In Situ Investigations of Bromine‐Storing Complex Formation in a Zinc‐Flow Battery at Gold Electrodes

The storage reactions of the zinc/bromine battery are the cathodic deposition of zinc and the anodic formation of a nonaqueous polybromide phase. Quaternary ammonium cations, N‐methylethylpyrrolidinium and N‐methylethylmorpholinium , store the bromine as polybromide complexes. The mechanism of this complicated reaction determines the polarization and self‐discharge rate of the bromine electrode. Electrochemical in situ techniques, phase‐stabilized electrochemical quartz microbalance, and in situ reflection‐absorption Fourier transform infrared spectroscopy were employed for the first time to investigate these electrode processes. It was shown that specifically adsorbed polybromide anions formed and ion pairs at gold electrodes, and that was adsorbed more strongly than . Therefore, a homogeneous chemical reaction of the dissolved cation with electrochemically generated bromine leads to the storage complex much more rapidly than the heterogeneous electrochemical reaction of the strongly adsorbed to . These results demonstrate that in situ techniques not only support the evaluation of the mechanism but also provide key information for battery development. © 1999 The Electrochemical Society. All rights reserved.