Axial Ligand Coordination Tuning of the Electrocatalytic Activity of Iron Porphyrin Electrografted onto Carbon Nanotubes for the Oxygen Reduction Reaction

Article abstract of DOI:10.1002/chem.202100736

The oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. It is desirable to design transition metal molecular catalysts inspired by enzymatic oxygen activation/reduction processes as an alternative to noble-metal-Pt-based ORR electrocatalysts, especially in view point of fuel cell commercialization. We have fabricated bio-inspired molecular catalysts electrografted onto multiwalled carbon nanotubes (MWCNTs) in which 5,10,15,20-tetra(pentafluorophenyl) iron porphyrin (iron porphyrin FeF₂₀TPP) is coordinated with covalently electrografted axial ligands varying from thiophene to imidazole on the MWCNTs’ surface. The catalysts’ electrocatalytic activity varied with the axial coordination environment (i. e., S-thiophene, N-imidazole, and O-carboxylate); the imidazole-coordinated catalyst MWCNTs-Im-FeF₂₀TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT-Im-FeF₂₀TPP was loaded onto the cathode of a zinc?air battery, an open-cell voltage (OCV) of 1.35 V and a maximum power density (P_max) of 110 mW cm^?2 were achieved; this was higher than those of MWCNTs-Thi-FeF₂₀TPP (OCV=1.30 V, P_max=100 mW cm^?2) and MWCNTs-Ox-FeF₂₀TPP (OCV=1.28 V, P_max=86 mW cm^?2) and comparable with a commercial Pt/C catalyst (OCV=1.45 V, P_max=120 mW cm^?2) under similar experimental conditions. This study provides a time-saving method to prepare covalently immobilized molecular electrocatalysts on carbon-based materials with structure–performance correlation that is also applicable to the design of other electrografted catalysts for energy conversion.

Full text of DOI:10.1002/chem.202100736

Accepted Article
Accepted Article
Title: Axial Ligand Coordination Tuning of Electrocatalytic Activity of
Iron Porphyrin Electrografted on Carbon Nanotubes for Oxygen
Reduction Reaction
Authors: Jin-Gang Liu, Xin-You Zhou, Chao Xu, Peng-Peng Guo, Wei-
Li Sun, and Ping-Jie Wei
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202100736
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1/2020

Chemistry - A European Journal
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Axial Ligand Coordination Tuning of Electrocatalytic Activity of
Iron Porphyrin Electrografted on Carbon Nanotubes for Oxygen
Reduction Reaction
Xin-You Zhou, Chao Xu, Peng-Peng Guo, Wei-Li Sun, Ping-Jie Wei, and Jin-Gang Liu*
[
a]
X.-Y. Zhou, C. Xu, P.-P. Guo, W.-L. Sun, Dr. P.-J. Wei, Prof. Dr. J.-G. Liu
Key Laboratory for Advanced Materials
School of Chemistry & Molecular Engineering
East China University of Science and Technology, Shanghai 200237, P. R. China
E-mail: E-mail: liujingang@ecust.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: Oxygen reduction reaction (ORR) is essential in many life
processes and energy conversion systems. Inspired by enzymatic
oxygen activation/reduction processes, it is desirable to design
transition metal molecular catalysts as an alternative to noble-metal-
Pt-based ORR electrocatalysts in view point of fuel cell
commercialization. Herein, we fabricate bio-inspired molecular
catalysts electrografted on multiwalled carbon nanotubes (MWCNTs),
where iron porphyrin FeF20TPP (5,10,15,20-tetra(pentafluorophenyl)
iron porphyrin) is coordinated with covalently electrografted axial
ligand on the MWCNTs’ surface varying from thiophene to imidazole.
