A Three-Decker Strategy Based on Multifunctional Layered Double

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Applications of Polymer, Composite, and Coating Materials

A 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01145 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

A 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*, 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 Tel:+86-10-64444919; Corresponding author: *E-mail: [email protected]

[email protected]

& These authors contributed equally to this paper.

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-6-azaspiro [5.5] undecane (OH-ASU) cations to promote it’s ion exchange capacity. The ASU-LDH is combined with triple-cations functionalized poly(phenylene oxide) (TC-PPO) to fabricate

a

three-decker

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 ASU-LDH/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 1

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alkaline stability test in 1 M NaOH at 80 °C for 588 hours, prolonging the lifetime of TC-PPO membrane. Furthermore, the ASU-LDH/TC-PPO hybrid membrane realizes a maximum power density of 0.209 W/cm2 under 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 next generation low-cost fuel cell technology due to 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 (PEMFCs).1–4 Nevertheless, the commercial application of the HEMFCs always hindered by its key component of hydroxide exchange membranes (HEMs). HEMs play dual functions in HEMFCs, which acts as an ion exchange medium as well as isolates the feed gas between 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 2


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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 to cut down the mechanical properties. Recently, numerous polyelectrolytes are explored to boost OH− conductivity and chemical stability of HEMs in past few years. But there still haven’t reached a consensus on the preparation of HEM.27–30 Marino and coworker have investigated the alkaline stability of quaternary ammonium (QA) and demonstrated that six-membraned 6-azonia-spiro[5.5]undecane (ASU) has the highest lifetime in 6 M NaOH at 160 oC than that of the other QA groups.31 However, the ASU groups have not been successfully utilized in HEM via an effective method, 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 coworkers have disclosed that efficient micro-phase separation in HEMs can induce to forming the ion transport channels and improve the hydroxide conductivity.32 Hickner and coworkers33 demonstrated that multiple-cations side chain HEMs possess much higher OH− conductivity and alkaline stability compared with benzyltrimethylammonium (BTMA) due to effective phase separation in the HEMs. Moreover, plenty of inorganic/organic 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, even chemical stability, because these methods 3



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 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 triple-cations functionalized poly(phenylene oxide) (TC-PPO) via an electrostatic-spraying method. Especially, the hexagonal ASU-LDH 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/TC-PPO hybrid membranes is observed via scanning electron microscope 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 4


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membrane.

2. Experimental section 2.1 Materials Poly(phenylene oxide) (PPO) was provided by Beijing Chemical Works. 4-Hydroxypiperidine, N, N, N’, N’-tetramethyl-1,6-hexanediamine (TMHA), dibenzoyl peroxide (BPO), N-bromobutanimide (NBS), 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 1HNMR spectra of BPTMA, DNTNU, and BPPO are shown in Figure S1, Figure 1(b), and Figure 1(a).The brominated ratio of BPPO can be calculated by 1HNMR spectrum as shown in Figure 1(a). BPPO (0.5 g) and DNTNU (0.3 g) were reacted in N, N-dimethylformamide solution at 60 °C for 96 hours. Subsequently, the polymer solution was precipitated in deionized water to remove the residual reagent, followed drying in an oven overnight to obtain a yellow triple-cations side chain PPO polyelectrolyte, named as TC-PPO polyelectrolyte. 1

HNMR spectrum is shown in Figure 1(c).

2.3 Synthesis of 3-hydroxy-6-azonia-spiro [5.5] undecane (OH-ASU) 5



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5.1 g of 4-hydroxypiperidine (50 mmol) and 6.8 mL of 1,5-dibromopentane (50 mmol) were respectively dissolved in 40 mL chloroform, and both of the solution were transferred to two dropping funnels. Then two solutions were simultaneously added to a flask containing 30 mL 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 for 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 hours. After evaporating the solvent, a residual product was carefully washed by diethyl ether for several times. Subsequently, the product was recrystallization by ethanol,

and

the

white

N,N,N-trimethyl-3-(triethoxysilyl)propan-1-aminium

precipitate bromide

(TMSPA)

of was

obtained. Hexagonal Mg/Al-LDHs were synthesized as previous literatures.46,47 The average particle size of hexagonal LDH is 90 nm, and OH-ASU and TMSPA were used to functionalized with as-prepared LDH. Typically, OH-ASU (0.1 g), TMSPA (0.1 g) and LDH (0.4 g) were dispersed in ethanol solution, the mixture was reacted at 110 °C for 8 h. Subsequently, the mixture solution was centrifuged and dried to get a 6


