Highly Electron-Deficient Ternary Metal Fluoride Nanocages for Overall Urea-Assisted Water Electrolysis

Abstract

Transition-metal fluoride has recently emerged as a promising metal-ion battery cathode; however, it has rarely been reported as an electrocatalyst, especially for the urea oxidation reaction (UOR). Herein, we report a facile and scalable approach for in situ transformation of polyhedron ZIF-67 nanocrystals into ternary metal fluoride nanocages as a bifunctional electrocatalyst for the overall urea-assisted water electrolysis. Metal-fluoride bonding offers the tunable binding abilities of CO* and NH* intermediates due to the highly electron-deficient metal center, accelerating both the hydrogen evolution reaction and UOR activities. As expected, the as-synthesized FeCoNiF₂ nanocages show a considerable improvement in the catalytic performance after the embedment of the fluorine component. For UOR, the FeCoNiF₂ nanocage catalysts achieve a 10 mA cm^–2 at low overpotential of 140 mV, which is considerably lower than that of oxygen evolution reaction. Operando Raman spectroscopy proved that a catalytic active NiOOH phase is evolved from the FeCoNiF₂ surface during the anodic reaction. Furthermore, a two-electrode cell constructed from FeCoNiF₂ as the anode and Pt/C as the cathode also shows an excellent urea-assisted overall water splitting activity, which requires only 1.4 and 1.63 V to reach 10 and 100 mA cm^–2. This discovery provides a promising strategy for developing effective urea electrolysis for large-scale hydrogen production.