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Iron-vanadium redox flow batteries with polybenzimidazole membranes: High coulomb efficiency and low capacity loss
Journal of Power Sources 439 (2019) 227079
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Iron-vanadium redox flow batteries with polybenzimidazole membranes: High coulomb efficiency and low capacity loss Wonmi Lee a, 1, Byeong Wan Kwon a, 1, Mina Jung a, b, Dmytro Serhiichuk b, c, d, Dirk Henkensmeier b, c, e, **, Yongchai Kwon a, * a
Graduate School of Energy and Environment, Seoul National University of Science and Technology, Nowon-gu, Seoul, 01811, Republic of Korea Fuel Cell Research Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea Division of Energy & Environment Technology, KIST School, University of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea d Chemical Technology Faculty, NTUU Igor Sykorsky Kyiv Polytechnic Institute, Kyiv, 03056, Ukraine e Green School, Korea University, Seongbukgu, Seoul, 02841, Republic of Korea b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Meta-polybenzimidazole (m-PBI) mem brane is used for Fe/V redox flow battery. � m-PBI shows lower active species permeability than Nafion 212. � m-PBI reveal a CE of 99% and a CLR of just 0.11 Ahr∙L 1 per cycle. � m-PBI induces three times better discharge capacity than Nafion 212. � 15 μm thick m-PBI costs 97% less than for a 50 μm thick Nafion.
A R T I C L E I N F O
A B S T R A C T
Keywords: Meta-polybenzimidazole Fe–V redox flow batteries New membrane Charge efficiency
An iron-vanadium redox flow battery utilizing 15 μm thick HCl doped meta-polybenzimidazole (m-PBI) mem branes is used. Ex-situ tests for m-PBI membranes show a much lower permeability for Fe2þ and V3þ ions than when using Nafion 212. Specifically, cells utilizing 50 μm thick Nafion 212 show a strong electrolyte imbalance (catholyte moving to anolyte), a low charge efficiency (CE) of 90%, and a high capacity loss rate (CLR) of 0.63 Ahr∙L 1 per cycle, indicating low energy efficiency and stability. In contrast to this, cells utilizing m-PBI reveal a CE of 99% and a CLR of just 0.11 Ahr∙L 1 per cycle. After 20 cycles, the discharge capacity is three times higher than for the cell with Nafion 212. Since the polymer needed for a 15 μm thick m-PBI membrane costs 97% less than for a 50 μm thick Nafion membrane, the utilization of m-PBI membranes is also economically advantageous.
* Corresponding author. ** Corresponding author. Fuel Cell Research Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea. E-mail addresses: [email protected] (D. Henkensmeier), [email protected] (Y. Kwon). 1 Wonmi Lee and Byeong Wan Kwon equally contributed to this work. https://doi.org/10.1016/j.jpowsour.2019.227079 Received 12 March 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 Available online 4 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
W. Lee et al.
Journal of Power Sources 439 (2019) 227079
1. Introduction
In this study, since FeVRFBs use HCl based electrolytes, we investi gate the effect of HCl on the properties of m-PBI membranes. It turns out that the m-PBI membranes quickly absorbs the diluted HCl solutions and the absorbed amount is high enough to render the m-PBI membranes conductive, and the compositions of these membranes (polymer, HCl and water) are quantitatively analysed. In turn, we test the permeability of Fe2þ and V3þ ions of the HCl doped m-PBI membranes, which is extremely low. Based on the data, the performance of FeVRFB using this new type of membrane is evaluated and as a result, the FeVRFB shows far higher CE and far lower capacity fade than those of the FeVRFB using conventionally used Nafion membrane and we report this result for the first time. In addition, we represent the picture of flow cell setup in Fig. S1.
