Conductive Two-Dimensional Phthalocyanine-based Metal–Organic Framework Nanosheets for Efficient Electroreduction of CO2

Article abstract of DOI:10.1002/anie.202104564

The electrocatalytic conversion of CO₂ into value-added chemicals is a promising approach to realize a carbon-energy balance. However, low current density still limits the application of the CO₂ electroreduction reaction (CO₂RR). Metal–organic frameworks (MOFs) are one class of promising alternatives for the CO₂RR due to their periodically arranged isolated metal active sites. However, the poor conductivity of traditional MOFs usually results in a low current density in CO₂RR. We have prepared conductive two-dimensional (2D) phthalocyanine-based MOF (NiPc-NiO₄) nanosheets linked by nickel-catecholate, which can be employed as highly efficient electrocatalysts for the CO₂RR to CO. The obtained NiPc-NiO₄ has a good conductivity and exhibited a very high selectivity of 98.4 % toward CO production and a large CO partial current density of 34.5 mA cm^?2, outperforming the reported MOF catalysts. This work highlights the potential of conductive crystalline frameworks in electrocatalysis.

Full text of DOI:10.1002/anie.202104564

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Conductive Two-Dimensional Phthalocyanine-based Metal-
Organic Framework Nanosheets for Efficient Electroreduction of
CO2
Authors: Rong Cao, Jun-Dong Yi, Duan-Hui Si, RuiKuan Xie, Qi Yin,
Meng-Di Zhang, Qiao Wu, Guo-Liang Chai, and Yuan-Biao
Huang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202104564
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10.1002/anie.202104564
Angewandte Chemie International Edition
RESEARCH ARTICLE
Conductive Two-Dimensional Phthalocyanine-based Metal-
Organic Framework Nanosheets for Efficient Electroreduction of
CO2
Jun-Dong Yi,[a] Duan-Hui Si,[a] Ruikuan Xie,[a] Qi Yin,[a] Meng-Di Zhang,[a] Qiao Wu,[a] Guo-Liang
Chai,[a,b] Yuan-Biao Huang,*[a,b] and Rong Cao*[a,b]
[a]
[b]
Dr. J. Yi, Dr. D. Si, Dr. R. Xie, Dr. Q. Yin, M. Zhang, Q. Wu, Prof. G. Chai, Prof. Y. Huang, Prof. R. Cao
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences
Fuzhou, 350002 (China)
E-mail: ybhuang@fjirsm.ac.cn; rcao@fjirsm.ac.cn
Prof. G. Chai, Prof. Y. Huang, Prof. R. Cao
University of Chinese Academy of Science
Beijing 100049 (China)
Supporting information for this article is given via a link at the end of the document.
Abstract: The electrocatalytic conversion of CO2 into value-added
chemicals using renewable electricity is a promising approach to
reduce atmospheric CO2 concentration and realize carbon-energy
balance. However, the low current density still limits CO2
electroreduction reaction (CO2RR) for commercial application.
Crystalline porous metal-organic frameworks (MOFs) are one class
of promising alternatives for CO2RR due to their high CO2 adsorption
uptakes and periodically arranged isolated metal active sites.
However, the poor conductivity and slow electron-transfer capability
of the traditional MOFs usually result in low current density in
CO2RR. Herein, conductive two-dimensional (2D) phthalocyanine-
based MOF (NiPc-NiO4) nanosheets were prepared by the
construction of nickel phthalocyanin-2,3,9,10,16,17,23,24-octaol
(NiPc-OH) and nickel(II) ions, which can be employed as highly
efficient electrocatalysts for CO2RR towards production of CO. The
obtained NiPc-NiO4 has a good conductivity with a high value of 4.8
 10-5 S m-1 due to the in-plane full π-d conjugation. Thus, the as-
prepared NiPc-NiO4 nanosheets exhibited a very high selectivity of
98.4% towards production of CO and a large CO partial current
density of 34.5 mA cm-2, outperforming the reported MOF catalysts.
