N,P-Doped carbon with encapsulated Co nanoparticles as efficient electrocatalysts for oxygen reduction reactions

Article abstract of DOI:10.1039/c8dt04650f

Exploring efficient non-noble ORR catalysts as alternatives to Pt-based catalysts are highly demanded for their possible application in fuel cells and rechargeable metal-air batteries. Herein, we demonstrate a rational design and synthesis of a N, P-doped carbon with encapsulated Co nanoparticles as efficient electrocatalysts for ORR. The catalyst is derived from a mixture of Co-MOF and triphenylphosphine with a mass ratio of 3?:?1 by pyrolysis in N₂ atmosphere at 700 °C. The catalyst exhibited a superior ORR catalytic performance among its counterparts in 0.1 M KOH with onset and half-wave potentials of 0.88 V and 0.80 V, a much larger limiting current density of ?5.93 mA cm^?2 that surpassed commercial 20% Pt/C, in addition to its durability and resistance to methanol. This enhanced ORR activity of the catalyst can be attributed to the synergistic effect between Co NPs and N, P atoms, the relatively large contact surface, more exposed active sites and good electrical conductivity. This study would provide some new ideas for the design and construction of promising carbon-based non-precious metal electrocatalysts for future practical fuel cell applications.

Full text of DOI:10.1039/c8dt04650f

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January 2016 Pages 1–398
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Efficient extraction of sulfate from water using
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DOI: 10.1039/C8DT04650F
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N, P‑Doped Carbon with Encapsulated Co Nanoparticles as
Efficient Electrocatalyst for Oxygen Reduction Reaction
Lili Fan,* Xinxin Du, Yuming Zhang, Mengfei Li, Meilian Wen, Xiyang Ge, Zixi Kang and Daofeng Sun*
Received 00th January 20xx,
Accepted 00th January 20xx
Exploring of efficient non-noble ORR catalysts alternative to Pt-based catalysts are highly demanded for fuel cells and
DOI: 10.1039/x0xx00000x
rechargeable metal-air batteries. Herein, we demonstrate a rational design and synthesis of a N, P‑doped carbon with
www.rsc.org/
encapsulated Co nanoparticles as efficient electrocatalyst for ORR. The catalyst is derived from a mixture of Co-MOF and
o
triphenylphosphine with mass ratio of 3:1 by pyrolysis in N
2
atomsphere at 700 C. The catalyst exhibits a superior ORR
catalytic performance among its counterparts in 0.1 M KOH with onset and half-wave potentials of 0.88 and 0.80 V, much
larger limiting current density of –5.93 mA cm-2 that surpasses the commercial 20 % Pt/C, in addition to the durability and
resistance to methanol. The enhanced ORR activity of the catalyst can be attributed to the synergistic effect between Co
NPs and N, P atoms, the relative large contact surface, more exposed active sites and good electrical conductivity. This work
would provide some new ideas for designing and constructing promising carbon based non-precious metal electrocatalysts
for future practical fuel-cell applications.
2
3, 36-40
Metal Co has been proved to be active in catalyzing ORR
Introduction
Worldwide concern about the energy shortage and
environmental issues stimulates the pursuit of clean and
sustainable energy storage and conversion technologies, such
and positive in promoting the graphitization degree of the
carbon, while the doped N atoms could modify the electronic
properties of neighbor carbon or transition-metal atoms,
facilitating the electron transport. Furthermore, abundant
defects and active sites could be generated by doping Co and N
into a carbon matrix, which play a crucial role in enhancing the
catalyst’s ORR performance.
1
-6
as renewable fuel cells and rechargeable metal-air batteries.
As one of the electrochemical conversion reactions that can
provide or store electrical energy efficiently, oxygen reduction
reaction (ORR) is of the great importance in these batteries.
However, the sluggish kinetics of ORR at the cathodes usually
To fabricate such M-N/C materials, the precursor method has
been adopted followed by a pyrolysis process in specific
atmosphere. A precursor can be the directly blended mixture
7
, 8
requires the use of electrocatalysts. Though platinum (Pt) and
its alloys are known to be highly efficient catalysts in boosting
ORR rate, commercialization of these catalysts is hindered by
their prohibitive cost and low stability. Hence, exploration of
cost-effective eletrocatalysts with competitive performance to
replace Pt-based materials has been a foremost subject of this
field.9
2
3, 41, 42
(individual metal, carbon and nitrogen sources),
the
renewable biomass resources (such as peanut shell, soy milk,
4
3, 44
45-52
etc.)
or the metal-organic frameworks (MOFs).
