From covalent triazine-based frameworks to N-doped porous carbon/reduced graphene oxide nanosheets: Efficient electrocatalysts for oxygen reduction

Article abstract of DOI:10.1039/c7ta07387a

By selecting 4,4′-dicyanobiphenyl (DCBP) as a building block, porous covalent triazine-based frameworks (CTFs) incorporating pyridinic N only have been deliberately fabricated. Upon pyrolysis, the CTF-templated N-doped and hierarchically porous carbons (NHCs) exhibit high surface area, adjustable pore structure and well-controlled doping of pyridinic and graphitic N species. The above CTFs have been rationally grown onto highly conductive reduced graphene oxide (rGO) to afford CTF/rGO hybrids. The subsequent pyrolysis gives sandwich-like NHC/rGO composites, which synergistically integrate respective advantages of both the components of NHC and rGO, thus offering excellent electrocatalytic performance for the oxygen reduction reaction (ORR). Significantly, the resultant NHC/rGO-950 exhibits a more positive onset and half-wave potential, higher diffusion-limited current density and better long-term stability than the state-of-the-art Pt/C and is among the best of previously reported metal-free electrocatalysts, in alkaline solution. Such rGO-templated NHCs based on CTFs offer a promising strategy to design highly efficient metal-free ORR electrocatalysts for fuel cells.

Full text of DOI:10.1039/c7ta07387a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

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From Covalent Triazine-based Framework to N-doped Porous
Carbon/Reduced Graphene Oxide Nanosheets: Efficient
Electrocatalysts for Oxygen Reduction
Received 00th January 20xx,
Accepted 00th January 20xx
†
,a
†,a
b
a
a
a
b
DOI: 10.1039/x0xx00000x
Long Jiao, Yingli Hu, Huanxin Ju, Chunde Wang, Min-Rui Gao, Qing Yang, Junfa Zhu, Shu-
a
*,a
Hong Yu and Hai-Long Jiang
www.rsc.org/
By selecting 4,4’-dicyanobiphenyl (DCBP) as a building block, porous covalent triazine-based frameworks (CTFs)
incorporating pyridinic N only have been deliberately fabricated. Upon pyrolysis, the CTF-templated N-doped and
hierarchically porous carbons (NHCs) exhibit high surface area, adjustable pore structure and well-controlled doping of
pyridinic and graphitic N species. The above CTFs have been rationally grown onto highly conductive reduced graphene
oxide (rGO) to afford CTF/rGO hybrids. The subsequent pyrolysis gives sandwich-like NHC/rGO composites, which,
synergistically integrate respective advantages of both components of NHC and rGO, thus offering excellent
electrocatalytic performance for oxygen reduction reaction (ORR). Significantly, the resultant NHC/rGO-950 exhibits a
more positive onset and half-wave potential, higher diffusion-limited current density and better long-term stability than
the state-of-the-art Pt/C and is among the best of previously reported metal-free electrocatalysts, in alkaline solution.
Such rGO-templated NHC based on CTFs offers a promising strategy to the design of highly efficient metal-free ORR
electrocatalysts for fuel cells.
incorporation of nonmetal heteroatoms (e.g. N, B, S, etc.) has
been well demonstrated to greatly improve the ORR
Introduction
[33-42]
performance of carbon materials in alkaline media.
With increasing concerns on global energy crisis, great efforts
have been made to develop energy conversion and storage
devices including fuel cells and metal-air batteries, in which
efficient electrocatalysts for the oxygen reduction reaction
Recently, much effort has been devoted on modulating the
charge density of carbons around heteroatoms to enhance the
[
33-42]
electrochemical performance.
In addition to tailoring the
electronic structure, the exposure of active sites through the
fabrication of porous nanostructure and enhancement of
electronic conductivity can further improve the ORR
(
ORR) are crucial to achieve optimal performance of these
[1-13]
devices.
Currently, Pt and its alloys are the most effective
electrocatalysts employed for the ORR, but they suffer from
[43]
[3,4,14-21]
performance.
high cost, poor stability, etc.
Much endeavor has been
To dope carbon-based materials, nitrogen is one ideal
candidate with its slightly smaller atomic radius to carbon but
much larger electronegativity. Nitrogen induces charge
delocalization of carbon and results in non-electroneutrality of
the catalysts, which are recognized as crucial factors for the
devoted to the exploration of Pt alternatives including metal
[
22-28]
oxides or sulfides
and metal/nitrogen co-doped carbon
particularly, the latter of which are
[29-32]
(M/N-C) materials,
very promising in both acidic and alkaline electrolytes.
