Functional Species Encapsulated in Nitrogen-Doped Porous Carbon as a Highly Efficient Catalyst for the Oxygen Reduction Reaction

Article abstract of DOI:10.1002/chem.201605026

The scarcity, high cost, and poor stability of precious metal-based electrocatalysts have stimulated the development of novel non-precious metal catalysts for the oxygen reduction reaction (ORR) for use in fuel cells and metal–air batteries. Here, we fabricated in situ a hybrid material (Co-W-C/N) with functional species (tungsten carbide and cobalt nanoparticles) encapsulated in an N-doped porous carbon framework, through a facile multi-constituent co-assembly method combined with subsequent annealing treatment. The unique structure favors the anchoring active nanoparticles and facilitates mass transfer steps. The homogenously distributed carbide nanoparticles and adjacent Co-N-C sites lead to the electrocatalytic synergism for the ORR. The existence of Co and W can promote the graphitization of the carbon matrix. Benefiting from its structural and material superiority, the Co-W-C/N electrocatalyst exhibits excellent electrocatalytic activity (with a half-wave potential of 0.774 V vs. reversible hydrogen electrode (RHE)), high stability (96.3 % of the initial current remaining after 9000 s of continuous operation), and good immunity against methanol in alkaline media.

Full text of DOI:10.1002/chem.201605026

A Journal of
Accepted Article
Title: Functional Species Encapsulated in Nitrogen-doped Porous
Carbon as a Highly Efficient Catalyst to Oxygen Reduction
Reaction
Authors: Li Song, Tao Wang, Yiou Ma, Hairong Xue, Hu Guo, Xiaoli
Fan, Wei Xia, Hao Gong, and Jianping He
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To be cited as: Chem. Eur. J. 10.1002/chem.201605026
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10.1002/chem.201605026
Chemistry - A European Journal
Functional Species Encapsulated in Nitrogen-doped Porous Carbon as a
Highly Efficient Catalyst to Oxygen Reduction Reaction
Li Song, Tao Wang, Yiou Ma, Hairong Xue, Hu Guo, Xiaoli Fan, Wei Xia, Hao Gong and Jianping He*
College of Materials Science and Technology, Jiangsu Key Laboratory of Materials and Technology for
Energy Conversion, Nanjing University of Aeronautics and Astronautics, 210016 Nanjing, PR China.
*Corresponding author. Tel: +86 25 52112906; Fax: +86 25 52112626.
E-mail address: jianph@nuaa.edu.cn
Abstract: The scarcity, high cost and poor stability of precious metal-based electrocatalysts have stimulated
the development of novel non-precious metal catalysts to oxygen reduction reaction (ORR) for the use in
fuel cells and metal-air batteries. Here, we in-situ fabricated a hybrid material (Co-W-C/N) with functional
species (tungsten carbide and cobalt nanoparticles) encapsulated in N-doped porous carbon framework,
through a facile multi-constituents co-assembly method combined with subsequent annealing treatment. The
unique structure is in favor of anchoring active nanoparticles and facilitating mass transfer steps. The
homogenously distributed carbide nanoparticles and adjacent Co-N-C sites lead to the electrocatalytic
synergism for ORR. The existence of Co and W can promote the graphitization of the carbon matrix.
Benefiting from its structural and material superiority, the Co-W-C/N electrocatalyst exhibits excellent
electrocatalytic activity (with a half-wave potential of 0.774 V vs. RHE), high stability (96.3% of the initial
current remaining after 9000 s of continuous operation) and prominent immunity against methanol in
alkaline media.
