Puffing Up Energetic Metal–Organic Frameworks to Large Carbon Networks with Hierarchical Porosity and Atomically Dispersed Metal Sites

Article abstract of DOI:10.1002/anie.201811126

Large carbon networks featuring hierarchical pores and atomically dispersed metal sites (ADMSs) are ideal materials for energy storage and conversion due to the spatially continuous conductive networks and highly active ADMSs. However, it is a challenge to synthesize such ADMS-decorated carbon networks. Here, an innovative fusion-foaming methodology is presented in which energetic metal–organic framework (EMOF) nanoparticles are puffed up to submillimeter-scaled ADMS-decorated carbon networks via a one-step pyrolysis. Their extraordinary catalytic performance towards oxygen reduction reaction verifies the practicability of this synthetic approach. Moreover, this approach can be readily applicable to a wide range of unexplored EMOFs, expanding scopes for future materials design.

Full text of DOI:10.1002/anie.201811126

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Puffing up Energetic Metal-organic Frameworks to Large Carbon
Networks with Hierarchical Porosity and Atomically Dispersed
Metal Sites
Authors: Ruo Zhao, Zibin Liang, Song Gao, Ce Yang, Bingjun Zhu,
Junliang Zhao, Chong Qu, Ruqiang Zou, and Qiang Xu
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811126
Angew. Chem. 10.1002/ange.201811126
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10.1002/anie.201811126
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Puffing up Energetic Metal-organic Frameworks to Large Carbon
Networks with Hierarchical Porosity and Atomically Dispersed
Metal Sites
Ruo Zhao+, Zibin Liang+, Song Gao, Ce Yang, Bingjun Zhu, Junliang Zhao, Chong Qu, Ruqiang Zou,*
and Qiang Xu*
Abstract: Large carbon networks featuring hierarchical pores and
atomically dispersed metal sites (ADMSs) are ideal materials for
energy storage and conversion due to the spatial-continuous
conductive networks and highly active ADMSs. However, it is a
challenge to synthesize such ADMS-decorated carbon networks.
Here, we innovatively present a fusion-foaming methodology to puff
up energetic metal-organic framework (EMOF) nanoparticles to
submillimetric ADMS-decorated carbon networks via one-step
pyrolysis. Their extraordinary catalytic performance towards oxygen
precursors for the fabrication of various nanostructured
materials.[6] Generally, carbonization of MOFs will produce
shrinked particles, sintered aggregations, or collapsed
structures.[6] Thus far, only a few ADMS-decorated materials
derived from MOFs have been reported, which are generally
limited to small particles due to the shrinkage upon pyrolysis.[7]
Breakthroughs, which are based on new type of MOFs and
synthetic concept, are urgently expected in order to fabricate
ADMS-decorated hierarchically porous carbon networks in a
reduction reaction verifies the practicability of this synthetic approach. large size.
Moreover, this approach can be readily applicable to a wide range of
unexplored EMOFs, expanding scopes for future materials design.
Herein, for the first time, we fabricate submillimeteric carbon
networks with hierarchical porosity and ADMSs from energetic
MOFs (EMOFs). Specifically, Zn-based triazole-rich EMOFs
doped with Co and/or Fe ions were used as precursors. Upon
one-step pyrolysis, Zn nodes are reduced and evaporated, while
the energetic triazole ligands decompose and release a large
amount of gases, foaming the hierarchicaly porous carbon
networks. Meanwhile, the homogeneously dispersed target
metal ions (Fe and/or Co ions) are transformed into ADMSs
(Scheme 1). The as-obtained CoFe@C is composed of
hierarchically porous carbon networks with high conductivity and
active Fe and Co ADMSs, which exhibits superior
electrocatalytic performance towards oxygen reduction reaction
(ORR).
Three-dimensional (3D) carbon networks with hierarchical
porosity are attracting considerable attention in the field of
energy storage and conversion.[1] Compared with typical
carbonaceous nanoparticles, which suffer from insufficient
contact and poor conductivity, 3D carbon networks with
hierarchical porosity can provide consecutive electrical
conductivity and porous channels for fast electron transfer and
effective mass transport.[2] To acquire desirable electrochemical
features, these carbon networks have been extensively explored
as supports to load various active components, especially metal
sites.[3] Recently, single-atom catalysts are attracting increasing
interests owing to their high catalytic activity from the atomically
dispersed metal sites (ADMSs).[4] Integration of ADMSs into 3D
carbon networks is a new frontier in material research; however,
it remains to be a challenge. Developing a simple and scalable
synthetic strategy is particularly desirable.
