# Design Principles for Air Tolerance in Pyridinium‐Based Flow Batteries

## TL;DR

This paper presents design principles for air tolerance in pyridinium-based flow batteries, enabling stable operation in air and long-term energy storage.

## Contribution

The study introduces equilibrium models to predict and validate air stability in methyl viologen-based flow batteries.

## Key findings

- Methyl viologen exhibits 94.9% capacity retention in air after 150 cycles when operating within its air stability threshold.
- Performance degrades dramatically when the system is diluted beyond the air stability threshold.
- The second Damköhler number is derived to inform optimal scaling of battery components for air-stable operation.

## Abstract

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.

Equilibrium models are derived for redox flow battery electrolytes that enable accurate prediction of the onset air stability in methyl viologen – a compound known to the community for over 100 years and one of the most well‐studied aqueous flow battery electrolytes.