Rice Paper Reinforced Sulfonated Poly(ether ether ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Rice Paper Reinforced Sulfonated Poly(ether ether ketone) as LowCost Membrane for Vanadium Flow Batteries Di Mu,† Lihong Yu,‡ Le Liu,† 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

‡

S Supporting Information *

ABSTRACT: Low-cost, highly efficient, and durable membrane is essential for practical application of vanadium flow battery (VFB). Sulfonated poly(ether ether ketone) (SPEEK) membrane is considered as potential candidate to replace the expensive Nafion membrane because of its high proton to vanadium ion selectivity. To overcome the poor mechanical/chemical stability of SPEEK membrane in VFB, a new strategy by using commercial paper (copy paper, rice paper, and filter paper) as the scaffold of SPEEK, and subsequently self-cross-linking the SPEEK through thermal treatment was conducted. The optimized rice paper reinforced SPEEK membrane exhibits superior VFB performances to that of the benchmark Nafion 115 membrane, such as 98.3% vs 92.8% for Coulombic efficiency, 79.5% vs 75.8% for energy efficiency, and 0.13% vs 0.16% for capacity decay per cycle at a high current density of 120 mA cm−2. Besides, S@RP-C6 shows an extremely stable performance in the temperature range of 0−70 °C. Combining the outstanding battery performances with the extremely low cost (estimated at about $10 per m2), the rice paper reinforced SPEEK membrane shows a very good application prospect in VFB. KEYWORDS: Vanadium flow battery, Paper reinforced membrane, Sulfonated poly(ether ether ketone), Cycling performance, Capacity retention

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INTRODUCTION In recent years, affordable and sustainable energy has been the cornerstone of economic prosperity and social development. Ever-expanded demand for energy requires a powerful support of large-scale energy storage technology.1−3 As one of the most widely spread large-scale energy storage technologies, vanadium flow battery (VFB) gets more attention due to the advantages of flexible design, fast response time and safety.4−7 VFB system uses VO2+/VO2+ and V3+/V2+ redox couples as positive and negative electrolytes respectively, and utilizes the redox reaction between different valence state of vanadium ions couples to achieve energy storage and release. A cell stack is primarily made up of three types of components, which are 1−2 M of vanadium ions and 1−3 M of sulfuric acid as electrolyte, carbon (graphite) felt as electrode and ion-exchange membrane (IEM) as separator.8−10 The importance of IEM in VFB system is self-evident.11−13 It separates positive and negative electrolytes, prevents cross mixing of vanadium ions, and provides the proton conduction © 2017 American Chemical Society

as well. Coulombic efficiency (CE), voltage efficiency (VE), energy efficiency (EE), and cycling stability are the important parameters in VFB. These parameters are all related to the physicochemical properties of IEM. However, in the process of operating, vanadium ion penetrating into IEM cause permeability and electrolyte imbalance which have a great influence on the battery efficiency and life span.14 In addition, since the IEM of VFB is always soaked in the strong oxidizing solution with constant erosion, the membrane may change its original structure and lose original function as well. Generally, an ideal IEM should not only have a perfect ion selectivity (low vanadium ion permeability and high proton conductivity), but meet the condition of high stability, good mechanical properties and low cost.15−17 Received: November 17, 2016 Revised: January 18, 2017 Published: February 3, 2017 2437

DOI: 10.1021/acssuschemeng.6b02784 ACS Sustainable Chem. Eng. 2017, 5, 2437−2444

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ACS Sustainable Chemistry & Engineering

linked by subsequent thermal treatment30 to improve further its cycling stability in VFB.

