Cobalt/nitrogen co-doped porous carbon nanosheets as highly efficient catalysts for the oxygen reduction reaction in both basic and acidic media

Article abstract of DOI:10.1039/c6ra16539g

Porous carbon materials have been widely developed as catalysts for the oxygen reduction reaction (ORR) under basic conditions but very few under acidic conditions. In this work, two-dimensional (2D) cobalt/nitrogen co-doped porous carbon nanosheets were prepared as catalysts for the ORR under both basic and acidic conditions by using a cobalt porphyrin based 2D conjugated microporous polymer as a precursor. Remarkably, the as-prepared porous carbon nanosheets exhibited excellent electrochemical catalytic performance for the ORR, with a low half-wave potential (E_1/2) at -0.146 V in 0.1 M KOH and 0.54 V in 0.5 M H₂SO₄ (versus Ag/AgCl) as well as a dominant four-electron transfer mechanism (n = 3.8 at -0.28 V in 0.1 M KOH; n = 3.8 at 0.55 V in 0.5 M H₂SO₄). The high catalytic ORR performance can be attributed to the high activity of CoN_x active sites as well as the high specific surface area that derived from the cobalt porphyrin blocks among the conjugated microporous polymer nanosheets. It's believed that this method opens up new avenues for metal/heteroatom co-doped porous carbon materials with promising performance for energy storage and conversion.

Full text of DOI:10.1039/c6ra16539g

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Cobalt/Nitrogen co-Doped Porous Carbon Nanosheets as Highly
Efficient Catalysts for Oxygen Reduction Reaction in Both Basic
and Acidic Media
Received 00th January 20xx,
Accepted 00th January 20xx
‡
a
‡a
a
a
*a
DOI: 10.1039/x0xx00000x
Zongsheng Hou, Chongqing Yang, Wenbei Zhang, Chenbao Lu, Fan Zhang, Xiaodong
Zhuang,
*a,b
www.rsc.org/
Porous carbon materials have been widely developed as catalysts for the oxygen reduction reaction (ORR) in basic
conditions but very few in acidic conditions. In this work, two-dimensional (2D) cobalt/nitrogen co-doped porous carbon
nanosheets were prepared as catalysts for ORR in both basic and acidic conditions by using cobalt porphyrin based 2D
conjugated microporous polymer as precursor. Remarkably, the as-prepared porous carbon nanosheets exhibited
excellent electrochemical catalytic performance for ORR, with a low half-wave potential (E1/2) at –0.146 V in 0.1 M KOH
and 0.54 V in 0.5 M H
2
SO
4
(versus Ag/AgCl) as well as a dominant four-electron transfer mechanism (n = 3.8 at –0.28 V in
SO ). The high catalytic ORR performance can be attributed to the high activity of
x
active sites as well as the high specific surface area that derived from cobalt porphyrin blocks among the conjugated
0
.1 M KOH; n = 3.8 at 0.55 V in 0.5 M H
2
4
CoN
microporous polymer nanosheets. It’s believed that this method opens up new avenues of metal/heteroatoms co-doped
porous carbon materials with promising performance for energy storage and conversion.
electronic properties and structural features. As the most
often studied system, Met/nitrogen (Met = Co and Fe in
particular) co-doped carbonaceous catalysts present activity
1
. Introduction
Now, more than ever before, the rising global energy
consumption demands rapid development of green energy
conversion and storage devices, such as fuel cells and Zn-air
and stability levels either superior or comparable to Pt-based
2, 16-17
1
catalysts in alkaline media.
However, few reports of
1
-4
these materials claim that they can compete with Pt/C
catalysts in terms of half-wave potential in acidic media. This
inhibits their implementation in practical energy
conversion/storage devices. To improve the performance of
non-precious metal catalysts in acid electrolytes, one must
simultaneously consider two decisive factors governing the
performance of carbon-based nonprecious metal catalysts: 1)
, 8- uniform surface functionalities, namely homogenous
distribution of doped elemental composition and interactions
between different components, which determine the intrinsic
nature of the active sites, like the turnover frequency (TOF)
per active site; 2) hierarchical porous structures, which provide
batteries.
