Abstract
Aqueous and non-aqueous redox flow batteries (RFBs) have limited energy and current densities, respectively, due to the nature of the electrolytes. New approaches to electrolyte design are needed to improve the performance of RFBs. In this work, we combined a highly conductive aqueous phase and an organic redox-active phase in a microemulsion to formulate a novel RFB electrolyte. As a proof-of-concept, we demonstrate an RFB using this microemulsion electrolyte with maximum current density of 17.5 mA·cm−2 with a 0.19 M posolyte and 0.09 M negolyte at a flow rate of only ∼2.5 ml·min−1, comparable to early vanadium electrolyte RFBs at similar flow rates on a per molar basis. The novel active negolyte component is an inexpensive oil-soluble vitamin (K3). By combining aqueous and organic phases, the solvent potential window and energy density may be increased without sacrificing current density and new redox couples may be accessed. Microemulsion electrolytes show great promise for improved performance and increased energy densities in aqueous RFBs but the path forward is complex. We end with discussion of areas that need work to achieve the potential of these electrolytes.
Highlights
Redox flow batteries (RFBs) are a promising energy storage technology
We show in Scheme 1 an illustration of a novel microemulsion RFB indicating simultaneous ion conduction through the aqueous phase and redox reactions in the oil phase
The RFB used for that experiment consisted of a zero-gap cell (Aaron et al, 2012) Fumasep F-930 cation exchange membrane (~30 μm thickness) with SGL GFD carbon felt electrodes (5 cm2 area, 2.5 mm thickness, 99% compression) where electrolytes passed through the cell at a flow rate of 2.4 ml·min−1
Summary
Redox flow batteries (RFBs) are a promising energy storage technology. Both aqueous and non-aqueous RFBs suffer from limitations imposed by electrolyte choice (Luo et al, 2019a). While progress has been made in the design and implementation of aqueous organic RFBs (Hu et al, 2017; Luo et al, 2019b), an alternative approach to is combine redox-containing oil phases with aqueous electrolyte phases in the same solution. Microemulsions are spontaneously forming, thermodynamically stable mixtures of oil, water, and emulsifiers, with complex and dynamic nanometer-scale structures. These structures can vary greatly from discrete droplets to bicontinuous networks, where both oil and water are continuous over longer length-scales.
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