Covalent Organic Frameworks Linked by Amine Bonding for Concerted Electrochemical Reduction of CO2

Article abstract of DOI:10.1016/j.chempr.2018.05.003

Covalent organic frameworks (COFs) with amine linkage in both three and two dimensions, COF-300-AR and COF-366-M-AR, were synthesized by direct reduction of their corresponding COFs with imine linkage, COF-300 and COF-366-M, respectively. The quantitative

Full text of DOI:10.1016/j.chempr.2018.05.003

Article
Covalent Organic Frameworks Linked by
Amine Bonding for Concerted
Electrochemical Reduction of CO2
Haoyu Liu, Jun Chu, Zhenglei
Yin, Xin Cai, Lin Zhuang,
Hexiang Deng
hdeng@whu.edu.cn
HIGHLIGHTS
Synthesis of amine-linked COFs in
both 2D and 3D forms
Stability in both strong acid and
base
Construction of molecularly
defined interface
Electrochemical reduction of CO
2
with high efficiency and selectivity
Amine-linked covalent organic frameworks (COFs) were synthesized by the
reduction of parent imine-linked COFs by crystal-to-crystal transformation. The
excellent chemical stability of these COFs in combination with the presence of a
large amount of amine functional groups led to a robust and molecularly defined
interface at the silver metal surface as an electrode for the electrochemical
2
reduction of CO . The concerted operation of COF and the metal surface resulted
in high conversion efficiency and excellent selectivity against the reduction of
water.
Liu et al., Chem 4, 1–14
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Article
Covalent Organic Frameworks Linked
by Amine Bonding for Concerted
Electrochemical Reduction of CO2
Haoyu Liu, Jun Chu, Zhenglei Yin, Xin Cai, Lin Zhuang, and Hexiang Deng1,2,3,
1
2
1
2
1,2
*
SUMMARY
The Bigger Picture
By extending covalent bonds to
two-dimensional (2D) and three-
dimensional (3D) frameworks, the
emergence of covalent organic
frameworks (COFs) represents a
new and exciting branch in porous
and crystalline materials.
Covalent organic frameworks (COFs) with amine linkage in both three and two
dimensions, COF-300-AR and COF-366-M-AR, were synthesized by direct reduc-
tion of their corresponding COFs with imine linkage, COF-300 and COF-366-M,
respectively. The quantitative reduction was confirmed by Fourier transform
1
3
15
infrared and cross-polarization magic angle spinning NMR (both C and N)
spectroscopy. These amine COFs were highly crystalline and exhibited excellent
chemical stability in strong acids and bases. The abundant amino groups in the
Currently, the discovery of new
linkages and improvement of
chemical stability are the two most
pressing challenges for the
COF-300-AR backbone facilitated the electrochemical reduction of CO
2
on
silver electrodes in a concerted manner and led to selective generation of CO.
Specifically, CO conversion efficiency was raised from 13% to 53% at À0.70 V
and from 43% to 80% at À0.85 V (versus a reversible hydrogen electrode) in
comparison with that of a bare silver electrode. The porosity of COFs favored
molecular diffusion to the electrode surface, and the amine functional groups
development of COFs. We report
the synthesis of an amine-linked
COFs in both 2D and 3D forms.
These COFs exhibited excellent
stability in strong acid and base.
Furthermore, by depositing COF-
close to the electrode surface promoted CO
carbamate intermediates.
2
conversion efficiency by forming
INTRODUCTION
3
00-AR on a flat silver electrode,
Covalent organic frameworks (COFs) are widely used for the storage, separation,
and conversion of gases, because they provide molecularly defined pores to interact
we were able to construct a
molecularly defined interface for
electrochemical reduction of CO
to CO with high efficiency and
1
–7
with guest molecules.
Although highly desirable for catalytic applications, chem-
2
ically stable COFs were hard to synthesize because of the dilemma between the
strength of the linkage and the microscopic reversibility required for the crystalliza-
selectivity. Spectroscopic studies
revealed that the concerted
8
–12
tion process.
usually resulted in amorphous phase rather than crystalline structure.
