Low overpotential water oxidation at neutral pH catalyzed by a copper(ii) porphyrin

Article abstract of DOI:10.1039/C8SC04529A

Low overpotential water oxidation under mild conditions is required for new energy conversion technologies with potential application prospects. Extensive studies on molecular catalysis have been performed to gain fundamental knowledge for the rational designing of cheap, efficient and robust catalysts. We herein report a water-soluble Cu^II complex of tetrakis(4-N-methylpyridyl)porphyrin (1), which catalyzes the oxygen evolution reaction (OER) in neutral aqueous solutions with small overpotentials: the onset potential of the catalytic water oxidation wave measured at current density j = 0.10 mA cm^?2 is 1.13 V versus a normal hydrogen electrode (NHE), which corresponds to an onset overpotential of 310 mV. Constant potential electrolysis of 1 at neutral pH and at 1.30 V versus NHE displayed a substantial and stable current for O₂ evolution with a faradaic efficiency of >93%. More importantly, in addition to the 4e water oxidation to O₂ at neutral pH, 1 can catalyze the 2e water oxidation to H₂O₂ in acidic solutions. The produced H₂O₂ is detected by rotating ring-disk electrode measurements and by the sodium iodide method after bulk electrolysis at pH 3.0. This work presents an efficient and robust Cu-based catalyst for water oxidation in both neutral and acidic solutions. The observation of H₂O₂ during water oxidation catalysis is rare and will provide new insights into the water oxidation mechanism.

Full text of DOI:10.1039/C8SC04529A

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Low overpotential water oxidation at neutral pH
catalyzed by a copper(II) porphyrin†
Cite this: DOI: 10.1039/c8sc04529a
a
a
a
b
a
All publication charges for this article Yanju Liu, Yongzhen Han, Zongyao Zhang, Wei Zhang,
Wenzhen Lai,
have been paid for by the Royal Society
of Chemistry
cd
ab
Yong Wang
and Rui Cao
*
Low overpotential water oxidation under mild conditions is required for new energy conversion
technologies with potential application prospects. Extensive studies on molecular catalysis have been
performed to gain fundamental knowledge for the rational designing of cheap, efficient and robust
II
catalysts. We herein report a water-soluble Cu complex of tetrakis(4-N-methylpyridyl)porphyrin (1),
which catalyzes the oxygen evolution reaction (OER) in neutral aqueous solutions with small
overpotentials: the onset potential of the catalytic water oxidation wave measured at current density j ¼
ꢀ2
0
.10 mA cm is 1.13 V versus a normal hydrogen electrode (NHE), which corresponds to an onset
overpotential of 310 mV. Constant potential electrolysis of 1 at neutral pH and at 1.30 V versus NHE
displayed a substantial and stable current for O evolution with a faradaic efficiency of >93%. More
importantly, in addition to the 4e water oxidation to O
oxidation to H in acidic solutions. The produced H
2
2
at neutral pH, 1 can catalyze the 2e water
O
2 2
2 2
O is detected by rotating ring–disk electrode
Received 11th October 2018
Accepted 7th January 2019
measurements and by the sodium iodide method after bulk electrolysis at pH 3.0. This work presents an
efficient and robust Cu-based catalyst for water oxidation in both neutral and acidic solutions. The
DOI: 10.1039/c8sc04529a
observation of H
2
O
2
during water oxidation catalysis is rare and will provide new insights into the water
rsc.li/chemical-science
oxidation mechanism.
problem by using photosystem II (PSII) to store solar energy as
chemical energy in a scale of sustaining the whole life circle on
Introduction
9–11
The growing threat of anthropogenic climate change and the the earth.
PSII is a membrane protein complex located in the
intense global energy demand on fossil fuels are among the thylakoid membranes and performs a series of light-induced
biggest challenges faced by human beings in the current electron transfer reactions leading to the splitting of water
6,12,13
century, which makes seeking efficient carbon-neutral sources into electrons/protons and molecular dioxygen.
Upon an
of renewable energy the most signicant and urgent task of the initial light-induced charge separation, chlorophyll (P680) with
1
–3
scientic society. The global total primary energy demand will a magnesium porphyrin core is converted to a cation radical
+
exceed 15 000 Mtoe (million tons of oil equivalent) in 2025 and species P680c , which subsequently abstracts an electron from
4 x
reach 18 000 Mtoe in 2040. Among all the renewable energy the Mn CaO cluster of the oxygen-evolving complex (OEC).
sources, sunlight is the ideal energy resource to meet such Aer four such successive steps, water is oxidized at the OEC to
a demand. One of the biggest challenges of sunlight utilization release dioxygen, and the generated electrons/protons are used
4
–8
is the large-scale economy efficient conversion and storage.
for the xation of carbon dioxide. Therefore, water oxidation
Nature, however, presents a fascinating scenario to solve this holds the central place of the natural energy-conversion
scheme.
a
Extensive studies on articial molecular catalysts for homoge-
Department of Chemistry, Renmin University of China, Beijing 100872, China. E-mail:
neous water oxidation have been performed to unveil its mecha-
ruicao@ruc.edu.cn
b
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, nistic mysteries, particularly the pivotal O–O bond formation
1,3,12,14
School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an process.
