Advanced Porous Membranes with Tunable Morphology Regulated by

Jun 12, 2019 - A simple salt-induced phase separation method is presented to prepare porous polybenzimidazole (PBI) membranes with tunable morphology ...
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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 24107−24113

Advanced Porous Membranes with Tunable Morphology Regulated by Ionic Strength of Nonsolvent for Flow Battery Lin Qiao,+,† Huamin Zhang,+,§ Wenjing Lu,+ Qing Dai,+,† and Xianfeng Li*,+,§ +

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Division of Energy Storage, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China † School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, China § Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), Dalian 116023, P. R. China S Supporting Information *

ABSTRACT: A simple salt-induced phase separation method is presented to prepare porous polybenzimidazole (PBI) membranes with tunable morphology for vanadium flow batteries (VFBs). This method is based on the traditional nonsolvent-induced phase separation (NIPS) method where salt is introduced into the coagulation bath to change the ionic strength of the nonsolvent. The change of ionic strength will affect the phase separation rate, and finally, the morphology of porous membranes is well tuned from finger-like voids to spongelike pores in site. The membrane with sponge-like pores created multiple barriers to the transfer of vanadium ions, offering the membrane with superhigh selectivity; meanwhile, spongy cells filled with sulfuric acid could provide the membrane with high proton conductivity. As a result, the membrane with sponge-like pores demonstrated a much better performance than that with finger-like voids. The resultant sponge-like porous PBI membrane exhibited a very impressive VFB performance with an energy efficiency of 89.9% at a current density of 80 mA cm−2, which was close to the highest values ever reported. The battery kept very stable performance even after continuously running for more than 10000 cycles at 160 mA cm−2, showing excellent stability. This paper provides an easy to scale up and environment-friendly method to fabricate high-performance porous membranes with tunable morphology. KEYWORDS: vanadium flow battery, polybenzimidazole, porous membranes, tunable morphology, ionic strength

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INTRODUCTION

Conventionally, perfluorinated sulfonated membranes with high ion conductivity and excellent chemical stability, such as Nafion, have been widely used for VFBs. Nevertheless, the poor ion selectivity on vanadium ions and extremely high cost of perfluorinated sulfonated membranes hamper their widespread application.2 Driven by the need to cut the cost and enhance ion selectivity of membranes, great efforts have been devoted to develop non-perflourinate ion exchange membranes with high ion selectivity and low cost.13−15 However, they are vulnerable to chemical attack from the highly oxidative pentavalent vanadium ions owing to the existence of ion exchange groups, which limit their practical application in VFBs.16,17 Porous membranes,18,19 which commonly have the features of low cost and high chemical stability, are very promising for VFBs. In addition, the balance of selectivity and conductivity could be achieved based on the regulation of membrane morphology. The feasibility of porous membranes was demonstrated in extensive research.12,20−26 Among them,

Concerns over the intermittent and random nature of renewable energy sources, such as wind and solar power, have led to increasing attention to large-scale energy storage.1−4 Among various energy storage technologies, a flow battery, vanadium flow battery (VFB) in particular, is one of the most promising candidates for large-scale energy storage due to its attractive characteristics of high energy efficiency, long cycle life, high safety, and uncouple energy and power rating.5−9 Nowadays, even though plenty of multi-megawatthour VFB systems have been demonstrated,10 they are still not ready for broad market penetration due to technical and economic obstacles. Many of these challenges are related to the membranes. Membranes play a role in preventing the crossmixing of the positive and negative active materials while allowing the transportation of protons.11 The high ion conductivity and selectivity of membranes are in favor of minimizing energy losses and achieving high energy efficiency, namely, accelerating the improvement in power density of a VFB. The excellent chemical stability and low cost of membranes are significantly vital for commercialization of VFB technology.12 © 2019 American Chemical Society

Received: April 8, 2019 Accepted: June 12, 2019 Published: June 12, 2019 24107

DOI: 10.1021/acsami.9b06142 ACS Appl. Mater. Interfaces 2019, 11, 24107−24113

Research Article

ACS Applied Materials & Interfaces

Figure 1. Cross section SEM images of NaCl-XM membranes. (a) NaCl-0M; (b) NaCl-1M; (c) NaCl-3M; (d) NaCl-4M; (e) NaCl-5M; (f) NaCl6M.

