A Stable and Conductive Metallophthalocyanine Framework for Electrocatalytic Carbon Dioxide Reduction in Water

Article abstract of DOI:10.1002/anie.202005274

Transformation of carbon dioxide to high value-added chemicals becomes a significant challenge for clean energy studies. Here a stable and conductive covalent organic framework was developed for electrocatalytic carbon dioxide reduction to carbon monoxide

Full text of DOI:10.1002/anie.202005274

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: A Stable and Conductive Metallophthalocyanine Framework for
Electrocatalytic Carbon Dioxide Reduction in Water
Authors: Donglin Jiang, Ning Huang, Ka Hung Lee, Stephan Irle,
Qiuhong Jiang, Yan Yue, and Xiaoyi Xu
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10.1002/anie.202005274
Angewandte Chemie International Edition
RESEARCH ARTICLE
A Stable and Conductive Metallophthalocyanine Framework for
Electrocatalytic Carbon Dioxide Reduction in Water
Ning Huang,[a],[b] Ka Hung Lee,[c],[d] Yan Yue,[b] Xiaoyi Xu,[b] Stefan Irle,[c],[d] Qiuhong Jiang,[a] and
Donglin Jiang*,[a],[e]
[a]
Prof. Dr. N. Huang, Dr. Q. Jiang, Prof. D. Jiang
Department of Chemistry, Faculty of Science
National University of Singapore,
3 Science Drive 3, Singapore 117543, Singapore
E-mail:
[b]
Prof. Dr. N. Huang, Dr. Y. Yue, X. Xu
MOE Key Laboratory of Macromolecular Synthesis and Functionalization
State Key Laboratory of Silicon Materials, Department of Polymer Science and Engineering
Zhejiang University
Hangzhou 310027, China
[c]
[d]
[e]
K. H. Lee, Prof. S. Irle
Bredesen Centre for Interdisciplinary Research and Graduate Education
University of Tennessee
Knoxville, TN 37996, U.S.A.
Prof. S. Irle
Computational Sciences and Engineering Division & Chemical Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831, U.S.A.
Prof. D. Jiang
Joint School of National University of Singapore and Tianjin University
International Campus of Tianjin University
Binhai New City, Fuzhou 350207, China
Supporting information for this article is given via a link at the end of the document.
Abstract: Transformation of carbon dioxide to high value-added
chemicals becomes a significant challenge for clean energy study.
Here we developed a stable and conductive covalent organic
framework for electrocatalytic carbon dioxide reduction to carbon
monoxide in aqueous solution. The cobalt(II) phthalocyanine catalysts
are topologically connected via robust phenazine linkage into two-
dimensional tetragonal framework that is stable under boiling water,
acid, or base conditions. The 2D lattice enables full π conjugation
along x and y directions well as π conduction along the z axis across
the π columns. With these structural features, the electrocatalytic
framework exhibits a faradaic efficiency of 96%, an exceptional
turnover number up to 320,000 and a long-term turnover frequency of
11,412 hour−1, which is 32-fold increment compared to molecular
catalyst. The combination of catalytic activity, selectivity, efficiency,
and durability is desirable for clean energy production.
from metals and their alloys to metal oxides and chalcogenides,
metal-organic complexes, and carbon materials have been
investigated for the electrocatalysis of CO2.[2] For example, metal-
organic complexes show much far superior turnover frequency to
others. However, they suffer from drawbacks of tedious synthesis,
high overpotential, and toxicity.[2a] On the other hand, noble
metals such as Au nanowires and Au-metal oxide hybrids
promote the reaction at the lowest overpotential. However, the
scarcity makes them unsuitable for application.[2c,2d] Exploring an
efficient electrocatalyst to combine activity (low overpotential high
current density, and high turnover frequency) with selectivity,
durability, and availability remains a substantial challenge.[2]
We sought to explore highly stable catalytic materials by
connecting molecular catalysts with robust covalent bond into
two-dimensional covalent organic frameworks (2D COFs). Owing
to the structural diversity, crystallinity, and porosity,[3] COFs have
been developed to show various functionalities, including
semiconductors,[4] light emitter,[5] heterogeneous catalysis,[6]
adsorption and separation,[7] and energy conversion.[8] The
unique structural features make COFs compelling candidates as
catalysts for electrochemical CO2 reduction.[9] We rationalized that
COFs could potentially achieve stability, conductivity, and
catalytic activity for electrochemical CO2 reduction: (1) The
stability and durability can be achieved by using strong covalent
bonds to connect catalytic sites into a robust framework, which
cannot leak the catalytic sites; (2) the conductivity can be realized
by using conjugated bonds to connect the catalytic sites and using
tetragonal topology to create a fully π-conjugated network; (3) the
catalytic activity and efficiency will be enhanced as electrons from
electrodes can be smoothly transported to the reaction center via
the shortest pathway in a highly conductive and ordered network.
