Selectively doping pyridinic and pyrrolic nitrogen into a 3D porous carbon matrix through template-induced edge engineering: Enhanced catalytic activity towards the oxygen reduction reaction

Article abstract of DOI:10.1039/c7ta07608h

Developing cost-effective and highly efficient oxygen reduction electrocatalysts, such as non-precious metal and metal-free catalysts, is undoubtedly crucial for the commercialization of lowerature fuel cells. Here, edge-rich nitrogen doped porous carbon catalysts for the oxygen reduction reaction (ORR) with a high proportion of pyridinic and pyrrolic N (up to 94%) were synthesized by an in situ released CO₂ activation method, using glucose and melamine as precursors and nano-CaCO₃ as the template. The catalysts exhibit a three-dimensional structure, hierarchical pores and large pore volumes. Benefiting from the increased active site density and structural advantage, the optimized catalyst shows excellent ORR activity with a half-wave potential of 0.853 V and long-term stability in alkaline media, which is among the best for metal-free catalysts reported to date.

Full text of DOI:10.1039/c7ta07608h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Selectively doping pyridinic and pyrrolic nitrogen into 3D porous
carbon matrix through template-induced edge engineering:
enhanced catalytic activity towards oxygen reduction reaction
Received 00th January 20xx,
Accepted 00th January 20xx
Ergui Luo,abc Meiling Xiao,ac Junjie Ge,*ac Changpeng Liu*ac and Wei Xing*acd
DOI: 10.1039/x0xx00000x
Developing cost-effective and highly efficient oxygen reduction electrocatalysts, such as non-precious metal and metal-
free catalysts, is undoubtedly crucial for the commercialization of low-temperature fuel cells. Here, edge-rich nitrogen
doped porous carbon catalyst for oxygen reduction reaction (ORR) with high proportion of pyridinic and pyrrolic N (up to
94%) was synthesized by an in-situ released CO2 activation method, using glucose and melamine as precursors, nano-
CaCO3 as template. The catalysts exhibit three-dimensional structure, hierarchical pores and large pore volumes.
Benefiting from the increased active site density and structural advantage, the optimized catalyst shows excellent ORR
activity with half-wave potential of 0.853 V and long-term stability in alkaline media, which is among the best in all
reported metal-free catalysts.
www.rsc.org/
structures are always located on exposed edge planes and are more
active towards ORR.21-24 Recently, Wei and co-workers synthesized
nitrogen-doped graphene with high yield of planar N using a space-
Introduction
The lack of high-performance and low-cost electrocatalysts for the
sluggish oxygen reduction reaction (ORR) is still the main obstacle
for the wide commercial application of fuel cells.1-3 Currently,
platinum and its alloys are remain as the most efficient ORR
catalysts, which suffer severely from the problems including scarcity,
high-cost and poor stability.4,5 Extensive studies have been
conducted to search for non-precious metal2, 3, 6-8and metal-free9-13
alternatives to noble metal catalysts for ORR. Among metal-free
catalysts, nitrogen-doped carbon materials have attracted much
attention owing to their intrinsic low cost, superior electrical
conductivity, excellent durability, and environmental friendliness.14-
confinement method as a metal-free catalyst which manifested
outstanding ORR activities.21 However, the reported strategy for the
preferred formation of planar N species was time-consuming and
complicated, which is not suitable for large-scale production. Apart
from nitrogen species, the pore structure of nitrogen-doped carbon
catalyst is another critical factor in fuel cells. Interconnected
hierarchical pores are believed to promote the mass transportation,
resulting in enhanced ORR activity.17, 25-27 To date, it remains a great
challenge to develop suitable strategies to maximize the exposure of
active nitrogen atoms as well as optimize the porosity of carbon
matrix.
