Two-Dimensional Core-Shelled Porous Hybrids as Highly Efficient Catalysts for the Oxygen Reduction Reaction

Article abstract of DOI:10.1002/anie.201600850

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have drawn much attention due to their unique physical and chemical properties. Using TMDs as templates for the generation of 2D sandwich-like materials with remarkable properties still remains

Full text of DOI:10.1002/anie.201600850

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201600850
German Edition: DOI: 10.1002/ange.201600850
Noble-Metal-Free Electrocatalysts
Two-Dimensional Core-Shelled Porous Hybrids as Highly Efficient
Catalysts for the Oxygen Reduction Reaction
+
+
Kai Yuan , Xiaodong Zhuang , Haiyan Fu, Gunther Brunklaus, Michael Forster, Yiwang Chen,
Xinliang Feng,* and Ullrich Scherf*
Abstract: Two-dimensional (2D) transition-metal dichalcoge-
nides (TMDs) have drawn much attention due to their unique
physical and chemical properties. Using TMDs as templates for
the generation of 2D sandwich-like materials with remarkable
properties still remains a great challenge due to their poor
Functionalized graphenes are promising templates for the
construction of 2D hybrid materials, such as 2D metal oxides,
metal sulfides, carbon nanosheets, and microporous polymer
[
3]
hybrids. Beyond graphene, more and more other 2D
nanomaterials attracted tremendous attentions in recent
[4]
solvent processability. Herein, MoS -coupled sandwich-like
years. Among these 2D materials, transition-metal dichal-
cogenides (TMDs) such as MoS , WS , MoSe and WSe are
2
conjugated microporous polymers (M-CMPs) with high
specific surface area were successfully developed by using
2
2
2
2
[
5]
one of the primary focuses of research. Because of their
unique electronic, mechanical, optical and chemical proper-
ties, TMDs showed high application potential in electronic
functionalized MoS nanosheets as template. As-prepared M-
2
CMPs were further used as precursors for preparation of
[4a]
MoS -embedded nitrogen-doped porous carbon nanosheets,
devices and for catalysis and energy storage. The covalent
functionalization of TMDs can further enhance the versatility
2
which were revealed as novel electrocatalysts for oxygen
reduction reaction with mainly four-electron transfer mecha-
nism and ultralow half-wave potential in comparison with
commercial Pt/C catalyst. Our strategy to core–shelled sand-
wich-like hybrids paves a way for a new class of 2D hybrids for
energy conversion and storage.
[
6]
of 2D TMDs. For example, improving the solubility in
common solvents facilitates their integration into polymer
[
7]
matrices. Furthermore, entirely new materials can be
[
8]
designed by attaching functional ligands. In contrast to
graphene, reports on the covalent functionalization of TMDs
[
6,7,9]
remain scarce.
Up to now, only a few covalently
[6,8,10]
G
raphene as the best-known two-dimensional (2D) nano-
connected polymer/MoS hybrids have been reported.
2
material has been intensively studied since 2004 due to its
Ultrathin MoS nanosheets are excellent candidates for
2
[
1]
exceptional physical and chemical properties. Covalent
functionalization of graphene has been demonstrated as
a viable approach for tailoring the electronic and chemical
properties of graphene-based materials, enabling the intro-
constructing hybrids with high specific surface area and high
[3b, 11]
flexibility.
Both sides of the MoS nanosheets can be used
2
as templates for the growth of functional composites with
[3b, 11b]
sandwich-like structure.
Heteroatom-doped carbons as
[2]
duction of reactive groups for subsequent functionalization.
promising materials for catalysis or energy conversion and
storage have attracted growing attention due to their unique
[12]
structural and electronic properties.
Conjugated micro-
[
+]
[
*] K. Yuan, Dr. M. Forster, Prof. Dr. U. Scherf
Macromolecular Chemistry Group (buwmakro) and Institute for
Polymer Technology, Bergische Universität Wuppertal
Gauss-Str. 20, 42119 Wuppertal (Germany)
porous polymers (CMPs) with high specific surface area and
hierarchical pore distribution are favored for a controllable
[3d,13]
construction of carbon-based materials.
