Extra Water- and Acid-Stable MOF-801 ... - ACS Publications

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 28656−28663

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Extra Water- and Acid-Stable MOF-801 with High Proton Conductivity and Its Composite Membrane for Proton-Exchange Membrane Jin Zhang,†,‡,⊥ Hui-Juan Bai,§,⊥ Qiu Ren,†,‡ Hong-Bin Luo,†,‡ Xiao-Ming Ren,*,†,‡ Zheng-Fang Tian,∥ and Shanfu Lu*,§ †

State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemistry and Molecular Engineering and College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, P. R. China § Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China ∥ Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, P. R. China

ACS Appl. Mater. Interfaces 2018.10:28656-28663. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/01/18. For personal use only.

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S Supporting Information *

ABSTRACT: Proton-exchange membranes (PEMs), characterized by selectively permitting the transfer of protons and acting as a separator in electrochemical devices, have attracted immense attention. The composite membrane, fabricated from organic polymer matrix and high proton-conducting metal−organic framework (MOF), integrates the excellent physical and chemical performances of the polymer and MOF, achieving collective properties for good-performance PEMs. In this study, we demonstrate that MOF-801 shows remarkable proton conductance with σ = 1.88 × 10−3 S cm−1 at 298 K and 98% relative humidity (RH), specifically, together with extra stability to hydrochloric acid or diluting sodium hydroxide aqueous solutions and boiling water. Furthermore, the composite membranes (denoted MOF-801@PP-X, where X represents the mass percentage of MOF-801 in the membrane) have been fabricated using the sub-micrometer-scale crystalline particles of MOF-801 and blending the poly(vinylidene fluoride)−poly(vinylpyrrolidone) matrix, and these PEMs display high proton conductivity, with σ = 1.84 × 10−3 S cm−1 at 325 K 98% RH. A composite membrane as PEM was assembled into H2/O2 fuel cell for tests, indicating that these membrane materials have vast potential for PEM application on electrochemical devices. KEYWORDS: metal−organic frameworks, MOF-polymer composite membrane, proton-exchange membrane, chemical stability, proton conductivity

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conductance (σ > 1.0 × 10−4 S cm−1), (2) excellent thermal stability and chemical stability in acid and water environments, (3) good mechanical performance, (4) low-cost production, and simplistic processes of preparation.6−8 In response, enormous numbers of studies are devoting to develop new types of PEMs; examples include modified Nafion, various polymers, and composite membranes.9−17 In recent years, a new type of crystalline porous material, called metal−organic frameworks (MOFs) or porous coordination polymers and featured with structure designability and pore size tunability, has attracted much attention and achieved great progress in proton conduction.18−29 Hitherto, thanks to

INTRODUCTION

Proton-exchange membranes (PEMs) are characterized by selectively permitting the transfer of protons and acting as separator in a range of electrochemical devices, for example, electrochemical reactors, fuel cells, electrochromic displays, and electrochemical sensors;1−3 thus, it is of interest from the viewpoint of both fundamental and applied research to explore new applicable PEMs. The most commonly used PEM is Nafion membranes, which show tremendous proton conductance (σ = 10−1−10−2 S cm−1) at 60−80 °C and 98% relative humidity (RH); however, they confront great challenges during large-scale realistic application resulting from low thermal stability, hazardous manufacturing process, and high cost.4,5 To date, these inherent flaws have not been settled yet. It is crucial to seek for potential alternative membranes that are accompanied by (1) good proton © 2018 American Chemical Society

