Sulfonated 2D Covalent Organic Frameworks for Efficient Proton Conduction

Article abstract of DOI:10.1002/chem.202004727

Open 1D channels found in covalent organic frameworks are unique and promising to serve as pathways for proton conduction; how to develop high-rate yet stable transporting systems remains a substantial challenge. Herein, this work reports a strategy for e

Full text of DOI:10.1002/chem.202004727

Accepted Article
Accepted Article
Title: Sulfonated 2D Covalent Organic Frameworks for Efficient Proton
Conduction
Authors: Zongfan Yang, Pei Chen, Wenjing Hao, Zhen Xie, Yu Feng,
Guolong Xing, and Long Chen
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0
1/2020

Chemistry - A European Journal
10.1002/chem.202004727
RESEARCH ARTICLE
Sulfonated 2D Covalent Organic Frameworks for Efficient Proton
Conduction
[
a]
[a]
[a]
[a]
[b]
[a]
Zongfan Yang, Pei Chen, Wenjing Hao, Zhen Xie, Yu Feng,* Guolong Xing, and Long
Chen*[a]
[
a]
Z. Yang, P. Chen, W. Hao, Z. Xie, W. Chen, Dr. G. Xing, Prof. L. Chen
Department of Chemistry
Institution of Molecular Plus
Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072 China
E-mail: long.chen@tju.edu.cn
[
b]
Prof. Y. Feng
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function
Institution of Chemistry
Chinese Academy of Sciences (CAS), Beijing 100190, P. R. China
E-mail: fengyu211@iccas.ac.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: Open 1D channels found in covalent organic frameworks
are unique and promising to serve as pathways for proton conduction;
how to develop high-rate yet stable transporting systems remains a
substantial challenge. Herein, we report a strategy for exploring
proton-conducting frameworks by engineering pore walls and
installing proton-containing polymers into the pores. Amide-linked and
sulfonated frameworks were synthesized from imine-linked
precursors via sequentially engineering to oxidize into amide linkages
and to further anchor sulfonic acid groups onto the pore walls,
enabling the creation of sulfonated frameworks with high crystallinity
and channel ordering. Integrating sulfonated polyether ether ketone
chains into the open channels enables proton hopping to across the
channels, greatly increases proton conductivity and enables a stable
continuous run. These results suggest a way to explore proton-
conducting COFs via systematic engineering of wall and space of the
open nanochannels.
transport.[7,8] Therefore, it is desirable to explore suitable
crystalline materials with both high proton conductivity and
excellent stability.
Covalent organic frameworks (COFs) represent an emerging
class of crystalline porous polymers which have been widely
explored in gas separation,[
9,10]
heterogeneous catalysis,[11]
sensors,[ ionic conduction, and many others. Noteworthy
features including the tunability of the pore size, the modifiability
of pore surface and the excellent stability are beneficial for proton
conduction. In the past decades, much attention has been paid to
employ COFs as a promising platform for proton-conducting
materials. In 2014, the first example of proton-conducting COF
was reported, where Tp-Azo COF was doped with guest
phosphoric acid molecules and applied as proton conductor under
both humid and anhydrous conditions.[ Later on, the same group
12]
[13]
[14]
7]
3
reported another hybrid COF of TpPa-(SO H-Py) with sulfonic
acid anchored on the pore surface and phytic acid impregnated in
the nanochannel.[15] Recently, Zhu and co-workers synthesized a
3−
novel cationic COF in which anionic PW12O40 was introduced by
static interactions.[16] Furthermore, Jiang’s group reported an
Introduction
extraordinary stable COF loaded with N-heterocyclic proton
Proton-conducting materials are a burgeoning type of solid
electrolyte, which have spurred tremendous interests in recent
years. Proton-conducting materials have been utilized as
separation membranes for hydrogen evolution via electrocatalytic
water splitting, hydrogen sensors, electrochemical reactors and
fuel cells, etc.[ Although extensive research efforts have been
devoted, the design and synthesis of proton-conducting materials
with both high proton conductivity and stability remain challenging.
