Mechanoassisted Synthesis of Sulfonated Covalent Organic

Jul 6, 2016 - It is challenging to introduce pendent sulfonic acid groups into modularly built crystalline porous frameworks for intrinsic proton cond...
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Mechanoassisted Synthesis of Sulfonated Covalent Organic Frameworks with High Intrinsic Proton Conductivity Yongwu Peng,†,⊥ Guodong Xu,‡,⊥ Zhigang Hu,† Youdong Cheng,† Chenglong Chi,† Daqiang Yuan,§ Hansong Cheng,*,‡ and Dan Zhao*,† †

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, University of Geosciences Wuhan, 388 Lumo RD, Wuhan, 430074 China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002 China ‡

S Supporting Information *

ABSTRACT: It is challenging to introduce pendent sulfonic acid groups into modularly built crystalline porous frameworks for intrinsic proton conduction. Herein, we report the mechanoassisted synthesis of two sulfonated covalent organic frameworks (COFs) possessing onedimensional nanoporous channels decorated with pendent sulfonic acid groups. These COFs exhibit high intrinsic proton conductivity as high as 3.96 × 10−2 S cm−1 with long-term stability at ambient temperature and 97% relative humidity (RH). In addition, they were blended with nonconductive polyvinylidene fluoride (PVDF) affording a series of mixed-matrix membranes (MMMs) with proton conductivity up to 1.58 × 10−2 S cm−1 and low activation energy of 0.21 eV suggesting the Grotthuss mechanism for proton conduction. Our study has demonstrated the high intrinsic proton conductivity of COFs shedding lights on their wide applications in proton exchange membranes. KEYWORDS: proton conductivity, covalent organic frameworks, mechanoassisted synthesis, mixed matrix membranes, proton exchange membranes

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INTRODUCTION Due to the growing concerns of fossil fuel exhaustion and environmental deterioration, hydrogen fuel cells have been considered as an important option for renewable and clean energy, in which the proton exchange membrane fuel cells (PEMFC) are the most widely studied ones.1,2 The research of proton exchange membranes,3 an important component of PEMFC, is mainly focused on polymeric materials, among which a sulfonated tetrafluoroethylene polymer named Nafion is considered as the benchmark material due to its high proton conductivity (ca. 10 −1 S cm −1 ). 4 However, the high manufacturing cost of Nafion as well as its narrow working conditions require further material development.5 In this context, many materials ranging from mineral to organic acids as well as organic−inorganic hybrids have been explored as possible candidates for proton conduction.6,7 Recently, modular crystalline materials such as metal−organic frameworks (MOFs) have attracted much attention due to their ordered structures and easily modified chemical compositions suitable for high proton conductivity.8−15 Nevertheless, MOFs as proton-conductive materials to be used in proton exchange membranes need to overcome several challenges such as © XXXX American Chemical Society

limited hydrothermal stability, poor mechanical strength, difficulty in forming compact membranes, etc.10 As a new member of crystalline materials, covalent organic frameworks (COFs) are porous crystalline polymers constructed from pure organic building blocks linked with robust covalent bonds.16−19 They are featured with periodic architectures, low densities, permanent porosity, and are widely investigated for various applications such as gas storage and separation,20−22 optoelectronics,23−26 heterogeneous catalysis,27−32 etc. Compared to MOFs, COFs have better hydrothermal stability due to their stable covalent bonds. In addition, their feature of organic polymers grants them better processability and compatibility to be used as membrane materials. These prominent properties render COFs as promising candidates for the next generation proton-conductive materials. However, COFs have rarely been explored for their proton conductivity yet. Before we embarked on this study, there was only one case demonstrated by Banerjee and co-workers, in which nonconductive COFs were Received: May 24, 2016 Accepted: June 27, 2016

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DOI: 10.1021/acsami.6b06189 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

