Multichannel Electrochemical Impedance Spectroscopy and equivalent circuit synthesis of a large-scale vanadium redox flow battery

An original multichannel Electrochemical Impedance Spectroscopy (EIS) system operating at high bias current and suitable for kW-class Vanadium Redox Flow Batteries (VRFBs) is presented. Power and signal connections, whose stray parameters affected measurements, required a careful optimization and calibration in the implementation of the measurement chain. In any cases, electromagnetic interferences set an upper limit to the applied stimulus frequency that limited the set of measurement data at each bias point. The paper provides early results obtained with this EIS system, which were used to identify an equivalent circuit of the whole stack in which each cell is represented with a dynamic Thévenin equivalent, i.e. the series of a voltage source, a variable resistor and a variable RC loop. The dependence of the equivalent parameters on the operating conditions has been experimentally analyzed, confirming that only the RC-loop resistance is strongly affected by mass transport. To our knowledge, this is the first dynamic equivalent circuit of a whole stack that has been validated against EIS measurements taken on a real industrial-scale VRFB. Building on the measurement methodology here presented, advanced online state of health of industrial flow batteries can be developed and implemented.