Research Article
Cite This: ACS Catal. 2019, 9, 4551−4560
pubs.acs.org/acscatalysis
Attaching Cobalt Corroles onto Carbon Nanotubes: Verification of
Four-Electron Oxygen Reduction by Mononuclear Cobalt Complexes
with Significantly Improved Efficiency
Jia Meng,† Haitao Lei,† Xialiang Li, Jing Qi, Wei Zhang, and Rui Cao*
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an 710119, China
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S Supporting Information
*
ABSTRACT: Cobalt complexes have been extensively explored in catalyzing
the oxygen reduction reaction (ORR), which is the cathode reaction in fuel
cells. Although they show high activities, mononuclear Co complexes typically
mediate the 2e reduction of O2 to H2O2, and two Co sites are generally
required to catalyze the 4e reduction of O2 to H2O. Herein we report the
significantly improved efficiency of covalently grafted Co corroles on carbon
nanotubes (CNTs) for the 4e ORR. Azide-containing Co corroles can be
attached to alkyne-modified CNTs via azide−alkyne cycloaddition. This
attachment can avoid the formation of dimeric face-to-face Co corroles. The
resulted hybrid catalyzes the 4e ORR in 0.5 M H2SO4 aqueous solutions with
a half-wave potential at 0.78 V versus reversible hydrogen electrode (RHE). This performance makes this hybrid one of the
most efficient Co-based molecular ORR electrocatalysts. Control studies using Co corrole analogues loaded on CNTs via
noncovalent interactions give ORR half-wave potentials at 0.68−0.61 V versus RHE. This work is significant in demonstrating
that with proper covalent bond interactions to CNTs mononuclear Co corroles can become intrinsically active for the 4e ORR
with significantly improved efficiency.
KEYWORDS: oxygen reduction, cobalt corrole, covalent immobilization, mononuclear, electrocatalysis
■
INTRODUCTION
Developing efficient and robust electrocatalysts for O2
reduction has attracted considerable interests because this
process is sluggish in kinetics but is important as a cathode
reaction in fuel cells.1−3 Platinum and its alloys are very active
for ORR,4 but their uses are limited due to the very low natural
abundance and high cost of this noble metal. As an alternative,
extensive efforts have been made in the past decade to make
ORR electrocatalysts derived from earth-abundant and cheap
transition metal elements.5−17 For mononuclear transition
metal catalysts, O2 can be reduced by four electrons to form
H2O or by two electrons to form H2O2 (Scheme 1).3 In
general, late transition metal complexes typically catalyze ORR
via the 2e pathway because it is challenging for the peroxo
intermediates M−OOH to undergo heterolytic OO bond
cleavage, leading to the generation of terminal metal-oxo
species that are high in energy. However, the 4e ORR can still
be accomplished by late transition metal complexes through
the formation of dinuclear peroxo intermediates.
Cobalt complexes have been widely studied and are shown
to be highly active for ORR.18−30 As inspired by cytochrome c
oxidases (CcOs) with heme active sites for the 4e ORR,31 a
variety of Co porphyrins and derivatives have been investigated
as ORR catalysts. Mononuclear and cofacial dinuclear Co
porphyrin catalysts are presented by Anson, Fukuzumi, Kadish,
and others.32−45 Mononuclear Co corroles and Co complexes
© 2019 American Chemical Society
Scheme 1. Reduction of O2 Catalyzed by Mononuclear
Transition Metal Complexes, Showing the Possible
Reaction Pathways for the 4e and 2e ORR
of porphyrin−corrole dyads and biscorroles are studied by
Kadish, Gross, Cao, and others.46−55 Although porphyrin and
corrole ligands are negatively charged and thus are effective in
stabilizing metal ions in their high valent states to benefit the
OO bond cleavage, dimeric Co-based forms are still usually
required for the 4e ORR.56−59 Xia and co-workers performed
Received: January 16, 2019
Revised: April 7, 2019
Published: April 10, 2019
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theoretical studies and suggested that the highest-occupied 3d
orbital of Co porphyrins has a relatively low energy level,
leading to the weak activity for the 4e ORR.60 It is necessary to
note that mononuclear Co complexes can form face-to-face
dimeric Co species in both solution and solid states to catalyze
the 4e ORR. This phenomenon has been demonstrated by
Girault and co-workers, who studied ORR using Co
porphyrins at liquid−liquid interfaces.61−63
Synthetic modeling studies of CcOs provide fundamental
knowledge for the rational design of efficient ORR
catalysts.64,65 Collman and co-workers revealed that rapid
electron transfer to heme was essential for CcOs to catalyze the
4e reduction of O2.66 In nature, this rapid electron transfer is
realized by involving extra redox centers (i.e., a tyrosine unit
and a Cu ion) located in the vicinity of the heme unit. For
biomimetic systems, Dey and co-workers reported that Fe
porphyrins with ferrocene units attached at the second
coordination sphere were able to catalyze the 4e ORR.67,68
The Dey’s group also showed the switch between 2e versus 4e
ORR by controlling the electron transfer rate from the
electrode to the Fe porphyrin unit through different linkers.69
On the other hand, Collman and co-workers revealed that
hydrogen bonding interactions were able to stabilize the O2adduct and to assist the OO bond cleavage, leading to much
improved 4e ORR selectivity.70 Similar effects of hydrogen
bonding interactions were reported by Mayer, Dey, and others
with the use of Fe porphyrin model complexes.71−76 Nocera
and co-workers demonstrated that intramolecular acid/base
groups of hangman Co porphyrins and corroles could lead to
increased 4e ORR activity.77,78 These results not only shed
light on understanding the reaction mechanism of CcOs for O2
reduction but also provide valuable information for designing
new efficient ORR catalysts.
