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Covalent bonding-triggered pore-filled membranes for alkaline fuel cells
Journal of Membrane Science 597 (2020) 117776
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Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Covalent bonding-triggered pore-filled membranes for alkaline fuel cells Xinle Xiao a, b, d, Muhammad A. Shehzad a, Aqsa Yasmin a, Zhenghui Zhang c, Xian Liang a, Liang Ge a, Jianjun Zhang a, Liang Wu a, *, Tongwen Xu a, ** a
CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, 230026, PR China b Key Laboratory of MicroNano Powder and Advanced Energy Materials of Anhui Higher Education Institute, School of Chemistry and Materials Engineering, Chizhou University, Chizhou, 247000, PR China c College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Wolong Street 1638, Nayang, Henan, 473061, PR China d Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei, 230601, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Pore-filling Interfacial reaction Anion exchange membranes Fuel cell
Highly conductive and physico-chemically stable ion transport channels (ITCs) are desirable for numerous membrane-integrated sustainable technologies such as fuel cells, redox flow batteries, reverse electrodialysis, etc. However, a “trade-off” exists among the ion exchange capacity, conductivity, and stability of the ITCs. In this context, we hereby propose a facile “covalent bonding-triggered (CBT)” strategy for filling of pre-formed membrane pores and formation of CBT-adopted ITCs (CITCs) and high-performance anion exchange mem branes (AEMs). The resulting CITCs-integrated AEMs offer high hydroxide conductivity (39.8 mS cm 1 at 30 � C) at low ion-exchange capacity (0.97 mmol g 1) and excellent chemical stability (<6% conductivity loss after being treated in 2 M NaOH aqueous solution at 60 � C for 7 days) due to aligned interfacial reactive sites within the CITCs. Therefore, the fabricated AEMs further exhibit high fuel cell performance, reaching a peak power density of 315 mW cm 2 at 60 � C. Thus, the proposed CBT pore-filling strategy guides the design for the for mation of high-performance and durable ITCs, required for fabrication of efficient AEMs.
1. Introduction Ion-exchange membranes (IEMs) have long been used in a wide range of industrial processes such as seawater desalination, treatment of industrial effluents, and chlorine-alkali production, etc [1]. Recently, the IEMs have also been effectually used in several sustainable energy conversion and storage systems including fuel cells (FCs) [2,3], redox flow batteries (RFB) [4,5], and reverse electrodialysis (RED) [6]. Effectual ion transportation and high membrane durability are essential properties of IEMs for high electrochemical performances in such membrane-integrated processes [7,8]. Fast transport of ions is achiev able by controlling the polymer architecture to form well-defined ge ometries. For example, the anion-exchange membrane (AEM), which is an essential element in FC systems possess a typical architecture in which the flexible side chains (containing the ion-conducting groups, e. g. ammonium cations) are covalently bonded to the hydrophobic poly mer chains. Herein, presence of both the highly hydrophobic polymer backbone and flexible side chains of high hydrophilicity drive the phase
separation and develop distinctive ion-conducting hydrophilic domains, which overlap to form interconnected ion transport channels (ITCs). Although the increasing concentration of the ion-conducting groups in an AEM can significantly help to reach a nebulous “higher conductivity”, but at the detriment of membrane durability such as undesired excessive membrane swelling on required hydration value [9]. Moreover, the side-chain type copolymers based membranes generally possess biaxial distribution of the ITCs, where the through-plane than the in-plane directional distribution is always beneficial for high membrane perfor mance [7]. Therefore, fabrication of effectual ITCs and their controlled distribution (especially across the membrane) are highly desirable to boost the electrochemical fuel cell performance [10]. In this context, the pore-filling concept of porous scaffolds, con taining highly ordered mono-dispersed ionic channels have been attracting researchers to manipulate ions conduction [7,11–14]. The approach of pore-filling with ion conductive polyelectrolytes generates aligned nanoscopic channels as ion transport pathways within a robust and durable scaffold, which mechanically prevents excessive swelling of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Wu), [email protected] (T. Xu). https://doi.org/10.1016/j.memsci.2019.117776 Received 4 October 2019; Received in revised form 18 December 2019; Accepted 21 December 2019 Available online 23 December 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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Journal of Membrane Science 597 (2020) 117776
the filling copolymer [7,8,15,16]. Yamaguchi et al. firstly prepared the pore-filling membranes by filling poly(acrylamide-tert-butylsulfonic acid) (PATBS), a gel-type polyelectrolyte, into cross-linked high- density polyethylene and polyimide porous substrates [17]. Several re searchers have also performed external crosslinking of the polymer inside the pre-formed pores to suppress the excessive membrane swelling and polymer leaching from the pores [16,18–20]. Although the available studies on the pore-filling concept for the fabrication of membranes reach a remarkable goal of arraying ion-conductive polymer domains through the membrane. However, the orientation of ion-exchange groups inside the domains (real ionic conductive channel) is still under the mystery. To address this challenge, we hereby report a “covalent bondingtriggered (CBT) pore-filling strategy” to produced aligned ITCs, mainly through the membrane. In contrast to the conventional physical porefilling approach, hereby tertiary amine groups of the filling copolymer anchor with bromomethyl groups in the porous membrane substrate, especially at pore walls via the Menshutkin reaction. The produced quaternary ammonium (QA) groups at the inner surface of the pores build symmetrically aligned ion transport channels for fast anion con duction. Therefore, the influence of the CBT pore-filling on several essential membrane characteristics such as percentage swelling on membrane hydration, ionic conductivity, membrane morphology, and H2/O2 fuel cell performance is discussed in detail.
2.3. Fabrication of porous BPPO membranes as the substrate Purified BPPO polymer (28 wt %) was dissolved in NMP, and the resulting uniform solution was cast on a clean glass plate using an adjustable film applicator with a stainless-steel blade (Jiangyin Jiatu technology co. Ltd. China). Herein, the gap between blade and glass plate was set as 20 μm. Adopting a famous Loeb-Sourirajan (L-S) phase inversion process, the casted film was immediately immersed into a pure isopropanol solvent and kept at room temperature for 24 h. The fabri cated porous BPPO membranes were thoroughly washed with DI water and then dried at 80 � C to obtain flexible and opaque membrane as porous substrate of almost 60 μm thickness (checked using SEM). 2.4. Preparation of covalent bonding-triggered pore-filled membranes Rectangular pieces of the porous BPPO membranes were immersed in the prepared homogeneous solutions of poly (DMAEMA-co-MMA) copolymers in ethanol (10 wt%). The immersed membranes were removed from the solutions and rinsed with ethanol (three times) to remove unreacted copolymers followed by air drying at room temper ature. The resulting membranes were denoted as D4M1BPPO, D2M1BPPO, D1M1BPPO, and D1M2BPPO, containing D4M1, D2M1, D1M1, and D1M2 copolymers, respectively. 2.5. Membrane characterizations
2. Experimental
2.5.1. Ion exchange capacity (IEC) of the pore-filled membranes The IEC of the pore-filled membranes was tested by well-established titration method [24]. Briefly, the membranes were dried to a constant weight (Wdry) and immersed in 1 M NaCl aqueous solution for 24 h at room temperature (25 � 1 � C). The membrane samples containing chloride ions were frequently (at least 15 times) washed with DI water during the next 24 h to remove the free sodium/chloride ions. The chloride-containing membrane samples were then immersed in 0.5 M Na2SO4 aqueous solution for next 24 h. Finally, the released Cl ions from the membranes into the Na2SO4 solution were titrated using a 0.01 M AgNO3 aqueous solution with K2CrO4 as the indicator (colour changes from bright green to earthy yellow at the endpoint). The IEC values were calculated from the volume of AgNO3 consumed (V(AgNO3)) and the weight of dry membrane samples (Wdry) using the following equation.
2.1. Materials Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Asahi Kasei Co. Ltd.. Bromomethylated PPO (BPPO) was synthesized according to our previous study [21]. 1H Nuclear Magnetic Resonance (NMR) analysis indicates a mole percentage of 58% bromobenzyl (Ph-CH2Br) per repeat unit. Monomers including Methyl methacrylate (MMA, C5H8O2, 99%) and 2-(dimethylamino) ethyl methacrylate (DMAEMA, C8H15NO2, 98%) were purchased from Energy Chemicals Co. Ltd. (Shanghai, PR China). Sodium chloride (NaCl), sodium sulfate (Na2SO4), and silver nitrate (AgNO3) were analytical grade and supplied by Sinopharm Chemicals Reagent Co. Ltd. Azobis(isobutyronitrile) (AIBN, C8H12N4, 99%), solvents including ethanol, chloroform, n-hex ane, N-methyl-2-pyrrolidone (NMP), and isopropanol were purchased from Sinopharm Chemicals Reagent Co. Ltd. Deionized (DI) water was used throughout the experiments.