Unfortunately, metal-based catalysts usually suffer from
dissolution and agglomeration more or less during the ORR
[36,44-49]
enhancement of oxygen adsorption and reduction.
The
[4,16,24]
precise control of nitrogen configurations is of prime
importance for the doping. It has been demonstrated that
both pyridinic and graphitic N species are effective at
process, leading to deteriorated activity.
In contrast,
metal-free porous carbon-based materials are inexpensive,
well accessible, corrosion resistant and eco-friendly. The
[45-47]
promoting ORR but pyrrolic N species is not.
The ORR
performance of carbon-based catalysts is also significantly
affected by the surface area and porosity, which govern the
[30,37]
exposure of active sites and mass transfer efficiency.
In
addition, the electric conductivity, regulated by the degree of
2
sp bonding of carbon materials, is essential for electron
[24,50,51]
transfer in electrocatalysts.
The introduction of foreign
nitrogen sources into carbon precursors often fails in the
[52-55]
control of active N species and pore structure,
and the
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generation of desired configuration of N sites in the resultant CTF/rGO composites were further converted to N-doped and
[18]
carbon materials is extremely challenging.
hierarchically porous carbon/rGO (denoted as NHC/rGO-T; T
We decided to tackle the above mentioned challenges by represents pyrolysis temperature) composites (Scheme 1). The
constructing N-doped porous carbon ORR electrocatalysts with abundant pyridinic N atoms in the CTF precursor result in only
dominant pyridinic and graphitic N incorporation via a single active pyridinic and graphitic N sites in NHC/rGO-T. Moreover,
porous precursor, covalent triazine-based frameworks (CTFs). our NHC/rGO-T features high surface area and optimized
CTFs feature tunable N doping and predictable 2D porous hierarchically porous structure, which are beneficial to the
structures through trimerization reaction by selecting exposure of active sites and fast diffusion of electrolytes and
appropriate aromatic nitriles as monomers, and they can be
2
O . Meanwhile, the close contact between NHC and rGO in
further interconnected to afford 3D porous networks with NHC/rGO-T leads to a consecutive conductive network, which
[56-63]
tunable pore structures.
The designable structure of is of great importance for electron transfer, and the rGO-
porous CTFs involving pyridinic N only makes them ideal induced thin layered structure further shortens the diffusion
precursors to afford N-doped porous carbons with inherited distance. We believe the above characters synergistically
active N species and optimized structure for ORR. However, contribute to the enhancement of ORR activity. Remarkably,
the electrical conductivity of CTFs and their derived carbon the resultant NHC/rGO-950 composite exhibits higher ORR
materials are commonly unsatisfied due to low degree of performance than the commercial benchmark Pt/C catalyst in
graphitization, which significantly limits the electrochemical 0.1 M KOH solution, including catalytic activity, long-term
[64,65]
application of CTF-based materials.
Reduced graphene stability and methanol tolerance.
oxide (rGO), which can be obtained from the reduction of
graphene oxide (GO), possesses a two-dimensional (2D) sheet-
like structure with superior electrical conductivity and
Experimental
[23,66-69]
excellent chemical stability.
The unique 2D structure,
Materials
high conductivity and oxygen-enriched groups on the surface
make rGO a promising candidate to serve as suitable template
and conductive substrate to induce the oriented growth of
CTF-based materials, enhancing their electrochemical
performance. For example, Su et al. have combined graphene
and porous polyaryltriazine-derived frameworks into a high-
All chemicals were purchased from commercial sources and
2
used without further treatment: zinc chloride (ZnCl ,
Sinopharm Chemical Reagent Co., Ltd., 98%), 4,4’-
dicyanobiphenyl (DCBP, Jinan Henghua Sci. & Tec. Co., Ltd.,
9
9%), graphene oxide (GO, XF Nano, 99.5%), hydrazine hydrate
[69]
(N ·xH O, Sinopharm Chemical Reagent Co., Ltd., 85%),
2
H
4
2
performance cathodes for lithium-ion batteries. Therefore,
it is highly desirable and would be feasible to develop an
effective method to couple CTF-based materials with rGO that
permits synergistically enhanced ORR performance.