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white powder, and named as ASU-LDH. X-ray diffraction (XRD) pattern and 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 were sonicated and uniformly dispersed in 200 mL ethanol to prepare the ASU-LDH ink, and then 0.08 g of TC-PPO polyelectrolyte was added to the ASU-LDH ink as 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 silicon wafer, and the hybrid membranes were named as ASU-LDH/TC-PPO-x, which x represents the thickness of ASU-LDH layer. 2.6 Structural Characterization 1

HNMR spectra of the OH-ASU and TC-PPO polyelectrolytes were obtained by

a Bruker Avance III 400 MHz spectrometer. CDCl3, D2O, and d6-DMSO were used as NMR

solvents.

Fourier

transformed

infrared

(FTIR)

spectra

of

the

ASU-LDH/TC-PPO hybrid membranes were recorded on a Nicoiet 8700 spectrometer. 2.7 Morphology Characterization The morphology of the membrane sample was acquired on a scanning electron microscope (Zeiss Supra 55). In addition, atomic force microscope (AFM, 7



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DMFASTSCAN2-SYS, Bruker Co.) was used to observe the micro-phase morphology of the TC-PPO samples. Besides, transmission electron microscope (TEM, Hitachi HT7700) and small angle X-ray scattering (SAXS, Xuess2-0) were further used to observe the micro-phase separation of the TC-PPO membrane. 2.8 IEC, Water Uptake (WU), and Swelling Ratio (SR) 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 follow. IEC =

× ×

(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; CHCl and CNaOH are the concentration of the HCl and NaOH solution. The dry weight of a membrane sample in OH− form was obtained after drying the membrane to a constant mass, and then one-direction size of the membrane sample was measured. Next, the dry sample was soaked in deionized water at different temperatures for 3 hours. 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 8


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by tissue paper. WU and SR can be calculated as follow: WU (%) = SR (%) =

"#$% "&'( "&'(

× 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), was calculated from: )=

*+++×,-

(4)

./ ×01

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 samples were tested for three times at elevated temperatures ranging from 30-80 °C. The high frequency region of the impedance was used to calculate the resistance and ionic conductivity, as follow: σ=

3

(5)

45

d (cm) represents the thickness of membrane sample, A (cm2) signifies contact area between the membrane and the electrodes, and R (Ω) is resistance. 2.10 Thermal Stability Thermogravimetric (TG) analysis of a membrane sample was performed by a TGA Q500 analyzer. A membrane sample was heated from 30 °C to 900 °C with a heating rate of 10 °C/min under a N2 environment. 9



2.11 Chemical Stability The oxidative stability of the ASU-LDH/TC-PPO hybrid membranes was evaluated by soaking the membrane in Fenton’s reagent (3.5% H2O2 containing 2 ppm FeSO4 at 80 °C) for 8 hours, 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 1HNMR 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 three days. 2.12 Fuel Cell Testing To fabricate catalyst-coated membrane (CCM), 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 ionomer to Pt/C in anode and cathode is 1:5, and the Pt loading is 0.55 mg/cm2 in both anode and cathode. The both electrode areas are 5 cm2. Subsequently, the CCM and two pieces of gas diffusion layers were pressed together to fabricate membrane electrode assembly (MEA). A Hephas Mini-L100 fuel cell system was used to measure the single cell performance of as-prepared MEA. Temperature is controlled at 60 oC with a humidity of 100% and back pressure is 100 kPa. The flow rates of H2 and O2 gases are 100 mL/min and 20 mL/min, respectively. 10