With the change of axial coordination environment (i.e., S-thiophene,
N-imidazole, and O-carboxylate), the catalysts’ electrocatalytic
activity varied accordingly, and the imidazole coordinated catalyst
MWCNTs-Im-FeF20TPP exhibited the highest ORR activity among the
prepared catalysts. When MWCNTs-Im-FeF20TPP was loaded on the
cathode of a zinc–air battery assembly, open-cell voltage (OCV) of
Because of their abundance and low cost, transition metal-based
macrocycles[ (e.g. porphyrins,
5]
[5a-f]
corroles
[5g]
and iron/cobalt
phthalocyanines,[5h-j]) have been intensively investigated for ORR,
which mimic the catalytic active center of cytochrome c oxidase
and heme-containing enzymes that efficiently activate/reduce
dioxygen with very low overpotential.[
6]
Metal macrocycles pyrolyzed at high temperatures are
usually applied to enhance catalyst performance.[7] Good
conductivity and durability were generally achieved after pyrolysis,
whereas the chemical structure of metal macrocycles might
experience an uncontrolled and unpredictable transformation
during the heat treatment.[8] Additionally, there is a trend for metal
aggregation under high-temperature treatment, which usually
leads to the formation of metal nanoparticles; this hampers the
further improvement of catalyst performance and makes it difficult
to realize the structure–performance-correlated catalyst design
for ORR.[9]
−2
1.35 V and the maximum power density (Pmax ) of 110 mW cm were
achieved, which was higher than those of MWCNTs-Thi-FeF20TPP
Immobilization of molecular catalysts on carbon-based
materials without pyrolysis allows to tune catalyst activity through
structure modification.[5b, 5g, 8a, 10, 11] Metal macrocycles, including
Fe porphyrin,[5b] Co corrole,[11] and Fe phthalocyanine,[12] have
been covalently grafted on graphene and carbon nanotubes as
an efficient electrocatalyst for ORR through the C–C bond
formation via the chemical reduction of diazonium salt precursors.
Electrografting is an alternative approach for surface covalent
functionalization, which has been widely used in the preparation
of sensors,[13] modified electrodes,[14] and capacitors.[15] Because
voltage is applied to the electrode, the diazonium salt is attracted
to the electrode surface by electrostatic interaction and is quickly
reduced. Hence, unlike the chemical reduction method, the
reaction sites are located only on the electrode surface where
−
2
(
OCV = 1.30 V, Pmax = 100 mW cm ) and MWCNTs-Ox-FeF20TPP
−2
(OCV = 1.28 V, Pmax = 86 mW cm ) and comparable with that loaded
−
2
with a Pt/C catalyst (OCV = 1.45 V, Pmax = 120 mW cm ) under similar
experimental conditions. This study provides a time-saving method to
prepare covalently immobilized molecular electrocatalysts on carbon-
based materials with structure–performance correlation, which is also
applicable to the design of other electrografted catalysts for energy
conversion.
Introduction
Activation and reduction of oxygen are very important processes
because oxygen reduction reaction (ORR) occurs in various life
processes and energy conversion devices such as zinc–air
batteries and fuel cells.[1] However, the kinetics of ORR in the
cathode of cells is sluggish, which requires high overpotential for
the catalyzed reaction,[2] and there is an urgent need for efficient
catalysts to reduce the activation energy barriers and improve the
overall performance.[3] Presently, the noble metal platinum (Pt)
catalysts are mainly applied in fuel cells, whereas its scarcity and
prohibitive costs still limit its large-scale commercialization.[4]
electrografting occurs. However, the generally used electrodes for
electrografting (e.g., glassy carbon,[16] Au,[17] and indium tin oxide
electrodes ) possess a small specific surface area, which
considerably limits the amount of prepared catalyst and its mass
production.
