(Invited) Membrane-Crossover Issues in Redox-Flow Batteries and Mitigation Strategies

Abstract

Inexpensive electrical-energy storage (EES) is critical for successful transformation of the electric grid [1], [2]. Redox-flow batteries (RFBs) possess compelling attributes for grid-scale EES [3]. Energy is stored using redox-active molecules in liquid electrolytes that are pumped through electrochemical reactors where the active species are oxidized and reduced to generate electricity. RFBs decouple power and energy, which permits cost-effective implementation of modular EES products based on active materials with low energy density. The concentrations of active materials in liquid electrolytes are typically on the order of 1 M. The low volumetric capacity of ~30 Wh/L, compared to conventional batteries like Li-ion, can support EES systems with attractive costs provided the active materials are stored in inexpensive vessels and the reactors operate at high area-specific power densities (approximately 0.5 W/cm2) to minimize costs associated with non-active components, such as electrodes, separators, and current collectors [4]. RFB systems are especially attractive for EES applications that require more than four hours of discharge per cycle at rated power.This symposium is in honor of Prof. Robert Savinell, who started working on RFBs more than 40 years ago [5] and continues to work in this area today [6]. Bob has provided a nice historical perspective of RFB technology [7].RFBs can, in principle, use a wide range of active materials. However, a number of simultaneous requirements must be met to make a chemistry attractive. One of the most challenging requirements is minimizing transport of active species through the membrane separating the positive and negative electrodes, and the ensuing inefficiency and capacity loss. Developing technologies to mitigate crossover, and strategies for recovering from its consequences, will enable development of successful systems with new active materials. The rate and impact of crossover depend on the nature of the active materials and their fate after they transport across the separator [8]. Accordingly, electrolyte solutions can be classified by what happens to the active species at the opposing electrolyte. This behavior also dictates what recovery strategies may be employed, and at what frequency. A major focus here shall be on describing desirable attributes for active materials and separators that help diminish crossover, and strategies that can be used to recover from its effects for each of the different classes of electrolytes. Acknowledgements The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR000994. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.