Composite Proton-Exchange Membrane with Highly Improved Proton

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Applications of Polymer, Composite, and Coating Materials

Composite Proton Exchange Membrane with Highly Improved Proton Conductivity Prepared by in Situ Crystallization of Porous Organic Cage Ruiyi Han, and Peiyi Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04311 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 10, 2018

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ACS Applied Materials & Interfaces

Composite Proton Exchange Membrane with Highly Improved Proton Conductivity Prepared by in Situ Crystallization of Porous Organic Cage Ruiyi Han and Peiyi Wu＊

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, 200433, Shanghai, P. R. China. KEYWORDS: proton exchange

membrane; soluble crystalline

porous organic cage; 3D proton transfer channel; in situ crystallization

1

ACS Paragon Plus Environment

porous materials;

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Porous organic cage, a kind of newly-emerging soluble crystalline porous material, is introduced to proton exchange membrane by in situ crystallization. The crystallized Cage 3 with intrinsic water-meditated three-dimensional interconnected proton pathways working together with Nafion matrix generates a composite membrane with highly improved proton conductivity. Different with inorganic crystalline porous materials like MOFs, the organic porous material shows better compatibility with Nafion matrix due to the absence of inorganic elements. In addition, Cage 3 can absorb water up to 20.1 wt%, which effectively facilitates proton conduction under both high and low humidity conditions. Meanwhile, the selectivity of Nafion-Cage 3 composite membrane is also elevated upon the loading of Cage 3. The proton conductivity is evidently enhanced without obvious increased methanol permeability. At 90 °C−95%RH, the proton conductivity of NC3-5 reaches 0.27 S·cm-1, highly improved compared to 0.08 S·cm-1 of recast Nafion under the same condition. This study offers a new strategy for modifying proton exchange membrane with crystalline porous materials.

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1. INTRODUCTION Solid electrolytes for ion conducting have drawn much attention because it’s widely used in fuel cells, flow batteries and lithium-ion batteries. Especially proton conduction in fuel cells is extensively studied.1 Direct methanol fuel cell (DMFC) has been one of the most promising power sources for its high energy density and low working temperature. Methanol used in DMFC is renewable and eco-friendly power source much better than traditional fossil fuels.2,3 As the core components of DMFC, the property of proton exchange membrane is of vital importance. Nafion is a commonly used proton exchange membrane for its good proton conduction and chemical stability.4 However, Nafion also has its disadvantages, its performance deteriorates under high temperature (>80 °C) and low humidity, which restricts its commercial use.5 Many works have been done to improve the proton transfer property. Using hydrothermal carbonization, Wang’s group obtained Nafion–Carbon nanocomposite membranes with higher proton conductivity and reduced methanol permeability.6 In order to elevate proton conductivity at low humidity, Jiang and coworkers incorporated imidazole microcapsules into polymer electrolyte membranes.7 Zhang’s group rearranged the ion-transport channels in PES/SPEEK blend membranes by using isopropanol treatment and sulfonated groups redistributed along hydrophilic channels to obtain a higher proton conductivity.8 Wang and Qiao’s group developed molecular-level hybridization of Nafion with quantum dots, the strategy can highly enhance proton conductivity without destroying Nafion hierarchical topology.9 Additionally, adding fillers such as functioned GO,10,11,12,13 modified CNT,14 SiO2 nanofibers,15 SPEEK nanofibers,16 zirconium oxide nanotube,17 Ti3C2Tx,18 into 3



membrane generate composite membrane with significantly improved proton transfer property. Besides, other hydrophobic–hydrophilic phase-separated polymer have also been synthesized as new kinds of proton exchange membrane though they didn’t perform as well as Nafion.19,20 Recently crystalline porous materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been widely studied for its high porosity, high specific surface area and chemical functionality.21,22 Unlike amorphous polymers, crystalline materials have tunable pore networks and regular frameworks, which make them a potential candidate for proton conduction. However, the fabrication of crystalline porous materials into a mechanically stable thin film is still challenging owing to their poor stability and

processability.23 In addition, their bulk phase and grain boundaries lead to a proton conduction pathway not consecutive enough, which restricts their proton conduction property.24,25 Porous organic cage is a kind of newly-emerging porous material formed by self-assembly using non-covalent forces,26 which is different from directional bonding in crystalline porous materials such as covalent organic frameworks and metal organic frameworks.27,28 Due to inefficient packing, these molecules form internal cavities and extrinsic porosity because of their covalent bonding and rigid shape. Controlled by the functional groups on the cage vertices, pore structures and the void volume can connect or disconnect.29,30 Due to inefficient packing, the covalent bonding and rigid shape make it easy to form internal cavities and extrinsic porosity.31 Cage 3 as a neutral organic cage packs in a window to window arrangement, which leads to an interconnected three-dimensional proton conduction pathways.32 Attributed to the 3D networks of Cage 3, the material showed well-defined pore structures, ultrahigh surface areas and chemical diversity. The specific structure make it 4