Herein we report the high efficiency of covalently grafted
mononuclear Co corroles on CNTs for the 4e reduction of O2.
Recently, we reported the covalent immobilization of azidecontaining Co corroles onto alkyne-modified CNTs through
azide−alkyne cycloaddition.79 CNTs are chosen as the support
for electrocatalysis due to their good electrical conductivities
and large surface areas.80 With the use of this method, we can
graft Co corrole 1 onto CNTs (Figure 1). The resulted hybrid
CNT-1 is highly efficient in catalyzing the 4e ORR in 0.5 M
H2SO4 aqueous solutions with a half-wave potential at 0.78 V
versus RHE. This performance is significantly improved in
comparison with Co corrole analogues immobilized on CNTs
through noncovalent interactions and is also outstanding as
compared with previously reported Co-based molecular ORR
catalysts. Because such covalently attached Co corroles cannot
form dimeric face-to-face Co species, this work provides direct
experimental evidence that mononuclear Co corroles can be
intrinsically active for the 4e ORR. As demonstrated above,
extensive efforts have been made to prove and improve the 4e
ORR activity of Fe porphyrins. However, the 4e ORR activity
of mononuclear Co complexes has been poorly understood,
although a large number of Co-based catalysts have been
reported to be able to catalyze ORR with high efficiency. This
work is therefore significant to show the significantly improved
4e ORR activity of mononuclear Co corroles and will bring
new insights into the ORR with complexes of late transition
metal elements.
Figure 1. (a) Molecular structures of Co corroles 1, 2, and 3. (b)
Illustration showing the preparation of CNT-1, CNT-2, and CNT-3.
■
EXPERIMENTAL SECTION
General Methods and Materials. Materials that are
sensitive to air and moisture were manipulated under N2 using
standard Schlenk line techniques. All reagents were commercially available and were used directly unless otherwise noted.
Dry solvents, including dichloromethane, tetrahydrofuran,
acetonitrile, and dimethylformamide were used in this work.
Co corroles were synthesized according to the method we
reported previously.79 All aqueous solutions were prepared
freshly using the Milli-Q water. Multiwalled CNTs (>95%
purity, <8 nm outside diameter, 3 nm intside diameter) were
commercially available. MALDI-TOF mass spectra were
obtained using Brüker MAXIS. UV−vis spectra were measured
using a Hitachi U-3310 spectrophotometer. 1H NMR spectra
were acquired on a Brü ker spectrometer. Transmission
electron microscopy (TEM) images were obtained using
JEOL JEM-2100 with a 200 kV accelerating voltage. Hitachi
SU8020 cold-emission field emission scanning electron
microscope (FE-SEM) was used to study the sample
morphologies with an accelerating voltage of 1 kV. Energy
dispersive X-ray (EDX) spectra were measured from three
areas of each sample to ensure the chemical homogeneity. Xray photoelectron spectroscopy (XPS) data were collected by
ESCALab 220i-XL electron spectrometer from VG Scientific
using 300 W Al Kα radiation. The C 1s peak at 284.6 eV
arising from adventitious hydrocarbon was used for the
correction of binding energies. All samples used were carefully
prepared via centrifugation and redispersion.
Synthesis of Co Corrole 1. To the solution of 5,15bis(pentafluorophenyl)-10-(4)-(1-azido)phenylcorrole (19
mg, 0.025 mmol) and anhydrous NaOAc (30 mg, 0.36
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mmol) in 10 mL of methanol were added Co(OAc)2·4H2O
(30 mg, 0.12 mmol) and triphenylphosphine (PPh3, 50 mg,
0.19 mmol). The resulted mixture was stirred for 1.5 h at room
temperature, and the solvent was evaporated. The residue was
then subjected to silica chromatography (petroleum ether/
dichloromethane = 5:1) to afford purple solids (22 mg, 85%).
1
H NMR (400 MHz, CDCl3): δ = 8.69 (d, J = 4.4 Hz, 2H),
8.34 (brs, 2H), 8.21 (d, J = 4.4 Hz, 2H), 8.01 (d, J = 4.4 Hz,
2H), 7.76−7.69 (m, 2H), 7.57−7.49 (m, 2H), 7.03 (t, J = 7.3
Hz, 3H), 6.69 (t, J = 6.6 Hz, 6H), 4.68 (s, 6H) (Figure 2a).
