Comparison of two water oxidation electrocatalysts by copper or zinc supermolecule complexes based on porphyrin ligand

Article abstract of DOI:10.1039/C8RA08338J

Based on the 5,10,15,20-tetra(cyanophenyl)porphyrin (CNTCPP) ligand, two supermolecule complexes formulated as [Cu(CNTCPP)]·2DMF (Cu-CNTCPP) and [Zn(CNTCPP)] (Zn-CNTCPP) have been synthesized and structurally characterized by single-crystal X-ray diffract

Full text of DOI:10.1039/C8RA08338J

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Comparison of two water oxidation
electrocatalysts by copper or zinc supermolecule
Cite this: RSC Adv., 2018, 8, 40054
complexes based on porphyrin ligand†
c
Zhaodi Huang,a Meixi Zhang,a Huan Lin,a Shuo Ding,a Bin Dong,a Di Liu,
b
a
a
*
*
Hong Wang, Fangna Dai
and Daofeng Sun
Based on the 5,10,15,20-tetra(cyanophenyl)porphyrin (CNTCPP) ligand, two supermolecule complexes
formulated as [Cu(CNTCPP)]$2DMF (Cu-CNTCPP) and [Zn(CNTCPP)] (Zn-CNTCPP) have been
synthesized and structurally characterized by single-crystal X-ray diffraction. Cu-CNTCPP features
Received 9th October 2018
Accepted 21st November 2018
a
0 dimensional (0D) supramoleculer structure, whereas Zn-CNTCPP is a 2D structure. With
coordination unsaturated points, Cu-CNTCPP exhibits electrocatalytic activity toward oxygen evolution
reaction (OER) with low potential in 1.0 M KOH, generating a current density of 10 mA cmꢀ2 at an
potential of 1.66 V vs. RHE, which means it an efficient electrocatalytic material for OER.
DOI: 10.1039/c8ra08338j
rsc.li/rsc-advances
perovskites, chalcogenides and so on,11–15 but there are scarce
reports of supermolecule complexes used as direct electro-
catalyst for OER.16
Introduction
With increasing energy consumption and the greenhouse
effect due to carbon dioxide emission, seeking renewable and
non-carbon energy alternatives is an urgent and important
issue.1,2 The water splitting reaction provides a convenient
chemical method for renewable energy storage.3,4 As the
oxidative half reaction of water splitting, the oxygen evolu-
tion reaction (OER) is kinetically sluggish and leads to
signicant overpotential and energy loss.5 Developing highly
efficient electrocatalysts for OER has emerged as one of the
most research hot spots in clean energy generation and
storage.6,7 The most reported precious-metal electrocatalysts
such as IrO2 and RuO2 are the state-of-art OER catalysts, but
they are limited by low abundance and high cost for large-
scale application.8,9 Therefore, there is tremendous interest
in developing inexpensive and earth-abundant OER cata-
lysts.10 Keeping the advantages of low cost, high chemical
stability, and electrocatalytic ability, the most reported
catalysts for OER are 3d transition metal (e.g. Fe, Cu, Co, Ni
and Mn) derived oxides or hydroxides, phosphates,
On the other hand, supermolecule complexes are a class of
interesting materials which have been intensively investigated
in diverse elds, such as luminescence, gas storage, sensing,
magnetics, catalysis, and so on.17–21 The supermolecule
complex structures and properties are mainly dependent on
functional ligands and metal nodes.22–24 As we know, porphy-
rins are valuable building ligands in supermolecule complex
systems; their extended square planar ligand favors the
formation of ordered frameworks.25–27 Besides that, porphy-
rins can be metalated in their center, and could function as
catalysis active centers,28–30 so porphyrins or metallporphyrin
molecules have been used as active constituents for con-
structing a variety of supermolecule complexes.31–33 For
example, reports on lithium-ion battery electrodes have been
recorded based on iron-porphyrin materials.34–36 Co-based
porphyrin and graphitic materials was predicted to be the
OER catalyst by density functional theory (DFT) calculation.37
Analysis research of porphyrines as catalysts for electro-
chemical reduction of O2 and oxidation of H2O have also been
reported.38 However, only a few supermolecule complexes
show sufficient electrocatalytic OER activity at the high electric
potential experienced during electrolysis (1.6–2.0 V vs. RHE).
