Axial Modification of Cobalt Complexes on Heterogeneous Surface with Enhanced Electron Transfer for Carbon Dioxide Reduction

Article abstract of DOI:10.1002/anie.202008759

Efficient electron communication between molecular catalyst and support is critical for heterogeneous molecular electrocatalysis and yet it is often overlooked during the catalyst design. Taking CO₂ electro-reduction on tetraphenylporphyrin cobalt (PCo) immobilized onto graphene as an example, we demonstrate that adding a relay molecule improves the interfacial electron communication. While the directly immobilized PCo on graphene exhibits relatively poor electron communications, it is found that diphenyl sulfide serves as an axial ligand for PCo and it improves the redox activity of PCo on the graphene surface to facilitate the generation of [PCo]^.- active sites for CO₂ reduction. Thus, the turnover frequencies of the immobilized Co complexes are increased. Systematic structural analysis indicates that the benzene rings of diphenyl sulfide exhibit strong face-to-face stacking with graphene, which is proposed as an efficient medium to facilitate the interfacial electron communication.

Full text of DOI:10.1002/anie.202008759

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Accepted Article
Title: Axial modification of cobalt complexes on heterogeneous surface
with enhanced electron transfer for carbon dioxide reduction
Authors: Xin Wang, Jiong Wang, Xiang Huang, Shibo Xi, and Hu Xu
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RESEARCH ARTICLE
Axial modification of cobalt complexes on heterogeneous surface
with enhanced electron transfer for carbon dioxide reduction
Jiong Wang,[a]# Xiang Huang,[b]# Shibo Xi, Hu Xu and Xin Wang*
[c]
[b]
[a]
[
a]
Dr J. Wang and Prof. Dr. X. Wang
School of chemical and biomedical Engineering
Nanyang Technological University
62 Nanyang Drive, Singapore 637459, Singapore
E-mail: WangXin@ntu.edu.sg
[
[
b]
c]
Dr. X. Huang and Prof. Dr. H. Xu
Department of Physics
Southern University of Science and Technology
Shenzhen 518055, China
Dr. S. Xi
Institute of Chemical and Engineering Sciences
Agency for Science, Technology and Research (A*STAR)
Singapore 627833, Singapore
Supporting information for this article is given via a link at the end of the document.
Abstract: Efficient electron communication between molecular
catalyst and support is critical for heterogeneous molecular
electrocatalysis and yet it is often overlooked during the catalyst
amide based pendant groups on the second sphere of complexes
to stabilize the adsorption of CO on the metal sites, which
facilitated the activation of CO
and boosted the catalysis.[5] To
2
2
design. Taking CO
2
electro-reduction on tetraphenylporphyrin cobalt
realize the heterogeneous molecular catalysis, metalloporphyrin
complexes are normally immobilized onto graphitic-type carbon
(PCo) immobilized onto graphene as an example, we demonstrate
3
3
[6]
that adding a relay molecule improves the interfacial electron
communication. While the directly immobilized PCo on graphene
exhibits relatively poor electron communications, it is found that
diphenyl sulfide serves as an axial ligand for PCo and it improves the
through various linking strategies, such as sp -sp C-C bonding,
pyrene based π-π stacking,[ electrostatic interaction and
7]
[8]
assembling in the forms of covalent or metal organic
frameworks[
2h, 9]
. Besides, single metal site can be directly
redox activity of PCo on the graphene surface to facilitate the
incorporated into N contained carbon matrices.[10] It exhibited the
analogous coordination structure as that in metalloporphyrin
complexes, and the N ligands of carbon matrices can be tunable
(e.g., replace by O, S, P atoms) for the further enhancement of
catalytic performance.[11] For electrochemical applications, the
carbon surfaces not just serve as the solid supports for the
complexes, more importantly, they can also affect the redox
activity of complexes through the interfacial electron
communication with the complexes. Various studies have
•
generation of [PCo] ‾ active sites for CO
2
reduction. The turnover
frequencies of the immobilized Co complexes are thus increased.
Systematic structural analysis indicates that the benzene rings of
diphenyl sulfide exhibit strong face-to-face stacking with graphene,
which is proposed as an efficient medium to facilitate the interfacial
electron communication.
suggested that electrocatalysis of CO
complexes starts with the lowering of their oxidation states.