Introduction
The global climate change and energy demands underpin
growing attention on exploring new technologies to convert
carbon dioxide (CO2) to carbon monoxide (CO) and other value-
added chemicals.[1] Electrochemical reduction of CO2 is an ideal
route to realize a carbon neutral clean energy. Progress over the
past decade has greatly deepened our understanding of the
electrochemical process involved in CO2 reduction.[2] An ideal
electrocatalyst to pursuit is the one that could combine catalytic
activity, selectivity, durability and availability in one material while
catalytic activity relies on overpotential, current density, and
turnover frequency and selectivity is determined by faradaic
efficiency (FE). Remarkably, a wide range of materials ranging
1
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10.1002/anie.202005274
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RESEARCH ARTICLE
Figure 1. Design and synthesis of a conductive and stable electrocatalytic COF. a) Synthesis of CoPc-PDQ-COF with phenazine linkage by condensing [NH2]8CoPc
with PDQ. Reproduced crystal structure of (b) top and (c) side views of CoPc-PDQ-COF (purple, N; red, Co; green, C; white, hydrogen). d) Experimentally observed
PXRD profiles of CoPc-PDQ-COF (black), Pawley refined profile (green) and their difference (orange), simulation results based on AA (blue) and AB (red) stacking
manners. e) Reconstructed structure of CoPc-PDQ-COF in the AA stacking manner. f) Reconstructed structure of CoPc-PDQ-COF in the AB stacking manner.
However, fully exploring these structural features to design COFs-
based electrocatalyst is still unprecedented.
high electrocatalytic activity, we selected metallophthalocyanines
as knots, which are well known for electrocatalytic reduction of
CO2. We designed the phenazine-linked metallophthalocyanine
COF by using pyrene units as linkers, which can extend π
conjugation along the x and y axis over the 2D sheets while the
stacking layer structure offers the conduction along the z direction
across the π columns, so that electrons directionally transport to
catalytic sites over the shortest distance. Moreover, the built-in
pores adsorb CO2 and this adsorption shortens the electron
transfer distance to CO2 and facilitates the reduction reaction.
This design strategy creates distinct interfaces that are key to the
reduction of CO2 as it combines durability, electric conductivity,
catalytic activity, and selectivity in one catalyst.
Here we report a unique catalytic COF for electrochemical
CO2 reduction in water, by exploring the robust phenazine linkage
to connect metallophthalocyanine catalytic sites into a fully π-
conjugated lattice. The phenazine linkage is critical as it endows
the resulting framework with stability and conductivity. It enables
topological link of metallophthalocyanine knots at a regular
distance of only 2.2 nm to form stacked tetragonal lattice.
Therefore, the catalytic sites are fused to allow electron
conduction along both the x and y axis while the π columns also
facilitate the electron transfer along the z direction perpendicular
to the 2D layer. With these structural features in one material, the
resulting COF catalyst enables the combination of durability,
conductivity, catalytic activity, and efficiency, which is hardly
accessible with other molecular frameworks and amorphous
polymers. We observed that it catalyzed the reduction of CO2 to
CO in water with high selectivity, exceptional turnover frequency,
and long-term catalytic durability, which outperform most state-of-
the-art solid-state and molecular electrocatalysts.[1, 2, 6]
Synthesis and characterization. The phenazine-linked
metallophthalocyanine CoPc-PDQ-COF (Figure 1a–c) was
synthesized
by
polycondensation
of
cobalt(II)
2,3,9,10,16,17,23,24-octakis(amino)
phthalocyanine
([NH2]8CoPc) with 4,5,9,10-pyrenediquinone (PDQ) in a mixture
of dimethylacetamide (DMAc) and ethylene glycol with acetic acid
catalyst at 200 °C. After one week, CoPc-PDQ-COF was collected
as shiny black powder in a yield of 85% (see Supporting
Information). The crystalline structure of CoPc-PDQ-COF was
resolved by PXRD analysis in combination with computational
simulations. The CoPc-PDQ-COF exhibited high crystallinity with
a series of strong PXRD peaks at 3.56°, 7.12°, 10.72°, and 25.84°,
which were assigned to (100), (200), (300), and (001) facets,
respectively (Figure 1d, black curve). Based on Scherrer equation,
the crystallite size of CoPc-PDQ-COF was calculated to be 13.6
nm, which was smaller than an average size of 19.8 nm observed
in electron microscopes. The tetragonal skeletons were simulated
with density functional tight-binding (DFTB+) calculation to obtain
optimum structure of layered lattice binding. Pawley refinement of
experimental PXRD patterns verified the assignment of these
diffraction peaks and exhibited good consistency with the
observed PXRD patterns (Figure 1d, black curve), as confirmed
Results and Discussion
Design strategy. COFs have been synthesized by using different
linkages ranging from boronate ester to imine and C=C bonds.[3]
Among various linkages, the phenazine ring is unique as it
enables the use of π-conjugated ring to fuse knot and linker units
into extended 2D π lattices.[4a,10] This linkage has two distinct
features: one is the stability as the phenazine rings are fused and
six-membered aromatic systems with exceptional chemical and
thermal stabilities; another one is that it enables the design of fully
π-conjugated lattice that is conductive. These structural
advantages are hardly accessible with other linkages or other
porous frameworks. To endow the phenazine-linked COFs with
2
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10.1002/anie.202005274
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RESEARCH ARTICLE
by their inappreciable difference (Figure 1d, orange curve).
Structural modeling using P4/M group space No. 83 with lattice
parameters α = β = γ = 90, a = b = 24.793 Å, and c = 3.36 Å gave
a set of PXRD patterns (Figure 1d, green curve), which are in
good accordance with the experimental data. The AA stacking
lattice structure is illustrated in Figure 1e. On the contrary, a
staggered lattice offset by the distance values of a/2 and b/2 (AB
stacking, Figure 1f) cannot regenerate the experimental PXRD
peaks (Figure 1d, blue curve). These results demonstrate that the
condensation between [NH2]8CoPc and PDQ forms CoPc-PDQ-
COF with AA-stacked 2D layers. The PXRD peak at 25.84°
reveals that CoPc-PDQ-COF is also ordered along the z axis with
a short interlayer distance of 3.36 Å.
activation energy was calculated to be 0.302 eV from the
Arrhenius plot based on variable temperature current
measurements (Figure 2c, inset). The high conductivity of CoPc-
PDQ-COF originates from the fully π-conjugated yet stacked
structure that allows electron transport over the whole skeleton.
