Unprecedented Capacity and Stability of Ammonium Ferrocyanide Catholyte in pH Neutral Aqueous Redox Flow Batteries

Abstract

Summary Aqueous organic/organometallic redox flow batteries (AORFBs) have gained increasing attention for large-scale storage of intermittent renewable energy (e.g., solar and wind) due to the advantages of decoupled energy and power, high current and power performance, safety features, and synthetic tunability of charge storage molecules. Here, we report the synthesis and physicochemical properties of highly water-soluble (NH4)3[Fe(CN)6] and (NH4)4[Fe(CN)6] catholyte materials and their application in pH neutral AORFBs. A 1.5 M half-cell (NH4)3[Fe(CN)6]/(NH4)4[Fe(CN)6] redox flow battery confirmed the high capacity and stability of ammonium ferrocyanide catholyte at pH neutral conditions. Paired with 1,1′-bis(3-sulfonatopropyl)-4,4′-bipyridinium ((SPr)2V) anolyte, a 0.9 M symmetric (NH4)4[Fe(CN)6]/(SPr)2V AORFB free of supporting electrolytes exhibited excellent cycling performance including nearly 100% capacity retention in 1,000 cycles (1,100 testing hours), 62.6% energy efficiency at 40 mA/cm2 operation current density, and a power density of 72.5 mW/cm2, representing the most stable AORFB reported so far.