Anodically deposited manganese-molybdenum oxide anodes with high selectivity for evolving oxygen in electrolysis of seawater

Manganese-molybdenum oxide electrodes were prepared by anodic deposition on an IrO₂-coated titanium substrate at a constant current density of 600 A m^-2 from baths containing 0.2 M MnSO₄ and 0-0.1 M Na₂MoO₄ at 90 °C and pH 0.5. These electrodes were characterized for oxygen evolution in the electrolysis at 1,000 A m^-2 in 0.5 M NaCl solution at 30 °C and pH 8 or 12. The most active and stable oxygen evolving anode exhibited 100% efficiency for oxygen evolution, and an efficiency of 98.5% for over 1,500 h at pH 12 and of 96.5% for over 2,800 h at pH 8 of continuous electrolysis. X-ray diffraction measurement and XPS analysis indicated that the deposits consist of a nanocrystalline single γ-MnO₂ type phase, and manganese and molybdenum in the deposits are in the Mn⁴⁺ and Mo⁶⁺ states. The electrochemical studies showed that the manganese-molybdenum oxide electrodes drastically reduced the electrocatalytic activity for chlorine evolution to the undetectable level, resulting in 100% efficiency for oxygen evolution, although the addition of molybdenum slightly increased the oxygen overpotential.