Constructing multilayered membranes with layer-by-layer self-assembly technique based on graphene oxide for anhydrous proton exchange membranes

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Constructing multilayered membranes with layer-by-layer self-assembly technique based on graphene oxide for anhydrous proton exchange membranes Tingting Jia, Si Shen, Libang Xiao, Jin Jin, Jing Zhao, Quantong Che

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Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Graphene oxide Polyurethane Multilayered membranes Layer-by-layer self-assembly Anhydrous proton conductivity

Layer-by-layer (LBL) self-assembly technique could nanoscopically construct the layered membranes and it has attracted more attention in the field of bionics, clean energy and microelectronics. The aim of this research to develop the multilayered membranes with the ordered deposition of components using LBL self-assembly technique. Successful preparation of anhydrous proton exchange membranes (PEMs) based on graphene oxide (GO) as polyanions, polyurethane (PU) and poly(diallyldimethylammonium chloride) (PDDA) as polycations have been demonstrated by identification of the ordered distribution of components and compact structure. While the prepared membranes were immersed into phosphoric acid (PA) solution, PA molecules were combined with the formation of PA doped membranes. In spite of PA dominating the proton conduction, the decreased proton conduction resistance is revealed from the lower activation energy owing to the multilayered structure. Specifically, the proton conductivity of 1.83 × 10−1 S/cm at 150 °C was obtained for (PU/GO/PDDA/GO)200/ 60%PA membranes. Moreover, the proton conductivity stability measurement demonstrated the component and mechanical stability, showing 1.47 × 10−1 S/cm at 120 °C and 1.83 × 10−1 S/cm at 140 °C. The result revealed that LBL self-assembly technique could provide a promising strategy to prepare multilayered membranes as membrane electrolytes using in high temperature proton exchange membrane fuel cells.

1. Introduction Proton exchange membrane fuel cells (PEMFCs) as the feasible energy conversion devices owing to high power density and low greenhouse emission have receiving the intensive attention for a variety of applications, especially in vehicular transportation [1]. PEMs as the key components of PEMFCs play an important role in conducting protons and separating hydrogen or methanol and oxidant, etc. [2,3]. In the past few decades, considerable efforts have been made to develop anhydrous PEMs for working in high temperature (> 100 °C) PEMFCs associated with the improved tolerance to CO poisoning, enhanced electrode kinetics, simplified water/heat management systems [4,5]. The materials and microstructures of anhydrous PEMs are the key factors to determine the performance working in high temperature PEMFCs [6–9]. Many materials such as phosphoric acid (PA), ionic liquids (ILs), inorganics and even functional polymers using as anhydrous proton carriers have been mixed with the supports of polymers for membrane electrolytes preparation [10–12]. Although the solution casting method has been recognized with the merits of convenient

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operation, rapid membrane forming and strong versatility, the performance of the prepared membranes were usually dragged from the irregular dispersion of components [13,14]. Recent years have witnessed the increasing interest in the construction of nanoscopically layered membranes with the LBL self-assembly technique through the alternate deposition of polyelectrolytes since it was firstly introduced by G. Decher in 1997 and further developed by many groups [15,16]. In theory, two or more polymers with the enough opposite electrical charges could form a polymer complex, composite membranes and hydrogels driven by the electrostatic interaction, van der Waals force and hydrogen bonds, etc. [17–19]. The LBL self-assembly technique is considered to open the new opportunity for preparing membranes with tailored composition and tunable properties [20,21]. In the field of PEMs, the studies were primarily focused on the membranes modification with depositing polymers or inorganics alternatively. Hammond PT has reported the pioneering work of depositing polyelectrolyte multilayers such as poly(ethylene oxide) (PEO) and poly(acrylic acid) (PPA) as PEMs [22]. Subsequently, Na H constructed a stable multilayer membranes of phosphotungstic acid and

Corresponding author. E-mail address: [email protected] (Q. Che).

https://doi.org/10.1016/j.eurpolymj.2019.109362 Received 26 August 2019; Received in revised form 25 October 2019; Accepted 9 November 2019 0014-3057/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Tingting Jia, et al., European Polymer Journal, https://doi.org/10.1016/j.eurpolymj.2019.109362

