Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 18246−18256
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Three-Decker Strategy Based on Multifunctional Layered Double Hydroxide to Realize High-Performance Hydroxide Exchange Membranes for Fuel Cell Applications Nanjun Chen,† Chuan Long,† Yunxi Li, Dong Wang, Chuanrui Lu, Hong Zhu,* and Jinghua Yu* State Key Laboratory of Chemical Resource Engineering, Institute of Modern Catalysis, Department of Organic Chemistry, School of Science, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *
ABSTRACT: Herein, we present a three-decker layered double hydroxide (LDH)/poly(phenylene oxide) (PPO) for hydroxide exchange membrane (HEM) applications. Hexagonal LDH is functionalized with highly stable 3-hydroxy-6azaspiro [5.5] undecane (OH-ASU) cations to promote it’s ion-exchange capacity. The ASU-LDH is combined with triplecations functionalized PPO (TC-PPO) to fabricate a threedecker ASU-LDH/TC-PPO hybrid membrane by an electrostatic-spraying method. Notably, the ASU-LDH layer with a porous structure shows many valuable properties for the ASULDH/TC-PPO hybrid membranes, such as improving hydroxide conductivity, dimensional stability, and alkaline stability. The maximum OH− conductivity of ASU-LDH/TC-PPO hybrid membranes achieves 0.113 S/cm at 80 °C. Only 11.5% drops in OH− conductivity was detected after an alkaline stability test in 1 M NaOH at 80 °C for 588 h, prolonging the lifetime of the TC-PPO membrane. Furthermore, the ASU-LDH/TC-PPO hybrid membrane realizes a maximum power density of 0.209 W/cm2 under a current density of 0.391 A/cm2. In summary, the ASU-LDH/TC-PPO hybrid membranes provide a reliable method for preparing high-performance HEMs. KEYWORDS: hydroxide exchange membrane, three-decker structure, layered double hydroxide, ion conductivity, durability, fuel cells
1. INTRODUCTION In the past few years, hydroxide exchange membrane fuel cells (HEMFCs) have received extensive academic attention as the next-generation low-cost fuel cell technology because of allowing the utilization of nonprecious metal catalysts and having fast cathode reaction kinetics, and the HEMFCs have been deemed as an alternative succedaneum to proton exchange membrane fuel cells.1−4 Nevertheless, the commercial application of the HEMFCs is always hindered by its key component of HEMs. HEMs play dual functions in HEMFCs, which acts as an ion-exchange medium as well as isolates the feed gas between the anode and cathode.5−9 Ideal HEMs should have good hydroxide conductivity, mechanical properties, and chemical stability. Generally, HEMs are composed of cationic groups and polymer backbone.10−12 Unfortunately, different cationic groups13−19 are prone to suffer from the nucleophilic substitution, Hofmann elimination, ylide, and ring opening degradation in alkali media, destroying OH− exchange capability of HEMs and reducing the lifetime.20−23 On the other hand, the insufficient hydroxide conductivity of HEMs still limits their application in fuel cells. The common strategy to boost hydroxide conductivity of HEMs is to improve their ion-exchange capacity (IEC).24−26 Nevertheless, excessive IECs always lead to severe membrane swelling so as to cut down the © 2018 American Chemical Society
mechanical properties. Recently, numerous polyelectrolytes are explored to boost OH− conductivity and chemical stability of HEMs in past few years. But, there still have not reached a consensus on the preparation of HEM.27−30 Marino and Kreuer have investigated the alkaline stability of quaternary ammonium (QA) and demonstrated that six-membraned 6-azoniaspiro[5.5]undecane (ASU) has the highest lifetime in 6 M NaOH at 160 °C than that of the other QA groups.31 However, the ASU groups have not been successfully utilized in HEM via an effective method, and thus, the appropriate method needs to be developed to prepare the ASU-based HEM. The hydroxide conductivity of HEMs has realized a breakthrough in recent years. Zhuang and co-workers have disclosed that efficient microphase separation in HEMs can induce forming the ion transport channels and improve the hydroxide conductivity.32 Hickner and co-workers33 demonstrated that multiple-cation side chain HEMs possess much higher OH− conductivity and alkaline stability compared with benzyltrimethylammonium because of effective phase separation in the HEMs. Moreover, plenty of inorganic−organic Received: January 21, 2018 Accepted: May 4, 2018 Published: May 4, 2018 18246
