Investigation of vanadium redox flow batteries performance through locally-resolved polarisation curves and impedance spectroscopy: Insight into the effects of electrolyte, flow field geometry and electrode thickness

The inhomogeneous distribution of electrolyte over the porous electrode is one of the main issues hindering vanadium flow battery performance, limiting the system power density. Moreover, the interplay between fluid dynamics in distribution channels and morphology of porous electrodes is not well understood at local level throughout the cell active area, where local reactant starvation may occur and further limit system performance. Therefore, a systematic local characterization is fundamental to improve the understanding of system operation and to enhance system power density. This work discusses the implementation of a 10 regions macro-segmented flow battery with 25 cm2 active area, that permits the local measurement of polarisation curves and impedance spectra. Both positive and negative symmetric cells, as well as all-vanadium configuration, are adopted, combining carbon paper electrodes of different thickness with single serpentine and interdigitated distributors. The characterization evidences that single serpentine presents poor performance towards cell outlet, while interdigitated geometry is limited in the central region of active area. In most of the investigated operating conditions, single serpentine induces a more heterogeneous operation, especially if coupled with thinner electrode and positive electrolyte. Moreover, the representativeness at local level of symmetric cells compared to all-vanadium one is analysed and discussed.