For example, numerous research works have been performed to
develop polymeric proton conductors such as polysulfones,[2]
3 4
carriers and H PO for efficient anhydrous proton conductions at
[8]
high temperatures. All these materials were constructed via de
novo syntheses and further incorporation of guest acid
3−
3 4
molecules/anions such as H PO , phytic acid, PW12O40 etc.,
which served as the proton carriers and played pivotal roles in
proton conduction. On the other hand, tailed-made functions can
be readily introduced to the well-established COFs by post-
synthetic modifications, which leads to completely new COFs
without "trial and error" processes.[17] Sulfonation is a widely used
strategy to improve the proton conductivity by introducing
abundant sulfonic acid groups, which not only provide proton
donors but also facilitate the formation of continuous hydrogen
bonding networks for proton migration.[5c] However, the reported
examples of sulfonated COFs are very rare and all prepared via
de novo syntheses using sulfonic acid functionalized monomers
because most COFs cannot survive in the harsh conditions of
sulfonation reactions.[15,18] It is highly challenging to maintain both
crystallinity and porosity of COFs upon sulfonation. Thus, direct
post-synthetic sulfonation of COFs remains largely unexplored.
1]
styrene-ethylene-butylene-styrene (SEBS) and poly(aryl ether
ketone)s.[
3,4]
However, such aromatic-based polymers require
further functionalization with sulfonic acid groups for efficient
proton conduction.[5b] Recently, metal−organic frameworks
(
MOFs) and conjugated microporous polymer (CMP) have been
[5,6]
employed as potential candidates for proton conduction.
Unfortunately, MOFs suffer from poor hydrolytic stability with low
acid tolerance of the doped guests, which limits its applicability in
operating conditions with high humidity. It is reported that one-
dimensional ordered channels can further facilitate proton
1
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Chemistry - A European Journal
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RESEARCH ARTICLE
Figure 1. a) Synthetic strategy for the sulfonated COFs through sequential imine formation, amide formation and sulfonation. b) Synthetic routes of the sulfonated
D COFs of BD-COF (black), Ox-BD-COF (red), Ox-BD-COF-SO H (blue). c) PXRD patterns and d) Nitrogen adsorption (solid) and desorption (open) isotherms of
BD-COF (black), Ox-BD-COF (red), Ox-BD-COF-SO H (blue). e) TEM images of Ox-BD-COF-SO H (scale bar: 20 nm).
2
3
3
3
Herein, we designed and synthesized two ultrastable imine
COFs annulated with cyclic ether moieties and further oxidized
and sulfonated by post-synthetic modification approach (Figure 1).
To our delight, these sulfonated COFs not only retain high
crystallinity but also exhibit enhanced proton conductivity. In
addition, an isostructural contrastive COF based on 1,2-
dimethoxybenzene (MB-COF) was synthesized for comparison to
explore the effect of the cyclic ether moieties on the crystallinity
and stability. The cyclic ether based imine COFs exhibited
excellent chemical stability toward strong acid (12 M HCl) and
base (9 M NaOH). In order to guarantee the COF skeletons can
retain both crystallinity and porosity upon sulfonation, the dynamic
imine linkages were further oxidized to more stable amide
linkages. Due to the presence of electron-donating cyclic ether
moieties, sulfonic acid groups were facilely grafted to the amide
COFs. Sulfonated polyether ether ketone (SPEEK) featuring
abundant sulfonic acid groups was infiltrated to the nanochannels
of these two sulfonated amide-linked COFs to further enhance the
proton conductivity. The hybrid composites of 50 wt%
Tri(4-aminophenyl)benzene (TAPB) was employed to synthesize
imine COFs (BD-COF, DBD-COF and MB-COF) with three
different dialdehydes (benzo[d][1,3]dioxole-4,7-dicarbaldehyde;
2,3-dihydrobenzo[b][1,4]dioxine-5,8-dicarbaldehyde and 2,3-
dimethoxybenzene-1,4-dicarboxaldehyde) under solvothermal
conditions at 120 °C for three days. The two cyclic ether
decorated imine-linked COFs (BD-COF and DBD-COF) were
further oxidized to corresponding amide COFs (Ox-BD-COF and
Ox-DBD-COF) using sodium chlorite (NaClO ) as the oxidant.
