Intrinsic Electrocatalytic Activity Regulation of R‑P Perovskite La_1.2Sr_0.8Ni_0.6Fe_0.4O_4+δ by Low-Temperature Fluorination

Ruddlesden–Popper (R-P) perovskite oxides have attracted much attention as highly active and stable bifunctional materials for the oxygen evolution reaction (OER)/oxygen reduction reaction (ORR) in alkaline solutions due to the nonuse of precious metal elements. Herein, a triple (H⁺, O^2–, and electron) conductive R-P perovskite oxide, La_1.2Sr_0.8Ni_0.6Fe_0.4O_4+δ, was prepared, and the valence state of transition metal cations and highly oxidized oxygen (O^–/O₂^2–) in the structure was tuned by a low-temperature fluorine substitution treatment. The homogeneous distribution of the fluorine elements across the particles of the R-P perovskite oxide after its fluorination was confirmed by high-resolution transmission electron microscopy (HRTEM) images. By regulation of the amount of highly oxidative state oxygen species and the valence state of transition metal cations in the R-P perovskite structure, the material exhibits a significant enhancement for both the OER and ORR electrocatalytic activities. The fluoridated La_1.2Sr_0.8Ni_0.6Fe_0.4O_4+δF_<i>y</i> (LSNF-OF) achieves a low OER overpotential of 308.1 mV in a 1 M KOH electrolyte at a current density of 10 mA cm^–2. This is superior to both commercial Co₃O₄ and the pristine sample without fluorination. The LSNF-OF electrode in an aqueous Zn-air battery (ZAB) exhibits a peak power density of 19.15 W g^–1 at a current density of 24 mA g^–1. The low-temperature trace fluorination can enhance the electrocatalytic efficiency of perovskite oxides. This technique can be applied to various types of metal oxides.