To date, perfluorosulfonic Nafion series membranes are the most widely used IEM in the VFB.18 Nafion series membranes have excellent proton conductivity and outstanding chemical stability, but fast vanadium ion crossover caused by the micelle structure19 and expensive price are the major problems of Nafion series membranes. These two main problems restrict its application in VFB to some degree. In order to solve the significant problems of Nafion membrane fundamentally, more researchers carry out the research about hydrocarbon framework of nonfluorine membranes.20−23 The development and application of aromatic ethers polymer membranes, such as sulfonated poly(ether ether ketone) (SPEEK), become a hotspot in VFB. On the one hand, the small hydrophilic/ hydrophobic microphase separation structure that makes the membrane shows a higher proton to vanadium ion selectivity than the Nafion membranes. On the other hand, the polymer material as one of the general engineering plastics demonstrates high temperature resistance, corrosion resistance and low cost. Where there are proper sulfonating agents and the suitable sulfonated conditions, there will be a better proton conductivity, stability, and ion selectivity.24−26 Nevertheless, comparing with the state-of-the-art Nafion membranes, the poor chemical stability and mechanical strength hinder SPEEK development and practical application in VFB system. Yan et al. has used porous polypropylene (PP) as an inner reinforced layer for SPEEK membrane to enhance its stability in VFB.27,28 In this work, three kinds of commercial paper (i.e., copy paper, rice paper, and filter paper) are chosen to act as the scaffold of SPEEK to enhance its mechanical/ chemical stability, and simultaneously reduce the cost of the membrane. As shown in Scheme 1a, paper is a three-

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EXPERIMENTAL SECTION

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RESULTS AND DISCUSSION

Materials. Poly(ether ether ketone) (PEEK 450G) was purchased from Victrex. Nafion 115 membrane (N115) was purchased from DuPont. The copy paper (A4, deli7401), rice paper (Φ11 cm, fs-wfgd007), filter paper (Shuangquan 203) and other regents were purchased from local suppliers and used as received. Membrane Preparation. SPEEK resin was prepared by sulfonating PEEK with H2SO4 (98 wt %) at 52 °C for 5 h.31 The sulfonation degree of SPEEK was 68% in this work. The paper reinforced SPEEK membranes were prepared by solution casting method. Briefly, 1.5 g of SPEEK resin was added into 10 mL of DMF and stirred for 12 h to obtain a uniform casting solution. Put the paper substrate onto a clean glass plate, poured proper amount of casting solution on the paper, scraped with a blade, and dried at 70 °C for 24 h. Finally, peeled off the membrane in deionized water. Pure SPEEK membrane was also prepared for comparison. The as-prepared membranes were soaked in deionized water for 24 h before use. To improve further the mechanical strength and chemical stability of paper reinforced SPEEK membranes, the optimized rice paper reinforced SPEEK membrane (S@RP) was chosen to thermal crosslinking at 140 °C32−34 in a vacuum for 6 h (S@RP-C6) and 12 h (S@ RP-C12), respectively. The benchmark N115 membrane was pretreated to remove impurities.16 The detailed process is as follows: boiled N115 membrane in 3 wt % H2O2, deionized water, 1 M H2SO4, and deionized water for 1 h in order. Characterization. The surface and cross section of the membranes were observed by scanning electron microscopy (SEM, ZEISS SUPRA 55) and energy-dispersive X-ray spectroscopy (EDX). The physicochemical properties, including water uptake, swelling ratio, VO2+ permeability,35 area resistance,36 contact angle, and mechanical properties were characterized as reported previously. The experimental details are given in the Supporting Information. Single-Cell Test. The VFB single-cell was assembled as our works reported previously.37,38 The test area of heat-treated graphite felt was 5 × 5 cm, and the membrane was cut into a square with side lengths of 7 cm. Both the positive and negative electrolytes were 50 mL of 1.5 M VO2+/V3+ (1:1) in 2.0 M free H2SO4 solution.39,40 All the cell tests were performed on a Neware CT-3008W battery testing system in the voltage window of 0.8−1.65 V. Broad temperature test was performed using a thermostat (Hongzhan, PU-80) to control the temperature range from 0 to 70 °C.36,40

Scheme 1. Schematic Illustration of the Cellulose Fibers in Paper (a) and Microstructure of Paper Reinforced SPEEK Membrane before and after Thermal Cross-Linking (b)

