Bifunctional Crosslinking Agents Enhance Anion Exchange Membrane Efficacy for Vanadium Redox Flow Batteries.

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Bifunctional Crosslinking Agents Enhance Anion Exchange Membrane Efficacy for Vanadium Redox Flow Batteries Wenpin Wang, Min Xu, Shubo Wang, Xiaofeng Xie, Yafei Lv, and Vijay K. Ramani ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/am501540g • Publication Date (Web): 02 Jun 2014 Downloaded from http://pubs.acs.org on June 9, 2014

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ACS Applied Materials & Interfaces

Bifunctional Crosslinking Agents Enhance Anion Exchange Membrane Efficacy for Vanadium Redox Flow Batteries Wenpin Wang,† Min Xu,‡ Shubo Wang,‡ Xiaofeng Xie,*,‡ Yafei Lv,*,† and Vijay Ramani§ †

College of Materials Science and Engineering, Beijing University of Chemical Technology,

Beijing, 100029, China ‡

Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China

§

Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago,

IL, 60616, USA

ABSTRACT: A series of cross-linked fluorinated poly (aryl ether oxadiazole) membranes (FPAEOM) derivatized with imidazolium groups were prepared. Poly (N-vinylimidazole) (PVI) was used as the bifunctional cross-linking agent to: a) lower vanadium permeability, b) enhance dimensional stability, and c) concomitantly provide added ion exchange capacity in the resultant anion exchange membranes. At a molar ratio of PVI to FPAEOM of 1.5, the resultant membrane (FPAEOM-1.5 PVI) had an ion exchange capacity of 2.2 meq g-1, a vanadium permeability of 6.8×10-7 cm2 min-1, a water uptake of 68 wt.%, and an ionic conductivity of 22.0 mS cm-1, all at 25°C. The FPAEOM-1.5 PVI membrane showed high chemical stability in highly acidic and oxidizing vanadium solution. Single cells prepared with the FPAEOM-1.5 PVI membrane

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exhibited a higher coulombic efficiency (> 92%) and energy efficiency (> 86%) after 40 test cycles in vanadium redox flow battery. KEYWORDS: vanadium redox flow battery, cross-linked AEM, bifunctional crosslinker, energy storage, density functional theory (DFT) 1.

INTRODUCTION

Recently, there has been much attention focused on vanadium redox flow batteries (VFB) from the perspective of large-scale (gird scale) electric energy storage. VFBs have several promising characteristics including long cycle life, millisecond response times (when primed), modular design, and deep-discharge capability, the ability to separate power and energy considerations, and relatively low storage costs.1-3 As a key component of the VFB, ion exchange membranes (IEMs) are employed to prevent the crossover and intermixing of vanadium ions between the positive and negative electrolytes and to complete the circuit by facilitating ion transport.4,5 To achieve high energy efficiency and long cycle life, the IEM should be designed with high ionic conductivity, low vanadium permeability, excellent mechanical and dimensional stability, and high oxidative stability. All of this should be achieved at low cost. The IEMs traditionally used in VFBs are perfluorosulfonic acid based cation exchange membranes. Although these membranes exhibit both high proton conductivity and excellent chemical stability, the high vanadium ion permeability through these membranes has limited their progress (These membranes cannot effectively discriminate between various cationic species. Hence, they are as likely to transport the various vanadium cations as they are to transport protons). The evaluation of several other candidate IEMs for VFB applications showed that most could not satisfy all of the above requirements primarily because of their poor conductivity, low permselectivity, or


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poor stability in the vanadium electrolyte, especially in the presence of the highly oxidizing vanadium (V) species.6,7 Alternative IEM materials are therefore required as a matter of great urgency for the successful commercial development of VFBs. Cation exchange membranes habitually suffer from high crossover of vanadium ions through the membrane, leading to self-discharge and, consequently, low columbic efficiency.8-10 To counter this issue, anion exchange membranes (AEMs) can be deployed effectively. Since the AEMs have tethered cations in their structure, they engender columbic repulsion of vanadium cations (Donnan effect).11,12

