Promoting High-Oxidation-State Metal Active Sites in a Hollow Ternary Metal Fluoride Nanoflake Array for Urea Electrolysis.

The adsorption ability of hydrogen, hydroxide, and oxygenic intermediates plays a crucial role in electrochemical water splitting. Electron-deficient metal-active sites can prompt electrocatalytic activity by improving the adsorption ability of intermediates. However, it remains a significant challenge to synthesize highly abundant and stable electron-deficient metal-active site electrocatalysts. Herein, we present a general approach to synthesizing a hollow ternary metal fluoride (FeCoNiF 2 ) nanoflake array as an efficient and robust bifunctional electrocatalyst for the hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). We find that the F anion withdraws electrons from the metal centers, inducing an electron-deficient metal center catalyst. The rationally designed hollow nanoflake array exhibits the overpotential of 30 mV for HER and 130 mV for UOR at a current density of 10 mA cm -2 and superior stability without decay events over 150 h at a large current density of up to 100 mA cm -2 . Remarkably, the assembled urea electrolyzer using a bifunctional hollow FeCoNiF 2 nanoflake array catalyst requires cell voltages of only 1.352 and 1.703 V to afford current densities of 10 and 100 mA cm -2 , respectively, which are 116 mV less compared with that required for overall water splitting.