Article Cite This: ACS Appl. Nano Mater. 2018, 1, 4537−4547
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Hybrid Poly(ionic liquid) Membranes with in Situ Grown Layered Double Hydroxide and Preserved Liquid Crystal Morphology for Hydroxide Transport Na Sun, Xinpei Gao,* Fei Lu, Panpan Sun, Aoli Wu, and Liqiang Zheng* Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, Shandong University, Jinan 250100, P. R. China
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ABSTRACT: Recently, alkaline polymer electrolytes have obtained widespread attention because of their increasing application for energy storage and conversion systems. In this work, a novel poly(ionic liquid) membrane preserving liquid crystal (LC) nanostructures composited with layered double hydroxide (LDH) was constructed. The LDH is in situ synthesized using hexagonal LC as a nanoreactor and exhibits a hierarchical structure in the membrane. The preserved LC nanostructures and in situ grown LDHs in the LC phase provide organic and inorganic pathways for hydroxide transport. The formation of continuous hydrogen bond networks among the hydroxide groups on the surface of LDHs distributed in hexagonal mesophases, and the positively charged imidazolium groups of the poly(ionic liquid) backbone lead to a high hydroxide conductivity according to the Grotthuss mechanism. Meanwhile, the hybrid membrane also exhibits a low swelling degree, enhanced chemical stability, and a comparable mechanical property. The novel combination of in situ synthesized LDHs and LC nanostructures in a poly(ionic liquid) film provides a feasible method to develop hybrid anion exchange membranes (AEMs) with enhanced performances. KEYWORDS: hybrid anion exchange membrane, layered double hydroxide, poly(ionic liquid), liquid crystal, ion transport channels
1. INTRODUCTION Anion exchange membranes (AEMs) are widely and increasingly used for energy storage and conversion systems, such as alkaline anion exchange membrane fuel cells, rechargeable zinc−air batteries, redox flow batteries, and electrolyzers.1−10 In practice, AEMs are required to have high hydroxide ion conductivity, good swelling resistance, and satisfactory alkaline stability.11−13 Therefore, the development of high-performance AEMs has emerged as a very promising research field in recent years.14−17 Because of the relatively low mobility of hydroxide ions, AEMs often suffer from insufficient ionic conductivity.18 Enhancing the ion exchange capacity (IEC) of AEMs is generally designed to improve the ionic conductivity. Unfortunately, high IEC is always accompanied by excessive water uptake and swelling degree which further results in declined dimensional stability and reduced mechanical strength.1 Thus, it still remains a challenge to develop AEMs with balanced conductivity and swelling degree.19 The incorporation of inorganic materials, such as silica, titanium oxide, graphene oxide, and montmorillonite with polymer matrix, is an effective way to construct inorganic− organic hybrid AEMs with improved performances.20−26 Because of the combination of the characteristics of inorganic materials and polyelectrolytes, this inorganic−organic strategy can effectively improve the comprehensive performances of © 2018 American Chemical Society
AEMs, including hydroxide conductivity, chemical stability, and mechanical property. However, the addition of these inorganic fillers at high loading into AEMs often leads to the blockage of the hydroxide transport channel and the sacrifice of hydroxide conductivity.27 Layered double hydroxides (LDHs) are a kind of important layered functional material composed of positively charged host layers and weakly bound charge-balancing interlayer anions, whose structures are generally expressed as [M2+1−xM3+ x(OH)2]x+·[An−x/n]x−· mH2O (M represents metal cations, and A represents anions).28 LDHs possess abundant hydroxyl groups covalently bonded on the surface of host layers, which facilitates the formation of hydrogen bond networks on the surface of hydroxide host layers with the assistance of absorbed water molecules. Hydroxide ions can be transported along the hydrogen bond networks through a plausible Grotthuss mechanism.29 As a kind of basic material, LDHs also exhibit excellent stability in alkaline environment. The introduced LDHs in hybrid AEMs can limit the loss of conductivity, which is beneficial for the alkaline stability of AEMs.30 Thus, LDHs have been proposed as a class of inorganic hydroxide ionic conductors to incorporate with the polymer matrix to Received: May 21, 2018 Accepted: August 28, 2018 Published: August 28, 2018 4537
DOI: 10.1021/acsanm.8b00850 ACS Appl. Nano Mater. 2018, 1, 4537−4547
Article
ACS Applied Nano Materials Scheme 1. Schematic Illustration for the Preparation Process of LDH Hybrid Membranes
based LDHs turned into aggregated assembly of corrugated LDH sheets from dispersed platelets. The IEC, swelling degree, water uptake, hydroxide ion conductivity, alkaline stability, and mechanical property of LDH hybrid membranes were characterized and compared with those of the pristine membrane. The hybrid AEMs composited with hierarchical LDHs exhibited higher ionic conductivity, alkaline stability, and mechanical property.