Heteroatoms can
be controllably introduced into CMPs network via hetero-
E-mail: scherf@uni-wuppertal.de
[14]
atom-containing tectons (building blocks). In this regard,
[
+]
Dr. X. Zhuang, Prof. Dr. X. Feng
the combination of MoS nanosheets and CMPs may generate
2
Center for Advancing Electronics Dresden (cfaed) and Department of
Chemistry and Food Chemistry, Technische Universität Dresden
Mommsenstrasse 4, 01062 Dresden (Germany)
2
D hybrid sandwiches for applications in catalysis and
electrochemical energy-related devices. However, using
E-mail: xinliang.feng@tu-dresden.de
MoS as templates for the generation of 2D sandwich-like
2
H. Fu, Prof. Dr. Y. Chen
College of Chemistry/Institute of Polymers, Nanchang University
materials with remarkable properties still remains a great
challenge due to their poor solvent processability.
9
99 Xuefu Avenue, Nanchang 330031 (China)
Herein, we demonstrate the fabrication of MoS -tem-
2
Dr. G. Brunklaus
plated conjugated microporous polymer (M-CMP) nano-
sheets by growing nitrogen-rich CMPs on 4-iodophenyl-
functionalized MoS templates. Unique polymer/MoS sand-
Institut für Physikalische Chemie
Westfälische Wilhelms-Universität Münster
Corrensstr. 28, 48149 Münster (Germany)
2
2
[
+]
wiches with high specific surface area and hierarchically
porous structure were achieved. As-prepared 2D porous
Dr. X. Zhuang
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University
polymer/MoS sandwiches can be further converted into the
2
Dongchuan Road 800, 200240, Shanghai (P.R. China)
corresponding 2D porous carbon hybrids by direct pyrolysis
of the M-CMP nanosheets. As proof-of-concept, oxygen
reduction reaction (ORR) and supercapacitor performances
were studied. The strong interaction between nitrogen-doped
+
[
] These authors contributed equally to this work.
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600850.
6
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porous carbon and MoS within the sandwich structure boosts
insoluble, crude products that were collected by filtration
2
the performance for ORR and electrochemical energy
storage. As a result, the 2D porous carbon hybrids display
and purified by Soxhlet extraction. Finally, the sandwich-like
M-CMPs, denoted as M-CMP1, M-CMP2 and M-CMP3 for
the pyridine-, pyrazine- and triazine-containing polymers,
respectively, were obtained after vacuum drying. The corre-
sponding conjugated microporous polymers (CMPs), denoted
as CMP1, CMP2 and CMP3 for the pyridine-, pyrazine- and
triazine-containing polymers, respectively, without the MoS2
template were also synthesized for comparison by using
similar procedure (Scheme S1).
Morphologies and microstructures of the M-CMPs were
investigated by scanning electron microscopy (SEM), trans-
mission electron microscopy (TEM), high-resolution TEM
(HRTEM) and atomic force microscopy (AFM). All of the
M-CMPs display similar sheet-like
a more positive half-wave potential (À0.14 V vs. À0.13 V for
À2
Pt/C) and a higher diffusion-limited current (5.4 mAcm ) in
ORR in comparison with MoS -free porous carbons derived
2
from CMPs. Moreover, the 2D hybrids deliver high capaci-
À1
À1
tance up to 344 Fg at 0.2 Ag , 45% higher than that of
À1
corresponding MoS -free porous carbons (237 Fg ).
2
The synthesis strategy for M-CMPs is illustrated in
Scheme 1. First, chemically exfoliated MoS (CE-MoS ) was
2
2
generated by reacting bulk MoS2 with n-butyllithium (n-
BuLi). Then, CE-MoS was functionalized with 4-iodophenyl
2
diazonium salt under aqueous conditions (detailed exper-
structures. Therefore, M-CMP2 will
be discussed as a typical example.
As shown in Figures 1a and 1b, M-
CMP2 possesses a wrinkled sheet
morphology similar to that of MoS2
layers. The surface of the M-CMP
sandwiches show a similar average
roughness as the non-templated
CMPs due to the coating of the
porous CMPs on MoS (Figure S8).
