This cutting-edge tracking will focus on an article by Misgina et al. on the optimization membrane of waterborne organic redox flow batteries (AORFB), hoping to also help everyone have a preliminary understanding of the field of organic flow batteries.
As is well known, flow batteries have received widespread attention in the field of energy storage due to their outstanding safety, with the most notable being all vanadium flow batteries. However, like lithium metal resources, the development of all vanadium flow batteries will also be limited and affected by the supply of vanadium resources. Therefore, in recent years, new organic water-soluble redox pairs have been used to replace transition metals, attracting increasing research interest due to their fast electrochemical reaction kinetics, high molecular structure adjustability, low cost, and no need for mineral extraction. Organic compounds that have been developed can be divided into aqueous organic redox flow batteries (AORFB), non-aqueous organic redox flow batteries (NAORFB), and aqueous/non-aqueous mixed active materials based on the properties of solvents. Among them, AORFB has more cost advantages, higher ion conductivity, faster kinetic processes, and is more suitable for large-scale industrial applications. This has also led to an endless stream of patents and start-ups related to AORFB in recent years. The following Figure 1 shows some typical cases, such as the TEMPTMA/methyl viologen all organic system (neutral pH) developed by Jena Batteries GmbH in Germany, the anthraquinone/ferrocyanide mixed system (alkaline pH) developed by KemiWatt in France, and the AQDS/bromine system (acidic pH) developed by Green Energy Storage in Italy.
At present, water-based organic redox flow batteries are an emerging technological solution in the field of grid scale energy storage. These batteries have long lifespan, safe operation, potential low cost, and environmental friendliness. The separator is an important component of a battery, as it affects the Ohmic resistance, power density, discharge depth, and lifespan of the battery, thus having a crucial impact on the storage cost of the battery. Figure 2 shows the cost analysis of a 250 kW flow battery pack using different membranes. It can be seen that in the same 250 kW flow battery pack, the cost of using sulfonated polyether ether ketone (SPEEK) film decreased from 37% to 8% compared to using perfluorosulfonic acid proton exchange membrane.
Figure 2 Cost analysis of 250 kW flow battery pack using different membranes
AORFB membrane performance and design method
The conductivity of ion exchange membranes is one of their most important properties, and ion conduction is believed to occur mainly through water channels formed by phase separation between hydrophilic (ionic groups) and hydrophobic regions (polymer main chains), which are related to their ability to transport ions from one side of the electrolyte to the other, ensuring electrical neutrality during electrochemical reactions and electron transfer processes. In addition, it is necessary to consider the chemical stability of the ion exchange membrane (the performance and structural changes of the membrane after working for a period of time), mechanical stability (perforation or tearing of the membrane), and so on. Figure 3 shows the direct correlation between the ion exchange membrane resistance and the polarization behavior of redox flow batteries, with significant differences in polarization behavior observed under different membrane resistances.
Figure 3 AORFB experiments and theoretical polarization curves under different membrane resistances
The impact of ion exchange membrane performance on AORFB system is mainly manifested through voltage efficiency (VE), Coulombic efficiency (CE), and energy efficiency (EE). Among them, the value of VE is the ratio of discharge voltage to charging voltage, the value of CE is the ratio of discharge quantity to charging quantity, and the value of EE is the product of VE and CE. The main reasons for the unsatisfactory Coulombic efficiency (<100%) are two: side reactions in the electrolyte and membrane selective permeability. When the selective permeation membrane is not in an ideal state, the ion current passing through the membrane is not only composed of conductive ions shared by the electrolytes on both sides, but also includes charged active electrolytes or supporting electrolytes. Figure 4 shows the changes in EE of different membranes under different current densities. Overall, EE decreases continuously with increasing current density. The linear part of the decrease can be attributed to the voltage loss in the battery caused by the fixed Ohmic impedance of the membrane, which is proportional to the current density. At present, many studies have also begun to calculate the free energy of interactions between different species in the membrane phase using molecular dynamics methods, in order to gain a more detailed understanding of the interactions between charged ions, water molecules, and membrane charged groups.
