Functionalized Poly(phenylene) Efficiency and Stability in Vanadium Redox Flow Batteries

Perflourinated polymers have long been considered for applications that take advantage of their high mechanical strength and innate thermochemical stability. Recently, a phenylated polyelectrolyte has been synthesized that shows great potential in applications such as in proton exchange membrane fuel cells, alkali fuel cells, and vanadium redox flow batteries (VRB’s). Typical membranes in VRB’s suffer from high vanadium ion permeability and membrane degradation. Poly(phenylene)s show promise in this area. Synthesis typically involves Diels Alder polymerization (scheme 1), followed by post-sulfonation. For the purely aromatic parent polymer, molecular weights were obtained in the range of 80-110 000 g/mol, with an average density of 1.058 g/cm³. IR reveals significant sulfonic group stretching frequencies at 1180, 1126 and 1035cm^-1. The materials show glass transition temperatures around 398 degrees Celsius. Thermal gravimetric analysis reveals that the functionalized polymers thermal stability is predominantly limited by the sulfonic functional group. It appears to have superior thermal stability over commercial Nafion 117, only showing minor degradation below 400^oC. Sulfonated poly(phenylene)’s with a targeted sulfonation stoichiometry of 4:1 displays a proton conductivity of roughly 96 mS/cm² at 35^oC, with water uptake of 79.7%. Work is being done to test these materials for VRB’s, most notably vanadium ion permeation and membrane degradation. These properties are tested as a function of molecular weight, polymer structure, and ion exchange capacity.