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attractive in catalysis, gas storage and molecular separations such as heterogeneous metal nanoparticle (MNP) catalysts and energy-efficient separation of commercially important chemicals like hydrocarbon isomers and chiral molecules.33,34 Owing to the intrinsic channel structure and topology, porous organic cage exhibits great potential for proton conduction. Unlike the one-dimensional proton transfer tendency in MOFs,35,36 the three-dimensional proton channel is more favorable in proton exchange membrane. Furthermore, Cage 3 can absorb water up to 20.1 wt%. The absorbed water helps building a consecutive hydrogen-bonding network along the intrinsic three dimensional channel, which facilitates the fast intermolecular proton transfer in Cage 3.32 Conventional crystalline materials like MOF usually have a poor compatibility with Nafion matrix, many works focusing on improving interface compatibility between polymer and MOF have been done.37,38 Unlike MOFs, porous organic cage containing only H, C, N and O atoms shows better compatibility with polymers. Porous organic cage can be dissolved in common solvent, which endows the ability of solution processing.31 In addition, Cage 3 prepared by in situ crystallization is finely distributed within the polymer bulk and shows better compatibility with Nafion matrix.39,40,41 Due to the absence of intermolecular covalent bonding, porous organic cage molecules shows unique mobility and flexible molecule structure, which may probably enhance the interaction between the hosts and guests.42,43 Porous organic cage can also be processed into thin film by spin-coating, though it is mechanically unstable compared to traditional polymer matrix membrane due to its weak intermolecular interaction.31 Herein, we prepared a mixed matrix membrane by in situ crystallization of porous organic cage. To the best of our knowledge, it is the first time that a kind of soluble crystalline porous 5



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material is introduced into proton exchange membrane. By in situ crystallization of porous organic cage, a composite PEM with highly improved proton transfer property was obtained. Containing no inorganic elements, porous organic cage shows better compatibility with Nafion matrix. Crystallized Cage 3 with intrinsic three-dimensional interconnected proton pathways dispersed within Nafion matrix helps facilitating the proton conduction of PEM. Owing to the hydrogen bond formed between Cage 3 and Nafion, more consecutive proton pathways are constructed probably due to the hydrogen-bond reorganization during self-assembly process of cage 3. Additionally, Cage 3 with high water absorption can help membrane stay hydrated, which guarantees the proton transfer under low humidity condition. The prepared membrane shows higher proton conductivity under both low humidity and high humidity conditions than recast Nafion. At 90 °C−95% relative humidity, the proton conductivity of NC3-5 reaches up to 0.27 S•cm-1, highly improved than 0.08 S•cm-1 of recast Nafion under the same condition.

2. EXPERIMENTAL SECTIONS 2.1

Materials

1,3,5-Triformylbenzene

was

bought

from

Sigma-Aldrich,

(R,R)-1,2-diaminocyclohexane was purchased from Aladdin, Nafion solution (perfluorinated resin solution, 5 wt%) was obtained from DuPont. H2O2, H2SO4 and DMF were Sinopharm Chemical Reagent Co. All reagents were used without further purification. 2.2 Synthesis of Cage 3 80mg 1,3,5-Triformylbenzene was dissolved in 3ml dichloromethane. Similarly, 80mg (R,R)-1,2-diaminocyclohexane was dissolved in 3ml dichloromethane.