°C under vacuum for 12 h to give NUS-9(G) (G indicates Grinding) as a red powder with 80% yield and NUS-10(G) as a dark red powder with 76% yield. Elemental analysis of NUS-9(G) (C12H8N2O5S)n % calc/found: C 49.31/49.84, H 2.76/2.72, N 9.58/9.12, S 10.97/11.36. Elemental analysis of NUS-10(G) (C12H8N2O8S2)n % calc/found: C 38.71/38.82, H 2.17/2.16, N 7.52/7.32, S 17.22/17.31. Recrystallization was applied in order to improve the crystallinity of COFs obtained via mechanosynthesis. In a typical procedure, NUS9(G) (25 mg), mesitylene/dioxane (1/9, v/v, 1 mL), and aqueous acetic acid (0.1 mL, 3 M) were added into a 10 mL Pyrex tube. The tube was flash frozen at 77 K using a liquid N2 bath and degassed by three freeze−pump−thaw cycles, sealed under vacuum, and then heated at 120 °C for 3 days. After cooling to room temperature, the red powder was collected by centrifugation and washed with anhydrous ethanol, anhydrous tetrahydrofuran, and anhydrous acetone. The product NUS-9(R) (R indicates Recrystallization) was subject to solvent exchange with anhydrous methanol for 3 days and dried at 150 °C under vacuum for 12 h before being characterized by PXRD and gas sorption analysis. NUS-10(R) was obtained similarly. The yields of recrystallization were almost quantitative. Proton Conductivity Measurements. The proton conductivity measurements of COFs were performed using COF pellets. Each COF pellet was prepared by finely grinding ca. 130 mg of NUS-9(R) or NUS-10(R) and pressing under a pressure of 10 t to obtain a uniform pellet with a diameter of 13 mm and a thickness of 0.10−0.20 cm (Figure S11). The COF-containing mixed matrix membranes (MMMs) denoted as COF@PVDF-X (X represents the weight percentage of COFs in MMMs) were prepared by conventional solution cast technique.40 Take NUS-9(R)@PVDF-50 for example. NUS-9(R) (75 mg) was dispersed in N-methyl-2-pyrrolidone (NMP, 5 mL) via sonication for 2 h, to which polyvinylidene fluoride (PVDF, 75 mg) dissolved in NMP (2 mL) was added, followed by stirring at 80 °C for another 2 h. The resultant homogeneous suspension was casted into a Petri dish, followed by drying at 80 °C for 24 h and further drying under vacuum at 80 °C for extra 12 h affording a membrane with a thickness of 80− 120 μm. The obtained membrane was washed for three times and placed in ultrapure water for 24 h prior to conductivity test. Other membranes were prepared similarly, with the total mass of COF and PVDF fixed at 150 mg for each membrane. The proton conductivity of COF pellets and COF-containing MMMs was measured by a quasi-four-probe method using a Solartron SI 1260 Impedance/Gain-Phase Analyzer and 1296 Dielectric Interface. Samples were equipped with two platinum blocking electrodes in a measurement cell and placed in a temperature and humidity controlled environment. For COF pellets, proton conductivity was measured in homemade humidity chambers with 19 cm height and 6 cm diameter (Figure S15). Different relative humidity (RH) was controlled by using various saturated aqueous salt solutions including MgCl2 (33% RH), K2CO3 (43% RH), Mg(NO3)2 (53% RH), NaCl (75% RH), KCl (85% RH), and K2SO4 (97% RH).41 Each saturated aqueous salt solution was placed in the chamber for 3 days prior to conductivity test to make sure the desired RH has stabilized inside the chamber. For each test, the sample was kept for at least 72 h under specific RH to ensure the establishment of equilibrium state. For COF-containing MMMs, proton conductivity was measured with MMMs directly soaked in ultrapure water under various temperatures recorded with a K-type thermocouple. Resistance was calculated from the semicircles of the Nyquist plots. Proton conductivity was calculated from the equation σ = L/(RA) where σ represents the proton conductivity, L stands for the thickness of the pellet or membrane, R denotes the resistance of the pellet or membrane, and A is the area of the pellet or membrane πr2 (r = radius of the pellet or membrane). The activation energy of proton conduction in MMMs was determined from the slope of Arrhenius plots by least-squares fitting.

