Phase-Dependent Effects of Fluorine Doping in Cobalt Oxides for Enhanced Oxygen Evolution Reaction Electrocatalysis

Fluorine modification is widely used to improve cobalt-based oxygen evolution reaction (OER) catalysts, yet its catalytic function is often obscured by simultaneous changes in morphology and crystal structure. Here, we establish a comparative platform using ZIF-67-derived CoO- and Co₃O₄-based catalysts (F-Co₃O₄/N-doped carbon (NC) and F-CoO/NC) to identify the roles of F dopants in different cobalt oxide phases. Experiments and theoretical calculations reveal that in F-CoO/NC, fluorination mainly causes surface etching and disordering, which increases the electrochemically accessible area but has little impact on the intrinsic activity of cobalt sites. In contrast, F dopants can be incorporated into the spinel Co₃O₄ lattice, leading to a higher Co³⁺/Co²⁺ ratio and substantially improved OER kinetics. As a result, F-Co₃O₄/NC achieves an overpotential of 288 mV at 10 mA cm^-2 and maintains stable operation for over 30 h in an alkaline electrolyte. This work elucidates the decisive role of lattice fluorine doping in governing cobalt valence states and establishes an effective anion-doping paradigm for designing high-performance, nonprecious OER catalysts.