Generally, the as-synthesized M-N/C materials are expected to
inherit some structural characteristics from the precursors.
Even the minimal retention of essential fragments sometimes
can greatly affect the final performance of the material. In this
respect, MOFs are more attractive candidates and efforts have
been devoted to converting them into non-precious
metal/carbon based materials. There are several advantages in
using MOFs as precursors to fabricate the M-N/C materials: 1)
there are numerous transition metal-centered MOFs to choose;
-16
So far, great efforts have been devoted to exploring non-
noble catalysts for ORR, including transition-metal-coordinating
1
7, 18
macrocyclic compounds,
metal-nitrogen-doped carbon (M-
N/C),4
,
19-25
transition-metal carbides or nitrides,
26-29
3
0-33
heteroatom doped metal-free carbon materials,
and even
etc. Among these materials, the M-
N/C (M = Mn, Fe, Co, Ni) have been regarded as a very promising
3
4, 35
conducting polymers,
2
) metal centers and organic ligands are highly dispersed in
MOFs structure; 3) MOFs show great diversity in metal centers
and functional groups; 4) heteroatoms, such as N, can be in situ
doped into carbon matrix during pyrolysis; 5) MOFs possess
high specific surface area and porosity that may be inherited by
the carbon material. Though many carbon-based M-N/C
materials have been derived from MOFs, substantial studies are
still demanded dedicated to the design and synthesis of new M-
N/C materials for catalyzing ORR.
1
5,
family of candidates for ORR, especially the Co-N/C materials.
State Key Laboratory of Heavy Oil Processing, Institute of New Energy, School of
Materials Science and Engineering, China University of Petroleum (East China),
Qingdao 266580, P. R. China
E-mail addresses: lilifan@upc.edu.cn; dfsun@upc.edu.cn
Electronic Supplementary Information (ESI) available: [PXRD, N
desorption curves, SEM and TEM images and LSV curves]. See DOI:
0.1039/x0xx00000x
2
adsorption-
1
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2
atmosphere. After naturally cooling
o
-1
Herein, we report on the rational design and synthesis of a N, speed of 5 C min under N
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P‑doped carbon with encapsulated Co nanoparticles (NPs) as to ambient temperature, the sample wDOaIs: 10ri.n10s3e9d/C8wDitTh046H5C0lF,
efficient electrocatalyst for ORR. The catalyst was prepared washed with water to neutral and dried at 333 K for 10 h. The
from a facile strategy of pyrolysis of a Co-MOF and final sample was denoted as Co@NC.
triphenylphosphine mixture with mass ratio of 3:1 in N
2
Preparation of Co@NC-P catalysts:
o
atomsphere at 700 C (denoted as Co@NC-P(3-1)). The catalyst The as-prepared Co-MOF (0.6 g) was firstly fully mixed with
exhibits a superior ORR catalytic performance in 0.1 M KOH with triphenylphosphine (0.2 g) and placed in a crucible. Then, the
onset and half-wave potentials of 0.88 and 0.80 V, limiting crucible was transferred into the tube furnace. The pyrolysis
-2
current density of –5.93 mA cm , and excellent durability and and sample treatment procedures were the same as Co@NC.
resistance to methanol. The good ORR catalytic activity can be The sample obtained at this mass ratio of Co-MOF and
attributed to the fact that the as-synthesized catalyst inherits triphenylphosphine was denoted as Co@NC-P(3-1). The
some original advantages of the Co-MOF in terms of Co@NC-P(5-1) and Co@NC-P(1-1) catalysts were prepared in
homogeneous distribution of Co centers, N-rich ligands, and the same way but with different initial amount of
large surface area. Besides Co NPs, the heteroatoms (N, P) in triphenylphosphine of 0.12 g and 0.6 g, respectively.
carbon structure also act as exposed active sites. The synergistic Characterization:
effect generated between them facilitates the kinetics of ORR Powder X-ray diffraction (PXRD) patterns were acquired at
process. This work would provide some new ideas for designing room temperature using a XPert Pro MPD diffractometer with
and constructing promising carbon based non-precious metal Cu Kα radiation (λ = 1.5418 Å). The morphology and structure of
electrocatalysts for future practical fuel-cell applications.
