2-Volt All-Iron Aqueous Flow Battery

All-iron flow batteries are attractive for use in large-scale energy storage systems due to the low cost and scalability of iron. However, iron flow batteries often use the Fe^2+/0 redox couple for the anodic reaction, which plates out iron onto an electrode during the charging process. This plating reaction limits the scalability of the all-iron flow battery for long duration because the amount of iron plated is related to the surface area of the electrode, cell, and stack, and the resulting power-energy ratio of the system cannot be scaled independently to long durations. In adddition, all-iron flow batteries usually operate at low voltages, limiting the power and efficiency of the system.<br/><br/>Here we show how the use of different coordinating organic ligands can modulate the potential of the Fe^3+/2+ redox couple over 2 volts in water at neutral pH. We further demonstrate the performance and cycling of an all-iron flow battery wherein both anolyte and catholyte solutions comprise iron in the +2 or +3 redox state, with different organic ligands, with an equilibrium cell potetential over 2 volts. The molecular design, bonding, and electronic structure of the organic materials will be discussed highlighting the critical requirments for organic materials to control metal-based redox reactions.