A Low-Cost and High-Performance Sulfonated Polyimide Proton

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A Low-Cost and High-Performance Sulfonated Polyimide Proton Conductive Membrane for Vanadium Redox Flow/Static Batteries Jinchao Li, Xiaodong Yuan, Suqin Liu, Zhen He, Zhi Zhou, and Aikui Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07437 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

A Low-Cost and High-Performance Sulfonated Polyimide Proton Conductive Membrane for Vanadium Redox Flow/Static Batteries Jinchao Li,†,‡ Xiaodong Yuan,§ Suqin Liu,*,† Zhen He,*,† Zhi Zhou,// and Aikui Li¶ †

College of Chemistry and Chemical Engineering and ‡Innovation Base of Energy and Chemical

Materials for Graduate Students Training, Central South University, Changsha, Hunan 410083, P.R. China. §

State Grid Jiangsu Electric Power Research Institute, Nanjing, Jiangsu 211103, P.R. China.

//

Science College of Hunan Agricultural University, Changsha, Hunan 410128, P.R. China.

¶

Wuhan NARI Limited Company of State Grid Electric Power Research Institute, Wuhan, Hubei

430074, P.R. China. KEYWORDS: vanadium redox battery, side-chain-type fluorinated sulfonated polyimide, membrane, proton selectivity, chemical stability

ABSTRACT: A novel side-chain-type fluorinated sulfonated polyimide (s-FSPI) membrane is synthesized by high-temperature polycondensation and grafting reactions for vanadium redox batteries (VRBs). The s-FSPI membrane shows a vanadium ion permeability that is over an order

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of magnitude lower and a proton selectivity that is 6.8 times higher compared to the Nafion 115 membrane. The s-FSPI membrane possesses superior chemical stability than most of the linear sulfonated aromatic polymer membranes reported for VRBs. Besides, the vanadium redox flow/static batteries (VRFB/VRSB) assembled with the s-FSPI membranes exhibit stable battery performance over 100- and 300-time charge-discharge cycling tests, respectively, with significantly higher battery efficiencies and lower self-discharge rates than those with the Nafion 115 membranes. The excellent physicochemical properties and VRB performance of the s-FSPI membrane could be attributed to the specifically designed molecular structure with the hydrophobic trifluoromethyl groups and flexible sulfoalkyl pendants being introduced on the main chains of the membrane. Moreover, the cost of the s-FSPI membrane is only one fourth of the commercial Nafion 115 membrane. This work opens up new possibilities for fabricating high-performance proton conductive membranes at low costs for VRBs.


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1. INTRODUCTION The vanadium redox battery (VRB) first proposed by M. Skyllas-Kazacos in 1985 has been acknowledged as a promising large-scale energy storage system due to its tunable power and capacity, excellent efficiencies, long cycle life, high safety and reliability, and environmental friendliness.1-5 The VRB mainly includes the vanadium redox flow battery (VRFB) and vanadium redox static battery (VRSB). The VRB operates by utilizing two redox reactions of VO2+/VO2+ and V3+/V2+ couples in the positive and negative electrolytes, respectively, as shown in the schematic illustration in Scheme 1. As a key component of VRB, the proton conductive membrane (PCM) affects the efficiencies, lifespan, and cost of the VRB system.6 An ideal PCM should possess a low vanadium ion permeability, high proton conductivity, excellent chemical stability, and low cost.7 Currently, the commonly used PCMs in VRBs are perfluorosulfonic acid membranes, such as the Nafion membranes, owing to their high proton conductivities and excellent chemical stability.8,9 However, the high vanadium ion permeability of the Nafion membranes lowers the efficiencies and results in fast self-discharge of the VRBs. Although numerous modification approaches have been developed to reduce the vanadium ion permeability of the Nafion membranes, the essentially high costs (i.e., $500 - $700 m-2, up to 40%

Scheme 1. Schematic illustrations of VRB operation and the mechanism of proton transport in the s-FSPI membrane.


