Activating and Stabilizing Lattice Oxygen in Spinel NiCo2O4 through F, Mn Dual Doping for Rechargeable Zinc–Air Batteries

Abstract

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are critical to the charge/discharge performance of zinc–air batteries (ZABs). The slow reaction kinetics of OER significantly limit the performance improvement of ZABs. Herein, an ingenious dual-doping strategy was devised, wherein Mn atoms substituted some of the metal ions in NiCo₂O₄ to induce lattice distortion and generate oxygen vacancies. F atoms were then substituted into these vacancies to synthesize NiMn_0.075Co₂O_4-XF_X. By activating and stabilizing lattice oxygen, stable synergistic effects between adsorbate evolution mechanism (AEM) and lattice oxygen mechanism (LOM) are achieved. Without generating any active impurity phases, the intrinsic activity of the catalyst was enhanced. Electrochemical tests exhibit that NiMn_0.075Co₂O_4-XF_X presents a small Tafel slope of 62.80 mV·dec⁻¹, low overpotential of 321 mV at 10 mA·cm⁻² for the OER, which is superior to that of the commercial RuO₂ catalyst. Furthermore, the ZABs assembled by the optimal catalyst (NiMn_0.075Co₂O_4-XF_X) deliver a peak power density of 257.6 mW·cm⁻², which exceeds that of the conventional Pt/C + RuO₂ catalyst. In situ test results indicate that the dual-doping strategy effectively activates lattice oxygen, reducing the voltage required to generate the surface active phase during the OER. Theoretical calculations demonstrate that this dual-element doping strategy shifts the O-2p band center toward the Fermi level and enhances hybridization of the metal 3d orbitals, promoting the evolution of AEM toward LOM. This work provides a strategic research for precisely regulating catalysts to enhance the intrinsic activity.