Polarization-Driven Hydrogen Evolution and Metal Dissolution in Electrocoagulation

Understanding hydrogen evolution in electrocoagulation remains challenging because most studies have not simultaneously quantified gas production and metal dissolution, limiting mechanistic insight into super-faradaic behavior. We investigate aluminum and iron electrodes under galvanostatic and potentiodynamic conditions, measuring the hydrogen production and metal dissolution rates. Iron is included as a baseline as its near-faradaic behavior enables validation of the experimental method. Simultaneous evaluation of cathodic and anodic processes revealed distinct electrode behaviors. Aluminum cathodes exhibited super-faradaic hydrogen evolution, reaching 2.5–3× theoretical values at low current densities (0.1–2.0 mA cm–2), while anodic hydrogen generation occurred only in chloride-containing electrolytes. Electrolyte composition strongly modulated aluminum: cathodic hydrogen was enhanced in NaCl and Na2SO4, moderate in synthetic groundwater (SGW), and suppressed in NaNO3, while anodic hydrogen appeared only in NaCl and SGW and was nearly absent in sulfate and nitrate due to passivation. Dissolved Al and an Al-assisted Volmer–Heyrovský pathway, where one electron is supplied electrochemically and the other by Al, explain the excess hydrogen and cathodic Al dissolution. Comparison of hydrogen evolution rates with polarization currents confirmed that combined electrochemical and chemical pathways drive aluminum’s super-faradaic behavior. These results clarify the combination of electrochemical and chemical reactions as the origin of aluminum’s anomalous efficiencies.