loaded with phosphoric acid to facilitate proton conduction in both hydrous (σ = 9.9 × 10−4 S cm−1) and anhydrous state (σ = 6.7 × 10−5 S cm−1).33 During the preparation of this manuscript, three more examples have been recently demonstrated by Banerjee’s group34,35 and Jiang’s group,36 independently. In most of the above examples, the demonstrated proton conductivity in COFs is extrinsically contributed by the loaded proton carriers, which may be susceptible to leakage resulting in unstable proton conduction under anhydrous and at hightemperature (T > 100 °C) working conditions.33,34,36 Therefore, it is more relevant to develop COFs with intrinsic proton conductivity that can ensure more stable proton-conductive performance.35 Intrinsic proton conductivity can be introduced by strong acid groups with high acid dissociation constants such as phosphoric acid or sulfonic acid groups. However, these functional groups are normally incompatible with the condensation reactions used for COF growth. Recently, Banerjee’s group has demonstrated remarkable chemical stability in COFs via irreversible enol-to-keto tautomerization.37 With the same chemistry, we have successfully synthesized the first fully sulfonated COF that was used as an efficient solid-acid catalyst for biobased chemical conversion.32 In this study, two fully sulfonated COFs were prepared by mechanosynthesis with much higher yields and greatly reduced synthetic costs. The intrinsic proton conductivity of these COFs and their composite membranes operated in the presence of water and at low-temperature (T < 100 °C) working conditions were systematically studied to evaluate their performance as proton-conductive materials.

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EXPERIMENTAL SECTION

Materials and Equipment. All chemicals and reagents were commercially available and used without further purification. 1,3,5Triformylphloroglucinol (TFP) and 2,5-diaminobenzene-1,4-disulfonic acid (DABDA) were synthesized according to the published procedure.38,39 Fourier transform infrared spectroscopy (FTIR) spectra were obtained with a Bio-Rad FTS-3500 ARX FTIR spectrometer. Solid-state nuclear magnetic resonance (NMR) data were collected on a Bruker Avance 400 MHz NMR spectrometer (DRX400) with cross-polarization magic-angle-spinning (CP/MAS). Elemental analyses were performed on a Vario MICRO series CHNOS elemental analyzer. Scanning electron microscopy (SEM) was conducted on a JEOL-JEM5600 Lab-SEM (15 kV) equipped with an energy dispersive spectrometer. Samples were treated via Pt sputtering before observation. The Brunauer−Emmett−Teller (BET) surface areas were calculated from N2 sorption isotherms at 77 K using a Micromeritics ASAP 2020 surface area and pore size analyzer. Transmission electron microscopy (TEM) was conducted on a JEOLJEM 3010 TEM. Powder X-ray diffraction (PXRD) patterns were obtained on a Bruker D8 Advance X-ray powder diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at a scan rate of 0.02° s−1. Thermogravimetric analyses (TGA) were performed using a Shimadzu DTG-60AH in the temperature range of 50 to 800 °C under flowing N2 (30 mL min−1) with a heating rate of 10 °C min−1. Synthetic Procedures. In a typical process of mechanosynthesis, a mortar (inner diameter =75 mm) was charged with TFP (21.0 mg, 0.1 mmol) and DABA (28.2 mg, 0.15 mmol) for NUS-9 or DABDA (40.2 mg, 0.15 mmol) for NUS-10 together with 50 μL of mixed solvent containing equal volume of mesitylene/dioxane/aqueous acetic acid (3 M). The mixture was ground using a pestle at room temperature for 45−60 min. The obtained solid was collected and washed with anhydrous dimethylformamide, anhydrous tetrahydrofuran, anhydrous acetone, and anhydrous dichloromethane, separately, to fully remove residual starting materials. The washed sample was further activated by solvent exchange with anhydrous methanol for 3 days and dried at 150 B

DOI: 10.1021/acsami.6b06189 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Material Synthesis. The two fully sulfonated COFs named NUS-9 and NUS-10 were prepared by condensation reactions between 1,3,5-triformylphloroglucinol (TFP) with 2,5-diaminobenzenesulfonic acid (DABA, for NUS-9) or 2,5-diaminobenzene1,4-disulfonic acid (DABDA, for NUS-10), respectively (Scheme 1). It is worth of noting that NUS-9 has also been

