Non-Aqueous Single-Metal Redox Flow Batteries

Redox flow batteries (RFBs) promise high-capacity energy storage useful for the leveling of power harvested from stochastic sources like sunlight or wind.  Unlike traditional batteries, whose chemical energy is stored in solid electrode materials, RFBs store energy by exchanging charge between chemically inert electrodes and a dissolved component of the liquid phase.  The choice of stable, reversible redox-active species is critical.¹  Ideally, the redox couples should be chosen to yield the highest possible potential during discharge. The use of single-metal disproportionation reactions also prevents irreversible side reactions that arise during active-species crossover.²

Research in our groups focuses on novel  nonaqueous RFB chemistries, which differ from the aqueous systems used commercially.  The water-splitting side reaction at 1.23 V (or slightly higher in strong-acid support) limits aqueous RFBs to a relatively narrow voltage range. Cyclic voltammetry indicates that the replacement of water by acetonitrile affords an operating voltage increase–and consequent power-density increase–of more than 50%, while suppressing solvent degradation. 

Sparingly soluble active species limit the energy density of the charge-storing liquid phase.  Thus, for nonaqueous RFB chemistries, a key technical issue is to increase of active-species concentration.  We have shown that this can be achieved by exploiting redox-active organometallic complexes whose ligands enhance solubility.³  For instance, the inset figure demonstrates how a vanadium(III) acetylacetonate, or V(acac)₃, active species can be used to successfully implement a nonaqueous single-metal RFB that achieves coulombic efficiencies of 85-90%, comparable to those achieved when water is the solvent.

But a transition from water poses significant design challenges: it alters the field of options for active species, supporting electrolytes, membrane separator materials, and electrodes.  In addition to discussing the new organometallic vanadium single-metal flow battery, we will discuss engineering aspects related to RFB design and optimization. 

This presentation will describe our work on the electrochemical analysis of system stability, thermodynamic investigations that elucidate the behavior of cell potential during battery operation, physicochemical characterization to establish the transport properties of liquids and membranes, and kinetic measurements showing how electrode materials catalyze the heterogeneous reactions at the liquid/metal interfaces.  Each of these aspects will be discussed in terms of its impact on the charge/discharge response of an RFB.

¹ C. Ponce de Leon et al. J. Power Sources 160 (2006) 716–732.

² E. Sum and M. Skyllas-Kazacos. J. Power Sources 15 (1985) 179–190.

³ Q. Liu et al. Electrochem. Comm. 11 (2009) 2312–2315.