Crystalline-Water/Coordination Induced Formation of 3D Highly Porous Heteroatom-Doped Ultrathin Carbon Nanosheet Networks for Oxygen Reduction Reaction

Article abstract of DOI:10.1002/cctc.201800890

Development of highly efficient electrocatalysts with low cost for oxygen reduction reaction (ORR) is crucial for their application in fuel cells and metal-air batteries. In this work, we report a synthesis of 3D heteroatom-doped ultrathin carbon nanosheet networks directly starting from solid raw materials. This method represents an operationally simple, general, and sustainable strategy to various ultrathin carbon nanosheet networks. Evaporation of crystalline water and coordination interaction are proposed to be responsible for the formation of the 3D carbon nanosheet networks. The carbon nanosheet networks possess high surface area with micro- and macropores, large pore volume, ultrathin nanosheet structure, and effective N/S-co-doping. The as-prepared materials show outstanding electrocatalytic ORR performance with more positive onset potential and half-wave potential, good methanol tolerance, and excellent stability, compared with those of the porous carbons derived from the ZIF counterpart and commercial Pt/C. This work not only provides highly active ORR electrocatalysts via an operationally simple and green process and also demonstrates a general method to prepare 3D ultrathin carbon nanosheet networks without any additional template and solvent.

Full text of DOI:10.1002/cctc.201800890

Accepted Article
Title: Crystalline-water/Coordination Induced Formation of 3D Highly
Porous Heteroatom-doped Ultrathin Carbon Nanosheet Networks
for Oxygen Reduction Reaction
Authors: Tingting Liu, Shi Feng, Jia Huo, Qiling Li, Chao Xie, and
shuangyin Wang
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Crystalline-water/Coordination Induced Formation of 3D Highly
Porous Heteroatom-doped Ultrathin Carbon Nanosheet Networks
for Oxygen Reduction Reaction
Tingting Liu,a,† Shi Feng,a,† Jia Huo,a,b,* Qiling Li,a Chao Xie,a and Shuangyin Wang a,*
Abstract: Development of highly efficient electrocatalysts with low
cost for oxygen reduction reaction (ORR) is crucial for their
application in fuel cells and metal-air batteries. In this work, we
especially, zeolitic imidazolate frameworks (ZIFs), contain
abundant heteroatoms, transition metals, and pores. These
materials have demonstrated tremendous potential for facile
preparation of highly porous heteroatom-doped carbon-based
electrocatalysts with ample catalytically active sites.15-19 To
further expose more active sites, it is desirable to construct two-
dimensional (2D) porous carbon nanosheets. Few examples
have been reported on synthesis of porous nanosheets starting
from MOFs and the synthetic processes required use of
template (such as graphene, NaCl) or preformation of 2D MOFs.
report
a synthesis of 3D heteroatom-doped ultrathin carbon
nanosheet networks directly starting from solid raw materials. This
method represents an operationally simple, general, and sustainable
strategy to various ultrathin carbon nanosheet networks. Evaporation
of crystalline water and coordination interaction are proposed to be
responsible for the formation of the 3D carbon nanosheet networks.
The carbon nanosheet networks possess high surface area with
micro- and macropores, large pore volume, ultrathin nanosheet
structure, and effective N/S-co-doping. The as-prepared materials
show outstanding electrocatalytic ORR performance with more
positive onset potential and half-wave potential, good methanol
tolerance, and excellent stability, compared with those of the porous
carbons derived from the ZIF counterpart and commercial Pt/C. This
work not only provides highly active ORR electrocatalysts via an
operationally simple and green process and also demonstrates a
general method to prepare 3D ultrathin carbon nanosheet networks
without any additional template and solvent.
20-26
However, these methods often require complicated
fabrication processes but also led to production of polluted
wastes, such as solvents and byproducts.20-26 Moreover, it is a
big challenge to prepare 3D porous carbons constructed with 2D
carbon nanosheets from MOFs, which are expected to avoid the
aggregation of carbon nanosheets and increase the substrate
diffusion during the electrocatalysis.
