Beyond steady-state conditions: Chronoamperometric state-of-charge and state-of-health measurements in flow battery electrolytes

Steady-state amperometry with microelectrodes based on potential step experiments arguably yields the best-in-class accuracy for the state-of-charge monitoring in redox flow battery (RFB) electrolytes. In fact, only a fraction of the current response obtained during the potential step was used in experiments for the state-of-charge (SOC) and state-of-health (SOH) assessment so far. This study explores the analysis of the transient chronoamperometric signal. It is demonstrated that the physicochemical information contained in the transient current signal enables both the replacement of microelectrodes with macroelectrodes for ex situ as well as in operando SOC measurements and for an ex situ concentration- and (theoretically) temperature-independent assessment of the electrolyte SOH with microelectrodes. The results demonstrate that chronoamperometry enables accurate, fast, and low-cost measurements of an electrolyte’s SOC. Unlike previous macroelectrode-based approaches, which required a limitation of electrolyte flow and reactant transport to the working electrode surface, the rapid chronoamperometric potential steps presented in this study enable an electrode placement directly in the free-flowing electrolyte. Additionally, the developed ex situ microelectrode-based chronoamperometric SOH measurement method allows for the investigation of the electrolyte lifetime in isolated vessels. Consequently, the presented SOH assessment technique holds a promise for establishing high-throughput, ex situ electrolyte stability measurements.