Covalently anchoring cobalt phthalocyanine on zeolitic imidazolate frameworks for efficient carbon dioxide electroreduction

Article abstract of DOI:10.1039/c9ce01517e

Transformation of CO₂ into fuels has drawn great attention due to increasing carbon emission in recent years. Coupling metal-organic frameworks (MOFs) with molecular catalysts is a promising technique for boosting the efficiency of carbon dioxide capture and conversion. Herein, a CoTAPc-ZIF-90 hybrid catalyst is synthesized by decorating cobalt phthalocyanine on the outer surface of ZIF-90 through a Schiff base reaction. We demonstrate that the ZIF-90 substrate can cooperate with the cobalt active center to boost the electrocatalytic CO₂ reduction performance. CoTAPc-ZIF-90 shows a large current density of 13 mA cm^-2 for effective conversion of CO₂ into CO in aqueous media at an overpotential of 0.86 V with a faradaic efficiency (FE) of 90%. What's more, the CoTAPc-ZIF-90 hybrid catalyst exhibits significantly higher catalyst stability compared with the free phthalocyanine molecule.

Full text of DOI:10.1039/c9ce01517e

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Covalently anchoring cobalt phthalocyanine on
zeolitic imidazolate frameworks for efficient
carbon dioxide electroreduction†
Cite this: DOI: 10.1039/c9ce01517e
Zhongjie Yang,ab Xiaofei Zhang,a Chang Long,ac Shuhao Yan,a Yanan Shi,a
ab
Jianyu Han,a Jing Zhang,d Pengfei An,d Lin Changab and Zhiyong Tang
*
Transformation of CO2 into fuels has drawn great attention due to increasing carbon emission in recent
years. Coupling metal–organic frameworks (MOFs) with molecular catalysts is a promising technique for
boosting the efficiency of carbon dioxide capture and conversion. Herein, a CoTAPc–ZIF-90 hybrid catalyst
is synthesized by decorating cobalt phthalocyanine on the outer surface of ZIF-90 through a Schiff base
reaction. We demonstrate that the ZIF-90 substrate can cooperate with the cobalt active center to boost
the electrocatalytic CO2 reduction performance. CoTAPc–ZIF-90 shows a large current density of 13 mA
cm−2 for effective conversion of CO2 into CO in aqueous media at an overpotential of 0.86 V with a
faradaic efficiency (FE) of 90%. What's more, the CoTAPc–ZIF-90 hybrid catalyst exhibits significantly higher
catalyst stability compared with the free phthalocyanine molecule.
Received 25th September 2019,
Accepted 13th November 2019
DOI: 10.1039/c9ce01517e
rsc.li/crystengcomm
roles in affording active sites and improving catalytic activity,
selectivity and stability, respectively.10 As for electrochemical
Introduction
Electrochemical reduction of CO2 to chemical fuels is a
promising way to alleviate the greenhouse effect and energy
reduction of CO2, molecular catalyst systems generally work
either in organic media or at low current density and faradaic
efficiency in aqueous solution, i.e. single component cobalt
phthalocyanine exhibits 0.98 mA cm−2 at −1.15 V and a
faradaic efficiency of 50%.11 Meanwhile, a composite CO2
reduction electrocatalyst, for instance, zeolitic imidazolate
framework-8 (ZIF-8) with high overpotential and low faradaic
efficiency (85% at η = 1.1 V vs. RHE), is developed via
coupling with a strong electron-donating molecule, 1,10-
phenanthroline. Interestingly, pristine zeolitic imidazolate
framework-8 (ZIF-8, a major subclass of MOFs) shows no
catalytic activity, and the sp2 carbon atoms in the imidazolate
ligand are determined to be the main catalytic sites after
ligand doping.12 Thus, design and application of composite
catalysts will dramatically benefit CO2 electrochemical
reduction.