DOI: 10.1021/acsami.8b01145 ACS Appl. Mater. Interfaces 2018, 10, 18246−18256
Research Article
ACS Applied Materials & Interfaces hybrid HEMs had been developed in past few years.34−39 These organic−inorganic strategies have been considered as effective methods to enhance the comprehensive performance of HEMs, including mechanical properties, hydroxide conductivity, and even chemical stability because these methods combine the both advantages of inorganic materials and polyelectrolytes. However, the performance of the hybrid HEMs is restricted by the doping amount of inorganic additives. The high content of additives in HEMs would embrittle the HEMs and block the hydroxide transport channels.39,40 Generally, the weight fraction of the inorganic additives are not allowed to exceed 15%. Consequently, more effective methods are needed to be explored to utilizing inorganic materials in HEMs. In this work, we use a new method to utilize additives in HEMs via designing three-decker-structure HEMs to address the conductivity and lifetime problem in organic−inorganic hybrid HEMs. Hexagonal layered double hydroxide (LDH) was prepared and functionalized with 3-hydroxy-6-azonia-spiro [5.5] undecane to prepare ASU-LDH composites, promoting the ion-exchange ability of the ASU-LDH. The three-decker HEMs were prepared by inserting the ASU-LDH into triplecations functionalized poly(phenylene oxide) (TC-PPO) via an electrostatic-spraying method. Especially, the hexagonal ASULDH hybrids can form a porous structure layer by controlling the spraying conditions during the membrane fabrication. The porous ASU-LDH layer shows many valuable properties for the three-decker ASU-LDH/TC-PPO hybrid membranes, such as improving the hydroxide conductivity, chemical stability, and mechanical properties. The OH− conductivity of ASU-LDH is measured, and the structure of three-decker ASU-LDH/TCPPO hybrid membranes is observed via scanning electron microscopy and linear elementals scanning. The performance evaluation demonstrates the three-decker ASU-LDH/TC-PPO hybrid membranes show much better comprehensive performance and lifetime than those of the pristine TC-PPO membrane.
Figure 1. 1H NMR spectra of (a) BPPO: (1) −CH2−Br, (2) Ph−H, and (3) −CH3; (b) triple-cations precursor: (1) N+−CH3, (2) N+− CH2−, (3) N−CH3, (4) N−C−C−CH2−, (5) N−CH2−, and (6) N− C−CH2−; and (c) TC-PPO polyelectrolyte: (1) Ph−CH2−N+, (2) Ph−H, (3) −CH2−Br, (4−6) N+−CH3, (7,8) N+−CH2−, (9,10) Ph− CH3, (11) N+−C−CH2−CH2−C−C−N+, and (12) N+−C−C− CH2−CH2−C−C−N+.
2. EXPERIMENTAL SECTION 2.1. Materials. PPO was provided by Beijing Chemical Works. 4Hydroxypiperidine, N,N,N′,N′-tetramethyl-1,6-hexanediamine, dibenzoyl peroxide, N-bromobutanimide, and 1,5-pentane dibromide were supplied by Sigma-Aldrich and used as received. Iodomethane, potassium carbonate, 3-aminopropyltriethoxysilane, and trimethylamine solution (33% in water) were provided by J&K Company. 2.2. Preparation of Triple-Cations Functionalized PPO Polyelectrolyte. The synthesis of (5-bromopentyl)trimethyl ammonium bromide (BPTMA), 1-(N′,N′-dimethylamino)-6-(N,N′-dimethylammonium)-11-(N,N′,N″-trimethyl ammonium)undecane bromide (DNTNU), and brominated PPO (BPPO) was according to our recent reports.41−45 1H NMR spectra of BPTMA, DNTNU, and BPPO are shown in Figures S1, 1b, and 1a. The brominated ratio of BPPO can be calculated by the 1H NMR spectrum as shown in Figure 1a. BPPO (0.5 g) and DNTNU (0.3 g) were reacted in N,Ndimethylformamide solution at 60 °C for 96 h. Subsequently, the polymer solution was precipitated in deionized water to remove the residual reagent, followed by drying in an oven overnight to obtain a yellow triple-cations side chain PPO polyelectrolyte, named as TCPPO polyelectrolyte. 1H NMR spectrum is shown in Figure 1c. 2.3. Synthesis of 3-Hydroxy-6-azonia-spiro [5.5] Undecane. 4-hydroxypiperidine (5.1 g, 50 mmol) and 6.8 mL of 1,5dibromopentane (50 mmol) were, respectively, dissolved in 40 mL of chloroform, and both of the solutions were transferred to two dropping funnels. Then, two solutions were simultaneously added to a flask containing 30 mL of chloroform, and then the solution was reacted at 60 °C for 12 h. After evaporating the chloroform, the product was purified by n-hexane and dichloromethane several times
