Crosslinked anion exchange membranes with primary diamine-based crosslinkers for vanadium redox flow battery application

Journal of Power Sources 363 (2017) 78e86

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Crosslinked anion exchange membranes with primary diamine-based crosslinkers for vanadium redox flow battery application Min Suc Cha a, b, Hwan Yeop Jeong a, Hee Young Shin a, Soo Hyun Hong a, c, Tae-Ho Kim a, Seong-Geun Oh b, Jang Yong Lee a, *, Young Taik Hong a, ** a b c

Center for Membranes, Korea Research Institute of Chemical Technology, 141 Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul 04763, Republic of Korea Department of Polymer Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


The crosslinking between polymer chains are constructed by 4,40 diaminobenzophenone.
The crosslinked AEMs were fabricated by commercial polysulfone (PSU; Udel® P-3500).
The crosslinked AEM based on PSU indicated good chemical and dimensional stability.
The performance of crosslinked AEM showed high EE (86%) with high capacity retention.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 May 2017 Received in revised form 24 June 2017 Accepted 19 July 2017

A series of polysulfone-based crosslinked anion exchange membranes (AEMs) with primary diaminebased crosslinkers has been prepared via simple a crosslinking process as low-cost and durable membranes for vanadium redox flow batteries (VRFBs). Chloromethylated polysulfone is used as a precursor polymer for crosslinked AEMs (CAPSU-x) with different degrees of crosslinking. Among the developed  AEMs, CAPSU-2.5 shows outstanding dimensional stability and anion (Cl, SO2 4 , and OH ) conductivity. Moreover, CAPSU-2.5 exhibits much lower vanadium ion permeability (2.72  108 cm2 min1) than Nafion 115 (2.88  106 cm2 min1), which results in an excellent coulombic efficiency of 100%. The chemical and operational stabilities of the membranes have been investigated via ex situ soaking tests in 0.1 M VOþ 2 solution and in situ operation tests for 100 cycles, respectively. The excellent chemical, physical, and electrochemical properties of the CAPSU-2.5 membrane make it suitable for use in VRFBs. © 2017 Elsevier B.V. All rights reserved.

Keywords: Anion exchange membrane Primary amine crosslinker Redox flow battery Vanadium poly(sulfone)s

Abbreviations: VRFB, vanadium redox flow battery; AEM, anion exchange membrane; AEMFC, anion exchange membrane fuel cell; PEM, proton exchange membrane; PEMFC, polymer electrolyte membrane fuel cell; PSU, poly(sulfone)s; cmPSU, chloromethylated PSU; APSU, quaternized cmPSU; CMME, chloromethyl methyl ether; DABP, 4,40 -diaminobenzophenone; TPP, triphenylphosphine; EIS, electrochemical impedance spectroscopy; FT-IR, Fourier transform infrared; HFR, high frequency resistance; IEC, ion exchange capacity; WU, water uptake; CE, coulombic efficiency; VE, voltage efficiency; EE, energy efficiency; IPA, isopropyl alcohol; NMP, N-methyl-2-pyrrolidine; PTFE, polytetrafluoroethylene; RH, relative humidity; SAXS, small-angle X-ray scattering. * Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (J.Y. Lee), [email protected] (Y.T. Hong). http://dx.doi.org/10.1016/j.jpowsour.2017.07.068 0378-7753/© 2017 Elsevier B.V. All rights reserved.