Inspired by the above reports, we choose the 5,10,15,20-
tetra(cyanophenyl)porphyrin (CNTCPP) ligand as organic
linker for building supermolecule complexes based on the
following reasons: (i) the ligand possesses delocalized p-elec-
tron system, which can provide an intense electrical conduc-
tivity;39 (ii) four cyanide bridged coordination groups and the
rigid phenyl in the ligand can afford more coordination
opportunities, which will leading to diverse structures;40 (iii)
aSchool of Materials Science and Engineering, College of Science, China University of
Petroleum (East China), Qingdao, Shandong, 266580, People's Republic of China
bSchool of Materials Science and Engineering, Beijing University of Chemical
Technology, Beijing, 100029, People's Republic of China
cSchool of Chemical and Environmental Engineering, Shandong University of Science
and Technology, Qingdao, Shandong, 266590, People's Republic of China. E-mail:
fndai@upc.edu.cn; wanghong@mail.buct.edu.cn
† Electronic supplementary information (ESI) available: Synthesis, PXRD, TGA
curves, crystal data and structure renement for complexes, method for
electrochemical measurements. CCDC 1450485 and 1449406. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c8ra08338j
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the electron withdrawing group of cyanide and high symmetry
framework are favor for electron transferring in the ordered
supermolecule structures; (iv) the central at surface structure
would help to increase the active surface area, which can
ensure efficient contact between the electrolyte and active
catalysts.41,42 Herein, we demonstrate two supermolecule
Results and discussion
Crystal Structure
Complex of Cu-CNTCPP crystallizes in the monoclinic space
group P2/c, and the asymmetric unit contains two Cu2+ (Cu1 and
Cu2) ions and two CNTCPP ligands. As displayed in Fig. 1a, Cu1
is surrounded by four N atoms from one CNTCPP ligands'
central porphyrin ring with the average Cu–N bond length of
complexes
[Cu(CNTCPP)]$2DMF
(Cu-CNTCPP)
and
[Zn(CNTCPP)] (Zn-CNTCPP) by assembling the designed
CNTCPP ligand with cupper (II) or zinc(II) ions, respectively.
Cu-CNTCPP are zero-dimensional (0D) complex with cupper
(II) metalated in the porphyrin center, Zn-CNTCPP is a 2D
structure. Although Zn-CNTCPP has been reported by Jiang
and coworkers, only its structure was focused upon in their
work.43 The OER properties of Cu-CNTCPP and Zn-CNTCPP
have been investigated. Particularly, due to the different
coordination situations, the Cu-CNTCPP materials afforded
better performance than Zn-CNTCPP in 1.0 M KOH, indicating
it a promising new class of non-noble catalyst for OER.
˚
2.006 A. Cu2 adopt similar coordination mode with the average
˚
Cu–N bond length of 2.003 A. The non-coordinated sites of Cu1
and Cu2 in the axial direction mean the possible presence of
large number of bridging and terminal hydroxyl or water
ligands, which could provide high concentration of potential
active sites. The angles between the side benzene rings and the
central Cu1-porphyrin ring are 85.828ꢁ, 69.302ꢁ, 73.563ꢁ, and
68.479ꢁ, respectively, the angles between the side benzene rings
and the central Cu2-porphyrin ring are 84.213ꢁ, 67.916ꢁ,
67.704ꢁ, and 68.709ꢁ, respectively (Fig. 1b). The CNTCPP
porphyrin ring are metallic by Cu2+ ion, and all the four cyanide
groups in the CNTCPP ligands are non-coordinated. The
distance between Cu1 and Cu2 in the two-porphyrin ring is
Materials and general methods
˚
˚
˚
8.599 A. There are weak C–H/N (3.695 A and 3.743 A) inter-
actions between the 0D Cu-porphyrin molecules to connect it
into 3D structure (Fig. 1c and d). There is microspore view along
c axis and the uncoordinated DMF molecules blocked the pores.
