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

Abstract

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 Co_xN 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 Co_xN 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.