Paper plays an indispensable role in our daily life. It is made by dewatering a dilute suspension of cellulose fibers, resulting in a three-dimensional porous structure with good wettability and flexibility.41,42 The unique composition and structure of paper inspired us to use it as the scaffold of SPEEK to enhance its mechanical strength and chemical stability for VFB application. We selected three kinds of commercially available and low-cost papers such as copy paper, rice paper, and filter paper in this study, as shown in Figure 1a. The parameters of these papers, including thickness, porosity, density (see Figure S1), and price, are listed in Table S1. Figure 1b,c illustrates the surface and cross section SEM images of three papers, respectively. All papers show a three-dimensional porous structure, which is composed of intertwined cellulose fibers with diameters around 10−22 μm. The interconnected cellulose fibers in paper are held together by hydrogen bonds between the hydroxyl groups nearby and make up a relatively flat surface.41 As expected, all three papers exhibit excellent wettability of the SPEEK casting solution (see Figure 1a) because of its hydrophilic nature. The contact angle follows the order of copy paper > rice paper >

dimensional porous structure, there are many hydroxyl groups (−OH) on the cellulose fibers. These abundant hydroxyl groups (−OH) can interaction with sulfonic groups (−SO3H) of SPEEK and form hydrogen bonds (see Scheme 1b), resulting in enhanced mechanical properties of the paper reinforced membrane. Copy paper reinforced SPEEK membrane (S@CP), rice paper reinforced SPEEK membrane (S@ RP), and filter paper reinforced SPEEK membrane (S@FP) are compared with the pure SPEEK membrane and the benchmark Nafion 115 membrane (N115)16,29 toward VFB application. Moreover, the optimized S@RP membrane was self-cross2438


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our membranes is uniform. Besides, different kinds of paper reinforced membranes appear various colors. S@CP is light blue, S@RP is bright yellow, S@FP is beige, and pure SPEEK membrane is transparent. All membranes display a similar value of contact angle toward vanadium electrolyte since all paper reinforced membranes are covered with the same SPEEK, as confirmed by the surface and cross section SEM images. The uniform distribution of the element S (−SO3H group from SPEEK) in the cross section of paper reinforced membranes demonstrates the successful filling of SPEEK in the paper substrate. The physicochemical properties of various membranes are compared in Table 1. The thickness of S@CP, S@RP, and S@ FP varies with its paper substrate (see Table S1). All paper reinforced SPEEK membranes show a lower water uptake and swelling ratio compared with pure SPEEK membrane, indicating that paper substrate could restrain the swelling of SPEEK membrane. Although the area resistance of paper reinforced SPEEK membranes increases owing to the nonconductive cellulose fibrous skeletons, its value is still comparable to the benchmark N115 membrane. Young’s modulus is one of the physical quantities about tensile or compressive ability within the elastic limit. It represents the rigidity of material, the bigger Young’s modulus is, the more difficult it deformed. Tensile strength is also one of the critical parameter of mechanical properties. Pure SPEEK has a higher Young’s modulus and adequate tensile strength compared with N115, and the paper reinforcement can further improve the mechanical performance through hydrogen bond interaction, as shown in Scheme 1. The S@FP exhibits opposite trend because of the relatively loose fibrous structure of filter paper as shown in cross section SEM image in Figure 1c, and the lowest density as shown in Table S1. The suppressed swelling and enhanced mechanical property will significantly improve the stability of the paper reinforced SPEEK membranes, which will be demonstrated in the subsequent battery testing. Figure 3 shows the VO2+ crossover plots of various membranes at room temperature, and the calculated VO2+ permeability is listed in Table 1. Pure SPEEK membrane displays lower VO2+ permeability compared with N115 membrane due to the smaller hydrophobic/hydrophilic microphase separation structure.44,45 All paper reinforced membranes exhibit much lower VO2+ permeability than that of the pure SPEEK membrane, because the paper cellulose fiber would block the transporting of VO2+. Particularly, S@RP has the smallest VO2+ permeability among all paper reinforced membranes. Coulombic efficiency (CE) and energy efficiency (EE) are very important parameters to evaluate the IEM in VFB system.4 As shown in Figure 4, the CE follows the order of S@RP > S@ FP > S@CP > SPEEK > N115, agreeing well with the VO2+ crossover result. Specifically, the CE of S@RP reaches 96.7% even at the low current density of 40 mA cm−2. The order of EE is S@RP > SPEEK > S@FP > S@CP > N115. S@RP demonstrates the highest EE (89.0%) among all test membranes owing to the comprehensive performance as shown in Table 1. To assess further the cycling stability of three paper reinforced SPEEK membranes, 200 cycles of continuous charge/discharge test were conducted under current density of 120 mA cm−2. S@RP exhibits the most stable efficiency (CE and EE) and lowest capacity fading during the long-term life test (see Figure S2).