sodium dodecyl sulfate (SDS, Sinopharm Chemical Reagent Co.,
Ltd., 86%), sodium (Na, Sinopharm Chemical Reagent Co., Ltd.,
9
9.5%), anhydrous tetrahydrofuran (THF, Sinopharm Chemical
Reagent Co., Ltd., 99.5%, anhydrous), acrylonitrile (AN, Chem
Greatwall, 99%), pyrrole (py, Aladdin industrial corporation,
In this work, by choosing 4,4’-dicyanobiphenyl (DCBP) as a
building block and polyacrylonitrile decorated rGO (PAN/rGO)
as a template, hierarchically porous CTFs were readily grown
on both sides of PAN/rGO nanosheets to afford sandwich-type
CTF/rGO composites, via an in situ “bottom-up” trimerization
9
9%), ammonium persulfate ((NH
Reagent Co., Ltd., 98%), ethanol (CH
Chemical Reagent Co., Ltd., 99.7%), methanol (CH
4
)
2
S
2
O
8
, Sinopharm Chemical
CH OH, Sinopharm
OH,
3
2
3
Sinopharm Chemical Reagent Co., Ltd., 99.5%), hydrogen
chloride (HCl, Sinopharm Chemical Reagent Co., Ltd., 37%),
potassium hydroxide (KOH, Sinopharm Chemical Reagent Co.,
Ltd., 85%). De-ionized water with the specific resistance of
of DCBP. Following facile pyrolysis under N atmosphere, the
2
1
8.25 MΩ·cm was obtained by reversed osmosis followed by
ion-exchange and filtration (Cleaned Water Treatment Co., Ltd.,
Hefei).
Synthesis of rGO
Graphene oxide (GO, 250 mg) were added into a 250 mL round
2
bottom flask with H O (125 mL), and subjected to ultrasonic
treatment for 2 h. Then, sodium dodecyl sulfate (1.0 g) and
hydrazine hydrate (3 mL) were added into above GO
dispersion, and reacted at 80 °C for 12 h with stirring. After the
reduction reaction, the mixture was treated by freeze-drying.
The freeze-dried solid was subsequently filtrated and washed
by water and ethanol for several times. Finally, the sample was
dried at 60 °C in vacuum to give the rGO.
Scheme 1 Illustration of rational stepwise fabrication of sandwich-like NHC/rGO-T
layered composite.
2
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Synthesis of PAN/rGO
electrode (RRDE) measurements were carried out to
determine the four-electron selectivity. The disk electrode was
scanned at a rate of 10 mV/s, and the ring electrode potential
was set to 1.2 V vs. RHE. All data are presented without iR
compensation.
rGO (80 mg) and sodium (0.05 g) were subjected to ultrasonic
treatment in anhydrous tetrahydrofuran (THF, 10 mL), thus
forming rGO polyanion salts (rGONa). Acrylonitrile (0.64 mL)
was added, followed by reaction at room temperature for 12 h.
The polymerization took place on the graphene surfaces,
resulting in an intimately mixed composite system. The
polymerization was terminated by adding methanol. The black
precipitation was centrifuged several times with THF followed
by vacuum drying at 60 °C overnight to obtain PAN/rGO as a
black powder.
The catalyst ink was prepared by dispersing 2 mg of catalyst
into 1 mL of ethanol containing 10 μL of 5 wt% Nafion and
sonicated for 30 min. Then 28 μL of the catalyst ink was loaded
onto a GCE of 5 mm diameter (loading amount: ~0.28
2
mg/cm ). For comparison, Pt/C (20 wt% platinum) was
conducted on the same electrochemical tests with a catalyst
2
loading of 0.1 mg/cm .
The mass activity J
m
is derived from Equation:
= J
k
is the kinetic current at 0.8 V and mcat represents the
Synthesis of pure CTFs and CTF/rGO composite
J
m
k
/mcat
In a typical experiment, DCBP monomer (200 mg), and a
where J
2
certain amount of ZnCl (1.4 g, 2.0 g or 2.6 g) were mixed
mass of the catalyst. mcat represents the total amount of
catalyst for all prepared samples. It is hard to calculate the real
active sites/composition because there is still no reliable
technique to evaluate the accurate amount of sites in N-doped
carbon system. Therefore, we calculated the mass activity of
the catalyst loaded onto the GCE, replacing the mass activity of
active site in catalysts.