to obtain a faint yellow solid. Finally, the solid was recrystallization in ethanol and dried. The white product was obtained with 34% yield and named as OH-ASU. 2.4. Synthesis of ASU-LDH Hybrids. To 10 mL of chloroform in flask, potassium carbonate (1.1 g, 8 mmol), 3-aminopropyltriethoxysilane (1.87 mL, 8 mmol), and iodomethane (1.49 mL, 24 mmol) were added together, and the reaction is controlled at room temperature for 48 h. After evaporating the solvent, a residual product was carefully washed by diethyl ether several times. Subsequently, the product was recrystallized by ethanol, and the white precipitate of N,N,N-trimethyl-3-(triethoxysilyl)propan-1-aminium bromide (TMSPA) was obtained. Hexagonal Mg/Al-LDHs were synthesized as described previous literatures.46,47 The average particle size of hexagonal LDH is 90 nm, and OH-ASU and TMSPA were used to functionalize with as-prepared LDH. Typically, OH-ASU (0.1 g), TMSPA (0.1 g), and LDH (0.4 g) were dispersed in ethanol solution, and the mixture was reacted at 110 °C for 8 h. Subsequently, the mixture solution was centrifuged and dried to get a white powder, and named as ASU-LDH. X-ray diffraction (XRD) pattern and transmission electron microscopy (TEM) images of ASU-LDH are shown in Figure 2. 2.5. Membrane Fabrication. TC-PPO polyelectrolyte (0.4 g) solution was prepared by dissolving the polymer in methanol. Similarly, 0.4 g of ASU-LDH was sonicated and uniformly dispersed in 200 mL of ethanol to prepare the ASU-LDH ink, and then 0.08 g of TC-PPO polyelectrolyte was added to the ASU-LDH ink as the 18247
DOI: 10.1021/acsami.8b01145 ACS Appl. Mater. Interfaces 2018, 10, 18246−18256
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ACS Applied Materials & Interfaces
Figure 2. XRD pattern and TEM images, (a,b) XRD patterns of OH-ASU, LDH, and ASU-LDH and TEM images of LDH (c) and ASU-LDH (d). binder. Subsequently, the TC-PPO solution and ASU-LDH ink were sprayed on silicon wafer layer by layer to prepare three-decker hybrid membranes by an electrostatic spraying machine from Sunlaite Company (SP201). The spraying speed was controlled at 0.1 mL/ min, and the spraying time was related to the thickness of the hybrid membrane. Finally, the hybrid membranes were stripped from a silicon wafer, and the hybrid membranes were named as ASU-LDH/TCPPO-x, in which x represents the thickness of ASU-LDH layer. 2.6. Structural Characterization. 1H NMR spectra of the OHASU and TC-PPO polyelectrolytes were obtained by a Bruker AVANCE III 400 MHz spectrometer. CDCl3, D2O, and DMSO-d6 were used as NMR solvents. Fourier transformed infrared (FTIR) spectra of the ASU-LDH/TC-PPO hybrid membranes were recorded on a Nicolet 8700 spectrometer. 2.7. Morphology Characterization. The morphology of the membrane sample was acquired on a scanning electron microscope (Zeiss SUPRA 55). In addition, an atomic force microscope (AFM, DMFASTSCAN2-SYS, Bruker Co.) was used to observe the microphase morphology of the TC-PPO samples. Besides, the transmission electron microscope (TEM, Hitachi HT7700) and small angle X-ray scattering (SAXS, Xuess2-0) were further used to observe the microphase separation of the TC-PPO membrane. 2.8. IEC, Water Uptake, and Swelling Ratio. The back-titration method was used to test the IEC of the ASU-LDH/TC-PPO membrane. In detail, a membrane sample was soaked in 1 mol/L NaOH solution for 2 days. Then, the sample was soaked in deionized water for 1 day and carefully purified to remove any residual surface salts. Subsequently, the membrane sample was soaked in 0.01 M HCl (40 mL) for 1 day, and then 0.01 M NaOH solution was used to titrate the above acid solution with a Mettler toledo pH meter to accurately detect the titration end-point near pH = 7.0. Finally, the dry mass of the membrane can be obtained after drying the membrane sample to a constant mass, as follows V × C HCl − VNaOH × C NaOH IEC = HCl mdry(Cl)