the samples were observed on field-emission scanning electron
microscope (SEM, HITACHI S4800) and Transmission electron
microscopy (TEM) images were collected on a FEI Tecnai F30
with an acceleration voltage of 300 kV. A SSA-4300 surface area
and pore size distribution analyzer was applied to measure the
Brunauer Emmett Teller (BET) surface area. Raman spectrum
was recorded on a HORIBA Evolution Raman Microscope
spectrometer with the excitation wavelength of 532 nm. X-ray
photoelectron spectrum (XPS) measurements was carried out
on a Thermo ESCALAB 250 Spectrometer and the binding
energies were calibrated based on the graphite C 1s peak (284.6
eV). Inductively coupled plasma optical emission (ICP-OES)
results were collected via a Varian 715 ICP optical emission
spectrometer.
Experimental
Materials: 18-crown-6 was procured from Energy Chemical.
KOH, Ni(NO ) ·6H O, Tetrafluoroboric acid (HBF ) and N,N-
3 2 2 4
dimethylacetamide (DMA) were purchased from Sinopharm
Chemical Reagent Co. Ltd (China). Biphenyl-4,4′-dicarboxylate
(H
2
bpdc) was supplied by Chemsoon Co. Ltd. 4-cyanopyridine
was obtained from Alfa Aesar. Methanol (CH OH) was procured
from Tianjin Fuyu chemical reagent. The platinum on carbon
Pt/C, 20wt %) and Nafion solution (5wt %) were obtained from
3
(
Sigma-Aldrich. All the chemicals were used as received without
any further purification.
Synthesis of 2,4,6-tri(4-pyridinyl)-1,3,5-triazine (tpt): 1.0 g of
Electrochemical measurements:
All electrochemical measurements were performed on a Gamry
electrochemical work station (INTERFACE 1000 T) with a
rotating electrode system (RDE710 Rotating Electrode) at room
1
8-crown-6 and 0.224 g of KOH were dissolved into a beaker
with 10 mL of CH OH. After stirred for 5 min, the solution was
3
transferred into a 20 mL Teflon-lined stainless steel autoclave,
which was opened and maintained at 323 K for 3 h. Then 8.32 g
of 4-cyanopyridine was added into the above solution and
stirred for another 5 min. Thereafter, the autoclave was sealed
and kept at 473 K for 10 h. After cooling down to room
temperature, the product was filtered, washed with water to
neutral and dried at room temperature for 3 days. Finally, about
temperature.
A three-electrode cell configuration was
employed by using a glassy carbon rotating disk (RED, 5.0 mm
in diameter) as the working electrode, a platinum foil as the
counter electrode and a saturated calomel electrode (SCE) as
the reference electrode. The potentials were converted to its
potential vs. the reversible hydrogen electrode (RHE) by using
θ
the equation ERHE = ESCE + 0.059pH + E SCE in 0.1 M KOH. The
4
.5 g of brown solid was received (54 % yield).
Preparation of Co-MOF [Co (µ -OH)(bpdc) (tpt)]: The Co-MOF
was synthesized according to the previous work. Typically,
catalyst inks were prepared by dispersing 4 mg of sample and
40 µL of 5 wt % Nafion solution in a mixture solvent of 800 uL
water and 200 uL ethanol by 1 h sonication. Then 15 μL of the
catalyst ink was pipetted onto the pre-cleaned glassy carbon
3
3
3
5
3
certain amount of Co(NO
3
)
2
·6H
bpdc (0.024 g, 0.1 mmol) were
OH (v/v, 1:1) in 20
(40% in water) was added
2
O (0.058 g, 0.2 mmol), tpt
(0.031 g, 0.1 mmol) and H
2
-2
electrode with a loading equivalent to 0.294 mg cm . The linear
sweep voltammograms (LSVs) were collected at a scan rate of
dispersed into a 10 mL solution of DMA/CH
mL glass vial. Then, 100 μL of HBF
into the solution. After sonicated for 30 min, the mixture was
sealed and heated to 393K for 3 days. The red crystals were
3
4
-1
1
0 mV s . Cyclic voltammetry (CV) measurements were
conducted in a N -saturated or O -saturated 0.1 M KOH with a
2
2
-1
scan rate of 10 mV s . The catalytic durability for ORR was
investigated by chronoamperometry at 0.812 V vs. RHE for 10 h
with a rotating speed of 400 rpm. The kinetic parameters were
calculated using the Koutecky–Levich (K–L) equation:
filtered, washed with DMA and CH
0 h.
