Ionic liquids as precursors for efficient mesoporous iron-nitrogen-doped oxygen reduction electrocatalysts

Article abstract of DOI:10.1002/anie.201409579

A ferrocene-based ionic liquid (Fe-IL) is used as a metal-containing feedstock with a nitrogen-enriched ionic liquid (N-IL) as a compatible nitrogen content modulator to prepare a novel type of non-precious-metal-nitrogen-carbon (M-N-C) catalysts, which feature ordered mesoporous structure consisting of uniform iron oxide nanoparticles embedded into N-enriched carbons. The catalyst Fesup10/sup@NOMC exhibits comparable catalytic activity but superior long-term stability to 20 wt% Pt/C for ORR with four-electron transfer pathway under alkaline conditions. Such outstanding catalytic performance is ascribed to the populated Fe (Feinf3/infOinf4/inf) and N (N2) active sites with synergetic chemical coupling as well as the ordered mesoporous structure and high surface area endowed by both the versatile precursors and the synthetic strategy, which also open new avenues for the development of M-N-C catalytic materials.

Full text of DOI:10.1002/anie.201409579

Angewandte
Chemie
DOI: 10.1002/anie.201409579
Heterogeneous Catalysis
Ionic Liquids as Precursors for Efficient Mesoporous Iron-Nitrogen-
Doped Oxygen Reduction Electrocatalyst**
Zelong Li, Guanglan Li, Luhua Jiang, Jinlei Li, Gongquan Sun,* Chungu Xia,* and Fuwei Li*
Abstract: A ferrocene-based ionic liquid (Fe-IL) is used as
a metal-containing feedstock with a nitrogen-enriched ionic
liquid (N-IL) as a compatible nitrogen content modulator to
prepare a novel type of non-precious-metal–nitrogen–carbon
(M-N-C) catalysts, which feature ordered mesoporous struc-
ture consisting of uniform iron oxide nanoparticles embedded
into N-enriched carbons. The catalyst Fe10@NOMC exhibits
comparable catalytic activity but superior long-term stability to
20 wt% Pt/C for ORR with four-electron transfer pathway
under alkaline conditions. Such outstanding catalytic perfor-
mance is ascribed to the populated Fe (Fe3O4) and N (N2)
active sites with synergetic chemical coupling as well as the
ordered mesoporous structure and high surface area endowed
by both the versatile precursors and the synthetic strategy,
which also open new avenues for the development of M-N-C
catalytic materials.
ity and durability at low cost with the state-of-the-art Pt/C
remains a challenge. In general, the M-N-C catalysts were
mainly prepared through direct pyrolyzation of the precursors
containing Fe or Co salts, nitrogen, and macrocyclic com-
pounds adsorbed on carbon black.[6] However, it was difficult
to efficiently construct strong interaction between the M and
N species and control porous structure of the materials.
Although the catalytically active sites in M-N-C catalyst
remain controversial,[7] it has been confirmed that the N-
doping of carbon could assist in a stronger coupling between
metal oxide and N-doped carbon, and thus lead to higher
ORR activity in alkaline solution.[8] Simultaneously, the
design of precursor structure at molecular level can effec-
tively tune the interaction among their components and
catalytic properties of materials, enabling enhancement both
in activity and durability.[9] Therefore, it is convinced that the
development of precursors at molecular level containing M,
N, and C resources with high thermal stability is crucial.[10]
The porosity and surface area of ORR catalyst are critical for
mass transport and exposure of active sites.[11] From this point
of view, the hard template method using ordered mesoporous
silica (SBA-15) as template can be especially feasible and
advantageous for preparing an ordered mesoporous M-N-C
catalyst with high specific surface area;[12] furthermore, the
formation of a metal-embedded nanophase through hard
template synthesis can enhance the stability of metal that
doped in carbon.[13]
I
n fuel cells, the oxygen reduction reaction (ORR) is
invariably involved at the cathode and currently dominated
by Pt-based electrocatalysts, which is also thought as the
major obstacle for a widespread deployment of these
technologies because of the high cost and limited reserves
of Pt.[1] As a consequence, the development of Pt-based
alloy[2] and cheaper metal-free[3] and non-precious metal
catalysts[4] (such as M-N-C, where M is mainly Fe or Co) with
high ORR activity and stability has become a major focus of
the fuel-cell research.