crisis, which would produce
a range of carbon-based
products, including C1 (CO, methane, methanol and formic
acid), C2 (ethylene, ethanol and acetic acid) and longer alkyl
chain products.1–7 Electrocatalytic CO2 reduction has good
economic benefit, since it works in aqueous media under
room temperature and ambient pressure conditions while the
energy is supplied from reproducible sources such as wind
and solar energy.8 However, it remains highly challenging to
design cost-efficient electrocatalysts with high catalytic
activity, selectivity and durability.9
Chemical reactions often require catalysts, which are able
to boost the reaction rate and improve the reaction
selectivity. In traditional composite catalyst systems, the
catalytic performance is directly related to their components
and structure. Primary and assistant catalysts play important
The cobalt phthalocyanine (CoPc) molecule on
a
conductive platform has been extensively studied as a hybrid
electrocatalyst for the CO2 reduction reaction. Meshitsuka
et al.13 reported for the first time that cobalt and nickel
phthalocyanine graphite hybrid electrodes in aqueous
electrolytes exhibited reduction peaks in the presence of CO2.
Recently, Wang et al.14 showed a uniform distribution of
CoPc molecules on carbon nanotubes via π–π interactions
a CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for
Excellence in Nanoscience, National Center for Nanoscience and Technology,
Beijing 100190, P. R. China
b University of Chinese Academy of Sciences, Beijing 100049, China
c School of Materials Science and Engineering, Harbin Institute of Technology,
Harbin 150080, China
d Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese
Academy of Sciences, Beijing 100049, P. R. China
† Electronic supplementary information (ESI) available: 1H NMR spectrum, ex
situ EXAFS data, CO and H2 faradaic efficiency, Co element content and fitting
parameters of Co K-edge EXAFS curves. See DOI: 10.1039/c9ce01517e
that possessed
a
faradaic efficiency of 90% at an
overpotential of 0.52 V in aqueous solution, and the low
durability (about 10 h at η = 0.52 V) was attributed to CoPc
aggregation under electronic energy conditions due to the
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rather weak π–π interactions. Obviously, a suitable catalytic
platform will play an important role in stabilizing molecular
catalysts. MOFs with accurate molecular structures, well-
dispersed pores and a high degree of crystallization have
displayed great potential as catalytic platforms.15 As
examples, some MOFs were used as platforms for loading
(1.9 g, 10.0 mmol), CoCl2·6H2O (0.6 g, 2.5 mmol), and urea
(4.8 g, 80.0 mmol) and a small amount of ammonium
molybdate catalyst were reacted in solid state at 160 °C for 5
h. The resulting crude product was ground and further
stirred in aqueous HCl solution (1 M, 150 mL) and aqueous
NaOH solution (1 M 150 mL) at 90 °C for 1 h, respectively.
The residue was then filtered, washed with water, and dried
under vacuum to obtain cobalt tetranitrophthalocyanine. The
crude cobalt tetranitrophthalocyanine (0.75 g, 1.0 mmol) was
then converted to CoTAPc by a reaction with Na2S·9H2O (3.0
g, 37.0 mmol) in DMF (20 mL) at 70 °C for 4 h. The resulting
solution was then poured into hot water (50 mL), and the
precipitate was filtered and repeatedly washed with water
until neutral. The product was dried under ambient vacuum
conditions to get bluish-green CoTAPc.
metal
nanoparticles
toward
electrocatalytic
CO2
reduction.16–24 Hupp et al.25 reported that copper
nanoparticles were embedded into a zirconium-based MOF
by in situ electrochemical deposition, and the hybrid catalyst
was electrocatalytically active with higher selectivity for
hydrogen evolution over CO2 reduction. Buonsanti et al.26
indicated that silver nanoparticles embedded in an Al-PMOF
([Al2IJOH)2-(TCPP)])
(tetrakisIJ4-carboxyphenyl)porphyrin
(TCPP)) were in intimate contact with a conductive substrate
to enhance electron transfer and suppress the hydrogen
evolution reaction. However, the practical application would
be limited by a low current density ( jCO = 0.15 mA cm−2) and
low faradaic efficiency (FECO = 60%).
