Carbon Nanotube

Research Article www.acsami.org

Two-Dimensional Zeolitic Imidazolate Framework/Carbon Nanotube Hybrid Networks Modified Proton Exchange Membranes for Improving Transport Properties Huazhen Sun, Beibei Tang, and Peiyi Wu* State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, People’s Republic of China S Supporting Information *

ABSTRACT: Metal−organic framework (MOF)/polymer composite proton exchange membranes (PEMs) are being intensively investigated due to their potentials for the systematic design of proton-conducting properties. However, the development of MOF/polymer composite PEMs possessing high selectivity remains exceedingly desirable and challenging for practical application. Herein, two-dimensional (2D) zeolitic imidazolate framework (ZIF-8)/carbon nanotube (CNT) hybrid crosslinked networks (ZCN) were synthesized via the rational design of the physical form of ZIF-8, and then a series of composite PEMs were prepared by hybridizing ZCN with sulfonated poly(ether ether ketone) (SPEEK) matrix. The effect of the incorporation of zero-dimensional (0D) raw ZIF-8 nanoparticles and 2D ZCN on the proton conduction and methanol permeability of the composite membranes was systemically studied. Benefiting from the morphological and compositional advantages of ZCN, the SPEEK/ZCN composite membranes displayed a significant enhancement in proton conductivity under various conditions. In particular, the proton conductivity of SPEEK/ZCN-2.5 membrane was up to 50.24 mS cm−1 at 120 °C-30% RH, which was 11.2 times that of the recast SPEEK membrane (4.50 mS cm−1) and 2.1 times that of SPEEK/ZIF membrane (24.1 mS cm−1) under the same condition. Meanwhile, the methanol permeability of the SPEEK/ZCN composite membranes was greatly reduced. Therefore, novel MOF/polymer composite PEMs with high selectivity were obtained. Our investigation results reveal that the proton conductivity and methanol permeability of the MOF/polymer composite membranes can be effectively tailored via creating more elaborate superstructures of MOFs rather than altering the chemical component. This effective strategy may provide a useful guideline to integrate with other interesting MOFs to design MOF/polymer composite membranes. KEYWORDS: metal−organic framework, superstructure, proton exchange membrane, ionic nanochannels, proton conduction, methanol permeability

1. INTRODUCTION Metal−organic frameworks (MOFs), constructed from the assembly of metal ions with organic ligands through the formation of coordination bonds, recently have emerged as an intriguing class of crystalline hybrid material.1−3 They have exhibited unique advantages in various fields, such as selective separation, gas storage, catalysis, sensors, and drug delivery.4−8 Proton conductivity now is regarded as a new functionality of MOFs because their proton conducting properties can be systematically tailored.9 Although several MOF materials with high proton conductivity have been reported, their practical application in fuel cells is far from being realized because of their crystal nature, which makes them difficult to process.10,11 The fabrication of MOF/polymer composite membranes is considered to be an alternative strategy to solve this plight.12 Hybridization of MOFs and polymers is producing new and versatile materials that exhibit peculiar properties hard to realize with the individual components. In a typical MOF/polymer © 2017 American Chemical Society

composite PEM system, the polymers will stabilize the mechanical properties of MOFs and fill their grain boundaries. Meanwhile, the high proton conductivity of MOFs will be transferred to the composite membranes through the interfaces between MOFs and polymers. The MOF/polymer composite proton exchange membranes (PEMs) were reported for the first time by Zhu’s research group in 2013 as a straightforward and universal strategy for practical utilization of MOF materials in fuel cells.13 Since then, many other research groups have attempted to further optimize the performance of MOF/ polymer composite PEMs, especially proton conductivity. A few MOF/polymer composite PEMs such as Fe-MIL-101-NH2SPPO,14 phytic@MIL/Nafion,15 sMOF-808/Nafion,16 sulMIL/SPEEK,17 PIL@MIL/SEBS,18 and acids@MIL-101/CS19 Received: August 28, 2017 Accepted: September 27, 2017 Published: September 27, 2017 35075

DOI: 10.1021/acsami.7b13013 ACS Appl. Mater. Interfaces 2017, 9, 35075−35085

Research Article

ACS Applied Materials & Interfaces

Figure 1. Illustration of the synthesis process of ZCN through in situ growth procedure.