Efforts to form strong covalent bonds in COFs via de novo synthesis
1
3,14
Recently,
behavior between COFs and the
silver electrode at their interface
was responsible for the promoted
performance. This molecularly
defined interface approach
Waller et al. reported a method for addressing this challenge, whereby the entire
framework structure of a two-dimensional (2D) COF made from labile linkages (im-
ines) is directly converted by oxidation reactions to highly stable linkages (amides)
1
5
without altering the original structural topology or order. In the present article,
we show that amine-based COFs, heretofore unknown, can be produced by the
application of reduction reactions directly on the imine-linked frameworks. We
demonstrated this reduction for both 2D and three-dimensional (3D) COFs,
improves the selectivity of the
electrochemical reaction without
sacrificing the overall efficiency.
1
6–18
COF-366-M and COF-300,
respectively, to generate their corresponding sec-
ondary amine frameworks, COF-366-M-AR (2D COF) and COF-300-AR (3D COF),
where AR stands for ‘‘after reduction,’’ and M = Co, Cu, and Zn. The crystallinity
and underlying topology of both 2D and 3D frameworks are fully retained
throughout the conversion process. The newly formed amine linkage provides
exceptional chemical and thermal stability for these COFs far beyond their imine
parent structures. Their crystallinity is unaltered after immersion in a 6 M HCl and
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6
M NaOH solution for more than 12 hr. The power of this solid-state conversion is
that it obviates the trial-and-error method intrinsic to de novo synthesis of COFs and
can be used to synthesize chemically robust COFs with new linkages that are not
easily obtained otherwise.
Furthermore, we applied one of the amine COFs, COF-300-AR, to the electrochem-
ical reduction of CO
framework and the amine functionality in the backbone play critical roles. COF-
00-AR particles were deposited on a silver electrode with a flat surface to form a
molecularly defined interface for the catalytic reaction. Compared with a bare silver
electrode, this interface provided excellent selectivity of the CO reduction reaction
CO RR) against the H evolution reaction (HER). Such improvement in selectivity
2
in aqueous media, where both the chemical stability of the
3
2
(
2
2
originates from the concerted environment endowed by the amine COFs and the
metal surface. The secondary amine linkages in the COF backbone provide chemi-
2
sorptive sites to selectively capture CO molecules by forming carbamates, as shown
1
3
by C solid-state nuclear magnetic resonance, and the silver surface activates the
carbamates and provides sufficient electrons for their reduction to CO.
2
Previous efforts to improve CO RR selectivity involved either the use of a molecular
catalytic center, where the electron-transfer efficiency was largely affected by the
conductivity of such material, or the use of small inactive molecules to modify the
surface of the metal electrode, where the accessibility of CO
2
was limited by their
In this study, we constructed an ordered porous environment on
the surface of a metal electrode to form a molecularly defined interface, where
CO diffusion was favored and excellent CO RR selectivity was provided without
1
9–24
coverage.
2
2
sacrificing the current of the original metal electrode.
RESULTS AND DISCUSSION
Reduction of 2D and 3D COFs and Characterizations
Amine linkages have been extensively used in polymer chemistry, and amine macro-
2
5,26
molecules have been widely used as surfactants and in biomedical applications.
Theoretically, amine linkages can be obtained by the reduction of imine linkages,
which are common in the formation of COFs. Although both oxidation and reduction
have been used extensively in organic molecules, such quantitative conversions in
extended solids are rare because of the limit in diffusion of the reactants and the
2
7–29
flexibility of the targeting functional groups.
In order to identify a suitable con-
dition to achieve solid-state conversion of imine to amine linkages in COFs, at first
we used a commercially available compound, benzylidene aniline, as a model.
Among various reductive conditions tested in this study, we identified that the
4
best method involved the use of NaBH , by which quantitative conversion of
benzylidene aniline to benzyl aniline was achieved (Figure 1A). This was confirmed
À1
by the appearance of new peaks at 1,603 and 1,266 cm from benzyl aniline
À1
and the disappearance of the 1,626 cm peak from benzylidene aniline in the
Fourier-transform infrared (FTIR) spectrum of the product (Figure 1C). The formation
1
of benzyl aniline was also verified by H nuclear magnetic resonance (NMR)
1UC Berkeley-Wuhan University Joint Innovative
Center, Institute for Advanced Studies, Wuhan
University, Luojiashan, Wuhan 430072, China
(
Figure S17).