Metal-organic frameworks (MOFs), an emerging class of
porous crystalline materials,[5] have flourished as attractive
[*]
R. Zhao,[+] Z. Liang,[+] Dr. S. Gao, Dr. B. Zhu, J. Zhao, Dr. C. Qu,
Prof. R. Zou
Department of Materials Science and Engineering, College of
Engineering, Peking University
Beijing 100871 (China)
E-mail: rzou@pku.edu.cn
Prof. Q. Xu
AIST-Kyoto University Chemical Energy Materials Open Innovation
Laboratory (ChEM-OIL), National Institute of Advanced Industrial
Science and Technology (AIST)
Sakyo-ku, Kyoto 606-8501 (Japan)
E-mail: q.xu@aist.go.jp
Prof. Q. Xu
School of Chemistry and Chemical Engineering, Yangzhou
University
Scheme 1. Synthesis of submilimetric CoFe@C via a fusion-foaming thermal
transformation of energetic CoFe@MET-6 nanoparticles.
Yangzhou, Jiangsu 225009 (China)
Dr. C. Yang
Chemical Science and Engineering Division
Argonne National Laboratory
Lemont, IL 60439 (USA)
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.
The reaction of zinc chloride and 1H-1,2,3-triazole produced
an octahedron-shaped triazole-based EMOF ([Zn(C2N3H2)2],
namely, MET-6),[8] in which each zinc ion was coordinated by six
nitrogen atoms (Supporting Information, Figure S1). Powder X-
[+]
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ray diffraction (PXRD) profile of the as-synthesized MET-6
matched well with the simulated diffraction patterns, indicating
the synthesis of phase-pure MET-6 with high crystallinity (Figure
1a). The N2 sorption isotherms revealed that MET-6 was
microporous with a Brunauer-Emmett-Teller surface area of 417
m2 g-1 (Supporting Information, Figure S1). The subsequent
incorporation of target metal ions (Fe and/or Co ions) via a wet-
chemistry method[9] produced transition metal-doped MET-6
(TM@MET-6, TM = Co, Fe, or CoFe). PXRD patterns (Figure
1a) and scanning electron microscopic (SEM) images
(Supporting Information, Figure S2) of TM@MET-6 verified their
structural and morphological maintenance after the impregnation
process. Thermogravimetric analyses (TGA) exhibited sharp
o
o
declines at ~400 C and continuous weight loss till 1,000 C for
all the samples (Figure 1b; Supporting Information, Table S1).
The significant loss in weight suggested that MET-6 and
TM@MET-6 were novel energetic compounds, whose
decomposition could generate a huge amount of gases.
Upon one-step carbonization, MET-6 and TM@MET-6 were
converted into un-decorated carbon network (u-carbon) and
transition
metal-decorated
carbon
networks
(TM@C),
respectively. In Figure 1c, PXRD profiles of u-carbon and
TM@C showed broad peaks at ~25 and ~44o, corresponding to
the (002) and (101) planes of graphitic carbon.[10] X-ray
photoelectron spectroscopic (XPS) analyses showed that the N
1s high-resolution spectra for u-carbon and TM@C could be
deconvoluted into several peaks,[11] including pyridinic-N (~398.3
eV), pyrrolic-N (~399.3 eV), graphitic-N (~400.8 eV) and
oxidized-N (~402.7 eV) (Figure 1d; Supporting Information,
Figure S3). For TM@C, the peak at ~398.3 eV might also
include a contribution from metal-nitrogen (M-N) bonding. The
binding energy of M-N and pyridinic-N is similar, which makes it
difficult to differentiate them quantitatively. The surface nitrogen
content in CoFe@C was 5.5 at.%, while no significant Co or Fe
2p peaks could be detected due to its ultralow content. The total
nitrogen content in CoFe@C determined by elemental analysis
was 7.1 wt.%, while the Co and Fe contents determined by
inductively coupled plasma atomic emission spectroscopic (ICP-
AES) measurements (Supporting Information, Table S2) were
0.5 wt.% and 0.37 wt.%, respectively. The N2 sorption curves of
u-carbon, Co@C, Fe@C, and CoFe@C exhibited typical type-IV
isotherms with specific surface areas of 1132, 805, 1095, and
950 m2 g-1, respectively (Figure 1e; Supporting Information,
Table S2). The large uptake at low relative pressure (P/P0 < 0.1)
and the distinct hysteresis loop between the adsorption and
desorption branches suggested the co-existence of micropores
and mesopores,[3a,6d] in accordance with their pore size
distribution. Raman spectroscopy showed that the intensity
ratios of disorder-induced D band (~1350 cm-1) to tangential
stretch G band (~1590 cm-1) for u-carbon, Co@C, Fe@C, and
CoFe@C were 1.10, 1.25, 1.21, and 1.13, respectively (Figure
1f). The structural defects (e.g., doped nitrogen atoms), referred
by D band, might serve as anchoring sites for ADMSs.