together and transferred into a quartz tube under an inert
atmosphere. The quartz tube was then evacuated, sealed, and
heated to 700 °C for 24 h in the muffle furnace (it took 5 h
from RT to 700 °C). After the quartz tube was cooled down to
room temperature and opened, the resulting CTFs were
obtained after washing several times with 1M HCl. Then the
samples were activated by refluxing in 1 M HCl and ethanol,
respectively, for 24 h. The CTF/rGO was synthesized via the
same procedure except for the addition of PAN/rGO (20 mg)
Rotating ring-disk electrode (RRDE) measurements were
carried out to determine the four-electron selectivity. The disk
electrode was scanned at a rate of 10 mV/s, and the ring
electrode potential was set to 1.2 V vs. RHE.
2
and a fixed amount of ZnCl (2.0 g).
Nyquist plots of the catalysts were all obtained by applying a
sine wave with amplitude of 5.0 mV over the frequency range
Synthesis of NHC/rGO-T and pure NHC-950
Typically, 20 mg of activated sample (CTF/rGO) was loaded 100 kHz to 0.01 Hz at an overpotential of 0.5 V in O
into tube furnace, and subsequently heated under an N atmosphere.
atmosphere. The sample was thermally treated at a heating
2
2
rate of 5 °C/min up to 850, 950 or 1050 °C and held at this Characterization
temperature for 2 hours. Thereafter, it was cooled down to
Powder X-ray diffraction patterns (PXRD) were collected on a
TM
Japan Rigaku SmartLab rotation anode X-ray diffractometer
room temperature to yield the final product. NHC-950 was
obtained following a similar procedure by replacing NHC/rGO-
equipped with graphite monochromatized Cu Kα radiation (λ =
950 with CTF-10.
1.54 Å). The transmission electron microscopy (TEM) and high-
resolution TEM were acquired on JEM-2010, and elemental
mapping was measured on JEOL ARM-200F with an electron
acceleration energy of 200 kV. Field-emission scanning
electron microscopy (FE-SEM) was carried out with a field
emission scanning electron microanalyzer (Zeiss Supra 40
scanning electron microscope at an acceleration voltage of 5
kV). Nitrogen sorption measurement was conducted using a
Micromeritics ASAP 2020 system at 77 K. Prior to nitrogen
adsorption/desorption measurement, the samples were dried
overnight at 160 °C under vacuum. X-ray photoelectron
spectroscopy (XPS) measurements were performed by an
ESCALAB 250 high-performance electron spectrometer using
monochromatized Al Kα (hν = 1486.7 eV) as the excitation
Synthesis of pyrrolic-N-NHC/rGO-950
Polypyrrole (Ppy) was firstly sythesized. In brief, pyrrole (py)
monomer (1 ml) was disolved in the mixture of ethonol (50 ml)
and water (50 ml) at 0-5 °C. 0.7 M (NH
20 ml) was then droped into the above py solution and stirred
for 24 h. Ppy was finally obtain by filtation. Then, DCBP
monomer (100 mg), Ppy (67 mg) and ZnCl (2.0 g) were mixed
4 2 2 8
) S O queous solution
(
2
together and transferred into a quartz tube under an inert
atmosphere. The latter heat treatment processes are similar to
the synthesis of NHC/rGO-950.
Electrochemical measurements
Electrochemical measurements were performed with a CHI source and at the Catalysis and Surface Science Endstation at
60E electrochemical analyzer (CH Instruments, Inc., Shanghai) the BL11U beamline in the National Synchrotron Radiation
7
and a rotating disk electrode (RDE) (Pine Instruments, Grove Laboratory (NSRL) in Hefei, China. The core level spectra of N
city, PA). All electrochemical measurements were conducted in 1s were recorded with a VG SCIENTA R4000 electron energy
a typical three-electrode setup with a graphite rod counter analyzer and using a monochromatic Al Kα X-ray source (hυ =
electrode and Hg/HgO reference electrode. LSV measurements 1486.7 eV) at normal incidence. The binding energy were
were conducted with scan rate of 5 mV/s. Rotating ring-disk calibrated using the C 1s from graphene oxide nanosheets
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located at 284.8 eV as the reference. The spectral S1†). Starting from free-standing sheets of PAN/rGO (Fig. 1a
decomposition was performed using the XPS Peak 41 program and S2†), the CTF/rGO composite based on an opꢀmized mass
with Gaussian functions after subtraction of
background.
