Letter www.acsami.org
Durable and Efficient PTFE Sandwiched SPEEK Membrane for Vanadium Flow Batteries Lihong Yu† and Jingyu Xi*,‡ †
School of Applied Chemistry and Biological Technology, Shenzhen Polytechnic, Shenzhen 518055, China Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
‡
S Supporting Information *
ABSTRACT: To overcome the poor cycling stability of sulfonated poly(ether ether ketone) (SPEEK) membrane in vanadium flow batteries (VFB), we demonstrate a facile and effective sandwich design by using hydrophilic porous poly(tetrafluoroethylene) (PTFE) films as a stress protective and electrolyte buffer layer for SPEEK membrane. VFB based on this novel sandwich PTFE/SPEEK/PTFE membrane exhibits super long-life properties, which can steadily run (98.5% of Coulombic efficiency and 85.0% of energy efficiency @ 80 mA cm−2) with 2.0 M vanadium electrolyte for more than 1000 cycles. This simple and powerful strategy may also be applied to other nonfluoride membranes. KEYWORDS: vanadium flow battery, membrane, SPEEK, cycling performance, capacity fading
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each one of these external damages can instantly tear up a membrane and destroy the VFB. To solve these problems, we first proposed a new design concept that the IEM (Figure 1a, b) is sandwiched and protected by a 3D porous, hydrophilic and particularly stable PTFE film (Figure 1c, d) on both sides, which would thoroughly change the interactions between IEM and other components, such as electrolytes, electrodes, and sealing materials. Although some kinds of multilayered or sandwiched membranes were investigated for VFB application,20,21 our strategy is completely different since the three components are directly superimposed to form a sandwich structure. This novel PTFE/IEM/PTFE design proved to be a very simple and effective way, showing unexpected but understandable successes: (1) prevent the IEM from being pierced by fibers in the electrode (Figure 1e, f); (2) relax the stress from the sealing gasket and prevent the IEM from tearing around the sealing gasket, which would generate electrolyte leakage channels; (3) protect the IEM from being directly scoured and corroded (degraded) by the continuously flowing electrolytes. Finally, porous PTFE film essentially is cheap and pliable and would bring no damage to the VFB. Under this new design concept, the VFB based on SPEEK membrane showed outstanding performances, which can steadily run at very high capacity retention for more than 1000 charge−discharge cycles. This study have not only verified the success of the new PTFE/
mong all energy storage systems (EES) from medium to large-scale, vanadium flow batteries (VFB) have many benefits over the others because of the super long lifespan, unlimited capacity, flexible design and safety, enabling them one of the most promising technologies in the present and the future.1−3 Ion exchange membrane (IEM) is the key component to separate the positive and negative electrolytes besides transporting protons to complete the electrical circuit in VFB, which mainly decides the performances (efficiencies, cycling stability, capacity fading, etc.) and the cost (up to 30− 50%) of VFB.4−7 To replace the widely used but very expensive perfluorinated Nafion membranes,8−10 great efforts have been made in the search of alternative nonperfluorinated skeleton based cation exchange membranes (CEM),4−6 anion exchange membranes (AEM),11 and the innovative porous membranes.12 Among all the candidates for IEMs, sulfonated poly(ether ether ketone) (SPEEK) based membrane is considered to be the most attractive one to replace Nafion membrane because of its lower cost8,13 and high proton to vanadium ion selectivity,14−17 if only the poor stability be solved.18,19 Through the literature survey, it is found that no matter for the perfluorinated or for the nonperfluorinated membranes, researchers have somehow focused only on the membrane internal structure and have indeed improved its essential stability in the harsh acid and oxidation vanadium electrolyte.1−6 However, they have ignored the damages on the membranes from the external forces, such as the exotic piercing force from graphite felt (GF) fiber, uneven pressure from sealing gasket, and the impact from the electrolyte flow. In fact, © XXXX American Chemical Society
