A Carbon Electrode Functionalized by a Tricopper Cluster Complex: Overcoming Overpotential and Production of Hydrogen Peroxide in the Oxygen Reduction Reaction

Article abstract of DOI:10.1002/anie.201712226

A study of the oxygen reduction reaction (ORR) on a screen printed carbon electrode surface mediated by the tricopper cluster complex Cu₃(7-N-Etppz(CH₂OH)) dispersed on electrochemically reduced carbon black, where 7-N-Etppz(CH₂OH) is the ligand 3,3′-(6-(hydroxymethyl)-1,4-diazepane-1,4-diyl)bis(1-(4-ethyl piperazin-1-yl)propan-2-ol), is described. Onset oxygen reduction potentials of about 0.92 V and about 0.77 V are observed at pH 13 and pH 7 vs. the reversible hydrogen electrode, which are comparable to the best values reported for any synthetic copper complex. Based on half-wave potentials (E_1/2), the corresponding overpotentials are about 0.42 V and about 0.68 V, respectively. Kinetic studies indicate that the trinuclear copper catalyst can accomplish the 4 e^? reduction of O₂ efficiently and the ORR is accompanied by the production of only small amounts of H₂O₂. The involvement of the copper triad in the O₂ activation process is also verified.

Full text of DOI:10.1002/anie.201712226

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Tricopper cluster complex functionalized carbon electrode: A
strategy to overcome the overpotential and the production of
hydrogen peroxide in the oxygen reduction reaction
Authors: Natarajan Thiyagarajan, Damodar Janmanchi, Yi-Fang Tsai,
Wondemagegn Hailemichael Wanna, Ravirala Ramu, Sunney
I Chan, Jyh-Myng Zen, and Steve S.-F. Yu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201712226
Angew. Chem. 10.1002/ange.201712226
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10.1002/anie.201712226
Angewandte Chemie International Edition
COMMUNICATION
Tricopper Cluster Complex Functionalized Carbon Electrode: A
Strategy to Overcome the Overpotential and Production of
Hydrogen Peroxide in the Oxygen Reduction Reaction
Natarajan Thiyagarajan, Damodar Janmanchi, Yi-Fang Tsai, Wondemagegn Hailemichael
Wanna, Ravirala Ramu, Sunney I. Chan,* Jyh-Myng Zen,* and Steve S.-F. Yu*
Abstract: A study of the oxygen reduction reaction (ORR) on a
screen printed carbon electrode surface mediated by
structures and mechanisms, leading to development of many
synthetic copper complexes mimicking the active sites of the
enzymes as part of a strategy to overcome the overpotential
(η).[6]
the tricopper cluster complex Cu3(7-N-Etppz(CH2OH)) dispersed on
electrochemically reduced carbon black, where 7-N-Etppz(CH2OH)
is the ligand 3,3'-(6-(hydroxymethyl)-1,4-diazepane-1,4-diyl)bis(1-(4-
ethyl piperazin-1-yl)propan-2-ol), is described. Onset oxygen
reduction potentials of ~0.92 V and ~0.77 V are observed at pH 13
Mononuclear Cu1+ complexes that largely use phenanthroline as
ligand show that electron withdrawing substituents and bulky
groups placed near the metal center decrease η in the ORR.
Also, proximally placed mononuclear Cu1+ phenanthroline
complexes with polynuclear assembly further reduce η.[7]
Similarly, mono and binuclear complexes with pyridyl-based
ligands have been developed to mimic the T3 center of
enzymes.[8] O2 activation of two mononuclear Cu1+ tris(2-
pyridylmethyl)amine in close juxtaposition shows the lowest
ORR η (at pH 1).2 Among binuclear complexes, Cu2-3,5-
diamino-1,2,4-triazole (Cu2-Hdartz) exhibits an ORR onset
reduction potential of 0.73 V and 0.86 V vs. RHE (at pH 7 and
13, respectively) making it the best performing catalyst of any
synthetic complex at pH 7. [9]
and pH
7 vs. the reversible hydrogen electrode, which are
comparable to the best values reported for any synthetic copper
complex. Based on half wave potentials (E1/2), the corresponding
overpotentials are ~0.42 V and ~0.68 V, respectively. Kinetic studies
indicate that the trinuclear copper catalyst can accomplish the 4-e−
reduction of O2 efficiently and the ORR is accompanied by the
production of only small amounts of H2O2. The involvement of the
copper triad in the O2 activation process is verified by comparing the
performances between electrodes modified by a Cu3 and Cu2
catalyst, underscoring the significance of having three reducing
equivalents readily available for ORR to alleviate the overpotential
as well as production of H2O2.