Synthesis of ZIF-90
The ZIF-90 nanoparticles (NPs) were synthesized according to
a
previously reported hydrothermal method with slight
To improve the performance of molecular catalysts in an
electrochemical CO2 reduction system, herein we report
molecule–ZIF hybrid materials as effective catalysts, where
cobalt tetraaminophthalocyanine (CoTAPc) is anchored onto
the ZIF-90 outer surface through a Schiff base reaction for
highly active, stable and selective CO2 reduction to CO. Based
on X-ray absorption spectroscopy results, the electronic
structure of cobalt element in CoTAPc does not change after
anchoring on the ZIF-90 outer surface. Significantly, ZIF-90 as
a catalytic platform can not only capture CO2 molecules from
the electrolyte,12 but also suppress the hydrogen evolution
reaction.27 As a result, the molecule–ZIF hybrid electrocatalyst
selectively reduces CO2 to CO with a large faradaic efficiency
of 90%, a high current density of 13 mA cm−2 at η = 0.86 V,
and excellent stability for at least 48 hours.
modification.29 Zn (NO3)2·6H2O (2.69 mmol) and ICA (8.10
mmol) were added to 45 mL of DMF in a round bottom flask
(150 mL). The resulting solution was sonicated for 15 min at
room temperature and stirred at 80 °C for 4 h. After the
solution was cooled to room temperature, methanol (45 mL)
was quickly poured into the solution while applying vigorous
stirring. After further stirring at room temperature for 60
min, the faint yellow solid products were recovered by
centrifugation and washed several times with DMF and
methanol (at least three times) to remove the excess
reactants. Then, the resulting ZIF-90 sample was vacuum
dried at 60 °C.
Synthesis of CoTAPc–ZIF-90
CoTAPc–ZIF-90 was synthesized according to a previous
report.29 0, 5, 10, 20 or 30 mg of CoTAPc and 100 mg of ZIF-
Experimental section
Chemical materials
90 were dissolved in
5 mL of anhydrous methanol,
respectively. A certain amount of CoTAPc was slowly added
into ZIF-90 solution with stirring at room temperature. The
resulting solution was vigorously stirred at 60 °C for 24 h.
The dark green solid samples were isolated by centrifugation
and washed with methanol three times. Then the hybrid
products were vacuum dried at 60 °C and denoted as ZIF-90
(0 mg CoTAPc), ZIF-90-1 (5 mg CoTAPc), ZIF-90-2 (10 mg
CoTAPc), ZIF-90-3 (20 mg CoTAPc) and ZIF-90-4 (30 mg
CoTAPc), respectively.
All chemicals and solvents were used as received without any
further purification. Urea, 4-nitrophthalonitrile and
imidazole-2-carboxaldehyde (ICA) were purchased from Alfa
Aesar. Sodium hydroxide (NaOH), sodium carbonate
(NaHCO3), potassium bromide (KBr), sodium sulfide (Na2S
·9H2O) and ammonium molybdate (H8MoN2O4) were bought
from Macklin. Cobaltous chloride hexahydrate (CoCl2·6H2O)
and zinc nitrate hexahydrate (Zn (NO3)2·6H2O) were obtained
from TCI (Shanghai) Development Co., Ltd. Anhydrous N,N-
dimethylacetamide and anhydrous methanol were obtained
from Innochem. Carbon dioxide (CO2, 99.999%) and argon
(Ar, 99.9999%) were supplied by Beijing Beiwen Gases
Company. Ultrapure water (Millipore Milli-Q grade) with a
resistivity of 18.2 MΩ was used in all experiments.
Material characterization
A Rigaku D/MAX-TTRIII (CBO) X-ray powder diffractometer
was used to get the powder X-ray diffraction (PXRD) patterns
by using Cu Kα radiation (λ = 1.5418 Å). A Hitachi S8220 field
emission scanning electron microscope (FESEM) was used to
obtain the SEM images. Fourier transform infrared (FTIR)
spectra were recorded on a Spectrum One in the spectral
range of 450–4000 cm−1 using the KBr disk method. The Co
content in different samples was determined by inductively
Synthesis of CoTAPc
CoTAPc was synthesized according to a previously reported
method.28 A uniformly mixed solid of 4-nitrophthalonitrile
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coupled plasma mass spectrometry (ICP-MS, Thermo Fisher
Scientific). Nuclear magnetic resonance (NMR) spectra were
recorded at 400 MHz with an Avance III Bruker Corporation
instrument.
jCO = jTot × FECO
where jCO is the partial current density of CO, jTot is the total
current density, and FECO is the faradaic efficiency of CO.