Figure 1. Morphology and wettability of various papers: (a) photographs and SPEEK casting solution wettability, (b) surface SEM images, (c) cross section SEM images.

filter paper, and the smaller the contact angle, the larger the droplet diffusion area in the same time (30 s). The mesoporous, fibrous structure and excellent wettability of the paper substrate allow fast polymer infiltration to form the paper/SPEEK hybrid. Figure 2a shows the fabrication process of paper reinforced SPEEK membrane. The resulting membrane is smooth, dense, and flexible, and it can be rolled in any direction to a small size without any wrinkles. Figure 2b−e exhibits the morphology and contact angle of S@CP, S@ RP, S@FP, and pure SPEEK membranes, respectively. From the four photographs in the left column, the whole texture of

Figure 2. (a) Fabrication processes of paper reinforced SPEEK membrane (S@RP as an example). Photograph, surface (inset shows contact angle of vanadium electrolyte), cross section, and S element mapping SEM images of various membranes: (b) S@CP, (c) S@RP, (d) S@FP, (e) SPEEK. 2439


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ACS Sustainable Chemistry & Engineering Table 1. Physicochemical Properties and Estimated Price of Various Membranes Membrane

Thickness (μm)

Water uptake (%)

Swelling ratio (%)

Area resistance (Ω cm2)

Young’s modulus (MPa)

Tensile strength (MPa)

VO2+ permeability (10−7 cm2 min−1)

Pricea ($ m−2)

N115 SPEEK S@CP S@RP S@FP

161 128 154 108 216

32.1 60.6 53.8 51.8 50.8

69.0 21.9 8.7 7.3 4.8

0.91 0.37 0.80 0.52 1.00

57.6 364.8 793.7 524.2 303.4

11.2 11.6 29.8 19.3 10.2

20.2 16.1 8.9 6.9 10.4

70043 40 12 10 20

a

The price of membrane was estimated based on the equations. Cost of N115 = purchase from DuPont/(1 + swelling ratio). Cost of SPEEK = PEEK price + H2SO4 price + DMF price. Cost of paper reinforced SPEEK membrane = SPEEK price × content + paper price × (1 − content).

Figure 3. VO2+ crossover plots of various membranes.

Figure 5. Cell performance of VFBs assembled with various membranes: (a) Coulombic efficiency, (b) energy efficiency, (c) cycling performance @ 120 mA cm−2.

densities, whereas pure SPEEK is better at high current densities. The EE of S@RP is lower than pure SPEEK at high current densities can be ascribed to the increased area resistance resulted from paper reinforcement (see Table 1). On the other hand, cycling stability is another very essential parameter other than the EE for the VFB application. Figure 5c shows the cycling performance of VFBs assembled with S@RP, SPEEK and N115 membranes at current density of 120 mA cm−2. All VFBs present stable CE during 200 cycles test, and the values of CE agree with the rate performance results in Figure 5a. Both pure SPEEK and S@RP membranes display slower capacity fading rate than that of N115 at the first 100 cycles due to their lower vanadium permeability. The accelerated capacity decay of pure SPEEK membrane after 100 cycles can be attributed to its poor stability in VFB.31,38,46 On the contrary, S@RP remains the highest capacity retention through the whole life testing process, indicating its excellent durability (beneficial from the hydrogen bond interaction between paper substrate and SPEEK, as shown in Scheme 1) in the harsh acid and oxidizing vanadium electrolyte. According to previous reports,30,32−34 thermal treatment is a simple and controllable method to self-cross-link SPEEK, resulting in enhanced mechanical properties. Therefore, to improve further the cycling stability in VFB, the optimized S@ RP membrane was self-cross-linked at 140 °C (see Figure S3), as shown in Scheme 1b. The heating time of 6 h (S@RP-C6) and 12 h (S@RP-C12) was used respectively, because too short of time will cause few sulfonic groups (−SO3H) self-crosslinking, whereas a long time will result in an significant increase

Figure 4. Coulombic and energy efficiency comparison of various membranes @ 40 mA cm−2.