Received: May 31, 2018 Accepted: August 2, 2018 Published: August 2, 2018 28656

DOI: 10.1021/acsami.8b09070 ACS Appl. Mater. Interfaces 2018, 10, 28656−28663

ACS Applied Materials & Interfaces

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Research Article

RESULTS AND DISCUSSION Crystal Structure of MOF-801. The crystal structure of MOF-801 was previously reported.49 For the convenience of better understanding the proton conductance, we describe simply the crystal structure of MOF-801 again. This MOF crystallizes in cubic space group Pn3̅, and the zirconium(IV) ion shows tetragonal antiprism coordination geometry with eight oxygen atoms (Figure S1a). The three-dimensional framework of MOF-801 is built from the zirconium oxide secondary building unit (SBU) with linear and ditopic linkers of fumarate. As shown in Figure S1b, a zirconium oxide SBU consists of six crystallographically equivalent zirconium ions, which are connected together through eight μ3-oxygen atoms, amongst them, four hydroxyl (OH) groups and four O2− ions. Each SBU is coordinated to 12 fumarates and each fumarate is coordinated to two SBUs to build a three-dimensional framework, which has a network of porous tetrahedral and octahedral cages (ref Figures S1c,d, and S2). The internal pore diameters in the framework were 7.4, 5.6, and 4.8 Å calculated using the Platon program.49 Synthesis and Chemical and Thermal Stabilities of MOF-801. Sub-micrometer-scale microcrystals of MOF-801 were achieved via a slight modification of the methodology previously reported.49 The sub-micrometer-scale crystalline sample was obtained. Transmission electron microscopy (TEM) photos of the sample of MOF-801 display that the microcrystals aggregate together and a single particle has a submicrometer scale (ref Figure S3), as found from the analysis from laser particle size measurement (ref Figure S4). The submicrometer-scale microcrystalline sample was also characterized by IR spectrum (Figure S5), XPS for C 1s, N 1s, O 1s, and Zr 3d core levels (ref Figures S6 and S7), and thermogravimetric (TG) technique (ref Figure S8a). The sample phase purity was examined through the powder X-ray diffraction (PXRD) technique. As shown in Figure 1, the experimental

the extensive study by researchers, some of the protonconducting MOFs show much high proton conductivity, which reaches up to 10−2 S cm−1,30−32 even 1.82 S cm−1 (at 70 °C and under 90% RH);33 nonetheless, most of them have poor chemical stability that can be ascribed to the coordination bonding construction characteristic between metal ion and ligand, which consequently hampers their realistic application in PEMs. In this regard, it is of great significance to explore excellent chemically stable MOFs with high proton conductivity for technique application.34−36 Noticeably, most of the proton-conducting MOFs reported were studied on the pellets or single crystal of materials, which are not directly usable as the electrolyte membrane in electrochemical devices. Thus, considering the practical application of protonconducting MOFs in electrochemical devices, it is necessary to make them as a membrane. Recent studies by us37,38 and other groups39−41 have demonstrated that the composite membrane of MOF with organic polymer integrates the exceptional chemical and physical properties of polymer and proton-conducting MOFs and may achieve collective properties to fabricate high-performance PEMs. Poly(vinylidene fluoride) (PVDF) shows outstanding chemical and thermal stabilities and high mechanical strength, together with the features of low cost and easily forming membranes, leading to it being one of the most widely used membrane materials in the industry.42−44 However, the hydrophobic nature of PVDF appears to be a major obstacle for PEM application under high relative humidity owing to the poor proton conductance, and several examples have validated that composite membranes based on PVDF matrix display considerable proton conductivity only when soaked in water.37,45 However, poly(vinylpyrrolidone) (PVP) is a typical hydrophilic polymer, and the hydrophilic PVP as the matrix can effectively improve water-assisted proton-transport performance of the composite membrane. Furthermore, the Nheterocycle of PVP can accept protons from acid, suggesting that PVP has a vast potential for PEM application39,46 since Zhu et al. reported the first study on polymer composite PEMs of MOF with PVP.39 In this context, a series of blended polymer membranes comprising PVP and other polymers show high proton conductivity and excellent performance in PEM application.7,47,48 Hence, if the sub-micrometer-scale high proton-conducting MOF particles are integrated into the blending PVDF−PVP matrix to fabricate composite membranes, this will bring together the best of individual components, achieving high proton conductance and excellent thermal and chemical stabilities. In this paper, we demonstrate high-performance protonconducting MOF-801, with steeply uptaking water vapor at low relative pressure and remarkable proton conductivity of 1.88 × 10−3 S cm−1 at 298 K and 98% RH. Particularly, MOF801 shows excellent chemical stability in hydrochloric acid or diluting sodium hydroxide aqueous solutions and boiling water. The membranes have been facilely prepared with the sub-micrometer-scale crystalline particles of MOF-801 and blending PVDF−PVP matrix, and the PEMs prepared display high proton conductance (1.84 × 10−3 S cm−1) at 325 K and 98% RH. As an illustration of concept application, the composite membranes as PEM were further assembled into H2/O2 fuel cell for tests, and the results suggest these membrane materials have vast potential of PEM application on electrochemical devices.

