Complementing the VRFB Membrane Performance Assessment Using Color Sensors for Self‐Discharge and Durability Stress Test

Reliable assessment of ion‐exchange membrane performance is essential for the design of VRFB systems, yet conventional accelerated cycling protocols assessing durability metrics depend on electrolyte volume and cycle duration, limiting their comparability and sensitivity to long‐term chemical degradation. This work proposes a stress‐test methodology implying the continuous color sensing of both electrolytes under 100% state‐of‐charge conditions, providing a durability indicator that is less dependent on cycle duration and electrolyte volume compared to conventional cycling‐based metrics and capable of resolving the temporal evolution of self‐discharge. The method was incorporated in the polarization and galvanostatic cycling tests of four representative membranes: Nafion‐212, Asahi, PBI, and SPEEK. The obtained results were validated by post‐test FTIR spectroscopy and compared with the Coulombic efficiency, permeability, and self‐discharge metrics obtained from open‐circuit voltage registration. The results demonstrate that the proposed approach allows for faster capture chemical degradation of the membrane compared to conventional tests. Rapid oxidative failure of SPEEK was observed within 1600 s of exposure to aggressive electrolyte media, followed by an increase in crossover examined in operando regime. FTIR analysis confirms SPEEK oxidation occurred during the stress test. The proposed framework provides insights for membrane durability assessment and material selection.