Heme, an iron porphyrin complex, is the cofactor of various
metalloenzymes. It has essential functions of transporting/storing,
[18]
activation/reduction of O
2
, and participating in various redox
reactions in biological systems. Of note, enzymes share the same
1
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Chemistry - A European Journal
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heme active center but with different axial ligand coordination. In
myoglobin/hemoglobin, heme oxygenase, and cytochrome c
oxidase, iron porphyrin is axially coordinated with the N atom of
imidazole from the histidine group; in cytochrome P450, cysteine
ligand coordinates with iron porphyrin through the S atom; in
catalase, tyrosine coordinates with iron porphyrin through the O
atom of the phenolic hydroxyl group. Hence, the activity and
functionalities of metalloenzymes are regulated by different axial
ligand coordination and peripheral environment around the active
paper to which the potential of −0.1 V vs. Ag/AgCl was applied for
1 min. Diazonium salts were attracted to the electrode by
electrostatic interaction and then reduced by electrons from the
2
working electrode, where many bubbles (N ) were produced
(Figure 2 and S1). Then, iron porphyrin FeF20TPPCl was loaded
on functionalized MWCNTs through axial coordination of
imidazole and thiophene, respectively. Considering that the
electron withdrawing F-substituted phenyl may reduce the
electron density of the central metal atom, downshifting the
site.[19] Recent studies showed that 5-coordinated metal (M-N
)
5
g
energy level of e -orbitals and anodically shifting the iron ion’s

macrocycles exhibited enhanced selectivity of the 4e reduction
pathway and thus of better performance for ORR than their
corresponding M-N
4
counterparts.[5a-b, 20]
To investigate the effect of axial ligand coordination with Fe
porphyrin on the catalyst ORR activity, herein, we report a time-
saving method of electrografting Fe porphyrin that is axially
coordinated with S-, N-, and O-containing ligand on the surface of
multiwalled carbon nanotubes (MWCNTs). Through the
electroreduction of different diazonium salts, we fabricated bio-
inspired ORR molecular catalysts composed of FeF20TPP
Figure 2. Illustration showing the preparation of MWCNTs-Im and MWCNTs-
Thi.
[
5,10,15,20-tetra (pentafluorophenyl) iron porphyrin] axially
coordinated with electrografted imidazole, thiophene, and
carboxylate ligand, respectively, on the surface of MWCNTs
redox potential, and improve the iron porphyrin catalyst
performance for ORR relative to their unsustituted one,[
5a, 5g, 21]
(
Figure 1). It was found that with the change of axial coordination
here a pentafluorophenyl iron porphyrin was then employed.
The characterization of functionalized MWCNTs was first
performed by transmission electron microscope (TEM).
Compared with pristine MWCNTs, the surface roughness of
MWCNTs-Im and MWCNTs-Thi increased (Figure S2a and 2b),
and small bumps along the sidewalls MWCNTs-Im-FeF20TPP and
MWCNTs-Thi-FeF20TPP were observed (Figure 3a and b), which
suggested surface modification of MWCNTs. Fourier transform
infrared (FTIR) spectroscopy was used to confirm additional
groups on the surface of MWCNTs. MWCNTs-Im exhibited
additional signals at 1580 and 1389 cm−1 (gray dash line)
compared with pristine MWCNTs (Figure 3c), which were
environment, the charge transfer resistance of the catalyst for
ORR varied accordingly, and the imidazole-coordinated catalyst
MWCNTs-Im-FeF20TPP exhibited the highest performance which
was comparable with a commercial Pt/C catalyst in 0.1 M KOH
solution as well as in a zinc-air cell assembly, demostrating the
application prospect of bio-inspired molecular catalysts for ORR.
+
assignable to the stretching vibrations of imidazole’s C=N H bond.
Those peaks shifted to 1540 and 1350 cm−1 after washing with a
+
NaOH solution, which indicated the deprotonation of the =N H
group in imidazole as previously observed by Ficher and co-
workers.[18]
2
Compared with 3-aminothiphene (3-NH -Thi), MWCNTs-Thi
−
1
exhibited similar signals between 1530 and 700 cm , as indicated
by a gray dashed line in Figure 3d, which showed the successful
functionalization of thiophene on MWCNTs.
