Nitrogen, sulfur and phosphorus-codoped carbon with a tunable nanostructure as an efficient electrocatalyst for the oxygen reduction reaction

Article abstract of DOI:10.1039/c6ra21540h

Novel types of N,S,P-codoped carbons with varied nanostructures are developed as efficient and stable electrocatalysts for the oxygen reduction reaction (ORR). The carbon catalysts were facilely prepared using poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanospheres as single precursors through a pyrolysis procedure in the presence or absence of melamine. The as-prepared microporous carbon nanospheres NSP-PC-1 with a high surface area (967 m² g^-1) exhibit a significantly enhanced ORR catalytic activity compared to their solely N-doped counterpart (N-PC-1), suggesting the remarkable contribution of additional S,P-doping on the ORR performance. More importantly, the as-prepared mesoporous carbon nanosheets NSP-PC-2 with a moderately high surface area (613 m² g^-1) and comparable overall NSP content show greatly improved ORR catalytic activity relative to that of NSP-PC-1, which is even slightly superior to that of commercial Pt/C catalysts. The excellent performance of NSP-PC-2 is mainly attributed to its proper N,S,P-codoping as well as advanced structural features.

Full text of DOI:10.1039/c6ra21540h

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Nitrogen, sulfur and phosphorus-codoped carbon with tunable
nanostructure as efficient electrocatalyst for the oxygen reduction
reaction
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
b
a
DOI: 10.1039/x0xx00000x
Kai Chen, Yajuan Hao, Meirong Zhang, Dongying Zhou, Yingjie Cao, Ying Wang, Lai Feng*
www.rsc.org/
A novel type of N, S, P-codoped carbons with varied nanostructures is developed as efficient and stable electrocatalyst for
the oxygen reduction reaction (ORR). The carbon catalysts were facilely prepared using poly(cyclotriphosphazene-co-4,4’ -
sulfonyldiphenol) (PZS) nanospheres as single precursors through a pyrolysis procedure with or without presence of
2
-1
melamine. The as-prepared microporous carbon nanospheres NSP-PC-1 with high surface area (967 m g ) exhibit a
significantly enhanced ORR catalytic activity compared to the solely N-doped counterpart (N-PC-1), suggesting the
remarkable contribution of additional S, P-doping on the ORR performance. More importantly, the as-prepared
2
-1
mesoporous carbon nanosheets NSP-PC-2 with moderately high surface area (613 m g ) and comparable overall NSP
content show greatly improved ORR catalytic activity relative to that of NSP-PC-1, which is even slightly superior to that of
commercial Pt/C catalyst. The excellent performance of NSP-PC-2 is mainly attributed to its proper N, S, P-codoping as well
as advanced structural features.aaa
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density of the active sites, which could be improved by proper
2
0,21
heteroatom-doping.
For instance, N-doping can introduce
1
. Introduction
catalytic active sites by breaking the electroneutrality of the carbon
Fuel cells have been of continuous interest in the field of clean
22-25
material, thus enhancing the ORR activity.
It was also suggested
energy owing to their high power density and low pollutant
that the co-doping of two or multiple elements, such as N/B, N/S,
N/P, N/S/P in carbon catalyst can introduce asymmetrical spin or
charge density due to their synergistic effects, therefore generating
1
emissions. Platinum (Pt) or Pt-based precious metal catalysts have
been widely used for promoting the electrochemical oxygen
2
-6
reduction reaction (ORR) on the electrode of fuel cell.
more active sites and higher catalytic activity relative to that of
Nevertheless, though Pt-based catalysts exhibit high efficiency for
ORR, their high cost and low tolerance to fuel crossover reduce
their potential in practical use and hinder the large-scale
26-30
unitary doped catalyst.
(2) The accessibility to the active sites
and mass transfer properties, which are closely related to the
structural feature of carbon catalyst (i.e. morphology, particle size
7-9
commercialization of fuel cells. In recent years, various Pt-free
31-32
and pore structure).