Despite some breakthroughs having been achieved by
selecting suitable metal, nitrogen, and carbon precursors
combined with the optimized synthetic strategies,[5] facile
preparation of M-N-C catalysts possessing comparable activ-
Recently, ionic liquids (ILs) with negligible volatility and
high thermal stability have been approved to be a new type of
precursor for the preparation of N-doping carbon materials
(Supporting Information, Table S1).[14] Nevertheless, metal-
based ionic liquids used as versatile precursors to prepare
porous metal-doped carbon materials are rarely reported.
Herein, we report our discovery on using the “task-specific”
Fe-IL ([FcN][NTf2]) as metal-containing precursor and N-IL
([MCNIm][N(CN)2]) as a compatible nitrogen content mod-
ulator to synthesize the Fe-N-doped ordered mesoporous
carbon catalysts (FeX@NOMC in Scheme 1, where X repre-
sents the mass percentage of Fe), among which the
Fe10@NOMC exhibits outstanding ORR performance under
alkaline conditions.
[*] Dr. Z. L. Li,[+] J. L. Li, Prof. C. G. Xia, Prof. F. W. Li
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, Lanzhou 730000 (China)
E-mail: cgxia@licp.cas.cn
fuweili@licp.cas.cn
Dr. G. L. Li,[+] Prof. L. H. Jiang, Prof. G. Q. Sun
Division of Fuel Cell & Battery
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics
Chinese Academy of Sciences, Dalian 116023 (China)
E-mail: gqsun@dicp.ac.cn
As shown in Scheme 1, FeX@NOMC (X = 25, 10, 5) were
prepared from Fe-IL or its homogeneous mixture with N-IL
by hard template method (see the Supporting Information).
Fe25@NOMC was initially synthesized using pure [FcN][NTf2]
as the Fe, N, and C feedstock with 1.4 wt% of N doping
revealed by elemental analysis. In contrast to the preparation
of Fe25@NOMC, Fe5@NOMC was obtained by treating the
carbon@silica composite with 10 wt% HF aqueous solution,
affording 1.8 wt% of N incorporation. Fe10@NOMC with N
[+] These authors contributed equally to this work.
[**] This work was supported by the Chinese Academy of Sciences, the
National Natural Science Foundation of China (21133011 and
21373246), and the “Strategic Priority Research Program” of the
Chinese Academy of Sciences (XDA09030104).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409579.
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Scheme 1. Illustration of the synthesis of FeX@NOMC.
contents as high as 11.9 wt% was fabricated by using the
mixture of [FcN][NTf2] and [MCNIm][N(CN)2] with 1:1 mass
ratio.
Figure 2. TEM (a, d, i), HRTEM (b, e, f), HAADF-STEM (c, g) and
elemental mapping (h) images of Fe25@NOMC (a–c), Fe10@NOMC
(d–h), and Fe5@NOMC (i).
The FeX@NOMC composites comprise of graphitic
carbon, as evidenced by the peak of wide-angle XRD
(WAXRD) patterns at around 258, which corresponds to
the diffraction from the (002) index of graphitic carbon
(Figure 1a). Only Fe25@NOMC displays four broad and weak
other hand, such FeONPs aggregation is also verified by the
appearance of honeycomb pore on the surface of the
Fe5@NOMC after acid leaching (Figure 2i). The small-
angle X-ray diffraction (SAXRD) pattern of the
Fe25@NOMC sample shows a weak diffraction peak at
about 18 (Figure 1b), which can be assigned to (100)
diffraction of the 2D hexagonal space group (p6mm),[15]
suggesting its ordered array of the interconnected Fe-doped
NOMC. With the leaching of FeONPs in Fe5@NOMC, the
diffraction peak at 18 disappears, indicating the ordered
structure is destroyed.
