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Title

Accumulation of Sulfonic Acid Groups Anchored in
Covalent Organic Frameworks as an Intrinsic
Proton-Conducting Electrolyte

Author(s)Zhai, Lipeng; Yao, Yuze; Ma, Baiwei; Hasan, Md.
Mahmudul; Han, Yuxi; Mi, Liwei; Nagao, Yuki; Li,
Zhongping

Citation

Macromolecular rapid communications, 43(1):2100590

Issue Date

2021-10-06

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18092

RightsThis is the peer reviewed version of the following
article: Copyright (C) 2021 Wiley-VCH. Lipeng Zhai,
Yuze Yao, Baiwei Ma, Md. Mahmudul Hasan, Yuxi
Han, Liwei Mi, Yuki Nagao, Zhongping Li,
Macromolecular rapid communications, 43(1),2021, pp.2100590, which has been published in
final form at
https://doi.org/10.1002/marc.202100590. This
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Use of Self-Archived Versions. This article may not
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Description

Japan Advanced Institute of Science and Technology

Accumulation of sulfonic acid groups anchored in covalent organic frameworks as an
intrinsic proton-conducting electrolyte
Lipeng Zhai, † Yuze Yao,† Baiwei Ma, Md Mahmudul Hasan, Yuxi Han, Liwei Mi, * Yuki Nagao,* and Zhongping Li *L. Zhai, B. Ma, L. Mi.
Country Henan Key Laboratory of Functional Salt Materials, Center for Advanced Materials
Research, Zhongyuan University of Technology, Zhengzhou, 450007 P. R. China.
E-mail: mlwzzu@163.com
Y. Yao, M. M. Hasan, Y. Nagao, Z. Li.
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1
Asahidai, Nomi, Ishikawa 923-1292, Japan.
E-mail: ynagao@jaist.ac.jp; lizhongping2020@gmail.com.
Y. Han.
School of Materials Science and Engineering, Shandong University of Science andTechnology, Qingdao, Shandong 266590, People’s Republic of China.
Keywords: Covalent organic frameworks, stability, nanochannel, proton conductivity.
Abstract: Covalent organic frameworks (COFs) are a novel class of crystalline porous
polymers, which possessed high porosity, excellent stability, and regular nanochannels. Twodimensional (2D) COFs provided 1D nanochannel to form the proton transport channels. The
abovementioned features afforded a powerful potential platform for designing materials as
proton transportation carriers. Herein, we incorporated sulfonic acid groups on the pore walls
as proton sources for enhancing proton transport conductivity in the 1D channel. Interestingly,
the sulfonic acid COFs (S-COFs) electrolytes with binder free exhibited excellent proton
conductivity about 1.5 × 10-2 S cm-1 at 25 ℃ and 95% relative humidity (RH), which ranked
the excellent performance in standard proton-conducting electrolytes. The S-COFs electrolytes
kept the high proton conduction over the 24 h. The activation energy was estimated to be as
low as 0.17 eV, which was much lower than most reported COFs. This research opens a new
window to evolve great potential of structural design for COFs as the high proton-conducting
electrolytes.