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2.2. Membrane construction

polyaniline depositing onto the surface of sulfonated poly(arylene ether ketone) bearing carboxyl groups membrane to minimize methanol permeation [23]. The composite membranes through the alternating deposition of poly(diallyl dimethyl ammonium chloride) and sulfonated poly(phenylene oxide) on Nation-212 membranes showed the lower methanol permeability and higher overall selectivity comparing to the pristine Nafion® membranes [18]. Besides that, the ternary multilayered PEMs based on different polymers, ILs and nanocrystals with the LBL self-assembly technique have been reported by our groups [24]. Graphene oxide (GO) as an electronically insulating correlated of the highly electrically conductive graphene has been considered as one of the most attractive materials for many application [25]. As already demonstrated, there are large number of oxidative functional groups such as hydroxyl and epoxy oxygen groups in its basal plane and edges [26,27]. The surrounding carboxylic groups could provide the binding sites to capture PA molecules with the formation of PA doped membranes. Therefore, GO has been investigated as proton carriers in the preparation of PEMs since these hydrophilic regions could provide more proton transport channels and hold more water molecules [28,29]. Furthermore, the functionalization of GO is considered to improve proton conductivity, mechanical strength, and other electrochemical properties of membranes [30,31]. Although the reported results have showed great promise in the application of GO in the development of novel membrane electrolytes, there remains a potential fear of the agglomeration and disordered distribution of GO in composite membranes. The main objective of this research is to construct the multilayered membranes with the well-ordered dispersion of GO and polymers using the LBL self-assembly technique, expecting to show an effective and facile way to improve the grade of PEMs. (PU/GO/PDDA/GO)200 membranes were thus prepared through alternate deposition of PU, PDDA with positive charges and GO with negative charges. Besides the electrostatic attraction, the intermolecular hydrogen bonds could drive the completion of the LBL selfassembly process. Moreover, PA molecules were captured with the formation of PA doped membranes owing to the intermolecular hydrogen bonds. As expected, (PU/GO/PDDA/GO)200/PA membranes showed high and stable proton conductivity, benefiting from the formed free PA molecule chain and the decreased proton conduction resistance.

Construction of LBL self-assembly membranes An ordinary glass substrate (GS) was treated with piranha solution (4 ppm Fe2+, 3 wt% H2O2) and was worn with numerous negative charges on the surface. The GS was sequentially immersed into 3 wt% of PU solution, 1 wt% of GO solution, 1 wt% of PDDA solution and 1 wt% of GO solution to complete the monolayer self-assembly process of components, denoting (PU/GO/PDDA/GO)1 membranes. Notably, the immersion time was controlled in 6 min and the GS was cleaned through rinsing in DI water for 30 s for each step. After repeating 200 times, free-standing (PU/GO/ PDDA/GO)200 membranes were obtained while they were automatically peeled off in 1 wt% of hydrofluoric acid solution. Moreover, (PU/GO/PDDA/GO)200 membranes were immersed into 40–85 wt% PA solution for 20 h with the formation of (PU/GO/PDDA/GO)200/ (40–85%)PA membranes [13]. For the preparation of (PU/GO/PDDA/ GO)200/bmimCl/60%PA membrane, (PU/GO/PDDA/GO)200 membranes were immersed into 40 wt% bmimCl solution for 60 h and 60 wt % PA solution for 20 h. 2.3. Characterization Zeta potential The zeta potential (ζ) values of PU, GO and PDDA were determined by dynamic light scattering (DLS) with Zetasizer Nano ZS90 (Malvern Instruments). Fourier transform infrared spectra The fourier transform infrared spectra (FTIR) of membranes were acquired using a Vertex 70 Spectrometer (Bruker optics). The curves were recorded in the range of 4000–500 cm−1 with an interval of 1 cm−1. Raman spectra A Horiba XploRA Spectrometer (Jobin Yvon, France) equipment was utilized to obtain the Raman spectra. Thermal stability For membranes, thermal degradation behaviors were assessed by thermogravimetric analysis (TGA) under an air flow rate of 30 mL/min on a TGA 290C system (Netzsch Company). Approximately 5 mg samples were heated from room temperature (RT) to 630 °C at a heating rate of 10 °C/min. All samples were preheated at 100 °C for 4 h to remove the residual solvent and the absorbed water during the samples preparation and transfer process. Fine microstructure The morphologies of membranes were observed on a Zeiss scanning electron microscope (SEM). Moreover, the transmission electron microscopy (TEM) images of GO nanosheets were collected by a JEM-2100Plus transmission electron microscope (JEOL). X-ray diffraction X-ray diffraction (XRD) scattering patterns were analyzed from 5ο to 80ο at 2θ angle under a scan speed of 5 ο/min by the Part Pro-MPD X-ray diffractometer (Panalytical B.V.) using Cu Kɑ radiation source (λ = 1.5418 Å). Methanol permeability The methanol permeation (p) values were measured using a dual flask apparatus (120 mL) with a sample as a separator fixing between them [34]. Methanol solutions with 10 M and 2 M were used as methanol sources. More details about the liquid permeation cell, the measurement procedures and the calculation according to Eq. S1 were shown in SI. Weight gain and dimension swelling of membranes doping with PA The pre-dried membranes were weighted and doped with PA in equilibrium while they were immersed into PA solution for 24 h. The wet membranes were then wiped with tissue paper and weighted quickly. Weight gain (WG) and volume swelling (VS) were calculated according to Eq. (1) and Eq. (2). Where, W0, W and V0, V refer to the weights and volume of the initial membranes, PA doped membranes.

