A low vanadium permeability sulfonated polybenzimidazole membrane with a metal-organic framework for vanadium redox flow batteries

Nature of Solvent/Nonsolvent Strategy in Achieving Superior Polybenzimidazole Membrane for Vanadium Redox Flow Battery

The tightly connected structure of polybenzimidazole (PBI) membrane can be relaxed by solvent/nonsolvent solution to achieve a high proton conductivity for vanadium redox flow battery (VRFB). However, the nature behind the solvent/nonsolvent strategy is not unraveled. This work proposes a guideline to analyze the effect of PBI membrane relaxing formulas based on the interactions between different components in membranes. The supreme-efficient PBI membrane derived by the DMSO/formamide formula according to the guideline displays a marvelous performance for VRFB, with the proton conductivity boosted by 4300 % (from 1.93 to 83.33 mS cm-1), and VRFB assembled with this membrane achieves an outstanding energy efficiency of 82.5 % under 200 mA cm-2. Moreover, this work profoundly unravels the structure, property and performance relationship of PBI membrane, which is of great value for the development of membranes.

Enhanced proton conductivity of main/side chain bi-sulfonated polybenzimidazoles via embedment of fluorinated modified MOF-801 for vanadium redox flow batteries

The development of low-cost membranes with high ion-selectivity is crucial for facilitating the practical application of vanadium redox flow batteries (VRFBs). In this study, sulfonated polybenzimidazole grafted with benzenesulfonic acid side chains was used as the polymer matrix, and fluorine-containing modified MOF-801 (FM) was incorporated to prepare a series of membranes for VRFB applications. The enhancement of proton conductivity and reduction of area resistance can be attributed to several factors: acid-base interactions, the microphase-separated structure of the grafted side chains, the intrinsic conductivity of MOF-801, and the extensive hydrogen-bonding network established through fluoride modification. Notably, the incorporation of fluorine or fluorine-containing groups into the hydrocarbon polymer matrix significantly improves its oxidative stability. The sPBIA5-FMX membrane, with a FM content of 3 wt%, demonstrated the most remarkable overall performance, achieving a proton conductivity of 33.5 mS cm-1 at 30 °C and 100 % relative humidity (RH). At the same time, it has outstanding oxidation stability and excellent ion selectivity (5.23 × 106 S min cm-3). In addition, the sPBIA5-FM3 membrane exhibited excellent cell performance, with energy efficiency (EE) from 84.80 % to 86.50 % at current densities of 40-120 mA cm-2. This study confirms the significant potential of fluoride-modified MOF-801 to enhance ion selectivity and provides an effective strategy for optimizing proton transport channels.

TiO2 Containing Hybrid Composite Polymer Membranes for Vanadium Redox Flow Batteries

In recent years, vanadium redox flow batteries (VRFB) have captured immense attraction in electrochemical energy storage systems due to their long cycle life, flexibility, high-energy efficiency, time, and reliability. In VRFB, polymer membranes play a significant role in transporting protons for current transmission and act as barriers between positive and negative electrodes/electrolytes. Commercial polymer membranes (such as Nafion) are the widely used IEM in VRFBs due to their outstanding chemical stability and proton conductivity. However, the membrane cost and increased vanadium ions permeability limit its commercial application. Therefore, various modified perfluorinated and non-perfluorinated membranes have been developed. This comprehensive review primarily focuses on recent developments of hybrid polymer composite membranes with inorganic TiO₂ nanofillers for VRFB applications. Hence, various fabrications are performed in the membrane with TiO₂ to alter their physicochemical properties for attaining perfect IEM. Additionally, embedding the -SO₃H groups by sulfonation on the nanofiller surface enhances membrane proton conductivity and mechanical strength. Incorporating TiO₂ and modified TiO₂ (sTiO₂, and organic silica modified TiO₂) into Nafion and other non-perfluorinated membranes (sPEEK and sPI) has effectively influenced the polymer membrane properties for better VRFB performances. This review provides an overall spotlight on the impact of TiO₂-based nanofillers in polymer matrix for VRFB applications.