In order to feed power from intermittent renewable energy sources into the grid, large MWh scale energy storage systems are needed. Among the possible solutions, redox flow batteries (RFB) are strong candidates [1–3]. One interesting feature of RFBs is that power is decoupled from energy: The former is determined by the size of the electrodes, while the latter is controlled by the type and concentration of electrolyte contained in the external reservoir, as well as its size [4,5]. To date, the most promising system seems to be the all vanadium redox flow battery (VRFB), mainly suggested by Skyllas-Kazacos [6]. The main advantage of the VRFB is that both electrolytes contain va nadium ions. Since cross-contamination of electrolytes is normally the largest source of capacity losses, in the case of VRFB, the initial capacity can be recovered by mixing and recharging the anolyte and catholyte [7]. However, the VRFB still has hurdles to overcome. First, V5þ pre cipitates into V2O5 at temperatures above 40 � C and V2þ precipitates into VSO4 at temperatures below 10 � C, which limits the operational temperature to a narrow window, and the concentration of the reactive species to 1.6 M [8,9]. Second, the high oxidative strength of V5þ ions causes a limitation on the selection of membranes [10]. Third, the most chemically stable membrane material to date, Nafion, is a cation ex change membrane and therefore tends to transport metal cations, resulting in a high crossover [11]. To overcome some of the above issues, there have been some studies associated with the iron-based redox flow batteries. First, the perfor mance of iron-chromium redox flow battery (ICRFB) was enhanced by Wei group in 2015. The main advantage of ICRFB compared with VRFB is its cheap capital cost when operated at high power densities. In addition, Fe(II)–Fe(III) redox reaction does not need catalyst to promote redox reactivity due to its naturally high redox reactivity. However, unlike the Fe(II)–Fe(III) redox reaction, in Cr(II)/Cr(III) redox reaction, the use of catalyst is necessary to improve redox reactivity and to sup press undesirable hydrogen evolution. Fe and Cr ions are also severely permeated through the membrane, and the severe permeation may induce a large capacity decay [12]. Second, Yan group also published zinc-iron redox flow battery (Zn–Fe RFB) [13]. The theoretical cell voltage of Zn–Fe RFB is extremely high (~1.99 V) and this property can lead to high power density (676 mW cm 2). In addition, a very low cost near $100 per kWh is possible. However, the dendrite formation of Zn lowers the stability of Zn–Fe RFB during cycling and Zn and Fe are differently dissolved in electrolyte with the use of two membranes (Zn is dissolved in alkaline electrolyte, while acidic electrolyte is used for the dissolution of Fe), and this requirement can induce a complicated cell system. Third, the FeVRFB was proposed by Wang et al. [14,15] In their research, hydrochloric acid (HCl) solutions with Fe2þ/Fe3þ and V2þ/V3þ were used as the positive and negative electrolyte, respectively, with an open circuit potential of 1.02 V. This configuration not only reduces the electrolyte costs (Fe is far cheaper than V), but also prevents reactions between components and the highly oxidative VOþ 2 ions. Furthermore, solutions containing Fe have a wider thermal stability window. How ever, Nafion based FeVRFBs still suffer severe crossover, and their discharge capacity was reported to decrease rapidly from 2.3 to 0.3 Ah within 100 cycles [14]. This indicates that different membranes are needed to overcome the issues of Nafion based FeVRFBs. Recently, the use of meta-polybenzimidazole (m-PBI) membranes were shown to perform well in sulfuric acid based VRFB [16–20]. When placed in contact with sulfuric acids, the imidazole groups of m-PBI are protonated, and the polymer backbone bears positive charges which are stabilized by mobile counter ions. Since protonated (positively charged) PBI repels cations and its low free volume (narrow channel size) limits the diffusion of metal ions in the membrane, these polymers can effi ciently repel metal cations as well as conduct anions [19,20]. Conse quently, PBI membranes can prevent the cross-over of vanadium ions (cations), leading to high charge efficiency (~99%).
2. Results and discussion As shown in Fig. S2, PBI absorbed approximately 3.2 mol HCl per mole polymer repeat unit when immersed in 2 M HCl solution. In addition, the conductivity of membranes doped in 2 M HCl increased up to 24% if the membrane is fabricated to absorb more water (Fig. S3). This absorption is fast: In EIS test, the resistance of FeVRFB cell using 15 μm thick PBI (which is non-conductive in its pristine form) reached 26 Ω cm2 within a few minutes and decreased further to 20 Ω cm2 over the course of several hours (Fig. S4). This value can be compared to the approximate value of 16 Ω cm2 for Nafion 212. This shows that thin HCl doped PBI membranes can reach similar resistances as the thicker Nafion membranes. Furthermore, the area specific resistance (ASR) was reduced with increasing temperature (Fig. S5). While Nafion mem branes of different thickness show a measurable trade-off between resistance and permeability [21,22] the VO2þ permeability of even thin sulfuric acid doped PBI membranes is so low that it cannot be analysed [18]. Furthermore, HCl swollen Nafion 212 showed a significant permeability of V3þ and Fe2þ, while permeation was so low for PBI, that it could not be quantified (Fig. 1). In addition, the possibility of oxida tion or precipitation of the Fe2þ ions was investigated and the result is presented in Figs. S6 and S7. To evaluate the effect of the membrane on the FeVRFB performance, charge-discharge curves of FeVRFB full cells using 15 μm thick PBI (PBIFeVRFB) and nominally 50 μm thick Nafion 212 (Nafion-FeVRFB) were measured at 80 mA cm 2 for 20 cycles (Fig. 2). The thickness of Nafion is thin in a comparison to the previous work, but also thicker membranes failed rapidly due to a high crossover of redox-active species [14,15]. On the other hand, PBI membranes can be used even in very thin qualities, due to their high mechanical strength and low metal ion permeability, and choosing thin PBI membranes allows to reduce the ASR [20]. As expected from the permeability measurements, the CE of PBIFeVRFB (99%) was significantly higher than that of Nafion-FeVRFB (90%, Fig. 2a). The VE of Nafion-FeVRFB (80%, Fig. 2b) was 7% higher than that of PBI-FeVRFB (75%). This is because VE is reciprocal to the ohmic resistance, which was observed to be lower for Nafion than for the tested PBI membrane. Additionally, the EE of the two RFBs were almost the same (Fig. 2c), with just a slight advantage to the PBIFeVRFB. However, since the FeVRFB uses two different metals in the electrolytes, the top priority is to avoid cross-contamination. Under these circumstances, the high CE of the PBI based system becomes the clear advantage. Beginning with the first charging cycle, the discharge capacity of PBI-FeVRFB (25 AhL 1, Fig. 3) was 47% higher than that of NafionFeVRFB (20 AhL 1), which is probably due to the excessive crossover of Fe2þ and V3þ ions through Nafion 212, as supported by Fig. 1. For the same reason, the capacity loss rate was also much higher for NafionFeVRFB than for PBI-FeVRFB. Over the first 20 cycles, the discharge capacity of Nafion-FeVRFB had already dropped 55%, making a continuation of the cycling test unnecessary, while the discharge ca pacity of PBI-FeVRFB decreased only 6%. This is an excellent result compared to previously reported data. For instance, Wang et al. reported 2
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Journal of Power Sources 439 (2019) 227079
Fig. 1. Absorbance and concentration of (a) V3þ ions and (c) Fe2þ ions permeated through Nafion 212 and (b) V3þ ions and (d) Fe2þ ions permeated through 15 μm thick PBI during permeability tests for 4 h duration.