Thus, these two considerations have been
6
10,11
catalysts have been developed, such as transition metal oxide,
combined to afford rational design in recent studies. A variety of
heteroatom-doped carbons with either meso or microporosity and
1
2-14
15,16
17-19
porous carbon,
graphene
and their composite.
Among
them, porous carbon catalysts have drawn considerable interests
owing to their evident advantages such as low cost, tunable
catalytic activity as well as high thermal/chemical stability.
Therefore, in recent years, intensive efforts have been devoted to
developing novel porous carbons with enhanced catalytic
performance.
large specific surface area have been prepared and employed as
33-36
ORR catalysts.
Typically, one of state-of-the-art carbon catalysts
was reported by Tour et al. very recently, which was highly
mesoporous with proper B, N codoping and showed excellent ORR
electrocatalytic activity, even much better than the commercial
30
Pt/C catalysts. However, the carbon catalysts with such desirable
It has been well established that two major considerations
shall be taken into account when designing and preparing high-
performance ORR carbon catalysts: (1) The catalytic nature and
ORR performance are very rare. Most of carbon catalysts reported
so far exhibited only moderate ORR performance, which is still
inferior to that of the commercial Pt/C catalysts.
Herein, we present a novel type of N, S, P-codoped carbon
catalysts with either micro- or mesoporosity. These catalysts can be
facilely
prepared
by
a
pyrolysis
process
using
(PZS)
poly(cyclotriphosphazene-co-4,4’-sulfonyldiphenol)
nanospheres as single precursor. It is demonstrated that the as-
prepared carbon catalysts exhibit much better catalytic
performance than the solely N-doped counterpart thanks to the
contribution of additional S, P doping. More importantly, the
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catalyst with mesoporous structure shows an excellent ORR
All electrochemical measurements were performed at room
performance very comparable to that of commercial Pt/C in alkaline temperature on a potentiostat (CHI 760E, CH Instrument, Shanghai,
media, probably due to the synergistic effect of the proper China) and a rotating ring disk electrode system (RRDE-3A, ALS Co.,
heteroatom-doping and its structural advantages.
Ltd, Japan) with a standard three electrode cell. A Pt wire and an
-
Hg/HgO/0.1 M OH electrode (0.164 V vs NHE) were used as the
counter and reference electrode, respectively. A RDE with glassy
carbon (GC) disk electrode (4 mm in diameter) and a rotating ring-
2
. Experimental
disk electrode (RRDE, ALS Co., Ltd, Japan) with a Pt ring (5 mm inner
2
.1. Material Preparation
diameter and 7 mm outer diameter) and a GC disk (4 mm diameter)
Crosslinking
poly(cyclotriphosphazene-co-4,4’- are used as the substrate for the working electrodes. Before use,
sulfonyldiphenol) nanospheres (PZS) were prepared according to the GC electrodes in RDE/RRDE are polished using aqueous alumina
37.38
the method as described in the literatures.
Briefly, 1.6 g of suspension on polishing pad. All electrode potentials were reported
phosphonitrilic chloride trimer and 3.455 g of 4,4'-sulfonyldiphenol relative to the reversible hydrogen electrode (RHE), which were
-
were dissolved in
ultrasonication. Then, 8 mL triethylamine was added slowly and the formula: ERHE=E(Hg/HgO)+φ(Hg/HgO/0.1 M OH )+0.0591*pH V.
reaction mixture was maintained for 10 min with ultrasonication,
To fabricate the catalyst-modified working electrode, the
a
acetonitrile solution (160 mL) with converted from the Hg/HgO/0.1 M OH electrode according to the
-
40
yielding a solution with white suspensions. Then, the reaction catalyst ink was prepared by ultrasonically dispersing the as-
mixture was centrifuged to wipe off the solvent and washed by prepared carbon catalyst (5 mg) in a solution containing 100 μL
using ethanol and distilled water. The product of PZS microspheres Nafion (5 wt%) solution and 900 μL deionized water. The Pt/C
was freeze-dried in vacuum overnight.
