Stable Dioxin-Linked Metallophthalocyanine Covalent Organic Frameworks (COFs) as Photo-Coupled Electrocatalysts for CO2 Reduction

Article abstract of DOI:10.1002/anie.202011722

In this work, we rationally designed a series of crystalline and stable dioxin-linked metallophthalocyanine covalent organic frameworks (COFs; MPc-TFPN COF, M=Ni, Co, Zn) under the guidance of reticular chemistry. As a novel single-site catalysts (SSCs), NiPc/CoPc-TFPN COF exhibited outstanding activity and selectivity for electrocatalytic CO₂ reduction (ECR; Faradaic efficiency of CO (FE_CO)=99.8(±1.24) %/ 96.1(±1.25) % for NiPc/CoPc-TFPN COF). More importantly, when coupled with light, the FE_CO and current density (j_CO) were further improved across the applied potential range (?0.6 to ?1.2 V vs. RHE) compared to the dark environment for NiPc-TFPN COF (j_CO increased from 14.1 to 17.5 A g^?1 at ?0.9 V; FE_CO reached up to ca. 100 % at ?0.8 to ?0.9 V). Furthermore, an in-depth mechanism study was established by density functional theory (DFT) simulation and experimental characterization. For the first time, this work explored the application of COFs as photo-coupled electrocatalysts to improve ECR efficiency, which showed the potential of using light-sensitive COFs in the field of electrocatalysis.

Full text of DOI:10.1002/anie.202011722

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
International Edition Chemie
www.angewandte.org
Accepted Article
Title: Ultrastable Dioxin-Linked Metallophthalocyanine Covalent
Organic Frameworks as Photo-Coupled Electrocatalysts for CO2
Reduction
Authors: Ya-Qian Lan, Meng Lu, Mi Zhang, Chun-Guang Liu, Jiang Liu,
Jia-Nan Chang, Lin-Jie Shang, Min Wang, and Shun-Li Li
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202011722
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Stable Dioxin-Linked Metallophthalocyanine Covalent Organic
Frameworks as Photo-Coupled Electrocatalysts for CO
2
Reduction
[
1+]
[2+]
[3+]
[1]
[1]
[1]
Meng Lu, Mi Zhang, Chun-Guang Liu, Jiang Liu, Lin-Jie Shang, Min Wang, Jia-Nan
[
1]
[1]
[1]
Chang, Shun-Li Li, and Ya-Qian Lan*
Abstract: In this work, we rationally designed a series of crystalline
and stable dioxin-linked metallophthalocyanine covalent organic
frameworks (MPc-TFPN COF, M = Ni, Co, Zn) under the guidance of
irradiation strongly interferes with the electronic properties of light
sensitive electrocatalysts, thus changing the ECR catalytic
process and activity.[5] However, the studies in this field are limited
and challenging.[5-6] Therefore, it is necessary to design and
develop new photo-coupled electrocatalyst system to further
explore this novel process. For a PECR catalyst, it demands an
efficient combination of electric and light energy, and the following
requirements must be met at the same time: 1) possess effective
reticular chemistry. As
NiPc/CoPc-TFPN COF exhibited outstanding activity and selectivity
for electrocatalytic CO reduction (ECR) (Faradaic efficiency of CO
FECO) = 99.8(± 1.24) % / 96.1(± 1.25) % for NiPc/CoPc-TFPN COF).
a novel single-site catalysts (SSCs),
2
(
More importantly, when coupled with light, the FECO and current
density (jCO) were further improved at all applied potential range (-0.6
to -1.2 V vs. RHE) compared to the dark environment for NiPc-TFPN
2
catalytic active site for CO reduction; 2) have efficient light
absorption and photo-electron conversion capabilities; 3) have
good structural stability when light irradiation and electric energy
were simultaneously applied.
-1
COF (jCO increased from 14.1 to 17.5 A g at -0.9 V; FECO reached up
to ~100% at -0.8 to -0.9 V). Furthermore, an in-depth
mechanism study was established by density functional theory (DFT)
simulation and experimental characterization. For the first time, this
work explored the application of COFs as photo-coupled
electrocatalysts to improve ECR efficiency, which showed the
potential of using light-sensitive COFs in the field of electrocatalysis.