80 mA cm 2 for 10 cycles at room temperature, 40 � C, and 60 � C, no precipitation was observed (Fig. 4). Moreover, the VE and EE improved with increasing temperature, due to ionic resistance decreasing with increasing temperature. This is also supported by the starting charging voltages, which decreased from 0.884 V at room temperature to 0.833 V and 0.816 V at 40 � C and 60 � C, respectively, while the starting discharge voltages increased from 0.944 V at room temperature to 1.016 V and 1.050 V at 40 � C and 60 � C, respectively. Furthermore, the charge and discharge capacities increased with the increase in temper ature (Fig. 4d). However, a slight decrease in CE was observed at higher temperatures, as the diffusion processes accelerate with increasing
a decrease of 35% over 20 cycles and a decrease of 85% after 90 cycles for a cell with Nafion 117, which is nominally 3.5 times thicker than Nafion 212 [14]. Furthermore, the mechanical properties of membranes are measured (Fig. S8). Current VRFBs cannot be operated above 40 � C, because of the pre cipitation of V5þ ions. In contrast to this, it is expected that FeVRFBs can be operated above 40 � C, because Fe2þ and Fe3þ solutions are stable even at temperatures above 40 � C. This would be very advanta geous for real RFB applications, because VRFB systems for outdoor use are temperature controlled; thus, reducing the round-trip efficiency of the energy storage system. When a PBI-FeVRFB was operated at 3
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Journal of Power Sources 439 (2019) 227079
Fig. 2. (a) Charge efficiencies, (b) voltage efficiencies, (c) energy efficiencies, and (d) charge and discharge curves at the 20th cycle of FeV RFB single cells using Nafion 212 and PBI membrane. The current density was kept constant at 80 mAcm 2.
Fig. 3. Capacities for charging and discharging Nafion-FeVRFB (a) and PBI-FeVRFB (b) cells. The current density was 80 mA cm
temperature. Hence, the performance of the PBI-FeVRFB can be even further improved when the operating temperature is increased, without risking cell failure, as is the case for the VRFB [23]. PBI based FeVRFBs perform much better than Nafion based FeVRFBs at a current density of 80 mA cm 2: While the EE is similar in both systems, with results above 70%, the PBI-Fe VRFB has a 10% higher CE and an approximately 3.4 times higher discharge capacity after 20 cy cles. Even in the first cycle, the PBI based cell has a 25% higher capacity. In contrast to VRFBs, which can only operate between 10 and 40 � C, the PBI-FeVRFB can be operated even at 60 � C. This eliminates the need
2
.
for cooling in outdoor installations, and increases the VE and thus, the EE to about 80%. This is with an insignificant tradeoff of a slight decrease of the CE due to the temperature-enhanced diffusion [15,24]. In addition to the technical advantages, the use of PBI membranes also has a strong impact on overall system costs. PBI membranes do not contain fluorinated components, which can potentially ease recycling of used RFBs. The cost of membranes – which are 13% of the overall RFB stack cost [25] - can also be reduced by changing from 50 μm thick Nafion to 15 μm thick PBI membranes, which reduces the weight of the membrane from 99 g/m2 to approximately 19 g/m2. Considering that 4
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Journal of Power Sources 439 (2019) 227079
Fig. 4. Effect of the operating temperature on a PBI-FeVRFB operating at 80 mAcm charge and discharge curves at the 10th charging cycle.
2
PBI is available on the lab scale for 0.67 USD/g (based on the price for 450 g), while Nafion powder is available for 4.70 USD/g (based on the price for 200 g), the polymer cost for producing 1 m2 membrane would be 465 USD/m2 for 50 μm thick Nafion, but only 12.70 USD/m2 for 15 μm thick PBI (97% less). This price is also 4–5 times lower than the minimum cost for Nafion 112 projected by Dupont for a high production volume.[26] In addition, iron is much cheaper and more abundant than vanadium.