Preparation of Co@NC catalyst：
3
OH, and dried at 333 K for
1
The as-prepared Co-MOF (0.6 g) was placed in a crucible. Then,
the crucible was transferred into the tube furnace and heated
to 473 K for 1 h and 973 K for 3 h sequentially with a heating
1
1
1
1
1
=
+
=
+
1
2
푗
푗퐿 푗푘
푗푘
퐵휔
2
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DOI: 10.1039/C8DT04650F
Scheme 1. Schematic representation of the synthesis of
Co@NC-P materials.
B = 0.62nF퐶 퐷 3υ―1 6
2
0
0
k
where j is the measured current density, j is the kinetic-limiting
current density, ω is the angular velocity of the disk, n is the
electron transfer number, F is the Faraday constant (F = 96485
-
1
-6
C mol ), C
0
is the bulk concentration of O
cm in 0.1 M KOH), D is the diffusion coefficient of O
.9×10 cm s in 0.1 M KOH), and υ is the kinematic viscosity of
2
(C
0
= 1.2×10 mol
-3
0
2
(D
0
=
-
5
-1
1
2
-1
the electrolyte (υ = 0.01 cm s in 0.1 M KOH).
Figure 1. TEM images (a, b), selected-area electron diffraction
pattern (c), high-resolution TEM images (d-f), HAADF-STEM
image (g) and corresponding EDX elemental mappings of C (h),
N (i), Co (j) and P (k) respectively of Co@NC-P(3-1).
Results and discussion
The synthesis of Co@NC-P materials is illustrated in Scheme 1.
Red octahedral crystals of Co-MOF with good crystallinity were
(
Figure 1f). To demonstrate the distribution of participating
elements, elemental mappings were also recorded for Co@NC-
P(3-1) (Figure 1g-k). As reaveled, the retained Co element
cluster around where the Co NPs are found. A considerable
amount of N atoms is detected to be preserved and distributed
homogeneously through out the carbon matrix, in which, the P
atoms have also been confirmed to be successfully doped.
Although the doping amount of P atoms is low, they also play
an important role in regulating the local electron configuration
of carbon and enhancing the catalytic performance.
obtained via solvothermal reaction of Co(NO
3 2 2
) ·6H O, tpt and
2
H bpdc in a mixed solvent of DMA/methanol (Figure S1 and S2).
This Co-MOF was chosen as the precursor owing to its Co
center, N-containing ligands and the open structure, which are
considered as important factors to derive efficient ORR
electrocatalysts with hierarchical structures. To in situ dope P
atoms in the final structure, the Co-MOF was throughly mixed
with triphenylphosphine before the subsequent pyrolysis,
o
which was performed at 700 C under N
2
atmosphere by using
The structural crystallinity and composition of Co@NC-P
materials were also characterized by powder X-ray diffraction
a tube furnace. The hybird materials obtained from different
amount of triphenylphosphine were further treated with HCl to
remove the accessible Co species, giving Co@NC-P composites.
The morphology and microstructure of the sample derived
from the mixture of Co-MOF and triphenylphosphine with a
mass ratio of 3:1 (Co@NC-P(3-1)) were characterized by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). From the SEM images (Figure S3a and 3b), it
can be observed that the original crystal outline was not firmly
preserved after HCl treatment and it presents no particular
morphology. More details of the structure is revealed by TEM
images (Figure 1a, 1b and S3c, 3d). As shown, the organic
(PXRD) measurement. As shown in Figure 2a, the broad peak
appears around 24 ~ 25.8° can be ascribed to the
characteristicdiffraction peak of carbon ((JCPDS 41-1487). The
four diffraction peaks presented at 44.3°, 51.5°, 75.9° and 92.4°
match well with the (111), (200), (220) and (311) crystal planes
of Co metal (JCPDS 15-0806), respectively, which confirms that
the Co element preserved mainly exists in the form of Co metal.