2College of Chemistry and Molecular Sciences,
Wuhan University, Luojiashan, Wuhan 430072,
China
Applying this optimized reduction condition, two classic imine COFs covering
both 2D and 3D topologies were successfully converted to amine COFs. The 3D
COF, COF-300, was obtained by linking tetra-(4-anilyl) methane and terephthalalde-
hyde (Figure 1B), and the 2D COF, COF-366-M (M = Co, Zn, and Cu), was
obtained by linking [5,10,15,20-tetrakis(4-aminophenyl) porphinato]-M and
3Lead Contact
*Correspondence: hdeng@whu.edu.cn
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Figure 1. Model Reaction and Reduction of COF-300 with Imine to Form COF-300-AR with Amine
Linkage
(
(
A) Scheme for model reaction.
B) Scheme for COF-300 reduction. BDA, benzene-1,4-dicarboxaldehyde; TAPM,
terta(4-aminophenyl)methane. In the space-filling diagrams, carbon and nitrogen atoms are
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Figure 1. Continued
represented as gray and blue spheres, respectively. Only the hydrogen atoms on the imine and
amine linkage are shown (in pink) for clarity.
(
(
(
C) FTIR spectra of the model compound before and after reduction.
D) FTIR spectra of COF-300 before and after reduction.
13
15
15
E and F) C (E) and N (F) CP-MAS NMR spectra of COF-300 before and after reduction. N-enriched
1
5
samples were used in the N CP-MAS NMR experiment. *, sideband; #, signal for carbon in ethanol.
terephthalaldehyde (Figure 2A). The successful conversion of the reduction reaction
was confirmed by FTIR spectroscopy (Figure 1D). Similar to that of the molecular
À1
À1
conversion, the new peaks at 1,251 cm and 1,608 cm represented C–N vibration
stretch and C–N–H bending stretch in the secondary amine linkage. The disappear-
À1
ance of the 1,620 cm peak, representing the C=N vibration stretch of imine in the
parent COF-300 structure, demonstrated the complete conversion of this reaction.
Similar phenomena were observed in FTIR spectra of COF-366-M and COF-366-M-
AR (Figures 2B, S3, and S4). In addition, the quantitative conversion of imine to
1
3
15
amine linkages was verified by both C and N cross-polarization magic angle spin-
ning (CP/MAS) NMR spectroscopy (Figures 1E and 1F). After reduction, the imine
carbon peak belonging to COF-300 at 159 ppm disappeared completely in the
1
3
C CP-MAS NMR spectrum, and a new peak at 50 ppm emerged, indicating the for-
mation of amine linkages (Figure 1E). A trace amount of ethanol was added to COF-
00-AR to increase the flexibility of the framework backbone; thus, sharper peaks
3
1
5
were observed in the CP-MAS NMR experiment (Figure 1E). In N CP-MAS NMR ex-
1
5
periments, both the COF-300 and COF-300-AR samples were N enriched with the
1
5
use of tetra-(4-anilyl) methane linkers labeled with N for the synthesis. The disap-
pearance of the imine nitrogen peak at 307 ppm and the emergence of an amine ni-
trogen peak at 53 ppm also demonstrated the successful conversion from imine to
amine linkage in the COF structure (Figure 1F). We noticed that there were multiple
peaks in addition to the major imine peak in the parent COF-300 structure, which
1
4
might originate from the unreacted linkers trapped in the framework. In contrast,
1
5
only one sharp peak was observed in the N CP-MAS NMR spectrum of COF-300-
AR, representing the uniform chemical environments of amine linkages in the COF
backbone. It is likely that the reduction process purged the impurities from the
COF structure.