Aside from the Cu triad in laccase being the active and vital
center for O2 binding and O−O bond cleavage, the spatial
arrangement of the Cu centers and the details of O2 activation
must be essential for ORR. Case in point, in synthetic models,
the T2 Cu mimic of the trinuclear Cu system often does not
participate in the reduction process and the electrochemistry
yields performances similar to the binuclear system.[10] Moreover,
many trinuclear copper systems have been synthesized and
found to be ineffective towards O2 activation.[11] Thus, it is
evident that ligand design is paramount for effective ORR.
The 4-e− reduction of dioxygen (O2) to water (H2O), i.e., the
oxygen reduction reaction (ORR), is biologically and
technologically significant, especially in fuel cell applications.[1]
The cathodic process affects the efficiency of fuel cells. Though
platinum has an onset reduction potential of ~1.0 V (vs. RHE),
given its sluggish ORR kinetics and cost, large-scale usage of
platinum is neither practical nor viable.[2] On the other hand, the
binding and cleavage of O2 in various processes is carried out
by many copper-containing proteins.[3] Multi-copper oxidases,
such as laccase, catalyze the 4-e− reduction of O2 to H2O
efficiently and show excellent electrochemical activity much
closer to the reversible oxygen reduction potential of 1.23 V (vs.
RHE).4 Laccase has 4 copper centers, including a type 1 Cu1+
site for electron transfer, and a Cu triad composed of a type 2
(T2) Cu1+ site and a type 3 (T3) dinuclear Cu1+ center for the
binding and cleavage of O2.[4] Despite superior performance,
enzymes are unstable, and have decreased current density due
to larger molecular sizes.[5] However, systematic studies of O2-
binding copper proteins, have provided key insights into their
CH2OH
N
N
N
N
+
1
+
Cu
1
Cu
(ClO4)
N
(ClO4)
N
O
O
O
O
N
N
+
1
+
1
1+
1+
Cu
Cu
N
Cu
Cu
N
N
N
N
2
N
1
N
N
+
1
Cu
(ClO4)2
(ClO4)
N
H
2O
O
O
HO
OH
N
N
N
N
N
N
N
+
O
1
+
1
Cu
N
Cu
N
N
+2Cu
+2
Cu
N
O
4
[*]
Dr. N. Thiyagarajan, Dr. D. Janmanchi, Dr. Y.-F. Tsai, W. H. Wanna,
Dr. R. Ramu, Prof. Dr. S. I. Chan, Prof. Dr. S. S.-F. Yu
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529
Taiwan, (R.O.C),
3
Scheme 1. Structures of various di- and trinuclear copper complexes used in
this work. Structures 1, 2, 3 are based on studies reported previously12 and 4
is derived from HR-ESI-MS data acquired during this study (SI).
E-mail: sfyu@gate.sinica.edu.tw, sunneychan@yahoo.com
Prof. Dr. J-M. Zen*
Department of Chemistry, National Chung Hsing University
Taichung City 402, Taiwan (R.O.C.)
E-mail: jmzen@dragon.nchu.edu.tw
We recently prepared several rationally designed trinuclear Cu1+
complexes that, upon O2 activation, efficiently oxidize methane
and other light alkanes to their corresponding alcohols at
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201712226
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COMMUNICATION
ambient conditions.[13] These complexes include Cu3(7-N-Etppz)
1 and Cu3(7-Dipy) 3, where 7-N-Etppz and 7-Dipy denote the
ligands 3,3'-(1,4-diazepane-1,4-diyl)bis(1-(4-ethylpiperazin-1-yl)
propan-2-ol) and 3,3′-(1,4-diazepane-1,4-diyl)bis(1-(bis(pyridin-
2-yl)methyl)amino)propan-2-ol), respectively. These tricopper
complexes are biomimics of the active site of the particulate
methane monooxygenase (pMMO) in methanotrophic bacteria.