construct hybrid AEMs. In most of the current studies, the hybrid AEMs were prepared by simple physical doping of LDH particles into the polymer matrix.30−32 The chaotic distribution of LDHs may increase the transmission distance and lower the efficiency of anion conduction. Recently, soft matter has been widely utilized to synthesize and control the growth of LDHs, including vesicles, microemulsions, and stearate monolayers.33−35 Among them, lyotropic LC has long-range order with a hydrophilic/hydrophobic domain and can be used as a nanoreactor to synthesize LDHs.36 As far as we know, there is no report on the preparation of LDH hybrid AEMs by in situ growth of LDHs directed by the LC nanostructure in the membrane. One of the valid and suitable strategies for improving the hydroxide ion conductivity is constructing ion-conductive channels in AEMs to increase the conductive efficiency of OH−.37,38 For example, design of polymer backbone structures with grafted hydrophobic alkyl chains can reform ionic clusters to promote microscopic hydrophilic/hydrophobic domain separation, which can effectively enhance the OH− conductivity.1 In addition, nanostructured liquid crystalline (LC) systems created by small-molecular self-assembly are considered as good materials for efficient ion transportation, for instance, of lithium ions and protons.39−41 However, studies on nanostructured LC systems for hydroxide ion transport are still rare.42 Our groups previously reported lyotropic LC systems based on a vinylimidazolium-type poly(ionic liquid) to construct AEMs with highly ordered ion-transporting nanochannels.43,44 The phase separation induced by LCs can provide hydrophilic ion channels for efficient OH− transportation. Meanwhile, the strong hydrophobic domain in the LC is beneficial to effectively restrain the swelling degree of membranes.45 In this work, we designed a polymerizable vinylimidazoliumbased ionic liquid with amphipathicity to construct lyotropic LC. Then, the polymerizable LC phase was used as a nanoreactor to synthesize LDHs. The Mg−Al Cl− LDH was selected because of the features of stability and a high OH− conductivity in alkaline media. After photopolymerization, a hybrid AEM retaining LC nanostructures and incorporated with in situ synthesized LDHs was successfully prepared. The preparation process of LDH hybrid membranes is illustrated in Scheme 1. From the images of SEM and TEM, the Mg−Al-
2. EXPERIMENTAL SECTION 2.1. Materials. The compounds 1-vinylimidazole (99%), 2hydroxy-2-methylpropiophenone (the photoinitiator, 97%), and 1chlorododecane (99%) were purchased from J&K Scientific Co. Ltd. MgCl2·6H2O (AR), AlCl3·6H2O (AR), and ammonia solution (AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. All materials were used without further purification. All the experiments used N2-saturated deionized water. 2.2. Synthesis of 1-Vinyl-3-dodecylimidazolium Chloride (C12 VIMCl). 46 1-Vinylimidazole (0.1 mol, 9.411 g) and 1chlorododecane (0.11 mol, 22.526 g) were dissolved in acetonitrile and refluxed at 80 °C for 48 h in a flask. After the evaporation of acetonitrile, the residue was purified by recrystallization in diethyl ether three times. The white powder was vacuum-dried at room temperature. 1H NMR (300 MHz, D2O, δ/ppm): 7.64 (1H), 7.44 (1H), 7.01 (1H), 5.64 (1H), 5.30 (1H), 4.10 (2H), 1.76 (2H), 1.15 (18H), 0.72 (3H). 