Then

the

solution

of

1,3,5-Triformylbenzene

was

added

to

(R,R)-1,2-diaminocyclohexane solution. The mixtures were leaved for two days without 6


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stirring finally crystals were formed. Another 8ml dichloromethane is added to re-dissolve crystals. The obtained solution is filtered through a PTFE filter with pore size 200 nm. Then solvent is removed and white crystals is obtained.31 2.3 Preparation of Nafion-Cage 3 Composite Membrane Nafion-Cage 3 composite membrane was prepared by a solution-casting method. Schematic illustration in Figure 1 shows the preparation process of Nafion-Cage 3 composite membrane. First, 4ml of as-received Nafion was taken and the original solvent was exchanged into DMF by rotary evaporation as described from our previous work.44 Second, 5.52mg Cage 3 was dissolved in DMF, the dissolving process is presented in Figure S3. Third, Cage 3 solution was added to Nafion/DMF solution. After ultrasonication for 6 hours, the mixtures were casting into a mold. The casting solution was left for some time then the solvent was removed by oven at 120 °C for 24 hours. A further treatment with 3 wt% H2O2 and 1M H2SO4 is needed. The obtained composite membrane is named as NC3-x, where x stands for the mass fraction of Cage 3 in Nafion matrix. NC3-3, NC3-5, NC3-7 were obtained by similar way. Besides, recast Nafion was prepared without Cage 3 adding and named as RN.

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Figure 1. Schematic illustration of preparation of Nafion-Cage 3 composite membrane 2.4 Characterizations The Fourier transform infrared (FTIR) spectra was collected on a NEXUS 6700 (ThermoFisher). The X-ray diffraction (XRD) patterns was acquired on a D8 ADVANCE and DAVINCI.DESIGN (Bruker) with Cu Kα radiation. A Perkin Elmer Thermal Analyzer was used to carry out the thermogravimetric analysis (TGA) experiments under N2 atmosphere with a heating rate of 20 °C /min from 50 to 800 °C. The field-emission scanning electron image was obtained from an Ultra 55 (Zeiss). A Multimode 8 (Bruker) was used to obtain the atomic force microscopy (AFM) images in QNM mode. Water uptake of the membrane was measured by the weight of membrane under humid conditions subtracting from dry membrane is shown in supporting information. A four-point probe device connected with electrochemical workstation CHI660D (Shanghai, China) was used to measure the proton conductivity detailed described in supporting information. Methanol permeability was obtained by ATR-FTIR technique on NEXUS 6700 (ThermoFisher) as previously reported.45 3. RESULTS AND DISCUSSION 3.1 Characterizations of Cage 3 8


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The morphology of Cage 3 is shown in Figure 2c. Cage 3 has a regular octahedron crystal structure with a general size of ca. 2µm. The XRD patterns of Cage 3 synthesized in dichloromethane are displayed in Figure 3a, confirming the crystalline structure. The characteristic peak diffraction pattern is consistent with that previously reported.26 Then we re-dissolve Cage 3 into DMF and Cage 3 was recrystallized from DMF, the diffraction pattern remained the same as that synthesized in dichloromethane. It obviously revealed the solution processability of Cage 3 by common solvents. After the recrystallized process from DMF, Cage 3 still remain the same crystal structure as shown in the XRD patterns. Nafion has a glass transition temperature (Tg) at 100 °C.46 Most of the commercialized Nafion membranes are annealed at a temperature around 120–130 °C.47 So the crystallization within Nafion will occur at higher temperature. While Cage 3 is redissolved in DMF with boiling point at 152.8 ℃ higher than dichloromethane at 39.75 °C, the removal of DMF was also operated at higher temperature. High-temperature will increase the crystallization velocity.48 Slow crystallization of cage molecules gives a crystalline structure with long range of order, while rapid precipitation can result in short range order.31 In our work, as is displayed in Figure 3a, the XRD pattern of Cage 3 synthesized in dichloromethane and recrystallized from DMF show the same diffraction pattern. Which confirms the same crystal structure from two different crystallization temperature.

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Figure 2. (a) The chemical formula of Cage 3, (b) 3D crystalline structure of Cage 3, (c) FESEM images of Cage 3. The chemical structure and 3D crystalline structure of Cage 3 is provided in Figure 2a and Figure 2b. Cage 3 is synthesized by the condensation reaction of 1,3,5-triformylbenzene with (R,R)-1,2-diaminocyclohexane in a [4 + 6] cycloimination, which formed imine-linked tetrahedral cages.26 The FTIR spectrum of Cage 3 is shown in Figure 3b, the characteristic peak of C=N bonding at around 1650 cm-1 clearly suggests the successful condensation reaction between -NH2 and –CHO.49 The 1H NMR data for Cage 3 in Figure S4 also identify the successful synthesis of Cage 3. For Cage 3, solvent dichloromethane loss at ambient temperature. And it was fully desolvated by heating to 125 °C.26 The Cage 3 sample was fully desolvated before TGA test. As is shown in Figure 3c, there is no obvious loss before 125 °C, which shows no solvent dichloromethane and physisorbed water in sample. Combined with DTG curves of Cage 3, the onset of decomposition for Cage 3 occurs at around 400 °C.26 Which shows that Cage 3 has relatively high thermal stability.

