Factors affecting the open-circuit voltage and electrode kinetics of some iron/titanium/redox flow cells

Performance of the iron-titanium redox flow cell was studied as a function of acid concentration. Anion permeable membranes separated the compartments. Electrodes were graphite cloth. Current densities ranged up to 25 mA/cm/sup 2/. Open-circuit and load voltages decreased as the acidity was increased on the iron side as predicted. On the titanium side, open-circuit voltages decreased as the acidity was increased in agreement with theory, but load voltages increased due to decreased polarization with increasing acidity. High acidity on the titanium side coupled with low acidity on the iron side gives the best load voltage, but such cells show voltage losses as they are repeatedly cycled. Analyse show that the bulk of the voltage losses are due to diffusion of acid through the membrane. No membrane tested so far will maintain differences in acidity. Chelating agents show some promise in reducing polarization at the Ti electrode and thus improving energy efficiency.