Metal-nitrogen-carbon (M-N-C) catalysts, especially metallo-
phthalocyanine based single-site catalysts (SSCs) have been
shown to have the ability to activate CO
2
.[7] In order to solve the
substrate accessibility and chemical stability of these SSCs, a
general method is to integrate these single-site molecules with
carbon substrates, such as carbon powder, carbon nanotubes, or
graphene to form hybrid catalytic materials.[8] However, this hybrid
method is still difficult to solve the problem of homogeneity of
catalytic sites and much improvement is still needed in their ECR
activity and stability.[7d]. Assembling single-site molecules into
highly ordered crystalline structures would be an ideal way to
overcome the above issues.[9] Covalent organic frameworks
Introduction
The excessive CO
environmental problems, especially the greenhouse effect and
sea-level rise.[1] Electrocatalytic CO
reduction (ECR) to high-
value-added chemicals is an efficient way to utilize CO and
2
emissions have triggered serious climate and
2
(COFs), a class of newly developed crystalline materials, which
2
have directional structural designability and high stabilities are a
promising platform for SSCs.[9-10] On the other hand,
mitigate greenhouse gas emissions, also offers opportunities for
large-scale and long-term energy storage.[2] Despite many efforts
metallophthalocyanine is
a
photosensitive 18π aromatic
devoted to the electrochemical conversion of CO
2
, the relatively
reduction are
macrocyclic compound, in addition to its efficient light absorption
capacity, its electronic properties can be greatly changed under
light driving.[11] Hence, the phthalocyanine-based materials
address the basic requirements of photo coupled electrocatalysts
for PECR. Therefore, based on the periodic structurale features
of COFs, assembling metallophthalocyanine molecules into
COFs not only ensures highly independent single metal active
sites, but also guarantees ideal uniform dispersion of them in the
whole platform. According to the above, rationally designed
crystalline metallophthalocyanine COFs will be an ideal model for
studying and realizing high efficient ECR and further applying to
photo coupled electrocatalytic reaction.
low efficiency, selectivity and poor stability of CO
still the main obstacles to the further development of ECR.[3] In
2
the field of ECR, except that CO
energy solely, the photo-coupled electrocatalytic CO
PECR) method further provides a promising way to realize its
high activity.[4] When coupled with photon energy, the light
2
reduction is driven by electric
2
reduction
(
[
1]
M. Lu,[+] Dr.J. Liu, L.-J. Shang, M. Wang, J.-N. Chang, Prof. S.-L.
Li, Prof.Y. -Q. Lan*
Jiangsu Collaborative Innovation Centre of Biomedical Functional
Materials, School of Chemistry and Materials Science, Nanjing
Normal University. No. 1, Wenyuan Road, Nanjing, 210023 (China)
E-mail: yqlan@njnu.edu.cn
Herein,
a
series
of
dioxin-linked
crystalline
[
[
2]
3]
M. Zhang,[+]
metallophthalocyanine covalent organic frameworks (MPc-TFPN
COF, M = Ni, Co, Zn) were first designed and synthesized under
the guidance of reticular chemistry. These COFs show high
stability and crystallinity even under harsh chemical environments
such as concentrated acids and base for at least 1 month. As an
exclusive SSCs for ECR, NiPc-TFPN COF and CoPc-TFPN COF
School of Chemistry, South China Normal University, Guangzhou
5
10006, P. R. China.
[+]
Prof. C.-G. Liu,
Department of Chemistry, Faculty of Science, Beihua University,
Jilin City, 132013, P. R. China
[
+]
These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202011722.
exhibited remarkable activity and selectivity for CO
conversion. The Faradaic efficiency of CO (FECO) up to 99.8(±
.24) % for NiPc-TFPN COF and 96.1(± 1.25) % for CoPc-TFPN
2
to CO
1
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RESEARCH ARTICLE
COF at -0.9 V vs. RHE, respectively, and remarkable cycle Results and Discussion
stability with neither an obvious current drop (an average of -16.3
mA cm-2 for NiPc-TFPN COF and -12.1 mA cm-2 for CoPc-TFPN
Synthesis of dioxin-linked metallophthalocyanine COFs.
As shown in Figure 1, an aromatic nucleophilic substitution
between o-difluorobenzene and catechol under the catalysis of
COF) nor large FE fluctuation was observed during 60 h ECR test.
More importantly, with the assistance of light, the FECO and the
current density were further enhanced over a wide potential range
triethylamine
(Et
,3,9,10,16,17,23,24-octahydroxyphthalocyaninato Metal (MPc-
OH, M = Ni, Co, Zn) with tetrafluorophthalonitrile (TFPN) to form
3
N)
was
applied
to
condense
(
-0.6 to -1.2 V vs. RHE) compared to the dark environment for
2
8
NiPc-TFPN COF (jCO increased from 14.1 to 17.5 mA cm-2 at -0.9
V; FECO reached up to ~100% at -0.8 to -0.9 V, which is one of
the highest in reported COFs), suggesting that the coupling of
light energy plays an important role. Besides, density functional
theory (DFT) calculations revealed the rate-determining step and
activity determining processes on the COFs based-SSCs, and
also revealed the mechanistic details of the photo coupled ECR
activity and selectivity of this family of catalysts.
three dioxin-linked phthalocyanine COFs, which were denoted as
NiPc-TFPN COF, CoPc-TFPN COF and ZnPc-TFPN COF,
respectively. The strong electron-withdrawing nitrile groups of
TFPN can enhance the electrophilicity of its C-F bond and
therefore the reactivity toward MPc-8OH.[12] Highly crystalline
MPc-TFPN COF were synthesized through the mixture of MPc-
8OH and TFPN in N, N-dimethylacetamide (DMA)/mesitylene
(
M)/Et N = 1:1:0.1 (v: v: v) at 150 °C for 3 days (Figure 1a).