Figure 1. Simulated and experimental PXRD patterns of MOF-801 samples, which correspond to the sample immersed in boiling water or dimethylformamide (DMF), HCl (6 mol L−1), or NaOH (0.1 mol L−1) aqueous solutions, used for alternating current impendence measurement, and the as-prepared one, respectively.

PXRD profile of MOF-801 is in good agreement with the simulated pattern of MOF-801, which is obtained from the single-crystal structure data using the program of Mercury 3.1, indicating that the microcrystalline sample of MOF-801 has high phase purity. To inspect the stability of MOF-801 in water, the microcrystalline samples of MOF-801 had been immersed in 28657


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Figure 2. (a) Nyquist plots of MOF-801 at selected temperatures and 98% RH; (b, c) plots of σ vs T and ln(σT) vs 1000/T for MOF-801; and (d) water vapor adsorption isotherms (298 K) of MOF-801. Filled and open symbols correspond to adsorption and desorption processes, respectively.

water at ambient temperature for 3 days and boiling water for 12 h as well as depicted in Figures 1 and S8b, respectively, and the PXRD patterns of the samples soaked in cooling or boiling water are nearly identical to that of the as-synthesized sample of MOF-801, demonstrating that MOF-801 possesses extra stability to water. The samples of MOF-801 had also been soaked in hydrochloric acid (6.0 mol L−1) and diluting sodium hydroxide (0.1 mol L−1) aqueous solutions for 3 days, separately. As shown in Figure 1, their PXRD patterns indicated that MOF-801 exhibits excellent acidic and diluting basic stability. It is worth mentioning that, to date, only few numbers of proton-conducting MOFs with good stability in both acidic and alkali circumstances have been reported. The PXRD profile of the sample used for the impedance test demonstrates that the structure of MOF-801 was maintained although it shows slightly poor crystallinity compared to the assynthesized sample. Furthermore, the variable-temperature PXRD patterns indicate that MOF-801 preserves its structural integrity up to 250 °C at least (ref Figure S9). The excellent chemical and thermal stabilities render MOF-801 to have practical applicability in the field of proton conduction. Proton Conductance of MOF-801. The impedance measurement was performed on the compressed pellet of a microcrystalline sample for assessing the proton conductivity of MOF-801, and the frequency spans from 102 to 106 Hz and the temperature ranges from 289 to 334 K at 98% relative humidity (RH) in the measurements. All of the Nyquist plots at different temperatures shown in Figure 2a display high similarity, which comprise an imperfect arc at the high frequency and a tail at the low frequency. In general, both bulk and grain boundary conduction might be expected. In this

study, in the high-frequency region, the Nyquist plots performed with an imperfect arc rather than two separated arcs, indicating that the proton transfer in bulk and grain boundary is difficult to separate from each other because the values of time constant τ of the two components are comparable and the analogous behaviors have been well elucidated in other proton-conducting materials.50−52 Noteworthy, this MOF exhibits much higher proton conductance with σ = 1.19 × 10−3 S cm−1 at lower temperature (289 K) and 98% RH. As depicted in Figure 2b, the proton conductivity of MOF-801 is increased with elevating temperature, and σ = 2.12 × 10−3 S cm−1 at 302 K, which is higher than the proton conductivity of well-studied proton-conducting MOFs, for example, PCMOF-5 (1.3 × 10−3 S cm−1 at 295 K and 98% RH),53 UiO-66(Zr)-(COOH)2 (0.85 × 10−3 S cm−1 at 298 K and 95% RH),32 and (NH4)2[MnCr2(ox)6]3·4H2O (1.1 × 10−3 S cm−1 at 295 K and 96% RH),54 and comparable to some efficient proton-conducting materials of Cd-5TIA (3.61 × 10−3 S cm−1 at 301 K and 98% RH),55 MOF-808 (2.65 × 10−3 S cm−1 at 290 K and 98% RH),37 and (NH4)2(adp)[Zn2(ox)3]· 3H2O (8 × 10−3 S cm−1 at 298 K and 98% RH).56 At 334 K and 98% RH, the maximum value of 4.16 × 10−3 S cm−1 is obtained, which is nearly more than 4-fold of the proton conductivity at 289 K. This remarkable proton conductivity value at 334 K is comparable with that of high proton conductivity water-assisted proton-conducting materials, such as UiO-66(Zr)-(COOH)2 (2.3 × 10−3 S cm−1 at 363 K and 95% RH)32 and Zr2(PO4)H5(L)2·H2O (1 × 10−3 S cm−1 at 413 K and 96% RH).57 Furthermore, the time-dependent impedance was investigated in the period of 8 days at room temperature and 98% RH, and the measurement was 28658