2. Experimental 2.1. Materials PDDA (20 wt% in water), average molecular weight (Mw): 400,000–500,000 was purchased from Sigma-Aldrich Corporation. 3 wt % of cationic PU aqueous solution was prepared by diluting 30 wt% of PU solution (Mw: 92,000, from Hepce Chem, South Korea). Graphite powder, sodium nitrate, potassium permanganate and glycidyltrimethylammonium chloride were supplied by J&K Scientific Ltd, China. Concentrated sulfuric acid (98 wt%) and PA solution (85 wt%) were provided from Sinopharm Chemical Reagent Co., Ltd, China. Moreover, 70–40 wt% PA solutions were obtained by diluting 85 wt% of PA solution, respectively. Ionic liquid of 1-butyl-3-methylimidazolium chloride (bmimCl) was supplied by Maya Reagent Co., Ltd, China. GO was prepared using the harsh oxidation of graphite with the modified Hummer's method by using KMnO4 and concentrated H2SO4 referring to the reported processes [28,32] and the procedures were shown in supplementary information (SI). As a result of the polar oxygen functional groups rendering GO with hydrophilicity, 1 wt% of GO solution was obtained by dispersing 1.0 g of GO in 100 mL deionized (DI) water with ultrasonication since GO could be exfoliated and dispersed particularly well in DI water [33]. All chemicals were used as received without further purification.

Weight gain (%) =

W − W0 × 100% W0

Volume swelling (%) =

V − V0 × 100% V0

(1)

(2)

Component stability The component stability of membranes was 2

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components could ensure the completion of LBL self-assembly process with the formation of multilayered membranes. Although the electrostatic interaction is substantially responsible interactions for the LBL self-assembly [37], the formed intermolecular hydrogen bonds between PU and GO, PDDA and GO could contribute to drive the self-assembly process.

studied according to the weight variation while the samples were placed in a closed oven at 100 °C with humidification for 96 h. The test was expected to simulate the continuous production of steam in high temperature PEMFCs. Proton conductivity Proton conductivity (σ) of membranes was measured in a test cell similar to the real full cell configuration. Membrane samples were cut into rectangle with a dimension of 2 cm × 5 cm and the tests were performed from 80 °C to 160 °C without humidification. Specifically, a pair of platinum probes supplying by the constant current were placed on both sides of membranes and the R was known through measuring the voltage drop between another pair of probes with Ampere's law. The through-plane σ values were calculated using Eq. (3) [35]. Where l and s mean the distance of current passing through membranes and the cross-sectional area of membranes.

