Characterization of iron(VI) compounds and their discharge products in strongly alkaline electrolyte

Iron(VI) compounds have been studied for stability over time and electrochemical activity in alkaline electrolytes. Fresh, aged, and discharged iron salts were characterized by Mossbauer spectroscopy and chronoamperometry. The materials were relatively stable over a few days if stored in a dry atmosphere, but the barium salt still decomposed completely to Fe3+ and Fe4+ by 3 months of storage. In alkaline solution, the barium ferrate salt was not stable, losing almost all of the Fe6+ oxidation state within the first 24 h. Chronoamperometry on freshly made cells showed that the Ba2+ and K+ salts discharged via different rate-determining steps. The chronoamperometry showed two distinct discharge steps for the barium salt and one for the potassium salt. The barium salt is believed to discharge through a stable intermediate, as there are stable perovskite structures that are available with the Ba2+ cation. In contrast, the initial discharge step for the potassium is the rate-determining step, and the unstable intermediate immediately converts to the Fe+3 state. These results help explain the superior rate capability for the barium ferrate salt. Discharge mechanisms for each salt are proposed in this paper. (C) 2005 The Electrochemical Society. All rights reserved.