Probing Heteroatomic Dopant-Activity Synergy over Co₃O₄/Doped Carbon Nanotube Electrocatalysts for Oxygen Reduction Reaction

Understanding and predicting how heteroatomic dopants of carbon nanotubes (CNTs)-based catalysts alter their catalytic performance at nanoscale is essential to design superior electrocatalysts for oxygen reduction reaction (ORR). This report describes findings of an investigation of the heteroatomic dopant-activity relationship for Co₃O₄/doped CNTs catalysts with different heteroatoms including N, O, and P atoms in ORR. By using an array of techniques to probe the structure and elementary valence of the catalysts, the incorporation of the Co₃O₄ nanoparticles can introduce defects into the doped CNTs, especially the N-CNTs, which should contribute to the generation of active sites. The Co₃O₄/N-CNTs are shown to exhibit both the highest ORR activity and stability compared with Co₃O₄/O-CNTs, Co₃O₄/P-CNTs, and Co₃O₄/CNTs, manifesting the synergistic correlation of Co₃O₄ nanoparticles, heteroatoms, and CNTs. This kind of synergy is assessed by density functional theory calculations based on the electronic properties and molecular orbitals. It is found that N, O, or P atoms can tune the charge distribution of CNTs by decreasing the lowest unoccupied molecular orbital–highest occupied molecular orbital energy gap, thus activating the adjacent C atoms. And the addition of Co₃O₄ will further redistribute the charge of CNTs from CNTs to Co₃O₄ toward enhanced ORR activity. Moreover, the Co₃O₄/N-CNTs catalyst exhibits a maximum structural stability due to the strong electronic interaction between Co²⁺ ions and N atoms, which is believed to result in its high ORR stability. Analysis of the results, along with a combined theoretical and experimental study, has provided implications for the design of catalysts with controlled activity and stability for ORR.