σ =

l R·s

3.3. FTIR and Raman spectra Fig. 2 (A) and (B) represented the FTIR spectra of GO, PU, PDDA, (PU/GO/PDDA/GO)200 and (PU/GO/PDDA/GO)200/60%PA membranes. For PU membranes, the peaks at 1230 cm−1 and 1090 cm−1 were respectively attributed to the stretching vibrations of CeN+ and the in-plane bending vibrations of CeH in quaternary ammonium groups [38]. The peaks at 1706 cm−1 and 1536 cm−1 were assigned to the C]O stretching and the NeH deformation, respectively [39]. The doublet bands at 2924 cm−1 and 2853 cm−1 were caused by the stretching vibration of the methyl group in PDDA [40]. Moreover, the identification of GO was confirmed by the broad peak of hydroxyl at 3700–3250 cm−1, the peak of the C]C at 1625 cm−1 and some small peaks from ether or epoxide at 1280–1000 cm−1 [24]. Complementarily to FTIR analysis, Raman spectra of these membranes were provided as shown in Fig. 2 (C). The D-band at 1360 cm−1 and G-band at 1600 cm−1 [41] were observed in the Raman spectra of GO and (PU/ GO/PDDA/GO)200 membranes. Moreover, it is hard to distinguish the tiny peaks at 1731 cm−1 and 1531 cm−1 from PU [42] as well as the peak at 1634 cm−1 from PDDA [43] due to the overlap from GO in (PU/ GO/PDDA/GO)200 membranes. These characteristic peaks could support the successful deposition of PU, GO and PDDA in (PU/GO/PDDA/ GO)200 membranes. As regards to the FTIR spectra of (PU/GO/PDDA/ GO)200/60%PA membranes as shown in Fig. 2 (B), there was no observable discrepancy even if the membranes went through the proton conductivity measurement at the elevated temperature.

(3)

Mechanical strength For membranes, the mechanical properties including tensile stress (E) and strain (ε) at break are measured using a CMT2000 tensile strength instrument (JNshijin Company, China) at RT. More details about the condition and process of measurements were shown in SI. 3. Results and discussion 3.1. Construction of LBL self-assembly membranes For the prepared membranes, the photographs of (PU/GO/PDDA/ GO)n membranes with different layers on GS, (PU/GO/PDDA/GO)200 and (PU/GO/PDDA/GO)200/60%PA membranes were shown in Fig. 1 (A)-(C) and Fig. 1 (D) schematically described the multilayered structure of (PU/GO/PDDA/GO)2 membranes. With more layers adhering to GS, (PU/GO/PDDA/GO)n membranes represented the increased homogeneity and the decreased transparency in the surface. After 200 layers deposition, the free-standing (PU/GO/PDDA/GO)200 membrane with good flexibility and stiffness was obtained without the fear of GO agglomeration visually. Although more PA molecules were doped with the formation of (PU/GO/PDDA/GO)200/60%PA membranes, it is difficult to find out the variation on the appearance. According to SEM images in the Section 3.5, the thicknesses of (PU/GO/PDDA/GO)200 and (PU/GO/PDDA/GO)200/60%PA membranes were statistically 13.2 ± 1.2 μm and 29.8 ± 1.6 μm, respectively.

3.4. Thermal stability The initial thermal decomposition temperature has been considered to be one of the most technical parameters to evaluate the application for the prepared membranes as high temperature PEMs. From the TGA curves as shown in Fig. 3, the initial mass loss occurred until 160 °C for (PU/GO/PDDA/GO)200 membranes due to the degradation of GO and PU at 150 °C and 160 °C, respectively [44,45]. For GO, the mass loss at 150–230 °C was caused by the decay of labile oxygen related functionalities and the sustained mass loss from 220 to 400 °C was due to the weakening of van der Waals forces between the GO layers [43,46]. The subsequent decomposition exceeding 400 °C was probably associated with the destruction of the carbon skeleton. As regards to (PU/ GO/PDDA/GO)200/60%PA membranes, the mass loss below 130 °C was assigned to the evaporation of the adsorbed water during the samples preparation and transfer process. Besides that, the followed mass loss stages were caused by the deformation of PA with the formation of pyrophosphoric acid at 200 °C and triphosphoric acid at 300 °C. The

3.2. Zeta potential The superficial charges of nanomaterials and polymers are generally evaluated by ζ values. According to the DLS results, the ζ values of the components were −46.2 mV for GO as polyanions, 43.4 mV and 54.6 mV for the polymers of PU and PDDA as polycations. As reported, the components with the absolute value of ζ exceeding 30 mV were suitable to participate the LBL self-assembly process [36]. The chemical formula of components were shown in Fig. S1. The abundant charges in