Unlike the amorphous product usually observed in previous studies to form amine
bonds in extended structure, the crystallinity and the underlying topology of the
parent COF were well preserved in this solid-state chemical conversion. The
reduced COF samples were examined by powder X-ray diffraction (PXRD) using
˚
a synchrotron radiation source (wavelength, l = 1.23980 A). Sharp peaks were
clearly observed in their PXRD patterns, indicating high crystallinity. PXRD patterns
of both COF-300-AR and COF-366-Cu-AR samples are in good agreement with
those of the original COFs tested under the same conditions (Figures 3A, 3B,
and S11). Pawley refinement was applied against the experimental PXRD patterns
to obtain the lattice parameters of the original and reduced COF-300 crystals. The
same tetragonal I41/a space group was identified in both crystals with a slight
˚
change in the lattice parameters from 27.2269 to 27.0632 A in a and from
˚
1
1.5153 to 11.5339 A in c for COF-300 and COF-300-AR, respectively. This varia-
tion can be attributed to the change in the hybridization of carbon and nitrogen
2
3
atoms, from sp in the imine linkage in COF-300 to sp in the amine linkage in
COF-300-AR. Similar observations can also be found in the reduction of COF-
3
66 (Figure S10B). Although the number of peaks was limited in the PXRD pattern
of COF-366-Cu-AR, it was clear that its crystallinity and the underlying topology
were unaltered.
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Figure 2. Reduction of COF-366-M Series with Imine Linkages to COF-366-M-AR Series with Amine Linkages
A) Scheme for the reduction of COF-366-M (M = Co, Cu, or Zn). In the space-filling diagrams, carbon and nitrogen atoms are represented as gray and
blue spheres, respectively. Only the hydrogen atoms on the imine and amine linkage are shown (in pink) for clarity.
(
(
(
B) FTIR spectra of COF-366-Cu before and after reduction.
1
3
15
15
15
C and D) C (C) and N (D) CP-MAS NMR spectra of COF-366-Zn and COF-366-Zn-AR. N enriched samples were used in the N CP-MAS NMR
experiment. *, sideband.
In order to confirm the preservation of the underlying pore structure of COF-300-AR,
inclusion experiments were conducted to introduce small organic molecules into the
COF pores. Here, we diffused tetraphenylethylene (TPE), a fluorescent molecule,
into the pores of COF-300-AR, and the inclusion of this fluorescent molecule was
characterized by laser scanning confocal microscopy. Blue fluorescence of TPE
was observed across the entire COF crystal, indicating the successful inclusion
˚
of TPE (largest dimension of 11.7 A) into the pores of COF-300-AR (Figures S30
and S31).
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Figure 3. Crystallinity and Chemical Stability of the Amine COFs
A and B) PXRD patterns of COF-300 before (A) and after (B) reduction. Pawley refinement was
(
performed against the diffraction patterns and the results showed that the crystallinity was well
maintained after conversion of imine linkages to amine linkages.
(
C) Unaltered PXRD patterns of COF-300-AR demonstrated excellent stability in both strong acid
˚
and base. Wavelength of the X-ray source is 1.23980 A.
Stability Test for Reduced COFs
The advantage of the amine linkages is their overwhelming chemical stability
compared with that of imine linkages. In order to demonstrate the chemical stability
of the amine COFs, we immersed COF-300-AR samples in extremely acidic and basic
aqueous solutions: 6 M HCl (pH < 0) and 6 M NaOH (pH > 14), respectively.
After immersion for 12 hr, PXRD patterns of COF samples were collected with a
˚
synchrotron X-ray source (wavelength, l = 1.23980 A). Sharp peaks identical to those
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of the pre-immersion samples were observed, demonstrating unaltered crystallinity
of the amine COFs after immersion (Figure 3C). In contrast, the imine COF, COF-
3
00, was dissolved completely in 6 M HCl under the same conditions. In the immer-
sion test in 6 M NaOH, the crystallinity of COF-300 was partially preserved
1
(
Figure S11). Upper clear solutions were also collected for H NMR study after the
immersion tests in acid. Multiple peaks present in the solution of the COF-300
test, which can be assigned as fingerprint peaks for two molecular building blocks
of this COF (tetraphenylmethane and benzene-1,4-dicarboxaldehyde). This further
indicated the decomposition of imine COF under acid conditions. However, no
observable peaks except for the solvent peaks were found in that of the COF-300-
AR test. It is clear that the chemical stability of COFs was drastically improved as
the imine linkages were converted to amine linkages (Figure S18).
