Electrodeposition of Crystalline Co₃O₄—A Catalyst for the Oxygen Evolution Reaction

Abstract

Crystalline films of Co3O4 are deposited by electrochemically oxidizing a tartrate complex of Co2+ in an aqueous, alkaline solution at elevated temperatures. The crystallinity and stability of the films are a strong function of the deposition temperature. Films deposited at temperatures from 50 to 90 °C are amorphous, but films deposited from refluxing solution at 103 °C are crystalline. The crystalline films adhere strongly to the substrate, whereas the amorphous films peel off of the substrate when dried due to drying stresses. The crystalline films deposit with the normal spinel structure, with a lattice parameter of 0.8097 nm and crystallite size of 26 nm. The catalytic activity of Co3O4 for the oxygen evolution reaction (OER) of the crystalline and amorphous films is compared by Tafel analysis in alkaline solution at pH 14. The crystalline Co3O4 film has a Tafel slope of 49 mV/decade and an exchange current density of 2.0 × 10–10 A cm–2, whereas an amorphous film deposited at 50 °C has a Tafel slope of 36 mV/decade and an exchange current density of 5.4 × 10–12 A cm–2. Because the films deposited from refluxing electrolyte deposit directly as crystalline films, it is possible to deposit them epitaxially on single-crystal Au(100). This opens up the possibility to study the catalytic activity of different Co3O4 planes exposed to the electrolyte.