M.S. Cha et al. / Journal of Power Sources 363 (2017) 78e86

1. Introduction Recently, with increased international interest in the development of sustainable and renewable energy technologies, ecofriendly and high-efficiency energy storage systems have received great attention. Among these energy storage systems, vanadium redox flow batteries (VRFBs) have been regarded as the most promising because of their outstanding efficiency, durability, and widespread availability [1e3]. Typically, VRFB systems consist of two external electrolyte tanks containing active materials, namely, 2þ 3þ VO2þ/VOþ redox couples as the catholyte and anolyte, 2 and V /V respectively, and a flow cell in which the redox reaction occurs [4]. In VRFBs, the membrane is a key component because it acts not only as a separator to restrict crossover of each active material, but also as a solid electrolyte to transport balancing ions for the overall redox reaction [5,6]. Therefore, a desirable electrolyte membrane in a VRFB requires low permeability of vanadium ions and high conductivity of balancing ions. The perfluorinated ionomer membrane Nafion® is the most widely used polymeric electrolyte membrane in VRFBs owing to its high proton conductivity and mechanical/chemical stability [7]. However, high cost and high vanadium ion crossover are regarded as critical problems that have to be resolved for practical use of VRFBs [8e10]. These limitations have brought about increased attention on alternative ionomer membrane materials with low cost, high performance, and high stability. Up to now, many alternative proton-conducting membranes that address the weaknesses of Nafion have been reported [11e13]. In particular, hydrocarbonbased cation exchange membrane (CEM) materials have been considered as the most effective solution for efficient nextgeneration membrane materials because of their low cost, ease of synthesis, structural diversity, and low permeability toward active materials. Nevertheless, CEMs are still limited by large dimensional variation and crossover of cationic active materials [14,15]. As a separator, anion exchange membranes (AEMs) are considered a more attractive alternative to the Nafion membrane than CEMs in VRFB applications because AEMs can fundamentally restrict the crossover of cationic active species by Donnan effects. Namely, vanadium ions do not penetrate AEMs because positively charged fixed ionic groups in the membranes repel positively charged vanadium ions. However, typical AEMs show low anion conductivity and a large degree of water uptake, which makes their utilization in VRFB applications difficult. Recently, the synthesis of block-type or rigidly structured AEM materials has been reported to improve their dimensional stability [16,17]. Although considerable improvement of dimensional stability was achieved, the complicated synthetic route is a critical drawback that results in increased material costs. Another synthetic strategy for increasing dimensional stability is interchain crosslinking, which is considered an efficient method owing to its simplicity [18e20]. The objective of this work was to develop a low-cost AEM with chemical, physical, and electrochemical durability for use in electrochemical applications. Thus, we report polysulfone-based AEMs crosslinked with a primary diamine-based crosslinker. Commercial polysulfone was used as a parent polymer for the development of low-cost materials. Moreover, a simple crosslinking method was introduced for improved durability. Unlike tertiary diamine-based crosslinkers, primary diamine-based crosslinkers are considered effective materials for increasing the chemical and physical stabilities of membranes because the crosslinker acts only as a crosslinker without increasing the ion exchange capacity. A series of polysulfone-based crosslinked AEMs (CAPSU-x) were fabricated via a simple synthetic route: 1) chloromethylation of the polymer backbone, 2) crosslinking between a well-controlled chloromethylated polysulfone and a diamine-based crosslinker, and 3)

79

quaternization. During quaternization, the chloromethylated polysulfone-based crosslinked membranes were immersed in an organic solution of trimethylamine to swell the membranes and thus facilitate penetration of trimethylamine molecules into the membranes. In VRFB applications, the developed AEMs demonstrated remarkable coulombic efficiencies (CEs) of almost 100%, resulting from low vanadium ion permeability, and energy efficiencies (EEs) of 86%, which is comparable to that of the Nafion membrane. 2. Experimental 2.1. Materials PSU Udel® P-3500 (Mw ¼ 79,000 g mol1) was purchased from Solvay Advanced Polymer and dried at 80  C under vacuum prior to use. Zinc chloride (ZnCl2), chloromethyl methyl ether (CMME), 4,40 diaminobenzophenone (DABP), trimethylamine (TMA, 45% aqueous solution), and anhydrous N,N-dimethylacetamide (DMAc) were purchased from Sigma Aldrich and used as received. 1,1,2,2Tetrachloroethane (TCE) was purchased from TCI and used as received. All other reagents were used without further purification. 2.2. Membrane preparation 2.2.1. Chloromethylation of PSU The following procedure is representative for preparation of PSU with 1.0 of degree of chloromethylation (DC). In a round bottom flask, PSU Udel® (34 g) was completely dissolved in TCE (140 mL) and ZnCl2 (2.97 g) was added to the solution. After stirring vigorously for 30 min, CMME (23 mL) was added dropwise to the mixture using a pressure-equalizing dropping funnel, and then the reaction mixture was heated at 40  C for 3.5 h [21]. Subsequently, the solution was poured into excess methanol to precipitate chloromethylated PSU (cmPSU), which was then washed with methanol and deionized water several times and dried at 80  C in a vacuum oven overnight. 1H NMR (400 Hz, CDCl3), d (ppm): 1.73 (s, 6H), 4.56 (s, 2H), 6.83e7.00 (m, 3H), 7.04 (m, 4H), 7.17e7.29 (m, 3H), 7.39 (s, 1H), 7.88 (m, 4H). 2.2.2. Preparation of a non-crosslinked PSU-based AEM (APSU-0) To prepare a non-crosslinked PSU-based AEM (APSU-0), cmPSU (15 g) was dissolved in DMAc (110 mL) in a round bottom flask. TMA (28 mL) was added to the solution dropwise through a pressureequalizing dropping funnel, and the solution was then stirred for 24 h. The reaction mixture was poured into excess deionized water to precipitate the aminated polymer. The precipitate was then washed with deionized water several times and dried at 80  C in a vacuum oven overnight. To prepare the membrane, the aminated polymer (2.5 g) was dissolved in DMSO (10 mL) [18,22]. The solution was filtered through a 5 mm PTFE membrane, cast onto a glass plate using a doctor blade, and dried in an oven at 80  C for 6 h. The membrane was detached from the glass plate by immersion in deionized water and then stored in deionized water prior to analysis. 1H NMR (400 Hz, DMSO-d6), d (ppm): 1.70 (s, 6H), 3.09 (d, 9H), 4.61 (d, 2H), 6.95e7.48 (m, 10H), 7.57e7.76 (d, 1H), 7.96 (m, 4H). 2.2.3. Preparation of crosslinked PSU-based AEMs (CAPSU-x) The crosslinked PSU-based membranes (CPSU-x, where x indicates the molar percentage of crosslinker added per chloromethyl groups) were prepared by in situ crosslinking of cmPSU with DABP. CPSU-1 was fabricated by dissolving cmPSU (1 g, 2.04 mmol of repeat units) in DMAc (10 mL) and filtering the solution through a PTFE membrane. DABP (4.3 mg, 0.02 mmol) was added to the filtrate and dissolved completely by stirring [18,21,22]. The solution