INTRODUCTION
Commercialization of new energy storage and conversion devices such as proton exchange membrane fuel
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cells (PEMFCs) and metal–air batteries is of global significance for renewable and sustainable energy
sources.[1-3] Currently, the major performance-limiting factor for these devices is the sluggish kinetics of the
oxygen reduction reaction (ORR) at the cathode.[4] Traditionally, platinum-based materials are regarded as
the most effective catalyst for the ORR, taking into consideration their high activity via the effective
four-electron-transfer pathway leading oxygen to water. But the prohibitive cost, low abundance and poor
durability severely hinder their large-scale application.[5, 6] The development of non-precious metal catalysts
(NPMCs) with both excellent activity and high stability, therefore, is pivotal to address this issue.[7, 8] Over
the past several decades, tremendous efforts have been devoted to the rational design and synthesis of
NPMCs as alternatives to Pt-based materials, including heteroatom-doped (N, P or S) carbonaceous
materials,[9-12] non-precious metal oxides,[13-15] carbides,[16-18] and transition-metal-N/C composites.[19-21]
Among the several promising catalysts, transition-metal carbide materials are extremely attractive due to
their high durability, favorable conductivity and superior corrosion resistance.[22, 23] Tungsten carbide (WC)
was discovered by Levy and Boudart to exhibit similar electronic structure and catalytic properties to
platinum group metals for several chemical reactions and regarded as a possible ORR catalyst.[24, 25]
Recently, great progress has been achieved in the exploration of the catalytic activity and stability of
WC-based materials. For example, an intermittent microwave heating assisted method was reported by Shen
to synthesize palladium and iron on nanocrystalline tungsten carbide (Pd/Fe-WC/C) as an ORR
electrocatalyst, which shows comparable activity to Pt/C.[16] In 2011, Liu et al. developed a new WC-based
catalyst via a molten solvent route with the high-surface-area WC as a noncarbon electrocatalyst support for
nanosized Pt clusters.[25] It was found that the activity of the Pt clusters is enhanced on WC compared with
supported on carbon, and the most likely cause of this enhancement is electron transfer between the catalyst
and support, leading to a rearrangement of the Pt d-band. In addition, WC shows extremely high stability
and anti-poisoning effect on carbon monoxide (CO) in electrocatalysis.[26] In spite of the intriguing
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characteristics, one limitation of previous studies is just utilizing WC as the electrocatalyst support of
precious metals,[16, 27] due to its inferior ORR performance in direct comparison to platinum. Non-precious
transition metal (cobalt or iron) and nitrogen doped carbon (Me−N−C) catalysts are another group of
promising NPMCs to ORR, whose active sites are generally speculated to be associated with N coordinated
metal structures (Me-Nx) with average coordination number x from 2 to 4 by X-ray absorption techniques or
Mössbauer spectroscopy.[7, 28, 29] But still, these reported Me-N-C catalysts are easy to deactivation in the
harsh electrolyte causing insufficient endurance, which limited the wide applications.[30]
One compromise is to integrate the two components WC and Me−N−C together. Herein, we report a
facile method to prepare a novel Co-W-C/N catalyst from the low-cost silicotungstic acid, cobalt nitrate,
dicyandiamide and phenol-formaldehyde through an evaporation-induced self-assembly (EISA) process and
subsequent thermal treatment. We use the highly volatile triblock copolymer Pluronic F127 (Mw =12 600)
as the template to construct the 3D porous system, in which the N-doped graphited carbon framework is
decorated by uniformly distributed functional species carbide and cobalt nanoparticles. The resulting
Co-W-C/N catalyst generated high activity and superior stability to ORR, due to abundant binary active sites
carbide nanoparticles and the adjacent Co-N-C species, the integrated synergism of porous structure, high
surface area and highly graphited conductive network.
RESULTS AND DISCUSSION
Figure 1 schematically illustrates the overall fabrication procedure of Co-W-C/N composite. It is
synthesized on the base of an EISA method using F127 as the scaffold template and resol as the carbon
source, which were co-mixed with silicotungstic acid, cobalt nitrate and dicyandiamide as the precursors.
Owing to the multiple phenolic hydroxyl groups, resol molecules can serve as a bridge to connect the
template F127 and the other precursors via hydrogen bonding and electrostatic interactions, which makes the
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three components co-assemble into ordered mesostructure during the EISA process.[31] During thermosetting
at 100 ºC for formation of the rigid phenolic resin and subsequent thermal treatment at 350 ºC for the
pyrolysis of the F127 template, dicyandiamide provides closed N species while resol can form a stable
framework. With this method, the metal ions and nitrogenous ligand would be in-situ incorporated into the
resultant carbon matrix, which would cause an electronic connection between the carbon framework and
metals.[32] After the carbonization at 750 ºC, the porous Co-W-C/N catalyst was successfully obtained with
the tungsten carbide and cobalt nanoparticles uniformly anchored in the 3D porous N-doped carbon
framework.
