Electrochemical behaviour of PBI-based vanadium redox flow battery membranes

Energy Storage System (ESS) has a critical role in ensuring the reliability and efficiency of energy supply. The Vanadium Redox Flow Batteries (VRFBs) are a promising solution because it is reliable, fire-safe, and a sustainable energy storage system. However, the widespread adoption of VRFB is limited by high costs and inefficiencies that are found in Nafion-based membranes. This study presents the synthesis and electrochemical evaluation of phosphoric acid-doped Polybenzimidazole (PBI) membranes blended with Sulfonated Polyether Ether Ketone (SPEEK) at varying weight percentages (3 wt%, 5 wt%, and 10 wt%) named 3M A, 5M A and 10M A respectfully, for application in VRFBs. The objective was to enhance electrochemical stability preserving chemical and structural integrity in acidic vanadium electrolyte environments. Comprehensive characterization including polarization corrosion testing, was done to evaluate the corrosion resistance and electrochemical impedance spectroscopy (EIS) testing used to evaluate the proton conductivity of the membranes via Nyquist plots to contribute, to the overall electrochemical properties of the composite membranes. Notably, the 5 wt% SPEEK-PBI blend demonstrated superior electrochemical performance, exhibiting a low corrosion current density = ~0.48 µA/cm², a stable corrosion potential = 0.081 V, SPEEK showed best proton conductivity at 1.61 mS/cm. The findings position the 5M A membrane, followed by SPEEK as a promising alternative to Nafion, offering improved electrochemical stability in VRFBs and recommendation to carefully cater for membrane thickness after solution casting.