Towards Cheaper Vanadium Flow Batteries: Porous Polyethylene

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Towards Cheaper Vanadium Flow Batteries: Porous Polyethylene Reinforced Membrane with Superior Durability Di Mu, Lihong Yu, Liwei Yu, and Jingyu Xi ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00083 • Publication Date (Web): 06 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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ACS Applied Energy Materials

Towards Cheaper Vanadium Flow Batteries: Porous Polyethylene Reinforced Membrane with Superior Durability Di Mu,†,§ Lihong Yu,‡,§ Liwei Yu,† and Jingyu Xi*,† †

Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China ‡ School of Applied Chemistry and Biological Technology, Shenzhen Polytechnic, Shenzhen 518055, China *E-mail: [email protected]

ABSTRACT Developing cheap and durable proton exchange membrane is crucial to promote the practical application of vanadium flow batteries (VFB). Here we report a simple and scalable method to fabricate reinforced sulfonated poly(ether ether ketone) (SPEEK) membrane using a lithium-ion battery separator, ceramic-coated porous polyethylene (CCP), as a robust scaffold. With the confinement effect of the extremely stable CCP substrate, the reinforced SPEEK membrane (S@CCP) shows significantly improved chemical/mechanical stability and reduced vanadium ion permeability compared to the control SPEEK membrane. Accordingly, the S@CCP membrane demonstrates excellent rate performance and cycling stability than those of the benchmark Nafion 212 membrane. It exhibits stable performance over 1500 cycles at 160 mA cm-2 with 99% of CE, 76% of EE and 0.126% of capacity decay per cycle. Meanwhile, the S @ CCP membrane is highly resistant to temperature fluctuations over a wide range of -20–60 oC. The superior durability, wide temperature adaptability along with low cost suggest that the S@CCP membrane offers great promise as an ideal membrane for VFB application.

KEYWORDS: Vanadium flow battery, Ceramic-coated polyethylene separator, Sulfonated poly(ether ether ketone), Durability, All-climate performance 1

ACS Paragon Plus Environment

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INTRODUCTION Large-scale energy storage systems are crucial to the highly efficient utilization of sustainable energy.1-3 Vanadium flow battery (VFB) has become one of the most promising energy storage technologies because of long cycle life, high efficiency and fast response.4-6 A typical VFB comprises VO2+/VO2+ and V2+/V3+ redox couples in H2SO4 solution as positive and negative electrolytes respectively, graphite (carbon) felt as electrode, and ion exchange membrane (IEM) as separator.7-9 As the core material of VFB, IEM is responsible for transporting ions between positive and negative electrolytes as well as preventing their cross mixing.10 The physical and chemical properties of IEM greatly affect the VFB performance and manufacturing cost.11 An ideal IEM should meet the requirements of low vanadium ion permeability, high proton conductivity, excellent mechanical/chemical stability and affordable price.12 Nowadays, the Nafion membranes, state-of-the-art perfluorosulfonic polymers,13 are widely employed as the benchmark in VFB because of their excellent chemical stability.14-16 However, the rapid crossover of vanadium ions and the expensive cost limit the practical use of Nafion membranes in VFB.17 In this case, low vanadium ion permeability and affordable membranes become is a hot topic. For example, as early as 1992, M. Skyllas-Kazacos et al. proposed that adsorbing polyelectrolyte/ion-exchange resin into the Daramic separator could greatly reduce the permeability of the vanadium ions.18 In recent years, sulfonated poly(ether ether ketone) (SPEEK) based membranes have drawn much attention because of low cost, simple fabrication and good tradeoff between proton conductivity and vanadium permebility.19-22 To achieve excellent proton/vanadium selectivity and acceptable chemical/mechanical stability of SPEEK membranes, various strategies have been employed, such as inorganic composite membranes,23-25 polymer blend membranes, 26-28 multilayer membranes,29-31 and substrate reinforced membranes.32,33 For 2


