Effect of doping order on metal-free heteroatoms dual-doped carbon as oxygen reduction electrocatalyst

Based on the differences in electronegativity and atomic radius of metal-free heteroatoms, Herein, we optimize the preparation process by doping order of metal-free heteroatoms to obtain the best electrocatalytic performance through three types of dual-doped carbon, including XC-N (first X doping then N doping), NC-X (first N doping then X doping) and NXC (N and X doping) (X = P, S and F). Metal-free heteroatoms dual-doped carbon has been recognized as one of the most promising Pt/C-substitutes for oxygen reduction reaction (ORR). Herein, we optimize the preparation process by doping order of metal-free heteroatoms to obtain the best electrocatalytic performance through three types of dual-doped carbon, including XC-N (first X doping then N doping), NC-X (first N doping then X doping) and NXC (N and X doping) (X = P, S and F). XC-N has more defect than the other two indicated by Raman spectra. X-ray photoelectron spectrom (XPS) measurements indicate that N and X have been dual-doped into the carbon matrix with different doping contents and modes. Electrocatalytic results, including the potential of ORR peak ( E p ), the half-wave potential, the diffusion-limiting current density mainly follows the order of XC-N > NC-X > NXC. Furthermore, the synergistic effect of second atom doping are also compared with the single doped carbon (NC, PC, SC and FC). The differences in electronegativity and atomic radius of these metal-free heteroatoms can affect the defect degree, the doping content and mode of heteroatoms on carbon matrix, induce polarization effect and space effect to affect O 2 adsorption and product desorption, ultimately to the ORR electrocatalytic performance.