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bonding networks of ···Zr6O4(OH)4···(H2O)n··· would be generated in high relative humidity, achieving high proton conductivity. The water adsorption profile of MOF-801 shows that the amount of water adsorbed increases with the rising relative pressure of water vapor (ref Figure 2d), implying that the MOF-801 is capable of adsorbing water at lower relative humidity. Additionally, the proton conductance of MOF-801 under anhydrous condition has also been measured; as shown in Figure S14, the MOF-801 exhibits negligible proton conductivity. This finding implies that the protons hop in the H-bond networks of ···Zr6O4(OH)4···(H2O)n··· but not on the surface of the cavities. Preparation and Characterization of Membranes MOF-801@PP-X. In terms of excellent stability and high proton conductivity, and as a step toward realistic application, we have successfully fabricated mixed matrix membranes in which the microcrystalline powders of MOF-801 as filler dispersed in blending PVP−PVDF supporting matrix. Here the composite membranes were denoted MOF-801@PP-X, where X is the mass percentage of MOF-801 (X%) in MOF-801@ PP-X and X = 0, 20, 40, and 60. The PXRD patterns of composite membranes MOF-801@PP-X together with the assynthesized MOF-801 are illustrated in Figure 3; all

performed every other day. As depicted in Figure S12, the impedance spectra obtained in different times show no significant change, indicating that MOF-801 possesses longterm proton-conducting stability. The proton-transport activation energy was determined by means of the Arrhenius equation (eq 1) ln(σT ) = ln A −

Ea kBT

(1)

where the symbols σ, A, Ea, and kB represent the proton conductivity, the pre-exponential factor, the proton-transport activation energy, and Boltzmann constant, respectively. The Arrhenius plot of MOF-801 is plotted in Figure 2c, which is linearly approximated in the temperature ranges of 289−334 K. The activation energy value was estimated as 0.256 eV, which lies in the range of Ea < 0.4 eV and corresponds to the typical Grotthuss mechanism.58 On the basis of the crystal structure analysis,49 the H-bonds form among the coordinated water molecules, OH groups in the Zr6O4(OH)4 SBUs, and the coordinated COO− groups on the surface of the cavities. Besides this, the water molecules at low uptake mainly reside in the tetrahedral cavities and form the H-bonds between water molecules in the cavities as well as between the OH groups in the Zr6O4(OH)4 SBUs and the water molecules in the cavities, whereas the water molecules at high uptake occupy two types of tetrahedral and octahedral cavities, and the H-bonds connect the water molecules in the tetrahedral and octahedral cavities and the OH groups in the Zr6O4(OH)4 SBUs. Obviously, the water molecules absorbed conduce to form well-established hydrogen-bonding network in the framework of MOF-801; the hopping process of protons could be realized by proton transfer in the H-bond networks of ···Zr6O4(OH)4··· (H2O)n··· or/and derived from the coordinated carboxylate groups and OH groups in Zr6O4(OH)4 SBUs, and this protonhopping process is similar to that observed in other hydrated proton conductors.32,59 To perform a closer investigation of the relationship between the relative humidity and proton conductivity in the MOF, the humidity-dependent proton conductivity has been measured at 298 K; as shown in Figure S13, the proton conductivity of MOF-801 is rapidly increased with increasing relative humidity in the range of 43−98% RH. The conductivity is found to be 1.92 × 10−7 S cm−1 at 43% RH and 1.64 × 10−5 S cm−1 at 75% RH, and the maximum value is 1.88 × 10−3 S cm−1 at 98% RH. The highly humiditydependent conductance indicates that the water molecules in cages of the MOF-801 framework play a crucial role in proton conduction for MOF-801, which is because the water molecules absorbed in octahedral and tetrahedral cavities of the framework are favorable to generating well-established hydrogen-bonding networks, and these hydrogen-bonding networks as effective pathways would give rise to MOF-801 showing higher proton conductivity under high relative humidity. This observation also agrees well with the aforementioned Grotthuss mechanism with the participation of water molecules. At lower relative humidity, the insufficient hydrogen-bonding networks, formed between the small amount of water molecules in the framework and the Zr6O4(OH)4 clusters, could not provide an efficient protonhopping pathway, thus resulting in poor proton conductivity, while the amount of adsorbed water in the framework increases with relative humidity, and thus more efficient hydrogen-