Electrochemical Reduction of CO
2
Performed by Concerted Electrode
reduction using metal electrodes
One of the key challenges for electrochemical CO
2
is the lack of selectivity toward the desirable product, although the kinetics of total
conversion are rather high. Intrinsically, this originates from the flat band structure of
the metal and the lower potential of the competing hydrogen evolution. In contrast,
2
molecular complex provides discrete frontier orbitals that can interact with CO mol-
ecules in specific orientations, thus providing excellent selectivity. When these
complexes were used in electrochemical catalysis, however, the overall reaction
kinetics was limited because of the electron-transfer barrier between the electrode
and the catalytic center in the molecule. In this study, we designed a molecularly
defined interface by combining the catalytic surface of the Ag electrode with the or-
dered porous amino environment of COFs, which is attractive for CO
Figure 4A). Ideally, CO conversion will take place between the metal surface and
the orderly arranged amino functional groups dangling above; thus, CO molecules
2
molecules
(
2
2
can directly accept electrons from the metal surface rather than through the COF
framework (Figure 4B). In this way, the reaction kinetics was well preserved, and
the selectivity was guaranteed by the molecularly defined environment. The inter-
face was constructed by a thin layer of COF-300-AR deposited homogeneously on
a flat silver electrode, as confirmed by scanning electron microscopy (SEM) and
energy dispersive X-ray spectrometer (EDX) mapping (Figures 4C–4F). The silver sur-
face was rather smooth, as visualized in the Ag La EDX image, and the thickness of
COF-300-AR layer was 15 mm, as revealed in the C Ka1 EDX image.
This COF-Ag interface was subjected to CO
2
electrochemical reduction tests. The
reduction was À0.50 V versus a
onset potential representing the initiation of CO
2
reversible hydrogen electrode (RHE) for COF-decorated silver electrode, which
was similar to that of the bare silver electrode (Figure S19). Compared with COF
on the bare silver electrode, COF on the silver electrode exhibited an increased
faradic efficiency (FE) of CO from 13% to 53% and 43% to 80% under the potential
of À0.70 and À0.85 V versus RHE, respectively (Figures 5C and 5D); on the other
hand, the HER was obviously suppressed from 80% to 22% and 60% to 9% under
À0.70 and À0.85 V versus RHE, respectively. Two control experiments were per-
formed under identical conditions. No obvious improvement in CO
selectivity was observed on the silver electrode using only Nafion binder (Figures
C and 5D), whereas almost no CO conversion took place on the glassy carbon
electrode decorated with COF-300-AR. In addition, COF-300 was used as another
control for a electrochemical CO conversion study, where the total current
2
conversion
5
2
2
decreased obviously (Figure S26), and there was no improvement in either catalytic
efficiency or selectivity (Figures S27 and S28). It is clear that the drastic increase in
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Figure 4. Molecularly Defined Interface Created by COF-300-AR on a Flat Silver Electrode
(
(
A) Illustration of the molecularly defined interface.
B) Scheme of the mechanism of concerted CO reduction taking place at the interface between COFs and the silver electrode through the critical
molecules that were activated by the amine functional groups in the COF structure.
C–F) SEM images and elemental mapping of the molecularly defined interface between COF and the silver electrode; the SEM image was taken at
2
carbamate formation. Electrons directly transferred to the CO
2
(
5
keV, and energy dispersive spectroscopy mapping was taken at 10 keV Ag La (E; green) and C Ka (F; red). Scale bars: 20 mm (C) and 10 mm (D–F).
2
CO RR activity on the silver electrode decorated with COF-300-AR originates from
the concerted effect between COFs and the surface of the silver electrode.