a
Shirley ratio (DCBP : ZnCl
2
=1:10) has been fabricated with a sandwich-
like layered structure, in which both sides of rGO sheets are
Raman scattering spectra were recorded with a Renishaw covered by porous CTFs (Fig. 1b and S3†). This is, as far as we
+
System 2000 spectrometer using the 514.5 nm line of Ar for know, the first rational assembly of CTF/rGO sheet-like
excitation. Fourier transform infrared (FT-IR) spectra were composite based on rGO template for electrocatalysis.
measured using a Nicolet 8700 FTIR E.S.D. and KBr pellet
samples. The sample for FT-IR test was obtained by stopping temperatures under N
the trimerization process of DCBP at 450 °C. Elemental analysis = 850, 950, and 1050 °C). The optimal pyrolysis temperature
The CTF/rGO then underwent pyrolysis at different
2
atmosphere to produce NHC/rGO-T (T
o
(
EA) was performed at VarioELIII analyzer. Nitrogen K-edge was determined to be 950 C, and the resultant NHC/rGO-950
near-edge X-ray absorption fine structure (NEXAFS) inherits sheet-like morphology with sharp and well-defined
spectroscopy were recorded at Soft X-Ray Magnetic Circular edges via the induction effect of 2D rGO template. Scanning
Dichroism Endstation (SXMCD) in total electron yield (TEY) electron microscopy (SEM) image shows that the porous NHC
mode in NSRL. The NEXAFS spectra were analyzed by a least- distributes on both sides of rGO homogeneously. Although
squares fitting using the WinXAS 3.1 program, and the peak possessing similar high BET surface area, the sheet-like
fitting procedure was accomplished with Gaussian functions in morphology of NHC/rGO-950 is quite different from the
conjunction with an arctan function background subtraction.
irregular bulk NHC-950 directly obtained from CTF-10 in the
absence of rGO (Fig. 1c, S4 and S5†). Transmission electron
microscopy (TEM) image clearly shows the existence of
mesopores with sizes of ~5 nm (Fig. 1d, indicated by dashed
yellow circles), which might be inherited from the mesopores
of CTF/rGO composite. Owing to the high porosity of CTF/rGO,
Results and discussion
To obtain relatively large pore sizes/volumes that are required
2
for efficient electrolyte transfer and O diffusion, DCBP, a
2
the BET surface area of NHC/rGO-950 reaches 1344 m /g with
monomer with molecular length of 1.3 nm, has been
deliberately employed to undergo trimerization to give 2D
sheets with 2 nm micropores (Scheme 1), which are
subsequently self-organized and cross-linked into porous 3D
networks with high surface areas. The pore structure can be
hierarchical pore sizes extending to ~80 nm (peak: ~50 nm),
which is similar to that of NHC/rGO-850 and NHC/rGO-1050,
revealing the good inheritance of the pore structure from
CTF/rGO (Fig. 2). The significant pore size distribution in the
range of 4~10 nm for NHC/rGO-950 agrees well with the TEM
observation mentioned above (Fig. S6†). The high-angle
annular dark-field scanning transmission electron microscopy
well tuned by changing the mass ratio of DCBP and ZnCl
2
, of
which the optimized mass ratio (1:10) of DCBP : ZnCl gives the
2
product CTF-10 with maximal pore volume, optimal pore
structure and a very high surface area among all CTFs (Fig.
(HADDF-STEM) and elemental mapping images show the
uniform nitrogen distribution throughout the carbon matrix,
demonstrating the advantage of CTFs in the controllable and
homogeneous N doping (Fig. 1e).
The X-ray photoelectron spectroscopy (XPS) study was
conducted to probe the chemical compositions and the
Fig. 1 SEM images of (a) PAN/rGO, (b) CTF/rGO and (c) NHC/rGO-950. (d) TEM image
and (e) HADDF-STEM image and C and N elemental mapping images of NHC/rGO-950.
The yellow arrows indicate the GO layers and the dashed yellow circles highlight the
pores involved in the structure.
Fig. 2 (a) N
2
sorption isotherms for CTF-10 and CTF/rGO at 77 K and (b) their
sorption
corresponding pore size distributions calculated by the DFT model. (c) N
2
isotherms for NHC/rGO-T at 77 K and (d) the corresponding pore size distributions
calculated by the DFT model.