AEMs with quaternary ammonium cations (perhaps the most

commonly used class of cations in AEMs) exhibit relatively low anion conductivity13,14 at low ion exchange capacities (IECs), and modest-to-poor thermal and chemical stability when their IEC is enhanced to boost conductivity.15-18 The higher the degree of amination, and consequently, IEC, the poorer the dimensional stability of these membranes. To resolve this trade-off, a crosslinking approach can be considered. Membrane properties can be effectively controlled by adjusting the extent of cross-linking.19-22 However, cross-linking can also result in a fine line between acceptable conductivity and acceptable mechanical stability unless an appropriate crosslinking agent is chosen. In our previous work, we had investigated a series of novel AEMs based on imidazolium cations. The physical and electrochemical properties of these AEMs were evaluated for alkaline fuel cell applications. The AEMs had high conductivity and ion exchange capacity, while their stability and performance needed improvement.23-27 We have also investigated other AEM variants, from the perspective of ionic conductivity and stability in alkaline solutions and have provided detailed mechanistic insights into the AEM backbone and cation degradation processes.28-32 In this study, we have synthesized a series of fluorinated poly (aryl ether


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oxadiazole) s membranes cross-linked with poly (N-vinylimidazole) (FPAEOM-x PVI) and derivatized with imidazolium cations. These membranes were prepared by quaternerization of bromomethylated poly (aryl ether oxadiazole) s (FPAEO-x BrTM) using 1-methylimidazole as the amination reagent and poly (N-vinylimidazole) (PVI) as the cross-linker. The novelty of this approach was that PVI served as a bifunctional crosslinker, to enhance dimensional and chemical stability, and simultaneously to enhance IEC, thereby facilitating acceptable ionic conductivity and circumventing the usual trade-off between conductivity and stability. The IEC, ionic conductivity, water uptake, oxidative stability, and vanadium ion permeability of these AEMs were measured in comparison with a Nafion 117® benchmark. The self-discharge rate, energy efficiency and columbic efficiency of single cells with an optimized cross-linked AEM (FPAEOM-1.5PVI) were measured across 40 charge-discharge cycles and contrasted against the Nafion® 117 benchmark. 2.

EXPERIMENTAL SECTION 2.1. Materials. Bromomethylated poly (aryl ether oxadiazole) (FPAEO- BrTM) was

synthesized from fluorinated poly (aryl ether oxadiazole)s containing tetramethyl groups (FPAEO-TM) as described in reference27; 99% pure 1-methylimidazole and 1-Vinylimidazole were purchased from Shanghai Jingchun Reagent Co. Ltd.; 99% pure dimethylacetamide (DMAc), ethyl acetate, N, N-dimethylformamide (DMF), 2，2′-azobisisobutyronitrile (AIBN) and benzene were purchased from Beijing Reagent Company. Vanadium sulfate was purchased from the fine chemical industry of Shenyang (China). All chemicals were used as received without further purification.


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Figure 1. (a) Synthesis of Poly (N-vinylimidazole) (PVI) and (b) Synthesis of FPAEOM-x PVI membranes.

2.2. Preparation of Poly (N-vinylimidazole) (PVI). PVI was synthesized by polymerization of 1-vinylimidazole using 2, 2′-azobisisobutyronitrile as an initiator. As shown in Figure 1a, 2.02 g 1-vinylimidazole was dissolved in 12 mL of benzene and polymerized with 0.10 g initiator at 75oC under a nitrogen atmosphere for 8 hours. The polymer was washed with ethyl acetate and


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precipitated as a white powder, which was collected by filtration followed by vacuum drying at 60oC for 24 hours.33 2.3. Preparation of cross-linked fluorinated poly (aryl ether oxadiazole) AEMs derivatized with imidazolium cations. As shown in Figure 1b, cross-linked AEM membranes (FPAEOM-x PVI) were prepared via amination of FPAEO-BrTM with 1-methylimidazole, followed by cross-linking using PVI as a cross linker. X refers to the theoretical molar ratio of PVI to FPAEOM. The degree of derivatization with 1-methylimidazole was determined by 1HNMR27 while the extent of crosslinking was determined by the gel fraction test. As an example, the procedure used to prepare FPAEOM-1.5PVI (which was subsequently identified to be the optimal formulation) was as follows: FPAEO-4 BrTM (0.35 g, 0.363 mmol) was dissolved in DMAc (4 mL) in a round-bottomed flask at room temperature to obtain a 10 wt. % solution. 1methylimidazole (0.0665 g, 0.8 mmol) was slowly added to this solution. The resultant mixture was stirred for 3 hours to obtain the casting solution (FPAEO-2.2 M, 2.2 refers to the theoretical molar ratio of 1-methylimidazole to FPAEOM); PVI (0.0512 g, 0.545 mmol) was then added and the mixture and stirred in for 5 minutes to promote crosslinking. This solution was cast onto a glass plate and dried at 40oC for 12 hours to complete the cross-linking reaction. Subsequently, the temperature was raised to 60oC and maintained until the residual solvent was removed. The membrane was peeled off from the glass plate to obtain the cross-linked FPAEOM-1.5 PVI AEM in the bromide counter ion form. To obtain the Cl- counter ion form of the membrane, the AEMs were immersed in 0.5 M NaCl solution for 48 hours and subsequently washed with deionized water several times. A representation of the cross-linked AEM is shown in Figure S1a (in the Supporting Information) and a photograph of the transparent FPAEOM-1.5 PVI membrane is shown in Figure S1b (in the Supporting Information).