Appendix A. Supplementary data
: (a) Charge efficiencies, (b) voltage efficiencies, (c) energy efficiencies and (d)
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227079. References [1] R. Ye, D. Henkensmeier, S.J. Yoon, Z. Huang, D.K. Kim, Z. Chang, S. Kim, R. Chen, Redox flow batteries for energy storage: a Technology review, J. Electrochem. Energy Storage Convers. 15 (2018), 010801. [2] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, Redox flow batteries: a review, J. Appl. Electrochem. 41 (2011) 1137. [3] W. Lee, C. Jo, S. Youk, H.Y. Shin, J. Lee, Y. Chung, Y. Kwon, Mesoporous tungsten oxynitride as electrocatalyst for promoting redox reactions of vanadium redox couple and performance of vanadium redox flow battery, Appl. Surf. Sci. 429 (2018) 187–195. [4] M. Rychcik, M. Skyllas-Kazacos, Characteristics of a new all-vanadium redox flow battery, J Power Sources 22 (1988) 59–67. [5] C. Noh, S. Moon, Y. Chung, Y. Kwon, Chelating functional group attached to carbon nanotubes prepared for performance enhancement of vanadium redox flow battery, J. Mater. Chem. 5 (2017) 21334–21342. [6] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green, New allvanadium redox flow cell, J. Electrochem. Soc. 133 (1986) 1057–1058. [7] F.J. Oldenburg, T.J. Schmidt, L. Gubler, Tackling capacity fading in vanadium flow batteries with amphoteric membranes, J Power Sources 368 (2017) 68–72. [8] Y. Chung, J. Jeong, H.T.T. Pham, J. Lee, Y. Kwon, Sulfenic acid doped mesocellular carbon foam as powerful catalyst for activation of V (II)/V (III) reaction in vanadium redox flow battery, J. Electrochem. Soc. 165 (2018) A2703–A2708. [9] W. Lee, B.W. Kwon, Y. Kwon, Effect of carboxylic acid doped carbon nanotube catalyst on the performance of aqueous organic redox flow battery using the modified alloxazine and ferrocyanide redox couple, ACS Appl. Mater. Interfaces 10 (2018) 36882–36891. [10] D. Chen, M.A. Hickner, V5þ degradation of sulfonated Radel membranes for vanadium redox flow batteries, Phys. Chem. Chem. Phys. 15 (2013) 11299–11305. [11] M. Vijayakumar, Q. Luo, R. Lloyd, Z. Nie, X. Wei, B. Li, V. Sprenkle, J.D. Londono, M. Unlu, W. Wang, Tuning the perfluorosulfonic acid membrane morphology for
3. Conclusion In summary, it can be concluded that the use of PBI membranes strongly enhances the economic and technical viability of FeVRFBs. Wang et al. showed that electrolytes containing both Fe and V ions strongly reduce the capacity losses of FeVRFBs [15]. With PBI mem branes, this system is expected to be even more stable, making it very attractive for larger scale testing. Acknowledgement This work was supported by the German Korean joint SME R&D projects program of MOTIE/KIAT (No. 20151732) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20172420108550) and by the National Research Foundation of Korea (NRF) and the Ministry of the Ministry of Education (MOE) (No. 2019R1A2C1005776).
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Journal of Power Sources 439 (2019) 227079
vanadium redox-flow batteries, ACS Appl. Mater. Interfaces 8 (2016) 34327–34334. Y.K. Zeng, T.S. Zhao, L. An, X.L. Zhou, L. Wei, A comparative study of all-vanadium and iron-chromium redox flow batteries for large-scale energy storage, J Power Sources 300 (2015) 438–443. K. Gong, X. Ma, K.M. Conforti, K.J. Kuttler, J.B. Grunewald, K.L. Yeager, M. Z. Bazant, S. Gu, Y. Yan, A zinc–iron redox-flow battery under $100 per kW h of system capital cost, Energy Environ. Sci. 8 (2015) 2941–2945. W. Wang, S. Kim, B. Chen, Z. Nie, J. Zhang, G.G. Xia, L. Li, Z. Yang, A new redox flow battery using Fe/V redox couples in chloride supporting electrolyte, Energy Environ. Sci. 4 (2011) 4068–4073. W. Wang, Z. Nie, B. Chen, F. Chen, Q. Luo, X. Wei, G.G. Xia, M. Skyllas-Kazacos, L. Li, Z. Yang, A new Fe/V redox flow battery using a sulfuric/chloric mixed-acid supporting electrolyte, Adv. Energy Mater. 2 (2012) 487–493. M. Jung, W. Lee, N.N. Krishnan, S. Kim, G. Gupta, L. Komsiyska, C. Harms, Y. Kwon, D. Henkensmeier, Porous-Nafion/PBI composite membranes and nafion/ PBI blend membranes for vanadium redox flow batteries, Appl. Surf. Sci. 450 (2018) 301–311. J.K. Jang, T.H. Kim, S.J. Yoon, J.Y. Lee, J.C. Lee, Y.T. Hong, Highly proton conductive, dense polybenzimidazole membranes with low permeability to vanadium and enhanced H2SO4 absorption capability for use in vanadium redox flow batteries, J. Mater. Chem. 4 (2016) 14342–14355.
