Experimental and modelling analyses of electrolytes volume variation in vanadium redox flow batteries: insight into water osmosis through the membrane

Electrolyte imbalance caused by undesired vanadium-ion crossover and water transport through the membrane remains one of the major challenges in vanadium redox flow batteries, leading to capacity decay and electrolyte volume variation. In this study, the evolution of electrolyte volume and vanadium crossover was systematically investigated over 400 charge-discharge cycles using a commercial-like electrolyte (1.6 M V in 2 M H2SO4). The evolution of vanadium concentration in both electrolytes was accurately determined via inductively coupled plasma mass spectrometry. While vanadium transport almost ceased after the first 50 hours of testing, water transport continued to modify the volumes of both positive and negative electrolytes. By the end of the test, the positive electrolyte volume increased by 15%, whereas the negative one decreased by 18%. To elucidate the relationship between volume variation and vanadium crossover, a one-dimensional physics-based model was employed. The model clarified the underlying mechanisms governing volume changes, identifying osmotic pressure as the predominant driving force during periods of significant electrolyte volume variation. Finally, the model was validated on charge-discharge cycles adopting an asymmetric electrolyte formulation (1.6 M VOSO₄ in 3.3 M H₂SO₄ for the positive electrolyte and 1.6 M VOSO₄ in 4.1 M H₂SO₄ for the negative electrolyte), demonstrating that the combined elimination of the initial osmotic gradient and the enhancement of coulombic efficiency effectively suppress electrolyte volume variation. These findings further emphasize osmosis as the main physical phenomenon contributing to electrolytes volume variation.