Challenges and Opportunities of Membrane-Free Redox Flow Batteries

Abstract

Redox Flow Batteries (RFB) have been recognized as a viable technology for the integration of renewable energies into the electricity grids due to their ability to decouple power and energy and the possibility of storing large amount of energy at competitive cost. However, wide-spread implementation of conventional RFBs is limited by some obstacles related to low abundance, toxicity and high cost of vanadium redox compounds, and the poor-performing and expensive ion exchange membranes used to separate catholyte and anolyte compartments. This key element can account for more than 30 % of the cost of the battery (2) and may limit the long-term battery performance due to unavoidable cross-contamination and insufficient mechanical, chemical and electrochemical stability.Here, we present an innovative concept of Membrane-Free Battery which proposes to eliminate any separator or membrane by using immiscible redox electrolytes (3). These electrolytes spontaneously form a biphasic system whose interphase functions as a “natural” barrier making the use of any membrane superfluous. Moreover, the vanadium redox species are replaced by cheap and abundant organic redox molecules that can be specifically designed to exhibit high solubility, adequate redox potentials and suitable partition coefficient between the two immiscible phases. It will be demonstrated that this disruptive technology is hugely versatile with respect to the type of organic redox species as well as the nature of immiscible electrolytes forming the biphasic system. We report on the electrochemical performance of Membrane Free batteries based on aqueous/non-aqueous immiscible electrolytes (4) but also a Total Aqueous Membrane-free Battery in which the two phases are aqueous (5), which brings some advantages in terms of cost, environmental issues and battery performance. New challenges such as the inherent selfdischarge in the liquid-liquid interface, the need of developing new reactors designs to keep the two immiscible liquids separated, and future opportunities of this innovative technology making use of thermodynamics and computational chemistry will be also discussed.