Raman spectra were further measured to analyze the structural
defects and disordered graphitic structures of carbon in
Co@NC-P composites (Figure 2b). Two sharp characteristic
−1
peaks appearing at around 1333 (D band) and1583 cm (G band)
correspond to the disordered carbon and graphitic sp
2
ligands of tpt and H bpdc and the Co center in Co-MOF has been
2
successfully transformed into nanosheet-like carbon matrix and
Co NPs respectively in Co@NC-P(3-1). The Co NPs are wrapped
by graphitic carbon and highly dispersed in the carbon material
with an average diameter of around 12.3 nm (Figure 1d and
S3e), suggesting the ability of Co metal in promoting the
graphitization degree of carbon. High-resolution TEM image of
the Co NPs presents the lattice fringes with an interplanar
distance of 0.200 nm that can be ascribed to the lattice distance
of the (111) plane of Co metal (Figure 1e). The lattice spacing of
around 0.344 nm is corresponding to the (002) planes of carbon
hybridized carbon. The intensity ratio of the D band to G band
(I / I ) for Co@NC, Co@NC-P(5-1), Co@NC-P(3-1), and Co@NC-
D G
P(1-1) are calculated to be 1.00, 1.02, 1.35, and 0.98,
respectively. The Co@NC-P(3-1) sample presents the highest
D G
I /I value, suggesting more defects are generated in the
structure. It is believed to be caused by more heteroatoms
incorporation into the carbon structure at a initial mass ratio of
3
:1 for Co-MOF and triphenylphosphine, which is also
supported by XPS data in Table S1. The accurate Co contents in
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DOI: 10.1039/C8DT04650F
2
Figure 2. (a) PXRD patterns, (b) Raman spectra, (c) N adsorption-desorption isotherms (inset: pore-size distributions), (d) C1s XPS
spectra, (e) N1s XPS spectra, (f) Co2p3/2 XPS spectra of Co@NC, Co@NC-P(5-1), Co@NC-P(3-1), and Co@NC-P(1-1); (g) P2p XPS
spectra of Co@NC-P(5-1), Co@NC-P(3-1), and Co@NC-P(1-1); (h) different kinds of doping N species in Co@NC, Co@NC-P(5-1),
Co@NC-P(3-1), and Co@NC-P(1-1).
these materials were further confirmed by ICP-OES (Table S2). (289.1 eV). Fittings of the N 1s spectra of Co@NC-P materials
It is 32.08 %, 9.18 %, 16.36 % and 33.12 % for Co@NC, Co@NC- (Figure 2e) show four peaks that are assigned to the pyridinic N
P(5-1), Co@NC-P(3-1), and Co@NC-P(1-1), respectively. The (398.3 eV), Co−N (399.1 eV), pyrrolic N (400.0 eV), and graphitic
Co@NC-P(5-1) sample shows the lowest Co content. That is N (400.8 eV). The total nitrogen contents in Co@NC, Co@NC-
because the electron donating effect of triphenylphosphine P(5-1), Co@NC-P(3-1), and Co@NC-P(1-1) are 5.16, 3.71, 5.88
induced more severe collapse of the MOF structure during the and 4.91 at %, respectively (Figure 2h). The nitrogen doping
pyrolysis. More Co centers escaped from the carbon matrix and efficiency of Co@NC-P(3-1) is enhanced compared with Co@NC
were washed away by HCl treatment. As the amount of due to the introduction of appropriate amount of
triphenylphosphine increases, the MOF crystals were protected triphenylphosphine. Co@NC-P(3-1) shows the highest total
better by the benzene rings of triphenylphosphine, and less Co content of the two major nitrogen components in all the
centers could escape during the pyrolysis, leading to samples: pyridinic N and graphitic N, which are regarded as
2
3, 54-56
incremental increasement of Co conten in Co@NC-P(3-1), and effcient actives sites for ORR.
Co@NC-P(1-1). The N adsorption- desorption isotherms and (Figure 2f), the peaks at 778.3, 780.6, and 784.5 eV correspond
pore-size distributions (Fig. 2c) of the resulting samples show to the binding energies of metallic Co, CoO /CoC , and Co-N
For the Co 2p3/2 spectra
2
x
N
x x
y
that all the samples display a type IV isotherm with hysteresis. species, respectively. By the XPS measurements, the P element
The total BET surface areas are calculated to be 256, 179, 225 is comfirmed to be low in content (Figure 2g). Three peaks are
2
-1
and 211 m g , respectively. The higher heteroatoms doping deconvoluted corresponding to the binding energies of P 2p3/2
and larger surface area of Co@NC-P(3-1) are favorable for orbital (128.1 eV), P 2p1/2 orbital (129.1 eV) and oxidized P
catalyzing the ORR process.