Received: June 27, 2016 Accepted: August 31, 2016
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DOI: 10.1021/acsami.6b07782 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
icochemical properties of naked SPEEK membrane and sandwiched PTFE30/SPEEK/PTFE30 membrane are compared in Table S1. Nafion 115 membrane is selected as the benchmark because of its similar thickness with the sandwiched PTFE30/SPEEK/PTFE30 membrane. The morphology and microstructure of the SPEEK membrane (70 μm), porous PTFE film (30 μm), and GF electrode were characterized and compared. Figure 1 a, c,and e shows the digital photograph of SPEEK membrane, PTFE film and GF, respectively. Figure 1 b, d, and f illustrates the SEM images corresponding to their left counterparts, respectively. From Figure 1 a-d, it is seen that the SPEEK membrane is transparent and light yellow, whereas the porous PTFE film is half-transparent and white, and both of them are homogeneous no matter in microscopy or in macroscopy. As shown in Figure 1e, f, GF has overspreading prickly fibers distributed on the surface and typically on the edge which may penetrate the IEMs during the assembling and operating process of VFB. By comparing Figure 1d, f, it is found that the pores in PTFE film are around 1−3 μm in size, much smaller than the GF fiber diameter (10−15 μm), which means the GF fiber could not pass through the PTFE pores and then prick the IEMs in the novel PTFE/IEM/PTFE design. This theoretically explains why PTFE can protect the IEM from being pierced by fibers in the GF. Before fabricating PTFE/IEM/PTFE architecture, performance comparison between the two typical IEMs, the commercial perfluorinated one (Nafion) and the nonfluoride one (SPEEK), should be conducted first. The VFBs based on these two naked IEMs were assembled22,23 and their profiles of Coulombic efficiency (CE), voltage efficiency (VE) and energy efficiency (EE) as the function of current densities are illustrated in Figure 2. Remarkable advantages of SPEEK over Nafion in terms of CE and EE are noticed confirming the good properties of SPEEK, agreeing well with the previous report.14−17 The VE of SPEEK based VFB slightly lower than that of Nafion can be attributed to the larger area resistance of SPEEK membrane (Table S1). To evaluate the stability of the SPEEK membrane, we conducted long-term charge−discharge cycling at the current density of 80 mA cm−2 (2 A). The cycling performance of VFBs with various membranes are displayed in Figure 3. Unfortunately, the charge capacity of SPEEK based VFB experienced a slow decaying process followed by a rather rapid decaying process and then suddenly fell at the 155 cycle. The change tendency of CE, such as slowly decay from 100 to 150 cycle and sharp decline after 150 cycle, is consistent with the capacity fading process. This phenomenon is mainly induced by VO2+ degradation18 and accidental ruptures of the naked SPEEK membrane caused by its poor chemical stability and mechanical property.19,22 The ruptures (marked by blue arrows) can be observed on the used SPEEK membrane, as shown in the left inset of Figure 3a. The visible cracks in the middle region are caused by the combination of electrolyte corrosion (degradation) and electrode stress, while the cracks located at the edge of effective area (50 × 50 mm) are also due to the addition stress from the sealing gasket.22 Under this circumstance, protecting IEMs by porous PTFE film to suppress the adverse effect of electrolyte, electrode, and sealing gasket as well as the torrential electrolyte flow is very essential. 3D porous structured PTFE film, as shown in Figure 1d, has a very important prerequisite to protect the IEMs in VFB that PTFE is completely hydrophilic. This is confirmed in Figure S3,
Figure 1. (a) Photograph of SPEEK membrane. (b) Cross-section SEM image of SPEEK membrane. (c) Photograph of porous PTFE film. (d) SEM image of porous PTFE film. Inset shows an enlarged image. (e) Photograph of graphite felt electrode. (f) Digital microscopy image of graphite felt with 50 times magnification. Inset shows the surface site and cross-section site SEM images of the graphite felt.