porous membranes23,27,28 with sponge-like pores are proven to be very promising for flow batteries since they can selectively transfer protons through multibarriers from the superthin pore walls. The sponge-like porous membranes were mostly prepared via the vapor-induced phase separation (VIPS) method. However, the traditional VIPS method is relatively tough (a humidity of 100% and a temperature of 50 °C), which makes the scale-up process more difficult. Compared with the VIPS method, the nonsolvent-induced phase separation (NIPS) method is more preferential for the scale-up process since it has been widely used in manufacturing traditional nanofiltration membranes. Nevertheless, when using deionized water as the nonsolvent, the resultant PBI porous membranes normally exhibit the finger-like voids in the cross section extending to fully evolution toward the underlayer of the membrane due to the rapid demixing process.24 In order to overcome the above problem, solvent treatment24 to asprepared membranes via NIPS has been developed, but the process still remains relatively complex. It will be perfect that porous membranes with sponge-like pores can be obtained by the one-step NIPS method. As reported, it can be achieved by adjusting the composition of the coagulation bath.29,30 However, the utilization of volatile and harmful organic solvents (like isopropyl)30 brings about much trouble in the industrialization of porous membranes. In this work, we report an easy to scale up and environmentfriendly one-step NIPS method to prepare high-performance porous membranes with sponge-like pores. Herein, polybenzimidazole (PBI) is chosen as the membrane material in view of its excellent stability23 and low cost.24 Based on the traditional NIPS method, green and nonvolatile inorganic salts are added to the nonsolvent. In the design, the rate of the phase separation could be well controlled by simply changing the concentrations (namely, ionic strength) of salt solution during the phase separation process. As the ionic strength of the nonsolvent increases, the phase separation process gradually changes from instantaneous demixing to delayed demixing. Therefore, the distinct transformation of the membrane morphology from finger-like voids to sponge-like pores was achieved via this simple salt-induced phase separation method without volatile organic solvents. Based on the excellent selectivity, great conductivity, and high

stability of sponge-like pores, the prepared membranes with such pores performed excellent VFB performance, fairly close to the highest values ever reported.

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EXPERIMENTAL SECTION

Materials. PBI was prepared based on our previous report.23 NaCl, MgCl2, and N,N-dimethylacetamide (DMAc) were obtained from Tianjin Damao Chemical Reagent Factory. Membrane Preparation. The porous PBI membranes were fabricated via a salt-induced phase separation process. At first, the PBI was dissolved in DMAc to form a 15 wt % solution. The solution was then cast onto a clean, dry, and smooth glass substrate using a scraper with a thickness of 250 μm at room temperature. Afterward, the solution on the glass substrate was soaked into the nonsolvent to form the PBI-based porous membranes. NaCl aqueous solutions with different concentrations (namely, 0, 1, 3, 4, and 5 M) were used as the nonsolvents. The resultant membranes were finally immersed in water to remove the DMAc before being used. The preparation of MgCl2XM (X = 1, 3, and 5) is similar to NaCl-XM. Membrane Morphology. The surface and cross section morphology of membranes were detected by scanning electron microscopy (SEM, JSM-7800F). The samples were fractured in liquid nitrogen and sprayed with gold before measurement. Area Resistance. The area resistance of the membrane was measured by a conductivity cell (Figure S1), which was separated into two compartments by a membrane with an effective area of 1 × 1 cm2. The conductivity cell was filled with 3 mol L−1 H2SO4 solution, and its electrical resistance was obtained by electrochemical impedance spectroscopy (Solartron Electrochemical System) over a frequency range of 1 MHz to10 kHz. The area resistances of the membranes were determined by subtracting the resistance of the cell without the membrane from that with the membrane. Vanadium Ion Permeability. The permeability of VO2+ was measured with a diffusion cell composed of two reservoirs, which was separated by a membrane. The effective area of a membrane was 3 × 3 cm2. The left reservoir was filled with 80 mL of 1.5 mol L−1 VOSO4 in 3.0 mol L−1 H2SO4 solution, and the right one was filled with the same volume of 1.5 mol L−1 MgSO4 in 3.0 mol L−1 H2SO4 solution. Solutions in both sides were stirred continuously to decrease concentration polarization. An aliquot of 3 mL solution was extracted from the right reservoir at a regular time interval; MgSO4 solution with the same volume was immediately added. The VO 2+ concentration in the samples was determined by a UV−vis spectrometer (JASCO, FT-IR 4100, Japan). VFB Performance. A VFB was constructed by sandwiching a membrane between two carbon felt electrodes, two graphite polar 24108