Figure 1. a) PXRD patterns of MET-6, TM@MET-6, and the simulated profile.
b) TGA of MET-6 and TM@MET-6 under an N2 atmosphere. c) PXRD patterns
of u-carbon and TM@C. d) The N 1s high-resolution spectrum of CoFe@C. e)
N2 adsorption (filled circles) and desorption (open circles) isotherms at 77 K for
u-carbon and TM@C. Inset shows the corresponding pore size distributions f)
Raman spectra for u-carbon and TM@C.
The morphologies of all the samples were thoroughly
examined (Figure 2; Supporting Information, Figures S4-S6). As
illustrated in Figure 2a, the interconnected channels and
hierarchical pores were clearly identified in u-carbon. These
features were maintained upon loading of different ADMSs
(Figure 2b-d; Supporting Information, Figures S4-S6). The
fracture surfaces of TM@C under Helium ion microscopy (HIM)
demonstrated the high porosity and bubble-like figures within the
carbonaceous walls (Figure 2e-g). Transmission electron
microscopic (TEM) analyses further revealed the existence of
abundant macro/mesopores (Figure 2h-m). The aforementioned
(N2 sorption, SEM, HIM, and TEM) analyses revealed that the
porous structures of u-carbon and TM@C could be classified
into four levels: (i) the integrated carbon networks (> 100 μm)
with interconnected channels; (ii) the porous carbon skeletons
(1~3 μm) with massive macropores (Figure 2e-g); (iii) the
abundant mesopores (10~40 nm) and (iv) micropores (< 2 nm)
within the entire structure.
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transformed Co K-edge EXAFS spectrum (Figure 3h) validated
the existence of Co ADMSs in Co-N4 structure.[11,13]
Figure 2. a) HIM images of u-carbon. Insets show HIM image with higher
magnification (top) and TEM image (bottom). b-d) SEM images of Co@C (b),
Fe@C (c), and CoFe@C (d). e-g) HIM images of cross section in Co@C (e),
Fe@C (f), and CoFe@C (g). h-m) TEM images of Co@C (h, k), Fe@C (i, l),
and CoFe@C (j, m).
Figure 3. a-d) Aberration-corrected HAADF-STEM images of CoFe@C (a,b),
Co@C (c), and Fe@C (d). e) Fe K-edge XANES spectra of CoFe@C, Fe2O3
and FeO. Inset shows the Fe K-edge pre-edge XANES spectra. f) Magnitude
of Fourier transformed Fe K-edge EXAFS spectrum and the corresponding
fitting curve of CoFe@C. g) Co K-edge XANES spectra of CoFe@C and
cobalt(III) acetylacetonate (Co(acac)3). Inset shows the Co K-edge pre-edge
XANES spectra. h) Magnitude of Fourier transformed Co K-edge EXAFS
spectrum and fitting curve of CoFe@C.