X-ray single-crystal diffraction reveals complex of Zn-
CNTCPP crystallizes in the monoclinic space group P21/n, and
the asymmetric unit contains one Zn2+ ion and one CNTCPP
ligand. As displayed in Fig. 2a, Zn2+ is surrounded by four N
atoms from one CNTCPP ligands' central porphyrin ring with
All of the chemicals were obtained from commercial sources
and were used without further purication, except CNTCPP
ligand was synthesized through the condensation of 4-for-
mylbenzonitrile with pyrrole, catalyzed by BF3$OEt2, followed by
oxidation with p-chloranil (Fig. S1†).44 C, H, N of elemental
analysis were measured on an EA 1110 elemental analyzer. The
powder diffraction data were collected on a Cu-Ka radiation
using an X-Pert PRO MPD diffractometer. Thermogravimetric
analyses were performed in the temperature ranged from 40 to
800 ꢁC with 10 ꢁC minꢀ1 rate of heat using a Mettler Toledo
Thermogravimetric analysis (TGA) instrument. The purity of
Cu-CNTCPP and Zn-CNTCPP were conrmed by comparison of
their simulated and experimental powder X-ray diffraction
(Fig. S2 and S3†). TGA measurement shows that Cu-CNTCPP
and Zn-CNTCPP lose uncoordinated solvents below 200 ꢁC and
˚
the average Zn–N bond length of 2.046 A and there are weak
interactions between the two cyanide groups from two different
CNTCPP ligands with the Zn2+ ions (Zn–N bond length of 2.645
˚
A). The CNTCPP porphyrin ring central are metallic by zinc ion
to form 0D molecule, and the weak Zn–N bond between the two
of cyanide groups in the CNTCPP ligands connect another two
zinc(II) ions from the metallic porphyrin ring to form 2D
ꢁ
can be stable up to 580 and 560 C, respectively. (Fig. S4 and
S5†). Method for electrochemical measurements are displayed
in ESI.†
The X-ray datum of complexes were obtained on an Agilent
Super nova with Cu-Ka radiation (l ¼ 1.54178) and MoKa (l ¼
0.71073) at room temperature, respectively. The absorption
corrections were decided by employing the SADABS program.
The structures and hydrogen atoms were rened to utilize the
SHELX-97 program 33–34 through the full-matrix least-squares
by tting on F2 and anisotropic thermal parameters, respec-
tively. There are many disordered solvent molecules existed in
the cavity of Cu-CNTCPP, which can not be achieved through
the reasonable modeling. Hence, the diffuse electron was
removed by the PLATON/SQUEEZE routine 35. The summary of
the structure data is displayed in Tables SI1 and SI2.† Crystal-
lographic data have been deposited at the Cambridge Crystal-
lographic Data Center (CCDC: 1450485 for Cu-CNTCPP,
1449406 for Zn-CNTCPP), which can be obtained from the
Cambridge Crystallographic Data Center, 12, Union Road,
Cambridge CB21EZ, U.K.
Fig. 1 (a and b) The angles between the side benzene rings and the
central Cu1 and Cu2-porphyrin ring. (c and d) 3D framework viewed
from c axis.
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Fig. 3 Electrocatalytic activity of the Cu-CNTCPP for OER. (a) Polar-
ization curves. (b) Tafel plots in 1.0 M KOH solution (5 mV sꢀ1). (c)
Nyquist plots examined at 0.5 V (vs. SCE). (d) Comparison of coordi-
nated and non-coordinated CNTCPP.
Fig.
