Effect of Boiling Pretreatment on Physicochemical and Transport Properties of Perfluorosulfonic Acid Membrane

An in-depth understanding of pretreatment effects on a perfluorosulfonic acid membrane is essential for its application to vanadium redox flow batteries (VRFBs). The physicochemical properties and transport properties of perfluorosulfonic acid membranes (PFSA) can be modified and controlled by conditioning treatments. This work investigates the effects of boiling pretreatment on the water uptake, acid uptake, membrane density, and conductivity of 3M PFSA membranes when they are in contact with H₂SO₄ or VOSO₄/H₂SO₄ electrolytes. Increased water and acid uptake and much higher membrane conductivity are outcomes of the boiling pretreatment. This is mainly attributed to the pretreatment-induced morphology change, which includes a higher normalized swelling volume and a larger number of continuous water channels. The reduction of the Donnan potential for boiled membrane contributes to a higher uptake and faster proton transport but also inevitably leads to a much higher vanadium permeability. The trade-off between membrane conductance and vanadium transport across the membrane is crucial and primarily driven by the propensity to absorb various solution components, as well as their intrinsic mobility. These findings shed light on addressing relationships among physicochemical properties, transport behavior, membrane morphology, and the pretreatment effects, all of which are crucial to the performance of PFSA membranes when applied in vanadium redox flow batteries.