Bearing this in mind, the present work attempts to design and synthesize a novel MOF-based material with more complex architectures and apply them in the fabrication of highly selective PEMs. ZIF-8, constructed from Zn2+ metal ions and 2methylimidazole as linker, was selected here to be used as building units to synthesize novel MOF-based materials with higher-order superstructures due to its excellent chemical stability, thermal stability, and confirmed potential in proton conduction.27,28 CNT is also being intensely explored in the field of PEMs because of its high aspect ratio, which benefits the formation of channel-like ionic clusters for proton transport.29,30 Nowadays, the modified CNT with carboxyl groups is taken as an ideal platform for the growth of MOFs because the carboxyl groups can coordinate with metal ions and facilitate the uniform nucleation and growth of MOFs.24,31 This would also greatly inhibit the agglomeration of MOFs. Interestingly, due to the interlaced nature of CNTs, a distinct ZIF/CNT network (ZCN) composed of cross-linked core− shell ZIF-8@CNT nanowires was successfully constructed here (Figure 1). By combining the morphological advantages of the 2D network and the internal compositional merits of ZIF-8, the obtained ZCN may act as an outstanding filler for fabricating composite PEMs. With respect to the polymer matrix, sulfonated poly(ether ether ketone) (SPEEK), a class of hydrocarbon-type PEMs, is selected here because of its simple preparation procedures, low cost, relatively good proton conductivity, and fuel-barrier properties.32 A series of composite PEMs then were prepared by hybridizing ZCN with SPEEK. The as-prepared SPEEK/ZCN composite PEMs showed significantly enhanced proton conductivity as well as greatly suppressed methanol permeability benefiting from the morphological and compositional advantages of ZCN. To be specific, the Hmim units originated from ZCN and the −SO3H groups of SPEEK are closely linked through electrostatic interaction, resulting in the formation of sulfonic acid-Hmim pairs. What is more, taking the unique 2D network superstructure of ZCN into consideration, it could be expected that the tighter cylindrical ionic nanochannels with good connectivity would be constructed along the interfaces between the ZCN and SPEEK matrix. In addition, the 0D ZIF-8 nanoparticles were also prepared for further studying the effect of the superstructure of MOFs on the composite membranes performance.

membranes have been successfully prepared. Generally, the further optimized proton conductivity of these MOF/polymer composite membranes is obtained through altering the chemical composition of MOFs, such as imbuing the pores of MOFs with different proton carriers (e.g., ionic liquids, imidazole, and acids) or modifying their organic ligands with functional groups (e.g., −SO3H, −COOH, −NH2). The researchers mainly focus on the structural designability of MOFs at the molecular length scale in the field of MOF/ polymer composite PEMs. As is well-known, besides the chemical component, the specific shape or morphology of fillers also plays an important role in the development of an ideal filler for improving the proton conductivity of composite PEMs.20,21 Wang’s group has systematically investigated the effect of fillers with different structures (from 0D to 2D) on the proton conductivity of composite PEMs.22 The results showed the asprepared composite PEMs incorporated with SSiO2 (0D), SHNT (1D), and SGO (2D) possessed conductivities of 24.9, 27.9, and 31.8 mS cm−1 (25 °C-100 RH%), respectively. It indicated that the fillers with 2D morphology had the strongest promotion ability for proton conductivity of membrane due to the constructed long-range pathways for proton transfer. In fact, creating more complex architectures at the mesoscopic/macroscopic scale, where MOF nanocrystals are used as building units to construct higher-order superstructures, has been proven to be an effective strategy for enhancing certain performance in a variety of fields via selection of superstructures of the appropriate dimensionality.23 For example, one-dimensional (1D) core−shell carbon nanotube (CNT)-MIL was meticulously synthesized via the in situ growth process and further used as fillers for fabricating mixedmatrix membranes. Because the in situ growth of MIL onto CNT surface can largely inhibit the agglomeration of MOFs, the obtained membranes exhibited not only a large CO2 permeability but also a high CO2/CH4 selectivity.24 Twodimensional (2D) zeolitic imidazole framework (ZIF-8)/GO hybrid nanosheets was prepared and further carbonized, which led to the formation of electrocatalysts with excellent oxygen reduction performance.25 Three-dimensional (3D) ZIF-8/ carbon nitrides foam was reasonably tailored for the efficient oil capture and chemical fixation of CO2.26 According to these successful examples, we believe that the structuring of MOFs in different hierarchical orders certainly opens a new opportunity to improve the overall performance of MOF/polymer composite PEMs via designing the physical form of MOFs instead of just altering the chemical component. 35076


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permeability (P) was measured at 40 °C with the aid of homemade equipment. The detailed operation process and equipment are also described in our previous work.40,41 The membrane selectivity (Φ) was calculated via eq 2: σ Φ= (2) P

