An Fe^III Azamacrocyclic Complex as a pH‐Tunable Catholyte and Anolyte for Redox‐Flow Battery Applications

Abstract

A reversible Fe³⁺ /Fe²⁺ redox couple of an azamacrocyclic complex is evaluated as an electrolyte with a pH-tunable potential range for aqueous redox-flow batteries (RFBs). The Fe^III complex is formed by 1,4,7-triazacyclononane (TACN) appended with three 2-methyl-imidazole donors, denoted as Fe(Tim). This complex exhibits pH-sensitive redox couples that span E_1/2 (Fe³⁺ /Fe²⁺ )=317 to -270 mV vs. NHE at pH 3.3 and pH 12.8, respectively. The 590 mV shift in potential and kinetic inertness are driven by ionization of the imidazoles at various pH values. The Fe³⁺ /Fe²⁺ redox is proton-coupled at alkaline conditions, and bulk electrolysis is non-destructive. The electrolyte demonstrates high charge/discharge capacities at both acidic and alkaline conditions throughout 100 cycles. Given its tunable redox, fast electrochemical kinetics, exceptional stability/cyclability, this complex is promising for the design of aqueous RFB catholytes and anolytes that utilize the earth-abundant element iron.