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of the VRB) of the Nafion membranes are another crucial issue that impedes their further commercialization in VRBs.10-15 Therefore, developing suitable PCMs to provide an optimal balance between the performance and cost is a focal point for VRB industrialization. To date, a series of sulfonated polyimide (SPI) membranes have been prepared and shown better proton selectivities and VRB performance compared with the commercial Nafion membranes. Unfortunately, the chemical stability of these SPI membranes is lower than that of the Nafion membranes.16-18 We have shown in our previous work that the semi-fluorinated SPI membrane with 50% sulfonation degree (6F-SPI-50) has excellent chemical stability.16 This can be attributed to the introduction of the hydrophobic trifluoromethyl groups (-CF3) with a strong electron-withdrawing nature into the 6F-SPI-50 membrane.16 The result shows that the introduction of -CF3 groups into SPI polymers is an effective method to improve the chemical stability of the sulfonated aromatic polymer membranes. In this study, we have designed and fabricated a novel side-chain-type fluorinated sulfonated polyimide (s-FSPI) membrane. The number of -CF3 groups on the polymer main chains of the sFSPI membrane are significantly increased compared with the 6F-SPI-50 membrane for further improving the chemical stability of the s-FSPI membrane. In addition, the flexible sulfoalkyl pendants are also introduced into the s-FSPI membrane for increasing the entanglement of the polymer chains, which could help to improve the mechanical properties of the s-FSPI membrane. Meanwhile, half of the -SO3H groups are attached on the main chains, whereas the other half of the -SO3H groups are located at the ends of the aliphatic side chains, which could effectively hinder the crossover of vanadium ions through the s-FSPI membrane. The molecular structure, morphologies, and physicochemical properties of the s-FSPI membrane are thoroughly investigated and discussed. Moreover, the s-FSPI membrane was applied to VRFB and VRSB to


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verify its battery performance. The physicochemical properties of the s-FSPI and Nafion 115 membranes as well as the performance of the VRFB and VRSB single cells equipped with the sFSPI or Nafion 115 membranes are listed and compared in Table S1 in the Supporting Information (SI). Owing to the specifically designed molecular structure, the s-FSPI membrane shows a significantly lower vanadium ion permeability, higher proton selectivity, longer selfdischarge duration, and better VRB performance compared to the Nafion 115 membrane, and superior chemical stability than most of the linear sulfonated aromatic polymer membranes reported for VRBs.

2. EXPERIMENTAL 2.1 Synthesis of s-FSPI polymer. The raw materials used for the synthesis of the s-FSPI polymer and the activation process for Nafion 115 membrane are described in the SI. The s-FSPI polymer containing -CF3 groups and flexible sulfoalkyl pendants was synthesized as follows (as shown in Scheme 2 and Scheme S1).17,19 Firstly, 2.0 mmol of 4, 4'-diamino-biphenyl 2, 2'disulphonic acid (BDSA), 55.0 mL of m-cresol, and 1.3 mL of triethylamine were added into a 250 mL three-necked flask equipped with a magnetic stirrer, a condenser, and an argon inlet, and heated at 50 °C for 0.5 h until BDSA was completely dissolved under vigorously stirring. Thereafter, 4.0 mmol of 2, 2-Bis[4-(4-aminophenoxy) phenyl]hexafluoropropane (HFBAPP) and 2.0 mmol of 2, 2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) were added to the flask and heated at 50 °C for about 0.5 h. Then, 8.0 mmol of 1, 4, 5, 8-naphthalenetetracarboxylic dianhydride (NTDA) and 16.0 mmol of benzoic acid were added successively. The mixture was stirred at 80 °C for 4.5 h, and then maintained at 180 °C for 20 h under vigorous stirring. After cooled down to 90 °C, the viscous solution was diluted with 10.0 mL of m-cresol.


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Scheme 2. Schematic illustration for the preparation of the s-FSPI membrane. When cooled to 50 °C, the viscous solution was slowly poured into 200.0 mL of acetone under stirring. The formed precipitate was filtered and further washed with methanol for twice to remove residual solvents, excess triethylamine, and unpolymerized monomers. Finally, the precipitate was collected after being dried at 80 °C in a vacuum oven for 24 h and nominated as FSPI-OH polymer. Subsequently, 5.0 g of the FSPI-OH polymer and 75.0 mL of dimethyl sulfoxide (DMSO) were added into a 250 mL three-necked flask equipped with a magnetic stirrer, a condenser, and an argon inlet, and heated at 40 °C for 4 h until the FSPI-OH polymer was completely dissolved under vigorous stirring. Then, 3.7 mmol of NaOH was added, and the mixture was allowed to react for 0.5 h. Excess 7.5 mmol of 1, 3-propane sultone (1, 3-PS) was added and the mixture was kept at 100 °C for 12 h. When cooled to 40 °C, the mixture was poured into 300.0 mL of deionized (D. I.) water under stirring. The formed precipitate was collected, washed for twice with anhydrous ethanol, and then dried in a vacuum oven at 80 °C for 24 h. The obtained product (about 4.7 g) was nominated as s-FSPI polymer. The yield of the s-FSPI polymer was about 94% based on the initial weight of the FSPI-OH polymer.