Additional evidence was provided by resonance Raman
spectroscopy. MWCNTs, MWCNTs-Im, and MWCNTs-Im-
FeF20TPP exhibited two main peaks at 1340 and 1584 cm−1
(
Figure 3e), which correspond to the D band of disordered sp3
2
carbons and G bond of sp hybrid carbons, respectively. The ratio
of D/G for MWCNTs-Im (I
D G
/I = 0.62) was greater than that for
MWCNTs (I /I 0.32), which suggested that diazonium
D
G
=
Figure 1. Structure illustration of MWCNTs-Im-FeF20TPP, MWCNTs-Thi-
FeF20TPP, MWCNTs-Ox-FeF20TPP, and MWCNTs+FeF20TPP.
treatment resulted in more defects on the MWCNTs surface, in
agreement with previous studies.[5b, 11] FeF TPP coordination
2
0
with imidazole on MWCNTs-Im produced insignificant changes in
the I /I value (0.64). The same trend of I /I change was
observed in MWCNTs-Thi and MWCNTs-Thi-FeF20TPP (Figure
f). Hence, Raman spectroscopic evidence further confirms that
D
G
D G
Results and Discussion
3
Functionalization of the surface of MWCNTs with imidazole and
thiophene was performed by electroreduction of corresponding
diazonium salts that were pre-adsorbed on MWCNTs/carbon
imidazole and thiophene groups have been covalently
functionalized on the surface of MWCNTs.
2
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Figure 4. (a) XPS survey high-resolution N 1s scan of MWCNTs-Im. (b) N 1s
scan of MWCNTs-Im-FeF20TPP. (c) N 1s scan of MWCNTs-Thi-FeF20TPP. (d)
UV/Vis absorption spectrum of FeF20TPP, MWCNTs-Im-FeF20TPP, MWCNTs-
Thi-FeF20TPP, and MWCNTs-Ox-FeF20TPP.
The high-resolution S 2p XPS spectrum of MWCNTs-Thi-
FeF20TPP (Figure S3c) revealed one main signal that could be
further deconvoluted into two peaks with binding energies at
Figure 3. TEM images of (a) MWCNTs-Im, (b) MWCNTs-Thi, (c) FTIR spectra
of MWCNTs, MWCNTs-Im, and MWCNTs-Im (washed with NaOH). (d) FTIR
spectra of MWCNTs, 3-NH -Thi, and MWCNTs-Thi. (e) Raman spectra of
2
1
65.2 and 164.1 eV, which correspond to C–S–C 2p1/2 and 2p3/2,
respectively.[ The content of S was determined to be 0.64 at %
22]
MWCNTs, MWCNTs-Im, and MWCNTs-Im-FeF20TPP. (f) Raman spectra of
MWCNTs, MWCNTs-Thi, and MWCNTs-Thi-FeF20TPP.
in MWCNTs-Thi-FeF TPP. The high-resolution Fe 2p XP spectra
2
0
of
MWCNT-Im-FeF20TPP
and
MWCNT-Thi-FeF20TPP
demonstrated similar signals at approximately 711.3 and 721.3
eV, which were assignable to the binding energy of Fe 2p3/2 and
Fe 2p1/2 (Figure S3a and 3d), with the Fe content of 0.12 and
X-ray photoelectron spectroscopy (XPS) was performed to
further analyze the chemical composition of the prepared
composites. Figure 4a shows that the N 1s high-resolution XP
spectrum of MWCNTs-Im exhibits two main peaks that can be
deconvoluted into three components at 401.8, 400.9, and 398.9
eV corresponding to protonated imine-, imine-, and amine-N of
imidazole group, respectively.[18] This observation agrees with the
abovementioned FTIR result and suggests the successful
electrografting of covalently attached imidazole group on
MWCNTs. There is an extra peak at 399.4 eV in the N 1s high-
resolution XP spectrum of MWCNT-Im-FeF20TPP (Figure 4b),
corresponding to the pyrrolic N in porphyrin.[1c, 5b] The nitrogen
content of MWCNT-Im was determined to be 0.61 at %, and it
increased to 0.99 at % after iron porphyrin loading on MWCNT-
Im. Of note, the N 1s XPS spectrum of thiophene-functionalized
MWCNTs-Thi-FeF20TPP exhibited only one pyrrolic N peak at
0.11 wt% for MWCNT-Im-FeF20TPP and MWCNT-Thi-FeF20TPP
determined by ICP-OES analysis (Table S1), respectively.
