Rational Design of Hierarchical, Porous, Co-Supported, N-Doped Carbon Architectures as Electrocatalyst for Oxygen Reduction

Article abstract of DOI:10.1002/cssc.201903053

Developing highly active nonprecious-metal catalysts for the oxygen reduction reaction (ORR) is of great significance for reducing the cost of fuel cells. 3D-ordered porous structures could substantially improve the performance of the catalysts because of their excellent mass-diffusion properties and high specific surface areas. Herein, ordered porous ZIF-67 was prepared by forced molding of a polystyrene template, and Co-supported, N-doped, 3D-ordered porous carbon (Co-NOPC) was obtained after further carbonization. Co-NOPC exhibited excellent performance for the ORR in an alkaline medium with a half-wave potential of 0.86 V vs. reversible hydrogen electrode (RHE), which is higher than that of the state-of-the-art Pt/C (0.85 V vs. RHE). Moreover, the substantially improved catalytic performance of Co-NOPC compared with Co-supported, N-doped carbon revealed the key role of its hierarchical porosity in boosting the ORR. Co-NOPC also exhibited a close-to-ideal four-electron transfer path, long-term durability, and resistance to methanol penetration, which make it promising for large-scale application.

Full text of DOI:10.1002/cssc.201903053

Accepted Article
Title: Rational Design of Hierarchical Porous Cobalt, Nitrogen-doped
Carbon Architectures as Efficient Oxygen Reduction Reaction
Electrocatalysts
Authors: Damien Voiry, Mengfei Qiao, Ying Wang, Xamxikamar
Mamat, Anran Chen, Guoan Zou, Lei Li, Guangzhi Hu,
Shusheng Zhang, and Xun Hu
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10.1002/cssc.201903053
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Rational Design of Hierarchical Porous Cobalt, Nitrogen-doped
Carbon Architectures as Efficient Oxygen Reduction Reaction
Electrocatalysts
Mengfei Qiao,‡[a,g] Ying Wang,‡[a] Xamxikamar Mamat,‡[a] Anran Chen,[b] Guoan Zou,[a] Lei Li,*[c]
Guangzhi Hu,*[a,b,c] Shusheng Zhang,[d] Xun Hu,[e] and Damien Voiry,*[f]
Abstract: Developing highly active non-precious metal catalysts for
the oxygen reduction reaction (ORR) is of great significance to
reduce the cost of fuel cells. Three-dimensional ordered porous
structures could substantially improve the performance of the
catalysts because of its excellent mass diffusion properties and high
specific surface area. Herein, we prepared ordered porous ZIF-67 by
forced molding of a polystyrene template and obtained cobalt-
supported, nitrogen-doped three-dimensional ordered porous carbon
(Co-NOPC) after further carbonation. The resultant Co-NOPC
exhibits excellent performance for the ORR in an alkaline medium
with a half-wave potential (E1/2) of 0.86 V vs. RHE, w hich is greater
than that of the state-of-the-art Pt/C (0.85V vs. RHE). Moreover, the
substantially improved catalytic performance of Co-NOPC compared
to cobalt-supported, nitrogen-doped carbon (Co-NC) reveals the key
role of its hierarchical porosity for boosting ORR. Co-NOPC also
performs a close-to-ideal four-electron transfer path, long-term
durability, and methanol penetration resistance, making it promising
for large-scale application.
exchange membrane fuel cells (PEMFCs) are expected to play
an important role in next-generation energy technologies.[2] One
major bottleneck to the commercial application of PEMFCs lies
in the insufficient activity and durability of the electrocatalysts to
accelerate the sluggish kinetics of the oxygen reduction reaction
(ORR) at the cathode.[3] For high-performance catalysts, most
theoretical and experimental studies believe that the active sites
should have an intermediate adsorption strength for the reactant
and its intermediates to reduce reaction barrier, and the
substrate should have a high specific surface area and rich pore
structure, thus achieving rapid mass transfer on electrode
surface.[4] Meeting all these conditions is a major bottleneck in
the search for ideal ORR catalysts. Notably, the control of the
porosity in order to optimize both the surface area and the
efficient diffusion of the O2 to the active sites request the
preparation of catalytic materials w ith hierarchical porosity.[5]
State-of-the-art catalysts Pt/C, w herein Pt nanoparticles are
supported on high-surface-area carbon, exhibit a relatively low
overpotential and ideal four-electron transmission path for the
ORR.[6] How ever, the high cost, resource constraints, poor
stability and nonresistance to methanol penetration, hinder its
large-scale application, necessitating the search for a better
alternative.[7] Based on current research, metal–nitrogen–carbon
(M–N–C) catalysts, w here M denotes a transition metal (typically
w ith M = Fe or Co), are the most promising low -cost substitutes
for Pt-based catalysts.[8] Fe-N-C catalysts show the highest
Introduction
In recent decades, issues such as energy shortage and
environmental pollution due to the extensive exploitation and
utilization of fossil fuels have stimulated the development of new
energy materials and energy conversion technologies.[1] With the
advantages of high energy density, environmental friendliness,
easy implementation, and unlimited sources of reactants, proton
[a]
M.F.Qiao, Y.Wang, X. Mamat, Prof. G.A.Zou, Prof. G.Z.hu
Key Laboratory of Chemistry of Plant Resources in Arid Regions
State Key Laboratory Basis of Xinjiang Indigenous Medicinal Plants
Resource Utilization, Xinjiang Technical,Institute of Physics and
Chemistry,Chinese Academy of Science
[d]
[e]
[f]
Prof. S.S.Zhang
College of Chemistry and Molecular Engineering, Zhengzhou
University Zhengzhou, 450000, China.
