Asymmetric Iron Complex as a Potential Catholyte for Alkaline Aqueous Redox Flow Battery

ABSTRACT An alkaline all‐iron complex aqueous redox flow battery (AAICARFB) is an RFB that utilizes iron complex as the active material for both its anolyte and catholyte. A key limitation of iron‐based RFBs is the use of solid Fe, which prevents full separation between the cell stack and the electrolytes, hindering scalability. This study investigates the properties and performance of iron dicarboxylic bipyridine cyanide electrolytes in alkaline conditions as potential catholytes for AAICARFB, evaluating them as alternatives to ferrocyanide (Fe(CN) 6 ). The higher redox potential and solubility of the complexes compared with Fe(CN) 6 at neutral pH mean that the AAICARFB that uses the iron dicarboxylic bipyridine cyanide catholyte has the potential to have higher cell voltage and energy potential compared with previous configurations, as energy density depends on both cell voltage and solubility. Catholytes made using Na 4 [Fe(Dcbpy) 3 ], K 4 [Fe(Dcbpy) 3 ], Na 4 [Fe(Dcbpy) 2 (CN) 2 ], K 4 [Fe(Dcbpy) 2 (CN) 2 ], and Na 4 [Fe(Dcbpy) (CN) 4 ] dissolved in strongly alkaline NaOH or KOH solution were studied. As a result, the Na 4 [Fe(Dcbpy)(CN) 4 ] was the only iron dicarboxylic bipyridine cyanide complex to exhibit redox activity in alkaline media. It demonstrated a higher redox potential than Fe(CN) 6 and better solubility than some Fe(CN) 6 electrolyte configurations in alkaline conditions. These findings suggest that the Na 4 [Fe(Dcbpy)(CN) 4 ] catholyte has the potential to enhance the energy density of AAICARFB systems.