2
Finally, these two amide COFs were directly sulfonated by
chlorosulfonic acid to afford the target sulfonated COFs (Ox-BD-
3 3
COF-SO H and Ox-DBD-COF-SO H, Figure 1b).
Fourier transform infrared spectroscopy (FT-IR), solid-state
NMR spectroscopy and elemental analysis were employed to
confirm the chemical composition and transformation of these
imine COFs, amide COFs and sulfonated COFs. In terms of BD-
-
1
COF, the disappearance of C=O stretching bands at 1674 cm
and N-H stretching bands at 3431 cm-1 compared to their
corresponding monomer reveals the high condensation efficiency
of these building units (Figure S1). The new appearance of
vibration bands at 1622 cm demonstrates the formation of imine
linkages which was also confirmed by the 13C cross-polarization
magic-angle spinning (CP-MAS) NMR spectra (Figure S5). The
chemical shift at 153 ppm is attributed to the carbon of C=N
double bonds. The verification of successful oxidation and
sulfonation of BD-COF can be collectively concluded by the
SPEEK@Ox-BD-COF-SO
3
H and 50 wt% SPEEK@Ox-DBD-
−3
COF-SO H exhibit much higher proton conductivity (6.22 × 10
3
−
1
−3
−1
S cm and 3.87 × 10 S cm at 90 °C and 98% RH, respectively)
than the pristine sulfonated COFs.
-
1
Results and Discussion
2
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Chemistry - A European Journal
10.1002/chem.202004727
RESEARCH ARTICLE
analysis of FT-IR and solid-state NMR spectra. For example,
network remains intact and adopts the same structure as BD-COF
and Ox-BD-COF upon sulfonation (Figure 1b). Similar structures
were also observed for DBD-COF, Ox-DBD-COF and Ox-DBD-
−
1
compared to BD-COF, the C=O stretching band (1651 cm ) was
−1
newly appeared and the C=N imine stretching band (1622 cm )
was disappeared in the FT-IR spectrum of Ox-BD-COF. In
3
COF-SO H (Figures S29 and S31).
addition, in the case of Ox-BD-COF-SO
3
H, new O=S=O
Permanent porosities of these imine COFs, amide COFs and
sulfonated COFs were assessed by nitrogen sorption
measurements at 77 K. The sorption profiles (Figures 1d and S7)
of all COFs exhibit typical characteristics of type IV isotherms,[
which reveal that these COFs are mesoporous. BD-COF exhibits
a Brunauer-Emmett-Teller (BET) surface area of 1580 m2 g-1
-
1
stretching bands at 1005, 1031, and 1058 cm were observed
which verified the presence of sulfonic acid groups. As illustrated
in Figure S5a, a new peak around 161-162 ppm in the CP-MAS
20]
1
3
C NMR spectra assignable to amide carbonyl was observed for
Ox-BD-COF, which is consistent with the reported results.[19]
Indeed, a new carbon signal attributed to the C-SO
observed at 166 ppm in CP-MAS 13C NMR spectra for Ox-BD-
COF-SO H. Furthermore, small shifts in the frequencies of the
aromatic carbon resonances among BD-COF, Ox-BD-COF and
Ox-BD-COF-SO H further support the conversion of imine to
3
H bond was
calculated from the N
region (P/P < 0.15) with a pore volume up to 2.10 cm g (P/P
0.99). Upon oxidation and sulfonation, although the BET surface
areas of Ox-BD-COF and Ox-BD-COF-SO H decreased to 853
and 630 m g , respectively. However, the uptake of water vapor
of Ox-BD-COF-SO H greatly increased due to the hydrophilicity
of the sulfonic acid substituents (Figure S11). The corresponding
2
adsorption isotherms in the low pressure