catalyst (10 wt% Pt on graphitized carbon, Sigma-Aldrich) ink was
The N, S, P co-doped porous carbon denoted as NSP-PC-1 was prepared in the same way. The above prepared catalyst ink was
obtained through directly pyrolyzing PZS nanospheres at 900 °C for casted onto the fresh surface of the GCE (a diameter of 4 mm). The
−
1
-2
-2
6
0 min with a heating rate of 10 °C min and then cooling down to catalyst loadings on RDE and RRDE were 400 μg cm (40 μgPt cm
room temperature under N
flow. Another co-doped porous carbon for the Pt/C catalyst).
of NSP-PC-2 were prepared by heating the mixture of melamine and
ORR performance of the catalysts was investigated via cyclic
PZS nanospheres (with a mass ratio of 10:1) in a semiclosed system voltammogram (CV) and linear sweep voltammogram (LSV) in either
2
−1
under N
2
atmosphere for minimizing the elution of decomposed
N or O saturated 0.1 M KOH solution. The scan rate was 50 mV s
2 2
−1
melamine at high temperature. In a typical run, 0.5 g PZS for CV and 10 mV s for LSV tests. Oxygen reduction current was
microspheres and 5 g melamine were dispersed in a hot water evaluated by subtracting the background capacitive current, which
o
solution (80 C) and stirred for more than 2 h. The resulting mixture was measured by scanning the electrode in a N
-saturated 0.1 M
2
o
was freeze-dried in vacuum overnight and pyrolyzed at 900 C for KOH solution under the same conditions. The electron transfer
0 min under N
atmosphere. The final products were washed by number (n) was calculated using Koutecky−Levich (K-L) equations:
ethanol and dried for use.
6
2
(
1)
2
.2. Structural Characterizations
(
2)
The as-prepared carbon catalysts were characterized by the
field emission scanning electron microscope (FESEM, Hitachi S-
800) and transmission electron microscope (TEM, FEI Tecnai F20)
where J is the measured current density, Jk is the kinetic
current density, B is the Levich constant, ω is the angular velocity of
the rotating electrode, n is the overall number of electrons
transferred in the ORR process, F is the Faraday constant (96485 C
4
for the morphology and microstructure identifications. Raman
spectra were recorded on a Micro-Raman spectroscopy system
(
Renishaw inVia-reflex, 532 nm excitation laser). X-ray
photoelectron spectroscopy (XPS) studies were carried out with an
ESCALAB 250 spectrometer using monochromated Al Kα
excitation source. Nitrogen adsorption–desorption isotherms at -
−1
−3
−1
mol ), C is the bulk concentration (1.2 × 10 mol L ) of O , D is
0
2
0
−5
2
−1
the diffusion coefficient (1.9 × 10 cm s ) of O in the KOH
2
2
a
−1
solution, and υ is the kinetic viscosity (0.01 cm s ) of the
4
1
o
electrolyte.
Rotating ring-disk electrode (RRDE) tests for ORR were
measured in the O saturated 0.1 M KOH solution with a scan rate
of 10 mV s , and the ring potential was set at 0.7 V for H
1
2
96 C were measured on an adsorption volumetric analyzer ASAP
020 manufactured by Micromeritics, Inc. (Norcross, Georgia, USA).
o
2
All samples were degassed at 200 C overnight prior to adsorption
measurements. A part of the N2 sorption isotherm in the P/P0
range of 0.005–0.05 was fitted to the BET equation to estimate the
−
1
2
O
2
4
0
production. The electron transfer number n and the production
−
−
3
9
yield of HO
2 2
(HO %) were determined by the following equations:
BET surface area. Total pore volume (Vtotal) was obtained at a
relative pressure of 0.985. The pore size distribution was obtained
using the NLDFT model in the Micromeritics ASAP 2020 software
package (assuming slit pore geometry). Micropore area (Smicro) and
volume (Vmicro) were calculated using the t-plot method.
(
3)
(4)
2
.3. Electrochemical Tests
2
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r
where I is the disk current, I is the ring current, and N is the
ARTICLE
d
current collection efficiency of the Pt ring, which was 0.424
provided by the manufacturer.
