The climate change threatens earth’s biosphere and motivates the transition towards renewable energy systems. Redox flow batteries (RFBs), particularly aqueous organic RFBs (AORFBs), are promising for large-scale storage. However, limited long-term stability of organic active materials remains critical. Reliable capacity fade rate assessment is therefore essential. This dissertation evaluates and improves three techniques: unbalanced, compositionally symmetric flow cell cycling (UCSFCC), full RFB cycling, and ex situ amperometric state-of-health (SOH) measurements. UCSFCC is examined as a state-of-the-art in operando method. A three-fold deviation of measured fade rate versus literature is observed in very similar conditions. Further evaluation of the technique proves that UCSFCC exhibits low precision. Multiple repetitions and prolonged cycling are recommended. Integration of flow-through SOC monitoring is applied for better analysis of irreproducibility caused by cross-over and charge imbalance and to improve the method. Full RFB cycling is evaluated. Combination with UCSFCC enables indication of the cross-over processes, which are undetectable by conventional analyses. Analysis of heating setups for thermal fade rate measurement demonstrates that localized heating causes temperature discrepancies and altered degradation behavior, whereas homogeneous heating ensures reliable results. Thermal modelling is introduced to optimize experiments. Finally, ex situ amperometric methods are studied as scalable alternatives. The steady-state ASOH method is temperature-sensitive and is corrected using the Stokes–Einstein relation, yielding results comparable to UCSFCC. Furthermore, a chronoamperometric SOH method is developed, making the measurement diffusion-independent and better suitable for highly concentrated electrolytes. The research results enhance accuracy, reproducibility, and scalability of capacity fade assessment for stable AORFB development.
Ivan A. Volodin (Thu,) studied this question.