IEC mmol: g
1
�
¼
VðAgNO3 Þ � CðAgNO3 Þ Wdry
(1)
2.5.2. Water uptake (WU) and swelling ratio (SR) The rectangular pieces of the membrane samples were dried to a constant weight (Wdry) and measured their length (Ldry). The dried membrane samples were immersed in DI water for 24 h at room tem perature (25 � 1 � C) to hydrate fully. Surfaces of the hydrated mem brane samples were quickly wiped with dry tissue and immediately weighed (Wwet) and measured the length (Lwet). The water uptake and swelling ratio were calculated using equations (2) and (3), respectively. � Wwet Wdry WUð%Þ ¼ � 100% (2) Wdry
2.2. Synthesis of poly (DMAEMA-co-MMA) as pore-filling copolymers Poly (2-dimethylamino ethyl methacrylate-co-methyl methacrylate) (poly (DMAEMA-co-MMA)) copolymers were synthesized by the free radical solution polymerization reaction [22,23]. Typically, 3.54 mL MMA and different amounts of DMAEMA, keeping the DMAEMA/MMA molar ratio of 4:1, 2:1, 1:1, and 1:2 were dissolved in ethanol in a 500 mL round bottom flask equipped with a magnetic stirring bar to form 10 wt% solution. The initiator (AIBN, 0.5 wt% to the total monomer weight) was added into a uniform solution of the monomers. The reac tion mixture was stirred at 70 � C for 24 h under dry nitrogen atmo sphere. The products were purified by repeated dissolution in chloroform and subsequent precipitation in hexane. The obtained poly (DMAEMA-co-MMA) precipitate was finally dried in a vacuum oven at 50 � C for 48 h. The molar ratio of DMAEMA/MMA in the copolymers was determined by 1H NMR and found 3.5:1, 2.0:1, 1:1.1, and 1:2.4, which are almost equal to the theoretical values (4:1, 2:1, 1:1, and 1:2, respectively). Therefore, the synthesized copolymers were labelled as D4M1, D2M1, D1M1 and D1M2. The synthesized poly (DMAEMA- co-MMA) copolymers were dissolved in ethanol (10 wt %) and obtained homogeneous solutions.
Swelling Ratioð%Þ ¼
� Lwet Ldry � 100% Ldry
(3)
2.5.3. Evaluation of hydroxide conductivity Adopting a well-established hydroxide conductivity evaluation method [24], the membrane samples as rectangular strips (1 cm � 4 cm) were initially converted into hydroxide form by soaking in 1 M NaOH solution for 12 h followed by thorough rinsing with plenty of DI water for to remove free hydroxide from the membranes. The electrochemical impedance of each membrane sample was measured in the longitudinal 2
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Scheme 1. The design concept of the covalent bonding-triggered pore-filling strategy. (a) Schematic illustration of polymer indicating hydrophobic main chain as yellow cord and hydrophilic tertiary amine groups as blue grappling hooks. (b) Schematic design of the membrane channel containing bromomethyl groups as pink balls. (c) Menshutkin reaction process and schematic diagram representing the covalent bonding between tertiary amine groups of the filling polymer and bro momethyl groups of the pre-formed membrane channel. (d) Schematic diagram of the covalent bonding-triggered pore-filling process and ionic conduction process. The blue region represents the hydrophilic domains formed by the aggregation of the hydrated filling polymer on the membrane surface.
membrane direction at their fully hydrated conditions within a Teflon cell under alternating current (AC) stimuli. Notably, the Teflon cell (immersed in water) comprised of two inner potential-sensing electrodes (platinum wire, 1 cm apart) and two outer current-carrying electrodes (flat stainless steel strips, 2 cm apart). The samples were tested using Autolab PGSTAT 30 (Eco Chemie, Netherland) over the frequency range from 1 MHz to 100 Hz in the galvanostatic mode, keeping the AC amplitude of 0.1 mA. The resulting complex impedance data was recorded as typical Nyquist plots and the membrane resistance was taken at zero phase shift. The hydroxide conductivity (σ) of the mem brane samples was calculated using the following equation (4).
σ¼
D R�T�W
assemble the anion exchange membrane with the cell electrodes for fuel cell testing. Pt/Ru/C or Pt/C catalysts (60 w/w in terms of metal con tent) were thoroughly suspended in DI water, isopropanol, and QPPO ionomer solution using ultrasonication to obtain a well-dispersed cata lyst inks that will yield catalyst layers containing 20% w/w of ionomer and 80% w/w of catalyst. The as-prepared ink was sprayed onto a larger area of the gas diffusion layer (GDL, Toray TGP-H-060) using an air spray gun to prepare the GDEs. The calculated metal loading in the catalyst layer was 0.5 mg cm 2 for a geometric surface area of 12.25 cm2. All the prepared GDEs and the test membranes were immersed in 1 M aqueous NaOH solution for 12 h followed by thorough washing with DI water before preparation of the membrane electrode assemblies (MEAs). 850E Multi Range fuel cell test station (Scribner Associates, USA) in the galvanic mode at multiple temperatures was used for fuel cell testing. The fuel cell temperature was controlled at 60 � C with a flow rate of 1000 cm3 min 1 for both H2 and O2 under full humidification and no backpressure. The cell voltage at each current density was deter mined under steady power conditions.
(4)
where R is the membrane resistance (obtained from the impedance analysis), D is a distance between the potential sensing electrodes (1 cm), whereas T and W are the thickness and width of the tested mem brane samples, respectively.
2.5.6. Miscellaneous measurements 1 H NMR spectra for copolymers were recorded using a Bruker 510 instrument (400 MHz for 1H) in CDCl3 as NMR solvent. The numberaverage molecular weight (Mn) of the synthesized copolymers was determined using gel permeation chromatography (GPC, Waters 1515), equipped with a refractive index detector (Waters 2414) and a series of Styragel HR1, HR3, and HR5 (DMF) columns, with the eluent flow rate of 1.0 mL min 1. Surface and cross-sectional morphology of the mem branes was visualized using a scanning electron microscope (SEM, Sirion200, Netherlands). Topographical surface and phase images were also examined by Atomic force microscope in tapping mode (AFM, Veeco di-Innova SPM). The chemical structure of the membranes was
2.5.4. Alkaline stability test To evaluate the stability of the membranes in alkaline media, the membrane samples as rectangular strips (1 cm � 4 cm) were immersed in 2 M NaOH solution at 60 � C for increasing periods such as one day followed by thorough washing with DI water. The IEC values and hy droxide conductivity of the membrane samples were tested following the above-stated methods for seven consecutive days. 2.5.5. Single-cell electrochemical fuel cell performance Preparation of membrane electrode assembly (MEA) and fuel cell test method are the same as reported in our previous work [24]. Briefly, the catalysed gas diffusion electrode (GDE) method was adopted to 3
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Journal of Membrane Science 597 (2020) 117776
Fig. 1. Synthesis and compositional characterization of poly (DMAEMA-co-MMA) copolymers. (a) Route for synthesis of the copolymers. (b) Schematic illus tration of the copolymer indicating tertiary amine groups in the polymer structure as the blue hooks entrenched with the hydrophobic polymer cord as the main chain. (c) 1H NMR spectra of the D4M1, D2M1, D1M1, and D1M2 copolymers. m: n represents the molar ratio of DMAEMA: MMA in the polymers calculated by the 1H NMR spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
for optimal ion transport performance. Therefore, poly (DMAEMA- co-MMA) copolymers with a similar number of average molecular weight (Mn) are carefully synthesized by varying the amount of MMA and DMAEMA monomers. The corresponding 1H NMR spectra of these copolymers (Fig. 1c) show proton signal (δH) at 2.3 and 3.6 ppm, which ascribe to the methyl groups in both the DMAEMA and MMA units, thus indicate successful copolymerization with the typical properties as listed in Table 1.