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the inorganic composite membranes and polymer blend membranes, the ion transport properties and mechanical stability of modified SPEEK membrane can be significantly enhanced by the interfacial interaction between SPEEK and inorganic fillers or other polymer segments. In the case of multilayer membranes, the SPEEK layer was protected by a more stable polymer layer to improve the chemical stability. The substrate reinforced membranes, also known as pore filling membranes, comprise a extremely stable 3D porous polymer scaffold and the proton-conducting SPEEK phase.23 Commercial available porous poly(tetrafluoroethylene) (PTFE) has been successfully used as the substrate of SPEEK, and the SPEEK@PTFE membrane exhibited remarkably improved chemical/mechanical stability under the harsh VFB operating condition.23,32,33 However, the relatively high cost of the porous PTFE film ($50 per m2) still hinders its large-scale application. Therefore, looking for a new porous substrate with the properties of low cost, suitable thickness/porosity and outstanding chemical/mechanical stability is critical to promote the application of SPEEK based membranes in VFB. Porous polyolefin separators, such as polyethylene (PE) and polypropylene (PP), are widely used in lithium-ion batteries (LIBs) because of its 3D porous structure and outstanding mechanical properties as well as low cost ($1-5 per m2).34-36 Given the good chemical stability of PE and PP in H2SO4 solution, these separators are considered to be suitable substrates for SPEEK membranes toward VFB application. Herein, we choose a ceramic (Al2O3)-coated PE separator (CCP) as a robust scaffold of SPEEK membrane. Compared with PE and PP separators, the CCP separator has some notable features, including improved wettability and thermal stability.37 The SPEEK casting solution wettability of the CCP separator can be further enhanced by sodium dodecyl benzene sulfonate (SDBS) modifying. The 3D porous structure and excellent wettability of the SDBS-modified CCP allow the quick infiltrating of SPEEK casting solution, resulting in a ultrathin, dense and stable CCP reinforced SPEEK (S@CCP) membrane. The 3



physicochemical properties, ion transport properties, oxidative stability and VFB single-cell performances of the S@CCP membrane are investigated and compared with those of the control SPEEK membrane and the Nafion 212 membrane. To meet the actual operating condition of VFB applications, the super long-term cycle stability, limit output power density and all-climate performance of the S@CCP membrane are also evaluated.

EXPERIMENTAL SECTION Materials. Poly(ether ether ketone) (PEEK 450G) was purchased from Victrex, and was pre-treated by washing and drying. CCP separator was supplied by Shenzhen Kejing, and its detail parameters were given in Table S1. Nafion 212 membrane (N212) was purchased form DuPont. The N212 membrane was treated by standard acid boiling process before use.15 Graphite felt (GF) with 5 mm of thickness was obtained from Gansu Haosi. The GF electrode was thermal activated in air at 420 oC for 10 h.38 Other chemicals were of analytical grade and used without further purification. Membrane Preparation. 25 g PEEK was sulfonated in 250 mL H2SO4 (98 wt%) with continuously mechanical stirring at 53 oC for 2.5 h to obtain the SPEEK resin.23 The degree of sulfonation of the synthesized SPEEK was 70% as determined by H+ titration method.22 The S@CCP membrane was prepared by solution casting method. Typically, 1 g SPEEK resin was added into 10 mL DMF and stirred for 12 h to obtain a homogeneous SPEEK casting solution. The as-received CCP separator was immersed in 0.75 mg/mL SDBS aqueous solution for 10 min, washed with deionized water and dried at room temperature. Then, the SDBS-modified CCP separator was immersed in the SPEEK casting solution for 12 h to ensure that the SPEEK solution completely entered the continuous pores of the CCP separator. Afterwards, the infiltrated CCP membrane was moved onto a clean glass plate and scraped with a blade (50 4