3
-1
3
0
0
=
3
3
amide, and final substitution of sulfonic acid groups. Similar
results were observed for the DBD-COF, Ox-DBD-COF and Ox-
2
-1
DBD-COF-SO
transformation of DBD-COF to the target Ox-DBD-COF-SO
Figures S2 and S5). On the other hand, the content of grafted
3
H as well, which confirmed the successful
3
3
H
3
-1
(
pore volumes reasonably decreased to 0.96 and 0.82 cm g for
Ox-BD-COF and Ox-BD-COF-SO H, respectively. Interestingly,
sulfonic acid groups in sulfonated COFs were further estimated
by the elemental analysis (Table S1), which revealed a sulfonated
degree of 6.2 and 5.8 wt% (X wt% is the mass ratio of sulfonic
3
the pore size distribution (PSD) calculated based on the nonlocal
density functional theory (NLDFT) method shows similar
dominant pore size centered at 3.2, 3.1 and 3.0 nm for BD-COF,
3
acid groups in the sulfonated COFs) for Ox-BD-COF-SO H and
H, respectively.[
3
6a]
Ox-DBD-COF-SO
3
Ox-BD-COF and Ox-BD-COF-SO H (Figure S9a) which match
well with the theoretical value (3.2 nm, Figure S28b). On the other
hand, DBD-COF, Ox-DBD-COF and Ox-DBD-COF-SO
3
H show
comparable BET surface areas of 1228, 620 and 524 m2 g ,
-1
respectively (Figure S7). The pore size distribution analyses of
the DBD-COF, Ox-DBD-COF and Ox-DBD-COF-SO
indicated a dominant pore size distribution of 3.2, 3.1 and 3.0 nm
Figure S9b), and the corresponding pore volumes are 1.67, 0.72
3
H also
(
3
-1
and 0.64 cm g , respectively. In sharp contrast, the contrastive
2
–1
MB-COF exhibits a much lower BET surface area of 228 m g
Figure S10). Unfortunately, such a low BET surface impedes
further post-functionalization on MB-COF.
(
Figure 2. a) TEM images and corresponding EDX elemental mapping images
of b) C, c) O, d) S for Ox-BD-COF-SO H. e) UV-visible spectra of BD-COF
black line), Ox-BD-COF (blue line) and Ox-BD-COF-SO H (red line), inset
shows photographs of the three COFs under ambient light.
The morphologies of these COFs were characterized by
scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). As revealed by SEM images (Figures S15–
S17), BD-COF and DBD-COF exhibit rod-like morphology while
MB-COF showcases sheet-like structures. Upon oxidation and
sulfonation, the morphologies of cyclic ether decorated COFs still
retained. Surprisingly, TEM images of all the imine COFs, amide
COFs and sulfonated COFs disclose clear lattice stripes, which
further reflected that these COFs are highly crystalline (Figures
S18–S20). As shown in Figure 2a-d, the EDX elemental mapping
demonstrated a homogeneous distribution of C, N, O and S,
indicating that the sulfonic acid groups were evenly distributed on
3
(
3
Solid-state UV/Vis spectra exhibit broad adsorption bands with
similar absorption maxima at 436 nm for BD-COF, Ox-BD-COF
and Ox-BD-COF-SO H (Figure 2e). Interestingly, the absorption
3
region of amide-linked and sulfonated COFs is broader than the
pristine imine COFs with increasing shoulder absorption around
5
20-530 nm, which are also corresponding to the color change
from imine COF (yellow) to amide COF (orange) and sulfonated
COF (red) as shown in the inset of Figure 2e.
The crystallinities of all imine COFs, amide COFs and
sulfonated COFs were evaluated by powder X-ray diffraction
3
the pore walls of Ox-BD-COF-SO H.