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2.4. Characterization of membranes. 1H-NMR spectra were recorded at room temperature on a BRUKER ARX400 MHz spectrometer using tetrahydrofuran as the internal standard and dimethyl sulfoxide-d6 as the solvent. FT-IR spectra were obtained with a Shimadzu-FTIR-8400 Fourier Transform Infrared Spectrophotometer to confirm the synthesis of the copolymer and the AEM. The gel fraction method was used to characterize the degree of polymer cross-linking. A certain quantity of cross-linked membrane sample (m1) was added to DMAc, heated at 90oC under reflux for 24 hours, and then dried in a vacuum oven until constant weight was attained. The final weight (m2) was accurately measured. The gel fraction was calculated according to the following equation:

x = (1 −

m1 − m2 ) ×100% m1

(1)

Where m1 (g) and m2 (g) are the weight of the samples before and after treatment respectively. The mechanical properties (stress-strain) of the crosslinked AEMs were studied at room temperature using a SHIMADZU AG-I 1KN instrument at a strain rate of 5 mm/min. All the samples were cut to 10 mm×50mm dimensions. For each film, five replicates were tested.34 The oxidative stability of the AEMs was initially evaluated by immersing the membrane sample in Fenton's reagent (3% H2O2 containing 2 ppm FeSO4) at 80oC. The relative stability of the membranes was estimated from the time taken for the membranes to begin to break up into pieces.35 Membrane IEC was measured by titration.36 The Cl- form of the FPAEOM-x PVI membrane was immersed in a fixed volume of 0.1 M NaNO3 aqueous solution for 48 hours. The amount of


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replaced chloride ions was titrated against 0.05 M AgNO3 solution using K2CrO4 (10%) as an indicator. The IEC was calculated using the following equation:

IEC ( mmol × g −1 ) =

C AgNO3 × VAgNO3 Wd

(2)

Where VAgNO3 (mL) is the volume of the AgNO3 solution, CAgNO3 (mol L-1) is the concentration of AgNO3 solution, and Wd (g) is the weight of the dried membrane sample. The water uptake and swelling ratio of FPAEOM-x PVI membranes were determined using the following procedure. The membrane was first dried under vacuum at 80oC for 24 hours, following which its weight and dimensions were accurately measured. The membrane was then immersed into deionized water at a given temperature. After equilibration for 24 hours, the membrane was recovered, surface water bolted with filter paper, and weight and dimensions measured. The water uptake (Wu) and swelling ratio (Sr) were determined using the following equations: Wu ( % ) =

(Ww − Wd ) × 100%

S r (%) =

(Sw − Sd ) × 100% Sd

(3)

Wd

(4)

Where Ww (g) and Wd (g) are the weight of the wet and dry membrane sample respectively, Sd (cm2) and Sw (cm2) respectively are the area (length * breadth) of the membrane before and after water absorption. The area-specific resistance of the membranes was measured using the method described in Ref.5. All the membranes were soaked in 1.5 M VOSO4 and 3 M H2SO4 solution for one day before measurement. A conductivity cell (Figure S2a in the Supporting Information) was separated into two compartments, filled respectively with 1.5 M VOSO4 and 3 M H2SO4


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solutions. The effective area (S) of the cell was 3.14 cm2. R1 and R2, which respectively represent the electric resistance of the cell with and without the membrane, were measured by electrochemical impedance spectroscopy (EIS) using a Zahner IM6ex electrochemical working station with an AC perturbation of 10 mV over the frequency range 100 kHz to 100 mHz. R was calculated using the following equation: R (Ω ⋅ cm 2 ) = ( R1 − R2 ) S