This work offers an opportunity to design conductive crystalline
frameworks for improving energy efficiency in electrocatalysis.
metal-free carbons.[31-33] Nevertheless, it still remains several
challenges for the CO2 electroreduction reaction (CO2RR),
including low energy conversion efficiency, poor selectivity and
stability. Therefore, it still needs design and fabrication of highly
active, selective and robust electrocatalysts towards CO2RR.
Compared with Au, Ag, Cu-based inorganic solid catalysts, the
tunable homogeneous molecular catalysts usually show
enhanced selectivity and specific activity in CO2RR.[34] However,
the extensive challenge for discrete metal complexes-based
electrocatalysts is to achieve high selectivity and large current
density in an aqueous medium. Loading electroactive metal
complexes into porous conductive substrates is a promising
approach to reduce CO2 with high current density because of
their fast electron transfer capacity and accessible active sites.
Compared with other substrates, crystalline porous metal-
organic frameworks (MOFs) with designable metal clusters and
functional organic linkers have shown high selectivity in
electrocatalytic CO2RR and other electrocatalytic reaction due to
their single active sites and large CO2 adsorption uptake.[35-51]
However, the poor electrical conductivity feature of the
traditionally insulating MOFs usually results in low current
density in CO2RR. Hence, developing conductive MOFs
(CMOFs) with highly single-active sites for CO2RR is extremely
desirable.
As we know, metalophthalocyanine complexes with M-N4 (e.g.
M = Ni, Fe, Co) structures have been considered as typical
active sites for CO2RR, but the selectivity and current density
still need to be improved.[52-54] To address these problems,
herein, Ni-phthalocyanine motif (NiPc) was integrated into a
porous intrinsically conductive nickel-catecholate-linked two-
dimensional (2D) MOF nanosheets (NiPc-NiO4, Figure 1) for the
highly efficient CO2RR towards production of CO in aqueous
medium. Notably, different from the traditional carboxylate-
based MOFs, the CMOF NiPc-NiO4, in which the planar NiPc
architecture was bridged by the nickel-catecholates, has good
electrical conductivity because of the high d-π orbital overlap
between the nickel node and the catechol. Thus, the
Introduction
Conversion of CO2 to value-added chemicals could help
reduce the atmospheric CO2 concentration and alleviate the
dependence on fossil fuels. Using electricity generated from
renewable energy sources, CO2 can be electrochemically
reduced to energy-rich products such as carbon monoxide (CO),
which is one of the most important feedstock to be used in the
Fischer-Tropsch industrial process.[1-5] Over the past decades,
various electrode materials for the conversion of CO2-to-CO
have been explored, such as Au, Ag, Cu metals,[6-9] metal
complexes[10-13] and single-atom Fe, Co, Ni based catalysts,[14-30]
1
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Figure 1. (a) Schematic illustration of the preparation of NiPc-NiO4. Top and side view of their structures with 2 × 2 square grids in AA-stacking mode. (b)
Experimental PXRD patterns for NiPc-NiO4 and simulated patterns with AA-packing and AB-stacking modes. (c) Current-voltage characteristic of NiPc-NiO4 using
two-contact probe method. (d) N2 adsorption–desorption isotherms and (e) the pore size distribution (PSD) profile of NiPc-NiO4.
activity. The N2 sorption measurement at 77 K revealed that the
2D NiPc-NiO4 has a high Brunauer-Emmett-Teller (BET) surface
area of 180 cm3 g-1 (Figure 1d). The pore size distribution
analysis (Figure 1e) indicated that NiPc-NiO4 possesses
micropore of 1.2 nm, which is consistent with the theoretical size
of the square shaped channels along the c direction (Figure 1a).
