*2.6. EIS Analysis*

For electrical impedance spectrometer (EIS) test, the felt and the electrode features were the same as the previous CV tests, but used a different process. In the test, 0.2 M VOSO<sup>4</sup> (purchased from Alfa) solution was used as the positive test electrolyte, with 0.17 M and 2.8–3 valance, and the vanadium and sulfuric acid mix solution made from the electrolysis reaction of the VRFB single cell test was used as the negative electrolyte. The SCE was used as the RE and a 2 mm thick glassy carbon rod used for the WE and CE. The starting voltage was automatically detected by the tool. EIS was performed, wherein an alternating current (AC) voltage of 10 mV in the frequency range of 105–10−<sup>2</sup> Hz was applied at the open circuit potential.