2
No free CMP particles or “naked”
Scheme 1. Idealized formula scheme depicting the chemical exfoliation of bulk MoS and subsequent
MoS2 sheets were observed, thus
2
functionalization with 4-iodophenyl substituents under formation of MoS -I as well as the preparation indicating that the majority of the
2
of MoS -templated conjugated microporous polymers (M-CMPs) and the corresponding MoS /
nitrogen-doped porous carbon (M-CMPs-T) hybrids. i) monomers: 1,3,5-triethynylbenzene and 2,5-
2
2
monomers have been grafted onto
the MoS surface. In the HRTEM
2
dibromopyridine, 2,5-dibromopyrazine, or 2,4,6-trichloro-1,3,5-triazine, argon, Pd(PPh ) , CuI, Et N,
3
4
3
image of M-CMP2, the alternating
bright and dark areas may be
attributed to the microporous struc-
À1
DMF, 1008C, 3 days; ii) argon, heating rate: 108Cmin , pyrolysis temperature: 700, 800, or 9008C,
h.
2
imental procedures are given in the
[
7,9]
Supporting Information).
The
obtained 4-iodophenyl-functional-
ized MoS (MoS -I) can be well-
2
2
dispersed in various organic sol-
vents, such as dimethylformamide
(
DMF, Figure S1) and toluene. The
successful functionalization of
MoS2 was evidenced by several
analytical techniques, the results
are presented in Figures S1–S7
(
for detailed discussions see the
Supporting Information). Next,
the arylacetylene building block
1
,3,5-triethynylbenzene
mixed
with an aryl di- or trihalide (2,5-
dibromopyridine, 2,5-dibromopyr-
azine, or 2,4,6-trichloro-1,3,5-tria-
zine) was reacted with MoS -I in
2
anhydrous DMF in the presence of
Pd(PPh ) , CuI and Et N under
3
4
3
inert atmosphere. This Sonoga-
shira–Hagihara reaction was car-
ried out at 1008C for 3 days under
Figure 1. a) TEM, b) SEM, and c) AFM images of M-CMP2. Inset of (a) is the selected-area electron
diffraction (SAED) pattern of a M-CMP2 nanosheet. d) Solid-state C CP/MAS spectrum of M-CMP1
and its peak assignments. e) Nitrogen adsorption/desorption isotherms, and f) corresponding pore
13
vigorous stirring and yielded size distribution profiles of M-CMPs.
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[
3d,13a,b, 15]
ture of the M-CMPs with homogenously distributed pores in
M-CMPs (Figure S8). AFM and thickness analyses (Fig-
ure 1c) indicate the presence of M-CMP sandwiches with
a uniform thickness of 35 Æ 6 nm. Oppositely, the non-
templated CMPs display a granular structure (Figure S9).
chemical catalysis.
Thermogravimetric analysis
(TGA) of M-CMPs revealed that they can be easily trans-
formed into MoS /carbon hybrid materials (ca. 80% residual
at 8008C, Figure S15). Therefore, M-CMP1, M-CMP2 and M-
CMP3 were pyrolyzed at 700, 800, or 9008C for 2 h under an
2
These results confirm the role of MoS -I as template for the
argon atmosphere (Scheme 1). The resulting MoS /nitrogen-
2
2
growth of the CMPs layers on both sides of MoS sheets.
doped porous carbon (M-CMPs-T) hybrids are denoted as M-
CMP1-T, M-CMP2-T and M-CMP3-T, respectively, in which
T represents the pyrolysis temperature. For comparison,
2
The chemical structure of the M-CMPs was analyzed by
13
FTIR and solid-state C cross-polarization/magic angle
spinning nuclear magnetic resonance (CP/MAS NMR) spec-
porous carbons without MoS were also prepared by pyrolysis
2
À1
troscopy. The disappearance of the CÀBr (at 1080 cm ) and
of the CMPs at T8C, denoted as the corresponding CMPn-T
(n = 1, 2, 3, T= 800), respectively.