In this work, we have developed an operationally simple and
general approach to prepare 3D highly porous heteroatom-
doped ultrathin carbon nanosheet networks. The synthesis is
directly starting from mechanically ground mixture of solid raw
materials without any additional template. The formation
mechanism of carbon nanosheet networks is proposed as the
induction of evaporation of the intrinsic crystalline water from
metal salts. Water actually acts as a pore-forming agent but also
an additive for formation of coordination bond between zinc ions
and imidazole-based ligands to stabilize the overall frameworks.
Additionally, the release of volatile components, especially Zn, at
high temperature resulted in the formation of micropores within
ultrathin walls to produce hierarchically porous carbon
nanosheet networks, which would create numerous defects at
the edges over micropores. The heteroatom-doping was easily
achieved through changing the ligand, for example, 2-
methylimidazole for N-doping and mixture of 2-methylimidazole
and 2-mercaptoimidazole for N/S-co-doping. Compared with
porous carbon derived from ZIF-8, the ultrathin carbon
nanosheet networks possess not only high surface area but also
abundant active sites and low diffusion resistance owing to the
3D ultrathin hierarchically porous structure. These features
result in unexpectedly high ORR activity, especially for the N/S-
co-doped carbon nanosheet network, which exhibits comparable
activity with Pt/C. To our knowledge, this is also the first
example that the crystalline water is used as a soft template to
produce highly porous ultrathin carbon nanosheet networks.
Introduction
Development of highly efficient oxygen reduction reaction (ORR)
electrocatalysts has received intensive attention, 1,2 since ORR is
a critical and also kinetically sluggish process for fuel cells and
metal-air batteries.3-6 Although Pt-based materials are the
currently well-known ORR catalysts, the exorbitant cost, CO
deactivation, and poor operational stability seriously hamper
their practical application.7,8 Henceforth, various strategies have
been developed to circumvent the above issues, such as
decreasing loading of Pt and employing non-precious metal
catalysts and even metal-free catalysts.9
Heteroatom-doping (such as N, S, or P) porous carbon-
based electrocatalysts, exhibiting high stability and resistance
toward CO poisoning, satisfactory cost, and more exposed
active sites, are regarded as one of the most promising Pt
alternatives.10-14 Various heteroatom-containing materials have
been used as precursors or templates to prepare heteroatom-
doping porous carbons, such as graphene, carbon nanotubes,
biopolymers, supramolecular polymers, and metal organic
frameworks (MOFs).10-19 Among these materials, MOFs,
a
T. Liu, S. Feng, Prof J. Huo, Q. Li, C. Xie, Prof S. Wang
Results and Discussion
State Key laboratory of Chem/Bio-sensing and Chemometrics
College of Chemistry and Chemical Engineering
Hunan University
Changsha, 410082, China
E-mail: jiahuo@hnu.edu.cn; shuangyinwang@hnu.edu.cn
Shenzhen Research Institute of Hunan University, Shenzhen
518057, Guangdong, China
These authors contributed equally to this work.
Nitrogen-doped ultrathin carbon nanosheet networks
(denoted as N-UCNN-n, where n is the ratio of Zn(NO3)2·H2O
and 2-methylimidazole), was prepared by direct pyrolysis of
mechanically ground mixture of zinc nitrate hexahydrate and 2-
methylimidazole under Ar atmosphere at 150 oC for 2 h and
b
[†]
o
then 1000 C for 6 h. The synthesis is quite straightaway and
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
does not require preformation of MOFs, and any additional
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template or precursors. The molar ratio of reactants, the
hydrated metal salts, and the pyrolysis temperature play very
important roles in the formation of carbon nanosheet networks
(the preparation procedure is shown in Scheme 1). As a
comparison, N-doped porous carbon nanoparticles (NPCN)
derived from ZIF-8 were prepared with the same starting
materials.
Scheme 1. Schematic illustration for preparation of heteroatom-doped ultrathin carbon nanosheet networks (UCNNs) and N-doped porous carbon nanoparticles
(NPCN).