Figure 4 Changes in EE of ion exchange membranes under different current densities
The first step in designing ion exchange membranes is to select the types of membranes, mainly porous membranes, cation exchange membranes (CEM), and anion exchange membranes (AEM). CEM typically exhibits poor separation performance (vanadium ions are positively charged) but better power density and voltage efficiency in all vanadium flow battery systems. Subsequently, the stability of the ion exchange membrane is considered. AEM is usually most susceptible to chemical degradation, and it is well known that Nafion ® membranes can work well in both acidic and alkaline environments, while commercial porous membranes (such as Celgard) ® 3501) and ceramic membranes have been shown to have the strongest chemical stability and are typically favored in NAORFB. Usually, in the design of membranes, it is necessary to consider the thickness of the membrane. The thicker the membrane, the better its stability and mechanical properties, but the poorer its conductivity. In addition, the thickness of the film directly affects the Ohmic resistance and battery performance, and adding low conductivity elements to the film usually reduces its conductivity. Therefore, a comprehensive balance must be made in terms of thickness, membrane conductivity, and mechanical properties. Finally, cost should also be considered. Porous membranes have the lowest cost, but due to their large pore size and poor ion selectivity, they are more suitable for active substances with a pore size that is one order of magnitude larger. The cost of AEM is slightly higher than that of porous membranes, but lower than CEM, and the ion selectivity of porous membranes is also higher.
Research progress
There is research [1] on three commonly used commercial AEMs on the market, namely Selemion ® DSV, AMV, and ASV (NaCl/KCl as supporting electrolyte) were tested in neutral FcNCl/MV based AORFB, and the experimental results are shown in Figure 5. Thinnest Semion ® The DSV film (which showed the lowest battery resistance in the tested film) produced the best battery performance: using Semion at 60 mA cm-2 ® DSV (76%) batteries exhibit better performance than Semion ® AMV (60%) and Semion ® ASV (44%) has a higher battery energy efficiency. Using Semion ® The DSV/NaCl battery provided the highest peak power density of 113 mW cm-2 at approximately 200 mA cm-2, while the same battery using AMV exhibited much lower peak power density (66 mW cm-2 at 114 mA cm-2). These observations are consistent with the correlation between energy efficiency and measured battery area resistance.
Figure 5: Neutral FcNCl/MV systems of three commercial AEMs: (A) Capacity and number of cycles; (B) Energy efficiency and number of cycles; (C) Voltage curve and capacity at 60 mA cm-2; (D) Polarization curve
There are also studies [2] using three different Nafion membranes: Nafion ® 212, Nafion ® 115, and Nafion ® 117 (represented as N212, N115, and N117 respectively) studied the performance of 2,5-dihydroxy-1,4-benzoquinone (DHBQ) as the active substance of AORFB under alkaline pH (paired with potassium ferrocyanide), as shown in Figure 6. Polarization results have shown that DHBQ/K4Fe (CN) 6 batteries made of N212, N115, and N117 provide peak power densities of 300, 164, and 137 mW cm-2.
Figure 6 Alkaline AORFB performance under three different Nafion membranes
Aziz et al. [3] first studied the cycling of quinone bromide (mixed) acidic AORFB, which was further developed by GES in Italy. Using Nafion ® The 212 membrane quinone bromide aqueous flow battery provides a peak power density of approximately 600 mW cm-2. According to reports, after 750 deep cycles, the average discharge capacity retention rate was 99.84% per cycle. In subsequent studies, N115 film was used to reconstruct the battery. By applying a current density of 250 mA cm-2, the average discharge capacity retention rate was 99.986% after 106 cycles from near zero SOC to near 100% SOC. Compared with N212 membrane, the difference in cycle times can explain the change in capacity retention rate, except for the decrease in water absorption rate of N115 membrane (38 wt% vs. 50 wt%).
Water based organic redox flow batteries (AORFB) are an energy storage technology worth exploring due to safety, low cost potential, and environmental friendliness of the supply chain. In the development of AORFB in the past few years, most attention has been focused on the study of redox active species and optimization of operating parameters (such as flow rate), with relatively little development of membranes. The development of membranes for this specific scenario may have great prospects in the future.
[1] B. Hu, C. Seefeldt, C. DeBruler, T.L. Liu, Boosting the energy efficiency and power performance of neutral acute organic redox flow batteries, J Mater Chem, 5 (42) (October 2017), 10.1039/C7TA06573F
[2] Z. Yang, et al. Alkaline benzoquinone acute flow battery for large scale storage of electrical energy, Adv Energy Material, 8 (8) (2018), 10.1002/aenm.201702056
[3] B. Huskinson, M.P. Marshak, M.R. Gerhardt, M.J. Aziz. Cycling of a quinone bromide flow battery for large scale electrochemical energy storage, ECS Trans, 61 (37) (Sep. 2014), 10.1149/06137.0027 ecst
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