Moreover, many mesopores ranging from 2 nm to 18 nm were
formed in NiPc-NiO4, which may be originated form the stacking
gaps and defects between microcrystalline particles. Besides,
NiPc-NiO4 shows a high CO2 adsorption uptake of 16.8 cm3 g-1
as-prepared NiPc-NiO4 nanosheets exhibit outstanding
electrocatalytic performance of CO2RR for production CO with
nearly 100% selectivity (98.4 %) and a partial current density of
34.5 mA cm-2 at -1.2 V versus the reversible hydrogen electrode
(vs. RHE). The turnover frequency (TOF) can reach to 2603 h-1
at -1.2 V vs. RHE and it also exhibits excellent long-term stability
with 86% CO selectivity maintained after 10 hours
chronoamperometry test. Moreover, the density functional theory
calculation proves that the nickel of phthalocyanine center is the
active site and NiPc-NiO4 performs better activity than NiPc-OH
due to the fast electron transfer capacity and excellent
reducibility, which is consistent with the experiment results.
Results and Discussion
The conjugated 2D CMOF NiPc-NiO4 was synthesized by
reaction of nickel phthalocyanin-2,3,9,10,16,17,23,24-octaol
(NiPc-OH) and Ni(OAc)2∙4H2O in water at 85 °C for 24 h (Figure
1a).[55] The powder X-ray diffraction (PXRD) pattern of the as-
prepared NiPc-NiO4 is well consistent with the AA-stacking
model of the simulated one (Figure 1b), instead of AB-stacking
model (Figure S1), suggesting the successful preparation of the
crystalline NiPc-NiO4. The peaks at 5.2°, 7.0°, 10.1°, and 27.0°
are assigned to (100), (110), (200), and (001) facets,
respectively. Among them, the (001) facets at 27.0° indicated
that NiPc-NiO4 possess a square-shaped channel of 14.5 Å in
diameter along the c-axis (Figure 1a-b). The layered
phthalocyanine-based MOF features good carrier transport
ability based on the large delocalized π system originating from
the macrocyclic phthalocyanine structure itself and the dπ-pπ
interaction between the nickel node and the catechol in these
square-planar moieties. As expected, the electrical conductivity
of NiPc-NiO4 has a high value of 4.8  10-5 S m-1 measured at
room temperature using a two-contact probe method (Figure
1c).[56] Such good electrical conductivity would be beneficial for
the electron transfer to the active sites during CO2RR, thereby
improving energy conversion efficiency and electrochemical
Figure 2. (a) TEM image and (b) HAADF-STEM image of NiPc-NiO4. (c) AFM
image of NiPc-NiO4. (d) The height profile along the white line in (c). (e) The
elemental mapping of Ni, O, N, and C for NiPc-NiO4.
2
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Figure 3. (a) LSV curves of NiPc-NiO4 and NiPc-OH in CO2-, and Ar-saturated 0.5 M KHCO3. (b) Faradaic efficiencies of CO, (c) CO partial current density, (d)
turnover frequency for NiPc-NiO4 and NiPc-OH. (e) Comparison of the TOF and maximum CO partial current density with reported MOF catalysts, including Co-
PMOF,[29] CoCp2@MOF-545-Co,[30] PcCu-O8-Zn,[31] MOF-1992/CB,[32] Re-MOF film,[33] Al2(OH)2TCPP-Co.[34] (f) Stability of NiPc-NiO4 at a potential of −0.85 V
versus RHE for 10 h.
at room temperature (Figure S2), which would facilitate in
promoting CO2RR.
potential range from -0.65 V to -1.1 V, reaching the maximum of
98.4% at -0.85 V, surpassing most of reported MOF catalysts
(Table S1).[35-48] Moreover, the CO partial current density of
NiPc-NiO4 gradually increases with the increase of the
overpotential, reaching 34.5 mA cm-2 at -1.2 V (Figure 3c),
which is the largest value among the reported MOFs (Table S1)
and about 4 times of that NiPc-OH monomer (Figure S4). The
excellent electrocatalytic CO2RR performance for NiPc-NiO4
nanosheets highlights the importance of their good electrical
conductivity, accessible single active sites, and large CO2
adsorption capacity.