Prof. X.Hu
School of Material Science and Engineering,University of Jinan, Jinan
250022, China.
Prof. D. Voiry
Urumqi 830011, China.
Emali : guangzhihu@ynu.edu.cn
[b]
[c]
Dr. A.R.Chen, Prof. G.Z.hu
Institut Européen des Membranes, IEM, UMR 5635, Université
Montpellier, ENSCM, CNRS, 34095 Montpellier Cedex5, France
Emali: damien.voiry@umontpellier.fr
School of Chemical Science and Technology, School of Energy,
Yunnan University ,Kunming 650091, China
Prof. L.li, Prof. G.Z.hu
College of Biological, Chemical Sciences and Engineering, Jiaxing
University
[g]
M.F.Qiao
University of Chinese Academy of Sciences, Beijing 100049, China.
Those authors contributed equally to the work.
‡
Emali : leili@mail.zjxu.edu.cn
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activity among all M–N–C materials, but the Fenton reactions
(Fe2+ + H2O2), w here dissolved Fe ions combine w ith H2O2, w ill
generate significant amounts of active oxygen-containing
hydroxyl and hydroperoxyl radicals, thereby leading to the
irreversible erosion and degradation of the electrodes and
diaphragm.[9] Therefore, Co-N-C based catalysts may be a
promising substitute for Pt/C w hen considering both, a high
catalytic activity and operational stability over reasonable service
life.[10]
Herein, w e demonstrate the successful synthesis of porous
ZIF-67 (OP-ZIF-67) w ith the accurate control of the macro and
microporosity by developing an original approach that combines
hard template method and ultrasonic nucleation.[18] The ordered
MOF superstructure translates, after carbonization, into
optimized hierarchical order macro-microporous carbon skeleton
for electrocatalytic materials, enabling efficient diffusion of the
reactant to the actives w hile maximizing the density of active
sites. The resultant carbonized product, cobalt-supported,
nitrogen-doped three-dimensional ordered porous carbon (Co-
NOPC), exhibits enhanced catalytic activity and long-term
stability tow ards ORR under both acidic and alkaline conditions.
The half-w ave potential of the obtained Co-NOPC w as 0.86 V vs.
RHE in 0.1M KOH electrolyte, w hich is substantially greater than
the 0.80 V vs. RHE of the cobalt-supported, nitrogen-doped
carbon catalysts (Co-NC) obtained by carbonizing w ell-defined
ZIF-67. The enhanced ORR activity of Co-NOPC is attributed to
the high intrinsic activity of the Co-Nx structure, adequate active
sites exposed by the higher specific surface area, and a higher
diffusion speed imparted by the orderly three-dimensional
porous structure. Significantly, our study shed light on an
innovative approach for enhancing the performance tow ards
ORR or other electrocatalytic reaction based on MOF-derived
materials.
In order to achieve a breakthrough in performance, many
inspiring w orks have focused on obtaining a perfect combination
of Co-N-C structure and excellent substrate materials.[11]
Recently, metal–organic framew orks (MOFs), w hich possess
structural diversity, easy functionalization and high specific
surface area, have been selected as
a self-sacrificing
precursor/template for preparing nanoporous carbons.[12] Among
the MOFs, ZIF-67, w hich consists of Co2+ and 2-medetomidine,
has received w ide attention ow ing to easy synthesis, high
porosity of the zeolite-like topology, and particularly it could be
an ideal source of carbon and nitrogen. The coordination
structure of Co2+ and 2-methylimidazole can be in situ-
transformed into a Co-Nx structure, and the pore structure can
be w ell inherited during the pyrolysis of ZIF-67, w hich endow s it
w ith high intrinsic activity and active site stability.[13] How ever,
their catalytic activity is limited because of deep burial of the
active sites and low mass transfer of inherit microporous
structure of ZIF-67, w hich stimulates the researcher to resolve
this problem by combining w ith excellent conductive and high
surface area carbon materials such as graphene, carbon
nanofibers etc..[14] Shen et al. reported a kind of ZIF-67 and
reduced graphene oxide derived carbon composite named ZIF-
C/rGO, w here the rGO integration led to better charge transfer
channels and more micropores to load active sites.[15] Yin et al.
