Redox flow batteries (RFBs) are a promising electrochemical technology for energy-intensive grid storage, but further cost reductions are needed for widespread adoption spurring research efforts into new redox chemistries and reactor designs¹. Notably the recent emergence of redox active organic molecules offers intriguing new pathways to low cost energy storage through tunable molecular structure and inexpensive synthesis routes. While the rapid and continued advancement of these redox couples is exciting, iterative design-test-improve cycles are slowed as molecules become increasingly robust and failure modes become more subtle. Thus, there is a growing need for more advanced diagnostic techniques to evaluate materials performance, particularly stability^2,3.

At present, common methods for characterizing the decay of RFB materials include extensive cycling in an electrochemical cell⁴ to correlate capacity fade to overall species decay or monitoring decay of a single reporter constituent (e.g. oxidized species) via different spectroscopy techniques (e.g. NMR³, EPR², or UV-Vis⁵). However, these methods are challenged by, in the case of cell cycling, disambiguation of the sources of capacity fade, and, in the case of spectroscopies, issues operating under RFB relevant conditions (e.g. high concentrations of active species and supporting salt). Here, I will present a new technique to quantitatively probe active species stability in RFB-type electrolytes using a microelectrode. The talk will include discussion of underlying theory, systematic evaluation of sources of error, method validation using model compounds, and, finally, demonstration with several new redox couples.

References

1. M. L. Perry and A. Z. Weber, J. Electrochem. Soc., 163, A5064–A5067 (2016).
2. J. Huang et al., Sci. Rep., 6, 32102 (2016).
3. C. S. Sevov et al., J. Am. Chem. Soc., 139, 2924–2927 (2017).
4. J. Huang et al., J. Mater. Chem. A, 3, 14971–14976 (2015).
5. J. D. Milshtein et al., Energy Environ. Sci., 9, 3531–3543 (2016).