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was cast onto a glass plate of about 10  10 cm2 in size using a silicone mold, and dried at 80  C for 20 h. The membrane was detached from the glass plate by immersing in deionized water, followed by treatment in a 1 M TMA organic solution (acetone:EtOH ¼ 50:50, v/v) for 24 h. Finally, the aminated membrane (CAPSU-1) was thoroughly washed with deionized water and stored in 0.5 M NaCl aqueous solution prior to analysis. 2.3. Characterization 1

H NMR spectra were obtained on a Bruker AVANCE III 400 spectrometer at 400.13 MHz using CDCl3 (d ¼ 7.27 ppm) or DMSOd6 (d ¼ 2.50 ppm) as a solvent and tetramethylsilane (TMS) as an internal reference. FT-IR spectra were obtained on a Bruker ALPHAP spectrometer. The gravimetric ion exchange capacity (IECw) was calculated by the back-titration method after soaking the membrane in 1 M NaOH aqueous solution for 24 h for ionic exchange. The membrane was then immersed in 0.01 M HCl aqueous solution for 24 h to exchange OH ions with Cl ions. The resulting solution was titrated with 0.01 M NaOH aqueous solution to neutralize residual Hþ ions. The membrane was washed with deionized water, dried at 80  C in a vacuum oven overnight, and weighed to determine the dry weight. The IECw was calculated using the following equation:

  IECW meq$g 1 ¼ CNaOH  DVNaOH =Wd ; where CNaOH, DVNaOH, and Wd are the concentration of NaOH aqueous solution, the volume of NaOH aqueous solution consumed, and the weight of the dried membrane, respectively. The water uptake (WU) was determined by determining the weight difference between the wet and dried membrane. The swelling ratio (SW) was obtained from the differences in the thickness and in-plane direction for the fully hydrated and dried membrane. The WU and SW were calculated using the following equations:

WUð%Þ ¼ ðWw  Wd Þ=Wd  100%;

SAXS analyses were performed at the PLS-II 3C beamline of the Pohang Accelerator Laboratory (PAL) in Korea. The scattering vector was calculated as follows:

q ¼ 4P=l sin 2 q where l is the wavelength of Cu-Ka radiation (0.154 nm) and 2q is the total scattering angle. The average ion cluster domain space (d) was calculated using the following equation:

d ¼ 2q=qmax ; The VO2þ permeability was evaluated using a membraneseparated diffusion cell that was filled with 2 M VOSO4 in 3 M H2SO4 aqueous solution in the left cell and 2 M MgSO4 in 3 M H2SO4 aqueous solution in the right cell. The amount of VO2þ in the MgSO4 solution was detected using UVeVis spectroscopy (Agilent Cary 8454). The VO2þ permeability was calculated using the following equation: V  dCt/dt ¼ A  P/L  (C0 - Ct), Where P is the permeability of VO2þ, V is the volume of MgSO4 solution, Ct is the concentration of VO2þ in the MgSO4 solution at time t, and A and L are the effective area and thickness of the membrane, respectively. To evaluate VRFB single-cell performance, a cell was fabricated by sandwiching the membrane between two carbon felt electrodes with an active area of 49 cm2, which were clamped with two graphite bipolar plates. The electrolyte was 1.65 M V3.5þ in 3 M H2SO4aqueous solution, and the volume and feed rate of the electrolyte was 80 mL and 60 mL min1respectively. Chargeedischarge tests were performed using an automatic battery cycler (WonATech WBCS3000M2) at a constant current density of 50 mA cm1 with a maximum voltage of 1.7 V for charging and minimum voltage for discharging. The coulombic efficiency (CE), voltage efficiency (VE), and energy efficiency (EE) were calculated using the following equations: CE (%) ¼ td/tc  100%,

where Wd and Ww are the weights of the dried and wet membrane, respectively.

VE (%) ¼ Vd/Vc  100%,

Dt ¼ ðtw  td Þ=td ;

EE (%) ¼ CE  VE,

Dl ¼ ðlw  ld Þ=ld ;

where td and Vd are the discharging time and voltage of the discharged cell, respectively, whereas tc and Vc are those of the cell in the charged state. The chemical stability of the membrane was examined by immersing the dried membrane (1  3 cm2) in 0.1 M VOþ 2 in 2þ 5 M H2SO4 at 40  C. The reduction of VOþ in the solution 2 to VO was detected using UVeVis spectroscopy (Agilent Cary 8454).

where td and ld are the thickness and length of the dried membrane, respectively, and tw and lw are those of the membrane after water absorption. The in-plane ionic conductivity was obtained from AC impedance spectroscopy (Solatron1280, impedance/gain phase analyzer) using a four-probe conductivity cell over a frequency range of 0.1 Hze20 kHz at various temperatures (25e80  C) and 100% relative humidity. Before measuring the conductivity of OH or SO2 4 , the membrane was treated with 1 M NaOH aqueous solution or 1 M Na2SO4 aqueous solution for 24 h. The ionic conductivity was calculated using the following equation: ionic conductivity (mS$cm1) ¼ l/R  S, where l is the distance between electrodes, R the impedance of the membrane, and S is the cross-sectional surface area of the membrane. Small-angle X-ray scattering (SAXS) measurements were used to characterize the morphology of the crosslinked membranes.