Figure 1. Schematics showing the formation of porous Co-W-C/N.
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Figure 2. (a) XRD patterns of Co-W-C/N as well as Co-N/C and W-C as references. The deconvoluted (b) W, (c) N 1s and
(d) C 1s spectra of the Co-W-C/N catalyst.
In order to identify the nature of the composites, XRD patterns of the three samples were recorded as shown
in Figure 2a. Several distinct diffraction peaks at 31.5°, 35.6°, 48.3°, 64.0°, 77.1° and 84.1° in the XRD
pattern of W-C can be indexed to (001), (100), (101), (110), (102) and (201) planes of WC (JCPDS No.
51-093) and four diffraction peaks at 40.2°, 58.2°, 73.1°and 87.0° are indexed to (110), (200), (211) and
(220) planes of metallic W (JCPDS No. 04-0806). The XRD pattern of Co-N-C sample shows the typical
diffraction peaks corresponding to metallic Co (JCPDS No. 15-0806), in which the diffraction peaks at
around 44° and 51° were assigned to the (111) and (200) facets of face-centered-cubic Co crystal. This result
indicates the conversion from Co2+ cations to metallic Co0 due to the carbothermal reduction, in good
accordance with the previous reports where Co nanoparticles was wrapped by the porous N-doped carbon
matrices under high temperature treatment.[33-36] The XRD pattern for Co-W-C/N exhibits the diffraction
peaks for both WC and metallic Co, as well as the peak of tiny amounts of W. Compared with the diffraction
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peaks in W-C and Co-N-C, the corresponding diffraction peaks in Co-W-C/N shows larger peak width at
half height, indicating the fine grain size. A broad diffraction peak is found in the XRD pattern of Co-W-C/N
at about 23o, which can be indexed to (002) plane of amorphous carbon framework.[33, 37] In addition, the
XRD pattern for Co-W-C/N shows a small diffraction peak of graphitic carbon at about 26o, demonstrating
the formation of graphitic structure,[38] which is ascribed to the effect of catalytic graphitization on
amorphous carbon by the transition metals.[34, 39, 40] One important thing to note is that we sucessfully obtain
WC at a relative low temperature of 750 oC, which is generally synthesized either by carbothermic reduction
at a high temperature above 1000 oC or at the assistance of reducing ambient.[25, 41] Actually the similar
result can be also observed for Co-W-C (Figure S1). So we can conclude that Co can accelerate the
formation of tungsten carbide.
The chemical environment of the catalyst was carefully investigated by the XPS. The XPS survey
spectrum is exhibited in Figure S2, clearly demonstrating the presences of W, Co, C, N and O elements.
Figure 2b shows the enlarged W spectrum of Co-W-C/N, which is deconvoluted into five major components.
According to previous reports, the three peaks located at 31.6, 33.7, and 37.7 are corresponding to W 4f7/2,
W 4f5/2 and W 5p3/2 peaks of tungsten carbide. The relatively higher ones at 35.8 and 38.3 eV should be
assigned to W 4f7/2 and W 4f5/2 orbits of W6+, respectively.[42-44] The formation of W oxide characteristic
peak can be ascribed to unavoidable oxidation in air atmosphere. The high-resolution N1s spectrum (Figure
2c) of the samples can be mainly fitted by three peaks centered at 398.6, 400.1 and 401.6 eV, which are
assignable to pyridinic-N, pyrrolic-N and graphitic-N in the pyrolyzed products.[36, 45] There exists large
amount of pyridinic-N and graphitic-N in Co-W-C/N, which are reported to play a crucial role in ORR
performance.[30, 46, 47] Pyridinic N can serve as metal-coordination sites due to the lone-pair electrons,
forming metal-N/C configurations.[48-50] The high-resolution spectrum C1s spectrum of Co-W-C/N (Figure
2d) centers at 284.8 eV with an asymmetric tail at the higher binding energies, which demonstrates that the
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carbon atoms are connected with N or O heteroatoms. The deconvolutions of the C1s spectrum are
corresponding to four peaks at 284.7, 285.2, 286.2 and 288.1 eV, which are associated with sp2 graphitic
lattice C=C&C–C, C–N&C–O, C=N&C=O and O-C=O, respectively.[51, 52] The XPS spectra are consistent
with the result of XRD, not only revealing the successful doping of N into the carbon framework but also
indicating the rich WC and Co-N-C active species in the catalyst.