Fig. 1. Photographs of (PU/GO/PDDA/GO)n membranes with different layers on glass substrates (A), (PU/GO/PDDA/GO)200 membranes (B), (PU/GO/PDDA/ GO)200/60%PA membranes (C) and the schematic representation of (PU/GO/PDDA/GO)2 membranes with the layered structure (D). 3

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Fig. 2. FTIR (A) and Raman (C) spectra of GO, PDDA, PU and (PU/GO/PDDA/GO)200 membranes, FTIR spectra (B) of (PU/GO/PDDA/GO)200/60%PA membranes.

magnified to 50000X in the insert image, a microphase separation was represented. As already described, the phenomenon of the compacted structure became loose due to the doping PA molecules have been reported in the systems of PA doped polybenzimidazole (PBI) membranes [2] and PA doped methylimidazolium group–modified polyvinyl chloride (PVC-MIMCl) [47]. The cross-section SEM images (Fig. 4 (E)) of (PU/GO/PDDA/GO)200 membranes presented the dense and multilayered structure of membranes. Fig. 4 (F) showed the overall morphologies were uniform without obvious defects in the cross-section even if a large amount of PA molecules were doped in (PU/GO/PDDA/ GO)200/60%PA membranes. Unlike the serious swelling of membranes from solution casting method, (PU/GO/PDDA/GO)200/60%PA membranes could keep the dimension and the compacted structure even a considerable amount of PA molecules doping. Fig. 3. Thermal stability of GO, PDDA, PU, (PU/GO/PDDA/GO)200 and (PU/ GO/PDDA/GO)200/60%PA membranes.

3.6. X-ray diffraction The existence of components in membranes was determined qualitatively by XRD measurements and the diffraction patterns were shown in Fig. 5. For graphite, the sharp diffraction peak at 2θ = 26.3° was identified with carbon (0 0 2) [48] with the interlayer spacing of 0.34 nm. Due to the oxidation, a new peak at 2θ = 11.5° (0.76 nm) indicated the oxidized graphene sheets loosely stacking in GO [49]. Notably, this characteristic peak moved to 2θ = 9.7° (0.91 nm) in the XRD pattern of (PU/GO/PDDA/GO)200 membranes. This shift with a lower angle probably was caused by the surrounding polyelectrolyte chains interspersing between the interlayers of GO. The similar phenomenon has been reported in GO/PDDA films by D.Z. Zhang [50]. Furthermore, the peak at 2θ = 26.3° was broadened in (PU/GO/PDDA/ GO)200 membranes. S.J. Kim has reported the broadening peak around

results indicated the prepared membranes are promising for the high temperature PEMFCs applications. 3.5. Fine structure of membranes SEM images could provide some key information to understand the microstructures of membranes. Fig. 4 (C) showed the GO with the foliated structure in (PU/GO/PDDA/GO)200 membranes. However, the smooth surface structure with tiny cracks was found in the surface of (PU/GO/PDDA/GO)200/60%PA membranes as shown in Fig. 4 (D), which probably caused by the PA molecules swelling the LBL self-assembly membranes even separating GO layers. When the images were

Fig. 4. TEM images (A) and (B) for GO nanosheets; SEM images for surface-section (C) (PU/GO/PDDA/GO)200, (D) (PU/GO/PDDA/GO)200/60%PA; SEM images for cross-section (E) (PU/GO/PDDA/GO)200, (F) (PU/GO/PDDA/GO)200/60%PA. 4

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Fig. 5. XRD patterns of (A) graphite, GO, PDDA, PU and (PU/GO/PDDA/GO)200 membranes, (B) (PU/GO/PDDA/GO)200/60%PA membranes.