The surface morphology and microstructure of N-UCNN-2
were characterized by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). SEM images (Figure
1a, 1b, S1a, and S1b) reveal that the N-UCNN-2 is indeed a 3D
carbon network consisting of macropores interconnected with
ultrathin carbon nanosheets. TEM images (Figure 1c and S1c)
indicate the formation of transparent vesicle structures implying
the carbon sheets are extremely thin. High resolution TEM
(HRTEM) image reveals that the ultrathin walls are constructed
with disordered graphene-like layers with a plenty of defects and
the corresponding FFT image (Figure 1d) confirms the formation
of disordered structure with abundant active sites. Atomic force
microscopy (AFM) was employed to detect the thickness of
nanosheets of the as-prepared carbon nanosheet networks. N-
UCNN-2 was strongly sonicated in ethanol to produce the
dispersion of carbon nanosheets suitable for AFM
characterization, as shown in Figure 1e. The average thickness
of tested nanosheets is about 3.7 nm (Figure 1f), validating the
ultrathin structure of N-UCNN-2. In addition, control samples
were synthesized at the same condition for synthesis of N-
UCNN-2, including materials prepared with different ratio of
Zn(NO3)2·6H2O and 2-methylimidazole (1:2.5 and 1:1.5) (N-
UCNN-2.5 and N-UCNN-1.5) and N-doped porous carbon
nanoparticles derived from ZIF-8 (NPCN). Pyrolyzing ZIF-8
produced porous carbon nano-polyhedrons without any
Figure 1. SEM images of N-UCNN-2: low magnification (a) and high
magnification (b); c) TEM image of N-UCNN-2; d) HRTEM image of N-UCNN-
macroporous interconnected networks (Figure S2). Interestingly,
only large bulk materials were formed when the ratio became
1:2.5 (although the resultant material is a bulk material, we still
named it as N-UCNN-2.5 for convenience). But N-UCNN-1.5
was a porous 3D carbon network similar with N-UCNN-2 (Figure
S3 and S4), suggesting that the content of crystalline water was
an important parameter to form the macroporous carbon
networks.
2 (inset in (d) shows the corresponding FFT pattern); e) AFM image of N-
UCNN-2; and f) Height profile a, b, and c derived from the white line a, b and c
in (e), respectively.
Raman spectroscopy was applied to analyze the surface
electronic properties of the N-UCNN-n and NPCN as shown in
Figure 2a and S5. The peaks at 1320 and 1590 cm-1 are
observed for both N-UCNN-n and NPCN in Figure 2a, which are
assigned to the D and G band, respectively, indicative of the
presence of both graphitic and disordered carbon atoms.27 The
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values of ID/IG of N-UCNN-2 and NPCN are 1.26 and 1.10,
respectively, declaring that more defects exist in N-UCNN-2 than
NPCN, consistent with the HRTEM results, which might result
from the ultrathin layered carbon structures within N-UCNN-2
physically adsorbed water or oxygen from external
environment.35,36 No signal of Zn was detected (Figure S8),
implying the obtained materials are the metal-free N-doped
porous carbons.37,38 From the high resolution C 1s spectrum
28,29. To further confirm the presence of disordered carbon layers, (Figure 2c), there are two main peaks assigned to the crystalline
the carbon samples were characterized by powder X-ray
diffraction (PXRD) (Figure S6). Only a broad (002) basal plane
diffraction is observed for N-UCNN-2, implying as-prepared
materials are made up of partly disordered structures.30,31 The
specific surface area and pore size distribution of N-UCNN-2
and NPCN were analyzed by the nitrogen adsorption-desorption
isotherms at 77 K. Both samples display typical type-I isotherms
(Figure 2b), indicating that they are predominantly microporous
materials and the average pore size is around 0.9
nm.32Importantly, compared with the surface area of NPCN, the
surface area of N-UCNN-2 does not decrease because of using
the solid raw materials as precursors. N-UCNN-2 exhibited a
ultrahigh surface area of 1921.4 m2 g-1, as estimated by the
Brunauer-Emmett-Teller (BET) method, exceeding those of
NPCN (1912.6 m2 g-1) and most of the MOF-derived carbon
materials.33,34 The above data confirm the formation of
hierarchically porous carbons with macro- and micropores. The
highly microporosity within the ultrathin carbon layers should
result from the evaporation of Zn atoms, which would also
graphitic sp2 carbon (C-C sp2) and the defect sp3 carbon (C-C
sp3). The ratio of sp2/sp3 of N-UCNN-2 and NPCN are 4.83 and
5.53, respectively, indicating that the N-UCNN-2 has more edge
defects than NPCN.28 The high-resolution N 1s spectrum (Figure
2d) suggests the presence of four configurations of nitrogen,
including pyridinic N (298.3 ± 0.2 eV), pyrrolic N (399.7 ± 0.2 eV),
graphitic N (401.1 ± 0.2 eV), and oxidized N (403.0 ± 0.3 eV).