17
It has been confirmed by both experimental studies9,15and
In this work, we present the design and fabrication of nitrogen-
doped carbon materials with high yield of pyridinic and pyrrolic N,
as efficient metal-free ORR electrocatalysts. The ultrathin
nanosheets assembled sponge-like nitrogen-doped porous carbon
(denoted as SNPC-X, where X represents the pyrolysis temperature
(°C)) was obtained through simple carbonization of a mixture of
melamine and glucose with nano-CaCO3 acting as template (Scheme
S1). Layered graphitic carbon nitride (g-C3N4) was formed during the
pyrolysis of melamine, which acted as a sacrificial template to direct
the formation of carbon nanosheets.28 In addition, CO2 released
from thermal decomposition of hard template contributed to the
exposure of more edges and efficient utilization of nitrogen atoms
(see the following XPS analysis).29 This synthetic strategy avoided
cumbersome processes and costly raw materials, making it
potentially applicable towards the facile fabrication of high-
performance N-doped carbon-based ORR catalysts.
theoretical calculations18-20that the ORR activity of nitrogen-doped
carbon materials is originated from the charge delocalization of the
carbon atoms due to nitrogen incorporation, which facilitates
oxygen adsorption and reduction. The activity was reported to be
highly dependent on the doping level and types of nitrogen atoms.
Previous studies revealed that pyridinic N and pyrrolic N with planar
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The electrochemical performances were conducted using the 750E
Bipotentiostat (CH Instruments ). SNPC catalyst ink was prepared by
ultrasonically dispersing 5 mg of SNPC catalyst in a suspension
containing 50μL Nafion (5wt%) solution and 950μL ethanol, while
Pt/C catalyst (20wt% Pt on Vulcan XC-72 carbon, Johnson Matthey)
ink was prepared by dispersing 5 mg catalyst ultrasonically in a
mixture solution containing 50μl Nafion (5wt%) solution, 550μL
isopropanol and 400μL Milli-Q water. The catalyst film coated
electrode was obtained by dispersing the catalyst ink on a glassy
carbon rotating disk electrode or rotating ring disk electrode
followed by drying in air. The catalyst loadings on RDE and RRDE in
all cases were 0.5 mg cm-2 except for Pt/C (30μgPt cm-2). A
conventional three-electrode cell was employed incorporating a
saturated calomel electrode (SCE) as the reference electrode, a Pt
foil as the counter electrode and the catalyst film coated RDE or
RRDE as the working electrode.
Experimental
Chemicals and materials
a-D-Glucose was purchased from Beijing Chemical Works. Nano-
CaCO3 (40-80 nm) was provided by Nanjing XFNano Company. High-
purity potassium hydroxide and melamine were purchased from
Aladdin Company. 5 wt% Nafion ionomer was obtained from Sigma-
Aldrich. Commercial state-of-the-art 20 wt% Pt/C (Johnson Matthey
Company, HiSPEC 3000) was used as the benchmark for comparison.
Ethanol and HCl were purchased from Beijing Chemical Works. All
the chemicals were used as delivered without further treatment.
Ultrapure water (Millipore, 18.23 MΩ cm) was used throughout all
experiments.
Catalyst preparation
In a typical synthesis, nano-CaCO3, glucose and melamine with a
mass ratio of 5:5:2 were first dissolved or dispersed into a mixture
of ultrapure water and alcohol (volume ratio of 2:1), and stirred at
room temperature for 5h. The solvents were evaporated at 85°C
The electrolyte was 0.1 M KOH solution for ORR test. RDE
measurements were conducted by liner sweep voltammetry (LSV)
from 1.1 V to 0.2 V at a scan rate of 10 mV s-1 at 1600rpm, while the
ring electrode of RRDE was held at 1.3 V vs. RHE. The H2O2
under vigorous stirring to obtain
a
homogeneous precursor.
pyrolysis and
Subsequently, the composite underwent
a
collection coefficient at the ring in RRDE experiments was 0.37, as
4-/3-
carbonization process under flowing argon at 700-1000℃ for 2h
with a heating rate of 5℃/min. The hard templates were leached
out in diluted HCl solution for 24h to isolate N-doped porous carbon
(SNPC-X). Control samples PC-X and NC-X were prepared using a
similar procedure, but in the absence of melamine and template,
respectively. The carbonization product of only glucose was labeled
as C-X. The same pyrolysis protocol of precursors without glucose
did not leave any carbonized products. The SNPC-SiO2 was prepared
at 900℃ with silica template instead of nano-CaCO3. Silica
nanoparticles (100-160 nm) were synthesized by the Stöber
method30. Samples obtained at 900℃ with nano-CaCO3/glucose
mass ratios of 1:5 and 3:5 were labeled as SNPC-900-0.2 and SNPC-
900-0.6, respectively.
measured using a Fe (CN)
redox couple. All the ORR currents
6
presented in the figures are faradaic currents, i.e. after correction
for the capacitive current and have already been normalized to the
electrode surface area. The following equations were used to
calculate n (the apparent number of electrons transferred during
ORR) and % H2O2 (the percentage of H2O2 released during ORR).