Durability of catalysts was evaluated from the i−t
chronoamperometric responses of as-prepared carbon catalyst and
2
Pt/C catalyst in O -saturated 0.1 M KOH at 0.6 V (vs RHE) for 5.5 h
with a rotation rate of 400 rpm. For fuel crossover effect tests, the
chronoamperometric response at 0.6 V (vs RHE) was recorded by
RDE tests with a rotation rate of 1600 rpm, and followed by the
introduction of methanol (3 M) into 0.1 M KOH solution.
Fig. 1. SEM images of (a) NSP-PC-1, (b) NSP-PC-2.
3
. Results and discussion
The catalysts NSP-PC-1 and NSP-PC-2 were prepared through a
two-step method (Scheme 1): (1) Highly cross-linked PZS
nanospheres were prepared according to the literature-reported
3
7,38
method;
(2) The as-prepared PZS nanospheres were carbonized
o
at 900 C with or without the presence of melamine under N
2
atmosphere to form N, S, P co-doped carbon catalysts. Particularly,
PZS nanospheres act as carbon source and N, S, P-doping sources,
which might result in a uniform distribution of N, S, P heteroatoms
in the carbon catalyst and simplify the preparation. In the case of
NSP-PC-2, melamine was added during pyrolysis for additional N-
doping and porosity adjustment. The following mechanism has
been proposed: in the temperature range of 300−600 °C, melamine
42
is decomposed into carbon nitrides, which diffuses into the matrix
o
of semi-carbonized PZS. At a higher temperature (i.e., >750 C),
Fig. 2. (a,b) TEM and HRTEM images of NSP-PC-2. (c) HADDF image
and compositional EDS mapping of NSP-PC-2 using scanning
transmission electron microscopy.
carbon nitrides are further decomposed into N2, NH
3
and some
+
+
+ 43-46
2 2 3 2 3 3
reactive species (i.e., C N , C N , C N ),
which are released
from or react with the carbon matrix to contribute to the porosity
adjustment or to introduce the N-containing functionalities in the
4
7
The X-ray photoelectron spectroscopy (XPS) analysis (Fig. 3 and
Fig. S2) further reveals the N, S, P-codoping in the catalyst NSP-PC-2
as well as NSP-PC-1. Particularly, the N 1s spectrum demonstrates
three types of N species in NSP-PC-2, corresponding to graphitic-N
carbon product.
The structures and morphologies of the as-prepared catalysts
were investigated by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). As shown in SEM images
4
8
49
(
401.0 eV), pyrrolic-N (399.8 eV), and pyridinic-N or P=N/P-N
398.2 eV). Among them, graphitic-N, pyrrolic-N and pyridinic-N or
(
Fig. 1), NSP-PC-2 has a sheet-like morphology, unlike the
(
nanospherical morphology of NSP-PC-1 or PZS (see Fig. S1a). Thus,
the formation of the sheet-like structures of NSP-PC-2 might be
attributed to the presence of melamine during pyrolysis, though the
mechanism is still unknown. The TEM image in Fig. 2a also reveals
the sheet-like feature of NSP-PC-2. Furthermore, a high-resolution
50-53
P=N/P-N are all considered to be favorable for ORR.
The S 2p
spectrum reveals only thiophene-S in NSP-PC-2, which is also
believed to play an important role in enhancing catalytic activity.
The two peaks positioned at 165.1 and 163.9 eV are consistent with
the S 2p and S 2p3/2 of thiophene-S, owing to their spin–orbit
(
HR)TEM image (Fig. 2b) suggests that NSP-PC-2 is amorphous in
1/2
5
4,55
coupling.
The P 2p spectrum can be fitted with two peaks at
nature. Fig. 2c shows a typical scanning transmission electron
microscopy (STEM) annular dark field (ADF) image and elemental
mapping of NSP-PC-2, where all the elements (C, O, N, P and S) are
homogeneously distributed throughout the carbon nanosheet.