Table 1 Copolymer composition (calculated from 1H NMR) and physical properties of the poly (DMAEMA-co-MMA) copolymers. Copolymer D4M1 D2M1 D1M1 D1M2
DMAEMA:MMAa 4:1 2:1 1:1 1:2
DMAEMA:MMAb
Mn
PDI
3.5:1 2.0:1 1:1.1 1:2.4
28018 26350 26234 20185
2.6 2.6 2.6 3.1
a
Molar ratio of monomers in the reaction solution. Molar ratio of DMAEMA and MMA units in the polymers determined by 1H NMR.
3.2. Successful fabrication of the CBT pore-filled membranes
b
BPPO porous substrate membranes, prepared via the Loeb-Sourirajan phase inversion process have a microporous spongey morphology, as shown in Fig. 2a and d. Notably, interfacial compatibility such as tuneable polarity difference between the filling polymer and the porous substrate is crucial for an effectual pore-filled membrane design. Pre dominant approaches usually use hydrophilic porous substrates (con taining ionic groups) to match the polarity with the filling polymers [25]. However, excessive swelling of such conventional pore-filled membranes (upon hydration) drives undesired leaching of the filled polymer out from the pores and causes membrane failure. Therefore, we have chosen a hydrophobic porous BPPO substrate (Fig. 2a and d) of the negligible swelling degree to immobilize the highly hydrophilic pore-filling poly (DMAEMA-co-MMA) copolymer (Fig. 1). Besides, the adopted Menshutkin reaction between bromomethyl groups of the BPPO substrate and tertiary amine groups of the filling copolymer further stabilize the filling polymer within the substrate pores. Besides, the produced quaternary ammonium groups (QA) within the cavities (dur ing the Menshutkin reaction) assembled at the pore walls and built a
recorded using X-ray photoelectron spectroscopy (XPS, Thermo ESCA LAB 250). 3. Results and discussion 3.1. Customized synthesis of poly (DMAEMA-co-MMA) copolymer as pore-filling material The poly (DMAEMA-co-MMA) copolymers were synthesized by conventional free-radical copolymerization of methyl methacrylate and 2-(dimethylamino)ethyl methacrylate [22]. The pendant tertiary amine groups anchor to the surface of cavities via the Menshutkin reaction with bromomethyl groups in the porous membrane substrate (generating quaternary ammonium cations in Scheme. 1c). Herein, DMAEMA/MMA molar ratio is a critical parameter, which drives the anchoring process as well as balance the hydrophobic/hydrophilic membrane characteristics 4
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Fig. 2. Morphological characterization of the membranes. (a) SEM surface micrograph of the porous BPPO mem brane substrate showing opening of the pores at the surface (inset shows high membrane opacity which is an indica tion of porous morphology). (b) SEM surface micrograph exhibiting a uniform distribution of the copolymer at the pore-filled D4M1BPPO membrane sur face. Inset showing high transparency of the membrane after the pore-filling process, as observable from the visibil ity of the university logo (which is invisible through porous BPPO mem brane). (c) EDX elemental mapping for D4M1BPPO membrane surface indi cating a uniform distribution of nitrogen element (which are in the quaternary ammonium group produced by the Menshutkin reaction and the unreacted tertiary amine group in the copolymer); it is a direct indication of the copolymer presence. (d) SEM cross-sectional micrograph of the unfilled BPPO mem brane showing highly porous morphology. (e) SEM cross-sectional micrograph of the D4M1BPPO mem brane indicating the formation of a dense membrane after the pore-filling process. (f) EDX elemental mapping for the cross-section of D4M1BPPO mem brane also showing a uniform distribu tion of nitrogen element of the produced quaternary ammonium and the unreac ted tertiary amine groups present in the copolymer (a direct indication of the copolymer presence). (g) AFM height image of the BPPO membrane showing smooth surface. (h) AFM height image of the D4M1BPPO membrane showing copolymer presence at the pore-filled membrane surface. (i) Schematic illus tration for the covalent bondingtriggered pore-filled membrane (the idea is extracted from the SEM and AFM analysis) indicating the filling copol ymer at the substrate surface and within the pores for the formation of CITCs.
continuous ion transport pathway alongside the cavities (Scheme 1c). The surface and cross-sectional SEM images of D4M1BPPO membrane show the successful filling of the substrate pores and producing the resulting pore-filled membrane (Fig. 2b and e). Briefly, Menshutkin re action between bromomethyl groups of the BPPO substrate and tertiary amine groups of the filling copolymer facilitates the permeation of the filling polymer into the pores, thus converting the porous substrate into the dense membranes (insets, Fig. 2a and b). The similar pore-filling phenomena have been observed in SEM images of D2M1BPPO, D1M1BPPO, and D1M2BPPO membranes, as given in the Supporting Information (Fig. S1). Typically, the pore-filled membrane exhibits patterned surface geometry (as shown in Fig. 2b and e). In addition to the Menshutkin reaction happening inside the pores, it also occurs on the membrane surface which contains a large amount of bromomethyl groups. Therefore, hydrated filling polymer chains tend to attach and aggregate onto the membrane surface. The polymer aggregate further shrinks to generate patterned geometry when being dehydrated. The SEM-EDX elemental mapping for nitrogen (a representative element of quaternary ammonium (QA) groups) at the surface (Fig. 2c) and within cross-section (Fig. 2f) of the D4M1BPPO membrane indicate uniform
distribution (negligible agglomeration) of the poly (DMAEMA-co-MMA) copolymer throughout the substrate membrane. Additionally, 3D AFM surface height images in Fig. 2g and h present change in surface morphology and roughness of the membranes after copolymer grafting (AFM height images of D2M1BPPO, D1M1BPPO and D1M2BPPO membranes are given in Supporting Information, Fig. S2). The benefit of the covalent bonding-triggered pore-filling strategy to membrane durability and the formation of ordered high-speed ITCs in the membrane can be confirmed by an interesting transparency-change phenomenon as observed for our pore-filled membranes during treat ment with ethanol. Fig. 3a shows that the dry D4M1BPPO membrane, which is transparent due to the filling of porous channels remained transparent after immersion in DI water at room temperature. Interest ingly, the membrane gradually became opaque after immersing in ethanol at room temperature within only 1 min (Fig. 3c). This is mainly due to the dissolution of filling copolymers within the porous channels in ethanol. However, the ethanol soaked membrane dramatically reverted to transparent after drying (Fig. 3d). We have confirmed the inversion of transparency in the membrane samples several times, which confirms perfect repeatability. It is certainly not surprising that the 5
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Journal of Membrane Science 597 (2020) 117776
Fig. 3. (a) A photograph of the dry D4M1BPPO membrane before immersing in DI water is transparent (the university logo is visible). (b) The photograph of the D4M1BPPO membrane after immersing in DI water is still transparent (the university logo is visible). (c) The photograph of the D4M1BPPO membrane after immersing in ethanol is opaque (the university logo is invisible). (d) The photograph of dried D4M1BPPO membrane after extraction from methanol is again transparent (the university logo is visible) and confirming inversion in the membrane states. (e) SEM cross-sectional micrograph of the BPPO porous membrane. (f) SEM cross-sectional micrograph of the D4M1BPPO membrane. (g) XPS-survey spectra of the D4M1 copolymer, substrate, and pore-filled membranes. (h) Highresolution XPS spectra of the membranes and copolymer indicating the presence of the N 1s peak in the binding energy region of 402-398 eV (as suggested with the black rectangle) demonstrates the appearance of the quaternary amine group after the Menshutkin reaction.