However, the lack of highly efficient and
inexpensive catalysts for the oxygen reduction reaction (ORR)
poses a great challenge to commercial implementation of
these technologies, which is a critical reaction for these green
5
-7
energy devices. Although platinum(Pt)-based catalysts have
been regarded as the most efficient catalysts for ORR, the
fragility, scarcity, and prohibitively high cost of Pt-based
6
catalysts seriously limit the practicality of their applications.
9
Great efforts have been made to search for readily available,
inexpensive, and efficient alternatives to Pt-based electrode
3
, 10
materials.
precious metals (e.g. iron, cobalt, and nickel)
heteroatoms (e.g. nitrogen, boron, phosphorus, and sulfur)
Carbonaceous materials doped with non-
5
,
11-12
high specific surface areas and accessible active sites, and
+
or
1
-
2
3- favor the fast transport of ORR-relevant species (H , O , e ,
1
5
2
and H O). The most traditional method to produce
with diverse nanostructures are notable candidates for ORR
that have been studied since 1964 because of their unique
Met/nitrogen co-doped porous carbon is the direct pyrolysis of
precursor mixtures that contain nitrogen and transition metals.
However, pyrolysis suffers from poor controllability of Met/N
1
8-19
4
active sites, such as the uniform distribution of FeN sites.
In this work, cobalt porphyrin-based two-dimensional (2D)
conjugated microporous polymer was first synthesized using
functionalized graphene as a 2D template, and was further
used as a precursor to prepare cobalt/nitrogen (Co/N) co-
doped porous carbon nanosheets by direct pyrolysis. The as-
prepared porous carbon nanosheets (named as GMC-CoPor-T,
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T = 700, 800, 900, corresponding to 700 °C, 800 °C, 900 °C) 85%). In control, cobalt porphyrin based conjugated
exhibited typical 2D morphology, high specific surface areas of microporous polymers without 2D morphology (CMP-CoPor)
2
-1
was also prepared by using the same method without using
RGO-Br template.
up to 466 m ·g and the uniform Co/N co-doping feature.
Benefiting from these characteristics, the prepared Co/N co-
doped porous carbon nanosheets exhibited excellent
electrochemical catalytic performance for ORR in 0.1 M KOH,
Pyrolysis of GMP-CoPor: As prepared GMP-CoPor was used as
precursor by pyrolysis at 700°C (or 800, 900°C) for 2h under
with a low half-wave potential (E1/2) at –0.146 V versus nitrogen atmosphere to produce the cobalt/nitrogen co-doped
Ag/AgCl and a dominant four-electron transfer mechanism (n = 2D porous carbons, which denoted as GMC-CoPor-T (T=700,
3
.8 at –0.28 V). Unlike most reported heteroatom-doped 800, 900). Similarly, CMP-CoPor was pyrolyzed by the same
porous carbons, they also exhibited high catalytic ORR procedure to produce cobalt/nitrogen co-doped porous
carbons (denoted as MC-CoPor-T, T=700, 800, 900).
performance in acidic conditions (0.5 M H
versus Ag/AgCl and n = 3.8 at 0.55 V), which can be attributed
to the high activity of CoN active sites that derived from
2 4
SO , E1/2 = 0.54V
Characterization: The morphology analysis of the samples was
acquired by field-emission scanning electron microscope
x
cobalt porphyrin blocks among the conjugated microporous
polymer nanosheets.