2. EXPERIMENTAL SECTION 2.1. Materials. Zn(NO3)2·6H2O and 2-methylimidazole (Hmim) were bought from Aladdin. Ethanol, methanol, dimethylformamide (DMF), and concentrated HCl, HNO3, and H2SO4 were provided by Sinopharm Chemical Reagent Co., Ltd. The MWCNT powder (external diameter, 20−30 nm; length, 5−30 μm; purity, >95%) was purchased from XFNANO (Nanjing, China). Poly(ether ether ketone) (VitrexPEEK, grade 381G) was provided by Yuanbang Engineering Plastics Co., Ltd. (Nanjing, China). 2.2. Preparation of ZCN. Before the preparation of ZCN, the CNTs were treated using an acid mixture (H2SO4 and HNO3 of 3:1 volume ratio) at 60 °C for 3 h with stirring.33 Thereafter, the sample was filtered, washed with deionized water, and dried under vacuum. The preparation of ZCN was realized by in situ growth of ZIF-8 in the presence of CNTs, as shown in Figure 1. In a typical procedure, 120 mg of CNTs was dispersed in a 60 mL methanol solution containing 297 mg of Zn(NO3)2·6H2O by repeating the sonication and stirring processes prior to adding 657 mg Hmim. The mixture then was heated inside a 100 mL autoclave at 90 °C for 12 h. After the reaction, the autoclave was cooled to room temperature, and the resulting precipitates were centrifuged, washed with anhydrous alcohol, and dried in air. For comparison, pure ZIF-8 nanoparticles were also synthesized without adding CNTs. 2.3. Preparation of SPEEK/ZCN Composite Membranes. SPEEK was obtained via the postsulfonation of PEEK, and the detailed synthesis process is shown in the Supporting Information.34 The degree of sulfonation (DS) of SPEEK in this study was determined using 1H-NMR spectroscopy (see Figure S1). The DS was calculated to be about 62%. SPEEK/ZCN composite membranes were prepared through the solution-casting method.35,36 The composite membranes with 1, 2.5, and 5 wt % loading of ZCN are marked as SPEEK/ZCN-1, SPEEK/ZCN-2.5, and SPEEK/ZCN-5, respectively. The membrane thickness was controlled at around 45 ± 5 μm. A recast SPEEK membrane and the composite membranes incorporated with ZIF-8 or CNTs were prepared for comparison as well. To confirm the stability of ZCN in membranes and the crystalline structure of membrane with high loading of ZCN, the composite membrane with 10% ZCN was also prepared. 2.4. Characterization of ZCN. Transmission electron microscopy (TEM) was carried out via a JEOL JEM2100 TEM instrument. Fourier transform infrared spectroscopy (FTIR) was performed on Nicolet Nexus 470 with a resolution of 4 cm−1 and 64 scans. X-ray diffraction (XRD) was measured on a PANalytical X’Pert diffractometer with Cu Kα radiation. Thermogravimetric analysis (TGA) was operated under air atmosphere with a PerkinElmer thermal analyzer at a heating rate of 20 °C/min. 2.5. Characterization of SPEEK/ZCN Composite Membranes. The membrane morphology was observed by field-emission scanning electron microscopy (FE-SEM, Zeiss, Ultra 55). The FTIR spectra were carried out on a Nicolet Nexus 470 spectrometer with an ATR accessory with a resolution of 4 cm−1 and 64 scans. The nanostructure of membranes was investigated by small-angle X-ray scattering (SAXS, Bruker D8 Advance ECO) and X-ray diffraction (XRD, PANalytical X’Pert diffractometer). TGA analyses were operated under a N2 atmosphere at a heating rate of 10 °C/min. Mechanical properties of the dry membranes were operated on an MTS mechanical tester (E43.104) with the elongation rate of 5.0 mm/min. Water uptake (WU) of the membranes at 30 and 70 °C in water was measured via a method described in previous works.37 The IEC of the membrane was determined by the classical titration method as reported elsewhere.38 The IEC value can be calculated according to eq 1: IEC (mmol g −1) =

0.01 × 1000 × VNaOH Wd

where σ and P are the proton conductivity and the methanol permeability of the PEMs at 40 °C, respectively.

3. RESULTS AND DISCUSSION 3.1. Characterization of ZCN. XRD patterns of CNT, ZCN, and ZIF-8 are displayed in Figure 2. CNT exhibits a

Figure 2. XRD patterns of CNT, ZCN, and ZIF-8.