À1
alkynyl CÀH (at 3275 cm ) stretch vibrations in the FTIR
spectra of M-CMP1 imply the occurrence of an efficient
The SEM and TEM images of the M-CMPs-T hybrids still
display the preserved nanosheet morphology (Figures 2a,b
and S16). On the contrary, the CMP-derived carbons exhibit
a particular morphology (Figure S16). HRTEM image and
corresponding SAED pattern of M-CMP2-800 indicate that
1
3
arylacetylene–aryl coupling (Figure S10). In the C NMR
spectrum of M-CMP1 the signal at 152 ppm can be assigned to
the pyridine carbons in the proximity of the nitrogen atom
(
Figure 1d). Moreover, diaryl CꢀC signals at 88–90 ppm are
observed again indicating successful formation of the CMP
the porous carbons in close connected with MoS templates
2
skeleton on the MoS surface. The FTIR and NMR spectra of
(Figure 2c). Scanning transmission electron microscopy
(STEM) element mapping images (Figures 2d and S17)
clearly show homogeneous distribution of C, N, Mo and S in
M-CMP2-800 nanosheets.
2
M-CMP2 and M-CMP3 (Figures S10 and S11) can be
interpreted in similar manner. M-CMPs were further charac-
terized by XPS (Figure S12). All M-CMPs are composed of
similar components, C, N, S and Mo, which are summarized in
Table S1. All structure characterization data indicate the
successful synthesis of microporous
After pyrolysis, the obtained MoS /porous carbon hybrids
2
2
À1
still show high BET surface area (790–850 m g ) indicating
MoS /polymer hybrid nanosheets
2
by utilizing MoS -I as template.
2
Consequently, nitrogen adsorp-
tion/desorption
measurements
were carried out for investigation
of the porous nature of M-CMPs.
The nitrogen sorption isotherms
and the corresponding pore size
distributions of the M-CMPs are
presented in Figures 1e,f. All M-
CMPs showed reversible type-IV
isotherms with BET surface areas
2
À1
of 1625, 1254, and 1058 m g for
M-CMP1, M-CMP2 and M-CMP3,
respectively. The pore size distribu-
tions based on non-local density
functional
theory
(NLDFT)
method (Figure 1 f) indicate the
presence of both meso- and micro-
pores in M-CMPs. Notably, M-
CMPs show much higher BET sur-
face areas than those for CMPs
without MoS2 (1086, 886 and
2
À1
3
92 m g ), respectively (Figur-
es S13 and S14). This may originate
from a synergetic effect of the MoS2
template on the CMP porosity.
CMPs-derived porous carbons
with integrated heteroatoms, metal
clusters or metal nanoparticles, or
metal oxides are promising materi-
als for various applications such as
gas storage and separation, energy
Figure 2. Characterization of MoS -based porous carbon nanosheets. a) SEM, b) TEM, and
2
c) HRTEM images of M-CMP2-800. Inset of (c) shows the corresponding SAED pattern. d) STEM
image of M-CMP2-800 and corresponding elemental mapping images for C, N, Mo and S.
e) Nitrogen adsorption/desorption isotherms and corresponding BET surface areas. f) Pore size
distributions of M-CMP1-800, M-CMP2-800, and M-CMP3-800. g) N1s core level XPS spectra of M-
conversion and storage, or electro- CMP1-800, M-CMP2-800, and M-CMP3-800.
6
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that the pore structure is mainly preserved after pyrolysis
Figures 2e and S18). Thereby, the surface areas of the MoS2-
templated hybrids are higher than those of the corresponding
CMP2-800 and M-CMP3-800, respectively (Table S3). The
(
pyridinic and graphitic nitrogen doping leads to a redistrib-
ution of charge and spin density of the adjacent carbons,
which is also favorable for carbon-based energy storage/
2
À1
porous carbons (681–828 m g ) derived from the MoS -free
2
[13a,19]
CMPs thus demonstrating the advantages of MoS templating
conversion, for example, ORR or supercapacitors.
