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Title

Intrinsic proton conduction in 2D sulfonatedcovalent organic framework through the postsynthetic strategy

Author(s)Zhang, Yuwei; Li, Chunzhi; Liu, Zhaohan; Yao, Yuze;
Hasan, Md. Mahmudul; Liu, Qianyu; Wan, Jieqiong;
Li, Zhongping; Li, He; Nagao, Yuki

Citation

CrystEngComm, 23: 6234-6238

Issue Date

2021-08-13

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/18026

RightsCopyright (C) 2021 Royal Society of Chemistry.
Yuwei Zhang, Chunzhi Li, Zhaohan Liu, Yuze Yao,
Md. Mahmudul Hasan, Qianyu Liu, Jieqiong Wan,
Zhongping Li, He Li and Yuki Nagao,
CrystEngComm, 23, 2021, 6234-6238.
https://doi.org/10.1039/D1CE00957E

Description

Japan Advanced Institute of Science and Technology

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COMMUNICATION
Intrinsic proton conduction in 2D sulfonated covalent organic
framework through the post-synthetic strategy
Received 00th January 20xx,
Accepted 00th January 20xx

Yuwei Zhang,a† Chunzhi Li,b† Zhaohan Liu,c Yuze Yao,c Md. Mahmudul Hasan,c Qianyu Liu,a Jieqiong
Wan,d Zhongping Li,c* He Li b* and Yuki Nagao c*

DOI: 10.1039/x0xx00000x

Two-dimensional covalent organic frameworks (2D COFs) haveattracted much attentions in proton conduction, owing to theirregular pore channels and easy functionalization. However, most of
COFs required the loading of proton carriers to achieve high proton
conductivity. Here, we report the immobilization of flexible sulfonicacid groups on the channel walls of PyTTA-DHTA-COF (synthesizedby condensation of 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetraaniline
and 2,5-dihydroxyterephthalaldehyde) via a simple post-synthetic
modification strategy. The sulfonated COF showed intrinsic proton
conductivity up to 10-3 at 25 °C and 100% relative humidity (RH),
and high conductivity up to 10-2 S cm-1 under at 70 °C and 100% RH
without introduction of any non-covalent acid molecules or
imidazole derivatives.

covalent organic frameworks (COFs) with high porosity havebeen extensively investigated for proton conductivity.3-5

There is a widely accepted realization that energy
consumption has been a serious problem for the developmentof modern civilization during the last past decades. In the faceof detrimental effects, effective technologies and new materials
are urgent and essential to exploitation. The last past has
witnessed the rapid development of proton-exchange
membrane fuel cells that serve as a typical representative ofclean energy.1 Nafion is a typical commercial high-proton
conductive membrane. However, high cost and contamination
are very serious owing to the fluorohydrocarbon structure.
Therefore, the design of alternative functional materials tofabricate proton exchange membrane is of high importance. 2-3
Porous materials, such as inorganic zeolite, metal-organic
frameworks (MOFs), porous organic polymers (POPs) anda. Laboratory of Preparation and Applications of Environmental Friendly Materials(Jilin Normal University), Ministry of Education, Changchun, 130103, China.

b. State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese

Academy of Sciences, Dalian, 116023, China. E-mail: lihe@dicp.ac.cn (H. Li).

c. School of Materials Science, Japan Advanced Institute of Science and Technology,

Ishikawa 923-1211, Japan. E-mail: lizhongping2021@yonsei.ac.kr (Z. Li);ynagao@jaist.ac.jp (Y. Nagao).
d. College of Chemistry and Chemical Engineering, Shanghai University ofEngineering Science, Shanghai, 201620, China.
† These authors contributed equally.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x.

Scheme 1. Design and synthesis of PyTTA-DHTA-COF and PyTTADHTA-COF-SO3H.
Comparing to other traditional porous materials, twodimensional COFs possess intriguing aesthetic architectures,
high porosity, good thermal and chemical stability.6-10 Especially,
owing to the periodic columnar π-arrays and uniform 1D
nanopores, 2D COFs have shown potential applications in ionsand proton conductivity.11-17 The pre-designed 2D COF made itmuch easier for the proton-containing carriers to be doped or

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Journal Nameanchored. For example, 2D imine-linked TPB-DMTP-COF withhigh porosity loaded plentiful triazoles to achieve high proton
conductivity of 1.1 × 10-3 S cm-1.18 Several phosphoric acid andazole doped COFs were also prepared and showed high proton
conductivity.11-13 However, most of COFs required the loadingof proton carriers to achieve high proton conductivity. Proton
conductivity origins from the external contribution of proton
carriers. Under high temperature, the proton carriers are easy
to leak, thus the proton conductivity would be reduced.
Therefore, it is highly desirable to develop instinct proton
conducting COFs without loading proton carriers to achieve
stable proton conductivity. In this research, we anchoredsulfonic acid groups on COF’s channel walls for proton
conduction via a post-synthetic modification, which is a reliable
method in COF’s field.19-21 The intrinsic proton conductivity ofsulfonated 2D COF was investigated.

