Electrolyte optimization of alkaline aqueous redox flow battery using iron-2,2-bis(hydroxymethyl)-2,2′,2′-nitrilotriethanol complex as active material for anolyte

In this study, optimal electrolyte condition of alkaline aqueous redox flow battery (ARFB) using iron-2,2-bis(hydroxymethyl)-2,2′,2′-nitrilotriethanol (Fe(BIS-TRIS)) and Ferrocyanide as redox couple is determined. Especially, the optimal condition of anolyte, including Fe(BIS-TRIS) and potassium hydroxide (KOH), is deeply explored. As a result, the stability of the anolyte is improved by controlling (i) the mixing ratio of metal to a ligand for the synthesis of Fe(BIS-TRIS) complex and (ii) the concentration of KOH, which is linked to hydroxyl (OH^–) ion concentration. By systematic analysis, it is revealed that 2.75 M OH^– ions are needed for the synthesis of 0.5 M Fe(BIS-TRIS) complex, while an additional 1.75 M OH^– ions are further required to remain the anolyte stable. When the performance of ARFB using the redox couple prepared with the optimized anolyte is measured, its energy efficiency and discharging capacities are 78.6 % and 40.4 Ah L^-1 at 80 mA cm⁻². This implies that its performance can be compatible with those of conventional vanadium RFB. Conclusively, it is confirmed that Fe(BIS-TRIS) is viable active material for ARFB and the ARFB performance benefit induced by Fe(BIS-TRIS) is outstanding.