synthesized ZIF-67/PAN by the in-situ grow th of ZIF-67
nanocrystals on PAN fibers, and further carbonized it at 800 °C
to obtain ZIF-67/PAN-800, w hich also show ed significantly
improved performance.[16] Although these attempts have
improved the catalytic performance of oxygen reduction to a
certain extent, they have failed to achieved benchmarks from
practical applications. Introducing macropores into ZIF-67
should be a good idea to achieve efficient utilization of active
sites by exposing more surfaces, meanw hile, macro/mesopores
w ill also improve the mass transfer of macromolecules near the
double layer.[17]
Results and Discussion
The method for the synthesis of Co-NOPC is illustrated in Figure
1a. First, polystyrene spheres (PS) templates w ere fully
immersed in a methanol solution of cobalt nitrate hexahydrate
and 2-methylimidazole. After ultrasonic nucleation and static
grow th at room temperature, PS@ZIF-67, w hich is a mixture of
PS and ZIF-67, w as prepared. Then, the precursor OP-ZIF-67
was then obtained by repeatedly w ashing w ith tetrahydrofuran
(THF) to remove the PS template. The OP-ZIF-67 engraved
reversely by the PS template has a three-dimensional ordered
porous structure. After further carbonation at 500 °C, the cobalt,
nitrogen-doped ordered porous carbon catalysts (Co-NOPC)
was finally obtained w ith w ell-preserved porous structure of the
precursor. The ordered porous structure of Co-NOPC w ith the
combination of macro/meso/micropores is expected to not only
provide a high specific surface area to load enough active sites,
but also facilitates the mass transfer process. On the other hand,
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dramatic migration and aggregation of
metallic Co occurs after pyrolysis, w hich
is attributed to the in-situ conversion of
coordination of Co and N under mild
conditions.[20]
For comparison, the
morphology of conventional ZIF-67 and
carbonation derivative (Co-NC) w ere also
investigated in Figure S3.
Figure 3a show s the X-ray diffraction
(XRD) patterns of ZIF-67, OP-ZIF-67 and
simulated ZIF-67. The consistency of
peak types among the three samples
confirms that ZIF-67 and OP-ZIF-67 both
formed a complete crystal structure w ith
no impurity phase in the materials.[21] The
XRD patterns of the final samples of Co-
NOPC and Co-NC are show n in Figure
Figure 1. (a) Schematic of the Co-NOPC synthesis. (b) SEM images of the Co-NOPC.
(c-e) SEM images of OP-ZIF-67. (f-h) SEM images of Co-NOPC.
the Co-Nx active sites obtained by the in-situ conversion of Co2+
3b. The diffraction peaks appearing at approximately 23.3° and
43.5° can be assigned to the (002) and (100) planes of the
graphite structure.[22] Moreover, there are no significant metal
peaks in XRD, w hich indicates that no obvious metal
nanoparticles w ere formed after annealing at 500 °C.
Furthermore, the XRD patterns of the catalysts synthesized at
different temperatures are compared in Figure S4. The Co-
and 2-methylimidazole complexes confer to the catalyst high
intrinsic activity and long-term stability.
To get information on the morphology and structure, the
pristine OP-ZIF-67 and the carbonized Co-NOPC w ere
investigated by field-emission scanning electron microscopy
(FESEM) and high-resolution transmission electron microscopy
(HRTEM). Figure 1c-e show s the typical SEM images of OP-ZIF-
67 at various magnifications. OP-ZIF-67 is found to adopt a w ell-
defined rhombic dodecahedron morphology w ith a particle size
of 1 μm, w here macropores of approximately 110 nm are
regularly distributed throughout the single-crystal particles.
Additionally, the Co-NOPC, obtained by the further carbonation
of OP-ZIF-67 at 500 °C, w as characterized by SEM (Figure 1b,
f-h). Although there is a certain degree of collapses, the pore
structure and morphology are vastly consistent w ith those of the
precursor. The internal structure of OP-ZIF-67 characterized by
HRTEM (Figure 2a-c) further revealed that the macropores are
regularly distributed throughout the single-crystal particles,
which w ere engraved reversely by the PS template (Figure S2).
After thermal treatment at 500 °C, the crystal surface (Figure 2d-
f) becomes no longer smooth as before, and the hierarchical
ordered macro-micropore transform into inter-connected carbon
skeleton. The relatively disordered carbon phase (Figure 2f)
implies that abundant defects, such as vacancies, dislocations
and shear defects in favor of ORR catalysis exist in the carbon
structure.[19] The EDS mapping in Figure 2g demonstrate that no
Figure 2. (a-c) HRTEM images of OP-ZIF-67. (d-f) HRTEM
images of Co-NOPC. (g) EDS mapping of Co-NOPC.