Such mechanistic information provides funda-
7
10119, China
mental knowledge for the rational design of efficient and robust
c
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
water oxidation catalysts. The natural Mn CaO of the OEC
4
x
encourages scientists to design biomimetic multinuclear metal
clusters with redox exibility suitable for the multi-electron water
oxidation reaction. In this context, several notable metal clusters
d
Institute of Drug Discovery Technology, Ningbo University, Ningbo 315211, China
†
Electronic supplementary information (ESI) available: Fig. S1–S17, Table S1,
crystallographic data in CIF format for 1, and the X-ray crystallographic
1
5,16
coordinates for the structure reported in this article. CCDC 1835967. For ESI have been studied as OER catalysts, including tetramanganese,
and crystallographic data in CIF or other electronic format see DOI:
0.1039/c8sc04529a
17,18
19
20,21
22
dimanganese,
tetracobalt, tetraruthenium,
diruthenium,
1
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23
76–78
and pentanuclear iron complexes. In these examples, intra- transportation.
The property of 1 to catalyze both O
2
molecular O–O bond formation through the coupling of two evolution at neutral pH and H O formation at acidic pH during
2
2
metal–oxo units is usually proposed. On the other hand, since the water oxidation is very rare and will provide new insights into
rst report of mononuclear Ru complexes as active catalysts for the catalyst design and also the mechanism for water oxidation.

24
water oxidation, many mononuclear metal complexes have been
25–52
identied to be active OER catalysts.
Unlike multinuclear
Results
systems, for mononuclear catalysts nucleophilic water attack to
terminal metal–oxo/oxyl units is the likely O–O bond formation Synthesis and characterization
route. Despite these achievements, however, cheap catalysts
functioning at neutral pH with low overpotentials for water
oxidation are still under development.
II
Complex 1 was synthesized by the reaction of Cu salts with
a porphyrin ligand (see Experimental section for details). Crys-
tals of a triate salt were isolated by slow evaporation from
a water/acetonitrile solution at room temperature, and were
used for characterization and electrochemical measurements.
The high-resolution mass spectrum (HRMS) of 1 showed ions at
mass-to-charge ratios of 184.8086 and 518.5703, which matched
the calculated values of 184.8090 for the tetravalent ion [Por-
As an alternative approach, metal porphyrins are promising
to serve as water oxidation catalysts. The cation radical species
+
P680c is the most oxidizing redox cofactor known in biology
6,12
with an estimated redox potential of 1.30 V versus NHE.
It
provides the initial oxidizing power for water oxidation
+
ꢀ
(2H
2
O / O
2
+ 4H + 4e , DE ¼ 1.23 V versus NHE). The
4+
2+
Cu] and of 518.5700 for the divalent ion [Por-Cu-(OTf) ] ,
2
oxidizing equivalent is stored in the porphyrin macrocycle.
Thus, it makes sense to design metal porphyrins to directly
catalyze water oxidation, given that the metal core with the
porphyrin macrocycle can provide one binding site for an aqua
ligand. Bioinspired by this, we and others have reported a series
respectively (Fig. S1†). Importantly, the observed isotopic
distributions matched those expected for these ions with
separations of 0.25 and 0.50 units between each peak for
tetravalent and divalent ions, respectively.
The structure of 1 was determined by single crystal X-ray
diffraction. Crystallographic studies revealed that 1 crystal-
3,49,53,54
of metal-porphyrin OER catalysts.
Herein, we present a signicant improvement on water
ꢀ
lized in the triclinic space group P1 with Z ¼ 3 (crystal data and
II
oxidation catalysis with water-soluble cationic Cu porphyrin 1
structure renement details are summarized in Table S1†). In
the X-ray structure of 1, one molecule is located at the special
position with the central Cu ion situated on the crystallo-
graphically required inversion center. The other two molecules
are located at the general position and are symmetry-related
through the inversion center. As a consequence, the asym-
metric unit contains one and a half molecules. The Cu ion is
coordinated by the porphyrin ligand through four N atoms to
give a distorted square-planar geometry (Fig. 1, right). No axial
ligands were found for the Cu ion in the solid state, although
several co-crystallized solvent water molecules were located in
the crystal lattice. Moreover, no changes were observed in the
UV-vis spectrum of 1 in propylene carbonate (PC, a weak coor-
dinating solvent) with the addition of water (Fig. S2†), further
conrming that water will not bind to the Cu ion of 1. The
(
Fig. 1). Copper plays vital roles in biological and biomimetic O
2
55–59
chemistry to mediate O–O bond formation and cleavage.