3. Results and discussion 3.1. Synthesis of polymers As shown in Scheme 1, cmPSU is prepared via FriedeleCrafts alkylation of PSU with CMME as a chloromethylation reagent in the presence of ZnCl2 as a Lewis acid catalyst [23]. The chloromethylation degree obtained for cmPSU is approximately 1.0, as determined from the integral ratio in the 1H NMR spectrum (Fig. S1) between the proton peak of the chloromethyl group (d ¼ 4.5 ppm) and the proton peak of the diphenylsulfone moiety in the PSU backbone (d ¼ 7.9 ppm). The non-crosslinked ionomer

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81

Scheme 1. Structures of APSU-0 and CAPSU-x.

(APSU-0) is synthesized via a homogeneous quaternization reaction between cmPSU and trimethylamine. CPSU-x (x ¼ 0, 1, 2.5, 5, 10, or 15) membranes with different crosslinking degrees are fabricated via N-alkylation reactions between cmPSU and DABP, which is a crosslinker [24]. Several research groups have reported the preparation of crosslinked ionomers using diamine-based crosslinkers [25,26]. Diamine-based crosslinkers have been considered efficient crosslinking reagents owing to their outstanding reactivity. However, with tertiary diamine-based crosslinkers, which are among the most commonly used crosslinking reagents, increases in the crosslinking degree lead to increased ion transporting groups via quaternization reactions, as well as improvement of the interchain polymer network. Thus, excessive amounts of tertiary diamine-based crosslinkers could decrease both chemical and mechanical stability, as an increased crosslinking degree results in an increased IEC value, which decreases dimensional stability. In contrast, primary diamine-based crosslinkers only act as crosslinkers without increasing the IEC value. Based on our previous research, DABP was employed as the crosslinker in this study because of its suitable reactivity for film fabrication processes [27]. As shown in the FT-IR spectra (Fig. S2), a C¼O stretching vibration peak is observed at ~1642 cm1 after the crosslinking reaction [28,29], which indicates successful crosslinking reactions between chloromethyl groups on the PSU backbone and primary amines on DABP. However, no new peaks are observed at ~3400 cm1, corresponding to the OH vibration peak, which shows that DABP acts only as a crosslinker without quaternization, as expected. Subsequently, the crosslinked membranes were quaternized with trimethylamine, with the remaining chloromethyl groups functionalized using a Menshutkin reaction to produce quaternary ammonium cations [30,31]. The crosslinked membranes were swelled by immersion in an organic solution of trimethylamine to facilitate penetration of trimethylamine molecules into the membrane. After quaternization under various organic

solvent conditions, the Cl conductivities of the quaternized crosslinked membranes (CAPSU-x) were investigated. As shown in Table S1, as a mixed acetone:EtOH solvent is most suitable for quaternization, all of the CAPSU-x membranes were quaternized in this solvent. Compared with the FTIR spectrum of CPSU-2.5, a broad band around 3400 cm1 corresponding to the OH vibration peak of water bound to quaternary ammonium cations is observed in the spectrum of CAPSU-2.5, which suggests successful quaternization of the membranes (Fig. S2) [32,33]. 3.2. Dimensional change and ion conductivity The water uptake and dimensional change of the CAPSU-x membranes at room temperature and 80  C are listed in Table 1. The IEC values decrease with increasing crosslinking degree owing to the consumption of chloromethyl groups during the crosslinking reaction. As the IEC values decrease and the crosslinking degrees increase, the water uptake and degree of dimensional change decrease. However, interestingly, APSU-0, the non-crosslinked membrane, shows less water uptake and a lower degree of dimensional change than the other crosslinked membranes at 25  C. This behavior likely originates from the different quaternization method used to prepared the membranes. APSU-0 was prepared using a homogeneous quaternization method, whereas all of the crosslinked polymer membranes, CAPSU-1eCAPSU-15, were prepared using a heterogeneous quaternization method because of their poor solubility in organic solvents. Immersion of the crosslinked cmPSU membranes in a mixed acetone and ethanol solvent promotes the quaternization reaction by swelling the membranes. Although CAPSU-x were successfully prepared, this process it is likely to also result in a higher degree of water uptake and dimensional change owing to swelling of membranes by organic solvents. Nevertheless, the poor dimensional stability of APSU-0 at 80  C indicates that the dimensional stability of the noncrosslinked ionomer is intrinsically limited.

