Practical flow battery diagnostics enabled by chemically mediated monitoring

Abstract

Aqueous organic flow batteries are a promising technology class for long-duration energy storage. However, the poor stability of redox-active components under the conditions frequently used in these batteries, coupled with the inherently high degree of active material chemical complexity, frequently gives rise to intricate degradation pathways that both limit attainable cycle life and are challenging to probe experimentally. Here, we utilize solution pH and bulk magnetic susceptibility to monitor the native minor equilibrium side reaction between water and the one-electron oxidized state of 2,2,6,6-tetramethyl-4-hydroxy-piperidin-1-oxyl (4-hydroxy-TEMPO)—an archetypical flow battery catholyte. This side reaction readily reports on both the main redox reaction of 4-hydroxy-TEMPO, which itself is not proton coupled, as well as on its principal self-discharge pathway. In so doing, it provides accurate, low-cost, and sensitive experimental insights into battery state of charge, state of health, and operating conditions for both flow and hybrid flow configurations.