11. Introduction
Due to the rapid progress and development of society, social modernization and industrial
development, fossil fuels are the main source of the utilized energy leading a major greenhousegas pollution and global warming. Therefore, developing novel and clean energy is an effective
and reasonable way to tackle these issues. Currently, owing to the lower or zero emissions
compared to fossil fuels, fuel cell has attracted scientists' wide attention. Meanwhile,developing materials with outstanding proton conductivity is essential and attractive forfabricating proton exchange membranes in electronic devices and fuel cells. In the past few
decades, owing to their distinct advantages including excellent physical and chemical stability
and outstanding proton conductivity, Nafion or Flemion is considered to be the widely protonconducting materials.1 However, inadequate durability and high water/methanol permeability
still restrict their further applications, which inspire us to explore and design new materials as
potential proton-conducting electrolyte candidates for replacing Nafion or Flemion.2-5
Owing to the high crystallinity, excellent porosity, remarkable stability, and pre-designablestructures, a rapid development and great progress were achieved for covalent organic
frameworks (COFs).6-11 Besides, COFs provided an ideal platform for precisely integratingorganic units into two- (2D) or three-dimensional (3D) structures with long-range periodic
skeletons and high-ordered porous structures. Therefore, these superiorities enable thempossessing wide areas including molecules storage and separation, catalysts, light-emitting
materials and sensing, and proton/ion conduction.12-35
Two-dimensional (2D) COFs possessed high-ordered π-structure and uniform 1D columns,
which provided effective channels for proton transportation. In this respect, COFs haveachieved a milestone using as proton-conducting electrolytes. However, up to now, mostresearchers have focused on employing COFs as carriers to provide proton resources such as
imidazole, phosphoric acid, and p-toluene sulfonic acid.30-37 In this strategy, host-guest
interactions lead to the inevitable leaching of protons from the pore structures. Long-term2excellent performance is hard to achieve since these molecules were aligned the channel.
Recently, in order to solve these issues, constructing proton-conducting COFs without adding
extra proton sources for directly fabricating electrolytes has been developed. Zhao and coauthors reported intrinsic proton-conducting COFs under water conditions with binder due totheir fragile style COFs pellet itself.38 Banerjee and co-authors constructed COFs as intrinsic
proton-conduction under anhydrous process. 39 However, the intrinsic proton-conducting COFs
as effective electrolytes with high stability, good durability and binder free has not been
explored under the humidity conditions. In this project, the electronic cloud of high-density
methoxy units over the vertexes delocalized from vertex to the imine-linkage through the p-π
conjugation in Scheme 1, which therefore enhanced the stability of frameworks. The richsulfonic acid groups have been anchored on the walls via the bottom-up strategy in Scheme 1.
These features afforded 2D sulfonic functionalized frameworks as excellent candidates for
proton conduction. The remarkable proton conductivity of S-COFs was as well as high 1.5 ×102

S cm-1 at 25 °C and 95% relative humidity (RH). Meanwhile, S-COFs not only possessed low

activation energy, but also achieved high durability for proton conduction in this work.
2. Results and discussion
The S-COF-1 and S-COF-2 were synthesized from 2,4,6-trimethoxybenzene-1,3,5tricarbaldehyde and 2,5-diaminobenzenesulfonic acid or 2,5-diaminobenzene-1,4-disulfonicacid using 1,4-dioxane and 1,3,5-trimethylbenzene as solvents and 6 M acetic acid as catalyst
under solvothermal reaction, respectively. The S-COF-1 and S-COF-2 were characterized by
Fourier-transform infrared spectroscopy (FT IR), Field emission scanning electron microscope(FE SEM), X-ray photoelectron spectroscopy (XPS), nitrogen sorption isotherm, elemental
analysis, water vapor sorption, and powder X-ray diffraction (PXRD).
Firstly, the chemical structure information of S-COFs was investigated by FT IR
measurement (Figure S1). The S-COF-1 and S-COF-2 showed the C=N bonds at 1580 and 1581
cm−1, respectively. The S-COF-1 and S-COF-2 displayed the methoxy units signals in 2917 and3