DOI: 10.1021/acsami.9b06142 ACS Appl. Mater. Interfaces 2019, 11, 24107−24113

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Figure 2. (a) Area resistances and (b) vanadium ion diffusion of the prepared NaCl-XM membranes and Nafion 115. plates, and two end plates. The carbon felt was purchased from Liaoyang Jingu Carbide Co., Ltd., and no additional treatment was carried out. The effective area of the membranes was 6 × 8 cm2. 60 mL of 1.5 M VO2+/VO2+ in 3 M H2SO4 solution and 60 mL of 1.5 M V2+/V3+ in 3 M H2SO4 solution were used as positive and negative electrolytes, respectively. The electrolytes were pumped into the electrodes with a flow rate of 50−60 mL min−1. Charge−discharge tests were conducted between 1 and 1.55 V using a test station (ArBin BT 2000) at current densities ranging from 40 to 200 mA cm−2.

protonated, which would assure the membranes with high ion conductivity.23 Thus, the porous PBI membranes with spongelike pores are expected to show excellent VFB performance. The morphology change derived from different salt concentrations is mainly due to the tunable ionic strength, which will be discussed in detail in the following section. Ion Conductivity. The ion conductivity of a membrane is an important parameter that significantly affects the voltage efficiency (VE) of a VFB. To characterize the ion conductivity of NaCl-XM, the area resistances are tested using electrochemical impedance spectroscopy (Figure 2a). The area resistances of NaCl-XM (X = 1, 3, 4, 5) are higher than that of NaCl-0M, and the area resistances of NaCl-XM increase with NaCl concentration in nonsolvents. That is attributed to the dramatic transformation of morphology from fully extending finger-like voids to sponge-like pores. Fortunately, the area resistances of NaCl-XM are all lower than that of Nafion 115. These results indicate that the transformation of morphology slightly breaks up the pore interconnectivity to a certain degree but not seriously. Protons can still pass rapidly through NaCl-XM, leading to a higher ion conductivity than Nafion 115. Traditionally, protons transport in PBI porous membranes via size exclusion and Grotthuss hopping mechanism. The abundant pores filled with sulfuric acid and adequately protonated nitrogen help protons hop more rapidly in the PBI backbone. As a result, NaCl-XM demonstrates high ion conductivity, and the VFBs with NaCl-XM are expected to achieve high voltage efficiency. Ion Selectivity. Ion selectivity of a membrane has a profound impact on the capacity retention and coulombic efficiency (CE) of a VFB. Ion selectivity of NaCl-XM is evaluated via measuring the VO2+ permeability of a diffusion cell. As shown in Figure 2b, the concentration of vanadium ions increases linearly with time for Nafion 115 and NaCl-XM membranes. NaCl-XM (X = 1, 3, 4, 5) demonstrates much lower vanadium ion permeation rates than NaCl-0M. When separated by NaCl-0M, the concentration in the blank chamber was as high as 0.054 mol L−1 after 30 h, while there was a rare change in chambers separated by NaCl-5M after 120 h. This significant decrease in the permeability rate is attributed to the dramatic transformation of morphology from finger-like voids to sponge-like pores. The sponge-like pores are disconnected, which form multiple barriers to the transfer of vanadium ions, resulting in high ion selectivity. It is worth noting that the thinner membranes have higher ion selectivity with increasing NaCl concentration. In general, the thinner membranes with the same structure have poor ion selectivity.

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RESULTS AND DISCUSSION Membrane Morphology. The porous PBI membranes were fabricated via the salt-induced phase separation method wherein aqueous solutions with different concentrations of sodium chloride (NaCl, the most common salt) (namely, 1 M, 3 M, 4 M, 5 M, and saturated) were used as nonsolvents. The resultant membranes are referred to as NaCl-XM, where X is the molar concentration of NaCl aqueous solution. The cross section morphologies of NaCl-XM are demonstrated in Figure 1. When using deionized water as the nonsolvent, NaCl-0M has typical finger-like voids, with the finger-like voids extending to fully evolution toward the underlayer (Figure 1a). However, as NaCl concentration increases, the elongation of finger-like voids is suppressed, while sponge-like pores are expanded (Figure 1a−f). When the concentration of NaCl solution reaches 5 mol L−1, the cross section of membranes is completely changed to a spongy structure. When further increasing the concertation of NaCl solution to being saturated (∼6 M), the as-prepared porous PBI membranes keep their sponge-like structure as well (Figure 1f). However, it is difficult to obtain a defect-free membrane due to NaCl precipitation in the saturated solution. Therefore, the distinct transformation from finger-like pores to sponge-like pores can be achieved via simply changing NaCl concentrations in nonsolvents. In Figure 1, we can observe that NaCl-XM differs greatly in thickness, which is driven by the dramatic transformation of morphology with the same thickness of a doctor blade. The surface morphologies of NaCl-XM are demonstrated in Figure S2. The top and bottom surfaces of NaCl-0M and NaCl-5M membranes are dense and relatively smooth. The dense surfaces of NaCl-5M membranes contribute to their high ion selectivities. The sponge-like porous structure (NaCl-5M) is somehow similar to that via the VIPS method.25,27,31,32 Its sponge-like pores are disconnected and expected to form multiple barriers to the transfer of vanadium ions, resulting in high ion selectivity. In addition, the sponge cells are filled with sulfuric acid, and the nitrogen in the PBI backbone is adequately 24109

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Figure 3. VFB performances of NaCl-XM and Nafion 115 membranes at a current density of 80 mA cm−2.

