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In vanadium flow batteries, electrolyte transfer across the membrane can lead to a volumetric imbalance between the two half-cell electrolytes and a subsequent loss of available capacity. However, the transfer mechanism has not been comprehensively understood and this lack of knowledge has significantly limited long-term discharge capacity and stability of the vanadium flow battery. To overcome this issue, the electrolyte transfer mechanism is systematically developed in this study by analyzing the pressure drop across the membrane in accordance with Darcy's law and further validated by experiments. The experimental results show that the viscosity difference between the two half-cell electrolytes contributes greatly to the electrolyte transfer from negative half-cell to positive half-cell, while a large flow rate applied to both half-cells may also exacerbate the electrolyte transfer. Moreover, further experiments also demonstrate that the electrolyte transfer in continuous charge-discharge operation can be effectively suppressed by optimizing the flow rates based on viscosity measurements, which subsequently yields a notable improvement in discharge capacity. Revealing the electrolyte transfer mechanism is not only beneficial to enhancing long-term performance and stability of the vanadium flow battery, but also highly valued for understanding the transport phenomena in other flow battery systems. Image 1 • Electrolyte transfer mechanism across a Nafion membrane is revealed for the VFB. • Pressure drop across the membrane arises from difference in electrolyte viscosity. • Appling a large flow rate may also accelerate the electrolyte crossover. • A Flow rate optimization strategy derived from Darcy's law is proposed. • Electrolyte transfer can be effectively mitigated by flow rate optimization. [ABSTRACT FROM AUTHOR]