A highly efficient axial coordinated CoN5 electrocatalyst via pyrolysis-free strategy for alkaline polymer electrolyte fuel cells

Abstract

Atomically dispersed Co-N-C materials represent the promising alternatives to precious metal-based catalysts for oxygen reduction reactions (ORR), due to the abundance and low cost of their constituent elements with maximized atom utilization. However, boosting their performances by modulating the active site structure still remains a challenge. Herein we have reported a pyrolysis-free synthetic method to prepare a new ORR catalyst with Co-N5 active sites by using nitrogen-functionalized reduced graphene oxide to anchor Co-Porphyrin molecules and provide the axial ligand for the cobalt center. The ORR half-wave potential of the obtained catalyst reached 0.908 V, and the maximum power density was significantly increased by 1.63 times compared with the catalyst with traditional Co-N4 active sites in alkaline polymer electrolyte fuel cells (APEFCs). Combined with density functional theory (DFT) calculations and electrochemical analysis, the results show that the electronic and geometric structure are remarkably changed after the Co 3d orbitals are rehybridized with the axially coordinated ligand orbitals, which greatly increases the rate of ORR. Importantly, in-situ Raman spectroscopy can dynamically track the conversion of Co-OH and Co-O species, further revealing that Co-N5 is the active site and electron localization strategy induced by axial Co-N coordination can enhance the O2 adsorption and activation, thus boosting ORR performance. Therefore, our research provides an atomic-level insight into the relationship between the electronic structure of the active center and the ORR performance and guidance for the rational design of high-efficiency electrocatalysts.