specific peak at around 25.8° that corresponded to the (002) plane of graphite.42 Its crystalline nature is not destroyed by the acid treatment. In terms of ZIF-8, the diffraction peaks at ca. 7.1° and 12.6° are observed, which is well consistent with that reported for the ZIF-8 topology.43 As for ZCN, the pattern shows sharp diffraction peaks similar to those of ZIF-8 nanoparticles. Notice that the characteristic diffraction peak of CNTs can also be observed in the pattern of ZCN, while the intensity is decreased as compared to that of pure CNTs. The XRD results indicate that the formation of linkages between Zn2+ and 2-methylimidazole goes smoothly in the presence of CNTs. The morphologies of the as-prepared samples were determined by TEM (Figure 3) and FESEM (Figure S2). The CNTs show the interlaced structures with an average diameter of about 25 nm and a smooth surface (Figure 3a,e). Figure 3b shows the whole morphology of the as-prepared ZCN. It demonstrates that the product is a 2D cross-linked network constructed from curved nanowires with a width of approximately 60 nm, which is much greater than the width of the CNTs (25 nm). FESEM observations (Figure S2) also demonstrate that the product contains curved nanowires with a much larger width in comparison with pure CNTs. The enlarged TEM (Figure 3f) indicates that this network consists of an internal CNT core and an outer ZIF-8 shell. The results from elemental mappings (Figure 3c) confirm the presence and homogeneous dispersion of C and Zn elements, further certifying the successful formation of ZIF-8 shell. CNTs appear to be an effective template for the nucleation and growth of MOF crystals. The carboxylic groups occurring on the CNT surfaces have strong coordination capability with Zn2+, thus benefiting the heterogeneous nucleation process and directing the ZIF-8 growth along the CNTs.44 Ultimately, a relatively continuous ZIF-8 shell is constructed on CNTs. As to the formation of peculiar network superstructure, it should be

(1)

where VNaOH was the volume of NaOH solution consumed for titration, and Wd was the weight of a dry sample. Proton conductivity (σ) under different conditions was measured with the same method as previously reported.39 The methanol 35077


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Figure 3. TEM images of CNT (a,e), ZCN (b,f), and ZIF-8 (d,g) at different magnifications; TEM-EDX mapping images of ZCN (c).

Figure 4. FTIR spectra (a) and TGA results (b) of CNT, ZCN, and ZIF-8.

Figure 5. Cross-sectional SEM images of the recast SPEEK membrane (a and a1), SPEEK/ZCN-1 (b and b1), -2.5 (c and c1), and -5 (d and d1) composite membranes.

100− 180 nm, which is consistent with the FESEM observation (Figure S2c). Figure 4a shows the FTIR spectra of CNT, ZCN, and ZIF-8, respectively. The FTIR spectrum of the pretreated CNT shows the characteristic absorption peaks of its functional groups at 3531 and 1717 cm−1, assigned to O−H stretching vibration and CO stretching vibration of carboxyl groups. In the case of ZCN, its FTIR spectrum exhibits the typical absorption peaks of ZIF-8 at 759 and 421 cm−1 assigned to the stretching vibration of Zn−O and Zn−N, respectively. The peak at 1177 cm−1 attributed to the C−N stretching vibration in the imidazole rings appears in the spectrum of ZCN as well.46 TGA diagrams of CNT, ZCN, and ZIF-8 are shown in Figure 4b. In addition to the decomposition of CNT in the temperature range of 150−390 and 492−575 °C, the typical weight loss at 390−492 °C is assigned to the removal of the

attributed to the interlaced nature of pretreated CNTs and the subsequent cross-linking of these nearby CNTs. During the formation of the core−shell ZIF-8/CNT nanowires in methanol, precursors first assemble to form ZIF-8 primary particles on the surface of the CNTs. Because of the crystal growth of the ZIF-8, some of the coated ZIF-8 primary particles in nearby CNTs tend to fuse and interconnect together to form the cross-linking points (Figure 3f).45 These knots reinforce the structural integrity of the hybrid networks during the processes of purification and membrane preparation. To gain a deep insight into the effect of the MOF superstructure on the membrane’s transport performance, ZIF-8 was also prepared because it possesses the similar chemical composition over ZCN but shows obvious differences in the architectures. As shown in Figure 3d,g, the as-prepared ZIF-8 exhibits a hexagonal morphology with a particle size in the range of 35078


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Figure 6. SAXS (a) and XRD (b) curves of the recast SPEEK membrane and SPEEK/ZCN composite membranes.