(
FESEM, FEI, Sirion 200, 25 KV) and transmission electron
microscope (TEM, JEOL, JEM-2010, 200 KV). Fourier transform
infrared spectroscopy (FT-IR) was performed on a Spectrum
1
00 (Perkin Elmer, Inc., USA) spectrometer with a scan range
-1
2
. Experimental
of 4000-400 cm . The sample powders were pulverized with
KBr, and pressed into disks. X-ray photo-electron spectroscopy
Materials and methods
(XPS) experiments were carried out on AXIS Ultra DLD system
from Kratos with Al Kα radiation as X-ray source for radiation.
Materials: The chemicals in this work were purchased from
Aldrich and Titan (Shanghai) Inc. The organic solvents were N adsorption-desorption isotherms at 77 K were determined
2
purified and distilled under dry nitrogen and the other by
a
Micromeritics ASAP 2010 instrument. Before
chemicals were used without purification. All reactions were measurement, the samples were degassed in vacuum at 150 °C
carried out under an inert atmosphere of nitrogen using for more than 10 h. Thermogravimetric analysis (TGA) of the
samples was measured using
a Perkin-Elmer Pyris 1
standard Schlenk techniques. The monomer tetra(4-
bromophenyl)cobalt(II)porphine (CoPor-4Br) was synthesized
thermogravimetric analyzer in flowing (100 mL/min) nitrogen
atmosphere. X-ray diffraction (XRD) was performed on a
Bruker D8 Advance powder diffractometer.
2
0
by using reported method.
Synthesis of RGBr: Graphene oxide (GO) was prepared from
natural graphite flakes through a modified Hummers method Electrochemical measurement
and then reduced by hydrazine hydrate in the presence of
sodium dodecyl benzene sulfonate (SDBS) under refluxing.
Catalyst ink fabrication: 4 mg of GMC-CoPor-T (or MC-CoPor-T)
was dispersed in 400 μL of 0.25 wt% Nafion ethanol solution,
then the mixture was sonicated for at least 2h to produce a
homogenous dispersion. Then, 6 μL of the catalyst dispersion
was pipetted onto a glassy carbon electrode (d=5.61 mm,
S=0.2472 cm ) for characterization. Commercial 20 wt%
platinum on carbon black (Pt/C, BASF) was used as control
catalyst.
21
Then as-obtained reduced graphene oxide (RGO) was
functionalized by using diazonium salt in water (5
2
2
equivalents). After 1 h stirring at room temperature, the
mixture was then poured into acetone, followed by filtration
2
and
washing
with
water,
acetone
and
N,N-
Dimethylmethanamide (DMF) three times and vacuum drying
to produce bromobenzene functionalized reduced graphene
oxide (RGBr) as a black powder.
ORR performance: All the electrochemical measurements were
carried out in a conventional three-electrode cell using the
Synthesis of GMP-CoPor and CMP-CoPor: Graphene-directed
cobalt porphyrin-based conjugated microporous polymer
PINE
electrochemical
workstation
(Pine
Research
Instrumentation, USA) at room temperature. Ag/AgCl and
platinum wire were used as the reference and counter
electrodes, respectively. A ring-disk electrode with a Pt ring
(GMP-CoPor) was synthesized through palladium-catalyzed
Sonogashira-Hagihara cross-coupling reaction of aryl
2
1, 23
ethynylenes and aryhalides.
Typically, RGO-Br (130 mg)
(
5.52 mm inner-diameter and 7.16 mm outer-diameter) and a
was firstly dispersed in dry DMF (200 mL) by ultrasonication
for a few hours to form a homogeneous solution. Then, 1,4-
diethynylbenzene (0.50 g, 4.0 mmol), CoPor-4Br (1.97 g, 2.0
mmol), tetrakis-(triphenylphosphine) palladium (70 mg, 0.06
2
2
glassy carbon disk (5.0 mm diameter, π(d/2) =0.196 cm ) was
used for evaluating the ORR activity of various catalysts.