(5)

Where S is the effective area of the ion exchange membrane (3.14 cm2). The ionic conductivity (σ) of the membrane was obtained by EIS using a Zahner IM6ex electrochemical working station with an AC perturbation of 10 mV and over the frequency ranges 100 Hz to 3 MHz. The membrane was immersed in deionized water for 24 hours before testing. As in the method described in Ref.37, the membrane sample was clamped between two Pt electrodes using two Teflon® blocks, placed in deionized water at 25oC and evaluated. The conductivity of the membranes was calculated using the following equation:

σ ( S / cm) =

d R×S

(6)

Where R (Ω) is the real impedance corresponding to zero phase angles in the impedance spectrum, d (cm) is the distance between two Pt electrodes, and S (cm2) is the cross-sectional area of the membrane orthogonal to l. The apparent activation energy (Ea) of the ion conduction process was calculated using the Arrhenius equation as:

Ea = −b × R

(7)

Where b is the slope of the linear regression of the ln (σ) vs. 1000/T plot, and R is the gas constant.


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The vanadium permeability was measured using a diffusion cell (Figure S2b in the Supporting Information) as described in Ref.38. The left side compartment of the cell was filled with 2 M VOSO4 in 3 M H2SO4 solution (solution A), while the right side compartment was filled with 2 M MgSO4 in 3 M H2SO4 solution (solution B) to equalize the ionic strengths and to minimize osmotic pressure effects. During the experiment, both cells were kept under mild stirring to avoid concentration polarization at the membrane surface. Samples from the right side compartment were collected at regular time intervals. The concentration of VO2+ in the samples was measured by using an UV-1750 UV-vis spectrometer. Vanadium permeability was calculated using the following equation:

VB dCt / dt = AP / L(C0 − Ct )

(8)

Where P is the permeability of VO2+, C0 is the VO2+ concentration in solution A, Ct is the VO2+ concentration in solution B at time t. VB, L and A are the volume of solution B, the thickness, and the effective area of the tested membrane, respectively. Cell self-discharge was tested using a computer controlled battery test system NEWARE BTS (5V, 6A). The apparatus used is shown in Figure S2c (in the Supporting Information). 1.5 M V2+/V3+ in 3.0 M H2SO4 solution and 1.5 M VO2+/VO2+ in 3.0 M H2SO4 solution were filled into the negative and positive half-cells, respectively. The self-discharge test was initiated at a state of charge (SOC) of 50% and a capacity of 3 Ah. The test was concluded when the open circuit voltage (OCV) fell below 0.8 V.39 The time taken to reach this OCV value was recorded for all the membranes tested. The chemical stability of crosslinked AEMs in highly oxidative acidic vanadium (V) was tested by immersing samples in 1.5M (VO2)2SO4 in 3M H2SO4 solution at 25oC for up to 30


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days. Post-immersion, the membrane samples were washed several times with deionized water and their key properties measured once again using the previously described methods. A VFB single cell was fabricated by sandwiching the desired membrane between two carbon felt electrodes; this assembly was clamped between two graphite monopolar plates. The structure of the VFB single cell is shown in Figure S3 (in the Supporting Information). 1.5 M V2+/V3+ in 3.0 M H2SO4 and 1.5 M VO2+/VO2+ in 3.0 M H2SO4 served as the negative and positive electrolytes, respectively, and were cyclically pumped through the corresponding half-cell. The active area of the cell was 75.6 cm2 and the volume of electrolyte solution was 100 mL in each half-cell. Charge-discharge tests were conducted using a NEWARE BTS (5 V 20 A) cycler at a constant current density of 40 mA/cm2. A maximum voltage of 1.65 V for charging and minimum voltage of 0.8 V for discharging were employed to avoid corrosion of the carbon felt and graphite plates. 40 charge-discharge cycles were performed. The columbic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) of the cell were calculated using the following equations:

CE (%) =

Qdis × 100% Qch

EE (%) =

Edis × 100% Ech

VE (%) =

EE × 100% CE

(9)

Where Qdis and Qch respectively were the cell’s discharge and charge capacity and Edis and Ech were the cell’s discharge and charge energy.