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2.2 Preparation of s-FSPI membrane. The FSPI polymer was dissolved in m-cresol to form a 4% w/v solution. Then, this solution was poured onto a clean and dry glass plate and heated at 60 °C for 48 h until the solvent was evaporated and the s-FSPI membrane was formed. The sFSPI membrane was peeled off by soaking the glass plate in D. I. water and then immersed in 1.0 mol L-1 H2SO4 at room temperature for 24 h to complete the proton exchange. Then, the membrane was immersed in D. I. water for 24 h to completely remove the residual H2SO4 and organic solvent and stored in D. I. water for further use. The pre-treatment of Nafion 115 membrane has a great impact on the VRB performance.20 Thus, the details about the pretreatment of Nafion 115 membrane in this work are described in Section 1 in the SI. The thicknesses of the prepared s-FSPI and pre-treated Nafion 115 membranes are about 60 and 120 μm, respectively. The densities of the s-FSPI and Nafion 115 membranes are 1.51 and 1.91 g cm3

in the dry state, respectively. Thus, the cost of the prepared s-FSPI membrane is estimated to be

about $167 m-2 (Table S2). The molecular weight distribution curve of the s-FSPI membrane has been determined by gel permeation chromatography (GPC) and is shown in Figure S1. The weight average molecular weight (Mw), number average molecular weight (Mn), and polydispersity index (PDI) values of the s-FSPI membrane are 213009 Da, 93849 Da, and 2.27, respectively. In addition, the characterizations of the membranes, including ATR-FTIR, 1H NMR, SEM-EDS, physicochemical properties, and VRB single cell tests, are described in the SI.

3. RESULTS AND DISCUSSION 3.1 Chemical structure. The chemical structures of the FSPI-OH and s-FSPI membranes were characterized by ATR-FTIR and the results are shown in Figure 1. The strong absorption bands around 1716.36 and 1675.87 cm-1 could be assigned to the stretching vibrations of carbonyl


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groups (C=O). The C-N asymmetric stretching vibration of the imide rings is observed around 1348.02 cm-1. The absorptions at 1101.17 and 983.53 cm-1 related to the stretching vibrations of sulfonic acid groups (O=S=O) are also observed.17 Besides, the absorption peak at 1120.46 cm-1 existing in both FSPI-OH and s-FSPI membranes corresponds to the C-F stretching of -CF3 groups, suggesting that the hydrophobic -CF3 groups with a strong electron-withdrawing capability have been incorporated into the membrane.21 In the ATR-FTIR spectrum of the s-FSPI membrane, a new peak corresponding to the vibration of methylene groups (-CH2) appears at 1469.52 cm-1. This peak suggests that the flexible aliphatic side chains have been successfully grafted to the main chains of the s-FSPI membrane.

Figure 1. ATR-FTIR spectra of the FSPI-OH and s-FSPI membranes in proton form in the wavenumber region of 700 to 2000 cm-1. In order to further confirm the chemical structure of the s-FSPI membrane, the 1H NMR spectra of the s-FSPI and FSPI-OH are presented in Figure 2 and Figure S2, respectively. The peaks of X (6.6 and 7.0 ppm), Y (2.5 ppm), and Z (2.2 ppm) represent the m-cresol, solvent (i.e., DMSO-d6), and H2O, respectively. The signals around 8.72 ppm (Ha and Hb) are associated with


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the protons on the naphthalene rings of the s-FSPI membrane.22 Compared to the spectrum of FSPI-OH polymer (Figure S2), the signal at 9.23 ppm (Hn) associated with the proton on the phenolic hydroxyl groups is absent in the spectrum of s-FSPI (Figure 2). Besides, the signals of the alkyl hydrogen are found between 2.0 and 2.8 ppm (Hk, Hm, and Hl). The peak of the hydrogen atoms (Hm) in aliphatic side chains connecting to -SO3H groups is overlapped with the DMSO-d6 peak.19 These results show that the flexible sulfoalkyl pendants have been successfully grafted onto the main chains of the s-FSPI membrane by a ring-opening grafting reaction.23,24 Furthermore, the signals between 7.1 and 8.2 ppm could be well assigned to the hydrogen atoms (Hc, Hd, He, Hf, Hg, Hh, Hi, and Hj) on the benzene rings of BDSA, HFBAPP, and APAF. The ATR-FTIR and 1H NMR results demonstrate that the s-FSPI membrane with the molecular structure as we designed has been fabricated.