SEM and TEM images in Figure 3a, 3b and S2a clearly disclose the well-dispersed nanoparticles in the
carbon matrix with the parallel cylindrical nanopores. Figure 3c illustrates the N2 adsorption-desorption
isotherm of Co-W-C/N composite and the corresponding pore size distribution in the inset. It exhibits a
type-IV curve and a H1-type hysteresis loop, suggesting the existence of typical cylindrical mesopores.[53, 54]
The pore size distribution plot demonstrates that Co-W-C/N possesses a narrow pore size distribution
centered at around 4.6 nm. The mesopores in the carbon framework endows the catalyst with high
accessibility to electrolyte infiltration and feasible diffusion path for reactants.[55] The BET surface area and
total pore volume are determined to be 558 m2 g-1 and 0.47 cm3 g-1 as shown in Table S1. From Figure 3d,
we can conclude that most of the nanoparticles are in the range of 5~20 nm. A deep look at the catalyst in Figure
3e, S2b and S2c demonstrates that the catalyst consists of the flocculent carbon (or quasi-nano-ring)
nanostructure, which is similar to the results of the relative researches.[56, 57] The bright-field high-resolution
TEM (HRTEM) image in the inset of Figure 3e reveals that the carbon atom layers are closely compacted with
well-ordered arrangement, forming the graphite shells, in keeping with the results of XRD. Figure 3f shows that
Co nanoparticles are well crystallized with the interlayer spacing of 0.198 nm, in accordance with the
theoretical value of the (111) plane, and surrounded by the graphite layers with a lattice distance of 0.331 nm.
The formation of the crystalline graphite shells structure was thought to remarkably improve the electrical
conductivity, being of importance for electrocatalytic activity.[34, 45, 58] Figure S3d and S3e clearly exhibits the
lattice overlap of WC (101) with a lattice distance of 0.187 nm and Co (200) with an interlayer spacing value of
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0.178 nm. The high-angle annular dark-field scanning TEM (HAADF-STEM) images and EDX elemental
mappings in Figure 3g shows that the Co, W and N elements were homogeneously distributed over the carbon
framework. The successful incorporation of N into the carbon framework would not only ameliorates the surface
wettability but also constitutes the Co-N-C configuration combined with the Co crystals.[34, 48] Thus, we endow
the porous carbon with rich WC and Co-N-C active species and unique structure with these nanoparticles
encapsulated by the graphitic carbon shell.
Figure 3. (a) SEM and (b) TEM images of Co-W-C/N. (c) N2 adsorption-desorption isotherms of Co-W-C/N. The inset of
(c) exhibits the corresponding pore size distribution. (d) The corresponding particle size distribution of the nanocrystals in
(b). (e, f) HRTEM images of Co-W-C/N. The fast Fourier transform (FFT) patterns taken from selected area in (f) is
employed to verify the crystalline structure. (g) HAADF STEM image and EDX mapping of Co-W-C/N.
To evaluate the electrocatalytic properties toward ORR, cyclic voltammetry (CV) measurements (Figure
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4a) were first performed for Co-W-C/N and commercial 20 % Pt/C catalysts in O2- and N2-saturated 0.1 M
KOH solution. Both the catalysts exhibit a pronounced cathodic peak in O2-saturated 0.1 M KOH solution,
while no peaks can be observed at the presence of N2, confirming a substantial ORR process. Shown in
Figure 4b are the steady-state linear sweep voltammetric (LSV) curves of W-C, Co-W-C, Co-N/C,
Co-W-C/N, Co-W-C/N-acid (Co-W-C/N after etching of 1 M hydrochloric acid)and Pt/C catalysts under a
same rotation rate of 1600 rpm in O2-saturated 0.1 M KOH solution. Among the investigated samples,
Co-W-C/N exhibits significantly enhanced ORR activity compared to those control catalysts W-C, Co-W-C,
Co-N/C and Co-W-C/N-acid. The values of onset potential Eonset and half-wave potential E1/2 for Co-W-C/N
are 0.960 V and 0.774 V, which are close to those of Pt/C. Note that the current density Co-W-C/N
outperforms that of Pt/C at high overpotential, which is due to the large specific surface area of Co-W-C/N.
