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+]O-4 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 CoII ion, the rate-determining step of ORR catalyzed by Co-III and Ni-III sites is altered. Moreover, this CoIII 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.