The contents of N for N-UCNN-2 and NPCN are 1.0 and 1.8 at%,
respectively, consistent with those of ZIF-derived carbons
obtained at the high temperature in the literatures.33,34 Moreover,
N-UCNN-2 possesses higher content of pyridinic-N and
graphitic-N (65.3 %) than NPCN (59.8 %), which were allegedly
beneficial for ORR.39
The effective N-doping together with abundant active sites
and hierarchically porous structures encourage us to investigate
the potential of N-UCNN-2 as ORR electrocatalyst. The
electrocatalytic ORR activity of N-UCNN-2 was characterized
firstly by the cyclic voltammogram (CV) measurements in 0.1 M
KOH solution saturated with oxygen at room temperature with a
expose more edge defects as the active sites for electrocatalysis. sweep rate of 50 mV s-1, with that of NPCN as a comparison. As
The macropores would facilitate the mass diffusion so that to
increase the electron transfer efficiency during
electrocatalysis.33,34
shown in Figure 3a, no obvious redox peak was detected in the
CV curves of N-UCNN-2 and NPCN measured in the N2-
saturated 0.1 M KOH solution. This phenomenon further proves
that zinc was entirely removed during the formation process of
N-UCNN-2 and NPCN. When tested in O2-saturated 0.1 M KOH,
the CV curves demonstrate sharp reduction peaks for both
carbon nanomaterial-modified electrodes, indicating their evident
ORR activities. Apparently, N-UCNN-2 presents a significant
electrocatalytic ORR activity with a more positive cathodic peak
potential (0.70 V) than NPCN (0.68 V), close to that of the
commercial 20 wt% Pt/C (Figure S9). In addition, with changing
the molar ratio of raw materials, the peak potential shifted
negatively from 0.70 V (N-UCNN-2) to 0.60 V (N-UCNN-1.5) and
0.56 V (N-UCNN-2.5) (Figure S11 and Figure S12), suggesting
that N-UCNN-2 has the highest ORR activity among the as
prepared N-doped carbon nanosheet networks.
To further investigate the electrocatalytic activity of N-UCNN-
2, linear sweep voltammograms (LSVs) were measured in O2-
saturated 0.1 M KOH solution with a scanning rate of 10 mV s-1
on a rotating disk electrode (RDE). For comparison, we also
prepared electrodes with NPCN or commercial 20wt% Pt/C
using the same procedure. As shown in Figure 3b, the
polarization curves tested with a rotation rate of 1600 rpm show
that the N-UCNN-2 electrode presents a more positive ORR
onset potential, Eonset (0.89 V) and half-wave potential, E1/2 (0.78
V) than those of NPCN (Eonset = 0.84 V, E1/2 = 0.72 V).
Meanwhile, the limiting diffusion current of N-UCNN-2 (5.15 mA
cm-2) is close to that of Pt/C (5.30 mA cm-2). A rotating ring-disk
electrode (RRDE) technique was applied to identify whether the
ORR process underwent a two-electron or four-electron pathway
and measure the quantity of H2O2 produced during the ORR
process. Figure S10a presents the disk and ring currents for N-
UCNN-2 and NPCN. The electron transfer number (n) was
calculated as 3.81-3.90 for N-UCNN-2 (Figure S10d) in the
Figure 2. a) Raman spectra of N-UCNN-2 and NPCN; b) Nitrogen
adsorption/desorption isotherms of N-UCNN-2 and NPCN at 77 K (Inset is the
corresponding micropore size distribution); and High-resolution C 1S (c) and N
1S (d) XPS spectra of N-UCNN-2 and NPCN.