direction size of the membrane sample was measured. Next, the dry sample was soaked in deionized water at different temperatures for 3 h. The wet mass and the direction size of the membrane were measured after quickly wiping away the water on the surface of the membrane sample by tissue paper. Water uptake (WU) and swelling ratio (SR) can be calculated as follows WU (%) =
SR (%) =
m wet − mdry mdry
Lwet − Ldry Ldry
× 100% (2)
× 100% (3)
where Lwet and Ldry represent the length of hydrated membrane and dry membrane, respectively. λ (hydration number) signifies the average number of H2O in per ammonium group (MH2O = 18.0 g/ mol) and was calculated from
λ=
1000 × WU M H2O × IEC
(4)
2.9. Ionic Conductivity. A two-electrode ac impedance analyzer on an electrochemical workstation (ZAHNER IM6EX) was used to measure the ionic conductivity (σ mS/cm) of the membrane sample with the frequency range from 100 kHz to 1 Hz. The membrane sample (size: 0.7 cm × 0.7 cm) was hydrated in deionized water before the conductivity test. Every sample was tested three times at elevated temperatures ranging from 30 to 80 °C. The high-frequency region of the impedance was used to calculate the resistance and ionic conductivity, as follows
σ=
d AR
(5)
d (cm) represents the thickness of membrane sample, A (cm2) signifies contact area between the membrane and the electrodes, and R (Ω) is the resistance. 2.10. Thermal Stability. Thermogravimetric analysis (TGA) of a membrane sample was performed by a TGA Q500 analyzer. A membrane sample was heated from 30 to 900 °C with a heating rate of 10 °C/min under a N2 environment. 2.11. Chemical Stability. The oxidative stability of the ASULDH/TC-PPO hybrid membranes was evaluated by soaking the membrane in Fenton’s reagent (3.5% H2O2 containing 2 ppm FeSO4
(1)
where the mdry(Cl) represents the dry weight of the membrane; VHCl and VNaOH are the initial volume of the HCl solution and the titrimetric volume of the NaOH solution, respectively; and CHCl and CNaOH are the concentrations of the HCl and NaOH solution, respectively. The dry weight of a membrane sample in the OH− form was obtained after drying the membrane to a constant mass, and then one18248
DOI: 10.1021/acsami.8b01145 ACS Appl. Mater. Interfaces 2018, 10, 18246−18256
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ACS Applied Materials & Interfaces Scheme 1. Synthetic Route of Multications Side Chain TC-PPO Polyelectrolyte
at 80 °C) for 8 h, and the residual weight of the membranes was recorded to evaluate the oxidative stability. OH-ASU was dissolved in 2 mol/L NaOH/D2O at 80 °C to evaluate the alkaline stability, and 1H NMR spectroscopy was used to periodically analyze the OH-ASU structure. Moreover, the alkaline stability of the ASU-LDH/TC-PPO hybrid membrane was evaluated in 1 mol/L NaOHaq solution at 80 °C. During the measurement, the OH− conductivity was measured periodically, and NaOH solution is regenerated every 3 days. 2.12. Fuel Cell Testing. To fabricate catalyst-coated membranes (CCMs), the Pt/C (40%) catalyst from Johnson Matthey Co was mixed with TC-PPO ionomer (IEC = 2.48 mmol/g) in water/ methanol solution. Then, as-prepared mixture solution was sprayed on the surface of a membrane sample. The weight ratio of the ionomer to Pt/C in the anode and cathode is 1:5, and the Pt loading is 0.55 mg/ cm2 in both the anode and cathode. Both electrode areas are 5 cm2. Subsequently, the CCM and two pieces of gas diffusion layers were pressed together to fabricate a membrane electrode assembly (MEA). A Hephas Mini-L100 fuel cell system was used to measure the singlecell performance of as-prepared MEA. Temperature is controlled at 60 °C with a humidity of 100% and the back pressure is 100 kPa. The flow rates of H2 and O2 gases are 100 and 20 mL/min, respectively.
