Highly Active Bifunctional Electrocatalysts for Oxygen Evolution and Reduction in Zn-Air Batteries

To realize the full performance of Zn-air batteries, the co-presence of a highly efficient oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) in the system is critical. Although copper and nickel are known to be bifunctional catalysts for ORR and OER, sluggish reactions as a result of the exceptionally strong O=O bond on the metal surface make it difficult to achieve high system efficiency. In this study, a metal carbide layer (CuC_x and NiC_x ) on dendritic copper and nickel is fabricated by a facile electrodeposition process to provide efficient catalytic active sites with moderate binding energy for easy electron transfer in both the OER and the ORR. The dendritic structure provides an enriched catalytic surface and the protective metal carbide layer offers an appropriate O binding energy and durability of Zn-air batteries. Owing to the presence of the stable metal carbide surface on the dendritic metal, the CuC_x /Cu and NiC_x /Ni catalysts exhibited well-defined limiting current densities of -5.19 and -5.11 mA cm^-2 , respectively, and improved ORR and OER activities with lower polarization than the corresponding metal catalysts. Density functional theory revealed a 0.74 eV decrease in the overpotential of NiC_x /Ni-catalyzed OER reactions compared with Ni-catalyzed OER reactions. The experimental and theoretical results prove that carbide layers on dendritic metal surfaces can greatly improve the activity of ORR and OER bifunctional electrocatalysts for Zn-air batteries.