2θ = 26.15° in the XRD pattern for GO since the lattice distortion occurs in the AB stacking order of the graphite lattice due to mild oxidation [32]. It is inferred that the oxidation of GO could decrease due to the interaction with PU and PDDA in (PU/GO/PDDA/GO)200 membranes. Simultaneously, the sharp peak from PDDA disappeared due to the decrement on crystallinity in membranes. As regards to (PU/GO/ PDDA/GO)200/60%PA membranes, the main peak at 2θ = 17.3° moved to a higher values while they went through the heated process at 120 °C, 140 °C and 160 °C.

membranes present the excellent performance on resisting methanol molecules permeation. In spite of the hydrophilic groups such as amino groups from PU, carboxyl groups and hydroxyl groups from GO and quaternary ammonium from PDDA combining water molecules with the formation of hydrophilic channels to conduct protons and methanol molecules, the layered structure and the sheet – like structure of GO [54] were considered to endow the membranes with the strong ability to resist methanol permeation [55]. 3.8. gain and dimension swelling of membranes doping with PA

3.7. Methanol permeation

An important factor in proton conduction for PA doped membranes is PA content, although the components of polymers could contribute to proton conduction with functional groups [56]. It is crucial to understand the WG associated with the dimension swelling for (PU/GO/ PDDA/GO)200/PA membranes. As already reported by P. Gómez-Romero [57], a higher PA concentrations could improve the WG values of PA doped membranes. As shown in Fig. 7, the maximum WG reached 558% for (PU/GO/PDDA/GO)200/85%PA membranes and the minimum value was 207% for (PU/GO/PDDA/GO)200/40%PA membranes. H. Na has reported that the pristine PBI membranes immersed into the 14.6 M PA solution exhibited high weight over 400% [58]. The components of GO, PDDA and PU could provide a considerable amount of sites to combine PA owing to the formed intermolecular hydrogen bonds in membranes. These membranes showed the TS in range of 273–65.1%, broadly similar to 247–63.5%, which revealed PA molecules arranging stereoscopically in (PU/GO/PDDA/GO)200/PA membranes. As already demonstrated, the use of PA could improve proton conductivity but thus degrade the stiffness of membranes. (PU/GO/ PDDA/GO)200/60%PA membranes with the suitable content of PA were recommended and avoided the occurrence of the dilemma of proton conductivity-mechanical strength.

Besides PEMFCs, the prepared membranes are also expected to work as membrane electrolytes in direct methanol fuel cells (DMFCs) with the premise of low methanol permeability (p). In DMFCs, the permeated methanol molecules could reduce the active sites through poisoning catalysts besides minimize the fuel utilization. So the p value is a key criterion in assessing fuel cell performance [51]. Fig. 6 showed the p values of (2.12–3.48) × 10−7 cm2/s and (0.366–1.25) × 10−7 cm2/s for (PU/GO/PDDA/GO)200 membranes with 10 M and 2 M methanol solution as source. In general, a higher p value for membranes is obtained with a higher concentration (c) of methanol solution for measurement [34]. The discrepancy on c of methanol solution caused the chemical potential (μ) gradient and c gradient, which was the exclusive force to perform the methanol permeation and more detailed explanations were shown in Fig. S2. There has been much published literature relating to the successful PEMs of Nafion® series membranes, such as 1.8 × 10−5 cm2/s of Nafion–117 membranes with 5 M methanol solution [52], 1.4 × 10−6 cm2/s of Nafion–112 membranes [20] and 2.45 × 10−6 cm2/s of Nafion–112 membranes with methanol gas [53]. According to the results, it is revealed that (PU/GO/PDDA/GO)200

3.9. Component stability Fig. 8 showed the similar trends of mass loss curves in (PU/GO/ PDDA/GO)200/60%PA and (PU/GO/PDDA/GO)200/bmimCl/60%PA membranes. The residual mass percents were 74.4% and 67.2% for (PU/GO/PDDA/GO)200/60%PA and (PU/GO/PDDA/GO)200/bmimCl/ 60%PA membranes while they were placed in a closed oven at 100 °C with humidification for 96 h. The mass loss in presence of bmimCl is slightly higher in (PU/GO/PDDA/GO)200/bmimCl/60%PA membranes, which reveled the occurrence of bmimCl leakage. Comparing to the reported 96.0% for PVC-MIMCl/PA (1/5) membranes [45], 97.4% for PA doped PBI membranes [59] and 95.0% for methylimidazolium group-modified polyvinyl chloride membranes [3], there remains a fear of the low mass retention ratio values in (PU/GO/PDDA/GO)200/60% PA and (PU/GO/PDDA/GO)200/bmimCl/60%PA membranes. In consideration of the main features of the polymers and GO immobilizing in