Figure 2. 1H NMR spectrum of the s-FSPI membrane. 3.2 Membrane morphology. The surface and cross-sectional morphologies of the s-FSPI membrane were investigated by SEM. The SEM images at different magnifications in Figure 3


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and Figure S3 show that the surface and cross-section of the s-FSPI membrane are dense with no pinholes or cracks. The surface of the s-FSPI membrane is quite smooth, whereas the crosssection of the s-FSPI membrane is coarse. In addition, the distribution of elements was also studied by EDS mapping (as shown in Figure 3d). All the elements including S (from -SO3H), and F (from -CF3) are uniformly distributed in the s-FSPI membrane. The EDS result agrees well with the ATR-FTIR and 1H NMR results, further demonstrating that the -SO3H and -CF3 groups have been successfully introduced into the s-FSPI membrane.

Figure 3. SEM images of the s-FSPI membrane: (a) surface morphology (with magnifications of 5000× and 10000×); (b-c) cross-section morphology (at magnifications of 2000×, 50000×, and 5000×). (d) The EDS analysis of the cross-section of the s-FPSI membrane. 3.3 Water uptake, swelling ratio, water contact angle, and mechanical property. Water uptake (WU) and swelling ratio (SR) have profound influences on the proton conduction and dimensional stability of the PCMs.25 Thus, the WU and SR of the s-FSPI and Nafion 115 membranes based on the weight, length, and thickness were studied (Table 1). The WU of the sFSPI membrane (17.78% at 20 °C and 22.22% at 40 °C) is much lower compared with that of


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the 6F-SPI-50 membrane (26.92% at 20 °C and 37.79% at 40 °C)16 because the existence of the hydrophobic alkyl side chains and more -CF3 groups in the s-FSPI membrane could repel the water molecules more effectively. The through-plane SR (SRΔt based on the change of thickness) of the s-FSPI membrane is higher compared to the in-plane SR (SRΔl based on the change of length) at both 20 and 40 °C. This could be attributed to that the s-FSPI membrane is an anisotropic membrane, and the rigid backbones (with -SO3H and -CF3 groups) of the s-FSPI polymer might align better along the in-plane directions. Similar results have been reported in other studies of SPI membranes.26,27 The s-FSPI membrane has lower SRΔt (14.51% at 20 °C and 17.72% at 40 °C) than the 6F-SPI-50 membrane (17.19% at 20 °C and 19.09% at 40 °C)16 due to the lower WU and the restrained movement of the molecular chains arising from the enhanced entanglement of the flexible side chains.19 In addition, the water contact angle of the s-FSPI membrane (83.5°, Figure 4a) is larger than that of the 6F-SPI-50 membrane (74.8°),16 which agrees well with their WU results. The Nafion 115 membrane has lower WU and SR and larger water contact angle (as shown in Figure 4b) than the s-FSPI membrane. This can be attributed to the hydrophobic poly(tetrafluoroethylene) main chains in the Nafion 115 membrane, which could effectively suppress the absorption of water.5 Table 1 Physicochemical properties of the s-FSPI and Nafion 115 membranes: water uptake, swelling ratio, ion exchange capacity, proton conductivity, vanadium ion permeability, proton selectivity, and mechanical properties.

WU/SRΔt/SRΔl IEC (meq g-1)

(%)

Membrane 20 °C

σ (×10-2 S cm-1)

40 °C

s-FSPI

17.78/14.51/2.87 22.22/17.72/4.17

1.50

2.28

Nafion 115

15.28/11.96/2.35 17.68/16.10/3.49

0.74

6.15

P (×10-7 cm2 min-1)

PS (×105 S min cm-3)

Ts (MPa)

Ym (GPa)

Eb (%)

20°C

40 °C

0.74

1.29

3.08

50.97

0.81

58.01

13.59 35.97

0.45

12.79

0.06

223.17


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Figure 4. The water contact angles of (a) s-FSPI and (b) Nafion 115 membranes. (c) Stressstrain properties of the s-FSPI and Nafion 115 membranes. The mechanical properties of the s-FSPI and Nafion 115 membranes were measured at room temperature. The stress-strain curves of the membranes (Figure 4c) include two stages. Before reaching the yield strength, the membrane undergoes elastic deformation. Then, the plastic deformation occurs, in which the stress increases again with the increase of the strain until the membrane fractures.28 The data (in Table 1) show that the tensile strength (Ts) and Young's modulus (Ym) of the s-FSPI membrane (50.97 MPa and 0.81 GPa) are much higher than those of the Nafion 115 membrane (12.79 MPa and 0.06 GPa) because of the robust aromatic backbone of the s-FSPI membrane.29 Besides, the s-FSPI and Nafion 115 membranes exhibit elongations at break (Eb) of 58.01% and 223.17%, respectively. The elongation at break of the s-FSPI membrane is surprisingly superior to that of the 6F-SPI-50 membrane (23.37%).16 This could be attributed to the enhanced entanglement of the polymer chains arising from the introduction of the flexible side chains (as shown in Scheme 1).19 This result shows that the introduction of