Study on Interaction and Mechanism Proposal
In order to glimpse into the mechanism of selective CO
concerted electrode, CO adsorption was tested on a COF-300-AR sample, and
was introduced in a CP/MAS NMR experiment for this COF. The
affinity between COF-300-AR and CO was demonstrated by CO isotherms
2
conversion on the
2
1
3
C-labeled CO
2
2
2
collected at both 273 K and 298 K (Figure 5A). The hysteresis between the desorp-
tion and adsorption curves indicated strong interaction between the amine
framework and CO
tween CO and COF-300-AR, C CP/MAS NMR spectroscopy was applied on
COF samples before and after absorbing CO (Figure 5B). A strong peak at 162.3
ppm emerged in the spectrum of COF samples after exposure to C-labeled CO
at 1 bar, corresponding to the formation of carbamate. This demonstrated the
presence of chemisorption between COF-300-AR and CO molecules. In addition,
2
molecules. In order to identify the type of this interaction be-
1
3
2
2
1
3
2
2
1
3
high-power decoupling C NMR spectra were performed on the same sample
without spinning. Compared with the dominating carbamate peak at 162.3 ppm,
2
a tiny peak at 125.3 ppm was observed, indicating physically absorbed CO mole-
3
0
cules also existed in the pores of COF-300-AR (Figure 5B). The dominating
chemisorption was also confirmed by FTIR spectroscopy of COF-300-AR exposed
À1
to CO
2
, where two new peaks at 1,629 and 1,650 cm appeared, representing
the formation of carbamate (Figure S14). These results demonstrated that the forma-
tion of carbamate is the key transition state before CO molecules receive electrons
from the Ag electrode to be reduced to CO (Figure 4B). Here, we propose a mech-
anism for the CO conversion on the concerted electrode. First, CO molecules were
2
2
2
attracted by the amine framework and then interacted with the amine functional
groups hanging above the electrode to form carbamates. Second, the formation
2
of carbamate further promoted the conversion of CO at the molecularly defined
Ag surface, and an excessive amount of electrons can be transferred without going
through the COF backbone. Third, facilitated by water molecules, the carbamates
À
were reduced to CO molecules and OH anions; thus, the amino functional groups
in the COF structure were regenerated to close the catalytic cycle (Figure 4B). The
À
OH anions generated in the aqueous solution diffused to the counter electrode
2
through an anionic membrane to generate O .
Conclusion
In this study, we synthesized highly crystalline amine COFs with both 3D and 2D
topologies through solid-state conversion from their corresponding imine COFs.
The amine linkage provided excellent chemical stability for the framework, and
the underlying topology was preserved. In principle, this conversion can be gener-
ally applied to any imine COFs. The amine COFs were applied in electrochemical
reduction of CO
frameworks with amine functional groups. Carbamate was identified to be the key
intermediate for an efficient CO reduction process. The high FE and drastically
improved selectivity of CO RR over HER originated from the molecularly defined
2
in this study, representing a promising application for porous
2
2
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Figure 5. Investigation of Interactions between COF-300-AR and CO
Performance of CO Reduction
adsorption isotherms of COF-300-AR at 273 and 298 K.
B) C CP-MAS NMR showing the formation of carbamate intermediate during CO
2
, and Electrochemical
2
(
(
A) CO
2
1
3
2
reduction
process.
2
C and D) Faradic efficiency for CO (C) and H (D) production on the concerted electrode of
(
COF-300-AR and Ag at various potentials. Bare silver foil and Nafion solution were used as controls.
Error bars represent the FE from two independent measurements.
interface constructed by amine COF and the silver electrode in a concerted manner.
The construction of a concerted electrode represents a promising direction for ac-
cessing high efficiency and selectivity in electrochemical catalytic reactions, espe-
cially for energy storage and conversion.
EXPERIMENTAL PROCEDURES
Reduction Procedure
A general procedure is presented below. The equivalences are calculated on the
basis of the number of imine functionalities in the COF.
General notes: all reagents must be of high purity. The imine COFs used in the
reduction conversion were prepared by sequential solvent exchange with 1,4-diox-
ine and acetone by Soxhlet extractor, followed by supercritical carbon dioxide
activation.
