In-situ phase transition induced nanoheterostructure for overall water splitting

Interface-engineering is an effective way to improve the electrocatalytic activities of the electrocatalysts. Multi-shell hollow structures containing multi-component metal ions provide better interface contact to improve the number of active sites of the catalyst itself. Herein, we successfully synthesized NiFe2O4 triple-shell hollow structures by one-step incorporating hydrothermal synthesis method. Hollow multi-shell microspherical NiFeOP nanoheterostructure composed of NiFe2O4, Ni2P and FeP2 is further fabricated by in-situ phase transition from the NiFe2O4 matrix induced by phosphating. The as-designed NiFeOP nanoheterostructure can be used as a bifunctional electrocatalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution with overpotentials of 153 and 217 mV, respectively, to reach a current density of 10 mA cm−2. Such superior electrocatalytic activities can be attributed to the abundant nano interfaces generated in the heterostructure and the rich nanopores on the shell structure, ensuring the large electrochemical surface area, highly-exposed active sites and fast charge transfer for electrocatalysis. In the oxygen evolution reaction, the NiOOH active intermediate was successfully detected. For hydrogen evolution reaction, DFT calculation shows that phosphating can adjust the hydrogen adsorption Gibbs free energy of atoms in the material, and greatly improve the hydrogen evolution performance. Finally, a two-electrode electrolyzer using NiFeOP as both cathode and anode is assembled for overall water splitting, which only requires a potential of 1.57 V to drive current of 10 mA cm−2. The strategy of fabricate electrocatalyst with rich nanoheterostructure provides a good template for future catalyst design and electrocatalytic performance improvements.