Electrochemical Polymerization Provides a Function-Integrated System for Water Oxidation

Article abstract of DOI:10.1002/anie.202015174

Water oxidation is a key reaction in natural and artificial photosynthesis. In nature, the reaction is efficiently catalyzed by a metal-complex-based catalyst surrounded by hole-transporting amino acid residues. However, in artificial systems, there is no example of a water oxidation system that has a catalytic center surrounded by hole transporters. Herein, we present a facile strategy to integrate catalytic centers and hole transporters in one system. Electrochemical polymerization of a metal-complex-based precursor afforded a polymer-based material (Poly-1). Poly-1 exhibited excellent hole-transporting ability and catalyzed water oxidation with high performance. It was also revealed that the catalytic activity was almost completely suppressed in the absence of the hole-transporting moieties. The present study provides a novel strategy for constructing efficient molecule-based systems for water oxidation.

Full text of DOI:10.1002/anie.202015174

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Accepted Article
Title: Electrochemical Polymerization Provides a Function-Integrated
System for Water Oxidation
Authors: Hikaru Iwami, Masaya Okamura, Mio Kondo, and Shigeyuki
Masaoka
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10.1002/anie.202015174
Angewandte Chemie International Edition
RESEARCH ARTICLE
Electrochemical Polymerization Provides a Function-Integrated
System for Water Oxidation
Hikaru Iwami,[a,b,c] Masaya Okamura,[b] Mio Kondo,*[a,d,e] and Shigeyuki Masaoka*[a,e]
[a]
Mr. H. Iwami, Prof. Dr. M. Kondo, Prof. Dr. S. Masaoka
Division of Applied Chemistry, Graduate School of Engineering
Osaka University
2-1 Yamadaoka, Suita Osaka 565-0871, Japan
E-mail: mio@chem.eng.osaka-u.ac.jp, masaoka@chem.eng.osaka-u.ac.jp
Mr. H. Iwami, Dr. M. Okamura
Department of Life and Coordination-Complex Molecular Science
Institute for Molecular Science (IMS)
[b]
[c]
5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan
Mr. H. Iwami
Department of Structural Molecular Sciences
SOKENDAI [The Graduate University for Advanced Studies]
Shonan village, Hayama, Kanagawa 240-0193, Japan
Prof. Dr. M. Kondo
JST, PRESTO, 4-1-8 Honcho, Kawaguchi, 332-0012, Japan
Prof. Dr. M. Kondo, Prof. Dr. S. Masaoka
[d]
[e]
Innovative Catalysis Science Division,
Institute for Open and Transdisciplinary Research Initiatives (OTRI),
Osaka University, Suita, Osaka 565-0871, Japan
Supporting information for this article is given via a link at the end of the document.
Abstract: Water oxidation is a key reaction in natural and artificial
photosynthesis. In nature, the reaction is efficiently catalyzed by a
metal-complex-based catalyst surrounded by hole-transporting amino
acid residues. However, in artificial systems, there is no example of
the water oxidation system which has a catalytic center surrounded
by hole transporters. Here, we present a facile strategy to integrate
catalytic centers and hole transporters in one system. Electrochemical
polymerization of a metal-complex-based precursor afforded the
polymer-based material (Poly-1). Poly-1 exhibits excellent hole-
transporting ability and catalyzes water oxidation with high
performance. It was also revealed that the catalytic activity was almost
suppressed in the absence of the hole-transporting moieties. The
present study provides a novel strategy for constructing efficient
molecule-based systems for water oxidation.
Scheme 1. (Top) Structure of the metal-complex-based precursor (1)
developed in this study. (Bottom) Schematic illustration of the preparation of
function-integrated catalytic system for water oxidation via the electrochemical
polymerization of 1.