[18] Z. Yuan, Y. Duan, H. Zhang, X. Li, H. Zhang, I. Vankelecom, Advanced porous membranes with ultra-high selectivity and stability for vanadium flow batteries, Energy Environ. Sci. 9 (2016) 441–447. [19] X.L. Zhou, T.S. Zhao, L. An, L. Wei, C. Zhang, The use of polybenzimidazole membranes in vanadium redox flow batteries leading to increased coulombic efficiency and cycling performance, Electrochim. Acta 153 (2015) 492–498. [20] C. Noh, M. Jung, D. Henkensmeier, S.W. Nam, Y. Kwon, Vanadium redox flow batteries using meta-polybenzimidazole based membranes of different thickness, ACS Appl. Mater. Interfaces 9 (2017) 36799–36809. [21] S. Jeong, L.H. Kim, Y. Kwon, S. Kim, Effect of nafion membrane thickness on performance of vanadium redox flow battery, Korean J. Chem. Eng. 31 (2014) 2081–2087. [22] B. Jiang, L. Wu, L. Yu, X. Qiu, J. Xi, A comparative study of Nafion series membranes for vanadium redox flow batteries, J. Membr. Sci. 510 (2016) 18–26. [23] C. Zhang, T.S. Zhao, Q. Xu, L. An, G. Zhao, Effects of operating temperature on the performance of vanadium redox flow batteries, Appl. Energy 155 (2015) 349–353. [24] R. Badrinarayanan, J. Zhao, K.J. Tseng, M. Skyllas-Kazacos, Extended dynamic model for ion diffusion in all-vanadium redox flow battery including the effects of temperature and bulk electrolyte transfer, J Power Sources 270 (2014) 576–586. [25] J. Noack, L. Wietschel, N. Roznyatovskaya, K. Pinkwart, J. Tübke, Technoeconomic modeling and analysis of redox flow battery systems, Energies 9 (2016) 627. [26] S. Banerjee, D.E. Curtin, Nafion perfluorinated membranes in fuel cells, J. Fluorine Chem. 125 (2004) 1211–1216.
6
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Short communication
Iron-vanadium redox flow batteries with polybenzimidazole membranes: High coulomb efficiency and low capacity loss Wonmi Lee a, 1, Byeong Wan Kwon a, 1, Mina Jung a, b, Dmytro Serhiichuk b, c, d, Dirk Henkensmeier b, c, e, **, Yongchai Kwon a, * a
Graduate School of Energy and Environment, Seoul National University of Science and Technology, Nowon-gu, Seoul, 01811, Republic of Korea Fuel Cell Research Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea Division of Energy & Environment Technology, KIST School, University of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea d Chemical Technology Faculty, NTUU Igor Sykorsky Kyiv Polytechnic Institute, Kyiv, 03056, Ukraine e Green School, Korea University, Seongbukgu, Seoul, 02841, Republic of Korea b c
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Meta-polybenzimidazole (m-PBI) mem brane is used for Fe/V redox flow battery. � m-PBI shows lower active species permeability than Nafion 212. � m-PBI reveal a CE of 99% and a CLR of just 0.11 Ahr∙L 1 per cycle. � m-PBI induces three times better discharge capacity than Nafion 212. � 15 μm thick m-PBI costs 97% less than for a 50 μm thick Nafion.
A R T I C L E I N F O
A B S T R A C T
Keywords: Meta-polybenzimidazole Fe–V redox flow batteries New membrane Charge efficiency
An iron-vanadium redox flow battery utilizing 15 μm thick HCl doped meta-polybenzimidazole (m-PBI) mem branes is used. Ex-situ tests for m-PBI membranes show a much lower permeability for Fe2þ and V3þ ions than when using Nafion 212. Specifically, cells utilizing 50 μm thick Nafion 212 show a strong electrolyte imbalance (catholyte moving to anolyte), a low charge efficiency (CE) of 90%, and a high capacity loss rate (CLR) of 0.63 Ahr∙L 1 per cycle, indicating low energy efficiency and stability. In contrast to this, cells utilizing m-PBI reveal a CE of 99% and a CLR of just 0.11 Ahr∙L 1 per cycle. After 20 cycles, the discharge capacity is three times higher than for the cell with Nafion 212. Since the polymer needed for a 15 μm thick m-PBI membrane costs 97% less than for a 50 μm thick Nafion membrane, the utilization of m-PBI membranes is also economically advantageous.
* Corresponding author. ** Corresponding author. Fuel Cell Research Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul, 02792, Republic of Korea. E-mail addresses: [email protected] (D. Henkensmeier), [email protected] (Y. Kwon). 1 Wonmi Lee and Byeong Wan Kwon equally contributed to this work. https://doi.org/10.1016/j.jpowsour.2019.227079 Received 12 March 2019; Received in revised form 26 August 2019; Accepted 27 August 2019 Available online 4 September 2019 0378-7753/© 2019 Elsevier B.V. All rights reserved.