2925 cm−1, respectively. The obvious peaks were found at 1441 and 1443 cm−1 for S-COF-1
and S-COF-2, respectively, which were assigned to the O=S=O groups stretching.
XPS analysis was further employed to investigate the structure information of these two
COFs (Figure 1). Based on the XPS spectra results, S-COF-1 and S-COF-2 displayed C 1s, O
1s, N 1s, S 2s, and 2p signals. S-COF-1 and S-COF-2 showed the bonding energies of S 2p3/2
and S 2p1/2 around 168.2 and 169.3 eV, and 168.2 and 169.3 eV, respectively. The element
analysis results displayed the sulfonic element of 9.76 and 15.88 wt% in Table S1, which isvery close to the theoretical values. The contents of sulfonic units for S-COF-1 and S-COF-2
were calculated to be 24.7% and 40.2%, respectively. The EDS analysis was observed for SCOF-1 (Figure S2) and S-COF-2 (Figure S3), which indicated a uniform distribution ofelemental Sulphur. FE-SEM images demonstrated that S-COF-1 and S-COF-2 exhibited
micrometerscale morphology (Figure S4).
Nitrogen (N2) adsorption-desorption isotherms at 77 K were utilized to investigate theporosity of S-COF-1 and S-COF-2 (Figure S5a-b). S-COF-1 and S-COF-2 exhibited type-1
sorption curves, which was typical of the microporous characteristics. The Brunauer-EmmettTeller (BET) surface areas were calculated to be 351 and 139 m2 g-1 for S-COF-1 and S-COF2, respectively. The pore size distribution of S-COFs was explored using non-local density
functional theory (NLDFT) method (Figure S5c-d). The S-COF-1 and S-COF-2 showed thepore size cantered at 1.30 and 1.26 nm, respectively.
The crystallinity of S-COFs was observed by PXRD measurement (Figure 2a-b). The
experimental PXRD pattern of S-COF-1 showed the strong peaks at 4.68, 8.78, 11.40, and26.60° (Figure 2a, red), which were assigned to 100, 200, 300, and 001 facets, respectively. As
shown in Figure 2a, the observed diffraction peaks were assigned by using the Pawley refinedpattern. Simulations based on the space group with a = b = 25.32 Å, c = 4.97 Å and α = β = 90°,
γ = 120° can reproduce the PXRD pattern that was in good agreement with the AA-stacking
experimentally observed pattern (Figure 2a, green). In contrast, the AB staggered stacking
4mode does not match the PXRD experimental result (Figure 2a, orange). The experimental
PXRD pattern of S-COF-2 displayed obvious peaks at 4.52, 7.64, and 25.43° (Figure 2b, purple),
which associated with 100, 200, and 001 facets, respectively. The S-COF-2 matched the AAstacking model (Figure 2b and 2c). After the Pawley refinement the optimize the lattice
parameters was gave for S-COF-2 with a = b = 25.35 Å, c = 5.23 Å and α = β = 90°, γ = 120°.
Humidity is an essential factor for proton transport system, which is beneficial to tune andpromote proton-exchange/transport efficiency. We investigated water vapor sorption isothermsof S-COFs at 298 K (Figure S6). The water adsorption capacities of S-COF-1 were 15.4 mmol
g−1 at P/P0 = 0.95 and 298 K (Figure S6a). The higher water uptake of S-COF-2 were found to
be 23.3 mmol g−1 (Figure S6b) due to its high sulfonic acid density in the frameworks comparedwith S-COF-1. To consider the long term for proton-conducting electrolyte, we kept the SCOFs samples in the water under the water system for 72 h. Then, the S-COFs were filtered,washed by water, and dried under the vacuum. As shown in Figure S7a-b, FT IR spectra and
PXRD patterns for S-COF-1-W and S-COF-2-W were also checked, the same characteristic
signals were found for as-synthesized COFs and S-COF-Ws. The good crystal peaks have beenshown in Figure S8a-b. The good stability of S-COFs was on the grounds that the electronic
cloud of the high-density methoxy units on the vertexes delocalized from vertex to the iminelinkage through the p-π conjugation to improve the stability of frameworks.26 These results
provided a guarantee for S-COFs using as electrolytes to work for a long time.
The S-COF-1 and S-COF-2 possessed high-density sulfonic acid groups on the wall and an
opened uniform 1D channel (Figure 3), which inspire us to explore the proton conductivity
under various humidity condition. Firstly, the humidity dependence on proton conductivity
were observed for S-COF-1 with binder free. The lower proton-conducting result was found at
1.4 × 10−7 S cm−1 under 40% and 25 °C (Figure 4a). With increasing humidity conditions, theconductivities of S-COF-1 were also enhanced to be 1.4 × 10−6, 2.4 × 10−5, and 1.8 × 10−4 S
cm−1, respectively, under 60%, 80%, and 90% RH at 25 °C (Figure 4a). The higher proton
5