2 (a) Coordination environment of the ligand and Zn(II) in
complex. (b) Angles between the side benzene rings and the central
Zn-porphyrin ring. (c) 2D layer viewed from b axis. (d) 3D space filling
representation viewed from a axis.
cmꢀ2 in 1.0 M KOH. The OER kinetics of these electrodes is
estimated by their corresponding Tafel plots. As shown ꢀi1n
supramolecular structure. The angles between the side benzene
rings and the central Zn-porphyrin ring are 86.849ꢁ, 64.595ꢁ,
86.849ꢁ, and 64.595ꢁ, respectively (Fig. 2b). Each ligand
connects three Zn(II) ions, and each Zn(II) attaches three
different CNTCPP ligands, generating a 2D network (Fig. 2c).
Fig. 3b, the Tafel slope of the Cu-CNTCPP is 83.9 mV dec
,
compared to 90.1 mV decꢀ1, 165.4 mV decꢀ1, 120.9 mV decꢀ1
and 267.4 mV decꢀ1 for CNTCPP, mixture of CNTCPP and Cu2+
,
amorphous Cu-CNTCPP and Cu(OH)2, respectively.
˚
There are weak C1–H/N8 (3.259 A) supramolecular hydrogen
To get further insight into the activity of as-synthesized Cu-
CNTCPP modied electrodes toward OER, electrochemical
impedance spectroscopy (EIS) analysis was also performed.
Fig. 3c describe the obtained Nyquist plots of Cu-CNTCPP and
the comparative catalysts, the charge-transfer resistance (Rct) at
the surface of the catalysts is determined from the diameter of
a semicircle at high frequencies in the Nyquist plot. Generally,
Rct value varies inversely with the electrocatalytic activity. That
is, smaller diameter corresponds to faster OER kinetics. The Rct
values of Cu-CNTCPP (60 U) are much lower than the other
catalysts. Thus, such a low Rct value of Cu-CNTCPP indicates
that its high electrocatalytic activity for OER could be ascribed
to the highly conductive Cu2+ hybrid CNTCPP ligands
improving the charge transfer characteristics of Cu-CNTCPP. All
the results show that the proprieties of Cu-CNTCPP are better
than the comparative catalysts. As we know the electrocatalytic
reaction is primarily a surface phenomenon, the axial positions
of coordinated Cu2+ ions are naked, so it could offer sufficient
active sites for OER. In the long-range ordered crystals of Cu-
CNTCPP, CNTCPP is supposed to promote electron transport at
heterogeneous surface due to its excellent electrical conductive
property, which act as an ideal platform for loading Cu2+, so it
showed higher catalytic activity than pure CNTCPP ligands,
mixture of CNTCPP and Cu2+ and amorphous Cu-CNTCPP.
From the crystal structure of Zn-CNTCPP, there are weak
interactions between the two cyanide groups from two different
CNTCPP ligands with the Zn2+ ions (Zn–N bond length of 2.645
bonds between the 2D layers to connect it into 3D structures.
View along b axis, there is micro-pores with dimension of 4.5 ꢂ
8.2 nm (Fig. 2d). The crystallographic data of the two complexes
are summarized in Tables S1 and S2.†
Electrocatalytic activity
Many researchs show promising electrocatalytic properties for
a myriad of electrochemical reactions such as OER. Different
studies reported various optimal compositions of copper, to
achieve maximum OER performance. But slightly literatures
reported the positive result got by supramolecular with copper
structures. As the Cu2+ ions are unsaturated coordinated in Cu-
CNTCPP, there are enriched active sites, we therefore investi-
gate the electrocatalytic activity of the Cu-CNTCPP. It is neces-
sary to understand the origination of the OER activity of the as-
prepared catalyst, for comparison, we have also measured the
electrocatalytic OER activity of the comparative catalysts under
the same conditions except Cu-CNTCPP: CNTCPP ligand,
mixture of CuCl2 and CNTCPP ligand by manual grinding
(named mixture of CNTCPP and Cu2+), mixture of CuCl2 and
CNTCPP ligand by reux in hot DMF solution (named amor-
phous Cu-CNTCPP), and Cu(OH)2 by conventional precipitin
reaction (named Cu(OH)2). Fig. 3a shows the polarization curves
of Cu-CNTCPP and the comparative catalysts. The potential
required for a current density of 10 mA cmꢀ2, which is a metric
associated with solar fuel synthesis.45 As shown, the Cu-
CNTCPP delivers a current density of 10 mA cmꢀ2 at a potential
of 1.66 V, whereas a potential as high as 1.74 V, 1.84 V and 1.78 V
are required for CNTCPP ligand, mixture of CNTCPP and Cu2+
and amorphous Cu-CNTCPP to achieve the same current
density, respectively. And for Cu(OH)2, it fails to reach 10 mA
˚
A), so it will partially block the catalytic active sites of Zn ions,
and the OER performance of it also veried our speculation.