under high temperature/pressure conditions yielding COFs as crystalline precipitates.17 Recently, mechanosynthesis has been successfully demonstrated in the preparation of crystalline porous materials such as MOFs and COFs.42−45 Possessing the advantages of short reaction time (within several hours), low synthetic costs, and high yields, mechanosynthesis is extremely suitable for the scaled-up production of functional materials. In this study, the previously developed mechanosynthetic method called liquid-assisted grinding (LAG)45 was used to prepare these COFs. In a typical procedure, NUS-9(G) was synthesized by suspending TFP and DABA with a molar ratio of 2:3 in a small amount of mixed solvent containing equal volume of mesitylene/dioxane/aqueous acetic acid (3 M) followed by grinding at room temperature for 45−60 min and isolated as a red powder in 80% yield. NUS-10(G) was synthesized similarly as a dark red powder in 76% yield. These two COFs were insoluble in water and common organic solvents such as ethanol, hexane, tetrahydrofuran (THF), dimethylformamide (DMF), and were formulated as (C12H8N2O5S)n for NUS-9(G) and (C12H8N2O8S2)n for NUS-10(G) based on elemental analyses performed on guest-free samples. Notably, the solvothermal synthesis of NUS-9(G) (previously denoted as TFP-DABA) was confirmed in our previous study,32 while NUS-10(G) could not be synthesized solvothermally due to the extremely low solubility of DABDA in organic solvents, indicating the indispensable role played by mechanosynthesis in preparing COFs involving highly polar monomers with limited solubility. Structural Characterization. Fourier transform infrared (FTIR) spectra of these two sulfonated COFs exhibit typical stretching bands at 1578 and 1238 cm−1 assignable to CC and C−N stretching bands, respectively (Figure S1), indicating the complete enol-to-keto tautomerization suggested in Scheme 1. The condensation reactions are almost complete as no

Scheme 1. Synthetic Scheme of Sulfonated COFs NUS-9(G) and NUS-10(G) via Liquid Assisted Grinding (LAG) at Room Temperaturea

a

Cavity size considering van der Waals radius is indicated.

reported as TpPa-SO3H during the preparation of this manuscript.35 The in situ enol-to-keto tautomerization helps to lock the COFs in structurally more stable conformations without being affected by the pendent sulfonic acid groups.32 COFs are typically prepared using solvothermal method, where monomers are dissolved in organic solvents and heated up

Figure 1. (a) Crystal structure of NUS-9 assuming eclipsed AA stacking. (b) Crystal structure of NUS-10 assuming eclipsed AA stacking. (c) Simulated and experimental PXRD patterns of NUS-9 under various conditions. (d) Simulated and experimental PXRD patterns of NUS-10 under various conditions. C

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ACS Applied Materials & Interfaces carbonyl stretching band corresponding to the starting monomer TFP (ca. 1643 cm−1) can be found. Furthermore, the FTIR peaks observed at 1026 and 1080 cm−1 along with a shoulder at 1438 cm−1 indicate the symmetric and asymmetric OSO stretching bands, confirming the existence of sulfonic acid groups. The amount of sulfonic acid groups in these COFs was determined to be 3.15 mmol g−1 for NUS9(G) and 5.20 mmol g−1 for NUS-10(G) via acid−base titration, which is comparable to the theoretical value of 3.42 mmol g−1 for NUS-9(G) and 5.61 mmol g−1 for NUS-10(G), suggesting full accessibility of these acid groups. The 13C crosspolarization magic angle spinning nuclear magnetic resonance (13C CP/MAS NMR) spectra further confirm the proposed chemical structures of NUS-9(G) and NUS-10(G), with the chemical shifts of keto-form carbonyl carbon at 181 ppm for NUS-9(G) and 182 ppm for NUS-10(G) (Figure S2). Both COFs were obtained in the morphology of nanoparticles with sizes ranging from 50 to 100 nm, as observed in the SEM images (Figure S5). TGA curves reveal that these COFs are thermally stability up to 300 °C (Figure S6). The crystallinity of COFs was evaluated by powder X-ray diffraction (PXRD). Surprisingly, NUS-9(G) directly obtained via mechanosynthesis in this study exhibits a much worse crystallinity compared to the one synthesized solvothermally,32 while mechanosynthesized NUS-10(G) is almost amorphous (Figure 1). It has been reported that COFs with 2D structures can be exfoliated with deteriorated crystallinity by mechanical force,44−46 which could be the reason for the poor crystallinity in this case. In order to repair crystallinity, the mechanosynthesized COFs were subjected to recrystallization, resulting in strong PXRD peaks at ca. 4.7° corresponding to (100) crystal plane of eclipsed 2D stacking models similar to the solvothermally synthesized NUS-9(S) (S indicates Solvothermal).32 Both NUS-9(R) and NUS-10(R) can be described as hexagonal structure with idealized P6̅ (no. 174) or P6/m (no. 175) symmetry for the typical (close-to) eclipsed AA layer stacking (Table S1−S2). More detailed unit cell parameters of NUS-9 can be found in our previous study.32 For NUS-10(R), the unit cell parameters were obtained as a = b = 23.2201 Å, c = 5.5656 Å, after a geometrical energy minimization using the universal force field (see the Supporting Information). Admittedly, the crystallinity of COFs reported herein is worse than other COFs such as those composed of boroxine bonds16 or imine bonds,47 possibly due to the irreversible enolto-keto tautomerization which hinders the effective establishment of long-range order during COF growth.48 Besides, NUS10(R) exhibits even worse crystallinity than NUS-9(R), possibly due to the steric hindrance from denser sulfonic acid groups preventing effective π−π stacking between successive COF layers resulting in impaired crystallinity.46 The poor crystallinity of NUS-9(R) and NUS-10(R) is confirmed in TEM images where no crystalline lattice can be found (Figure S7). Surface Area and Porosity Analyses. The porosity of recrystallized COFs was checked by N2 sorption isotherms collected at 77 K. Both COFs adopt reversible Type IV isotherms with hystereses between adsorption and desorption branches (Figure 2). The Brunauer−Emmett−Teller (BET) surface area calculated over the P/P0 range of 0.05−0.14 is 102 m2 g−1 for NUS-9(R) and 69 m2 g−1 for NUS-10(R). The BET surface area of NUS-9(R) synthesized herein is lower than that of the one prepared solvothermally (170 m2 g−1),32 possibly due to the reduced crystallinity caused by mechanosynthesis.