NOPC-500, Co-NOPC-600, and Co-NOPC-700 w ere derived
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from the same precursor OP-ZIF-67-3:4, w here 3:4 represents
the mass ratio of Co(NO3)2·6H2O and 2-methylimidazole, but
annealed at different temperatures of 500 °C, 600 °C, and
700 °C, respectively. The apparent metal peaks of Co for Co-
NOPC-600 and Co-NOPC-700 suggest an intensity reduction
from Co2+ to Co0 and inevitable aggregation of the metal atoms
when the carbonation temperature exceeds 600 °C, w hich w ill
result in a decrease of the active sites.[23]
originates from the structure of the ZIF-67 and forms during the
graphitization process. How ever, a higher specific surface area
of Co-NOPC (418 m2 g-1) compared w ith that of Co-NC (214 m2
g-1) confirms that ordered macropores are responsible for the
increased specific surface area. Raman spectra w ere analyzed
to characterize the change in the carbon phase w ith the
synthesis temperature (Figure S6). The higher ID/IG ratio of Co-
NOPC-500 reveals that an over-high temperature w ill reduce the
defect positions w hich are typically active site centers.[25] This
result agrees w ell w ith that of XRD, in that Co2+ w ill be quickly
reduced to Co0 w ith increasing temperature, and that the
recombination of chemical bonds w ill also reduce the number of
active sites.[26]
The catalysts obtained at different temperature w ere
characterized and compared
by
X-ray
photoelectron
spectroscopy (XPS).[27] The survey of Co-NOPC (Figure S7,
Table S1 and Table S2) reveals the presence of C 1s, N 1s, O
1s, and Co 2p elements, w ithout any impurities. The content of
N 1s, O 1s, and Co 2p decrease w hile the content of C 1s
increases w ith the increasing pyrolysis temperature. The high-
resolution XPS spectrum of Co 2p3/2 (Figure 4a) can be fitted
into tw o components, corresponding to Co0 (776.9 eV) and Co2+
(779.1 eV).[28] Co2+ is know n to be more beneficial to the
catalytic activity w hen in the form of coordinated Co-Nx obtained
through the in-situ transformation of the coordination structure of
Co 2+ and 2-methylimidazole. On the other hand, Co0, obtained
through the reduction product of Co2+, tends to agglomeration
and lose its ORR activity.[29] The relative proportion of Co2+ over
Co(0) reduces from 91.5% to 68.6% w hen the annealing
temperature increases from 500 °C to 700 °C, verifying the
advantage of mild-temperature graphitization (Figure 4c). To
precisely determine the close relationship betw een active site
centers and chemical environment of the doped nitrogen, the N
1s spectra of all the samples w ith different pyrolysis
temperatures w ere analyzed. Figure 4b show s that the high-
resolution N 1s spectrum split into four subpeaks at 397.3 eV,
398.1 eV, 399.2 eV and 401.6 eV, corresponding to pyridinic
(Py-N), pyrrolic (Pyr-N), graphitic like (Gra-N), and oxidized type
(Oxi-N) nitrogen, respectively. According to the literature, Py-N
and Gra-N can effectively boost oxygen adsorption to reduce the
ORR overpotential because of their excellent electron-accepting
abilities.30] According to the literature, Py-N and Gra-N can
effectively boost oxygen adsorption to reduce the ORR
overpotential because of their excellent electron-accepting
Figure 3. (a) XRD patterns of the precursors ZIF-67 and OP-
ZIF-67. (b) XRD patterns of the catalysts Co-NC and Co-NOPC.
(c) Nitrogen adsorption-desorption isotherms and (d)
corresponding pore size distributions of Co-NOPC and Co-NC.
To further investigate the influence of the pyrolysis
temperature on the structure of the catalyst, nitrogen adsorption
experiments w ere performed for comparing the specific surface
area and pore size distribution of the samples synthesized at
different temperatures. Co-NOPC-500, w hich w as annealed at
500 °C, had a better nitrogen physical adsorption capacity
compared w ith the samples synthesized at 600 °C or 700 °C
(Figure S5). The BET surface areas of Co-NOPC-500, Co-
NOPC-600, and Co-NOPC-700 are 418 m2 g-1, 362 m2 g-1, and
314 m2 g-1, respectively. The similar isotherms for Co-NOPC and
Co-NC at low pressure indicate a similar microporous structure
(Figure 3c), w hereas the pore size distribution curves (Figure
3d) illustrate that both Co-NOPC and Co-NC have abundant
mesopores w ith a pore size of ~3.6 nm, w hich are formed during
pyrolysis. These results show that the introduction of
macropores has no significant effect on the formation of
micropores and mesopores. We believe that the mesoporosity
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to the grow th of single crystal MOF particles w hile a high cobalt
mass ratio may lead to the uncontrolled shapes of crystal
particles (Figure S8). Furthermore, w e evaluated the effect of
feed amount of Co(NO3)2·6H2O to active sites using XPS (Figure
S9). The proportion of Co-Nx also tends to increase w ith
increasing Co content, and reaches the maximum w hen the ratio
of Co(NO3)2·6H2O and 2-methylimidazole is 3:4. In addition,
compared to controlled sample Co-NC, Co-NOPC-3:4 had a
higher fitting Co2+/Co0 ratio, w hich might be related to a better
coordination structures. All these evidences suggest that Co-
NOPC-3:4 is the most suitable candidate for the oxygen
evolution reaction.