The electrochemical experiments were conducted in O2
2 4
saturated 0.1 M KOH and 0.5 M H SO electrolyte. The RRDE
3
mmol), copper iodide (14 mg, 0.06 mmol) together with Et N(8
measurements were conducted at a rotating speed of 1600
rpm with a sweep rate of 10 mV/s. In order to estimate the
double layer capacitance, the electrolyte was deaerated by
bubbling with nitrogen for 30 min, and then the
voltammogram was evaluated again in the deaerated
electrolyte. On the basis of ring current and disk current, the
electron transfer number (n) and H O yield for catalysts were
mL) were added to the RGO-Br dispersion under nitrogen
atomsphere. The mixture was then heated to 80 °C and stirred
for 3 days under nitrogen atmosphere. Finally, the precipitate
was filtered and washed several times with chloroform, water
and acetone to remove the unreacted residues and
monomers. Further purification was carried out by Soxhlet
extraction with acetone for 48 h. Then the black powder was
dried and recovered in vacuum to produce GMP-CoPor (yield:
2
2
calculated from the following equations:
ꢁ
ꢂ
ꢃ
ꢀ
= ꢂ
(1)
ꢃ
ꢄꢂ
ꢅ
⁄ꢆ
2
| J. Name., 2012, 00, 1-3
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ARTICLE
ꢉꢍꢍꢎꢂ
ꢅ
⁄ꢆ
⁄ꢆ
ꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢈ
ꢉ
ꢊ
ꢉ
ꢋ%ꢌ = ꢂ
(2)
transmission electron microscopy (TEM). It was observed that
all the porous polymer nanosheets showed similar sheet-like
ꢏ
ꢄꢂ
ꢅ
where, I
and N = 0.37 is the ring collection efficiency of the Pt ring, (Fig. 2a–2c). Neither free porous polymers nor naked graphene
which was provided by manufacture.
were visible, which suggests that almost all of the monomers
The RDE measurements were conducted at a scan of 10 were grafted onto the surface of graphene. The control sample
D R
and I
are the disk and ring currents, respectively, morphologies with sizes from 500 nm to several micrometers
-1
mV·S with the rotating speed from 225 to 1600 rpm. The of CMP-CoPor exhibited rod-like morphologies (Fig. 2d). All
slopes of the linear fitted lines were used to calculate the these results demonstrated that the graphene had successfully
transferred electron number (n) per oxygen molecule in the directed the conjugated microporous polymers into a 2D form.
ORR process on the basis of the Koutecky-Levich equations:
Notably, the introduction of highly dispersible RGO-Br not only
produces a competitive template for the growth of porous
polymers and prevents aggregation problems, but also greatly
enhances long-distance conductivity for GMC-Por.
ꢐ
ꢑ
ꢐ
ꢐ
ꢐ
ꢗ/ꢘ
ꢐ
= ꢑ ꢓ ꢑ = ꢕꢖ ꢓ ꢑ
(3)
(4)
(5)
ꢒ
ꢔ
ꢔ
ꢍ ꢍ
ꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢙ = 0.62nFC ꢋD
ꢌꢉ/ꢚꢛꢜꢐ/ꢝ
ꢟ ꢍ
ꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢇꢞ = ꢀꢠꢡC ꢇ
k L
where J is the measured current density, J and J are the
kinetic and diffusion limiting current densities, ω is the angular
velocity (ω=2πN, where N is the linear rotation speed); F is the
-1
Faraday constant (96486 C·cm ); C is the bulk concentration
0
-
3
-1
of O
2
(C
(1.9
0
= 1.2ꢇꢎ102 mol·L ); D
0
is the diffusion coefficient of
-5
-1
O
2
ꢎ10 cm ·s ); n is the kinematic viscosity of the
electrolyte (0.01 cm ·s ), and k is the electron-transfer rate
constant.
2
-1
3
. Results and Discussion
Fig. 2 a) SEM and b,c) TEM images of GMP-CoPor; d) SEM image of CMP-
CoPor.