membrane became opaque after immersing in ethanol because of the dissolution of poly (DMAEMA-co-MMA) in ethanol (as described in the synthesis section) which is making the membrane porous (the reason behind opacity). However, reverting the membrane into transparent is very important and confirming the presence of the filling copolymer after removal of the pore-filled membrane from ethanol. This retrieval in the transparency of the ethanol soaked membrane after drying is sug gesting that the filling copolymer can dissolve in ethanol but cannot undergo diffusive loss from the porous substrate because of the inter facial chemical bonding (as illustrated in Scheme 1c and confirmed using XPS analysis in Fig. 3g). To further confirm this conclusion, We have conducted the comparing study on OH conductivity and IEC of the pore-filled membrane before and after ethanol treatment. As shown in Table S2 and Fig. S3, the membranes sustain their performances after 7 days of treatment in the ethanol, indicating that the membranes have high stability and do not undergo diffusive loss from the porous substrate. The thickness change of the membrane before and after the porefilling process (Fig. 3e and f) further supports our stance of covalent bonding-triggered pore-filling strategy. As highlighted above, the Men shutkin reaction between bromomethyl groups of the BPPO substrate
and tertiary amine groups of the filling copolymer occurs inside the pores. Poly (DMAEMA-co-MMA) copolymer containing a large number of tertiary amine groups acts as the macromolecular crosslinker to convert the porous structure into the dense membrane. Accordingly, the inter-crosslinking reaction can significantly decrease the thickness of the resultant membrane. Furthermore, XPS analysis indicates that the pro duced quaternary ammonium groups (QA) within the cavities (during the Menshutkin reaction) are expected to assemble at the pore walls and built a continuous ion transport pathway alongside the cavities. The XPS spectra of all the pore-filled membranes (Fig. 3g) show four distinctive peaks at binding energies of 533.1, 285.0, 182.0, and 69.0 eV which can be assigned to O 1s, C 1s, Br 3p3/2, and Br 3d, respectively. More importantly, the N 1s peak in the binding energy region of 402-398 eV (marked by the black rectangle) demonstrates the appearance of the quaternary ammonium groups after the Menshutkin reaction, which plays an essential role in the formation of CITCs. 3.3. IEC, water uptake, swelling ratio and conductivity The XPS analysis (Fig. 3g and h) and the membrane transparency change phenomena infer that the quaternary ammonium (QA) groups 6
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displays an almost linearly increasing trend, which exceeds up to 65 mS cm 1 at 60 � C. This result suggests that the ITCs constructed by the array of the covalent bonding-triggered QA groups at the interfacial region of the substrate pores are beneficial for fast ion conduction. To further evaluate this benefit, conductivities of Cl and HCO3 anions with much lower mobility relative to OH as a function of temperature was measured and given in Fig. 4c. The Cl conductivity of D4M1BPPO membrane at 30 � C is 25 mS cm 1 which is higher than our previously reported membranes (Cl conductivity of 16 mS cm 1 for AEMs bearing densely grafted, dual cation side chains with a IEC value of 1.94 mmol g 1) [47]. Thus, the ion transport channels constructed by covalent bonding-triggered pore-filling strategy dramatically improves the ionic conductivity.
Table 2 IEC, water uptake, swelling ratio, and conductivities (OH ) of the pore-filled membranes. Membranes
OH Conductivity (mS cm 1)
IEC (mmol g 1)
Water Uptake (%)
Swelling Ratio (%)
D4M1BPPO D2M1BPPO D1M1BPPO D1M2BPPO AMX [26]
39.8 32.7 28.3 26.5 12.0
0.97 0.92 0.90 0.87 1.70
45.5 43.7 35.7 30.1 32.0
15 12 8 5 2
are arrayed regularly within the interfacial region of the pore walls and the copolymers, thus triggering the formation of the continuous ion transport channels. The increased OH conductivity of the pore-filled AEMs at 30 � C (measured under a fully hydrated state using a fourelectrode AC impedance method) linearly relate with the IEC values (representing the amount of QA groups) and water uptake for these membranes (Table 2). Briefly, large content of the QA groups adsorbs more water molecules, thus facilitating fast OH transportation [24,27]. Consequently, the representative D4M1BPPO membrane provides a highest ever reported OH conductivity (39.8 mS cm 1, Fig. 4b) at a corresponding very low IEC value (0.97 mmol g 1), which compared with recently published and standard AEMs at different IEC value [28–46]. Moreover, its conductivity-temperature curve in Fig. 4a
3.4. High electrochemical single-cell performance It is well recognized that degradation of quaternary ammonium (QA)-type AEMs is unavoidable due to the OH attack to the QA groups via β-hydrogen elimination or direct nucleophilic substitution at α-car bon [48,49]. Therefore, in addition to the fast OH conduction, high alkaline stability is another desirable characteristic of the AEMs for their effective use in AEMFCs. Herein, alkaline stability of the pore-filled AEMs in terms of their loss in hydroxide conductivity (presented in Fig. 5a) is tested at 60 � C in 2 M NaOH aqueous solution for 7 days. The
Fig. 4. Anion transport characteristics of the pore-filled AEMs. (a, c, and d) The conductivities of OH , Cl , and HCO3 in the pore-filled membranes indicate a linear increase in conductivities with increased testing temperature from 30 to 60 � C. (b) A comparison for OH conductivity of D4M1BPPO membrane at 30 � C with recently published and standard commercial AEMs at different IEC values (details in Supporting Information, Table S1). 7
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Fig. 5. Chemical stability (as a decrease in hydroxide conductivity) of the pore-filled membranes. The membrane samples were immersed in 2 M NaOH at 60 � C for increased chemical treatment time. (a) The hydroxide conductivities measured at 25 � C exhibit high chemical stability of the representative D4M1BPPO membrane. A negligible decrease in OH conductivity (only 5.6% decrease) is observed after seven days of chemical treatment at 60 � C in 2 M NaOH solution. (b) The polarization and power density curves of a single-cell AEMFC indicating comparatively highest power output for the representative D4M1BPPO membranes.
conductivity-time relations in Fig. 5a indicate a prolonged decline in conductivity for the representative D4M1BPPO membrane (only 5.6% conductivity loss after 168 h), thus ensuring its high chemical stability. The high alkaline stability of D4M1BPPO is due to self-assembling of QA groups of the filling copolymer at the interface of the pore-walls within BPPO substrate (Scheme 1c). Besides, densely located cationic QA groups in the interface region between the porous matrix and the filling component (as illustrated in Scheme 1) in D4M1BPPO maintain higher water content within the self-assembled CITCs (inset of Fig. 5a) to dilute the local alkali concentration within the channels and mitigate the degradation of the QA groups. In contrast, the D1M2BPPO exhibited 30% conductivity loss after 168 h due to lesser DMAEMA content, which is insufficient for the formation of uniform CITCs and water uptake. The H2/O2 single-cell AEMFC test, conducted at 60 � C (Fig. 5b) indicates high power output (315 mW cm 2) for D4M1BPPO membrane, which is significantly higher (2.1 � ) than the power output using the D1M2BPPO membrane (150 mW cm 2). Briefly, the CBT pore-filled D4M1BPPO membrane offer highest ever reported hydroxide conductivity at the low IEC (Fig. 4b and Supporting Information Table 2), large water uptake to keep the ions hydrated while their transportation (Table 1), excellent alkaline stability (Fig. 5a), and significantly high AEMFC single-cell power output (Fig. 5b). Thus, the proposed strategy for filling of the pores within hydrophobic BPPO substrate using hydrophilic poly (DMAEMA-co-MMA) and self-assembling of the ion-conducting QA groups envisions construction of highly efficient and physico-chemically stable AEMs for their effectual use in sustainable electrochemical pro cesses such as AEMFCs, redox flow batteries (RFBs), and artificial photosynthesis (AP), etc.
CITCs and guides the design for fabrication of highly anion conductive and chemically stable AEMs for potential use in AEMFCs, RFBs, and AP devices.
4. Conclusions
Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117776.