3
Figure 1. Synthesis and structure of dioxin-linked metallophthalocyanine COFs. a. Schematic of the synthesis and structure of MPc-TFPN COF through the
condensation of MPc-8OH and TFPN. b. 13C ssNMR spectrum of NiPc-TFPN COF. c. Experimental, Pawley refined and simulated PXRD patterns (AA stacking
and AB stacking mode) of NiPc-TFPN COF. d. Top view of simulated AA stacking mode and e. AB stacking mode for NiPc-TFPN COF crystal structure. f. Side
view of AA stacking mode for NiPc-TFPN COF crystal structure.
bands at 1277 cm-1 and 1010 cm , which belong to the dioxin
C−O asymmetric and symmetric stretching modes (Figures S3-
5).[12] All these results confirmed the successful condensation
reaction required for the dioxin-linked MPc-TFPN COFs structure.
Crystal Structure. The crystal structure of MPc-TFPN COF
was determined by the powder X-ray diffraction (PXRD)
combined with theoretical structural simulations using Materials
Studio package (Figure 1c). The AA (eclipsed) stacking model
(Figure 1d) with P4/mmm (No.123) space group based on NiPc-
TFPN COF was built and then carried out Pawley refinements of
-1
The formation of the dioxin linkages in the MPc-TFPN COF
were first confirmed by solid state 13C cross-polarization magic
angle spinning (ss 13C CP/MAS) NMR spectroscopy and Fourier
Transform Infrared Spectroscopy (FTIR). The resonance signals
at 141 and 150 ppm in NMR as shown in Figure 1b, which are
characteristic of the carbons of C−O and the signal at 95 ppm is
assigned to aromatic carbons connected to nitriles, whereas
those at 109 and 135 ppm are assigned to the nitrile carbons and
aromatic carbons. Similarly, the 13C CP-MAS spectrum of CoPc-
TFPN COF and ZnPc-TFPN COF can also be fully assigned
(Figures S1-2). The FTIR spectrum revealed stretching vibration
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RESEARCH ARTICLE
77.3 K (Figure 2b, Figures S18-19). The measured pore sizes
(
~1.2 nm) for NiPc-TFPN COF were consistent with the
theoretical results (Figure 2b, inset). The Brunauer–Emmett–
Teller (BET) surface area of NiPc-TFPN COF was found to be
2
−1
2
2
52.375 m g . Appreciable amounts of CO adsorption on MPc-
TFPN COF were observed (Figures S20-22), which should be
attributed to not only the combination of microporous character
and the heteroatom-rich channels in the MPc-TFPN COF, but also
a Lewis acid-base interaction between the coordinated metal ions
and absorbed CO
2
molecules.[14]
Figure 2. Characterization and morphology of MPc-TFPN COF. a. Chemical
2
stability of NiPc-TFPN COF. b. N adsorption isotherm of NiPc-TFPN COF. Inset
is the pore size distribution of NiPc-TFPN COF. c. HRTEM image and lattice
fringes of NiPc-TFPN COF. d. Element mapping of NiPc-TFPN COF.
the PXRD patterns for full profile fitting against the proposed
models, which provided a unit cell parameter of a = b = 20.3180
Å and c = 3.4545 Å, α = β= γ = 90°. The calculated PXRD
diffraction patterns matched well with the experimental results,
with Rwp = 3.63% and Rp = 2.85%. The AB (staggered) stacking
structure was also simulated (Figure 1e), while the comparison of
its PXRD profile with the experimental one showed significant
deviations, ruling out the AB stacking model. NiPc-TFPN COF
exhibited diffraction peaks at 4.37°, 6.17° and 8.71°, which are
assigned to the (100), (110) and (200) planes, respectively. The
simulated crystal structure showed the layer distance of NiPc-
TFPN COF is 3.455 Å (Figure 1f).
