The Influence of Supporting Electrolytes on Zinc Half-Cell Performance in Zinc/Bromine Flow Batteries

Five supporting electrolytes were studied for their viability as alternatives in the zinc half-cell of a zinc/bromine (Zn/Br) flow battery. The secondary electrolytes studied included sodium salts of the following anions: Br−, SO42-, H2PO4− and NO3−, which were compared against the conventionally employed Cl−. Cyclic voltammetry and Tafel analysis showed improved electrochemical performance from electrolytes containing NaBr, Na2SO4 and NaH2PO4. Consequently, these chemicals are proposed as potential alternatives in future Zn/Br design work. Electrochemical impedance spectroscopy revealed that the lowering of charge-transfer resistance and diffusion limitation was the contributing reason toward improved performance from those electrolytes. Scanning electron microscopy and X-ray diffraction of zinc electrodeposits obtained during charging showed the type of supporting electrolyte present alters zinc crystallinity. Generation of smaller crystals was related to observations of good half-cell performance during voltammetry. Mossy deposits were linked with higher nucleation overpotentials between zinc plating/de-plating. The well-performing Na2SO4 supporting electrolyte produced mossy deposits, suggesting that contrary to common assumption, such deposition behavior is possibly unrelated to poor zinc-side performance. While the proposed compounds are intended for Zn/Br flow battery applications, they are possibly adaptable to other types of flow batteries utilizing the Zn2+/Zn redox couple.