Figure 3. (a) XRD patterns of Cage 3 as synthesized in dichloromethane and Cage 3 recrystallized from DMF, (b) FTIR spectra of Cage 3, (c) TGA and DTG curves of Cage 3. 3.2 Characterization of the PEMs

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After the loading of Cage 3, the appearance of Nafion-Cage 3 composite membrane changed to pale yellow compared to transparent Recast Nafion membrane as shown in Figure S5. The XRD patterns of Recast Nafion and Nafion-Cage 3 composite membrane is shown in Figure 4a. The character peak at around 17°is shown in Figure 4a, which is an integration of amorphous and crystalline domains of Nafion matrix.50,51 When the loading of Cage 3 is less than 5 wt%, only weak characteristic peaks of Cage 3 could be identified in composite membrane probably due to the limited amount of Cage 3 in composite membrane. As the loading of Cage 3 reaches 7 wt%, the characteristic peak of Cage 3 appears clearly, which demonstrates the successfully crystallization of Cage 3 in composite membrane. The Fourier-Transform Infrared (FTIR) Spectra of recast Nafion membrane and Nafion-Cage 3 composite membrane are displayed in Figure 4b. The major vibrational bands attributed to Nafion matrix including the C–F stretching vibrations of perfluorosulfonate backbone from 1000 cm-1 to 1400 cm-1 and bands at 1057 cm-1 and 980 cm-1 corresponding to SO3−, C–O–C respectively, are found in all Nafion-Cage 3 composite membranes.52,53 Compared to RN, NC3 composite membranes have a weaker characteristic vibrations. In addition, the vibrational band at around 1650 cm-1 attributed to C=N bonding also suggests the successful loading of Cage 3. There shows the hydrogen bond interaction between Cage 3 and Nafion in magnified FTIR spectrum in Figure S7, RN has stretching vibration peak of sulfonic groups at around 1055 cm-1, while the peak of sulfonic groups in NC3-7 shift to 1051 cm-1. At the same time, Cage 3 has a characteristic peak of C=N at 1648 cm-1, when Cage 3 is added into Nafion matrix and the characteristic peak of C=N in Cage 3 shifts to 1642 cm-1. The slight shift of characteristic peak of sulfonic groups in Nafion and C=N in Cage 3 shows that there is 11



hydrogen bond interaction between Nafion and Cage 3. The presence of hydrogen bond is attributed to comparatively large electron density and polarity of the lone-pair orbital of its sp2 nitrogen in Cage 3.54,55

Figure 4. (a) XRD patterns, (b) FTIR spectra of RN, NC3 composite membrane. The surface and cross section of Recast Nafion and Nafion-Cage 3 composite membrane are shown in Figure 5. The recast Nafion has a relatively smooth surface and cross section as expected. With a loading of 3 wt% of Cage 3, NC3-3 appears a similar clean surface morphology with recast Nafion. Meanwhile the cross section becomes rough and Cage 3 particles are observed in the cross section. As the Cage 3 loads up to 5 wt%, the crystalline Cage 3 is clearly observed in the surface and cross section in Nafion-Cage 3 composite membrane, which further confirms the successful crystallization of Cage 3 in Nafion matrix. As we can see from the surface and cross section with larger magnification in Figure S6, the Cage 3 in the membrane still has obvious crystalline structure. And the crystallized Cage 3 in the membrane also has a general size of ca. 2 µm, which is similar with individual crystallized Cage 3. As the Cage 3 is loaded up to 7 wt%, the Cage 3 begins aggregating as shown in Figure 5d, which may have negative effect on the continuous Nafion matrix and deteriorate the mechanical property and proton conduction ability. As there is only very small amount of 12


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Cage 3 on the membrane surface, the Cage 3 on the surface of membrane is just easy for identifying the crystallization of Cage 3 with membrane. It is the Cage 3 inside membrane that play an important role on improving the membrane property.