The Tafel slope was calculated as follows:
Electrochemical measurements
η = a + b × log|jCO
|
According to our previous work,30 all electrochemical
measurements were carried out using a CHI 660E potentiostat
in a three-electrode configuration using hydrophobic carbon
paper as the working electrode, a gauze platinum electrode as
the counter electrode and Ag/AgCl (3.5 M KCl) as the reference
electrode, respectively. All potentials were converted to
reversible hydrogen electrode (RHE) values using the
equation: E (vs. RHE) = E (vs. Ag/AgCl) + 0.205 V + 0.059 V ×
pH. For the preparation of the working electrode, 5 mg of
catalyst and 30 μL of 5 wt% Nafion solution were dispersed in
1 mL of isopropanol and vigorously sonicated for 60 min to
form a uniform catalyst ink. Then 150 μL of catalyst ink was
uniformly dropped onto the carbon paper electrode with an
area of 1 × 1 cm2. Electrochemical measurements were carried
out in a gas-tight two-compartment electrochemical cell with
a piece of Nafion 117 cation-exchange membrane as the
separator. A 0.5 M NaHCO3 aqueous solution was used as the
electrolyte and was purged with CO2 (99.999%) at a speed of
20 mL min−1 for at least 30 minutes prior to electrolysis.
Linear sweep voltammetry (LSV) measurements were
performed at a scan rate of 10 mV s−1. In order to determine
the reduction products and their faradaic efficiency, bulk
electrolysis was conducted at selected potentials (−0.77 V to
−1.07 V vs. RHE). All electrochemical curves were obtained
without iR corrections.
where η is the overpotential, a is the Tafel constant and b is
the Tafel slope.
X-ray absorption data collection and analysis
Ex situ Co K-edge XAS was carried out under ambient
conditions in transmission mode at beamline 1W1B of BSRF,
using a Si (111) double-crystal monochromator. The storage
ring of BSRF was operated at 2.5 GeV with a maximum
current of 250 mA in decay mode. The energy was calibrated
using a Co foil, and the intensity of the incident and
transmitted X-rays was monitored by standard N2-filled ion
chambers. The powder samples (the pristine CoTAPc and
ZIF-90-4) were pressed to a pellet to determine the best signal
to noise ratio.
The obtained X-ray absorption fine structure (XAFS) data
were processed in Athena (version 0.9.25) for background,
pre-edge line and post-edge line calibrations. Then Fourier
transformed fitting was carried out in Artemis (version
0.9.25). The k2 weighted data, in the k-range of 3–12 Å−1 and
R range of 1–3 Å, were used for fitting. CoTAPc was used to
calculate the simulated scattering paths. Four parameters,
namely the coordination number, bond length, Debye–Waller
factor and E0 shift (CN, R, σ2, and ΔE0), were fitted without
being fixed, constrained, or correlated.
The evolved gaseous products were flowed into the gas-
sampling loop of a gas chromatograph (GC, Shimadzu GC-
2014C) equipped with a packed Molecular Sieve 5A capillary
column (Agilent) and a Porapak Q column (Agilent, 80–100
mesh). Both a thermal conductivity detector (TCD) and flame
ionization detector (FID) were installed on the GC to detect
the evolved gaseous products. The liquid products were
determined by 1HNMR spectroscopy (Bruker Advance 400
spectrometer, 400 MHz) via water suppression using a
presaturation method. A 500 μL electrolyte was mixed with
100 μL of D2O, and 1.0 ppm dimethyl sulfoxide (DMSO) was
added as the internal standard.
