Insight into the Active Contribution of N-Coordinated Cobalt Phosphate Nanocrystals Coupled with Carbon Nanotubes for Oxygen Electrochemistry

Abstract

The exploration of efficient nonprecious bifunctional electrocatalysts toward both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly desirable for the development of the rechargeable metal–air battery. Herein, “diamond necklace”-like N-coordinated cobalt phosphate nanocrystals coupled with carbon nanotubes (CoPiC-DP/CNTs) are synthesized through the facile hydrothermal-carbonization approach and employed as the bifunctional electrocatalyst for ORR and OER. The CNTs act like the “chain” as support, the “diamond”-like N-coordinated cobalt phosphate nanocrystals serve as catalytic active sites, and the N-doped carbon derived from the phosphonate ligand-like “linker” can not only protect the “diamond” from leaching but also promote the mass transfer. Such advanced structure and composition features endow CoPiC-DP/CNTs with impressive ORR and OER electrochemical activities. To probe the active contribution of CoPiC-DP/CNTs toward ORR, a series of cobalt phosphate–carbon hybrids are prepared by adjusting the types of phosphonate ligands of the cobalt phosphonate precursor to rationally control their active components. The comprehensive investigation from structural characterizations and electrochemical measurements of these cobalt phosphate–carbon hybrids verifies that both N-coordinated cobalt sites and the suitable carbon substrate are essential for the enhanced electrocatalytic performance. Furthermore, the liquid- and solid-state zinc–air batteries with these fabricated CoPiC-DP/CNTs employed as the air cathode exhibit decreased charge–discharge potential gaps, high power density, and considerable cycling durability, which are beneficial for the potential development of flexible/portable electronic devices.