HRMS: calcd for C55H28CoF10N5P [1-N2+H+]+, 1038.1249;
found, 1038.1245 (Figure 2b).
line was mixed with 5 mg of Co corrole 1 in 5 mL of
dimethylformamide, and the mixture was stirred at 85 °C
overnight. The suspension was kept in a sonication bath for 1
h, and the crude CNT-1 was collected by centrifugation. The
solids were resuspended in dichloromethane to remove simply
adsorbed complex 1 on CNTs, and then were collected by
centrifugation again and dried in the dark at room temperature.
The Co content in the hybrid CNT-1 was determined by using
inductively coupled plasma mass spectrometry (ICP-MS,
Bruker aurora). CNT-1 was immersed in a concentrated
high purity HNO3 solution under ultrasonication to dissolve
the Co species. The HNO3 solution was filtered and was then
diluted into 20 mL for ICP-MS measurements. The Co ICPMS standards (AccuStandard) with a concentration of 50, 100,
200, 300, 400, and 500 ppb were used for the calibration.
Preparation of Catalyst-Loaded Electrodes. In order to
prepare CNT-1-loaded electrode, 1.0 mg of CNT-1 was added
to 1.0 mL of dimethylformamide (DMF), and the resulted
mixture was treated with an ultrasonic cleaner to give a
homogeneous CNT-1 suspension (1 mg mL−1). The glassy
carbon (GC) electrode was rinsed with ethanol and then dried.
The CNT-1 DMF suspension was added onto the GC
electrode (5 μL) or GC disk electrode (16 μL) using a pipet,
and the electrode was dried slowly in the dark. In order to
prepare electrodes loaded with noncovalently immobilized Co
corroles, 5 mg of CNTs were added to 5 mL of DMF, and the
resulted mixture was treated with an ultrasonic cleaner to give
a homogeneous CNT suspension (1 mg mL−1), which was
then added onto the GC electrode (5 μL) or GC disk
electrode (16 μL) using a pipet. The electrode was then dried
slowly. The acetonitrile solution of Co corrole 2 or 3 (1 mg
mL−1) was dropwisely added to the CNT-coated GC electrode
(5 μL) or GC disk electrode (16 μL), and the electrode was
dried in the dark at room temperature. The electrode was then
gently rinsed with acetonitrile to remove weakly adsorbed
corroles and was dried in the dark at room temperature. The
Co contents in these samples of noncovalent immobilization
were also determined by ICP-MS.
Electrochemistry. Electrochemical measurements were
carried out using CH Instruments (model CHI660D Electrochemical Analyzer). Cyclic voltammogram was collected in 0.1
M Bu4NPF6 acetonitrile solution using a three-compartment
cell with a 0.07 cm2 GC working electrode, graphite rod
counter electrode, and Ag/AgNO3 reference electrode. At the
end of each measurement, ferrocene was added as an internal
standard to calibrate the potential. In aqueous solutions, Ag/
AgCl reference electrode was used. Bipotentiostat (model
DY2300 Electrochemical Analyzer) was used for rotating ringdisk electrode measurements. The rotating ring-disk electrode
has a GC disk electrode (0.125 cm2) and a platinum ring
electrode (0.188 cm2, ALS RRDE-2). The collection efficiency
of 0.425 was evaluated for this ring-disk electrode using the
[Fe(CN)6]3−/4− redox couple.
Figure 2. (a) 1H NMR spectrum of Co corrole 1 in CDCl3. (b)
HRMS of 1 in methanol. The observed isotopic distribution pattern is
identical to that calculated using the formula C55H28CoF10N5P, [1N2+H+]+. (c) UV−vis absorption spectrum of 1 in DMF at room
temperature. (d) CVs of Co corroles 1, 2, and 3 in 0.1 M Bu4NPF6
acetonitrile. Conditions: GC electrode, 100 mV s−1 scan rate, 20 °C.
Preparation of CNTs Modified with 4-Ethynylaniline.
Sonication was performed to disperse CNTs to avoid any
agglomeration and sedimentation. CNTs (30 mg) were mixed
with 4-ethynylaniline (307 mg, 2.6 mmol) and sonicated
together in 70 mL of hydrochloric acid solution (0.5 M).
Sodium nitrite (1.5 equiv to 4-ethynylaniline) dissolved in 15
mL of deionized water was then added to generate the
diazonium salt from 4-ethynylaniline. The addition of iron
powder (500 mg) could reduce the diazonium salt to generate
the corresponding aryl radical. Generated aryl radicals can be
grafted onto available surfaces, and successive radical
substitutions finally yielded dehydrobenzene coatings on
CNTs. The crude products were stirred in excessive hydrochloric acid to remove the residue iron and were washed with
ethanol, acetone, and water for several times until no Fe
content was detected in both the washing solution and the
hybrid. The modified CNTs can be obtained after drying at
room temperature.
Caution! The addition of iron powder to the hydrochloric acid
solution is potentially explosive and should be handled with care
and in small amounts.