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RESULTS AND DISCUSSION 3.1. Polymer synthesis and membrane preparation. Bromomethylated poly (aryl ether

oxadiazole) was prepared using NBS as the bromination reagent and BPO as initiator. The 1HNMR spectrums for FPAEO-TM is shown in Figure S4a (in the Supporting Information), while that of FPAEO-BrTM is shown in Figure S4b (in the Supporting Information). In Figure S4a, the peaks at δ 6.89 were assigned to the protons of the Ar-H group (a), the peaks at δ 2.20 were attributed to the protons of the ph-CH3 group (b), and the peaks at δ 1.64 were attributed to the protons of the –C(CH3)2– group (c). In Figure S4b, new peaks were observed at δ 4.43-4.48 (assigned to the protons of ph-CH2Br (d)), the peaks at δ 2.20 (attributed to the protons of phCH3 (b)) decreased in size, and the peaks at δ 6.89 (assigned to the protons of Ar-H (a and a’)) split into three peaks at δ 6.88, δ 7.00 and δ 7.12. These changes unequivocally confirmed the bromination of the benzylmethyl sites in the FPAEO-TM. FT-IR spectroscopy was used to confirm the synthesis of PVI. The FT-IR spectrum of PVI is shown in Figure S5 (in the Supporting Information). Compared to the standard spectrum of Nvinylimidazole, the peaks for the –CH=CH2 groups (990 cm-1, 1660 cm-1) decreased, and new peaks appeared at 722, 2931, and 2875.4 cm-1. These were attributed to the vibration of the −(CH2-CH2)−n groups. The other characteristic peaks remained the same. This evidence confirmed that the C=C bond had indeed opened and that the synthesis of PVI was successful. The formation of quaternary ammonium groups within the cross-linked membranes was also confirmed by FTIR. The FT-IR spectra of non cross-linked and cross-linked membranes (FPAEO-2.2 M and FPAEOM-1.5 PVI membranes) are shown in Figure S6 (in the Supporting Information). Both membranes showed absorption bands at 2980-2840 cm-1 due to the –CH3 or –


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CH2– groups. The peaks at 1648 cm-1 and 1573 cm-1 were attributed to the vibrations of C=C in benzene and the vibrations of C=N in oxadiazole. The peak at 1163 cm-1 was attributed to the fluorine substituted benzene groups. Compared with Figure S6a, the peaks at 1225 cm-1 and 615 cm-1, which were assigned to the wagging vibration of –CH2Br groups and the stretching vibration of C-Br bonds respectively, were found to have disappeared in Figure S6b. Meanwhile, an absorption band around 722 cm-1 appeared, which was attributed to the −(CH2-CH2)−n groups in PVI. The peaks at 3433 cm-1 increased and was attributed to the stretching vibrations of N–H groups in the imidazole ring. These above observations confirmed the successful synthesis of the cross-linked FPAEOM-x PVI membrane. Cross-linking was also confirmed by gel fraction measurements, the results of which are reported in Table S1 (in the Supporting Information). With increasing molar ratio of PVI, the degree of crosslinking of the membrane, and hence the gel fraction, increased due to interactions and node formation between the FPAOEM and the PVI molecular chains. Above a molar ratio X=1.5, 100% crosslinking was achieved and any additional PVI addition could not further reinforce the membrane due to an absence of excess functional –CH2Br groups to promote additional crosslinking. The PVI in this case served both as the cross-linking agent and as the functional reagent promoting ionic conductivity, and all the above results demonstrated the successful synthesis of the FPAEOM-x PVI membrane. 3.2. Oxidative stability and mechanical properties. The oxidative stability of the membranes was initially investigated in a practical manner by measuring the time at which the membrane began to break apart into pieces after immersion into Fenton’s reagent (3% H2O2 containing 2 ppm FeSO4) at 80oC. The results obtained are shown in Table S1. All of the crosslinked membranes showed higher oxidative stability than the FPAOEM membrane. The


