Design principles for air tolerance in pyridinium-based flow batteries

Pyridinium compounds represent promising electrolyte candidates for aqueous flow batteries. Recently, their ability to afford air-stability was demonstrated, unlocking potential avenues both for relaxed system constraints and for high voltage operation, when paired with a suitable catholyte. Here, we develop simple models for pyridinium electrolytes, which we leverage 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, even at oxygen reducing potentials. 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 which informs optimal scaling of battery components to minimise parasitic processes with air. 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 electrolyte to associate, we anticipate that the findings presented can be generalised to many other current and future systems.