Chemical and Electrochemical Behavior of the Cr(III)/Cr(II) Half‐Cell in the Iron‐Chromium Redox Energy Storage System

The Cr(III) complexes present in the acidified chromium solutions used in the iron‐chromium redox energy storage system have been isolated and identified as and by ion‐exchange chromatography and visible spectrophotometry. The cell reactions during charge‐discharge cycles have been followed by means of visible spectrophotometry. The spectral bands were resolved into component peaks and concentrations of the Cr(III) species calculated using Beer's law. During the charge mode, is reduced to , and during the discharge mode is oxidized back to . Electrode potential measurements also support this interpretation. Hysteresis effects in the charge‐discharge curves can be explained by the slow attainment of equilibrium between and .