a consequence, Cu complexes have attracted increasing atten-
tion as catalysts for water oxidation.
catalyze water oxidation in neutral aqueous solution with
impressively small overpotentials. The onset potential of the
catalytic water oxidation wave measured at the current density j
As
60–75
II
Cu porphyrin 1 can
ꢀ
2
¼
0.10 mA cm is 1.13 V versus NHE, which corresponds to an
onset overpotential h ¼ 310 mV. This value is remarkable in
comparison with that of any other reported molecular OER
catalysts working at neutral pH. More importantly, 1 can also
catalyze the 2e water oxidation to H
selective H generation is signicant because H O
extensively used as an oxidant in industry and is regarded as
an alternative fuel with the benets of easy storage and
2
O
2
in acidic solutions. The
has been
2
O
2
2 2
9
II
˚
average Cu–N bond distance of 1.998(4) A indicates a d Cu
electronic structure. In addition, for each molecule of 1, four
ꢀ
CF SO counter anions were successfully located in the X-ray
3
3
II
structure, further conrming the Cu oxidation state.
Electrocatalytic water oxidation
The cyclic voltammogram (CV) of 1.0 mM 1 in 0.10 M pH 7.0
phosphate buffer obtained using a uorine-doped tin oxide (FTO)
working electrode displayed a pronounced catalytic wave. The
ꢀ
2
onset of this catalytic wave, measured at j ¼ 0.10 mA cm , is at
1.13 V versus NHE (all potentials reported in this work are refer-
enced to NHE unless otherwise noted), which corresponds to an
onset h ¼ 310 mV (Fig. 2a). CVs of 1 in neutral H O and D O
2
2
phosphate buffers showed a kinetic isotope effect (KIE ¼ 1.81,
1
/2
Fig. S3†). The normalized catalytic currents (i /n ) decreased
cat
Fig. 1 (Left) Molecular structure of 1. (Right) Thermal ellipsoid plot of
the X-ray structure of 1 (50% probability). Hydrogen atoms and counter
anions are omitted for clarity.
with increasing scan rates (Fig. 2b), indicating that the current is
associated with a catalytic process. Comparison of 1 with
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ꢀ
1
ꢀ2
2
4
Fig. 2 (a) CVs of 1, CuSO , and buffer-only solution at a 100 mV s scan rate. (b) Normalized CVs of 0.50 mM 1 at different scan rates in mA cm
ꢀ1/2 1/2
V
0
s
. (c) CVs of 1 at different concentrations. (d) Plot of catalytic peak current versus [1]. Conditions: FTO working electrode (S ¼ 0.25 cm ),
ꢁ
.10 M pH 7.0 phosphate buffer, and 20 C.
reported Cu-based molecular catalysts underpins its remarkable
The molecular nature of 1 for water oxidation was veried.
efficiency. As shown in Table 1, Cu complexes usually require No catalytic currents were observed up to 1.45 V in a buffer-only
strong alkaline conditions and large onset h values >400 mV to solution and in a 1.0 mM CuSO4 solution under identical
function as active OER catalysts. Only a few Cu catalysts are able conditions (Fig. 2a). The catalytic currents increased with
to work at neutral pH. However, they always require large onset h increasing concentration of 1 (Fig. 2c), exhibiting a linear
>
600 mV to exhibit evident catalytic activities.
