The Role of the Lower Metal Oxide/Higher Metal Oxide Couple in Oxygen Evolution Reactions

The minimum potential for the evolution of oxygen on six oxides ( , , , , and ) in has been studied by potentiostatic steady‐state and galvanostatic charging experiments. The results show that there is an excellent correlation between the minimum potential for oxygen evolution on the first three oxides and their lower oxide/higher oxide couples ( ; ; ) listed in Pourbaix diagrams. For , there was good agreement between the lower oxide/higher oxide couple determined in this work by galvanostatic charging. Galvanostatic charging on and indicates that oxygen evolution takes place at 1.76 and 1.71V, respectively. However, these two values do not correlate with their lower oxide/higher oxide couple values. It is therefore suggested that oxygen evolution on such oxides proceeds via the couple (1.77V), where the reaction intermediates are physisorbed. We also discuss the stability of the lower and higher oxides of the six metals considered from the point of view of the crystal field concept.