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µm). Finally, the S@CCP membrane was dried at 40 °C and 100 °C for 12 h, respectively. Control SPEEK membrane was also prepared for comparison. The as-prepared membranes were peeled off from the glass plate and soaked in 1 M H2SO4 for 12 h. Subsequently, the membranes were immersed into deionized water for another 12 h to remove the excess acid. All the membranes were stored in deionized water before use. Membrane Characterization. Contact angle measurements were performed on a POWEREACH® JC2000D1 goniometer (Shanghai Zhongchen). 5 µL of the SPEEK casting solution was dropped on the as-received and SDBS-modified CCP separators respectively to observe the contact angle. The morphology of as-received and SDBS-modified CCP separators as well as the N212, control SPEEK and S@CCP membranes were thoroughly recorded by digital camera, scanning electron microscopy (SEM, ZEISS SUPRA 55) and energy-dispersive X-ray spectroscopy (EDX). The physicochemical properties of the membranes, including thickness (in wet and dry state), water uptake, swelling ratio (length and thickness), area resistance, VO2+ permeability and mechanical properties, were all characterized according to our previous work.16,22 The ex-situ oxidative stability test was conducted by immersing the membranes in 0.1 M VO2+ + 3 M H2SO4 and 1.5 M VO2+ + 3 M H2SO4 solutions, respectively.22 All the experimental details were given in the Supporting Information. Single Cell Test. The VFB single-cell was assembled by sandwiching a membrane between two GF electrodes (5 × 5 cm2) as reported in our previous work.23,27 The initial positive and negative electrolytes were both 50 mL of 1.5

M

VO2+/V3+ (1:1) in 2

M

H2SO4 solution.39 All the cell tests were performed on a

Neware CT-3008W-5V10A battery testing system with the cut-off voltage of 0.8-1.65 V. Self-discharge test was started from 50% state of charge (SoC). All-climate performance was evaluated using a thermostat (Hongzhan, PU-80) in the temperature range of -20 to 60 °C.40 Polarization and power density curves were tested according to previous report,38 and the effective area was 3 × 3 cm2. The single-cell was discharged 5



at each current density (0 to 800 mA cm-2 with an increase step of 25 mA cm-2) for 60 s from 100% SoC and then the cell voltage was recorded.

RESULTS AND DISCUSSION The wettability of the CCP separator to the SPEEK casting solution is critical to fabricate a dense S@CCP membrane with uniform properties. Therefore, the wettability of as-received and SDBS-modified CCP separators was firstly evaluated. As shown in Figure 1 a and b, the contact angle for both CCP separators is around 10o, indicating excellent SPEEK casting solution wettability of these separators. This is due to the hydrophilicity of the surface Al2O3 coating layer.37 In contrast, when the same amount of casting solution was dropped onto the surfaces of the two separators, the solution rapidly passed through the SDBS-modified CCP separator but failed to penetrate the as-received CCP separator, indicating that SDBS improved the wettability of the internal porous PE layer of the CCP separator. This can be further confirmed by wettability test in deionized water with and without SDBS. As shown in Figure S1, the CCP separator floats on the deionized water, but rapidly sinks into the SDBS aqueous solution.

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Figure 1. Preparation of S@CCP membrane: (a) and (b) the casting solution wettability of as-received and SDBS-modified CCP separators, respectively; (c) the fabrication process of S@CCP membrane; (d) the photogragh of a piece of super long S@CCP membrane.