dependence (Fig. 2d). Further analysis of the catalytic water
Table 1 Comparison of catalytic performances and conditions for a variety of Cu-based OER molecular catalysts
a
ꢀ1
Catalyst
Catalyst concentration (mM)
pH
Onset h (mV)
TOF (s
)
Reference
Cu porphyrin 1
1.0
1.0
0.89
0.7
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
7.0
12.5
11
310
540
520
400
330
400
30
100
33
This work
[
[
[
[
[
[
[
[
(bpy)Cu(OH)
(TGG )Cu (OH
2
]
60
62
67
61
64
4ꢀ
II
2ꢀ
2
)]
ꢀ
(Py
3
P)Cu(OH)]
8.0
20
(dhbp)Cu(OH)
2
]
12.4
11.5
11.5
7.0
0.4
3.58
0.16
30
0.4
0.7
20.1
0.6
33
2
ꢀ
ꢀ
(L2)Cu]2
(L4)Cu]
170 (not stable)
2+
Cu(TMC)(H
2
O)]
600
530
460
650
800
720
73
72
66
69
63
68
2+
Cu(en)
2
]
8.0
Cu(pyalk)
2
13.3
10.0
7.0
II
ꢀ
[
[
[
pdca–Cu –CO
Cu (BPMAN)(m-OH)]
(TMPA–Cu )
3
H]
3
+
2
II
2+
2
–(m-OH)
2
]
12.5
a
The onset overpotential values for water oxidation mediated by molecular catalysts are generally reported and are used for comparison in
0
the literature. Abbreviations used in this table: bpy, 2,2 -bipyridine; TGG, triglycylglycine; Py
dicarboxamidate; dhbp, 6,6 -dihydroxy-2,2 -bipyridine; L2 and L4, N
tetramethyl-1,4,8,11-tetraazacyclotetradecane; en, 1,2-ethylenediamine; pyalk, 2-pyridyl-2-propanoate; pdca, N,N -2,6-dimethylphenyl-2,6-
3
P, N,N-bis(2-(2-pyridyl)ethyl)pyridine-2,6-
0
0
0
1
,N -(1,2-phenylene)bis(N
1
2
-methyloxalamide) derivatives; TMC, 1,4,8,11-
0
pyridinedicarboxamidate;
2-pyridylmethyl)amine.
BPMAN,
2,7-[bis(2-pyridylmethyl)-aminomethyl]-1,8-naphthyridine;
TMPA,
bis((6-methyl-2-pyridyl)methyl)-
(
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64,79–83
ꢀ1/2
oxidation wave by using foot of the wave analysis (FOWA,
information, the plot of icat/i
d
versus n
shows a linear rela-
Fig. S4†) showed that the observed reaction rate constant tionship (Fig. S10†), which is consistent with a pure kinetic
remained almost unchanged at different concentrations of 1. behavior in this scan rate range. A turnover frequency (TOF)
ꢀ1
84
These results together indicate the rst-order dependence of the value of 30 s can be calculated using established methods.
catalytic reaction kinetics on 1. In addition, the coordination- This value is remarkable because it is obtained at neutral pH
caused inhibition by either phosphate anions (Fig. S5†) or and is comparable to the values presented by those Cu-based
acetonitrile molecules (Fig. S6†) is strong evidence for Cu- molecular catalysts working in alkaline solutions. Moreover,
centered homogeneous catalysis.
this TOF value with 1 is obtained at much smaller over-
The stability of 1 under catalytic conditions was evaluated. potentials as compared to that of reported Cu-based catalysts
Successive CV scans of 1 showed no current increase (Fig. 3a). (Table 1).
The peak current slightly decreased in the rst several scans and
then remained almost constant. The initial small drop of the
pH-Dependent electrochemical studies
peak current is due to the generation and accumulation of O
2
gas bubbles on the surface of the FTO working electrode. Aer establishing the molecular nature of 1 for catalysis, we
Constant potential electrolysis (CPE) of 1 for 10 h at 1.30 V next carried out pH-dependent studies. Fig. 4a displayed the
displayed a substantial and stable current (Fig. 3b). The FTO linear sweep voltammetry (LSV) curves of 0.50 mM 1 in 0.10 M
working electrodes aer 20 CV scans and aer CPE displayed no pH 2.0–7.0 phosphate buffers obtained using glassy carbon
catalytic current in freshly prepared buffer-only solutions (GC) electrodes. Signicantly, 1 was active even in strong
(Fig. 3c). Surface analysis of the electrode aer CPE by scanning acidic solutions (i.e., pH < 3). This result is additional support
electron microscopy (SEM) and energy dispersive X-ray spec- for the molecular nature of 1 as the real water oxidation
troscopy (EDX) showed no sign of the deposition of any catalyst because any Cu oxides that may possibly be generated
heterogeneous Cu-based materials (Fig. S7†). In addition, the upon the decomposition of 1 are unstable in acidic solutions.
ꢀ
2
solution aer CPE was analyzed by UV-vis spectroscopy, The plot of the potential as measured at j ¼ 0.45 mA cm
showing a negligible change as compared to that before CPE versus pH gives a linear relationship (Fig. 4b). The slope is
(
Fig. 3d). These results strongly supported the molecular nature ꢀ0.029 V per pH unit, indicating that the catalytic cycle
ꢀ +
of 1 and its stability under catalytic OER conditions.
involves an unusual 2e /1H redox step. Notably, in strong
The evolved O during CPE was then determined, revealing acidic solutions at pH 2.0–3.0, a shoulder peak at 1.29 V prior
2
a faradaic efficiency of >93% for O evolution (Fig. S8†). The to the catalytic water oxidation wave can be identied, which is
2
turnover number was calculated to be 5.0 based on all catalyst assigned to the 1e oxidation of 1.
