Evaluation Study of Phosphonate and Phosphonate/Carboxylate-Based Iron Complexes as Anolytes in Aqueous All-Iron Redox Flow Batteries

The increasing adoption of the renewable energy systems such as wind and solar systems have prompted the need for efficient, long duration energy storage device, to broaden and maximize the usage of these renewables. Redox flow battery (RFB), a promising electrochemical energy storage technology, has shown potential for long term energy storage solution owing to its unique features such as flexibility, scalability, durability and most notably, the decoupled power and energy density, allowing for customer focused deployment for specific purpose application.
Iron-ligand complexes based redox flow batteries (Fe-RFBs) capitalizing on the coordination chemistry between the ligands and low-cost transition metal, Fe, are gaining traction for their distinctive characteristics. The redox potential, solubility and stability of the iron redox active material can be fine-tuned by the coordinating ligands, facilitating advancement of Fe-RFBs. Primarily, Fe-RFBs utilizes resource abundant and low-cost Fe as the redox active material in its electrolytes resulting in cost-effective RFBs.
In this work, we avail new set of nitrogenous phosphonate ligands - nitrilotri(methylphosphonic acid), (NTMPA), and N, N-Bis(phosphonomethyl)glycine, (BPMG) in preparation of iron complex as anolytes for aqueous Fe-RFBs. These ligands share a similarity having phosphonate (-PO(OH)₂) and a stark difference from replacing one of phosphonate with carboxylate (-COOH) functional group that proved to be consequential in the electronic structures of the complexes. A comprehensive comparative analysis results exhibit different speciation features of the two complexes, and , and their impacts on the electrochemical properties when employed as negative redox active materials in aqueous redox flow battery. Full cell testing results when / anolytes were paired with ferrocyanide as catholyte demonstrated over 700 consecutive charge and discharge cycles with a coulombic efficiency (C.E.) of 100% at a current density of 20 mA/cm².