W. Lee et al.
Journal of Power Sources 439 (2019) 227079
1. Introduction
In this study, since FeVRFBs use HCl based electrolytes, we investi gate the effect of HCl on the properties of m-PBI membranes. It turns out that the m-PBI membranes quickly absorbs the diluted HCl solutions and the absorbed amount is high enough to render the m-PBI membranes conductive, and the compositions of these membranes (polymer, HCl and water) are quantitatively analysed. In turn, we test the permeability of Fe2þ and V3þ ions of the HCl doped m-PBI membranes, which is extremely low. Based on the data, the performance of FeVRFB using this new type of membrane is evaluated and as a result, the FeVRFB shows far higher CE and far lower capacity fade than those of the FeVRFB using conventionally used Nafion membrane and we report this result for the first time. In addition, we represent the picture of flow cell setup in Fig. S1.
In order to feed power from intermittent renewable energy sources into the grid, large MWh scale energy storage systems are needed. Among the possible solutions, redox flow batteries (RFB) are strong candidates [1–3]. One interesting feature of RFBs is that power is decoupled from energy: The former is determined by the size of the electrodes, while the latter is controlled by the type and concentration of electrolyte contained in the external reservoir, as well as its size [4,5]. To date, the most promising system seems to be the all vanadium redox flow battery (VRFB), mainly suggested by Skyllas-Kazacos [6]. The main advantage of the VRFB is that both electrolytes contain va nadium ions. Since cross-contamination of electrolytes is normally the largest source of capacity losses, in the case of VRFB, the initial capacity can be recovered by mixing and recharging the anolyte and catholyte [7]. However, the VRFB still has hurdles to overcome. First, V5þ pre cipitates into V2O5 at temperatures above 40 � C and V2þ precipitates into VSO4 at temperatures below 10 � C, which limits the operational temperature to a narrow window, and the concentration of the reactive species to 1.6 M [8,9]. Second, the high oxidative strength of V5þ ions causes a limitation on the selection of membranes [10]. Third, the most chemically stable membrane material to date, Nafion, is a cation ex change membrane and therefore tends to transport metal cations, resulting in a high crossover [11]. To overcome some of the above issues, there have been some studies associated with the iron-based redox flow batteries. First, the perfor mance of iron-chromium redox flow battery (ICRFB) was enhanced by Wei group in 2015. The main advantage of ICRFB compared with VRFB is its cheap capital cost when operated at high power densities. In addition, Fe(II)–Fe(III) redox reaction does not need catalyst to promote redox reactivity due to its naturally high redox reactivity. However, unlike the Fe(II)–Fe(III) redox reaction, in Cr(II)/Cr(III) redox reaction, the use of catalyst is necessary to improve redox reactivity and to sup press undesirable hydrogen evolution. Fe and Cr ions are also severely permeated through the membrane, and the severe permeation may induce a large capacity decay [12]. Second, Yan group also published zinc-iron redox flow battery (Zn–Fe RFB) [13]. The theoretical cell voltage of Zn–Fe RFB is extremely high (~1.99 V) and this property can lead to high power density (676 mW cm 2). In addition, a very low cost near $100 per kWh is possible. However, the dendrite formation of Zn lowers the stability of Zn–Fe RFB during cycling and Zn and Fe are differently dissolved in electrolyte with the use of two membranes (Zn is dissolved in alkaline electrolyte, while acidic electrolyte is used for the dissolution of Fe), and this requirement can induce a complicated cell system. Third, the FeVRFB was proposed by Wang et al. [14,15] In their research, hydrochloric acid (HCl) solutions with Fe2þ/Fe3þ and V2þ/V3þ were used as the positive and negative electrolyte, respectively, with an open circuit potential of 1.02 V. This configuration not only reduces the electrolyte costs (Fe is far cheaper than V), but also prevents reactions between components and the highly oxidative VOþ 2 ions. Furthermore, solutions containing Fe have a wider thermal stability window. How ever, Nafion based FeVRFBs still suffer severe crossover, and their discharge capacity was reported to decrease rapidly from 2.3 to 0.3 Ah within 100 cycles [14]. This indicates that different membranes are needed to overcome the issues of Nafion based FeVRFBs. Recently, the use of meta-polybenzimidazole (m-PBI) membranes were shown to perform well in sulfuric acid based VRFB [16–20]. When placed in contact with sulfuric acids, the imidazole groups of m-PBI are protonated, and the polymer backbone bears positive charges which are stabilized by mobile counter ions. Since protonated (positively charged) PBI repels cations and its low free volume (narrow channel size) limits the diffusion of metal ions in the membrane, these polymers can effi ciently repel metal cations as well as conduct anions [19,20]. Conse quently, PBI membranes can prevent the cross-over of vanadium ions (cations), leading to high charge efficiency (~99%).