II
I
molecules in the solution. A diffusion-controlled Cu /Cu redox
couple of 1 was found at E1/2 ¼ 0.06 V (Fig. S9†). With this electrochemistry by using the differential pulse voltammetry
DPV) method. The DPV results of 1 displayed two oxidation
events in 0.1 M pH 2.52–4.45 phosphate buffers (Fig. 4c). At pH
On the basis of these results, we studied the pH-dependent
(
2
.52, these two oxidation events are at Ep,a ¼ 1.27 and 1.44 V,
respectively. Signicantly, the rst oxidation is pH-
independent, while the second one is pH-dependent. The
plots of the rst and second oxidation peak potentials versus
pH values are depicted in Fig. 4d, showing that the Ep,a values
of the rst oxidation remain nearly identical but those of the
second oxidation show a monotonic decrease with a slope of
ꢀ
0.028 V per pH unit. This number is the same as that ob-
tained in the abovementioned LSV studies, suggesting that the
ꢀ
+
second oxidation corresponds to the 2e /1H process. With
the further increase of the solution pH, the second oxidation
wave steadily moves towards the rst oxidation wave
(
Fig. S11†), and they eventually become superimposed under
near neutral conditions. Notably, in the literature, over-
potentials at the initial foot of the DPV oxidation wave and at
the half-peak position of the catalytic CV oxidation wave are
also reported. In the case of 1 at pH 7.0, the potential at the
initial foot of the DPV oxidation wave is 1.12 V, while the half-
Fig. 3 (a) 20 successive CV cycles of 0.75 mM 1 with the FTO working
2
ꢀ1
electrode (S ¼ 0.25 cm ) at a 50 mV s scan rate. (b) Catalytic currents peak potential of the catalytic CV oxidation wave is 1.26 V.
in CPE with or without 1 (0.75 mM) using the FTO electrode (S ¼ 1.0
These give h ¼ 300 mV (at the initial foot of DPV) and 440 mV
2
cm ) at an applied potential of 1.30 V. (c) CVs of the FTO electrode after
(
at the half-peak position of the CV wave) for catalytic water
1
1
0 h CPE with 1, the FTO electrode after 20 successive CV cycles with
, and a freshly cleaned FTO electrode in the buffer-only solution.
oxidation, respectively. The overpotential values determined
from the initial foot of both the CV and DPV oxidation waves of
1 are identical in our experiments.
ꢀ1
Conditions: 0.10 M pH 7.0 phosphate buffer, 100 mV s scan rate, and
2
ꢁ
0 C. (d) UV-vis spectra of 1 before and after 10 h CPE.
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ꢀ2
Fig. 4 (a) LSV curves of 0.50 mM 1 in 0.10 M pH 2.0–7.0 phosphate buffers. (b) Plot of potential measured at j ¼ 0.45 mA cm versus pH. (c) DPV
curves of 0.50 mM 1 in 0.10 M pH 2.52–4.45 phosphate buffers. (d) Plots of the first and second oxidation peak potentials versus pH from 2.52 to
ꢀ1
ꢁ
4
.45. Conditions: GC working electrode, scan rate 100 mV s , and 20 C.
It is worth noting that at pH 2.52, the current of the second current increases with the scan rate. Moreover, the ratio of the
oxidation is much smaller than that of the rst oxidation in the reduction peak current versus the oxidation peak current
DPV plot of 1 (Fig. 4c). With increasing pH, the second oxidation increases with the scan rate (Fig. 5). This result further supports
moves towards the rst oxidation, and its current also increases the chemical reaction/transformation of the 1e-oxidized species
signicantly. This trend implies a pH-dependent chemical of 1, leading to its corresponding reduction wave only being
reaction/transformation of the 1e-oxidized species of 1. In order detectable on reversing the scan with large scan rates.
to get more insights into the rst oxidation process, we
Next, we performed electrolysis of 1 at 1.40 V in a pH 3.0
measured the CV of 1 at pH 2.52 by reversing the scan prior to phosphate buffer solution. Thin-layer spectroelectrochemical
the catalytic water oxidation wave (i.e., <1.35 V). As shown analysis showed an absorbance decrease at 425 and 550 nm but
in Fig. 5, almost no reduction peak can be observed at the absorbance increases in the 600–800 nm range (Fig. S12†).
ꢀ
1
a 100 mV s
scan rate. However, a small but signicant These results together suggest that the rst oxidation of 1 is
ꢀ
1
II
reduction peak appears at a 200 mV s scan rate and the ligand-centered to form a Cu porphyrin cation radical species.
