An Amperometric, Temperature-Independent, and Calibration-Free Method for the Real-Time State-of-Charge Monitoring of Redox Flow Battery Electrolytes

A simple mathematical framework for a calibration-free real-time state-of-charge (SOC) monitoring of redox flow battery (RFB) electrolytes, which is based on mass-transfer limited amperometry, is theoretically derived. The equations are initially validated with literature data for a vanadium-based RFB electrolyte, revealing absolute root-mean-square deviations (RMSD) of 4–6% for the obtained SOC estimates. Subsequently, our own experimental results based on a microelectrode as an amperometric sensor are presented. Absolute RMSDs of 2–7% are obtained for ferri-/ferrocyanide-based RFB electrolytes at two different concentrations in an offline measurement. Furthermore, the real-time SOC monitoring capability is demonstrated for the capacity limiting half-cell of a symmetric aqueous RFB utilizing N,N,N-2,2,6,6-heptamethylpiperidinyloxy-4-ammonium chloride (TEMPTMA) as an active organic molecule, yielding an absolute RMSD of below 3%. The theoretical considerations and the experimental results imply a complete independence of the presented approach from, e.g., the type of mass-transfer limiting process, the temperature, the RFB type (organic/inorganic, aqueous/nonaqueous), and the electrolyte composition.