(133.2 eV). In addition to N atoms, the P-doping also can induce
X-ray photoelectron spectrum (XPS) measurements were defects in the carbon framework and increase the electron
further performed to provide insights into the detailed chemical delocalization due to its good electron donating properties,
composition and bonding configuration of the as-synthesized promoting active sites towards ORR.57, 58
composites. The XPS spectra of the composites verify the
presence of elemental C, N, Co, P and O. Figure 2d shows the were evaluated by using the rotating disk electrode (RDE)
high-resolution C 1s spectra, which can be deconvoluted into measurements in 0.1 KOH electrolyte. Cyclic
C−C (284.6 eV), C=N (285.5 eV), C=O (286.5 eV), and O−C=O voltammograms(CV) were recorded to verify their ORR
The ORR kinetic processes in the as-synthesized composites
M
4
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key factors contributing to its good ORR performance. LSVs of
View Article Online
DOI: 10.1039/C8DT04650F
the samples at different rotating speeds from 400 to 2500 rpm
were also investigated (Figure 3d and S4-6). The corresponding
Koutecky-Levich (K-L) plots of Co@NC-P(3-1) show a good linear
relationship between J- and ω . The number of electrons
transferred (n) per oxygen molecule (at 0.39 V) derived from the
slopes indicates that Co@NC-P(3-1) (n = 3.97) participates in an
1
-1/2
efficient 4-electron pathway resulting in the direct reduction of
-
O
2
to HO in 0.1 M KOH solution.
For commercialization, the electrocatalytic durability of the
catalyst and its resistance to methanol crossover effect are two
very important aspects to be considered. To probe the
durability of Co@NC-P(3-1) in constant ORR process,
chronoamperometry at 0.812 V vs. RHE was conducted for 10 h.
The time-dependent current density curve is shown in Figure 3e
in comparison with 20 % Pt/C, which reveals better
electrochemical durability of Co@NC-P(3-1) with a decrement
of 9.4 % over 10 h, while the 20 % Pt/C presents a more obvious
degraded ORR performance with a current density drop of 29.4
%
. To discuss the reasons of the durability reduction of the
catalyst, PXRD and TEM was used to characterize the
morphology and structure of the catalyst after
chronoamperometry test (Figure S8). From the TEM images we
can see that the structures of carbon was well maintained
without any obvious changes. However, a certain degree of
agglomeration of the encapsulated Co nanoparticles and the
lattice fringes with an interplanar distance of 0.238 nm that can
be ascribed to the (311) plane of Co O phase are found in the
3 4
catalyst after the chronoamperometry test. To acquire the
PXRD pattern of the tested catalyst, we deposited the catalyst
2 2
Figure 3. (a) CV curves in N and O -saturated 0.1 M KOH
electrolyte, (b) LSV curves and (c) Tafel slopes of Co@NC,
Co@NC-P(5-1), Co@NC-P(3-1), Co@NC-P(1-1) and 20 % Pt/C,
respectively; (d) LSV curves of Co@NC-P(3-1) at different
-1
-1
rotation rates (inset: corresponding K-L plots of J vs. ω at
different potentials); (e) Chronoamperometric responses of
Co@NC-P(3-1) and 20 wt % Pt/C at + 0.812 V vs. RHE in an O
2
-
saturated 0.1 KOH solution (400 rpm); (f)
M
Chronoamperometric responses of Co@NC-P(3-1) and 20 wt % ink on a piece of titanium plate with the same loading density
Pt/C at + 0.812 V vs. RHE in an O -saturated 0.1 M KOH solution as the glassy carbon electrode. In the PXRD pattern, diffraction
2
-3
with the addition of 0.2 mol dm methanol (400 rpm).
peaks of Co metal are preserved, and small new peaks that can
be attributed to the Co phase also show up. From these
2 2
response in N - and O -saturated electrolyte (Figure 3a). As results, we can conclude that the agglomeration and oxidation
3 4
O
shown, all the samples present featureless slopes for of the encapsulated Co nanoparticles is the cause of the 9.4 %
cathodiccurrent within the entire potential range when the reduction of the ORR performance of the catalyst after 10 h
electrolyte is N
are observed in O
2
-saturated, while well-defined cathodic peak chronoamperometry test.