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flexible alkyl side chains can effectively improve the elongation at break of the sulfonated aromatic polymer membranes. 3.4 Ion exchange capacity, proton conductivity, vanadium ion permeability, and proton selectivity. The ion exchange capacities (IECs) of the s-FSPI and Nafion 115 membranes are presented in Table 1. The IEC is directly related to the amount of the -SO3H groups per gram of membrane in the dry state.19 The IEC of the s-FSPI membrane (1.50 meq g-1) is about twice as large as that of the Nafion 115 membrane (0.74 meq g-1), meaning that the s-FSPI membrane can obtain more ion exchange groups than the Nafion 115 membrane. This result further indicates that the -SO3H groups have been grafted into the s-FSPI polymer by the grafting reaction. The sFSPI membrane with an excellent IEC can provide a satisfactory proton conductivity (σ), which directly affects the performance of the VRB. The configuration of the device for measuring the proton conductivity is shown in Scheme S2a. As shown in Table 1, the proton conductivity of the s-FSPI membrane (2.28×10-2 S cm-1) is lower than that of the Nafion 115 membrane (6.15×10-2 S cm-1). This could be mainly attributed to the flexible perfluoroalkyl side chains terminated with the hydrophilic -SO3H groups and the hydrophobic perfluorinated backbones in the Nafion 115 membrane, which result in a unique phase separation that is beneficial for its proton conductivity.5,30 Generally, the proton conduction for sulfonated aromatic polymer membranes follows a combination of the Vehicular and Grotthuss mechanisms. The Vehicle mechanism describes the protons diffusing in the form of H3O+, H5O2+, and H9O4+, while the Grotthuss mechanism governs the protons hoping from one carrier site to the neighboring one.31 The sFSPI membrane has a low water content, which in turn will limit the Vehicle-type proton transport. However, the two types of -SO3H groups (existing on the ends of the flexible alkyl side chains and attaching to the main chains) could synergistically increase the amount of proton-


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hoping sites and promote the Grotthuss-type proton transport (as shown in Scheme 1).31 Thus, the proton conductivity of the s-FSPI membrane is twice as large as the commercially acceptable value of 0.01 S cm-1, meaning that our s-FSPI membrane is applicable for the VRB system.32 The test setup of the vanadium ion permeability is shown in Scheme S2b. The change of the vanadium ion concentration as a function of time is plotted in Figure 5a, and the calculated vanadium ion permeabilities (P) of the membranes are listed in Table 1. Since the VRBs generally operate below 40 °C in order to avoid the precipitation of VO2+ species, the vanadium ion permeability is measured at 20 and 40 °C in this work.33 The vanadium ion permeabilities of the s-FSPI membrane are 0.74×10-7 cm2 min-1 at 20 °C and 1.29×10-7 cm2 min-1 at 40 °C, which are about 18.4 times and 27.9 times lower compared to those of the Nafion 115 membrane (i.e., 13.59×10-7 cm2 min-1 at 20 °C and 35.97×10-7 cm2 min-1 at 40 °C), respectively. The substantially low vanadium ion permeability of the s-FSPI membrane could be due to the increased chain packing density of the s-FSPI membrane from the introduction of the flexible sulfoalkyl pendants and the narrowed transport channels for vanadium ions in the sulfonated aromatic polymer membranes.30,34 Moreover, the -SO3H groups not only exist on the ends of the flexible alkyl side chains but also attach to the main chains in the s-FSPI polymer, leading to the poorer phase separation than the Nafion 115 membrane.19 These results show that the prepared sFSPI membrane, as a PCM for VRBs, has a strong ability to suppress the crossover of vanadium ions between the positive and negative electrolytes at the temperatures of 20 to 40 °C. In addition to the low vanadium ion permeability, an ideal PCM should also possess high proton conductivity. The proton selectivity (PS), defined as the ratio of proton conductivity to vanadium ion permeability, is used to evaluate the comprehensive performance of the PCMs. Normally, a higher proton selectivity results in a better VRB performance.35 The proton