To further investigate the coordination between FeF20TPP
and modified MWCNTs, diffuse reflectance UV/Vis spectra were
obtained. The spectrum of FeF20TPP shows an intense band at
411 nm and broad bands at 505, 569, and 633 nm that correspond
to porphyrin Soret and Q-bands, respectively (Figure 4d). After
coordination with different functional groups on the MWCNTs’
surface, the Soret band shifted to 403, 404, and 407 nm for
MWCNTs-Thi-FeF20TPP,
MWCNTs-Im-FeF20TPP,
and
MWCNTs-Ox-FeF20TPP, respectively, and Q-bonds shifted to
approximately 570 nm (Figure 4d). This observation is consistent
with our previous report[5b] and suggests that iron porphyrin is
anchored to MWCNTs through the axial coordination of surface
functionalized groups (imidazole, thiophene, and carboxylate).
399.5 eV (Figure 4c) with a corresponding N content of 0.63 at %.
3
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E
1/2 = 0.75 V). The Tafel slopes of MWCNTs-Im-FeF20TPP (55
-1
-1
mV dec ) and MWCNTs-Thi-FeF20TPP (57 mV dec ) were
comparable to Pt/C (68 mV dec ), indicating their similar ORR
-1
kinetics in an alkaline solution (Figure 5d).
−
Additionally, the plot of HO
2
yield versus potential revealed
that a significantly reduced amount of hydroperoxide species was
produced for the axially coordinated catalysts (MWCNTs-Im-
FeF20TPP,
MWCNTs-Thi-FeF20TPP,
and
MWCNTs-Ox-
FeF20TPP) compared with that of the control sample
MWCNTs+FeF20TPP (Figure 5e). MWCNTs-Im-FeF20TPP-
catalyzed ORR generated the lowest amount of peroxide species
(
less than 5%) in the potential range of 0.2–1.0 V, where the
electron transfer number was calculated to be approximately 4.0
Figure 5f), suggesting high selectivity of the 4e reduction
(
pathway. These results indicate the important contribution of axial
ligand coordination to the resultant iron porphyrin catalyst’s ORR
activity, which renders the catalyst with the more positive onset

and half-wave potential and remarkably enhanced 4e reduction
selectivity for ORR.
To further explore the electrochemical kinetics for ORR, the
electrochemical impedance spectra (EIS) were measured in the
frequency ranging from 100 kHz to 0.01 Hz with an alternating
current (AC) perturbation of 5 mV. To investigate the interfacial
phenomena of disk electrodes (Figure 6a), the equivalent circuit
model was set as shown in Figure 6b, which consists of the
internal resistance of electrolyte solution (R
layer capacitor (C ), and Faraday impedance (Z
charge transfer resistance (Rct) and Warburg resistance for ionic
diffusion processes (Ζ ). Warburg resistance will disappear when
the frequency is very high because mass transfer occurs too late.