The schematic procedure for preparation of GMC-CoPor-T is
shown in Fig. 1. First, bromo-functionalized graphene
nanosheets (RGO-Br) were prepared through the reduction of
graphene oxide and further functionalized using p-
bromobenzene diazonium salt in aqueous conditions (see the
experimental section). Then, with RGO-Br and CoPor-4Br as 2D
template and key monomer, 2D graphene-based conjugated
microporous polymer was synthesized through the
Sonogashira–Hagihara coupling reaction.
The chemical characteristics of GMP-CoPors and CMP-
CoPors were studied by Fourier transform infrared
spectroscopy (FT-IR). As shown in Fig. 3a, the vibration peak
bridging -C≡C- in the porous polymers was detected at 2330
-1
-1
-1
-1
cm . The peaks at 1631 cm , 1508 cm , 1384 cm , and 999
cm can be attributed to the -C=C-, -C=N, -Co-N- bonds of
24-25
benzene, and -Co-N- bonds of porphyrin rings respectively.
-1
All these results confirmed that the porous polymer was
successfully prepared from the in situ polymerization of
ethynylenes-aryhalides on RGO-Br. In order to examine the
chemical structure, Co and N in GMP-CoPor were further
investigated by X-ray photoelectron spectroscopy (XPS). The
high-resolution spectra of Co were fitted to four peaks (Fig.
3b), among which the peaks at 780.3 eV (2p3/2) and 795.9 eV
(
2p1/2) corresponded to Co (III) and the peaks at 782.1 eV
2p3/2) and 804.1 eV (2p1/2) corresponded to the Co (II) of the
(
cobalt porphyrin blocks.
2
6-27
The high-resolution N 1s spectra
of GMP-CoPor (Fig. 3c) were fitted to two peaks centered at
3
pyrrolic N-Co, respectively.
successful introduction of Co and
microstructures.
98.9 and 400.3 eV, corresponding to the pyridinic N-Co and
5
, 12, 28
These results indicated the
N into GMP-CoPor
Fig. 1 Preparation of graphene-directed Co/N co-doped microporous
carbons (GMC-CoPor-T, T=700, 800 and 900). i) 4-bromobenzenediazonium
tetrafluoroborate, 0°C to RT, 2 h; ii) N
2
, Pd(PPh
3
)
4
, CuI, Et
3
N, DMF, 80°C, 3
days
(M1=1,4-diethynylbenzene; M2=
cobalt(II)
tetra(p-
-
1
bromophenyl)porphine); iii) N
h.
2
, RT to 700 °C (or 800 °C, 900 °C), 5°C·min , 2
The morphologies of the as-prepared GMP-CoPor were
examined by scanning electron microscopy (SEM) and
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electrochemical catalysis. Thermogravimetric analysis (TGA)
was performed for GMP-CoPor and CMP-CoPor to evaluate
their thermal stability. As shown in Fig. 4a, GMP-CoPor was
feasibly transformed into carbon materials with a carbon yield
Fig. 3 a) FT-IR spectra of GMP-CoPor and CMP-CoPor; b) XPS Co 2p and of of 62% at 800°C, and the control sample CMP-CoPor exhibited
1s; c) core level spectra of GMP-CoPor; d) nitrogen
adsorption/desorption isotherms of GMP-CoPor and CMP-CoPor.
N
a great weight loss in the temperature range from 350 to
50°C, leading to a low carbon yield of 10%. The enhanced
6
thermal stability of GMP-CoPor could be attributed to the
stabilization effect caused by covalent functionalization of
graphene as well as by weak interactions (e.g. π-π interation)
between the conjugated polymers and graphene template,
which suggests the combination of conjugated polymers and
graphene in the hybrid GMP-CoPor was notable for its
The porous natures of GMP-CoPor and CMP-CoPor were
further revealed by N
2
adsorption-desorption isotherms (Fig.