The 3D porous structure and excellent wettability of the SDBS-modified CCP scaffold allow the quickly infiltrate of SPEEK casting solution to form a dense and stable S@CCP membrane. Figure 1c shows the simple and controllable fabrication process of the S@CCP membrane. Using this simple method, we also developed a 50 cm long S@CCP membrane (Figure 1d), which is uniform, flexible, and can be rolled in any direction to a small size. This newly developed S@CCP membrane can also be fabricated via a scalable process in the well-developed roll-to-roll production line. The SDBS modification, SPEEK infiltration and drying processes can be easily incorporated into the roll-to-roll production line.41 Figure 2 shows the micro-morphology of various samples. In order to make a reasonable comparison between the different membranes, the control SPEEK membrane was fabricated with a thickness close to that of the benchmark N212 membrane. As shown in Figure 2 a and b, both the N212 and the SPEEK membranes are dense, smooth and uniform in the cross-section. The sandwich structure of the as-received 7



CCP separator can be clearly identified from Figure 2 c-e, and the ceramic (Al2O3) coating layers are about 2 µm in thickness. This unique structure successfully improves the thermal stability of the CCP separator for power LIBs application.37 In this work, the thermal stability of the CCP is beneficial to maintain the macroscopic structure of S@CCP membrane during high temperature drying process (Figure 1c). After the SDBS modification, the morphology, structure and thickness of the CCP separator do not change (Figure 2 f and g). The even distribution of the elements Na and S in the cross-section of CCP (Figure 2 h) indicates that the SDBS molecules are uniformly attached to the hydrophobic porous PE interlayer, resulting in significantly improved wettability to SPEEK casting solution (Figure 1b). After the filling of SPEEK, a flexiable and transparent S@CCP membrane is obtained. As shown in Figure 2 i and j, the SPEEK resin completely covers the gap of the ceramic coating layer and fills the pores of the porous PE interlayer, forming a smooth surface and a dense cross-section morphology. The uniform distribution of the elements O and S (−SO3H group from SPEEK) in the cross-section of S@CCP membrane (Figure 2k) demonstrates the successful filling of SPEEK in the SDBS-modified CCP scaffold.

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Figure 2. Cross-section SEM images of N212 (a) and SPEEK (b) membranes. Surface SEM image, cross-section SEM image and EDX spectra of as-received CCP separator (c)-(e), SDBS-modified CCP separator (f)-(h), and S@CCP membrane (i)-(k).

The mechnical properties of various samples are compared in Table 1. The CCP scaffold itself shows higher elongation percentage and breaking strength than that of control SPEEK membrane. Therefore, the S@CCP membrane exhibits significantly improved mechanical properties compared to control SPEEK 9



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membrane. Owing to the low cost of mass prduction CCP separator ($4 per m2), the estimated price for the S@CCP membrane is only $20 per m2 based on lab-scale fabrication (Table S2). This makes the S@CCP membrane very competitive in VFB application.

Table 1. Mechanical properties of various samples. Membrane

Elongation percentage (%)

CCP

Breaking strength (MPa)

103.1

15.5

SPEEK

42.5

12.5

S@CCP

67.5

19.2

Table 2 summarizes the physicochemical and ion transport properties of various membranes. The thickness of the SPEEK membrane is close to the N212 membrane, while the S@CCP membrane is much thinner. The S@CCP membrane comprises an inner SPEEK@PE layer and two outer SPEEK coating Al2O3 layers (Figure 2 g and j), resulting a thickness of around 33 µm compared to the 16 µm of the CCP separator. The SPEEK membrane shows higher water uptake value compared with the N212 membrane, agreeing well with previous reports.42,43 With the confinement effect of the porous CCP substrate, the S@CCP membrane exhibits significantly reduced water uptake than the control SPEEK membrane. Meanwhile, the S@CCP membrane displays an extremely lower swelling ratio in length direction (4.1%), indicating that CCP substrate could suppress the in-plane swelling of the SPEEK membrane. On the contrary, the through-plane (thickness) swelling ratio of the S@CCP membrane is slightly higher than that of the control SPEEK membrane (21.2% vs. 17.1%), suggesting that the outer SPEEK coating Al2O3 layer is favorable to absorb water. Non-conductive 3D CCP scaffold combined with swelling-inhibited SPEEK component will significantly affect the ion transport properties of the S@CCP membrane. As a result, the area resistance of the S@CCP membrane is much larger than that of control SPEEK membrane (0.82 vs.