2. Results and discussion As shown in Fig. S2, PBI absorbed approximately 3.2 mol HCl per mole polymer repeat unit when immersed in 2 M HCl solution. In addition, the conductivity of membranes doped in 2 M HCl increased up to 24% if the membrane is fabricated to absorb more water (Fig. S3). This absorption is fast: In EIS test, the resistance of FeVRFB cell using 15 μm thick PBI (which is non-conductive in its pristine form) reached 26 Ω cm2 within a few minutes and decreased further to 20 Ω cm2 over the course of several hours (Fig. S4). This value can be compared to the approximate value of 16 Ω cm2 for Nafion 212. This shows that thin HCl doped PBI membranes can reach similar resistances as the thicker Nafion membranes. Furthermore, the area specific resistance (ASR) was reduced with increasing temperature (Fig. S5). While Nafion mem branes of different thickness show a measurable trade-off between resistance and permeability [21,22] the VO2þ permeability of even thin sulfuric acid doped PBI membranes is so low that it cannot be analysed [18]. Furthermore, HCl swollen Nafion 212 showed a significant permeability of V3þ and Fe2þ, while permeation was so low for PBI, that it could not be quantified (Fig. 1). In addition, the possibility of oxida tion or precipitation of the Fe2þ ions was investigated and the result is presented in Figs. S6 and S7. To evaluate the effect of the membrane on the FeVRFB performance, charge-discharge curves of FeVRFB full cells using 15 μm thick PBI (PBIFeVRFB) and nominally 50 μm thick Nafion 212 (Nafion-FeVRFB) were measured at 80 mA cm 2 for 20 cycles (Fig. 2). The thickness of Nafion is thin in a comparison to the previous work, but also thicker membranes failed rapidly due to a high crossover of redox-active species [14,15]. On the other hand, PBI membranes can be used even in very thin qualities, due to their high mechanical strength and low metal ion permeability, and choosing thin PBI membranes allows to reduce the ASR [20]. As expected from the permeability measurements, the CE of PBIFeVRFB (99%) was significantly higher than that of Nafion-FeVRFB (90%, Fig. 2a). The VE of Nafion-FeVRFB (80%, Fig. 2b) was 7% higher than that of PBI-FeVRFB (75%). This is because VE is reciprocal to the ohmic resistance, which was observed to be lower for Nafion than for the tested PBI membrane. Additionally, the EE of the two RFBs were almost the same (Fig. 2c), with just a slight advantage to the PBIFeVRFB. However, since the FeVRFB uses two different metals in the electrolytes, the top priority is to avoid cross-contamination. Under these circumstances, the high CE of the PBI based system becomes the clear advantage. Beginning with the first charging cycle, the discharge capacity of PBI-FeVRFB (25 AhL 1, Fig. 3) was 47% higher than that of NafionFeVRFB (20 AhL 1), which is probably due to the excessive crossover of Fe2þ and V3þ ions through Nafion 212, as supported by Fig. 1. For the same reason, the capacity loss rate was also much higher for NafionFeVRFB than for PBI-FeVRFB. Over the first 20 cycles, the discharge capacity of Nafion-FeVRFB had already dropped 55%, making a continuation of the cycling test unnecessary, while the discharge ca pacity of PBI-FeVRFB decreased only 6%. This is an excellent result compared to previously reported data. For instance, Wang et al. reported 2
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Fig. 1. Absorbance and concentration of (a) V3þ ions and (c) Fe2þ ions permeated through Nafion 212 and (b) V3þ ions and (d) Fe2þ ions permeated through 15 μm thick PBI during permeability tests for 4 h duration.
80 mA cm 2 for 10 cycles at room temperature, 40 � C, and 60 � C, no precipitation was observed (Fig. 4). Moreover, the VE and EE improved with increasing temperature, due to ionic resistance decreasing with increasing temperature. This is also supported by the starting charging voltages, which decreased from 0.884 V at room temperature to 0.833 V and 0.816 V at 40 � C and 60 � C, respectively, while the starting discharge voltages increased from 0.944 V at room temperature to 1.016 V and 1.050 V at 40 � C and 60 � C, respectively. Furthermore, the charge and discharge capacities increased with the increase in temper ature (Fig. 4d). However, a slight decrease in CE was observed at higher temperatures, as the diffusion processes accelerate with increasing
a decrease of 35% over 20 cycles and a decrease of 85% after 90 cycles for a cell with Nafion 117, which is nominally 3.5 times thicker than Nafion 212 [14]. Furthermore, the mechanical properties of membranes are measured (Fig. S8). Current VRFBs cannot be operated above 40 � C, because of the pre cipitation of V5þ ions. In contrast to this, it is expected that FeVRFBs can be operated above 40 � C, because Fe2þ and Fe3þ solutions are stable even at temperatures above 40 � C. This would be very advanta geous for real RFB applications, because VRFB systems for outdoor use are temperature controlled; thus, reducing the round-trip efficiency of the energy storage system. When a PBI-FeVRFB was operated at 3
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Fig. 2. (a) Charge efficiencies, (b) voltage efficiencies, (c) energy efficiencies, and (d) charge and discharge curves at the 20th cycle of FeV RFB single cells using Nafion 212 and PBI membrane. The current density was kept constant at 80 mAcm 2.