The thermal stabilities of these COFs were evaluated by
thermogravimetric analysis (TGA). TGA profiles reveal that BD-
COF could be thermally stable up to about 400 °C and retained
(
PXRD) measurements (Figure 1b, Figure S26). The PXRD
pattern of BD-COF showed prominent peaks at 2.75°, 4.78°, 5.53°,
.34°, 9.59° and 25.23° corresponding to the (100), (110), (200),
210), (220) and (001) facets, respectively (Figure S28a, black
7
(
3% of its initial mass even at 800 °C under N
Figure S22a). In contrast, the amide-linked COF (Ox-BD-COF)
and sulfonated COF (Ox-BD-COF-SO H) exhibit lower
thermostability and showcase weight loss started at around
00 °C and 200 °C, respectively. This is probably due to the less
2
atmosphere
7
(
3
line). BD-COF was determined to be fitted in P3 space group with
unit cell parameters of a = b = 36.9244 Å, c = 3.5369 Å, α = β =
3
90° and γ = 120° (Table S2) upon Pawley refinement. The
simulated PXRD pattern based on the AA stacking mode (Figure
S28a, magenta line) is in good agreement with the experimental
results with the R and Rwp converged to 5.29% and 4.05%,
p
respectively. Ox-BD-COF retained the same space group and
eclipsed stacking configuration, but with slight difference in the
unit cell parameters (a = b = 36.7132 Å, c = 3.8793 Å) comparing
with BD-COF (Figure S30a). The nearly identical PXRD pattern of
conjugated skeleton and the grafted sulfonic acid groups. Similar
results were also observed for DBD-COF, Ox-DBD-COF and Ox-
DBD-COF-SO H (Figure S22b). However, the contrastive MB-
3
COF showcased weight loss below 350 °C (Figure S22c),
indicative of a worse thermostability. On the other hand, the
chemical stability tests of imine COFs were performed by soaking
3
the sulfonated Ox-BD-COF-SO H indicates that the covalent
3
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RESEARCH ARTICLE
Figure 4. a) Nitrogen sorption profiles, b) pore size distribution and c) PXRD
3 3 3
patterns of Ox-BD-COF-SO H, 50 wt% Ox-BD-COF-SO H, Ox-BD-COF-SO H-
MIX and Ox-BD-COF-SO H-WAS.
3
Figure 3. a) PXRD patterns and b) Nitrogen adsorption (solid) and desorption
attributed to the disorder induced by the flexible SPEEK chains
inside the 1D channels. However, the peak at 2.75° in the PXRD
(open) isotherms of BD-COF before (black) and after 12 M HCl (red), 9 M NaOH
blue) and boiling water (magenta) treatments.
(
pattern of Ox-BD-COF-SO
sample of 50 wt% SPEEK and Ox-BD-COF-SO
same with that of the pristine Ox-BD-COF-SO H, because the
SPEEK is mainly located outside the one-dimensional channels
and causes little effect on the periodicity of Ox-BD-COF-SO H.
After removal of SPEEK from the COF by thoroughly washing with
solvent (denoted as Ox-BD-COF-SO H-WAS), the peak intensity
of (100) facets is recovered, illustrating the structure of the COF
was retained during the whole process (Figure 4c). N adsorption
measurements were further carried out to confirm the successful
incorporation of SPEEK inside the regular channels. N uptake
and BET surface area of Ox-BD-COF-SO H-MIX exhibited a
3
H-MIX (i.e. mechanically mixed
3
H) is basically
these COFs in boiling water, strong acid (12 M HCl, 25 °C), and
corrosive base (9 M NaOH, 25 °C) for 3 days. After treatments in
these harsh conditions, the isolated COF samples were again
3
3
characterized by PXRD, N
2
sorption, FT-IR spectroscopy (Figures
3, S23, S24 and S25). All COFs maintained their crystallinity,
3
porosity and chemical composition in these harsh conditions
except the contrastive MB-COF which exhibited severe
decomposition. Furthermore, for justifying the chemical stability of
cyclic ether moieties, monomer 3 and 4 were treated in same
conditions (12 M HCl and 9 M NaOH, 25 °C) for 3 days. The
resulting mixtures were extracted with ethyl acetate, dried over
2
2
3
2
-1
significant reduction (248 m g ) but are still much higher than
H (21 m2 g ). The
-1
anhydrous MgSO
4
and evaporated without purification. Solution
that of 50 wt% SPEEK@Ox-BD-COF-SO
3
1
nearly nonporous feature of 50 wt% SPEEK@Ox-BD-COF-SO H
state H NMR spectra show that cyclic ether moieties are stable
in these conditions. These results indicate that the COFs with
cyclic ether moieties are much more stable than MB-COF bearing
o-methoxy groups in terms of both thermal and chemical stability.
