Eric Fell1,Michael Aziz1
Harvard University1
Aqueous Organic Redox Flow Batteries (AORFBs) have emerged as potentially disruptive technologies for the storage of electrical energy from intermittent renewable sources for use over long discharge durations when the wind isn’t blowing, and the sun isn’t shining. AORFBs could become preferred solutions for grid-scale storage due to their potentially low-cost active materials, inherent non-flammability, and intrinsic decoupling of energy and power capacities. Our group has previously 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 holds to avoid artifacts caused by drifts in internal resistance and employ volumetrically unbalanced, compositionally symmetric cell configurations to distinguish molecular fade from membrane crossover. Synthetic efforts have continued to improve molecular design strategies to the point that the most stable molecules degrade at less than 1% per year [2]. With further lifetime increases, the measurement of lower capacity fade rates now necessitates higher precision coulometry methods [3] and the use of thermally accelerated degradation protocols [4,5] to determine which stabilizing approaches are most effective without waiting for multi-month cycling tests to quantify capacity fade.<br/><br/>We have developed a high-throughput setup for cycling AORFBs at elevated temperatures, providing a new dimension in the flow battery characterization space to explore. An in-depth study comparing previously published redox-active organic molecules has been performed using the protocol discussed above. Capacity fade rates as functions of temperature are evaluated in the high-throughput setup using high-precision coulometry, providing the ability to extrapolate fade rates to lower operating temperatures. Collectively, these results highlight the importance of accelerated decomposition protocols to expedite the screening process of candidate molecules for long lifetime AORFBs, which may enable massive grid penetration of intermittent renewable energy.<br/><br/>[1] M.-A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods,” <i>Journal of The Electrochemical Society</i><i>,</i> <b>165</b>, A1466 (2018).<br/><br/>[2] M. Wu, Y. Jing, A. A. Wong, E. M. Fell, S. Jin, Z. Tang, R. G. Gordon, M. J. Aziz, “Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors,” <i>Chem</i>, <b>6</b>, 1432 (2020).<br/><br/>[3] T. M. Bond, J. C. Burns, D. A. Stevens, H. M. Dahn, and J. R. Dahn, "Improving Precision and Accuracy in Coulombic Efficiency Measurements of Li-Ion Batteries", <i>Journal of The Electrochemical Society</i>, <b>160</b>, A521 (2013).<br/><br/>[4] D. G. Kwabi, K. Lin, Y. Ji, E. F. Kerr, M.-A. Goulet, D. De Porcellinis, D. P. Tabor, D. A. Pollack, A. Aspuru-Guzik, R. G. Gordon, and M. J. Aziz, “Alkaline Quinone Flow Battery with Long Lifetime at pH 12,” <i>Joule</i>, <b>2</b>, 1894 (2018).<br/><br/>[5] D. A. Stevens, R. Y. Ying, R. Fathi, J. N. Reimers, J. E. Harlow, and J. R. Dahn, "Using High Precision Coulometry Measurements to Compare the Degradation Mechanisms of NMC/LMO and NMC-Only Automotive Scale Pouch Cells", <i>Journal of The Electrochemical Society</i>, <b>161</b>, A1364 (2014).<br/><br/>[6] Y. Ji, M.-A. Goulet, D. A. Pollack, D. G. Kwabi, S. Jin, D. De Porcellinis, E. F. Kerr, R. G. Gordon, and M. J. Aziz, “A Phosphonate-Functionalized Quinone Redox Flow Battery at Near-Neutral pH with Record Capacity Retention Rate,” <i>Advanced Energy Materials</i>, <b>9</b>, 1900039 (2019).