The CoPc-PDQ-COF was unambiguously characterized using
various methods (Figures S1–S13). Fourier transform infrared
(FT-IR) spectroscopy revealed the phenazine linkage formation
with characteristic vibration peaks at 1,521, 1,433 and 1,354 cm–
1. At the same time, the vibration bands of amino groups at 3,384
and 3,313 cm–1 and quinone units at 1,676 cm–1 disappeared
(Figure S1). Elemental analysis exhibited the contents of C, H,
and N were 68.84%, 3.12%, and 18.79%, respectively, which
were on the verge of the theoretical values of 71.17%, 2.99%, and
19.15%, respectively (Table S1). Inductively coupled plasma
mass spectrometry revealed that the cobalt content was 4.35%,
which was slightly lower than the theoretical value of 5.52%.
Thermogravimetric analysis demonstrated that the CoPc-PDQ-
COF did not lose weight even at 470 °C under nitrogen (Figure
S2). Scanning electron microscopy (SEM) revealed bundles of
belts, which extended to several tens of micrometers and are
formed from elementary belt with high aspect ratios up to 35 and
diameters of 12–44 nm (Figure 2a). High-resolution transmission
electron microscopy (TEM) displayed that CoPc-PDQ-COF
consisted of parallel arrays of 2D sheets with an intralayer
distance of 3.5 Å (Figure 2b), which revealed a high order in its
arrangement and was consistent with the (001) peak in PXRD
patterns. Solid-state 13C cross-polarization magic angle spinning
nuclear magnetic resonance (CP/MAS NMR) spectroscopy
revealed four characteristic signals at δ = 141.88, 138.61, 128.44,
and 126.25 ppm, which were attributable to the aromatic carbons
of cobalt phthalocyanine knot (Figure S3). The signals at δ =
142.42, 141.15, 137.72, 135.02, and 131.64 ppm are assigned to
the carbons on the pyrene linker (Figure S3). Solid-state
Figure 2. a) SEM images of CoPc-PDQ-COF at 100 nm scale bar. b) TEM
image of CoPc-PDQ-COF. c) Temperature-dependent conductivity plot of
CoPc-PDQ-COF pellet measured by a two-point probe at temperatures from
293 to 393 K with an applied voltage of 1 V. Inset shows the Arrhenius plot of
the current versus temperature. d) Nitrogen sorption isotherm profile of CoPc-
PDQ-COF measured at 77 K.
electronic absorption spectroscopy revealed
a wide band
covering from ultraviolet regions to near infrared regions (Figure
S4), suggesting an extended π cloud delocalization in the
framework. Frontier molecular orbital calculation also
demonstrated the delocalization of π electronic clouds over all the
network (Figures S5−S9). In order to quantitatively measure the
interlayer-stacking mode, we utilized the self-consistent charge
DFTB method together with UFF (Universal Force Field)
dispersion. Table S2 summarized the computational calculation
results. The AA-stacking manner (atomic coordinates are listed in
the Table S3) has single-layer stacking energy of 4.33ꢀeV per
lattice, which is much higher than that of the staggered AB
manner (3.07ꢀeV, atomic coordinates are given in Table S4).
The conductivity of CoPc-PDQ-COF was investigated by
utilizing a four-point probe approach on a pressed 1.0 mm thick
film. Notably, the bulk conductivity of CoPc-PDQ-COF was
calculated up to 3.68 × 10−3 S m–1 at 298 K (Figure 2c, Table S5),
which ranks as the highest one among all COF materials.[3] The
Figure 3. Chemical stability. a) Residual weight of CoPc-PDQ-COF after
treatment in various solvents. Residual weight in DMF (99%), DMSO (98%),
THF (97%), MeOH (98%), cyclohexane (99%), 25 °C water (98%), 100 °C water
(98%), HCl (97%), and NaOH (98%). b) PXRD patterns of CoPc-PDQ-COF after
40-day treatment under different conditions.
3
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RESEARCH ARTICLE
Figure 4. Electrocatalytic CO2 reduction. a) LSV curves of CoPc-PDQ-COF (red curve), monomeric [NH2]8CoPc (blue curve), and commercial CoPc (black curve)
in 0.5 M KHCO3 solution saturated with CO2. b) Chronoamperometric responses of CoPc-PDQ-COF at different potentials (red: −0.32 V; orange: −0.36 V; yellow:
−0.43 V; green: −0.45 V; cyan: −0.49 V; blue: −0.54 V; purple: −0.58 V; pink: −0.61 V; brown: −0.65 V; black: −0.66 V). c) Faradaic efficiencies of CoPc-PDQ-COF
(red curve), monomeric [NH2]8CoPc (blue curve), commercial CoPc (green curve), and carbon fiber (black curve). Error bars signify the standard deviation of three
independent samples. d) Tafel plots of CoPc-PDQ-COF (red curve), monomeric [NH2]8CoPc (blue curve) ,and commercial CoPc (black curve). e) TOF of CoPc-
PDQ-COF. f) Long-term operation stability of CoPc-PDQ-COF in current density (blue curve) and faradaic efficiency (red curve) over 24 h at −0.66 V (h = 0.56 V).