Figure 2. Cell performance of VFBs with Nafion and SPEEK membranes under different current densities.
IEM/PTFE design concept, but confirmed the bright future of SPEEK membrane used in VFB. SPEEK was fabricated by the sulfonation of commercial PEEK and the molecular structures of PEEK and SPEEK are shown in Figure S1. The optimized preparation details19 are described in the Supporting Information and the preparation procedure of SPEEK membrane is shown in Figure S2. It can be seen that the procedure of fabricating SPEEK membranes is simple and controllable, allowing large-scale production of the low-cost and homogeneous SPEEK membranes. The physB
DOI: 10.1021/acsami.6b07782 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 3. Long-term cycling performance of VFBs with Nafion, PTFE30/Nafion/PTFE30, SPEEK, and PTFE30/SPEEK/PTFE30 membranes: (a) Coulombic efficiency, inset shows the photographs of SPEEK membrane after use; (b) charge capacity.
Figure 4. Charge−discharge curves for VFBs with PTFE protected PTFE30/Nafion/PTFE30 and PTFE30/SPEEK/PTFE30 membranes over 200 cycles.
Figure 5. Self-discharge curves of VFBs with Nafion, SPEEK, and PTFE30/SPEEK/PTFE30 membranes.
that the pure PTFE film and the PTFE/SPEEK/PTFE sandwiched membrane are totally transparent when water is added in. It is encouraging that attaching porous PTFE film on SPEEK membrane would not affect the capacities or efficiencies of the VFB single cell, since area resistance of PTFE/SPEEK/ PTFE sandwiched membrane (0.89 Ω cm2) is only slightly larger than that of naked SPEEK membrane (0.87 Ω cm2, see Table S1). The CE, VE, and EE of VFBs based on SPEEK with or without PTFE films are typically the same and so does for the Nafion, which is verified in Figure S4. What PTFE affects most is the cycling lifetime of the VFBs. As shown in Figure 3, the charge capacities of VFBs based on Nafion and PTFE30/ Nafion/PTFE30 similarly underwent a rapid decaying process and the capacity retention after only 200 cycles is 22.9 and 26.6%, respectively, because of the rapid vanadium ions crossover.5,11 In fact, Nafion does not require a PTFE
protective layer since Nafion itself has an extremely high chemical stability and good mechanical properties. Besides, the fast vanadium ion crossover issue of Nafion can not be solved by the PTFE protective strategy. By contrast, with the protection of PTFE layer, VFB based on PTFE30/SPEEK/ PTFE30 avoided the degradation and rupturing of SPEEK. It steadily ran for 1000 cycles with a relatively stable CE near to 99% and a final capacity retention of 41%. This is absolutely a great success of the pure SPEEK membrane based VFB to run 1000 cycles (1624 h) without being broken. What’s more, the SPEEK membrane remained intact after 1000 cycles as shown in the right inset of Figure 3a. After 1000 cycles test, the porous PTFE film also remains good hydrophilic and intact micro 3D connected porous structure, as shown in Figure S5, which confirms the reliable stability and reusable of porous PTFE film used in VFB. C
DOI: 10.1021/acsami.6b07782 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
Figure 6. Cycling performance of VFBs assembled with Nafion, SPEEK and PTFE protected SPEEK membranes. Inset shows the various protect strategies including positive side protective (PTFE30/SPEEK/none), negative side protective (none/SPEEK/PTFE30), and both sides protective (PTFE30/SPEEK/PTFE30).
