Deciphering\nthe Underlying Mechanism of the Fourth\nEntity in Medium-Entropy NiCoFeMP toward Boosting Oxygen Evolution\nElectrocatalysis

High-/medium-entropy\nmaterials have been explored as promising\nelectrocatalysts for water splitting due to their unique physical\nand chemical properties. Unfortunately, state-of-the-art materials\nface the dilemma of explaining the enhancement mechanism, which is\nnow limited to theoretical models or an unclear cocktail effect. Herein,\na medium-entropy NiCoFeMnP with an advanced hierarchical particle-nanosheet-tumbleweed\nnanostructure has been synthesized via simple precursor preparation\nand subsequent phosphorization. Evaluated as the electrocatalyst for\noxygen evolution reaction (OER), the medium-entropy NiCoFeMnP displays\na lower overpotential of 272 mV at a current density of 10 mA cm<sup>–2</sup>, and more favorable kinetics than the binary NiFeP,\nternary NiCoFeP, quaternary NiCoFeCuP and NiCoFeCrP counterparts,\nand other reported high-/medium-entropy electrocatalysts. Careful\nexperimental analyses reveal that the incorporation of Mn can significantly\nregulate the electronic structure of Ni, Co, and Fe sites. More importantly,\nthe Mn introduction and entropy stabilization effect in the reconstructed\nmetal (oxy)hydroxide simultaneously promote the lattice oxygen mechanism,\nimproving the activity. This work sheds new light on the design of\nhigh-/medium-entropy materials from an in-depth understanding of the\nunderlying mechanism for improving energy conversion efficiency.