In MPc-TFPN COF, the four nitrogen atoms located in the
center of phthalocyanine can coordinate with different metal
atoms, thus realizing the modulation of COFs structure at the
atomic level. As shown in the PXRD patterns in Figure S6,
CoPc/ZnPc-TFPN COF also displayed crystalline structure. The
comparison of PXRD patterns of MPc-TFPN COF with those of
TFPN and MPc-8OH monomers were also conducted (Figures
S7-9), which proved that MPc-TFPN COF has high purity. More
details and raw data are given in the Supporting Information. As
we know, most of the previously reported phthalocyanine COFs
exhibited poor structural chemical stability[13], while as dioxin-
linked phthalocyanine COFs formed from irreversible reactions,
MPc-TFPN COFs displayed high thermal and chemical stability.
The thermal stabilities of MPc-TFPN COFs were confirmed by
thermogravimetric analysis (TGA, Figures S12-17). Take NiPc-
TFPN COF as an example, it showed no obvious change up to
2
Figure 3. Electrocatalytic CO reduction performances of MPc-TFPN COF. a.
LSV curves for the MPc-TFPN COF and contrast catalysts. b. The faradic
efficiency of carbon monoxide calculated overpotential range from -0.5 to -1.2
V. Inset is the FECO and FEH2 of different COFs. c. Tafel plots. d. Partial CO
-
1
current density and TOF(h ). e. Stability of NiPc-TFPN COF and CoPc-TFPN
COF at -0.9 V vs. RHE. The error bars represent the standard deviation of three
independent measurements. All the above tests were conducted in a CO
saturated 0.5 M KHCO aqueous solution under dark environment.
2
-
3
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were performed to characterize the
morphology of MPc-TFPN COF, which showed that NiPc-TFPN
COF was composed of microcrystalline particles with 200-300 nm
in diameter (Figures S23-25). The structural characteristics of
NiPc-TFPN COF were visualized by high resolution transmission
electron microscopy (HRTEM). As shown in Figure 2c, NiPc-
TFPN COF displayed clear lattice fringes of (001) crystal face,
also confirmed the simulated crystal structure. Energy-dispersive
X-ray spectroscopy (EDX) analysis revealed that C, N, O, and Ni
are uniformly distributed over NiPc-TFPN COF (Figure 2d).
Additionally, from EDX, Co and Zn are also uniformly distributed
over CoPc-TFPN COF and ZnPc-TFPN COF (Figures S26-27),
respectively. X-ray photoelectron spectroscopy (XPS) was
performed to explore the metal valence states in MPc-TFPN COF
~
400 °C under nitrogen and ~300 °C under oxygen atmosphere.
The chemical stability of NiPc-TFPN COF was examined by
immersing it into different solvents (including different commonly
used organic solvents such as N-Methyl-2-pyrrolidone (NMP), N,
N-dimethylformamide
(DMF),
tetrahydrofuran
(THF),
dichloromethane (DCM), ethanol (EtOH) and acid/base such as
concentrated HCl (12 M), concentrated NaOH (12 M) and 0.5 M
KHCO
differences were observed from the PXRD after the treatment
Figure 2a), indicating its structural robustness. The porosity of
these COFs was assessed by nitrogen sorption measurements at
3
)) at room temperature for at least 1 month. No obvious
(
(Figures S28-33). In the XPS spectrum of Ni 2p for NiPc-TFPN
COF, the peaks at 855.7 eV (Ni 2p 3/2) and 873.0 eV (Ni 2p 1/2)
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RESEARCH ARTICLE
correspond to the +2 state of Ni. For CoPc-TFPN COF and ZnPc-
TFPN COF, both cobalt and zinc are also divalent from XPS
results. The metal content in NiPc-TFPN COF, CoPc-TFPN COF
and ZnPc-TFPN COF were determined to be 5.39 wt.%, 5.48
wt.% and 5.08 wt.% respectively by inductively coupled plasma
optical emission spectrometry (ICP-OES).
impedance spectroscopy (EIS) was also conducted to investigate
the charge transfer resistance (Figure S37). The Nyquist plots
demonstrated that NiPc-TFPN COF has much smaller charge
transfer resistance than others due to its smallest semicircle in the
high frequency region. We then calculated the partial current
2
densities of CO (jCO) and H (jH2) at a wide range of potentials
Electrochemical CO
2
reduction activity of COFs. Based on
(Figure 3d). The results demonstrated that jCO of NiPc-TFPN COF
is larger than those of CoPc-TFPN COF at -0.7 to 1.1 V, whilst at
lower potentials (< -0.7 V), CoPc-TFPN COF performs better.