2.2 and 3.1 ppm represent −N−CH3 and N+−CH3 groups, respectively. Preparation of the TC-PPO polyelectrolyte and membrane is shown in Scheme 1. In Figure 1c, the peaks between 3.1 and 3.3 ppm belong to −CH2−N+ groups in side chain, indicating that triple-ammonium precursor sufficiently reacted with BPPO. The functionalized degree of TC-PPO polyelectrolytes is 40% at the benzyl position, which is calculated by the relative integral areas between −CH3 and −CH2−Br groups in Figure 1a. Synthesis route of OH-ASU and ASU-LDH hybrids is shown in Scheme 2. OH-ASU was synthesized by a cyclization Scheme 2. Synthetic Route of Modified ASU-LDH Hybrids
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. Four steps needed to fabricate the three-decker ASU-LDH/TC-PPO hybrid membrane are as follows: (1) synthesis of the triple-cations precursor, (2) preparation of the TC-PPO polyelectrolyte and membrane, (3) synthesis of OH-ASU and ASU-LDH hybrids, and (4) fabrication of the ASU-LDH/TC-PPO membrane by electrostatic spraying. Synthesis of the triple-cations precursor is explained as follows. BPTMA was synthesized as an intermediate product by the nucleophilic substitution. To prevent bis-substituted byproduct, tetrahydrofuran was chosen as the solvent so that the monosubstituted product would be precipitated immediately once it was formed. Then, mono BPTMA was reacted with diamine to prepare the triple-cations precursor. 1H NMR spectra of the intermediate product and triple-cations precursor are shown in Figures S1 and 1b. In Figure S1, the arisen peaks between 3.1 and 3.3 ppm are assigned to the N+−CH3 and N+−CH2− groups. The peak around 3.5 ppm corresponds to the Br−CH2− group. In Figure 1b, the peaks around 3.2−3.4 ppm are attributed to −CH2−N+ groups. The chemical shifts at
reaction, and 1H NMR spectrum of the OH-ASU is shown in Figure S2. OH-ASU is soaked in 2 M NaOH at 80 °C to assess the alkaline stability, and the 1H NMR result shows superb alkaline stability of the OH-ASU after 500 h (as shown in Figure S3). Then, the OH-ASU and TMSPA are modified with hexagonal LDH to prepare the ASU-LDH composites, which prominently improves the ion-exchange ability of the ASULDH. Figure 2 shows the XRD analysis of pristine LDH and ASU-LDH; XRD patterns clearly show the characteristic peaks (2θ = 9.9: (003) crystal faces, 2θ = 19.8: (006) crystal faces, 2θ = 34.4: (009) crystal faces, and 2θ = 60.7: (110) crystal faces) 18249
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ACS Applied Materials & Interfaces Scheme 3. Fabrication of ASU-LDH/TC-PPO Hybrid Membrane
Figure 3. FTIR spectra of the TC-PPO, ASU-LDH/TC-PPO membranes, and ASU-LDH. (a) Wavenumber from 800 to 3750 cm−1 and (b) the partial enlarged picture from 700 to 1800 cm−1.
Figure 4. SEM images of the hybrid membrane. (a) Surface section, (b) cross section, (c) surface of ASU-LDH layer, and (d) EDX linear-scanning of ASU-LDH/TC-PPO-60 hybrid membrane in cross section.
of Mg/Al-LDH. In addition, the 003 crystal face in ASU-LDH shifts to a low 2θ, demonstrating that the interlayer distance of the ASU-LDH becomes larger, which forcefully verifies that the LDH is successfully modified with ASU and TMSPA. Besides, the size of d can be calculated by the Scherrer formula based on the 003 crystal face, and the size of d in LDH and ASU-LDH is
about 15.3 and 18.7 nm, respectively. TEM images of the LDH and ASU-LDH are shown in Figure 2c,d, the hexagonal and layered structure of the LDH is clearly observed. After surface modification, the ASU-LDH still maintains the hexagonal structure, and the particle size of the ASU-LDH slightly 18250
DOI: 10.1021/acsami.8b01145 ACS Appl. Mater. Interfaces 2018, 10, 18246−18256
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ACS Applied Materials & Interfaces
Figure 5. (a) AFM 2D image of the TC-PPO membrane, (b) AFM 3D image of the TC-PPO membrane, (c) TEM image of the TC-PPO membrane, and (d) SAXS analysis of the TC-PPO membrane.
