Electrochemical characterization of polypyridine iron(II) and cobalt(II) complexes for organic redox flow batteries

The electrochemical properties of a series of complexes based on chelate ligands were explored with the aim of finding promising candidates for applications in high voltage organic flow cells. The combination of Co(II)/(I) and Fe(III)/(II) as the cathodic and the anodic couple leads to open circuit voltages close to 2 V and to limiting current of the order of 30 mA/cm2 in 0.3 M concentration in a mixture of Ethylene and Propylene carbonates. The best solubility was found for bis-cationic complexes as triflate (OTf) salts, reaching >0.6 M at room temperature. Potentiostatic and potentiodynamic experiments point to a substantial chemical and electrochemical stability at carbon based electrodes as well.