Figure 3. PXRD profiles of the as-synthesized MOF-801 and the membranes of MOF-801@PP-X (X = 0, 20, 40, and 60).

membranes showed MOF-801 characteristic diffractions, and the relative intensities of MOF-801 diffractions gradually enhanced on increasing its relative amount in the membranes, demonstrating that the crystalline structure of MOF-801 is maintained without any change in the supporting matrix. The morphology and cross section of each composite membrane were characterized by scanning electron microscopy (SEM), and the corresponding SEM images are shown in Figures 4 and S16. The microcrystals of MOF-801 are evenly mixed and distributed in the membranes, and this can be further validated by energy-dispersive X-ray spectroscopy. The elemental mapping shows the oxygen and zirconium elements homogeneously distributed in the membranes (ref Figure S17). Besides, with increasing content of MOF-801 in the composite membranes, the microcrystals of MOF-801 are more closely spaced and the surface roughness of composite membranes increases. Interestingly, the flexibility of composite membranes speedily increased when they were exposed to the high relative humidity (98% RH) environment, and this is because PVP absorbs water from the high relative humidity environment, resulting in the increase of flexibility of the membranes. 28659


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Figure 4. (a) SEM image of MOF-801@PP-60 and the elemental mapping images for (b) F, (c) O, and (d) Zr in MOF-801@PP-60.

Proton Conductance of Composite Membranes MOF801@PP-X. The proton conductivity of membranes MOF801@PP-X was evaluated by impedance spectra, and the membranes were exposed to 98% RH environment and temperature range of 289−334 K during the measurement. The representative Nyquist plots are displayed in Figure S20, and the proton conductivities were obtained. As depicted in Figures 5 and S20h, all of the membranes display increased

at 298 K in contrast with that of MOF-801-incorporated membranes. Apparently, the proton conductivity increases with rising mass percentage of MOF-801 in a membrane, and this observation demonstrates that MOF-801 improves directly the proton-transport performance of membranes because of its high intrinsic proton conductance. PEMFC Performance of Composite Membranes. The high proton conductivity of the membranes encourages us to examine their single fuel cell performance. The composite membrane MOF-801@PP-60 was chosen as the representative to fabricate a membrane electrode assembly (MEA). As depicted in Figure 6a, the open-circuit voltage is 0.95 V at 303 K and 100% RH, which is comparable to that obtained in other composite membranes reported, such as CS/H3PO4@MIL101-6 (0.91 V), CS/S-MIL-101-6 (0.91 V), and CS/H2SO4@ -MIL-101-8 (0.95 V),60 and less than Nafion-based membranes (1.01 V).61 Additionally, the open-circuit voltage showed no clear change after the membrane electrode assembly worked for 26 h. Meanwhile, as displayed in Figure 6b, the MOF-801@PP-60-based MEA shows the maximum power density of 2.2 mW cm−2 at 4.53 mA cm−2, which means that the membrane is promising for PEM application on electrochemical devices.

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Figure 5. ln(σT) vs 1000/T plots of membranes MOF-801@PP-X (X = 0, 20, 40, and 60).