Figure 2. N2 sorption isotherms of NUS-9(R) and NUS-10(R) at 77 K: adsorption, closed; desorption, open. (inset) Pore size distribution data.

Unlike other applications, the reduced surface area of sulfonated COFs may be beneficial for them to be used in proton exchange membranes to alleviate the unwanted fuel crossover, which is regarded as one of the most severe problems in fuel cells.49,50 Pore size distribution (PSD) of NUS-9(R) calculated using nonlocal DFT method reveals two pore sizes centered at 14.2 and 17.0 Å which are close to the theoretical pore size of 1.4 nm measured from the simulated crystal structure. The PSD of NUS-10(R) exhibits a much smaller pore of 8.4 Å, possibly due to partial AB-stacking that is consistent with its lower surface area and worse crystallinity.48 Proton Conductivity Measurements. High proton conductivity is expected in NUS-9(R) and NUS-10(R) due to their special structure of massive sulfonic acid groups neatly aligned within one-dimensional nanoporous channels serving as favorable pathways for proton conduction.6 Considering the fact that the proton conduction of sulfonated materials such as Nafion is due to the ionization of sulfonic acid groups in the presence of water,4 the proton conductivity of recrystallized COFs was measured under hydrous state with various degrees of relative humidity (RH). As shown in Figure 3a and b, the room temperature intrinsic proton conductivity of NUS-9(R) starts from a relatively low value of 1.5 × 10−4 S cm−1 under 33% RH, but jumps 2 orders of magnitude to a value 1.24 × 10−2 S cm−1 under 97% RH, indicating the important role of water molecules as proton carriers.51 Because of its denser content of sulfonic acid groups, NUS-10(R) exhibits even higher proton conductivity under both 33% RH (σ = 2.8 × 10−4 S cm−1) and 97% RH (σ = 3.96 × 10−2 S cm−1). Notably, the proton conductivity of sulfonated COFs reported herein is among the highest of all the modularly built crystalline porous frameworks (e.g., MOFs and COFs) reported so far (Table S3). In addition, time-dependent proton conductivity of NUS-10(R) was recorded at 298 K and 97% RH, showing excellent stability with negligible loss of conductivity even after 15 days of continuous assessment (Figure 3c). The above results shed light on the enormous potential of sulfonated COFs as protonconductive materials. As has been suggested, the remarkably high proton conductivity of NUS-9(R) and NUS-10(R) may originate from the abundant sulfonic acid groups in the confined nanospaces, which not only facilitate the preferential adsorption of water molecules (Figure 3d, Figure S10), but also enable the organization of hydrophilic domains to establish favorable pathways for proton transportation that is similar to those observed in Nafion.4 To confirm that the high proton D

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Figure 3. (a) Proton conductivity of NUS-9(R) and NUS-10(R) measured at 298 K under different relative humidity. (b) Nyquist plots of NUS9(R) and NUS-10(R) at 298 K and 97% RH. (c) Time-dependent proton conductivity of NUS-10(R) recorded at 298 K and 97% RH. (d) Water vapor sorption isotherms of NUS-9(R) and NUS-10(R) at 273 K. Each proton conductivity test was repeated at least 3 times and an average value was reported.