Fig. 3. Capacities for charging and discharging Nafion-FeVRFB (a) and PBI-FeVRFB (b) cells. The current density was 80 mA cm
temperature. Hence, the performance of the PBI-FeVRFB can be even further improved when the operating temperature is increased, without risking cell failure, as is the case for the VRFB [23]. PBI based FeVRFBs perform much better than Nafion based FeVRFBs at a current density of 80 mA cm 2: While the EE is similar in both systems, with results above 70%, the PBI-Fe VRFB has a 10% higher CE and an approximately 3.4 times higher discharge capacity after 20 cy cles. Even in the first cycle, the PBI based cell has a 25% higher capacity. In contrast to VRFBs, which can only operate between 10 and 40 � C, the PBI-FeVRFB can be operated even at 60 � C. This eliminates the need
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for cooling in outdoor installations, and increases the VE and thus, the EE to about 80%. This is with an insignificant tradeoff of a slight decrease of the CE due to the temperature-enhanced diffusion [15,24]. In addition to the technical advantages, the use of PBI membranes also has a strong impact on overall system costs. PBI membranes do not contain fluorinated components, which can potentially ease recycling of used RFBs. The cost of membranes – which are 13% of the overall RFB stack cost [25] - can also be reduced by changing from 50 μm thick Nafion to 15 μm thick PBI membranes, which reduces the weight of the membrane from 99 g/m2 to approximately 19 g/m2. Considering that 4
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Fig. 4. Effect of the operating temperature on a PBI-FeVRFB operating at 80 mAcm charge and discharge curves at the 10th charging cycle.
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PBI is available on the lab scale for 0.67 USD/g (based on the price for 450 g), while Nafion powder is available for 4.70 USD/g (based on the price for 200 g), the polymer cost for producing 1 m2 membrane would be 465 USD/m2 for 50 μm thick Nafion, but only 12.70 USD/m2 for 15 μm thick PBI (97% less). This price is also 4–5 times lower than the minimum cost for Nafion 112 projected by Dupont for a high production volume.[26] In addition, iron is much cheaper and more abundant than vanadium.
Appendix A. Supplementary data
: (a) Charge efficiencies, (b) voltage efficiencies, (c) energy efficiencies and (d)
Supplementary data to this article can be found online at https://doi. org/10.1016/j.jpowsour.2019.227079. References [1] R. Ye, D. Henkensmeier, S.J. Yoon, Z. Huang, D.K. Kim, Z. Chang, S. Kim, R. Chen, Redox flow batteries for energy storage: a Technology review, J. Electrochem. Energy Storage Convers. 15 (2018), 010801. [2] A.Z. Weber, M.M. Mench, J.P. Meyers, P.N. Ross, J.T. Gostick, Q. Liu, Redox flow batteries: a review, J. Appl. Electrochem. 41 (2011) 1137. [3] W. Lee, C. Jo, S. Youk, H.Y. Shin, J. Lee, Y. Chung, Y. Kwon, Mesoporous tungsten oxynitride as electrocatalyst for promoting redox reactions of vanadium redox couple and performance of vanadium redox flow battery, Appl. Surf. Sci. 429 (2018) 187–195. [4] M. Rychcik, M. Skyllas-Kazacos, Characteristics of a new all-vanadium redox flow battery, J Power Sources 22 (1988) 59–67. [5] C. Noh, S. Moon, Y. Chung, Y. Kwon, Chelating functional group attached to carbon nanotubes prepared for performance enhancement of vanadium redox flow battery, J. Mater. Chem. 5 (2017) 21334–21342. [6] M. Skyllas-Kazacos, M. Rychcik, R.G. Robins, A.G. Fane, M.A. Green, New allvanadium redox flow cell, J. Electrochem. Soc. 133 (1986) 1057–1058. [7] F.J. Oldenburg, T.J. Schmidt, L. Gubler, Tackling capacity fading in vanadium flow batteries with amphoteric membranes, J Power Sources 368 (2017) 68–72. [8] Y. Chung, J. Jeong, H.T.T. Pham, J. Lee, Y. Kwon, Sulfenic acid doped mesocellular carbon foam as powerful catalyst for activation of V (II)/V (III) reaction in vanadium redox flow battery, J. Electrochem. Soc. 165 (2018) A2703–A2708. [9] W. Lee, B.W. Kwon, Y. Kwon, Effect of carboxylic acid doped carbon nanotube catalyst on the performance of aqueous organic redox flow battery using the modified alloxazine and ferrocyanide redox couple, ACS Appl. Mater. Interfaces 10 (2018) 36882–36891. [10] D. Chen, M.A. Hickner, V5þ degradation of sulfonated Radel membranes for vanadium redox flow batteries, Phys. Chem. Chem. Phys. 15 (2013) 11299–11305. [11] M. Vijayakumar, Q. Luo, R. Lloyd, Z. Nie, X. Wei, B. Li, V. Sprenkle, J.D. Londono, M. Unlu, W. Wang, Tuning the perfluorosulfonic acid membrane morphology for
3. Conclusion In summary, it can be concluded that the use of PBI membranes strongly enhances the economic and technical viability of FeVRFBs. Wang et al. showed that electrolytes containing both Fe and V ions strongly reduce the capacity losses of FeVRFBs [15]. With PBI mem branes, this system is expected to be even more stable, making it very attractive for larger scale testing. Acknowledgement This work was supported by the German Korean joint SME R&D projects program of MOTIE/KIAT (No. 20151732) and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20172420108550) and by the National Research Foundation of Korea (NRF) and the Ministry of the Ministry of Education (MOE) (No. 2019R1A2C1005776).
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