Three-Chamber Design for Aqueous Acid–Base Redox Flow Batteries
Amy S. Metlay, Brandon Chyi, Yein Yoon, Ryszard Wycisk, Peter N. Pintauro, Thomas E. Mallouk
Abstract
Aqueous acid–base redox flow batteries exploit a pH gradient maintained by a bipolar membrane to increase the energy storage capacity of the cell. An earlier study using a ferro-/ferricyanide redox couple revealed that redox molecule transport processes rapidly degrade the bipolar membrane and cell cycle life. Here, we mitigate this problem using a three-chamber cell design that interposes a cation exchange membrane between the positive electrolyte and an inner KOH electrolyte on the basic side of the bipolar membrane. This arrangement improves the stability and cycle life of the cell. In addition, the KOH solution in the central chamber maintains the pH gradient across the bipolar membrane, eliminating the requirement for a high pH positive electrolyte. Highly positive and soluble redox couples such as bromine/bromide that are unstable under strongly alkaline conditions can be used with the three-chamber design against 2,7-disulfonate-9,10-anthraquinone, achieving an open circuit voltage of ∼1.7 V.