1
32.6 and 133.6 eV, corresponding to P-C and P=N/P-N bonding,
5
1,56
respectively,
indicating that P atoms were incorporated into the
N-doped carbon framework. Such a doping way was found to create
more active sites and trigger synergistic effects for ORR
1
6
electrocatalysis . NSP-PC-1 also exhibits similar XPS spectra (Fig.
S2). However, the N, S, P contents of the carbon catalysts are very
different (see Table S1 in ESI). Particularly, NSP-PC-2 has 9.05 at.%
N, 0.6 at.% S and 1 at.% P, while NSP-PC-1 has 3.75 at.% N, 4 at.% S
and 0.1 at.% P. The higher N-content and lower S-content in NSP-
PC-2 may be attributed to the additional precursor of melamine,
which not only provides additional N source but also results in
5
7
desulfurization to some extent. On the other hand, the higher P
content in NSP-PC-2 might be related to the increase of N-doping.
Scheme 1. Preparations of NSP-PC-1 and NSP-PC-2.
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As suggested in P 2p spectrum (Fig. 3), the higher P-doping in NSP- Table 1. Textural properties of NSP-PC-1 and NSP-PC-2, compared
PC-2 occurs with bonding configuration of C-P=N/C-P-N. with those of control catalysts N-PC-1 and NSP-PC-3.
Raman spectroscopy is adopted to further investigate the Catalysts
structural feature of the carbon catalysts. As shown in Fig. S4 (in
SBET
2
(m g )
967
Vtotal
Smicro
2
Vmicro
Smeso
Vmeso
-1
a
3
-1
-1
3
-1
2
-1
3
-1
(cm g )
0.38
(m g )
898
807
49
(cm g )
0.346
0.307
0.019
0.009
(m g )
69
(cm g )
0.036
0.023
1.40
-1
ESI), the two characteristic peaks at around 1350 and 1580 cm are
assigned to the D band and G band, respectively, confirming the
NSP-PC-1
N-PC-1
845
0.33
38
5
8
presence of disordered carbon and graphitic carbon. Particularly,
NSP-PC-1 and NSP-PC-2 show similar I /I band intensity ratio (i.e.,
.925 and 0.923), indicating their similar disorder degree in spite of
NSP-PC-2
613
1.42
564
621
D
G
NSP-PC-3
648
1.56
27
1.55
0
a
Surface areas (SBET) were calculated by the BET method.
their different pyrolysis conditions (i.e., with or without melamine).
b
The total pore volumes (Vtotal) were estimated from the adsorbed amount at a
0
relative pressure P/P = 0.985.
c
Micropore area (Smicro) and volume (Vmicro) were calculated using the t-plot
method.
d
Mesopore areas (Smeso
) and volume (Vmeso) were calculated from the
difference of the BET surface area and micropore area (Smeso = SBET-Smicro), and the
total pore volume and micropore volume (Vmeso = Vtotal -Vmicro), respectively.
The porous characteristics of the catalysts NSP-PC-2 and NSP-PC-
1
2
were investigated by N adsorption-desorption measurements.
The textural properties were summarized in Table 1. As presented
in Fig. 4a, NSP-PC-2 presents a typical type-IV isotherm according to
the IUPAC classification, which shows a small N
relative pressure (P/P < 0.001), a clear hysteresis loop and an
evident rise in the medium and high pressure regions (P/P = 0.8-
.0), indicative of mainly mesoporous structures with a moderately
2
uptake at low
0
0
1
2
-1
high BET surface area of 613 m g and a large pore volume of 1.42
3
-1
cm g . In contrast, the isotherm for NSP-PC-1 is assigned to type-I,
suggesting sheer microporous structures (up to 93% microporosity)
Fig. 3. XPS spectra of NSP-PC-2.