10


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0.37 Ω cm2).

Table 2. Physicochemical and ion transport properties of various membranes. Area resistance VO2+ permeability

Thickness

Water uptake

Swelling ratio

Swelling ratio

(dry, µm)

(%)

(Length, %)

(Thickness, %)

(Ω cm2)

(10−7 cm2 min−1)

N212

52±1

32.7

17.5

17.3

0.22

38.2

SPEEK

41±2

54.6

17.0

17.1

0.37

10.0

S@CCP

33±2

34.0

4.1

21.2

0.82

2.2

Membrane

To further investigate the impact of CCP scaffold on the vanadium ion crossover behavior of the SPEEK membrane, the VO2+ diffusion and the self-discharge measurements were carried out. As shown in Figure 3a, the color changes in the right part of different diffusion cells after 60 h are different, and the crossover rate of VO2+ follows the order of N212 >> SPEEK > S@CCP. The calculated VO2+ permeability values are listed in Table 2. The VO2+ permeability reduction of the S@CCP membrane is due to the 3D CCP scaffold occupying a portion of the membrane space and inhibiting the swelling of SPEEK component. This effective improvement can also be confirmed by the self-discharge results shown in Figure 3b. The initial open circuit voltage (OCV) of the N212 is obviously lower than other membranes because of fast vanadium crossover during charging process. The OCV decay rate of the S@CCP membrane is much slower than that of the N212 and SPEEK membranes. The self-discharge time of the N212, SPEEK and S@CCP membranes is 5.5 h, 44.5 h and 91.7 h respectively, agreeing well with the VO2+ crossover results.

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Figure 3. (a) Time-dependent concentration changes of VO2+ plots of various membranes. The right photographs illustrate the color changes of different membrane-separated diffusion cells after 60 hours. (b) Open circuit voltage curves of VFBs with various membranes.

Chemical stability of the membrane is critical for VFB application.9,12 We carried out the ex-situ chemical stability test of various membranes before single-cell evaluation. Two groups of samples were immersed in dilute (0.1 M) and concentrate (1.5 M) VO2+ solutions for 60 days, respectively.44,45 As shown in Figure 4a, no obvious color change was observed within 60 days for all sample-soaked 0.1

M

VO2+

solutions, indicating excellent oxidative stability of the CCP separator and various membranes. After 60 days, all samples were taken out from the 0.1

M

VO2+ solution and washed by deionized water, and the

digital photos of these samples are shown in Figure 4b. The CCP separator, N212 membrane and S@CCP membrane remain intact, while the SPEEK membrane is slightly broken. This result indicates that the mechanical stability of S@CCP membrane is improved remarkably by the CCP scaffold. Figure 4c illustrates the weight loss of various samples after immersing in 1.5 M VO2+ solution for 60 days. The CCP separator exhibits the smallest weight loss, demonstrating its superior chemical stability in vanadium electrolyte. The merits of chemical and mechanical stability reveal that the CCP can be used as a robust scaffold for the SPEEK membrane. Through the CCP reinforcement, the weight loss of the S@CCP membrane is reduced more than half compared to the control SPEEK membrane.

12


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Figure 4. Room temperature (25±2 oC) oxidative stability test: (a) color change of various membranes immersed 0.1 M VO2+ electrolytes with time, (b) photographs of various membranes after immersed in 0.1 + + M VO2 electrolyte for 60 days, (c) weight loss of various membranes after immersed in 1.5 M VO2 electrolyte for 60 days.