3d). The hysteresis loops of the isotherms can be classified as
Type I according to IUPAC classifications. The specific
Brunauer–Emmett–Teller (BET) surface area of GMP-CoPors
2
was 685 m /g, which was slightly higher than that of the
2
control sample, 673 m /g (CMP-CoPor, Table 1). These stabilizing properties. Therefore, graphene-directed Co and N
phenomena may be attributed to the high surface areas of co-doped microporous carbons can be generated by the direct
graphene templates. Notably, the micropore surface area pyrolysis of GMP-CoPor at various temperatures (denoted as
2
(
denoted as Smicro) for GMP-CoPor (Smicro = 438 m /g, 64% of GMC-CoPor-T, T = 700, 800, 900, corresponding to 700°C,
total surface area) was lower than that of CMP-CoPors (Smicro
2
=
92 m /g, 73% of total surface area), indicating that the
800°C, 900°C) under nitrogen atmosphere. For comparison,
Co/N co-doped porous carbons (CMC-CoPor-T, T =700, 800,
4
introduction of graphene may contribute to in-plane long
distance polymerization, but may affect the degree of out-of-
plane polymerization. The narrow micropore size distribution
was found according to the Barrett−Joyer−Halenda (BJH)
model (insets in Fig. 2d). It was centered at 1.6 nm for both
GMP-CoPors and CMP-CoPors.
9
00) were also prepared by the same procedure using CMP-
CoPor as the precursor.
Table 1. Nitrogen physisorption properties for CMP-CoPor,
GMP-CoPor and their pyrolyzed products.
[
a]
b]
[c]
av
[d]
[e]
[f]
S
BET
S
Lang[
D
S
micro
V
micro
3
V
tot
2
2
2
3
[
m /g] [m /g] [nm]
[m /g] [cm /g] [cm /g]
A
B
C
D
E
673
540
400
568
685
466
463
401
786
582
446
605
864
531
541
468
4.37
4.90
4.54
3.87
8.06
2.96
2.96
3.23
492
397
285
472
438
301
255
227
0.22
0.17
0.12
0.19
0.20
0.13
0.11
0.10
2.70
2.43
1.67
2.02
1.38
1.27
1.26
1.19
Fig. 4 a) TGA curves of CMP-CoPor and GMP-CoPor; b) SEM image of MC-
F
CoPor-800; c) SEM image of GMP-CoPor-800; d) TEM image of GMC-CoPor-
8
00; e) XPS of N1s; f) Co2p core level spectra of GMC-CoPor-800.
G
H
GMC-CoPor-800 was chosen as a typical example for
morphological examination because it showed results similar
to those of GMC-CoPor-700 and GMC-CoPor-900. The SEM and
TEM images suggested that the GMC-CoPor-800 maintained
Note: Surface areas calculated from the nitrogen adsorption
based on [a] BET model and [b] Langmuir model; [c] average
pore diameter; [d] surface area; [e] pore volume of micropore;
2D morphology with a large aspect ratio because of its
effective restriction of agglomeration, while the control
sample CMC- CoPors presented rod-like morphology similar to
that of earlier samples (Fig. 4b-4d). XRD pattern of the
0
[f] the total pore volume calculated at P/P = 0.99. A-H: CMP-
CoPor, CMC-CoPor-700, CMC-CoPor-800, CMC-CoPor-900,
GMP-CoPor, GMC-CoPor-700, GMC-CoPor-800, GMC-CoPor-
o
composite (Figure S1) exhibited two broad peaks at 25.7 and
4
o
3.4 , which can be attributed to the corresponding (002) and
101) facets of the sample respectively, illustrating the
900.
(
The obtained 2D conjugated microporous polymers can
2
1, 29-32
amorphous structure of GMC-CoPor-800. Then XPS was
performed to analyze the N and Co in CMC-CoPors. High-
resolution Co 2p3/2 spectra were divided into two peaks with
further serve as a type of carbon-rich precursor.