The elemental compositions and chemical states of N-
UCNN-2 were examined by X-ray photoelectron spectroscopy
(XPS). The survey spectrum (Figure S8) of N-UCNN-2 explicitly
exhibits a mainly narrow graphtic C 1s peak at around 285 eV
accompanied by a N 1s peak at 400 eV. Furthermore, an O 1s
peak also appears at around 540 eV, possibly owing to the
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potential range of 0.0-0.75 V, indicating that the ORR on N-
and S15). Instead, the mixture of hydrated zinc salt and 2-
methylimidazole would form a paste after grinding and the
subsequent calcination at 150 C led to expansion and upwrap,
UCNN-2 proceeds with
pathway.
a dominant four-electron reaction
o
where water was evaporated (Figure S13). SEM image of the
as-obtained material shows that the macroporous carbon
networks formed at this stage, which indicates that the
evaporation of the crystalline water is the primary reason for the
formation of macroporous networks (Figure S23). Interestingly,
when the dehydrated mixture of Zn(NO3)2 and 2-methylimidazole
was rehydrated with the same amount of water for N-UCNN-2,
ultrathin carbon nanosheet networks were regenerated, further
confirming the role of water (Figure S16 and S17).
The coordination interaction is also important for the
formation of final carbon nanosheet networks. PXRD patterns,
FT-IR spectra, and N2 sorption isotherms prove that grinding
mixture of hydrated zinc salt and 2-methylimidazole actually
produces nonporous zinc-imidazolate coordination polymer
(Figure S18-S21), but no any coordination polymer was formed
after grinding dehydrated Zn(NO3)2 and 2-methylimidazole
(Figure S18), which is reasonable considering that minute
amount of solvent is required during mechanochemical
synthesis of metal organic frameworks.19 Calcination of this
mixture only produces very small amounts of powders (NC)
without macroporous carbon networks, which clarifies the
importance of coordination interaction on the stabilization of
overall frameworks during the pyrolysis. We further
demonstrated the role of coordination interaction by annealing
the grinded products of Zn(NO3)2∙6H2O and 1-ethyl-3-
methylimidazolium chloride, where the N-position of organic
ligand was capped with an ethyl group to prohibit the formation
of infinite coordination polymer. Similar with the case of
dehydrated zinc salt, almost no product left finally after
calcination at 1000 oC (Figure S22).
Figure 3. a) CV curves of N-UCNN-2 and NPCN in N2-saturated and O2-
saturated 0.1 M KOH; b) ORR polarization curves for N-UCNN-2, NPCN and
Pt/C at 1600 rpm; c) Current-time (I-t) chronoamperometric responses of N-N-
UCNN-2, NPCN, and Pt/C in O2-saturated 0.1 M KOH at -0.15 V; and d)
Current-time (i-t) chronoamperometric responses of N-UCNN-2, NPCN, and
Pt/C (1600 rpm) in O2-saturated 0.1 M KOH at -0.15 V followed by the
introduction of methanol (3 mL) at 100 s.
Based on the values of the disk and ring currents, the H2O2
yield of the N-UCNN-2 is less than 10 % (Figure S10e), while
the NPCN has a relatively higher H2O2 yield (14 %). Those
results demonstrate that the ORR activity of N-UCNN-2 is
superior to that of NPCN. By consideration of the difference of
the morphology between N-UCNN-2 and NPCN, the excellent
ORR performance of N-UCNN-2 could be attributed to the
ultrathin carbon layers with abundant micropores and unique
hierarchical porous structures, which would increase active
surface area and enhance the mass transfer. Furthermore, as
displayed in Figure 3c, N-UCNN-2 exhibits only a slight current
The above results inform us that the evaporation of
crystalline water from zinc nitrate hexahydrate induces the
formation of macroporous carbon networks and the coordination
interaction stabilizes the overall framework during the pyrolysis.
