Decoupling mass-transport contributions in vanadium flow battery using electrochemical impedance spectroscopy

The low-frequency mass-transport region of flow battery (FB) impedance spectra is commonly attributed to finite-diffusion processes, however, previous studies have not clearly distinguished between diffusion- and convection-dominated features. Here, two VFB cells with identical geometric electrode area but different flow orientations are studied by electrochemical impedance spectroscopy, exploiting a fourfold difference in cross-sectional area perpendicular to flow direction to differentiate the impact of those two effects. Comparison of the two orientations reveals that the low-frequency region contains two distinct contributions that respond differently to volumetric flow rate and mean linear velocity and are therefore consistent with different physical processes. These findings allow identification of the diffusion-dominated and convection-dominated contributions, which are essential for flow cell modelling and optimisation.