Although the structural arrangements of the trinuclear Cu
clusters and specifics of the O2 activation in pMMO and in these
pMMO biomimetics differ from that of the T2/T3 Cu triad in
laccases, the O-O bond is cleaved at the 3-e− level of reduction
in both incidences.[6a,14] Accordingly, we can expect the designed
trinuclear copper complexes to behave similar to laccase on the
electrode surface during electrocatalytic reduction of O2. This is
indeed the case. Here we demonstrate the ability of the tricopper
cluster complex Cu3(7-N-Etppz(CH2OH)) 2, where 7-N-
Etppz(CH2OH) stands for the ligand 3,3'-(6-(hydroxymethyl)-1,4-
diazepane-1,4-diyl)bis(1-(4-ethylpiperazin-1-yl)propan-2-ol), to-
wards efficient reduction of O2 at pH 13 and 7 with onset
reduction potentials of ~0.92 V (η = 0.42 at E1/2) and ~0.77 V (η
= 0.68 at E1/2) vs. RHE, respectively, in the ORR. In experiments
comparing the performance of the Cu3(7-Dipy) complex 3 under
different pHs with that of the Cu2(7-Dipy) complex 4, we find η to
be significantly higher in the latter complex (by ca. 70 mV at pH
7), verifying the prominent role of the Cu triad in reducing η in
ORR. Details of the synthetic procedures and characterization of
various catalysts are provided in the Supporting Information (SI).
The Cu triad and dicopper complexes are confirmed by high-
resolution ESI-MS. To study the ORR, 7 μL of the catalyst
dissolved in acetonitrile (concentration ~20 mM) is drop casted
on to an electrochemically reduced carbon black (CB) modified
screen printed carbon electrode (SPCE) and allowed to dry.
The cyclic voltammogram (CV) of SPCE modified by catalyst 2,
in N2 saturated Britton-Robinson (B-R) buffer (pH 7), exhibits a
quasi-reversible peak with a half wave potential (E1/2) of ~0.55 V
(vs. RHE) with additional small shoulder anodic and cathodic
waves at 0.65 V and 0.45 V, respectively (Figure 1a). From the
slope value of the plot of reduction peak current vs. scan rate, a
surface coverage (m) of 1.1 × 10−12 mol/cm2 is estimated for the
active tricopper catalyst (Figure S1b; equation (1) in SI).[15] The
anodic and cathodic slope values in the plot of log (Ip) vs. log
(scan rate) obtained from the scan rate study (Figure S1c),
indicate a surface-confined redox process with our tricopper
catalyst. The heterogeneous electron transfer rate constant (ks)
of 2.4 s−1 is obtained by Laviron’s method (equation (2) in SI for
nΔEp >200 mV/s), indicating reasonably fast ET to the modified
electrode (Figure S1).[16] In this analysis, we assume n = 3 for
the number of electrons transferred to the catalyst per cycle, as
the redox potentials of the three copper ions are similar.
would still be meaningful for comparison of different molecular
catalysts. For the catalyst 2 modified SPCE at pH 7, ηCu/ORR
=
EO2/H2O − ECu(ll/l) = ~0.68 V vs. RHE at pH 7 (EO2/H2O = 1.23 V vs.
RHE). The LSV curves of CB and the catalyst modified SPCE,
and the Tafel plot (with a slope of 97 mV/decade at 300 RPM)
for the catalyst 2 modified SPCE, are given in Figures S2a and
S2b. From these data, we deduce a turn-over frequency (TOF)
of 12 s−1 at E1/2 of the active tricopper sites. (TOF = JA/4Fm,
where J is current density, A is electrode geometric area, F is
Faraday constant, and m is the active catalyst surface coverage
mentioned earlier.[18]
)
To estimate the fraction of H2O2 (p) generated during the ORR,
we use a screen printed ring-disk electrode (SPRDE)-flow
injection analysis (FIA) system following previous work (Figure
S2c).[19] The 2 modified disk and MnO2 ring electrode potentials
are fixed at 0.42 V and 1.2 V vs. RHE, respectively. With N2
saturated B-R buffer (pH 7) as the mobile phase, 20 μL of O2
saturated B-R buffer is injected at regular intervals. Currents
measured at the ring (Ir) and disk (Id) electrodes are 0.23 µA
and 14.05 µA, respectively, corresponding to ~27% H2O2
(equation (6) in SI; Table S1). From Ir/Id, the n value is 3.46
(equation (7) in SI). For comparison, the simple CB/SPCE
generates 88% H2O2 (n = 2.24) at pH 7 (Figure S2d).
