Nitrogen-doped carbon nanosheets and nanoflowers with holey mesopores for efficient oxygen reduction catalysis

Article abstract of DOI:10.1039/c8ta02319k

Efficient structure optimization is one of the key factors for improving the oxygen reduction reaction (ORR) catalytic performance of carbon materials. This paper describes a dual-template method for fabrication of new carbon materials as ORR catalysts, including N-doped carbon nanosheets and nanoflowers with holey mesopores, by employing a polystyrene-b-poly(ethylene oxide) block copolymer as the pore-forming agent, layered double hydroxide (LDH) nanosheets or nanoflowers as the sacrificial morphology-directing template, and m-phenylenediamine as the carbon precursor. The resultant carbon materials possess a honeycomb-like mesoporous structure with similar nitrogen contents of ～4 wt%, an average pore size of ～14 nm and a specific surface area of ～260 m² g^-1. Due to the presence of the holey mesopores that facilitate mass transfer and may shorten the diffusion distance of O₂ molecules to the active sites, the nanosheets and the nanoflowers exhibit excellent electrocatalytic performance when serving as metal-free ORR catalysts in basic media with high half-wave-potentials (+0.80 V) and limiting current densities (5.5 mA cm^-2) which surpass those of many reported carbon-based materials with much higher surface areas but without holey pores. Moreover, the porous nanoflowers show better electrocatalytic activity than that of the nanosheets, profiting from their 3D structure that can prevent the blockage of partial holey pores caused by the preferential layer-by-layer stacking of the nanosheets.

Full text of DOI:10.1039/c8ta02319k

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Nitrogen-Doped Carbon Nanosheets and Nanoflowers with Holey
Mesopores for Efficient Oxygen Reduction Catalysis
Hao Tiana, Nan Wanga, Fugui Xua, Pengfei Zhanga, Dan Houa, Yiyong Maia*, and Xinliang Fengb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Efficient structure optimization is one of key factors for improving oxygen reduction reaction (ORR) catalytic performance
of carbon materials. This paper describes a dual-template method for fabrication of new carbon materials for ORR
catalysts, including N-doped carbon nanosheets and nanoflowers with holey mesopores, by employing polystyrene-b-
poly(ethylene oxide) block copolymer as the pore-forming agent, layered double hydroxide (LDH) nanosheets or
nanoflowers as the sacrificial morphology-directing template, and m-phenylenediamine as the carbon precusor. The
resultant carbon materials possess a honeycomb-like mesoporous structure with similar nitrogen content of 4 wt%,
average pore size of 14 nm and specific surface area of 260 m2 g-1. Due to the presence of the holey mesopores that
facilitate mass transfer and may shorten the diffusion distance of O2 molecules to the active sites, the nanosheets and the
nanoflowers exhibit excellent electrocatalytic performance when serving as metal-free ORR catalysts in basic media, with
high half-wave-potentials (+0.80 V) and limiting current densities (5.5 mA cm-2), which surpass those of many reported
carbon-based materials with much higher surface areas but without holey pores. Moreover, the porous nanoflowers
shows better electrocatalytic activity than that of the nanosheets, profiting from their 3D structure that can prevent the
blockage of partial holey pores caused by the preferential layer-by-layer stacking of the nanosheets.