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Figure 5. (a) Concentration changes of VO2+ ions as a function of time. (b) Ex-situ chemical stability study of the s-FSPI and Nafion 115 membranes (The variation of VO2+ ion concentration as functions of time in 0.1 mol L-1 VO2+ + 3.0 mol L-1 H2SO4 solution at 40 oC and 1.5 mol L-1 VO2+ + 3.0 mol L-1 H2SO4 solution at room temperature.). selectivity of the s-FSPI membrane is 3.08×105 S min cm-3, which is around 6.8 times higher than that of the Nafion 115 membrane (0.45×105 S min cm-3). Thus, we infer that the VRB with the s-FSPI membrane should display excellent VRB performance, which will be verified in Section 3.6.


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3.5 Ex-situ chemical stability. The chemical stability of the PCMs is of vital importance for VRBs because the stability of the PMCs significantly affects the lifetime and performance of VRBs. The ex-situ chemical stability of the s-FSPI and Nafion 115 membranes were investigated by both Method 1 (i.e., soaking the membrane in the 0.1 mol L-1 VO2+ + 3.0 mol L-1 H2SO4 solution at 40 °C) and Method 2 (i.e., soaking the membrane in the 1.5 mol L-1 VO2+ + 3.0 mol L-1 H2SO4 solution at room temperature). The highly oxidizing VO2+ ions in the solution could oxidize the membranes (which causes the degradation of the membranes) and be reduced to VO2+ ions. Therefore, the degradation of the membranes could be estimated by measuring the concentration change of the VO2+ ions. For both Method 1 and 2, the concentrations of VO2+ in the solutions gradually increase as the immersing time increases, as shown in Figure 5b. The VO2+ ion concentration increases faster in Method 2 compared to that in Method 1 since the concentration of VO2+ ions in the solution of Method 2 is higher, which causes faster degradation of the membranes.17-19,36 The comparison of the ex-situ chemical stability between the linear sulfonated aromatic polymer membranes are presented in Table S3.16-18,30,37-39 The ex-situ chemical stability of the s-FSPI membrane is superior to most of the reported linear sulfonated aromatic polymer membranes due to two possible reasons. First, the low water uptake of the sFSPI membrane is beneficial for protecting the imide rings on the polymer main chains from being attacked by the hydrolytic species like H+ ions.21,36 Second, the s-FSPI membrane possesses abundant electron-withdrawing groups (i.e., -CF3 groups), which facilitates the formation of the electron-deficient aromatic main chains with enhanced resistance towards the destructive species with positive charges.19,40 The Nafion 115 membrane exhibits excellent chemical stability because of its very stable poly(tetrafluoroethylene) main chain structure.6


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3.6 VRFB and VRSB single cell performance. The VRFB single cell and its internal structure are shown in Scheme S3. The cycling performance of the VRFB assembled with the sFSPI membrane was measured and compared to that with Nafion 115 membrane at the current density of 60 mA cm-2. As illustrated in Figure 6a, the VRFB with the s-FSPI membrane shows stable performance for 100 cycles and has a higher coulombic efficiency (CE) of around 99.6% than that with the Nafion 115 membrane (96.5%). The excellent CE is mainly due to the substantially low vanadium ion permeability of the s-FSPI membrane (in Figure 5a).41 The energy efficiency (EE) and voltage efficiency (VE) of the VRFB with the s-FSPI membrane remain around 77.0% and 77.3%, respectively, higher than those of the VRFB with Nafion 115 (around 66.4% and 68.8%, respectively). This can be attributed to the obviously higher proton selectivity of the s-FSPI membrane compared to Nafion 115 membrane (in Table 1).19 In addition, the discharge capacity retentions of the VRFBs with the s-FSPI and Nafion 115 membranes are also shown in Figure S4. The discharge capacity decay of the VRFB with the sFSPI membrane is slower than that with the Nafion 115 membrane. After the 100-time cycling tests, the discharge capacity retentions of the VRFBs assembled with the s-FSPI and Nafion 115 membranes are 77.1% and 66.2%, respectively. The higher discharge capacity retention of VRFB with the s-FSPI membrane could be attributed to lower transport of the active vanadium species across the membrane during the cycling. The 500-time charge-discharge cycling tests at 60 mA cm-2 are also performed in order to further verify the cycling performance of the VRFB with the s-FSPI membrane. As presented in Figure S5, the CE (around 99.0%) of the VRFB assembled with the s-FSPI membrane is highly stable for over 500 cycles. The EE starts to decay slowly after ~299 cycles due to the unbalancing of the positive and negative electrolytes with the increase of cycle number.36 The EE recovers to the initial value (around 77.0%) after the positive