Introduction
Demand for a sustainable society and clean energy
production has increased in recent years.1 Four-electron oxidation
of water (2H2O → O2 + 4H+ + 4e−) is an important reaction for
suppling the protons and electrons required to generate reduced
products (i.e., chemical energy) from abundant resources in a
clean manner.2 In nature, the reaction is catalyzed by a metal-
complex-based catalyst, the oxygen-evolving complex (OEC),
embedded in photosystem II.3,4 The OEC is known as a highly
active and robust catalyst that can drive the reaction under mild
condition. In the OEC, a manganese-based multinuclear cluster,
which serves as the catalytic center, is surrounded by several
amino acid residues, and these residues play an important role in
transferring electrons/holes to the catalytic center.3,4 This fact
indicates that the presence of hole transporters close to the metal-
complex based catalytic center is a key to efficiently promoting
water oxidation reactions. Thus far, there have been many reports
on the molecule-based catalytic system for water oxidation.
However, the works mainly focus solely on the construction of
catalytic centers2a-g,5,6 or the immobilization of catalytic centers on
electrode surfaces2h,7. Thus, the literature contains no report on
the development of catalytic systems that have a catalytic center
surrounded by hole transporting units.
Here, we disclose a novel approach for developing an
artificial catalytic system for water oxidation. Inspired by the
natural water oxidation system, we hypothesized that the
integration of catalytic centers and hole transporters into one
material could be a promising strategy. Based on this strategy, a
1
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10.1002/anie.202015174
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RESEARCH ARTICLE
polymer-based catalytic system was constructed by assembling
catalytic center and hole transporter units. The several
spectroscopic measurements clearly demonstrated that these
functional units are successfully incorporated in the assembled
system (Poly-1). It was also revealed that Poly-1 exhibited high
hole transporting ability. Moreover, Poly-1 can promote
electrocatalytic water oxidation with a high faradaic efficiency
(>90%) and a low overpotential (η = 413 mV) in aqueous media.
This performance of Poly-1 is of the highest class compared to
the relevant systems.
of crystallization. In 1, all the acetate ligands were exchanged for
czp− ligands. The average bond distances between the Co ions
and the oxygen atoms in the μ-oxo bridging ligand (Co-Ooxo) and
between the Co ions and the oxygen atoms of czp− ligands (Co-
Ocpz) are 1.855(3)-1.867(2) and 1.947(3)-1.995(3) Å, respectively.
These values are comparable to those reported previously.13
The electrochemical behaviors of 1 were investigated by
cyclic voltammetry (CV) in an organic medium (0.1 M tetra-n-butyl
ammonium hexafluorophosphate (TBAPF6) in CH2Cl2), and the
results are shown in Figure 1. In the first cycle (Figure 1a), the
oxidation peak was observed at approximately 0.278 V (vs.
Fc/Fc+), and the peak was assigned to the oxidation of the cobalt
R
a)
N
center, CoIII3CoIV/CoIII 13. Upon further sweeping of the potential
R
N
R
N
R
N
R
N
4
-e-
dimerize
to the positive region, a large peak attributed to the oxidation of
the carbazole moieties was observed at approximately 0.7 V. In
the reverse scan, two new redox waves were observed at 0.323
and 0.601 V. These redox waves were due to the formation of a
biscarbazole structure.10 In the second and subsequent cycles
(Figure 1b), the intensity of the current attributed to the redox
N
R
b)
R
N
R
N
R
N
-e-
-e-
N
R
N
R
N
R
couple of biscarbazole (E = 0.432 and 0.755 V) was increased,
½
indicating the formation of further biscarbazole moieties upon
electrochemical oxidation. Note that the similar trend was also
observed when an ITO was used as a working electrode (Figure
Scheme 2. (a) Oxidative dimerization of carbazole derivatives to form
biscarbazole derivatives. (b) Redox properties of biscarbazole derivatives.
S2). These results indicate that
1
is electrochemically
polymerized via the dimerization of czp− ligands.
