Towards Totally Aqueous Membrane-Free Flow Batteries: Fundamentals and Challenges

Abstract

The implementation of renewable energies (solar, wind, etc) is driven the development of new energy storage systems that can mitigate their inherent intermittency solving the mismatch between the energy demand and production. Among different systems, electrochemical energy storage devices stand out due to their efficiency, flexibility and modularity. Specifically, Redox Flow Batteries (RFB) are the unique type of battery able to decouple energy and power densities which makes them versatile and adaptable to different applications. Nevertheless, although they are commercially available, their implementation is limited by the high cost and issues associated with some components such as the vanadium-based electrolytes and Nafion ion-exchange membranes. In this regard, our approach to overcome these limitations is the development of a membrane-free flow battery based on immiscible/biphasic electrolytes with organic active species1,2. In this talk, we will present a totally aqueous membrane-free battery based on a sustainable and safe aqueous biphasic system with near neutral pH and highly soluble organic active species. The aqueous biphasic electrolytes allow a selective thermodynamic separation of the active species instead of using any membrane or physical barrier. Thanks to a new flow-cell design3, it becomes the first example of aqueous membrane-free flow battery (Figure 1). The results show high coulombic efficiency (94%), capacity utilization (98%), and stable long-term performance over 250 cycles4. The fundamental aspects and the last advances in this innovative concept will be discussed.Figure 1 Schematic representation of the development of a membrane-free flow battery based on two aqueous biphasic electrolytes with organic active species.