According to the above experimental results, we can see that
the above illustration of the MPc-TFPN COF crystal structure, it
can be seen that MPc-TFPN COF represent highly uniformly
dispersed single metal sites (the distance between the two
adjacent single metal atoms is 2.03 nm), making these COFs an
ideal SSCs for ECR. The ECR activities for NiPc-TFPN COF,
CoPc-TFPN COF, ZnPc-TFPN COF and COF-316 were first
2
NiPc-TFPN COF's CO to CO selectivity and current density are
higher than CoPc-TFPN COF in a wide potential range, which
leads us to the conclusion that NiPc-TFPN COF shows optimal
performance for ECR among these COFs materials. This result is
evaluated, respectively, in
separated by Nafion ®212) with a CO
a
two-compartment H-type cell
-saturated 0.5 M KHCO
(
2
3
consistent with the results previously reported for NiPc or Ni-N
4
solution (pH = 7.2) as electrolyte in dark environment. In classic
ECR experiments, we use current density (j) and faradaic
efficiency of the reduction products as the indices to evaluate the
performance.[15] As revealed by the linear sweep voltammetry
curves (LSV) (Figure 3a), all MPc-TFPN COF has a small onset
potential (-0.42 V for CoPc-TFPN COF, -0.48 V for NiPc-TFPN
based catalysts.[7a, 7d, 8c] Besides, the turnover frequency (TOF) of
NiPc-TFPN COF was calculated to be 490 h−1 at −0.9 V, large
than CoPc-TFPN COF(369 h−1 at −0.9 V).
COF and -0.22 V for ZnPc-TFPN COF, versus RHE) on CO
2
-
saturated 0.5 M KHCO solution. Interestingly, at low (< -0.7 V)
3
and high potentials (> -1.1 V), the total current density (jtotal) of
CoPc-TFPN COF is higher than that of NiPc-TFPN COF, while at
wide range medium potentials (-0.7 to -1.1 V), NiPc-TFPN COF
performs better than CoPc-TFPN COF. In addition to current
density, the selectivity in ECR was also investigated. We tested
all the COFs under wide potentials ranges from -0.5 V to -1.2 V,
and obtained the corresponding FE values (Figure 3b). It was
2
noted that the total FE of CO and H was found to be ~100% over
the entire tested potential range, and no other products were
detected. Good reproducibility was observed in measurements of
three independently prepared samples. Our results showed that
both CoPc-TFPN COF and NiPc-TFPN COF exhibited excellent
ECR catalytic activity and selectivity. In particular, the maximal
FECO can reach up to 99.8(± 1.24) % with a partial current density
CO) of ~14.1 mA cm-2 (A g ) at -0.9 V versus RHE (Figure 3b
-1
(j
and d) for NiPc-TFPN COF. While as for CoPc-TFPN COF, the
maximal FECO can reach to 96.1(± 1.25) % with a partial current
density (jCO) of ~10.6 mA cm-2 at the same conditions for NiPc-
TFPN COF, which is consistent with other reports of using Co-
phthalocyanine as catalyst for ECR.[8a, 8b, 16] However, for contrast
samples, COF-316 (without metal phthalocyanine but with TFPN
unit, also known as JUC-505, Figure S11) [12] and ZnPc-TFPN
COF only showed relatively low catalytic efficiency (FECO = 10.4%
and 22.9% for COF-316 and ZnPc-TFPN COF, respectively)
2
Figure 4. Evaluation of CO reduction performance of NiPc-TFPN COF by
photo-coupled electrochemical measurements. a. Schematic of photo-coupled
electrochemical measurements method. b. Photo-current response curve of
NiPc-TFPN COF at different bias voltage in H-cell. c. Comparison of LSV curves
of NiPc-TFPN COF under dark and light. d. FECO, e. Tafel plot of NiPc-TFPN
COF under dark and light. f. jCO, and TOF at different potentials under dark and
light. The error bars represent the standard deviation of three independent
measurements.
(
Figure 3b, inset). The ECR activity of MPc-8OH monomers was
also tested (Figures S34-36), while the highest FECO of them only
reaching ~85 % due to the sluggish diffusion of CO . These
results show that the crystalline structure of COFs and its
2
metallophthalocyanine as SSCs are particularly important for CO
reduction.
2
Long-term stability remains
phthalocyanine based catalysts despite their effectiveness in the
to CO.[16a] Time-dependent total current
electroreduction of CO
density of NiPc-TFPN COF was first tested at wide potentials from
0.5 to -1.2 V. All of them remained stable during 1 h, suggesting
a
challenge for metallo-
Tafel slopes were calculated to elucidate the dynamic activity
of MPc-TFPN COF for ECR (Figure 3c). The results showed that
the Tafel slope for NiPc-TFPN COF is 209.9 mV dec −1, which is
much smaller than that of CoPc-TFPN COF (570 mV dec −1) and
ZnPc-TFPN COF (230.9 mV dec −1). This indicates that NiPc-
2
-
the sufficient stability of the catalyst (Figures S38-39).