SPEEK, featuring high mechanical and thermal stability, low
3
revealed that the channels of sulfonated COFs were almost fully
2
occupied by SPEEK. After the removal of SPEEK, the N uptake
2
-1
and BET surface area can be recovered up to 401 m g . The loss
in the BET surface area compared to that of the pristine Ox-BD-
2
-1
price and high proton conductivity, has been widely used as
proton-conducting materials and proton exchange membranes.[21]
3
COF-SO H (630 m g ) might be attributed the SPEEK residue
remained in the channels (Figure 4a). In order to investigate the
effect of SPEEK content on proton conductivity, different amounts
of SPEEK were loaded to prepare corresponding hybrid
composites (Figures S25 and S28). The hybrid composites were
denoted as X wt% SPEEK@Ox-BD-COF (X is the mass ratio of
SPEEK in the hybrid material). The results demonstrate that the
proton conductivity increased rapidly and gradually saturated as
the content increased (Figures S34d and S36e). Considering the
cost-effectiveness, we selected and fixed the loading amount of
50 wt% SPEEK into the hybrid composites for systematical
investigation.
However, SPEEK exhibits large solubility in water attributed to the
high sulfonation degree, which hinders its practical application as
proton conductors at high temperatures and humidity. On the
other hand, COFs are normally obtained as insoluble powders
which is difficult for direct processing. Fortunately, the large
porosity and excellent stability enable COFs as a versatile
platform for post-synthetic hybridization with various guest
polymers, which afford processable composites for tailor-made
applications.[
22]
As a kind of hydrophilic polymer, the ability of
+
SPEEK to hinder the loss of proton (H ) under high humidity is
much better than commonly used guest acid molecules/anions
The proton conductivity of the hybrids was evaluated by
alternating current impedance measurements. Figure 5b shows
403−. Stable proton-
such as H PO , phytic acid and PW12O
3 4
conducting materials under high humidity were prepared by
infiltration of SPEEK into the one-dimensional nanochannels of
sulfonated COFs. The successful incorporation of SPEEK into
sulfonated COFs was clearly demonstrated by SEM, PXRD and
the Nyquist plots of 50 wt% SPEEK@Ox-BD-COF-SO
3
H at 98%
RH from 50 to 90 °C. The proton conductivity increases with
−
3
−1
elevating the temperature and reaches 6.22 × 10 S cm at
90 °C . To illustrate the universality of this doping approach, the
same test was also performed for other hybrid composite 50 wt%
N
2
sorption measurements. SEM images indicated that the
morphology of the hybrid composites obviously changed when
increases the amount of SPEEK (Figures S16 and S17). As for
the PXRD patterns (Figure 4c), the peak at 2.75° corresponding
to the (100) facets is significantly weakened for 50 wt%
3
SPEEK@Ox-DBD-COF-SO H and comparable conductivity of
−
3
−1
3.87 × 10 S cm at 90 °C was obtained (Figure S35c).