Figure 4. Electrochemical behaviors of VFBs. (a) VFB performance of NaCl-5M at various operation temperatures (−5 to 50 °C) at a current density of 80 mA cm−2. (b)VFB performance of NaCl-5M at different current densities (80−200 mA cm−2) at ambient temperature.

temperature window often necessitates heat management, which will lead to additional energy loss and increasing operating cost. The temperature-dependent behaviors of VFBs with NaCl-5M were tested at the temperature range from −5 to 50 °C at a current density of 80 mA cm−2 (Figure 4a). At higher temperatures, the CE decreases slightly owing to the more serious ion crossover. However, the rising temperature is propitious to the transportation of protons, resulting in the increase in VE. As shown in Figure 4a, a VFB with NaCl-5M exhibited high performance at the temperature range from −5 to 50 °C where an EE of over 75% could even be realized at a temperature as low as −5 °C. Furthermore, the performance of a VFB with NaCl-5M was conducted in a wide current density range of 40−200 mA cm−2 at ambient temperature (Figure 4b). With the increase of current density, CE increases, while VE decreases owing to the higher polarization at a higher current density. In Figure 4b, a VFB based on NaCl-5M shows a stable efficiency at a certain setting of current density. In addition, a high EE of up to 80% is achieved at a high current density of 160 mA cm−2, which demonstrates a very good rate capability. Overall, a VFB using NaCl-5M demonstrates very impressive performance at different temperatures and current densities. As demonstrated in Figure S5, considerable capacity fading of a VFB with the Nafion 115 membrane is found over 100 cycles due to its low ion selectivity.33 In contrast, the spongelike pores in NaCl-5M effectively obstruct the transfer of vanadium ions, resulting in a much better capacity retention. The charge−discharge cycling test was carried out to evaluate the long-term stability of NaCl-5 M in the VFB

However, the thinner membranes with sponge-like structures exhibited higher ion selectivity than the thicker membranes with finger-like structures. The result shows that the ion selectivity of NaCl-XM is remarkably improved via the simple salt-induced separation method. Therefore, VFBs using NaClXM with sponge-like pores are expected to perform high CE and low capacity fade. VFB Performance. To assess the charge/discharge performance of NaCl-XM, VFBs were assembled with NaClXM, and their performances were compared to those for VFB using Nafion 115. All the VFB performances were conducted under a voltage window of 1−1.55 V to avoid the corrosion of carbon felts and graphite polar plates. Typical galvanostatic charge/discharge profiles of VFBs with NaCl-5M and Nafion 115 at ambient temperature are shown in Figures S3 and S4. A VFB with NaCl-5M has a low IR drop and an average discharge voltage of 1.30 V, indicating the high conductivity of NaCl-5M. As shown in Figure 3, with increasing NaCl concentration in nonsolvents, VFBs assembled with NaCl-XM present an immense improvement in CE and a slight decrease in VE. The results are in agreement with those from area resistance and vanadium ion permeability. That is attributed to the increase in ionic strength in nonsolvent, which suppresses the formation of finger-like voids and helps to form typical sponge-like pores. It is noteworthy that a VFB with NaCl-5M achieves a pretty high CE of 99.3% and EE of 89.9% at 80 mA cm−2, which is significantly higher than that with a Nafion 115 membrane (EE of 82.5%). Practically, VFBs will face various harsh environments such as the fluctuation in temperatures. This narrow operational 24110

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these results indicate the superstability of prepared membranes. Morphology Formation Mechanism. The morphology transformation is explained by the mechanism as shown in Scheme 1. In deionized water, water molecules exist in the form of free water, and the exchange between the solvent and nonsolvent is faster, owing to small dimensions and rapid movement of water molecules. The instantaneous demixing process contributes to forming the typical finger-like void structure. By contrast, in NaCl aqueous solution, water molecules were combined with the ions, which decrease the activity and chemical potential (μ) of water due to the salt effect. Thus, the driving force of the nonsolvent into the casting solution is reduced, which postpones the demixing process and suppresses the formation of finger-like voids. In addition, the increase of salt concentration, ultimately mounting the ionic strength, results in a delayed demixing phase process and forms typical sponge-like pores. The ionic strength is calculated via the following formula

medium. Figure 5 demonstrates the CE, VE, and EE as a function of cycling numbers at a current density of 160 mA

Figure 5. Cycling performance of a VFB assembled with NaCl-5M at a current density of 160 mA cm−2.