-saturated electrolyte. The Co@NC-P(3-1)
The influence of methanol crossover on Co@NC-P(3-1) and
2
shows the largest current density and most positive oxygen 20 % Pt/C was investigated by injecting equivalent of 0.2 mol
reduction peak centered around 0.82 V vs. reversible hydrogen dm-3 methanol into the 0.1 M KOH solution. As shown in Figure
electrode (RHE), indicating its superior ORR activity among all 3f, the addition of methanol triggers a drastic and sharp surge
the as-synthesized samples. Figure 3b compares the linear of current density of 20 % Pt/C that cannot be recovered to the
sweep voltammetry (LSV) curves recorded for these materials initial level. By contrast, the Co@NC-P(3-1) exhibits much
and that of the commercial 20 % Pt/C under the same mass stronger tolerance against chemical corrosion as compared to
loading in O -saturated 0.1 M KOH at 1600 rpm. Notably, 20 % Pt/C since no obvious decay in current density has been
2
Co@NC-P(3-1) exhibits prominent ORR activity, surpassing its observed. The graphitic carbon shells of Co NPs protect them
counterparts, with more positive values of onset potential of from corrosion in the alkaline solution, providing promising
0
.88 V and half-wave potential of 0.80 V. The diffusion-limited potential for future commercial applications.
-2
current density of –5.93 mA cm is larger than that of the
The above results suggest that the as-synthesized Co@NC-
commercial 20 % Pt/C (-5.46 mA cm ). The small Tafel slope of P(3-1) material is an efficient electrocatalyst for ORR. The good
Co@NC-P(3-1) (66 mV dec ) also demonstrates that the ORR performance can be attributed to the following three
Co@NC-P(3-1) catalyst possesses a faster electron transfer rate aspects: 1) The appropriate amount of encapsulated Co NPs
compared to that of 20 % Pt/C (88 mV dec ) (Figure 3c). From could facilitate the protonation of O and enable the full four-
-2
-1
-1
2
the Nyquist plots recorded at −150 mV vs. SCE (Figure S7), electron reduction pathway on the outer metal-free active
Co@NC-P(3-1) shows the smallest charge-transfer resistance, sites;36 2) N- and P-doping induces defects and increases the
suggesting its good electrical conductivity, which is one of the electron delocalization, promoting active sites towards ORR; 3)
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Journal Name
Relative large contact surface and good electrical conductivity 7.
also boosts the electrocatalytic performance. Compared with
Co@NC, the P atoms doping in Co@NC-P(3-1) increases the
D. Wang, H. L. Xin, R. Hovden, H. Wang, Y. YuV,ieDw.AArt.icMle Oulnlleinre,
F. J. DiSalvo and H. D. Abruna, NatDuOreI:m10a.1t0e3ri9a/lCs,8D20T01436, 5102F,
8
1-87.
8
.
Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier and H.
Dai, Nature materials, 2011, 10, 780-786.
exposed active sites and induces synergistic effect between N
and P atoms, enhancing its ORR activity. As for Co@NC-P(5-1)
and Co@NC-P(1-1), the limited N- and P-doping affects their
kinetic processes of ORR.
9
.
J. Greeley, I. E. Stephens, A. S. Bondarenko, T. P. Johansson,
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Conclusions
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1
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M. K. Debe, Nature, 2012, 486, 43-51.
In summary, a N, P‑doped carbon with encapsulated Co NPs was
synthesized as efficient ORR catalyst from pyrolysis of a mixture
of Co-MOF and triphenylphosphine with mass ratio of 3:1. In
this catalyst, Co NPs are wrapped by graphitic carbon shell, free
from corrosion in the alkaline solution. The N and P atoms were
in situ doped into the carbon matrix, promoting active sites
towards ORR. Compared with its counterparts, the catalyst
D. S. Su and G. Sun, Angewandte Chemie, 2011, 50, 11570-
1
1572.
1
3.