Figure 1. a) CV of the Cu3(7-N-Etppz(CH2OH)) CB/SPCE in N2 saturated B-
R buffer, pH 7. Inset: LSV curves obtained with 2/SPCE, scan rate 10 mV/s,
pH 7. b) CV of the Cu3(7-N-Etppz(CH2OH)) CB/SPCE in N2 saturated 0.1 M
NaOH. Inset: LSV curves obtained with CB/SPCE and 2/ SPCE, scan rate 10
mV/s, pH 13.
Given the less corrosive nature and other advantages of an
alkaline medium,[20] we have also evaluated the ORR kinetics at
pH 13. The 2 modified SPCE in N2 saturated 0.1 M NaOH
solution exhibits a quasi-reversible peak with E1/2 of 0.81 V vs.
RHE (Figure 1b). The inset in Figure 1b shows an onset
reduction potential of ~0.92 V vs. RHE, which is more positive
than the value of 0.86 V reported for Cu2-Hdartz at the same
pH.[9] The corrresponding ηCu/ORR and TOF at E1/2 are ~0.42 V
vs. RHE and 12 s−1, respectively. For comparison, the simple
CB/SPCE has an onset reduction potential of 0.85 V vs. RHE at
pH 13.
The ORR activity of the catalyst 2 modified SPCE at pH 7 is
shown in the inset of Figure 1a. The linear sweep voltammetry
(LSV) curve of the modified electrode exhibits an onset
reduction potential of ~0.77 V vs. RHE (Table S1). This value is
more positive than the onset reduction potential reported for any
Cu complex modified electrode at this pH.[9] For a molecular
catalyst that exhibits a redox peak, η at E1/2 is a better indicator
To elucidate the ORR kinetics at pH 13, we obtain LSV curves at
different rotation rates with the 2 modified electrode in O2
saturated 0.1 M NaOH solution using the rotating-disk electrode
of performance as it can be systematically measured from the
[17]
E1/2
.
Although it is more appropriate for a homogenous
system, where the current density is constant at η beyond E1/2, it
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Figure 3. CVs of the Cu3(7-Dipy)- and Cu2(7-Dipy)-CB/SPCEs in the
presence of O2 in B-R buffer, pH 7, at a scan rate of 10 mV/s. *denotes
electrochemically pre-treated electrodes.
Figure 2. a) LSV curves of the Cu3(7-N-Etppz(CH2OH)) modified CB/GCE in
O2 saturated 0.1 M NaOH (pH 13) obtained at different rotation rates, scan
rate 10 mV/s. b) Koutecky-Levich plots of the LSV data. c) Tafel slope at 300
RPM. d) FIA performed with a MnO2/Cu3(7-N-Etppz(CH2OH)) modified ring-
disk electrode in 0.1 M NaOH at flow rate of 0.4 mL/min.
study,10 we electrochemically pretreat the Cu2Ag(Dipy) complex
on a CB/SPCEs in N2 saturated buffer, where upon continuous
cycling (40 cycles) the Ag1+ ions are removed to form the
dicopper system 4 (Figure S5). We compare the 3 and 4
modified SPCEs for their activity towards ORR using B-R buffer
(pH 7). The CVs of catalyst 3 and 4 modified CB/SPCEs under
N2 atmosphere are shown in Figure S6. When these SPCEs are
tested in an O2 saturated buffer, the system with the dicopper
center 4 shows a less positive onset reduction potential by ~70
mV compared to 3. The 3 and 4 modified SPCEs generate 25%
and 58% of H2O2 at pH 7, respectively. The Tafel slope
obtained at 300 rpm for the 4 modified SPCE is 165 mV/decade,
which is higher than that of the catalyst 3 modified SPCE (117
(Figure 2a). The limiting current density increases with the
rotation speed due to the higher O2 flux available to the
electrode surface. Using the Koutecky-Levich (K-L) equations
(3)−(5) in SI, the kinetics of the catalyst 2 modified electrode is
explored.[21] From the plot of J−1 vs. ω-1/2 (Figure 2b), the n value
of 3.3 at 0.6 V vs. RHE is obtained, which is close to the
theoretical n value of 4 for O2 reduction to H2O. Based on
equation (5) in SI, the k value for the catalyst 2 modified carbon
black/glassy carbon electrode (CB/GCE) at 0.6 V is 7.94 x
10−3 cm s−1. The Tafel slope of 77 mV/decade obtained from the
LSV curve at 300 RPM (Figure 2c) indicates electron transfer as
the rate determining step.2
The H2O2 (p) generated during the ORR at pH 13 is 21% (Table
S1) from the SPRDE-FIA experiment (Figure 2d). The 2
modified disk and MnO2 modified ring electrode potentials are
fixed at 0.65 V and 1.1 V vs. RHE respectively. With N2
saturated 0.1 M NaOH as the mobile phase, 20 μL of O2
saturated 0.1 M NaOH is injected at regular intervals (pH 13).
