Sulfonated Ionomer (Bio)Composites for Use in Vanadium Redox Flow Batteries

Sulfonated aromatic polymers, such as sulfonated poly (ether ether ketone) (SPEEK), have emerged as promising proton exchange membranes (PEMs) for vanadium redox flow batteries (VRFBs) due to their high thermal and mechanical stability, low cost, and tunable conductivity via adjusting the sulfonation degree of the polymer. Herein, the impact of both inorganic and organic fillers on the performance properties of SPEEK-based membranes were investigated. Specifically, inorganic silica nanoparticles (SiNPs) functionalized with both cationic (i.e., amine-containing) and anionic (i.e., sulfonic acid-containing) surfaces, as well as the biopolymer lignin, of varying molecular weight, were used to fabricate SPEEK-based ionomer (bio)composites. Membrane performance properties – e.g., proton conductivity and vanadium permeability – of these films were characterized, where SPEEK-lignin composites, with up to 15 mass % lignin, demonstrated significantly enhanced proton selectivity when compared to their neat SPEEK counterparts. Further, to improve our understanding of the interplay of the water transport and polymer swelling kinetics, water diffusion and ionomer swelling kinetics of SPEEK-lignin composites were studied using in situ time-resolved attenuated total reflectance-Fourier transform infrared (tATR-FTIR). Finally, the impact of sulfonic acid group dispersion on membrane properties was investigated, where both random and “blocky” SPEEK membranes were fabricated and compared.