Such
precursors excel at uniformly integrating metal and
heteroatom atoms (Co and N) into carbon frameworks for
4
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the binding energies of 780 eV and 784 eV. These peaks were polarization curves (Fig. 5b) showed that the half-wave
5
, 12
assigned to N-Co and O-Co.
The N 1s spectra of GMC- potential (E1/2) and the diffusion-limiting current densities (JDL)
-2
CoPor-800 were divided into two signals with binding energies of the GMC-CoPor-800 were –0.146 V and 4.3 mA·cm ,
of 398.9 eV and 401.0 eV, corresponding to pyridinic N and respectively, both of which were better than those of CMC-
-2
graphitic N, respectively. Notably, these peaks should include CoPor-800 (E1/2 = –0.168 V; JDL = 3.6 mA·cm ). These results
the contribution of N-Co because only a small difference of indicated that the GMC-CoPors with graphene exhibited
binding energy exists between pyridinic N and cobalt N (Figure electrocatalytic activity superior to that of samples without
4
e-f). The elemental ratio of N/Co for the obtained samples graphene.
varies from 7.5-9.1 when calcination at different temperatures
Table S1). These results show that Co and N atoms could form
into the structure-uncertained complex CoN after calcination,
In addition, the ORR polarization curves for GMC-CoPor-700,
GMC-CoPor-800, and GMC-CoPor-900 (Fig. 5c) indicate that
the high temperature had no obvious effect on the catalytic
(
x
indicating possible high ORR catalytic activities.
The porosity of GMC-CoPor-T was confirmed by its nitrogen
physisorption. The BET surface areas of GMC-CoPor-700, GMC-
ORR performance; the E1/2 values were at –0.147, –0.146 and –
0
-2
.145 V; JDL values were 4.6, 4.3, and 4.2 mA·cm at 0.9 V,
respectively. The E1/2 values for GMC-CoPors were only 10–13
mV lower than commercial precious metal Pt/C catalyst.
Because the various types of GMC-CoPors had only negligible
differences between their ORR polarization curves, GMC-
CoPor-700 was chosen as a typical example for further RRDE
study (Fig. 5d).
2
CoPor-800, and GMC-CoPor-900 were 466, 463 and 401 m /g,
respectively, all of which were slightly smaller than those of
2
the control samples of CMC-CoPor-T (400-560 m /g) (Table 1).
The slight difference of GMC-CoPor-T in BET surface area may
have been caused by the degradation of polymers and
recombination of fragments under the different pyrolysis
The calculated electron transfer number and
2 2
H O
2
1, 33-39
temperatures.
concentration were approximately 3.8 and 13% at –0.28 V vs.
Ag/AgCl (Fig. 5d inset). This result highlights that the ORR
proceeds through a primary four-electron pathway. The RDE
2
voltammetric profiles in O -saturated 0.1 M KOH solution
showed that current density was increased when the rotation
rate increased (from 225 to 1600 rpm, Fig. 5e). The kinetic
current density based on Koutecky–Levich (K–L) plots derived
from RDE curves for GMC-CoPor-700 was high, the value fo
-2
which was up to 26 mA·cm (Fig. 5f). This superior
performance might result from the high density of active sites
x
CoN exposed to the electrochemical interface, high electrical
conductivity, and long-distance 2D electron transport of
porous carbon nanosheets.
Fig. 5 ORR performance of GMC-CoPor-T in 0.1 M KOH. a) CV curves of
GMC-CoPor-800 and CMC-CoPor-800 in N -saturated and O -saturated 0.1
M KOH at a scan rate of 50 mV·s ; b) ORR polarization curves GMC-CoPor-
00 and CMC-CoPor-800 at 1600 rpm; c) ORR polarization curves of GMC-
CoPor-X and Pt-C at 1600 rpm; d) RRDE curve of GMC-CoPor-800 at 1600
rpm (the inset shows electron transfer number and the percentage of H O
2
2
2
-
1
8
2
as a function of potential); e) LSV curves of GMC-CoPor-800 at different
scan rates; f) calculated kinetic-limiting current as a function of potential
based on K–L plot derived from RDE measurement.
