Oxygen evolution anodes composed of anodically deposited Mn–Mo–Fe oxides for seawater electrolysis

Abstract

Preparation of anodes for oxygen evolution in seawater electrolysis was carried out. Manganese–molybdenum double oxides, Mn 1− x Mo x O 2+ x , prepared by anodic deposition from MnSO 4–Na 2MoO 4 solutions showed the 100% oxygen evolution efficiency at a current density of 1000 A m −2 in 0.5 M NaCl at 30 °C and pH 12, but an increase in solution temperature resulted in dissolution of the oxides as molybdate and permanganate ions. In order to increase the stability of the electrodes at higher temperatures the addition of iron to the manganese–molybdenum oxides was performed by anodic deposition in MnSO 4–Na 2MoO 4–FeNH 4(SO 4) 2 solutions. The electrodes thus prepared showed the 100% oxygen evolution efficiency at 1000 A m −2 in 0.5 M NaCl at 30–90 °C, when proper amounts of molybdenum and iron were contained. The iron addition also enhanced the oxygen evolution efficiency. The electrodes were not composed of oxide mixtures but triple oxides, Mn 1− x− y Mo x Fe y O 2+ x−0.5 y , consisting of Mn 4+, Mo 6+ and Fe 3+. The formation of the triple oxides seemed responsible for enhancement of both oxygen evolution efficiency and stability.