o
However, after heat treatment of the ground product at 150 C,
the walls of networks are still quite thick (around 200 nm) (Figure
S23). To investigate the reason for the formation of ultrathin wall,
the pyrolysis process was monitored by preparing samples at
the different stages, including samples with different calcination
temperature (400, 500, 600, 700, 800, 900, and 1000 oC) without
holding time (named as N-UCNN-2-400-0, -500-0, -600-0, -700-
0, -800-0, -900-0, and -1000-0) and samples calcined at 1000 oC
with different holding time (3 and 6 h) (Shorten as N-UCNN-2-
1000-3, and -1000-6). SEM images of these samples in Figure
S23-S31 indicate that the wall thickness becomes thinner
gradually from 200 nm to 100 nm with increasing the
temperature from 150 to 700 oC possibly because of slow
gasification of organic ligands. Continual increasing the
temperature results in the pronounced reduction of wall
thickness mainly owing to vaporization of zinc atoms and
remaining organic components (the wall thickness decreases to
reduction with 90
%
retention after
a
20000
s
of
chronoamperometry test while Pt/C shows a more current loss
with only 75 % retention, emphasizing the higher long-term
stability for N-UCNN-2. Meanwhile, the N-UCNN-2 also shows
better methanol tolerance than commercial Pt/C (Figure 3d).
Since both N-UCNN-2 and NPCN are N-doped carbon materials,
NPCN exhibits similar ORR stability and methanol tolerance with
N-UCNN-2.
To understand the formation mechanism of N-UCNN-2, a
series of control experiments were carried out. The influence of
molar ratio of raw materials on the formation of carbon
nanosheet networks has suggested the importance of crystalline
water. Therefore, we first investigate the effect of crystalline
water during the preparation of macropoorus carbon networks.
Zn(NO3)2∙6H2O was dehydrated by heating in a flask, quickly
transferred into a glove box filled with N2, and then grinded with
2-equimolar 2-methylimidazole, which was calcined under the
same condition for preparation of N-UCNN-2. It was found only
small amount of bulk powders were obtained finally (Figure S14
o
~12 nm at 900 C). The wall thickness was then slowly reduced
to 3.7 nm after pyrolyzing the sample at 1000 oC for 6 h because
of the loss of residual ingredients, such as zinc and carbon. The
relationship between wall thickness and temperature is shown in
Figure S32, which matches well with the TGA curve of grinded
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mixture of Zn(NO3)2∙6H2O and 2-methylimidazole (Figure S33),
confirming the formation process mentioned above.
where the crystalline water accelerated
interaction between zinc ions and 2-methylimidazole (step a).
The water would evaporate after heating the paste at 150 C to
form the macroporous carbon networks, where the water served
as a pore-filling agent (step b). Carbonization and evaporation of
zinc and other organic components (step c-d) would rapidly
reduce the wall thickness to form the ultrathin carbon nanosheet
networks finally.
the coordination
o
Based on the above results, the formation of ultrathin carbon
nanosheet networks originates from the evaporation of
crystalline water, formation of coordination polymers, and
vaporization of ingredients including zinc, carbon, and hydrogen.
The formation mechanism is depicted in Scheme 2. Firstly, a
paste of zinc- imidazolate coordination polymer was formed after
grinding the mixture of Zn(NO3)2∙6H2O and 2-methylimidazole,
Scheme 2. Proposed formation mechanism of the ultrathin carbon nanosheet networks induced by evaporation of crystalline water, coordination interaction and
vaporization of ingredients (step a: grinding for formation of coordination polymers, step b: water evaporation for formation of macroporous carbon networks, step
c: carbonization, step d: continual evaporation of organic components and zinc for formation of ultrathin carbon nanosheet networks; and SEM images under the
proposed formation mechanism represent the grinded mixture, samples pyrolyzed at 150 oC and 700 oC without holding time, and 1000 oC for 6 h, respectively).
(2-methylimidazole is denoted as Hmim in the Scheme)
The simplicity of the method and the superior electrocatalytic
activity of dual-doping push us to extend this method to prepare
dual-heteroatom-doped carbon networks, such as N/S-co-
doping.40,41 The N/S-co-doped ultrathin carbon nanosheet
network (NS-UCNN) was prepared using the same procedure
for preparation of N-UCNN-2 except the mixture of 2-
methylimidazole and 2-mercaptoimidazole (molar ration is 5:1)
were used instead of pure 2-methylimidazole. NS-UCNN has a
hierarchically porous 3D carbon network interconnected with
ultrathin walls similar with N-UCNN-2 and a high specific surface
area of 1028.7 m2 g-1(Figure 4 and S34). Too much 2-
mercaptoimidazole would produce bulk ZnS and less S-
containing imidazole would decrease the ORR activity (Figure
S35).