PTFE30/SPEEK/PTFE30, which further confirms the unique of the sandwiched PTFE/SPEEK/PTFE design concept. Besides the above-discussed protection effect, the porous PTFE film can also serve as a buffer layer to avoid direct erosion of electrolyte on SPEEK membrane. This can be proved by the ex-situ VO2+ ion permeation experiment and in situ self-discharge test. As shown in Figure S7, SPEEK membrane exhibits very slow VO2+ ion crossover compared to Nafion, resulting in the high CE value of SPEEK-based VFB (see Figure 2). It is worth noting that the VO2+ ion permeation through PTFE30/SPEEK/PTFE30 is slower than that of naked SPEEK. In addition, the VFB assembled with PTFE30/SPEEK/ PTFE30 also displays slightly longer self-discharge time than that of SPEEK, as shown in Figure 5. The slow ion diffusion phenomena of PTFE30/SPEEK/PTFE30 in case of no external power drive (i.e., electric field) should be assigned to that the porous PTFE film can hold a relatively stable electrolyte layer. This buffer layer would also contribute to the super long-life of PTFE30/SPEEK/PTFE30 for VFB application (Figure 3). To further investigate the detail protection mechanism of porous PTFE film, we conducted the cycle life test of VFBs assembled with Nafion, SPEEK, one side PTFE protected SPEEK, and PTFE sandwiched SPEEK membranes and the results are compared in Figure 6. All VFBs were stopped when the capacity retention decays to 60%. The CE of Nafion is the lowest one among all these tested membranes and it costs only 43 cycles for capacity retention decaying to 60%. The CE of all the VFBs based on SPEEK whether or not protected by PTFE on one or both sides maintains a higher value than Nafion and shows no decline through all the whole cycling test. It is noticed that the thickness of porous PTFE film would influence the cycling results of the VFBs. The PTFE15/SPEEK/PTFE15, PTFE30/SPEEK/PTFE30, and PTFE60/SPEEK/PTFE60 membranes can persist 478, 620, and 605 cycles respectively before the capacity retention decays to 60%, demonstrating that the protection is probably constant once the PTFE layers reach a given thickness. On the contrary, SPEEK without protection by
A detailed charge−discharge investigation of the assembled VFBs is a typical way to examine how well the IEMs work. Figure 4 compares the charge−discharge curves for VFBs based on PTFE 30/Nafion/PTFE 30 and PTFE30 /SPEEK/PTFE30 membranes to see how porous PTFE film affects different IEMs. The charge and discharge platforms of VFBs based on both membranes are shortened with the increase of cycle number. However, the former fades much faster than the latter. The capacity retention of VFB based on PTFE30/SPEEK/ PTFE30 is much higher than that on PTFE30/Nafion/PTFE30 by 82.9% vs 26.6% after 200 cycles, which means a much lower vanadium ion crossover through PTFE30/SPEEK/PTFE30 than PTFE30/Nafion/PTFE30. The advantages of the PTFE30/ SPEEK/PTFE30 come from the combined results of the core SPEEK membrane (see Figure 2) and the protecting PTFE film (see Figure 3). On the other hand, a traditional pore filling membrane24,25 SPEEK@PTFE30 was compared with this novel sandwiched PTFE30/SPEEK/PTFE30 architecture to examine whether the skeleton reinforced structure would exceed this sandwich structure through VFB cycling test as shown in Figure S6. It can be seen from Figure S6b that SPEEK@PTFE30 membrane shows a constant CE (∼99%) during the whole 500 cycles test, suggesting that the PTFE pore filling method can also enhance the stability of SPEEK membrane. However, because of the