Furthermore, we found NiPc/CoPc-TFPN COF catalysts can be
2
TFPN COF has good CO to CO kinetics, which may be attributed
to the high initial electron transfer efficiency and large active
surface during the catalytic process.[17] The electrochemical
continuously operated at least 60 h with well maintained current
density of ~16.3 mA cm−2 / ~12.1 mA cm−2 and a stable FECO of
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
9
8.0 ~ 99.9% / 94.5 ~ 97% in an H-cell at a constant potential of
of NiPc-TFPN COF were retained after the PECR reaction, as
confirmed by PXRD, FTIR, and XPS characterizations (Figures
S44-46). All of these confirmed that NiPc-TFPN COF materials
are efficient and selective photo-coupled electrocatalyst for ECR.
Tafel slopes were calculated to elucidate the catalytic kinetics
of these electrocatalysts under dark and light environments,
respectively. The results showed that the Tafel slope for NiPc-
−0.9 V, suggesting their remarkable stability for long-term
operations (Figure 3e). In order to confirm that the produced CO
indeed comes from the reduction of CO
, the 13C isotope labeling
experiments were performed. When using 13CO
as substrates,
13CO (m/z = 29) was finally detected by using mass spectrometry,
which confirmed that the produced CO originates from the
2
2
−
1
reactant CO
Photo-coupled electrochemical CO
COFs. Phthalocyanine, as photosensitive 18π aromatic
2
instead of decomposition of catalysts (Figure S40).
TFPN COF under light is 173.7 mV dec , which is much smaller
−
1
2
reduction activity of
than that under dark (209.9 mV dec ) (Figure 4e). This value is
-1
a
higher than 118 mV dec , suggesting that the adsorbed
+
macrocyclic compound, its electronic properties can be
dramatically altered under light driving, thus may alter its intrinsic
activity and energy efficiency.[11, 18] Phthalocyanine moieties also
possess efficient light absorption capabilities.[11, 19] Besides, many
previous works have proved that the light field effect plays an
important role in electrocatalytic processes.[5, 6b, 20] In view of this,
we proposed that light irradiation probably facilitates
CO
2
receiving an electron and combining with H from the
−
electrolyte to form *COOH (the adsorbed *CO
2
+ [H+ + e ] →
*COOH) is the rate determine step (RDS).[21] The Tafel slope
drastically decreased under light indicated that external light-field
can accelerate the electron transfer in phthalocyanine COFs.[5] To
better investigate the effect of light irradiation on the catalytic
active sites, we calculated the TOF and
jCO at different
electrochemical activation of CO
2
molecule over phthalocyanine
overpotentials (Figure 4f). These results showed that both TOF
and jCO are significantly enhanced in a wide potential range (-0.5
V to -1.2 V), and the activation effect by light irradiation was more
obvious with the increase of overpotential. Based on the above
experimental results, we can concluded that coupled with light can
strongly enhance the ECR activity by interfering with the electron
transfer characteristics of phthalocyanine COFs, which is
COFs catalyst. To verify this assumption, we carried out the
PECR test for MPc-TFPN COF as shown in Figure 4a. Before
evaluating the PECR activity of these photo coupled
electrocatalysts, we conducted a photo-current response test of
the COFs material at different potentials in a CO
2
saturated 0.5 M
KHCO solution (NiPc-TFPN COF was taken as an example due
3
to its strongest light absorption capacity as shown in Figure S41,
as well as its optimal ECR performance). The experiment results
showed that the current density increases obviously under the
trigger of light excitation (Figure 4b). For comparison, we also
tested the photo-current response of the NiPc-TFPN COF in an
2
conducive to the reduction of CO .
We further studied the effect of light irradiation on ECR activity
of CoPc-TFPN COF and ZnPc-TFPN COF (Figures S47-54). The
photo-current response test of the CoPc-TFPN COF and ZnPc-
TFPN COF material at different potentials showed that the total
current density also increases obviously under the trigger of light
excitation. For CoPc-TFPN COF, the ECR test results show that
when coupled with light, the FECO only exhibited slightly higher
compared to the dark environment in the narrow potential range
2 3
N -bubbling 0.5 M KHCO solution, which only showed a very
small photo-current response (Figure S42). The above results
indicate that light excitation can indeed enhance the activation of
CO
2
, thus increasing the catalytic efficiency of electrocatalytic
reduction reaction.