Results and Discussion
To elucidate the polymerization behavior of 1,
electrochemical measurements were also conducted using a
transparent electrode (ITO plate) as the working electrode. In this
experiment, a constant potential (1.3 V vs. Ag/Ag+) was applied to
the ITO electrode in a 0.1 M TBAPF6-CH2Cl2 solution containing
The key to our success is the utilization of a newly designed
metal-complex-based precursor, 1, shown in Scheme 1. 1 is
comprised of two distinct functional units: a tetranuclear cobalt
complex and carbazole moieties (Scheme 1, top). The
tetranuclear cobalt complex has a cubane-shaped structure,
which is also observed in the OEC, and it serves as an artificial
catalyst for water oxidation.8 Carbazole derivatives are known to
dimerize under oxidative conditions (Scheme 2a).9 In addition, the
formed biscarbazole exhibits hole-transfer ability (Scheme 2b),10
which should favor the oxidative catalytic reaction. Therefore,
materials with catalytic centers and hole transporters should be
accessible (Poly-1, Scheme 1, bottom) by the electrochemical
polymerization of 1.
The metal-complex-based precursor, 1, was synthesized by
following Scheme S1 in the Supporting Information (SI). Initially,
carbazole-9-propionic acid (Hczp) was synthesized by modifying
a
previously reported procedure11 (yield: 57%) and was
characterized by 1H NMR spectroscopy. Subsequently, Hczp was
reacted with Co4O4(OAc)4(py)4 (−OAc = acetate anion, py =
pyridine)12 in MeCN at 60 °C for 1 h. The reaction was monitored
by thin-layer chromatography and quenched after the
consumption of Co4O4(OAc)4(py)4. Notably, the ligand substitution
reaction at 90 °C led to the decomposition of the starting materials,
and thus, the reaction was conducted at a moderate temperature
(60 °C). The crude product was further purified by silica-gel
column chromatography, and the desired complex 1 was obtained
in 66% yield. The complex was characterized by 1H NMR
spectroscopy, single-crystal X-ray structural analysis and
elemental analysis.
Single crystals of
1
suitable for X-ray diffraction
Figure 1. Cyclic voltammograms of a 0.19 mM solution of 1. (a) 1st scan and
(b) 2nd to 6th scans. Measurements were performed in a CH2Cl2 solution
containing 0.1 M TBAPF6 under Ar at a scan rate of 100 mV s–1. Working
electrode, glassy carbon; counter electrode, Pt wire; reference electrode,
Ag/Ag+. CV scans were initiated from the open-circuit potential.
measurements were obtained by vapor diffusion of Et2O into a
CH2Cl2 solution of 1 at room temperature. An ORTEP diagram of
1 is shown in Figure S1. 1 crystallized in P , and the asymmetric
unit contains one complex and 2.5 CH2Cl2 molecules as solvents
2
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10.1002/anie.202015174
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RESEARCH ARTICLE
Figure 2. UV-vis absorption spectra of the ITO electrodes electrolyzed at 1.3 V
vs. Ag/Ag+ for 10-90 s using a 0.1 M TBAPF6-CH2Cl2 solution containing 0.19
mM of 1. (Inset) Photographs of the ITO electrodes after the electrolysis.
0.19 mM 1 for 10 – 90 s, and after electrolysis, the electrodes
were analyzed by UV-vis absorption spectroscopy. The UV-vis
absorption spectra of the electrodes are shown in Figures 2 and
S3. A band center at approximately 420 nm and a broad band at
approximately 800 nm were observed in all cases, and the
intensities of the bands gradually increased as the electrolysis
time increased. The band at 800 nm can mainly be attributed to
the intervalence charge transfer (IVCT) transitions of mixed-
valent biscarbazole units or charge-resonance bands in the case
of Robin-Day class III systems.10,14 These results clearly
demonstrate that the carbazole moieties of 1 were dimerized upon
oxidation to afford a polymer with a biscarbazole backbone. To
further verify the structure of the polymer, the infrared spectra of
1 before and after polymerization were measured (Figure S4).