Preparation of CNT-1 through the Azide−Alkyne
Cycloaddition. CNTs (10 mg) modified with 4-ethynylani-
■
RESULTS AND DISCUSSION
Synthesis and Characterization of Co Corroles. Co
corrole 1 (Figure 1a) was synthesized by the reaction of Co
acetate and 5,15-bis(pentafluorophenyl)-10-(4)-(1-azido)phenylcorrole. Triphenylphosphine was added as an axial
ligand during the reaction. Purple crystals of 1 were obtained
with a yield of 85% and were used for subsequent reaction with
the alkyne-modified CNTs. Complex 1 was characterized by
1
H NMR (Figure 2a) and high-resolution mass spectrometry
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(HRMS, Figure 2b). In the 1H NMR spectrum, the eight βhydrogen atoms of the corrole ring and the four hydrogen
atoms of the 10-(4)-(1-azido)phenyl substituent are found as
sharp peaks in the range of 8.80−7.50 ppm. The rest three
hydrogen peaks at 7.03 (3 H), 6.69 (6 H), and 4.68 (6H) ppm
are due to the axial PPh3 unit. The large upfield movement of
the PPh3 hydrogen peaks is caused by the aromatic effect of the
corrole ring, which is strong evidence to support the binding of
PPh3 at the Co axial site. The HRMS of 1 shows an ion with
the mass-to-charge ratio of 1038.1245. This value matches the
calculated value of 1038.1249 for the monocation of [1N2+H+]+ with the expected isotopic distribution. This result
suggested that during the HRMS measurement, the axial PPh3
ligand was retained but the azide group decomposed to lose a
N2 molecule. Additionally, the UV−vis spectrum of 1 shows
characteristic corrole-based Soret bands at 375 and 411 nm
and Q bands at 556 and 587 nm (Figure 2c).81 On the basis of
these results, we can confirm the identity and purity of Co
corrole 1.
Co complexes of 5,15-bis(pentafluorophenyl)-10-(4)-(1pyrenyl)phenylcorrole (2) and 5,10,15-tris(pentafluorophenyl)corrole (3) were synthesized according
to our previously reported methods.81 The redox properties of
Co corroles 1-3 were then studied by using cyclic voltammetry
in acetonitrile. As shown in Figure 2d, the cyclic voltammogram (CV) of 1 on a GC electrode displays two quasireversible reduction couples at E1/2 = −0.86 and −1.87 V
versus ferrocene in acetonitrile, which are ascribed to the
formal CoIII/CoII and CoII/CoI couples, respectively. These
values are very similar to those of Co corroles 2 and 3. For
complex 2, the two reduction couples have E1/2 = −0.76 and
−1.88 V versus ferrocene, and for complex 3, they are E1/2 =
−0.72 and −1.83 V versus ferrocene. These results
demonstrate that all three Co corroles have very similar
redox behaviors, which are important for direct comparison of
their ORR activities when loaded on CNTs.
Covalent Immobilization of 1 on CNTs. To graft the
azide-containing Co corrole 1 onto CNTs through covalent
bonds, CNTs were first functionalized with 4-ethynylaniline to
introduce surface alkyne groups.79,82 The as-prepared alkynefunctionalized CNTs were denoted as CNT-alkyne hereafter.
Notably, iron powder used in this synthesis was carefully
removed by stirring the products in hydrochloric acid and then
washing with ethanol, acetone, and water. This process was
repeated for several times until no Fe content was detected in
the washing solution by using ICP-MS. The as-prepared CNTalkyne was further analyzed by XPS to exclude the presence of
residual Fe. This washing step is essential because Fe-doped
CNTs are known to have some activities for ORR. The alkynefunctionalized CNTs were analyzed by Raman, showing the
decrease of the ID/IG ratio (Figure 3a, ID and IG represents the
intensity of the D and G bands at 1341 and 1583 cm−1,
respectively). Although diazonium treatment will usually cause
the increase of the ID/IG ratio, but the surface-modification of
CNTs with alkyne groups, which contain abundant graphitelike C atoms, will lead to the decrease of the ID/IG ratio.83−85
Subsequently, Co corrole 1 was grafted onto the alkynemodified CNTs through the azide−alkyne cycloaddition to
make hybrid CNT-1 (Figure 1b). Notably, during this
cycloaddition, no metal catalysts were added to avoid the
contamination of the CNT hybrid with other metal species.
The successful attaching of molecules of 1 onto CNTs was
confirmed. The Raman spectrum of CNT-1 showed further
Figure 3. (a) Raman spectra of CNT-1, CNT-alkyne, and unmodified
CNT. (b) CV of GC electrode loaded with CNT-1 in acetonitrile,
CVs of GC electrode loaded with or without blank CNTs in the
acetonitrile solution of 1. (c) FTIR spectra of CNT-1, 1, and CNT.