In order to determine whether the catalytically active site is the
nickel of phthalocyanine center or NiO4 node, the typical 2D
CMOF Ni3(HHTP)2 containing only NiO4 node was synthesized
for CO2RR under the same conditions.[57] As shown in Figure S5,
the main product of Ni3(HHTP)2 is H2 with selectivity of around
95% throughout the testing range of -0.7 to -1.2 V, indicating
that the NiO4 node cannot catalyze the CO2RR and can be
excluded as the active site in NiPc-NiO4 for CO2RR. Besides,
pure ketjen black and carbon paper electrode also only produce
H2 (Figure S6 and S7), while the ligand NiPc-OH shows certain
CO2RR activity, indicating that the active site in NiPc-NiO4 is the
NiN4 sites of NiPc moiety. Thus, on the basis of the NiPc motif
as active site, the turnover frequency (TOF) of the NiPc-NiO4
was calculated and summarized in Figure 3d. The TOF of NiPc-
The morphology of the as-prepared NiPc-NiO4 was
characterized by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). The SEM images in
Figure S3 show that the obtained NiPc-NiO4 was composed of
nanoparticles (NPs) of less than 100 nm. The TEM (Figure 2a)
and high angle annular dark-field scanning transmission electron
microscopy (HAADF-STEM) images (Figure 2b) display no
aggregated Ni NPs. To ensure the accessible of the Ni active
sites in the slipped AA stacking frameworks for the electrolytes
and CO2, the ultrathin 2D NiPc-NiO4 nanosheets were
successfully fabricated by exfoliating from its bulk via high-
frequency sonication at room temperature. As shown the atomic
force microscopy (AFM) image and its height profile in Figure
2c-2d, a uniform thickness of ca. 1.65 nm was clearly observed,
which is close to five layers of NiPc-NiO4 nanosheets. Such
ultrathin 2D NiPc-NiO4 nanosheets would facilitate in exposure
of more the Ni active sites for the CO2 and electrolytes, thus
improving CO2RR activity. Furthermore, the corresponding
element mapping analysis of NiPc-NiO4 indicated Ni, N, O and C
elements were homogeneously distributed over the entire
structure (Figure 2e). The results indicated that the Ni species in
NiPc-NiO4 are positioned in isolated sites.
To examine the catalytic performance of the as-prepared
NiPc-NiO4 nanosheets towards CO2RR in 0.5 M KHCO3 solution, NiO4 gradually increases with the increasing overpotential,
a conductive ink containing NiPc-NiO4 was prepared and drop-
casted on carbon paper electrode. The catalytic activity was
evaluated in a three-electrode system using a two-compartment
H-type cell that separated by a proton-exchange membrane. All
the potentials mentioned in this work are with reference to the
reversible hydrogen electrode (RHE). As shown in Figure 3a,
the onset potentials of both NiPc-OH and NiPc-NiO4 in CO2
atmosphere are more positive than that of in Ar atmosphere,
indicating that their activities are originated from the CO2RR.
Furthermore, as shown in Figure 3b, NiPc-NiO4 nanosheets
show high Faradaic efficiencies of CO with above 90% in a wide
reaching a maximum of 2603 h-1 at -1.2 V. To the best of our
knowledge, such high TOF and CO partial current density are
better than any state-of-the-art MOF catalysts (Figure 3e). The
long-term stability is another important factor for evaluating the
practicability of catalysts. As shown in Figure 3f, after 10 hours
durability test at –0.85 V, the current density of NiPc-NiO4 did
not change significantly by decreasing only 0.8 mA cm-2, while
the high CO selectivity of 86% for NiPc-NiO4 was still maintained.