The high-resolution N 1s and S 2p spectra, as shown in
Figure 5a and 5b, verify the successful doping of N and S atoms
into the NS-UCNN.42,43 The
S 2p spectrum confirms the
presence of C-SX-C and -C=S- bonds at 163.9 eV and 165.1 eV,
respectively. Besides, the sulfur peaks at 168.2 eV were
attributed to C–SOx–C.44 No signal of Zn is discovered,
indicating that the NS-UCNN was a metal-free carbon material
(Figure S36). The amounts of N and S are 1.5 and 1.4 at%,
respectively. NS-UCNN exhibits remarkable ORR catalytic
performance with a high cathodic peak potential (0.75 V) and
positive ORR onset potential (0.93 V) and half-wave potential
(0.83V), which makes it among the best metal-free ORR
electrocatalysts in the literatures, comparable with Pt/C (Figure
S37a and Figure 5c), and better than other ZIF-derived metal-
containing porous carbons in terms of the onset potential and
half-wave potential (Table S1).10,11,15-19 The electron transfer
number was 3.73-3.97 for NS-UCNN with extremely low H2O2
yield (Figure 5d). NS-UCNN presents an excellent durability with
90 % retention of the current density after a 20000 s of
chronoamperometry test (Figure S37b). The unexpected ORR
activity of NS-UCNN can be attributed to the effective doping of
N and S plus the hierarchically porous structures with ultrathin
walls.
Figure 4. High magnification (a) and low magnification (b) SEM images of NS-
UCNN.
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Materials and chemicals. All chemicals were from commercial and used
without further purification: Zinc nitrate hexahydrate (Zn(NO3)2·6H2O,
98.5%, Sinopharm Chemical Reagent Co., Ltd.), 2-methylimidazole
(99.0%, Aladdin), 2-mercaptoimidazole (99.0%, Energy).
Preparation of N-UCNNs. Typically, the molar ratios of Zn(NO3)2·6H2O
and 2-methylimidazole were 1:1.5, 1:2, and 1:2.5, respectively. The
mixture was ground together in an agate mortar for 5 min. Next, the
precursors were transfer into a quartz boat and placed inside a tube
furnace. The furnace was heated with a heating ramp of 5 °C min-1 and
maintained at 150 °C for 2 h under continuous Ar, then heated to
1000 °C for 6 h. The corresponding samples made from various molar
ratio of 1:1.5, 1:2, and 1:2.5 were named as N-UCNN-1.5, N-UCNN-2,
and N-UCNN-2.5, respectively.
Preparation of NPCN. 0.2 mg ZIF-8 powers, which were prepared
according to the literature,18 were transferred into a quartz boat and
placed inside a tube furnace. The furnace was heated with a heating
ramp of 5 °C min-1 and maintained at 150 °C for 2 h under continuous Ar,
then heated to 1000 °C for 6 h.
Figure 5. a) High-resolution N 1S XPS spectrum of NS-UCNN; b) High-
resolution S 2p XPS spectrum of NS-UCNN; c) ORR polarization curves for
NS-UCNN and Pt/C at 1600 rpm; and
chronoamperometric response of NS-UCNN in O2-saturated 0.1 M KOH at -
0.15V (black) and H2O2 yields of NS-UCNN (red).
d) Current-time (I-t)
Preparation of NS-UCNN. Typically, the molar ratios of Zn(NO3)2·6H2O
and the mixture of 2-methylimidazole and 2-mercaptoimidazole were 1:2.
The molar ratios of 2-methylimidazole and 2-mercaptoimidazole were 1:1,
3:1, 5:1, 7:1, respectively. The mixture was ground together in an agate
mortar for 5 min. Next, the precursors were transfer into a quartz boat
and placed in a tube furnace. The furnace was heated with a heating
ramp of 5 °C min-1 and maintained at 150 °C for 2 h under continuous Ar,
then heated to 1000 °C for 6 h. The corresponding sample made from
molar ratio of 5:1 was named as NS-UCNN.