The catalytic performance of Co-NOPC and Co-NC samples
for ORR w ere firstly evaluated in O2-saturated 0.1 M KOH on a
rotating disk electrode (RDE) at a sw eep rate of 10 mV s−1 and a
rotation rate of 1600 rpm. The cyclic voltammetry (CV)
measurements w ere employed to evaluate the ORR catalytic
activity of the as-prepared catalysts and the benchmark Pt/C (20
wt.%) catalyst (Figure S10). The CV curves of all the tested
catalysts show ed obvious oxygen reduction peaks in O2-
saturated electrolytes, w hereas featureless CV curves w ere
obtained in N2-saturated electrolytes. Notably, the more positive
reduction peak potential of Co-NOPC (0.83 V), compared to
0.80 V and 0.83 V (vs. RHE) of Co-NC and Pt/C, respectively,
suggests higher ORR catalytic activity. To further assess the
ORR activities of the tested catalysts, w e recorded the linear
sw eep voltammetry (LSV) polarization curves (Figure 5a). The
onset potentials (Eonset) of Co-NOPC, Co-NC, and Pt/C w ere
0.95 V, 0.92 V, and 0.98 V (vs. RHE) respectively. The potential
at w hich the reduction current sharply decreases on the LSV
curves is know n as Eonset. Experimentally, w e used cross-point
potential of tangent lines from the rising current and baseline
current to determine the onset potential of the reaction. The half-
wave potential (E1/2) of Co-NOPC (0.86 V) is obviously improved
compared to that of Co-NC (0.82 V) and even compares
favorably to commercial Pt/C catalyst (0.85 V). Additionally, the
Tafel slope – calculated from the LSV curves – of Co-NOPC (72
mV) is significantly low er than that of Pt/C (91 mV) confirming
Co-NOPC is an efficient ORR catalyst w ith improved reaction
kinetics compared to Pt/C (Figure 5b). The enhanced steady-
state diffusion current density (jL = 5.2 mA cm−2) of Co-NOPC is
comparable to that of commercial Pt/C (5.3 mA cm−2). The low er
overpotential and larger limit current density from Co-NOPC is
attributed to its ordered hierarchical macro/meso/microporosity
Figure 4. The fine spectra of (a) Co 2p and (b) N 1s of Co-
NOPC synthesized at different temperatures. (c) Corresponding
fitting mass ratio of Co2+ and Co0. (d) Corresponding fitting
mass ratio of N w ith different chemical structures.
[31]
abilities.
In addition, part of pyridinic N and pyrrolic N atoms
can serve as coordination sites for Co in the form of N-
coordinated Co structures: Co-Nx (x = 2 and 4) as previously
identified using X-ray absorption spectroscopy (XAS) and
Mössbauer spectroscopy[.[19,32] Density functional theory (DFT)
in agreement w ith experimental studies has suggested that Co-
Nx has a low er energy barrier for oxygen dissociation and thus
acts as active site in the catalytic ORR process from Co-N-C
based catalysts [10b, 11, 29] As show n in the Figure 4d, the ratios of
Py -N and Pyr-N in Co-NOPC rapidly decrease w ith the increase
of annealing temperature. On the other hand, the rapid grow th of
Gra-N signals at increasing annealing temperature verifies the
results of Raman spectra. In order to optimize catalytic
performance, w e adjusted the ratio of precursors for the
synthesis of Co-NOPC. A series of OP-ZIF-67 typed materials
was synthesized w ith different ratios of Co2+ and 2-
methylimidazole precursor and named as OP-ZIF-67-n:m, w here
n:m represents the feeding mass ratios of Co( NO3)2·6H2O:2-
methylimidazole. We found that the regular single crystal
structure of ZIF-67 w ith ordered porosity w as only formed at the
mass ratios of 3:4 of Co(NO3)2·6H2O:2-methylimidazole. Our
analyses also reveal that a low cobalt mass ratio does not lead
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temperature. To get additional understanding of the influence of
temperature, w e characterized the samples synthesized at
different temperatures by XRD analysis, BET surface area
determination, Raman spectroscopy, and XPS analysis. The
presence of apparent Co patterns of Co-NOPC-600 and Co-
NOPC-700 (Figure S4) indicates that the reduction of the Co2+ to
Co 0, implying a significant reduction of active sites Co-Nx (Figure
4d) w hen the annealing temperature exceeded 600 °C, w hich
matches w ell w ith the XPS (Figure 4) and Raman spectra
observations (Figure S6), w here the defect sites in the carbon
phase decrease w ith increasing temperature.[34] In addition, the
reductions of specific surface area and average pore size w ith
enhanced temperature (Figure S5) suggest that higher
temperature is responsible for the partial shrinkage of the
material structural frame resulting on the reduced mass transfer.
Besides the annealing temperature, the mass ratio of
Co(NO3)2·6H2O:2-methylimidazole also influences the density of
active sites because the coordination structure of ZIF-67 w ill
translates, at the end, into tunable catalytic activity. Our
analyses of the relationship betw een pyrrolic/pyridinic N content
(Figure S9c) and the half-w ave potential (Figure S12) support
the fact that Co-Nx active sites play a central role in delivering
high ORR catalytic performance.
