Evaluation of iodide and titanium halide redox couple combinations for common electrolyte redox flow cell systems

The electrochemical behaviour of a range of halide and titanium halide redox couple combinations has been evaluated for potential application in redox flow cells employing common elements in both half-cells. Cyclic voltammetry and cell cycling measurements were employed to establish the reversibility of the redox couples and the expected cell voltage and performance in a redox flow cell. An all iodine cell was found to be impractical due to the formation of an insoluble iodine deposit in the cell that caused cell blockage. A titanium polyhalide cell that uses a solution of TiCl4 in HBr/HCl supporting electrolyte in both half-cells was, however, shown to produce an open circuit voltage of 0.9 V and high coulombic efficiencies. Although the voltage efficiency of the cell was low, improvements in cell design may reduce ohmic losses allowing much higher energy efficiencies to be achieved. This cell is equivalent to the vanadium bromide cell that employs a vanadium bromide solution in both half-cells. The lower price of titanium, however, opens the possibility of a significant cost benefit for the titanium polyhalide redox flow cell.