In Situ Synthesis of Superhydrophilic Amorphous NiFe Prussian Blue Analogues for the Oxygen Evolution Reaction at a High Current Density

Synthesis of efficient and low-cost catalysts for the oxygen evolution reaction (OER) is a pivotal process for large-scale electrocatalytic water splitting to produce hydrogen. Prussian blue analogues (PBAs) prepared by the conventional co-precipitation method, with a less active site density and a poor electrical transport, are often used as precursors for further preparation of PBA derivatives, such as metal oxides, metal alloys, metal phosphides, and so on, due to their poor OER activity. In this report, controllable synthesis of NiFe PBA with Fe2O3 byproducts on a Ni foam substrate was achieved through a facile one-step hydrothermal reaction by adjusting the amount of urea and potassium ferricyanide. After chemical etching and electrochemical activation, NiFe PBA was entirely transformed into amorphous superhydrophilic NiFe PBA (denoted a-NiHCF), which exhibited a remarkable OER performance at a large current density. To drive high current densities of 400 and 800 mA cm–2, only ultralow overpotentials of 280 and 309 mV were required, respectively, which far exceed many recently reported OER catalysts. The superior performance can be attributed to the following: (1) in situ growth on a metal foam substrate can improve the structural stability and provide a faster charge transfer as well as oxygen bubble release; (2) chemical etching allows exposing more surface active sites; (3) an electrochemical activation-induced amorphous surface possesses a larger Brunauer–Emmett–Teller surface area, more high-valent oxidation states, and higher intrinsic OER activity; and (4) the superhydrophilic surface structure is conducive to the adsorption of water molecules. These advantages make a-NiHCF a promising candidate for application in the field of electrocatalytic water splitting.