2
strategy (Figures S19, S20 and Table S2). The large gas uptake
Based on the above discussed hierarchical porous struc-
ture, their heteroatom-doping feature and 2D morphology,
M-CMPs-T are promising candidates for electrochemically
catalyzed ORR. The ORR activity of M-CMPs-Twas studied
under alkaline condition (0.1m KOH). A scheme depicting of
the ORR process at the M-CMPs-T hybrids is shown in
Figure 3a, with the active sites homogeneously distributed
at low relative pressure (below p/p = 0.1) and the hysteretic
0
loops in the high p/p region of the isotherms indicate the
0
coexistence of micro- and mesopores in the M-CMPs-T
[
16]
hybrids. The peaks for diameters around 1.1 and 3.5 nm in
the pore size distribution profiles (Figures 2 f and S18) further
confirm the hierarchical porous structure. Such large specific
surface areas together with the hierarchical porous structure
are crucial for accelerating electron- and mass-transport for
device related applications, such as electrochemically cata-
lyzed ORR and supercapacitors.
throughout the M-CMPs-T nanosheets. O molecules can be
2
À
directly reduced into OH at the active sites. Cyclic voltam-
metry (CV) revealed a well-defined cathodic ORR peak for
all the M-CMPs-T hybrids in O -saturated solutions in
2
In the Raman spectra of M-CMPs-T (Figure S21), the
contrast to N -saturated solutions, indicating a pronounced
2
À1
MoS -related part (300–500 cm ) and two peaks around 1330
catalytic activity of the hybrids. Thereby, M-CMP2-800
exhibited the highest peak current density at most positive
potential (À0.21 V versus Ag/AgCl) in comparison with other
2D hybrids (Figures 3b and S23) and CMP2-800 (Figure S24).
To gain further insight into the ORR process, linear sweep
voltammetry (LSV) measurements were recorded on the
rotating disk electrode (RDE). Based on the LSV curves
under a rotation of 1600 rpm (Figures S25–S27), the diffusion-
2
À1
and 1590 cm assigned to disordered (D-band) and ordered
graphitic carbons (G-band), respectively, are clearly observed
[
17]
for all hybrids. The observed relatively high intensity of the
D-band (I /I ratio: ca. 1.2) for all M-CMPs-T hybrids can be
D
G
[13a,18]
attributed to disorder associated with edge defects.
The
introduction of nitrogen centers into the porous carbons also
[13a]
contributes to the D-band intensity.
XPS measurements
À2
were carried out for identifying the elemental components of
M-CMPs-T hybrids. Based on the XPS analysis (Figures 2g
and S22), pyridinic and graphitic N were found at a nominal
nitrogen level of 2.8, 3.6 and 3.1 wt% for M-CMP1-800, M-
limited currents at À0.6 V were 5.5, 4.7, 5.4, and 3.4 mAcm
for commercial Pt/C, M-CMP1-800, M-CMP2-800 and M-
CMP3-800, respectively. M-CMP2-800 again showed the
highest diffusion-limited current and the most positive onset
Figure 3. a) ORR process taking place at the surface of the M-CMPs-T hybrids in alkaline condition. b) CV curves in N - and O -saturated 0.1m
2
2
À1
KOH. c) LSV curves for M-CMP2-800 at different rotation rates in O -saturated 0.1m KOH at 5 mVs . The inset shows the Koutecky–Levich (K-L)
2
plots. d) RRDE curve for M-CMP2-800 for a rotation speed of 1600 rpm (inset: calculated electron transfer number (n) against the potential).
À1
e) LSV curves for CMP2-800, M-CMP2-800, and Pt/C at 1600 rpm and a scan rate of 5 mVs . f) Galvanostatic charge/discharge curves of CMP2-
À1
8
00 and M-CMP2-800 at a current density of 0.2 Ag
.
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potential among M-CMPs-T hybrids, possibly as a result of
the high specific surface area and N content (3.6 wt%,
Table S3).
ture, providing channels for effective mass transport, and thus
facilitating electrolyte penetration and ion diffusion.
In conclusion, a series of sandwich-like MoS -templated
2
The electron transfer number per oxygen molecule (n) for
ORR was calculated based on the LSV curves at different
rotation rates (400–1600 rpm) and potentials (À0.4--0.9 V) by
conjugated microporous polymer nanosheets (M-CMPs) with
high specific surface area and aspect ratio was successfully
developed by utilizing 4-iodophenyl-functionalized MoS as
2
[20]
using the Koutechy–Levich (K-L) equation
(Supporting
template. By direct pyrolysis, 2D MoS /nitrogen-doped
2
Information). The K-L plots show a good linearity for all
potentials (Figures 3c and S28, S29). The electron transfer
number n is calculated as ca. 4.0 between À0.4 and À0.9 V for
M-CMP2-800. Based on the rotating ring-disk electrode
porous carbon (M-CMPs-T) hybrids were obtained with
large specific surface areas and hierarchical porous structure.