4ID
(1)
n =
ID + (IR / N)
2IR / N
(2)
ID + (IR / N)
%H2O2 =100
where ID is the faradaic current at the disk, IR the faradaic current at
the ring and N is the H2O2 collection coefficient at the ring. The
apparent number of electrons transferred for ORR was also
determined by the Koutechy-Levich equation given blow:
Material characterization
Scanning electron microscopy (SEM) measurements were
performed with an XL 30 ESEMFEG field emission scanning electron
microscope. Transmission electron microscopy (TEM), high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) and element mapping analysis were conducted on
Philips TECNAI G2 electron microscope operating at 200 kV. Power
X-ray diffraction (XRD) measurements were performed on PW1700
diffractometer (Philips Co.) with a Cu Kα (λ=0.15405 nm) radiation
source operating at 40 kV and 30 mA. Raman spectra were collected
on a J-Y T64000 Raman spectrometer with 514.5 nm wavelength
incident laser light. The textural and morphological features of the
various carbon supports and catalysts prepared were determined by
nitrogen physisorption at 77 K in a Micromeritics ASAP 2020. X-ray
photoelectron spectroscopy (XPS) measurements were carried out
on Mg Kα radiation source (Kratos XSAM-800 spectrometer). The
binding energy was calibrated against the C 1s line. Thermo
gravimetric analysis (TGA) was performed with a Pyres 1 TGA
apparatus (Perkin Elmer, MA), with a heating rate of 5 °C min−1 from
50 to 1000 °C in Ar atmosphere.
1
1
1
1
1
(3)
=
+
=
+
J
JL JK
B
ω1/ 2 JK
1
6
B = 0.62nFCO (D0 )2 / 3ν −
(4)
where J is the measured current density, JK is the kinetic current
density, JL is the diffusion limited current density, ω is the electrode
rotation rate, F is Faraday constant (96485 C mol-1), C0 is the bulk
concentration of O2 (1.2×10-3 mol L-1), D0 is the diffusion coefficient
of O2 (1.9×10-5 cm2 s-1) and ν is the kinetic viscosity of the
electrolyte (0.01 cm2 s-1)31. In all figures, the potentials were
converted to values versus the reversible hydrogen electrode (RHE).
The conversion from SCE to RHE is done by measuring the voltage
ΔE between the SCE and a Pt-black coated Pt wire immersed in the
same electrolyte saturated with H2. The measured ΔE was 0.998 V
in 0.1 M KOH solution. The accelerated durability tests (ADTs) were
performed at room temperature in O2-saturated 0.1 M KOH
solution by applying cyclic potential sweeps between 0.6 and 1.0 V
versus RHE at a sweep rate of 200 mV s-1 for 10000 cycles.
Electrochemical measurements
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Results and discussion
Representative scanning electron microscopy (SEM) and
transmission electron microscopy (TEM) images at different
magnifications are shown in Fig. 1, revealing the formation of
sponge-like structure composed of ultrathin nanosheets and non-
uniform pores of the final SNPC-900 catalyst. STEM image and
associated elemental mapping image (Fig. 1f) reveal the
homogeneous distribution of nitrogen element in carbon matrix.
Fig. 2 Desorption isotherms of (a) SNPC-900 and (b) NC-900 samples; The
corresponding pore size distribution curves calculated from the desorption
branches by utilizing (c) nonlocal density functional theory (NLDFT) and (d)
Barrett-Joyner-Halenda (BJH) method.
(Fig. 2c-d) further confirms the microporosity. By introducing
template, the SNPC-900 exhibits lower specific surface area but
larger total pore volume than NC-900. As mentioned above, the
released CO2 activating gas can enlarge or connect up pores,
resulting in fewer micropores in SNPC-900, which play the greatest
role in evaluating specific surface area. Meanwhile, abundant meso-
and macropores increase the total pore volume. All these
morphology and structural features (3D structure, hierarchical pores
and large pore volume) can provide SNPC-900 more contact
interfaces, facilitating the transfer of electrolyte ions and electrons,
thereby improving its catalytic activity.
Fig.
1 (a) SEM image; (b-c) TEM images; (d) scanning transmission
microscopy (STEM) image and (e–g) elemental mappings of SNPC-900.