4
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configurations of N species in NHC/rGO-950. As shown in Fig.
Powder X-ray diffraction (PXRD) pattern of NHC/rGO-950 as
S7, the XPS spectrum demonstrates the existence of C, N and well as NHC shows two prominent diffraction peaks at 26.0°
O. The high-resolution N1s spectrum can be mainly fitted into and 43.7°, corresponding to the (002) and (101) reflections of
two peaks, corresponding to pyridinic N and graphitic N (Fig. graphitized carbon, respectively (Fig. 3c). The existence of
3
a). It is interesting that no pyrrolic N was detected, while it is these two diffraction peaks suggests a relatively high
[70-
present in most other reported N-doped carbon materials.
graphitization degree of NHC/rGO-950 and NHC, which has
7
3]
Similar results can be also obtained for NHC/rGO-850 and been further characterized by Raman spectra. The D and G
-
1
NHC/rGO-1050, as well as NHC-950 directly pyrolyzed from bands at 1332 and 1584 cm correspond to the disordered
2
pure CTF-10 (Fig. 3a, S8 and S9†). This phenomenon is and sp hybridized graphitic carbon, respectively, and the
attributed to the exclusive existence of pyridinic nitrogen in intensity ratio of I
D G
/I is determined by graphitization degree
the structure of CTFs, which can be transformed into pyridinic of carbon. The I /I
D
G
values of NHC/rGO-950 and NHC-950 are
and graphitic nitrogens but not pyrrolic nitrogen during the 1.02 and 1.05, respectively, supporting the PXRD results and
pyrolysis. The very small oxidized-N peak in NHC/rGO-950 suggesting their good graphitization (Fig. 3c and 3d).
might come from reduced graphene oxide. To glean further
Encouraged by the above results, electrocatalytic ORR
experimental evidence on N configurations, N K-edge near- performance of the NHC/rGO catalysts was evaluated. To this
edge X-ray absorption fine structure (NEXAFS) spectroscopy, end, thin films were prepared on glassy carbon electrodes for
which is chemical state sensitive, was employed to investigate cyclic voltammetry (CV) in O -saturated 0.1 M KOH in a
2
NHC/rGO-950 (Fig. 3b). There are two π* peaks at 398.5 and homemade polytetrafluoroethylene (PTFE) electrochemical
4
02 eV, corresponding to pyridinic
N
and graphitic N, cell. As shown in Fig. 4a, no obvious redox peak is observed for
[22]
respectively. The peak at 399.4 eV suggests a well-defined NHC/rGO-950 in N -saturated electrolyte; in contrast, when
2
[74]
structure of C=N. The broaden peak centered at 406 eV is the solution is saturated with O , a well-defined cathodic peak
2
the general transition from the N1s core level to the C-N σ* clearly appears at 0.80 V, highlighting its electrocatalytic ORR
[22,55]
state.
Obviously, no pyrrolic N signal (~400 eV) is activity comparable to that of the benchmark Pt/C catalyst (Fig.
74]
[
detectable. This result is fully consistent with the XPS results S10†). The linear scanning voltammetry (LSV) curves collected
above and further confirms the presence of pyridinic N and on rotating disk electrode (RDE) after the elimination of
graphitic N, but not pyrrolic N species. It has been generally capacitive currents show that NHC/rGO-950 possesses a
accepted that only pyridinic N and graphitic N can behave as prominent catalytic activity with an onset potential of 0.95 V, a
active sites for ORR catalysis. Pyridinic N atoms can bind with half-wave potential of 0.83 V and diffusion-limited current
and activate oxygen molecules, and graphitic N can promote
the electron transfer and facilitate the oxygen dissolution on
75-78]
[
its adjacent carbon atoms.
The absence of inactive
pyrrolic N species in NHC/rGO-950 highlights the advantage of
pyridinic N enriched CTFs as precursors in the controlled
doping of highly active N sites in the carbon materials, which is
significant to enhance ORR activity.