Fig. 6. Methanol permeability of (PU/GO/PDDA/GO)200 membranes at 2 M and 10 M as a function of time. The data of (PU/CNT-CdTe/PU/CS)200 membrane from our previous work. 5

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Fig. 7. Weight gain (WG), thickness swelling (TS) and volume swelling (VS) of (PU/GO/PDDA/GO)200 membranes immersing into PA solutions at RT.

the σ values of membranes increasing gradually with temperature rising due to the accelerated mobility of protons at elevated temperature. Meanwhile, the σ values were also improved with increasing the concentration of PA solution in relation to the content of PA in membranes. Specifically, the highest σ was 3.91 × 10−1 S/cm for (PU/GO/PDDA/ GO)200/85%PA membranes with a maximum WG of 558%. Notably, bmimCl could further improve σ values associated with the contribution to absorb PA molecules. By contrast, the σ values were respectively 1.87 × 10−1 S/cm and 2.48 × 10−1 S/cm at 150 °C for (PU/GO/ PDDA/GO)200/60%PA and (PU/GO/PDDA/GO)200/bmimCl/60%PA membranes. For PEMs, the σ value is a substantial process, which is affected by the proton conduction behaviors and the mechanism. It is crucial to understand how the proton conduction in the prepared PA doped membranes. As the demonstration of PA conducting protons, the proton conduction resistance is thus reduced owing to the high content of PA and the uniform dispersion of components, which is reflected in the reduction of activation energy (Ea). As shown in Fig. 9 (B), the relation between σ values and temperature matched well with Arrhenius’ law and the Ea values were calculated by multiplying the slope (k) with constant R (8.314 J·mol−1·K−1) in SI. Specifically, the calculated Ea values were 15.0 kJ/mol, 16.1 kJ/mol, 16.3 kJ/mol, 22.3 kJ/mol and 36.3 kJ/mol for (PU/GO/PDDA/GO)200/(85%−40%)PA membranes and 13.5 kJ/mol for (PU/GO/PDDA/GO)200/bmimCl/60%PA membranes, respectively. As reported, sulfonated poly(ether sulfone)/sulfonated graphene oxide membranes showed the increased membrane

Fig. 8. The mass variation of (PU/GO/PDDA/GO)200/60%PA and (PU/GO/ PDDA/GO)200/bmimCl/60%PA membranes at 100 °C with humidification for 96 h.

the prepared LBL self-assembly membranes, the leakage of PA and the absorbed water could cause the obvious downtrend, especially within the initial 20 h.

3.10. Proton conductivity Proton conductivity (σ) is a key factor in distinguishing PEMs materials, which correlates to the output power efficiency of PEMFCs. So the σ values of the prepared LBL self-assembly membranes were measured from 80 °C to 150 °C without humidification. Fig. 9 (A) showed

Fig. 9. Anhydrous proton conductivities of (PU/GO/PDDA/GO)200/PA membranes and (PU/GO/PDDA/GO)200/bmimCl/60%PA membranes as a function of temperature (A) and Arrhenius plots of these membranes (B). (PU/GO/PDDA/GO)200/PA membranes were prepared while (PU/GO/PDDA/GO)200 membranes were immersed into 40 wt%−85 wt% PA solution. 6

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membranes achieved the increment on E values with the addition of GO [62], (PU/GO/PDDA/GO)200 membranes actually showed the ordinary performance on E values. Although the polymers of PU and PDDA are in charge of mechanical property of the prepared LBL self-assembly membranes, they are always known as proton conduction carriers rather than stiff membrane supports. Moreover, (PU/GO/PDDA/GO)200 membranes with the thickness of 14 μm could not bear the exertion on force during the test. The reduction on E values and the invariance on ε values were observed in (PU/GO/PDDA/GO)200/PA membranes. As reported by H. Na [56] and J.C. Zhao [5], the strategies of crosslinking and grafting polymers were an effective to improve the stiffness of membranes. The separation of polymer backbones due to PA molecules could play a dominant role in the reduction of E values even if the stiffness was benefited from the compact and stable structure in LBL self-assembly membranes. In consideration of the rise on σ with PA at the expense of stiffness, the higher contents of macromolecule polymers could be given the priority to prepare anhydrous PEMs with LBL selfassembly technique in the followed work.