(d) XPS survey scan spectra of CNT-1 and CNT. (e) SEM image of
CNT-1. (f) TEM image of CNT-1.
decrease of the ID/IG ratio to 0.64 (Figure 3a), which is
consistent with the surface-modification with corrole moieties
that contain abundant sp2 C atoms. When a GC electrode was
loaded with blank CNTs and was immersed in an acetonitrile
solution of 1, the CV showed two quasi-reversible reduction
couples at E1/2 = −0.89 and −1.91 V versus ferrocene (Figure
3b, blue line). These values match very well with the reduction
couples of 1 measured by using a freshly cleaned GC electrode
(Figure 3b, black line).
Importantly, the CV of GC electrode loaded with CNT-1
showed two small but significant reduction processes at similar
potentials in acetonitrile (Figure 3b, red line). This result
suggests the covalent attachment of 1 on CNTs, although the
small CV current implies the very low loading of Co corrole
molecules. Notably, if GC electrodes loaded with CNT-2 or
CNT-3 were immersed in acetonitrile, no signals from Co
corroles could be identified in CV measurements. We
rationalize that Co corrole molecules in CNT-2 and CNT-3
have noncovalent interactions with CNTs and will redissolve
into acetonitrile, making its detection by CV very challenging.
This difference is noteworthy and strongly supports the
covalent immobilization of 1 on CNTs in the hybrid CNT-1.
Additional evidence for the covalent attachment of
molecules of 1 in CNT-1 was provided by infrared spectroscopy. As shown in Figure 3c, complex 1 presents a
characteristic azido group resonance band at 2113 cm−1.
This band disappears in the infrared spectrum of CNT-1,
indicating the reaction of this azido group with alkyne groups
on the surface of CNTs. The resonance bands of 1 are well
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same Co corrole core structures and they have almost identical
redox behaviors, this improved ORR performance of CNT-1
suggested the synergistic effect between molecules of 1 and
carbon supports for ORR. More importantly, CNT-1 retained
the initial ORR activity after ultra-sonication in dichloromethane, but both CNT-2 and CNT-3 almost completely lost
their activities due to the redissolution of Co corrole molecules
into organic solvents (Figure S8). This result provides strong
support for the covalent immobilization of Co corrole 1 on
CNTs in CNT-1.
The selectivity of O2 reduction with CNT-1 was then
evaluated by using rotating ring-disk electrode (RRDE)
measurements. As shown in Figure 5a,b, CNT-1 displayed a
reserved in the infrared spectrum of CNT-1, confirming the
presence of intact molecules of 1 in the hybrid. Notably, in the
infrared spectrum of unmodified CNTs, no such resonance
bands are found in the same range.
In addition, CNT-1 was characterized by XPS (Figure 3d)
and EDX (Figure S4). Comparison of the XPS spectra of
CNTs before and after cycloaddition clearly showed the
appearance of Co, N, and F signals from 1. The corresponding
Co 2p narrow scan spectrum was depicted in Figure S5,
showing Co 2p3/2 and Co 2p1/2 signals at 778 and 790 eV. This
result implies the presence of CoIII ions in the hybrid, which is
consistent with the Co oxidation state of Co corrole 1.
Moreover, CNT-1 was analyzed by FE-SEM (Figure 3e) and
TEM (Figure 3f), excluding the presence of aggregated
particles on CNT surfaces. We also loaded Co corroles 2
and 3 onto CNTs through noncovalent interactions to give
CNT-2 and CNT-3, respectively (Figure 1b). As we
mentioned above, the coordination structures of Co corroles
1-3 and their redox behaviors are almost identical. These
factors are essential to analyze the effect of covalent and
noncovalent immobilization methods on ORR electrocatalysis.
Electrocatalytic Oxygen Reduction Studies. The
electrocatalytic ORR features of the three hybrids were then
examined in 0.5 M H2SO4 aqueous solutions. The CV of a GC
electrode loaded with CNT-1 showed no catalytic current
under N2 but showed a large catalytic current under O2 with
the half-wave potential at 0.78 V versus RHE (Figure 4a, all
Figure 4. (a) CVs of GC electrode loaded with CNT-1 and CNT
under O2 in 0.5 M H2SO4 solution. (b) Normalized LSVs of GC
electrode loaded with CNT-1, CNT-2, and CNT-3 under O2 in 0.5 M
H2SO4 solution. Conditions: 50 mV s−1 scan rate, 20 °C.
Figure 5. (a) O2 reduction in RDE at the GC disk electrode loaded
with CNT-1, CNT-2, and CNT-3 in O2-saturated 0.5 M H2SO4
solutions at 2500 rpm. O2 reduction in RRDE at the GC disk
electrode loaded with CNT-1 (b), CNT-2 (c),CNT-3 (d), and CNTalkyne (e) in O2-saturated 0.5 M H2SO4 solutions at 2500 rpm. The
Pt ring electrode was polarized at 1.20 V. (f) The ORR n value with
CNT-1, CNT-2, CNT-3, and CNT-alkyne determined from RRDE.