The impedance diagrams were simulated using an equivalent
Ω
), electric double-
d
f
). Z includes
f
2
Figure 5. (a) Cyclic voltammograms of MWCNTs-Im-FeF20TPP in O - and Ar-
saturated 0.1
MWCNTs + FeF20TPP, MWCNTs-Im-FeF20TPP, MWCNTs-Thi-FeF20TPP, and
MWCNTs-Ox-FeF20TPP in -saturated 0.1 KOH solutions. (c) ORR
M
KOH solutions. (b) Cyclic voltammograms of
ω
O
2
M
polarization curves and (d) Tafel slopes of MWCNTs+FeF20TPP, MWCNTs-Im-
FeF20TPP, MWCNTs-Thi-FeF20TPP, MWCNTs-Ox-FeF20TPP and Pt/C (20
Ω
circuit (Figure 6b) and the values of R and Rct for catalyst
wt%) at 1600 rpm in O
2 2 2
-saturated 0.1 M KOH solutions. (e) H O % yields and
(f) Electron transfer number of MWCNTs+FeF20TPP, MWCNTs-Im-FeF20TPP,
composites were listed in Table 1. All catalysts had approximately
MWCNTs-Thi-FeF20TPP, MWCNTs-Ox-FeF20TPP and Pt/C (20 wt%).
the same RΩ because they have similar solutions. For R , which
ct
reflects the charge transfer’s resistance, a notable difference was
observed between axial-coordinated-catalysts and the control
sample MWCNTs+FeF TPP without axial coordination (Figure
6c). Significantly higher Rct (656 Ω) was obtained for the
The ORR catalytic activity of prepared catalysts was
evaluated using cyclic voltammetry (CV) that was conducted in
2
0
O
2
- and Ar-saturated 0.1 M KOH solutions, respectively. A
MWCNTs+FeF TPP catalyst relative to MWCNTs-Im-FeF TPP
2
0
20
featureless CV was obtained for MWCNTs-Im-FeF20TPP in an Ar-
saturated solution, while a prominent reduction peak at 0.89 V (vs.
20
(334 Ω) and MWCNTs-Thi-FeF TPP (396 Ω). The relatively low
Rct value observed for MWCNTs-Im-FeF TPP indicated an easy
20
RHE) was observed in the O
2
-saturated solution (Figure 5a),
pathway for rapid electron/charge transport, which demonstrated
the importance of electrografted-imidazole axial coordination for
promoting the electron/charge transport between the catalyst
active center and the electrode.
Theoritical DFT calculations have shown that the axial ligand
can provide an extra electron “push” effect and produce the
hybridization of the 3d orbitals of the Fe metal center with the
orbitals of the axial ligand, leading to significant geometric and
suggesting its good electrocatalytic activity toward ORR. The
shape of the reduction wave is asymmetrically deformed might
due to the electrode surface using MWCNTs. And the reduction
potential (0.89 V) was 60 and 90 mV more positive than those of
MWCNTs-Thi-FeF20TPP (0.83 V) and MWCNTs-Ox-FeF20TPP
(
0.80 V), respectively (Figure 5b). Meanwhile, the control sample
of the physical mixtures of MWCNTs+FeF20TPP showed the
lowest potential at 0.79 V among the catalysts tested. Linear
scanning voltammograms (LSVs) obtained using a rotating ring-
disk electrode (RRDE) were also recorded to examine the onset
electronic structure changes in the metal center and thus of
enhanced catalyst performance for ORR.[
20 c-g]
As can be found in
this work that axial ligand (imidazole, thiophene and carboxylate)
coordination to the iron porphyrin led to both increased rate of the
(
E
onset) and half-wave potential (E1/2) of the catalysts for ORR.
