Single-Atom Ir Reserved in Lower Oxidation State by Divalent Iron for Efficient Alkaline Water Oxidation

Ir in a lower oxidation state exhibits superior intrinsic activity for oxygen evolution reaction (OER) due to its low Ir-O connectivity in alkaline media. However, it is more prone to being oxidized to a high oxidation state and dissolving inactivation during the OER process. Herein, single-atom Ir in a lower oxidation state is dispersed on a sea urchin-like FeNi-based spinel support (NiFe₂O₄) possessing super-hydrophilicity and super-aerophobicity. The as-prepared Ir/NiFe₂O₄, loading of only 0.087 mg_Ir cm^-2 Ir element, shows 196 mV overpotential at 10 mA cm^-2, and it can work stably up to 400 h at 200 mA cm^-2 in 1 m KOH solution. The OER performance of Ir/NiFe₂O₄ surpasses that of commercial IrO₂ catalysts and most currently reported Ir/Ru-based single-atom catalysts (SACs), which can be attributed to Fe²⁺ in the support acting as an electron reservoir, maintaining the lower oxidation state of single-atom Ir with high activity and stability during the OER process. Furthermore, the catalytic interface exhibits both super-hydrophilicity and super-aerophobicity, which not only facilitates electron-ion coupling transport but also promotes the rapid removal of generated oxygen bubbles.