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oxidative stability gradually increased with increasing of PVI content. This enhancement in oxidative stability was attributed to the formation of an increasing number of strengthening cross-link nodes between the FPAOEM and the PVI chains. Above a molar ratio of 1.5, the gel fraction of FPAEOM-x PVI membranes remained almost unchanged, suggesting no further cross-linking. However, the oxidative stability decreased due to the higher water uptake and swelling (and hence higher ingress of the oxidizing medium into the membrane) engendered by the excess PVI (which also served to increase IEC). Hence, cross-linking with PVI decreased water uptake and improved chemical stability (by a factor of ca. 9) up until a molar ratio of PVI to FPAEOM of 1.5 was reached. The mechanical properties of all the membranes are also presented in Table S1. All the crosslinked membranes showed higher tensile strength and Young’s Modulus compared to the original FPAEOM membrane, confirming the positive effect of cross-linking with PVI. In particular, FPAEOM-1.5 PVI proved to be a robust membrane that exhibited a 7-fold higher Young’s modulus and tensile strength. It also exhibited a relatively lower elongation at break (21% vs. 32 % for FPAEOM-0.5 PVI and 26% for FPAEOM-1.0 PVI) due to increasing rigidity and lower chain flexibility with increasing cross-linking.

Figure 2. IEC, water uptake, and swelling ratio of FPAEOM-x PVI membranes at 25oC.


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3.3. IEC, water uptake and swelling ratio. The effect of PVI content on the IEC of the membrane is shown in Figure 2. As expected, membrane IEC increased from 1.86 to 2.44 meq g1

with increasing of PVI content (from FPAEOM-0.0 PVI to FPAEOM-2.0 PVI). The IEC is

known to have a high impact on ionic conductivity, water uptake (and hence dimensional stability and mechanical properties) and on long-term performance AEM-based devices.28 While high IEC and water uptake in AEMs can enhance ion conductivity up to a point, excessive water uptake results in unacceptable dimensional (in)stability and a lowering of conductivity due to gel formation and reduction in ionic mobility. The effect of PVI content on membrane water uptake and dimensional swelling ratio at 25oC are presented in Figure 2. The water uptake and swelling ratio of FPAEOM-x PVI membranes decreased from 128% to 68% and 32% to 7.5%, respectively, as the PVI molar ratio was increased up to 1.5 with a concomitant increase in extent of cross-linking. Cross-linking hinders chains mobility, resulting in more compact membranes with lower free volume and hence lower water uptake and superior dimensional stability.40-42 Above a PVI molar ratio of 1.5, no additional increase in cross-link density was achieved, but the added PVI did contribute to enhanced water sorption (more hydrophilic cation groups) and hence these membranes exhibited an upturn in swelling and in water uptake, rendering them less suitable for VFB applications.


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Figure 3. Conductivities of FPAEOM-x PVI membranes at different temperatures.

3.4. Area-specific resistance and ionic conductivity. The ionic conductivity of FPAEOM-x PVI membranes as a function of PVI content is shown in Figure 3. The conductivity of the AEMs increased with increasing PVI content, an excellent trend considering that most crosslinking approaches lead to a trade-off between water uptake, dimensional stability, and conductivity. Since the PVI used in this study served a bifunctional role, it simultaneously enhanced stability (by increasing crosslink density) and conductivity (by providing additional cation sites). All FPAEOM-x PVI membranes exhibited a room temperature ionic conductivity higher than 15 mS cm-1. The ionic conductivity of the membranes expectedly increased with temperature, consistent with the free-volume model.43,44 The calculated activation energy values (listed in Table S1) ranged from 11.80-15.05 kJ mol-1, which was consistent with values previously reported in the literature.45,46 The ionic conductivity of the FPAEOM-1.5 PVI membrane in deionized water was 42.5 mS cm-1 at 60oC which was similar to AEMs based on imidazolium salts described in the literature - Ref.45 (33.3 mS cm-1, at 30oC). In conjunction with the higher chemical and mechanical stability and lower vanadium permeability (discussed below), this represented an excellent membrane for VFB applications.


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Figure 4. Area-specific resistances (R) of FPAEOM-x PVI and Nafion117® membranes.

The area-specific resistance (R) has a profound influence on the single cell efficiency, especially the energy efficiency. As shown in Figure 4, the area-specific resistance of FPEOM-x PVI membranes gradually decreased with increasing PVI content. FPEOM-1.5 PVI membranes possessed a low R of about 2 Ω cm2. The R-value for Nafion 117® is also provided as a reference and was evaluated to be about 1.5 Ω cm2. While this value was lower, the FPEOM-1.5 PVI membrane exhibited significant advantages in terms of vanadium permeability as discussed below. Further increase in PVI content above a molar ratio of 1.5 resulted in R values approaching 1 Ω cm2, but the resultant membranes were not as mechanically or chemically stable and we do not believe they would be good candidates for VFBs.