Fig. 5 CVs of 1.0 mM 1 in 0.10 M pH 2.52 phosphate buffer by reversing the scan at 1.35 V with different scan rates (Left) and the ratio of the
ꢁ
reduction peak current versus the oxidation peak current ired/ioxi (Right). Conditions: GC working electrode and 20 C.
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In addition, we can oxidize 1 in a PC solution using ceric CPE of the GC disk electrode at 1.40 V and CPE of the Pt ring
ammonium nitrate (CAN) as the chemical oxidant. As shown in electrode at 0.70 V in a pH 3.0 phosphate buffer solution. Stable
Fig. S13,† similar changes were observed in this chemical currents were observed on both electrodes (Fig. S16†), giving
oxidation process: the absorbance decreases at 544 nm but a faradaic efficiency of 45%. This value is much smaller than
increases in the 590–800 nm range, which is characteristic for 100%, and is likely caused by some level of oxidation of H
the ligand-centered oxidation to give porphyrin cation radical the disk electrode upon its formation. This provides strong
species. evidence for the electrocatalytic features of 1 to catalyze 2e water
Interestingly, the electrochemical behaviors of this rst oxidation to generate H in acidic solutions.
oxidation event of 1 are different in aqueous solutions versus Moreover, the CVs of 1 with addition of authentic H
those in organic solutions. In aqueous solutions, the rst remarkably increased catalytic waves (Fig. 6a). As a control, the
2 2
O at
2 2
O
2 2
O show
ꢀ1
oxidation wave is irreversible at a 100 mV s
However, in organic solutions, complex 1 displays a quasi- a negligible current (Fig. S17†). We next studied the DPV of 1
reversible oxidation process even at slow scan rates (i.e., with H in 0.10 M pH 3.6 phosphate buffer (Fig. 6b). Signif-
0 mV s , Fig. S14†). We propose that upon the rst oxidation, icantly, with the addition of H , the rst oxidation wave
scan rate. CV of H O under the same conditions without 1 showed
2 2
2 2
O
ꢀ
1
3
2 2
O
the 1e-oxidized species of 1 can bind an aqua ligand at the axial remained almost unchanged, but the second oxidation wave
position of the Cu ion, which can then initiate the water became much more pronounced, conrming that the second
oxidation process. This chemical reaction/transformation is oxidation was associated with the H O -related species.
2 2
unlikely to happen for the 1e-oxidized species of 1 in the organic
More importantly, the association of the second oxidation in
PC solution.
the DPV of 1 with H O -related species can be further veried by
2
2
using RRDE analysis. Electrochemical studies of 1 were con-
ducted in 0.10 M pH 7.0 or pH 3.0 phosphate buffer solutions
using LSV of the GC disk electrode with a scan rate of 10 mV s
H
2
O
2
-related electrochemical studies
ꢀ1
Signicantly, during the electrolysis of 1 in pH 3.0 buffer solu- and CPE of the Pt ring electrode at 0.70 V. At pH 7.0, no anodic
tion with applied potentials <1.40 V, H
was generated and current of H oxidation could be detected at the Pt ring
was detected by the addition of sodium iodide (Fig. S15†). In electrode (Fig. 6c). However, in the pH 3.0 solution, a large
order to determine the faradaic efficiency of H O generation, current due to H oxidation was observed at the Pt ring
we used a rotating ring (Pt)–disk (GC) electrode (RRDE) with electrode (Fig. 6d). This difference is likely caused by the
2
O
2
2 2
O
O
2 2
2
2
ꢀ
1
Fig. 6 (a) CVs of 0.75 mM 1 with the addition of H
without 1.0 mM H
2
O
2
using the GC electrode at a 100 mV s scan rate. (b) DPV curves of 0.50 mM 1 with or
ꢁ
2
O
2
. Conditions: 0.10 M phosphate buffer and 20 C. (c) RRDE analysis of 0.5 mM 1 in 0.10 M pH 7.0 phosphate buffer. (d) RRDE
analysis of 0.50 mM 1 in 0.10 M pH 3.0 phosphate buffer. The potential of the ring electrode is maintained at 0.70 V. Rotation rate is 1000 rpm,
ꢀ1
and scan rate is 10 mV s
.