cluster dimension (d) in the membranes could be calculated from the following equations: d = 2π/q, where q is the scattering vector.51,52 On account of the results in Figure 6a, the incorporation of ZCN leads to an increase of the q value from 0.28 nm−1 to 0.31, 0.33, and 0.34 nm−1, respectively, when 1, 2.5, and 5 wt % ZCN is embedded. It indicates that the incorporation of ZCN decreases the average ionic cluster dimension in the composite membrane. Meanwhile, the increased loading content of ZCN would further decrease the ionic cluster size under identical conditions. In an amphiphilic PEM such as SPEEK, the size of the ionic clusters is determined by the equilibrium between the internal osmotic pressure of the clusters and the counteracting elasticity of the organic matrix.53−55 In this typical MOF/polymer system, ZCN with abundant Hmim units originating from the ZIF-8 shell can change this equilibrium, resulting in the formation of tighter nanochannels during the phase separation through the strong interactions between SPEEK and ZCN. Recently, Jacob’s groups also demonstrated that the incorporation of cellulose nanowhiskers (CWs) resulted in the formation of nanochannels with narrower size due to the existence of a considerable interaction between CWs and Nafion polymer.21 For another, an obvious reduction of scattering intensity is observed with ZCN being incorporated into the SPEEK matrix. The compatibility between ZCN and SPEEK should be responsible for this reduction, which weakens the electron density by way of mutual interactions. Hydrophobic domains formed from backbones were tested by XRD. As shown in Figure 6b, all of the membranes show a very broad crystalline peak at around 2θ = 19°. This peak exhibits a broadening and intensity decline phenomenon, while the crystalline structure is still there even when 10 wt % ZCN was incorporated (Figure S6). Such phenomenon should be attributed to the presence of ZCN, which hinders the crystallization of the SPEEK backbones by the strong interactions between ZCN and the SPEEK matrix.56 For another, the presence of a high content of ZCN (10 wt %) gives the composite membrane the characteristic diffraction peaks of ZCN (Figure S6). Meanwhile, these peaks are well preserved after proton conductivity measurement, which indicates that the crystalline ZCN structure is stable in the membrane matrix. These results indicate that ZCNs could be considered as effective and elegant nanostructures to regulate the microstructure of SPEEK, which ultimately will affect the overall performances of as-prepared MOF/polymer composite PEMs. 3.3. Thermal and Mechanical Properties of the SPEEK/ ZCN Composite PEMs. Benefiting from the interfacial interactions between ZCN and SPEEK matrix, the composite membranes are expected to show improved thermal and

organic linker molecules and the collapse of ZIF-8. It can be roughly calculated that the content of ZIF-8 in the hybrid network is 47.4 wt % from the TGA analysis.47 In summary, these results demonstrate that ZCN was successfully synthesized, and their unique superstructure and proper chemical composition make them an ideal MOF-based material for fabricating composite PEMs. 3.2. Microstructure and Physicochemical Properties of the SPEEK/ZCN Composite Membranes. In light of the architecture of composite membranes, the main superiority of MOFs in comparison with traditional inorganic nanoparticles lies in their better affinity with the soft polymer matrix.48 Figure 5 depicts the cross-sectional SEM images of the recast SPEEK membrane and SPEEK/ZCN composite membranes with different ZCN loading contents. Overall, all of the membranes have a dense cross section without cracks or pinholes. It is also observed that the SPEEK/ZCN composite membranes show rougher cross sections, as compared to the recast SPEEK membranes. As the ZCN content increases, this promotion would be exacerbated, resulting in a much rougher cross section. These phenomena can be attributed to the strong interfacial interactions provided by the linker functionality of ZIF-8.49 To be specific, such interactions probably come from (1) the electrostatic interaction between N atoms/N−H (ZCN)···−SO3H (SPEKK) and (2) the π−π interactions between Hmim rings in ZIF-8 and benzene rings in SPEEK. The slight red-shift of the symmetric stretching vibration peak of −SO3H groups at 1078 cm−1 in the FTIR spectra of SPEEK/ ZCN composite membranes verifies the existence of such interactions to some extent (Figure S3). This facilitates the compatibility between ZCN and SPEEK matrix and the subsequent formation of the uniform MOF/polymer composite membranes. As shown in the cross-sectional SEM images of the composite membranes at higher magnification (Figure S4), ZCN is uniformly dispersed in the SPEEK matrix and wrapped tightly by the chains of SPEEK due to the favorable interactions. The cross-sectional TEM image (Figure S5) of the SPEEK/ZCN-2.5 composite membrane also shows that ZCN is uniformly dispersed in SPEEK matrix and the morphology of ZCN in the SPEEK matrix conforms with the morphology as discussed above, which can be highly beneficial to promote proton conductivity of the composite membranes.50 To investigate the influence of ZCN incorporation on the microstructure of the as-prepared membranes, SAXS and XRD were performed. According to the SAXS patterns (Figure 6a), both the recast SPEEK and the SPEEK/ZCN composite membranes exhibit a typical ionomer peak, suggesting the presence of self-organized ionic clusters. The average ionic 35079


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Figure 7. TGA (a) and stress−strain (b) curves of the recast SPEEK membrane and SPEEK/ZCN composite membranes.

Figure 8. Humidity-dependent (at 70 °C, a) and temperature-dependent proton conductivities (at 30% RH, b) of the recast SPEEK membrane and SPEEK/ZCN composite membranes.

conductivity of SPEEK/ZCN-2.5 is 0.206 S cm−1 at 70 °C95% RH, 2.3 times that of the recast SPEEK (0.090 S cm−1). Especially, the proton conductivity enhancement is much more evident in the high-temperature/lower-humidity circumstances. The proton conductivities of SPEEK/ZCN-2.5 are found to increase with the rise in temperature and reach up to 50.24 mS cm−1 at 120 °C-30% RH, 11.2 times of the recast SPEEK (4.50 mS cm−1) under the same condition (Figure 8b). It shows strong competitiveness as compared to the previously reported works (Table S2). The significant enhancement in the proton conductivity of SPEEK/ZCN composite membranes should be attributed to the proper chemical composition and well-tailored superstructure of ZCN. To further investigate the effect of more complex superstructures of MOF on the proton conduction of MOF/polymer composite membranes, the proton conduction mechanism will be comprehensively discussed in the next section by comparing with the raw ZIF8, which possesses similar chemical composition but different superstructures.59 3.5. Proton Conduction Mechanism of the Composite Membranes. To gain a deep insight into the effect of the MOF superstructure on the membrane’s transport performance, ZIF-8 was also prepared because it possesses similar chemical composition but shows obvious differences in the architecture in comparison with ZCN. Considering that the SPEEK/ZCN-2.5 membrane possesses a relatively higher proton conductivity, the composite PEMs incorporated with 2.5 wt % of ZIF-8 or CNTs were also prepared. A certain filler is clearly observed in the cross-sectional SEM or TEM images of the corresponding composite membranes (Figure S7). The properties of the recast SPEEK membrane and the composite membranes (incorporated with 2.5 wt % fillers) were compared in terms of SR, WU, IEC, proton conductivity, as well as methanol permeability (discussed later).

mechanical stabilities. As shown in Figure 7a, the weight loss of all of the membranes consists of three stages, which is consistent with the previous literature.57 In comparison with the recast SPEEK, an obvious increase in desulfonation temperature can be observed in SPEEK/ZCN composite membranes. The strong electrostatic interaction between −SO3H groups and Hmim units inhibits the thermal decomposition of the −SO3H groups of SPEEK matrix, thus resulting in enhanced thermal stability. To investigate the mechanical stability of these composite membranes, the stress− strain curves of them are collected and presented in Figure 7b. According to the results, it is found that the incorporation of ZCN leads to a significantly increased mechanical stability. For instance, the tensile strength is elevated from 39.3 MPa for recast SPEEK to 50.8 MPa for SPEEK/ZCN-2.5, together with the Yong’s modulus increasing from 1291 to 1853 MPa (Table S1). The enhanced mechanical properties could be attributed to the interfacial interactions between the functional ligands of ZIF-8 and the matrix, which suppresses the mobility of the SPEEK chains. The intrinsically superior mechanical strength of ZIF-8/CNT network can be efficiently transferred to the composite membranes through the defect-free interfaces between ZCN and SPEEK.58 Collectively, these results indicate that the thermal and mechanical properties of the composite membranes can be effectively tailored by the incorporation of ZCN. 3.4. Proton Conductivity of the SPEEK/ZCN Composite Membranes. As shown in Figure 8, the proton conductivities of the SPEEK/ZCN composite membranes are significantly increased in comparison with those of the recast SPEEK membrane under various conditions. The SPEEK/ ZCN-2.5 membrane exhibits a typical proton conduction property and the highest proton conductivity among the composite PEMs under various conditions. The proton 35080


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SPEEK/ZIF composite membranes are significantly increased, as compared to that of the recast SPEEK membrane. This common increase for these two composite membranes should be attributed to the similar chemical composition of these fillers. In other words, the Hmim units originated from ZIF-8 or ZCN play a significant role in the proton conductivity improvement of the composite membranes. Linker termination of the ZIF-8 shell can yield a large number of exposed Hmim units, which can act as both proton donors (sharing protons on N−H) and proton acceptors (through noncoordinated N atoms).27,62 These Hmim units and the −SO3H groups of SPEEK are closely linked through electrostatic interaction, resulting in the formation of sulfonic acid−Hmim pairs at filler−SPEEK interfaces (illustrated in Figure 10a). Taking the SAXS results in Figure 6a into consideration, the incorporation of ZCN leads to an increase of the q value from 0.28 nm−1 to 0.31, 0.33, and 0.34 nm−1, respectively, when 1, 2.5, and 5 wt % ZCN are embedded. It indicates that the incorporation of ZCN leads to tighter ionic clusters that resulted from these sulfonic acid−Hmim pairs. For these pairs, proton donor and acceptor are closely linked. As a result, protons can transfer between the donor and acceptor via the Grotthuss mechanism with a low energy barrier in the absence of water. By comparison, SPEEK/ CNT composite membrane exhibits tiny improvement in proton conductivity in comparison with the recast SPEEK membrane under such conditions due to the absence of acid− base pairs. The Arrhenius activation energy (Ea) values of the membranes at 30% RH are calculated and shown in Figure 9b. Both SPEEK/ZCN and SPEEK/ZIF composite membranes show much lower Ea values than that of the recast SPEEK membrane and SPEEK/CNT composite membrane. This implies the formation of acid−base pairs, which effectively decreases the energy barrier for proton transfer in the composite membranes. Collectively, these results indicate that the significant improvement of proton conductivity of SPEEK/ ZCN and SPEEK/ZIF composite membranes should be attributed to the existence of ZIF-8 units, and the CNTs in ZCN only work as templates for the growth of ZIF-8 nanocomposite with more complex superstructures. It is noteworthy that the proton conductivity of the SPEEK/ ZCN composite membrane is also much higher than that of SPEEK/ZIF composite membrane, although both of them possess sulfonic acid−Hmim pairs in the membranes and show comparable IEC, WU, and SR. This observation makes us believe that the superstructure of MOFs does play a significant role in elevating the proton conductivity of MOF/polymer membranes. In fact, when fillers have a considerable interaction with the SPEEK or other similar polymeric PEMs, the ionic

As shown in Table 1, all of the composite membranes incorporated with ZIF-8 or ZCN exhibit much lower SR than Table 1. IEC, WU, and SR of the As-Prepared Membranes WU (%)

SR (%)

PEMs

IEC (mmol/g)

30 °C

70 °C

30 °C

70 °C

SPEEK SPEEK/ZIF SPEEK/ZCN SPEEK/CNT

1.62 1.47 1.48 1.54

22.5 20.8 19.4 26.5

56.4 40.6 40.2 58.9

18.9 12.8 6.7 23.5

49.8 21.2 8.6 55.4

that of the recast SPEEK membrane. Because all of these fillers contain ZIF-8, the possible strong interactions between ZIF-8 of fillers on the two domains of SPEEK (as mentioned above) mainly lead to this phenomenon. This greatly increases the usability of the SPEEK membrane as the PEM because the recast SPEEK membranes are readily swellable in the aqueous, which lowers both the proton transport and the methanol resistance. Correspondingly, the SPEEK/ZIF and SPEEK/ZCN composite membranes have a relatively low water uptake in comparison with the recast SPEEK membrane. Apart from the interactions between fillers and polymer matrix, the hydrophobicity of ZIF-8 mainly contributes to the reduction of WU. IEC is also a significant parameter of PEMs, which strongly affects proton transfer via the Grotthuss mechanism. Although the introduction of additional proton donors (N−H in exposed Hmim units) in these fillers should confer a higher IEC value to the composite membranes than that of the recast SPEEK membrane, the tested data in Table 1 give much lower IEC values to the SPEEK/ZCN and SPEEK/ZIF-8 composite membranes. The decreased IEC value reasonably results from the strong electrostatic interaction between the −SO3H groups of SPEEK and Hmim of fillers, which allows fewer −SO3H groups available for the titration.60 By comparison, SPEEK/ CNT composite membrane shows an obvious difference in these properties due to the different chemical composition of modified CNTs (Table 1). The corresponding results have been well investigated in previous works, and redundant discussion will not be included here. Herein, the as-prepared SPEEK/ZCN composite PEMs show much more obvious superiority under the conditions of low humidity and/or elevated temperature; thus the proton conductivity of these composite membranes is measured and compared under 30% RH. Meanwhile, it is always a major concern for researchers to achieve a PEM with enhanced proton conductivity under such a condition.61 As shown in Figure 9a, the proton conductivities of SPEEK/ZCN and

Figure 9. Temperature-dependent proton conductivities (at 30% RH, a) and Arrhenius plots (at 30% RH, b) of the recast SPEEK membrane and the composite membranes incorporated with different nanoparticles. 35081


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Figure 10. Acid−base interaction between SPEEK and ZIF-8 (a), and the schematic illustration for the interface design of SPEEK/ZIF or SPEEK/ ZCN composite membranes (b).

Table 2. Transport Properties of the Recast SPEEK and Composite PEMs at 40 °C PEMs SPEEK SPEEK/ZCN-1 SPEEK/ZCN-2.5 SPEEK/ZCN-5 SPEEK/ZIF