More impressively, the estimated cost of paper reinforced SPEEK membranes based on lab-scale production is about $10−20 per m2 (see Table 1), which is much cheaper than that of pure SPEEK membrane ($40 per m2) and commercial N115 membrane ($700 per m2).43 This is because that the very cheap paper substrate occupies a large volume ratio (60−90%, see Table S1) in the reinforced membrane. Through the above analysis, it is apparent that S@RP owns excellent mechanical properties, smallest VO2+ permeability, best VFB performance and the cheapest price among three paper reinforced SPEEK membranes. Therefore, S@RP was selected for the subsequent study. Rate performances of VFBs with S@RP, SPEEK, and N115 are compared in Figure 5. The efficiencies of all batteries show the typical change trend of VFB system with the current density. CE increases with the current density, whereas EE decreases with the current density. The CE of S@RP is higher than that of SPEEK and N115 at all current densities owing to its suppressed vanadium ions crossover (see Figure 3). The EE of S@RP has advantages in the low and middle current 2440


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ACS Sustainable Chemistry & Engineering of membrane resistance. Figure 6 shows the charge−discharge curves of VFBs assembled with S@RP before and after thermal

Figure 6. Charge−discharge curves of VFBs assembled with various membranes. Inset shows the corresponding efficiencies.

cross-linking. All the thermal cross-linking membranes possess a lower charge voltage and a higher discharge voltage compared with S@RP at the current density of 120 mA cm−2. The high discharge platforms of S@RP-6 and S@RP-C12 are probably due to their lower vanadium ion permeability, revealing the higher utilization of electrolyte. As shown in the inset of Figure 6, the CE and EE improve slightly with the increase of thermal cross-linking time. S@RP-C12 has the highest CE and EE @ 120 mA cm−2. However, S@RP-C12 is not as stable as S@RPC6 in the subsequent life test (see Figure S4). Hence, we choose S@RP-6 for the further study. The S@RP-C6 is compared with the benchmark N115 in terms of long-term cycling under a high current density of 120 mA cm−2. As shown in Figure 7, S@RP-C6 exhibits very stable and much higher CE and EE than that of N115 during life cycling test. The CE of S@RP-C6 and N115 is about 98.3% and 92.8%, respectively. The EE of S@RP-C6 remains stable at 79.5% throughout the test, whereas N115 shows a decay of 1.9% compared with its initial value of 75.8%. When the capacity retention decays to 60%, the VFBs with S@RP-C6 and N115 ran for 308 and 247 cycles respectively, as shown in Figure 7c. For a more comprehensive assessment of the VFB performance of S@RP-C6 membrane, a series of former reported membranes, including cation exchange membranes (Nafion 212,47 SPEEK,24 SPI,48 and PBI49), anion exchange membranes (QBPPEK50), and porous membranes (PES51) are selected for comparison in Table 2. Obviously, the S@RP-C6 demonstrates the best VFB performances in terms of working current density (higher up to 240 mA cm−2), efficiencies (CE and EE), and cycling stability (capacity fading of 0.13% per cycle). The extremely slower capacity decay strongly demonstrates the superior stability of S@RP-C6 due to the synergistic effect of paper reinforcement and thermal crosslinking, as described in Scheme 1. The excellent stability of S@RP-C6 membrane can also be confirmed by the micromorphology after life test. As shown in Figure 8, no matter which side of half-cell the membrane faces, the surface of S@RP-C6 shows no traces of structure damage even after 308 cycles test at a high current density of 120 mA cm−2. The dense cross section and uniform distribution of

Figure 7. Cycling performance of S@RP-C6 and N115 membranes: (a) Coulombic efficiency, (b) energy efficiency, (c) discharge capacity retention.