2
RR on metalloporphyrin
Introduction
[2c, 3a, 7,
12]
Thus, the electron communication between the complexes and
carbon surfaces could play critical role in the CO RR activities,
The electrolysis of carbon dioxide (CO
represents a promising strategy for CO
reduction reaction (CO RR) at the cathode of electrolysis is
2
) using renewable energy
2
2
utilization. The CO
2
and it can be affected by the factors like molecular configurations
of linking bonds, distance and electron conjugation degree
between the complexes and carbon surfaces. The correlation
of linking strategies with the redox activity of metalloporphyrin or
other complexes is so far commonly overlooked and there are
fewer investigations in analyzing the influence of electron
2
kinetically sluggish and requires efficient electrocatalysts to
promote its reaction kinetics.[1] Recent studies showed that the
immobilization of homogeneous molecular catalysts onto
conductive substrates derived various heterogeneous molecular
structures as a type of novel electrocatalyst. This strategy
combines the advantages of both heterogeneous and
homogeneous catalysis and shows the great potential in the
electrocatalytic field.[2] In particular, tetraphenylporphyrin cobalt
and iron complexes represent a very important type of molecular
[13]
2
communication in heterogeneous molecular catalysis for CO RR.
In this work, tetraphenylporphyrin cobalt (PCo) was
immobilized onto graphene simply through π-π interaction. This
linking strategy is considered as the most convenient to construct
the heterogeneous molecular catalysts, which also preserves the
pristine properties of graphene. Through both experimental and
theoretical investigations, it was demonstrated that a [PCo] ‾ state
achieved from one electron reduction of PCo served as the
efficient active site for the chemisorption and subsequent
catalyst for CO
2
RR.[2h, 3] Note that their ligands typically carry
tetraphenyl groups that can be systematically modified to tune the
catalytic properties of the complexes. For instance, the
tetraphenyl groups of tetraphenylporphyrin were appended with
positively charged trimethylammonio groups to polarize the
complexes and thus promote the CO
water.[4] The tetraphenyl groups were also grafted with phenol or
•
2
reduction of CO in the heterogeneous molecular catalysis. While
the directly immobilized PCo on graphene exhibited relatively low
redox activity, we showed that diphenyl sulfide (Di-S) can be an
2
RR catalytic activity in
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
axial ligand to the Co sites of PCo. The axial coordination was
suggested to promote the electron conjugation degree between
PCo and graphene. It improved the electron communication
between them to make more amounts of immobilized PCo
immobilized molecular Co sites. The corresponding energy-
dispersive X-ray spectroscopic (EDX) analysis confirmed the
uniform distribution of Co element over the graphene layers
(Figure S3c). In the normalized X-ray absorption near edge
spectra (XANES, Figure 2c), from SCo to SCo-G, we observed
the increase of white line intensity and negative shift of absorption
edge. Such variations indicated that the electrons of Co sites were
delocalized after the immobilization by the electron conjugation
with graphene. While in the corresponding Fourier-transformed
extended X-ray absorption fine structure spectra (FT-EXAFS,
Figure 2d), the first shell of Co sites was not shifted after the
electrochemically active and thus facilitate the generation of
•
[
PCo] ‾ active sites. As a result, it achieved higher turnover
frequencies (TOFs) of the heterogeneous Co sites on graphene
comparing to the cases without the axial coordination. By
structural analysis, we found that the benzene rings of the axial
Di-S had a stable and parallel stacking geometry relative to
graphene, and thus were proposed as the relay for electron
transfer with the heterogeneous Co complexes.
immobilization. Thus, there should not be any chemical bonds
between SCo and graphene,[
2c, 16]
suggesting that the interaction
between SCo and graphene was Van der Waals type. For PCo-
G, it showed the similar morphologies with the uniform distribution
of molecular Co sites over the graphene (Figure S4, S5), and the
immobilization of PCo was also realized by Van der Waals
interaction (Figure S6).