Figure 5. FESEM images of the surface of (a) RN, (b) NC3-3, (c) NC3-5, (d) NC3-7. the cross section of (e) RN, (f) NC3-3, (g) NC3-5, (h) NC3-7. The water retention capacity is highly improved by incorporating Cage 3 into Nafion matrix as shown in Figure 6(a). Due to the intrinsic three dimensional channels, Cage 3 has a nearly 20.1 wt% water retention ability.32 To study the water absorption ability of membrane, two temperature at 40 °C and 90 °C are typically chosen as experiment temperature. Especially at 40 °C, Nafion-Cage 3 composite membrane has a water absorption ability three times of that in recast Nafion. At higher temperature 90 °C, the water absorption ability has also been slightly improved. Generally speaking, proton conduction of composite membrane has a guarantee by the increased water retention, which is also proved in our following proton conduction data. Thermogravimetric analysis was used to study the degradation temperature of membranes. Figure 6(b) shows the TGA curves of recast Nafion and Nafion-Cage 3 composite membrane. There is no obvious weight loss from 100 to 320 °C. The first stage of weight loss begins from 320 to 410 °C, which is attributed to desulfonation process. The 13



second stage from 410 to 470 °C and the third stage from 470 to 550 °C may be correspoded to side chain and PTFE-backbone decomposition.6 Different from recast Nafion, Nafion-Cage 3 composite membrane has a smaller and slower weight loss during decomposition process, which clearly indicates that the loading of Cage 3 confers a higher thermal stability of composite membrane.

Figure 6. (a) WU (wt％), (b) TGA of recast Nafion and Nafion-Cage 3 composite membrane. The proton conductivity of Nafion-Cage 3 composite membrane and recast Nafion is shown in Figure 7a. Composite membrane NC3 has higher proton conductivity than that of recast Nafion. Besides, NC3-5 membrane has the highest proton conductivity among the composite membrane. At 90 °C−40% RH, the proton conductivity of NC3-5 can reach 0.085 S·cm−1, highly increased than that of recast Nafion 0.010 S·cm−1 under the same condition. Additionally, the loading of Cage 3 into Nafion matrix can obviously increase the proton conductivity at higher temperature probably owing to the water retention ability of the intristic three-dimensional channels in Cage 3. While as the Cage 3 loads up to 7 wt%, the proton conductivity of NC3-7 drops slightly than that of NC3-5,which is probably attributed to the deterioration of continuous structure of Nafion matrix by larger amounts of Cage 3.56,57 In other words, excessive loading of Cage 3 decreases the proton conductivity of Nafion 14


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matrix. At high relative humidity, Nafion-Cage 3 membrane also has better proton transfer performance than recast Nafion. As is shown in Figure 7b, the proton conductivity of NC3-5 membrane reaches 0.271 S·cm−1 at 90 °C−95% RH, which is much higher than 0.083 S·cm−1 of the recast Nafion membrane under the same condition.

Figure 7. (a) Temperature-dependent (40％ RH), (b) Humidity-dependent (90 ℃) proton conductivity of recast Nafion and Nafion-Cage 3 composite membrane. As is shown in Figure 8, the phase separation of RN and the NC3 composite membrane is obtained by AFM in QNM mode. In the AFM images, the lighter areas represent to the hydrophobic areas of Nafion backbone, while darker areas denote hydrophillic areas containing −SO3− ionic clusters because of its low modulus.58,59 As is shown in Figure 8, NC3-3 has a more concentrated distribution dark areas than RN, which demonstrates that the loading of Cage 3 into Nafion exhibits more obvious phase separation structure. The clear phase separation is better for proton conduction. As Cage 3 loads up to 5 wt%, the dark areas become more concentrated and consecutive in NC3-5 shown in Figure 8c. It provide more consecutive proton transfer pathways than RN and NC3-3. When excess Cage 3 was added into Nafion matrix, as we can see from Figure 8d, NC3-7 still has more concentrated ionic distribution. But the ionic distribution seems more isolated than NC3-5, which goes against high proton conduction. The excess amount of Cage 3 loading will lead to inevitable local 15



agglomeration. The agglomeration may have destroy the intrinsic phase separation of Nafion and deteriorate the proton conduction ability.56

Figure 8. LogDMTModulus AFM images of (a) RN, (b) NC3-3, (c) NC3-5, (d) NC3-7 Nafion-Cage 3 composite membrane has better performance on proton conduction property than previously reported membranes as shown in Table S1. Three probable reasons could be accounted for the highly increased proton transfer ability : (i) The Cage 3 has a water absorption up to 20.1 wt%, which guarantees the water content in Nafion matrix and facilitates the proton pathways in Nafion especially in water-rich area around Cage 3. (ii) The Cage 3 has intrinsic three dimensional interconnected proton channels, the hydrated 3D channels working together with Nafion exhibited well-performanced proton transfer property. The hydrated Cage 3 with 3D proton transfer channels can conduct proton itself. To maximize proton transfer, 3D channels is preferred to 1D channels. It can also provide additional proton 16