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Figure 5. Permeability (P) of vanadium ions through FPAEOM-x PVI and Nafion117® membranes.

3.5. Vanadium permeability. The permeability (P) of vanadium ions through the various membranes is shown in Figure 5. The permeability values for FPAEOM-x PVI membranes were much lower than that through Nafion117®. As the molar ratio of PVI increased, the P decreased as a result of the significantly stronger Donnan exclusion effect between the fixed cations in the AEM and the vanadium ions. P through the FPAEOM-1.5 PVI membrane was an order of magnitude lower than that through the Nafion117® membrane (ca. 7 x 10-7 vs. 7 x 10-6 cm2 min1

). Above this ratio of PVI, an increase in vanadium permeability was seen due to enhanced

water uptake and swelling (discussed above), which provided additional transport pathways for the vanadium ions through the polymer matrix. 3.6. Self-discharge tests. The OCV is an important indicator of capacity loss in a VFB and reflects the imbalance in the concentration of vanadium ions between the positive and negative electrolytes caused by vanadium permeation. Self-discharge tests were conducted in a simulated cell by monitoring the decay in OCV with time with various membranes. As shown in Figure 6, the OCV of VFB single cells with Nafion117® and FPAEOM-x PVI membranes decreased


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slowly at first. After 40 hours, the OCV of the single cell with the Nafion117® separator dropped rapidly below 0.8 V, while the single cell prepared with the FPAEOM-1.5 PVI membrane could maintain a flat OCV decay curve for over 160 hours The much lower OCV decay rates of FPAEOM-x PVI membranes compared to Nafion117® provided added confirmation that these membranes could effectively exclude vanadium ions. Consistent with the trend in permeability, the self-discharge rate for the FPAEOM-x PVI increased significantly above a molar ratio of PVI of 1.5, again confirming a PVI molar ratio of 1.5 being optimal.

Figure 6. OCV decay in VFB cells with Nafion117® and FPAEOM-x PVI membranes.

3.7. Chemical (oxidative) stability. The advanced oxidative stability test was performed only for the FPAEOM-1.5 PVI membrane, as it was optimal in all other aspects. This membrane was tested again using the above described methods to evaluate the change in key properties after 30 days of immersion in highly oxidizing 1.5 M (VO2)2SO4 in 3 M H2SO4 solution (results shown in Table S2 (in the Supporting Information)). The thicknesses, IEC, ionic conductivity and the areaspecific resistance of FPAEOM-1.5 PVI membranes were nearly identical before and after soaking in this highly corrosive solution. This indicated that the FPAEOM-1.5 PVI membranes


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had excellent oxidative stability in vanadium (V) and that it was highly suitable for VFB applications. This test also revealed that the imidazolium cations were relatively stable in this oxidative acid solution. This is in contrast to the perceived instability of imidazolium cations in alkaline solutions. It has been reported in the literature that imidazolium cations are likely to be attacked by OH- and degraded through a ring-opening mechanism under alkaline conditions.47,48

3.8. VFB single cell performance. The optimal FPAEOM-1.5 PVI membrane showed outstanding overall performance and was chosen for VFB testing. Figure 7 and Table S3 (in the Supporting Information) show the results of the tests in terms of the coulombic efficiency, the voltage efficiency, the energy efficiency, and the durability of the ion-exchange membrane. As shown in Table S3, the CE, VE, and EE of the FPAEOM-1.5 PVI membrane were 93.5%, 92.5%, and 86.5%, respectively, which were comparable to Nafion 117® (91.5%，93%，85%) despite the lower ionic conductivity of these membranes. As shown in Figure 7, no change in efficiency was seen after 40 charge–discharge cycles, confirming that the durability of the membrane was promising for use in a VFB in the presence of highly oxidizing vanadium (V).


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Figure 7. VFB efficiencies obtained with a FPAEOM-1.5 PVI AEM during charge-discharge cycling (current density: 40 mA/cm2); (a) coulombic efficiency, (b) voltage efficiency and (c) energy efficiency.

4.

