Optimization of the iron-ion/hydrogen redox flow cell with iron chloride catholyte salt

A redox flow cell utilizing the Fe²⁺/Fe³⁺ and H₂/H⁺ couples is investigated as an energy-storage device. A conventional polymer-electrolyte-fuel-cell anode and membrane design is employed, with a cathode containing a carbon porous electrode flooded with iron chloride in an aqueous acidic solution. Foam, paper, and fabric carbon electrodes are studied, and it is found that SGL Sigracet 10AA carbon paper provides the best performance. This carbon paper is then impregnated with a wide variety of carbon powders, and it is found that none improve performance significantly, with several reducing it. Membranes of varying thickness and composition are studied, and there is a trade-off between charge/discharge performance and self-discharge. It is found that the concentration of HCl supporting electrolyte has a dramatic impact on charging performance and OCV. Charge currents in excess of 1 A cm⁻² are achieved for 4 and 6 M HCl. The maximum discharge power density, 257 mW cm⁻², is achieved for 0.9 M iron chloride with 0.9 M HCl.