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Figure 6. Comparison of the cycling performance of (a) VRFB and (b) VRSB with the s-FSPI and Nafion 115 membranes. and negative electrolytes are replaced at the 326 cycle, suggesting that the s-FSPI membrane has a stable cycling performance for VRFBs. Besides, the cycling performance of the VRSBs assembled with the s-FSPI and Nafion 115 membranes were also measured under various current densities (20, 40, and 60 mA cm-2) for 300 cycles, as shown in Figure 6b. The EE and VE of the VRSB decrease with the increase of current density from 20 to 60 mA cm-2. This is due to the


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stronger polarization effect at higher current densities.42 The CE slightly increases from 98.1% to 99.5% as the current density increases. This could be due to the severer vanadium ion permeation at lower current densities since the charge-discharge time is longer.43 However, the CE, EE, and VE of the VRSB with the s-FSPI membrane are higher than those with Nafion 115 membrane at each tested current densities. Furthermore, this low vanadium ion permeability of the s-FSPI membrane is also further verified by studying the open circuit voltage (OCV) of the VRBs, as shown in Figure S6.44 The self-discharge durations (for OCV above 0.8 V) of the VRFB and VRSB assembled with the sFSPI membrane are more than 239.9 and 103.3 h, respectively, which are 4.1 and 2.8 times longer than those with Nafion 115 membrane (58.2 and 37.4 h, respectively). The OCV results reveal a slower rate of vanadium ions crossing through the s-FSPI membrane than Nafion 115 membrane. The VRFB and VRSB charge-discharge cycling and OCV test results indicate that the s-FSPI membrane has an excellent resistance towards the vanadium ion permeation, stable proton conduction, and outstanding chemical stability. In order to further investigate the stability of the s-FSPI membrane in VRFB and VRSB under the strong oxidizing and acidic conditions, SEM study was carried out on the s-FSPI membrane after 100-time charge-discharge cycling test (for about 200 h), as shown in Figure 7 and S7. The surface of the used s-FSPI membrane facing the positive (Figure 7a and S7a-c) and negative (Figure 7b and S7d-f) electrodes show almost no change except for some dents (as marked by the yellow circles) after the long-term cycling test. As shown in Figure 7c and S7g-h, the crosssectional morphology of the s-FSPI membrane after cycling test shows no pinholes or cracks, and is almost the same as the fresh s-FSPI membrane (in Figure 3b-c). The cross-sectional


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Figure 7. SEM images of the s-FSPI membrane after 100-time VRFB cycling test: (a) surface facing positive electrode, (b) surface facing negative electrode, (c) cross-section. (The yellow circles mark the dents). (d) The EDS analysis of the cross-section of the s-FPSI membrane after 100-time VRFB cycling test. elemental analysis of the s-FSPI membrane after 100-time cycling test was also conducted by EDS mapping (in Figure 7d). The elements C, N, O, F, S, and V are uniformly distributed in the s-FSPI membrane after the cycling test, which further supports that the s-FSPI membrane is still dense and uniform. Based on the obtained SEM-EDS results, it is reasonable to infer that the sFSPI membrane has a stable chemical structure for VRB applications. The photographs of the s-FSPI membrane after the 100- and 300-time VRFB and VRSB cycling tests are shown in Figure 8a and S8. The s-FSPI membrane is intact and its color is not changed, meaning that the as-prepared s-FSPI membrane is durable in the VRFB and VRSB applications. However, the surface of the s-FSPI membrane after the cycling test looks slightly corrugated because the two pieces of graphite felt electrodes squeezed the membrane surface during the long-term cycling test. In addition, the water contact angles, ATR-FTIR spectra, and


