Article pubs.acs.org/IC
Proton Conductance of a Superior Water-Stable Metal−Organic Framework and Its Composite Membrane with Poly(vinylidene fluoride) Hong-Bin Luo,†,‡ Mei Wang,†,‡ Shao-Xian Liu,†,‡ Chen Xue,†,‡ Zheng-Fang Tian,*,§ Yang Zou,†,‡ and Xiao-Ming Ren*,†,‡,∥ †
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 § Hubei Key Laboratory for Processing and Application of Catalytic Materials, Huanggang Normal University, Huanggang 438000, P. R. China ∥ State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China ‡
S Supporting Information *
ABSTRACT: Proton-exchange membranes (PEMs) as separators have important technological applications in electrochemical devices, including fuel cells, electrochemical sensors, electrochemical reactors, and electrochromic displays. The composite membrane of a protonconducting metal−organic framework (MOF) and an organic polymer combines the unique physical and chemical nature of the polymer and the high proton conductivity of the MOF, bringing together the best of both components to potentially fabricate high-performance PEMs. In this study, we have investigated the proton-transport nature of a zirconium(IV) MOF, MOF-808 (1). This superior-water-stability MOF shows striking proton conductivity with σ = 7.58 × 10−3 S·cm−1 at 315 K and 99% relative humidity. The composite membranes of 1 and poly(vinylidene fluoride) (PVDF) have further been fabricated and are labeled as 1@PVDF-X, where X represents the mass percentage of 1 (as X%) in 1@PVDF-X and X = 10−55%. The composite membranes exhibit good mechanical features and durability for practical application and a considerable proton conductivity of 1.56 × 10−4 S·cm−1 in deionized water at 338 K as well. Thus, the composite membranes show promising applications as alternative PEMs in diverse electrochemical devices.
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above,9 excellent thermal and chemical stability together with good mechanical performance,10,11 and low-cost and facile manufacturing processes.12 Metal−organic frameworks (MOFs) or porous coordination polymers are a class of new crystalline porous materials, emerging as the most promising candidates for next-generation proton-conducting materials because of the advantages of tremendous structural flexibility, large pore volume, and poresize tenability.13−16 In recent years, the MOF-based protonconducting materials have been widely studied, some of which show much high proton conductivity, e.g., UiO-66(SO3H)2 (8.4 × 10 − 2 S·cm − 1 at 80 °C and 90% RH), 1 7 Mg2(H2O)4(H2L)·H2O (3.55 × 10−2 S·cm−1 at 70 °C and 95% RH),18 and [Cr4In4(Himdc)12]·H2O (5.8 × 10−2 S·cm−1 at 22.5 °C and 98% RH).19 These new types of functional materials could compete with the traditional proton conductor Nafion but are immature for practical application because of the proton-conducting materials used in the form of membranes in
INTRODUCTION Proton-exchange membranes (PEMs), typical ionic sieve membranes, has received immense attention owing to their important applications as separators in electrochemical devices, including fuel cells, electrochemical sensors, electrochemical reactors, and electrochromic displays.1−3 In addition, the wide availabilities of proton-conducting materials have also stimulated continuous exploration of novel PEMs. Perfluorosulfonic acid (PFSA) polymer membranes are the most commonly and widely used types of PEMs, and the Nafion membrane is a typical example. The Nafion membrane shows high proton conductivity (10−1−10−2 S·cm−1) at moderate temperature (60−80 °C) and high relative humidity (98% RH) but high-cost and hazardous manufacturing processes and low thermal stability.4,5 In this context, a number of efforts have been dedicated to exploring alternative types of PEMs, e.g., modified PFSA membranes, acid−base polymer membranes, and inorganic−organic composite membranes.6−8 Nevertheless, the challenge still remains to develop new types of PEMs that meet the following requirements simultaneously: high proton conductivity with a value of 1.0 × 10−4 S·cm−1 or © 2017 American Chemical Society
Received: January 20, 2017 Published: March 22, 2017 4169
DOI: 10.1021/acs.inorgchem.7b00122 Inorg. Chem. 2017, 56, 4169−4175
Article
Inorganic Chemistry