Declaration of competing interest The authors declared that they have no conflict of interest to this work and warrant that this manuscript is not under consideration elsewhere. CRediT authorship contribution statement Xinle Xiao: Investigation, Methodology, Writing - original draft. Muhammad A. Shehzad: Investigation, Visualization. Aqsa Yasmin: Investigation, Visualization. Zhenghui Zhang: Formal analysis, Vali dation. Xian Liang: Investigation. Liang Ge: Validation. Jianjun Zhang: Investigation. Liang Wu: Supervision, Conceptualization, Writing - review & editing. Tongwen Xu: Supervision, Writing - review & editing. Acknowledgements This research was supported by National Key R&D Program of China (No. 2018YFB1502301), the National Natural Science Foundation of China (Nos. 21720102003, 21875233), Key Technologies R&D Program of Anhui Province (No. 18030901079). Appendix A. Supplementary data
Our results demonstrate that the proposed “covalent bondingtriggered pore-filled” an effective strategy for the construction of high ly conductive and durable ion transfer channels in AEMs. Consequently, 39.8 mS cm 1 OH conductivity at 30 � C and 65 mS cm 1 at 60 � C is achieved at only 0.97 mmol g 1 IEC. Also, excellent chemical stability (<6% of OH conductivity decayed throughout 168 h of alkaline treatment at 60 � C) further signify the importance of the proposed porefilling strategy in the fabrication of durable AEMs. The resulting CBT pore-filled AEMs also exhibited a significantly high peak power density of 315 mW cm 2 during single-cell AEMFC performance analysis. Thus, the CBT pore-filling strategy is highly beneficial for the construction of
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Covalent bonding-triggered pore-filled membranes for alkaline fuel cells Xinle Xiao a, b, d, Muhammad A. Shehzad a, Aqsa Yasmin a, Zhenghui Zhang c, Xian Liang a, Liang Ge a, Jianjun Zhang a, Liang Wu a, *, Tongwen Xu a, ** a
CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, 230026, PR China b Key Laboratory of MicroNano Powder and Advanced Energy Materials of Anhui Higher Education Institute, School of Chemistry and Materials Engineering, Chizhou University, Chizhou, 247000, PR China c College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Wolong Street 1638, Nayang, Henan, 473061, PR China d Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei, 230601, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: Pore-filling Interfacial reaction Anion exchange membranes Fuel cell
Highly conductive and physico-chemically stable ion transport channels (ITCs) are desirable for numerous membrane-integrated sustainable technologies such as fuel cells, redox flow batteries, reverse electrodialysis, etc. However, a “trade-off” exists among the ion exchange capacity, conductivity, and stability of the ITCs. In this context, we hereby propose a facile “covalent bonding-triggered (CBT)” strategy for filling of pre-formed membrane pores and formation of CBT-adopted ITCs (CITCs) and high-performance anion exchange mem branes (AEMs). The resulting CITCs-integrated AEMs offer high hydroxide conductivity (39.8 mS cm 1 at 30 � C) at low ion-exchange capacity (0.97 mmol g 1) and excellent chemical stability (<6% conductivity loss after being treated in 2 M NaOH aqueous solution at 60 � C for 7 days) due to aligned interfacial reactive sites within the CITCs. Therefore, the fabricated AEMs further exhibit high fuel cell performance, reaching a peak power density of 315 mW cm 2 at 60 � C. Thus, the proposed CBT pore-filling strategy guides the design for the for mation of high-performance and durable ITCs, required for fabrication of efficient AEMs.
1. Introduction Ion-exchange membranes (IEMs) have long been used in a wide range of industrial processes such as seawater desalination, treatment of industrial effluents, and chlorine-alkali production, etc [1]. Recently, the IEMs have also been effectually used in several sustainable energy conversion and storage systems including fuel cells (FCs) [2,3], redox flow batteries (RFB) [4,5], and reverse electrodialysis (RED) [6]. Effectual ion transportation and high membrane durability are essential properties of IEMs for high electrochemical performances in such membrane-integrated processes [7,8]. Fast transport of ions is achiev able by controlling the polymer architecture to form well-defined ge ometries. For example, the anion-exchange membrane (AEM), which is an essential element in FC systems possess a typical architecture in which the flexible side chains (containing the ion-conducting groups, e. g. ammonium cations) are covalently bonded to the hydrophobic poly mer chains. Herein, presence of both the highly hydrophobic polymer backbone and flexible side chains of high hydrophilicity drive the phase
separation and develop distinctive ion-conducting hydrophilic domains, which overlap to form interconnected ion transport channels (ITCs). Although the increasing concentration of the ion-conducting groups in an AEM can significantly help to reach a nebulous “higher conductivity”, but at the detriment of membrane durability such as undesired excessive membrane swelling on required hydration value [9]. Moreover, the side-chain type copolymers based membranes generally possess biaxial distribution of the ITCs, where the through-plane than the in-plane directional distribution is always beneficial for high membrane perfor mance [7]. Therefore, fabrication of effectual ITCs and their controlled distribution (especially across the membrane) are highly desirable to boost the electrochemical fuel cell performance [10]. In this context, the pore-filling concept of porous scaffolds, con taining highly ordered mono-dispersed ionic channels have been attracting researchers to manipulate ions conduction [7,11–14]. The approach of pore-filling with ion conductive polyelectrolytes generates aligned nanoscopic channels as ion transport pathways within a robust and durable scaffold, which mechanically prevents excessive swelling of
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Wu), [email protected] (T. Xu). https://doi.org/10.1016/j.memsci.2019.117776 Received 4 October 2019; Received in revised form 18 December 2019; Accepted 21 December 2019 Available online 23 December 2019 0376-7388/© 2019 Elsevier B.V. All rights reserved.
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the filling copolymer [7,8,15,16]. Yamaguchi et al. firstly prepared the pore-filling membranes by filling poly(acrylamide-tert-butylsulfonic acid) (PATBS), a gel-type polyelectrolyte, into cross-linked high- density polyethylene and polyimide porous substrates [17]. Several re searchers have also performed external crosslinking of the polymer inside the pre-formed pores to suppress the excessive membrane swelling and polymer leaching from the pores [16,18–20]. Although the available studies on the pore-filling concept for the fabrication of membranes reach a remarkable goal of arraying ion-conductive polymer domains through the membrane. However, the orientation of ion-exchange groups inside the domains (real ionic conductive channel) is still under the mystery. To address this challenge, we hereby report a “covalent bondingtriggered (CBT) pore-filling strategy” to produced aligned ITCs, mainly through the membrane. In contrast to the conventional physical porefilling approach, hereby tertiary amine groups of the filling copolymer anchor with bromomethyl groups in the porous membrane substrate, especially at pore walls via the Menshutkin reaction. The produced quaternary ammonium (QA) groups at the inner surface of the pores build symmetrically aligned ion transport channels for fast anion con duction. Therefore, the influence of the CBT pore-filling on several essential membrane characteristics such as percentage swelling on membrane hydration, ionic conductivity, membrane morphology, and H2/O2 fuel cell performance is discussed in detail.
2.3. Fabrication of porous BPPO membranes as the substrate Purified BPPO polymer (28 wt %) was dissolved in NMP, and the resulting uniform solution was cast on a clean glass plate using an adjustable film applicator with a stainless-steel blade (Jiangyin Jiatu technology co. Ltd. China). Herein, the gap between blade and glass plate was set as 20 μm. Adopting a famous Loeb-Sourirajan (L-S) phase inversion process, the casted film was immediately immersed into a pure isopropanol solvent and kept at room temperature for 24 h. The fabri cated porous BPPO membranes were thoroughly washed with DI water and then dried at 80 � C to obtain flexible and opaque membrane as porous substrate of almost 60 μm thickness (checked using SEM). 2.4. Preparation of covalent bonding-triggered pore-filled membranes Rectangular pieces of the porous BPPO membranes were immersed in the prepared homogeneous solutions of poly (DMAEMA-co-MMA) copolymers in ethanol (10 wt%). The immersed membranes were removed from the solutions and rinsed with ethanol (three times) to remove unreacted copolymers followed by air drying at room temper ature. The resulting membranes were denoted as D4M1BPPO, D2M1BPPO, D1M1BPPO, and D1M2BPPO, containing D4M1, D2M1, D1M1, and D1M2 copolymers, respectively. 2.5. Membrane characterizations
2. Experimental
2.5.1. Ion exchange capacity (IEC) of the pore-filled membranes The IEC of the pore-filled membranes was tested by well-established titration method [24]. Briefly, the membranes were dried to a constant weight (Wdry) and immersed in 1 M NaCl aqueous solution for 24 h at room temperature (25 � 1 � C). The membrane samples containing chloride ions were frequently (at least 15 times) washed with DI water during the next 24 h to remove the free sodium/chloride ions. The chloride-containing membrane samples were then immersed in 0.5 M Na2SO4 aqueous solution for next 24 h. Finally, the released Cl ions from the membranes into the Na2SO4 solution were titrated using a 0.01 M AgNO3 aqueous solution with K2CrO4 as the indicator (colour changes from bright green to earthy yellow at the endpoint). The IEC values were calculated from the volume of AgNO3 consumed (V(AgNO3)) and the weight of dry membrane samples (Wdry) using the following equation.
2.1. Materials Poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) was purchased from Asahi Kasei Co. Ltd.. Bromomethylated PPO (BPPO) was synthesized according to our previous study [21]. 1H Nuclear Magnetic Resonance (NMR) analysis indicates a mole percentage of 58% bromobenzyl (Ph-CH2Br) per repeat unit. Monomers including Methyl methacrylate (MMA, C5H8O2, 99%) and 2-(dimethylamino) ethyl methacrylate (DMAEMA, C8H15NO2, 98%) were purchased from Energy Chemicals Co. Ltd. (Shanghai, PR China). Sodium chloride (NaCl), sodium sulfate (Na2SO4), and silver nitrate (AgNO3) were analytical grade and supplied by Sinopharm Chemicals Reagent Co. Ltd. Azobis(isobutyronitrile) (AIBN, C8H12N4, 99%), solvents including ethanol, chloroform, n-hex ane, N-methyl-2-pyrrolidone (NMP), and isopropanol were purchased from Sinopharm Chemicals Reagent Co. Ltd. Deionized (DI) water was used throughout the experiments.