CO
2
of -0.8 to -1.1 V vs. RHE. While for ZnPc-TFPN COF, the FECO
CO and TON all showed much higher than that measured under
dark at all applied potentials, implying that the photo-excited
phthalocyanine COFs is beneficial to CO reduction. According to
,
LSV curves were then tested and compared under dark and
light conditions, respectively (Figure 4c). The current density of
NiPc-TFPN COF in N -saturated electrolyte (black curve) was
relatively low but was far increased in CO -saturated electrolyte
blue curve), which proved that CO participates in the reaction,
j
2
2
2
the ECR performance of these three kinds of COFs under light
excitation, the following conclusions can be obtained: 1). For
NiPc-TFPN COF and ZnPc-TFPN COF, the ECR performance of
both of them were greatly improved in the relatively low
overpotential range (from -0.5 to -0.7 V), while the performances
of CoPc-TFPN COF were almost unchanged. Previous reports
have shown that external light irradiation can facilitate internal
electrons transfer for light sensitive catalysts [5] However, the Tafel
plot test results imply that CoPc-TFPN COF has a very large Tafel
slope (Figure S50), indicating that it has intrinsic low dynamic
activity and low electron transfer efficiency. Hence, the effect of
light excitation on its electron transfer is very limited, making it
hardly to improve the performance under light irradiation.
However, at more negative potential (from -0.8 to -1.2 V), the jCO
and TOF are increased obviously, which indicates that the
synergistic effect of high potential and photoexcited electrons are
more significant, therefore accelerate the catalytic efficiency. 2).
For the FECO, jCO and TOF of ZnPc-TFPN COF, the general trend
is that the performance peak position moves to more positive
potential, confirming that external light-field can reduce the
(
2
that is, the ECR process occurs. More importantly, with the
assistance of light (red curve), the current density of NiPc-TFPN
COF was further improved obviously (increased from ~14.1 to
~
17.5 mA cm-2 at -0.9 V, versus RHE). All the above results
suggested that the ECR activity of the catalyst will be enhanced
under light assistance. To further prove the above conclusion, the
effect of light on FE at different potentials was also investigated.
When coupled with light, the FECO of NiPc-TFPN COF exhibited
higher than 90% in a wide potential range from −0.6 to −1.2 V and
the maximum FECO can reach up to almost 100% at −0.8 V to -
0
.9V (Figure 4d), which is one of the highest among reported
COFs (Table S1). In contrast, the potential range of when FECO
0% in the dark environment was only -0.8 to -1.0 V. Moreover,
>
9
the results also indicated that the overpotential for the maximum
FECO has a positive shift about 0.1 V, implying that the photo-
2
excited Ni-phthalocyanine COFs is beneficial to CO reduction
under relatively low potential. The photo-coupled ECR stability of
NiPc-TFPN COF was tested under light irradiation at -0.9 V vs
RHE, although its current density showed a very small decrease
within 16 hours, which may be caused by photo corrosion or
sample shedding; meanwhile, the FECO remained almost
unchanged (Figure S43). The crystallinity and structural integrity
2
overpotential for CO activation by affect electronic property of
catalysts.
Investigating structure-property relationships. In view of
the precise crystal structure of MPc-TFPN COF, we then studied
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Angewandte Chemie International Edition
10.1002/anie.202011722
RESEARCH ARTICLE
its structure-property relationships for ECR by combining
between S
0
and S
1
is 2.05 eV, which is consistent with the band
theoretical calculations with experiments. Among these
gap calculated from the measured UV-vis absorption spectrum,
which also proved the accuracy of the calculation model systems.
elementary reaction steps, the adsorption of CO
then accompanied by a proton-coupled electron transfer step to
generate *COOH was calculated to be the RDS for CO to CO on
2
on the MPc and
1
The optical transition energy of T state is 0.80 eV (vs. ground
2
state), indicating that under light conditions, the phthalocyanine
COFs catalysts will be excited to a higher energy excited state. In
addition to enhancing the electron transfer from the ligand to the
MPc-TFPN COFs. DFT calculation results indicate that in our
active center, the COFs catalyst in the T
1
excited state is also
more conducive to the activation of CO
2
to *COOH due to the
reduced free energy change compared to the ground state.[5] This
explains the inherent reason why NiPc-TFPN COF performers
better ECR activity under the irradiation of light.
Conclusion
In this work,
a series of highly stable dioxin-linked
metallophthalocyanine 2D-COFs were rationally designed. The
crystal structure suggested that MPc-TFPN COF possesses
highly uniformly dispersed single metal sites, which coordinate in
the center of phthalocyanine, making these COFs ideal SSCs. At
first, its performance for ECR was systematically studied, and the
results showed that NiPc-TFPN COF and CoPc-TFPN COF
exhibited superior activity and selectivity (FECO = 99.8(± 1.24) %
for NiPc-TFPN COF and 96.1(± 1.25) % for CoPc-TFPN COF at -
0.9 V vs. RHE, respectively), also shown ultra-long cyclic stability
(60 h). More importantly, by coupling with light, the jCO and FECO
Figure 5. Mechanism and DFT calculations. a. Comparison of the relative
+ 2H+ + 2e → *CO
-
+ 2H+
adsorption energy for first step of ECR (* + CO
2
2
+
e-) on metal site (red) and nitrogen site (blue) on MPc-TFPN COF. b. The free
2
energy diagrams for CO
2
reduction to CO on MPc-TFPN COF (Note that the
further showed obvious enhancement compared to tested in the
dark environment (jCO increased from 14.1 to 17.5 mA cm-2 at -0.9
symbol in brackets represent the reaction sites). The rate-determining step and
the corresponding free energy on each COFs are indicated. c. Schematic
representation of the excited states of NiPc-TFPN COF.
