Accelerated Stress Test Method for the Assessment of Membrane Lifetime in Vanadium Redox Flow Batteries

An accelerated stress test (AST) method was developed to estimate the lifetime of ion-exchange membranes in a vanadium redox flow battery. The oxidative VO₂⁺ ions present in the charged positive electrolyte are the predominant stressor causing loss of functional groups and membrane conductivity. Membrane aging was accelerated in ex situ tests by exploiting elevated temperatures and the increased oxidative strength of Ce⁴⁺. Acceleration factors were determined on the basis of the analysis of aged radiation grafted g(S-AN) membranes. The degradation in a Ce⁴⁺ solution was found to be ∼4 times faster than in VO₂⁺. The highest acceleration factor of ∼200 was found for the degradation with Ce⁴⁺ at 80 °C. The degradation reaction present in the accelerated stress test showed a similar activation energy of ∼50 kJ/mol in VO₂⁺ and Ce⁴⁺ solutions, suggesting a similar reaction pathway. The applicability of the test was further evaluated with the second membrane, g(AMS-MAN). Its lifetime was estimated based on the accelerated stress test and acceleration factors previously determined for the g(S-AN) membrane and compared to the lifetime projected from an extended cycling experiment in the cell. The two values were in the same range of ∼4000 h. The proposed AST can serve as a basis for predictive modeling of membrane lifetime in vanadium redox flow batteries. The potential of the method and the limitations are discussed.