The unique Co/N co-doping feature, its 2D morphology, and
its porous nature all suggest that as-prepared GMC-CoPor-T
might demonstrate excellent electrochemical catalytic
performance toward ORR. The ORR catalytic activities of the
as-prepared GMC-CoPor-T were first evaluated by cyclic
voltammetry (CV, Fig. 5a). The ORR peak potential of –0.21 V
versus Ag/AgCl for GMC-CoPor-800 was slightly higher than
that for the control sample CMC-CoPor-800 (–0.22 V). The ORR
Fig 6. ORR performance of GMC-CoPor-700 in 0.5 M H
-saturated and O -saturated 0.5 M H SO ; b) RRDE curve of GMC-CoPor-
00 at 1600 rpm; c) electron transfer number and percentage of H O2 as a
2 4
SO . a) CV curves in
N
2
2
2
4
7
2
function of potential; d) LSV curves at different scan rates; e) K–L plots
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ARTICLE
Journal Name
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, e1500564.
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00 was chosen as a typical example for further analysis of the
catalytic activity in acidic conditions (0.5 M H SO ). First, the
ORR catalytic activity of GMC-CoPor-700 was evaluated by CV
Fig. 6a) in N - and O -saturated 0.5 M H SO . The oxygen
reduction peak for GMC-CoPor-700 was observed at 0.65 V,
whereas the signal vanished in N -saturated 0.5 M H SO . The
1
2
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half-wave potential (E1/2) of the GMC-CoPor-700 in an RDE
voltammogram was at approximately 0.54V versus Ag/AgCl
8
9
1
1
1
.
.
2 2
(Fig. 6b), and the electron transfer number and H O
C. Sealy, Mater. Today, 2008, 11, 65.
concentration were calculated, approximately, as 3.8 and 10%
at 0.55 V versus Ag/AgCl (Fig. 6c), which was better than the
performance under basic conditions. The RDE voltammetric
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current density was increased by an increase in the rotation
rate (from 225 to 1600 rpm, Fig. 6d). K–L plots (Fig. 6e) with a
well-fitting linear relationship for GMC-CoPor-700 were
calculated from linear sweep voltammetry (LSV) curves (Fig.
1
1
7
8
6d) at various rotation rates. Linearity and parallelism of the
1
1
plots are usually interpreted as an indication of first-order
reaction kinetics with respect to the concentration of dissolved
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2
2
K
-2
mA·cm at 0.4 V (Fig. 6f), therefore GMC-CoPor-700 delivered
superior ORR performance under acidic conditions.
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2
-1
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doped porous carbon nanosheets exhibited excellent
21, 5284.
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x
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3
The authors thank the financial support from 973 Programs of
China (2013CBA01602, 2012CB933400), Natural Science
Foundation of China (51403126, 21574080, 21102091), the
Shanghai Committee of Science and Technology
(15JC1490500) and ERC Grant on 2DMATER and EU Graphene
Flagship.
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3
Notes and references
6
| J. Name., 2012, 00, 1-3
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PleaseRd So Cn oA t da vd aj un s ct ems argins
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Journal Name
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5
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6
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DOI: 10.1039/C6RA16539G
Cobalt/Nitrogen co-Doped Porous Carbon Nanosheets as Highly
Efficient Catalysts for Oxygen Reduction Reaction in Both Basic and
Acidic Media
A two-dimensional (2D) cobalt/nitrogen co-doped porous carbon nanosheets as high-efficient
catalysts for ORR in both base and acid electrolytes was prepared by using cobalt porphyrin
based conjugated microporous polymer with GO nanosheet as precursor