Comparative Study of Organic Radical Cation Stability and Coulombic Efficiency for Nonaqueous Redox Flow Battery Applications

When building energy dense redox flow batteries, the cell voltage of the flow cell is critical. However, the stability of the charged forms of redox-active species often suffers at extreme redox potentials due to either self-discharge in electrolyte solutions or molecular degradation, presenting a challenge for achieving high cell voltages. In this work, we explore the chemical and cycling stability of a series of organic posolytes with a 0.8 V vs Cp2Fe0/+ range in oxidation potentials in nonaqueous electrolyte systems. The relationship between the coulombic efficiency of posolytes during galvanostatic cycling and the chemical stability of their charged forms was compared by performing bulk electrolysis and UV–vis spectroscopy experiments. In general, these studies show that organic posolytes with high oxidation potentials exhibit either self-discharge and/or molecular degradation in their charged forms, lowering coulombic efficiency and/or cycle lifetime in galvanostatic cycling.