W-C exhibits very poor ORR activity in terms of Eonset and E1/2, similar to the results of the related
researches.[42, 44] On introducing cobalt nitrate, the ORR electrocatalytic performance of the corresponding
catalyst can be greatly improved. It is noteworthy that while both cobalt and nitrogen were introduced into
the catalyst, the ORR activity of the catalyst Co-W-C/N tremendously leaped forward in terms of a
remarkably positively shifted E1/2, which is 94 and 63 mV positive than that of the catalysts W-C and
Co-N/C, respectively. This is ascribed to the coexistence of rich Co-N/C active sites and carbide species.
Combined with the results of XRD, we can conclude that the strong synergetic coupled effect of carbide and
Co-N-C active sites endows the catalyst higher activity than either of the single configuration. What's
interesting is that the ORR activity of Co-W-N/C after etching of 1 M hydrochloric acid is significantly
degraded. This can be explained the loss of the most Co species, with only WC remained, according the
XRD pattern (Figure S4). The comparison of electrocatalytic ORR activity, including Eonset and E1/2 for
Co-W-C/N and other reported nonprecious electrocatalysts, are also shown in Table S2. The Tafel plots
(Figure S5) can be constructed from the RDE polarization curves by taking the logarithm of the kinetic
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current density. The corresponding Tafel slope of Co-W-C/N is calculated to be 86 mV dec-1 and close to
that of Pt/C (69 mV dec-1), indicating that Co-W-C/N undergoes an analogous ORR mechanism to Pt/C.[59]
The lower Tafel slope given by the cobaltic samples compared to W-C should be due to the catalytic
graphitization effect of Co, which would lead to enhanced electrical conductivity.[34, 60, 61] The low Tafel
slope makes the ORR overpotential rise slowly with current density increasing, suggesting that oxygen is
easily adsorbed and activated at its surface.[15] To gain deep insights into the ORR mechanism of Co-W-C/N,
rotating ring-disc electrode (RRDE) measurements were conducted at room temperature in O2-saturated 0.1
M KOH solution. The ring current (ir) (the oxidation of H2O2) of Co-W-C/N is negligible in comparison to
the disc current (id), indicating the strikingly suppressed H2O2 evolution during the ORR process (Figure 4c).
The H2O2 yield and the electron transfer number (n) of Co-W-C/N and Pt/C are derived from the ring and
disc current according to Eqs. (5) and (6), as presented in Figure 4e and 4f. It is revealed that the H2O2 yield
on Co-W-C/N were below 16% over the scanned potential range. The corresponding electron transfer
number n during the reaction is almost constant at 3.91 from 0.9 to 0.6 V and gradually decreases to ~3.64 at
potentials higher than 0.4 V. Specifically, the H2O2 yield on Co-W-C/N was ~4% at E1/2, compared with 1.73%
on Pt/C. The low H2O2 yield and near optimal four electron pathway are close to the RRDE results of Pt/C,
suggesting a near 4-electron reaction dominant pathway.
The RDE polarization curves of the Co-W-C/N at various rotation speeds from 400 to 2025 rpm were also
measured to get more information about the ORR kinetics and to further validate whether it catalyzes the
ORR via a four electron ORR process. Figure S6 shows that the voltammetric profiles for both Co-W-C/N
and Pt/C exhibit obvious diffusion-limiting currents after a mixed kinetic diffusion region and an increased
current intensity with increasing rotating rate due to the shortened diffusion distance.[62] The good linearity
between J−1 and ω−1/2 and the nice parallelism of the fitting lines of the Koutecky-Levich (K-L) plots for
Co-W-C/N suggest the first-order reaction kinetics toward the concentration of dissolved O2 and similar
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values of electron transfer number over the potential range. On the basis of K−L equations, described in the
supporting information in detail, the average electron transfer number (n) for Pt/C and Co-W-C/N at 0.3-0.6
V can be determined from the fitting slopes of the K-L plots to be about 4.01 and 3.89, respectively,
approximate to 4. This conclusion is in agreement with the calculated results of RRDE measurements,
further confirming the outstanding ORR activity of Co-W-C/N.