pattern displayed that the structure of 1 remained integrated, typical amorphic character was shown. To determine the proton conductivity, measurements of alternating-current (ac) impedance spectroscopy were carried out on the compressed pellet of the as-synthesized, powdered crystalline sample in the temperature range of 290−327 K, and the ac frequencies span from 102 to 106 Hz at 99% RH. The representative Nyquist plots at selected temperatures and 99% RH are shown in Figures 2a,b and S4; all of the Nyquist plots display an imperfect arc, where the intercept value at the Z′ axis is regarded as the resistance of proton-conducting materials.28 In the temperature range of 290−315 K, the resistance decreases upon heating, implying that the proton conductivity is enhanced at elevated temperature owing to thermal activation of ion motion. It is worth mentioning that the resistance value increases with elevating temperature when the temperature was over 315 K. The conductivity (σ) as a function of the temperature (T) is plotted in Figure 2c, at 99% RH; the σ value starts to gradually increase upon heating, from 2.65 × 10−3 S·cm−1 at 290 K to 5.35 × 10−3 S·cm−1 at 309 K, further achieves a maximum value of 7.58 × 10−3 S·cm−1 at 315 K, then decreases gradually upon increasing temperature, and drops to 5.69 × 10−3 S·cm−1 at 318 K and 3.55 × 10−3 S·cm−1 at 327 K. The abnormal drop of the conductivity at a temperature over 315 K is related to the process of water molecules exiting the lattice to bring about the destruction of hydrogen-bonding networks along the proton-transport pathways at temperatures above 315 K, and this is confirmed by thermogravimetric analysis (TGA) and variable-temperature PXRD measurements for 1. As shown in Figures S2 and S3, TGA discloses an obvious mass-loss process between 298 and 423 K, and the variabletemperature PXRD patterns in 293−353 K show diffractions with 2θ at ca. 26.1, 27.1, and 27.9°, shifting to the bigger angle side when the temperature is over 313 K, indicating that the change occurs for the arrangements of guest molecules in the cavity. Surprisingly, the MOF 1 shows much higher conductivity (2.65 × 10−3 S·cm−1) over 10−3 S·cm−1 at lower temperature (290 K) and 99% RH; the conductivity is also pronounced, with a value of 3.14 × 10−3 S·cm−1, at room temperature and 99% RH, and these σ values are comparable to those of the most efficient MOF-based proton-conducting materials of InIA-2D-1 (3.4 × 10−3 S·cm−1 at 300 K and 98% RH),29 V[Cr(CN)6]2/3·zH2O (1.7 × 10−3 S·cm−1 at 293 K and 100% RH),30 Na-HPPA (5.6 × 10−3 S·cm−1 at 297 K and 98% RH),31 and [{(Zn0.25)8(O)}Zn6(L)12(H2O)29(DMF)69(NO3)2]n (2.2 × 10−3 S·cm−1 at 298 K and 95% RH).32 Notably, at 315 K and 99% RH, the proton conductivity reaches 7.58 × 10−3 S·cm−1, which is more than twice the value at room temperature and compatible with that of water-facilitated proton-conducting materials, such as Zr6O4(OH)6(L)5 with σ = 6.93 × 10−3 S· cm−1 at 338 K and 95% RH,33 (NH4)2(adp)[Zn2(ox)3]·3H2O with σ = 8.0 × 10−3 S·cm−1 at 298 K and 95% RH,34 and CaPiPhtA-NH3 with σ = 6.6 × 10−3 S·cm−1 at 297 K and 98% RH.35 To gain more insight into the proton-transport information, we calculated the proton-transport activation energy derived from the Arrhenius equation, expressed as follows:
electrochemical devices. The fabrication of MOFs into composite membranes is considered to be a significant step toward their practical application in electrochemical devices. To date, only a few MOF−polymer composite membranes have been reported to achieve PEMs.20−22 A MOF−polymer composite membrane combines the unique physical and chemical properties of a polymer and the high proton conductivity of MOFs, bringing together the best of both to potentially fabricate high-performance PEMs. Poly(vinylidene fluoride) (PVDF) is an excellent organic polymer supporting matrix because of the following striking features: (1) excellent chemical and thermal stability as well as high mechanical strength with sufficient resistance against the grim conditions in electrochemical devices;23 (2) solubility in some polar solvents (thus, a simple slurry casting method can be used to prepare composite membranes);24 (3) lower cost compared with the conventional fully fluorinated polymer electrolyte.25 It is possible to integrate the high proton conductivity of MOF crystalline particles into PVDF to prepare composite membranes for achieve high-performance PEMs. Herein we present a study of the proton conductance on a zirconium(IV) MOF, MOF-808 (1),26,27 which possesses high thermal stability, superior water stability, and high waterassisted proton conductivity, with σ = 7.58 × 10−3 S·cm−1 at 315 K and 99% RH. The composite membranes have further been easily and successfully fabricated using PVDF and crystalline particles of 1. More importantly, the composite membranes show good mechanical performance and durability for practical application, as well as considerable proton conductivity, reaching to 1.56 × 10−4 S·cm−1 at 338 K in deionized water.
