Thorn-shaped NiCo2O4 nanoparticles as multi-functional electrocatalysts for electrochemical applications

Identification of the Active Sites of NiCo2O4 and the Support Effect with Carbon Nanotubes for Oxygen Reduction Catalysis

Since the active site of spinel NiCo₂O₄ as an oxygen reduction reaction (ORR) catalyst is under debate, the optimal active sites of NiCo₂O₄ are identified by investigating the catalytic properties of two models of NiCo₂O₄. The results indicate that the ORR activity of five studied active sites of isolated NiCo₂O₄ is primarily limited by excessively binding of *OH, and the Co^III and Ni^III active sites of Co²⁺[Co³⁺Ni³⁺]O₄ are identified as the optimal catalytic sites with overpotentials of 0.69 and 0.76 V, respectively. Moreover, for the investigation of the support effect, the isolated NiCo₂O₄ is supported on pristine and N-doped carbon nanotube (CNT and NCNT). Encouragingly, the ORR activity of Co^III and Ni^III active sites is improved after NiCo₂O₄ is supported on CNT and NCNT due to the weakened binding of *OH. When NiCo₂O₄ is supported on NCNT via the Co^II ion, the rate-determining step of ORR catalyzed by Co^III and Ni^III sites is altered. Moreover, this Co^III site exhibits the highest ORR activity and displays an overpotential of only 0.45 V. Therefore, the support effect can improve the catalytic activity as well as change the mechanism of ORR. This study presents more insights into the active sites and ORR activity of NiCo₂O₄.