The CoPc-PDQ-COF possesses tetragonal channels with a
theoretical diameter of 2.2 nm. The porosity property of the
phenazine-linked COF was evaluated by the nitrogen sorption
analysis at 77 K. As shown in Figure 2d, the sorption isotherm of
CoPc-PDQ-COF was classified into typical type IV, which was
characteristic in mesoporous materials. The Brunauer-Emmett-
Teller (BET) surface area and pore volume values were
calculated to be 762 m2 g–1 and 0.73 cm3 g−1, respectively. Based
on the nonlocal density functional theory model, the pore size
distribution of CoPc-PDQ-COF was centered at 2.2 nm (Figure
S10), which was highly close to the theoretical value. The surface
area value of CoPc-PDQ-COF is similar with those of other
phthalocyanine-based COFs, such as 2D-NiPc-BTDA COF,[4b]
NiPc COF,[11a] and DZnPc-ANDI-COF.[11b]
Chemical stability. Owing to the particularly chemical stability of
phenazine linkage, the CoPc-PDQ-COF is exceptionally robust
against various harsh conditions. To inspect the chemical
robustness of CoPc-PDQ-COF, we soaked CoPc-PDQ-COF
samples for 40 days in different solvents, including
dimethylformamide (DMF), dimethyl sulfoxide (DMSO),
tetrahydrofuran (THF), methanol (MeOH), cyclohexane, water
(100 and 25ꢀ°C), concentrated HCl (12 M), and NaOH solution (14
M). Figure 3a shows the weight proportion of the residual COF
samples. In aqueous solutions and organic solvents, CoPc-PDQ-
COF showed little weight loss (< 4 wt%). Even under the ultra-
harsh acid (concentrated HCl) or base (14 M NaOH) conditions,
the residual weight proportion was as high as 97 wt% and 98 wt%,
respectively. In addition, the CoPc-PDQ-COF kept 98 wt% of its
original weight after one-week hydrolysis in boiling water.
The CoPc-PDQ-COF retained its pristine crystalline network,
as revealed by unchanged positions and intensities of its PXRD
peaks after soaking in DMF, DMSO, THF, MeOH, cyclohexanone,
water (25 and 100 °C), NaOH solution (14 M), concentrated HCl
(12 M) aqueous solutions for 40 days (Figure 3b). We assessed
the crystallinity of above COF samples with its full width at half-
maximum value of the (100) peak. Their width was calculated to
be 100%, 99%, 100%, 99%, 100%, 100%, 99%, and 98% that of
as-synthesized samples, respectively. These results indicate that
the crystallinity of these COF samples is retained. The surface
area values were 760, 759, and 762 m2ꢀg–1 for the treated COF
samples in boiling water, concentrated HCl (12 M), and aqueous
NaOH solution (14 M) for 40 days, respectively (Figure S11);
these values are in close proximity to that of freshly synthesized
COF (762 m2ꢀg–1). Moreover, the FT-IR spectra verified that these
chemical bonds were preserved quite well (Figure S12). As far as
we know, this phenazine-linked COF ranks as one of the most
stable COFs to date.[3] The stability originates from the robust
phenazine ring that serves as the linkage to connect CoPc and
PDQ consisting of stable phthalocyanine and pyrene backbones.
Phenazine linkage has been widely used to construct stable
heterocyclic polymers[11c–11e] and COFs.[4a,10,12d] Compared to
other phenazine-linked analogue, CoPc-PDQ-COF decreases
structural defects by improving crystallinity as evidenced by a
4
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10.1002/anie.202005274
Angewandte Chemie International Edition
RESEARCH ARTICLE
narrow PXRD peak width. All these factors work together to
enable an exceptional stability.
Electrocatalytic activity. The catalytic activity of CoPc-PDQ-
COF was investigated in KHCO3 aqueous solutions (0.5 M, pH =
small Tafel slope of CoPc-PDQ-COF compared to those of
molecular catalysts indicates that less overpotential is required to
get high current. These differences originate from the specific π
structures of CoPc-PDQ-COF which consists of fully
π
7.2) saturated with CO2 using a standard two-component conjugation along x and y axis and π conduction along the z
electrochemical cell. The CoPc-PDQ-COF was thoroughly mixed
with black carbon and dispersed in Nafion solution as binder to
direction, so that electrons are much easier to transport to the
catalytic sites (Figure 5a). Notably, the surface concentration of
form a black homogeneous paste, and then coated on carbon electrocatalytically active CoPc species was calculated as high as
fiber paper as a working electrode. In linear sweep voltammetry
(LSV), CoPc-PDQ-COF exhibited a steep increase in reduction
current density beyond −0.40 V based on a reversible hydrogen
electrode (RHE), indicating a successful CO2 reduction on CoPc-
PDQ-COF (Figure 4a, red curve). The geometric cathodic current
density reaches up to 49.4 mA cm−2 at −0.66 V (RHE, Figure 4a).
In contrast, the monomeric [NH2]8CoPc (Figure 4a, blue curve)
and commercial CoPc (Figure 4a, black curve) exhibited much
lower currents of only 7.2 and 11.2 mA cm–2, respectively.
4.72% of total cobalt atoms by integrating the cyclic voltammetry
peak area (Figure S12). This result is better than that (4%) of
COF-367-Co,[6f] indicating the high degree of accessibility to
cobalt phthalocyanine active sites in the 2D CoPc-PDQ-COF.
