An Investigation into Factors Affecting the Iron Plating Reaction for an All-Iron Flow Battery

The all-Iron flow battery utilizes the iron II/III redox couple at the positive electrode and the iron II/0 reaction at the negative electrode. The standard reduction potential of the iron II/0 reaction is at −0.44 V vs. NHE, suggesting that hydrogen evolution could be a significant factor in coulombic losses on the negative electrode. Methods of increasing the coulombic efficiency of iron plating are considered, such as anion concentration and electrolyte additives. The use of a chloride anion containing electrolyte showed less hydrogen evolution rates and faster plating kinetics than an electrolyte containing the same concentration of sulfate anions. Increasing the chloride concentration significantly reduced the hydrogen evolution observed on an iron electrode, and plating efficiencies of 97% were demonstrated on a rotating rod electrode. The effect of complexing ligands on plating and hydrogen evolution was also investigated.