi level,
thus enhancing O2 reduction.[14] Therefore, a high amount of
pyridinic N together with highly dispersed Fe-Nx active sites in
Fe3C/b-NCNT is expected to enhance the intrinsic ORR activity.
The structure of Fe3C/b-NCNT can therefore be described as
crystalline Fe3C nanoparticles being entirely encapsulated in
highly graphitic N- and Fe-Nx-doped porous bamboo-like carbon
nanotubes. Previous studies showed that despite the encapsu-
lated Fe3C nanoparticles are shielded from direct contact with
electrolytes, they are able to activate surrounding graphitic
carbon toward the ORR,[5c,9] most likely through electronic
interaction. Therefore, the unique confinement of Fe3C
nanoparticles within graphitic porous b-NCNTs is believed to
efficiently improve the ORR electrocatalysis. Additionally,
synergistic interaction between the two species Fe-Nx and Fe3C
can further enhance the ORR activity. To the best of our
knowledge, this is a rare report of homogeneous bamboo-like
CNTs based on a MOF as precursor.
carbon structure is formed. TEM images confirm that the
assembled carbon structure was not hollow in nature, and
carbon particles were mostly connected in a one-dimensional
o
chain (Figure 3a). Increasing the temperature to 800 C led to
homogeneous Fe3C/b-NCNT (Figure 1b), whereas a higher
carbonization temperature (900 oC) only produced collapsed
Fe3C/b-NCNT nanostructures (Figure 3b). The sample prepared
without using ZIF-8 did not produce any CNTs (Figure S5). The
o
mechanism of formation of porous hollow b-NCNTs at 800 C
was further investigated by adjusting the iron content, because
iron is known to catalyze CNT formation.[15]. As shown in Figure
o
3c, hollow carbon capsules were generated at 800 C when a
lower amount of Fe3+ was used (2 mL of 0.1 M FeCl3.6H2O).
These findings suggest that the composite of polypyrrole with
ZIF-8 synergistically produced the hollow structure at 800 oC,
requiring a sufficient amount of iron to catalyze homogeneous
growth of the b-NCNTs. Such coupling/conversion of hollow
structures into b-NCNTs has been observed recently.5b
Therefore, we conclude that pyrolysis protocol, polypyrrole/ZIF-8
composite, effective release (vaporization) of the zinc by-product
and the adjusted iron content are all together responsible for the
formation of the novel Fe3C/b-NCNT catalyst.
b)
a)
700 o
C
900 o
C
a)
b)
c)
c)
d)
200 nm
d)
lower Fe3+ content
800 o
C
MOF/pyrrole/Fe3+
~600 o
Hollow capsules
C
Figure 4. a) Voltammograms of Fe3C/b-NCNT and 20 wtꢀ% Pt/C. b) Rotating
ring-disk voltammogram showing the hydrogen peroxide ring current and the
electron transfer number (n) of a Fe3C/b-NCNT modified electrode. c) Tafel
plots of Fe3C/b-NCNT and 20 wtꢀ% Pt/C modified electrodes. d) Chronoam-
perometric ORR stability measurement of Fe3C/b-NCNT at 0.78 V vs. RHE. All
measurements were obtained in O2-saturated 0.1 M KOH solution at a rotation
speed of 1600 rpm, except the chronoamperometric stability measurement
which was performed at 900 rpm. The catalyst loading was 0.42 mgꢀcm−2 for
Fe3C/b-NCNT and 0.21 mgꢀcm−2 (42 μg(Pt) cm-2) for Pt/C. The scan rate was 5
800 o
C
700 o
C
Assembled structure
Bamboo-like CNT
Carbon
Figure 3. TEM images of samples synthesized a) at 800 oC, b) at 900 oC, and
c) at 800 oC using a lower Fe3+ content. d) Schematic representation of
Fe3C/b-NCNT formation.
mVꢀs−1
.
A possible mechanism of the formation of Fe3C/b-NCNT is
suggested in Figure 3. The final carbonization temperature to
The ORR activity of Fe3C/b-NCNT was assessed in both
alkaline and acidic electrolytes using rotating disk electrode
(RDE) and rotating ring-disk electrode (RRDE) techniques. To
o
form hollow b-NCNTs is critical. At 600 C, the ZIF-8 structure
collapses to generate carbon only. At 700 oC, a one-dimensional
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perform these tests, 0.42 mg cm-2 of Fe3C/b-NCNT was loaded
on the surface of a glassy carbon electrode. As shown in Figure
4a, Fe3C/b-NCNT displayed impressive ORR activity in 0.1 M
KOH with 0.96 V onset potential, and 0.83 V at a current density
of -3 mA cm-2. This performance is very close to that of commer-
cially available state-of-art 20% Pt/C catalyst (Pt-ETEK) with a
potential of 0.86 V at a current density of -3 mA cm-2. These data
are comparable to the previous activity benchmarks of Fe3C-
based ORR electrocatalysts.[3,5] Moreover, extremely low H2O2
oxidation currents were detected at the ring in the RRDE
measurements corresponding to less than 1ꢀ% H2O2 yield at all
potentials (Figure 4b), which is much lower as compared to pre-
viously reported benchmark Fe3C-based NPMC.[5b] The number
of electrons transferred per O2 molecule (n) during the ORR was
calculated to be close to 4, suggesting a predominantly four
electron oxygen reduction pathway to produce water as the main
product (Figure 4b). Tafel plots of the kinetic currents corrected
for the background and mass transport effects yielded a Tafel
slope of 71.2 mV/dec for F3C/b-NCNT at low overpotentials,
close to 62.9 mV/dec for 20% Pt/C catalyst (Figure 4c). These
Tafel slopes do not vary significantly from low to high overpoten-
tials suggesting that the mechanism of oxygen reduction at
Fe3C/b-NCNT in 0.1 M KOH follows a similar mechanism as at
Pt/C. Therefore, corresponding to the known ORR processes on
Pt/C the transfer of the first electron is most probably the rate-
determining step of the ORR on Fe3C/b-NCNT.[5c] Chronoam-
perometric stability evaluation (at 0.78 V vs RHE) showed no
significant loss in ORR activity after ~70 h of continuous mea-
surement indicating that Fe3C/b-NCNT is able to sustain a stable
ORR performance over a long period of time (Figure 4d).