Both spectra showed several common vibration peaks, COO−
asymmetric vibration (1550 cm−1), pyridine ring vibration (760
cm−1) and Co-Ooxo stretching vibration (630 cm−1).13 The data
strongly indicates that the cobalt-cubane structure is preserved in
the polymer. We also analyzed the obtained polymer by scanning
electron microscopy (SEM). The SEM image of the polymer
shows a flat surface structure, indicating the formation of a
homogeneous film on the electrode (Figure S5, top). It should be
noted that the morphology of the film is completely distinct from
that of oxidized Co phosphate inorganic materials (Figure S8, top),
which indicates that the oxidized Co phosphate inorganic
materials are not formed during the polymerization of 1. The
energy dispersive X-ray (EDX) elemental map of the film showed
Figure 3. (a) Nyquist plots of a bare GC (black), Co4O4(PhCOO)4(py)4@GC
(blue) and Poly-1@GC (red). Measurements were performed in 0.2
M
a
uniform distribution of cobalt, which also suggests the
phosphate buffer (pH = 7.06) under an argon atmosphere. (Inset) the equivalent
circuit for fitting data. Applied potential: 1.30 V vs. Ag/AgCl, measurement
range: 0.1 Hz -100 kHz. 20 nmol of Co4O4(PhCOO)4(py)4 was mounted from a
CH2Cl2 solution of Co4O4(PhCOO)4(py)4 by drop-cast method to prepare
Co4O4(PhCOO)4(py)4@GC. (b) Controlled potential electrolysis data of Poly-
1@GC (red line), Co4O4(PhCOO)4(py)4@GC (blue line) and a bare GC (black
line) in phosphate buffer solutions (pH = 7.06) under an argon atmosphere.
Applied potential: 1.30 V vs. Ag/AgCl. (c) Schematics of the comparison of
properties between Poly-1@GC, Co4O4(PhCOO)4(py)4@GC.
homogeneity of the polymer (Figure S5, bottom). In the EDX
spectrum (Figure S6), peaks attributable to fluorine and
phosphorus were also detected, suggesting that PF6− was doped
into the polymer. The control experiments using the oxidized Co
phosphate inorganic materials revealed stronger peak intensities
of Co and P (Figure S8, bottom) than Poly-1 (Figure S6),
supporting that the film has different composition from oxidized
Co phosphate inorganic materials. These observations indicate
that 1 is electrochemically polymerized to uniform polymer, Poly-
1, which has both biscarbazole and cobalt cubane moieties.
To clarify charge transfer ability of Poly-1, electrochemical
impedance spectra (EIS) were measured. EIS were measured for
a bare glassy carbon (GC) electrode, the GC electrode modified
with the cobalt cubane complex without biscarbazole moieties,
Co4O4(PhCOO)4(py)4 (Co4O4(PhCOO)4(py)4@GC, PhCOOH =
benzoic acid, for the details of the sample preparation, see the
Supporting Information (P.S14)), and the GC electrode modified
with Poly-1 (Poly-1@GC). Figure 3a shows the EIS measured at
1.30 V vs. Ag/AgCl of the bias potential. The diameters of
semicircles in Nyquist plots were decreased in the order of a bare
GC, Co4O4(PhCOO)4(py)4@GC and Poly-1@GC. Fitted charge
3
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10.1002/anie.202015174
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RESEARCH ARTICLE
transfer resistance (RCT) values with the equivalent circuit (Figure
3a inset) of a bare GC, Co4O4(PhCOO)4(py)4@GC, and Poly-
1@GC were 4606, 1706 and 679 Ω respectively (Table S2). RCT
of Poly-1@GC was reduced to 15% of bare GC and 40% of
Co4O4(PhCOO)4(py)4@GC. These results clearly demonstrate
that charge transfer ability is largely enhanced due to the
biscarbazole moieties embedded in Poly-1.
afforded a film-based material (Poly-1). The characterization of
Poly-1 was performed by various experimental techniques,
including UV-vis absorption spectroscopy IR spectroscopy, SEM
and EDX, and the integration of two functional units were
confirmed. The properties of Poly-1 was analyzed by EIS and the
high transporting ability was clarified. Moreover, Poly-1 can
catalyze the water oxidation reaction with a high faradaic
efficiency and a low over potential. Notably, the results of control
experiments disclosed that the integration of two functional units
are essential to achieve excellent activity of Poly-1 (Figure 3c).