elements, such as C, O, and S (−SO3H group from SPEEK), clearly reveal the superior chemical and mechanical stability of S@RP-C6 membrane under the operating condition of VFB. In addition, the existence of element V in the membrane can be ascribed to the absorbed vanadium ion during VFB test. Besides, FTIR spectrum of S@RP-C6 before and after life test (see Figure S5) confirms that the stability of membrane can adequately meet the demand of VFB as well. Figure 9 displays wide temperature performance of VFB assembled with S@RP-C6. The cell was run for 10 cycles at each temperature from 0 to 70 °C under the current density of 120 mA cm−2. With the increase of temperature, the CE decreases owning to the accelerate of vanadium ion crossover.36 Meanwhile, attributed to the improved electrochemical reaction kinetics and the decreased Ohmic resistance,40,52 the VE increases with the temperature from 0 to 70 °C. The S@RP-C6 shows highest EE between 30 and 50 °C due to the mutual effect of CE and VE. It can be seen that the S@RP-C6 keeps stable at all testing temperature range, demonstrating the superior stability of S@RP-C6 membrane.

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CONCLUSION Three kinds of paper (i.e., copy paper, rice paper, and filter paper) reinforced SPEEK membranes are prepared and evaluated for VFB application. The mesoporous, fibrous structure, and excellent wettability of the paper substrate allow fast polymer infiltration to form the uniform paper/ SPEEK hybrid. The abundant hydroxyl groups (−OH) on the surface of paper cellulose fiber can interact with the sulfonic 2441


Research Article

ACS Sustainable Chemistry & Engineering Table 2. Performances of VFB with S@RP-C6 in Comparison with Previous Work Test conditions

Efficiency @ j = 80

Life testing

Membrane

Thickness (μm)

Voltage (V)

j (mA cm−2)

CE

EE

j

cycles

decay per cycle

ref.

Nafion 212 SPEEK SPI PBI QBPPEK PES S@RP-C6

60 60 50 30 47 115 ± 5 108

0.80−1.65 0.80−1.65 1.00−1.70 1.00−1.70 0.80−1.80 0.80−1.65 0.80−1.65

40−80 40−120 20−100 20−80 20−80 80 40−240

92% 97% 99% 99% 99% 92% 98%

72% 85% 62% 65% 82% 76% 84%

80 80 60 40 60 80 120

100 150 100 20 100 150 308

0.57% 0.16% 0.21% 0.20% / / 0.13%

47 24 48 49 50 51 This work

performance, as well as low cost make the rice paper reinforced SPEEK membrane more competitive for 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/acssuschemeng.6b02784. Additional information, including experimental details, parameters of various papers, cycling performance of various paper reinforced SPEEK membranes, TGA and DTA curves, cycling performance of thermal cross-linked S@RP membranes, and FTIR spectrum (PDF)

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AUTHOR INFORMATION

Corresponding Author

Figure 8. SEM and EDX images of S@RP-C6 after life test.

*J. Xi. E-mail: [email protected]. ORCID

Jingyu Xi: 0000-0002-7618-8500 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Guangdong Province (2016A030310025 and 2015A030313894) and the Basic Research Project of Shenzhen City (JCYJ20150630114140630 and JCYJ20150331151358143).

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REFERENCES

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Figure 9. Broad temperature performance of the VFB assembled with S@RP-C6. Inset shows the photograph of our all-climate test platform.

groups (−SO3H) of SPEEK and form hydrogen bonds, resulting in enhanced mechanical properties, suppressed swelling ratio and vanadium ion permeability of the paper reinforced membrane. Rice paper reinforced SPEEK membrane (S@RP) exhibits the best VFB performances among the three paper reinforced membranes, and its stability can be further improved by self-cross-linking at 140 °C for 6 h. The resulted S@RP-C6 membrane demonstrates greatly enhanced VFB cycling performance (98.3% of CE and 79.5% of EE) and extremely slower capacity fading rate (0.13% per cycle) during long-term life test at current density of 120 mA cm−2. Moreover, S@RP-C6 remains extremely stable on the broad temperature (0−70 °C) test. The stable and highly efficient 2442


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