CONCLUSIONS In summary, the thorough investigation disclosed that MOF801 shows impressive proton conductivity of 1.88 × 10−3 S cm−1 at 298 K and 98% RH and ultrastability to hydrochloric acid or sodium hydroxide aqueous solutions and boiling water. These excellent performances in both proton conductance and stability are better than those of many up-to-date protonconducting MOFs recently reported. As a significant step toward the practical application in PEMs, the membranes were facilely and successfully fabricated using the sub-micrometerscale crystalline particles of MOF-801 with blending PVDF−

proton conductivity with elevating temperature; with temperature increasing from 289 to 321 K, the proton conductivity increases from 8.20 × 10−6 and 4.35 × 10−5 to 3.13 × 10−5 and 2.43 × 10−4 S cm−1 for MOF-801@PP-20 and MOF-801@ PP-40, respectively. As for MOF-801@PP-60, the proton conductivity σ = 9.05 × 10−4 S cm−1 at 289 K, and the maximum of 1.84 × 10−3 S cm−1 is achieved at 325 K. However, the bare composite membrane MOF-801@PP-0 shows a much lower proton conductance (4.46 × 10−6 S cm−1) 28660


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Figure 6. (a) Plot of open-circuit voltage versus time. (b) Polarization and power density curves of MOF-801@PP-60 at 303 K and 100% RH. composite membranes were investigated using Hitachi S-3400 field emission SEM and JEM-2800. The proton conductivity at a certain humidity was measured using a CHI 660D electrochemical workstation with a conventional threeelectrode method, and the alternating current frequency spans from 100 Hz to 1 MHz with 5 mV signal amplitude. The proton conductivity in the anhydrous environment was evaluated by impedance measurements under N2 atmosphere, which were collected using a Concept 80 system, and the frequency ranges from 1 Hz to 10 MHz. The membrane electrode assembly (MEA) was finally prepared by cold pressing the electrodes using a 4.0 cm2 (2.0 cm × 2.0 cm) membrane, and the sample was fixed between two graphite plates with parallel gas flow channels. The Pt loaded on both the cathode and anode was of 0.5 mg cm−2. The fuel cell test system was operated at 30 °C with humidified H2 and O2 fed into the cell at the flow rates of 100 and 200 mL min−1, respectively. The single cell was firstly activated via constant-voltage charge−discharge for 1 h to ensure the fuel cell performance reaches the steady state.

PVP matrix, and the studies demonstrated that the composite membrane MOF-801@PP-60 showed much high proton conductance with σ = 1.84 × 10−3 S cm−1 at 325 K and 98% RH, together with great potential to be applied as PEMs. Our study gives rise to a fresh impetus for the practical application of proton-conducting MOF composite materials as PEMs in electrochemical devices.

