Carbon-supported Co_1.67Te₂ nanoparticles as electrocatalysts for oxygen reduction reaction in alkaline electrolyte

Well-dispersed Co_1.67Te₂ nanoparticles supported on carbon black have been synthesized via a solid-state reaction using Co and Te precursors in an autoclave at elevated temperatures. Their oxygen reduction reaction (ORR) activity and selectivity as a function of heating temperatures in catalyst synthesis were evaluated by rotating disk (RDE) and ring-disk electrodes (RRDE). It was found that the best performing catalyst (CoTe/C-900) was synthesized at a temperature of 900 °C, with regard to the most positive RDE onset (-0.18 V vs Ag/AgCl) and half-wave potentials (-0.35 V vs Ag/AgCl) as well as the lowest peroxide yield (ca. 5%) in alkaline solution (0.1 M KOH, pH = 13). Meanwhile, well-defined limiting currents were reached in the mass transfer-controlled potential range at various rotating speeds, attesting to the high density and uniform distribution of ORR active sites on the catalyst. The average electron transfer number of ORR was determined to be 3.5 for the CoTe/C-900 catalyst by using a modified Koutecky-Levich equation, nearly providing a four-electron pathway for the ORR. A transition of the Tafel slope from ca. -60 mV/dec to ca. -120 mV/dec with overpotential is directly associated with oxide formation and their coverage variation onto catalysts, suggesting a change of the rate-determining step in the ORR mechanism from intermediate-migration to charge-transfer. Extensive physical characterizations including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), and X-ray photoelectron spectroscopy (XPS) were conducted for each CoTe/C sample prepared at various heating temperatures to provide insights into the origins of active sites, and Co_1.67Te₂ chalcogenide nanoparticles supported on carbon were found to be highly active toward ORR in alkaline electrolytes.