To confirm the existence of ADMSs in TM@C, aberration-
corrected high-angle annular dark-field scanning transmission
electron microscopic (HAADF-STEM) measurements were
performed. For CoFe@C, abundant mesopores were observed
from the high-resolution image (Figure 3a). The isolated bright
dots indicated the existence of Fe and Co ADMSs (Figure 3b).[12]
Energy-dispersive X-ray spectroscopy (EDS) mapping analysis
elucidated the uniform elemental distribution of C, N, Fe, and Co
in CoFe@C (Supporting Information, Figure S7). Similarly, Co
and Fe ADMSs could be identified in Co@C and Fe@C,
respectively (Figure 3c-d; Supporting Information, Figure S8).
X-ray absorption near edge structure (XANES) and extended
X-ray absorption fine structure (EXAFS) were performed to
investigate the oxidation states and local environment of ADMSs
in CoFe@C. The Fe K-edge XANES spectrum in Figure 3e
showed that the Fe oxidation state was between FeII and FeIII. In
Figure 3f, the magnitude of Fourier transformed k2-weighted Fe
K-edge EXAFS spectrum exhibited a main peak at ~1.5 Å, which
was assigned to Fe-N coordination.[7b] The absence of metallic
Fe-Fe coordination certified that Fe sites were atomically
dispersed in CoFe@C. The fitting results of Fourier transformed
Fe K-edge EXAFS spectrum (Supporting Information, Table S3)
suggested that the Fe ADMSs existed in a Fe-N4-C structure.[7]
Llikewise, the Co K-edge XANES (Figure 3g) and Fourier
To gain insight into the thermal transformation process of
CoFe@MET-6, we carbonized CoFe@MET-6 at different
temperatures and obtained CoFe@C-T (T denoted the
o
carbonization temperature, T = 700, 800, or 900 C). CoFe@C-
700 preserved the morphology of CoFe@MET-6, while
CoFe@C-800 and CoFe@C-900 had much larger size with
abundant pores, similar to CoFe@C (carbonized at 1000 oC)
(Supporting Information, Figure S10). It suggested a violent
transformation between 700 and 800 oC, where the small-size
particles were fused into large carbon networks with a foam-like
morphology.
Proton exchange membrane fuel cells and metal-air batteries
have attracted great attentions in recent years.[14] However, their
applications are hindered by the sluggish ORR in cathode.[15]
The development of noble-metal-free electrocatalysts is
important. To evaluate the catalytic performance of CoFe@C,
we constructed a three-electrode configuration using 0.1 M KOH
and 0.5 M H2SO4 as alkaline and acidic electrolytes, respectively.
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As depicted in Supplementary Figure S11 (Supporting
Information), the contrast between the featureless cyclic
voltammetry (CV) curves in N2-saturated electrolyte and well-
defined cathodic peaks in O2-saturated electrolyte revealed the
high catalytic activity of CoFe@C. In alkaline electrolyte,
CoFe@C exhibited a more positive onset potential (at J = -0.1
mA cm-2) and half-wave potential than those of commercial Pt/C
and most of the previously reported noble-metal-free
electrocatalysts (Figure 4a; Supporting Information, Figure S11
and Table S4). Rotating ring disk electrode (RRDE) investigation
showed that the electron transfer number (n) of CoFe@C was
above 3.9 (0.2 ~1.0 V) with a low peroxide yield (Figure 4b;
Supporting Information, Figure S12). Additionally, Koutecky-
Levich plots (Supporting Information, Figure S11) presented
parallel fitting lines, indicating the first-order reaction kinetics
with respect to the concentration of the dissolved O2. The
calculated n was 3.98 at 0.7 V, which was in agreement with the
RRDE results. Notably, CoFe@C also achieved a high catalytic
activity towards ORR in the acidic electrolyte, which has always
been a challenge for noble-metal-free electrocatalysts.[16] The
half-wave potential approached that of Pt/C with a gap of only 32
mV (Figure 4c), when the RRDE analysis demonstrated a high n
(n >3.8) and a low peroxide yield (Figure 4b; Supporting
Information, Figure S12).
The stability of CoFe@C in both electrolytes was analyzed
by chronoamperometric measurements (Figure 4d). For
CoFe@C, the current retentions were as high as 88 % in 0.1 M
KOH and 87 % in 0.5 M H2SO4 after continuous operation for
20000 s. In comparison, Pt/C showed obvious deteriorations
with only retentions of 48 % and 39 %, respectively. In CoFe@C,
the nitrogen-rich 3D hierarchically porous carbon networks
stabilized the ADMSs and prevented ADMSs from
agglomeration via the strong interaction with the ADMSs, which
accounted for the high electrochemical stability of CoFe@C
catalyst.