Figure 4. Temperature-dependent proton conductivity of (a) NUS-9(R)@PVDF and (b) NUS-10(R)@PVDF with different COF loading measured in pure water. Arrhenius-type plots between proton conductivity and temperature of (c) NUS-9(R)@PVDF and (d) NUS-10(R)@PVDF MMMs.

conductivity of these two COFs is from the sulfonic acid groups instead of the backbone oxidation, the proton conductivity of the COF TpPa-1 was measured, which is isostructural with NUS-9 but without any sulfonic acid groups.37 The proton conductivity test demonstrates that TpPa-1 exhibits a negligible proton conductivity with the σ value of 2.40 × 10−5 S cm−1 under 97% RH and room temperature (Figure S9), which is

much lower than that of COFs with sulfonic acid groups [NUS9(R) and NUS-10(R)] suggesting that the observed high proton conductivity should come from the sulfonic acid groups instead of oxidized π-electron network. To further evaluate the sulfonated COFs as protonconductive materials for proton exchange membranes, a series of mixed matrix membranes (MMMs) were prepared by E

DOI: 10.1021/acsami.6b06189 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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blending various amounts of COFs with nonconductive polyvinylidene fluoride (PVDF) through solution casting. Thanks to the organic feature of COF fillers, the resultant MMMs exhibit good mechanical property, smooth texture, and great compatibility between COF fillers and PVDF matrix (Figure S11 and 12).52 The crystallinity of COF fillers is partially retained in these MMMs (Figure S13). Due to their great mechanical property, these MMMs can be directly soaked in ultrapure water for proton conductivity test without disintegration. As can be seen from Figure 4a and b, increasing COF loading is beneficial for the proton conductivity of MMMs, with the highest σ value of 2.06 × 10−3 S cm−1 for NUS-9(R)@PVDF-50 and 5.16 × 10−3 S cm−1 for NUS-10(R) @PVDF-50 obtained at 298 K. Further increase of COF loading was unsuccessful due to the brittleness of obtained MMMs preventing extra process. The proton conductivities of MMMs measured at various temperatures display a typical Arrhenius behavior, with the highest σ value reaching 5.06 × 10−3 S cm−1 for NUS-9(R)@PVDF-50 and 1.58 × 10−2 S cm−1 for NUS-10(R)@PVDF-50 measured at 353 K. To the best of our knowledge, crystalline porous frameworks (e.g., MOFs and COFs) have rarely been processed into composite membranes for proton conduction,34,53 and the ultrahigh proton conductivity of 1.58 × 10−2 S cm−1 displayed by NUS-10(R)@ PVDF-50 at 353 K in this work sheds light on these studies. The activation energy Ea of proton conduction in MMMs was determined from linear least-squares fits of the slopes of Arrhenius plots shown in Figure 4c and d. Interestingly, MMMs loaded with various amounts of COFs display almost the same Ea, indicating the close contact of COF fillers in MMMs forming continuous pathways for proton transportation. The Ea was determined to be 0.20 eV for NUS-9(R)@PVDF and 0.21 eV for NUS-10(R)@PVDF, suggesting the Grotthuss mechanism for proton conduction (Ea < 0.4 eV) by which protons hop within the network formed by sulfonic acid and water molecules through hydrogen bonding interactions that agrees well with the crystal models of sulfonated COFs.6

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.Z.). *E-mail: [email protected] (H.C.). Author Contributions ⊥

Y.P. and G.X. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.Z. acknowledges the support from National University of Singapore (CENGas R-261-508-001-646) and Singapore Ministry of Education (MOE AcRF Tier 1 R-279-000-410112). H.C. acknowledges the support from National Natural Science Foundation of China (No. 21233006, 21473164, and 21403202).