V. FECO reached up to ~100% at -0.8 to -0.9 V and the
overpotential for the maximum FECO has a positive shift about 100
mV), indicating higher performance for PECR. Mechanism study
revealed that external light-field can enhance the electron transfer
COFs system, the N atom in the Pc ligand plays an important role
in adsorbing and activating CO
CoPc-TFPN COF, the free energy change for CO
central metal is much higher than N site (N , which means N in
MPc, which is highlighted with blue background in Figure 5a).
Therefore, according to the minimum energy principle, CO
molecule tends to be adsorbed and activated on N atoms in the
2
. As for NiPc-TFPN COF and
2
adsorption on
to the adsorbed CO
in phthalocyanine COFs, which is conducive
. This is the first report of dioxin-linked
2
M
to the reduction of CO
2
metallophthalocyanine COFs, which show excellent performance
for ECR and PECR. Our results also highlight that external light
irradiation has a significant effect on the ECR activity and product
selectivity for light-sensitive electrocatalysts.
2
M
+
-
+
-
first step of ECR (* + CO
these two catalysts. While Zn atom on ZnPc-TFPN COF showed
lower free energy change of CO to *CO compared to N site (NZn).
Thus it is more reasonable that the first step of CO activation is
carried out on Zn atom for ZnPc-TFPN COF. The calculated
Gibbs free energy diagram for CO reduction processes on MPc-
TFPN COF was shown in Figure 5b and S57-59. The calculated
ΔG for NiPc-TFPN COF from adsorbed *CO to *COOH is lower
than that of CoPc-TFPN COF and ZnPc-TFPN COF, indicating a
higher CO reduction activity, which is in line with our
2 2
+ 2H + 2e → *CO + 2H + 2e ) on
2
2
2
Acknowledgements
2
This work was financially supported by NSFC (No. 21871141,
21871142 ， 21701085 and 21901122); the NSF of Jiangsu
2
Province of China (No. BK20171032); the Natural Science
Research of Jiangsu Higher Education Institutions of China (No.
17KJB150025 and 19KJB150011) and Project funded by China
2
experimental results. We also studied the possible transfer
process of *COOH between metal and N in MPc during the
reaction, as shown in Figures S60-61. However, these results
indicate that transferring of *COOH group from the metal center
to the N atom of the MPc ligand is kinetically unfavorable because
of the high activation energy barrier, see Supporting information
for details.
Postdoctoral Science Foundation (No.
019M651873 and 2020M682748); Postgraduate Research &
Practice Innovation Program of Jiangsu Province
KYCX20_1169); Priority Academic Program Development of
2018M630572,
2
(
Jiangsu Higher Education Institutions and the Foundation of
Jiangsu Collaborative Innovation Center of Biomedical Functional
Materials.
In order to understand the effect of light on catalytic activity, the
electronic properties of S
phthalocyanine COFs catalysts were calculated. After light
irradiation, the NiPc-TFPN COF was first excited from S to S
state and then spontaneously transformed to T state through the
process of intersystem crossing (Figure 5c). The energy gap
1 1
and T excited states of these
0
1
Conflict of interest
1
The authors declare no conflict of interest.
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Angewandte Chemie International Edition
10.1002/anie.202011722
RESEARCH ARTICLE
Keywords: covalent organic frameworks • electrocatalytic CO
reduction • metallophthalocyanine • photo-coupled
electrocatalysts
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RESEARCH ARTICLE
RESEARCH ARTICLE
Text for Table of Contents
Meng Lu, Mi Zhang, Chun-Guang Liu,
Jiang Liu, Lin-Jie Shang, Min Wang,
Jia-Nan Chang, Shun-Li Li, and Ya-
Qian Lan*
A
series of stable dioxin-linked
metallophthalocyanine covalent
organic frameworks were developed
and applied for photo-coupled
Page No. – Page No.
2
electrocatalytic CO reduction. This
work represents a new insight for the
future rational design of light sensitive
Stable Dioxin-Linked
Metallophthalocyanine Covalent
Organic Frameworks as Photo-
crystalline
reduction.
materials
for
CO
2
Coupled Electrocatalysts for CO
2
Reduction
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