(Battery Division Student Research Award Sponsored by Mercedes-Benz Research & Development) High-Throughput Electrochemical Characterization of Aqueous Organic Redox Flow Batteries

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Aqueous organic redox flow batteries (AORFBs) have emerged as potentially disruptive technologies for the storage of electrical energy from intermittent renewable sources. With the goal of cost-effective, safe, and scalable stationary long duration energy storage systems, AORFBs could become preferred over Li-ion batteries for grid-scale stationary storage due to their inherent non-flammability, lack of materials scarcity fluctuations, and intrinsic decoupling of energy and power capacities. Our group has demonstrated that calendar life, rather than cycle life, limits molecular lifetimes in AORFBs due to various molecular instabilities that lead to side reactions, thus inhibiting performance [1]. To accurately determine molecular fade rates, we utilize potentiostatic cycling to avoid artifacts caused by drifts in internal resistance and employ volumetrically unbalanced compositionally symmetric cell configurations to distinguish molecular fade from membrane crossover or cell unbalancing. Redox-active organic molecule stability has improved to the point that the most stable chemistries degrade at less than 1% per year [2]. With further lifetime increases, the measurement of lower capacity fade rates necessitates higher precision coulometry methods and thermally accelerated degradation protocols to determine which stabilizing approaches are most effective without waiting for multi-month cycling tests to quantify capacity fade.We developed a high-throughput setup for cycling AORFBs at elevated temperatures, providing a new dimension in the flow battery characterization space to explore [3,4]. Capacity fade rates of previously published redox-active organic molecules, as functions of temperature, were evaluated in the high-throughput setup. Demonstrated Arrhenius-like behavior in the temporal capacity fade rates of multiple AORFB electrolytes provided the ability to extrapolate fade rates to lower operating temperatures.Complemented by open source zero-dimensional modelling that incorporates redox-active material degradation [5], we explored temporal capacity evolution in symmetric cells driven by different capacity fade mechanisms such as active species degradation, self-discharge [6], and membrane crossover. Collectively, these results highlight the relevance of electrochemical techniques to understand molecular degradation and expedite the screening process of candidate molecules for long lifetime AORFBs, which may enable massive grid penetration of intermittent renewable energy.