The time-dependent total geometric current density of CoPc-
PDQ-COF was further estimated at different electrode potentials
ranging from −0.32 to −0.66 V (Figure 4b). Notably, CoPc-PDQ-
COF exhibited an excellent stability in the whole CO2 reduction
process under all the potentials. The reduction products were
determined as CO and H2 based on gas chromatography
measurement. There is no other liquid products detected using
nuclear magnetic resonance spectroscopy of the electrolyte. The
electrochemical catalysis generated CO as the primary product,
coincided with a small quantity of H2 as a by-product. The faradaic
efficiency for CO (FECO) was evaluated over the entire potential
range (Figure 4c). The CO production was detected at a potential
of −0.28 V (RHE) with a partial current density of 0.52 mA cm–2.
The FECO constantly increased with the upward overpotential and
reached 60% at −0.36 V (h = 260 mV) and 96% at −0.66 V (h =
560 mV), respectively (Figure 4c, red curve). Remarkably, the
FECO value of CoPc-PDQ-COF is higher than that of cobalt
porphyrin COF-367-Co (53% at −0.67 V, h = 560 mV, Table S6)[23]
Therefore, CoPc-PDQ-COF achieves an excellent selectivity as
high as 96% in the electrochemical reduction of CO2 into CO other
than the reduction of water into H2, while COF-367-Co has a
selectivity of only 53%. As control experiments, the FECO values
of monomeric [NH2]8CoPc, commercial CoPc and carbon fiber
were evaluated. The [NH2]8CoPc (Figure 4c, blue curve) and
CoPc (Figure 4c, green curve) exhibited a similar FECO with that
of CoPc-PDQ-COF, while its performance was inferior, especially
in the low overpotential range. The carbon fiber (Figure 4c, black
curve) showed almost no electrocatalytic activity for CO2
reduction, confirming that the catalytically active sites originate
from the cobalt phthalocyanine units of CoPc-PDQ-COF.
To investigate the catalytic mechanism, we calculated the Tafel
slope from a characteristic curve of the overpotential versus a
logarithm of the steady partial current density of CO (Figure 4d).
The linear Tafel slope was evaluated as 112 mV decade−1 from
0.24 V to 0.42 V (Figure 4d, red curve). This result is consistent
with other reported values, indicating that the formation of CO2−
intermediate via electron transport from COF skeletons to CO2 is
the rate-determining step.[12] Figure 5a shows the π structures of
conductive CoPc-PDQ-COF and its catalytic cycle of
electrochemical CO2 reduction by cobalt phthalocyanine, which is
also supported by CV measurements (Figure S13). Meanwhile,
the monomeric [NH2]8CoPc and commercial CoPc exhibited
inferior Tafel slopes of 122 mV decade−1 (Figure 4d, blue curve)
and 125 mV decade−1 (Figure 4d, black curve), respectively. The
Figure 5. π structure of COF and catalytic cycle. a) The π structure of CoPc-
PDQ-COF with π conjugation along the x and y axis and π conduction along the
z axis which facilitate the electron transport to the cobalt(II) phthalocyanine
catalytic centre. b) The electrocatalytic cycle of CO2 reduction to CO by cobalt(II)
phthalocyanine.
The specific activity of CoPc-PDQ-COF was quantified by
standardizing the CO partial current density relative to the weight
of active sites. The mass activity of CoPc-PDQ-COF was
calculated up to 762 mA mg–1 at −0.66 V (h = 0.56 V).
Furthermore, the turnover frequency (TOF) was evaluated given
that all the cobalt active centers get involved in the reduction
course of CO2. Notably, the TOF value for CO production was
calculated as high as 11,412 h−1 at h = 0.56 V (Figure 4e), while
the turnover number reaches 320,000. These values are 32-fold
as high as those of molecular catalyst [NH2]8CoPc. The TOF and
turnover number of CoPc-PDQ-COF are superior to those (9,396
h−1 and 290,000 at h = 0.55 V) of imine-linked cobalt porphyrin
COF-367-Co (1%), which is the best one reported to date.[6f] Note
that COF-367-Co (1%) specifically reduces the content of cobalt
porphyrin sites in the framework to only 1% by diluting with
inactive copper porphyrin, which raises the TOF value. Indeed,
when the cobalt porphyrin content is 100%, the resulting COF-
367-Co exhibits an average TOF of only 165 h−1. Compared to
CoPc-CN/CNT[2a] and COF-367-Co,[6f] CoPc-PDQ-COF achieves
a much lower overpotential and a much higher current density. In
comparison with other COFs, including TTF-Por(Co)-COF,[13a]
COF-Re_Fe,[2e] COF-366-F-Co,[13b] COF-2,2’-bpy-Re,[13c] and
COF-300-AR,[13d] CoPc-PDQ-COF[2a] features the best overall
performance. As shown in Table S6, CoPc-PDQ-COF surpasses
the state-of-the-art organic and inorganic molecular catalysts
including perfluorinated CoPc,[14] nanoporous Ag,[12c] Au
nanowires,[15a] and Pd nanoparticles.[15b] Moreover, the CoPc-
PDQ-COF catalyst exhibited much higher geometric current
density than other catalysts under the same conditions (Table S6).
.
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Beside the catalytic activity, the lifetime and capability of
repetitive use are key to high-performance electrocatalysis. The
robust CoPc-PDQ-COF assumes a long-term catalytic durability.