Results and Discussion
Heterogenization of Co complexes. The heterogeneous Co
complexes were achieved following two steps. First, PCo were
reacted with Di-S in N, N-dimethylformamide (DMF) to form
modified Co complexes (Figure 1a, denoted as SCo, the detailed
procedures were provided in the experiments of supporting
information, [SI]). In the UV-visible spectra (Figure S1a), PCo
exhibited two absorbance bands centered at 416 nm and 535 nm
for the characteristic B and Q bands of tetraphenylporphyrin,
respectively. For comparison, the B and Q bands of SCo shifted
bathochromically, while the main molecular skeleton was
maintained as PCo according to X-ray photoelectron
spectroscopic surveys (XPS, Figure S1b, c). The similar variation
of UV-visible absorbance for the axial modification of PCo based
complexes was reported previously,[14] which implied that the axial
position of Co sites of PCo could be occupied by Di-S.[15] By
CHNS elemental analysis combining with inductively coupled
plasma spectrometric (ICP) measurements, we found that the
molar ratio of Co:N in PCo was ~1:5.1, which was larger than the
stoichiometric 1:4. Thus, except for the tetraphenylporphyrin
ligand, there were other N based molecules serving as the liable
Figure 1. (a) Fully relaxed structure of PCo interacting with Di-S through the
formation of axial Co-S bonds. The brown, white, yellow, blue and light blue
spheres represent C, H, S, Co and N atoms, respectively. (b) Scheme illustrates
that SCo was immobilized onto graphene by van der Waals interactions.
3
ligands for PCo, which could be either the DMF solvent or NO ‾
ions from the Co salts precursor (Figure S2). After the
modification by Di-S, the Co:N molar ratio in SCo decreased to
1:4.6-4.8, meanwhile, S was observed in the SCo and the Co:S
molar ratio was measured to be ~1:0.13. This result suggested
that Di-S connected with PCo through replacing parts of those
axial N containing ligands (Figure 1a).
Secondly, the resultant SCo was immobilized onto graphene in
o
a mild condition (i.e., DMF, 80 C and 12 h) to generate
heterogeneous Co complexes (denoted as SCo-G, Figure 1b). As
the control sample, PCo was heterogenized by graphene
following the same steps and the product was denoted as PCo-G.
In the UV-visible spectra (Figure S1d), the π-π* transition of
graphene was at 269.6 nm, exhibiting a bathochromic shift
comparing to those of SCo-G (274.3 nm) and PCo-G (272.8 nm),
respectively. This can be ascribed to the electron conjugation
between graphene and the complexes, and the spectra of PCo-G
Figure 2. TEM (a) and HAADF-STEM (b) images of SCo-G. Scale bar: 200 nm
(a) and 5 nm (b). (c) Normalized Co K-edge XANES data of SCo and SCo-G.
(d) Nonphase-shift-corrected Fourier transform (FT) data for SCo, SCo-G and
Co(NO •6H O.
3
)
2
2
and SCo-G also showed the characteristic
Q bands of
Electrochemical measurements of Co complexes on
graphene. The redox behaviors of the heterogeneous Co
complexes were firstly investigated by cyclic voltammograms
tetraphenylporphyrin. In the transmission electron microscopic
images (TEM, Figure 2a and S3a, b), SCo-G only exhibited the
typical graphene characters and no metal-based nano or bulk
structures can be observed. In the high-angle annular dark-field-
scanning transmission electron microscopic images (HAADF-
STEM, Figure 2b), there were dispersed bright dots with sizes of
(CVs) by depositing the samples onto planar glassy carbon
electrodes (GCE). For SCo-G and PCo-G, their redox peaks both
occurred near -0.1 V (Figure 3a, all potentials were quoted against
the reversible hydrogen electrode, RHE, unless stated otherwise)
0.2-0.3 nm distributing over SCo-G, which were assigned to the
•
assigning to the [PCo]/[PCo] ‾ couple (For the case that such a
2
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RESEARCH ARTICLE
redox couple occurring on SCo-G, it was labeled as [SCo]/[SCo]•‾
for convenience).[17] The peak current densities exhibited linear
correlations with the scan rates, reflecting the immobilized states
of complexes on graphene (Figure 3b). We next used differential
pulse voltammograms (DPVs) to minimize the contribution from
the capacitive currents and have a clearer observation of those
redox signals. As shown in Figure 3c, SCo-G afforded higher DPV
currents than PCo-G. Since the Co contents of SCo and PCo on
graphene were almost the same (5.1 × 10-8 mol mg SCo-G and 5.0
terms of Co sites). The results showed that the TOFs of Co sites
of SCo-G were improved comparing to that of PCo-G (Figure 3f).