Figure 4. (a) CV curves of Co-W-C/N and 20 wt% Pt/C at a scan rate of 20 mV s-1 in O2-saturated (solid line) or
N2-saturated (dash line) 0.1 M KOH solution. (b) Steady-state ORR polarization curves of different catalysts at 1600 rpm
with a sweep rate of 5 mV s-1 in O2-saturated 0.1 M KOH solution. RRDE voltammograms for (c) Co-W-C/N and (d) 20 wt%
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Pt/C. Hydrogen peroxide yield and corresponding electron transfer number during ORR for (e) Co-W-C/N and (f) 20 wt%
Pt/C.
Chronoamperometry was performed by holding the electrodes for 9 000 s at a constant potential of 0.2 V
in O2-saturated 0.1 M KOH solution to evaluate the stability of the catalysts. Figure 5a shows that about
96.3% relative current for Co-W-C/N still remains in comparison with up to 16.4 % loss of its initial current
over Pt/C catalyst, demonstrating a much better stability of Co-W-C/N than Pt/C. An accelerated durability
test was carried out to further testify the long-term stability of Co-W-C/N in catalyzing ORR by cycling the
potential over the range 1.0-0.4 V in O2-saturated 0.1 M KOH at a scan rate of 50 mV s-1 for 5 000
continuous cycles. As shown in Figure 5b, Pt/C exhibited 81 mV negative shift of E1/2 and significant
attenuation in diffusion-limited current density, which is commonly observed in Pt/C caused by the
dissolution of Pt nanoparticles.[30, 63] However, Co-W-C/N shows almost no changes of Eonset and only very
slight shift of E1/2 (9 mV) after 5 000 cycles. The slight shift for Eonset and E1/2 indicates the superior stability
of the active sites in Co-W-C/N. The small decrease of limited current density can be found for Co-W-C/N,
which should be attributed to the unavoidable oxidation of the carbon matrix.[30] These results are well
consistent to the features of the I-t plots and confirm the superior durability of Co-W-C/N to that of Pt/C.
Methanol tolerance is one of the major concerns for the potential application in direct methanol fuel cells,
where methanol may cross over the membrane and deteriorate the ORR. Figure 5d shows the influence of
methanol addition into O2-saturated 0.1 M KOH solution on the instant current curves of Co-W-C/N and
Pt/C catalysts under identical experimental conditions. With the injection of 3 M methanol at 500 s, the
current slightly decreases over the Co-W-C/N while a rapid current falloff is observed in the ORR current of
the Pt/C electrode, suggesting that Co-W-C/N demonstrates a much stronger tolerance to methanol
crossover.
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Figure 5. (a) I–t chronoamperometric stability measurements (at 0.2 V) of Co-W-C/N and 20 wt% Pt/C in O2-saturated
0.1 M KOH solution at a rotating speed of 1600 rpm. Polarization curves at 1600 rpm for (b) Pt/C and (c) Co-W-C/N before
and after 5000 cycles. (d) Methanol crossover tests for Co-W-C/N and Pt/C catalysts performed on the RDE (1600 rpm)
by injecting of 3 M methanol into the electrolyte.
All the elctrocatalytic measurements indicate that Co-W-C/N is a promising candidate to Pt/C as the
ORR catalyst. Such excellent properties can be ascribed to the rational design of Co-W-C/N: (1) Both WC
nanoparticles and Co-N-C species are uniformly distributed in the interior of rigid porous carbon matrix.
