Cycling performance of the iron-chromium redox energy storage system

Extended charge-discharge cycling of this electrochemical storage system at 65 C was performed on 14.5 sq cm single cells and a four cell, 867 sq cm bipolar stack. Both the anolyte and catholyte reactant fluids contained 1 molar concentrations of iron and chromium chlorides in hydrochloric acid and were separated by a low-selectivity, cation-exchange membrane. The effect of cycling on the chromium electrode and the cation-exchange membrane was determined. Bismuth and bismuth-lead catalyzed chromium electrodes and a radiation-grafted polyethylene membrane were evaluated by cycling between 5 and 85 percent state-of-charge at 80 mA/sq cm and by periodic charge-discharge polarization measurements to 140 mA/sq cm. Gradual performance losses were observed during cycling but were recoverable by completely discharging the system. Good scale-up to the 867 sq cm stack was achieved. The only difference appeared to be an unexplained resistive-type loss which resulted in a 75 percent W-hr efficiency (at 80 mA/sq cm versus 81 percent for the 14.5 sq cm cell). A new rebalance cell was developed to maintain reactant ionic balance. The cell successfully reduced ferric ions in the iron reactant stream to ferrous ions while chloride ions were oxidized to chlorine gas.