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Figure 8. (a) The photograph of a bended s-FSPI membrane after 100-time VRFB cycling test. Comparison of the s-FSPI membrane before and after 100-time VRFB cycling test: (b-c) Water contact angles of s-FSPI membrane facing positive and negative electrodes; (d) ATR-FTIR spectra of s-FSPI membrane in the wavenumber region of 700 to 2000 cm-1; (e) Stress-strain properties. mechanical property tests of the s-FSPI membrane after 100-time charge-discharge are shown in Figure 8b-e. The water contact angles of the s-FSPI membrane facing either positive or negative electrodes slightly decrease after the VRFB and VRSB cycling tests (as shown in Figure 8b-c and S9a-b), suggesting that the surface hydrophilicity of this membrane is slightly enhanced. Similar results are also obtained for the water contact angles of the Nafion 115 membrane after the VRFB and VRSB cycling tests (as shown in Figure S9c-f). The ATR-FTIR spectra (in Figure 8d) of the s-FSPI membrane facing the positive and negative electrodes after the 100-time VRFB cycling test show neither new peaks nor peak shifts, implying that the chemical environments of all the functional groups in the s-FSPI membrane have not been changed. In Figure 8e, the tensile strength of the s-FSPI membrane (62.37 MPa) after 100-time VRFB cycling test is stronger than that of the fresh s-FSPI membrane (50.97 MPa). This indicates that the s-FSPI membrane has become more rigid after the cycling test. The cross-linking density of the polymer


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chains in the s-FSPI membrane is increased after the 100-time VRFB cycling test, leading to the improvement of the elastic deformation capacity and deterioration of the plastic deformation capacity for the used s-FSPI membrane. More importantly, the elongation at break of the s-FSPI membrane still can attain 48.99% after the cycling test. The mechanical properties of the s-FSPI membrane were further measured after 300-time VRSB cycling test, as shown in Figure S10. The tensile strength is slightly increased by 3.21 MPa and elongation at break is slightly reduced by 3.82% compared with those of the fresh s-FSPI membrane. These results show that the mechanical properties of the s-FSPI membrane can satisfy the long-term VRB application. All these promising results confirm that the s-FSPI membrane possesses excellent stability for VRB applications.

4. CONCLUSIONS In conclusion, we have designed and prepared a novel s-FSPI membrane with flexible sulfoalkyl pendants and trifluoromethyl groups by high-temperature polycondensation and grafting reactions for VRBs. The overall physicochemical properties of the s-FSPI membrane have been investigated and compared with the commercial Nafion 115 membrane. The low-cost s-FSPI membrane exhibits a high ion exchange capacity, outstanding tensile strength, and excellent proton selectivity. The VRFB and VRSB cells with the s-FSPI membranes show higher CEs, EEs, and VEs than those with the Nafion 115 membrane at all tested current densities. Besides, the s-FSPI membrane has a significantly low vanadium ion permeability, resulting in longer self-discharge durations of the VRFB and VRSB with the s-FSPI membranes compared to those with the Nafion 115 membranes. Furthermore, the VRFB and VRSB assembled with the sFSPI membranes show stable battery efficiencies over 100- and 300-time charge-discharge


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cycles. After the cycling tests, the used s-FSPI membrane still shows remarkable chemical and structural stability. Considering these results, the s-FSPI membrane is expected to be a promising candidate membrane for VRB applications. In addition, since the flexible sulfoalkyl pendants and the number of -CF3 groups have been shown to be of great importance to the physicochemical properties of the SPI membranes, it will be interesting to adjust the length of flexible sulfoalkyl pendants and the number and position of -CF3 groups for further optimizing the SPI membranes in future research.

ASSOCIATED CONTENT Supporting Information. The raw materials and characterization methods of the membranes; synthetic route of the s-FSPI membrane; cost estimation of the prepared 1.0 m2 s-FSPI membrane; comparisons between the ex-situ chemical stability of the linear sulfonated aromatic polymer membranes; GPC study of the s-FSPI membrane; 1H NMR spectrum study of the FSPIOH polymer; discharge capacity retentions of the VRFBs assembled with the s-FSPI and Nafion 115 membranes; 500-time cycling performance of the VRFB assembled with the s-FSPI membrane; open circuit voltages of VRFB and VRSB assembled with s-FSPI and Nafion 115 membranes; SEM, water contact angle, and mechanical property studies of the s-FSPI membrane before and after the cycling test. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Authors *E-mail addresses: [email protected] (Liu, S.); [email protected] (He, Z.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Plan Project (grant no. 2017YFB0903502), National Natural Science Foundation of China (grant nos. 51772332 and 51372278), Science and Technology Major Special Project of Hunan Province (grant no. 2016GK1003-1), Innovation-Driven Project

of Central South University (grant no.

2016CXS031), and Science and Technology Project of SGCC “Key techniques research of ion exchange membrane for vanadium redox flow battery”.

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Table of Contents Graphic:


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A Low-Cost and High-Performance Sulfonated Polyimide Proton

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