Results and discussion
A series of CoTAPc–ZIF-90 hybrid catalysts are synthesized
through
a solvothermal reaction. ZIF-90 (100 mg) and
CoTAPc (0, 5, 10, 20 or 30 mg) were dispersed into anhydrous
methanol at 60 °C for 24 h to get ZIF-90, ZIF-90-1, ZIF-90-2,
ZIF-90-3, and ZIF-90-4, respectively (see the Experimental
section). The structures of the obtained CoTAPc–ZIF-90
hybrid catalysts were analyzed by XRD. As shown in Fig. 3A,
all of the diffraction peaks are in good agreement with those
The faradaic efficiency (FE) was calculated from the
concentration determined by GC using the following
equation:
−1
FE% = ppm × flow rate × (nFp0/RT) × ( jTot
)
× 100
where ppm is the concentration of the gas (CO or H2)
determined by GC, n is the electron transfer number, F is the
faradaic constant, p0 is the pressure, T = 273.15 K and jTot is
the total current density.
The partial current density of CO was calculated as
follows:
Fig. 1 Schematic illustration of the synthesis of CoTAPc–ZIF-90.
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coupled plasma (ICP) spectroscopy. These loadings are close
to theoretical calculations of the Co content (Table S1†),
demonstrating that all the added CoTAPc molecules are
anchored on the outer surface of ZIF-90 (Fig. 1).
To further verify the structure of CoTAPc molecules on
ZIF-90, the XAS technique was used to evaluate the electronic
structure of the Co center (corresponding fitting results are
shown in Fig. S2 and Table S2†). The Fourier transform (FT)
R space of the extended X-ray absorption fine structure
(EXAFS) spectra of the pristine and anchored CoTAPc is
presented in Fig. 3C. Evidently, the EXAFS of the anchored
CoTAPc at the Co K-edge are well consistent with those of
pristine CoTAPc. Also, the X-ray absorption near edge spectra
(XANES) of the Co K-edge energy of the pristine and
anchored CoTAPc show nearly the same absorption shoulders
(Fig. 3D). Note that the absorption shoulders in the XANES
are the fingerprints of the Co–N4 structure, where the first
peak at 7708.6 eV is specified as the dipole forbidden 1s to
3d transition and the second peak at 7714.2 eV is attributed
to the shakedown satellite 1s to 4pz transition.30,31 In other
words, the CoTAPc molecules are successfully anchored onto
the outer surface of ZIF-90, which might be a good platform
to explore the electrochemical CO2 reduction performance.
The electrochemical CO2 reduction performance of the
series of CoTAPc–ZIF-90 hybrid catalysts was evaluated in 0.5
M NaHCO3 in an airtight electrochemical H-type cell (the
compartments are separated by a piece of Nafion 117 cation-
exchange membrane). All the CoTAPc–ZIF-90 hybrid (i.e., ZIF-
90, ZIF-90-1, ZIF-90-2, ZIF-90-3 and ZIF-90-4) catalysts were
dropped cast onto carbon paper and subjected to
examination. In this work, all the potentials were measured
vs. the Ag/AgCl (3.5 M KCl) electrode and then converted to
values vs. the reversible hydrogen electrode (RHE) without iR
compensation.
Fig. 2 SEM images of CoTAPc–ZIF-90 hybrid crystals at different
added amounts of CoTAPc: (A) 0 mg, (B) 5 mg, (C) 10 mg, (D) 20 mg,
and (E) 30 mg. Scale bars: 500 nm.