shown in Figure 4d, with the increase of the scanning distance, the content of Mg, Al, and O elements increases in the middle part and then significantly decreases in two edge part, indicating that the middle part is the ASU-LDH layer. Besides, the content of C element in the edge part is higher than that in the middle part, which mainly represents the TC-PPO is in both sides of the hybrid membrane. Furthermore, the thickness of the ASU-LDH layer and TC-PPO layers also can be disclosed in EDX linear scanning. The thickness of the ASU-LDH layer is approximately 50 μm, and the TC-PPO layers are about 20 μm per side. Currently, the microphase structure of HEMs has been extensively studied because the microphase separation significantly influences the ion conductivity of the HEMs. It is believed that HEMs with effective phase separation is beneficial to inducing the ion transport channels in the HEMs and improving the hydroxide conductivity. Before the AFM test, the TC-PPO membrane in the Br− form was completely dried to insure the low water content in the membrane. AFM images of the TC-PPO membrane in two and three dimensions are shown in Figure 5a,b; the TC-PPO membrane shows a clear hydrophilic/hydrophobic phase separation. The red domains in Figure 5a and yellow domains in Figure 5b are assigned to the hydrophilic ion clusters, and the pink domains in Figure 5a and blue domains in Figure 5b correspond to the hydrophobic polymer backbones. The hydrophilic phase domains are almost interconnected in the membrane so as to construct the continuous ionic channels in the TC-PPO membrane; these ionic channels can be served as highways for OH− transport, which can effectively improve the hydroxide conductivity of the HEM. The microphase structure of the TC-PPO membrane was further studied by TEM. To better observe the TC-PPO microphase structure, the TC-PPO sample was stained by sodium tungstate. As shown in Figure 5c, the TC-PPO shows obvious microphase-separated morphology. The dark domains
increases. The particle size of the LDH and ASU-LDH is about 90 and 100 nm, respectively. The fabrication of the three-decker ASU-LDH/TC-PPO hybrid membranes is achieved by an electrostatic-spraying method, as shown in Scheme 3. The low toxicity and boilingpoint ethanol is chosen as a solvent. Besides, the different spraying speed and structure of the ASU-LDH would influence the morphology of the ASU-LDH layer. The hexagonal LDH was designed to preferably construct the porous structure in the ASU-LDH layer so as to maximally improve ionic conductivity. A series of ASU-LDH/TC-PPO-x hybrid membranes were prepared (where x signifies the thicknesses of ASU-LDH layer). Specifically, all of polymer layers are 20 μm, and the thicknesses of ASU-LDH layer are controlled at 30−60 μm. Notably, the ASU-LDH/TC-PPO membrane can be designed much thinner when the polyelectrolyte with better mechanical strength is developed, even ASU-LDH-based inorganic membrane would be realized after adjusting the ASU-LDH ink. As per the FTIR analysis (Figure 3), the ASU-LDH hybrid membranes show the characteristic absorption band of Mg/Al-LDH around 1349 cm−1, and the intensity of the peak becomes strong with the content of the ASU-LDH, confirming the existence of the ASULDH. 3.2. Membrane Morphology. Figure 4 shows SEM images of the ASU-LDH/TC-PPO-60 hybrid membrane. Figure 4a shows that the surface of the hybrid membrane was homogeneous and dense, which avoids the gas permeation during fuel cell operation. Besides, Figure 4b shows the cross section image of ASU-LDH/TC-PPO membrane and clearly shows the three-decker structure, and the interface between ASU-LDH layer and TC-PPO is tight. Moreover, Figure 4c exhibits a porous surface of the ASU-LDH layer, which is beneficial to improving the ion conductivity of the ASU-LDH layer. To qualitatively characterize the three-decker layers, energy-dispersive X-ray spectroscopy (EDX) linear scanning was chose to analyze elementals in the three-decker layers. As 18251
DOI: 10.1021/acsami.8b01145 ACS Appl. Mater. Interfaces 2018, 10, 18246−18256
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ACS Applied Materials & Interfaces
branes so as to improve the IEC values. The WU of the hybrid membrane exhibits the same rule with the IEC values. However, it is noteworthy that SR does not follow this rule and even shows a little decrease with the thickness of ASULDH. This unusual and impressive phenomenon in SR is attributed to the existence of the ASU-LDH layer. The ASULDH layer still acts as an inorganic material and is difficult to swell. Besides, a small number of TC-PPO ionomers in the ASU-LDH layer improve the electrostatic force interaction in two-phase boundary between the ASU-LDH layer and TCPPO membranes, and this electrostatic force can slightly restrict the swelling of the membranes. Moreover, the temperature dependence on WU and SR was studied in Figure S4, the WU and SR of the ASU-LDH/TC-PPO hybrid membrane show an increasing tendency at elevated temperatures. Notably, the ASU-LDH/TC-PPO hybrid membranes with high IEC values did not show an excessive SR (