Similar with Cu-CNTCPP, we have also measured the electro-
catalytic OER activity of Zn-CNTCPP and the comparative cata-
lysts under the same conditions for comparison: ZnCl2 and
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Fig.
5
Cyclic voltammograms (CV) of (a) Cu-CNTCPP. (b) Zn-
Fig. 4 Electrocatalytic activity of the Zn-CNTCPP for OER. (a) Polar-
ization curves. (b) Tafel plots in 1.0 M KOH solution (5 mV sꢀ1). (c)
Comparison of coordinated and non-coordinated CNTCPP.
CNTCPP. (c) CNTCPP. (d) Estimated Cdl.
monitored over 10 h. Moreover, taking further insight from the
i–t curves obtained at 10 mA cmꢀ2, it can be seen that the Cu-
CNTCPP keeps approximately 83% compared with the Zn-
CNTCPP of 54%. Notably, Cu-CNTCPP exhibits a slight variation
over the entire time, indicating the outstanding stability.
(Fig. 6b and d). In a word, Cu-CNTCPP with lower potential,
Tafel slope and higher estimated Cdl, compared to others as
depicted in Table 1, has the potential to be developed to be
superior catalytic oxygen production from electrolyzed water.
In a word, the OER performance of Zn-CNTCPP with coor-
dination saturation is inferior to the Cu-CNTCPP with coordi-
nation unsaturation. The main reason for the difference
between Cu-CNTCPP and Zn-CNTCPP in OER performance is
due to the presence of weak Zn–N bonds of Zn-CNTCPP, which
will partially block the catalytic active sites of Zn ions. Based on
the structures and its outstanding OER performance of Cu-
CNTCPP ligand mixture by manual grinding (named mixture of
CNTCPP and Zn2+), ZnCl2 and CNTCPP ligand mixture by reux
in hot DMF solution (named amorphous Zn-CNTCPP), and
Zn(OH)2 by conventional precipitin reaction (named Zn(OH)2).
Fig. 4a shows the polarization curves of Zn-CNTCPP and the
comparative catalysts. As shown, the Zn-CNTCPP deliver
a current density of 10 mA cmꢀ2 at a potential of 1.71 V, whereas
a potential as high as 1.74 V are required for CNTCPP ligand to
achieve the same current density. And for mixture of CNTCPP
and Zn2+, amorphous Zn-CNTCPP and Zn(OH)2, it fails to reach
10 mA cmꢀ2 in 1.0 M KOH., the Tafel slope of the Zn-CNTCPP is
87.5 mV decꢀ1, compared to 272.1 mV decꢀ1 and 348.2 mV
decꢀ1 for mixture of CNTCPP and Zn2+, amorphous Zn-
CNTCPP, respectively (Fig. 4b). The Rct values of Zn-CNTCPP are
140 U (Fig. S6†). All the results show the OER properties of Zn-
CNTCPP are inferior to Cu-CNTCPP, that coincides with our
speculation, that is the weak interactions of Zn–N bond of
central Zn(II) ions partially block the catalytic active sites of Zn
ions, but the axial positions of coordinated Cu2+ ions are naked,
so it offer sufficient active sites for OER and afford more
convenient electrocatalytic reaction.