In these hybrids, both sides of the single-layer MoS nano-
2
sheets were uniformly decorated with the nitrogen-doped
porous carbon layers for good interfacial contact. The M-
CMPs-T hybrids have been used for electrochemical cata-
lyzed ORR with high activity and selectivity as well as for the
construction of excellently performing supercapacitors. We
expect that our novel strategy can be extended for the
generation of various other layered hybrid materials for
energy conversion and storage.
(
(
RRDE) curves (Figure 3d) a similar value for n is calculated
ca. 3.8, inset of Figure 3d), which is higher than the values for
M-CMP1-800 (3.6) and M-CMP3-800 (3.3). Additionally, M-
CMP2-800 shown the lowest peroxide yield (Figure S30),
suggesting that the ORR catalyzed by M-CMP2-800 is mainly
dominated by a four-electron transfer pathway. From the
intercept of the K-L plots, the kinetic current density Jk
À2
corrected for mass transport is calculated as 26.3 mAcm
for M-CMP2-800, which is also higher than the values for the
À2
other samples (19.2 mAcm
for M-CMP1-800 and Acknowledgements
À2
9
.1 mAcm
for M-CMP3-800). The half-wave potential
(
HWP) of M-CMP2-800 occurred at À0.14 V, 108 mV lower
K.Y. and X.Z. would like to thank the fellowship program of
the China Scholarship Council for financial support. The
authors thank the Natural Science Foundation of China
(51403126), the ERC (grant 2DMATER), and the EU
Graphene Flagship program for financial support.
than that for MoS -free CMP2-800 (Figure 3e), and only
1
demonstrating its high catalytic activity. M-CMP1-800 and M-
CMP3-800 display a similar decrease of the HWP values
2
1 mV higher than the value of commercial Pt/C, impressively
(
Figures S31–S33) in comparison with those of the MoS -free
2
CMP1-800 (by 194 mV) and CMP3-800 (by 106 mV), respec-
tively. These results indicate a lower energy consumption and
operation voltage for M-CMPs-T hybrids in comparison with
Keywords: conjugated microporous polymers ·
hybrid materials · molybdenum disulfide ·
noble-metal-free electrocatalysts · supercapacitors
corresponding MoS -free counterparts.
2
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6858–6863
Angew. Chem. 2016, 128, 6972–6977
MoS -based materials can efficiently store charges by
2
formation of intersheet and intrasheet double-layers or
pseudocapacitances (faradaic charge-transfer process on the
[21]
Mo center). Regarding this point, M-CMPs-T hybrids have
also been examined as electrode materials of supercapacitors
for electrochemical energy storage (Figure 3 f). The capaci-
tive performance was investigated by CV and galvanostatic
charge/discharge (GCD) measurements (Figures 3 f, S34 and
S35, more detailed discussions are given in the Supporting
Information). Near-rectangular shape CV curves were
observed for M-CMP2-800, indicating an ideal capacitive
behavior. Remarkably, the current density for M-CMP2-800
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44 Fg at 0.2 Ag , 45% higher than the capacitance of
À1
CMP2-800 (237 Fg ). The specific capacitance of M-CMP2-
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8
00 outperforms the values for other MoS -based electrodes,
2
[
22]
such as metallic 1T phase MoS ,
hollow MoS2 nano-
and is comparable to
the performance of heteroatom-doped graphene and porous
2
[
23]
[21a,24]
spheres
and MoS /graphene,
2
[3d,13a]
carbon-based electrodes
(Table S4). The superior ORR
and supercapacitor performances of M-CMPs-T hybrids
should be significantly associated with a maximized syner-
gistic interaction between MoS and porous carbon layers
2
combined with the presence of a hierarchical porous struc-
6
862 www.angewandte.org
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Received: January 26, 2016
Published online: April 21, 2016
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