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different potentials required for the second oxidation in pH 7.0 Liao, Zhang and co-workers described another dinuclear Cu
II
2
3+
and pH 3.0 solutions. At neutral pH, the second oxidation wave complex [Cu –(m-OH)] , which catalyzed water oxidation at
is superimposed with the rst oxidation wave of 1 (Fig. S11†). As neutral pH with an onset h ¼ 800 mV. Aer 2e oxidation, the
III
a consequence, this H O -related species is instantly oxidized oxygen atom of the terminal Cu –OH unit coupled with the
2
2
III
4+
on the GC disk electrode upon its formation. At pH 3.0, bridging oxo atom of [Cu
2
–(m-O)] to form the O–O bond. For
IV
however, more positive potentials (i.e., >1.40 V) are required for these dinuclear Cu complexes, although high-energy Cu (i.e.,
the oxidation of this H O -related species (Fig. 4c). Therefore, it Cu –oxo) active species were avoided in the proposed mecha-
2 2
IV
can be detected at the Pt ring electrode. Importantly, as shown nisms, dinuclear forms are intrinsically more difficult to be
in Fig. 6d, along with the generation and accumulation of the oxidized than mononuclear forms because of their larger overall
H O -related species in LSV, the current at the Pt ring electrode positive charges. As a consequence, these dinuclear Cu catalysts
2
2
constantly increases. However, at more positive potentials (i.e., still worked at high overpotentials.
1.40 V), the current at the Pt ring electrode starts to decrease. In the present article, we report on a water-soluble cationic
This phenomenon is consistent with the oxidation/degradation mononuclear Cu porphyrin 1 as a highly efficient and stable
of the H -related species at the GC disk electrode at more molecular electrocatalyst for water oxidation. Complex 1 can
positive potentials. The synchronization between the catalyze the 4e water oxidation to generate O in neutral
>
II
2 2
O
2
generation/degradation of the peroxide species at the disk aqueous solution with a small onset h ¼ 310 mV. This value is
electrode and the increase/decrease of the current at the ring remarkable in comparison with that with any other reported
electrode during RRDE measurements is supportive of (1) the molecular OER catalysts working at neutral pH. More impor-
formation of H O2 during water oxidation with 1 in acidic tantly, 1 can also catalyze the 2e water oxidation to produce
2
media and (2) the association of the second oxidation in the H O in acidic solutions. The impressively small overpotentials
2
2
DPV of 1 with H
2
O
2
-related species.
for water oxidation, and the features of both O
2
evolution at
neutral pH and H formation at acidic pH during water
2 2
O
oxidation catalysis, are unique for complex 1 as a water oxida-
tion catalyst.
Discussion
Since the rst report by Mayer and co-workers in 2012, Cu-based
Although the complete catalytic cycle of water oxidation with
molecular OER catalysts have grown rapidly due to the advan- 1 is difficult to be fully understood based on the current
tages of high earth abundance, low cost and rich redox features experimental results, we can observe H generation during
of copper. On the other hand, copper enzymes play signicant water oxidation with 1 in acidic solutions and can also provide
roles in numerous O
activation processes in nature, and strong evidence to support the fact that the second oxidation of
biomimetic Cu complexes have also been extensively investi- 1 is associated with H -related species. This result suggests
O
2 2
2
2 2
O
gated for such O chemistry. These features make Cu complexes that the 1e-oxidized species of 1 triggers O–O bond formation.
2
the most attractive candidates as OER catalysts. For the re- This proposal may explain why complex 1 can catalyze water
ported mononuclear Cu complexes, however, their catalytic oxidation in neutral aqueous solutions with small over-
activity is mostly limited to functioning at high overpotentials potentials. For example, one can imagine that the initiation of
and in alkaline solutions. A water nucleophilic attack (WNA) to O–O bond formation through 1e-oxidized species, instead of
IV
III
IV
III
high-valent Cu –oxo or Cu –oxyl units is commonly suggested involving high-valent Cu –oxo/Cu –oxyl species, should
for the O–O bond formation mechanism. Electrochemical certainly decrease the potentials required for water oxidation.
evidence was provided by Meyer and co-workers to support the More mechanistic studies are in progress with aims to better
II
presence of the nascent Cu –OOH species. Very recently, understand the catalytic cycle, in particular, O–O bond forma-
Maseras, Llobet and co-workers proposed an alternative single tion, of water oxidation with this Cu porphyrin catalyst.
II
electron transfer–water nucleophilic attack (SET–WNA) mech-
IV
anism. In this mechanism, a formal Cu –OH species was
Experimental section
proposed as the active species to react with a hydroxyl ion for
O–O bond formation. Instead of transferring two electrons in
General materials and methods
a direct WNA step, two single electron transfer steps from All reagents were purchased from commercial suppliers and
IV
a hydroxyl ion to Cu –OH take place with the involvement of an used as received unless otherwise noted. Deionized water was
ꢀ
(
HO/OH) moiety. This mechanism avoids the involvement of used in all experiments. Tetrakis(4-N-methylpyridyl)porphyrin
IV
49
high-energy Cu –oxo, but alkaline conditions are still required. was prepared according to modied literature procedures.