I=

cm−2. In coincidence with the excellent stabilities of PBI in the literature,23,28 the battery using NaCl-5M could run continuously for over 10000 cycles and kept stable performance with an energy efficiency of about 80%. These results show a decent stability of NaCl-5M under the VFB medium. To further confirm its chemical stability, the cross section SEM image of the tested membranes was compared with the initial one (Figure S6). After over 10000 charge−discharge cycles, no obvious destruction and degradation can be observed where the difference of the thickness is caused by the squeeze during the assembly of the VFB. All

1 2

∑ cizi 2

(1)

i

where I is the ionic strength, ci is the molar concentration of ion i (M, mol L−1), and zi is the charge number of that ion. To prove the mechanism, magnesium chloride (MgCl2) aqueous solution (concentrations: 1, 3, and 5 M) was chosen as the nonsolvent. Similarly, the resultant membranes are referred to as MgCl2-XM, where X is the molar concentration of MgCl2 aqueous solution. For NaCl, where each ion is singly charged, the ionic strength is equal to the concentration. As for MgCl2, the magnesium ion is doubly charged, leading to ionic strength that is 3 times higher than an equivalent concentration of NaCl. Namely, the ionic strength of 1 M MgCl2 aqueous

Scheme 1. Principle of Porous Membranes with Tunable Morphology Regulated by Ionic Strength of Nonsolvent for Vanadium Flow Battery

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Figure 6. Cross section SEM images of MgCl2-XM membranes. (a) MgCl2-1M; (b) MgCl2-3M; (c) MgCl2-5M.

ORCID

solution is equal to 3 M NaCl aqueous solution. The cross section SEM images of MgCl2-XM are listed in Figure 6 and Figure S7. As the ionic strength increases from 3 (MgCl2-1M) to 9 mol L−1 (MgCl2-3M), the cross section structures of MgCl2-XM demonstrate the distinct transformation from finger-like voids to sponge-like pores. Furthermore, surprisingly, MgCl2-5M becomes a relatively compact nonporous membrane when the ionic strength increases to 15 mol L−1. The results indicate that regulating the morphology from porous to compact structures can be achieved by simply increasing the ionic strength of the nonsolvent. The one-step and manageable strategy could be applied to other fields, such as gas separation, seawater desalination, wastewater treatment, and so on.

Xianfeng Li: 0000-0002-8541-5779 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The authors acknowledge financial support from the Natural Science Foundation of China, NSFC (U1808209), Strategic Priority Research Program of the CAS (XDA21070100), Key Project of Frontier Science, CAS (QYZDB-SSW-JSC032), and Key R & D Project of Da Lian (2018YF17GX020). Notes

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The authors declare no competing financial interest.

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CONCLUSIONS Based on the traditional NIPS method, porous membranes with sponge-like pores could be prepared via simply adding inorganic salt to the nonsolvent. The essence of this method lies in tuning ionic strength of the nonsolvent and the rate of the phase separation process; the morphology of porous PBI membranes could be well tuned from finger-like voids to sponge-like pores. Furthermore, the work makes it possible to regulate the morphology from porous to compact structures. Membranes with sponge-like pores exhibited optimized VFB performance owing to their high proton conductivity and excellent ion selectivity. A VFB assembled with NaCl-5M showed a CE of 99.3% and an EE of 89.9% at a current density of 80 mA cm−2, which was in comparison to the highest values ever reported. It is worth noting that the salt-induced phase separation method is operated at room temperature and the process is finished in 2 min, superior to the VIPS method. Thus, the simple, facile, and effective method is easy to scale up, which is expected to provide a new inspiration for membrane research in other fields.

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ABBREVIATIONS PBI, polybenzimidazole VFB, vanadium flow battery NIPS, nonsolvent-induced phase separation VIPS, vapor-induced phase separation VE, voltage efficiency CE, coulombic efficiency EE, energy efficiency DMAc, dimethylacetamide

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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.9b06142.

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REFERENCES

SEM images of NaCl-XM membranes, the galvanostatic charge−discharge curves, and the capacity fading of VFBs with NaCl-XM membranes (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 24112

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