From the above discussion, it can be concluded that the robust CCP scaffold can significantly boost the SPEEK membrane properties such as improving mechanical strength, reducing swelling and vanadium ion crossover, and enhancing chemical/mechanical stability. These characteristics will be helpful to obtain an efficient and durable VFB based on the S@CCP membrane. A VFB single-cell test was therefore conducted to investigate the rate performance and cycling stability of the S@CCP membrane comparing with the N212 and SPEEK membranes. Figure 5 reveals the rate performance of various membranes over a current density range of 40-200 mA cm-2. Due to the shorter charge/discharge times, the Coulombic efficiency (CE) of all membranes increases with the increasing current density (Figure 5a). The CE follows the order of S@CCP > SPEEK >> N212, agreeing well with the vanadium crossover and self-discharge results (Figure 3). Notably, the S@CCP membrane exhibits a CE higher than 95% even at a current density of 40 mA cm-2, and the CE exceeds 98% when the current density reaches 100 mA cm-2. Such a high CE can be achieved with this ultrathin (33 µm) membrane, which has rarely been reported in previous work.9-12 The voltage efficiency (VE) of all membranes decreases with increasing current density because of

13



increased polarization (Figure 5b). The S@CCP membrane shows a lower VE than the SPEEK membrane due to the large area resistance, and the gap increasing with the current density. Under the mutual effect of CE and VE, the S@CCP membrane exhibits the highest energy efficiency (EE) at a current density of 40 mA cm-2, while the SPEEK membrane shows the highest EE at the current density range of 80-200 mA cm-2 (Figure 5c). Besides, the S@CCP membrane owns higher EE than the commercial N212 membrane at all current densities. At a moderate current density of 80 mA cm-2, the EE of the S@CCP and control SPEEK membranes is 86.8% and 88.3% respectively, whereas the EE of the N212 membrane is only 77.7%.

Figure 5. Rate performance of VFBs with N212, SPEEK and S@CCP membranes: (a) Coulombic efficiency, (b) voltage efficiency, (c) energy efficiency. 14


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Apart from rate performance, cycling stability is another important metric to evaluate a membrane for VFB. A 100-cycles charge/discharge test is conducted, and the CE and discharge capacity retention of various membranes are shown in Figure 6. The initial CE of all membranes is in accordance with the value in rate performance test (Figure 5a). The S@CPP and N212 membranes maintain a stable CE within 100 cycles, demonstrating their excellent chemical/mechanical stability. The CE of the control SPEEK membrane decreases rapidly from the 77th cycle, indicating a partial membrane rupture. As a result, the SPEEK membrane shows a very fast capacity decay and approaches 0 after 100 cycles. The failure of the SPPEK membrane in such a short run time is mainly due to its poor mechanical stability as it is much thinner than previous work (41 vs. 70-100 µm).22,31 The S@CCP membrane exhibits slower capacity decay rate than the N212 membrane, and the capacity retention after 100 cycles for the S@CCP and N212 membranes is 80.4% and 41.9%, respectively. The excellent stability of the S@CCP membrane can also be verified by the unchanged micro-morphology after the cycling performance test (Figure S2). It can be concluded that S@CCP membrane possesses a superior chemical/mechanical stability in vanadium electrolyte under strong oxidizing and acidic condition, which is consistent with aforementioned ex-situ oxidative stability results.

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Figure 6. Cycling stability of VFBs with N212, SPEEK and S@CCP membranes at a moderate current density of 80 mA cm-2.

In order to examine the durability of the S@CCP membrane, we carried out a super long-term charge/discharge test at a high current density of 160 mA cm-2. After each 500 cycles test, the positive and negative electrolytes of the VFB were refreshed, followed by another 500 cycles test. The ambient temperature was controlled at 26±2 oC. As illustrated in Figure 7a, the S@CCP membrane shows extremely stable CE (~99%) and EE (~76%) over the whole 1500 cycles (around 40 days) test. The discharge capacity follows the same fading trend in three rounds of testing, and the capacity retention is 32.1%, 44.1% and 35.3% respectively after 500 cycles (Figure 7b). The average capacity decay rate is only 0.126% per cycle over 1500 cycles test, demonstrating the superior durability of the S@CCP membrane for application in long lifespan VFB.