MWCNTs-Im-FeF20TPP demonstrated the most positive Eonset
1.04 V vs. RHE) and E1/2 (0.87 V vs. RHE) among the axial

ORR and enhanced 4e reduction selectivity when compared with
(
the catalyst without axial coordination. And the promotion effect
of the imidazole ligand is more significant than the thiophene and
carboxylate ligand, respectively; this might suggest a much
stronger binding ability of the imidazole ligand to iron porphyrin
with strenthened back-bonding that led to promoted modulating
ability for the heterolytic O–O bond cleavage as have been found
coordinated samples of MWCNTs-Im-FeF20TPP, MWCNTs-Thi-
FeF20TPP (Eonset = 0.93 V, E1/2 = 0.81 V), and MWCNTs-Ox-
FeF20TPP (Eonset = 0.92 V, E1/2 = 0.77 V, Figure 5c), and the
values of Eonset and E1/2 of MWCNTs-Im-FeF20TPP for ORR
are130 and 120 mV more positive, respectively, than those of the
physical mixture sample MWCNTs+FeF20TPP (Eonset = 0.91 V,
4
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Chemistry - A European Journal
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−2
in previous biomimetic studies on chemical models of the active
site of cytochrome c oxidase.[5c-f]
sample MWCNTs+FeF20TPP (79 mW cm , Table 2), which is
consistent with the observation from the solution electrochemical
activity evaluation.
Regarding
the
long-term
catalyst
stability,
the
chronoamperometric responses of MWCNTs-Im-FeF20TPP,
MWCNTs-Thi-FeF20TPP, MWCNTs-Ox-FeF20TPP, and 20 wt.%
Pt/C were recorded at 0.55 V vs. RHE in an O -saturated 0.1 M
2
KOH solution with a rotation rate of 900 rpm. MWCNTs-Im-
FeF20TPP maintained 93% of its initial current after 24 h, as
shown in Figure 6e, which was the highest ORR durability among
the prepared catalysts, and MWCNTs-Thi-FeF20TPP (80%)
showed durability that was similar to that of Pt/C (81%), whereas
MWCNTs-Ox-FeF20TPP possessed approximately 73% of its
initial current. Long-time durability of the zinc–air cell with cathode
loading of MWCNTs-Im-FeF20TPP and MWCNTs-Thi-FeF20TPP
was also tested. MWCNTs-Im-FeF20TPP maintained the voltage
at 1.2 V for 30 h, and MWCNTs-Thi-FeF20TPP could run
continuously for 50 h, whereas the output voltage declined from
1.10 V to 0.92 V (Figure 6f).
Table 2. Electrocatalytic performance of prepared catalysts in the zinc–air cell.
Catalysts
P
cm
max / mW
OCV / V
1.35
−2
MWCNTs-Im-
FeF20TPP
110
100
86
MWCNTs-Thi-
FeF20TPP
1.30
Figure 6. (a) Schematic diagram of the disk electrode interface. (b) Schematic
diagram of the equivalent circuit model. (c) EIS of MWCNTs-Im-FeF20TPP,
MWCNTs-Thi-FeF20TPP, MWCNTs-Ox-FeF20TPP, and MWCNTs+FeF20TPP.
MWCNTs-Ox-
FeF20TPP
1.28
(
d) Zinc–air performance while using MWCNTs-Im-FeF20TPP, MWCNTs-Thi-
FeF20TPP, MWCNTs-Ox-FeF20TPP, MWCNTs+FeF20TPP (1.0 mg cm−2), and
Pt/C catalyst (JM 20 wt%, 1.0 mg cm−2
as cathode catalysts. (e)
MWCNTs
Pt/C
+
FeF20TPP
79
1.27
1.45
)
Chronoamperometric responses of MWCNTs-Im-FeF20TPP, MWCNTs-Thi-
FeF20TPP, MWCNTs-Ox-FeF20TPP, and 20 wt.% Pt/C at 0.55 V vs. RHE in an
120
O
2
-saturated 0.1 M KOH solution. (f) Long-time durability of the zinc–air cell
loaded MWCNTs-Im-FeF20TPP and MWCNTs-Thi-FeF20TPP at 5 mA cm−2
.
Conclusion
Table 1. Fitted data for R
Ω
and Rct
/ Ω
.