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REFERENCES

(1) Jacobson, M. Z.; Colella, W. G.; Golden, D. M. Cleaning the Air and Improving Health with Hydrogen Fuel-Cell Vehicles. Science 2005, 308 (5730), 1901−1905. (2) Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications, and Needs on Fundamental Research. Appl. Energy 2011, 88 (4), 981−1007. (3) Zhang, H. W.; Shen, P. K. Recent Development of Polymer Electrolyte Membranes for Fuel Cells. Chem. Rev. 2012, 112 (5), 2780−2832. (4) Mauritz, K. A.; Moore, R. B. State of Understanding of Nafion. Chem. Rev. 2004, 104 (10), 4535−4585. (5) Nasef, M. M. Radiation-Grafted Membranes for Polymer Electrolyte Fuel Cells: Current Trends and Future Directions. Chem. Rev. 2014, 114 (24), 12278−12329. (6) Kreuer, K. D. Proton Conductivity: Materials and Applications. Chem. Mater. 1996, 8 (3), 610−641. (7) Kreuer, K. D.; Paddison, S. J.; Spohr, E.; Schuster, M. Transport in Proton Conductors for Fuel-Cell Applications: Simulations, Elementary Reactions, and Phenomenology. Chem. Rev. 2004, 104 (10), 4637−4678. (8) Hurd, J. A.; Vaidhyanathan, R.; Thangadurai, V.; Ratcliffe, C. I.; Moudrakovski, I. L.; Shimizu, G. K. H. Anhydrous Proton Conduction at 150 °C in a Crystalline Metal-Organic Framework. Nat. Chem. 2009, 1 (9), 705−710. (9) Bureekaew, S.; Horike, S.; Higuchi, M.; Mizuno, M.; Kawamura, T.; Tanaka, D.; Yanai, N.; Kitagawa, S. One-Dimensional Imidazole Aggregate in Aluminium Porous Coordination Polymers With High Proton Conductivity. Nat. Mater. 2009, 8 (10), 831−836. (10) Shimizu, G. K. H.; Taylor, J. M.; Kim, S. Proton Conduction with Metal-Organic Frameworks. Science 2013, 341 (6144), 354−355. (11) Yamada, T.; Otsubo, K.; Makiura, R.; Kitagawa, H. Designer Coordination Polymers: Dimensional Crossover Architectures and Proton Conduction. Chem. Soc. Rev. 2013, 42 (16), 6655−6669. (12) Yoon, M.; Suh, K.; Natarajan, S.; Kim, K. Proton Conduction in Metal-Organic Frameworks and Related Modularly Built Porous Solids. Angew. Chem., Int. Ed. 2013, 52 (10), 2688−2700. (13) Ramaswamy, P.; Wong, N. E.; Shimizu, G. K. H. MOFs as Proton Conductors - Challenges and Opportunities. Chem. Soc. Rev. 2014, 43 (16), 5913−5932. (14) Ramaswamy, P.; Matsuda, R.; Kosaka, W.; Akiyama, G.; Jeon, H. J.; Kitagawa, S. Highly Proton Conductive Nanoporous Coordination Polymers with Sulfonic Acid Groups on the Pore Surface. Chem. Commun. 2014, 50 (9), 1144−1146. (15) Matoga, D.; Oszajca, M.; Molenda, M. Ground To Conduct: Mechanochemical Synthesis of a Metal-Organic Framework with High Proton Conductivity. Chem. Commun. 2015, 51 (36), 7637−7640.

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CONCLUSIONS In summary, we have demonstrated high intrinsic proton conductivity in sulfonated COFs obtained through mechanosynthesis. One of the COFs NUS-10(R) exhibits stable proton conductivity of 3.96 × 10−2 S cm−1 at 298 K and 97% RH, which is among the highest of all the MOFs and COFs reported so far. More interestingly, we have fabricated composite membranes containing sulfonated COFs, among which NUS-10(R)@PVDF-50 shows high proton conductivity of 1.58 × 10−2 S cm−1 in water at 353 K. Our study has revealed the huge potentials of sulfonated COFs as proton-conductive materials and will encourage further research of proton conduction in modularly built crystalline porous frameworks.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06189. Structure determination and additional figures and tables (PDF) Simulated crystal structure of NUS-9 assuming eclipsed conformation (CIF) Simulated crystal structure of NUS-10 assuming eclipsed conformation (CIF) F

DOI: 10.1021/acsami.6b06189 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.6b06189 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Frameworks (COFs) for Efficient CO2 Separation. Chem. Mater. 2016, 28 (5), 1277−1285. (53) Liang, X. Q.; Zhang, F.; Feng, W.; Zou, X. Q.; Zhao, C. J.; Na, H.; Liu, C.; Sun, F. X.; Zhu, G. S. From Metal-Organic Framework (MOF) to MOF-Polymer Composite Membrane: Enhancement of Low-Humidity Proton Conductivity. Chem. Sci. 2013, 4 (3), 983−992.

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DOI: 10.1021/acsami.6b06189 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX