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Rational Design of S-UiO-66@GO Hybrid Nanosheets for Proton Exchange Membranes with Significantly Enhanced Transport Performance Huazhen Sun, Beibei Tang, and Peiyi Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07651 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 17, 2017

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ACS Applied Materials & Interfaces

Rational Design of S-UiO-66@GO Hybrid Nanosheets for Proton Exchange Membranes with Significantly Enhanced Transport Performance 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.

Keywords: Metal-organic framework; In situ growth; Proton exchange membrane; Optimized ionic nanochannels; Proton conduction Abstract: Metal-organic frameworks (MOFs) are being intensively explored as filler materials for polymeric proton exchange membranes (PEMs) due to their potentials for the systematic design and modification of proton-conducting properties. S-UiO-66, a stable MOF with functional groups of –SO3H in its ligands, was selected here to prepare S-UiO-66@graphene oxide (GO) hybrid nanosheets via facile in situ growth procedure, and then a series of composite PEMs were prepared by hybridizing S-UiO-66@GO and sulfonated poly(ether ether ketone) (SPEEK). The resultant hybrid nanosheets not only possessed abundant –SO3H groups derived from the ligands of S-UiO-66 but also yielded a uniform dispersion of S-UiO-66 onto GO nanosheets, thus effectively eliminating the agglomeration of S-UiO-66 in the 1

ACS Paragon Plus Environment


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membrane matrix. Thanks to the well-tailored chemical composition and nanostructure of S-UiO-66@GO, the as-prepared SPEEK/S-UiO-66@GO composite PEMs present a significant increase in its proton conductivity under various conditions. In particular, the proton conductivity of SPEEK/S-UiO-66@GO-10 membrane was up to 0.268 S·cm-1 and 16.57 mS·cm-1 at 70 oC-95% RH and 100 o

C-40% RH (2.6 and 6.0 times of recast SPEEK under the same condition),

respectively. Moreover, the mechanical property of composite membranes was substantially strengthened and the methanol penetration was well suppressed. Our investigation indicates the great potential of S-UiO-66@GO in fabricating composite PEMs, and also reveals that the high proton conductivity of MOFs can be fully utilized by means of MOF/polymer composite membranes.

Introduction Constructed from metal ions and organic linkers, metal-organic framework (MOF) materials exhibit designable framework architectures and specific porous surfaces, and significant focus has been placed on their applications in gas storage, catalysis, separation, and sensors.1-6 Proton conductivity is now considered as a new functionality of MOFs.7 Several MOFs with high proton-conduction ability have been reported.8-10 However, their practical application in fuel cells is seriously restricted. The bulk phase and brittleness of MOFs not only make them difficult to process but also lead to poor distance conductivity. Converting MOFs into a film and introducing them into polymer membranes are considered to be a crucial step toward their 2


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practical application in electrochemical devices.11-14 A MOF/polymer composite membrane integrates the merits of both MOFs and polymer membranes. In a typical polymer/MOF system, the polymers can fill the grain boundaries and improve the mechanical stability of MOFs, and MOFs will endow the polymer membranes with increased proton conductivity. Besides, the new proton conduction pathways between the interfaces of MOFs and polymer matrixes can be constructed. As a result, the high proton conductivity of MOFs can also be fully utilized. To date, a few of MOF/polymer composite membranes have been reported. Zhu et al. first fabricated a PVP/Ca-MOF composite membrane, which showed highly increased proton conductivity when compared to that of pure MOF crystals or PVP matrix.15 Since this pioneering work, several groups have tried to further optimize the performance of MOF/polymer composite PEMs, especially the proton conductivity, through the rational design of MOFs.16 Generally, the rational design of MOFs with improved proton conductivity can be divided into two distinct approaches. The first approach is to imbue the pores of MOFs with different proton carriers. For example, phytic@MIL,17 PIL@MIL,18 and acids@MIL-10119 have been successfully prepared, and then the composite membranes were obtained by incorporating these MOF materials into different polymer matrixes. Functionalization of the organic ligands (e.g., -SO3H, -COOH, -OH) of MOFs to enhance their acidity and hydrophilicity is another approach. For example, MOF-808 and sulfated MOF-808 were incorporated into PVDF20 and Nafion21, respectively, to promote the proton conductivity under low humidity. Besides, the controllable integration of 3



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MOFs with other nanoparticles such as graphene oxide (GO) has also been proven to be an effective way to improve the proton conductivity of the resulting membranes.22 These exciting developments show that through deliberately tailoring the chemical compositions and nanostructures of MOFs before embarking on synthesis, the desired purpose of improving the proton conductivity of composite membranes could be readily realized.23-24 Nevertheless, the published works related to this class of composite membranes is far from enough and consistent. Besides, apart from the well-studied MOFs mentioned above, few of other charming and interesting MOFs have been exploited for PEMs. Therefore, research on the design of novel and distinctive MOF-based material is highly desirable, especially for the application as PEMs up to this point. Derived from the rational design concept as discussed above, the present work attempts to design and synthesize a novel MOF-based composite material and apply them in the fabrication of high-performance composite PEMs. S-UiO-66, a stable MOF with functional groups of –SO3H in its ligands, was selected to synthesize S-UiO-66@GO hybrid nanosheets via facile in situ growth procedure because of its excellent chemical stability, thermal stability, and relatively high proton conductivity.25 GO, a well-known nanosheet with carboxyl, hydroxyl, and epoxide functional groups, is also being intensely explored in the field of PEMs.26-28 Recently, it has been considered as an ideal platform for the growth of MOFs.29 In fact, it is often difficult to achieve a homogeneous MOF dispersion and corresponding a uniform film by direct dispersion of neat MOF particles in a polymer solution.30 Even 4


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if the MOF dispersion is pre-treated by sonication, aggregates may still be inevitable due to the hydrophobic nature and/or strong interparticle interactions.31-32 Nevertheless, the in situ growth of S-UiO-66 onto GO surface can largely inhibit the agglomeration of MOF in the membranes. Since the carboxyl groups of GO can coordinate with Zr ions and facilitate the uniform nucleation and growth of S-UiO-66 on the surface of GO (Figure 1B). Compared with MOF-808, MIL-101, ZIF-8 and other unmodified MOFs, inherently acidic MOF S-UiO-66 itself shows quite high proton conductivity of 0.34×10-2 S cm-1 at 30 oC and 97% relative humidity (RH), thus fussy modifications may be dispensable.33 By superimposing the high proton conductivity and the internal merits of both S-UiO-66 and GO, S-UiO-66@GO may act as an outstanding proton conductor as well as fillers for polymer membranes. In respect to the polymer matrix, sulfonated poly(ether ether ketone) (SPEEK), a class of hydrocarbon-type PEMs, is chosen here due to its relatively good proton conductivity and fuel-barrier properties as well as its simple preparation procedures and low cost.34 Therefore, the enhancement in overall performances of hybrid PEMs can be expected, especially proton conductivity. Herein, in situ growth of S-UiO-66@GO hybrid nanosheet were first synthesized, and then a series of composite PEMs were prepared by hybridizing S-UiO-66@GO and SPEEK. The synthesis of S-UiO-66@GO was confirmed by TEM, XRD, FTIR and TGA. The membrane cross-sectional morphologies, thermal and mechanical properties, chemical stability, nanophase separation, water uptake, proton conductivity, and methanol permeability of SPEEK/S-UiO-66@GO PEMs were investigated in 5



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detail.

2 Experimental section 2.1 Materials Graphite powders were purchased from Yingtai Co., Ltd. Poly(ether ether ketone) (VitrexPEEK, grade 381G) was supplied by Nanjing Yuanbang Engineering Plastics CO., Ltd. Sodium 2,5-dicarboxybenzenesulfonate (BDC-SO3Na) was purchased from TCI. ZrOCl2·8H2O, formic acid were bought from Aladdin. Absolute ethanol, dimethylacetamide (DMA), dimethylformamide (DMF) and concentrated HCl were provided by Sinopharm Chemical Reagent Co., Ltd. 2.2 Preparation of S-UiO-66@GO and SPEEK Graphene oxide (GO) was prepared from natural graphite through a modified Hummer’s method.35 S-UiO-66@GO hybrid nanosheets were prepared by in situ growth of S-UiO-66 onto the GO nanosheets and the typical synthesis procedure is demonstrated in Figure 1B. To fabricate the S-UiO-66@GO hybrid nanosheets, 150 mg of GO powder was first dispersed into a mixed solution consisting of 11.7 mL of formic acid and 40 mL of DMA; the resulting solution was then sonicated to obtain a homogeneous GO suspension solution. Next, 1.00 g of ZrOCl2·8H2O and 0.83 g of BDC-SO3Na were added into the above GO suspension solution and stirred the contents for 30 min to obtain a homogeneous solution. Then the solution was heated inside a 100 mL autoclave at 150 oC for 24 h. After cooling down the reaction system, the dark precipitate was collected by filtration and dried in air. The yield was 1.25 g, it can be roughly calculated that the percentage of S-UiO-66 in the composite is 88%. 6


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For comparison, pure S-UiO-66 nanoparticles were prepared via the same process without adding GO nanosheets (Figure 1A). SPEEK was obtained via the post sulfonation of PEEK36 and the sulfonation degree (DS) of SPEEK in this study was about 62% as determined using 1H-NMR spectroscopy (see Figure S1).

Figure 1. Illustration of the synthesis process of S-UiO-66 (A) and the in situ growth of S-UiO-66@GO hybrid nanosheets (B). 2.3 Preparation of SPEEK/S-UiO-66@GO composite membranes SPEEK/S-UiO-66@GO

composite

membranes

were

prepared

by

the

solution-casting method.37-38 First, SPEEK (200 mg) was dissolved in DMF (3 mL) under stirring. Furthermore, the desired amount of S-UiO-66@GO was added into the SPEEK/DMF solution. The mixture was ultrasonicated for 4 h and stirred vigorously for another 12 h at room temperature to obtain a homogeneous dispersion. The concentration of S-UiO-66@GO was determined according to the weight of the SPEEK polymer. Second, the S-UiO-66@GO/SPEEK/DMF dispersion was carefully 7



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cast into a rectangular mould in a vacuum oven at 60 oC for 12 h. Then, the membrane was further dried under vacuum at 80 oC for another 12 h. Third, the membrane was peeled off from the mould, followed by being treated in 1 M HCl solution at room temperature for 48 h to convert the membrane into H+ form. Finally, the prepared SPEEK/S-UiO-66@GO composite membranes were carefully rinsed with deionized water for several times before the characterizations. The resultant membranes were designated as SPEEK/S-UiO-66@GO-X, where X (X = 5, 10 or 15) was the weight ratio of the S-UiO-66@GO to SPEEK. The membrane thickness was controlled at around 40 ± 10µm. The recast SPEEK membrane and the composite membranes with other fillers were also prepared through the same method for comparison. 2.4 Characterization of S-UiO-66@GO Fourier transform infrared spectroscopy (FTIR) was measured on Nicolet Nexus 470 with a resolution of 4 cm-1 and 64 scans. X-Ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, Al Kα) was applied to characterize the chemical components. X-ray diffraction (XRD) was conducted on a PANalytical X’Pert diffractometer with Cu Kα radiation. High-resolution transmission electron microscopy (HRTEM) equipped with an energy-dispersive X-ray spectrometer (EDX) was investigated via a JEOL JEM2011 TEM instrument operated at 200 eV. Thermogravimetric analysis (TGA) was carried out under N2 atmosphere with a PerkinElmer thermal analyzer at a heating rate of 20 oC/min. Element analysis was measured by an energy-dispersive spectrometer (EDS) equipped by the field-emission scanning electron microscope (FE-SEM, Zeiss, Ultra 55).

8


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2.5 Characterization of SPEEK/S-UiO-66@GO composite membranes The membrane morphology was observed by FE-SEM mentioned above. The FTIR spectra were performed on a Thermofisher NEXUS 470 spectrometer with an ATR accessory with a resolution of 1 cm-1 and 64 scans. The nanostructure of membranes was explored by small-angle X-ray scattering (SAXS) using a Bruker D8 Advance ECO in the ranges of 0.1~4o. X-ray diffraction（XRD）was carried on a PANalytical X’Pert diffractometer in the ranges of 5~60o. TGA analyses were performed at a heating rate of 10 oC/min under a N2 atmosphere with a PerkinElmer Thermal Analyzer. Mechanical properties of the dry membranes were tested on MTS mechanical tester (E43.104) with the elongation rate of 5.0 mm/min. The oxidative stability of the membranes was also investigated using Fenton’s reagent (3% H2O2 containing 2 ppm Fe2+) at 60 oC until it started to rupture. The stability of the membranes was estimated from the rupture time and remaining weight after using the Fenton's reagent for 1h.39 Water uptake (WU) of the PEMs at 30 oC in water and 100 o

C-40% RH conditions was measured with a method described in our previous

report.40 WU was calculated from Eqn (1):

WU% =

× 100%

(1)

where Wdry and Wwet refer to the weight of the dry and wet membranes. The IEC of the membrane was determined by classical titration method: a dry pre-weighted sample (Wd, g) was immersed in a 2 M NaCl solution for 48 h to completely replace the H+ with the Na+ ions. The liberated H+ was titrated with 0.01 M NaOH solution using phenolphthalein as an indicator. The IEC value can be 9



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calculated according to Eqn (2):

IEC mmol =

.×× !

(2)

where VNaOH was the volume of NaOH solution consumed for titration. Proton conductivity (σ) was measured through a four-electrode method using AC impedance spectroscopy between 0.1 MHz and 1 Hz with potentiostat control (CHI660d Model). The temperature and humidity were controlled by a temperature and humidity test chamber during the measurements. All the samples were placed under the desired condition for 4 h before the measurements. The methanol permeability (P) was measured at 30 oC in an ATR cell (Thermofisher NEXUS 470). The detailed operation process and equipment can be found in our previous works.41-42 To evaluate the overall performance of the PEMs, the membrane selectivity (Φ) at 30 oC was calculated by the Eqn (3)

" =

#

(3)

$

where σ and P are the proton conductivity and the methanol permeability of the PEMs at 30 °C, respectively.

3 Results and discussion 3.1 Characterizations of S-UiO-66@GO XRD patterns of GO, S-UiO-66 and S-UiO-66@GO hybrid nanosheets are displayed in Figure 2A. GO shows its characteristic peak at around 2θ = 9.3o, corresponded to the interlayer spacing of 0.95 nm.43 In the case of S-UiO-66, various sharp diffraction patterns of S-UiO-66 are preserved, which is consistent with that reported for the UiO-66 topology.44-45 It demonstrates the successful synthesis of 10


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S-UiO-66. As for S-UiO-66@GO hybrid nanosheets, the pattern exhibits similar well-resolved diffraction peaks to those of S-UiO-66 nanoparticles, evidently identifying the generation of S-UiO-66 crystals. Meanwhile, the major diffraction peak of GO completely disappears in the XRD pattern of S-UiO-66@GO, which is attributed to the loss of long-range order in GO sheets46 or the reduction of GO47 after the solvent-thermal process. The XRD results reveal that the formation of linkages between Zr4+ and BDC-SO3Na was not prevented by the presence of GO and the S-UiO-66@GO hybrid nanosheets were successfully prepared.

Figure 2. XRD patterns (A) and FTIR spectra (B) of GO, S-UiO-66 and S-UiO-66 @GO hybrid nanosheets. Figure 2B shows the FTIR spectra of GO, S-UiO-66 and S-UiO-66@GO hybrid nanosheets, respectively. The FTIR spectrum of GO exhibits the typical absorption peaks of its functional groups at 1225, 1400 and 1729 cm-1, corresponded to phenolic C–OH stretching, carboxylic C–OH stretching and C=O stretching vibrations of carboxyl, respectively.48 These oxygen-containing functional peaks decrease to a large extent or even disappear in the spectrum of S-UiO-66@GO hybrid nanosheets, owing to the coordination between the carboxyl groups of GO and Zr4+ as well as the 11



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partially reduction of GO. Besides, the new peaks at 1227 and 1179 cm-1 attributed to the symmetric and asymmetric stretching vibration of S=O, appear in the spectrum of S-UiO-66@GO, as well as the peak at 657 cm-1 attributed to the S-O stretching vibration. The newly observed peaks in the IR spectrum of S-UiO-66@GO clearly suggest the presence of -SO3H groups in the hybrid nanosheets.49 Moreover, the intense doublet at 1592 and 1407 cm-1 can be assigned to the carboxylate groups of BDC-SO3Na ligands in S-UiO-66. The peaks at 775 and 578 cm-1, attributed to Zr-O2 longitudinal and transverse modes, emerges, further confirming the successful growth of S-UiO-66 on GO surface. XPS spectra of S-UiO-66 and S-UiO-66@GO were also measured to confirm their chemical composition. The results demonstrate that both of them contain C, O, S and Zr elements (Figure S2). Compared to that of S-UiO-66, the content of Zr and S elementals in S-UiO-66@GO are relatively lower, which indicates the existence of GO (Table S1). Besides, the capacity of –SO3H groups in S-UiO-66 and S-UiO-66@GO can be calculated about 1.1 and 0.6 mmol/g based on the sulfur component, respectively.50 FESEM-EDS results also reveal the presence of Zr, S, O and C elements in S-UiO-66@GO (Figure S3). What’s more. TGA results show that the functional structures of S-UiO-66@GO hybrid nanosheets are thermally stable to around 400 oC (Figure S4). The morphology of S-UiO-66@GO hybrid nanosheets is indicated by the TEM images presented in Figure 3. For comparison, the TEM images of GO and S-UiO-66 are included as well. GO exhibits a distinctive transparent and laminar structure with some wrinkles.29 In the absence of GO, the as-prepared S-UiO-66 nanoparticles 12


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display the irregular squares with various particle sizes smaller than 400 nm. In the case of S-UiO-66@GO hybrid nanosheets, S-UiO-66 particles with around 200 nm size and some smaller particles with 40~50 nm size were densely and uniformly distributed on the GO layers. Besides, the HAADF-STEM in Figure 3E as well and the corresponding elementary mapping in Figure 3F also indicate that S-UiO-66 nanoparticles are laid on the surface of GO. GO sheets appear to provide an ideal support for the nucleation and growth of MOF crystals since the phenolic and carboxylic groups occurring on the GO sheets have strong coordination capability with metal ions. These oxygen-containing groups can act as anchoring sites to combine with Zr4+, which can initiate the nucleation and growth of S-UiO-66 crystals on GO sheets. Therefore, S-UiO-66 can be homogeneously distributed on the surface of GO using the in situ growth technology. In summary, these results demonstrate that S-UiO-66@GO hybrid nanosheets were well synthesized, and their unique structure and chemical composition make them an ideal MOF-based material for fabricating hybrid PEMs.

13



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Figure 3. TEM images of GO (A), S-UiO-66 (B) and S-UiO-66@GO hybrid nanosheets (C, D); HAADF-STEM (E) and corresponding EDX mapping images (F) of S-UiO-66@GO hybrid nanosheet.

3.2 Characterizations of SPEEK/S-UiO-66@GO composite membranes

Figure 4. Cross-sectional SEM images of the recast SPEEK membrane (A and A1), SPEEK/S-UiO-66@GO-5 (B and B1), -10 (C and C1), -15 (D and D1) membranes

14


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and SPEEK/S-UiO-66-10 membrane (E and F). The internal morphologies of the membranes are visualized by the cross-sectional SEM images, as shown in Figure 4. The recast SPEEK has a relatively uniform and dense cross section without defects. By comparison, the incorporation of S-UiO-66@GO makes SPEEK/S-UiO-66@GO composite membranes become rougher due to the mutual interactions between the SPEEK matrix and the S-UiO-66@GO hybrid nanosheets. In particular, it can be clearly seen that S-UiO-66@GO still retains sheet structure and remain exfoliated in the cross section of SPEEK/S-UiO-66@GO-10. In fact, it is often difficult to achieve a homogeneous MOF dispersion and corresponding a uniform film by direct dispersion of neat MOF particles in a polymer solution due to the hydrophobic nature and/or strong interparticle interactions. As shown in Figure 4F, the cross-sectional SEM images of SPEEK/S-UiO-66-10 composite membrane show the large sizes of S-UiO-66 aggregates. On the contrary, the unique structure of S-UiO-66@GO hybrid nanosheets where nano-sized S-UiO-66 are uniformly dispersed on the surface of GO with no aggregation helps the dispersion of S-UiO-66 in the SPEEK matrix.51-52 Figure 5A and B show that S-UiO-66@GO are tightly embedded in the SPEEK matrix and the morphology of S-UiO-66@GO in the SPEEK matrix conforms with its morphology discussed above. The good compatibility between S-UiO-66@GO and SPEEK matrix can be attributed to the mutual interactions between them. Such interactions probably come

from

(1)

the

hydrogen

bonding

interactions

between

-SO3H

(S-UiO-66@GO)...-SO3H (SPEKK) and (2) the π-π interactions between unsaturated 15



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C–C bonds in graphene oxide and benzene rings in SPEEK.53 The FTIR results of these membranes can verify this inference to some extent. The peak at 1078 cm-1 in the FTIR spectra of recast SPEEK membranes is corresponded to the symmetric stretching vibration of –SO3H groups of the SPEEK chains. This peak presents a red-shift upon the incorporation of S-UiO-66@GO and is decreased to ca. 1076 cm-1 for SPEEK/S-UiO-66@GO-15 composite membrane (Figure S5).54 Nevertheless, the aggregation of S-UiO-66@GO appears in the SPEEK/S-UiO-66@GO-15 membrane (Figure 5C).

Figure 5. TEM images of the SPEEK/S-UiO-66@GO-5 (A), -10 (B) and -15 (C) composite membranes.

Figure 6. TGA (A) and stress-strain (B) curves of the recast SPEEK membrane and SPEEK/S-UiO-66@GO-X composite membranes.

16


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Benefited from the interfacial interactions between S-UiO-66@GO and SPEEK matrix, the composite membranes are expected to show improved thermal and mechanical stabilities. As shown in Figure 6A, all the SPEEK-based membranes show typical three weight-loss stages, which is consistent with the previous works.55 Upon S-UiO-66@GO incorporation, an obvious increase in desulfonation and backbone decomposition temperature can be obtained in S-UiO-66@GO/SPEEK composite membranes (marked by black dash lines for comparison). Besides, S-UiO-66@GO hybrid nanosheets may capture the free radicals generated during thermal decomposition and suppress the diffusion of decomposed products, which may also contribute to this phenomenon.56 The stress-strain curves of SPEEK/S-UiO-66@GO composite membranes are shown in Figure 6B. As expected, S-UiO-66@GO as an effective reinforcer increases the mechanical stabilities of composite membranes. For example, the Yong’s modulus is elevated from 1511 MPa for recast SPEEK to 2190 MPa for SPEEK/S-UiO-66@GO-10, together with the tensile strength increasing from 41.1 to 53.5 MPa (Table S2). The enhanced mechanical properties can be ascribed to the interfacial interactions between S-UiO-66@GO and the SPEEK matrix, which inhibits the mobility of the SPEEK chains. In particular, the good dispersion of S-UiO-66@GO plays a critical role in improving the mechanical performances because the intrinsically superior mechanical strength of GO can be efficiently transferred to the composite membranes through the large interfaces between S-UiO-66@GO

and

SPEEK.

The

elongation

at

break

values

of

SPEEK/S-UiO-66@GO composite membranes was smaller than that of recast SPEEK 17



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due to the presence of rigid inorganic fillers.57 What’s more, the Fenton’s test which simulates the fuel cell oxidative environment was used to evaluate the relative durability of the as-prepared membrane, as listed in Figure S6. Overall, the weight loss of all the membranes are less than 5 % and all the membranes are stable up to 3.5 h, suggesting their good chemical durability.58 Collectively, these results indicate that the thermal and mechanical properties, as well as the chemical durability, of the composite membranes could be effectively tailored through the incorporation of S-UiO-66@GO.

Figure 7. XRD (A) and SAXS (B) curves of the recast SPEEK membrane and SPEEK/S-UiO-66@GO-X composite membranes. The nanophase separation of the as-prepared membranes was probed by SAXS and XRD (Figure 7). As a kind of typically amphipathic polymer, SPEEK usually presents a characteristic nanophase-separation structure, where the backbones form hydrophobic domains (tested by XRD) and the –SO3H groups aggregate into ionic clusters (tested by SAXS).59-60 A very broad crystalline peak is usually observed in the 18


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XRD patterns of SPEEK-based membranes at around 2θ = 19o.61 Compared to that of the recast SPEEK membrane, the composite membranes show a broadening and intensity decline of this peak. Such phenomena are attributed to the crystallization hindering of the SPEEK backbones by the strong interactions between S-UiO-66@GO and the SPEEK matrix, as well as space interference. Besides, the presence of S-UiO-66@GO gives the composite membranes a characteristic diffraction peak at 2θ = 8o. For practical applications, a PEM usually works under humidified condition, thus the SAXS datum was collected under the wet state (Figure 7B). The recast SPEEK shows a clear ionomer peak at q = 0.28 nm-1 resulted from the existence of self-organized ionic clusters.57 By comparison, the incorporation of S-UiO-66@GO into the SPEEK matrix shifts this peak to a smaller value in the range of 0.24~0.27 nm-1. Given that the Bragg spacing (d), which indicates the average size of the hydrophilic ionic cluster, is calculated from d = 2π/q, the reduction of q values of the composite membranes indicates the increment of ionic cluster size.62 Meanwhile, the increased loading amount of fillers would exacerbate this promotion, resulted in a further increase of ionic cluster size under identical condition. In an amphiphilic proton exchange membrane like SPEEK, the dimensions of the ionic clusters are controlled by the equilibrium between the internal osmotic pressure of the clusters and the counteracting elasticity of the organic matrix.63 The S-UiO-66@GO hybrid nanosheets with abundant –SO3H groups could change the osmotic pressure of the ionic clusters, resulting in larger ionic clusters in the membrane matrix. Unlike peak location, the scattering intensity is proportional to the difference in the electron 19



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density between backbones and clusters.64-65 With S-UiO-66@GO being incorporated into the SPEEK matrix, a clear reduction of scattering intensity is observed. This phenomenon can be attributed to the compatibility effect of amphiphilic S-UiO-66@GO on the two domains of SPEKK by means of the mutual interactions as discussed above.

Figure 8. Temperature-dependent proton conductivities under 95% RH (A), Arrhenius plots under 95% RH (B), temperature-dependent proton conductivities under 40% RH (C), and Arrhenius plots under 40% RH (D) of the recast SPEEK and SPEEK/S-UiO-66@GO-X membranes. As presented in Figure 8 and Figure S7, the proton conductivities of the S-UiO-66@GO/SPEEK composite membranes are significantly increased, compared to that of the recast SPEEK membrane under various conditions. Among the composite PEMs, SPEEK/S-UiO-66@GO-10 displays the highest proton conductivity. 20


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The proton conductivity of SPEEK/S-UiO-66@GO-10 reaches 0.268 S·cm-1 at 70 o

C-95% RH, 2.6 times of recast SPEEK (0.105 S·cm-1). At 100 oC-40% RH, the

proton conductivity of SPEE/S-UiO-66@GO-10 is 16.57 mS·cm-1, up to 6 times of the recast SPEEK under the same condition. It shows strong competitiveness compared to the previously reported works (Table S3). The Arrhenius activation energy (Ea) values are calculated and depicted in Figure 8B and D. All the composite membranes exhibit lower Ea values than that of the recast SPEEK membrane under different relative humidity. Especially, the Ea of SPEEK/S-UiO-66@GO-10 membrane under 95% is 9.0 kJ/mol, much lower than that of the recast SPEEK membrane (18.4 kJ/mol). This implies the incorporation of S-UiO-66@GO efficiently decreases the energy barrier and accelerate proton transfer in the composite membranes. Nevertheless, the hybrid membrane's conductivity decreases when the loading of S-UiO-66@GO in SPEEK increases to 15% (w/w). This may result from the aggregation of excessive S-UiO-66@GO. At the same time, the proton conductivities of SPEEK/S-UiO-66@GO-10 are also much higher than those of the SPEEK-based membranes incorporated with different material-GO, S-UiO-66, S-UiO-66＆GO (a mixture of S-UiO-66 and GO) under the same conditions (Figure S8).

21



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Figure 9. WU (A) and IEC (B) of the recast SPEEK membrane and SPEEK/S-UiO-66@GO-X composite membranes. The

impressive

enhancement

in

the

proton

conductivity

of

SPEEK/S-UiO-66@GO composite membranes should be attributed to the well-tailored nanostructure and chemical composition of S-UiO-66@GO. To be specific, it can be explained via the following aspects. First, S-UiO-66 are uniformly dispersed on the surface of GO without aggregation, which can largely inhibit the agglomeration of S-UiO-66 in SPEEK matrix. At the same time, the compatibility effect of amphiphilic S-UiO-66@GO on the two domains of SPEEK by means of the mutual interactions guarantees the good dispersion of S-UiO-66@GO in SPEEK matrix. Both these are the crucial preconditions of obtaining a high-performance MOF/polymer composite membrane. Second, S-UiO-66@GO possesses highly acidic and strong hydrophilic functional groups -SO3H derived from the ligands of S-UiO-66. As a result, water retention capability of the SPEEK/S-UiO-66@GO composite membranes is obviously improved (Figure 9A). The available water molecules and the abundant -SO3H groups of S-UiO-66@GO not only work as proton carriers for proton transfer via the vehicle 22


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mechanism but also facilitate the proton hopping in the Grotthuss mechanism.66 Water evaporation drastically increases the electrolyte ohmic resistance at higher temperatures. Hence, it is always a major concern for researchers to eliminate such a drawback. Accordingly, PEMs with more bond water are more desirable to maintain high proton conductivity at relatively low humidity and/or high temperature. Here, SPEEK/S-UiO-66@GO-10 composite membrane shows excellent water retention ability at high-temperature and low-humidity conditions, which can be attributed to the presence of strongly bonded water molecules at Brønsted acidic sites within S-UiO-66 frameworks.21,33 In other words, S-UiO-66@GO plays an effective role in keeping water molecules in the bonded state in the ionomeric domains of composite membranes. This guarantees the great increase in proton conductivity of SPEEK/S-UiO-66@GO-10 membrane at high-temperature and low-humidity conditions. Third, optimized ionic channels (clusters) are constructed in the composite membranes via the mutual interactions between SPEEK and S-UiO-66@GO hybrid nanosheets. On one hand, S-UiO-66@GO with abundant sulfonic acid groups could change the osmotic pressure of the ionic clusters, which results in larger ionic clusters in the membrane matrix as discussed above. What’s more, the abundant –SO3H groups in S-UiO-66@GO have hydrophilic and hydrogen-bonding interactions with the ionic clusters of SPEEK matrix. These interactions can promote the assembly of -SO3H groups of S-UiO-66@GO together with -SO3H groups of SPEEK to ionic clusters along the interfaces between two components during the membrane-forming 23



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process. Ultimately, well-connected ionic nanochannels may be constructed along the interfaces between the S-UiO-66@GO and SPEEK matrix (as shown in Figure 10).67-68 The interesting IEC behavior can verify this inference to some extent. As shown in Figure 9B, SPEEK/S-UiO-66@GO-10 composite membrane shows the highest IEC value, which can be attributed to the optimized ionic clusters that allow the largest number of –SO3H groups available for the titration. While this may not be realized by direct incorporation of S-UiO-66 into SPEEK matrix due to the aggregation and 0-dimensional structure of S-UiO-66.

Figure 10. Schematic illustration of the enhanced transport properties of the SPEEK/S-UiO-66@GO composite membranes. Methanol permeability is another indispensable parameter for PEMs being applied in DMFCs. As is well known, there is a trade-off between the proton conductivity and the methanol resistance. In other words, the high proton conductivity of the as-prepared composite membranes here may be obtained at the sacrifice of methanol

resistance

ability.

However,

the

methanol

permeability

of

the

SPEEK/S-UiO-66@GO composite membranes only increases faintly compared to 24


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that of recast SPEEK, according to the results of methanol permeability tests listed in Table 1. The methanol permeability of SPEEK/S-UiO-66@GO-10 is just 10% higher than that of recast SPEEK. When S-UiO-66@GO being incorporated into SPEEK matrix, the positive effect of a more facile ionic channel on enhancing the methanol permeability is probably offset by the barrier effect of two-dimensional S-UiO-66@GO hybrid nanosheets on suppressing the methanol crossover. 48 A slight increase in methanol permeability is observed with the increasing S-UiO-66@GO content. This may originate from the rigidity and overloading of S-UiO-66@GO, which induces defects in the composite membranes.22,69 Consequently, the selectivity of SPEEK/S-UiO-66@GO composite membranes is obviously enhanced benefitted from the significantly increased proton conductivity and relatively low methanol crossover. This further proves that our approach is a very effective way to obtain high-performance MOF/polymer composite PEMs with low methanol permeation in conjunction with excellent proton conductivity. Table 1. Transport properties of recast SPEEK and composite PEMs at 30 °C Methanol permeability (P, ×10-7 cm2/s)

Proton conductivity (σ, S/cm)

Selectivity (Φ= σ/P, ×105 S*s/cm-3)

SPEEK

5.14 ± 0.21

0.047 ± 0.005

0.914 ± 0.0433

SPEEK/S-UiO-66@GO-5

5.59 ± 0.33

0.105 ± 0.005

1.88 ± 0.1754

SPEEK/S-UiO-66@GO-10

5.67 ± 0.15

0.173 ± 0.002

3.05 ± 0.4832

SPEEK/S-UiO-66@GO-15

6.24 ± 0.59

0.076 ± 0.001

1.22 ± 0.2253

PEMs

4 Conclusions A novel MOF-based material S-UiO-66@GO was first synthesized and then a 25



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series of composite PEMs were prepared by hybridizing S-UiO-66@GO and SPEEK. The as-prepared SPEEK/S-UiO-66@GO composite membranes display excellent proton conductivity under different conditions. For instance, the proton conductivity of SPEEK/S-UiO-66@GO-10 membrane reaches 0.268 S·cm-1 and 16.57 mS·cm-1 at 70 oC-95% RH and 100 oC-40% RH (2.6 and 6.0 times of recast SPEEK under the same condition), respectively. This remarkable enhancement in proton conductivity should be ascribed to the well-tailored nanostructure and chemical composition of S-UiO-66@GO. First, the uniform dispersion of S-UiO-66 on GO surface can ensure the good dispersibility of S-UiO-66 inside the membranes. Second, the available water and -SO3H groups for proton transfer in composite membranes are significantly enhanced due to the existence of inherent Brønsted acidic sites in S-UiO-66 networks. Ultimately, enlarged ionic clusters and well-connected ionic nanochannels are constructed along the interfaces between S-UiO-66@GO and SPEEK matrix in composite membranes via hydrophilic and hydrogen-bonding interactions. Meanwhile, the methanol crossover of composite membranes is well suppressed due to the barrier effect of 2d S-UiO-66@GO nanosheets. Our investigation reveals that the rational design of MOF materials is an efficient method to optimize the utilization of MOFs in the field of PEMs. It also indicates that the high proton conductivity of MOFs can be fully utilized by means of MOF/polymer composite membranes.

ASSOCIATED CONTENT Supporting Information. Preparation method and 1H-NMR spectrum of SPEEK 26


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sample; XPS spectra and elemental percentage of S-UiO-66 and S-UiO-66@GO hybrid nanosheets; Element mapping and EDS of S-UiO-66@GO; TGA curves of GO, S-UiO-66 and S-UiO-66@GO; FTIR spectra, mechanical properties, Nyquist plots and oxidative stabilities of the recast SPEEK and SPEEK/S-UiO-66@GO composite membranes; Proton conductivities of the composite membranes incorporated with other inorganic nanoparticles; Comparison of transport properties with the previously reported work. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected].

Notes 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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