Fig. 1 (a) FT-IR spectra of PyTTA-DHTA-COF (black curve) and
PyTTA-DHTA-COF-SO3H (red). Full XPS scan survey of (b) PyTTADHTA-COF and (c) PyTTA-DHTA-COF-SO3H.
PyTTA-DHTA-COF was synthesized by condensation of
4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetraaniline
and
2,5dihydroxyterephthalaldehyde through Schiff base reaction
under solvothermal conditions (Scheme 1). The PyTTA-DHTACOF kept excellent crystallinity under 1 M NaOH at room
temperature (Fig. S1). PyTTA-DHTA-COF-SO3H that decorated
sulfonic acid groups was prepared by reaction with 1,3-propane
sulfone under base conditions (Scheme 1). Comparing the red
powder of original COF, PyTTA-DHTA-COF-SO3H showed black
powder. Firstly, Fourier transforms infrared spectroscopy (FTIR) measurements were performed for the as-prepared COFs
(Fig. 1a). The characteristic C=N stretching vibration at around1616 cm–1 confirmed the imine bond of COFs was found. Aftermodification, the signal of O═S═O symmetric and asymmetricstretching modes of PyTTA-DHTA-COF-SO3H was observed at1178 and 1387 cm–1, respectively. Additionally, a new strong
peak for PyTTA-DHTA-COF-SO3H was observed at 1641 cm–1 andseveral peaks appeared at around 2800 to 3000 cm–1,

attributing to the -CH2- stretching vibration of ethylene groups.
The X-ray photoelectron spectroscopy (XPS) analysis was
performed for PyTTA-DHTA-COF, the C 1s, O 1s, and N 1s signals
were clearly observed in the full XPS scan survey (Fig. 1b). The
signals for sulfur have been found in PyTTA-DHTA-COF-SO3H(Fig. 1c). The binding energies of the S 2p3/2 and S 2p1/2
appeared at 168.6 and 169.1 eV, respectively (Fig. S2). Field
emission scanning electron microscopy (FE SEM) images were
implemented to investigate the morphology of COFs (Fig. S3).
The morphology is no obvious difference between two COFs
and this indicated the morphology remained after thesulfonation process. We observed the elemental mapping byenergy dispersive spectroscopy (EDS) analysis for COF (Fig. S4).
Interestingly, the EDX mapping indicated that the sulfur
element is evenly dispersed in the skeleton of sulfonated COF(Fig. S4b). PyTTA-DHTA-COF-SO3H exhibited moderate sulfur
acid group amount of 19.5 wt%, which was confirmed throughelemental analysis (Table S1).

Fig. 2 (a) PXRD patterns of the PyTTA-DHTA-COF(experimentally observed pattern: red; Pawley refinement:
black; their difference: pink; AA staking: blue). (b) Top and (c)side views of the crystal structure of PyTTA-DHTA-COF derived
using the AA stacking model.
The crystalline nature of COF was determined by powder X-ray
diffraction (PXRD) measurement. The PXRD pattern of PyTTA-DHTACOF displayed obvious diffraction peaks at two theta of 3.70, 5.30,
7.31, 11.29, 15.17 and 23.69°, which were assignable to (110), (020),(220), (040), (060), and (001) facets, respectively (Fig. 2). The Pawley
refined pattern (Fig. 2a, black) confirmed the diffraction peakassignments of the observed diffraction peaks (Fig. 2a, red). FromXRD patterns, it is matched well with AA-mode. Simulations using the
P1 space group with a = 32.3826 Å, b = 39.3389 Å, c = 3.6909 Å and
α = β = 90°, γ = 120° gave a PXRD pattern that was in good agreementwith the experimentally observed pattern. The staggered AA mode
(blue) primely meted the experimental results with Rwp of 5.65%
and Rp of 3.85%. PyTTA-DHTA-COF-SO3H also remained the crystal
structure after post modification and showed broad peaks incomparison with original COF (Fig. S5).
We inspected the porosity of COFs through nitrogen sorption
measurements at 77 K (Fig. 3). PyTTA-DHTA-COF has the highest
Brunauer–Emmett–Teller (BET) surface area of 2013 m2 g–1 witha pore size of 2.1 nm, which is adjacent to the theoretical result
by AA stacking mode of 2.1 nm. This result proved the high

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regular 1D channel of PyTTA-DHTA-COF. After the postmodification process, a lower BET surface area of 493 m2 g–1 for
PyTTA-DHTA-COF-SO3H was obtained. PyTTA-DHTA-COF-SO3H
also decreased the pore volume from 1.91 cm3 g–1 (PyTTADHTA-COF) to 0.75 cm3 g–1. Pore size was also decreased from
2.1 nm to 1.7 nm due to the pore block by the post-modification
process. For a humidity-mediated proton-transport system, it
promotes andstrengthens
proton-exchange/transport
efficiency owing to extended hydrogen-bonded frameworks
and water molecules. Therefore, the water vapor sorption
isotherm of sulfonated COF was measured at 298 K (Fig. S6).
PyTTA-DHTA-COF-SO3H showed a high water uptake of 22.4mmol g-1, which is comparable to those of reported COFs.22-25