Conclusions
We have successfully developed an operationally simple,
general, and sustainable strategy to prepare 3D heteroatom-
doped ultrathin carbon nanosheet networks as ORR
electrocatalysts via direct pyrolysis of the ground mixture of raw
materials. We propose that the evaporation of crystalline water
and coordination interaction are responsible for the formation of
3D carbon networks. The method can be generalized to prepare
different heteroatom-doped carbon networks, such as N and/or
S-doping. The heteroatom-doped ultrathin carbon nanosheet
networks, especially the N/S-co-doped one, show outstanding
ORR catalytic performance with more positive onset potential
(0.93 V) and a half-wave potential (0.83V) comparable with that
of commercial Pt/C, and better methanol tolerance and long-
term stability. The excellent activity can be ascribed to the
effective doping with N and S, ultrathin carbon layers with highly
microporosity, and unique hierarchical porous structures, which
would increase electrocatalytic active sites and enhance the
mass transfer. The strategy is very simple and green, which
could extremely simplify the process and thus reduce the total
cost, and alleviate the damage to the nature. Considering the
convenience, generalizability, and special structure, this method
has the potential to replace the current MOF-route to prepare a
series of heteroatom-doping carbons, single atom catalysts,
noble metal/transition metal-based carbon, and so on. This work
not only provides highly active electrocatalysts and also
demonstrates a facile and sustainable strategy to prepare 3D
carbon nanosheet networks for various applications using solely
crystalline water as the pore-forming agent.
Electrocatalytic Performance Investigations. All the electrochemical
measurements were conducted in a typical three electrode setup with an
electrolyte solution of 0.1 M KOH, a working electrode, a Pt counter
electrode, and a saturated calomel electrode (SCE) reference electrode.
In all measurements, the SCE reference electrode was calibrated with
respect to reversible hydrogen electrode (RHE). LSV measurements
were conducted in 0.1 M KOH with scan rate of 5 mV s-1. All the
potentials reported in our work were vs. the reversible hydrogen
electrode (RHE). E(RHE) = E(SCE) + 0.059lgpH+0.244. The onset
potential (Eonset) for ORR is defined as the critical potential where the
reduction current density reaches 1% of the limiting current density. The
working electrodes were prepared as follow: Catalyst ink was prepared
by dispersing 4 mg of the catalyst into 1 mL of ethanol solvent containing
50 µL of 5 wt% Nafion through sonication for 90 min. Then 6.4 µL of the
catalyst ink was loaded onto a glass-carbon electrode (GCE) of 4 mm in
diameter.
Characterization. The morphology of all electrocatalysts was
investigated by scanning electron microscope (SEM, Hitachi, S-4800)
and transmission electron microscope (TEM, FEI Tecnai G20). The size
and thickness of electrocatalysts were determined by atomic force
microscope (AFM, Brucker Bioscope system). The crystal structures of
the samples were characterized using powder X-ray diffraction (XRD,
Bruker D8 Advance diffractometer, Cu Kα1). The X-ray photoelectron
spectroscopy (XPS) analysis was performed on an ESCALAB 250Xi X-
ray photoelectron spectrometer using Mg as the excitation source. The
nitrogen adsorption- desorption isotherms were measured at 77 K with a
JW-BK200C (JWGB SCI&TECH) gas adsorption device.
.
Acknowledgements
Experimental Section
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10.1002/cctc.201800890
ChemCatChem
FULL PAPER
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10.1002/cctc.201800890
ChemCatChem
FULL PAPER
Entry for the Table of Contents
FULL PAPER
3D highly porous heteroatom-doped
ultrathin carbon nanosheet networks
have been synthesized directly
Tingting Liu,† Shi Feng,† Jia Huo,* Qiling
Li, Chao Xie, and Shuangyin Wang*
starting from solid raw materials
without any additional template. The
ultrathin structure, hierarchical
porosity, and heteroatom-doping
endow as-prepared ultrathin carbon
nanosheet networks with outstanding
eletrocatalytic ORR performance.
Page No. – Page No.
Crystalline-water/Coordination
Induced Formation of 3D Highly
Porous Heteroatom-doped Ultrathin
Carbon Nanosheet Networks for
Oxygen Reduction Reaction
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