A Mechanistic Study of Oxygen Evolution on NiCo2 O 4 : I . Formation of Higher Oxides

The formation of high valence oxides of Ni and Co on at potentials before oxygen evolution commences was studied by controlled potential coulometry, cyclic voltammetry, and by galvanostatic charging. Three oxides are formed at 1.25, 1.31, and 1.40V vs. a dynamic hydrogen electrode (DHE) in at 25°C, and the charges passed in their formation are approximately in the ratio 1:1:2. It is suggested that the transitions are , , and either all or tetrahedrally coordinated . The total charge passed to form higher oxides is 9.7% of the maximum possible charge if all the cations in the electrode are oxidized to the +4 state. For of 43 m 2 g −1 , approximately 8% of the unit cells are found at the surface. This suggests that the formation of higher oxides is limited to the surface of the particles. Analysis of the controlled potential coulometry results indicates that oxide formation obeys first‐order kinetics.