With the increasing of N content for Fe10@NOMC, the
absence of iron phases in the WAXRD patterns together
with the evidence of its TEM image might indicate its
FeONPs are homogeneously dispersed in the carbonous
structure (Figure 2d), and the intensity of the (100)
diffraction peak increases (Figure 1b). Both XRD and
TEM analyses indicate that incorporating more N into
carbon can enhance the mesostructure regularity of
Fe10@NOMC and promote the FeONP dispersion. Such
deduction can also find some clues from their results of N2
sorption isotherms and the pore size distribution.
FeX@NOMC possess a mesoporous structure (Figure 1c),
high surface areas (411–506 m2 gÀ1), and large pore volumes
(0.71–1.05 m3 gÀ1; Supporting Information, Table S2). The
pore size distribution of Fe10@NOMC is highly uniform
centered at 5.6 nm (Figure 1d), which is narrower than that of
Fe25@NOMC (centered at 6.8 nm). As expected, Fe5@NOMC
presents a broader pore size distribution because of the
partially leaching of different sized FeONPs.
Figure 1. a) WAXRD, b) SAXRD, c) N2 sorption isotherms, and d) the
corresponding pore size distribution curves of Fe25@NOMC,
Fe10@NOMC, and Fe5@NOMC.
peaks at around 30.2, 35.6, 43.2, and 62.98, indicating the
formation of g-Fe2O3 (JCPDS no. 39-1346) nanoparticles
(Fe2O3NPs). Although the iron oxide nanoparticles (FeONPs,
the coexistence of g-Fe2O3 and Fe3O4 is verified by the later
XPS analysis) in Fe25@NOMC are mostly well-dispersed on
the carbon walls revealed by transmission electron microsco-
py (TEM) and high-angle annular dark-field scanning trans-
mission electron microscopy (HAADF-STEM) (Figure 2a,c),
the aggregated FeONPs are also detected (Figure 2b). On the
HRTEM images show that the most FeONPs of
Fe10@NOMC form the embedded nanophase, which has
unique semiexposure morphology, with one part of the NPs
being partially exposed to pore channels and the other part
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being tightly trapped in the carbons (Figure 2e,f), and the
ultrafine FeONPs with uniform particle size are verified by
the homogeneous distribution of white dots along the
mesoporous carbon walls (Figure 2g). Elemental mapping
analysis reveals that the C, Fe, N, and O in Fe10@NOMC are
all uniformly distributed (Figure 2h). It is interesting to note
from HAADF-STEM/EDS line scan of Fe10@NOMC that the
distribution profile of Fe is highly accordant with that of N
(Supporting Information, Figure S3), which strongly suggests
certain interaction between Fe and N.
As reported previously, the composition of M, N, and their
interaction are the key factors determining the catalytic
performance of M-N-C catalysts.[16] The Fe2p XPS spectra for
FeX@NOMC are deconvoluted into a higher binding energy
(BE) peak II attributed to g-Fe2O3 and a lower BE peak I
assigned to the Fe cations of Fe3O4 (Fe3O4NPs) that coordi-
nated with nitrogen (Fe-N; Supporting Information, Fig-
ure S4 and Table S3),[17] and the Fe-N sites possibly constitute
the catalytic centers.[18] Notably, the Fe (Fe3O4) content in the
surface of Fe10@NOMC is much higher than that of
Fe5@NOMC and Fe25@NOMC (Supporting Information,
Table S3), and the N content in Fe10@NOMC is also
significantly higher than the others (Supporting Information,
Table S4), suggesting the incorporation of more N is benefi-
cial to the formation of Fe3O4 with the help of certain Fe
nucleation around N and strong Fe-N interaction, thus
provides uniform and highly populated active sites in
Fe10@NOMC. Moreover, the core-level peaks of Fe2p for
Fe10@NOMC shift to higher BEs compared with the other two
samples, indicating somehow different iron electronic states.
The N1s spectra are deconvoluted to three peaks locating at
398.3 eV (N1), 399.5 eV (N2), and 400.5 eV (N3) (Supporting
Information, Figure S5 and Table S4). N1 and N3 are assigned
to pyridinic and pyrrolic nitrogen, and N2 is regarded as the
nitrogen associated with Fe3O4.[19] Interestingly, the N2
content for Fe5@NOMC also decreases with the reduction
of iron loading by HF etching, experimentally confirming N2
being bonded to Fe.