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pathways than recast Nafion.32 The Nafion-Cage 3 composite membrane has a proton conductivity much higher than individual Cage 3 or individual Nafion. It is the synergistic effect between Nafion and Cage 3 that improve the proton transfer ability. (iii) The in situ crystallization process of Cage 3 may influence the Nafion casting process. It is demonstrated that there is hydrogen-bond interaction between Nafion and Cage 3. The in situ crystallization of Cage 3 is a process that Cage 3 molecular gather and self-assemble. During the crystallization of Cage 3, the sulfonic acid group in Nafion will get gathered along with Cage 3 molecular due to the hydrogen-bond interaction. So the obtained Nafion-Cage 3 membrane has more concentrated and consecutive ionic distributions than recast Nafion. It is also demonstrated by AFM images in Figure 8. Schematic illustration of proposed proton transfer mechanism of NC3 composite membrane is shown in Figure 9, Cage 3 packs in a window to window arrangement controlled by the cyclohexyl groups. This results in a interconnected three dimensional channel structure. And the hydrated channels provide proton transfer pathways. In addition, the channels showed high water retention ability. The water-rich areas around Cage 3 can guarantee the proton transfer property especially under low humidity condition.

Figure 9. Schematic illustration of proposed proton transfer mechanism of Nafion-Cage 3 17



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composite membrane Table 1 Transport properties of the recast Nafion membrane and Nafion-Cage 3 composite membranes. PEMs

methanol permeability

Selectivity

(10-8 cm2 /s, 40 oC)

(105 S s /cm3, 40 oC)

RN

5.48 ± 0.27

0.71 ± 0.01

NC3-3

5.67 ± 0.33

2.44 ± 0.02

NC3-5

5.82± 0.27

6.53 ± 0.03

NC3-7

6.21 ± 0.25

4.03 ± 0.02

Methanol permeation is also a crucial factor that influences the property of PEM.60 Due to the same transfer channnels of proton and methnol, the increase of proton conductivity means a more severe methnol permeation to some extent.45 The methanol permeation and selectivity of membrane is shown in Table 1. After the loading of Cage 3, Nafion-Cage 3 composite membrane has only a slightly increase of mathanol permeation than recast Nafion. Because of the obviously increased proton conductivity of composite membrane, the selectivity of NC3-5 membrane is greatly elevated by nearly one order of magnitude than that of recast Nafion.40 The hydrated channels in Cage 3 facilitates the proton conduction in Nafion composite membrane. While the added Cage 3 cannot permeate mathanol easily and effectively reduce methanol crossover in composite membrane at some extent.61 4. Conclusion 18


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A novel proton exchange membrane is obtained by in situ crystallization of porous organic cage, a kind of newly-emerging crystalline porous materials with solution processability. Cage 3 packing in a window to window arrangement results in intrinsic three-dimensional interconnected channels for proton transport. Our study showed that the loading of Cage 3 into Nafion matrix highly improved proton conductivity of Nafion-Cage 3 composite membrane than that of recast Nafion. And NC3-5 has the highest proton conductivity 0.27 S·cm−1 at 90 °C−95% RH, which is highly above 0.08 S·cm−1 of recast Nafion. At the same time, the selectivity of membrane also increased significantly as the Cage 3 loaded. This study offers a novel method for modifying Nafion membrane with improved proton transfer property.

ASSOCIATED CONTENT Supporting Information: Measurement of WU; Measurement of Proton Conductivity; Measurement of Methanol Permeability; Schematic diagram and digital photo used to measure the methanol permeability; The dissolving process of Cage 3 in DMF; 1H NMR data for Cage 3; The image of recast Nafion and Nafion-Cage 3 composite membrane. FESEM images of the surface and cross section of NC3-5 with larger magnification; Magnified FTIR spectrum of (a) RN and NC3-7, (b) NC3-7 and Cage 3. AUTHOR INFORMATION Corresponding Author E-mail: [email protected]. Notes 19



The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the Ministry of Science & Technology of China (No. 2016YFA0203302)

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Composite Proton-Exchange Membrane with Highly Improved Proton

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