www.rsc.org/
Moreover, the dimensionality of materials also affects their
ORR performance due to the different packing mode in the
electrode. Two-dimensional (2D) mesoporous carbon materials
Introduction
The oxygen reduction reaction (ORR) is a key step for fuel
cells, though its sluggish kinetics limits the energy conversion
efficiency.1,2 Conventionally, the mainstream choice of
catalytic materials for ORR is Pt metal, but its large-scale
production is hampered by high cost, limited resource, and
inferior durability.3-5 The past few decades have witnessed
enormous efforts devoted to solving these problems, among
which rational design of cost-effective alternatives, such as
heteroatom-doped (e.g. N atoms) metal-free carbon catalysts, is
of particular interest.6-12, which can improve O2 adsorption and
lead to the enhancement of ORR performance.13-15
The ORR performance of heteroatom-doped carbonaceous
catalysts is also governed by their mesostructures, which may
determine the accessibility of internal active sites.16, 17 Many
studies have focused on tailoring the mesostructures of carbon
materials in order to efficiently use their active sites and thus to
improve their ORR performance.18-20 For example, mesopores
in carbon materials can reserve electrolyte and reactants, and
thereby shorten their diffusion distances to the active sites.21-23
are promising candidates for ORR applications, as 2D structure
facilitates the exposure of active sites.24-31 Currently, graphene
template-assisted method is a primary strategy towards 2D
porous carbon materials, which results in sandwich-like
structures.24-26 Pyrolysis of some biomass materials followed by
KOH activation is another approach for the synthesis of 2D
mesoporous carbon nanosheets in spite of its multistep
procedure and uncontrollable mesopore structure.27-28 In
addition, there was only one case regarding the fabrication of
2D mesoporous carbon nanosheets by using active-salt-
templating.32 Nevertheless, most of the reported 2D
mesoporous carbon nanosheets for ORR application were scant
of holey pores (i.e. through holes),33-35 which may facilitate
mass transfer and shorten the diffusion distance of oxygen
molecules to the interior active sites of the nanomaterials, thus
improving ORR performance (see Figure S1). Therefore, these
materials usually exhibit unsatisfactory ORR performance. To
solve this problem, introduction of holey pores in 2D carbon
materials is highly desirable. However, it has remained a great
challenge as it is difficult to find appropriate templates.
Self-assembly of block copolymers (BCPs) in solution has
a. School of Chemistry and Chemical Engineering, Shanghai Key Laboratory of
Electrical Insulation and Thermal Ageing, Shanghai Jiao Tong University, 800
Dongchuan RD, Shanghai 200240, China
proven to be a facile approach for building up diverse ordered
37
nanostructures,36,
which are ideal soft templates for the
*E-mail: mai@sjtu.edu.cn
construction of mesopouous materials with tailorable pore
structure/size.38-41 Layered double hydroxide (LDH)
nanosheets, which are a unique type of 2D inorganic
b.Department of Chemistry and Food Chemistry, Technische Universität Dresden,
Mommsenstrasse 4, 01062 Dresden, Germany
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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Route A:
Co-assembly
Polymerization
Carbonization
LDH nanosheet
2D PS133-b-PEO114/PmPD
NMHCSs
composite
PEO corona
PS core
Self-assembly
PS-b-PEO
in solution
Route B:
Polymerization
Carbonization
Co-assembly
LDH nanoflower
PS133-b-PEO114/PmPD
NMCFs
nanoflower
Figure 1. Schematic illustration of the dual-template self-assembly strategy towards the holey mesoporos carbon nanosheets (NMHCSs, Route A) and
the holey mesoporos carbon nanoflowers (NMCFs, Route B). During the polymerization of mPD, the LDH templates were etched by the released acid.
nanomaterials with a hexagonal structure, can act as desired in- Figure S1). This study not only develops a dual-template
situ sacrificial templates as they are easy to dissolve in acidic approach for the synthesis of new carbon nanomaterials with
43
solutions.42,
In the present work, by combining the high ORR catalytic activity, but also unveils the influence of
advantages of LDH and BCP self-assembly, we developed a packing mode on the ORR performance of porous carbon
facile dual-template approach for the construction of 2D N- nanosheets, which is helpful for the design of novel porous
doped carbon nanosheets with holey mesopores, using materials as potential ORR catalysts.
polystyrene-b-poly(ethylene oxide) (PS-b-PEO) spherical
micelles as the pore-building soft template, hexagonal LDH
nanosheets as the 2D sacrificial template, and m-
Results and discussion
phenylenediamine (mPD) as the carbon precursor (Figure 1).
The synergistic co-assembly of the block copolymer micelles,
The PS133-b-PEO114 copolymer (the subscript denotes the
degree of polymerization) with a narrow molecular weight
LDH and mPD and the subsequent polymerization of mPD
polydispersity index (PDI) of 1.05 is employed as the soft
yielded a 2D PS-b-PEO/PmPD nanocomposite. Pyrolysis of the
template in the present work. The synthesis procedures and
characterizations are given in the ESI (see experimental section
and Figure S2).48
PS-b-PEO/PmPD nanocomposite at 800 oC under inert
atmosphere produced N-doped mesoporous hexagonal carbon
nanosheets with holey pores (NMHCSs). The nitrogen content
of NMHCSs is 4 wt%, while their average pore size and
specific surface area are 14 nm and 256 m2 g-1, respectively.
Benefitting from the presence of the holey pores, NMHCSs
exhibit high electrocatalytic activity when severing as ORR
catalyst in basic media. Their half-wave-potential (E1/2) and
limiting current density (JL) are 0.77 V and 5.0 mA cm-2,
respectively, which are superior to those of many reported 2D
carbon-based materials of much higher surface areas (>400 m2
g-1) but lack of holey pores.26, 27, 44-47
To understand the effect of the packing mode of holey
porous carbon nanosheets in electrodes on their ORR
performance, we also synthesized 3D N-doped carbon
nanoflowers (NMCFs) consisting of holey mesoporous carbon
nanosheets by employing the dual-template method, however,
with LDH nanoflowers as the sacrificial templates (Figure 1).
Compared with NMHCSs, NMCFs exhibit better
electrocatalytic activity (E1/2 = 0.80 V, JL = 5.5 mA cm-2) and
excellent stability. The improved activity is profiting from the
3D packing of holey porous nanosheets, which may prevent the
unavoidable blockage of partial holey pores caused by the
preferential layer-by-layer stacking of the nanosheets (see
The Co2+-Ni2+ derived LDH nanosheets and Ni2+-Mn3+
derived nanoflowers were fabricated by a co-precipitation
method, which involved employing different metal ions as the
precursors and hexamethylenetetramine (HMT) as a hydrolysis
agent (experimental section in the ESI).49,50 Scanning electron
microscopy (SEM) images show hexagonal LDH nanosheets
with diameters of 2-5 μm (Figure S3a). The thickness of the
LDH nanosheets, measured from atomic force microscopy
(AFM) height profile, is about 20 nm (Figure S3b). X-ray
diffraction (XRD) pattern (Figure S3c) of the Co2+-Ni2+ derived
LDH nanosheets show typical signals attributed to brucite LDH
crystals.49 On the other hand, the resultant LDH nanoflowers
possess diameters of 2-5 μm, which consist of numerous thin
2D petals with an average thickness of 30 nm (Figure S4a,b).
Typical signals from hydrotalcite crystals are observed in the
XRD curve of the Ni2+-Mn3+ derived LDH nanoflowers (Figure
S4c), indicating their hydrotalcite crystalline structure.50 The
fabrication procedures of NMHCSs and NMCFs are illustrated
in Figure 1 (see ESI for detail procedures). First, spherical
micelles with an average diameter of 18 ± 1 nm were
constructed by the self-assembly of PS133-b-PEO114 in a
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Table 1. Structural parameters, chemical compositions and electrocatalytic activity of the porous carbon materials in the present work.
c
e
Surface Areab
(m2 g-1)
PBJH
Cd
Nd
Od
E1/2
JLe
electron transfer
a
PTEM
Samples
(nm)
(nm)
(wt %)
(wt %)
(wt %)
(V)
(mA cm-2
number (n) f
)
NMHCSs
NCSs
16 ± 1
256
232
266
14
91.4
91.7
91.4
4.2
4.2
4.4
4.4
4.1
4.2
0.77
0.67
0.80
5.0
3.0
3.7
3.9
4
4.0
-
NMCFs
15 ± 2
15
5.5
a
Average pore sizes obtained from TEM images. b Calculated from nitrogen adsorption–desorption isotherms. cAverage pore sizes calculated by the BJH
method based on the the adsorption branches of the isotherms. d The contents of C , N and O elements measured by XPS. e Half-wave potential (E1/2) and
limiting current intensity (JL) obtained from Figure 5b. f Electron transfer number (n) calculated from the RRDE results.
water/ethanol/tetrahydrofuran mixture (Figure S5a). After leading to the formation of holey pores after the elimination of
mixed with mPD monomers and LDH template, the micelles the LDH and micelle templates. The LDH templates were
could be adsorbed and closely packed on both surfaces of LDH gradually etched by the hydrion released during the mPD
via the electrostatic and H-bonding interactions,38,41 (TEM polymerization (to be proved below), yielding sheet-like or
images in Figure S5b,c). Meanwhile, the mPD molecules could flower-like PS133-b-PEO114/PmPD composites. Finally, the
be adsorbed in the PEO coronae of the micelles through H- carbonization of the composites along with the removal of the
bonding, which converted to PmPD network after PS133-b-PEO114 micelle template was conducted by thermal
o
polymerization. Driven by an attractive force produced during treatment at 800 C under N2 atmosphere, producing NMHCSs
the polymerization, the mPD molecules would transfer from or NMCFs with holey mesoporous structure. As a control
both the top and bottom of the micelles to the empty space of sample, N-doped carbon nanosheets without holey pores,
38,41
neighboring micelles.
Thus, the micelle top and bottom denoted as NCSs, were prepared under the similar experimental
were vacant of PmPD moiety after the polymerization,38,41 conditions by employing LDH nanosheets as the 2D template
but without the addition of the copolymer.
Before carbonization, the morphology of the 2D PS133-b-
a
c
b
PEO114/PmPD composites, which are the precursor of
NMHCSs, was examined by SEM. Figure 2a presents a typical
SEM image of the 2D PS133-b-PEO114/PmPD composites with
spherical micelles uniformly packed on the surfaces. XRD
pattern demonstrates that the diffraction peaks attributed to 2D
crystalline LDH completely vanish after the polymerization
(Figure 2b), indicating the disappearance of crystalline structure
of LDH during polymerization process. Moreover, inductively
coupled plasma atomic emission spectrometry (ICP-AES)
measurement did not detect any metal ions from LDH in the 2D
PS133-b-PEO114/PmPD composites, demonstrating the complete
removal of the LDH template during the polymerization of
mPD. Thermogravimetric analysis (TGA) reveals that the
carbon yield of the PS133-b-PEO114/PmPD nanocomposites at
nitrogen atmosphere remained nearly constant at ca. 25 wt%
200nm
d
50nm
500nm
200nm
o
over 800 C (Figure S6). Thereby, the nanocomposites were
e
treated at 800 oC under nitrogen atmosphere for complete
f
carbonization.
After carbonization, SEM and transmission electron
microscopy (TEM) micrographs show that the hexagonal sheet-
like structure perfectly retained for the resultant NMHCSs,
indicating their pronounced thermal stability (Figure 2c,d and
Figure S7a). Closely-packed spherical pores are observed on
NMHCSs, and their average diameter is 16 ± 1 nm (Figure 2c,d
and Table 1). Electron diffraction (ED) pattern shows that there
are no diffraction signals attributed to LDH or its derived
metal/metal oxides in NMHCSs (inset of Figure 2d). XRD
pattern of NMHCSs shows two characteristic diffraction peaks
at 2θ of about 22 and 44°, which are attributed to amorphous
200nm
Figure 2. (a) SEM image of the 2D PS-b-PEO/PmPD nanocomposite; (b)
XRD patterns of the Co2+-Ni2+ derived LDH nanosheet, the 2D PS-b-
PEO/PmPD composite and NMHCSs; (c) SEM image of NMHCSs, the
inset shows a high-magnification SEM image; (d) TEM image of
NMHCSs, the inset shows the ED pattern; (e) AFM height image of
NMHCSs; (f) Nitrogen sorption-desorption isotherm of NMHCSs, the
inset gives the pore size distribution.
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carbon (Figure 2b). These results further confirm the complete thickness of NCSs is measured to be ca. 30 nm by VAieFwMArti(cFleiOgnulirnee
elimination of the LDH template and the corresponding metal S8c), which is much larger than the mesDoOpoI:r1e0.1s0iz3e9,/Ci8nTdAi0c2a3ti1n9gK
ions. AFM profile gives a 35 nm thickness for NMHCSs that most of the mesopores are embedded in NCSs and thus are
(Figure 2e), which is nearly twice the diameter (18 nm) of the not holey pores.
spherical micelles, manifesting that NMHCSs consist of
double-layer mesoporous carbon nanosheets. The double-layer
structure was formed due to the association of the two single
layers on both sides of a LDH nanosheet after the LDH
removal. Moreover, as the top and bottom of the copolymer
micelles were short of PmPD after the polymerization of mPD,
the removal of the LDH sheet and the micelles resulted in the
formation of holey mesopores in the double-layer carbon
nanosheets. NMHCSs shows a type-IV nitrogen adsorption-
desorption isotherm (Figure 2f), which is a characteristics of a
mesoporous structure. Based on the Brunauer-Emmett-Teller
(BET) protocol, the specific surface area of NMHCSs is
calculated to be 256 m2 g-1 (Table 1). The pore size distribution
is centered at about 14 nm (inset of Figure 2f), supporting the
SEM and TEM results.
Figure 4. Characterizations of structural and chemical nature of NMHCSs,
NMCFs and NCSs. (a) Raman spectra; (b) XPS spectra; (c-e) high-resolution
N1s spectra for NMHCSs (c), NMCFs (d), NCSs (e); (d) weight percentages of
the four nitrogen species in NMHCSs, NMCFs and NCSs.
The NMCFs with a flower-like structure were characterized
by methods similar to those applied to NMHCSs. Before
carbonization, SEM images show that PS133-b-PEO114 spherical
micelles closely-packed on the nanoflower composites (Figure
3a). XRD (Figure 3b) and ICP-AES measurements did not
detect the signals attributed to LDH in the PS-b-PEO/PmPD
nanoflower composites, demonstrating the complete removal of
the LDH template and the corresponding metal ions. Through
the thermal treatment, the composites were carbonized to
NMCFs that consist of numerous holey mesoporous carbon
petals bearing an average pore size of 15 ± 2 nm (Figure 3c,d
and Figure S7b). The average thickness of the petals, measured
Figure 3. (a) SEM image of the PS-b-PEO/PmPD nanoflower composite;
(b) XRD patterns of the Ni2+-Mn3+ derived LDH nanoflower, the PS-b-
from TEM images, is about 40 nm (Figure 3e), which is twice
the average diameter (18 nm) of the spherical micelles,
demonstrating a double-layer structure of the petals. The
nitrogen adsorption-desorption isotherm demonstrates the
mesoporous structure for NMCFs (Figure 3f). The uptake at the
high relative pressures (P/P0 > 0.9) suggests the presence of
macropores, which derive from the 3D packing of flower-like
nanostructures.51 The BET specific surface area of NMCFs is
266 m2 g-1 and its average pore size is 15 nm (Table 1), which
are basically identical to those of NMHCSs owing to the
utilization of the same copolymer template.
PEO/PmPD composite and NMCFs; (c) SEM image of NMCFs; (d, e) TEM
image of NMCFs, the inset shows the SEAD pattern; (f) Nitrogen
sorption-desorption isotherm of NMCFs, the inset gives the pore size
distribution.
The control sample, NCSs, also possesses a hexagonal sheet-
like structure but lack of obvious closely-arranged spherical
mesopores (TEM image in Figure S8a). Nitrogen adsorption-
desorption isotherm reveals a mesoporous structure for NCSs
(Figure S8b), and their average pore size and specific surface
area are 4 nm and 232 m2 g-1, respectively (Table 1). Note that
the mesopores in NCSs were formed during the carbonization
due to the partial degradation of the PmPD polymer. The
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Raman spectra of NMHCSs, NMCFs and NCSs show two adsorption and act as the active sites for ORR13-14. Apparently,
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strong signals at 1345 and 1585 cm-1 (Figure 4a), which NMHCSs, NMCFs and NCSs have similDarOIw: 1e0i.g10h3t9p/Ce8rcTeAn0t2a3g19eKs
correspond to the D (disordered) and G (graphitized) bands of for each type of N atoms, which suggests that these carbon
carbon materials. The IG/ID values of these carbon materials materials possess similar active site contents. The high angle
were calculated to be ca. 0.83, suggesting their high degree of annular dark field scanning transmission electron microscope
graphitization. X-Ray photoelectron spectroscopy (XPS) was (HAADF-STEM) images and the corresponding elemental
employed to probe the chemical nature of these N-doped mapping of the carbon materials show that the active sites
carbon materials. There were three pronounced peaks at 285, uniformly distribute in the catalysts (Figure S9).
401 and 529 eV for all samples, which may be assigned to C 1s,
With the similar chemical nature but different structural
N 1s, and O 1s, respectively (Figure 4b). The different carbon parameters, the ORR performance of the carbon materials was
materials possess similar carbon (~91.5 wt%), nitrogen (~4.2 evaluated. Their catalytic activity was initially tested by cyclic
wt%), and oxygen contents (~4.3 wt%), respectively (Table 1). voltammogram (CV) at a scan rate of 100 mVꢀs−1 in a 0.1 M
High resolution N 1s spectra are deconvoluted into four peaks KOH electrolyte (Figure 5a). For all of the three samples, well-
centered at 398.2, 399.8, 400.9 and 404.1 eV (Figure 4c-e), defined ORR peaks were observed in O2-saturated electrolyte,
which can be assigned to pyridinic nitrogen (N1), pyrrolic indicating their distinct ORR activity. Then, the comparison of
nitrogen (N2), graphitic nitrogen (N3) and –N+-O- (N4), the ORR activity of the different carbon materials was
respectively. The contents of these types of nitrogen atoms conducted by rotating disk electrode (RDE) analysis (Figure
were calculated based on the N 1s spectra and shown in Figure 5b). The linear sweep voltammetry (LSV) results reveal that
4f. N1 and N2 are believed to affect the charge density of NMCFs exhibits a much higher half-wave potential (E1/2 = 0.8
adjacent carbon atoms, which can generally improve oxygen V vs. RHE) and a larger limiting current density (JL = 5.5 mA
NMHCSs
a
b
NMCFs
NCSs
c
e
d
f
NMHCSs
NMCFs
Figure 5. ORR performance of NMHCSs, NMCFs and NCSs in 0.1 M KOH electrolyte. (a) CV curves in N2 or O2-saturated KOH solution at a scan rate of
100 mV s-1; (b) RDE plots of NMHCSs, NMCFs, NCSs and a commercial Pt/C catalyst at a scan rate of 10 mV s-1 and a rotation rate of 1600 rpm; (c) K–L
plots obtained from RDE measurements at 0.2 V vs. RHE; (d) electron transfer number (n) and the H2O2 yield; (e) durability comparison of NMHCSs,
NMCFs, NCSs and the commercial Pt/C catalyst at 0.40 V and a rotation speed of 1600 rpm; (f) chronoamperometric responses of NMHCSs, NMCFs and
the commercial Pt/C at 0.40 V and a rotation rate of 1600 rpm, the arrow stresses the addition of 2 % (v/v) methanol at 300 s.
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cm-2). These values surpass those of many N-doped carbon active sites buried in the packing space of NMHCSs can be
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catalysts (Table S1) and are very close to that of the more efficiently used. Nevertheless, it iDsOsI:ti1l0l.1i0n3e9v/Cit8aTbAl0e23th19aKt
commercial Pt/C catalyst (E1/2 = 0.81 V, JL = 5.0 mA cm-2).52,53 partial holey pores will be blocked after the stacking of the
In comparison, E1/2 and JL of NMHCSs are 0.77 V and 5.0 mA porous carbon nanosheets (Figure S1b). Thus, the 3D packing
cm-2, respectively. Although lower than those of NMCFs, these fashion of holey porous carbon nanosheets in NMCFs may
values still apparently surpass those of NCSs without holey avoid this problem such that much more active sites can be
pores (E1/2 = 0.67 V, JL = 4.0 mA cm-2) as well as those of accessible by oxygen and electrolyte, which results in the best
numerous reported N-containing carbon nanosheets of ORR performance of NMCFs compared with those of
apparently higher specific surface areas (>400 m2 g-1) but lack NMHCSs and NCSs.
of holey pores (Table S1), highlighting the contribution of the
holey pores on the improvement of the electrocatalytic activity.