C. Chen, Y. Kang, Z. Huo, Z. Zhu, W. Huang, H. L. Xin, J.
D.Snyder, D. Li, JeffreyA.Herron, ManosMavrikakis,
MiaofangChi, K. L.More, Y. Li, NenadM.Markovic, G.
Somorjai, P. Yang and V. Stamenkovic, Science, 2014, 343,
1
339-1343.
shows a superior ORR performance with more positive onset 14.
and half-wave potentials of 0.88 and 0.80 V and much larger
limiting current density of –5.93 mA cm , which surpasses the 15.
H. L. Liu, F. Nosheen and X. Wang, Chemical Society
reviews, 2015, 44, 3056-3078.
F. Hu, H. Yang, C. Wang, Y. Zhang, H. Lu and Q. Wang, Small,
-2
2
017, 13.
commercial 20 % Pt/C, in addition to the durability and
1
1
1
1
6.
7.
8.
9.
Y. Tang, B. L. Allen, D. R. Kauffman and A. Star, J. AM.
CHEM. SOC., 2009, 131, 13200–13201.
resistance to methanol in 0.1 M KOH. The enhanced ORR
activity of Co@NC-P(3-1) can be attributed to the synergistic
effect between Co NPs and N, P atoms, the larger contact
surface, more exposed active sites and good electrical
conductivity. This work would provide some new ideas for
designing and constructing promising carbon based non-
precious metal electrocatalysts for future practical fuel-cell
applications.
H. Singh, N. Iyengar, R. Yadav, A. Rai and A. K. Sinha,
Sustainable Energy & Fuels, 2018, 2, 1699-1706.
H. Tang, H. Yin, J. Wang, N. Yang, D. Wang and Z. Tang,
Angewandte Chemie, 2013, 52, 5585-5589.
G. Wu, K. L. More, C. M. Johnston and P. Zelenay, Science,
2
011, 332, 443-447.
20.
S. Yasuda, A. Furuya, Y. Uchibori, J. Kim and K. Murakoshi,
Advanced Functional Materials, 2016, 26, 738-744.
S. Zhang, Y. Zhang, W. Jiang, X. Liu, S. Xu, R. Huo, F. Zhang
and J.-S. Hu, Carbon, 2016, 107, 162-170.
2
2
2
2
1.
2.
3.
4.
Conflicts of interest
There are no conflicts to declare.
M. Sun, D. Davenport, H. Liu, J. Qu, M. Elimelech and J. Li,
Journal of Materials Chemistry A, 2018, 6, 2527-2539.
Q. Cheng, L. Yang, L. Zou, Z. Zou, C. Chen, Z. Hu and H. Yang,
ACS Catalysis, 2017, 7, 6864-6871.
L. C. Pardo Pérez, N. R. Sahraie, J. Melke, P. Elsässer, D.
Teschner, X. Huang, R. Kraehnert, R. J. White, S. Enthaler,
P. Strasser and A. Fischer, Advanced Functional Materials,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant Nos. 21601205, 21571187,
2
018, 28, 1707551.
2
1501198), Taishan Scholar Foundation (ts201511019), and the
Fundamental Research Funds for the Central Universities
18CX02047A, 18CX07001A)
2
2
2
5.
6.
7.
S. Chen, J. Cheng, L. Ma, S. Zhou, X. Xu, C. Zhi, W. Zhang, L.
Zhi and J. A. Zapien, Nanoscale, 2018, 10, 10412-10419.
A. M. Alexander and J. S. Hargreaves, Chemical Society
reviews, 2010, 39, 4388-4401.
(
Y. N. Regmi, G. R. Waetzig, K. D. Duffee, S. M. Schmuecker,
J. M. Thode and B. M. Leonard, Journal of Materials
Chemistry A, 2015, 3, 10085-10091.
J. Luo, X. Tian, J. Zeng, Y. Li, H. Song and S. Liao, ACS
Catalysis, 2016, 6, 6165-6174.
M. D. Meganathan, S. Mao, T. Huang and G. Sun, Journal of
Materials Chemistry A, 2017, 5, 2972-2980.
S. Wang, L. Zhang, Z. Xia, A. Roy, D. W. Chang, J. B. Baek
and L. Dai, Angewandte Chemie, 2012, 51, 4209-4212.
J. Duan, S. Chen, M. Jaroniec and S. Z. Qiao, ACS Catalysis,
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