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Table 1 IEC, water uptake, and swelling ratio of the prepared AEMs at different temperatures. Membrane

APSU-0 CAPSU-1 CAPSU-2.5 CAPSU-5 CAPSU-10 CAPSU-15 a b c

IECa (meq$g1)

1.76c 1.99 1.91 1.79 1.56 1.34

IECb (meq$g1)

1.7c 1.95 1.74 1.50 1.32 1.14

25  C

80  C

Water uptake (%)

Swelling ratio (%)

Dl

Dt

Dv

38.7 80.6 60.3 59.7 38.7 34.4

11.1 22.7 18.3 17.3 10.9 8.9

5.7 11.2 11.4 12.6 6.6 7.5

30.5 67.4 55.9 55.0 31.1 27.5

Water uptake (%)

Swelling ratio (%)

Dl

Dt

Dv

dissolved 173.3 90.5 85.2 46.6 38.9

dissolved 42.5 27.5 26.0 14.2 11.4

dissolved 25.4 22.2 15.0 8.5 9.3

dissolved 154.6 98.7 82.6 41.5 35.6

Theoretical IEC values. Experimental IEC values determined by back-titration. Theoretical and experimental IEC values when DC is 0.85.

As shown in Table 1, the water uptake and dimensional change degree of the crosslinked membranes gradually decrease as the crosslinking degree increases. In the case of CAPSU-1, even though it is crosslinked, a relatively high water uptake is obtained, especially at 80  C (173%), which results from the low crosslinking degree and high IEC value of this membrane. With the exception of CAPSU-1, the crosslinked ionomer membranes show appropriate water uptake values for practical use of below 60%. In addition, these membranes might be applicable to fuel cell applications as an electrolyte membrane because they show relatively stable dimensional changes of below 100%. The anionic conductivities of the synthesized ionomer membranes were measured for chloride, sulfate, and hydroxide ions at various temperatures and 100% relative humidity. As indicated in Fig. 1, the anionic conductivities increase with increasing temperature in all of the membranes. Moreover, similar trends are observed for the anionic conductivities of the synthesized membranes for each anion. In spite of a relatively low IEC value, CAPSU2.5 has higher ion conductivity than CAPSU-1 at temperatures over 60  C. Even at lower temperatures of 25 and 40  C, the anionic conductivities of CAPSU-2.5 are comparable to those of CAPSU-1. This behavior likely originates from differences in water uptake and dimensional variation. As mentioned above, although CAPSU-1 has a higher IEC value than CAPSU-2.5, the water uptake and dimensional variation of these membranes are similar at room temperature. However, at 80  C, the water uptake and dimensional variation of CAPSU-2.5 show moderate increases, whereas those of CAPSU-1 exhibit considerable increases, which results in a decreased concentration of ion conducting functional groups per unit volume of the membrane. Thus, compared with CAPSU-1, the relatively low dimensional variation of CAPSU-2.5 should result in better anionic conductivities. Interestingly, the non-crosslinked membrane APSU-0 has lower anionic conductivities than CAPSU-1 and CAPSU-2.5. Moreover, APSU-0 exhibits lower water uptake and anion conductivities than CAPSU-5, despite its higher IEC value. This behavior likely results from differences in morphology originating from different quaternization method. Thus, swelling of the crosslinked membranes during the heterogeneous quaternization reaction significantly influences water uptake, conductivity, and morphology properties. 3.3. Morphology The morphological features of the synthesized membranes were investigated by SAXS. The average distance between the hydrophilic and hydrophobic domains could be estimated from the SAXS peak (qmax). As shown in Fig. 2, the ionomer peak for CAPSU-10 is weak, and no ionomer peak is observed for CAPSU-15. These

morphological features originate from the increasingly amorphous nature of these membranes resulting from increased crosslinking. In contrast, APSU-0 and CAPSU-1, CAPSU-2.5, and CAPSU-5 have almost identical d-spacings (d ¼ ~12 nm), as calculated using the Bragg equation (d ¼ 2p/qmax, where qmax is the maxium peak position). However, as the ionomer peak intensity is too weak to identify the position of qmax clearly, phase separation between the hydrophilic and hydrophobic domains is not well developed. These morphological features are more strongly affected by polymer structure than by crosslinking degree because APSU-0, CAPSU-1, CAPSU-2.5, and CAPSU-5 exhibit similar SAXS spectra, despite their different crosslinking degrees.

