Integrating cobalt phosphide and cobalt nitride-embedded nitrogen-rich nanocarbons: high-performance bifunctional electrocatalysts for oxygen reduction and evolution

The demand for cost-effective bifunctional oxygen electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for application in rechargeable metal-air batteries and fuel cells operated in alkaline solutions has increased over the decades. We report for the first time an easy procedure for a unique nitrogen-rich sandwich-architectured catalyst (CoNP@NC/NG) as a highly efficient bifunctional electrocatalyst for ORR and OER. Physical characterizations confirmed the coexistence of Co2P and CoxN crystal phases in the nanostructure. The as-prepared CoNP@NC/NG exhibited potent bifunctional electrochemical performance with superior positive onset potential, large kinetic current density, and outstanding stability toward both ORR and OER, thereby showing excellent activities compared with Pt/C and state-of-the-art nonprecious catalysts. The excellent performance could have originated from the robust conjugation between the Co2P and CoxN crystal structures leading to a synergistic effect of the two interfaces, and the carbon shell also increased the number of nitrogen active sites. Moreover, the integrated structure of CoNP@NC/NG provided high electrical conductivity and facilitated electron transfer. Furthermore, the rechargeable zinc-air battery testing of CoNP@NC/NG-700 revealed good performance and long-term stability. The current work provided a new pathway to design bifunctional catalysts with multiple crystal phases for energy conversion and storage.