conductivity value reached 7.5 × 10−4 S cm−1 at 95% RH. The proton-conducting values of SCOF-2 were also promoted with the increasing of humidity (Figure 4b). The excellent proton
conductivity of 1.5 × 10−2 S cm−1 were achieved for S-COF-2, which was 20 times than that of
S-COF-1. The remarkable conductivity of S-COF-2 was attributed to high-density protonsource on the walls, which among the excellent performance in the reported COFs and sulfonicporous polymers. 40-42
We also investigated the proton conductivity of S-COFs under different temperatures and
95% RH. With improvement of temperature, the proton conductivities of S-COF-1 and S-COF2 were slightly enhanced. For example, the proton conductivity values of S-COF-1 increasedslightly to 1.1 × 10–4 (35 °C), 1.4 × 10–4 (45 °C), 1.7 × 10–4 (55 °C), and 5.1 × 10–3 S cm–1
(65 °C) under 95% RH (Figure 5a). The highest value was as high as 2.9 × 10–2 S cm–1 for SCOF-2 (Figure 5b). To our best knowledge, this result shows similarly high conductivity
performance, which could be comparable to the standard materials such as Nafion 115 (4.5 ×
10-2 S cm–1, 65 °C, water)43a and Nafion 112 (3.1 × 10–2 S cm–1, 65°C, water).43b With theconsideration of durability, we inspected the time-dependent proton conduction for S-COF-2
for 24 h in Figure S9. There was no obvious decline in proton conductivity values for S-COF2. The remarkable proton-conducting performance not only dependent on the high-density
sulfonic acid groups on the wall contributed the proton source, but also an opened 1D channel
formed effective proton transportation pathway.
Linear least-squares fitting of the slopes of Arrhenius plots at different temperatures were
employed to investigate the activation energy (Ea) of proton-conducting electrolyte. The Ea
values were found to be 0.25 eV for S-COF-1 (Figure 5c) and 0.17 eV for S-COF-2 (Figure 5d),respectively. Therefore, the mechanism for proton conduction is transported by the hopping
pathway through extended hydrogen-bonding interaction between sulfonic acid units on thewalls and water molecules. In the respect, the proton-transfer in the 1D channel of 2D COFs
was great improvement.
63. Conclusion
In summary, we designed and synthesized two new sulfonated COFs as intrinsic protonconducting electrolytes. The 2D S-COFs had rich sulfonic acid groups on the wall and a uniform
1D channel to afford the excellent proton-conducting performance with binder free. The SCOF-2 exhibited excellent proton conductivity of 1.5 ×10-2 S cm-1 at 25 °C and 95% relative
humidity (RH), which ranked the excellent performance in standard proton-conducting
electrolytes. The S-COF-2 electrolyte kept the high proton conduction during the 24 h.
Moreover, S-COF-2 also displayed the lower activation energy at 0.17 eV. This research opens
the new design of proton-conducting COFs electrolytes for a new benchmark that combineshigh conductivity, durability, and binder free.
Author Contributions
L. Zhai and Y. Yao contributed equally. L. Z. and Y. Yao, B. Ma, N. Qin, Y. Han, Md. M.
Hasan, and Z. Li conducted experiments and collected data collection. L. Zhai, L. Mi, Y. Nagao
and Z. Li wrote the manuscript and discussed the results with all authors.

7

Scheme 1. Design and synthesis of S-COF-1 and S-COF-2.

8Figure. 1. X-ray photoelectron spectroscopy spectrum of (a) S-COF-1 and (e) S-COF-2. XPS
analysis of C 1s of (b) S-COF-1 and (f) S-COF-2. XPS analysis of N 1s of (c) S-COF-1 and (g)S-COF-2. XPS analysis of S 2p of (d) S-COF-1 and (h) S-COF-2.

9Figure 2. PXRD patterns of (a) S-COF-1 and (b) S-COF-2 (Observed pattern: purple; Pawley
refinement: red; Their difference: black; AA-stacking model: green; AB-stacking model:
orange). The unit cell structure of (c) (d) S-COF-1 and (e) (f) S-COF-2.

10Figure 3. Proton framework in 1D channel of (a) S-COF-1 and (b) S-COF-2.

11

Figure 4. Proton conductivities and Nyquist plots of (a) S-COF-1 and (b) S-COF-2 under
different RH and 25 °C.

12Figure 5. Nyquist plots of (a) (c) S-COF-1 and (b) (d) S-COF-2 under 95% RH and different
temperature. (308 K: blue curves; 318 K: sky-blue curves; 328 K: green curves; 338 K: red
curves).
Acknowledgements
L. Z. appreciates support from National Natural Science Foundation of China (52103277), Key
Projects of Science and Technology of Henan Province (No. 212102210208), N. Q. appreciates
support Key Projects of Science and Technology of Henan Province (No. 202102210054). L.
M. thanks the National Natural Science Foundation of China (Grant No. 21671205, U1804126,
21771164, and U1804129). Y. N. appreciated the support by JSPS KAKENHI grant numbers
JP21H00020 and JP21H01997 and The Murata Science Foundation. Y. H. appreciates support
from Shandong Provincial Natural Science Foundation (No. ZR2020QB023).
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
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Two new sulfonated COFs as intrinsic proton-conducting electrolyte were constructed. The 2D
COFs have rich sulfonic acid groups on the uniform 1D channel to afford the excellent protonconducting performance with binder free. Excellent proton conductivity of 1.5 ×10-2 S cm-1 at
25 °C and 95% relative humidity (RH) were achieved, which ranked the excellent performancein proton-conducting electrolyte.
L. Zhai, Y. Yao, B. Ma, M. M. Hasan, Y. Han, L. Mi, * Y. Nagao,* Z. Li*
Accumulation of sulfonic acid groups anchored in covalent organic frameworks as an
intrinsic proton-conducting electrolyte

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