Regulation of Reconstruction Sites in Spinel NiCo 2 O 4 via Morphology Modulation to Enhance OER Performance

Study of dynamic reconstruction of active sites in spinel oxide-catalyzed oxygen evolution reactions (OER) is of great importance; however, the mechanism of how morphology modulation affects surface reconstruction and the performance of OER is still not well understood. Herein, the effect of morphology modulation (nanorods, nanoflowers, and hierarchical nanoflowers) in spinel NiCo 2 O 4 on the kinetic process and surface reconstruction of metal active sites during OER was studied. Through in situ Raman spectral and EIS characterizations, we found that the metal active sites in NiCo 2 O 4 with different morphologies underwent a surface reconstruction at high potentials, which resulted in faster *OH adsorption than deprotonation. Among them, NiCo 2 O 4 -NR exhibited dominantly the [M-O 4 ] reconstruction sites, whereas NiCo 2 O 4 -HNF demonstrated mainly [M-O 6 ] sites, which would accelerate *OH adsorption due to the abundant vacancies and hierarchical structure at a lower potential, thus enabling a surface reconstruction. The DFT calculations further revealed that [M-O 6 ] in NiCo 2 O 4 -HNF is the primary adsorption site, while the active site is mainly [M-O 4 ] in NiCo 2 O 4 -NR and NiCo 2 O 4 -NF. Also, the OER theoretical overpotential of HNF (0.40 V) is lower than that of NR (0.83 V) and NF (0.51 V). Overall, NiCo 2 O 4 -HNF demonstrates a better OER performance (η 10 = 236 mV) than NiCo 2 O 4 -NR (η 10 = 366 mV) and NiCo 2 O 4 -NF (η 10 = 302 mV). In summary, this work suggests that morphology modulation in hierarchical structures might be a viable way for improving the performance of electrocatalysts in OER design.