-
1
Typically, at -0.87 V, the TOF of Co sites reached 2.4 s on SCo-
-
1
G and it was only 1.6 s on PCo-G. Through the density functional
theory (DFT) calculations, it was found that the Di-S modification
of PCo did not promote the intrinsic CO
complexes (Figure S9). On PCo, the theoretical overpotential of
CO RR was 0.18 V, while on SCo, the overpotential even slightly
2
RR activity of the
2
-1
increased to 0.20 V. We thus proposed that the apparent different
TOFs of Co sites in PCo and SCo should be attributed to the tuned
efficiency of electron transfer between the Co complexes and
graphene.
×
10-8 mol mg-1PCo-G
, respectively), as detected by ICP
measurements (Table S1), the higher DPV currents suggested
that SCo-G exhibits better dispersion of Co complexes or better
interfacial electron transfer between the Co complexes and
graphene than PCo-G, making more Co complexes
electrochemically active to serve as the efficient heterogeneous
catalytic sites.
CO
mechanism of CO
interfacial electron transfer, we simulated the electrochemical
reducing environments of CO RR on SCo by introducing an
electrochemical double layer (EDL) structure above the molecular
plane of SCo. The EDL is composed of four H O molecules and
one excess H atom. Upon the optimization of the electronic
properties of EDL, this excess atom spontaneously
2
Reduction Mechanism. To gain an insight into the
2
RR on SCo, which was possibly relevant to the
2
2
H
decomposed into one solvated proton in the water layer with the
countercharge transferred to SCo.[ This countercharge induced
the formation of reduced state of SCo, which corresponded to the
18]
•
redox event of [SCo]/[SCo] ‾ (Figure 3c) as observed in the DPV
•
measurements. On the molecular plane of [SCo] ‾, it was found
that CO
2
spontaneously chemisorbed onto the Co site with the
bond angel of O=C=O tilting from 180 o to 127.1 . The charge
density difference combined with the calculated Bader charge
analysis reveal that there are 0.83 e being transferred from the
o
complex to CO
CO
2
(Figure 4a). These results clearly indicate that
was activated on [SCo] ‾. In contrast, in the absence of EDL,
•
2
Figure 3. (a) CVs of PCo-G and SCo-G, 0.1 M KHCO
3
, Ar atmosphere, 50
i.e., in a nonreductive environment, CO
physisorption on SCo as verified by its pristine linear geometry
2
only exhibited the
mV s-1 scan rate. (b) Their anodic peak current densities of [PCo]/[PCo] ‾ redox
•
-1
at scan rates ranging from 10, 20, 50, 80, 100, 150 to 200 mV s . (c) The
(
Figure 4b). The same results for CO
2
activation was observed on
•
corresponding DPVs for [PCo] ‾ to PCo oxidation. (d) FEs of CO production on
•
[PCo] ‾ and PCo (Figure S10). These results well supported that
3 2
PCo-G and SCo-G, respectively, 0.1 M KHCO , CO atmosphere. (e) Their
current densities for CO production. (e) Their TOFs of Co sites at various
potentials.
the interfacial electron transfer between SCo (PCo) and graphene
determined the formation of reduced states of SCo (PCo), namely,
•
•
SCo ‾ ([PCo] ‾), which served as the active sites for the
chemisorption of CO to commence the CO RR.
2
2
The electrocatalysis of CO
complexes was conducted by chronoamperometry with stepwise
changing potentials in CO saturated 0.1 M KHCO . The reaction
products were analyzed by in-line gas chromatography (GC) and
high-performance liquid chromatography (HPLC). At the
potentials ranging from -0.6 V to -1 V, the only product detected
2
RR on those heterogeneous Co
To further understand the facilitated redox of Co complexes by
Di-S axial coordination, we explored the molecular configuration
of SCo and PCo immobilized on graphene using the DFT
calculations. For these systems, the van der Waals interaction of
π-π stacking between the complexes and graphene primarily
contributed to their structural stabilities. In the fully relaxed
configuration of PCo-G, the porphin plane floated above the
graphene and the surrounding tetraphenyl groups were tilted with
2
3
was CO. SCo-G afforded both higher Faradic efficiencies (FE-CO
,
Figure 3d and S7) and catalytic current densities (J-CO, Figure 3e)
than PCo-G. For comparison, we took the advantage of the
insolubility of SCo and PCo, and directly cast the complexes onto
the electrodes. In DPVs, they did not exhibit the redox features of
o
angles larger than 40 relative to graphene plane (Figure S11).