Figure 7. Durability of the S@CCP membrane over three rounds of 500 cycles test at a high current density of 160 mA cm-2: (a) the efficiencies, (b) discharge capacity retention.

As a large-scale energy storage battery, instantaneous high-power output capability is an important index of VFB.10 Figure 8 presents the room temperature polarization and power density curves of a VFB

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with the S@CCP membrane. The voltage of the VFB decreases linearly with the increasing discharge current density due to gradually increased Ohmic and electrochemical polarization.38 Meanwhile, a maximum power density of 377 mW cm-2 is achieved at a current density of 550 mA cm-2. This high-power ability ensures that the S@CCP membrane is suitable for grid-scale VFB.

Figure 8. Polarization and power density curves of the VFB with the S@CCP membrane. Inset shows the photograph of the single cell.

Temperature-tolerance is essential for VFB since it is mainly used as energy storage system of sustainable energy.39,40 Figure 9 and Figure S3 display the all-climate performance of a VFB with the S@CCP membrane. The VFB was ran for 10 cycles at each temperature from -20 to 60 °C under a current density of 80 mA cm-2. The S@CCP membrane shows stable cycling performance over the wide testing temperature ranges. The CE decreases gradually with increasing temperature because of the accelerated vanadium crossover.16 On the contrary, the VE increases with temperature due to the improved electrochemical reaction kinetics.46 Under the joint action of CE and VE, the EE reaches the highest value of around 86% at the temperature range of 30-60 oC. When the temperature returns to 20 oC, the CE, VE and EE of the S@CCP membrane almost go back to its initial value, demonstrating the excellent temperature-tolerance of the S@CCP membrane. 17



Figure 9. All-climate performance of the VFB with the S@CCP membrane. Inset shows the photograph of the wide-temperature test platform.

CONCLUSION In summary, we developed a facile sodium dodecyl benzene sulfonate (SDBS) modifying method to fabricate a ceramic-coated porous PE (CCP) reinforced SPEEK membrane (S@CCP) for VFB application. The 3D porous structure and excellent wettability of the SDBS-modified CCP allow the quick infiltrating of SPEEK solution, resulting in a ultrathin, uniform and dense S@CCP membrane. The robust CCP scaffold can significantly boost the SPEEK membrane properties such as improving mechanical strength, reducing swelling and vanadium ion crossover, and enhancing chemical/mechanical stability. As a result, the S@CCP membrane demonstrates excellent rate performance and cycling stability superior to the commercial Nafion 212 membrane and the control SPEEK membrane. The S@CCP membrane also exhibits outstanding durability over 1500 cycles with a stable CE (~99%) and EE (~76%) as well as slow capacity decay rate (0.126% per cycle) at a high current density of 160 mA cm-2. In addition, the S@CCP membrane shows stable cycling performance in a wide temperature range of -20–60 oC. This newly developed S@CCP membrane can also be fabricated via a scalable process in the well-developed roll-to-roll production line, resulting in a much lower cost (< $20 per m2). Therefore, this efficient, durable 18


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and cheap S@CCP membrane is very promising in VFB practical application.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional information, including experimental details, parameters of as-received CCP separator, CCP wettability in SDS aqueous solution and DI water, estimated price of the S@CCP membrane and SEM images of the S@CCP membrane after life test (PDF)

■ AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Author Contributions §

(D.M., L.H.Y.) These authors contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21576154), the Shenzhen Basic Research Project (No. JCYJ20170412170756603, JCYJ20170307152754218 and JCYJ20170818115018000) and the Natural Science Foundation of Guangdong Province (No. 2015A030313894 and 2016A030310025). 19



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(46) Wu, L.; Shen, Y.; Yu, L.; Xi, J.; Qiu, X. Boosting Vanadium Flow Battery Performance by Nitrogen-Doped Carbon Nanospheres Electrocatalyst. Nano Energy 2016, 28, 19-28.

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Towards Cheaper Vanadium Flow Batteries: Porous Polyethylene

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