Searching for new redox-complexes in organic flow batteries

The study of redox couples based on Fe(III)/(II) and Co(II)/(I) organic complexes has demonstrated chemically reversible redox processes as well as good stability in organic solvents. These active complexes, obtained with polypyridine ligands, present low cost, low toxicity and good chemical stability. Moreover, they demonstrated fast redox kinetics and for that, they are candidate for active species in redox flow cells. A wide library of polypyridine complexes have been prepared and tested as acceptor ligands to reach an open circuit voltage up to 2 V, in a mixture of ethylene carbonate and propylene carbonate (EC/PC) chosen for their low volatility and electrochemical stability. Solubility data are presented after tuning ligand design to optimize metal-complex solubility. The best compounds were [Fe(bpy)3]Tf2 (Tf = CF3SO3−, bpy = 2.2′-bipyridine) and [Co(bpy)3]Tf2 which generated current densities of the order of 30 mA/cm2 in thin layer static cells. These complexes were also preliminary tested in a complete flow cell equipped with a Nafion membrane, with LiTf electrolyte, and ca. 90% coulombic efficiency was observed. The decrease of performance observed after 8 h is under investigation and assigned, for now, to membrane degradation. A change of membrane characteristics should be considered to exploit the full potentiality of these redox mediators.