Magneli phase Ti4O7 electrode for oxygen reduction reaction and its implication for zinc-air rechargeable batteries

In this paper, Magneli phase Ti4O7 was successfully synthesized using a TiO2 reduction method, and characterized using X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The electrode coated with this Ti4O7 material showed activities for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). For the ORR, several parameters, including overall electron transfer number, kinetic constants, electron transfer coefficient, and percentage H2O2 production, were obtained using the rotating ring-disk electrode (RRDE) technique and the Koutecky–Levich theory. The overall electron transfer number was found to be between 2.3 and 2.9 in 1, 4, and 6 M KOH electrolytes, suggesting that the ORR process on the Ti4O7 electrode was a mixed process of 2- and 4-electron transfer pathways. Electrochemical durability tests, carried out in highly concentrated KOH electrolyte, confirmed that this Ti4O7 is a stable electrode material, suggesting that it should be a feasible candidate for the air-cathodes of zinc-air batteries. To understand the stability of this material, Raman and XPS spectra were also collected for the Ti4O7 samples before and after the stability tests. The results and analysis revealed that a thin layer of TiO2 formed on the Ti4O7 surface, which may have prevented further oxidation into the bulk of the Ti4O7 electrode.