The chronoamperometric current density and Faradaic efficiency
versus time were recorded over 24 h (Figure 4f). There was no
significant loss in these two parameters. Notably, both the
catalytic efficiency and selectivity retained the same values as the
original ones. This is distinct from that of COF-366-Co, which has
an initial TOF of 98 h−1 and further decreased to 56 h−1 after 24
h.[6f] The remarkable long-term catalytic durability attributes to the
stable yet fully π-conjugated structure, which produces high
catalytic current densities. Moreover, the robust CoPc-PDQ-COF
precludes the possibility of cobalt phthalocyanine detachment
from the framework and thus boosts its performance stability.
To understand the CO2 reduction process, we performed spin-
polarized DFT calculations on single layer of CoPc-PDQ-COF.
The results indicate that the two-electron involved CO2 reduction
is a two-step process; the first step is one-electron in involved
transformation of CO2 to COOH* while the transformation from
COOH* to CO constitutes the second one-electron process
(Figure S14). Their free energy change (ΔG) was determined to
be –0.48 and –0.06 eV, respectively at 298 K. Subsequently, the
anchored CO was spontaneously released from the COF surface
with a ΔG value of –0.41 eV. Therefore, the first step is the rate-
determining step, which is in accordance with the Tafel analysis.
Its theoretical overpotential (ηt) was calculated to be 0.36 V.
Moreover, we investigated the sequential proton-electron transfer
processes for CO2 reduction on CoPc-PDQ-COF (Figure S15).
The valance change from Co(II) to Co(I) occurs in the first step
upon the electron injection, which is mostly distributed on the Co-
dz2 orbital and partly on the C-pz orbital of proximate carbons on
Pc ring. The charge on the central Co(I) was +0.87 |e|, which was
lower than +1.17 |e| of Co(II). The formation of CO2− anion was
realized via a charge transfer (0.55 |e|) from Co(I) to the
coordinated CO2 molecule. The following formation of
intermediate COOH* involved the proton transfer. Finally, CO*
was generated from COOH* through the concerted proton-
electron transfer. As shown in Figure 5b, the periodic cycling
between Co(II) and Co(I) promotes the conversion of CO2 to CO.
and Engineering Fellowship by the Bredesen Centre for
Interdisciplinary Research and Graduate Education at the
University of Tennessee, Knoxville. S.I. acknowledges support by
the Laboratory Directed Research and Development (LDRD)
Program of Oak Ridge National Laboratory. ORNL is managed by
UT-Battelle, LLC, for DOE under contract DE-AC05-00OR22725.
D.J. acknowledges supports by MOE tier 1 grant (R-143-000-
A71-114) and NUS start-up grant (R-143-000-A28-133).
Conflict of interest
The authors declare no conflict of interest.
Keywords: covalent organic frameworks • 2D polymers • π
conjugation • electrocatalysis • CO2 reduction
[1]
[2]
O. S. Bushuyev, P. Luna, C. T. Dinh, L. Tao, G. Saur, J. de Lagemaat,
S. O. Kelley, E. H. Sargent, Joule 2018, 2, 825–832.
a) C. W. Machan, M. D. Sampson, C. P. Kubiak, J. Am. Chem. Soc. 2015,
137, 8564–8571; b) X. Zhang, Z. Wu, X. Zhang, L. Li, Y. Li, H. Xu, X. Li,
X. Yu, Z. Zhang, Y. Liang, H. Wang, Nat. Commun. 2018, 8, 14675; c)
W. L. Zhu, Y. J. Zhang, H. Y. Zhang, H. F. Lv, Q. Li, R. Michalsky, A. A.
Peterson, S. H. Sun, J. Am. Chem. Soc. 2014, 136, 16132–16135; d) M.
Ma, B. J. Trześniewski, J. Xie, W. A. Smith, Angew. Chem. Int. Ed. 2016,
55, 9748–9752; Angew. Chem. 2016, 128, 9900–9904; e) E. M. Johnson,
R. Haiges, S. C. Marinescu, ACS Appl. Mater. Interfaces 2018, 10,
37919–37927; f) Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff,
J. K. Nørskov, T. F. Jaramillo, Science 2017, 355, eaad4998; g) A. Y.
Khodakov, W. Chu, P. Fongarland, Chem. Rev. 2007, 107, 1692–1744;
h) D. Leckel, Energy Fuels 2009, 23, 2342–2358.
[3]
[4]
a) N. Huang, P. Wang, D. Jiang, Nat. Rev. Mater. 2016, 1, 16068; b) C.
S. Diercks, O. M. Yaghi, Science 2017, 355, eaal1585; c) S.-Y. Ding, W.
Wang, Chem. Soc. Rev. 2013, 42, 548–568; d) M. S. Lohse, T. Bein, Adv.
Funct. Mater. 2018, 28, 1705553.
a) M. C. Wang, M. Ballabio, M. Wang, H.-H. Lin, B. P. Biswal, X. Han, S.
Paasch, E. Brunner, P. Liu, M. Chen, M. Bonn, T. Heine, S. Zhou, E.
Cánovas, R. Dong, X. Feng, J. Am. Chem. Soc. 2019, 141, 16810–
16816; b) X. Ding, L. Chen, Y. Honsho, X. Feng, O. Saengsawang, J.
Guo, A. Saeki, S. Seki, S. Irle, S. Nagase, V. Parasuk, D. Jiang. J. Am.