of pristine ZIF-90.29 The morphology of the samples was
characterized by SEM imaging. As shown in Fig. S3,† Zn, Co
and N elements are uniformly distributed in ZIF-90-4 with a
dodecahedron morphology. This result demonstrates that the
CoTAPc units are uniformly distributed in the outer surface
of ZIF-90. High-resolution SEM images show that the outer
surface of pristine ZIF-90 is smooth and the CoTAPc–ZIF-90
hybrid catalysts become rougher with increasing amount of
added CoTAPc (Fig. 2A–E). It is worth mentioning that the
increased roughness of the electrodes benefits the
improvement of the electrochemical performance.29 FT-IR
analysis was carried out to verify the formation of imine
linkages through the Schiff base reaction. As shown in
Fig. 3B, a new peak at 1620 cm−1 appears in the series of
CoTAPc–ZIF-90 catalysts compared with pristine ZIF-90,
which is assigned to the bending vibration of CN, while
the peak at 1671 cm−1 is ascribed to the stretching vibration
of the remaining CO in pristine ZIF-90.29 The Co content is
determined to be 0.46, 0.83, 1.81 and 2.61 wt% by inductively
The linear sweep voltammetry (LSV) curves of ZIF-90 and
ZIF-90-4 were recorded at a scan speed of 10 mV s−1 in argon
(Ar) and in CO2 saturated 0.5 M NaHCO3 solution (pHAr = 8.4
and pHCO = 7.4), respectively. Notably, to reach a current
2
density of 3 mA cm−2, potentials of −1.16 V and −0.65 V vs.
RHE are required for the pristine ZIF-90 and ZIF-90-4 in the
CO2 saturated NaHCO3 solution, respectively (black and red
curves in Fig. 4A). Correspondingly, the LSV curve of ZIF-90-4
in the Ar saturated NaHCO3 solution shows a more negative
potential of −0.97 V vs. RHE to reach 3 mA cm−2 (blue curve
in Fig. 4A), which confirms the increased current density in
the CO2 saturated NaHCO3 solution from catalyzing CO2
conversion. Clearly, the CoTAPc–ZIF-90 hybrid catalyst has an
excellent catalytic performance toward CO2 reduction under
aqueous conditions. Fig. 4B shows that with increasing
amount of anchored CoTAPc, the electrochemical
performance gradually becomes better. To further obtain an
appropriate reduction potential of the series of molecule–ZIF
hybrid catalysts for CO2 reduction, all of the catalysts are
electrolyzed for 2 hours at gradually increasing potential from
−0.77 to −1.17 V vs. RHE. Generally, gaseous products (CO
and H2) are detected by gas chromatography (GC), and no
Fig. 3 XRD patterns (A) and FT-IR spectra (B) of CoTAPc–ZIF-90 hybrid
crystals at different added amounts of CoTAPc (ZIF-90, ZIF-90-1, ZIF-
90-2 ZIF-90-3, or ZIF-90-4). Ex situ EXAFS in R space (C) and Co
K-edge XANES (D) of pristine CoTAPc and anchored CoTAPc.
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macrocycles. Fig. S7B† shows that ZIF-90-4 has superior
durability compared with CoTAPc, in good agreement with
the fact that the anchored CoTAPc molecules are uniformly
dispersed on the outer surface of ZIF-90 to provide the
catalytic sites. As shown in Fig. S8† the specific activities of
ZIF-90-1, ZIF-90-2, ZIF-90-3 and ZIF-90-4 are quantified by
normalizing the CO partial current density with respect to
the area of electrodes. At −0.97 V, the activity of ZIF-90-4
(12.85 mA cm−2) is 2–22 times larger than that of the other
four samples. What is more, ZIF-90-4 is the electrocatalyst
with the highest content of CoTAPc, which is anchored on
the outer surface of ZIF-90 through imine bonds. In the field
of electrocatalytic carbon dioxide reduction, the functional
groups adjacent to the active centers greatly affect the
electrocatalytic performance.32,33 In our case, ZIF-90-4
possesses the highest number of imine bonds and shows the
best catalytic activity, mainly due to the increase of the local
carbon dioxide concentration on the surface of the
catalyst.34,35
To explore the possible electrochemical mechanism
pathway, Tafel analysis is performed and data are plotted
with the overpotential (η) against the logarithm of the steady-
state CO partial current density [logIJ jCO)] (Fig. 4C). The Tafel
slope of ZIF-90-4 is calculated to be 171 mV dec−1 by linear
fitting logIJjCO) ranging from 0.57 V to 0.87 V. This value
reveals that the initial electron transfer to CO2 to form the
*COO− intermediate is the possible rate-determining
step.36–41 The larger Tafel slope value of the other molecule–
ZIF-90 hybrid catalysts is 219 mV dec−1, 267 mV dec−1, 367
mV dec−1 and 389 mV dec−1, respectively, attributed to the
mass transport limitations at a high current density. The
durability of ZIF-90-4 is investigated at a potential of −0.97 V
vs. RHE (Fig. 4D). The cathodic current density is maintained
at −13 mA cm−2 for 48 h with negligible activity decay and
the corresponding FECO is kept around 90% for the whole
electrolysis period, demonstrating the superior durability of
ZIF-90-4 compared with those of CoTAPc, CoPc/CNT,14
CoPPc/CNT,31 and defective polymeric-CoPc.42
Fig. 4 (A) LSV curves of ZIF-90–CO2, ZIF-90-4–Ar, and ZIF-90–CO2.