methanol permeability (P, ×10−7 cm2/s) 20.23 10.74 2.45 1.86 14.59

± ± ± ± ±

2.01 0.68 0.19 0.23 0.37

selectivity (Φ = σ/P, ×104 S·s/cm−3) 0.0305 0.686 5.924 2.154 0.367

± ± ± ± ±

0.0026 0.0531 0.4680 0.2146 0.0353

application potential of the SPEEK/ZCN composite membranes in DMFC, their methanol permeability was measured. According to the results in Table 2, the methanol permeability of the SPEEK/ZCN composite membranes is much lower than that of the recast SPEEK membrane, indicating the incorporation of ZCN could effectively reduce the methanol crossover through the membrane. This is in good agreement with the lower SR and WU of the SPEEK/ZCN composite membranes (Table 1). According to the SAXS results of membranes (Figure 6a), the incorporation of ZCN leads to a decrease of the average ionic cluster dimension from 22.4 nm for recast SPEEK membrane to 20.2, 19.0, and 18.5 nm for SPEEK/ZCN composite membranes, respectively, when 1, 2.5, and 5 wt % ZCN are embedded. The formation of tighter hydrophilic domains, which makes methanol permeation channels more obstructed, leads to the considerably lower methanol permeability of SPEEK/ZCN composite membranes. Meanwhile, the increased loading content of ZCN would further decrease the methanol permeability under identical condition. Besides, the barrier effect of nanoparticles that bestows an increased tortuosity of the methanol transport channels upon composite membranes should also be taken into account.21 It is worth noting that the methanol permeability of the SPEEK/ZCN membrane is also lower than that of SPEEK/ ZIF composite membrane. This probably results from the improved dispersion of ZIF-8 in the SPEEK matrix through the in situ growth of ZIF-8 on the external surface of CNTs. This observation also makes us believe that the methanol permeability of MOF/polymer composite membranes can be effectively tailored by the structuring of MOFs.

clusters can be adjusted during the membrane-forming process. What is more, the adjusted ionic clusters always emerge around the fillers.63,64 In general, ZCN has a moderate interaction with the ionic clusters of SPEEK matrix as discussed above. During the membrane-forming process, this interaction can promote the rearrangement of −SO3H groups of SPEEK together with Hmim of ZCN, making the hydrophilic parts of SPEEK (−SO3H groups) aggregate around the ZCN to lower the energy of the system. Considering the unique 2D network superstructure of ZCN, it could be suggested that the tighter cylindrical ionic nanochannels with good connectivity would be constructed along the interfaces between the ZCN and SPEEK matrix.65 By comparison, the 0D ZIF-8 is believed to lead to the formation of isolated spherical ionic domains. This inference is schematically shown in Figure 10b. The Ea values can verify the inference to some extent. It is found that the SPEEK/ZCN composite membrane shows the lowest Ea value (15.93 kJ/ mol), which implies the incorporation of ZCN most efficiently decreases the energy barrier for proton transfer (Figure 9b). For another, the holes between the neighboring ZIF-8@CNT nanowires in ZCN might be able to construct additional paths for the transfer of protons, avoiding the blockage of proton transport by 2D nanosheets, which are usually used as fillers for PEMs. These results indicate that the performance, especially the proton conductivity, of MOF/polymer composite PEMs can be effectively and easily tailored through creating more complex architectures. 3.6. Methanol Permeability of the SPEEK/ZCN Membranes. As a vital building block of the direct methanol fuel cell (DMFC), PEMs should not only allow proton transportation but also make sure no methanol crosses over from the anode to the cathode, because any methanol crossover would deteriorate the activity of the cathodic catalyst and hence dramatically reduce the overall fuel efficiency. Therefore, it is always a major concern for researchers to minimize the methanol crossover.66,67 To further evaluate the practical

4. CONCLUSION In summary, we have demonstrated a novel strategy to further optimize the proton conductivity of MOF/polymer composite PEMs through designing the physical form of MOFs. For the MOF/polymer composite membranes, ZIF-8-based materials 35082


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with different morphologies, 0D nanoparticle and 2D network, were dispersed in the SPEEK matrix. The better results were obtained for the ZIF-8/CNT cross-linked networks (ZCN), which led to an enhancement of the proton conductivity in comparison with the recast SPEEK membrane and the composite membranes containing raw ZIF-8 nanoparticles. Moreover, the methanol crossover of the composite membrane incorporated with ZCN was also greatly suppressed. Our investigation results reveal that the proton conductivity and methanol permeability of the MOF/polymer composite PEMs can be effectively modulated via designing the physical form of MOFs. This strategy may also promote the application of MOFs in other membrane fields by selecting appropriate MOFs and creating proper superstructures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13013. Preparation method and 1H NMR spectrum of SPEEK sample; FESEM image of CNT, ZCN, and ZIF-8; FTIR spectra of the recast SPEEK membrane and SPEEK/ ZCN composite membranes; cross-sectional SEM images of the SPEEK/ZCN composite membranes at higher magnification; cross-sectional TEM image of the SPEEK/ZCN-2.5 composite membrane; cross-sectional SEM (TEM) images of SPEEK/ZIF and SPEEK/CNT composite membranes; XRD curves of the SPEEK/ ZCN-10 composite membrane before and after proton conductivity test; mechanical properties of the recast SPEEK membrane and SPEEK/ZCN composite membranes; and comparison of transport properties with the previously reported work (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peiyi Wu: 0000-0001-7235-210X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the Ministry of Science & Technology of China (no. 2016YFA0203302).

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REFERENCES

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