Conditions: GC disk electrode (area 0.125 cm2), Pt ring electrode
(area 0.188 cm2), 10 mV s−1 scan rate, 20 °C.
potentials reported in 0.5 M H2SO4 aqueous solution in this
work are referenced to RHE unless otherwise noted). This
result indicates electrocatalytic reduction of O2 by CNT-1.
The linear sweep voltammetry (LSV) of CNT-1 loaded on a
GC electrode was also measured, giving the same result as
obtained by CV (Figure S7). This ORR performance of CNT1 is close to that obtained with the commercial Pt/C material,
which shows the half-wave potential at 0.84 V in 0.5 M H2SO4
solution. The LSV of GC electrodes loaded with CNT-2 and
CNT-3 showed catalytic ORR currents with the half-wave
potential at 0.68 and 0.61 V, respectively. In comparison with
CNT-2 and CNT-3, the catalytic ORR wave with CNT-1
shifts to the anodic direction by more than 100 mV under the
same conditions. Moreover, the loading of Co corrole
molecules on CNTs can be determined by ICP-MS, giving
2.81, 11.93, and 11.17 μg Co per mg of CNT-1, CNT-2, and
CNT-3, respectively (Table S1). By normalizing the LSV data
per Co content, we can see that the catalytic performance of
CNT-1 is much improved in comparison with that of CNT-2
and CNT-3 regarding both the overpotentials and the catalytic
currents (Figure 4b). Because complexes 1−3 contain the
pronounced catalytic ORR activity in RRDE. The GC disk
electrode was scanned by LSV from 1.0 to 0.20 V, whereas the
Pt ring electrode was applied with a constant potential at 1.20
V to detect H2O2 that might be produced by partially reduced
O2 at the disk electrode (Figure S9). The H2O2 yield can be
calculated according to current densities at the disk (Id) and
the ring (Ir) electrodes using the following equation,2,50,51,86,87
%H 2O2 = 200
Ir
N
Id +
Ir
N
in which N = 0.425 is the current collection efficiency of the Pt
ring. For CNT-1, this result gives the number of electrons (n)
transferred per O2 molecule to be 3.73 (Figure 5f). Moreover,
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Table 1. Comparison of Electrocatalytic ORR Performances for Co-Based Complexes
catalysts
CNT-1
CNT-2
CNT-3
PloyCoTAC
Oxa-Co
MWCNT/Co(TCPP)pyr4
Vc/Nf/Co2Pc2
rGo/(Co-THPP)7
CoTNPc/PGr
MWCNT-CoP-1
C−COP−P-Co
GO-CorCo
[Co(tpfcBr8)]/BP2000
CoHPX-1
CoTPFC/MWCNT
Co-1/MWCNT
CoCOF-py-0.05rGO
MWCNT-Co-porphyrin
conditions
half-wave potentials (versus RHE)
0.5 M H2SO4
0.5
0.1
0.5
0.1
0.5
0.1
0.1
0.5
0.1
0.1
0.1
0.5
0.5
0.5
0.5
0.1
1.0
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
M
0.78
0.68
0.61
0.58
0.70
0.63
0.85
0.25
0.74
0.80
0.65
0.82
0.56
0.73
0.75
0.45
0.62
0.62
0.76
0.51
H2SO4
KOH
H2SO4
KOH
H2SO4
KOH
NaOH
H2SO4
KOH
HClO4
KOH
H2SO4
H2SO4
H2SO4
H2SO4
KOH
H2SO4
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
V
n values from RRDE
references
3.73
3.68
3.61
2.14
3.10
3.96
3.10
3.84
3.85
3.90
3.93
3.65
3.61
2.00
3.60
3.40
3.60
3.80
3.80
3.70
this work
47
59
45
28
21
29
42
30
46
49
78
50
51
43
41
necessary to note that the Levich analysis is in general only
applicable under the totally mass-transfer-limited ORR
conditions, which could be regarded at the applied potential
range of 0.20−0.40 V for CNT-1. The n values for CNT-2
(Figure S15) and CNT-3 (Figure S16) were also examined
from RDE measurements, giving similar values to those from
RRDE.
The durability of CNT-1 for ORR was examined by
chronoamperometric tests. During 10-h measurements under
potential 0.60 V, the current with CNT-1 decreased by only
10% in an air-saturated solution (Figure 6a), while the current
with Pt/C decreased by 24%. It is worth noting that although
the catalytic feature of CNT-1 for ORR was studied in 0.1−0.5
M H2SO4 solutions. The ionic strength of these H2SO4
solutions is maintained the same with addition of Na2SO4.
As shown in Figure S10, the half-wave potential shows a linear
relationship with the solution pH, giving a slope of −63 mV
per pH. This is consistent with the proton-coupled electron
transfer process of ORR. For selectivity, similar n values were
obtained in 0.1−0.5 M H2SO4 solutions (Figure S11). In
addition, CNT-2 (Figure 5c), CNT-3 (Figure 5d), and CNTalkyne (Figure 5e) were subjected to RRDE measurements.