Figure 5. (a) ORR polarization curves of samples in O2
saturated 0.1 M KOH solution w ith a scan rate of 10 mV s−1 and
(b) Tafel plot of corresponding polarization curves. (c) LSV
curves of Co-NOPC at different rotating speeds w ith a scan rate
of 10 mV s−1 and (d) the corresponding Koutecky-Levich plots.
(e) Chronoamperometric response and (f) methanol crossover
tests chronoamperometric response of Co-NOPC and Pt/C at
half-w ave potential in 0.1 M KOH.
Next, w e investigated the reaction mechanism from Co-NOPC
by determining the number of electrons involved in the reaction
of oxygen. According to the Damjanovic model, a direct 4e-
pathw ay, involving reduction of O2 to H2O, is more efficient than
a 2e- pathw ay, in w hich O2 is reduced to H2O2 and further to H2O
sequentially. Hence LSV curves tested at various rotating
speeds w ere used to evaluate the ORR kinetics of Co-NOPC
and Pt/C (Figure 5c). The fitted Koutecky-Levich (K-L) plots of
Co-NOPC ranged from 0.78 to 0.58 V (vs. RHE) indicates that
the reaction on Co-NOPC follow s the desirable 4e- reaction
pathw ay and negligible formation of H2O2 during the ORR
process (Figure 5d). The electron transfer mechanism and the
formation of peroxide intermediates on Co-NOPC during ORR
were also monitored by rotating ring-disk electrode (RRDE)
measurements. Based on the recorded ring and disk and ring
currents (Figure S13a, b), the average transferred electron
number (n) w as 3.93 and the yield of H2O2 is limited to less than
5% (Figure S13c, d), proving that the four-electron reaction
pathw ay is proceeding under alkaline conditions.
leading to more exposed active sites and accelerated mass
transfer.[33] The influence of the hierarchical porosity is clearly
noticeable w hen comparing the steady state current from Co-
NOPC and Co-NC. On the other hand, the small Tafel slope and
the positive Eonset may originate from the high intrinsic activity of
Co-Nx (the coordinated structure of Co w ith the pyrrolic/pyridinic
N), w here N-coordinated Co sites serve as the ORR catalytic
active sites.
To further improve the catalytic activity, w e explored the
influence of the annealing temperature and of the ratio of
precursors: Co(NO3)2·6H2O and 2-methylimidazole. The control
of annealing temperature is indeed likely to modify the formation
of active sites and carbon phase of the substrate material.
Figure S11 show s that both the onset potential and the half-
wave potential are reduced w ith increasing annealing
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To verify the practical applications of the catalysts,
chronoamperometric measurements w ere used to evaluate their
selectivity and long-term durability in O2-saturated 0.1 M KOH
solution at the half-w ave potential of 0.86 V.[35] For long-term
durability, Co-NOPC exhibits minimal loss of activity w ith a
retention of 85% after 24 h, w hich is significantly larger than
commercial Pt/C that exhibits an 69% retention, corresponding
to 2-fold larger loss of activity (Figure 5e).[36] To assess the
consequences of methanol crossover on the performance of Co-
NOPC, 10 mL of methanol w as instantaneously added to 100
mL of 0.1 M KOH solution to investigate methanol tolerance of
catalysts during the chronoamperometric measurements (Figure
5f).[37] Upon the addition of methanol, the current of commercial
Pt/C encounter a reversal from negative to positive, w here the
reaction on the electrode is transformed from ORR into
methanol oxidation reaction (MOR). On the other hand, only a
slight vibration w as observed for Co-NOPC, implying its greater
resistance to methanol oxidation. In conclusion, the enhanced
catalytic performance from Co-NOPC tow ards ORR in terms of a
low er overpotential, a larger diffusion controlled current density,
and a reduced Tafel slope, are attributed to the large surface
area exposed superior density of exposed active sites and the
ordered hierarchical macro-microporous structure promoted
mass transfer. On the other hand, the almost ideal four-electron
transfer pathw ay and excellent long-term durability of Co-NOPC
is due to the preference formation of Co-Nx by in-situ pyrolysis at
mild temperature. Excessive annealing temperature leads to
progressive inactivation of active sites, w hile optimized ratio of
precursor is found beneficial for the formation of Co-Nx structure.
While the investigation of the ORR activity in alkaline
environment allow s screening the electrocatalyst candidates,
proton-exchange membrane fuel cells (PEMFCs) requires the
use of ORR electrocatalysts that can also efficiently operate in
acid medium. This inspired us to further explore the
electrocatalytic ORR performance of the Co-NOPC in acid
solutions using O2-saturated 0.5 M H2SO4. The CV curves of the
different samples are show n in Figure S14, and the LSV
polarization curves and corresponding Tafel slopes are
illustrated in Figure 6a and Figure 6b. The half-w ave potentials
of Pt/C, Co-NOPC, and Co-NC w ere 0.82 V, 0.76 V, and 0.72 V,
respectively. Remarkably minimal gap betw een the catalytic
activities of Co-NOPC compared to that of commercial Pt/C w as
controlled porosity can be an efficient catalyst for ORR operated
for operating ORR in acidic conditions. Both RDE (Figure S15a,
b) and RRDE (Figure 6c, d) investigations w ere employed to
reveal the desirable 4e- reaction pathw ay of Co-NOPC for ORR.
