pH signalling and regulation in pyridinium redox flow batteries

Pyridinium compounds are promising electrolyte candidates for aqueous flow batteries yet undergo complex parasitic processes involving free radical species that remain difficult to monitor, track and regulate. Inspired by biological redox signaling, we developed operando pH monitoring to accurately track native pyridinium electrolyte processes across nine orders of magnitude (i.e., pH 3 – 12), elucidating trace (nanomolar) state-of-charge-dependent equilibria that can be used for continuous monitoring of state of charge and health in materials that do not feature a principal proton coupled redox process. By monitoring pH, we gain simultaneous real-time insights into direct and mediated water splitting, direct and mediated dioxygen reduction, oxidation-statedependent pyridinium pKa effects, and trans-membrane OH– transport. These insights inform the development of a global regulation strategy, based on a single radical dependent dimensionless parameter, towards complete suppression of parasitic processes under practical battery cycling conditions. We also clarify the nature of the reaction between dioxygen and reduced viologen towards improved system engineering for long duration energy storage applications.