Our study is a significant example of a molecule-based catalytic
system that mimics the key features of the OEC; the systems with
Finally, the electrocatalytic activity of Poly-1 for water
oxidation was evaluated. To investigate the catalytic activity,
Poly-1 was immobilized on a glassy carbon electrode (Poly-
1@GC, for details of the preparation of the film, see the SI). Figure
S9 shows the cyclic voltammograms of Poly-1@GC in a
phosphate buffer solution (pH = 7.06). A large irreversible current
was clearly observed at approximately 1.0 V (vs. Ag/AgCl), which
confirms the water oxidation ability of Poly-1. The overpotential
(η) was estimated to be 413 mV. To further verify the water
oxidation performance of Poly-1, controlled potential electrolysis
of Poly-1 was then performed. After 1 h of electrolysis of Poly-
1@GC at 1.30 V (vs. Ag/AgCl), 1.55 C of charge was passed, and
3.68 μmol of O2 as the major product was detected by gas
chromatography (Figure 3b and Table S3). The maximum
faradaic efficiency of the reaction based on the 4e− process was
estimated to be 91.5%. The isotope labelling experiments using
18O-labelled water, H218O (97%) were also performed. After the
30 min of the electrolysis at 1.30 V (vs. Ag/AgCl), gaseous phase
was analyzed by GC-MS. As a result, the formation of 18O2 as a
major product was confirmed (Figure S11). Moreover, the ratio
between 18O2 and 18O16O was in good agreement with the
a
catalytic center surrounded by hole transporting units.
Collectively, the present study offers a novel concept for obtaining
efficient catalytic system for water oxidation.
Acknowledgements
This work was supported by KAKENHI (17H06444, 19H00903,
and 20K21209 (S. M.) and 15H05480, 17K19185, 17H05391,
19H04602, and 20H02754 (M. K)) from the Japan Society for the
Promotion of Science. This work was also supported by Research
Grants in the Natural Sciences from the Mitsubishi Foundation
and the Tokuyama Science Foundation.
Conflict of Interest
theoretical values (exp. 18O2
: :
18O16O = 100 :7.8, theor. 18O2
18O16O = 100 : 6.1). These results clearly demonstrate that the
origin of oxygen atoms in evolved dioxygen is water in the
catalysis mediated by Poly-1. To investigate the stability of Poly-
1, a long-term electrolysis was carried out. During 6 h of
electrolysis at 1.30 V (vs. Ag/AgCl), the current passed
continuously (Figure S12), and the formation of dioxygen with
88.6% of Faradaic efficiency was confirmed (Table S4), which
suggests that Poly-1 was stable under the catalytic condition.
These results clearly demonstrate that Poly-1 can promote
electrocatalytic water oxidation. It should be noted that the
catalytic activity is almost suppressed under identical
experimental condition when the electrode modified with the
complex without biscarbazole moieties, Co4O4(PhCOO)4(py)4
@GC, was used (Figure 3b, blue line). The result together with
the results of the EIS measurements clearly demonstrate that the
existence of biscarbazole moieties as hole transporters is
essential to achieve the efficient catalysis. In other words, the
importance of the integration of hole transporting units close to
the catalytic center is successfully confirmed. In addition, the
catalytic performance of Poly-1 was of the highest class among
the reported cobalt-complex-based catalytic system for water
oxidation in terms of overpotentials and faradaic efficiencies (for
the details of the comparisons with other relevant catalysts, see
Table S5).5g, 15-23
The authors declare no competing financial interest.
Keywords: cobalt • electro catalysis • heterogeneous catalysis •
water oxidation • electrochemical polymerization
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10.1002/anie.202015174
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
Four-electron oxidation of water with the function-integrated polymer is reported. Electrochemical polymerization provided a material
with molecule-based active sites and hole-transporting moieties. Electrochemical impedance analysis revealed that hole transporting
polymer reduced charge transfer resistance. The material catalyzed electrochemical water oxidation with a high faradaic efficiency
and a low overpotential.
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This article is protected by copyright. All rights reserved.