High Accuracy Measurements for Rapid Evaluation of Extremely Low Decomposition Rates of Redox Active Molecules

Abstract

Aqueous Organic Redox Flow Batteries (AORFBs) have potential to disrupt the energy storage technology market for renewable energy sources. To combat the issue of intermittency[i] for these energy sources, AORFBs use non-volatile, potentially cheap, and abundant materials to deliver decadal time scale energy storage with the advantage of decoupled energy and power density by scaling the former with the size of electrolyte tank volumes and scaling the latter with the area of the electrode surface where the reactions take place. Because many candidates have been discovered and remain to be discovered for applications as redox active molecules with long chemical lifetimes in AORFBs, in this work we developed a setup for quickly and accurately evaluating molecular decomposition.To make AORFBs more competitive for commercialization, our group has fine-tuned the electrochemical properties of active organic molecules via new synthetic approaches to achieve capacity fade rates lower than 1% per year.[ii] To better understand the decomposition mechanisms of these redox active species in AORFBs, we have also developed volumetrically unbalanced, compositionally symmetric cell cycling methods to evaluate capacity losses solely from decomposition and not from active species crossover through the membrane or changes in internal resistance of the battery.[iii] While the symmetric cell cycling technique allows isolating molecular decomposition as the source of capacity fade for AORFBs, for evaluating extremely low decomposition rates, flow batteries can require run times from several weeks to months due to noise in capacity measurements from temperature and pressure fluctuations or droplets in the batteries losing contact with solution being pumped to react in the cell. Additionally, since we have experienced slight variations in fade rates across cells with the same experimental conditions, decomposition rates should be determined from several cycling experiments, not one.[iv] In order to more quickly and more accurately evaluate extremely low decomposition rates, we have developed a simple static cell into which electrolyte is injected into porous electrodes and sandwiched together by current collectors without a flow system. By removing the complexities of the flow system, we may determine capacity fade rates in much shorter times and with lower uncertainties. [i] J. Rugolo and M. J. Aziz, “Electricity storage for intermittent renewable sources.” Energy Environ. Sci., 2012, 5, 7151 [ii] M. Wu et al., “Extremely Stable Anthraquinone Negolytes Synthesized from Common Precursors”. Chem 6,1–11 June 11, 2020 [iii] M.A. Goulet and M. J. Aziz, “Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods”. Journal of The Electrochemical Society, 165 (7) A1466-A1477 (2018) [iv] Eric Michael Fell and Michael J. Aziz 2021 Meet. Abstr. MA2021-01 207