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RESULTS AND DISCUSSION MOF-808 (1) was synthesized using the slightly modified procedure reported previously.26,27 The powder X-ray diffraction (PXRD) profiles are illustrated in Figure 1 for 1,
Figure 1. PXRD patterns of 1, which correspond to the sample immersed in water or DMF for 7 days, and the as-synthesized sample together with the simulated PXRD profile of 1, respectively.
which correspond to those of the samples immersed in water or N,N-dimethylformamide (DMF) for 7 days, and for the assynthesized sample together with the simulated PXRD profile of 1 from single-crystal structure data. All PXRD profiles show quite high similarity, suggesting that we have successfully obtained the target sample of 1, and this MOF is highly stable to water and DMF. However, the crystals of 1 show poor stability in hot water. As shown in Figure S1, the sample of 1 was immersed in hot water at 353 K for 4 h; even if the PXRD
ln(σT ) = ln A − 4170
Ea kBT
(1) DOI: 10.1021/acs.inorgchem.7b00122 Inorg. Chem. 2017, 56, 4169−4175
Article
Inorganic Chemistry
Figure 2. (a and b) Nyquist plots of 1 at selected temperatures and 99% RH. (c and d) Temperature-dependent proton conductivity in the forms of σ−T and ln(σT) against 1000/T for 1.
Figure 3. Optical images of the pure PVDF membrane and the composite membranes of 1@PVDF-X (X = 10, 25, 40, and 55, respectively).
and 3.90 × 10−5 S·cm−1 at 67% RH, followed by a conductivity jump, 2.04 × 10−4 S·cm−1 at 84% RH, and reaches a maximum of 3.34 × 10−3 S·cm−1 at 99% RH as well. The proton conductivity is quite sensitive to the humidity of the environment, indicating that proton transfer relies heavily on the water content in the conduction pathway; that is because the adsorbed water in the lattice could assist in building wellestablished hydrogen-bonding networks along the protontransport pathways, which would provide effective pathways for proton conduction and result in the improvement of conductivity. This observation was consistent with a proton conduction process, which follows a Grotthuss mechanism under water-rich conditions. Moreover, it was further confirmed by low proton conductivity under anhydrous conditions, where the corresponding Nyquist plots are almost linear in a wide range of temperatures from 283 to 333 K (Figure S6), implying the existence of negligible proton conductivity under an anhydrous environment for 1. Considering 1 as proton-conducting material for practical application in PEMs, the microparticles of 1 were used as fillers and combined with a PVDF supporting matrix to form mixedmatrix membranes (MMMs) through a slurry casting method.
where σ is the proton conductivity, Ea represents the protontransport activation energy, kB is the Boltzmann constant, and A is the preexponential factor. Figure 2d shows the plot of ln(σT) against 1000/T according to the Arrhenius law. The activation energy for proton transfer in 1 is 0.37 eV, and this value lies within the range (Ea < 0.4 eV) corresponding to the conventional Grotthuss mechanism36 and suggesting that the protons in 1 transfer within a hydrogen-bonding network. The protons originating from the coordinated carboxylate groups transport through water channels by means of the hopping process between neighboring −COO−···H3O+···(H2O)n units. A series of well-known hydrated proton-conducting materials with similar Ea values have been interpreted as an analogous proton-hopping process, e.g., UiO-66(SO3H)2 (0.32 eV),17 NDTBP (0.32 eV),37 Fe(ox)·2H 2 O (0.37 eV), 38 and HUO2PO4·4H2O (0.32 eV).39 The humidity dependence of the proton conductivity was further measured at 298 K and a selected relative humidity to elucidate the correlation between the proton conductivity and relative humidity (Figure S5). This MOF exhibits increased conductivity with increasing relative humidity, with values of 3.41 × 10−6 S·cm−1 at 43% RH, 7.09 × 10−6 S·cm−1 at 54% RH, 4171
DOI: 10.1021/acs.inorgchem.7b00122 Inorg. Chem. 2017, 56, 4169−4175
Article
Inorganic Chemistry