Figure 3. a) RDE voltammetric response for the ORR in O2-saturated
0.1m NaOH at a scan rate of 10 mVsÀ1. b) RDE voltammograms
recorded for Fe10@NOMC electrode in an O2-saturated 0.1m NaOH
solution with different rotation rates at a scan rate of 10 mVsÀ1. c) CV
curves of Fe10@NOMC and Pt/C catalysts at a scan rate of 50 mVsÀ1
in O2-or N2-saturated 0.1m NaOH solutions as well as O2-saturated
0.1m NaOH solution with 1m CH3OH. d) Current density degradation
at 0 V (vs. Hg/HgO) in the potential cycling process from À0.25 to
0.10 V at a scan rate of 100 mVsÀ1
.
much less active toward the ORR (Figure 3a). Further XPS
analysis of Fe@FDU3-N reveals that its nitrogen are mainly in
graphitic state (Supporting Information, Figure S9a and
Table S4),[21] no N2 species and the crucial Fe-N2 interaction
as existence in FeX@NOMC are observed. This result certifies
that the post-loading of [FcN][NTf2] into the N-enriched
FDU3-N cannot generate Fe-N2 interaction, and the forma-
tion of Fe-N2 active sites is not only related to the N content
but also dependent on the synthetic procedure. Furthermore,
Fe@CN with finely dispersive Fe2C5 and Fe3O4 NPs (Support-
ing Information, Figures S6, S7b, S10)[22] and high surface area
(280 m2 gÀ1; Supporting Information, Table S2) was also
prepared by direct pyrolysis of [FcN][NTf2] under N2 (see
Supporting Information); however, it displayed more nega-
tive onset potential and half-wave potential (Figure 3a)
compared with the less-active Fe25@NOMC, which is prob-
ably due to the negligible interaction between Fe and N (Fe-
N2) as observed (Supporting Information, Figure S9b,
Table S4). In comparison with the Fe25@NOMC, its analogues
Fe5@NOMC also demonstrated lower ORR reactivity owing
to the etching of the Fe species, which leads to the decrease of
Fe-N2 active sites. Based on these results, it is concluded that
the present synthetic strategy involving the unique Fe-N-C IL
precursors and the one-pot synthesis through hard-template
can not only generate high surface area and mesoporous
structure, but also facilitate the formation of Fe-N2 species,
which are all crucial for an efficient ORR catalyst.
The electrocatalytic activities of FeX@NOMC toward the
ORR were first investigated by linear sweep voltammetry
(LSV) using a rotating disk electrode in 0.1m NaOH solution
saturated with O2. For comparison, commercial 20 wt% Pt/C
catalyst was also examined under the same conditions.[20] As
shown in Figure 3a, both the ORR onset potential and the
half-wave potential (0 V vs. MMO) for the Fe10@NOMC are
closer to that for the Pt/C and comparable to the Fe/Fe3C-
melamine/N-KB composite catalyst (Ar-800) under similar
test conditions,[5c] but significantly more positive than that for
Fe5@NOMC and Fe25@NOMC. The facile ORR process over
the Fe10@NOMC could be due to the increased content of Fe-
N2 with the enhancement of N content.
