Three-dimensional self-supporting NiFe-X (X = OH, O, P) nanosheet arrays for high-efficiency overall water splitting

Three dimensional (3D) self-supporting materials are a class of promising catalysts for electrochemical water splitting, because 3D materials can not only effectively prevent the catalyst from falling off by the generated vigorous H₂ or O₂ bubbles, but also provide large catalytically active surface areas by designing porous structures. However, the relationship between anion species and their electrocatalytic performances of such materials in alkaline media has not been clearly known. Herein, a simple strategy is used to grow NiFe-based hydroxides/oxides/phosphides (NiFe-X, X = OH, O, P) nanosheet arrays directly on nickel foam (NF) as electrode materials for overall water splitting. We found that the electrocatalytic activity is strongly dependent on the composition of the prepared 3D composites. The electrochemical results reveal that the NiFeP-NF shows the highest electrocatalytic activity for oxygen evolution reaction (OER) with ultra-low overpotentials of 194 and 220 mV to reach the current densities of 20 and 50 mA cm⁻², while the NiFeO-NF has the highest catalytic activity for hydrogen evolution reaction (HER) with an overpotential of 162 mV to reach the current density of 10 mA cm⁻². In addition, both materials exhibit high long-term durability. Based on the excellent catalytic properties for OER and HER, the two catalysts are then assembled as cathode and anode, respectively, and only 1.58 V is needed to achieve the current density of 10 mA cm⁻² for overall water splitting. This work presents a strategy to fabricate excellent OER and HER catalysts for electrochemical water splitting and demonstrates the effects of various anion species on the catalytic activities of transition metal-based nanosheet materials. (paper)