In addition, both NMCFs and NMHCSs possess a wide plateau
Conclusion
of limiting current density in the polarization curve (Figure 5b),
This study developed a facile dual-template strategy for the
indicating a diffusion-controlled xprocess in ORR, which
synthesis of 2D and 3D N-doped carbon nanomaterials with
benefits from the presence of holey pores. RDE measurements
holey mesopores, by combining the advantages of block
copolymer self-assembly and in-situ sacrificial LDH template.
The resultant porous carbon nanosheets and nanoflowers
exhibit high ORR catalytic activity comparable to that of the
at a scan rate of 10 mV s-1 and various rotating speeds (400 –
2025 rpm) were also carried out (Figure S10a-c), and the
kinetic parameters were analyzed by utilizing the Koutecky–
Levich (K-L) equation (see Page S5). The K-L plots display
commercial Pt/C thanks to the presence of the holey pores,
excellent linearity (Figure 5c), indicating a first-order reaction
which may shorten the diffusion distance of O2 to active sites.
kinetics for the three different carbon materials. Tafel plots give
Furthermore, the 3D nanoflowers show better ORR
similar slopes for NMHCSs and NMCFs, suggesting a similar
electrocatalytic activity, profiting from the 3D packing of holey
ORR mechanism occurred in these catalysts (Figure S10d).
porous carbon nanosheets, which may avoid the inevitable
The rotating ring-disk electrode (RRDE) technique was
blockage of partial holey pores caused by the layer-by-layer
applied to determine the electron transfer number (n) and the
yield of H2O2 (Figure 5d). Clearly, NMCFs have the highest n
value (3.9) in the voltage window of 0.2−0.7 V, which is close
to that of the commercial Pt/C. Meanwhile, the H2O2 yield by
stacking of porous 2D structures. The dual-template strategy is
promising in the synthesis of a variety of holey porous
materials for energy-related potential applications.
NMCFs is reduced to 8%. In contrast, NMHCSs possess a
slightly smaller n value (3.7), whereas NCSs exhibit the lowest Conflicts of interest
n value (3) and their H2O2 yield is as high as 50%.
Durability and methanol tolerance of the catalyst is another
There are no conflicts to declare.
major concern in ORR, which was investigated by the current–
time (i–t) chronoamperometry testing (Figure 5e). NMHCSs
and NMCFs exhibit a slow current decrease with a ~94 % high
Acknowledgements
This work was supported by the Natural Science Foundation of
retention over 4-hour continuous operation, which is better than
China (51573091, 21774076, 21320102006 and 91527304),
that of Pt/C (88% after 4 hours). In sharp contrast, NCSs show
Program of the Shanghai Committee of Science and
a faster current loss with only 60 % retention under the similar
Technology (17JC1403200), and Program of Shanghai Eastern
condition. On the other hand, the catalytic selectivity of
Scholar.
NMHCSs and NMCFs is tested by adding 2 vol.% methanol to
evaluate their methanol tolerance (Figure 5f). The current
density for NMHCSs and NMCFs shows negligible decay,
whereas the current on Pt/C suffers from a sharp loss,
demonstrating the excellent methanol tolerance of NMHCSs
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Journal of Materials Chemistry A
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DOI: 10.1039/C8TA02319K
Nitrogen-Doped Carbon Nanosheets and Nanoflowers with Holey Mesopores
for Efficient Oxygen Reduction Catalysis
Hao Tian, Nan Wang, Fugui Xu, Pengfei Zhang, Dan Hou, Yiyong Mai*, and Xinliang Feng
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