Detection and analysis of inner potential dynamics in vanadium redox flow batteries

•An in situ probing method for the inner potential dynamics of flow batteries is developed.•A physics-based theoretical model for probing and analysis of the battery performance is established.•Collective quantification studies are conducted using the experimental data and the theoretical model.•The complex interconnectedness of internal potential dynamics and battery performance is unveiled.•The studies provide a quantitative method for real time probing, understanding, and predicting flow battery performance. The interconnectedness of the inner potential dynamics during the charge–discharge operation of a vanadium redox flow battery is studied by in-situ measurements of the through-plane potential distribution of the battery cell and the states of charge of the electrolytes. For a quantitative investigation, experimentally probed information is integrated and analyzed using simplified physics-based theoretical models, which collectively enable accurate detection and interpretatation of the inner potential dynamics from the individual components to the single-cell battery level. It has been demonstrated that this collective analysis makes quantifying the ohmic-kinetic and mass transport resistances of the electrodes possible. In particular, the sluggish transport of vanadium ions near the membrane interface of both electrodes leads to additional diffusion overpotential at the beginning of charging and discharging in the electrode. The membrane potentials, which can be detected experimentally, reflect highly coupled conditions of the electrolytes on both sides that affect the voltage of each electrode and the state of charges of the battery. This work demonstrates that unveiling the internal dynamics within a cell leads to an in-depth understanding of the charge–discharge behavior of vanadium redox flow batteries.