Raman spectra (Fig. 3a) show the typical D-band at roughly 1327
cm−1 and G-band at about 1586cm−1, resulting from the disordered
and sp2-hybridized graphitic carbon atoms, respectively. The ratio of
integrated intensities ID/IG is found to decrease with increasing
pyrolysis temperature, indicating that higher degree of
graphitization is obtained for SNPC-X at higher temperatures.
Similar results can be obtained from the powder X-ray diffraction
(XRD) patterns (Fig. 3b) with a peak at ca.26.5° corresponding to the
Nevertheless, no obvious flake structure can be observed for the
C-900 and PC-900 samples (Fig. S1), indicating an indispensible role
of melamine in the growth of carbon nanosheets. Furthermore,
three-dimensional structure and much thinner nanosheets are
found for the SNPC-900 compared with NC-900 (Fig. S1c, f),
suggesting that nano-CaCO3 template may prevent the nanosheets
from aggregation to some extent.
We further investigate the porous structure and specific surface (002) planes of the graphitic carbon. Moreover, SNPC-900 exhibits a
areas of samples by N2 adsorption-desorption technique. The SNPC- higher ID/IG ratio than NC-900, which can be ascribed to the
900 shows a type-
Ⅳ
isotherm with a distinct hysteresis loop at a increased edges in the carbon matrix.
relative pressure
P
/P0 of 0.4-1 (Fig. 2a), indicating the existence of
X-ray photoelectron spectroscopy (XPS) analysis confirms the
lots of mesopores. The BET specific surface area is measured to be successful introduction of nitrogen atoms into the carbon-based
621.4 m2/g with total pore volume of 2.2 cm3/g. Pore size materials (Fig. 3c). As the pyrolysis temperature increases from 700
distribution analysis confirms the hierarchical porous structure with to 1000
℃, total nitrogen content of the SNPC-X reduces
micropore (1.2 nm) by DFT method as well as meso- and significantly from 10.18 at% to 2.22 at% due to the loss of unstable
macropores by BJH method (Fig. 2c-d). Specifically, the pore size nitrogen (Table S1). The atomic contents of N dopant are estimated
distribution of SNPC-900 (20-180 nm) is much wider than particle to be 5.39 at% and 2.13 at% for SNPC-900 and NC-900, respectively.
size distribution of nano-CaCO3 template (40-80 nm), which can be Meanwhile, high resolution N 1s spectra (Fig. 3d-e) are further
attributed to: i) the CaCO3 template decomposition is accompanied deconvoluted into three main signals corresponding to pyridinic N
with releasing of CO2, which further reacts with carbon atoms (398.2±0.1 eV), pyrrolic N (399.6±0.2 eV) and quaternary N (400.9
according to the equation: CO2+C
→
2CO.29 This effect enlarges the eV). The relative contents of quaternary N and pyridinic N increase
pores or creates new pores; ii) some meso- and macropores come with elevating pyrolysis temperature, whereas pyrrolic N decreases
from micropores that connected up by CO2 activating agent. For (Table S1). It is worth noting that SNPC-900 possesses a much higher
comparison, the NC-900 obtained without template exhibits typical
type-I isotherm characteristics for only microporous materials with
specific surface area of 1029 m2/g and total pore volume of 0.56
cm3/g (Fig. 2b).The pore size distribution analysis
percentage of pyridinic and pyrrolic N (94 %), which exhibit higher
electrocatalytic activity for ORR, compared with NC-900 (44.5 %), as
shown in Fig. 3d-e and Table S1, suggesting that nano-CaCO3
template can effectively promote the N doping as well as alter the
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SNPC-900-0.2, SNPC-900-0.6 and SNPC-900 which were prepared
with different amount of nano-CaCO3. As shown in Fig. 3f, total
nitrogen atomic content as well as pyridinic N and pyrrolic N
percentages are positively associated with the nano-CaCO3/ glucose
mass ratio, while it is opposite for the quaternary N percentage. All
these results strongly reveal the critical role of nano-CaCO3
template to promote the preferred formation of pyridinic and
pyrrolic N.
