A Carbon-Free Ag–Co 3 O 4 Composite as a Bifunctional Catalyst for Oxygen Reduction and Evolution: Spectroscopic, Microscopic and Electrochemical Characterization

Abstract

A key challenge for rechargeable metal–air batteries is the development of a cost-effective bifunctional catalyst for both oxygen evolution (OER) and reduction (ORR) reactions. Here, we took the advantages of high OER activity of Co₃O₄ spinel and high ORR activity of Ag to develop a carbon-free oxygen electrode, e.g., for Li–air batteries. The optimized Ag + Co₃O₄ catalyst was further characterized and exhibited a good bifunctional activity in alkaline media. From rotating ring-disk electrode results, the mixed Ag + Co₃O₄ catalyst revealed significantly lower (∼320 mV) overpotential for ORR than single Co₃O₄, and a slightly lower overpotential than pure Ag. A four-electron pathway was also elucidated. The OER activity of the mixed catalyst is 1.5-fold compared to pure Co₃O₄, although the Co₃O₄ loading is only 10%, suggesting a large synergistic effect. The potential difference between OER and ORR (i.e., the sum of the overpotentials at 1 mA cm⁻²) is ca. 0.85 V, which is comparable to noble metal based catalysts. To better understand the origin of this synergism, an XPS analysis was performed, demonstrating that only after oxidation of the mixed catalyst, Co₃O₄ was reduced to Co(OH)₂ at potentials of the ORR, probably due to the presence of Ag⁺. This redox switching, which was not observed for pure Co₃O₄, is a probable explanation for the increased catalytic activity. The morphology and the electrochemically active surface area of Ag on the surface were examined by electron microscopy and lead-underpotential deposition, respectively. These results also show that when 88% of the Ag surface is blocked by Co₃O₄ particles, the residual 12% free Ag sites altogether have a higher activity for ORR than the (100%) pure Ag surface, i.e., the activity per Ag site is increased by more than a factor of 10. The combination of low cost and high performance endows this catalyst as a promising candidate for energy devices, and the present synergistic effect opens a new track for high activity.

Graphical Abstract