This unique structure on the one hand improves the transportation of the reactants, intermediates, products,
and on the other hand inhibits the self-aggregation during the catalysis process, thus preventing the loss of
available active sites. (2) The synergistic effect between the stable WC nanoparticles and the active Co-N-C
species accelerates the ORR. (3) Highly graphitized carbon shell outside the active species can provide the
fast electron-transport pathway. (4) The introduced heteroatom N would cause the polarized distribution of
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spin and charge density, which is considered as the one of the reasons for the high activity.[64] The
synergistic effect between lone electron pairs of N and the π-conjugated system of graphite would cause a
unique electronic properties: the replacement of the carbon (C) atom with n-type dopant in graphitized
carbon results in the donation of electrons to O2, leading to the improvement of ORR activity.[65] Figure 6
described the schematic microstructure and ORR process on the Co-W-C/N, in which WC and Co-N-C
species are firmly anchored inside the N-doped porous carbon channel and well protected by the graphitized
carbon shell. The integral porous system provides the thorough route for the infiltration of electrolyte and
transportation of reactants. The metal nanocrystal encased in graphitized carbon layers (M@C) structure has
been confirmed to be an efficient design for electrocatalysis.[45, 66] The special structure usually shows high
chemical durability because the outer carbon layers can protect the inner active species from direct contact
with external environment. It can avoid the oxidization and corrosion of the active nanoparticles during the
long-time ORR process, and furthermore prevent agglomeration with the adjacent nanoparticles.
Figure 6. Schematic illustration of the microstructure and ORR process on the Co-W-C/N.
CONCLUSION
In summary, a hybrid ORR catalyst was successfully fabricated through a simple procedure involving the
process of multi-constituents co-assembly and the subsequent carbonization process. The resultant catalyst
(Co-W-C/N) consists of tungsten carbide and Co-N-C species, which decorate the porous N-doped carbon
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framework together. Benefiting from the high active sites yield and enhanced electron transfer capacity, this
catalyst exhibits excellent electrocatalytic performance for ORR and superior durability to Pt/C. Co-W-C/N
also shows better tolerance against methanol than Pt/C. The strong synergetic coupled effect between
tungsten carbide and Co-N-C species was testified to enhance the ORR activity. These results lead us to
firmly believe that Co-W-C/N hybrid catalyst is a promising non-precious metal electrocatalyst for fuel cells
and metal–air batteries.
Experimental
Fabrication of porous Co-W-C/N
The porous Co-W-C/N nanocomposite was synthesized through a multi-component co-assembly pathway. A
typical synthetic process is shown as follows: 1.0 g of F127 was dissolved in 10 mL ethanol to form a clear
solution. Simultaneously, a certain amount of silicotungstic acid (H4SiW12O40), cobalt nitrate (Co(NO3)2
9H2O) and dicyandiamide (C2H4N4) were dissolved into ethanol to form uniform solutions, respectively.
After 30 min, 5.0 g of soluble resol solution (20 wt% in ethanol) was added dropwise into the F127 solution
and vigorously stirred for 10 min. Subsequently, the other precursor solutions were added into the above
mixture in turns. The obtained mixture solution was further stirred for 1 h and cast onto Petri dishes to
evaporate ethanol at room temperature for 12 h. Then, the obtained sticky membrane was subjected to
thermo-polymerization in an oven at 100 oC for 24 h. At last, the as-prepared orange membrane was scraped
off and pyrolyzed in tube furnace at nitrogen atmosphere. The membrane was calcined at 350 oC for 3 h and
then at 750 oC for 3 h. In order to better understand the catalytic performances and reveal the active sites in
the catalyst Co-W-C/N, three control catalysts were prepared under the similar procedures as Co-W-C/N
except for no addition of dicyandiamide, silicotungstic acid, both cobalt nitrate and dicyandiamide, noted as
Co-W-C, Co-N/C and W-C, respectively.
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10.1002/chem.201605026
Chemistry - A European Journal
Structural and morphological characterization
The crystalline phases of Co-W-C/N were identified by Powder X-ray diffraction (XRD) using a Bruker D8
advance diffractometer with a Cu Kα radiation (λ= 0.154056 nm) at 40 kV and 40 mA, operated at a scan
rate of 6° min-1. Transmission electron microscope (TEM, FEI Tecnai G2 at 200 kV) was used to analyze
the distribution and size of the nanoparticles involved in the N-doped ordered mesoporous carbon matrix.
