Origin of the Deactivation of Spinel CuxCo3−xO4/Ti Anodes Prepared by Thermal Decomposition

Thin films of spinel Cu x Co 3− x O 4 with nominal composition 0.0 ≤ x ≤ 1.0 were supported on Ti by the thermal decomposition method. The resulting electrodes were deactivated by prolonged anodic polarization in 1 M NaOH. The ensuing changes in the surface morphology, chemical composition, and crystalline properties were studied by means of scanning electron microscopy, energy dispersive X-ray microanalysis, X-ray diffraction, and X-ray photoelectron spectroscopy. The electrochemical response was inspected by cyclic voltammetry. Surface imaging shows grain sharpening and extensive loss of the oxide coating, which becomes more marked with the increasing proportion of lattice Cu 2+ ions. Diffraction patterns show the rise in the relative intensity of the underlying Ti reflections and the loss of the cobalt spinel diffraction peaks. Lattice Cu 2+ ions dissolve preferentially and are virtually absent in deactivated mixed oxide anodes. Photoelectron spectroscopy indicates the presence of a highly hydrated Co(II)-containing surface layer. The reduction of the spinel oxide surface layer into an inactive hydrated CoO or a Co(OH) 2 layer after oxidative electrolysis is discussed.