UHV preparation and electrochemical/-catalytic properties of well-defined Co– and Fe-containing unary and binary oxide model cathodes for the oxygen reduction and oxygen evolution reaction in Zn-air batteries

Abstract

• Coupled surface science/electrochemistry characterization of mixed oxide electrodes. • Oxygen reduction and oxygen evolution on well-defined Co– and Fe-containing oxides. • STM and XPS show atomically flat unary and binary oxide thin film electrodes. • Co doping improves OER and overall activity of Fe 3 O 4 electrodes. • Activity order: Fe 3 O 4 (0 0 1) < CoFe 2 O 4 /Fe 2 O 3 (1 1 1) < CoFe 2 O 4 /CoO(1 1 1) ≪ CoO(1 1 1) The performance of structurally and chemically well-defined single-crystalline cobalt- and iron-containing mixed oxide thin film model electrodes as bifunctional catalyst in the oxygen reduction and oxygen evolution reactions (ORR and OER) was investigated and compared with those of unary CoO(111) and Fe 3 O 4 (001) oxides in a combined surface science and electrochemistry approach. Pure and mixed cobalt- and iron- containing film electrodes were prepared by vapor deposition in an O 2 atmosphere and characterized under ultrahigh vacuum (UHV) conditions by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Electro-chemical/catalytical measurements were performed in an electrochemical cell directly coupled to the UHV system. XPS measurements of mixed binary oxides with different Co:Fe atomic ratios reveal solely Co 2+ and Fe 3+ states, pointing to CoFe 2 O 4 in combination with excess Co or Fe either in a CoO or a Fe 2 O 3 phase. For the CoFe 2 O 4 /CoO binary metal oxide electrodes the base CVs in 0.5 M KOH show clear differences compared to Fe 3 O 4 (001) and the CoFe 2 O 4 /Fe 2 O 3 electrodes at potentials > 0.5 V, reflecting the formation of Co 2+ to Co 3+ transition. The mixed cobalt- and iron-containing thin film electrodes show a higher overpotential for the OER than pristine CoO(111); it is, however significantly lower compared to magnetite Fe 3 O 4 (001). Together with changes in the ORR performance Fe doping is found to lower the overall efficiency as bifunctional catalyst, as compared to the pure CoO(111) and Co 3 O 4 (111) electrodes, but it is much higher compared to that of a pristine Fe 3 O 4 (001) electrode.