STEM-EDX mapping was performed to qualitatively investigate the local element composition of the
catalysts through the energy dispersive X-ray spectrum attached to the FEI Tecnai G2 system. Nitrogen
adsorption-desorption isotherms (Micromeritics ASAP 2020, 77 K) were measured to confirm the porous
morphology. The surface areas were estimated utilizing the Brunauer-Emmett-Teller (BET) method based
on the adsorption data. The pore size distributions derived from the adsorption branche of isotherms were
estimated based on the Barrett-Joyner-Halenda (BJH) model and the total pore volumes (Vtotal) were
calculated from the adsorbed amount at a relative pressure P/P0 of 0.993. X-ray photoelectron spectroscopy
(XPS) spectra were recorded on a PHI Quantera SXM surface analyzer (ULVAC-PHI Inc., Japan) with an
Al Ka source.
Electrochemical measurements
All electrochemical measurements were performed on a CHI760D potentiostat with a typical three-electrode
cell. The three-electrode system consists of a glassy carbon (GC) electrode (Pine Instrumentation, 5 mm for
RDE and 5.6 mm for RRDE) coated with the catalyst ink as the working electrode, a saturated calomel
electrode (SCE) as the reference and a Pt foil as the counter electrodes, respectively. All potentials reported
in this work have been converted to the RHE scale. Catalysts were loaded on the GC electrode by dropwise
casting the catalyst ethanol suspension, which consists of catalyst powder, conductive carbon black (Vulcan
o
XC-72), 5 wt.% Nafion ethanol solution and absolute ethanol, and dried at 50 C to form a thin layer. The
loading is controlled at 0.625 mg cm-2. Vulcan XC-72-supported 20 wt.% Pt catalyst with a loading of 25 µg
16
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10.1002/chem.201605026
Chemistry - A European Journal
Pt cm-2 was also measured for comparison. All the electrochemical measurements were carried out in N2- or
O2-saturated 0.1 M KOH at room temperature. The cyclic voltammetry measurements (CVs) were scanned
between 1.2 and 0.4 V with a scan rate of 20 mV s-1. The linear sweep voltammetrys (LSVs) was performed
via RDE technique to collect the polarization curves at a scan rate of 5 mV s-1 from 1.2 to 0.1 V at various
rotating speeds from 400 to 2025 rpm. In chronoamperometric responses (I−t plots), the potential was set to
0.2 V in O2-saturated 0.1 mol L-1 KOH for 9000 s. The stability tests were carried out in the O2-saturated
electrolyte for 5000 CV cycles in the potential range from 1 to 0.4 V and the RDE polarization curves before
and after 5000 CV cycles were recorded to test the durability of Co-W-C/N catalyst. Methanol resistance
tests were performed through the chronoamperometric response by adding 3 M methanol into the electrolyte.
For RRDE tests, the disk electrode was scanned at a rate of 5 mV s-1 and the ring electrode potential was set
to 1.4 V. The hydrogen peroxide yield [H2O2(%)] and the electron transfer number (n) were determined
from the following equations:
I
r
N
H2O（%）= 200
2
Id + I
r
N
Id
Id + I
n = 4
r
N
where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring,
which was determined to be 0.40 from the reduction of K3Fe[CN]6.
Acknowledgements
Our sincere thanks go to the financial support of National Natural Science Foundation of China (51372115
& 11575084) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD).
Keywords: Tungsten carbide, nitrogen-doping, oxygen reduction reaction, non-noble metal catalysts
17
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10.1002/chem.201605026
Chemistry - A European Journal
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Graphical Abstract
This structure with functional species encapsulated in nitrogen-doped porous carbon is in favor of
anchoring active nanoparticles and facilitating mass transfer steps. The homogenously distributed tungsten
carbide nanoparticles and the adjacent Co-N-C sites lead to the electrocatalytic synergism for ORR.
Benefiting from its structural and material superiority, the Co-W-C/N electrocatalyst exhibits excellent
electrocatalytic activity, high stability and prominent immunity against methanol.
20
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