composite membranes, and all of the characteristic diffractions are visible when the content of 1 increases by up to 55%. On the basis of the above PXRD analysis, it can be concluded that the crystalline structure of 1 remains integrated without any change in the composite membranes. Scanning electron microscopy (SEM) was employed to inspect the morphologies of crystals of 1 and composite membranes; it can be seen that the crystal particles of 1 perform in a uniform size, with the dimensions being less than 1 μm (Figure S8a,b); as shown in Figures 5 and S8, the crystals of 1 are well mixed and homogeneously distributed in the PVDF matrix, which can be further confirmed by the energy-dispersive X-ray spectroscopy (EDS) result, and the elemental mapping of the composite membrane indicated a homogeneous distribution of oxygen and zirconium (Figures 5c,d and S9). The proton conductivity is an important evaluation criterion of well-performance PEMs. All of the membranes of 1@PVDFX (X = 10, 25, 40, and 55) were directly soaked in deionized water during the proton conductivity measurement. Typical Nyquist plots at selected temperatures are displayed in Figures 6a,b and S10, and the temperature-dependent proton conductivity has been calculated, as shown in Figure 6c; all of the composite membranes perform positive temperature− conductivity correlations; the σ value increases from 8.4 × 10−6 and 1.93 × 10−5 S·cm−1 at 293 K to 1.76 × 10−5 and 3.69 × 10−5 S·cm−1 at 338 K for the lower-mass-percentage composite membranes 1@PVDF-10 and 1@PVDF-25, respectively. For the higher-mass-percentage composite membranes, the proton conductivity of 1@PVDF-40 is improved from 3.86 × 10−5 S· cm−1 at 293 K to 8.60 × 10−5 S·cm−1 at 338 K; for 1@PVDF55, the proton conductivity is 7.55 × 10−5 S·cm−1 at 293 K and reaches a maximum value of 1.56 × 10−4 S·cm−1 at 338 K,
Figure 3 shows the optical images of the pure PVDF membrane and the composite membranes 1@PVDF-X (X = 10, 25, 40, and 55, respectively), indicating that the composite membrane can be transparent when the mass percentage of MOF 1 is less than 40%. As shown in Figure S7, the composite membranes display excellent mechanical performance even in the case of the mass percentage of 1 reaching 55%. Composite membranes with a higher loading amount of 1 have not been prepared because they are easily breakable. The PXRD profiles of pure PVDF and composite membranes of 1@PVDF-X are shown in Figure 4 together with the as-synthesized 1, indicating that
Figure 4. PXRD patterns of the as-synthesized 1 and the composite membranes of 1@PVDF-X (X = 10, 25, 40, and 55, respectively).
several characteristic diffractions of 1 are visible even though the mass percentage of 1 in the PVDF matrix has been as low as 10%. The relative intensity of the characteristic diffractions of 1 gradually enhances with an increase in the amount of 1 in the
Figure 5. (a and b) SEM images of 1@PVDF-40 and the elemental mapping images of (c) oxygen and (d) zirconium in the sample 1@PVDF-40. 4172
DOI: 10.1021/acs.inorgchem.7b00122 Inorg. Chem. 2017, 56, 4169−4175
Article
Inorganic Chemistry
Figure 6. (a and b) Nyquist plots of 1@PVDF-55 at selected temperatures. (c) Temperature-dependent proton conductivity of 1@PVDF composite membranes. (d) Plots of ln(σT) against 1000/T for 1@PVDF composite membranes.
Figure 7. Time-dependent (a) Nyquist plots and (b) proton conductivity of 1@PVDF-40 measured at 338 K in deionized water.
conductivity (>10−4 S·cm−1) even if it is soaked in deionized water for 5 days (see Figure S12), demonstrating the durability of the composite membrane, which is essential for practical applications.
which can satisfy the requirement of practical application (>10−4 S·cm−1). Obviously, the proton conductivity increases with increasing amount of 1 incorporated in the composite membranes. It is worth noting that the conductivity of the pure PVDF membrane also has been inspected at similar conditions, and the value of σ = 7.38 × 10−6 S·cm−1 at 300 K (Figure S11) is much lower than the value for 1@PVDF-10 (3.69 × 10−5 S· cm−1), indicating that 1 directly benefits from the improvement of the proton conductivity of composite membranes. Plots of ln(σT) versus 1000/T are shown in Figure 6d, and the approximate linear relationship between ln(σT) and 1000/ T complies with the typical Arrhenius behavior. The activation energy (Ea) was determined to be 0.167 eV for 1@PVDF-10, 0.145 eV for 1@PVDF-25, 0.172 eV for 1@PVDF-40, and 0.167 eV for 1@PVDF-55. All of the composite membranes show a lower Ea value (