improved (Figs. 4a and S8). PyTTA-DHTA-COF-SO3H showed theproton conductivities of 6.2 × 10−3, 1.1 × 10−2, and 1.6 ×
10−2 S cm−1 at 40, 50 and 60 °C, respectively. When increasing
the temperature to 70 °C, the conductivity value was as high as
2.0 × 10−2 S cm−1, which ascribes uniform 1D channel affords theway to improve proton transport. This result is comparable towell-performed materials with sulfonic acid groups such aspoly(fluorenyl ether ketone) (5.4 × 10–3 S cm–1, 80 °C, water),26
and sulfonated poly(aryl ether) (1.0× 10–2 S cm–1, 80 °C,
water).27 The time-dependent proton conduction of PyTTADHTA-COF-SO3H was measured and the proton conductivity
didn’t show any change after 8 h (Fig. S10).

Fig. 4 Proton conductivity of (a) PyTTA-DHTA-COF (black) and
PyTTA-DHTA-COF-SO3H (red) at different temperature and100% RH. Arrhenius plots for (b) PyTTA-DHTA-COF (black) and
PyTTA-DHTA-COF-SO3H (red).

Fig. 3 Nitrogen sorption isotherms of (a) PyTTA-DHTA-COF and(c) PyTTA-DHTA-COF-SO3H measured at 77 K (●: adsorption, ○:desorption). Pore size distribution profiles of (b) PyTTA-DHTACOF and (d) PyTTA-DHTA-COF-SO3H.
PyTTA-DHTA-COF-SO3H exhibited good porosity, remarkablewater uptakes, and plenty of sulfonic acid units, which triggered
us to measure the water-mediated proton conductivity. With
the increasing humidity, the proton conductivities of PyTTADHTA-COF-SO3H have been improved of 1.0 × 10−6, 3.2 × 10−5,
1.4 × 10−4 and 4.9 × 10−4 S cm−1, respectively, under 43%, 53%,75%, and 85% humidity (Fig. S7). This indicates the crucial roleof humidity in the proton conductive process. The proton
conductivity dependence on temperature was investigated by
testing the materials from 25 to 70 °C under 100% RH (Figs. 4aand S8). PyTTA-DHTA-COF-SO3H showed the proton
conductivities of 3.0 × 10−3 S cm−1 at 25 °C and 100% RH, which
is much higher than that of PyTTA-DHTA-COF (9.5 × 10-5 S cm−1,
Figs. 4a and S8) and PyTTA-TA0.5-DHTA0.5-COF-SO3H0.5 under thiscondition (4.1 × 10-4 S cm−1, Fig. S9). This indicates that
increasing the sulfonic acid groups of PyTTA-DHTA-COF cangreatly enhance proton conductivity. We also investigated thetemperature dependence on proton conductivity for COFs from25 to 70 °C under 100% RH. With the increment of temperature,
the proton-conducting values of PyTTA-DHTA-COF-SO3H were

From linear least-squares fits of the slopes of Arrhenius plotsat different temperatures, we calculated the activation energy(EA) of proton-conducting COFs (Fig. 4b). A higher activationenergy was determined to be 0.50 eV for PyTTA-DHTA-COF. On
the contrary, the PyTTA-DHTA-COF-SO3H exhibited a lower
activation energy of 0.38 eV. These results indicated Grotthuss
proton-transport mechanism is shown in the sulfonated COF
system. Owing to extended hydrogen-bonding frameworks
towards water molecules, a hooping pathway affords atransportation way of the proton and improved the protontransfer efficiency.
In short, we report the immobilization of flexible sulfonic acidgroups on the channel walls of PyTTA-DHTA-COF via a simple
post-synthetic modification strategy. The sulfonated COF
showed intrinsic proton conductivity up to 10-3 at 25 °C and100% RH, and high conductivity up to 10-2 S cm-1 under at 70 °C
and 100% RH without introduction of any acid, imidazole ortriazole derivatives. This research evolves a new way to design
high proton-conductive COFs.

Author Contributions
Y. Zhang and C. Li contributed equally. Y. Zhang, C. Li, Z. Liu, Y. Yao,
Md. M. Hasan, Q. Liu, H. Li, J. Wan and Z. Li conducted experiments
and collected data collection. Y. Zhang, H. Li, Y. Nagao and Z. Li wrote
the manuscript and discussed the results with all authors.

Conflicts of interest
J. Name., 2013, 00, 1-3 | 3This journal is © The Royal Society of Chemistry 20xx

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There are no conflicts to declare.

Acknowledgements
Y. Z. appreciated the support by National Natural Science
Foundation of China (grant no. 21805110) and the projects of
Jilin Province Department of Education (JJKH20210441KJ). H. L.
appreciate the support National Natural Science Foundation ofChina (grant no. 22002162). Y. N. appreciated the support by
JSPS KAKENHI grant numbers JP21H00020 and JP21H01997 andThe Murata Science Foundation. J. Wan appreciated support
from The National Natural Science Foundation of China (No.
51902195)

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