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3. Results and discussion 3.1 Synthesis and Characterization Five steps need to fabricate the three-decker ASU-LDH/TC-PPO hybrid membrane. (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, (4) fabrication of the ASU-LDH/TC-PPO membrane by electrostatic spraying. Synthesis of the triple-cations precursor. BPTMA was synthesized as an intermediate product by the nucleophilic substitution. In order to prevent bis-substituted byproduct, THF was chosen as solvent so that the mono-substituted product would be precipitated immediately once it was formed. Then mono BPTMA was reacted with diamine to prepare the triple-cations precursor. 1HNMR spectra of the intermediate product and triple-cations precursor are shown in Figure S1 and Figure 1(b). In Figure S1, the arisen peaks between 3.1 ppm and 3.3 ppm are assigned to the N+-CH3 and N+-CH2- groups. The peak around 3.5 ppm corresponds to Br-CH2group. In Figure 1(b), the peaks around 3.2-3.4 ppm are attributed to -CH2-N+ groups. The chemical shifts at 2.2 and 3.1 ppm represent to -N-CH3 and N+-CH3 groups, respectively. Preparation of the TC-PPO polyelectrolyte and membrane, as shown in Scheme 1. In Figure 1 (c), the peaks between 3.1 to 3.3 ppm belong to -CH2-N+ groups in side chain, indicating that triple-ammonium precursor is sufficiently reacted with BPPO. The functionalized degree of TC-PPO polyelectrolytes is 40 % at the benzyl position, 11



which is calculated by the relative integral areas between -CH3 and -CH2-Br groups in Figure 1 (a). Scheme 1. Synthetic route of multi-cations side chain TC-PPO polyelectrolyte.

12


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Figure 1. 1HNMR spectra of (a) brominated PPO: 1:-CH2-Br, 2: Ph-H, 3: -CH3 (b) triple-cations precursor: 1: N+-CH3, 2: N+-CH2-, 3: N-CH3, 4: N-C-C-CH2-, 5: N-CH2-, 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+, 12: N+-C-C-CH2-CH2-C-C-N+.

Synthesis route of OH-ASU and ASU-LDH hybrids is shown in Scheme 2. OH-ASU was synthesized by a cyclization reaction, and 1HNMR spectrum of the OH-ASU is shown in Figure S2. OH-ASU is soaked in 2 M NaOH at 80 oC to assess the alkaline stability, and the 1HNMR result shows superb alkaline stability of the OH-ASU after 500 hours (as shown in Figure S3). Then the OH-ASU and TMSPA are 13



modified with hexagonal LDH to prepare the ASU-LDH composites, which prominently improves the ion exchange ability of the ASU-LDH. 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) of Mg/Al-LDH. In addition, the 003 crystal face in ASU-LDH shifts to a low 2θ, demonstrating the interlayer distance of the ASU-LDH becomes larger which forcefully verifies the LDH is successfully modified with ASU and TMSPA. Besides, the size of d can be calculated by the Scherrer Formula based on 003 crystal face, and the size of d in LDH and ASU-LDH is about 15.3 nm and 18.7 nm, respectively. TEM images of the LDH and ASU-LDH are shown in Figure 2(c) and (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 increases. The particle size of the LDH and ASU-LDH is about 90 nm and 100 nm, respectively.

14


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Scheme 2. Synthetic route of modified ASU-LDH hybrids.

Figure 2. XRD pattern and TEM images, (a) and (b) XRD patterns of OH-ASU, LDH, and ASU-LDH, TEM images of LDH (c) and ASU-LDH (d).

The fabrication of the three-decker ASU-LDH/TC-PPO hybrid membranes is 15



achieved by an electrostatic-spraying method, as shown in Scheme 3. The low toxicity and boiling-point 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 ASU-LDH layer so 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 to 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 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 ASU-LDH. Scheme 3. Fabrication of ASU-LDH/TC-PPO hybrid membrane.

16


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Figure 3. FTIR spectra of the TC-PPO, ASU-LDH/TC-PPO membranes, and ASU-LDH. (a) Wavenumber from 800 cm-1 to 3750 cm-1 and (b) the partial enlarged picture form 700 cm-1 to 1800 cm-1.