3.4. Vanadium ion (VO2þ) permeability The VO2þ permeabilities of CAPSU-x and Nafion 115 for VRFB applications are demonstrated in Fig. S3. For high performance in VRFB systems, a membrane with low vanadium ion permeation is essential for high CE and a low self-discharge rate [34]. In this study, the VO2þ permeabilities were investigated by measuring the diffusion coefficient of each membrane using a diffusion cell and UVeVis spectroscopy. As shown in Fig. S3, the Nafion 115 membrane shows the highest degree of the permeation among the investigated membranes. The diffusion coefficients were calculated using the molar concentrations of VO2þ that permeate the membrane over time (Table 2). The diffusion coefficient of Nafion 115 is two orders of magnitude greater than those of CAPSU-x. The synthesized membranes show low diffusion coefficients from 7.18  108 cm2 min1 for CAPSU-1 to 2.26  108 cm2 min1 for CAPSU-5. Furthermore, no permeation of vanadium ions is observed for CAPSU-10 and CAPSU-15, even after 7 days, owing to Donnan exclusion effects, where repulsion by cationic functional groups in the AEMs prevent transportation of VO2þ, and the low degree of water uptake originating from the crosslinked polymer structure [5]. In general, a higher crosslinking degree leads to a lower diffusion coefficient in the synthesized membranes because the IEC value and water uptake decrease as the crosslinking degree increases. Similar to protons, as vanadium ions are transferred through water channels inside a membrane, controlling the size of ion transport channels is very important for preventing crossover of active ions. Crosslinking is regarded as an efficient method for controlling ion transport channels and enhancing the mechanical properties of ionomers by increasing intermolecular interactions between polymer chains. The above results indicate that crosslinking could induce improved intermolecular interactions, which reduces the VO2þ permeabilities of the CAPSU-x membranes.

Fig. 1. (a) Chloride ion conductivity, (b) sulfate ion conductivity, and (c) hydroxide ion conductivity of prepared AEMs.

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Fig. 2. SAXS profiles of APSU-0 and CAPSU-x.

Table 2 Vanadium ion permeabilities and cell efficiencies of CAPSU-x and Nafion 115. Membrane

VO2þ permeability (cm2$min1)

CAPSU-1 CAPSU-2.5 CAPSU-5 CAPSU-10 CAPSU-15 Nafion 115

7.18  2.72  2.26  None None 2.88 

108 108 108

106

CE (%)

VE (%)

EE (%)

98 100 100 100 e 96

89 86 80 76 e 89

87 86 81 77 e 86

3.5. Chemical stability The chemical stabilities of the synthesized membranes were investigated via ex situ soaking tests by determining the concentration of reduced VO2þ in 0.1 M VOþ 2 /5 M H2SO4 solution. The concentration of VO2þ, which is obtained by reduction of VOþ 2 , can be estimated quantitatively using UVeVis spectroscopy. The chemical stabilities of Nafion 115 and SPAES 50, which is one of the most typical hydrocarbon-based ion exchange membrane materials, were also evaluated as references. As shown in Fig. 3, among these membranes, SPAES 50 undergoes the fastest chemical degradation, whereas the CAPSU-x membranes show higher chemical stability. In particular, although CAPSU-1 and SPAES 50 have similar IEC values, CAPSU-1 shows much better oxidative stability than SPAES 50. Moreover, SPAES 50 was broken into tiny pieces after the chemical stability test, whereas the CAPSU-x membranes maintained their shape. Considering that CAPSU-1 and SPAES 50 have similar backbone structures, the better oxidative stability of CAPSU-1 originates from Donnan exclusion effects. The fixed cations in CAPSU-1 restrict penetration of VOþ 2 into the membrane, which reduces the frequency of chemical interactions between the ionomer material and the strong oxidizing agent (VOþ 2 ). In addition, the chemical stability of CAPSU-x improves as the crosslinking degree increases, owing to the decreased IEC values and water uptake. 3.6. Vanadium redox flow battery performance and durability The VRFB performance of single cells assembled with CAPSU-x (thickness: ~70 mm) or Nafion 115 (thickness: ~125 mm) was investigated at a current density of 50 mA cm2 for 30 cycles. Typically, the thickness of membrane has a great effect to the cell performance. Increase of membrane thickness leads to decrease of

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Fig. 3. Chemical resistance of Nafion 115, SPAES 50, and CAPSU-x to 0.1 M VOþ 2/ 5 M H2SO4 solution at 40  C over 500 h.

vanadium ions crossover and increase of membrane resistance, which results in increase of CE and decrease of VE, while decrease of membrane thickness gives rise to increase of vanadium ions crossover and decrease of membrane resistance, which results in decrease of CE and increase of VE. Therefore, if the membrane materials showed low vanadium permeability and outstanding chemical stability, thin membrane is considered to have more advantages than thick one in terms of cell efficiency and membrane cost. In this study, since the CAPSU-x membranes showed extremely low vanadium ion permeability and outstanding chemical stability, the CAPSU-x membranes were fabricated thinner than Nafion 115 for increase of cell efficiency. The chargeedischarge behaviors in the 1st and 30th cycle of the CAPSU-x membranes and Nafion 115 are depicted in Fig. S4 (a) ~ Fig. S4 (e). Furthermore, charge and discharge capacity values of the CAPSU-x membranes and Nafion 115 in every operation cycles were indicated in Fig. S4 (f). The initial discharge capacity of Nafion 115 was as high as 3.7 Ahr. Although the CAPSU-x membranes showed somewhat lower initial discharge capacities than that of Nafion 115, the CAPSU-1, 2.5, 5 membranes indicated outstanding initial discharge capacities over 3.2 Ahr. However, after 18th cycle, the CAPSU-2.5, 5 membranes showed higher charge and discharge capacities than those of the Nafion 115 membrane. Interestingly, the Nafion 115 membrane exhibited large gap between charge capacity and discharge capacity, furthermore, charge capacity and discharge capacity decreased continuously during 30 cycles operation. It is derived from crossover of vanadium ions, which affects decrease of CE and capacity retention. Whereas the CAPSU-2.5, 5 membranes displayed nearly no decline of capacities because of their low vanadium ions permeability, which resulted in high values of CE and capacity retention. The chargeedischarge curves of CAPSU-15 are not shown because incomplete chargeedischarge behavior was observed, originating from high membrane resistance. With the exception of the CAPSU-15 membrane, the CAPSU-x membranes show good stability over 30 cycles. Typically, a membrane with high vanadium ion crossover exhibits a lower CE. Loss of CE primarily occurs through capacity loss owing to crossover of active materials during chargeedischarge cycling. The CAPSU-x membranes exhibit almost perfect CE values (100%), with the exception of CAPSU-1 (98%), whereas Nafion 115 has the lowest CE (96%) among the measured membranes. This CE data is consistent with the result of the vanadium permeability tests. The VE is governed by ohmic resistance between the cathode and the anode, and typically, membrane resistance has the greatest

