State and Prospects of Unbalanced, Compositionally Symmetric Flow Battery Cycling and Steady-State Amperometry Techniques for Electrolyte Stability Assessment: The Case of Methyl Viologen

An increasing number of redox-active substances with improved electrochemical stability is currently being reported in the literature for potential applications in redox flow batteries (RFBs). Consequently, the demand for methods capable of estimating the long-term stability of redox couples is increasing as well. It seems to be of particular importance to understand the limitations of existing stability assessment techniques. In this work, two recently established and highly promising stability assessment techniques are directly compared for the first time to measure the capacity fade rate of a N,N′-dimethyl-4,4′-bipyridinium dichloride (methyl viologen or MV) molecule as a benchmark anolyte substance. These two methods are the unbalanced, compositionally symmetric flow cell cycling (symmetric cycling or UCSFCC) and the steady-state amperometric state of health (ASOH) measurement. The results of the symmetric cycling indicate a 3-fold higher stability for MV compared to a previous literature report as well as a high standard deviation for the measured capacity fade rate. More comprehensive investigations of the technique revealed that the influence of the MV purity, electrolyte leakage, and membrane cross-over could not cause the difference between the measured and earlier reported capacity fades and, thus, indicated insufficiency of the technique’s accuracy for the measurement of highly stable redox species. ASOH as the second technique exhibited similar capacity fade rates like the symmetric cycling as well as comparably high standard deviations. Furthermore, it is prone to temperature fluctuations and variations of the electrode radius. Thus, we demonstrate an approach for a temperature correction in this method, which enables significantly higher accuracy and reliability. Despite this, the variation of the electrode radius remains the main concern for this technique. Based on the obtained results for the MV anolyte, the state and prospects of both techniques for the stability assessment in RFB research are discussed in detail.