(B) LSV curves of CoTAPc–ZIF-90 hybrid crystals at different added
amounts of CoTAPc in 0.5 M NaHCO3 saturated with CO2 at 10 mV s−1
,
(ZIF-90, ZIF-90-1, ZIF-90-2, ZIF-90-3 and ZIF-90-4). (C) Tafel plots of
ZIF-90, ZIF-90-1, ZIF-90-2, ZIF-90-3 and ZIF-90-4, respectively. (D)
Chronoamperograms and faradaic efficiency of CO and H2 for ZIF-90-
4 over 48 h at −0.97 V vs. RHE.
liquid product is found by nuclear magnetic resonance
(NMR) spectroscopy at the end of electrolysis (Fig. S4†). The
relationship between selectivity and the applied potential is
deduced for ZIF-90, ZIF-90-1, ZIF-90-2, ZIF-90-3 and ZIF-90-4
(Fig. S5A–E†). Remarkably, the highest CO faradaic efficiency
(FECO) of 90% 1.5% for ZIF-90-4 is obtained at a potential
of −0.97 V vs. RHE. As shown in Fig. 5, a more positive or
negative potential would not largely set off water splitting,
and hence the FECO of ZIF-90-4 remains beyond 65% in a
wide range of preferences of CO2 reduction. In sharp
contrast, ZIF-90 shows a very low FECO (9.6% 2%–35.5%
3%) in the range of applied potentials. As shown in Fig. S7A,†
when the applied potential is increased to −0.97 V, the CO
faradaic efficiency continuously increases and reaches 81%
and 90% for CoTAPc and ZIF-90-4, respectively. This result
not only confirms that ZIF-90 could suppress the hydrogen
evolution reaction, but also reveals that the high CO2 RR
activity of ZIF-90-4 mainly originates from Co-centered
Conclusion
In summary, we have successfully synthesized CoTAPc–ZIF-90
hybrid materials through a Schiff base reaction for high-
performance electrocatalysis toward CO2 reduction.
Impressively, the ZIF-90-4 electrocatalyst (anchoring CoTAPc
molecules as active moieties) exhibits a large current density
of 13 mA cm−2 for electrochemical reduction of CO2 to CO in
aqueous solution at an overpotential of 0.86 V with a faradaic
efficiency (FE) of 90%, while the excellent stability of ZIF-90-4
is distinguished with a negligible change within 48 hours of
electrolysis. This study will open an avenue for immobilizing
molecular catalysts on the surface of MOFs by covalent bond
linkage to enhance catalyst durability as well as understand
the synergistic effect of active sites and supports, which
eventually guide the precise construction of highly efficient
catalysts for energy conversion and storage.
Fig. 5 CO and H2 faradaic efficiency of ZIF-90-4 and ZIF-90-4 at
different applied potentials.
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Acknowledgements
The authors acknowledge financial support from National
Key Basic Research Program of China (2016YFA0200700, Z. Y.
T.), National Natural Science Foundation of China (21890381
and 21721002, Z. Y. T.), Frontier Science Key Project of
Chinese Academy of Sciences (QYZDJ-SSW-SLH038, Z. Y. T.),
and K. C. Wong Education Foundation (Z. Y. T.).
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