Repeated conductivity measurements were conducted and
confirmed that the conductivity of 50 wt% SPEEK@Ox-BD-COF-
3
SPEEK@Ox-BD-COF-SO H, which could be reasonably
3
SO H basically remained unchanged on a 48-h-continuous run at
4
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Chemistry - A European Journal
10.1002/chem.202004727
RESEARCH ARTICLE
lower than 10−12 S cm , which suggests that post-sulfonation of
COFs is crucial for proton conduction enhancement. To further
confirm the versatility of the post-modification strategy, the same
−1
9
0 °C under 98% RH (Figure S34d). It is well accepted that the
Grotthuss and vehicular mechanisms are two main mechanisms
for proton transport, which are primarily identified by the activation
energy.[
23]
Based on the temperature-dependent conductivity
3
tests were carried out for 50 wt% SPEEK@Ox-DBD-COF-SO H
profiles (Figures 5c and S36d), the activation energy of 50 wt%
SPEEK@Ox-BD-COF-SO H and 50 wt% SPEEK@Ox-DBD-
COF-SO H for the proton conduction was calculated to be as low
as 0.199 eV and 0.336 eV, respectively. Such low barriers (< 0.4
and similar results of proton conductivity enhancement was
confirmed (Figure S36).
3
3
eV) imply a Grotthuss-type hopping mechanism for the proton Conclusion
conduction in these SPEEK@sulfonated COFs.
In summary, two highly crystalline and sulfonated 2D covalent
organic frameworks with cyclic ether moieties on the pore walls
were synthesized by sequential post-synthetic oxidation and
sulfonation of the corresponding imine COFs. Integrating SPEEK
into the nanochannels of sulfonated COFs triggers a profound
synergistic effect on proton conductivity. The 2D imine-linked
COFs with cyclic ether moieties exhibited higher porosity and are
much more stable than the isostructural imine COF with o-
dimethoxy groups. Further introduction of sulfonic acid groups
and SPEEK are conducive for releasing more protons for proton
conduction, therefore leading to a great enhancement of proton
conductivity. Owing to the synergistic effect of sulfonic acid
groups and SPEEK, the resulting 50 wt% SPEEK@Ox-BD-COF-
SO
3
H showcases excellent proton conductivity up to 6.22 × 10−3
S cm-1 at 90 °C and 98% RH, which is higher than many proton-
conducting COFs (Table S7). The current work thus shows the
importance of engineering pore walls and open space of the
nanochannels to create stable high-rate transporting systems and
offers a new phase in developing proton-conducting molecular
frameworks.
Acknowledgements
+
Figure 5. a) Schematic illustrations of H transport in the hybrid material. b) EIS
spectra of 50 wt% SPEEK@Ox-BD-COF-SO H. c) Arrhenius plots of 50 wt%
SPEEK@OX-BD-COF-SO H.
This work was financially supported by National Key Research
and Development Program of China (2017YFA0207500),
National Natural Science Foundation of China (51973153), and
the Natural Science Foundation of Tianjin City (17JCJQJC44600).
3
3
To differentiate the contributions of sulfonic acid groups in the
COF skeletons and the infiltrated guest SPEEK on the proton
conduction, the conductivity of 50 wt% SPEEK@Ox-BD-COF and
Keywords: covalent organic frameworks • sulfonation • proton
conduction • post-synthesis • stability
Ox-BD-COF-SO
same conditions. The proton conductivities of Ox-BD-COF-SO
i.e. sulfonated COF without SPEEK) and 50 wt% SPEEK@Ox-
BD-COF (i.e. amide-linked COF without sulfonic acid groups but
3
H were investigated and compared under the
3
H
[
1]
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−1
BD-COF-SO
3
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6
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Chemistry - A European Journal
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RESEARCH ARTICLE
Entry for the Table of Contents
Highly crystalline sulfonated COFs annulated with cyclic ether moieties were constructed by direct post-synthetic sulfonation for the
first time. Sulfonated polyether ether ketone (SPEEK) was further infiltrated into the 1D regular channels of these sulfonated COFs to
+
improve the proton conductivity and reduce the loss of protons (H ). The resulting hybrid material achieves excellent proton conductivity
up to 6.22 × 10−3 S cm−1.
7
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