nonproton-conducting PTFE skeleton occupies a part (∼20%) of the membrane matrix (Figure S6), it leads to the increase in the area resistance of the SPEEK@PTFE30 (1.15 Ω cm2 vs 0.87 Ω cm2 of naked SPEEK). Hence, the EE of VFB with SPEEK@PTFE30 membrane is only about 76%, which is obviously lower than that of naked SPEEK (∼85%). On the contrary, the surface attaching PTFE protected layer would not affect the cell efficiencies of PTFE30/Nafion/PTFE30 based VFB, as discussed in Figure S4. According to the previous study, capacity decay of VFB is mainly caused by electrolyte imbalance and side reaction. 26−29 The SPEEK@PTFE30 presents an accelerated capacity fading trend compared to D
DOI: 10.1021/acsami.6b07782 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
(3) Xi, J.; Wu, Z.; Qiu, X.; Chen, L. Nafion/SiO2 Hybrid Membrane for Vanadium Redox Flow Battery. J. Power Sources 2007, 166, 531− 536. (4) Li, X.; Zhang, H.; Mai, Z.; Zhang, H.; Vankelecom, I. Ion Exchange Membranes for Vanadium Redox Battery (VRB) Applications. Energy Environ. Sci. 2011, 4, 1147−1160. (5) Schwenzer, B.; Zhang, J.; Kim, S.; Li, L.; Liu, J.; Yang, Z. Membrane Development for Vanadium Redox Flow Batteries. ChemSusChem 2011, 4, 1388−1406. (6) Doan, N. L.; Hoang, T. K. A.; Chen, P. Recent Development of Polymer Membranes as Separators for All-Vanadium Redox Flow Batteries. RSC Adv. 2015, 5, 72805−72815. (7) Jiang, B.; Yu, L.; Wu, L.; Mu, D.; Liu, L.; Xi, J.; Qiu, X. Insights into the Impact of the Nafion Membrane Pretreatment Process on Vanadium Flow Battery Performance. ACS Appl. Mater. Interfaces 2016, 8, 12228−12238. (8) Minke, C.; Turek, T. Economics of Vanadium Redox Flow Battery Membranes. J. Power Sources 2015, 286, 247−257. (9) Reed, D.; Thomsen, E.; Wang, W.; Nie, Z.; Li, B.; Wei, X.; Koeppel, B.; Sprenkle, V. Performance of Nafion® N115, Nafion® NR-212, and Nafion® NR-211 in a 1 kW Class All Vanadium Mixed Acid Redox Flow Battery. J. Power Sources 2015, 285, 425−430. (10) Xi, J.; Wu, Z.; Teng, X.; Zhao, Y.; Chen, L.; Qiu, X. SelfAssembled Polyelectrolyte Multilayer Modified Nafion Membrane with Suppressed Vanadium Ion Crossover for Vanadium Redox Flow Batteries. J. Mater. Chem. 2008, 18, 1232−1238. (11) Maurya, S.; Shin, S. H.; Kim, Y.; Moon, S. H. A Review on Recent Developments of Anion Exchange Membranes for Fuel Cells and Redox Flow Batteries. RSC Adv. 2015, 5, 37206−37230. (12) Zhang, H.; Zhang, H.; Li, X.; Mai, Z.; Zhang, J. Nanofiltration (NF) Membranes: the Next Generation Separators for All Vanadium Redox Flow Batteries (VRBs). Energy Environ. Sci. 2011, 4, 1676− 1679. (13) Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Alternative Polymer Systems for Proton Exchange Membranes (PEMs). Chem. Rev. 2004, 104, 4587−4612. (14) Li, Z.; Xi, J.; Zhou, H.; Liu, L.; Wu, Z.; Qiu, X.; Chen, L. Preparation and Characterization of Sulfonated Poly(Ether Ether Ketone)/Polyvinylidene Fluoride Blend Membrane for Vanadium Redox Flow Battery Application. J. Power Sources 2013, 237, 132−140. (15) Winardi, S.; Raghu, S. C.; Oo, M. O.; Yan, Q.; Wai, N.; Lim, T. M.; Skyllas-Kazacos, M. Sulfonated Poly(Ether Ether Ketone)-Based Proton Exchange Membranes for Vanadium Redox Flow Battery Applications. J. Membr. Sci. 2014, 450, 313−322. (16) Li, Z.; Dai, W.; Yu, L.; Xi, J.; Qiu, X.; Chen, L. Sulfonated Poly(Ether Ether Ketone)/Mesoporous Silica Hybrid Membrane for High Performance Vanadium Redox Flow Battery. J. Power Sources 2014, 257, 221−229. (17) Dai, W.; Yu, L.; Li, Z.; Yan, J.; Liu, L.; Xi, J.; Qiu, X. Sulfonated Poly(Ether Ether Ketone)/Graphene