This is ascribed to the significant steric effect from the neighboring
porphin ring. Reasonably, the tilted geometry of benzene rings
should weaken the π-π stacking of the complex with graphene.
The electron transfer was realized from graphene onto either the
porphin or benzene rings of PCo. In the case of SCo-G, it was
found that the Di-S group had the face-to-face stacking geometry
relative to graphene. Both two benzene rings of Di-S showed very
complexes and their CO
S8). These results verified that CO
2
RR activities were relatively low (Figure
RR was enabled by the
2
heterogeneous Co complexes with redox activity. Therefore, the
redox of Co complexes was facilitated by graphene that enabled
the interfacial electron transfer during the overall electrocatalytic
process. In the absence of Co complexes, the pristine graphene
o
small tilting angles of less than 10 relative to graphene surface
or Di-S modified graphene afforded insignificant CO
2
RR currents,
and the distance between the benzene ring of Di-S and graphene
was about 3.3 Å (Figure 4c). This indicated that the Di-S part of
SCo had very strong π-π stacking with graphene.[19] Thus, it is
believed that from PCo-G to SCo-G, the electron transfer chains
suggesting that CO RR was realized by the Co complexes. We
2
therefore normalized J-CO with the Co contents on graphene as
determined by ICP and calculated the TOFs of Co complexes (in
3
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
between PCo and graphene can be tuned as Di-S can serve as
the electron relay (Figure 4d). The derived result can be
analogous to those pyrene moieties mediated interfacial electron
transfer, where the pyrene moieties also exhibited the close face-
to-face stacking geometry between their benzene rings and
graphitic carbon, and the improved transfer efficiencies were
observed.[ The electron transfer pathways in PCo-G and SCo-
G could adopt either the tunneling or hopping mechanism.
Because of the non-covalent interaction of Co complexes with
graphene and the lack of a molecular bridge as the medium to
migrate the electron from the donor to acceptor [21], the possibility
of hopping mechanism can be excluded The efficiency of electron
transfer then was determined by the distance and electron
conjugation degree between the Co complexes and graphene. As
the transfer distance in PCo-G and SCo-G were calculated to be
very close (near 3.3 Å), we therefore suggested that the
introduced Di-S moiety promoted the electron conjugation degree
of Co complexes and graphene.[13, 22] This made the more
amounts of the immobilized Co on graphene to be
electrochemically active. Indeed, the electron homogeneity of
graphene can be broken due to any structural defects formed
during the chemical synthesis of graphene,[23] it is very likely that
the insufficient interfacial electron conjugation could lead to the
redox-inactivity of some Co complexes on graphene.
diphenyl sulfone (S
bonds (denoted as S
13). By ICP measurements, from S
5
) to axially coordinate PCo through Co-O=S
4
Co-G and S
5
Co-G, respectively, Figure S12,
Co-G to S Co-G, the amounts
2
5
of the immobilized Co complexes on graphene slightly varied from
4.6 × 10-8 to 6.2 × 10 mol/mgsamples, which were very close to the
cases of SCo-G and PCo-G (Table S1). For all these control
samples, CVs showed that their capacitive currents were close to
the cases of PCo-G and SCo-G, and the redox potentials of
complexes all occurred near -0.1 V. Their anodic peak current
densities exhibited linear correlations with the various CV scan
rates indicating the immobilized states of Co sites (Figure S14).
-8
20]
In the DPV measurements, as expected, S
also showed the improved redox currents comparing to PCo-G
(Figure 5b). Especially in the case of S Co-G, the DPV currents
were even higher than that of SCo-G, reflecting that the benzene
rings of those axial ligands facilitated the electron communication
between PCo and graphene. Besides, with the axial coordination
bonds changing from Co-S to Co-O=S bonds, the redox currents
of Co complexes on graphene also varied as can be seen from
2 3
Co-G and S Co-G
2
4 5
the DPVs of S Co-G and S Co-G. It indicates that the
configurations of axial bonds also affect the electron transfer. In
addition, DFT study and ICP measurements suggested that on
the graphene surfaces, PCo, SCo or the other modified
complexes should not stack to affect the electron transfer
efficiencies (Figure S15).
