Molybdenum and fluorine co-doping induces lattice oxygen activation in Ni-Fe spinel oxides for enhanced oxygen evolution 

The oxygen evolution reaction (OER) is a critical process in electrochemical water splitting, yet challenging in activation of lattice oxygen oxidation mechanism (LOM) for cost-effective transition metal oxides, in which strong metal-oxygen (M-O) bonds inherently inhibit lattice oxygen reactivity. Here, we design a molybdenum/fluorine (Mo/F) co-dopant in NiFe₂O₄ spinel to engineer the electronic structure via an LOM pathway. The incorporation of high-valence Mo and highly electronegative F collaboratively optimizes the electronic configuration of Ni/Fe sites, facilitating the formation of stable high-valent metal species and effectively weakening the M-O bonds. This synergy not only results in faster OER kinetics but also promotes oxygen vacancy formation, thereby enabling direct lattice oxygen involvement. Real-time ¹⁸O-labeled differential electrochemical mass spectrometry (DEMS) coordinates with in-situ electrochemical impedance spectroscopy conclusively verify the activation of the LOM. The Mo/F-NiFe₂O₄ catalyst exhibits outstanding OER performance, requiring low overpotentials of 247 and 311 mV to achieve current densities of 50 and 100 mA cm^-2, respectively. Remarkably, it demonstrates exceptional durability in seawater electrolytes, operating steadily for over 300 h at a high current density of 100 mA cm^-2. This work provides a general and effective doping strategy to activate the LOM in robust oxide catalysts, paving the way for efficient hydrogen production from both pure water and seawater resources.