Oxygen evolution on manganese-molybdenum oxide anodes in seawater electrolysis

MnO₂-type manganese-molybdenum oxide electrodes with extremely high oxygen evolution efficiency in chloride-containing solutions have been prepared by anodic deposition on IrO₂-coated titanium substrates. The anodic activity and the durability of electrodes have been examined in a 0.5 M NaCl solution at pH 12 and 30°C. For all prepared compositions, the manganese-molybdenum oxide electrodes show an oxygen evolution efficiency of almost 100%, when adequately thick oxides are deposited on the substrate. The galvanostatic polarization reveals that an increase in molybdenum content results in an increase in overpotential at current densities lower than 100 Am^-2. However, at a current density of 1000 Am^-2, often used for practical electrolysis, the overpotential is almost independent of the molybdenum content. The manganese-molybdenum oxide electrodes have significantly higher durability than the manganese oxide electrodes. During electrolysis at 1000 Am^-2, the manganese-molybdenum oxide electrodes show only a slight decrease in oxygen evolution efficiency. Surface observation indicates that the slight decrease in the oxygen evolution efficiency results from the fact that the oxide layer is partly peeled off after electrolysis, even though a thin manganese-molybdenum oxide layer remains in the peeled-off regions.