This paper investigates the redox behavior of pyridinium electrolytes in redox flow batteries (RFBs), focusing on their air tolerance and cycling stability. A library of extended bispyridinium compounds was synthesized and their performance was evaluated across a range of potentials. The singlet-triplet free energy gap (ΔEST) was identified as a key descriptor predicting capacity fade mechanisms. Operando NMR and EPR spectroscopies revealed two electrochemical performance regimes (narrow and wide energy gaps), both linked to free radical formation. π-dimerization was found to suppress radical reactivity with oxygen, enabling air-tolerant operation. These findings challenge previous understandings of π-dimers' role in RFBs and offer pathways for designing air-stable electrolytes.

Mark E. Carrington, Kamil Sokołowski, Erlendur Jónsson, Evan Wenbo Zhao, Anton M. Graf, Israel Temprano, Jade A. McCune, Clare P. Grey, Oren A. Scherman