The result indicates the superior structural stability of NiPc-NiO4
in this neutral aqueous medium. Moreover, no Ni NPs were
observed in the TEM images of the NiPc-NiO4 after
3
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Figure 4. (a) Normalized Ni K-edge XANES spectra of Ni foil, NiO, NiPc-OH, NiPc-NiO4 and NiPc-NiO4-tested. (b) XPS spectra of the Ni 2p region of NiPc-OH,
NiPc-NiO4 and NiPc-NiO4-tested. (c) Fourier transform EXAFS spectra of different samples. (d) Wavelet transforms for the k2-weighted EXAFS signals of different
samples. The corresponding EXAFS fitting curves of (e) NiPc-OH, (f) NiPc-NiO4 (g) NiPc-NiO4-tested. The inset in (e) is the molecule architecture of NiPc-OH. (h)
Schematic illustration of the transformation process throughout the CO2RR reaction.
electrocatalysis (Figure S9-S10), and the post-reaction SEM
images showed no significant changes in the size and shape
(Figure S11), suggesting that no structural collapse of NiPc-
NiO4 was occurred and no crystalline metal impurities generated
during the CO2RR process. Furthermore, the major peaks were
remained in the PXRD of NiPc-NiO4 after the CO2RR
measurement (NiPc-NiO4-tested) (Figure S12), indicating that
the crystalline framework of NiPc-NiO4 was still maintained in
this neutral electrochemical testing system. To further determine
whether CO was produced from the CO2RR, the 13C-labeling
CO2 isotope experiments were conducted via gas
chromatography-mass spectrometry (GC-MS). As shown in
Figure S13, when conducting CO2RR with 13C-labeling CO2 in
0.5 M KH12CO3, both the signal of 12CO and 13CO was observed,
while only 13CO was observed in KCl electrolyte. These results
demonstrate that CO was originated from the CO2 in equilibrium
with bicarbonate anions in CO2-saturated KHCO3 aqueous
solution.[58-60]
NiPc-NiO4-tested almost overlap together, indicating that the
structure of NiPc-NiO4 maintained well after the CO2RR process.
In addition, two fingerprint peaks of NiN4 symmetrical structure
were found at 8334 eV and 8340 eV in NiPc-OH, NiPc-NiO4 and
NiPc-NiO4-tested. One at 8334 eV is assignable to the dipole
forbidden 1s to 3d transition, and the other one at 8340 eV is
attributed to the shakedown satellite 1s to 4pz transition.[61-62]
The presence of these two NiN4 fingerprint peaks in NiPc-NiO4-
tested reveals that the nickel atom in the center of
phthalocyanine maintains the NiN4 structure after CO2RR,
instead of detaching from the phthalocyanine center and
aggregating together. Both of the white line positions of NiPc-
NiO4 and NiPc-NiO4-tested are very close to that of NiO,
indicating that the valence state of the Ni atoms in these
samples is close to +2.[61-62] The XPS results also indicated the
dominance of Ni2+ in all the samples, consistent with the XANES
analysis. As shown in Figure 4b, the Ni 2p XPS spectra of the
three samples display the similar patterns with two peaks at
873.0 eV and 855.7 eV, which can be assigned to Ni 2p1/2 and
Ni 2p3/2, respectively.[61-63] What’s more, cyclic voltammetry (CV)
measurement could be used to analyze the dynamic change
process of oxidation state. As shown in Figure S14, the CV
curves of NiPc-NiO4 were conducted in an anhydrous system
(0.1 M TBAPF6/acetonitrile) in order to eliminate the effect of the
In order to further figure out the coordination environment and
electronic structure of Ni active sites during the CO2RR, the ex
situ synchrotron based X-ray absorption spectroscopy (XAS)
and X-ray photoelectron spectroscopy (XPS) were carried out.