3.2 Membrane Morphology Figure 4 shows SEM images of ASU-LDH/TC-PPO-60 hybrid membrane. Figure 4(a) shows that the surface of the hybrid membrane was homogeneous and dense, which avoids the gas permeation during fuel cell operation. Besides, Figure 4(b) shows the cross-section image of ASU-LDH/TC-PPO membrane clearly shows the three-decker structure, and the interface between ASU-LDH layer and TC-PPO is tight. Moreover, Figure 4(c) exhibits a porous surface of ASU-LDH layer, which is beneficial to improving the ion conductivity of the ASU-LDH layer. In order to 17



qualitatively characterize the three-decker layers, EDX linear scanning was chose to analyze elementals in the three-decker layers. As shown in Figure 4(d), with the increasing of scanning distance, the content of Mg, Al, and O elements increases in the middle part and then significantly decreased in two edges part, indicating that the middle part is the ASU-LDH layer. Besides, the content of C element in the edges part is higher than in 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 ASU-LDH layer is approximately 50 µm and the TC-PPO layers are about 20 µm per side. Currently, the micro-phase structure of HEMs has been extensively studied because the micro-phase 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 AFM test, the TC-PPO membrane in 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 5 (a) and (b), the TC-PPO membrane shows a clear hydrophilic/hydrophobic phase separation. The red domains in Figure 5(a) and yellow domains in Figure (b) are assigned to the hydrophilic ion clusters, and the pink domains in Figure 5(a) and blue domains in Figure (b) correspond to the hydrophobic polymer backbones. The hydrophilic phase domains are almost interconnected in membrane so to construct the continuous ionic channels in the TC-PPO membrane, these ionic channels can be served as highways for OH− 18


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transport, which can effectively improve the hydroxide conductivity of the HEM. The micro-phase structure of the TC-PPO membrane was further studied by TEM. To better observe TC-PPO micro-phase structure, the TC-PPO sample was stained by sodium tungstate. As shown in Figure 5 (c), the TC-PPO shows obvious micro-phase separated morphology. The dark domains are assigned to tungstate ion-stained hydrophilic ion clusters, and the bright domains belong to unstained hydrophobic polymer backbone. The sizes of the hydrophilic domains can be estimated to 7-12 nm by TEM. The micro-phase morphology of the TC-PPO membrane was further studied by SAXS. SAXS pattern of the TC-PPO membrane is shown in Figure 5(d), the domain spacing (d, nm) inside ion regions can be calculated from the Bragg’s equation: d=2π/q, where q signifies the ionomer peak which is assigned to interparticle scattering. A scattering peak around 0.491 nm-1 was observed in the membrane sample, thus d value is around 12.8 nm. This values is closed to the size of ion domains in TEM.

19



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.

Figure 5. (a) AFM 2D image of the TC-PPO membrane, (b) AFM 3D image of the TC-PPO 20


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membrane, (c) TEM image of the TC-PPO membrane, and (d) SAXS analysis of the TC-PPO membrane.

3.3 IEC, WU, SR, λ, and Mechanical properties. IEC represents the ion exchange capability of HEMs. In general, the IEC significantly influences the WU and SR of HEMs. Large IEC values would lead to high WU which usually results in the overlarge SR of HEMs. Certainly, the high IEC is beneficial for improving the ion conductivity. Table 1 shows the IEC, WU, SR, and λ of the three-decker ASU-LDH/TC-PPO hybrid membranes, surprisingly, the ASU-LDH layer exhibits many positive effects on the ASU-LDH/TC-PPO hybrid membrane. The hybrid membranes show a relatively high IEC values (3.11 to 3.90 mmol/g).In addition, the IEC is prominently increased along with the thickness of ASU-LDH layer due to the additional cationic groups in ASU-LDH layer. Furthermore, extra TC-PPO ionomer in ASU-LDH layer also can improve the ion exchange ability of it. The ASU-LDH layer provides additional ion exchange sites for the hybrid membranes so to improve the IEC values. The WU of the hybrid membrane exhibits a same rule with the IEC values. However, it is noteworthy that SR doesn’t follow this rule and even shows a little decrease with the thickness of ASU-LDH. This unusual and impressive phenomenon in SR is attributed to the existence of ASU-LDH layer. The ASU-LDH layer still acts as an inorganic material and is difficult to swell. Besides, a small number of TC-PPO ionomers in ASU-LDH layer improve the electrostatic force interaction in two-phase boundary between the ASU-LDH layer and TC-PPO membranes, and this electrostatic force can slightly restrict the swelling of the membranes. Moreover, the temperature dependence on WU 21



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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 didn’t show an excessive SR (

A Three-Decker Strategy Based on Multifunctional Layered Double

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