Kieber-Emmons and co-workers recently described a dinuclear High-resolution mass spectrometric measurements were made
Cu complex [Cu
II
2+
2 2
–(m-OH)
] , which catalyzed water oxidation on a Br u¨ ker Fourier transform ion cyclotron resonance mass
with an onset h ¼ 720 mV at pH ¼ 12.5. The 2e-oxidized form spectrometer APEX IV. Electronic absorption spectra were
III
2+
[
Cu
atoms to form a “side-on” m-(h :h )-peroxo dinuclear Cu
intermediate. Subsequent redox isomerization converts this using a JSM-6700F eld emission scanning electron microscope
side-on” structure to an “end-on” m-(1,2)-peroxo structure. (FE-SEM). The EDX spectra were collected from three randomly
However, the authors mentioned that WNA to one oxo group of selected areas of each sample. In addition, the materials
2
–(m-O)
2
]
was considered to couple the two bridging oxo recorded on a Cary 50 spectrophotometer. The morphologies of
2
2
II
the FTO surface before and aer electrolysis were examined
“
III
2+
[
Cu
2
–(m-O)
2
]
was also possible for O–O bond formation. were analyzed at several local spots to ensure their chemical
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homogeneity. Images were obtained with an acceleration measured potential. The working electrode was ultrasonically
voltage of 5 kV.
washed with deionized water. Electrolysis was recorded in
.10 M pH 7.0 phosphate buffer containing 0.75 mM 1 at 1.30 V
in a three-compartment cell with a 1.0 cm FTO working elec-
trode, a graphite rod auxiliary electrode, and an Ag/AgCl refer-
0
2
Synthesis of [Por-Cu](OTf) (1)
4
Tetrakis(4-N-methylpyridyl)porphyrin tetratosylate (0.68 g, 0.50
mmol) and CuCl2 (0.47 g, 3.5 mmol) were added into H O
ence electrode. The produced O
Optics FOXY probe (Model NeoFox). The generated H
2
was analyzed using an Ocean
was
2
2 2
O
(
50 mL). The mixture solution was then reuxed for 5 h in the
dark. Aer cooling to room temperature, the reaction solution
was precipitated by adding it to a saturated aqueous NH PF
detected by electrochemical measurements using RRDE anal-
ysis. Electrochemical studies of 1 were conducted in 0.10 M pH
4
6
7
.0 or pH 3.0 phosphate buffer using LSV of the disk electrode
solution under stirring. The precipitate that formed was ltered
off, washed with water, and dried in a vacuum to give red
crystalline solids of [Por-Cu](PF
ꢀ1
with a scan rate of 10 mV s and electrolysis of the ring elec-
trode at 0.70 V.
6 4
) (0.53 g, 0.40 mmol, yield
8
0%). These red solids were added into a mixture of CH CN/
3
ꢀ
ꢀ
H O (1 : 1), and the PF anions were exchanged by CF SO
3
Conflicts of interest
2
6
3
anions on an anion exchange resin by slow stirring at room
temperature for three days. The solution was ltered, and the
The authors declare no competing nancial interests.
3
resin was washed with CH CN. The solvents were then removed
under reduced pressure to give red crystalline solids. Red Acknowledgements
needle-like crystals of 1 were obtained by slow evaporation from
We are grateful for the support from the “Thousand Talents
Program” in China, the Fok Ying-Tong Education Foundation
for Outstanding Young Teachers in University (by the Ministry
of Education of China), the National Natural Science Founda-
tion of China under Grants 21101170, 21573139, 21773146 and
CH
3 2
CN and H O at room temperature with a yield of 85%
(0.45 g, 0.35 mmol). HRMS: calcd for the tetravalent ion
4
+
[
[
Por-Cu] , 184.8090; found, 184.8086; calcd for the divalent ion
Por-Cu-(OTf)2] 518.5700; found, 518.5703 (Fig. S1†). Anal.
)](CF SO : C, 43.13; H, 2.72; N, 8.38.
2
+
calcd for [Cu(C44
H
36
N
8
3
3 4
)
21873052, the Fundamental Research Funds for the Central
Found: C, 43.32; H, 2.81; N, 8.50.
Universities, the 3315-program of the Ningbo government, and
the Research Funds of Renmin University of China.
Crystallographic studies
The complete dataset for 1 (CCDC 1835967†) was collected. A
single crystal suitable for X-ray analysis was coated with
Paratone-N oil, suspended in a small ber loop, and placed on
a Bruker D8 Venture X-ray diffractometer at 150(2) K. Diffraction
intensities were measured using graphite monochromated Mo
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