Toward the electrocatalysis of CO
those various control samples remained only as CO. With the
facilitated redox activity of Co complexes on graphene, the FE-CO
2
RR, the reaction products on
,
J
-CO and TOFs of Co sites were all increased on S
2
Co-G, S
4
Co-G comparing to those on PCo-G (Figure S16, S17a). At
3
Co-G
and S
-
1
-0.87 V, the highest TOF (2.75 s ) was observed on S
2
Co-G,
while S Co-G exhibited the lowest activity because of its
5
decreased redox activity. Accordingly, for those various
heterogeneous Co complexes, their TOFs of Co sites exhibited a
positive correlation with the DPV currents (Figure S17b). This well
2
verified that, for the heterogeneous Co complexes, the CO RR
electrocatalysis was commenced by the reduced forms of PCo.
Thus, it is important to establish efficient electron relays to
promote the interfacial redox of PCo.
2
Figure 4. (a) Charge density difference plot showing CO activation on the Co
3
site of SCo within a EDL structure. Isosurfaces of 0.015 e/Å (charge
3
accumulation) and -0.015 e/Å (chargedepletion) are visualized in red and green,
2
respectively. The fully relaxed configurations of CO adsorption on SCo in the
absence of EDL (b) and SCo on graphene (c). The inset of Figure 4c indicated
the relative angle between benzene rings of Di-S and graphene. (d) Schematic
illustration of the possible electron transfer pathways from graphene, SCo to
CO toward the activation of CO . The charge density difference is calculated
2 2
by ρtotal-ρCO2-ρEDL/SCo, where ρtotal, ρCO2 and ρEDL/SCo represent the charge
2 2
densities of CO chemisorbed on SCo within an EDL structure, CO molecule,
and SCo with an EDL structure, respectively.
Figure 5. (a) The various derivatives of Di-S as the control samples to modify
PCo. (b) Comparison of DPVs of SCo-G, S
and PCo-G.
2 3 4 5
Co-G, S Co-G, S Co-G, S Co-G,
Our observations suggested that the benzene rings of Di-S
could mediate the interfacial electron transfer and facilitate the
generation of CO
observation, we examined the derivatives of Di-S (Figure 5a) of
-biphenylyl phenyl sulfide (S ) and dibenzothiophene (S ) to
axially coordinate PCo following the same steps to achieve SCo.
The resultant products were denoted as S Co and S Co, which
were immobilized onto graphene to derive S Co-G and S Co-G,
respectively, for tuning the interfacial electron transfer.
Additionally, considering that the S sites of Di-S could also
possibly mediate the electron transfer, we thereby changed the
2
RR active sites. To further verify such an
Conclusion
4
2
3
In summary, we showed that Di-S can be an efficient ligand for
PCo through formation of coordination bond at the axial position
Co sites. Such a modification strategy enhanced the redox activity
of PCo upon immobilizing onto graphene surface, which
facilitated the generation of [PCo] ‾ state as the active sites for
chemisorption of CO
2
3
2
3
•
2 2
and commenced the electrochemical CO
4
nature of S binding sites and used diphenyl sulfoxide (S ) and
to CO conversion in the aqueous solution. The TOF of Co sites
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
achieved 2.4 s-1 at -0.87 V comparing to that of 1.6 s-1 for pristine
PCo on graphene. Using a derivative axial ligand of 4-biphenylyl
phenyl sulfide, the TOF of Co sites was further increased to 2.75
s-1 with higher redox activity of Co complexes. We demonstrated
that the benzene rings of Di-S groups bonding with PCo had
stable face-to-face stacking geometry with graphene, which thus
could serve as an efficient mediator to tune the electron transfer
chains between PCo and graphene. These results should provide
a new insight into understanding the electrocatalytic performance
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Keywords: heterogeneous molecular catalysis• interfacial
electron transfer • axial coordination • carbon dioxide reduction •
catalytic mechanism
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Angewandte Chemie International Edition
10.1002/anie.202008759
RESEARCH ARTICLE
Entry for the Table of Contents
We report that diphenyl sulfide can be an axial ligand for tetraphenylporphyrin cobalt. The axial coordiantion promotes the redox
activity of tetraphenylporphyrin cobalt immobilized onto graphene surface through van der Waals interaction, and faciliates the
generation of a reduced state of tetraphenylporphyrin cobalt to serve as the active site for electrochemical CO reduction.
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This article is protected by copyright. All rights reserved.