The durability of manganese–molybdenum oxide anodes for oxygen evolution in seawater electrolysis

Abstract

MnO 2-type manganese–molybdenum oxides have been anodically deposited under various conditions on IrO 2-coated titanium substrates, and the durability of the oxide anodes for oxygen evolution in seawater electrolysis has been examined. When sufficiently thick oxides are deposited, all the oxide anodes containing 8 mol% or more molybdenum in cationic percentage show an initial oxygen evolution efficiency of almost 100% at a constant current density of 1000 A·m −2 in 0.5 M NaC1 solution at pH 12 and 30°C. However, the oxygen evolution efficiency decreases gradually during the electrolysis mainly due to the exfoliation of oxides. It has been found that the exfoliation is remarkably suppressed when the oxide is deposited under vigorous oxygen evolution at high temperature of 90°C, high current density of 600 A·m −2 and low electrolyte pH of 0. The anode prepared under this condition reveals an oxygen evolution efficiency of more than 99.6% even after 1500 h of the electrolysis at 1000 A·m −2. The anodic deposition at lower temperatures at and below 50°C results in the electrochemical dissolution as permanganate ions during the electrolysis. Such dissolution does not occur for the oxides deposited at 90°C, even when the electrolysis is performed at 10000 A·m −2 at which the potential of the anode exceeds 3 V from the equilibrium potential of the formation of permanganate from MnO 2.