COF-300-AR: p-phthalic acid (57.6 mg, 0.347 mmol, 1 equiv) was added to the
suspension of COF-300 samples (50 mg, 0.347 mmol by imine) in 25 mL of MeOH.
ꢀ
The mixture was stirred at À15 C for 5 min before NaBH
4
(0.5 g, 13.22 mmol, 38
ꢀ
equiv) was added in small portions over a period of 10 min. After stirring at À15 C
for 1 hr, the mixture was stirred at room temperature for another 10 hr. A light-yellow
solid could be collected either by centrifugation or filtration. The reduced COF,
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COF-300-AR, samples were then washed with water three times, extracted with
ethanol for 24 hr (with a Soxhlet extractor), and then dried under dynamic vacuum
at room temperature. Elemental analysis (EA) of activated COF-300-AR samples is
as follows: calcd C 84.25%, H 6.16%, N 9.59%; found C 81.93%, H 5.86%, N 9.44%.
COF-366-M-AR: the synthesis of COF-366-MAR was carried out according to a pro-
tocol similar to that used for COF-300-AR. In the synthesis, the COF-300 sample was
replaced with COF-366-Co (50 mg, 0.054 mmol by imine), COF-366-Cu (50 mg,
0
.054 mmol by imine), and COF-366-Zn (50 mg, 0.054 mmol by imine), and the
rest of the synthetic procedure was the same as for COF-300-AR. EA of the activated
COF-366-Co-AR sample is as follows: calcd C 77.00%, H 4.70%, N 11.98%; found
C 76.62%, H 4.70%, N 12.37%. EA of COF-366-Cu-AR is as follows: calcd C
7
3
6.78%, H 4.51%, N 11.94%; found C 72.84%, H 4.70%, N 11.02%. EA of COF-
66-Zn-AR is as follows: calcd C 76.63%, H 4.50%, N 11.92%; found C 76.08%,
H 4.52%, N 9.81%.
Material Characterizations
PXRD data were recorded on both synchrotron and laboratory-based X-ray sources.
The laboratory-based PXRD patterns were collected on a Rigaku Smartlab 9 kW
diffractometer in reflection mode operated at 45 kV, 200 mA for Cu Ka
˚
ꢀ
ꢀ
(
l = 1.5406 A) with a scan speed of 1 /min and a step of 0.01 in 2q at ambient tem-
perature and pressure. The synchrotron-based PXRD patterns were performed in
transmission mode at beamline BL14B1 in the Shanghai Synchrotron Radiation
˚
Facility (SSRF) with a wavelength of 1.2398 A. Samples were held in a sealed capillary
with a drop of ethanol inside. Thermogravimetric analysis (TGA) was performed on a
TA Instruments Q-500 series thermal gravimetric analyzer with samples held in plat-
inum pans in a continuous nitrogen or air flow atmosphere. The heating rate was
ꢀ
1
0 C/min throughout the TGA experiments. Elemental microanalyses were
performed in the Center of Material Research and Analysis at Wuhan University of
Technology with an Elementar Vario EL cube elemental analyzer. FTIR spectra
were recorded on a Nicolet NEXUS670 IR spectroscope, and samples were tableted
with KBr as support. Liquid-state NMR spectra were referenced with the chemical
shift of solvent peaks.
Electrochemical Experiments
Preparation of Ag Electrode and COF-Ag Electrode
The Ag electrode was prepared by polishing Ag foil bought from Alfa Aesar and
subsequent washing with deionized (DI) water and ethanol three times each. Ten
milligrams of COF-300-AR was mixed with 40 mL of Nafion solution, 250 mL of
ethanol, and 750 mL of DI water and then placed in an ultrasonic bath for 2 min.
This mixture was dropped onto the Ag foil and dried with an infrared lamp to prepare
it for testing. In the control experiment, 40 mL of Nafion solution, 250 mL of ethanol,
and 750 mL of DI water were mixed and then dropped on another Ag foil under iden-
tical conditions.