effect on the ohmic resistance in a cell. Thus, the VEs of the single cells correspond to the ionic conductivity of the membrane. As shown in Fig. 4, CAPSU-1 and Nafion 115 have the highest VEs (89%) among the membranes. CAPSU-2.5 also shows a high VE of 86%, but CAPSU-5 and CAPSU-10 have VEs of 80% and 76%, respectively. Namely, the membranes with higher ion conductivities exhibit higher VEs in VRFB single cells. The EE is defined as the product of CE and VE. Thus, because of its excellent VE, CAPSU-1 shows the highest EE (89%), despite having the lowest CE. CAPSU-2.5 also shows an outstanding EE of 86%, which is comparable to that of Nafion 115. In contrast, CAPSU-5 and CAPSU-10 show poor EEs of 81% and 77%, respectively, owing to their low VEs. The capacity retention of CAPSU-x and Nafion 115 during cell operation is shown in Fig. 4. In comparison with the discharge capacity in the 2nd cycle, those of CAPSU-2.5, CAPSU-10, and CAPSU-5 decrease to 272, 50, and 8 mA h1 in the 30th cycle, which correspond to losses of 8%, 2%, and 0.3%, respectively, whereas CAPSU-1 and Nafion 115 show clear capacity losses of 49% and 28%, respectively, after 30 cycles. Typically, capacity fade is caused by an imbalance of the vanadium ion concentration between the anode and the cathode owing to vanadium ion crossover. Thus, decreasing vanadium ion crossover, which is indicated by the VO2þ permeabilities of the membranes, results in lower capacity fade. However, the capacity decay of CAPSU-1 is higher than that of Nafion 115 despite the much lower vanadium ion permeability (diffusion coefficient) of CAPSU-1. This behavior may be caused by a combination of factors, such as water uptake by the membrane, electrolyte flow conditions, and SOC. Although CAPSU-1 shows the highest EE among the synthesized membranes, it is not a promising membrane for practical applications owing to high capacity decay. CAPSU-2.5 is regarded as the most suitable AEM for VRFBs because it exhibits a high EE, which is comparable to that of Nafion 115, as well as outstanding capacity retention. The cycling efficiencies of the VRFB single cell with CAPSU-2.5 at various current densities (50, 80, 100, 120, and 150 mA cm2) were investigated to compare the rate capabilities. Typically, operation at a high current density is desired to improve the economical and technical feasibility of VRFB systems, as high operating current densities lead to high output power densities, which allows reduced stack sizes and lowers the capital cost of VRFB systems. In this system, the CAPSU-2.5 membrane with a thickness of as little as 50 mm improves the VE because the membrane exhibits outstanding vanadium ion blocking properties. As shown in Fig. 5, the VRFB single cell with the CAPSU-2.5 membrane shows an excellent CE of 100% over the whole range of applied current densities. In contrast, the Nafion 115 membrane shows lower CEs. As the current density increases, the VEs of both membranes decrease. However, the Nafion 115 membrane exhibits a larger decrease than the CAPSU-2.5 membrane. The improved behavior of CAPSU-2.5 is a result of its low ohmic resistance, which originates from its outstanding ion conductivity and the thinness of the membrane. On applying a high current density of 150 mA cm2, the single cell with the CAPSU-2.5 membrane shows an outstanding EE of 73%, which is 7% higher than that of the single cell with the Nafion 115 membrane. A cyclic operation test of the single VRFB cell with the CAPSU-2.5 membrane was performed over 100 cycles to investigate the longterm operation durability of the membrane. The high frequency resistance (HFR) was measured every cycle to identify membrane degradation. As illustrated in Fig. 6, the VRFB cell with the CAPSU2.5 membrane exhibits an excellent CE of almost 100% up to the 100th chargeedischarge cycle, with the exception of the 3 cycles at the beginning of the test. Moreover, high VEs and EEs of 86%e87% are observed up to the 100th cycle, without any decreases. In

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Fig. 4. VRFB single-cell efficiency and discharge capacity retention for cells with CAPSU-x or Nafion 115 membranes under a current density of 50 mA cm2: (a) coulombic efficiency, (b) voltage efficiency, (c) energy efficiency, and (d) capacity retention.

