Electrolyte Solvent Mixtures for a Symmetric, Non-Aqueous Redox Flow Battery Based on [Fe(bpy)₃][Fsi]₂

Redox flow batteries (RFBs) store energy in liquid redox-active electrolytes which are charged and discharged while flowing through an electrochemical cell. This separation of storage and reaction sites enables flexible scalability of power and energy density. [1] Research suggests that non-aqueous RFB systems offer a promising combination of cost and performance for stationary applications, such as storage of excess renewable energy from domestic- to grid-scale [2]. In this work, we investigate a RFB based on the iron(II)-tris(2,2’-bipyridine) bis(fluorosulfonyl)imide complex, which shows one metal-centred oxidation process and up to three ligand-centred reduction processes, providing 2.4 V cell potential [3]. Using the same active material on both positive and negative sides of the cell mitigates the detrimental effects of cross-over through the separator membrane. These redox processes are solvent-dependent [3] and their electrochemistry was studied in a range of aprotic single and mixed solvents, some already used in lithium-ion batteries [4]. Since energy density is determined by the cell potential and the concentration of the active material, the solubility of the reduced and oxidated states of the complex was examined by UV/vis spectroscopy. Furthermore, conductivity and viscosity were measured over a range of temperatures. In RFBs, the dynamic viscosity of the electrolyte influences not just the conductivity but also affects the power required to pump the electrolyte. Cost, safety and environmental impact were considered as well. References: [1] L. F. Arenas, C. Ponce de León, F. C. Walsh, J. Energy Storage 2017, 11, 119-153. [2] R. Dmello, J. D. Milshtein, F. R. Brushett, K. C. Smith, J. Power Sources 2016, 330, 261-272. [3] D. M. Cabral, P. C. Howlett, D. R. MacFarlane, Electrochim. Acta 2016, 220, 347-353. [4] K. Xu, Chem. Rev. 2004, 104, 4303-4417. Figure 1