Chem. Soc. 2011, 133, 14510–14513; c) D. D. Medina, T. Sick, T. Bein,
Adv. Energy Mater. 2017, 7, 1700387; d) E. L. Spitler, J. W. Colson, F.
J. Uribe-Romo, A. R. Woll, M. R. Giovino, A. Saldivar, W. R. Dichtel,
Angew. Chem. Int. Ed. 2012, 51, 2623–2627; Angew. Chem. 2012, 124,
2677–2681; e) S. Wan, J. Guo, J. Kim, H. Ihee, D. Jiang, Angew. Chem.
Int. Ed. 2009, 48, 5439–5442; Angew. Chem. 2009, 121, 5547–5550; f)
N. Huang, L. Zhai, D. E. Coupry, M. A. Addicoat, K. Okushita, K.
Nishimura, T. Heine, D. Jiang, Nat. Commun. 2015, 7, 12325.
Conclusion
In conclusion, we have explored a phenazine-linked stable and
conductive metallophthalocyanine COF that catalyzes reduction
of CO2 to CO in water. This electrocatalyst is unique as it enables
the full use of structural features of COFs to integrate full π
conjugation, stable skeleton, and catalytic sites into one lattice,
endowing the framework with stability, conductivity, and catalytic
activity. The COF electrocatalyst achieves high efficiency and
selectivity in catalyzing the reduction of CO2 to CO with
exceptional turnover number and frequency as well as long-term
durability. This design strategy thus offers a platform based on 2D
conjugated frameworks for tailoring robust yet well-defined
catalysts to convert CO2 into various valuable chemicals.
[5]
[6]
a) S. Dalapati, E. Jin, M. Addicoat, T. Heine, D. Jiang, J. Am. Chem. Soc.
2013, 138, 5797–5800; b) J. W. Crowe, L. A. Baldwin, P. L. McGrier, J.
Am. Chem. Soc. 2016, 138, 10120–10123; c) Z. Li, A. M. Evans, I.
Castano, M. J. Strauss, W. R. Dichtel, J.-L. Bredas, J. Am. Chem. Soc.
2018, 140, 12374–12377.
a) W. Cao, W. D. Wang, H.-S. Xu, I. V. Sergeyev, J. Struppe, X. Wang,
F. Mentink-Vigier, Z. Gan, M.-X. Xiao, L.-Y. Wang, G.-P. Chen, S.-Y.
Ding, S. Bai, W. Wang, J. Am. Chem. Soc. 2018, 140, 6969–6977; b) Q.
Sun, B. Aguila, J. Perman, N. Nguyen, S. Ma, J. Am. Chem. Soc. 2016,
138, 15790–15796; c) J. Zhang, X. Han, X. Wu, Y. Liu, Y. Cui, J. Am.
Chem. Soc. 2017, 139, 8277–8285; d) H. Xu, J. Gao, D. Jiang, Nat.
Chem. 2015, 7, 905–912; e) H. S. Xu, S.Y. Ding, W. K. An, H. Wu, W.
Wang, J. Am. Chem. Soc. 2016, 138, 11489–11492; f) S. Lin, C. S.
Diercks, Y.-B. Zhang, N. Kornienko, E. M. Nichols, Y. Zhao, A. R. Paris,
D. Kim, P. Yang, O. M. Yaghi, C. J. Chang, Science 2015, 349, 1208–
1213; g) Y. Zhi, P. Shao, X. Feng, H. Xia, Y. Zhang, Z. Shi, Y. Mu, X. Liu,
J. Mater. Chem. A 2018, 6, 374–382; h) Y. Zhang, Y. Hu, J. Zhao, E.
Park, Y. Jin, Q. Liu, W. Zhang, J. Mater. Chem. A 2019, 7, 16364–16371.
Acknowledgements
N.H. acknowledges support by the research start-up fund of
Zhejiang University. K.H. was supported by an Energy Science
6
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[7]
a) N. Huang, X. Chen, R. Krishna, D. Jiang, Angew. Chem. Int. Ed. 2015,
54, 2986–2990; Angew. Chem. 2015, 129, 3029–3033; b) N. Huang, R.
Krishna, D. Jiang, J. Am. Chem. Soc. 2015, 137, 7079–7082; c) H. Fan,
A. Mundstock, A. Feldhoff, A. Knebel, J. Gu, H. Meng, J. Caro, J. Am.
Chem. Soc. 2018, 140, 10094–10098; d) S. Yuan, X. Li, J. Zhu, G. Zhang,
P. V. Puyvelde, B. V. der Bruggen, Chem. Soc. Rev. 2019, 48, 2665–
2681; e) H. Fan, J. Gu, H. Meng, A. Knebel, J. Caro, Angew. Chem. Int.
Ed. 2018, 57, 4083–4087; Angew. Chem. 2018, 130, 4147–4151.
a) E. Jin, Z. Lan, Q. Jiang, K. Geng, G. Li, X. Wang, D. Jiang, Chem 2019,
5, 1632–1647; b) X. Wang, L. Chen, S. Y. Chong, M. A. Little, Y. Wu, W.-
H. Zhu, R. Clowes, Y. Yan, M. A. Zwijnenburg, R. S. Sprick, A. I. Cooper,
Nat. Chem. 2018, 10, 1180–1189; c) C.-Y. Lin, D. Zhang, Z. Zhao, Z. Xia,
Adv. Mater. 2017, 30, 1703646; d) S. Haldar, K. Roy, R. Kushwaha, S.