IEC mmol: g
1
�
¼
VðAgNO3 Þ � CðAgNO3 Þ Wdry
(1)
2.5.2. Water uptake (WU) and swelling ratio (SR) The rectangular pieces of the membrane samples were dried to a constant weight (Wdry) and measured their length (Ldry). The dried membrane samples were immersed in DI water for 24 h at room tem perature (25 � 1 � C) to hydrate fully. Surfaces of the hydrated mem brane samples were quickly wiped with dry tissue and immediately weighed (Wwet) and measured the length (Lwet). The water uptake and swelling ratio were calculated using equations (2) and (3), respectively. � Wwet Wdry WUð%Þ ¼ � 100% (2) Wdry
2.2. Synthesis of poly (DMAEMA-co-MMA) as pore-filling copolymers Poly (2-dimethylamino ethyl methacrylate-co-methyl methacrylate) (poly (DMAEMA-co-MMA)) copolymers were synthesized by the free radical solution polymerization reaction [22,23]. Typically, 3.54 mL MMA and different amounts of DMAEMA, keeping the DMAEMA/MMA molar ratio of 4:1, 2:1, 1:1, and 1:2 were dissolved in ethanol in a 500 mL round bottom flask equipped with a magnetic stirring bar to form 10 wt% solution. The initiator (AIBN, 0.5 wt% to the total monomer weight) was added into a uniform solution of the monomers. The reac tion mixture was stirred at 70 � C for 24 h under dry nitrogen atmo sphere. The products were purified by repeated dissolution in chloroform and subsequent precipitation in hexane. The obtained poly (DMAEMA-co-MMA) precipitate was finally dried in a vacuum oven at 50 � C for 48 h. The molar ratio of DMAEMA/MMA in the copolymers was determined by 1H NMR and found 3.5:1, 2.0:1, 1:1.1, and 1:2.4, which are almost equal to the theoretical values (4:1, 2:1, 1:1, and 1:2, respectively). Therefore, the synthesized copolymers were labelled as D4M1, D2M1, D1M1 and D1M2. The synthesized poly (DMAEMA- co-MMA) copolymers were dissolved in ethanol (10 wt %) and obtained homogeneous solutions.
Swelling Ratioð%Þ ¼
� Lwet Ldry � 100% Ldry
(3)
2.5.3. Evaluation of hydroxide conductivity Adopting a well-established hydroxide conductivity evaluation method [24], the membrane samples as rectangular strips (1 cm � 4 cm) were initially converted into hydroxide form by soaking in 1 M NaOH solution for 12 h followed by thorough rinsing with plenty of DI water for to remove free hydroxide from the membranes. The electrochemical impedance of each membrane sample was measured in the longitudinal 2
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Scheme 1. The design concept of the covalent bonding-triggered pore-filling strategy. (a) Schematic illustration of polymer indicating hydrophobic main chain as yellow cord and hydrophilic tertiary amine groups as blue grappling hooks. (b) Schematic design of the membrane channel containing bromomethyl groups as pink balls. (c) Menshutkin reaction process and schematic diagram representing the covalent bonding between tertiary amine groups of the filling polymer and bro momethyl groups of the pre-formed membrane channel. (d) Schematic diagram of the covalent bonding-triggered pore-filling process and ionic conduction process. The blue region represents the hydrophilic domains formed by the aggregation of the hydrated filling polymer on the membrane surface.
membrane direction at their fully hydrated conditions within a Teflon cell under alternating current (AC) stimuli. Notably, the Teflon cell (immersed in water) comprised of two inner potential-sensing electrodes (platinum wire, 1 cm apart) and two outer current-carrying electrodes (flat stainless steel strips, 2 cm apart). The samples were tested using Autolab PGSTAT 30 (Eco Chemie, Netherland) over the frequency range from 1 MHz to 100 Hz in the galvanostatic mode, keeping the AC amplitude of 0.1 mA. The resulting complex impedance data was recorded as typical Nyquist plots and the membrane resistance was taken at zero phase shift. The hydroxide conductivity (σ) of the mem brane samples was calculated using the following equation (4).
σ¼
D R�T�W
assemble the anion exchange membrane with the cell electrodes for fuel cell testing. Pt/Ru/C or Pt/C catalysts (60 w/w in terms of metal con tent) were thoroughly suspended in DI water, isopropanol, and QPPO ionomer solution using ultrasonication to obtain a well-dispersed cata lyst inks that will yield catalyst layers containing 20% w/w of ionomer and 80% w/w of catalyst. The as-prepared ink was sprayed onto a larger area of the gas diffusion layer (GDL, Toray TGP-H-060) using an air spray gun to prepare the GDEs. The calculated metal loading in the catalyst layer was 0.5 mg cm 2 for a geometric surface area of 12.25 cm2. All the prepared GDEs and the test membranes were immersed in 1 M aqueous NaOH solution for 12 h followed by thorough washing with DI water before preparation of the membrane electrode assemblies (MEAs). 850E Multi Range fuel cell test station (Scribner Associates, USA) in the galvanic mode at multiple temperatures was used for fuel cell testing. The fuel cell temperature was controlled at 60 � C with a flow rate of 1000 cm3 min 1 for both H2 and O2 under full humidification and no backpressure. The cell voltage at each current density was deter mined under steady power conditions.
(4)
where R is the membrane resistance (obtained from the impedance analysis), D is a distance between the potential sensing electrodes (1 cm), whereas T and W are the thickness and width of the tested mem brane samples, respectively.
2.5.6. Miscellaneous measurements 1 H NMR spectra for copolymers were recorded using a Bruker 510 instrument (400 MHz for 1H) in CDCl3 as NMR solvent. The numberaverage molecular weight (Mn) of the synthesized copolymers was determined using gel permeation chromatography (GPC, Waters 1515), equipped with a refractive index detector (Waters 2414) and a series of Styragel HR1, HR3, and HR5 (DMF) columns, with the eluent flow rate of 1.0 mL min 1. Surface and cross-sectional morphology of the mem branes was visualized using a scanning electron microscope (SEM, Sirion200, Netherlands). Topographical surface and phase images were also examined by Atomic force microscope in tapping mode (AFM, Veeco di-Innova SPM). The chemical structure of the membranes was
2.5.4. Alkaline stability test To evaluate the stability of the membranes in alkaline media, the membrane samples as rectangular strips (1 cm � 4 cm) were immersed in 2 M NaOH solution at 60 � C for increasing periods such as one day followed by thorough washing with DI water. The IEC values and hy droxide conductivity of the membrane samples were tested following the above-stated methods for seven consecutive days. 2.5.5. Single-cell electrochemical fuel cell performance Preparation of membrane electrode assembly (MEA) and fuel cell test method are the same as reported in our previous work [24]. Briefly, the catalysed gas diffusion electrode (GDE) method was adopted to 3
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Fig. 1. Synthesis and compositional characterization of poly (DMAEMA-co-MMA) copolymers. (a) Route for synthesis of the copolymers. (b) Schematic illus tration of the copolymer indicating tertiary amine groups in the polymer structure as the blue hooks entrenched with the hydrophobic polymer cord as the main chain. (c) 1H NMR spectra of the D4M1, D2M1, D1M1, and D1M2 copolymers. m: n represents the molar ratio of DMAEMA: MMA in the polymers calculated by the 1H NMR spectra. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
for optimal ion transport performance. Therefore, poly (DMAEMA- co-MMA) copolymers with a similar number of average molecular weight (Mn) are carefully synthesized by varying the amount of MMA and DMAEMA monomers. The corresponding 1H NMR spectra of these copolymers (Fig. 1c) show proton signal (δH) at 2.3 and 3.6 ppm, which ascribe to the methyl groups in both the DMAEMA and MMA units, thus indicate successful copolymerization with the typical properties as listed in Table 1.
