Anodically deposited manganese–molybdenum–tungsten oxide anodes for oxygen evolution in seawater electrolysis

Abstract

Ternary manganese–molybdenum–tungsten oxides were anodically deposited on to IrO₂-coated titanium substrates at current densities of 60–600 A m⁻² in 0.4 M MnSO₄ solutions containing 0.003 M Na₂MoO₄ and 0.003 M Na₂WO₄ at pH 0–1.5 and at 30–90 °C. The effect of anodic deposition conditions on the activity, selectivity and durability of the anodes for oxygen evolution in 0.5 M NaCl solution was investigated. Most of the oxide anodes prepared initially gave an oxygen evolution efficiency of almost 100%. When the anodic deposition was performed at temperatures lower than 90 °C, gradual oxidative dissolution occurred during the electrolysis in the NaCl solution, due to formation of poorly crystalline oxides. In contrast, the oxide deposited at 90 °C revealed no obvious dissolution during electrolysis for more than 1500 h. The oxygen evolution efficiency, however, decreased gradually with time of electrolysis because of partial detachment of the deposited oxide. The anodic deposition in electrolytes at lower pH and at higher current density resulted in the formation of oxides with better adhesion to the substrate, resulting in improved anode durability. The durability was further improved by repeated anodic deposition with the oxide surface washed during intervals.