Fig. 4 Electrochemical ORR performance of NHC/rGO-950. (a) CV in N
.1 M KOH solution at a scan rate of 100 mV/s, (b) LSV in O -saturated 0.1 M KOH
solution at a rotation speed of 1600 rpm, (c) LSV at different RDE rotation rates in O
2 2
and O -saturated
0
2
2
-
Fig. 3 (a) The high-resolution N1s XPS spectra of NHC/rGO-950 and NHC-950 (pyrrolic
N: 399.5 eV). (b) N K-edge NEXAFS spectrum of NHC/rGO-950 (pyrrolic N: ~ 400 eV). (c)
Powder XRD patterns and (d) Raman scattering spectra of NHC/rGO-950 and NHC-950.
saturated 0.1 M KOH solution, (d) K-L plots and electron transfer numbers at different
potentials. Peroxide yield with regard to the total oxygen reduction products and the
calculated electron transfer number of (e) NHC/rGO-950 and (f) Pt/C in O
2
-saturated
0
.1 M KOH.
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2
density of -5.64 mA/cm (Fig. 4b), which are better than those
of commercial Pt/C. In addition, CTF-10, CTF/rGO, NHC-950
and CTF/rGO-T (T = 850, 950, 1050) were also carefully
examined for comparison. As expected, NHC/rGO-950 showed
the highest activity among studied catalysts (Fig. 4b and S11†).
Through pore size distribution analysis, elemental analysis, and
electrochemical impedance spectroscopy (EIS), the essential
influences by both pyrolysis temperature and rGO introduction
on the ORR performance are manifested. All NHC/rGO-T show
Fig.
6
(a) Time-dependent current density curves and (b) methanol-tolerance
similar surface area while NHC/rGO-950 presents the largest evaluation of NHC/rGO-950 in 0.1 M KOH solution.
pore volume, most of which is ascribed to the pores with sizes
beyond 40 nm that are favorable for mass transfer (Fig. 2c and
2d). The content of N species, especially the most active
NHC/rGO-950 toward the four-electron reduction of oxygen
with a low H yield at about 4%, which is comparable to that
2 2
O
pyridinic N, in NHC/rGO-950 is comparable to that in
NHC/rGO-850 but larger than that of NHC/rGO-1050, which
accounts for the inferior performance of the latter (Fig. S12
and Table S1†). Meanwhile, compared to pure NHC-950,
NHC/rGO-950 possesses similar N content but a smaller radius
in Nyquist plot, reflecting the higher conductivity of NHC/rGO-
of Pt/C (Fig. 4e and 4f). In addition, the kinetic current density
2
and mass activity for NHC/rGO-950 at 0.8 V are 14.5 mA/cm
and 51.6 mA/mgcat respectively, demonstrating a better kinetic
behavior of NHC/rGO-950 than most of reported carbon-based
catalysts but inferior to Pt-based catalysts (Table S2†).
Moreover, the excellent ORR activity of NHC/rGO-950 can be
further supported by the faster current growth along with the
increase of overpotential and the smaller Tafel slope (74
mV/decade) than Pt/C (80 mV/decade), illustrating the more
favorable ORR kinetics of the former (Fig. S19†). Our studies
suggest that NHC/rGO-950 is among the best of all reported
metal-free carbon-based ORR catalysts (Fig. 5 and Table S2†).
In view of the excellent ORR in alkaline solution, the ORR
950, which results in its much better ORR activity and this also
explains the inferior performance of CTFs materials (Fig. 4b,
S13, S14 and Table S1†). Furthermore, the rGO-induced 2D
lamellar structure of NHC/rGO-950 nanosheets makes it more
favorable to conduct mass transfer than the bulk NHC-950 (Fig.
1c and S4†). The combination of the above characters make
NHC/rGO-950 the best ORR catalyst in our investigation. In
addition, to further clarify the effect of pyrrolic N, pyrrolic-N-
containing NHC/rGO-950 (pyrrolic-N-NHC/rGO-950), with
similar N content, pore size distribution and conductivity to
NHC/rGO-950, has been also prepared (Fig S15-S17†). The
inferior performance of pyrrolic-N-NHC/rGO-950 to NHC/rGO-
4
activity of NHC/rGO-950 in 0.1 M HClO has been also tested.
However, it presents low ORR activity due to the protonation
of the surface N-groups in acidic condition, which is consistent
[93]
with most reported results on N-doped carbon (Fig. S20†).
In addition to high ORR activity, excellent stability and
methanol tolerance are also required for electrocatalysts to be
practically useful for fuel cells. We evaluated the stability of
NHC/rGO-950 at a constant voltage of 0.8 V for 20000 seconds
9
50 clearly exhibits that the absence of pyrrolic N in NHC/rGO-
50 plays significant role on the enhancement of ORR activity
9
(Fig. S18†).
