Today, ionic liquids are more and more present in many fields and more particularly in electrochemistry. Indeed, their physical and chemical properties are appealing and attractive. They are conductive solvents in which organic and inorganic salts can be dissolved depending on the nature of the anion and cation that make up the ionic liquid. However, only very few studies have reported their use in membraneless redox flow batteries (RFBs) for the storage of renewable energy ^{1, 2}. The concept of membraneless redox-flow batteries was first reported by Ferrigno et al.³ in 2002, with the development of a millimeter-scale redox fuel cell based on the vanadium aqueous electrolyte solutions.

In this work, we have developed an ionic liquid membraneless RFB by using 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (C₂mimTFSI) as supporting electrolyte and Quinone (Q) and iron chloride (FeCl₂) as electroactive species in a microfluidic system. Polarization curve and cyclic voltammetry were used to characterize the electrochemical properties as well as the performance of the microbattery. The proof-of-concept of the system has been shown with an open circuit potential of 0.6 V, obtained with both polarization curve and cyclic voltammetry, and with a current density ranging from 0.3 to 0.65 mA cm^-2 for total flow rates of 10 to 20 µL min^-1. As shown on fig. 1(b), a maximum of power of 40 µW cm^-2 has been obtained. Such a technology is promising and performances can be enhanced by using 3D electrodes and optimizing the choice of the redox mediators (concentration, potential, etc.)

Figure 1: (a) Experimental set-up and polarization curves of the cell for a total flow rate of (b) 20 µL.min^-1

References

1. Navalpotro, P.; Palma, J.; Anderson, M.; Marcilla, R., A Membrane-Free Redox Flow Battery with Two Immiscible Redox Electrolytes. Angew. Chem. Int. Ed. Engl. 2017, 56 (41), 12460-12465.

2. Chen, R.; Bresser, D.; Saraf, M.; Gerlach, P.; Balducci, A.; Kunz, S.; Schroder, D.; Passerini, S.; Chen, J., A Comparative Review of Electrolytes for Organic-Material-Based Energy-Storage Devices Employing Solid Electrodes and Redox Fluids. ChemSusChem 2020, 13 (9), 2205-2219.

3. Ferrigno, R.; Stroock, A. D.; Clark, T. D.; Mayer, M.; Whitesides, G. M., Membraneless vanadium redox fuel cell using laminar flow. Journal of the American Chemical Society 2002, 124 (44), 12930-12931.