Toward pH Versatile, Imidazole‐Based Posolytes for Aqueous Organic Redox Flow Batteries

The use of organic redox species has emerged as a promising strategy to improve the sustainability of redox flow batteries (RFBs). Several organic redox species have been developed for the negative half‐cell of aqueous RFBs. However, the number of organic species available for the positive half‐cell—so‐called posolytes—is limited. In this study, different imidazole‐based compounds were synthesized from inexpensive starting materials and investigated for their suitability as posolytes in aqueous RFBs. Out of the imidazole‐based compounds assessed, 4‐(4,5‐diphenyl‐1H‐imidazol‐2‐yl)phenol was synthesized on a multigram scale and demonstrated sufficient solubility (0.53 M) in aqueous media. The aforementioned compound was found to display reversible electrochemical performance with standard reduction potential of +0.22 and +0.73 V versus standard hydrogen electrode (SHE), in 1 M KOH and 1 M AcOH, respectively. While chemical degradation of the oxidized species was observed during RFB cycling in 1 M KOH media no chemical degradation was perceived in 1 M AcOH, highlighting a pH‐dependent stability.