Composite Membrane for Vanadium Redox Flow Battery. Electrochim. Acta 2014, 132, 200−207. (18) Yuan, Z.; Li, X.; Hu, J.; Xu, W.; Cao, J.; Zhang, H. Degradation Mechanism of Sulfonated Poly(Ether Ether Ketone) (SPEEK) Ion Exchange Membranes Under Vanadium Flow Battery Medium. Phys. Chem. Chem. Phys. 2014, 16, 19841−19847. (19) Xi, J.; Li, Z.; Yu, L.; Yin, B.; Wang, L.; Liu, L.; Qiu, X.; Chen, L. Effect of Degree of Sulfonation and Casting Solvent on Sulfonated Poly(Ether Ether Ketone) Membrane for Vanadium Redox Flow Battery. J. Power Sources 2015, 285, 195−204. (20) Jia, C.; Liu, J.; Yan, C. A multilayered membrane for vanadium redox flow battery. J. Power Sources 2012, 203, 190−194. (21) Jia, C.; Liu, J.; Yan, C. A significantly improved membrane for vanadium redox flow battery. J. Power Sources 2010, 195, 4380−4383. (22) Li, Z.; Dai, W.; Yu, L.; Liu, L.; Xi, J.; Qiu, X.; Chen, L. Properties Investigation of Sulfonated Poly (Ether Ether Ketone)/Polyacrylonitrile Acid−Base Blend Membrane for Vanadium Redox Flow Battery Application. ACS Appl. Mater. Interfaces 2014, 6, 18885−18893.
PTFE can only run for 124 cycles under the same condition. In addition, only one side protection by PTFE also decreases the capacity fading rate effectively. PTFE30/SPEEK/None (protected at positive side) shows better effect than None/SPEEK/ PTFE30 (protected at negative side) by 309 vs 230 cycles when capacity decays to 60%, because of the stronger oxidation of VO2+ in the positive electrolyte.30 The above results indicate that the protection of the IEM on both sides is very necessary. More impressively, the estimated cost of PTFE/SPEEK/ PTFE sandwiched membrane based on lab-scale production is about 100 US dollars per m2, which is much cheaper than that of commercial Nafion series membranes (500−700 US dollars per m2).4,5 The success of the PTFE/SPEEK/PTFE is just one example of the design concept because any of the components in the PTFE/IEM/PTFE design can be replaced once a more advanced membrane, especially nonfluoride membrane, is invented and the further improved VFB performances can be achieved. In summary, a new design concept PTFE/IEM/PTFE is first proposed that the porous abundant and reliable PTFE film can sandwich the IEM preventing the external damage to the IEM from the electrolytes, electrodes and sealing materials. The design typically benefits from that PTFE can prevent the pricks from the fibers in the GF electrode, relax the stress on the IEM from the sealing gasket and protect the IEM from being directly scoured and corroded by the flowing electrolytes. By this design concept, the VFB based on homemade SPEEK membrane showed greatly promoted performances and steadily ran at a very high capacity retention for more than 1000 charge− discharge cycles. Therefore, the new design concept makes the low-cost nonperfluorinated IEMs more competitive for practical VFB application.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b07782. Additional information, including experimental details, preparation and characterization of SPEEK membrane, wettability of porous PTFE film, VFB performance of various membranes, and VO2+ crossover curves (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors appreciate financial support from the NSFC (21576154), NSF of Guangdong Province (2016A030310025 and 2015A030313894) and Basic research project of Shenzhen City (JCYJ20150630114140630 and JCYJ20150331151358143).
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REFERENCES
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