Faradaic efficiency and anodic electrochemical behavior of Fe electrocoagulation: The role of co-existing ions and ultrasonic assistance

Anodic passivation is a key factor affecting the Faradaic efficiency of Fe electrocoagulation. In practice the electrolyte nature of water matrix played a critical role in governing the Faradaic efficiency. In this study the effect of co-existing ions and ultrasonic field on the Faradaic efficiency and the electrochemical behavior of iron anodes was investigated. The results showed that in Cl− and SO42− solution Fe anode exhibited a typical characteristic of dissolution with a high Faradaic efficiency (>90 %), but severe passivation appeared in NO3− solution with a low Faradaic efficiency (4 %). However, adding Cl− can impair the passivation effect of NO3−. As the Cl−: NO3− ratio increased to 1:10, the Faradaic efficiency significantly increased to 90.65 %. Potentiodynamic polarization and electrochemical impedance spectroscopy analysis showed that the thickness of the passive film decreased and the corrosion rate increased, and thus promoting the anodic dissolution. In addition, ultrasonic field was introduced to assist electrocoagulation. In 50 mM NaNO3 + 2.5 mM NaCl solution, the Faradaic efficiency was only 12 % without ultrasonic assistance, but it increased to 60.9 % when the ultrasonic power was 180 W. Therefore, ultrasonic assistance was beneficial to reduce the anodic passivation and increase the Faradaic efficiency.