Table 1 Copolymer composition (calculated from 1H NMR) and physical properties of the poly (DMAEMA-co-MMA) copolymers. Copolymer D4M1 D2M1 D1M1 D1M2
DMAEMA:MMAa 4:1 2:1 1:1 1:2
DMAEMA:MMAb
Mn
PDI
3.5:1 2.0:1 1:1.1 1:2.4
28018 26350 26234 20185
2.6 2.6 2.6 3.1
a
Molar ratio of monomers in the reaction solution. Molar ratio of DMAEMA and MMA units in the polymers determined by 1H NMR.
3.2. Successful fabrication of the CBT pore-filled membranes
b
BPPO porous substrate membranes, prepared via the Loeb-Sourirajan phase inversion process have a microporous spongey morphology, as shown in Fig. 2a and d. Notably, interfacial compatibility such as tuneable polarity difference between the filling polymer and the porous substrate is crucial for an effectual pore-filled membrane design. Pre dominant approaches usually use hydrophilic porous substrates (con taining ionic groups) to match the polarity with the filling polymers [25]. However, excessive swelling of such conventional pore-filled membranes (upon hydration) drives undesired leaching of the filled polymer out from the pores and causes membrane failure. Therefore, we have chosen a hydrophobic porous BPPO substrate (Fig. 2a and d) of the negligible swelling degree to immobilize the highly hydrophilic pore-filling poly (DMAEMA-co-MMA) copolymer (Fig. 1). Besides, the adopted Menshutkin reaction between bromomethyl groups of the BPPO substrate and tertiary amine groups of the filling copolymer further stabilize the filling polymer within the substrate pores. Besides, the produced quaternary ammonium groups (QA) within the cavities (dur ing the Menshutkin reaction) assembled at the pore walls and built a
recorded using X-ray photoelectron spectroscopy (XPS, Thermo ESCA LAB 250). 3. Results and discussion 3.1. Customized synthesis of poly (DMAEMA-co-MMA) copolymer as pore-filling material The poly (DMAEMA-co-MMA) copolymers were synthesized by conventional free-radical copolymerization of methyl methacrylate and 2-(dimethylamino)ethyl methacrylate [22]. The pendant tertiary amine groups anchor to the surface of cavities via the Menshutkin reaction with bromomethyl groups in the porous membrane substrate (generating quaternary ammonium cations in Scheme. 1c). Herein, DMAEMA/MMA molar ratio is a critical parameter, which drives the anchoring process as well as balance the hydrophobic/hydrophilic membrane characteristics 4
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Fig. 2. Morphological characterization of the membranes. (a) SEM surface micrograph of the porous BPPO mem brane substrate showing opening of the pores at the surface (inset shows high membrane opacity which is an indica tion of porous morphology). (b) SEM surface micrograph exhibiting a uniform distribution of the copolymer at the pore-filled D4M1BPPO membrane sur face. Inset showing high transparency of the membrane after the pore-filling process, as observable from the visibil ity of the university logo (which is invisible through porous BPPO mem brane). (c) EDX elemental mapping for D4M1BPPO membrane surface indi cating a uniform distribution of nitrogen element (which are in the quaternary ammonium group produced by the Menshutkin reaction and the unreacted tertiary amine group in the copolymer); it is a direct indication of the copolymer presence. (d) SEM cross-sectional micrograph of the unfilled BPPO mem brane showing highly porous morphology. (e) SEM cross-sectional micrograph of the D4M1BPPO mem brane indicating the formation of a dense membrane after the pore-filling process. (f) EDX elemental mapping for the cross-section of D4M1BPPO mem brane also showing a uniform distribu tion of nitrogen element of the produced quaternary ammonium and the unreac ted tertiary amine groups present in the copolymer (a direct indication of the copolymer presence). (g) AFM height image of the BPPO membrane showing smooth surface. (h) AFM height image of the D4M1BPPO membrane showing copolymer presence at the pore-filled membrane surface. (i) Schematic illus tration for the covalent bondingtriggered pore-filled membrane (the idea is extracted from the SEM and AFM analysis) indicating the filling copol ymer at the substrate surface and within the pores for the formation of CITCs.
continuous ion transport pathway alongside the cavities (Scheme 1c). The surface and cross-sectional SEM images of D4M1BPPO membrane show the successful filling of the substrate pores and producing the resulting pore-filled membrane (Fig. 2b and e). Briefly, Menshutkin re action between bromomethyl groups of the BPPO substrate and tertiary amine groups of the filling copolymer facilitates the permeation of the filling polymer into the pores, thus converting the porous substrate into the dense membranes (insets, Fig. 2a and b). The similar pore-filling phenomena have been observed in SEM images of D2M1BPPO, D1M1BPPO, and D1M2BPPO membranes, as given in the Supporting Information (Fig. S1). Typically, the pore-filled membrane exhibits patterned surface geometry (as shown in Fig. 2b and e). In addition to the Menshutkin reaction happening inside the pores, it also occurs on the membrane surface which contains a large amount of bromomethyl groups. Therefore, hydrated filling polymer chains tend to attach and aggregate onto the membrane surface. The polymer aggregate further shrinks to generate patterned geometry when being dehydrated. The SEM-EDX elemental mapping for nitrogen (a representative element of quaternary ammonium (QA) groups) at the surface (Fig. 2c) and within cross-section (Fig. 2f) of the D4M1BPPO membrane indicate uniform
distribution (negligible agglomeration) of the poly (DMAEMA-co-MMA) copolymer throughout the substrate membrane. Additionally, 3D AFM surface height images in Fig. 2g and h present change in surface morphology and roughness of the membranes after copolymer grafting (AFM height images of D2M1BPPO, D1M1BPPO and D1M2BPPO membranes are given in Supporting Information, Fig. S2). The benefit of the covalent bonding-triggered pore-filling strategy to membrane durability and the formation of ordered high-speed ITCs in the membrane can be confirmed by an interesting transparency-change phenomenon as observed for our pore-filled membranes during treat ment with ethanol. Fig. 3a shows that the dry D4M1BPPO membrane, which is transparent due to the filling of porous channels remained transparent after immersion in DI water at room temperature. Interest ingly, the membrane gradually became opaque after immersing in ethanol at room temperature within only 1 min (Fig. 3c). This is mainly due to the dissolution of filling copolymers within the porous channels in ethanol. However, the ethanol soaked membrane dramatically reverted to transparent after drying (Fig. 3d). We have confirmed the inversion of transparency in the membrane samples several times, which confirms perfect repeatability. It is certainly not surprising that the 5
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Fig. 3. (a) A photograph of the dry D4M1BPPO membrane before immersing in DI water is transparent (the university logo is visible). (b) The photograph of the D4M1BPPO membrane after immersing in DI water is still transparent (the university logo is visible). (c) The photograph of the D4M1BPPO membrane after immersing in ethanol is opaque (the university logo is invisible). (d) The photograph of dried D4M1BPPO membrane after extraction from methanol is again transparent (the university logo is visible) and confirming inversion in the membrane states. (e) SEM cross-sectional micrograph of the BPPO porous membrane. (f) SEM cross-sectional micrograph of the D4M1BPPO membrane. (g) XPS-survey spectra of the D4M1 copolymer, substrate, and pore-filled membranes. (h) Highresolution XPS spectra of the membranes and copolymer indicating the presence of the N 1s peak in the binding energy region of 402-398 eV (as suggested with the black rectangle) demonstrates the appearance of the quaternary amine group after the Menshutkin reaction.