A series of controlled electrochemical measurements were
conducted under the same condition for u-carbon, Co@C and
Fe@C (Supporting Information, Figures S13-S16). In the
alkaline electrolyte, CoFe@C, Co@C and Fe@C exhibited
higher ORR catalytic activities than that of u-carbon due to the
incorporation of active ADMSs. CoFe@C demonstrated the best
ORR activity, which could be ascribed to the following factors
(Figure 4e): (i) The large carbon network with hierarchical pores
benefited electron transfer, mass transport and electrolyte
penetration. (ii) The carbon networks could serve as effective
supports for ADMSs, which guaranteed the high exposure and
accessibility of ADMSs. (iii) The active ADMSs and their
interactions with carbon matrix contributed to the overall activity
towards ORR.[17] The extraordinary catalytic performances of
CoFe@C indicate that this synthetic approach can effectively
produce useful materials. Considering the extensive class of
EMOFs[18] and the controllable immobilization of single/multiple
ADMSs, this strategy demonstrates a broad scientific scope.
In summary, we have fabricated submillimeteric ADMS-
decorated hierarchically porous carbon networks via a simple
thermal transformation of EMOFs. Upon pyrolysis, the energetic
triazole ligands and volatile zinc nodes puffed up EMOFs to the
hierarchically porous carbon networks in a large size. By
introducing different target metal source via a wet-chemistry
method, various ADMSs can be generated. As a proof of
concept, we demonstrated that Co and Fe ADMS-decorated
carbon network (CoFe@C) is high-performance ORR catalyst.
This work not only expands the scope of MOF utilization for
advanced material design, but also generates useful materials
with great potential for a variety of applications, such as energy
storage and conversion.
Acknowledgements
The authors thank the reviewers for valuable suggestions. This
work was financially supported by the National Natural Science
Foundation of China (51772008), the National Key Research
and Development Program of China (2017YFA0206701),
National Program for Support of Top-notch Young Professionals,
Changjiang Scholar Program and AIST. Use of the Advanced
Photon Source was supported by the U.S. Department of
Energy, Office of Basic Energy Sciences [grant number DE-
AC02-06CH11357]. MRCAT operations, beamline 10-BM, are
supported by the Department of Energy and the MRCAT
member institutions.
Figure 4. a) ORR polarization curves of CoFe@C and Pt/C at 10 mV s−1 and
1,600 rpm (revolutions per minute) in 0.1 M KOH. b) Electron transfer number
(n) of CoFe@C in 0.1 M KOH (top) and 0.5 M H2SO4 (bottom). c) ORR
polarization curves of CoFe@C and Pt/C at 10 mV s−1 and 1,600 rpm in 0.5 M
H2SO4. d) Long-term chronoamperometric responses of CoFe@C and Pt/C at
1,600 rpm. Top: at 0.77 V in 0.1 M KOH. Bottom: at 0.50 V in 0.5 M H2SO4. e)
Schematic illustration of the active CoFe@C for efficient ORR electrocatalysis.
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Conflict of interest
The authors declare no conflict of interest.
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frameworks • single sites catalyst • transition metals
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Entry for the Table of Contents
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R. Zhao, Z. Liang, S. Gao, C. Yang, B.
Zhu, J. Zhao, C. Qu, R. Zou,* and Q.
Xu*
Page No. – Page No.
Puffing up Energetic Metal-organic
Frameworks to Large Carbon
Networks with Hierarchical Porosity
and Atomically Dispersed Metal Sites
Text for Table of Contents
Large carbon networks with hierarchical pores and atomically doped metal sites (ADMSs) are obtained from an energetic metal-organic
framework (MOF). The energetic ligands (1H-1,2,3-triazole) and volatile zinc nodes in MOFs decompose/evaporate upon pyrolysis, foaming the
carbon networks, while the pre-imbedded Fe and Co ions are transformed into active ADMSs. These ADMS-decorated carbon networks are highly
active catalysts for oxygen reduction reaction.
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