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EXPERIMENTAL SECTION

Materials and Reagents. Solvents and chemicals used in this study were of analytical grade and used without further purification. Preparation of MOF-801. The sub-micrometer-scale microcrystals of MOF-801 were harvested following a slightly modifying procedure published in the literature.49 Fumaric acid (1 mmol, 116 mg), ZrOCl2·8H2O (1 mmol, 322 mg), and the solvent of DMF/ formic acid (4:1.4 mL) were mixed and stirred at ambient temperature; then, the mixture was transferred to a 10 mL Teflonlined autoclave and kept at 130 °C for 6 h. White precipitate of MOF801 was achieved when the Teflon-lined autoclave was cooled down to ambient temperature. The precipitate was separated by suction, washed with deionized water for three times (10 mL each time), and finally dried at 80 °C under vacuum for 24 h for all measurements. Yield: ca. 68% calculated according to the reactant of fumaric acid. Elemental microanalysis calculated for [Zr6O4(OH)4(fumarate)6](DMF)1.9(H2O)22: C, 18.79; H, 3.89; N, 1.40. Found: C, 18.83; H, 3.42; N, 1.47. Preparation of Composite Membranes. In this study, the composite membrane (MOF-801@PP-X) was fabricated by a slurry casting method. The sub-micrometer-scale microcrystals of MOF-801 were mixed with PVP (70 wt %) and PVDF (30 wt %) for the corresponding composite membrane, and the amount of MOF-801 was 20, 40, and 60 wt % in each membrane. A similar procedure was utilized for all composite membranes’ preparations; the typical preparation process is described here for the membrane with 20 wt % of MOF-801, and the sample is labeled as MOF-801@PP-20. MOF-801 microcrystals (100 mg) were sonically dispersed in DMF (3 mL) for 1 h to produce a suspension solution; PVP−PVDF powders (280 :120 mg) were added to the suspension solution, and the mixture was stirred at room temperature for 4 h to give a homogeneous jelly. This homogeneous jelly was poured onto a glass slide, which was dried under vacuum at 70 °C for 2 h for removing DMF. The solidified membrane was removed from the slide and washed with deionized water for three times (10 mL per time) and dried in air for all kinds of measurements. General Methods. Elemental analyses were performed for C, H, and N using an Elementar Vario EL III analytical instrument. PXRD data were collected on a Bruker D8 diffractometer, operated at 40 kV and 40 mA, with Cu Kα radiation (λ = 1.5418 Å) in the range of 2θ = 5−50° with 0.01° per step. TG analysis was carried out using a TA2000/2960 instrument from 20 to 800 °C in N2 atmosphere. The morphologies of sub-micrometer-scale crystals MOF-801 and the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09070. Tetragonal-antiprismatic coordination geometry of ZrO 8 ; octahedral SBU of Zr 6 O 4 (OH) 4 (COO) 12 ; octahedral cage and tetrahedral cage in the crystal structure of MOF-801; packing diagram showing octahedral and tetrahedral cages in MOF-801 viewed along the ⟨110⟩ direction; TEM photos of the assynthesized sample of MOF-801; particle size distribution of MOF-801 obtained using laser particle size analyzer; IR spectrum of the as-synthesized sample of MOF-801; XPS spectrum of MOF-801; XPS spectra of C 1s, N 1s, O 1s, and Zr 3d in MOF-801; TG curve of MOF-801; PXRD patterns of the samples as-synthesized and soaked in DMF for 3 days together with the simulated pattern of MOF-801; variable-temperature PXRD patterns of MOF-801 at 30−190 and 30−400 °C; PXRD patterns of MOF-801 before and after treatment with 0.1 M H3PO4, 0.2 M NaOH, and 6 M HCl; Nyquist plots of MOF-801 at 298 K with different relative humidity and humidity-dependent proton conductivity at 298 K; circle fitting of MOF-801 at 298 K; time-dependent Nyquist plots of MOF-801 at room temperature and 98% RH; Nyquist plots of MOF801 at selected temperatures under anhydrous conditions; optical images of MOF-801@PP-X (X = 0, 20, 28661


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40, and 60); SEM images of the surface morphology and cross section of membrane MOF-801@PP-X (X = 0, 20, 40, and 60); SEM images of MOF-801@PP-60 and the elemental mapping images of C, F, O, and Zr in MOF801@PP-60; TG plots of MOF-801@PP-X (X = 0, 20, 40, and 60); swelling ratio of MOF-801@PP-X (X = 0, 20, 40, and 60) at 30 °C, H2 crossover current density of MOF-801@PP-60 composite membrane at 303 K and at 100% RH; current density at a constant cell voltage of 0.60 V at 303 K and at 100% RH for 1440 min in a single cell; stress−strain curves of MOF-801@PP-X (X = 40 and 60) composite membrane; Nyquist plots of MOF-801@PP-X (X = 0, 20, 40, and 60) at 98% RH at selected temperatures; plots of lg(σ) versus T of composite membranes MOF-801@PP-X (X = 0, 20, 40, and 60) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-M.R.). *E-mail: [email protected]. Tel: 86-25-58139476. Fax: 86-2558139481 (S.L.). ORCID

Hong-Bin Luo: 0000-0002-2225-7072 Xiao-Ming Ren: 0000-0003-0848-6503 Author Contributions ⊥

J.Z. and H.-J.B. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the support of the Priority Academic Program Development of Jiangsu Higher Education Institutions, the National Nature Science Foundation of China (21671100), and the Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX17_0930).

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REFERENCES

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