Monitoring of vanadium mass transfer using redox potential probes inside membranes during charge and discharge of flow batteries: An experimental study

It widely recognized that vanadium crossover is the primary source of capacity fade in vanadium flow batteries (VFB). Transport models that could help understand and improve the capacity fade have largely overlooked the role of self-discharge reactions between the vanadium species inside the membrane. This neglect could be the reason why published diffusion coefficients differ significantly. In a previous study, a time-resolved in operando method was developed, which monitors all the vanadium redox couples inside the membrane. However, information about the mass transfer inside the membrane under charging and discharging conditions is not yet available. In the present study, the previously developed method is used to gain valuable insights into the transport process during battery operation. It is shown that the dynamic behavior of the membrane resistance is caused by the vanadium crossover processes dependent on current density and state of charge. This interaction between crossover and membrane resistance demonstrates conclusively that reactions within membranes must be considered for a proper description of membrane properties and crossover processes. •New method monitors vanadium ions across membrane layers using potential probes.•In operando method for locally resolved membrane resistance and vanadium valence.•Locally resolved measurements performed during charging and discharging.•Electrical potential gradient influences location of reaction zones inside membrane.•Unexpected temporary maximum of membrane resistance observed during discharge.