In summary, we developed a time-saving electrografting method
for the covalent functionalization of MWCNTs surface with
imidazole and thiophene ligand, respectively, that is axially
coordinated with iron porphyrin (FeF20TPP) as electrocatalyst for
ORR. The electroactivity of catalysts with axial ligand varied from
imidazole, thiophene, to carboxylate has been evaluated with a
half-cell reaction in 0.1 M KOH solutions and Zn–air battery
assembly. The experimental results showed that the catalyst with
imidazole coordination, MWCNTs-Im-FeF20TPP, exhibited the
highest electrocatalytic activity for ORR among the catalysts
prepared in this work. When MWCNTs-Im-FeF20TPP was applied
Catalysts
R
Ω
Rct / Ω
MWCNTs-Im-
FeF20TPP
126
134
137
170
334
396
460
656
MWCNTs-Thi-
FeF20TPP
MWCNTs-Ox-
FeF20TPP
+
MWCNTs FeF20TPP
−
2
to a zinc–air cell assembly, it generated Pmax (110 mW cm ) and
OCV (1.35 V) that were comparable with those of commercial
Pt/C catalyst under similar conditions. This study provides a facile
approach to the covalent immobilization of molecular catalysts on
carbon-based materials, which may have significant implications
on the design and preparation of efficient molecular
electrocatalysts for energy conversion.
The performance of catalysts toward ORR was further
evaluated in a zinc–air battery assembly (Figure 6d). The
MWCNTs-Im-FeF20TPP loaded cell produced an OCV of 1.35 V
−
2
and a maximum power density (Pmax) of 110 mW cm , which is
comparable with that of the commercial Pt/C catalyst (JM 20 wt%,
−
2
−2
1
.0 mg cm ) with Pmax = 120 mW cm and OCV = 1.45 V under
similar experimental conditions. Additionally, MWCNTs-Thi-
−
2
FeF20TPP displayed a higher power density (100 mW cm ) than
Experimental Section
−
2
MWCNTs-Ox-FeF20TPP (86 mW cm ) and the physical mixture
5
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Chemistry - A European Journal
10.1002/chem.202100736
FULL PAPER
Multi-walled carbon nanotubes (MWCNTs) and carboxyl multi-walled
carbon nanotubes (MWCNTs-Ox, -COOH 3.86 wt%) were commercially
available (Chengdu Organic Chemical Co. Ltd, Chinese Academy of
Sciences), and were purified in a mixed acid solution (VH2SO4: VHNO3 = 1:2)
under stirring at room temperature for 24 h and then thoroughly washed
by deionized water. The purified MWCNTs were dried under vacuum
overnight at 80 °C. FeF20TPPCl was commercially available from Sigma-
Aldrich. 4-imidazolylphenylamine (4-Im-Am) was prepared according to
literature (Figure S6).[18]
used as the anode. An electrolyte consisting of ZnAc
(6.0 M) was used.
2
(0.2 M) and KOH
Acknowledgements
This study was financially supported by the National Nature
Science Foundation of China (No. 21571062, 21271072 to JGL),
the Program for Professor of Special Appointment (Eastern
Scholar) at the Shanghai Institutions of Higher Learning to JGL.
Material preparation
Preparation of MWCNTs-Im: MWCNTs (50.0 mg) and 4-Im-Am (76.0 mg)
were ultrasonically dispersed in ethanol (2.0 mL), and then the mixture
were dropped on carbon paper (3.0 cm  2.5 cm). After evaporating the
solvent, the carbon paper was dried at room temperature which to be used
as working electrode. The working electrode was then soaked in a solution
Keywords: oxygen reduction reaction • electrochemistry •
covalent immobilization • iron porphyrin • energy conversion
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Entry for the Table of Contents
Axial ligand coordination tuning the catalyst ORR activity: A time-saving electrografting method (within couple of minutes) for the
covalent functionalization of carbon nanotubes surface with imidazole or thiophene ligand that is axially coordinated with iron porphyrin
as electrocatalyst for ORR was demonstrated. The N-imidazole coordinated catalyst MWCNTs-Im-FeF20TPP exhibited the highest
ORR activity followed by the S-thiophene and O-carboxylate axial coordinated catalyst, respectively.
8
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