Electrochemically Anodized FeCoNi Alloy for Oxygen Evolution Reaction in Alkaline Electrolyte

Most reported FeCoNi ternary composite catalysts are supported, plagued by large catalyst-substrate interfacial charge-transfer resistance and weak adhesion, necessitating in situ derived catalysts. Herein, an FeCoNi alloy-based catalytic electrode is fabricated by potentiostatic anodization in an NH₄F-containing ethylene glycol (EG)/glycerol (GLY) electrolyte with trace water, followed by cyclic voltammetry (CV) activation in alkaline media. The effects of solvent composition and alloy elemental ratio on oxygen evolution reaction (OER) performance are systematically explored. Under optimized conditions (EG:GLY:H₂O = 6:4:0.025 by volume, Fe:Co:Ni = 4:3:3 by molar ratio, 30 V anodization for 1 h, and 15 CV cycles), the electrode delivers excellent OER activity in 1 M KOH, with overpotentials of 170/222 mV at 10/100 mA cm^-2, a Tafel slope of 57.45 mV dec^-1, and satisfactory long-term stability. Anodization forms crystalline fluorides, as well as low-crystalline F-layered double hydroxide (F-LDH) embedded with (Fe, Co, Ni)₃O₄ spinel nanodomains on the alloy substrate surface. These fluoride species supply abundant preactive sites that are converted into highly active LDH-type species during CV activation. Meanwhile, the alloy composition tunes the chemical composition, intrinsic activity, morphology, and ECSA of the anodized layer. Optimizing the Fe:Co:Ni ratio is therefore essential for balancing these factors and maximizing the overall OER activity.