To further clarify the active sites of such Fe-N-C catalysts,
a controllable material Fe@FDU3-N (FDU3-N denotes the N-
doped ordered mesoporous carbon) with dispersive g-Fe2O3
and Fe3O4 NPs[17a,b] (Supporting Information, Figures S6, S7a,
S8) and high surface area (522 m2 gÀ1; Supporting Informa-
tion, Table S2) was prepared by loading [FcN][NTf2] into the
pore of FDU3-N and then direct carbonization under N2 (see
the Supporting Information). Unfortunately, Fe@FDU3-N is
To gain more information on the FeX@NOMC-catalyzed
ORR, the RDE experiments at different rotating speeds
(100–2500 rpm) were performed, and their exact kinetic
parameters including the electron-transfer number (n) and
the kinetic limiting current (Jk) were calculated on the basis of
Koutecky–Levich equation and RDE measurements (Fig-
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ure 3b; Supporting Information, Figure S12). Remarkably,
the ORR over the Fe10@NOMC proceeds via a four-electron
transfer pathway (n = 4), which is same with that over the Pt/
C. While for the Fe25@NOMC (n = 3.7) and Fe5@NOMC (n =
3.9), the electron transfer number is a little lower. The Jk value
at À0.5 V vs. MMO for Fe10@NOMC (Jk = 5.30 mAcmÀ2) is
close to that for the Pt/C catalyst (Jk = 5.38 mAcmÀ2) and
higher than the values for Fe25@NOMC (Jk = 4.50 mAcmÀ2)
and Fe5@NOMC (Jk = 4.27 mAcmÀ2). In contrast, the elec-
tron-transfer number of the Fe@FDU3-N and Fe@CN are 2.5
and 2.3, respectively, suggesting a two electron-transfer
involving peroxide as the intermediate is dominant in these
two ORRs.
Methanol tolerance is a crucial factor for a good ORR
catalyst in direct methanol fuel cells. As illustrated in
Figure 3c, the Fe10@NOMC catalyst showed perfect ORR
selectivity in methanol containing electrolyte, whereas a sig-
nificantly high methanol oxidation current is detected for the
Pt/C catalyst under the same conditions. Furthermore, the
stabilities of the Fe10@NOMC and Pt/C were further eval-
uated by an accelerated aging test procedure as described in
the Supporting Information.[23] After 20000 cycles scanning
from À0.25 to 0.10 V, the current density at 0 V vs. MMO only
decreased by 16.5% for Fe10@NOMC, which is much lower
than that of the Pt/C (74.8%) (Figure 3d). Such outstanding
stability of the as-prepared Fe10@NOMC catalyst might be
ascribed to not only the strong interaction between FeONPs
and N2 species but also the formation of FeONPs-embedded
nanophase that could retard their leaching into the alkaline
media during long-term operation.
In summary, a new family of mesoporous Fe-N-C catalyst
has been fabricated from versatile and structurally defined
Fe-IL/N-IL precursors, the introduction of another IL with
high N content as co-precursor can facilely tune the N content
of the Fe-N-C catalysts. It was found that doping more N was
beneficial to the formation of active Fe (Fe3O4) and N (N2)
species as well as enhancement of their interaction. Com-
pared with the post-loading and direct pyrolysis procedures,
the synthetic strategy using unique ionic liquid precursors and
the one-pot synthesis through hard-template showed signifi-
cant advantages in the construction of mesoporous morphol-
ogy with high surface area and the formation of Fe-N2 species,
which played important roles in the development of an
efficient ORR catalyst. With these crucial characters,
Fe10@NOMC displayed comparable reaction current density
and onset potential with the commercial Pt/C, but much
better fuel crossover resistance and long-term durability in
alkaline condition. The facile N, M tuning of the present
method and outstanding ORR performance of the resultant
Fe10@NOMC indicate its wide application in the development
of M-N-C materials for heterogeneous catalysis, batteries, and
supercapacitors.
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Keywords: heterogeneous catalysis · ionic liquids · iron ·
ordered mesoporous carbon · oxygen reduction reaction
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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.
Angewandte
Communications
Communications
Heterogeneous Catalysis
N-enriched and Fe-embedded ordered
mesoporous carbon electrocatalysts were
fabricated from an ionic liquid precursor
by hard template synthesis. This strategy
provides a high surface area and popu-
lated Fe-N active sites with synergetic
interaction for an optimized
Z. L. Li, G. L. Li, L. H. Jiang, J. L. Li,
G. Q. Sun,* C. G. Xia,*
F. W. Li*
&&&& — &&&&
Ionic Liquids as Precursors for Efficient
Mesoporous Iron-Nitrogen-Doped
Oxygen Reduction Electrocatalyst
Fe10@NOMC catalyst. The catalyst shows
excellent electrocatalytic efficiency and
durability for the oxygen reduction in
alkaline media.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!