2
-1
with a high BET surface area of 967 m g and a pore volume of
3
-1
0
.38 cm g . The different porosities of these carbon catalysts were
also demonstrated by pore size distribution (PSD) results calculated
using NLDFT method. As shown in Fig. 4b, the pore volume of NSP-
PC-1 is mainly contributed by the micropores (with sizes smaller
than 2 nm), while the porosity of NSP-PC-2 is mainly distributed into
a mesopore system with sizes ranging from 2.5 nm to 50 nm (～
9
0% mesoporosity). Thus, it appears that the melamine plays a
significant role in achieving the mesoporosity of NSP-PC-2. This may
be attributed to the generation of gaseous agents (i.e., N and NH
due to the decomposition of melamine or its derivative (i.e., carbon
2
3
)
4
4,45
nitride) at high temperature (i.e., 900 oC),
which might swell the
carbon matrix and induces the formation of mesopores.
The ORR catalytic activity of the catalysts NSP-PC-1 and NSP-PC-2
was first assessed by means of cyclic voltammetry (CV) and linear
2
sweep voltammetry (LSV) in 0.1 M KOH solution saturated with O .
In order to better understand the catalytic performances and reveal
the role of heteroatom-doping, a control catalyst was prepared by
5
9,60
pyrolyzing melamine-doped polymeric nanospheres,
noted as
N-PC-1 (see ESI for preparation details). Though N-PC-1 was derived
from the precursor different from that of NSP-PC-1, its structural
features including the nanospherical morphology, microporous
characteristics and BET area are very similar to those of NSP-PC-1
(
see Fig. S1a, S5 in ESI and Table 1). Moreover, as suggested by XPS
Fig. 4. (a) N
2
adsorption–desorption isotherms and (b) pore size
analysis, N-PC-1 contains no S, P but only N species. The identified
N-compositions and content in N-PC-1 are similar to those found in
NSP-PC-1. (Fig. S3 and Table S1 in ESI).
distributions of NSP-PC-1 and NSP-PC-2 calculated using NLDFT.
As shown in Fig. 5a, the CV results indicate that the control
catalyst N-PC-1 shows a poorer ORR catalytic activity in terms of the
4
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more negative oxygen reduction peak potential as compared to that degraded catalytic performance might be attributed to the
of NSP-PC-1 or NSP-PC-2. This trend is again demonstrated by the reduction of electrical conductivity with higher NPS content, which
LSV results (Fig. 5b). The control catalyst N-PC-1 exhibits an obstructs the ORR process in NSP-PC-3.
apparently less enhanced catalytic activity for ORR in view of an E1/2
It is noteworthy that the E1/2 of NSP-PC-2 is even 10 mV higher
of 0.71 V, which is 70 and 140 mV lower than those of N, S, P co- than that of the commercial Pt/C catalyst (i.e., a value of 0.84 V
6
1-63
doped catalysts NSP-PC-1 and NSP-PC-2, respectively. Considering comparable to the reference data with similar Pt loading),
the similar structural features of N-PC-1 and NSP-PC-1, the better suggesting an excellent ORR activity for NSP-PC-2. This E1/2 value is
catalytic activity of the latter can be safely attributed to the also more positive than those of recently reported NSP-codoped
contribution of additional S, P-doping. It is also clear seen that the porous carbons, which are remarkably lower than that of
2
7,28
−1
−1/2
catalyst NSP-PC-2 shows the best catalytic activity among three as- commercial Pt/C catalyst.
The K-L plots (J vs. ω
in Fig. 5e
prepared carbon catalysts in terms of the most positive E1/2 of 0.85 and S11) derived from the LSVs of NSP-PC-2 show an excellent
V. The improved catalytic activity of NSP-PC-2 relative to that of linearity, implying a first-order reaction in this potential range. The
NSP-PC-1 might be due to its slightly higher overall NSP content and calculated average electron-transfer number (n) is 3.84. This value
more importantly, the distinct structural feature (such as the is close to that obtained from the rotating ring-disk electrode
mesoporosity), which makes the active sites very accessible for the (RRDE) measurements (n=4.02~3.99, see Fig. 5f), suggesting a one-
reactants and facilitates the mass-transfer despite of the lower step, four-electron oxygen reduction process for this carbon
surface area of NSP-PC-2 relative to that of microporous NSP-PC-1.