Similar to LSV studies mentioned above, the catalytic ORR
wave with CNT-1 shifts to the positive direction by more than
100 mV in comparison with that with CNT-2 and CNT-3 as
observed in RRDE. The n values for CNT-2 and CNT-3 are
3.68 and 3.61, respectively. Importantly, CNT-alkyne displayed
much smaller ORR catalytic current under much more
negative potentials, confirming that the ORR activity of
CNT-1 is due to covalently attached Co corroles instead of the
alkyne groups on the surface of CNTs. In addition, we
examined the catalytic ORR feature of complex 1 loaded
directly on the GC electrode. As shown in Figure S12,
molecules of 1 on GC electrode showed very poor ORR
activity regarding both the overpotentials and catalytic
currents. Moreover, the n value of 1 on GC electrode was
determined to be 2.93. These results confirmed the
significantly improved ORR performance of 1 by attaching
to CNTs through covalent bonds. As shown in Table 1, CNT1 is one of the most efficient Co-based ORR electrocatalysts
regarding both the half-wave potentials and the n value.
Additionally, the n value was further determined using the
Koutecky−Levich (K-L) and the Levich analysis in rotating
disk electrode (RDE) experiments. The n value for CNT-1
calculated from the K-L and Levich analysis was very similar to
that obtained from RRDE measurements under the applied
potential range of 0.20−0.40 V (Figures S13 and S14). The
linearity and the near-parallelism as observed in the K-L plots
in the potential range of 0.20 to 0.40 V suggested a first-order
reaction kinetics on dissolved O2 concentration and similar
ORR n values at different potentials (Figure S13b). It is
Figure 6. (a) Controlled-potential electrolysis of GC electrode loaded
with CNT-1 and Pt/C at 0.60 V in air-saturated 0.5 M H2SO4
solutions. (b) Chronoamperometric responses of CNT-1 and Pt/C
upon adding 2.0 M methanol. (c) CV of GC electrode loaded with
CNT-1 in N2-saturated 0.5 M H2SO4 after adding H2O2. (d)
Histogram illustration of currents of GC electrodes loaded with CNT1 at different electrolysis potentials under O2-saturated or N2saturated 0.5 M H2SO4 solutions. In N2-saturated 0.5 M H2SO4
solution, H2O2 was added to get the final concentration of 2.0 mM.
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ACS Catalysis
disproportionation as catalyzed by CNT-1, making its
detection at the ring electrode unsuccessful. Moreover, as
shown in Figure S17−S21 during the electrolysis with H2O2
under N2, we did not observe the increase of currents,
indicating that the disproportionation of H2O2 by CNT-1 was
also not likely during the long time electrolysis studies.
Mononuclear late transition metal complexes, such as Co
complexes, are in general thought to catalyze the 2e ORR
because the heterolytic OO bond cleavage of their M−
OOH units toward the 4e ORR activity leads to the generation
of terminal late transition metal oxo species, which are high in
energy. However, mononuclear Co porphyrins and corroles
have still shown 4e ORR activities.48−51 In these studies, selfassembled dimers or oligomers of Co porphyrins and corroles
are suggested to form on the surface of electrode materials,
which play crucial roles in improving the 4e ORR selectivity.
For example, Girault and co-workers studied and established
that the formation of self-assembled face-to-face dimeric Co
porphyrins would improve the 4e selectivity of ORR.61−63
Consequently, dimeric face-to-face Co complexes of bisporphyrins, biscorroles, and porphyrin−corrole dyads have been
designed and extensively studied as 4e ORR catalysts.33−36,54,56−59
In our studies, by using the ICP-MS result, we can calculate
that each Co atom of CNT-1 is supported by about 400 C
atoms of the outmost wall of CNTs (please see Table S1 and
Figure S24 and their corresponding discussion for details).
Because the diazonium of CNT surfaces is a nonselective
process, it is unlikely to form dimeric face-to-face Co corrole
species in subsequent azide−alkyne cycloaddition, which gives
very low loading density of Co corrole molecules on the
surface of CNTs. This is further supported by the absence of
blue shift for the Soret band of Co corroles in CNT-1 (Figure
S23), which is characteristic for the formation of cofacial “faceto-face” structures.91 Although we cannot completely rule out
the presence of possible trace amount of dimeric face-to-face
Co corrole species, the majority of covalently attached Co
corrole molecules have to be presented as monomers.
Therefore, our results demonstrate that the mononuclear Co
corrole molecules grafted on CNTs can indeed catalyze the 4e
ORR. We rationalized that the short conjugated linkers enable
rapid electron transfer between Co corroles and electrode
materials. As the reduction of peroxo species and the
dissociation of peroxo from Co sites are two competing
reactions, the increased electron transfer rates will eventually
improve the 4e ORR activity.
Pt/C shows high durability for electrocatalytic ORR in alkaline
media, it is much less stable in acidic solutions.30,88,89
Moreover, CNT-1 exhibited a very small current response in
the methanol crossover test (Figure 6b), but Pt/C showed an
apparent increase in the electrocatalytic current upon the
addition of methanol, which corresponds to the methanol
oxidation.