Currents measured are 0.47 µA and 38.72 µA at Ir and Id,
respectively. The n value is 3.58, which is in close agreement
with the result obtained by the K-L method and consistent with
4-e− reduction pathway of O2.
For comparison, the K-L plot obtained for the simple CB/GCE
exhibits a 2-e− reduction pathway with n value of 2.56. SPRDE-
FIA study also indicates substantial production of H2O2 (71%)
with n value of 2.58 (Figure S3a). The catalyst 1 modified
CB/SPCE produced 12% H2O2 with n value of 3.76 but lower ks
= 1.6 s-1 than 2 (Table S1). Stability of the 2 modified electrode
tested under constant bubbling of O2 at 0.67 V for 24 hr, retains
~80% of the initial current (Figure S4).
Finally, the involvement of all three copper sites in the O2
reduction process is validated by comparing the performance of
electrodes modified by the Cu3-complex with that of the Cu2-
complex. For this study, we select Dipy as the ligand to
assemble the complexes, since the Cu2+ ions have higher affinity
toward dipicolylamine than Ag1+,[22] a property that we exploit to
replace the Cu2+ in the apical position of the tricopper triad with
Ag1+ to prepare the Cu2Ag(Dipy) complex. Following a previous
mV/decade) at pH
7
(Figure S7). These observations
underscore the key role played by the Cu center at the apical
position in the Cu triad in reducing η. Transfer of the third
electron (possibly proton-coupled)[23] to the tricopper cluster
complex to cleave the O−O bond, is the rate limiting step in ORR
kinetics, and presumably the binding of O2 to the complex does
not limit turnover of the catalyst. As control, electrochemically
pretreating catalyst 3 shows no significant change in the onset
potential of this electrode (Figure 3), indicating that the
electrochemical treatment does not alter the complex notably.
However, the pretreatment can lead to some disproportionation
of the complex without migration of the copper ions. The catalyst
3 modified SPCE has an onset reduction potential of ~0.71 V vs.
RHE at pH 7 (Table S1). Experiments performed under pH 5
and 10 exhibit similar behavior albeit slight differences in the
onset reduction potentials between the catalysts 3 and 4
modified SPCE, with the catalyst 3 always exhibiting a more
positive onset reduction potential (Figure S8).
To summarize, the involvement of a trinuclear copper cluster in
O2 activation substantially reduces the overpotential and
production of H2O2. The results reveal a 4-e− reduction of O2
with the transfer of the third electron to the tricopper center as
the rate-limiting step. The complex exhibits one of the lowest
onset reduction potentials for any synthetic copper complex at
pH 7. Improved oxygen reduction kinetics and further reduction
of the overpotential are expected with an optimized ligand
system.
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Acknowledgements
This work was supported by Academia Sinica including the
Research Program on Nanoscience and Nanotechnology (2393-
105-0200), grants from the “Taiwan’s Deep Decarbonization
Pathways towards a Sustainable Society” (106-0210-02-11-05)
and Ministry of Science and Technology, R.O.C. (MOST 101-
2113-M-001-007-MY3 and 104-2113-M-001-013-MY3 to S.S.-
F.Y.; MOST 103-2113-M-005-002-MY3 to J.-M.Z.). The authors
would also like to thank Dr. Arun Prakash Periasamy for useful
discussions.
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Keywords: alkaline oxygen reduction • copper triad •
overpotential • screen printed carbon electrode • tricopper
cluster complex
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10.1002/anie.201712226
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Involvement of a synthetic copper
triad in binding and reduction of O2 is
demonstrated. The presence of the
third Cu ion is crucial to supplying the
third reducing equivalent for O−O
bond cleavage and reduces the
overpotential.
Natarajan Thiyagarajan, Damodar
Janmanchi, Yi-Fang
Tsai, Wondemagegn Hailemichael
Wanna, Ravirala Ramu, Sunney I. Chan,
Jyh-Myng Zen and Steve S-F Yu
Page No. – Page No.
Tricopper Cluster Complex
Functionalized Carbon Electrode: A
Strategy to Overcome the
Overpotential and Production of
Hydrogen Peroxide in the Oxygen
Reduction Reaction
This article is protected by copyright. All rights reserved.