Dissecting the Quinone-Bromide Flow Battery

The excitement brought by the quinone-bromide flow battery resides not only in the low cost of materials, but also in the high performance, reaching 1.0 V open circuit voltage and 1.0 W/cm² peak galvanic power density to date. Further performance improvements require better understanding of the loss mechanisms. Here, we present a detailed examination of the losses in the system, via installation of a reference electrode, electrochemical impedance spectroscopy, and ex-situ measurements of ohmic resistance. Cell overvoltage is then dissected into contributions from charge transfer kinetics, mass transport, membrane resistance, etc. and interpreted. We address current efficiency through measurements of active species cross-over rates and molecular stability.