The number of transferred electrons and the percentage of H2O
is found to be close to 4e- and low er than 5 % respectively at low
overpotential – closed to the values measured for commercial
Pt/C. Finally, the long-term tolerance and anti-methanol
performance of Co-NOPC reveal its potential for practical
applications for ORR in acid solution (Figure S15c, d).[15]
Figure 6. (a) ORR polarization curves of samples in O2
saturated 0.5 M H2SO4 solution w ith a scan rate of 10 mV s−1
and (b) the Tafel plot of corresponding polarization curves. (c)
The disk current and the ring current of Co-NOPC and Pt/C. (d)
The electron transfer number and the hydrogen peroxide yield
calculated based on the corresponding RRDE data.
Conclusions
In summary, w e demonstrated the synthetic method of
ordered porous ZIF-67 using hard template and ultrasonic
nucleation, follow ed by carbonization to obtain cobalt-supported
nitrogen-doped three-dimensional ordered porous carbon w ith
controlled hierarchical porosity. Compared to Co-NC derived
from the original ZIF-67, the three-dimensional ordered porous
structure of Co-NOPC display ordered hierarchical macro-
microporous structure w ith higher specific surface area to
maximize the density of exposed active sites w hile allow ing
excellent mass transfer. ORR performance tests show ed that
observed. We also observed
a substantial improvement
compared to Co-NC clearly demonstrating the Co-NOPC w ith
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10.1002/cssc.201903053
ChemSusChem
FULL PAPER
by stirring. Then, the PS template was soaked in the above solution and
treated by ultrasound for 30 s to nucleate. After 24 h of static growth at
room temperature, the bulk composite of ZIF-67@PS was formed after
further dried at 40 °C. The obtained ZIF-67@PS was then dissolved in
THF for 12 h to removed PS. After multiple centrifuging, washing and
drying at 110 °C, the ordered porous ZIF-67 (OP-ZIF-67) was collected.
Under nitrogen, the blue precursor was carbonized at 500 °C for 2 h to
obtain cobalt-supported, nitrogen-doped 3D ordered porous carbon (Co-
NOPC). For comparison, control samples, named as Co-NOPC-1:4, Co-
NOPC-2:4, Co-NOPC-3:4, Co-NOPC-4:4 were prepared by adding
Co(NO3)2·6H2O (n) and 2-methylimidazole (m) with different mass ratios:
n:m. Co-NOPC-500, Co-NOPC-600 and Co-NOPC-700 was synthesized
with the same mass ratio at 3:4 of Co(NO3)2·6H2O and 2-methylimidazole
but different temperaturesof 500 °C, 600°C and 700°C, respectively.
Synthesis of ZIF-67 and Co-NC. For comparison, conventional ZIF-67
and carbonation derivative (Co-NC) were also synthesized according to
the following procedure. Firstly, 4.5 g of cobalt nitrate hexahydrate and 6
g of 2-methylimidazole were dissolved in 250 mL of methanol. After full
mixing, the mixed solution was treated under ultrasonic for 20 min to
nucleate and then stirred at room temperature and pressure for 12 h.
Finally, ZIF-67 was obtained by several times washing with methanol,
centrifugal collection and drying at 110 °C. The precursor of conventional
ZIF-67 was annealed at 500 °C for 2 h with nitrogen protection to obtain
Co-NC.
the half-w ave potential of the final catalyst Co-NOPC (0.86 V)
surpasses that of the state-of-the-art Pt/C catalyst (0.85 V) in
alkaline solutions. Remarkably, the ORR activity in acidic have
demonstrated a substantial improvement of the ORR for Co-
NOPC compared to Co-NC. The superior ORR activity originates
from the optimization of the density of Co-Nx active sites in the
materials and the controlled hierarchical porosity of the structure
that induce both improved half-w ave potential and diffusion
limited current density. Long-term tolerance and stability against
methanol crossover in acidic and basic media make Co-NOPC
an affordable alternative of Pt/C. Overall our results demonstrate
a novel practical strategy for modifying MOF-derived materials
for ORR. We expect such strategy can be extended to other
electrocatalytic reactions for w hich the performances are limited
by poor diffusion of the reactants to the active sites.