CONCLUSIONS

A series of cross-linked fluorinated poly (aryl ether oxadiazole)s membranes based on imidazolium salts (FPAEOM-x PVI) were prepared. As the amount of bifunctional (crosslinking + added IEC) agent (PVI) was increased, the IEC, dimensional stability, water uptake, mechanical properties, ionic conductivity, and vanadium permeability of the resultant AEMs were improved up until a PVI to FPAEOM molar ratio of 1.5. When the PVI content was increased beyond this optimal point, the resultant AEMs had higher water uptake, poorer dimensional stability and higher vanadium permeability. This trend was attributed to a maximization of crosslinking density at a PVI molar ratio of 1.5. Further addition of PVI increased IEC (and hence swelling in the absence of additional stabilizing crosslink nodes), and yielded AEMs with poorer mechanical properties. The FPAEOM-1.5 PVI AEMs had a vanadium permeability of 6.8×10-7 cm2 min-1, a water uptake of 68%, an ionic conductivity of 22.0 mS cm-1, an IEC of 2.2 meq g-1, and a tensile strength of 22.7 MPa at 25°C. It was stable upon immersion for 30 days in highly oxidizing vanadium (V) solution. In the self-discharge test, the single cell with the FPAEOM-1.5 PVI membrane lasted four times longer than the cell prepared with Nafion117®. The single cell VFB prepared with the optimized AEM showed high-energy efficiency (> 86%) and coulombic efficiency (> 92%), both comparable to the much more expensive Nafion117® separator. Based on the above results, we propose that the FPAEOM-1.5 PVI AEM has exceptional promise for VFB and other redox flow battery applications.


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ASSOCIATED CONTENT Supporting Information Structure and photograph of the membrane, device for measuring membranes, NMR spectra of polymer, FT-IR spectra and basic data of membranes. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86 10-6278-4827. * E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National High Technology R&D Program of China (2012AA053401), the National Natural Science Foundation of China (21176140) and State Key Laboratory of Automobile Safety and Energy (No. KF14162). VR would like to acknowledge the Hyosung S. R. Cho Endowed Chair for partially supporting this collaboration with Tsinghua University. REFERENCES (1) Rychcik, M.; Skyllas-Kazacos, M. Characteristics of a New All-Vanadium Redox Flow Battery. J. Power Sources 1988, 22, 59-67. (2) Shibata, A.; Sato, K. Development of Vanadium Redox Flow Battery for Electricity Storage. Power Eng. J. 1999, 13, 130-135.


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Cross-link points, and concomitantly providing added IEC


28

A zeolite ion exchange membrane for redox flow batteries.

Graphite Felts as Advanced Electrode Materials for Vanadium Redox Flow Batteries.

Vanadium Redox Flow Batteries Using meta-Polybenzimidazole-Based Membranes of Different Thicknesses.

The relationship between anion exchange and net anion flow across the human red blood cell membrane.

Bifunctional crosslinking ligands for transthyretin.

bromine redox flow batteries.

Redox-Flow Batteries: From Metals to Organic Redox-Active Materials.

Ferrocene and cobaltocene derivatives for non-aqueous redox flow batteries.

polyacrylonitrile acid-base blend membrane for vanadium redox flow battery application.

High-energy redox-flow batteries with hybrid metal foam electrodes.

Nanorod niobium oxide as powerful catalysts for an all vanadium redox flow battery.

An electrochemical quartz crystal microbalance study of a prospective alkaline anion exchange membrane material for fuel cells: anion exchange dynamics and membrane swelling.

Crosslinking of antibody molecules by bifunctional antigens.

Molecular dosimetry for sister-chromatid exchange induction and cytotoxicity by monofunctional and bifunctional alkylating agents.

Rechargeables: Vanadium batteries will be cost-effective.

TEMPO-based catholyte for high-energy density nonaqueous redox flow batteries.

The lightest organic radical cation for charge storage in redox flow batteries.

A solution-phase bifunctional catalyst for lithium-oxygen batteries.

Anion-exchange chromatography of phosphopeptides: weak anion exchange versus strong anion exchange and anion-exchange chromatography versus electrostatic repulsion-hydrophilic interaction chromatography.

A radiometric anion-exchange method for acetylcholinesterase.

Highly conductive anion exchange membrane for high power density fuel-cell performance.

Bifunctional redox tagging of carbon nanoparticles.

Letter: Induction of tumours and bifunctional crosslinking metabolites of nitrosamines.

Air Redox Flow Battery.