Iron-based Redox Flow Battery

Redox flow batteries (RFBs) are seen as an alternative to lithium-ion technology for the storage of excess renewable energy from domestic- to grid-scale. Positive and negative electrolytes are (dis)charged on electrodes separated by a membrane and stored in external tanks, which enables flexible scalability of power and energy. Symmetric cells utilizing the same electrolyte on both sides avoid cross-contamination and only lose efficiency in case of mixing through the membrane, but require materials with at least three accessible redox states. Metal complexes with non-innocent ligands can meet this requirement and utilize the electrochemical window of aprotic solvents for increased energy density, but often suffer from low solubility [1].
We are working on a system based on the iron-tris(2,2’-bipyridine) complex [2] which shows one metal-centered oxidation and up to three ligand-centered reductions, providing a symmetric RFB with 2.4 V cell potential. However, these processes have been shown to be solvent-dependent [3] and the electrochemistry in different solvents and mixtures was studied. Properties like density, viscosity and conductivity were measured over a temperature range as well as the solubility of the active material in all states of charge. Further points like cost, safety and environmental impact were considered as well.

[1] D. Cabral et al., Electrochim. Acta 2015, 180, 419–426.
[2] J. Mun et al., J. Electrochem. Soc. 2018, 165, 215-219.
[3] D. M. Cabral, P. C. Howlett, D. R. MacFarlane, Electrochim. Acta 2016, 220, 347-353.