In order to distinguish the two possible pathways of the 4e
reduction of O2, namely the direct 4e ORR (i.e., O2 → 2H2O)
and the stepwise 2e + 2e ORR (i.e., O2 → H2O2 → 2H2O),90
we examined the electrocatalytic activity of CNT-1 for the
reduction of H2O2. As shown in Figure 6c, CNT-1 is virtually
inactive in catalyzing the H2O2 reduction. This result excludes
the possibility that H2O2 is first generated from O2 reduction
and is subsequently reduced to H2O by CNT-1. In other
words, CNT-1 is able to catalyze the direct 4e reduction of O2
to H2O. Importantly, in the reverse scan of this test, we can see
a pronounced catalytic current, which is due to the oxidation of
H2O2 to evolve O2 (Figure 6c). In the subsequent cathodic
scan, the generated O2 is reduced by CNT-1, confirming the
generation of O2 from the H2O2 oxidation. Moreover, we did
controlled potential electrolysis with CNT-1 at 0.40−0.80 V in
either an O2-saturated solution or an N2-protected solution
with addition of 2.0 mM H2O2 (Figures S17−S21). The
concentration of H2O2 used here is more than the 1.30 mM
concentration of dissolved O2 in an aqueous solution under 1.0
atm O2 at 23 °C. As shown in Figure 6d, the currents observed
in O2-saturated solutions in the potential range of 0.40−0.80 V
are much larger than those observed in N2-protected solutions
with addition of H2O2. These results provide strong evidence
that CNT-1 is ineffective in catalyzing the reduction of H2O2.
Consequently, CNT-1 is an efficient electrocatalyst for the
direct 4e reduction of O2 to H2O. In addition, chemical O2
reduction by ferrocene (Fc) with CNT-1 in acetonitrile gives n
value of 3.2 based on generated Fc+ and H2O2 (Figure S22).
This relatively smaller n value in the chemical reduction studies
is likely due to the less efficient electron transfer between Fc
and CNTs in comparison with that between the electrode and
CNTs.
By further analyzing the results mentioned above, we can
also exclude the possibility that small ring currents during the
RRDE measurements of CNT-1 are due to the trapping of
H2O2 by CNTs or the disproportionation of H2O2 as catalyzed
by CNT-1. First, as shown in Figure 5e, the RRDE
measurement of CNT-alkyne shows large ring currents. This
result confirms the reduction of O2 to form H2O2 at the disk
electrode by CNT-alkyne and suggests that CNTs are not
likely to trap H2O2, as large ring currents are observed. It is
worthy noting that the disk current of CNT-alkyne is much
smaller than that of CNT-1 but the ring current of CNTalkyne is similar with that of CNT-1 under the same
conditions. Second, data from Figure 5e and Figure S12
suggest that the disproportionation of H2O2 by CNTs and
complex 1 is at least not significant. Even with these small disk
currents as observed for CNT-alkyne and complex 1, large
currents can be observed at the ring electrode. In addition, as
shown in Figure 6c, if the disproportionation of H2O2 by
CNT-1 was quick and significant, we should observe large
catalytic ORR currents during the first run of CV (O2 is
generated in situ from the disproportionation of H2O2).
However, no such current is observed in the first CV run in
Figure 6c. This result strongly argues against the possibility
that the formed H2O2 at the disk electrode undergoes
■
CONCLUSIONS
In summary, we report the electrocatalytic ORR features of Co
corrole 1 covalently immobilized on CNTs. The resulted
CNT-1 is highly efficient to catalyze the 4e ORR in 0.5 M
H2SO4 aqueous solutions with a half-wave potential at 0.78 V.
As compared with noncovalent immobilization of Co corrole
analogues on CNTs, this covalent immobilization causes more
than 100 mV anodic shift for ORR with significantly improved
activity and stability. More importantly, the covalent attachment with short linkers and very low loading density prevents
the formation of dimeric face-to-face Co corrole species. This
result demonstrates that with rapid electron transfer rates,
mononuclear Co corroles can become intrinsically active for
the direct 4e ORR. Although it is shown previously that rapid
electron transfer abilities can considerably improve the 4e
ORR activity of Fe porphyrins, it is very rare to demonstrate
4557
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ACS Catal. 2019, 9, 4551−4560
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similar effect for Co corroles because of the challenging in
molecular design and synthesis. This work is therefore
significant to demonstrate that with sufficient electron transfer
abilities mononuclear Co complexes can display significantly
improved activity for the 4e ORR.
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.9b00213.
■
Figures S1−S24 and Table S1 (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: ruicao@ruc.edu.cn.
ORCID
Rui Cao: 0000-0002-1821-9583
Author Contributions
†
J.M. and H.L. contributed equally to this work.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We are grateful for support from the “Thousand Talents
Program” of China, the Fok Ying-Tong Education Foundation
for Outstanding Young Teachers in University, the National
Natural Science Foundation of China (Grants 21101170,
21573139, and 21773146), the Fundamental Research Funds
for the Central Universities, and the Research Funds of
Shaanxi Normal University.
■
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