Currently, energy storage technologies are becoming essential in the transition of replacing fossil fuels with more renewable electricity production means. Among storage technologies, redox flow batteries (RFBs) can represent a valid option due to their unique characteristic of decoupling energy storage from power output. To push RFBs further into the market, it is essential to include low-cost materials such as new generation membranes with low ohmic resistance, high transport selectivity, and long durability. This work proposes a composite membrane for vanadium RFBs and a method of preparation. The membrane was prepared starting from two polymers, meta-polybenzimidazole (6 mm) and porous polypropylene (30 μm), through a gluing approach by hot-pressing. In a vanadium RFB, the composite membrane exhibited a high energy efficiency (~84%) and discharge capacity (~90%) with a 99% capacity retention over 90 cycles at 120 mA∙cm−2, exceeding commercial Nafion® NR212 (~82% efficiency, capacity drop from 90% to 40%) and Fumasep® FAP-450 (~76% efficiency, capacity drop from 80 to 65%).

The vanadium redox flow battery (VRFB) is one of the most promising secondary batteries as a large-capacity energy storage device for storing renewable energy.1 Perfluorinated-based ion exchange membranes such as Nafion are the most widely used membranes in VRFB due to their high proton conductivity. However, the extremely high cost and low ion selectivity of Nafion have limited their further application in VRFB. Among the aromatic hydrocarbon-based membranes that combine both high ion selectivity and high proton conductivity, polybenzimidazole- based membranes have served as one of the most promising alternatives to Nafion due to their excellent chemical and mechanical stability and low cost.2 In the present work m-PBI membranes of varying thickness and modified PBI were prepared, and their properties and VRFB performances were compared. The voltage efficiency (VE) increased with decreasing membrane thickness because of the decreasing ohmic resistance. The excellent current efficiency (CE) of VRFBs using meta-PBI based membranes is attributed to the negligible vanadium crossover observed in ex-situ permeability tests. The stability of the membranes after long term cycling is one of the most challenging problems in large-scale VRB applications. The durability of PBI membrane in VRFB cell testing and the capacity decay was measured by operating the cells for 100 cycles under a high current density of 200 mA cm−2. Fig 1: Flow battery efficiencies using electrodes: 2.5 mm SGL, CR = 20%, membrane: PBI 7 µm at current density of 200 mA/cm2, the electrolyte solution V(IV):V(III) (50:50) in 2.0 M H2SO4 and 0.05 M H3PO4 . The chemical stability of PBI membranes was analyzed after immersion of the PBI samples in various highly oxidizing and reducing sulfuric acid based vanadium and cerium solutions. The membranes stayed unchanged for more than 4 months. This result suggests that the PBI membranes demonstrate excellent ex-situ chemical stability in the harsh acid, oxidizing and reducing conditions. References: 1) Noh, M. Jung, D. Henkensmeier, S. W. Nam and Y. Kwon, ACS Appl. Mater. Interfaces, 9 (2017) 36799-36809. 2) Chen, H. Qi, T. Sun, C. Yan, Y. He, C. Kang, Z. Yuan, X. Li, J. Membr. Sci., 586 (2019) 202-210. Figure 1

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The global goal to combat climate change is growing year by year, as for example indicated by the latest climate conference by the United Nations, COP26. Hereby, one of the main goals concerns the transition from fossil fuels to more renewable energy means. In doing so, the importance of highly efficient energy storage technologies, such as supercapacitors, Li-ion batteries and/or redox flow batteries that both store and release energy on a short-, mid- and long-term scale, grows alongside it1. Among storage technologies, redox flow batteries (RFBs) are considered as promising candidates for mid-term storage (daily discharge for 2-12 h) due to their unique characteristic of a decoupled energy storage and power output2. To enable the incorporation of these systems into society further and thus smoothen the transition towards renewable energy technologies, highly efficient and low cost membranes in terms of efficiency and durability are needed, that will simultaneously improve the efficiency and reduce the cost of energy storage. One of the membrane types for vanadium redox flow battery (VRFB) applications that gained the interest of industry and the scientific community are of the polybenzimidazole (PBI) type. Although these materials possess excellent chemical stability and vanadium barrier properties, they suffer from a poor proton conductivity compared to commercial standards such as Nafion® 212. Several strategies were proposed to tackle this issue, ranging from the preparation of composite membranes with a thin PBI skin layer3, alkaline pretreatments that improve the swelling characteristics of the PBI film4 and chemical modifications to attach ionic conducting groups on the polymer backbone, thus directly improving the conductivity5. This latter approach showed that a highly conductive PBI membrane could be obtained by the addition of sidechains containing charged moieties such as sulfonic acid5. However, this improved conductivity comes at a price, specifically, a decrease in its vanadium barrier properties due to a significant crossover of vanadium ions, thus losing one of the main benefits of PBI type membranes5. The work presented herein describes the preparation of multiple PBI derivatives containing small and readily available non-charged sidechains, such as alkylated or benzylated PBI. Herein, it was envisaged that the addition of non-charged sidechains to the PBI backbone could increase the free volume of the membrane and thus enable the incorporation of electrolyte into the film without significantly affecting its repelling effect for vanadium ions. With this in mind, the influence of these non-charged sidechains on the ex-situ properties under VRFB conditions was studied in terms of dimensional swelling, acid uptake, vanadium barrier properties and mechanical strength. Hereby, key differences in the behavior of the membranes were observed between each individual type of functionalization, such as swelling, uptake and composition in acidic media. Subsequently, the correlation between the ex-situ properties of the PBI derivatives and in-situ VRFB cycling performance was studied to further understand their significance. Through this process, valuable knowledge was obtained that showed a clear correlation between the ex-situ properties and the observed in-situ area specific resistance, energy efficiency and capacity fading of the various materials, with the best material outperforming Nafion® 212 in terms of energy efficiency by 1% at a current density of 200 mA∙cm2, a current density regime that is normally dominated by cation exchange membranes due to their low resistance. Thereby proving that even a small modification of the PBI backbone can have a significant influence on the performance in a VRFB cell. Yuan, Z.; Duan, Y.; Zhang, H.; Li, X.; Zhang, H.; Vankelecom, I., Advanced porous membranes with ultra-high selectivity and stability for vanadium flow batteries. Energy & Environmental Science 2016, 9 (2), 441-447. Chae, I. S.; Luo, T.; Moon, G. H.; Ogieglo, W.; Kang, Y. S.; Wessling, M., Ultra-High Proton/Vanadium Selectivity for Hydrophobic Polymer Membranes with Intrinsic Nanopores for Redox Flow Battery. Advanced Energy Materials 2016, 6 (16), 1600517. Gubler, L.; Vonlanthen, D.; Schneider, A.; Oldenburg, F. J., Composite Membranes Containing a Porous Separator and a Polybenzimidazole Thin Film for Vanadium Redox Flow Batteries. Journal of The Electrochemical Society 2020, 167 (10), 100502. Noh, C.; Serhiichuk, D.; Malikah, N.; Kwon, Y.; Henkensmeier, D., Optimizing the performance of meta-polybenzimidazole membranes in vanadium redox flow batteries by adding an alkaline pre-swelling step. Chemical Engineering Journal 2020, 126574. Yan, X.; Dong, Z.; Di, M.; Hu, L.; Zhang, C.; Pan, Y.; Zhang, N.; Jiang, X.; Wu, X.; Wang, J.; He, G., A highly proton-conductive and vanadium-rejected long-side-chain sulfonated polybenzimidazole membrane for redox flow battery. Journal of Membrane Science 2020, 596, 117616.

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