Ogale, R. Vaidhyanathan, Adv. Energy Mater. 2019, 1902428; e) J. Zhao,
P. Pachfule, S. Li, T. Langenhahn, M. Ye, C. Schlesiger, S. Praetz, J.
Schmidt, A. Thomas, J. Am. Chem. Soc. 2019, 141, 6623–6630; f) T.
Sick, A. G. Hufnagel, J. Kampmann, I. Kondofersky, M. Calik, J. M. Rotter,
A. Evans, M. Döblinger, S. Herbert, K. Peters, D. Böhm, P. Knochel, D.
D. Medina, D. Fattakhova-Rohlfing, T. Bein, J. Am. Chem. Soc. 2018,
140, 2085–2092; g) Y. Zhang, Y. Dong, J. Li, S. Li, P. Shao, X. Feng, B.
Wang, J. Am. Chem. Soc. 2019, 141, 1923–1927.
[8]
[9]
D. Voiry, H. S. Shin, K. P. Loh, M. Chhowalla, Nat. Rev. Chem. 2018, 2,
0105.
[10] a) J. Guo, Y. Xu, S. Jin, L. Chen, T. Kaji, Y. Honsho, M. A. Addicoat, J.
Kim, A. Saeki, H. Ihee, S. Seki, S. Irle, M. Hiramoto, J. Gao, D. Jiang,
Nat. Commun. 2013, 4, 2736; b) V. A. Kuehl, J. Yin, P. H. H. Duong, B.
Mastorovich, B. Newell, K. D. Li-Oakey, B. A. Parkinson, J. O. Hoberg,
J. Am. Chem. Soc. 2018, 140, 18200−18207.
[11] a) X. Ding, J. Guo, X. Feng, Y. Honsho, J. Guo, S. Seki, P. Maitarad, A.
Saeki, S. Nagase, D. Jiang, Angew. Chem. Int. Ed. 2011, 50, 1289–1293;
Angew. Chem. 2011, 123, 1325–1329; b) S. Jin, X. Ding, X. Feng, M.
Supur, K. Furukawa, S. Takahashi, M. Addicoat, M. E. El-Khouly, T.
Nakamura, S. Irle, S. Fukuzumi, A. Nagai, D. Jiang, Angew. Chem. Int.
Ed. 2013, 52, 2017–2021; Angew. Chem. 2013, 125, 2071–2075; c) C.
Sun, F. Pan, H. Bin, J. Zhang, L. Xue, B. Qiu, Z. Wei, Z.-G. Zhang, Y. Li,
Nat. Commun. 2018, 9, 743; d) S. Yang, M. Chu, Q. Miao, J. Mater.
Chem. C 2018, 6, 3651−3657; e) J. Heiska, M. Nisula, M. Karppinen, J.
Mater. Chem. A 2019, 7, 18735−18758.
[12] a) S. Kapusta, N. Hackerman, J. Electrochem. Soc. 1984, 131, 1511–
1514; b) J. Shen, R. Kortlever, R. Kas, Y. Y. Birdja, O. Diaz-Morales, Y.
Kwon, I. Ledezma-Yanez, K. J. P. Schouten, G. Mul, M. T. M. Koper, Nat.
Commun. 2015, 6, 8177; c) Q. Lu, J. Rosen, Y. Zhou, G. S. Hutchings,
Y. C. Kimmel, J. G. Chen, F. Jiao, Nat. Commun. 2014, 5, 3242; d) Z.
Meng, R. M. Stolz, K. A. Mirica, J. Am. Chem. Soc. 2019, 141,
11929−11937.
[13] a) Q. Wu, R.-K. Xie, M.-J. Mao, G.-L. Chai, J.-D. Yi, S.-S. Zhao, Y.-B.
Huang, R. Cao, ACS Energy Lett. 2020, 5, 1005–1012; b) D. A. Popov,
J. M. Luna, N. M. Orchanian, R. Haiges, C. A. Downes, S. C. Marinescu,
Dalton Trans. 2018, 47, 17450–17460; c) C. S. Diercks, S. Lin, N.
Kornienko, E. A. Kapustin, E. M. Nichols, C. Zhu, Y. Zhao, C. J. Chang,
O. M. Yaghi, J. Am. Chem. Soc. 2018, 140, 1116–1122; d) H. Liu, J. Chu,
Z. Yin, X. Cai, L. Zhuang, H. Deng, Chem 2018, 4, 1696–1709
[14]
N. Morlanés, K. Takanabe, V. Rodionov, ACS Catal. 2016, 6, 3092–
3095.
[15] a) W. Zhu, W. Zhu, Y.-J. Zhang, H. Zhang, H. Lv, Q. Li, R. Michalsky, A.
A. Peterson, S. Sun, J. Am. Chem. Soc. 2014, 136, 16132–16135; b) D.
Gao, H. Zhou, J. Wang, S. Miao, F. Yang, G. Wang, J. Wang, X. Bao, J.
Am. Chem. Soc. 2015, 137, 4288–4291.
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Entry for the Table of Contents
Pumping electrons to produce fuel. A fully π-conjugated 2D covalent organic framework with aligned metallophthalocyanine
electrocatalytic sites enable electron pump to CO2 reduction centers. The 2D π lattice catalyzes two electron reduction of CO2 to CO
in water to achieve an exceptional turnover number up to 320,000 hour−1 and a faraday efficiency of 96%.
Institute and/or researcher Twitter usernames: chmjd@nus.edu.sg
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