Oxygen Reduction and Evolution on Ni-modified Co₃ O₄ (1 1 1) Cathodes for Zn-Air Batteries: A Combined Surface...

The performance of structurally and chemically well-defined Ni-free and Ni-modified single-crystalline Co₃ O₄ (1 1 1) thin-film electrodes in the oxygen reduction and evolution reactions (ORR and OER) was investigated in a combined surface science and electrochemistry approach. Pure and Ni-modified Co₃ O₄ (1 1 1) film electrodes were prepared and characterized under ultrahigh-vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy. Both Ni decoration (by post-deposition of Ni) and Ni doping (by simultaneous vapor deposition of Ni, Co, and O₂ ) induced distinct differences in the base cyclic voltammograms in 0.5 m KOH at potentials higher than 0.7 V compared with Co₃ O₄ (1 1 1) electrodes. Also, all oxide film electrodes showed a higher overpotential for the ORR but a lower one for the OER than polycrystalline Pt. Ni modification significantly improved the ORR current densities by increasing the electrical conductivity, whereas the OER onset of approximately 1.47 V_RHE (RHE: reversible hydrogen electrode) at 0.1 mA cm^-2 was almost unchanged.