3D Hierarchical MOF-Derived Defect-Rich NiFe Spinel Ferrite as a Highly Efficient Electrocatalyst for Oxygen Redox Reactions in Zinc–Air Batteries

The strategy of defect engineering is increasingly recognized for its pivotal role in modulating the electronic structure, thereby significantly improving the electrocatalytic performance of materials. In this study, we present defect-enriched nickel and iron oxides as highly active and cost-effective electrocatalysts, denoted as Ni_0.6Fe_2.4O₄@NC, derived from NiFe-based metal-organic frameworks (MOFs) for oxygen reduction reactions (ORR) and oxygen evolution reactions (OER). XANES and EXAFS confirm that the crystals have a distorted structure and metal vacancies. The cation defect-rich Ni_0.6Fe_2.4O₄@NC electrocatalyst exhibits exceptional ORR and OER activities (ΔE = 0.68 V). Mechanistic pathways of electrochemical reactions are studied by DFT calculations. Furthermore, a rechargeable zinc-air battery (RZAB) using the Ni_0.6Fe_2.4O₄@NC catalyst demonstrates a peak power density of 187 mW cm^-2 and remarkable long-term cycling stability. The flexible solid-state ZAB using the Ni_0.6Fe_2.4O₄@NC catalyst exhibits a power density of 66 mW cm^-2. The proposed structural design strategy allows for the rational design of electronic delocalization of cation defect-rich NiFe spinel ferrite attached to ultrathin N-doped graphitic carbon sheets in order to enhance active site availability and facilitate mass and electron transport.