Composite Anion Exchange Membranes Fabricated by Coating and UV Crosslinking of Low-Cost Precursors Tested in a Redox Flow Battery

Impact of Formulation of Photocurable Precursor Mixtures on the Performance and Dimensional Stability of Hierarchical Cation Exchange Membranes

This work presents a systematic approach to formulating UV curable ionomer coatings that can be used as ion-exchange membranes when they are applied on porous substrates. Ion-exchange membranes fabricated in this way can be a cost-effective alternative to perfluorosulfonic acid membranes, such as Nafion and similar thin ionomer film membranes. Hierarchically structured coated membranes find applications for energy storage and conversion (organic redox flow batteries and artificial photosynthesis cells) and separation processes (electrodialysis). Designing the ionomer precursor for membrane formulation requires the introduction of compounds with drastically different properties into a liquid mixture. Hansen solubility theory was used to find the solvents to compatibilize main formulation components: acrylic sulfone salt (3-sulfopropyl methacrylate potassium salt) and hexafunctional polyester acrylate cross-linker (Ebecryl 830), otherwise nonmiscible or mutely soluble. Among the identified suitable solvents, acrylic acid and acetic acid allowed for optimal mixing of the components and reaching the highest levels of sulfonic group content, providing the desired ion-exchange capacity. Interestingly, they represented a case of a reactive and nonreactive solvent since acrylic acid was built into the ionomer during the UV curing step. Properties of the two membrane variants were compared. Samples fabricated with acetic acid exhibit improved handleability compared with the case of acrylic acid. Acetic acid yielded a lower area-specific resistance (6.4 ± 0.17 Ohm·cm2) compared to acrylic acid (12.1 ± 0.16 Ohm·cm2 in 0.5 M NaCl). This was achieved without severely suppressing the selectivity of the membrane, which was standing at 93.4 and 96.4% for preparation with acetic and acrylic acid, respectively.

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.

Thin Reinforced Ion-Exchange Membranes Containing Fluorine Moiety for All-Vanadium Redox Flow Battery

In this work, we developed pore-filled ion-exchange membranes (PFIEMs) fabricated for the application to an all-vanadium redox flow battery (VRFB) by filling a hydrocarbon-based ionomer containing a fluorine moiety into the pores of a porous polyethylene (PE) substrate having excellent physical and chemical stabilities. The prepared PFIEMs were shown to possess superior tensile strength (i.e., 136.6 MPa for anion-exchange membrane; 129.9 MPa for cation-exchange membrane) and lower electrical resistance compared with commercial membranes by employing a thin porous PE substrate as a reinforcing material. In addition, by introducing a fluorine moiety into the filling ionomer along with the use of the porous PE substrate, the oxidation stability of the PFIEMs could be greatly improved, and the permeability of vanadium ions could also be significantly reduced. As a result of the evaluation of the charge-discharge performance in the VRFB, it was revealed that the higher the fluorine content in the PFIEMs was, the higher the current efficiency was. Moreover, the voltage efficiency of the PFIEMs was shown to be higher than those of the commercial membranes due to the lower electrical resistance. Consequently, both of the pore-filled anion- and cation-exchange membranes showed superior charge-discharge performances in the VRFB compared with those of hydrocarbon-based commercial membranes.