Electrochemical Measurements
The electrochemical experiments were carried out in a homemade H-type electro-
lytic cell equipped with three electrodes with the working and counter electrode
compartments separated by a Nafion membrane. The Ag foil served as the working
electrode, and the counter electrode and the reference electrode were a sheet of
carbon paper and the saturated calomel electrode (SCE), respectively. Electrochem-
ical reduction of CO
2 2
was conducted in a CO -saturated atmosphere in the presence
of 0.1 M KHCO solution (pH 6.8), and the electrode potentials were converted to
3
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CO , Chem (2018), https://doi.org/10.1016/j.chempr.2018.05.003
2
the RHE reference scale with E (versus RHE) = E (versus SCE) + 0.2415 V + 0.0591 3
pH, where 0.2415 V represents the standard potential of SCE with saturated KCl
2
solution. The geometric area of the electrode was 0.50 cm .
Product Analysis
The major products of electrochemical reduction of CO
gaseous products such as CO and H , in accordance with previous studies. The
amount of H and CO generated was quantified by online gas chromatography
GC) analysis (Shimadzu, GC-2010 plus) equipped with a ShinCarbon column
Restek) and a Barrier discharge ionization detector. In general, the cathodic
gas of
electrochemical
was steadily bubbled to the cell at 5 sccm. The product
gases went through the sampling loop and were analyzed by GC continuously dur-
ing the potentiostatic electrolysis. The FE of the electrochemical reduction of CO
2
on the Ag electrode were
2
2
(
(
compartment of the gas-tight H-type electrolytic cell was purged with CO
high purity for at least 30 min before each measurement. In the CO
conversion process, CO
2
2
2
2
process was calculated as follows, by the corresponding peak area present in the
chromatogram, which is calibrated by a certified standard gas.
I
p
FE = 3 100%;
I
t
V
p
3 P 3 nF
I
p
=
;
RT
V
p
= 4p 3 V
t
;
where 4
p
is the volume fraction of the product derived from GC calibration, V is total
t
gas flow rate, and R, T, P, n, and F represent the ideal gas constant, temperature,
pressure, the number of electrons transferred, and faradic constant, respectively,
plus the total current obtained from a potentiostat (CHI-720E). I
product calculated from GC data; I is the total current.
p
is the current of
p
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 31 fig-
ures, and 1 table and can be found with this article online at https://doi.org/10.1016/
j.chempr.2018.05.003.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China
(
21471118, 91545205, and 91622103), National Science Foundation of Jiangsu
Province of China (ZXG201446 and BK20140410), National Key Basic Research Pro-
gram of China (2014CB239203), Key Program of Hubei Provence (2015CFA126), and
Innovation Team of Wuhan University (WHU) (2042017kf0232). We thank the Test
Center and Core Research Facilities of WHU and Wuhan National Laboratory for Op-
toelectronics for providing technical support, beamline BL14B1 (SSRF) for providing
the beam time and help during experiments, and Profs. F. Deng and J. Xu (National
Center for Magnetic Resonance in Wuhan, Wuhan Institute of Physics and Mathe-
matics, Chinese Academy of Sciences) for help with solid-state NMR experiments.
Special thanks go to Prof. Y.-B. Zhang (School of Physical Science and Technology,
ShanghaiTech University) for help with the crystal structure analysis, X. Wei (College
of Chemistry and Molecular Science, WHU) for help with the FTIR test, and M. Tang
(
College of Life Science, WHU) for the fluorescence test. We also thank all group
members’ support and help in Deng’s lab.
12
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CO , Chem (2018), https://doi.org/10.1016/j.chempr.2018.05.003
2
AUTHOR CONTRIBUTIONS
H.D. conceived the idea and led the project; H.L. and H.D. designed the COF reduc-
tion experiments; H.L. performed the reduction of COF-300; J.C. performed the
reduction of the COF-366 series and, in a joint effort, characterized the structure
and stability of the amine COFs; H.D. and L.Z. designed the electrochemical CO
2
reduction experiment; Z.Y., X.C., and H.L. performed the electrochemical experi-
ments. H.L., X.C., and H.D. prepared the first version of the manuscript, and all au-
thors contributed to the discussion of results and the final version.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: December 11, 2017
Revised: January 21, 2018
Accepted: May 4, 2018
Published: June 14, 2018
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2
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