Rationalizing (in)stability of pyridinium ketones in aqueous organic flow batteries

Rationalizing the stability of organic molecules for aqueous flow battery applications is crucial for development of sustainable large scale energy storage technologies. Herein, the properties of two molecules featuring a pyridinium group next to a ketone either as an open structure (benzoylpyridinium) or as a fused ring structure (azoniafluorenone) were compared. The electrochemical investigations and flow battery experiments reveal that benzoylpyridinium electrolytes show very poor to poor stability depending on the pH while azoniafluorenones show good stability in neutral pH and poor stability in alkaline pH. The root of the problem appears to lie in the irreversible protonation of the benzylic position that takes place as the last step of the redox cycle of benzoylpyridinium electrolytes. The benzylic C-H proton of the doubly reduced molecule is readily protonated with a pK _a of 10.3, while the corresponding pK _a for the fused azoniaflurenone is only 6.0. Additionally, the computational studies indicate that the barrier for the deprotonation of the reduced form of azoniafluorenone appears to be on the order of 7-10 kcal mol^-1 lower than that of benzoylpyridinium, and results from a structural transformation of the open benzoylpyridinium with a significantly widening dihedral angle from 6.5 to 70.5° for the two aromatic rings. This structural transformation strongly stabilizes the doubly reduced protonated species, preventing deprotonation and reoxidation in the battery, while the protonation of the reduced fused azoniafluorenone only takes place at much lower pH without significant structural changes. This structural destabilization allows reversible ketone/alcohol transition similarly to what has been observed earlier for fluorenones. The poor cycling stability under alkaline conditions results from decomposition of the oxidized species. The lessons learned in these studies strongly underline the need to consider complete redox cycles in the design of electrolytes for aqueous organic flow batteries.