2.3. Preparation of LC Samples. For the preparation of LC samples, a designed amount of C12VIMCl and water was weighted into stoppered glass vials (the weight percent of C12VIMCl was 60− 80 wt %). For the synthesis of Mg−Al LDH, MgCl2 and AlCl3 solution (the molar ratio of Mg:Al was fixed at 2:1, the total concentration of metal cations is 0.12, 0.24, and 0.36 mol/L) was used instead of water. After repeated stirring with a vortex mixer and centrifuging, the prepared LC samples were homogenized in a thermostat (25 °C) for at least 1 month to reach an equilibrium state. 2.4. Fabrication of Hybrid Membranes. After equilibration, the LC samples were used as a nanoreactor to synthesize LDHs.47 The dilute ammonia (6 wt %) was used as coprecipitant and added into the LC samples. The dosage of dilute ammonia was four times the theoretical amount calculated from the cations. After the in situ synthesis of Mg−Al LDH in LC samples, the photopolymerizable samples (0.5 wt % 2-hydroxy-2-methylpropiophenone was used as photoinitiator) were loaded between two pieces of quartz substrate and equilibrated at 25 °C for 1 h before UV irradiation. After irradiating by UV light for 30 min at room temperature, hybrid membranes were obtained. For the acquisition of membranes in OH− 4538
DOI: 10.1021/acsanm.8b00850 ACS Appl. Nano Mater. 2018, 1, 4537−4547
Article
ACS Applied Nano Materials
Figure 1. (a) SAXS patterns of LCs with increasing content of C12VIMCl. (b−d) SAXS patterns of C12VIMCl LCs with different concentrations of salts at 25 °C. form, the membranes in Cl− form were immersed in 1 M aqueous potassium hydroxide (KOH) solution at 25 °C for 48 h to achieve the complete exchange. The ion exchange process was repeated three times. After the ion exchange reaction, the membranes in OH− form were washed entirely with N2-saturated deionized water. The membranes in HCO3− form were obtained by immersing the membranes in Cl− form in 1 M potassium bicarbonate solution for 48 h to ensure a complete displacement. 2.5. Characterization. 1H NMR spectroscopy was recorded on a Bruker Avance II 300 MHz NMR spectrometer. The textures of the LC phase were obtained by a polarized optical microscope (POM, Olympus BX51p) equipped with a cooled CCD (Evolution MP5.1RTV, Q-imaging). Small-angle X-ray scattering (SAXS) measurements were conducted on the SAXSess mc2 X-ray scattering system (Anton Paar) with Cu Κα radiation (0.154 nm) operating at 50 kV and 40 mA. The morphologies of membranes and Mg−Al LDHs were characterized using scanning electron microscopy (SEM, JEOL JSM-7600F). The morphologies of LDHs were also characterized by transmission electron microscopy (TEM, JEOL, JEM-100CX II). Powder X-ray diffraction (PXRD) patterns were measured on an X-ray diffractometer (Rigaku, D/max-Ra) with Cu Κα radiation source (λ = 0.154 nm, 40 kV). Fourier transform infrared (FT-IR) spectra of freeze-dried LDH samples, LC samples, and membranes were obtained by using an FT-IR spectrometer (PerkinElmer Spectrum Two) in the range 4000−450 cm−1. The thermal stability of LDHs and membranes was measured by thermogravimetric analysis (TGA, Rheometric Scientific TGA1500, Piscataway, NJ) under inert atmosphere of nitrogen in the range 40− 600 °C. The tensile tests were carried out on a commercial tensile tester (Tensile Tester AG-2000A, Shimadzu). The wet membranes were cut into the size of 25 mm × 5 mm (the thickness around 0.5 mm), and stretched at an elongation rate of 5 mm/min. 2.6. Determination of Ion Exchange Capacity (IEC) Values. The IEC values were measured by a back-titration method according to the previous study.38 The membranes in OH− form were immersed in 0.1 mol/L hydrochloric acid solution (HCl, 25 mL) at room temperature for 24 h. Then, the resulting solution was titrated with a
