In situ investigations of bromine-storing complex formation in a zinc-flow battery at gold electrodes

One of the most promising candidates for affordable energy storage systems for electric vehicles is the zinc/bromine battery. The reversible cell voltage of 1.82 V, a theoretical specific energy of 430 Wh kg{sup {minus}1}, robustness, high cycle life, unlimited standby periods in the charged or discharged state, low self-discharge rates, and operation at ambient temperature explain the scientific and industrial interest in this system. 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 (MEP{sup +}) and N-methylethylmorpholinium (MEM{sup +}), 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 (Br{sub n}{minus}) formed MEM-Br{sub n}. Therefore, a homogeneous chemical reaction of the dissolved MEP{sup +} cation with electrochemically generated bromine leads to the storage complex MEP-Br{sub n} much more rapidly than the heterogeneous electrochemical reaction of the strongly adsorbed MEM{sup +} to MEM-Br{sub n}. These results demonstrate that in situ techniques not only support the evaluation of the mechanism but also provide key information for battery development.