The electrochemical double-layer capacitance (Cdl) was
investigated under non-faradaic condition to estimate the
electrochemical surface area (ECSA). The cyclic voltammograms
at various scan rates of the Cu-CNTCPP, Zn-CNTCPP and
CNTCPP were shown in Fig. 5a–c, according to which the Cdl
was calculated. As shown in Fig. 5d, the Cdl of the Cu-CNTCPP,
Zn-CNTCPP and CNTCPP are 0.531, 0.254 and 0.104 mF cmꢀ2
,
suggesting that the Cu-CNTCPP may have an advantage in
active surface area and active sites over others for oxygen
production. Catalytic activity and stability towards OER are two
valid parameters for oxygen production. In order to investigate
the stability in alkaline solution of the prepared Cu-CNTCPP
and Zn-CNTCPP, continuous CV was performed within the
potential range of 1.45 to 1.7 V (vs. RHE) for 2000 cycles at scan
rate of 50 mV sꢀ1. As shown in Fig. 6a and c, clearly, Cu-CNTCPP
exhibits slight loss compared with Zn-CNTCPP. Furthermore,
the continuous electrolysis at different current density were
Fig. 6 (a) Polarization curves of Cu-CNTCPP, for initially and after
2000 potential sweeps. (b) Long-term stability test carried out at
constant current densities of 10, 20, 30 mA cmꢀ2 for Cu-CNTCPP
(inset: time dependent current density curves at current densities of 10
mA cmꢀ2). (c) Polarization curves of Zn-CNTCPP, for initially and after
2000 potential sweeps. (d) Time dependent current density curves for
Zn-CNTCPP.
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Table 1 Comparison for different non-precious metal catalysts in
1.0 M KOH for OER
Conflicts of interest
There are no conicts to declare.
h10
Electrolytes (mV) [mV decꢀ1
Tafel slope
Electrocatalyst
]
References
Acknowledgements
Cu-CNTCPP
Zn-CNTCPP
CNTCPP
Mn3O4/CoSe2
Zn-Co-LDH
Fe doped Co3O4
1 M KOH
1 M KOH
1 M KOH
1 M KOH
0.1 M KOH
1 M KOH
430
480
510
450
460 101
370
400
83.9
87.5
90.1
49
This work
This work
This work
1 (ref. 46)
2 (ref. 47)
3 (ref. 48)
4 (ref. 49)
This work was supported by the NSF of China (Grant No.
21771191), Taishan Scholar Foundation (ts201511019), the
Shandong Natural Science Fund (ZR2017QB012), the Funda-
mental Research Funds for the Central Universities
(14CX02213A, 16CX05015A), the Foundation of State Key
Laboratory of Structural Chemistry (20160006), and the Applied
Basic Research Projects of Qingdao (16-5-1-95-jch).
60
54.5
Fe3O4@Co9S8/rGO-2 1 M KOH
CNTCPP, we supposed its reaction mechanism in the OER
process, as the axial direction of Cu2+ ions are unsaturated-
coordinated, which provides mainly catalytically active sites
used for fast dissipation of the electrons generated during OER
(Scheme 1): a Cu atom with an oxidation state of 2+ serves as the
center, it could be connected by solvent –OH and H2O ligands in
1.0 M KOH solution. As H2O adsorption on the surface is
favorable thermodynamically, the OER directly splits H2O on
a Cu2+ site to produce an adsorbed –OH. –OH then loses
a proton to form an O atom, so a Cu–Oꢀ group would form
(steps I and II). Aer the nucleophilic attack of a solvent
molecule on the Cu–Oꢀ group, a hydroperoxo CuOOH group
was yielded (step III). Then a superoxo (step IV) and molecular
O2 (step V) could formed. Finally, O2 is evolved, and the catalyst
is recovered when a H2O molecule connects to the Cu site (step
VI).
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