Fig. 6. Long-term operation of a VRFB single cell with a CAPSU-2.5 membrane under a current density of 50 mA cm2. Fig. 5. Rate capabilities of CAPSU-2.5 and Nafion 115 under current densities of 50e150 mA cm2.

4. Conclusion

addition, no large change in the HFR is observed up to the 100th cycle, which indicates that the membrane and VRFB cell do not undergo chemical or physical degradation. This long-term operation data and outstanding cycling efficiency indicate the potential of the CAPSU-2.5 membrane for practical use in VRFB systems.

Polysulfone-based crosslinked AEMs (CAPSU-x) with a primary diamine-based crosslinker have been successfully developed. Among the developed membranes, CAPSU-2.5 shows outstanding dimensional stability and anion conductivity. The VRFB single cell with the CAPSU-2.5 membrane exhibits outstanding cell

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efficiencies, in particular, an excellent CE of 100%, resulting from the excellent vanadium ion blocking properties of the membrane. Moreover, no degradation of the CAPSU-2.5 membrane is observed during cyclic operation over 100 cycles. In addition, the fuel cell with the CAPSU-2.5 membrane shows a superb power density of 138 mW cm2. The outstanding chemical and electrochemical properties of the CAPSU-2.5 membrane indicate that this membrane is a suitable candidate for VRFB application. Acknowledgement This work was supported by the Energy Efficiency & Resources Core Technology Program (no. 20152010103210) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry & Energy, Republic of Korea. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.07.068. References [1] M. Rychcik, M. Skyllas-Kazacos, Characteristics of a new all-vanadium redox flow battery, J. Power Sources 22 (1988) 59e61. [2] Z. Yang, J. Zhang, M.C.W. Kintner-Meyer, X. Lu, D. Choi, J.P. Lemmon, J. Liu, Electrochemical energy storage for green grid, Chem. Rev. 111 (2011) 3577e3613. [3] M. Tanaka, K. Fukasawa, E. Nishino, S. Yamaguchi, K. Yamada, H. Tanaka, B. Bae, K. Miyatake, M. Watanabe, Anion conductive block poly(arylene ether) s: synthesis, properties, and application in alkaline fuel cells, J. Am. Chem. Soc. 133 (2011) 10646e10654. [4] X. Li, H. Zhang, Z. Mai, H. Zhang, I. Vankelecom, Ion exchange membranes for vanadium redox flow battery (VRB) applications, Energy Environ. Sci. 4 (2011) 1147e1160. [5] D. Chen, M.A. Hickner, E. Agar, E.C. Kumbur, Selective anion exchange membranes for high coulombic efficiency vanadium redox flow batteries, Electrochem. Commun. 26 (2013) 37e40. [6] C. Wu, S. Lu, H. Wang, X. Xu, S. Peng, Q. Tan, Y. Xiang, A novel polysulfoneepolyvinylpyrrolidone membrane with superior proton-to-vanadium ion selectivity for vanadium redox flow batteries, J. Mater. Chem. A 4 (2016) 1174e1179. [7] G. Merle, M. Wessling, K. Nijmeijer, Anion exchange membranes for alkaline fuel cells: a review, J. Membr. Sci. 377 (2011) 1e35. [8] J.R. Varcoe, P. Atanassov, D.R. Dekel, A.M. Herring, M.A. Hickner, P.A. Kohl, A.R. Kucernak, W.E. Mustain, K. Nijmeijer, K. Scott, T. Xu, L. Zhuang, Anionexchange membranes in electrochemical energy systems, Energy Environ. Sci. 7 (2014) 3135e3191. [9] Y.J. Wang, J. Qiao, R. Baker, J. Zhang, Alkaline polymer electrolyte membranes for fuel cell applications, Chem. Soc. Rev. 42 (2013) 5768e5787. [10] M.A. Hickner, A.M. Herring, E.B. Coughlin, Anion exchange membranes: current status and moving forward, J. Polym. Sci. Part B Polym. Phys. 51 (2013) 1727e1735. [11] Z. Yuan, X. Zhu, M. Li, W. Lu, X. Li, H. Zhang, A highly ion-selective zeolite flake layer on porous membranes for flow battery applications, Angew. Chem. Int. Ed. 55 (2016) 3058e3062. [12] S.G. Jo, T.H. Kim, S.J. Yoon, S.G. Oh, M.S. Cha, H.Y. Shin, J.M. Ahn, J.Y. Lee, Y.T. Hong, Synthesis and investigation of random-structured ionomers with highly sulfonated multi-phenyl pendants for electrochemical applications, J. Membr. Sci. 510 (2016) 326e337. [13] J.Y. Lee, D.M. Yu, T.H. Kim, S.J. Yoon, Y.T. Hong, Multi-block copolymers based on poly(p-phenylene)s with excellent durability and fuel cell performance,

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