Bromine Complexation in Zinc‐Bromine Circulating Batteries

Upon electrolysis of aqueous zinc bromide solutions containing unsymmetrically substituted, cyclic quaternary ammonium bromides, both zinc metal and a bromine‐rich liquid are produced. Data, reported for N‐ethyl, N‐methylmorpholinium (1), N‐methoxymethyl, N‐methylpiperidinium (2), and N‐chloromethyl, N‐methylpyrrolidinium (3) bromides suggest that the bromine‐rich liquid is a fused salt and separates from the aqueous solution because of insolubility and density differences. The separation process of the bromine fused salt from the aqueous solution is represented as a partitioning of bromine between two phases and is dependent upon electrolysis state‐of‐charge, quaternary ammonium bromide, bromide ion concentration, and temperature. The bromine‐fused salts have properties dependent on composition. Selected examples of densities, specific resistances, viscosities, and polarization are given. Bromine, in the form of a dense, stable, conductive liquid, is useful in circulating zinc‐bromine batteries.