Experimental Section
Materials. All the chemicals used in the experiment were analytical purity
and no further purification was made. 2-methylimidazole (98%), cobalt
nitrate hexahydrate (Co(NO3)2·6H2O, 98%), potassium persulphate
(K2S2O8, 99%) and potassium hydroxide (KOH, 99.99%) were purchased
from Aladdin (Shanghai, China). Nafion solution (5%), polyvinyl
pyrrolidone (PVP, 30 k), sodium hydrate (NaOH, 96%) and isopropanol
(99.7%) were supplied by Sigma-Aldrich (Shanghai, China). Methanol
(CH3OH, 99%), sulphuric acid (H2SO4, 98%), styrene (99%) and
tetrahydrofuran (THF, 99%) were obtained from Sheng Jie Yi Xuan Trade
Co. Ltd. (Urumchi, China). Pt/Vulcan (Pt loading 20%, Premetek, U.S.)
sample labeled as Pt/C obtained from Sigma-Aldrich (Shanghai, China)
was used to evaluate catalytic activity as benchmark for ORR. Deionized
water (≥18.2 MΩ) was prepared using a Simplicity®UV system and used
for the preparation of all aqueoussolution.
Structural characterization. Morphology and the microstructure of the
prepared samples were characterized on a field emission scanning
electron microscope (SEM; Zeiss Supra55 VP, Germany) and a high-
resolution transmission electron microscope (HRTEM; Bruker Nano
GmbH Berlin, Germany), which were both equipped with an energy
dispersive X-ray spectroscope (EDS). Wide-angle X-ray diffraction (XRD)
was performed by a Bruker D8 Advance X-ray diffractometer (Germany)
with Cu Kα (λ
= 0.15418 nm). Nitrogen adsorption-desorption
measurements were performed on an Autosrob-1 (Quanta chrome
INSTRUMENTS). The pore size distributions and specific surface areas
were calculated by BJH method based on desorption branch, and the
Brunauer-Emmett-Teller
(BET)
method,
respectively.
Raman
Synthesis of 3D ordered PS template. The 3D ordered polystyrene
spectroscopy measurements were characterized with a high-resolution
laser confocal microscopy Raman spectrometer (Horiba evolution) with a
532 nm excitation laser. X-ray photoelectron spectroscopy (XPS) was
recorded on a Thermo SCIENTIFIC ESCALAB 250 using 150 W Al Kα
radiation.
sphere (PS) template with
a particle diameter of 110 nm were
synthesized according to the following procedure. Firstly, 50 mL of
washed styrene, 400 mL of deionized water and 1.5 g of PVP were
added into a 1 L flask. Then the mixture was bubbled with nitrogen for 10
min and stirred at 92 °C for 30 min. Subsequently, 50 mL of 1 wt.%
K2S2O8 was added into the flask to initiate the polymerization reaction of
styrene. After keeping stirring at a rotation rate of 400 rpm for 24 h at this
temperature, the mixture was cooled down and the obtained milk-like
product was the monodispersed colloidal polystyrene spheres. The
monodispersed polystyrene spheres were further filtered to form filter
cakes. Finally, the 3D ordered PS templates were obtained after further
washing with deionized water and drying at 60 °C for overnight.
Electrochemical characterization. All electrochemical measurements
were carried on PGSTAT 302N (Metrohm Auto Lab) and a standard
electrochemical cell (Pine Instrument) in an O2 saturated electrolyte at
room temperature (18‐ 25 °C). The counter and reference electrodes
were a platinum wire and Ag/AgCl/3M KCl, respectively. RHE calibration
was performed in the high purity hydrogen saturated electrolyte with a Pt
foil as the working electrode and a Pt wire as the counter electrode.
Cyclic voltammetry (CV) was ran at a scan rate of 1 mV s-1, and the
average of two potentials at which the current cross zero was taken as
Synthesis of OP-ZIF-67 and Co-NOPC. 4.5 g of Co(NO3)2·6H2O and 6
g of 2-methylimidazole were added to 60 mL of methanol and fully mixed
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10.1002/cssc.201903053
ChemSusChem
FULL PAPER
thermodynamic potential for the hydrogen electrode reactions (Figure S1).
The potential measured against an Ag/AgCl electrode was converted to
the potential versus the reversible hydrogen electrode (RHE) according
to E(RHE) = E(Ag/AgCl) + 0.976 V in 0.1 M KOH, and E(RHE) =
E(Ag/AgCl) + 0.240 V in 0.5 M H2SO4. All data were presented without IR
compensated.
Keywords:Cobalt • Electron transfer • Fuel cell • Oxygen
reduction reaction • ZIF-67
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[
(
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[(
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This w orkw as supported by the National Natural Science
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FULL PAPER
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FULL PAPER
Entry for the Table of Contents
Layout 2:
FULL PAPER
Mengfei Qiao,YingWang, Xamxikamar
Mamat, Anran Chen, Guoan Zou, Lei Li*,
Shusheng Zhang, Xun Hu, Damien
Voiry* ,and Guangzhi Hu*
Page No. – Page No.
Rational Design of Hierarchical Porous
Cobalt,
Nitrogen-doped
Carbon
Architectures
as Efficient
Oxygen
Reduction Reaction Electrocatalysts
A hierarchical porous cobalt, nitrogen-doped carbon were obtained by pyrolysis of ordered
porous ZIF-67, that is prepared by forced molding of a polystyrene template. The hierarchical
porosity result in a significant improvement in the performance of ZIF-67 derived carbon to
oxygen reductionreaction inbothalkalineand acidic conditions.
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