Fig. 10. Anhydrous proton conductivity of (PU/GO/PDDA/GO)200/60%PA membranes at 120 °C and 140 °C as a function of time. Table 1 Mechanical properties of the prepared LBL self-assembly membranes at RT. Membranes

Thickness/μm

Tensile stress/ MPa

Strain/%

(PU/GO/PDDA/GO)200 (PU/GO/PDDA/GO)200/40%PA (PU/GO/PDDA/GO)200/60%PA (PU/GO/PDDA/GO)200/bmimCl/ 60%PA (PU/GO/PDDA/GO)200/85%PA

14 23 30 48

2.65 2.28 1.01 0.73

26.4 20.1 18.1 21.0

49

0.41

4.70

4. Conclusion On the basis of the previously reported technique of solution casting to prepare anhydrous PEMs, this work proposed the strategy to construct (PU/GO/PDDA/GO)200 membranes with the LBL self-assembly technique through the alternative deposition of PU, GO and PDDA. The results have demonstrated that the PA doped membrane denoting (PU/ GO/PDDA/GO)200/60%PA with high and stable proto conductivity owing to the formed PA molecule chains and the decreased proton conduction resistance. Moreover, the LBL self-assembly membrane with GO possessed the strong ability to hinder methanol permeation with a methanol permeability of (0.366–1.25) × 10−7 cm2/s, revealing the improvement on the grade of PEMs with the ordered dispersion of components and the layered structure. The use of GO improved the proton conduction with capturing more PA molecules and showed a promise in the optimization of proton conduction–mechanical property. As a result of the fine performance on proton conductivity, components and mechanical stability, there was an optimistic prospect of LBL selfassembly technique providing a new strategy for the use of GO in the field of anhydrous PEMs. Further study on LBL self-assembly technique is under way to determine the universality of anhydrous PEMs with the fine performance owing to the well-ordered dispersion of GO with functional polymers.

conductivities of (3.5–5.80) × 10−2 S/cm at 30 °C as well as (8.21–16.5) × 10−2 S/cm at 90 °C with the decreased Ea values of 17.50–12.36 kJ/mol [46]. The results revealed that there was no obvious discrepancy on proton conduction resistance for (PU/GO/PDDA/ GO)200/85%PA, (PU/GO/PDDA/GO)200/60%PA and (PU/GO/PDDA/ GO)200/bmimCl/60%PA membranes. The improvement on σ values was primarily attributed to more proton carriers participating in proton conduction process. For (PU/GO/PDDA/GO)200/50%PA and (PU/GO/ PDDA/GO)200/40%PA membranes, the Ea values were confined by the increased resistance to proton conduction as a result of the relatively incomplete proton conduction network reflecting on higher Ea values. 3.11. Proton conductivity stability As well as the relation between σ values and temperature, the σ values stability was also measured for estimating the practical relevance of membranes. Specifically, (PU/GO/PDDA/GO)200/60%PA membranes as the recommended candidate were selected and the σ values at 120 °C and 140 °C were measured with the experiment lasting for 300 h. Fig. 10 showed the slight drop on σ values and the stable σ value of 1.47 × 10−1 S/cm at 120 °C and 1.83 × 10−1 S/cm at 140 °C were observed experiments were terminated voluntarily. In our view, the dimensional change of membrane samples contacting with Pt wires [24], PA leaking from membranes [60] and even the structure change led to the drop on σ values of (PU/GO/PDDA/GO)200/60%PA membranes. Similarly, the σ value has been reported to decrease from 1.3 × 10−2 S/cm to 1.1 × 10−2 S/cm at 130 °C PA doped membranes [61]. According to the results, it was inferred that the layered structure of membranes could improve the stability on σ values owing to the fine stability on the multilayered structure. The performance of (PU/GO/ PDDA/GO)200/60%PA membrane was satisfactory in a long-time σ values even at high temperature.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements We are grateful for the financial supports by the National Natural Science Foundation of China (21703029) and Natural science foundation of Liaoning province (20180550033). Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.eurpolymj.2019.109362. References

3.12. Mechanical strength Besides proton conduction, mechanical properties including E and ε have been frequently discussed to assesses membrane electrolytes. Measurements of the prepared LBL self-assembly were performed at RT and the results were summarized as in Table 1. Although the reported

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