Design Principles for Air Tolerance in Pyridinium-Based Flow Batteries.

Pyridinium compounds represent promising electrolyte candidates for aqueous redox flow batteries. Recently, their ability to afford air-stability was demonstrated, unlocking potential avenues both for relaxed system constraints and for high voltage operation. Here, simple equilibrium models for pyridinium electrolytes are developed, which are leveraged to predict and successfully validate the air stability of methyl viologen - the lowest cost and most well-studied pyridinium system to date. By controlling the degree of π-association of active species, the total fraction of radicals can be kept below a critical threshold, from which air-stable operation can be accessed. The resulting system exhibits 94.9% capacity retention in air after 150 cycles but undergoes dramatic losses in performance once diluted outside of its air stability threshold. We tie this behaviour to rates of oxygen consumption in solution and further derive the second Damköhler number, a dimensionless parameter which informs optimal scaling of battery components. On this basis, air stability is shown to be compatible with scaling requirements needed for applications in long-duration energy storage. Given the known tendency for broader classes of organic electrolytes to associate, it is anticipated that the findings presented can be generalized to many other current and future systems.