membrane became opaque after immersing in ethanol because of the dissolution of poly (DMAEMA-co-MMA) in ethanol (as described in the synthesis section) which is making the membrane porous (the reason behind opacity). However, reverting the membrane into transparent is very important and confirming the presence of the filling copolymer after removal of the pore-filled membrane from ethanol. This retrieval in the transparency of the ethanol soaked membrane after drying is sug gesting that the filling copolymer can dissolve in ethanol but cannot undergo diffusive loss from the porous substrate because of the inter facial chemical bonding (as illustrated in Scheme 1c and confirmed using XPS analysis in Fig. 3g). To further confirm this conclusion, We have conducted the comparing study on OH conductivity and IEC of the pore-filled membrane before and after ethanol treatment. As shown in Table S2 and Fig. S3, the membranes sustain their performances after 7 days of treatment in the ethanol, indicating that the membranes have high stability and do not undergo diffusive loss from the porous substrate. The thickness change of the membrane before and after the porefilling process (Fig. 3e and f) further supports our stance of covalent bonding-triggered pore-filling strategy. As highlighted above, the Men shutkin reaction between bromomethyl groups of the BPPO substrate
and tertiary amine groups of the filling copolymer occurs inside the pores. Poly (DMAEMA-co-MMA) copolymer containing a large number of tertiary amine groups acts as the macromolecular crosslinker to convert the porous structure into the dense membrane. Accordingly, the inter-crosslinking reaction can significantly decrease the thickness of the resultant membrane. Furthermore, XPS analysis indicates that the pro duced quaternary ammonium groups (QA) within the cavities (during the Menshutkin reaction) are expected to assemble at the pore walls and built a continuous ion transport pathway alongside the cavities. The XPS spectra of all the pore-filled membranes (Fig. 3g) show four distinctive peaks at binding energies of 533.1, 285.0, 182.0, and 69.0 eV which can be assigned to O 1s, C 1s, Br 3p3/2, and Br 3d, respectively. More importantly, the N 1s peak in the binding energy region of 402-398 eV (marked by the black rectangle) demonstrates the appearance of the quaternary ammonium groups after the Menshutkin reaction, which plays an essential role in the formation of CITCs. 3.3. IEC, water uptake, swelling ratio and conductivity The XPS analysis (Fig. 3g and h) and the membrane transparency change phenomena infer that the quaternary ammonium (QA) groups 6
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displays an almost linearly increasing trend, which exceeds up to 65 mS cm 1 at 60 � C. This result suggests that the ITCs constructed by the array of the covalent bonding-triggered QA groups at the interfacial region of the substrate pores are beneficial for fast ion conduction. To further evaluate this benefit, conductivities of Cl and HCO3 anions with much lower mobility relative to OH as a function of temperature was measured and given in Fig. 4c. The Cl conductivity of D4M1BPPO membrane at 30 � C is 25 mS cm 1 which is higher than our previously reported membranes (Cl conductivity of 16 mS cm 1 for AEMs bearing densely grafted, dual cation side chains with a IEC value of 1.94 mmol g 1) [47]. Thus, the ion transport channels constructed by covalent bonding-triggered pore-filling strategy dramatically improves the ionic conductivity.
Table 2 IEC, water uptake, swelling ratio, and conductivities (OH ) of the pore-filled membranes. Membranes
OH Conductivity (mS cm 1)
IEC (mmol g 1)
Water Uptake (%)
Swelling Ratio (%)
D4M1BPPO D2M1BPPO D1M1BPPO D1M2BPPO AMX [26]
39.8 32.7 28.3 26.5 12.0
0.97 0.92 0.90 0.87 1.70
45.5 43.7 35.7 30.1 32.0
15 12 8 5 2
are arrayed regularly within the interfacial region of the pore walls and the copolymers, thus triggering the formation of the continuous ion transport channels. The increased OH conductivity of the pore-filled AEMs at 30 � C (measured under a fully hydrated state using a fourelectrode AC impedance method) linearly relate with the IEC values (representing the amount of QA groups) and water uptake for these membranes (Table 2). Briefly, large content of the QA groups adsorbs more water molecules, thus facilitating fast OH transportation [24,27]. Consequently, the representative D4M1BPPO membrane provides a highest ever reported OH conductivity (39.8 mS cm 1, Fig. 4b) at a corresponding very low IEC value (0.97 mmol g 1), which compared with recently published and standard AEMs at different IEC value [28–46]. Moreover, its conductivity-temperature curve in Fig. 4a
3.4. High electrochemical single-cell performance It is well recognized that degradation of quaternary ammonium (QA)-type AEMs is unavoidable due to the OH attack to the QA groups via β-hydrogen elimination or direct nucleophilic substitution at α-car bon [48,49]. Therefore, in addition to the fast OH conduction, high alkaline stability is another desirable characteristic of the AEMs for their effective use in AEMFCs. Herein, alkaline stability of the pore-filled AEMs in terms of their loss in hydroxide conductivity (presented in Fig. 5a) is tested at 60 � C in 2 M NaOH aqueous solution for 7 days. The
Fig. 4. Anion transport characteristics of the pore-filled AEMs. (a, c, and d) The conductivities of OH , Cl , and HCO3 in the pore-filled membranes indicate a linear increase in conductivities with increased testing temperature from 30 to 60 � C. (b) A comparison for OH conductivity of D4M1BPPO membrane at 30 � C with recently published and standard commercial AEMs at different IEC values (details in Supporting Information, Table S1). 7
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Fig. 5. Chemical stability (as a decrease in hydroxide conductivity) of the pore-filled membranes. The membrane samples were immersed in 2 M NaOH at 60 � C for increased chemical treatment time. (a) The hydroxide conductivities measured at 25 � C exhibit high chemical stability of the representative D4M1BPPO membrane. A negligible decrease in OH conductivity (only 5.6% decrease) is observed after seven days of chemical treatment at 60 � C in 2 M NaOH solution. (b) The polarization and power density curves of a single-cell AEMFC indicating comparatively highest power output for the representative D4M1BPPO membranes.
conductivity-time relations in Fig. 5a indicate a prolonged decline in conductivity for the representative D4M1BPPO membrane (only 5.6% conductivity loss after 168 h), thus ensuring its high chemical stability. The high alkaline stability of D4M1BPPO is due to self-assembling of QA groups of the filling copolymer at the interface of the pore-walls within BPPO substrate (Scheme 1c). Besides, densely located cationic QA groups in the interface region between the porous matrix and the filling component (as illustrated in Scheme 1) in D4M1BPPO maintain higher water content within the self-assembled CITCs (inset of Fig. 5a) to dilute the local alkali concentration within the channels and mitigate the degradation of the QA groups. In contrast, the D1M2BPPO exhibited 30% conductivity loss after 168 h due to lesser DMAEMA content, which is insufficient for the formation of uniform CITCs and water uptake. The H2/O2 single-cell AEMFC test, conducted at 60 � C (Fig. 5b) indicates high power output (315 mW cm 2) for D4M1BPPO membrane, which is significantly higher (2.1 � ) than the power output using the D1M2BPPO membrane (150 mW cm 2). Briefly, the CBT pore-filled D4M1BPPO membrane offer highest ever reported hydroxide conductivity at the low IEC (Fig. 4b and Supporting Information Table 2), large water uptake to keep the ions hydrated while their transportation (Table 1), excellent alkaline stability (Fig. 5a), and significantly high AEMFC single-cell power output (Fig. 5b). Thus, the proposed strategy for filling of the pores within hydrophobic BPPO substrate using hydrophilic poly (DMAEMA-co-MMA) and self-assembling of the ion-conducting QA groups envisions construction of highly efficient and physico-chemically stable AEMs for their effectual use in sustainable electrochemical pro cesses such as AEMFCs, redox flow batteries (RFBs), and artificial photosynthesis (AP), etc.
CITCs and guides the design for fabrication of highly anion conductive and chemically stable AEMs for potential use in AEMFCs, RFBs, and AP devices.
4. Conclusions
Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117776.
Declaration of competing interest The authors declared that they have no conflict of interest to this work and warrant that this manuscript is not under consideration elsewhere. CRediT authorship contribution statement Xinle Xiao: Investigation, Methodology, Writing - original draft. Muhammad A. Shehzad: Investigation, Visualization. Aqsa Yasmin: Investigation, Visualization. Zhenghui Zhang: Formal analysis, Vali dation. Xian Liang: Investigation. Liang Ge: Validation. Jianjun Zhang: Investigation. Liang Wu: Supervision, Conceptualization, Writing - review & editing. Tongwen Xu: Supervision, Writing - review & editing. Acknowledgements This research was supported by National Key R&D Program of China (No. 2018YFB1502301), the National Natural Science Foundation of China (Nos. 21720102003, 21875233), Key Technologies R&D Program of Anhui Province (No. 18030901079). Appendix A. Supplementary data
Our results demonstrate that the proposed “covalent bondingtriggered pore-filled” an effective strategy for the construction of high ly conductive and durable ion transfer channels in AEMs. Consequently, 39.8 mS cm 1 OH conductivity at 30 � C and 65 mS cm 1 at 60 � C is achieved at only 0.97 mmol g 1 IEC. Also, excellent chemical stability (<6% of OH conductivity decayed throughout 168 h of alkaline treatment at 60 � C) further signify the importance of the proposed porefilling strategy in the fabrication of durable AEMs. The resulting CBT pore-filled AEMs also exhibited a significantly high peak power density of 315 mW cm 2 during single-cell AEMFC performance analysis. Thus, the CBT pore-filling strategy is highly beneficial for the construction of
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