The X-ray absorption near edge structure (XANES) of Ni K edge
in Figure 4a shows that the XANES curves of NiPc-NiO4 and
4
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redox peaks of water. There is a pair of redox peaks assigned to
the conversion between Ni(II) and Ni(I) in the CV tested in argon
atmosphere, while no obvious redox peaks are observed in CO2
atmosphere, which may be due to the electrons are quickly
transferred from the formed Ni(I) species to the CO2 substrate
molecules. According to the CV results, it is suggested that Ni(II)
could be reduced to Ni(I) under cathodic bias during CO2RR,
which would be quickly oxidized back to Ni(II) after the electrons
transferred from the formed Ni(I) species to the CO2 substrate
molecules.[64]
The coordination environment of the nickel atoms in the
samples are further analyzed by extended X-ray absorption fine
structure (EXAFS). As shown in Figure 4c, compared with Ni foil
sample, no Ni-Ni bond at 2.2 Å was observed in NiPc-OH, NiPc-
NiO4 and NiPc-NiO4-tested, indicating that all the nickel atoms in
these samples exist as single Ni site without the presence of
metal-derived aggregates. The results revealed that the NiPc
sites and NiNO4 nodes in NiPc-NiO4 were not reduced to NPs
during CO2RR process. Besides, the NiPc-OH, NiPc-NiO4 and
NiPc-NiO4-tested samples have one main peak at 1.4 Å that
attributed to the NiN bond, while the latter two samples show
another peak centered at 1.6 Å, which overlaps with the NiO
bond in NiO and can therefore be attributed to the NiO4 node.[65]
These results indicated that the NiPc sites and NiNO4 nodes in
NiPc-NiO4 were well maintained after CO2RR.
isolated atomic-level dispersion. To obtain quantitative structural
parameters of the Ni atoms, a least-squares EXAFS fitting was
carried out and the fitting results are shown in Figure 4e-4g,
Figure S15-S16 and Table S2. The fitting result of NiPc-NiO4
indicated that the average coordination number of Ni-N and
NiO bond are 4.0 and 5.5, respectively. The larger coordination
number of NiO bond in comparison with the theoretical value
can be reasonably ascribed to the axial coordinated H2O
molecules adsorbed on NiO4 nodes. In general, the analysis of
the oxidation states and the precise coordination environment of
Ni atoms throughout the CO2RR process demonstrate the
superior structure stability of NiPc-NiO4 and the full exposure of
active sites.
To further determine whether the catalytically active site is the
nickel of phthalocyanine center (NiPC) or NiO4 node, we
calculated the energy for CO2RR and hydrogen evolution
reaction (HER) at two different Ni active sites in NiPc-NiO4
(Table S3). For the NiPC, the energies of the rate-determining
step (RDS) for CO2RR (formation of COOH* intermediate) and
HER (formation of H*) are 1.93 eV (Figure 5a) and 1.98 eV
(Figure S17), respectively, suggesting that CO2RR occurs
preferentially at NiPC sites. In contrast, the corresponding
energy for CO2RR (2.53 eV, Figure 5a) in the nickel of the NiO4
node is larger than that of HER (1.58 eV, Figure S17), which
indicated that HER takes precedence at NiO4 nodes. Moreover,
the energy for RDS on the NiPC (1.93 eV) is significantly lower
than that on NiO4 node (2.53 eV), which clearly shows that NiPC
is the active site for CO2RR. The above calculated results are in
agreement with the experiment results that NiPc-OH showed
active for CO2RR (Figure 3b) while Ni3(HHTP)2 exhibited only
catalytic HER performance (Figure S5).
The high activity and selectivity of NiPC in the CO2RR can be
further explained in terms of adsorption capacity, charge
distribution, and reducibility. Firstly, the adsorption energies of
CO2 on NiPC and NiO4 node are 0.23 eV and 0.02 eV (Table
S3), respectively. Thus, the CO2 molecular was easier to
combine with NiPC, which is also revealed by noncovalent-
interaction. As shown in Figure 5b, there is stronger Van der
Waals interaction between CO2 molecule and NiPC than that of
NiO4 node. Secondly, the Mulliken population analysis of the two
Ni atoms indicates the more electron-rich environment of NiPC
compared with Ni in NiO4 node, no matter that whether Ni atom
was or not adsorbed by CO2 molecule (Figure 5c). Thirdly, the
lowest unoccupied molecular orbital (LUMO) energy level of
NiPc-NiO4 shifts from -4.22 eV to -4.62 eV when the CO2
molecule moves from NiPC to NiO4 node (Figure 5d), indicating
the excellent reducibility of NiPC. Thus, the Ni of phthalocyanine
center is an excellent active site on account of its strong
adsorption capacity of CO2, electron-rich environment and
excellent reducibility.