To gain more insight into the reaction kinetics of NHC/rGO-
2
in O -saturated 0.1 M KOH solution. The 97% of the current
9
50, LSV curves were measured under different rotating
density remained, which is higher than that of the commercial
Pt/C (95%) (Fig. 6a). To exclude the possibility of ORR activities
induced by any possible metal contaminants, inductively
coupled plasma-mass spectrometry (ICP-MS) analysis was
conducted on NHC/rGO-950 before and after long-term
electrocatalytic tests. Results show that negligible metal (Fe,
speeds from 400 to 1600 rpm, showing well-defined platform
of diffusion-limited current density (Fig. 4c). The electron
transfer number (n) calculated from the LSV curves according
to Koutechy-Levich (K-L) equation was determined to be 3.91,
suggesting an ideal four-electron pathway of NHC/rGO-950 for
ORR (Fig. 4d). Rotating ring-disk electrode (RRDE) test was
adopted to further demonstrate the high selectivity of the
Co, Pt, Zn) contamination (≤20 ppm) is present in the final
products (Table S3†), revealing that N heteroatoms are
[94]
responsible for ORR catalysis. The washed electrode after
long-term a measurement was restarted the electrocatalytic
test in a fresh portion of 0.1 M KOH (Fig. S21†). The activity
improvement in fresh electrolyte was not observed, revealing
that the above slight deterioration in performance might not
be due to the contamination. Furthermore, the reproducibility
tests for 3 different samples of NHC/rGO-950 on LSV and
stability have been also investigated and the results clearly
show the reliable performance of the optimal NHC/rGO-950
catalyst (Fig. S22†). Moreover, the methanol crossover effect
Fig. 5 Comparison of ORR performance for NHC/rGO-950 with other metal-free
electrocatalysts previously reported in 0.1 M KOH solution based on (a) kinetic current should be considered given that methanol in anode can
density at 0.8 V vs. half-wave potential, and (b) mass activity (calculated by the total sometimes permeate through the polymer membrane to the
[
16,20,40-42,50,54,75,79-92]
mass loading of the catalysts) at 0.8 V vs. half-wave potential.
cathode and seriously lowers the ORR performance of cathode
catalysts in direct methanol fuel cell. Therefore, the tolerance
6
| J. Name., 2012, 00, 1-3
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ARTICLE
of NHC/rGO-950 against methanol poisoning was studied.
After the addition of 3 M methanol, almost no noticeable
current variation was observed for NHC/rGO-950, which is in
stark contrast to the significant decline of Pt/C activity (Fig.
6
7
8
9
1
1
1
1
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2
,
2
6b). These results unambiguously suggest higher stability,
greater durability and much better tolerance to methanol
crossover effect of NHC/rGO-950 than Pt/C.
M. Zhou, H.-L. Wang and S. Guo, Chem. Soc. Rev., 2016, 45
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a
rGO-templated DCBP self-
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highly efficient ORR catalyst. Based on the porous CTF
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precursor judiciously selected, active
N species can be
conveniently controlled in the resultant carbon materials. This
5
Soc., 2013, 135, 7823-7826.
introducing N species in which control of N configurations is
difficult. Thanks to the synergistic effect between CTF-derived
NHC and 2D rGO in terms of high surface area, hierarchical
pores, controllable incorporation of active N species, favorable
mass transfer and high conductivity, the layered NHC/rGO-950
exhibits superb ORR activity and ultrahigh stability as well as
excellent methanol tolerance. To the best of our knowledge,
the overall catalytic performance of NHC/rGO-950 surpasses
the state-of-the-art Pt/C and almost all metal-free catalysts in
1
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2
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We are grateful to Dr. Zhiyong U. Wang at Troy Univ. (USA) for 29 Y. Zhu, B. Zhang, X. Liu, D.-W. Wang and D. S. Su, Angew.
helpful discussions and manuscript refinement. This work is
supported by the NSFC (21371162, 21673213, and 21521001),
the National Research Fund for Fundamental Key Project
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA07387A
Graphical Abstract
Based on a covalent triazine-based framework, N-doped porous carbon/rGO
nanosheets are prepared and exhibit excellent electrocatalytic performance toward
oxygen reduction.