standard 0.1 mol/L KOH solution to pH 7. The IEC of the membrane is calculated by
IEC =
ni(H+) − nf(H+) mdry
where ni(H+) is the initial amount of H+ in HCl solution, nf(H+) is the residual amount of H+ in the HCl solution after the titration, mdry is the weight of dry membranes in Cl− form. The final result was conducted three times to reach an average value. 2.7. Water Uptake and Swelling Degree Measurements. The water uptake and swelling degree were obtained by measuring the weight and dimension of membranes (OH− form) in both dry and wet states and obtained from the percentage of the change of weight and dimension to the membranes in the dry state.43 The water uptake (WU) and swelling degree (SD) of membranes can be calculated by
ww − wd × 100 wd
WU (%) =
SD (%) =
x w − xd × 100 xd
where ww and wd are the weight of the wet and dry membranes, and xw and xd are the length of the wet and dry membranes, respectively. 2.8. Ionic Conductivities for Membranes. The through-plane ion conductivity of membranes was carried out on the CHI760D electrochemical workstation using a two-probe ac impedance method. The membrane samples (the thicknesses of membranes were 100 μm) were placed in the homemade test fixture which was put into a water bath placed in an oven. Measurements were carried out at temperatures ranging from 30 to 80 °C under fully hydrated conditions. The membrane impedance was measured in the frequency range 1−106 Hz. The resistance of the membrane was acquired from the Nyquist plot. The conductivity was calculated by the following equation:
σ = l /(AR ) 4539
DOI: 10.1021/acsanm.8b00850 ACS Appl. Nano Mater. 2018, 1, 4537−4547
Article
ACS Applied Nano Materials
Figure 2. SEM images of LDHs synthesized in hexagonal LC (70%) with different concentrations of salts (the total cation concentration was 0.12, 0.24, and 0.36 mol/L, respectively) after (a−c) 48 h and (d−f) 96 h. TEM images of LDHs synthesized in hexagonal LC with 0.36 M salt solution after (g) 48 h and (h) 96 h and (i) synthesized from coprecipitation in solution without LC template. where l is the distance between the electrodes, and A and R are the cross-sectional area and the measured resistance of the membrane, respectively. 2.9. Alkaline Stability Test. The alkaline stability of the obtained membranes was evaluated by immersing the membranes in N2saturated 1 M KOH solution at 60 °C for 300 h. During the testing period, the KOH solution was replaced every 24 h, and the change of membrane hydroxide ion conductivity was recorded. For the accelerated alkaline stability test, a piece of membrane was put into 2 M KOH solution and maintained at 80 °C for different times. Then, the membranes were taken out and washed several times with deionized water and then dried under vacuum. The 1H NMR spectroscopy was used to investigate the chemical structural change of poly(ionic liquid) during the alkaline stability test. 2.10. Zinc−Air Battery Testing. Zinc−air batteries were fabricated according to the previous work.8 The battery was assembled through layer-by-layer method, in which the prepared membranes were sandwiched between a zinc pellet electrode and a bifunctional air electrode. The zinc pellet was prepared by pure zinc power (Sigma-Aldrich) and used as zinc electrode. The air electrode was made by spraying commercial cobalt oxide nanoparticles (