We also calculated the energy required for CO2RR of NiPc
molecule. As shown in Figure S18, the energy for RDS towards
CO2RR by NiPc molecule is 3.12 eV, which is quite larger than
that of the Ni of phthalocyanine center (1.93 eV) in NiPc-NiO4.
The result indicated that the catalytic capacity of the Ni active
site has been significantly improved after the phthalocyanine
molecules are inserted into the 2D conductive MOF material.
This improvement can be reasonably ascribed to the dπ-pπ
interaction between the nickel node and the catechol in the
square-planar NiPc-NiO4.
Figure 5. (a) Calculated energy diagrams for CO2-to-CO conversion on two
proposed active sites in NiPc-NiO4. (b) The noncovalent interaction (NCI)
between CO2 and NiPc-NiO4 structure. (c) The Mulliken charge of different Ni
atoms in NiPc-NiO4. (d) The energy level of HOMO and LUMO of different Ni
atoms in NiPc-NiO4 with introduction of CO2.
EXAFS wavelet transform (WT) analysis is powerful for
discriminating the backscattering atoms in R-space, by providing
radial distance resolution and k-space resolution.[65-67] In the
EXAFS WT contour plots of Ni foil (Figure 4d), only one
intensity maximum at 7.4 Å-1 ascribed to the NiNi bond was
observed. While two intensity maximum at 4.8 and 6.9 Å-1 were
observed in NiO standard sample, which can be ascribed to the
NiO and NiNi bond, respectively. Notably, only one intensity
maximum at 3.4 Å-1 ascribed to the NiN(O) bonding was
observed and no intensity maximum corresponded to NiNi
bonding existed in NiPc-NiO4 and NiPc-NiO4-tested, further
demonstrating that the nickel species were presented with
5
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RESEARCH ARTICLE
[7]
F. P. G. De Arquer, C. Dinh, A. Ozden, J. Wicks, C. Mccallum, A. R.
Kirmani, D. Nam, C. Gabardo, A. Seifitokaldani, X. Wang, Y. C. Li, F. Li,
J. Edwards, L. J. Richter, S. J. Thorpe, D. Sinton, E. H. Sargent,
Science 2020, 367, 661–666.
Conclusion
In summary, nickel phthalocyanine molecules as active sites
were installed into nickel-catecholate-linked 2D conductive
metal-organic framework (NiPc-NiO4) nanosheets for efficiently
CO2 electroreduction reaction. Due to the in-plane full π-d
conjugation between the phthalocyanine molecule and the NiO4
nodes, NiPc-NiO4 has good conductivity of 4.8  10-5 S m-1.
Hence, the as-prepared 2D NiPc-NiO4 nanosheets show
outstanding activity towards CO2 electroreduction with nearly
100% CO selectivity, large CO partial current density up to 34.5
mA cm-2, high turnover frequency up to 2603 h-1, and excellent
long-term durability. To the best of our knowledge, such high
TOF and CO partial current density is better than any state-of-
the-art MOF catalysts. The density functional theory calculation
proves that the nickel of phthalocyanine center is the active sites
and NiPc-NiO4 performs better activity than the phthalocyanine
molecules due to the fast electron transfer capacity and
excellent reducibility, which is consistent with the experiment
results. Our work provides an effective strategy to improve the
CO2 electroreduction performance by designing conductive
crystalline frameworks with uniformly distributed phthalocyanine
active sites. It also builds a bridge between the homogeneous
molecule catalysts and heterogeneous porous catalysts.
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Keywords: Conductive metal-organic framework • CO2
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Entry for the Table of Contents
Nickel phthalocyanine molecules as active sites were installed into nickel-catecholate-linked 2D conductive metal-organic framework
nanosheets for efficiently CO2 electroreduction reaction with nearly 100% CO selectivity.
8
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