The catalytic performance of the resulting materials towards ORR
was then assessed on a three-electrode electrochemical station by
rotating disk electrode (RDE) voltammetry in 0.1 M KOH solution,
and compared against that of a state-of-the-art commercial Pt/C
catalyst (Johnson Matthey Company, Pt/C, 20 wt%). Results are
shown in Fig. 4a-b. The catalytic performances of samples in terms
of half-wave potential and limiting current density are gradually
enhanced by increasing the nano-CaCO3/glucose mass ratio, and all
the SNPC-X catalysts show superior performances to NC-900 and
SNPC-SiO2. These are ascribed to high doping levels of pyridinic and
pyrrolic N species of the SNPC-Xs, thus providing sufficient reactive
sites, as well as the advanced pore structures to facilitate mass
transfer. Especially, the SNPC-900 manifests the most positive half-
wave potential of 0.853 V, just 15mV lower than that of commercial
Pt/C catalyst, and exhibits one of the highest ORR electrocatalytic
activities compared with other heteroatom-doped carbon catalysts
reported in the literatures32-36. However, relatively poor catalytic
performance is observed for the SNPC-700 which possesses the
Fig. 3 (a) Raman spectra for the different catalysts; (b) XRD patterns of each
sample; (c) XPS survey spectra of different catalysts; (d-e) High resolution N
1s spectra of SNPC-900 and NC-900; (f) Total nitrogen atomic contents and N
species percentages of NC-900, SNPC-900-0.2, SNPC-900-0.6 and SNPC-900.
N doping types. Considering pyridinic N and pyrrolic N are always
located on exposed edge planes, we conclude that nano-CaCO3
templates play an important role in promoting edge exposure,
which is in consistence with the Raman analysis result mentioned
above. The mechanism can also be explained by “CO2 inner-
activation effect’’. During the carbonization process, CO2 gas
released from CaCO3 template reacted with carbon atoms during its
diffusion, forming abundant exposed-edges. Nitrogen atoms were
readily anchored to these defective edge sites, resulting in more
pyridinic and pyrrolic
N species, as illustrated in Scheme 1.
Moreover, because of the loss of carbon atoms, total N doping level
is elevated. Thermal gravity analysis (TGA) of nano-CaCO3 and XRD
pattern of carbonization product before HCl washing further
confirm the decomposition of CaCO3 (Fig. S2). To highlight the
unique role of nano-CaCO3 template, we also synthesized SNPC-SiO2
with silica template instead for comparison. As anticipated, SNPC-
SiO2 (Fig. S3f) is observed to have almost the same N content as NC-
900 and much lower percentage of pyridinic and pyrrolic N (25 %).
Further comparative studies were conducted on NC-900,
Fig. 4 ORR performance for the synthesized catalysts. (a) RDE polarization
curves of nitrogen-doped carbon catalysts prepared at 900℃ and Pt/C in O2-
saturated 0.1 M KOH with scan rate of 10 mV s-1 and rotation speed of 1600
rpm; (b) RDE polarization curves of C-900, PC-900 and catalysts prepared at
different temperatures; (c) LSVs for oxygen reduction on SNPC-900 in O2-
saturated 0.1M KOH at various rotation speeds with scan rate of 10 mV s-1.
Inset: K–L plots at different potentials; (d) The H2O2 yielding and
corresponding electron transfer number for SNPC-900 in 0.1 M KOH. ORR
polarization plots of (e) SNPC-900 and (f) Pt/C before and after 10000
potential cycles in O2-saturated 0.1 M KOH.
Scheme 1 Schematic view revealing the selectively formation of planar
edge-N in the SNPC catalyst.
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measurement (Fig. 4d) with peroxide yield calculated to be less than
5% and n ranges from 3.9 to 4.0 at potentials ranging from 0.2 to 0.8
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Conclusions
In summary, highly pyridinic and pyrrolic nitrogen doped porous
carbon assembled of thin nanosheets was fabricated by a simple
one-pot, scalable method and acted as an excellent electrocatalyst
for ORR. Both melamine and template were proven to play vital
roles in the formation of final carbon nanostructure. The ultrahigh
doping level of planar exposed-edge N was induced by CO2
activation from nano-CaCO3 template. The outstanding ORR
catalytic activity and superior stability were ascribed to the
increased active N species and advanced pore structure, which
facilitates mass transportation and prevents structural collapse. This
work may give researchers great inspirations in low-cost and large-
scale production of porous carbon-based catalysts for fuel cells,
even extended fields of water splitting, adsorbents and
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Acknowledgements
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The work is supported by the National Natural Science Foundation
of China (21433003, 21633008), National Science and Technology
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J. Name., 2013, 00, 1-3 | 5
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