Bromine Retention Versus Electrochemical Accessibility in a Ferri‐/Ferrocyanide–Bromine Flow Battery

Abstract

Bromine retention in aqueous redox flow batteries can be improved by complexation, but the resulting gain in stability is often accompanied by losses in electrochemical accessibility and transport. Here, we investigate a ferri‐/ferrocyanide–bromine flow battery based on soluble ferri‐/ferrocyanide and bromine/bromide redox electrolytes. Three bromine/bromide positive‐electrolyte conditions were compared: a pristine bromide electrolyte, an electrolyte containing the bromine‐complexing agent (BCA) N ‐ethyl‐ N ‐methylpyrrolidinium bromide ([EMPyrr]Br), and the same electrolyte after nitrogen presparging. Charge–capacity retention after 70 cycles increased from 23.4% for the pristine electrolyte to 34.9% with [EMPyrr]Br and 66.0% with [EMPyrr]Br and nitrogen presparging. Because neither a nitrogen‐pre‐sparged BCA‐free control nor an alternative inert‐gas control was examined, the higher retention cannot be attributed uniquely to nitrogen; deaeration and sparging‐induced redistribution of bromine‐rich phases may also contribute. Improved retention was accompanied by increased polarization, higher ex situ electrolyte impedance, and an initial voltage rise associated with bromine‐rich phase formation. Postcycling cyclic voltammetry provides qualitative evidence of bromine crossover, while Raman spectroscopy shows condition‐dependent bromine‐related signatures in the sampled liquid phase. These findings show that improved charge–capacity retention does not necessarily translate into better cell performance unless the retained bromine remains electrochemically accessible.