1600 rpm except chronoamperometric stability measurement which was per-
formed at 900 rpm. The catalyst loading was 0.42 mgꢀcm−2 for Fe3C/b-NCNT
and 0.21 mgꢀcm−2 (42 μg(Pt) cm-2) for Pt/C. The scan rate was 5 mVꢀs−1
.
Furthermore, we evaluated the ORR performance of Fe3C/b-
NCNT in 0.5 M H2SO4 (Figure 5). The ORR polarization curve of
Fe3C/b-NCNT exhibited a potential of 0.63 V at a current density
of -3 mA cm-2. In contrast, the potential at -3 mA cm-2 obtained
with 20% Pt/C was 0.75 V (Figure 5a). The H2O2 yield during
ORR on Fe3C/b-NCNT in 0.5 M H2SO4 was below 1% over the
entire investigated potential range (Figure 5b), which is lower
than in 0.1 M KOH. The very low H2O2 yield indicates that this
catalyst reduces O2 nearly exclusively to H2O via the 4-electron
transfer process. Kinetic currents derived from the curves in
Figure 5a generated Tafel plots of Fe3C/b-NCNT and 20% Pt/C
are shown in Figure 5c. Although the hydrogen peroxide genera-
tion curves are similar in both 0.1 M KOH and 0.5 M H2SO4, the
Tafel slope of the ORR on Fe3C/b-NCNT in 0.5 M H2SO4 varied
significantly from 27 mV/dec in the lower overpotential region to
83 mV/dec at higher overpotentials. This indicates that the
mechanism of the ORR on Fe3C/b-NCNT in acidic electrolyte
varied significantly with potential in this narrow potential range.
Moreover, the mechanism of the ORR on Fe3C/b-NCNT is
different in both electrolytes. Chronoamperometric stability
studies performed at a potential of 0.61 V vs RHE, showed
minimal loss in the ORR current after ~70 h of continuous
polarization (Figure 5d). The novel Fe3C/b-NCNT catalyst thus
exhibits promising durability even in the more challenging acidic
electrolytes.
Based on the structural and compositional characterization
of Fe3C/b-NCNT, we conclude that five important aspects are
responsible for its excellent ORR activity and stability: 1) High
a)
b)
content of
N
(8.6 wt%) enhances hydrophilicity, thus
strengthening the electrode-electrolyte interaction. 2) High
percentage of pyridinic N (59%) promotes O2 adsorption owing
to the reduced local work function of carbon.[9c] 3) Unique
bamboo-like CNTs offer a higher number of edges, thus
introducing a higher number of ORR active sites. 4) High content
of N-doping in b-NCNTs as a prerequisite for the formation of
active Fe-Nx sites. 5) Encapsulated metal NPs can lead to
unique host-guest electronic interactions which make the outer
surface of the carbon layer more active in the ORR.[9]
In summary, we developed a novel NPMC-based ORR elec-
trocatalyst consisting of Fe3C nanoparticles encapsulated in po-
rous bamboo-like N-doped CNTs, synthesized by a new MOF-
templated assembly approach. The excellent electrocatalytic
activity, negligible hydrogen peroxide formation, and long-term
stability in both acidic and basic media render Fe3C/b-NCNT a
promising low-cost, highly efficient ORR catalyst. This work
provides a new protocol to construct pure-phase Fe3C hybrids
from MOF, and opens new avenues for further development of
active and durable ORR catalysts.
c)
d)
Figure 5. a) Voltammograms of Fe3C/b-NCNT and 20 wtꢀ% Pt/C. b) Hydrogen
peroxide ring current and electron transfer number (n) of Fe3C/b-NCNT. c)
Tafel plots of Fe3C/b-NCNT and 20 wtꢀ% Pt/C. d) Chronoamperometric ORR
stability measurement of Fe3C/b-NCNT at 0.61 V vs. RHE. All measurements
were obtained in O2-saturated 0.5 M H2SO4 solution at a rotation speed of
Acknowledgements
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Financial support from the DFG (Deutsche Forschungsgemein-
schaft) in the framework of the Cluster of Excellence RESOLV
(EXC1069) is gratefully acknowledged. A.A. acknowledges the
Alexander von Humboldt Stiftung for a postdoctoral fellowship.
The authors thank the ICAN team at the Universität Duisburg-
Essen for the electron microscopy support.
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10.1002/chem.201701389
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Fe3C nanoparticles encapsulated
homogeneously in N-doped bamboo-
like carbon nanotubes are fabricated
from a pyrrole-loaded MOF using a
well-controlled pyrolytic approach. The
obtained catalyst shows high activity
and stability towards the oxygen
reduction reaction in both acidic and
basic electrolytes.
Arshad Aijaz, Justus Masa, Christoph
Rösler, Hendrik Antoni, Roland A.
Fischer, Wolfgang Schuhmann,* and
Martin Muhler*
Page No. – Page No.
MOF-Templated Assembly Approach
for Fe3C Nanoparticles Encapsulated
in Bamboo-Like N-doped CNTs:
Highly Efficient Oxygen Reduction
under both Acidic and Basic
Conditions
This article is protected by copyright. All rights reserved.