catalyst. The RRDE measurements also reveal that the production of
-
To further explore the effect of the dosage of introduced HO2 was suppressed on the catalyst NSP-PC-2 with a yield below
heteroatoms on the electrocatalytic activity of the as-prepared ~7.3% over the potential range of 0.4~0.9 V (see Fig, S12 in ESI),
carbon catalyst, another control catalyst NSP-PC-3 was prepared by which is closely comparable to that reported for Pt/C catalyst (4-
3
0
the same synthetic route as NSP-PC-2 except for lower pyrolysis 5%). Moreover, the Tafel slope of NSP-PC-2 was calculated to be
o
-1
temperature (i.e., 800 C). As suggested by N2 adsorption- 84.1 mV dec (see Fig. S13 in ESI), very close to that of Pt/C (83.8
-1
desorption measurement, Raman and XPS analysis (see Fig. S1b, S4, mV dec ) measured under the same condition, indicating the
S6, S7 and Table S1 in ESI), NSP-PC-3 exhibits similar structural transfer of the first electron is probably the rate-determining step
64
feature as that of NSP-PC-2 but much higher overall NSP content. in ORR catalyzed by NSP-PC-2, similar to platinum. Thus, all these
However, NSP-PC-3 shows a poorer catalytic activity in terms of results suggest an excellent ORR activity for the catalyst NSP-PC-2,
smaller diffusion limiting current density and an E1/2 at 0.83 V, which is very comparable to that of commercial Pt/C catalyst.
which is 20 mV lower than that of NSP-PC-2 (see Fig. 5d). Such a
Fig. 5. (a) CV curves of different carbon catalysts recorded in O
curves of NSP-PC-1 and NSP-PC-2 compared to Pt/C and control catalysts in O
at different rotation speeds (rpm) in 0.1 M KOH. (e) The Koutecky−Levich (K-L) plots for NSP-PC-2 at different potentials (see Fig. S11 for
theoretical K-L plots for 2 and 4 electron-transfer, respectively). (f) RRDE test of the ORR on NSP-PC-2 in O -saturated 0.1 M KOH at 1600
rpm. The inset shows the n value against electrode potential. The loading of all nonprecious catalysts is 400 μg cm . The Pt loading is 40
2
-saturated (red line) or N
2
-saturated (black line) in 0.1 M KOH. (b, c) LSV
2
-saturated 0.1 M KOH at 1600 rpm. (d) LSV curves of NSP-PC-
2
2
−2
−
2
μgPt cm .a
aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
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Journal Name
PC-1 exhibits a significantly enhanced ORR catalytic activity
compared to solely N-doped counterpart, it unambiguously
suggests that additional S, P-doping can dramatically promote the
ORR activity of carbon catalyst. Moreover, the mesoporous catalyst
NSP-PC-2 shows an optimal catalytic activity in terms of an E1/2 at
0
.85 V (vs RHE) in alkaline media, which is closely comparable to
that of Pt/C and 70 mV higher than that of microporous NSP-PC-1.
The superior catalytic activity of NSP-PC-2 is mainly attributed to its
proper N, S, P-codoping (~11at.%) as well as its structural
advantages such as the high mesoporosity and moderately high
2
-1
surface area (613 m g ), which might make the active sites very
2
accessible and facilitates the O mass transfer.
Acknowledgements
This work is the supported in part by the Natural Science
Foundation of China (51372158), Jiangsu Specially Appointed
Professor Program (SR10800113), and the Project for Jiangsu
Scientific and Technological Innovation Team (2013).
Notes and references
Fig. 6. (a) The durability of the catalyst-modified electrodes for the
ORR in oxygen-saturated 0.1 M KOH. (b) i–t chronoamperometric
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a rotating speed of 1600 rpm in oxygen-saturated 0.1 M KOH. The
arrow indicates the addition of methanol into the electrolytic cell.
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TOC
The N/S/P-codoped carbon catalysts with varied nanostructures were facilely
prepared. The mesoporous carbon nanosheets exhibit the optimal catalytic activity for
ORR.
1